Ï Interaction in Dibromo-9,9

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Investigation of the intra-CH/# interaction in dibromo-9,9’-di-alkylfluorenes Nozomu Suzuki, Takashi Matsuda, Toshiki Nagai, Kazuki Yamazaki, and Michiya Fujiki Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01290 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 7, 2016

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Crystal Growth & Design

Investigation of the intra-CH/π interaction in dibromo9,9’-di-alkylfluorenes Nozomu Suzuki,†,‡ * Takashi Matsuda, ‡ Toshiki Nagai, ‡ Kazuki Yamazaki, ‡ and Michiya Fujiki‡* †

Department of Chemistry, Faculty of Science, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima,

Tokyo 171-8501, Japan. ‡

Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama,

Ikoma, Nara 630-0192, Japan.

Polyfluorene are well-known derivatives that exhibit electrical conductivity, electroluminescence, and liquid crystallinity but its crystallization process is not well understood. To investigate the role of intraCH/π interaction in the process, 2,7-dibromo-9,9’-dialkylfluorenes (DBFs) with various dialkyl side chains (alkyl group = methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-octyl, 2-methylpropyl, and 2ethylbutyl) were prepared. Systematic observation of the single-crystal structure revealed that DBF with a methyl group does not possess intra-CH/π interactions, whereas DBFs with straight side chains that are equal to or longer than an ethyl group exhibit CH/π interactions between Hβ and the fluorene ring. For DBFs with branched side chains, one Hβ and one Hγ interact with the fluorene ring. The computations at the MP2/cc-pVTZ//MP2/6-31G(d) level with a polarizable continuum model revealed that the close proximity of Hγ and the fluorene ring in the branched DBF crystals was caused by the geometrical feature of the side chain and that the interaction was repulsive rather than attractive. The conformational stability was estimated by both calculations and VT-NMR.

1. Introduction ACS Paragon Plus Environment


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The CH/π interaction1-8 is a type of XH/π interaction (e.g., OH/π,9,10 NH/π,9-11 and SH/π12) that plays key roles in crystal formation,13-15 conformational change,16-22 and molecular recognition.23,24 Unlike other XH/π interactions, recent computational calculations revealed that the major stabilizing energy of the CH/π interaction is dispersion energy and that the contribution of electrostatic energy is relatively small.25 Several molecular design strategies can be used to enhance the CH/π interaction: (1) attaching an electron-withdrawing group to the carbon atom of the CH group, (2) increasing the s character of the carbon atom, (3) and/or increasing the polarizability of the sp2 carbon with the interacting πorbital.10,21,26-30 Notably, the energy of the interaction between benzene and chloroform, whose hydrogen is activated by electron-withdrawing chlorides (-5.6 kcal mol-1), is even larger than that of the water dimer (-5.0 kcal mol-1).30 These strategies can be coupled with the cooperativity of the CH/π interaction to enable tuning of the occurrence of a specific conformation.31 Careful observations of the hydrocarbon geometry facilitate elucidation of the interactions of the CH/π moiety, and the CH/π interaction can enrich the molecular design strategy and processability of compounds. Fluorene is hydrocarbon with attractive luminescent properties and high quantum yields. Its polymers, which are termed polyfluorenes, are well-known derivatives that exhibit electrical conductivity, electroluminescence, and liquid crystallinity. Poly[bis(2-ethyl)hexylfluorene] (PF-2EtHex) and poly(9,9-dioctylfluorene) (PF-Oct) are two typical polyfluorenes, but their side chain configurations and conformations result in distinctly different single-stranded and assembled structures. For instance, PF-2EtHex shows hexagonal packing in the crystal because of the formation of a 5/2 helix32-34 (found to be a 21/4 helix in a recent study35), which aligns the side chain parallel to the fluorene ring and enables the bis(2-ethyl)hexylfluorene units to interact with the neighboring units. The side-chain conformation is also important for PF-Oct. This polymer assumes various backbone torsional angles: the Cα conformer (135°); Cγ conformer (155°); and approximately planar conformer, Cβ, (165°) as observed in the β-phase.36 The β-phase exhibits long-wavelength absorption and emission, and controlling the abundance of the β-phase enables tuning of the material’s emission wavelength and quantum yield. Knaapila and coworkers studied side chain length affect of polydialkyllfluorene (alkyl ACS Paragon Plus Environment

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group = n-hexyl, n-heptyl, n-octyl, n-nonyl, n-octyl) in methylcyclohexane solution, and revealed that the amount of the β-phase varies nonlinearly among those polymers and reaches maximum at PF-Oct.37 For PF-Oct side-chain conformations, T-shape and Y-shape have been discussed,38 and the antigauche-gauche (AGG) conformation at ω1ω2ω3 (Figure 1) is thought to be important for the formation of the β-phase. However, discussions of the intra- and inter-polymer interactions have been limited to ππ stacking and steric hindrance, and much less work has focused on the conformation of dialkylfluorene monomers, although some reports exist for monophenylfluorenes.39-42 Thus, a study of dialkylfluorenes that accounts for CH/π interactions should facilitate achieving control over the intra- and inter-polymer interactions of these polyfluorenes.38 Previously, we suggested that the intra-CH/π interactions between the alkyl chain and fluorene ring in 9,9’-dialkylfluorene systems could be identified based on the up-field shift of β-methylene in 1H-nuclear magnetic resonance (NMR) and 1H–1H nuclear Overhauser enhancement spectroscopy (NOESY) experiments.43 This finding suggested the possible origin of the side-chain conformational stability and a method to control the side-chain conformation using hydrogen-activated solvents. However, some questions remained unanswered. (1) Can the CH/π interaction also be observed in the single-crystal form of fluorene monomers? (2) Which hydrogen atoms show this interaction in straight and branched side chain? (3) How close are the interacting hydrogen and sp2 carbon? (4) How stable are these sidechain conformers in the solution state? (5) Finally, how efficiently can we control the conformation of the side chain in the fluorene unit? Noting the importance of the solution-state structure for controlling the amount of the β-phase, questions (4) and (5) are especially important. In this work, we tried to answer these questions by systematically observing the single-crystal structure of 2,7-dibromo-9,9’-dialkylfluorenes (DBFs) and carrying out a quantum mechanics (QM) method and variable-temperature 1H-NMR (VT-NMR) with relatively simple DBF structures. We chose DBFs as the model compounds because they are the simplest monomer unit and exist as solids at room temperature. Therefore, they are more readily obtained as crystals than 9,9’-dialkylfluorenes. We employed a vapor annealing method to relax the structure of PF-Oct, utilizing various organic solvents, ACS Paragon Plus Environment


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including those inducing CH/π and π/π interactions with fluorene units. The amount of the β-phase of PF-Oct was measured by photoluminescence (PL) and PL excitation (PLE).

Figure 1. Chemical structures of fluorene derivatives: (a) compounds studied in this work and (b) positions of carbon atoms. For DBF-2MePr and DBF-3EtBu, ω2 is defined as the C9-Cα-Cβ-Hβ torsion angle. 2. Results and Discussion 2.1. Single-crystal structures of DBFs Observation of the X-ray crystal structure is a relatively more direct method of determining whether a CH/π interaction exists than NMR (e.g., observations of the chemical shift or nuclear Overhauser effect), because the distances between interacting atoms can be measured and compared with the van der Waals distances. The criterion for identifying CH/π interactions in crystal structures was established by Nishio et al. and has been applied to various compounds and proteins by other researchers.1,6,15,44 Nishio’s method divides the surrounding space around a benzene ring into three parts (regions I, II, and III) and determines whether CH/π interactions exist depending on the interacting region (Figure S1). The smallest distance between a hydrogen atom and the plane of the phenyl ring (Dpln, region I), the CC bond of the ring (Dlin, region II), or the closest carbon atom (Datm, region III) was estimated (Table 1). We used the van der Waals radii of C and H (Dpln, Dlin, or Datm < Dmax, Dmax = 2.9 Å as the criteria for CH/π interactions. For the CH/π interaction in region III, the fulfillment of an additional condition related to the dihedral angle (-ωmax < ω C1 for DBF-Et and C3 > C1 > C2 for DBF-Pr. High surface area of fluorene ring ACS Paragon Plus Environment

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means that the solvent can access to the surface by inter-CH/π. Therefore, the stabilization effect of the solvent in the experiment is higher for C2 than C1 for DBF-Et, and, the effect is high for C3, C1, and C2 in this order for DBF-Pr. Furthermore, solvents with activated CH groups (TCE) should preferentially interact with the fluorene ring compared to solvents with normal CH groups (c-Hex), and this tendency is confirmed by the comparison of the conformational energy in TCE and c-Hex. Thus, DBFs with fluorene rings with larger surface areas were stabilized to a greater degree than those with relatively small surface areas in the experiment, and this tendency was stronger in TCE than c-Hex.

Table 2. Conformational energies determined at various calculation levels and the experimental values estimated by VT-NMR combined with the GIAO method. DBF-Et

∆Ga

∆Gb

DBF-Pr

DBF-2MePr

C1

C2

C3

C4

C1

C2

C3

C4

C1

C2

C3

C4

MP2/6-31G(d)

0.00

2.32

5.43

7.53

0.00

1.37

2.36

3.50

0.00

0.66

2.70

3.37

MP2/cc-pVTZ

0.00

2.32

5.08

7.35

0.00

1.35

2.36

3.46

0.00

0.70

2.31

3.62

MP2/cc-pVTZ+PCM(TBP)

0.00

2.29

5.02

7.28

0.00

1.39

2.33

3.51

0.00

0.69

2.32

3.59

MP2/cc-pVTZ+PCM(c-Hex)

0.00

2.53

5.50

7.68

0.00

1.37

2.34

3.49

0.00

0.70

2.32

3.60

exp. (TCE)

0.00

1.30

-

-

0.00

1.82

1.07

-

-c

-c

-c

-c

exp. (c-Hex)

0.00

1.75

-

-

0.00

1.71

1.37

-

-c

-c

-c

-c

a

Gibbs free energy [kcal mol-1] at 298.15 K calculated at various QM levels. bGibbs free energy [kcal mol-1] at 298.15 K obtained experimentally and via the GIAO method at the B3LYP/cc-pVTZ level of theory. cWe could not determine the conformational energies of DBF-2MePr possibly because of the coexistence of three stable conformations, two of which are almost identical (C1 and C2) in terms of their chemical shifts and coupling constants.

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2.4. PL and PLE measurements of PF-Oct To determine whether TCE and c-Hex show distinct differences in luminescence, PF-Oct (Mn = 2.69×104, PDI = 3.23) was adsorbed onto silica and vapor-annealed with various solvents (tetrahydrofuran (THF), TCE, toluene, chloroform, c-Hex, and benzene) (Figure S8). The luminescent peak was shifted by 25 nm after annealing. No peak was evident near 420 nm for TCE, and therefore, we could not obtain the ratio of peaks at 420 and 470 nm, as reported earlier.58 Therefore, instead of determining this ratio, we measured the relative amount of the βphase by calculating the intensity ratio of the peaks at 470 nm and 440 nm. PF-Oct annealed with TCE showed a relatively strong β-phase emission (1.5), whereas annealing with c-Hex resulted in a moderate value (0.90). THF produced the highest value (1.9), whereas benzene showed the lowest value (0.53). We expected that chloroform would exhibit a tendency similar to that of TCE, but instead, a moderate value (0.91) was found. The actual ratio could have been underestimated by a shoulder peak at c.a. 420 nm caused by a disordered phase.58 The solvent-βphase relationship as well as crystal formation process in various solutions should be examined in detail as has been done by Knaapila and coworkers and may be the next logical step in this line of research.37 3. Conclusion We observed CH/π interactions in the single-crystal form of fluorene monomers. DBF-Me was found to not have an intra-CH/π interaction because Cα lies between Hα and the fluorene ring, whereas DBFs with straight side chains that are equal to or longer than an ethyl group exhibit a CH/π interaction between Hβ and the fluorene ring. For DBFs with branched side chains, one Hβ and one Hγ interacted with the fluorene ring (DBF-2MePr and DBF-3EtBu). The distances between the hydrogen and carbon were 2.674–2.804 Å for straight-chain DBFs and 2.549–2.768


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Å for branched-chain DBFs. The computations revealed that the close proximity of Hγ and the fluorene ring in branched DBF crystals was caused by the geometrical feature of the side chain and that the interaction was expected to be repulsive rather than attractive. The conformational stability was estimated by both calculations and VT-NMR. For DBF-Et in TCE and c-Hex, the energy difference between the most stable and second-most stable conformations were 1.30 kcal mol-1 and 1.75 kcal mol-1, respectively. For DBF-Pr in TCE and c-Hex, the energy differences between the most stable and second-most stable conformations were 1.96 and 1.89 kcal mol-1, and those between the most stable and third-most stable conformations were 1.06 and 1.37 kcal mol-1, respectively. Considering the CH/π interaction in different solvents could lead to changes in the abundances of specific conformations according to the accessible surface area (e.g., ω1ω2ω3 = AGG conformations that induce the β-phase).

ASSOCIATED CONTENT Supporting Information. Description of experimental and computational methods; characterization of compounds; conformational energies obtained from MM, quantum mechanics, and VT-NMR experiments; and results of PL and PLE measurements. AUTHOR INFORMATION Corresponding Author *Nozomu Suzuki: [email protected] *Michiya Fujiki: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Funding Sources This research is funded by Japan Society for the Promotion of Science (JSPS) KAKENHI.


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ACKNOWLEDGMENT The authors are thankful for Mr. Fumio Asanoma, who provided advice on the VT-NMR measurements, and Shouhei Katao, who measured the single-crystal X-ray crystallography. We thank Prof. Shinichi Yamabe (Emeritus Prof. Nara University of Education) who encouraged us to study CH/pi interactions in a series of organic compounds. The authors appreciate Prof. Hiroko Yamada and Hironari Kamikubo for fruitful discussions. MF acknowledges the financial support from JSPS KAKENHI Grant Number 16H04155. REFERENCES (1)

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"For Table of Contents Use Only" Investigation of the intra-CH/π interaction in dibromo-9,9’-di-alkylfluorenes Nozomu Suzuki*, Takashi Matsuda, Toshiki Nagai, Kazuki Yamazaki, and Michiya Fujiki*

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To investigate the role of intra-CH/π interaction in the crystallization process of polyfluorene, 2,7-dibromo-9,9’-dialkylfluorenes with various dialkyl side chains were prepared. Systematic observation of the single-crystal structure, computations at the MP2/cc-pVTZ//MP2/6-31G(d) level with a polarizable continuum model and VT-NMR revealed that Hβ shows attractive interaction with fluorene ring while Hγ shows repulsive interaction.


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Ï Interaction in Dibromo-9,9

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