The role of πσ* states in the photochemistry of heteroaromatic biomolecules and their subunits: insights from gas-phase femtosecond spectroscopy

In an effort to illuminate why nature has chosen a particular set of bio-molecular `building-blocks' for life, recent years have seen an up-surge of gas-phase spectroscopy experiments aimed at understanding why the DNA/RNA nucleobases, aromatic amino acids and their corresponding chromophore subunits, exhibit a resistance to photochemical damage (photostability) following the absorption of ultraviolet (UV) radiation. The research considered in this Perspective article specifically focuses on the role of electronically excited dissociative (1)pi sigma states (formed through the photo-induced promotion of an electron in a sigma <- pi molecular orbital transition), which have been implicated to contribute to this photostable behaviour. In particular, we review the application of gas-phase femtosecond pump-probe spectroscopies to gain insights into (1)pi sigma state driven relaxation dynamics in a number of heteroaromatic biomolecules and their UV chromophore subunits. This Perspective article also discusses how the information obtained from these studies can be used as a `stepping-stone' for extending this research to larger, more complex biomolecules and ultimately, more realistic systems in solution.