Proton-Controlled Organic Microlaser Switch

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Proton-Controlled Organic Microlaser Switch Zhenhua Gao, Wei Zhang, Yongli Yan, Jun Yi, Haiyun Dong, Kang Wang, Jiannian Yao, and Yong Sheng Zhao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01607 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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Proton-Controlled Organic Microlaser Switch Zhenhua Gao,†,§ Wei Zhang,† Yongli Yan,*,† Jun Yi,‡ Haiyun Dong,†,§ Kang Wang,†,§ Jiannian Yao,†,§ and Yong Sheng Zhao*,†,§ †

Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences,

Beijing 100190, China ‡

Department of Chemistry, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji, Tokyo

192-0397, Japan §

University of Chinese Academy of Sciences, Beijing 100049, China

*Email: [email protected];[email protected]

ABSTRACT: Microscale laser switches have been playing irreplaceable roles in the development of photonic devices with high integration levels. However, it remains a challenge to switch the lasing wavelengths across a wide range due to relatively fixed energy bands in traditional semiconductors. Here, we report a strategy to switch the lasing wavelengths among multiple states based on proton-controlled intramolecular charge-transfer (ICT) process in organic dye-doped flexible microsphere resonant cavities. The protonic acids can effectively bind onto the ICT molecules, which thus enhance the ICT strength of the dyes, and lead to a redshifted gain behaviors. On this basis, the gain region was effectively modulated by using acids

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with different proton donating ability, and as a result, laser switching among multiple wavelengths was achieved. The results will provide guidance for the rational design of miniaturized lasers with performances based on the characteristic of organic optoelectronic materials.

KEYWORDS: organic nanomaterial, laser switch, smart responsiveness, proton acid, nanophotonic material

Miniaturized lasers have attracted broad research interest for their potential applications ranging from biological and chemical sensing to on-chip optical communication.1-3 The everincreasing demands on information density and bandwidth in highly integrated photonic devices urgently require microlasers capable of multi-wavelength emitting and wavelength switching.4-9 Recently, laser switch have been realized by manipulating the resonant mode through altering the size or refractive index of the microcavities.10,11 However, these ways suffer from a rather limited tunable range, which cannot satisfy the requirements of applications in multicolor detection and multiband communication. An alternative approach to switching the lasing wavelength across a wide range is to tailor the gain region of lasing material.12 Nevertheless, the energy band structures in traditional semiconductors are generally fixed, making it difficult to dynamically modulate their gain region. Organic optoelectronic materials, with abundant energy levels and tailorable excited-state processes, afford an ideal platform to achieve dynamically tunable optical gain.13,14 These compounds have shown great potential for tailoring the gain region based on the cooperative gain processes between two distinctstates,15,16 which inspires us to further exploit multistates by


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chemically modifying the excited-state processes. The gain region of the intramolecular chargetransfer (ICT) molecules strongly relies on the electron-accepting or electron-donating strength in the molecular skeleton,17 which can be effectively modulated by interacting with the surrounding environment, thereby endowing these molecules with smart responsiveness to external stimuli.18-20 The stimulated ICT molecules may produce multiple emissive states, which thus provide an opportunity for controllably switching the lasing wavelength. Herein, we propose a strategy to switch the lasing wavelengths among multiple states based on proton-controlled ICT process in organic dye-doped flexible microcavities. The proton-donating acid can effectively bind onto the molecules in the organic whispering-gallery-mode (WGM) cavities, which were synthesized with an emulsion–solvent–evaporation self-assembly strategy. The acid binding enhances the ICT strength of the dye, and leads to a red-shifted optical gain. On this basis, the gain region was effectively modulatedbyusing acidswith different proton donating ability, andas a result, laser switching among multiple states was achieved. More generally, the lasing action not only offers a insight into the charge transfer process of organic materials, but provides a guidance for the development of miniaturized lasers with specific functionalities compared with the traditional semiconductor materials.

RESULTS AND DISCUSSION The working mechanism of the responsive ICT dye is displayed in Figure 1A. Organic polarized molecules, where a π-conjugated bridge is end-capped with an electron-donor (D) and an electron-acceptor (A) group, usually exhibit an obvious ICT characteristic.21 Strengthening the electron-donating or electron-accepting abilities would directly enhance the ICT strength and


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hence decrease the exciton bandgap.22 In addition, these dyes can interact with surrounding molecules through intermolecular interactions such as hydrogen bonds, π-π stacking, van der Waals forces etc., which would affect their energy levels.23,24 Since the energy levels are closely related to the gain regions of the dyes, such a responsiveness offers a chance to tune the gain region via external stimuli. 3-(2-benzothiazoly)-7-diethylaminocoumarin (C6, Figure 1B) was selected as the model compound to modulate its gain region. Theemission band of C6 monomers gradually red-shifts with the increase of the solvent polarity, which suggests an obvious solvatochromic effect, demonstrating that the C6 moleculesare of strong ICT characteristics (Figure S1).25 It is worth noting that there is a benzothiazole group in C6 molecule, in which the nitrogen atom can be viewed as a good recognition element.26 The lone-pair electrons on the nitrogen atom can interact with various protonic acids because of the Lewis base character, which would affect the ICT strength of C6 dye and thus give rise to marked changes in photoluminescence (PL), providing an opportunity to tailor the gain regions by various protonic acids. Moreover, the good compatibility of organic small molecules with polymers ensures the effective doping of the C6 into flexible polymer matrix, which can readily assemble into mcirodisks or microspheres,27-31 possibly triggering a low-threshold WGM lasing emission. An emulsion-solvent-evaporation method was utilized to fabricate C6 dye doped organic WGM resonators with high quality (Q) factors.32 In a typical preparation (Figure 1C), well mixed C6/Polystyrene(PS)/CH2Cl2 solution was added into cetyltrimethylammonium bromide (CTAB) aqueous solution. Under vigorous stirring, an oil-in-water emulsion was formed (Figure S2). Hydrophobic C6/PS/CH2Cl2 solution would be encapsulated into the hydrophobic interior of the


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CTAB micelles. Driven by interfacial tension, PS molecules with low crystallinity prefer to aggregate into spherical structures after complete evaporation of CH2Cl2 solvent.

Figure 1.Design and preparation of ICT dye doped microlasers. (A) Illustration of a stimuliresponsive ICT molecule. (B) Molecular structure of C6. (C) Schematic diagram of the fabrication processes for the C6 doped microspheres. (D) PL spectra of an individual C6 doped microsphere under different pump fluences. Inset: PL image of the microsphere under laser excitation. Scale bar is 5 μm. (E) The PL peak intensities and FWHM at 525 nm as a function of pump fluence. As shown in the top- and side-view scanning electron microscopy (SEM) images (Figure S3), the obtained microspheres have perfect circular boundary and ultrasmooth surfaces, which are


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favorable for WGM resonance (Figure S4). When a dye-doped microsphere was excited locally with a focused pulsed laser beam (400 nm, 200 fs) in a homemade microphotoluminescence system (Figure S5), a bright ring shape pattern was observed at the outer boundary (Figure 1D, inset), indicating total internal reflection of the emitted light along the edge of the sphere. With increasing pump fluence, the PL intensity in the gain region were dramatically amplified, manifesting lasing action from the dye doped microstructure (Figure 1D). The spectra show two separated lasing envelopes as a result of the breakdown of the degeneracy of the transverse electric (TE) and transverse magnetic (TM) modes in the microspheres. This was further verified by the spatially resolved polarization-sensitive PL spectra (Figure S6).The corresponding dependence of the PL intensity on pump fluence showed a nonlinear behavior at the threshold of 162 nJ cm-2 (Figure 1E).Above the lasing threshold, the full-width at half-maximum (FWHM) at 525 nm dramatically narrows down to about 0.4 nm, revealing a sharp increase of temporal coherence.33We then measured input-output intensity profiles of 20 different mcirospheres and all the samples showed obviously nonlinear amplification behavior with low lasing thresholds of 155-171 nJ cm-2 (Figure S7), further verifying that C6 functions as a good lasing medium in these microspheres. Furthermore, the C6 doped mcirospheres demonstrate high photostability under laser operation, which may be attributed to the high-quality microcavity and efficient energy level of the C6 dye (Figure S8). The lone-pair electrons on the benzothiazole segment of the C6 dye can interact with acids, which might be utilized to tailor the lasing wavelength from the microsphere. Here, we selected hydrochloride (HCl) to modulate the ICT strength due to its good proton-donating ability. Under HCl gas atmosphere, the as-prepared microspheres changed their fluorescence color from cyan (Figure 2A) to green (Figure 2B). The confocal microscopy images of a microsphere recorded


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under exposure of HCl vapor indicate that the acid vapor can penetrate inside the microcavities and bind to the C6 molecules efficiently (Figure S9).The corresponding PL spectra further confirm that the HCl gas did promote C6 to its protonated state (Figure 2C).34 The NMR spectra clearly reveal a significant downfield shift of the protons close to thiazole moiety and negligible shifts of other protons (Figure S10), indicative of protonation of the nitrogen on the thiazole unit. The electronic structures of the C6 and protonated C6 (C6-HCl complex) were investigated using density functional theory (DFT) in Gaussian 09, where the geometry had been optimized in vacuum (Figure 2D). The optimized steric configuration of the C6-HCl complex reveals that the distance between N atom in C6 and H atom in HCl is 1.11 Å, which is much shorter than the distance of a moderate hydrogen bond (~2.0 Å).35 This results originatefrom the relatively strong acid-base interaction between C6 and HCl molecules, which has a direct impact on the electroncloud density in C6 dye. As shown in Figure 2E, the highest occupied molecular orbital (HOMO) density of neutral C6 is delocalized on the entire molecule, while the lowest unoccupied molecular orbital (LUMO) situates mainly on the benzothiazole and the adjacent lactone group. The HCl binding leaves the LUMO relatively unchanged, while the HOMO’s electron cloud on the benzothiazole groups decreased, indicating that the ICT character of the protonated C6 is enhanced compared with that of the neutral C6. Consequently, protonation reduces the HOMO-LUMO gap from 3.33 eV to 3.04 eV, which is in good accordance with the red-shift in optical spectra after treatment with HCl gas. Benefitting from their distinct proton donating abilities, different acids may have varied effects on the ICT process of C6 dye. HF was selected as another proton acid to tailor the gain region of the C6. The bond lengths of HF is 0.92 Å, which is a bit shorter than that of HCl (1.29 Å, Figure 2F), suggesting a relative weaker proton donating ability.36 The optimized steric


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configuration of the C6-HF complex reveals that the distance between N and H in C6-HF complex is 1.50 Å (Figure 2G), which is much longer than that of C6-HCl complex (1.11 Å), indicating that HF would have less impact on the energy levels of C6 dye. Consequently, the calculated HOMO-LUMO gap in C6-HF complex (3.24 eV, Figure 2H) is higher than that in C6HCl complex (3.04 eV), agreeing well with the corresponding optical spectra after the treatment with the two acid vapors (Figure S11). Therefore, it is reasonable to believe that distinct energy levels of the C6 dye can be achieved via combining with acids of different proton donating ability.

Figure 2. Effects of different acids on the electronic structures of the C6 dye. (A, B) PL images of microspheres of neutral C6 (A) and C6-HCl complex (B), respectively, under UV band


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(330−380 nm) excitation. Scale bars are 5 μm. (C) Corresponding PL spectra of the microspheres shown in a and b. (D) Optimized molecular structure of the C6-HCl complex. (E) Pictorial presentations of the HOMOs and LUMOs in C6 molecule and C6-HCl complex. (F) Optimized molecular structures of HF and HCl molecules. The bond length of HF molecule is 0.92 Å, while that of HCl is 1.29 Å. (G) Optimized molecular structures of C6-HF complex. (H) Corresponding pictorial presentations of the HOMO and LUMO in C6-HF complex. The protonic acids controlled ICT process in C6 providesan effective way to broadly modulate the gain region. As shown in Figure 3A, the C6 dye undergoes a transition from the groundstate (S0) to the high vibronic level of the first singlet excited state (S1) under optical pump. Then it vibrationally cools fast to the bottom of the first singletexcited state. The lasing transition occurs down to the vibronic levels of the ground state, followed by a rapid relaxation to the bottom vibronic state.37,38 The HCl or HF molecules can affect the energy levels of C6 to different extents, which would bring multiple well-defined optical gain regions, affording an ideal platform to tune the lasing wavelength of C6 dyes via different acids (Figure 3B). Under HCl atmosphere, the lasing action at ~580 nm was observed in an individual dyedoped microsphere (Figure 3C). Since the refractive index of HCl vapors (n=1.01) is similar to that of the air (n=1.00),39 so we could rule out the influence of refractive index changing on the wavelength red-shift. The lasing wavelength can be switched within 1s and then keep unchanged upon exposure to HCl atmosphere. Such high switching speed might be attributed tothe acid-base interaction between C6 and HCl molecules as well as high diffusion rate of the small acid molecule into the polymer mcirosperes(Figure S12). Replacing HCl with HF vapor resulted in the lasing wavelength switching to ~560 nm (Figure 3E). On this basis, multi-gain


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regions have been successfully achieved under acids stimuli in the C6 doped microscale WGM resonant cavities.

Figure 3. Effects of different acids on the lasing property of the microlasers. (A) Potential energy diagrams and corresponding lasing processes of C6, C6-HF, and C6-HCl complexes. (B) Schematic illustration for the interaction of C6 molecules with acid vapor in a typical organic WGM cavity. (C, D) Pump fluence-dependent PL spectra of microspheres with C6-HCl (C) and their PL peak intensities and FWHM at ~580 nm as a function of pump fluence (D). (E, F) Pump fluence-dependent PL spectra of microspheres with C6-HF (E) and their PL peak intensities and


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FWHM at ~560 nm as a function of pump fluence (F). Insets: corresponding PL images of the microspheres under laser excitation. Scale bars are 5 μm. The lasing thresholds were determined to be 180 nJ cm-2 and 168 nJ cm-2 for C6-HCl and C6-HF microspheres, respectively (Figure 3D, F). In comparison with that of the pristine microsphere (162 nJ cm-2), the HCl and HF did not bring about an obvious increase in the lasing threshold. Furthermore, the FWHM at 580 nm and 560 nm dramatically narrows down to about 0.4 nm above their lasing thresholds, confirming that the microspheres still keep a high quality after exposure to the HCl and HF vapors. The WGM cavity, providing efficient optical feedback without bandwidth limitation,40 can simultaneously support the lasing actions in the C6, C6-HF and C6-HCl emission bands. Therefore, it is possible toreversibly switch the lasing wavelength in a single microsphere cavity by adjusting the balance among the emissions from the three species. As the HCl or HF vapor in the surrounding medium evaporated in air, the C6 doped microsphere would recover to the initial stateafter 2 minutes (Figure 4A). We subsequently utilized a gas flowing system to realize a multi-wavelength switchable microlaser. As schematically illustrated in Figure 4B, a C6 doped microsphere was sealed in the chamber, where different acid vapors and air could be introduced in real time. The microsphere with a diameter of 3.6 μm was excited with the 400 nm pulse laser and the lasing spectra were measured in response to the variation of the atmosphere inside the chamber.


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Figure 4. Organic microlaserswitch with multistates. (A) Fluorescence microscopy images of a typical microsphere under alternate exposure to HCl, HF and air, respectively. Scale bars are 5 μm. (B) Schematic of the microlaser measurement under exposure to different atmospheres. (C) The evolution of lasing spectra of a typical sphere under the alternate exposure to HF, HCl, and air respectively. (D) Plot of the lasing wavelength (528 nm, 561 nm and 582 nm) against the switching cycles. The measurement was conducted at a fixed pump fluence, a little bit higher than the highest threshold among the three species. Figure 4C shows that the lasing emission can be switched among three distinct wavelengths (λAir=528 nm, λHF=561 nm, and λHCl=582 nm) by alternating plugs of the three gases. Furthermore, the lasing spectral shift remained almost unchanged after tens of continuous cycles (Figure 4D), indicating the significant stability and reliability of


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the multi-wavelength switchable microlasers. The wavelength tuning mechanism was attributed to the remote chemical stimulated ICT behaviors, which would advance the fundamental understanding of the well-controlled ICT processes. Furthermore, the lasing spectra of the microspheres exhibit a series of narrow lasing emission which can allow for easily distinguishable readout. Such a feature constitutes a signature of the microsphere and allows for definition of a barcode-type sensor. Therefore, the barcode-type system can distinguish different protonic acids based on proton-controlled intramolecular charge-transfer (ICT) process, which would enlighten the development of tunable miniaturized lasers and other responsive nanophotonic devices that can be applied in chemical sensing or environmental monitoring.

CONCLUSIONS In conclusion, the lasing wavelengths have been switched among multiple states based on the proton controlled ICT process in dye-doped organic WGM microcavities. The ICT strength of laser dye can be enhanced through the binding with proton acid, which further affects the gain region of the dye. As a result, multiple gain regions were achieved through manipulating the ICT process by changing the proton donating ability of acid. On this basis, we have achieved microscale lasers with three distinct reversibly states. More generally, the lasing behavior offers an insight into the gain process of the organic materials and provides guidance for the development of photonic devices with specific functionalities based on the characteristic of organic materials.


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METERIALS AND METHODS Materials. The polystyrene (PS, M.W. 250,000; CAS number, 9003-53-6) and Coumarin 6 (C6, C20H18N2O2S; CAS number, 38215-36-0) were purchased from Acros Organics. The cetyltrimethylammonium bromide (CTAB, C19H42BrN; CAS number, 57-09-0) was purchased from Innochem. All chemicals were used without further treatment. Preparation of C6 doped PS microspheres. The C6 doped PS microspheres were prepared through an emulsion-solvent-evaporation method. In a typical preparation, 50 μL well mixed C6/PS/dichloromethane (CH2Cl2) solution was added into 500 μL CTAB aqueous solution (2 mmol), which was subsequently treated with vigorous stirring. After aging for 2h, the C6 doped PS microspheres were obtained in the colloid solutions. Later, the surfactant CTAB was removed through filtration and washing. The precipitate was redispersed in aqueous solution and then used to prepare samples for further characterizations by drop-casting. The diameters of obtained spheres can be well tuned from 3 to 20 μm through increasing the concentration of PS from 10 to 50 mg mL−1. The C6 dye was added to the polymer solution at a concentration of ∼1 wt % relative to PS. Characterization. The morphology of the composite microspheres was examined through scanning electron microscopy (Hitachi S-4800). The fluorescence spectra were measured on a fluorescence spectrometer (Hitachi F-7000). Bright-field optical images and fluorescence microscopy images were taken with an inverted fluorescence microscope (Nikon Ti-U). A focused 400 nm pulse laser beam (200 fs, 1000 Hz) was employed to locally excite the composite microspheres and the spatially resolved spectra were recorded with a monochrometer (Princeton Instrument Acton SP 2300i) connected with an EMCCD (Princeton Instrument


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ProEM 1600B). Laser confocal fluorescence microscope (Olympus FV1000-IX81) equipped with 405 nm laser was used for micro-area photoluminescence intensity analysis. Calculation methods. Theoretical calculations were carried out with the Gaussian 09 software. The geometry optimizations were performed by B3LYP function41,42 with 6-311G (d,p) basis sets.43 After the optimization by the density functional theory (DFT) calculation, the electroncloud density of HOMO and LUMO and their corresponding orbital energies were obtained.

ASSOCIATED CONTENT Conflict of Interest: The authors declare no competing financial interest. Supporting Information. The Supporting Information is available online. PL spectra of C6 molecules in different solvents; Emulsification process images; SEM images; Electric field distribution of resonant cavity modes; Experimental setup for the optical characterization; Spatially resolved polarization-sensitive lasing spectra; Lasing thresholds of 20 different microspheres; Lasing intensity as a function of the number of pump pulses; Confocal microscopy images of a microsphere; 1H NMR spectra of C6 and C6+HCl; PL spectra of microspheres after treatment with HF and HCl vapors; Lasing signals of the microspheres with different exposure time of acid vapors are given in Figures S1-S12. AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected]


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Author Contributions Y.S.Z. conceived the idea. Y.S.Z., J.Y. and Y.Y. supervised the project. Z.G. designed the experiments and prepared the materials. Z.G. and Y.Y. performed the optical measurements. Z.G., W.Z., Y.Y. J.Y. and K.W. put forward the theoretical model and contributed to the theoretical calculations. Z.G., W.Z., Y.Y., H.D. and Y.S.Z. analyzed the data. Z.G.,Y.Y. and Y.S.Z. wrote the manuscript. All authors discussed the results and commented on the manuscript.

ACKNOWLEDGMENT This work was supported financially by the Ministry of Science and Technology of China (Grant Nos. 2017YFA0204502 and 2015CB932404), the National Natural Science Foundation of China (Grant Nos. 21773265, 21790364 and 21533013), and the Youth Innovation Promotion Association CAS (2014028). Moreover, we would like to thank the Research Center for Computer Science/Institute for Molecular Science, Okazaki, Japan for providing the computational resources used in this study.

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Graphical Table of Contents


Proton-Controlled Organic Microlaser Switch

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