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

Abstract

Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ* to a dark nπ* state. The ππ*/nπ* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ*/nπ* internal conversion takes place within (60 ± 30) fs. High-level-coupled cluster calculations confirm the method’s impressive electronic structure sensitivity for excited-state investigations.

A New and Efficient Equation-of-Motion Coupled-Cluster Framework for Core-Excited and Core-Ionized States

“…The NTOs of these two states, obtained at the Franck-Condon geometry, are shown in Table 7. Theory and Computation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 similar to what has been observed for thymine, 8 one can expect that the TR-NEXAFS measurements at the O-edge are the best to probe the population of the n O π * due to ultrafast internal conversion. Indeed, we show in Figs.…”

“…28, has been devised and used, for instance, to simulate and interpret TR-NEXAFS experiments in thymine. 8 The study aimed at assessing the ability of K-edge resonant absorption spectroscopy to probe ultrafast ππ /nπ internal conversion in organic 37 Page 37 of 58…”

“…TR-NEXAFS spectra of uracil have not been experimentally measured yet, but they are expected to bear strong similarities with those of thymine, whose O-edge TR-NEXAFS was measured in Ref. 8. Two valence excited states were considered, the first bright ππ * state (S 2 at FC geometry) and the first dark n O π * (S 1 at FC geometry) state.…”

“…The experimental advances over the past decade have been driving the interest in developing highly accurate theoretical methods for X-ray spectroscopy and, in particular, for time-resolved XAS, which is proving to be a powerful mean to investigate molecular dynamics. 5,[8][9][10] Owing to its low computational cost relative to ab initio methods, time-dependent (TD) density functional theory (DFT) has been among the most commonly used techniques for modeling absorption spectra, including X-ray absorption. 11 However, TDDFT often fails to deliver an accurate description of spectroscopic properties.…”

“…16 The focus of this contribution is on extending coupled-cluster (CC) approaches, 17-24 which are among the most successful ab initio methods for molecular properties and electronic spectra in the UV-vis region, to absorption and ionization phenomena in the X-ray domain. 8,[25][26][27][28] Although ionization or excitations of core electrons superficially appear to be similar to valence transitions, the numerical experiments have shown that direct application of standard approaches to core-level transitions leads to unsatisfactory results, 5 due to the following essential features of core-ionized and core-excited states. First, these states have open-shell character.…”