Article pubs.acs.org/cm
High Hole Mobility in Triindole-Based Columnar phases: Removing the Bottleneck of Homogeneous Macroscopic Orientation Angela Benito-Hernández,† Upendra K. Pandey,‡ Emma Cavero,†,§ Roberto Termine,‡ Eva M. García-Frutos,† José L. Serrano,∥ Attilio Golemme,‡,* and Berta Gómez-Lor†,* †
Instituto de Ciencia de Materiales de Madrid, CSIC, C/Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain Centro di Eccellenza CEMIF.CAL, LASCAMM CR-INSTM, CNR-IPCF UOS CS, Dipartimento di Chimica, Università della Calabria, 87036 Rende, Italy ∥ Departamento de Química Orgánica, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain ‡
S Supporting Information *
ABSTRACT: We report the synthesis, mesomorphic behavior, and mobility values of a series of highly ordered N-substituted triindole-based columnar liquid crystals. Shortening the length of N-alkylic substituents from N-dodecyl to N-methyl chains results in a drastic approach of the disks within the columns and in an impressive increase in charge carrier mobility. An study of aggregation in solution provide insights into the intermolecular forces responsible of the reduction of the intrastack distance as the size of the N-alkyl chains is decreased and offer evidence of stabilization of the columns by the contribution of cooperative CH−π interactions. The materials presented here exhibit mobility values, even in totally misaligned columnar phases, that may compete with those of the best polycrystalline organic semiconductors, without the need of costly vacuum evaporation processes. KEYWORDS: liquid crystals, self-assembly, flexible electronics, semiconducting materials
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INTRODUCTION The opportunities offered by organic semiconductors in the construction of soft, flexible and low-cost electronic devices are remarkable and the functionalities that nowadays are only being envisioned may eventually play an increasingly larger technological role. However, some issues still need to be addressed before at least some of these ambitious goals can be achieved. One of them is the development of high mobility organic semiconductors meeting the easy processability standards required for the high throughput manufacturing of devices. In fact, charge carrier mobility in organic semiconductors has already reached values surpassing those of amorphous silicon (∼ 1 cm2 V−1 s−1), but although such values are already acceptable for many of the intended applications, the implementation of high mobility organic semiconductors in the fabrication of devices still shows serious process limitations.1−4 In this respect, a main drawback lies in the high degree of macroscopic order required for efficient charge transport, which imposes complex process standards for fabrication. In the search for the ideal trade-off between processing requirements and mobility values, discotic liquid crystals, with their soft and self-organizing properties, were targeted as promising candidates for easy-to-process, high-mobility semiconductors.5−7 Discotic mesogens, with a rigid inner disk-like core surrounded by a number of flexible chains, form highly ordered columnar superstructures, where the π-orbital overlap between adjacent molecules favors a one-dimensional migration of charge carriers. In addition, the inherent fluidity of liquid © 2012 American Chemical Society
crystals offers the potential for self-healing of structural defects over macroscopic areas. However, fluidity is also associated with fluctuations of molecules within the stacks, that may drastically reduce carrier mobility in the bulk.8−10 The preferred strategy for improving mobility in discotic mesophases has then been the enhancement of the intermolecular order within the stacks, by locking translation and rotation of discotic units. Unfortunately, increasing the intracolumnar order significantly increases the viscosity of the mesophases, hindering the achievement of macroscopic alignment and thus reducing bulk mobility, which is highly sensitive to domain boundaries. In this work, we show that it is instead possible to obtain high hole mobility values in highly ordered, even totally misaligned columnar phases. We have previously introduced heptacyclic 10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (triindole), as a central core for the construction of discotic liquid crystals.11 Attachment of six peripheral decyl chains to this electron-rich platform successfully induces the formation of columnar hexagonal mesophases, although no stacking periodicity could be observed. Hexadecyltriindole liquid crystals have shown mobilities up to μ = 0.02 cm2 V−1 s−1, a relatively high value in spite of the absence of a stacking periodicity.12 In an attempt to raise the mobility values of trindole-related liquid crystals, the π-conjugation area of the mesogens has been extended by functionalizing the core with peripheral alkynyl Received: August 1, 2012 Revised: December 18, 2012 Published: December 21, 2012 117
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Method 2. A mixture of N-substituted hexabromotriindole (0.116 mmol), Pd(PPh3)4 (0.07 mmol) and 4-nonylbenzeneboronic (0.93 mmol) was degassed. Then, 0.7 mL of 2 M aqueous K2CO3 and 6 mL of THF were added. The mixture was heated at 110 °C for 2 days under nitrogen, extracted with CH2Cl2 and washed with brine. The organic layer was separated, dried (MgSO4) and the solvent was evaporated in vacuum. The residue was purified by column chromatography. Compound 2. 49% (Method 1); yellow pale waxy; 1H NMR (200 MHz, CDCl3, δ): 8.30 (s, 3H, ArH), 7.61 (s, 3H, ArH), 7.26−7.06 (m, 12H, ArH), 7.12−7.06 (m, 12H, ArH), 4.86 (m, 6H, CH2), 2.67−2.59 (m, 12H, CH2), 2.09 (m, 6H, CH2), 1.66 (m, 12H, CH2), 1.19 (m, 102H, CH2), 0.92−0.88 (m, 27H, CH3); 13C NMR (50 MHz, CDCl3, δ): δ 140.7, 140.2, 140.1, 139.8,139.4, 135.8, 133.0, 130.2, 128.9, 127.8, 127.2, 123.3, 122.4, 111.6, 102.8, 47.1, 35.6, 31.9, 31.8, 31.4, 30.9, 30.3, 29.6, 29.4, 29.1, 26.7, 22.7, 14.1, . UV−vis (CH2Cl2, 25 °C): λmax (ε) = 339 (130396) nm; MALDI-TOF MS m/z 1895 [M++H]; HRMS (MALDI-TOF) calcd for C138H195N3: 1894.5371, found: 1894.5346. Compound 3. 80% (Method 2); yellow pale solid; 1H NMR (200 MHz, CDCl3, δ): δ 8.31 (s, 3H, ArH), 7.62 (s, 3H, ArH), 7.25−7.18 (m, 12H, ArH), 7.11−7.05 (m, 12H, ArH), 4.94 (m, 6H, CH2), 2.66− 2.58 (m, 12H, CH2), 2.11 (m, 6H, CH2), 1.64 (m, 12H, CH2), 1.29 (m, 78H, CH2), 0.90 (m, 18H, CH3), 0.82 (t, J = 7 Hz, 9H, CH3). 13C NMR (50 MHz, CDCl3, δ): δ 140.7, 140.5, 140.3, 140.2, 139.9, 139.6, 135.9, 133.2, 130.2, 127.8, 123.5, 122.6, 111.7, 103.0, 47.2, 35.6, 31.9, 31.5, 29.6, 29.4, 29.3, 22.8, 22.7, 20.0, 14.1, 13.9; UV (CH2Cl2, 25 °C): λmax (ε) = 340 (82609) nm; MALDI-TOF MS m/z 1727 (M+), HRMS (MALDI-TOF) calcd for C126H171N3: 1727.3501, found: 1727.3468. Compound 4. 86% (Method 1); yellow pale waxy; 1H NMR (200 MHz, CDCl3, δ): 8.49 (s, 3H, ArH), 7.56 (s, 3H, ArH), 7.23−7.17 (m, 12H, ArH), 7.10−7.06 (m, 12H, ArH), 4.48 (m, 9H, CH2), 2.62 (m, 12H, CH2), 1.64 (m, 12H, CH2), 1.30 (m, 81H, CH2), 0.9 (m, 9H, CH3); 13C NMR (50 MHz, CDCl3, δ): δ 141.5, 140.8, 140.5, 140.3, 139.9, 136.2, 133.0, 130.3, 130.1, 127.9, 123.8, 122.1, 111.3, 102.8, 36.2, 35.6, 31.9, 31.4, 29.6, 29.4, 22.7, 14.1; UV−vis (CH2Cl2, 25 °C): λmax (ε) = 338 (54400) nm; MALDI-TOF MS m/z 1601 [M++H], HRMS (MALDI-TOF) calcd for C117H153N3: 1601.2092, found: 1601.2107. Compound 5. 73% (Method 1); yellow pale waxy; 1H NMR (200 MHz, CDCl3, δ): 8.62 (s, 3H, ArH), 8.04 (s, 3H, NH), 7.61 (s, 3H, ArH) 7.22−7.16 (m, 12H, ArH), 7.09−7.05 (m, 12H, ArH), 2.62 (m, 12H, CH2), 1.64 (m, 12H, CH2), 1.30 (m, 81H, CH2), 0.9 (m, 9H, CH3); 13C NMR (200 MHz, CDCl3, δ): 140.6, 140.0, 139.6, 137.7, 136.3, 134.9, 133.6, 130.3, 130.2, 127.9, 127.8, 121.9, 121.1, 112.5, 101.5, 35.6, 31.9, 31.5, 31.4, 29.6, 29.4, 22.7, 14.1; UV−vis (CH2Cl2, 25 °C): λmax (ε) = 328 (53200) nm; MALDI-TOF MS m/z 1559 [M++H], HRMS (MALDI-TOF) calcd for C114H147N3: 1559.1615, found: 1559.1623. Mesomorphic Properties. The optical textures of the mesophases were studied with an Olympus polarizing microscope BX51 equipped with a Linkam hot-stage and Linkam TMS 91 central processor and microphotographs were taken with an Olympus DP12 digital camera. The transition temperatures and enthalpies were measured by differential scanning calorimetry with a TA Instrument Q20 and Q2000 calorimeter operated at a scanning rate of 10 °C min−1 on both heating and cooling. The apparatus was calibrated with indium (156.6 °C; 28.4 J g1−) as a standard. The XRD patterns were obtained with a pinhole camera (Anton-Paar) operating with a point-focused Nifiltered Cu−Kα beam. The sample was held in Lindemann glass capillaries (1 mm diameter) and heated, when necessary, with a variable-temperature oven. The capillary axis is perpendicular to the Xray beam and the pattern is collected on flat photographic film perpendicular to the X-ray beam. Spacing was obtained via Bragg’s law. Sample Preparation and SCLC Measurements. To prepare cells for mobility measurements, we cleaned lithographically patterned (several 1 mm wide stripes on each substrate) ITO coated glasses (UNAXIS, 110 nm ITO thickness) by successive ultrasonication using lightly soapy water, distilled water, acetone, and isopropyl alcohol. The cleaned glasses were then transferred to a vacuum oven for overnight
chains or by means of expanded trindoles as central cores. Such strategies yield more ordered mesophases rendering hole mobilities as high as 1.4 cm2/(V s)13 and 0.69 cm2/(V s),14 respectively, and qualifying triindoles among the highest mobility mesogenic semiconductors. In efforts to obtain even more ordered mesophases, bulky phenyl spacers were found to be effective, yielding highly ordered columnar hexagonal mesophases in broad temperature ranges. Linking the cores to peripheral alkylic chains through bulky phenyl moieties can efficiently interlock the molecules within the columns10 and it can even induce helical columnar arrangements, providing a high degree of order.15 However, the high steric demand of these connecting groups induces large stacking distances in triindole liquid crystals (c = 4.4 Å), hindering charge transport. In fact, hexa(4-nonylphenyl) triindole 1 (see Scheme 1) shows a hole mobility of only μ ∼ 6 × 10−4 cm2 V−1 s−1 in spite of the higher order achieved.13 Scheme 1. Differently N-Substituted Hexa(4-nonylphenyl) Triindoles 1−5
In this contribution, we show how N-substitution drastically affects the self-assembly of hexa(4-nonylphenyl) triindoles both in solution and in the mesophases. In particular, we show that by varying N-functionalization we can significantly reduce the intrastack distance in the mesophases, obtaining an impressive increase in charge carrier mobility in macroscopically disordered samples, processed by simply squeezing a hot melt of the active material between two electrodes.
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EXPERIMENTAL SECTION
Materials Synthesis. The synthesis of the hexaaryl triindole 1 has been previously reported in our group.13 Compounds 2-5 were prepared by 6-fold Suzuki coupling of the corresponding N-substituted hexabromotrindoles with 4-nonylbenzeneboronic acid using microwave (Method 1) or thermal activation (Method 2). NMR spectra were recorded at 23 °C. Routine 1H and 13C NMR spectra were recorded on Bruker AMX 300 and Bruker AC-200 spectrometers. Chromatography purifications were carried out by using a gradient of eluents (hexane → CH2Cl2/hexane 20%) on a Biotage SP1 flash chromatography system. Method 1. To a solution of N-substituted hexabromotriindole (0.05 mmol) in 2.5 mL of THF was added 4-nonylbenzeneboronic (0.39 mmol) and 2 M aqueous K2CO3 (0.75 mmol) in a 10 mL pressurized vessel that was carefully degassed before and after adding Pd(PPh3)4 (0.03 mmol). The vessel was then sealed and heated at 150 °C in a MW system during 1 h with stirring. After 1 h, water was added, the mixture was extracted with CH2Cl2 and washed with brine, the organic layer was separated and dried (MgSO4), and the solvent was evaporated in vacuum. The residue was purified by column chromatography. 118
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drying at 90 °C. Some of the cleaned substrates were treated with air plasma (Diener Electronics, Model Femto, working at 100 W and 40 kHz) for 5 min, in order to tune the work function of ITO. The plasma-cleaned glass was immediately transferred inside a glovebox with a nitrogen atmosphere (O2 < 0.1 ppm, H2O < 0.1 ppm) where cells were prepared by squeezing the hot melt between two ITO electrodes (one of them plasma-treated). The temperature was always kept well below the decomposition temperature of each substance. Samples were then allowed to slowly cool to room temperature before being sealed using epoxy glue and taken out of the glovebox for thickness and SCLC measurements. The thickness (between 9 and 12 μm) was obtained from the interference maxima and minima in the near-IR region when the light intensity transmitted by the samples was recorded by an AGILENT 8453 UV−vis spectrometer.
shifting. Similar alternated stacking has been observed in different triindole aggregates both in solution16 and in the solid state.17 The mesomorphic behavior of all the new compounds was studied by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) (Table 1) (see Figures S1−S4 in the Supporting Information) Table 1. Mesomorphic Properties and Transition Temperatures (°C) of compounds 1−5 compd
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RESULTS AND DISCUSSION The synthesis of the hexaaryl triindole 1 (Scheme 1) has been previously reported by our group.13 Compounds 2−5 were prepared by 6-fold Suzuki coupling of the corresponding Nsubstituted hexabromotriindoles with 4-nonylbenzeneboronic acid in the presence of Pd(PPh3)4, 2 M aqueous K2CO3 using THF as solvent. Although 2−4 could be obtained in good yield under conventional thermal cross-coupling conditions, unsubstituted hexabromotriindole suffers partial N-arilation. Compound 5 was therefore obtained in low yield as a complex mixture difficult to purify. Use of microwave-assisted conditions significantly reduced side reactions rendering 5 in good yield. The structure of all new compounds has been confirmed by NMR and exact mass spectrometry. 1H-NMR spectra of compounds 4 and 5 in chloroform solutions are concentration dependent, with the aromatic proton signals of the triindole core moving upfield with increasing concentration, as illustrated in Figure 1. This effect, due to the magnetic interaction
transition temp (°C) ΔH(kJ mol−1)a c
1
C 39.3 (39.7) Colho 153−160 I
2
C 45.8 (4.7) Colho 128.8 (0.61) I
3 4
Cr 48.9 (13.6) I Colho ∼190 (dec.)d
5
Colho ∼110 (dec.)e
XRD data (Å) at 25 °Cb a = 29.4 c = 4.4 a = 26.6 c = 4.4 a = 29.8 c = 3.46 a = 31.5 c = 3.30
Data taken from the first heating cycle at 10 °C min−1. Cr, crystal; Colho, ordered hexagonal columnar; I, isotropic liquid. bAfter thermal treatment heating at 94 °C (2), 175 °C (4), and 105 °C (5), cooling at room temperature. cTransition temperature determined by polarized optical microscopy (POM). dAlthough the loss of weight (1%) in the TGA takes place at very high temperatures >350 °C and no change is observed in POM, in DSC a small exothermic peak at ∼190 °C indicates the beginning of the decomposition (dec.) of the product. e Although the loss of weight (1%) in the TGA takes place at ∼150 °C and no change is observed in POM, in DSC a small exothermic peak at ∼110 °C indicates the beginning of the decomposition (dec.) of the product. a
showing that mesomorphism does not vary monotonically with chain length. Shortening the size of the chains from 12 to 8 carbon atoms induces no significant change: the same stacking distances are observed in 1 and 2 and only transition temperatures are slightly influenced. Both 1 and 2 exhibit a hysteresis phenomenon in the crystallization process upon cooling, showing a supercooled state that maintains the structural features of the mesophases. On further reducing the length of the N-alkyl chain to four carbon atoms, no mesomorphism is observed: 3 is a crystalline material that enters an isotropic state nearly at room temperature. Upon reducing the N-alkyl chains length even further to N-CH3 or NH (compounds 4 and 5) columnar hexagonal mesomorphism is observed again in a very broad range of temperatures. Moreover, a striking shortening of the intrastack distance, approximately 1 Å shorter than in 1, is also observed. N-Alkyl chains may introduce two opposite effects, as it was previously observed in series of liquid crystals functionalized with lateral chains:18,19 a steric hindrance to organize in columns, due to the tendency of alkyl chains to be localized out of the plane of the molecule, and an increase in the interaction between the molecular units due to van der Waals attraction involving alkyl chains. When the length of substituents is very large (as in compounds 1 and 2) the steric hindrance introduced by the alkyl chains and responsible of the larger intrastack distance is compensated by the positive effect of intermolecular attraction, while on derivatives with very short substituents both effects are minimal. Compound 3, substituted with three N-butyl chains, appears to be in an intermediate
Figure 1. Concentration-dependent 1H NMR spectra of 5 in CDCl3.
between two aromatic rings in close proximity, suggests a strong tendency of these two derivatives to self-assemble in solution. A clear upfield effect can also be observed for the NCH3 proton signals in compound 4 and for the N-H signal in 5. In fact, both signals suffer the most pronounced shifting. In contrast, 1−3 show no concentration-dependent chemical shift, indicating that these molecules only exist as monomers in chloroform solutions. The study of association in solution by NMR, provides interesting structural information about the supramolecular arrangement of the aggregates. In this particular case, the fact that only aromatic protons of triindole and N-H/N-CH3 signals are affected, suggests an alternated arrangement in which each molecule is rotated by 60° with respect to the next molecular unit. This arrangement would place affected signals in the shielding cone of the aromatic system, explaining their strong 119
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Figure 2. X-ray diffractograms of the Colh mesophase recorded at room temperature on compound 4 and crystallographic packing of the parent Ntrimethyltriindole.
second nearest neighbor), between the mesophase of compound 4 and the crystal structure of the parent N-trimethyl triindole, point to a similar role of the methyl groups in the stabilization of the columnar arrangement. In view of the favorable supramolecular order found in compounds 4 and 5, charge mobility measurements were performed by the Space Charge Limited Current (SCLC) method.21−24 In such measurements, substances are confined between two electrodes, with one of them injecting charges of a given sign (ohmic contact) while the other one should be noninjecting for charges of the opposite sign.25 Since HOMO values of 4 and 5 are 5.0 and 5.1 eV (see Table S2 in the Supporting Information), respectively,26 one plasma-treated ITO electrode (with a work function ∼4.9 − 5.1 eV)27,28 was used as positive electrode. By measuring the current as a function of the applied potential difference it is possible to observe a transition from a ohmic region (at lower voltages) where the current depends linearly on voltage, to a so-called space charge limited current region,29 where such dependence is quadratic:
situation, because the short alkyl moieties exert an important steric effect that is not compensated by attractive interactions. Compounds 4 and 5 are obtained as waxy solids and they show mesomorphism at room temperature (Table 1). Typical polarized optical microscopy (POM) textures were not observed for these mesophases, neither was the transition to isotropic liquid. Upon cooling, fluidity gradually decreases but no evidence of crystallization or glass transition could be observed by POM and DSC. However, the X-diffraction patterns of compounds 4 and 5 (both at room and at high temperatures) (see Table S1 in the Supporting Information) show a set of maxima at low angles that unequivocally identify the mesophases as hexagonal columnar. A maximum at high angles, corresponding to the periodic stacking of the cores, can also be observed for both derivatives at 3.46 and 3.30 Å, respectively. This scattering peak is sharper in the case of 4, indicating a longer range correlation distance for the order within the columns (Figure 2). The XRD pattern of 4 shows an additional diffuse maximum at middle angles, corresponding to about twice the stacking distance, consistent with a certain local order within the column. This result agrees with a model in which each molecule is rotated by 60° with respect to the next molecular unit within the columns, which would lead to a correlation of every second molecule along the stacks. The impressive shortening in the intrastack distance found in 5, when compared to 1 and 2, is not surprising, because Nfunctionalization of trindole induces a twisting of the platform,16,17 whereas unsubstituted triindole has a planar core.20 This feature may be at the origin of the tighter packing of the units within the stacks. Less anticipated is instead the effect exerted by the methyl groups. In order to shed light on the influence of the N-methyl moieties in mesomorphism, we have revisited the crystal structure of the parent Ntrimethyltriindole, previously published by some of us.17 This molecule crystallizes, forming a highly ordered columnar structure in which stacked molecules are rotated by 60° with respect to each other and are situated at two alternating distances (3.53 and 3.68 Å) along the column, as determined by single-crystal X-ray diffraction analysis. Each methyl group shows short contacts with the aromatic rings that lie above and below it, suggesting that CH-π interactions are involved in the stabilization of the columnar alternated arrangement. Such interactions, although very weak, act cooperatively, becoming significant. The interesting structural similarities (intrastack distance, periodicity every
J=
9 V2 ε0εrμ 3 8 d
(1)
Here J is the measured current density, ε0 is the free space permittivity, εr is the dielectric constant of the material, μ is the charge mobility, V is the applied voltage, and d is the thickness of the device. Such regime is obtained when the charge density is so high that the electric field responsible for the drift of charges is not only due to the applied potential difference but also to the injected charge. After measuring the dielectric constant and the thickness of the samples, charge mobility can then be extracted from current/voltage curves. Measurements were carried out at room temperature on samples prepared as reported in the Experimental Section. Given the waxy nature of the phases, with no real phase transition, it was not possible to homogeneously align the columnar symmetry axis throughout the samples volume. In all cases, polarized optical microscopy actually revealed that the structure was totally misaligned, as illustrated by the pictures included in the Supporting Information (Figure S7). A typical current/voltage curve is shown in Figure 3. The resulting mobilities were μ = 2.8 ± 1.6 cm2 V−1 s−1 for 4 and μ = 2.4 ± 1.3 cm2 V−1 s−1 for 5. Considering that the technique used is very sensitive to the presence of defects and distortions in the alignment of the columnar structure, and given the total lack of 120
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alignment in the present case, such high mobilities are remarkable.
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CONCLUSION The materials presented here highlight the importance of the chemical versatility of the triindole central core in tuning the electronic and morphological properties of columnar mesophases. In addition, they qualify the N-methyl derivative as a most promising platform in the search for organic semiconductors for the development of high throughput devices by low-cost processing methods, such as printing or solution/melt deposition. In fact, these materials present mobility values that may compete with those of the best polycrystalline organic semiconductors, without the need of costly vacuum evaporation processes. The picture emerging from the results illustrated above is that columnar phases with high stacking order and short stacking distance can be a promising answer to the easyto-process requirements for the manufacturing of organic electronic devices. ASSOCIATED CONTENT
S Supporting Information *
Details on the mesomorphic properties and SCLC measurements. This material is available free of charge via the Internet at http://pubs.acs.org.”
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REFERENCES
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Figure 3. Current/applied voltage curve for a sample of 5 with a 12 μm thickness. The straight lines, with slopes 1 and 2, are not fittings of the experimental points.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (A.G.);
[email protected] (B.G.L.). Phone: (+34) 91-3349031 (B.G.-L.). Fax: (+39) 0984492138 (A.G.); (+34) 91-3720623 (B.G.-L.). Present Address §
Instituto de Nanociencia de Aragón, Universidad de Zaragoza. Mariano Esquillor s/n, 50018 Zaragoza, Spain
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support by Comunidad Autónoma de Madrid under project S2009/MAT-1756 and by Ministerio de Ciencia e Innovación (Spain) under projects CTQ2010-18813, CTQ2009-09030/ BQU, MAT2011-27978-C02-01 are gratefully acknowledged. 121
dx.doi.org/10.1021/cm3033548 | Chem. Mater. 2013, 25, 117−121
