Mesoscopic Organization of Two-Dimensional J ... - ACS Publications

Mesoscopic Organization of Two-Dimensional J-Aggregates of Thiacyanine in Langmuir-Schaefer Films. Chun-Hong Tian, Dao-Jun Liu, Roel Gronheid, Mark ...
0 downloads 0 Views 702KB Size
Download PDF
Langmuir 2004, 20, 11569-11576

11569

Mesoscopic Organization of Two-Dimensional J-Aggregates of Thiacyanine in Langmuir-Schaefer Films Chun-Hong Tian, Dao-Jun Liu, Roel Gronheid, Mark Van der Auweraer,* and Frans C. De Schryver Laboratory for Photochemistry and Spectroscopy, Department of Chemistry, Katholieke University Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium Received June 23, 2004. In Final Form: September 30, 2004 When dioctadecyl dimethylammonium bromide (DODAB) is compressed on a subphase containing 3,3′disulfopropyl-5,5′-dichlorothiacyanine (THIAMS), adsorption of the dye to the DODAB monolayer results in the formation of J-aggregates which spontaneously organize into polygonal domains of micron size. The features of the domains depend on the surface pressure. The fluorescence of the individual domains is polarized. The shapes of the domains determined by fluorescence microscopy and atomic force microscopy (AFM) are identical. The domains differ however significantly from those observed after injection of a 3,3′-disulfopropyl-5,5′-dichloro-9-ethylthiacarbocyanine (THIATS) or THIAMS solution below a precompressed DODAB film, as well as from the domains observed upon compression of a DODAB monolayer on a subphase containing 10-6 M THIATS.

Introduction Studies of dye aggregates have attracted considerable attention due to their role in photographic processes,1 energy transfer in photosynthesis,2 photodynamic therapy,3 laser technologies,4 and light-harvesting arrays in artificial photosynthetic systems.5 Most technological applications of dye aggregates exploit the delocalization of the Frenkel exciton over chromophores arranged in a one- or twodimensional structure.6,7 J-aggregates of cyanine dyes have been fabricated in a variety of matrixes, such as liquid solutions,8-10 rigid solutions,11-13 polymer films,14,15 silver halide emulsions,16,17 and polyelectrolyte multilayers.18-20 The Langmuir-Blodgett (LB) technique has been used to prepare * To whom all correspondence should be addressed. E-mail: [email protected]. Fax: +32(0)16 32 79 90. (1) Chatterjee, S.; Gottschalk, P.; Davis, P. D.; Schuster, G. B. J. Am. Chem. Soc. 1988, 110, 2326. (2) Bioenergetics of photosynthesis; Govindjee, Ed.; Academic Press: New York, 1975. (3) Dougherty, T. J.; Thoma, R. E. In Lasers in Photomedicine and Photobiology; Pratesi, R., Sachi, C. A., Eds.; Springer-Verlag: Berlin, Germany, 1980; p 67. (4) Maeda, M. Lasers Dyes; Academic Press: Tokyo, 1984. (5) Minuro, M.; Nozawa, T.; Tamai, N.; Shimada, K.; Yamazaki, I.; Lin, S.; Knox, R. S.; Wittmershaus, B. P.; Brune, D. C.; Blankenship, R. E. J. Phys. Chem. 1989, 93, 7503. (6) Knapp, E. W. Chem. Phys. 1984, 85, 73. (7) Makhov, D. V.; Egorov, V. V.; Bagatur’yants, A. A.; Alfimov, M. V. Chem. Phys. Lett. 1995, 246, 371. (8) Jelley, E. E. Nature 1936, 138, 1009. (9) Scheibe, G. Z. Angew. Chem. 1936, 49, 563. (10) Khairutdinov, R. F.; Serpone, N. J. Phys. Chem. B 1997, 101, 2602. (11) Cooper, W. Chem. Phys. Lett. 1970, 7, 73. (12) Scheblykin, I. G.; Varnavsky, O. P.; Bataiev, M. M.; Sliusarenko, O.; Van der Auweraer, M.; Vitukhnovsky, A. G. Chem. Phys. Lett. 1998, 298, 341. (13) Scheblykin, I. G.; Sliusarenko, O. Y.; Lepnev, L. S.; Vitukhnovsky, A. G.; Van der Auweraer, M. J. Phys. Chem. B 2001, 105, 4636. (14) Misawa, K.; Ono, H.; Minoshima, K.; Kobayashi, T. Appl. Phys. Lett. 1993, 63, 577. (15) Sluch, M. I.; Vitukhnovsky, A. G.; Yonezawa, Y.; Sata, T.; Kunisawa, T. Opt. Mater. 1996, 6, 261. (16) Tani, T.; Suzumoto, T.; Kemnitz, K.; Yoshihara, K. J. Phys. Chem. 1992, 96, 2778. (17) Muenter, A. A.; Brumbaugh, D. V.; Apolito, J.; Horn, L. A.; Spano, F. C.; Mukamel, S. J. Phys. Chem. 1992, 96, 2783.

J-aggregates formed by the adsorption of negatively charged cyanine dyes onto positively charged lipid monolayers at the air/water interface.21-25 It has been demonstrated that dye aggregate formation depends on the surface phase, the surface charge density of the amphiphilic monolayers,26 and the history of the sample.27-30 This could explain why different combinations of lipids and dyes induce different aggregate morphologies and spectral characteristics of the adsorbed cyanine dyes. The monolayers obtained in this way can be transferred to the surface of a suitable hydrophilic or hydrophobic substrate by the LB or Langmuir-Schaefer (LS) technique without changing the structure of the domains formed at the air/ water interface.31 Previously, J-aggregates formed by the adsorption of 3,3′-disulfopropyl-5,5′-dichlorothiacyanine sodium salt (THIAMS) to a compressed monolayer of the amphiphile dioctadecylammonium bromide (DODAB) were studied.26 Upon injection of an aqueous solution of THIAMS into the subphase under the compressed DODAB monolayer, 2D J-aggregate domains of THIAMS with distinct sizes and shapes were formed at different surface pressures. (18) Rousseau, E.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 2000, 16, 8865. (19) Rousseau, E.; Van der Auweraer, M.; De Schryver, F. C. Photochem. Photobiol. Sci., submitted for publication. (20) De Feyter, S.; Hofkens, J.; Van der Auweraer, M.; Nolte, R. J. M.; Mu¨llen, K.; De Schryver, F. C. Chem. Commun. 2001, 585. (21) Czikkely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11. (22) Bu¨cher, H.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 183. (23) Lehmann, U. Thin Solid Films 1988, 160, 257. (24) Bliznyuk, V. N.; Kirstein, S.; Mo¨hwald, H. J. Phys. Chem. 1993, 97, 569. (25) Kirstein, K.; Steitz, R.; Garbella, R.; Mo¨hwald, H. J. Chem. Phys. 1995, 103, 818. (26) Tian, C. H.; Zoriniants, G.; Gronheid, R.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 2003, 19, 9831. (27) Angelova, A.; Vollhardt, D.; Ionov, R. Thin Solid Films 1996, 284/285, 85. (28) Vranken, N.; Van der Auweraer, M.; De Schryver, F. C.; Lavoie, H.; Salesse, C. Langmuir 2002, 18, 1641. (29) Vranken, N.; Van der Auweraer, M.; De Schryver, F. C.; Lavoie, H.; Be´langer, P.; Salesse, C. Langmuir 2000, 16, 9518. (30) Vranken, N.; Foubert, P.; Ko¨hn, F.; Gronheid, R.; Scheblybin, I.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 2002, 18, 8407. (31) Harrison, W. J.; Mateer, D. L.; Tiddy. G. J. T. J. Phys. Chem. 1996, 100, 2310.

10.1021/la048449j CCC: $27.50 © 2004 American Chemical Society Published on Web 11/19/2004

11570

Langmuir, Vol. 20, No. 26, 2004

Figure 1. π-A isotherms of DODAB (‚ ‚ ‚) on a subphase of 10-6 M THIAMS aqueous solution and (s) on water. Inset: chemical structures of DODAB and THIAMS.

This effect was attributed to the different rigidity and charge density of DODAB monolayers compressed to different surface pressures.26 The results obtained with the larger dye 3,3′-disulfopropyl-5,5′-dichloro-9-ethylthiacarbocyanine (THIATS)28,29 indicated that the morphology of the aggregates observed on a micron size depends on the experimental procedure used to prepare the aggregates. We will discuss in this contribution the morphology of J-aggregates formed upon the compression of a DODAB monolayer on a subphase containing THIAMS. After deposition of the monolayers onto the hydrophobic glass surface by the LS technique, confocal fluorescence microscopy (CFM) and atomic force microscopy (AFM) were used to study the luminescent and topographic properties of the THIAMS J-aggregates in the LS films. It was found that THIAMS molecules formed J-aggregate domains with a totally different appearance (cfr. infra) to those formed by the adsorption of THIAMS to a compressed monolayer of DODAB.26 Experimental Section The dye THIAMS (its chemical structure is shown in the inset of Figure 1) was purchased from FEW Chemicals GMBH (Germany) and was used as received. The amphiphile DODAB (its chemical structure is shown in the inset of Figure 1) was purchased from Sigma-Aldrich (purity 95%) and recrystallized from CHCl3/Et2O before use to remove a fluorescence impurity. The water with a resistivity of 18.2 MΩ‚cm-1 and a pH of 5.5 used for all experiments and all cleaning steps was obtained by treating distilled water using a filter system with several filtration steps (Millipore, catalog no. CFOF 012 05). Dichlorodimethylsilane was purchased from Acros. Other solvents used in the experiments were of spectrophotometric grade and used as received. The deposited films were prepared on a commercially available LB trough (KSV Instruments Ltd., size 150 × 528 mm). The surface pressure was measured with a Wilhelmy type balance with a Pt plate. 60 µL of 10-3 M DODAB in chloroform solution was spread on the subphase of 10-6 M THIAMS aqueous solution. After 15 min of evaporation time of the chloroform, the DODAB film was compressed to the target surface pressure (15, 25, and 30 mN/m) at a speed of 5 mm/min (π-A isotherms are shown in Figure 1, dotted line). Compression took ∼30-45 min. As observed by Kirstein32 and Angelova27 and for a THIATS/DODAB Langmuir film,28 addition of THIAMS to the subphase shifts the onset of the lipid-expanded-liquid-condensed phase transition to a larger area per molecule (125 Å2/molecule) and a smaller surface pressure (3-4 mN/m). Then, the monolayers were deposited onto hydrophobic glass substrates by the LS technique after 30 min of adsorption time. All experiments were performed at 20.5 ( 0.1 °C. The methods to clean and hydrophobize the glass substrates and to deposit LS film have been described in detail previously.26,30 (32) Kirstein, S.; Mo¨hwald, H.; Shinomura, M. Chem. Phys. Lett. 1989, 154, 303.

Tian et al. Steady-state absorption spectra were recorded on a doublebeam ultraviolet-visible (UV-vis) spectrophotometer of the Lambda 40 series (Perkin-Elmer Instruments). Fluorescence spectra were recorded with a fluorimeter (Spex Fluorolog model 1691) excited at 458 nm in front face conditions (26°). The wide range fluorescence microscopy of the LS films was monitored visually via an optical microscope (Nikon OPTIPHOT-2) with an episcopic fluorescence attachment (Nikon EFD-3), and the micrographs were recorded using a digital camera (Nikon 995). A continuous wave (CW) argon-ion laser (Spectra physics, model 2025) was used for fluorescence excitation at 458 nm with circular polarized light. In the experiments, an MPlan dry objective lens (Nikon, NA 0.5, 40×), a dichroic mirror (DM 460, Nikon), and a notch filter (N 457.5, Kaiser Optics) that prevented the scattered light from passing into the registration port of the microscope were used. White light was used as a light source when measuring the polarized micrographs, where a polarizer was put between the light source and the sample while a perpendicular oriented polarizer was placed before the detector. The high-resolution fluorescence microscopy of the LS films was performed using a confocal microscope (Nikon Diaphot 200) with an oil immersion, high-numerical aperture lens (Nikon, NA 1.4, 60×). In the fluorescence polarization measurements, a 458 nm line of an argon-ion laser with circular polarization was used to excite the samples. A long pass filter (LP 460, Omega) and a glass plate (thickness 0.17 mm) were used instead of a dichroic mirror.26 The emission was split with a polarizing beam splitter cube and detected by two independent avalanche photodiodes (APDs, EG&G, Canada). The first APD detected the s-polarized component, and the second one detected the p-polarized component of the fluorescent light. For the combined measurements (spectra and micrographs), the linear polarized 410 nm line of a mode-locked and frequency-doubled picosecond Ti:sapphire laser (Tsunami Spectra Physics, 82 MHz repetition rate) pumped by an Ar+ laser (Beam Lock 2080 Spectra Physics) was used to excite the samples. A dichroic mirror of 450 nm (Kaiser Optics) and a 450 nm long pass filter (Omega) were used to suppress excitation light. The emission of the samples was split with a beam splitter cube, guiding 50% of the light to the liquid-nitrogencooled, back-illuminated charge-coupled device (CCD) camera (Princeton Instruments) and 50% to the APD. The exposure time for the acquisition of each spectrum was 5 s. The morphology and thickness of the films was investigated with atomic force microscopy (AFM) using a Discoverer TMX2010 AFM system (ThermoMicroscopes, San Francisco, CA) operating in noncontact mode using silicon probes (ThermoMicroscopes, San Francisco, CA) with a spring constant of 34-47 N/m and a resonance frequency of 174-191 kHz. A calibration silicon grating (TGZ01, pitch 3 µm, ∆z ) 26 ( 1 nm, MicroMasch, Tallinn, Estonia) was used to calibrate the piezo scanner. Measurements were done under ambient conditions. Image analysis was performed with Topometrix SPMLab 5.0 software. To determine the geometry of THIAMS and THIATS, ab initio calculations (6-31G** basis) were performed using the SPARTAN (Wavefunction Inc., Irvine, CA) package on 3,3′-diethyl-5,5′dichlorothiacyannine perchlorate and 3,3′,9-triethyl-5,5′-dichlorothiacarbocyanine perchlorate as model compounds for THIAMS and THIATS, respectively.

Results and Discussion Fluorescence Microscopy. The absorption and emission spectra of THIAMS/DODAB LS films deposited at 15 and 30 mN/m are shown in Figure 2. The absorption maxima of the films deposited at 15 and 30 mN/m are situated at 469 and 467 nm. This means that in the LB film the J-aggregate is shifted 4-6 nm to longer wavelengths compared to an aqueous solution of THIAMS.31 The emission maxima of both films are at 471 nm. The sharp bands (FW2/3M26 ) 430 ( 30 cm-1) and small Stokes shift (2 and 4 nm) indicate the formation of a J-aggregate of THIAMS in the LS films. Under otherwise the same experimental conditions, the absorbance and fluorescence intensities of the J-band were lower for the film deposited at 15 mN/m than for the film deposited at 30 mN/m.

J-Aggregates of Thiacyanine in LS Films

Langmuir, Vol. 20, No. 26, 2004 11571

Figure 2. Absorption (left) and emission (right, excited at 458 nm) spectra of THIAMS/DODAB LS films deposited at (‚ ‚ ‚) 15 mN/m and (s) 30 mN/m.

Figure 4. Transmission micrograph of a THIAMS/DODAB LS film deposited at 30 mN/m recorded with an epifluorescence microscope using crossed polarizers under irradiation by white light (size ∼200 × 200 µm).

Figure 3. Fluorescence micrographs of THIAMS/DODAB LS films recorded with an epifluorescence microscope deposited at (a) 15 mN/m and (b) 30 mN/m (excitation wavelength: 458 nm, size ∼200 × 200 µm).

Wide range fluorescence micrographs of the films were obtained by the epifluorescence microscope where excitation occurred by a CW argon-ion laser at 458 nm with circular polarized light. The fluorescence micrograph from the film deposited at 15 mN/m shows a homogeneous fluorescence with striation structures with increased intensity (Figure 3a). A fluorescence micrograph of the film deposited at a surface pressure of 30 mN/m (Figure 3b) shows a patchwork of polygonal intensely fluorescent domains with a size of 2-5 µm. Most adjacent fluorescence domains have a straight boundary. We no longer find the agglomeration of spindle-shaped domains formed upon the adsorption of THIAMS to a DODAB monolayer compressed to 30 mN/m26 or the large circular domains formed upon the compression of DODAB on top of a THIATS subphase.30 For the layer deposited at 30 mN/m, those domains can be seen even more clearly by looking at the transmission micrograph of the sample placed between two perpendicular polarizers (Figure 4). A pattern of quadrangular optical domains (or agglomerations thereof) with a size of several microns showing different color and brightness is observed. Such a pattern is according to Wolthaus33 characteristic of microcrystalline domains of different orientations. It is unclear to what extent this pattern points to birefringence as observed for lyotropic smectic phases of the same dyes.31,33-34 In the latter case, the sample contains a large number (up to at least several hundred) (33) Wolthaus, L.; Schaper, A.; Mo¨bius, D. Chem. Phys. Lett. 1994, 225, 322. (34) Tiddy, G. J. T.; Mateer, D. L.; Ormerod, A. P.; Harrison, W. J.; Edwards, D. J. Langmuir 1995, 11, 390.

Figure 5. Fluorescence micrographs of THIAMS/DODAB LS films deposited at (a) 15 mN/m (size 20 × 20 µm) and (b) 30 mN/m (size 40 × 40 µm), excited by a circular polarized laser at 458 nm (the arrows show the polarization direction of the detectors).

of dye layers in the focal plane of the microscope which differs strongly from the monolayers studied here. To investigate the structure of the domains with a larger spatial resolution and to obtain information upon the orientation of transition dipole moments, fluorescent images of the THIAMS J-aggregate in THIAMS/DODAB LS films were obtained using a confocal fluorescence microscope. Excitation occurred using a circular polarized laser at 458 nm, while the emission was detected by two APDs with perpendicular polarization, as indicated by the arrows in Figure 5. The fluorescence micrographs of the film deposited at 15 mN/m show bright domains with ellipsoidal shape. The size of the domains varies from about 2 × 3 µm to 10 × 20 µm (Figure 5a). The polarization of the fluorescence of the domains is rather homogeneous in contrast to the “butterfly” structures or “spherulite” structures observed for THIATS J-aggregates deposited

11572

Langmuir, Vol. 20, No. 26, 2004

Figure 6. Schematic structure of a domain of THIAMS J-aggregates formed at 30 mN/m.

at 30 mN/m using an otherwise identical experimental procedure.30 For the film deposited at 30 mN/m, a conglomerate of polygonal domains with straight boundaries was observed. The size of the domains is 10-20 µm in one dimension and 2-5 µm in the other dimension (Figure 5b). The different domains contain furthermore smaller granular structures, with a size of 1-3 µm, with significantly higher fluorescence intensity. Those spots are perhaps single crystalline aggregate domains with only a very small number of defects (cfr. infra). While in solution, Jaggregates of the related cyanine dye THIATS show a fluorescence quantum yield close to 1;12,13 this is for the same dye much less in Langmuir films28-30 or selforganized films.18,35 In the sample section shown, most domains have a “horizontal” orientation and show a higher intensity for “vertical” polarized light. This means that at least the orientation of neighboring domains is correlated. Although domains with similar shapes were also observed on a large scale, Figures 3 and 4 no longer indicate a preferred orientation of the domains. Within each domain, the fluorescence polarization is apparently constant. This suggests that most transition dipoles in each domain have the same orientation. Although in most domains the transition dipoles are perpendicular to the longest dimension, this observation is not valid for all domains. This means that the preferential growth direction of most domains is perpendicular to the transition dipole of the aggregate and the transition dipoles of the individual dyes therefore perpendicular to the molecular plane where the π-interactions will be maximal.36,37 The presence of a single narrow red-shifted absorption and emission spectrum is compatible with a 2D brick-wall38-40 packing. A possible structure for the domains is given in Figure 6. Fluorescence spectra recorded at different positions in the films with an excitation wavelength of 410 nm are shown in Figure 7. Sharp bands with a maximum at 472 nm and a long tail at longer wavelengths (dotted lines in Figure 7a and b, FW2/3M are 473 and 423 cm-1, respectively) were obtained from the center of bright domains in the films deposited at both 15 and 30 mN/m. The spectra have the same shape as those measured from THIAMS J-aggregates made by adsorption to a precompressed DODAB film,26 although we used another excita(35) Rousseau, E.; Van der Auweraer, M.; De Schryver, F. C. Photochem. Photobiol. Sci. 2002, 1, 395. (36) Baraldi, I.; Caseli, M.; Momicchioli, F.; Ponterini, G.; Vanossi, D. Chem. Phys. 2002, 275, 149. (37) Markov, I. V. Crystal Growth for Beginners; World Scientific Publishing Co.: Singapore, 1998. (38) Czikkelly, V.; Fo¨rsterling , H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207. (39) Domı`nguez-Adame, F.; Malyshev, V. A.; Rodriguez, A. J. Chem. Phys. 2000, 112, 3023. (40) Potma, E. O.; Wiersma, D. A. J. Chem. Phys. 1998, 108, 4894.

Tian et al.

tion wavelength (458 nm) for those samples. The emission spectra obtained from sites between bright domains in the film deposited at 15 mN/m (solid line in Figure 7a) shows beside the J-band at 472 nm a broad shoulder with three peaks at 510, 530, and 555 nm. The latter are attributed to the emission of THIAMS dimers or oligomers with a sandwich packing.19,41-43 U. Lehmann23 also found using reflection spectroscopy that the dye pseudoisocyanine (PIC) formed oligomers or dimers at low surface pressure. This indicates that, at a surface pressure of 15 mN/m, the adsorbed THIAMS molecules form beside J-aggregates (in the bright domains) also dimers, trimers, or oligomers (between the bright aggregate domains). For the emission spectra obtained from sites between the bright domains in a film deposited at 30 mN/m (solid line in Figure 7b), also a shoulder (500-550 nm) beside the J-band at 474 nm was found in the fluorescence spectrum. We also measured the emission spectra with excitation light of 410 nm in the center of the domains and between the big bright domains in films made by the adsorption of THIAMS to a precompressed DODAB monolayer.26 The spectra obtained in the center of the bright domains (not shown) are similar to the dotted line in Figure 7a and b. However, the spectra of the material between the bright domains, obtained upon excitation at 410 nm (Figure 7c and d), differ strongly from those obtained upon excitation at 458 nm (Figures 5 and 7 in ref 26). This we can attribute to the larger absorption cross section of monomers and dimers compared to J-aggregates at 410 nm.19 When we excited the films made by adsorption of the dye to a precompressed monolayer at 410 nm, the emission shows a broad band with a maximum at 492 nm for the film deposited at 15 mN/m and a broad band with maxima at 475 and 490 nm from the film deposited at 30 mN/m. This confirms our previous conclusion26 that at 15 mN/m a large fraction of the adsorbed THIAMS molecules remained as monomers and dimers in the dark areas between the luminescent domains. This indicates a larger density of defects in THIAMS J-aggregates formed by adsorption to a precompressed DODAB film (Figure 7c and d) compared to films formed by the compression of DODAB on a THIAMS subphase (dotted lines in Figure 7a and b). Atomic Force Microscopy (AFM). To study the morphology and thickness of the THIAMS/DODAB LS films, we investigated the AFM micrographs of films made at three different surface pressures: 15, 25, and 30 mN/ m. To interpret the AFM images of DODAB films deposited from a THIAMS subphase at different surface pressures, one has to consider the π-A isotherms. The slope of the π-A isotherm (dotted line in Figure 1) shows that when a DODAB monolayer is compressed on the THIAMS subphase to surface pressures of 15, 25, and 30 mN/m it is still in a liquid state. Waiting for ∼30 min before the onset of compression to stabilize the THIAMS/DODAB monolayer on the air/water interface and to equilibrate the adsorption of THIAMS molecules does not alter the shape of the π-A isotherm. The morphology of the film deposited at 15 mN/m (Figure 8a) shows a striation structure with some ellipsoidal or circular domains. The striations correspond with the structures seen in Figure 3a, while the ellipsoidal or circular domains correspond very well with the very intense domains seen in Figure 5a. This suggests they are the origin of the J-aggregate (41) Van der Auweraer, M.; Biesmans, G.; De Schryver, F. C. Chem. Phys. 1988, 119, 355. (42) Van der Auweraer, M.; Scheblykin, I. Chem. Phys. 2001, 275, 285. (43) Kuhn, H.; Mo¨bius, D.; Bu¨cher, N. In Physical Methods in Chemistry; Weissberger, A., Rossiter, B., Eds.; Wiley: New York, 1972; Vol. I, Part 3B.

J-Aggregates of Thiacyanine in LS Films

Langmuir, Vol. 20, No. 26, 2004 11573

Figure 7. Emission spectra recorded at different positions of THIAMS/DODAB LS films excited at 410 nm: (a) film deposited at 15 mN/m ((‚ ‚ ‚) in the center of bright domains, (s) between bright domains); (b) film deposited at 30 mN/m ((‚ ‚ ‚) in the center of bright domains, (s) between bright domains); (c) between bright domains in film deposited at 15 mN/m by the method of ref 26 (adsorption of THIAMS to a precompressed DODAB monolayer); (d) between bright domains in film deposited at 30 mN/m by the method of ref 26 (adsorption of THIAMS to a precompressed DODAB monolayer).

Figure 8. Topography images of a THIAMS/DODAB LS film deposited at (a) 15 mN/m, (b and c) 25 mN/m, and (d) 30 mN/m (size 15 × 15 µm).

fluorescence. The striations and the domains have a height of 5-6 nm (Figure 9a) which is larger compared to the height of the domains observed in LS films of DODAB and THIATS (4.5 ( 0.8 nm)30 or the films investigated by Kirstein.44,45 The observed height corresponds however to the height of the irregular interdomain structures observed in THIATS/DODAB LS films deposited at 30 mN/m. The wide areas uncovered by the LS film indicate that at 15 mN/m the film is not fully compressed. The average area of 0.85 nm2/molecule observed from the π-A curve (Figure 1) corresponds (at least in the deposited films) to areas that are complete uncovered and areas covered with a film of 6 nm height. (44) Tsukruk, V. V.; Renecker, D. H.; Bliznyuk, V. N.; Kirstein, S.; Mo¨hwald, H. Thin Solid Films 1994, 244, 763. (45) Kirstein, S.; Bliznyuk, K.; Mo¨hwald, H. Physica A 1993, 200, 759.

While the morphology of the THIAMS/DODAB LS film deposited at a higher surface pressure of 25 mN/m is in some areas (Figure 8b) very similar to that in the film deposited at 15 mN/m, in other areas, a more uniform film with holes and wide crevices (Figure 8c) is formed. This indicates that at 25 mN/m the LS film on the air/ water interface is still not fully compressed and in some areas the coverage of film is the same as that at 15 mN/m while in some other areas the coverage is much higher. Upon increasing the surface pressure to 30 mN/m, a more compact film with smaller holes and more crevices was observed (Figure 8d). This corresponds with the smaller average molecular area observed for the π-A isotherm (0.65 nm2). Also, in the films obtained at 25 or 30 mN/m, the thickness of the film is between 5 and 6 nm (Figure 9b and c). At 30 mN/m, furthermore, some small structures raising 3-3.5 nm above the film surface are observed. At 25 or 30 mN/m, the roughness of the domains is considerably smaller than that at 15 mN/m. The formation of the large circular and ellipsoidal domains observed at 15 mN/m using both AFM (Figures 8a and 9a) or scanning confocal microscopy (Figure 5a) can be related to the transport of DODAB and THIAMS to small monocrystalline and highly ordered domains (Ostwald ripening) in the fluid film. At higher surface pressures, the film becomes too rigid (viscous) to allow this transport. The polygonal and quadrangular structures observed at 25 mN/m using AFM correspond in shape and size to those observed at 30 mN/m using confocal microscopy. They can however no longer be observed at 30 mN/m using AFM. This indicates that the domains are closely packed and that some annealing of their boundaries occurred. At the (partially annealed) boundaries, the J-aggregates still contain a large number of defects, leading to decreased fluorescence intensities and a long wavelength shoulder in the emission spectra. At 25 mN/m and to a smaller extent at 30 mN/m, inside the domains (or the annealed domains), still some uncovered area is observed. This corresponds to the observation that the fluorescence of the different domains is not homogeneous. Whether this inhomogeneity of the fluorescence of the domains is limited

11574

Langmuir, Vol. 20, No. 26, 2004

Tian et al.

Figure 9. Expanded topography images (left) and line scans (right) recorded at the position of the horizontal line in the images of THIAMS/DODAB LS film deposited at (a) 15 mN/m, (b) 25 mN/m, and (c) 30 mN/m (size 5 × 5 µm).

to missing material (as indicated in Figure 8d or 9c) or by a larger defect density around the holes cannot be determined. Comparison of THIAMS J-Aggregates Formed on the DODAB Monolayer by Different Methods. In this contribution, the same dye (THIAMS) and amphiphile (DODAB) as in our earlier work were used.26 Hence, the different morphology and fluorescence of the J-aggregates of THIAMS should be attributed to the differences in the experimental procedure. In our earlier experiments,26 the dye was injected into the subphase after the DODAB monolayer was compressed to the target surface pressure, and the J-aggregates had to adapt to existing structures (liquid-condensed structures in a liquid-expanded matrix32) in the DODAB layer. This is supported by the resemblance of the dye microcrystallites formed at 15 mN/m and the shape and size of the structure of the condensed domains observed by fluorescence microscopy in DODAB films doped with a long chain substituted rhodamine after annealing at 20 mN/m.46 As the dye is injected under an already compressed monolayer, the large surface potential, especially at 30 mN/m, will lead to a large local concentration and supersaturation for the formation of 2D crystallites.26,32 This leads to a rapid and frequent formation of the 2D J-aggregates which can be considered as 2D crystallites and, hence, the large density of small spindle-shaped domains with a size smaller than the liquid-condensed domains of DODAB.46 (46) Shimomura, M.; Fujii, K.; Shimamura, T.; Oguchi, M.; Shinohara, E.; Nagata, Y.; Matsubura, M.; Koshiishi K. Thin Solid Films 1992, 210/211, 98.

As in this contribution, DODAB was compressed on the dye subphase and changes in packing of the DODAB and the dye occurred simultaneously. Due to the strong π-interactions36 between the dyes, one can expect that the dye molecules determined the packing of the organized domains of the dye and that the DODAB molecules followed the packing of the dye molecules, leading to the formation of much larger mesoscopic structures. The latter is suggested by the changes in the π-A isotherm in the presence of THIAMS. This correlates with the influence of the counterion on the morphology of DODAB monolayers as determined by Brewster angle microscopy (BAM).47 On the other hand, the small spectral shift of the aggregate compared to solution31 indicates that the DODAB film influences in its term the molecular packing and hence the exciton interaction. In this respect, THIAMS/DODAB is analogous to the combination studied by Kirstein et al.24,25,45 where the lattice of the alkyl chain of DODAB was epitaxial with the dye and where the “more rigid dye lattice”, although in itself exhibiting a peculiar defect structure, commands the arrangement of the “aliphatic tails”.45 While the size of the largest intensely fluorescent domains observed at 15 mN/m corresponds to those observed by Kirstein et al. for the adsorption of a similar 9-methylthiacarbocyanine dye before annealing,32 most domains are much smaller and resemble the domains initially observed using BAM upon the compression of (47) Ajuha, R. C.; Caruso, P.-L.; Mo¨bius, D. Thin Solid Films 1994, 242, 195.

J-Aggregates of Thiacyanine in LS Films

DODAB on a subphase containing an imidazolocarbocyanine.27 To what extent the stripelike structures observed in Figures 3, 8, and 9 are comparable with those observed by Angelova27 is less clear. While for THIATS/DODAB at 15 mN/m the area of the dark structures (no deposited material) is much larger than those containing deposited material, the situation is inverted for a floating film observed by Angelova. This suggests that at 15 mN/m the liquid-expanded part of the deposited film reorganizes after deposition. The smaller number of larger domains observed here at 30 mN/m compared to previous work26 indicates that nucleation already occurs at a much lower surface pressure and that upon further compression further dye adsorption to the domains occurs rather than the formation of new domains. At 30 mN/m (and a fortiori at 15 mN/m), the area per DODAB molecule (0.65 nm2) is considerably larger than twice the cross section of an alkyl chain (2 × 0.18 nm2),48 allowing either for some conformational freedom of those chains or for some tilt.44,49 As the cross section of a THIAMS molecule, which is adsorbed edgeon, amounts to 0.66 ( 0.05 nm2,50 nearly one THIAMS molecule is adsorbed per DODAB molecule. Following this line of thought, it is amazing that during the compression from 15 to 30 mN/m such strong reorganization of the micron-size dye assemblies could occur when a DODAB layer was compressed on a THIAMS subphase. Kirstein et al.25,32,45,51 observed upon first compression of DODAB in the presence of 9-methylthiacarbocyanine the formation of nearly circular domains, which were in contrast with our data for THIATS28-30 not polarized. Stopping the compression for 70 min after the initial compression resulted in the growth of tangentially oriented rod-shaped domains of 50 × 10 µm at the border of the circular domains.25,32,45,51 Upon decompression and recompression, those domains became predominant. Although the size and shape of those domains25,32,51 correspond to some extent with the domains we observe at 30 mN/m, they were more elongated and regular. Also the orientational correlation of the domains was more outspoken than observed for THIAMS at 30 mN/m.51 Furthermore, in agreement with the domains we observed, the fluorescence was polarized parallel to the short axis of the domains. Using 9-methylselenocyanine instead of 9-methylthiacarbocyanine led to the formation of shorter and broader domains.24,25 One should keep in mind that domains observed for 9-methylthiacarbocyanine or 9-methylselemocarbocyanine are made up of differently packed dye molecules, as they show an absorption spectrum characteristic of a herringbone aggregate.25,32 While cyanines (such as THIAMS)52,53 or carbocyanines with a 9-ethyl substituent adopt mainly a brick-stone packing, 9-methyl-substituted carbocyanines25 adopt a columnar packing (H-aggregates) or a herringbone packing.54 However, it is not yet clear how this has to be related to the mesoscopic structures of 2D J-aggregates. (48) Dutta, P.; Peng, J. B.; Lin, B.; Ketterson, J. B.; Prakash, M.; Georegopoulos, P.; Ehrlich, S. Phys. Rev. Lett. 1987, 58, 2228. (49) Okuyama, K.; Soboi, Y.; Iijima, N.; Hirabayashi, K.; Kunitake, T.; Kajiyama, T. Bull. Chem. Soc. Jpn. 1988, 61, 1485. (50) Calculated on the basis of an ab initio calculation on the parent 3,3-diethyl′-5,5′-dichlorothiacyanine perchlorate using 1.782 × 10-10 m as the van der Waals radius for chlorine. (51) Kirstein, S.; Mo¨hwald, H. Chem. Phys. Lett. 1992, 189, 408. (52) Yao, H.; Sugiyama, S.; Kawabata, R.; Ikeda, H.; Matsuoka, O.; Yamamoto, S.; Kitamura, N. J. Phys. Chem. B 1999, 103, 4452. (53) Ono, S. S.; Yao, H.; Matsuoka, O.; Kawabata, R.; Kitamura, N.; Yamamoto, S. J. Phys. Chem. B 1999, 103, 6909. (54) Janssens, G.; Touhari, F.; Gerritsen, J. W.; van Kempen, H.; Callant, P.; Deroover, G.; Vandenbroucke, D. Chem. Phys. Lett. 2001, 344, 1.

Langmuir, Vol. 20, No. 26, 2004 11575

For the corresponding nonmesosubstituted thiacarbocyanine on the other hand, quadrangular domains with a nearly 1/1 ratio of both axes were formed.25 In the latter case, the angle between molecules in neighboring columns in the herringbone aggregate are larger and the molecules have an orientation between edge-on and flat-on. For the meso-ethylsubstituted thiacarbocyanine THIATS on the other hand,28-30 micron-size circular domains with a central defect radial polarization of the emission were observed. J-aggregates of pseudoisocyanine (PIC) adsorbed at the mica/water interface also form elongated domains of micron size. While the shape and size of the domains formed at 0.1 mM correspond to those observed upon the adsorption of THIAMS to a DODAB layer compressed to 30 mN/m,26 the domains observed for a solution of 3.0 mM correspond to those observed upon the compression of a DODAB layer on a THIAMS subphase to 30 mN/m.52,53 However, contrary to the domains observed here at 30 mN/m, the fluorescence is polarized along the long axis.52,53 This growth in a direction where the π-interactions are not maximal was attributed to specific dye-mica interactions. AFM for these systems suggests the formation of multilayers with a thickness of 3-6 nm. This could also be the case for THIAMS, taking into account that the observed layer thickness of 5-6 nm (Figure 9) is much larger than the thickness of a DODAB layer (1.8 nm) and a THIAMS monolayer (1.5 nm).44 The fluorescence and AFM micrographs show that the THIAMS aggregates are less homogeneous than those of THIATS.28,30 This is probably due to the formation of crevices during deposition or the formation of energy traps.55,56 Both factors suggest that the aggregates of THIAMS have less cohesion or stability than those of THIATS.32 This is in contradiction with the fact that the dye concentration where J-aggregate formation occurs in aqueous solution is 100 times lower for THIAMS than for a molecule resembling THIATS.31 The stronger bonding and increased stability could be related to the larger area of the π-system,36 leading to larger van der Waals interactions. The strong difference between the morphology of THIAMS and THIATS aggregates could be related to the dihedral angle of