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 direct meso–meso coupled dimer, and trimer are measured in the $B$ (Soret) band region by sub-30 fs laser pulses. It is shown that detailed information on the electronic structures and energy transfer dynamics can be obtained from the anisotropy and the magic angle data. The anisotropies of all three molecules exhibit remarkable behaviors in the first 200 fs region. Experimental observations can be accounted for adequately by treating the transient absorption signal as an explicit sum of ground state bleach and excited state stimulated emission contributions. In the monomer, the anisotropy decay denotes an oscillatory feature followed by a 100 fs time constant exponential decay. It is argued that the $B_x$ and $B_y$ transitions of the porphyrin monomer are nondegenerate with an energy splitting of $\text{∼170\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}}.$ Furthermore, equilibration of the excitation energy within the $B$ states is slightly underdamped. Excitation of the monomerlike $B$ band of the dimer (trimer) leads to ultrafast $\text{∼30\hspace{0.05em}}\mathrm{fs}$ (60 fs) anisotropy decay and a subsequent rise with $\text{∼60\hspace{0.05em}}\mathrm{fs}$ (70 fs) time constant. It is concluded that the anisotropy decay is due to the ultrafast energy transfer to the low-energy exciton split $B$ band, while the rise is due to the redshift of the excited state stimulated emission spectrum thereby decreasing the contribution of the excited state stimulated emission component in the overall transient absorption signal.