Journal of Fluorine Chemistry 221 (2019) 1–7

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Journal of Fluorine Chemistry

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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 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 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 significant 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. A plot of θ vs. φ for all nine structures is shown in Fig. 2. For cupancies that vary from 10% to 50% are 2,4,6-C6H2(CF3)2(C(CF3)2OH) comparison, an acene bend angle of 18° is approximately 50% of the [39], 2-chloro-1,3-(2,6-diisopropylphenyl)-4,5-dihydro-1H-imidazol-3- 36.6° bend angle in the X-ray structure of 9,10-dihydroanthracene, in ium tetrakis-(3,5-trifluoromethyl-phenyl)borate [40], and 3-bromo- which C9 and C10 are sp3 hybridized (see Fig. S16) [45]. The corre- methyl-2-trifluoromethylchromone [41]. sponding value of θ in the X-ray structure 6,13-dihydropentacene is In this paper we report the structure of ANTH-6-1 and the 1/2 30.0° [46]. The structures of the two polymorphs of 6,13-PENT(CF3)2 donor/acceptor co-crystal structure of it with pyrene (PYRN), viz. and 6,13-dihydropentacene are compared in Fig. S17. The θ and φ PYRN/(ANTH-6-1)2. The CF3 groups are not disordered in ANTH-6-1 values for all of the X-ray and DFT structures discussed in this paper are and only one of the six unique CF3 groups in PYRN/(ANTH-6-1)2 was listed in Table S3. This includes the X-ray structure of 9,10-ANTH disordered and split into two sets of F atoms with 75% and 25% oc- (CF2C6F5)2 [47], with θ = 19.5° and φ = 5.1°, which certainly supports cupancies. This has allowed us to observe that the conformations of the the correlation, but this structure is not included in Figs. 1 and 2 be- CF3 groups on C9 and C10 are correlated with the degree of aromatic cause the C6F5 moiety is sterically more-demanding than an F atom or a core bending with respect to a hypothetical hinge connecting C9 and CF2 moiety. Note that when θ = 0° the acene may exhibit a small de- C10. New DFT calculations and an analysis of published experimental gree of nonplanarity due to twisting of the aromatic core. and DFT-predicted structures of ANTH and PENT derivatives with CF3, Crystal packing forces in general, and π–π interactions in particular, 2 n-C6F13, and n-C8F17 groups on the central C(sp ) atoms indicate that can affect the molecular structures of polycyclic aromatic hydrocarbon this is a general phenomenon. Furthermore, we report that bending of derivatives. Therefore, the X-ray structures discussed above cannot be the ANTH core has an effect on the DFT relative energies and EAs of used to determine if crystal packing forces are responsible for a parti- ANTH-6-1 and 9,10-ANTH(CF3)2, with the lowest energy and highest cular set of θ or φ values. In other words, could a θ = 0°/φ = 60° EA found when the central CF3 groups are eclipsed and the aromatic molecular structure be lower in energy than a θ ≈ 18°/φ ≈ 0° structure, core is bent by ca. 17°. yet result in a θ ≈ 18°/φ ≈ 0° crystal structure due to the compensating energetics of crystal packing forces? DFT-predicted relative energies of 2. Results and discussion isolated molecules with various values of θ and φ can answer this question. 2.1. X-ray structures 2.2. DFT-predicted structures and their relative energies and electron The structures of ANTH-6-1 and PYRN/(ANTH-6-1)2 were de- affinities (EAs) termined by single-crystal X-ray diffraction. Data collection and refinement parameters are listed in Table S2. A thermal ellipsoid plotof The changes in DFT relative energies and EAs discussed below are ANTH-6-1 is shown in Fig. S5. Drawings of the π–π overlap and the small, ≤11 kJ mol−1 and ≤50 meV, respectively. However, the trends packing of ANTH-6-1 molecules in the structure of ANTH-6-1 are shown in these values as the acene core is bent, not necessarily the magnitudes in Figs. S6 and S7. A thermal ellipsoid plot of the ANTH-6-1/PYRN/ of the changes, are what we believe are meaningful. DFT calculations ANTH-6-1 sandwich in the co-crystal structure of PYRN/(ANTH-6-1)2 is have been used by several authors (including the authors of this paper) shown in Fig. S10, and the packing of the sandwiches in the unit cell in to predict relative energies and EAs in PAHs and their derivatives, and the structure of PYRN/(ANTH-6-1)2 is shown in Fig. S11. in some cases the effects of different functionals and/or basis sets have In this paper the steric figure of merit of greatest interest for 9,10- also been reported (see Supplementary data for several leading refer- ANTH(RF)2 and 6,13-PENT(RF)2 derivatives (with or without other ences). However, there are few cases where experimental relative en- substituents) is the bend angle, θ, which is defined in Fig. S12 ergies or EAs for a series of closely-related molecules are known, pre- (RF = CF3, n-C6F13, or n-C8F17). It is the angle that the two halves of the cluding comparisons with DFT predicted values. Nevertheless, two

2 N.J. DeWeerd, et al. Journal of Fluorine Chemistry 221 (2019) 1–7

Fig. 1. X-ray structures of (top to bottom) 9,10-ANTH(CF3)(n- C8F17) (A, ref. [4]), P21/n 6,13-PENT(CF3)2 (B, ref. [44]), 2-phe-

nylethynyl-9,10-ANTH(CF3)2 (C, ref. [4]), ANTH-6-1 in PYRN/

(ANTH-6-1)2 (D, this work; the C–F bonds of the minor component of the disordered CF3 group on C10 are shown with dashed lines), ANTH-6-1 (E, this work), 2,6-bis((4-methoxyphenyl)ethynyl)-

9,10-ANTH(CF3)2 (F, ref. [4]), and P21/c 6,13-PENT(CF3)2 (G, ref. [43]). Fluorine atoms are highlighted in yellow. The bend angles, θ, defined in Fig. S8 and discussed in the text, are shown nextto each structure. The small red dots connected with dashed lines are the C–C and C⋯C centroids that define θ (some are hidden from view). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

cases where the reader can judge the reliability of the DFT methods 27 meV (ca. 2%) as the core is bent by 17°. Interestingly, the minimum used in the present study are (i) the good agreement between the small EA, 1.375 eV, occurs when φ = 40° and θ = 6.8° (relative energy 8.5 kJ −1 changes in experimental EAs for two series of PAH(RF)n compounds and mol ). The DFT EA of unsubstituted ANTH increased by 48 meV (from the corresponding DFT predicted values [17] and (ii) the good agree- 0.397 to 0.445 eV), nearly 2 times as much as 9,10-ANTH(CF3)2, as the ment between the experimental and DFT EAs for ANTH-6-1 (2.81(2) vs. core was bent by 17°, as shown in Fig. S18. To our knowledge, the 2.804 eV), which will be discussed later in this section. reason why bending the core of an acene such as ANTH would raise its DFT relative energies of ANTH, with θ = 0–24°, and 9,10-ANTH electron affinity has not been addressed in the literature, and further (CF3)2, with φ = 0° and θ = 0–30°, are shown as plots in Fig. 3 (the study of this phenomenon was beyond the scope of this investigation. energies were sampled in 1° increments of θ). Not surprisingly, the Fig. S18 also shows the relative energies and vertical EAs of 9,10- lowest energy structure of ANTH has θ = 0°. On the other hand, the ANTH(CF3)2 as φ is changed without the CF3 groups constrained to lowest energy structure of 9,10-ANTH(CF3)2 has θ = 17°. The relative their φ = 0° positions. This is the first time that DFT calculations have −1 energy of 9,10-ANTH(CF3)2 with θ = 0° is 10.5 kJ mol . The energy of shown that uniform changes in the conformations of CF3 groups might −1 unsubstituted ANTH with θ = 17° is 18.0 kJ mol . Near θ = 17°, the affect the EAs of some CF3-substituted aromatic molecules, although the energy of ANTH is changing almost linearly with a slope of ca. changes would probably be rather small. −1 −1 2.3 kJ mol deg . A previous DFT optimization of 9,10-ANTH(CF3)2 The intramolecular interactions that lead to bending of the core are reported θ = 14.6° and φ = 0.3° [2]. Similarly, the DFT-optimized subtle. The F⋯C and F⋯H distances for three conformers of 9,10-ANTH structure of 6,13-PENT(CF3)2, with eclipsed CF3 groups and θ = 19.1°, (CF3)2 are shown in Fig. S19. These are (i) the lowest energy structure, −1 was found to be 9.0 kJ mol more stable than the structure with with eclipsed CF3 groups and θ = 17°, (ii) the structure with eclipsed −1 staggered CF3 groups and θ = 0.0° [44]. CF3 groups but with θ = 0° (relative energy 10.3 kJ mol ), and (iii) Fig. 4 shows plots of DFT relative energies and vertical EAs for 9,10- the structure with staggered CF3 groups and θ = 0° (relative energy −1 ANTH(CF3)2 as φ is varied from 0 to 60° with one CF3 group con- 9.1 kJ mol ). The F⋯C interatomic distances range from 2.71 to strained to its position when φ = 0°. As in the results shown in Fig. 3, 3.38 Å (the sum of van der Waals radii for F and C is 3.17 Å [48]) and the molecule is most stable when φ = 0° and θ = 17° and least stable the F⋯H interatomic distances range from 1.94 to 2.78 Å (the sum of −1 when φ = 60° and θ = 0° (relative energy 10.5 kJ mol ). The EAs also van der Waals radii for F and H is 2.67 Å). It is clear that when the CF3 vary as φ is varied, with a minimum value at φ = 40°. The EAs for groups are eclipsed and oriented as in Fig. S19, the repulsive intera- φ = 60° and 0° are 1.385 and 1.412 eV, respectively, a increase of tomic interactions involving F91 and F93 on one CF3 group and F101

3 N.J. DeWeerd, et al. Journal of Fluorine Chemistry 221 (2019) 1–7

Fig. 4. DFT relative energies and vertical electron affinities (EAs) for 9,10-

ANTH(CF3)2 as a function of the relative conformation angle φ, with one CF3 group constrained to its position at φ = 0°. The molecule is most stable when φ = 0° and θ = 17° and least stable when φ = 60° and θ = 0° (relative energy 10.5 kJ mol−1). The EAs for φ = 0° and 60° are 1.412 and 1.385 eV, respectively. The minimum EA, 1.375 eV, occurs when φ = 40° and θ = 6.8°, at which point the relative energy is 8.5 kJ mol−1. Fig. 2. Plot of acene bend angle θ vs. the average F–C⋯C–F torsion angle (the relative conformation angle φ) in nine acene X-ray structures with CF3 and/or 2 n-RF groups on the acene central C(sp ) atoms. The two 9,10-ANTH(CF3)(RF) and F103 on the other CF3 group act on the same side of the ANTH compounds, with RF = n-C6F13 and n-C8F17, have the same values of θ and φ, as plane, and the result is that the ANTH core is bent. When the CF3 groups do the three compounds located at the bottom of the plot. For 2-alkynyl-9,10- are eclipsed but oriented as in Fig. S19, F91 and F93 are on opposite ANTH(CF3)2, the alkynyl substituent is phenylethynyl. For 2,6-(alkynyl)2-9,10- sides of the ANTH plane, as are F101 and F103, the repulsive interac- ANTH(CF ) , the alkynyl substituent is (4-methoxyphenyl)ethynyl. 3 2 tions are offset and the result is that the ANTH core is planar. Whenthe CF3 groups are staggered, the repulsive interactions are always offset and the core is planar regardless of the way that the pair of CF3 groups is oriented. These DFT structures are compared with the structures of the ANTH-6-1 molecules in the X-ray structures of ANTH-6-1 and PYRN/(ANTH-6-1)2 in Figs. S20 and S21. The DFT structures of ANTH-6-1 were optimized (i) starting with semi-eclipsed CF3 groups on C9 and C10 (resulting in eclipsed optimized structure a; the orthogonal coordinates of ANTH-6-1 molecule in the structure of PYRN/(ANTH-6-1)2 was used as the starting point) and (ii) starting with nearly staggered CF3 groups on C9 and C10 (resulting in staggered optimized structure b). The optimized structure ANTH-6-1- a has θ = 17.5° and φ = 0.7°. The optimized structure ANTH-6-1-b has θ = 0.0° and φ = 60.0°, and is 5.7 kJ mol−1 higher in energy than ANTH-6-1-a (i.e., ANTH-6-1-b is an energetic local minimum). A previous DFT optimization of ANTH-6-1 reported θ = 13.9° and φ = 0.4° [2]. It is now clear that the stable molecular structure of ANTH-6-1 (i.e., its intrinsic, gas-phase structure) probably has a significantly bent ANTH core. As stated in the Introduction, we reported the 2.81(2) eV gas-phase EA of ANTH-6-1 in 2013, and we can now say with some confidence that this is most likely the EA of a significantly bent aromatic molecule. We calculated the vertical and adiabatic EAs with our DFT code, and the results are 2.687 and 2.804 eV, respectively, for ANTH-6-1-a and 2.644 and 2.754 eV, respectively, for ANTH-6-1-b. Conceptually, bending the ANTH core from θ = 0° to θ = 17° raised the DFT adiabatic and vertical EA values by 50 and 43 meV, respectively. (The previously reported DFT vertical and adiabatic EAs of ANTH-6-1, with θ = 14.6°, are 2.46 and 2.73 eV, respectively [2]). Note that a Fig. 3. Plots of DFT relative energies of ANTH and 9-10-ANTH(CF3)2 with eclipsed CF3 groups vs. the aromatic core bend angle θ. The relative energy of difference of 50 mV in E1/2 values for a pair of redox reagents can unsubstituted ANTH is lowest when the core is planar and 18.0 kJ mol−1 when change the equilibrium constant for a redox reaction by a factor of 7.

θ = 17°. In contrast, the relative energy of 9,10-ANTH(CF3)2 with eclipsed CF3 The structures of most of the X-ray and DFT structures discussed groups is lowest when θ = 17° and is 10.5 kJ mol−1 when θ = 0°. above are shown in Fig. 5. The orientation of each molecule is looking

4 N.J. DeWeerd, et al. Journal of Fluorine Chemistry 221 (2019) 1–7

bent structure. This possibility would probably have been counter- intuitive to most chemists before this work. It is likely that most chemists would have assumed the opposite, that crystal packing forces were responsible for orienting the central CF3 or n-RF groups away from the assumed stable staggered conformation, thereby forcing an in- trinsically planar molecule of this type to be bent in the solid state. Of course it is also possible that crystal packing forces cause an in- trinsically planar molecule of this type to be non-planar in the solid state, and the non-planarity secondarily causes the central CF3 or n-RF groups to be eclipsed or semi-eclipsed. This "chicken-and-egg" argument cannot be resolved by simply examining the structures. What can be unambiguously concluded from an examination of the structures discussed in this paper is that the conformations of the central CF3 or n-RF groups and the degree of planarity of the acene core are strongly correlated.

2.3. Structures of other 9- and 9,10-ANTH derivatives

Several structures of ANTH derivatives with one or two bulky tri- podal substituents on C9 and/or C10 and bent aromatic cores have been reported, including 9-ANTH(t-Bu) (θ = 17.3°, see Fig. S22) [49], 1,8- Cl2-10-ANTH(t-Bu) (θ = 17.8°) [50], 9,10-ANTH(SiMe3)2 (nearly- eclipsed, θ = 15.6°; see Fig. S23) [51], and 9,10-ANTH(Br)(P(O)Ph2) (θ = 14.6°) [52]. The first three of these are included in Table S3. The Gaussian 94 (3-21 G) predicted barrier to rotation of the t-Bu group in an isolated 9-ANTH(t-Bu) molecule is 4.7 kJ mol−1 if a concerted t-Bu rotation and butterfly-like inversion of the bent ANTH core is con- sidered [53]. In the solid state, the barrier to rotation, determined by solid-state 13C NMR spectroscopy, is much higher, 63(3) kJ mol−1 [53]. Derivatives with sterically more innocent ligands in C9 and C10 have planar aromatic cores with θ values of 0.0–0.5°. Examples are 9,10-ANTH(Me)2 [54], 9,10-ANTH(NO2)2 [55], and 2,6-(OSO2CF3)2- 9,10-ANTH(CN)2 [56]. Drawings of the latter two structures are shown in Fig. S24.

3. Conclusions

The X-ray structures of ANTH-6-1 and PYRN/(ANTH-6-1)2, extensive DFT calculations, and a review of previously published structures of ANTH and PENT derivatives with central CF3 or n-RF groups show that bending their aromatic cores away from planarity lowers their energy relative to the corresponding hypothetical planar structures. Notwithstanding that the differences in DFT energies are small, we propose that molecules of this type are probably bent in the gas phase.

Fig. 5. DFT and X-ray structures of ANTH(X)n and PENT(X)n derivatives with n- (They may be bent in solution as well, but the relative energies of these 2 C8F17 and/or CF3 groups bonded to the central C(sp ) atoms. The abbreviations molecules in solution were not calculated in this study, so this remains used in this figure are RF = n-C7F15 and R = (4-methoxyphenyl)ethynyl. The an open question.) The degree of bending is strongly correlated with the structures not explicitly labelled DFT are single-crystal X-ray structures. DFT relative conformations of the central CF3 or n-RF groups as they vary and P21/n 6,13-PENT(CF3)2 are from ref. [44], 9,10-ANTH(CF3)-(C8F17) and from eclipsed (most stable, most bent) to staggered (always planar). 2,6-R2-9,10-ANTH(CF3)2 are from ref. [4], DFT 9,10-ANTH(CF3)2 is from ref. Since bending unsubstituted anthracene from planarity increases its [2], 2,6,9,10-ANTH(n-C8F17)4 is from ref. [42], P21/c 6,13-PENT(CF3)2 is from energy, it is the intramolecular steric strain of the central eclipsed or ref. [43], and the other structures are from this work. The acene bend angles θ semi-eclipsed CF3 or n-RF groups that causes the bending (i.e., the are shown next to each structure. eclipsed conformation, with two F atoms on each CF3 or n-RF group on the same side of the acene core, is the most stable conformation). We down the F3C⋯CF3,F3C⋯CF2RF, or F2(RF)C⋯CF2RF, direction, so that propose that crystal packing forces may not be responsible for bending the relative conformations of the central CF3 and/or CF2RF groups can the aromatic core in these molecules when bent structures like 9,10- be readily seen. As discussed above, the compounds with θ ≥ 13.5° ANTH(CF3)(n-C8F17), the P21/n polymorph of 6,13-PENT(CF3)2, and have eclipsed or nearly eclipsed CF3 or CF2RF groups, the compound PYRN/(ANTH-6-1)2 are observed in the solid state. In fact, the opposite with θ = 7.4° has semi-eclipsed CF3 groups, and all of the compounds may be true: intermolecular crystal packing forces would be responsible with θ < 7.4° have almost perfectly staggered CF3 or CF2RF groups. for counteracting the intrinsic stability of bent molecules of this type by The conformations of substituents such as CF3 or CF2RF groups in forcing the central CF3 or n-RF groups to be staggered, which in turn solid state crystal structures are a subtle convolution of intramolecular forces the core to be planar in the solid state (e.g., 2,6,9,10-ANTH and intermolecular packing forces. In those cases where planar struc- (C8F17)4, 2.6-((4-methoxyphenyl)ethynyl)2-9,10-ANTH(CF3)2, and the tures are observed for real compounds of this type in the solid state (the P21/c polymorph of 6,13-PENT(CF3)2). bottom three compounds in Fig. 5), crystal packing forces may be Finally, with the caveat that the differences in DFT predicted EAs counteracting the presumed intrinsic (i.e., gas-phase) stability of the are small, our results show for the first time that changes in the

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conformations of CF3 groups may affect the EAs of some CF3-substituted Appendix A. Supplementary data aromatic molecules. Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jfluchem.2019.02.010. 4. Experimental References 4.1. Materials

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