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

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

Beijing Key Laboratory for Optical Materials and Photonic Devices, Department of Chemistry, Capital Normal University, Beijing 100048, People’s Republic of China § Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of 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, People’s Republic of China S Supporting Information *

ABSTRACT: Controlled fabrication of organic polymorphisms with welldefined dimensions and tunable luminescent properties plays an important role in developing optoelectronic devices, sensors, and biolabeling agents but remains a challenge due to the weak intermolecular interactions among organic molecules. Herein, we developed a two-step solution self-assembly method for the controlled preparation of blue-emissive or green-emissive microribbons (MRs) of difluoroboron avobenzone (BF2AVB) by adjusting the cluster-mediated nucleation and subsequent one-dimensional growth processes. We found that blue-emissive MRs belong to the monoclinic phase, in which BF2AVB molecules form slipped π-stacks, resulting in J-aggregates with the solid-state photoluminescence efficiency φ = 68%. Meanwhile, green-emissive MRs are ascribed to the orthorhombic phase and exhibit cofacial π-stacks, which lead to H-aggregates with φ = 24%. Furthermore, these as-prepared MRs can both act as polymorph-dependent Fabry−Pérot resonators for lasing oscillators. The strategy described here might offer significant promise for the coherent light source of optoelectronic devices. KEYWORDS: cluster-mediated nucleation, polymorphism, polymorph-dependent properties, organic laser, optoelectronic devices 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. In addition to the gain medium, a variety of optical resonators have been also adopted for the realization of the laser and modulation of its wavelength.4,22−26 Note that organic polymorphisms could easily provide different microresonators 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 that made by thin-film counterparts.29 Therefore, organic polymorphisms provide a material to combine high carrier transport, efficient optical gain, and various microresonator structures together on the way to develop electrically pumped organic lasers. It is

S

elf-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, biolabeling and bioimaging agents, and organic light-emitting diodes (OLEDs).1−7 The optoelectronic properties of self-assembled organic nanostructures, such as energy or electron transfer, emission color, luminescence quantum yield, carrier transport, and so on, are not only determined by the chemical structure of the constituent molecules but also sensitively related to the molecular-stacking arrangement in the solid state.8−15 In this sense, organic microcrystals with different crystalline forms (socalled polymorphisms) have different external crystal morphologies and directly influence the device performance.16−19 This has provided an attractive approach to tune the organic solidstate optoelectronic behaviors for device applications. For example, one-dimensional (1D) organic nanostructures ranging from wires to ribbons and tubes have been readily prepared through π−π stacking of conjugated molecules, which enabled © XXXX American Chemical Society

Received: January 7, 2018 Accepted: April 26, 2018 Published: April 26, 2018 A

DOI: 10.1021/acsnano.8b00150 ACS Nano XXXX, XXX, XXX−XXX

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evaporation of THF, cyclohexane, and isopropanol, microribbon crystals of BF2AVB were obtained finally. Figure 1a and 1b show the fluorescence microscopy images of typical microcrystals obtained from samples of t = 3, 30 min,

important to realize the controllable preparation of organic polymorphisms to develop organic microlasers. However, the controlled fabrication of organic polymorphisms with welldefined dimensions and different aggregation types (H or J aggregation) has met with limited success.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, surfactants, and so on,21,33−36 leading to kinetic pathways toward different complex aggregations. How to rationally controllably prepared organic polymorphisms with a particular crystal phase and morphology has become a critical challenge of current polymorphous organics. The nucleation and growth processes have been recently considered as important factors to regulate organic selfassembly processes. We reported the preparation of organic microcrystals with well-defined morphology or different crystal phase by means of dynamically and thermodynamically controlled growth.19,28,37 However, these two processes were always entangled with each other, which makes it 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 a 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 direct cluster-mediated nucleation and subsequent one-dimensional growth processes toward blue-emissive and/or green-emissive microribbons (MRs), respectively. We found that blue-emissive MRs belong to the monoclinic phase, while the green-emissive MRs can be ascribed to the orthorhombic phase. Single-crystal data reveal that, in blue-emissive MRs, BF2AVB molecules stack into slipped π-stacks, resulting in J-type aggregates that exhibit the solid-state photoluminescence efficiency φ = 68%. Meanwhile, green-emissive MRs exhibit cofacial π-stacks, which lead to Haggregates with φ = 24%. We demonstrated that these easily fabricated organic microscale polymorphisms with controlled dimensions and crystalline forms can behave as optical resonators for laser applications.

Figure 1. (a, b) Fluorescence microscopy images of BF2AVB MRs obtained from samples of t = 3, 30 min, respectively, excited with unfocused UV light (330−380 nm). (c, d) SEM images of blueemissive and green-emissive MRs, respectively. (e, f) SAED patterns of single blue-emissive and green-emissive MRs. Insets show the corresponding TEM images. (g) XRD profiles of blueemissive and green-emissive MRs grown on silicon substrates. The top and bottom insets show cartoons of blue-emissive and greenemissive MRs.

respectively. Notably, these microcrystals exhibited distinctly different luminescent behaviors upon excitation with unfocused UV light (330−380 nm): intense blue and bright green emissions from samples of t = 3, 30 min, respectively. Figure 1c,d present scanning electron microscopy (SEM) images of two samples, respective. It can be seen that both blue- and green-emissive microcrystals are indeed two-dimensional (2D) MRs with smooth outer surfaces and sharp edges. In combination with atomic force microscopy (AFM) measurements (Figure S4), the blue-emissive MRs have a length (l) of 35−40 μm, a width (w) of 6−8 μm, and a height (h) about 740 nm (Figure 1c). Meanwhile, the green-emissive MRs have a length (l) ranging from 20 to 25 μm, an average width (w) of 5 μm, and a thickness (h) of about 800 nm (Figure 1d). 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 MR (Figure 1f). The sharp spots in SAED images reveal that these two MRs are single crystalline rather than polycrystalline. It is noted that BF2AVB crystals have two main crystalline forms: a monoclinic phase with lattice parameters of a = 24.448(3) Å, b = 7.0594(8) Å, c = 10.290(1) Å, α = γ = 90°, and β =

RESULTS AND DISCUSSION The small organic molecule BF2AVB was synthesized via a single-step reaction of BF3·OEt2 boronation in CH2Cl2.40 A mixture of avobenzone (AVB) and boron trifluoride diethyl etherate was added into a flask equipped with a reflux condenser and heated in an oil bath at 60 °C (see the Supporting Information for details and Figure S1). After silicagel 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 (1/1 v/v) and shaken by hand. Then this mixture was aged for different times (t): for instance, t = 3, 10, 30 min, respectively. In the second step, 100 μL of aged samples with different t values was drop-casted onto a quartz substrate. Accompanied by B


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Å. Along the slipped π-stack, two 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-totail fashion. In sharp contrast, as shown by the pink arrow in Figure 2b, BF2AVB molecules in the green-emissive MRs form cofacial π-stacks along the b axis of the 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 an H-type aggregate. Therefore, it is expected that Jtype coupling occurs in blue-emissive MRs and H-type coupling in green-emissive MRs. Spectroscopy Characterizations of J Aggregation in Blue-Emissive MRs and H Aggregation in GreenEmissive MRs. Figure 3a presents the normalized steadystate absorption (dashed line) and PL (solid line) spectra of blue-emissive MRs (middle panel) and green-emissive 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 dilute solution (5 × 10−6 M) exhibits 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, the 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. In contrast to that of the monomers, the absorption spectrum of green-emissive MRs exhibits an apparently blueshifted absorption with a maximum at 355 nm (bottom panel of Figure 3a). An additional band at 440 nm is due to the 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, which is much greater 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 nonrigid 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 photogenerated excitons relax rapidly into the lowest energy state by coupling with vibrational phonons (upper wavy arrow in Figure 3b). Because of the forbidden 0−0 emission and allowed 0−n (n > 1) sideband emissions, a 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 from the green-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 blueemissive MRs is at 421 nm, showing a typical exciton red shift.

94.303(4)° (CCDC No. 780003) and an orthorhombic phase with cell parameters of a = 14.5159(2) Å, b = 6.9687(1) Å, c = 17.3259(2) Å, and α = β = γ = 90° (CCDC No. 780002).40 Further analysis of the SAED pattern of a blue-emissive MR (Figure 1e) reveals that the sets of spots marked with triangles and circles are due to (001) and (010) Bragg reflections with dspacing values of 10.3 and 7.1 Å, respectively, in good agreement with the cell parameters of a 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 a monoclinic crystal structure and adopt a lamellar structure with the crystal (100) plane parallel to the substrate. Taking into account the image from transmission electron microscopy (TEM) (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 sets of SAED spots of green-emissive MRs marked with triangles and circles (Figure 1f) can be ascribed to the reflections from (200) and (010) planes of an orthorhombic structure with d-spacing values of 7.2 and 7.0 Å, respectively. Indeed, the XRD curve of green-emissive MRs in Figure 1g shows the relative abundance of (001) facets (d = 17 Å) on the surfaces of orthorhombic MRs. Combining 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). Combining the above analysis of XRD and SAED data with the help of monoclinic and orthorhombic crystal structures enables the molecular packings within blue-emissive and greenemissive BF2AVB MRs to be obtained, and they are 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 the monoclinic crystal, as shown by the pink arrow in the blueemissive MRs, therefore forming slipped π-stacks along the MR length direction with the shortest π−π separation of about 3.4

Figure 2. Molecular packing arrangement of BF2AVB molecules in blue-emissive MRs (a) and in green-emissive MRs (b), respectively. C


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

which is apparently 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) has 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 a coherent excited domain due to the coupling of the transition dipole moments of aggregated chromophores.42,50 The temperature-dependent PL spectra were also measured to further understand the assignment of J and H aggregates. In the case of green-emissive H aggregates, the 0−0 emission band was found to be enhanced with an increase in the temperature from 77 to 295 K (Figure S5a). However, in the case of blue-emissive J aggregates, the 0−0 emission band was found to be decreased with an increase in 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 Spano.51,52 Cluster-Mediated Nucleation and Subsequent OneDimensional Growth Mechanism toward 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 example, drop-casting the first-step samples with t ≤ 3 min and/or t ≥ 30 min on quartz plates gives rise to exclusively blue-emissive and/or green-emissive MRs, respectively. To probe the formation mechanism, we performed several more

Table 1. Photophysical Parameters of BF2AVB Monomers in Dilute Solution and Solid-State MRs sample

λabs (nm)

λem (nm)

Φa

τb (ns)

kfc (ns−1)

monomers

387 401 377 421 355 432

424 441 461 492 492

0.82

1.99

0.41

0.68

1.85

0.37

0.24

6.44

0.04

blue-MRs green-MRs

Φ of monomer solution in THF measured through a relative method by using Rhodamine 6G as a standard and Φ of MRs measured through an absolute method by using an integration sphere. b Fluorescence lifetime. cRadiative decay rate calculated according to kf = Φ/τ. a

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 of about 1900 cm−1 that is slightly closer to that of the monomers (about 1400 cm−1). These typical characteristics of a J-aggregate 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 information on the nature of the excited states, we performed time-resolved fluorescence measurements by a streak camera (Figure 3d). The emission of BF2AVB monomers at 424 nm decays monoexponentially (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, D


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Scheme 1. (a) Emission Spectra of the Aged Mixtures at Aging Times of t = 0, 3, and 30 min, Respectively,a (b) 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 a Xenon Lamp Equipped with a Band-Pass Filter, and (c) Schematic Model of Solution Self-Assembly for the Formation of Blue-Emissive and Green-Emissive MRs for Precursor Solution with t = 3, 10, and 30 min, Respectively

a

The excitation wavelength is 355 nm.

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 the 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 and 30 min underwent red shifts and changed in intensity apparently in comparison with those of the monomers (t = 0 min), which indicated that new clusters had been generated. (iii) We then monitored the growth process of MRs on quartz plates by using a PL microscope. Taking the green-emissive MRs as an example (Figure S6), at an early stage about 1.2 min after drop-casting the first-step sample with t = 30 min on a 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 by the evaporation of the solvents. Such a 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, 30 min, respectively) together. Drop-casting of this mixture on a 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. By combining the above experimental results, we propose a cluster-mediated nucleation followed by subsequent onedimensional growth for the formation of blue-emissive or green-emissive MRs (Scheme 1c). Jonkheijm and co-workers reported that aggregated molecular clusters can serve as high-

energy prenuclei 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 a 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 exhibit a PL maximum at 437 nm (blue curve, Scheme 1a) red-shifted by a value of 13 nm in comparison to 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, 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) are 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 green-emissive MRs (middle path in Scheme 1c). This suggests that J-type clusters convert into H-type clusters as the t value is prolonged. The key reason for controlled BF2AVB polymorphisms is the time-dependent 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 features of an active optical waveguide, such as brighter PL emission at the two end faces and little PL from the bodies of MRs (Figure 1a,b). It suggests that two self-assembled MRs can E


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width at half-maximum (fwhm) of the 0−1 peak dramatically decreases from 53 nm below the threshold to 5 nm above the threshold. When pump density is increased exceeding the threshold, strong laser emission at the 0−1 peak quickly dominates the whole spectrum. This agrees with the fact that the 0−1 transition has the maximum net gain in a 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, 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 a 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 an FP mode cavity.26,37,54 To further understand the cavity mode, the mode spacing Δλ between two adjacent resonance peaks (Supporting Information, Figure S8) is given by

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 homemade microphotoluminescence (μ-PL) system (Figure S7 in the Supporting Information). 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). Figure 4a reveals the PL spectra of an isolated blue-emissive MR (l = 30 μm) with an increase in the pump intensity (E) of

Δλ =

λ2 L[n − λ(dn/dλ)]

(1)

where n is the phase refractive index of the crystal, L is the 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 Figure 4c. The best-fit line (black line) is clearly linear, which confirms that the blue-emissive MRs operate as FP mode resonators along the length rather than the other dimensions of the blue-emissive MRs. Furthermore, a simulation of the 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. Therefore, the cavity Q factor is estimated to be as high as ∼985, which suggests that this high Q value is reasonably responsible for high-quality lasing emission. Similarly, the laser action of individual green-emissive MR was observed, as shown 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 fact that 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 greenemissive MRs. From this μ-PL image (the inset of Figure 5a), the brighter green spots at two end faces and weaker

Figure 4. (a) PL spectra of a single blue-emissive MR with l = 30 μm under different pump energies 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) Mode spacing Δλ at λ = 485 nm versus 1/L of blue-emissive MRs, clearly showing a linear relationship. Inset: simulated 2D normalized electric field (λ 485 nm, n = 1.80) in the cross section of blue-emissive MR.

the 400 nm laser. At a 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 the 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 F


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Figure 6. Variation of the lasing threshold for blue-emissive MRs (black squares) and green-emissive MRs (red triangles) as a function of different lengths, respectively.

shows that the results of the laser threshold Eth of the blueemissive MRs are illustrated by black squares and those of the green-emissive MRs are demonstrated by red triangles. The square dots are always below the triangles at equivalent optical path lengths, indicating that the thresholds Eth of the blueemissive MRs are always lower than those 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 the two MR cavities (Q ≈ 985 and 923, respectively). We think that the main reason might come from their different PL quantum yields. It can be seen from Table 1 that J-aggregation blue-emissive MRs exhibit a PL quantum yield of Φ = 0.68, much higher that of Haggregation green-emissive MRs (Φ = 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 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) Mode spacing Δλ at λ 495 nm versus 1/L of green-emissive MRs, clearly showing a linear relationship. Inset: simulated 2D normalized electric field (λ 495 nm, n = 1.80) in the cross section of green-emissive MR.

CONCLUSION In conclusion, we have demonstrated 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 the 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 effects 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 in understanding the structure−property relationship and enrich the functionality of high-performance organic solid-state coherent microscale light sources.

luminescence in the body indicates that the the green-emissive MRs may also be FP mode cavities. The mode spacing Δλ being inversely proportional to L (Figure 5c) demonstrates that the green-emissive MRs also function as the FP mode resonators along the length. A simulation of the 2D electric field distribution along the cross section of green-emission MRs also confirms that the FP mode indeed exists along the length direction of the individual MR (inset of Figure 5c). The cavity Q factor of the 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 the blue-emissive MR cavity. Furthermore, laser operation of two MRs can both be done without fatigue for 4 h (Figure S9), suggesting their excellent stability. Moreover, these two kinds of polymorphisms were further compared by their laser threshold (Figure 6) under the different optical cavity lengths. The laser threshold is one of the basic conditions of the lasing formation and is a significant parameter in evaluating a laser. We selected 13 blue-emissive MRs and green-emissive MRs with different lengths for the experiments of the lasing threshold, respectively. Figure 6

EXPERIMENTAL SECTION Molecular Synthesis. An analytical balance was used to measure 310 mg of avobenzene (1 mmol) into a 100 mL three-necked flask, and a clean magnet was placed into the flask. About 20 mL of dichloromethane was placed in the flask at one end with a syringe under argon, about 0.5 mL of boron trifluoride (diethyl ether) was placed at the other end, and the mixture was stirred at room temperature for 12 h. The solution slowly became pale yellow and had a yellowish green fluorescence. After completion of the reaction, the G


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ACS Nano solvent was spin-dried and the product was purified by column chromatography. The product was then recrystallized from ethanol to give a yellow powdery product, which was weighed; the calculated yield was about 80%. Structural and Spectral Characterization of MRs. TEM images were obtained with a JEOL JEM-1011 transmission electron microscope (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 sizes of microribbons were also examined by field emission scanning electron microscopy (FESEM, Hitachi, 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 with an Olympus Research inverted system microscope (FV1000IX81, Tokyo, Japan) equipped with a charge-coupled device (CCD, Olympus DP 71, Tokyo, Japan) camera. The excitation source was 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 with 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 on 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 was 0.1 nm. By placing a movable aperture on the PL collection optical path, spatially resolved PL spectra from individual MRs could 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 of 1 nm and a time resolution of 10 ps.

ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China (Grant No. 2017YFA0204503), the National Natural Science Foundation of China (Grant Nos. 21503139, 21521062, and 21673144), the Beijing Natural Science Foundation of China (Grant No. 2162011), High-level Teachers in Beijing Municipal Universities in the Period of 13th Five-year Plan (Grant Nos. IDHT20180517 and CIT&TCD20180331), and the Youth Innovative Research Team of Capital Normal University. REFERENCES (1) Murphy, A. R.; Frechet, J. M. J. Organic Semiconducting Oligomers for Use in Thin Film Transistors. Chem. Rev. 2007, 107, 1066−1096. (2) Liu, S.; Wang, W. M.; Briseno, A. L.; Mannsfeld, S. C. E.; Bao, Z. Controlled Deposition of Crystalline Organic Semiconductors for Field-Effect-Transistor Applications. Adv. Mater. 2009, 21, 1217− 1232. (3) Ichikawa, M.; Hibino, R.; Inoue, M.; Haritani, T.; Hotta, S.; Araki, K.; Koyama, T.; Taniguchi, Y. Laser Oscillation in Monolithic Molecular Single Crystals. Adv. Mater. 2005, 17, 2073−2077. (4) Samuel, I.; Turnbull, G. Organic Semiconductor Lasers. Chem. Rev. 2007, 107, 1272−1295. (5) Coleman, A. C.; Beierle, J. M.; Stuart, M. C. A.; Macia, B.; Caroli, G.; Mika, J. T.; van Dijken, D. J.; Chen, J.; Browne, W. R.; Feringa, B. L. Light-Induced Disassembly of Self-Assembled Vesicle-Capped Nanotubes Observed in Real Time. Nat. Nanotechnol. 2011, 6, 547− 552. (6) Rybtchinski, B. Adaptive Supramolecular Nanomaterials Based on Strong Noncovalent Interactions. ACS Nano 2011, 5, 6791−6818. (7) Chen, Z.; Lohr, A.; Saha-Moeller, C. R.; Würthner, F. SelfAssembled Pi-Stacks of Functional Dyes in Solution: Structural and Thermodynamic Features. Chem. Soc. Rev. 2009, 38, 564−584. (8) Elemans, J.; Van Hameren, R.; Nolte, R. J. M.; Rowan, A. E. Molecular Materials by Self-Assembly of Porphyrins, Phthalocyanines, and Perylenes. Adv. Mater. 2006, 18, 1251−1266. (9) Würthner, F.; Kaiser, T. E.; Saha-Moeller, C. R. J-Aggregates: From Serendipitous Discovery to Supramolecular Engineering of Functional Dye Materials. Angew. Chem. Int. Ed. 2011, 50, 3376−3410. (10) Wei, L.; Yao, J.; Fu, H. Solvent-Assisted Self-Assembly of Fullerene into Single-Crystal Ultrathin Microribbons as Highly Sensitive UV-Visible Photodetectors. ACS Nano 2013, 7, 7573−7582. (11) Park, C.; Park, J. E.; Choi, H. C. Crystallization-Induced Properties from Morphology-Controlled Organic Crystals. Acc. Chem. Res. 2014, 47, 2353−2364. (12) Liao, Q.; Jin, X.; Zhang, H.; Xu, Z.; Yao, J.; Fu, H. An Organic Microlaser Array Based on a Lateral Microcavity of a Single Jaggregation Microbelt. Angew. Chem. Int. Ed. 2015, 54, 7037−7041. (13) Zhang, W.; Yao, J.; Zhao, Y. S. Organic Micro/Nanoscale Lasers. Acc. Chem. Res. 2016, 49, 1691−1700. (14) Dong, H.; Wei, Y.; Zhang, W.; Wei, C.; Zhang, C.; Yao, J.; Zhao, Y. S. Broadband Tunable Microlasers Based on Controlled Intramolecular Charge-Transfer Process in Organic Supramolecular Microcrystals. J. Am. Chem. Soc. 2016, 138, 1118−1121. (15) Li, Q.; Li, Z. The Strong Light-Emission Materials in the Aggregated State: What Happens from a Single Molecule to the Collective Group. Adv. Sci. 2017, 4, 1600484. (16) Desiraju, G. R. Polymorphism: The Same and Not Quite the Same. Cryst. Growth Des. 2008, 8, 3−5. (17) Pick, A.; Klues, M.; Rinn, A.; Harms, K.; Chatterjee, S.; Witte, G. Polymorph-Selective Preparation and Structural Characterization of Perylene Single Crystals. Cryst. Growth Des. 2015, 15, 5495−5504. (18) Lei, Y. L.; Liao, Q.; Fu, H. B.; Yao, J. N. Phase- and ShapeControlled Synthesis of Single Crystalline Perylene Nanosheets and Its Optical Properties. J. Phys. Chem. C 2009, 113, 10038−10043.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b00150. General experimental details and spectral data (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail for Q.L.: [email protected]. *E-mail for H.F.: [email protected]. ORCID

Hongbing Fu: 0000-0003-4528-189X Author Contributions

Q.L. and H.F. conceived the idea. X.G.W., S.L., and Y.Z. performed experiments, X.G.W., Z.X., Q.L., and H.F. analyzed the results. X.G.W., Q.L., and H.F. prepared the manuscript. All authors reviewed the manuscript. Notes

The authors declare no competing financial interest. H


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

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