Ultrafast two-electron transfer drives 2MLCT to 2LMCT excited state conversion in a novel iron(III) dual charge transfer absorber complex

The development of sustainable photosensitiser complexes based on Earth-abundant metallic elements is important for sustainable solar energy conversion and green synthesis through photocatalysis. We report the synthesis and characterisation of a new bis(tripodal) iron(III) N-heterocyclic carbene complex [Fe(PhB{impy}3)2]+ (PhB{impy}3 = phenyltris(imidazo-[1,5-a]-pyridinylidene)borate) featuring -extended imidazo-[1,5-a¬]-pyridinylidene donors. This complex exhibits dual charge transfer absorption through well-resolved 2LMCT and 2MLCT absorption bands at 600-800 and 400-500 nm wavelengths, respectively. The dual charge transfer absorber motif combines ligands with a comparatively narrow -* gap, together with a central Fe(III) metal ion with a set of partially filled metal t2g levels capable of acting as both electron acceptors and donors. Ultrafast transient absorption spectroscopy reveals that ground state recovery is dominated by conversion from the lower 2LMCT state with a 13 ps time constant, independently of which of the two charge transfer bands is initially excited. The measurements further reveal that the initial dynamics following excitation into the higher 2MLCT state through 400 nm excitation results in efficient internal conversion into the lower 2LMCT state on a sub-ps timescale. This ultrafast conversion involves a two-electron internal conversion process that changes the orbital configuration from (L)2(t2g)4(L*)1 in the 2MLCT state to (L)1(t2g)6(L*)0 in the 2LMCT state. Excitation into the 2MLCT band thus drives a rare case of double reduction of the metal from Fe(IV) to Fe(II) that efficiently outcompetes alternative ultrafast decay mechanisms via metal centered (4MC) scavenger states. The presence of two charge transfer bands with opposite charge transfer directionality provides an opportunity to provide detailed insight into the excited state dynamics of a prototype dual charge transfer absorber, and the broader implications of the results for further development and potential utilization of dual charge transfer absorbers are finally discussed.