UC Santa Barbara

Resonant excitation multiphoton ionization (REMPI) with TOF mass spectrometry detection was used to study and characterize organic chromophores which play a role in our prebiotic origins, the medicines we use, and in the paintings and other cultural artifacts we treasure.

For our molecules which are prepared in a molecular beam (i.e. cold, neutral, and isolated) we are in the opportune position to compare with state of the art computation. This analysis was invaluable when differentiating alternative nucleobase structures in the chapters on isocytosine and 6-thioguanine, and understanding the effects of intramolecular hydrogen bonding environments when working with anthraquinone dyes.

Of great interest was understanding excited state chromophore deactivation from low energy electronic states. The deactivation pathways a molecule takes, be them radiative or non-radiative, have severe implications on the form and function of that molecule. I continued to expand on the photostability centered hypothesis of prebiotic chemistry; that the aromaticity of genetic molecules leave them prone to harsh UV absorption in an ozone-less environment, however, a predisposition to convert electronic to ground state vibrational energy (heat) on an ultrafast timescale supports the prevalence of our canonical bases A, T, G, C, U. The work on isocytosine is exciting both in analogy to biological (keto) G and as a possible xeno or pre-RNA/DNA nucleobase (when paired with complimentary isoguanine). Secondly, 6-thioguanine converts with near unity to a long lived dark state and due to the thiooxo guanine substitution this intersystem crossing happens at an abnormally high rate. The work on anthraquinone dyes was an effort to understand the photostability of these molecules within their historical usage in cultural heritage materials and here I will present our case study on the these dyes and whether they undergo an excited state intramolecular proton transfer (ESIPT) to greatly reduce excited state lifetimes, promoting colorfastness as a function of structure.