Environmental effects on the photophysics of thienyl ketones investigated by transient absorption and phosphorescence emission in polarized light

Abstract

Environmental effects on the triplet states of some thienyl ketones have been investigated by nanosecond laser flash photolysis, as well as stationary and time-resolved phosphorescence techniques. The compounds studied were the 2,3 ′-di-thienyl, 2-thienyl-4 ′-pyridyl, 3,3 ′-di-thienyl and 3-thienyl-4 ′-pyridyl ketones. Triplet properties (absorption spectra, lifetimes, quenching by oxygen and hydrogen abstraction), determined in ethanol and 3-methylpentane, indicated that the n, π ∗/ π, π ∗ character of the lowest triplet is modulated by the solvent and by structural characteristics. The spectral distribution and lifetime of phosphorescence emission in a rigid matrix at 80 K were affected by the nature of the matrix, rate of freezing, excitation and emission wavelengths and sample concentration. While the matrix influences the excited state energy and therefore the nature of the emitting state, concentration changes are responsible for aggregation. The latter effect is explained in terms of the formation of a triplet excimer (for the 3-thienyl-4 ′-pyridyl ketone) and a ground state dimer/aggregate (for the 2,3 ′-di-thienyl ketone) formation. Measurements of phosphorescence and phosphorescence excitation in polarized light, carried out by the photoselection method at 80 K in a matrix of suitable viscosity (1/4 iso-pentane-3-methylpentane, η=6.2×10 10 P), provided valuable information about the polarization of the electronic transitions involved.