Cluster-Mediated Nucleation and Growth of J- and H-Type

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Cluster-Mediated Nucleation and Growth of J- and H-type Polymorphs of Difluoroboron Avobenzone for Organic Microribbon Lasers Qing Liao, Xin Guo Wang, Shaokai Lv, Zhenzhen Xu, Yi Zhang, and Hongbing Fu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00150 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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Cluster-Mediated Nucleation and Growth of J- and H-type Polymorphs of Difluoroboron Avobenzone for Organic Microribbon Lasers Qing Liao,*,† Xin Guo Wang,† Shaokai Lv,† Zhenzhen Xu,† Yi Zhang,‡ Hongbing Fu*,†, § †

Beijing Key Laboratory for Optical Materials and Photonic Devices, Department of Chemistry,

Capital Normal University, Beijing 100048, P. R. China §

Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry,

School of Sciences, Tianjin University. Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, P. R. China ‡

Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of chemistry, Chinese

Academy of Sciences, Beijing, 100190, P. R. China E-mail address: [email protected]; [email protected]

KEYWORDS: cluster mediated nucleation, polymorphism, polymorph-dependent properties, organic laser, optoelectronic devices

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ABSTRACT

Controlled fabrication of organic polymorphisms with well-defined dimensions and tunable luminescent properties plays an important role in developing optoelectronic devices, sensors and bio-labeling agents, but remains a challenge due to the weak intermolecular interactions among organic molecules. Herein, we developed a two-step solution self-assemble method for controlled preparation of blue-emissive or green-emissive micro-ribbons (MRs) of difluoroboron avobenzone (BF2AVB) by adjusting the cluster-mediated nucleation and subsequent onedimensional growth processes. We found that blue-emissive MRs belong to monoclinic phase, in which BF2AVB molecules form slipped π-stacks, resulting in J-aggregates with a solid-state photoluminescence efficiency φ = 68%. Meanwhile, green-emissive MRs are ascribed to orthorhombic phase and exhibit co-facial π-stacks, which lead to H-aggregates with φ = 24%. Furthermore, these as-prepared MRs can both act as polymorph-dependent Fabry-Pérot resonator for lasing oscillator. The strategy described here might offer significantly promise for the coherent light source of optoelectronic devices.


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Self-assembled organic nanostructures exhibiting efficient luminescence have attracted a great deal of research interest due to their promising potential in a variety of applications, such as fluorescent sensors, bio-labeling and bio-imaging agents, and organic light-emitting diodes (OLEDs).1-7 The optoelectronic properties of self-assembled organic nanostructures, such as the energy or electron transfer, the emission color, luminescence quantum yield, the carrier transport, and so on, are not only determined by chemical structure of the constituent molecules, but also sensitively related to molecular-stacking arrangement in the solid state.8-10, 11-15 In this sense, organic microcrystals with different crystalline forms (so-called polymorphisms) have different external crystal morphology and directly influence the device performance.16-19 This had provided an attractive approach to tune the organic solid-state optoelectronic behaviors for device applications. For example, one-dimensional (1D) organic nanostructures ranging from wires, ribbons to tubes have been readily prepared through π-π stacking of conjugated molecules, which enabled the realization of field effect transistors (FETs), optical waveguides, and photodetectors.7, 20, 21 During the past decades, organic solid-state lasers have made great progress. Besides the gain medium, a variety of optical resonators have been also adopted for the realization of laser and modulation of its wavelength.4, 22-26 Note that organic polymorphisms could easily provide different micro-resonator for the fundamental investigation of the effect of optical resonators on laser behaviors.27, 28 On the other hand, Bao and co-workers reported that the performance of FETs made by single-crystalline microcrystals is superior to those made by thin-film counterparts.29 Therefore, organic polymorphisms provide a material to combine high carrier transport, efficient optical gain, and various micro-resonator structures together on the way to develop electrically pumped organic lasers. It is an important to realize the controllable prepare


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of organic polymorphisms for developing organic micro-lasers. However, controlled fabrication of organic polymorphisms with well-defined dimensions and different aggregation types (H- or J-aggregation) has been met with limited successes.30, 31 The main reason is that, different from strong chemical-bonding interactions in inorganic semiconductors (ca. 200 kJ/mol), organic microcrystals are held together by weak intermolecular interactions of van der Waals type (ca. 5 kJ/mol).32 As a result, the nucleation and growth processes are easily altered by external conditions, such as solvents, temperature, solution concentration, and surfactants, and so on,21, 3336

leading to kinetic pathways toward different complex aggregations. How to rationally

controllably prepared organic polymorphisms with a particular crystal phase and morphology becomes a critical challenge of current polymorphous organics. The nucleation and growth processes have been recently considered as important factors to regulate organic self-assembly processes. We had reported the preparation of organic microcrystal with well-defined morphology or different crystal phase by means of the dynamically and thermodynamically controlled growth.19, 28, 37 However, these two processes always entangled with each other, which induces that it is hard to figure out the dominating factor in the controllable assembly of organic polymorphisms and the corresponding model system has not yet been reported. It is key issue to develop an approach for the controllable fabrication of organic polymorphisms. Here, by employing difluoroboron avobenzone (BF2AVB)38-41 as the model compound, we report a two-step solution self-assembly approach to directing the cluster-mediated nucleation and subsequent one-dimensional growth processes towards blue emissive and/or green emissive micro-ribbons (MRs), respectively. We found that blue-emissive MRs belong to the monoclinic phase, while the green-emissive MRs are ascribed to orthorhombic phase. Single-crystal data reveal that in blue-emissive MRs, BF2AVB molecules


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stack into slipped π-stacks, resulting in J-type aggregates that exhibit a solid-state photoluminescence efficiency φ = 68%. Meanwhile, green-emissive MRs exhibit co-facial πstacks, which lead to H-aggregates with φ = 24%. We demonstrated that these easily fabricated organic micro-scale polymorphisms with controlled dimensions and crystalline forms can behave as the optical resonators for laser applications. RESULTS AND DISCUSSION Small organic molecule BF2AVB was synthesized via a single-step reaction of BF3OEt2 boronation in CH2Cl2.40 A mixture of avobenzone (AVB) and boron trifluoride diethyl esterate was added into flask equipped with a reflux condenser and heated in oil bath at 60 °C (see the Supporting Information for details and Figure S1). After silica-gel separation, BF2AVB was obtained in 85% yield and was characterized by 1H NMR (Figure S2) and MALDI-MS (Figure S3). Two-step solution self-assembly of blue-emissive MRs and green-emissive MRs of BF2AVB. In the first step, 200 µL of a stock solution of BF2AVB (10 mM) in tetrahydrofuran (THF) was rapidly injected into 1 mL of mixed antisolvents of cyclohexane and isopropanol (v:v = 1:1) under hand-shaking. Then this mixture was aged for different times (t), for instance, t = 3, 10, and 30 min, respectively. In the second step, 100 µL of aged samples with different t values was drop-casted onto a quartz substrate. Accompanied with evaporation of THF, cyclohexane and isopropanol, micro-ribbon crystals of BF2AVB were obtained finally. Figure 1a and b show the fluorescence microscopy images of typical microcrystals obtained from samples of t = 3 and 30 min, respectively. Notably, these microcrystals exhibited distinctly different luminescent behaviors upon excitation with unfocused UV light (330−380 nm): intense


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blue and bright green emissions from samples of t = 3 and 30 min, respectively. Figure 1c and d present scanning electron microscopy (SEM) images. It can be seen that both blue and green emissive microcrystals are indeed two-dimensional (2D) MRs with smooth outer surfaces and sharp edges. Combined with atomic force microscopy (AFM) measurements (Figure S4), those blue-emissive MRs have the length (l) of 35−40 µm, the width (w) of 6−8 µm and the height (h) about 740 nm (Figure 1c). Meanwhile, those green-emissive MRs have a length (l) ranging from 20 to 25 µm, an average width (w) of 5 µm, and the thickness (h) about 800 nm (Figure 1d).

Figure 1. (a) and (b) Fluorescence microscopy images of BF2AVB MRs obtained from samples of t = 3 and 30 min, respectively, excited with unfocused UV light (330-380 nm). (c) and (d) SEM images of blue-emissive and green-emissive MRs, respectively. (e, f) SAED patterns of single blue-emissive and green-emissive MRs. Insets shows corresponding TEM images. (g)


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XRD profiles of blue-emissive and green-emissive MRs grown on silicon substrates. The top and bottom insets show cartoons of blue-emissive and green-emissive MRs. In order to understand the packing arrangement of BF2AVB molecules within these two MRs, selected-area electron diffraction (SAED) and X-ray diffraction (XRD) measurements were carried out. The SAED patterns were recorded by directing the electron beam perpendicular to the top facet of a single blue-emissive MR (Figure 1e) or a green-emissive one (Figure 1f). The sharp spots in SAED images reveal these two MRs are single-crystalline rather than polycrystalline. It is noted that BF2AVB crystals have two main crystalline forms: the monoclinic phase with the lattice parameters of a = 24.448(3) Å, b = 7.0594(8) Å, c =10.290(1) Å, α = γ = 90°, β = 94.303(4)° (CCDC No. 780003), and the orthorhombic phase with the cell parameters of a = 14.5159(2) Å, b = 6.9687(1) Å, c = 17.3259(2) Å, α = β = γ = 90° (CCDC No. 780002).40 Further analysis of the SAED pattern of a blue-emissive MR (Figure 1e) reveals that the triangled and circled sets of spots are due to (001) and (010) Bragg reflections with d-spacing values of 10.3 and 7.1 Å, respectively, in good agreement with the cell parameters of monoclinic crystal structure. Moreover, the XRD curve (blue curve in Figure 1g) is dominated by a series of peaks corresponding to the crystal plane (100) with d = 25 Å, such as (200), (300), (400), (500) and (600) peaks. This also suggests that blue-emissive MRs belong to monoclinic crystal and adopt a lamellar structure with the crystal (100) plane parallel to the substrate. Taking into account transmission electron microscopy (TEM) image (the inset of Figure 1e), it can be concluded that blue-emissive MRs grow preferentially along the crystal [001] direction (see the top inset in Figure 1g). In contrast, the triangled and circled sets of SAED spots of greenemissive MRs (Figure 1f) can be ascribed to the reflections from (200) and (010) planes of orthorhombic structure with d-spacing values of 7.2 and 7.0 Å, respectively. Indeed, the XRD


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curve of green-emissive MRs in Figure 1g shows the relative abundance of (001) facets (d = 17 Å) on the surfaces of orthorhombic MRs. Combined SAED, XRD and TEM results together, we can conclude that green-emissive MRs grow preferentially along the crystal [010] direction (see the bottom insets in Figure 1g).

Figure 2. Molecular packing arrangement of BF2AVB molecules in blue-emissive MRs (a) and in the green-emissive MRs (b), respectively. Combining the above analysis of XRD and SAED data with the help of monoclinic and orthorhombic crystal structures, the molecular packing within blue-emissive and green-emissive BF2AVB MRs are obtained and shown in Figure 2. It can be seen from Figure 2a that BF2AVB molecules are stacked along the c-axis (i.e., the [001] direction) of monoclinic crystal as shown by the pink arrow in the blue-emissive MRs, therefore forming slipped π-stacks along the MR length direction with the shortest π-π separation about 3.4 Å. Along the slipped π-stack, two


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neighboring molecules are packed with their short molecular-axes pointing to opposite directions. The roll angle and transverse displacement are negligible, while the pitch angle and the longitudinal displacement are 44.5° and 3.7 Å, respectively. Therefore, adjacent molecules along the slipped π-stack are arranged in a head-to-tail fashion. In sharp contrast, as shown by the pink arrow in Figure 2b, BF2AVB molecules in the green-emissive MRs form co-facial π-stacks along the b-axis of monoclinic crystal (i.e., the [010] direction). The shortest π-π separation is about 3.5 Å, similar to that in blue-emissive MRs. However, the pitch angle is 90° and there is no the longitudinal displacement. According to the molecular exciton model,42 the pitch angle, which defines the angle between the molecular transition-dipole and the π-stack direction, is < 54.7° in a J-type aggregate and > 54.7° in an H-type aggregate. Therefore, it is expected that J-type coupling in blue-emissive MRs and H-type coupling in green-emissive ones. Table 1. Photophysical parameters of BF2AVB monomers in the dilute solution and solid-state MRs. c kf −1 (ns ) 0.41

0.68

1.85

0.37

0.24

6.44

0.04

λabs (nm)

λem (nm)

Фa

Monomers

387 401 377 421 355 432

424 441 461 492 492

Blue-MRs Green-MRs a

0.82

τb (ns) 1.99

samples

Ф of monomer solution in THF measured through a relative method by using Rhodamine 6G as a standard and Ф

of MRs measured through an absolute method by using an integration sphere. b Fluorescence lifetime. c Radiative decay rate calculated according to kf = Ф/τ.

Spectroscopy characterizations of J-aggregation in blue-emissive MRs and Haggregation in green-emissive MRs. Figure 3a presents the normalized steady-state absorption


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(dashed line) and PL (solid line) spectra of blue-emissive MRs (middle panel) and greenemissive MRs (bottom panel) on a quartz substrate, in comparison with those of pure BF2AVB monomers in THF dilute solution (top panel). Table 1 summarizes the related photophysical parameters. The lowest S0→S1 transition of BF2AVB monomers in the dilute solution (5 × 10-6 M) exhibits a vibronic progression bands at 401 and 384 nm (top panel of Figure 3a) with a molar extinction coefficient of ε = 61800 M-1 cm-1. Similarly, PL spectrum of the monomers shows a mirror structure of its absorption with a subband spacing around 900 cm-1. The maximum emission peak is at 424 nm, giving rise to a Stokes shift of about 1400 cm-1.

Figure 3. (a) Normalized absorption (dashed lines) and PL (solid lines) spectra of monomers in THF dilute solution (top panel), blue-emissive MRs (middle panel) and green-emissive MRs (bottom panel) on quartz plate. (b) and (c) are approximate energy level schemes for J-


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aggregation of blue-emissive MRs and H-aggregation of green-emissive MRs, respectively. Blue energy levels correspond to nodeless excitons, while magenta energy levels are node excitons. In (c), photoexcited nodeless excitons vibrationally relax to the lowest node states as indicated by the upper waved arrow. Because of optical rules, the 0−0 transition is forbidden, while 0−n (n≥1) transitions are allowed. In contrast to H-aggregates (b), the 0−0 transition is allowed and enhanced according to optical rules in J-aggregates. (d) PL decay and fitted curves of BF2AVB monomeric solution in THF, blue-emissive and green-emissive MRs on quartz plate, respectively. In contrast to that of the monomers, the absorption spectrum of green-emissive MRs exhibits an apparently blue-shifted absorption with the maximum at 355 nm (bottom panel of Figure 3a). And an additional band at 440 nm is due to aggregate state of BF2AVB molecules. The PL spectrum of green-emissive MRs is dominated by the 0-1 transition: the intensity ratio between 0-1 and 0-0 emissions is around 14:1. The Stokes shift is approximately 8000 cm-1 that is much bigger than that of the monomers (about 1400 cm-1). This is a specific fingerprint of H-type aggregates of π–conjugated oligomers with a “face-to-face” molecular-packing arrangement.43-44 For such H-type aggregates composed of non-rigid molecules, only nodeless states are optically excited from the vibrationless ground state, as indicated by the absorption transition in Figure 3b.44-45 Then these photo-generated excitons relax rapidly into the lowest energy state by coupling with vibrational phonons (upper waved arrow in Figure 3b). Because of the forbidden 0-0 emission and allowed 0-n (n > 1) side-band emissions, PL spectrum caused by the dominant 0-1 transition can be observed. In most cases, H-aggregates of π-conjugated oligomers in thin films and crystals are characterized by poor PL φ.45 In fact, the PL φ = 0.24 of green-emissive MRs is much smaller than that of the monomers (φ = 0.82) (Table 1). Different to the green-


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emissive MRs, the absorption and PL spectra of the blue-emissive MRs exhibit slightly bathochromically shifted and narrow bands of greater intensity in comparison to those of the monomers (middle panel of Figure 3a). The maximum absorption peak of blue-emissive MRs is at 421 nm, showing typical exciton red shifted. The blue-emissive MRs are strongly emissive in the solid state with φ = 0.68, which is only slightly smaller than that of the monomers (Table 1). The emission maximum also red-shifts to 461 nm, yielding a Stokes shift about 1900 cm-1 that is slightly closer to that of the monomers (about 1400 cm-1). These typical characteristics of Jaggregates model, such as the red-shifted absorption and PL spectra, evident and strong 0-0 and 0-1 emissions (Figure 3c), high φ and negligible stokes shift, indicate that BF2AVB molecules may be arranged in “head-to-tail” orientations in the blue-emissive MRs. To further obtain the information on the nature of the excited states, we performed timeresolved fluorescence measurements by the streak camera (Figure 3d). The emission of BF2AVB monomers at 424 nm decays mono-exponentially (red line), yielding a lifetime of τm = 1.99 ns. The fluorescence decay of green-emissive MRs at 492 nm (green line) was also fitted monoexponentially with a time constant of τgreen = 6.44 ns, which is apparent longer than that of the monomers. According to the equation kf = φ/τ,46-49 the radiative decay rates (k) are calculated to be km = 0.41 ns-1 and kgreen = 0.04 ns-1 for monomers and green-emissive MRs, respectively (Table 1). The fact that kgreen < km is also consistent with the H-aggregation model.45 Similarly, PL kinetics of blue-emissive MRs at 461 nm (blue line) with a time constant of τblue = 1.85 ns and k is calculated to be kblue = 0.37 ns-1. Such enhanced radiative decay (known as superradiance) is a key characteristic of J-aggregates, which arises from coherent excited domain due to the coupling of the transition dipole moments of aggregated chromophores.42, 50 The temperature-dependent PL spectra are also measured for further understanding the assignment of


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J- and H-aggregates. In the case of green-emissive H-aggregates, the 0-0 emission band was found to be enhanced with increasing the temperature from 77 to 295 K (Figure S5a). While in the case of blue-emissive J-aggregates, the 0-0 emission band was found to be decreased with increasing the temperature caused by the strongly quenched super-radiance (Figure S5b). That is, raising the temperature leads to enhanced 0-0 emission in H-aggregates and the opposite behavior in J-aggregates (Figure S5), in good agreement with the exciton model developed by F. C. Spano.51, 52 Cluster-mediated nucleation and subsequent one-dimensional growth mechanism towards controlled preparation of blue-emissive and green-emissive MRs. In our preparation, the parameter that determines blue-emissive or green-emissive MRs is the aging time t used in the first step. For examples, drop-casting the first-step samples with t ≤ 3 min and/or t ≥ 30 min on quartz plates give rise to exclusively blue-emissive and/or green-emissive MRs, respectively. To probe the formation mechanism, we performed several more experiments. (i) We filtered the aged mixtures obtained in the first step with different t values through an AAO template (pore size: 20 nm), and found nothing left on the surface of AAO filter. (ii) We contrasted the emission spectra of the aged mixtures at different t = 0, 3, and 30 min (Scheme 1a). The emission peaks at t = 3 min and 30 min had red-shifted and intensity-changed apparently in comparison with that of the monomers (t = 0 min), which indicated new clusters had been generated. (iii)We then monitored the growth process of MRs on quartz plate by using a PL microscope. Taking the green-emissive MRs as an example (Figure S6), at an early stage about 1.2 min after dropcasting the first-step sample with t = 30 min on quartz plate, short MRs are quickly formed with a length of 3 µm. These primary short MRs then undergo a rapid growth within 1 min, leaving final products of green-emissive MRs accompanied with the evaporation of the solvents. Such


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kind of one-dimensional growth was also observed during the formation of blue-emissive MRs. (iv) We further did two parallel experiments, and mixed two first-step samples (with t = 3 and 30 min respectively) together. Drop-casting of this mixture on quartz plate produces blue-emissive and green-emissive MRs simultaneously. (Scheme 1b) Therefore, we reasonably think that the formation of blue-emissive or green-emissive MRs is determined by the nucleation process in the first step rather than the growth process in the second step.

Scheme 1. (a) The emission spectra of the aged mixtures at the aging time of t = 0, 3, and 30 min, respectively. The excitation wavelength is 355 nm. (b) The fluorescence microscopy images of BF2AVB MRs obtained from the mixture of two precursor solutions with t = 3 and 30 min respectively, excited with unfocused UV-light of 330~380 nm from Xenon lamp equipped with a


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band-pass filter. (c) A schematic model of solution self-assembly for the formation of blueemissive and green-emissive MRs for precursor solution with t = 3, 10 and 30 min, respectively. By combining the above experimental results, we propose a cluster-mediated nucleation followed by subsequent one-dimensional growth for the formation of blue-emissive or greenemissive MRs (Scheme 1c). P. Jonkheijm and co-workers reported that aggregated molecular clusters can serve as high-energy pre-nucleus initiating the nucleation-growth pathway.53 We speculate that in the first step, the dispersion of THF into the mixed antisolvents of cyclohexane and isopropanol induces the aggregation of BF2AVB molecules into clusters as nuclei for subsequent self-assembly process (Scheme 1c). At room temperature, BF2AVB molecules only form loosely-packed monoclinic J-type clusters (CCDC density: 1.343 g/cm3) within t = 3 min, which exhibits a PL maximum at 437 nm (blue curve, Scheme 1a) red-shifted by a value of 13 nm than the solution PL peak (black curve, Scheme 1a). If this mixed solution is dropped onto the substrate, only J-type clusters exist as high-energy nuclei for further nucleating aggregation and one-dimensional growth of blue-emissive MRs (top path in scheme 1c). In the case of t ≥ 30 min, there is only green-emissive MRs can form (bottom path in Scheme 1c), because only the closely-packed orthorhombic H-type clusters (CCDC density: 1.357 g/cm3) is left in the mixture as evidenced by a PL shoulder > 500 nm (green curve, Scheme 1a). Interestingly, an intermediate aging time of t = 10 min in the first step generates a mixture of blue-emissive MRs or greenemissive MRs (middle path in scheme 1c). This suggests that J-type clusters convert into H-type clusters as the t is prolonged. The key reason of controlled BF2AVB polymorphisms is the timedependent conversion of two clusters. Microcavity effect and lasing characterizations. Both blue-emissive and green-emissive MRs exhibit strong PL emission under excitation of unfocused UV light, with typical feature of


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an active optical waveguide, such as brighter PL emission at the two end-faces and little PL from the bodies of MRs (Figure 1a and b). It suggests that two self-assembled MRs can both confine the PL inside their ribbon-structures with regular shapes and work as active optical microresonators. To further investigate the microresonator effect, we characterized an isolated single MR using a home-made micro-photoluminescence (µ-PL) system (Supporting Information, Figure S7). The second harmonic (λ = 400 nm, pulse width 150 fs) of a 1 KHz Ti:sapphire regenerative amplifier was focused to a 50 µm diameter spot to uniformly excite the selected isolated MR on a 2D movable table. Spatially resolved PL spectra were collected underneath by using a three-dimensional (3D) movable objective (50×0.9 NA) coupled to an optical fiber and detected using a liquid-nitrogen cooled charge-coupled device (CCD).


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Figure 4. (a) PL spectra of a single blue-emissive MR with l = 30 µm under different pump energy at room temperature. (b) Integrated area of the 0-1 PL peak as a function of pump density. The lasing threshold is identified as the intersection between the linear and superlinear regions. (c) The mode spacing ∆λ at λ = 485 nm versus 1/L of blue-emissive MRs, showing clearly a linear relationship. Inset: simulated 2D normalized electric field (λ = 485 nm, n = 1.80) in the cross section of blue-emissive MR.


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Figure 4a reveals the PL spectra of an isolated blue-emissive MR (l = 30 µm) with increasing the pump intensity (E) of 400 nm laser. Under low pump density of E = 324 µJ/cm2, the PL spectrum of blue-emissive MR is dominated by a broad spontaneous emission (black curve). When pump density reaches a threshold of E = 535 µJ/cm2 (red line), a set of sharp peaks at the position of the 0-1 peak emerges and the full width at half maximum (FWHM) of the 0-1 peak dramatically decreases from 53 nm below the threshold to 5 nm above the threshold. Continuing to increase pump density exceeding the threshold, strong laser emission at the 0-1 peak is quickly dominated the whole spectrum. This agrees with the fact that the 0-1 transition has the maximum net gain in four-level organic laser system,4 because unoccupied vibronic sublevels of the ground state in thermal equilibrium facilitates the buildup of population inversion.22 Figure 4b shows the integrated area intensity of the 0-1 peak as a function of pump density. Clearly, the experimental data are divided into two regions. The intensity dependence of these two areas is separately fitted to a power law xp with p = 0.64 and 3.13, respectively. Therefore, the lasing threshold is identified as Eth = 530 µJ/cm2, at the intersection between the sublinear and superlinear regions. The former p = 0.64 below the threshold indicates the presence of bimolecular exciton−exciton annihilation process, while the latter p = 3.13 represents a superlinear increase of PL output, characteristic of laser emission. Moreover, the µ-PL image (the inset of Figure 4a) exposes apparently brighter blue PL at two end-faces of the MR and weaker luminescence in the body, showing a characteristic of FP mode cavity.26, 37, 54 To further understand the cavity mode, the mode spacing ∆λ between two adjacent resonance peaks (Supporting Information, Figure S8) is given by

(eq. 1)


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where n is the phase refractive index of the crystal, L is round-trip distance within a cavity mode (a function of the length of MR: L = 2l), and dn/dλ is the dispersion relation.55-57 A plot of 1/L of the blue-emissive MRs versus the mode spacing dλ at 485 nm is shown in Fig. 4c. The best-fit line (black line) is clearly linear, which confirms that the blue-emissive MRs operates as FP mode resonator along the length rather than the other dimensions of the blue-emissive MRs. Furthermore, the simulation of two-dimensional (2D) electric field distribution confirms that the FP-mode indeed exists along the length direction of the individual MR (inset of Figure 4c). We also measured the quality factor (Q) experimentally according to the definition Q = λ/δλ, where λ is the lasing wavelength and δλ is the FWHM of individual lasing peaks just above the threshold. It can be seen from Figure 4a that δλ around λ = 485 nm can be fitted to 0.493 nm using a Gaussian function. So the cavity Q factor is estimated to be as high as ~ 985, which suggests this high Q is reasonably responsible for high quality lasing emission.


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Figure 5. (a) PL spectra of the individual green-emissive MR with l = 25 µm under different pump pulse densities at room temperature. (b) Integrated area of the 0-1 PL peak as a function of pump density. The lasing threshold is identified as the intersection between the linear and superlinear regions. (c) The mode spacing ∆λ at λ = 495 nm versus 1/L of green-emissive MRs, showing clearly a linear relationship. Inset: simulated 2D normalized electric field (λ = 495 nm, n = 1.80) in the cross section of green-emissive MR.


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Similarly, the laser action of individual green-emissive MR was observed in Figure 5a. Strong laser emission develops as a set of sharp peaks on the top of the 0-1 peak when the pump density exceeds a threshold from E = 1138 µJ/cm2 (red curve) to 1562 µJ/cm2 (green curve). The lasing threshold (Eth = 1126 µJ/cm2) is identified as the intersection between the sublinear regions (p = 0.41) and superlinear areas (p = 3.75) in Figure 5b. Due to the 0-0 fluorescence transition is optically forbidden in green-emissive H-aggregate MRs, (Figure 3c), the optically allowed 0-1 transitions naturally provide a four-level scheme. Therefore, the observed lasing peak is around 495 nm along the 0-1 transition, which coincides with the PL maximum in green-emissive MRs. From this µ-PL image (the inset of Figure 5a), the brighter green spots at two end-faces and weaker luminescence in the body indicates the green-emissive MRs may also be FP mode cavity. The mode spacing ∆λ inversely proportional to L (Figure 5c) demonstrates that the greenemissive MRs also function as the FP mode resonator along the length. The simulation of 2D electric field distribution along the cross section of green-emission MRs also confirm that the FP-mode indeed exists along the length direction of the individual MR (inset of Figure 5c). The cavity Q factor of green-emissive MR cavity is determined to be ~ 923 according to the result of δλ ≈ 0.536 nm at λ = 495 nm in Figure 5a, which is comparable to that of blue-emissive MR one. Furthermore, laser operation of two MRs can both be done without fatigue for 4 hours (Figure S9), suggesting their excellent stability. Moreover, these two kinds of the polymorphisms were further compare by their laser threshold (Figure 6) under the different optical cavity length. The laser threshold is one of the basic conditions of the lasing formation and a significant parameter to evaluate a laser. We selected 13 blue-emissive MRs and green-emissive MRs with the different lengths for the experiments of the lasing threshold, respectively. Figure 6 shows the results of the laser threshold Eth of the blue-


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emissive MRs are illustrated by black squares and those of the green-emissive MRs are demonstrated by red triangle. The square dots are always below the triangles under the equivalent optical path length, indicating that the thresholds Eth of the blue-emissive MRs are always lower than that of the green-emissive MRs. The laser threshold relies on both the cavity quality factor and the optical gain coefficients of the gain materials. There is almost no difference in the Q factors of two MR cavities (Q ~985 and 923, respectively). We think that the main reason might come from their different PL quantum yield. It can be seen from Table 1 that J-aggregation blue-emissive MRs exhibit a PL quantum yield of Ф = 0.68, much higher that of H-aggregation green-emissive MRs (Ф = 0.24). Therefore, the gain coefficient of J-aggregation blue-emissive MRs is larger than that of H-aggregation green-emissive MRs. It is concluded that the dimensions and polymorphisms of microcrystals directly modulate lasing behaviors such as the formed optical microcavity, lasing thresholds.

Figure 6. The variation of the lasing threshold for blue-emissive MRs (black squares) and greenemissive MRs (red triangle) as a function of different length, respectively. CONCLUSION


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In conclusion, we demonstrate that the preparation of two MRs with different dimensions and polymorphisms through a cluster-mediated controlled self-assembly method. We have identified that J-type clusters form within 3 min while H-type clusters form after 10 min of nucleation process. The different clusters play a major role in solution self-assembly for nucleation and growth of two MRs. Both blue-emissive and green-emissive MRs exhibit good optical microresonator effect and laser behaviors. Significantly, it is found that the dimensions and polymorphisms of the microstructures actually have a fundamental effect on the lasing threshold. Our work can help understand the structure–property relationship and enrich the functionality of the high-performance organic solid-state coherent micro-scale light sources. EXPERIMENTAL SECTION Molecular Synthesis. Use the analytical balance to take 310 mg of Avobenzene (1 mmol) into a 100 mL three-necked flask and take a clean magnet into the flask. About 20 mL of dichloromethane was added to the flask at one end with a syringe under argon, and about 0.5 mL of boron trifluoride (diethyl ether) was added at the other end and stirred at room temperature for 12 hours. The solution slowly becomes pale yellow and has yellowish green fluorescence. After completion of the reaction, the solvent was spin dried and the product was purified by column chromatography. And then recrystallized from ethanol to give a yellow powdery product which was weighed and the calculated yield was about 80%. Structural and spectral characterization of MRs. TEM images were obtained by a JEOL JEM-1011 transmission electron microscopy (TEM). One drop of the as-prepared mixed solution was deposited on a carbon-coated copper grid, and dried under high vacuum. TEM measurement was performed at room temperature at an accelerating voltage of 100 kV. The morphologies and


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sizes of micro-ribbons were also examined by field emission scanning electron microscopy (FESEM, Hitach, S-4300) by dropping on a quartz slide and the samples were coated with a thin platinum layer using an Edwards Sputter Coater. The height of MRs was examined by Atomic force microscopy (AFM, Santa Barbara, CA). The X-ray diffraction (XRD) patterns were measured by a D/max 2400 X-ray diffractometer with Cu Kα radiation (λ=1.54050 Å) operated in the 2θ range from 3° to 30°, by using the samples of MRs filtered on the surface of an AAO membrane. Fluorescence images of ensemble samples were recorded through an Olympus research inverted system microscope (FV1000-IX81, Tokyo, Japan) equipped with a charge couple device (CCD, Olympus DP 71, Tokyo, Japan) camera. The excitation source is a Xenon lamp equipped with a band-pass filter (330~380 nm for UV-light, 460-490 nm for blue light) and the samples were deposited onto a quartz slide. The absorption and emission spectra were measured by Shimidazu UV-3600 UV-VIS-NIR and Horiba FluoroMax-4-NIR spectrophotometers, respectively. The quantum yields were measured absolutely by using an integrating sphere. Lasing Characterization of MR Lasers. The second harmonic (400 nm, 150 fs, 1 kHz) of a regenerative amplifier (Spitfire, Spectra Physics) seeded with a mode-locked Ti:sapphire laser (Tsunami, Spectra Physics) was focused to a 100 µm diameter spot to excite the MRs uniformly. Then PL spectra were collected under a reflection mode through the same 50 × 0.9 NA objective that was mounted a 3D movable stage. A 420 nm long-wave pass dielectric filter was used to block any scattered excitation light. Finally, the collected PL was coupled to an optical fiber and detected using a liquid-nitrogen-cooled CCD (SPEC-10-400B/LbN, Roper Scientific) attached to a polychromator (Spectropro-550i, Acton). The spectral resolution is 0.1 nm. By placing a movable aperture on the PL collection optical path, spatially resolved PL spectra from individual


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MRs can be selectively recorded (Figure S6). Using the same detection geometry shown in Figure S6, time-resolved PL was detected with a streak camera (C5680, Hamamatsu Photonics) dispersed by a polychromator (250is, Chromex) with a spectral resolution 1 nm and a time resolution of 10 ps. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.” General experimental details and spectral data (PDF). AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Author Contributions Q. Liao and H. B. Fu conceived the idea. X. G. Wang, S. K. Lv and Y. Zhang performed experiments. X. G. Wang, Z. Z. Xu, Q. Liao and H. B. Fu analyzed the results. X. G. Wang, Q. Liao and H. B. Fu prepared the manuscript. All authors reviewed the manuscript. ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of China (Grant No. 2017YFA0204503), the National Natural Science Foundation of China (Grant No. 21503139, 21521062 and 21673144), the Beijing Natural Science Foundation of China (Grant No.


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2162011), High-level Teachers in Beijing Municipal Universities in the Period of 13th Five–year Plan (Grant No. IDHT20180517 and CIT&TCD20180331), the Youth Innovative Research Team of Capital Normal University. REFERENCES 1.

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BRIEFS We developed a two-step solution self-assemble method for controlled preparation of blueemissive or green-emissive micro-ribbons of BF2AVB by adjusting the cluster-mediated nucleation and subsequent one-dimensional growth processes. These two micro-ribbons have different molecular stacking arrangement and can both act as polymorph-dependent Fabry-Pérot resonator for laser oscillator. TOC.


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Cluster-Mediated Nucleation and Growth of J- and H-Type

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