On the behavior of the carboxyphenylterpyridine(8-quinolinolate) thiocyanatoruthenium(II) complex as a new black dye in TiO2 solar cells modified with carboxymethyl-beta-cyclodextrin

A terpyridine ligand encompassing a terminal 4-carboxyphenyl group (cptpy), was employed in a new Ru(II) black dye, in the presence of 8-quinolinolate (Q) and SCN− as ancillary ligands. Such compound, here referred as [Ru(cptpy) (Q) (NCS)], was designed aiming its inclusion into carboxymethyl-beta-cyclodextrin, anchored on TiO2. This host–guest strategy was employed to prevent the formation of aggregates and protect the photoinjecting moiety against parallel deactivation events. Such expectation has indeed been fulfilled by the system. On the other hand, 8-quinolinolate as a strong electron donor ligand, effectively enhanced the light harvesting behavior of the dye, shifting and spreading the IPCE peaks over the entire visible region. Unfortunately, the red shift of visible charge-transfer bands was compensated by a decrease of the Ru(III)/(II) potentials, slowing down the electron transfer kinetics with the I3/I− redox mediator. Therefore, the observed counterbalance between charge transfer energies and redox potentials imposes a critical limit in the design of better mononuclear ruthenium-polypyridine dyes.