Article pubs.acs.org/JPCC
Evidence of Exciton Self-Trapping in Pseudoisocyanine J‑Aggregates Formed in Layered Polymer Films Alexander V. Sorokin,* Nikita V. Pereverzev, Irina I. Grankina, Svetlana L. Yefimova, and Yury V. Malyukin Institute for Scintillation Materials, STC “Institute for Single Crystals”, NAS of Ukraine, 60 Lenin Avenue, 61001 Kharkov, Ukraine S Supporting Information *
ABSTRACT: 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.
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INTRODUCTION Exciton self-trapping is a very important phenomenon in photophysics of molecular crystals, alkali halides, semiconductors, and other excitonic materials.1−5 It is well known that because of exciton−phonon interaction in deformable lattices exciton causes a displacement of molecules or ions from their lattices sites forming polarons.1−3 Depending on a lattice type and exciton−phonon interaction strength, polarons could be large or small. Typically, large polarons are formed in polar crystals due to the exciton interaction with the optical phonons and are described using Fröhlich potential, while small polarons form in molecular crystals due to the exciton interaction with acoustical phonons and are described using the deformation potential.3 When the lattice distortion caused by the excitons is too large, the excitons appear to be trapped in a self-induced potential well. The idea of exciton self-trapping (EST) was first proposed by Landau and further developed by Pekar, Fröhlich, Feynman, Rashba, Toyozawa, and others.1−5 According to the Rashba and Toyozawa theory, in 3D and 2D crystals with strong enough exciton−phonon coupling, a free exciton metastable state and a self-trapped (ST) exciton state can coexist and should be separated by a self-trapping barrier.4,5 In such case, two emission bands could be revealed: a narrow resonant band of free excitons and a wide, strongly Stokesshifted band of self-trapped excitons.1−5 As opposed to that, in 1D systems excitons are self-trapped at any exciton−phonon coupling strength.4,5 There are many theoretical and experimental studies devoted to the EST in molecular crystals with deep understanding of its feature.1−5 For example, it was shown that an excimer formation in the crystals of aromatic molecules such as pyrene and some others is a particular case of exciton self-trapping;6 © 2015 American Chemical Society
however, in the case of molecular aggregates such as Jaggregates a situation is much more complicated. Molecular aggregates, called J-aggregates, are well-ordered luminescent organic nanoclusters of noncovalently coupled luminophores such as cyanines, porphyrines, and some other dyes.7−10 A high degree of dye molecules ordering in Jaggregate’s chains results in the appearance of an excitonic narrow absorption band (so-called J-band), which is bathochromically shifted with respect to a monomer band.7−10 Spectral properties of J-aggregates are governed by the exciton delocalization length, which is usually equal to up to tens of monomers rather than the physical length of aggregates.10 Because of the excitonic nature of electronic excitations, Jaggregates reveal a number of unique spectral properties such as very narrow for organic molecules spectral lines (tens of cm−1 at liquid helium temperatures), large extinction coefficients (hundreds of thousands of cm−1·M−1), giant third-order optical nonlinearities up to 10−5 esu, and so on.7−13 Such optical properties allow considering the Jaggregates as very promising objects for many applications, for example, spectral sensitization in photovoltaics, nonlinear optical devices, luminescenct probes in biology and medicine, and so on.7−13 The exciton−phonon interaction in J-aggregates is usually suggested to be weak, so the EST in J-aggregates is not taken into consideration.10 Nevertheless, because the J-aggregates structure is often described as 1D molecular chains,7−10,14−16 the barrierless EST should take place.4,5 Indeed, the EST idea was successfully used to explain a large Stokes shift, radiative Received: October 11, 2015 Revised: November 17, 2015 Published: November 19, 2015 27865
DOI: 10.1021/acs.jpcc.5b09940 J. Phys. Chem. C 2015, 119, 27865−27873
Article
The Journal of Physical Chemistry C
increasing PIC amount in the mixture. The luminescence lifetime detected at the red-shifted band is much longer, and the band was assumed to be originated from sandwich dimers acting as trapping defect states.39 A strong exciton coupling with low-frequency phonons and weak superradiance enhancement was also found, which could not be explained by large disorder only.39 The authors of ref 40 tried to find the origin of the low-temperature red emission band in PIC J-aggregates prepared by spin-coating in poly(vinyl sulfate) (PVS) films. They found that the intensity of the red band strongly depends on the J-aggregate preparation conditions and the intensity of the luminescence excitation. A noticeable shift of the red band maximum from 591 to 620 nm was revealed with a temperature rise.40 The temperature dependence of the maximum shift was fitted by single exponential law Δ ≈ exp(−hΩ/kBT), where Ω = 39 cm−1 was considered as low-frequency thermally activated phonon mode interacted with the trap.40 Trapping dimeric states was suggested as an origin of the long-wavelength band, which are common for all segments of the J-aggregate film.40 The room-temperature studies of aqueous solutions of PVSbounded PIC J-aggregates41 also should be mentioned here. Despite the lack of low-temperature experiments, a long-living bottleneck trapping state was found that was responsible for short lifetime and small luminescence quantum yield of the Jaggregates.41 Recently the heavy low-energy tail of perylene bisimide (PBI) J-aggregates precipitated on a Si/SiO2 substrate and the appearance of the broad strongly red-shifted band in the luminescence spectrum of the individual J-aggregates at 77 K has been reported.42 The observed spectral features were associated with the Levy low-energy exciton states induced by the disorder and acting as the exciton traps emitting in the long-wavelength spectral range.42 Using fluorescence superresolution microscopy technique, the exciton migration over 100 nm distance was found.42 The EST was not considered as a possible reason for the exciton trapping. We consider the EST as an origin of the red emission band of PIC J-aggregates formed in layered polymer films at low temperature. The analysis of the exciton−phonon coupling, luminescence decay, and time-resolved emission spectra testifies to the barrier-type exciton self-trapping in PIC Jaggregates due to the effect of high topological disorder in Jaggregate chains formed in the film.
dynamics, and nonlinear optical properties for some Jaggregates.17−21 Moreover, for some J-aggregates the evaluation of exciton−phonon coupling strength revealed a large value of the exciton−phonon coupling constant,22−25 which is the condition for EST.1,5 The great significance of EST is confirmed by the fact of EST responsibility for low luminescence quantum yield of some J-aggregates,23−25 whereas its suppression leads to the significant luminescence quantum yield enhancement.26 Up to now, a barrier-like EST with coexisting free and ST excitons has been described only for amphi-PIC (S120) Jaggregates at low temperatures.27−31 Let us briefly describe spectral features of amphi-PIC J-aggregates because it is important for understanding the experimental results presented in a current research. Amphi-PIC is an amphiphilic molecule, and in binary solutions containing water the aggregation of amphi-PIC proceeds similarly to the formation of surfactant micelles.27−31 Amphi-PIC J-aggregates exhibit a broad J-band (650 cm−1 at room temperature32 and 380 cm−1 at 1.5 K29,30) with a maximum centered near 580 nm. Any changes of a dimethylformamide (DMF) part in a binary DMF/water solution used for the J-aggregates preparation strongly affect the J-aggregates spectral properties.29−31 If the DMF part in a binary DMF/water solution exceeds 25%, significant static disorder is observed at low temperatures, followed by the change in the low-energy edge of the J-band from Gaussian form to Lorentzian one.33 That is the distinctive feature of high topological disorder in a molecular chain of the J-aggregate.34 At 1.5 K, the J-aggregate excitation within the J-band highenergy edge or at its maximum leads to the appearance of a broad luminescence band, which splits into two bands at 77 K: a narrow resonant to J-band and a broad strongly red-shifted with respect to J-band.27−31 Luminescence decay analysis at different registration wavelengths showed that the bands could be ascribed to free and ST excitons, respectively.28−31 At the excitation within the J-band low-energy edge formed by the localized excitons,30,31,35 only the free exciton emission band was observed.29−31 In the solutions with a small part of DMF (J-aggregates without topological disorder), the same situation was observed at any excitation wavelengths.28−31 The barrierlike EST in quasi-1D amphi-PIC J-aggregates with topological disorder was explained using a model of 3D deformation of the aggregate structure by a glass matrix.29,36 It was proposed that EST in such a system possesses a threshold character and occurs when the exciton delocalization length is less than a critical value,29,36 similarly to the EST model in C60 clusters proposed by Rashba.37 Some authors reported the appearance of a similar longwavelength luminescence band for some pseudoisocyanine (PIC) J-aggregates at low temperatures, but they did not associate this band with the EST state.38−40 The authors of ref 38 saw a novel red emission band in a Langmuir−Blodgett (LB) monolayer of PIC J-aggregates at 77 K, whereas in the LB bilayer only a typical free exciton luminescence was observed; however, in both cases, the J-bands exhibit the same pattern with heavy-tail low-energy edges due to strong topological disorder. It was supposed that the red emission is originated from dimer-like states formed below the exciton band as a result of disorder.38 In ref 39, the size-depending exciton dynamics in mixtures of PIC and its azacyanine analogue on silver halides microcrystals was studied. It was found that at low temperatures (∼90 K) the additional long-wavelength luminescence band appears, whose intensity decreases with
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EXPERIMENTAL SECTION Pseudoisocyanine (1,1′-diethyl-2,2′-cyanine iodide, PIC, Chart 1a) dye, cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA, average Mw < 100 000 g/mol, solution 35 wt % in H2O, Chart 1b), and anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS, average Mw ≈ 70 000 g/mol, powder, Chart 1c) were purchased from Sigma-Aldrich (USA) and used as received. PIC J-aggregates were prepared by dissolving the PIC (0.5 mM) in an aqueous NaCl (0.2 M) solution under moderate heating (
