AmericanChemicalSociety.com

Our efforts in the third year of this project focused on semiclassical dynamics simulation study of the following photochemical reactions:

1. Photoisomerization of methyl substituted azobenzenes
   

Photoinduced surface relief grating (SRG) formation is an important photomechanical effect and has attracted considerable research attentions.  The SRG formation is produced by the mass transport in the surface and is a unique property of azopolymer film. It is believed that the photoinduced mass transport in the surface of azopolymer film is induced by many trans-cis isomerization cycles of the azobenzene or its derivatives in the film. It is therefore expected that the trans-cis photoisomerization cycle plays an important role in the SRG formation. Our recent semiclassical dynamics simulation study for four methyl substituted azobenzenes shows that the photoisomerization times for four methyl substituted molecules are in the same order as the times for the surface relief grating formation of four represented molecules. The results provide a base for the understanding of photoinduced mass transport, specifically the surface relief grating formation of azobenzene-based materials upon irradiation with laser beams. The manuscript based on this work is currently under review for publication in J. Phys. Chem. C.

2. Photoinduced thymine dimerization
   

Exposure of DNA to UV radiation in the 200-300 nm causes the DNA damage mainly because of the formation of cyclobutane pyrimidine dimers (CPD) between two adjacent pyrimidine bases within the same DNA strand. The formation of CPDs has been linked with a range of human health problems, including cell lethality, mutagenesis, and the development of skin cancers. Since the first isolation of CPDs a half century ago ^[1], there have been intensive experimental ^[2-5] and theoretical ^[6-13] investigations in understanding how and to what extent the initially photon excited thymine molecules lead to the formation of CPDs.
We simulated the response to ultrashort laser pulses of two stacked thymine molecules by semiclassical dynamics simulation with laser radiation explicitly incorporated. The laser pulses used to excite the thymine molecule have a 25 (FWHM) fs Gaussian shape with a photon energy of 4.0 eV. Simulation follows two different reaction paths produced by the laser pulses with two different fluences.  In one reaction, the stacked thymine molecules form a cyclobutane pyrimidine dimer, which is the main course of photoinduced DNA damage, and the formation of two chemical bonds linking two thymines occurs nonsynchronously after the excimer decays to electronic ground state. In other reaction, only one bond is formed between two thymine molecules. In the second reaction, the bond breaks about 50 fs after formation and then two molecules move away from each other. This reaction leads to the DNA damage repair. The simulation finds that the deformation of pyrimidine ring plays an important role in cleaving this bond. The manuscript from this work has been accepted for publication in J. Phys. Chem. C.

3. Ring opening reaction of cyclohexadiene
   

Photochemical interconversion between 1,3-cyclohexadiene (CHD) and hexatriene (HT) is a model system for studying electrocyclic ring opening reactions and has attracted considerable research interest both experimentally and theoretically over the last several years.We have investigated the photoisomerization reaction of cyclohexadiene ring to cZc-hexatriene conformers induced by a femtosecond-scale laser pulse using the semiclassical dynamics simulation technique. Different from the most theoretical investigations and the dynamical pictures with all the detailed structure information for the ring opening reaction have been given clearly. Along the simulation path the potential energy curves (PECs) of the ground (S₀), first (S₁) and second (S₂) excited state have been calculated using the complete-active-space self-consistent-field and multi-reference second-order perturbation theory (CASSCF/MRPT2). The intersection and avoided crossing found on the PECs confirm the rationality of CHT to cZc-HT photoisomerization path obtained by the simulation. The time scale for the transition from S₁ to S₂ via S₁/S₂ intersection and then S₂ radiationless decays to S₀ via S₂/S₀ avoided crossing are respectively 56fs and 74fs, which could be comparable with experimental and other theoretical values. The simulation path, of course, is obtained under the specific laser pulse and the given initial conditions. Changing the laser pulse or initial conditions the SERID simulation may give birth to some else dynamic paths, which is obviously meaningful for further experimental investigations. This work has been published in Chem. Phys. Lett.

4. Laser-induced nonthermal fragmentation of C₆₀
   

Fullerene (C₆₀) consists of 20 hexagonal and 12 pentagonal rings as the basis of an icosohedral symmetry closed cage structure. Each carbon atom is bonded to three others through sp2 hybridization. With this unique structure, C₆₀ exhibits an extremely fast response upon laser excitation and therefore becomes a model system for studying the electronic and nuclear dynamics induced by intense laser pulses. The photon fragmentation of C₆₀, one of the relaxation channels for the excited fullerene, has been an interesting topic in this field since the C₆₀ fullerene was discovered.
We have performed semiclassical dynamics simulation study for photofragmentation of the C₆₀ fullerene induced by ultrafast laser pulses. The simulation study is focused on the excitation below the continuum levels. A laser pulse of 40 fs (FWHM) with an effective photon energy of 2.0 eV and different intensities was selected to interact with the C₆₀ fullerene. The simulation results show that averaged fragmentation size distribution over groups of initial geometries selected at random exhibits a power law pattern with the peak at C₂ at high laser pulse intensities. The threshold for the C₆₀ fragmentation was determined. The simulation finds that as many as 55 electrons are excited from the occupied molecular orbitals to unoccupied molecular orbitals upon the laser irradiation and that the number of the fragments significantly depends on the number of electrons excited. Finally, the temperature examination seems to suggest that the nonthermal effect may play a significant role in laser fragmentation of the C₆₀ fullerene. This work has been published in Molecular Physics.