J. Phys. Chem. 1993,97, 13736-13741
13736
Spectroscopic Characterization of Aggregation Behavior in Hemicyanine Dye Monolayer and Multilayer Systems Q.Song, C. E. Evans, and P. W. Bohn’ Department of Chemistry and Beckman Institute, University of Illinois at Urbana-Champaign, 600 S. Mathews St., Urbana, Illinois 61801 Received: June 22, 1993: In Final Form: September 17, 1993”
The aggregation behavior of the hemicyanine dye N-(3-sulfopropyl)-4-(p-(dioctylamino)styryl)pyridinium, fabricated in Langmuir-Blodgett (LB) monolayer and multilayer systems and directly physisorbed onto quartz substrates has been studied by absorption and fluorescence spectrocopy. The LB monolayer films prepared on quartz substrates show entirely H-aggregate structures, characterized by a large blue-shifted (AI3 9000 cm-*) electronic transition at 340 nm, not only for the pure-dye system but also for the dye diluted in stearic acid at molar ratios down to 17%. The LB multilayer systems with pure dye show an identical absorption band (340 nm) to the LB monolayers. However, excitation of the aggregate bands yields fluorescence emission at 530 nm for monolayers, and two emission bands, at 400 and 530 nm, for multilayers. The presence of the blue emission band indicates that a twisted intramolecular charge-transfer (TICT) state is accessed in multilayer assemblies. In addition, submonolayer coverages of the dye on fused-quartz substrates are obtained by physisorption from C H C h solution. Similar to the LB film, physisorbed dye self-aggregates at the air-quartz interface with a rate of aggregation, which is critically temperature dependent. N
I. Introduction The properties and structure of organic molecules incorporated into monolayer and multilayer systems have been the subject of substantial research in recent years.lr2 The interest is motivated by the possible applications of these systems to optoelectronic and molecular electronic devices. With the development of the self-assembly (SA) and Langmuir-Blodgett (LB) techniques, organic molecules with unique properties can be incorporated into artificial molecular assemblies with preferred spatial and orientational order to achieve specific functional goals.3d As an example of the dichotomy between molecular and bulk properties, consider the development of efficient nonlinear optical devices based on organic chromophores. Although a large variety of organic molecules have large and fast nonlinear molecular polarizability, 8, many cannot be used, because they form centrosymmetric bulk materials, in which the individual effects are canceled, and the overall bulk second-order susceptibilities, ~ ( 2 are ) ~zero.’ To exploit the inherent nonlinear optical properties, such organic molecules need to be arranged in a highly ordered, noncentrosymmetric fashion. This structural constraint can be implemented in highly oriented Langmuir-Blodgett or selfassembled monolayer and multilayer thin films.*-I1 Much attention has been given to hemicyanine dye molecules containing long alkyl chains. This type of molecule not only has a large second-order polarizability,I2as well as a variety of other interesting molecular properties, but they also form very stable monolayer and multilayer films.I3 Due to these properties, hemicyanine monolayer and multilayer systems have been widely used in studies of second harmonic However, evaluation of the properties of hemicyanine monolayer and multilayer films is complicated by the formation of H aggregates; collections of closely packed molecules which have well-ordered orientations, associate with each other electronically, and act collectively to exhibit spectral and electronic properties distinct from that of the isolated monomer species. Since hemicyanine dyes have a strong tendency to aggregate in monolayer and multilayer systems, it is necessary to understand the nature of the ~
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To whom correspondence should be addressed.
* Abstract published in Aduance ACS Abstracts, November
15, 1993.
0022-3654/93/2097-13736S04.00/0
aggregationphenomenon,so monolayer and multilayer assemblies with controlled properties can be successfully designed and fabricated. Aggregation in monolayers and multilayers is a well-studied ~ h e n o m e n o n . ~Depending ~-~~ on the sign of the interactionenergy, aggregates are classified as H or J type. These two types of aggregates exhibit completely different spectroscopic behavior: H aggregates show a characteristic blue-shifted optical specJ aggregates display a band that is narrowed t r ~ m , ~whereas ”~ and red-shifted compared with monomer species.2s-28 A number of studies concerning the aggregation behavior of hemicyanine dyes have been reported. Schildkraut et al.” investigated the optical spectra of pure and mixed LB monolayersof dye1 (Scheme I) with fatty acids. The electronic spectra were found to be concentration dependent, with pure dye monolayers exhibiting only the blue-shifted band and diluted films showing only the monomer transition. Monolayers were also investigated using second harmonic generation (SHG), and the mixed monolayers were shown to have substantially larger second-order nonlinear susceptibility than pure dye monolayers. Young et and Anderson et aL9 studied multilayers of polymeric hemicyanine dyes and found only small amounts of aggregates in these systems. It was also found that counterions in the subphase substantially affected the packing of the coated LB films, favoring nonaggregated structure^.^^ Cross et al.I9 investigated SHG for LB monolayers of I and 11,observing significantlydifferent nonlinear susceptibilitiesfor the two dyes. Marowsky et al.I4studied SHG of LB monolayers of I and 111 and found a larger susceptibility in the former than in the latter. Differencesin the SHG behavior of hemicyanines have been attributed by different authors to a variety of factors including local field factors, effects of changing the chain lengths of the di-N-substituted alkyl chains on the electronicstructure of the chromophore, and aggregation-induced changes in the electronic structure. H-aggregate formation in hemicyanine monolayers has been shown to be very sensitive to preparation conditions, with small changes in preparation procedures substantially affecting the aggregation behavior.18J0J4 In a recent study from this laboratory, the hemicyanine dye N-(3-sulfopropyl)-4-(p-(dioctylamino)styry1)pyridinium (IV), was incorporated into LB monolayers and used for studying a variety of phenomena in organic thin (8
1993 American Chemical Society
Hemicyanine Dye Mono- and Multilayer Systems
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13737
SCHEME I
300
films.35936Like other hemicyanine dyes, IV has a strong transition in the optical region, which can be used as a spectroscopic probe. In contrast to other hemicyanine dyes (I, II, and III), IV is a zwitterion, which may be used to advantage in perturbing the structure of the thin film with an external electric field. Due to the zwitterionic headgroup, the properties, especially the aggregation behavior of the dye in monolayers and multilayers, are expected to differ appreciably from other hemicyanine dyes. Indeed, recent studies indicate that this molecule exhibits significant self-aggregationat the air-water interface.36 In this paper, we contrast the aggregation and spectroscopic behavior of IV in mixed monolayers and multilayers as well as in selfassembled submonolayers with that of dyes EIII.
II.
ExperimeatalSection
Sample Preparation. All Langmuir-Blodgett monolayer and multilayer samples were prepared on a KSV Instruments Langmuir-Blodgett trough (Model 5000). A 1 mM solution of IV (MolecularProbes) or appropriatemolar ratio of IV and stearic acid (Sigma Chemical Co.) was prepared in CHCl3 (Baxter Healthcare) and spread onto a deionized water subphase (Millipore Corp., Model Mini-Q UV plus). An initial surface concentration of 85 A2/moleculewas utilized in all experiments. Fifteen minutes was allowed for CHCl3 to evaporate before compression. The compression rates were typically 15 cm2/min, limited by a maximum surface pressure change of 1 mN/(m min). Monolayers at the air-water interface were transferred to 25 X 25 X 0.15 mm fused-silica substrates (Heraeus-Amersil) or 25 X 50 X 1 mm fused-quartz substrates (Quartz Scientific) at a surface pressure of 30 mN/m. The dipping rates were typically 15 mm/min for upstroke deposition and 2.5 mm/min for downstrokedeposition, resulting in reproducible transfer ratios (TR= 1.10 f 0.05). The preparation of mixed monolayers was identical to that of pure monolayers, except for a deposition rate of 12 mm/min in the former case ( TR= 1.1 f 0.05). Physisorbed submonolayer films were prepared by immersing 50 X 25 X 1 mm fused-quartz slides in a CHCl3 solution of IV for 2 min and then withdrawing slowly from the solution followed by rinsing with chloroform. Self-aggregation was then allowed to proceed at the air-quartz interface. In all cases, substrates were cleaned in three sequential hot H2S04 baths, each for 5 min, followed by rinsing and immersion in a 4:l mixture of NH40H:H202. After this, the substrates were rinsed in deionized water. Dye solutions and substrates were prepared the same day the LB films were coated. If substrates were not used immediately after cleaning, they were stored in deionized water to prevent contamination. All chemicals were used as received without further purification. Measurement. Absorption spectra were measured using a commercial double-beam grating UV-visible spectrophotmeter (Cary-3, Varian Instruments) with normal incidence to the surface. Absorption linear dichroism (LD) measurements, however, used an incident angle of 45O and were accomplished by positioning a Glan-Taylor polarizing prism (Melles-Griot) in
400
500
Wavelength (nm)
600
Figure 1. Absorption spectra for IV in a transferred LB monolayer on quartz substrate (a) and in methanol solution (3 X lo-' M) (b). The spectrum for the LB monolayer was measured immediately after
deposition. thesample beam to obtains- and p-polarized UV-visibleradiation. Fluorescence measurements were completed using a dualmonochromator system with a thermoelectricallycooled photomultiplier tube (Spex, Model DMlB). The incident angle for the excitation beam was 45O, relative to the surface normal of sample films, with collection of emission at 30° relative to the excitation beam. Absorbance measurements at the air-quartz interface were also performed using a multipass multichannel spectrometer to shorten the spectral acquisition time in kinetics runs (
