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

Abstract

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 significant lower barrier height than S 0 1 and T 1 excited states, indicates that the S state, showing also a S 1 /S 0 1 state exhibits a 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.