Dramatic solvent effects on thioxanthone fluorescence lifetime

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

“…In any case, most of the population that continues in the singlet manifold remains at the S pp* state, which, independently of the solvent, can also be considered as the source of the fluorescence. In fact, using the data for ACN (f F = 0.0041 and t F = 0.07 ns), [4,5] a radiative lifetime (t rad = t F /f F ) of 17 ns is obtained for the emitting state, in very good correspondence with our computed value of 27 ns for the S pp* state. Its dipole moment, which is near 1-2 D larger than that of the ground state, explains the observed red shift in the emission spectrum.…”

“…[5] As clearly illustrated in Figure 6 b, the stabilization of the S pp* state in the more polar solvents can be expected to favor the fluorescence emission, the quantum yield of which increases by three orders of magnitude on going from THF to ethylene glycol. [4,5] The decay toward the ground state is clearly less favorable in polar solvents because the deactivation path through the final (gs/np*) CI becomes less accessible due to the increase of the energy barriers.…”

“…[1][2][3][4][5][6][7] Large increases in the fluorescence quantum yield with the polarity have been reported previously, related to the concomitant decrease in the internal conversion (IC) rate from the excited spectroscopic state to the ground state. Although initially such a behavior was attributed to the "proximity effect" [8] and subsequent vibronic coupling caused by the presence of close-lying S pp* and S np* singlet states in polar environments, it is now well established that such a model, designed just considering the electronic states and their related energy gaps at the Franck-Condon (FC) region (ground-state geometry), is not generally applicable to explain ultrafast IC processes and fluorescence yield diminution.…”

“…f F (calc) is calculated from the equation in ref. [4] that relates it to the solvent properties. All rate constants are given in ns À1 .…”

Singlet Exciplexes between a Thioxanthone Derivative and Substituted Aromatic Quenchers: Role of the Resonance Integral

“…Addition of HMB results in a new red-shifted emission with respect to that of ETX, and simultaneously, the ETX fluorescence decreased. Since the solvatochromism of ETX fluorescence indicates that light emission would have shifted towards blue wavelengths by a reduction of polarity, [39] the red-shifted emission cannot be attributed to a local decrease of medium polarity under addition of HMB. Moreover, the dispersion effect due to a change in the refractive index was ruled out since adding cyclohexane (up to 1 m) to a solution of ETX in acetonitrile did not induce any significant change in the fluorescence spectrum.…”