ARTICLE pubs.acs.org/Langmuir
Self-Assembly of Pyrazine-Containing Tetrachloroacenes Kyoungmi Jang, Lacie V. Brownell, Paul M. Forster, and Dong-Chan Lee* Department of Chemistry, University of Nevada, Las Vegas, 4505 South Maryland Parkway, Las Vegas, Nevada 89154-4003, United States
bS Supporting Information ABSTRACT: In this paper, we report self-assembly of tetrachloroacenes containing pyrazine moieties. The title compounds, phenazine and bisphenazine substituted with four chlorine atoms for increased electron deficiency and alkyloxy side groups for solubility, demonstrated excellent gelation ability in select organic solvents. The assembled structure of these two series of compounds exhibited a morphological difference. Tetrachlorophenazine containing hexadecyloxy side groups induced rigid microbelts, while more extensive entanglement of thinner, more flexible fibers was observed from tetrachlorobisphenazine compounds, characterized by scanning electron microscopy. Tetrachlorophenazine and tetrachlorobisphenazine gels showed quite different emission behavior compared to their solution state. A strong, red-shifted emission compared to that of its diluted solution state was observed from the gel of tetrachlorophenazine. We have ascertained this is a result of J-aggregate formation. From the crystal structure of a model compound, it was found that tetrachlorophenazine cores adopt π π stacking with a short stacking distance of 3.38 Å, enabling significant intermolecular π-orbital overlap. In addition, the π-cores were displaced longitudinally, indicative of J-aggregate formation. Surprisingly, the gel of tetrachlorobisphenazine showed fluorescence comparable to that of its dilute solution, suggesting that such a close packing of the π-cores may not be possible due to the bulky tert-butyl substituents.
’ INTRODUCTION Although fluorine has been one of the most popular substituents in the preparation of electron-deficient π-conjugated materials,1 chlorine is now considered a viable and potentially preferable alternative. A recent study demonstrated the efficacy of Cl in increasing the electron affinity of n-type organic semiconductors, making this substituent more attractive than F for a variety of reasons, including lower overall cost as well as synthetic accessibility.2 In addition to the enhanced electronic properties upon chlorination, it has been reported that Cl substituents are more effective in promoting π-stacking than F, which can enhance charge mobility.3 Available single-crystal X-ray crystallographic data on chlorinated perylenebisimide, naphthalenebisimide, copper phthalocyanine, and pentacene have also demonstrated molecular packing that promotes π π interactions.2b d Interestingly, significant changes in optical properties have been observed for chlorinated diazaacenes, suggesting that pyrazine may be necessary in addition to chlorination to effectively engineer optical properties by stabilizing lowest unoccupied molecular orbital (LUMO) energy levels.4 A comparable suggestion was made by theoretical evaluations.5 We have also investigated the effect of Cl substitution on phenazine (P) and bisphenazine (BP) derivatives which contain pyrazine units.6 Experimentally, the LUMO energy levels were found to be 3.31 eV for tetrachlorophenazine and 3.41 eV for tetrachlorobisphenazine with respect to vacuum. The tetrachlorination lowered LUMO energy levels by 0.37 and 0.29 eV r 2011 American Chemical Society
for P and BP, respectively, when compared to the unsubstituted versions. It should be noted that theoretical evaluations predicted a more effective LUMO lowering effect with chlorination than fluorination. N-Heteroacenes have been considered as potential electrontransporting materials (n-type) depending upon the number of electron-deficient imine nitrogens present.7 Reports have indicated that chlorinated pyrazine-containing acenes are also potentially useful electron-transporting materials;4,5 however, these properties depend largely on solid-state molecular packing. In that regard, self-assembly of n-type organic semiconductors8 is particularly important since effective π π overlap may enhance electron-transporting properties. Nevertheless, there are limited studies available on solid-state molecular packing of chlorinated pyrazine-containing acenes.4b,5,6 The crystal structure of 2,3,7, 8-tetrachlorophenazine with two Cl atoms on each side of the π-core revealed effective π π interactions with a stacking distance of 3.42 Å.5 However, such effective π-stacking was not observed for 2,3-dichloro-7,8-bis(decyloxy)phenazine.8 This asymmetrically substituted phenazine π-core adopts off-face stacking with an antiparallel molecular arrangement. Although structural differences between 2,3,7,8-tetrachlorophenazine and 2,3-dichloro-7,8-bis(decyloxy)phenazine seem rather trivial, the Received: September 15, 2011 Revised: October 25, 2011 Published: October 27, 2011 14615
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Table 1. Gelation Properties of P and BP Compounds in Various Organic Solventsa solvent
P-1
P-2
P-3
BP-1
BP-3
cyclohexane
ppt
ppt
ppt
PG
PG
PG
hexane decane
ppt ppt
ppt ppt
ppt ppt
NS ppt
NS ppt
NS ppt
hexadecane
ppt
ppt
ppt
ppt
ppt
ppt
toluene
S
S
S
ppt
ppt
ppt
ethyl acetate
ppt
ppt
ppt
ppt
ppt
ppt
THF
S
S
S
G (10),
G (5),
ppt
TCE
S
S
ppt
G (3),
G (4),
Gb (5),
DCE
ppt
ppt
G (8),
39 °C PG
36 °C G (10),
60 °C ppt
TCTFE
ppt
ppt
ppt
NS
NS
NS
CCl4
S
S
S
PG
G (10),
ppt
29 °C
Figure 1. Structures of tetrachlorophenazines and tetrachlorobisphenazines.
BP-2
50 °C
65 °C
85 °C
56 °C ethanol
ppt
ppt
ppt
NS
NS
NS
propanol
ppt
ppt
ppt
NS
NS
NS
a
Abbreviations: G, gel; PG, partial gel; ppt, precipitation upon cooling; S, soluble after cooling; NS, not soluble; THF, tetrahydrofuran; TCE, 1,1,1-trichloroethane; DCE, 1,1-dichloroethane; TCTFE, 1,1,2-tricholoro-2,2,1-trifluoroethane. b Fast cooling is required. The CGC is shown in parentheses (mM). For gels, Tgel is provided.
Figure 2. UV vis absorption and fluorescence spectra of compounds P-3 (dashed line) and BP-3 (solid line) in CHCl3. Excitation wavelengths for P-3 and BP-3 were 410 and 421 nm, respectively. For easy comparison, the spectra are normalized at their maxima.
differences in their solid-state molecular packing cannot be ignored. In this work, we investigated how phenazine or bisphenazine with four Cl atoms on one side and alkoxy groups on the other side of the π-cores assemble. It should be noted that all aromatic hydrogens on one side of the π-core are replaced by Cl atoms, limiting possible hydrogen-bonding sites. It is uncertain if π π stacking can be maintained given that the π-cores are crowded with Cl atoms. Their ability to generate one-dimensional (1D) nano- or microfibers and their optical properties are of particular interest, as they are crucial for future nanodevice applications. To the best of our knowledge, this is the first study on self-assembly properties of tetrachloroacenes with pyrazine units.
’ RESULTS AND DISCUSSION The molecular structures of tetrachlorinated P and BP compounds are shown in Figure 1. One side of the π-core was substituted with four Cl atoms, while the other side was modified with solubilizing alkyloxy groups. The alkyloxy side groups varied from decyloxy to hexadecyloxy groups so that the effect of the alkyl chain length on the assemblies could be investigated. The target molecules were synthesized on the basis of procedures
developed in our research laboratory.9,10 The final compounds were also fully characterized with 1H and 13C NMR spectroscopy and mass spectrometry. Optical properties of the tetrachlorinated heteroaromatic compounds are provided in Figure 2. The absorption and emission properties were unaffected by the length of the alkyl side groups (see the Supporting Information), and therefore, only one example from each of the P and BP compounds is presented in the figure. In CHCl3 solution, P-3 showed an absorption maximum (λmax) at 410 nm while BP-3 exhibited a red-shifted λmax at 421 nm as a result of the extension of the π-core. In the case of P, tetrachlorination led to a red-shifted λmax when compared to the P without Cl atoms.6 BP-3 also showed an additional shoulder centered at ca. 455 nm which was a result of chlorination. Note that the bisphenazine without the Cl substituents did not show such a shoulder.10 The molar extinction coefficient of P-3 at 410 nm was 3.3 10 4 L 3 mol 1 3 cm 1, while that of BP-3 at 421 nm was 5.5 10 4 L 3 mol 1 3 cm 1. A similar trend between phenazine and bisphenazine was also observed in the fluorescence spectra. The fluorescence maximum of P-3 was 473 nm (excitation at 410 nm), while BP-3 showed red-shifted fluorescence at 523 nm (excitation at 421 nm). The fluorescence quantum efficiency measured in CHCl3 was found to be 4% and 23% for P-3 and BP-3, respectively. Our primary interest in self-assembly of the title compounds is the formation of organogels, which is evidence of fibrillization. Organogels have been the focus of recent studies in self-assembly due to their ability to generate 1D nanofibers quickly and reproducibly.11 In an appropriate solvent, low molecular weight organogelators (LMOGs) form 1D nanofibers through intermolecular forces such as hydrogen bonding, π π interactions, and van der Waals interactions. Subsequent three-dimensional 14616
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Figure 3. SEM images of the xerogels of P-3 (a) and BP-3 (b). Scale bars: 5 µm (a) and 1 µm (b).
(3D) entanglement of the 1D nanofibers with trapped solvent molecules produces organogels. Gel formation can normally be achieved by simple heating and cooling processes. Among others, π-organogelators have been of particular interest because of their potential electro-optical applications.11b,12 Therefore, we investigated the gelation properties of the title compounds in various organic solvents. As summarized in Table 1, a variety of solvents, including nonpolar, polar, aromatic, and halogenated solvents, were used for the test. In the case of the P series, only P-3 gelled in 1,1-dichloroethane (DCE), while all the BP series (BP-1, BP-2, and BP-3) formed gels in at least one solvent. Initial investigation on the self-assembly ability of the P series molecules indicated that all the P compounds are potential candidates for gelation as manifested by polarized optical microscopy (POM) images of cast films. Although the cast films of P-1 and P-2, prepared by slow evaporation of CH2Cl2 solutions, were not homogeneous, long fibers were easily found in the films (see the Supporting Information). However, in the solvents tested for gelation, P-1 and P-2 may not have the necessary balance between gelator gelator and gelator solvent interactions which is crucial for gelation. As a result, P-1 and P-2 either were soluble or formed precipitates in the tested solvent after cooling to room temperature. An appropriate combination of the tetrachlorophenazine π-core with hexadecyloxy side groups afforded such balanced interactions in DCE, leading to gelation. The critical gel concentration (CGC) of the DCE gel was 8 mM, and the gelling temperature (Tgel) was 50 °C. In the case of the BP series, all of the target title compounds formed a gel in 1,1, 1-trichloroethane (TCE). BP-1, which has decyloxy side groups, additionally gelled in THF, while BP-2 containing undecyloxy groups also formed gels in THF, DCE, and CCl4. The CGC and Tgel of each gel are presented in the table. The morphological study of the P and BP gels was carried out with scanning electron microscopy (SEM). The gels were transferred onto a clean gold/mica substrate, and the solvents were slowly evaporated to make dried gels (xerogels) for SEM experiments. Since TCE was the common gelation solvent for the BP compounds, further characterization of the TCE gels was conducted for the BP series, while a DCE gel was used for P-3. As shown in Figure 3, a clear morphological difference was observed between the xerogels of P-3 and BP-3. Note that the xerogels of BP-1 and BP-2 showed morphologies similar to that of BP-3 (see the Supporting Information). Gelation of P-3 produced microbelts with a randomly distributed width between 0.2 and 0.7 μm. The microbelts possessed a layered structure with bundles of different widths. In the case of the BP compounds, more extensive entanglement of thinner and more flexible fiber bundles was observed. A relatively uniform thickness of ca. 100 nm was measured from the fibers. Initially, the
Figure 4. UV vis absorption spectra of P-3 in solution (5 10 6 M, dashed line) and in the gel state (8 mM, solid line). In both cases, DCE was used as the solvent.
self-assembly ability of both the P and BP compounds was investigated by the preparation of cast films. POM images of the cast films showed that the BP compounds produced a network of curved fibers, while relatively straight fibers were observed for P compounds, which is consistent with the morphology of xerogels as shown in Figure 3. This result demonstrated the tendency of bisphenazines to assemble into more flexible and entangled fibers when compared to P compounds. The morphological difference between the P and BP series may result from the different molecular packing structures about the π-core. Molecular aggregation behavior was further investigated in the gel state with UV vis absorption spectroscopy. The gel sample for UV vis measurements was prepared by sandwiching the gels between cover slides. The absorption spectra obtained from the organogels were then compared to those in the solution state. Note that the same solvent was used for both the gel and solution states. In addition, the path length for the gel is a lot shorter than that of the solution (1 cm). Therefore, the comparison is only qualitative, and we focus on the overall absorption shape and maxima. In Figure 4, P-3 in the gel state showed a new noticeable shoulder at 432 nm and the overall spectral feature became much broader than that in solution. In the case of BP-3, UV vis absorption in the gel state exhibited a λmax at 432 nm, which was red-shifted from 421 nm, and a more pronounced red-shifted shoulder at ca. 480 nm (see the Supporting Information). Other BP gels showed similar absorption features. Those spectral changes in both P-3 and all BP compounds suggest the presence of intermolecular π π interactions in the gel state, especially the possibility of a J-type aggregate formation.13 We also compared fluorescence of P-3 and BP-3 in the gel and solution states using the same solvent for both states, i.e., DCE for P-3 and TCE for BP compounds. The fluorescence study was conducted in a 1 10 mm cell under right angle and front face geometries since the gels were semitransparent (see the Supporting Information for the detailed experimental setup) and compared to that of their diluted solutions. As depicted in Figure 5, under right angle geometry, P-3 in DCE at a low concentration (10 5 M, curve I) showed a fluorescence maximum (λem) at ca. 471 nm when excited at 14617
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Figure 5. Fluorescence spectra of P-3 in DCE solution at 10 5 M (I) under right angle geometry. For the gel in DCE at 8 mM, fluorescence was measured under right angle geometry (II, III) and front face geometry (IV). Excitation wavelengths were 410 nm for curves I, II, and IV and 430 nm for curve III.
Figure 6. Photograph of the DCE solution (10 5 M, left) and DCE gel (8 mM, right) of P-3 under illumination (365 nm).
410 nm. Surprisingly, the gel showed a new, strong emission at 578 nm (a remarkable red shift by 107 nm) in addition to the emission at 473 nm with reduced intensity (curve II). When the excitation wavelength became 430 nm, the relative intensity at 578 nm became stronger (curve III). Note that the DCE solution (10 5 M) showed negligible fluorescence when excited at 430 nm. The fluorescence of P-3 gel under front face geometry (curve IV) showed a reduced fluorescence compared to that under right angle geometry (curve II) when excited at the same wavelength (410 nm), presumably due to the shorter path length. However, the fluorescence maxima and shape remained the same. The change in the emission color due to the large red shift upon gelation is presented in Figure 6. While the DCE solution of P-3 (10 5 M) emitted a weak blue color, a bright yellow emission was observed from the DCE gel (8 mM) under
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Figure 7. Fluorescence spectra of BP-3 in TCE at 10 5 M (I) under right angle geometry. For the gel in TCE at 5 mM, fluorescence was measured under right angle geometry (II) and front face geometry (III). The excitation wavelength was 421 nm.
excitation at 365 nm. This interesting, strong fluorescence with a prominent red shift of λem from the organogel of P-3 may originate from J-aggregation.14 The fluorescence of BP-3 was quite different from that of P-3 (Figure 7). The BP-3 gel (8 mM) showed fluorescence only comparable to that of its diluted solution (10 5 M). A dramatic red shift in the fluorescence maximum like that for the P-3 gel was not observed. Under different geometries, the fluorescence intensity showed a trend similar to that observed for the P-3 gel. However, it should be noted that there is diminished fluorescence at lower wavelength regions and the fluorescence maximum is slightly more red-shifted under right angle geometry (curve II, λem = 531 nm) compared to front face geometry (curve III, λem = 528 nm), which is indicative of self-absorption by the gel.15 Overall, the fact that BP-3 gel did not show an increased fluorescence compared to its diluted solution indicates that tight packing as hypothesized for P-3 was not available for BP-3 due to the presence of bulky tert-butyl groups. To gain additional insights into how these types of molecules self-assemble, we attempted to grow single crystals suitable for X-ray diffraction. Attempts to grow single crystals for P-1, P-2, P-3, BP-1, BP-2, BP-3 were unsuccessful due to their tendency to form flexible 1D assembled structures. Therefore, we prepared a model compound, P-4, which has hexyloxy side groups to encourage more interaction between solutes and less interaction between solvent and solute.16 Following the synthetic approaches for the P series, P-4 was prepared. We were unable to prepare a bisphenazine containing hexyloxy side groups since one of the intermediates, 1,2-bis(hexyloxy)-4,5-diaminobenzene, was unstable for purification. By slow evaporation of a hexane/ CH2Cl2 solution of P-4, we were able to grow single crystals. A needle-shaped crystal was selected under paratone oil, mounted on a glass fiber, and cooled in a nitrogen cold stream to 100 K. A full hemisphere of data were then collected on a Bruker APEX II diffractometer. The structure was solved by direct methods and refined against F2 using SHELX-97.17 Hydrogen atoms were added geometrically and refined using the riding model. Details of the refinement are presented in the Supporting Information. As shown in Figure 8a, the π-cores of the P-4 molecules pack in opposite (or antiparallel) orientations. The shortest 14618
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Figure 8. Intermolecular interactions of the crystal of compound P-4 (a). Interatom distances: Cl(1) 3 3 3 H C(10), 2.818 Å; Cl(1) 3 3 3 N(2), 3.416 Å; Cl(1) 3 3 3 Cl(4), 3.596 Å; N(1) 3 3 3 Cl(4), 3.544 Å; Cl(4) 3 3 3 H C(7), 2.978 Å. π π stacking of P-4 in the crystal (top view (b), side view (c)). Hydrogen atoms in the alkyloxy side groups are omitted in (b) and (c) for clarity.
intermolecular contact was found between aromatic protons and Cl atoms in the neighboring phenazines with distances of 2.818(1) and 2.978(1) Å. The intermolecular distances between imine nitrogens and neighboring Cl atoms were found to be 3.416(1) and 3.544(1) Å. Unlike previous crystallographic results from 2,3-dichloro-7,8-bis(decyloxy)phenazine where C H 3 3 3 N plays a significant role in crystal packing,9 a C H 3 3 3 N interaction was not observed from P-4 molecules since aromatic C H was not available for the interaction due to Cl substitutions. In addition, Cl 3 3 3 Cl interaction may not play a major role as the distance was rather long, 3.596(1) Å. From the crystal structure, we found there are significant intermolecular π π interactions. As shown in Figure 8b, the tetrachlorophenazine π-cores have cofacial stacking with longitudinal displacement of ca. 11/2 benzene ring length, indicating J-aggregate formation. The π π stacking distance was quite short (3.38 Å) (Figure 8c), which is comparable to those of other reported chlorinated organic semiconductors.2b d Overall, P-4 formed a herringbone structure in the crystal (see the Supporting Information). Although we cannot be certain that the same interactions are present in gels, it is reasonable to assume the strong, red-shifted fluorescence from the P-3 gel originates from J-aggregate formation with a strong π π interaction. We further investigated the xerogels of P-3 and BP-3 with X-ray powder diffraction (XRD) experiments. Although we were unable to deduce the molecular packing mode from the XRD patterns, a clear difference between the crystallinity of the xerogels of P-3 and BP-3 was observed. The result is consistent with the SEM results. In the case of the P-3 xerogel, the XRD pattern showed well-defined diffraction patterns. However, the BP-3 xerogel exhibited amorphous character with a broad peak centered at 2θ = 18° corresponding to a d spacing of ca. 5.0 Å. The lack of well-defined peaks in the XRD pattern of BP-3 can be attributed to the presence of the bulky tert-butyl groups. Although both P and BP π-cores adopt a planar geometry, the presence of bulky tert-butyl groups in BP may affect the close packing of the molecules.
’ CONCLUSIONS In this paper, we have demonstrated that tetrachlorinated P and BP substituted with alkyloxy side groups can self-assemble
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into 1D fibers. In the case of the P molecules, only P-3 with hexadecyloxy side groups showed gelation properties. However, it should be noted the other P series molecules with decyloxy and undecyloxy groups formed fibers in cast films. Meanwhile, all the BP compounds in this study formed gels in at least one solvent regardless of the length of the alkyl side groups. The morphology of the fibers obtained from P and BP gels showed noticeable differences. P-3 induced rather straight, rigid, and bundled microbelts, while more extensive entanglement of thinner and flexible fibers was observed for all BP compounds. We also report an interesting fluorescence behavior. The DCE gel of P-3 showed strong, red-shifted fluorescence compared to diluted solutions. In the case of BP gels, the fluorescence was comparable to that of the dilute solution with only a slightly altered emission maximum wavelength. From the crystal structure of a model compound, P-4, the tetrachlorophenazine π-cores were found to form J-aggregates with a short inter-π-core distance of 3.38 Å, enough to allow significant π-orbital overlap. We ascribe the strong redshifted fluorescence in the P-3 gel to J-aggregate formation. Such a close packing of the π-cores may not be present in the BP series due to the bulky tert-butyl groups.
’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed synthetic procedures, characterizations of all tetrachlorophenazines and tetrachlorobisphenazines, UV vis and fluorescence spectra of P-1, P-2, BP1, and BP-2, Beer’s plot of P-3 and BP-3, POM images of cast films, additional SEM images, UV vis absorption of BP-3 gel, herringbone structure of the crystal of P-4, XRD powder patterns, and checkCIF/PLATON report. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT D.-C.L. gratefully acknowledges financial support from a National Science Foundation Career Award (Grant DMR0846479). Prof. Kenneth Czerwinski and Mr. Edward Mausolf are greatly acknowledged for their help with the X-ray powder diffraction analysis. We also thank Prof. David W. Hatchett for granting us access to the fluorometer. ’ REFERENCES (1) (a) Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.; Tokito, S.; Taga, Y. J. Am. Chem. Soc. 2000, 122, 1832–1833. (b) Heidenhain, S. B.; Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.; Mori, T.; Tokito, S.; Taga, Y. J. Am. Chem. Soc. 2000, 122, 10240–10241. (c) Sakamoto, Y.; Komatsu, S.; Suzuki, T. J. Am. Chem. Soc. 2001, 123, 4643–4644. (d) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Katz, H. E.; Marks, T. J. Angew. Chem., Int. Ed. 2003, 42, 3900–3903. (e) Schmidt, R.; Ling, M. M.; Oh, J. H.; Winkler, M.; K€onemann, M.; Bao, Z.; W€urthner, F. Adv. Mater. 2007, 19, 3692–3695. (f) Chen, H. Z.; Ling, M. M.; Mo, X.; Shi, M. M.; Wang, M.; Bao, Z. Chem. Mater. 2007, 19, 816–824. (2) (a) Ling, M.-M.; Erk, P.; Gomez, M.; Koenemann, M.; Locklin, J.; Bao, Z. Adv. Mater. 2007, 19, 1123–1127. (b) Tanag, M. L.; Oh, J. H.; Reichardt, A. D.; Bao, Z. J. Am. Chem. Soc. 2009, 131, 3733–3740. (c) Oh, J. H.; Suraru, S.-L.; Lee, W.-Y.; K€onemann, M.; H€offken, H. W.; R€oger, C.; Schmidt, R.; Chung, Y.; Chen, W.-C.; W€urthner, F.; Bao, Z. 14619
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