Photochromism in Bianthrone and Related Compounds. Part VI. The structure of the photocromic isomers of bianthrones

Abstract

Partie experimentale. -GLndraZitds. L'acktonitrile (Fluka) a 6t6 sCch6 sur tamis mol6culaires puis distill6 sous azote sec. Le bromure de t6tra6thylammonium (Fluka) a 6t6 recristallise dans un melange ethanol-ac6tate d'6thyle. Les 6lectrolyses ont 6t6 faites, sous azote. B une cathode de rnercure s6par6e d'une anode en platine par un diaphragme poreux (Diapor). Le potentiel de la cathode Ctait fix6 par rapport au couple Ago/Ag+ (KC1 sat.) B l'aide d'un potentiostat construit dans notre laboratoire. Les polarogrammes ont 6t6 relevis avec un appareil Polarecord Metrohm E 261 R B temps de goutte contral6.PhLnylcyclopropane; Une solution de 15 g (0,054 mole) de ph6nyl-l-dibromo-l, 3-propane [5] dans 400 ml d'acdtonitrile-Et,NBr ( 0 , 3~) a 6t6 ClectrolysCe sous un courant cathodique moyen de 1 A. Le potentiel de la cathode a Ct6 tout d'abord fix6 3i -0,9 V, puis B -1,4 V en fin d'dlectrolyse afin de maintenir un courant d'intensiti convenahle. L'Blectrolyse a 6t6 interrompue lorsque le courant est tomb6 en dessous de 0,l A. On a enregistre le passage d'env. 9600 coulombs. La temp6rature a 6t6 maintenue entre 20-25" pendant la durCe de 1'6lectrolyse.Environ 250 ml d'ac6tonitrile ont Ct6 chassCs du melange final par distillation. AprBs addition de 250 ml d'eau et extraction B l'kther, 1'6vaporation des extraits 6th6r6s a abandonn6 un r6sidu qui a 6t6 distill6 sous pression r6duite. 4,3 g de ph6nylcyclopropane ont 6t6 receuillis 3i 60-61" 15 Tom sous forme d'un liquide incolore: n g = 1,5322. La structure a 6t6 confirm6e par comparaison du spectre de RMN. avec celui d'un Cchantillon authentique.Summary. The cyclic structure I11 is assigned to the C photochromic isomer of 1, S'-diniethylbianthrones on the basis of spectral and chemical evidence. Similar considerations indicate that this does not apply to the B photoisomers of bianthrones. l) 2, 3,Parts IV and V: cf. ref. [3h].

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

“…[7,54,55] The 1,1'-cyclized valence isomer 6 has been identified as the photochromic C form, which is distinct from the B form. [2,31,56] The photocyclization of BAEs is analogous to that of (Z)-stilbene to give 4a,4b-dihydrophenanthrene. [8,57] Substitution with bulky substituents at the 1-and 1'-positions prevents thermochromism.…”

“…[26] The thermochromic, photochromic, and piezochromic B forms are identical. [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.…”

Biradicaloid Behavior of a Twisted Double Bond

“…Bianthrone 1 is a well-studied overcrowded ethylene that contains steric repulsion arising from the bulkiness of the tricyclic aromatic rings substituted at both ends (Figure b). − The bianthrones have two structural isomers, a twisted form with a twisted CC bond and a folded form with an untwisted CC bond. The twisted form only exists as a transient species upon heating or light irradiation because the folded form is more energetically stable by about 15 kJ mol –1 . , Therefore, the twisted form has not been isolated and the electronic structure of the twisted CC bond has remained unknown. , …”