Progress and Challenges in the Calculation of Electronic Excited States

Computational Photochemistry and Photophysics: the state of the art

This review starts with the most basic concepts in photochemistry and photophysics, followed by a chronological introduction of theoretical methods and relevant applications in the history of computational photochemistry, along with the authors’ comments on the methodologies currently available for photochemical studies. Recent advances in the field are next summarized and discussed, focusing separately on methodology and computational techniques and some highlighted applied works carried out during the last two years on the topics of photodissociations, photostability, photodimerizations, photoisomerizations, proton/hydrogen transfer, photodecarboxylations, charge transport, bioexcimers, chemiluminescence and bioluminescence. We finish this review by conclusions and an outlook of the future.

Electronic interactions in helical stacked arrays of the modified DNA base pyrrolocytosine

The thermal stability and ultraviolet and circular dichroism spectra of nine synthetic DNA hairpins possessing one or more (P)C-G base pairs ((P)C = pyrrolocytosine) have been investigated. One group of hairpins possess 1-5 sequential (P)C-G base pairs while another group possess two (P)C-G base pairs separated by 1-3 A-T base pairs. The first group displays a nearly linear dependence of UV and exciton-coupled circular dichroism (EC-CD) band intensity upon the number of neighboring chromophores, whereas the second group shows weak EC-CD only at the shortest distances between non-neighboring chromophores. This result stands in marked contrast to the exciton coupling seen between stilbene chromophores separated by as many as a dozen base pairs. The weak exciton coupling between non-neighboring (P)C chromophores, like that of the natural nucelobases, is attributed to their relatively weak electronic transition dipoles.

Experimentally measured permanent dipoles induced by hydrogen bonding. The Stark spectrum of indole-NH3

Hydrogen bond pairs involving the chromophore indole have been extensively studied in the gas phase. Here, we report high resolution electronic spectroscopy experiments on the indole-NH(3) hydrogen bond pair in the absence and presence of an electric field. The S(1)-S(0) origin band of this complex recorded in zero field at high resolution reveals two overlapping spectra; a consequence of NH(3) hindered internal rotation. The barrier to internal rotation is predicted by theory to be less than 20 cm(-1) in the ground state, therefore requiring a non-rigid rotor Hamiltonian to interpret the spectra. Conducting the experiment in the presence of an applied electric field further perturbs the already congested spectrum of the complex, but makes possible the measurement of the permanent electric dipole moments in its S(0) and S(1) states. These values reveal significant changes in electron distribution that arise from hydrogen bonding effects.