1. Introduction

Thermally activated delayed fluorescence (TADF) materials have attracted much attention in recent years because of their delayed fluorescence, which are realized by perfect internal up-conversion from the lowest triplet excited state excitons to the lowest singlet excited state excitons through reverse intersystem crossing [1–7]. Owing to such excellent TADF mechanism, they are actively studied and applied for emissive materials of lighting applications aiding development of more energy saving devices. However, there are many technical challenges for TADF based lighting devices to reach a stage of commercial applications, especially molecular aggregation caused quenching is the urgent issue of all organic fluorescent dye materials based devices [8, 9]. Hence, it is critical that TADF molecules are packed in suitable size host materials to suppress the efficiency roll-off for the TADF based light emitting devices.

In terms of the molecular arrangement and restriction of molecular interaction, there is considerable scientific and technical interest in metal organic framework (MOF), which is a kind of porous materials that is simply constructed with metal ion containing nodes cross-linked by organic linkers [10–12]. These MOFs are featured for the large surface area, easy synthesis, self-assembling, tunable crystal size and thermal stability; these benefits promise application development in diverse research fields such as gas storage/separation [13–15], chemical reactor [16, 17], encapsulation of nano size particles/drugs [18–21], micro cavity for light amplification [22, 23], and so on. The individual MOF pores can isolate the guest molecules and restrict inter/intra-molecular motions by their rigid frameworks, so that they can decrease non-radiative transitions and increase the external quantum efficiency. Up to now, many types of dye molecules encapsulated within MOF pores have been reported, for example, fluorescein [24], coronene [25], pyridinium hemicyanine dyes [26]. Concerning the possibility of encapsulation, it is important to choose the appropriate type of MOFs having larger cavities and smaller pore windows than objective dye molecules.

The present study of MOFs are mainly focusing on the zinc based zeolitic imidazolate frameworks (ZIFs) [27–30], which are a subclass of MOFs having an excellent high chemical and thermal stability. Among many type of ZIFs, zeolitic imidazolate framework-11 (ZIF-11) [28, 30–33], which is composed of zinc ions and benzmethlimidazole (bIm) linkers, is especially ideal for host materials because of its large porecavity size (14.9 Å) and narrow aperture (3.3 Å) is suitable for stable entrapment of the fluorescent dye molecules. Herein, we report a direct synthesis of 1,2,3,5-Tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) [1], one of the most popular TADF materials, molecular encapsulation into ZIF-11 pores using one-pot method. Interestingly, 4CzIPN has a similar kinetic molecular diameter (14.5 Å) to ZIF-11, therefore ZIF-11 pore cage can securely hold 4CzIPN molecules.

In this letter we demonstrate the direct encapsulation of TADF molecules into MOF pores (Fig. 1), and also investigate the change of optical properties of 4CzIPN molecules on the from the perspective of intramolecular rotation/vibration/libration. In combination of Fourier transfer infrared spectroscopy (FTIR), terahertz time-domain spectroscopy (THz-TDS), powder x-ray diffraction infrared spectroscopy (FTIR), terahertz time-domain spectroscopy (THz-TDS), powder x-ray diffraction (PXRD) measurement, fluorescence spectroscopy and Thermo gravimetric analysis (TGA), it was confirmed that the most non-radiative decay of 4CzIPN single molecule was suppressed by the ZIF-11 pore wall and its quantum efficiency is increased. Ultimately, the presented results are expected to extend the use of TADF materials for commercial applications of TADF based lighting devices and promote our knowledge of dye molecular interactions with MOFs.

 

Fig. 1 Flow chart of the one-pot synthesis of 4CzIPN included ZIF-11 micro crystal.

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2. Experimental section

2.1. Chemicals

ZIF-11 containing 4CzIPN was synthesized with reagents to form ZIF-11 : benzimidazole (bIm, 98 % purity, Tokyo Chemical Industry), zinc acetate dihydrate (Zn(COO)${ }_2\cdot$2H₂O, 98 % grade, Sigma-Aldrich), and 4CzIPN. Ethanol ($\ge$ 99.9 % purity), Toluene ($\ge$ 99.5 % purity) and Ammonium hydroxide (25.0 – 27.9 %, aqueous solution) was used as solvents. All of these solvent was purchased from FUJIFILM Wako Pure Chemical Corporation. polydimethylsiloxane (PDMS) SIM-360 was purchased from ShinEtsu Sillicon Co., Ltd.

2.2. ZIF-11 synthesis

The synthesis method of ZIF-11 adopted ordinary toluene assisted ethanol solvent procedure. 126.9 mg of bIm was dissolved in 2.906 g ethanol, together with the 2.352 g of toluene and 110.4 mg of ammonium hydroxide in a 20 mL glass bottle (bIm solution). On the other hand, 137.8 mg of zinc acetate dihydrate was dissolved in 2.906 g ethanol and 2.352 g toluene in another 10 mL glass bottle (Zn${ }^{+2}$ solution). Each bIm and Zn${ }^{+2}$ solutions were treated with 10 minutes sonication and 10 minutes warm-up at 60 °C. After that, the Zn${ }^{+2}$ solution were softly poured into bIm solution with gentle stirring with glass rod. The synthesis reaction took place on the stirrer in 120 rpm for 12 hours at 60 °C. The products looks like white mud precipitated on the bottom was collected, rinsed (stirred with 720 rpm revolution and rotation for 60 seconds together with 5 ml ethanol), and centrifuged with 1440 rpm revolution for 120 seconds. This procedure was repeated for 5 times to completely get rid of unreacted residue. Thereafter, the white mud were dried at 80 °C in the teflon beaker for 24 hours to volatilize solvent molecules. Then the powder ZIF-11 microcrystals were collected in a 5 mL glass bottle. The molar composition of this synthesis was Zn²⁺ : bIm : NH₃ : H₂O : Ethanol : Toluene = 1 : 1.5 : 2.5 : 2 : 258 : 81.

2.3. 4CzIPN$\subset$ZIF-11 synthesis

The basic procedure of the synthesis of ZIF-11 microcrystals was the same as the former genuine ZIF-11 microcrystals. However, 2.5 mg of 4CzIPN was added into the bIm solutions before the pouring of Zn²⁺ solutions. The purification process of 4CzIPN$\subset$ZIF-11 is the same with ZIF-11. In this procedure, the encapsulation of 4CzIPN molecules into ZIF-11 pours stochastically goes, so that the encapsulation ratio can be estimated to be very low. The molar composition of the mixture was Zn²⁺ : bIm : 4CzIPN : NH₃ : H₂O : Ethanol : Toluene = 1 : 1.5 : 0.003 : 2.5 : 2 : 258 : 81.

2.4. Characterization

The scanning electron microscopy (SEM) images were taken with a Shimazu superscan SS-550 model at 20 kV. The powder samples were applied on the carbon tape and coated with platinum under vacuum condition. The powder X-ray diffraction analysis was performed using a Rigaku SmartLab with a copper anode and graphite monochromator to select Cu K-α radiation (λ = 1.541862 Å). The sample powders were measured between 3° and 40° of 2θ, with an angular step of ${0.05}^{\circ }$. Also, the simulation of the powder X-ray diffraction pattern was simulated from structural data of ZIF-11 using Rietveld analysis with VESTA software. fluorescence lifetime measurement system at room temperature with the excitation wavelength of 372 nm. 40 gr./mm grating was chosen for the wide range and short wavelength fluorescence measurement.

The time resolved fluorescence signal was accumulated with exposing the detector for 200 seconds. Fluorescent measurement was carried out at room temperature with Jasco FP-8200 spectrometer with excitation wavelength at 370 nm. The signal was accumulated for 20 times with 1 nm resolution.

Photoluminescent quantum yields (PLQY) of 4CzIPN$\subset$ZIF-11 was measured with QE-2000 (Otsuka Electronics, Co., Ltd. The measurement was performed at room temeperature. The excitation wavelength was changed from 300 to 420 nm by 10 nm, and the emitted photons were counted in the range of 195 to 950 nm. The sample powder was dispersed in the polymer (PDMS SIM-360) with 0.5 wt.%, and deposited with 1 mm film.

Thermogravimetric analysis was performed with HITACHI TG/DTA-7300 at room temperature. 50 $\mu \mathcal{l}$ aluminum pan was used for sample container, and dry N₂ was used as purge gas. Al₂O₃ powder was used as a reference. The experiment was performed in the range of 30 – 500°C by heating speed of 10 °/minute.

The Fourier transform infrared spectroscopy absorption spectra were measured with Bomem DA-8 at room temperature in the range of mid-infrared (400 – 1800 cm⁻¹). Spectra of the sample were accumulated for 10 times at a resolution of 0.5 cm⁻¹, using KBr pellet technique. Powder sample was ground with KBr grains in 0.5 wt.% and pressed with 13 mm diameter die set with the pressure of 11.7 kgf/mm².

The terahertz spectroscopy absorption spectra were measured with Advantest TAS-7500 at a room temperature in the range of 0.5–4.5 THz. Powder samples were directly placed on the sample holder and measured. The signals were accumulated for 4096 times.

To assign the spectroscopic data of FTIR and terahertz absorption measurement, density functional theory (DFT) based calculation was performed with Gaussian16 package. The geometry optimization and resonant frequency calculation were carried out using the functional of M06-2X, which is the highly parametrized empirical exchange-correlation functionals, and 6−31G(d,p) basis set.

3. Results and discussion

The powder products of 4CzIPN included ZIF-11 micro crystals look like yellow same with the original 4CzIPN powder sample, while the genuine ZIF-11 micro crystals were colored white. This result means that the 4CzIPN in the ZIF-11 micro crystals has not lost its absorption properties of 4CzIPN molecules at least.

 

Fig. 2 SEM images of (a) ZIF-11 and (b) 4CzIPN$\subset$ZIF-11. c) Measured powder–XRD patterns of 4CzIPN, 4CzIPN$\subset$ZIF-11, and calculated XRD pattern of ZIF-11. Dotted line shows 8.28° of 2θ.

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3.1. SEM and PXRD analysis

To confirm the encapsulation of 4CzIPN molecules into ZIF-11 pores, 4CzIPN$\subset$ZIF-11 samples were analyzed several methods. Because microcrystal sizes of 4CzIPN$\subset$ZIF-11 are relatively smaller than ZIF-11s’, the condition (acceleration voltage and magnification range) was changed to get crisp images. Fig. 2 shows the SEM images and XRD pattern of ZIF-11 and 4CzIPN$\subset$ZIF-11. In our synthesis method, the rhombic dodecahedral microcrystals were successfully fabricated (a). The crystal size of 4CzIPN$\subset$ZIF-11 microcrystals shown in (b) were relatively small and collapsed compared to pure ZIF-11 ones nevertheless the amount of zinc ion and bIm molecules in the precursors are almost the same. There are two reasons for the difference of crystal size; (1)one is that 4CzIPN molecules works as a template of ZIF-11 frameworks, so that the crystal grew into an irregular shape, (2)the other is the adsorbed 4CzIPN molecules on the surface of ZIF-11 micro crystals inhibit the formation of another cell. To confirm the surface adsorption, the powder XRD measured and Fig. 2(c) were obtained. Synthesized ZIF-11 pattern agrees well with simulated ZIF-11 one at each labeled $2\theta$ in the Fig. 2(c), demonstrating the successful formation of ZIF-11 micro crystals. The unknown phase at 2 θ=8.28° can be explained by the appearance of the solution mediated distortion of rhombic dodecahedron crystal structures into cubic, chamfered cube, diamond like structure, strombic icositetrahedron or some other crystal structures [30, 34, 35]. The XRD from 4CzIPN$\subset$ZIF-11 showed different profile from 4CzIPN powder but agreed well to pure-ZIF-11. Thus, the surface adsorbed 4CzIPN seemed to be negligible.

 

Fig. 3 Measured TGA curves of 4CzIPN$\subset$ZIF-11 and ZIF-11 for comparison. The data was normalized by weight.

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Fig. 4 Concentration vs. Fluorescence intensity diagram in Chloroform solutions of 4CzIPN (red dots) and 4CzIPN$\subset$ZIF-11 (blue dot). The linear fit was shown as blue line.

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3.2. Loading efficiency analysis

The TGA curve of 4CzIPN$\subset$ZIF-11 showed a plateau curve up to 400 °C while ZIF-11’s one steeply decreased over 100 °C in Fig. 3. The sublimation point of 4CzIPN is over 450 °C, so that the difference of weight loss between 100 °C to 200 °C is fully depends on the water vapor desorption. The difference of the weight loss between the 100 to 200 degree of Celsius indicates that the 4CzIPN molecules are securely loaded in the ZIF-11 pores, and adsorption area of the water vapor in the ZIF-11 pore cavity was replaced by 4CzIPN molecules.

To measure the proportion of 4CzIPN molecules to ZIF-11 pores, we adopted the linear regression analysis with the fluorescence vs. intensity relationship. Fig. 4 shows the concentration vs. fluorescence intensity diagrams. In this measurement, we prepared two types of the solvent: ones are the several micro-molar (μM) concentrations (10⁻⁶mol/cm³) of 4CzIPN in chloroform solvents, and the other is 3.0 mg 4CzIPN$\subset$ZIF-11 samples suspended in 10 m$\mathcal{l}$ chloroform. Each solution was poured to 1 cm length quartz cuvette, and then measured with the fluorescence spectrometer at excitation wavelength of 370 nm. The calibration straight line was defined from the linear fitting passing through the origin of 4CzIPN at chloroform measurement results, and we got the correlation coefficient R2 of 0.993. With comparing the 4CzIPN$\subset$ZIF-11 signal and the calibration line, it is assumed that the 4CzIPN$\subset$ZIF-11 at Chloroform suspension corresponds to 0.8234 μM 4CzIPN at Chloroform solution. Finally, we obtained a loading efficiency of 4CzIPN into ZIF-11 pores of 0.116 4CzIPN molecules per ZIF-11’s unit cell.

 

Fig. 5 The mid–infrared absorption spectrum of 4CzIPN, 4CzIPN$\subset$ZIF-11 and ZIF-11. The experimental conditions were 0.04 Torr at room temperature.

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3.3. Intra & inter–molecular investigation

Generally, organic molecules have many resonant motions like intramolecular bending, stretching and vibration in the mid-infrared range. To confirm the intramolecular motions restriction effects, mid-infrared spectroscopy was performed by FTIR spectroscopy. Fig. 5 shows the IR absorbance of 4CzIPN, 4CzIPN$\subset$ZIF-11 and ZIF-11. The FTIR spectra of 4CzIPN$\subset$ZIF-11 and ZIF-11 are similar to each other, and their absorption peaks were assigned that the band at 1611 cm⁻¹ and 1465 cm⁻¹ are assigned to the C–C stretching of the aromatic ring, 1287 cm⁻¹ is corresponded to the imidazole ring breathing, 1287 cm⁻¹ is the imidazole ring breathing, 1244 cm⁻¹ is in-plane C–H deformation of the disubstituted imidazole ring, 1181, 1118 and 776 cm⁻¹ is are the benzimidazole in-plane C–H and N–H bending and the imidazole in-plane ring bending, 1005 cm⁻¹ is the benzene-ring vibration and 910 cm⁻¹ is is the C–H out of plane bending of single hydrogen in substituted benzene rings, respectively. Despite ZIF-11 have lots of very intensive absorption band in 600 – 2000 cm⁻¹ is, we can distinguish some spectral features specific for 4CzIPN molecules in 4CzIPN$\subset$ZIF-11 spectrum: 1547 cm⁻¹ is in-plane motions of dicyanobenzene ring and 723 cm⁻¹ is the breathing motions of carbazole groups. This result means that in-plane vibrational motions of 4CzIPN molecule are hard to be restricted by the encapsulation into ZIF-11 close cavity. Terahertz spectroscopy is very useful tool to investigate the intramolecular symmetric (Raman active) and asymmetric (IR active) vibration and libration, rotation and libration but not intermolecular motions. Fig. 6 shows the terahertz absorption results of 4CzIPN, 4CzIPN$\subset$ZIF-11 and ZIF-11 in 0.5 – 3.0 THz range. There are many wavy structures in overall spectrum because of the fringe patterns arisen from sample thickness. Interestingly, we observed the gate opening motions of imidazole pore windows, which was predicted with ab initio calculation by some researchers [28]. Our previous work revealed that 4CzIPN molecule have intramolecular resonances in 0.75 – 0.95 THz range derived from the rotational and librational modes of carbazole group [36]. However, experimental results show that the absorption band disappeared with the encapsulation of ZIF-11. This is the direct evidence that the ZIF-11 pores restrict the intramolecular rotational and librational modes of 4CzIPN.

 

Fig. 6 Terahertz absorption spectrum of 4CzIPN, 4CzIPN$\subset$ZIF-11 and ZIF-11 0.5 – 3.0 THz range. The measurement conditions are, below 1 % humidity at room temperature.

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Fig. 7 Time resolved spectrum of a) 4CzIPN’s fluorescence, b) 4CzIPN$\subset$ZIF-11’s fluorescence, d) 4CzIPN’s phosphorescence and e) 4CzIPN$\subset$ZIF-11’s phosphorescence. (c) and (f) shows the photon counts of (a), (b), (d), (e) as a function of decay time.

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Fig. 8 Temperature dependence of 4CzIPN$\subset$ZIF-11’s fluorescence spectrum from 50 K to 300 K by 50 K with cryogenic spectroscopy.

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Fig. 9 Photoluminescence quantum yields of $\subset$ZIF-11 at room temperature.

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3.4. Change of photoluminescence lifetime

FTIR and THz spectroscopic analysis confirmed that ZIF-11 pores restrict the intramolecular rotational motions of 4CzIPN but not the intramolecular in-plane distort motions. Molecular rotational and vibrational motions are corresponding to the non-radiative transition, so that we assume the fluorescence condition can be changed by the encapsulation. https://www.overleaf.com/project/ Fig. 7 shows the fluorescence lifetime measurement results of 4CzIPN and 4CzIPN$\subset$ZIF-11. Surprisingly, to see Fig. 7(a), (b) and (c), the lifetime of 4CzIPN molecules about 2.37 ns were elongated to 14.8 ns by encapsulation into ZIF-11. The intramolecular motion was restricted by close MOF pores because the kinetic molecular size of 4CzIPN is much similar to the cavity size of ZIF-11. The rigid ZIF-11 pore walls restrict the non-radiative decay, and leads to enhancement of internal quantum efficiency. It can be seen in Fig. 7 (d) (e) and (f) that the lifetime of the 4CzIPN$\subset$ZIF-11’s phosphorescence became shorter than that of 4CzIPN powder samples, from 2.15 μs to 1.65 μs, respectively. To confirm the TADF ability of 4CzIPN$\subset$ZIF-11’s, we measured the temperature dependence of fluorescent spectrum from 50 K to 300 K by 50 K using cryogenic system, and Fig. 8 is the result of it. The fluorescence intensity of 4CzIPN is increased according to the temperature. Clearly, 4CzIPN$\subset$ZIF-11 was not lost its TADF ability.

Fig. 9 shows the PLQY results of 4CzIPN$\subset$ZIF-11 at room temperature. The peak of the PLQY is around 0.77 with the excitation wavelength of 420 nm, and this value relatively small compared to that of 4CzIPN (generally $\ge$0.9). In this measurement, 4CzIPN$\subset$ZIF-11 film was deposited with PDMS (SIM-360, transparent polymer in the visible light). It can be assume that the difference of PLQY stem from the attenuation of the SIM-360.

With these fluorescence and phosphorescence lifetime differences while 4CzIPN$\subset$ZIF-11 does not lost its TADF emission, we can assume that the 4CzIPN molecule’s reverse intersystem crossing from the lowest excited triplet state to the lowest excited singlet state was accelerated by the ZIF-11 encapsulation. Since the steric hindrance, or the torsion between the donor and acceptor planar (carbazoles and the dicyanobenzene in this case) of TADF molecules characterize the small energy gap between the lowest excited singlet state and the lowest excited triplet state, intramolecular motions affect to the (reverse) intersystem crossing. The close ZIF-11 pore wall restricts 4CzIPN’s non-radiative decay, and this effect increase the population of the excitons and simplifies the emission process. Therefore, the reversed intersystem crossing rate of 4CzIPN molecules were accelerated by the restriction of intramolecular motions.

4. Conclusion

We demonstrated the fluorescence lifetime elongation of the 4CzIPN for encapsulation into ZIF-11 pores using one step synthesis method with a loading efficiency of 4CzIPN into ZIF-11 pores of 0.064 of the proportion of 4CzIPN/ZIF-11. FTIR and Terahertz spectroscopy shows that the non-radiative emission of 4CzIPN was reduced because of the restriction of intramolecular motions by rigid ZIF-11 framework. With this restriction effect, the florescence lifetime of 4CzIPN was considerably elongated from 2.37 ns to 14.8 ns. On the contrary, the phosphorescence photon counts was decreased and the lifetime became a little bit short, this results indicated that the reverse intersystem crossing rate was also increased by the restriction of the intramolecular motions. This work gives important information on how the photoluminescence of the fluorescent dye molecule, especially the TADF molecules can be changed. This results will contribute to the development of organic light emitting devices based on luminescent MOFs.