Antenna-coupled infrared nano-spectroscopy of intra-molecular vibrational interaction

Ultrafast intramolecular vibrational energy transfer and redistribution (IVR) in the complex many-body environment of polyatomic molecules has a profound influence on molecular properties and chemical reactivity. In particular it limits coherence in applications like control of chemical reactivity or of single quantum levels. In order to investigate IVR, nonlinear multidimensional spectroscopy is the method of choice for large ensembles, but it is challenging to extend its sensitivity to probe small molecular ensembles and achieve nanoscale spatial resolution. Here we study the intramolecular coupling with IR nanospectroscopy in small ensembles in an IR-resonant nanoantenna coated with a molecular monolayer of a metal-carbonyl complex. By a combination of antenna resonance tuning and selective laser excitation of symmetric and asymmetric carbonyl modes we resolve the intramolecular vibrational coupling pathways from the effective vibrational lifetimes. A coupled quantum oscillator model quantifies the competition between intrinsic intra-molecular, extrinsic antenna-mediated vibrational energy transfer and Purcell enhancement of the vibrational lifetimes. As a nano-optical approach it allows to probe and control the low energy intramolecular vibrational energy landscape, with nanoscale spatial resolution and enhanced sensitivity with perspective into the single molecule regime.