Solvent and temperature effects on the phosphorescence of 9,10-phenanthrenequinone in fluid solution

The phosphorescence of 9,10-phenanthrenequinone (PQ) was observed and studied in carbon tetrachloride (CCl4) and acetonitrile (MeCN) using steady-state and time-resolved emission spectroscopy. From the estimated phosphorescence quantum yields and the measured triplet lifetimes at room temperature the natural radiative lifetime of the PQ triplet was calculated to be 10±4 ms in CCl4 and 39±8 ms in MeCN. The emission showed vibrational fine structure in both solvents and a hypsochromic shift when going to the more polar solvent. Both are typical of the n,π∗ state. Examination of the temperature effect on the intensity of the initial emission in time-resolved experiments indicated an activation energy of 2.38±0.39 kcal/mol in MeCN, and a negative activation energy of −0.25±0.04 kcal/mol in CCl4. It was concluded that the emissive state is n,π∗ in nature and is in thermal equilibrium with a nearby non-emissive π,π∗ state in these solvents. In CCl4 the lowest triplet state is n,π∗ with an energy of 49.1 kcal/mol, and the second excited triplet is π,π∗ and is almost isoenergetic with an energy that is only 0.25 kcal/mol higher. In MeCN there is an inversion of these states, the lowest state is π,π∗ with an energy of 47.6 kcal/mol, and is in equilibrium with a higher n,π∗ state of 50.0 kcal/mol. This is the first time that it has been possible to show an inversion of states due to changes in the solvent at room temperature and measure the energies of both the lowest n,π∗ and π,π∗ triplet states.