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Supramolecular Solid-State Microlaser Constructed from Pillar[5]arene-Based Host-Guest Complex Microcrystals Bin Hua, Wei Zhou, Zhaoliang Yang, Zhihua Zhang, Li Shao, Haiming Zhu, and Feihe Huang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11156 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018
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Supramolecular Solid-State Microlaser Constructed Pillar[5]arene-Based Host−Guest Complex Microcrystals
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Bin Hua,† Wei Zhou,† Zhaoliang Yang, Zhihua Zhang, Li Shao, Haiming Zhu* and Feihe Huang* State Key Laboratory of Chemical Engineering, Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China; Fax and Tel: +86-571-8795-3189; Email:
[email protected];
[email protected]. ic microlasers and successfully prepared host−guest complex microcrystals with the organic laser dye DASP and host matrix P5 (Figure 1). Due to mono-dispersion and geometric restriction induced by the scaffold of P5, the host−guest complex microcrystal 2P5DASP shows much higher photoluminescence efficiency than individual DASP microcrystals. Moreover, by virtue of its regular shape and smooth surfaces, each 2P5DASP microcrystal forms a Fabry-Pérot (F-P) cavity itself and outputs lasing emission with a high Q factor and a low threshold.
ABSTRACT: Herein we report a novel host−guest complex single crystal material based on a pillar[5]arene (P5) and a laser dye trans-4-[p-(dimethylamino)styryl]-1methylpyridinium iodide (DASP). Single crystal structure characterization shows that two P5 molecules act as a supramolecular nanocapsule for one isolated DASP molecule. This spatial confinement effectively restricts aggregation-caused quenching (ACQ) effects and other nonradiative channels (e.g., twisted-intramolecular-charge-transfer), leading to high luminescent efficiency of the isolated DASP. Together with its regular shape and smooth surface, each 2P5DASP microcrystal forms a Fabry-Pérot (F-P) microcavity and produces a stable laser output with a high Q factor and a low threshold. In recent decades, organic solid-state microlasers have attracted wide research attention due to their tremendous potential in areas ranging from on-chip optical communication to chemical and biological sensing.1 Compared with inorganic counterparts, organic materials possess unique advantages for the construction of high performance microlasers, including easy fabrication, mechanical flexibility, excellent biocompatibility, and wide spectral coverage.2 Organic luminescent molecules such as laser dyes generally exhibit high emission efficiency and excellent lasing performance in dilute solutions. However, most of them cannot be directly integrated as gain media in solid-state lasers due to the notorious aggregation-caused quenching (ACQ) and other nonradiative decay channels. These quenching effects prevent facile population inversion in the solid state, posing significant limitations on the advancement of organic solid-state microlasers.3
Figure 1. Chemical structures of P5 and DASP. We first investigated host−guest interactions between P5 and DASP in solution using 1H NMR. As shown in Figure 2, compared with the spectrum of free DASP, significant upfield shifts corresponding to proton signals of DASP occur in the presence of P5. Meanwhile, broadening effects related to protons Hdh on DASP are observed due to complexation dynamics. These results indicate that DASP is encapsulated by the cavity of P5 and the protons on DASP are shielded by the electron-rich cyclic structure of P5 upon formation of an inclusion complex.8 A 2D NOESY NMR investigation was conducted to study the relative positions of the components in the inclusion complex. Strong correlations are observed between proton H4 on P5 and proton Hf on DASP (Figure S1), indicating that proton Hf resides in the cavity of P5, consistent with 1H NMR characterization.
One effective strategy to circumvent these intermolecular quenching effects is to spatially separate the luminescent molecules by incorporating them as guests in host matrices.4 To date, materials scientists have developed a variety of host matrices, including polymers, molecular/ionic crystals, gelatins, and sol–gel glasses for organic solid-state microlasers.5 These host materials usually lack specific interactions with guest dye molecules and only act as typical dispersion media. Therefore, the orientation of dye molecules and stoichiometric ratio between host/guest molecules cannot be easily controlled in a precise manner. The dye concentration in these matrices is usually intentionally kept low to avoid the ACQ effect, which limits the optical gain and the output power of microlasers. Aiming to construct solid-state microlasers with a high concentration of doped dye molecules, while suppressing the self-quenching effects, novel, appropriate and smart host materials are urgently needed to precisely sequester every single dye molecule. In search of an ideal host material, we focus on a novel class of macrocyclic hosts, pillararenes.6 Recently, our group reported per-ethylated pillar[6]arenes as excellent solid-state host matrices for iodine capture with high chemical and thermal stability.7 Inspired by this, we explored the possibility of dispersing laser dyes into pillararene-based host matrices to construct solid-state organ-
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Figure 3. (a) Bright-field optical microscopic and (b) fluorescence microscopic images of 2P5DASP microcrystals. (c) Side view of the X-ray single crystal structure of 2P5DASP. (d) Packing structure of 2P5DASP.
Figure 2. Partial 1H NMR spectra (500 MHz, CDCl3:DMSO-d6 = 1:1, 295 K): (a) DASP; (b) 5.00 mM DASP and 5.00 mM P5; (c) P5.
As expected, these microcrystals emit a strong red fluorescence at 637 nm, as can be seen in Figure 3b and the photoluminescence (PL) spectrum in Figure 4a. The absolute fluorescence quantum yield from 2P5DASP microcrystals is measured to be 13.4%, much stronger than pure DASP powders (~1%) and DASP in solution (~0.2%).14 The photoluminescence excitation (PLE) spectrum of an individual 2P5DASP microcrystal shows the lowest energy absorption peak at 610 nm, yielding a Stokes shift of ~85 meV. The dramatically enhanced photoluminescence and much smaller Stokes shift suggest the effective suppression of TICT formation and ACQ effect in 2P5DASP microcrystals. Fluorescence titration experiments were performed to confirm it. Upon gradual addition of P5 to a solution of DASP, the peak corresponding to DASP showed a marked increase in intensity and a continuous blue shift (Figure S7). Additionally, we performed transient absorption (TA) measurements to probe the excited state evolution process. The position of the bleach peak on the TA spectrum directly reflects the transient energetics of the occupied excited states. As shown in Figures S8 and S9, TA spectra on 2P5DASP microcrystals show a near-constant bleach peak with only a slight redshift in ~100 ps, while that of DASP in methanol exhibits a large redshift in ~10 ps. The TA results confirmed the localization to the low energy TICT state in the DASP solution, but not in 2P5DASP microcrystals, consistent with PL results.15
After confirming the formation of an inclusion complex between P5 and DASP, we developed a simple vapor diffusion method to obtain a large number of high-quality host−guest complex microcrystals. Optical microscopic images of a representative batch of microcrystals are shown in Figure 3a and S2. These microcrystals have a regular rhomboidal shape with smooth surfaces and flat end facets, which can serve as superior optical cavities. Single crystal X-ray crystallography yielded the crystal structure of the hostguest complex between P5 and DASP. Surprisingly, DASP and P5 did not form a common 1:1 host−guest threaded structure. Instead, a supramolecular nanocapsule structure was formed with a 2:1 host−guest stoichiometric ratio (2P5DASP). As shown in Figure 3c, each of two centrosymmetric P5 molecules extends one outer edge to accommodate one isolated DASP molecule. The electron-poor pyridinium group of DASP resides in the electron-rich cavity of a pillar[5]arene molecule bound through donor-acceptor interactions and cation/π interactions.9 Multiple intermolecular C-H/Br hydrogen bonds drive two P5 molecules to form a nanocapsule (Figures S3 and S4). As we know, it is very rare to see such a supramolecular nanocapsular structure by pillararene-based host−guest recognition motifs.10 The packing structure of 2P5DASP in the solid state is shown in Figure 3d. The laser dye DASP molecules are uniformly oriented in the host−guest complex single crystals, which was confirmed below by polarization measurements. DASP is a typical intramolecular charge transfer (ICT) molecule containing an electron donor unit and an electron acceptor moiety.11 Upon photoexcitation, the locally-excited (LE) state can transfer electron from the electron-donating aniline moiety to the electron-accepting pyridinium unit, forming the ICT state.12 This charge transfer is accompanied by single/double bond twisting, which thus is also called the twist ICT (TICT) state. Due to the Frank-Condon principle, molecules in the TICT state generally show a large Stokes shift and weak fluorescence. This is exactly what we observed for DASP in methanol (polar solvent) and chloroform (non-polar solvent), exhibiting Stokes shifts of 640 and 420 meV (Figure S5 and S6), respectively, and extremely weak emission.13 However, in the 2P5DASP crystal structure, the supramolecular nanocapsule encapsulation should effectively restrict the twisting of DASP molecules and at the same time, minimize the ACQ effect, leading to efficient photoluminescence.
Figure 4. (a) PL and PLE spectra of a 2P5DASP microcrystal; (b) PL intensity of the 2P5DASP microcrystal as a function of detection polarization angle (θ). Detection polarization perpendicular to microcrystal long axis denoted as 0°. Solid line is fit to cos2θ. Inset: optical image of the microcrystal. We expected polarized emission from individual 2P5DASP microcrystals because of the uniform molecular ori-
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crystals was observed when excited at 1030 nm (Figure S11), indicating their potential in up-conversion processes.22
entation of DASP molecules in the host−guest complex single crystals.16 We measured the PL spectra of a single 2P5DASP microcrystal as a function of detection polarization angle (θ). As shown in Figure 4b, the PL intensity as a function of θ can be well described with a cos2θ function. The degree of polarization is defined as DOP = (lmax − lmin)/(lmax + lmin), which is about 80% for the 2P5DASP microcrystal. The maximum PL intensity was observed when the polarization was along the crystal’s long axis. The strong PL and regular shape qualify 2P5DASP microcrystals as a good candidate for high-performance lasing. Lasing measurements were performed on a home-built epi-fluorescence microscope at room temperature (Figure S10). Figure 5a shows the PL spectra of a representative 2P5DASP microcrystal at different pump fluences. Below a certain threshold (PTh ~ 230 μJ/cm2), the PL spectra were dominated by a weak and broad spontaneous emission (SPE). With increasing pump fluence above that, several equally-spacing sharp peaks around 630 nm emerged, while the intensity of the broad SPE remained almost constant, indicating multiple lasing modes from stimulated emission. In general, the lasing modes compete and the one with the highest gain dominates. The multiple lasing modes observed here could be due to cavity inhomogeneity and/or additional nonlinear effects.17 As shown in Figure 5b, the plot of emission intensity versus pump fluence exhibits a clear knee behavior at the lasing threshold. Accompanying that, the FWHM (full width at halfmaximum) of the emission spectrum drops from ~40 nm to ~0.5 nm once above the lasing threshold.
Figure 5. (a) Lasing spectra of a 2P5DASP microcrystal pumped at 515 nm with different fluences (microcrystal L = 4.68 μm). (b) PL intensity and the full-width at half maximum (FWHM) as a function of pump fluence. (c) Lasing spectra of 2P5DASP microcrystals with different cavity lengths. (d) Mode spacing versus reciprocal cavity length. Solid line is a linear fit with intercept at zero. (e) Time-resolved PL decay kinetics of a 2P5DASP microcrystal below and above PTh. The solid lines are exponential fits and the gray line is IRF.
An appropriate resonator is essential for lasing. Several feedback mechanisms have been reported for micro- and nanolasers, including Fabry-Pérot (F-P) cavities, Whispering-GalleryMode (WGM) cavities and Distributed Feed-Back (DFB) cavities.18 Herein the two parallel facets along the length of a 2P5DASP microcrystal could function as the end mirrors of a Fabry-Pérot cavity to reflect the guided emission. The smooth surfaces of microcrystals minimize the optical scattering losses. For a Fabry-Pérot cavity, the lasing mode spacing (Δλ) obeys a simple relationship, Δλ = λ2/2Lng, where L is the cavity length, and ng is the group refractive index. We measured lasing behavior in microcrystals with different cavity lengths. As shown in Figure 5c and 5d, the lasing peaks get closer with increasing cavity length and a clear linear relationship between mode spacing (Δλ) and reciprocal of microcrystal cavity length (1/L) was observed, confirming Fabry-Pérot-type cavity resonance. The fitted ng value was 18.26, which is high enough for the confinement of light.19 The Q factor is important to describe the quality of the cavity. According to the definition Q = λ/δλ (where λ is the resonant wavelength and δλ is the full-width at half-maximum), the Q factor here is estimated to be ~1300. Such high Q factor for organic microcrystals results from the high crystal quality and high emission efficiency.
In conclusion, we explored a pillar[5]arene as a novel host matrix for laser dyes to construct supramolecular solid-state microlasers. With a spontaneous supramolecular self-assembly process, we fabricated microlasers with high crystal quality and precise stoichiometric ratio in a very facile manner. The nanocapsular confinement and isolation effect from the pillararene molecules effectively restrict the bond twisting of dye molecules and the ACQ effect, endowing the microlaser with high fluorescence efficiency and excellent laser performance. This work not only provides a new avenue to design organic photonic materials, but also accelerates the development of pillararene-based self-assembled functional materials. Further work will address fabrication of pillararene-based microlasers with tunable broad spectra by encapsulating various dye molecules into pillararene host matrices.
To further confirm the lasing behavior of 2P5DASP microcrystals, we measured the PL decay processes below and above PTh (Figure 5e). The PL decay below PTh exhibited a single exponential process with a lifetime of ~1.2 ns, corresponding to an SPE process. Above PTh, the PL decay kinetics were dominated by a much faster process with a lifetime of ~80 ps, shorter than the FWHM (~100 ps) of our instrument-response function (IRF), corresponding to the lasing process. The emerging picosecond PL decay process above PTh clearly indicates the occurrence of stimulated amplification once population inversion is achieved, i.e., lasing.20
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We also investigated two-photon pumped lasing in 2P5DASP microcrystals. Interestingly, due to the large twophoton absorption cross-section of laser dye DASP (about 104 GM),21 a two-photon pumped lasing signal of 2P5DASP micro-
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[email protected].
Supporting Information Experimental details, a NOESY NMR spectrum, X-ray crystal data, and other materials. These materials are available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author Author Contributions B. H. and W. Z. contributed equally to this work.
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lation of Organic Dye in Metal-Organic Frameworks. Adv. Mater. 2016, 28, 7424–7429; (c) 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) Cesaretti, A.; Carlotti, B.; Germani, R.; Spalletti, A.; Elisei, F. Inclusion of Push–Pull N-Methylpyridinium Salts within Surfactant Hydrogels: Is Their Excited State Intramolecular Charge Transfer Mediated by Twisting? Phys. Chem. Chem. Phys. 2015, 17, 17214−17220. (16) Gozhyk, I.; Clavier, G.; Méallet-Renault, R.; Dvorko, M.; Pansu, R.; Audibert, J.-F.; Brosseau, A.; Lafargue, C.; Tsvirkun, V.; Lozenko, S.; Forget, S.; Chénais, S.; Ulysse, C.; Zyss, J.; Lebental, M. Polarization Properties of Solid-State Organic Lasers. Phys. Rev. A 2012, 86, 043817. (17) Casperson, L. W. Threshold Characteristics of Multimode Laser Oscillators. J. Appl. Phys. 1975, 46, 5194−5201. (18) Samuel, I. D. W.; Turnbull, G. A. Organic Semiconductor Lasers. Chem. Rev. 2007, 107, 1272−1295. (19) Zhang, W.; Yan, Y.; Gu, J.; Yao, J.; Zhao, Y. S. LowThreshold Wavelength-Switchable Organic Nanowire Lasers Based on Excited-State Intramolecular Proton Transfer. Angew. Chem. Int. Ed. 2015, 54, 7125−7129. (20) He, H.; Ma, E.; Cui, Y.; Yu, J.; Yang, Y.; Song, T.; Wu, C.-D.; Chen, X.; Chen, B.; Qian, G. Confinement of Pyridinium Hemicyanine Dye within An Anionic Metal-Organic Framework for Two-PhotonPumped Lasing. Nat. Commun. 2016, 7, 11087–11093. (21) Yu, J.; Cui, Y.; Xu, H.; Yang, Y.; Wang, Z.; Chen, B.; Qian, G. Polarized Three-Photon-Pumped Laser in A Single MOF Microcrystal. Nat. Commun. 2013, 4, 2719–2725. (22) Chen, G.; Agren, H.; Ohulchanskyy, T. Y.; Prasad, P. N. Light Upconverting Core–Shell Nanostructures: Nanophotonic Control for Emerging Applications. Chem. Soc. Rev. 2015, 44, 1680−1713.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21434005, 91527301, 21773208). REFERENCES (1) Kuehne, A. J.; Gather, M. C. Organic Lasers: Recent Developments on Materials, Device Geometries, and Fabrication Techniques. Chem. Rev. 2016, 116, 12823–12864. (2) Zhang, W.; Yao, J.; Zhao, Y. S. Organic Micro/Nanoscale Lasers. Acc. Chem. Res. 2016, 49, 1691–1700. (3) Photophysics of Aromatic Molecules; Birks, J. B. Ed.; Wiley: London, 1970. (4) Dong, H.; Zhang, C.; Zhao, Y. S. Host–Guest Composite Organic Microlasers. J. Mater. Chem. C. 2017, 5, 5600–5609. (5) (a) Peterson, O. G.; Snavely, B. B. Stimulated Emission From Flashlamp-Excited Organic Dyes in Polymethyl Methacrylate. Appl. Phys. Lett. 1968, 12, 238–240; (b) Kogelnik, H.; Shank, C. V. Stimulated Emission in A Periodic Structure. Appl. Phys. Lett. 1971, 18, 152–154; (c) Knobbe, E. T.; Dunn, B.; Fuqua, P. D.; Nishida, F. Laser Behavior and Photostability Characteristics of Organic Dye Doped Silicate Gel Materials. Appl. Opt. 1990, 29, 2729–2733; (d) Lo, D.; Parris, J. E.; Lawless, J. L. Multi-Megawatt Superradiant Emissions from Coumarin-Doped Sol-Gel Derived Silica. Appl. Phys. B. 1992, 55, 365–367. (6) Ogoshi, T.; Kanai, S.; Fujinami, S.; Yamagishi, T.; Nakamoto, Y. para-Bridged Symmetrical Pillar[5]arenes: Their Lewis Acid Catalyzed Synthesis and Host–Guest Property. J. Am. Chem. Soc. 2008, 130, 5022−5023. (7) Jie, K.; Zhou, Y.; Li, E.; Zhao, R.; Li, Z.; Huang, F. Reversible Iodine Capture by Non-Porous Pillar[6]arene Crystals. J. Am. Chem. Soc. 2017, 139, 15320−15323. (8) Li, C.; Zhao, L.; Li, J.; Ding, X.; Chen, S.; Zhang, Q.; Yu, Y.; Jia, X. Self-Assembly of [2]Pseudorotaxanes Based on Pillar[5]arene and Bis(imidazolium) Cations. Chem. Commun. 2010, 46, 9016– 9018. (9) Xue, M.; Yang, Y.; Chi, X.; Zhang, Z.; Huang, F. Pillararenes, A New Class of Macrocycles for Supramolecular Chemistry. Acc. Chem. Res. 2012, 45, 1294–1308. (10) Ogoshi, T.; Yamagishi, T.; Nakamoto, Y. Pillar-Shaped Macrocyclic Hosts Pillar[n]arenes: New Key Players for Supramolecular Chemistry. Chem. Rev. 2016, 116, 7937–8002. (11) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures. Chem. Rev. 2003, 103, 3899−4031. (12) (a) Strehmel, B.; Seifert, H.; Rettig, W. Photophysical Properties of Fluorescence Probes. 2. A Model of Multiple Fluorescence for Stilbazolium Dyes Studied by Global Analysis and Quantum Chemical Calculations. J. Phys. Chem. B 1997, 101, 2232−2243; (b) Huang, Y.; Cheng, T.; Li, F.; Huang, C. H.; Wang, S.; Huang, W.; Gong, Q. Photophysical Studies on the Mono- and Dichromophoric Hemicyanine Dyes III. Ultrafast Fluorescence Up-conversion in Methanol: Twisting Intramolecular Charge Transfer and “Two-State Three-Mode” Model. J. Phys. Chem. B 2002, 106, 10041−10050. (13) (a) Glasbeek, G.; Zhang, H. Femtosecond Studies of Solvation and Intramolecular Configurational Dynamics of Fluorophores in Liquid Solution. Chem. Rev. 2004, 104, 1929–1954; (b) Carlotti, B.; Elisei, F.; Fortuna, C. G.; Mazzucato, U.; Spalletti, A. Intramolecular Charge Transfer of Push–Pull Pyridinium Salts in the Singlet Manifold. J. Phys. Chem. A 2014, 118, 3580−3592. (14) (a) Bojtár, M.; Szakács, Z.; Hessz, D.; Kubinyi, M.; Bitter, I. Optical Spectroscopic Studies on The Complexation of Stilbazolium Dyes with A Water Soluble Pillar[5]arene. RSC Adv. 2015, 5, 26504−26508; (b) Wei, Y.; Dong, H.; Wei, C.; Zhang, W.; Yan, Y.; Zhao, Y. S. Wavelength-Tunable Microlasers Based on The Encapsu-
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