Self-assembled nanostructures of linear arylacetylenes and their aza-substituted analogues

Xu, Jia-Ju; Yang, Xiong-Bo; Feng, Hua; Wang, Yu-Long; Shan, Hai-Quan; Xu, Zong-Xiang;Ng, Chi-On; Huang, Long-Biao; Ko, Chi-Chiu; Roy, V. A L

Published in:AIP Advances

Published: 01/06/2016

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Published version (DOI):10.1063/1.4954036

Publication details:Xu, J-J., Yang, X-B., Feng, H., Wang, Y-L., Shan, H-Q., Xu, Z-X., Ng, C-O., Huang, L-B., Ko, C-C., & Roy, V. A.L. (2016). Self-assembled nanostructures of linear arylacetylenes and their aza-substituted analogues. AIPAdvances, 6(6), [65210]. https://doi.org/10.1063/1.4954036

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Self-assembled nanostructures of linear arylacetylenes and their aza-substitutedanaloguesJia-Ju Xu, Xiong-Bo Yang, Hua Feng, Yu-Long Wang, Hai-Quan Shan, Zong-Xiang Xu, Chi-On Ng, Long-Biao Huang, Chi-Chiu Ko, and V. A. L. Roy

Citation: AIP Advances 6, 065210 (2016); doi: 10.1063/1.4954036View online: https://doi.org/10.1063/1.4954036View Table of Contents: http://aip.scitation.org/toc/adv/6/6Published by the American Institute of Physics

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AIP ADVANCES 6, 065210 (2016)

Self-assembled nanostructures of linear arylacetylenesand their aza-substituted analogues

Jia-Ju Xu,1,2 Xiong-Bo Yang,2 Hua Feng,3 Yu-Long Wang,1 Hai-Quan Shan,1Zong-Xiang Xu,1,a Chi-On Ng,3 Long-Biao Huang,2 Chi-Chiu Ko,3,aand V. A. L. Roy2,a1Department of Chemistry, South University of Science and Technology of China,Shenzhen, P. R. China, 5180552Department of Physics and Materials Science and Centre of Super Diamond and AdvancedFilms (COSDAF), City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong3Department of Biology and Chemistry, City University of Hong Kong,Tat Chee Avenue, Kowloon, Hong Kong

(Received 12 May 2016; accepted 2 June 2016; published online 10 June 2016)

A series of linear phenylene ethynylene molecules have been synthesized, andaza-substitution has been used as a strategy to fine-tune the properties of themolecules. All the compounds exhibited pure blue emission both in solution andsolid state, and fluorescence quantum yields as high as 0.66, 0.63 and 0.82 werefound in chloroform solutions. The well-defined nanostructures such as quasi-cubes,cubes and rods were fabricated by self-assembly method from these compounds.The photophysical properties and aggregation behavior of self-assembled struc-tures were analyzed in detail. The morphology as well as photophysical propertiesof these nanostructures have been tuned with selective requirements. C 2016 Au-thor(s). All article content, except where otherwise noted, is licensed under a CreativeCommons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4954036]

Arylacetylene materials have received considerable attention as optoelectronic or electronicmaterials due to their abundant π-conjugation.1–3 Over the last decade, significant progress has beenmade for the development of this class of functional materials, and some of them (linear phenyleneethynylene molecules) have been found to exhibit desirable properties in charge transport, energytransfer and luminescence.4–11 One of the most appealing characteristic of phenyl acetylene mole-cules is their linear structure that potentially offers efficient π-stack between neighbouring mole-cules.10 It has been reported that in the solid state phenyl acetylene molecules can self-organize viacofacial interactions, resulting in close packed molecular aggregates.10 On the other hand, structuralmodification of such materials can be easily achieved by controlling the length of π-conjugation ofthe molecules or introducing substituents or hetero-atoms to the linear primary frameworks, leadingto the systematical change in the physical, electronic, and optoelectronic properties. With this inmind, a series of linear phenylene ethynylene molecules as well as their aza-substituted analogues(Scheme 1) have been synthesized and studied in this report.

As a matter of fact, the properties of materials not only depend on intrinsic properties ofmolecules but also depend on molecular aggregation state, particularly, when molecules were fabri-cated into nanostructures, the morphologies of nanoscale semiconductors usually bring significantchanges to electronic properties, and play an important role in electronic devices.12–14 Recentself-assembly studies reveal that weak noncovalent intermolecular interactions can be employedto fine-tune molecular alignments, and create well-defined supramolecular structures.15–18 How-ever, studies of linear molecules with extended π-conjugated system that assemble into well-defined nanostructures are relatively uncommon. Such linear molecules are particularly attractive

aAuthor to whom correspondence should be addressed. Electronic mail: xuzx@sustc.edu.cn, vinccko@cityu.edu.hk, val.roy@cityu.edu.hk

2158-3226/2016/6(6)/065210/9 6, 065210-1 ©Author(s) 2016.

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065210-2 Xu et al. AIP Advances 6, 065210 (2016)

SCHEME 1. Linear molecules studied herein.

building blocks for supramolecular chemistry because: 1) their phenyl, pyrimidine, and C≡C triplebond moieties offer π-π interaction to enhance the stacking of molecules in aggregated state;19,20

2) the introduction of hetero-atoms to linear primary frameworks provides C–H· · ·X (X: O, N,and etc.) or dipole-dipole interaction between molecules, and thus could lead to molecules thatform well-defined arrays in solution; and 3) such self-organized nanostructures are expected toshow interesting performance in optoelectronics, sensors, and electronic circuits21 because of theirwell-defined channels for charge carriers. Herein, we describe self-assembled nanostructures fabri-cated from the above materials (2, 3, 4, and 5, Scheme 1).

In order to better understand the electronic structures of the synthesized compounds, weexplored their properties using density functional theory (DFT). Molecular-orbital calculations of2, 3, 4 and 5 were carried out at the B3LYP/6-31G (d) level, and all the compounds were predictedto adopt planar geometries (optimized geometries) (Fig. S1 in the supplementary material22). Thecalculated results showed that with the replacement of CH groups by nitrogen atoms, both HOMOand LUMO energy levels of pyrimidine-containing molecules (HOMO level -5.80 eV and LUMOlevel -2.12 eV for 2, and HOMO level -5.71 eV and LUMO level -2.53 eV for 3) decreased signif-icantly compared with their corresponding phenylene ethynylene molecules (HOMO level -5.42 eVand LUMO level -1.71 eV for 4, and HOMO level -5.25 eV and LUMO level -2.07 eV for 5). Also,the reduction of HOMO-LUMO energy band gaps was observed by the introduction of pyrimidinemoieties to the linear molecules, and the calculated band gap (Egap) was found to decrease in thefollowing order of 4 > 2 > 5 ∼ 3 (Table S1 in the supplementary material22).

The calculated results were consistent with UV-Vis and fluorescence spectra in dilute solutions.As shown in Fig. 1(a), the lowest energy bands in the absorption spectra of 2-5 in dilute chloroformsolutions occurred between 250 and 402 nm, the absorption spectra of 2 showed structured absorptionwith maximum (λmax) at 326 nm and a shoulder at ca. 341 nm, undergoing a slight red-shift (6 nm) inthe absorption maximum relative to that of 4 (λmax at 320 nm). Similarly, the introduction of nitrogenatoms to the linear phenylene ethynylene structure resulted in the reduction of HOMO-LUMO energygap, and led to a slight red-shift (4 nm) in the absorption maximum from 3 to 5. According to theUV-Vis spectra, the optical band gaps determined from the initial absorption were 3.42, 3.09, 3.52,

065210-3 Xu et al. AIP Advances 6, 065210 (2016)

FIG. 1. (a) Normalized UV-Vis absorption spectra of 2, 3, 4 and 5 in chloroform solutions. (b) Normalized emission spectraof 2, 3, 4 and 5 in chloroform solutions. The emission spectra were measured upon excitation at 310 nm.

and 3.17 eV for 2-5, respectively, which are in good agreement with the calculated data (Table S1 inthe supplementary material22). In addition, red-shifts of emission bands in fluorescence spectra havebeen observed due to aza-substitution (Fig. 1(b)). Upon excitation at 310 nm, compounds 3, 4 and5 exhibited deep blue photoluminescence with highly structured emission band peaking at 393, 347and 387 nm for 3, 4 and 5, respectively. Fluorescence quantum yields were determined by excitationat 310 nm in dilute chloroform solutions (1×10−6 M) at 298 K, and quantum yields as high as 0.66,0.63 and 0.82 were found for 3-5, respectively (Table S2 in the supplementary material22). Compound2 in contrast exhibited a broad and slightly structured emission band with maximum at 387 nm anda shoulder at ca. 364 nm. The broadening of the emission band is due to the formation of molecularaggregation in solution, which can be further supported by the non-linear concentration-dependenteffect of the emission in solutions (Fig. S4 in the supplementary material22). Moreover, a relativelylow quantum yield of 0.04 observed for 2 (Table S2 in the supplementary material22) is suggestive ofan efficient non-radiative decay occurring in solution.

Recent research on the conformation-dependent photophysical properties indicates that thetwisting angle for the aryl versus the linear enthynyl units plays an important role on the spectralvariations, therefore, investigating the factors that affect the changes of conformation as well as theformation and characteristics of molecular aggregation becomes basically crucial.23 For phenyleneethynylene molecules, for instance, 4 and 5, the cylindrical symmetry of the C≡C triple bondcan maintain conjugation between adjacent phenyl units regardless of the relative orientations oftheir aromatic planes,10 and one interesting representation of this structural feature was the rela-tively free rotation along the aryl-alkyne single bonds,24 typically resulting in coexistence andrapid equilibration of coplanar and twisted structures.10 For nitrogen-free compounds (4 and 5),intermolecular interaction in solution can be attributed, at least partly, to π-π interaction betweenadjacent molecules, and such close association of π system often causes a substantial decrease inthe photoluminescence quantum yield compared with isolated chromophores.25 The existence oftwisted structures suppressed the self-quenching effects through hindering or minimizing the cofa-cial aggregations,23 and thus higher quantum yields than that of 2 were observed for 4 and 5. Partic-ularly, a relatively high quantum yield of 0.82 was observed for 5 due to the increase of twistedconformal population because of the increase of number of triple bonds within the molecule, whichcaused a substantial increase in the fluorescence quantum yield relative to 4 (0.62). As revealedin the theoretical studies C–H· · ·N intermolecular interactions are common in various purine andpyrimidine based molecules.26 Thus, unlike 4 and 5, the intermolecular C–H· · ·N interaction in 2would minimize the intermolecular distance, which lead to the increase of π-π interaction betweenmolecules and molecular aggregation, resulting in efficient self-quenching effect of the photolumi-nescence. In contrast to 2, a relatively high emission quantum yield (0.66) was observed in solutionof another nitrogen-containing compound (3). This can be explained as follows. Although intermo-lecular distance could also be decreased due to C–H· · ·N interaction between adjacent moleculesof 3, the effective cofacial π-π interaction between molecules is hindered in some degree due to the

065210-4 Xu et al. AIP Advances 6, 065210 (2016)

FIG. 2. Caption SEM images of self-assembled nanostructures fabricated from linear molecules: (a) compound 2,acetone/H2O = 1/125; (b) compound 3, DMSO/H2O = 1/5; (c) compound 4, acetone/H2O = 1/125; and (d) compound 5,DMSO/H2O = 1/5.

increase of twisted conformational population with the increase in the number of the triple bonds(similar to 5), which suppressed aggregation and the resulting self-quenching effect.

In order to study the aggregation behavior of such linear molecules, a phase transfer (PT)method with an acetone/H2O binary solvent system was adopted to prepare the nanostructures fromcompound 2 and 4. Due to the lower solubility of compound 3 and 5, a DMSO/H2O binary solventsystem was employed for the fabrication of their nanostructures. The resulting nano-products wereinvestigated by SEM. Samples were prepared by casting a drop of solution onto a silicon wafer, andthen dried in a vacuum box prior to analysis. As illustrated in Fig. 2(a), quasi-cubic nanosturetureswith the size ranging from 180 to 300 nm were found for 2. For investigation on the morphologyof self-assembled nanostructures due to the influence on the extension of π-conjugation of thepyrimidine-containing linear molecules, compound 3 was also fabricated into aggregates using thesimilar method. As shown in Fig. 2(b), well-defined nano-cubes with the size ranging from 400 to600 nm is observed from compound 3. Unlike 2 and 3, rod-shaped aggregates with the length ofca. 850 nm and diameter of ca. 330 nm is observed for 4 (Fig. 2(c)). The SEM images (Fig. 2(d))revealed that the aggregates of 5 consisted of a large quantity of rice-like rods with the length ofseveral micrometers, while their maximum diameters were in the range of 700 nm to 1.2 µm.

Self-assembly is a complicated process that is affected by various factors such as moleculesize and structure, intermolecular interaction, solvent effect, temperature, solubility of the sample,etc. Therefore, comprehensive understanding of the above factors to construct different morphol-ogies has been the focus of current research interests on this area. In this report, nanostructuresof phenylene ethynylene molecule and its corresponding aza-substituted analogue were fabricatedunder the same condition (i.e. acetone/H2O = 1/125 for 2 and 4, and DMSO/H2O = 1/5 for 3 and 5).Thus molecular interplay depending on molecular structures should play an important role on thebehavior of self-organization and so the morphologies of the resulting nanostructures. Phenyleneethynylene molecules, for instance, 4 and 5, are of typical extended π-conjugated structures withintrinsic intermolecular interaction via π-π stacking. Tuning intermolecular interaction of such kindof linear molecules can be easily achieved by introducing pyrimidine moieties onto the primarybackbones of the molecules, which provides a way to enhance C–H· · ·N interaction between mole-cules during the self-assembly process. The morphology of nanostructures fabricated from 4 was

065210-5 Xu et al. AIP Advances 6, 065210 (2016)

FIG. 3. (a) TEM image of self-assembled nanostructures of 4 fabricated by acetone/H2O = 1/125. (b) SAED pattern of thenanostructure of 4. (c) Schematic illustration of possible mechanism of the formation of nanostructures fabricated from 4.

confirmed by TEM (Fig. 3(a)), and the measurement of the selected area electronic diffraction(SAED) pattern suggested the crystalline characteristics of the sample (Fig. 3(b)). Compound 4 wasreported to possess a linear, coplanar structure and adopt a S-shaped crystal packing in the solidstate,27 as shown in Fig. 3(c). Hence, we describe the possible formation mechanism of nano-rodsof 4 in Fig. 3(c). The S-shaped π-π stacking facilitated the extension of the molecule aggregatesalong the a direction, while molecules were weakly held together by van der Waals force in theb direction, which hindered the extension of the aggregates in this direction, and thus rod-shapednanostructure was formed. Similar crystalline feature was observed for the nanostructures fabri-cated from 2 (Fig. 4(b)), and according to the literature23 the molecules should adopt a linear andcoplanar structure in crystalline state. Due to the existence of nitrogen atoms within the molecule,the main driving force operating in self-organization was assigned to the C–H· · ·N interaction aswell as π-π stacking of molecules. The possible mechanism of the formation of the quasi-cubicnanostructures fabricated from 2 was depicted in Fig. 4(c). Molecular blocks adopted a face-to-faceπ stack along the b direction, while in a direction, unlike 4, molecules were bound together viaC–H· · ·N interaction rather than the relatively weak interaction of van der Waals force, whichfacilitated the efficient growth of the aggregates along this direction, and as a result, quasi-cubicnanostructures is formed. For compounds 3 and 5, due to the lack of crystal data, DFT calculationwas employed to predict the molecule packing in solid state. The result obtained on the basis ofgeometry optimization and energy minimized molecular structures of 3 and 5 utilizing Gaussian 03program at B3LYP/6-31G (d) level (Fig. S1 in the supplementary material22) suggested that 3 and5 should also adopt a linear and coplanar structure in solid state. Based on this, we proposed thatmolecules could thermodynamically find an energy minimized molecular packing conformation

065210-6 Xu et al. AIP Advances 6, 065210 (2016)

FIG. 4. (a) TEM image of self-assembled nanostructures of 2 fabricated by acetone/H2O = 1/125. (b) SAED pattern of thenanostructures of 2. (c) Schematic illustration of possible mechanism of the formation of nanostructures fabricated from 2.

(i.e. linear coplanar structure) to enhance effective π-π stacking and maximized orbital overlappingduring self-organization. Similar to 4, the main driving force operating in the self-assembly processis π-π stacking interaction for 5, where the aggregates tended to grow in one main direction, andthus rice-shaped nano-rods is formed. Unlike 5, C–H· · ·N interaction would also play an importantrole on the self-organization of molecules for 3, and the formation mechanism should be similar to2, and therefore the nano-cubes were formed.

The samples for fluorescence measurement of self-assembled nanostructures fabricated from2-5 were prepared by casting drops of solution onto a silicon wafer and dried in vacuum box priorto use. For the purpose of comparison, emission spectra of bulk solid as well as solution sampleshave also been examined, and the results were illustrated in Fig. 5. As expected, all the emissionspectra of nanostructures were significantly different from those of the corresponding solution andbulk solid samples. The emission spectra of nanostructures of 2 displayed a broad emission bandwith maximum at 405 nm (Fig. 5(a)), undergoing a red shift of 28 nm relative to that of the solution

065210-7 Xu et al. AIP Advances 6, 065210 (2016)

FIG. 5. Normalized emission spectra of 2, 3, 4 and 5 in solution, nanostructures and bulk solid state. The emission spectrawere measured upon excitation at 310 nm. The self-assembled nanostructures were fabricated as follows: (a) compound 2,acetone/H2O = 1/125; (b) compound 3, DMSO/H2O = 1/5; (c) compound 4, acetone/H2O = 1/125; and (d) compound 5,DMSO/H2O = 1/5.

sample and a blue shift of 45 nm relative to that of the bulk solid. Similarly, the emission maximumof the nanostructures fabricated from 3 was found to appear at 403 nm with a red-shift of 11 nmrelative to that of the solution and a blue-shift of 52 nm relative to that of the bulk solid (Fig. 5(b)).Typically, red shift and broadening of emission bands are attributed to the formation or increase ofmolecular aggregation in solid state. The red shift at the emission band of bulk solids relative tothat of nanostructures suggested the increase of π-π stacking in bulk solid state with the increase ofparticle sizes from nanoscale to microscale. Similar results are observed for 4 and 5. The emissionmaximum of nanostructures of 4 exhibited a red-shift of 48 nm relative to that of the solution aswell as a blue-shift of 33 nm relative to that of bulk solid (Fig. 5(c)). Molecular aggregation alsobrought significant spectral change when molecules of 5 were fabricated into nanostructures, thebroadening and blue-shift of emission band relative to that of the solution is observed. It shouldbe noted that smaller spectral changes of the nanostructures relative to the solution-phase sampleswere observed than those to the bulk solid samples. Since the properties of the nanostructure areaffected by various factors including molecular aggregation, molecular packing, sizes and shapes ofthe nanostructures and so on, the spectral change becomes complicated and the reason is still underdiscussion. Generally, such self-assembled nanostructures acting as intermediates between mole-cules and bulk solids were found to exhibit different photophysical properties from their solutionsand bulk solids, which made them the promising materials that are expected to exhibit interestingelectronic and optical properties for device application in future.

A series of nanostructures with different sizes and shapes were fabricated from 2 by changingthe preparation conditions, as illustrated in Fig. 6. The experimental results showed that by adjust-ing the ratio of the acetone/H2O, different nanostructures such as particles, quasi-cubes and rodsare obtained. Particles with the size of ca. 60 nm were observed in the condition of acetone/water= 1/500 and 1/250 (Fig. 6(a), 6(b)), quasi-cubes with the size in the range of 180–300 nm wereformed at acetone/water = 1/125 (Fig. 6(c)), rod-shaped nanostructures with the width of ca.200 nm and the length in the range of 270–400 nm were found at acetone/water = 2/125 (Fig. 6(d)),and it has also been observed that with the increase of the ratio of acetone/water (from 2/125 to

065210-8 Xu et al. AIP Advances 6, 065210 (2016)

FIG. 6. TEM images and fluorescence emission spectra of nanostructures of 2. Nanostructures were prepared under thefollowing conditions: (a) acetone/H2O = 1/500; (b) acetone/H2O = 1/250; (c) acetone/H2O = 1/125; (d) acetone/H2O= 2/125; and (e) acetone/H2O = 1/25.

1/25), the rod-shaped nanostructures with the increased size (i.e. the width of ca. 400 nm and thelength of ca. 750 nm) is obtained (Fig. 6(e)). The emission spectra have also been measured to findout the influence of the morphologies of such nanomaterials on the photophysical properties. Thefluorescence behavior was found to exhibit a morphology-dependent characteristics. As illustratedin Fig. 6(a)–6(e), the emission bands of such self-assembled nanostructures were shifted to longerwavelength with the change of their shapes as well as the increase of their sizes, namely, maximumsat 394, 395, 405, 410 and 420 nm were found corresponding to different aggregation modes. Notethat photoluminescence properties of the self-assembled nanostructures can be tuned by adjustingthe preparation conditions, therefore the application of such materials for optoelectronic deviceswith selective requirements in the photophysical properties of semiconductors is promising.

In conclusion, a series of linear phenylene ethynylene molecules have been synthesized, andaza-substitution has been used as a strategy to fine-tune the properties of the molecules. All thecompounds exhibited pure blue emission both in solution and solid state, fluorescence quantumyields of 0.66, 0.63 and 0.82 were found for compound 3–5 in chloroform solutions, respec-tively. The well-defined nanostructures such as quasi-cubes, cubes and rods were fabricated byself-assembly method, and all the nanostructures demonstrated distinct luminescence propertiesin comparison with their solution and bulk solid samples. Moreover, it has been found that themorphologies as well as the photophysical properties of nanostructures fabricated from 2,5-bis(2-phenylethynyl)pyrimidine (2) can be tuned by adjusting the preparation conditions, suggesting thepotential application of such materials for optoelectronic devices with selective requirements on thephotophysical properties of semiconductors.

We acknowledge the Special Funds for the Development of Strategic Emerging Industries inShenzhen City, China (Grant No. JCYJ20120830154526537), grants from City University of HongKong’s Applied Research Grant Project no. 7004198 and the Research Grants Council of the HongKong Special Administrative Region (Project No.T23-713/11).

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