1,3-Dianiline squarate (DSQ) is an interesting squaraine
frequently
used as an intermediary reactant in organic synthesis aiming products
1,3-(R
1
R
2)-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 DSQ planarity by interacting with the phenyl rings; they promote
the molecule to adopt a non-planar tilted orientation on the metal
nanoparticle surface.