title: Spectroscopic characterization and modeling of quadrupolar charge-transfer dyes with bulky substituents.
abstract: Joint experimental and theoretical work is presented on two quadrupolar D-π-A-π-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.
authors.affiliation: Dipartimento di Chimica GIAF and INSTM-UdR Parma, Università di Parma , Parco Area delle Scienze 17/a, 43124 Parma, Italy.
authors.forename: Cristina
authors.initials: C
authors.lastname: Sissa
authors.forename: Francesca
authors.initials: F
authors.lastname: Terenziani
authors.forename: Anna
authors.initials: A
authors.lastname: Painelli
authors.forename: Raja Bhaskar Kanth
authors.initials: RB
authors.lastname: Siram
authors.forename: Satish
authors.initials: S
authors.lastname: Patil