Triplet Harvesting Using Two-Photon Absorption in Substituted Naphthalimides for Their Application as Heavy-Atom-Free Photosensitizers
Pralok K. Samanta, Niall J. English
Abstract
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 1π–π* → 3n–π*; the crossing channel for thiocarbonyl naphthalimides is 1n–π* → 3π–π*. The latter, with nonradiative S1 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 ∼696–1092 nm; we predict that combining both TPA and superior intersystem crossing renders naphthalimides as ideal PDT photosensitizers.