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Effect of Methylation on the Photodynamical Behavior of Arylazoimidazoles: New Insight from Theoretical ab Initio Potential Energy Calculations and Molecular Dynamics Simulations 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: Arylazoimidazoles are a series of azobenzene derivatives possessing the ability to undergo photoinduced trans−cis isomerization. Their isomerization quantum yields are found to be dependent on the excitation wavelength and chemical substituents. The current work investigated the ultrafast nonadiabatic decay behaviors of three arylazoimidazoles (Pai-H, Tai-H, and Tai-Me) after being photoexcited to the S1 and S2 states by means of high-level ab initio potential energy calculations and on-the-fly surface hopping dynamical simulations in gas phase to explore the effect of the methylation. The results found that the Pai-H with no methylation substituents only decay along a NNC bending reaction pathway for both the S1 and S2 states. The Tai-H with a methylation substituent on the six-membered ring can decay along both the NNC bending and twisting motion pathways for the S1 and S2 states. The Tai-Me has methylation substituents on both the six- and five-membered rings prefers to decay by a twisting motion in the S1 state, while a NNC bending channel is activated following excitation to the S2 state. The position and numbers of methylation substituents has important influence on the dynamical behaviors of arylazoimidazoles. The current work provides fundamental knowledge of the arylazoimidazoles and will be helpful for advanced and further exploration and application.

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INTRODUCTION Azobenzene and its derivatives have the ability to undergo light or thermal induced reversible trans−cis isomerization. The amazing characteristics attracted numerous research interests and have made them good candidates for molecular switches, storage devices, molecular motors, etc.1−7 Most azobenzene derivatives that have been widely studied and applied are those that have substituents on the azobenzene scaffold,5,7−16 much less attention has been paid to its analogues having the phenyl group displaced by a heterocyclic ring. Arylazoimidazoles, as such kind of compounds with a heterocyclic ring,17 have a great potential in applications in molecular devices and biological usages because the imidazole part is a ubiquitous and vital group in biological field. They can serve as coordination sites to metal ion or hydrogen bonding, which enabled them to be used to modulate or change the properties of metal-complexes or hydrogen-bonded supramolecules.18,19 Another noticeable point is that the photoinduced isomerization quantum yield of most arylazoimidazoles are generally higher than that of azobenzene, indicating the arylazoimidazoles should be more efficient photochromic compounds. However, very few studies concerning the photochromic reactions have been performed to explore the arylazoimidazoles.3,17−19 Majima and co-workers20 observed the reversible trans−cis isomerization process of (deoxy)ribofuranosyl derivatives of 2-(phenylazo)imidazole (Pai-H). © XXXX American Chemical Society

The isomerization quantum yield was observed to be lower than those of azobenzene. However, a new question whether these properties are general for arylazoimidazoles or significantly influenced by the (deoxy)ribofuranosyl substituents has been proposed. Fukuda et al.21 prepared polymers containing the arylazoimidazole dyes possessing photoinduced birefringence phenomena to explore the photochromic characteristics of arylazoimidazoles. In 2005, Sinha and co-workers18 reported the photoisomerization and thermal isomerization behaviors of 2-(phenylazo)imidazole (Pai-H) and 1-N-methyl-2-(phenylazo)imidazole (Pai-Me) by X-ray crystallography, absorption spectroscopic analysis, and density functional theory (DFT) calculations. They found that the Pai-Me undergoes reversible trans−cis photoisomerization process. The isomerization quantum yield is wavelength-dependent and much higher than those for azobenzene. However, the Pai-H responds poorly to the irradiation with a much lower photoisomerization quantum yield than azobenzene, but shows a much faster thermal cis−trans isomerization process. It is still an open question whether the different photoisomerization and thermal isomerization behaviors are intrinsic to Pai-H and Pai-Me or general for arylazoimidazoles Received: November 1, 2016 Revised: December 13, 2016 Published: December 13, 2016 A

DOI: 10.1021/acs.jpca.6b10968 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

calculations were also carried out at CASSCF level. It should be noted that the energy gaps between two relevant adiabatic states of all MECIs located in this work are less than 0.2 eV confirmed by different CASSCF levels. Nonadiabatic Dynamics Simulation. The photodynamical behavior of the three molecules after being excited to the S1 and S2 states were investigated at SA-CASSCF level including the nonadiabatic transition method based on Zhu−Nakamura theory,24,25 which has been developed into a program package termed as NAIMD-DICP.26,27 This method has been employed to deal with nonadiabatic processes in many molecular systems successfully.12,28−34 The accuracy of different SA-CASSCF level used for each molecule is examined prior to the dynamical simulations by comparing the results with high-level CASSCF and CASPT2 methods. Details of the CASSCF parameters (included electronic states, basis set, and active spaces) are given in the Supporting Information. The gradients and relevant energies are evaluated on-the-fly. The equation of motion for nuclei is described by the Velocity−Verlet35 algorithm with a time step of 0.5 fs. The time step was changed to 0.1 fs when the molecule was propagated near the intersection seams. When the potential energy difference between two relevant electronic states is less than 0.02 hartree and is located as a local minimum, the nonadiabatic coupling vector and transition probability will be evaluated. The hopping events will take place if the transition probability is larger than a generated uniform random number, and the nuclei velocities will be adjusted to conserve the total energy. The initial geometries and velocities were generated from a 500 ps ground-state molecular dynamics simulation, with a constant temperature of 300 K. The number of trajectories computed for the S1 and S2 states for each molecule is also given in the Supporting Information.

with or without substituent on the imidazole nitrogen. Later, they investigated photoisomerization and thermal isomerization behaviors of an extensive series of arylazoimidazoles to study the substituent effect on the isomerization processes.17 They concluded that the trans−cis quantum yields of arylazoimidazoles are general higher than those of azobenzene and are wavelength dependent similar to azobenzene. The thermal cis− trans isomerization process is faster than that of azobenzene, especially those without substituent on the imidazole nitrogen. These preliminary data bode well for a potential application of arylazoimidazoles in chemical or molecular device fields. These remarkable advances stimulated a need for a detailed and deeper understanding of the underlying mechanism for a series of arylazoimidazoles structures. In this article, we choose three typical structures (named as Pai-H, Tai-H, and Tai-Me) aimed to shed light on the effect of substituents on the photodynamical behavior from a theoretical point of view. As we know, all the previous investigations performed are in the liquid phase. The calculations were performed in gas phase in order to exclude the hydrogen bond or other influences from the solvents. Considering the wavelength-dependent properties of the arylazoimidazoles, the dynamical behavior of the first and second excited states were both explored. It is worth mentioning that we have reported the wavelength-dependent photoisomerization behavior of Tai-Me,22 and more attention will be devoted to the other two molecules and the different dynamical behaviors. We employed on-the-fly surface hopping excited state dynamical simulations and high-level ab initio potential energy calculations to provide a comprehensive picture of the ultrafast photoinduced dynamical behavior of the three geometries. The outline of the article is concluded as follows. The next section describes the computation details, and then the results and discussion are presented. Conclusions are given in the final section, and the further research directions are also included.

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RESULTS AND DISCUSSION A. Static Calculations. The ground state stable geometries of the three molecules optimized at SA3-CASSCF(12,10)/ 6-31G* level are shown in Figure 1, along with the atomic

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CALCULATION METHODS Static Calculations. The equilibrium geometries of the ground (S0), first (S1), and second (S2) excited states were optimized without any symmetry restrictions by means of the state averaged complete active space self-consistent field (CASSCF) method as implemented in MOLPRO 2010.1 code.23 The minimum energy conical intersections (MECIs) located on the intersection seams play vital roles in the nonadiabatic transitions. The search for MECIs connecting S1/S0 and S2/S1 were also performed at state-averaged CASSCF levels. For the geometry optimization, the reasonable choice of active space, basis sets, and number of included electronic states in the averaging procedure determines the reasonableness and credibility of the results. Therefore, a judicious choice of calculation level for each structure balancing the computation cost and accuracy should be a crucial step. For equilibrium geometries and MECIs optimizations of the three molecules, three adjacent electronic states were included and the active space was composed of 12 electrons in 10 orbitals (SA3-CASSCF(12,10)). More information about the active spaces can be found in the Supporting Information (Figure S1). The 6-31G* basis set was employed for all atoms. In addition, the energy profiles constructed by linearly interpolated internal coordinate (LIIC) connecting the Franck−Condon region and MECIs were performed at SA3-CASSCF(12,10)/6-31G* level. After geometries optimizations, the single-point energy

Figure 1. Numbering scheme for the three stable arylazoimidazoles structures on the S0 state. Left, Pai-H; middle, Tai-H; and right, TaiMe. Here and in other figures: white, H; gray, C; blue, N.

numbering scheme. The three molecular structures are all almost planar. The Tai-Me form is the most energetically stable structure among the three molecules. The energetic order is Tai-Me < Tai-H < Pai-H, which is consistent with the previous experimental observations and theoretical results obtained by Sinha et al.17 The most relevant geometrical parameters of the three molecules in different electronic states and the minimum energy conical intersections (MECIs) are summarized in Tables 1 and 2, respectively. The relevant energies properties B


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Table 1. Most Relevant Geometrical Parameters of Pai-H, Tai-H, and Tai-Me (from Ref 22) Calculated at SA3-CASSCF(12,10)/ 6-31G* Level, with Bond Length in Å and Angles in Degrees geometry

states

C2−N3

N3−N4

N4−C5

C2−N3−N4

N3−N4−C5

C2−N3−N4-C5

Pai-H

S0-min S1-min S2-min S0-min S1-min S2-min S0-min S1-min S2-min

1.419 1.362 1.309 1.419 1.359 1.306 1.418 1.359 1.308

1.246 1.259 1.352 1.243 1.257 1.340 1.244 1.257 1.361

1.400 1.363 1.355 1.408 1.374 1.383 1.405 1.359 1.341

115.3 128.6 113.2 115.2 130.5 113.3 115.2 130.5 113.4

113.1 126.5 111.6 114.3 124.6 113.0 114.4 124.6 112.8

180.0 180.0 180.0 −180.0 180.0 −180.0 −180.0 180.0 180.0

Tai-H

Tai-Me22

Table 2. Most Relevant Geometrical Parameters of the Minimum Energy Conical Intersections of Pai-H, Tai-H, and Tai-Me (from Ref 22) Calculated at SA3-CASSCF(12,10)/6-31G* Level, with Bond Length in Å and Angles in Degrees geometry Pai-H

Tai-H

Tai-Me22

MECIs

C2−N3

N3−N4

N4−C5

C2−N3−N4

N3−N4−C5

C2−N3−N4-C5

PlanarS2/S1 NNCS1/S0 TWIST1S1/S0 TWIST2S1/S0 TWIST3S1/S0 TWIST4S1/S0 PlanarS2/S1 NNCS1/S0 TWIST1S1/S0 TWIST2S1/S0 TWIST3S1/S0 TWIST4S1/S0 PlanarS2/S1 NNCS1/S0 TWIST1S1/S0 TWIST2S1/S0 TWIST3S1/S0 TWIST4S1/S0

1.337 1.287 1.360 1.361 1.401 1.400 1.336 1.302 1.356 1.356 1.395 1.402 1.317 1.286 1.359 1.357 1.391 1.398

1.366 1.210 1.263 1.262 1.268 1.268 1.370 1.206 1.255 1.255 1.265 1.259 1.348 1.210 1.254 1.253 1.272 1.265

1.297 1.347 1.388 1.402 1.358 1.358 1.294 1.332 1.388 1.388 1.353 1.343 1.334 1.350 1.405 1.406 1.358 1.356

112.1 156.6 140.4 140.1 118.4 118.3 110.8 156.9 144.3 144.3 118.8 118.4 111.7 156.5 143.6 144.1 118.9 118.9

110.6 144.0 116.3 116.1 135.2 135.2 112.1 146.1 117.1 117.1 136.4 140.9 113.9 144.1 116.5 116.5 135.7 135.7

−179.7 179.9 92.4 −92.3 92.9 −92.9 −180.0 179.8 95.6 −95.5 94.0 −97.1 −179.9 179.9 95.4 −95.7 94.1 −93.2

planar structure with a dihedral angle 179.9°. The next four MECIs are characterized by a twisting motion around the NN bond with nearly 90° of the absolute values of C2N3N4C5. TWIST1(3)S1/S0 is a minor structure of TWIST2(4)S1/S0 with an opposite rotation direction. The main differences of the two kinds of MECIs lie in the bond angles C2N3N4 and N3N4C5. The C2N3N4 and N3N4C5 are nearly 140° and 116°, respectively, for the TWIST1(2)S1/S0, while 118° and 135° for the TWIST3(4)S1/S0. The vertical excitation energies of Pai-H to the S1 and S2 states were calculated to be 3.41 and 5.47 eV, respectively, as shown in Table 3. The calculated values are bigger than the experimental observations in toluene because of the lack of dynamic electron correlation of CASSCF level and the effect of the solvents. For the Tai-H and Tai-Me, the geometrical changes from the S0-min to S1-min and S2-min are similar to those of Pai-H. Six minimum energy conical intersections were also located between relevant crossing seams, with a PlanarS2/S1 between the S2 and S1 states. The remaining five MECIs are located between the energy crossing seams between the S1 and S0 states, with one NNC bending MECI and four related with the torsional motion around the NN bond. These geometries are similar to those of Pai-H. More values information can be found in Tables 1 and 2. As presented in Table 3, the vertical energies of Tai-H to the S1 and S2 states are 3.33 and 5.63 eV, respectively, and the experimental value is 2.73 eV.

Table 3. Vertical Excitation Energies of the Pai-H, Tai-H, and Tai-Me to the S1 and S2 States Calculated at SA3-CASSCF(12,10)/6-31G* Level, Together with Experimental Observations in Toluene17,19

a

geometry

S1

S2

exptl S1

exptl S2

Pai-H Tai-H Tai-Me

3.41 3.33 3.24

5.47 5.63 5.59

2.75a 2.73a 3.39b

3.42a 3.49a 3.49b

Ref 17. bRef 19.

are collected in Table 3. The detailed analysis for each molecule is discussed separately as follows. For the Pai-H, the main differences between the geometries of S0, S1, and S2 states exist in that the C2N3 and N3C4 bond lengths are getting shorter, while the N3N4 bond becomes longer from the S0 to the S1 and S2 states. In addition, the bond angles C2N3N4 and N3N4C5 of S1-min are about 10° bigger than those of S0-min and S2-min. Such bond alternation activities would contribute to the rotation motion around the N3N4 bond. Six minimum energy conical intersections were located between two adjacent electronic states. The PlanarS2/S1 is located between the S1 and S2 states, with a planar structure. This geometry is similar to the S2-min of Pai-H, except for a much longer C2N3 and N3N4 bond lengths. The NNCS1/S0 is characterized by a broadening of the C2N3N4 and N3N4C5 bond angles, and it is nearly a C


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The Journal of Physical Chemistry A As discussed in our previous report,22 there are also six MECIs located for the Tai-Me. The most relevant geometrical parameters are summarized in Table 2. Obviously, the geometrical properties of the six MECIs are similar for the three targeted molecules. The structures are provided in the Supporting Information Figure S4−S6, and the relevant Cartesian coordinates of geometries are provided in Supporting Information. B. On-the-fly Nonadiabatic MD Simulation. As stated repeatedly, the calculation details of the dynamical simulations for the three molecules are presented in the Supporting Information. The simulation time lasts in the region between 1.7 to 1.8 ps. The dynamical behaviors for the three molecules will be discussed separately as follows. Pai-H. S1 State Dynamics Simulation. A total of 35 trajectories are used for analysis of the S1 state dynamics simulations. There are 12 trajectories decayed to the ground state. The time evolution population of the S1 and S0 states is shown in Figure 2. Obviously, nearly 30% molecules finish the

Figure 3. Diagram (left) for bond angles N3N4C5 (ordinate) and C2N3N4 (abscissa), and (right) for dihedral angle C2N3N4C5 (ordinate) and bond angle C2N3N4 (abscissa) at hopping points of Pai-H from S1 to S0 states. The six MECIs were optimized at the SA3-CASSCF(12,10)/6-31G* level.

Figure 2. Time evolution of the S1 and S0 states of the Pai-H following excitation to the S1 state.

internal conversion process to decay to the ground state, and remaining 70% still on the S1 state. The averaging decay time is calculated to be 800 fs. In order to clarify the relevant importance of the five S1/S0 MECIs, we summarized the S1/S0 hopping events and the six located MECIs in Figure 3. Combining the population of the two bond angles and dihedral angle CNNC, we can conclude that all hopping points possess a nearly planar structure rather than a twisting motion around the NN bond. That is, the hopping points are all around the NNCS1/S0 MECI, which plays vital importance in the S1−S0 decay process. We choose a typical trajectory to present a detailed analysis of the decay mechanism, as shown in Figure 4. The molecule decays nearly 975 fs, along with the elongation of the NN bond and the contraction of the C2N3 and N4C5 bonds, which would contribute to the torsional motion around the NN bond. At the hopping points, the C2N3N4 and N3N4C5 bond angles change to nearly 130°, and C2N3N4C5 dihedral angle changes to −164°. The geometrical parameters are in the NNCS1/S0 MECI region. S2 State Dynamics Simulation. We summarized the hopping points of Pai-H following excitation to the S2 states and the six located MECIs in Figures 5 and 6. As shown in Figure 5, all S2−S1 hopping points are around the PlanarS2/S1,

Figure 4. Geometrical parameter changes of Pai-H as a function of the simulation time for a typical trajectory following the NNC bending channel after excitation to the S1 state.

meaning that the PlanarS2/S1 serves as the gateway for the S2−S1 internal conversion process. After decaying to the S1 state, the structures change along the reaction coordinate to approach the crossing seams to decay to the ground state. Figure 6 presents the hopping events of S1−S0 internal conversion process. The CNNC dihedral angles of the hopping points all around the NNCS1/S0 indicate an effective decay process through the NNCS1/S0. A typical trajectory is presented in Figure 7. As shown, the system undergoes an ultrafast S2−S1 decay process in about 100 fs, keeping a planar structure. Following that, the relevant geometrical parameters begin to adjust to follow the reaction channel. For the S1−S0 decay process at about 1450 fs, the C2N3N4 and N3N4C5 bond angles become larger, accompanied by the three bond lengths adjustment. The combination of the bonds and angles drives the molecule to approach the NNCS1/S0 region and then funnel to the ground state. By combining the decay behaviors of Pai-H in the S1 and S2 states as discussed above, it is not hard to conclude that the D


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Figure 5. Diagram (left) for bond angles N3N4C5 (ordinate) and C2N3N4 (abscissa), and (right) for dihedral angle C2N3N4C5 (ordinate) and bond angle C2N3N4 (abscissa) at hopping points of Pai-H from the S2 to S1 states. The six MECIs were optimized at the SA3-CASSCF(12,10)/6-31G* level.

Figure 7. Geometrical parameter changes of Pai-H as a function of the simulation time for a typical trajectory following the NNC bending channel after excitation to the S2 state.

Scheme 1. Decay Scheme of Pai-H Following Excitation to the S1 and S2 States, Respectively

Figure 6. Diagram (left) for bond angles N3N4C5 (ordinate) and C2N3N4 (abscissa), and (right) for dihedral angle C2N3N4C5 (ordinate) and bond angle C2N3N4 (abscissa) at hopping points of Pai-H from the S1 to S0 states following excitation to the S2 state. The six MECIs were optimized at the SA3-CASSCF(12,10)/6-31G* level.

hopping events mainly occur around the NNCS1/S0 MECI, and the MECIs characterized by twisting motion are not included in the decay process. The decay mechanism of Pai-H is summarized in Scheme 1. Tai-H. S1 State Dynamics Simulation. There are 42 trajectories included in the dynamics simulations for the TaiH when directly excited to the S1 state, and the S1 and S0 state populations as a function of the simulation time are shown in Figure 8. Obviously, all trajectories finish the internal conversion process at the end of the simulation, which is more efficient than Pai-H. The averaged lifetime of the S1 state is calculated to be 798 fs. In order to check the relevant importance of the five S1/S0 MECIs, we summarized the timeevolution of the C2N3N4C5 dihedral angles in Figure 9. Most hopping events occurs near C2N3N4C5 = 90°, and a small fraction are around C2N3N4C5 = 180°. For a better intuitive understanding of the roles, we summarized the hopping events and the MECIs in Figure 10 as a function of the C2N3N4,

Figure 8. Time-evolution population of the S0 and S1 states of Tai-H following excitation to the S1 state.

N3N4C5, and C2N3N34C5 angles. Clearly, consistent with the results in Figure 9, the hopping structures can be divided into two branches around the twisting-motion-characterized MECIs, and the other one around the NNC bending MECI. The results indicated that there are at least two reaction coordinates of the Tai-H following excitation to the S1 state. We present each reaction pattern a typical trajectory to give a more detailed view of the reaction mechanism in Figures 11 and 12. E


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Figure 9. Time evolution of the C2N3N4C5 dihedral angles of the Tai-H following excitation to the S1 state, and the yellow states are the hopping events of every analyzed trajectories.

Figure 11. Geometrical parameters changes of Tai-H as a function of the simulation time for a typical trajectory following the NNC bending channel after excitation to the S1 state.

Figure 10. Diagram (left) for bond angles N3N4C5 (ordinate) and C2N3N4 (abscissa), and (right) for dihedral angle C2N3N4C5 (ordinate) and bond angle C2N3N4 (abscissa) at hopping points of Tai-H from the S1 to S0 states following excitation to the S1 state. The six MECIs were optimized at the SA3-CASSCF(12,10)/6-31G* level.

Figure 12. Geometrical parameters changes of Tai-H as a function of the simulation time for a typical trajectory following the twisting motion channel after excitation to the S1 state.

Figure 11 presents a typical trajectory of Tai-H following the NNC bending reaction coordinate. The system quickly decays to the S0 state in less than 100 fs. At the hopping point, the C2N3N4 and N3N4C5 bond angles extend to 154° and 165°, respectively, accompanied by the C2N3N4C5 dihedral angle of nearly 152°. This hopping point is located in the NNCS1/S0 region, which serves as the gateway to deactivate to the ground state. Figure 12 shows a typical trajectory following the twisting motion reaction channel, with the hopping time nearly 590 fs. At the hopping point, the C2N3N4 and N3N4C5 bond angles adjust to 116° and 137°, respectively, and the C2N3N4C5 dihedral angles is located around 106°. The hopping point is in the region of TWIST3S1/S0, which contributes to the decay process. As discussed above, the Tai-H can decay to the ground state from the S1 state by two reaction channels characterized by the NNC bending and NN bond twisting motion. S2 State Dynamics Simulation. The C2N3N4C5 dihedral changes of Tai-H as a function of the simulation time following excitation to the S2 state is presented in Figure 13. The S2−S1 internal conversion activities occur in less than 50 fs keeping

Figure 13. Individual trajectories of C2N3N4C5 dihedral angles of Tai-H as a function of simulation time. The green and yellow stars indicate the hopping events of the S2−S1 and S1−S0, respectively. F


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The Journal of Physical Chemistry A the structure planar. After that, the S1−S0 hopping events can be divided into two branches similar to the properties mentioned above. Most C2N3N4C5 values of hopping structures are around the 90°, and the remaining dihedral angles are around the 180°. The relationship of the six MECIs is concluded in Figures 14 and 15. As shown in Figure 14,

Figure 16. Geometrical parameter changes of Tai-H as a function of the simulation time for a typical trajectory following the NNC bending channel after excitation to the S2 state.


Figure 17. Geometrical parameter changes of Tai-H as a function of the simulation time for a typical trajectory following the twisting motion channel after excitation to the S2 state.

Scheme 2. Decay Scheme of Tai-H Following Excitation to the S1 and S2 States, Respectively


the S2−S1 hopping points are all around the PlanarS2/S1, serving as the only gateway to the S1 state. For the following S1−S0 decay process, the hopping events can also be divided into two branches. Most hopping points are around the twisting-motioncharacterized MECIs, and the remaining fractions are around the NNCS1/S0, indicating that there are also two decay channels of Tai-H after excitation to the S2 state. Figures 16 and 17 present each decay pattern as a typical trajectory for a better clarification of the mechanism. The details are similar to Figures 12 and 13. Overall, we can conclude that the Tai-H molecule would follow two decay reaction coordinates for both

the S1 and S2 states. The details of the decay mechanism can be found in Scheme 2. Tai-Me. The detailed analysis of the decay mechanism of Tai-Me after photoexcitation to the S1 and S2 states can be found in our previous article.22 Here, we only summarize the main decay mechanism in Scheme 3. Mechanism Analysis. As discussed above, the methylation substitute has obvious effect on the deactivation behaviors of G


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The Journal of Physical Chemistry A Scheme 3. Decay Scheme of Tai-Me22 Following Excitation to the S1 and S2 States, Respectively

the three targeted molecules. The roles played by the two possible reaction pathways characterized by twisting motion around the NN bridging bonds and the NNC bending process are different in the nonadiabatic processes of the Pai-H, Tai-H, and Tai-Me. In order to clarify the reasons for the different decay mechanisms, we constructed the reaction pathways for the internal conversion process by means of linear interpolation of internal coordinate (LIIC) connecting the Franck−Condon region and NNCS1/S0 and TWISTS1/S0 at SA3-CASSCF(12,10)/ 6-31G* level, as shown in Figures 18−20. In Figure 18, we can

Figure 19. Energy profiles of Tai-H connecting the FC point and TWIST (a) and NNCS1/S0 (b) constructed by linearly interpolated internal coordinate (LIIC) at SA3-CASSCF(12,10)/6-31G* level.

Figure 20. Energy profiles of Tai-Me connecting the FC point and TWIST (a) and NNCS1/S0 (b) constructed by linearly interpolated internal coordinate (LIIC) at SA3-CASSCF(12,10)/6-31G* level.

photoisomerization quantum yield of Pai-H is low on both the S1 and S2 states. As shown in Figure 19, we can conclude that the pathways connecting the FC points and NNCS1/S0 and FC points and TWISTS1/S0 are quite similar, indicating that both the mentioned pathways would play important roles in the decay processes for Tai-H. For Tai-Me in Figure 20, as discussed before,22 the NNCS1/S0 is located on an uphill energy compared to the FC point, which is not energetically accessible on the S1 state. Therefore, the Tai-Me prefers to decay to the ground state by an energetically accessible TWISTS1/S0 when being excited to the S1 state. However, when Tai-Me is excited to the S2 state, the energy would be enough to access the NNCS1/S0 energy barrier to activate the NNC bending pathway. Therefore, the NNC bending action and NN twisting motion pathways played combined roles in the S2 state deactivation process. We would like to present a speculation about the effect of the methyl group on the dynamical behaviors of the three targeted molecules by combination of the current calculation results and a previous research work.17 The methyl group on the imidazole ring would make the energy of NNCS1/S0 much higher than that

Figure 18. Energy profiles of Pai-H connecting the FC point and TWIST (a) and NNCS1/S0 (b) constructed by linearly interpolated internal coordinate (LIIC) at SA3-CASSCF(12,10)/6-31G* level.

find the TWIST S1/S0 is energetically accessible for the Pai-H after excitation to the S1 state, while the S1 state energy of the NNC S1/S0 is slightly higher than the FC point. However, at the beginning of the constructed pathways, the big slope for the energy changes of the NNC bending pathway will contribute to the molecules preferring to decay along this channel rather than the relatively flat profile along the twisting motion. When energies of the molecule are enough to overcome the energy barrier of the NNCS1/S0, the S1/S0 internal conversion process will occur. A part of the molecules will be trapped to remain on the S1 state, which will explain why the decay ratio of the Pai-H is quite lower than the other two systems. When the molecule was excited to the S2 state, there is an energy barrier along the twisting motion channel, indicating that the photoisomerization channel will not be preferred even on the S2 state. This energy profile can well explain the reason why the H


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The Journal of Physical Chemistry A Notes

of Franck−Condon point of S1 state, making the NNC bending become an unfavorable decay channel, whereas the methyl group on the phenyl ring could make the energy surface more favorable for the NN twisting rotation decay channel. Based on the experimental findings in ref 17, we expected that the wavelength-dependent isomerization property would be general for the arylazoimidazoles. The substitution on the imidazole ring would slower the cis−trans isomerization process of the arylazoimidazoles. Further understanding and explanation of the complex photochemistry of a series of arylazoimidazoles need a thorough analysis of the specific systems.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21203187 and 21473195).

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CONCLUSION A comprehensive picture of the ultrafast nonadiabatic decay behavior of Pai-H, Tai-H, and Tai-Me after photoexcitation to the S1 and S2 states has been revealed by optimization of the critical geometrical structures and on-the-fly surface hopping dynamics simulations. The excited state potential energy profiles connecting the Franck−Condon and MECIs were constructed in order to compare in detail the deactivation pathways of the three molecules. The theoretical results suggest that the number and location of the methylation have evident influence on the photoinduced decay behavior of the three molecules. For the Pai-H, the molecule prefers to decay to the ground state by a NNC bending channel rather than the twisting motion for either the S1 and S2 states. For the Tai-H, the molecule has two decay channels characterized by a twisting motion and a NNC bending process for both the S1 and S2 states. However, for the Tai-Me molecule, the deactivation processes for the S1 and S2 states are different. The molecule prefers to decay by a torsional motion on the S1 state, while another competitive channel characterized by a NNC bending process would be activated following excitation to the S2 state. The different dynamical behaviors can attribute to the different energy profiles connecting the Franck−Condon regions and the minimum energy conical intersections. This finding gives solid support to previous experimental−theoretical investigations17,18 and clearly describes that the position and number of the substitutes attached to the main skeleton would have significant influence on the photoinduced deactivation behavior and photoisomerization processes. The current work could provide useful information for the design of phototriggered molecule devices. For more detailed and comprehensive understanding of the series of azobenzene derivatives in different environments, one should wait for the surface hopping dynamics simulations taking the solvents into consideration.

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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.6b10968. Active orbitals, details of calculation methods, Cartesian coordinates, and figures of optimized structures (six minimal energy conical intersections for the three molecules) (PDF)

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