Triplet Harvesting Using Two-Photon Absorption in Substituted Naphthalimides for Their Application as Heavy-Atom-Free Photosensitizers

The role of photodynamic therapy (PDT) in cancer treatment arises due to its specificity and sensitivity for tumor cells. For a series of thiocarbonyl-naphthalimide derivatives of potential PDT interest as heavy-atom free photosensitizers, optical (one- and two-photon) absorption, and fluorescence properties, along with intersystem-crossing rates, have been estimated by density-functional theory (DFT) and time-dependent DFT (TD-DFT). Absorption and fluorescence energies and intersystem crossing rates agreed well with the available experimental data. Our results predict that the effective intersystem crossing for conventional naphthalimides occurs as (1)pi-pi* -> (3)n-pi*; the crossing channel for thiocarbonyl naphthalimides is (1)n-pi* -> (3)pi-pi*. The latter, with nonradiative S-1 state, transforms to the triplet state via an intersystem crossing. Encouragingly for photosensitizer prospecting, both conventional and thiocarbonyl naphthalimides exhibit strong two-photon absorption (TPA) in the near-infrared region at similar to 696-1092 nm; we predict that combining both TPA and superior intersystem crossing renders naphthalimides as ideal PDT photosensitizers.