Deciphering the Mechanism of Aggregation-Induced Emission (AIE) of

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Deciphering the Mechanism of Aggregation-Induced Emission (AIE) of a Quinazolinone Derivative Displaying Excited-State Intramolecular Proton-Transfer Properties: A QM, QM/MM and MD study Hongjuan Wang, Qianqian Gong, Gang Wang, Jingshuang Dang, and Fengyi Liu J. Chem. Theory Comput., Just Accepted Manuscript • DOI: 10.1021/acs.jctc.9b00421 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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Journal of Chemical Theory and Computation

Deciphering the Mechanism of Aggregation-Induced Emission (AIE) of a Quinazolinone Derivative Displaying Excited-State Intramolecular Proton-Transfer Properties: A QM, QM/MM and MD study Hongjuan Wang#, Qianqian Gong#, Gang Wang, Jingshuang Dang, 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.

Corresponding Author: Fengyi Liu. E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Combined

excited-state

intramolecular

proton-transfer

(ESIPT)

to

aggregation-induced emission (AIE) has opened new opportunities to develop color-tunable luminescent materials with high quantum yield. Understanding the emission mechanism of these luminophores is essential for the molecular design and construction of functional system. Herein, we report a QM (MS-CASPT2//TD-DFT, MS-CASPT2//CASSCF) and ONIOM (QM:MM) studies on the fluorescence quenching and AIE mechanism of 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone (HPQ) with typical characteristics of AIE and ESIPT as an exemplar case. The computational results indicate that in THF solution, once being excited to the S1 state, the molecule tends to undergo an ultrafast, barrierless ESIPT from enol to keto tautomer, and then accesses a S1/S0 conical intersection in the vicinity of a C=C bond twisted intramolecular charge-transfer (TICT) intermediate, leading to nonradiative decay from excited to ground state. Hence, the TICT-induced nonadiabatic transition, which has been further confirmed by the on-the-fly trajectory surface hopping (TSH) dynamics simulations, accounts for the fluorescence quenching in solution. Contrarily, in the solid state, the non-radiative relaxation pathway via C=C bond rotation is suppressed due to environmental hindrance, leaving the ESIPT-induced enol-keto tautomerization as the only excited-decay channel, thus the fluorescence is observably enhanced in crystal.


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1. INTRODUCTION Organic luminescent materials have broad applications in the fields of photoelectricity, sensing, and photobiology.1-4 However, most of the organic luminophores present remarkable luminescence in their dilute solution but become weakly luminescent in concentrated solutions and solid state.5-7 This is known as “aggregation-caused quenching (ACQ)”, an ubiquitous photophysical character of aromatic conjugated compounds. Such a phenomenon is one of the major obstacles that prevents organic materials from practical applications, since these materials are usually expected to be utilized in their solid state. Contrastingly, aggregation-induced emission (AIE) is an exotic photophysical phenomenon where the organic luminescent materials are faintly emissive in solution but strongly luminescent in their aggregates. The concept of AIE was coined in 2001 by Tang group,8 and now is widely-accepted as a feasible strategy to facilitate the development of organic luminescence materials. Circumventing the drawbacks of ACQ, AIE offers a route for an array of possibilities with huge potential in high-tech innovations. Since then, researchers have shown persistent desires to uncover new AIE systems and gain mechanistic knowledge on the AIE phenomena. Tang proposed a restriction of intramolecular motions (RIM) mechanism as the origin of the AIE phenomena based on theoretical and experimental finding.9-11 As the research continues to deepen, a variety of mechanisms were put forward, such as restricted access to the conical intersection (RACI)12-13, excimer emission14, J-aggregate formation15, and twisted intramolecular charge transfer (TICT)16, etc.



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Recently, excited-state intramolecular proton transfer (ESIPT) coupled to AIE have potential applications in the optical memory, optoelectronics, and fluorescence sensors and probes.17-19 The ESIPT is a unique photochemical process involving a photo-induced tautomerization from enol to keto tautomer, leading to significant variations in the electronic structure and photophysical properties from those observed in their original structure. As a result, the emission spectrum attributed to keto tautomer has no overlap with the absorption spectrum of enol form, which is beneficial to avoid unwanted self-absorption and inner-filter effects, and increase luminous efficiency.20-22 However, due to the transient nature of the enol-keto tautomerization process, the ESIPT emission is highly sensitive to environment, which can be overcome by combining AIE and ESIPT.23

Figure 1. The structure of 2-(2-Hydroxy-phenyl)-4(3H) quinazolinone (HPQ) in Z-enol configuration.

Up to date, mechanistic investigation of the coupling of “AIE+ESIPT” has been a hot topic for both experimentalist and theoreticians. Among many of the “AIE+ESIPT”

luminophores,

2-(2-Hydroxy-phenyl)-4(3H)

quinazolinone24-25

(abbreviated as HPQ, see Figure 1) is chosen as an ideal candidate to study the mechanisms. HPQ exhibits high quantum yield and has broad applications in sensor and bioimaging.26-28 Here, we aimed at investigating the differing photochemical


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behavior of HPQ in THF solution and in solid phase, as well as clarifying the role of ESIPT process in AIE mechanisms. The time-dependent density functional theory (TD-DFT)29-30 and complete active space self-consistent field (CASSCF)31 calculations were carried out to explore the fluorescent quenching in THF solution, followed by the trajectory surface hopping (TSH)32-33 molecular dynamics (MD) validation of the nonradiative decay, ; while the QM:MM calculations with our own N-layered integrated molecular orbital and molecular mechanics (ONIOM)34-36 scheme were applied to simulate the solid-state photophysics and photochemistry. These results are expected to deepen the understanding of photophysics and photochemistry of “ESIPT+AIE” luminophore, meanwhile, and also provide insight on future design of high-efficient luminescent materials.

2. COMPUTATIONAL DETAILS 2.1. QM Calculations for Solvated HPQ Firstly, we investigated the quenching mechanism of HPQ in THF solution. Both the (TD)PBE037 and CASSCF methods, with a 6-31G(d,p) basis set38, were employed to provide a reliable description of the ground and excited states. The polarizable continuum model (PCM)39-40 is employed to consider the solvent (THF, ε=7.4257) contribution of solvated molecule. In TD-PBE0 calculations, PCM with state-specific (SS)41 solvation correction is used in predicting the absorption and emission maxima, while the computationally efficient linear-response (LR) formalism42 is employed for geometry optimizations and minimal-energy path (MEP) searching. In CASSCF calculations, we used a three-state-averaged wave function and an active space



constructed with 12 electrons distributed over 11 frontier orbitals, namely SA3-CASSCF(12e,11o). Details of the CAS(12e,11o) active orbitals are available in Supporting Information (Figure S1 and S2). Based on the optimized minima, we explored the S0- and S1-state MEPs of the ESIPT and the possible isomerization process, that is, the rotational around the C5-C6 bond (in according to dihedral angle , see Figure 1 for definitions). The MEPs were obtained by a series of constrained geometry optimizations, such strategies have been adopted in our previous research43-45 and proved as an efficient and reliable approximation. The consistency between the (TD)PBE0 and CASSCF results proves the suitability and reliability of the calculations. Since the TDDFT, as a single-reference method, breaks down in the nearly degenerate ground and excited states situation. Thus, we only implemented the state-averaged CASSCF method to locate the conical intersections (CI) that are expected to play vital roles in fluorescent quenching.46-47 2.2. MD Setup for Fluorescent Quenching To confirm the fluorescent quenching mechanism of HPQ, we further employed trajectory surface hopping molecular dynamics to simulate the excited-state process of HPQ. The potential energies, gradients and nonadiabatic coupling vectors are calculated on-the-fly at the (TD)PBE0/6-31G(d,p) level. The initial conditions were obtained by a Wigner distribution for the quantum harmonic.48 Trajectories were initially excited using an energy window of 0-10 eV. 189 of total 234 trajectories are populated to the bright S1 state, which is then assigned as the starting state based on


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simulated absorption spectra. The simulation time was set to 2000 fs with a time step of 0.5 fs. The number of substeps for integration of the electronic equation-of-motion (EOM) was set in 25. The Velocity–Verlet algorithm49-50 was applied to solve the Newton’s equations of nuclear motion. And the decoherence corrections were taken into account following the energy-based method with the relevant parameter α=0.1 Hartree.51 The local diabatization scheme was used to describe non-adiabatic effects between the states.52-53 In order to conserve total energy, during a surface hop, the kinetic energy was adjusted by rescaling the velocity vectors. As a hopping scheme, the ‘‘Standard SHARC surface hopping probabilities’’ scheme were employed.54 Surface hopping dynamics based on DFT (TSH/DFT) has become an essential tool for the investigation of nonadiabatic processes, even if the known drawbacks of DFT (especially linear response DFT employed in this study) in describing the multireference character in the regions of surface crossing between the excited and ground states, which make the S1-to-S0 state hops less reliable. 55 On top of that, the lack of long-range correction in PBE0 functional that may cause poor description of CT nature of the hopping structure will be another issue. Fortunately, the TSH/DFT still provides sufficiently good results for excited-state-only evolution in the region far from near-degeneracy situation. Therefore, the simulation aims to track the S1-state evolution of the fluorophore so as to provide information of the ESIPT and initial-stage C-C rotation processes (geometry evolution and excited-state lifetime, while the latter is still semi-quantitative), rather than to obtain the less reliable results for nonadiabatic events (hopping probabilities, hopping time and product yields).



The TSH MD was carried out using SHARC (Surface Hopping in the Adiabatic Representation Including Arbitrary Couplings) program suite54,56-57. 2.3. ONIOM Calculation for Solid-State Luminophore Crystallization of HPQ from THF can produce two polymorphs emitting a blue (B) and blue-green (BG) light, which are referred to as HPQ-B and HPQ-BG, respectively.24 The most remarkable conformational difference of the fluorophore in two polymorphs is the C5-C6 torsional angle  between the phenol and quinazolinone rings (3.7 and -10.0° for HPQ-B and HPQ-BG, respectively, as seen in Figure 2).

Figure 2. Crystal structures of the two polymorphs under investigation. Polymorph HPQ-B (left) shows blue fluorescence, whereas HPQ-BG (right) gives blue-green fluorescence.

In order to provide a complete picture of AIE mechanisms in crystal, we used the ONIOM(QM:MM) approach with the two-layered model, combining the quantum mechanical (TD)PBE0/6-31G(d,p) with the UFF force field58. The clusters for ONIOM calculations were cut from the crystal structures in experiment, and our multi-model approach ensures size consistency of the models: B and BG clusters consist of 36 and 34 molecules, respectively. The model part, treated at the QM level,


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was placed in the center of the model and allowed to move. Whilst the surrounding molecules were computed by low-level UFF force field with QEQ59 charges and kept frozen during the optimizations. In ONIOM, the extrapolated energy is given by the expression: ONIOM QM UFF UFF Eextrap = Emodel + Ereal − Emodel

where model refers to the first layer and real to the whole system32. All energies are refined at the multi-state complete active space second order perturbation (MS-CASPT2)60 level to consider both the multi-reference effect and dynamics electron correlation. An imaginary level shift 0.1 Hartree is used in all CASPT2 calculations. The (TD)DFT and ONIOM calculations were performed using the Gaussian 09 program61, while and CASSCF and CASPT2 calculations were done by MOLCAS 8.0 package62-63.

3. RESULTS AND DISCUSSIONS In the following sections, we first present the important stationary points in THF solution and in crystal. Next, we show the excited-state quenching channel in THF solution, which is explained by a nonadiabatic transition mechanism combining essential ESIPT and conical intersection model. Finally, we show a restricted quenching channel in crystal that accounts for the AIE enhancement of HPQ. 3.1 Optimized Minima and Electronic Excitations 3.1.1 HPQ in Solvated Phase We first optimized geometries of solvated HPQ in the S0 and S1 states at the



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(TD)PBE0/6-31G(d,p) and SA3-CASSCF(12e,11o)/6-31G(d,p) levels. The key geometry parameters and relative energies are shown in Table 1. Table 1. The key geometry parameters and relative energies in THF solution and in crystal. Dihedral angles are in degree, bond lengths are in angstrom, relative energies are in kcal·mol-1 and eV (in parentheses). Experimental data were obtained from ref 24. Structure

Method

Geometric Parameters



 R(8-9)R(4-9)

ΔE(opt) S0

ΔE(CASPT2) S1

S0

S1

In THF solution Z-enol

PBE0

1.7 -179.9 1.006 1.615 0.0(0.00) 88.7(3.85) 0.0(0.00) 95.4(4.14)

CASSCF 0.0 180.0 0.951 1.847 0.0(0.00) 108.6(4.71) 0.0(0.00) 105.4(4.57) Z-keto*

TD-PBE0 26.8 -176.3 1.809 1.031 16.0(0.70) 74.3(3.22) 14.7(0.64) 79.9(3.46) CASSCF 0.0 -180.0 2.041 0.988 37.0(1.60) 82.6(3.58) 11.9(0.51) 94.0(4.08)

CT-keto* TD-PBE0 104.5 -150.9 3.672 1.007 46.1(2.00) 69.4(3.01) 55.9(2.42) 77.6(3.36) CASSCF 97.7 -163.4 3.495 0.993 72.9(3.16) 73.3(3.18) 45.6(1.98) 72.5(3.14) S1/S0-CI E-enol

CASSCF 99.2 163.0 3.483 0.993 72.8(3.16) 73.3(3.18) 44.5(1.93) 73.2(3.18) PBE0 -180.0-180.0 0.964 5.172 15.1(0.66)108.3(4.04) 3.8(0.17) 108.5(4.70)

E-enol* TD-PBE0-180.0-180.0 0.965 5.207 14.0(0.61) 88.4(3.83) 10.8(0.55) 95.2(4.95) E-keto

PBE0 180.0 180.0 4.758 1.009 8.1(0.35) 83.1(3.25) 11.0(0.48) 89.7(3.89)

E-keto* TD-PBE0 158.1 -176.6 4.643 1.010 17.3(0.75) 75.2(3.26) 16.3(0.71) 81.4(3.53) In Crystal HPQ-B

Exp.

3.7 179.9 0.840 1.812

Z-enol(B) ONIOMa -2.4 -179.5 1.005 1.616 0.0(0.00) 92.7(4.02) 0.0(0.00) 99.4(4.31) Z-keto(B)* ONIOM -4.6 -180.0 1.809 1.002 28.7(1.24) 71.1(3.06) 28.7(1.25) 95.3(4.13) HPQ-BG

Exp.

-10.0 178.8 0.956 1.715

Z-enol(BG) ONIOM -10.6 -177.7 1.003 1.614 0.0(0.00) 92.9(4.03) 0.0(0.00) 95.1(4.12) Z-keto(BG)* ONIOM -21.0 -176.8 1.746 1.035 21.1(0.92) 78.9(3.42) 20.5(0.89) 77.8(3.37) a.

ONIOM stands for ONIOM-EE(TD-PBE0/6-31G(d,p):UFF) calculation.

Our calculations at either the PBE0 or CASSCF level, as seen in Table 1, confirm Z-enol as the most stable configuration in S0 state, in agreement with the reported crystal structure (HPQ-B and BG)24. The only difference exists in the dihedral angles shown in Table 1: In THF solution, Z-enol tends to be a more planar structure


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(i.e.,=1.7 and 0.0for PBE0 and CASSCF, respectively), while the structures in crystal deviate slightly from the planar configuration (=3.7 and -10.0 for HPQ-B and BG, respectively)24, which may owe to the environment-induced distortion in crystal (especially by the intermolecular hydrogen-bond interaction). Starting from Z-enol, possible configurations of HPQ including Z-keto (unstable in S0 state), E-enol and E-keto, which can be obtained via proton transfer or Z/E isomerization, are also summarized in Table 1. We also calculated the vertical absorption energy at Z-enol to predict its absorption spectra. The S0→S1 excitation at FC point, leading to a spectroscopically bright state with ππ* character, mainly results from HOMO to LUMO excitation (See Supporting Information, Table S3). The S0→S1 vertical excitation energy calculated by TD-PBE0 with LR-PCM approach is 322 nm (3.85 eV, f=0.46), while the one computed by the SS-PCM model is 321 nm (3.86 eV). Both of those results are close to the experimentally measured absorption maximum of 333 nm (3.72 eV)25. Since the Z-enol* fails to be optimized as a local excitation (LE) minimum in S1 state due to the spontaneous ESIPT to Z-keto*, we turn to other configurations to locate the emissive intermediate. The tautomerized product, Z-keto*, showing a vertical emission energy of 490 nm (2.53 eV) at the TD-PBE0 level with the LR-PCM approach (and 496 nm, 2.50 eV with SS-PCM calculation), are in excellent agreement with experimentally reported weak fluorescence maximum 495 nm (2.51 eV, see Table S3)26, therefore, Z-keto* is assigned as the most likely emissive structure on the S1 state.



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Furthermore, we obtained an energetically more favorable TICT intermediate CT-keto* in S1 state, by about 4.8 kcal·mol-1 (0.21 eV) lower than Z-keto* at the TD-PBE0 level (2.3 kcal·mol-1, 0.10 eV after MS3-CASPT2 correction). The quasi-perpendicular structure around C5-C6 bond of CT-keto* dramatically changed the distributions of involved orbitals, generating a charge transfer from the phenol to the quinazolinone moiety (Table S3). The relatively low S1-state energy and charge-separated structure suggest CT-keto* acts as a funnel for nonradiative decay from S1 to S0 state; moreover, the narrow S1−S0 energy gap (about 23.4 and 21.7 kcal·mol-1, 1.01 and 0.94 in eV at the TD-PBE0 and MS3-CASPT2 level, respectively) at CT-keto* also implies a S1−S0 is possibly exists nearby (as will be confirmed in section 3.2). 3.1.2 Crystalline Molecule As seen in Table 1 (and Figure S3), the geometry parameters of HPQ-B and HPQ-BG

for

Z-enol(B)

and

Z-enol(BG)

models,

calculated

by

ONIOM-EE(TD-PBE0/6-31G(d):UFF) methodology, are consistent well with experimental values, which indicates that the methodologies employed in the current study are reliable in producing the geometries in crystal. In ONIOM calculations, Z-enol and its ESIPT tautomer Z-keto* was found to be the only S0 and S1-state minimum, respectively. In the case of the Z-keto* for B and BG, one can find that  dihedral angles for Z-keto(B)* and Z-keto(BG)* are smaller than that optimized in THF solution, which reflects the restriction of rotation in crystal environment. Besides, the intermolecular


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charge-transfer intermediate CT-keto* that has been obtained in THF solution is failed to be located in crystal. We then employed ONIOM-EE(TD-PBE0:UFF) to compute the vertical absorption maximum at Z-enol for B (317 nm, 3.91 eV) and BG (318 nm, 3.90 eV), both of which are blue-shifted from the reported experimental solid-state absorption maximum (370 nm, 3.35 eV)24. It is clearly that the bright states are both in S1 state that results from a HOMO to LUMO excitation. Meanwhile, the emission maximum for Z-keto(B)* (441 nm, 2.81 eV) and Z-keto(BG)* (495 nm, 2.51 eV) at LE state, computed by ONIOM-EE(TD-PBE0:UFF) method, are in accordance with the one in the reference (497 nm, 2.49 eV for B and 511 nm, 2.43 eV for BG, respectively).24 In general, the non-covalent interactions from the surrounding molecules help to stabilize the crystal packing and restrict the freedom degree of center molecule to some extent. Hence, the proton transfer or Z/E isomerization in solid state may be different from that in THF solution. To validate this proposal, we show the simulated photochemical process of HPQ in solvated state in subsection 3.2, compared with that in crystal shown in the subsection 3.3. 3.2 Fluorescent Quenching Mechanism Starting from the S1-state Franck-Condon structure of Z-enol, we simulated three possible nonradiative decay channels consuming its excited-state energy, and explored the S1-profile via a series of constrained geometry optimizations, including: (a) the rotational motions of solvated HPQ started from the FC(Z-enol) structure along the  torsional coordinates, without the ESIPT involved, (b) the ESIPT process along the



O-H distances and (c) the rotational motion of solvated HPQ started from Z-keto* along the  torsional coordinates. The TD-PBE0/6-31G(d,p) optimized S1-MEPs, along with the MS3-CASPT2 corrected ones, are shown in Figure 3.

Figure 3. The TD-PBE0/6-31G(d,p) optimized S1 energy profiles along the (a)  dihedral angles at enol form; (b) O-H distances; (c)  dihedral angles at keto form; (d) diagrammatic description of the decay channels in THF solution. (S1: TDDFT optimized S1-MEP; S0//S1: vertically projected S0 energy on top of optimized S1 geometries; PT2//TD: MS3-CASPT2 computed energy profiles on top of TD-PBE0 optimized geometries).

As seen in Figure 3a, starting from FC(Z-enol), the S1 energy rises from right to left-side by about 15.6 kcal·mol-1 (0.68 eV) at TD-PBE0 level (20.6 kcal·mol-1 and 0.89 eV after MS3-CASPT2 correction) with the increase of  dihedral angle, then drops again after C=C rotation angle about 130.0. They evidently uphill tendency of


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S1-MEP from FC suggests the rotational relaxation around C5-C6 bond at enol form (enol rotation in Figure 3a) is unfavorable. Unlike the enol rotation, the S1-MEP of ESIPT process (shown in Figure 3b) shows a monotonically decreasing trend from the FC region to the tautomerization product, Z-keto*. Therefore, the ESIPT, instead of enol rotation, is confirmed to be the initial step in excited-state relaxation of solvated HPQ. Subsequently, started from the Z-keto* tautomer, we observed an essentially barrierless rotational relaxing path (around C5-C6 bond) to CT-keto*, as shown in Figure 3c (keto rotation). The MS3-CASPT2//TD-PBE0 method predicted a consistent trend with TD-PBE0. As a consequence, the emissive structure Z-keto* is kinetically metastable and readily decay to CT-keto* in THF solution. The favorable relaxations as well as the narrow S1-S0 gap near the CT-keto* are expected to account for the fluorescence quenching of solvated HPQ. An analogous behavior can be seen in the multireference CASSCF and MS3-CASPT2//CASSCF results. Figure S5 illustrates the (a) unfavorable enol rotation, barrierless (b) ESIPT and (c) keto rotation, respectively. The topologies of the CASSCF-PESs are similar to the TD-PBE0-computed ones, except for a much better description of state-crossing region being observed in CASSCF results (see Figure S5c). The S1 and S0 states approach each other and finally become degenerate in the vicinity of the CT-keto*, where a S1/S0 conical intersection is expected to be optimized. In nonadiabatic photochemistry, conical intersection plays an vital role as the funnel for nonradiative relaxation.43-47 To identify the path for the S1 → S0



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nonadiabatic decay, we carried out the SA3-CASSCF(12e,11o)/6-31G(d,p) MECPs optimization to locate the CI in the vicinity of CT-keto* and successfully obtained a S1/S0-CI (shown as yellow funnels in the Figure 3c and S4c).

Figure 4. The optimized and S1/S0-CI (show in yellow) and CT-keto* (show in red and blue) in THF solution. Different geometries are superimposed together by aligning the coordinates of quinazolinone moiety.

As shown in Figure 4, the CASSCF-optimized S1/S0-CI is structurally similar to CT-keto* obtained at both the CASSCF and TDDFT levels, as being proved by the small root-mean-square deviation (RMSD) values between S1/S0-CI (taken as reference structure) and CT-keto* structures, as well as other geometry parameters. The

S1/S0-CI

is

slightly

higher

in

energy

than

CT-keto*

at

the

MS3-CASPT2/CASSCF level by only 0.8 kcal·mol-1 (0.03 eV). Meanwhile, the S1/S0-CI is well below the FC(Z-enol*) in energy about 32.2 kcal·mol-1 (1.39 eV) at the same level. Considering the barrierless ESPIT and keto rotation steps, as well as the great similarity between S1/S0-CI and CT-keto*, it is sufficient to conclude that accessing to the funnel and nonadiabatically transition to ground state are facile, which well account for the weak fluorescence of HPQ in THF solution.


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The fluorescence quenching channels mentioned above have been further verified by the TSH dynamic simulations. Starting from the FC(Z-enol), 188 trajectories are successfully finished and used in evaluating the photodynamics of HPQ. The statistics of trajectories are illustrated in Supporting Information, from Figure S7 to S12. It is found that the trajectories finish an ultrafast ESIPT process, in average, within 20 fs (the completion of ESIPT is determined by R([O-H]-[N-H]) greater than 0.8 Å, as seen in Figure S8a). Subsequently, the formed Z-keto* tautomers start to rotate around C5=C6 bond, the torsional angle (absolute value, ||) is observed to gradually increase until 729 fs; after that, 96% of trajectories are maintained at regions where ||=90.0° within the simulation time (Figure S8b). Regarding the specially concerned S1→S0 hopping events, due to the wrong description of S1/S0 conical intersection region by TDDFT, unreliable results are obtained. For instance, it is observed that 43 trajectories (23%) once hop from S1 to S0 state, while their stay in S0 state are transient: In several fs, most of the trajectories switch back to S1 state, leaving only 4 out of 43 computed trajectories in S0 state until the end of simulation time. The unsuccessful hops by TSH/DFT lead to a low S0 state population shown in Figure S9, and thus is unable to provide a physically meaningful description of the fluorescent quenching. The average time for the S1-to-S0 hops is 1235 fs (Note, which is, strictly speaking, not equals to the commonly accepted hopping time in TSH dynamics). Fortunately, the geometries of S1→S0 hopping points (see a averaged || of 93.5° in Figure S10, and a representing trajectory in Figure S11 and S12) are found to be in good consistent with S1/S0-CI (θ=99.2°) obtained in the



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CASSCF electronic-structure calculation. More detailed discussions of the simulation are found in Supporting Information. To provide more reliable description of the hopping events, TSH with multireference method, such as the CASSCF as being employed in the electronic-structure calculations, are required, unfortunately which is computationally too expensive for dynamics. In short, by using the electronic structure calculations combined with TSH dynamic

simulations,

we

have

comprehensively

investigated

the

possible

nonadiabatic decay channels of HPQ, thus confirmed that the fluorescence quenching is ascribed to the unification of the ESIPT and subsequent bond-rotation relaxation, both are indispensable (see Figure 3d). 3.3 AIE Mechanism for Crystalline Solid To rationalize the AIE enhancement for HPQ-B and HPQ-BG, we simulated the excited-state photochemical processes (that has been explored for solvated HPQ) at the ONIOM(TDDFT:UFF) and ONIOM(CASSCF:UFF) levels, respectively. The results are illustrated in Figure 5 for HPQ-B and Figure S5 for HPQ-BG.


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Figure 5. The ONIOM-EE(TD-PBE0/6-31G(d,p):UFF)-optimized S1 energy profiles along the (a)  dihedral angles at enol form; (b) O-H distances; (c)  dihedral angles at keto form; (d) diagrammatic description of the decay channels for HPQ-B in solid state.

Taking HPQ-B for example, started from FC point of Z-enol(B), as seen in Figure 5a (the middle of the S1-MEP), the out-of-plane C5-C6 rotation towards either left or right side leads to an uphill slope in S1-MEP, which is in good agreement with the enol rotation shown in Figure 3a and again suggests the C5-C6 rotation relaxation is unfavorable, either in THF solution or in crystal. For the ESIPT process shown in Figure 5b, it is still found to be a barrierless relaxing path that is similar to that observed in solvated HPQ and has not been restricted by the environment. Sharp difference between the crystalline and solvated HPQs are found in Figure 5c, in which



the S1-MEP of keto rotation is illustrated. Unlike the downhill MEP from Z-keto* to CT-keto* shown in Figure 3a, the ONIOM extrapolated energy curve rise sharply with the increase of  torsional angle in Z-keto(B)*, therefore the keto rotation is suppressed in crystal. In consequence, ESIPT process became the dominant decay paths and its tautomerization intermediate thus determine the solid luminescence properties. An analogous behavior can be seen for HPQ-BG (See supporting information, Figure S5). From above discussions we can draw conclusion that the AIE enhancement mechanism is the cooperation of an ESIPT and restricted rotation mechanisms, and they are not mutually exclusive but instead can work together to bring about the AIE enhancement phenomenon (see Figure 5d).

4. CONCLUSIONS We have carried out the QM (TD-DFT and CASSCF), ONIOM (QM:MM) calculations and trajectory surface-hopping molecular dynamics to investigate the fluorescence quenching mechanism of HPQ in THF solution and its AIE enhancement mechanism. As proposed, in THF solution, the HPQ molecule displays almost no emission as the unification of the ESIPT mechanism and the rotational relaxation of the excited keto tautomer. A conical intersection in the vicinity of TICT intermediate plays a key role for the nonradiative decay from excited to ground state. In its solid state, the non-radiative relaxation pathway via C=C bond rotation mentioned above is strongly blocked due to environmental hindrance, hence, fluorescence enhancement is observed in crystal. And the ESIPT becomes the


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dominant excited-state decay channel, and its tautomerization intermediate determines the crystalline fluorescence. This mechanistic understanding is helpful to provide design strategies for the development of novel organic luminophores. With the above knowledge, it is highly possible to mediate the luminescence properties of such materials via regulating the ESIPT process.

Supporting Information More detailed results of the CASSCF and TSH calculations; as well as the cartesian coordinates of optimized stationary points. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT This work is supported by grants from the National Natural Science Foundation of China (Grant Nos. 21873060, 21473107, 21636006), Fundamental Research Funds for the Central Universities (Grant No. GK201901007).

Author Contributions #

H.W. and Q.G. contributed equally to this work.

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Deciphering the Mechanism of Aggregation-Induced Emission (AIE) of

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