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The electrochemical surface-enhanced Raman spectra (SERS) of 1,4-bis(4-vinylpyridyl)benzene (bvpb) recorded on silver electrodes at different potentials, using three excitation wavelengths (785, 532, and 473 nm), reveal the presence of a resonance process at negative potentials under 785 nm excitation. This process leads to the appearance of two strongly enhanced SERS bands at ca. 1500 and 1150 cm⁻¹. These findings are consistent with the VIS–NIR transient absorption spectrum, which displays a prominent band at 607 nm and a weaker band at 1163 nm, assigned to the first singlet (S₁) and triplet (T₁) excited electronic states, respectively.
Density functional theory (DFT) calculations of the potential energy profiles for the trans–cis (E–Z) isomerization indicate that, in the S₁ state, the energy barrier is significantly lower than in the ground state (S₀). Moreover, a conical intersection between S₁ and S₀ is identified at a geometry characterized by a 90° twisting of the vinyl double bond. Time-dependent DFT (TD-DFT) simulations of resonance Raman spectra, based on a simplified surface complex model (Ag₂₀–bvpb), show that the twisted conformation accurately reproduces the selective enhancement of the two observed SERS bands.
These results suggest that bvpb may function as an electroactive conformational molecular switch under suitable laser excitation conditions, with potential applications in nanoelectronic devices.