Controlling the non-resonant chemical mechanism of SERS using a molecular photoswitch

In this work we present a detailed investigation of the Raman properties of a dithienylethene photoswitch interacting with a small gold cluster (Au ₁₉⁺) using time-dependent density functional theory (TD-DFT). The enhancement mechanism (CHEM) due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) has been characterized for this system. We demonstrate that it is possible to control the CHEM enhancement by switching the photoswitch from its closed form to its open form. The open form of the photoswitch is found to be the strongest Raman scatterer when adsorbed on the surface whereas the opposite is found for the free molecule. This trend is explained using a simple two-state approximation. In this model the CHEM enhancement scales roughly as (ω_X/_e) ⁴, where ω_X is the HOMO-LUMO gap of the free molecule and _e is an average excitation between the HOMO of the photoswitch and the LUMO of the metal. We propose that the ability of this photoswitch to switch reversibly from open to closed will make it an excellent probe to control the CHEM enhancement of SERS.