Attenuated antiaromaticity and redirected excited-state dynamics upon Cu-metallation of a benzene-annulated phenanthriporphyrin

Carbaporphyrinoids provide unique opportunities to modulate macrocyclic conjugation, molecular aromaticity, and metal-ligand interactions through core modification and [Formula: see text]-extension. Here, we investigated the electronic structure and excited-state dynamics of a benzene-annulated phenanthriporphyrin and its Cu-metallated analogue. The free-base framework retains phenanthriporphyrin-centered antiaromatic character, as evidenced by characteristic 1 H NMR results and weak NIR absorption features, as supported by DFT calculations. However, benzene annulation attenuates the antiaromatic character by perturbing the macrocyclic [Formula: see text]-electronic structure. Upon Cu-metallation, substantial metal-ligand interactions further attenuate the paratropic ring-current effect and markedly alter the excited-state dynamics. Femtosecond transient absorption measurements show that the free-base compound undergoes ultrafast excited-state decay dominated by its antiaromatic nature, whereas the Cu-metallated compound efficiently evolves into a ligand-to-metal charge-transfer state. TDDFT calculations further support the involvement of Cu-centered orbitals in FMO levels. These results demonstrate that [Formula: see text]-extension and metallation provide potential molecular handles for controlling antiaromaticity, metal-ligand interactions, and excited-state behavior in phenanthriporphyrin-based organometallic [Formula: see text]-systems. Notably, although the paratropic ring current is only partially attenuated upon Cu-metallation, the resulting metal-ligand orbital mixing opens a new, energetically competitive LMCT channel that overrides the ultrafast decay pathway intrinsic to the free-base framework’s antiaromatic electronic structure.