pubs.acs.org/Langmuir © 2009 American Chemical Society
Mixing Alternating Copolymers Containing Fluorenyl Groups with Phospholipids to Obtain Langmuir and Langmuir-Blodgett Films Thays C. F. Santos,† Laura O. Peres,† Shu H. Wang,‡ Osvaldo N. Oliveira Jr.,§ and Luciano Caseli*,† †
Laborat orio de Materiais Hı´bridos, Universidade Federal de S~ ao Paulo (UNIFESP), Diadema, SP, Brazil, Departamento de Engenharia Metal urgica e de Materiais, Escola Polit ecnica, Universidade de S~ ao Paulo (USP), S~ ao Paulo, SP, Brazil, and §Instituto de Fı´sica de S~ ao Carlos, Universidade de S~ ao Paulo (USP), S~ ao Carlos, SP, Brazil ‡
Received October 8, 2009. Revised Manuscript Received November 3, 2009 The control of molecular architectures may be essential to optimize materials properties for producing luminescent devices from polymers, especially in the blue region of the spectrum. In this Article, we report on the fabrication of LangmuirBlodgett (LB) films of polyfluorene copolymers mixed with the phospholipid dimyristoyl phosphatidic acid (DMPA). The copolymers poly(9,9-dioctylfluorene)-co-phenylene (copolymer 1) and poly(9,9-dioctylfluorene)-co-quaterphenylene) (copolymer 2) were synthesized via Suzuki reaction. Copolymer 1 could not form a monolayer on its own, but it yielded stable films when mixed with DMPA. In contrast, Langmuir monolayers could be formed from either the neat copolymer 2 or when mixed with DMPA. The surface pressure and surface potential measurements, in addition to Brewster angle microscopy, indicated that DMPA provided a suitable matrix for copolymer 1 to form a stable Langmuir film, amenable to transfer as LB films, while enhancing the ability of copolymer 2 to form LB films with enhanced emission, as indicated by fluorescence spectroscopy. Because a high emission was obtained with the mixed LB films and since the molecular-level interactions between the film components can be tuned by changing the experimental conditions to allow for further optimization, one may envisage applications of these films in optical devices such as organic light-emitting diodes (OLEDs).
Introduction The discovery of conductivity1 and electroluminescence2 in conjugated polymers has placed these materials in the forefront of research for a number of applications, including light-emitting diodes (LEDs), flexible displays, and solar cells. This has given birth to the new area of plastic electronics, which is promising for reaching a low manufacturing cost for electronic products in general. With regard to the electroluminescent devices, one major challenge was the identification of polymers with efficient emission in the blue region of the spectrum. Polyfluorenes (PFs) have been important in this respect3-6 owing to their good emission properties in the blue, in addition to good mechanical properties and processability, making them amenable to optoelectronic applications.7 Perhaps the most notable advantage of conjugated polymers such as PFs is the possible tuning of properties either by chemical modification or by a judicious choice of molecular architectures. The optical and mechanical properties of semiconducting polymers may be strongly affected by modifications of the side chains, especially due to changes in packing of the chains in *To whom correspondence should be addressed. E-mail: lcaseli@ unifesp.br. (1) Shirakava, H.; Irakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. J. Chem. Soc., Chem. Commun. 1977, 16, 578–580. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539–541. (3) Akcelrud, L. Prog. Polym. Sci. 2003, 28, 875–962. (4) Peres, L. O.; Errien, N.; Faulques, E.; Athalin, H.; Lefrant, S.; Massuyeau, F.; Wery, J.; Froyer, G.; Wang, S. H. Polymer 2007, 48, 98–104. (5) Cirpan, A.; Ding, L.; Karasz, F. E. Polymer 2005, 46, 811–817. (6) Veldman, D.; Ipek, O.; Meskers, S. C. J.; Sweelssen, J.; Koetse, M. M.; Veenstra, J. C.; Kroon, J. K.; van Bavel, S. S.; Loos, J.; Janssen, R. A. J. J. Am. Chem. Soc. 2008, 130, 7721–7735. (7) Gu, E.; Zhang, H. X.; Sun, H. D.; Dawson, M. D.; Mackintosh, A. R.; Kuehne, A. J. C.; Pethrick, R. A.; Belton, C.; Bradley, D. D. C. Appl. Phys. Lett. 2007, 90, 031116.
Langmuir 2010, 26(8), 5869–5875
the solid state. For instance, through functionalization at the 9-position of fluorene, it is possible to control interactions among chains, leading to tailored electrical and optical properties. This is the reason why poly(9,9-dioctylfluorene) (PFO) has become the prototypical member of a family with commercial potential as the active layers in organic light-emitting diodes.8 Distinct routes have been used to synthesize PFs, among which the Suzuki reaction involving carbon-carbon bond formation offers several advantages, as it is not affected by the presence of water and tolerates a wide range of functional groups. Fabricating ultrathin polymers has been proven effective to control at the molecular level their optical properties,9-12 particularly with the Langmuir-Blodgett (LB)13 and the layer-by-layer (LbL)14 techniques that allow precise control of thickness and film architectures. Usually, these techniques allow controlled fabrication of nanometer-scale objects at self-assembly interfaces.15-18 The use of these methods for PF derivatives is scarce, and we were (8) Bernius, M. T.; Inbasekaran, M.; O’Brien, J.; Wu, W. S. Adv. Mater. 2000, 12, 1737–1750. (9) Marx, K. A.; Samuelson, L. A.; Kamath, M.; Lim, J. O.; Sengupta, S.; Kaplan, D.; Kumar, J.; Tripathy, S. K. Mol. Biomol. Electron. 1990, 240, 395–412. (10) Marletta, A.; Goncalves, D.; Oliveira, O. N., Jr.; Faria, R. M.; Guimaraes, F. E. G. Macromolecules 2000, 33, 5886–5890. (11) Marletta, A.; Olivati, C. A.; Ferreira, M.; Vega, M. L.; Balogh, D. T.; Faria, R. M.; Oliveira, O. N., Jr. Braz. J. Phys. 2006, 36, 496–498. (12) Pavinatto, F. J.; Barletta, J. Y.; Sanfelice, R. C.; Cardoso, M. R.; Balogh, D. T.; Mendonca, C. R.; Oliveira, O. N., Jr. Polymer 2009, 50, 491–498. (13) Tsarkova, L. A.; Protsenko, P. V.; Klein, J. Colloid J. 2004, 66, 84–94. (14) Zhang, N.; Schweiss, R.; Knoll, W. J. Solid State Electrochem. 2007, 11, 451–456. (15) Ariga, K.; Hill, J. P.; Lee, M. V.; Vinu, A.; Charvet, R.; Acharya, S. Sci. Technol. Adv. Mater. 2008, 9, 014109. (16) Proust, A.; Thouvenot, R.; Gouzerh, P. Chem. Commun. 2008, 16, 1837– 1852. (17) Wang, Y.; Angelatos, A. S.; Caruso, F. Chem. Mater. 2008, 20, 848–858. (18) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Chem. Soc. Rev. 2008, 109–122.
Published on Web 11/18/2009
DOI: 10.1021/la9038107
5869
Article
Santos et al.
Figure 1. Structure
of poly(9,9-dioctylfluorene)-co-phenylene (copolymer 1) and poly(9,9-dioctylfluorene)-co-quaterphenylene) (copolymer 2).
able to find only three papers reporting on LB films with PFs. Worsfold et al.19 produced Langmuir and LB films of the commercial PFO, with an efficient transfer onto the solid substrates, according to the film characterization made with UV-vis absorption and photoluminescence spectroscopies. Ferreira et al. demonstrated that LB films could be transferred onto various types of substrate using two structures of PFs.20 In the paper in ref 20, the LB films were obtained from the Langmuir monolayers of the neat polymers, in spite of their hydrophobic nature. Park et al.21 showed that polyfluorene-b-polythiophene diblock copolymer adsorbed as Langmuir-Blodgett films exhibits spectral shifts due to changes in morphology. It is therefore possible that optimized properties may be obtained if mixed films are fabricated with PFs embedded in a protecting matrix in order to facilitate spreading at the air-water interface. This is a well-known strategy, especially using amphiphilic lipids.22 In this study, two new PFs whose structures are shown in Figure 1 have been used to form Langmuir monolayers along with dimyristoyl phosphatidic acid (DMPA). LB films could be deposited onto solid substrates, and they exhibited intense fluorescence emission, which makes them promising for applications in optical devices.
Materials and Methods Synthesis of the Polymers. The synthesis of poly(9,9-dioctylfluorene)-co-phenylene, referred to here as copolymer 1, was performed by mixing 3.475 g (6.342 mmol) of 9,9-dioctylfluorene-2,7-dibromofluorene (Aldrich 96%), 1.0 g (6.04 mmol) of 1,4phenylenebisboronic acid, and 0.001 g (0.009 mmol) of tetrakis(triphenylphosphine) palladium (P(Ph3)4Pd) (Acros 99%) in a 50 mL three-necked round-bottom flask with 15 mL of xylene under a nitrogen atmosphere, in a drybox. A pressure-equalizing dropping funnel with 10 mL of K2CO3 solution (2 mol/L) and a reflux condenser were attached to the reaction flask, in which the mixture was allowed to react under the nitrogen atmosphere and magnetic stirring for 48 h in the dark. Then 1.65 g (6.04 mmol) of phenylboronic acid was added, and the reaction proceeded for an additional 24 h. The solution was filtered and washed with chloroform and methanol. The solvent was evaporated to dryness, and the yellow residue was characterized as copolymer 1. η = 69%. 1H NMR (CDCl3, ppm): δ = 7.8-7.3 (m, 10H, Ar-H); 1.98-1.9 (m, 4H, CH2); 1.05 (s, 24H, CH2); 0.8-0.5 (19) Worsfold, O.; Hill, J.; Heriot, S. Y.; Fox, A. M.; Bradley, D. D. C.; Richardson, T. H. Mater. Sci. Eng., C 2003, 23, 541–544. (20) Ferreira, M.; Olivati, C. A.; Machado, A. M.; Assaka, A. M.; Giacometti, J. A.; Akcelrud, L.; Oliveira, O. N., Jr. J. Polym. Res. 2007, 14, 39–44. (21) Park, J. Y.; Koenen, N.; Forster, M.; Ponnapati, R.; Scherf, U.; Advincula, R. Macromolecules 2008, 41, 6169–6175. (22) Caseli, L.; Nobre, T. M.; Silva, D. A. K.; Loh, W.; Zaniquelli, M. E. D. Colloids Surf., B 2001, 22, 309–321.
5870 DOI: 10.1021/la9038107
(m, 6H, CH3). The synthesis of poly(9,9-dioctylfluorene-co-pquaterphenylene), referred to as copolymer 2, was prepared as described in the literature.4 η = 84%. 1H NMR (CDCl3, ppm): δ = 7.8-7.2 (m, 22H, Ar-H); 1.98 (m, 4H, CH2); 1.08 (s, 20H, CH2); 0.81 (m, 6H, CH3). Both of the polymers are readily soluble in organic solvents such as chloroform and toluene. Characterization. The UV absorption spectra were collected on a Varian Cary 50 spectrophotometer. Copolymer samples were dissolved in chloroform, with the solution being poured into a 10 mm square quartz cell. The transmission spectrum was collected in the range λ = 200-600 nm. The photoluminescence (PL) spectra in chloroform solution were taken with a Varian Eclipse fluorescence spectrophotometer, with excitation at the wavelength of the maximum absorption according to the UV-vis spectra. The molar mass and the polydispersity were determined by gel permeation chromatography (GPC) using a Waters HPLC system comprising two columns in series, PLgel mixed-B and PLgel mixed-C, at 35 °C and with 1 mL/min of tetrahydrofuran (THF) as eluent. GPC measurements were carried out using monodisperse polystyrene samples as standards. Further details on the polymer characterization can be found in ref 4. Spreading of Langmuir Monolayers. Ultrapure water supplied by a Millipore system, with resistivity ≈ 18.2 MΩ cm, pH ≈ 5.5-6.0, surface tension ≈ 72.5 mN/m, was used as the subphase. PFs and DMPA (Avanti Polar Lipids) were dissolved in chloroform to yield a 0.5 mg/mL solution. Aliquots of these solutions were spread carefully drop by drop on the surface of the aqueous subphase. After the spreading, 10-15 min were allowed for solvent evaporation, before starting the film compression with two movable barriers at a rate of 25 cm2/min. The surface pressure and the surface potential were measured during compression using a Wilhelmy plate and a Kelvin probe, respectively. The mixed PF-DMPA Langmuir films were formed by first spreading the DMPA monolayer and then the polymer solution onto DMPA. A KSV mini-trough (System 2, total volume of 220 mL) was employed for surface-pressure-mean-moleculararea and surface-potential-mean-molecular-area isotherms. Images of the air-water interface were obtained using a Brewster angle microscope (BAM2Plus system from Nanofilm Technology) adapted to a NIMA Langmuir trough, model 601 M, with total volume of 300 mL. All monolayers were produced at a constant temperature of 23 ( 0.1 °C. DMPA was chosen because of its simple structure and its easiness to be deposited in multilayers as LB films.23 Deposition as LB Films. Quartz slides or an AT-cut quartz crystal coated with Au (Stanford Research Systems Inc.) were used as substrates for the transfer of the LB films. The quartz substrates were cleaned by treating with a KOH 5% ethanol solution in an ultrasonic bath for 5 min. The LB film transfer was performed with a dipping rate of 5 mm/min and a constant surface pressure of 20 mN/m, with the first layer obtained by raising the substrate from the aqueous subphase. At this surface pressure, the monolayer is already in the liquid condensed phase and can be transferred properly. For multilayer Y-type LB films, an interval of 20 min elapsed before the subsequent dipping with the plate at the most upward position for drying. The LB films were characterized using UV-vis spectroscopy (Hitachi U 2001), fluorescence spectroscopy (Shimadzu RF-5301PC), and a quartz crystal microbalance (QCM, Stanford Research Systems Inc.). All LB films were produced at a constant temperature of 23.0 ( 0.1 °C.
Results and Discussion Langmuir Monolayers. Both copolymer 1 and copolymer 2 are hydrophobic. Copolymer 1 does not spread by itself on the air-water interface to form Langmuir monolayers. In order to produce stable films of this polymer, the Langmuir films were prepared by first spreading DMPA on the interface, which is easy (23) Engel, M.; Riegler, R. Makromol. Chem. 1991, 46, 395–400.
Langmuir 2010, 26(8), 5869–5875
Santos et al.
Figure 2. Surface pressure (A) and surface potential (B) isotherms for DMPA monolayers on water mixed with copolymer 1. Insets show the expansion caused by an increasing amount of copolymer spread (1.0 μmol of unit/L).
to transfer as multilayers and serves as a support of LB films.23 Then the copolymer solution was spread onto the already-formed DMPA film at a zero surface pressure, thus yielding a stable mixed Langmuir monolayer. Figure 2A shows the surface pressure isotherms for DMPA with varying amounts of PF cospread. The incorporation of the copolymer affected the mean molecular area of the amphiphilic molecules, causing a monolayer expansion. For the pure DMPA monolayer, a typical liquid-expandedto-liquid-condensed transition was observed at approximately 45-70 A˚2, featuring a plateau with a surface pressure of about 5 mN/m,24 which does not alter even with addition of copolymer 1. This indicates that probably these two components tend to be phase-separated. In contrast, the corresponding pressure seems to decrease upon addition of copolymer 1 to DMPA, which may suggest some strong interactions between these components. Interestingly, when the copolymer was introduced, a new plateau appeared at 14-17 mN/m. An intermediate transition state has been reported on DMPA monolayers with Mn(II) porphyrins incorporated,25 which was ascribed to conformational changes of the heme group, leading to a pyramidal configuration upon compression. In our case, after the main transition at 10 mN/m, a region of high compressibility existed, which indicates that the liquid-condensed state had not been reached. One then infers that the first plateau at lower surface pressures occurs (24) Ahuja, R. C.; Caruso, P. L.; Mobius, D. Langmuir 1993, 9, 1534–1544. (25) Caseli, L.; Vinhado, F. S.; Iamamoto, Y.; Zaniquelli, M. E. D. Colloids Surf., A 2003, 229, 169–180.
Langmuir 2010, 26(8), 5869–5875
Article
between two liquid-expanded phases, while the liquid-condensed phase is only attained at high surface pressures (>17 mN/m). The high surface pressure is indicative of a more condensed state due to the addition of a bulky material (the copolymer), with interpenetration within the loosely packed DMPA molecules. A similar behavior was observed for natural polymers mixed with DMPA monolayers.26,27 Note also from Figure 2A that, with an increasing concentration of the copolymer, the onset of the collapse appears at an increasing area per DMPA molecule; so, with a higher amount of copolymer, the monolayer tends to collapse at lower molecular areas. The scattering points in the inset graphs of Figure 2 are related to the expansion of DMPA molecules caused by copolymer molecules that are flexible and do not spread well on water. Therefore, the molecular area for DMPA does not grow exactly linearly. The surface-potential-area isotherm is typical of pure DMPA on water,23 varying from negative (∼ -125 mV) to positive (∼200 mV) values. The negative surface potential at large areas is due to the contribution of the double layer. As Figure 2b shows, the introduction of the copolymer led to increasing values of surface potential. This increase can be ascribed to various factors, including the contribution from the stacking of the polymer phenyl rings, tilting of the DMPA dipoles, interaction of the polymer with DMPA tails, and reorganization of the water molecules close to the interface. With the surface potential data alone, it is not possible to distinguish between these possibilities. The change in slope in the region of smaller areas indicates the onset of collapse. We should emphasize that the monolayer features in Figure 2 were preserved when the procedures for preparing the films changed. For instance, no significant changes were observed upon cospreading the lipid and the copolymer at the interface in the same solution (premixing them before spreading) or by compressing at different compression rates (5-150 cm2/min). Furthermore, surface-pressure-area isotherms for successive cycles of compression-decompression were reproducible for many cycles. We also did not observe any change in the morphology and in the first and second cycle of compression by using Brewster angle microscopy. Therefore, the monolayers with the lipid-polymer mixture are kinetically stable. By contrast, copolymer 2 spread out easily over the surface when the polymer solution was deposited on the air-water interface, as could be observed visually during the drop-by-drop deposition procedure. Also, Figure 3 shows typical surfacepressure-area and surface-potential-area isotherms, as expected for an insoluble Langmuir monolayer. There is no obvious explanation why copolymer 2 forms Langmuir monolayers on its own and copolymer 1 does not. It is possible that the conformation imposed to the macromolecule by the additional phenyl groups may facilitate spreading at the air-water interface. The low molecular area is reasonable, since its random coil polymeric structure allows two or more repeating units to occupy the same section of area projected on the air-water interface. A region with high relative compressibility was observed between 11 and 7 A˚2, with surface pressure ranging only from 8 to 10 mN/m. This region may be ascribed to a phase transition from the liquidexpanded to the liquid-condensed state. The monolayer can withstand high surface pressures, with the collapse at 59 mN/m. (26) Pavinatto, F. J.; Pavinatto, A.; Caseli, L.; dos Santos, D. S., Jr.; Nobre, T. M.; Zaniquelli, M. E. D.; Oliveira, O. N., Jr. Biomacromolecules 2007, 8, 1633– 1640. (27) Pavinatto, F. J.; Caseli, L.; Pavinato, A.; dos Santos, D. S., Jr.; Nobre, T. M.; Zaniquelli, M. E. D.; Silva, H. S.; Miranda, P. B.; de Oliveira, O. N., Jr. Langmuir 2007, 23, 7666–7671.
DOI: 10.1021/la9038107
5871
Article
Santos et al.
Figure 3. Surface-pressure-area and surface-potential-area isotherms for copolymer 2.
Figure 4. Surface pressure (A) and surface potential (B) isotherms for DMPA monolayers on water mixed with copolymer 2. Insets show the expansion caused by increasing amounts of the copolymer (0.7 μmol of unit/L).
The surface potential isotherm had a critical area of 20 A˚2, below which the potential increased sharply up to 190 mV. This value was reached already in the so-called gaseous phase (or transition between the gaseous and the liquid-condensed phases) and remained constant until the collapse. Therefore, the effective, normal component of the dipole moment, μ (μ = ΔV/εε0A, where ΔV is the surface potential, ε is the effective dielectric constant of 5872 DOI: 10.1021/la9038107
Figure 5. BAM images (430 642 μm2) for DMPA monolayer at 5 mN/m (A); DMPA-copolymer 1 monolayer at 5 mN/m (B) and 15 mN/m (C); copolymer 2 monolayer at 5 mN/m (D) and 20 mN/m (E); and DMPA-copolymer 2 monolayer at 5 mN/m (F).
the monolayer, ε0 is the vacuum permittivity, and A is the molecular area), decreased upon compression, indicating some molecular reorientation of the polar groups of the copolymer. Unfortunately, no quantitative analysis can be made for monolayers of copolymers, as one cannot identify the contributions of all dipoles. We also observed that the lifting point for the surface-potentialarea isotherm is much larger than that for the surface-pressurearea isotherm. Surface potential is more sensitive than surface Langmuir 2010, 26(8), 5869–5875
Santos et al.
Article
Figure 6. Adsorbed mass measured with a quartz crystal microbalance versus number of layers of the LB films made with the copolymers on quartz crystals (0.67 cm2). Five micrograms of copolymer was utilized for the mixed monolayers with DMPA.
Figure 8. Emission spectra in solution (CHCl3), as cast film, and as LB film of copolymer 1 (A) and copolymer 2 (B).
Figure 7. Absorption spectra in solution (CHCl3), as cast film, and as mixed DMPA LB film of copolymer 1 (A) and copolymer 2 (B). Inset shows the spectrum for the LB film of neat copolymer 2.
pressure because it indicates the orientation of dipoles, which occurs in larger molecular areas. The incorporation of copolymer 2 into DMPA monolayers induced a more significant expansion than for copolymer 1, as shown in Figure 4, probably owing to the higher molecular weight of its repeating unit. However, there was no additional phase transition, with the shape of the isotherm resembling that of neat DMPA. There was a slight increase in the compressibility in the liquid-condensed region and collapse at 50 mN/m. This reveals Langmuir 2010, 26(8), 5869–5875
that the copolymer causes the monolayer to be less resistant to compression; that is, the surface pressure increased less steeply with the decrease in area. Also, the incorporation of copolymer 2 increased the surface potential to positive values at larger molecular areas, with the shape of the isotherm being the same as that for neat DMPA. The maximum surface potential was within 180-250 mV for all cases. The morphology of the monolayers was examined using Brewster angle microscopy (BAM). Typical domains of neat DMPA monolayers at the transition surface pressure are seen in Figure 5A, with liquid-condensed domains dispersed in liquid-expanded ones.28 Figure 5B shows highly dense domains with strong reflectivity for mixed monolayers of DMPA and copolymer 1, indicating a considerable effect from the copolymer on the phase transition of DMPA. For the second surface pressure transition at ca. 15 mN/m, fractal domains are observed in Figure 5C, which have lower reflectivity than the domains at 5 mN/m. It seems that at 15 mN/m the copolymer molecules are better dispersed at the interface, probably being entrapped by the DMPA molecules forming a highly packed structure. Reorientation of the rigid platelet segments in the copolymer and interactions between lipid and polymer may take place, which explains the fractal shape of the domains, pointing to the existence of intermolecular repulsions among the molecular components of the interfacial film. (28) Wu, F.; Gericke, A.; Flach, C. R.; Mealy, T. R.; Seaton, B. A.; Mendelsohn, R. Biophys. J. 1998, 74, 3273–3281.
DOI: 10.1021/la9038107
5873
Article
Santos et al.
Figure 9. Emission spectra of copolymer 2 (A) and mixed with DMPA (B). Insets show the increase in fluorescence with the number of deposited layers.
The molecular weights of copolymers 1 and 2 are 723 and 494 g/mol, respectively. The fact that the mass for copolymer 2 is lower can be one of the reasons that it forms Langmuir monolayers even without the presence of the phospholipid matrix. For copolymer 2, a high contrast of domains is observed at low surface pressures in Figure 5D, possibly reflecting the aggregation of the copolymer at the air-water interface. Upon compression and ensuing decrease in the available area, the molecules tend to self-organize. Distinguishing the domains via reflectivity is more difficult now, as illustrated in Figure 5E. Additionally, Figure 5F shows that copolymer 2 affected the morphology of the DMPA monolayer in the phase transition region, with more interconnected domains being observed. This is probably due to the coalescence of liquid-condensed domains of the lipids, which involves the self-organization of the polymer around the lipid molecules. In other words, DMPA provides a suitable matrix for copolymer 1 to form a stable Langmuir film while enhancing the capacity of copolymer 2 to form insoluble monolayers at the air-water interface. Langmuir-Blodgett Films. The monolayers from neat copolymer 2 and from mixtures with DMPA could be transferred from the water surface onto solid supports, as confirmed with nanogravimetric measurements on Y-type LB films, deposited with the substrate initially submerged in the aqueous subphase. Figure 6 shows the steady increase in mass as each monolayer was transferred up to seven layers. The transfer ratio varied from 0.98 to 1.09 for these layers, which reflects the regularity of the transfer process. If one attempted to transfer more than seven layers, little mass change was measured, and the transfer ratio dropped to the range 0.05-0.25. This result indicates a limit of seven layers for the transfer process described here. Since 109.5 ng of DMPA is transferred to a solid substrate with the same area as used here when the lipid is alone at the interface,22 the mass transferred per layer was estimated roughly as 400 ng for copolymer 1 and 150 ng for copolymer 2, both when mixed with DMPA. A pure copolymer 2 monolayer was deposited at the solid support at an average mass of 300 ng per layer. The higher mass transferred for copolymer 1 mixed with DMPA, in comparison to copolymer 2, is probably due to more significant aggregation for copolymer 1. Figure 7 shows the optical absorption of copolymers 1 and 2 in solution (CHCl3) and in cast films and LB films with DMPA. For copolymer 1, the spectrum of the cast film is the broadest, with the peaks at 285 and 315 nm, assigned to the πfπ* transitions for the aromatic ring, almost amalgamated with each other. The spectrum in solution is similar to the one in the cast film, but narrower 5874 DOI: 10.1021/la9038107
and with the peaks well-defined. The differences are ascribed to aggregation in the cast film. For the LB film, in contrast, a fine structured spectrum is observed with well-defined peaks, which indicates that the DMPA matrix prevented copolymer 1 from aggregating strongly. If one takes the onset for the transitions (at the edge) as representing the band gap energies (Eg), one obtains 3.34, 3.47, and 3.73 eV for the copolymer 1 as cast film, in solution, and as LB film, respectively. The most distinguishing feature of the absorption spectra in Figure 7B for copolymer 2 is the similarity between the spectra for the LB and cast films. All spectra presented a single broad band, indicating a wide distribution of electron energy states, irrespective of the aggregation origin from cast or LB film, and both concomitantly red shift in comparison with the solution spectrum, from 345 to 352 nm. As described earlier,4 the peak at 345 nm is already red-shifted in comparison with the monomers, namely, 300 nm for 4,40 -dibromo-p-quaterphenyl and 315 and 285 nm for 9,9-dioctylfluorene-2,7-diboronic acid. The onset for the transitions at 315-285 nm region also differs from that for copolymer 1, but in this case the band gap energies are practically the same for the cast and LB films (2.95 eV), while in the CHCl3 solution it is 3.15 eV. The red shift in the spectra for copolymer 2, in comparison with copolymer 1, may be attributed to their different structures of the polymers and their different molar mass. The fluorescence emission spectra of copolymers 1 and 2 in Figure 8 were collected in CHCl3 solutions and as cast and LB films. For copolymer 1, with one aromatic ring, the curves have different profiles and shifted maxima (360 and 380 nm in CHCl3, 380 for LB film, and 400 and 420 nm for the cast film), according to the aggregation degree. Moreover, a broad band appears for the LB film at wavelengths longer than 500 nm, which can be due to a scattering light effect. For copolymer 2, no significant difference was observed with the shape of the emission band remaining the same. There was nevertheless a red shift in the emission peak, from 400 nm in solution to 418 nm for the LB and cast films. The similarity of the spectra for LB films and cast films from copolymer 2 was preserved when DMPA was added. A comparison of the emission spectra collected from copolymers 1 and 2 showed that the latter one is red-shifted, that can be attributed to the new structure of the polymer when adsorbed as LB films with DMPA. The emission properties of the LB films from both copolymers indicate that they are suitable for optical applications. Efficient emission was indeed expected on the basis of the reported quantum yields of 86.4% and 55% for copolymer 2 Langmuir 2010, 26(8), 5869–5875
Santos et al.
and copolymer 1, respectively.4 The lower yield for copolymer 1 is attributed to a smaller number of aromatic rings. We studied the increase in intensity with the number of deposited layers in LB films made with copolymer 2. Figure 9 shows the emission spectra, with the insets illustrating an almost linear increase in intensity at 418 nm with the number of deposited layers. Interestingly, the slope in Figure 9B is higher than that in Figure 9A, which points to a positive effect from the mixture of the copolymer with DMPA. Apparently, DMPA forms a protecting matrix, thus enhancing the fluorescence of copolymer 2, which can be due to a deaggregation effect. In subsidiary experiments, we observed that the emission intensity also increased linearly with the number of layers for the mixed LB film with copolymer 1 and DMPA (results not shown).
Langmuir 2010, 26(8), 5869–5875
Article
Conclusions We have demonstrated that the phospholipid DMPA serves as a matrix to form stable Langmuir films with two fluorene copolymers, with molecular-level interactions between the components leading to marked changes in film properties. The LB films with up to seven layers transferred from mixed monolayers exhibited optical (photoluminescence) properties that could be enhanced and controlled. These films can be used in the future as active layers in light-emitting devices, with the emission properties tuned by varying the relative concentration of DMPA in the mixture with the copolymer. Acknowledgment. This work was supported by CNPq, Capes, and FAPESP (Brazil).
DOI: 10.1021/la9038107
5875
