Towards panchromatic Fe(ii) NHC sensitizers via HOMO inversion

A combined experimental and theoretical study of iron(ii) complexes with pyridyl N-heterocyclic carbene ligands is presented, with a new focus on the effect of extending the ligands with thiophenes of variable number. In agreement with recent theoretical predictions by the Jakubikova group, these ligands induce a substantial mixture of the occupied t(2g) and pi HOMO orbitals, as manifested by a near 80 nm red-shift of the (MLCT)-M-1 transition and a 2-3 fold increase of the molar extinction coefficient. The thiophene moieties permit delocalisation of the MLCT state on the ligands, which results in excited state lifetimes in the 13-18 ps range, almost twice as much as for the reference compound lacking the thienyl substituents. Relaxation into (MC)-M-3 states remains the main (MLCT)-M-3 quenching mechanism, and the effect of the number of thiophenes in lengthening the (MLCT)-M-3 lifetime is qualitatively consistent with a reduction in the (MLCT)-M-3-(MC)-M-3 energy gap.

Automated summary

A study on iron(ii) complexes with pyridyl N-heterocyclic carbene ligands, using both experimental and theoretical approaches, has been conducted. The focus of this study is to investigate the influence of extending the ligands with different numbers of thiophenes. The results show that these thiophene moieties induce a mixture of the occupied t(2g) and pi HOMO orbitals, leading to a red-shift of the (MLCT)-M-1 transition and an increase in the molar extinction coefficient. Additionally, the introduction of thiophenes prolongs the excited state lifetimes, which can be qualitatively explained by a reduction in the (MLCT)-M-3-(MC)-M-3 energy gap.