Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Direct Formation of Carbocyanine J‑Aggregates in Organic Solvent Sivan Harazi,†,‡,§,∥ Omree Kapon,‡,§ Amos Sharoni,†,§ and Ya’akov R. Tischler*,‡,§ Department of Physics, ‡Department of Chemistry, and §Institute for Nanotechnology and Advanced Materials, Bar Ilan University, Ramat Gan 5290002, Israel
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ABSTRACT: J-aggregates are molecular nanoscale selfassemblies with distinct excitonic properties, such as highly delocalized excitons, making them main candidates for designing excitonic energy transfer systems. Here, we show a technique to form J-aggregates of the cyanine dye 1,1′dioctyl-3,3′-di(4sulfobutyl)-5,5′,6,6′-tetrachloro-benzimidazolocarbocyanine (C8S4) in organic solutions of chloroform and acetone. These J-aggregates have a spectrum with three redshifted absorption peaks and two sharp emission peaks. Spincoated thin films of C8S4 aggregates have similar spectra. Investigation of the thin films via electron microscopy shows two morphologies of aggregates, which partially explains the complicated spectra. We have shown two ways for isolating the type of aggregate with the lowest transition energy. Atomic force microscopy and high-resolution transmission electron microscopy images of the aggregates reveal a 2D brickstone arrangement. Finally, J-aggregating C8S4 from organic solvent will enable mixing J-aggregates directly with various fluorescent organic compounds, thereby expanding the options for designing energy transfer systems based on J-aggregates.
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INTRODUCTION Many photosensitizing dyes can self-assemble in solution or in certain interfaces to form quasi-1D or quasi-2D nano or microcrystals, usually referred to as molecular aggregates. When these molecular aggregates display a new sharp, redshifted band, they are called J-aggregates after E.E. Jelley who first observed this phenomenon.1 The optical properties of Jaggregates include large oscillator strength, a very small Stokes shift (excluding porphyrin dyes), a sharp decrease in the fluorescence lifetime compared to the molecular one,2−4 and a high third-order nonlinear susceptibility.5,6 These optical characteristics are the result of the strong coupling between the transition dipoles of the molecules in the aggregates, which generates new cooperative excitonic energy bands (the Frenkel delocalized exciton).7 The coupling term between the transition dipoles of the molecules in the aggregates is the main factor that determines the amount of shift of the energy bands, and it strongly depends on the relative orientation between the molecules in the aggregate.8 Hence, the spectrum of the aggregate is a direct consequence of the molecular arrangement in the aggregates. J-aggregates were first used in the photographic process as photosensitizers with silver halides.9 Currently, they are used as membrane-potential probes10 and as two-level systems in strong light−matter coupling.11−13 Because of the delocalization of their excitations, J-aggregates can act as efficient donors and acceptors in excitation energy transfer (EET) systems. In photosynthetic systems, EET between different types of lightharvesting molecular aggregates is part of how energy is transferred to the reaction centers of the system.14 In synthetic © XXXX American Chemical Society
systems, EET in hybrid organic/inorganic systems was also demonstrated using J-aggregates combined with quantum dots or nanowires. Cyanine dyes have some outstanding features making them attractive for various J-aggregates’ applications:15 their spectrum can be altered by changing only the length of the polymethine chain; the cyanine molecules are predominantly rigid and planar, which optimizes the aggregation process; the high polarizability of the molecules’ ground state causes strong attractive forces that give rise to high stability of the aggregates. Two intensely investigated cyanine dyes are BIC and TDBC, which aggregate in relatively low concentration and present a red-shifted, high, and narrow absorption peak.15−17 These dyes are TBC derivatives, usually designated as CmRn, where m and n indicate the length of the alkyl chains and R describes the ionic group (e.g., TDBC = C2S4). Other TBC derivatives, surfactant-like, were synthesized by Daehne and co-workers,17,18 via adding hydrophobic substituents to the TBC molecule. In aqueous solutions, some of these surfactant-like TBC dyes form double-layered tubular assemblies. Each layer in the double-layer structure is a different j-aggregate; therefore, it has a different excitonic band structure, which is reflected in its spectrum with multi, red-shifted peaks. Moreover, Aviv and Tischler19 synthesized a long-chained TBC (C18S4) and used its high degree of amphiphilicity to form a stable Langmuir−Blodgett (LB) film. Received: February 7, 2019 Revised: May 29, 2019 Published: July 2, 2019 A
DOI: 10.1021/acs.jpcc.9b01116 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. (a) Chemical structure of C8S4. (b) Normalized absorbance and photoluminescence spectra of the C8S4 monomer in CHCl3 0.005 mM (dashed lines) and C8S4/CHCl3 aggregates prepared via the acetonic route to get 0.1 mM (solid lines). (c) Normalized absorbance and photoluminescence spectra of a C8S4 thin film on glass, which were spin-coated out of acetonic solution of 0.1 mg/mL with a spin speed of 1000 rpm.
The formation of J-aggregates usually occurs in aqueous solutions. An aqueous environment limits the design of EET systems, which combine J-aggregates with organic compounds, such as conjugated polymers and organic dyes. Hence, sophisticated deposition techniques were previously used to build a J-aggregate/organic EET systems. Yet, there is research on the formation of J-aggregates in organic solvents. Chibisov et al.20 showed the formation of a red-shifted band in the absorption and fluorescence spectra of N-sulfobutyl oxacarbocyanine in binary mixtures of different organic solvents with DMSO. In addition, when a small quantity of water is added to organic solution of some porphyrin dyes, these dyes can form J-aggregates in the organic surrounding.21,22 Also, derivatives of perylene bisimide form J-aggregates in methylcyclohexane,23,24 and derivatives of diketopyrrolopyrrole (DPP) dyes present some J-aggregation in organic solvents under certain conditions.25 However, to the best of our knowledge, for 5,5′,6,6′-tetrachlorobenzimidacarbocyanines (TBCs), a class of cyanine dyes, there are no reports of J-aggregate formation in organic solvents. In this paper, we present the formation of J-aggregates of the surfactant TBC dye C8S4 in chloroform mixed with acetone (95:5%). The optical spectra of the solution and spin-coated thin films were characterized, and the effects of two experimental factors on the spectrum were explored. We suggest a molecular structure that can explain the obtained
results and present a high-resolution TEM image to verify the suggested structure.
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EXPERIMENTAL SECTION Materials. The Cyanine dye 1,1′-dioctyl-3,3′-di(4-sulfobutyl)-5,5′,6,6′-tetrachloro-benzimidazolocarbocyanine (C8S4) was purchased from FEW Chemicals GmbH, Germany and were stored in a vacuum desiccator. Stabilized chloroform (CHCl3, 99%, 0.006% amylene) and acetone (99.9%) were purchased from commercial source and kept under standard conditions. Preparation of the C8S4 J-Aggregates in Organic Solvent Solutions. First, fresh CHCl3 was illuminated with long-wavelength UV lamp for 24 h in a glass vial. It is well known that CHCl3 in air can degrade to phosgene and hydrochloric acid, and we assume that the low-energy UV radiation increases this process by photodegradation of the stabilizer. In many cases, the presence of ions in dye solution induces or increases the aggregation,15 therefore, the radiation of the solvent is essential. All of the solutions described here were prepared from the illuminated fresh CHCl3. The C8S4 was dissolved in two types of solutions. One is CHCl3, and the second is a mixture of CHCl3 and acetone (95%:5%), which allowed the preparation of higher concentration solutions. For the CHCl3 solution, the needed volume B
DOI: 10.1021/acs.jpcc.9b01116 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. (a) Changes with time of the absorption spectrum of films prepared in different spin-coating speeds and accelerations: black solid (dashed) - fresh (1 day old) film prepared in a speed of 1000 rpm and an acceleration of 500 rpm/s and red solid (dashed) - fresh (1 day old) film prepared in a speed of 3000 rpm and an acceleration of 5000 rpm/s. The concentration of starting solution for both films was 0.4 mg/mL. (b) Normalized absorption spectra of a C8S4 J-aggregate thin film on glasses with different pretreatments, spin-coated out of an acetonic solution of 0.2 mg/mL, at 1000 rpm.
therefore, the AFM imaging was conducted for films on silicon wafers. The thin films were also examined under a high-resolution scanning electron microscope. For this purpose, the films were coated with a 1.5 nm iridium film in a sputtering system. For the transmission electron microscopy (TEM, FEI Spirit G12, USA), a lacy copper grid (400 mesh) was attached to a glass substrate by two small pieces of indium. This assembly was spin-coated with 100 μL of acetonic solution of 0.05 mg/ mL. However, for the high-resolution TEM (HRTEM, JEOL2100, US), the density of the aggregates on the grid was increased by dropping 0.1 mg/mL J-aggregate solution on the grid without spinning. The coated grid was then dried for an hour in a vacuum desiccator.
of the solvent was added to a small amount of dye powder (
