Diolefin crystals, photodimerization and

Diffusion control and pre-association in nitrosation, nitration, and halogenation, 16, 1 Dimethyl sulphoxide, physical organic chemistry of reactions, in, 14, 133 Diolefin crystals, photodimerization and photopolymerization of, 30, 117 Dipolar aptotic and protic solvents, rates of bimolecular substitution reactions in,... 

In the crystal of 1,4-dicinnamoylbenzene (1,4-DCB) (see Fig. 12), the distances between the intermolecular photoadductive carbons are 3.973 and 4.086 A for one cyclobutane ring, and 3.903 and 3.955 A for the other. The two topochemical pathways may occur competitively in a single crystal of 1,4-DCB at the initial stage of reaction. Then, both intramolecular photodimerization and intermolecular photopolymerization of the diolefinic mono-cyclobutane intermediate occur competitively to give tricyclic dimer 21,22,23,24-tetraphenyl-l,4,ll,14-tetraoxo-2(13),12(13-diethanol, [4.4] para-cyclophane or oligomers (Hasegawa et al., (1985). On photoirridation at room temperature the 1,4-DCB crystal gives >90% of the tricylic... 

H-stacking interactions have also been exploited to orientate olefinic moieties in a geometry suitable for photochemical cycloaddition reactions, and have been invoked by Coates et al. to explain the photodimerization and photopolymerization of mono- and diolefins carrying phenyl and perfiuorophenyl groups [43]. Matsumoto et al. reported the photodimerization of 2-pyridone in co-crystals with naphthalene-substituted monocarboxyhc acids, where the stacking of the naphthalene rings provides carbon-carbon distances appropriate for [4+4] cycloaddition [44]. 

It was first reported that the topochemical photopolymerization of diolefin crystals gave rise to cracks and deformation [7]. An atomic force microscopic (AFM) study made possible the observation that the photodimerizations of trans-cinnamic acids and anthracenes in the crystalline state induced surface morphological changes at the tens and hundreds of nanometers level by the transportation and rebuilding of the surface molecules [8]. The appearance of a surface relief grating on the single crystal of 4-(dimethylamino)azobenzene was demonstrated by repeated irradiation with two coherent laser beams [9]. 

---