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 –CO 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.