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Pseudoisocyanine J-Aggregate to Optical Waveguiding Crystallite Transition: Microscopic and Microspectroscopic Exploration Alexander N. Lebedenko, Gleb Ya. Guralchuk, Alexander V. Sorokin,* Svetlana L. Yefimova, and Yuri V. Malyukin Institute for Scintillation Materials, STC “Institute for Single Crystals”, 60 Lenin AVenue, 61001 KharkoV, Ukraine ReceiVed: March 30, 2006; In Final Form: July 12, 2006
Using fluorescent microscopy and microspectroscopy, optical properties and morphology transformations in individual pseudoisocyanine (PIC) J-aggregates in aqueous electrolyte solutions have been explored. A stringlike structure of J-aggregates with a string diameter much less than 1 µm has been observed. Photodestruction of the strings under short-wavelength excitation has been revealed. Rodlike PIC crystallites, about 1 µm in diameter, have been observed with time. The fluorescence spectrum of rodlike crystallites has been found to differ from that of stringlike J-aggregate and from PIC crystal powder spectra. The crystallites are very stable, and their photodestruction has not been observed under any excitation conditions. It has been found that rodlike crystallites in contrast to stringlike J-aggregates possess optical waveguide properties. The luminescence of crystallites can be observed only at the excitation spot and at butt ends located up to hundreds of micrometers from the excitation spot.
Introduction More than 70 years ago Jelley and Scheibe discovered independently that cyanine and merocyanine dyes can form ordered aggregates, which were named J-aggregates or Spolymers in honor of their discoverers. J-aggregates are assemblies of noncovalently coupled luminophores in the form of linear or circular chains, which, in turn, can form complex cylindrical patterns.1-3 J-aggregates are characterized by a narrow absorption band red-shifted with respect to the relevant monomer band. The main optical properties of J-aggregates such as superradiance and coherent exciton transport are welldescribed on the basis of a one-dimensional exciton model.1,2,4 However, J-aggregate structure and morphology have been poorly understood for a long time due to the impossibility of observing J-aggregates in solution in situ. Recently a series of experiments on J-aggregate structure exploration have been carried out using such advanced techniques as near-field scanning optical spectroscopy (NSOM), atomic force microscopy (AFM), cryotransmission electron microscopy (cryo-TEM), fluorescent microscopy, and microspectroscopy.3,5-16 1,1′-Diethyl-2,2′-cyanine (pseudoisocyanine, PIC) is one of the most studied organic dyes and forms J-aggregates both in aqueous solution and at the solid/liquid interface. Higgins and Barbara were among the first to investigate PIC J-aggregate structure in a poly(vinyl sulfate) (PVS) film using NSOM.5,6 PIC J-aggregates were found to form a supramolecular network from fibers with lengths of up to hundreds of micrometers and widths of about 50 nm. The localized photodestruction of the J-aggregates was used as a measure of the physical extent of exciton migration along the J-aggregate. The excitons were found to migrate less than about 50 nm from the NSOM tip. It should be noted that these data contradict results cited in ref 1, where strong quenching of PIC J-aggregate luminescence by energy traps was observed at the ratio PIC molecules:trap ) 2600:1. * Corresponding author. E-mail:
[email protected].
Using AFM, Yao et al. observed PIC J-aggregate formation at a mica/solution interface in solution.7 J-aggregates were shown to possess a three-dimensional island structure. The size of the islands ranges about 400-600 nm long, 80-100 nm wide, and 3-6 nm high. Von Berlepsch and co-workers studied PIC J-aggregate structure in aqueous solutions by cryotransmission electron microscopy (cryo-TEM) and polarized-light optical microscopy.9 They observed a homogeneous and closely packed network of long J-aggregate fibers, with fiber diameter of 2.3 nm and length on the order of 350 nm. It was revealed that J-aggregate structure strongly depends on dye concentration and at high concentration the transition to the more concentrated hexagonal phase was observed. They also found a formation of complex fiber bundles from isolated fibers for PIC J-aggregates in aqueous sodium chloride solution.10 It is clear now that the supramolecular structure of Jaggregates depends strongly on the type of dye, substitutes, and preparation conditions and can change in time (hours, days, and months). Moreover, despite a relatively long history of Jaggregate investigation, some questions concerning J-aggregate structure (i.e., aggregation number, geometric shape, and size) and the effect of structure on exciton properties and exciton energy migration still remain even for the most investigated PIC J-aggregates. In this paper we report on the structural transformation of pseudoisocyanine iodide (PIC) J-aggregates in aqueous electrolyte solution using fluorescent microscopy and microspectroscopic techniques, as it was found that adding electrolyte in aqueous solution can significantly affect J-aggregate morphology.8,10 Experimental Section 1,1′-Diethyl-2,2′-cyanine iodide (Sigma Aldrich) was used without further purification. Sample solutions were prepared by
10.1021/jp061965t CCC: $33.50 © 2006 American Chemical Society Published on Web 08/22/2006
PIC J-Aggregate Structural Transformation
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17773
dissolving PIC (0.5 mM) in an aqueous NaCl (0.2 M) solution under moderate heating (
