Excited State Dynamics of Trans-Cis Photoisomerization in Photoactive Yellow Protein Chromophores

218 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008.As an efficient and accurate quantum chemical method for photobiological and condensed phase applications, pseudospectral time-dependent density functional theory (PS-TDDFT) is presented, which alleviates the O(N4) scaling problem in two-electron integral calculations. For the test cases in this work, PS-TDDFT is up to 10 times faster than the conventional TDDFT with hybrid functionals without sacrificing chemical accuracy.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Charge-Transfer Excited States and Proton Transfer in Model Guanine-Cytosine DNA Duplexes in Water

Characterization of the excited electronic states and relaxation processes in DNA systems is critical for understanding the physical basis of radiation damage. Spectroscopic studies have shown evidence of coupling between the relaxation dynamics of photoinduced charge-transfer states and interstrand proton transfer in DNA duplexes, where a deuterium isotope effect was observed for duplexes with alternating sequences but not with nonalternating sequences. We performed quantum mechanical/molecular mechanical (QM/MM) calculations of the vertical excitation energies and excited state proton potential energy curves for model DNA duplexes comprised of three guanine-cytosine pairs with alternating and nonalternating sequences in aqueous solution. Our calculations indicate that the intrastrand charge-transfer states are lower in energy for the alternating sequence than for the nonalternating sequence. The more accessible intrastrand charge-transfer states could provide a relaxation pathway coupled to interstrand proton transfer, thereby providing a possible explanation for the experimentally observed deuterium isotope effect in duplexes with alternating sequences

Photoinduced Proton-Coupled Electron Transfer of Hydrogen-Bonded <i>p</i>‑Nitrophenylphenol–Methylamine Complex in Solution

Proton-coupled electron transfer can occur through concerted (electron–proton transfer, EPT) or sequential mechanisms, but this distinction becomes less well-defined for photoinduced reactions. These issues have been examined with transient absorption experiments on a hydrogen-bonded complex consisting of <i>p</i>-nitrophenylphenol and <i>t</i>-butylamine. These experiments revealed two spectroscopically distinct states: the higher-energy excited state was interpreted to be a conventional intramolecular charge transfer (ICT) state within the <i>p</i>-nitrophenylphenol, whereas the lower-energy state was interpreted to be an ICT-EPT state, where photoexcitation resulted in both ICT and the shifting of electronic density corresponding to effective proton transfer from the phenol to the amine. In the present work, the singlet excited states of the hydrogen-bonded <i>p</i>-nitrophenylphenol–methylamine complex in 1,2-dichloroethane are studied with time-dependent density functional theory and higher-level ab initio methods. The calculations suggest that the ππ* state, which is the S₁ state at the Franck–Condon geometry, corresponds to the state denoted ICT-EPT in the experimental analysis, whereas the <i>n</i>π* state, which is the S₂ state at this geometry, likely corresponds to the state denoted ICT in the experimental analysis. According to the calculations, the ππ* state has charge-transfer character, as well as a change in electronic density on the amine, with the minimum-energy structure corresponding to the proton bonded to the nitrogen acceptor, consistent with proton transfer. The <i>n</i>π* state has little charge-transfer character, as well as negligible change in electronic density on the amine, with the minimum-energy structure corresponding to the proton bonded to the oxygen donor. The calculations also provide evidence of an avoided crossing between these two states located energetically close to the Franck–Condon point. These calculations provide the foundation for future nonadiabatic molecular dynamics studies of the relaxation process