Theoretical determination of electronic spectra and intersystem spin-orbit coupling: The case of isoindole-bodipy dyes

Density functional theory and its time-dependent extension (DFT, TDDFT) has been herein employed to elucidate the structural and electronic properties for a series of isoindole-boron dipyrromethene (isoindole-BODIPY) derivatives. The role played by both the nature and the positions of the substituents on intersystem spin-crossing has been investigated computing the spin-orbit matrix elements between singlet and triplet excited state wave functions weighted by the TDDFT transition coefficients. Their potential therapeutic use as photosensitizers in photodynamic therapy (PDT) is proposed on the basis of their strong absorbance in the red part of the visible spectrum, vertical triplet energies resulting higher than 0.98 eV, and the spin-orbit matrix elements that result to be comparable with different drugs already used in PDT.