Steric and electronic effects of CF3 conformations in acene(CF3)n derivatives

Abstract

The X-ray and DFT-optimized structures of the electron acceptor 2,3,6,7,9,10-anthra-cene(CF3)6 (ANTH-6-1) and the 1/2 donor/acceptor co-crystal pyrene/(ANTH-6-1)2 are reported. These structures, along with extensive DFT calculations on various conformations of ANTH-6-1 and 9,10-ANTH(CF3)2, suggest that the degree of bending of the aromatic core in ANTH-6-1 and the DFT-predicted energies of ANTH-6-1 and 9,10-ANTH(CF3)2 are strongly correlated with the relative eclipsed vs. staggered conformations of the CF3 groups attached to the central C(sp2) carbon atoms (C9 and C10). Other literature X-ray and DFT-optimized structures of anthracene and pentacene derivatives with n-C8F17 and/or CF3 substituents on the central C(sp2) atoms are analyzed and show (i) that the eclipsed vs. staggered correlation with the degree of bending of the aromatic cores may be a general phenomenon and (ii) that molecules of this type are probably more stable when the acene core is bent than when it is planar. DFT predicted modest changes in the electron affinities of ANTH, 9,10-ANTH(CF3)2, and ANTH-6-1 as the aromatic cores are bent and the relative eclipsed vs. staggered conformations of the central CF3 groups are changed are also reported. This is the first time that DFT calculations have shown that uniform changes in the conformations of CF3 groups may affect the EAs of some CF3-substituted aromatic molecules. In addition, the tendency of CF3 substituents to exhibit 2-, 3-, and 4-fold rotational disorder in a variety of molecules, and to affect the degree of planarity and electronic properties of PAH(CF3)n derivatives, are briefly reviewed (PAH = polycyclic aromatic hydrocarbon).