Photoaccumulation of Long-lived Reactive Electrons in the Microporous Ti(IV) Oxocluster Based Metal-organic Framework for Light and Dark Photocatalysis

The microporous Ti12 oxocluster based metal-organic framework MIP-177(Ti)-LT exhibits excellent stability and photoactivity, making it highly promising for photocatalytic applications (LT stands for low temperature synthesis). This study employs transient and photoinduced absorption spectroscopies to analyse the behaviour of reactive electrons in MIP-177(Ti)-LT across femtosecond-to-second timescales. Over these timescales, MIP-177(Ti)-LT shows effective charge separation and relatively slow decay kinetics, with photogenerated charges persisting through to microsecond-second timescales and exhibiting significantly higher yields and slower decay kinetics than two benchmark metal(IV) oxoclusters based photoactive MOFs, MIL-125(Ti)-NH2 and UiO-66(Zr)-NH2. Photogenerated holes in MIP-177(Ti)-LT are found to be able to oxidise water to produce oxygen at a yield of 335 µmol g-1⋅ h-1 in the presence of electron scavengers. Meanwhile, long-lived photogenerated electrons accumulate under continuous irradiation, with this accumulation being enhanced by the presence of a hole scavenger, methanol. Notably, these photogenerated electrons can persist for over 48 hours post-photoexcitation under sealed conditions in argon, causing a reversible colour shift from white to black. These accumulated long-lived electrons are shown to be redox-active, efficiently reducing both added oxygen and methyl-viologen. The subsequent dark addition of a platinum co-catalyst to the photocharged MIP-177(Ti)-LT results in hydrogen evolution with a yield of circa 300 µmol/g (~58 C/g), corresponding to an accumulated electron density of one electron per 12 Ti atoms (i.e., one electron per titania oxocluster). These results highlight the photocharging properties of MIP-177(Ti)-LT and its potential for sustainable and efficient photocatalytic processes.