A new bisboronate with five six-membered ring heterobicycles was prepared by reaction of [2-(2-hydroxy-4-diethylaminobenzylidene)amino]-5-nitroaminophenol and 1,4-phenyldiboronic acid, and its quadratic nonlinear optical (NLO) response was compared to that of a related monoboronate species. A computational investigation conducted within the framework of the DFT theory, at the B3PW91/6-31G* level, indicates that, while the diboronate derivative exhibits a centrosymmetric conformation in the ground state, and hence a vanishing quadratic hyperpolarizability (β), the application of an external electric field provides a gradual intramolecular rotation of the two “push–pull” sub-units which reaches 128° for a field intensity of 10⁻³ a.u. and leads to a β value equal to 49.2 × 10⁻³⁰ cm⁵ esu⁻¹. This result suggests a possibility for a molecular NLO switch induced by an electric field in such systems. Experimentally, the NLO response measured by the electric field induced second harmonic (EFISH) technique indicates an NLO response (expressed as the μ × β product) 1.95 times larger for the bisboronate than for the monoboronate analogue, thus suggesting that, although the ground state conformation is centrosymmetric, the “push–pull” sub-units are in quasi-free rotation at room temperature.

BODIPY-derived molecular fluorescent rotors are one of the most useful families of fluorescent probes for the quantitative measurement of microviscosity. Here, we report the photophysical properties of two molecular rotors as ratiometric viscosity sensors, meso-(4-(9H-fluoren-2-yl)ethynyl)phenyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (1) and meso-(5-(9H-fluoren-2-yl)ethynyl)thiophen-2-yl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (2) having two independent chromophores; one (fluorene) is not influenced by viscosity and is used to determine concentration properties, and the other chromophore (BODIPY) acts as a molecular rotor. The fluorescence ratiometric calibration against microviscosity for the most sensitive probe (1), was obtained in a wide linear dynamic range. The sensitivity of the spectroscopic observables for 1 and 2 towards solvent viscosity and the typical analytical interfering solvent polarity and polarizability parameters were investigated by means of two different solvent scales, namely Lippert–Mataga and Catalán, finding consistent results. Quantum chemical calculations were conducted to analyze the probe mechanism in terms of Natural Transition Orbitals (NTOs) and the spatial extent of charge transfer excitations.

Boron and tin complexes have been a versatile and very interesting scaffold for the design of nonlinear optical (NLO) chromophores. In this paper we present a wide range of reports since the 1990s to date, which include second-order (e.g., second harmonic generation) and third-order (e.g., two-photon absorption) NLO properties. After a short introduction on the origin of the NLO response in molecules, the different features associated with the introduction of these inorganic motifs in the organic-based NLO materials are discussed: Their effect on the accepting/donating capabilities of the substituents, on the efficiency of the π-conjugated linkage, and on the topology of the chromophores which can be tuned from the first generation of “push-pull” chromophores to more sophisticated two- or three-dimensional architectures.

A series of electronic and physicochemical parameters were explored to determine their effect on experimental spectroscopic and photophysical data. Through a systematic obtention of a series of meso-(o-aryl)-BODIPYs, ¹⁹F NMR spectra were analyzed and their fluorescence quantum yields in several solvents were measured. Experimental values of ¹⁹F chemical shift difference Δδ_F correlate well with σ-Hammett constants, which is indicative of the inductive nature of the functional groups on the fluorine atoms. A computational DFT exploration of rotational energy barriers, electrostatic potential maps, group electronegativity, charge partitions and hardness/softness provided insight into how those traits can be directly related to the measured features. Expanded understanding of such characteristics provides design arguments and a structure–property relationship, which in a more advantageous way, would help to understand the properties of the synthesized molecules and of future attempts that are structurally related.