Temperature dependence of the dual phosphorescence from xanthone in n-hexane matrices

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Abstract
The phosphoresence spectrum and lifetime of xanthone embedded in an n-hexane matrix has been investigated as a function of temperature (1·6-100 K). Vibrational analyses of the spectra reveal that emission occurs from three sites, two of which are dominant. Emission from one site (B) occurs from the 3 nπ* state of a planar xanthone molecule and is characterized by strong totally symmetric carbonyl stretching vibrations and a short lifetime (2·4 ms). Emission from the other site (C) is shown to arise from the 3ππ* state of an out-of-plane distorted xanthone molecule. It displays a vibrational structure rich in modes of a 1, b 1, and b 2 (C 2v notation) symmetry and a long lifetime (115 ms). Both direct spin orbit coupling via configurational mixing of the nπ* and ππ* states due to the non-planarity of the molecule in its 3ππ* state and spin-orbit vibronic interaction involving 3 A 1(ππ*)-1 A 2(nπ*) spin-orbit and 1 A 2(nπ*)-1 B 2(ππ*) vibronic interaction via out-of-plane b 1 vibrations are shown to be responsible for the C site emission intensity. Vibronic mixing between the 3ππ* and 3 nπ* states is not important. With increasing temperature, the phosphorescence intensity from the B site (3 nπ*) emitters increases at first, reaches a maximum at ∼20 K and then decreases. The C site (3ππ*) intensity simply decreases with rising temperature. At a given temperature, the phosphorescence lifetimes are identical and exponential for all emission bands regardless of site origin. These observations indicate an equilibrium between emitters in the two sites throughout the lattice. A phonon-assisted energy transfer mechanism is proposed to account for these observations.