Mechanisms for light emission enhancement from low lying doublet states in copper porphyrin <i>H</i> -aggregates

Open-shell molecules are emerging as promising candidates for quantum information science, yet a fundamental understanding of how aggregation influences their optical properties remains limited, largely due to the lack of well-defined model systems. Here, we investigate H-aggregate formation in two copper porphyrin monomers with different peripheral meso substituents and examine how aggregation modulates light emission processes through exciton delocalization. We identify spectral signatures consistent with delocalized B-state excitons arising from H-aggregate formation, which lead to enhanced light emission from monomeric Cu-porphyrins relative to dimeric counterparts. A model incorporating non-Condon vibronic coupling was developed to propose that Q-state emission becomes enhanced by a factor that depends quadratically on the number of molecules over which the B-state exciton delocalizes. In addition, enhanced emission from the 2T state is assigned to a thermally activated delayed fluorescence mechanism that depends on triplet exciton delocalization that is mediated by exchange interactions between the unpaired Cu2+ d electron and the eg porphyrin orbitals. We develop a kinetic model based on simple Hamiltonians that yields a good agreement between simulated and experimentally measured 2T emission decay dynamics. Together, these results demonstrate how controlled aggregation can be used to tune exciton delocalization and excited-state dynamics in open-shell metalloporphyrin systems that may play a role in their application to quantum information.