Dichroism of the triplet–triplet transient absorption of copper(II) porphyrins in liquid solution. New interpretation of the subnanosecond relaxation component

For copper(II) porphyrins CuOEP and CuTPP in liquid solution the subnanosecond decay component was observed in picosecond transient absorption experiments at parallel polarization setting of the pump and probe beams. The saturation effect, i.e. reduction of the subnanosecond component amplitude with increase of the pump pulse intensity was demonstrated. This component was ascribed to the decay of the triplet–triplet transient absorption anisotropy of the copper(II) porphyrin solutions due to rotational relaxation, but not to 2T1 → 4T1 equilibration kinetics as was considered before [J. Chem. Phys. 70 (1979) 1720]. Time constants of the rotational relaxation were estimated as 96 ± 30 ps for CuOEP and 140 ± 30 ps for CuTPP, both in benzene, with initial anisotropy ≈0.1.