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Asymmetric Molecular Conformation of Steric Terfluorene toward Constructing Polyhedral Microcrystals for Deep-Blue Laser Xiang An, Mengna Yu, Lu-Bing Bai, Li-Li Sun, Ning Sun, Ya-Min Han, Chuan-Xin Wei, Wei Xu, Jinyi Lin, Chang-Jin Ou, Ling-Hai Xie, Xue-Hua Ding, Chunxiang Xu, and Wei Huang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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The Journal of Physical Chemistry

Asymmetric Molecular Conformation of Steric Terfluorene toward Constructing Polyhedral Microcrystals for Deep-blue Laser Xiang An,†■ Meng-Na Yu,‡■ Lu-Bing Bai,†■ Li-Li Sun,† Ning Sun,† Ya-Min Han,† Chuan-Xin Wei,† Wei Xu,⊥ Jin-Yi Lin,*†,§ Chang-Jin Ou,*† Ling-Hai Xie,‡, § Xue-Hua Ding,† Chun-Xiang Xu,⊥ and Wei Huang*†,§ †Key

Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM) &

School of Physical and Mathematical Sciences, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China. ‡Centre

for Molecular Systems and Organic Devices (CMSOD), Key Laboratory for Organic

Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China. §Shaanxi

Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU),

127 West Youyi Road, Xi'an710072, Shaanxi, China. ⊥State

Key Laboratory of Bioelectronics, School of Electronic Science and Medical

Engineering, Southeast University, Nanjing 210096, China

ACS Paragon Plus Environment

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ABSTRACT. Molecular conformation is crucial for precisely controlling photophysical processing and self-assembly behavior of organic conjugated semiconductors for optoelectronic applications. Herein, we obtained a novel asymmetric molecular conformation of steric terfluorene with torsion angles of -172° & 44° to enable its molecules self-assembling into a polyhedral microcrystal for deep-blue laser. Interestingly, significantly different to previous quasi-symmetrical ones, novel asymmetric secondary interactions, included C-H…π and O…HC hydrogen-bonds, are easily observed in our novel crystalline structure, and enable DOSFXSFX molecules to self-organize into an octahedral microcrystal, which is an excellent optical microcavity for organic lasers. Meanwhile, supramolecular steric encapsulation layer can be induced by large hindrance SFX nano-group and simultaneously effectively inhibited intermolecular π-π interactions, allow for intramolecular photophysical behavior in crystal states. As an intermediated and asymmetric conformation, our DOSFX-SFX molecules exhibited longer effective conjugated length with a red-shifted 0-1 emission peak at 430 nm than controlled ones (417 nm) with a larger torsional adjacent angels but shorter than the quasi-planar conformational ones (445 nm), confirmed the key role of molecular conformation in controlling photophysical behavior. Finally, DOSFX-SFX octahedron-based low threshold microlasers are also constructed with a 0-1 band emission peak of 417±2 nm, suggested intramolecular photophysical behavior in crystal states. Our work supported an assumption that molecular conformation is a key factor to dominate material’s properties and self-assembly behaviors.


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Introduction Organic conjugated aromatic molecules are attracted widely attentions for their potential applications in various organic optoelectronic devices.1-12 In general, organic semiconductors are molecules featuring an aromatic backbone unit with a system of delocalized π-electrons, which can induce a wide range of intramolecular excitonic or photoelectrical processes.5-6,

13-14

The intrinsic linear topological backbone structure

allows possible anisotropic optoelectronic properties due to their oriented p-orbital overlap along the conjugated backbone, which also can be enhanced by optimizing the condensed structure in the solid states.6, 15-18 Therefore, molecular conformation is a key factor to not only precisely tune optoelectronic processing, such as charge transport, photophysic behavior, but also programably control molecular self-organization and arrangement in condensed structure.5,

15, 19-21

However, compared to abundantly and

systematically study on the effect of electronic structure on the optolelectronic property, it is rarely attentions to systematically and comprehensively investigate the diverse and variable molecular conformation of conjugated aromatic molecules, especial uncover their role in determining materials self-assembly behavior, optoelectronic property and device applications. In last decades, fluorene-based semiconductors are widely application in deep-blue organic lightemitting diodes (OLEDs), organic laser and organic light-emitting transistor (OLETs) for their widely band-gap, deep-blue emission and high luminance efficiency in the condensed states. In fact, its derivatives also exhibited “programmable“ diverse molecular conformation22-26 and further induce variable phase formation in solution and film states, included amorphous, liquid crystalline, semicrystalline phases and complex gel states,22, 27-30 to meet the unique requirements


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of varous optoelectronic devices. For the energic motion of σ-bonds in conjugated backbone structures,6 it is not surprising that the adjcent angle between fluorene monomers can be easily tuned (Scheme 1A), as a respond to the external conditions, which are dierctly influenced on the effective conjugation length (ECL), and molecular regular packing mode in solid states.13, 115, 22, 31

Significantly different to the unpredictable and complex conformations in polyfluorenes,

fluorene-based dimers and trimers are excellent preliminary elements and platform to investigate the conformation–property relationship for their definite molecular structures and unique define molecular conformation in single crystals.5 Besides, it is necessary to uncover the photophysical processing of materials at various conformations under single molecule states in order to avoid stronger intermolecular aggregation effect and electronic coupling. Fortunately, we obtained a series of quasi-symmetrical molecular conformation of steric fluorene-based dimer and trimer, included defined large adjacent angle (~-45° in dimer and ~45° & ~-45° in trimer),32-37 coplanar conformation (~180° in dimer and ~180° & ~180° in trimer) at single molecular levels via molecular encapsulation strategy (Scheme 1A). As we expected, different conformations also presented identical optical property, included emission and lasing behaviour, and a series of similar one-dimensional crystalline structures ( ≤ Hexahedra), as shown in Scheme 1A and Figure S1. In this letter, we constructed a novel asymmetric molecular conformation with two different torsion angles of 172° (monomer 1 and 2) & ~-44° (monomer 2 and 3) in steric terfluorene (DOSFX-SFX) octahedral microcrystals (Scheme 1B and Figure 1).33,

38-39

Interestingly, for the asymmetric intermolecular non-covalent bonds, octahedral microcrystals with a high photoluminescent quantumyield (PLQY) of 60% are also manufactured via simple interface self-assembly method.40-41 Interestinly, our polyhedron show a deep-blue lasing peak at 417 nm (0-1 vibronic transition), attributed to the single-molecular excitonic behaviour in the


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well-defined microcrystal, which are redshifted about 10 nm than symmetrical conformational DOF-SFSO (45° & -45°) and blue-shifted about 23 nm than 2O8-DPFOH-SFX (180° & -180°). Therefore, molecular conformation is a key factor to dominate the molecular arrangement and photophysical processing of oragnic semiconductors.

Scheme 1. (A) Molecular conformation of fluorene-based semiconductor: dimers and trimers. All materials show quasi-symmetrical conformation in the single crystal states, which enable it derivatives to self-assemble into one-dimensional micro or nanocrystals (≤Hexahedra) (Figure S1).5, 14, 17-20, 22, 29-32 Lasing peak (0-1) values of various conformations are also demonstrated. Pink dashed line and circle present the symmetrical plane and center. (B) Designed model of steric fluorene-based trimer molecules: DOSFX-SFX. DOSFX-SFX molecules had an asymmetric conformation (~180° and ~-45°). The asymmetric intermolecular interaction induced molecules to self-assemble into octahedral microcrystals for deep-blue organic laser.33-34 Its effective conjugated length is slightly longer than BSFX and BSBF for the one more fluorene monomer (3) but also longer than TDMeF and DOF-SFSO, associated with the slightly planar conformation (Monomer 1 and 2). Blue plates present the planar monomer 1 and 2 and pink ones signify monomer 3. Red dashed lines the noncovalent bonds, include π…π stacking, hydrogenbonding and C-H…π interaction. Results and Discussion As reported in our previous works,33, 42 steric SFX monomers are obtained via one-pot reaction.


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Therefore, our DOSFX-SFX was easily prepared by Suzuki reaction, and its accurate chemical structure were screened by single crystal X-ray diffraction measurements.33 In addition, DOSFXSFX exhibited excellent thermal stability with a high thermal decomposition temperature (Td) up to 414 °C (Figure S1). Similar to other steric fluorene-based semiconductors, no phase-transition can be observed from 30 °C to 200 °C in the DSC curve of DOSFX-SFX (Figure S2). Therefore, our terfluorene had excellent thermal stability for optoelectronic applications. Single crystal structure is an effective tool to accurately reveal the most stable molecular conformation and optimized molecular packing mode in well-defined solid states. Significantly different to the one-dimensional crystal shape (≤Hexahedra) of BSFX, TDMeF and DOF-SFSO,32, 35, 37 two crystal polymorphs (needle and plate) of DOSFXSFX were obtained by a solvent evaporation method (Figure S3). However, the needle crystal (c-DOSFX-SFX) is instable even under room temperature for the mixed alcohol molecules in lattice. Therefore, we take more attempts to study the plate one (t-DOSFXSFX). As expected, the DOSFX-SFX molecules in plate crystal show excellent stability. And two different torsion angles of 172° and -44° are observed, which suggested the nonplanar conformation of DOSFX-SFX molecules in single crystal state [Scheme 1(B), Figures 1(A), S4-S6], and further induced a shorter ECL than planar ones (2O8DPSFOH-SFX). In addition, molecules exhibited uniaxial orientation and homogeneous alignment in the single crystalline structure, which can minimize scattering loss and favourable to optical waveguide and organic lasers. Meanwhile, extremely weaker intermolecular π-π interactions are also observed for the large steric hindrance interaction induced by steric SFX units, which may display quasi-unimolecular photophysical behaviour in the crystal.41 More interestingly, DOSFX-SFX molecules are intercalated


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packing in single crystal, that the two fluorenes (1 and 2, Figures 1B and 1C) are packing together but the side fluorene backbone structure is at the side ends (3 in Figures 1B, 1C and S6). As we expected, stronger supramolecular C-O (7)…H-C (43) hydrogen-bonding interactions with a characteristic distance of 2.37 Å are observed at side fluorene 3 (to 3 ones at other molecules), but weaker various C-O … H-C hydrogen-bonds (>2.60 Å) at fluorenes 1 and 2. What is more, it also shows C-H … π interaction (2.68 Å) between fluorene 3 in DOSFX-SFX dimer (Figure S7), stronger than those between fluorenes 1 and 2 (> 2.75 Å). In this regard, our non-planar conformation enables our DOSFX-SFX molecules to show asymmetric intermolecular C-O (7) … H-C and C-H … π interactions, which may allow for self-assembling into polyhedral microcrystals.

Figure 1. (A) DOSFX-SFX crystallographic structure. DOSFX-SFX molecules consisted of three units: monomer 1 (Purple) and 2 (Red) with an adjacent angle of -172° (quasiplanarity) but nonplanarity among 1/2 and 3 (Pink) (44°). A schematic illustration (Right at bottom) is also shown here to screen fluorene units from sectional view. Centered ring represent the deep-blue emissive fluorene backbone structures. (B) The predicted equilibrium 2D morphology. Inset show the photograph of our DOSFX-SFX polyhedral single crystal. (C) Molecular packing of DOSFX-SFX in single crystals. It is easy observed a range of asymmetric intermolecular interactions (Both C-O…H-C and C-H… π interaction).


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As a range of widely studied light-emitting conjugated materials, fluorene-based spirocyclic aromatic hydrocarbons have been proved to be the most successful and stable deep-blue organic emitters.5 In fact, well-resolved emission behaviour of oligomer is also mostly determined to the precise molecular conformation, which may visually screen by the adjacent angle between fluorenes. And single crystal structure is an effective mean to visually present this conformation. In fact, compared to the 0-0 energy band in PL spectra, 0-1 emission peak is a more proper probe to investigate the photophysical process, ascribes to the weak influenced by self-absorbance effect. For example, DPFOH single molecule with an expected adjacent angle of ~45° in diluted solution or PMMA solid solution,34 always had 0-1 emission peak at 385 nm and a similar ones for organic laser. However, for the long ECL, 0-1 emission or probable lasing peaks of BSFX (planar conformation, similar to BSFSO) are both red-shifted to around ~400 nm,37 similar to the those of terfluorenes (TDMeF and DOF-SFSO) with a larger torsional angle of (~45° and ~-45°).32,

35

Interestingly, coplanar conformation (~180° and ~-180°) of terfluorenes

(2O8-DPFOH-SFX) had longest ECL with a 0-1 emission peak of 445 nm compared to materials above, that lasing peak are reasonably expected about 440 nm (Confirm but unpublished now).36 As an intermediate conformation, the optical behaviour of DOSFXSFX single molecules was also systematically investigated here (Figure 2). As a result of the similar backbone structure, all our materials diluted solution (DOSFX-SFX, DOFSFSO and 2O8-DPFOH-SFX) had a similar maximum peak at around 355 nm in the absorption spectra and corresponding PL spectra consisted of three feature peaks at 399 nm (0-0), 423 nm (0-1), and 449 nm (0-2) (Figure 2A), respectively.24 Similar to previous works,32-33,

35

three oligomers pristine spin-coated films are red-shifted about 6 nm in


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absorption spectra compared to diluted solution states. Besides, PL spectra of three materials’ pristine spin-coated films also consisted of three well-resolved peaks at about 407, 426 and 450 nm, red-shifted about ~8 nm than those of diluted solution, attributed to the change of dielectric constant. Meanwhile, DOSFX-SFX spin-coated film shows a stable deep-blue emission without any green-band emission, even under thermal annealing at 200 °C. As we know, molecules show stabilized torsional conformation with a largeadjacent angle (~45°) in the diluted solution for the steric hindrance and freedom motion (Figure S8). As we expected, DOSFX-SFX neat spin-coated film showed an ASE peak of 428 nm, associated to 0-1 vibronic band emission.33 Compared to 2O8-DPFOHSFX, TDMeF and DOF-SFSO, our DOSFX-SFX is an intermediate and asymmetric conformational state with a 0-1 emission peak at 415 nm in crystal states (Figure 2B). More interestingly, 0-1 vibronic band emission of DOSFX-SFX crystal presented a slightly blue-shifted of ~8 nm compared to diluted solution, indicated a unimolecular excitonic behaviour with a large torsional angle (shorter ECL). Stronger self-absorption effect in thick crystal sample (~5 μm) can reasonably explains the disappearance of the 00 emission peak in PL spectra of DOSFX-SFX crystal. Besides, time-resolved transient PL measurements are also explored here to check the intrachain photophysical behaviour. It is easily observed that there is only one lifetime obtained for DOSFX-SFX diluted CHCl3 solution (τ=0.42 ns) and crystal (τ=0.68 ns). But film had two lifetimes with a 0.67 (75%) and 0.89 ns (25%), respectively. Our DOSFX-SFX had PLQYs of 95%±2%, 48%±2% and 60%±2% for diluted solution, film and crystal. It is believed that our DOSFX-SFX molecules exhibited unimolecular excitonic behaviour and stable emission in crystal states.


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Figure 2. (A) Absorption andemission spectra of DOSFX-SFX, together with DOF-SFSO, and BSFX in diluted solution, pristine film (about ~85 nm) and crystal states. (B) Lasing peak and 0-1 peak of fluorene-based dimer and trimer molecules in solid solution and crystal states. Lasing peak of the trimer with a (180o, -45o) and (180o, -180o) are also proposed in this table. Lasing peak of 2O8-DPFOH-SFX crystal are about at 440 nm in our recent work (Unpublished now). Own to the asymmetric intermolecular interactions, DOSFX-SFX polyhedral microcrystals can be easily obtained via solid-liquid molecular self-assembly behaviour (Figure 3). A schematic of the experimental setup is shown in Figure S9, and the detailed preparation procedures of the microcrystals are illustrated as follow: DOSFX-SFX polyhedral microcrystals were produced by slowly self-assembled from a CH2Cl2/n-hexane solution in a closed container. Scanning electron microscopy (SEM) image revealed the average length of the welldefined octahedral microcrystal is ~45 μm, and the width is about ~40 μm (Figures 3A). The emission colour of our microcrystal is deep blue under UV lamp (365 nm) irradiation (Figure 3B). XRD pattern clearly shows the layered organization with a series of evenly spaced peaks ascribed to {002} facets (Figure 3C). The appearance of (002) peak illustrates that the largest (002) crystal plane is parallel with the substrate. The pattern has been indexed according


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to the crystallographic data. It can be proved that the out-of-plane belongs to {002}s. As shown in Figure 1B, by the surface free energy minimization principle, the predicted morphology of DOSFX-SFX crystal is simulated using Materials Studio software. The morphology exactly matches with the actual polyhedral profile shape, which is composed of eight faces. These results implied that DOSFX-SFX molecules adopt the typical interdigital lipid bilayer-like (ILB) packing mode (Figure 3D). The single layer thickness is measured to be estimated about 2.12 nm in modeling, which is coincide well with the d-spacing value of the (002) plane. In this regard, DOSFX-SFX polyhedral microcrystals with an well-defined profile can be manufactured via simple solid-interface self-assembly.

Figure 3. (A) SEM image of DOSFX-SFX polyhedral microcrystals. (B) Fluorescence microscope (FLM) images of DOSFX-SFX octahedral microcrystals. Scale bar: 50 μ m.(C) XRD patterns of the microcrystals. (D) Ordered molecular layer arrangement of DOSFX-SFX with an ILB manner viewed from a-axis of the crystal lattice.


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In general, molecular conformation has direct effect on the photophysical process of organic conjugated molecules and their detailedly applications.6,

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In order to further

confirm this assumption, DOSFX-SFX polyhedral microcrystals were also used as a microcavity resonator for organic lasers. As shown in Figure 4A, broad emission spectra are obtained at lower excitation energy of pumped laser but sharp and narrower peaks are appeared with increasing the pump density above threshold. It is easy to find that a range of lasing peaks of DOSFX-SFX microcrystal is appears around at 417 nm, associated to the 0-1 emission band of microcrytals, similar to 0-1 vibronic band of DOSFX-SFX diluted solutions, further confirmed the unimolecular photophyscial behaviour in crystal states. In fact, these multi-model sharp lasing peaks became clearly with a linewidth Δλ = 0.45 nm and centred at 417 nm (0-1 transition), red-shifted about 12 nm than those of DOF-SFSO and TDMeF (405 nm), associated with the longer ECL. Besides, our polyhedral microcrystals emitted a bright and continuous deep-blue light from the edge area of octahedron (Inset shown in Figure 4B), strongly supported that our microcavity effect is belongs to Fabry-Pérot (FP) optical microresonator. The integrated intensity from 400 to 440 nm (0-1 band) of emission spectral profile as function of pump density is also demonstrated in Figure 4B. Lasing threshold

(IthLaser)

value of our DOSFX-SFX

microcrystals is estimated to be 53 W/cm2 which is much lower than those of TDMeF and DSFX, BDPhF and BSBF32, 37 but slightly higher than DOF-SFSO.35 In fact, this IthLaser value of the microcrystal is influenced by some key factors, such as emission efficiency, scattering losses, exciton annihilation and absorption at excited states, etc. Meanwhile, we cannot obtain two organic lasers with a range of completely same lasing peaks based on this polyhedral microresonator. In a word, lasing peak of DOSFX-SFX at 417 nm, similar


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to 0-1 band emission of single molecular, are effective confirmed weaker intermolecular excited coupling in the crystal states.

Figure 4. (A) PL spectra of DOSFX-SFX polyhedral microcrystals excited at different laser pump density. (B) PL integrated area of the lasing peak as a function of pump densities for DOSFX-SFX microcrystals. Inset: Dark field optical image of the DOSFXSFX microcrystal. Conclusions In summary, we demonstrated a novel asymmetric conformation of terfluorene in crystal state for low-threshold deep-blue polyhedral microlasers via steric molecular design. DOSFX-SFX molecules show asymmetric conformation with two torsional angles of (monomer 1 and 2) 172 and (monomer 2 and 3) -44°, allow for asymmetric intermolecular interactions, included C-H…π and O…H-C hydrogen-bonding interactions. And these synergistic effects also enable DOSFXSFX molecules to self-assemble into regular octahedral crystals via simple solid-interface selfassembly method. Impressively, our polyhedral microcrystals show a deep-blue lasing peak at 01 vibronic band (417±2 nm) with a low threshold of 53 W/cm2, supported the intramolecular photophysical behavior in crystal states. Therefore, molecular conformation is a key role to not


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only precisely tune optoelectronic behavior but also allow for controlling molecular selfassembly behavior for the fabrication of optoelectronic devices. ASSOCIATED CONTENT Supporting Information. A listing of the contents of each file supplied as Supporting Information should be included. For instructions on what should be included in the Supporting Information as well as how to prepare this material for publications, refer to the journal’s Instructions for Authors. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected]; [email protected] Author Contributions Xiang An, Meng-Na Yu and Lu-Bing Bai are equally contributed to this work. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT The work was supported by the National Natural Science Foundation of China (61874053, 61805117, 21504041, 21502091, 21502092, 21774061), National Key Basic Research Program of China (973) (2015CB932200), Natural Science Funds of the Education Committee of Jiangsu Province (18KJA430009), Natural Science Foundation of Jiangsu Province (BK20171470), “High-Level Talents in Six Industries” of Jiangsu Province (XYDXX-019), the open research


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