1. Introduction

Intensive development of photonics and optoelectronics requires the creation of new materials, as well as micro- and nanostructures with unique optical and photoelectric properties. To this end, extensive studies of various excitonic [1], plasmonic [2], and hybrid structures and materials [3] were performed in the last decade. These studies were mainly aimed at the development of a variety of photonic and optoelectronic devices. In particular, many efforts have been devoted to the investigation and fabrication of nanophotonic integrated circuits [4], nanolasers [5], solar cells [6], sensors [7], light-emitting diodes [8,9], nanowaveguides [10,11], and near-field optical probes that convert optical radiation into light fields localized on nanometer scales [12,13]. A separate direction here is represented by fundamental and applied research in the field of organic and hybrid organo/inorganic photonics. Use of organic materials have attracted considerable research interest since, for many applications, they are less expensive alternatives to inorganic counterparts. In particular, as a molecular component of organic photodetectors and solar cells, cyanine dyes are attractive [14, 15] owing to their remarkable properties. Cyanine dyes differ from the dyes of other classes by the maximal absorption of light and the ability to form highly-ordered aggregates and molecular crystals. In such aggregates electronic excitations of individual molecules are collectivized, forming Frenkel excitons. Molecular aggregates have unique optical properties, including the extremely high oscillator strength in the J-band, the absorption maximum being red shifted relative to the maximum of the monomer molecule absorption. J-aggregates of cyanine dyes have a large nonlinear-optical susceptibility and possess a high quantum yield of generation of photoexcited charge carriers [16,17]. We also note that in recent years, due to a number of applications in optoelectronics, studies of various J-aggregates in the crystalline phase have attracted considerable interest [18,19].

In the past 10-15 years, molecular aggregates of cyanine dyes have found a wide range of applications as an organic component of hybrid organometallic nanostructures of various compositions, shapes and sizes. We should note here a series of works on studies of the optical properties of two-layer and three-layer nanostructures consisting of a metallic core and an outer shell of ordered molecular dye aggregates [20–23]. Some authors have performed the analysis of plasmon-exciton coupling effects and simulation of spectral characteristics of core/shell and core/double-shell organometallic nanospheres [24–28], nanorods [29], nanodisks [30], nanostars [31], and nanoplatelets [32]. When interacting with light, such hybrid structures combine the advantages of an excitonic subsystem associated with the large oscillator strength of the radiation transition and narrow absorption width of the J-band, and the plasmonic subsystem that can lead to a strong local field enhancement near the surface of the metallic nanoparticle compared with the incident electromagnetic radiation. Therefore such structures are promising for their use in the development of hybrid photonic and optoelectronic devices of next generation.

It is important to stress that J-aggregates of cyanine dyes usually studied are monochromic supramolecular systems, i.e. they have one rather narrow and intense absorption peak located, as a rule, in the visible or near-IR spectral range. The main goal of this work is to synthesize and study the multilayer molecular nanocrystals of new type that consist of J-aggregates of the anionic cyanine dyes and two J-aggregates of the cationic dyes. We shall demonstrate that such organic crystals are multichromic systems, i.e. they are capable of effectively absorbing light in several spectral ranges. Previous studies [33] have shown that aggregation is stimulated by multiply-charged metal cations and this leads to the formation of metal complex J-aggregates. Thermodynamic studies of the assembly of aggregates of anionic dyes under the action of multiply-charged cations have indicated a decrease in the enthalpy and entropy for J-aggregation. It was found in our preliminary experiments that in aqueous dispersions, J-aggregates of anionic cyanine dyes in the form of metal complexes, may have a negative electrokinetic potential. On this basis we recently proposed [34] a method of matrix assembly of J-aggregates of cationic dyes on the surface of negatively charged J-aggregates of anionic dyes.

In the present work we have essentially developed this method and utilized it for obtaining the multichromic organic nanocrystals of various compositions having attractive features. Main attention will be paid to the study of photoluminescent, photoabsorption, spectral-kinetic and photoelectric properties of such crystals. We will also carry out a thorough investigation of the morphology and optical properties of metal-complex J-aggregates of three different anionic cyanine dyes, that form the so-called “anionic platforms”, on the surfaces of which a self-assembly of J-aggregates of two different cationic dyes occurs using matrix synthesis.

2. Materials and experimental methods

To obtain each sample of multichromic crystals, one anionic and two cationic cyanine dyes were used (see Table 1). At the first stage, using magnesium sulfate (MgSO₄) and anionic cyanine dyes (AD-1, AD-2 or AD-3), negatively charged J-aggregates of magnesium complexes were synthesized. Negative charge accumulated on the J-aggregate surface is associated with a negative value of the electrokinetic potential up to −42 mV in the aqueous medium. J-aggregates of magnesium complexes will further be referred to as anionic platforms. At the second stage of the synthesis, J-aggregates of two different cationic dyes CD-4 and CD-5 were grown on the surface of anionic platform (see Fig. 1(f)). The synthesis conditions are chosen so that, the formation of standalone cationic J-aggregates is excluded, while the negatively charged surface of the anionic platform plays the role of an effective catalyst for the growth. Thus, the formation of cationic J-aggregates occurs on the surface of the anionic platform only. As cationic dyes, we used two cyanine dyes (CD-4 and CD-5, see Table 1), which absorb light in the green and red regions of the spectrum, respectively. Note that the formation of layers of J-aggregated cationic dyes in the form of islands on the surface of the anionic platform was monitored via measurements of their optical absorption spectra at each stage.

Table 1. Abbreviations and names of anionic and cationic dyes used in this work.

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Fig. 1 (a)–(e) Chemical structures of the anionic and cationic dyes used in the synthesis of multichromic crystals: MC-1, MC-2, and MC-3. (f) Nanoarchitecture of such crystals in which the J-aggregates of two different cationic dyes (purple and green bricks) are distributed in the form of islands over the two opposite surfaces of the J-aggregate platform of magnesium complexes of an anionic dye (blue balk). (g) Schematic view of photocell with a photosensitive layer (multichromic crystal of 0.1 μm thickness), hole-transporting layer of PVC (0.1 μm), Ag-cathode (200 μm), ITO-anode (0.2 μm), and glass substrate.

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Measurements of the absorption and luminescence spectra of the aqueous solutions were carried out in glass cuvettes using a spectrophotometer Ocean Optics USB2000 (USA). Luminescence spectra were recorded on a Cary Eclipse spectrophotometer (USA). In the study of the luminescence of single dye crystals, the latter were deposited on the glass and excited by radiation from a stationary semiconductor laser (λ = 405 nm). Using micro- and correcting lenses the magnified (by 2 – 20 times) image of the luminescence spot was focused on the input slit of a grating spectrograph (Princeton Instruments Acton SP2500) equipped with nitrogen cooled CCD detector. To measure the polarization degree of the luminescence signal, a polarizing beam-splitter cube, placed between the micro- and correcting lenses was used. At maximum magnification the spatial resolution of the system was 2 μm, while the spectral resolution was not worse than 0.03 nm. A description of the method is given in [35]. Measurements of the luminescence kinetics were performed upon excitation by the second harmonic of a mode-locked titanium-sapphire laser. The pulse duration was 2.5 ps, the repetition rate was 76 MHz. The data were obtained by excitation with radiation at λ = 400 nm. The spectral-temporal dynamics of the signal was recorded using a monochromator combined with a Hamamatsu C5680 strik-camera. The spectral and temporal resolution was 0.3 nm and 10 ps, respectively.

The schematic view of an organic photocell fabricated on the basis of multichromic molecular crystals synthesized in this work is illustrated in Fig. 1(g). As a transparent electrode, glass plates with a sprayed electrically conductive transparent ITO-layer (Indium Tin Oxide) with a surface resistivity of 7 Ω·cm⁻² were used. Then, layer of poly-9-vinylcarbazole (PVC), acting as a hole-transporting layer, was applied over the resulting dye layer.

3. Results and discussions

3.1. Optical properties and morphology of anionic J-aggregate platforms

The advantage of our method for synthesizing the anionic platforms of molecular crystals of J-aggregates from metal complexes of cyanine dyes is the possibility of obtaining highly ordered organic crystals of various morphology (balk or strip and brick) both in nano- and micro-scales. This is achieved by varying the conditions of synthesis and by controllable crystallization, in particular by choosing the appropriate structure of the dye molecules, the dye and metal salt concentrations, the rate and conditions of mixing the solution, temperature of synthesis, cooling and crystallization time, and the chemical composition of the solvent. It was found that in viscous water-organic mixtures with slow cooling of solution and crystallization, microcrystals are formed mainly in the shape of long balk, whereas rapid crystallization forms nanocrystals in the shape of brick or strip. Using atomic-force, electron and optical microscopy we showed that the sizes of the synthesized J-aggregate platforms of anionic dye magnesium complexes can be varied from 1 nm to 100 nm in thickness, from 100 nm to 1 μm in width and from 1 μm to 300 μm in length. In this work the anionic platforms in the shape of long balk and strip were used to synthesize multichromic J-aggregated structures. We note that the crystalline phase of anionic platforms (i.e. the presence of a regular lattice) was additionally confirmed by the X-ray powder phase analysis.

Below we represent several examples of images of anionic platforms obtained in this work using different methods. So, Fig. 2 shows the SEM-images of two J-aggregate platforms of magnesium complexes of the anionic dyes, AD-1 (Fig. 2(a)) and AD-2 (Fig. 2(b)), synthesized from aqueous solutions of methanol with a low concentration of dye molecules. It is seen that the fabricated crystals are in the shape of a balk with a relatively small dispersion of transverse dimensions. Another examples of anionic platforms in the shape of a strip are given in Figs. 2(c) and 2(d). These J-aggregates of magnesium complexes of cyanine dye, AD-1, were obtained by crystallizing a solution of this dye in methanol. Figure 2(c) represents an optical image of a manifold of such J-aggregate platforms obtained using the optical microscopy. As in the case of balks shown in Figs. 2(a) and 2(b), the J-aggregate balks in Fig. 2(c) demonstrate a relatively small size dispersion. Figure 2(d) illustrates an AFM scan of a thin J-aggregate strip with a height of ∼ 2 nm. It proves a high stability of the transverse crystal sizes on submicron scales.

 

Fig. 2 (a) and (b) SEM-images (obtained using a scanning electron microscope JSM 7500F) of two J-aggregate platforms fabricated from the magnesium complexes of the anionic dyes, AD-1 and AD-2, respectively. (c) and (d) Images of the J-aggregate platforms, AD-1, obtained using optical microscopy (Fig. 2(c)) and atomic force microscopy (Fig. 2(d)). Inset shows AFM-scan in the direction perpendicular to the long axis of a platform.

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We discuss now main features in the photoabsorption and photoluminescence spectra of monochromic anionic and cationic cyanine dyes by presenting our experimental results in Fig. 3. Figures 3(a)–3(c) illustrate the behavior of such spectra for the anionic dyes solutions at a concentration of 2.5·10⁻⁵ M. These solutions contain dyes, AD-1, AD-2, or AD-3, in monomeric (M) and dimeric (D) forms (blue curves) and do not contain molecular J-aggregates. Addition of magnesium sulfate, MgSO₄, with a concentration of 1·10⁻⁴ M into these solutions leads to the formation of J-aggregates for which one can clearly observe rather narrow excitonic absorption band. The corresponding J-band in the absorption spectra, usually described within the framework of the Frenkel exciton model, is depicted by a green curve on each of the Figs. 3(a)–3. Each of the J-absorption bands is shifted to the long-wavelength region with respect to the spectral peak of its dye in monomeric and dimeric forms. We note that J-aggregates of anionic cyanine dyes, AD-1, AD-2 and AD-3, exhibit maxima of optical excitonic absorption at wavelengths of λ_max = 465, 492, and 790 nm, respectively. A summary of the absorption properties of cyanine dyes studied in this work can be obtained from Fig. 3(f). It shows the values of the molar absorption coefficient obtained at maxima of excitonic absorption.

 

Fig. 3 Absorption spectra of initial dye solutions (blue curves) as well as absorption (green curves) and luminescence (red curves) spectra of aqueous solutions containing of J-aggregates (J) of all dyes under study. (a)–(e) Spectra of anionic and cationic dyes. Main excitonic absorption and luminescence peaks are marked by green and red arrows, respectively. Black arrows indicate defect-related luminescence peaks. (f) Luminescence spectra of the water solution of the AD-1 dye containing only monomers (M) and dimers (D). (g) Structure of the resulting luminescence and absorption spectra of AD-1 dye: J+D+M (Abs.) and J+D+M (PL) – photoabsorption and photoluminescence spectra containing contributions from J-aggregate anionic platforms as well as monomers and dimers, respectively; M+D (PL) –photoluminescence spectrum containing only monomers and dimers of AD-1 dye; J (PL) – photoluminescence spectrum of J-aggregate platforms obtained by subtracting the J + D + M (PL) and M+D (PL) curves. (h) Molar extinction coefficient (in units of 10⁻⁵·L mol⁻¹ cm⁻¹) at the maxima of the spectral peaks of monomers, dimers, and J-aggregates.

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Red curves in Figs. 3(a)–3(c) correspond to the luminescence spectra of aqueous solutions containing synthesized J-aggregates of magnesium complexes. It should be noted that such spectra for J-aggregates of anionic dyes are characterized by the presence of two spectral peaks. The left shorter-wavelength peak (labeled with a red arrow) is an excitonic peak in luminescence of J-aggregates. It corresponds to a long-wavelength wing in the spectra of excitonic absorption. The luminescence peaks of monomers and dimers are superimposed on this peak. Therefore, in order to correctly reconstruct the shape of the J-aggregate band, it is necessary to subtract from the full spectrum the contributions from dimers and monomers. This is done in Fig. 3(f) and 3 (g) for the anionic AD-1 dye. The black curve in Fig. 3(g) is the resulting J-aggregate band with the full width of about 16 nm. We also note that for J-aggregates of magnesium complexes, along with the exciton-related luminescence, an intense band is observed in the long-wavelength region. It is indicated by a black arrow in Figs. 3(a)–3(c). The presence of such a second long-wavelength peak with large Stokes shift is typical for a radiative transition caused by impurity or defect-related states in some organic crystals and J-aggregates (see, e.g., [17,18]).

As follows from a comparison of the results presented in Figs. 3(a)–3(c), the ratio of the intensities of the left and right peaks in these panels is significantly different for the J-aggregates of the three dyes studied by us. The defect-related and excitonic peaks are pronounced for J-aggregates of dyes, AD-1 and AD-2, with the intensity of the defect peak being particularly high in the second case. However, for the J-aggregate of the dye, AD-3, the defect and excitonic peaks practically merge, forming a single and fairly intense spectral peak.

One can see from Fig. 3 that there are no noticeable features corresponding to defect-related states in the absorption spectra. Note that it is quite possible that defects in the structure of J-aggregate platforms can manifest themselves in a significant way in luminescence and in no appreciable way affect their absorption spectra. Indeed, the appreciable role of the defect-related states in the luminescence spectra is usually governed by the Boltzmann factor exp (−ΔE/kT). Its value can be estimated from the Stokes shift ΔE, which can be found from the energy shift of the maximum of the long-wavelength emission band relative to the maximum of the excitonic peak. The value of ΔE for different anionic platforms varies in the range from −0.1 eV to −0.4 eV. Thus, at thermal energy kT = 0.026 eV for the Boltzmann factor we obtain values in the range of ∼ 4·10¹ – 9·10³. This enhancing factor gives a significant increase in the role of defect states in luminescence, which practically do not manifest themselves in absorption.

We now turn briefly on the behavior of the absorption and luminescence spectra for J-aggregates of cationic dyes, CD-4 and CD-5 (see Figs. 3(d) and 3(e)). It is seen that the excitonic peaks in the absorption bands of these J-aggregates appear at λ_max = 545 nm and λ_max = 640 nm, respectively. Note that, on the whole, the physical picture observed in the spectra of the cationic J-aggregates is largely similar to that discussed above for anionic dyes. The only difference is that the second long-wavelength band is absent in the luminescence spectra.

In figures 4(a) and 4(b) we present the luminescent microphotographs of two crossed microcrystals made from the multilyer J-aggregates of the magnesium complexes of the anionic AD-1 dye with two different polarizations of the luminescence signal. They were obtained by the method described in [35] for the spectral range of 0.45 – 0.90 μm. Red arrows in the figures show the direction of electric field passed through the polarizer. The maximal intensity of luminescence is observed when the polarizer is oriented parallel to the long axis of the given J-aggregate. Notice that the oriented growth of the J-aggregate occurred along this axis. Thus, J- aggregate optical anisotropy is tied to its growth axis over a wide spectral range and the J-aggregate growth direction coincides with the long axes of the dye molecules chromophores entering the J-aggregate. This feature of the synthesized J-aggregates is in good agreement with some previous data [36].

 

Fig. 4 Photoluminescent (PL) micrographs of two crossed organic crystals, fabricated from the multilayer J-aggregates of metal complexes of anionic AD-1 dye, with parallel (a) and perpendicular (b) polarizations of the luminescence signal along the long axis of one of the crystals. Red arrows show the direction of the electric field strength vector passed through the polarizer. (c) PL spectra of a single crystal from the regions marked with yellow arrows on panels (a) and (b) for the polarizer orientation along the crystal axis (curve 1) and perpendicular to it (curve 2).

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Figure 4(c) additionally represents the luminescence spectra obtained for parallel (curve 1) and perpendicular (curve 2) polarizations of the luminescence signal with respect to the long axis of a J-aggregate platform fabricated from magnesium complexes of anionic dye, AD-1. The corresponding area on the crystal is marked by an arrow in Figs. 4(a) and 4(b). It is seen that for both polarizations modulation of the luminescence spectra is observed. The modulation arises apparently as a result of interference associated with the reflection of light from different faces of J-aggregated microcrystal. In the short-wavelength part of the spectra, the modulation is suppressed due to a noticeable absorption of light near the bottom of excitonic band.

In addition to the ordinary excitonic luminescence of J-aggregates, the long-wavelength band located at λ = 625 nm can be clearly observed. Presence of this band in the spectra of a separate microcrystal proves its relationship with the electronic states of J-aggregate platform. Thus, the above mentioned long-wavelength luminescence signal (marked by black arrow in Fig. 3) is due to some peculiar states in the synthesized J-aggregates of magnesium complexes. As one can see from Fig 4(c), the long-wavelength band is also characterized by an appreciable degree of linear polarization with orientation of the polarization plane being the same as in the case of excitonic luminescence. Spectra composed of two luminescence bands, similar to that presented in Fig 4(c) as well as appreciable degree of linear polarization for both bands along the growth axis, are typical for J-aggregates of a magnesium complex synthesized in this work using anionic dyes, AD-2 or AD-3. Since the growth axis of J-aggregate coincides with the orientation of the dye molecules therein, it can be stated that for the J-aggregates of the magnesium complex, the luminescence spectra are governed by two main types of transitions for which the orientation of dipole matrix element reflects the orientation of chromophore molecules.

There are no features in the absorption spectra corresponding to the long-wavelength band of anionic platforms (see Fig. 3), which indicates a low density of the corresponding electronic states and their defect origin. Thus, in spite of similar polarization properties the two bands observed in photoluminescence spectra of anionic platforms have quite different nature. According to results of some works [37, 38], additional luminescence band in J-aggregates, which has a significant Stokes shift relative to the excitonic absorption maximum, is explained either by the presence of defects in the packing of dye molecules, or by the presence of defects on the surface of J-aggregate. Defect-related origin of the long-wavelength peaks in the luminescence spectra of anionic platforms is further supported by the in-situ optical measurements of J-aggregate nanoparticles being recrystallized. Figures 5(a) and 5(b) depict the optical absorption and luminescence spectra of AD-1 anionic platforms as functions of time elapsed after their synthesis. In the course of the liquid dispersion of nanocrystals of J-aggregates, they become recrystallized into highly ordered J-aggregates of a larger size. As one can see from Fig. 5, this recrystallization is not spurred by noticeable changes in the absorption spectra. Unlike this in the luminescence spectra a drastic rearrangement occurs: with increasing the average crystal size, a decrease in the intensity of the long-wavelength band accompanied by the growth of ordinary excitonic luminescence is observed. Thus, the role of states responsible for the long-wavelength luminescence increases noticeably with the decrease of the crystal size. This observation points to the predominant role of the surface in the formation of the defect-related states discussed above.

 

Fig. 5 (a) and (b) Photoabsorption and photoluminescence (PL) spectra of a solution of the J-aggregate of the magnesium complexes of anionic dye, AD-1, obtained after different time intervals (indicated in the figures) after the synthesis completion. (c) Temporal dynamics of the PL signal for the anionic platform, AD-1, recorded after excitation of J-aggregates by a picosecond light pulse with a central wavelength of 400 nm. Curves 1 and 2 are the responses in the spectral regions of 468 – 472 nm (excitonic emission peak) and 598 602 nm (long-wavelength luminescence peak), respectively.

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The processes of population and radiative recombination for the excitonic states and states responsible for the long-wavelength peaks in luminescence spectra of the anionic platforms differ drastically from each other. Figure 5(c) shows temporal dynamics of the luminescence signal for AD-1 based anionic platform recorded after excitation of J-aggregates by a picosecond light pulse with a central wavelength of 400 nm. Curve 1 illustrates the response in the spectral region of 468 – 472 nm (narrow spectral fragment of excitonic emission). As one can see, after the absorption of excitation pulse, the excitonic photoluminescence grows, that corresponds to the relaxation of excitons to the band bottom due to the exciton-phonon coupling. This process takes several tens of picoseconds. Further, luminescence decay with a time constant of ~500 ps is observed. Curve 2 of Fig. 5(c) shows the luminescence response in the spectral region of 598 – 602 nm corresponding to the maximum of the long-wavelength luminescence peak. Long-wavelength luminescence reveals the rise time of more than 2 ns. The characteristic decay time corresponding to curve 2 in Fig. 5(c) falls close to the time delay between two excitation pulses of ∼ 13 ns (a non-zero luminescence signal excited by the previous pulse can be observed on the right-hand side of curve 2 before the arrival of the excitation pulse). Thus, the decay time as well as the rise time for long-wavelength peak is more than order of magnitude greater than the same value for ordinary excitonic emission. This observation confirms the connection of the long-wavelength luminescence band with the electronic states, the nature of which differs markedly from the excitonic states being typical for J-aggregates.

A qualitatively similar behavior of the luminescence kinetics was observed for the other types of anionic platforms. For the short-wavelength luminescence band, the decay time varied in the range of 100 – 500 ps, while for the long-wavelength band it varied in the range of 2 – 14 ns. The ratio between slow and fast decay times exceeded an order of magnitude for all samples studied.

3.2. Absorption, luminescent and photoelectric properties of multichromic organic crystals

Schematic view of the nano-architecture of the obtained multichromic organic molecular crystals is shown in Fig. 1(f). Thus, each of the multichromic molecular crystals synthesized in the work is a multilayer highly-ordered system based on monochromic anionic J-aggregated “platform”, on the surface of which “self-assembly” of two different J-aggregates of cationic cyanine dyes was performed using the matrix synthesis.

Absorption spectra of multichromic crystals including J-aggregates of cationic dyes, CD-4 and CD-5, on the anionic platform of magnesium complexes prepared using one of the dyes, AD-1, AD-2 or AD-3, are shown in Fig. 6. It is evident that the multichromic systems synthesized, including J-aggregates of two cationic and one anionic dyes, have three intense maxima of excitonic absorption – in the blue, green, and red (or infrared) spectral regions. Spectral positions of these bands reproduce the positions of the excitonic absorption peaks for the parent monochromic cationic and anionic J-aggregates with an accuracy within 10 nm. This means that excitonic states of J-aggregated dyes retain their properties to a certain extent being a part of a multichromic system. The positions of the excitonic absorption peaks and the ratio of their intensities for aqueous dispersions of multichromic J-aggregates and their thin layers on a glass substrate practically coincide with each other.

 

Fig. 6 (a) Absorption spectra of monochromic J-aggregates of cyanine dyes: AD-1, AD-2, CD-4, and CD-5. (b), (c) and (d) Absorption spectra of multichromic crystals, MC-1, MC-2, and MC-3. (e) The values of the absorption coefficients of multichromic crystals.

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A more complicated situation occurs in the luminescence spectra of multichromic crystals, when the luminescence bands of the J-aggregates of the three different dyes composing it turn out to be substantially shifted relative to the corresponding bands of monochromic J-aggregates and overlap significantly with each other. Some example of the luminescence spectrum of the multichromic crystal synthesized in this work is shown in Fig. 7. It is seen that in contrast to the case of monochromic J-aggregates, the luminescence spectrum of an individual multichromic crystal is characterized by the presence of several bands. Their spectral positions depend on the specific composition of a set of J-aggregated dyes a multichromic crystal is made of. This observation confirms the presence of several types of excitonic states in a multichromic crystal. Note that the nature of the observed bands in the emission spectra of multichromic crystals turns out to be different. These bands are apparently related both to the resonance excitonic luminescence and to the luminescence caused by the surface defects of the anionic platform.

 

Fig. 7 (a) Luminescence spectra of monochromic J-aggregates of cyanine dyes: AD-1, AD-2, CD-4, and CD-5. (b) and (c) Luminescence spectra of individual multichromic crystals (MC-1 and MC-2), synthesized on their basis. (d) Luminescence spectra obtained from two different points of MC-1 crystal at a distance of 10 μm from each other.

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We discuss this point in more detail and emphasize some differences from the case of bands appearing in absorption spectra. The absorption spectrum is determined mainly by the density of excitonic states and the related oscillator strengths. As can be seen from Fig. 6, the peak positions in the absorption spectra of multichromic J-aggregated crystals agree well with the peaks observed in the absorption spectra of the corresponding monochromic J-aggregates. This means that, as a first approximation, the structure of the excitonic states for a multichromic crystal can be treated in terms of the set of monochromic J-aggregates. On the other hand, its emission spectrum is mainly governed by the long-wavelength tail corresponding to the exitonic band bottom. Therefore, a noticeable shift of peaks in the emission spectra for multichromic crystal (with respect to the peaks in the emission spectra of the corresponding monochromic J-aggregates) indicates a change in the structure of the excitonic states located close to the low-frequency edge of the exitonic band. Most probable cause of these changes is the influence of interfaces between the various components of multichromic crystals.

We also performed some studies that explicitly demonstrate that upon transition from one region of a multichromic nanocrystal to another, there is no noticeable change in its optical properties. The corresponding results of measurements of the spectral distribution of the photoluminescence intensity, which were obtained from two small regions of a multichromic MC-1 crystal with 10 μm separation between them, are shown in Fig. 7(d). It is seen that the small variations in the luminescence spectra are observed only for the relative contributions of different spectral bands.

Due to the intense and narrow absorption bands of J-aggregates they are attractive in photodetector applications where wavelength specificity is important. Accordingly, there are a number of studies on the photoconductivity in J-aggregating cyanine dyes upon their excitonic absorption of light and in the use of this phenomenon to create the narrow-spectrum photosensitive layers of photodetectors [14, 15]. At the same time, the multiband absorption spectra of the synthesized multichromic crystals cover a significant part of the visible spectral range. In Figure 6(e) we present the results of our experiments on measuring the absorption coefficients (in units of 10⁵ cm⁻¹) of organic photosensitive layers of 100 nm thickness consisting of multichromic nanocrystals synthesized here. For each nanocrystal (MC-1, MC-2 and MC-3), the data are given for wavelengths corresponding to three different absorption peaks (i.e., in fact for those magnitudes of λ that are determined by the positions of the maxima in the absorption spectra of J-aggregates of one anionic and two cationic dyes that make up this crystal). We note that for each photosensitive layer made of one of the synthesized multichromic crystals, its absorption coefficient lies between 10⁵ and 10⁶ cm⁻¹. Thus, the characteristic values of the absorption coefficient turn out to be more than an order of magnitude higher for these organic crystals than for silicon. Along with the relatively low cost of such kind of organic multichromic crystals fabricated from the anionic and cationic J-aggregates of cyanine dyes, this makes the use of thin layers of such crystals particularly promising for the development of thin-film organic photocells with a large photosensitive area and varied photoabsorption spectra.

We have also performed some experiments to measure the external quantum efficiency (EQE) of the created samples of photocells based on the multichromic crystals (MC-1, MC-2 and MC-3) as a photosensitive layer. It is defined as a percentage ratio of the number of registered charge carriers to the number of photons, absorbed by the photosensitive area: EQE = (S j_pchν/eP)·100 %. Here j_pc is the photocurrent density flowing through the photosensitive area of the photocell; S is the light-sensitive (working) area of the photocell; hν is the photon energy, P is the power of light incident on the sample; e is the electron charge. The results of measuring the photocurrent densities, j_pc, and external quantum efficiencies of photoconductivity are shown in Fig. 8. The dark current densities, j_dark, are 0.10, 0.11 and 0.14 μA·cm⁻² for the photocell samples made of crystals MC-1, MC-2, and MC-3, respectively. The quantum efficiency is varied from 2.7 % to 8.8 %, depending on the specific anionic dye constituting the multichromic organic crystal. These are only the first experiments. In our opinion, the EQE can be significantly increased in the future by, for example, selecting the optimal thickness of the photosensitive layer and the materials used for the electron and hole transporting layers of the photocell, just like it was done in some previous works, e.g., [14].

 

Fig. 8 Photocurrent densities (a) and external quantum efficiency (b) of the created samples of photocells fabricated from the multichromic crystals (MC-n=AD-n+CD-4+CD-5, n=1, 2, and 3) as a photosensitive layer of 100 nm thickness.

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4. Conclusions

We developed a method of controlled formation of organic nanocrystals of monochromic metal-complex J-aggregates of anionic cyanine dyes. It was shown that the luminescence spectra of such anionic platforms are characterized by the presence of the shorter-wavelength peak associated with the Frenkel excitons and the defect-related long-wavelength peak. We also found that the decay time of excitonic luminescence in anionic platforms is determined by the subnanosecond scale typical for J-aggregates, while for the luminescence of defects the characteristic luminescence decay times are of the order of 10 ns.

A key point of the work is the synthesis and study of the optical properties of multichromic crystals that have been obtained by the matrix method of self-assembly of J-aggregates of two cationic dyes on a negatively charged platform of J-aggregates of magnesium complexes of anionic dye. We have shown that the absorption spectrum of such crystals is close to the superposition of the absorption spectra of a set of monochromic J-aggregates assembled from the same components. The emission spectrum of a single multichromic crystal includes a set of several (usually four) peaks that can be preliminarily interpreted as excitonic (three peaks) and defect-related (one peak) luminescence of two cationic J-aggregates and one anionic platform.

On the basis of multichromic crystals, prototypes of organic photocells were fabricated. Their spectral sensitivity can be tuned in the visible and near-IR range due to the possibility of selecting the original components constituting the crystal. The potential advantage of synthesized structures compared with the organic photocells based on conventional J-aggregates of dyes, is in the greater spectral sensitivity zone and higher thermal stability of the photosensitive layer owing to the use of metal complex J-aggregates.

Multichromic J-aggregates can also find applications in the development of excitonic waveguides, nanoscale optical switches and light-emitting devices with a “tunable” emission spectrum. Due to the presence of three absorption bands, they have a number of advantages for the fabrication on their basis of hybrid metalorganic nanostructures with the desirable optical properties over the monochromic J-aggregates of cyanine dyes used earlier for this purpose. This will make it possible to investigate at a qualitatively new level the effects of plasmon-exciton interaction and near-field electromagnetic coupling in hybrid systems containing a metal component and molecular J-aggregates of dyes.