Vibrational Cooling in Oligomeric Viologens of Different Sizes and Topologies

Abstract

Vibrational cooling was investigated in a set of homologous dimers and trimers with methyl viologen repeat units (MV²⁺). The rapid, <500 fs decay of the D₁ excited state of monoreduced viologen (MV^+•) via a conical intersection allows the preparation of a vibrationally hot D₀ ground state with a large excess energy of 1.7 eV, which is equivalent to the initial effective temperature of ∼800 K. Pump-probe spectroscopy was used to monitor the disappearance of the characteristic D₀ → D₁ hot absorption band, which appears at longer wavelengths than the steady-state spectrum of "cold" MV^+• in equilibrium with the solvent. It is assumed that the vibrational excitation of the ground is initially confined to the same monoreduced viologen repeat unit, which was optically excited to the localized electronic D₁ state, although some degree of redistribution may occur already in the excited state. The observed cooling rates depend on the size and topology of the oligomer, with the linear trimer exhibiting significantly faster thermalization than the branched one. The experimental results were corroborated by molecular dynamics simulations carried out in the harmonic approximation. The dynamics of the thermal equilibration in these systems appears to be consistent with primarily ballistic initial propagation of the vibrational excess energy over distances as large as ∼4 nm and suggests the presence of interference between the equivalent pathways in the branched trimer.