Tuning the electronic properties of Fe(II)-NHC sensitizers with thienyl π-extended ligands

Heteroleptic Fe(II)-NHC sensitizers NS158, NS271 and NS252 bearing fused thiophenes and carboxylic or thienylcyanoacrylic anchoring moieties were designed to enhance light harvesting via HOMO inversion in DSSCs. The complexes were comprehensively characterized in solution and when immobilized on TiO₂ photoanodes using electrochemical measurements, transient absorption spectroscopy, and ab initio calculations. All tested DSSC samples from these sensitizers exhibited a power conversion efficiency of at least 1% on 18 μm photoanodes. The NS252 sensitizer combining a thienylcyanoacrylic anchor and a fused dithienyl moiety, showed the broadest absorption up to 750 nm but the lowest IPCE (25%) with 1.07% PCE and a photocurrent of 4.8 mA cm^-2. NS158 and NS271 bearing respectively a dithienyl and terthienyl fused moiety associated to a carboxylic anchor delivered respectively 1.32% and 0.93% PCE and 5.75 and 4.67 mA cm^-2 photocurrents. The best efficiency (1.5%) and photocurrent (6.46 mA cm^-2) were obtained with ARM15, fitted with a single thiophene. The increased conjugation, while enhancing the panchromaticity, reduced dramatically the excited-state directionality, localizing charge away from the anchoring units leading to a lowered coupling for injection into TiO₂. This work highlights the delicate balance to be found between light-harvesting ability, excited-state lifetime, and interfacial charge-transfer efficiency in the design of Fe(II)-based DSSC sensitizers.