Two-dimensional electronic spectroscopy of gold nanorods: Nodal line slope analysis and spectral interference

Two-dimensional (2D) electronic spectroscopy (ES) has been used to investigate the electronic coupling and dynamics of coupled multi-chromophore systems. In particular, off-diagonal features on 2D ES spectra provide critical information, such as on the electronic couplings, excitation transfer rates and molecular conformational changes between the involved electronic states. Recently, a 2D ES study has been performed on plasmonic gold nanorods with the surface plasmonic resonance excitation of free electrons. To gain quantitative information on the degree of dynamical inhomogeneity of polydisperse gold nanorods, nodal line slope analyses of the time-resolved 2D ES spectra with varying center wavelengths of pump and probe pulses have been carried out and shown to be of use for investigating broad distributions of dimensionally heterogeneous gold nanorods. In addition, due to the strong nonlinear surface plasmonic resonance response of gold nanorods, we observed unusual spectral fringe patterns in their 2D ES spectra, which were not observed in organic chromophores, in a reverse time-ordered pulse sequence. This was shown to be further critical evidence of the inhomogeneously broadened longitudinal surface plasmonic resonance band of gold nanorods. In this chapter, we present a detailed description of the 2D ES responses of gold nanorods that are associated with ultrafast electron heating, coherent electron dynamics, and the subsequent relaxation processes.