Photophysics of molecular plasmonics

The fascinating feature of plasmonic metal nanoantennas of being able to confine light far beyond the diffraction limit is attracting rapidly increasing attention in solar applications, nanoimaging, ultrasensitive detectors and nonlinear processes. An intriguing unexplored question is whether the plasmonic antenna alters the photodynamics of the molecule in the vicinity.

We are studying the photophysics of plasmonic antennas coated with Ru[(dpp)3]2+ (tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) dichloride) molecules, a well-known model system for Ru-ligand-metal based H2-evolving photocatalysts. The phosphorescence of Ru[(dpp)3]2+ in water shows a decay which is well described by a mono-exponential function with a time constant of 746 +/- 1 ns (black curve in Figure 1). We found that electrostatic attachment of a monolayer of Ru[(dpp)3]2+ molecules on the surface of an antenna (Ag sphere with a radius of 50 nm and 8 nm silica shell) alters the phosphorescence decay (red curve), with the appearance of a 15 +/- 1 ns decay (38 % amplitude). This fast decay may correspond to molecules in the field-enhanced regions of the antenna, while the long-lived component is assigned to molecules outside this region for which the decay is slowed down slightly as compared to the aqueous environment. Ultrafast transient absorption measurements are underway to establish the nature of this fast excited state deactivation.

figure 1. Phosphorescence decay of Ru[(dpp)3]2+ in H2O, and electrostatically attached to a plasmonic antenna. Fits to mono- and bi-exponential decay functions and resulting time constants are included.