Probing ultrafast \pi\pi*/n\pi* internal conversion in organic chromophores via K-edge resonant absorption

TL;DR

This study uses element-specific soft x-ray absorption spectroscopy to directly observe ultrafast internal conversion from
* to n
* states in organic chromophores, exemplified by thymine, within approximately 60 femtoseconds.

Contribution

It introduces a novel application of time-resolved NEXAFS spectroscopy at the K-edge for tracking ultrafast excited state dynamics in heteroatomic organic molecules.

Findings

Internal conversion occurs within (60 30) fs in thymine.

NEXAFS spectra reveal the formation of a hole in the n orbital during conversion.

High-level calculations confirm the electronic structure sensitivity of the method.

Abstract

Organic chromophores with heteroatoms possess an important excited state relaxation channel from an optically allowed {\pi}{\pi}* to a dark n{\pi}*state. We exploit the element and site specificity of soft x-ray absorption spectroscopy to selectively follow the electronic change during the {\pi}{\pi}*/n{\pi}* internal conversion. As a hole forms in the n orbital during {\pi}{\pi}*/n{\pi}* 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 {\pi}{\pi}*/n{\pi}* 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.