The main photophysical properties of a series of recently synthetized 1,2- and 1,3-squaraines, including absorption electronic spectra, singlet-triplet energy gaps, and spin-orbit matrix elements, have been investigated by means of density functional theory (DFT) and time-dependent DFT approaches. A benchmark of three exchange-correlation functionals has been performed in six different solvent environments. The investigated 1,2 squaraines have been found to possess two excited triplet states (T1 and T2) that lie below the energy of the excited singlet one (S1). The radiationless intersystem spin crossing efficiency is thus enhanced in both the studied systems and both the transitions could contribute to the excited singlet oxygen production. Moreover, they have a singlet-triplet energy gap higher than that required to generate the cytotoxic singlet oxygen species. According to our data, these compounds could be used in photodynamic therapy applications that do not require high tissue penetration.
The molecular structures and absorption electronic spectra of two novel phototoxic pentapyrrolic expanded porphyrins (a isopentaphyrin derivative and its lutetium complex) have been studied at the density functional level and its time-dependent extension (TDDFT). The geometries were optimized with three different exchange-correlation functionals (PBE0, B3LYP, and ωB97XD) and the SV(P) basis set plus the pseudopotential method for the complex. With respect to the porphyrin, the structure of [1.1.1.1.1]-pentaphyrin and its lutetium complex are predicted much distorted due to the lack of conjugation. The lowest excitation energy band (experimental at 814 nm) for the free-base isopentaphyrin is well predicted by the ωB97XD at 772 nm. The possible photodynamic reaction mechanisms (types I and II) were studied through the calculation of the electron affinity and ionization potentials in solvent, using the COSMO model.
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