Spectroscopic Characterization and Modeling of Quadrupolar Charge-Transfer Dyes with Bulky Substituents

Joint experimental and theoretical work is presented on two quadrupolar D-pi-A-pi-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.

Peptide delta-Turn: Literature Survey and Recent Progress

Among the various types of a-peptide folding motifs, delta-turn, which requires a central cis-amide disposition, has been one of the least extensively investigated. In particular, this main-chain reversal topology has been studied in-depth neither in linear/cyclic peptides nor in proteins. This Minireview article assembles and critically analyzes relevant data from a literature survey on the d-turn conformation in those compounds. Unpublished results from recent conformational energy calculations and a preliminary solution-state analysis on a small model peptide, currently ongoing in our laboratories, are also briefly outlined.