Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2020 Sep 18;5(38):24379–24388. doi: 10.1021/acsomega.0c02752

Oligo(3-methoxythiophene)s as Water-Soluble Dyes for Highly Lustrous Gold- and Bronze-like Metal-Effect Coatings and Printings

Minako Tachiki 1, Reo Tagawa 1, Katsuyoshi Hoshino 1,*
PMCID: PMC7528169  PMID: 33015454

Abstract

graphic file with name ao0c02752_0007.jpg

To overcome various shortcomings associated with commercial metal-effect gloss paints containing metal flakes, we examine possible candidates to be used as novel organic-only metal-effect dyes for both paints and inks. Recently, one of the authors developed a potential candidate (ClO4–-doped oligo(3-methoxythiophene)), but the required solvent was not industrially acceptable and the cured paint displayed low glossiness. Herein, we synthesized and characterized Cl–-doped oligo(3-methoxythiophene) dyes that were water-soluble and displayed a highly lustrous gold- and bronze-like hue upon curing. Additionally, we found that films derived from these oligomers form extremely regular and compact edge-on lamella crystallites through self-organization; these films also display a highly glossy metallic appearance due to the extremely high optical constants of the crystallites. These as-prepared films were easily soluble in water, but we also found that the films become insoluble in water upon dehydration, making industrial implementation feasible.

Introduction

Lustrous optical effects contribute to a multitude of industrial products; for instance, lustrous properties are desired for decorative purposes in products such as security printings, cosmetics, and car paintings.1 Industrially, metal-effect pigments are used to fabricate such products as printings and paintings.1−3 Typically, the production of metal-effect pigments incorporate metallic flakes (sized from a few to tens of micrometers; thickness, <0.1–1 μm) of aluminum, copper, copper-zinc alloys, zinc, etc. During the preparation of printing inks and paints, these metallic flakes are dispersed in binder matrices.1,2 However, these flakes have a propensity to settle in paints since their specific gravities are higher than that of the binder matrices; this settling necessitates the continuous stirring of these paints during storage. Coating layers, resulting from the use of such paints, display high weights due to both a high coating thickness required to obscure the sublayer and the high specific gravity of the incorporated flakes.1,4,5 Additional drawbacks involved in the use of these metal flake-based paints include a need to maintain control over the orientation of the flakes during coating and the corrosive nature of the flakes.1,2,5

Thus, nonmetallic organic materials that display metal-like luster are currently being developed to circumvent these problems. In the approaches using films as the building blocks,6−9 laminated films comprising alternating layers with high and low refractive indices were developed, showing metal-like reflection based on the structural color. In the other approaches, nonmetallic paint candidates have been generated from polymers such as polyacetylene10 and polythiazyl11 and from low-molecular-weight compounds such as thiophene-pyrrole-thiophene structures,12−17 azobenzene derivatives,18,19 polyaniline analogues,20,21 dimeric compound of fused thiophene,22 and thienyl-furyl pentacycle.23 However, none of the aforementioned paint candidates have displayed all the following desiderata: solubility in solvents, good film-forming property, and excellent stability during aging.

Recently, it was shown that a ClO4–-doped 3-methoxythiophene oligomer produced gold-like lustrous films while satisfying all of the aforementioned desiderata.24−26 This oligomer is possibly the first nonmetallic material that is likely to be used during the industrial production of lustrous inks and paintings. However, the oligomer was solvated in either nitromethane or acetonitrile that are considered to be employee-unsafe.27,28 Typically, solvents deemed suitable for employee-safe environments are preferred for use (e.g., water). If its solvents can be replaced by the employee-safe solvents, then the oligomer is likely to have a very high significance with respect to its applications and chemistry. To date, poly(3-alkylthiophene) molecules, with an alkyl group bound to the 3-position of the thiophene ring, have displayed self-organizing properties, capable of forming lamellar crystal structures, attributed to intermolecular interactions. This unique self-orientating property is promising for use in applied organic electronics devices.29−31 The aforementioned 3-methoxythiophene oligomer also forms self-organizing lamellar crystal structures similar to those of poly(3-alkylthiophene), which have been studied extensively.24−26 However, this oligomer displays specific properties arising from compactness of the methoxy group, high levels of electron donation, and multiple supramolecular interaction points; the oligomer shows specific morphological, optical, and chemical properties different from those of poly(3-alkylthiophene). For example, the former has larger optical constants (refractive index and extinction coefficient) than the latter.32,33

In the present study, we synthesized Cl–-doped 3-methoxythiophene oligomers under specific conditions, resulting in a cast film displaying a highly regular orientation compared to the aforementioned ClO4–-doped 3-methoxythiophene oligomer film. This highly regular orientation induced extremely large optical constants, producing a high metal-like luster. Furthermore, Cl–-doped 3-methoxythiophene oligomers are easily dissolved in water, promising a new generation of water-based nonmetallic paints that display a metallic luster. Interestingly, we observed that although the oligomers were readily soluble in water, the resultant coating film became insoluble in water upon dehydration. We report on those vital properties expressed by Cl–-doped 3-methoxythiophene oligomers and their films.

Results and Discussion

External Appearance and Surface Topology of Coating Films

3-Methoxythiophene underwent oxidative polymerization with FeCl3, FeCl3·6H2O, and Fe(ClO4)3 to obtain oligomers 1, 2, and 3, respectively (see Methods and the Supplementary Note with heading “Materials”, Figures S1–S3, and Tables S1 and S2 in the Supporting Information). The common structural moieties between oligomers 1, 2, and 3 are that the 3-methoxythiophene units in each system display a linearly coupled structure and that the anions are doped. Three compositional differences highlighted are that (1) the weight-average degree of polymerization for oligomer 1 is ∼75% of that of the other oligomers, (2) the polydispersity of oligomers 1 and 2 is ∼50% that of oligomer 3, and (3) the three oligomers are composed of different dopant species (Table 1; see the Supplementary Note with heading “Comment on the Dopant Species”).

Table 1. Structural Characteristics of Oligomers.

oligomer Mwa Mnb polydispersityc nwd nne dopantf doping level (%)f
1 1.70 × 103 1.29 × 103 1.31 15.1 11.5 Cl– 29
2 2.32 × 103 1.87 × 103 1.24 20.7 16.6 FeCl4–, Cl– 30
3 2.29 × 103 9.9 × 102 2.31 20.4 8.81 ClO4– 30
Open in a new tab
a

Weight-average molecular weight of polymer skeleton measured by gel permeation chromatography. The contribution of dopant is excluded.

b

Number-average molecular weight of polymer skeleton measured by gel permeation chromatography. The contribution of dopant is excluded.

c

Mw/Mn.

d

Weight-average polymerization degree measured by gel permeation chromatography.

e

Number-average polymerization degree measured by gel permeation chromatography.

f

Determined by energy dispersive X-ray analysis.

To prepare coating solutions, oligomers 1 and 2 were both dissolved in water (∼0.20–0.40 wt %), whereas oligomer 3 was dissolved in nitromethane (1.0 wt %). Films 1, 2, and 3 were obtained by applying each respective oligomer coating solution onto an individual glass plate and then allowed it to cure. Figure 1a–c shows digital microscopy images of films 1–3, respectively. Figure 1d (included for reference) is a microscopy image of a metallic gold film prepared on a glass plate by the vacuum evaporation method. As an initial binary, qualitative measurement of the glossy reflection properties, a stainless-steel ruler was placed next to each film and reflections were observed on all films. The glossiness of films 1 and 2 was observed to be slightly darker than, yet still comparable to, that of the evaporated metal film. As the appearance of glossy objects is greatly affected by the ambient lighting, photographs of films 1 and 2 were taken under reduced illumination; photographs are provided in Figure S4, Supporting Information. The overall film-forming process (drying process) in air is also shown in Movie S1, Supporting Information. Film 3 featured a uniform gold-like gloss over the entire film, while a nonglossy dark stain-like region was observed close to the center of films 1 and 2. The formation mechanism of the stain-like region has been examined in detail using coffee stain generation analogy34−36 (see the Supplementary Note with heading “Formation of Stain Region” in the Supporting Information). All physical measurements described below were performed on the glossy region of each coating film, rather than the stain-like region.

Figure 1.

Figure 1

Open in a new tab

(a) Photograph of film 1 (t = 1.6 μm) prepared by dropping 0.30 wt % aqueous solution of oligomer 1 (0.60 mL) on a glass plate (25 mm × 15 mm area) using a micropipette. (b) Photograph of film 2 (t = 2.0 μm) prepared by dropping 0.30 wt % aqueous solution of oligomer 2 (0.60 mL). (c) Photograph of film 3 (t = 1.7 μm) prepared by dropping 1.0 wt % nitromethane solution of oligomer 3 (0.070 mL). (d) Photograph of metallic gold film (t = 0.20 μm) prepared by the vacuum evaporation method. Dynamic force mode AFM images of (e) film 1, (f) film 2, and (g) film 3.

Figure 1e–g shows the surface topology of each film as observed with atomic force microscopy (AFM). Films 1 and 2 display granular surface structures, while film 3 is formed from a fibrillar structure. The actual fibril thickness of oligomer 3 was approximately 10 nm, calculated by correcting the apparent fibril thickness considering the tip diameter and angle of aperture of the AFM cantilever37 (see Methods and Figure S5 in the Supporting Information). The length of a single oligomer chain was calculated to be approximately 8 nm based on the distance between thiophene units, either as determined by scanning tunneling microscopy (STM) observations38 (0.38 nm) or as calculated using molecular mechanics calculations39 (0.39 nm), and the average polymerization degree of oligomer 3 (nw = 20.4, Table 1). That value is almost equal to the aforementioned fibril thickness. As previously reported for poly(3-hexylthiophene) nanofibers,40−42 it is most likely that the extended chains present in oligomer 3 are also packed parallel to each other with their long axis oriented perpendicular to the long fibril axis. Films 1 and 2, displaying granular morphologies, are composed of granules with a corrected actual size of ≤10 nm (Figure S5, Supporting Information). The lengths of a single oligomer chain within films 1 and 2 were estimated from the average degrees of polymerization of oligomer 1 (nw = 15.1) and oligomer 2 (nw = 20.7), respectively (see Table 1). Calculated lengths of ∼6–8 nm indicate that the elongated oligomer molecules are packed in parallel to each other, thereby forming particles. However, these molecular stacking interactions do not propagate over a long distance, preventing them from forming fibril shapes similar to those in oligomer 3. The formation of the crystallites responsible for the above structural model will be described later.

Optical and Surface Properties of Coating Films

To find the cause of the high luster property, we conducted specular reflectance measurements, color measurements, ellipsometry measurements, and surface roughness measurements on both films 1 and 2. The specular reflection spectra of films 1 and 2 are shown in Figure 2a; the spectrum24,25 of film 3 and both evaporated metallic gold and copper films (200 nm thick) are also shown for comparison (Figure 2b). Films 1 and 3 strongly reflected yellow light (570–590 nm), orange light (590–620 nm), and red light (620–750 nm) yet only slightly reflected green light (495–570 nm). These reflective characteristics are similar to those of the evaporated metallic gold film, justifying the gold-like appearances of films 1 and 3. Film 2 exhibited reflective characteristics more similar to metallic copper, with only small reflection of green light; this justifies the fact that film 2 presents a copper-like tone. The differences in color between films 1 and 2 can be attributed to differences in the effective conjugation lengths of oligomers 1 and 2 (see Figure S6, Supporting Information). Focusing on the maximum reflectance, the reflectance of both films 1 and 2 (28 and 40%, respectively) significantly exceeds the reflectance of film 3 (18%),24,25 also exceeding that of previously reported metal-free organic gloss films or aggregates (11–22%).18−21 Also, the excellent reflectance of both films 1 and 2 is comparable to the reflectance of coating films cured from metal-effect pigments.2,3

Figure 2.

Figure 2

Open in a new tab

(a) Specular reflection spectra of film 1 (solid red line, t = 1.6 μm), film 2 (blue solid line, t = 2.0 μm), and film 3 (black broken line, t = 1.7 μm). (b) Specular reflection spectra of vacuum-evaporated metal gold film (red solid line, t = 0.1 μm) and metal copper film (blue solid line, t = 0.1 μm). (c) Values of a*, b*, and L* according to a CIELab color system for film 1 (solid red circle), film 2 (solid blue up-pointing triangle), film 3 (black solid square), evaporated metal gold film (black empty circle), and metal copper film (black empty up-pointing triangle).

Figure 2c shows the values of coordinates L*, a*, and b* in the L*a*b* color space for films 1, 2, and 3; values for evaporated gold and copper films are also shown as a comparison. As a reference, the chromaticity of evaporated gold film is (a*, b*) = (7, 37), very close to the chromaticities of both film 1 ((a*, b*) = (8, 36)) and film 3 ((a*, b*) = (11, 37)); this measurement confirms the observed hue similarity between these three specimens. Alternatively, the chromaticity of film 2 was determined to be (a*, b*) = (23, 21), fairly closer to the chromaticity of evaporated copper film ((a*, b*) = (17, 25)); this measurement confirms the observed hue similarity between film 2 and evaporated copper. In terms of the lightness metric, all the coating films have a lightness of L* = ∼35–45; in contrast, the lightness measures of evaporated gold film and evaporated copper film were L* = ∼80–90. If the reflectance and L* values of these coating films are made equal to those of the metals, then the optical characteristics would be almost completely equivalent, enabling the fabrication of metal-free metal-effect coatings with extremely low specific gravities.

The magnitude and wavelength dependence of both the refractive index (n) and extinction coefficient (κ) determine both the reflectance and chromaticity of a material. We therefore measured both n and κ via ellipsometry. Figure 3a–c shows the wavelength dependence of the optical constants, as measured by ellipsometry, for films 1–3. The reflectance (R) between air and film material, in case of normal incidence, is expressed by eq 1;21R is calculated after measuring values for the optical constants and placing them into eq 1 (see Figure 3d).

graphic file with name ao0c02752_m001.jpg 1

Figure 3.

Figure 3

Open in a new tab

Refractive index (n, black solid line) and extinction coefficient (κ, red solid line) plotted against the wavelength of light for (a) film 1 (t = 1.6 μm), (b) film 2 (t = 1.9 μm), and (c) film 3 (t = 1.7 μm). (d) Calculated reflection spectra for film 1 (red solid line), film 2 (blue solid line), and film 3 (black broken line).

The refractive index displayed a significant dependence on wavelength, changing from normal dispersion to anomalous dispersion modes.43 The extinction coefficient reached its maximum at a midpoint wavelength in the anomalous dispersion. As can be noted from eq 1, R at a certain wavelength is dependent on both n and κ; its value increases with any increase in n or κ. As conventional transparent polymers show no photoabsorption in the visible region (i.e., κ = 0) and the value of n is in the range of 1.3–1.7,44 they do not present a metal-like reflection. As a reference material to frame discussion, the polyacetylene developed by Shirakawa et al.10 displays a large refractive index (n = 9.0 at 641 nm for the cis form and n = 10 at 641 nm for the trans form) and extinction coefficient (κ = 0.65 at 541 nm for the cis form and κ = 0.77 at 645 nm for the trans form),45 resulting in strong silvery luster (R = 32% at 606 nm for the cis form and R = 28% at 858 nm for the trans form), although it shows low storage stability in air. The maximum value of n for films 1 and 2 is relatively small compared to the n of polyacetylene, while the maximum value of κ is larger for films 1 and 2; as a result, the luster displayed by films 1 and 2 is comparable to or greater than that of polyacetylene, which has the greatest reflectance among all previously reported metal-free organic gloss films.

The calculated value of the specular reflectance for each film (Figure 3d) correlates well with the measured values (Figure 2a); the only significant deviation is a shoulder peak seen at ∼500 nm not present in the measured spectra, attributed to optical anisotropy generated by the molecular orientation of the oligomers described below. That is, it may be that the optical constants perceived by light vary depending on the incident angle of light. The agreement between calculated and measured reflectance values implies that the order of magnitude associated with n and/or κ in the wavelength range where reflection occurs increases in the order film 3 < film 1 < film 2; the results of Figure 3a–c support this assumption. Furthermore, the agreement implies that the roughness of the films is very small (i.e., mirror reflection) and therefore does not perturb the specular reflectance. Table 2 shows the line roughness (root-mean-square roughness, Rq) of films 1–3 as measured with a stylus-type surface roughness tester and AFM. When comparing the Rq values of the films with that of a glass plate (taken to be a reference value), the Rq values of the films can be regarded to be quite small, although they are larger than that of the glass plate, indicating that the surface roughness does not affect specular reflection.

Table 2. Root-Mean-Square Roughness Rq of Films.

film Rqa (nm) Rqb (nm) t (μm)
film 1 12 ± 4 9 ± 4 1.6
film 2 6 ± 3 4 ± 2 1.7
film 3 15 ± 1 20 ± 2 1.7
glass plate 2.3 ± 0.8 0.3 ± 1  
Open in a new tab
a

Measured by the stylus method.

b

Measured by atomic force microscopy.

Molecular Alignment in Coating Films

X-ray diffraction measurements were taken to determine molecular structure factors responsible for the high n and high κ values. The gold-like lustrous regions of both films 1 and 2 were removed by abrasion (see Methods and Figure S7 in the Supporting Information) and pulverized; the sample spectra were measured under the 2θ/ω mode and compared to the spectra (2θ/ω mode) of the lustrous regions of films 1 and 2 taken on a glass plate. Powder scraped from film 1 (powder 1) appeared to be brown and partially lustrous (see the Supplementary Note with heading “Coloration of Oligomer Powder” in the Supporting Information). The XRD pattern contained signals at 2θ = 7.41°, ∼15° (shoulder), and 26.2° (see Figure 4a). The former two signals correspond to reflections from the (100) and (200) planes of the edge-on lamella (Figure 4e), formed by the thiophene ring being oriented perpendicular to the plate; the latter peak corresponds to reflection from the face-on lamella (020) plane, as shown in Figure 4f.24−26 The interlayer distances that corresponded to 2θ = 7.41° and 26.2° are a = 1.19 and b = 0.34 nm, respectively. These values are similar to those obtained from the XRD pattern of as-synthesized oligomer 1 powder (a = 1.18 and b = 0.34 nm). As seen from the XRD patterns in Figure 4a, the signal areas corresponding to both the (100) and (020) planes are almost similar, demonstrating a nearly 1:1 ratio of the two lamella crystallites in the sample. However, in the XRD pattern of the lustrous region of film 1 (Figure 4c), the observed signals (2θ = 7.96°, a = 1.11 nm) are almost exclusive from the (100) plane, indicating a dominant formation of edge-on lamella. Interpretations of the XRD spectra of powder 2 (derived from film 2, see Figure 4b) and the bronze-like lustrous region of film 2 (Figure 4d) are nearly identical; however, the diffraction corresponding to the edge-on lamella of film 2 (2θ = 8.27°) is much greater than that of film 1. The solid black line spectra within Figure 4c,d correlate to the XRD patterns25,26 of film 3 measured under the same conditions as the patterns of films 1 and 2. The diffraction intensity (2θ = 7.85°, d = 1.13 nm) of the edge-on lamella in film 3 was the lowest of all three coating films. Hence, an ordering of samples by diffraction intensity, corresponding to the edge-on lamella, matched the aforementioned orders of specular reflectance and optical constants. As for the reflection from the (020) plane, almost no signals were observed in the XRD patterns of films 1 and 2 (Figure 4c,d), indicating that almost no face-on lamellar crystallites are formed in the films.

Figure 4.

Figure 4

Open in a new tab

X-ray diffraction patterns of powders obtained by scratching the metal-like lustrous area of (a) film 1 and (b) film 2. Twenty samples of film 1 were prepared by applying 0.30 wt % coating solution (3.0 mL) to a glass plate (26 mm × 76 mm area) and then placing the sample in a desiccator with silica gel for 17 h. Nineteen samples of film 2 were prepared by applying 0.22 wt % coating solution (3.0 mL) to a glass plate (26 mm × 76 mm area) and then placing the sample in a desiccator with silica gel for 17 h. X-ray diffraction patterns of the metal-like lustrous area of (c) film 1 (red solid line, t = 1.7 μm) and (d) film 2 (blue solid line, t = 2.0 μm). The black lines in (c) and (d) indicate the XRD pattern of film 3 (t = 1.7 μm). (e) Edge-on lamellar crystallites. (f) Face-on lamellar crystallites.

The above experimental results indicate that the observed luster is largely caused by edge-on lamella formed in the film. That is, the lamellar structure present in the film displays anisotropy in both the refractive index and extinction coefficient depending on its orientation relative to the glass plate. The refractive index and extinction coefficient are both greater in magnitude when the incident light is parallel to the (100) axis than when parallel to (020), that is, larger optical constants when incident light is perpendicular, rather than parallel, to the lamellar surface. As a result, the greater amount of edge-on lamella, relative to face-on lamella, in the film, the greater the gold-like luster.

The reason films 1 and 2 contain more edge-on lamella than does film 3 is likely due to differing curing times.42,46 As reported with poly(3-hexylthiophene), the additional curing time promotes molecular alignment and crystallization, resulting in greater crystallinity of the film. Film 3 requires ∼10 min to cure due to nitromethane evaporation. Alternatively, films 1 and 2 were cured in a desiccator with 6% humidity for approximately 10 h. Thus, the observed difference in crystallinity for the edge-on lamella may be due to the difference in the curing time caused by the large discrepancy between the solvent evaporation rates.

Induced Dissolution Resistance of Cured Films to Water

Film 1 became insoluble in water by induced dehydration after curing. We prepared three film samples derived from film 1 under various dehydration conditions. Dehydration was induced by the following: (1) One hour exposure of film 1 to an atmospheric oven set to 80 °C (thermal annealing) results in a film referred to as Film 1-TA. Thermogravimetric analysis (TG) indicates that the skeleton of oligomer 1 is stable when heated to 196 °C (Figure S8, Supporting Information). (2) Leaving film 1 in a desiccator, maintained at a humidity of 6% RH, 20–25 °C, and atmospheric pressure, for 48 h (low-humidity annealing) results in a film referred to as Film 1-LH. (3) Finally, leaving film 1 in a vacuum oven maintained at ≤1000 Pa at 20–25 °C for 48 h (low-pressure annealing) results in a film referred to as Film 1-LP. The specular reflection spectra indicate that the dehydration treatments cause a blue shift in the onset wavelength of reflectance and a declination of the reflectance in the long wavelength regime (Figure 5a). In particular, Film 1-TA displayed a blue shift of 19 nm from a baseline of film 1; this resulted in an increased reflection of light in the green region, producing a more yellowish gold-like hue. This observation was supported by the optical constant measurements for each film; note that the thermal annealing procedure produced the greatest changes in both the refractive index and extinction coefficient (Figure S9, Supporting Information).

Figure 5.

Figure 5

Open in a new tab

(a) Specular reflection spectra of Film 1-TA (red solid line), Film 1-LP (blue solid line), and Film 1-LH (green solid line). For comparison, the spectrum of film 1 (i.e., as-prepared film) is also shown as a black broken line. Film 1 was prepared by applying 0.20 wt % coating solution (0.90 mL) to a glass plate (25 mm × 15 mm area) and then placing the sample in a desiccator with silica gel for 17 h. The film thicknesses of film 1, Film 1-LH, Film 1-LP, and Film 1-TA were 0.97, 0.90, 0.88, and 0.79 mm, respectively. Note that the film thickness was reduced by the dehydration treatments. (b) XRD patterns of film 1 (black broken line), Film 1-TA (red solid line), Film 1-LP (blue solid line), and Film 1-LH (green solid line). Film 1 was prepared by applying 0.30 wt % coating solution (0.60 mL) to a glass plate (25 mm × 15 mm area) and placed in a desiccator with silica gel for 17 h. (c) Photographs of film 1 (as-prepared), Film 1-TA, Film 1-LP, and Film 1-LH before (upper images) and after washing with water for 1 min (lower images). The film samples were prepared in the same manner as in (a).

The above results indicate that, from an engineering perspective, it is possible to control the color tone within a range from reddish-gold to yellowish-gold by application of specific dehydration routines after curing the films. Scientifically, the results demonstrate that water molecules are incorporated as building blocks within the lamella crystals. To ascertain the relationship between water and lamella crystals, we took XRD measurements of the films both before and after dehydration treatments (Figure 5b). The values of 2θ for a signal corresponding to the edge-on lamella interlayer distance increased in the order film 1 (as-prepared) < film 1-LP ≈ Film 1-LH < Film 1-TA, while intensity I decreased, and the half value width FWHD increased (Table 3). This indicates that water molecules contribute to the formation of lamellar crystallites and that the removal of water by dehydration resulted in both a reduction in the lamella interlayer distance a and reduction in the volume and size of the lamella crystallites. This reduction of the interlayer distance may be responsible for the change in color tone, and the reduction in the both crystal volume and size may cause the reduction in reflectance.

Table 3. Crystallographic Properties of Films.

film 2θ (degree)a a (nm)b I (cps)c FWHD (degree)d t (μm)e
film 1 7.96 1.11 1.05 × 103 0.85 1.7
Film 1-LH 8.88 1.00 8.16 × 102 0.98 1.6
Film 1-LP 8.91 0.99 8.16 × 102 0.98 1.6
Film 1-TA 9.83 0.90 4.36 × 102 1.09 1.5
Open in a new tab
a

Peak position corresponding to the lamellar interlayer spacing.

b

The value of the lamellar interlayer spacing.

c

Diffracted X-ray intensity for the peak corresponding to the lamellar interlayer spacing.

d

The full width at half-depth of the peak corresponding to the lamellar interlayer spacing.

e

Thickness of films for the XRD measurements. Film 1 was prepared by applying 0.30 wt % coating solution (0.60 mL) to a glass plate (25 × 15 × 1.1 mm) and then placing the sample in a desiccator with silica gel for 17 h. Note that the film thickness was reduced by the annealing treatments as was also observed in Figure 5a.

Next, each film was placed in water contained in a Petri dish while stirring for 1 min, and then it was removed and air-dried. The photograph of the obtained film before and after insertion in water is shown in Figure 5c. The as-prepared film 1 completely dissolved when placed in water, but the gold-like lustrous region of the dehydrated films did not dissolve (Figure S10, Supporting Information). This demonstrates a hydrophilic character for films containing water, while the dehydrated films were hydrophobic in character.

The same thermal annealing process (80 °C, 1 h) was applied to film 2, resulting in Film 2-TA. Similar analysis steps to those above showed that the specular reflection spectrum, refractive index spectrum, extinction spectrum (Figure S9, Supporting Information), and XRD pattern (Figure S11, Supporting Information) displayed similar shifts in character as observed in film 1. However, unlike Film 1-TA, when film 2 and Film 2-TA were placed in water, both films were dissolved, although when treated by long-time thermal annealing (17 h), the bronze-like lustrous region did not dissolve in water (Figure S12, Supporting Information). The differences in water solubility between Film 1-TA and Film 2-TA are partially attributed to a difference in the amount of adsorbed water. TG measurements revealed that oligomers 1 and 2 contain 0.89 and 1.32 wt % adsorbed water, respectively. The greatest difference between the two oligomers is that oligomer 2 contains both FeCl4– and Cl– as dopant species. It is possible that the FeCl4– dopant may adsorb more moisture,47 leading to film 2 retaining more water and greater hydrophilicity, suggesting that longer annealing times are required to increase the hydrophobicity of the resulting cured film.

Conclusions

In summary, we developed water-soluble thiophene oligomer dyes that yield paints and coating films having gold- and bronze-like colors without the addition of metal-effect pigments. Oligo(3-methoxythiophene), the thiophene oligomer, soluble in nitromethane and acetonitrile was developed recently,24−26 yet it requires solvents that are desirable from an industrial perspective; additionally, it displays an inadequate specular reflectance (∼20%). The aforementioned dyes developed in this report are soluble in water, making industrial implementation feasible, and yields a coating film with high reflectance (28% for the gold-like film and 40% for the bronze-like film). An urgent problem in the field of imaging technologies when using conventional metal-effect pigments is that they clog the ink ejection nozzles when used with ink-jet printers. In contrast, our newly developed oligomer dyes molecularly dissolve in water, relieving any concern that ink will clog the nozzle; this feature, likely the first dyes for ink-jet ink displaying these properties, makes our inks easily deployable in commercial grade ink-jet printers, possibly leading to the production of new commercial ink products for in-home use. Furthermore, through sufficient characterizations, we attribute the lustrous appearance to the presence of highly ordered and compact lamella crystals developed through self-organization and displaying extremely large optical constants. From an application perspective, the highly ordered structures of the aforementioned oligomer dyes may also serve as components in organic electronic systems; please refer to Table S3 in the Supporting Information for information on the electrical conduction characteristics of films 1 to 3.

Methods

Synthesis of Oligomers

Oligomers 1 and 2 were prepared by reference to the method for the preparation of poly(3-methoxythiophene) doped with chloride anion,48 but with certain modifications. To 20 mL of stirred acetonitrile solution of 3-methoxythiophene (0.10 M) was added 20 mL of acetonitrile solution containing 0.20 M FeCl3 (or FeCl3·6H2O) under a nitrogen atmosphere at 21 °C. The color of the solution rapidly changed from colorless to dark purple (or dark blue). The obtained purple solution was then stored for 2 h while stirring, followed by evaporation of the solvent. Then, the residue was repeatedly washed with ethanol to remove the oxidant and monomeric 3-methoxythiophene and dried at 60 °C in vacuum to yield oligomer 1. The obtained blue solution was also stored for 2 h while stirring. Then, the dissolved components were removed by decantation. The residue and adherent materials to the inner wall of a reaction vessel were collected, washed with ethanol, and dried at 60 °C in vacuum, thereby obtaining oligomer 2. The weight-based yields of oligomers 1 and 2 were 43 and 20%, respectively; these yields were appreciably low in comparison with that for the ClO4–-doped oligo(3-methoxythiophene) (96% yield). Details concerning the characterization of oligomers are provided within the Supplementary Discussion in the Supporting Information (Figures S1–S3, S6, and S8 and Tables S1–S3).

Film Formation

A sample of oligomer 1 or 2 was dissolved in water to prepare an ∼0.20–0.40 wt % coating solution. The coating solution was drop-cast onto a glass plate using a micropipette (Nichipet EXII, Nichiryo). The typical substrate size was 15 mm × 25 mm or 25 mm × 76 mm, and the film thickness was controlled by changing the amount of coating. Drying after casting was performed by placing the sample in a desiccator containing silica gel for 17 h, thereby obtaining film 1 or 2 from the respective oligomer sample. A 1.0 wt % oligomer 3 coating solution was obtained by dissolving oligomer 3 in nitromethane. Film 3 was then obtained by applying the coating solution onto a glass plate in the same manner as above and drying in air at 20 ± 1 °C for 10 min.24−26 The concentrations of coating solutions of oligomers 1, 2, and 3 were adjusted such that the film thicknesses of films 1, 2, and 3 were nearly the same. The reason for the different concentrations should be the solvent effect.

Observation of Films

A digital microscope (VHX-5000, KEYENCE) and atomic force microscope (SPA300, SII) were used for the macroscopic and microscopic observations of films, respectively. The AFM measurements were carried out in the tapping mode using a silicon cantilever with a spring constant of 1.3 N m–1 and a tip radius of 10 nm. In the case where a sample with a radius R is observed by a cantilever tip (with radius r), the corresponding radius in the obtained AFM image, D, is given by eqs 2 and 3 based on geometrical factors.37

graphic file with name ao0c02752_m002.jpg 2
graphic file with name ao0c02752_m003.jpg 3

Here, 2θ is the tip opening angle. With the cantilever tip used in this study, r = 8 nm and 2θ = 40°, allowing the actual size of the fibrils or granules formed by the oligomers to be estimated using eqs 2 and 3.

Measurements of Optical Properties

UV–vis reflection spectra were obtained using a spectrometer (MSV-379, Jasco) with incident and reflection angles of 23° from normal. The spectra were recorded at 23 °C using a vacuum-evaporated aluminum film as the reference material. The film color was analyzed using a colorimeter (CM-600d, Konica Minolta) with a D65 illuminant and an observer angle of 10° (CIE 1964 Standard Observer). The CIELab parameters were a* for redness-greenness, b* for yellowness-blueness, and L* for lightness (black-white). Optical constants of films were measured using a variable-angle spectroscopic ellipsometer (alpha-SE, J. A. Woollam Co.), which covers 380–900 nm and permits a scanning of incident angle from 65° to 75° with an accuracy of 0.01°. Ellipsometric data were fitted using J. A. Woolam CompleteEASE software.

General Instrumentation and Analytical Methods

The film thickness t and root-mean-square surface roughness Rq were measured using a surface profile measuring system (Dektak 3030, Sloan, now Veeco). The latter value was also measured by AFM. Out-of-plane X-ray diffraction (XRD) measurements of film samples were performed using a diffractometer (X’Pert MRD, Malvern Panalytical) equipped with a Cu-Kα radiation source (λ = 1.5406 Å, 45 kV, 40 mA) set to the 2θ/ω scan mode. X-ray powder diffraction studies were undertaken using Bruker D8 Advance (Bruker AXS) equipped with a Cu-Kα radiation source (λ = 1.5418 Å, 40 kV, 40 mA). TG analysis of the oligomers was carried out under a nitrogen stream using a thermal analyzer (TG8010D, Shimadzu) in the range of 16–500 °C with a heating rate 20 °C min–1. Further information on the instrumentation and methods can be found in Supplementary Note in the Supporting Information.

Acknowledgments

This work was financially supported by the Ogasawara Foundation for the Promotion of Science & Engineering (to K.H.) and JSPS KAKENHI (grant 17K06815 to K.H.). The authors thank the Center for Analytical Instrumentation of Chiba University and Prof. Shigeru Takahara for the measurements of reflection spectra, 1H NMR spectra, and GPC charts.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c02752.

  • Materials; general instrumentation and analytical methods; instrumentation and method of film characterization; formation of stain region; coloration of oligomer powder; 1H NMR and FT-IR measurements of oligomers (Figures S1 and S2); GPC analyses of oligomers (Figure S3 and Table S1); EDX analyses of films (Table S2); external appearance of coating films under a low illumination condition (Figure S4); cross-sectional analyses of films (Figure S5); preparation and optical characterization of coating solutions (Figure S6); preparation of samples for XRD measurements (Figure S7); thermogravimetric analysis (Figure S8); measurements of optical constants for films subjected to dehydrating treatments (Figure S9); dissolution behavior of films with and without the dehydrating treatments (Figures S10 and S12); effect of dehydration treatment on the molecular orientation characteristic in film 2 (Figure S11); electric conductivity of films (Table S3); supplementary references (PDF)

  • Dropping of a coating solution of oligomer 1 on a glass plate measuring 25 mm × 15 mm and the subsequent curing process in the ambient atmosphere (24 °C, 37% RH) (MP4)

The authors declare no competing financial interest.

Supplementary Material

ao0c02752_si_001.pdf (1.2MB, pdf)
ao0c02752_si_002.mp4 (2.1MB, mp4)

References

  1. Liu H.; Ye H.; Zhang Y. Preparation and Characterization of PMMA/flaky Aluminum Composite Particle in the Presence of MPS. Colloids Surf., A 2008, 315, 1–6. 10.1016/j.colsurfa.2007.06.057. [DOI] [Google Scholar]
  2. Pfaff G.; Reynders P. Angle-Dependent Optical Effects Deriving from Submicron Structures of Films and Pigments. Chem. Rev. 1999, 99, 1963–1982. 10.1021/cr970075u. [DOI] [PubMed] [Google Scholar]
  3. Smith G. B.; Gentle A.; Swift P. D.; Earp A.; Mronga N. Coloured Paints Based on Iron Oxide and Silicon Oxide Coated Flakes of Aluminum as the Pigment, for Energy Efficient Paint: Optical and Thermal Experiments. Sol. Energy Mater. Sol. Cells 2003, 79, 179–197. 10.1016/S0927-0248(02)00410-5. [DOI] [Google Scholar]
  4. Maile F. J.; Pfaff G.; Reynders P. Effect Pigments—Past, Present and Future. Prog. Org. Coat. 2005, 54, 150–163. 10.1016/j.porgcoat.2005.07.003. [DOI] [Google Scholar]
  5. Schoppe R.Aluminum Pigments for Plastics. https://www.silberline.com/wp-content/uploads/2019/03/White-Paper-Aluminum-Pigments-for-Plastics.pdf (accessed July 27, 2020), August, 2012, pp 1–18.
  6. Goto H. An Optically Active Polythiophene Exhibiting Electrochemically Driven Light-Interference Modulation. Adv. Funct. Mater. 2009, 19, 1335–1342. 10.1002/adfm.200801782. [DOI] [Google Scholar]
  7. Moritsugu M.; Ishikawa T.; Kawata T.; Ogata T.; Kuwahara Y.; Kurihara S. Thermal and Photochemical Control of Molecular Orientation of Azo-Functionalized Polymer Liquid Crystals and Application for Photo-Rewritable Paper. Macromol. Rapid Commun. 2011, 32, 1546–1550. 10.1002/marc.201100244. [DOI] [PubMed] [Google Scholar]
  8. Nano-multilayer Film PICASUSTM. https://www.films.toray/en/products/picasus/ (accessed July 27, 2020).
  9. Goto H. Crystal–Liquid Crystal Ordered Double Layer Electroactive Polymer Prepared with Phase Transition Sequential Polymerization, Showing Metallic Electrochromism–Bronze, Silver, and Gold. J. Polym. Sci. Part A: Polym. Chem. 2013, 51, 3097–3102. 10.1002/pola.26729. [DOI] [Google Scholar]
  10. Shirakawa H.; Louis E. J.; Macdiarmid A. G.; Chiang C. K.; Heeger A. J. Synthesis of Electrically Conducting Organic Polymers: Halogen Derivatives of Polyacetylene, (CH)x. J. Chem. Soc., Chem. Commun. 1977, 578–580. 10.1039/C39770000578. [DOI] [Google Scholar]
  11. Mikulski C. M.; Russo P. J.; Saran M. S.; MacDiarmid A. G.; Garito A. F.; Heeger A. J. Synthesis and Structure of Metallic Polymeric Sulfur Nitride, (SN)x, and Its Precursor, Disulfur Dinitride, S2N2. J. Am. Chem. Soc. 1975, 97, 6358–6363. 10.1021/ja00855a012. [DOI] [Google Scholar]
  12. Ogura K.; Zhao R.; Yanai H.; Maeda K.; Tozawa R.; Matsumoto S.; Akazome M. Facile Formation of Stable Crystals with Gold-Like Metallic Luster from Organic Molecules: 1-Aryl-2-(2-thienyl)-5-[5-(tricyanoethenyl)-2-thienyl]pyrroles. Bull. Chem. Soc. Jpn. 2002, 75, 2359–2370. 10.1246/bcsj.75.2359. [DOI] [Google Scholar]
  13. Zhao R.; Akazome M.; Matsumoto S.; Ogura K. Novel Organic Crystals with Red–Violet Metallic Luster: 1-aryl-2-(2-thienyl)-5-[5-(tricyanoethenyl)-2-thienyl]pyrrole Derivatives Bearing a Heteroatom Combined Methyl Substituent. Tetrahedron 2002, 58, 10225–10231. 10.1016/S0040-4020(02)01354-6. [DOI] [Google Scholar]
  14. Zhao R.; Matsumoto S.; Akazome M.; Ogura K. Supramolecular Architecture of Metal-Lustrous Inclusion Crystals Based on Aromatic CH−π Interaction: Versatile Inclusion of 1-(p-ethoxyphenyl)-2-(2-thienyl)-5-[5-(tricyanoethenyl)-2-thienyl]pyrrole Host with Various Electron-Rich Aromatic Guest Molecules. Tetrahedron 2002, 58, 10233–10241. 10.1016/S0040-4020(02)01355-8. [DOI] [Google Scholar]
  15. Ogura K.; Zhao R.; Jiang M.; Akazome M.; Matsumoto S.; Yamaguchi K. Easy Formation of Gold-Like Lustrous Crystals with a High Melting Point from 1-aryl-2,5-bis[5-(tricyanoethenyl)-2-thienyl]pyrroles. Tetrahedron Lett. 2003, 44, 3595–3598. 10.1016/S0040-4039(03)00716-0. [DOI] [Google Scholar]
  16. Ogura K.; Zhao R.; Mizuoka T.; Akazome M.; Matsumoto S. Formation of Gold-Like Metal-Lustrous Inclusion Crystals from 1-phenyl-2,5-bis[5-(tricyanoethenyl)-2-thienyl]pyrrole Host and an Electron-Donating Aromatic Guest. Org. Biomol. Chem. 2003, 1, 3845–3850. 10.1039/b302689b. [DOI] [PubMed] [Google Scholar]
  17. Ogura K.; Ooshima K.; Akazome M.; Matsumoto S. Formation of Metal-Lustrous Organic Crystals from 2-aryl-1-(4-methoxyphenyl)-5-(5-tricyanoethenyl-2-thienyl)pyrroles. Tetrahedron 2006, 62, 2484–2491. 10.1016/j.tet.2005.12.055. [DOI] [Google Scholar]
  18. Matsumoto A.; Kawaharazuka M.; Takahashi Y.; Yoshino N.; Kawai T.; Kondo Y. Gold-Colored Organic Crystals Formed from an Azobenzene Derivative. J. Oleo Sci. 2010, 59, 151–156. 10.5650/jos.59.151. [DOI] [PubMed] [Google Scholar]
  19. Kondo Y.; Matsumoto A.; Fukuyasu K.; Nakajima K.; Takahashi Y. Gold-Colored Organic Crystals of an Azobenzene Derivative. Langmuir 2014, 30, 4422–4426. 10.1021/la500035h. [DOI] [PubMed] [Google Scholar]
  20. Kukino M.; Kuwabara J.; Matsuishi K.; Fukuda T.; Kanbara T. Synthesis and Metal-Like Luster of Novel Polyaniline Analogs Containing Azobenzene Unit. Chem. Lett. 2010, 39, 1248–1250. 10.1246/cl.2010.1248. [DOI] [Google Scholar]
  21. Yamada H.; Kukino M.; Wang Z. A.; Miyabara R.; Fujimoto N.; Kuwabara J.; Matsuishi K.; Kanbara T. Preparation and characterization of green reflective films of polyaniline analogs containing azobenzene units. J. Appl. Polym. Sci. 2015, 132, 41275. 10.1002/app.41275. [DOI] [Google Scholar]
  22. Li X.-C.; Sirringhaus H.; Garnier F.; Holmes A. B.; Moratti S. C.; Feeder N.; Clegg W.; Teat S. J.; Friend R. H. A highly π-stacked organic semiconductor for thin film transistors based on fused thiophenes. J. Am. Chem. Soc. 1998, 120, 2206–2207. 10.1021/ja9735968. [DOI] [Google Scholar]
  23. Parakka J. P.; Cava M. P. Furan- and Pyrrole-Containing Analogs of ce-Quinquethiophene: Spectroscopic and Electrochemical Properties. Synth. Met. 1995, 68, 275–279. 10.1016/0379-6779(94)02297-C. [DOI] [Google Scholar]
  24. Tagawa R.; Masu H.; Itoh T.; Hoshino K. Solution-Cast Self-Assembled Films of Perchlorate-doped Oligo(3-methoxythiophene) Showing a Gold-Like Luster. RSC Adv. 2014, 4, 24053–24058. 10.1039/c4ra03548h. [DOI] [Google Scholar]
  25. Takashina Y.; Mitogawa T.; Saito K.; Hoshino K. Chemical Events in Oligo(3-methoxythiophene) Coating Solutions and Their Effect on the Goldlike Coating Film Properties. Langmuir 2018, 34, 3049–3057. 10.1021/acs.langmuir.7b04182. [DOI] [PubMed] [Google Scholar]
  26. Takashina Y.; Hoshino K. Effect of π–π Interaction-Induced Secondary Doping on the Gold-Like Luster of Oligo(3-methoxythiophene) Cast Films. Polym. J. 2019, 51, 591–599. 10.1038/s41428-019-0172-9. [DOI] [Google Scholar]
  27. Wen Y.; Wen W.; Zhang X.; Wang S. Highly Sensitive Amperometric Biosensor Based on Electrochemically-Reduced Graphene Oxide-Chitosan/Hemoglobin Nanocomposite for Nitromethane Determination. Biosens. Bioelectron. 2016, 79, 894–900. 10.1016/j.bios.2016.01.028. [DOI] [PubMed] [Google Scholar]
  28. El-Harairy A.; Yiliqi; Lai B.; Vaccaro L.; Li M.; Gu Y. A Sulfone-Containing Imidazolium-Based Brønsted Acid Ionic Liquid Catalyst Enables Replacing Dipolar Aprotic Solvents with Butyl Acetate. Adv. Synth. Catal. 2019, 361, 3342–3350. 10.1002/adsc.201900246. [DOI] [Google Scholar]
  29. Sirringhaus H.; Brown P. J.; Friend R. H.; Nielsen M. M.; Bechgaard K.; Langeveld-Voss B. M. W.; Spiering A. J. H.; Janssen R. A. J.; Meijer E. W.; Herwig P.; de Leeuw D. M. Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685–688. 10.1038/44359. [DOI] [Google Scholar]
  30. Tsao N. H.; Müllen K. Improving Polymer Transistor Performance via Morphology Control. Chem. Soc. Rev. 2010, 39, 2372–2386. 10.1039/b918151m. [DOI] [PubMed] [Google Scholar]
  31. Sirringhaus H.; Tessler N.; Friend R. H. Integrated Optoelectronic Devices Based on Conjugated Polymers. Science 1998, 280, 1741–1744. 10.1126/science.280.5370.1741. [DOI] [PubMed] [Google Scholar]
  32. Musa I.; Eccleston W. Determination of the Optical Constants of Thin Films of Regioregular Poly (3-alkylthiophene) on Transparent Substrates. Synth. Met. 1998, 97, 69–72. 10.1016/S0379-6779(98)00116-7. [DOI] [Google Scholar]
  33. Tokuda T.; Hoshino K. Electropolymerized Films of 3-Methoxythiophene with a Potential Sweep-Induced Gold-Like Luster. Polym. J. 2016, 48, 1141–1149. 10.1038/pj.2016.86. [DOI] [Google Scholar]
  34. Deegan R. D.; Bakajin O.; Dupont T. F.; Huber G.; Nagel S. R.; Witten T. A. Capillary Flow as the Cause of Ring Stains from Dried Liquid Drops. Nature 1997, 389, 827–829. 10.1038/39827. [DOI] [Google Scholar]
  35. Kajiya T.; Nishitani E.; Yamaue T.; Doi M. Piling-to-Buckling Transition in the Drying Process of Polymer Solution Drop on Substrate Having a Large Contact Angle. Phys. Rev. E 2006, 73, 011601 10.1103/PhysRevE.73.011601. [DOI] [PubMed] [Google Scholar]
  36. Yunker P. J.; Still T.; Lohr M. A.; Yodh A. G. Suppression of the Coffee-Ring Effect by Shape-Dependent Capillary Interactions. Nature 2011, 476, 308–311. 10.1038/nature10344. [DOI] [PubMed] [Google Scholar]
  37. Rao A.; Gnecco E.; Marchetto D.; Mougin K.; Schönenberger M.; Valeri S.; Meyer E. The Analytical Relations Between Particles and Probe Trajectories in Atomic Force Microscope Nanomanipulation. Nanotechnology 2009, 20, 115706. 10.1088/0957-4484/20/11/115706. [DOI] [PubMed] [Google Scholar]
  38. Grévin B.; Rannou P.; Payerne R.; Pron A.; Travers J. P. Multi-Scale Scanning Tunneling Microscopy Imaging of Self-Organized Regioregular Poly(3-hexylthiophene) Films. J. Chem. Phys. 2003, 118, 7097–7102. 10.1063/1.1561435. [DOI] [Google Scholar]
  39. Kasai H.; Tanaka H.; Okada S.; Oikawa H.; Kawai T.; Nakanishi H. STM Observation of Single Molecular Chains of π-Conjugated Polymers. Chem. Lett. 2002, 31, 696–697. 10.1246/cl.2002.696. [DOI] [Google Scholar]
  40. Verilhac J.-M.; LeBlevennec G.; Djurado D.; Rieutord F.; Chouiki M.; Travers J.-P.; Pron A. Effect of Macromolecular Parameters and Processing Conditions on Supramolecular Organization, Morphology and Electrical Transport Properties in Thin Layers of Regioregular Poly(3-hexylthiophene). Synth. Met. 2006, 156, 815–823. 10.1016/j.synthmet.2006.04.012. [DOI] [Google Scholar]
  41. Kline R. J.; McGehee M. D.; Kadnikova E. N.; Liu J.; Fréchet J. M. J.; Toney M. F. Dependence of Regioregular Poly(3-hexylthiophene) Film Morphology and Field-Effect Mobility on Molecular Weight. Macromolecules 2005, 38, 3312–3319. 10.1021/ma047415f. [DOI] [Google Scholar]
  42. Kim D. H.; Park Y. D.; Jang Y.; Kim S.; Cho K. Solvent Vapor-Induced Nanowire Formation in Poly(3-hexylthiophene) Thin Films. Macromol. Rapid Commun. 2005, 26, 834–839. 10.1002/marc.200400647. [DOI] [Google Scholar]
  43. Sorar I.; Kasım Şener M.; Tepehan F. Z.; Gül A.; Burkut Koçak M. Structural and Optical Studies of Tetra(tricarbethoxy)-Substituted Metallophthalocyanines. Thin Solid Films 2008, 516, 2894–2898. 10.1016/j.tsf.2007.05.058. [DOI] [Google Scholar]
  44. Brandrup I.; Immergut E. H.; Grulke E. A.. Polymer Handbook; John Wiley & Sons, Inc.: New York, 1999; pp. VI-571 – VI-582. [Google Scholar]
  45. Fujimoto H.; Kamiya K.; Tanaka J.; Tanaka M. Optical Constants of Polyacetylene. Synth. Met. 1985, 10, 367–375. 10.1016/0379-6779(85)90196-1. [DOI] [Google Scholar]
  46. Yang H.; LeFevre S. W.; Ryu C. Y.; Bao Z. Solubility-Driven Thin Flm Structures of Regioregular Poly(3-hexythiophene) Using Volatile Solvents. Appl. Phys. Lett. 2007, 90, 172116. 10.1063/1.2734387. [DOI] [Google Scholar]
  47. Gogoi P.; Dutta A. K.; Sarma P.; Borah R. Development of Brönsted–Lewis Acidic Solid Catalytic System of 3-methyl-1-sulfonic acid imidazolium Transition Metal Chlorides for the Preparation of bis(indolyl)methanes. Appl. Catal. A-Gen. 2015, 492, 133–139. 10.1016/j.apcata.2014.12.013. [DOI] [Google Scholar]
  48. Aaron J. J.; Fall M. Photopysical Properties of sexi(3-methoxythiophene). Evidence for Energy Migration in a Conducting Oligomer, Based on Fluorescence Quenching. Spectrochim. Acta, Part. A 2000, 56, 1391–1397. 10.1016/S1386-1425(99)00264-4. [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ao0c02752_si_001.pdf (1.2MB, pdf)
ao0c02752_si_002.mp4 (2.1MB, mp4)

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES