Self-Assembly and Near Perfect Macroscopic Alignment of

Apr 29, 2013 - Self-Assembly and Near Perfect Macroscopic Alignment of. Fluorescent Triangulenium Salt in Spin-Cast Thin Films on PTFE...
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Self-Assembly and Near Perfect Macroscopic Alignment of Fluorescent Triangulenium Salt in Spin-Cast Thin Films on PTFE Fredrik Westerlund,*,†,‡ Henrik T. Lemke,§ Tue Hassenkam,† Jens B. Simonsen,† and Bo W. Laursen*,† †

Nano-Science Center and Department of Chemistry, University of Copenhagen, Universitetsparken 5, Copenhagen 2100 Ø, Denmark ‡ Department of Chemical and Biological Engineering, Chalmers University of Technology, Kemivägen 10, 412 96 Gothenburg, Sweden § Centre For Molecular Movies, Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, Copenhagen 2100 Ø, Denmark S Supporting Information *

ABSTRACT: Highly fluorescent, discotic trioxatriangulenium dyes were aligned by simple spin-casting on substrates with friction transferred PTFE layers. The fluorescent crystalline thin films show near perfect macroscopic alignment on centimeter large areas directly from spin-casting. Gracing Incidence X-ray Diffraction (GIXD) unambiguously allowed the determination of a long-range order unit cell as well as its orientation with respect to the PTFE fibers. Further analysis of the X-ray data, in conjunction with polarized absorption spectroscopy, suggest a lamellar packing model with alternating layers of alkyl chains and ionic dyes oriented parallel to the substrate. This structure results in a highly anisotropic electrostatic potential around the cationic chromophore, causing significant shifts in energy and orientation of the optical transitions. Thus, the optical properties of the material are, to a large extent, controlled by the position of the otherwise inert PF6− counterions. The bright fluorescence from the films is also polarized parallel to the PTFE alignment layer. Doping of the thin films with fluorescent energy acceptor traps shows that efficient exciton migration takes place in the thin films. The excellent exciton transfer capabilities, in conjunction with the perfect alignment, might be of interest in future applications in solar energy harvesting or as thin film sensors.

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INTRODUCTION Self-assembly of discotic molecular π-systems is an important strategy for formation of electroactive organic materials.1−3 The discotic building blocks find use in bulk materials, thin films, and nano objects such as tubes and fibers.4,5 Applications are also very diverse and include transistors,6−8 photovoltaics,9,10 and light emitting diodes.11−13 In all cases, the properties are highly dependent on the structure of the material, both on the macroscopic (device) level and the nanoscopic level. On the device level, macroscopic alignment of aggregates/domains is required in order to obtain efficient charge transport or for selective interaction with polarized light. Consequently, development of various alignment techniques plays an important role for these molecular materials.2,3 On the nano scale, the local organization of the molecular components has strong impact on charge transport through coupling of frontier orbitals.14−16 The optical properties may be even more sensitive to the local structure, since both energy and probability of optical transitions in supramolecular aggregates are strongly dependent on the exact orientation, and thus coupling, of the molecular transition moments, as illustrated for example by H- and J-aggregates.16,17 The packing structure of molecular materials is determined by the complementarity of the molecular units in terms of shape, electrostatics, and orbital overlap. To control the packing structure, these properties can © XXXX American Chemical Society

be varied in the design of the discotic units and their substituents.2,5 There has recently been an increased interest in charged systems, which allows for ionic self-assembly (ISA).18,19 By employing charged discotics, additional tools for engineering of the packing structure become available; e.g. the molecular packing may be tuned by simply changing the counterions,20 just as strong ionic interactions may stabilize liquid crystalline mesophases.18,21,22 The introduction of charge can also impose amphiphilic properties on the systems that may facilitate self-assembly of nanostructures from solution,23,24 and formation of Langmuir and Langmuir−Blodgett (LB) films.25,26 Salts of the positively charged tris(dialkylamino)trioxatriangulenium ion (ATOTA+, Figure 1) are particularly interesting units in self-assembled ionic materials for a number of reasons, as follows: (1) The ATOTA+ dyes exhibit attractive photophysical properties, such as high oscillator strength and high fluorescence quantum yields.27−29 (2) The ATOTA+ ions display extreme cation stability due to efficient delocalization of the positive charge over the trioxatriangulenium π-system and the three conjugated dialkylamino groups.30,31 (3) The large πsystem and efficient charge delocalization facilitate close Received: March 2, 2013 Revised: April 25, 2013

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Figure 1. Chemical structure of the ATOTA+ derivative used in this study, bis(didecylamino)dimethylamino-trioxatriangulenium (1·PF6), as well as related systems previously studied in LB films (2·PF6) and (3·PF6).

columnar π−π stacking of the ATOTA+ cores.26,30 (4) The flexible synthetic protocol allows for independent variation of the three dialkylamino substituents at the corners of the ATOTA+ system.28,30,31 We have recently taken advantage of these properties to synthesize amphiphilic ATOTA+ derivatives by attaching n-decyl chains at only one or two of the three amino groups.25,32 Derivatives with two long alkyl chains per ATOTA+ core (2·PF6 and 3·PF6, Figure 1) yield high quality Langmuir and LB multilayer films with a close cofacial packing of the ATOTA+ cores25 and significant uniaxial alignment of the columnar aggregates along the dipping direction of the LB films.26 However, for optoelectronic applications the degree of alignment in the LB films is insufficient. Furthermore, the LB films display the somewhat unattractive optical properties typical for H-type aggregates formed by the nontilted cofacial packing of the ATOTA+ chromophores in the columnar LB structures. An efficient way to align small molecules in organic thin films was introduced in 1991 by Wittmann and Smith33 who showed that slowly pulling a heated piece of poly(tetrafluoroethylene) (PTFE) over a cleaned glass slide yields very thin PTFE fibers that are perfectly aligned with the pulling direction. The fibers are of nanometer height and width and can act as templates for crystallization of small molecules in one specific direction. Early studies of alignment of conjugated materials34−36 and of discotic materials37−39 on PTFE relied mostly on vacuum deposition or slow crystal growth. However, Gearba et al. showed that liquid crystals of a hydrophobically modified discotic phtalocyanine can be aligned on PTFE from solution casting by subsequent heating to the isotropic phase (>166 °C) and slow cooling.40 Herein we demonstrate that bis(didecylamino)dimethylamino-trioxatriangulenium hexafluorophosphate (1·PF6, Figure 1) with four n-decyl chains can be processed into fluorescent thin films with high crystalline order and macroscopic alignment on centimeter large areas by simple spin-casting on friction transferred PTFE layers. Importantly, no subsequent annealing is needed to obtain the near perfect alignment. The detailed structure of the 1·PF6 films was investigated by Gracing Incidence X-ray diffraction (GIXD),41 which provided the unit cell and its orientation on the PTFE layer. Further qualitative analysis of the X-ray data, in

conjunction with polarized absorption spectroscopy, led to a packing model where the ATOTA+ units are organized in ionic layers parallel with the substrate, and aligned to the PTFE fibers. This structure is a product of a nanoscale phase separation where electrostatic interactions and the space-filling requirements for the four long alkyl chains favor a highly anisotropic electrostatic potential around the cationic chromophore, which results in orientation of the optical transitions and formation of a J-aggregated structure. Hence, the optical properties of the material are largely controlled by the position of the otherwise inert PF6− ions. Fluorescence from the 1·PF6 films is polarized parallel to the PTFE alignment layer. Doping of the thin films with fluorescent energy acceptor traps shows that efficient exciton migration takes place in the thin films, making the system potentially interesting for light harvesting and sensor applications.

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EXPERIMENTAL METHODS

The synthesis and bulk characterization of compound 1·PF6 is reported in ref 25. Preparation of Thin Films. Films were prepared by casting of 20 μL chloroform solutions of 1·PF6 (1−2 mg/mL) directly onto a PTFE treated microscope slide spinning at 2000 rpm. To prepare very thin films of highly aligned PTFE the friction-deposition technique was utilized,33 in which a heated bar of PTFE is slid across a thoroughly cleaned glass substrate. The deposition was performed at a velocity of 1 mm/s, a temperature of 280 °C, and an applied pressure of 0.4 MPa. Microscopy. The micrographs were taken in a Zeiss Axiotech microscope with polarizers. Spectroscopy. The absorption studies where performed on a Perkin-Elmer Lambda 800 equipped with depolarizer and Glan-Taylor polarizers. The fluorescence measurements were done using a Fluorolog-3 from Horiba. X-ray Measurements. A single-crystal κ-diffractometer was used to map the scattering signal in three dimensions from a thin film of 1·PF6 deposited on PTFE. For that purpose, the thin film sample was aligned horizontally at grazing angle (0.3°) with the incoming X-ray (1 Å, 12.40 keV). The scattered signal was collected by a plane CCDdetector (marmosaic 300, 4096 × 4096 pixels, 73.242 μm pixel size) at 300 mm distance and aligned perpendicularly to the incoming X-ray beam. B

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RESULTS AND DISCUSSION Thin films of 1·PF6 were prepared by casting chloroform solutions directly on glass slides coated with friction transferred PTFE spinning at 2000 rpm. Homogeneous, reddish transparent films were obtained without any further annealing. Figure 2 shows micrographs taken under crossed polarizers;

Figure 2. (A) Micrograph images taken under crossed polarizers of a 1·PF6 film spin-cast on a PTFE-treated glass with the sample rotated 0° (left) and 45° (right). (B) Fluorescence micrograph images of a 1·PF6 film spin-cast on a PTFE-treated glass with a polarizer collecting the fluorescence parallel (left) and perpendicular (right) to the PTFE fibers. The micrographs were taken through an optical microscope; the scale bar represents 50 μm and the arrow points out the PTFE direction. A 500 nm cut off filter was used in B.

maximum transmitted light is observed when the film is positioned with the PTFE alignment layer ±45° to the polarizers (right panel), while the image is completely dark when the angle is 0° or 90° (left panel). This indicates an anisotropic structure that is well-aligned with respect to the underlying PTFE fibers.42 The films show clear fluorescence that, to a very large extent, is polarized parallel with the PTFE fibers, meaning that it can be virtually turned on and off by rotating the film or the polarizer 90° (Figure 2B). The fact that practically no emission is observed with the polarizer perpendicular to the PTFE fibers shows that the dark streaks in Figure 2A and 2B correspond to regions not covered with 1·PF6, rather than isotropic regions. These defects in the film coverage are most likely related to poor wettability of the cast solution on the PTFE. This was further demonstrated by the fact that drop casting completely failed to give thin films of an appreciable quality due to dewetting. In fact the applied method of very fast film formation by casting directly on a spinning substrate seems to be a condition to suppress dewetting. Films prepared from solutions of 1 mg/mL gave an average film thickness of 15 nm according to AFM measurements (Figure S1, Supporting Information (SI)). Polarized Absorption. Figure 3A shows polarized absorption spectra of a 1·PF6 film when changing the polarization from parallel to perpendicular to the PTFE fibers at normal incidence. The film is highly anisotropic with two separate transitions orthogonal to each other. The absorption spectrum polarized parallel to the PTFE fibers peaks at 509 nm with a distinct vibrational shoulder at 483 nm, and is significantly red-shifted compared to the solution spectrum (λmax ≈ 470 nm).26 The spectrum perpendicular to the PTFE fibers is blue-shifted compared to the solution spectrum and

Figure 3. Absorption spectra of a 1·PF6 film spin-cast on a PTFEtreated glass: (A) measured with polarized light, rotating the polarizer 5° for each spectrum. The arrows point out the spectral evolution when the polarizer is rotated from parallel to perpendicular to the PTFE fibers. Inset: The contribution from the red-shifted spectrum (full squares) and the blue-shifted spectrum (open squares) plotted against the angle between the PTFE fibers and the polarizer. The solid lines are cos2(ω) and 1 − cos2(ω), respectively. (B) measured with light polarized parallel to the PTFE fibers on a sample that is tilted 0° (dashed line) and 60° (solid line) relative to the incoming light. Inset: The difference between the normalized spectra at 0° tilt and 60° tilt (solid line) and the spectrum perpendicular to the PTFE fibers at 0° tilt (dashed line), normalized to the maximum absorption of the former.

centered at 443 nm. By projecting the spectra at different intermediate polarization angles on the spectra parallel and perpendicular to the PTFE fibers, the angle-dependent contribution from the two transitions can be obtained (Figure 3A, inset). As can be expected for perfectly oriented transitions, the red-shifted spectrum falls off as cos2(ω), where ω is the angle between the polarizer and the PTFE fibers, whereas the blue-shifted spectra increases as 1 − cos2(ω). This is also seen directly in the spectra where there is a very distinct isosbestic point and very little contribution from the blue spectrum at 0° (parallel to the PTFE direction) and from the red spectrum at 90°. Calculation of the dichroic ratio for the main absorption band at 509 nm gives a value of D ≈ 18. C

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The films were further investigated by changing the tilt angle between the sample and the incoming light (Figure 3B). For light polarized parallel to the transition moment of the red peak (the PTFE direction) the intensity falls off significantly when the sample is tilted 60°. This is expected for a transition that has a low angle relative to the substrate, and hence a small outof-plane component. Interestingly, a peak emerges on the blue side of the spectrum at high tilt angles. Normalizing the spectra at 0° and 60° tilt at 509 nm and subtracting the former from the latter, gives a spectrum that resembles the blue-shifted spectrum from Figure 3A perfectly with a maximum absorption at 442 nm (Figure 3B inset). This is expected if the blue-shifted transition has a significant tilt relative to the substrate, and hence a component that is perpendicular to the substrate. Doing the same tilting parallel to the blue-shifted transition does not show any contribution from the red-shifted peak (not shown), again supporting that the transition moment of the red component has an orientation very close to the plane of the substrate. Alignment and Packing Structure. The molecular packing structure in the spin-cast films was studied by Gracing Incidence X-ray diffraction (GIXD). The diffraction intensity was collected as a function of rotation of the film around the surface normal (φ), as illustrated in Figure 4. Well-defined Bragg reflections overlaid with a broad diffuse background from the substrate are observed.

Figure 5. Bragg peak intensity as a function of in-plane rotation φ at (QII, Q⊥) = (0.952, 0.502) Å−1 and (QII, Q⊥) = (1.237, 1.113) Å−1 (black and gray, respectively). The peaks are labeled by the Miller indices according to the suggested unit cell.

hexabenzocorenene (HBC) materials obtained from solution by very slow crystallization on PTFE or by zone casting.38,42,44 However, it was particularly noted for HBC that direct spin coating did not yield aligned films.42 Bragg reflections with similar positions in Q-space and similar intensity relations were found for bulk powder, indicating that the same crystal structure is present both in bulk and in thin films. By use of the Dirax program,45 consistent unit cell parameters of the 1·PF6 crystal structure were determined independently from the powder data and the φdependent GIXD measurement of a thin film on PTFE. All 32 reflections used for the refinement (see Table S1 in SI) could be attributed to a single monoclinic unit cell with a 2-fold multiplicity corresponding to a 180° in-plane rotation (SI Figure S2). The 2-fold symmetry is expected since there is no directionality in either the spin-casting technique or in the structure of the PTFE layers. The b-axis of the unit cell is found to be parallel to the PTFE fibers. The obtained unit cell has a volume of 3105 Å3, and a realistic density of 1.14 g/cm3 is obtained with the assumption of two 1·PF6 per unit cell. This density is in good agreement with the densities of organic materials containing both saturated alkyl chains and aromatic units. The by far most intense Bragg reflection from the oriented thin film is the −2,0,3 reflection (see SI Table S1), and corresponds to a predominant modulation of the electron density with a repetition period of 4.3 Å. The wave vector of this repetition lies in the ac-plane and is tilted by 31.5° with respect to the substrate plane, thus corresponding to periodic planes in real space oriented perpendicularly to the repeat direction. The most likely molecular unit responsible for this feature is the planar ATOTA+ core. The repeat distances of these planes along the unit cell a-axis is 4.3 Å/cos(31.5°) = 5.04 Å. This length cuts a (= 10.2 Å) approximately in half which makes the model of stacking planes a reasonable guess considering the unit cell size. However, an interplane distance of 4.3 Å is much larger than the distances expected in planar πstacked molecules (≈ 3.5 Å).46 For π-stacked ATOTA+ systems distances of 3.3 and 3.4 Å have been found for single crystals and LB films.25,26,30 This excludes a simple columnar π-stack of the ATOTA+ planes and instead suggests alternating packing of ATOTA+ cations and PF6− anions in the a-direction, as illustrated in Figure 6. In this alternating stack, the distance between two parallel ATOTA+ planes becomes 8.6 Å (see Figure 6B). This length is in perfect agreement with the reported single crystal structure of a PF6− salt of a closely

Figure 4. Schematic drawing of the geometry used for measuring the GIXD pattern of a thin film of 1·PF6 (spin-cast on PTFE) as function of the in-plane rotation angle φ. Red arrows indicate the incoming and outgoing (scattered) X-ray beam. The shown scattering intensity image corresponds to a single orientation of the film/substrate.

φ-Dependent diffraction measurements of the films on friction transferred PTFE gives the opportunity to obtain the diffraction peak positions in three dimensions and perform a detailed analysis of the degree of alignment. In the out-of-plane direction an upper limit for the distribution width is found to be as low as 2.7°. The high degree of in-plane alignment documented by the high dichroic ratio in the optical measurements is clearly confirmed by the φ-scan for several Bragg peaks (Figure 5). The φ-scan gives an upper limit for the in-plane alignment of 4.25°, which is comparable to the alignment found for perylene crystals grown on PTFE alignment layers by chemical vapor deposition (3.3°),43 and significantly better than that found in shear aligned LB films of the amphiphilic ATOTA+ analogues 2·PF6 and 3·PF6 (∼ 30°).26 The high degree of alignment in the 1·PF6 film direct from spin-casting may be compared to aligned electroactive D

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Figure 6. Cross sections of the proposed structure model, unit cell indicated in red, PF6− ions as green circles, ATOTA+ ions as yellow/gray triangles/rectangles: (A) top view along the surface normal (ab projection); alkyl chains left out for clarity; (B) side view along the unit cell b axis (= the PTFE direction), a and c axes of the unit cell shown in red; (C) front view along the unit cell a axis.

related planar trioxatriangulenium (TOTA+) salt where a distance of 8.47 Å was found for an alternating stack.47 While an exact structure model cannot be derived, these findings lead us to suggest the simplified packing model shown in Figure 6. The general structure consists of ionic layers separated by aliphatic layers. The layers are parallel to the substrate and have a height of 17.8 Å (c × sin β), in good agreement with the terrace step height observed in the AFM studies (see below). The detailed structuring of the alkyl chains is not resolved, yet it is clear that alkyl chains must occupy the volume above and below the ionic layer in each unit cell. Parallel to the substrate this lamellar structure has a unit cell area of a × b = 175 Å2 (Figure 6A), to accommodate the eight decyl chains (≈ 22 Å2 per alkyl chain). With a minimum area of ∼19 Å2 per upright standing alkyl chain48 this area relation corroborates the model and indicates an average tilt of the alkyl chains of approximately 30° from the film normal (19 Å2/ sin(60) = 22 Å2). Within the ionic layers, two interdigitated stacks of alternating ATOTA+ and PF6− ions extend along the a-axis with the ATOTA+ planes parallel to the b-axis (Figure 6A). Thus the length of the b-axis is defined by the width of the ATOTA+ core, which was found to be exactly 17 Å in columnar structures in LB films.25 The ATOTA+ planes are tilted by 58.5° with respect to the substrate and the ab-plane (Figure 6B). The rotational orientation of the ATOTA+ with respect to the a-axis is unknown, however based on simple space filling arguments we suggest that the two unique molecules in the unit cell are packed in an antiparallel fashion sketched in Figure 6C. The proposed packing model may in part be viewed as a result of nanoscale phase segregation between aliphatic and aromatic/ionic sub units. The electrostatic forces favor a structure where the anions are surrounded by a maximal number of polarizable ATOTA+ ions. For 1·PF6 the relative large number of attached alkyl chains (four) requires a large interface to the aliphatic regions favoring the highly tilted packing, and prohibiting extension of the ionic layer into the third dimension. This is contrary to the double layers found in LB films of 2·PF6 and 3·PF6 both carrying only two alkyl chains per ATOTA+ unit.26 For 2·PF6 and 3·PF6 a close nontilted columnar π-stacking is favorable, in agreement with the Langmuir studies showing that the nontilted ATOTA+ core has a foot print of ∼58 Å2 which is less than the two attached alkyl chains.

Film Morphology. AFM measurements were performed on 1·PF6 films on PTFE to investigate the surface morphology. The main part of the film surface (Figure 7A) is characterized

Figure 7. Tapping mode AFM pictures of 1·PF6 spin-cast on PTFEtreated glass. (A) Representative picture of the general surface structure of the film. (B) The edge of the film where the uncovered PTFE fibers can be seen in the bottom of the picture. (C) Zoom-in on the shaded part of B. (D) Section analysis along the black line in C with the characteristic ∼2 nm terraces.

by a fibril-like structure running in the horizontal direction of the image with a long axis that coincides perfectly with the direction of the underlying PTFE fibers. These fibrils do not have any specific length or width, but the width is in the range of a few hundred nanometers. The fibril-like structure appears relatively rough and unordered, considering the very high degree of crystalline order and alignment documented by the optical measurements and X-ray data. The roughness could be E

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explained by very fast crystallization, where efficient nucleation on the PTFE fibers leads to formation of small crystals of different heights. Thus the fibrils should not be regarded as single crystal domains, but rather as macroscopic features resulting from the anisotropic PTFE surface. Figure 7B shows the edge region of a 1·PF6 film, which displays several interesting features: In the lower part of Figure 7B, PTFE fibers that are not covered with 1·PF6 can be seen. The thickness of these fibers is typically 10−40 nm and similar to fiber widths measured on pure PTFE substrates, where also some defects49 in the alignment layer are observed (SI Figure S4). Small areas with a clearly layered structure are found in the edge region of the film (Figure 7B). We suggest that this structure is identical to the dominant structure, but originates from domains with larger crystallites (slower crystallization) that are formed where the underlying PTFE layer is missing. Close to the PTFE fibers, the layered structure shows a clear directionality, whereas the directionality is decreasing further away, suggesting that this structure is growing from the PTFE covered regions, where nucleation is fast. This finding also explains why small defects in the PTFE layer as those observed in the AFM do not compromise the alignment in the thin film of 1·PF6. A zoom-in on the layered structure is shown in Figure 7C, and section analysis along the black line is given in Figure 7D. Each layer is approximately 2 nm high, which corresponds well to the projected height of the unit cell, 17.8 Å, determined by X-ray diffraction (see above). Additional AFM images and crosssection analysis of such terrace regions are given in SI Figure S4. Optical Transitions. The polarized absorption spectra discussed above show two electronic transitions clearly resolved by orientation and energy (Figure 3A). This is in clear contrast to the solution spectra where the ATOTA+ chromophore is characterized by a single degenerated transition at ∼470 nm as expected from the 3-fold symmetry (D3h).27,29 In LB films of 2·PF6 and 3·PF6, where the ATOTA+ cores are packed in a cofacial columnar structure segregated from the PF6− ions, Htype exciton coupling results in a splitting of the degenerated energy level into two blue-shifted absorption bands at 420 and 460 nm, and red-shifted fluorescence (600 nm).26 From the polarized absorption measurements and the GIXD studies the intense red transition at 509 nm is found to be parallel to the PTFE fibers and unit cell b-axis. The blue-shifted 443 nm transition is polarized perpendicular to the b-axis, that is, in the ac plane, with a significant component along the film normal. Considering that all allowed transitions must be polarized in the plane of the ATOTA+ π-system, this is in good agreement with the suggested packing model outlined in Figure 6. These observations strongly suggest that the two optical transitions are oriented in the ATOTA+ framework as shown in Figure 8. The splitting and energetic order of the two transitions may be ascribed to two effects: (1) the highly asymmetric electric field generated by the localization of the PF6− anions, and (2) the different exciton coupling between transitions polarized along the b-axis and transitions polarized along the a-axis. In the derived structure model all the PF6− ions are localized in the same plane in the lamellar layer (Figure 6). As the lower electronic transitions in the ATOTA+ chromophore are of charge transfer character, moving more of the positive charge to the amino groups,27,29 the transition polarized along the b-axis toward the anions (red arrow, Figure 8) will experience a stabilizing Stark effect. The perpendicular polarized transition

Figure 8. ATOTA+ chromophore and its nearest neighbors in the thin film crystal. (A) Transition moments for the 509 and 443 nm transitions shown as red and blue arrows respectively. The x and y axes coincide with the a and b axes of the unit cell. The z axis is perpendicular to the film/substrate. (B) ATOTA+ chromophore highlighted in yellow surrounded by its nearest neighbors. View along the molecular plane in direction of the 443 nm transition (blue arrow in A). The J-type exciton coupling is illustrated by the two red arrows indicating the S1 transition moments.

(blue arrow, Figure 8), on the contrary, is working against the electric field of the PF6− ions by moving positive charge to the amino group farthest away from the anions, leading to a destabilizing Stark effect. It has been shown that a single PF6− ion in a close ion pair with a ATOTA+ dye in solution may generate a local field of close to 1 GV/m resulting in a Stark effect splitting the electronic transitions by ∼1700 cm−1 (∼40 nm).27,29 On top of the Stark splitting of the molecular transitions, the interdigitated ATOTA+ stacks are expected to favor J-type exciton coupling between the molecular (S0→S1, red arrow) transitions. The extent of this coupling can be estimated based on Kasha’s exciton theory.17 Thus, by considering the interaction between two colinear transition moments with a center-to-center distance of 17 Å (Figure 8b) and a magnitude corresponding to an oscillator strength f = 0.6,27 an interaction energy of ≈ 600 cm−1 is estimated, emphasizing that significant J-type exciton coupling is expected in the proposed structure. This is in agreement with the narrow and intense 509 nm absorption band of the film. On the contrary, any exciton coupling between the S0→S2 (blue arrow in Figure 8a) transitions will be of H-type. All together these arguments provide a good explanation of the absorption properties of the 1·PF6 film in agreement with the proposed packing model. An energy diagram illustrating the combined effects of Stark and exciton splitting is given in SI Figure S5. Fluorescence and Energy Migration. To further substantiate the packing model, and investigate whether this new material is a good candidate in organic thin film optoelectronic applications, we were interested in investigating F

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its fluorescence properties. The polarized fluorescence microscopy images (Figure 2B) demonstrate that also the emission to a large extent is polarized parallel with the PTFE fibers. Emission spectra excited parallel and perpendicular to the PTFE alignment layer reveal some interesting details (Figure 9A). The shape of the spectra are significantly different

shifted part of the emission spectrum is much less polarized with a much larger Stokes shift, and resembles the emission from H-aggregated ATOTA+ dyes in LB films.26 We suggest that this red part of the emission originates from a very small fraction of H-aggregated molecules at domain boundaries acting as “traps” and collecting a large fraction of the excitation energy. These molecules are not visible in the absorption spectrum, which indicates that they constitute far less than 1% of the molecules in the film. That such a small fraction of molecules contribute to the emission spectrum to such a large extent implies that highly efficient exciton migration takes place in the film. While short-range (Dexter) energy transfer may be a dominating mechanism in columnar H-aggregates, energy transfer in the present alternating structure will be dominated by J-type exciton interactions between S1 states. Based on the estimated coupling very efficient energy transfer with rates exceeding 1013 s−1 can be expected.17 The high oscillator strength and small Stokes shift of the 1·PF6 film will, even if the effect of exciton coupling is ignored, facilitate Förster type energy transfer, which can be effective even for very weakly coupled systems.17,50 To further elucidate the exciton transfer efficiency, we introduced an extrinsic trap in the films. The trap molecule used, DiD, is shown in the inset of Figure 9B. DiD has a strong absorption in the red part of the UV−vis spectrum that overlaps well with the emission of the 1·PF6 film. Films with only 0.1 mol % DiD (i.e., 1 trap per 1000 ATOTA+ dyes) have an emission spectrum where approximately 50% of the emission of the film comes from the trap (Figure 9B), indicating a very efficient exciton transfer in the films. Subtracting the spectra with and without the trap shows that the emission is uniformly quenched from the two peaks discussed above. These results confirm the conclusions from the emission spectra of pure 1·PF6 films that the energy transfer is so efficient that a vast majority of the emission can originate from molecules that are present in numbers much less than 1%. The ease of incorporating a trap and the efficient energy transfer to the trap in the ATOTA film could be utilized in sensor systems in which the energy transfer to a sensor molecule (trap) could provide amplified response.

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CONCLUSIONS We have demonstrated excellent alignment of the cationic discotic 1·PF6 by simple spin-casting on PTFE substrates. The degree of alignment, based on optical and structural (polarized absorption and GIXD) measurements, is comparable to that traditionally obtained from vapor deposition and slow crystal growth. This is remarkable considering the very fast process and absence of any further annealing. Further studies to elucidate the detailed alignment mechanism are in progress. As a result of a nanoscale phase segregation between the ionic ATOTA dyes and the four appended aliphatic alkyl chains 1·PF6 organizes into a layered structure with columns of alternating ATOTA+ ions and PF 6− ions. The highly anisotropic distribution of the PF6− counterions dictates the orientation and coupling of the optical transitions. This highlights that ionic π-systems not only provide additional features in the self-assembly process but also that large electronic effects can be imposed by the exact position of the otherwise inactive counterions. By use of a fluorescent dopant, we have demonstrated that very efficient energy migration takes place in the thin film and

Figure 9. (A) Emission spectrum of a 1·PF6 film on PTFE, excited at 500 nm parallel (solid line) and perpendicular (dashed line) to the PTFE alignment layer at 45° angle of incidence. (B) Emission spectrum of a 1·PF6 film spin-cast on PTFE with ∼0.1 mol % DiD (inset) excited at 460 nm (solid line) as well as the corresponding spectrum without DiD (dashed) and the difference between the two (dotted line). The spectra are normalized at the maximum emission of 1·PF6.

for the two polarization directions; there is a distinct shoulder on the blue side of the spectrum (∼543 nm) when exciting parallel to the PTFE that is absent for the perpendicular direction. Furthermore the emission maximum is red-shifted approximately 10 nm for the perpendicular direction (595 vs 585 nm). We explain the blue-shifted shoulder as being direct emission from the S1→S0 transition (absorbing at 509 nm). This explanation is supported by the perfect polarization parallel to the PTFE fibers and is in agreement with the small Stokes shift (