1,2,3]Triazolo[4,5-b]porphyrins: New Building Blocks for Porphyrinic Materials (original) (raw)

There has been a strong interest in recent years in developing multiporphyrin arrays through covalent or noncovalent synthetic methodologies. The synthesis of porphyrinbased framework solids is also an active field. These multiporphyrinic systems are of interest because of their potential application in a range of areas, such as electronics, materials, catalysis, and medicine. In fact, porphyrins are one of the most attractive building blocks for supramolecular systems. They offer a variety of desirable features, such as rigid planar geometry, high stability, intense electronic absorption and emission, a small HOMO-LUMO energy gap, and the ability to tune their optical and redox properties by changing the metal center. During the past decade, various synthetic strategies have been developed to make multiporphyrin oligomers with linear, cyclic, and cross-linked geometries. An elegant and versatile method developed by Osuka and Shimidzu consists of a Ag I -promoted coupling reaction of meso-unsubstituted porphyrins. This method allows the preparation of porphyrin arrays in which the constituent porphyrins are connected directly at their meso positions without any extra linking atoms. By using this method, Osukas group prepared a wide range of covalently linked porphyrin arrays, including meso-meso-linked linear porphyrin arrays, two-dimensional windmill arrays, three-dimensional gridlike arrays, strapped meso-mesolinked diporphyrins, and cyclic arrays. Further oxidation of the meso-meso-linked porphyrin arrays under strong conditions (DDQ and Sc(OTf) 3 ) provides porphyrin tapes and sheets (DDQ = 2,3-dichloro-5,6-dicyano-1,4benzoquinone, OTf = trifluoromethanesulfonate). These fully conjugated porphyrin systems have planar structures and display drastically red-shifted absorption spectra that reach into the far-IR region, which reflects the extensive p conjugation over the molecules. Other very interesting linear, dendritic, and cyclic multiporphyrin systems, as well as organometallic porphyrin arrays have been published in recent years. All of these well-defined multiporphyrin arrangements are interesting for application such as molecular photonic and electronic wires. In particular, the cyclic porphyrin arrays are excellent light-harvesting antenna systems that mimic those found in the photosynthetic system. Herein we report the synthesis of the novel heterocyclic system [1,2,3]triazolo[4,5-b]porphyrin and its use in the preparation of porphyrinic dimers and pentamers. The 1,2,3triazoles are an important type of heterocyclic compounds and are being studied by many research groups because of their theoretical interest and synthetic usefulness. They also find numerous applications in industry, in medicine, and as agrochemicals. 1,2,3-Triazoles can be synthesized by many approaches; one method involves the reaction of sodium azide with alkenes bearing strongly electron-withdrawing substituents to afford N-unsubstituted 1H-1,2,3triazoles in good yields. We have considered a synthetic route involving the reaction of b-nitro-meso-tetraarylporphyrins with sodium azide; in these reactions the porphyrins should behave as nitroalkenes and the formation of triazoloporphyrins was anticipated. Indeed, the [1,2,3]triazolo[4,5b]porphyrins 2 were obtained in moderate to good yields from the reaction of nitroporphyrins 1 with NaN 3 . The alkylation and arylation of the triazole moiety allowed easy access to new porphyrinic materials with potentially interesting applications.