Triplet Excitions in Crystalline Trans-Stilbene I. Spectroscopy of the S_o → T₁ Transition

Abstract

The first singlet → triplet absorption of trans-stilbene has been studied between 10 and 300 K. The triplet exciton energy (energy of the lowest 0–0 line) is 17380 cm⁻¹ at 10 K, and does not change significantly with temperature. Vibrational A_g modes of 206. 1250 and 1570 cm⁻¹ are active. The Franck-Condon factor of the origin region is small. The low temperature spectra of the 0–0 and 206 cm⁻¹ regions show a doublet structure with a splitting of 82 ± 1 cm⁻¹ which is attributed to the site splitting. Further structure shown by the lines is discussed. The lines are approximately lorentzian. From curve fitting, line-widths and exciton-phonon coupling constants, increasing linearly with T, are deduced. Exciton-phonon coupling appears to be different on the two sites. The product of absorption coefficient at 4880 Å, α, by the total triplettriplet interaction rate constant has been measured at 295 K: αγ=2.3 ± 0.3. 10⁻¹⁵ cm² sec⁻¹, corresponding for γ_total to a value of a few 10⁻¹² cm³ sec⁻¹.