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Spectroscopic Studies of the Interactions between a Cationic Cyanine Dye and a Synthetic Phyllosilicate: From Photophysics to Hybrid Materials Christian Ley,*,† Jocelyne Brendlé,‡ Moise Miranda,† and Xavier Allonas† †

Laboratoire de Photochimie et d’Ingénierie Macromoléculaires and ‡Institut de Science des Matériaux de Mulhouse, CNRS-UMR7361, Université de Haute-Alsace, 3b rue Alfred Werner, 68093 Mulhouse Cedex, France S Supporting Information *

ABSTRACT: The interaction of the cationic organic dye Astrazon orange R (AO-R) with the synthetic phyllosilicate Laponite leads to very interesting hybrid materials. Indeed, the Laponite nanoparticles modify the photophysical properties of AO-R, inducing a stabilization of its excited emissive state by preventing ultrafast isomerization. The long-lived emissive clay−dye hybrid complex can be used to develop efficient photoinitiating systems, leading to organic−inorganic hybrid crosslinked polymer materials.

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INTRODUCTION 2:1 Phyllosilicates have a lamellar structure in which an octahedral sheet (O) containing one or more hexacoordinated elements (aluminum, magnesium, etc.) is sandwiched between two tetrahedral (T) sheets containing one or more tetracoordinated elements (silicon and aluminum). Depending on the occupancy of the different sheets, the framework can be neutral, like in talc or pyrophyllite, or negatively charged because of the isomorphic substitutions in the tetrahedral and/ or octahedral sheets like in montmorillonite, hectorite, or beidellite. In this case, the interlayer cations (such as sodium) balance the negative charges of the sheet (see Scheme 1). Smectites, which belong to this group, are known for their ionexchange properties with applications in many fields such as water depollution, catalysis, or drug delivery systems1−4 The interlayer compensation cations can be replaced by a wide range of cations, either mineral or organic, such as organic cationic dyes.1,5−7 In this case, the interactions of the clay mineral surface with the molecules of the dyes can modify their photochemistry and optical properties.4,8−16 Recently, we have shown that phyllosilicates can be used to control the photophysics of the triarylmethane dye crystal violet.17 The interaction of the cationic dye, attracted by the negative octahedral charge of a commercially available synthetic 2:1 phyllosilicate (Laponite), with the platelet surface turns the nonemissive dye into a fluorescent compound.17 In the present work, we studied the modification of the photophysics of a cationic cyanine dye, Astrazon orange R (AO-R), by Laponite nanoplatelets. Indeed, AO-R exhibits complex photophysics involving isomerization in the excited state.18 This ultrafast © XXXX American Chemical Society

isomerization process, which is responsible for the quenching of the AO-R singlet excited state, is controlled by solvent viscosity. Recently, we have shown that this viscosity control of the isomerization can be used to conceive an interesting photoinitiating system (PIS).19 Thus, in this paper, we will demonstrate that the photophysics of the excited state of AO-R in interaction with the mineral-charged Laponite nanoparticles is completely modified leading to a long-lived fluorescence, as observed in other systems.20−23 The effect of the clay−dye interaction is studied by time-resolved and steadystate spectroscopy: the ultrafast excited state behavior is greatly modified when the dye is in interaction with the clay surface. Furthermore, the interaction of a molecule with clay produces a longer-lived excited state that could help develop a new type of PISs, that is, to convert the light energy into a chemical radical to perform free-radical photopolymerization. With this in mind, we explore the effect of clay contents on the efficiency of the AO-R PISs for free-radical photopolymerization of a model acrylate monomer.

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EXPERIMENTAL SECTION

Steady-state UV−visible spectra were measured on a Cary 4000 spectrophotometer. Steady-state fluorescence and excitation spectra were measured with a HORIBA-Jobin-Yvon FluoroMax 4 instrument. The time-correlated single photon counting (TCSPC) module of the spectrofluorimeter permits emission lifetime measurements. A 1.5 ns 460 nm nano-LED was used as a pulsed source. Spectra were corrected Received: April 18, 2017 Revised: June 11, 2017 Published: June 12, 2017 A

DOI: 10.1021/acs.langmuir.7b01330 Langmuir XXXX, XXX, XXX−XXX

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Scheme 1. Laponite, AO-R, N-Phenylglycine (NPG), Iodonium Salt (I250), and SR 349 Diacrylate Monomer Molecular Structures

to take into account the nonreciprocity of the absorption and emission phenomena according to refs 24 and 25 [transition dipole moment (TDM) representation]. Femtosecond pump−probe measurements were performed on a CDP Corp ExciPro system, the femtosecond laser excitation wavelength was adjusted to 590 nm with an optical parametric amplifier as described elsewhere. A pump−probe crosscorrelation under 200 fs was obtained.18 AO-R was purchased from TCI; its structure was checked by NMR, and it was used as received (see Scheme 1). Laponite RD was obtained from BYK Additives & Instruments former (Rockwood). The monomer formulations use a commercial diacrylate (SR 349, Sartomer, see Scheme 1) with variable amounts of dimethyl sulfoxide (DMSO) to adjust the viscosity of the resin. Their molecular structures were checked by NMR, and they were used as received. DMSO and N-phenylglycine (NPG) were purchased from Aldrich and used as received. 4-Methylphenyl[4-(2methylpropyl)phenyl]-iodonium hexafluorophosphate (I250) was a gift from Ciba Spa (Switzerland). The viscosities of the formulations were measured with a Brookfield Model DV-II+ viscometer at 20 °C. The kinetics of free radical photopolymerization were obtained by following the disappearance of the CC acrylate bond stretching signal at 1637 cm−1 by real time fourier transformed infrared (RTFTIR) spectroscopy using a Vertex 70 FTIR spectrophotometer (Bruker Optik), equipped with an MCT detector working in the rapid scan mode. The degree of the acrylate double bond conversion (C) was calculated from the decrease of the IR absorption peak area between the samples using the following equation: C(%) = (A0 − At)/ A0 × 100, where A0 is the initial peak area before irradiation and At represents the peak area of the acrylic double bond at time t. A good estimate of the maximum rate of conversion Rc was determined by the slope of the conversion kinetics at the inflection point or by taking the maximum of the time derivative of conversion curves (in s−1).26,27 For all experiments, the PIS was dissolved in the SR 349 diacrylate monomer resin in the weight ratio of 0.1 wt % of the dye AO-R, 1 wt % of the electron donor NPG, and 2 wt % of the electron acceptor I250. A 473 nm laser diode (Roithner Lasertechnik) was used as the irradiation source. A Philips CM200 microscope equipped with a LaB6 filament was used to collect transmission electron microscopy images under an accelerating voltage of 200 kV. A LKB 8800 Ultrotone III was used to obtain 50−100 nm thick slices.

exfoliated, leading to the presence of nanoparticles, that is, nanolayers (see Scheme 1).31−33 The two external surfaces are identical siloxane planes that are nonpolar and cannot form hydrogen bonds.28 It is thus awaited that electrostatic interactions between the cationic dye and the anionic nanolayers will lead to the adsorption of the dye on the basal surface of the nanoplatelets.12,15,16 Steady-state absorption and emission spectroscopies were performed on the dye in water and in a 2 g·L−1 Laponite solution. In these conditions, the amount of AO-R is less than 3.6% of the synthetic clay cation exchange capacity (CEC). The spectra are displayed in a TDM25 representation in Figure 1 (the uncorrected spectra are

Figure 1. Normalized absorption, emission, and excitation spectra (TDM25) of AO-R in water and 2 g·L−1 clay solution.

given in Supporting Information), and the maximum of emission and absorption are given in Table 1. It is clear that the presence of mineral nanoparticles in the solution influences the optical properties of the dye. First, an enhancement of the fluorescence intensity is observed, because of the increased singlet-excited state lifetime (vide infra) as was reported for other compounds.17,20−23 Second, a bathochromic shift of absorption (−476 cm−1) and emission (−140 cm−1) bands is observed when compared with water. As a consequence, the Stokes shift (i.e., the difference between the maximum of the absorption and the emission) is reduced in the presence of clay particles. Moreover, the interaction of the dye with Laponite leads to the formation of a vibrational shoulder in the

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RESULTS AND DISCUSSION Laponite presents an isomorphic substitution in the octahedral sheets, leading to a charge unbalance that is compensated by the presence of sodium cations in the interlayer space (see Scheme 1). In the presence of positively charged AO-R molecules in water, an ion exchange reaction can occur between these latter and interlayer sodium cations, leading to the possible adsorption of the dye on mineral nanoparticles.5,28−30 Depending on its concentration in water, Laponite can be B


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Table 1. Spectroscopic Properties of AO-R in the Presence of Laponite Nanoparticles (2 g·L−1), νmax(Fluo) and νmax(Abs) Indicates Maximum Wavenumbers (cm−1) of Fluorescence and Absorption, Respectively; FWHM Represents Full Width at Half Maximum of Absorption Spectra; and Δν Is the Stokes Shift H2O LAP

ν(abs)max

FWHM

ν(exc)max

FWHM

ν(em)max

FWHM

Δν

20 260 19 784

3013 2780

20 280 19 900

2951 2720

18 760 18 620

2802 2715

1500 1164

in 2 g·L−1 Laponite solution. In this experiment, UV−vis spectra are measured as a function of the delay between the pump (i.e., excitation) femtosecond laser pulse and the femtosecond white probe pulse, which permit the calculation of the absorption differential spectra at each pump−probe delay. The obtained differential spectra are displayed in Figure 3 for relevant pump−probe time delays. Besides the clear differences between the two experiments, positive bands and negative bands can be observed. For both experiments (and according to ref 17), the positive band observed in the blue region (390−430 nm) is ascribed to the absorbance of the singlet excited state of the AO-R molecule, and the negative band from 430 to 490 is ascribed to the photobleaching, that is, the ground state depletion. The negative band around 530−600 nm that overlaps the emission spectra (in black lines) is the stimulated emission band, which is due to the stimulated deactivation of the excited singlet state of the dye by the probe photons. This band allows following the time-resolved fluorescence of the AO-R singlet excited state. The results in water are very close to that of the experiment that was carried out in acetonitrile in ref 17. An ultrafast isomerization is observed in the first picoseconds: a positive absorption band with a maximum around 520 nm completely overlaps the stimulated emission band (i.e., the fluorescence). In around 50 ps, no more fluorescence is observed and a longlived positive signal is obtained together with a permanent photobleaching around 475 nm. This behavior was ascribed to an ultrafast isomerization of AO-R in its excited state toward a ground state Z isomer, whose absorption band is around 520 nm.17 It is clear that the ultrafast photophysics of AO-R is greatly influenced by the interaction with the clay surface. Indeed, in the presence of clay particles in water, the picture is completely different. First, the decays are slower; second, no Z isomer formation is observed: the stimulated emission band is not overlapped and does not go back to zero. At the same time, the absorption band in the blue part of the spectra, that is, the singlet state absorption band of AO-R, decays in a homothetic manner and a permanent positive signal is maintained. This strongly indicates that a long-lived emissive state is formed when the AO-R molecules are in interaction with the clay surface. A global analysis procedure by singular value decomposition was performed: this mathematical treatment allows the extraction of all the relevant kinetic information of pump− probe experiments in the form of some so-called orthogonal kinetics (KOP).35,36 These orthogonal kinetics are then simultaneously fitted by a function F(t), which is the sum of the exponential decays and a step function according to

absorption band, which does not completely overlap the excitation spectra, contrary to the case of pure water. Moreover, the full width at half maximum (FWHM) of the absorption and emission bands are thinner in the presence of clay, and the mirror image symmetry between the absorption/ excitation and emission spectra is better. All of these phenomena can be explained in terms of the structure-fixing effect and the structure-resembling effect: the electrostatic interaction of AO-R on the basal surface of the Laponite platelets prevents a strong reorganization of the molecular structure between the ground and excited states, leading to more structured, narrower and thinner bands.20,21,34 To further characterize the emission properties of AO-R in the presence of clay and to explain the increase in the fluorescence intensity, the emission lifetime was measured by TCSPC as shown in Figure 2.

Figure 2. TCSPC experiment (AO-R < 3.6% CEC, 2 g·L−1 Laponite solution). In black squares, the instrument response function (IRF); in blue dots, the emission decay; and in plain red line, its fitting with a 3exponential decay function.

Multiexponential fit, that is, a sum of up to 3-exponential decay functions, was needed to correctly reproduce the emission decay indicating some possible heterogeneities. A very small ultrafast decay, below the time resolution of the TCSPC system, was observed. However, in a 2 g·L−1 clay solution, the decay emission signal was dominated by a 3 ns fluorescence lifetime. This lifetime value of 3 ns is the attended value in the case of a very viscous solvent as described in ref 19 and very close to the natural radiative lifetimes of AO-R measured in ref 18. As a conclusion, it seems that the electrostatic interaction between the cationic dye and the clay surface are strong enough to prevent the deactivation of the excited emissive state by the ultrafast isomerization of the dye in its excited state. To get further information on the modification of the photophysics of the dye, we performed time-resolved femtosecond pump−probe UV−vis spectroscopy in pure water and

n

F (t ) =

∑ e−(t− t )/τ 0

i

+ step(t − t0)

i=1

where t0 is the moment where the pump and the probe pulses overlap, τi is the shared time constant, and n is the number of C


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Figure 3. Pump−probe time-resolved femtosecond spectroscopy of AO-R in the Laponite solution (2 g·L−1) (left) and pure water (right). (Up) Short time scale (1−10 ps). (Down) Long time scale (10 ps to 1.8 ns).

the needed time constant for an accurate fit (from two to three vide infra). The step function allows the reproduction of the time constant that is too long to be observed by the pump− probe experiment (i.e. >1 ns). The four first orthogonal kinetics (KOP 1−4) of the experiments, in pure water and in the presence of the synthetic phyllosilicate, are displayed in Figure 4. In pure water, the best fits of the KOP required a sum of two exponential decay functions and a step function with time constants of 2.54 ± 0.15 and 11.54 ± 0.20 ps, as described in ref 18, whereas the sum of the three exponential decay functions and a step function was necessary to reproduce the kinetics in the presence of the Laponite nanoparticles with lifetimes of 0.482 ± 0.02, 17.41 ± 0.75, and 417 ± 33 ps. Except for the first time constant, which can be ascribed to an ultrafast relaxation of the Franck−Condon state reached directly after light absorption, the presence of Laponite nanoparticles leads to a lowering of all time constants compared with pure water (or in other solvents, see ref 18), underlying the slowdown of the photophysics of the dye by interaction with the mineral nanoparticle surface. The most interesting observation is the final negative stimulated emission signal (together with the positive absorption band in blue), which is attributed to the measured 3 ns fluorescence lifetime by the TCSPC system. This indicates that the singlet excited state of AO-R is stabilized by the dye−Laponite interactions: the observed lifetime is very close to the natural fluorescence lifetime of the AO-R singlet state (i.e., the longest possible). As a conclusion, the optical properties and the photophysics of the organic dye AO-R are modified when the molecules are in interaction with the synthetic phyllosilicate nanosurface, leading to a long-lived singlet excited state hybrid.

In the last part of the paper, we discuss the use of the kinetic control of the AO-R excited state by the clay surface to improve the conversion of light into chemical energy. Indeed, in free radical photopolymerization, the photoinitiator (PI) is responsible for the conversion of irradiating light into initiating radicals. To develop PI, it is possible to combine a visible dye (which will absorb the light) with an electron donor (such as NPG) and/or acceptor (such as I250) coinitiator to form two and/or three-component PIS (see refs 37−39 for more detailed information about the photoinitiator). After light absorption by the dye, a photoinduced electron transfer (PET) reaction occurs between the dye’s excited state and the coinitiators leading to initiating radicals. These PET reactions are bimolecular; thus, the longer the excited state lifetime, the higher the probability of PET. Consequently, a quantum yield of the initiating radical production is higher for long-lived excited states, leading to a higher rate of monomer conversion into the crosslinked polymer network. Indeed, it was shown that the polymerization efficiency of the PIS can be directly linked to the photochemistry of the dye-coinitiators.40 In a recent paper,19 we have shown that the slowdown of the AO-R photophysical excited state deactivation in a viscous monomer tunes the performance of the PISs based on AO-R combined to an electron donor and an electron acceptor: when the viscosity of the resin increases, the lifetime of the AO-R singlet excited state also increases, and the radical production becomes more efficient. However, it was also shown that at too high a viscosity, the bimolecular PET reaction becomes diffusion controlled, that is, the reaction rates decrease, and the benefit of a longer singlet excited state lifetime cannot compensate for the slower diffusion of coinitiators toward the excited dye molecules. As a consequence, the quantum yield of the radical D


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Figure 5. Effect of the DMSO content (in wt %) on the viscosity of the different mixtures of monomers and Laponite (from 0.17 to 1.8 wt % of clay).

Table 2. Effect of Synthetic Phyllosilicate Contents on the Maximum Rate of Conversion Rc (s−1) and Final Conversion of the Photoinitiating Systems Based on AO-R/NPG/I250 clay (wt %)

DMSO (wt %)

Rc (s−1)

final conversion (%)

0 0.18 1.8

10 20 28

0.6 1.1 2.3

58 60 70

Figure 4. Orthogonal kinetics (KOP) of the pump−probe experiments in pure water (up) and in the presence of 2 g·L−1 synthetic phyllosilicate (down). The plain lines are the best fit of the KOP with a sum of up to three exponential decay functions and a step function.

decreases, and the efficiency of free radical polymerization is lowered. To circumvent this problem, we propose to use Laponite nanoparticles to fix the AO-R photophysics while maintaining a constant (low) viscosity in the monomer−clay resin. However, when a certain amount of Laponite is introduced in the SR349 monomer, the mixture viscosity increases (see Figure 5). To compensate for the viscosity increase, some quantity of DMSO is added in the monomer. Figure 5 displays the evolution of the monomer/Laponite mixture viscosity as a function of wt % of DMSO, for different contents of Laponite. It can be seen that the addition of the DMSO decreases the resin viscosity even in the presence of the synthetic phyllosilicate. A quite linear relationship was obtained for the different clay content (in wt %). In the standard photopolymerization experiment, 10 wt % of DMSO is added in the formulation to permit good solubilization of the compounds (the dye AO-R, the electron donor NPG, and the electron acceptor I250). With 10 wt % DMSO (the reference experiment), the viscosity of the resin is 0.25 Pa·s; thus, we decided to adjust all formulation viscosity to this value for the following experiments (see Table 2) by adjusting the DMSO wt % of the formulations. The evolution of the monomer to polymer conversion as a function of irradiation time is displayed in Figure 6. It can be seen that the addition of the synthetic phyllosilicate into the formulation leads to an increase in both the final conversion

Figure 6. Effect of Laponite contents on the polymerization kinetics followed by RT-FTIR.

and the rate of conversion: the highest clay content presents the highest reactivity. The value of the final conversion and the maximum rate of conversion Rc are given in Table 2. It can be seen that Rc is multiplied by 3.8 when 1.8 wt % of Laponite is added to the photopolymerizable formulation. As the viscosity of the resin is kept constant, this effect cannot be ascribed to a higher viscosity of the monomer but to the presence of the Laponite nanoparticles. Thus, it is shown here that the stabilization of the singlet excited state of AO-R by the electrostatic interactions of the cationic dye molecules and the clay surface induce great improvement of the photoreactivity of the PIS. To gather information on the dispersion state of the synthetic phyllosilicate in the final hybrid crosslinked polymer, E


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Langmuir TEM analyses were carried out on the cured sample containing 1.8 wt % of Laponite. It can be seen in Figure 7 that the layers

Xavier Allonas: 0000-0003-2194-939X Notes

The authors declare no competing financial interest.

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Figure 7. TEM image of the polymer composite prepared with the SR 349 acrylate monomer, the AO-R/NPG/I250 PIS, and 1.8 wt % of Laponite.

are stacked in a quite regular way, evidencing that the synthetic phyllosilicate is not completely exfoliated. However, in the obtained composite, the dye can be intercalated in the interlayer space, located at the edges or at the surfaces of the stacked layers, which is enough to control the photophysics of some AO-R molecules as indicated by the 4-fold increase in conversion rates.

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CONCLUSIONS In this paper, we presented an in-depth study of the clay−dye interaction by different steady-state and time-resolved spectroscopic techniques. It is shown that the photophysics of the cationic cyanine dye is influenced by the presence of Laponite nanosheets. The AO-R−clay surface electrostatic interaction prevents the isomerization of the dye after light absorption, stabilizing the emissive singlet excited state of the dye. It was then possible to improve the photoinitiating ability of AO-R/ NPG/I250 PISs by adding the synthetic phyllosilicate to monomer formulation. The 4-fold increase in the conversion rates at a constant resin viscosity proves that the hybrid clay− dye photoinitiator is also formed in the resin, even if the Laponite is not completely exfoliated as revealed by the TEM experiment. It is worthy to note that Laponite−AO-R forms very efficient hybrid photoinitiators. Studies are underway to decipher the mechanism of the formation of the Laponite−dye hybrid.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01330. Effect of the synthetic phyllosilicate on optical properties of the cationic dye AO-R, normalized absorption, emission and excitation spectra of AO-R in water and 2 g·L−1 laponite solution (PDF)

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REFERENCES

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*E-mail: [email protected]. ORCID

Christian Ley: 0000-0001-7052-4692 F


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