45446-B6
Computer Simulation of Photochemical Reactions
Our efforts in the first year of this project focused on two important photochemical reactions described below:
1. Photoisomerization of azobenzene: Azobenzene (Ab) has two stable forms, namely the cis (Z) and trans (E) isomers. When subjected to ultraviolet or visible radiation, Ab undergoes E « Z isomerization. This makes Ab and its derivatives excellent candidates for many applications, including molecular switches, image storage devices, as well as the recently designed light-driven molecular shuttle. One of the challenges in understanding this reaction is to interpret the quantum yield dependence on the excitation wavelength, which provides a case of violation of the Kasha rule that the quantum yield is independent of the excitation energy. A debatable explanation is that the Ab molecule takes inversion path from the reactant to the product for the np* excitation and rotation path for the pp* excitation. The goal of our research for this topic is to examine the reaction mechanisms under different laser excitations using semiclassical dynamics simulation technique. This work has been conducted in collaboration with Prof. Zhenyi Wen (Chongqing University of Posts and
(1) The simulation results show that for both excitations the reaction path is predominated by the rotation coordinate of the NN bond. The simulation finds that CNN inversion angles expand as soon as the rotation starts. The expansion of the CNN bond angles permits the molecule to rotate efficiently. It is therefore suggested that the photoisomerization of trans-azobenzene follows an inversion-assisted rotation path. These simulation results are significant for understanding the mechanism of this important process. This work has been submitted to Chem. Phys and Chin. Chem. Lett. (in press). (2) The simulations find that for pp* excitation, the relaxation of the S(pp*) state is immediately followed by double excitation, (p) 2(p*) 2. The decay from the S((p) 2(p*) 2) state to the S0 state can occur at partially twisted structure, which favors the formation of the trans isomer. Multiple decay channels are found at about twisted structure for both np* and pp* excitations. Decay at about twisted geometry leads to the formation of either cis or trans isomer. Opening of the decay channel at partially twisted structure accounts for the smaller isomerization yield for the pp* excitation. The result of this work has been accepted for publication in J. Phys. Chem. A.
(3) The simulations also demonstrate that the photoisomerization process can be held back by an external resisting force of 90-200 picoNewtons depending on the frequency and intensity of the lasers and that a pure mechanical isomerization is possible from the cis to trans state if the azobenzene molecule is stretched by an external force of ~ 1250 – 1650 pN. The load-bearing capacity of photoisomerizing azobenzene predicted generally agrees with the findings of previous single-molecule mechanical measurements using AFM and the stopping forces found explains the robust mechanical outputs observed in a broad range of azobenzene-bazed molecular optomechanical devices. This work has been accepted for publication in J. Chem. Phys.
2. Nonradiative deactivation of adenine: The high photostability of DNA as a primary UV chromophore of the genetic material of life is attributed to the presence of ultrafast nonradiative deactivation pathways. Through the nonradiative deactivation, electrically excited nucleobases in DNA decays to vibrationally excited ground state on a subpicosecond time scale. The fast internal conversion prevents the destruction of DNA. Adenine is one of most important nucleobases in DNA and 9H-adenine is the most stable tautomer. Numerous experimental investigations have been performed to character the ultrafast radiationless deactivation of 9H-adenine in gas phase. The simulation follows two different excitations induced by two 80 fs (FWHM) laser pulses that are different in energy: one has a photon energy of 5.0 eV and other 4.8 eV. The simulation finds that the excited molecule decays to the electronic ground state from the 1pp* state in both excitations, but through two different radiationless pathways: in the 5.0 eV excitation the decay channel involves the out-of-plane vibration of the amino group while in the 4.8 eV excitation the decay strongly associates with the deformation of the pyrimidine at the C2 atom. The lifetime of the 1np* state determined in the simulation study is 630 fs for the 5.0 eV excitation and 1120 fs for the 4.8 eV excitation. These are consistent with the experimental values of 750 fs and 1000 fs. We conclude that the experimentally observed difference in the lifetime of the 1np* state at various excitations results from the different radiationless deactivation pathways of the excited molecule to the electronic ground state. This work has been performed in collaboration with Prof. Keli Han The result of this work has been accepted by published in J. Phys. Chem. A 112, 8497 (2008).
