Fluoromaticity: The Molecular Orbital Contributions of Fluorine Substituents to the π-Systems of Aromatic Rings

Abstract

The addition of fluorine atoms to an aromatic ring brings about an additional set of π-bonding and antibonding orbitals culminating after the addition of the sixth fluorine with a new set of π-aromatic-like orbitals that affect the molecule in a way that we will refer to hereafter as "fluoromaticity". Depending on the number and position of the fluorine atoms, the contributed πorbitals can even further stabilize the ring leading to smaller bond lengths within the ring and higher resistance to addition reactions. This added ring stability partially explains the high thermostability and chemical resistance found in polymers containing fluorinated aromatics in their architecture. A similar molecular orbital effect is seen with the addition of other halogen atoms to aromatic rings, though to a much smaller degree and not resulting in the additional ring stability.

Isomer-Dependent Electron Affinities of Fluorophenyl Radicals, •C6H5–xFx (2 ≤ x ≤ 4)

“…Lying below the HOMO are the close-lying π orbitals, which are labeled π 2 and π 3 (which correlate to the π 2 and π 3 orbitals in the phenyl radical and the degenerate e 1g orbitals of benzene), which are C–C bonding and C–F antibonding. As described by Fuhrer et al, the π orbitals that are C–F bonding lie much lower in energy, and every F substitution is accompanied by an additional doubly occupied π orbital. Strictly speaking, the orbitals labeled π 2 and π 3 in Figure are actually π 4 and π 5 , but for the sake of a simple comparison between the di-, tri-, and tetrafluorophenide anions, we will adhere to labeling the frontier orbitals as shown.…”

“…The phenyl radical is also an important reactant in astrochemistry. 11−13 The reactivity 14 and electronic structures 15 of aromatic rings are known to be strongly influenced by substituents on the ring. For example, a fluorine substituent on benzene contributes a lone pair to resonance stabilization but is highly electronegative.…”

Anion Photoelectron Imaging Spectroscopy of C6HF5–, C6F6–, and the Absence of C6H2F4–

“…Fuhrer et al recently described the evolution of the π systems in fluorinated benzenes. 65 Each F-substitution on benzene is associated with an additional C−F π bonding orbital that lies energetically lower in energy than the other six π orbitals (including the C−F π* orbital), the latter of which correlates closely to the six π orbitals of unsubstituted benzene. In forming a negative ion, the excess electron typically occupies the LUMO of the neutral precursor.…”

Intrinsic Charge Transport for DTzTI-Based All-Acceptor Homopolymer n-Type Organic Semiconductors: Roles of Conjugation Length and Orbital Delocalization

“…Even though C–H and C–F bonds have about identical lengths, C–F bonds are more stable and have stronger electron-absorbing capabilities, making them very desired in molecular design . We switched the hydrogen atom to a fluorine atom to increase the molecule’s local orbital delocalization . We substituted a selenium atom for the sulfur atom since it has more electrons and is better suited for electron delocalization .…”

“…27 We switched the hydrogen atom to a fluorine atom to increase the molecule's local orbital delocalization. 28 We substituted a selenium atom for the sulfur atom since it has more electrons and is better suited for electron delocalization. 29 These improvements are made to improve the electron mobility and ambient stability of DTzTI-based polymers as well.…”