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Dec 11, 2017 - range of applications including gas storage,1 catalysis,2 and sensing.3 In recent years, MOFs and CPs are also emerging as a good mater...
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Multiphoton Absorption and Two-Photon-Pumped Random Lasing in Crystallites of a Coordination Polymer Min Liu,†,‡ Hong Sheng Quah,§ Shuangchun Wen,†,∥ Ying Li,† Jagadese J. Vittal,*,§ and Wei Ji*,†,‡ †

SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China 518060 ‡ Department of Physics, National University of Singapore, Singapore 117551 § Department of Chemistry, National University of Singapore, Singapore 117543 ∥ Key Laboratory for Micro-/Nano- Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha, China 410082 ABSTRACT: Functional materials with multiple optical properties have been investigated extensively in recent years because of their potential applications. Here we demonstrate a ladder polymer, [Zn2(benzoate)4(An2Py)2], for its efficient light frequency up-conversion via multiphoton excitation at wavelengths ranging from 800 to 1500 nm. The zinc-based coordination polymer can be self-assembled from zinc(II) nitrate, trans,trans-9,10-bis(4-pyridylethenyl) anthracene (An2Py), and benzoate ligands furnishing a one-dimensional ladder structure. The coordination polymer exhibits a high photoluminescence (PL) quantum yield of 70%, a twophoton-absorption cross section of 890 GM at 800 nm, and a three-photon-absorption cross section of 3.1 × 10−78 cm6 s2 photon−2 at 1200 nm. Furthermore, the coordination polymer in the form of nanoscale crystallites is capable of random lasing when pumped by 800 nm laser pulses with a threshold of 1.5 mJ/cm2.

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INTRODUCTION Metal−organic frameworks (MOFs) and coordination polymers (CPs) are a class of crystalline materials composed of coordination bonds between metal ions and multidentate organic molecules. MOFs and CPs can be utilized for a wide range of applications including gas storage,1 catalysis,2 and sensing.3 In recent years, MOFs and CPs are also emerging as a good material candidate for nonlinear optical and lasing technologies. Large second and third harmonic generation and multiphoton absorption have been observed in MOFs,4−7 which are fabricated via two different approaches: (i) taking advantage of the porous structures that MOFs provide by incorporating nonlinear dye molecules into their pores and (ii) using nonlinear organic molecules as linkers when designing the MOF. Because of the nonlinear dyes and organic linkers, the nonlinear optical properties of MOFs are enhanced considerably. In our design, we adopted the latter approach to synthesize MOFs and CPs for light frequency up-conversion. Light frequency up-conversion is an anti-Stokes process that converts light with lower frequency to a higher one. Since the first report in 1959,8 infrared-to-visible up-conversion phosphors are becoming increasingly important because of their technological applications in lasing, bioanalytics, medical therapy, and light harvesting.9,10 In our previous work,5,6 we © XXXX American Chemical Society

successfully synthesized three-dimensional MOFs with trans,trans-9,10-bis(4-pyridylethenyl) anthracene (An2Py) as linkers between the zinc ions. An2Py is an organic molecule with large two-, three-, and four-photon-absorption coefficients, as well as third-order nonlinear optical susceptibility (∼1 × 10−11 esu) for third harmonic generation. Very recently, we also synthesized a one-dimensional ladder coordination polymer with the spacer ligand An2Py and found that the third-order nonlinearity,11 χ(3), has been enhanced by ∼8-fold, due to the reduced dimensionality that results in the quantum confinement, similar to nanoscale semiconductors (such as quantum dots, quantum wires, etc.). Here, we aim at exploring its multiphoton-excited fluorescence and random lasing. Twophoton-absorption (2PA) and two-photon-pumped (2PP) lasing have attracted great attention because of their advantage in converting near-infrared laser light to the visible region.12−14 As emerging materials, MOFs have also been demonstrated for these applications. An indium(III)-MOF was reported for its 2PA cross section up to 3072 GM,15 greater than most commercial materials and stimulated emission.16−18 Chen and Qian’s laboratory reported a dye incorporated into the voids of Received: November 1, 2017 Revised: December 10, 2017 Published: December 11, 2017 A

DOI: 10.1021/acs.jpcc.7b10789 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C a MOF single crystal to exhibit 2PP lasing.19 Moreover, the same group also reported three-photon-pumped lasing with a different MOF.20 It should be noted that the up-conversion lasing can also be realized with random lasing in nanoscale crystallites of fluorescent materials. The random effect is stimulated when multiple light scattering becomes significant to provide the feedback mechanism necessary for optical gain to occur.21,22 The process has been observed in closely packed semiconducting quantum dots (CdSe/CdS core−shell)23 and ZnO powder24 with multiphoton excitation. However, it has not been realized in CP or MOF crystallites. The synthesis and structure characterization of the onedimensional (1D) CP, [Zn2(benzoate)4(An2Py)2], CP-1 was previously documented.11 As the ligand, An2Py has a symmetrical acceptor−π−donor−π−acceptor structure and a singlet biradical electronic ground state,6 which has potential for giant optical nonlinearities. More importantly, the nonlinearities can be enhanced further if the An2Py linkers are confined into a rigid one- or two-dimensional structure. It has been reported that the relative packing of the fluorescent ligands in CPs or MOFs can significantly influence their optical properties. Through the rigidifying fluorescence linkers in CPs or MOFs, the photophysical properties can be enhanced. This kind of packing has advantages such as producing different fluorescence and/or absorption energies; a larger intermolecular distance can increase the fluorescence quantum yield due to the decrease of self-quenching.25−28 The An2Py ligands from adjacent chains are not aligned and thus will lead to the diminishing of self-quenching, or the increasing of fluorescence quantum yield. On the basis of a combination of the above-discussed enhancements, we observe that the CP crystallites yield two-photon-pumped random lasing with a threshold as low as 1.5 mJ/cm2. The details are discussed below.

Figure 1. (a) 1-D ladder structure of CP-1. (b) SEM photo of CP-1 crystallites. (c) Size dispersion of CP-1 crystallites. The inset shows a photo of CP-1 crystallites in a quartz cuvette. (d) PL intensity of CP1 and the background for the determination of quantum yield (λex = 400 nm). (e) PL spectra excited by 1PA (λex = 473 nm), 2PA (λex = 800 nm), and 3PA (λex = 1200 nm). (f) Optical setup for upconversion measurements. BPF, NDF, and SPF stand for band-pass filter, neutral density filter, and short pass filter, respectively. CL1 is a cylindrical lens with a focal length of 60 mm. L2 and L3 are lenses with a focal length of 100 mm. The red box represents the laser that emits femtosecond laser pulses.

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RESULTS AND DISCUSSION The structure of CP-1 is shown in Figure 1a, and a scanning electron microscopic (SEM) photo of CP-1 is depicted in Figure 1b, with the size distribution in Figure 1c. Using a fluorescence spectrometer (FSP920-C, Edinburgh), the fluorescence quantum yield (η) of CP-1 was determined to be 70% (Figure 1d) which is comparable to the MOF-1a reported in ref 6. The one-photon-absorption (1PA) induced PL spectrum in Figure 1e was recorded by a confocal microspectroscopy system (NTEGRA Spectra, NT-MDT). The PL signal spans from 525 to 800 nm, revealing its potential for white light emitting diodes.29 Figure 1f illustrates our optical setup for the PL spectra excited by both two-photon absorption (2PA) and three-photon absorption (3PA). The laser excitation was provided by femtosecond laser pulses of a wavelength in the range from 800 to 1500 nm (pulse duration, 150 fs; repetition rate, 1 kHz). Details on the 2PA- and 3PA-induced PL measurements can be found in our previous publication.11 The 2PA- and 3PA-induced PL spectra are similar to the 1PAinduced PL spectrum, with a width (fwhm) ∼30 nm narrower than the 1PA-induced PL profile. It is indicative of the same emission process regardless of 1PA, 2PA, or 3PA. Being excited by the infrared laser pulses in the wavelength range from 1200 to 1500 nm, CP-1 exhibits a strong PL (Figure 2). The PL peak intensity vs excitation fluence (see one of the insets in Figure 2) shows the characteristics of 3PA

as the slopes are ∼3. The PL signal excited at 1200 nm is the strongest. Besides the PL, there are appearances of weak second harmonic generation (SHG) and strong third harmonic generation (THG) signals. The origin of the SHG has been attributed to the defects in CP-1.11,30 The pronounced THG is anticipated, as An2Py was previously identified5,8 as an efficient nonlinear optical ligand. A multiphoton-absorption-induced PL signal, Fn, can be obtained by integrating Δf n over the entire laser focused volume, ds dz, and time, t. Δf n is given by31,32 Δf n = (1/ n)ϕησnρInr ds dz dt/(ℏω)n, where n accounts for the fact that n photons are absorbed from the near-infrared (NIR) laser light for each fluorophore excitation generated; ℏω is the photon energy of the NIR incident laser beam; ϕ is the fluorescence collection efficiency of the experimental setup; η is the PL quantum yield; σn is the n-photon-absorption cross section; ρ is the sample molar concentration, ds dz is the small volume of the focused laser beam considered; and Ir is the nearly constant B

DOI: 10.1021/acs.jpcc.7b10789 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C σ2CP =

F2CPη MOF ‐ 1aρ MOF ‐ 1a F2MOF ‐ 1aηCPρCP

σ2MOF ‐ 1a

(1)

and σ3CP =

F3CPη MOF ‐ 1aρ MOF ‐ 1a F3MOF ‐ 1aηCPρCP

σ3MOF ‐ 1a

(2)

From Figure 3, we infer that the 2PA and 3PA cross sections of CP-1 are 890 GM and 3.1 × 10−78 cm6 s2 photon−2, respectively. In comparison to MOF-1a,6 these cross sections are 2 orders of magnitude greater. The quantum confinement into 1D is believed to be the observed enhancement in the multiphoton absorption. In a recent paper,33 a series of highly fluorescent organic molecules for 2PA PL were reported with pyridinium salt LI2 (λex = 750 nm), which has a 2PA cross section of 309 GM. Our CP is favorably compared to this value. As for the 3PA cross section, the best value reported so far is 1500 × 10−78 cm6 s2 photon−2 at 1300 nm for CdSe/CdS core/shell nanoplates.34 It should be noted that such a large 3PA cross section is achieved with an average volume of 1072 nm3, in which there exist 1.9 × 104 unit cells of CdSe. By dividing the 3PA cross section of 1500 × 10−78 cm6 s2 photon−2 by 1.9 × 104, we find that the 3PA cross section of CdSe per unit cell is less than that of CP-1 by an order of magnitude. To demonstrate random lasing, the crystallites of CP-1 were placed in a sample holder illustrated in Figure 4a (dimensions about 25 × 1 × 1 mm3). A cylindrical lens (focal length 60 mm) was used to focus the pumping on CP-1 with a stripe shape. The size of the excited region was about 8 mm in length and 150 μm in width. The PL/random lasing signals were collected at an angle of 90° to the propagation direction of excitation laser pulses. Two-photon-pumped PL spectra and random lasing (λex = 800 nm) are shown in Figure 4b. The slope is ∼2, with excitation laser fluences below 1.5 mJ/cm2, clearly evident for 2PA-induced PL. As the excitation laser fluence was increased above 1.5 mJ/cm2, several narrow peaks (or spikes) evolved. In addition, the gradient of emission vs excitation changes from 2.1 to 9.5 (see the inset in Figure 4b), which is a direct evidence for random lasing. Figure 4c shows the photo of fluorescence below the threshold of 1.5 mJ/cm2. With an excitation laser fluence being increased above the threshold, the brightness increased sharply, indicating an amplified stimulated emission (or lasing action). Figure 4d is the photo of random lasing when excitation fluence is above the threshold. The spacing between the neighboring spikes is ∼10 nm, and the line width is ∼4 nm. The threshold of 1.5 mJ/cm2 is less than 8.2 mJ/cm2 at 800 nm for closely packed CdSe/CdS/ZnS QDs35 and ∼2.4 mJ/cm2 (derived from 0.55 μJ and an excitation region of 7 mm × 3.2 μm) at 710 nm for ZnO powder.23 In addition, we have also found that CP-1 is better in terms of long-term chemical stability and photostability when compared to organic molecules. The thermal stability of CP-1 is measured using thermal gravimetric analysis and is found to be stable up to 300 °C before decomposing.11 Random lasing can be categorized in terms of coherent and incoherent feedback mechanisms.20 Pronounced spikes in Figure 4b supported coherent random lasing from CP-1. The particle sizes in the crystallites of CP-1 (Figure 1c) are in the range 50−700 nm, and the central wavelengths of the spikes are in the range 550−650 nm. These lasing wavelengths are so close to the particle sizes that Mie scattering dominates.

Figure 2. 3PA-induced PL spectra (broad peaks between 525 and 800 nm) and THG (narrow peaks between 400 and 500 nm) of CP-1 at excitation of 1200, 1300, 1400, and 1500 nm, recorded with the setup in Figure 1f. The PL signal excited at 1200 nm is divided by 5 for comparing at the same scale. Insets are the plots of PL and THG peak vs excitation fluence on the log−log scale.

laser intensity at this small volume. The multiphoton absorption cross section of CP-1 can be determined by comparing its signal to the one from a known sample under the same experimental setup. We used MOF-1a as the known sample because it was investigated in detail in our previous work.5,6 Figure 3 displays our measurements of CP-1 and MOF-1a under the same conditions. As such, the 2PA and 3PA cross sections of CP-1 are determined by

Figure 3. (a) 2PA-induced PL spectra of CP-1 and MOF-1a and (b) 3PA-induced PL spectra of CP-1 and MOF-1a. C

DOI: 10.1021/acs.jpcc.7b10789 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 5. (a) SEM photo and (b) PL spectrum of micrometer scale crystallites. The excitation fluence is 2.5 mJ/cm2.

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CONCLUSION We have reported a very high multiphoton absorption in [Zn2(benzoate)4(An2Py)2], with a PL quantum yield of 70%. By taking advantages of these excellent nonlinear optical and luminescent properties, two-photon-pumped random lasing has been demonstrated in the CP crystallites of several hundred nanometers at a comparatively low threshold of 1.5 mJ/cm2 at 800 nm.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Min Liu: 0000-0001-5671-9653 Jagadese J. Vittal: 0000-0001-8302-0733 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Education, Singapore, for financial support through NUS FRC Grant Nos. R-143-000-678-112 and R144-000-327-112 and National Research Foundation through NRF-CRP Grant No. 10-2012-03. M.L. and H.S.Q. sincerely acknowledge her financial support from Shenzhen University and his NGS scholarship from National University of Singapore, respectively.

Figure 4. (a) Experimental setup for the observation of random lasing. (b) PL and lasing spectra of CP-1 at 800 nm excitation, recorded with the setup in part a. The magenta curve shows the lasing peaks at excitation above the threshold. The cyan cure is at the threshold. The blue line is the PL spectrum below the threshold. (c) Photo of PL with excitation below the threshold. (d) Photo of random lasing above the threshold.

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Theoretical studies20,21 reveal that random lasing is preferred in strongly scattering media (klt ≤ 1, where k is the wavevector and lt is the transport mean free path). klt ≤ 1 is also known as the Ioffe−Regel region. lt is related to the scattering mean path, ls, by lt =

ls , 1 − ⟨cos θ ⟩

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where cos θ ≈ 0.5 for Mie scattering.20

Since the crystallites of CP-1 are closely packed, it is reasonable to assume that the scattering mean path is the distance between the opposite facets of the particle (i.e., the particle size). As a result, we have klt ≈ 1−16, which is close to the Ioffe−Regel region for the occurrence of random lasing. We also used crystallites of a larger size (up to several micrometers, see Figure 5a) at an excitation wavelength of 800 nm using the same setup. The PL signal is shown in Figure 5b. No pronounced lasing spike was observed. The larger size of CP-1 crystallites diminishes light scattering effects, thus resulting in less likelihood for random lasing. D

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DOI: 10.1021/acs.jpcc.7b10789 J. Phys. Chem. C XXXX, XXX, XXX−XXX