Porphyrin Aggregation Revisited: From the Four-Orbital Gouterman Model to an Eight-Orbital Framework in Porphin H-Dimers.

Porphyrin aggregation fundamentally alters photophysical properties, yet the underlying electronic structure reorganization remains poorly understood. Porphin H-type dimerization induces a characteristic blueshift of the B-band, explained within Kasha's exciton model through dipole-dipole interactions, but this phenomenological picture does not resolve the frontier-orbital reorganization driving these changes. UV-vis spectroscopy and time-dependent density functional theory show spectral changes originate from systematic frontier-orbital splitting. The Gouterman four-orbital model extends to an eight-orbital framework in dimers through symmetric and antisymmetric combinations of the monomer HOMO-1, HOMO, LUMO, and LUMO+1. Configuration interaction within this expanded manifold governs B-band splitting, while natural transition orbital analysis shows Q-band excitations remain largely localized to single macrocycles. Nitrogen K-edge TDDFT reveals nearly unchanged core-level NEXAFS, confirming preserved local electronic structure despite valence reorganization. Aggregation thus extends Kasha's exciton picture and the Gouterman model, providing an orbital-resolved foundation for excitonic effects in porphyrin assemblies.