On the viability of ruthenium (II) N-heterocyclic carbene complexes as dye-sensitized solar cell (DSSCs): A theoretical study

Changes in the ground state and excited state properties of the [Ru(bpy)3]2+ complex induced by functionalization of bpy ligands with N-heterocyclic carbene ligand (imidazolylidene, benzimidazolylidene and 1,2,3-triazolylidene) and substituted methyl wingtip group in protonated and deprotonated forms were studied theoretically using DFT and TDDFT methods. The absorption band is broader and features approximately a 26 nm more bathochromic tail than the [Ru(bpy)3]2+. Our calculations show that red shift for 1,2,3-triazolylidene ligand in the protonated series is only due to the affect of the energies of LUMOs which is localized on the substituted ligands. Consistent with previously observed trends in absorption spectra of Ru (II) complexes, a blue shift is observed for the deprotonated series. The effect originates from interplay of positive and negative solvatochromism in 1–3 and 1a–3a complexes respectively. This results in more delocalized character of the electron transition orbitals in the deprotonated complexes. We performed an NTO analysis based on the calculated transition density matrices to analyze the calculated singlet states. The calculated triplet excited state can either be weakly optically allowed 3MLCT or optically forbidden Ru 3d–d transitions. The ability to reduce the HOMO–LUMO gap by the carbene ligand is expected to be beneficial for photovoltaic applications.