Anchor group effects in ruthenium(II) photosensitizers bearing N-heterocyclic carbene and polypyridyl ligands for DSSCs: a computational evaluation

In this work, we evaluate the bonding characteristics and interfacial electron transfer dynamics of three heteroleptic ruthenium photosensitizers featuring different terpyridine (tpy) acceptor end configurations through quantum-chemical calculations. These photosensitizers feature tpy ligands with a mono/tricarboxylic acid or a phenyl-carboxylic acid anchor and a trans-disposed -NCS and N-heterocyclic carbene ligand in a C<^>N donor set. Adiabatic transitions reveal significant overlap between higher-excited and ground-state orbitals and correlate with the experimentally observed anti-Kasha fluorescence. In all complexes, two carboxylate-O atoms of the central anchor effectively bind with TiO2 by bridging two titanium sites. However, appending an additional anchor on each peripheral pyridine of tpy renders binding through only one anchor's -C = O atom to TiO2. Contrary to the complex bearing three anchors, those featuring monocarboxylate-functionalized tpy exhibit faster electron injection within 100 fs. In all, this study provides insights into the photosensitization attributes of the ruthenium complexes for dye-sensitized solar cells.