Light as a Switch: EPR Evidence for the Wavelength-Modulated Generation of Intrinsic and Defect-Associated Ti3+...

Titanium-based metal-organic frameworks (Ti-MOFs) are widely investigated for their potential in photocatalytic applications owing to their unique structural and electronic properties. However, the local identity of long-lived photoinduced Ti(III) species remains unresolved. In this study, the nature of photogenerated Ti(III) species in the stable Ti-MOF MUV-10(Ca) was examined by electron paramagnetic resonance (EPR) spectroscopy. The results of X- and Q-band (9/34 GHz) continuous wave (CW) EPR provided unequivocal evidence for the presence of two chemically distinct Ti(III) sites, with the selectivity of Ti(IV) photoreduction governed by the hydration state of the material and the wavelength of the incident light. Consolidation of CW EPR and 1H Davies electron-nuclear double resonance (ENDOR) data allowed us to clearly determine the structure of both centers: one associated with an ideal Ti2Ca2-cluster and another one assigned to the missing-linker defect of the MUV-10(Ca) structure. This first example of such hydration state and wavelength-controlled selectivity in photoactive Ti-MOFs contributes to a deeper understanding of the nature of photoactive centers in Ti-MOFs, highlighting the importance of defect engineering for advanced applications.