Novel “Potentially Antiaromatic”, Acidichromic Quinonediimines with Tunable Delocalization of Their 6π-Electron Subunits

Abstract

We present a novel family of "potentially antiaromatic" alkyl-substituted p-benzoquinonediimine pH-dependent chromophores. It appears from the structural data that these overall 12 pi-electron molecules should be better considered as constituted by two chemically connected but electronically not conjugated 6 pi-electron subunits. Molecule 5 appears to be the first example of two separated, conjugated, and localized 6 pi-electron systems that can be tuned by reversible protonation to become delocalized. The mono- and diprotonated derivatives have been characterized by spectroscopic methods and X-ray diffraction. These systems develop supramolecular interactions in the solid state that clearly reflect the degree of protonation and depend on the nature of the counterion. These compounds constitute new chromophores for which the color can be tuned depending on the degree of protonation, going in solution from yellow for 5 to red for 5.HCl and blue for 5.2HCl. Theoretical calculations have provided a deeper insight into the electronic structure of these molecules and allowed an assignment of the experimental UV-vis spectra. The visible and near-UV spectrum of the neutral and protonated benzoquinonediimines can be classically assigned from the coupling of two 6 pi-electron polymethine units. TD-DFT calculations confirm the observed red shift of the two lowest pi --> pi* transitions of the benzoquinonediimines upon protonation and relate it to the moderate energy lowering of the HOMO --> LUMO transition induced by the delocalization of the polymethine pi system.

On the Electronic Structure of NiII Complexes That Feature Chelating Bisguanidine Ligands

“…[7] Combined experimental and theoretical investigations on such systems showed that the ligand plays a significant role in the electron transfer inside the complex. [8,9] In particular, the analysis of the different electronic states and their frontier orbitals and spin-density distributions provided valuable insight into the properties of non-innocent diimine ligands.…”

Spatial magnetic properties subject to lone pair and π electron delocalization in benzenoid and quinoid structures. Are quinoid tautomers really nonaromatic?

“…1.5 Å in length both from X-ray measurements as well as by DFT calculations). 47 If the spatial magnetic properties of 3 are carefully examined, however, there is noted to be no difference between the TSNMRS of para-benzoquinone 2 and 3. In both cases the conjugated quinoid moiety is described by similar, if not identical, TSNMRS due to the anisotropic effects of the former C=C double and C-C single bonds which are now partially conjugated C=C double bonds (cf.…”

“…Figure 2) and compared with the spatial magnetic properties of the iso-electronic para-benzoquinone 2. The former compound is expected to be potentially antiaromatic 46 and, due to the special substitution, is estimated from the X-ray structure to consist of two independent 6π-electron trimethine cyanine subunits 47 which are connected via two C-C single bonds (ca. 1.5 Å in length both from X-ray measurements as well as by DFT calculations).…”

Design and Synthesis of a Coumarin-based Acidichromic Colorant

“…The term acidichromic colorants refers to compounds exhibiting reversible color changes depending on either the pH in the solution or on alternating exposure to HCl and NH 3 in films [1][2][3][4][5][6][7][8][9]. These compounds are highly promising for use in acid-base sensors [2], photo-and chemicalswitching systems [10] and gas-controlled reversible color-change devices [11].…”

“…These compounds are highly promising for use in acid-base sensors [2], photo-and chemicalswitching systems [10] and gas-controlled reversible color-change devices [11]. Given that most reported acidichromic colorants contain only one pH-sensitive functional group in their molecules, their color changes are limited to a narrow pH range.…”