Exploring the Mechanism of Fluorescence Quenching and

Article pubs.acs.org/JPCC

Exploring the Mechanism of Fluorescence Quenching and Aggregation-Induced Emission of a Phenylethylene Derivative by QM (CASSCF and TDDFT) and ONIOM (QM:MM) Calculations Bin Wang, Xiaojuan Wang, Wenliang Wang, and Fengyi Liu* Key Laboratory for Macromolecular Science of Shaanxi Province, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi 710062, P. R. China S Supporting Information *

ABSTRACT: We report a QM (including TD-DFT and CASSCF) and ONIOM (QM:MM) study on the fluorescence quenching in methanol solution and fluorescence enhancement in crystal for a styrene derivative, namely 4-diethylamino-2 benzylidene malonic acid dimethyl ester (BIM) that possesses push−pull structure and AIE properties. The results showed that in methanol solution the weakening of ethylenic CC bond after photoexcitation initiates a barrierless relaxation via one-bond rotation around it, until the reactive molecule reaches a lowenergy intermediate with strong charge-transfer character, then a S1/S0 conical intersection optimized near the charge-transfer intermediate is responsible for the fluorescence quenching in the dilute solution. The existences of charge-transfer intermediate as well as the conical intersection in the vicinity, which has not been observed in other symmetric (or less polar) phenylethylenebased luminophores, are the major features of BIM in solution. While in crystalline phase, the excited-state deactivation channels via torsional motions, either via one-bond rotation or via hula-twist mechanism, are restricted by steric hindrance and electrostatic repulsion from surrounding molecules, and thus fluorescence is enhanced.

1. INTRODUCTION Organic luminescent materials have attracted an enormous amount of attention in recent years due to their various advantages with respect to their inorganic counterparts, including cheaper production, easy fabrication, tunable emission wavelength, etc.1−4 In the meantime, the drawback is evident: For most organic fluorophores, the strong luminescence measured in diluted solution becomes weakened or even totally quenched at high concertation. Such phenomena are known as “concentration quenching (CQ)” or “aggregation-caused quenching (ACQ)”.5,6 The wide existence of ACQ phenomena has been a major obstacle that prevents organic luminophores from practical applications in solid phase. In 2001, an unusual phenomenon of 1-methyl-1,2,3,4,5pentaphenylsilole was observed by Tang et al., that is, the luminescence is weak in solution while it is greatly enhanced in molecular aggregates.7 The finding of “aggregation-induced emission (AIE)” lifted the development of organic luminescence materials out of the difficulty caused by ACQ,8−10 and since then, diverse luminophores with AIE properties have been synthesized11 and successfully applied as organic light-emitting diodes (OLEDs),12,13 chemo- and biosensors and probes,14−17 etc. Among vast number of AIE luminophores, the phenylethylene derivatives have drawn special attention,18−22 not only © 2016 American Chemical Society

due to their simplicity in backbone but also because of their flexibilities in structural modulation and spectroscopic tuning. Therefore, it is not surprising that AIE mechanisms of phenylethylene-based luminophores have been a hot topic for both experimentalist and theoreticians. So far, a restriction of intramolecular motions (RIM) mechanism is widely recognized as the origin of the AIE phenomena.9,10 Theoretical calculations can provide detailed mechanistic and dynamical information at molecular level, and therefore have been used in exploring the fluorescent quenching as well as aggregation-induced emission processes. For example, the quenching mechanism of multiluminescent acene derivatives in solution and solid has been reported,23 and molecular dynamics simulations on the excitedstate fluorescent deactivation and AIE of tetraphenylethylene (TPE) have been reported by Zhao24 and Sun et al.25 Recently, AIE mechanisms for some similar compounds, such as diphenyldibenzofulvene (DPDBF) and dimethyl tetraphenylsilole (DMTPS), have been rationalized by both a surface hopping molecular dynamics26,27 and a conical intersection (CI) model.28,29 All these calculations revealed the important role of intramolecular rotation around ethylenic CC bond Received: August 7, 2016 Revised: August 28, 2016 Published: August 30, 2016 21850

DOI: 10.1021/acs.jpcc.6b07963 J. Phys. Chem. C 2016, 120, 21850−21857

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The Journal of Physical Chemistry C

Next, we used the computationally less-expensive TDDFT method to explore the reaction path on the excited-state potential energy surface (PES), while only taking advantage of the capabilities of multireference CASSCF wave function to locate and characterize the critical points such as the minima and minimal-energy crossing points (MECPs). The good consistency between the two levels of methods confirms the validation of computational results. Specifically, a series of constraint geometry optimizations around the α and β dihedral angles (see Figure 1 for their definitions) were done at the TDPBE0/6-31G(d) level on the S1 PES to mimic the possible fluorescent quenching pathways caused by intramolecular rotations around the ethylenic CC and its neighboring bonds, which show the largest fluctuations in bond length before and after electron excitation (see the Supporting Information, Figure S2a). Subsequently, the MECP optimization42,43 at CASSCF level were carried out in the regions where the S0-S1 gap becomes narrow, in order to locate the conical intersections that are expected to play important roles in fluorescent quenching of luminophores. 2.2. ONIOM Setup for Solid State Luminophore. The AIE mechanisms were calculated with a similar strategy at the ONIOM (TDDFT:UFF) and ONIOM (CASSCF:UFF) levels, respectively. The initial structures of ONIOM calculations were set up by subtracting a 34-molecule cluster (1224 atoms in total) from crystal structure, in which one BIM luminophore in the middle was set as model part and treated by high-level QM calculations, while the surrounding molecules were computed by low-level UFF force field44 with QEQ45 charges. During geometry optimization, only the QM molecule was allowed to move, thus the electronic excitation and excited-state quenching are approximated by a single-molecule model. This electron-localized model is incapable of precisely describing the physical picture of the excited crystal, but it is supposed to be sufficient for the purpose of the study, that is, to understand why fluorescent quenching paths are prohibited in solid state from the single-molecule view. In ONIOM calculations, an electronic embedding46 (ONIOM-EE) scheme, which incorporates the electrostatic interaction in the QM Hamiltonian and allows the wave function to be polarized by the charge distribution of MM region, was employed. Single-point energies for either the solvated or crystalline molecules were refined by the multistate complete active space second order perturbation (MS-CASPT2)47,48 calculation to consider the dynamic and multireference correlations. In CASPT2 calculation, an imaginary level shift of 0.1 hartree is used.49 The (TD)DFT calculations were performed using the Gaussian 09 program,50 and CASSCF and CASPT2 calculations were done by MOLCAS@UU package.51,52

and confirmed the restriction of intramolecular rotation (RIR) mechanism for AIE. Recently, a benzylidene malonates molecule, i.e., 4dimethylamino-2-benzylidene malonic acid dimethyl ester (see Figure 1; for easy of discussion, hereafter shortened to

Figure 1. 4-Dimethylamino-2-benzylidene malonic acid dimethyl ester (BIM).

BIM), has been synthesized by Cariati et al. and shows AIE (or crystallization-induced emission, CIE) properties.30 Unlike the above-mentioned TPE and DPDBF molecules that carry charge-equivalent moieties on either sides of the ethylenic CC bond, in BIM the substitutions of electron-donating amino and electron-withdrawing ester groups make the ethylenic bond more polarized. Therefore, it would be an optimal candidate to study the influence of push−pull substitution on the fluorescence quenching in solution as well as the emission in solid state. Actually, in addition to their syntheses and spectroscopic evidence that confirm the RIM mechanism, Cariati and co-workers also theoretically studied the ground- and excited-state geometry of BIM, and proposed a possible RIM mechanism (around bond in the ester moiety) on the basis of geometric differences in S0 and S1-state minima.31 Due to the absence of detailed information about S0 and S1 PESs as well as the critical points between them (such as conical intersection), besides the missing of excited-state charge-transfer intermediate that has been found to be energetically more favorable than the located one, the suggested mechanism is still speculative and worthy of a comprehensive study with adequate level of theory. In the current study, we carried out the complete active space selfconsistent field (CASSCF)32 and time-dependent density functional theory (TDDFT)33 calculations to systematically explore the fluorescent quenching pathways in methanol solution, as well as the QM:MM calculations with our own N-layered integrated molecular orbital and molecular mechanics (ONIOM) scheme34−36 to reveal the AIE mechanism. The results will be expected to deepen the understanding of photophysics for luminophore in solution and solid state and provide insight on future design of high-efficient AIE compounds.

2. COMPUTATIONAL DETAILS 2.1. PCM Calculation for Solvated Molecule. We first optimized the ground-state (S0) and excited-state (S1) minima at the CASSCF and (TD)DFT-PBE037 level with 6-31G(d) basis sets,38 starting from the reported crystal structure.30 In CASSCF calculations, a state-average wave function and (12e, 12o) active space were used (the illustrations of active orbitals are found in Figure S1, see the Supporting Information). In TDDFT calculations, the polarizable continuum model (PCM)39 with computationally efficient linear-response (LR) formalisms40 is utilized in order to consider the solvent (methanol) contribution to energy and geometry of solvated molecule, while in predicting the absorption and emission maxima, PCM with state-specific (SS) solvation correction41 is employed.

3. RESULTS AND DISCUSSION In the following section, we first present the optimized groundstate and excited-state stationary points in methanol solution and in crystal, with respect to the experimentally measured one. Then in subsection 3.2, the excited-state quenching path in solution is shown, followed by a proposed nonadiabatic transition mechanism using a conical intersection model. Finally, the restriction of intramolecular rotation path in crystal, which is expected to account for the AIE of BIM luminophore, is presented in subsection 3.3. 3.1. Optimized Minima in Methanol Solution and in Crystal. The optimized geometries of solvated BIM in the 21851


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state are named as S1-EM (emissive intermediate), S1−CTa (charge-transfer intermediate with torsion around α bond), S1− CTb (CT intermediate with torsion around β bond), and S1− CTab (CT intermediate with simultaneous torsion around α and β bonds), respectively. 3.1.1. Solvated Molecule. The superimposed ground-state structures in Figure 2a show that the S0 minima (mS0-MIN) optimized at both the PBE0/6-31G(d) (illustrated as red tube) and CASSCF(12e,12o)/6-31G(d) (in green) level show relatively small deviations from that in crystal (in dark gray). It suggests that the geometry relaxations of BIM in methanol solution are moderate, and in addition, the methodologies employed in the current study are sufficiently reliable in describing the geometry of BIM. Specifically, both methods provide substantially consistent bond lengths with crystal (seen in Figure S2b, Supporting Information), while the small deviations mainly come from the flexible diester moieties. As found in Table 1, the PBE0 and CASSCF calculations both slightly overestimate the dihedral angle α (around the C5−C6 single bond) by 6−9°, while the former predicts a smaller and the latter predicts a larger γ (around the C7−C8 single bond), respectively, with deviations of −13.8 and +4.3°. Considering the geometric flexibilities of linear-chain diester moieties, such deviations are acceptable; therefore, PBE0 and CASSCF are employed as method of choice in solution and high-level QM method in ONIOM methodology in the crystalline phase. The excited-state geometries of the solvated molecule are shown in Figure 2b. Three stationary points are obtained at the TD-PBE0/6-31G(d) level. The vertical excitation energy computed by TD-PBE0 with LR-PCM approach at mS0-MIN is 359 nm (3.45 eV), while the one obtained with the SS-PCM model is red-shifted to 375 nm (3.31 eV), which is in excellent agreement with experimentally measured absorption maximum 375 nm (3.31 eV). The S0 → S1 excitation at Franck-Condon (FC) point mainly results from HOMO to LUMO excitation, with an oscillator strength (f) of 0.96. In the vicinity of FC, a S1 minimum (mS1-EM) that is 42.6 kJ/mol energetically more stable than FC is located; comparing the geometric parameters shown in Table 1, one can see that mS1-EM has similar structure with mS0-MIN, except for some differences in γ and β dihedral angles. The evidently smaller γ and slightly increased β dihedral angles suggest that the diester moiety becomes more coplanar than in mS0-MIN, due to the photoinduced weakening of the C6−C7 bond as well as strengthening of the C7−C8 bond. The TD-PBE0 predicted vertical excitation energy at mS1-EM, with the LR-PCM approach, is 444 nm (2.79 eV, f = 1.0), in line with the reported weak fluorescence maximum (465 nm, 2.67 eV). Therefore, mS1-EM is assigned as the emissive structure on the S1 state. In the meantime, the TDPBE0 and SS-PCM calculation obtained a vertical excitation energy of 432 nm (2.87 eV, f = 1.0), which is even more blueshifted with respect to the experimentally measured weak emission maximum. Therefore, the LR-PCM model was used to explore the quenching mechanism in solution. In the others two S1 minima (mS1−CTa and mS1−CTb) optimized by TD-PBE0, remarkable torsions around α and β bonds, respectively, are observed. The nearly perpendicular α and β bonds plus the push−pull substituents in BIM substantially change the distributions of involved orbitals, i.e., the HOMOs are localized in benzene and LUMOs in diester moieties, respectively. Thus, the HOMO→LUMO excitations (∼70% in configuration weights) of mS1−CTa and mS1−CTb lead to charge-transfer excited state (See Supporting

ground and excited states are illustrated in Figure 2 and those in crystal are shown in Figure 3, with their important geometric

Figure 2. Optimized (a) S0 and (b) S1 state minima in methanol solution. Different geometries are superimposed together by aligning the coordinates of benzyl moiety.

Figure 3. Optimized S0 and S1 state minima of BIM in crystal by ONIOM-EE (PBE0:UFF) methodology. The optimized crystalline QM geometries are shown as tube in the cavity surrounded by other MM molecules (illustrated as van der Waals surfaces).

parameters summarized in Table 1. The nomenclatures are explained as follow: Stationary points starting with “m” indicate that they are optimized with PCM model in methanol solution, while those starting with “c” refer to geometries obtained in crystal by ONIOM (QM:MM) method. The minima in ground state are labeled as S0-MIN, and intermediates in S1 excited 21852


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Table 1. Key Geometry Parameters and Relative Energies of S0 and S1 State Minima Optimized in Methanol Solution and in Crystala ΔE(opt)

geometric parameters structure mS0-MIN mS1-EM mS1−CTa mS1−CTb mS1/S0-CIb cS0-MIN cS1-EM cS1−CTab expb a

method

α

β

PBE0 CASSCF TD-PBE0 TD-PBE0 TD-PBE0 CASSCF CASSCF

14.6 17.1 20.0 87.8 0.3 −0.5 −0.8

2.4 1.2 24.6 8.1 86.8 97.0 102.9

PBE0:UFF TD-PBE0:UFF TD-PBE0:UFF

10.3 9.5 79.5 8.4

−2.4 9.2 −52.9 −3.7

γ in methanol 73.2 91.3 27.4 1.9 26.2 −3.4 −8.5 in crystal 87.8 55.5 136.9 87.0

ΔE(CASPT2)

γ′

S0

S1

S0

S1

−175.5 −178.6 −170.1 −179.1 −164.5 −175.4 −171.7

0.0 0.0 21.4 83.9 177.2 273.6 363.6

333.3 444.9 290.7 271.5 211.4 337.8 364.4

0.0 0.0 40.2 154.2 215.8 173.0 183.3

331.9 340.0 294.5 290.5 244.0 213.2 285.3

−179.9 −174.7 155.6 −177.1

0.0 15.7 162.3

350.3 342.5 331.6

0.0 17.9 151.9

348.7 355.8 409.7

Dihedral angles are in degree, bond lengths are in angstrom, and relative energies are in kJ/mol. bCrystal structure from ref 30.

Figure 4. Optimized S1 energy profiles along the α (a) and β (b) dihedral angles in methanol solution (TD-DFT: TD-PBE0/6-31G(d) optimized S1-MEP; S0//S1: vertical projected energy of S0 state on the basis of optimized S1 geometries; PT2//TD: MS-CASPT2 energy profile computed on the basis of TD-PBE0 optimized geometries).

Information, Table S1). It is noticed in mS1−CTb, the S1−S0 gaps are about 34.2 and 40.2 kJ/mol at the TD-PBE0 and MSCASPT2 level, respectively, narrow enough to act as a possible nonadiabatic funnel. Both mS1−CTa and mS1−CTb are energetically more stable than mS1-EM by about 19.2 and 79.3 kJ/mol at the TD-PBE0 level (4.0 and 50.5 kJ/mol after MS-CASPT2 energy correction), respectively. It implies that the emissive state of BIM is thermodynamically unstable and may able to decay to other intermediates, i.e., mS1−CTa via α bond rotation and mS1−CTb via β bond rotation, respectively, as will be discussed in section 3.2. The locating of chargetransfer intermediates with narrow S1−S0 gap is the major feature of the BIM system compared with other less-polar AIE luminophores (for instance, TPE and DPDBF). The CASSCF excited-state calculation finds the global minimum on excited-state, mS1−CTb, which has similar structure and relative energy with that by TD-PBE0. However, neither mS1-EM nor mS1−CTa, were located as stationary points at the CASSCF level, again suggesting that solvated BIM readily undergoes fluorescent quenching path via β bond rotation rather than being trapped in emissive state. 3.1.2. Crystalline Molecule. Figure 3 shows the optimized structures of the crystalline BIM molecule by ONIOM methodology. As seen in Figure 3 and Table 1, the ONIOM-

EE (PBE0:UFF) calculations well mimic the crystalline structure of BIM (cS0-MIN) in both its geometry and spatial orientation in the cavity, which provides a solid base for predicting the AIE mechanism taking place in solid phase. Also, ONIOM-EE (PBE0:UFF) successfully locates an emissive structure in S1 state (cS1-EM), in which the diester moiety shows a frustrated torsion toward coplanar (as was observed in solvated mS1-EM). The smaller torsion of γ dihedral angle in cS1-EM (∼32.3°, with respect to cS0-MIN) than that in mS1EM (45.8°) reflects restriction of rotation in crystalline environment. We also tried to optimize the CT structures as those being found in methanol solution. Unlike in solution where both mS1−CTa (via α bond rotation) and mS1−CTb (via β bond rotation) were located, in crystalline phase, only one S1 structure with CT nature (cS1−CTab), in which both α and β dihedral angles vary substantially from those in mS1-EM, was obtained. It is interesting to see from Figure 3 that the terminal groups of BIM (both the dimethylamino and ester groups) hardly move, while C6 atom tilts up and leads to a frustrated hula-twist structure that has been observed in Z/E isomerization of polyenes in confined space.53,54 Although TD-PBE0 calculations suggest that the geometry relaxations from cS1-EM to cS1−CTab are slightly exoenergetic with mild energy release 21853


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The Journal of Physical Chemistry C of 10.9 kJ/mol, the CASPT2 energy corrections significantly alter their relative energies and disfavor such a trend. Considering the good consistency between TD-PBE0 and CASPT2 results at S0-MIN and S1-EM (where excited-state PES is well-separated from the ground-state one), we account for such a discrepancy to the poor description of nearly degenerate ground and excited states for single-reference TDDFT method. The ONIOM-EE (TD-PBE0:UFF) computed vertical excitation maximum at cS0-MIN is 341 nm (3.64 eV), ∼0.19 eV blue-shifted with respect to that in methanol solvent. The emission maximum at cS1-EM computed at the same level is 366 nm (3.39 eV), showing a Stokes shift of 0.25 eV. Although the calculated Stokes shift is comparable with the experimental measured value 0.17 eV, the absolute absorption and emission maxima show evident discrepancies with experimental values by about 0.82 and 0.74 eV, respectively. It indicates the computational scheme employed in the current study, especially the UFF force field with untamed parameters and QEQ charges, is somehow incapable of quantitatively predicting the electronic spectroscopy of the BIM crystal. The ground- and excited-state stationary-point calculations suggest that the solvated and crystalline BIM molecules have similar ground-state and excited-state structures, therefore, which are expected to have similar absorption and emission behavior; however, the tendency of geometry relaxations toward a charge-transfer state caused by push−pull substituents is different: The favorable relaxations in solution are expected to be responsible for the fluorescence quenching, while the frustrated relaxations in crystal may account for the AIE phenomena. To validate this proposal, we carried out reaction path studies in both solution and crystal and present the results in the following subsections. 3.2. Fluorescent Quenching Mechanism in Methanol Solution. Figure 4a,b illustrates the S1-state energy profiles of solvated BIM along the α and β torsional coordinates, respectively, through which we try to mimic the possible excited-state decay channels to mS1−CTa (via α bond rotation) and mS1−CTb (via β bond rotation). Along either the α or β bond rotation, we observed an essentially barrierless geometry relaxing path from mS1-EM to CT structure (seen in Figure 4a,b, from left to right-side of the S1-MEPs); the barrier heights for both paths are lower than 2.0 kJ/mol at the TD-PBE0 and the MS-CASPT2//TD-PBE0 level. As a consequence the emissive structure mS1-EM is kinetically metastable and fluorescent emission from mS1-EM is expected to very weak. Comparably, the β-bond rotation path is much steeper than the α-bond rotation path and leads to energetically more favorable product mS1−CTb, suggesting that the former is the dominant S1 decay channel. The feasible decay via β bond rotation as well as the narrow S1−S0 gap at mS1−CTb are expected to account for the fluorescence quenching of BIM in methanol solution. Analyses of the main configurations in TD-PBE0 calculations show that the S0 and S1 states are evidently mixed at mS1−CTb, which implies the strong interaction between ground and excited states. While such a strong interaction has not been observed in mS1−CTa, which results from the rotation of α bond that has been proposed to be responsible for the nonradiative relaxation process.30 Therefore, we carried out geometry optimization at the CASSCF(12e, 12o)/6-31G(d) level to locate the S0-S1 MECP in the vicinity of mS1−CTb. The optimized crossing-point structure of S1/S0-CIb, along with those of mS1−CTb, are superposed in Figure 5. S1/S0-CIb

Figure 5. Optimized S1/S0 MECP and S1 state minima in methanol solution. Different geometries are superimposed together by aligning the coordinates of the benzyl moiety.

is structurally similar to mS1−CTb, while it is 72.1 kJ/mol higher in energy than mS1−CTb. Still, S1/S0-CIb is 54.7 kJ/mol energetically more favorable than mFC at the MS-CASPT2/ CASSCF level; therefore, excited molecules can easily access the funnel and nonadiabatically decay to ground state. It has to be mentioned that the evident discrepancies between the CASSCF and MS-CASPT2//CASSCF predicted S1−S0 gaps at S1/S0-CIb, due to the lack of dynamical correlation in CASSCF wave function, will somehow affect the reliability of S1/S0-CIb as a critical crossing. However, the consistently narrow S1−S0 gaps near mS1−CTb, at both CASSCF and TDDFT levels, strongly support that nonadiabatic transition takes place via surface crossings in its close vicinity. Moreover, we also attempted to optimize S0-S1 MECP along other motions; however, the optimizations either go to the same crossing point, or reach S1/S0-CI with higher relative energy (e.g., a S1−S0 crossing point along the benzyl-carbon tilting out coordinate at the electron-deficient side of excited BIM, named as mS1/S0-CIt, can be seen in the Supporting Information). In summary, we rationalized the fluorescence quenching mechanism through a bond rotation and conical intersection mechanism on the basis of CASSCF and TD-DFT calculations, which is helpful for understanding the AIE mechanism of crystalline BIM, as will be discussed in the next subsection. 3.3. AIE Mechanism in Crystal. The bond rotations of BIM in crystalline phase differ from that in methanol solution, due to the steric and electrostatic confinements of the crystalline environments. The energy difference between FC and the nearby emissive structure (cS1-EM) decreases from 42.6 kJ/mol in solution (TD-DFT results) to 7.8 kJ/mol in crystal (ONIOM (TD-PBE0:UFF) results), therefore in crystal the less excessive energy generates smaller driving force for bond rotation. As seen in Table 1, after CASPT2 correction, the energy of FC is even lower than cS1-EM by 7.1 kJ/mol, the unphysically lower FC energy again implies the excited-state molecule will be trapped in the vicinity of FC instead of relaxation to other conformations via bond rotation. Actually, even we “force” the bond rotation occurring by carrying out a relaxed geometry scan along either α or β torsional coordinates, and the ONIOM extrapolated energy will continuously increase (as seen in Figure 6a and b), in sharp contrast to that observed in Figure 4 for solvated BIM. Taking the α rotation path in Figure 6a as an example, started from FC points in the middle, with the decrease of α torsional angle (α-/ β+ bond rotation, from middle to left side), both the ONIOM (TD-PBE0:UFF) and ONIOM (MS-CASPT2:UFF)-corrected S1-MEPs show a uphill trend, suggesting a unfavorable rotary 21854


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Figure 6. Optimized S1 energy profiles along the (a) α and (b) β dihedral angles in crystalline phase. In panel a, the concerted rotation of α and β bonds toward two directions are denoted as α-/β+ and α+/β- bond rotation paths, which are essentially the same as β+/α- and β-/α+ paths in panel b. For notations of TD-DFT, S0//S1 and PT2//TD, see the caption of Figure 4.

process, while the S1-MEP from FC to cS1−CTab (α+/β- bond rotation, from middle to right side) is slightly downhill at ONIOM (TD-PBE0:UFF) level with the increase of α torsional angle, but becomes evidently uphill after ONIOM (MSCASPT2:UFF) energy correction. As was explained in section 3.1, the MS-CASPT2 wave function is supposed to provide more reliable descriptions for excited state with multiconfigurational nature; therefore, the FC to cS1−CTab is considered to be energetically unfavorable, too. At the left and right ends of α rotation path, the energy is more than 60 kJ/mol above the FC points, which is sufficient to hinder the photoinduced rotary relaxation, while the S1−S0 energy gaps are still large. Therefore, nonadiabatic funnel in the vicinity of cS1−CTab is unlikely to exist and computational effort to locate a S1/S0 MECP is unnecessary. By close looking at the geometry variation along the α or β bond rotation, we found that the bond torsions always take place in a concerted manner, that is, the increase of α dihedral angle is always accompanied by the simultaneous decrease of β torsion, and vice visa. Such a hula-twist type of rotation (as discussed in mS1-EM) generates very different rotary geometries compared with those by one-bond-rotation55 mechanism observed in solvated BIM. During the bond rotations in the crystalline phase, the spatial orientations of the terminal dimethylamino and ester groups of BIM have hardly been changed due to the strong confinements, while only the smaller bridging C6 atom is tilted from the molecular plane. Even though C6 atom is flexible with respect to the backbone of the molecule, its movement is limited; once it tilts far from the equilibrium structure or too close to other neighboring BIM molecule, the energy increases significantly and further hulatwist is restricted. From above discussions we can draw conclusion that the bond rotation relaxation of BIM molecule in solution become energetically unfavorable in the crystalline phase, which will increase the fluorescent quantum yields in BIM crystal and account for the AIE phenomena.

(1) In methanol solution, the excited-state BIM molecule, rather than being trapped in its emissive state, will barrierlessly decay to a charge-transfer intermediate via one-bond-rotation relaxation channel around the ethylenic CC bond, due to the contribution of push−pull substitution. A conical intersection in the vicinity of CT intermediate is responsible for the radiationless decay from excited state to ground state, and consequently the fluorescence quenching in solution. (2) In crystal, the observed one-bond-rotation mechanism is restricted by environmental confinements, and instead, the very limited geometry relaxations obey a frustrated hula-twist pattern. From either the energetic and structural points of view, the excited-state BIM molecule is tightly trapped in its emissive state, and thus high fluorescent quantum yields are expected. The results will be expected to deepen the understanding of photophysics for luminophore in solution and solid state and provide insight on future design of high-efficient AIE compounds.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07963. Full citations for refs 7, 14, 34, and 50−52; detailed information about electronic state analyses and geometry comparisons, as well as Cartesian coordinates of important stationary and crossing points. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS We carried out the QM (CASSCF and TD-DFT) and ONIOM (QM:MM) calculations for BIM molecules in methanol solution and in crystal, in order to understand the fluorescent quenching in solution as well as the AIE mechanism in solid phase. The major conclusions are drawn as below:

ACKNOWLEDGMENTS This work is supported by grants from the NSFC (Grant Nos. 21473107 and 21473108), Natural Science Basic Research Plan in Shaanxi Province of China (Grant No. 2015JM2056), Fundamental Research Funds for the Central Universities (Grant No. GK201502002), and Program for Changjiang 21855


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Scholars and Innovative Research Team in University (IRT_14R33).

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