Article pubs.acs.org/cm
Efficient Third Harmonic Generation in a Metal−Organic Framework Min Liu,†,‡ Hong Sheng Quah,§ Shuangchun Wen,†,∥ Zhangshou Yu,‡ 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 518060, China ‡ Department of Physics, National University of Singapore, 117551, Singapore § Department of Chemistry, National University of Singapore, 117543, Singapore ∥ Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, China S Supporting Information *
ABSTRACT: As a recently emerged class of inorganic−organic hybrid crystals, metal−organic frameworks (MOFs) have been investigated for their capabilities for doubling light frequency. We report, for the first time, our observation of an efficient coherent up-conversion via third harmonic generation (THG) in a MOF. It is achieved by utilizing a MOF with a constituent ligand that possesses significant third-order nonlinear optical (NLO) responses. Our THG measurements at fundamental wavelengths ranging from 1200 to 1600 nm show that the MOF constructed with the An2Py ligand exhibits an effective χ(3) value that is 3 orders of magnitude greater than α-quartz. Our findings demonstrate a new avenue to realize efficient third-order NLO responses by utilizing MOFs consisting of strong third-order NLO ligands.
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INTRODUCTION Nonlinear optical (NLO) processes in materials, such as second harmonic generation (SHG) and third harmonic generation (THG), allow laser frequency up-conversion, which has found a wide range of applications in laser and biomedical imaging technologies. In SHG-active (or THG-active) materials, two (or three) photons of the irradiating monochromatic light are coherently converted to a photon having a doubled (or tripled) frequency. To enable viable NLO applications, materials possessing efficient optical nonlinearities (χ(2), χ(3), etc.) are greatly desirable. Furthermore, many applications prefer NLO materials in the solid state. SHG and THG in inorganic crystals, solid-state polymers, and organic glasses have been investigated extensively.1 Recently, there has been a great deal of effort in understanding and exploring THG in nanoscale crystals. THG in one-dimensional crystals (e.g., single-wall carbon nanotubes2), two-dimensional systems (e.g., graphene,3−13 monolayer MoS2,14 and multilayer GaSe15), or oxide nanolayers16−18 has attracted attention. It has also been reported that both SHG and THG are observed in a ferroelectric liquid crystal.19 Herein we report our observation of an efficient THG in a metal− organic framework (MOF) material. MOFs are a recently emerged class of inorganic−organic hybrid materials which can crystallize in one-, two-, or threedimensional structures. In MOFs, metal ions connected by ligands form solid-state frameworks. By selecting suitable constituent ligands or encapsulating desirable guest molecules in the voids of MOFs, with a combination of crystal engineering © XXXX American Chemical Society
and synthetic chemistry, the modular synthetic nature of MOFs allows researchers to achieve advanced composite crystals with highly efficient optical nonlinearities. Other advantages of using MOFs include cost-effective fabrication processes as compared to expensive synthesis techniques for inorganic crystals,20 and long-term chemical stability, as compared to that of most organic crystals.20 SHG-active MOFs have been welldocumented,21 and the magnitude of χ(2) in MOFs of Zn((E)-4-pyv-4bza)2 has been found to be 2 orders of magnitude greater than that of α-quartz. Two-photon-excited lasing of dye molecules encapsulated in the voids of MOFs has also been achieved.22 Efficient light frequency up-conversion via multiphoton excitation and consequent fluorescence has been reported very recently.23,24 Previously, we have observed strong two-, three-, and fourphoton absorption (which correspond to the imaginary part of χ(3), χ(5), and χ(7) optical nonlinearities, respectively) in threedimensional crystals of MOFs, due to the constituent ligand, trans, trans-9,10-bis(4-pyridylethenyl) anthracene (An2Py). The An2Py ligand possesses a symmetrical acceptor−π− donor−π−acceptor structure and a singlet biradical electronic ground state, thus boosting its χ(3), χ(5), and χ(7) optical nonlinearities.23 As illustrated in Figure 1, each An2Py ligand is bridging two zinc(II) ions in the framework and keeps a Received: February 11, 2016 Revised: April 25, 2016
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DOI: 10.1021/acs.chemmater.6b00632 Chem. Mater. XXXX, XXX, XXX−XXX
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graph in Figure 3a revealed that the crystallite sizes were in the range 0.2−1.3 μm. The size distribution is displayed in Figure
Figure 1. View of MOF-1a showing the connectivity in the threedimensional framework.
distance (>15 Å) from neighboring An2Py ligands, hence overcoming the quenching in optical nonlinearities which is common in organic solids assembled from molecular chromophores. Furthermore, the rigidifying effect in the MOF also results in an enhancement in multiphoton-excited photoluminescence (PL) of An2Py;23 it is also consistent with a recent report25 in which the restriction of intramolecular rotations is attributed to aggregation-induced multiphotonexcited PL. However, the studies23 are limited to incoherent processes whereby light frequency up-conversion is demonstrated through multiphoton excitation and consequent fluorescence. In this paper, for the first time, we present our observation of efficient coherent frequency up-conversion via THG scattering in crystallites of [Zn2(SDC)2(An2Py)], MOF1a. The effective χ(3) (3ω) optical nonlinearities of MOF-1a are 3 orders of magnitude greater than that of α-quartz. These properties may have potential applications in bioimaging and laser technologies.
Figure 3. (a) SEM photo and (b) a size dispersion of MOF-1a crystallites.
3b, showing 75% of the crystallites with sizes of less than 1 μm. MOF-1a exhibits an absorption band (200−800 nm) with a maximum around 450 nm, see Figure 4a. For comparison, the
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RESULTS AND DISCUSSION Solid-State Structures. MOF-1 and its desolvated form, MOF-1a, crystals were synthesized with details summarized in the Methods section. It was found that MOF-1 crystals became unstable when exposed to laser radiation, while MOF-1a crystals were stable. Therefore, MOF-1a crystals were investigated for their χ(3) response. The powder X-ray diffraction (PXRD) of MOF-1a in Figure 2 reveals that its crystalline structure belongs to triclinic P1̅. This space group is centrosymmetric, and hence, the second-order nonlinearity vanishes. The crystal structure consists of [Zn2(SDC)2] layers made up of paddlewheel units and pillared by An2Py ligands. The as-synthesized MOF-1a compound was then in the form of crystallites. The scanning electron microscopic (SEM) view
Figure 4. (a) Diffuse reflectance spectrum of MOF-1a and An2Py crystallites, and (b) one-photon absorption spectrum and one-photonexcited photoluminescence (PL) spectrum of An2Py dissolved in CH3Cl solution. The arrows indicate three-photon and four-photon absorption.
Figure 2. PXRD comparison between the as-synthesized MOF-1a and simulated crystal powder patterns. B
DOI: 10.1021/acs.chemmater.6b00632 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials diffuse reflectance spectrum of An2Py crystallites was also measured (Figure 4a). It is apparent from Figure 4a that the optical absorption of MOF-1a is dominated mainly by the An2Py ligand in the spectral range from 400 to 800 nm. The one-photon absorption spectrum and one-photon-excited PL spectrum of An2Py dissolved in CHCl3 solution were also measured. Figure 4b shows that the one-photon absorption (with the lowest-energy transition: S0 → S1) peaks at 414 nm, while the maximum intensity of PL is at ∼580 nm. Up-Conversion Results. As for frequency up-conversion experiments, we excited an ensemble of MOF-1a crystallites by femtosecond laser pulses of wavelength varying from 1200 to 1600 nm, see Figure 5a. The THG and multiphoton-excited PL
Figure 6. (a) Multiphoton-excited PL and THG in MOF-1a crystallites and (b) THG in α-quartz crystallites. Note that the THG signals in part b are multiplied by 10 in order to compare with the THG signals in part a on the same scale.
excited PL dominates because the slopes are nearly 4, clearly showing a quartic power dependence. All these observations are in agreement with our previous data23 on the three-photon excitation or four-photon excitation to the first excited state (S1) of An2Py maximized at ∼414 nm, as indicated by vertical arrows in Figure 4b. The plots in Figure 7b confirm THG signals at 400, 433, 466, 500, and 533 nm as they exhibit the cubic power dependence. To evaluate the nonlinearities of MOF-1a, we consider the case of THG excited by 1600 nm laser pulses. In this particular case, it is obvious from Figures 6 and 7 that the four-photonexcited PL is considerably weaker, as compared to the THG. Thus, the PL can be ignored, and the THG intensity is governed by eq 1 as shown below.19
Figure 5. (a) Experimental setup for multiphoton-excited PL and THG measurements on the crystallites, and (b) the scattered THG signals measured as a function of the scattered angles for MOF-1a crystallites. The black squares are the measurements, and the red curve is the fit by the cos2(θ)-dependence.
signals were collected simultaneously at an angle slightly away from the reflection angle. We also measured the THG signal at other angles. As shown in Figure 5b, the THG signals were scattered in a fashion described by the Lambert law.26 Note that our measurements confirmed no THG signal when the quartz cuvette was empty. Figure 6a displays both multiphoton-excited PL spectra and THG at an excitation pulse energy density of ∼6 mJ/cm2 for MOF-1a. To the best of our knowledge, for the first time, both incoherent (i.e., multiphoton-excited PL) and coherent (i.e., THG) frequency up-conversion are observed simultaneously from MOFs, although there were previous reports on organic aggregates,24 gold/silver nanostructures,27 and inorganic crystals (GaN)28 with microscopic techniques. MOF-1a possesses a broad band (500−750 nm) of multiphoton-excited PL and THG at wavelengths corresponding to one-third of the excitation laser wavelengths. The PL maxima were observed at ∼580 nm, consistent with the onephoton-excited PL characteristics of An2Py in solution (Figure 4b). The excitation power dependence of the PL signals is depicted in Figure 7a. As for the PL excited by the laser pulses of 1200, 1300, and 1400 nm wavelengths, the slopes of PL maxima versus excitation powers on the log−log scale confirm a cubic power dependence, and these are indicative of PL induced by three-photon absorption. When exposed to the laser pulses at wavelengths of 1500 and 1600 nm, four-photon-
2
(3) 2
I(3ω , r ) ∝ r |χ |
sin 2
( Δ2kr ) + sinh2(αr /4) e−αr/2I 3(ω) 2 2 ( Δ2kr ) + ( α4r ) (1)
Here r is the crystal size, and Δk = k(3ω) − 3k(ω) is the difference in the wavenumber; α is the absorption coefficient at 3ω. I(3ω) and I(ω) is the light irradiance at 3ω and ω, respectively. Equation 1 is derived with the following assumptions: (i) the incident light is a plane wave; (ii) the nonlinear medium is isotropic (validated by experiments in sections 3 and 4 of the Supporting Information); and (iii) the crystal is absorptive at 3ω. In general, cubic crystals are anisotropic, containing two independent elements (χ(3) 1111 and (3) χ(3) susceptibility.29 In our measurements, the crystals 1212) in χ are randomly oriented; hence, χ(3) in eq 1 is an averaging (or effective) value over these two components, and the crystals are approximated as isotropic. C
DOI: 10.1021/acs.chemmater.6b00632 Chem. Mater. XXXX, XXX, XXX−XXX
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where χs(3) is the third-order nonlinear susceptibility of α6π quartz, Δk m = λ (m3ω − mω) is the difference in the wavenumber in α-quartz, mω and m3ω are the known refractive index of α-quartz at ω and 3ω,30,31 and rs is the crystal size of αquartz. Note that α-quartz is nonabsorptive at 3ω. The cubic power dependence of THG from the α-quartz crystallites is shown in Figure 7c. By considering that there are Ns values of α-quartz crystallites, we can sum eq 3 over the size range in a similar way to eq 2. Finally, we obtain the ratio as follows: N ∑ w(ri)I(3ω , ri) I ′(3ω) = Is′(3ω) Ns ∑ ws(rs)Is(3ω , rs)
Here Is′(3ω) is the detected THG signal from the ensemble of α-quartz crystallites. By analyzing the data in Figure 6a,b with eq 4, the effective χ(3) value of MOF-1a can be estimated. In the estimation, an −14 average value of χ(3) esu is utilized for α-quartz.32 s = 2.6 × 10 On the basis of some indication from ref 22, and our measurements on the refractive index of MOF-1a (see section 2 in Supporting Information), we have |n3ω − nω | = 0.6 ± 0.4. The absorption coefficient, α = 40 cm−1 at 533 nm, can be extrapolated from the product of NAn2Pyσ, where NAn2Py is the An2Py density in MOF-1a, and σ is the absorption crosssection that has been measured from An2Py dissolved in CHCl3 solution (C = 10−4 M, Figure 4b). The ratio of N/Ns ≈ 5.1 can be determined from our SEM studies. With these known parameters, we calculate that the effective value of χ(3) is (1.5−0.8) × 10−11 esu for MOF-1a, 3 orders of magnitude greater than quartz. In section 4 of the Supporting Information, we have determined the effective χ(3) value, directly from the single crystal of MOF-1a, to be (2.8−2.0) × 10−11 esu. It is within the same order of magnitude, as compared to the above result from MOF-1a crystallites. Here, it is demonstrated that the analysis based on eq 4 is an effective way to estimate (within 1 order of magnitude) the χ(3)-nonlinearity from the crystallites. Such a large χ(3) value is consistent with our previous studies,23 and it is due to the constituent ligand, An2Py, that possesses a symmetrical acceptor−π−donor−π−acceptor structure and a singlet biradical electronic ground state, thus boosting its χ(3) optical nonlinearities. The χ(3) values could be enhanced if crystal sizes are reduced to be less than a few tens of nanometers, whereby the secondary quantum confinement becomes pronounced. Because of the crystallite sizes at 0.2 μm or greater, it is not anticipated that the second quantum confinement might boost the optical nonlinearities. It should be pointed out that both bilayer graphene flakes33 and few-layer graphite films3 exhibit large χ(3) values, which are 6 orders of magnitude greater than α-quartz. As the fundamental wavelength decreases from 1600 nm, a part of the pulsed laser excitation is converted to incoherent processes: four- or three-photon-excited PL, as illustrated by Figure 8. As a consequence, the THG signal decreases considerably, as shown in Figure 6a. In addition, more THG signals are reabsorbed as shown by Figure 8b as 3ω approaches the S0 → S1 resonance of An2Py, thus resulting in a further decrease in the detected THG. There is another possible mechanism for the decrease of the THG signal with the decrease of excitation wavelength. Since the photon energy of THG (excited by 1600 nm laser pulses) is on the resonance with the S1′ state, the THG signal should be enhanced due to the resonance effect. As the excitation wavelength decreases,
Figure 7. Log−log plots for the PL maxima or THG signals vs the excitation laser power.
Next, we consider that our signals are collected from an ensemble of the crystallites with the size distribution shown in Figure 3b. The detected THG signal should be given by eq 2 as shown below. I ′(3ω) ∝ cos2(θ )N ∑ w(ri)I(3ω , ri)
(2)
Here w(ri) is the percentage of the crystallite at a size of ri, which can be evaluated from Figure 3b; θ is the scattered angle, and N is the number of the crystallites involved. To acquire the effective χ(3) value of MOF-1a, we compare it to the THG signal from quartz crystallites. Figure 6b displays the measured THG signals from α-quartz crystallites (in sizes of 0.5−16 μm obtained from our SEM study in a similar fashion to Figure 3b and contained in a 1-mm-thick quartz cuvette under the same conditions of the THG experiments, see section 1 in the Supporting Information). The THG signal from each α-quartz crystallite can be expressed by the following equation: Is(3ω , rs) ∝
2
rs |χs(3) |2
Δk mrs 2 Δk mrs 2 2
( ) I (ω) ( )
sin 2
(4)
3
(3) D
DOI: 10.1021/acs.chemmater.6b00632 Chem. Mater. XXXX, XXX, XXX−XXX
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Laser Excitation and Spectroscopy. The laser pulses (wavelength range, 1200−1600 nm; repetition rate, 1 kHz; and pulse width, 150−250 fs) were generated by an optical parametric amplifier (OPA, TOPAS-C, Light-Conversion). The OPA was pumped by a regenerative amplified femtosecond Ti:sapphire laser system (wavelength, 800 nm; repetition rate, 1 kHz; pulse energy, 3 mJ; and pulse width,
