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 (H_rp) 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 H_rp 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 H_rp value exponentially decreases with increasing electron transfer distance (r_AB), as follows: H_rp = H_rp° exp[−β(r_AB − r_AB°)/2] with β = 11 nm⁻¹, where H_rp° is the electronic coupling matrix element when r_AB is the closest distance (r_AB°) 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.