syn-Diarylphthalimidoporphyrins: Effects of Symmetry Breaking on Two-Photon Absorption and Linear Photophysical Properties

Aromatically π-extended porphyrins possess exceptionally intense one-photon (1P) and sometimes two-photon (2P) absorption bands, presenting interest for construction of optical imaging probes and photodynamic agents. Here we investigated how breaking the molecular symmetry affects linear and 2PA properties of π-extended porphyrins. First, we developed the synthesis of porphyrins fused with two phthalimide fragments, termed syn-diarylphthalimidoporphyrins (DAPIP). Second, the photophysical properties of H₂, Zn, Pd, and Pt DAPIP were measured and compared to those of fully symmetric tetraarylphthalimidoporphyrins (TAPIP). The data were interpreted using DFT/TDDFT calculations and sum-over-states (SOS) formalism. Overall, the picture of 2PA in DAPIP was found to resemble that in centrosymmetric porphyrins, indicating that symmetry breaking, even as significant as by syn-phthalimido-fusion, induces a relatively small perturbation to the porphyrin electronic structure. Collectively, the compact size, versatile synthesis, high 1PA and 2PA cross sections, and bright luminescence make DAPIP valuable chromophores for construction of imaging probes and other bioapplications.