Synthesis of new benzimidazole based ruthenium (II) dyes for application in dye-sensitized solar cells with detailed spectroscopic and theoretical evaluation

In this work, four ancillary ligands named SL-1 to SL-4, having core nucleus of 2-(2-pyridyl) benzimidazole, augmented with electron donating and electron withdrawing groups were successfully synthesized in good yields. The characterization of ligands was assured by FTIR, 1HNMR, 13CNMR, and X-ray crystallographic analysis. The as-synthesized ligands were subjected to complexation with 2,2 & PRIME;-bipyridyl-4,4 & PRIME;-dicarboxylic acid and potassium thiocyanate to prepare four new ruthenium (II) dyes. All the synthesized ruthenium (II) dyes were characterized by infrared spectroscopy and their photophysical and electrochemical studies were compared to the benchmarked dye (N719). Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were performed to gain deep insights and better understanding of the quantitative structure-property relationships of the named ruthenium dyes (SL1-4). Results showed excellent correlation between the experi-mental and theoretical results; all the four new dyes exhibited narrow band gap and low singlet excitation en-ergies (1.33 - 1.72 eV) indicating good light harvesting ability in DSSCs. Mulliken charge analysis reveals the intramolecular charge transfer between the highly electronegative nitrogen and oxygen donor centers and the strongly electropositive ruthenium acceptor in the synthesized dyes.