Femtosecond Time-Resolved Dynamics of trans-Azobenzene on Gold Nanoparticles

We report a first femtosecond time-resolved transient absorption study of the photoinduced ultrafast dynamics of trans-azobenzene (AB) on gold nanoparticles (AuNPs). The observed changes in optical density following excitation at λ = 357 nm were analyzed by using temperature-dependent Mie theory and by Lorentzian band fitting to disentangle the ultrafast relaxation of the local surface plasmon resonance (LSPR) excitation of the Au core and the electronic deactivation of the attached AB ligands. The analysis of the dynamics associated with the AB photochrome yielded lifetime constants of τ1 = 1.2 ± 0.2 ps and τ2 = 4.7 ± 1.1 ps. Both values together indicate surprisingly little difference in the dynamics of the AB ligand on the AuNPs vs in solution. Our results thus highlight the extraordinarily efficient electronic decoupling of the azo chromophore and the Au core by the alkyl linker chain.