LETTER pubs.acs.org/JPCL
Self-Assembly of Perylenediimide Nanobelts and Their Size-Tunable Exciton Dynamic Properties Xinqiang Cao, Yishi Wu, Hongbing Fu,* and Jiannian Yao* Beijing National Laboratory for Molecular Science (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, and Graduate University of Chinese Academy of Sciences (GUCAS), Beijing 100049, P. R. China
bS Supporting Information ABSTRACT: Upon the oxidation of perylenediimide dianion precursors, controlled release of neutral units paves the way for the solution-phase self-assembly of nanobelts via synergistic ππ stacking and hydrogen-bonding interactions. The obtained belt size has been regulated through adjusting the precursor supersaturation. This controlled synthesis also offers us an opportunity to explore size-tunable exciton dynamics features in the nanobelt, in which the competitive evolution to H-like exciton or excimer is found to be in strong relevance to the molecular packing and crystal size. SECTION: Nanoparticles and Nanostructures
elf-assembly of π-conjugated molecules from solution provides a powerful approach for generating supramolecular nanostructures with long-range order, which are of great importance in the development of organic optoelectronic devices.15 Perylenediimide derivatives (PDIs) are particularly attractive building blocks for self-assembly because chemical modifications of the PDI core at either imide or bay positions offer great opportunities to alter the molecular packing in the solid state, which ensuingly results in rich electronic612 and photonic properties.1315 For example, W€urthner et al. reported that introduction of trialkoxyphenyl wedges at bay positions of PDI led to highly fluorescent J-aggregate assembly in a strongly slipped arrangement, whereas attachment of aminoethylbenzamide at imide positions of PDI as a gelator afforded helical nanofibers and bundles.1618 Zang and coworkers demonstrated that nanostructures of PDIs with different shapes can be prepared via endfunctionalization at imide positions, such as 0D particle and 1D belt or rod.19 Moreover, by covalent incorporation of PDIs with different kinds of molecular recognition units, a myriad of supramolecular architectures had been constructed through ππ stacking, metal ion coordination, and hydrogen bonding interactions.15,2022 Nonetheless, self-assembly of unmodified parent chromophore, PDINH, remains a great challenge because of its intrinsic insolubility in most common organic solvents.15 The solid-state molecular packing has a significant effect on both exciton migration and carrier mobility, which are pivotal for optoelectronic devices, for example, organic light-emitting diodes (OLEDs), field-effect transistors (FETs), and photovoltaics.10,2325 In this regard, assemblies of unmodified PDINH hold a special position in understanding the intrinsic electronic
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and photophysical properties of PDI-related materials.26,27 Recently, Ji and coworkers reported the preparation of 1D nanowires of PDINH on glass substrates using a gas-phase physical vapor deposition method.28 However, controlling the formation and morphology of PDINH assembly is still elusive. Factually, uncovering and mapping the evolution of structural, electronic, and optical properties from molecular level to bulk phase requires synthetic routes to prepare a homologous series of monodisperse nanoarrays in the nanometer range.2 This still remains a formidable task by employing conventional solutionor gas-phase self-assembling techniques because molecular components can easily get trapped into various kinetically stable topologies.29,30 Herein, we describe a two-step wet chemical reaction method to the fabrication of ultrathin nanobelts of unmodified PDINH with well-controlled sizes in a homogeneous solution. First, soluble precursors of PDINH, PDINH dianions (PDINH2), were prepared according to reaction 1 through reduction of PDINH using dithionite as the reducing agent. (See the Supporting Information for details.)31,32 2 PDINH þ Na2 S2 O4 þ 4NaOH f Naþ 2 PDINH þ 2Na2 SO3 þ 2H2 O
ð1Þ
The solution of PDINH2 in tetrahydrofuran (THF) is transparent and magenta in color with an absorption spectrum across 420 to 650 nm, consistent with previous reports (Figure S1 of the Received: July 12, 2011 Accepted: August 10, 2011 Published: August 10, 2011 2163
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Figure 2. (a,b) Equilibrium morphology of PDINH crystal for minimum total surface energy calculated by using the software of Material Studio package. (c) Packing arrangement by viewing onto the {020} plane. Figure 1. SEM (a,d), TEM (b,e), and AFM (c,f) images of PDINH nanobelts prepared with precursor concentration 0.16 (ac) and 0.044 mM (df). Insets in b, e, c, and f are the SAED patterns and corresponding AFM cross-section profiles.
Supporting Information). Second, 0.5 mL of stock solution of 0.48 mM PDINH2 in THF was injected into different volumes (V) of THF saturated with oxygen. After injection, the mixture was sealed in the vessel and kept in the dark for 12 h. In this mixture, neutral PDINH molecules were gradually released via oxidation of PDINH2 by molecular oxygen according to reaction 2. 2PDINH2 þ O2 þ 2H2 O f 2PDINHðsÞ þ 4OH 2
ð2Þ
Although ionic precursors of PDINH are well-dissolved in THF, neutral PDINH molecules are almost insoluble. Once the concentration of newly generated neutral molecules reaches the nucleation threshold, self-assembly of PDINH molecules is initiated probably through a nucleation burst, followed by growth. As long as the consumption of feedstock by the growing assemblies is not exceeded by the release rate of neutral units via reaction 2, no new nuclei form.33 Therefore, this wet chemical method facilitates the separation between the nucleation and growth stages. In our experiments, we changed the total volume of the reaction mixture to adjust the monomer concentration of CPDINH, according to CPDINH = (0.48 0.5)/(0.5 + V), paving the way to manipulate the growth kinetics.29 Figure 1a,b presents the scanning and transmission electron microscopy (SEM, TEM) images of nanoassemblies prepared at CPDINH = 0.16 mM (V = 1.0 mL). It can be seen from Figure 1a that uniform 1D structures were obtained with a width of ∼220 nm and a length of tens of micrometers. Moreover, the TEM image shows that these 1D structures are thin with a beltlike morphology (Figure 1b). Atomic force microscopy (AFM) measurement reveals that the belt thickness is ∼40 nm
(Figure 1c). By increasing V to 3.0 mL, thus decreasing CPDINH to 0.07 mM, the belt-morphology was well-preserved; however, the width and thickness were reduced to ∼120 and ∼30 nm, respectively (Figure S2 of the Supporting Information). At CPDINH = 0.044 mM (V = 5.0 mL), we obtained nanobelts with even smaller width (∼35 nm) and thickness (∼5 nm) in large quantity, as shown in Figure 1df. (The belt widths and thicknesses reported for each sample have a narrow distribution of 58%.) These results demonstrate that PDINH nanobelts have been successfully prepared with good controllability by using this wet chemical reaction method. To probe the internal structure, we measured the selected-area electron diffraction (SAED) pattern by directing the electron beam perpendicular to the flat faces of a single belt. The symmetric SAED pattern in the inset of Figure 1b demonstrates the single-crystalline nature of the obtained nanobelt. On the basis of the monoclinic PDINH crystal (obtained from CCDC, entry ID: LENPEZ01),34 data with cell parameters of a = 4.872(3) Å, b = 14.726(8) Å, c = 10.882(6) Å, R = γ = 90°, β = 91.48(4)°, and the square and triangle sets of spots with d spacing values of 4.9 and 5.4 Å, respectively, are due to {100} and {002} Bragg reflections; thereby, the circled set of spots with a d spacing of 3.7 Å is assigned to reflections from {102} crystal planes. Correlation of Bragg reflections with the orientation of the single belt shown in Figure 1b makes it clear that nanobelt preferentially grows along the [102] direction and is bounded by {020} facets on the top and bottom flat surfaces. Moreover, the X-ray diffraction (XRD) pattern of nanobelts is dominated by the {020} diffraction peak (Figure S5 of the Supporting Information) in the absence of others. To gain further insight into the mechanistic basis behind nanobelt self-assembly, we have calculated the equilibrium shape of PDINH crystal for minimum total surface energy by using the software of Material Studio package. The calculated surface energies γ{hkl} of the various crystal faces {hkl} follow the order: 2164
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Figure 3. Normalized excitonic absorption (a) and fluorescence spectra (b) of PDINH nanobelts. Black, blue, and red lines represent belts with width of 35, 120, and 220 nm, respectively. Spectra of monomer solution in DMSO (dashed line) are also shown for easy comparison.
Figure 4. Decay profiles of fluorescence lifetimes for monomer (square), belt emission at 630 nm (circled), and 770 nm (triangle) with 220 nm belt width. Black, red, and blue lines are the corresponding fitting curves.
γ{200} > γ{002} > γ{102} > γ{020} (Table S1 of the Supporting Information), yielding a polyhedron in shape (Figure 2a,b). Note that in both the calculated polyhedron and the self-assembled belt the most abundant faces on their surfaces are {020} faces that have the lowest surface energy. Figure 2c shows that PDINH molecules are stacked in a parallel fashion within {020} planes, forming a π-stacked layer structure. The green dashed lines in Figure 2c indicate the closet intermolecular contacts between the imide hydrogen and the carbonyl oxygen atom with a separation of 1.96 Å. Those molecules connected by NH 3 3 3 OC hydrogen bonds, such as those labeled by 1, 2, and 3 in Figure 2c, are in the same plane, which factually overlays along the {102} crystal face (Figure S6 of the Supporting Information). Bidirectional extension along 1 and 3 molecules via hydrogen bonding gives rise to chain-like structures. As shown by the yellow arrow, these chains can further stack along the [102] direction, leading to a 2-D layer with the shortest ππ interactions of ∼3.33 Å (Figure 2c).10,15 The longitudinal (along the molecular long-axis) and transverse (along the molecular shortaxis) displacements were calculated to be 3.37 and 1.08 Å, respectively (Figure S7 of the Supporting Information), indicating a large molecular overlap in the π-stacked layer. It is well known that the high-energy faces grow faster than the low-energy faces because the growth kinetic barrier of {hkl} is inversely proportional to the surface energy of γ{hkl}. It can be seen from Figure 2a that the growth of {020} faces is driven by weak van der Waals interactions between 2-D π-stacked layers, whereas ππ
stacking and hydrogen bonding interactions cooperatively contribute to the growth of {102} faces. Therefore, the growth kinetic barrier of {020} faces should be higher than that of {102} faces. In our wet chemical method, smoothly controlled release of neutral PDINH molecules might not be able to provide a supersaturation that is high enough to ensure the growth of {020} faces. Although it has not been fully understand yet, we speculate that the restriction of the growth along [020] direction and the preferential growth along [102] direction together result in the observed belt-like morphology. The spectroscopic studies (Figure 3), when correlated with the SEM observations, further clarify the formation of PDINH nanobelts with different sizes. In Figure 3, spectra (m) correspond to monomers in dilute dimethyl sulphoxide solution (
