A time-resolved electron paramagnetic resonance study of the photoexcited triplet state of free-base porphycene

The photoexcited triplet state of free-base porphycene randomly oriented in a glassy toluene matrix has been investigated by time-resolved EPR over the temperature range 4-200 K. The spectra show a remarkable temperature dependence. It is concluded that two triplet states are present in the EPR spectra: only one is present at 4 K, and the other is populated at higher temperatures. The energy difference between the two states is estimated to be 0.4 kJ mol-1. The behaviour is explained in terms of fast intramolecular NH tautomerism present in the porphycene macrocycle. As the temperature is raised the exchange between the two states increases and one effective state is observed. In addition, at even higher temperatures where the solvent is close to the liquid-glass transition, the triplet spectrum is affected by anisotropic relaxation caused by rotation of the porphycene molecule in the molecular plane.