Molecular and Interfacial Calculations of Iron(II) Light Harvesters.

Iron-carbene complexes show considerable promise as earth-abundant light-harvesters, and adsorption onto nanostructured TiO2 is a crucial step for developing solar energy applications. Intrinsic electron injection capabilities of such promising Fe(II) N-heterocyclic complexes (Fe-NHC) to TiO2 are calculated here, and found to correlate well with recent experimental findings of highly efficient interfacial injection. First, we examine the special bonding characteristics of Fe-NHC light harvesters. The excited-state surfaces are examined using density functional theory (DFT) and time-dependent DFT (TD-DFT) to explore relaxed excited-state properties. Finally, by relaxing an Fe-NHC adsorbed on a TiO2 nanocluster, we show favorable injection properties in terms of interfacial energy level alignment and electronic coupling suitable for efficient electron injection of excited electrons from the Fe complex into the TiO2 conduction band on ∼100 fs time scales.