Phthalonitriles is a compound with a large conjugated system of 18 electrons, and its structure is very similar to the porphyrins widely present in nature. However, unlike porphyrin, phthalonitrile is a completely synthetic compound.
Naphthalonitriles can be regarded as a kind of phthalonitrile derivative, which is formed by combining four benzene rings on the four benzene rings around the phthalonitrile ring. There are twenty-four positions around the naphthalonitrile ring that can connect various types of substituents, so the conjugated system is larger than phthalonitrile. Naphthalonitrile is also a large π conjugated system with 18 electron composition, which conforms to the Huckel rule and is aromatic. Compared with phthalonitrile, due to the larger planar conjugated structure, the redox potential, photoconductivity and catalytic ability of naphthalonitrile are significantly improved, but the solubility of naphthalonitrile is also significantly reduced.
Figure 1. Planar structure of phthalonitrile and naphthalonitrile
Because each position on the benzene ring of the molecular structure can be substituted by a substituent, there are a wide variety of phthalonitrile and naphthalonitrile compounds. In addition, the photoelectric performance of phthalonitrile and naphthalonitrile is very excellent. Therefore, phthalonitrile and naphthalonitrile have very wide application value in the fields of photoelectric materials, electrochemistry, dyes, resins and biomedicine.
- Photoelectric materials: Various groups can be introduced around the phthalonitrile molecules, including polar groups and polarizable groups. The interaction between the molecules can keep the molecules oriented and ordered. At the same time, phthalonitrile also has a conjugated structure with a large π bond, which has a certain rigidity, which structurally ensures that the phthalonitrile molecules have liquid crystal properties. The study found that when the phthalocyanine is substituted with an alkyl group and the alkyl chain exceeds four carbons, the phthalonitrile has liquid crystal properties, and the stability of the liquid crystal phase increases with the increase of the carbon chain. Depending on the phthalonitrile ring substituents, columnar or discotic liquid crystals can be formed. Both peripherally substituted and non-peripheral alkyl chain substituted phthalonitriles can exhibit liquid crystal properties. Therefore, phthalonitrile can be used as a raw material for liquid crystal display materials, and thus used for making various liquid crystal display screens. Naphthalocyanine is also widely used in the field of photoelectric materials, especially in optical limiting materials. Since the optical limiting material can reduce the laser intensity to an acceptable range for human eyes, it can be used to protect the eyes from strong laser damage. The naphthalonitrile complex has a larger electron conjugation system and a larger absorption cross section than the cyanocyanine complex. At the same time, the absorption peak of the band of naphthalonitrile is significantly red-shifted than that of phthalonitrile, and the high light transmission window between the bands increases, which improves the light stability in the visible light range. Therefore, naphthalonitrile is an optical limiting material with good performance.
- Electrochemistry: Phthalonitrile has a wide range of applications in electrochemistry. Multi-walled carbon nanotubes with high electrical conductivity are common raw materials for the preparation of electrochemical composite materials, but their own high surface energy leads to the disadvantage of easy aggregation. Doping phthalonitrile in multi-walled carbon nanotubes can play a role in reducing surface energy, thereby improving the performance of multi-walled carbon nanotubes. For example, 3-APN/MWCNTs can be made by adding 3-aminophenoxyphthalonitrile (3-APN) to the multi-walled carbon nanotubes (MWCNTs) by solvent method. Then 3-APN/MWCNTs were added to polyarylene ether nitrile (PEN) to make a composite material 3-APN/MWCNTs/PEN. This composite material has good thermal stability, extremely high initial decomposition temperature and greater dielectric constant.
- Resin: Epoxy resin is a general-purpose thermosetting resin and has a pivotal position in commerce. Such resins have good thermal properties, mechanical properties and excellent electrical properties, but their low softening point limits their practicality. Based on this situation, phthalonitrile was added to the epoxy to form the copolymer biphenyl PN/epoxy. Compared with pure epoxy resin, as the content of biphenyl PN increases, the reactivity of biphenyl PN/ epoxy gradually increases, the softening point gradually increases, and the thermal stability increases. Therefore, the epoxy resin modified by phthalonitrile has better processability and greater application value.
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