Relaxation in excited states of porphycene in low-temperature argon and nitrogen matrices

Picosecond transient spectroscopy was used to study excited state deactivation processes of porphycene embedded in solid argon and nitrogen matrices at low temperatures. Comparison of time profiles of the signals due to ground state bleaching and stimulated fluorescence reveals that the latter is delayed by about 100 ps with respect to the former when the molecule is excited into the S3 or S4 electronic states, corresponding to Soret bands. No delay between the two signals is observed for excitations into Q bands, corresponding to S1 and S2 transitions. This is evidence of slow vibrational relaxation from S3 and S4 into S1 and S2, due to the considerable S1–S3 energy gap and similarity of the potential energy profiles in different electronic states. On the contrary, for nonrigid dibenzo-derivatives of porphycene which have an `intruderʹ state located between the Q and Soret bands, the relaxation from excited electronic states is faster than the experimental time resolution of about 30 ps.