The photoexcited triplet state of free-base porphycene:: a time-resolved EPR and electron spin echo investigation

The photoexcited triplet state of free-base porphycene randomly oriented in a glassy toluene matrix has been investigated by time-resolved electron paramagnetic resonance and electron spin echo spectroscopy over the temperature range 4-100 K. Spectral broadening and anisotropic relaxation are both observed as the temperature is raised, demonstrating the complementary use of these techniques. It is concluded that two triplet states are present in the EPR spectra: only one at 4 K, and the other becoming populated at higher temperatures. This behaviour is explained in terms of intramolecular N-H tautomerism in the porphycene macrocycle. The N-H tautomerism is estimated to have a maximum rate of 7x10(8) s(-1) at 100 K.