Cu(I) photosensitizers with alkylated diphosphines: Towards enhancing photostability and architecture extension

Abstract

Homogeneous copper(I) photosensitizers (PSs) have garnered attention as viable alternatives to the use of noble metal-containing complexes in photoredox processes. More specifically, the heteroleptic architecture [CuI(PP)(NN)]+ (PP = diphosphine; NN = diimine) is attractive as it permits directional charge transfer, an important consideration when integrating PSs into heterogeneous structures. Towards that goal, three new Cu(I) PSs were synthesized and characterized, each containing an alkylated-diphosphine. The diphosphine ligands possess a Nixantphos backbone (Nixantphos = 4,6-bis(diphenylphosphino)-10H-phenoxazine) that has been alkylated at the phenoxazine’s nitrogen atom using ethyl, n-butyl, and n-hexyl groups (labeled Et-Nixantphos, Bu-Nixantphos, and Hex-Nixantphos, respectively). While not capable of direct surface attachment, these alkylated-diphosphines serve as structural and electronic controls for future Cu(I) PSs with anchoring groups. Similar to the [CuI(Nixantphos)(biq)]+ (biq = 2,2′-biquinoline) parent complex, all three alkyl-Nixantphos Cu(I) complexes absorb visible light and populate redox-active excited states (ESs) of sufficient lifetime to undergo bimolecular electron transfer. In the presence of an electron donor and H2-evolving catalyst, 447 ± 10 nm irradiation results in H2 gas production, albeit to a lesser degree than the previously reported PP = Xantphos analogue (Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene). The results highlight that alkylated-Nixantphos Cu(I) PSs maintain favorable properties for visible-light-induced electron transfer; however, long-term photostability remains an issue to prevent formation of the inactive, homoleptic [CuI(biq)2]+ species.