Lophyl radicals

Addition of hydrogen peroxide to the triphenylimidazolyl (lophyl) radical (49), generated from its piezochromic dimer, yields 43 tertiary hydroperoxides likewise give 50 (R = Me, Ph). °... 

The Dylux system operates somewhat like the above photoxidation of leuco crystal violet, but the initiation step is different. Triarylimidazolyl radicals are generated by ultraviolet (365 nm) irradiation of their stable dimers (HABI s hexaarylbiimidazoles, 11) [44]. The persistent free radicals (called lophyl radicals because they were historically derived from the parent lophine, triphenylimidazolyl... 

Another initiator system that can operate using electron transfer pathways involvers the HABI s discussed above in connection with color formation systems (see Sect. 6). Lophyl radicals were shown to be able to oxidize various leuco dye species to their colored forms. Similarly, such radicals can extract an electron from an arylamine containing hydrogens on a carbon alpha to N to form a-amino radicals capable of initiation [Eqs. (21,22)] of... 

It was mentioned earlier that the lophyl radical is a poor initiator of free-radical polymerization however, it is an excellent hydrogen atom abstractor. The design of HABI compounds for photopolymerization has been optimized to exploit this difference [164]. 

The reaction mechanisms have been explored in the coumarin, ketocoumarin, or titanocene derivative/bis-aryl imidazole Cl-HABPmercaptobenzoxazole system [240]. For example (10.67), when the sensitizer is the coumarin Cl, an electron transfer occurs and the formed Cl-HABI radical anion leads to a lophyl radical that undergoes a hydrogen transfer with the thiol SH (the lophyl radical anion reacts with the thiol radical cation). The Cl/SH interaction can also participate in the initiation process. The role of new thiols and disulfides has been recently studied [241]. 

Figure 5-9. Disordered structure after photoirradiation. The two molecules with white bonds are initial 1,2 -dimer molecules whereas the molecules with black bonds are one 2,2 -dimer and two lophyl radicals...

The 2,2 -dimer, which is known as a piezodimer, has a C2—C2 bond instead of the Nl—C2 bond of the original 1,2 -dimer. The structure of the 2,2 -dimer was determined for the first time, although the existence of the 2,2 -dimer was deduced spectroscopically. On warming to 25 °C, the black crystal returned to the original pale yellow crystal. The ESR spectra of a single crystal of HABI after irradiation showed a broad doublet signal corresponding to free lophyl radicals. Infrared spectra before and after irradiation indicate the decrease and increase of 1,2 - and 2,2 -dimers, respectively. 

The structure after irradiation at 50 K was analyzed at beamline BL02B1 of SPring-8. The 2,2 -dimer was not observed but four lophyl radicals appeared in the disordered structure. On warming to 100 K, the 2,2 -dimer was observed with X-rays. Thermal motion of the lophyl radicals and a void space around the radicals might be important to produce the 2,2 -dimer [18],... 

Keywords Pyridine derivatives Salicylideneaniline derivatives Structure of excited molecule Structure of lophyl radical Unstable species of photochromism... 

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]. 

Fig. 7.3 A pair of the produced lophyl radicals viewed along the normal to the radical plane. The dotted line between C4 and N2 indicates the bond before the irradiation...

Scheme 7.2 Formation of a piezodimer and a peiir of lophyl radicals from two HABI molecules...

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... 

The IR spectra after irradiation were assigned to the original l,2 -dimer, produced 2,2 -dimer and lophyl radical. Moreover, the spectra also indicate that this photochemical process is reversible. The existence of the 2,2 -dimer has been assumed from the spectroscopy but the structure analysis has not been successful, because the 2,2 -dimer is easily transformed to the l,2 -dimer. The 2,2 -dimer with 4n-4n electron system is less stable than the l,2 -dimer with 4n.-6% system. The theoretical calculation indicated that the l,2 -dimer is stabler by 5.9 kcal moH than the 2,2 -dimer. The in situ X-ray crystallography shown here is a very powerful method to capture the light-induced metastable intermediates such as a 2,2 -dimer. 

After irradiation, the HABI molecules decompose into lophyl radicals which subsequently abstract protons from a donor, e.g. thiols [BER 14] (Figure 4.9), heteroaromatic thiols, benzothiazoles, benzooxazoles, tertiary amines, alcohols, thiocarboxylic acids [BLA 09] or dyes [BEN 02], If used in a dental composite, they lead to improved compressive, yield and flexural strengths compared to conventional composites with CQ/DMABE. Furthermore, the sensitivity to ambient light could be decreased [BLA 09]. 

Figure 4.9. Reaction scheme for the formation of lophyl radicals followed by the abstraction ofprotons from a thiol [BER 14]...

A review of the various properties of different TEMPO derivatives in several ILs was recently presented. This included UV-VIS investigations in ILs of the formation and recombination with other radicals, like the lophyl radical generated by photolysis of o-chlorohexaatylbisimidazole. 

Energy from the excited sensitizer molecule may be transferred to a HABI initiator molecule L-L (L = lophyl radical) if certain photochemical criteria are satisfied ... 

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