Selective Q‑Band Excitation Modulates Exciton-Exciton Annihilation in Tetra(p‑Sulfonatophenyl)porphyrin J‑Aggregates.

The ultrafast photophysics of the Q-band manifold of tetra-(p-sulfonatophenyl)-porphyrin (H2TPPS) is investigated by two-dimensional electronic spectroscopy (2DES) in both its monomeric and J-aggregated forms. By selectively exciting different regions of the Q bands, we uncover strongly excitation-dependent relaxation dynamics in the aggregates. Photoexcitation centered on the lower-energy Q transition leads to pronounced interband coupling and clear signatures of exciton-exciton annihilation, whereas selective excitation of the higher-energy side of the Q-band manifold limits annihilation. These findings provide new insight into how the mixed Q-band structure governs excitation flow in porphyrin aggregates and open avenues for the rational design of materials with tailored annihilation properties.