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Enhanced Optical Nonlinearity of Polyaniline-Porphyrin Nanocomposite Rakesh K. Pandey, C. S. Suchand Sandeep, Reji Philip, and V. Lakshminarayanan* Raman Research Institute, C.V. Raman AVenue, Bangalores560080, India ReceiVed: October 1, 2008; ReVised Manuscript ReceiVed: March 11, 2009
We observe that the porphyrin derivative 4,4′,4′′,4′′′-(porphine-5,10,15,20- tetrayl)tetrakis(benzoic acid) (PTBA) forms nanoparticles when mixed with the electrochemically formed conducting polymer polyaniline (PANI) in a DMF/water mixture. From atomic force microscopy (AFM) studies, the nanoparticles are found to be fairly uniform in size, with a dimension of around 80-100 nm and a z-height of 3-4 nm. The nonlinear optical absorption of this nanocomposite is measured at an excitation wavelength of 532 nm using the open aperture Z-scan experiment. The nanocomposite is found to show an enhanced optical limiting property compared to its constituent compounds PTBA and PANI. 1. Introduction Porphyrins are derived from four pyrroline subunits and are highly conjugated aromatic macrocycles. They are widely studied for their potential applications as molecular electronics materials in light harvesting devices,1,2 catalysts,3 sensors,4 and supramolecular chemistry.5,6 In recent times, there has been an immense interest in the formation and study of nanostructures of porphyrins, which show interesting chemical, physical, and optoelectronic properties.7-11 Just as in the case of metal nanoparticles, the challenge involved in the synthesis of porphyrin nanoparticles is to control the size and prevent their agglomeration. There have been reports of the use of polymers such as poly(ethylene glycol) (PEG) as a stabilizer for the nanoparticles of porphyrin7 and also counterion-dependent aggregates of porphyrin nanostructures in aqueous acidic solutions.8 Rotomskis et al. have shown how the porphyrin molecules attain ring shape by J-aggregation, which is similar to the “spread deck of cards”-like conformation. They hypothesized the formation of tubular structures from porphyrin J-aggregates, which was also confirmed by AFM images on silicon substrate.12a The ability of porphyrin derivatives to form interesting structures in the form of J- and H-aggregates is the key to the formation of nanostructures. The J-aggregates form due to the interaction between the negatively charged functional group attached to the porphyrin and positive charge at the center of the adjacent porphyrin. H-aggregates form when porphyrin molecules are stacked in a face-to-face configuration.7 Here we report a simple method of preparation of porphyrin nanoparticles and the study of their nonlinear optical properties. We have used 4,4′,4′′,4′′′-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid) (PTBA) as the precursor. The nanoparticles were prepared by mixing the PTBA solution with a dilute solution of polyaniline (PANI), which is a well-known conducting polymer. These nanoparticles show long time stability. PANI acts as an effective stabilizing agent which prevents further aggregation of the nanoparticles, which otherwise would result in precipitation. While the nonlinear optical properties of porphyrins are well studied in literature,12b-d we show that the porphyrin-polyaniline nanocomposite described here exhibits * To whom correspondence should be addressed. E-mail: narayan@ rri.res.in.
an enhanced nonlinear optical property compared to the parent compounds PTBA and PANI. 2. Experimental Section PANI was prepared by electrochemical potential cycling in an electrolyte containing 0.1 M H2SO4 and 0.5 M aniline at a potential range of -0.2 to 1.0 V. An indium tin oxide (ITO) electrode was used for the deposition of the PANI film. 4,4′,4′′,4′′′-(Porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid) (PTBA) was purchased from Sigma-Aldrich and used as received. It is known that PANI has a higher solubility in DMF when synthesized through electrochemical route than by chemical method.13 Ultrasonicating the electropolymerized PANI film for 10 min in DMF/water mixture provides a good dispersion of PANI. To prepare the nanoparticles, a 12 µM solution of PANI and a 0.25 mM solution of PTBA, both prepared in DMF/ water (1:1), were used as the precursors. When these two solutions were mixed in a 1:1 proportion, the PANI-PTBA nanocomposite was formed. The UV-Vis absorption spectra of PANI, PTBA, and the PANI-PTBA mixture were measured using a spectrophotometer (Perkin-Elmer-Lambda 35). Photoluminescence measurements were carried out using a standard spectrofluorometer (Fluoromax 4, Horiba Jobin Yvon). We carried out controlled experiments to investigate the formation of PTBA nanoparticles in the mixture. A drop of the nanoparticle solution prepared in the DMF/water system was deposited on mica substrate and allowed to evaporate. For AFM studies, we used a muscovite mica substrate, which was freshly cleaved by means of Scotch tape. AFM studies were carried out using Pico plus (Molecular Imaging) AFM in ac (tapping) mode with an n-doped silicon tip. The images obtained were raw images, which were plane corrected using the scanning probe image processor (SPIP) software (Image Metrology, Denmark). The nonlinear absorption measurements were done at the excitation wavelength of 532 nm, using laser pulses of 7 ns duration obtained from a Q-switched, frequency doubled Nd: YAG laser (Quanta Ray, Spectra Physics). The technique of open aperture Z-scan14 was used for the measurements. In the Z-scan experiment, a laser beam is first focused using a lens, and the focal point is visually determined. The direction of beam propagation is taken as the z-axis, and the focal point is considered as z ) 0. The z-value increases toward either side
10.1021/jp808691v CCC: $40.75 2009 American Chemical Society Published on Web 04/23/2009
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Figure 1. Photograph of the solutions of PANI, PTBA, and PTBA nanoparticles (a) and UV-Vis absorption spectra of the samples showing the splitting in the Soret band (b). Inset shows the red shift in the Q bands.
of the focal point, but the sign will be negative on one side and positive on the other (similar to a number line). The sample is now placed in the beam at a position (z) between the lens and the focal point, and the transmitted energy is measured. Then the sample is moved in small steps toward the focus and beyond, and the transmission is measured at each step. At each of these positions, the sample will experience a different laser intensity, and the intensity will be a maximum at the focus. Thus in practice the open aperture Z-scan is essentially a sample transmission measurement, the data being continuously taken while the sample is slowly translated from a position before the focus to a position after the focus. If a spatially Gaussian laser beam is used, then each z-position will correspond to an input laser energy density (fluence) of 4(ln 2)1/2Ein/π3/2ω(z)2, where Ein is the input laser pulse energy and ω(z) is the beam radius. ω(z) is given by ω(0)*[1+ (z/z0)2]1/2, where ω(0) is the beam radius at the focus and z0 ) πω(0)2/λ is the Rayleigh range (diffraction length), where λ is the excitation wavelength. Thus from the open aperture Z-scan data, it is possible to draw a graph between the input laser fluence and the sample transmission. The nature of this graph will reveal the absorptive nonlinearity of the system. In our experiment a planoconvex lens of 20 cm focal length was used to focus the laser, and the focal spot radius was about 18 µm. The laser pulse energy was 30 µJ. The samples were taken in a 1 mm thick glass cuvette (Hellma GmBH) and mounted on a programmable linear translation stage. The input energy reaching the sample and the energy transmitted by the sample were measured using two pyroelectric energy probes (RjP 735, Laser Probe Inc.). The interval between successive laser pulses was kept sufficiently large (about 1 s) to allow for complete thermal relaxation of the sample between adjacent laser pulses. The whole experiment was automated using a PC.
Figure 2. AFM topographic image (2 µm × 2 µm) of porphyrin nanoparticles formed in DMF/water showing the height and size of the individual nanoparticle.
3. Results and Discussion The samples and the UV-Vis spectra of PANI-PTBA solution are shown in parts a and b, respectively, of Figure 1. The spectra show a clear splitting in the Soret band. It is also seen that the Soret band is broadened and the Q bands are slightly shifted toward the longer wavelengths which indicates the formation of the porphyrin aggregates in the solution.7,15a-f Usually the porphyrin aggregates fall into the two categories of H-type and J-type aggregates. In H-type aggregation porphyrin molecules are stacked in a sandwich-like manner, which
is also called the face-to-face configuration. In J-type aggregation the porphyrin molecules are stacked in a spread deck of cards like arrangement. The absorption spectrum of the PANI-PTBA mixture gives a clear indication of the formation of PTBA nanoparticles, which are essentially the PANI stabilized aggregates of PTBA. Figure 2 shows the tapping mode AFM topographic image of nanoparticles dispersed on mica. It shows a dense distribution of nanoparticles on mica substrate. The typical particle size is
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Figure 3. AFM phase image (10 µm × 10 µm) of porphyrin nanoparticles formed in DMF/water. Inset shows the zoomed 1 µm × 1 µm part. Phase image is sharper than the topographic image.
Figure 4. Laser fluence vs normalized transmittance curve for the samples. The lines are theoretical fits to the data calculated using eq 1.
TABLE 1: Linear and Nonlinear Optical Coefficients for the Samples sample
linear transmission
PANI PTBA PANI-PTBA
0.89 0.81 0.85
linear absorption effective 2PA coefficient (R0) coefficient (β) (m-1) (m/W) 116.5 210.7 162.5
9 × 10-11 11 × 10-11 23 × 10-11
(β/R0) (m2/W) 7.73 × 10-13 5.22 × 10-13 14.2 × 10-13
found to be 80-100 nm, and the z-height corresponding to the thickness of the nanocomposite is 3-4 nm. In addition, a few bigger clusters are also observed in the topographic image. We have carried out AFM phase imaging, as it is known to yield much sharper features than the topographic images. Figure 3 shows the 10 µm × 10 µm AFM phase image of the nanoparticles on mica. The inset in Figure 3a shows the zoomed image where the well-resolved individual nanoparticles can be clearly seen. The phase image confirms that most of the nanoparticles are of uniform size ranging from 80 to 100 nm. It can also be seen from the phase image in Figure 3b that the bigger clusters, which are found in the topographic image in Figure 3c, are actually made up of smaller sized nanoparticles. The presence of water in the case of the DMF/water system enhances the polarity of the solvent which induces the aggrega-
Figure 5. Normalized nonlinear scattering from C60 and PANI-PTBA sample under the same experimental conditions.
tion of porphyrin cores. This polarity driven aggregation may be helpful in controlling the size of the porphyrin nanoparticles. The driving forces behind the formation of the nanoparticles are π-stacking effect and hydrophobic interaction. Some of the PTBA molecules can also stack together to form J-aggregates due to the intermolecular association between the positively charged porphyrin rings and the negatively charged benzoic acid groups of the neighboring porphyrin molecules. The presence of four negatively charged groups attached on the periphery of the porphyrin may not cause repulsion since the negative charge of the attached groups is delocalized over the porphyrin ring.15g,h The assembly of such J-aggregates driven by hydrophobic interaction and π-stacking leads to the formation of the nanoparticles. This stacking of porphyrin has also been reported elsewhere.12a,16 The assemblies of meso-tetrakis(4-sulfonatophenyl)porphine (TPPS4)12a and meso-tetrakis(4-hydroxyphenyl)porphyrin (H2THPP)16 have been shown to form J-aggregates. We have also deposited PTBA and PANI separately on mica substrate and imaged them using tapping mode AFM. In the case of PANI on mica, the AFM image (Figure S2A) shows a few lumps of polymer clustered together in some regions. In the case of PTBA (Figure S2B), we observed more uniform disclike structures of about 300 nm in size and 6-8 nm in height, which are possibly the bigger aggregates of PTBA stacked together. From the AFM images it is clear that the
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Figure 6. Emission spectra (a) and excitation spectra (b) of the PANI-PTBA mixture.
features we obtained in PTBA (Figure S2B) are larger than the particles obtained in the case of PANI-PTBA mixture. The open aperture Z-scan data is given in Figure S3, where the normalized transmittance of the samples (measured transmission normalized to the linear transmission of the sample) is plotted as a function of the sample position z. From this data, the input fluence vs normalized transmittance graph is drawn (Figure 4). The experimental data is found to fit well to a twophoton type absorption (2PA) given by the equation17
T ) ((1 - R)2 exp(-R0L) ⁄ √πq0)
∫-∞+∞ ln[1 + q0 exp(-t2)] dt (1)
where T is the net transmission of the samples, L and R are the sample length and surface reflectivity, respectively, and R0 is the linear absorption coefficient. q0 in eq 1 is given by β(1 R)I0Leff, where I0 is the on-axis peak intensity, Leff is given by [1 - exp(-R0L)]/R0, and β is the two-photon absorption coefficient. This two-photon type nonlinearity can originate from a genuine two-photon absorption or a two-step absorption (also known as reverse saturable absorption).18a-d Considering the fact that the sample shows some residual absorption at the excitation wavelength (532 nm) and there exists a strong absorption at the two-photon level of 266 nm (as depicted from the UV-Vis absorption spectrum of the samples), it can be deduced that both the above-mentioned processes have contributions to the nonlinear absorption. Therefore we attribute the optical limiting mechanism in the present case to an effectiVe two-photon absorption, in accordance with the nomenclature used in earlier reports in literature.18b,c The effective two-photon absorption coefficients (β) numerically calculated from fitting the experimental data to eq 1 are given in Table 1. To learn whether induced thermal scattering (nonlinear scattering) has a contribution to the observed nonlinear transmission, we did a measurement of the laser light scattered by the samples during the Z-scan. This was done by keeping a sensitive photodiode close to the sample during the experiment. A small amount of nonlinear scattering could be seen when the sample was at and near the beam focus. To get an estimate of the magnitude of this nonlinear scattering, we measured the nonlinear scattering from a C60-toluene solution under identical conditions. The linear transmission of the C60-toluene solution was so chosen that it gave the same optical limiting efficiency as the PANI-PTBA solution. It was found that the nonlinear scattering found in our samples is either comparable to, or slightly lower than, that seen in the C60-toluene sample. Figure 5 shows the normalized nonlinear scattering vs z-position plot for C60-toluene and PANI-PTBA samples. It is generally
accepted that excited-state absorption is the dominant mechanism for limiting in C60 samples,19a,b and the contribution from nonlinear scattering is relatively lower. Therefore our comparative measurements indicate that the contribution from nonlinear scattering is low in the present samples. Since the PANI-PTBA nanocomposite is prepared by mixing the solutions of PANI and PTBA in equal volumes, the concentration of PANI and PTBA in the final solution is only half of that in their individual solutions. In the case of PANI, the linear transmission (R0) at 532 nm is 89%, and for PTBA it is 81%. For the PANI-PTBA nanocomposite, it is at the intermediate value of 85%. To check whether the observed nonlinearity enhancement in PANI-PTBA arises simply from an addition of the individual nonlinearities of PANI and PTBA, we calculated the β/R0 values for all the samples (given in Table 1). The β/R0 value of PANI-PTBA is found to be nearly two times that of PANI and three times that of PTBA. If the nonlinearity of PANI-PTBA was the sum of the nonlinearities of PANI and PTBA, then the β/R0 value should have been nearly the same for all the three samples. Since this is not the case, it is clear that there is an obvious enhancement in the optical nonlinearity of PANI-PTBA, compared to pure PANI or PTBA. This is in agreement with earlier reports that in nanostructured media, the optical nonlinearities can be enhanced vis-a`-vis their bulk counterparts.20,21 The enhancement in the nonlinear properties is attributed to the formation of PANI stabilized PTBA aggregates in the solution which have a different electronic structure compared to the precursor PTBA molecules. There are a few reports on the enhancement of the nonlinear optical property in the case of charge-transfer complex formation of porphyrins and phthalocyanines.12c,22a-c We carried out photoluminescence measurements in the PTBA-PANI nanocomposite in order to get information about possible chargetransfer interaction between PTBA and PANI. However, no quenching was observed in the PTBA photoluminescence upon 420 nm excitation (position of the Soret band), implying the absence of a charge-transfer interaction. The photoluminescence spectra of the nanocomposite show the two-band red-shifted luminescence, which is similar to the spectra obtained by Serpone et al. for J- and H- aggregates in the case of mesotetraphenylporphyrin (H2TPP).15c,e-g Figure 6 shows the emission and excitation spectra of the nanocomposite sample. The excitation spectrum was taken at the fixed emission wavelength of 650 nm (maximum emission wavelength), and it is almost identical to the absorption spectrum of the sample.
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4. Conclusion We have shown that the porphyrin derivative 4,4′,4′′,4′′′(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid) (PTBA) forms nanoparticles when mixed with the conducting polymer polyaniline (PANI) in a DMF/water mixture. Nonlinear optical absorption measurements at 532 nm show that the nanocomposite has an enhanced optical limiting property compared to the precursor compounds PTBA and PANI. This enhancement in the nonlinear absorption is due to the formation of aggregates of PTBA within the PANI-PTBA nanocomposite, which is also confirmed from AFM studies. The present experiment is a clear demonstration of the fact that under favorable conditions for nanoparticle formation, even a simple procedure like mixing of two media can lead to a substantial modification of the net nonlinear optical property of a given chemical system. Acknowledgment. The authors thank Mr. A. Dhason for his help in AFM imaging. Supporting Information Available: FTIR spectroscopy results, AFM images of PANI deposited on mica and PTBA deposited on mica, and Z-scan curves. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Takahashi, R.; Kobuke, Y. J. Am. Chem. Soc. 2003, 125, 2372– 2373. (2) Lin, V.; DiMagno, S.; Therien, M. Science 1994, 264, 1105–1111. (3) Zhou, Q.; Li, C. M.; Li, J.; Cui, X.; Gervasio, D. J. Phys. Chem. C 2007, 111, 11216–11222. (4) Dunbar, A. D. F.; Richardson, T. H.; McNaughton, A. J.; Hutchinson, J.; Hunter, C. A. J. Phys. Chem. B 2006, 110, 16646–16651. (5) Anderson, S.; Anderson, H. L.; Bashall, A.; McPartlin, M.; Sanders, K. M. Angew. Chem., Int. Ed. Engl. 2003, 34, 1096. (6) Sun, D.; Tham, F. S.; Reed, C. A.; Boyd, P. D. W. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5088–5092. (7) Gong, X.; Milic, T.; Xu, C.; Batteas, J. D.; Drain, M. C. J. Am. Chem. Soc. 2002, 124, 14290–14291. (8) Doan, C. S.; Shanmugham, S.; Aston, D. E.; McHale, J. L. J. Am. Chem. Soc. 2005, 127, 5885–5892. (9) Collini, E.; Ferrante, C.; Bozio, R. J. Phys. Chem. B 2005, 109, 2–5.
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