Effect of amino group on the excited-state intramolecular proton transfer (ESIPT) mechanisms of 2-(2′-hydroxyphenyl)benzoxazole and its amino derivatives

Abstract

The excited-state intramolecular proton transfer (ESIPT) reactions of 2-(2′-hydroxyphenyl)benzoxazole (HBO), 5-amino-2-(2′-hydroxyphenyl)benzoxazole (5A-HBO) and 6-amino-2-(2′-hydroxyphenyl)benzoxazole (6A-HBO) were investigated with the time-dependent density functional theory (TD-DFT) method at the B3LYP/6-31G(d,p) theoretical level. The primary bond lengths and infrared (IR) vibrational spectra show that the intramolecular hydrogen bond is significantly strengthened in S₁ state. The Mulliken's charge distribution and the frontier molecular orbitals (MOs) were analyzed. The result is consistent with the ESIPT mechanism proposed by Han and co-workers. Upon photo-excitation, the intramolecular hydrogen bond of 5A-HBO-enol (1.73 Å) and 6A-HBO-enol (1.74 Å) in the S₁ state is weaker than that of HBO-enol (1.69 Å) due to the influence of the amino group in the HBO framework. After vertical excitation to the S₁ state, the electronic density redistributes and migrates from the phenol ring to the benzoxazole ring of HBO. While for 5A-HBO and 6A-HBO, it transfers from the amino-benzoxazole moiety to the phenol ring. The analysis of the potential energy curves of HBO, 5A-HBO and 6A-HBO indicates that the ESIPT process of HBO occurs most easily. It is demonstrated that the presence and the position of the amino group in the HBO framework can change the behavior of the intramolecular hydrogen bonds O–H⋯N in the S₁ state and thus hinder the ESIPT processes to some extent.

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1. Introduction

Excited-state proton transfer (ESPT) reaction has been discovered and recognized to be one of the most elementary and important photochemical reactions in a variety of biological and physicochemical processes.^1–8 The excited state intramolecular proton transfer (ESIPT) reaction dynamic has been investigated both experimentally and theoretically by many groups since the first experimental observation of the phenomenon by Weller and co-workers in 1965.^9–17 A mechanism of an excited state H-atom-transfer reaction along a hydrogen-bonded “wire” has been proposed and widely studied in photochemical^18–20 and biological^21,22 processes. Though the hydrogen bonding in the ground state has been studied by different experimental and theoretical methods, little is known about electronic excited state hydrogen bonding mainly due to the extremely short time scales involved.²³ Recently, Han and co-workers have investigated the intermolecular and intramolecular hydrogen bonds between proton donors and proton acceptors in ground states and excited states.^24–30 The intermolecular and intramolecular hydrogen bonds are significantly strengthened in the excited state, which provides a driving force in facilitating the ESIPT reaction.^31,32 The proton transfer along with hydrogen bonding have drawn more and more attention in recent years because of the important role hydrogen bonds play in the ESIPT reaction.^33–35 Although the spectroscopic technique was performed to investigate the PT processes in the ground and excited state, there still exist some unsolved problems.³⁶ To better elucidate the transfer mechanisms, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were adopted to research the PT and ESIPT reactions.²³ The mechanism of the ESIPT generally includes the transfer of a hydroxyl (or amino) proton to an oxygen or nitrogen acceptor atom.²³ Upon photo-excitation, the molecule is projected on the excited state potential energy surface, leading to the proton in an unstable state.¹⁷ The difference between the locally excited and relaxed excited state energies provides the driving force for the transformation.³⁶

As a type of proton transfer dye, 2-(2′-hydroxyphenyl)benzoxazole (HBO) exhibits remarkable changes in the ESIPT reaction and related spectroscopy when an acceptor or a donor group is present in its molecular framework.³⁷ A number of investigations have been done on HBO derivatives, which contain an electron acceptor substituent (–NO₂, –COOH, –COOR or –RCN) in the benzazole ring.^38–40 For these well-known dyes, the proposed mechanism is a proton motion followed by an intramolecular charge transfer process.^38–40 On the contrast, the intramolecular charge transfer process occurs prior to the ESIPT when the substituent is an electron donor group (–NH₂, –NR₂, or –CH₃).⁴¹ In polar and hydroxylic media, HBO exhibits an ESIPT reaction, giving rise to the corresponding anionic species.⁴² To explore the anion formation in both the S₀ and S₁ states, the steady-state UV-visible spectra and picoseconds time-resolved fluorescence behavior of HBO and its amino derivatives (5A-HBO and 6A-HBO) have been investigated by Alarcos and co-workers.⁴³ Spectroscopic techniques, such as time-resolved fluorescence spectroscopy, can only provide indirect information about some photo-physical properties.

This paper explored the effect of the presence and position of the amino group in the HBO moiety by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The configurations of the S₀ and S₁ states were optimized, and the IR vibrational spectra, absorption and fluorescence spectra, frontier molecular orbitals (MOs) and potential energy curves were obtained. These calculation results could provide direct information on the ESIPT process, which show that the presence and the position of the amino group in the HBO framework can largely change the behavior of the S₀ and S₁ state.

2. Computational details

In the present work, all calculations were performed on the Gaussian 09 program suite.⁴⁴ The ground state and the electronic excited state geometric optimizations of HBO and its amino derivatives were performed using DFT and TD-DFT, respectively. The TD-DFT method has been confirmed to be a reliable tool to gain insight into the hydrogen bonding interaction in the excited state of hydrogen-bonded systems.²⁴ Becke's three-parameter hybrid exchange functional with Lee–Yang–Parr gradient-corrected correlation functional (B3LYP) and the 6-31G(d,p) basis set have been used in all the calculations.^45–47 All the local minima were confirmed by the absence of an imaginary mode in the vibrational analysis calculations. In addition to the unconstrained optimization, constrained calculations on the S₀ and S₁ potential energy curves of the HBO and its amino derivatives have been scanned by fixing the O–H bond length a series of values.

The dichloromethane (DCM) was selected as the solvent in the calculations based on the Polarizable Continuum Model (PCM) using the integral equation formalism of the Polarizable Continuum Model (IEFPCM).^48–50 Fine quadrature grids 4 were also employed. The self-consistent field (SCF) convergence thresholds for the energy in both the ground state and excited state optimizations were set to 10⁻⁸ (default settings are 10⁻⁶). Harmonic vibrational frequencies in both the ground-state and excited state were determined by diagonalization of the Hessian.⁵¹ The excited state Hessian was obtained by numerical differentiation of the analytical gradients using central differences and default displacements of 0.02 Bohr.⁵² The infrared intensities were determined using the gradients of the dipole moment.

3. Results and discussion

3.1. Optimized geometric structures

We studied the HBO and its amino derivatives which contain an electron donor group (–NH₂) on the benzoxazole moiety: 5-amino-2-(2′-hydroxyphenyl)benzoxazole (5A-HBO) and 6-amino-2-(2′-hydroxyphenyl)benzoxazole (6A-HBO) based on the DFT and TD-DFT methods for the S₀ and S₁ states. The optimized enol tautomer and keto tautomer structures of HBO and its amino derivatives were obtained at the B3LYP/6-31G(d,p) basis set level (shown in Fig. 1), with a subsequent vibrational frequency analysis to ensure the validity of the stationary points. The structure parameters are listed in Table 1. The calculated bond lengths of O–H and H–N for HBO-enol, 5A-HBO-enol and 6A-HBO-enol are 0.99 and 1.76 Å, 0.99 and 1.75 Å, and 0.99 and 1.76 Å, respectively. While, in S₁ state, the O–H bond lengths elongate to 1.01, 1.00 and 1.01 Å, and the H–N bond lengths shorten to 1.69, 1.73 and 1.74 Å, respectively. These results clearly show that, upon photo-excitation, the intramolecular hydrogen bonds O–H⋯N is significantly strengthened, which can facilitate the ESIPT reaction. This phenomenon is in agreement with previous finding drawn by Zhao and co-workers.^26,27 It should be noted that the H–N bonds lengths of the HBO-enol, 5A-HBO-enol and 6A-HBO-enol in S₁ state are different, which are 1.69, 1.73 and 1.74 Å, as described above. The intramolecular hydrogen bond of HBO-enol in S₁ state is stronger than that of 5A-HBO-enol and 6A-HBO-enol, demonstrating that the presence and position of the amino group in HBO framework can change the behaviors of the intramolecular hydrogen bonds O–H⋯N in S₁ state and thus hinder the ESIPT reactions to some extent. In addition, the bond lengths of O–H for HBO-keto, 5A-HBO-keto and 6A-HBO-keto increase to 1.93, 1.92 and 1.92 Å, and the H–N bond lengths reduce to 1.02, 1.02 and 1.02 Å in S₁ state, respectively. The H–N bonds are more stable in S₁ states. That is to say, for the ESIPT form HBO-keto, 5A-HBO-keto and 6A-HBO-keto, the stable structures exist only in S₁ state, which can be found in the discussion of part 3.3.

Fig. 1 The optimized enol and keto-tautomer structures of the HBO and its amino derivatives at the B3LYP/6-31G(d,p) theoretical level.

Table 1 The calculated geometric parameters (bond lengths in Å and angles in °) for HBO, 5A-HBO and 6A-HBO in ground state and excited state

	HBO-enol		HBO-keto		5A-HBO-enol		5A-HBO-keto		6A-HBO-enol		6A-HBO-keto
Electronic state	S0	S1	S0	S1	S0	S1	S0	S1	S0	S1	S0	S1
O–H	0.99	1.01	1.60	1.93	0.99	1.00	1.63	1.92	0.99	1.01	1.59	1.92
H–N	1.76	1.69	1.06	1.02	1.75	1.73	1.06	1.02	1.76	1.74	1.07	1.02
δ(O–H–N)	147	150	139	126	147	149	138	127	147	149	139	127

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The vibrational frequencies of the O–H stretching vibration can provide evidence for excited state intramolecular hydrogen bond strengthening or weakening through the electronic spectra red-shift or blue-shift.²⁸ Therefore, we used this rule to detect whether the intramolecular hydrogen bonds O–H⋯N are strengthened or not. The calculated infrared spectra of the O–H bonds both in S₀ and S₁ states are shown in Fig. 2. It can be noted that, for HBO in S₀ state, the calculated O–H stretching vibrational frequency is located at 3306 cm⁻¹, whereas the mode in S₁ state is 2915 cm⁻¹. The strong red-shift of 391 cm⁻¹ for the O–H stretching mode suggests that the intramolecular hydrogen bond O–H⋯N is significantly strengthened in S₁ state. Analogously, the O–H stretching band red-shifts from 3295 cm⁻¹ in S₀ state to 3269 cm⁻¹ in S₁ state for 5A-HBO and from 3299 cm⁻¹ to 3214 cm⁻¹ for 6A-HBO. The stretching mode of O–H group in S₀ state for HBO is red-shift of 391 cm⁻¹, which is larger than that of 5A-HBO (26 cm⁻¹) and 6A-HBO (85 cm⁻¹). It can also be concluded that the intramolecular hydrogen bond of 5A-HBO and 6A-HBO is weaker than that of HBO due to the influence of amino group in HBO framework.

Fig. 2 Calculated IR spectra of HBO (a), 5A-HBO (b) and 6A-HBO (c) in the spectral region of O–H stretching band based on the B3LYP/6-31G(d,p) theoretical level.

3.2. Electronic spectra and frontier molecular orbitals (MOs)

The calculated absorption and fluorescence spectra of HBO, 5A-HBO and 6A-HBO at the TD-DFT/B3LYP/6-31G(d,p) theoretical level are shown in Fig. 3. The absorption peak of HBO was at 318.1 nm, which is in good agreement with the experimental result of 280–330 nm.⁵³ The calculated fluorescence peak of HBO was at 325.3 nm. A red-shift of 7.2 nm relative to the absorption peak should be ascribed to the Stokes shift. In addition, the absorption peaks of 5A-HBO and 6A-HBO were at 369.7 and 348.1 nm, which is consistent with the experimental result of 346 and 339 nm.^42,54 Similarly, the red-shift of 5.7 nm for 5A-HBO and 10.0 nm for 6A-HBO are ascribed to the Stokes shift. The agreement with the experimental values justifies the validity of our calculation method and the red-shift of the electronic emissive spectra is believed to influence the ESIPT process. The amino group on the benzoxazole rings gives rise to a red-shift in the absorption and fluorescence spectra of 5A-HBO and 6A-HBO when compared with HBO. However, 5A-HBO and 6A-HBO exhibit different spectra bands, reflecting that the different position of the amino groups in HBO molecular structure may influence the distribution of the spectra bands.

Fig. 3 The calculated electronic spectra of HBO (a), 5A-HBO (b) and 6A-HBO (c) at B3LYP/6-31G(d,p) theoretical level.

To further investigate the nature of the charge distribution and charge transfer in the electronic excited state, the frontier MOs of HBO, 5A-HBO and 6A-HBO in DCM are depicted in Fig. 4. The calculated electronic excitation energies and the corresponding oscillator strengths of the first six states for the HBO, 5A-HBO and 6A-HBO are listed in Table 2. The oscillator strength of S₀ → S₁ transition of HBO, 5A-HBO and 6A-HBO are 0.544, 0.276 and 0.744, respectively, showing that the amino group has a positive effect on 6A-HBO. This paper only discussed the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) since the first excited is associated with the two orbitals. It is noted that the HOMO orbitals of the studied molecular system are π type orbitals while the LUMO orbitals are π* character, indicating that the first excited states are of the ππ*-type transition from HOMO to LUMO. The HOMO and LUMO are localized on different parts of the molecules. In addition, the H⋯N bonds in the LUMO orbital has σ character, showing that the N atoms with strong electron pair donation ability can form a covalent bond with the H atoms after photo-excitation to the S₁ state. The calculated Mulliken charges of the hydroxyl moiety O atom and the neighboring N atom for HBO, 5A-HBO and 6A-HBO are shown in Table 3. Upon transition from the HOMO to LUMO, the decrease of electron density in the hydroxyl moiety and the increase in N atoms are expected to directly influence the intramolecular hydrogen bonding. That is, the H⋯N bonds length is shortened upon excitation to S₁ state. From the viewpoint of the valence bond theory, the transition from S₀ to S₁ state leads to more negative charge distributed on the N atoms, and the enlarged interaction between the lone pair electron of N atoms and the σ* (O–H) orbital will facilitate ESIPT processes from O atom to N atom. Therefore, the ESIPT is expected to occur due to the intramolecular charge transfer.

Fig. 4 Calculated frontier molecular orbitals HOMO and LUMO for HBO (a), 5A-HBO (b) and 6A-HBO (c) at TD-DFT/B3LYP/6-31G(d,p) theoretical level.

Table 2 The calculated electronic excitation energy (nm) and corresponding oscillator strengths (OS) of HBO, 5A-HBO and 6A-HBO at TD-DFT/B3LYP/6-31G(d,p) level. The orbital transitions and contributions are also listed. H: highest occupied molecular orbital (HOMO); L: lowest unoccupied molecular orbital (LUMO)

	HBO		5A-HBO		6A-HBO
	E (nm)	OS	E (nm)	OS	E (nm)	OS
S1	318	0.544	370	0.276	348	0.744
	H → L (0.953)		H → L (0.987)		H → L (0.987)
Exp.	280–330		346		339
S2	279	0.329	309	0.295	293	0.025
S3	264	0.033	275	0.336	273	0.021
S4	236	0.004	260	0.024	252	0.055
S5	228	0.042	248	0.009	248	0.086
S6	219	0.001	233	0.075	236	0.206

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Table 3 The calculated Mulliken charges (in a.u.) of the hydroxyl moiety O atom and the neighboring N atom for HBO, 5A-HBO and 6A-HBO at TD-DFT/B3LYP/6-31G(d,p) level

	HBO		5A-HBO		6A-HBO
	S0	S1	S0	S1	S0	S1
O	−0.589	−0.542	−0.592	−0.591	−0.592	−0.581
N	−0.662	−0.684	−0.673	−0.709	−0.663	−0.704

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The total Mulliken charge distribution analysis and natural bond orbital (NBO) analysis of the phenol moiety for HBO, 5A-HBO and 6A-HBO are shown in Table 4. It is noted that, the charge of the phenol moiety of HBO increases from 0.071 in S₀ state to 0.133 in S₁ state. For 5A-HBO and 6A-HBO, the charges of the phenol moiety decrease from 0.051 to −0.218 and 0.045 to −0.149, respectively. In addition, the NBO analysis method has also been used to investigate the charge distribution. The charge of the phenol moiety increases from 0.033 in S₀ state to 0.118 in S₁ state for HBO, while it decrease from 0.017 to −0.259 and 0.009 to −0.187 respectively for 5A-HBO and 6A-HBO. The analogous results between Mulliken charge distribution analysis and NBO analysis were obtained. It can be concluded that the electronic density migrates from phenol ring to the benzoxazole ring of HBO after a vertical excitation to S₁ state, while for 5A-HBO and 6A-HBO it moves from the amino-benzoxazole moiety to the phenol ring. On the whole, the presence of the amino group modifies the electronic density of the molecule, which can influence the ESIPT reaction in S₁ state.

Table 4 The calculated total Mulliken (no parentheses) and NBO (in parentheses) charges (in a.u.) of the phenol moiety for HBO, 5A-HBO and 6A-HBO at TD-DFT/B3LYP/6-31G(d,p) level

	S0	S1
HBO	0.071(0.033)	0.133(0.118)
5A-HBO	0.051(0.017)	−0.218(−0.259)
6A-HBO	0.045(0.009)	−0.149(−0.187)

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3.3. Potential energy curves

To further reveal the nature of the ESIPT process, the S₀ state and S₁ state potential energy curves of the HBO, 5A-HBO and 6A-HBO were scanned based on constrained optimizations in their corresponding electronic states at fixed O–H distance for a series of values. Previous research indicates that the TD-DFT/B3LYP calculation method is reliable in exploring the shape of the hydrogen-transfer potential energy curves of ESIPT process, though this method may be expected to be inaccurate in surmounting the correct ordering of the closely spaced excited states.^55–57 The potential energy curves were constructed, keeping the O–H bond distance fixed at values in the range from 0.99 Å to 2.15 Å in steps of 0.05 Å, as shown in Fig. 5. These potential energy curves can provide qualitative description for the ESIPT process. It can be seen clearly that the energy of the S₀ state increases along with the lengthening of the O–H bond from the initial length without a stable energy point and that the potential barriers are 10.47, 10.01 and 9.65 kcal mol⁻¹, respectively. While, the S₁ state potential energy curves exhibit a barrier of 2.30, 8.74 and 8.73 kcal mol⁻¹, respectively. The ESIPT processes are therefore more likely to proceed in S₁ state than in S₀ state. After crossing the potential barrier, the energy of the S₁ state decreases without any barrier with the increase of the O–H bond length until the energy reaches a stable point at the O–H bond length about 1.891, 1.893 and 1.891 Å, which corresponds to the stable optimized geometry of the HBO-keto, 5A-HBO-keto and 6A-HBO-keto tautomers.

Fig. 5 Potential energy curves of S₀ and S₁ states for 6A-HBO–MeOH complex along with O–H bond length of 6A-HBO. The insert shows the corresponding optimized geometries.

In order to reveal the potential energies more visually, the precise Hartree–Fock energy values of the S₀ state and the S₁ state for HBO, 5A-HBO and 6A-HBO are shown in Table 5. It is noted that the S₀ state is more stable than the S₁ state, indicating that the proton transfer processes are unlikely to occur in the S₀ state. Thus, it can be concluded that in S₀ state the N atom can capture the proton of the hydroxyl group to form an intramolecular hydrogen bond O–H⋯N. Through photo-excitation, the intramolecular hydrogen bonds were strengthened, which provides a driving force in facilitating the ESIPT reaction. Subsequently, the ESIPT occurs with the hydroxyl group acting as a proton donor and the neighboring N atom acting as a proton acceptor. The lower potential barriers of the S₁ state can be easily overcome to reach the stable optimized geometry of the HBO-keto, 5A-HBO-keto and 6A-HBO-keto tautomers.

Table 5 Potential energies (hartree) of these stable structures on the potential energy curves of the S₀ and S₁ states based on the DFT and TD-DFT methods, respectively

	HBO		5A-HBO		6A-HBO
	S0	S1	S0	S1	S0	S1
Energy	−706.043	−706.034	−761.401	−761.392	−761.404	−761.395

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As described above, the S₁ state potential energy curves exhibit a barrier of 2.30, 8.74 and 8.73 kcal mol⁻¹ for HBO, 5A-HBO and 6A-HBO, respectively. This result indicates that the ESIPT process can more easily occur for HBO by undergoing a relatively low barrier of 2.30 kcal mol⁻¹. While for 5A-HBO and 6A-HBO, the ESIPT processes should overcome the larger barriers of 8.74 and 8.73 kcal mol⁻¹, respectively. This finding shows that the presence and position of the amino group in the HBO framework may hinder the ESIPT processes to some extent.

4. Conclusions

In this work, an investigation has been performed to study the ESIPT processes of the HBO and its amino derivatives based on DFT and TD-DFT methods using B3LYP exchange-correlation functional at the basis set of 6-31G(d,p). The DCM was selected as the solvent in the calculations using the IEFPCM. The analysis of the bond lengths and IR vibrational spectra indicates that the intramolecular hydrogen bond O–H⋯N is significantly strengthened in the electronically excited state, which provides a driving force in facilitating the ESIPT reaction. In addition, the calculations of the absorption spectra are consistent with the experimental results, showing that the TD-DFT method can be used to decipher the experiment reasonably. The analysis of the Mulliken's charge distribution and the frontier MOs reveals that the arrangement of electronic density is an important positive factor for the proton transfer and that the ESIPT reaction occurs after the intramolecular charge transfer in S₁ state. The analysis of the potential energy curves and Hartree–Fock energy show that the ESIPT reaction is unlikely occurs in S₀ state.

What's more, the intramolecular hydrogen bond of 5A-HBO-enol (1.73 Å) and 6A-HBO-enol (1.74 Å) in S₁ state is weaker than that of HBO-enol (1.69 Å) due to the influence of amino group in HBO framework. After a vertical excitation to S₁ state, the electronic density redistributes and migrates from phenol ring to benzoxazole ring of HBO. For 5A-HBO and 6A-HBO, it transfers from the amino-benzoxazole moiety to the phenol ring. The analysis of the potential energy curves of HBO, 5A-HBO and 6A-HBO indicate that the ESIPT process of HBO most easily occurs. It is demonstrated that the presence and position of the amino group in the HBO framework can change the behaviors of the intramolecular hydrogen bonds O–H⋯N in S₁ state and thus hinder the ESIPT processes to some extent. Our findings might be enlightening in the design of proton transfer materials and related fields.

Acknowledgements

This work is supported by National Natural science Foundation of China (Grant No. 11274096), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 124200510013) and Innovative Research Team in Science and Technology in University of Henan Province (Grant No. 13IRTSTHN016).

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