Laura
Le Bras
a,
Karen
Chaitou
a,
Stéphane
Aloïse
b,
Carlo
Adamo
ac and
Aurélie
Perrier
*ad
aChimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris (IRCP), F-75005 Paris, France
bUniv. Lille, CNRS, UMR 8516, LASIR, Laboratoire de Spectrochimie Infrarouge et Raman, F59 000 Lille, France
cInstitut Universitaire de France, 103 Boulevard Saint Michel, F-75005 Paris, France
dUniversité Paris Diderot, Sorbonne Paris Cité, 5 rue Thomas Mann, F-75205 Paris Cedex 13, France. E-mail: aurelie.perrier-pineau@univ-paris-diderot.fr
First published on 10th October 2018
We report a QM (TD-DFT) and QM/QM′ (ONIOM) study of the modulation of emission in a series of thiazolo[5,4,b]thieno[3,2-e]pyridine (TTP) derivatives [Huang et al., J. Mater. Chem. C, 2017, 14, 3456]. By computing the excitation energy transfer couplings and the Huang–Rhys (HR) factors, we rationalize the aggregation-caused quenching (ACQ) observed for the parent molecule and the crystallization-induced emission (CIE) observed for the derivatives presenting intra-molecular H-bonding. We also show that the CIE strategy relying on the rigidification of the arch-bridge-like stator should be considered with caution since it can promote the energy dissipation through vibrational motions.
In combination with experimental works, theoretical studies have been undertaken to provide a better comprehension of the ACQ and AIE/CIE phenomena. Due to the necessity to describe diverse environments (solution, amorphous phases, aggregates, and crystals), different approaches have been considered ranging from quantum mechanics (QM) calculations relying on Density Functional Theory (DFT) and its Time-Dependent counterpart (TD-DFT) in molecular or crystalline environments, to molecular dynamics (MD). Some recent works have shown that aggregation effects can block the low-frequency movements and thus hinder non-radiative decay channels.12–14 The non-radiative decay of isolated molecules was also attributed to the existence of a conical intersection between the ground and the first excited states, as demonstrated by excited state dynamics combined with QM calculations.15 In a recent work, we have investigated the modulation of the emission behavior of a dipyrrolyldiphenylethene molecule in three different phases (solution, crystals, and aggregates) with the help of MD, (TD-)DFT and hybrid QM/QM′ calculation. We have demonstrated that in crystals and within the aggregates, the steric hindrance effect strongly modifies the topology of the first excited-state potential energy surface and this results in a restriction of the vibrational modes involved in the energy dissipation.16
In this work, we are interested in the properties of a new family of CIE luminogens. A fluorophore family based on the same core, namely a thiazolo[5,4-b]thieno[3,2-e]pyridine (TTP) stator (Fig. 1), has been found to exhibit interesting luminescent properties.17 The parent molecule 0 shows weak PL efficiency in benzene (Φf = 10%, see Table 1) and a very weak solid PL efficiency (only 1%). An intramolecular H bond between the stator and the rotor (molecule 1, Fig. 1) was introduced to rigidify the structure, restrict the intramolecular rotation and thus increase the solution PL efficiency. However, Huang et al. have shown that the rigidification of the structure leads to a decrease of the PL efficiency in a non-polar solvent (benzene). At the same time, an enhancement of the emission properties in the solid state was observed. While the relative PL efficiency of 0 and 1 in solution has not be rationalized, the particular arrangement of the molecules in the condensed phase is a possible explanation for the ACQ for molecule 0 and for the CIE behavior observed for 1. Based on this first study, Wan and coworkers have synthesized a large number of molecules which are able to emit all along the visible spectra (403 nm ≤ λem ≤ 655 nm).18 The molecules present subtle different structures thanks to various substitutions on the rotor (the A-ring in Fig. 1). Among the pool of synthesized compounds, molecules 2–5, displayed in Fig. 1, exhibit much stronger PL efficiencies in the solid phase than in solution. One can also notice that for compounds 1 to 5, a large Stokes shift is observed and can be attributed to an excited state intramolecular proton transfer (ESIPT) phenomenon.
Fig. 1 Representation of the structures and selected atom numbering for molecules 0 to 5. The dihedral angles ϕ and ψ are also defined. |
Solution | Solid state | |||||
---|---|---|---|---|---|---|
λ abs | λ em | Φ f | λ abs | λ em | Φ f | |
0 | 360 | 387 | 10 | 394 | 452 | 1 |
1 | 373 | 545 | 2 | 374 | 550 | 26 |
2 | 375 | 530 | 12 | 393 | 530 | 55 |
3 | 373 | 545 | 4 | 400 | 532 | 60 |
4 | 377 | 570 | 4 | 400 | 550 | 60 |
5 | 376 | 565 | 4 | 402 | 555 | 62 |
In their experimental work, to rationalize the ACQ or CIE effects, Huang et al. provide some hypotheses based on the mechanisms described in the literature19 as well as the molecular packing in the crystalline phase. However, the behavior of molecule 1 in benzene solution is not discussed and the relationship between the minor structural modifications and the solid state properties could not be fully assessed. In this work, we thus propose to rely on (TD-)DFT calculations combined with the QM/QM′ approach to investigate the structural and optical properties of molecules 0–5 both in solution and in their crystalline environment. We aim at understanding (i) the ACQ phenomenon for 0, (ii) the decrease of the solution PL efficiency of 1 compared to 0 and the moderate CIE effect for the former system and (iii) the larger CIE phenomenon observed for compounds 2–5, in order to formulate structure–photoproperty relationships for this family of luminogens. Our goal is to identify the different processes that can contribute to the non-radiative decay rate and to describe the competition between these effects for the six different molecules in two different phases (solution and crystals). For this purpose, the absorption and emission properties of the different molecules will first be theoretically investigated in solution, with an implicit solvent model, and then used as a reference to rationalize the modulation of emission in the crystalline phase.
A benchmark study of the geometry and absorption properties of molecules 0 and 1 is provided in the ESI.† The absorption properties were calculated with the TDi//optimizationj protocol (i and j corresponding to PBE0, B3LYP or CAM-B3LYP), that is to say 9 different computational strategies. As discussed in the ESI,† while the GS optimized structures are globally not impacted by the choice of the functional, the value of the calculated maximum absorption wavelength depends on the choice of the XC functional. The energy differences between the experimental and calculated excitation energies (ΔEcalc–exp) are gathered in Fig. S1 (ESI†) for the different computational strategies. These results show that both the TD-B3LYP//B3LYP and TD-PBE0//B3LYP computational schemes provide an accurate reproduction of the experimental data with a |ΔEcalc–exp| equal to 0.12 eV for both 0 and 1. For the sake of simplicity, we have thus decided to investigate the structural and optical properties of 0 to 5 with the same XC functional, namely B3LYP.
Huang–Rhys (HR) factors, electron-vibration coupling constants, were also calculated. For an emission process, the dimensionless HR factor illustrates the variation of the jth vibrational mode in the course of the Sn (n = 1 in our case) → S0 deexcitation:
(1) |
(2) |
State | 0 | 1 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|---|
S0 | ||||||
ϕ (°) | −19 | 0 | 0 | 0 | 0 | 1 |
ψ (°) | −117 | −101 | −77 | −103 | −103 | −81 |
Cb–Cc (Å) | 1.466 | 1.457 | 1.457 | 1.454 | 1.456 | 1.456 |
Nd⋯HO (Å) | 1.761 | 1.754 | 1.764 | 1.766 | 1.767 |
State | 0 | 1 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|---|
S1*(E) | S1*(K) | |||||
ϕ (°) | −8 | 4 | −21 | 3 | 2 | 11 |
ψ (°) | −125 | −121 | −65 | −121 | −121 | −59 |
Cb–Cc (Å) | 1.461 | 1.460 | 1.457 | 1.459 | 1.460 | 1.462 |
Nd–H⋯O (Å) | 1.810 | 1.909 | 1.818 | 1.790 | 1.821 |
The value of the maximum absorption wavelength (λabs), given in Table 3, is a direct consequence of the electronic conjugation modulation. Indeed, there is a bathochromic shift from 0 (λabs = 349 nm) to 1–5 (360 nm ≤ λabs ≤ 368 nm). For all the molecules, Table 3 shows a good agreement between calculations and experimental data with the largest excitation energy difference, ΔEcalc–exp, obtained for molecule 3 (0.13 eV). The calculated λabs systematically corresponds to the S0 → S1 transition as shown for molecules 0 and 1 in Table SII (ESI†). This transition can be ascribed to an electronic excitation from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO). Fig. 3 shows that there is no charge transfer for this electronic transition, the frontier orbitals being mainly located both on the stator and the A-ring. A representation of the HOMO and LUMO for molecules 2–5, given in the ESI† (Fig. S2), confirms this conclusion for the other systems.
Molecule | λ (nm) | f | Δλcalc–exp (nm) | λ exp (nm) | |
---|---|---|---|---|---|
0 | abs | 349 | 0.983 | −11 | 360 |
em | 411 | 0.984 | +24 | 387 | |
1 | abs | 360 | 0.740 | −13 | 373 |
em | 525 | 0.321 | −20 | 545 | |
2 | abs | 361 | 0.919 | −14 | 375 |
em | 533 | 0.331 | +3 | 530 | |
3 | abs | 361 | 0.850 | −13 | 374 |
em | 534 | 0.292 | −11 | 545 | |
4 | abs | 369 | 0.648 | −8 | 377 |
em | 557 | 0.295 | −13 | 570 | |
5 | abs | 368 | 0.681 | −8 | 376 |
em | 559 | 0.295 | −6 | 565 |
Fig. 4 Impact of the proton transfer, from an enol to a keto form of 1, on the energetics and optical properties (absorption and emission wavelengths in nm). |
For molecule 0, due to the para position of the hydroxyl moiety, there is no ESIPT phenomenon. The emission signal arises from a relaxation from the enol form S1*(E), thus leading to a small Stokes shift (exp.: 27 nm, calculated: 62 nm). A superposition of the optimized ground S0(E) and first excited S1*(E) state geometries in Fig. 5 shows that the structure becomes more planar with a decrease of the dihedral angle Φ between the stator and the A-ring (−19° to −8°). Besides, there is a rotation around the Cf–Cg bond which leads to an orientation modification for the B-ring. This rotation is illustrated with the evolution of Ψ values in Table 2 (Ψ = −117° for S0 and −125° for S1*(E)). The latter conclusion holds for molecules 1 and 3–5. For these molecules, the rotation of the B-ring is accompanied by the proton transfer and the structure remains planar. On the opposite, for molecule 2, the relaxation on the ES also leads to a loss of planarity: the electron-withdrawing character of the 4-Cl substituent on the A-ring weakens the interaction between the NdH and the keto groups. This induces a rotation of the A-ring with respect to the stator and the ϕ dihedral angle reaches −21° (see Fig. S3, ESI†).
To explain the non-radiative behavior of 0 in solution, Huang et al. proposed (i) the formation of intermolecular hydrogen bonding and (ii) the participation of photo-induced proton transfer.17 However, the formation of intermolecular hydrogen bonds between the solvent and the solute is unlikely in an aprotic solvent. In the same vein, the concentration of the solution (2 × 10−5 M) should not promote the formation of H bonds between solute molecules. The same conclusions hold for the intermolecular photo-induced proton transfer: the concentration of the solution as well as the nature of the solvent should not favor this phenomenon. Hence, the existence of non-radiative energy-dissipating paths should not arise from intermolecular interactions, as originally stated in the experimental study, but from intramolecular-scale processes.
In this framework, recent works have proposed an interpretation of the origin of the non-emissive behavior of AIE/CIE molecules in solution.14,15,38 In some cases, the weak PL efficiency is due to the presence of an energetically accessible conical intersection (CI) leading to an ultrafast internal conversion from the excited state to the ground state. The identification of conical intersections for the considered systems is far from trivial since (i) the molecules are large, (ii) the electronic structure is thus described within the TD-DFT framework and (iii) the environment should be properly described to account for the emission modulation phenomena. At that stage, the partial characterization of the fundamental and first excited potential energy surfaces (minima and transition states as shown in Fig. 4 for 1) has not revealed the existence of S1/S0 crossing.
Finally, the role of photophysical energy dissipation caused by vibrational motions was investigated to rationalize the weak emission signal in solution. For this purpose, the HR factors were computed and used as an indicator of the contribution of vibrational modes in the non-radiative relaxation pathways. Indeed, the non-radiative decay rate is influenced by the internal conversion (IC) rate which directly relies on these factors.14,16 The calculated HR factors are presented in Fig. 6 for 0 and 1 and also in the ESI† for the four other molecules. For all the compounds, one vibrational mode is of major importance and is always associated with one of the lowest frequencies (ω = 20 cm−1 for 0 and 21 cm−1 for 1). A large HR factor is found for this mode, HR = 5 and 12 for 0 and 1 respectively (the complete list of HR factors and the corresponding reorganization energies is given in the ESI†). One can notice that for all the structures, this mode corresponds to the same motion (see Fig. 7 for a schematic representation of this vibration mode).
Fig. 6 Calculated HR factors versus the normal-mode wave numbers (ω) in the S1 state for 0 (left) and 1 (right). Vibration modes yielding the largest HR factors are also represented. |
Fig. 7 Schematic representation of the vibrational mode corresponding to the large HR value (ω = 20 cm−1, HR = 5 for molecule 0). |
One can notice that 0 is more emissive than 1, with a PL efficiency of 10% and 2% respectively. For 1, the intramolecular hydrogen bond was originally introduced to rigidify the molecular structure, activate the Restriction of Intramolecular Motion (RIR) and enhance PL efficiency. The counter-intuitive behaviour observed for 1 can be rationalized by the identification of the vibrational mode contributing to the energy dissipation: this mode implies bending of the conjugated core (stator and A-ring) above and under the molecular plane (Fig. 7). For molecule 1, the “A-ring–stator” moiety becomes more conjugated. The rigidification of the structure thus induces an enhancement of the HR factor and an increase of the reorganization energy (105 cm−1 for 0 and 275 cm−1 for 1). As a matter of fact, the intramolecular H-bond induces less structural reorganization while relaxing on the S1 potential energy surface and this should enhance the HR value according to eqn (1).
The same conclusion holds for molecules 3, 4 and 5. They exhibit a relatively small fluorescence quantum yield which is due to the participation of the same low-frequency vibration mode (Fig. S5, ESI†), with a high HR factor (HR = 12, 12 and 23 for 3, 4 and 5 respectively), to the IC and thus the non-radiative deexcitation processes. We can also deduce that for alkyl substituents, neither the substitution position (4-Me for 3 and 5-Me for 4) nor the bulkiness of the group (5-Me for 4 ad 5-t-Bu for 5) influence the emission properties. For compound 2 with a 4-Cl substituent, there is not only one low-frequency vibration mode with a high HR factor but two (HR = 7 and 6 for ω = 20 cm−1 and ω = 30 cm−1, see Table SII in the ESI†). One can notice that these frequencies correspond to the same nearly degenerate vibrational mode. One frequency mainly implies the stator (ω = 20 cm−1) while for the second one (ω = 30 cm−1), the A-ring is mainly involved (Fig. S5, ESI†). This different behavior and the decrease of the HR values compared to molecules 3–5 might be rationalized by the twist of the A-ring with respect to the stator for the S1*(K) structure of 2. Therefore, one can observe a slight modulation of PL efficiency between 2 (Φf = 12%) and 3, and 4 and 5 (Φf = 4% for the three molecules).
Fig. 8 Crystal structure of compound 0 with the three monomers A, B and C represented. The intermolecular distances d1 to d5 are defined. |
In the course of the geometry optimizations, we have optimized the positions of respectively one, two and three molecules for the 0-M, 0-D and 0-T models, the positions of the other molecules (respectively 25, 24 and 23 molecules) being frozen.
In Table 4, selected optimized geometrical parameters obtained with the three models are compared to experiments. For the intramolecular structural parameters, namely the two dihedral angles and the Cb–Cc bond distance, one can conclude that (i) the three different models provide the same results and (ii) there is no major difference between the experimental and calculated values. The same conclusions hold for the intermolecular π–π interactions that are described by the distances between two ring centers (d1 to d5 in Fig. 8). The small intermolecular distances d1 to d3 (∼3.75 Å) show that the two monomers A and B are close enough to allow the emergence of short range interactions.39 The interaction between A and C is weaker with only one π-stacking distance (d4) shorter than 4 Å. We can also note that our computational strategy, that does not include dispersion corrections, provides intermolecular distances in agreement with experiments. The crystal packing is thus controlled by steric effects rather than dispersion.
Exp. | 0-M | 0-D | 0-T | |
---|---|---|---|---|
ϕ (°) | −19 | −17 | −18, −18 | −18, −18, −19 |
ψ (°) | −117 | −114 | −116, −115 | −114, −115, −114 |
Cb–Cc (Å) | 1.465 | 1.468 | 1.467, 1.467 | 1.466, 1.467, 1.466 |
d 1 (Å) | 3.784 | 3.782 | 3.779 | 3.781 |
d 2 (Å) | 3.768 | 3.766 | 3.762 | 3.691 |
d 3 (Å) | 3.690 | 3.778 | 3.606 | 3.667 |
d 4 (Å) | 3.766 | 4.022 | 3.695 | 3.703 |
d 5 (Å) | 4.424 | 4.256 | 4.091 | 4.054 |
To explore the possible excitonic coupling, we have computed the absorption spectra for the three different models (Table 5). For monomer 0-M, compared to the isolated molecule in solution (Table 2 and Table SII, ESI†), the maximum absorption band is also ascribed to a HOMO → LUMO electronic excitation, with the frontier orbitals provided in Fig. S8 (ESI†) identical to the solvated monomer ones. The crystal environment shifts this state to the blue (λ = 338 nm) compared to the absorption in solution (λ = 349 nm). This finding is in disagreement with experiments: a red-shift of the absorption band has been observed while going from the solution (λ = 360 nm) to the crystal (λ = 400 nm). For the 0-D model, the absorption spectrum is dominated by two transitions. The frontier orbitals involved in the corresponding electronic excitations are delocalized over the two chromophores (Fig. 9). Even if the brightest state is also blue-shifted compared to the solution (λ = 335 nm), the convoluted absorption spectrum shows an enlargement of the absorption band towards larger wavelengths compared to the 0-M model (Fig. 10). The same conclusions can be drawn for the 0-T model: (i) the frontier orbitals involved in the main transitions are delocalised over the three chromophores (Fig. S8, ESI†) and (ii) there is a global broadening of the spectrum for λ ≥ 350 nm that is due to numerous low-energetic transitions (with small f values) arising from the excitonic coupling.
System | State | λ abs (nm) | f | Assignment |
---|---|---|---|---|
0-M | S1 | 344 | 0.116 | HOMO−1 → LUMO |
S2 | 338 | 0.557 | HOMO → LUMO | |
0-D | S3 | 351 | 0.221 | HOMO−1 → LUMO |
S7 | 335 | 0.586 | HOMO−2 → LUMO+1 | |
0-T | S3 | 362 | 0.127 | HOMO−1 → LUMO |
S6 | 349 | 0.553 | HOMO−2 → LUMO | |
S11 | 338 | 0.508 | HOMO−3 → LUMO+1 | |
1-M | S1 | 360 | 0.312 | HOMO → LUMO |
S2 | 355 | 0.225 | HOMO−1 → LUMO | |
1-D | S2 | 370 | 0.094 | HOMO → LUMO |
S4 | 359 | 0.257 | HOMO → LUMO+1 | |
S5 | 351 | 0.133 | HOMO−2 → LUMO |
Fig. 9 Molecular orbitals calculated for the 0-D model. Only chromophores A and B are represented (isodensity = 0.025 a.u.). |
Fig. 10 Calculated absorption spectra of the 0-M (black), 0-D (red) and 0-T (blue) models. The sticks have been convoluted with a Gaussian presenting a FWHM of 0.3 eV. |
Further understanding can be achieved by calculating the excitonic couplings between the three chromophores. In Table 6, the largest coupling, on the order of −100 meV, corresponds to the A–B interaction and the negative sign shows the existence of J-aggregates at the origin of the bathochromically shifted bands.40 The JA–C coupling is also negative and reaches ∼−60 meV. A recent work has demonstrated that excitonic coupling always enhances the non-radiative decay constants, regardless of the sign of J.41 Therefore, for 0, in going from the solution to the crystal environment, the complex molecular packing with two different π-stacking arrangements promotes a non-radiative relaxation pathway through excitonic couplings and this phenomenon is at the origin of the observed ACQ.
Molecule | 0 | 1 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|---|
J A–B | 0.097 | −0.024 | −0.021 | 0.015 | 0.015 | 0.001 |
J A–C | −0.057 | 0.017 |
State | 0 | 1 | 2 | 3 | 4 | 5 | |
---|---|---|---|---|---|---|---|
S0 | |||||||
ϕ (°) | Exp. | −19 | 6 | 1 | −5 | −1 | 4 |
Cal. | −17 | 6 | −2 | −4 | −1 | 4 | |
ψ (°) | Exp. | −117 | −116 | −63 | −116 | −112 | −63 |
Calc. | −114 | −115 | −67 | −118 | −112 | −63 | |
Cb–Cc (Å) | Exp. | 1.465 | 1.452 | 1.460 | 1.451 | 1.453 | 1.458 |
Calc. | 1.468 | 1.455 | 1.457 | 1.449 | 1.452 | 1.451 | |
Nd⋯HO (Å) | Exp. | 1.951 | 1.892 | 1.898 | 1.874 | 1.900 | |
Calc. | 1.781 | 1.724 | 1.749 | 1.753 | 1.781 |
State | 0 | 1 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|---|
S1*(E) | S1*(K) | |||||
ϕ (°) | −11 | 6 | −1 | −6 | −8 | 4 |
ψ (°) | −120 | −120 | −64 | −126 | −119 | −58 |
Cb–Cc (Å) | 1.422 | 1.451 | 1.464 | 1.471 | 1.476 | 1.474 |
Nd–H⋯O (Å) | 1.852 | 1.880 | 1.935 | 1.807 | 1.943 |
The comparison of the calculated emission energy in Table 8 with experiments reveals a reasonable blue-shift deviation (−0.25 eV). After calculating the vibrational spectrum of the S1*(E) structure, we have carefully investigated the low-frequency vibrational modes and compared them with the ones obtained in solution. This comparison reveals that the mode contributing to the energy dissipation in solution (20 cm−1) can be retrieved in the crystalline environment. As illustrated in Fig. 11, despite the intermolecular interactions arising from the π–π stacking, the packing structure for 0 does not restrict this vibrational motion.
Molecule | λ (nm) | f | ΔEcalc–exp (eV) | λ exp (nm) | |
---|---|---|---|---|---|
0 | abs | 338 | 0.557 | −0.52 | 394 |
em | 415 | 0.182 | −0.25 | 452 | |
1 | abs | 360 | 0.313 | −0.13 | 374 |
em | 567 | 0.141 | 0.07 | 550 | |
2 | abs | 360 | 0.625 | −0.29 | 393 |
em | 486 | 0.303 | −0.21 | 530 | |
3 | abs | 361 | 0.557 | −0.34 | 400 |
em | 601 | 0.124 | 0.27 | 532 | |
4 | abs | 366 | 0.433 | −0.29 | 400 |
em | 605 | 0.134 | 0.20 | 550 | |
5 | abs | 365 | 0.481 | −0.31 | 402 |
em | 611 | 0.152 | 0.29 | 555 |
Fig. 11 Low-frequency vibrational spectrum (0–500 cm−1) calculated for the S1*(E) structure for 0 in the crystalline phase. Vibrational modes presenting similarities to the mode with a large HR value in solution (Fig. 7 and 8) are given. |
We can thus conclude that the ACQ phenomenon arises from the combination of two processes: the energy loss through excitation energy transfers between the molecules in the crystal, as proposed by Huang et al. in their experimental work, and through vibrational motions (rather than intramolecular rotations).
Fig. 12 Representation of the packing structure of 1. Distances (in angstroms) between the two monomers are also indicated. The calculated values correspond to the 1-D model. |
In Table 7, the comparison of selected optimized geometrical parameters with X-ray crystallographic data shows that the methodology we have used is appropriate to mimic the crystalline structure. Globally, for all the molecules, there is no major difference between the experimental and calculated dihedral angles ϕ and ψ. Our calculation scheme also provides an accurate description of the Cb–Cc bond distance that accounts for the conjugation between the stator and the A-ring. There is small variation of the H-bonding between the Nd atom of the stator and the hydroxyl group: our calculated values are systematically shorter than the experimental ones, with an average underestimation of 0.15 Å. Besides, due to the relative rigidity of the conjugated core (stator and A-ring), the optimized structures in the crystalline phase are slightly modified compared to those in solution.
We have then investigated the absorption spectra of the different systems. For 1, we have considered both the 1-M and 1-D models with respectively the monomer A and the dimer A–B defining the QM region. In going from a monomer chromophore to a dimer system, the brightest state shifts from S1 to S4 as shown in Table 5 but there is no modification of the maximum absorption wavelength (∼360 nm). This finding is in agreement with the experimental data collected in Table 1. For the dimer, the last occupied molecular orbitals that are involved in the main electronic transitions are delocalized on the two chromophores whereas the first low-lying virtual orbitals are localized on the stator and the A-ring moieties of only one molecule (Fig. S9, ESI†). Calculations of the J coupling show that the excitonic coupling is four times smaller than the one calculated for 0. Therefore, one should expect an impact of the excitonic coupling on the non-radiative decay constant but this effect should be less significant than 0.
For the systems i = 3–5, the absorption energies calculated for the different model systems (monomers i-M and dimers i-M) are given in Table SX (ESI†). The value of the J coupling in Table 6 is positive, which might indicate the formation of H-aggregates. More importantly, these values are close to zero. As a consequence, the non-radiative decay constant should not be impacted by the intermolecular interactions for these systems.
For system 2, the excited-state properties of the trimer model 2-T in Table SX (ESI†) combined with the value of the J coupling reveal a more complicated situation with a larger modification of the spectrum compared to the 2-M model (there is a large decrease of the oscillator strength value) and two J coupling values, one positive and one negative.
Finally, the first excited state was optimized for all the molecules and the resulting structures were compared to those obtained in solution. There is no major difference between the ground state and S1*(K) structures except for the proton transfer. Compared to the solvated molecules, there is no rotation of the B-ring upon relaxation on the S1 state. Moreover, contrary to the solution, there is no loss of planarity for 2 upon relaxation on the S1 state.
Calculated emission energies for 1–5 are reported in Table 8. They exhibit either a blue (0.21 eV for 2) or a red (0.07, 0.27, 0.20 and 0.29 for 1, 3, 4, 5 respectively) shift compared to experiments. Even if the ΔEcalc–exp values in the crystal are globally larger than the ones calculated in solution, they remain in the range of values obtained with the approximation that has been made (a central molecule with a frozen crystalline environment) and that are reported in the literature for TD-DFT calculations in molecular crystals.42,43
The low-frequency vibrational mode of the S1*(K) structure has then been investigated. For 1, we have identified one vibrational mode potentially responsible for the non-emissive behavior in solution. However, in the crystalline environment, this vibrational mode could not be identified. As depicted in Fig. S8 (ESI†), the particular packing of 1 leads to strong restrictions of molecular motions previously allowed in solution. Therefore, for 1, there are two opposing phenomena: (i) the small but non-negligible value of the exciton coupling J that should enhance the non-radiative decay rate and (ii) the hindrance in the molecular plane bending motions that should lead to a decrease of the same non-radiative decay rate through reducing the IC rate. This competition is in favor of the second effect thus leading to an enhancement of the PL efficiency: experimentally, Φf increases from 2% to 26% and 1 presents a moderate CIE effect. This conclusion is in agreement with a recent study that has demonstrated that for CIE luminogens, the exciton coupling does not have a major effect on the non-radiative decay rate.14 It can enhance this rate by about 12–33% but does not modify the order of magnitudes of this constant. Finally, one should note that these two competitive effects both arise from the molecular packing of 1 in the crystalline phase. Contrary to the hypotheses of Huang et al.,18 the intramolecular H-bonding is not directly responsible for the increase of the PL efficiency.
In the same vein, we have calculated the vibrational spectrum of S1*(K) for 2–5 and inspected the low-frequency vibrational modes. Due to the crystal packing, the vibrational mode potentially contributing to the energy dissipation in solution is no longer present in the crystal phase. Since the exciton coupling effects are negligible for these systems, this finding can rationalize the observed emission modulation and should explain the strong increase of Φf.
In crystals, for molecule 0, the ACQ phenomenon arises from the combination of two processes contributing to the non-radiative decay: (i) the energy loss through excitonic couplings that is due to a particular π–π stacking arrangement and (ii) the energy dissipation during the internal conversion process through the low-frequency vibration mode identified in solution. On the opposite, for the other systems, the crystal environment results in the blocking of this bending vibrational mode. For molecule 1, the non-radiative relaxation pathway through excitonic coupling is also possible and the competition between these two antagonist effects finally leads to a moderate CIE effect. For the other systems, we have demonstrated that the excitonic coupling is negligible and thus rationalized the larger CIE phenomenon observed for 2–5.
Therefore, this study reveals that the combination of adequate theoretical strategies enables a careful investigation of the different photophysical processes at the origin of the ACQ/CIE effects and a qualitative understanding of the experimental observations. The appropriate modeling of the environmental effects is crucial to pave the way towards the development of novel CIE functional materials.
Footnote |
† Electronic supplementary information (ESI) available: Choice of the XC functional, selected molecular orbitals; frequencies and HR factors; packing structures in single crystals. See DOI: 10.1039/c8cp04730h |
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