Exploring the Te(II)/Te(IV) Redox Couple of a Tellurorosamine Chromophore: Photophysical, Photochemical, and Electrochemical Studies

A tellurorosamine dye [Te(II)] undergoes aerobic photooxidation. Although Te(IV) species have been used in a number of oxidations, key Te(IV)-oxo and Te(IV)-bis(hydroxy) intermediates are challenging to study. Under aerobic irradiation with visible light, Te(II) (λ_max = 600 nm) transforms into a Te(IV) species (λ_max = 669 nm). The resultant Te(IV) species is not stable in the dark or at −20 °C, decomposing back to Te(II) and other byproducts over many hours. To eliminate the structural ambiguity of the Te(IV) photoproduct, we used spectroelectrochemistry, wherein the bis(hydroxy) Te(IV)-(OH)₂ was electrochemically generated under anaerobic conditions. The absorption of Te(IV)-(OH)₂ matches that of the Te(IV) photoproduct. Because isosbestic points are maintained both photochemically and electrochemically, the oxo core formed photochemically must rapidly equilibrate with Te(IV)-(OH)₂. Calculations on the bis(hydroxy) versus oxo species further corroborate that the equilibration is rapid and the spectra of the two species are similar. To further explore Te(IV) cores, two novel compounds, Te(IV)-Cl₂ and Te(IV)-Br₂, were synthesized. Characterization of Te(IV)-X₂ was simplified because these cores have no analogue to the Te(IV)-(O)/Te(IV)-(OH)₂ equilibrium. This work provides insights into the photophysical and electrochemical behavior of Te analogues of chalcogenoxanthylium dyes, which are relevant for a broad range of photochemical applications.