Controlling Intramolecular Conformation through Nonbonding

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Controlling Intramolecular Conformation Through Nonbonding Interaction for Soft-Conjugated Materials: Molecular Design and Optoelectronic Properties Yuanfang Cheng, Yuanyuan Qi, Yuting Tang, Chao Zheng, Yifang Wan, Wei Huang, and Runfeng Chen J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01695 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on August 30, 2016

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The Journal of Physical Chemistry Letters

Controlling Intramolecular Conformation through Nonbonding Interaction for Soft-Conjugated Materials: Molecular Design and Optoelectronic Properties Yuanfang Cheng1, Yuanyuan Qi1, Yuting Tang1, Chao Zheng1*, Yifang Wan1, Wei Huang2*, Runfeng Chen1* 1

Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced

Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing, 210023, P.R. China. 2

Key Laboratory of Flexible Electronics & Institute of Advanced Materials, Jiangsu National

Synergetic Innovation Center for Advanced Materials, Nanjing Tech University (Nanjing Tech), Nanjing 211816, P.R. China.

ACS Paragon Plus Environment

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ABSTRACT: To address the intrinsic contradictory between high optoelectronic properties and good processabilities in organic π-conjugated molecules, we propose that soft-conjugated molecules (SCMs), conformationally locked by intramolecular nonbonding interactions, can benefit from both non-planar molecular structures in solution for processing and rigid coplanar structures in solid states for enhanced optoelectronic properties. Computational results reveal that nonbonding pairs of S···N, N···H, and F···S are strong enough to prevail thermal fluctuations, steric effects, and other repulsive interactions to force the adjacent aromatic rings planar; thus constructed SCMs display delocalized frontier molecular orbitals with comparable frontier orbital energy levels, band-gaps, reorganization energies, and photophyscial properties to that of rigid-conjugated molecules, due to their stable planar soft-conjugation at both ground and excited states. These understandings gained from the theoretical investigations of SCMs provide keen insights into construction and modification of soft-conjugations to harmonize the optoelectronic property and processability in conjugated molecules for advanced optoelectronic applications.


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Organic optoelectronic materials, designed in planar π-conjugated architecture, have been actively studied and integrated in a large variety of semiconducting devices, including organic light-emitting diodes (OLEDs), field effect transistors, organic photovoltaics (OPVs), and chemical and biological sensors.1-4 To achieve desired electrical and optical properties for highperformance device applications, organic molecules are generally needed to be designed in rigid molecular structures to increase the conjugation length and suppress the non-radiative decay with improved photon absorption and emission properties and to promote the intermolecular π-π interactions for enhanced charge transfer and transport properties.5-6 As a typical example, heterofluorenes, constructed by bridging the two benzene rings in biphenyl into a plane with heteroatoms, show much improved optical and electronic activities than the non-planar biphenyl and have been widely applied as important build blocks in organic electronics.7-8 However, the flat and rigid molecular framework essentially required for prominent optoelectronic properties usually leads to poor solubility in solvents and heavy intermolecular interactions in solid films, arising many difficulties in material preparation and purification, device fabrication, and film morphology control, which in turn may lead to poor device performance.9-12 This dilemma is even more apparent in preparing narrow band-gap materials with multi-fused aromatic architectures for OPVs.13-15 Inspired by the intramolecular nonbonding interactions that can induce a planar geometry at solid state as observed in fluorine substituted OPV donors,16-18 we reason that the forced planar conformation by nonbonding interactions should be equally effective in producing expanded π-conjugation with expected optical and electronic properties to that by heteroatom bridging as in heterofluorenes. Compared to the heteroatom bridging strategy in configurationally constructing large planar rigid aromatic compounds, the nonbonding


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interaction planarized molecules are conformationally locked,19-21 exhibiting rather high flexibility and softness that they can be tuned to non-planar in solution or at high temperature by disturbing the weak nonbonding interactions through solvent effects or thermal fluctuations.22-23 Therefore, it is possible to design a kind of soft-conjugated molecules (SCMs) by carefully adjusting the strength of the intramolecular nonbonding interactions to an optimal extent: not too strong to hinder the transfer to non-planar conformation with high solubility in solvents for facilitated material preparation and device fabrication; and also not too weak so that the molecules can be planar with promising optical and electronic properties at solid state for device applications. Such an approach extracts the most adversary properties from both sides of conjugated and non-conjugated materials and abandons their drawbacks, successfully overcoming the intrinsic contradictories in designing high-performance optoelectronic molecules with good material and device processability and high optoelectronic properties simultaneously.23-24 Here, we set out to investigate SCMs based on six nonbonding interaction pairs of F···S, F···N, F···H, S···N, S···H, and N···H in three kinds of molecular architectures containing 5-5, 5-6, and 6-6 aromatic rings, respectively (Figure S1, Supporting Information), focusing on evaluating how and how strong the interaction pair influences the conformation of adjacent aromatic rings and the optoelectronic properties of the resulted SCMs. The additional nonbonding interactions can act as a second bond to rigidify the 5-5 rings of 2,2'-bithiophene (BT), 5-6 rings of 2-phenylthiophene (PT), and 6-6 rings of 1,1'-biphenyl (BP) to achieve conformationally locked SCMs through soft bonding (Figures 1a-1c). To compare with the actual bonding in heteroatom-bridged compounds, the rigid-conjugated molecules of 4Hcyclopenta[1,2-b:5,4-b']dithiophene (CT), 4H-indeno[1,2-b]thiophene (IT) and 9H-fluorene


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(FL) via C-bridging were also studied. Furthermore, non-planar molecules (MeT-MeT, MePMeT and P-DMeP) designed by deliberately hindering the planarization with methyl substituents were investigated to figure out the effects of soft-conjugation on the molecular structures and optoelectronic properties.

Figure 1. Soft-conjugated molecules (SCMs) locked conformationally by intramolecular nonbonding interactions (in red square), rigid-conjugated molecules locked configurationally by CH2, and non-planar molecules sterically hindered by CH3 in 5-5 (a), 5-6 (b), and 6-6 (c) aromatic ring architectures. To correctly predict the molecular conformation affected by the soft nonbonding interactions, various calculation methods and basis sets were tested and compared with the experimental single crystal data of 2-2’-bipyridine (NP-NP).25 From Table S1, all the methods suggest the planar molecular geometry of NP-NP with very small angle between the two pyridine rings; the close values to the experimental results are achieved by using density functional theory (DFT) of both B3LYP/6-31G(d) and expensive long-range corrected hybrid


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density functions of CAM-B3LYP, LC-BLYP, and ωB97XD with better considerations of noncovalent weak interactions. Considering that B3LYP method approaches experimental structural value at relatively small basis sets with excellent performance in energy prediction, B3LYP/631G(d) was selected for the following computational studies.26 The optimized ground-state geometries of BT, PT and BP in 5-5, 5-6, and 6-6 molecular architectures are non-planar with large angles (θ = 23.7°, 28.3° and 38.4°, respectively) between the adjacent two aromatic rings, indicating that S···H is too weak for the planarization of BT and PT.27 But after the introduction of the strong and attractive intramolecular nonbonding interaction pairs, the molecules are tuned to be planar (Figures 1 and S1).28-31 And, it seems that the 5-5 architecture is more apt to be in planar than the 5-6 and 6-6 systems under the effects of the same interaction pair. For example, the FT-T and FP-T have the same major nonbonding interaction of F···S, but the θ of the 5-6 type (FP-T) is much large than that of the 5-5 type (FTT) (Figure S1). Moreover, all the studied 5-5 type molecules containing nonbonding interaction pairs are planar with θ ≈ 0°, the majority of 5-6 type molecules are planar, and only 3 molecules in 6-6 type are planar. To assess the relative strength of the non-bonding interactions, the torsion angle of θ is rather indicative; smaller θ means larger attractive non-bonding interaction in forcing the aromatic rings coplanar.32-35 From Figure S1a of 5-5 SCMs, very small θ (0°) is observed, indicating obvious attractive interactions of S···N and F···S; these two attractive interactions also lead to planar 5-6 SCMs (Figure S1b), and it seems that S···N is stronger than F···S, showing more planar molecules with lower θ. Impressively, the θs of S···N-based molecules are all zero for perfect planarity. From Figure S1c, only N···H-based 6-6 molecules were found to be planar, suggesting the strong attractive interactions of N···H in constructing SCMs. In


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contrast, F···N-based 6-6 molecules (typically in DNP-DFP and FNP-FNP) exhibit larger θs than that of BP, demonstrating even repulsive interaction of F···N. Similarly, F···H seems to be repulsive too, as indicated by the larger θs of P-DFP, FP-FP, and P-FP than that of BP; however, in 5-6 molecules, smaller θs were observed when comparing P-FT to PT and DFP-T or FP-FT to FP-T, indicating otherwise the attractive interaction feature of F···H. This different effects of F···H on the intramolecular planarity were further investigated via nonbonding covalent interaction (NCI) analysis with reduced density gradient (RDG) method. From Figures S2a-S2d, both attractive and repulsive interactions of F···H, S···H, and H···H can be observed; the stronger attractive interactions of S···H and F···H are responsible for the smaller θs in PT and P-FT; the replacement of H with F leads to both enhanced attractive and repulsive interactions, but the enhancement of attractive interactions dominates in 5-6 molecules, resulting in smaller θ in P-FT. Besides the influence of the different aromatic skeletons, steric effects are also important in determining θ for the construction of SCMs. Typically in DFP-T and FP-FT, strong attractive interaction of F···S overcomes the repulsive effects of F···H, propelling the molecules to be almost planar; when F is in benzene (DFP-T), the longer distance between F and H due to larger steric hindrance will be resulted, leading to increased repulsive interactions for larger θ (Figures S3a and S3b). Therefore, it can be concluded that the attractive interactions of S···N, N···H, and F···S are strong enough and can even overcome repulsive interactions to conformationally lock the nearby aromatic rings to form SCMs, while S···H, F···H, and F···N are weak and not applicable to planarize the molecular conformation alone. To shed in-depth insights on the nonbonding interactions, bond order and atomic dipole moment corrected Hirshfeld population (ADCH) charge analyses were performed.36-38 Expect for S···H, the other five pairs of weak interactions show shorter intramolecular distance than the


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sum of their respective van der Waals radium, indicating apparent intramolecular interactions (Table S2). The bond order of S···N was found to be the highest (0.0393~0.0502), followed by that of N···H (0.0383~0.0404) and F···S (0.0334~0.0397). Very low and even minus bond order of F···N was observed (-0.003~0.001), suggesting its weak and even anti-bonding feature. The bond order of F···H varies in a wide range (0.0260~0.0455), which is coincident with the θ analysis that F···H is weak and sensitive to other factors. Although the bond order of the nonbonding interactions are very low, the higher bond order undoubtedly represents stronger interaction. In this sense, the attractive interactions of the intramolecular conformational locks decease in the order of S···N > N···H > F···S. This order can be further confirmed by the ADCH method. In Figure 2a, the calculated ADCH charges of S, N, F, and H are 0.042~0.090, -0.277~0.251, -0.088~-0.073, and 0.113~0.149, respectively. It is clear that N has the highest negative charges and S has the highest positive charges. Therefore, S···N should have the strongest attractive electrostatic interactions between the differently charged atoms. Following this criteria further, N···H and F···S should have the next strong attractive interactions. It should be also noticed that S···H and F···N have the same positive and negative ADCH charges, revealing the origin of their repulsive interaction features as found in θ analysis. The total pattern of nonbonding interactions can be obtained through NCI analysis (Figure 2b).39-40 From the color-filled RDG isosurface of FT-NT (inset of Figure 2b), the deeper blue means the stronger interactive interaction; the region between S and N shows light blue color, suggesting a strong attractive interaction between them; the interaction region marked by a green circle corresponding to F···S interaction is in both green and light brown, indicating weak attraction and repulsion respectively; the regions in the centers of the aromatic rings show strong


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steric effect, since they are filled by red. The NCI results are well in line with θ and ADCH charge analyses.

Figure 2. ADCH charges (a) and reduced density gradient (RDG) versus sign(λ2)ρ with RGD isosurface of FT-NT (b). Positive charges are in red, while negative charges are in blue. The NCI analysis was then used to evaluate the influence of intermolecular interactions on the molecular planarity in the solid state based on the single crystal structure of bithiophene.41 From Figures S4a and S4b, FT-FT exhibits stronger attractive interaction (sign(λ2)ρ = -0.011)


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than repulsive interaction (sign(λ2)ρ = -0.010), leading to its planar molecular structure; while the similar attractive and repulsive interactions of BT (sign(λ2)ρ = -0.006) correspond to the nonplanar BT in single molecular state. In the solid crystal state, BT turns to be planar due to enhanced intermolecular interactions, while FT-FT, which is already planar in single molecular state, shows further enhanced intermolecular attractions (Figures S4c and S4d). Nevertheless, the intramolecular attractions (sign(λ2)ρ = -0.011) are stronger than the intermolecular attractions (sign(λ2)ρ = -0.003 ~ -0.006), suggesting clearly the dominative role of intramolecular attractions in promoting the formation of planar structures of SCMs in solid state. It is of fundamental importance to figure out the softness of the soft-conjugation to ensure that SCMs are strong enough that can overweight other stimuli to form planar molecular conformation via nonbonding intramolecular interactions. Strength of ~2kT (0.05 eV at room temperature) are generally considered as the lower limit for conformational control, and any interaction weaker than that will likely be washed out by thermal fluctuations. To this end, we tend to calculate the torsional barriers of the SCMs by twisting θ from -180° to 180° (Figures 3a-3c). In accordance with the literature report, BT reaches its absolute minimum at transdistorted (also known as anti-gauche) conformation with a S-C-C-S dihedral angle of ±150° (θ ≈ ±30°);42 the second pair of minima appears at θ ≈ ±140° (cis-distorted conformation, also known as syn-gauche); there are two transition states at θ ≈ ±90°, which act as barriers to separate the first and second energy minima. Based on the ~2kT (0.05 eV) criteria, the θ of BT interchanges flexibly between ±58° at room temperature. The 5-5-type SCMs behave similarly to BT, but show higher torsion energy barriers (Eb) at θ ≈ ±90° and eliminated transition state at θ = 0° (cisplanar of BT) due to the effects of the nonbonding interactions to lock the nearby thiophenes (Figure 3a). The Eb increases in the order: BT < FT-T < FT-FT < FT-NT, which is in line with


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the weak interaction strength increase in these molecules: 2S···H

Controlling Intramolecular Conformation through Nonbonding

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