spectroelectro-chemical methods.101a,103 The starting 4,5-difluorophthalonitrile can be prepared using the aromatic nucleophilic substitution reaction between commercially available 4,5- dichlorophthalonitrile and potassium fluoride, or by a palladium-catalyzed reaction between 1,2- dibromo-4,5-difluorobenzene and Zn(CN)2.39,101 Metal-free and transition-metal 1,2,3,4,8,9,10, 11,15,16,17,18,22, 23,24,25-hexadecafluoro phthalocyanines, prepared in the normal way using commercially available 3,4,5,6-tetrafluorophthalonitrile, have been intensively studied during last decade because of their potential application as low-voltage thin-film transistors.104 Unlike unsubstituted phthalocyanine, these compounds are soluble to some extent in a variety of organic solvents such as DMF, chlorobenzene, acetone, and chloroform.
One of the first reports on the preparation of chlorinated copper phthalocyanines with different degrees of chlorination was published in 1959.105 It was found that the copper complexes of 1,8(11),15(18),22(25)-tetrachloro-, 2,9(10),16(17),23(24)-tetrachloro-, 1,4,8,11,15, 18,22,25-octachloro-, 1,2,8,9(10,11),15,16(17,18),22,23(24,25)-octachloro-, 2,3,9,10,16,17,23, 24-octachloro-, 1,2(3),4,8,9(10),11,15,16(17),18,22, 23(24),25-dodecachloro-, 1,2,3,8(11),9,10, 15(18),16,17,22(25),23,24-dodecachloro-, and hexadecachlorophthalocyanine can be prepared in excellent yields from the corresponding chlorinated phthalic anhydrides, urea, and copper salt. Again, when comparisons were possible, the yields of the less sterically crowded polychloro- containing phthalocyanines were higher compared with those with chlorine atoms located at the a-positions.5,105 Another interesting observation was that formation of hexadecachlorophthalocyanine from tetrachlorophthalic anhydride and urea leads to the partial dechlorination of the phthalocyanine core. The pure hexadecachlorophthalocyanine could be prepared from the commercially available tetrachlorophthalonitrile, avoiding urea and other potential nucleophiles in the reaction mixture.106,107 After the initial report, a large number of the other chlorinated phthalocyanines were published. In the majority of cases, chlorine-containing phthalonitriles were used as the key precursors in the cyclotetramerization reaction. It has been recently shown108 that yields of chlorinated transition-metal phthalocyanines from chlorinated anhydrides can be improved by using an ionic liquid as the reaction media.
In general, introduction of chlorine atoms into the phthalocyanine core increases the first oxidation potential of the respective phthalocyanine,109 and changes the color of the target compounds from blue to green. Indeed, commercial green-colored phthalocyanine-based pigments usually contain 14-15 chlorine atoms.110 The position of long wavelength Q-band in UV-vis spectra of chlorinated phthalocyanines depends on number and position of chlorine atoms in macrocycle – it is red shifted in the case of substituents at a-position, especially in polysubstituted compounds (Figure 4).
Use of chloromaleonitrile111 for preparation of the methyl 2,3,4-trichloro-5,6- dicyanobenzoate is a rare example of the Diels-Alder reaction being used for synthesis of precursors for phthalocyanines with electron-withdrawing groups.79 The interaction of commercially available 1,2,3,4-tetrachloro-5,5-dimethoxycyclopenta-1,3-diene and chloromaleonitrile leads to formation of a bicyclic intermediate, which can be aromatized in two
