Dipole moments of conjugated donor-acceptor substituted systems: calculations vs. experiments

Synthesis and Strong Solvatochromism of Push-Pull Thienylthiazole Boron Complexes

The solvatochromic behavior of two donor-π bridge-acceptor (D-π-A) compounds based on the 2-(3-boryl-2-thienyl)thiazole π-linker and indandione acceptor moiety are investigated. DFT/TD-DFT calculations were performed in combination with steady-state absorption and emission measurements, along with electrochemical studies, to elucidate the effect of two different strongly electron-donating hydrazonyl units on the solvatochromic and fluorescence behavior of these compounds. The Lippert–Mataga equation was used to estimate the change in dipole moments (Δµ) between ground and excited states based on the measured spectroscopic properties in solvents of varying polarity with the data being supported by theoretical studies. The two asymmetrical D-π-A molecules feature strong solvatochromic shifts in fluorescence of up to ~4300 cm⁻¹ and a concomitant change of the emission color from yellow to red. These changes were accompanied by an increase in Stokes shift to reach values as large as ~5700–5800 cm⁻¹. Quantum yields of ca. 0.75 could be observed for the N,N-dimethylhydrazonyl derivative in nonpolar solvents, which gradually decreased along with increasing solvent polarity, as opposed to the consistently reduced values obtained for the N,N-diphenylhydrazonyl derivative of up to ca. 0.20 in nonpolar solvents. These two push–pull molecules are contrasted with a structurally similar acceptor-π bridge-acceptor (A-π-A) compound.

Conjugated donor–acceptor substituted systems involving the 1,3-indandione-derived electron accepting moieties

Conjugated donor–acceptor molecules are the focus of research owing to their unusual photo- and electro-physical properties. At the same time, several unusual features of these compounds are difficult to explain or predict. Here we present our results on the synthesis, X-ray structures and D-NMR spectra providing a deeper insight into the conjugation within the derivatives involving the 1,3-indandione-derived series of compounds with varying electron acceptor strength and conjugating bridge length. The X-ray structures show the presence of several intermolecular short contacts strongly affecting the molecular geometries. In solution, the coalescence temperatures corresponding to the rotation of the phenylamino moiety of all derivatives do not exceed 246 K indicating the unhindered rotation at room temperature. Using B3LYP/aug-cc-pVDZ, the calculated model chemistry barriers to rotation, dipole moments and first hyperpolarizabilities are within experimental error. We conclude that neglecting the electron donating properties of bridges themselves and internal rotation about the single bonds taking part in conjugation can result, for instance, in misinterpretation of their room temperature NMR spectra and overestimation of the computed molecular dipole moments by more than 5 D.

Determination of barriers to rotation using the D-NMR technique within a series of push–pull derivatives sheds light on the inability of stronger acceptors to considerably increase the dipole moments and nonlinear optical response.