Time-Resolved Silica Luminescence Probes Photoinduced Electron Transfer from Heterobinuclear Units to 2,2′-Bipyridine and O₂

Abstract

Heterobinuclear units consisting of early and late transition metal ion pairs supported on a silica surface strongly absorb visible light, producing metal-to-metal charge transfer (MMCT) states with applications in renewable energy when units are coupled to electron donors or acceptors in integrated devices. Challenges to measuring these charge transfer pathways on subnanosecond time scales were surmounted using time-correlated single-photon counting (TCSPC) on self-supporting pressed pellets mounted in a 3D-printed aluminum gas cell. Following synthesis and characterization with spectroscopy and DFT, electron transfer from Ti IV OCo II and Zr IV OCo II units to molecular electron shuttles and subsequently to luminescent silica trap states was measured on the sub-100 ps time scale using TCSPC. Zr IV OCo II units produced silica luminescence following excitation to a Zr III OCo III MMCT state by transferring electrons via either 2,2′-bipyridine (bpy) or O 2 shuttles; by comparison, analogous Ti IV OCo II units excited to a Ti III OCo III MMCT state could produce luminescence with O 2 but not bpy, despite showing equivalent coordination. This is evidence for rectifying behavior, in which electron transfer proceeds only in a single direction and depends on the relative arrangement of energy levels, because Zr(III) centers are capable of reducing bpy, but Ti(III) centers are not. Transport through the bpy-ZrOCo electron transfer chain takes place in 110 ± 20 ps; analogous samples showed similar time scales. The resulting silica luminescence exhibited peaks at 450 nm (2.8 eV) and 560 nm (2.2 eV), which decayed, as fit with an Albery-type function, with lifetimes of around 200 ps and 2 ns and dispersions of 3 and 4, respectively. Quenching of emission from bpy-ZrOCo samples by O 2 supported its crossover from an S = 3/2 to an S = 1/2 state along the charge transfer pathway. These results demonstrate the application of rectifying principles for the development of devices for artificial photosynthesis as well as the use of silica photoluminescence as a low-noise probe of electron transfer processes in systems that do not otherwise luminesce.