Vibrational Spectroscopic Analysis of 1,3-Dianiline Squarate: Infrared, Normal Raman, Surface-Enhanced Raman Scattering, and Density Functional Theory Calculations

1,3-Dianiline squarate (DSQ) is an interesting squaraine frequently used as an intermediary reactant in organic synthesis aiming products 1,3-(R1R2)-squarate type, as well as a building block of photoinitiators under visible light widely used in photopolymerization reactions. In this work, structural investigations of DSQ were performed using spectroscopic methods such as solid-state 13C nuclear magnetic resonance and several vibrational techniques. Therefore, a detailed vibrational spectroscopic analysis of DSQ was carried out by infrared, normal Raman, and surface-enhanced Raman scattering (SERS) combined with density functional theory calculations. SERS data suggest two preferential interactions between squaraine and the silver nanoparticle. It is proposed that squaraine is adsorbed mainly through a regular Ag-O interaction involving the carbonyl group; that connection impacts the oscillators around them and consequently their vibrational spectra. In addition, the silver atoms may perturb the DSQplanarity by interacting with the phenyl rings; they promote the molecule to adopt a non-planar tilted orientation on the metal nanoparticle surface.