Photophysics, photochemistry and thermally-induced redox reactions of a (Pterin)rhenium(I) complex

Abstract

In this work, we present a whole and deep study on the thermal redox and the photophysical and photochemical reactions of a tricarbonyl Re(I) complex coordinating Pterin, fac-ReI(CO)3(pterin)(H2O) (pterin = 2-amino-4-oxo-3H-pteridine). In aqueous solutions, the fluorescence of the complex is attributed to the emitting 1IL state (τemi ∼ 7.6 ns). In MeCN, however, the luminescence was ascribed to an overlapping dual emission from 1IL and 3MLCT states (τemi1 = 8.0 ns and τemi2 = 1.0 μs). Oxygen quenching of the 3MLCT based luminescence occurred with kq = 1.6 × 109 M−1 s−1. In glasses at 77 K, nevertheless, the 3MLCT prevailed over 1IL states. Flash photolysis experiments in aqueous solutions showed the spectrum of 3IL while in MeCN the presence of 3MLCT was evident. Pulse radiolysis experiments under oxidizing and reducing environments were performed in aqueous solutions of the Re-Pterin complex. By performing multivariate curve resolution − alternating least-squares (MCR-ALS), two species were identified under reducing conditions: an intermediate related to the semireduced radical of pterin ligand and the dihydrogenated [Re(CO)3(7,8-PtrH2)(H2O)] product. TD-DFT calculations helped to ascertain the nature of these species. Flash photolysis experiments where the excited states were reduced with triethylamine were in good agreement with pulse radiolysis experiments under reductive conditions. The oxidized transient spectrum was also obtained by pulse radiolysis, which compares very well with those published for Re(II) species, leading us to propose oxidation in the metal core as the reaction product of the transient under oxidizing conditions. The combination of different spectroscopic techniques along with the theoretical calculation allowed elucidating the nature, dynamics and reactivity of the excited states prevail in a Re-Pterin complex. This is of the particular importance considering that equivalent studies have not been reported for any other rhenium complex of substituted pterins nor for complexes of other transition metal ions of pterin derivatives.