Electrochemical and Photophysical Study of Homoleptic and Heteroleptic Methylated Ru(II) Bis‐terpyridine Complexes

Abstract

In this study, we investigate the impact of N‐methylation on the electronic and photophysical properties of both homoleptic and heteroleptic Ru(II) bis‐terpyridine complexes based on the recently reported ligand 4’‐(4‐bromophenyl)‐4,4’’’ : 4’’,4’’’’‐dipyridinyl‐2,2’ : 6’,2’’‐terpyridine (Bipytpy), with pyridine substituents in the 4‐ and 4’’‐position. The first reduction of the methylated complexes takes place at the pyridinium site and is observed as multi‐electron process. Following N‐methylation, the complexes exhibit higher luminescence quantum yields and longer excited‐state lifetimes. Interestingly, the photophysical properties of the heteroleptic and homoleptic complexes are rather similar. TD‐DFT calculations support the experimental results. Furthermore, the complexes are tested as photosensitizers for photocatalytic hydrogen production, as the parent complex 1 [Ru(Bipytpy)(Tolyltpy)](PF6)2 (Tolyltpy: 4′‐tolyl‐2,2′ : 6′,2′’‐terpyridine) was recently shown to be active and highly stable under photocatalytic conditions. However, the methylated complexes reported herein are inactive as photosensitizers under the chosen conditions, presumably due to loss of the methyl groups, converting them to the non‐methylated parent complexes.

Spectro-electrochemical study of iron and ruthenium bis-terpyridine complexes with methyl viologen-like subunits as models for supramolecular polymers

“…In addition, the HOMO-stabilizing effect of more electron-withdrawing ligands can be observed by the anodic shift of homoleptic complexes compared to the heteroleptic complexes, as observed in comparable studies. 41,42…”

“…This reduction process is assumed to correspond to the reduction of the pyridinium unit, based on our previous study. 41 In the case of the homoleptic complexes, the first observed reduction wave (highlighted by an asterisk) in cyclic voltammetry is two closely spaced one-electron processes, as determined by square wave voltammetry (see Fig. 3 for Ru1 as well as in Fig.…”

“…In addition, the HOMO-stabilizing effect of more electronwithdrawing ligands can be observed by the anodic shift of homoleptic complexes compared to the heteroleptic complexes, as observed in comparable studies. 41,42 The first reduction, i.e., the reduction occurring at least negative potentials, in all complexes occurs around −0.95 V vs. Fc/Fc + . This reduction process is assumed to correspond to the reduction of the pyridinium unit, based on our previous study.…”