Excitation wavelength-dependent SERS and DFT study to probe Herzberg-Teller selection rules on charge-transfer effect

In the present work, Herzberg-Teller selection rules on the charge-transfer (CT) effect in surface-enhanced Raman spectroscopy (SERS) are explored for the 3,4,9,10-perylene tetracarboxylic diimide (PTCDI) adsorbed on the Ag nano-island film (AgNIF) using several Raman excitation wavelengths. The UV/VIS/NIR spectrum of PTCDI adsorbed on the AgNIF indicates that excitation wavelengths of 514.5 nm, 633 nm, and 785 nm are in resonance with CT states of the complex, PTCDI adsorbed on the AgNIF. This CT resonance results in intensity enhancement of non-totally symmetric vibrational modes with b₁ symmetry in SERS. The three resonances (molecular, localized surface plasmon, and charge transfer) are observed for SERS with 514.5 nm. The totally symmetric Raman bands with symmetry a₁ exhibit maximum enhancement in SERS with a 514.5 nm wavelength and suggest the maximum electromagnetic mechanism in SERS with 514.5 nm. Few Raman-forbidden modes and silent modes are allowed in the SERS spectra due to the reduced symmetry of PTCDI in the proximity of the AgNIF. Moreover, density functional theory computation is also carried out to calculate vibrational modes and electronic transitions.