Time-resolved resonance Raman spectroscopy of excited-state porphyrins

Time-resolved resonance Raman (TR3) and absorbance difference studies of the excited states of Cu(TPP) (TPP = 5, 10, 15, 20-tetraphenylporphyrin) have been carried out with < 10 ps times resolution in THF and pyridine solvents. In THF the distinctive transient Raman bands in the nu(2) and nu(4) regions, previously observed with ns laser pulses, grow in the first 55 ps before decaying in 100's of ps. The Delta A spectra also show biphasic decay. This behaviour is associated with attack by solvent on the 4-coordinate excited state to form the longer lived species observed in TR3 experiments. In pyridine two component decay is also observed but it is the shorter-lived species which gives the transient Raman bands seen previously with ns laser excitation. This state is different from that seen in THF. At 5 ps delay nu(4) is broader than in the ground state and, more importantly, there is a significant shia in the two pyridine bands at ca. 1000 cm(-1). This implies a significant involvement of the pyridine-based orbitals in the excited state.