Tuning the photophysical properties of ESIPT active unsymmetrical azine dyes by the change in the substituent and solvent: TD-PBE0 and TD-CAM-B3LYP studies

Abstract

In this study, the effects of the substituent and solvent on the photophysical properties of the designed ESIPT active as well as donor–acceptor structured unsymmetrical azine dyes L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃ and –CN, respectively) were investigated at PBE0/6-31++G(d,p) and CAM-B3LYP/6-31++G(d,p) levels of theory in the gas phase and three solvent media. The structural parameters, relative energies, vibrational spectra, photophysical properties, potential energy curves, natural bond orbital (NBO) charges, charge transfer (CT) indices, electron density properties, and reduced density gradient (RDG) spikes were computed. The results of vibrational spectra, structural parameters and electron density analysis demonstrated that the O–H⋯N H-bonding interaction is strengthened in all dyes upon photoexcitation from the S₀ to S₁ state which can facilitate the ESIPT process. All dyes exhibited both enol and keto emissions, in good agreement with the reported experimental results. The largest Stokes shift for keto emissions in solvent media was observed in MeOH solvent and is in the order 143 nm (L5) > 138 (L4) > 133 (L1) > 120 (L3) > 115 (L2) at the PBE0/6-31++G(d,p) level of theory. Introducing electron-withdrawing groups can increase the absorption and emission wavelengths as well as the red shift in fluorescence emission of L4 and L5, but hinder the occurrence of the ESIPT process compared with L2 and L3. The results demonstrated that the substituent effect is more significant in changing the molecular optical properties than the solvent effect. Our designed ESIPT molecules can simultaneously show enol and keto emissions and thus can be regarded as candidates to design single-molecule white-light emission materials.

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Design, System, Application

Azines are a class of Schiff base compounds that undergo a wide variety of chemical processes and have interesting optical properties. Particularly, ESIPT-active symmetrical and unsymmetrical azines coupled with AIE units have been developed and applied in cell imaging, fluorescent probes for the detection of cations, antibacterial activity and textile applications. The photophysical properties of azine dyes can be tuned by substitution on the ESIPT moiety. In this work, we have utilized a quantum engineering strategy for designing a suitable molecular photo-switch based on azine derivatives. It was found that all dyes exhibit double enol and keto fluorescence emissions.

1. Introduction

Luminescent organic materials exhibiting the unique excited-state intramolecular proton transfer (ESIPT) process have technological and biological roles in applications such as fluorescent probes,^1–7 organic light-emitting diodes (OLEDs),^8–11 laser dyes^12,13 and drug delivery systems (DDSs) or bioactive donors.^14,15 Since the ESIPT mechanism was first reported in an experimental study in methyl salicylate by Weller,¹⁶ numerous investigations have been performed experimentally and theoretically due to its special photophysical and photochemical properties.^17–21 ESIPT molecules feature an intramolecular hydrogen bond between the proton donor (–OH, –NH₂) and the proton acceptor (–C=O, –C=N–) groups close to each other allowing proton transfer to occur upon photoexcitation. Generally, ESIPT is a kind of enol–keto phototautomerization in which the excited molecule undergoes a proton transfer process, resulting in the formation of a new tautomeric form. Some ESIPT chromophores show dual emissions, short wavelength emission due to the enol form (normal emission), and a longer one due to the keto form (ESIPT emission) through the photocycle. The single ESIPT happens in a four-level photocycle including absorption, ESIPT, emission, and ground-state intramolecular proton transfer (GSIPT). Large Stokes shifts, tunable and dual emission, ultrafast processes, and spectral sensitivity to the environment conditions are the desirable properties of ESIPT fluorophores.^22–25 ESIPT fluorophores have long been attractive as solid-state emitters and this interesting characteristic can be achieved by the introduction of ESIPT centers on the core of aggregation-induced emission (AIE) dyes.²⁶

Azines are a class of Schiff base compounds that undergo a wide variety of chemical processes and have interesting optical properties.^27,28 Particularly, ESIPT-active symmetrical and unsymmetrical azines coupled with AIE units have been developed and applied in cell imaging,^29,30 fluorescent probes for the detection of cations,^31,32 antibacterial activity³³ and textile applications.³⁴ Both ESIPT and AIE processes can be found in salicylaldehyde azines and Schiff bases, leading to efficient emission and large Stokes shifts in the solid state. Tong et al.³⁵ have developed an alternative building block, keto-salicylaldehyde azine (KSA), for constructing various AIEgens via an ESIPT process to detect some cellular organelles and specific metal ions. They have shown that their intrinsic electronic structure can be easily affected by the asymmetric substitution effect. Mathivanan and coworkers designed and synthesized donor–acceptor-structured triphenylamine (TPA) functionalized unsymmetrical azines containing various salicylaldehyde derivatives which exhibited intramolecular charge transfer (ICT), ESIPT and tuned multi-color AIE features.³⁶ By changing the electron-donating ability at the ESIPT moiety, they could tune the optical properties of these newly synthesized molecules. All compounds exhibited the AIE behavior in a THF/water mixture and the ESIPT. In addition, different emission colors in the aggregated state were observed by the tuning of a peripheral substituent in the salicylaldehyde moiety. The crystal structure of the molecules confirmed the existence of strong intramolecular hydrogen bonding between the imine nitrogen C=N and phenolic OH groups, which promotes the ESIPT process in the excited state. In addition, Mathivanan et al.³³ have also reported the synthesis of four donor–acceptor structured symmetrical triphenylamine supported bis unsymmetrical azine derivatives. They have found that all compounds show interesting solvatochromic features and substituent-dependent multi-color aggregation-induced emission behavior in a THF–water mixture. The experimental results confirmed AIE behavior in the compounds and it might be due to the restriction of the intramolecular rotation (RIR) mechanism.

The excited state hydrogen bonding dynamics and excited state intramolecular proton transfer (ESIPT) mechanism of a salicylaldehyde derivative with a para-position electron-withdrawing cyano group (CN-SAA) have been explored.³⁷ It has been demonstrated that dual intramolecular hydrogen bonds of CN-SAA can be enhanced in the S₁ state and charge reorganization plays an important role in promoting ESIPT behavior for CN-SAA. In another study, four organic fluorophores with two intramolecular H-bonding functionalities and unsymmetrical structures (salicylaldehyde–imine and benzophenone–imine) have been designed and explored.³⁸ All dyes showed tunable solid-state fluorescence between 536 and 645 nm dependent on inter/intramolecular H-bonding interaction. Four novel salicylaldehyde–diphenyl–azine (SDPA) skeleton-based luminogens with tunable emission have been synthesized by Jain et al.³⁹ The photophysical studies in different solvents elucidated their ESIPT and AIE characteristics. Also, this study demonstrated that changing the substituents on the salicylaldehyde does not affect the selectivity and sensitivity towards Cu²⁺ ions. Instead, this significantly tunes emission properties towards the longer wavelength region. Recently,⁴⁰ a comprehensive photophysical study of a novel unsymmetrical monosubstituted salicylaldehyde azine-based organic functional molecule named 4-chloro-2-((E)-((E)-(pyren-1-ylmethylene)hydrazono)methyl)phenol (PHCS) has been reported. A prevalence of the synergistic effect of aggregation-induced emission enhancement (AIEE) and ESIPT in the PHCS molecular system has been rationalized at extremely high water concentrations compared to organic solutions with low to moderate water concentrations.

However, the information on ESIPT dynamics and the effect of the substituent as well as solvent on the ESIPT process of unsymmetrical azine derivatives cannot be provided by the current spectroscopic techniques. Computational chemistry as a reasonable and reliable tool can be used to deeply explore the mechanism of ESIPT at the molecular level. Theoretical studies on the mechanism of ESIPT-based chromophores not only help the researcher to understand the underlying mechanism for tuning the fluorescence behavior but also provide a theoretical guide for further developing new ESIPT-based dyes. Despite the reported experimental research on salicylaldehyde azine-based ESIPT dyes, based on the best of our knowledge, there was no detailed study conducted on the mechanism of ESIPT and the origin of the changes in photophysical properties of unsymmetrical azines containing various salicylaldehyde derivatives as the acceptor group and triphenylamine as the donor one in various solvents. The study of the ESIPT mechanism can provide insights into the design of materials with desired electronic and photophysical properties. Accordingly, in this work, we have investigated the excited state mechanism and photophysical properties of five ESIPT-based salicylaldehyde azine derivatives.

We have previously reported the influence of different substituents on the ESIPT reactions of a series of coumarin–benzothiazole derivatives based on theoretical calculations.⁴¹ In this study, we have explored the ESIPT mechanism and the effect of the substituent and solvent on the photophysical properties of the designed L1–L5 azine-based dyes (Scheme 1) by using the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT) approaches at PBE0/6-31++G(d,p) and CAM-B3LYP/6-31++G(d,p) levels of theory in the gas phase and three solvents with various polarities. In the main synthesized³⁶ molecule L1, triphenylamine is a well-known π-electron-donating group with high donor tendency, and salicylaldimine (SAI) is a common acceptor as well as an ESIPT-active unit. All the compounds possess the main donor (TPA)–acceptor (SAI) structure with additional substituents (–H, –NH₂, –OCH₃, –CF₃, and –CN) on the proton donor part. In the designed L2–L5 molecules, since the electron-donating TPA group is located on the proton acceptor side, the introduction of electron-donor and acceptor substituents at the proton donor side can more significantly cause the push/pull substituent effect on electrons, thereby tuning the ESIPT reaction as well as photophysical properties. The study of the ESIPT mechanism can provide insights into the design of materials with desired electronic and photophysical properties. The purpose of the current work is to design and develop azine-based ESIPT molecules with potential applications in light-emitting diodes, optical sensors and optoelectronic devices. In addition, our designed ESIPT molecules can simultaneously show two emission bands, including enol-form and keto-form emissions, and thus can be regarded as candidates to design single-molecule white-light emission materials owing to the combination of the dual emission of enol/keto forms.

Scheme 1 Eno1–keto tautomerism and chemical structures of L1–L5 compounds.

2. Computational details

Time-dependent density functional theory (TD-DFT) has become a popular tool for computing the signatures of electronically excited states, and more specifically, the properties directly related to the optical (absorption and emission) spectra of molecules.⁴² The density functional theory^43,44 and the time-dependent density functional theory (TDDFT)⁴⁵ have been employed in the S₀ and the S₁ states, respectively. In this work, the experimental data of absorption and emission for L1 (ref. 36) were considered as a benchmark to validate CAM-B3LYP⁴⁶ and PBE0 (ref. 47) functionals in conjunction with the 6-31++G(d,p) basis set.⁴⁸ The ground and first excited state optimized geometries were computed by using the PBE0 (consisting of 25% of the exact exchange of HF) and CAM-B3LYP (long-range-corrected version of the B3LYP hybrid functional consisting of 19–65% of the exact exchange of HF) functionals in conjunction with the 6-31++G(d,p) basis set using the Gaussian 16 program package.⁴⁹

The selection of exchange–correlation (XC) functionals incorporating different fractions of Hartree–Fock (HF) exchange is based on the recognized influence of HF exchange on excitation energies^50–52 and oscillator strengths.^52–55 The CAM-B3LYP functional is a long-range-corrected version of the B3LYP hybrid functional, while PBE0 mixes the PBE exchange energy and Hartree–Fock exchange energy in a set 3 : 1 ratio, along with the full PBE correlation energy. The main difference between the two functionals is the way they handle the exchange–correlation energy. CAM-B3LYP uses a different switching function than B3LYP, while PBE0 mixes the PBE exchange energy and Hartree–Fock exchange energy. Both functionals have been shown to be accurate in DFT calculations, but the choice of functional depends on the specific system being studied and the desired level of accuracy. The reported photophysical results revealed that for both local n → π* and π → π* transitions, global hybrids such as PBE0 will generally provide accurate estimates whereas for CT or Rydberg EES, it is mandatory to use range separated hybrids (recommended CAMB3LYP or wB97X-D) to reach physically meaningful estimates.^42,45 Therefore, a direct comparison of the results of CAM-B3LYP and PBE0 functionals can provide insights into the application of these functionals in luminescence property calculations.

The absorption and emission spectra were calculated by using both functionals at TD-DFT/6-31++G(d,p)//DFT/6-31++G(d,p) and TD-DFT/6-31++G(d,p) levels of theory, respectively. The vertical absorption E(vert-abs) and vertical fluorescence E(vert-flu) energies are calculated by the following equations:⁴²

E(vert-abs) = E^EES(R^GS) − E^GS(R^GS) (1)

E(vert-flu) = E^EES(R^ES) − E^GS(R^ES) (2)

where E^EES(R^ES) is the energy of the excited state at the ground state geometry, E^GS(R^GS) is the energy of the optimized ground state, E^EES(R^ES) is the energy of the optimized excited state and E^GS(R^ES) is the energy of the ground state at the excited state geometry.

Dimethylsulfoxide (DMSO: ε = 46.826), methanol (MeOH: ε = 32.613) and toluene (ε = 2.3741) solvents were selected to study the effect of solvent polarity on the absorption and fluorescence radiation based on the polarizable continuum model (PCM).⁵⁶ To further investigate the ESIPT process, the potential energy curves (PECs) along the proton transfer path were constructed by freezing the O–H coordinate to a series of values and then optimizing the rest of the coordinates using the PBE0 method at the S₀ state and the TD-PBE0 method at the S₁ state.^42,57,58 To characterize the hydrogen bonds with different strengths, the quantum theory of atoms in molecules (QTAIM) analysis^59,60 was performed at the PBE0/6-31++G(d,p) level of theory by using the AIM2000 program package.⁶¹ Besides, for revealing the intramolecular hydrogen bonding interactions in real space and to compare the hydrogen bond strength, the reduced density gradient (RDG) isosurfaces⁶² were calculated using Multiwfn software⁶³ and plotted using the (VMD) program.⁶⁴ Analysis of charge transfer during electron excitation based on the electron density difference (EDD)⁶⁵ was done using Multiwfn software. The natural bond orbital (NBO) analysis^66,67 was carried out at the PBE0/6-31++G(d,p) level to realize the charge distributions during the ESIPT process.

3. Results and discussion

3.1. Geometric structures

Fig. 1 shows the geometric structures of azine derivatives L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) in enol (E) and keto (K) forms at S₀-E, S₁-E and S₁-K states at the PBE0/6-311++G(d,p) level. Intramolecular hydrogen bond (IHB) interactions are the important driving force of the ESIPT reaction in a molecule. Thus, for a deeper understanding of the proton transfer behavior, the main parameters such as the bond length and bond angle involved in the hydrogen bonds in the S₀ and S₁ states were calculated and are reported in Table 1. For L1–L5, the O–H bond length at the S₀-E state is in the order L3 (0.998 Å) > L2 (0.997 Å) > L4 and L5 (0.996 Å) > L1 (0.994 Å). The order of the O–H⋯N H-bonding distance is L1 (1.724 Å) > L4 (1.717 Å) > L5 (1.715 Å) > L2 (1.712 Å) > L3 (1.709 Å). The δ(O–H⋯N) bond angle can be arranged in the following order: L2 (147.8°) > L3 (147.7°) > L1 (147.2°) > L5 and L4 (147.1°). Based on the structural parameters, the strength of IHB interaction in the S₀-E state molecules can be arranged as follows: L3 (–OCH₃) > L2 (–NH₂) > L5 (–CN) > L4 (–CF₃) > L1 (–H). The O⋯N distance in the O–H⋯N H-bonding structure for L1 to L5 at the S₀-E state is 2.616, 2.611, 2.607, 2.610 and 2.608 Å, respectively. The shortest O⋯N and H-bonding distance corresponds to the L3 molecule with the electron-donating –OCH₃ substituent.

Fig. 1 The PBE0 optimized structures of L1–L5 at S₀ and S₁ states (the key bond lengths in Å are also shown).

Table 1 The main bond lengths (Å) and angles (degree) of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) calculated at the PBE0/6-31++G(d,p) level at the S₀ and S₁ states

	State	O–H	N–H	δ(O–H–N)	N–O
L1	S0-E	0.994	1.724	147.2	2.616
	S1-E	1.002	1.684	149.4	2.597
	S0-K	1.555	1.075	142.7	2.496
	S1-K	1.525	1.084	145.7	2.496
L2	S0-E	0.997	1.712	147.8	2.611
	S1-E	1.005	1.673	149.7	2.591
	S0-K	1.601	1.065	141.0	2.519
	S1-K	1.817	1.038	136.3	2.666
L3	S0-E	0.998	1.709	147.7	2.607
	S1-E	1.006	1.670	149.7	2.588
	S0-K	1.626	1.059	140.0	2.530
	S1-K	1.593	1.067	143.3	2.531
L4	S0-E	0.996	1.717	147.1	2.610
	S1-E	1.003	1.678	149.4	2.592
	S0-K	1.571	1.071	142.1	2.504
	S1-K	1.537	1.080	145.3	2.501
L5	S0-E	0.996	1.715	147.2	2.608
	S1-E	1.001	1.681	149.3	2.593
	S0-K	1.550	1.076	143.0	2.494
	S1-K	1.527	1.082	145.4	2.495

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The comparison of geometric parameters reveals the changes in IHB strength after the photoexcitation process. As seen in Table 1, for L1–L5, the O–H bond length and δ(O–H⋯N) bond angle increase, whereas the N⋯H and N⋯O bond lengths decrease after the photoexcitation process. The O–H bonds of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) are elongated from 0.994, 0.997, 0.998, 0.996 and 0.996 Å at the S₀-E state to 1.002, 1.005, 1.006, 1.003 and 1.001 Å at the S₁-E state, respectively. The N⋯H H-bond distance is 1.724, 1.712, 1.709, 1.717 and 1.715 Å at the S₀-E state which decreases to 1.684, 1.673, 1.670 and 1.678 and 1.681 Å at the S₁-E state, respectively. The decrease in the N⋯H H-bond distance upon photoexcitation is 0.040, 0.039, 0.039, 0.039 and 0.034 Å in L1–L5, respectively. The O⋯N distance is also reduced by 0.019, 0.019, 0.019, 0.018 and 0.016 Å upon excitation from S₀-E to S₁-E. A decrease in the O⋯N distance makes the H-bonded ring smaller in the S₁-E state than in the S₀-E one and, in turn, makes S₁-E unstable and proton transfer faster. The δ(O–H⋯N) bond angle in L1–L5 increases from 147.2, 147.8, 147.7, 147.1 and 147.2° at the S₀-E state to 149.4, 149.7, 149.7, 149.4 and 149.3° at the S₁-E state, respectively. These results show that the intramolecular hydrogen bond is strengthened at the S₁ state which can facilitate the occurrence of the proton transfer process. Based on hydrogen bond structural parameters, it is predicted that the strength of IHB interaction at the S₁-E state is in the order L3 (–OCH₃) > L2 (–NH₂) > L4 (–CF₃) > L5 (–CN) > L1 (–H).

In addition, to judge the configurational changes of molecules going from the S₀-E state to the S₁-E one, the optimized structures of the molecules at the S₀ and S₁ states were evaluated by root mean square deviation (RMSD). Greater RMSD implies greater structural changes during the photoexcitation. Analysis of RMSD showed that its value is more sensitive to dihedral angles of the donor part of molecules than to bond lengths and bond angles. The RMSD values for the structural changes of the L1–L5 molecules at the PBE0/6-31++G(d,p) level of theory are 5.88, 4.95, 5.2, 7.25 and 37.8, respectively. Its value at the CAM-B3LYP/6-31++G(d,p) level of theory is 1.02, 0.73, 0.86, 1.23, and 29.38, respectively, that is, lower than those calculated using the PBE0 functional; the predicted trend of both functionals is the same. These results indicate that the RMSD value for the L4 and L5 molecules with electron-withdrawing substituents is greater than those found for the L1–L3 molecules. Besides, the structural variations between S₀ and S₁ states can correlate with the normal emission wavelength of molecules which will be discussed in the next section.

The O–H⋯N H-bonding at the S₁-E state is switched by O⋯H–N H-bonding at the S₁-K state upon the ESIPT process. In the S₁ excited state, the O⋯HN H-bond distance corresponding to the S₁-K form of L1, L2, L3, L4 and L5 is 1.525, 1.817, 1.593, 1.527 and 1.537 Å, respectively, indicating that the strength of IHB decreases as follows: L1 (–H) > L4 (–CF₃) > L5 (–CN) > L3 (–OCH₃) > L2 (–NH₂). The H-bond distance in the S₁-K (O⋯H–N) forms of L1–L5 is smaller than those found for the S₁-E (O–H⋯N) forms (except for L2). This reveals that O⋯H–N H-bonding in the keto form is stronger than the O–H⋯N one in the enol form at the S₁ state. The values of the δ(O–H⋯N) bond angle at the S₁-K form of L1, L2, L3, L4 and L5 are 145.7, 136.3, 143.3, 145.4 and 145.3°, respectively, which are smaller than those at the S₁-E form.

The variations of the (N–H and O⋯N) bond distances and δ(O–H⋯N) bond angle versus RC (RC = d_OH) along the proton transfer pathway of selected L1–L5 molecules at the S₁ state are illustrated in Fig. 2. The graphs in the S₀ state are shown in Fig. S1 of the ESI.† The plots of the variations of the (O⋯N) distance and δ(O–H⋯N) angle are similar in both states, but the O⋯N distance is different in the crossover point (COP) of the ground and excited states. As can be observed, the O⋯N distance first decreases to a minimum and the δ(O–H⋯N) bond angle increases to a maximum in the COP before the proton transfer occurs. The COP between the enol and keto forms of L1 to L5 is located at R(O–H) = 1.300, 1.305, 1.306, 1.303 and 1.301 Å at the S₁ state, respectively. At the COP, the O⋯N distance in the L1–L5 is 2.430, 2.422, 2.421, 2.420 and 2.419 Å so that L5 with the electron-withdrawing –CN group shows the shortest O⋯N distance at the COP. All of these intrinsic distortions in the structures happen to facilitate the proton transfer in the first S₁ excited state.

Fig. 2 Variations of the N–H and O–N bond distances and O–H–N bond angle versus RC (d_OH) along the proton transfer path for L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) in the S₁ state at the PBE0/6-31++G(d,p) level.

3.2. Relative energies

The relative energies of the optimized structures in the gas phase and solvent media at the PBE0/6-31++G(d,p) level are given in Table 2 and those at the CAM-B3LYP/6-31++G(d,p) level are listed in Table S1 of the ESI.† In the gas phase, at the ground state, the keto forms of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) are less stable than the enol forms due to the higher energies. Compared with S₀-E, the energies of the S₀-K forms for L1–L5 are 7.46, 6.14, 5.63, 6.87 and 7.03 kcal mol⁻¹ based on the PBE0 and 8.67, 7.13, 6.57, 8.08, and 8.30 kcal mol⁻¹ based on the CAM-B3LYP functional. The S₁-(FC) states of L1–L5 are 69.04, 69.64, 69.58, 66.43 and 63.89 kcal mol⁻¹ using the PBE0 and 81.44, 80.97, 81.25, 79.84 and 78.35 kcal mol⁻¹ based on the CAM-B3LYP functional higher than the S₀-E state. In addition, compared with S₀-E, the S₁-E state energy of the five L1–L5 molecules is predicted to be 61.10, 64.04, 63.05, 57.29 and 55.10 kcal mol⁻¹ by using PBE0 and 75.42, 74.90, 75.13, 73.91 and 72.45 kcal mol⁻¹ based on the CAM-B3LYP method, respectively. Therefore, it can be concluded that the conversion of S₁-(FC) to S₁-E in L1–L5 is an exergonic process by releasing the energy of −7.94, −5.60, −6.53, −9.14 and −8.79 kcal mol⁻¹ based on PBE0 and −6.02, −6.07, −6.12, −5.93 and −5.9 kcal mol⁻¹ by using the CAM-B3LYP method. The formation of the keto tautomers is driven by the difference in energy between the S₁-(FC) and keto structures at the S₁ state. The results in Tables 2 and S1† show that for all compounds, the PBE0 method provides a smaller energy difference between the two S₁-(FC) and S₁-K states of L1 (−2.44), L2 (−3.13), L3 (−2.66), L4 (−4.02) and L5 (−3.43 kcal mol⁻¹) compared with the CAM-B3LYP method, which is L1 (−5.04), L2 (−3.22), L3 (−4.34), L4 (−4.48) and L5 (−4.05 kcal mol⁻¹). However, these results indicate that the proton transfer for all azine derivatives is an exergonic process at the S₁ state. A comparison of relative energies calculated using the two functionals demonstrates that the CAM-B3LYP energies are greater than the PBE0 ones.

Table 2 Relative energies of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) at the S₀ and S₁ states calculated at the PBE0/6-31++G(d,p) level

Media	Molecule	S0-E	S1(FC)	S1-E	S0-K	S1-K
Gas	L1	0.00	69.04	61.10	7.46	66.60
	L2	0.00	69.64	64.04	6.14	66.51
	L3	0.00	69.58	63.05	5.63	66.92
	L4	0.00	66.43	57.29	6.87	62.41
	L5	0.00	63.89	55.10	7.03	60.46
DMSO	L1	0.00	65.71	57.88	5.01	59.05
	L2	0.00	66.28	58.27	3.65	60.07
	L3	0.00	66.42	58.60	3.34	59.19
	L4	0.00	63.10	55.42	4.40	56.42
	L5	0.00	59.98	52.79	4.57	54.14
MeOH	L1	0.00	66.03	57.97	5.05	59.17
	L2	0.00	66.64	58.38	3.69	60.19
	L3	0.00	66.74	58.69	3.37	59.31
	L4	0.00	63.40	55.55	4.43	56.54
	L5	0.00	60.27	52.86	4.61	54.27
Toluene	L1	0.00	66.07	60.57	6.33	63.20
	L2	0.00	66.64	61.86	5.03	64.35
	L3	0.00	66.71	61.65	4.57	63.34
	L4	0.00	63.39	57.43	5.72	60.54
	L5	0.00	60.54	55.02	5.87	58.46

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Fig. 3 shows the schematic energy diagram of the photo-tautomerization for the L1–L5 molecules in the gas phase. In the ground state, the dyes are mostly stable in the enol (S₀-E) form. Excitation of this form by photons leads to the transition of the molecules to the Franck–Condon excited state S₁-E(FC), which rapidly decays to the local enol form (S₁-E) minimum. There are two deactivation paths from the minimum of S₁-E. The excited molecules can immediately undergo the ESIPT process to provide the new tautomer keto (S₁-K) or return to the S₀-E state by emitting radiation. If the ESIPT process happens, the S₁-K keto tautomer reverts to the ground state via fluorescence emission to form S₀-K. Finally, the S₀-E enol form is recovered by ground state proton transfer (GSIPT). In addition, all five compounds are stable at the S₀-K state. Thus, it is predicted to exhibit a vertical transition to the Franck–Condon region S₁-K(FC). From this point, the molecules can regress to the S₁-K state with the structural rearrangement and then rapidly return to the ground state S₀-K by fluorescence emission.

Fig. 3 The schematic representation of the ESIPT process involving enol–keto phototautomerization.

The impact of polar and nonpolar solvents on the relative energy of the five molecules was investigated (Tables 2 and S1†). Similar to the gas phase, the keto structures in the three solvents at the S₀ state are less stable than the enol forms. The S₀-K form of L1–L5 is more stable in the polar DMSO solvent than in the gas phase and other solvents by 3.34 to 5.01 (4 to 5.96 kcal mol⁻¹) by using the PBE0 (CAM-B3LYP) methods. From Table 2, the S₁-(FC) state of L1–L5 has less energy in DMSO than in the gas phase and other solvents based on PBE0, indicating that these dyes are energetically more stable in the polar DMSO solvent. The CAM-B3LYP energy difference between the S₁-E and S₁-(FC) states of the L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) azine-based dyes given in Table S1† is in the ranges of −10.67 to −15.38 in DMSO, −10.88 to −11.9 in MeOH and −6.98 to −7.29 kcal mol⁻¹ in toluene, which are greater than the PBE0 ones. These values indicate that the conversion of the S₁-(FC) to S₁-E state in polar solvents is more exergonic than in the gas phase. The differences in energy between the two S₁-(FC) and S₁-K states are in the ranges of −5.84 to −7.23 kcal mol⁻¹ (−8.87 to −14.68) in DMSO, −6.00 to −7.43 kcal mol⁻¹ (−9.07 to −10.53) in MeOH and −2.08 to −3.37 kcal mol⁻¹ (−4.68 to −6.02) in toluene based on the PBE0 (CAM-B3LYP) methods, indicating that the S₁-(FC) to S₁-K tautomerization in polar solvents is more exergonic than in the gas phase and the L4 and L5 dyes have the highest values.

3.3. The potential energy curves

To further investigate the influence of different substituents on the ESIPT process and elucidate the dynamics of ESIPT in the five studied molecules, the potential energy curves (PECs) along the proton transfer pathway as a function of the reaction coordinate (RC = d_OH) at the S₀ and S₁ states of the L1–L5 molecules were computed at the PBE0/6-31++G(d,p) and TD-PBE0/6-31++G(d,p) levels of theory in the gas phase, respectively (Fig. 4). It can be seen that in the S₀ state, the enol (S₀-E) form is the most stable tautomeric form. As discussed previously, the molecular geometry optimization shows that the keto forms of all five molecules with O⋯H–N H-bonding are stable at the S₀ state. However, it can be seen that the O–H⋯N enol forms of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) in the ground state are more stable than the corresponding O⋯H–N keto tautomers. From Fig. 4, the energy barrier at the S₀ state for enol → keto transformation in the L1–L5 molecules is in the range of 6.14 to 7.53 kcal mol⁻¹. However, the energy barrier for the reverse process in L1–L5 is 0.06, 0.30, 0.50, 0.13 and 0.08 kcal mol⁻¹, respectively, indicating that the keto → enol GSIPT is nearly a barrier-less process at the S₀ state. The potential energy curves show that the proton transfer energy barrier changes by adding substituents to the meta position of the OH group compared with the unsubstituted L1. In the excited state, the energy barrier is lower than the ground state, which indicates that the proton transfer is more facile in the S₁ state than in the S₀ one. The energy barrier order at the S₁ state is L3 (4.43 kcal mol⁻¹) < L2 (4.65 kcal mol⁻¹) < L4 (5.29 kcal mol⁻¹) < L5 (7.11 kcal mol⁻¹) < L1 (5.60 kcal mol⁻¹). This trend implies that the energy barrier at the S₁ state for substituted molecules is smaller than the unsubstituted structure L1 so the electron-donating groups facilitate the ESIPT process.

Fig. 4 The PBE0/6-31++G(d,p) PECs of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) as functions of the reaction coordinate, RC (O–H), along the proton transfer at the S₀ and S₁ states.

Most ESIPT molecules possess a strong H-bonding interaction, so ESIPT is either a barrier-less process or a small barrier one. The time scale for the ESIPT process is generally less than 1 ps, resulting in a lack of normal emission or weak normal emission, of course, there are exceptions.^68–71 The higher endergonic ESIPT can lead to weaker tautomer emission due to the lower population of the S₁-K keto species. In molecules in which the energy difference between S₁-E enol and S₁-K keto in the excited state and the corresponding barrier energies are small, an equilibrium between the two forms (reversible ESIPT) can be established before their respective emission.⁷² The PECs and data given in Table 2 reveal that the S₁-K form of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) is less stable than S₁-E by 5.5, 2.5, 3.9, 5.1 and 5.4 kcal mol⁻¹, respectively. Since the conversion of S₁-(FC) to S₁-E in L1–L5 is an exergonic process by releasing the energy of −7.94, −5.60, −6.53, −9.14 and −8.79 kcal mol⁻¹ based on PBE0, the formation of the keto tautomers is driven by the difference in energy between the S₁-(FC) and keto structures at the S₁ state. However, despite the reverse ESIPT being a barrier-less process, it is predicted that an equilibrium between the two enol and keto forms with the dominance of the enol form can be established before their corresponding emission. Accordingly, this process can affect the population of the enol and keto states and their respective contributions to the overall fluorescence and lead to changes in the relative intensities of the two bands and the emission characteristics of the system.

3.4. Vibrational spectra

Theoretical analysis of vibrational spectra is a well-established tool to assess the strength of H-bonding interactions. We reported the CAM-B3LYP results because the calculated O–H vibrational wavenumber for the L1 dye (3338.1 cm⁻¹) was closer to the experimental result (3338.1 cm⁻¹) compared with PBE0 (3228.8 cm⁻¹). The CAM-B3LYP calculated vibrational spectra of azine derivatives L1–L5 in the enol form at the S₀ state are given in Fig. S2.† The vibrational wavenumber of the O–H group involved in H-bonding can provide a specific measure of the H-bonding strength. The O–H vibrational wavenumber for L1–L5 at the S₀-E state is 3338.05, 3292.33, 3283.35, 3314.87 and 3311.47 cm⁻¹, respectively, indicating that the O–H⋯N H-bonding strength is stronger in L2 and L3 with electron-donating substituents than in the other ones. There is a correlation between the O–H vibrational wavenumber and the O–H bond distance; the greater the O–H bond distance, the lower the corresponding vibrational wavenumber.

The vibrational spectra of keto tautomers with O⋯H–N H-bonding at the S₀ state are given in Fig. S3.† As can be seen, the intensity of the N–H band in the keto form is lower than the O–H one in the enol form. The N–H vibrational wavenumbers in the L1–L5 dyes are 2927.40, 3012.01, 3060.07, 2947.25 and 2904.63 cm⁻¹, respectively. A comparison of O–H and N–H vibrational frequencies demonstrates that the O⋯H–N H-bonding in the keto form is stronger than the O–H⋯N one in the enol form, in good agreement with the corresponding H-bonding distances.

3.5. Photophysical properties

The photophysical characteristics of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) including the absorption and emission wavelengths, oscillator strengths f and the vertical transition energies of the L1–L5 molecules in the enol and keto forms calculated in the gas phase and three solvents at the PBE0/6-31++G(d,p) level are given in Tables 3 and 4 and those at the CAM-B3LYP/6-31++G(d,p) level are given in Tables S2 and S3.† As can be seen in Tables 3 and S2,† the calculated absorption wavelength (λ₁) for the S₀-E form of L1 is 414 nm (351 nm) based on the PBE0 (CAM-B3LYP) methods. This reveals that the PBE0 wavelength for L1 is closer to the experimental result (395 nm),³⁶ compared with the CAM-B3LYP one. The absorption wavelengths calculated for L2, L3, L4 and L5 are 410 (353), 410 (351), 430 (358) and 447 (364 nm) in the gas phase, suggesting that the absorption maxima are shifted to longer wavelengths by electron-withdrawing substituents. The first S₀ → S₁ vertical excitation energies (VEEs) of L1 to L5 are 2.99 (3.53), 3.02 (3.51), 3.02 (3.52) 2.88 (3.46) and 2.77 eV (3.40 eV), respectively. The PBE0 VEEs are smaller than the CAM-B3LYP ones, in good agreement with the reported results.^45,52

Table 3 The calculated absorption and emission wavelengths (λ) and oscillator strength (f) of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) in the gas phase at the PBE0/6-31++G(d,p) level

	S0-E	S1-E	S0-K	S1-K	S0-E	S1-E	S0-K	S1-K
	L1				L2
λ 1 (nm)	414	545	452	558	410	505	434	563
λ 1 (eV)	2.99	2.27	2.74	2.22	3.02	2.45	2.85	2.2
λ 2 (nm)	339	390	391	468	336	392	388	458
λ 3 (nm)	337	387	371	402	332	377	356	423
f 1	1.195	0.138	1.064	0.213	1.493	0.311	1.261	0.209
f 2	0.076	0.769	0.123	0.356	0.167	1.213	0.222	1.211
f 3	0.001	0.008	0	0	0.018	0.001	0.001	0
	L3				L4
λ 1 (nm)	410	519	437	528	430	599	461	616
λ 1 (eV)	3.02	2.39	2.83	2.34	2.88	2.07	2.69	2.01
λ 2 (nm)	334	390	377	446	342	395	393	456
λ 3 (nm)	331	378	353	382	336	386	371	403
f 1	1.395	0.222	1.299	0.344	1.155	0.102	1.172	0.121
f 2	0.166	1.167	0.064	0.385	0.002	0.003	0.017	0.533
f 3	0.002	0	0.0002	0	1.005	1.005	0	0
	L5
λ 1 (nm)	447	626	474	645
λ 1 (eV)	2.77	1.98	2.61	1.92
λ 2 (nm)	348	400	402	458
λ 3 (nm)	343	393	378	411
f 1	1.158	0.107	1.176	0.118
f 2	0.003	0.003	0.015	0.558
f 3	0.295	1.207	0.0001	0

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Table 4 The calculated absorption and fluorescence emission properties of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) in different solvents at the PBE0/6-31++G(d,p) level. The experimental values are given in parentheses

	S0-E	S1-E	S0-K	S1-K	S0-E	S1-E	S0-K	S1-K
DMSO	L1				L2
λ 1 (nm)	435 (393)	578 (536)	471	606 (566)	431	534	450	545
f 1	1.357	1.53	1.307	1.533	1.73	2.07	1.602	1.855
MeOH
λ 1 (nm)	432 (392)	535	468	565 (555)	429	534	448	544
f 1	1.332	1.518	1.284	1.526	1.7	2.061	1.572	1.847
Toluene
λ 1 (nm)	432 (391)	514 (548)	471	534 (568)	429	495	451	513
f 1	1.376	0.891	1.294	1.259	1.723	1.676	1.572	1.521
DMSO	L3				L4
λ 1 (nm)	430	531	456	549	453	562	488	589
f 1	1.558	1.817	1.499	1.723	1.301	1.387	1.288	1.437
MeOH
λ 1 (nm)	428	530	453	548	450	560	485	588
f 1	1.532	1.808	1.476	1.717	1.277	1.376	1.267	1.428
Toluene
λ 1 (nm)	428	498	456	517	451	557	485	563
f 1	1.577	1.37	1.509	1.457	1.428	0.598	1.323	0.985
DMSO	L5
λ 1 (nm)	476	590	511	618
f 1	1.288	1.461	1.26	1.415
MeOH
λ 1 (nm)	474	588	448	617
f 1	1.263	1.444	1.572	1.406
Toluene
λ 1 (nm)	472	579	456	590
f 1	1.322	0.682	1.509	0.918

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The magnitude of the oscillator strength f for an electronic transition is proportional to the square of the transition dipole moment produced by the interaction between electromagnetic radiation and an electric dipole. The greater the transition dipole moment, the greater the transition probability between quantum states and in turn the greater the f. Thus, the intensity of the absorption peaks is proportional to the oscillator strength f. The PBE0 calculated oscillator strength f for the S₀ → S₁ vertical transition of the L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) molecules is in the order 1.493 (L2) > 1.395 (L3) > 1.195 (L1) > 1.158 (L5) > 1.155 (L4), and the CAM-B3LYP calculated order is 1.693 (L2) > 1.612 (L3) > 1.555 (L5) > 1.464 (L4) > 1.431 (L1), indicating that intensity of the S₀ → S₁ transition peak for the L2 and L3 molecules with electron-donating groups is greater than that for the L4 and L5 molecules with electron-withdrawing substituents. As can be seen, the CAM-B3LYP calculated oscillator strengths are greater than the PBE0 ones.

There is an interpretation of longer wavelengths or lower VEEs observed for the L4 and L5 molecules compared to the other ones. Our results (see Fig. 6) show that the LUMO energy of L4 and L5 with the electron-withdrawing substituents CN and CF₃ decreases and that of L2 and L3 (R2 and 3 = NH₂ and OMe) increases relative to L1 (R = H), leading to lower H–L energy gaps and longer wavelengths for them. It is clear that as the degree of electron delocalization increases, the energy gap between the HOMO and LUMO decreases, resulting in a shift in the electronic absorption spectrum towards longer wavelengths. To understand the effect of the substituents on the electron distribution in the phenyl ring, we used NBO analysis data. The results of NBO population analysis show that the oxygen lone pairs of the O–H group are mainly delocalized to vicinal anti-bonding orbitals BD*(C6–C4) and BD*(C6–C7), resulting in the decrease of LP population of the O atom (LP(O)). The average value of occupancy of LP(O) is 1.7932 in L1, 1.7942 in L2, 1.7938 in L3, 1.7921 in L4 and 1.7923 in L5. This indicates that the delocalization of LP(O) towards BD* of the phenyl ring for L4 and L5 with electron-withdrawing substituents is greater than that for L1–L3. This is due to the inductive effect of the electron-withdrawing substituents which leads to the greater withdrawal of LP(O) electrons. This electron pull can increase the π-electron conjugation (electron density) of the phenol ring, thereby causing a bathochromic shift in absorption λ.^73,74 These LP(O) → BD*(C–C) charge transfer interactions increase the electron density on the phenyl ring and contribute to the π–π* interactions which shift the λ to longer wavelengths. The AIM results also show that the electron density at the ring critical point (RCP) of the phenol ring for L2 (0.0203 au) and L3 (0.0204 au) is lower than that for L4 (0.0208 au) and L5 (0.0206 au). The higher electron density and, in turn, the total electron energy density (H) in the RCP of the phenol ring suggest that the electrons are strongly delocalized in the center of the ring.⁷⁵

After photoexcitation, the S₁-E form produced by relaxation of S₁-(FC) can either return to the S₀-E ground state via the fluorescence emission or is converted to the keto form through the ESIPT process. The PBE0 calculated S₁-E emission wavelengths (Table 3 and S2†) for L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) are 545 nm, 505, 519, 599 and 626 nm, respectively, which are longer than the CAM-B3LYP ones, similar to the absorption wavelengths. Moreover, all the molecules exhibit redshifted S₁-E emission relative to their vertical excitation wavelengths. The PBE0 Stokes shift for the S₁-E emission is in the order L5 (179 nm) > L4 (169 nm) > L1 (131 nm) > L3 (109 nm) > L2 (95 nm). This indicates that the Stokes shift for the molecules with electron-withdrawing substituents is smaller than the other ones. According to the oscillator strength f values given in Table 3, the trend of the intensity of S₁-E emission bands by using the PBE0 functional is L2 (0.311) > L3 (0.222) > L1 (0.138) > L5 (0.107) > L4 (0.102) and that by using CAM-B3LYP is L2 (1.823) > L3 (1.749) > L4 (1.691) > L5 (1.574) > L1 (1.563), indicating that the highest intensity of the S₁-E → S₀-E fluorescence emission corresponds to L2 with the electron-donating –NH₂ group. As can be seen, the oscillator strength of the normal fluorescence emission calculated using the CAM-B3LYP functional is greater than the PBE0 one, similar to the absorption intensity.

The S₁-E form can be transformed into S₁-K through the ESIPT process and then return to the S₀-K state via fluorescence radiation emission. From Tables 3 and S2,† the calculated S₁-K fluorescence emission wavelengths for L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) by using the PBE0 (CAM-B3LYP) methods are 558 (451), 563 (432), 528 (433), 616 (450) and 645 nm (457 nm), respectively. These results reveal that the PBE0 emission wavelengths are greater than the CAM-B3LYP ones and the value for the L1 dye is closer to the experimental value (566 nm). The calculated PBE0 Stokes shifts are higher than the CAM-B3LYP ones and follow the order: L5 (198 nm) > L4 (186 nm) > L2 (153 nm) > L1 > (144 nm) > L3 (118 nm). Similar to enol emissions, the largest Stokes shifts are obtained for electron-withdrawing substituents. According to the results given in Table 3, the values of f for S₁-K → S₀-K transitions in L1–L5 are in the ranges of 0.118–0.344 and 0.939–1.408 based on the PBE0 and CAM-B3LYP methods, respectively. It can be seen that except for L2, the PBE0 intensities of the S₁-K fluorescence emission bands are higher than those of S₁-E. The trend of the intensities of S₁-K emission bands is L3 > L1 > L2 > L4 > L5 by the PBE0 method and L3 > L2 > L5 > L4 > L1 by the CAM-B3LYP one, indicating that the highest intensity of the S₁-K fluorescence emission corresponds to L3 with the electron-donating –OCH₃ group.

The effect of solvents on the absorption and emission peaks of the studied molecules was explored. The solvents with different dielectric constants namely dimethylsulfoxide (DMSO: ε = 46.826), methanol (MeOH: ε = 32.613) and toluene (ε = 2.3741) were selected and the results are listed in Tables 4 and S3.† The PBE0 calculated absorption and emission spectra in DMSO solvent are shown in Fig. 5. The corresponding spectra in MeOH and toluene solvents are shown in Fig. S4 and S5 of the ESI.† The highest absorption wavelengths calculated for L1–L5 in the three solvents by using the PBE0 (CAM-B3LYP) methods are in the ranges of 432 to 435 nm (362 to 363 nm) for L1, 429 to 431 nm (365 to 367 nm) for L2, 428 to 430 nm (362 to 364 nm) for L3, 450 to 453 nm (368 to 370 nm) for L4 and 472 to 476 nm (376 to 378 nm) for L5. The PBE0 (CAM-B3LYP) calculated absorption wavelength for L1 is 435 nm (363 nm) in DMSO and 432 nm (362 nm) in MeOH and toluene. Compared with the experimental results given for L1, the PBE0 functional overestimates the absorption wavelengths in all solvents and the CAM-B3LYP functional underestimates them. However, both CAM-B3LYP and PBE0 values for L1 are close to the experimental values (393 nm in DMSO, 392 nm in MeOH and 391 nm in toluene).³⁶ It can be seen that the bathochromic shift in the absorption wavelength values for L4 and L5 is greater than those for L1, L2 and L3. The CAM-B3LYP oscillator strengths f for the S₀ → S₁ vertical transition are greater than the PBE0 ones and in DMSO, MeOH and toluene follow the order: L2 > L3 > L5 > L4 > L1.

Fig. 5 The absorption and emission spectra of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) in DMSO solvent at the PBE0/6-31++G(d,p) level. The values in parentheses are experimental data (abs = absorption spectrum; flu = fluorescence spectrum).

The calculated enol S₁-E emission wavelengths for L1–L5 in solvent media are in the ranges of 514–536 nm (443–481 nm) for L1, 495–534 nm (450–492 nm) for L2, 498–531 nm (446–484 nm) for L3, 557–562 nm (452–493 nm) for L4 and 579–590 nm (465–512 nm) for L5 based on the PBE0 (CAM-B3LYP) functionals. From Tables 4 and S3,† a slight red shift can be observed in enol emission for all the molecules with an increase in the polarity of the solvent, in good agreement with the bathochromic shift observed by an increase in the polarity of the solvent.³⁶ In addition, the enol emission wavelengths of the S₁-E form for L1 are 536 nm (481) in DMSO, 535 nm (480 nm) in MeOH and 514 nm (443 nm) in toluene. The PBE0 calculated values are closer to the experimental values (578 nm in DMSO and 548 nm in toluene).³⁶ Compared with the experimental results given for L1, the PBE0 functional overestimates the emission wavelengths in all solvents and the CAM-B3LYP functional underestimates them and both functionals underestimate the enol emission wavelength in toluene solvent.

The PBE0 (CAM-B3LYP) calculated S₁-K fluorescence emission bands of L1–L5 in the three solvents are in the ranges of 534–566 nm (470–500 nm) for L1, 513–545 nm (458–490 nm) for L2, 517–549 nm (459–490 nm) for L3, 563–589 nm (477–512 nm) for L4 and 590–618 nm (488–526 nm) for L5. The calculated keto emission of the S₁-K state for L1 appears at 566 nm (500 nm) in DMSO, 565 nm (499 nm) in MeOH and 534 nm (470 nm) in toluene by using the PBE0 (CAM-B3LYP) methods. These results are well comparative with the experimental data (606 nm in DMSO, 555 nm in MeOH and 568 nm in toluene).³⁶ However, the PBE0 results are closer to the experimental values available for L1. In addition, the largest PBE0 Stokes shifts computed in MeOH solvent follow the order 143 nm (L5) > 138 (L4) > 133 (L1) > 120 (L3) > 115 (L2). It can be concluded that the Stokes shifts for L4 and L5 containing electron-withdrawing groups are greater than those for L2 and L3 possessing electron-donating substituents. According to the f values given in Tables 4 and S3,† the intensities of S₁-K and S₁-E fluorescence emission bands in solvent media are higher than those in the gas phase.

The dipole moments for azine derivatives L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) in the gas phase and solvent media are listed in Table 5. The change in dipole moment upon photoexcitation is associated with the reorganization of the electron density and the redistribution of charges within the molecule through intramolecular charge transfer (ICT). The strength of the push–pull substituents can affect the charge distribution and, consequently, the molecular dipole moment and their photophysical properties in solvent media. The charge separation is proportional to the electron-donating/withdrawing strength. It should be noted that –NH₂ is a better push group than –OMe because of the lower electronegativity of the nitrogen atom; at the same time, the –CN substituent pulls electrons more than –CF₃. According to the PBE0 results, in the gas phase, the ground-state dipole moment values range from 1.3 to 10.2 D and 2.7 to 12.3 D for the enol and keto forms, respectively, which (except for L2) decrease upon the S₀ to S₁ transition. The different behavior of the L2 molecule can be attributed to the lower value of RMSD (4.95) upon the S₀ to S₁ transition, compared with the other molecules (5.2 to 37.8). The dipole moment values are in the range of 1.0 to 5.9 D in the S₁-E form and 0.8 to 7.5 D in the S₁-K form. The differences between the ground and excited state dipole moments in the L4 and L5 dyes with electron-withdrawing substituents are higher than the other ones. In addition, L4 and L5 exhibit larger dipole moment changes than the other ones upon enol to keto tautomerization in the S₁ state.

Table 5 The PBE0 calculated dipole moments (Debye) of the azine dyes L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN)

	S0-E	S1-E	S0-K	S1-K	S0-E	S1-E	S0-K	S1-K
	Gas				DMSO
L1	3.6	1.0	5.6	2.3	4.9	4.2	8.3	7.9
L2	1.3	3.2	2.7	4.0	1.6	1.7	4.1	4.1
L3	1.8	1.5	3.8	0.8	2.9	2.4	6.0	5.8
L4	7.9	4.3	10.0	5.9	9.7	9.1	13.1	12.5
L5	10.2	5.9	12.3	7.5	12.6	11.9	16.1	15.1
	MeOH				Toluene
L1	4.9	4.1	8.2	7.9	4.3	2.1	6.9	6.2
L2	1.6	1.7	4.1	4.0	1.5	1.9	3.3	3.2
L3	2.9	2.3	6.0	5.7	2.3	0.9	4.8	4.1
L4	9.6	8.9	13.1	12.4	8.8	6.1	11.6	9.3
L5	12.5	11.8	16.0	15.0	11.5	8.4	14.2	11.3

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The change in dipole moment upon photoexcitation can provide valuable information about the solvatochromic effect. The PBE0 dipole moment values (Table 5) at the S₀-E state range from 1.6 D for L2 to 12.5 D for L5 in the protic CH₃OH polar solvent, 1.6 D for L2 to 12.6 D for L5 in the aprotic DMSO polar solvent and 1.5 D for L2 to 11.5 D for L5 in the nonpolar toluene solvent, indicating that the dipole moment slightly decreases going from the polar solvent to the nonpolar one. From the small changes of the dipole moment with the change of solvent, it is predicted that the absorption wavelength does not significantly change when the polarity of the solvent changes, as the results of Table 4 show (weak solvatochromism in the absorption band).

The PBE0 dipole moment values at the S₁-E state are in the range of 1.7 D for L2 to 11.8 D for L5 in CH₃OH, 1.7 D for L2 to 11.9 D for L5 in DMSO and 0.9 D for L3 to 8.4 D for L5 in the toluene solvent, indicating that the dipole moment slightly decreases going from the polar solvent to the nonpolar one at the S₁-E state. The small increase in the dipole moment of the enol form (except for L2) going from S₁-E to S₀-E in the polar solvents DMSO and CH₃OH is consistent with the results of the small solvation effect for the emission bands of the enol form. However, the increase in dipole moment for the enol form upon the S₁-E to S₀-E transition in toluene is greater than that in the polar solvents, which results in the shorter enol emission wavelength in toluene, as given in Table 4. The increase in dipole moment for the L4 and L5 molecules in toluene is greater than the other ones.

For keto forms, the dipole moment increases in all solvents upon the S₁-K to S₀-K transition so its value in the toluene solvent is greater than the other ones. Therefore, a decrease in solvent polarity is conducive to stabilizing the S₀-K state due to a larger increase in dipole moment. Accordingly, a red shift in keto fluorescence emission (positive solvatochromic effect) is estimated by an increase in the polarity of the solvent, as can be seen in Table 4 (458–488 nm in toluene and 489–525 nm in polar solvents), in good agreement with results given in ref. 36. In addition, an increase in dipole moment in the three solvents is found upon enol to keto switching through the ESIPT process in the S₁ state, showing that the keto form is more stabilized than the enol form and can emit in longer wavelength.

Based on the results given in Tables 6 and 7, the S₀ → S₁ and S₁ → S₀ transitions in L1 to L5 are mainly described as HOMO (H) → LUMO (L) and LUMO (L) → HOMO (H) transitions, respectively, with more than 69% contribution, demonstrating that the HOMO–LUMO contribution is dominant. Accordingly, the calculated highest occupied molecular orbitals (HOMOs) and lowest occupied molecular orbitals (LUMOs) for the enol and keto forms of L1–L5 in the gas phase are presented in Fig. 6. It can be seen that the first transition to the excited state has a ππ* feature due to the π character of the HOMO as well as the π* character of the LUMO. The electron density in the HOMO of all molecules at the S₀-E state is distributed on the whole of the molecule including the triphenylamine (donor), azine and phenol units, and that in the LUMO is located mainly on the azine and substituted phenol units. A comparison of the electron density distribution of the HOMO and LUMO reveals that intramolecular charge transfer (ICT) occurs from triphenylamine to salicylaldimine (phenol-C=NH) as an acceptor unit upon excitation from S₀ to S₁. This ICT process can facilitate the proton transfer.

Table 6 The calculated absorption properties of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) at the PBE0/6-31++G(d,p) level of theory

	Media	Transition	E (eV)	λ abs (nm)	f	Component	CI (%)
L1	Gas	S0 → S1	2.99	414	1.195	H → L	69.84
		S0 → S2	3.65	339	0.076	H-1 → L	67.71
		S0 → S3	3.68	337	0.001	H-2 → L	68.64
	DMSO	S0 → S1	2.85	435	1.357	H → L	69.77
	MeOH		2.86	432	1.332	H → L	69.81
	Toluene		2.87	432	1.375	H → L	69.69
L2	Gas	S0 → S1	3.02	410	1.493	H → L	69.86
		S0 → S2	3.68	336	0.167	H-1 → L	67.98
		S0 → S3	3.73	332	0.017	H → L + 1	66.34
	DMSO	S0 → S1	2.87	431	1.73	H → L	69.93
	MeOH		2.89	429	1.7	H → L	69.94
	Toluene		2.89	429	1.723	H → L	69.89
L3	Gas	S0 → S1	3.02	411	1.395	H → L	69.89
		S0 → S2	3.7	334	0.165	H-1 → L	67.77
		S0 → S3	3.73	331	0.002	H-3 → L	68.65
	DMSO	S0 → S1	2.88	430	1.557	H → L	69.8
	MeOH		2.89	428	1.532	H → L	69.83
	Toluene		2.89	428	1.577	H → L	69.77
L4	Gas	S0 → S1	2.88	430	1.155	H → L	69.98
		S0 → S2	3.62	342	0.002	H-3 → L	69.55
		S0 → S3	3.68	336	0.183	H-1 → L	67.46
	DMSO	S0 → S1	2.75	450	1.277	H → L	69.9
	MeOH		2.74	453	1.301	H → L	69.86
	Toluene		2.75	451	1.318	H → L	69.86
L5	Gas	S0 → S1	2.77	447	1.157	H → L	70.06
		S0 → S2	3.56	348	0.002	H-3 → L	69.26
		S0 → S3	3.61	343	0.294	H-1 → L	65.15
	DMSO	S0 → S1	1.29	476	1.288	H → L	69.88
	MeOH		2.61	474	1.262	H → L	69.93
	Toluene		2.62	472	1.322	H → L	69.77

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Table 7 The calculated fluorescence properties and composition index (CI) of molecular orbitals involved in transitions in the enol and keto (given in parentheses) forms of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) at the PBE0/6-31++G(d,p) level

	Media	Transition	E flu (eV)	λ flu (nm)	f	Component	CI (%)
L1	Gas	S1 → S0	2.27 (2.22)	545 (558)	0.138 (0.213)	L → H (L → H)	69.90 (69.84)
	DMSO	S1 → S0	2.31 (2.18)	535 (566)	1.530 (1.533)	L → H (L → H)	69.86 (69.42)
	MeOH	S1 → S0	2.32 (2.19)	535 (565)	1.518 (1.526)	L → H (L → H)	69.86 (69.43)
	Toluene	S1 → S0	2.41 (2.32)	514 (534)	0.891 (1.259)	L → H (L → H)	69.99 (69.89)
L2	Gas	S1 → S0	2.45 (2.20)	505 (563)	0.311 (0.209)	L → H (L → H)	69.72 (69.48)
	DMSO	S1 → S0	2.31 (2.27)	536 (545)	2.070 (1.855)	L → H (L → H)	70.19 (69.80)
	MeOH	S1 → S0	2.32 (2.28)	534 (544)	2.061 (1.847)	L → H (L → H)	70.19 (69.80)
	Toluene	S1 → S0	2.50 (2.42)	495 (513)	1.676 (1.521)	L → H (L → H)	70.13 (69.87)
L3	Gas	S1 → S0	2.39 (2.34)	519 (528)	0.222 (0.344)	L → H (L → H)	69.87 (69.77)
	DMSO	S1 → S0	2.33 (2.26)	531 (549)	1.817 (1.723)	L → H (L → H)	70.02 (69.68)
	MeOH	S1 → S0	2.34 (2.26)	530 (548)	1.808 (1.717)	L → H (L → H)	70.03 (69.69)
	Toluene	S1 → S0	2.49 (2.40)	498 (517)	1.370 (1.457)	L → H (L → H)	70.09 (69.98)
L4	Gas	S1 → S0	2.07 (2.01)	599 (616)	0.102 (0.121)	L → H (L → H)	69.90 (69.88)
	DMSO	S1 → S0	2.21 (2.10)	562 (589)	1.387 (1.437)	L → H (L → H)	69.79 (69.29)
	MeOH	S1 → S0	2.21 (2.11)	560 (588)	1.376 (1.428)	L → H (L → H)	69.80 (69.30)
	Toluene	S1 → S0	2.22 (2.20)	557 (563)	0.598 (0.985)	L → H (L → H)	69.97 (69.76)
L5	Gas	S1 → S0	1.98 (1.92)	626 (645)	0.107 (0.118)	L → H (L → H)	69.87 (69.90)
	DMSO	S1 → S0	2.10 (2.00)	590 (618)	1.461 (1.415)	L → H (L → H)	69.71 (69.27)
	MeOH	S1 → S0	2.11 (2.01)	588 (617)	1.444 (1.406)	L → H (L → H)	69.72 (69.28)
	Toluene	S1 → S0	2.14 (2.10)	579 (590)	0.682 (0.918)	L → H (L → H)	69.97 (69.81)

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Fig. 6 Isodensity surface plots of the HOMOs and LUMOs of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) involved in the first singlet excitation calculated at the PBE0/6-31++G(d,p) level. The numerical values are the HOMO–LUMO energy gap. Isosurfaces are set to 0.02.

The HOMO–LUMO (H–L) energy gap for L1 to L5 at the S₀-E state follows the order L2 and L3 (3.54 eV) > L1 (3.49 eV) > L4 (3.35 eV) > L5 (3.21 eV), in good agreement with the lower excitation energy or greater absorption wavelength calculated for L4 and L5 having electron-withdrawing substituents. The simultaneous destabilization of the π HOMO by the electron-donating substituent and stabilization of the π* LUMO by the electron-withdrawing substituent in L4 and L5 result in the decrease of the H–L gap and, in turn, the red shift of the ππ* absorption.

As Fig. 6 shows, the electron density in the LUMO of the S₁-E state is mainly distributed on the azine and substituted phenol units and that in the HOMO is located in the triphenylamine donor group. This indicates that the ICT process takes place upon enol emission. The H–L energy gap for L1–L5 at the S₁-E state is 2.86, 3.03, 2.98, 2.61 and 2.48 eV respectively, which are in good agreement with the corresponding normal emission wavelengths.

The (H–L) energy gaps of keto tautomers at the S₁-K state for L1 (2.77), L2 (2.79), L3 (2.90), L4 (2.53) and L5 (2.41 eV) are smaller than those of their enol structures, which are L1 (2.86), L2 (3.03), L3 (2.98) L4 (2.61) and L5 (2.48 eV) at the S₁-E state, indicating that the keto forms of these molecules will emit longer fluorescence emission wavelengths. Moreover, the transition properties involving the electronic excitation energy, corresponding oscillator strengths (f) and composition index for absorption transitions in the gas phase and solvent media are summarized in Table 6 and the fluorescence properties are listed in Table 7. It can be seen that L1–L5 show absorption maxima in the ranges of 410 to 447 nm (f = 1.155 to 1.493) which are mainly due to the HOMO (H) → LUMO (L) transition with the largest contributions in the ranges of 69.84 to 70.06% in the gas phase. The S₀ → S₂ transition which involves HOMO-1 (H-1) → LUMO (L) for L1, L2, and L3 and HOMO-3 (H-3) → LUMO (L) for L4 and L5 is also allowed, and despite the large orbital contributions, their oscillator strengths are in the ranges of 0.002 to 0.167. Thus, it is predicted that S₀ → S₁ is the main light transition in these compounds. In solution, we mainly focus on the S₀ → S₁ transition. Thus, it can be seen that in DMSO, MeOH and toluene media, the HOMO (H) → LUMO (L) transition contributes more than 69% for L1 to L5. According to the results given in Table 7, the S₁ → S₀ enol and keto emissions of L1 to L5 are related to LUMO (L) → HOMO (H) transitions with more than 69% contribution in the gas phase and solvent media.

The fluorescence rate constants, k_f, are calculated for the S₁-E and S₁-K forms of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) via Einstein's emission transition probability equation: (ref. 76), where ν is the fluorescence wavenumber and f is the oscillator strength. The calculated fluorescence rate constants, k_f, and fluorescence lifetimes (τ = 1/k_f) in the gas phase and solvent media are reported in Table 8. For L1–L5, the fluorescence lifetime of the S₁-E state follows the order: L5 > L4 > L1 > L3 > L2. The most significant effect of electron-withdrawing substitutions in L4 and L5 is the smaller k_f and, in turn, the longer ππ* lifetime (τ_f) compared with the other molecules in the gas phase and all solvent media, where the S₁-E state in L4 and L5 has a lifetime three times longer than that in L2 and L3 in the gas phase and toluene solvent, although the lifetime of the S₁-E state of L4 and L5 is greater than that of L2 and L3 in the polar solvent. The results show that the lifetime of the S₁-E state in all molecules is nearly the same in polar solvents and is longer in toluene solvent than in the polar ones.

Table 8 The S₁-E and S₁-K fluorescence rate constants (k_f) and radiation lifetimes (τ_f) for L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) calculated at the PBE0/6-31++G(d,p) level

Media	Molecule	S1-E	τ f (ns)	S1-K	τ f (ns)
		k f (s^−1)/10^6		k f (s^−1)/10^6
Gas	L1	30.94	32.32	45.41	22.02
	L2	81.10	12.33	43.90	22.77
	L3	54.82	18.24	81.96	12.20
	L4	18.87	52.94	21.27	47.01
	L5	18.09	55.27	18.88	52.96
DMSO	L1	354.31	2.82	318.17	3.14
	L2	479.59	2.08	415.19	2.40
	L3	428.81	2.32	415.07	2.41
	L4	292.53	3.41	275.99	3.62
	L5	279.88	3.57	348.53	2.86
MeOH	L1	352.86	2.83	476.89	2.09
	L2	480.10	2.08	415.07	2.40
	L3	428.57	2.33	380.24	2.62
	L4	292.25	3.42	275.38	3.63
	L5	277.58	3.60	245.84	4.06
Toluene	L1	224.75	4.45	293.79	3.41
	L2	454.6	2.19	385.13	2.60
	L3	367.2	2.72	363.22	2.75
	L4	128.28	7.79	207.00	4.83
	L5	135.46	7.38	175.71	5.69

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Similar to the S₁-E state, the lifetime of the S₁-K state for L4 and L5 in all media is greater than that found in L2 and L3, indicating that L4 and L5 containing electron-withdrawing groups have longer lifetimes compared with L2 and L3 having electron-donating substituents. The comparison of rate constants for the studied molecules reveals that the emission rate constants of the S₁-E form for L1, L4 and L5 are smaller than those of S₁-K in the gas phase and toluene and it is the opposite for L2 and L3. In the polar solvent media, there is no particular order.

3.6. Hole–electron excitation properties

After photoexcitation, electron transfer occurs from the donor group (D) to the acceptor (A). In the studied molecules, the excitation mainly occurs from the HOMO to the LUMO. However, the substituent effects on ICT degrees cannot be quantitatively given via HOMO and LUMO isosurfaces. Furthermore, orbital analysis may be difficult to be directly connected to simple chemical parameters and concepts such as the charge transfer extent. To solve this problem, several approaches have been derived to analyze the character of the excited states, specifically, to quantify the degree of locality and nonlocality of an excitation, meaning by computing the hole–electron properties upon photoexcitation. Herein, the “hole” signifies the region where the excited electron leaves, and the “electron” is the region where the excited electron eventually goes.⁶⁵

The hole–electron excitation descriptors including the charge transfer (CT) excitation distance (D_CT), transferred charge (q_CT), variation in dipole moment between the ground and excited states (μ_CT), H_CT (Å), H index (Å), t index (Å) and overlap integral between ρ₋ and ρ₊ regions (S_±) computed at the PBE0/6-31++G(d,p) level for L1–L5 in the gas phase and DMSO solvent are given in Table 9.

Table 9 Computed CT indices for vertical transition of L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) calculated at the PBE0/6-31++G(d,p) level in the gas and DMSO media

	D CT/Å	q CT/|e|	∥μCT∥/Debye	H CT/Å	H index/Å	t index/Å	S ± index
L1 (gas)	3.996	0.643	12.337	4.218	4.694	−0.222	0.858
L1 (DMSO)	4.862	0.708	16.53	4.123	4.603	0.739	0.808
L2 (gas)	2.857	0.56	7.681	4.292	4.713	−1.435	0.917
L2 (DMSO)	2.858	0.596	8.185	4.287	4.68	−1.430	0.911
L3 (gas)	3.385	0.593	9.634	4.311	4.755	−0.926	0.896
L3 (DMSO)	4.073	0.649	12.700	4.132	4.587	−0.059	0.857
L4 (gas)	4.549	0.71	15.517	4.479	4.936	0.070	0.843
L4 (DMSO)	5.454	0.769	20.144	4.339	4.809	1.115	0.791
L5 (gas)	5.075	0.746	18.178	4.624	5.071	0.451	0.820
L5 (DMSO)	6.18	0.811	24.100	4.552	5.004	1.628	0.762

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The process of single-electron excitation involves an electron moving from a hole to an electron. The simultaneous distribution of holes and electrons on the dyes is illustrated in Fig. 7. The green isosurface represents the electron distribution, and the blue isosurface represents the hole distribution; the isovalue has been set to 0.002. This indicates that the hole isosurface of these dyes is mainly distributed over the electron-donating part, TPA, and the electron delocalized mainly over the acceptor moiety. The separated distribution of holes and electrons leads to the electron movement from the donor group to the acceptor one. In L2 and L3 with electron-withdrawing substituents, the holes are also extended to the substituents, leading to a decrease in CT. From the distribution of holes and electrons, we can infer that the hole and electron distributions should be composed of π and π* orbitals, respectively. Therefore, it can be concluded that S₀ → S₁ is a CT excitation with a π → π* feature. If an excitation is perfectly described as a HOMO → LUMO transition, then the hole and electron could be simply represented by the HOMO and LUMO, respectively, as can be observed for the S₀ → S₁ transition in these dyes.

Fig. 7 The simultaneous distribution of hole and electron isosurfaces on the L1–L5 dyes calculated at the PBE0/6-31++G(d,p) level in the gas phase. The isovalue has been set to 0.002. The blue and green isosurfaces are related to the hole and electron distributions, respectively.

The widespread application of the D_CT index in a wide range of very different molecular systems is due to its simplicity, effectiveness, and straightforward interpretation. The calculation of the D_CT index requires only the calculation of the electronic density of the ground (ρ^GS) and excited (ρ^ES) states of interest. The density change associated with the electronic transition is given by Δρ(r) = ρ^ES − ρ^GS. From Δρ(r), the first two ρ₊ and ρ₋ functions are defined which show the points in space where an increment or a depletion of the electron density occurs upon excitation. Then, from these two ρ₊ and ρ₋ charge distribution functions, D_CT can be expressed as the spatial distance between the two barycenters of the positive (R₊) and negative (R₋) density distributions: D_CT = |R₊ − R₋|. D_CT has a vectorial character which is the difference between the positions of the two barycenters. This descriptor can be used as a measure of the effective distance of the electron and the hole upon excitation.^65,77

From Table 9, the D_CT for L2 (2.857 Å) and L3 (3.385 Å) having the electron-donor groups NH₂ and OMe is lower than that for L1 (3.996 Å) and the D_CT for L4 (4.549 Å) and L5 (5.075 Å) having the electron acceptor groups CF₃ and CN is greater than that for L1. Therefore, the distance between the hole and electron centroids (charge transfer length) in L4 and L5 is greater than the other ones. Nevertheless, the D_CT values are relatively high, indicating that there is a significant separation of the hole and electron distributions. Therefore, the S₀ → S₁ transition in the L1–L5 compounds is attributed to CT excitation rather than locally excited (the D_CT value is small), in good agreement with the predictions based on HOMO and LUMO isosurfaces.

The transferred charge (q_CT) can be obtained by integrating over all space ρ₊ and ρ₋. For one electron excitation, q_CT can receive values between 0 and 1.⁶⁵ In agreement with D_CT, the transferred charge, q_CT, value increases as the substituents vary from the electron-donating group to the electron-withdrawing group. For one electron excitation, q_CT can assume values between 0 and 1. The q_CT values are 0.710 |e| in L4 and 0.746 |e| in L5, indicating that the CT character of transition in these molecules is stronger than the other ones. It should be noted that q_CT is the magnitude of the integral of ρ₊ (regions of electronic density increase) and ρ₋ (regions of electronic density decrease) over the whole space. It only corresponds to the total amount of charge whose distribution is perturbed during electron excitation; it does not correspond to the net charge transfer from the donor group to the acceptor group.⁶⁵ The D_CT and q_CT are inversely proportional to the HOMO–LUMO energy gap, that is an increase in these CT indices is accompanied by a decrease in VEE and, in turn, an increase in absorption wavelength as observed. In the DMSO solvent, the D_CT and q_CT indices are smaller than those found in the gas phase, indicating that the absorption wavelength must increase going from the gas phase to DMSO, in good agreement with the S₀ → S₁ excitation wavelength given in Table 6.

The μ_CT (variation in dipole moment between the ground and excited states) can be computed by ∥μ_CT∥ = q_CTD_CT.⁶⁵ Similar to q_CT and D_CT, due to the larger charge excitation length, the μ_CT for L4 and L5 is also greater than that for the other molecules in both gas and DMSO media. In addition, μ_CT increases going from the gas phase to the DMSO medium.

The H_CT is the average degree of spatial extension of hole and electron distribution in the CT direction, and the H index is an overall measure of their extension that represents the half sum of the centroid axis length along the D–A direction. It can be seen from the hole and electron maps in Fig. 9 that both the holes and electrons of the S₀ → S₁ transition in L4 and L5 are distributed in wider regions than in the other molecules. This is why the H indices for these molecules are bigger than the other ones.

The t index represents the difference between the D_CT and H_CT values. If the t index >0, it means that the ρ₋ and ρ₊ regions are significantly separated due to CT. The negative value of the t index indicates that the ρ₋ and ρ₊ regions are not substantially separated owing to CT. The t index for L1–L3 is negative and for that L4 and L5 is positive so the L5 molecule has the greatest positive t index, suggesting that the degree of separation of holes and electrons is very low in L1–L3. Besides, the overlap integral between the ρ₋ and ρ₊ regions (S_± index) for L4 and L5 is less than that for L1–L3, in agreement with other CT indices.

3.7. AIM analysis and RDG isosurfaces

The strength of intramolecular H-bonding also can be investigated by the topological analysis and the reduced density gradient (RDG) analysis. The topological analysis is based on electron density and is an important part of atoms in molecules theory that was first proposed by Bader.^78,79 The corresponding topological parameters including the electron density ρ(r), the Laplacian of electron density ∇²ρ(r), the energy density H(r) and the ratio of −G(r)/V(r) at the bond critical points (BCPs) for ground and excited states are given in Table 10. Here, we focus on the main BCPs at N⋯H and O⋯H H-bonds in enol and keto structures, respectively. The value of ρ(r) is strongly related to the strength of IHB. In addition, the potential energy density V(r) highly correlates with the hydrogen bond energy (E_HB) which is described as Espinosa's equation: E_HB = 1/2|V(r)BCP|.⁸⁰ From Table 10, the ρ(r) value at the N⋯H BCP of L1 to L5 is 0.0499, 0.0513, 0.0518, 0.0509 and 0.0511 a.u. in the S₀-E state, respectively. The corresponding value of ∇²ρ(r) at the N⋯H BCP is 0.1163, 0.1168, 0.1173, 0.1171 and 0.1174 a.u. These results show that the intramolecular H-bond strength follows the order: L3 > L2 > L4 > L5 > L1, in good agreement with the calculated H-bonding distances. Based on the ρ(r) and ∇²ρ(r) values of L1–L5 at the O⋯H BCP in the S₀-K state, the IHB strength follows the order L5 > L1 > L4 > L2 > L3. The nature of interactions can be determined in terms of the ∇²ρ(r) and H(r). Moreover, the ratio −G(r)/V(r) in the non-covalent nature of the interactions is greater than 1 and that for the covalent nature is smaller than 1. From Table 10, the values of ∇²ρ(r) and H(r) at the N⋯H H-bond critical point (HBCP) of the L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) compounds are positive and negative, respectively, and the −G(r)/V(r) values are smaller than 1 in both S₀-E and S₀-K states, indicating that the nature of HB interaction in all compounds is partially covalent.

Table 10 Bond critical point data (in a.u.) in the S₀ and S₁ states for all L1–L5 (R1–5 = –H, NH₂, –OCH₃, –CF₃, and –CN) molecules at the PBE0/6-31++G(d,p) level

	S0-E		S1-E		S0-K		S1-K
	O–H	N–H	O–H	N–H	O–H	N–H	O–H	N–H
L1
ρ(r)	0.3291	0.0499	0.3218	0.0553	0.0710	0.2851	0.0767	0.2787
∇ ^2 ρ(r)	−1.9061	0.1163	−1.8186	0.1210	0.1676	−1.4570	0.1650	−1.4023
H(r)	−0.5511	−0.0045	−0.5304	−0.0067	−0.0115	−0.4160	−0.0157	−0.4039
−G(r)/V(r)	0.1192	0.8816	0.1250	0.8467	0.8230	0.1106	0.7836	0.1166
E HB		0.0191		0.0218	0.0324		0.0364
L2
ρ(r)	0.3260	0.0513	0.3183	0.0569	0.0633	0.2936	0.0392	0.3189
∇ ^2 ρ(r)	−1.8766	0.1168	−1.7835	0.1205	0.1625	−1.5282	0.1046	−1.7376
H(r)	−0.5438	−0.0051	−0.5217	−0.0075	−0.0071	−0.4318	−0.0018	−0.4791
−G(r)/V(r)	0.1207	0.8710	0.1269	0.8336	0.8711	0.1034	0.9393	0.0853
E HB		0.0197		0.0226	0.0274		0.0274
L3
ρ(r)	0.3254	0.0518	0.3175	0.0574	0.0595	0.2986	0.0645	0.293
∇ ^2 ρ(r)	−1.8712	0.1173	−1.7756	0.1207	0.1589	−1.5695	0.1622	−1.5223
H(r)	0.5425	−0.0053	−0.5198	−0.0078	0.0595	−0.4412	−0.0078	−0.4306
−G(r)/V(r)	0.1210	0.8681	0.1274	0.8299	0.8943	0.0997	0.8414	0.1041
E HB		0.0199		0.0229	0.0252		0.0280
L4
ρ(r)	0.3275	0.0509	0.3208	0.0561	0.0680	0.2888	0.0742	0.2823
∇ ^2 ρ(r)	−1.8928	0.1171	−1.8119	0.1216	0.1669	−1.4892	0.1673	−1.4324
H(r)	−0.5478	−0.0049	−0.5288	−0.0071	−0.0096	−0.4231	−0.0138	−0.4106
−G(r)/V(r)	0.1198	0.8750	0.1255	0.8413	0.8427	0.1071	0.8009	0.1133
E HB		0.0195		0.0223	0.0304		0.0347
L5
ρ(r)	0.3271	0.0511	0.3216	0.0557	0.0718	0.2842	0.0760	0.2805
∇ ^2 ρ(r)	−1.8897	0.1174	−1.8240	0.1216	0.1679	−1.4508	0.1676	−1.4167
H(r)	−0.5470	−0.0050	−0.5318	−0.0068	−0.0121	−0.4144	−0.0151	−0.4071
−G(r)/V(r)	0.1200	0.8733	0.1248	0.8448	0.8176	0.1110	0.7903	0.1151
E HB		0.0197		0.0220	0.0330		0.0361

---

The ρ(r) at the N⋯H BCP of the L1–L5 molecules at the S₁-E state is 0.0553, 0.0569, 0.0574, 0.0561 and 0.0557 au, respectively, indicating that its value for L2 and L3 is greater than that for the other molecules. The comparison of ρ(r) at the N⋯H BCP reveals that after photoexcitation from S₀-E to S₁-E, the electron density ρ(r) at the N⋯H BCP of all L1–L5 molecules increases. An increase in ρ(r) at the N⋯H BCP upon photoexcitation from S₀-E to S₁-E is accompanied by a decrease in ρ(r) at the O–H BCP.

These changes in ρ(r) reveal that the H-bonding interaction is strengthened at the S₁-E state which can promote the proton transfer. The value of E_HB is also highly related to the strength of the H-bonding. The larger the value of E_HB, the stronger the H-bond. The enhancement in the H-bond strength in the S₁-E state is in the order L3 > L2 > L4 > L5 > L1, in good consistency with the H-bond structural parameters. The sign and values of ∇²ρ(r) and H(r) at the N⋯H BCP in the S₁-E state present an increase in the covalent nature of the N⋯H BCP under S₀ → S₁ transition in the enol structures.

Upon the ESIPT process at the S₁ state, the OH⋯N H-bonding is switched by the O⋯HN one. The ρ(r) at the O⋯HN BCP of the L1–L5 molecules at the S₁-K state is 0.0767, 0.0392, 0.0645, 0.0742 and 0.0760 au, respectively, indicating that its value for L2 and L3 is less than that for the other molecules. The comparison of the ρ(r) value reveals that the O⋯HN interaction at the S₁-K state of the molecules is stronger than that of OH⋯N in the corresponding S₁-E state. The comparison of the values of ∇²ρ(r) and H(r) at the O⋯H BCP shows that the covalent character in the S₁-K structures is greater than that in the S₁-E forms with N⋯H H-bond interaction.

The non-covalent interaction (NCI) method, which is also known as the reduced density gradient (RDG) method, is a very important method for studying weak interactions.⁸¹ The NCI analysis method can be regarded as an extension of the AIM theory for visual studies. RDG is a simple dimensionless function of the electron density, ρ(r), and its gradient ∇ρ(r). It can be utilized to elucidate the effect of different factors on the hydrogen bond strength. To further study the IHB in real space, the scatter graphs of the reduced density gradient (RDG) versus sign(λ₂)ρ and the corresponding isosurfaces of L1, L2 and L5 at the S₀ and S₁ states are depicted in Fig. 8, and those of the L3 and L4 structures are given in Fig. S6.† The relationship between ρ(r) and λ₂(r) is obtained in Bader's theory of atoms in molecules,⁵⁹ where λ₂(r) represents the second largest eigenvalue of electron density of the Hessian matrix. The sign(λ₂(r))ρ(r) is the product of the sign of λ₂(r) and the electron density of the Hessian matrix at position r. The negative values of sign(λ₂)ρ refer to the H-bonding interactions; positive sign(λ₂)ρ stands for the steric effects and values near zero display the van der Waals (VDW) interactions.⁸¹ The blue circled spike in the RDG isosurfaces shows a strong IHB interaction for these molecules. The spikes located at around −0.05 a.u. for the five molecules in both S₀ and S₁ states confirm the existence of the O–H⋯N H-bonding interaction in the enol form of the five molecules. In the NCI plots, the light blue color elliptical slab between the oxygen and hydrogen atoms indicates that there is a hydrogen bond.

Fig. 8 RDG scatter plots and NCI isosurfaces of L1, L2 and L5 at the S₀ and S₁ states calculated at PBE0/6-31++G(d,p).

3.8. NBO charge analysis

To investigate the impact of electron-donating and withdrawing groups on the redistribution of atomic charges in the L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) compounds and, in turn, on the proton transfer, the NBO charges were calculated at the S₀ and S₁ states in the gas phase at the PBE0/6-31++G(d,p) theoretical level. The selected natural charges of atoms involved in H-bonding interaction are listed in Table 11. The positive charge of the H atom in the S₀-E state increases as L1 (0.5211 e) < L2 (0.5216 e) < L4 (0.5221 e) < L5 (0.5222 e) < L3 (0.5225 e) and that in the S₀-K state follows the order: L3 (0.4650 e) < L2 (0.4656 e) < L1 (0.4672 e) < L4 (0.4674 e) < L5 (0.4681 e), indicating that the acidity of the H atom in the enol forms is higher than that in the keto ones. The results show that the positive charge on the H atom in the S₁-E state is L2 (0.5171 e) < L1 (0.5174 e) < L3 (0.5178) < L4 (0.5180 e) < L5 (0.5190), which are lower than those found at the S₀-E state (L1 (0.5211 e) < L2 (0.5216 e) < L4 (0.5221 e) < L5 (0.5222 e) < L3 (0.5225 e)). This indicates that the positive charge of the H atom involved in H-bonding in the gas phase decreases upon photoexcitation from the S₀ to S₁ state. Besides, the positive charge of the H atom at the S₁-K state is in the order L2 (0.4609 e) < L3 (0.4627 e) < L1 (0.4645 e) < L4 (0.4646 e) < L5 (0.4652 e), which are smaller than those of the corresponding enol ones at the S₁-E state. Moreover, in both states, the highest positive charge of the H atom corresponds to L4 and L5 having electron acceptor groups.

Table 11 The selected NBO charges (e) of the L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) molecules at the S₀ and S₁ states in the gas phase at the PBE0/6-31++G(d,p) level

State	q H	q N	q O	q H	q N	q O
	L1			L2
S0-E	0.5211	−0.3741	−0.7059	0.5216	−0.3912	−0.7082
S1-E	0.5174	−0.3719	−0.7059	0.5171	−0.3857	−0.7084
S0-K	0.4672	−0.3817	−0.6937	0.4656	−0.3917	−0.7038
S1-K	0.4645	−0.3706	−0.6936	0.4609	−0.3659	−0.7183
	L3			L4
S0-E	0.5225	−0.3864	−0.7071	0.5221	−0.3641	−0.6993
S1-E	0.5178	−0.3826	−0.7073	0.518	−0.3608	−0.699
S0-K	0.465	−0.3886	−0.6952	0.4674	−0.3728	−0.6837
S1-K	0.4627	−0.3758	−0.697	0.4646	−0.3631	−0.6813
	L5
S0-E	0.5222	−0.3599	−0.6982
S1-E	0.519	−0.358	−0.6974
S0-K	0.4681	−0.3686	−0.6849
S1-K	0.4652	−0.3612	−0.6805

---

The negative charge of the N atom (q_N) involved in O–H⋯N H-bonding in the S₀-E state increases in the following order: L5 (−0.3599 e) < L4 (−0.3641 e) < L1 (−0.3741 e) < L3 (−0.3864 e) < L2 (−0.3912). It can be seen that the q_N in the L2 (R = –NH₂) and L3 (R = –OCH₃) molecules is greater than that in the other molecules. Besides, the presence of two electron-donating groups in both sides of the N–N group in the L2 and L3 molecules collectively leads to a greater increase in the electron density of the N atom in the CH=N group of the salicylaldimine moiety and is also involved in O–H⋯N H-bonding.³⁶ Upon photoexcitation from S₀-E to S₁-E, the negative charge of the N atom decreases as follows: L5 (−0.3580 e) < L4 (−0.3608 e) < L1 (−0.3717 e) < L3 (−0.3826 e) < L2 (−0.3857). Compared with L1, the presence of the typical electron-withdrawing groups (–CN and –CF₃) in L4 and L5 can attract lone pair electrons of the N atom, thus weakening the ability of the N atom to capture protons. In contrast, introducing the typical electron-donating groups (–NH₂ and –OMe) in L2 and L3 has the opposite effect, in good agreement with the lower O–H⋯N H-bonding distance obtained for L2 (1.673 Å) and L3 (1.670 Å) compared with L4 (1.678 Å) and L5 (1.681 Å). The results also show that the S₁-E to S₁-K process leads to a decrease and an increase in the negative charge of the N atom in the L1–L3 and L4–L5 molecules, respectively.

The negative charge of atom O involved in hydrogen bonding at the S₀-E state is in the order L5 (−0.6982 e) < L4 (−0.6993 e) < L1 (−0.7059 e) < L3 (−0.7071 e) < L2 (−0.7082 e) which changes to L5 (−0.6974 e) < L4 (−0.6990 e) < L1 (−0.7059 e), L3 (−0.7073 e) and L2 (−0.7084 e) at the S₁-E state. It is well known that –NH₂ is a better push group than –OMe because of the lower electronegativity of the nitrogen atom; at the same time, the –CN substituent pulls electrons more than –CF₃. Compared with L1, the negative charge of the O atom (i.e. electronegativity of the O atom) in L4 and L5 decreases and that of L2 and L3 increases in both states. That is, the ability of the O atom to attract protons in L4 and L5 becomes less than that in L2 and L3, in good agreement with the greater O–H bond length obtained for L2 (1.005 Å) and L3 (1.006 Å) than for L4 (1.003 Å) and L5 (1.001 Å) at the S₁ state. Consequently, the natural charge values of the N and O atoms are in good agreement with the elongation of the O–H bond length and the shortening of the H⋯N H-bonding distance confirms that the electron-donating substituents can facilitate proton transfer at the S₁ state. Besides, this conclusion is consistent with the electron density at the corresponding BCP.

The negative charge of atom O at the S₁-K state is in the order L5 (−0.6805 e) < L4 (−0.6813 e) < L1 (−0.6936 e) < L2 (−0.6970 e) < L3 (−0.7183 e), demonstrating that the highest negative charge value is obtained by adding electron-donating groups. In addition, the negative charge of the O atom decreases upon S₁-E → S₁-K switching. The behaviors of the natural atomic charges of H, O and N involved in proton transfer versus RC at the S₁ state for selected L1, L3 and L5 compounds are illustrated in Fig. 9. It can be seen that the positive charge of the H atom decreases during the PT process. On the other hand, the negative charge of N and O atoms first increases to a minimum and then decreases before relaxing to the keto form. The change in the natural charge of atoms is caused by the change in the structural parameters upon photoexcitation and phototautomerization.

Fig. 9 Changes of the natural atomic charges (H, O and N) in terms of RC along the PT pathway of the selected L1, L3 and L5 molecules at the S₁ state.

The results of NBO population analysis indicate that the intramolecular charge transfer (ICT) takes place between donor TPA and acceptor salicylaldimine moieties. The sum of atomic natural charges of the donor TPA part can be a good measure of ICT in these molecules. The ICT values in the L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃, and –CN) molecules are +0.345 e, +0.310 e, +0.002 e, +0.373 e and +0.383 e at the S₀-E state which change to +0.301 e, +0.271 e, −0.042 e, +0.327 e and 0.340 e at the S₁-E state, respectively. The species possesses strong charge transfer (L4 and L5), giving a greater difference in dipole moment between the normal and tautomer forms (1.6 D in the gas phase). The ICT results confirm the electron-donating character of TPA for all L1–L5 at both states, except for L3 at the S₁ state. As results show, electrons are transferred from the electron-donating TPA part to the electron-accepting part of molecules (except for L3 at the S₁ state), which leads to the positive charge of the TPA part of molecules, in good agreement with the electron density distribution of the HOMO on the azine and phenol units. Besides, it is found that the ICT value decreases going from the S₀ state to the S₁ one (except for L3). In the case of L3, the presence of two different electron-donating groups OMe and TPA on both sides of the N–N group nearly prevents ICT from the TPA part of the molecule so the calculated ICT becomes approximately zero (0.002 e at S₀ and −0.014 e at the S₁ sate), compared to the other molecules.

4. Conclusion

In this work, the ESIPT process of a series of donor–acceptor triphenylamine–salicylaldehyde unsymmetrical azine derivatives was investigated by using DFT and TDDFT methods. The effects of different substituents on the photophysical properties of the azine dyes in the gas phase and DMSO, MeOH and toluene solvents were explored. The change in structural parameters, vibrational wavenumbers, electron densities at the BCP, and RDG isosurfaces confirmed that the hydrogen bond interaction is strengthened upon S₀ to S₁ excitation. The greatest H-bonding strength in both states was obtained for L2 and L3 having electron-donating groups. The results indicated that the RMSD values for the L4 and L5 molecules having electron-withdrawing substituents are greater than those found for the L1–L3 molecules. Constructing the potential energy curves exhibited that the electron-donating and electron-withdrawing substituents could decrease the energy barrier in the first excited state and promote the intramolecular proton transfer compared to the original L1 molecule. The results indicated that the red shift in the absorption band in the L4 and L5 molecules with electron acceptor groups is greater than the other ones. Both S₁-E and S₁-K fluorescence emissions were observed for all studied molecules. The longest emission wavelengths in the gas phase and three solvents are found for L3 and L4 with electron-withdrawing groups. Introducing electron-withdrawing groups increases the absorption and emission wavelengths as well as the red shift in fluorescence emission of L4 and L5, but hinders the occurrence of the ESIPT process compared with L2 and L3. The fluorescence rate constants were greater in solution than those in the gas phase. The analysis of frontier molecular orbitals and the hole–electron analysis confirmed the occurrence of ICT during the photoexcitation. The results of NBO charge analysis were in good agreement with the results of H-bonding structural parameters and electron density analysis implying that the ability of the N atom to capture protons in the O–H⋯N network in L4 and L5 is lower than that of L2 and L3 at the S₁ state.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4me00039k

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