Polymorphism Versus Thermochromism: Interrelation of Color and Conformation in Overcrowded Bistricyclic Aromatic Enes

Abstract

The nature of the thermochromic form of overcrowded bistricyclic aromatic enes (BAEs) has been controversial for a century. We report the single-crystal X-ray structure analysis of the deep-purple and yellow polymorphs of 9-(2,7-dimethyl-9H-fluoren-9-ylidene)-9H-xanthene (11), which revealed the molecules in a twisted and a folded conformation, respectively. Therefore, the deeply colored thermochromic form B of BAEs is identified as having a twisted conformation and the ambient-temperature form A as having a folded conformation. This relationship between the color and the conformation is further supported by the X-ray structures of the deep-purple crystals of the twisted 9-(9H-fluoren-9-ylidene)-9H-xanthene (10), and of the yellow crystals of the folded 9-(11H-benzo[b]fluoren-11-ylidene)-9H-xanthene (12). Based on this conclusive crystallographic evidence, eleven previously proposed rationales of thermochromism in BAEs are refuted. In the twisted structures, the tricyclic moieties are nearly planar and the central double bond is elongated to 1.40 A and twisted by 42 degrees . In the folded structures, the xanthylidene moieties are folded by 45 degrees and the fluorenylidene moieties by 18-20 degrees . Factors stabilizing the twisted and folded conformations are discussed, including twisting of formal single or double bonds, intramolecular overcrowding, and the significance of a dipolar aromatic "xanthenylium-fluorenide" push-pull structure.

Formation of a leuco Spirolactone from 4‐(2‐Carboxyphenyl)‐7‐diethylamino‐4′‐dimethylamino‐1‐benzopyrylium: Design of a Phase‐Change Thermochromic System Based on a Flavylium Dye

“…Thermochromism: Some molecules exhibit the property of changing the color according to the temperature. These molecules comprise spiroheterocyclic compounds, [11,[17][18][19] salicylSchiff bases, [20][21][22][23] overcrowded ethylenes [24][25][26] and phasechange thermochromic materials (PCTM, organic thermochromic mixtures) that include crystal violet lactone and related compounds. [27][28][29][30][31] Thermochromism involving the present flavylium compound is a PCTM sample and was achieved by using EDIPA as a color developer, and different solvents, namely acetonitrile and n-pentadecanonitrile ( Figure 5).…”

Variations of bistricyclic aromatic enes: mono-bridged tetraarylethene naphthologs

“…• the pure ethylenic twist angle x around the central double bond (defined as the average of the torsion angles C 9a -C 9 -C 9 0 -C 1 0 and C 8a -C 9 -C 9 0 -C 1 0 ' for 11-15 and of C 9a -C 9 -C 9 0 -C 2 0 and C 8a -C 9 -C 9 0 -C 2 0 ' for [16][17][18][19][20] • the folding angle u of the tricyclic moiety is the dihedral angle between the benzene rings of each tricyclic moiety for 11-20 • the dihedral angle h between the least-square planes of the naphthalenyl substituents. • the pyramidalization angles v at C 9 /C 9 0 (defined as the improper torsion angles C 9a -C 9 -C 9 0 -C 8a /C 1 0 -C 9 0 -C 9 -C 1 0 ' for 11-15 and C 9a -C 9 -C 9 0 -C 8a /C 2 0 -C 9 0 -C 9 -C 2 0 ' for 16-20 MOD 360 minus 180°) • the naphthalenyl torsion angles s'(C 9 -C 9 0 -C 1 0 -C 8a' )/ s''(C 9 -C 9 0 -C 1 00 -C 8a'' ) for 11-15 and s'(C 9 -C 9 0 -C 2 0 -C 1 0 )/s''(C 9 -C 9 0 -C 2 0 ' -C 1 0 ' ) for 16-20) • torsion angles t'(C 4a -C 9a -C 1 0 -C 8a' )/t''(C 4b -C 8a -C 1 0 ' -C 8a'' ) for 11-15 and t'(C 4a -C 9a -C 2 0 -C 1 0 )/t''(C 4b -C 8a -C 2 0 ' -C 1 0 ' ) for 16-20.…”

“…Each conformation represents a compromise between p-delocalization and steric strain. The bridges X and Y play an important role in the relative stability of the three conformations, leading in favorable cases to thermochromism [16][17][18][19]. The stereochemistry of BAEs has very recently been reviewed and analyzed [4].…”

The Effects of Fluorine and Chlorine Substituents across the Fjords of Bifluorenylidenes: Overcrowding and Stereochemistry

“…The bistricyclic aromatic enes (BAEs) [1,2] (Figure 1) have fascinated chemists since bifluorenylidene (BF) (Figure 1; X, Y: -) was synthesized in 1875, dixanthylene ( Figure 1, X,Y: O) was synthesized in 1895, and thermochromism [3] and photochromism were revealed in bianthrone ( Figure 1; X, Y: C=O). [4] They can be classified into homomerous bistricyclic aromatic enes ( Figure 1, X = Y) and heteromerous bistricyclic aromatic enes ( Figure 1, X ϶ Y).…”