Evidence as conformational molecular switch of 1,4-bis(4-vinylpyridyl)benzene adsorbed on a nanostructured silver surface: A synergetic study of femtosecond transient absorption spectra, electrochemical SERS records and DFT calculations

• 1,4-bis(4-vinylpyridyl)benzene (bvpb), a double heterostilbene, was synthesized and characterized. • Electrochemical Surface-Enhanced Raman Spectra (SERS) of bvpb put in evidence a resonance process at −1.0 V with 785 nm excitation line. • The study of dynamic relaxation processes by femtosecond Transient Absorption Spectra (TAS) shows a first excited state (S 1 ) at 607 nm in which a trans–cis rotation of the vinyl bond is involved. • DFT potential energy curve of vinyl torsion predicts a low barrier height between trans- and cis -conformers in S 1 state with a S 1 /S 0 conical intersection corresponding to a twisted vinylpyridyl bond. • The synergy of experimental data, SERS and TAS, and DFT calculations shows bvpb can act as a conformational molecular switch addressed by electric potential under a selected wavelength. The electrochemical surface-enhanced Raman spectra (SERS) of 1,4-bis(4-vinylpyridyl)benzene (bvpb) recorded at different electrode potentials with three excitation wavelengths (785, 532 and 473 nm) point out there exists a resonance process involved at more negative voltage with the 785 nm line, giving two strongly enhanced SERS bands at about 1500 and 1150 cm −1 . This result agrees with the VIS-NIR transient absorption spectrum characterized by a strong band at 607 nm and a weaker one at 1163 nm, corresponding to the first singlet (S 1 ) and triplet (T 1 ) excited electronic states. The DFT potential energy curve corresponding to the trans – cis ( E-Z ) isomerization in the S 0 state and in the two lowest S 1 and T 1 excited states, indicates that the S 1 state exhibits a significant lower barrier height than S 0 state, showing also a S 1 /S 0 conical intersection at a structural conformation in which the vinyl double bond twists by 90°. TD-DFT resonance Raman spectra of the planar and twisted conformation in a simple model of surface complex, Ag 2 0 -bvpb, yield that the twisted molecule is able to predict the selected enhancement of the two SERS bands. Therefore, bvpb could act as an electroactive conformational molecular switch under a selected wavelength in nanoelectronic devices.