Nonlinear Optical Response of Chloroaluminiumphthalocyanine

Jun 12, 2008 - ... Chennai-600036, Institute of Physics, Bhubaneswar-751005, India ... Preparation and Application for Enantioselective Crystallizatio...
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Langmuir 2008, 24, 7485-7491

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Nonlinear Optical Response of Chloroaluminiumphthalocyanine Encapsulated by Silica Core-Shell Particles K. Sathiyamoorthy, C. Vijayan,* and Shikha Varma Department of Physics, Indian Institute of Technology Madras, Chennai-600036, Institute of Physics, Bhubaneswar-751005, India ReceiVed January 31, 2008. ReVised Manuscript ReceiVed April 11, 2008 We report for the first time on the synthesis of core-shell particles containing chloroaluminiumphthalocyanine (ClAlPc) prepared using a sol-gel technique. These particles have the dye molecules at the core, encapsulated by silica shell. The mean size of the particle is determined from HRTEM studies and is found to be approximately 0.08 µm. The surface and bulk compositions of the core-shell particles are studied by XPS and EDAX measurements, respectively. Time-resolved fluorescent measurements indicate a decrease in fluorescence lifetime for the core-shell particles as compared to that of bare dye dissolved in ethanol. This is analyzed on the basis of available theoretical models. Third-order nonlinear optical effects are investigated by the Z-scan technique using 8 ns pulses at a wavelength of 532 nm from a frequency-doubled Nd:YAG laser. The analysis indicates that both singlet and triplet excited-state absorption contribute to nonlinear absorption.

Introduction Surface coating of nanoparticles with appropriate materials to design core-shell structures is currently an active area of research, as coating allows abundant scope for modification and tailoring of the physical and chemical properties of core materials depending on the nature of the shell and conditions of synthesis.1–4 Though several studies have been made on metallic or semiconducting particles encapsulated by insulating shells, the interest in dyes in the form of shell-protected core particles is very recent.5–10 The incorporation of organic dyes into solid matrices is a topic of wide interest because of its scope in designing stable and efficient materials for applications such as laser materials, nonlinear optical materials, optical memories, and light concentrators in solar cells.12–15 One of the recent observations has been that core-shell composites can increase the luminescent quantum yield due to improved passivation of the surface while being physically more robust than the bare clusters. Kityk studied the effect of the nanointerfaces (between the nanoparticles and * Corresponding author. E-mail: [email protected]. (1) Heyes, C. D.; Kobitski, A. Y.; Breus, V. V.; Nienhaus, G. U. Phys. ReV. B 2007, 75, 125431. (2) Titova, L. V.; Hoang, T. B.; Jackson, H. E.; Smith, L. M.; Yarrison-Rice, J. M.; Kim, Y.; Joyce, H. J.; Tan, H. H.; Jagadish, C. Appl. Phys. Lett. 2006, 89, 173126. (3) Prasher, R. Appl. Phys. Lett. 2006, 89, 063121. (4) Chavez, J. L.; Wong, J. W.; Duran, R. S. Langmuir 2008, 24, 2064. (5) Penninkhof, J. J.; Graf, C.; Dillen, T. V.; Vredenberg, A. M.; Blaaderen, A. V.; Polman, A. AdV. Mater. 2006, 18(21), 2802. (6) Lin, Y. W.; Liu, C. W.; Chang, H. T. J. Nanosci. Nanotech. 2006, 6(4), 1092. (7) Tapec, R.; Zhao, X. J.; Tan, W. J. Nanosci. Nanotech. 2002, 2, 405. (8) Ethiraj, A. S.; Hebalkar, N.; Kharrazi, S.; Urban, J.; Sainkar, S. R.; Kulkarnai, S. K. J. Lumin. 2005, 114, 15. (9) Ow, H.; Larson, D. R.; Srivastava, M.; Barbara, A.; Baird, B. A.; Webb, W. W.; Wiesner, U. Nano.Lett. 2005, 5, 113. (10) Lee, M. H.; Beyer, F. L.; Furst, E. M. J. Colloid Interface Sci. 2005, 288, 114. (11) Pfeiffer, M.; Beyer, A.; Plonnigs, B.; Nollau, A.; Fritz, T.; Leo, K.; Schlettwein, D.; Hiller, S.; Wohrle, D. Solar Energy Mater. Solar Cells 2000, 63, 83. (12) Shen, Y.; Swiatkiewicz, J.; Prasad, P. N.; Vaia, R. A. Opt. Commun. 2001, 200, 9. (13) Duchowicz, R.; Scaffardi, L. B.; Costela, A.; Garcia-Moreno, I.; Sastre, R.; Acuna, A. U. Appl. Opt. 2003, 42(6), 1029. (14) Sathiyamoorthy, K.; Vijayan, C.; Kothiyal, M. P. J. Phys. D: Appl. Phys. 2007, 40, 6121. (15) Kityk, I. V. J. Non-Cryst. Solids 2001, 292, 184.

the surrounding matrix) on the second-order nonlinear susceptibilities15 and found that both nanoconfined effects as well as electron-phonon anharmonicity play a dominant role in determining second-order susceptibility. So, efforts directed toward the study of nanoparticles with embedding background optimally matched to the third-order susceptibilities would be welcome. Earlier, spontaneous emission in photonic colloidal crystals was studied by dispersing active dye molecules (the sources of emission) in a liquid medium that also contained dispersed colloidal particles of silica.16,17 Later it was recognized that this leads to complications due to chemical interaction of the dye with the colloidal particle surfaces. A cleaner way to study photonic effects would be to shield the dye by incorporating it inside colloidal particles such as silica spheres. Imhof et al. studied the spectroscopic properties of fluorescein incorporated in colloidal silica spheres and showed that colloids with low dye concentration are useful for photonic applications, whereas those with high dye concentrations are of interest to optical experiments in colloid science.18 The present work is on the synthesis and characterization of the nonlinear optical behavior of a composite material where phthalocyanine dye molecules are encapsulated by silica shells. Phthalocyanine is a synthetic dye known to possess large optical nonlinearity due to delocalized π orbitals.19–22 The encapsulation is done by a room temperature chemical route based on the sol-gel technique, which incorporates the hydrophobic dye molecules within the silica matrix. Important parameters of fluorescence such as the singlet state lifetime and triplet excitedstate absorption of core-shell particles are estimated by timeresolved fluorescence measurements and laser flash photolysis. (16) Blaaderen, A. V.; Vrij, A. Langmuir 1992, 8, 2921. (17) Blaaderen, A. V.; Vrij, A. J. Colloid Interface Sci. 1993, 156, 1. (18) Imhof, A.; Megens, M.; Engelberts, J. J.; deLang, D. T. N.; Sprik, R.; Vos, W. L. J. Phys. Chem. B 1999, 103, 1408. (19) Shirk, J. S.; Pong, R. G. S.; Flom, S. R.; Heckmann, H.; Hanack, M. J. Phys. Chem. A 2000, 104, 1438. (20) Perry, J. W.; Mansour, K.; Lee, L-Y. S.; Wu, X-L.; Bedworth, P. V.; Chen, C.-T.; Ng, D.; Marder, S. R.; Miles, P.; Wada, T.; Tain, M.; Sasabe, H. Science 1996, 273, 1533. (21) Li, C.; Zhang, L.; Yang, M.; Wang, H.; Wang, Y. Phys. ReV. A 1994, l49, 1149. (22) Wei, T.-H.; Huang, T.-H. Appl. Phys. Lett. 1998, 72, 2505.

10.1021/la800340t CCC: $40.75  2008 American Chemical Society Published on Web 06/12/2008

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Experimental Section Synthesis of Core-Shell Particles. Tetraethylorthosilicate (TEOS) from Lancaster, ethanol from Hayman, and ammoniumhydroxide from Sisco were used. Doubly deionized water is used for our synthesis. Monodispersed spherical shaped silica particles in nanometer size range can be synthesized by the method suggested by Stober et al.,23 which involves hydrolysis of tetraethylorthosilicate (TEOS) in the presence of certain bases. This method is utilized by several other groups to entrap nanomaterials made of semiconductors, metals, and dyes within a silica shell.8,9,24–26 Hydrophilic dyes can be easily incorporated into a silica matrix by this method, whereas it is difficult to entrap hydrophobic dyes. Tapec et al. used a relatively highly hydrophobic precursor, phenylethoxysilane (PTES), to trap organic flourophores.8 This method has been used for the synthesis of hydrophobic dye-doped microspheres. Anita et al. modified Stober’s method in the form of a two-step process for the synthesis of hydrophobic dye-doped microspheres.9 The first step involves the hydrolysis of two of the four ethyl groups of TEOS by choosing an appropriate concentration of the ammonium hydroxide base. The other two unhydrolyzed ethoxy groups show affinity for hydrophobic dye. This solution is subsequently mixed with a silica-forming solution to get the dye-entrapped silica shell. The method of synthesis used in the present work is basically similar to the above method with a few modifications to entrap chloroaluminiumphthalocyanine (ClAlPc) molecules in silica shell efficiently. ClAlPc solution is prepared in ethanol with about 1 mM concentration. Then 10 mL of this solution is added to 1 mL of TEOS and stirred for about 10 min. To this, 0.78 mL of 1 M of aqueous ammonium hydroxide is added as a catalyst and stirring is continued for about 1 h to hydrolyze only two of the four ethoxy group of TEOS. Another solution is prepared with 0.6 mL of TEOS in 30 mL of ethanol and mixed with the first solution in a dropwise manner. A third solution containing 0.78 mL of 1 M of ammoniumhydroxide in 30 mL of water is prepared separately. This solution is added to the solution obtained in the previous step, after the addition of the second solution, and stirring is continued for 3 more hours. Finally, a mixture containing 4.5 mL of TEOS and 4.68 mL of NH4OH is added and the stirring is extended 12 h more. The reaction is stopped by adding ethanol and the end solution is centrifuged. The resultant white precipitate is washed with ethanol until clear supernatant powder is obtained. Synthesis of core-shell particles with base of other concentrations results in conglomeration of silica and dye molecules. Surface Characterizations. TEM. Morphology studies on the core-shell particles are performed with a JEOL 3010 high-resolution transmission electron microscope (HRTEM) operated at 300 keV. EDAX measurement is done with a Philips CM12 transmission electron microscope equipped with an energy dispersive spectroscopy (EDS) detector for microanalysis. XPS. Surface characterization of the material is done with the help of X-ray photoelectron spectroscopy (XPS). XPS experiments are conducted in a UHV VG system with the base pressure of 10-10 Torr. The analyzer is operated with a pass energy of 200 eV for a large size survey scan of the 0-1000 eV binding energy region. For high-resolution scans, a pass energy of 20 eV is utilized. The XPS system is equipped with a dual Mg-Al anode and a hemispherical analyzer. Non-monochromatic Mg KR 1256.6 eV radiation is used at 300 W for all the XPS measurements reported here. Time-ResolVed Fluorescence Measurements. A time-correlated single-photon counting technique is used to determine the florescence lifetimes of the samples. The measurement is made using an IBH5000 single photon counting spectrometer. A nano-LED from Spectra Physics with a resolution of 200 ps is used to excite the sample and a Hamamatsu R 3809 U-50 microchannel plate photomultiplier is (23) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (24) Wang, Z.; Yuan, J.; Han, D.; Niu, L.; Ivaska, A. Nanotechnology 2007, 18, 115610. (25) Velikov, K. P.; Moroz, A.; Blaaderen, A. V. Appl. Phys. Lett. 2002, 80, 49. (26) Lu, S-Y.; Wu, M-L.; Chen, H-L. J. Appl. Phys 2003, 93, 5789.

Figure 1. TEM picture of core-shell particles. Table 1. Elemental Composition of the Core-Shell Particles weight % element

SiO2

ClAlPc

core-shell

Al Si Cl O N C

– 37.54 – 62.46 – –

04.79 – 03.71 05.97 16.44 69.09

00.37 36.96 00.04 49.05 08.36 05.22

used as the detector. The fluorescence decay curves are analyzed using an iterative fitting program provided by IBH. The singlet excited-state lifetime is determined from the decay profile. The excitation and emission wavelengths are fixed in the setup at 640 and 694 nm, respectively. Laser Flash Photolysis. Conventional flash photolysis experiments are carried out using an Applied Photophysics KN-020 series flash kinetic spectrometer fitted with a Czerney Turner monochromator and a Hamamatsu R928 photomultiplier tube as detector and equipped with an IE-567 Digitest Laboratory oscilloscope for signal capture and display. Laser flash photolysis experiments are carried out using the third harmonic output from a Quanta Ray GCR-2 Nd:YAG laser (355 nm, 8 ns). The monitoring source is a 250 W pulsed xenon lamp. The transient signals are captured with a 54201A HewlettPackard digital storage oscilloscope interfaced to a PC via GPIBIEEE 488 board. Z-Scan. Nonlinear optical response is characterized by the open Z-scan technique20 using a Q-switched Nd:YAG laser with a pulse width 8 ns at 532 nm as the source. The laser beam is focused on the sample by a lens of focal length 10 cm. The beam waist ωo at the focus is ≈13 µm. The thickness of the sample is kept at L ≈ 1 mm so that it is well within the Raleigh range of our system.

Results and Discussion Figure 1 shows the TEM picture of core-shell particles. The dark core represents the dye molecules encapsulated by brighter silica shell, which shows up as a bright periphery. The average size of the core-shell particles is about 0.08 µm. The elemental composition of the core-shell particles is obtained from EDAX measurement (Table 1). EDAX analysis of core-shell particles reveals the presence of elements such as Si, O, Al, Cl, C, and N. This compared well with the composition of ClAlPc and pure silica. Core-shell particles containing the composition of both

ClAlPc Encapsulated by Silica Core-Shell Particles

Figure 2. (a) Optical absorption spectra of core-shell particle in ethanol (solid line) and in DMF (dotted line). (b) Optical absorption spectra of bare dye dissolved in ethanol (solid line) and in DMF (dotted line).

ClAlPc and silica confirms the presence of both materials. The weight percent of oxygen in the core-shell particles is more as compared to bare ClAlPc due to the presence of the silica shell. The carbon composition of bare ClAlPc is found to be reasonably more as compared to silica and core-shell particles. This could be due to the usage of carbon tape, above which the sample is deposited, in the SEM measurement. The silica used in the present study is commercially obtained from Degussa. Figure 2a shows the optical absorption spectra of core-shell particles in ethanol (solid line) and in DMF (dotted line). The spectra show two major peaks, the Soret band in the UV and the Q-band located in the longer wavelength region of the visible spectrum. The Soret band, at 355 nm, arises due to electronic transitions in the molecule. The Q bands, near 674 nm, arise due to the transition between vibration levels of the first and second excited electronic states. Both spectra have peak positions at the same values of wavelength. Figure 2b represents the optical absorption spectra of bare dye dissolved in ethanol (solid line) and in DMF (dotted line). Slight shifts are observed in the peak positions of the bare dye with a change in the solvent. This shift is caused by the use of solvents with different polarity. This solvochromatic shift is not observed in the case of core-shell particles. The reason is that the dye is prevented from interacting with solvent when it is encapsulated by the silica shell. The

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nonzero baseline of core-shell particles is due to extinction resulting from scattering. On the other hand, if we compare the spectra of bare dye and core-shell particles in the same solvent, a small red shift of 2 nm (with respect to the bare dye) is observed for core-shell particles with ethanol as solvent. A blue shift of 4 nm is observed (with respect to the bare dye) in the case of core-shell particles with DMF as solvent. No appreciable shift is observed in the case of the Soret band. Blue shifts have been reported in earlier studies on hydrophobic dye encapsulation.4 The observed shifts in the present work appear to be due to solvochromatic effects. Any further insight can be obtained only after further experimentation. Figure 3a shows the XPS spectrum obtained from the core-shell particles and from the bare ClAlPc molecules using Mg KR radiation. The XPS spectrum corresponding to pure dye molecules shows three main peaks at binding energies 533, 400, and 286 eV, respectively. The most intense peak occurs at a binding energy of 286 eV due to the presence of a high percentage of carbon constituents in the ClAlPc molecule. The broad C1s spectrum arises due to eight chemically nonequivalent carbon atoms in the molecule, six of the benzene-type and two of pyrroletype. The inset represents the N1s core-level spectrum with a peak at 400 eV, which stem from the three nonequivalent nitrogen atoms in the molecule, which is absent in the case of SiO2encapsulated ClAlPc molecules. Figure 3b represents Cl spectrum of the ClAlPc molecule. Si2s, Si2p, and O1s are the only peaks that appear in the XPS spectrum of core-shell particles, indicating the absence of dye molecules on the surface and implying that the dye molecules are encapsulated within the silica shell. Figure 4 represents the dependence of emission lifetime on the concentration of ClAlPc in ethanol. The excited-state lifetime is found to decrease with increasing concentration. This may be due to concentration quenching arising form the interaction of some of the excited molecules with ground-state molecules. The linear plot is extrapolated to zero concentration to obtain hypothetical luminescent lifetime at infinite dilution, where there is no concentration quenching and the value is estimated to be τD ) 1 × 10-8 s. If the concentration-dependent red shift in the absorption and fluorescence spectra is the result of self-quenching due to interactions between close pairs of dye molecules, this red shift should be accompanied by a reduction of the fluorescence lifetimes. This is because a high probability of energy transfer between dye molecules shortens the lifetime during which molecules remain in their excited state. If the quenching were due only to the formation of nonfluorescence dimers, then no change in lifetime would be expected, because these dimers do not contribute to the signal. The data points represented by circles in Figure 5 are obtained from a measurement of the intensity decay profile of the core-shell particles dissolved in ethanol at excitation 640 nm. For comparison, the decay profile (represented by stars in the figure) of ClAlPc at a concentration 1 × 10-4 M in ethanol is incorporated. The measured singlet state lifetime of core-shell particles dispersed in ethanol is 7.4 × 10-9 s. This value is found to be reasonably lower than the hypothetical fluorescence decay time of pure ClAlPc in ethanol (1 × 10-8 s) at infinite dilution, where there is no concentration quenching. This shortening of excited-state lifetime of core-shell particles must be due to effects of ultrafast Forster energy transfer between the molecules. These intermolecular energy transfers plays a dominant role in determining the response time of third-order nonlinear optical effects. The rate of homotransfer of energy (k)

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Figure 3. (a) XPS spectra of the core-shell particles and bare ClAlPc molecules obtained using Mg KR radiation. The inset represents the N1s XPS spectrum of core-shell particles and bare ClAlPc molecule. (b) Cl XPS spectrum of core-shell particles and bare ClAlPc molecule.

between the dye molecules can be estimated by using the expression27

k)

( )

1 R0 τ r

6

(1)

where τ is the decay time of the excited dye molecule, r is the distance between the nearest neighbor molecules, and R0 is the critical Foster distance. The intensity decay is given by27

I(r, t) ) I0 exp[(-t/τ) - kt]

(2)

where τ is the decay time of the excited molecule in the absence of quencher and I0 is the emission intensity at t ) 0. The above equation is used to fit the fluorescence decay data of core-shell (27) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 2006.

particles with I0 and k as running parameters. We assume the value of τ to be equal to the fluorescence decay time at infinite dilution, as there is no concentration quenching. The reason is at low concentration the intermolecular distance will be reasonably high and hence the effect of fluorescence quenching due to molecular aggregates will be absent. The obtained value of k is approximately equal to 3.3 × 109 s-1. The rate of energy transfer depends on several mechanisms. One major factor that affects the energy transfer is quenching due to annihilation between excited-state molecules. These excited-state annihilations could be due to either singlet-singlet annihilation or singlet-triplet annihilation as the quenching of excitations at the spheres surface is not possible due to the micron size of the core-shell particles. Further, the singlet lifetime with a sufficient background triplet population has a significant effect on the decay rate of the singlet excitons. Hence, singlet-triplet annihilation is recognized as another main loss process for singlet

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Figure 4. Dependence of emission lifetime on the concentration of ClAlPc in ethanol.

Figure 5. Fluorescence emission intensity decay profile at 694 nm due to excitation at 640 nm. The data points shown by circles refer to core-shell particles and the data points shown by stars refer to bare ClAlPc.

excitons. In the case of singlet-singlet annihilation in a system where rapid migration homogenizes the density of the excitations, the corresponding rate equation is18,28–30

(

)

1 1 t⁄τ0 e - κτ0 ) κτ0 + u(t) u0

(3)

where u(t) and u0 are the fluorescence emission intensities at time t and t ) 0, respectively, and κ is the rate constant of the singlet-singlet annihilation process. The above equation is used to fit the fluorescence decay data of core-shell particles. Figure 6 is obtained by plotting the inverse of the fluorescence emission intensity as a function of decay time. The fit obtained (shown by the solid line in Figure 6) is reasonable and the corresponding values are τ0 ) 7.7 × 10-9 s and κ ) 0.4 × 109 s-1. Thus, the singlet-singlet annihilation (28) Larsen, J.; Bruggemann, B.; Polıvka, T.; Sundstrom, V.; Akesson, E.; Sly, J.; Crossley, M. J. J. Phys. Chem. A 2005, 109, 10654. (29) Barzda, V.; Gulbinas, V.; Kananavicius, R.; Cervinskas, V.; Amerongen, H. V.; Grondelle, R. V.; Valkunas, L. Biophys. J. 2001, 80, 2409. (30) King, S. M.; Dai, D.; Rothe, C.; Monkman, A. P. Phys. ReV. B 2007, 76, 085204.

Figure 6. Fluorescence emission intensity decay profile of the core-shell particles dissolved in ethanol at 694 nm due to excitation at 640 nm. The solid line shows the fit obtained using the excitation annihilation model represented by eq 3.

has significant influence on the energy transfer rate. Further, the coefficient of the second-order process is an order of magnitude lower than that of the first-order process. The excitation source used in the fluorescence lifetime measurement was a light-emitting diode with rather low output power; hence, the effect of intensity variation on singlet-singlet annihilation could not be studied. Further the order of magnitude of the first-order process is about 15 times higher as compared to second-order process; hence, it is very difficult to study separately the singlet-singlet annihilations at such low power. Figure 7a shows the transient absorption spectra of core-shell particles in ethanol (data points represented by circles) as well as bare ClAlPc in ethanol (data points represented by stars). This is a time-resolved absorption spectrum, which is obtained by measuring the absorption decays at a series of wavelengths between 300 and 800 nm separately and plotting the data in a “point by point” fashion. Figure 7b shows one such absorption decay of transient species at wavelength 480 nm. The time resolution of the transient absorption measurement is about 20 ns. The plot for core-shell particles shows triplet excited-state absorption in the wavelength range from 380 to 600 nm, with maximum transient absorption at 470 nm. Features due to the ground-state photobleaching of the Q and Soret bands of the core-shell can be observed at the corresponding wavelengths, 355 and 672 nm, respectively. The lifetimes of the triplet excited states are evaluated to be within the broad range from 0.6 to 3 µs. The lifetime is obtained from the absorption decay experiment of transient species done at each wavelength. The result of one such experiment is shown in Figure 7b. The lifetime is defined as the time taken by the triplet state population to fall to 1/e times of its initial value. For example, from Figure 7b, the lifetime is found to be about 2.78 µs. Under the same conditions, the transient absorption due to the core-shell particles is weaker in comparison with the transient absorption due to bare ClAlPc in ethanol, indicating lower concentrations of the molecules in the triplet state. This may be due to singlet-triplet annihilation when there is a background reasonable triplet density comparable to the singlet density required for singlet-triplet annihilation. Triplet-triplet energy transfer can be ruled out because of the low intersystem crossing quantum yield of phthalocyanine molecules. Further, it can be

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Figure 8. Normalized transmittance of core-shell particles measured using the open-aperture Z-scan. Solid lines represent theoretical fits obtained using eq 4.

Figure 7. (a) Transient absorption spectrum. The data points shown by circles refer to core-shell and those by stars refer to bare ClAlPc. (b) Triplet state absorption decay curve of core-shell particles at 480 nm.

seen from the figure that the transient absorption spectrum of core-shell particles is red-shifted relative to that of bare dye. This spectral shift is due to the effect of intermolecular interaction due to aggregation of the core-shell particles. Figure 8 shows the normalized transmittance of core-shell particles measured using the open-aperture Z-scan technique. The nonlinear absorption coefficient β is calculated from the experimental data by fitting the equation for the normalized transmittance as a function of the position along the z axis31

Tnorm(z) )

loge[1 + q0(z)] q0(z)

(4)

where q0(z) is given by

q0(z) )

q 1 + (z/z0)2

and z0 is the diffraction length of the beam and q ) βeffI0Leff, whereLeff ) (1 - e-RL)/R, R0 is the effective intensity-dependent nonlinear absorption coefficient, and I0 is the intensity of the light at focus. Leff is known as the effective length of the sample, (31) Garcia-Frutos, E. M.; O’Flaherty, S. M.; Maya, E. M.; Torre, G. D. L.; Blau, W.; Vazquez, P.; Torres, T. J. Mater. Chem. 2003, 13, 749.

defined in terms of the linear absorption coefficient, R0, and the true optical path length through the sample, L. The experiment is performed at intensity (0.16 GW/cm2) and the corresponding value of β is found to be 2.2 cm/GW respectively. Phthalocyanines (Pc) are known to possess interesting nonlinear optical properties and have been proposed as materials for optical limiting.20–22,32–37 The photophysical properties of Pc can be altered significantly by incorporating different central metal ions into the macrocycle (MPc). These materials exhibit a variety of nonlinear optical mechanisms, depending on the pulse width and wavelength of excitation. The molecule also permits structure modification to engineer the parameters of nonlinear response. It is important to examine different structural variants of the basic phthalocyanine with a view to developing materials with optimized performance at different wavelengths of interest. Nonlinear absorption processes in these materials are governed by the nature of singlet and triplet transitions and their lifetimes. Thus, a proper understanding of the nonlinear optical processes in these materials requires detailed investigations on these parameters. Nonlinear absorption processes in MPc’s is generally described by a five-level model.22,23 Depending on the input pulse duration, the possible modes of nonlinear absorption in these materials are through transitions from (a) S0 to S2 state by an instantaneous two-photon process (TPA), (b) S0 f S1 f S2 states by a two-step resonant TPA, or (c) T1 f T2 states by means of excited-state absorption (ESA). Reverse saturable absorption generally arises in a molecular system when the ESA cross section is larger than the ground-state cross section. It has been found that though ClAlPc has significant singlet-singlet excited-state absorption for subnanosecond pulses, triplet-triplet dynamics begins to dominate in the case of excitation with longer pulses. The measured excited singlet state lifetime of core-shell particles (32) Gundy, S.; Putten, W. V. D.; Shearer, A.; Buckton, D.; Ryder, A. G.; Ball, M. Phys. Med. Biol. 2004, 49, 359. (33) Coulter, D. R.; Miskowski, V. M.; Perry, J. W.; Wei, T. H.; Stryland, E. V. V.; Hagan, D. J. Proc. SPIE 1989, 1105, 42. (34) Torre, G. D. L.; Vazquez, P.; Lopez, F. A.; Torres, T. J. Mater. Chem. 1998, 8, 1671. (35) Song, L.; Lee, W.-K. Opt. Commun. 2006, 259, 293. (36) Shevchenko, A.; Buchter, S. C.; Tabiryan, N. V.; Kaivola, M. Opt. Commun. 2004, 232, 77. (37) Brugioni, S.; Meucci, R. Opt. Commun. 2002, 206, 445.

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is found to be 7.4 ns. For phthalocyanine systems, the values of σS/σG and σT/σG are known to be greater than 10 and 30, respectively.19 ClAlPc also has short intersystem cross timing of about