TY - JOUR
T1 - Probing highly efficient photoisomerization of a bridged azobenzene by a combination of CASPT2//CASSCF calculation with semiclassical dynamics simulation
AU - Liu, Lihong
AU - Yuan, Shuai
AU - Fang, Wei Hai
AU - Zhang, Yong
PY - 2011/9/8
Y1 - 2011/9/8
N2 - Mechanism of phototriggered isomerization of azobenzene and its derivatives is of broad interest. In this paper, the S0 and S1 potential energy surfaces of the ethylene-bridged azobenzene (1) that was recently reported to have highly efficient photoisomerization were determined by ab initio electronic structure calculations at different levels and further investigated by a semiclassical dynamics simulation. Unlike azobenzene, the cis isomer of 1 was found to be more stable than the trans isomer, consistent with the experimental observation. The thermal isomerization between cis and trans isomers proceeds via an inversion mechanism with a high barrier. Interestingly, only one minimum-energy conical intersection was determined between the S 0 and S1 states (CI) for both cis → trans and trans → cis photoisomerization processes and confirmed to act as the S 1 → S0 decay funnel. The S1 state lifetime is ∼30 fs for the trans isomer, while that for the cis isomer is much longer, due to a redistribution of the initial excitation energies. The S 1 relaxation dynamics investigated here provides a good account for the higher efficiency observed experimentally for the trans → cis photoisomerization than the reverse process. Once the system decays to the S0 state via CI, formation of the trans product occurs as the downhill motion on the S0 surface, while formation of the cis isomer needs to overcome small barriers on the pathways of the azo-moiety isomerization and rotation of the phenyl ring. These features support the larger experimental quantum yield for the cis → trans photoisomerization than the trans → cis process.
AB - Mechanism of phototriggered isomerization of azobenzene and its derivatives is of broad interest. In this paper, the S0 and S1 potential energy surfaces of the ethylene-bridged azobenzene (1) that was recently reported to have highly efficient photoisomerization were determined by ab initio electronic structure calculations at different levels and further investigated by a semiclassical dynamics simulation. Unlike azobenzene, the cis isomer of 1 was found to be more stable than the trans isomer, consistent with the experimental observation. The thermal isomerization between cis and trans isomers proceeds via an inversion mechanism with a high barrier. Interestingly, only one minimum-energy conical intersection was determined between the S 0 and S1 states (CI) for both cis → trans and trans → cis photoisomerization processes and confirmed to act as the S 1 → S0 decay funnel. The S1 state lifetime is ∼30 fs for the trans isomer, while that for the cis isomer is much longer, due to a redistribution of the initial excitation energies. The S 1 relaxation dynamics investigated here provides a good account for the higher efficiency observed experimentally for the trans → cis photoisomerization than the reverse process. Once the system decays to the S0 state via CI, formation of the trans product occurs as the downhill motion on the S0 surface, while formation of the cis isomer needs to overcome small barriers on the pathways of the azo-moiety isomerization and rotation of the phenyl ring. These features support the larger experimental quantum yield for the cis → trans photoisomerization than the trans → cis process.
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U2 - 10.1021/jp203704x
DO - 10.1021/jp203704x
M3 - Article
AN - SCOPUS:80052316923
SN - 1089-5639
VL - 115
SP - 10027
EP - 10034
JO - Journal of Physical Chemistry A
JF - Journal of Physical Chemistry A
IS - 35
ER -