New Insight into the Photoisomerization Process of the Salicylidene

Article pubs.acs.org/JPCA

New Insight into the Photoisomerization Process of the Salicylidene Methylamine under Vacuum Li Zhao,†,‡ Jianyong Liu,† and Panwang Zhou*,† †

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡ University of the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: The deactivation process of salicylidene methylamine in the gas phase has been explored using static calculations (CASSCF, CASPT2, and CC2) and on-the-fly surface hopping dynamics simulations (CASSCF). Five minimum energy conical intersections (MECIs) were located upon the geometry optimization calculations. One corresponds to the excited state intramolecular proton transfer (ESIPT) process, and the remaining four arise from CN bond rotational motion. Our calculation results found that the molecule prefers to decay to the ground state through the four rotational motion related MECIs rather than the ESIPT related one. This mechanistic scenario is verified by the energy profiles connecting the Franck−Condon point and the MECIs at CASSCF, CASPT2, and CC2 levels. Our proposed new decay mechanism can explain the previous experimental findings of femtosecond pump−probe photoionization spectroscopy and can provide additional guidance to the rational design of photochemically switchable molecules.

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INTRODUCTION The photochromism phenomenon in organic molecules has inspired numerous experimental and theoretical studies for their enormous application potential.1−9 In particular, considerable interest has been placed on the family of aromatic Schiff bases for their photochromic and thermochromic properties.10−16 The presence of intramolecular hydrogen bonding in such compounds can lead to the proton transfer process and then result in keto and enol tautomers. The photochromism and thermochromism phenomena result from the reversible change between keto and enol tautomers. The photochromism mechanism of Schiff bases has been studied both experimentally and theoretically, particularly for 2-(1-(phenylimino) methyl)-phenol (SA) and N, N′-bis(salicylidene)-p-phenylenediamine (BSP).17−22 It was found that after photoexcitation, the enol tautomer of these bases can form a keto tautomer by a proton transfer process from the OH group to the imine nitrogen along an intramolecular hydrogen bond. The S1 state cis-keto tautomer may undergo subsequent isomerization to a trans-keto form as the final product or generate the original enol form by a ground state recovery process. It is worth © XXXX American Chemical Society

mentioning that the substituents in the phenyl ring or at the nitrogen atom may affect intramolecular hydrogen bonding and then influence the tautomerism of the Schiff bases14,23−25 and the deactivation pathways after photoexcitation. Therefore, there could be different photodynamical behaviors for different Schiff bases. Salicylidene methylamine (SMA) is one of the simplest aromatic Schiff bases, as depicted in Figure 1. Despite its compact size, fewer studies were devoted to investigate the properties and behavior of SMA than other aromatic Schiff bases such as SA.18−20,26−28 After synthesis,29 a study about the enol-keto equilibrium at ground state as a function of temperature in protic or aprotic solvents was carried out by electronic absorption or emission and Raman spectroscopy.30−32 Later, the infrared and Raman spectra in the gas phase, the transient absorption spectra of SMA,22 the inelastic neutron scattering, and vibrational spectra33 were reported. TheoretReceived: June 7, 2016 Revised: August 18, 2016

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DOI: 10.1021/acs.jpca.6b05719 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

electronic structure computations and surface hopping dynamics simulations at higher calculation levels (CASSCF/ CASPT2), which have been used to deal with the excited state proton transfer and isomerization processes successfully.39−43 In this paper, we employed the ab initio on-the-fly surface hopping method to simulate the singlet excited state dynamics of SMA in the gas phase. By tracking the real-time nuclear motion, we present a detailed deactivation process of SMA. This work can provide an important reference to understand the photodynamical behavior of Schiff bases and provide useful information on the rational design of photochemically switchable molecules.

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CALCULATION DETAILS Electronic Structure Calculation. The most stable structures of SMA tautomers on the S0 and S1 states and the minimum energy conical intersections (MECIs) between the S0 and S1 states were all optimized at complete active space selfconsistent field (CASSCF) method without any symmetry restrictions. The active space in the CASSCF calculations was composed by 14 electrons in 12 orbitals. The plots of these included orbitals are provided in the Supporting Information, Figure S1. Two electronic states were included with equal state weights, which will be sufficient to deal with the photodynamical processes of SMA.37,44 The 6-31G** basis set was employed for all atoms. Vertical excitation energies of different SMA tautomers were calculated at SA2-CASSCF(14,12)/631G** level and were corrected by the complete active space second-order perturbation theory CASPT2 method because of the lack of dynamic electron correlation of CASSCF level,45 and an imaginary shift of 0.2 au was applied to avoid intruderstate issues in the CASPT2 calculations. Reaction pathways for the internal conversion process of SMA constructed by linearly interpolated internal coordinate (LIIC) reaction paths connecting the S0 state geometry and minimum energy conical intersections were calculated at CASSCF and CASPT2 levels. The CC2 method with the ccpVDZ basis set was also performed mainly to compare the results to those published previously.36 All the CASSCF/ CASPT2 calculations were executed using MOLPRO 2010.1 program,46 and the CC2 calculations were carried out with TURBOMOLE 6.4.47 Nonadiabatic Dynamics Simulations. The dynamics simulations were performed using on-the-fly surface hopping method at the SA2-CASSCF(8,8)/6-31G* level with all relevant energies and gradients calculated on the fly as needed. The active space was confirmed to be sufficient to deal with the dynamical behaviors of SMA, especially the important conical intersection regions corresponding to both the excited state proton transfer and photoisomerization processes.36 In addition, the accuracy and applicability of this calculation level has also been confirmed by comparing the results from SA2-CASSCF(14,12)/6-31G**, SA2-CASSCF(12,10)/631G**, SA2-CASSCF(10,8) /6-31G**, SA2-CASSCF(8,8)/631G**, SA2-CASSCF(8,6)/6-31G**, SA2-CASSCF (6,6)/631G**, SA2-CASSCF (14,12)/6-31G*, SA2-CASSCF(12,10)/ 6-31G*, SA2-CASSCF(10,8)/6-31G*, SA2-CASSCF(8,8)/631G*,SA2-CASSCF(8,6)/6-31G*, and SA2-CASSCF(6,6)/631G* levels prior to the dynamics simulations. The nuclear motions were described by classical trajectories using the velocity Verlet algorithm48 with a time step of 0.5 fs and a 0.1 fs time step near conical intersection regions. A total of 150 trajectories were calculated for a maximal simulation time of

Figure 1. Geometries and numbering scheme of SMA tautomers in the S0 state.

ically, Zgierski and Grabowska34 performed a thorough study of SMA on the ground and the first excited states at HF/6-31G(d) and CIS/6-31G(d) levels. Five tautomers were located; the cisenol form was found to be the most stable tautomer in the ground state, and cis-keto form is more stable than the cis-enol form on the first excited state. A small energy barrier of 1.62 kcal mol−1 was calculated along the ESIPT pathway on the first excited state. Ortiz-Sánchez and co-workers explored the dynamic study of SMA in the ground state and first excited state by means of the Heidelberg multiconfiguration timedependent Hartree method (MCTDH).35 They found that the proton transfer process is endoergic in the ground state with a 5.9 kcal mol−1energy barrier and is exoergic in the S1 state. Jankowska and co-workers36 explored the photophysical behavior of SMA by means of CC2, TDDFT, and CASPT2 calculation methods. Two kinds of conical intersections were located, corresponding to the ESIPT process and rotational motion around the CC bond. The two conical intersections have been identified as the main gates for the relaxation to the ground state. Recently, Lasse and co-workers37 investigated the detailed dynamics behaviors by on-the-fly photodynamics simulation at a semiempirical OM2/MRCI level. They found that the molecule prefers to decay to the ground state along the ESIPT pathway. Most publications devoted to the ESIPT process of SMA were reported on the results of calculation investigations. In 2001, Grzegorzek and co-workers38 studied the photochemical behavior of SMA experimentally. They investigated the photoisomerization of SMA and its chlorosubstituted derivative (SMAC) in low temperature argon matrixes by infrared matrix isolation spectroscopy and DFT quantum chemical calculations. However, only the enol tautomers which were generated by the rotation around the CC bond were observed, but no keto tautomers related to ESIPT process were observed. The inconsistency between theoretical and experimental results provokes new questions about the mechanism of this phenomenon which governs the photodynamic behavior of SMA. The explanation and understanding of the photochemical behavior of SMA requires a better description of the energy surfaces and conical intersections. It should be noted that neither the TDDFT nor CC2 method used in previous studies is applicable when dealing with the conical intersections or avoided crossing regions. Given this situation, we decided to study the mechanistic photochemistry behavior of SMA with B


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Table 1. Key Parameters of Optimal Structures of Different Tautomers of SMA at the S0 and S1 States Calculated at the SA2CASSCF(14,12)/6-31G** Levela

a

geometry

C4C7

C7N8

C8N9

O10H11

N8H11

C4C7N8C9

C3C4C7N8

C4C3O10H11

S0-min-α S1-min-α S0-min-β S1-min-β S0-min-ζ S1-min-ζ S0-min-γ S1-min-γ S0-min-δ S1-min-δ

1.474 1.434 1.369 1.426 1.483 1.443 1.360 1.419 1.495 1.475

1.292 1.311 1.346 1.394 1.272 1.306 1.375 1.395 1.294 1.299

1.448 1.445 1.441 1.445 1.445 1.446 1.447 1.446 1.452 1.452

0.951 0.957 2.043 2.469 0.947 0.942 4.738 4.639 0.943 0.943

1.918 1.873 0.995 0.997 4.993 5.047 0.994 0.998 2.673 2.613

180.0 −180.0 −168.4 174.7 −180.0 −180.0 159.1 −178.4 −2.4 −5.3

0.0 0.0 2.9 −5.1 −180.0 180.0 177.8 −174.3 −65.5 −51.3

0.0 0.0 0.91 22.7 −180.0 −180.0 3.0 13.1 −10.2 −9.1

Bond distances are in Å; angles are in degrees.

Table 2. Vertical Excitation Energy Information of the Five SMA Tautomers Computed at CASSCF(14,12)/6-31G** and CASPT2 Levels and Compared with Previous Results at CASSCF(8,8)/cc-PVDZ,36 CC2/cc-PVDZ,36 and OM2/MRCI Levels37 with Available Experimental Data22

a

geometry

CAS(14,12)a

CASPT2a

CAS(8,8)b

CC2b

OM2/MRCIc

exptld

α β γ ζ δ

4.67 3.47 3.19 4.72 4.88

4.31 3.31 3.11 4.44 3.11

5.28

4.21

3.97

3.56

3.07

4.33 3.02 3.28

4.55

4.59

6-31G**. bcc-PVDZ, ref 36. cRef 37. dref 22.

twisting motion around C4C7 bond with the dihedral angle C4C7N8C9 changing from 0°(β) to 180°(γ). Moreover, the nominal α form may also isomerize to a δ conformer with a torsional motion around the C7N8 bond. The ζ form is generated by a rotational motion around both the C4C7 and O10H11 bonds. It is worth mentioning that, different from previous publications, we pay additional attention to the ζ form to discuss the proposed scenario in the experimental observations.38 The SMA tautomers can be divided into three classes, and the tautomers in one particular class are interconvertible by twisting motion around certain single bonds with only little energy.36,37 The geometrical scheme of the three classes of SMA tautomers can be found in ref 36. The vertical excitation energies for the five SMA tautomers at CASSCF(14,12)/6-31G** and CASPT2 levels are summarized in Table 2 together with previous calculation results at CASSCF(8,8)/cc-PVDZ, OM2/MRCI, and CC2 levels35−37 for comparison. The vertical excitation energies are overestimated at the CASSCF level due to the lack of dynamic electron correlation. As expected, the values are corrected at CASPT2, which is consistent with the MP2/MRIC and CC2 values and much closer to the experimental results obtained in solutions.22 2. Excited State Geometry Optimization. At the CASSCF level, the stable structure of S1-α has been located, which is found to be 4.44 eV above the S0-α energy. However, such an S1-α minimum has not been located at CC2/cc-PVDZ and OM2/MRCI calculation levels because the S1-α form can spontaneously relax to the S1-β through the ESIPT path in the course of optimization.36,37 The inconsistency between the CASSCF and OM2 methods has been reported previously.61 The main differences between the S0-α and S1-α are found in the C4C7 and C7N8 bond lengths, which are, respectively, shrunken by 0.04 Å and stretched by 0.02 Å of S1-α. The S1-β was located to be the most stable structure in the S1 state,

1000 fs. In 100 trajectories, the initial geometries and velocities were generated from a 500 ps ground-state molecular dynamics simulation of the SMA at room temperature. In the remaining 50 trajectories, the initial conditions were selected from a harmonic oscillator Wigner distribution49,50 to compare the effect on the dynamics simulation results of the initial conditions. The nonadiabatic dynamics simulations were performed with the NAIMD-DICP procedure,51,52 which has been developed as a program package based on the Zhu-Nakamura theory.53,54 The procedure can monitor the energy fluctuations and track geometry changes as a function of the simulation time. When a local minimum of the energy gap was located and the energy space was less than 0.02 hartree, the transition probability between the two electronic states is predicted by ZhuNakamura formulas. The probability will be compared to a generated uniform random number to judge whether a hopping event is taken. If the hopping occurs, the nucleus velocities will be corrected immediately to preserve the energy conservation. Further technical details are provided in previous publications.55−60

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RESULTS AND DISCUSSION 1. Ground State Geometry Optimization and Vertical Excitation Energies. The ground state equilibrium structures and atom enumeration scheme of the five main tautomers of SMA are displayed in Figure 1. The most relevant geometry parameters are summarized in Table 1, and related Cartesian Coordinates are provided in the Supporting Information. To compare the results easily, we employed the same name scheme as in the previous theoretical publication.36 α is an enol-form with a planar structure, which has been confirmed to be a global minimum in the ground state. β is a keto-form with a nearly planar structure, which is formed by an ESIPT process of α. The β form can further isomerize to a trans-keto form (γ) by a C


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Figure 2. Geometries of the five minimum energy conical intersections optimized at the SA2-CASSCF(14,12)/6-31G** level.

Table 3. Key Parameters of the Five Located S1/S0 Minimum Energy Conical Intersections Calculated at the SA2CASSCF(14,12)/6-31G** Level Together with the Results Obtained at the SA2-CASSCF(6,6)/cc-PVDZ Level in Reference 36a

a

geometry

C4C7

C7N8

C8N9

O10H11

N8H11

C4C7N8C9

C3C4C7N8

C4C3O10H11

ESIPTS1/S0

1.457

1.359

1.444

3.350

0.995

−157.5

86.7

−37.2

ref 36 TWin1 TWin2 ref 36 TWout1 TWout2

1.457 1.434 1.433 1.414 1.438 1.438

1.298 1.397 1.397 1.374 1.377 1.377

1.460 1.447 1.447 1.448 1.442 1.442

3.126 0.945 0.945 0.948 0.942 0.942

1.025 2.163 2.163 2.091 3.663 3.663

−168.5 90.8 −90.8 −90.3 96.9 −96.9

87.7 1.8 −1.9 −3.5 −8.8 8.7

−38.4 30.9 −30.9 −25.3 174.8 −174.8

Bond distances are in Å; dihedral angles are in degrees.

MECIs (TWout1 and TWout2) were located in our calculations, which have not been reported yet. TWout1 is also located in the enol region, related with the rotational motion around the C7N8 and C3O10 bonds. The dihedral angles C4C7N8C9 and C4C3O10H11 are 97° and 174.8°, respectively. The OH bond is back to the N8 atom. Similarly, TWout2 is a mirror structure of TWout1 with the dihedral angles of C4C7N8C9 and C4C3O10H11 nearly −97° and −174.8°, respectively. It should be mentioned that the rotation of the C3O10 bond will also increase the N8H11 bond. The four twisting related MECIs have similar energies, which are 3.28 eV above the S0-α. The key parameters of the five MECIs are presented in Table 3, and the Cartesian coordinates are given in the Supporting Information. 4. Reaction Paths. The reaction pathways for the internal conversion at the five MECIs have been calculated at CASSCF, CC2 (see Figures S2−S3 in the Supporting Information), and CASPT2 levels. The reactions paths were computed by linearly interpolated internal coordinates connecting the ground state minimum geometries to the conical intersections. Because of the similarity of the reaction pathways of the twisting motion, we present only a representative one in this article (structure information on the end points is shown in the Supporting Information, Tables S3−S5). The three calculation levels presented essentially the same qualitative properties and should result to similar dynamics behaviors. Here, we discuss only the reaction paths calculated at CASPT2 level. As presented in Figure 3, the potential energy curve of the S1 state is almost flat with a small energy barrier of ∼0.2 eV along the twist motion pathway. However, the energy barrier (2.2 eV) is much higher along the ESIPT process. The barrier height for the reaction pathways was also confirmed by the CC2 calculations. Considering the high-energy barrier along the pathway to the ESIPT process related MECI, we expect that twisting motion characterized MECIs are expected to play a comparatively

which lies about 2.60 eV above the S0-α and 0.87 eV below the S1-α. According to the CASSCF optimization, the S1-γ is a nearly planar structure generated by an ESIPT process and twisting motion, which lies 1.44 eV below the S1-α. The main difference between the S1-min-ζ and S0-min-ζ was found in the distances of the C4C7 and N7C8 bonds, which are, respectively, shrunken by 0.04 Å and stretched by 0.03 Å. The S1-min-δ is found to have a C3C4C7N8 bond smaller than that of the S0-min-δ. The S1-min-ζ and S1-min-δ were located about 0.05 and 0.47 eV above the S1 state energy of the S0-min, respectively. The key parameters of the geometries mentioned above are also provided in Table 1, and the related Cartesian coordinates are given in the Supporting Information. 3. Minimum Energy Conical Intersections. Five important S1/S0 minimum energy conical intersections for SMA at the CASSCF(14,12)/6-31G** level were located, which can be divided into three types. The first MECI (donated as ESIPT) is located in the keto region, which is about 3.04 eV above the S0-α. It is characterized by an ESIPT process from the O atom to the N atom and a twisting motion around the C4C7 bond with the dihedral angle of C3C4C7N8 of nearly 87°. The ESIPT corresponds to an increase in the O1H11 length and a decrease in the N8H11 bond length. A similar but somewhat more planar structure was found in the recent SA2CASSCF(6,6)/cc-PVDZ level37 with a dihedral angle of C4C7N8C9 of −168.5°. TWin1 is located in the enol region, related to the rotational motion around the C7N8 bond with the dihedral angle of C4C7N8C9 at nearly 90° and the OH bond pointing to the N atom (as depicted in Figure 2). TWin2 is a mirror structure of TWin1 with a different rotation direction of C7N8 bond, and the dihedral angle of C4C7N8C9 is nearly −90°. TWin1 and TWin2 can be classified into one type. A similar structure has also been found in previous publications.36,37 Apart from the two kinds of MECIs mentioned above and reported previously,36,37 two more D


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depopulated at the end of the simulation. The average decay time was calculated to be 540 fs. In Figure 5, we summarized

Figure 3. Linearly interpolated pathways connecting the S1-min-α and the ESIPT (left) and Twin1 (right) constructed at the CASPT2/SA2CASSCF(14,12)/6-31G** level.

major role in the S1 decay process of SMA. This is also borne out by the CASSCF trajectories that are discussed in the next section. In addition, to avoid the overestimation of the energy barrier of the LIIC method employed here, we also recalculated the relaxed potential energy profiles scan at the CASPT2// SA3-CASSCF(14,12)/6-31G** level as a function of the O−H bond length and the C4C7N8C9 dihedral angle. The results are shown in Figure S4 in the Supporting Information. Obviously, these energy profiles present a qualitative agreement with the LIIC results. There is still an energy barrier along the ESIPT process, which is much higher than that of the C7N8 rotational motion pathway, indicating that the molecule would prefer the rotational motion decay channel. This calculation result once again proved the accuracy of our dynamics simulation results. 5. Dynamics Results. A total of 138 trajectories which satisfy the energy conservation criterion were used for the final evaluations (90 sampling from the ground state molecular dynamics and 48 sampling from Wigner distribution). The two initial condition sampling methods produce consistent results; here, we present the analysis results of only the 90 ground state sampling trajectories. More information about the trajectories with the Wigner distribution sampling method is presented in the Supporting Information, Figures S4 and S5. Figure 4 shows the evolution of the occupation of the S0 and S1 states. At time zero, the SMA is occupied in the S1 state and begins to decay to the ground state after 400 fs. The S1 state is completely

Figure 5. Bond length changes of O10H11 and N8H11 as a function of simulation time.

the OH and NH bond length changes as a function of the simulation time. It is evident that the OH bond shows little changes during the simulation period, and the NH bond becomes longer after about 300 fs. If the excited state proton transfer occurs, the OH bond should become longer and the NH bond will become shorter, and they will intersect with each other at one certain point. Clearly, there are no line intersections of the NH and OH bonds, indicating that the ESIPT process is not supported by our dynamics results. The increased lengths of the NH bonds here can be attributed to the rotational motion of the C3O10 bond. Figure 6 presents

Figure 6. C4C7N8C9 dihedral angle changes as a function of simulation time; the yellow stars correspond to the C4C7N8C9 values of hopping structures.

the changes of the dihedral angle of C4C7N8C9 as a function of simulation time. The yellow stars are the hopping points for the 90 trajectories. Clearly, all the hopping events occur around C4C7N8C9 = 90°. Similarly, the time dependent values of C3C4C7N8 for the 90 trajectories are also shown in Figure 7. The C3C4C7N8 values of the hopping structures are around 0°, indicating that rotation around the C4C7 bond is not obvious at the hopping points. Combined with the results shown in Figure 6, we can conclude that the twisting motion

Figure 4. Time evolution of the population of the S0 and S1 states. E


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Figure 7. C3C4C7N8 dihedral angle changes as a function of simulation time; the yellow stars correspond to the C3C4C7N8 values of hopping structures.

Figure 9. Key parameter changes of a typical trajectory passing the TWin2 conical intersection. The vertical line is the hopping time of the molecule.

around the C7N8 bond rather than the C4C7 bond is the dominant decay process for SMA. In addition, we also summarized the hopping point structures as functions of NH bond length, C4C7N8C9, and C4C3O10H11 in Figure 8. As

Figure 10. Key parameter changes of a typical trajectory passing the TWout2 conical intersection. The vertical line is the hopping time of the molecule. Figure 8. Diagram (left) for dihedral angles C4C7N8C9 (ordinate) and NH bond length (abscissa), and (right) for dihedral angle C4N3O10H11 (ordinate) and bond length NH (abscissa) at hopping points from the S1 to S0 state. The balls are the five MECIs that were located, and the red stars are the hopping structures of the trajectories.

on a descending pathway to the S1/S0 region at about 640 fs, which is characterized by a rotational motion of both C7N8 and C3O10 bonds. The TWout2 serves as the gateway for the deactivation process. The dynamics simulation products are summarized in Figure 11. The details of the class definition are provided in the Supporting Information, Table S2, according to ref 37. Depending on the three class definitions mentioned above, the products can be assigned into two classes, the C4C7N8C9−0° (δ set) and C4C7N8C9−180° (α set), with a splitting ratio nearly 1:1, but no γ set was observed. In the α set, there are about 36% percent ζ molecules. The statistical analysis of the dynamics simulations lends support to the proposed reaction mechanism in our static electronic structure calculations. The ESIPT process is not supported in our calculations. The generation of the δ set and the ζ-type molecules are in remarkable agreement with the observations in the experimental publication.38 The ESIPT process observed previously is mainly in the solutions, which may be attributed to the hydrogen bond environments. The results here stress the need for femtosecond time-resolved photoelectron spectroscopy to investigate the ultrafast excited state dynamics behavior of SMA in the gas phase.

shown, all the hopping structures are centralized around the four twisting MECIs but are far away from MECI related with the ESIPT process. The results indicated that the four MECIs characterized by a twisting motion around the C7N8 bond rather than the ESIPT conical intersection play important roles in the deactivation process. Among them, a majority of the hopping points surround the Twin1 and Twin2. A typical trajectory is shown in Figure 9. The system propagates in the S1 state before 480 fs. During this period, the OH and NH bonds and two dihedral angles C4C7N8C9 and C4C3O10H11 do not show evident change. At about 480 fs, the NH bond becomes slightly shorter, and C4C7N8C9 and C4C3O10H11 change to about −80° and −19°, respectively. The molecule approaches the region near the TWin2, which will serve as a gateway to funnel to the ground state. The remaining trajectories follow a relaxation pathway which decays to the ground state by the Twout1 and Twout2, as depicted in Figure 10. The molecule again moves F


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Figure 11. Product distribution as a function of the dihedral angles C4C7N8C9 and C3C4C7N8. Red, δ set; blue, α set; blue star, ζ.

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CONCLUSION The ultrafast radiationless decay process of SMA has been revealed by optimizations of stable structures and minimum energy conical intersections, determination of reaction pathways, and ab initio on-the-fly surface hopping dynamics simulations. The simulation predicted that, after photoexcitation to the S1 state, the system prefers to decay to the ground state by a photoisomerization process rather than an ESIPT process, which correlates well with the experimental findings. The different reaction mechanisms obtained are results of the discrepant potential energy surfaces at different calculation levels. As most of the hydrogen bonds of SMA could be affected by the solvent effect in different solutions, further investigation of the dynamics behavior of SMA in complex environments should be the next step. We hope the current study can provide new insight into interpretation of the experimental observations and can pave the way for the advanced investigation of the intricate photochemistry in different environments.

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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.jpca.6b05719. Active orbitals, energy profiles, Cartesian coordinates, and figures of optimized structures (S0, S1 minimumenergy structures, and five minimal energy conical intersections) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +86-411-84379930. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21203187 and 21473195). REFERENCES

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New Insight into the Photoisomerization Process of the Salicylidene

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