Thiophosgene monomer–dimer equilibria in perfluoroalkane solutions: spectroscopic and photophysical implications of self-association in the ground state

Abstract

The S2–S0 absorption, emission, and emission excitation spectra of thiophosgene in a perfluoroalkane solvent have been measured as a function of the analytical concentration, C, of the solute. Strong evidence of the formation of van der Waals dimers at 10−4 M < C < 10−1 M and of higher oligomers at C > 10−1 M has been found. Variations in the absorption spectra at C < 10−1 M have been analyzed successfully in terms of a single [Formula: see text] equilibrium, for which the value of K is (5.0 ± 0.4) × 102 M−1 at 295 K. The mole fractions of monomer and dimer have been calculated at each C, and have been used to construct the absorption spectrum of the dimer. Both the monomer and dimer have similar S0 → S1, and S0 → S2 absorption spectra, but the dimer has an oscillator strength that is about twice that of the monomer. The effects of self-aggregation on the S2–S0 emission and excitation spectra have been analyzed. Absorption by the dimer in the ultraviolet also leads to emission, but with a different quantum efficency compared with direct excitation of the monomer. The implications of these observations on the interpretation of the spectroscopy, photophysics, and photochemistry of thiophosgene in solution are assessed.