A benzoperylene probe self-assembled on polyethyleneimine/manganese doped ZnS quantum dots: a new nanocomposite for bright and tunable white light emission

Abstract

A bright white light emitting benzoperylene probe-polyethyleneimine capped quantum dot nanocomposite has been synthesized for the first time. The negatively charged benzoperylene probe with a carboxylic acid functional group tends to self-assemble in the presence of a polycation, and exhibits distinct induced excimer fluorescence. Phosphorescence emitting manganese doped zinc sulfide quantum dots (Mn-ZnS QDs) were capped with polyethyleneimine (PEI). The PEI capped Mn-ZnS QDs were used to induce the benzoperylene probe self-assembly. A bright white light emitting nanocomposite was obtained both in solution and in solid state. The white emission has Commission Internationale de l'Eclairage (CIE) coordinates of (0.33, 0.32), a color-rendering index (CRI) of 72.61 and correlated color temperature (CCT) of 5697 K. Multiple color emissions from blue to orange could also be obtained in a controllable manner via simple adjustment of the ratio of the PEI capped Mn-ZnS QDs and the benzoperylene probe. The nanocomposite displays good photo, chemical and long-term stability. A strong white light emitting LED device based on the nanocomposite was demonstrated.

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Introduction

Light emitting devices account for 20% of the worldwide energy consumption.¹ The synthesis of white light emitting materials (WLEMs) has attracted enormous interest due to the extensive applications in both lighting and display.^2,3 Considerable efforts have been made to prepare various kinds of phosphor materials for a white light-emitting diode (WLED). There are two main methods to produce the phosphor based WLED, one is mixing individual emitters that emit primary blue, green and red (RGB) color under UV light excitation to form a hybrid luminescent white light device. The other is based on the coating of a yellow phosphor, or combination of a green and a red phosphor, onto a blue light emitting LED; or coating directly a blue and a yellow phosphor onto the UV light emitting LED.⁴ However, the commercial phosphors are usually expensive rare earth based materials. The complicated fabrication processes and the fine control of multiple emitting materials and colors are challenging problems.

Many new types of phosphor materials for the construction of WLEDs have been reported in recent years, such as metal–organic frameworks,^5,6 organic compounds,^7,8 organometallic complexes,⁹ and inorganic materials.^10,11 Nevertheless, each of the existing type of WLEM has its own characteristics and drawbacks as shown in Table S1 (ESI†). Thus, WLEMs with low cost, simple and mild fabrication process are still challenging and in great need.

Organic materials have great potential as WLEMs due to their low cost, good processability, flexible device fabrication, and the existance of many alternative choices. White light emission based on organic materials was usually obtained by the combination of different organic dye emitters or dyes at different aggregate states. However, organic dyes often suffer from concentration quenching which seriously limits their use as white light emitting phosphor materials.¹² In addition, although generally limited to solution states, simple mixing of several organic emitters is often unsuccessful because of the negative effects of the intermolecular interactions and complex energy transfer processes. Fine tuning of the emission property of the fluorophore, the resonance energy transfer between the fluorophore pairs, and suppression of undesirable intermolecular interactions are usually required to get white light emission.^13–19 An alternative approach for white light emission is to use hybrid organic–inorganic materials.¹³

Perylene derivatives have been used as fluorescent dyes due to their excellent thermal stability, photo stability, chemical inertness and high fluorescence quantum yield.^20,21 Most of the perylene derivatives have a strong tendency to self-aggregate through π–π stacking and hydrophobic interactions.²⁰ Under the right conditions, the controlled self-assembly of the perylene derivatives can result in distinct excimer emission. The excimer emission gives a very broad spectrum with large stokes shift, which provides a unique advantage for the preparation of WLEMs.²² The benzoperylene derivatives, as a specific class of perylene dye, can give strong excimer emission.²³ However, no excimer emitting WLEMs based on benzoperylene derivatives have been reported.

Quantum dots (QDs) are considered one of the most promising nanomaterials as color converting centers for lighting and display, due to their high luminescence efficiency, high photostability, and excellent color tunability.^24,25 QDs have been used for the construction of WLEDs in a number of cases.^26–38

In this work, we report the preparation of a white light emitting nanocomposite (WLEN) based on a benzoperylene probe and the polyethyleneimine capped Mn²⁺ doped ZnS QDs (PEI/Mn-ZnS QDs) for the first time (Scheme 1). The Mn-ZnS QDs and a negatively charged benzoperylene probe (probe 1) were synthesized. The PEI/Mn-ZnS QDs induced efficient probe 1 self-assembly which gave strong blue excimer emission. The mixing of blue excimer fluorescence and orange PEI/Mn-ZnS QDs phosphorescence resulted in bright white light emission. The white emission possesses a Commission Internationale de l'Eclairage (CIE) coordinates of (x = 0.33, y = 0.32). The WLEN showed bright white light emission, good photo, chemical and long-term stability. The emission color of the final product could be finely tuned from orange to blue by simple adjustment of the ratio of the two components.

Scheme 1 Schematic illustration of probe 1 self-assembly onto the PEI/Mn-ZnS QDs and the formation of the WLEN.

Experimental

Reagents

Perylene, chloranil, maleic anhydride, 6-aminocaproic acid, benzamidine hydrochloride, ZnSO₄·7H₂O and Na₂S·9H₂O were purchased from Aladdin (Shanghai, China). Mn(CH₃COO)₂·4H₂O was purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Polyethyleneimine (MW: 10 000 or 1800 g mol⁻¹, branched) was purchased from Alfa Aesar (Tianjin, P. R. China). Probe 1 was synthesized according to literature procedures.²¹ The buffer solutions were prepared using ultrapure water purified using a Milli-Q A10 filtration system (Millipore, Billerica, MA, USA). All reagents were of analytical grade.

Apparatus

Transmission electron microscopy (TEM) measurements were performed on a FEI TECNAI G2 high-resolution transmission electron microscope (the Netherlands) operating at an accelerating voltage of 200 kV. Emission spectra were recorded using a Fluoromax-4 spectrofluorometer (Horiba Jobin Yvon Inc., USA), a 360 nm longpass optical filter was placed at the emission light path to remove the overtone bands. Slit widths of excitation and emission were both set at 3 nm, and the excitation wavelength was set at 300 nm. For photographs, the sample solution and solid WLEN were illuminated under 302 nm UV light. The UV-vis spectra were recorded on a Cary 50 Bio Spectrophotometer (Varian Inc., CA, USA). Quartz cuvettes (10 mm × 2 mm) were used for UV-vis and PL measurements. The luminescence decay lifetime was detected with a Lecroy Wave Runner 6100 Digital Osilloscope (1 GHz), a tunable laser (Contimuum Sunlite OPO, pulse width = 4 ns, gate = 50 ns) was used as the excitation source.

PEI and PEI/Mn-ZnS QDs induced probe 1 self-assembly

The PEI/Mn-ZnS QDs (10000PEI-QDs and 1800PEI-QDs) were prepared according to reported literature methods.^39,40 PEI/Mn-ZnS QDs and Tris–HCl buffer were added to a fluorescence quartz cuvette. Various amounts of water and probe 1 were added (total sample volume: 400 μL; final concentrations: QDs: 50 μg mL⁻¹; buffer: 5 mM, pH 7.4; probe 1: 0–10 μM). The sample solution was mixed thoroughly before the emission measurements. The PEI induced probe 1 self-assembly experiments followed similar procedures, with 0.2 mg mL⁻¹ PEI (MW = 10 000 g mol⁻¹) aqueous solution used instead of the PEI/Mn-ZnS QDs.

Preparation of the freeze-dried WLENs

Appropriate amounts of probe 1 (400 μM), the PEI/Mn-ZnS QDs (2 mg mL⁻¹) stock solution and certain amount of water were mixed, the intensity of the orange emission at 580 nm was made approximately equal to that of the blue emission at 470 nm. The sample solution was frozen at −20 °C and lyophilized. The resulted dry solid was stored at 4 °C before use.

Preparation of the WLEN gel

0.4 mL 10 × TBE (mixture of Tris base, boric acid and EDTA, pH 8.0) buffer, 0.4 mL WLEN solution (1 mg mL⁻¹) and 2 mL acrylamide (40 wt%) were added to 1.17 mL water, followed by 25 μL ammonium persulfate (10 wt%) and 10 μL tetramethylethylenediamine (TEMED). The sample solution was stirred briefly and left standing at ambient temperature, and gradually formation of the WLEN gel was observed.

Fabrication of the WLEN based LED device

The LED device was fabricated by coating the UV LED chips (Zhuhai Tianhui Electronic Co., Ltd, China, λ_em = 300–310 nm) with the WLEN, and the measurements were taken on a Starspec SSP6612.

Results and discussion

Synthesis and characterization of PEI/Mn-ZnS QDs and probe 1

Fig. 1 shows the emission spectra of probe 1 monomer and the PEI/Mn-ZnS QDs. Probe 1 monomer emitted at 419 nm and 441 nm as blue fluorescence, and the PEI/Mn-ZnS QDs gave an orange phosphorescence emission at 580 nm as a result of Mn²⁺ doping.³⁹

Fig. 1 Emission spectra of probe 1 (left) and the PEI/Mn-ZnS QDs (right). Inset: photographs of the aqueous sample solutions of probe 1 (left) and PEI/Mn-ZnS QDs (right).

Self-assembly of probe 1 induced by PEI

Probe 1 could be adsorbed onto the surface of the PEI/Mn-ZnS QDs via electrostatic attractive interactions between the negatively charged carboxylic functional group of probe 1 and the positively charged amine functional group of PEI. Changes in probe 1 emission spectrum in the presence of different concentrations of PEI were studied (Fig. 2 and S1, ESI†). Strong red shifted excimer fluorescence at 470 nm in the presence of PEI was observed, although the induced aggregation reduced with the decrease of the PEI chain length (MW = 1800 and 600 g mol⁻¹) compared to PEI (MW = 10 000 g mol⁻¹, Fig. S1†). Consequently 10000-PEI was used in the following experiments. The ratio of the maximum emission of the excimer (at 470 nm) over that of the monomer (at 419 nm) was defined as the I₄₇₀/I₄₁₉ value. And the I₄₇₀/I₄₁₉ value increased more than 7 times with the increase of probe 1 concentration from 0.1 μM to 10 μM in the presence of 0.2 mg mL⁻¹ PEI. At 10 μM probe 1 concentration, the I₄₇₀/I₄₁₉ value of the excimer was 14 times higher than that of the monomer emission, which clearly demonstrates the significant changes of the emission spectrum of the benzoperylene probe. However, no excimer fluorescence could be observed in the presence of probe 1 alone even at considerably higher probe concentrations, as a result, no changes in the I₄₇₀/I₄₁₉ value was observed. The results clearly show that PEI could induce extensive self-assembly of probe 1 which resulted in strong excimer fluorescence emission.

Fig. 2 (a) Changes in emission spectrum of probe 1 with probe concentration in the presence of 0.2 mg mL⁻¹ PEI (MW = 10 000 g mol⁻¹). (b) I₄₇₀/I₄₁₉ value changes of (a) at different probe 1 concentrations. (c) Changes in emission spectrum of probe 1 with probe concentration. (d) I₄₇₀/I₄₁₉ value changes of (c) at different probe concentrations. Conditions: 5 mM Tris–HCl buffer (pH 7.4).

Self-assembly of probe 1 induced by PEI/Mn-ZnS QDs

The PEI/Mn-ZnS QDs induced self-assembly of probe 1 was studied. The PL emission intensity of PEI/Mn-ZnS QDs at 580 nm decreased slightly with the addition of increasing concentrations of probe 1, whilst the excimer emission intensity of probe 1 gradually enhanced (Fig. 3a and b and S2†). Distinct probe 1 excimer fluorescence appeared at low probe concentrations. And significant increase in excimer fluorescence intensity was observed with the increase of probe 1 concentration. The conditions were adjusted to give about the same intensity of the blue and orange emission, and from which the WLEN was obtained.

Fig. 3 Changes in photoluminescence emission of the WLENs with probe 1 concentration. Conditions: (a) and (b), with 10000PEI-QDs; (c) and (d), with 1800PEI-QDs. Sample solution contained 5 mM Tris–HCl buffer (pH 7.4) and 50 μg mL⁻¹ PEI/Mn-ZnS QDs.

We also prepared the PEI/Mn-ZnS QDs with PEI of shorter chain length (MW = 1800 g mol⁻¹), and studied the interactions with probe 1 (Fig. 3c and d and S2†). However, the performance was less satisfactory. When 10000PEI-QDs was used, the concentration of probe 1 needed to induce probe 1 excimer emission with intensity equal to that of the QDs was about 0.45 μM (Fig. 3b). However, when the 1800PEI-QDs was used, 1.2 μM probe 1 was needed (Fig. 3d). The results showed that PEI with shorter chain length induced weaker probe 1 self-assembly, which was consistent with that of pure PEI, suggesting that the performance of the WLENs was related to the chain length of PEI.

Synthesis and characterization of WLENs

The WLEN was prepared by simple probe 1 self-assembly onto the PEI/Mn-ZnS QDs. Fig. 4a gives the PL emission spectrum and photographs of the WLEN in solution and in solid state. The emission spectrum is composed of the excimer emission of the benzoperylene probe and the PL of the QDs. The WLEN shows clear bright white light emission, and its aqueous solution also displays bright white emission. Fig. 4b shows the UV-vis absorption spectra of probe 1, the PEI/Mn-ZnS QDs and the WLEN. The WLEM shows a clear shoulder peak at 300 nm. And a clear broad absorption band at 350–400 nm was observed with intensity higher than that of the PEI/Mn-ZnS QDs, which clearly indicates the adsorption of the benzoperylene probe onto the surface of the QDs. In addition, the fine structure of probe 1 UV-vis absorption in this region almost disappeared completely, which suggests strong aggregation of the probe.²⁰ The results confirm the successful self-assembly of probe 1 onto the PEI/Mn-ZnS QDs.

Fig. 4 (a) Emission spectrum of the WLEN. Inset: photographs of the lyophilized WLEN solid and its aqueous solution. (b) UV-vis absorption spectra of probe 1, the PEI/Mn-ZnS QDs, and the WLEN.

HR-TEM images reveal that the absorption of probe 1 hardly affected the dispersity of the PEI/Mn-ZnS QDs (Fig. S3a and b†). The nanoparticles dispersed well with many small ZnS nanocrystals (3–4 nm) evenly distributed in each PEI nanoparticle (Fig. S3c and d†). Dynamic light scattering results indicate that the average hydrodynamic diameter appreas to reduce slightly upon binging to probe 1 (Fig. S4†). And a decrease in zeta potential value from 9.0 ± 1.2 to 1.9 ± 0.7 mV was also observed (Fig. S5†). The results suggest that once the negatively charged probe 1 bound to the positively charged PEI, the positive charge on the PEI polymer decreased.

The PL emission decay of probe 1 and the nanomaterials was studied. Fig. S6† shows that the PEI/Mn-ZnS QDs had a long-lived emission at 580 nm (1.19 ± 0.01 ms) arising from the triplet transition of Mn²⁺ (⁴T₁–⁶A₁),⁴¹ and the lifetime reduced slightly to 1.14 ± 0.02 ms after the conjugation with probe 1. The results indicate that conjugation with probe 1 had little influence on the core PEI/Mn-ZnS QDs emission. However, the lifetime of probe 1 changed significantly. Probe 1 monomer gave a lifetime value of 18.96 ± 0.10 ns, and the probe 1 aggregates gave a considerably longer lifetime of 41.78 ± 0.17 ns. This seems no surprising since the microenvironment of the benzoperylene probe changed considerably, and in addition, strong probe 1 aggregation could also influence the emission lifetime significantly.

CIE coordinates of the WLENs

CIE coordinates of the 10000PEI-QDs-probe 1 nanocomposite are shown in Fig. 5. Pure probe 1 and QDs emissions were in the blue and orange sections, respectively. The best WLEN was obtained at the probe 1 concentration of 0.45 μM, where the intensity of the two emission peaks was about the same value. The CIE 1931 chromaticity coordinates were calculated (x = 0.33, y = 0.32), which is quite close to the pure white coordinate (x = 0.33, y = 0.33), and the correlated color temperature (CCT) was 5697 K.

Fig. 5 CIE coordinates of the WLENs. Conditions: probe 1 concentration: 0–10 μM, PEI/Mn-ZnS QDs: 50 μg mL⁻¹, direction of the arrow indicates increase in probe concentration. Inset: photograph of the samples containing different concentrations of probe 1 (0–2 μM).

In addition, when probe 1 was at a concentration of below 0.45 μM, the 580 nm emission of the QDs dominated, and an orange emitting material was obtained, whilst at a probe 1 concentration of above 0.45 μM, excimer fluorescence of the probe dominated, and cyan emitting materials were obtained. Blue emission materials could also be obtained by simple increase of probe 1 concentration. The emission color of the material could thus be easily tuned from orange to blue with simple adjustment of the ratio of the two components.

Photostability of the WLENs

Photostability of the synthesized WLEN was examined. The freeze-dried solid was radiated under an UV lamp at 302 nm for an hour, and bright white light emission maintained without noticeable changes (Fig. S7†). In addition, the CIE coordinates of the WLENs kept mostly unchanged in the white light region for at least three months in open air ambient conditions. The results clearly suggest that the new WLEN has good photostability.

Fabrication of the WLENs based LED device

In order to study the illumination properties of the synthesized WLEN, a LED device coated with the WLEN was fabricated. Strong white light emission from the LED was clearly observed (Fig. 6) under 100 mA forward current. We also tested the illumination properties of the WLEN solution as shown in Fig. S8.† Bright white light emission could be obtained from the device under 30 mA forward current. The results show that the new WLEN could be fabricated in different formats. The color-rendering index (CRI) value of the WLENs was 72.61.

Fig. 6 Photographs of the LED device based on WLEN [(a) current off, (b) current on].

Synthesis of the white-light emitting gel

The WLEN could also be transformed into other forms of materials. For example, we incorporated the WLEN into the polyacrylamide gel (Fig. S9†). Although the gel synthesis is a complicated process which involved the use of monomer, initiator, accelerator, and the generation of a large amount of free radicals, the obtained gel emitted strong white light under UV illumination. The results clearly indicate that the WLEN is stable under different conditions and in different environments, and has a good potential for applications in different white-light emitting formats.

Conclusions

In summary, a new WLEN based on the self-assembly of the benzoperylene probe onto the PEI/Mn-ZnS QDs was successfully developed. The blue excimer fluorescence of the benzoperylene probe combined with the strong orange emission of the QDs endowed the nanocomposite color tunable properties. And pure white-light emission was obtained both in solution and in the solid state through simple synthesis process. The WLEN displays good photo, chemical and long-term stability, and could also be prepared in different forms. Strong white-light emitting WLEN based LED device was also successfully demonstrated. We envision that the combination of the small molecule probe excimer fluorescence and the PEI/Mn-ZnS QDs distinct PL could provides new ways for the construction of various types of white-light emitting luminescent materials and devices.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21561162004, 21275139), the Natural Science Foundation of Jilin Province (20150101183JC), the Jilin Provincial Strategic Economic Infrastructure Adjustment fund (2014Y077), the Pillar Program of Changchun Municipal Bureau of Science and Technology (14KG062).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Table for comparison of phosphor materials, changes in PL emission of probe 1 with different PEI and QDs, hydrodynamic diameter, zeta potential, lifetime, HR-TEM of QDs and WLEN, white light emitting LED device and gel based on WLEN. See DOI: 10.1039/c6ra18465k

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