Optical limiting properties of zinc phthalocyanines in solution and solid PMMA composite films

Optics Communications 283 (2010) 4749–4753

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Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m

Optical limiting properties of zinc phthalocyanines in solution and solid PMMA composite films Sezen Tekin a, Ulaş Kürüm a, Mahmut Durmuş b, H. Gul Yaglioglu a, Tebello Nyokong c, Ayhan Elmali a,⁎ a b c

Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Beşevler, Ankara, Turkey Gebze Institute of Technology, Department of Chemistry, Gebze, Kocaeli, 41400, Turkey Department of Chemistry, Rhodes University, Grahamstown, 6140, South Africa

a r t i c l e

i n f o

Article history: Received 15 February 2010 Received in revised form 1 July 2010 Accepted 1 July 2010 Keywords: Z-scan Nonlinear absorption Optical limiting Polymer film Phthalocyanine

a b s t r a c t The nonlinear absorption and optical limiting (OL) performance of tetra- and octasubstituted zinc phthalocyanine complexes were described in solution and in the solid state using the open-aperture Z-scan technique. The measurements were performed using collimated 4 ns pulses generated from a frequencydoubled Nd:YAG laser at 532 nm wavelength. The polymeric films exhibit a much larger effective nonlinear absorption coefficient in comparison with solution. However, the parameters of the ratio of the excited to ground state absorption cross section and energy-dependent saturation in solution are much better compared to properties in the polymeric film. In terms of the ratio of the excited to ground state absorption cross section, the peripherally substituted complexes show better OL performance than the nonperipherally substituted derivative. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Optical limiting (OL) is an important application of nonlinear optics. It is useful for the protection of optical instruments including the human eye. An efficient optical limiter strongly attenuates intense laser pulses, while exhibiting high transmittance for low-intensity ambient light. Among many functional materials such as fullerenes [1,2], carbon nanotubes [3,4], polymer/nanotube composites [5] and phthalocyanines (Pcs) optically limit nanosecond light pulses in a fairly wide range of the UV/Vis spectrum via strong reverse saturable absorption (RSA) [6]. The nonlinear optical absorption of Pcs can be modified by incorporating different metal atoms into the ring or substitution of peripheral or axial position with different groups [7]. Pcs have low solubility in most organic solvents and they aggregate both in solution and in the solid state [7]. To overcome the solubility problem, a variety of substituents have been attached to the macrocycle in varying numbers and different substitution patterns [7–10]. The peripheral [11] or axial [12] substitution could enhance the solubility of Pcs and thus improve their usability. It was reported that strong donor peripheral alkoxy substituents improve the OL response with low OL threshold. On the other hand, axial ligands lead to a considerable enhancement in optical limiting response due to the presence of a dipole moment perpendicular to

⁎ Corresponding author. Tel.: +90 312 2033423; fax: +90 312 2127343. E-mail address: [email protected] (A. Elmali). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.07.003

the macrocycle in the axially substituted phthalocyanines. Very recently OL properties of octasubstituted gallium and indium phthalocyanines were studied in both solution and host poly (methyl methacrylate) (PMMA) polymer film [13]. It was found that 4-benzyloxyphenoxy group as substituent on the phthalocyanine framework increased the solubility of gallium and indium phthalocyanines. Thus, in this paper, we choose the 4-benzyloxyphenoxy group as substituent on the zinc phthalocyanine framework to increase the solubility of the studied Pcs. Increasing solubility makes the thin film preparation in polymer matrix easy and may improve the optical limiting properties of the studied phthalocyanine complexes. A practical optical limiting device requires the casting of the optically active compounds in the solid state. Most of the OL studies of Pcs have been done in solution [6,7] while there are only fewer reports investigating passive solid state nonlinear optical devices [13–17]. Thus, in an attempt to optimize the OL properties of Pcs we have investigated the OL properties of 4benzyloxyphenoxy-substituted zinc phthalocyanines (Fig. 1), tetrasubstituted at the non-peripheral (1) and peripheral (2) positions and octasubstituted at the peripheral (3) position with the 4benzyloxyphenoxy group both in solution and incorporated in a host PMMA polymer film. 2. Experimental The synthesis and structural characterization of the investigated compounds have been previously reported [18,19]. For the fabrication of the solid state Pc/polymer films of all investigated

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phthalocyanine-PMMA composite film were found to be about 1.20 GW/cm2 and 0.32 mW/cm2, respectively. 3. Results and discussion

Fig. 1. Chemical structures of investigated compounds.

phthalocyanines, 110 g/L solution of PMMA in cyclohexanone was placed in a low power sonic bath for 48 h until completely dissolved. The zinc phthalocyanines were then added, and were also sonically agitated until they were completely dissolved and homogeneous solution was formed. Solid state films of samples were formed on quartz substrates by using multilayer conventional spin casting technique (SCS-Spin Coat G3P). Each layer of Pc/PMMA film was formed by sequential spinning of substrate while dropping prepared composite solution. Prepared films were baked approximately 1 h at 100 °C between each layer to facilitate the removal of residual solvent. Three layers for compound 1 and two layers for compound 2 were deposited to obtain the final film thickness. The good quality polymer film of 3 could not be obtained. Thicknesses of the Pc/ polymer films were determined using spectroscopic ellipsometer (SE) M2000V (J.A. Woollam Co.). SE experiments were done in the photon energy range from 1.24 to 3.34 eV (from 370 to 999 nm). All the spectra were taken at three angles of incidence (60°, 65° and 70°) to increase the fitting accuracy. The best fits were obtained by using multilayer analysis with Cauchy model [20]. The UV–Vis absorption spectra were recorded using a scanning spectrophotometer (Shimadzu UV-1800). The open-aperture Z-scan technique was used to measure the nonlinear absorption [21]. This measures the total transmittance through the sample as a function of incident laser intensity while the sample is gradually moved through the focus of a lens (along the z-axis). A Q-switched Nd:YAG laser (Quantel Brillant) was used as the light source with a repetition rate of 10 Hz, pulse duration of 4 ns and wavelength of 532 nm. The lens with 20 cm focal length (f-number is 8) was used to focus the beam. The solution based Z-scan experiments were undertaken in quartz cuvettes with 1 mm path length. No evidence of film fatigue or degradation was observed in the film, after numerous scans at different laser intensities. The damage thresholds of the input irradiances for the pure PMMA film and zinc

Fig. 1 shows the chemical structure of the 4-benzyloxyphenoxysubstituted zinc phthalocyanines (1 to 3), tetrasubstituted at the nonperipheral and peripheral positions and octasubstituted at the peripheral position with the 4-benzyloxyphenoxy group. The details of the synthesis and characterization of these compounds were reported earlier in the literature [18,19]. The ground state electronic absorption spectra in chloroform solutions showed monomeric behavior for complexes 2 and 3 as evidenced by a single (narrow) Q band, typical of metallated phthalocyanine complexes, Fig. 2A. As reported before [18,19], the non-peripherally substituted complex 1 showed a new absorption band in chloroform at 744 nm in addition to the main Q band, due to protonation in chloroform which contains small amounts of acid. The red-shifting of the Q band of complex 1 is typical of non-peripherally substituted phthalocyanines complexes. Aggregation in MPc complexes is due to a coplanar association of rings, resulting in splitting and broadening of spectra, with a blue shifted peak (~ 630 nm) being due to the H aggregate, and the lower energy band (~ 670 nm) being due to the monomer. The solution spectrum in Fig. 2A does not show aggregation, however at higher concentrations in chloroform, there is considerable aggregation, Fig. 2B (a). This is evidenced by the presence of a broad band near 630 nm for complexes 2 and 3, accompanying a narrower band due to the monomer at ~ 670 nm. For complex 1 in chloroform at high concentration (linear absorption coefficient α0 = 0.80 cm− 1), a broad peak is observed, due to the overlap of both the monomer and the aggregate absorption peaks. The overlap of monomer and aggregate peaks is also observed for all complexes in cyclohexanone at α0 = 0.80 cm− 1, Fig. 2B (b). More severe aggregation is observed for complex 2/PMMA thin films (Fig. 2C), with the band due to the aggregate being more pronounced that the monomer peak. For complex 1/PMMA thin films (Fig. 2C), there is less aggregation due to non-peripheral substitution which prevents aggregation more effectively than peripheral substitution. All open-aperture Z-scan experiments performed exhibited a decrease of transmittance around the focus which is typical of an induced nonlinear absorption of incident light. This behavior is the evidence of intensity dependent absorption of the incident light. Effective absorption coefficient βeff is calculated using theory previously reported [21]. The normalized transmittance TNorm(z) as a function of position z is given by TNorm ðzÞ =

loge ½1 + q0 ðzÞ : q0 ðzÞ

ð1Þ

In the above equation q0(z) is given by q0 ðzÞ =

q00 ; 1 + ðz =z0 Þ2

q00 ∼βeff I0 Leff

ð2Þ

where z0 is the diffraction length of the beam, βeff is the effective nonlinear absorption coefficient and I0 is the intensity of the light at focus. Leff ~ [1 − exp(−α0L)] / α0, is known as the effective length of the sample and defined in terms of the linear absorption coefficient, α0, and the true optical path length through the sample, L. All openaperture Z-scan data sets were fitted using the method of leastsquares regression. OL performance of Pcs can be improved by RSA from excited triplet state absorption [22]. According to this model each molecule at the S0 state absorbs the first photon to the S1 state. Electrons in S1 state can go to the second excited singlet state S2. Since the lifetime

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Fig. 2. A. Absorption spectra of 1, 2 and 3 in chloroform. Concentration = 4 × 10− 6 M. B. Absorption spectra of 1, 2 and 3: a) in chloroform (α0 = 0.80 cm− 1), b) in cyclohexanone (α0 = 0.80 cm− 1). C. Absorption spectra of 1 and 2 in Pc/PMMA films.

of the S2 state is very short, electrons relax back to the S1 state almost immediately and the first excited triplet state T1 is populated via intersystem crossing with a time constant τisc. The lifetime of the T1 state (τph) of the investigated compounds is reported previously and ranged between 200 and 210 μs for 1, 2 and 3 [19]. T1 population causes excitations and relaxations to from T2 triplet

state. The nonlinear absorption coefficient given by Eq. (3) was derived using rate equations [6,22] and steady state approximation. Normalized transmittance data collected from open-aperture Z-scan experiment were fitted using least-squares regression. κ is the ratio of the triplet to ground state absorption cross sections. Fsat is the energy density saturation which is the energy density at which the

Fig. 3. Plot of normalized transmittance for 1, 2 and 3 in cyclohexanone against incident pulse energy density.

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output reaches its saturated value. Note that a good optical limiter must have high κ, low FSat, low α0, high βeff values. αðI; Isat ; κÞ =

α0 1 + FF

sat

  F 1+κ Fsat

ð3Þ

The OL data plotted with the normalized transmission against the incident energy density per pulse are depicted in Figs. 3, 4 and 5. The parameters κ and Fsat were treated as free parameters in the fitting algorithm. A low Fsat is indicative of fast and efficient NLO response to incident irradiation. These OL parameters were used in the literature to estimate the effectiveness of optical limiting materials. For solution it was reported that βeff ranges in the order of 10− 9 to 10− 8 cm W− 1, Fsat in the order of 5 to 150 J cm− 2 and κ in the order of 2 to 27 in [6] while for the polymeric films βeff ranges in the order of 10− 4 to 10− 6 cm W− 1, Fsat in the order of 1.6 to 20.2 J cm− 2 and κ in the order of 3.2 to 23.8 [13– 17,23]. The OL parameters of Pcs in the literature were widely determined using the same mass concentration such as 0.5 and 1 g L− 1 to compare the optical limiting performance of the different Pcs [6]. However, very recently it was found that κ and βeff are largely dependent on the linear absorption coefficient [13]. Therefore, we performed all the measurements with the same linear absorption coefficient (see Table 1) in cyclohexanone. For the solution the results showed that OL parameters of the investigated compounds are close to each other with the same linear absorption coefficient. As seen from Table 1, OL parameters of 2 and 3 do not show significant differences, but are higher than 1. The effective nonlinear absorption coefficient βeff of all investigated compounds are almost same. One can compare the nonlinear absorption coefficient βeff of the investigated compound with zinc phthalocyanines [24]. The investigated compounds both in solution and film have larger βeff values than zinc phthalocyanine polymer films although they have the remarkably low linear absorption coefficient. This clearly indicates that the substituent 4-benzyloxyphenoxy improves NLO. Compound 2 exhibits the largest excited to singlet state absorption cross section ratio κ and the lowest saturation energy density Fsat. By making comparison with the literature it is not easy to estimate which Pcs will be the better optical limiter since most of the materials do not satisfy all the conditions for good optical limiting behavior. In phthalocyanine complexes the largest κ value reported was 33 for tBu4PcInCl [12]. In contrast to this result in solution, in the solid state tBu4PcInCl/PMMA film the measured κ is 4.06, only 0.17 times that of the former. As seen from Table 1, the investigated compounds have a relatively high βeff and κ values and a relatively low Fsat value if one compares with the literature [6]. Table 1 shows

that peripheral substitution (complexes 2 and 3) causes better OL behavior than non-peripheral substitution. Since the good quality polymer film of compound 3 could not be obtained, only the OL response of compounds 1 and 2 was investigated. The OL parameters of Pc/PMMA films were determined and given in Table 2. Open-aperture Z-scan spectra with normalized transmittance plotted as a function of sample position z and plot of normalized transmittance against incident pulse energy density were given in Figs. 4 and 5 for Pc/PMMA films. As seen from Table 2, the magnitudes βeff of 1 and 2 are slightly different from solution. This may indicate that Pcs in polymers increase the magnitude of the nonlinear absorption at appropriate intensities as observed in the literature [13–17]. In solid state, compound 2 has higher α0 and larger βeff values than compound 1. Similar behavior of βeff values is also reported in Pc/ PMMA films in the literature [13–17]. By making Pc/PMMA films, the number density of Pcs molecules can be increased dramatically in comparison with the solution. Since the concentration of absorbing molecules in the Pc/PMMA is significantly higher than that of in solution, it is expected that the Pc/PMMA system shows higher nonlinear absorption than the PC/solution system. In our experiments, κ values of Pc/PMMA films for 1 and 2 are lower than those in cyclohexanone (see Table 2). This may indicate that the influence of molecular aggregation and also host lattice interactions in film are considerably strong. Although the Pc/PMMA films for 1 and 2 have higher energy density Fsat values than their solution, Fsat values of 1 and 2 in polymeric films are relatively low in comparison with the literature [13,17]. The ability to control reductions in the saturation energy density is important from the point of view of realizing applications. Very recently we reported the optical limiting performance of GaPc and InPc complexes which were substituted with the same substituent in this work [13]. GaPc and InPc showed better optical limiting behavior in comparison with ZnPc in this work for both solution and PMMA matrix. It is known that the insertion of heavy atoms into Pcs ring causes a significant improvement in RSA, owing to an increased rate of intersystem crossing from the lowest excited singlet state to the strongly absorbing triplet state [8,9,12,25]. Thus, the heavy atom effect reinforces the absorption of the triplet state by increased population. In summary, we studied nonlinear absorption and optical limiting performance of 4-benzyloxyphenoxy-substituted zinc phthalocyanines, tetrasubstituted at the non-peripheral, peripheral positions and octasubstituted at the peripheral position with the 4-benzyloxyphenoxy group both in cyclohexanone and in a host PMMA polymer matrix. While the substitution position has no considerable effect on NLO of the investigated compounds, the OL parameters show remarkable differences. The investigated compounds in the solution

Fig. 4. For Pc/PMMA film for 1 (A) Open-aperture Z-scan spectra with normalized transmittance plotted as a function of sample position z (B) Plot of normalized transmittance against incident pulse energy density.

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Fig. 5. For Pc/PMMA film for 2 (A) Open-aperture Z-scan spectra with normalized transmittance plotted as a function of sample position z (B) Plot of normalized transmittance against incident pulse energy density.

Table 1 Linear and nonlinear coefficients and optical limiting parameters for 1, 2 and 3 at the same linear absorption coefficient in cyclohexanone solution (at focal intensity 0.5 GW cm− 2). α0 [cm− 1]

Materials 1 2 3

βeff [cm W− 1] −7

0.80 0.80 0.80

2.25 × 10 2.67 × 10− 7 2.55 × 10− 7

Fsat [J cm− 2]

κ [σex/σ0]

5.2 4.1 4.4

10.6 13.7 13.3

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

Table 2 Linear optical properties, nonlinear coefficients and optical limiting parameters for 1 and 2 in PMMA (at focal intensity 0.2 GW cm− 2). Materials

Thickn. [μm]

α0 [cm− 1]

βeff [cm W− 1]

Fsat [J cm− 2]

κ [σex/σ0]

1 2

7.5 4.7

69 170

6.8 × 10− 6 1.3 × 10− 5

6.3 5.1

7.5 9.1

[10] [11] [12] [13] [14] [15] [16] [17]

show better optical limiting behavior than the Pc/PMMA films. Embedding Pcs as inclusion in PMMA host reduced κ values and increased the saturation energy density Fsat values.

Acknowledgment We gratefully acknowledge the financial support by the Scientific and Technical Research Council of Turkey (TUBITAK) (no. 107T708).

[18] [19] [20] [21] [22] [23] [24] [25]

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