Article pubs.acs.org/Langmuir
Highly Oriented J-Aggregates of Nitroazo Dye and Its SurfaceInduced Chromism Toshihiko Tanaka,*,†,‡,§,∥ Masamitsu Ishitobi,‡ Tetsuya Aoyama,§ and Shinya Matsumoto∥ †
Department of Chemistry and Biochemistry, Fukushima College, National Institute of Technology, 30 Aza-Nagao, Tairakamiarakawa, Iwaki, Fukushima 970-8034, Japan ‡ ASET Sumitomo Chemical laboratory, Tsukuba Research Laboratory, Sumitomo Chemical Co. Ltd., 6 Kitahara, Tsukuba, Ibaraki 300-3294, Japan § RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ∥ Department of Environment and Natural Sciences, Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan S Supporting Information *
ABSTRACT: Highly oriented J-aggregates of a nitroazo dye were obtained in solid thin films on aligned poly(tetrafluoroethylene) surfaces. During film deposition on a friction-transferred poly(tetrafluoroethylene) layer, a sharp peak grew in the polarized absorption spectra around 613 nm, which was red-shifted 117 nm from the peak in dilute dichloromethane solution. The peak showed remarkable optical anisotropy: dichroic ratios D of up to 22 were observed, and the intrinsic D value should substantially exceed this value. These results indicate that the peak is attributable to highly oriented J-aggregates. On glass, however, H-like aggregates grew, exhibiting an absorption peak at 410 nm. Hence, the substrate surface induced the remarkable chromism observed as a 203 nm red shift.
1. INTRODUCTION Highly oriented J-aggregates are fascinating materials because of their scientific significance and possible applications. The physics of J-aggregates can be explained by Bose condensation, a remarkable demonstration of quantum physics. The condensation of excitons provides a resonance model that is widely accepted to account for the peculiar physical properties of J-aggregates.1 The resonance also plays a photosensitizing role in photosynthesis.2,3 Because of the peculiar low dimensionality of J-aggregates, macroscopic orientation enhances their properties in a certain direction, thereby enabling the elucidation of their intrinsic functions. Such directional enhancement makes the orientation of J-aggregates quite favorable in device applications such as nonlinear optics,4−7 lasers,8−11 and semiconducting devices including organic fieldeffect transistors.12,13 Oriented growth on aligned poly(tetrafluoroethylene) (PTFE) layers is a promising process for obtaining highly oriented J-aggregates. Since the seminal work of Witttmann and Smith,14 this process has been used to orient a wide variety of materials, including organic molecules,15−23 polymers,14,24−31 liquid crystals,32−38 inorganic crystals,14 carbon nanotubes,39 metal nanoparticles,40,41 and nanocomposites.42 Furthermore, we found that growth on aligned PTFE can also be used to prepare highly oriented J-aggregates of a bisazomethine dye.43−45 Although the aggregates are partly grown in films, © 2016 American Chemical Society
their dichroic ratio (D) exceeds the value of 20 reported for a monolayer of cyanine J-aggregates ordered on a gypsum crystal surface.46 On friction-transferred (FT) PTFE layers modified with further PTFE deposition and rubbing, D reaches a remarkably high value of 44.44 Highly oriented J-aggregates are uncommon, and to the best of our knowledge, D values of greater of 20 have not been reported except in the two examples mentioned above. D values of less than 10 have been reported for other alignment methods, such as vertical spin coating,47 electrospinning,48 anisotropic evaporation of solution,49 application of magnetic fields,50,51 use of Langmuir− Blodgett (LB) films,52 adsorption on a single-crystal surface,53 and laser microfixation.54 Two other bisazomethine derivatives also provide oriented J-aggregates on aligned PTFE layers, but their D values do not exceed 20.45 We report here highly oriented J-aggregates of monoazo dye 1 (Figure 1) in films deposited on aligned PTFE layers. These J-aggregates grew during film deposition and showed remarkable D values of up to 22. We reported that 1 undergoes oriented growth on aligned FT PTFE layers and that the oriented dye film shows a large bathochromic shift of over 100 nm for an absorption peak compared to the spectrum of 1 in Received: January 25, 2016 Revised: April 17, 2016 Published: April 18, 2016 4710
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The spectra of dye powder dispersed in KBr disks were obtained in transmission mode at normal incidence without polarizers. Polarized optical micrographs were recorded on a Carl Zeiss Axioimager equipped with an Axiocam HRC (or MRc 5s) camera through crossed nicols or a single nicol. The sliding direction of the FT layers was oriented at nearly 45° to the two polarizations in crossed nicols observation or was oriented parallel to the light polarization in single nicol observation.
Figure 1. Molecular structure of dye 1.
dichloromethane solution.55 Considering the highly oriented Jaggregates of bisazomethine dyes, we investigated oriented films of 1 that are slightly thinner than those prepared previously. Results for a 77-nm-thick film indicate that the peak should be assigned to J-aggregates. This is the first report of oriented J-aggregates of an azo dye. J-aggregates that are not oriented macroscopically are formed from various azo dyes in some interphases, including LB films,56 bilayers,57,58 and adsorbates on the surface of a dendrimer.59
3. RESULTS AND DISCUSSION 3.1. Orientation of Long Axes and Surface-Induced Chromism. The polarized UV−visible spectra of the dye films revealed that the long axes of dye 1 molecules were oriented parallel to the PTFE chains. On both the FT and ER layers, absorbance in the polarized spectra was substantially larger when the incident light was polarized parallel to the sliding direction than when it was polarized perpendicular to the sliding direction (Figures 2 and 3). Because each absorbance
2. EXPERIMENTAL SECTION 2.1. Preparation of Substrates Modified with PTFE. Slide glasses were used as substrates. Aligned PTFE thin layers were deposited on the substrates by a previously reported friction-transfer method60 or by evaporation and rubbing (ER).23,43 FT layers were deposited with an Tribotrack ultrathin PTFE film coater (DACA Instruments). The substrates were rubbed at a pressure of about 5 kg/ cm2 with the curved surface of a PTFE rod (ϕ, 10 mm; length, 20 mm) sliding in a direction at a constant speed of 1 mm/s at 300 °C. For ER layers, a PTFE film (50 nm thick) was deposited on a glass substrate under vacuum by evaporating PTFE (molecular weight 5000−20 000; Wako Pure Chemical Co., Ltd.). Then, the film was rubbed at a pressure of about 5 kg/cm2 with a cloth that was slid in a single direction at a constant speed of 5 mm/s. The PTFE was thereby oriented with its chains parallel to the sliding direction in both the FT and ER layers.23 2.2. Dye Deposition. Oriented thin films of dye 1 (Figure 1) were deposited on the aligned PTFE layers by evaporating the dye in a vacuum of about 10−5 Torr at a growth rate of 0.05−0.3 nm/s. The average thickness was monitored by the frequency change of a quartz resonator. The dye was synthesized by a conventional azo coupling prodcudure.61 The absorption spectrum of 1 in dichloromethane solution (10−5 M) was measured using a quartz cell. 2.3. Characterization of the Dye Films. The polarized UV− visible absorption spectra of the dye films on the glass substrates were recorded on a Cary 5E UV−visible−NIR spectrophotometer with a highly efficient Glan−Thompson polarizer placed before the sample on the optical path. Spectra were acquired at normal incidence in transmission mode with polarization separately parallel and perpendicular to the sliding direction. For each polarization, a reference spectrum of a PTFE layer on a glass substrate was taken and subtracted from each sample spectrum during data processing. For the measurement of polarized IR absorption spectra, dye 1 films on ER layers were deposited on silicon substrates in the same manner as for the films on the glass substrates. The dye 1 films on FT layers were transferred from glass substrates to silicon substrates as follows. Some droplets of 10 wt % aqueous pullulan (Hayasibara Co.) were placed on the dye film and then spread over it. After drying, the pullulan together with the dye and PTFE was carefully peeled off of the glass and allowed to float on the surface of water with the pullulan side down. After the pullulan dissolved, the dye film with PTFE was skimmed from the surface onto a piece of silicon wafer. The polarized IR absorption spectra were recorded on a Nicolet Magna 860 FT-IR spectrometer equipped with a TGS detector. The IR beam was polarized using an Al wire grid KRS-5 polarizer. The spectra were obtained at normal incidence in transmission mode with polarization separately parallel and perpendicular to the sliding direction. A reference spectrum of an ER PTFE layer on a silicon substrate was taken for each polarization and subtracted from each sample spectrum during data processing. All IR spectra were taken at 2 cm−1 resolution by collecting 500 interferograms for each spectrum.
Figure 2. UV−visible polarized absorption spectra of dye 1 oriented films on FT PTFE layers: film thicknesses of (a) 95, (b) 77, (c) 56, (d) 34, (e) 17, (f) 95, (g) 77, (h) 56, (i) 34, and (j) 17 nm. Light polarization is parallel to the aligned PTFE chains in a−e, and it is perpendicular to the chains in f−j.
peak is maximized for the polarization direction exactly parallel to the sliding direction, the mean directions of the transition moments agree with those of the PTFE chains. A molecule of dye 1 has a transition moment parallel to its long axis, as is well known for dichroic azo dyes: they have one-dimensional conjugation of their π orbitals, and their electronic transitions are accompanied by some intramolecular push−pull charge transfer along the long axis from one end (amine group) to the other (nitro group). This electronic structure of azo chromophores has been theoretically shown by molecular orbital (MO) calculations, such as Pariser−Parr−Pople MO calculations.62,63 4711
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77 nm (Figure 2b) on an FT layer, the peak grows and shifts from 584 to 613 nm, tapering to a sharp peak for the 77-nmthick film. This peak becomes slightly less sharp upon further deposition to a thickness of 95 nm (Figure 2a). On ER layers, however, the peak is appreciably weaker, becoming a shoulder (Figure 3b−d) or a broad peak (Figure 3a) at around 593 nm. The second zone is located from 530 to 570 nm. A peak or shoulder grows for parallel polarization on both the FT and ER layers (Figures 2a−e and 3a,b). The last type is located in the third zone below 500 nm. On ER layers, the peak grows and shifts with increasing film thickness. Here, the shoulders on FT layers (Figure 2a−e) correspond to this peak, but they do not appear as peaks because of overlap with the peaks in the second zone at around 540 nm. For an isotropic film deposited on glass without any PTFE, a distinctive sharp peak is observed that is attributed to H-like aggregates. As shown in Figure 4a, a peak appears at 410 nm
Figure 3. UV−visible polarized absorption spectra of dye 1 oriented films on ER PTFE layers: film thicknesses of (a) 95, (b) 77, (c) 34, (d) 17, (e) 95, (f) 77, (g) 34, and (h) 17 nm. Light polarization is parallel to the aligned PTFE chains in a−d, and it is perpendicular to the chains in e−h.
The degree of uniaxial orientation can be estimated from the polarized spectra. The dichroic ratio is defined for each peak in the polarized spectra as A D= A+ (1)
Figure 4. UV−visible absorption spectra of dye 1: (a) film deposited on glass, (b) dichloromethane solution, and (c) oriented film deposited on an FT PTFE layer with light polarization parallel to the PTFE chains. The film thickness is 77 nm for a and c.
where A∥ and A+, respectively, are the absorbance of light polarized parallel and perpendicular to the sliding direction. D values exceed 5.8 for all A∥ peaks in Figures 2 and 3 and reach 22 at the main peak in Figure 2a for the 95-nm-thick film on an FT layer. We define three zones for the peaks in these spectra, as summarized in Table 1. The first zone is located above 570 nm. When the film thickness increases from 17 nm (Figure 2e) to
that is distinct from the third zone peaks in terms of both shape and wavelength. The peak’s shoulder (510 nm) in turn seems to correspond to the peak or shoulder mentioned for the second zone for the films on PTFE. The 86 nm hypsochromic shift relative to the peak in dichloromethane solution (Figure
Table 1. Wavelengths and D Values of Peaks in the UV−Visible Spectra of Dye 1a sample
third zone
first zone
second zone
state
substrate
thickness (nm)
wavelength (nm)
D
wavelength (nm)
D
wavelength (nm)
D
solution film film film film film film film film film film
glass FT FT FT FT FT ER ER ER ER
77 17 34 56 77 95 17 34 77 95
496 410, 510 sd 468 sd 468 sd 468 sd 456 sd, (441) 430 sd, (435) 426 449 475 483
10 8.9 9.7 9.0 10 5.8 6.7 7.7 8.1
537 534 539 548, (538) 551, (529)
17 13 14 11 15
535 sd 535
8.3 9.6
584 594 600 613,b (610) 606, (604) 593 sd 593 sd 593 sd 593 sd
26 19 21 19, (42) 21, (51) 7.3 6.5 11 13
Peaks in A∥ spectra are shown for the films on aligned FT or ER PTFE layers. Bold numbers indicate the analyzed peaks according to Figure 7. sd, shoulder. bSharp peak. a
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(Figure 4a). As shown in Figure 5b, the bright crystalline domains (blue or magenta) tend to grow, with sizes reaching a few micrometers. However, we have not yet succeeded in preparing a single crystal that can be analyzed by X-ray crystallography. The significant axial orientation is also seen in the polarized IR spectra. Several absorption peaks between 1106 and 1604 cm−1 show strong polarization dependence, as seen in the typical spectra shown in Figure 6b,c for an FT layer. As
4b) and the peak shape are comparable to those reported for the H-aggregates of a few 1-amino-4′-nitroazobenzene derivatives.64 They have the same chromophore as 1, thus indicating that the peak can be attributed to similar H-like aggregates. Another similar shift was reported for H-like aggregates from methyl orange (4-dimethylaminoazobenzene4′-sulfonate), which is also a similar push−pull linear azobenzene.65,66 A drastic change in color was evident across the boundary between films deposited on bare glass and on FT PTFE under microscope observation with the polarization parallel to the PTFE chains (Figure 5a). To the naked eye, the color of the
Figure 6. IR absorption spectra of 1: (a) powder in a KBr disk with no polarizer; (b, c) oriented dye film (thickness 77 nm) on an FT layer. Light polarization is parallel to the aligned PTFE chains in b, and it is perpendicular to the chains in c.
summarized in Table 2, some peaks can be assigned according to detailed peak assignments in the spectra of azobenzene and its isotope68 or according to normal-mode analysis of 4(dimethylamino)azobenzene.69 We attribute five peaks to vibrational modes originating from asymmetric p-phenylene (8a, 19a, 9a, and 11) and from coupled C−N stretching in the azobenzene moiety (e1 and d2). A few major peaks (8a, 19a, and 9a) are thought to arise from the transition moments nearly parallel to the long axis because they correspond to the irreducible representation of A1 in C2h. Two peaks (1332−1334 and 1382−1383 cm−1) obviously arise from some combination of local vibrations along the long axis.69 These five major peaks exhibit a polarization dependence where D > 1, thus demonstrating the axial orientation. The D values from the 19a peak (1519 cm−1) should be appreciably influenced by a neighboring peak (1506 cm−1), which shows a polarization dependence for 0 < D < 1. Hence, the D values without them, from 6.6 to 7.9 for an FT layer or from 7.1 to 11 for an ER layer, reflect the degree of axial orientation. 3.2. Planar Orientation. The dye molecules do not have an appreciable planar orientation in the anisotropic films on the PTFE layer. The peak at 860 cm−1 can be assigned to an out-ofplane mode of p-phenylene moiety CH (11[B1]) according to the assignments mentioned above. The planar orientation should attenuate the absorption of this mode, and six peak intensities (8a and 11, one set for powder and two sets for the film under both polarization directions) allow an estimation of the planar orientation according to a previously reported
Figure 5. Polarized transmission optical micrographs through dye 1 films with a thickness of 77 nm: (a) film deposited around the boundary between the FT PTFE layer and bare glass through a single nicol with the light polarization parallel to the sliding direction and (b) film on an FT PTFE layer through crossed nicols polarizers. F−H indicate the sliding direction, the film deposited on bare glass, and the film deposited on the FT PTFE layer, respectively.
film was blue-violet on the FT layer and light brown on bare glass. Hence, the color depends on the surface where the dye is deposited, and the change is a kind of surface-induced chromism.67 The deposited 1 films have highly anisotropic crystalline domains. Figure 5a also shows many dense blue dots on the FT layer that appear to be crystalline domains where dye molecules are highly oriented. Some dots are also visible on the bare glass, thus contributing to the spectral shoulder extending to 800 nm 4713
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Langmuir Table 2. Vibrational Frequencies (cm−1) of Assigned Peaks in the IR Spectra and their D Values powder frequency b
1602 s 1517 s 1506 sd 1382 s 1335 s 1140 s 860 mb 823 m
on an FT layer frequency
D
1603 s 1519 m 1506 m 1382 s 1332 s 1145 s 861 m 823 m
6.6 5.7 0.5 7.9 6.9 7.1 0.91 0.52
on an ER layer ⟨cos2ϕ′⟩
0.51
frequency
D
1604 s 1519 m 1506 m 1383 s 1334 s 1143 s 860 m 824 m
7.7 4.3 0.4 11 7.1 9.3 1 0.4
⟨cos2ϕ′
0.51
PLa ∥ ∥ + ∥ ∥ ∥ + +
attribution 8a[A1] 19a[A1], v(C−N) v(NN), v(C−N) v(N−O), v(C−N) 9a[A1], e1[Ag] 11[B1], d2[Au] 11[B1], d2[Au]]
PL indicates the polarization direction where an IR peak is stronger. ∥ indicates polarization parallel to the sliding direction, and + indicates polarization perpendicular to the sliding direction. e1 and d2 are vibrational modes coupled with C−N stretching.68 s, strong; m, medium; sd, shoulder; v, stretch. The dye films are 77 nm thick. bPeak intensities are used for ⟨cos2 ϕ′⟩ calculations according to the model.23 a
Figure 7. Typical deconvolution results of the polarized spectra for oriented films of dye 1 on FT PTFE layers: film thicknesses of (a, b) 95 nm and (c, d) 77 nm. Light polarization is parallel to the aligned PTFE chains in a and c, and it is perpendicular to the chains in b and d. Each spectrum was demonstrated by the sum of three Gaussian peaks (violet, green, and orange) through optimum fitting by Microsoft Excel, and each colored number (a percentage) shows the ratio of each peak area to the sum of the three peak areas.
method,23 giving a ⟨cos2 ϕ′⟩ value of 0.51 for the films on both the FT layer and the ER layer. In a uniaxial orientation, ϕ′ essentially expresses the angle between the substrate surface and the director normal to the phenyl plane. Hence, ϕ′ would be near 44° if all molecules in the film had the same ϕ′ value, showing negligible planar orientation. 3.3. J-Aggregates. Without J-aggregates, there are no known species that can account for all three of the following results regarding the sharp first-zone peak around 613 nm: (1) the large bathochromic shift of 117 nm from the peak in solution; (2) the peak sharpness; and (3) the very high intrinsic D values, which are mentioned below. The intrinsic D value for the sharp peak at around 613 nm probably exceeds 40 because overlap with peaks in other zones should lead to a significant underestimation of D. Figure 2g
shows that only a small shoulder appears at 613 nm in A+, thus indicating that most of A+ is the contribution from peaks in other zones. Figure 2b shows that A∥ at 613 nm also includes a notable contribution from the second-zone peak. In Figure 7, the deconvolution with three Gaussian peaks for two polarized spectra (Figure 2a,b,f,g) shows such a contribution, thus indicating remarkably intrinsic D up to 51 for their first-zone peaks. More Gaussian peaks from 4 to 7 give better fitting results, and they do not significantly influence the D values (from 41 to 58). Hence, we noticed that the intrinsic D values for them exceed at least 40. The transition moment of the species is highly oriented: its uniaxial orientational order S = (D − 1)/(D + 2) should be at least 0.93. D spectra (Figure 9b,d) illustrate that only the sharp peak has very high D values. The second-zone peaks appear to be 4714
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deviation is likely to decrease. Moreover, there is no part where A+ is larger than A∥ in Figure 2b−g, so we can rule out Davydov splitting from bent dimers, which is another possible mechanism for displaying high D. Figure 7b,d also shows that there is no appreciable contribution of the second-zone peak to the perpendicular polarization. These spectra can be fitted substantially with two peaks in the first and third zones, as shown in Figure 7d. Davydov splitting probably takes place for the second peak to the third zone, and the Figure 7d spectrum was hardly analyzed with three peaks because of the closeness between the second and third peaks shown in Figure 7b. We unfortunately have not found a way to estimate the exact content of J-aggregates in the films. Certain molecules, however, forms J-aggregates, and the other molecules do not. Perhaps J-aggregates are located in the aforementioned crystalline domains observed in the micrographs with crossed polarizers because of the remarkable anisotropy of the domains. J-aggregates also grow on the ER layers, as indicated by the peaks shown in their D spectra. The peaks appear at 602 nm (Figure 6c) and 603 nm (Figure 6d) whereas their corresponding absorption peak or shoulder appears at 593 nm in Figure 3a,b. The J-aggregate content should therefore be considerably smaller on an ER layer than on an FT layer, leading to a shift of about 10 nm through the overlap. We think that some speculation on the growth mechanism of J-aggregates on PTFE is not futile here. The driving force for oriented growth is probably the atomic groove effect shown with an MD simulation previously:55 a snapshot for one MD trajectory is included in the Supporting Information (Figure S1). In the simulation, linear aromatic molecules including 1 are trapped along the atomic grooves between adjacent PTFE chains, where the plane of the aromatic molecules is also parallel to the PTFE surface. The negligible planar orientation mentioned earlier hardly neglects such planar orientation on the surface because the significant roughness on the nanometer scale44 should increase the ⟨cos2 ϕ′⟩ values. We think that such a trapped molecule, oriented uniaxially and lying flat on the surface, also plays a key role in the growth of J-aggregates. In the oriented films, the molecules attaching PTFE should be trapped, thus preventing aggregation where its long molecular axis on the unit cell is aligned along one direction or the other direction biaxially as p-nitrophenol15 or aggregation where molecular planes are aligned parallel to one plane or to the other plane as sexithiophene.18 Such a uniaxial or planar inducing effect at the interface on PTFE is illustrated schematically in Figure 9a,b. Considering the probable Davydov splitting of the second peaks, the uniaxial effect would take place at the interface. Furthermore, at least one of the two effects would accounts for the growth of J-aggregates from some other linear dyes (three bisazomethines45 and a pyrazine71) on PTFE. The inducing effects should take place at the interface on PTFE, thus growing J-aggregates near the three-phase contact among vacuum, solid dye, and PTFE. Further deposition beyond 77 nm thick did not increase Jaggregates as shown in Figure 7a,b, and the three-phase contact probably disappears when the dye of 77 nm is deposited. A typical staircase model is shown on a PTFE chain in Figure 9c. Considering the moderate bulkiness of the pyrrolidinyl group, some slip angle is reasonable for avoiding unfavorable steric interactions between two adjacent molecules. The pyrrolidinyl and nitro groups should provoke a strong push− pull electronic transition. A slip angle with a strong electronic
Figure 8. D spectra of oriented films of dye 1 on PTFE layers: film thicknesses of (a) 95 nm on an FT layer, (b) 77 nm on an FT layer, (c) 95 nm on an ER layer, and (d) 77 nm on an ER layer.
Figure 9. Possible schematic models for the mechanism in inducing Jaggregates of 1 on an aligned PTFE surface: (a) an uniaxial inducing effect on the long molecular axis of 1, (b) a planar inducing effect on the molecular plane of 1, and (c) one of several possible 1D chain models for an induced J-aggregate of 1 on aligned PTFE chains. The aggregate is a linear staircase of dye molecules stacked face to face, growing upward with a slip angle.
overestimated by overlap, whereas the third-zone peaks do not: D ≈ 7 for 77 nm thickness and D ≈ 10 for 95 nm thickness. These values are also close to those from polarized IR spectra (Figure 9), where vibrational modes are formed on each molecule, thus indicating the average axial orientation of the whole film. For this reason, the outstanding values should be due to a highly oriented component, J-aggregates, in the films. The outstanding intrinsic D should therefore be due to the molecular arrangement of J-aggregates together with their motional narrowing effect.70 As shown in the ladder, staircase, and brickwork models (Figure 9), molecules are aligned parallel to each other with a very small deviation in the axial direction. Furthermore, the motional narrowing effect makes the deviation negligible in each aggregate. Hence, the deviation depends only on that of the aggregates in the films, so the 4715
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Langmuir transition is favorable for J-aggregates, thus probably accounting for the J-aggregate formation from dye 1. However, we cannot rule out other possible models,72 such as ladders, brickwork, and double strands. Without the inducing effects by atomic grooves, almost everything is still an open question for the growth process of the J- or H-like aggregates in the films. In particular, it is not clear as to which kinds of PTFE layers are preferable for Jaggregates, ER, or FT. It depends on dye molecules: 1 and a pyrazine71 prefer FT, but a buthoxybisazomethine prefers ER,45 and two alkylaminobisazomethines prefer both layers.43−45 The kind of layer also influences the growth of the third-zone peak mentioned earlier. Nevertheless, it should be noted that the difference in aggregate formation between ER and FT layers would arise from their morphology. Both layers consist of linear microscopic ridges. The ridges on ER layers, however, are not as straight as those on FT layers, undulating gently.44 Moreover, we have to confirm the structure of the H-like aggregates to elucidate the growth mechanism on glass. Obviously, for a challenging analysis on glass by X-ray diffraction, we need a macroscopic single crystal of 1 as a reference, which would also give a more sophisticated analysis for J-aggregates as shown for one of the alkylaminobisazomethines.43
ACKNOWLEDGMENTS
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00289.
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Part of this work was performed under the management of the Association of Super-Advanced Electronic Technologies (ASET) in the Ministry of International Trade and Industry Program of Super-Advanced Electronic Technologies supported by the New Energy and Industrial Technology Development Organization. We thank Mr. Toru Tanaka of Carl Zeiss Microscopy Co., Ltd. for taking the high-resolution photographs.
4. CONCLUSIONS We have demonstrated that an anisotropic film containing highly oriented J-aggregates can be obtained by depositing dye 1 on aligned PTFE layers through evaporation. We reached the following five conclusions: (1) the long axes of the dye molecules are oriented in the films along the PTFE chains; (2) dye molecules form J-aggregates that produce D values of up to 22, with intrinsic D values exceeding this value; (3) FT PTFE layers provide many more J-aggregates than ER PTFE layers do; (4) on bare glass, however, the deposited dye forms H-like aggregates; and (5) large surface-induced chromism is observed as a 203 nm blue shift from the oriented J-aggregate (613 nm) to the H-like aggregates (410 nm). Such drastic differences in aggregate growth arising from the surfaces are fascinating. How do these surfaces control aggregation occurring on them? What features of the surfaces govern the aggregation? Although we hardly give a plausible explanation for these questions except the inducing effects by atomic grooves, we believe that there is another significant matter to be revealed.
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Article
Snapshot from the MD simulation for the orientation of 1 on an aligned PTFE surface55 (PDF)
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