3A18
シクロヘキサジエン/ヘキサトリエン間光異性化反応に対する理論的研究(上智大学院理工 1,個人 2)○太田亜由美 1，小林理 1，石田俊正 2，南部伸孝 1

Theoretical study of photoisomerization reaction between 1,3-cyclohexadiene and hexatrienein both of vapor and liquid phase(Sophia Univ.1, Individual2)○Ayumi Ohta1,Osamu Kobayashi1,Toshimasa Ishida2,Shinkoh Nanbu1【Introduction】
Photoisomerization between 1,3-cyclohexadiene (CHD)
and 1,3,5-cis-hexatriene (HT) has been attracting greatinterest as the model system of large-size photochromic
molecules like diarylethenes.
Thisisomerization

has

been

experimentallyinvestigated in vapor phase [1,2] and in liquid phase [3].
In

2009,

Kosma

etal.

has

measured

Figure.1

1,3-cyclohexadiene(left)
1,3,5-cis-hexatriene(right)

theenergy-relaxation time after photoexcitation to the first excited state (S1) in vapor phase, and they havefound that the relaxation time is 136 fs, which is an only experimental data for the energy-relaxation
time from CHD.
On the other hand, several groups have performed ab initio calculations and discussed the reaction
mechanism and the product branching ratio of CHD to HT so far. [4,5] The first quantum dynamics
using the two-dimensional reaction model based on Jacobi coordinates has been executed by Tamura
et al. in 2006.

They have suggested that the branching ratio of CHD to HT is approximately 50 : 50,

and also they have found that the S1-lifetime approximates 130 fs, which has been proven by Kosma etal. However, the used two-dimensional model is still open to question about whether or not this simple
model is reasonable to describe the photochromic process.
In this study, firstly semiclassical dynamics of the photoisomerization of CHD in vapor phase wascarried out based on the full-dimensional model which consists of 42 degrees of freedom. thephotoexcitation process from CHD was assumed to go along with Franck-Condon (FC) principle.
Specifically, the energy-relaxation processes after the photoexcitation to not only S1 but also S2 wereexplored by ab initio molecular dynamics (MD) simulation. Tully’s Surface Hopping (TSH) method[6] with Zhu-Nakamura formula [7] was employed to treat the non-adiabatic transition.

The used abinitio method was at multi-reference configuration interaction (MRCI) and complete active space 2ndperturbation(CASPT2)levelswithTable 1 analysis for transition dipole moments

cc-pVDZ basis set. For the excitation to S1

MRCICASPT2

d𝑆1−𝑆0 /Bohr

0.063999151

0.016761036

d𝑆2−𝑆0 /Bohr

1.080936402

1.196021555

and S2, 15 and 17 trajectory-computations
wereexecutedbyourIntelXEON(Westmere, Nehalem, and Sandybridgecores) computers.

First, the transition dipole moments wereexplored by MRCI and CASPT2 methods
around the equilibrium geometry of S0. Table 1shows the dependence analysis for transition
dipole moments on these methods. Since thetransition dipole moments for the excitation toPotential energy(eV)【Result】

the S2 state have a certain amount, theexcitation to S2 should be explored to8642S₂S₀S₁0050

100150Time(fs)understand the photoisomerization.
Figure.2 is the typical trajectory decaying

Figure.2 result of the typical trajectory in vapor phasefrom S2 in vapor phase. The product branchingratio of CHD to HT after photoexcitation to S1 is found to be 50 : 50, equivalence. Contrary to ourexpectation, the S1-lifetime is 78.5 fs on an average, which is shorter than previous studies.

However,since the number of our trajectories is not enough to get the converged result, we still need moretrajectories.
The ratio of CHD to HT after excitation to S2 is 70 : 30; the photoisomerization prefers CHD product.
Figure.3 shows that the points where the non-adiabatic transitions occurred at conical intersection
between S2 and S1 states (CoIn-S2/S1) and CoIn-S1/S0. The decay time to reach S0 is 57.8 fs on anaverage. This shorter lifetime than S1 would be reasonable if we consider the excess energy comparing
with the case of the transition to S1, and moreover S1 and S2 states are lying close to each other.
On the other hand, the conformational feature when the non-adiabatic transition occurs is found to bequite similar in every case even for CoIn-S2/S1 and CoIn-S1/S0. The actual changing structure seemed
to be a stretching motion for each of C-C bonds, because the corresponding non-adiabatic vectors also
show such a stretching motion.
Ab initio MD simulation in liquid phase was performed with QM/MM method. The fluctuation ofpotential energy was smaller than that in vapor phase, but the non-adiabatic transition has happened at
the shorter lifetime in the excited states than the vapor phase.
【Reference】[1] K. Kosma, S. A. Trushin, W. Fuß, W. E. Schmid, Phys. Chem. Chem. Phys.11, 172 (2009). [2] J. L. White, J. Kim, V. S.
Petrović, P. H. Bucksbaum, J. Chem. Phys.136, 054303 (2012). [3] E. C. Carroll, B. J. Pearson, A. C. Florean, P. H.
Bucksbaum ,R. J. Sension, J. Chem. Phys., 124, 114506 (2006). [4] H. Tamura, S. Nanbu, T. Ishida, H. Nakamura, J. Chem.
Phys. 124, 084313 (2006). [5] J. B. Schönborn, J. Sielk, B. Hartke, J. Phys. Chem. A, 114, 4036 (2010). [6] J. C. Tully, R. K.
Preston, J. Chem. Phys, 55, 562 (1971). [7] H. Nakamura, Nonadiabatic Transition: Concept, Basic Theories andApplications ,2nd Edition(World Scientific, Singapore, 2012)