4,4 ,5,5 -Tetraphenyl-2,2 -biimidazole

Stabilized radicals, and so on [1]. An unprecedented example is the in situ observation of a light-induced radical pair produced in a crystal of 2,2 -di (p-chlorophenyl)-A,A, 5,5 -tetraphenyl-biimidazole (o-Cl-HABI) by X-ray diffraction, as shown in Scheme 7.1 [2], HABl derivatives are well known as the photo- and thermochromic compounds, the polymerization photo-initiators in imaging materials, and holographic photopolymers since it was prepared in 1960 [3-5], It has been proposed that the photolysis of the H ABI derivatives in benzene solution instantaneously produces a pair of lophyl (triarylimidazoyl) radicals. The lophyl radical has been proposed to be an initiator for the radical polymerization. Despite the intriguing properties of HABI derivatives, there is very few report [6]. 

The direct observation of the radical pair formation was performed for the crystal of the HABI mother compound, 2,2 -diphenyM,4, 5,5 -tetraphenyl-biimidazole as shown in Scheme 7.2 [9]. Since the molecule is composed of two monomer molecules bonded between N1 and C2, this is called l,2 -dimer. The molecular structure before photo-irradiation is similar to that of o-Cl-HABI. A pale yellow crystal of HABI was irradiated with a high-pressure Hg lamp for 30 min at low temperatures under 108 K. The disordered molecular structures of l,2 -dimers and the photo-produced ones around an inversion center are shown in Fig. 7.4. The original 1,2 -dimer is drawn with white bonds. The new molecules with black bonds appeared, which have the occupancy factor of 0.139(2). There are two lophyl radicals in both sides and... 

Appl. May 13, 1968. An image-forming composition of (1) a dye in its leuco form, and (2) a 2,2, 4,4, 5,5 -hexaarylbiimidazole. The composition can be combined with a binder and coated upon a substrate from a solvent solution. A representative composition is tris(4-diethylamino-o-tolyl)methane trihydrochloride and 2,2 -bis(o-chlorophenyl)-4,4 5,5 -tetraphenyl biimidazole. 

The Patent Office suspended prosecution of the application for several years while a structure determination was carried out. The structure of the o-bromophenyl dimer [2,2 -bis(o—bromophenyl)-4,4 5,5 -tetraphenyl-l,2 -biimidazole] was subsequently determined and submitted to the Patent Office, and the claims of the application were allowed. Consequently, the basic HABI patent, U.S. Patent 3,784,557 (to Lawrence Anthony Cescon), Phototropic 2,4,5-Triphenylimidazolyl Radicals and Dimers Thereof, did not issue until January 8,1974, nearly twelve years after the filing of the initial application. 

Upon oxidation with potassium ferricyanide the imidazole was converted in 91% yield to 2,2 -bis(2,4-dime-thoxyphenyl)-4,4, 5,5 -tetraphenyl-I,2 -biimidazole, a pale green phototropic solid. Analysis of a portion of the product which was isolated in an unsolvated state was ... 

The above dimer of 2-(o-bromophenyl)-4,5-diphenyl-imidazolyl radical recrystallized from methanol is linked in the 1,2 positions and the compound is, therefore, 2,2 -bis(o-bromophenyl) - 4,4, 5,5 - tetraphenyl - 1,2 - biimidazole. The molecular configuration was determined by X-ray crystal structure analysis. The measured atomic coordinates are given in Table m, the bond lengths in angstrom units and bond angles are listed in Table TV and dihedral angles between various planes of the dimer molecule are stated in Table V. 

Bond lengths and bond angles in 2,2 -bis(o-bromo-phenyl) -4,4, 5,5 -tetraphenyl-1,2 -biimidazole... 

When 1.0 pari of the imidazole was reacted with potassium ferricyanide in an ethanol solution of potassium hydroxide as was done in Example I, 2,2 -bis(o-fluorophen-yl)-4,4, 5,5 -tetraphenyl-l,2 -biimidazole was produced in a 78% yield. This biimidazole is a pale yellow solid which g softens at 139 -I49° C., forms a gel at 158 C., and changes to a yellow-brown liquid at 165 C. Recrystallization from ethyl ether gives unsolvated dimer, M.P. 216.5-217 C. 

Additional dimer compounds prepared by the procedure of Example I include 2,2 -bis(o-methylphenyl)-4,4, 5,5 -tetraphenyl-l,2 -biimidazole, M.P. 120 C., giving a violet radical with a 3.5 color fading value as measured in benzene by the procedure described in Example 1 2,2 -bis-(2,4,6 - trimethylphenyl) - 4,4, 5,5 - tetraphenyl-l,2 -bi-imidazole giving a violet color, having a color life of about 7 minutes and a fading rate of 1.0 2,2 -bis(o-methoxy-phenyl) - 4,4 - bis(p - methoxyphenyl) - 5,5 - diphenyl-l,2 -biimidazoIe giving a blue color having a color life of... 

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