Probing ultrafast ππ*/nπ* internal conversion in organic chromophores via K-edge resonant absorption

Abstract

Organic chromophores with heteroatoms possess an important excited state relaxation channel from an optically allowed ππ* to a dark nπ*state. We exploit the element and site specificity of soft x-ray absorption spectroscopy to selectively follow the electronic change during the ππ*/nπ* internal conversion. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the near edge x-ray absorption fine structure (NEXAFS) spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept with the nucleobase thymine, a prototypical heteroatomic chromophore. With the help of time resolved NEXAFS spectroscopy at the oxygen K-edge, we unambiguously show that ππ*/nπ* internal conversion takes place within (60 \pm 30) fs. High-level coupled cluster calculations on the isolated molecules used in the experiment confirm the superb electronic structure sensitivity of this new method for excited state investigations.