171. Thermochromism and related effects in bixanthenylidenes and bianthronylidenes. Part II. General observations

Mills, Jack F.; Nyburg, S. C.

doi:10.1039/jr9630000927

Cited by 13 publications

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“…[25,41] 6) A thermally populated triplet excited state of the molecule. [26,32,42] 7) A coplanar (or nearly planar) conformation with enhanced p delocalization and a stretched central double bond. [35,43] 8) A twisted conformation (singlet state).…”

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confidence: 99%

“…[46] Kortüm and co-workers argued that, at the temperature of sublimation, only 1 % of the dixanthylene molecules would be in the thermochromic B form, sufficient for the green color of the crystals, but not detectable by X-ray diffraction. [32,48] As the B form has not yet been crystallized and subjected to X-ray structure analysis, one has to rely on indirect and inconclusive arguments regarding its structure. [2] An analysis of the previously reported X-ray structures indicated that BAEs with central five-membered rings E and F have twisted conformations, whereas BAEs with central six-membered rings have anti-folded conformations.…”

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confidence: 99%

“…[8,[28][29][30][31] The reversible formation of color raises the question of the nature of the colored species. [2,27,32] In fact, the enigma of thermochromism in BAEs has played an important role in the development of the theory of organic chromophores. [33][34][35] Various rationales have been suggested to explain the thermochromic phenomenon in BAEs:…”

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

Biedermann

Stezowski

Agranat

2006

Chemistry A European J

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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.

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

Biedermann

Stezowski

Agranat

2006

Chemistry A European J

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“…The concave shape adopted by the truxene core perfectly mirrors the convex surface of fullerenes, indicating that van der Waals and concave-convex πÀπ interactions between them should be maximized. Indeed, the association of trux-TTF and fullerenes in solution was investigated by 1 H NMR titrations with C 60 and C 70 as guests affording binding constants of log K a ¼ 3.1 and 3.9 for C 60 and C 70 in CDCl 3 /CS 2 , respectively [155].…”

Section: On Concave-convex Interactionsmentioning

confidence: 99%

“…Concerning the applicability on CNTs, it is a powerful tool when an assessment of the purity and three-dimensional morphological data are needed. In this direction, SEM is the instrument for the visualization of patterned surfaces where different CNTs topologies are grown [155]. On the other hand, the relatively low resolution cannot provide information about functionalization on processed tubes or accurate details about their structure.…”

Section: Scanning Electron Microscopymentioning

confidence: 99%

Polyarenes II

2014

Topics in Current Chemistry

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Conformational Space and Dynamic Stereochemistry of Overcrowded Homomerous Bistricyclic Aromatic Enes − A Theoretical Study

2001

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The conformational spaces and dynamic stereochemistry of representative overcrowded homomerous bistricyclic aromatic enes (1, X = Y) are investigated, applying the semiempirical PM3 method. The experimental energy barriers for E,Z isomerizations, enantiomerizations, and conformational inversions of 1 and related compounds, derived from DNMR and other kinetic studies, are reviewed. This study focuses on the analysis of the minima, transition states, and dynamic mechanisms of the conformational isomerizations of bifluorenylidene (2), dixanthylene (3), dithioxanthylene (9), and bi-5H-dibenzo[a,d]cyclohepten-5-ylidene (11). The four representative bistricyclic enes differ in the sizes of their central rings and in their bridging groups. The mechanisms of the interconversions

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171. Thermochromism and related effects in bixanthenylidenes and bianthronylidenes. Part II. General observations

Cited by 13 publications

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

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

Polyarenes II

Conformational Space and Dynamic Stereochemistry of Overcrowded Homomerous Bistricyclic Aromatic Enes − A Theoretical Study

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