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Columnar Self-Assembly and Alignment of Planar Carbenium Ions in Langmuir-Blodgett Films Jens B. Simonsen,*,†,‡ Fredrik Westerlund,†,§ Dag W. Breiby, Niels Harrit,† and Bo W. Laursen*,† †
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Nano-Science Center and Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark, ‡Department of Basic Sciences and Environment, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark, §Chemical and Biological Engineering, Chalmers University of Technology, Kemiv€ agen 10, S-412 96 Gothenburg, Sweden, and Department of Physics, Norwegian University of Science and Technology, Høgskoleringen 5, N-7491 Trondheim, Norway Received September 21, 2010. Revised Manuscript Received November 4, 2010 Structural and optical properties of multilayer Langmuir-Blodgett (LB) films of two amphiphilic carbenium salts 2-didecylamino-6,10-bis(dimethylamino)-4,8,12-trioxatriangulenium hexafluorophosphate (ATOTA-1) and 2,6-bis(decylmethylamino)-10-dimethylamino-4,8,12-trioxatriangulenium hexafluorophosphate (ATOTA-2) are described. The LB films were prepared on lipophilic glass by standard vertical dipping. Grazing incidence X-ray diffraction (GIXD) measurements show that the planar organic cores, in spite of their positive charge, form closely packed columns with a repeating distance of ∼3.45 A˚. Specular X-ray reflectivity (SXR) reveals the LB multilayers to consist of Y-type bilayers with thickness 31 A˚ for ATOTA-1 and 41 A˚ for ATOTA-2. This significant difference is ascribed to the different packing motifs of the alkyl chains in the two LB films. GIXD and polarized UV-vis absorption and emission spectroscopy show that the columnar aggregates in the LB films are oriented along the dipping direction. This alignment is attributed to shear effects during LB transfer. The main absorption band of the LB films is blue-shifted compared to that in solution, while the fluorescence is red-shifted by more than 100 nm. These findings suggest the presence of H-aggregates in agreement with the cofacial packing derived from the X-ray measurements. Polarized absorption spectroscopy with variable angle of incidence was used to resolve two perpendicular optical transitions in the visible range, one at 460 nm polarized perpendicular to the columnar direction, in the plane of the film, and one at 420 nm polarized along the film normal.
*To whom correspondence should be addressed. E-mail:
[email protected] (J. B.S.);
[email protected] (B.W.L.).
features of a discotic π-system and does indeed form columnar structures in the crystalline state.9 We recently reported how two ATOTAþ systems equipped with two long alkyl chains (Figure 1) self-assemble into closely packed columnar aggregates at the airwater interface.10 Both derivatives, 2-didecylamino-6,10-bis(dimethylamino)-4,8,12-trioxatriangulenium hexafluorophosphate (ATOTA-1) and 2,6-bis(decylmethylamino)-10-dimethylamino4,8,12-trioxatriangulenium hexafluorophosphate (ATOTA-2), form nontilted columnar aggregates with a distance of 3.4 A˚ between the ATOTAþ ions, and a columnar width of ∼17 A˚. The ATOTAþ π-system constitutes a class of strongly absorbing dyes with high fluorescence quantum yields.11,12 This combination of optical properties and columnar aggregation suggests that these dyes may be interesting materials for energy (exciton) transport. Generally, charge and exciton transport in columnar systems is only efficient in the π-π stacking direction and thus is highly anisotropic. Consequently, alignment is essential for the use of these materials in devices.13-18 The Langmuir-Blodgett (LB) technique is well-known to provide lateral organization of columnar aggregates
(1) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. Rev. 2007, 36(12), 1902– 1929. (2) Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Hagele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; Tosoni, M. Angew. Chem., Int. Ed. 2007, 46(26), 4832–4887. (3) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45(1), 38–68. (4) Cammidge, A. N.; Bushby, R. J. Synthesis and structural features of discotic liquid crystals. Handbook of Liquid Crystals 2B; Demus, D., Ed.; Wiley-VCH: Weinheim, 1998; pp 693-748. (5) Binnemans, K. Ionic liquid crystals. Chem. Rev. 2005, 105(11), 4148–4204. (6) Wu, D. Q.; Pisula, W.; Haberecht, M. C.; Feng, X. L.; Mullen, K. Org. Lett. 2009, 11(24), 5686–5689. (7) Wu, D. Q.; Feng, X. L.; Takase, M.; Haberecht, M. C.; Mullen, K. Tetrahedron 2008, 64(50), 11379–11386. (8) Takase, M.; Enkelmann, V.; Sebastiani, D.; Baumgarten, M.; Mullen, K. Angew. Chem., Int. Ed. 2007, 46(29), 5524–5527.
(9) Laursen, B. W.; Krebs, F. C.; Nielsen, M. F.; Bechgaard, K.; Christensen, J. B.; Harrit, N. J. Am. Chem. Soc. 1998, 120(47), 12255–12263. (10) Simonsen, J. B.; Kjaer, K.; Howes, P.; Norgaard, K.; Bjornholm, T.; Harrit, N.; Laursen, B. W. Langmuir 2009, 25(6), 3584–3592. (11) Laursen, B. W.; Reynisson, J.; Mikkelsen, K. V.; Bechgaard, K.; Harrit, N. Photochem. Photobiol. Sci. 2005, 4(8), 568–576. (12) Laursen, B. W.; Sorensen, T. J. J. Org. Chem. 2009, 74(8), 3183–3185. (13) Bunk, O.; Nielsen, M. M.; Solling, T. I.; van de Craats, A. M.; Stutzmann, N. J. Am. Chem. Soc. 2003, 125(8), 2252–2258. (14) Kastler, M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; Mullen, K. J. Am. Chem. Soc. 2005, 127(12), 4286–4296. (15) Piris, J.; Debije, M. G.; Stutzmann, N.; van de Craats, A. M.; Watson, M. D.; Mullen, K.; Warman, J. M. Adv. Mater. 2003, 15(20), 1736–1740. (16) Pisula, W.; Menon, A.; Stepputat, M.; Lieberwirth, I.; Kolb, U.; Tracz, A.; Sirringhaus, H.; Pakula, T.; Mullen, K. Adv. Mater. 2005, 17(6), 684–689.
Introduction Self-assembly of discotic π-conjugated molecules, into columns of cofacially stacked molecules, has been studied as a general organizational principle and specifically for applications in organic electronics. These π-π stacked columnar aggregates provide noncovalent supermolecular channels for charge and energy transport and have been used in thin film field effect transistors, photoconductors, and photovoltaics.1-3 Most of the molecular systems designed for columnar aggregation are based on neutral π-systems. Until recently, only few studies of columnar aggregates of charged π-systems have been reported.4,5 However, cationic π-systems offer additional options for modification of the self-assembly process and the aggregate structure via ionic interactions and space requirements of the counterions.6-8 The dialkylamino substituted trioxatriangulenium system ATOTAþ shown in Figure 1 has the structural
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Figure 1. Molecular structures of ATOTAþ derivatives. R1-R6 represent arbitrary alkyl groups. ATOTA-1: 2-Didecylamino-6,10bis(dimethylamino)-4,8,12-trioxatriangulenium hexafluorophosphate. ATOTA-2: 2,6-Bis-(decylmethylamino)-10-dimethylamino-4,8,12-trioxatriangulenium hexafluorophosphate.
and excellent structural control in the direction normal to the substrate, due to the layer-by-layer deposition.19-23 In this study, we show that close columnar structures are formed in LB films of ATOTA-1 and ATOTA-2 and that these aggregates display significant macroscopic uniaxial alignment parallel to the LB dipping direction. The structural studies are based on grazing incidence X-ray diffraction (GIXD), specular X-ray reflectivity (SXR), polarized UV-vis absorption, and emission spectroscopy.
Results and Discussion Preparation of Langmuir-Blodgett Films. ATOTA-1 and ATOTA-2 were selected for fabricating LB films because they form highly ordered columnar aggregates at the air-water interface.10 Initial studies of the LB transfer process for these two compounds showed that transfer by the standard vertical LB-dipping technique to polar substrates such as glass and quartz was inefficient. When hydrophobic OTS-covered substrates (see Experimental Section) were used, full transfer of the first layer was observed in the downstroke. However, this layer was lost again in the subsequent upstroke. Addition of KPF6 salt (1.0 mM) to the subphase resulted in transfer ratios of 1.0 ( 0.1 for at least 10 cycles. (LBtransfer profiles of ATOTA-1 and ATOTA-2 are shown in the Supporting Information, Figure S1.) These findings can be understood with reference to our previous X-ray studies of the Langmuir films at the air-water interface which showed that a substantial amount of the PF6- anions are distributed in the water phase and not embedded in the Langmuir film.10 Thus, the presence of extra PF6ions in the subphase allows the Langmuir film to be separated from the water phase. After complete transfer, the LB films were dried in air for 30 min and subsequently immersed in pure water for 7 min in order to dissolve and remove microcrystallites of KPF6 formed on top of the LB films when the water evaporated. By applying this procedure, (17) van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson, M.; Mullen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. Adv. Mater. 2003, 15(6), 495–499. (18) Wittmann, J. C.; Smith, P. Nature 1991, 352(6334), 414–417. (19) Bjornholm, T.; Hassenkam, T.; Reitzel, N. J. Mater. Chem. 1999, 9(9), 1975–1990. (20) Norgaard, K.; Bjornholm, T. Chem. Commun. 2005, 14, 1812–1823. (21) Nuckolls, C.; Katz, T. J.; Verbiest, T.; Van Elshocht, S.; Kuball, H. G.; Kiesewalter, S.; Lovinger, A. J.; Persoons, A. J. Am. Chem. Soc. 1998, 120(34), 8656–8660. (22) Reitzel, N.; Hassenkam, T.; Balashev, K.; Jensen, T. R.; Howes, P. B.; Kjaer, K.; Fechtenkotter, A.; Tchebotareva, N.; Ito, S.; Mullen, K.; Bjornholm, T. Chem.;Eur. J. 2001, 7(22), 4894–4901. (23) Verbiest, T.; Van Elshocht, S.; Kauranen, M.; Hellemans, L.; Snauwaert, J.; Nuckolls, C.; Katz, T. J.; Persoons, A. Science 1998, 282(5390), 913–915.
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Figure 2. Illustration of the LB sample geometry in the GIXD experiment. x-, y-, and z-axes are defined relative to the LB-dipping direction. Vectors representing the incoming (ki) and outgoing scattered (kf) X-ray beams, the corresponding scattering vector (Q), the sample rotation (Φ), and the total scattering angle (2θ). The solid yellow lines represent the proposed dominating orientation of the ATOTAþ cores, with π-π stacking along the dipping direction. The vectors and the molecular line representation are not to scale.
good quality LB-multilayer films with 1-20 layers of ATOTA-1 and ATOTA-2 were obtained. Further information on the preparation of the LB films can be found in the Supporting Information. Structure of LB Films. GIXD was conducted in order to obtain information about the molecular packing and structure of the LB films. In-plane diffraction data were collected for two perpendicular directions, x and y. The x-axis was defined to be parallel to the LB-dipping direction (see Figure 2). The incident angle of the beam was chosen to be Ri = 0.16, and the scattering vector component perpendicular to the surface, Qz, was kept constant at 0.038 A˚-1. Diffraction intensities of Q-scans in the x and y directions are plotted for the LB film of ATOTA-1 and ATOTA-2 in Figure 3A and C, respectively, and the relevant peak information is summarized in Table 1. Starting with ATOTA-2, a pronounced peak is observed in the Qx-scan (Figure 3C) at Qx = 1.83 A˚-1 corresponding to a repeat distance in real space (d) of 3.43 A˚ (d = 2π/Q and Q = (Qx2 þ Qy2 þ Qz2)1/2). This peak is assigned to an intracolumnar π-π stacking distance, which was also observed in the Langmuir film (d ≈ 3.45 A˚).10 The relatively short π-π stacking distance excludes significant tilt of the ATOTAþ cores relative to the stacking direction. This observation implies that the upright nontilted orientation of the π-systems found in the Langmuir film is maintained in the LB film. No further peaks are observed in the x direction. In the y direction, two distinct peaks are found at Qy=0.225 A˚-1 (27.5 A˚) and 0.45 A˚-1 (13.9 A˚) (Figure 3A). These peaks are assigned as first and second order peaks of the same ∼28 A˚ scattering unit. The perpendicular orientation between this structure and the π-π stacking direction indicates that this d-spacing is related to the width of the columnar aggregates. In the compressed Langmuir film, we found a columnar width of 16-17 A˚ for both ATOTA-1 and ATOTA-2, in good agreement with the 16 A˚ width of the ATOTAþ core deduced from single crystal X-ray diffraction and semiempirical calculations.9,10,27 (24) Laursen, B. W.; Norgaard, K.; Reitzel, N.; Simonsen, J. B.; Nielsen, C. B.; Als-Nielsen, J.; Bjornholm, T.; Solling, T. I.; Nielsen, M. M.; Bunk, O.; Kjaer, K.; Tchebotareva, N.; Watson, M. D.; Mullen, K.; Piris, J. Langmuir 2004, 20(10), 4139–4146. (25) Piris, J.; Debije, M. G.; Stutzmann, N.; Laursen, B. W.; Pisula, W.; Watson, M. D.; Bjornholm, T.; Mullen, K.; Warman, J. M. Adv. Funct. Mater. 2004, 14(11), 1053–1061. (26) Smolenyak, P.; Peterson, R.; Nebesny, K.; Torker, M.; O’Brien, D. F.; Armstrong, N. R. J. Am. Chem. Soc. 1999, 121(37), 8628–8636. (27) Krebs, F. C.; Laursen, B. W.; Johannsen, I.; Faldt, A.; Bechgaard, K.; Jacobsen, C. S.; Thorup, N.; Boubekeur, K. Acta Crystallogr., Sect. B: Struct. Sci. 1999, 55, 410–423.
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Figure 3. Diffractograms of 20-monolayer LB films of (A) ATOTA-1 and (C) ATOTA-2 recorded with Q along the x- and y-axis. The rocking curves (B and D) demonstrate the high degree of in-plane alignment. The broken lines indicate the Bragg angles for alignment along (for Qxy = 1.83 A˚-1, Φ = 10.4), and perpendicular (for Qxy = 0.23 A˚-1, Φ = 1.3 - 90 and Qxy = 0.42 A˚-1, Φ = 2.4 - 90) to the dipping direction. Table 1. GIXD Data: Characteristic X-ray Peaks/Distances for the Langmuir Films and LB Films of ATOTA-1 and ATOTA-2 compd
Π (mN/m)
lamellar distance (A˚)
Llamellar (A˚)c
π-stacking distance (A˚)
Lπ-stacking (A˚)c
ATOTA-1 LB filma 20 14.9 147 (10) 3.47 73 (21) 20 16.9 598 (35) 3.46 91 (26) ATOTA-1 Langmuir filmb 20 27.5d/13.9 313 (11)d/278 (20) 3.43 71 (21) ATOTA-2 LB filma 20 17.2 531 (31) 3.48 77 (22) ATOTA-2 Langmuir filmb a Data extracted from graphs in Figure 3. b Data presented in ref 10. c Coherence length (L) followed by the number of units corresponding to the repetition length in paranthesis. d Corresponds to the length and repetition of the dimer.
The distinct difference between the Qx and Qy scans shows that the columnar structure is aligned in the LB film. Usually, crystalline Langmuir films are expected to consist of domains with no preferential in-plane orientation (a 2D powder). However, alignment of these domains may occur during vertical LB transfer due to the nonuniform force gradients experienced by the monolayer upon pulling the substrate from the water subphase. Such flow alignment28,29 has been reported for several columnar structures.24-26,29-31 In the Qy scan shown in Figure 3C, the π-π stacking peak is completely absent, while only a small remnant of the lamellar peak is seen in the (28) Tabe, Y.; Ikegami, K.; Sugi, M. J. Appl. Phys. 1993, 73(2), 905–913. (29) Schwiegk, S.; Vahlenkamp, T.; Xu, Y. Z.; Wegner, G. Macromolecules 1992, 25(9), 2513–2525. (30) Silerova, R.; Kalvoda, L.; Neher, D.; Ferencz, A.; Wu, J.; Wegner, G. Chem. Mater. 1998, 10(8), 2284–2292. (31) Kubowicz, S.; Pietsch, U.; Watson, M. D.; Tchebotareva, N.; Mullen, K.; Thunemann, A. F. Langmuir 2003, 19(12), 5036–5041.
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Qx scan. This indicates a high degree of alignment in the LB film. In order to further investigate the degree of alignment, the scattering intensity of the 3.43 A˚ π-π stacking peak (at Qxy = (Qx2 þ Qy2)1/2 = 1.83 A˚-1) was recorded while rotating the sample around the surface normal (z-axis). This rocking-curve scan (rotating the sample around the film normal with the detector at constant Q) is shown in Figure 3D. Maximum intensity is observed parallel to the y-axis, confirming the preferential alignment of the π-stacks along the dipping direction (x-axis). The intensity difference and slight offsets from the expected 180 symmetry between the two peaks are due to nonoptimal centering of the sample, leading to variations in the footprint of the X-ray beam, during sample rotation. The degree of anisotropy of the π-π stacked crystallites is quantified by the fullwidth at half-maximum (fwhm) of the rocking curve peaks (∼31). The rocking-curve scan performed at Qxy = 0.225 A˚-1 (d = 27.5 A˚) confirms that the lamellar packing is perpendicular (90) to the π-π stacking. The fwhm value of ∼35 is similar to the value obtained for Langmuir 2011, 27(2), 792–799
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Figure 4. Specular X-ray reflectivity from 20 monolayers of ATOTA-1 (A) and ATOTA-2 (B) on OTS-treated glass.
the π-π stacking peak. The broad feature at Qxy = 1.5 A˚-1 (d = 4.2 A˚) is equally intense in the Qx and Qy scans and is assigned to amorphous scattering from the glass substrate, which also is observed from pure glass substrates without LB film. The Qx diffractogram of ATOTA-1 LB film (Figure 3A) also exhibits a single intense peak at Qx=1.81 A˚-1 (d=3.47 A˚) assigned to π-π stacking. An almost vanished π-stack repetition feature in the Qy-scan indicates that the columnar aggregates are aligned preferentially along the dipping direction, as also found for the ATOTA-2 LB film. Three distinct peaks appear in the Qy-scan. We have not been able to assign the two peaks corresponding to repetition lengths of 7.94 and 21.5 A˚. However, the most intense peak, at Qy=0.42 A˚-1 (d=14.9 A˚), is assigned to the lamellar spacing, which is close to the lamellar spacing observed in the Langmuir film and the width of the ATOTA core. Characteristic distances for Langmuir and LB films of ATOTA-1 and ATOTA-2 are summarized in Table 1. The preferential alignment of the ATOTA-1 crystallites was confirmed by the rocking-curve scans shown in Figure 3B based on the 3.47 and 15.3 A˚-1 repetition distances. The main difference between the ATOTA-1 and the ATOTA-2 LB films is that the transfer of the ATOTA-2 film induces a dimerization along the lamellar packing distance. We suggest that the flanking positions of the two long alkyl chains in ATOTA-2 favor the more close packed dimer due to full exclusion of the PF6- ions from the columnar layer as schematically shown in Figure 5. A trend when going from an ATOTA Langmuir film to an ATOTA LB film seems to be a shortening of the coherence lengths along the lamellar direction of the crystallites (see Table 1). The coherence length (L) can be estimated by use of Scherrer’s formula: L = 0.9(2π/wxy), where wxy is the width (fwhm) of the peak.32 Correcting for the instrumental resolution is attempted using wobs2 =wxy2 þ winstr2, with winstr =0.015 A˚-1. For the ATOTA-1 Langmuir film, L ∼ 600 A˚,10 while it is reduced to 150 A˚ when the film is transferred to a LB-multilayer. The reduction of the crystallite size could be due to the crystallites in the Langmuir film breaking during the transfer to the LB substrate. On the contrary, the length of the crystallites along the π-π stacking is conserved. This is most likely due to the rather strong interactions along the π-π stacking compared to the rather weak forces along the lamellar packing direction. Information about the vertical ordering in a 10 down- and upstroke ATOTA-1 LB film was obtained from SXR measurements (Figure 4A). The first, second, and third order Bragg peaks (32) Guinier, A. X-ray Diffraction; Freeman: San Francisco, 1968.
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seen for ATOTA-1 in Figure 4A correspond to a 41.1 A˚ structural repeat unit along the out-of-plane direction (z). The full length of the organic ATOTA-1 cation is around 20 A˚. Hence, the repeat unit is a bilayer, as is also seen from the eight bimaxima (Kiessig fringes) between the Bragg peaks, in agreement with a 10-layer structure. The bilayer conformation is consistent with the transfer ratio of 1.0 ( 0.1 in the down- and upstroke (see the Supporting Information, Figure S1). All of these observations confirm a Y-type LB deposition,35 with alternating orientation of the amphiphiles along the out-of-plane direction (z). The total thickness (Ltotal) of the LB film can be derived from the Kiessig fringes,33 since the Kiessig fringe spacing is given by ΔQz = 2π/Ltotal. Thus, the total organic layer thickness is ∼430 A˚ in the case of the ATOTA-1 LB film. This is in very good agreement with an ATOTA-1 LB film built up by 10 bilayers of 41.1 A˚ plus one layer of OTS (20 A˚). SXR data of a 10 bilayer ATOTA-2 LB film on OTS-treated glass is shown in Figure 4B. For this sample, no valid SXR data was obtained in the low Q-range. A very weak peak was observed at Qz=0.387 A˚-1 (d=16.2 A˚), which we assign to a second order Bragg peak corresponding to a bilayer thickness of 32.4 A˚. We obtained a total film thickness of ∼350 A˚ from the clear Kiessig fringes. The lack of intense Bragg peaks despite the good transfer ratios indicates a film with a much more homogeneous vertical distribution of the electron density than the ATOTA-1 LB film. This may in turn relate to the relatively extensive reorganization that takes place during LB transfer. The total thickness of the ATOTA-2 film is significantly smaller than that for the ATOTA-1 film and suggests a bilayer thickness of approximately 33 A˚, which is in good agreement with the bilayer deduced from the weak second order Bragg peak. The smaller bilayer thickness of ATOTA-2 compared to ATOTA-1 could be explained by the more flanking positions of the long alkyl chains in ATOTA-2 versus ATOTA-1. This molecular structure may enable a larger degree of interdigitation of the alkyl chains between neighboring layers, resulting in a denser film, possibly with full exclusion of the PF6- ions from the columnar layer. A model summarizing the structural investigations is given in Figure 5. In order to explain the significant increase in the columnar width when going from the Langmuir film at the air-water interface to the multilayer LB film in the case of the ATOTA-2 LB film, we suggest a dimerization along the lamellar direction, as illustrated for ATOTA-2. In the dimeric structure, the average (33) Als-Nielsen, J.; McMorrow, D. Elements of Modern X-Ray Physics; Wiley: New York, 2001. (34) Kasha, M. Radiat. Res. 1963, 20(1), 55–70. (35) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: London, 1991.
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Figure 5. Schematic representations of the suggested lamellar packing in the compressed Langmuir films10 and proposed packing in the LB films of ATOTA-1 and ATOTA-2. It should be emphasized that we have not been able to determine the exact positions of the anions in the LB film. Thus, the shown positions of the anions in the LB films are qualitative suggestions based on electrostatic and space filling arguments. The dashed lines illustrate the different degrees of interdigitation of the neighboring alkyl chains in the ATOTA-1 and ATOTA-2 LB films.
column width is thus only 14 A˚, that is, less than the width of the ATOTAþ core. Since any significant tilting of the ATOTAþ cores can be excluded, we suggest that the columnar packing is compressed by a rotation of the ATOTA-2 molecules around the stacking axis (see Figure 5). Such reorganization would favor optimal space filling in the solid film, as water and PF6- ions leave the columnar layer during LB-transfer. In support for this model, it should be noted that the size of the PF6- ion is not compatible with the 3.45 A˚ crystalline packing of the ATOTAþ cores and the ions are thus assumed to be disordered in the Langmuir films. The thinner bilayer thickness and smaller vertical electron density variation of the ATOTA-2 LB films versus the ATOTA-1 LB film is most likely due to the enfolding of the neighboring alkyl chains that is possible in the case of the ATOTA-2 LB film as described above. Optical Properties of LB Films. The ATOTAþ cation is a strong light absorber and an efficient fluorescent dye in polar organic solvents. In many respects, the ATOTAþ chromophore may be considered an extended threefold symmetric version of rhodamine.11 The absorption and fluorescence spectra of ATOTA-1 in dichloromethane solution and in an LB film are shown in Figure 6. In dichloromethane solution, the ATOTAþ chromophore is characterized by a relatively narrow and intense absorption with a maximum at 471 nm and an intense fluorescence with a maximum at 494 nm. This absorption has been assigned to a doubly degenerate transition inherent to the threefold symmetric cation.11 In the LB film, the absorption is significantly broadened and the absorption maximum blue-shifted to 460 nm. Furthermore, the fluorescence from the LB film is drastically red-shifted to 600 nm, 796 DOI: 10.1021/la103785z
Figure 6. (A) Normalized absorption and emission of ATOTA-1 in dichloromethane. (B) Normalized absorption and emission of ATOTA-1 in LB film.
extending its red tail to 800 nm. Such broadening and opposite shift in absorption and fluorescence spectra is in good agreement with Langmuir 2011, 27(2), 792–799
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Figure 7. Micrographs of a birefringent 12-layer LB film of ATOTA-1 on an OTS-treated glass substrate, viewed through crossed polarizers in reflection mode at different sample orientations. Arrows indicate the LB dipping direction, and black lines the directions of the stationary polarizers. The dotted lines mark the edge between LB film and bare substrate. The dimensions of the micrographs are approximately 0.5 mm 0.6 mm.
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an H-aggregate,11,34 as expected for the cofacial columnar structure proposed from the X-ray data. The GIXD data very clearly demonstrate that the columnar aggregates are aligned along the dipping direction. As discussed above, the relatively short π-π stacking distance excludes any large tilting of the planar ATOTA system relative to the columnar direction. Since all allowed transitions in the ATOTAþ molecule must be polarized in the plane of the molecule, this structure should also give rise to anisotropic optical properties of the LB film. A first indication of this is found using polarized optical microscopy, showing that the LB films are highly birefringent (see Figure 7). Polarized optical microscopy shows the consecutive bilayer steps in the region between the uncovered substrate and the 12-layer LB film. The stepwise transition behavior could be due to evaporation of water from the trough during transfer. Further information about structure and alignment of the columnar aggregates in the LB film was obtained by polarized UV-vis absorption and emission spectroscopy. Linearly polarized UV-vis absorbance spectroscopy can reveal the orientation of the absorption transition moment relative to the orientation of the sample. The dependence of the absorbance (A) and the angle (θ) between the transition moment and the polarization plane of the light beam is given by A = Amax cos2 θ. Polarized absorption spectra of a 10-monolayer ATOTA-1 LB film are shown in Figure 8. Corresponding data for ATOTA-2 LB films can be found in the Supporting Information. Figure 8A shows the absorption of polarized light parallel and perpendicular to the dipping direction in a front face geometry, as illustrated in Figure 8C. The absorbance is significantly stronger perpendicular to the dipping direction than parallel to the dipping direction, in agreement with the GIXD study. A linear dichroic (LD) ratio (A^/A ) of approximately 4 is obtained for both ATOTA-1 and ATOTA-2 (see the Supporting Information). The shape of the spectra is identical for both polarization directions and both ATOTA analogues (λmax = 460 nm). Based on the high degree of alignment found in the X-ray measurements (Figure 3C), one would expect a higher optical LD value. However, a discrepancy between the optical and GIXD anisotropies is believed to derive from the fact that GIXD only probes ATOTA molecules ordered in crystallites, while optical spectroscopy probes all chromophores. The modest optical LD thus indicates that a fraction of the ATOTA molecules are not present in highly ordered crystalline domains. Randomly ordered ATOTA molecules may, for example, be distributed in crystallite domain boundaries, crystallite defects, and amorphous domains. Linearly polarized absorption was also measured with variable angle of incidence (θ) obtained by rotation of the sample around an axis in the plane of the surface and perpendicular to the dipping direction. The schematic drawing in Figure 8F illustrates the Langmuir 2011, 27(2), 792–799
sample geometry. This setup permits probing transition moments oriented along the surface normal. Figure 8D shows the variation of the absorption as θV varies. Two interesting features can be seen: (1) The peak at 460 nm decreases in intensity as the angle of incidence increases, and (2) there is a new peak emerging on the blue side of the 460 nm peak. This blue-shifted peak must arise from a transition polarized predominantly along the z direction, that is, normal to the surface. In order to decompose the two differently oriented transitions, we assume that the spectrum at θV =0 represents a transition polarized parallel to the surface of the substrate and hence that its intensity falls of as cos2 θ. This allows us to isolate the spectrum corresponding to θV = 90. Figure 8B illustrates this decomposition into two components at an angle of incidence of 45. One spectral component is centered at approximately 420 nm and the other at 460 nm. Figure 8E shows the normalized contributions of the two generated spectra as a function of θV. The intensity of the 460 nm peak follows a cos2 θ dependence, as defined by the initial assumptions, while the 420 nm peak increases following (1 - cos2 θ). Figure 8G shows the spectral evolution at increasing incidence angles when a horizontal polarization was used as illustrated in Figure 8I, and the sample was rotated around an axis parallel to the dipping direction. The contribution from the 420 nm peak is not as distinct, since the absorption from the 460 nm is much stronger (due to the alignment along the dipping direction discussed above). When the previously decomposed spectra (from sample geometry (F)) are applied to this data set (geometry I) identical cos2 θ and 1 cos2 θ dependencies are obtained both for the 460 and 420 nm bands, as shown in Figure 8H. The fact that the same spectrum/ angle dependence of the 420 nm transition is obtained for both sample orientations (both (F) and (I)) shows that this transition indeed is normal to the surface and not a superposition of two transitions at an angle ( x to the surface normal. Similarly, this finding also gives strong support to the original assumption of the 460 nm transition being parallel to the film surface. The angle dependent linear polarized absorption data clearly shows that two main transitions are present in the ATOTA-1 LB films. One is polarized in the plane of the film and polarized mainly perpendicular to the columnar direction (460 nm band), and the other is polarized along the surface normal (420 nm band), as illustrated in Figure 9A. In an isotropic environment, the threefold symmetry of the ATOTAþ ion implies double degeneracy of the lowest excited state. However, this degeneracy is broken and the lowest transitions split into two orthogonal transitions in the plane of the molecule when an ion-pair is formed with the anion in an asymmetric position.11 These two first transitions are accompanied by considerable transfer of charge as electron density is shifted from the amino DOI: 10.1021/la103785z
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Figure 8. Absorbance of a 10-monolayer ATOTA-1 LB film using polarized light. (A) Polarization of the light parallel (solid) and perpendicular (dashed) to the dipping direction (sample geometry illustrated in (C)). (B) Solid black line shows the absorption obtained at an angle of incidence of 45 (geometry shown in (F)). Dashed and dotted lines show the decomposed spectra for the absorbance polarized in the plane of the film (dashed line) and along the surface normal (dotted line). (D) Absorption using a vertical polarization at varying incidence angles (geometry (F)); θV = 0 to 55; arrows indicate the spectral evolution when θV increases. (E) Relative contribution from the 460 nm band (full squares) and the 420 nm band (open circles) at different angles, based on the spectra in Figure 8D; lines correspond to cos2 θV and 1 - cos2 θV, respectively. (G) Absorption using horizontally polarized light at different angles of incidence (geometry (I)); θH = 0 to 55; arrows indicate the spectral evolution when θH increases. (H) Normalized contribution from the 460 nm band (full squares) and the 420 nm band (open circles) at different angles, based on the spectra in Figure 8G; lines correspond to cos2 θH and 1 - cos2 θH, respectively.
donor groups to the cation center.11 Of the two transitions, the lowest (S0fS1) is polarized along the axis connecting the two amino groups closest to the anion, while S0fS2 is polarized orthogonal to this, as illustrated in Figure 9B. Thus, the observed linear dichroism of the LB films indicates a structure where the anions mainly are localized in between the ATOTA layers, as illustrated in Figure 5. As shown in Figures 5 and 9A, the ATOTAþ ions are most likely rotated in the columnar aggregate relative to their nearest neighbor due to spatial repulsion.10 As outlined above, such a supermolecular geometry is expected to station the transitions not strictly parallel with the substrate and surface normal. However, the coupling between electronic transitions in the close cofacial H-aggregate is expected to result in transitions polarized strictly along the axis of symmetry for the aggregate, and thus aligned with the macroscopic axis of the substrate, as illustrated in Figure 9A. Similar arguments apply to the proposed aggregate structure of the ATOTA-2 LB films (Figure 5). 798 DOI: 10.1021/la103785z
Figure 10 shows the emission from a 10-monolayer ATOTA-1 film LB film measured in a fluorescence microscope (0 between excitation and emission light path) observed through a polarizer on the analyzing side oriented either parallel or perpendicular to the sample dipping direction. The emission is mainly polarized perpendicular to the dipping direction, just as the lowest transitions in the absorption measurements. Interestingly, the emission parallel to the dipping not only is weaker but also has a slight red-shift (see inset in Figure 10). One possible explanation for this could be that the ATOTAþ ions in the crystallites have a slightly more blue-shifted emission than the amorphous ATOTAþ ions. The amorphous ATOTAþ ions could then act as intrinsic traps, accepting the energy from the higher lying S1 state in the crystalline aggregates. This would also explain why the dichroic ratio is smaller for emission than absorption. Langmuir 2011, 27(2), 792–799
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Experimental Section
Figure 9. (A) Sketch indicating the orientation of the two main transition bands in the ATOTA-1 LB film. Red arrow indicates the 460 nm transition, and blue arrow indicates the 420 nm transition polarized along the z-axis. (B) Orientation of the S1 (red arrow) and S2 (blue arrow) transitions in a single ATOTAþ chromophore when exposed to the electric field of an anion.
Figure 10. Emission spectra obtained from a 10-monolayer ATOTA-1 film parallel (dashed) and perpendicular (solid) to the LB dipping direction. Inset: The same spectra normalized.
Conclusions In this work, it has been shown that amphiphilic derivatives of the highly stabilized ATOTA carbenium salt form well organized Langmuir-Blodgett films with close π-π stacked columns of the planar carbenium ions. The π-π stacked repeating distance of ∼3.45 A˚ in the Langmuir film10 is maintained when the film is transferred to solid substrates. SXR measurements reveal that the layer thicknesses of the LB films are in good agreement with the number of monolayers transferred to the lipophilic substrate in a Y-type fashion and an upright orientation of the ATOTA salts within each layer. The LB transfer induces a macroscopic alignment of the π-π stacked column along the dipping direction according to the GIXD data. The observation of a significant blue-shift in the absorption spectrum and red-shift of the fluorescence in the LB film compared to in solution indicates a strongly coupled H-type aggregate, in good agreement with the structure derived from GIXD. The observations from linear dichroism and polarized fluorescence measurements of the films confirm that the π-π stacked columns are macroscopically aligned along the LB-dipping direction. The orientation and relative energy of the optical transitions imply a supramolecular structure with the anions localized in a layer between two layers of ATOTA columns. The close π-π stacking and simple alignment of the amphiphilic ATOTAþ salts in LB films make them potential materials for optical and electronic devices.
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The synthesis and bulk characterization of compounds ATOTA-1 and ATOTA-2 are reported in ref 10. OTS Treatment. (1) The glass substrates were rinsed with Milli-Q water, isopropanol, and finally acetone and then dried by a flow of nitrogen. (2) The glass substrates were immersed into a 0.1 vol % OTS solution in heptane for 1 h under nitrogen. (3) The excess of OTS was washed out with heptane, ethanol, and finally Milli-Q water. (4) The OTS-treated glasses were ultrasonicated in a 50:50 water/ethanol mixture for several minutes. (5) The OTStreated glass substrates were rinsed with Milli-Q water. Langmuir Films. Monolayers of the amphiphilic salts ATOTA-1 and ATOTA-2 were spread from chloroform solution (1 mg/mL) onto a Milli-Q purified water subphase (18.2 MΩ 3 cm) at room temperature (rt). Compression was conducted after 20-30 min using a barrier speed of 5 mm/min. More details on structure and properties of the Langmuir films are given in ref 10. Preparation of the LB Films. The amphiphilic PF6- salts ATOTA-1 and ATOTA-2 were spread from chloroform (1 mg/mL) solution onto a 1 mM KPF6(aq) subphase in the LB-trough at rt. LB-multilayer films were obtained by transferring multiple number of monolayer films to OTS-treated substrate, by the standard Langmuir-Blodgett technique with vertical dipping. All transfers were performed with dipping rates of 2 mm/min and constant surface pressure of 20 mN/m. The LB films for the UV-vis absorption and emission measurements were deposited on only one side of the OTS-treated glass by placing two OTS glasses backto-back during transfer of the Langmuir film. GIXD and SXR. The GIXD studies were performed with a Cyberstar point detector using the z-axis diffractometer at the wiggler beamline BW2 at HASYLAB, DESY. A wavelength λ = 1.24 A˚ corresponding to a photon energy of 10.0 keV was employed, with a beam cross section (hor vert) of 1.0 0.1 mm2. The sample, being on its glass substrate, was mounted horizontally with an additional rotational axis Φ about the sample normal. The sample cell was made of X-ray transparent kapton and flushed with helium. k = |kf| = |ki| = 2π/λ, where ki and kf are the incoming and outgoing wave vectors, respectively. A Qxscan corresponds to scanning |Q| = 4π sin θ/λ with fixed values of Qy and Qz, using the automated routines of the diffractometer. For the rocking scans, the sample was rotated about its sample normal for fixed Q. Specular X-ray reflectometry implies scanning Qz for Qx = Qy = 0, which requires the incidence angle Ri to be equal to the exit angle Rf, rather than the 0.16 used for the GIXD scans. Spectroscopy. The absorptions studies were performed on a Perkin-Elmer Lambda 800 equipped with depolarizer and GlanTaylor polarizers. The fluorescence measurements of LB films were performed in front face geometry in an epifluorescence microscope equipped with a polarizer, a filter removing the excitation light and an Ocean Optics QE65000 diode array spectrometer .
Acknowledgment. This work was supported by the DanishChinese Center for Self-Assembled Molecular Nano-Electronics, funded by the Danish Basic Research Foundation, and the Danish Natural Science Research Council’s DANSCAT program. F.W. acknowledges the Knut & Alice Wallenberg foundation for funding. The HASYLAB staff is gratefully acknowledged for technical support. Supporting Information Available: Transfer and surface pressure versus layer profiles for LB films of ATOTA-1 and ATOTA-2; absorption and emission spectra for a 10 monolayer ATOTA-2 film. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la103785z
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