Redox and photoinduced electron-transfer properties in short distance organoboryl ferrocene-subphthalocyanine dyads

Reaction between ferrocene lithium or ethynylferrocene magnesium bromide and (chloro)boronsubphthalocyanine leads to formation of ferrocene- (2) and ethynylferrocene- (3) containing subphthalocyanine dyads with a direct organometallic B-C bond. New donor-acceptor dyads were characterized using UV-vis and magnetic circular dichroism (MCD) spectroscopies, NMR method, and X-ray crystallography. Redox potentials of the rigid donor-acceptor dyads 2 and 3 were studied using the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) approaches and compared to the parent subphthalocyanine 1 and conformationally flexible subphthalocyanine ferrocenenylmethoxide (4) and ferrocenyl carboxylate (5) dyads reported earlier. It was found that the first oxidation process in dyads 2 and 3 is ferrocene-centered, while the first reduction as well as the second oxidation are centered at the subphthalocyanine ligand. Density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to probe the electronic structures and explain the UV-vis and MCD spectra of complexes 1-5. DFT-PCM calculations suggest that the LUMO, LUMO+1, and HOMO-3 in new dyads 2 and 3 are centered at the subphthalocyanine ligand, while the HOMO to HOMO-2 in both dyads are predominantly ferrocene-centered. TDDFT-PCM calculations on compounds 1-5 are indicative of the π → π* transitions dominance in their UV-vis spectra, which is consistent with the experimental data. The excited state dynamics of the parent subphthalocyanine 1 and dyads 2-5 were investigated using time-resolved transient spectroscopy. In the dyads 2-5, the initially excited state is rapidly (<2 ps) quenched by electron transfer from the ferrocene ligand. The lifetime of the charge transfer state demonstrates a systematic dependence on the structure of the bridge between the subphthalocyanine and ferrocene.