Molecular Arrangement in Cyanine Dye J-Aggregates Formed on

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Molecular Arrangement in Cyanine Dye JAggregates Formed on CeO Nanoparticles 2

Svetlana L. Yefimova, Ganna V. Grygorova, Vladimir K. Klochkov, Igor A. Borovoy, Alexander V. Sorokin, and Yuri V. Malyukin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06590 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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The Journal of Physical Chemistry

Molecular Arrangement in Cyanine Dye J-aggregates Formed on CeO2 Nanoparticles

Svetlana L. Yefimova*, Ganna V. Grygorova, Vladimir K. Klochkov, Igor A. Borovoy, Alexander V. Sorokin, Yuri V. Malyukin

Institute for Scintillation Materials National Academy of Sciences of Ukraine, 60 Nauky ave., 61072 Kharkiv, Ukraine *e-mail: [email protected]

Abstract The aggregation behavior of 1-methyl-1’-octadecyl-2,2’-cyanine perchlorate (amphi-PIC) and 3,3’-dimethyl-9-(2-thienyl)-thiacarbocyanine iodide (L-21) dyes in binary DMF (DMSO)/water (1:9) solution containing CeO2 nanoparticles (NPs) has been studied by optical absorption and fluorescent spectroscopy, as well as dynamic light scattering method. Formation of stable hybrid complexes between NPs and dye J-aggregates due to Coulomb and stacking interactions has been revealed. The dye molecules arrangement in aggregates (slipping angle θ, twisting angle α, the center-to-center distance R, and the exciton splitting energy Δ) was analyzed within he Kasha exciton model based on oblique transition dipoles orientation. For amphi-PIC, formation of twisted face-to-face and oblique head-to-tail J-aggregates has been revealed depending on CeO2 NPs concentration, whereas L-21dye forms aggregates of a twisted face-to-face fashion independently on NPs concentration.

Introduction Ordered organic fluorescent nanoclusters, so-called J-aggregates, composed of dye molecules (cyanines, merocyanines or porphyrins) possess properties quite different from that of constituting molecules and provide unique possibility in developing novel fluorescence-based materials for sensing and imaging technologies1-3. J-aggregates (Jelley’s aggregates named after their first 1 ACS Paragon Plus Environment

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inventor E. Jelley)4 exhibit an intense and narrow absorption band (J-band), large extinction coefficients (up to 106 cm−1·M−1) and near-resonant fluorescence with an ultra-short radiative lifetime1-6. The J-band is red-shifted with respect to the monomer one. Due to translational symmetry within the J-aggregate molecular chains, the electronic excitations of monomers delocalize over chain segments and molecular (Frenkel) excitons form. So, excitonic origin of the Jband is the feature of J-aggregates1-6. It is known that the monomer arrangement in J-aggregate chains and, consequently, optical (excitonic) properties of J-aggregates are very sensitive to the environment1-3,5. For instance, in solutions containing templates for J-aggregates growths, such as nucleic acids and others, J-aggregate chains could be several tens of monomer molecules7,8. At the same time, often the templates provoke some distortions in the J-aggregated molecular chains with Davydov’s splitting of J-aggregate’s excited state level and appearance of two blue- (H-band) and red-shifted (J-band) excitonic bands1,2,5,8,9. In some cases, due to template heterogeneity or complexity, formation of J-aggregates with various lengths could be favorable providing several J-bands in the absorption spectra. For example, L-21 J-aggregates formation on DNA, RNA and vanadate NPs surfaces leads to simultaneous appearance of J-type dimers and spatially elongated J-aggregates910,11. As a result, both J-bands for dimers (called D-band) and long J-aggregates are presented in ensemble absorption and, in some cases, fluorescence, spectra10,11. It was shown, that the dimers are the building blocks of extended Jaggregates, assuming the dimer exhibition is caused by existing spatial restrictions of the templates10,11. Thus, J-aggregates show great potential for a number of applications such as photographic process, new laser materials, molecular electronics, nonlinear optical devices1,2,6-13. In recent years, there are also some reports on J-aggregates applications in bio-imaging and theranostics (therapy and diagnostics) of cancer14-17. There is a growing interest in the study of organic dyes conjugates with inorganic nanoparticles (noble

metal,

semiconducting

nanoparticles,

quantum

dots,

magnetic

nanoparticles),

photophysicochemical properties, aggregations, and absorption behaviors of organic dye molecules 2 ACS Paragon Plus Environment

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on the surface of nanoparticles (NPs) due to their new applications for nano-photonics, chemical sensing, catalysis, drug delivery and theranostics3,18-24. The combination of unique properties of Jaggregates and nanoparticles (NPs) allow novel effects to be observed in such systems. Nanocrystals of cerium oxide CeO2 (nanoceria) exhibit high capacity to absorb and release oxygen due to ceria ion ability to cycle between the +3 and +4 valence states that promotes the numerous utilizations of CeO2 materials in the industrial processes and apparatus25-30. Owing to the ability of CeO2 NPs to work as a regenerative scavenger of reactive oxygen species31-35, nanoceria is prospective for theranostic applications. Aggregation behavior of 1-methyl-1’-octadecyl-2,2’-cyanine perchlorate (amphi-PIC) and 3,3dimethyl-9-(2-thienyl)-thiacarbocyanine iodide (L-21) dyes (Chart 1) and exciton properties of their J-aggregates were studied in details previously10,36-38. The dye aggregation behavior, J-aggregate architecture and optical properties were shown to be strongly affected by the various conditions, including additives and presence of templates for the J-aggregate growth. In present study, we report the amphi-PIC and L-21 dyes aggregation in water solutions containing CeO2 NPs and analyze the dye molecule arrangement in formed aggregates within the Davydov and Kasha molecular exciton theory.

Chart 1. Molecular structures of amphi-PIC (a) and L-21 (b) dyes.

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Experimental section L-21 was purchased from Otava Ltd (Ukraine) and used as received. Amphi-PIC was synthesized by Dr. Igor Borovoy (Institute for Scintillation Materials NAS of Ukraine). The purity of the dye was controlled by NMR and thin layer chromatography. Organic solvents dimethylformamide (DMF) and dimethylsulfoxide (DMSO) of spectroscopic grade were purchased from

Sigma-Aldrich

and

used

as-received.

Sodium

citrate

Na3C6H5O7

99%,

hexamethylenetetramine ((CH2)6N4, 99%), hydrogen peroxide (H2O2, 35%) (wt/vol) and ammonium hydroxide (NH4OH, 25%) were from “Macrokhim” Co. Ltd. Cerium chloride (CeCl3 99.9%, Acros organic Company) and calcium chloride (CaCl2 anhydrous, 93%, Sigma–Aldrich) were used as received. Aqueous colloidal solutions of CeO2 NPs were synthesized by the method reported earlier39. 100 ml of solution CeCl3 (2 mM) were mixed with 100 ml of solution hexamethylenetetramine (4 mM) and stirred by means of a magnetic stirrer for 3 h at room temperature. After that, 1.8 ml NH4OH and 0.6 ml of H2O2 were added into the solution. Then, the solution was put in roundbottom flask and refluxed for 5 h. As a result, a transparent colorless solution was obtained. The solution was evaporated in a rotary evaporator flask under vacuum at the bath temperature of 70°C to 30 ml. A solution of 1 M CaCl2 was added to the obtained solution until the resulting solution became turbid. Then coagulated ceria nanoparticles were isolated by centrifugation and redispersed in solution of 1 M CaCl2. The procedure of precipitate cleaning was repeated three times. After the last stage of centrifugation, a solution of sodium citrate with molar ratio CeO2 /NaCt as 1:1 was added to the precipitate. The solutions were additionally dialyzed for 24 h against deionized water to remove the excess of ions and organics species. Dialysis membrane tubing with a molecular weight cutoff of “Cellu Sep H1” 3.5 KDa was used, and water was renewed each 6 h (the water/colloid volume ratio is 40). Thus, the impurity-free solution was found to be transparent in transmitted light at the concentration CeO2 1g/L with a poorly yellow color. Particle morphology and size distribution were determined by Transmission Electron Microscopy (TEM-125K electron microscope, Selmi, Ukraine). Specimens were prepared by 4 ACS Paragon Plus Environment

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placing a drop of colloidal solution onto a 200-mesh holey carbon copper grid. The grid was airdried under dust-free conditions and examined at 100 kV. Infrared spectra (IR) of the samples were recorded by SPECTRUM ONE (Perkin-Elmer, USA) IR-Fourier (IR-Fourier) spectrophotometer. The FT-IR spectrum of the CeO2 NPs was obtained in the wave number range of 450–4000 cm−1. Samples were prepared by spreading colloids on silicon wafers. NPs sizes in a colloidal solution before and after addition of the dye were determined by dynamic light scattering (DLS). Hydrodynamic diameter and ζ-potential were measured with ZetaPALS (Brookhaven Instruments) analyzer operating with a He−Ne laser at 657 nm wavelength. For each test, at least ten records were obtained and then averaged. All investigations were performed at 25°C. The samples for investigations were prepared as follows. First, stock solutions of amphi-PIC (0.1 mM) in DMF and L-21 (0.1 mM) in DMSO were prepared. Then, required amount of the dye stock solutions and aqueous solution of CeO2 NPs (1 g/L) were mixed in a flask and left for about 1 hour for equilibria. The final dye concentration in the solutions was 5x10-6 M, whereas the concentration of CeO2 NPs was varied within the 0.05 – 0.5 g/L range. The absorption spectra were measured with a SPECORD 200 (Analytik Jena) spectrophotometer. The temperature inside the sample compartment of the spectrophotometer was 25 ± 0.25◦C. The luminescence spectra were measured with a Lumina (Thermo Scientific) spectrofluorimeter. For registration of the absorption spectra and luminescence spectra, a quartz cuvette with optical path length of 1.0 cm was used. Results and Discussion

Characterization of synthesized CeO2 NPs. Figure 1 and Table 1 represent the TEM image of as-prepared CeO2 NPs with a size distribution histogram, IR spectrum and DLS measurement data. As evident from Figure 1 a, the particles are spherical in shape and well-dispersed with average particle size of about 2 nm. Obtained values of polydispersity index indicate that the sample has

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rather narrow size distribution. Hydrodynamic diameter of synthesized CeO2 NPs is 9 nm (Table 1). The larger value of dh as compared with that obtained from the size distribution histogram is due to a stabilized citrate shell formed around CeO2 NPs during the synthesis. CeO2 NPs possess negative charge due to carboxylate groups of the stabilizing citrate shell. The large values of ζ-potential and electrophoretic mobility suggest that the colloidal solution of CeO2 NPs should be stable for a long time that is in agreement with our observation.

Figure 1. TEM image with a size distribution histogram (a) and FT-IR-spectrum of synthesized CeO2 NPs (b).

Table 1. DLS measurements of hydrodynamic diameter (dh), polydispersity index (PdI), zeta potential (ζ-potential), conductivity and mobility of CeO2 colloidal solutions (0.5 g/L) in the absence and presence of amphi-PIC or L-21 dyes (C= 5x10-6 M) CeO2 NPs рН dh(nm) PdI ζ-potential (mV) Conductivity (µS) Mobility

7.30 9.0±0.4 0.217 –19.21±0.5 1550 –0.70

CeO2NPs+ amphi-PIC 7.40 119.1±1.4 0.229 –30.66±0.63 2716 -2.40

CeO2 NPs+ L-21 7.45 123.9±1.8 0.363 –34.55±0.67 2477 -2.70

FT-IR spectrum of as-prepared CeO2 NPs is shown in Figure 1 b. The small bands obtained at 745, 615 and 540 cm-1 are corresponding to Ce-O stretching mode40,41, indicating the formation of CeO2. The two bands at 1390 and 1585 cm-1 are respectively ascribed to the symmetrical and 6 ACS Paragon Plus Environment

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asymmetrical valence vibrations of the carboxylate groups40,41. The two peaks at 1078 and 1270 cm1

correspond to C-O stretching bands of the citrate41. Two sharp absorptions at 2920 and 2860 cm-1

were assigned to the asymmetric vibrations and asymmetric vibrations of C-H groups for hydrocarbon chain, respectively41. This clearly attests the presence of the citrate ligands on the surface of the nanoparticles. A broad band in the range of 3200–3500 cm-1 and a weak sharp band at 1665 cm-1 are characteristic for stretching vibrations and bending vibration of the O–H groups, indicating that the CeO2 NPs contain hydroxyl groups or H2O on their surfaces. As CeO2 NPs are synthesized in aqueous solution, their surface is capped with a large number of hydroxyl groups either chemically bonded or physically adsorbed to the surface.

Amphi-PIC aggregation in the solution containing CeO2 NPs. Amphi-PIC is an amphiphilic analogue of pseudoisocyanine dye (PIC)42. Figure 2 a presents the absorption spectrum of amphiPIC dye in DMF. At low dye concentrations, three absorption bands could be observed:

Figure 2. Absorption (a) and fluorescence, λexc = 530 nm (b) spectra of amphi-PIC in binary DMF/water (1:9) solutions containing CeO2 NPs of various concentrations. [amphi-PIC] = 5x10-6 M. Dashed line represents the absorption spectrum of amphi-PIC in DMF.

the main maximum centered at 530 nm (18860 cm-1) corresponded to the 0 → 0 vibronic transition between the electronic molecular ground state and the first excited state, and two less intense bands 7 ACS Paragon Plus Environment


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with maxima centered at 496 nm (20160 cm-1) and 460 nm (21740 cm-1) corresponded to the 0 → 1 and 0 → 2 vibronic transitions, respectively. The energy differences between the main absorption maximum and 0 → 1 and 0 → 2 vibronic transitions are 1300 cm-1 and 1580 cm-1 that is typical for carbocyanine dyes43. In binary DMF/water (1:9) solutions amphi-PIC dye is known to undergo self-aggregation36,37, that could be detected by appearance of new very intense red-shifted band (J-band) with λmax = 575 nm (17390 cm-1) associated with the dye aggregated species, Figure 2 a. The appearance of red-shifted, blue-shifted or both bands belonging to the dye aggregates depends on the relative orientation of the transition moment vector of the monomeric units in aggregate’s chain and could be explain within the Kasha and Davydov exciton model built on the electrostatic interaction approximation of the point transition dipoles in the excited state1,6,44,45. The shifts of the absorption maximum of the aggregate with respect to that of the monomer depends on the angle between the direction of the dipole moments and the line linking the molecular centers (slipping angle θ), as well as the angle between the transition moments of the monomers in the aggregate (twisting angle α), see Chart 2.

Chart 2. Schematic energy splitting diagram for aggregates with different monomer electric dipole orientation (arrow in ellipses) based on Kasha molecular exciton coupling theory. Reprinted with permission from Ref. 36. Copyright 2014 American Chemical Society. 8 ACS Paragon Plus Environment

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As shown in Chart 2, for face-to face packing of molecules in the aggregate (H-type aggregate, 54.7° < θ ≤ 90° and α = 0), the allowed transition energy gap is larger than that of monomer. It should be noted that the excitation from upper splitting level undergoes fast internal conversion to the lower one, from which the emissive transition to the ground state is forbidden44,45. Thus, the quantum yield of H-aggregate fluorescence is negligible. For head-to-tail molecular packing (J-type aggregate, 0° ≤ θ < 54.7° and α = 0), allowed transition energy gap is smaller than that of monomer and emissive transition to the ground state is allowed. So, J-aggregates possess intense fluorescence band. Thus, for these two limiting cases, a blue-shifted band (H-band) and a red-shifted band (Jband) could be observed in the absorption spectrum, respectively. It is clear that such division into H- and J-aggregates is simplification. Real systems tends to form more complex geometry, such as twisted face-to-face aggregates (54.7° < θ ≤ 90° and 0° < α ≤ 90°) or oblique head-to-tail aggregates (0° ≤ θ < 54.7° and 0° < α ≤ 90°), for which transitions to both upper and lower levels of the split excited state are allowed and, consequently, both H- and J-bands could be observed in the absorption spectra (Chart 2)44-46. Let us note, that the division proposed in Ref.46 depending on slipping angle θ, namely, twisted face-to-face and oblique head-to-tail fashions is also relative and based on the Kasha exciton band theory for oblique transition dipoles45,46. Moreover, for both types of molecular packing, the transition from the lower split excited level to ground state is allowed, so both aggregates types are fluorescent. So, both types could be named as J-aggregates (fluorescent aggregates). Thus, in a binary DMF/water (1:9) solution, amphi-PIC dye undergoes J-type aggregation that is revealed by the appearance of the J-band and strong fluorescence one (Figure 2a,b). The part of the aggregated species can be estimated compared the areas of the monomer band with that belonging to the aggregated species. For this, the aphmi-PIC absorption spectrum in diluted DMF solution (dashed line in Fig. 2a) was inscribed into the spectrum in a binary DMF/water (1:9) solution and subtracted. Obtained curve represents the absorption spectrum of J-aggregates. The calculated parts of amphi-PIC aggregated species is about 60 %. 9 ACS Paragon Plus Environment


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The full width at half maximum value of the J-band (∆ ) is 815 cm-1 that is rather large value. The J-band width is known to be depended on the exciton delocalization length ( )5:

=

∙(∆ )

∙(∆ )

− 1,

(1)

! ! where ∆ is full width at half maximum of the monomer band. Taking ∆ = 1135 cm-1

(obtained from the amphi-PIC monomer absorption spectrum at low concentration in DMF after the sustraction of bands corresponding to 0 → 1 and 0 → 2 vibronic transitions), we obtaine = 2. Obtained value for amphi-PIC J-aggregated formed in a DMF/water (1:9) solution at room temperature is very small and explained by high degree of static disorder in J-aggregate’s47-49. In a DMF/water (1:9) solution containing CeO2 NPs, the absorption spectrum of amphi-PIC undergoes remarkable transformation (Figure 2 a). The J-band decreases in intensity, becomes narrow and an additional band with λmax= 490 nm, blue-shifted with respect to the monomer one is observed. The relative intensities of blue- and red-shifted bands depend on CeO2 NPs concentration in the solution. Such spectral transformation points to the remarkable changes in J-aggregates structure and monomer molecules arrangement within the J-aggregate chain due to the dye adsorption on the NPs surface. Adsorption of dyes on various surfaces and then the formation of adsorbed dye aggregates is well-studied phenomenon. Indeed, electrostatic interactions between the dye cations and negatively charged CeO2 NPs as a result of a stabilizing citrate shell cause the dye adsorption within a citrate shell: "# + %&'( ) ⇆ ( " … %&'(). Forced dye molecules concentration within the NPs surface provokes their aggregation due to dye’s π-systems interaction with a formation of hybrid complexes consisting of several NPs and dye aggregates

, ( " … %&'() ⇆ ( " … %&'()

11,50

. This statement is supported by the sufficient increase of

NPs hydrodynamic diameter and ζ-potential value (Table 1). At the same time, conductivity and electrophoretic mobility parameters are also increased that indicates stability of formed hybrid complexes at used dye concentration, which is below the critical coagulation concentration11. Appearance of two absorption bands belonging to aggregate species points to the complex structure of aggregates with two non-equivalent monomer molecules in aggregate’s unit that causes 10 ACS Paragon Plus Environment

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exciton band splitting (Davydov splitting, see Chart 2)45. Using Kasha exciton band theory for oblique transition dipole44-46, we can estimate the geometry of aggregates formed on CeO2 NPs and effects of NPs concentrations on dye arrangement in the aggregates. Two CeO2 NPs concentrations (0.05 g/L and 0.5 g/L) were used for calculations. Absorption spectra of amphi-PIC aggregates formed at this two NPs concentrations in a DMF/water (1:9) solution (blue curves) were obtained as ascribed above by subtracting the inscribed monomer band (red dashed line) from the absorption spectra presented in Figure 2a (the black curves) (see Supporting information, Fig. S1). The angles α and . can be calculated as follows50,51: 2

/0 1 3 =

4

4

, . =

567∘ #2

,

(2)

where 9 and 9 are the oscillation strengths of the corresponding electronic transition, the ratio

9 ⁄9 is proportional to the ratio of areas under the corresponding bands in the absorption spectrum. The center-to-center distance (R) between to dye molecules in aggregate’s unit could be calculated from the following Equation44,45:

Δ =

||

Molecular Arrangement in Cyanine Dye J-Aggregates Formed on

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