Excited-State Absorption and Ultrafast Relaxation Dynamics of Porphyrin, Diprotonated Porphyrin, and Tetraoxaporphyrin Dication

The relaxation dynamics of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and tetraoxaporphyrindication (TOxP2+) has been investigated in the femtosecond-nanosecond time domain uponphotoexcitation in the Soret band with pulses of femtosecond duration. By probing with spectrally broadfemtosecond pulses, we have observed transient absorption spectra at delay times up to 1.5 ns. The kineticprofiles corresponding with the band maxima due to excited-state absorption have been determined for thethree species. Four components of the relaxation process are distinguished for H2P: the unresolvably shortB f Qy internal conversion is followed by the Qy f Qx process, vibrational relaxation, and thermalization inthe Qx state with time constant 150 fs, 1.8 ps, and 24.9 ps, respectively. Going from H2P to TOxP2+, twoprocesses are resolved, i.e., B f Q internal conversion and thermal equilibration in the Q state. The B f Qtime constant has been determined to be 25 ps. The large difference with respect to the B f Qy time constantof H2P has been related to the increased energy gap between the coupled states, 9370 cm-1 in TOxP2+ vs6100 cm-1 in H2P. The relaxation dynamics of H4P2+ has a first ultrafast component of 300 fs assigned asinternal conversion between the B (or Soret) state and charge-transfer (CT) states of the H4P2+ complex withtwo trifluoroacetate counterions. This process is followed by internal CT f Q conversion (time constant 9ps) and thermalization in the Q state (time constant 22 ps).