Synthesis and Photophysical Properties of Difluoroboron Complexes of Curcuminoid Derivatives Bearing Different Terminal Aromatic Units and a meso-Aryl Ring

Abstract

The synthesis of nine curcuminoids and their difluoroboron complexes is described, with seven of them containing a meso-phenyl ring. Dynamic (19)F NMR confirmed the fact that rotation of that meso-aryl fragment is restricted in the latter systems at room temperature and become allowed at higher temperature (>45 °C). The molecular structure of a meso-substituted derivative in the solid state showed that the phenyl ring lies in a highly twisted plane with respect to the mean curcuminoid plane. The photophysical properties of the nine compounds were investigated in solvents of different polarity. Meso-substitution with a phenyl ring has little influence on fluorescence emission properties in solution, radiative and nonradiative kinetic constants being similar for meso- and nonsubstituted compounds, which is in contrast to the case of BODIPY derivatives. However, introduction of an electron donor p-methoxy group at the meso-phenyl ring leads to small perturbation of the curcuminoid π-system fluorescence emission. We also report the influence of the meso-phenyl group on the emission properties of the aggregated solids.

Influence of the electron donor groups on the optical and electrochemical properties of borondifluoride complexes of curcuminoid derivatives: a joint theoretical and experimental study

“…The data for 4, 9, 12, 13, 14, 17, 24, 25, 26, 27, 29 were reported previously by some of us. [32][33][34][35][36] Those for 6, 7, 8 were published by other groups. 27,28 For the sake of comparison, we recorded the spectra of the last three compounds under the same experimental conditions using DCM as solvent and the data we obtained are fully consistent with the published ones.…”

“…In our recent studies, [32][33][34][35] we showed that the unique linear and nonlinear optical properties of borondiuoride complexes of curcuminoids also stem from the combination of electron donor (D) and acceptor (A) units in a D-A-D quadrupolar-like architecture in which A is the central dioxaborine ring and D is a terminal aromatic moiety acting as an electron donor substituent. As such, these compounds are highly solvatochromic dyes and their emission wavelengths can be tuned over the entire visible spectrum up to NIR region by controlling the strength of the terminal donor group.…”

“…52 Previous studies have shown the solvatochromic dependence of absorption and emission properties of borondiuoride complexes of curcuminoids, which is consistent with the partial CT character of the transitions. [32][33][34][35][36] In the case of 1 and 2, containing electronwithdrawing groups, such CT behavior is not observed, their absorption and emission spectra being well-structured and lying at higher energies. As noticed elsewhere, the introduction of a meso aryl unit systematically shis the spectra to the red (ca.…”

“…32 A variety of symmetrical and unsymmetrical D-A-D molecules have been investigated, which provided valuable insights into the photophysics of those systems. [37][38][39][40] In order to provide comprehensive structure-property relationships, we sought to establish correlations between Hammett parameters 41 and optical and electrochemical properties in series of borondiuoride complexes of curcuminoids.…”

Effect of Methoxy Substitution on Supramolecular Arrangement of Borondifluorides of 2'-Hydroxy Chalcones and their Solid-State NIR Fluorescence

“…This sequence is found consistent with that noticed for BF2 complexes of curcuminoids bearing the same series of terminal aryl groups. [16] However, steady-state fluorescence spectroscopy shows a peculiar behavior for 2 and even more for 3. Compound 2 emits at the longest wavelength (576 nm) while the spectrum of 3 is considerably blue-shifted relative to that of 1.…”