Two-Dimensional Organic Single Crystals with Scale Regulated

Letter pubs.acs.org/JPCL

Two-Dimensional Organic Single Crystals with Scale Regulated, Phase-Switchable, Polymorphism-Dependent, and Amplified Spontaneous Emission Properties Zhenyu Zhang, Xiaoxian Song, Shipan Wang, Feng Li, Hongyu Zhang, Kaiqi Ye,* and Yue Wang* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, 2699 Qianjin Avenue, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: The successful preparation of two-dimensional (2D) single crystals can promote the development of organic optoelectronic devices with excellent performance. A Schiff base compound salicylidene(4-dimethylamino)aniline with aggregation induced emission (AIE) property was employed as the building block to fabricate 2D thin single crystal plates with scales from around 50 μm to 1.5 cm. Yellow and red emissive polymorphs were concomitantly obtained during crystallization. The single-crystal-tosingle-crystal (SC-to-SC) transformation from yellow polymorph to red one was demonstrated. Furthermore, both polymorphs exhibited amplified spontaneous emission (ASE) properties. Interestingly, the red polymorph displayed size-dependent ASE characteristics. The larger red polymorph showed near-infrared ASE with maximum at 706 nm, whereas the smaller one presented red ASE with maximum at 610 nm. These results suggest that the different scale single crystalline thin films with perfect optoelectronic properties may be fabricated by using the organic molecules with 2D assembly feature.

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unaddressed issue. So far, the design and synthesis of organic functional building blocks, which have a 2D self-assembly crystallization feature, is still a great challenge. The development of large scale 2D single crystalline architecture based on organic luminescent and semiconductive molecules will open up an avenue toward organic devices with excellent properties. In the huge family of organic crystals, organic polymorphism is a unique branch and has recently attracted great attention as this kind of crystals provide desired mode systems to study the molecular-packing-dependent optoelectronic properties.23−26 The investigation on polymorphism can disclose the relationship between supramolecular structures and properties of organic materials. On the other hand, the transformation behavior between different polymorphs can promote the development of stimuli-responsive materials.27−29 More importantly, the successful preparation of 2D organic polymorph single crystals may offer the opportunity to fabricate devices with different properties based on one kind of organic molecule. However, so far, the pursuit of qualified building blocks for assembly of 2D organic polymorphs remains no desired decision. In this contribution, we present a Schiff base moleculesbased, salicylidene(4-dimethylamino)aniline (SADA)30,31 and its derivatives. This class organic molecules were employed building blocks to fabricate thin 2D single crystals with different

t is well-known that solution processed organic emitter and semiconductor based thin films have the extremely attractive potential for constructing high performance electronic and optoelectronic devices, such as organic light emitting devices (OLEDs),1,2 organic photovoltaic cells (OPVs),3,4 organic field effect transistors (OFETs),5,6 organic solid-state lasers (OSSLs),7−9 and so on.10,11 In these types of devices, the organic emitting and semiconducting films are critical components that dominate the performance of the devices. To fabricate highly efficient devices, the organic thin films with intensive light emission or high charge mobility are indispensable. To design and prepare strongly luminescent organic solids, the aggregation induced emission (AIE) concept was proposed and established by Tang and co-workers and demonstrated an successful strategy for constructing organic luminescent materials with high quantum yield in solid state.12,13 It was confirmed that some single crystals displayed high carrier mobility or strong emission properties.14−19 In the long-range ordered organic single crystals, desired intermolecular noncovalent interactions can result in the formation of unblocked charge transport channel and elimination of vibration induced emission quenching. Therefore, the large scale organic single crystalline thin films are considered to be the cornerstone for establishing organic electronic devices with high performance and special function such as laser emission. Unfortunately, very limited organic molecules exhibited twodimensional (2D) self-assembly characteristic20−22 and the assembly of area-controllable single crystalline thin films with significant semiconductive and emissive properties remains an © XXXX American Chemical Society

Received: March 29, 2016 Accepted: April 20, 2016

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crystals (Figure 1b and c) formed in the solution and then the thin plate-like crystals gradually grew up. One day later, large diamond-shaped crystals with the length of over 10 mm and thickness of about 0.1 mm were obtained (Figure 1d). The SADA crystals with different dimensions also could be easily achieved by slow solvent evaporation from the DCM solution. It is worth noting that during crystallization, the red emissive crystals are dominant products and about 70% of total obtained crystals are the red ones. The micrometer scale crystals have been prepared by drop-casting the SADA DCM solution (0.5 mg/mL) on glass substrate in air (Figure 1e). Upon solvent evaporation, the red luminescent microcrystals formed first and then yellow ones generated on the substrate under ambient atmosphere. Therefore, the SADA thin 2D crystal pieces with different dimensions from around 50 μm to 1.5 cm have been facilely achieved by controlling the crystallization time and process. Some other SADA derivatives have been synthesized and showed 2D crystallization property (Figure S2), suggesting the general supramolecular assembly characteristic of salicylidene(4-dimethylamino)aniline derivatives. The room temperature and low temperature (77 K) emission spectra of red and yellow emitting crystals are presented in Figures 1f and S3, respectively. The crystals exhibit obviously different structural characteristics and red-shifted emission compared to the DCM solution. The yellow polymorph exhibits bright yellow emission with the fluorescence quantum efficiency (Φf) of 0.29 and maximum at 580 nm. For red polymorph, the red emission (Φf = 0.26) is observed with maximum at 611 nm and a shoulder at around 705 nm. The fluorescence quantum efficiencies were measured by emission spectrometer with an integrating sphere. Time-resolved photoluminescence (PL) characterizations revealed that the yellow and red polymorphs showed the excited state lifetimes (τF) of 2.62 and 3.79 ns, respectively (Figure S4). The low temperature emission spectrum of yellow crystal showed similar profile to that of SADA toluene solution at 77 K (Figure S5). Therefore, in yellow crystal the SADA molecules exhibit the emissive feature, which is similar to that of single molecule. To evaluate the molecular structure and packing-dependent emission properties of red and yellow crystals, the single crystal

scales and polymorph properties. The plate crystals exhibited polymorphism-dependent emission, single-crystal-to-singlecrystal (SC-to-SC) transformation and elegant amplified spontaneous emission (ASE) behavior. The solution of SADA in dichloromethane (DCM, 1 × 10−5 M) displayed a UV−vis absorption maximum at 390 nm and very weak green emission (Figure S1). Upon standing the DCM solution (10 mg/mL), which was covered with ethanol under ambient conditions (Figure 1a), red and yellow emissive

Figure 1. (a) Molecular structure of SADA and its CH2Cl2 solution covered with ethanol. (b) Small plate-like crystals on a quartz circular plate with diameter of 2 cm. (c) Large plate-like crystals on a quartz circular plate with diameter of 2 cm. (d) Very large diamond-shaped crystals under UV irradiation. (e) Microcrystals under UV irradiation. (f) Emission spectra of the two polymorphs at room temperature.

thin plate-like crystals simultaneously generated in the interfacial region. Therefore, SADA showed typical AIE property.32 At the crystalline initial stage, small pieces of

Figure 2. Crystal packing structures of the red polymorph (a) and yellow polymorph (b). Experimental wide-angle X-ray diffraction patterns and the calculated diffraction patterns based on the single crystal data of the red polymorph (c) and yellow polymorph (d). Layered packing structures of the red polymorph (e) and yellow polymorph (f). 1698

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The Journal of Physical Chemistry Letters X-ray diffraction studies were performed. In two polymorphs, the SADA molecules display planar structure feature and identical geometry parameters except the intramolecular hydrogen bonding distances of OH···NC. In red polymorph, the hydrogen bonding distance is 1.66 Å, which is smaller than that (1.81 Å) in the yellow polymorph (Figure S6 and S7). The two polymorphs present obviously distinguished molecular packing modes (Figure 2a and b). In the yellow polymorph, the intermolecular C−H···π interactions between CH3 and benzene ring resulted in the formation of molecular chains that lined up together leading to the molecular layers. In the absence of intermolecular π···π interaction, therefore, yellow polymorph displayed similar emissive property to the molecules that were discrete existence in solution glass state. For the red polymorph, molecular dimers exist based on intermolecular π···π stacking interactions with contact distance of 3.53 Å. In a π···π stacking dimer, the two molecules adopt parallel head-to-tail arrangement and the two π-conjugated frameworks almost completely overlaps each other without side-slipping. The significant intermolecular π···π interactions can lead to the emission spectrum shifts to long wavelength region.33−35 In both polymorphs, SADA molecules exhibit layered assembly structural feature and no intermolecular interaction was observed between the layers. Therefore, the two polymorphs have strong 2D growth trend along the crystallographic a−b plane and plate-like crystals were easily obtained upon crystallization. Indeed, the two crystals adopt similar layered structure characteristic. To confirm the layered structure characteristic of the two polymorphs, wide-angle X-ray diffraction experiments were carried by horizontally fixing crystal slices on the substrate surface. The X-ray diffraction patterns of red and yellow crystal slices presented perfect layered structure feature and the upper surfaces of crystals are attributed to (002) plane (Figure 2c and d). The calculated period distances of the layer structures are 13.6 Å for the red crystal (Figure 2e) and 12.5 Å for the yellow one (Figure 2f), respectively, which are identical to the thickness of single molecule layers in the two crystals. To understand the 2D growth property, the theoretical simulation of the crystal equilibrium shapes was performed by calculating the total surface energy with the Materials Studio package.36 The crystal data and ORTEP drawings of the two single crystals were used for the calculations. The calculated results reveal the growth pattern and the thermodynamically stable architectures of the two polymorphs, suggesting that two-dimensional platelike structure is the superior morphologies (Figure S8). During crystallization, the (002) face shows larger attachment energy compared with other crystal faces indicating the weak growth tendency on (002) face (Table S1 and S2). The similar layered molecular arrangement feature inspired us to investigate the phase transformation behavior between the two polymorphs. Surprisingly, when the yellow emissive crystals were heated at 105 °C for 3 min and cooled down naturally to room temperature, red emissive crystals were obtained, suggesting the SC-to-SC transformation from a yellow polymorph to a red one. Interestingly, when the melted sample of either yellow or red polymorph cooled down to room temperature, slowly (0.5 K min−1) and quickly (10 K min−1) cooling down processes resulted in red and yellow emissive crystals, respectively (Figure 3). The X-ray diffraction and emission spectra (Figure S9 and S10) demonstrate that the red and yellow solids obtained by cooling the melted samples are identical to the crystallization-generated red and yellow

Figure 3. (a) Digital images of crystal transformation recorded under UV light. (b) Illustration of the changes of the molecular packing modes from the SC-to-SC process. (I, heating to melt; II, solidification in 0.5 K min−1; III, solidification in 10 K min−1; IV, heating 3 min at 105 °C without melt.)

polymorphs. The red polymorph cannot transform into yellow one by heating treatment before the sample was melted. In other words, SC-to-SC transformation from a red polymorph to a yellow one is impossible. The differential scanning calorimetric (DSC) curve (Figure S11) of the yellow polymorph showed an exothermic peak at around 105 °C and an endothermic peak at 140 °C, corresponding to its phase transformation and melting temperatures, respectively. However, for the red polymorph, only melting process appeared at 140 °C and there were no other endothermic and exothermic processes. Therefore, the SC-to-SC conversion from yellow polymorph to red one is permitted and the reverse SC-to-SC conversion is forbidden. The above results demonstrated that the red emissive crystal is a thermodynamically stable phase compared with the yellow emissive one. However, their stability difference is very minor. Therefore, the red emissive crystal is the thermodynamically stable polymorph and yellow emissive one is the kinetically stable polymorph. To obtain the organic emissive polymorphs based on variable intermolecular interactions, a rational molecule design strategy, is that to integrate the electron donor and acceptor function moieties through single bonds.23,25 This flexible molecular framework may lead to the formation of different polymorphs. Under UV irradiation, the edges of the two polymorphs exhibit stronger emission than the bodies, suggesting that the light is highly confined inside the crystals. In thin plate-like crystals, all molecules adopt the orientation along the c axis and layered packing mode, which are beneficial to the occurrence of ASE phenomenon.37−42 To study the ASE property, a set of isolated crystal slices with regular shapes were selected and excited with a pulsed laser and the emissions were subsequently collected from the edge area. Unexpectedly, the red polymorph displayed unique size-dependent ASE characteristic. For the larger rhomboid red crystal with sides of 1−2 mm, the ASE maximum appeared at 706 nm and the threshold value was 12.6 kW cm−2. However, the microcrystals with side length of around 100 μm generated on a silicon substrate showed ASE maximum at 610 nm and the threshold value of the microcrystals was 26.4 kW cm−2. On the basis of careful 1699

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Figure 4. Large size red crystals: PL spectra as the function of the pump laser energy (a). Pump laser energy-dependent peak intensity and fwhm of the emission spectra (b). Red polymorph microcrystals: PL spectra as the function of the laser energy (c). Pump laser energy-dependent peak intensity and fwhm of the emission spectra (d).

function and the difference between the two angles corresponding to the maximum and minimum was near to 90°. The yellow polymorph with different sizes showed unfavorable ASE characteristics. Although their PL spectra displayed narrow tendency at certain excitation laser powers, the base lines remained broad profile (Figure S16). This probably originates from the slightly different molecular packing and orientation in yellow polymorph compared with red one. The yellow emissive crystal plates with different sizes displayed similar UV−vis absorption spectra (Figure S17) and identical ASE property. It is worth mentioning that the red polymorph, which was achieved by annealing the yellow polymorph, exhibited similar ASE behavior with the red one obtained directly by crystallization from solution (Figure S18). These results provided the additional proof for the presence of SC-to-SC transition. In summary, we have demonstrated that the SADA molecules can self-assemble into 2D crystals with different sizes and the large area thin single crystal plates with sides of over 1.5 cm have been successfully prepared by crystallization. The strong 2D crystallization feature of SADA molecules are ascribed to their 2D intermolecular noncovalent interaction characteristic in crystals. The unique 2D oriented nucleation property of this kind of molecules provided the driving force for the construction of large area organic single crystal thin plates with promising luminescence properties. Moreover, during crystallization, two polymorphs were obtained and they exhibit similar layered structure and distinguished emission characteristics, which are attributed to the different intermolecular interactions in the two kinds of crystals. The interconversions between the two polymorphs were realized based on thermal treatment and SC-to-SC transition from a yellow polymorph to a red one was achieved. The polymorph transformation properties of SADA-based solids may be used to develop stimuli responsive materials. Uniquely, the red polymorph

examination of the low temperature emission spectrum (Figure S3) of the red polymorph, the 610 and 706 emission peaks are attributed to the different vibration coupling excited states corresponding to the same electronic transition excited state. To understand the size-dependent ASE phenomenon, the UV− vis absorption spectra of the red crystals with different sizes were recorded by the diffuse reflectance approach (Figure S12). The larger crystals exhibited red-shifted absorption compared with the smaller ones. For the larger crystals, the edge of the main absorption band localized beyond 600 nm. Conversely, that of the smaller crystals was restricted within 600 nm. In the ASE process, there is competition between absorption and emission. It was demonstrated that the self-absorption characteristic could determine the emission maximum of organic crystals.43,44 Therefore, the various ASE peaks for the red polymorphs with different sizes should be attributed to their distinguished absorption properties. The different size red emissive crystalline samples used for ASE study have identical XRD patterns (Figure S13), suggesting they belong to the same phase. The nonlinear curves characterized by the peak intensity and pump energy demonstrate the ASE behavior of the crystal. The slope of the widths at half-maximum (fwhm) versus pump energy changes at the pump energies of 12.6 kW cm−2 for the larger red polymorph and 26.4 kW cm−2 for the smaller one, which are usually known as the ASE threshold values (Figure 4). Moreover, the optical gain measurement was carried out using the variable stripe length method, in which the length of the pump stripe was adjusted by a slit (Figure S14). For a larger red polymorph, the net gain coefficient at 706 nm is 65 cm−1. The polarization of the emission was measured with the pumping laser energy at around 10 kW cm−2. A scatter plot (Figure S15) depicted the PL intensity fluctuation characteristic upon changing the relative angles between the crystal and the polarizer. The fitting curve was in accordance with the sine 1700

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displayed size-dependent ASE behaviors suggesting the possibility to achieve ASE organic crystals with different luminescent colors based on manipulating polymorphic forms. The 2D crystallization characteristic of SADA molecule may promote to develop the fabrication approach of organic single crystal thin films with excellent luminescent and semiconduting properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00704. Additional details of the synthesis, spectra, photographic, DSC curves, XRD patterns and other information. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.Y.). *E-mail: [email protected] (Y.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (91333201), National Basic Research Program of China (2015CB655000) and Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT101713018).

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