Ruthenium (II) complexes bearing N‐heterocyclic carbene‐based C^N donor sets in dye‐sensitized solar cells.

We present the syntheses of ruthenium (II) complexes bearing an N‐heterocyclic carbene (NHC)‐based C^N donor set and an NCS ligand and evaluate their use as photosensitizers in dye‐sensitized solar cells (DSSCs). These complexes deploy a monocarboxylic acid‐functionalized terpyridine (tpy)/phenyl‐tpy to anchor with the TiO2 of the photoanodes. Results show that the complexes devoid of the phenyl spacer between the acid anchor and the tpy harvest the visible light more effectively. Absorption of the visible light transfers the electron density from the ruthenium, NHC, and the NCS donors to the tpy acceptor (1MRuLdonorLacceptorCT). Stronger MRuLdonorσ‐bonds in the complexes with opposite NHC and tpy ligand configuration render facile ruthenium‐centered oxidation. In contrast, the metal‐centered oxidation of complexes with trans‐oriented NHC and NCS ligands is relatively difficult. However, these complexes display higher photon conversion efficiency (PCE) in DSSCs. One of them shows PCE of 3.44%, which is ~70% of the standard N3 dye, under similar conditions. A longer electron lifetime and the lowest charge transfer resistance at the TiO2/electrolyte interface derived from the electrochemical impedance spectra accounts for the enhanced PCE. Insights into the oxidized dye regeneration in a DSSC setup, obtained from the computed Hirshfeld charges and spin density, depict the essential role of iodide anion in dye regeneration. This report summarizes our investigation of photophysics, electronic structure calculations, and the electrochemical study of all newly prepared complexes and their use as photosensitizers in DSSCs. [ABSTRACT FROM AUTHOR]