Nanoplasmonic sensing is a very active and diverse field with a wide variety of applications in chemistry, biomolecular and materials science . Optically resonant molecular systems often display what is called a strong coupling to the nanophotonic systems. This is primarily explored for the nanophotonics active control and in the studies of quantum optics . At the same time, the strong coupling of the molecular resonances to the nanoplasmonic antennas has not been addressed to follow the light-induced molecular processes. Here we combine an exemplary molecular photo-switch, from the spiropyran photochromic family, with anisotropic nanoplasmonic antennas to earn the monitoring tool for the light-activated processes using molecular and nanoplasmonic resonances strong coupling regime. We follow the reversible photo-isomerization of the spiropyran photoswitch from the spiro form to the merocyanine form by tuning in the nanoplasmon antenna to the excitonic state of the merocyanine form (at 570 nm), prompting the formation of a hybrid excitonic-plasmonic state. Our anisotropic nanoantenna provides two polarization-dependent spectrally separated resonances in the visible region, allowing for separate monitoring of the plasmon-exciton strong coupling and the conventional enhanced optical near-field refractive index sensing. This system uncovers a new modality in polaritonic chemistry and optical label-free monitoring of the photo-activated processes and can find applications in photocatalysis, biosensing and in hybrid molecular-nanoantenna actively modulated systems.
 M. I. Stockman, Science 348, 287 (2015); A. Dmitriev (Ed.), Nanoplasmonic Sensors, Springer NY (2012).
 Yoshie, T. et al. Nature 432, 200 (2004) ; Kasprzak, J. et al. Nature Mater. 9, 304 (2010); Reinhard, A. et al. Nature Photon. 6, 93 (2012).