A ‘bottom up’, ab initio computational approach to understanding fundamental photophysical processes in nitrogen containing heterocycles, DNA bases and base pairs

A systematic computational study of non-radiative decay pathways following UV excitation of selected heterocycles, DNA bases, nucleosides and base-pairs in the gas phase.</p

A Multipronged Comparative Study of the Ultraviolet Photochemistry of 2-, 3-, and 4-Chlorophenol in the Gas Phase

The S1(1ππ*) state of the (dominant) syn-conformer of 2-chlorophenol (2-ClPhOH) in the gas phase has a subpicosecond lifetime, whereas the corresponding S1 states of 3- and 4-ClPhOH have lifetimes that are, respectively, ∼2 and ∼3-orders of magnitude longer. A range of experimental techniques–electronic spectroscopy, ultrafast time-resolved photoion and photoelectron spectroscopies, H Rydberg atom photofragment translational spectroscopy, velocity map imaging, and time-resolved Fourier transform infrared emission spectroscopy–as well as electronic structure calculations (of key regions of the multidimensional ground (S0) state potential energy surface (PES) and selected cuts through the first few excited singlet PESs) have been used in the quest to explain these striking differences in excited state lifetime. The intramolecular O–H···Cl hydrogen bond specific to syn-2-ClPhOH is key. It encourages partial charge transfer and preferential stabilization of the diabatic 1πσ* potential (relative to that of the 1ππ* state) upon stretching the C–Cl bond, with the result that initial C–Cl bond extension on the adiabatic S1 PES offers an essentially barrierless internal conversion pathway via regions of conical intersection with the S0 PES. Intramolecular hydrogen bonding is thus seen to facilitate the type of heterolytic dissociation more typically encountered in solution studies

Photophysics of the sunscreen ingredient menthyl anthranilate and its precursor methyl anthranilate : a bottom-up approach to photoprotection

The ultrafast excited state dynamics of the sunscreen ingredient menthyl anthranilate (MenA) and its precursor methyl anthranilate (MA) were studied in vacuum (using time-resolved ion yield spectroscopy) and in solution (using transient electronic absorption spectroscopy). MenA and MA both show long-lived dynamics, with the observation of a kinetic isotope effect suggesting that hydrogen motion acts as the rate determining process in the overall decay. Complementary computational studies exploring the intuitive decay pathways of MA revealed a bound S1 state with a shallow ‘up-hill’ gradient with respect to proton transfer. From these results, it is suggested that photoexcited population is trapped in this excited state from which luminescence occurs as a prominent decay pathway. This work has shown that the photophysics of MA and MenA – and hence their photoprotection capabilities – are not drastically influenced by aliphatic structure or solvent environment alone. A bottom-up approach, such as the one described herein, is essential to understand the combination of factors that afford optimum photoprotection and to develop a new generation of tailor made, efficacious sunscreens