Pinpoint-fluorinated polycyclic aromatic hydrocarbons (F-PAHs) and their heteroaromatic analogs: syntheses, reactivities, and properties

Pinpoint-fluorinated polycyclic aromatic hydrocarbons (F-PAHs) and their heteroaromatic analogs, which are regioselectively substituted by one or two fluorine atoms, were systematically synthesized by Friedel-Crafts cyclization of fluoroalkenes (2-trifluoromethyl-1-alkenes, 1,1-difluoro-1-alkenes, 1,1,2-trifluoro-1-alkenes, and 1,1-difluoroallenes) that were Al(III)-mediated, Pd(II)-catalyzed, and In(III)-catalyzed. The key feature of these reactions is ring closures that proceed via alpha-fluorine-stabilized CF2 cations and related species. Using an array of synthesized F-PAHs and their heteroaromatic analogs, their chemical reactivities and physical properties were investigated. (i) The reactivities of F-PAHs and their heteroaromatic analogs were used for further ring construction. (ii) The crystal structures of F-PAHs revealed that the planarity of their pi-conjugated systems was not compromised because of the low steric demand of fluorine atoms. (iii) The introduction of fluorine atom(s) into PAH molecules increased their solubility in organic solvents, which was best exemplified by the high solubility of 6-fluoropicene (5.3 wt%) and 6,7-difluoropicene (5.4 wt%) in THF. (iv) One of the F-PAHs, 13-fluoropicene exhibited p-type semiconducting behavior (mobility 6.6 x 10-2 cm2/Vs by vacuum deposition; 1.3 x 10-4 cm2/Vs by spin casting using toluene as a solvent). (v) The HOMO-LUMO energy gaps of the F-PAHs were smaller than those of the corresponding fluorine-free PAH (i.e. picene) by 0.02 to 0.26 eV, and the HOMO and LUMO energy levels were lowered by 0.10 to 0.22 and 0.12 to 0.41 eV, respectively. F-PAHs and their heteroaromatic analogs were synthesized via alpha-fluorine-stabilized CF2 cations and related species. (i) The ring construction based on F-PAHs and heteroaromatic analogs, (ii) the planarity of F-PAHs, (iii) the solubility of F-PAHs in organic solvents, (iv) the semiconducting behavior of F-PAHs, and (v) the HOMO-LUMO energy gaps and the HOMO and LUMO energy levels of F-PAHs are also presented.