A time-resolved EPR study of short-lived triplet states: Host and temperature dependences of triplet properties of phthalazine

The triplet properties of phthalazine are investigated by means of the time-resolved EPR technique with laser excitation in various hosts over a wide range of temperature. The host and temperature dependences are interpreted satisfactorily in terms of the vibronic mixing between the 3nπ* and 3ππ* states. From the host dependences of the EPR parameters the zfs of the pure 3nπ* state of phthalazine is deduced to be Xn =0.027 cm−1, Yn =−0.058 cm−1, and Zn =0.031 cm−1. The fraction of the 3nπ* character of the T1 state is estimated to be ∼0% in ethanol, benzoic acid (BAC), and 1, 4-dichlorobenzene (DCB), 14% in 1, 2, 4, 5-tetrachlorobenzene (TCB), 60% in biphenyl (BP), and 84% in durene. The temperature dependences of the zfs, linewidth, phosphorescence spectrum, and lifetime are investigated and the results are explained using a continuum model. From the simulation of the temperature dependence of the triplet lifetime in the DCB host the decay rate constant of the second excited triplet (3nπ*) state is estimated to be 2.5×104 s−1. A drastic increase of the decay rate at higher temperatures suggests the presence of a higher 3nπ* state with a decay rate constant of ∼106 s−1. Possible causes for the lack of phosphorescence and fast radiationless decay are also discussed.