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

A series of linear phenylene ethynylene molecules have been synthesized, and aza-substitution has been used as a strategy to fine-tune the properties of the molecules. All the compounds exhibited pure blue emission both in solution and solid state, and fluorescence quantum yields as high as 0.66, 0.63 and 0.82 were found 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 structures were analyzed in detail. The morphology as well as photophysical properties of these nanostructures have been tuned with selective requirements. C 2016 Au-thor(s). All article content, except where otherwise noted, is licensed under a Creative Commons

][6][7][8][9][10][11] One of the most appealing characteristic of phenyl acetylene molecules is their linear structure that potentially offers efficient π-stack between neighbouring molecules. 10It has been reported that in the solid state phenyl acetylene molecules can self-organize via cofacial interactions, resulting in close packed molecular aggregates. 10On the other hand, structural modification of such materials can be easily achieved by controlling the length of π-conjugation of the molecules or introducing substituents or hetero-atoms to the linear primary frameworks, leading to the systematical change in the physical, electronic, and optoelectronic properties.With this in mind, a series of linear phenylene ethynylene molecules as well as their aza-substituted analogues (Scheme 1) have been synthesized and studied in this report.
6][17][18] However, studies of linear molecules with extended π-conjugated system that assemble into welldefined nanostructures are relatively uncommon.Such linear molecules are particularly attractive SCHEME 1. Linear molecules studied herein.
building blocks for supramolecular chemistry because: 1) their phenyl, pyrimidine, and C≡C triple bond 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 that form well-defined arrays in solution; and 3) such self-organized nanostructures are expected to show interesting performance in optoelectronics, sensors, and electronic circuits 21 because of their well-defined channels for charge carriers.Herein, we describe self-assembled nanostructures fabricated from the above materials (2, 3, 4, and 5, Scheme 1).
In order to better understand the electronic structures of the synthesized compounds, we explored their properties using density functional theory (DFT).Molecular-orbital calculations of 2, 3, 4 and 5 were carried out at the B3LYP/6-31G (d) level, and all the compounds were predicted to adopt planar geometries (optimized geometries) (Fig. S1 in the supplementary material 22 ).The calculated results showed that with the replacement of CH groups by nitrogen atoms, both HOMO and LUMO energy levels of pyrimidine-containing molecules (HOMO level -5.80 eV and LUMO level -2.12 eV for 2, and HOMO level -5.71 eV and LUMO level -2.53 eV for 3) decreased significantly compared with their corresponding phenylene ethynylene molecules (HOMO level -5.42 eV and 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 pyrimidine moieties to the linear molecules, and the calculated band gap (E gap ) was found to decrease in the following order of 4 > 2 > 5 ∼ 3 (Table S1 in the supplementary material 22 ).
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 chloroform solutions occurred between 250 and 402 nm, the absorption spectra of 2 showed structured absorption with maximum (λ max ) at 326 nm and a shoulder at ca. 341 nm, undergoing a slight red-shift (6 nm) in the absorption maximum relative to that of 4 (λ max at 320 nm).Similarly, the introduction of nitrogen atoms to the linear phenylene ethynylene structure resulted in the reduction of HOMO-LUMO energy gap, and led to a slight red-shift (4 nm) in the absorption maximum from 3 to 5. According to the UV-Vis spectra, the optical band gaps determined from the initial absorption were 3.42, 3.09, 3.52, and 3.17 eV for 2-5, respectively, which are in good agreement with the calculated data (Table S1 in the supplementary material 22 ).In addition, red-shifts of emission bands in fluorescence spectra have been observed due to aza-substitution (Fig. 1(b)).Upon excitation at 310 nm, compounds 3, 4 and 5 exhibited deep blue photoluminescence with highly structured emission band peaking at 393, 347 and 387 nm for 3, 4 and 5, respectively.Fluorescence quantum yields were determined by excitation at 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 material 22 ).Compound 2 in contrast exhibited a broad and slightly structured emission band with maximum at 387 nm and a shoulder at ca. 364 nm.The broadening of the emission band is due to the formation of molecular aggregation in solution, which can be further supported by the non-linear concentration-dependent effect of the emission in solutions (Fig. S4 in the supplementary material 22 ).Moreover, a relatively low quantum yield of 0.04 observed for 2 (Table S2 in the supplementary material 22 ) is suggestive of an efficient non-radiative decay occurring in solution.
Recent research on the conformation-dependent photophysical properties indicates that the twisting angle for the aryl versus the linear enthynyl units plays an important role on the spectral variations, therefore, investigating the factors that affect the changes of conformation as well as the formation and characteristics of molecular aggregation becomes basically crucial. 23For phenylene ethynylene molecules, for instance, 4 and 5, the cylindrical symmetry of the C≡C triple bond can maintain conjugation between adjacent phenyl units regardless of the relative orientations of their aromatic planes, 10 and one interesting representation of this structural feature was the relatively free rotation along the aryl-alkyne single bonds, 24 typically resulting in coexistence and rapid equilibration of coplanar and twisted structures. 10For nitrogen-free compounds (4 and 5), intermolecular interaction in solution can be attributed, at least partly, to π-π interaction between adjacent molecules, and such close association of π system often causes a substantial decrease in the photoluminescence quantum yield compared with isolated chromophores. 25The existence of twisted structures suppressed the self-quenching effects through hindering or minimizing the cofacial aggregations, 23 and thus higher quantum yields than that of 2 were observed for 4 and 5. Particularly, a relatively high quantum yield of 0.82 was observed for 5 due to the increase of twisted conformal population because of the increase of number of triple bonds within the molecule, which caused a substantial increase in the fluorescence quantum yield relative to 4 (0.62).As revealed in the theoretical studies C-H• • • N intermolecular interactions are common in various purine and pyrimidine based molecules. 26Thus, unlike 4 and 5, the intermolecular C-H• • • N interaction in 2 would minimize the intermolecular distance, which lead to the increase of π-π interaction between molecules and molecular aggregation, resulting in efficient self-quenching effect of the photoluminescence.In contrast to 2, a relatively high emission quantum yield (0.66) was observed in solution of another nitrogen-containing compound (3).This can be explained as follows.Although intermolecular distance could also be decreased due to C-H• • • N interaction between adjacent molecules of 3, the effective cofacial π-π interaction between molecules is hindered in some degree due to the 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/H 2 O binary solvent system was adopted to prepare the nanostructures from compound 2 and 4. Due to the lower solubility of compound 3 and 5, a DMSO/H 2 O binary solvent system was employed for the fabrication of their nanostructures.The resulting nano-products were investigated by SEM.Samples were prepared by casting a drop of solution onto a silicon wafer, and then dried in a vacuum box prior to analysis.As illustrated in Fig. 2(a), quasi-cubic nanosturetures with the size ranging from 180 to 300 nm were found for 2. For investigation on the morphology of self-assembled nanostructures due to the influence on the extension of π-conjugation of the pyrimidine-containing linear molecules, compound 3 was also fabricated into aggregates using the similar method.As shown in Fig. 2(b), well-defined nano-cubes with the size ranging from 400 to 600 nm is observed from compound 3. Unlike 2 and 3, rod-shaped aggregates with the length of ca.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 of several 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 molecule size and structure, intermolecular interaction, solvent effect, temperature, solubility of the sample, etc.Therefore, comprehensive understanding of the above factors to construct different morphologies has been the focus of current research interests on this area.In this report, nanostructures of phenylene ethynylene molecule and its corresponding aza-substituted analogue were fabricated under the same condition (i.e.acetone/H 2 O = 1/125 for 2 and 4, and DMSO/H 2 O = 1/5 for 3 and 5).Thus molecular interplay depending on molecular structures should play an important role on the behavior of self-organization and so the morphologies of the resulting nanostructures.Phenylene ethynylene molecules, for instance, 4 and 5, are of typical extended π-conjugated structures with intrinsic intermolecular interaction via π-π stacking.Tuning intermolecular interaction of such kind of linear molecules can be easily achieved by introducing pyrimidine moieties onto the primary backbones of the molecules, which provides a way to enhance C-H• • • N interaction between molecules during the self-assembly process.The morphology of nanostructures fabricated from 4 was 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 was reported to possess a linear, coplanar structure and adopt a S-shaped crystal packing in the solid state, 27 as shown in Fig. 3(c).Hence, we describe the possible formation mechanism of nano-rods of 4 in Fig. 3(c).The S-shaped π-π stacking facilitated the extension of the molecule aggregates along the a direction, while molecules were weakly held together by van der Waals force in the b direction, which hindered the extension of the aggregates in this direction, and thus rod-shaped nanostructure was formed.Similar crystalline feature was observed for the nanostructures fabricated from 2 (Fig. (i.e.linear coplanar structure) to enhance effective π-π stacking and maximized orbital overlapping during self-organization.Similar to 4, the main driving force operating in the self-assembly process is π-π stacking interaction for 5, where the aggregates tended to grow in one main direction, and thus rice-shaped nano-rods is formed.Unlike 5, C-H• • • N interaction would also play an important role on the self-organization of molecules for 3, and the formation mechanism should be similar to 2, and therefore the nano-cubes were formed.The samples for fluorescence measurement of self-assembled nanostructures fabricated from 2-5 were prepared by casting drops of solution onto a silicon wafer and dried in vacuum box prior to use.For the purpose of comparison, emission spectra of bulk solid as well as solution samples have also been examined, and the results were illustrated in Fig. 5.As expected, all the emission spectra of nanostructures were significantly different from those of the corresponding solution and bulk solid samples.The emission spectra of nanostructures of 2 displayed a broad emission band with maximum at 405 nm (Fig. 5(a)), undergoing a red shift of 28 nm relative to that of the solution sample and a blue shift of 45 nm relative to that of the bulk solid.Similarly, the emission maximum of the nanostructures fabricated from 3 was found to appear at 403 nm with a red-shift of 11 nm relative 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 of molecular aggregation in solid state.The red shift at the emission band of bulk solids relative to that of nanostructures suggested the increase of π-π stacking in bulk solid state with the increase of particle sizes from nanoscale to microscale.Similar results are observed for 4 and 5.The emission maximum of nanostructures of 4 exhibited a red-shift of 48 nm relative to that of the solution as well as a blue-shift of 33 nm relative to that of bulk solid (Fig. 5(c)).Molecular aggregation also brought significant spectral change when molecules of 5 were fabricated into nanostructures, the broadening and blue-shift of emission band relative to that of the solution is observed.It should be noted that smaller spectral changes of the nanostructures relative to the solution-phase samples were observed than those to the bulk solid samples.Since the properties of the nanostructure are affected by various factors including molecular aggregation, molecular packing, sizes and shapes of the nanostructures and so on, the spectral change becomes complicated and the reason is still under discussion.Generally, such self-assembled nanostructures acting as intermediates between molecules and bulk solids were found to exhibit different photophysical properties from their solutions and bulk solids, which made them the promising materials that are expected to exhibit interesting electronic and optical properties for device application in future.
A series of nanostructures with different sizes and shapes were fabricated from 2 by changing the preparation conditions, as illustrated in Fig. 6.The experimental results showed that by adjusting the ratio of the acetone/H 2 O, different nanostructures such as particles, quasi-cubes and rods are 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 were formed 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 1/25), the rod-shaped nanostructures with the increased size (i.e. the width of ca.400 nm and the length of ca.750 nm) is obtained (Fig. 6(e)).The emission spectra have also been measured to find out the influence of the morphologies of such nanomaterials on the photophysical properties.The fluorescence behavior was found to exhibit a morphology-dependent characteristics.As illustrated in Fig. 6(a)-6(e), the emission bands of such self-assembled nanostructures were shifted to longer wavelength with the change of their shapes as well as the increase of their sizes, namely, maximums at 394, 395, 405, 410 and 420 nm were found corresponding to different aggregation modes.Note that photoluminescence properties of the self-assembled nanostructures can be tuned by adjusting the preparation conditions, therefore the application of such materials for optoelectronic devices with selective requirements in the photophysical properties of semiconductors is promising.
In conclusion, a series of linear phenylene ethynylene molecules have been synthesized, and aza-substitution has been used as a strategy to fine-tune the properties of the molecules.All the compounds exhibited pure blue emission both in solution and solid state, fluorescence quantum yields of 0.66, 0.63 and 0.82 were found for compound 3-5 in chloroform solutions, respectively.The well-defined nanostructures such as quasi-cubes, cubes and rods were fabricated by self-assembly method, and all the nanostructures demonstrated distinct luminescence properties in comparison with their solution and bulk solid samples.Moreover, it has been found that the morphologies as well as the photophysical properties of nanostructures fabricated from 2,5-bis(2phenylethynyl)pyrimidine (2) can be tuned by adjusting the preparation conditions, suggesting the potential application of such materials for optoelectronic devices with selective requirements on the photophysical properties of semiconductors.

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FIG. 3. (a) TEM image of self-assembled nanostructures of 4 fabricated by acetone/H 2 O = 1/125.(b) SAED pattern of the nanostructure of 4. (c) Schematic illustration of possible mechanism of the formation of nanostructures fabricated from 4.

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