Ultrafast Decay of the Excited Singlet States of Thioxanthone by Internal Conversion and Intersystem Crossing

Abstract

The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high‐level ab initio CASPT2 calculations of the singlet‐ and triplet‐state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin–orbit coupling terms. The initially populated singlet ππ* state is shown to decay through internal conversion and intersystem crossing processes via intermediate nπ* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet–triplet crossing near the singlet ππ* minimum and the large spin–orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity.

Isolated and Solvated Thioxanthone: A Photophysical Study

“…Experiments conducted at room temperature in acetonitrile 13 determine comparable IC (5.5 Â 10 9 s À1 ) and ISC (10.6 Â 10 9 s À1 ) rates, thus, supporting our explanation. A recent work by Angulo et al 34 reports the ISC and IC rates in AcN to be 8 Â 10 9 s À1 and 4.1 Â 10 9 s À1 , which are also in good agreement with our results and further support the qualitative picture provided by our calculations. The calculated vibronic spinÀorbit coupling is of the order of 10 7 s À1 .…”

Thioxanthone in apolar solvents: ultrafast internal conversion precedes fast intersystem crossing

“…Femtosecond (fs) transient absorption spectroscopy has already been applied to TX in cH. 15 In that study, a time constant of B5 ps was observed and ascribed to the depletion of the primary 1 pp* excitation. Based on SE signatures in fs-transient absorption data as well as fs-fluorescence spectroscopy, we will show that in this study 15 a process on the time scale of 100 fs was missed.…”

Time-Resolved Resonance Raman Spectroscopic Studies on the Triplet Excited State of Thioxanthone

“…15À22 It has attracted considerable attention of both the photophysicists and the photochemists due to its solvatochromic behavior. 15À20, 22,23 The intensity and decay characteristics of the TX fluorescence strongly depend on the nature of the solvent. It has also been found that the tripletÀtriplet (TÀT) absorption maximum is very sensitive to solvent polarity.…”