Equilibrium and thermodynamic studies of chromic overcrowded fluorenylidene-acridanes with modified fluorene moieties

Abstract

Chromic materials, an important class of stimuli-responsive materials, have aroused extensive attention in recent years. Normally, their color is based on changes in morphology. Few examples of chromic material based on conformational isomerization, such as in overcrowded alkenes, have been reported previously. Furthemore, experimental thermodynamic studies of overcrowded bistricyclic aromatic enes have not been carried out to our knowledge. Here, we show that N-phenyl-substituted fluorenylidene-acridanes, with a properly modified fluorene moiety, performs chromisms originating from conformational changes. Thermodynamic studies determine equilibrium constants, changes in enthalpy, entropy, and free energy in solution, enabling in-depth understanding of the equilibrium behavior of overcrowded alkenes and providing useful information for designing functional chromic compounds. Single-crystal X-ray diffraction analysis of fluorenylidene-acridanes in this work clearly shows well-tuned charge transfer from the acridane to the fluorene moiety. Various chromic behaviors such as mechanochromism, thermochromism, solvatochromism, vapochromism, and proton-induced chromism also support understanding of conformational isomerism.

Quantitative and high-resolution mechanical pressure sensing functions of mechanochromic fluorenylidene-acridane

“…17 Single crystal analysis for 1 also was done and indicated that the folded conformer existed in the crystalline state. 19 In the present work, to support these previous reports, we studied why this transformation was induced by mechanical pressure by performing DFT calculations with a B3LYP/6-31G(d) method/basis set using the Gaus-sian09 package. Monomers and dimers of 1 in the folded and twisted conformers were calculated (Fig.…”

“…From among FA derivatives, we selected dimethoxy fluorenylidene acridane (1) [17][18][19] because the electron-donating dimethoxy groups on the acceptor part inhibit charge transfer, which occurs only in the twisted conformer. Therefore, dimethoxy groups cause the molecule to adopt the folded conformation as the initial state.…”

“…1). [17][18][19] Single crystal X-ray analysis explained that this color change occurs because of a conformational change from the folded conformer to the twisted conformer that exhibits charge transfer absorption from the electron-donating acridane moiety to the electron-withdrawing fluorene moiety. As the pure twisted conformer appears blue, the visual color of stimulated FA is green due to the coexistence of the twisted conformer and unconverted folded conformer exhibiting yellow color.…”

Sterically Frustrated Aromatic Enes with Various Colors Originating from Multiple Folded and Twisted Conformations in Crystal Polymorphs**

“…Similar to those of other OCEs, FAE 1 and 2 display mechanochromic behavior in the solid state [4f–h,13] . For example, grinding an orange crystal of FAE 1 or yellow crystal of FAE 2 causes the colors to change to dark red and orange color, respectively (Figures 9 and S22).…”

Biradicaloid Behavior of a Twisted Double Bond

“…There are two approaches to weakening the bonding interaction: stretching the bond and twisting the bond (Figure a). , Quinoidal and cumulenic biradicals can be regarded as the result of the bond-stretching operation, although the scaffold is extended. − The bond-twisting operation has been attained by enforcing the twisted structure through steric repulsion between bulky substituents at both ends of the ethylene moiety. These ethylenes with bulky substituents are termed overcrowded ethylenes and are known for structures with significantly twisted and elongated CC bonds. − However, only a few reports have been published on their biradicaloid behavior. , …”