Unusual Low-Energy Near-Infrared Bands for Ferrocenyl–Naphthalimide Donor–Acceptor Dyads with Aromatic Spacer Groups: Prediction by Time-Dependent DFT and Observation by OTTLE Spectroscopy

Abstract

Time-dependent density functional theory (TDDFT) calculations for a series of donor–spacer–acceptor (D–S–A) molecules with phenyl (1), biphenyl (2), and anthryl (3) spacers interpolated between the ferrocenylalkene donor and −C≡C-4-naphthalimido acceptor components predicted the presence of weak, very low energy near-IR (NIR) transitions in the electronic spectra of the oxidized species Fc+–S–A. Subsequent optically transparent thin layer electrolysis (OTTLE) spectroscopy experiments in the spectral range 1500–3200 nm reveal that, in each case, very low energy NIR bands are indeed observed at approximately 2500 nm. The TDDFT calculations not only allow the prediction of these low energy bands but also, in concert with recently developed natural transition orbital (NTO) analysis, give detailed insights into the nature of the transitions involved. It is found that these bands around 2500 nm correspond to forbidden e1″ → e1″ transitions localized on the ferrocenium subunit with very low oscillator strengths.