1 research outputs found
Spectroscopic Characterization and Modeling of Quadrupolar Charge-Transfer Dyes with Bulky Substituents
Joint experimental and theoretical work is presented
on two quadrupolar
D-Ï€-A-Ï€-D chromophores characterized by the same bulky
donor (D) group and two different central cores. The first chromophore,
a newly synthesized species with a malononitrile-based acceptor (A)
group, has a V-shaped structure that makes its absorption spectrum
very broad, covering most of the visible region. The second chromophore
has a squaraine-based core and therefore a linear structure, as also
evinced from its absorption spectra. Both chromophores show an anomalous
red shift of the absorption band upon increasing solvent polarity,
a feature that is ascribed to the large, bulky structure of the molecules.
For these molecules, the basic description of polar solvation in terms
of a uniform reaction field fails. Indeed, a simple extension of the
model to account for two independent reaction fields associated with
the two molecular arms quantitatively reproduces the observed linear
absorption and fluorescence as well as fluorescence anisotropy spectra,
fully rationalizing their nontrivial dependence on solvent polarity.
The model derived from the analysis of linear spectra is adopted to
predict nonlinear spectra and specifically hyper-Rayleigh scattering
and two-photon absorption spectra. In polar solvents, the V-shaped
chromophore is predicted to have a large HRS response in a wide spectral
region (approximately 600–1300 nm). Anomalously large and largely
solvent-dependent HRS responses for the linear chromophores are ascribed
to symmetry lowering induced by polar solvation and amplified in this
bulky system by the presence of two reaction fields