Synthesis of Panchromatic Ru(II) Thienyl-Dipyrrin Complexes and Evaluation of Their Light-Harvesting Capacity

Ru­(II) complexes with 5-(3-thienyl)-4,6-dipyrrin (3-TDP),
containing
2,2′-bipyridine (bpy) or 4,4′-bis­(methoxycarbonyl)-2,2′-bipyridine
(dcmb) as coligands, have been prepared and extensively characterized.
Crystal structure determination of [Ru­(bpy)2(3-TDP)]­PF6 (1a) and [Ru­(bpy)­(3-TDP)2] (2) reveals that the 3-thienyl substituent is rotated with
respect to the plane of the dipyrrinato moiety. These complexes, as
well as [Ru­(dcmb)2(3-TDP)]­PF6 (1b), act as panchromatic light absorbers in the visible range, with
two strong absorption bands observable in each case. A comparison
to known Ru­(II) complexes and quantum-chemical calculations at the
density functional theory (DFT) level indicate that the lower-energy
band is due to metal-to-ligand charge transfer (MLCT) excitation,
although the frontier occupied metal-based molecular orbitals (MOs)
contain significant contributions from the 3-TDP moiety. The higher
energy band is assigned to the π–π* transition
of the 3-TDP ligand. Each complex exhibits an easily accessible one-electron
oxidation. According to DFT calculations and spectroelectrochemical
experiments, the first oxidation takes place at the RuII center in 1a, but is shifted to the 3-TDP ligand in 1b. An analysis of MO energy diagrams suggests that complex 1b has potential to be used for light harvesting in the dye-sensitized
(Grätzel) solar cell.