Cu(i) vs. Ru(ii) photosensitizers: elucidation of electron transfer processes within a series of structurally related complexes containing an extended π-system

Abstract

Heteroleptic Cu(i) complexes are a promising alternative towards traditional Ru(ii) photosensitizers. In particular, Cu(i) complexes of the type [Cu(P^P)(N^N)]+, where N^N represents a diimine and P^P a bulky diphosphine ligand, are already successfully applied for photocatalysis, organic light-emitting diodes or dye-sensitized solar cells. Therefore, this study aims for the systematic comparison of three novel heteroleptic Cu(i) compounds, composed of xantphos (xant) as P^P ligand and different diimine ligands with an extended π-system in the backbone, with their structurally related Ru(ii) analogues. In these Ru(ii) photosensitizers [Ru(bpy)2(N^N)]2+ (bpy = 2,2'-bipyridine) the same N^N ligands were used, namely, dipyrido[3,2-f:2',3'-h]quinoxaline (dpq) and dipyrido[3,2-a:2',3'-c]phenazine (dppz). To gain an in-depth understanding of the photoinduced charge transfer processes, the photophysical features of these complexes and their electrochemically oxidized/reduced species were studied by a combination of UV-vis absorption, resonance Raman and spectroelectrochemistry. (TD)DFT calculations were applied to qualitatively analyze these measurements. As a result, the heteroleptic Cu(i) complexes exhibit comparable charge transfer properties to their Ru(ii) analogues, i.e. upon visible light excitation they undergo a metal-to-ligand charge transfer to the diimine ligand(s). In contrast, the reduced Cu(i)- and Ru(ii)-dppz complexes show considerably different electronic transitions. The singly reduced Cu(i)-dppz complexes are able to accumulate an additional electron at the phenanthroline moiety upon blue-light excitation, which is beneficial for multi-electron-transfer reactions. Upon low-energy light irradiation electronic transitions from the dppz- anion to the xant ligand are excited, which could shorten the lifetime of the photosensitizer intermediates in an unwanted way.

Ligand electronic fine-tuning and its repercussion on the photocatalytic activity and mechanistic pathways of the copper-photocatalysed aza-Henry reaction

“…420 nm, which is reminiscent to the absorption spectrum of an analogous reduced copper complex reported by Dietzek and coworkers. 70 However, these small changes in the UV spectra do not provide irrefutable evidence of a contribution from a reductive quenching pathway.…”

“…S18b †). 70 Noting that 6 is itself photoactive under these conditions, we next investigated NEt 3 as a non-chromophore surrogate of 6, which possesses a similar E ox to 6 (E ox = 0.83 for NEt 3 , 71 E red = 0.82 V for 6). When NEt 3 is used as the quencher, only small absorption changes were found at ∼455 nm (Fig.…”

Cu(i) diimine complexes as immobilised antibacterial photosensitisers operating in water under visible light

“…The analytical data agree with those published previously. 51,59,66,70,72 The properties of complex 1 are summarised in Table S1 in the ESI; † the absorption and emission spectra in acetonitrile solution, in the solid state, and when immobilised on silica, are shown in Fig. 2.…”

“…30,45,[49][50][51][52][53][54][55] Recent studies have shown that Cu(I) complexes containing one diimine ligand, and one diphosphine ligand such as 4,5-bis(diphenylphosphino)-9,9dimethyl-xanthine (xantphos), which has a large bite angle and high steric hindrance, can have even longer excited state lifetimes (ns-ms). [56][57][58][59][60] Cu(I) complexes are being increasingly used as photosensitisers in photocatalysis, 24,25,30,51,[61][62][63][64][65][66][67][68][69][70][71][72][73] and dyes-sensitised solar cells. 74 They are also used as antitumor [75][76][77][78] and antibacterial agents [79][80][81][82][83][84][85] due to their innate dark toxicity.…”

“…We present the first example of a Cu(I) complex used as photosensitiser for antibacterial action in water, immobilised on silica support. We demonstrate the high light-induced antibacterial activity of the well-known complex [Cu(I)(xantphos)(dmp)]tfpb (1) [tfpb = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate] 51,59,66,70,72 which has been previously examined as an oxygen sensor. 59 The antibacterial action of 1 was tested on a Gram-negative bacterium, Escherichia coli (E. coli, strain MG1655), 5,86,87 and a Gram-positive bacterium Staphylococcus aureus (S. aureus, the Methicillin Resistant strain MRSA 315), both of which commonly occur in water.…”