Electromagnetic Field of Plasmonic Nanoparticles Extends the Photoisomerization Lifetime of Azobenzene

The thermodynamically stable azobenzene trans conformation photoisomerizes to the cis form on an ultrashort time scale when photoirradiated with photons of sufficient energy. Irradiating p,p'-dimercaptoazobenzene (DMAB) adsorbed on the surface of plasmonic nanoparticles with a 532 nm laser led to the appearance of Raman bands corresponding to a ring stretching. The torsion of DMAB during the photoisomerization is responsible for the appearance of the ring stretching, which disappeared after several minutes. Colloidally prepared gold nanorods (AuNRs) are assembled into two-dimensional (2-D) arrays on the surface of a glass substrate. AuNR 2-D arrays on a glass substrate are coated with silver half shells (AgHAuNRs) of different thicknesses. The irradiation time required to induce the isomerization of DMAB adsorbed on the surface of AgHAuNRs and the lifetime of the isomerization is increased by increasing the thickness of the silver layer. The plasmon field calculated with the discrete-dipole approximation technique showed that the plasmon field intensity of AgHAuNR excited at 532 nm is decreased upon increasing the thickness of the silver layer. This suggests that the plasmon field intensity is responsible for the photoisomerization of DMAB.