Redox Exciton Quenching in Cadmium Selenide Quantum Dots by Methyl Viologen: Quantum Yield of Charge Separation

Abstract

Using femtosecond laser spectroscopy with a time resolution of 25 fs, it has been shown that methyl viologen (MV²⁺) quenches the lower exciton state X1 in CdSe quantum dots in toluene. The quenching kinetics is approximated by a three-exponential function with time constants of τ₁ = 350 fs (79%), τ₂ = 1.3 ps (15%), and τ₃ = 4 ps (6%). The quantum yield of charge separation to produce the h⁺//MV⁺ pair possessing a lifetime of more than 1 ns does not exceed 24%. The low quantum yield of the radical ion MV⁺ is explained in terms of fast recombination of MV⁺ and the hole. The methyl viologen adsorbed on the quantum dot surface mainly acts as the center of nonradiative electron–hole recombination.