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 ...

In the present work controlled plasmon enhanced fluorescence of thiacyanine dye J-aggregates in water solution has been demonstrated. To control a distance between J-aggregates and silver nanoparticles the latter have been covered by a polymer shell of variable thickness using the layer-bylayer assembly method. The best 2-fold fluorescence enhancement has been observed for the 16 nm thick polymer shell. Transmission electron microscopy (TEM) images have revealed an insufficient contact between Jaggregates and NPs that could be the main reason for the small fluorescence enhancement. Experimental results have been described using a model of twolevel system affected by the local plasmon resonances field. According to the model more than 20-fold enhancement of J-aggregates fluorescence could be expected under optimal conditions. Besides, strong fluorescence enhancement dependence on an exciton coherence length has been predicted. According to it, significant fluorescence response should be observed for metal nanoparticles interacting J-aggregates with large exciton coherence length such as pseudoisocyanine J-aggregates and some others. ■ INTRODUCTIONIn recent decades, the field of nanoplasmonics dealing with localized surface plasmon (LSP) resonances in noble metal nanoparticles has attracted a considerable interest. 1−3 Strong enhancement of electromagnetic field near metal nanoparticles in combination with tunable large extinction in the visible and near-infrared region results in very attractive possibilities for manipulation by optical species properties such as the surfaceenhanced Raman scattering (SERS) 4,5 and plasmon-enhanced fluorescence (PEF). 2,3,6,7 The latter appears to be strongly dependent on a distance between fluorescent species and metal surface because both radiative and nonradiative decay rates are influenced by LSP. 1−3 As the nonradiative decay rate is usually caused by resonance energy transfer or electron transfer from fluorescent species to metal nanoparticles, it dominates on short distances and leads to strong fluorescence quenching. The distance increase can lead to more than 10-fold fluorescence enhancement due to the radiative decay rates enhancement. 1−3The exciton−plasmon interaction in composites based on excitonic materials such as quantum dots, 8,9 conjugated polymers 10 or molecular aggregates 11−19 could provide a much more interesting case study as compared to the plasmonic interaction with a localized excitation. For example, fluorescent molecular aggregates, the so-called J-aggregates, exhibit unique spectral properties such as narrow spectral bands, high extinction coefficients, and excellent nonlinear properties. 20−22 The PEF phenomenon could be very useful in order to improve further the spectral characteristics of Jaggregates and to design novel optical materials and devices. Jaggregate formation on metal nanoparticles leads to new hybrid electronic states appearing as a result of exciton−plasmon coupling. 13−18 However, such composites are not fluorescent due to nonradiat...

Using luminescent exciton traps, an efficiency of the exciton migration in J-aggregates of an amphiphilic analogue of pseudoisocyanine (amphi-PIC) dye in solutions, has been investigated. Applying a modified Stern−Volmer equation for analysis of the J-aggregate luminescence quenching by the trap, the quenching of 50% of amphi-PIC J-aggregate luminescence accessible for trapping at the ratio amphi-PIC/DiD = 120:1 has been revealed. To increase the exciton migration efficiency, the J-aggregate structure was improved by the formation of a “J-aggregate−surfactant” complex. The J-aggregate structure improvement is confirmed by the about 3 times increase in the exciton delocalization length that leads to the 1.3 times enhancement in the exciton migration efficiency in solutions with the surfactant. To the best of our knowledge, such a control of the exciton transport parameters in J-aggregates in solutions has not been demonstrated yet.

The diindolenine derivative of squaraine dye Sq-2Me has been used as an exciton energy trap (an energy acceptor) for amphi-PIC J-aggregates in a solution. Using the modified Stern-Volmer equation, parameters of the energy transfer have been obtained. It has been revealed that 50% of J-aggregate luminescence is quenched in a binary DMF/water solution at the ratio trap/amphi-PIC ) 1:80 that corresponds to the exciton migration over 20 delocalization segments within the J-aggregate.

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Using a luminescent exciton trap, a mechanism of the exciton migration in disordered J-aggregates of amphiphilic analogue of pseudoisocyanine (amphi-PIC) dye has been investigated in the temperature range of 80−300 K. Due to a strong topological disorder in amphi-PIC J-aggregates observed in a binary dimethylformamide−water (DMF/W) solution with a low water content, two types of excitonic states have been revealed, delocalized exciton state that forms the main part of the J-aggregates absorption band (J-band) and a state of strongly localized excitons that forms the long-wavelength edge of the J-band. These excitonic states are characterized by the different mechanism of the exciton transport: a coherent mechanism for delocalized excitons and an incoherent one for localized excitons. As localized excitons provide a small contribution to the J-band and appear only at high degree of topological disorder, the coherent mechanism of the exciton transport in amphi-PIC J-aggregates has been concluded. Such a result is nontrivial due to a small delocalization length of excitons in amphi-PIC J-aggregates (11 monomers at 80 K) provided by the moderate energetic disorder and strong exciton−phonon coupling.