A TDDFT investigation of bay substituted perylenediimides: Absorption and intersystem crossing

The conformational structure and electronic spectra properties of a series of bay substituted perylenediimides (PDI) derivatives have been investigated by means of density functional theory (DFT) and time-dependent DFT. The B3LYP and PBE0 hybrid exchange-correlation functionals were applied in conjunction with the double-ζ quality SVP basis set. These compounds are interesting for organic materials science and as photosensitizers in cancer phototherapy (PDT), because of their intense absorption in the visible region. Results show that the substitution at the bay position of the PDI parent molecule with N-alkyl groups shifts the absorption maxima towards the red part of the visible spectrum (around 650-700 nm) as required for the applications in PDT. The main PDT action mechanisms have been investigated by computing of electron affinities, ionization potentials, triplet energies and spin-orbit matrix elements between singlet and triplet excited states.