Position-Dependent Extension of π-Conjugation in D-π-A Dye Sensitizers and the Impact on the Charge-Transfer Properties

A series of five organic donor-π-bridge-acceptor (D-π-A) sensitizers is investigated within the context of their photoinduced charge-transfer properties. Thereby, the focus is set on the impact of structural modifications of the molecular architecture on the π-systems of the dyes. In particular, two different modes of systematic extension of the sensitizers’ π-systems, namely, (i) within the electron donating site and (ii) within the π-bridge, are investigated by means of steady-state and time-resolved spectroscopic methods. The photophysical studies of the molecules in solution and as deposited on Al 2 O 3 or TiO 2 films reveal that different effects on the charge-transfer characteristics evolve dependent where – within the molecular structure – the modification of the π-system is performed. Hence, π-extension of the donor sites, for instance, leads to a strong red shift of the absorption features and a variation of light-harvesting properties. Modifying the π-bridges results in a spatial decoupling of the HOMO and LUMO orbitals, which goes along with changes of the electronic coupling to TiO 2 . Furthermore, solution studies show that the electronic structure of the dyes governs their singlet excited-state features. As shown, the results obtained from these studies then allow important predictions about the deactivation of the excited states of these molecules adsorbed on TiO 2 . Finally, quantum chemical methods – among others, time-dependent density functional theory calculations – provide conclusive insight into the relationship between the electronic structure of the dyes and its impact on the photoinduced charge-transfer characteristics.