The Heteroleptic Cu(I) Photosensitizer‐Containing 3,8‐Disubstituted Phenanthroline: Synthesis, Photophysical Properties and Photocatalytic Hydrogen Evolution from Water

Abstract

Four novel heteroleptic Cu(I) photosensitizers with 3,8‐diaryl‐2,9‐diisopropyl‐1,10‐phenanthroline derivatives as electron‐donating bidentate N‐ligands have been designed and synthesized. The metal copper complexes are studied in light‐driven water reduction system. And the substituent effect at the 3,8‐positions of N‐ligands impacting on photosensitive activity and photoelectric chemical properties are explored and discussed. Surprisingly, with the enhancement of electron‐donating capacity of substituent group at 3,8‐sites of 1,10‐phenanthroline, the photosensitive activity of photosensitizer linearly increases, and photocatalytic performance up to 817 turnover numbers (TON) was reached combining with water reduction catalyst (WRC), Fe3(CO)12. The photocatalytic performance of these photosensitizers dramatically increased compared to 2,9‐diispropyl‐1,10‐phenanthroline complex (TON = 89). Moreover, the introduction of methoxy substituent to copper complexes, which could increase the fluorescence lifetimes (τmax = 209 ns) and promote hydrogen evolution.

Cross-Coupled Phenyl- and Alkynyl-Based Phenanthrolines and Their Effect on the Photophysical and Electrochemical Properties of Heteroleptic Cu(I) Photosensitizers

“…All phenyl-based complexes C P 2−C P 7 show one reversible single-electron reduction (Figure 4 and Table 3) at −1.94 to −2.11 V versus Fc/Fc + , which can be assigned to a reduction of the diimine ligand. 24,35,51,87 This behavior is also in agreement with the structurally related reference complex ref, which possesses the unsubstituted dmp ligand. 51,87 The maximum deviation of 170 mV in the reduction potentials between the different phenyl-based complexes and the reference complex (i.e., −1.94 V for C P 7, −2.11 V for C P 5, and −2.10 V for ref) is comparably small and suggests only a small impact of the substitution on the orbital energy levels of the diimine core.…”

“…Phenylbased ligands L P 2−L P 7 (Figure 2) were synthesized via Suzuki−Miyaura cross-coupling, which is one of the most broadly applied catalyses for C−C bond formation (Scheme 1). 35,59,60 This is partly because of the great variety of boronic acids (and other related reagents) that are readily available. 61−63 For these reasons, Suzuki−Miyaura cross-coupling allowed us to study the influence of different substituents and functional groups by granting access to a library of structurally related derivatives.…”

“…A similar effect has already been observed for phenyl substitution in the 4,7-position (λ abs = 387 nm) 87 and the 3,8-position (λ abs = 384 nm). 35…”

“…Phenyl-based ligands L P 2–L P 7 (Figure ) were synthesized via Suzuki–Miyaura cross-coupling, which is one of the most broadly applied catalyses for C–C bond formation (Scheme ). ,, This is partly because of the great variety of boronic acids (and other related reagents) that are readily available. − For these reasons, Suzuki–Miyaura cross-coupling allowed us to study the influence of different substituents and functional groups by granting access to a library of structurally related derivatives. In our initial attempts, L1 was reacted toward L P 7 under rather classical Suzuki–Miyaura reaction conditions with [Pd­(PPh 3 ) 4 ] as the catalyst (12 mol %) and the corresponding boronic acid (3.0 equiv) in a biphasic system consisting of THF and an aqueous 2 M Na 2 CO 3 solution.…”

“…Bringing such systems into practice, however, requires a combination of favorable characteristics and high resource efficiency. In addition to some recent and successful examples of photosensitizers based on Cr­(III), , Fe­(II/III), − and Mo(0), especially heteroleptic diimine-diphosphine Cu­(I) complexes of the type [(P^P)­Cu­(N^N)] + are promising alternatives to their traditional counterparts. − …”