Exciton Dynamics and Self-Trapping of Carbocyanine J-Aggregates in

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C: Physical Processes in Nanomaterials and Nanostructures

Exciton Dynamics and Self-Trapping of Carbocyanine J-Aggregates in Polymer Films Alexander V. Sorokin, Irina Yu. Ropakova, Steffen Wolter, Regina Lange, Ingo Barke, Sylvia Speller, Svetlana L. Yefimova, Yuri V. Malyukin, and Stefan Lochbrunner J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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The Journal of Physical Chemistry

Exciton Dynamics and Self-Trapping of Carbocyanine J-Aggregates in Polymer Films Alexander V. Sorokin1*, Irina Yu. Ropakova1, Steffen Wolter2, Regina Lange2, Ingo Barke2, Sylvia Speller2, Svetlana L. Yefimova1, Yuri V. Malyukin1, Stefan Lochbrunner2 1Institute

for Scintillation Materials of NAS of Ukraine, SSI “Institute for Single Crystals” of NAS of Ukraine, 60 Nauky ave., 61072 Kharkiv, Ukraine

2Institute

of Physics and Department of Life, Light and Matter, University of Rostock, 18051 Rostock, Germany

e-mail: [email protected], [email protected]

Abstract Spectral properties and morphologies of TDBC J-aggregates formed in two different types of polymer films (neutral spin-coated and charged layered) were studied using steady-state, picosecond, and femtosecond spectroscopy as well as optical and atomic force microscopy. It was found that the J-aggregates adopt different morphologies in the films: quasi one-dimensional rodlike in the spin-coated films and two-dimensional island-like in the layered films. The TDBC Jaggregates exhibit very similar absorption and fluorescence spectra in the different films, but their fluorescence lifetimes and quantum yields differ strongly (~ 4% for spin-coated films and ~ 0.5% for layered films). By pump-probe spectroscopy three different contributions to the population dynamics and relaxation to lower lying levels are identified with exciton-exciton annihilation dominating the observed kinetics. Low temperature experiments reveal strong exciton self-trapping in the J-aggregates due to large exciton-phonon coupling. However, the different morphologies of the TDBC J-aggregates in the films result in different behaviors of exciton self-trapping. While in spin-coated films barrierless self-trapping is dominant, in layered films it is associated with overcoming an energy barrier.

Introduction Organic materials, especially, nanostructured ones, have attracted intense research interest as a very promising subject for optoelectronic and photonic applications.1,2 Organic molecules exhibit high fluorescence efficiencies at low particle densities, a wide variety of optical properties, and are easy and cheap to process, resulting in an extreme large design flexibility, very good possibilities for integration into devices, and an impressive performance of organic materials.1 In active organic ACS Paragon Plus Environment

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2 devices, including organic light-emitting diodes, organic photovoltaic cells, organic field-effect transistors, electrochromic devices, and organic nonlinear optics, the material is typically applied in form of thin films.2 In this case special efforts should be taken to prevent concentration quenching due to energy transfer to weakly or non-fluorescent states like dimers or excimers.1–4 However, there is an example of aggregates consisting of organic fluorophores which, contrary to concentration quenching, exhibit strong fluorescence, namely J-aggregates.5–9 J-aggregates are highly ordered nanostructures of non-covalently coupled dyes such as cyanines, porphyrins, merocyanines, perylenes, and others.5–9 They were independently discovered in the late 1930s by E. Jelley10 and G. Scheibe11 by the appearance of a novel bathochromically shifted band (called Jband) in the absorption spectra of pseudoisocyanine dyes in water solutions upon increasing their concentration. The observed changes were ascribed to the formation of dye aggregates which were henceforth called J-aggregates (Jelley’s aggregates)6–9 or sometimes Scheibe aggregates (Scheibe polymers).5 Due to the translational symmetry within the molecular chains of a J-aggregate, the electronic excitations of the monomers are delocalized over chain segments and molecular (Frenkel) excitons are formed.5–9,12,13 The distinctive feature of J-aggregates is the J-band which results from electric dipole transitions into the low energy edge of the exciton band.5–9,12,13 Depending on the molecular packing within the chains also a hypsochromic excitonic band (H-band) or both J- and Hbands can appear.5–9,12 J-aggregates exhibit a number of unique spectroscopic properties, which are distinctly different from those of the individual molecules constituting the aggregates: very narrow absorption and fluorescence line widths as for organic compounds (down to tens of cm–1 at low temperatures), near-resonant fluorescence, large extinction coefficients (up to 106 cm−1·M−1), giant third-order optical nonlinearities (up to 10–5 esu), exciton superradiance, and energy migration up to micron distances.5–9,12,13 Thus they are ideal candidates for novel photonic materials, especially for thinfilm applications.14–16 Unfortunately, pure aggregate films are often not stable enough.14–16 Polymer-based thin films with incorporated J-aggregates could be more durable and might be an attractive alternative.16–23 There are two main approaches to form polymer films containing J-aggregates: spin-coating and layer-by-layer assembly.16 In both cases the optical properties of the J-aggregates are affected by the polymer medium, resulting, for example, in a strong decrease of the fluorescence quantum yield or a transformation of the aggregate structure.16,19,24–26 Spin-coating is a quite simple and convenient method for preparing thin polymer films.27 A solution containing the dissolved polymer and the aggregating dye is dropped onto a fast rotating substrate. The central force leads to spreading of the solution over the substrate and because of the large surface to a quick evaporation of the solvent. A thin film is formed within several minutes with a thickness depending on the ACS Paragon Plus Environment

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3 rotation speed.27 In the layer-by-layer self-assembly (LbL) technique alternating multilayer polymer films can be deposited onto an electrically charged substrate by means of electrostatic attraction dipping the substrate sequentially in aqueous solutions of polycations and anions, here the aggregating dye molecules.27,28 The samples consist of a sequence of successive monolayers of a polymer and of the aggregates, which are within their layers densely packed. There is a representative of the carbocyanine dyes, namely 1,1'-disulfobutyl-3,3'-diethyl5,5',6,6'-tetrachlorobenzimidazolylcarbo-cyanine sodium salt (TDBC, Chart 1a), which easily forms J-aggregates both in water solutions and in polymer films.17,18,20–23,29–31 J-aggregates of the TDBC dye appear in water at concentrations exceeding 10–6 M.30,31 At high dye concentrations such as 10–3 M almost all dye molecules are aggregating and the monomer band is nearly absent in the absorption spectrum.30,31 The spectral properties and exciton dynamics of TDBC J-aggregates in water solution are well studied.30–34 However, this does not hold for TDBC J-aggregates in polymer films despite the large interest in such composites.17,18,20–23,29 In some studies, it is assumed that TDBC J-aggregates in different types of polymer films have the same spectral properties22,23 although such a statement hasn’t been proved experimentally yet. The aim of the article is to compare the features of the exciton dynamics and spectral properties of TDBC J-aggregates prepared in neutral spin-coated and charged LbL polymer films.

Experimental Section The TDBC dye was purchased from Few Chemicals GmbH (Germany) and used asreceived. The cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA, average Mw = 200,000 – 350,000 g/mol, solution 20 wt. % in H2O, Chart 1b) and the polymer poly(vinyl alcohol) (PVA, Mw = 89,000 – 98,000 g/mol, Chart 1c), were obtained from Sigma Aldrich (USA) and also used as-received. Before film preparation, the glass substrates were cleaned for 30 min by a water solution of hydrogen peroxide (30%) heated up to 70 °C. Thereafter, the substrates were successively rinsed by water, isopropanol, and acetone, at least three times by each solvent, and dried by dry air. To obtain an aqueous stock solution of TDBC (C = 10–3 M) J-aggregates and PVA, the dye was dissolved in an aqueous PVA (4 wt.%) solution by placing the sample for a couple of hours in an ultrasonic bath at a temperature of 60 °C. The stock J-aggregate-PVA solution was quite stable and could be stored in the refrigerator at least for one month without precipitation. To prepare spin-coated films with a thickness of about 50 nm, 100 L of the stock J-aggregate-PVA solution was dropped onto a quiescent substrate, spread on its surface. Then the substrate was rotated at 2000 rpm for 3 min using a Spin 150 spin-coater (SPS, the Netherlands). In this way a uniform distribution of the film ACS Paragon Plus Environment


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4 over substrate can be obtained. If the solution is dropped onto a rapidly rotating substrate, donutshaped films are typically obtained (see Figure S1 of Supporting Information). For the preparation of LbL polymer films an aqueous PDDA (0.5 wt.%) solution and a stock solution of TDBC (C = 10–3 M) J-aggregates in water were used. First, the clean substrate was immersed for 20 seconds in the aqueous PDDA solution agitated by a magnetic stirrer (at 1000 rpm)35 to deposit a positively charged polymer monolayer. After that, the substrate covered by the PDDA film was dipped into the J-aggregate stock solution for 10 min (without agitation) to deposit a layer of J-aggregates. Finally, the J-aggregate layer was covered by a PDDA layer immersing the sample for 20 seconds in the agitated PDDA solution. Each layer deposition was followed by rinsing the substrate for 10 seconds in agitated distilled water and drying it with dry air. To obtain multilayer films the procedure was repeated several times. As we got the films on both sides of the substrate, one of them was cleaned by a water-wetted cloth before the measurements. To increase the stability of the LbL films they were heated at 60 °C for 5 min36 and covered by a spin-coated pure PVA film.

a)

b)

c)

Chart 1. Structural formulas of the dye and polymers: a) TDBC, b) PDDA, c) PVA. Absorption spectra were measured by a Specord 50 spectrophotometer (Analytik Jena, Gemany). Fluorescence spectra were recorded by a FluoroMax-4 spectrofluorimeter (Horiba Scientific, Japan) using a solid sample holder. Fluorescence decay spectra were obtained by a FluoTime 200 fluorescence lifetime spectrometer (PicoQuant, Germany) equipped with a solid sample holder and for excitation a picosecond laser diode emitting at 531 nm with a pulse energy of 13 pJ. The width of the instrument response function (IRF) for the whole setup was 100 ps and below. For the analysis of the decay curves, the FluoFit software (PicoQuant, Germany) was used. For the collection of time-resolved emission spectra (TRES) a set of luminescence decay curves ACS Paragon Plus Environment

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5 was recorded covering the spectral range from 575 to 720 nm with a step size of 1 nm and keeping the acquisition time of all curves the same via the TRES option of the PicoHarp software (PicoQuant, Germany). For the analysis of the TRES data the FluoPlot software (PicoQuant, Germany) was used. Low-temperature spectra were measured using a nitrogen cryostat. The temperature was controlled within 1 K. Absorption spectra at low temperatures were obtained by means of an incandescent lamp and a fiber coupled USB4000 microspectrometer (OceanOptics, USA) using a home-made fiber-optic adapter attached to a 20X eyepiece. Fluorescence spectra at low temperatures were recorded applying a monochromator (MDR-23, LOMO, Russia) and a diodepumped Nd3+:YAG laser (exc = 532 nm, 5 mW) or a xenon lamp as excitation source. The spectral output of the monochromator was calibrated using a calibrated tungsten halogen lamp (HL-2000CAL, OceanOptics, USA). Absolute fluorescence quantum yields were measured by a home-made integrating sphere (diameter of 100 mm), which provides a reflectance > 99% over the spectral range of 300–1000 nm. For excitation, the laser line at  = 514.5 nm was selected from the spectrum of a multi-line Ar-ion laser (Stellar-PRO 150 mW, Modu-Laser, USA) by a small grating monochromator (MUM, LOMO, Russia). For fluorescence detection the MDR-23 monochromator with a calibrated spectral output was used. The absolute quantum yield was calculated using the two-measurement method37 and correcting for self-absorption38 of the fluorescence which is quite significant for J-aggregates due to the very small Stokes shift. The experimental setup was adjusted and tested with rhodamine 6G (in ethanol, C = 10–5 M, Qlit = 0.9435)37 resulting in an accuracy of ± 5% which is typical for such a setup.37 Transient absorption measurements were performed with a setup based on a noncollinearly phase-matched optical parametric amplifier (NOPA) pumped by a regenerative Ti:sapphire laser system (CPA 2001; Clark MXR) which provides 180 fs long pulses at a repetition rate of 1 kHz.39 The NOPA was tuned to a central wavelength of 580 nm and its output pulses were compressed to a length of 20 fs by a sequence of fused silica prisms. The pulses were split into two beams for excitation and probing. The time delay between pump and probe pulses was adjusted by a retro reflector attached to a motorized linear stage in the optical path of the excitation beam. The polarization of the excitation pulses was set to magic angle, i.e. 54.7° with respect to the probe polarization by an achromatic half wave plate to prevent that pump induced anisotropies influence the transient absorption dynamics. The intensity of the pump pulses was adjusted by neutral density filters and a combination of two wire-grid polarizers for fine tuning. Pump and probe beam were focused onto the sample substrate with spot sizes of about 200 µm and 100 µm in diameter, ACS Paragon Plus Environment


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6 respectively. The energy of the excitation pulses, which were applied with a repetition rate of 500 Hz, was varied between 4 nJ and 32 nJ. From the spot diameter and the spectral overlap of the ultrashort laser pulses with the J-band an excitation probability of 1.2% to 9.8% per TDBC molecule was estimated. To prevent artifacts from photodegradation, the sample was rotated with constant speed during the measurement using a home-made motorized sample holder. After the sample the probe beam was dispersed by a fused silica prism and spectrally resolved detected with a photodiode array. Optical dark-field microscopy was conducted with a Zeiss Axio Scope A1 microscope using an objective lens with 100x magnification. Illumination was done with a broadband halogen lamp. Atomic force microscopy images were acquired using a beam deflection microscope (XE-100, Park Systems) in the dynamic mode. Cantilevers with eigenfrequencies around ~ 300 kHz were used (ACTA, Applied NanoStructures, Inc.). Setpoint and oscillation amplitude were chosen to ensure gentle imaging conditions. Topographic data were background-corrected by a 2D polynomial fit of 2nd order (for drift correction) and by a constant offset for each line (to account for short-term fluctuations) which was chosen such that the median of the difference between two consecutive lines is minimized.

Results and Discussion Before analyzing the spectral features of the J-aggregates formed in polymer films, it is interesting to compare their visual and microscopic characteristics. Indeed, even for the naked eye the difference between the two different polymer films containing the J-aggregates is visible (Figure 1). One could see, while the optical transmittance images are very similar for both films, their reflectance images are quite different (Figure 1). The LbL films, especially those with several Jaggregates layers, look like coloured metal mirrors (Figure 1c). Such a behavior is associated with a strong negative real part of the permittivity in the vicinity of the J-band leading to the appearance of surface exciton-polariton modes due to the intense J-band and the very high aggregate density.22,23 The spin-coated films are thicker and exhibit a lower aggregate concentration and a correspondingly weaker permittivity. As a result, no mirror like behavior appears (Figure 1d).

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7

a)

b)

d) c)

Figure 1. Transmittance (a,b) and reflectance (c,d) images of a LbL film with six layers TDBC Jaggregates (a,c) and a spin-coated film (b,d). The samples are illuminated by standard room daylight lamps. Optical dark-field microscopy images reveal further differences between the two types of polymer films containing TDBC J-aggregates (Figure 2).

a)

b)

Figure 2. Optical dark-field microscopy images of TDBC J-aggregates in spin-coated (a) and LbL (b) films. The bright spot in b) is likely due to an impurity.



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8 In the spin-coated film the J-aggregates show a rod-like morphology (Figure 2a) similar to that in solution.31 Note that the apparent feature width is limited by the optical resolution, which is ~ 500 nm in this case. AFM images show some features with lengths of about 1 µm and heights of 30-40 nm (Figure 3, left) which may be attributed to the lengthy structures (probably aggregates or agglomerated aggregates) seen in optical microscopy (Figure 2a), occasionally penetrating the surface. Since the J-aggregates are mostly located within the PVA film, they are hardly accessible by our scanning microscopy techniques. On the other hand, smaller features with a grainy appearance can be observed in images with higher resolution (Figure 3, right). In the absence of additional data like spectroscopic ones, we cannot explicitly assign any of these features to the Jaggregates or polymer stuff. Thus, it would be desirable to apply additional imaging methods like near-filed scanning optical microscopy techniques to obtain more information about the morphology of the TDBC aggregates in spin-coated films.

a)

b) Figure 3. AFM image (a) and typical profiles (b) taken along the dashed lines of TDBC Jaggregates in a spin-coated film. ACS Paragon Plus Environment

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9 Nevertheless, the rod-like morphology is clearly seen in the optical microscopy images (Figure 2a), so it should be possible to align the J-aggregates in spin-coated films by means of the flow of the original solution. Indeed, when the aggregates and PVA containing solution is dropped onto a rapidly rotating substrate, highly polarized films with an orientation along the centrifugal force field are obtained (Figure S1 of Supporting Information). There is a small difference in the preparation allowing to prepare less or more oriented spin-coated films. For typical spin-coated films the PVA containing solution is dropped onto and then spread over a quiescent substrate which is rotated thereafter. In this case we obtained less oriented samples (Figure 2a) homogeneously covering the substrate. For a better alignment the solution was dropped onto the quickly rotating substrate resulting in donut-shaped films with film depletion in the centre (Figure S1a of Supporting Information). In this case the J-aggregates appeared to be much more aligned. A possible, speculative explanation of the effect is that in the first case some aggregates or polymer strands have time to fix to the substrate and become a template in the further film growing process. Contrary to the spin-coated film, the LbL film shows spots of irregular shapes in the optical microscopy images, which should correspond to TDBC aggregates (Figure 2b). In dark-field microscopy images of the LbL fims the actual aggregate layers appear to be black since the fluorescence yield of the aggregates in the LbL films is very low. Their structure and morphology, which might cause scattering of the illuminating light, cannot be seen neither since scattering objects smaller than the diffraction limit are hard to detect and most of the J-aggregates in the LbL films are quite small and below this limit. The spots seen on the microscope images are probably due to large agglomerates formed accidently. Contrary, for the spin-coated films we see the individual large agglomerates of the J-aggregates (Figure 2a). AFM images of the uncovered J-aggregates reveal a relatively homogeneous, grainy appearance (Figure 4). Occasionally large features of non-uniform size are observed (Figure 4) which are probably due to impurities or larger agglomerates. The measured roughness is around 1.5 nm (root mean square, rms) and thereby much larger than of the rather flat pure polymer film exhibiting corrugation amplitudes of only 0.4 nm (rms) (Figure S2 of Supporting Information). After six days of storage, larger grains were found on the film (Figure S3 of Supporting Information) pointing to an agglomeration process. In a previous AFM study of TDBC aggregates in LbL films we observed similar featureless pictures.17 We concluded that in the LbL films the TDBC aggregates exhibits an island-like two-dimensional morphology similar to the one of PIC aggregates in LbL films.26



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10

a)

b) Figure 4. AFM image (a) and an exemplary line profile (b) taken along the dashed line of a freshlyprepared LbL film with TDBC aggregates. The J-aggregates are placed on top of a PDDA layer and are not covered by another PDDA layer.

Thus, TDBC aggregates have different morphologies in different types of the polymer films: quasi one-dimensional in the spin-coated film and two-dimensional in the LbL film. However, we have no direct information about the microscopic structure of the aggregates. Such information might be obtained by X-ray diffraction or high resolution electron microscopy. Unfortunately, since the J-aggregates are dispersed in polymer films, our samples are not suitable for these techniques and transferring the aggregates to other substrates or environments would most probably change their structure. Thus, some assumptions concerning the J-aggregate structure can be made only based on the spectral properties. It should be mentioned, that the TDBC J-aggregates show a significantly different stability in the two polymer films. The spin-coated PVA films are quite stable when stored under normal ACS Paragon Plus Environment

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11 conditions and their optical density showed only a minor decrease after 6 months storage (Figure S4 of Supporting Information). Contrary, the original LbL films exhibited a very low stability and their optical density strongly decreased after just 1 day of storage at normal conditions (Figure S5a of Supporting Information). Two ways were found to improve the stability of the LbL films: covering the film by a spin-coated film of pure PVA (Figure S5b of Supporting Information) or preparing a film containing several layers (more than 4) of TDBC J-aggregates similar to the approach found for Langmuir-Blodgett films of J-aggregates.36 However, in the latter case we obtain films with a very high optical density (more than 1) which would be unsuitable for various measurements and applications. Despite the different morphologies, the steady-state spectra of the TDBC J-aggregates in the polymer films (Figure 5) are very similar to each other and to those in water solution (Figure S6 of Supporting Information).

a)

b)

Figure 5. Absorption (1) and fluorescence (2, exc = 530 nm) spectra of TDBC J-aggregates in a) spin-coated and b) LbL films.

In water solution (Figure S6 of Supporting Information) TDBC J-aggregates exhibit a very narrow J-band (max = 585 nm, FWHM = 280 cm–1) and a nearly resonant, narrow fluorescence band (max = 586.5 nm, FWHM = 320 cm–1). The J-aggregate fluorescence is bright with a quantum yield of  ~ 0.31 and a quite short lifetime of av ~ 60 ps.31 It should be noted that the observed Jband is narrower than in less concentrated TDBC water solutions (C = 10–4 M, FWHM = 360 cm– 1),31

possibly due to an effective agglomeration of the J-aggregates40–42 as also found by scanning

electron microscopy.31 ACS Paragon Plus Environment


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12 The absorption spectra of the polymer-bound J-aggregates (Figure 5, curves 1) are very similar to that for J-aggregates in solution (Figure S6 of Supporting Information) although the spincoated films exhibit a larger optical density (OD ~ 0.3 – 0.5) compared with LbL films containing a single layer of J-aggregates (OD ~ 0.1 – 0.2). This can be attributed to the different film thicknesses: 50 nm for the spin-coated PVA film and 1–2 nm for the J-aggregate layer in the LbL PDDA film (estimated by AFM). Despite significant differences in the film thickness one should keep in mind that in the case of LbL films the J-aggregates are very densely packed in the thin layer resulting in an extremely large absorption constant (106 cm–1 for a film with three J-aggregate layers)17 while in the spin-coated film the J-aggregates are uniformly distributed within the whole film (Figure 2a). Only small differences in the spectral position, width, and shape of the J-band are found for spin-coated and LbL films despite their different morphologies (Figures 5 and S7 of Supporting Information). In the spin-coated film the absorption maximum appears at maxspin = 590 nm and the bandwidth is Jspin = 255 cm–1 while in the LbL film the absorption maximum is at

maxLbL = 587.5 nm and the bandwidth amounts JLbL = 300 cm–1 (Figure 5). The shape of the absorption spectra of the aggregates in water, spin-coated films, and LbL films are very similar indicating that the coherence lengths and the arrangement and packing of the molecules in the aggregates are comparable. In all cases the monomeric transition moments are parallel oriented. Otherwise one would expect a kind of Davydov splitting.5–9 The dipole-dipole interaction strength J varies only little (see below) indicating that the separation and relative orientation of the chromophores are also similar. We can use literature data for similar cases to draw some conclusions about the structure of the aggregates. The TDBC dye belongs to the 5,5’,6,6’-tetrachlorobenzimidacarbocyanine (TBC) dye family. Some representatives of which are well studied including structural details.7,43–45 For amphiphilic derivatives (like C8O3 or C8S3 which possess two long hydrophobic tails C8H17)7,43 and TTBC (derivative with four small hydrocarbon tails C2H5)44,45 applying highresolution cryo-TEM tubular morphologies were found: hollow tubes with an outer diameter of ~ 10 nm for amphiphilic derivatives7,43 and solid tubes with an outer diameter of ~ 3.5 nm for TTBC. 44,45

These single tubes are microns long and tend in solution to agglomerate to bundles, which can

also be twisted around their axis.7,43 Using theoretical modelling, the structure of these tubular Jaggregates was established. For hollow tubes a rolled-up 2D brick-wall structure was proposed.7,43,46 This results in the appearance of two J-bands one of them polarized along the tube axis and one perpendicularly to it.46 For TTBC J-aggregates a tube consisting of rings of six chromophores with rotational symmetry was proposed.45 The inter-chromophore coupling in the tube leads to a striking change in the optical properties. Namely two main absorption bands, the


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13 transition-dipoles of which are orthogonally polarized, appear: a sharp low-energy J-band that is polarized along the symmetry axis of the tube and a much broader high-energy H-band.45 For our case, namely TDBC J-aggregates, modelling of the structure was not performed and is a future task. Nevertheless, in previous work31 it was found that TDBC J-aggregates precipitated from solution exhibit the morphology of solid tubes with strong agglomeration of the tubes to bundles, some of which are twisted around their axis. In the present work, using dark-field microscopy a rod-like morphology was found for TDBC J-aggregates in the spin-coated PVA films (Figure 2a) with a single narrow J-band polarized mainly along the rod axis (Figure S1 of Supporting Information). Comparing the J-aggregate morphology for the species precipitated from solutions the rods can be ascribed to J-aggregate agglomerates (tube bundles).31 Since only one Jband is present, contrary to C8O3 and TTBC J-aggregates,7,43–46 and since it is polarized along the rod axis 1D molecular packing can be reasonably suggested as main motive for the TDBC Jaggregates. For TDBC J-aggregates in the layered LbL films, an island-like (or plate-like) morphology was found. Such a morphology is often present when J-aggregates form on surfaces, especially on charged or well-ordered ones.5–9,26,47–49 Typically, a plate-like morphology is described using the commonly accepted brickwork (brick-stone) 2D structure.5–9 Despite the 2D structure, flat brickstone J-aggregates mostly reveal a single J-band.5–9 So, for island-like TDBC J-aggregates in LbL films we can assume a 2D structure. The narrow J-band is an evidence for a large exciton coherence (or delocalization) length Ncoh, which denotes the number of coherently coupled monomers in an aggregate segment and which can be estimated as:12

N coh 

3  ( mon ) 2 1, 2  ( J ) 2

(1)

where mon and J are the full widths at half maximum of the monomer and J-band, respectively. Using monet = 1140 cm–1 for TDBC in ethanol31 and monPVA = 1085 cm–1 for TDBC in PVA (Figure S8 of Supporting Information) we obtain Ncohsol ~ 23 5, Ncohspin ~ 26 5 and NcohLbL ~ 18 5 in water solution, spin-coated and LbL films, respectively. Two main error sources appear in estimating the exciton coherent length. The determined band width of the J-band has an uncertainty of 1 nm due to the spectral resolution of the UV/Vis spectrophotometer. The monomer band in the polymer films is not accurately known since it was not possible to prepare TDBC monomers in the LbL films and in the spin-coated films only a mixture of monomers and aggregates was accessible.12 These uncertainties result in an error of  5 for the determined coherence lengths. Accordingly, the differences in the coherence lengths observed for the different samples are close to the experimental error. For different J-aggregates in solution and at room temperature exciton ACS Paragon Plus Environment


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14 coherent lengths of 5 to 50 (some estimates even at 100) monomers were reported depending on their structures.5–9 Thus, the coherence lengths found for our samples can therefore be considered to be typical. Despite the much more rigid environment, TDBC J-aggregates reveal a low static disorder in both polymeric films and in the spin-coated film it seems to be even smaller than in solution as indicated by the increase of the exciton coherence length.12,50,51 On the other hand, the long wavelength (low energy) wing of the J-band is slightly flatter in the spin-coated film compared to the steeper one in the LbL film and in water (Figures 5 and S9 of Supporting Information). This is an indication for a noticeable contribution of off-diagonal static disorder.12,50–55 Indeed, while diagonal (energetic) disorder doesn’t change the low energy J-band tail, which is well described by a Gaussian contour, off-diagonal (topological or orientational) disorder leads to a Lorentzian profile of the tail.12,50,52 The contour of the low energy J-band tail is often described using a Lévy α-stable distribution, taking into account that both, Gaussian and Lorentzian distributions are special cases of it.53–56 In this case the flatter low energy tail of the J-band can be referred to as heavy tail and can be described not only by a Lorentzian, but also by an intermediate distribution.53–56 Recently, the appearance of heavy tails has been proven for TDBC J-aggregates immobilized in a gel matrix similar to our case.55 It was shown that it causes a static segmentation of the one-dimensional chains.55 The heavy tail found for the J-aggregates in the spin-coated films suggests large static disorder which should decrease the exciton coherence length and, respectively, make the J-band wider. Thus, the similarity of the exciton coherence lengths in different environments (water and spin-coated and LbL films) is nontrivial. Currently, we are not able to explain this phenomenon in detail due to a lack of structural information about the aggregates. However, we can speculate that the methods for generating the thin films allow preparing highly ordered J-aggregates: in the spincoated films the aggregates form densely packed bundles (agglomerates) and in the LbL films densely packed brickwork structures similar to that in Langmuir–Blodgett films. If this is true, one could expect a narrowing of the J-band and an increase of the exciton coherence length in the more perfectly arranged aggregate structure in the polymer films. However, the increasing excitonphonon coupling competes with this process and we obtain nearly the same exciton coherence lengths for different exciton-phonon couplings. To support this speculation we can compare the Jbands for TDBC J-aggregates in PVA films prepared by drop-casting and by spin-coating (Figure S10 of Supporting Information). Both films were prepared from the same stock solution of TDBC Jaggregates in aqueous PVA (4 wt.%) solution. The spin-coated film was prepared within 3 minutes and has a thickness of about 50 nm while the drop-casted film has a thickness of several microns and was casted into a ring-like form and left for drying at normal conditions for 12 hours until all ACS Paragon Plus Environment

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15 solvent was evaporated. Although the optical density of the films is comparable (some amount of the solution spread away during the spin-coating procedure), the J-band widths are strongly different (Figure S10 of Supporting Information). The J-band to monomer band relation is still very high for the drop-casted film so the disaggregation process of the J-aggregates is still minimal. Thus, the main difference is placing the J-aggregates in films of different thicknesses. We can assume that in the drop-casted film the distribution of the J-aggregates is similar to the water solution but the environment of the J-aggregates is more rigid in the polymer films and, consequently, stronger static disorder is expected. For spin-coated films, a stronger agglomeration of single aggregates can be assumed providing higher order within the J-aggregates and leading to a large exciton coherence length even though the static disorder in the polymer films is stronger than in a water solution. To prove this idea a further characterization of the structure of the J-aggregates in the films should be done. Some recent studies on the agglomeration of J-aggregates show indeed improved exciton properties like larger diffusion lengths or quantum yields compared to single Jaggregates.40–42 Compared to the solution case the spectral position of the J-band maximum appears to be slightly red-shifted by 2.5 nm for the LbL film and 5 nm for the spin-coated film (Figure 5). This could be a result of a red-shift of the monomer band in the polymer film compared to water due to the solvatochromic effect.3 Indeed, in thick PVA films with a low dye concentration the maximum of the monomer band is at maxfilm = 526.5 nm (Figure S8 of Supporting Information) while in water it is at maxH2O = 514 nm.31 Unfortunately, due to the high aggregation ability of the TDBC dye and the very small thickness of the LbL films we were unsuccessful to obtain monomers in a PDDA film. It’s well-known that the shift of the J-band with respect to the monomer band depends on the dipole-dipole interaction strength J, which can be obtained by Eq. 2:12 J

 mon  J 2.4

,

(2)

where mon and J are the maxima of the monomer and the J-band expressed in cm–1, respectively. Thus, for the aqueous solution we find Jsol = 980 cm–1 and for the films JLbL = 820 cm–1 and Jspin = 835 cm–1 (we assume the same spectral position of the monomer band in PDDA as in PVA films). Therefore, similarly to PIC J-aggregates in LbL films,25 for TDBC J-aggregates the dipole-dipole interaction strength is less in the films than in aqueous solution. It can be assumed that the polarizability, the dielectric constant (80 in water and 2-3 in polymers) and the index of refraction (1.33 for water and about 1.5 in polymers) of the environment have only little influence on the coupling since they are quite different in water and in the polymers while for the different samples ACS Paragon Plus Environment


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16 the spectral positions of the J-bands are similar and the dipole-dipole interaction strengths J vary by less than 20%. The aggregates seem to act as their own environment at least for the coupling between neighbouring chromophores. The variation of the dipole-dipole interaction strength J results probably mostly from small variations in the microscopic structure. As a summary, it can be concluded that the absorption spectra of TDBC J-aggregates in polymer films are similar to that in water solution and exhibit only little differences between each other. At first sight, the fluorescence spectra of the films behave in a similar way as the absorption spectra (Figure 5). Indeed, the fluorescence bands of TDBC J-aggregates in LbL and spin-coated films and their maxima at maxLbL = 588.5 nm and maxspin = 591.5 nm, respectively, are spectrally shifted relative to the aggregate emission in aqueous solution in a similar manner as the J-bands (Figure 5). Interestingly, for the spin-coated film the J-aggregate fluorescence band is slightly broader than the aggregate emission of the LbL fims (FWHMspin = 325 cm–1 vs. FWHMLbL = 270 cm–1) and shows a slightly lager Stokes shift (Stokesspin = 45 cm–1 vs. StokesLbL = 30 cm–1) (Figures 5 and S11 of Supporting Information) contrary to the behavior of the J-bands. However, the fluorescence intensity of the spin-coated films at comparable optical densities of the J-band is much larger than that of LbL films (Figure S12 of Supporting Information). Indeed, direct measurements of the fluorescence quantum yield by an integrating sphere resulted in values of spin ~ 4% and LbL ~ 0.5%. Thus, in the spin-coated film the J-aggregates exhibit a quantum yield which is 8 times larger than that in the LbL film. But both of them are much smaller than in the solution case (sol ~ 31%).31 A similar situation was found earlier for PIC J-aggregates in LbL films.25 Such a strong difference in the quantum yields can contribute in addition to the variation in permittivity to the different appearance of the films in the optical reflectance images (Figure 1). Indeed, the spin-coated film is highly fluorescent despite the smaller quantum yield compared to water (Figure 1d). This can be understood taking into account the high absorption coefficient and the small thickness of the film which minimizes reabsorption. The latter has a strong influence due to the very small Stokes shift (Figure 5). The strong difference in the fluorescence quantum yields of the TDBC J-aggregates in the polymer films is also reflected by the fluorescence lifetimes (Figure 6).


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17

Figure 6. Fluorescence decays (exc = 531 nm, reg = 590 nm) for TDBC J-aggregates in different polymer films: 1– spin-coated, 2 – LbL. Curve 3 is the IRF.

As one can see from the time-resolved fluorescence measurements presented in Figure 6 (curve 1) the emission of the J-aggregates in the spin-coating film exhibits a double-exponential decay with the two time constants (and fractional amplitudes) 1spin ~ 65 ps (98.5%) and 2spin ~ 1.27 ns (1.5%) and an average lifetime of avspin ~ 80 ps taking the amplitudes into account (Figure S13 of Supporting Information). It is possible that the dynamics is non-exponential and the fit represents an approximation. In this case the obtained time constants have to be considered as a parameterization of the dynamics. However, they indicate the relevant time scales of the involved processes. The dominant lifetime component as well as the spectral parameters of the fluorescence band for TDBC aggregates in the spin-coated film are very similar to the corresponding values for TDBC aggregates in water.31 The only significant difference between the spin-coated film and water is the presence of a low intensity tail with the lifetime 2spin ~ 1.27 ns in the former case (see Figure S14 in the supporting information). However, the fluorescence quantum yield is by a factor of eight smaller. This triggers the conjecture that maybe only a fraction of about 13% of the excitons are in the emitting free exciton state while the rest is in a state with a negligible emission yield such as exciton traps (i.e. defects and recombination sites at the J-aggregate/polymer interface)57,58 or an exciton self-trapped state.51,59–61 It is known, that due to exciton interaction with the lattice phonons, novel hybrid states appear, called polarons, which could be described as excitons “dressed” by lattice deformations.51,59–61 Depending on the exciton-phonon coupling the radius of the polarons can be different. Large radius polarons behave in a similar way as free excitons and could be revealed by ACS Paragon Plus Environment


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18 an increase of the Stokes shift and a broadening the fluorescence band.51,59–61 For small radius polarons the lattice deformation can be so large that it is trapped by a self-induced potential well.51,59–6151,59,60 Such exciton states are referred to as self-trapped excitons. Exciton self-trapping, first predicted by Landau, is for crystalline systems a well-known and studied phenomenon both theoretically and experimentally.51,59–61 Theory predicts a threshold character of exciton selftrapping for two- and three-dimensional lattices depending on the exciton-phonon coupling constant. In the case of a coupling constant g > gcr ~ 1 both free and self-trapped exciton states appear to be separated by a potential barrier (self-trapping barrier) and reveal themselves as two luminescence bands: a narrow band nearly resonant to the exciton absorption band corresponds to free excitons and a broad band with a large Stokes shift corresponds to self-trapped excitons.51,59–61 For an one-dimensional lattice, the threshold is typically absent and all excitons are self-trapped at any exciton-phonon coupling constants.51,59–61 For the special case of closed one-dimensional systems (e.g. ring-like) the self-trapping threshold was found with (gl)cr = 2/2, where 2l is the circumference length of the ring.62 Thus in rings with a small diameter and weak exciton-phonon coupling the excitons stay free. However, despite the threshold of exciton self-trapping in this case a self-trapping barrier does not exist.62 Barrierless exciton self-trapping reveals itself as an increasing Stokes shift and a widening of the free exciton luminescence band.51,59–61 Exciton self-trapping was initially studied for alkali haloid and molecular crystals.59–61 Further, exciton self-trapping was also confirmed for different excitonic materials, including thin molecular films,63,64 conjugated polymers and oligomers,65–67 photosynthetic antennas

68–70

and so

on. J-aggregates are a special case of molecular crystals with the strongest differences in a typically lower dimensionality (1D or 2D vs. 3D typical for molecular crystals) and a more pronounced influence of static disorder on the optical properties.5–9 So, exciton self-trapping is an expected phenomenon for J-aggregates.51,71,72 However, due to the mostly one-dimensional structure of Jaggregates and weak exciton-phonon coupling typical for J-aggregates in solutions this phenomenon is not widely discussed and often not taken into consideration. This holds also for TDBC aggregates in water, which is a very flexible environment and does not contribute to excitonphonon coupling. In this case the self-trapped excitons are very similar to free excitons. On the other hand, it was clearly demonstrated that in some cases exciton self-trapping is strongly affecting the photophysics of J-aggregates.51,71 Assuming thermal equilibrium at room temperature one finds from the ratio of the quantum yields r = 0.13 that such a self-trapped state would be about 430 cm–1 lower in energy than the free exciton state. This self-tapping energy EST results from EST = –ln(r)∙kBT. The 65 ps would be the lifetime of the free and of the self-trapped excitons, which are in thermal equilibrium. The long component (1.27 ns) is then due to other trapped states. Red shifted from the resonance ACS Paragon Plus Environment

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19 fluorescence one might expect an emission band from the self-trapped exciton. However, at room temperature the red shifted band is hard to see since the self-trapped and the free exciton are in thermal equilibrium and the resonance fluorescence is dominating. From the quantum yield and fluorescence decay time of the J-aggregates in water, a fluorescence rate constant of about 5x109 s–1 can be obtained.31 This is about 20 times larger than the value expected for a monomer. Hence, the coherence length obtained from the fluorescent rate constant and the width of the bands match for aggregates in solution.31 However, with an average singlet decay time of 80 ps and a fluorescence quantum yield of 4 %, the fluorescence rate constant of the J-aggregates in the spin-coated films is only about 5x108 s–1. This would correspond to a coherence length of about two which is much lower than the values found from the band width (Eq. 1 and Figure 5). This difference in coherence lengths can be easily understood within the selftrapping model. The J-band and, hence, the exciton coherence length, reflects the exciton generation process and the optical transition from the ground state to the excited state. Accordingly, the coherence length deduced from the J-band characterizes the delocalization in the unrelaxed excited state which has still the ground state geometry.70 Contrary, exciton self-trapping takes place in the excited state after the optical excitation.59–61,70 In thermal equilibrium the major part of the population is in the weakly emitting self-trapped state while only a small fraction of the population exists as strongly fluorescing free excitons. This leads to the low quantum yield. The fluorescence lifetime of TDBC J-aggregates in the LbL films is very short (less than 20 ps) and the decay curve cannot be distinguished from the IRF of our setup (Figure 6, curve 2). This could be an indication, that exciton self-trapping is more pronounced and accelerates also the nonradiative deactivation path.51,59–61 Another possibility can be supposed comparing the obtained results with those of THIATS J-aggregates in LbL films.57,58 THIATS aggregates in LbL films prepared from different polyelectrolytes show all similar optical properties and mediate exciton coherence lengths (~10). The latter are comparable to Langmuir–Blodgett films of THIATS aggregates, lower than that in an aqueous NaCl solution (~ 20) but larger than in aqueous polyelectrolyte solutions (~1-3).57,58 In all cases the LbL films exhibit small fluorescence quantum yields (about 3%) and fast fluorescence decays (~ 20 ps). The florescence data were explained by exciton trapping due to non-fluorescent traps as a result of exciton diffusion.57,58 In case of absence of traps, the fluorescence lifetime of the J-aggregates is quite long. Only spectroscopic and no structural data were obtained and the authors are not sure about the dimensionality of the aggregates in the LbL films (1D or 2D).57,58 However, in the exciton self-trapping model the excitons can move along the aggregate during the time window between optical excitation and self-trapping. So, the exciton self-trapping model is not in conflict with the exciton diffusion obtained from the fluorescence data for THIATS J-aggregates in LbL films.57,58 Thus, applying an exciton selfACS Paragon Plus Environment


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20 trapping model for the description of the optical properties of J-aggregates with different geometries and embedded in polymer films is also reasonable. As the fluorescence decays are very short (Figure 6), experiments with ultrashort laser pulses could be useful to understand the exciton dynamics in the J-aggregates.13,32,33,73–78 Figure 7 shows transient absorption spectra of TDBC aggregates in spin-coated and LbL films recorded by means of ultrafast pump-probe spectroscopy. It turned out that the aggregates degrade under irradiation with ultrashort laser pulses, in particular in the LbL samples at higher excitation powers. To reduce this effect a rotating sample holder was implemented to replace continuously irradiated sample area by fresh one. Using it, measurements can be performed over a sufficiently long time to achieve a good signal-to-noise ratio as it is demonstrated by the data presented in Figure 7.

a)

b)

Figure 7. Transient absorption spectra for TDBC J-aggregates in spin-coated (a) and LbL (b) films at different excitation powers. For comparison the stationary absorption of the aggregate films and the spectrum of the excitation pulses are depicted too.


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21 The transient spectra of the TDBC aggregates in spin-coated and LbL films exhibit a similar shape and time evolution (Figure 7). They are dominated by the ground state bleach but show in addition stimulated emission (SE) at the long wavelength side of the ground state absorption and excited state absorption (ESA) at the short wavelength side due to transitions from the one- to the two-exciton manifold.13,32,75 Very similar transient absorption spectra have been also observed for TDBC aggregates in solution.44 Most of the signal decays within the first ten picoseconds and only a weak component remains for longer times. In addition, the ESA shifts with time somewhat to the red. This effect was previously observed for BIC J-aggregates in solution and was assigned to a redistribution between negative and positive signals with time.79 With increasing excitation power the bleach signal becomes only a little bit stronger in the case of the spin coated films and stays even almost constant in case of the LbL films. However, the ESA gains significantly intensity towards the blue in both types of films (Figure 7). These spectral changes can be attributed to saturation of the transition to the lower band edge of the one-exciton manifold, which carries most of the oscillator strength, and the increasing excitation of exciton states with a smaller coherence length which absorb in the blue wing of the J-band.77 To characterize the time dependence more quantitatively we performed a global fit of a triple exponential decay to the transient spectra. The decay associated difference spectra (DADS) and time constants obtained for the data shown in Figure 7 are depicted in Figure 8. The dominant contribution of the dynamics is described by a sum of an exponential decay with a short time constant in the range of several hundred femtoseconds to a picosecond and a second, slower exponential component with a time constant of a few picoseconds (Figure 8). The corresponding DADSs exhibit a shape similar to the original transient spectra and reflect also a combination of the ground state bleach with SE and ESA bands located at the red and blue side of the bleach, respectively. The DADS of the faster component is thereby blue shifted with respect to the slower one. This indicates that higher lying excitonic states contribute particularly to the earliest phase of the dynamics. The third exponential component is much weaker and decays on a timescale in the order of a nanosecond. Due to its weakness and the limited scanning range (2 ns) of the setup, the time constant cannot be accurately determined. The associated DADSs consist merely of ground state bleach. This points to a species which itself exhibits no pronounced absorption or stimulated emission signal and which relaxes back to the original ground state on a timescale of a nanosecond.



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22

a)

b)

Figure 8. Decay associated difference spectra of a global fit with three exponential decays for the transient absorption spectra shown in Figure 7. The associated time constants are given in the legend.

At higher excitation levels, the DADSs of the shortest time constant become more dominant (Figure 8) and simultaneously a shortening of all time constants is observed. This indicates that exciton-exciton annihilation dominates the decay dynamics (Figure 8) as it was also found for TDBC aggregates in solution.44 In an annihilation event one exciton hops onto an aggregate segment which is already occupied by another exciton. Thereby a highly excited state is populated which relaxes quickly back to the lowest electronically excited state, i.e. the one-exciton state. This results in the deactivation of one of the two involved excitons. The hopping process is probably of Förster type and the rate should scale with the spectral overlap between the donor emission, i. e. the exciton fluorescence and the acceptor absorption, here the ESA of a populated exciton state.60,62 Due to the large transition dipoles of the excitons and the good spectral overlap resulting from the small Stokes shift, large hopping rates are expected. In particular, the fluorescence of higher lying exciton states contributing to the blue wing of the J-band overlaps almost perfectly with the ESA of ACS Paragon Plus Environment

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23 excitons at the lower band edge. It can therefore be expected that the higher lying excitons are involved in particularly fast annihilation events. We think that these annihilation events are dominantly responsible for the shortest decay time. Thereafter the exciton population is focused at the band edge and the subsequent phase is characterized by annihilation among these excitons. Due to the larger shift between ESA and fluorescence, the spectral overlap is now smaller and the annihilation dynamics slower. This phase is more or less resembled by the second decay time. However, it has to be noted that the dynamics is most probably non-exponential and the description by a triple exponential decay is just a parameterization. A clear assignment of the decay constants to specific steps is therefore not possible. Beside of exciton-exciton annihilation, also other processes should contribute to the dynamics like non-radiative decay, as the rather short luminescence lifetimes indicate, and self-trapping as discussed below. We also cannot rule out the contribution of bi-exciton states and the fast decay component can contain contributions from a decay of biexcitons to the single exciton state. However, we think the contributions from higher lying states in the single exciton band dominate. The decay of a bi-exciton to a single free exciton should dominantly shift the stimulated emission but not change its strength. Furthermore with increasing excitation intensity the relative strength of the bi-exciton contribution should strongly increase. However, both signatures are not clearly seen in the transient spectra. In the ultrafast absorption measurements the excitation probability was varied from 1.2% to 9.8% per TDBC molecule (see above). Due to the delocalization of the excitons these excitation levels lead already to a pronounced interaction between the excitons and to exciton-exciton annihilation. In the picosecond measurements a pulse energy of 13 pJ was applied which is three orders of magnitude less. Accordingly exciton-exciton annihilation does not significantly contribute to the dynamics observed in the picosecond measurements. Comparing the decay constants obtained by the pump-probe measurements for the Jaggregates in the spin-coated and LbL films, one finds that the time constants for the LbL films are shorter (Figure 8). This can be explained by the smaller Stokes shift of the fluorescence in the LbL films resulting in a larger spectral overlap and in faster hopping processes than in spin coated films (Figure 6). At higher excitation levels, the decay constants for the spin-coated and LbL films become more similar probably due to the increasing contribution of exciton states above the band edge (Figure 8). Summarizing the femtosecond data, we can conclude that the fast dynamics within the first 10 ps is dominated by exciton-exciton annihilation and that thereafter some population is observed in states, which exhibit largely reduced transition dipoles compared to fully delocalized excitons. The origin of these states is not yet clear. At room temperature, we have a quite complicated ACS Paragon Plus Environment


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24 situation with respect to the dynamics due to the strong influence of thermal processes. Therefore, lowering the sample temperature could provide additional information.13,26,33,51,75,76 At low temperatures (77 K) the optical spectra of TDBC J-aggregates in polymer films exhibit some differences to the room temperature spectra (Figure 9). The J-bands and, correspondingly, also the fluorescence bands are blue-shifted and narrowed due to weaker linear exciton-phonon scattering.33,80 The J-band maximum is at LTspin = 587 nm in the spin-coated film corresponding to a blue-shift of spin = – 85 cm–1 with respect to room temperature and at LTLbL = 585.5 nm in the LbL film corresponding to a shift of LbL= – 60 cm–1 (Figure 9). It should be noted that the LbL film contained four aggregate layers to obtain a more intense fluorescence signal. The J-band of such a multilayer film is broader compared to a single aggregate layer.17 Thus, we will not compare the band widths at room and low temperature.

a)

b)

Figure 9. Absorption (1) and fluorescence (2, exc = 530 nm) spectra of TDBC J-aggregates in a) spin-coated and b) LbL (four J-aggregate layers) films at T = 77 K.

There are two obvious and important changes in the steady-state spectra for TDBC Jaggregates in the polymer films at low temperatures (Figure 9). First, in the spin-coated film the heavy tail of the J-band is much flatter and wider compared to the room temperature situation (Figure 9a). A heavy tail can be found for the LbL film, too, although it is less pronounced (Figures 9b and S15 of Supporting Information). This is evidence of strong off-diagonal disorder at least in the spin-coated film.12,50–55 Second, in the fluorescence of the LbL film a strong long wavelength shoulder (maxshoulder = 635 nm) is observed beside of the resonance fluorescence (maxmain = 591.5 nm) contrary to the emission spectra at room temperature (Figure 5b). The fluorescence of the spinACS Paragon Plus Environment

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25 coated film exhibits also a shoulder at the red wing, although it is less pronounced (Figure 9a). This feature could be an indication for exciton self-trapping.26,51 Unfortunately, accurate measurements of the fluorescent quantum yields at low temperatures are impossible with our experimental setup. We can roughly estimate changes of the quantum yield via the relative changes of the integral intensity of the fluorescence band at different temperatures. For this purpose, two fluorescence spectra of the films were recorded under the same experimental conditions using a cryostat: at room temperature and at liquid nitrogen temperature (Figure S16 of Supporting Information). The fluorescence spectra of the spin-coated film have nearly the same integral intensity (Figure S16a of Supporting Information), but in case of the LbL film the fluorescence signal is at low temperature about twice (1.8 times) as large as at room temperature with an intensity growth of both bands (Figure S16b of Supporting Information). This observation disproves the hypothesis that the red-shifted fluorescence band originates from some energy accepting state, because an intensity redistribution should be expected between donor and acceptor emission if the energy transfer efficiency decreases. Beside of self-trapping, an explanation for the low fluorescence yield in the polymer films might be additional non-radiative decay channels or dark states induced by the matrix.57,58 In the first case a strongly reduced fluorescence lifetime is expected. This is in contrast to the observation of similar lifetimes of 65 ps and 60 ps in the spin-coated films at room temperature and in water, respectively (Figures 6 and S14 of Supporting Information). The very fast fluorescence decay beyond the time resolution of the picosecond measurements found in the LbL films might be interpreted as signature of such a fast deactivation channel or a fast transfer to a non-emissive state other than the exciton selftrapping.57,58 Indeed, we cannot completely rule out that such matrix induced non-emissive states exist. However, we think it is unlikely that they are primarily responsible for the low fluorescence yield and the red-shifted emission at low temperatures. In the pump-probe experiments we didn't see signatures for other deactivation paths such as the formation of charge transfer or ionic states, which might play a role in recombination processes. We also think that the additional emission band appearing at cold temperatures does not result from trap states in the polymer or at the interface of the aggregate to the polymer since its spectral position is close to the resonance fluorescence indicating that it is related to excitonic states of the aggregates. In addition, the red-shifted emission band is similar in both polymer films even though they are made off very different polymers. Furthermore, in case of efficient energy migration to weakly fluorescing traps as main reason for fluorescent quenching one should expect another character of the spectral changes with varying temperature. In case of dominant coherent exciton transport81 the diffusion rate increases with decreasing temperature, so one would expect a decrease of the J-aggregate fluorescence intensity and a growth of the trap fluorescence with decreasing temperature.82 In case of incoherent ACS Paragon Plus Environment


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26 (hopping) exciton transport vice versa, at lower temperatures one would expect a growth of the Jaggregate fluorescence and a decrease of the trap fluorescence.82 However, we see a growth of both bands for the LbL films and a stronger shoulder for the spin-coated films (Figure S16 of Supporting Information). For J-aggregates it is known that large static disorder, especially an off-diagonal one, can provoke exciton self-trapping.51 To clarify the possibility of exciton self-trapping the excitonphonon coupling constant should be estimated.26,51,59,60 This can be done applying the well-known Urbach rule to the temperature dependence of the long wavelength wing of the J-band (see details in the Supporting Information, particularly, Figure S17).26,51,59,60 In the case of TDBC J-aggregates in polymer films, we obtained in this way estimations for the exciton-phonon coupling constants of gspin ~ 1.15 and gLbL ~ 1.35. Thus, for both films the coupling is strong leading to the formation of small radius polarons and exciton self-trapping should be expected.26,51,59,60 Since in spin-coated films the TDBC J-aggregates are quasi-one-dimensional, exciton self-trapping can result in this case in a larger Stokes shift, broadening of the fluorescence bandwidth and a smaller emission yield as it is found at room temperature (Figure 5a).71 Contrary, in the two-dimensional J-aggregates in the LbL films a barrier for exciton self-trapping should exist which could be revealed by two band emission at low temperatures (Figure 9b).26,51 Besides the steady-state spectra, also modifications in the fluorescence decays were observed at low temperatures, especially for the LbL films (Figure 10).

Figure 10. Fluorescence decays (exc = 531 nm, reg = 590 nm) for TDBC J-aggregates in different polymer films at T = 77 K: 1 – spin-coated, 2 – LbL (four J-aggregate layers). Curve 3 is the IRF. ACS Paragon Plus Environment

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27 Contrary to the room temperature situation (Figure 6), the fluorescence decay curves for TDBC J-aggregates in spin-coated and LbL films are very similar (Figure 10). They can be approximated by three exponential components with time constants (and fractional amplitudes) of

1spin ~ 30 ps (96,9%), 2spin ~ 0.21 ns (2.7%) and 3spin ~ 1.24 ns (0.4%) for the spin-coated film (Figure S18 of Supporting Information) and 1LbL ~ 30 ps (95.4%), 2 LbL ~ 0.19 ns (4.1%) and 3 LbL ~ 0.91 ns (0.5%) for the LbL film (Figure S19 of Supporting Information). Taking the fractional amplitudes into account results in average lifetimes of ~ 40 ps for both films. Due to the time resolution of the picosecond apparatus one cannot exclude that faster components exist. However, they should not dominate the fluorescence since otherwise the shape of time-resolved emission spectra after the instrument response function should be very different from the stationary fluorescence which is not the case (see below). Using three-exponential fits instead of twoexponential ones, also in view of the small fractional contribution of the longest components (~ 0.5%), is reasonable due to the visible contribution of the long components to the stationary intensities (Figures S18 and S19 of Supporting Information). Also, applying three-exponential fitting is approved by 2 ~ 1 and a random distribution of deviations on the residual plots (Figures S18 and S19 of Supporting Information).3 However, we have no clear evidence for three different emitting states existing in our systems. In analogy to the lifetime distribution found for self-trapped states in PIC J-aggregates,26 we suppose that the shortest and dominating contribution belongs to free excitons and the longer components originate from self-trapped states which exhibit probably a distribution of lifetimes. Often, non-exponential distributions can be fitted by two exponents.3 Since we have overlapping emissions from free and self-trapped excitons it is difficult to determine the lifetime distribution for the latter states accurately. The shortening of the fluorescence lifetime upon lowering the temperature, as it is revealed for the spin-coated film, is typical for J-aggregates and caused by an increase of the coherence length (Figure 9) due to weaker exciton-phonon scattering.33 Indeed, the exciton lifetime for Jaggregates is proportional to Ncoh–1, and this effect is often called exciton superradiance.12 The growth of the lifetime for the LbL film might result from a reduced accessibility of the nonradiative relaxation channel due to the lower thermal energy. To analyze the long living emission components and the impact of self-trapping in more detail, fluorescence decays were recorded at different wavelengths within the fluorescence band of the TDBC aggregates in the polymer films (Figure 11).



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28

a)

b)

Figure 11. Fluorescence decays (exc = 531 nm) for TDBC J-aggregates at T = 77 K in a) spincoated and b) LbL (four J-aggregate layers) films at different detection wavelengths.

One can see, that for the spin-coated film the contribution of the main, fast component decreases by tuning the detection wavelength from the center of the narrow fluorescence band to the long wavelength tail while the second, slower component is more or less preserved (Figure 11a). Thus the short living component, which is dominant in the fluorescence decay, can be associated with the narrow near-resonant fluorescence band (maxmain = 592 nm), and the second, weaker component can be attributed to a band centered at longer wavelengths and seems to contribute strongly to the long wavelength shoulder (maxshoulder = 630 nm) of the fluorescence band (Figure 9a). Species responsible for such an emission can be self-trapped excitons for which the lifetime is typically larger than the free exciton lifetime.26,51,60 A similar decrease of the short decay component at longer wavelengths is also observed in the LbL film (Figure 11b). However, the decay time of the second component apparently increases at longer wavelengths (Figure 11b), contrary to the spin-coated film pointing to a difference in the exciton dynamics. To reveal the contribution of self-trapped excitons, time-resolved emission spectra (TRES) appeared to be suitable.26,51 Indeed, TRES of the spin-coated and the LbL films exhibit a different evolution of the spectra with time (Figure 12).


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29

a)

b)

Figure 12. Normalized time-resolved emission spectra at T = 77 K of TDBC J-aggregates in a) spin-coated and b) LbL (4 J-aggregate layers) films at different delay times after excitation.

For the spin-coated film the narrow near-resonant band (maxmain = 592 nm) is dominant during the whole decay (Figure 12a). However, at larger delay times the relative contribution of the broad shoulder (maxshoulder = 630 nm) increases leading also to a broadening of the main band and a small red-shift to maxmain (at 2.4 ns) = 594.5 nm (see Figure S20a in the Supporting Information). For the LbL film a redistribution of the bands is observed (Figures 12b and S20b in the Supporting Information) with the broad and red shifted band of the self-trapped excitons (maxSTE = 638 nm) becoming dominant at later delay times instead of the narrow band of the free-excitons (maxFE = 591.5 nm). It should be noted that the actual intensity of the emission at large delay times is quite low (Figure S21 in the Supporting Information) and the red-shifted bands contribute only little to the steady-state fluorescence spectra (Figure 9). Now we can consider the features of the exciton dynamics in the TDBC J-aggregates in different polymer films. In both, spin-coated and polymer films, exciton self-trapping occurs due to strong exciton-phonon coupling. In both cases it leads to a decrease of the fluorescence quantum yield. As in spin-coated films TDBC J-aggregates preserve their quasi one-dimensional structure, ACS Paragon Plus Environment


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30 exciton self-trapping is in this case barrierless (Figure 13a).26,51,59,60 Exciton self-trapping manifests itself in a larger Stokes shift, a broadening of the near-resonance fluorescence band and a decrease of the emission yield.26,51,59,60 At decreasing temperatures this process competes with a weakening of exciton-phonon scattering, which tends to increase the exciton coherence length, and, hence to narrow the fluorescence band and to decrease the exciton lifetime. However, some aggregate agglomerates of non-one-dimensional character are probably also present in the spin-coated films. They can lead to contributions to the fluorescence, which are responsible for the weak red-shifted band and the small slowly decaying component (Figures 9a and 10).

a)

b) Figure 13. Schemes of barrierless (a) and barrier-type (b) exciton self-trapping in form of

adiabatic potential energy surfaces. Legend: FE – free excitons, STE – self-trapped excitons, B – half-width of the exciton band, EST – self-trapping depth, EB – self-trapping barrier height.

Contrary, in the LbL films the J-aggregates possess a two-dimensional geometry and exciton self-trapping is associated with a barrier (Figure 13b).26,51,59,60 The height EB of the self-trapping barrier (Figure 13b) can be obtained by:59 EB = B2/4ELR ,


(3)

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31 where B = 2J is the half-width of the exciton band and ELR the lattice relaxation energy (Figure 13b). The latter is given by ELR = EST + B, where EST is the energy reduction upon self-trapping or the energy difference between the fluorescence maxima of the free and of the self-trapped exciton (Figure 13b).59 Using Eq. 2 and the data of Figure 9b we obtain JLbL (at 77 K) = 785 cm–1 and ESTLbL = 1230 cm–1, and thus B = 1570 cm–1 and ELR = 2800 cm–1. This results for TDBC J-aggregates in LbL films in a barrier height for self-trapping of EB = 220 cm–1 which is twice as large as the one for PIC J-aggregates in LbL films.26 Using a schematic representation of exciton self-trapping by adiabatic potential curves as shown in Figure 13, one can understand the emission properties of the TDBC J-aggregates in the polymer films.61,64,83 At low temperatures the emission of the free and the self-trapped exciton are observed at 592 nm and 630 nm, respectively. Using the trap depth of 430 cm–1 estimated above for the spin-coated films the emission of the self-trapped state would be expected at (592–1 nm–1 – 430 cm–1)–1 = 607 nm. However, this calculation implies that the ground state does not change its energy along the self-trapping coordinate although the latter is associated with structural deformations. This is unrealistic and the ground state energy should rise significantly with the structural deformation, as shown in Figure 13. If one assumes that the energetic rise of the ground state is comparable to the self-trapping energy one expects at cold temperatures the emission of the self-trapped exciton at 624 nm which is already close to the observation. The temporal behaviour is governed by the potential energy barriers. For quasi-one-dimensional J-aggregates in spin-coated films the crossing point of the adiabatic self-trapped and ground state potential lay above the lowest point of the free exciton potential (Figure 13a).83 Therefore, even at high temperatures the nonradiative transition of self-trapped excitons to the ground state is less probable.83 Contrary, for twodimensional J-aggregates in LbL films the crossing point of the adiabatic self-trapped and ground state potential lays lower than the lowest point of the free exciton potential and the highest point of the self-trapping barrier (Figure 13b).83 After the optical excitation excess energy is stored in the aggregates. This allows at early times to cross the self-trapping barrier of the aggregates in the LbL films even at low temperatures. Later cooling processes reduce strongly the probability for barrier crossing and decouple the population of free excitons from the self-trapped ones. Since the lifetime of the free excitons is at 77 K only 30 ps the self-trapped exciton emission dominates at late times the fluorescence spectrum. At room temperature barrier crossing is possible all the time and an equilibrium between the free and the self-trapped excitons exists. Emission from the free and the self-trapped excitons decays then simultaneously and rapidly. In addition, the self-trapped exciton can be depopulated via a non-radiative decay channel provided by a crossing of the ground state potential with the potential of the self-trapped exciton, as it is also indicated in Figure 13.61,64,83 However, this channel exhibits a barrier given by the crossing point. Due to this barrier the channel ACS Paragon Plus Environment


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32 is suppressed at 77K. This contributes to a prolongation of the lifetime of the self-trapped exciton and an increased emission yield. The different nonradiative transition probabilities for the selftrapped excitons in TDBC J-aggregates formed in the two types of polymer films cause the differences in quantum yields and spectroscopic properties found in the study.

Conclusions We studied the exciton dynamics of TDBC J-aggregates formed in two types of polymer films, namely uncharged spin-coated PVA films and charged layer-by-layer PDDA films. It was found that despite the similarity of the main steady-state spectral characteristics of the J-aggregates in the polymer films their fluorescence quantum yields are strongly different (spin ~ 4% and LbL ~ 0.5%) and both are much smaller than in solution (sol ~ 31%). Using optical microscopy and AFM images the different morphologies of the TDBC aggregates in the two types of polymer films were characterized. For spin-coated films a quasi-onedimensional, rod-like morphology of the J-aggregates was found similar to that in solution. However, for LbL films the morphology of the aggregates transforms to two-dimensional, islandlike structures. Due to the more rigid environment in the polymer films the exciton-phonon coupling constant of the J-aggregates becomes quite large resulting in efficient exciton self-trapping. The latter is supposed to be responsible for the lower quantum yields in the films compared to solution. Exciton self-trapping manifests itself at low temperatures (~ 80 K) in a red-shifted fluorescent band with a long decay time. The population of self-trapped exciton states can probably be assigned to the weak long living component of the transient absorption spectra. It reflects only ground state bleach indicating that the populated states are only weakly emitting. The fast dynamics within the first 10 ps observed by pump-probe spectroscopy is dominated by exciton-exciton annihilation and associated with a faster deactivation of higher lying exciton states. The population transfer to the self-trapped states is hidden under this dynamics. Using time-resolved emission spectra it was shown that the features of the self-trapping processes are different for both types of the studied polymer films. For the quasi 1D TDBC aggregates in the spin-coated films barrierless exciton self-trapping dominates. For the 2D TDBC aggregates in the LbL films exciton self-trapping occurs via a barrier with a height of 220 cm–1. As a result fluorescence quenching is for TDBC J-aggregates in LbL films at room temperature much stronger than at low temperatures. ACS Paragon Plus Environment

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33 Acknowledgements Financial support by the German Science Foundation via the grant "Control of Exciton Dynamics in Molecular Aggregates by Plasmonic Coupling to Nanoparticles" for initiating an international collaboration, reference number L0714/9-1, is gratefully acknowledged. Furthermore, we thank Franziska Fennel for her help in the pump-probe measurements and her comments on the manuscript. Supporting Information Additional spectral data and microscope images are provided to support the presented results. This material is available free of charge via the internet at http://pubs.acs.org.

References (1)

Organic Nanostructures for next Generation Devices; Al-Shamery, K. H., Rubahn, H.-G., Sitter, H., Eds.; Springer: Berlin, Heidelberg, 2008.

(2)

Organic Optoelectronics; Hu, W., Ed.; Wiley-VCH: Weinheim, Germany, 2013.

(3)

Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer, 2006.

(4)

Demchenko, A. P. Introduction to Fluorescence Sensing, Second Edi.; Springer International Publishing: Cham, 2015.

(5)

Möbius, D. Scheibe Aggregates. Adv. Mater. 1995, 7 (5), 437–444.

(6)

J-Aggregates; Kobayashi, T., Ed.; World Scientific: Singapore, 1996.

(7)

J-Aggregates. Volume 2; Kobayashi, T., Ed.; World Scientific: Singapore, 2012.

(8)

Würthner, F.; Kaiser, T. E.; Saha-Möller, C. R. J-Aggregates: From Serendipitous Discovery to Supramolecular Engineering of Functional Dye Materials. Angew. Chemie - Int. Ed. 2011, 50 (15), 3376–3410.

(9)

Bricks, J. L.; Slominskii, Y. L.; Panas, I. D.; Demchenko, A. P. Fluorescent J-Aggregates of Cyanine Dyes: Basic Research and Applications Review. Methods Appl. Fluoresc. 2017, 6 (1), 012001.

(10)

Jelley, E. E. Spectral Absorption and Fluorescence of Dyes in the Molecular State. Nature 1936, 138 (3502), 1009–1010.

(11)

Scheibe, G.; Kandler, L.; Ecker, H. Polymerisation Und Polymere Adsorption Als Ursache Neuartiger Absorptionsbanden von Organischen Farbstoffen. Naturwissenschaften 1937, 25 (5), 75.

(12)

Knoester, J.; Agranovich, V. M. Frenkel and Charge-Transfer Excitons in Organic Solids. In Thin Films and Nanostructures: Electronic Excitations in Organic Based Nanostructures, ACS Paragon Plus Environment


Page 34 of 41

34 Volume 31; Agranovich, V. M., Bassani, G. F., Eds.; 2003; Vol. 31, pp 1–96. (13)

Kühn, O.; Lochbrunner, S. Quantum Dynamics and Spectroscopy of Excitons in Molecular Aggregates. In Quantum Efficiency in Complex Systems, Part II: From Molecular Aggregates to Organic Solar Cells, Volume 85; Wurfel, U., Thorwart, M., Weber, E., Eds.; Academic Press, 2011; pp 47–81.

(14)

Shelkovnikov, V. V.; Plekhanov, A. I. Optical and Resonant Non-Linear Optical Properties of J-Aggregates of Pseudoisocyanine Derivatives in Thin Solid Films. In Macro To Nano Spectroscopy; Uddin, J., Ed.; InTech, 2012; p 317.

(15)

Tisserant, J. N.; Brönnimann, R.; Hany, R.; Jenatsch, S.; Nüesch, F. A.; Mezzenga, R.; Bona, G. L.; Heier, J. Resonance Light Scattering in Dye-Aggregates Forming in Dewetting Droplets. ACS Nano 2014, 8 (10), 10057–10065.

(16)

Sorokin, A. V.; Yefimova, S. L.; Malyukin, Y. V. Polymer-Bound Supramolecular JAggregates: Optical Properties and Applications. In Encyclopedia of Polymer Science and Technology; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2018; pp 1–33.

(17)

Bradley, M. S.; Tischler, J. R.; Bulović, V. Layer-by-Layer J-Aggregate Thin Films with a Peak Absorption Constant of 106 Cm-1. Adv. Mater. 2005, 17 (15), 1881–1886.

(18)

Kirstein, S.; Bourbon, S.; Gao, M.; De Rossi, U. Layer-by-Layer Deposition of J-Aggregates and Polyelectrolytes for Electroluminescence Applications: A Spectroscopic Study. Isr. J. Chem. 2000, 40 (2), 129–138.

(19)

Scheblykin, I.; Lepnev, L.; Vitukhnovsky, A.; Van der Auweraer, M. Electroluminescence and Optical Properties of Poly(Phenylenevinylene)/J-Aggregate Composites. J. Lumin. 2001, 94–95, 461–464.

(20)

Tischler, J. R.; Bradley, M. S.; Bulović, V.; Song, J. H.; Nurmikko, A. Strong Coupling in a Microcavity LED. Phys. Rev. Lett. 2005, 95 (3), 036401.

(21)

Tischler, J. R.; Scott Bradley, M.; Zhang, Q.; Atay, T.; Nurmikko, A.; Bulović, V. Solid State Cavity QED: Strong Coupling in Organic Thin Films. Org. Electron. 2007, 8 (2–3), 94–113.

(22)

Gentile, M. J.; Núñez-Sánchez, S.; Barnes, W. L. Optical Field-Enhancement and Subwavelength Field-Confinement Using Excitonic Nanostructures. Nano Lett. 2014, 14 (5), 2339–2344.

(23)

Cacciola, A.; Triolo, C.; Di Stefano, O.; Genco, A.; Mazzeo, M.; Saija, R.; Patanè, S.; Savasta, S. Subdiffraction Light Concentration by J-Aggregate Nanostructures. ACS Photonics 2015, 2 (7), 971–979.

(24)

Obara, Y.; Saitoh, K.; Oda, M.; Tani, T. Room-Temperature Fluorescence Lifetime of Pseudoisocyanine (PIC) J Excitons with Various Aggregate Morphologies in Relation to ACS Paragon Plus Environment

Page 35 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35 Microcavity Polariton Formation. Int. J. Mol. Sci. 2012, 13 (5), 5851–5865. (25)

Sorokin, A. V.; Zabolotskii, A. A.; Pereverzev, N. V.; Bespalova, I. I.; Yefimova, S. L.; Malyukin, Y. V.; Plekhanov, A. I. Metal-Enhanced Fluorescence of Pseudoisocyanine JAggregates Formed in Layer-by-Layer Assembled Films. J. Phys. Chem. C 2015, 119 (5), 2743–2751.

(26)

Sorokin, A. V.; Pereverzev, N. V.; Grankina, I. I.; Yefimova, S. L.; Malyukin, Y. V. Evidence of Exciton Self-Trapping in Pseudoisocyanine J-Aggregates Formed in Layered Polymer Films. J. Phys. Chem. C 2015, 119 (49), 27865–27873.

(27)

Nabok, A. Organic and Inorganic Nanostructures; Artech House, 2005.

(28)

Xiang, Y.; Lu, S.; Jiang, S. P. Layer-by-Layer Self-Assembly in the Development of Electrochemical Energy Conversion and Storage Devices from Fuel Cells to Supercapacitors. Chem. Soc. Rev. 2012, 41 (21), 7291.

(29)

Zhang, Q.; Atay, T.; Tischler, J. R.; Bradley, M. S.; Bulović, V.; Nurmikko, A. V. Highly Efficient Resonant Coupling of Optical Excitations in Hybrid Organic/Inorganic Semiconductor Nanostructures. Nat. Nanotechnol. 2007, 2 (9), 555–559.

(30)

Makio, S.; Kanamaru, N.; Tanaka, J. The J-Aggregate 5,5′,6,6′-Tetrachloro-1,1′-Diethyl-3,3′Bis(4-Sulfobutyl)Benzimidazolocarbocyanine Sodium Salt in Aqueous Solution. Bull. Chem. Soc. Jpn. 1980, 53 (11), 3120–3124.

(31)

Sorokin, A. V.; Ropakova, I. Y.; Grynyov, R. S.; Vilkisky, M. M.; Liakh, V. M.; Borovoy, I. A.; Yefimova, S. L.; Malyukin, Y. V. Strong Difference between Optical Properties and Morphologies for J-Aggregates of Similar Cyanine Dyes. Dye. Pigment. 2018, 152, 49–53.

(32)

van Burgel, M.; Wiersma, D. A.; Duppen, K. The Dynamics of One-Dimensional Excitons in Liquids. J. Chem. Phys. 1995, 102 (1), 20–33.

(33)

Moll, J.; Daehne, S.; Durrant, J. R.; Wiersma, D. A. Optical Dynamics of Excitons in J Aggregates of a Carbocyanine Dye. J. Chem. Phys. 1995, 102 (16), 6362–6370.

(34)

Pawlik, A.; Ouart, A.; Kirstein, S.; Abraham, H.-W.; Daehne, S. Synthesis and UV/Vis Spectra of J-Aggregating 5,5′,6,6′-Tetrachlorobenzimidacarbocyanine Dyes for Artificial Light-Harvesting Systems and for Asymmetrical Generation of Supramolecular Helices. European J. Org. Chem. 2003, 2003 (16), 3065–3080.

(35)

Fu, Y.; Li, S.-J.; Xu, J.; Yang, M.; Zhang, J.-D.; Jiao, Y.-H.; Zhang, J.-C.; Zhang, K.; Jia, Y.G. Facile and Efficient Approach to Speed up Layer-by-Layer Assembly: Dipping in Agitated Solutions. Langmuir 2011, 27 (2), 672–677.

(36)

Debnath, P.; Chakraborty, S.; Deb, S.; Nath, J.; Dey, B.; Bhattacharjee, D.; Hussain, S. A. Stability of J-Aggregated Species in an Indocarbocyanine Dye in Langmuir–Blodgett Films. J. Lumin. 2016, 179, 287–296. ACS Paragon Plus Environment


Page 36 of 41

36 (37)

Faulkner, D. O.; McDowell, J. J.; Price, A. J.; Perovic, D. D.; Kherani, N. P.; Ozin, G. A. Measurement of Absolute Photoluminescence Quantum Yields Using Integrating Spheres Which Way to Go? Laser Photon. Rev. 2012, 6 (6), 802–806.

(38)

Ahn, T.-S.; Al-Kaysi, R. O.; Müller, A. M.; Wentz, K. M.; Bardeen, C. J. Self-Absorption Correction for Solid-State Photoluminescence Quantum Yields Obtained from Integrating Sphere Measurements. Rev. Sci. Instrum. 2007, 78 (8), 086105.

(39)

Riedle, E.; Beutter, M.; Lochbrunner, S.; Piel, J.; Schenkl, S.; Spörlein, S.; Zinth, W. Generation of 10 to 50 Fs Pulses Tunable through All of the Visible and the NIR. Appl. Phys. B 2000, 71 (3), 457–465.

(40)

Clark, K. A.; Krueger, E. L.; Vanden Bout, D. A. Direct Measurement of Energy Migration in Supramolecular. J. Phys. Chem. Lett. 2014, 5, 2274–2282.

(41)

Eisele, D. M.; Arias, D. H.; Fu, X.; Bloemsma, E. A.; Steiner, C. P.; Jensen, R. A.; Rebentrost, P.; Eisele, H.; Tokmakoff, A.; Lloyd, S.; et al. Robust Excitons Inhabit Soft Supramolecular Nanotubes. Proc. Natl. Acad. Sci. 2014, 111 (33), E3367–E3375.

(42)

Sorokin, A. V.; Ropakova, I. Y.; Yefimova, S. L.; Malyukin, Y. V. Influence of Pseudoisocyanine J-Aggregate Agglomeration on the Optical Properties. Funct. Mater. 2018, 25 (1), 088–092.

(43)

Kirstein, S.; Daehne, S. J-Aggregates of Amphiphilic Cyanine Dyes: Self-Organization of Artificial Light Harvesting Complexes. Int. J. Photoenergy 2006, 2006, 1–21.

(44)

v. Berlepsch, H.; Böttcher, C. Supramolecular Structure of TTBC J-Aggregates in Solution and on Surface. Langmuir 2013, 29 (16), 4948–4958.

(45)

Friedl, C.; Renger, T.; Berlepsch, H. V.; Ludwig, K.; Schmidt am Busch, M.; Megow, J. Structure Prediction of Self-Assembled Dye Aggregates from Cryogenic Transmission Electron Microscopy, Molecular Mechanics, and Theory of Optical Spectra. J. Phys. Chem. C 2016, 120 (34), 19416–19433.

(46)

Didraga, C.; Pugžlys, A.; Hania, P. R.; von Berlepsch, H.; Duppen, K.; Knoester, J. Structure, Spectroscopy, and Microscopic Model of Tubular Carbocyanine Dye Aggregates. J. Phys. Chem. B 2004, 108 (39), 14976–14985.

(47)

Yao, H.; Sugiyama, S.; Kawabata, R.; Ikeda, H.; Matsuoka, O.; Yamamoto, S.; Kitamura, N. Spectroscopic and AFM Studies on the Structures of Pseudoisocyanine J Aggregates at a Mica/Water Interface. J. Phys. Chem. B 1999, 103 (21), 4452–4456.

(48)

Maskasky, J. E. Molecular Orientation of Individual J Aggregates on Gelatin-Grown AgBr Tabular Microcrystals. Langmuir 1991, 7 (2), 407–421.

(49)

Wolthaus, L.; Schaper, A.; Möbius, D. Brickstone Arrangement in J-Aggregates on an Amphiphilic Merocyanine Dye. Chem. Phys. Lett. 1994, 225 (4–6), 322–326. ACS Paragon Plus Environment

Page 37 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


37 (50)

Malyukin, Y. V.; Tovmachenko, O. G.; Katrich, G. S.; Kemnitz, K. Optical Spectroscopy of Disorder in Molecular Chains (J -Aggregates). Low Temp. Phys. 1998, 24 (12), 879–886.

(51)

Malyukin, Y. V.; Sorokin, A. V.; Semynozhenko, V. P. Features of Exciton Dynamics in Molecular Nanoclusters (J-Aggregates): Exciton Self-Trapping (Review Article). Low Temp. Phys. 2016, 42 (6), 429–440.

(52)

Fidder, H.; Knoester, J.; Wiersma, D. A. Optical Properties of Disordered Molecular Aggregates: A Numerical Study. J. Chem. Phys. 1991, 95 (11), 7880–7890.

(53)

Eisfeld, A.; Vlaming, S. M.; Malyshev, V. A.; Knoester, J. Excitons in Molecular Aggregates with Lévy-Type Disorder: Anomalous Localization and Exchange Broadening of Optical Spectra. Phys. Rev. Lett. 2010, 105 (13), 137402.

(54)

Merdasa, A.; Jiménez, Á. J.; Camacho, R.; Meyer, M.; Würthner, F.; Scheblykin, I. G. Single Lévy States–Disorder Induced Energy Funnels in Molecular Aggregates. Nano Lett. 2014, 14 (12), 6774–6781.

(55)

Lüer, L.; Rajendran, S. K.; Stoll, T.; Ganzer, L.; Rehault, J.; Coles, D. M.; Lidzey, D.; Virgili, T.; Cerullo, G. Lévy Defects in Matrix-Immobilized J Aggregates: Tracing Intra-and Intersegmental Exciton Relaxation. J. Phys. Chem. Lett. 2017, 8 (3), 547–552.

(56)

Vlaming, S. M.; Malyshev, V. A.; Eisfeld, A.; Knoester, J. Subdiffusive Exciton Motion in Systems with Heavy-Tailed Disorder. J. Chem. Phys. 2013, 138 (21), 214316.

(57)

Rousseau, E.; Van Der Auweraer, M.; De Schryver, F. C. Steady-State and Time-Resolved Spectroscopy of a Self-Assembled Cyanine Dye Multilayer. Langmuir 2000, 16 (23), 8865– 8870.

(58)

Rousseau, E.; Koetse, M. M.; Van der Auweraer, M.; De Schryver, F. C. Comparison between J-Aggregates in a Self-Assembled Multilayer and Polymer-Bound J-Aggregates in Solution: A Steady-State and Time-Resolved Spectroscopic Study. Photochem. Photobiol. Sci. 2002, 1 (6), 395–406.

(59)

Singh, J. Excitation Energy Transfer Processes in Condensed Matter; Springer US: Boston, MA, 1994.

(60)

Song, K. S.; Williams, R. T. Self-Trapped Excitons; Springer Berlin Heidelberg, 1993.

(61)

Toyozawa, Y. Optical Processes in Solids; Cambridge University Press, 2003.

(62)

Rashba, E. I. Critical Length for Self-Trapping in One Dimension. Synth. Met. 1994, 64 (2– 3), 255–257.

(63)

Jursenas, S.; Gruodis, A.; Kodis, G.; Chachisvilis, M.; Gulbinas, V.; Silinsh, E. A.; Valkunas, L. Free and Self-Trapped Charge-Transfer Excitons in Crystals of Dipolar Molecules of N , N -Dimethylaminobenzylidene 1,3-Indandione. J. Phys. Chem. B 1998, 102 (7), 1086–1094.

(64)

Jursenas, S.; Gulbinas, V.; Gustavsson, T.; Pommeret, S.; Mialocq, J.-C.; Valkunas, L. ACS Paragon Plus Environment


Page 38 of 41

38 Excitation Relaxation in Films of Dipolar N,N-Dimethylaminobenzylidene 1,3-Indandione Molecules. Chem. Phys. 2002, 275 (1–3), 231–242. (65)

Kobrak, M. N.; Bittner, E. R. A Dynamic Model for Exciton Self-Trapping in Conjugated Polymers. I. Theory. J. Chem. Phys. 2000, 112 (12), 5399–5409.

(66)

Kobrak, M. N.; Bittner, E. R. A Dynamic Model for Exciton Self-Trapping in Conjugated Polymers. II. Implementation. J. Chem. Phys. 2000, 112 (12), 5410–5419.

(67)

Schumacher, S.; Ruseckas, A.; Montgomery, N. A.; Skabara, P. J.; Kanibolotsky, A. L.; Paterson, M. J.; Galbraith, I.; Turnbull, G. A.; Samuel, I. D. W. Effect of Exciton SelfTrapping and Molecular Conformation on Photophysical Properties of Oligofluorenes. J. Chem. Phys. 2009, 131 (15), 154906.

(68)

Timpmann, K.; Katiliene, Z.; Woodbury, N. W.; Freiberg, A. Exciton Self Trapping in OneDimensional Photosynthetic Antennas. J. Phys. Chem. B 2001, 105 (49), 12223–12225.

(69)

Kunz, R.; Timpmann, K.; Southall, J.; Cogdell, R. J.; Freiberg, A.; Köhler, J. Exciton Self Trapping in Photosynthetic Pigment–Protein Complexes Studied by Single-Molecule Spectroscopy. J. Phys. Chem. B 2012, 116 (36), 11017–11023.

(70)

The Biophysics of Photosynthesis; Golbeck, J., van der Est, A., Eds.; Springer New York: New York, NY, 2014.

(71)

Embriaco, D.; Balagurov, D. B.; La Rocca, G. C.; Agranovich, V. M. Topical Questions in the Photophysics of J Aggregates. Phys. Status Solidi C Conf. 2004, 1 (6), 1429–1438.

(72)

Drobizhev, M. A.; Sapozhnikov, M. N.; Scheblykin, I. G.; Varnavsky, O. P.; Auweraer, M. Van der; Vitukhnovsky, A. G. Exciton Dynamics and Trapping in J-Aggregates of Carbocyanine Dyes. Pure Appl. Opt. J. Eur. Opt. Soc. Part A 1996, 5 (5), 569–581.

(73)

Marciniak, H.; Li, X.-Q.; Würthner, F.; Lochbrunner, S. One-Dimensional Exciton Diffusion in Perylene Bisimide Aggregates. J. Phys. Chem. A 2011, 115 (5), 648–654.

(74)

Schubert, A.; Settels, V.; Liu, W.; Würthner, F.; Meier, C.; Fink, R. F.; Schindlbeck, S.; Lochbrunner, S.; Engels, B.; Engel, V. Ultrafast Exciton Self-Trapping upon Geometry Deformation in Perylene-Based Molecular Aggregates. J. Phys. Chem. Lett. 2013, 4 (5), 792–796.

(75)

Wolter, S.; Magnus Westphal, K.; Hempel, M.; Würthner, F.; Kühn, O.; Lochbrunner, S. Low Temperature Exciton Dynamics and Structural Changes in Perylene Bisimide Aggregates. J. Phys. B At. Mol. Opt. Phys. 2017, 50 (18), 184005.

(76)

Bakalis, L. D.; Knoester, J. Pump−Probe Spectroscopy and the Exciton Delocalization Length in Molecular Aggregates. J. Phys. Chem. B 1999, 103 (31), 6620–6628.

(77)

Minoshima, K.; Taiji, M.; Misawa, K.; Kobayashi, T. Femtosecond Nonlinear Optical Dynamics of Excitons in J-Aggregates. Chem. Phys. Lett. 1994, 218 (1–2), 67–72. ACS Paragon Plus Environment

Page 39 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


39 (78)

Heijs, D. J.; Dijkstra, A. G.; Knoester, J. Ultrafast Pump-Probe Spectroscopy of Linear Molecular Aggregates: Effects of Exciton Coherence and Thermal Dephasing. Chem. Phys. 2007, 341 (1–3), 230–239.

(79)

Johnson, A. E.; Kumazaki, S.; Yoshihara, K. Pump-Probe Spectroscopy and Exciton Dynamics of J Aggregates at High Pump Intensities. Chem. Phys. Lett. 1993, 211 (6), 511– 515.

(80)

Fidder, H.; Terpstra, J.; Wiersma, D. A. Dynamics of Frenkel Excitons in J-Aggregates. J. Chem. Phys 1991, 94 (10), 6895–6907.

(81)

Scheblykin, I. G.; Sliusarenko, O. Y.; Lepnev, L. S.; Vitukhnovsky, A. G.; Van der Auweraer, M. Excitons in Molecular Aggregates of 3,3‘-Bis-[3-Sulfopropyl]-5,5‘-Dichloro9- Ethylthiacarbocyanine (THIATS): Temperature Dependent Properties. J. Phys. Chem. B 2001, 105 (20), 4636–4646.

(82)

Lebedenko, A. N.; Grynyov, R. S.; Guralchuk, G. Y.; Sorokin, A. V.; Yefimova, S. L.; Malyukin, Y. V. Coherent Mechanism of Exciton Transport in Disordered J-Aggregates. J. Phys. Chem. C 2009, 113 (29), 12883–12887.

(83)

Toyozawa, Y. Dynamics and Instabilities of an Exciton in the Phonon Field and the Correlated Absorption-Emission Spectra. Pure Appl. Chem. 1997, 69 (6), 1171–1178.



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40 TOC Graphic


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TOC graph 80x39mm (300 x 300 DPI)


Exciton Dynamics and Self-Trapping of Carbocyanine J-Aggregates in

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