Fluorinated building blocks comprise a very important family of fluorine-containing compounds. fluorine atoms possess the largest electronegativity (Fig 1) and the smallest atomic radius except hydrogen, which determines that they have some unique physicochemical properties. fluorine is the most abundant halogen group element in the earth, but it mainly exists in the form of inorganic salts (CaF2, etc.). the content of organic fluorides is very small in nature, and almost all the organic fluorides are artificially synthesized now. Because of the large bond strength of the C-F bond, the electron cloud distribution is not easily affected by the environment, endowing fluorine with weak polarization ability and low reactivity with other substances.
Fig.1. Comparation of electronegativity and bond strength between fluorine and other elements.
Fluorinated building blocks are almost not water-soluble, because excellent stability of the C-F bond and the interaction between fluorocarbon molecules can be negligible, so the fluorinated building blocks have very low surface energy and great waterproof and oil-proof property. Compared with non-fluorine analogues, the boiling point of fluorine-containing organic compounds is usually lower owing to the poor polarization ability and weak intermolecular force. Given that fluorinated substances possess some unique physicochemical properties, such as outstanding physical and chemical stability and unique biological activity, which determine their wide application in materials, medicine, and energy. According to statistics, 20-25% of pharmaceuticals and 30% of pesticide molecules worldwide contain at least one fluorine atom, and nearly half of high-end materials are currently fluorinated.
Fluorinated building blocks are very important for life science. In pharmaceutical chemistry, the introduction of fluorine atoms can regulate the lipophilicity, pKa, conformation and bioavailability of drug molecules. fluorine atoms have a strong ability to absorb electrons, so they can reduce the electron cloud density of the linked aromatic rings and enhance the antioxidant ability of compounds, which is usually used to improve metabolic stability. Moreover, the isotope 18F is increasingly used in PET imaging techniques due to its suitable half-life period (Fig 2).
Fig.2. Positron emission tomography (PET) imaging with fluorinated compounds (J. Med. Chem. 2016, 59, 3705-3718.)
Owing to the weak interaction between water molecules and fluorine atoms, Fluorinated substances can be utilized as a natural waterproof barrier to prevent water molecules soaking inside the material (Fig 3).
Fig.3. Surface fabrication by photopolymerization of fluorinated materials.
(Polym. Chem. 2018, 9, 4161-4171.)
Various fluorinated commercialized products such as Halar, Lumiflon and Tefzel have excellent properties such as high temperature resistance, cold resistance, acid and alkali resistance, which can be widely used in extreme conditions. Related high-end fluorinated polymers have been synthetized for further application (Fig 4).
Fig.4. Copolymerization of chlorotrifluoroethylene (CTFE) and vinyl ethers (VEs). (J. Am. Chem. Soc. 2020, 142, 7108-7115.)