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J. Phys. Chem. C 2009, 113, 3862–3868
Tunable Fluorescence Emission and Efficient Energy Transfer in Doped Organic Nanoparticles Xiuping Li, Yan Qian, Shuangqing Wang, Shayu Li,* and Guoqiang Yang* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China ReceiVed: NoVember 10, 2008; ReVised Manuscript ReceiVed: December 20, 2008
Luminescent organic nanoparticles consisting of a green-emitting ESIPT compound DHBIA doped with a red-emitting dye MAFN have been prepared by the reprecipitation method. It was found that emission of nanoparticles can be tuned gradually from green to red by increasing doping levels. These nanoparticles could be analyzed as a system of densely packed chromophores that showed efficient energy transfer from DHBIA to MAFN. The energy transfer in these systems was assigned to the Fo¨rster resonance mechanism. A simple model was modified and used to analyze quantitatively this energy transfer behavior in doped nanoparticles. The bathochromic of guest emission was presumably due to solid state solvation effects (SSSE). Introduction Over the past decade, many kinds of luminescent organic materials have been prepared and investigated for applications such as light-emitting nanodevices, tagging applications, optical sensors, etc. on account of their advantages including facilitation of structural design, low cost, and unique properties.1-4 It is difficult to find a single chromophore that exhibited more than one kind of emissive color. To obtain various colorful luminescences, as far as is known, the most facile method is to simply mix several compounds with different emitting colors; this the strategy is complicated and it is difficult to control the long-term color balance of multicolor emitting devices. Therefore, it is beneficial to design a simple system with a color tunable characteristic. As a general consideration, the doped system with energy transfer is a probable choice. Energy transfer is based on the distance dependence between a donor and an acceptor via radiative, nonradiative dipole-dipole, or electron exchange interaction, which results in a decrease of donor emission and an increase of acceptor emission. Thus, it is facile to adjust emission color through a variety of donor/acceptor ratios.5 However, this approach induces two problems in practical application. The first one is the quantum efficiency of the organic emitter in the solid state. Most organic light emitting materials exhibit strong nonradiative decay in the solid state, which presents the luminescence quantum efficiency decreasing exponentially with peak broadening and bathochromic shift in comparison with that in dilute solution. This case limits to a large extent the practical application of organic materials in the solid-state light-emitting devices. The second problem is the efficiency of the energy transfer between donor and accepter. Thus, it is significantly important to research and develop organic materials with strong luminescence and guest-host systems with highly efficient energy transfer in the solid.6 In our previous work, a series of new excited state intramolecular proton transfer (ESIPT) compounds based on 2,2′hydrophenyl benzothiazole have been studied,7 and the results indicated that these compounds exhibited obvious enhancement * Corresponding author. S.L.: phone 86-10-82617263, fax 86-1082617315, e-mail
[email protected]. G.Y.: phone 86-10-62562693, fax 86-10-62562693, e-mail
[email protected].
of luminescent intensity in their aggregation that was the socalled aggregation induced emission enhancement (AIEE).8-11 In this work, aiming at obtaining highly luminescent materials with tunable color emission in the condensed solid state, we chose an ESIPT compound N,N′-di[3-hydroxy-4-(2′-benzothiazole)phenyl]-5-tert-butylisophthalic amide (DHBIA) as host material due to its AIEE characteristic and a red light emission compound (2-((1,2-dihydro-1,2,2,4-tetramethylquinolin-6-yl)methyleneamino)-3-((10E)-(1,2-dihydro-1,2,2,4- tetramethylquinolin-6-ylimino)methyl)fumaronitrile)) (MAFN) as the guest for its solvatochromism. The structures of two compounds are shown in Scheme 1. Several kinds of organic nanoparticles were prepared with different doping levels by the reprecipitation method and emissions with different color were observed. Furthermore, some photophysical characteristics in these nanoparticles were also investigated and a modified model was used to analyze quantitatively the energy transfer process in this multiobject system. Experimental Section Sample Preparation. DHBIA, ivory-white solid-state powders (yield 70%) were synthesized by using a previous published method.7 The structure of the compound was characterized by NMR, MS, and element analysis, which correspond to the reported literature. MAFN, metallic luster solid was synthesized following a published method.12 A solution of 1,2,2,4-tetramethyl-1,2dihydro-6-quinolinecarbaldehyde (2.26 g), diaminomaleonitrile (0.54 g), glacial acetic acid (10 mL), and a few drops of acetic anhydride was refluxed at 120 °C for 3 days under Ar. After cooling to room temperature, the solution was poured into water (60 mL) and neutralized with NaHCO3. The solution was extracted with dichloromethane. The combined organic layer was washed with water and dried with magnesium sulfate. After the solvent was evaporated under reduced pressure, the residue was separated by aluminum oxide column chromatography, using dicholomethane/cyclohexane ) 1/1 (v/v) as eluent. Yield: 40%. Melting point: 305-308 °C. EI-MS: m/z 502, found 502. 1 H NMR (300 MHz, CDCl3, ppm): δ 8.57 (s, 2H), 7.74 (d, 2H, J ) 8.4 Hz), 7.64 (s, 2H), 6.56 (d, 2H, J ) 8.4 Hz), 5.34 (s,
10.1021/jp809911s CCC: $40.75 2009 American Chemical Society Published on Web 02/05/2009
Energy Transfer in Doped Organic Nanoparticles
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3863
SCHEME 1: Molecular Structures of DHBIA and MAFN
2H), 2.96 (s, 6H), 2.07 (s, 6H), 1.42 (s, 12H). Anal. Calcd for C32H34N6: C 76.46, H 6.82, N 16.72. Found: C 76.43, H 6.89, N 16.58. The nanoparticles of MAFN-doped DHBIA were prepared by rapidly injecting 100 µL of THF solution of MAFN/DHBIA with different mixture levels (DHBIA was 1.0 × 10-3 M, MAFN/DHBIA mole ratio was 0-4%) into 10 mL of deionized water with vigorous stirring for 60 s at room temperature. The samples were immediately taken for UV-vis absorption, fluorescence detection, and fluorescence decay. The scanning electron microscope (SEM) samples were prepared by dropping the mixture solutions onto a silicon plate and immediately evaporating the solvent under vacuum. Spectra Measurements. The absorption spectra were recorded on a Hitachi UV-3010. The fluorescence spectra were collected on a Hitachi F-2500 excited at 355 nm. The concentration of the solution was 1.0 × 10-5 M (DHBIA). The fluorescence decay curves were performed on an Edinburgh FLS-920 instrument, using single photon counting measurement (Samples were irradiated by 370 nm pulse LED). The solvents used in the spectrum were HPLC grade reagent (Beijing Chemical Reagents). Microscopy Measurements. The images of organic nanoparticles were characterized on a Hitachi S-4300 field emission scanning electron microscope. The samples used in the electron microscope were prepared by the following procedure. A drop of dilute H2O solution containing nanoparticles was dripped onto the clean Si substrate, and then recorded by electron microscope when the water had been vaporized. Photograph. The fluorescence photograph of the nanoparticles in H2O was taken by a digital cameral under an ultraviolet lamp (365 nm).
from DHBIA to MAFN occurred. Meanwhile, the doped systems showed a highly efficient antenna effect.13-15 When the nanoparticles with 4% doping concentration were excited at 355 nm, the fluorescence intensity of MAFN around 659 nm was 29 times higher than that with excitation at 560 nm, which was irradiated directly to MAFN and shown in the inset of Figure 1a. Additionally, we could see directly from the photograph that the fluorescence of doped nanoparticles could be tuned from green to red (Figure 1b) when the doping concentration was increased. FE-SEM images of DHBIA nanoparticles were shown in Figure 2. The appearance of doped organic nanoparticles presented little change in the whole range of doping concentration. Both DHBIA nanoparticles and the doped nanoparticles were sphere-like with an average diameter of about 200 nm, which indicated that MAFN molecules apparently did not affect the shape of the doped nanoparticles. Thus, it was rational to assume that MAFN molecules were monodispersed (e.g., dissolved) in DHBIA aggregates.
Results and Discussion Tunable Fluorescence Emission and Morphology of Doped Nanoparticles. Fluorescence spectra of organic nanoparticles for MAFN/DHBIA with different doping levels (MAFN:DHBIA ) 0-4%) were presented in Figure 1a. Pure DHBIA nanoparticles showed one peak with a maximum at 510 nm. Different from that of pure DHBIA nanoparticles solution, the fluorescence of doped nanoparticles exhibited two peaks. The peak at 510 nm without location changes in the range of doping was attributed to the emission of the donor-DHBIA, while the other peak that was changed from 620 to 659 nm with increased doping concentration should be ascribed to emission from acceptor-MAFN. It should be noted from Figure 1a that the emission of DHBIA was decreasing rapidly with gradual increasing of the doping concentration. As the doping concentration was increased to 4%, the fluorescence of DHBIA was almost quenched, which meant that a complete energy transfer
Figure 1. (a) The fluorescence spectra of the nanoparticles with different doping concentrations (excitation with 355 nm). (b) The photograph of nanoparticles under a ultraviolet lamp (365 nm).
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Li et al.
Figure 2. FE-SEM of DHBIA nanoparticles (left) and doped DHBIA nanoparticles with the doping level of 4% (right) obtained from suspensions of H2O (the concentration of DHBIA: 1.0 × 10-5 M).
Figure 3. Absorption spectra of DHBIA in THF solution and its aggregates in H2O (1.0 × 10-5 M).
To understand the tunable luminescence, in the following section, the energy transfer process in MAFN/DHBIA doped nanoparticles and factors for the tunable emission are discussed. Spectroscopic Characteristics of Donor-DHBIA and Acceptor-MAFN. Figure 3 showed the absorption spectra of DHBIA in THF solution and the aggregates in water. In THF solution, DHBIA exhibited an apparent absorption band with a maximum at 353 nm that was assigned to the π-π* transition. The absorption of DHBIA aggregates was red-shifted from that in the dilute solution and a shoulder peak emerged at the range of 375 to 380 nm. The shoulder peak could also be observed in the excitation spectra (see below). It could be assigned to the formation of J-aggregation.7,16,17 As shown clearly in Figure 4, DHBIA exhibited obvious aggregation induced emission enhancement (AIEE) phenomenon in water, which was analogous to the case where DHBIA was dispersed in THF/H2O (v/v ) 38:62) mixture.7 The fluorescence quantum yield of DHBIA in H2O (ΦF,H2O ) 0.072) was 36 times higher than that of THF (ΦF,THF ) 0.002) solution. The possible mechanisms for the phenomenon were suggested as the combined effects of a larger population of intramolecular H-bonds and special aggregation mode formationsa less optimal method of J-aggregation, as well as the resulting restriction of energy-consuming motions.7,17-22 Such compounds with high fluorescence quantum yield in the aggregate state may be used as good energy donors for solid state luminescent devices. MAFN exhibited apparent intramolecular charge transfer (ICT) character for its typical D-π-A-π-D structure. Figure S1a (shown in the Supporting Information) showed the absorption spectra of MAFN in different polar solvents. The data in
Figure 4. Fluorescence spectra (excited at 355nm) and photograph under UV lamp of DHBIA in THF solution and its aggregates in H2O (1.0 × 10-5 M). In the photograph, the samples were irradiated with 365 nm (left: aggregates in H2O; right: aggregates in THF solution).
TABLE 1: Photophysical Parameters of MAFN in Different Solvents solvent
λabsmax (nm)
λemmax (nm)
Φf
τ (ns)
kf (109 s-1)
n-hexane EtOEt CH3COOEt THF CH3CN
531 547 556 563 563
553 589 608 614 641
0.03 0.18 0.27 0.29 0.09
-a 0.12 0.40 0.48 0.52
1.5 0.68 0.60 0.17
a The fluorescence decay of MAFN in n-hexane was too fast to be detected with the setup.
protic and halogen solvents were excluded to avoid specific solute-solvent interactions. From the spectra, it could be seen that the absorption band was polarity-sensitive and changed gradually from 531 to 563 nm with increasing solvent polarity. It should be noted that the absorbance of MAFN in n-hexane was unusual compared with that of the others because of its poor solubility. The fluorescence spectra of MAFN in solvent with various polarities were shown in Figure S1b (shown in the Supporting Information). In nonpolar solvent, the emission of MAFN showed a peak with a maximum at 553 nm. When the solvent polarity was increased, the emission exhibited an obvious bathochromic shift to 589 (EtOEt) and 641 nm (CH3CN). It could be seen from Table 1 that the quantum yield of MAFN emission increased in turn with solvent polarity increasing up to THF, and then decreased in higher polarity solvent (CH3CN). The former stage was the so-called “negative solvatokinetic effect”, which was explained by several mechanisms such as biradicaloid charge transfer, proximity effect, and
Energy Transfer in Doped Organic Nanoparticles
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3865 TABLE 2: Fluorescence Decay Lifetimes of DHBIA in Doped Nanoparticles with Various MAFN/DHBIA Molar Ratios MAFN/DHBIA molar ratio ) a
τ (ns) a
0
0.0025
0.005
0.01
0.02
0.04
4.5
3.5
2.6
1.3
0.3
