Kinetic study of the photo-induced electron transfer reaction between ruthenium(ii) complexes of 2,2′-bipyridine derivatives and methyl viologen. Effects of bulky substituents introduced onto 2,2′-bipyridine

Abstract

Ruthenium(II) complexes of 2,2′-bipyridine derivatives, [Ru(Rbpy)3]2+ (Rbpy = 4,4′-di(alkylaminocarbonyl)-2,2′-bipyridine; alkyl = propyl, hexyl or adamantyl), have been newly synthesized and their photo-induced electron transfer (ET) reaction with methyl viologen (MV2+) has been investigated. The rate constant of the ET reaction decreases in the order alkyl = propyl > hexyl > adamantyl, which is opposite to the increasing order of the size of the alkylaminocarbonyl substituent introduced onto 2,2′-bipyridine. After correction of the diffusion rate, the ET reaction in the exciplex, [*Ru(Rbpy)32+⋯MV2+] → [Ru(Rbpy)33+⋯MV˙+], was analyzed on the basis of Marcus' theory. The electronic coupling matrix element (Hrp) decreases in the order propyl (2.28 × 10−3 eV) > hexyl (1.86 × 10−3 eV) > adamantyl (1.37 × 10−3 eV), while the reorganization energy (λ) depends little on the alkyl group of Rbpy; λ = 0.772 eV, 0.767 eV and 0.798 eV for R = propyl, hexyl and adamantyl, respectively. Thus, not the λ value but the Hrp value is responsible for the above-mentioned decreasing order of the rate constant. This means that the bulky substituent decreases the orbital overlap between donor and acceptor to suppress the ET reaction. The Hrp value exponentially decreases with increasing electron transfer distance (rAB), as follows: Hrp = Hrp° exp[−β(rAB − rAB°)/2] with β = 11 nm−1, where Hrp° is the electronic coupling matrix element when rAB is the closest distance (rAB°) for effective contact between donor and acceptor. This β value is almost the same as the value (12 nm−1) reported for the thermal ET reaction between aromatic compounds.