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 T1(ππ*) and T2(nπ*). The two coupling constants obtained are: (for the spin–orbit coupling) ‖〈1ξ‖Hso‖2η〉‖ =15.70 cm−1 (ξ=x, η=y or vice versa); (for environmental direct coupling) 〈1ζ‖Henv‖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.