Structural, Thermo-Optical, and Photophysical Properties of Highly

Article pubs.acs.org/JPCC

Structural, Thermo-Optical, and Photophysical Properties of Highly Oriented Thin Films of Quinoxalinophenanthrophenazine Derivative Tomasz Makowski,*,† Rasha M. Moustafa,‡ Pawel Uznanski,† Wojciech Zajaczkowski,§ Wojciech Pisula,§ Adam Tracz,† and Bilal R. Kaafarani*,‡ †

Centre of Molecular and Macromolecular Studies, Polish Academy of Science, Sienkiewicza 112, 90-363 Lodz, Poland Department of Chemistry, American University of Beirut, Beirut 1107-2020, Lebanon § Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany ‡

S Supporting Information *

ABSTRACT: The structural, thermo-optical, and anisotropic photophysical properties of highly oriented thin solid films of 2,11-bis(1,1-dimethylethyl)-6,7,15,16-tetrakis(dodecyloxy)quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazine (TQPP-OC12) prepared by a zone-casting method are discussed. The films were obtained on glass substrate by solution processing. The in-plane alignment of TQPP-OC12 molecules in the formed layers was studied by X-ray diffraction methods, optical polarized microscopy combined with thermooptical analysis, atomic force microscopy (AFM), and UV−vis absorption and fluorescence spectroscopy both with polarized light methods. The high molecular order of zone-casted TQPP-OC12 was studied in conjunction with the observed abundance of phase transitions as a function of thermal conditions to assess the material’s suitability for optoelectronic device applications. The molecular disk planes in the as-cast samples are parallel to the casting direction and orient almost perpendicularly (∼96°) to the substrate. Continuous films with a thickness of 200−300 nm are formed from lamellas arranged parallel to the surface. All observed phase transitions have crystal−crystal character; however, molecular primary arrangement remains basically identical for all processing conditions. Sample annealing destroys the multiple polymorphs observed in the as-cast sample and leads to an increase of molecular ordering.

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field-effect transistors15,16,27−40 and organic photovoltaic devices.41 Differential scanning calorimetry (DSC) studies of the bulk material of newly synthesized pyrene-based discoid 2,11bis(1,1-dimethylethyl)-6,7,15,16-tetrakis(dodecyloxy)quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazine, TQPP[t-Bu]2-[OC12H25]442 (called hereafter TQPP-OC12), Figure 1, showed at least five phase transitions before the material undergoes isotropization above 200 °C. Thus, this material can serve as a well-suited model for studying the influence of these multiple transitions on optical properties and morphology changes in highly ordered thin films. Highly oriented thin films were obtained on glass substrates using solution-based zonecasting, which has been successively applied to discotic derivatives of hexabenzocoronenes 16,43 and phthalocyanines.44,45 The effects of heating−cooling cycles on the transmittance of polarized light in thermo-optical analysis are discussed in relation to DSC data obtained for the bulk material and

INTRODUCTION The molecular structure of organic semiconductors controls their macroscopic order and enhances their charge carrier transport in thin films, leading to improved performance of electronic devices.1,2 The anisotropy of charge transport is related to the molecular packing of small organic semiconductors such as rubrene3−5 and pentacene;6 alignment of polymers such as poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4phenylenevinylene],7 poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene),8 and poly(3-hexylthiophene);2,9 and liquid crystals.10 Discotic liquid crystals compounds (DLCs) forming columnar mesophases are considered as potential charge and exciton transport layers in the field of optoelectronics.11−13 DLCs are usually composed of a discoid core to which sidechain substituents (typically alkyl) are attached.14 DLCs were exploited in electronic devices such as organic field-effect transistors,15,16 solar cells,17−21 and light-emitting diodes.22,23 Different techniques were reported to align DLCs in the optimized arrangement to achieve an efficient functioning device.24 The zone-casting technique was first reported in 198125,26 and has been widely used to align materials (even if they do not exhibit self-assembly behavior) over a large area. Indeed, many materials were efficiently processed for organic © 2014 American Chemical Society

Received: May 14, 2014 Revised: July 18, 2014 Published: July 24, 2014 18736

dx.doi.org/10.1021/jp504750x | J. Phys. Chem. C 2014, 118, 18736−18745

The Journal of Physical Chemistry C

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Figure 1. Chemical structure of TQPP-OC12.

changes in film morphology. In particular, the morphology of zone-cast films at different temperatures was studied at the nanoscale by using in situ hot stage atomic force microscope (AFM). This method allows the analysis of the morphological changes as observed concurrently with the phase transition. In this work, the application of the zone-casting technique (Figure 2) for depositing oriented films of TQPP-OC12 is discussed.

The X-ray diffraction (XRD) measurements were performed using a θ−θ Siemens D8 apparatus and a graphitemonochromatized Cu Kα1 X-ray beam. The diffraction patterns were recorded in the 2θ range from 2° to 30° and are presented as functions of the scattering vector q, with q = 4π sin θ/λ, where θ is the scattering angle. Grazing incidence wide-angle X-ray scattering (GIWAXS) experiments were performed by means of a solid anode X-ray tube (Siemens Kristalloflex X-ray source, copper anode X-ray tube operated at 30 kV and 20 mA), osmic confocal MaxFlux optics, X-ray beam with pinhole collimation, and a MAR345 image plate detector. The samples were prepared as thin films and were irradiated at the incident angle (αi) of 0.20°. Changes of the transmission of polarized light as a function of temperature (TOA) were recorded and visualized using an author’s software TOAPLOTS. At the same time, the morphology of the films was observed under a Nikon Eclipse E400 Pol microscope equipped with polarizing filters and a SANYO VCC-3770P camera. The images were acquired using Leadtek TV Tuner WinFast PVR2. Heating and cooling of the samples at a controlled rate was accomplished using a Mettler FP82 hot stage equipped with an FP90 controller and a photo detector. UV−vis absorption spectra were collected at ambient conditions in air at room temperature (25 °C) using a Hewlett−Packard 8453 spectrophotometer, and the polarization of incidence light was selected using a polarizing Glan− Thompson prism. Emission spectra were measured using a Fluorolog-3 22 Horiba Jobin-Yvone instrument, with a solidsample holder tilted at 30° to the incident light and automated polarizers FL-1044. Linear dichroism studies of TQPP-OC12 molecules were conducted in an oriented low-density polyethylene matrix (PE). The molecules were introduced into the polymer in hot toluene, and after evaporation of the solvent, the sample was dried in a high vacuum at 70 °C. Polymer films of 0.5 mm in thickness were prepared by compression molding at 150 °C following fast cooling to 0 °C to diminish a fine spherulitic structure of the PE matrix and the amount of stray light. Stretching was done on a homemade stretcher mechanical device. The final drawing ratio of the PE sheets was 400%. Separate dichroic spectra of the dopant were recorded with the electric vector of the analyzing light along the stretching direction (A||) and with the electric vector perpendicular to sample axis (A⊥).

Figure 2. Schematic illustration of the zone-casting apparatus and a course of the alignment process.

Herein, we demonstrate a strategy for characterizing the oriented thin organic layer TQPP-OC12 displaying multiple phase transitions using thermo-optical analysis (TOA), DSC, AFM and UV−vis absorption and fluorescence spectroscopy with polarized light methods.

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EXPERIMENTAL METHODS 2,11-Bis(1,1-dimethylethyl)-6,7,15,16-tetrakis(dodecyloxy)quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazine (TQPPOC12), Figure 1, was synthesized as described earlier.46 Zonecasting (ZC) of TQPP-OC12 was performed by means of a home-built apparatus used earlier for ordered deposition of other organic systems.47−49 Fresh solutions of TQPP-OC12 were filtered through a PTFE membrane (0.2 μm) before use. A solution of TQPP-OC12 in toluene at a concentration of 2 mg/mL was cast through a flat nozzle of 3 cm width (slit-like) onto a moving glass support at 60 °C. The solution supply to the substrate (Figure 2) was achieved by pushing the piston of an injector at a constant rate. The morphology of the films was investigated by means of atomic force microscopy (AFM). Images were recorded under ambient atmosphere at different temperatures using a Nanoscope IIIa, MultiMode (Veeco, Santa Barbara, CA) microscope equipped with a hot stage probe. The probes were commercially available rectangular silicon cantilevers (RTESP from Veeco) with nominal radius of curvature in the 10 nm range with a spring constant of 20−80 N/m and resonance frequency lying between 264 and 369 kHz. The images were recorded with the highest available sampling resolution of 512 × 512 data points. Differential scanning calorimetry studies were performed using TA Instruments Q 20 and TA Instruments DSC-2920 instruments, calibrated with an indium standard at a rate of 10 °C/min under nitrogen flow.

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RESULTS AND DISCUSSION Zone-cast films of TQPP-OC12 were about 200−300 nm thick and were uniform over several square centimeters as was evident from polarizing optical microscope images spaced out between some elongated dark lines perpendicular to the casting direction (Figure 3a). Interestingly, these are not film 18737



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Figure 5. X-ray diffractogram of the TQPP-OC12 zone-cast layer thermally treated on a glass substrate.

diffraction plane (00l), and higher-order peaks up to (006) are clearly visible. Taking into account Bragg’s law (nλ = 2d sin θ; λ = 0.154 nm), it is found that the primary (001) diffraction peak at 2θ = 3.9° corresponds to a d-spacing of 2.26 nm, which is in exact agreement with the height of one single monolayer. This is confirmed by AFM measurements (Figure 6). The very weak

Figure 3. Optical microscopy images of an aligned zone-cast TQPPOC12 film in crossed polarizers with the analyzer/polarizer axes oriented at the angle of (a) 45° and (b) polarizer parallel to the zonecasting direction (white arrow indicates the zone-casting direction). (c) Tapping mode AFM image of folds appearing as dark lines perpendicular to the casting direction in panel a (85 nm high and width of 500 nm) . White arrow indicates the zone-casting direction. The length of the white bar on the images in panels a and b corresponds to 100 μm, whereas that in panel c corresponds to 10 μm.

discontinuities but elongated folds possessing a height of about 85 nm as detected by AFM examinations (Figure 3c). It is assumed that these folds are formed because of some solidification instabilities during evaporation of the solvent. However, the exact mechanism still remains unclear. The films showed high birefringence (Figure 3a) and strong optical anisotropy (Figure 3b) with maximum birefringence at an angle of approximately 45° between the alignment direction and the analyzer/polarizer axes. This optical behavior was confirmed by UV−vis absorption and emission measurements which showed pronounced anisotropy with higher absorption of polarized light for a perpendicular orientation of the polarizer and the film orientation (Figure 4).

Figure 6. AFM image showing a thickness of a single monolayer of TQPP-OC12. White bar on the image corresponds to 100 nm.

diffraction intensities for the as-cast film, denoted as “a” and “b”, correspond to the d-spacing of 1.78 and 1.42 nm, respectively, and suggest the coexistence of a second polymorphic form of TQPP-OC12. The “a” peak might be correlated with the second-order Bragg reflection of the K1 phase, which is based on a monoclinic unit cell, whereby in this case the axis of the c parameter of 3.56 nm is likely aligned normal to the surface equal.50 The disappearance of this peak at high temperature suggests a crystal−crystal phase transition. Peak “b” is assigned to the disarrangement of the crystal lattice toward the surface and therefore disappears after annealing of the sample. This behavior might suggest that the multiple polymorphs observed in the as-cast sample have vanished. Because no other diffraction peaks appear during annealing, it can be assumed that the molecular arrangement remains basically identical for all processing conditions. The slight shift of the Bragg peak corresponds to a small increase of the interlayer distance from 2.26 nm at room temperature to 2.34 nm at 150 °C. Grazing incidence wide-angle X-ray scattering (GIWAXS) patterns of the as-cast film aligned samples are consistent with the lamellar structure arranged parallel to the surface observed in XRD. The GIWAXS pattern shown in Figure 7a was acquired with the incident beam aligned parallel to the casting direction. The periodicity determined from the position of the first meridional reflection in the GIWAXS pattern is assigned to the (001) reflection (qxy = 0 Å−1; qz = 0.284 Å−1) and a dspacing of 2.24 nm. This value is attributed to the lamellar

Figure 4. UV−vis absorption (left panel) and emission (right panel) spectra in polarized light recorded parallel and perpendicular to the processing direction for zone-cast TQPP-OC12 films.

The zone-cast film of TQPP-OC12 on the glass substrate was investigated by X-ray diffraction (XRD) to attain an understanding of the structural transformations occurring upon annealing. Figure 5 shows a set of XRD patterns recorded at different temperatures with the following order: as-cast sample, during annealing at 150 °C, and cooled back to 30 °C. It has to be emphasized that XRD measurements in reflection mode provide information only about the out-of-plane structure with respect to the surface. The high intensity Bragg peaks in the diffractogram indicate high crystallinity for the zone-cast layer. The dominating peak belongs to the same family of the 18738



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the TQPP-OC12 sample cooled at a rate of 10 °C/min to −50 °C, the DSC heating thermodiagram showed several phase transitions upon heating at a rate of 10 °C/min within the temperature range of −50 to 250 °C. The enthalpy of some transitions is quite small as compared to the enthalpy of the high-temperature transitions at around 204 °C (before isotropization above 209 °C). The transitions at 3.6, 33, 47, and 75 °C are observed only when the diagram is considerably enlarged (Figure 8a). The DSC scan for TQPP-OC12 powder that was rapidly cooled to −50 °C is considerably different (Figure 8b). The phase transitions at ca. 4 and 12 °C did not appear, whereas a phase transition at ca. 45 °C, which was not seen previously, was present. This is accompanied by a phase transition of smaller enthalpy at ca. 55 °C. On the other hand, the phase transitions at 118 and 134 °C were not detected. Moreover, there are some differences in transitions, as evidenced in the DSC traces before isotropization (compare Figure 8a and inset in Figure 8b). The results presented in Figure 8 clearly show that the structure of TQPP-OC12 at room temperature is strongly dependent on the thermal history. Thermo-optical (TOA) analysis51 correlates well with the results of differential scanning calorimetry; additionally, TOA allows the observation of changes in textures (in situ) in oriented layers. TOA studies were carried out on TQPP-OC12 of different thermal history (Figure 9). When the pristine layer

Figure 7. GIWAXS patterns for zone-cast film of TQPP-OC12 acquired with the incident beam aligned (a) parallel and (b) perpendicular to the casting direction.

repeating distance as observed by XRD. Reflections on the equatorial (qxy = 0.424 Å−1; qz = 0 Å−1) and off-equatorial plane (qxy = 0.424 Å−1; qz = 0.335 Å−1) correspond to a d-spacing of 1.48 and 1.22 nm and are related to (100) and (101) indexes, respectively. The position of the reflections in the pattern indicates an almost perpendicular (γ = 96°) arrangement of the molecular aromatic plane to the surface, whereby the stacking axis is parallel to the zone-casting direction. The GIWAXS pattern recorded with the incident beam aligned perpendicular to the casting direction is presented in Figure 7b. The first reflection on the meridional plane (qxy = 0 Å−1; qz = 0.284 Å−1) and its higher orders confirm the long-range lamellar structure along the film thickness. The additional peak on the same plane (qxy = 0 Å−1; qz = 0.359 Å−1) corresponding to the distance of 1.75 nm is consistent with the second-order peak denoted as “a” in the XRD diffractogram. The (010) reflection on the equatorial plane in middle-angle range (qxy = 0.910 Å−1; qz = 0 Å−1) corresponds to a real distance of 0.69 nm parallel to the casting direction between molecules tilted by ∼57°. On the basis of this structural data, a monoclinic unit was derived with a = 1.56 nm, b = 0.69 nm, c = 2.21 nm, and β = 96° between the a and c axes, whereby axis b is oriented parallel to the zonecasting direction and the surface. TQPP-OC12 exhibits a very rich polymorphism. The DSC spectra of TQPP-OC12 powder encapsulated in a DSC pan cooled to −50 °C at different rates are shown in Figure 8. For

Figure 9. TOA curves (dI/dT) demonstrating changes in intensity of transmitted light through the zone-cast oriented TQPP-OC12 film of different thermal history: (a) pristine layer, (b) heated to 150 °C and then cooled to 30 °C, (c) heated to 203 °C and then cooled 30 °C, and (d) cooled to −5 °C and then heated to 30 °C. Images of the corresponding layers as observed under the polarizing optical microscope. The white bar on the images represents 100 μm. The heating rate for sample (b) was 5 °C/min.

is heated in the temperature range of 30−150 °C, a highly oriented TQPP-OC12 exhibited two phase transitions (Figure 9a). The first and second phase transitions occurred at ca. 55 °C and ca. 134 °C, respectively (Figure 9a). The phase transition at 134 °C was also observed in DSC measurement after cooling (Figure 8a). The temperature alone is, therefore, not a sufficient parameter to determine the TQPP-OC12 structure during its solidification from toluene solution. This means that during zone-casting, the aggregation of TQPP-OC12 proceeded under specific conditions resulting in formation of the layer exhibiting specific morphology and structure. The phase transitions at 55 and 134 °C are accompanied by some morphological changes of the layer. Parallel dark lines perpendicular to the casting direction can be seen in the pristine layer (Figure 9a). These dark lines are linear folds (elevations) on the layer surface as observed by AFM investigations (see Figure 3c). During cooling from 150 to 30 °C after a cycle temperature change of 30 to 150 to 30 °C, the

Figure 8. DSC heating scans for TQPP-OC12: (a) powder cooled to −50 °C at a rate of 10 °C/min and (b) powder quenched to −50 °C following heating at a rate of 10 °C/min. 18739



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Figure 10. AFM topography images of TQPP-OC12 surface morphology registered at different temperatures: (a) during heating (30−150 °C) and (b) during cooling (150−30 °C). (c) Comparison of topography of as-obtained layer and the same layer after heating−cooling cycle. Schemes show the orthogonal network of linear folds. The arrow indicates the zone-casting direction. The white bar on the images represents 20 μm.

polarizers are shown in Figure 9c. When the temperature reached 203 °C, blue domains appeared. These domains started to grow very fast, rendering the entire layer blue within seconds. This spectacular change of color after the phase transition at ca. 203 °C is related to the change of layer structure. After the sample was cooled from 204 to 30 °C, the pristine orientation in the blue layer was preserved. However, the density of the linear folds perpendicular to the casting direction increased considerably (Figure 9c). Upon further heating, the phase transition in the blue domains occurred at 124 °C (red curve dI/dT in Figure 9c). This transition was not accompanied by changes in morphology (as seen under the microscope). The phase transition at 124 °C presumably corresponds to the transition occurring during heating at 134 °C (compare to DSC traces in Figure 8a). From the DSC trace shown in Figure 8a, it is clear that TQPP-OC12 underwent phase transitions below room temperature, i.e., at ca. 4 and 12 °C. During cooling from 25 °C to −5 °C, a layer undertook a phase transition at ca. 4 °C, which is an expected transition according to the DSC trace (Figure 8a). The strong decrease of the light intensity, which accompanied the transition, is related to the appearance of brown domains. The angle between the domain walls and the direction of the zone-casting is ca. 38° (image in Figure 9d). However, if the layer was kept at −5 °C for 2 h, reorganization occurred uniformly within the whole layer. The characteristic morphological feature of the transformed layer has a high density of small domains with an orthogonal network of linear folds (compare image d in Figure 9). During heating from −5 °C, the brown areas were transformed into domains of blue color. The color was similar to the color of the pristine sample. However, the layer, which experienced a phase transition at 4 °C, exhibited upon subsequent heating to 150 °C a phase transition at temperature of ca. 45 °C (dI/dT curve in Figure 9d), whereas the pristine

folds perpendicular to the casting direction are more uniformly (periodically) distributed than those in the pristine sample (Figure 9b). Phase transition at 55 °C results in reorganization of these folds. Some pristine folds perpendicular to the casting direction flatten (become hardly visible). Simultaneously, many shorter folds perpendicular to the zone-casting direction emerge (image “60 °C” in Figure 10). Finally, the folds perpendicular to the casting direction are more uniformly (periodically) distributed than those in the pristine sample. After the phase transition at 134 °C, the folds perpendicular to the casting direction disappear but instead, some folds parallel to the casting direction are formed (image “150 °C” in Figure 10). This suggests that changes of the structure related to the phase transition lead to expansion of the layer in the direction perpendicular to the casting direction. The stress within the layer is relaxed by formation of the folds parallel to the casting direction. Reorganization of folds caused by the phase transitions in the temperature range 30 to 150 °C can be detected in situ using AFM (Figure 10). The structure formed during the zone-casting process was irreversibly lost when it underwent the phase transition at 134 °C. Indeed, during the second heating from 30 to 150 °C, only one transition at 52 °C (corresponding to the transition during heating at 55 °C) occurred (Figure 9b). The above results imply that TQPP-OC12 in the form of a highly oriented layer obtained by zone-casting at 60 °C exists in two structures, which exhibit endothermic phase transitions at 55 and 134 °C. This conclusion is fully supported by the presence of two diffraction reflexes in XRD diffractograms (Figure 5). It was important to check whether the orientation of the layer was preserved after heating the layer to 204 °C (i.e., above the last phase transitions before isotropization, Figure 8). The results of the TOA analysis during the heating−cooling cycle 30 to 204 to 30 °C and images of the layers as seen under crossed 18740



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Figure 11. Absorption and fluorescence spectra of TQPP-OC12 in cyclohexane (black), dichloromethane (red), and dichlorometane cast film on glass substrate (blue); λex = 340 nm. The dotted line is an excitation spectrum in cyclohexane collected at λem = 460 nm.

Figure 12. UV−vis absorption dichroic spectra of TQPP-OC12 (left panel) and polarized fluorescence emission spectra (right panel) in stretched polyethylene. The four pairs of polarized emission curves excited at 405 nm [(|| ||), (|| ⊥), (⊥ ⊥), and (⊥ ||)] are referred to the polarization direction of excitation and emission with respect to the stretching of the polymer.

layer showed two phase transitions at 55 and 135 °C (dI/dT curves in Figure 8a). This suggests that once the layer was cooled below 4 °C, the structure of the pristine structure was not recovered. The phase transition at 46 °C was also detected using DSC when TQPP-OC12 was rapidly cooled to −50 °C (DSC trace in Figure 8b). Detailed studies on the photophysical properties of TQPPOC12 in solution were reported earlier.46 Herein, we revisit the photophysical properties in comparison to those observed for aligned solid films. Absorption spectra of dilute TQPP-OC12 in cyclohexane solutions are very structured (Figure 11). Three bands of comparable intensity are observed with maxima at 243, 286, and 344 nm. At lower energy, one high-intensity band at 426 nm is present. Three vibronic bands at 426, 402, and 382 nm separated by 1400 and 1303 cm−1, respectively, can be clearly seen. These are attributed to the aromatic C−H bending in the core skeletal vibration52 and are accompanied by their sidebands separated by ∼590 cm−1. The excitation spectrum overlaps the absorption spectrum, showing that the internal conversion from the upper excited singlet state to the lowest one occurs with unit quantum efficiency. The emission spectrum, which is a mirror image of the absorption spectrum, peaks at 426 nm. The Stokes shift determined as the energy difference between the emission maximum and the maximum of the lowest-energy absorption band is below 20 cm−1, indicating that the ground state and the fluorescent state have similar geometries. The solvent polarity has an evident influence on the shape of absorption and emission spectra. In dichloromethane (dielectric constant 8.9), the vibronic substructure of the lowest absorption state broadens and the spectral position is slightly affected (shift is smaller than 110 cm−1). In the emission spectrum, the vibronic structure is not present any longer although the band profile is practically the same. Moreover, the Stokes shift has increased to 950 cm−1, indicating that the relaxation in the lowest excited state is getting more pronounced in this solvent.

In polarized light, the absorption and emission studies of TQPP-OC12 embedded in an oriented PE film were conducted to demonstrate polarization direction of the first electronic transition. Knowledge of the polarization of this transition is important for comparing molecular alignment deduced from photophysical studies with GIWAX research results. In symmetrical molecules, only some transition moment directions are possible. TQPP-OC12 core belongs to D2v point group symmetry and its transition moments coincide with the three molecular axes.53 Polarized UV−vis spectra in stretched PE film are shown in Figure 12 and indicate that the B1u ← Ag transition is polarized along the long molecular axis. The calculated dichroic ratio d = A||/A⊥, which is defined as the ratio of the absorbance with polarized light parallel to the axis of stretching direction (A||) to the absorbance measured with polarization perpendicular to this axis (A⊥), is 11, and this corresponds to a perfect alignment of the molecular plane with the stretching direction.53 Moreover, the observed polarized emission intensities indicate that emission from the third excited state is also polarized along the long aromatic core axis. Absorption spectrum of solution-cast films of TQPP-OC12 show similar vibronic progression of the first 0−0 transition (B1u ← Ag transition in a molecule of D2v symmetry). This is consistent with coupling to a 1300 cm−1 core skeletal vibration and a red shift from 428 to 437 nm with respect to its position in the dichloromethane spectrum, although, in the former case the peaks are less resolved. This red-shift of the discussed 0−0 transition cannot be explained by strong exciton coupling between the transition moments of the stacked molecules, which should have a place if we have accepted the columnar structure of the solution-cast film with molecular cores aligned perpendicularly to the column axis.54 On the contrary, the bathochromic shift can be related to the formation of exciton states when the stacking axis forms with the normal to the molecular planes an angle larger than the magic angle,54 as in the case of J aggregates. Indeed, such a shift can be explained by 18741



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Figure 13. Top panel: schematic representation of the four experimental configurations used in the fluorescence (PL) polarization measurements. Bottom panels: polarized optical microscopy images of ZC-layers with different thermal history taken at room temperature and their UV−vis absorption and fluorescence spectra measured in polarized light parallel and perpendicular to the direction of moving substrate. (a) Pristine layer, (b) layer after heating to 203 °C and cooling to 30 °C, (c) layer after being held at −5 °C for 1 h and subsequent heating back to 30 °C. White arrow indicates the zone-casting direction. Samples were heated and cooled at the rate of 5 °C/min. Emission spectra of TQPP-OC12 were excited at λ = 390 nm. White bar on the images corresponds to 100 μm.

along the core z-axis. For the as-cast sample, the intensity of the absorption spectrum with light polarized perpendicularly to the zone-casting direction is higher than that of the spectrum for analyzing light polarized parallel. Alignments of transition moments of TQPP-OC12 molecules imply that the molecular planes have to be oriented more or less perpendicular to the zone-casting direction with the long molecular axis parallel to the substrate surface. Taking into account that the electronic transitions lie in the plane of the heteroaromatic core of D2v symmetry and that cores are aligned parallel, because the samples are dichroic, it is deduced that the central cores are rotated relative to the ZC direction. Otherwise, if molecules are arranged “edge-on” the surface with aromatic cores perfectly perpendicular to casting direction, the dichroic effect should be more pronounced and at least one polarized component would disappear from the spectrum as for the alignment of TQPPOC12 in oriented PE matrix. In fact, some changes in dichroism were observed for samples that were thermally modified.

the presence of two bulky tert-butyl substituents at the pyrene ring, which presumably cause molecular displacement within the column. Emission spectrum of the TQPP-OC12 solutioncast sample measured at 25 °C shows further red-shift from 437 to 448 nm as compared to dichloromethane solution, although beside this maximum one more intense and broad band peaking at 486 nm is observed. This band is ascribed to excimer emission similar to that in the emission of model pyrene molecule.52,55−57 Shown in Figure 13 are polarized UV−vis absorption and fluorescence spectra of zone-casting TQPP-OC12 samples with different thermal history measured at 25 °C for the as-cast layer, after heating it to temperature just before isotropic transition (203 °C), and after thermal conditioning at −5 °C for 1 h. The observed absorption dichroism of the films can be quantitatively related to the orientation of molecules via the known orientation of the transition moment dipole within the molecular framework. Therein, the first transition is polarized 18742



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Heating the sample to 203 °C reverses the intensity of the polarized spectra and strengthens the absorption anisotropy within the zone-casting layer. This means that upon temperature change, a reorientation of columns occurs so that molecular planes are oriented parallel to the casting direction. The calculated dichroic ratio at 412 nm is d = 1.85 for a sample obtained directly after fabrication (Figure 13a) and d = 3.24 for a sample after high-temperature treatment (Figure 13b). One can expect that the biaxial alignment of TQPP-OC12 molecules within columns should cause anisotropy of the dielectric constant of the aligned film. This effect manifests itself in a relative absorption shift of mutually perpendicular polarized absorption spectra in which the perpendicular component of the absorbance is red-shifted by 2 nm as compared to the parallel component. Different molecular alignment of the zone-cast films due to temperature transformations is also clearly visible in the emission spectra. For the as-cast ZC sample, the intensity of the spectral feature ascribed to monomeric emission at ∼450 nm does not depend on the polarization of the excitation and emission, whereas the excimer emission at 489 nm measured with perpendicular polarization is higher than the one collected at parallel polarization. For the sample heated to 203 °C and cooled rapidly to room temperature, the excimer intensity relations for the polarized components remain unchanged while it increases the intensity of monomeric fluorescence observed for emission spectra with parallel polarization of excitation. The polarized absorption and emission spectra of the sample cooled to −5 °C are clearly distinguishable from those of the films processed at temperatures above 30 °C wherein the long-range alignment that was obtained during zone-casting is now lost. The crystalline phase formed because of storage at −5 °C is stable at room temperature (vide DSC studies). Thus, upon cooling, a new solid-state transformation occurred and all the spectral properties of the room temperature phase disappeared, Figure 13c. We have also found that the emission spectrum of the TQPP-OC 12 powder compound obtained through crystallization from a solvent corresponds to the spectrum of pristine ZC sample and not to that developed at negative temperatures. Our spectroscopic results concerning the thermal behavior of TQPP-OC12 are also confirmed by polarizing optical microscopy. Thus, for the phase formed after the low-temperature treatment, the absorption spectrum of the zone-casting film regains its vibronic structure similar to that of the solution spectra. It can be seen in Figure 13c that, for the frozen film, the peak intensity at 433 nm which was measured at perpendicular polarization increased and the dichroic ratio became slightly negative. This happens because the transition dipoles, initially perfectly aligned in columns, orientate in a different way. Moreover, the spectral changes can be related to different conformations of the side chains near the aromatic core in the formed phase at low temperature and in the as-cast film, which may influence the energy of the electronic transitions. A change in the ordering of the crystalline phase is visible in the emission spectra, in which polarization has little effect on the shape and intensity. As such, the revealed vibronic progression for the monomeric region is a consequence of changes in the absorption and is similar to that observed in nonpolar solvents.

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CONCLUSIONS The fabrication of highly oriented layers of TQPP-OC12 was achieved using a zone-casting technique. The morphology and structure of these oriented films can be changed upon heating or cooling. In spite of phase transitions, the pristine overall orientation resulting from zone-casting solidification is preserved. Thus, oriented layers exhibiting different anisotropic optical properties can be obtained by a certain thermal treatment of the pristine layer. Our studies showed that using the technique thermo-optical analysis correlates very well with the results of differential scanning calorimetry. An important advantage of these measurements (TOA) is the low demand for the material (about 0.1 mg). The highly oriented TQPP-OC12 layers, which were obtained by zone-casting at 60 °C, exist in two structures, which exhibit endothermic phase transitions at temperatures of 55 and 134 °C. The conclusion is fully supported by the presence of two diffraction reflexes in XRD diffractograms. Despite some morphological changes accompanying phase transitions after heating−cooling cycles of 30 to 150 to 30 °C, the macroscopic overall orientation of the zonecast layer was preserved. AFM studies have provided information consistent with X-ray diffraction. From these results, it appears that the material has many phase transitions from room temperature to 200 °C and is thus not suitable for building operating systems such as organic field-effect transistors. However, this material has very interesting optical properties that can be controlled by thermal history without chemical modifications.

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ASSOCIATED CONTENT

S Supporting Information *

The full citations of references 35, 39, 40, 42, 49, and 50. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +48-42-6803228. E-mail: [email protected]. *Phone: +961-3151451. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Petroleum Research Fund (PRF) of the American Chemical Society (Grant 47343-B10), the Ministry of Science and Higher Education Republic of Poland (Grant 5155/B/T02/2010/39). The authors are grateful for this support. W.Z. acknowledges the support from the ERC Advanced Grant NANOGRAPH (AdG-2010267160). This paper is dedicated to Prof. Adam Tracz, who passed away on Dec 23, 2013.

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REFERENCES

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