Novel two-photon absorbing dyes: Their possible use in photoactive applications

In this study, the linear and non-linear optical properties of two recently synthe- sized polythiophene based dyes, DTP1 and DTP2, were investigated. In the first part, a thorough conformational search was performed, the absorption spectra have been obtained at time-dependent density functional theory (TD-DFT) level considering vibrational and dynamical effects via a Wigner exploration of the potential energy surface. Furthermore, the excited state topology and electronic density reorganization have been characterized using natural transition orbitals and the charge transfer character quantified through recently developed descriptors, also allowing for the rationalization of the poor interfacial electron injection properties exhibited by the dyes when grafted on TiO2 surfaces. Finally, two-photon absorption (TPA) spectra have been calculated, extremely high cross sections have been obtained in the infrared region paving the way to the possible exploitation of the dyes for the development of photoactive smart materials or photodynamic therapy. In the second part, the possible pathways leading to the intersystem crossing and triplet manifold population were examined. In particular, using high level state- of-the-art molecular modeling methodologies, the remarkable two-photon absorption (TPA) cross-section was underlined. Intersystem crossing pathways were elucidated by considering the energy difference between the relevant triplet and singlet states on the potential energy surfaces as the key critical points. The spin-orbit coupling is also assessed, and the results globally point to a possible, albeit probably slow, intersystem crossing that could allow the use of the two dyes as singlet oxygen photosensitizers, for instance in photodynamic therapy, owing to their high TPA cross-sections.