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Journal of Fluorine Chemistry 221 (2019) 1–7

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Journal of Fluorine Chemistry 221 (2019) 1–7 Journal of Fluorine Chemistry 221 (2019) 1–7 Contents lists available at ScienceDirect Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor Steric and electronic effects of3 CF conformations in acene(CF3)n derivatives T Nicholas J. DeWeerda, Eric V. Bukovskya, Karlee P. Castroa, Igor V. Kuvychkoa, ⁎ ⁎ ⁎ Alexey A. Popovb, , Steven H. Straussa, , Olga V. Boltalinaa, a Department of Chemistry, Colorado State University, Fort Collins, CO 80523, United States b Liebniz Institute for Solid State and Materials Research (IFW), Dresden 01069, Germany ARTICLE INFO ABSTRACT Keywords: 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 Trifluoromethyl the 1/2 donor/acceptor co-crystal pyrene/(ANTH-6-1)2 are reported. These structures, along with extensive DFT PAH calculations on various conformations of ANTH-6-1 and 9,10-ANTH(CF3)2, suggest that the degree of bending of Acene the aromatic core in ANTH-6-1 and the DFT-predicted energies of ANTH-6-1 and 9,10-ANTH(CF3)2 are strongly Conformations 2 correlated with the relative eclipsed vs. staggered conformations of the CF3 groups attached to the central C(sp ) Bent pi systems carbon atoms (C9 and C10). Other literature X-ray and DFT-optimized structures of anthracene and pentacene X-ray structures 2 DFT calculations derivatives with n-C8F17 and/or CF3 substituents on the central C(sp ) 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 phenom- enon 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 inthe 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 properties3 ofPAH(CF )n derivatives, are briefly reviewed (PAH = polycyclic aromatic hydrocarbon). 1. Introduction [6,12,16,18–23] are listed in Table S1 (Supplementary data) and shown in Fig. S1. An interesting result is that the DFT-predicted HOMO–LUMO Perfluoroalkyl groups in general, and3 CF groups in particular, are gaps of PAH(CF3)n derivatives with n = 0, 2, 4, and 6 are very similar important substituents for both organic [1–11] and inorganic com- (Table S1) [2]. We can now confirm this prediction experimentally: the pounds [12–16]. They are much stronger electron-withdrawing groups lowest energy λmax values in the UV spectra of ANTH and ANTH-6-1 than F atoms when they are attached to C(sp2) atoms [2,17,18]. For only differ by 0.042 eV, as shown in Fig. S2[29]. example, the DFT-predicted electron affinities (EAs) of per- F atoms and CF3 groups have different, but equally problematic, fluoroanthracene (ANTH(F)10) and ANTH(CF3)10 are 1.84 [19,20] and solid-state disorder issues. An example of an F atom disorder is in the X- 4.01 eV [2], respectively. Even with only six CF3 groups, the experi- ray structure of 2,3,4-trifluorobenzo[b]triphenylene, shown in Fig. S3 mental EA of 2,3,6,7,9,10-ANTH(CF3)6 (hereinafter abbreviated ANTH- (Supplementary data) [30]. More extensive F/H disorders were ob- 6-1), at 2.81(2) eV [21], is 1 eV higher than ANTH(F)10 and is the same served in the X-ray structures of 2-fluoronaphthalene [31] and tetra- as the 2.78(6) eV EA of the common electron acceptor chloranil fluorophenanthrene [32]. It is well known that CF3 substituents are (2,3,4,5-tetrachlorobenzoquinone) [22]. The monotonic, approximately prone to 2-, 3-, and 4-fold rotational disorder about their CeCF3 bonds linear, increase in the electron affinities (EAs) of PAH(CF3)n derivatives in solid state structures [33]. A search of the Cambridge Structural as n increases, and the dependence of the magnitude of the slopes of n Database in December 2018 determined that there are nearly 40,000 vs. EA plots on the size of the polycyclic aromatic hydrocarbon (PAH) published X-ray structures of organic molecules with at least one CF3 core, have been studied by Sun et al. and by Boltalina, Strauss et al. group, and of these more than 20,000 exhibit rotational or positional [2,17,21,23–28]. Selected data and figures adapted from refs. CF3 disorder (see Supplementary data for a discussion of how the CSD ⁎ Corresponding authors. E-mail addresses: [email protected] (A.A. Popov), [email protected] (S.H. Strauss), [email protected] (O.V. Boltalina). https://doi.org/10.1016/j.jfluchem.2019.02.010 Received 21 December 2018; Received in revised form 26 February 2019; Accepted 26 February 2019 Available online 28 February 2019 0022-1139/ © 2019 Elsevier B.V. All rights reserved. N.J. DeWeerd, et al. Journal of Fluorine Chemistry 221 (2019) 1–7 was searched). This is not surprising in view of the fact that barriers to aromatic core have been bent away from planarity at the ANTH rotation of CF3 groups in solids are small, and the relative energies of C9⋯C10 hinge or the PENT C6⋯C13 hinge (the numbering schemes for different orientation minima can be even smaller. For example, the ANTH and PENT derivatives are shown in the Supplementary data barrier to rotation of the CF3 group in crystalline 3-phenanthrene(CF3), PDF). Defined in this way, θ is essentially the same as the dihedral angle which exhibits 2-fold rotational disorder [34], is 11.5(7) kJ mol−1 [35]. made by the two halves of the aromatic cores, but does not suffer from In the isolated molecule, the DFT-predicted barrier is only 1.7 kJ mol−1 the inclusion of the twisting of the core that occurs in some derivatives. [34]. The bend angle θ is 7.4° in crystalline ANTH-6-1 and 13.4° for the Additional examples of 2-fold disorder are: (i) two CF3 groups in the ANTH-6-1 molecules in co-crystalline PYRN/(ANTH-6-1)2. structure of C70(CF3)10 exhibit 2-fold rotational disorder [36]; (ii) four There are five related X-ray structures of ANTH derivatives: of the CF3 groups in the structure of Sc3N@(C80-Ih(7))(CF3)14 exhibit 2- 2,6,9,10-ANTH(n-C8F17)4, shown in Fig. S13, with θ = 0.0° [42]; 9,10- fold rotational disorder (three are 50% staggered and 50% eclipsed ANTH(CF3)(n-C8F17), shown in Fig. S14, and 9,10-ANTH(CF3)(n-C6F13), with respect to the cage CeC bonds that radiate from the cage C atom to both with θ = 18.0° [4]; 2-phenylethynyl-9,10-ANTH(CF3)2, with which they are attached, and one is 89% staggered and 11% eclipsed) θ = 14.6° [4]; and 2,6-bis((4-methoxy-phenyl)ethynyl)-9,10-ANTH [37]; and (iii) the CF3 group in the structure of [4′-CF3BzPy][Ni(mnt)2] (CF3)2, shown in Fig. S15, with θ = 0.0° [4]. In addition, there are two is 2-fold disordered in the high-temperature phase but is ordered in the related X-ray structures of 6,13-PENT(CF3)2. The acene bend angle θ is low-temperature phase (4′-CF3BzPy = 1-(4′-(trifluoromethyl)benzyl) 0.0° in the P21/c polymorph [43] and 16.8° in the P21/n polymorph 2− pyridinium; mnt = maleonitriledithiolate) [38]. [44]. The relative conformations of the central CF3 and/or n-RF groups An example of a CF3 group with 3-fold disorder is in the structure of (i.e., the substituents on C9 and C10 in the ANTH derivatives or on C6 1,3,6,8,10-perylene(CF3)5 (each F atom was split into three positions and C13 in the PENT derivatives) can be defined by another angle, φ, with occupancies of 50%, 28%, and 22%) [21]. This structure, which is which is also defined in Fig. S12. 3The CF and/or n-RF substituents are shown in Fig. S4, also shows the tendency of CF3 substituents to affect staggered when φ = 60° and eclipsed when φ = 0°. the degree of planarity of the aromatic cores in some PAH(CF3)n deri- Seven of these nine X-ray structures are shown with similar or- vatives (PAH = polycyclic aromatic hydrocarbon; see ref. [26] for other ientations in Fig. 1. The correlation between θ and the average of the examples). The CF3 groups on C1 and C7 (bay positions) cause sig- three individual values of φ is readily apparent: the acenes are sig- nificant non-planarity of the 16-membered perylene aromatic core (the nificantly bent, with θ ≥ 14°, when the 3central CF and/or n-RF sub- dihedral angle between the two nearly-planar naphthalene moieties is stituents are eclipsed, with φ = 0, and are not bent with respect to the 2 2 30°). Examples of compounds in which one or more CF3 groups exhibit central C(sp ) ) hinge (i.e., θ = 0.0°) when the central groups are 4-fold disorder and were modeled as 12 partial F atoms with site oc- staggered.
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