Zur Frage der Thermochromie; ein Beispiel für eine echte Valenztautomerie

Theilacker, Walter; Kortüm, G.; Friedheim, Gerd

doi:10.1002/cber.19500830603

Cited by 51 publications

(11 citation statements)

References 7 publications

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“…[23] Thermochromism is a reversible, temperature-dependent change of color. [1] In BAEs, the phenomenon is based on a unimolecular [24] equilibrium [25,26] between a colorless or yellow ambient-temperature form A and a deep-blue or deep-green high-temperature form B: [27] A kT ! B Although A absorbs in the UV region (and close to it), the thermochromic form has a new absorption band at l = 600-700 nm.…”

Section: Introductionmentioning

confidence: 99%

“…[25,26,28] DH is independent of the solvent. [26] The thermochromic, photochromic, and piezochromic B forms are identical.…”

mentioning

confidence: 97%

“…[39] 4) A diradical state in which the central bond linking the planar tricyclic moieties is twisted by 908 and is "not a true double bond". [24,25,40] 5) A "biradikalet": planar molecular halves twisted by less than 908 about the central double bond, the p electrons have parallel spins with a detectable contribution from the triplet state. [25,41] 6) A thermally populated triplet excited state of the molecule.…”

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confidence: 99%

“…[24,25,40] 5) A "biradikalet": planar molecular halves twisted by less than 908 about the central double bond, the p electrons have parallel spins with a detectable contribution from the triplet state. [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.…”

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confidence: 99%

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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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Section: Introductionmentioning

confidence: 99%

“…[25,26,28] DH is independent of the solvent. [26] The thermochromic, photochromic, and piezochromic B forms are identical.…”

mentioning

confidence: 97%

mentioning

confidence: 99%

mentioning

confidence: 99%

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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 fluorescence yields of (I), (11), and (111) crystals a t 25" C and -183" C were independent of exciting light. Alcoholic solutions and rigid glasses of (I) and (111) also retained constant yields of emission over the range 2500 A-3500 A.…”

Section: Resultsmentioning

confidence: 90%

The Luminescence of Complex Molecules in Relation to the Internal Conversion of Excitation Energy. Part I

Hochstrasser¹

1959

Can. J. Chem.

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The relative fluorescence yields of a variety of double molecules have been m e a s~~r e d as a function of the \vavelength of exciting light. I n some cases the yield from the second excited state is lower than from the first. This effect is attributed to either photodecomposition or to the enhancement of intramolec~~lar energy transfer processes which can compete with the internal conversion between the excited singlet states. The experiments on bianthrone have suggestecl a method of elucidating the mechanism of the photochromic iso~nerization of this compound. INTRODUCTIONThe fluorescelice of tlie vast majority of aromatic compounds arises from the radiative transition -first excited singlet to ground (singlet) state. On excitation to higher electronic states this same luminescence is observed and usually with unchanged yield. This implies that the rate of internal co~lversio~i to the radiating level is fast co~npared to the rate of intersystem crossing between upper excited states of different n~ultiplicity. I t has been suggested (3) that the fall-off in fluorescence yield of dibromoanthrace~ie excited with light below 3000 A is due to heavy atom-induced i~ltersystem crossing to the second triplet level. This indicates comparable rates for these co~npeting processes. Preliminary studies have indicated that a variety of molecules wliich are apparently quite stable photochemically have luminescence efficiencies wliich are anolllolous in ternis of the above rule.In this work the relative luminescence yields of a series of double molecules have been measured as a function of the exciting wavelength. A double molecule is one co~isisting of two identical parts coupled through a conjugating liillctge. E X P E R I M E N T A LThe fluorescence measurements were liiade by illuminating the samples contained in a Vycor tube or silica cell with the bean1 from a hydrogen lamp and Beck~lian D.U. monochromater. The luminescence was detected over a 20" region centering a t 70" to the main beam by liieans of an end-on-type pllotolnultiplier with S11 response. The incident radiation was filtered out by 1.5 c~i i of a saturated solution of sodium nitrite in water. By using suitable Corning filters, the cutoff could be extended to longer wavelengths. This combination provided good cutoff a t 3950 A. The relative quanta1 output of the hydrogen lamp was l;no\vrn a t each wavelength from 2500-3800 A in ternis of the luminescence of pure anthracene, u~hich is lcno~v~i to be collstallt over this region (3,4). T h e range was extended to 4100 -4 by calibrating tlie lanlp output against the luminescence intensity from a totally (99y0) absorbing solutioli of tetracene. The fluorescelice yield of tetracene is independent of the exciting light wavelength in this region of its absorption.The hydrogen lamp becornes less conti~iuous above 3700 A but good reproducibility could be obtained by opening up the mollochrolnater slit for this range of excitation. All the solutions urhich were studied absorbed a t least 99% of the incident li...

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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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Zur Frage der Thermochromie; ein Beispiel für eine echte Valenztautomerie

Cited by 51 publications

References 7 publications

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

The Luminescence of Complex Molecules in Relation to the Internal Conversion of Excitation Energy. Part I

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

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