Excited-State Behavior of Thermally Stable Radical Ions

Abstract

The excited states of several families of thermally stable radical ions in solution are surveyed by transient absorption and steady-state fluorescence spectroscopy to determine prevalent deactivation mode(s). Of the species investigated, weak fluorescence can be observed only for substituted triarylamine radical cations, presumably from their lowest excited doublet states. The primary excited-state deactivation pathway for radical anions of the quinones, aryl ketones, and cyanoarene hydrocarbons examined here is internal conversion from the lowest excited doublet state to the ground-state doublet. The efficiency of this deactivation mode arises typically from a low D r D 1 energy gap, as demonstrated by near-infrared absorbance in each species. Contrary to a prior literature report, the 9,lO-anthraquinone radical anion and the 9,lO-dicyanoanthracene radical anion are nonluminescent in solution. Luminescent side products generated in ground-state reactions of these radical anions are identified as 9,9-bianthrone dianion (from the dimerization and deoxygenation of the anthraquinone radical anion) and 10-cyanoanthrolate (from the reaction of dicyanoanthracene radical anion with molecular oxygen).

A luminescent organic radical with two pyridyl groups: high photostability and dual stimuli-responsive properties, with theoretical analyses of photophysical processes

“… 14 Thus, k nr is determined mainly by the rate of internal conversion. Internal conversion has been suggested as a major pathway in the non-radiative decay of some radicals, 21 , 28 , 34 however, no theoretical calculations have been performed to clarify the deactivation mechanism. In this study, VCD analysis of the luminescent radicals elucidated the mechanism of their non-radiative decay.…”

Pump‐Pump‐Probe Spectroscopy of a Molecular Triad Monitoring Detrimental Processes for Photoinduced Charge Accumulation

“…[19] However, photoexcited TAA + is expected to be a very strong acceptor while photoexcited AQ − should be a very strong donor. [20] It is conceivable that these species are in fact sufficiently potent redox reagents for (temporary) oxidation or reduction of the central Ru(bpy) 3 2+ photosensitizer unit, and this would provide an efficient charge recombination pathway resembling a hopping mechanism …”

Investigations of bimolecular photoinduced electron transfer reactions in polar solvents using ultrafast spectroscopy

“…A major reason for this is probably the difficulty to observe excited radical ions in the condensed phase. Indeed, only a very small number of radical ions are known to fluoresce [80,[92][93][94][95][96][97]. Moreover, the very few investigations on their excited state dynamics have shown that the ground state recovery of radical ions is ultrafast and takes place in picosecond timescale [80,81,98,99].…”