We use non-adiabatic ab initio molecular dynamics to study the influence of substituent side groups on the photoactive unit (Z)-hexa-1,3,5-triene (HT). The Time-Dependent Density Functional Theory Surface Hopping method (TDDFT-SH) is used to investigate the influence of substituted isopropyl and methyl groups on the excited state dynamics. The 1,4 and 2,5-substituted molecules are simulated: 2,5-dimethylhexa-1,3,5-triene (DMHT), 2-isopropyl-5-methyl-1,3,5-hexatriene (2,5-IMHT), 3,7-dimethylocta-1,3,5-triene (1,4-IMHT), and 2,5-diisopropyl-1,3,5-hexatriene (DIHT). We find that HT and 1,4-IMHT have the lowest ring-closing branching ratios of 5.3% and 1.0%, respectively. For the 2,5-substituted derivatives, the branching ratio increases with increasing size of the substituents, exhibiting yields of 9.78%, 19%, and 24% for DMHT, 2,5-IMHT, and DIHT, respectively. The reaction channels are shown to prefer certain conformation configurations at excitation, where the ring-closing reaction tends to originate from the gauche-Z-gauche (gZg) rotamer almost exclusively. In addition, there is a conformational dependency on absorption, gZg conformers have on average lower S1 ← S0 excitation energies that the other rotamers. Furthermore, we develop a method to calculate a predicted quantum yield that is in agreement with the wavelength-dependence observed in experiment for DMHT. In addition, the quantum yield method also predicts DIHT to have the highest CHD yield of 0.176 at 254 nm and 0.390 at 290 nm.
Additionally, we study the vitamin D derivative Tachysterol (Tachy) which exhibits similar photochemical properties as HT and its derivatives. We find the reaction channels of Tachy also have a conformation dependency, where the reactive products toxisterol-D1 (2.3%), previtamin D (1.4%) and cyclobutene toxisterol (0.7%) prefer cEc, cEt, and tEc configurations at excitation, leaving the tEt completely non-reactive. The rotamers similarly have a dependence on absorption as well, where the cEc configuration has the lowest energy S 1 ← S0 excitation of the rotamers. The wavelength dependence of the rotamers should lead to selective properties of these molecules at excitation. An excitation to the red-shifted side of the maximum absorption peak will on average lead to excitations of the gZg rotamers more exclusively.
|Commitee:||Gredig, Thomas, Tapavicza, Enrico|
|School:||California State University, Long Beach|
|Department:||Physics and Astronomy|
|School Location:||United States -- California|
|Source:||MAI 58/02M(E), Masters Abstracts International|
|Subjects:||Computational physics, Physical chemistry, Physics|
|Keywords:||Hexatriene, Quantum yield, Surface hopping, TDDFT, Tachysterol, Time-dependent density functional theory|
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