ARTICLE pubs.acs.org/JPCC
Efficient Two-Photon Absorption Materials Consisting of Cationic Dyes and Clay Minerals Yasutaka Suzuki,†,§ Yuta Tenma,†,§ Yukihiro Nishioka,†,§ Kenji Kamada,‡,§ Koji Ohta,‡,§ and Jun Kawamata*,† † ‡
Graduate School of Medicine, Yamaguchi University, Yoshida, Yamaguchi 753-8512, Japan Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan ABSTRACT: We systematically investigated enhancement of the molecular two-photon absorption (TPA) cross sections (σ(2)) of organic dyes confined in the interlayer space of clay films. Several possible mechanisms have been proposed to explain the enhanced TPA of dyes in the interlayer spaces of clays. The present study comprehensively investigates two of these mechanisms, namely, (1) enhanced molecular planarity and the consequent extension of the π-electron system and (2) enhanced molecular orientation. We fabricated transparent dyeclay films consisting of a synthetic saponite clay and four cationic dyes. Two of these dyes are expected to have enhanced planarity when they are hybridized with a clay mineral. The other two dyes are considered to retain the planarity of their conjugated π-electron systems when they are in solution or in the interlayer space of a clay. We experimentally measured the σ(2) of these dyes in a clay film and in solution and found that the σ(2) of all four dyes is enhanced in a clay film but that this enhancement is greater for the former two dyes. These results are explained in terms of the effects of (a) extension of the π-conjugated system due to the enhanced planarity, (b) reduced detuning energy due to the enhanced planarity, (c) enhanced molecular orientation, and (d) the occasionally hydrophobic environment of the interlayer space of clay minerals. The highest enhancement of σ(2) was observed for a porphyrin derivative, tetrakis(1-methylpyridinium-4-yl)porphyrin ptoluenesulfonate, for which σ(2) in a hybrid film was 13 times greater than that in a dimethyl sulfoxide solution. Therefore, hybridization of cationic dyes with a clay mineral is an effective design strategy for TPA materials.
1. INTRODUCTION Two-photon absorption (TPA) occurs when a material absorbs two photons simultaneously. The TPA intensity increases quadratically with increasing excitation intensity. Consequently, by using a tightly focused laser beam, TPA can be induced in a submicrometer volume centered on the focal spot. This characteristic enables TPA to be utilized for 3D optical data storage,15 twophoton fluorescence microscopy,68 3D microfabrication,1,9,10 two-photon photodynamic therapy,11,12 and optical limiting.1316 However, the TPA cross-section (denoted by σ(2); i.e., the TPA efficiency per molecule) of most materials is too small for practical applications. An intense pulsed laser (e.g., a femtosecond pulsed laser) is required to observe TPA in most materials. Therefore, it is important to develop materials with large σ(2) to enable TPA to be excited using commonly used lasers. According to perturbation expansion theory,17,18 σ(2) at the lowest-energy TPA transition (σ(2)peak) for a quadrupolar molecule can be written as ð2Þ σpeak
jμkg j2 jμe1 k j2 ΔE2 Γe1 g
ð1Þ
E2e1 g
where μkg is the transition dipole moment from the ground state (g) to the lowest one-photon absorption allowed energy level (k), μe1k is the transition dipole moment from k to the lowest r 2011 American Chemical Society
TPA allowed energy level (e1), μe1g is the energy gap between g and e1, ΔE (i.e., the detuning energy) is the difference between the photon energy of the two-photon excitation and the energy gap between g and k (ΔE = (Ekg 1/2Ee1g)), and Γe1g is a damping factor. On the basis of eq 1, a considerable number of molecules have been developed with large transition moments1924 a small detuning energies (ΔE), or both.17,25,26 A widely employed strategy to enhance the transition dipole moment is to introduce electron donor (D) or acceptor (A) groups at opposite ends of a πconjugated system.1728 Another effective strategy for obtaining molecules with large transition dipole moments is to enhance the planarity of a π-conjugated system.27,28 To decrease ΔE, for example, incorporation of azulenyl noiety in a π-conjugated system is known as an effective strategy.26 Hybridizing a cationic dye with a clay mineral is an alternative way to produce efficient TPA materials.29 Hybrid films consisting of 1,4-bis(2,5-dimethoxy-4-{2-[4-(N-methyl)pyridinium] ethenyl}phenyl)butadiyne triflate (MPPBT) (see Figure 1a) and a synthetic clay (Smecton SA or SSA) have been prepared. At a 20% loading level relative to the cation-exchange capacity (%CEC), σ(2)peak was 2.5 times larger than that in a dimethyl Received: April 24, 2011 Revised: September 12, 2011 Published: September 12, 2011 20653
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Figure 1. Chemical structures of (a) MPPBT, (b) TMPyP, (c) FL, and (d) PIC.
sulfoxide (DMSO) solution. Several explanations have been proposed for this enhanced TPA.29 Torsional motion of the diacetylene moiety of MPPBT can be thermally activated at room temperature, and the two planar styrylpyridyl groups can rotate almost freely around the diacetylene axis while in solution. MPPBT molecules can thus be considered as consisting of the two separate π-electron systems. Confinement of the chromophore to the 2D nanospace formed between the silicate nanosheets of the clay by intercalation is expected to suppress torsional motion of the MPPBT (and thus shorten the πconjugated system). This confinement should also ensure that the π-plane of the molecules and the electric field of the incident light can lie in the same plane when a film is irradiated by a laser beam that propagates normal to the film. Consequently, the incident irradiation should be utilized more effectively for excitation than in isotropic systems. However, it is unclear why the σ(2)peak of MPPBT is enhanced in the MPPBTSSA composite. The difficulty in fabricating dyeclay hybrid materials suitable for TPA measurements has hindered the clarification of the cause of this effect. Consequently, considerable enhancement of σ(2)peak for a dye hybridized with a clay mineral has been reported only for the MPPBTSSA composites. We have recently developed a method for fabricating lowlight-scattering dyeclay composite films.30,31 Films fabricated by this method exhibit definite layer-by-layer characteristics and are transparent to high optical densities (>6) over a wide range of %CEC values. These characteristics make them suitable for systematically investigating the TPA properties of dyeclay composite films. The present study compares the TPA properties of several dyeclay films fabricated by our method. Of the two above-mentioned explanations that are expected to enhance the σ(2)peak of dyes in composite films, the molecular orientation of the chromophore between clay layers should be enhanced in all dyeclay films fabricated by our method because the composites are stacked in a layer-by-layer manner over the films. For composites containing dyes whose planarities are expected to be improved by intercalation (e.g., MPPBT), not only will the molecular orientation be enhanced but also the effective π-conjugation length of the dye should be extended. Therefore, this study compares the spectroscopic properties of dyes whose planarities are thought to be improved by intercalation with those whose planarities are considered to remain
unchanged by intercalation both in solution and in films consisting of the dyes and SSA. For the former dyes, we selected MPPBT and tetrakis(1-methylpyridinium-4-yl)porphyrin p-toluenesulfonate (TMPyP, Figure 1b). The four pyridyl rings and the central porphyrin ring of TMPyP lie in the same plane when the molecule forms a composite with SSA, whereas the pyridine rings of TMPyP are perpendicular to the porphyrin ring plane in solution.3234 Therefore, similar to MPPBT, the π-electron system of TMPyP should be extended in composite films. We selected 1,30 -diethyl-4,40 -(9,9-diethyl-2,7-fluorenediyl-2,1-ethenediyl)dipyridinium perchlorate (FL; Figure 1c) and 1,30 -diethyl-2,20 -carbocyanine iodide (pseudoisocyanine; PIC; Figure 1d) as dyes whose planarities are not thought to be improved by intercalation. These dyes have rigid, planar π-electron systems, even in solution. Therefore, they are expected to exhibit similar degrees of conjugation in solution and in films. The mode of adsorption onto a clay surface and the spectroscopic characteristics of the composite in an aqueous dispersion state were investigated for the four dyes used in the present study. TPA was generally enhanced considerably in dyes whose planarities are considered to be enhanced by confinement. In particular, the σ(2)peak of TMPyP in a hybrid film was nearly 13 times greater than that in a solution. These results are explained in terms of the effects of molecular orientation and planarity.
2. EXPERIMENTAL SECTION 2.1. Materials. MPPBT and FL were synthesized according to the reported method.35,36 TMPyP and PIC were purchased, respectively, from Aldrich and Kanto Kagaku and were used without further purification. Synthetic saponite (Smecton SA (SSA); synthesized by Kunimine Industries) used in this study was purchased from the Clay Science Society of Japan. The stoichiometric formula of SSA is [(Si7.20Al0.80)(Mg5.97Al0.03) O20(OH)4]0.77 3 (Na0.49Mg0.14)0.77+. Takagi et al.37 reported that SSA has a CEC of 0.997 mEq g1. In previous atomic force microscopy measurements,31 we estimated the average SSA particle diameter in the in-plane direction to be ∼30 nm. 2.2. Fabrication of Composite Film. As described above, extremely low-light-scattering composite films were fabricated according to the method reported in our previous paper.31 DyeSSA composites were prepared by an ion-exchange reaction by mixing an aqueous dispersion of SSA with solutions of the 20654
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Table 1. Optimized Parameters for Preparing DyeSSA Composites dye concentration
amount of solution
concentration
amount of SSA dispersion
%CEC (%)
(mM)
mixed with SSA dispersion (mL)
of SSA dispersion (g/L)
mixed with solution (mL)
Ru(phen)3/water
60
0.10
6.0
0.017
120
MPPBT/DMSO
10
1.0
0.10
0.10
20
20
1.0
0.20
0.050
40
40
1.0
0.40
0.025
80
60
1.0
0.60
0.017
120
80
1.0
0.80
0.013
160
10 20
0.050 0.050
1.0 2.0
0.10 0.050
20 40
40
0.050
4.0
0.025
80
60
0.050
6.0
0.017
120
80
0.050
8.0
0.013
160
10
1.0
0.10
0.10
20
20
1.0
0.20
0.050
40
40
1.0
0.40
0.025
80
60 80
1.0 1.0
0.60 0.80
0.017 0.013
120 160
10
0.20
1.0
0.050
40
20
0.20
2.0
0.025
80
40
0.20
4.0
0.013
160
60
0.20
6.0
0.0084
240
80
0.20
8.0
0.0063
360
dye/solvent
TMPyP/water
FL/DMSO
PIC/water
dyes. Composite films were prepared by filtering the aqueous dispersions of dyeclay composites under suction through a mixed cellulose ester membrane filter (Advantec, A010A025A; pore size: 100 nm; diameter: 25 mm). The residue was colored, whereas filtrates were colorless. This indicates that all dye molecules were adsorbed on the clay. MPPBT and FL had solubilities in water of ∼105 mol L1. If water solutions of these dyes had been employed, then relatively large amounts of dispersions would be generated, which would require a considerable time to suction. This may prevent the fabrication of lowlight-scattering, homogeneous films. Therefore, we employed DMSO as the solvent because it can dissolve MPPBT and FL at high concentrations (ca. 103 mol L1) and is miscible with water. TMPyP and PIC have sufficiently high solubilities in water for fabricating composite films. Therefore, water solutions of these dyes were mixed with clay dispersions. The deposited films were peeled off the membrane filter as self-standing films. Table 1 lists the optimized values for the concentration of the clay dispersion, the polarity of the dispersion, and the dye loading level for low-scattering films. As we previously reported, this procedure provides with a high reproducibility38 2.3. Measurements of UVvis Spectra. Absorption spectra of the solutions and the dispersion of composites with dye concentrations of 105 mol L1 were measured with a UVvis spectrometer (Jasco, U-670) using 10 mm quartz cuvettes. The same spectrometer equipped with an attachment for films (Jasco, VTA-752) was used to obtain absorption spectra of the films. 2.4. Measurements of Two-Photon Absorption Spectra. TPA spectra were obtained by plotting σ(2) as a function of wavelength measured by the open-aperture Z-scan technique.18,39 A femtosecond pulsed beam from an optical parametric amplifier (Spectra-Physics, OPA-800C) pumped by a beam from a regenerative amplifier (Spectra-Physics, Spitfire) was used as the light source. The pulse duration was typically 150200 fs, and the
repetition rate was 1 kHz. The incident beam was focused by a plano-convex lens (f = 150 mm), and the sample was scanned along the incident beam axis. The average incident power was varied from 0.01 to 0.4 mW, corresponding to on-axis peak powers (I0) of 6 to 240 GW/cm2. The measurements revealed that the TPA absorbance (q0) is proportional to the incident power (and hence I0). This is a reliable indication that the transmittance changes observed from the Z-scan measurements were purely due to TPA and not to any other nonlinear optical process.18,40 To perform reliable Z-scan measurements, the sample must be thinner than the Rayleigh length and must contain a number of dye molecules in the optical path. Because the dyes were not dissolved in water at the required concentration, Z-scan measurements of homogeneous solutions were performed using a DMSO solution of dyes with concentrations over 2 103 mol L1. One-photon absorption spectra of these solutions were essentially the same as those measured for a solution with a concentration of 106 mol L1, indicating that the dyes were in monomer form and did not form H- or J-aggregates even at high concentrations. For evaluation σ(2), we estimated the concentrations of dyes in composite films by area of films, film thickness, and quantity of dyes. The area of the composite films was estimated by measuring the length with a ruler. Film thickness was measured by the Swanepoel method.41 The quantity of dyes in the film was estimated from the loaded amount and concentration of dye of initial dye solution. 2.5. X-ray Diffraction Measurements. X-ray diffraction (XRD) data were collected using a Rigaku Ultima-IV diffractometer with monochromatized Cu Kα radiation (λ = 0.154 nm).
3. RESULTS AND DISCUSSION 3.1. One-Photon Absorption Properties. The UVvis absorption spectra of dyes were investigated in pure solvents of water, ethanol, DMSO, acetone, and chloroform. Table 2 lists the 20655
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Table 2. Absorption Maxima (λmax) and Molar Absorption Coefficient of Dyes in Various Solventsa molar absorption coefficients ( 103 L mol1 cm1)
λmax (nm) solvents
a
ET(30)
MPPBT
TMPyP
FL
PIC
MPPBT
TMPyP
FL
PIC
water
63.1
453
423
420
523
66.3
226
43.4
64.0
ethanol
51.9
465
424
430
524
70.4
228
59.0
75.4
DMSO
45.1
466
424
429
525
66.0
276
63.9
70.5
acetone
42.2
462
422
427
524
70.8
233
61.7
74.0
chloroform
39.1
insoluble
insoluble
445
529
insoluble
insoluble
59.4
75.0
Polarity parameters (ET(30)) of the solvents are also shown.
Figure 2. Absorption spectra of (a) MPPBT, (b) TMPyP, (c) FL, and (d) PIC in dispersions of composites with SSA prepared at various %CEC and in water (TMPyP and PIC) and a DMSO (0.5% v/v)-water mixed solvent (MPPBT and FL).
polarity parameters (ET(30)),42 the wavelengths of the absorption maxima (λmax), and the molar absorption coefficients at the λmax of the dyes dissolved in these solvents. The λmax of MPPBT, FL, and PIC tended to red shift with decreasing ET(30) of the solvent. The value of molar absorption coefficients at λmax tends to increase with decreasing the ET(30) of solvents. In contrast, λmax of TMPyP was almost constant even when the solvent was varied. However, the molar absorption coefficients tended to increase with decreasing ET(30). These observed behaviors are typical of compounds exhibiting positive solvatochromism (e.g., ref 43). Figure 2 shows UVvis spectra of the composites with SSA dispersed in water (TMPyP and PIC) or a mixed solvent (hereafter denoted by dispersion/dye) of DMSO and water (MPPBT and FL). As described in the Experimental Section, dispersion/ MPPBT and dispersion/FL composites were fabricated by mixing
a water dispersion of SSA and a DMSO solution of dye. To determine the contribution of DMSO to the spectra, we recorded spectra of MPPBT and FL without clay in a mixed solvent of DMSO and water at the same mixture ratio as those of the dispersion. To obtain absorption spectra of the clay systems, we prepared composites at loading levels of 10, 40, and 80%CEC. Table 3 summarizes the spectral results. For MPPBT, TMPyP, and FL, the absorption maxima of the dispersion/dye showed shifted to longer wavelengths than the dye in a homogeneous solution at the same composition. This clearly indicates that these dye molecules interact with SSA particles. They were probably adsorbed onto clay layer surfaces through an ion-exchange reaction. In contrast with dispersion/MPPBT, dispersion/TMPyP, and dispersion/FL, no obvious spectral difference was observed in the absorption maximum of the main absorption band between a 20656
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Table 3. Wavelengths of Absorption Maxima (nanometers) of Dyes in Homogeneous Solution, in Composite Dispersion, and Composite Films MPPBT
TMPyP
FL
PIC
in homogeneous
466 (in DMSO
420
420 (in DMSO
523
solution
water mixed solvent)
(in water)
water mixed solvent)
(in water)
in dispersion
in composite film
10%CEC
482
448
435
521
20 40
482 482
448 448
435 435
523
60
482
448
435
80
476
448
432
10%CEC
505
479
444
524
20
500
478
444
525
40
486
475
440
60
479
468
436
80
472
460
436
Figure 3. Absorption spectra of films of (a) MPPBT, (b) TMPyP, (c) FL, and (d) PIC fabricated from composites with SSA prepared with 10, 40, and 80%CEC.
homogeneous solution of PIC and dispersion/PIC. Instead, the growing up of an additional absorption band at 567 nm was seen with increasing loading (Figure 2d). It is reported that PIC forms J-aggregates on the surface of a clay at high loadings.44 Therefore, the additional absorption band at 567 nm is thought to be due to J-aggregate formation, which is interesting because such aggregation significantly affects the nonlinear optical properties of dyes.4547 However, because the present study seeks to perform a comparative study of the TPA properties of dyes in films and solutions, PICSSA composites prepared at loadings higher than 40%CEC were not further investigated.
The absorption bands of dispersion/MPPBT and dispersion/ FL blue-shifted with increasing loading, whereas no dependence on loading was observed for dispersion/TMPyP. The blue shift observed for dispersion/MPPBT and dispersion/FL is probably due to the formation of H-aggregates.48 TMPyP is reported to adsorb on SSA with no aggregation even at a loading level of 100% in a composite dispersion.34 Figure 3 shows UVvis spectra of film samples of MPPBT, TMPyP, FL, and PIC (hereafter denoted film/dye). These films were prepared with loadings of 10, 40, and 80%CEC. In all cases, dyes in hybrid films had a longer λmax than the corresponding 20657
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Figure 4. X-ray diffraction patterns of (a) SSA/MPPBT and (b) SSA/TMPyP films fabricated from composites prepared at various %CEC.
composites in a dispersed state. This was ascribed to the dyes in the films being in a hydrophobic environment, whereas the dyes were in contact with the water phase in aqueous dispersions. Film/MPPBT and film/TMPyP had a much larger red shift than film/FL and film/PIC. For example, at 10%CEC loading, the shifts of λmax of MPPBT and TMPyP were, respectively, 39 and 59 nm, whereas those of FL and PIC were, respectively, 24 and 1 nm. This difference suggests that the planarities of MPPBT and TMPyP improved in films, whereas FL and PIC had the same planarities in films and solutions. A similar red shift of the Soret band has been reported for freeze-dried composite films of TMPyP and SSA.37 In that case, the bathochromic shifts of the Soret band were explained in terms of flattening of the porphyrin molecules due to confinement in the 2D interlayer spaces of the clay.3234 At higher loadings, the λmax of films blue-shifted for MPPBT, TMPyP, and FL. This shift was ascribed to the formation of H-aggregates. PIC did not exhibit a blue shift because it formed J-aggregates. A similar shift was observed for dispersions of MPPBT and FL when %CEC was >80%. In contrast, blue shifts were observed at 10 and 20%CEC in the films. Another possible explanation for the blue shifts of MPPBT and TMPyP may be unflattening of the molecular planes at a higher %CEC due to extension of the interlayer spacing as the result of accommodating many molecules. Figure 4 shows the XRD results for film samples. The interlayer spacing of film/MPPBT increased when the loading was >40%CEC, whereas that of film/TMPyP was nearly constant even when the loading reached 100%CEC. Surface coverage is
Figure 5. Dependence of interlayer spacing of (a) SSA/MPPBT and (b) SSA/TMPyP composite films on surface coverage.
calculated as coverage ð%Þ ¼
%CEC cross-sectional area of a dye per cationic moiety surface area of SSA per anionic site
Therefore, the coverage represents the percentage of a clay surface covered by dye. It depends on both the molecular size and the valence of the dye molecule. Here we assumed that the molecular planes covering the π-electron systems are completely flattened; the cross-sectional areas of MPPBT and TMPyP viewed normal to the π-electron system are estimated to be about 3.5 and 2.5 nm2, respectively. MPPBT (divalent) and TMPyP (tetravalent) are calculated to have cross-sectional areas per cationic moiety of 1.8 and 0.6 nm2, respectively. Therefore, MPPBT has a three times larger coverage than TMPyP at the same loading. SSA is calculated to have a surface area per anionic site of 1.3 nm2. Figure 5 shows the interlayer spacing of the films as a function of coverage. When the coverage is
