Browsing by Author "Li, Anyang"

Now showing 1 - 2 of 2

Excited-state E -> Z photoisomerization mechanism unveiled by ab initio nonadiabatic molecular dynamics simulation for hemithioindigo-hemistilbene
(2017-05-21)
Yang, Meihong
;
Huo, Chunyan
;
Li, Anyang
;
Lei, Yibo
;
Yu, Le
The Zhu-Nakamura formulas based on on-the-fly trajectory surface hopping dynamics simulations at the two-state-averaged CASSCF level were employed to investigate the E -> Z photoisomerization mechanisms of hemithioindigo-hemistilbene (HTI) upon S-1 excitation. Seven conical intersections were observed along the isomerization pathways, which were composed of double bond torsion, benzene ring torsion, inversion and pyramidalization motions, and only three of them were found to play a role in the dynamics simulations started at S-1 E-HTI. The dominant isomerization pathway proceeds via central double bond torsion together with pyramidal and tilt motions to some extent (hop via CI5) and accounts for all the reactive trajectories. On the other hand, the two pathways that involve the conical zones lie in the vicinity of the E-form Franck-Condon region (CI7) and proceed along the combined central double bond and benzene ring torsion route (CI3/CI4) with generation of the E products. Within the 332 simulated trajectories, 66 hop to the ground state and only 19 switch to the Z product. The estimated quantum yield of 0.057 (19 in 332) agrees well with the reported experimental value of 0.053 +/- 0.016. The excited-state lifetimes span a wide region from hundreds of femtoseconds to several picoseconds, depending on the time for vibrational relaxation and number of cycles for periodical mixed mode torsion.
Methyl substitution enhanced photoisomerization of trans,trans-1,4-diphenyl-1,3-butadiene: direct ab initio trajectory surface hopping dynamic simulations
(2018-01-28)
Fan, Yueqian
;
Chen, Juan
;
Yu, Le
;
Li, Anyang
;
Zhai, Gaohong
Single methyl group substitution on the p-position of the phenyl ring (tt-DPB-me1) or the conjugated CQC bond (tt-DPB-me2) has been found to enhance the photoisomerization efficiency for two trans, trans-1,4-diphenyl-1,3-butadiene (tt-DPB) derivatives by performing direct ab initio trajectory surface hopping dynamics simulations. With implementation of the Zhu-Nakamura global switching algorithm, on-the-fly trajectory surface hopping dynamics simulations based on the ground state and first excited state potential energies and their gradients calculated by the two state averaged complete active space self-consistent field method with basis set 6-31G were propagated up to 3000 femtoseconds. Four-hundred sampling trajectories have been performed for both tt-DPB-me1 and tt-DPB-me2, and five distinctive photoisomerization pathways were observed for both of them. Among which, One Bond Flipping (OBF) and Hula-Twist (HT) are the dominant photoisomerization mechanisms. The lifetime of the S1 state is estimated to be 1423.0 fs (819.0 fs), and the photoisomerization quantum yields are 0.088 (0.378) in tt -> ct , 0.070 (0.015) in tt -> ct and 0.073 (0.065) in tt -> cc for tt-DPB-me1 (tt-DPB-me2). By analyzing the dynamics simulation data, it can be concluded that closer methyl substitution with respect to the central C=C double bond results in a higher percentage of the corresponding photoisomerization products. The present simulation results are in agreement with the ultrafast spectroscopy measurements, which unveil the photoisomerization mechanisms of tt-DPB derivatives and present useful physical insights on how to tune the photoisomerization of the substituted DPB.