Interstate couplings in the triplet manifold of 9-xanthone

Using a heat-pulse modulation technique, phosphorescence spectra of 9-xanthone have been studied in n-pentane at 77 and 2.1 K. The three legitimate phosphorescent levels with different lifetimes are detected at 25 751, 25 766, and 25 774 cm^-1 above the zero-vibrational level of the ground state. The origin of these multiple levels is explained in terms of spin-orbit interaction superimposed on environmental electrostatic coupling between T ₁ (ππ*) and T₂(nπ*). The two coupling constants obtained are: (for the spin-orbit coupling) |〈1ξ|H _so|2η〉| = 15.70 cm^-1 (ξ = x, η = y or vice versa); (for environmental direct coupling) 〈1ζ|H _env|2ζ〉 = 3.14 cm^-1 (ζ = x, y, or z). For comparison, the T←S phosphorescence excitation spectrum, measured in the same matrix, are in good agreement with the calculated ones.