At low temperatures in the luminescence spectrum of pseudoisocyanine (PIC) J-aggregates formed in a layered polymer film an unusual broad red-shifted band appears. The analysis of spectral properties of PIC J-aggregates allowed us to ascribe the additional red band to the exciton self-trapped state. In a layered polymer film, PIC J-aggregates are found to possess a 2D island-like structure, which results in a barrier type of the exciton self-trapping with coexisting free and self-trapped excitons. Both the strong topological disorder and exciton−phonon coupling are suggested to be the reason for the exciton self-trapping in J-aggregates. Nonradiative relaxation of self-trapped excitons at room temperature has been proposed to be responsible for a very low luminescence quantum yield and giant nonradiative rate constant for PIC J-aggregates formed in a layered film.

Formation of the "J-aggregate-surfactant" complex for three cyanine dyes (L-21, LC-1 and PIC) in binary solutions containing cationic surfactant CPB at the concentration higher than the critical micelle concentration has been observed. The complex formation causes a significant increase of J-aggregate luminescence quantum yield and a decrease of radiative lifetime. The model of exciton self-trapping suppression in the "J-aggregatesurfactant" complex that causes changes of luminescence parameters has been proposed.

A formation of pseudoisocyanine (PIC) dye Jaggregates in the polyelectrolyte film by the layer-by-layer (LbL) assembly method has been studied. It has been shown that this process leads to significant J-band widening and fluorescence quenching as a result of increasing static disorder. To enhance the J-aggregate fluorescence properties, the effect of J-aggregate interaction with plasmon resonances of gold nanoparticles has been used. It is found that the maximal 8-fold fluorescence enhancement for PIC J-aggregates in the LbL films could be achieved at 16 nm distance between Au nanoparticles (NPs) and the J-aggregates. Plasmon influence on the J-aggregate fluorescence has been analyzed using a two-level system in the local plasmon field approximation. The model gives a good correlation with the experimental results and could be used for further studying the exciton−plasmon interaction in J-aggregates. ■ INTRODUCTIONWell-ordered molecular nanoclusters called J-aggregates attract great attention due to their unique optical properties, distinctly different from those of the individual molecules constituting the aggregate: narrow absorption band, high oscillator strength, giant third-order susceptibility, resonant fluorescence, etc. 1−4 Such optical properties of J-aggregates are explained by the strong interactions between the molecules within the aggregates. 1−4 The resulting delocalization of electronic excitations over certain molecules on the chain leads to the formation of collective eigenstates for all molecules, the exciton state (Frenkel exciton formation). 1−5 Depending on a type of molecular packing within the aggregate chain, one can observe a blue-shifted exciton band (H-band, the "face-to-face" arrangement), a red-shifted band (J-band, the "face-to-tail" arrangement), or both J-and H-bands due to the "herringbone"-type molecular packing. 1−4 The distinct feature of J-aggregates is a close correlation between J-aggregate excitonic properties and structure that opens up possibilities for the manipulation of J-aggregate optical characteristics by changing the condition of nanocluster formation. 1−4 Jaggregates have proved themselves as a perspective material for a number of applications such as photography, nonlinear optical devices, optical memory, and some others. 1−8 Low photostability in solutions resulting in photodegradation and photoreorganization processes is a considerable disadvantage of J-aggregates. 9,10 One of the ways to overcome this problem is using solid samples of J-aggregates especially in the form of polymer films suitable for many applications. 11−13 There are two main ways to form J-aggregates in polymer films: spin-coating 11−16 and layer-by-layer assembly (LbL). 17−19 Unfortunately, both ways cause a significant decrease in Jaggregate fluorescence quantum yield. 15,16,19 So, special efforts should be made to increase the fluorescence quantum yield of Jaggregates formed in polymer films.A very attractive way to improve the optical properties of Jaggregates is using the effect of ...