Time-resolved fluorescence spectroscopy of high-lying electronic states of Zn-tetraphenylporphyrin

Depopulation of the S-2 excited electronic state of the Zn-tetraphenylporphyrin (ZnTPP) was monitored by measuring the decay of S-2-->S-0 and the rise of S-1-->S-0 fluorescence using the up-conversion fluorescence technique with a time-resolution of 120 fs. The lifetime of the S-2 electronic state, measured for ZnTPP in ethanol (tau(S2)=2.35 ps) correlates with the risetime of S-1-->S-0 fluorescence. This result demonstrates the depopulation of S-2 to S-1 via (vibrational) states with lifetimes much shorter than that of S-2. The rise time of S-2 fluorescence (tau(v) = 60-90 fs) was attributed to vibrational relaxation (in S-2) Fluorescence anisotropy decay of the S-2 state was also studied by measuring the parallel and perpendicular fluorescence components. The high initial anisotropy of r greater than or equal to 0.7 is interpreted as due to the existence of a degenerate excited electronic state S-2 and the corresponding fast decay time tau(1) = 0.2 ps to the electronic dephasing of the degenerate level pair. The long component of the anisotropy decay (tau(2) much greater than 10 ps) is due to rotational diffusion. (C) 1998 American Institute of Physics.