Transient absorption anisotropy study of ultrafast energy transfer in porphyrin monomer, its direct meso-meso coupled dimer and trimer

Transient absorption anisotropies of the Zn(II)porphyrin monomer, its direc t meso-meso coupled dimer, and trimer are measured in the B (Soret) band re gion by sub-30 fs laser pulses. It is shown that detailed information on th e electronic structures and energy transfer dynamics can be obtained from t he anisotropy and the magic angle data. The anisotropies of all three molec ules exhibit remarkable behaviors in the first 200 fs region. Experimental observations can be accounted for adequately by treating the transient abso rption signal as an explicit sum of ground state bleach and excited state s timulated emission contributions. In the monomer, the anisotropy decay deno tes an oscillatory feature followed by a 100 fs time constant exponential d ecay. It is argued that the B-x and B-y transitions of the porphyrin monome r are nondegenerate with an energy splitting of similar to 170 cm(-1). Furt hermore, equilibration of the excitation energy within the B states is slig htly underdamped. Excitation of the monomerlike B band of the dimer (trimer ) leads to ultrafast similar to 30 fs (60 fs) anisotropy decay and a subseq uent rise with similar to 60 fs (70 fs) time constant. It is concluded that the anisotropy decay is due to the ultrafast energy transfer to the low-en ergy exciton split B band, while the rise is due to the redshift of the exc ited state stimulated emission spectrum thereby decreasing the contribution of the excited state stimulated emission component in the overall transien t absorption signal. (C) 2001 American Institute of Physics.