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Optical limiting performance of meso-tetraferrocenyl porphyrin and its metal derivatives
Journal of Photochemistry and Photobiology A: Chemistry 239 (2012) 24–27
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Short note
Optical limiting performance of meso-tetraferrocenyl porphyrin and its metal derivatives Prabhat Gautam a , Bhausaheb Dhokale a , Vijay Shukla b , Chandra Pal Singh b , Kushvinder Singh Bindra c , Rajneesh Misra a,∗ a
Department of Chemistry, Indian Institute of Technology Indore, MP 452017, India Laser Physics Applications Section, Raja Ramanna Centre for Advanced Technology, Indore, MP 452013, India c Solid State Laser Division, Raja Ramanna Centre for Advanced Technology, Indore, MP 452013, India b
a r t i c l e
i n f o
Article history: Received 14 March 2012 Received in revised form 23 April 2012 Accepted 26 April 2012 Available online 4 May 2012 Keywords: NLO Reverse saturable absorption Porphyrin Ferrocene
a b s t r a c t meso-Tetraferrocenyl porphyrin and its metal derivatives were found to be excellent optical limiters towards second harmonic of Q-switched Nd:YAG nanosecond laser. meso-Tetraferrocenyl porphyrin 3a, its zinc 3b, and copper 3c derivatives exhibited superior optical limiting performance than the benchmark fullerene C60 , and rest of the metal derivatives 3d–3e are comparable. Reverse saturable absorption phenomena is mainly responsible for the optical limiting behaviour in these compounds. The ratio ex / 0 was estimated from the nonlinear transmission characteristics of 3a–3e. The ratio ex / 0 more than 7 was obtained in case of 3a, which makes them attractive candidates as optical limiting material. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In order to protect human eyes and sensitive optical systems from the lasers, the intensity of incoming laser must be reduced. Optical limiters are the materials, which reduce the intensity of hostile lasers by its instantaneous filtering action. The incident laser light alters the absorptive and refractive properties of the material (optical limiting material) in such a manner that the resulting transmitted intensity is drastically reduced. Optical limiting materials based on reverse saturable absorption (RSA) are very useful for this purpose [1]. Recently several materials and device configurations have been proposed, and developed to meet this challenge. Among new materials, organic and organometallic compounds with nonlinear optical (NLO) properties have emerged as promising candidates [2]. These materials include porphyrins, phthalocyanines, fullerenes, and organometallic compounds. Among them, porphyrins and phthalocyanines are of interest because their NLO properties can be tuned by suitable structural modifications [3,4]. The optical limiting (OL) behaviour of phthalocyanines is widely studied, and they have shown some promising results [5]. On the other hand few reports are available on the OL behaviour of porphyrins [6–8].
∗ Corresponding author. Tel.: +91 7312438710; fax: +91 7312361482. E-mail addresses: [email protected] (C.P. Singh), [email protected] (R. Misra). 1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2012.04.020
Porphyrins and metalloporphyrins are effective optical limiters, because their ground state absorption is confined to narrow regions, which allows high transmission between Soret and Q bands. They exhibit large excited state cross-section ex and long triplet excited state life time. These properties of porphyrin make it material of choice for OL effect. The literature reveals that peripheral substitution on the meso or ˇ-positions and central atom influences the OL behaviour of the porphyrins [9–12]. Organometallic compounds are the other promising group of materials that have recently been studied for optical limiting effect [13]. Many organometallic compounds exhibit RSA under condition of intense optical excitation. The RSA in these materials, which is a consequence of the excited state absorption cross-section being larger than the ground state, can occur either in the singlet or triplet state. The RSA process in porphyrins, organometallics, and other dyes is often explained by simplified five level model (Fig. 1). Ferrocene is the most widely studied organometallic compound. The ferrocene derivatives are thermally and photochemically stable, and show large NLO effect [14]. To the best of our knowledge, there are only few reports which deal with the OL behaviour of ferrocenyl derivatives. One manuscript published by Paras N. Prasad group reported three-photon absorption based OL behaviour of ferrocenyl derivatives 1a and 1b (Chart 1) using subpicosecond IR laser pulse [15]. Moreover Tang group explored that molecules with more ferrocenyl groups 2a and 2b are better optical limiting materials (Chart 1). They reported two-photon absorption based OL behaviour of ferrocenyl derivative using nanosecond (ns) pulsed
P. Gautam et al. / Journal of Photochemistry and Photobiology A: Chemistry 239 (2012) 24–27
S2
25
T2 S
KISC
S1 E
T1 0
KS0 KT0
S0 Fig. 2. meso-Tetraferrocenyl porphyrin and its metal complexes 3a–3e. Fig. 1. The five-level reverse saturable absorption model for nonlinear absorptive optical limiting.
laser [16]. From these literature reports, it is evident that, ferrocenyl derivatives show good OL behaviour, and as the number of ferrocenyl group in the molecule increases the OL performance also increases. Encouraged by these literature reports, we were interested to incorporate four ferrocenyl moieties in the molecule and to explore its OL behaviour. The smart way to incorporate the four ferrocenyl moieties into the molecular system was to substitute the ferrocenyl group at the meso-positions of the porphyrin. Therefore we designed meso-tetraferrocenyl porphyrin, 3a. We were also interested to see the effect of metallation on the OL behaviour of meso-tetraferrocenyl porphyrin, 3b–3e. meso-Tetraferrocenyl porphyrin and its metal complexes 3a–3e (Fig. 2) were synthesised by reported procedure [17–19]. The ferrocenyl group act as an electron donor, as it pumps the electron density towards the porphyrin ring [20]. The meso-tetraferrocenyl porphyrin shows no sign of fluorescence. It has very low quantum
yield of singlet and triplet states, due to fast back electron transfer in the excited state [21]. meso-Tetraferrocenyl porphyrin and its metal derivatives 3a–3e are stable in air, and with laser light under experimental conditions. The electronic absorption spectrum of 3a–3e is shown in Fig. 3, which consists of Soret and Q bands. The Soret band in meso-tetraferrocenyl porphyrins 3a–3e is red shifted as compared to meso-tetraphenyl porphyrin, which shows former is more conjugated than the latter. The metal derivatives of meso-tetraferrocenyl porphyrin 3b–3e exhibit Soret band between 420 and 435 nm, with a shoulder around 490 nm and the Q band between 660 and 680 nm, with a shoulder at higher energy. 2. Experimental method The experimental set-up used is shown in Fig. 4. The optical limiting experiments were carried out with frequency doubled Q-switched Nd:YAG laser at 532 nm wavelength, delivering 30 nanosecond (FWHM) laser pulses at 1 Hz repetition rate. Beam was focused using a 20 cm focal length lens on a 5 mm path length cell containing the sample. Incident and transmitted energies were measured with calibrated photodiodes in two geometries. Geometry (i), for optical limiting measurements, in which transmitted beam was collected through a 90% transmission aperture placed in front of a photodiode in the far field. In this geometry decrease
3a 3b 3c 3d 3e
Absorbance
1
0 400
500
600
700
Wavelength (nm)
Chart 1.
Fig. 3. Normalized UV–vis absorption spectra of 3a–3e in chloroform at room temperature.
26
P. Gautam et al. / Journal of Photochemistry and Photobiology A: Chemistry 239 (2012) 24–27
1.0
3b C60
Fig. 4. Layout of the experimental setup used for the transmission measurements. L: lens; F: neutral density filters; BS: beam splitter; A: aperture; D: photo diode; and S: sample.
Transmission
0.8
3a 3c
0.6
0.4
0.2 in the transmission would account for absorption, defocusing, and scattering originating from absorption induced thermal effects. Geometry (ii), in which transmitted light was measured simultaneously by putting a beam splitter near to the sample to exclude defocusing and small angle scattering in the forward direction. The input energy was varied with neutral density filters. Linear transmission of all the samples was kept 70% at excitation wavelength. Samples including fullerene C60 were dissolved in toluene to exclude the contribution of relative thermal effect in the optical limiting behaviour arising due to temperature dependent refractive index variation of the solvent. 3. Results and discussion Fig. 5 shows the OL response of 3a, 3b, and 3c along with C60 (see ESI for the OL response of 3a–3e). In all the plots, a linear increase in output intensity is followed by a decrease in the slope, and finally the emergence of flat region at high input intensities. This is a typical OL behaviour. For 3a, the deviation from linearity occurs at 20 J, whereas, for 3b, 3c, 3d, and 3e it occurs at 15 J, 25 J, 35 J, and 40 J respectively. Therefore 3a–3c show a better limiting action, which is superior to 3d and 3e in terms of onset of OL and flattening of output at high input intensities. The OL behaviour and its efficiency in 3a–3e were estimated by measuring the transmittance change on going from 0 to 350 J input energy. The data of final transmission in Geometry (i), which is through 90% transmission aperture (the optical limiting effect) and Geometry (ii), which is without aperture (the RSA effect) at 350 J energy are shown in Table 1. In both the geometries there is a drastic reduction in the transmittance of meso-tetraferrocenyl porphyrins and its metal derivatives 3a–3e, Fig. 6 (see ESI for the 100
Output Energy ( J)
80
70 % LT
60
3b 70% linear transmission C60 3a 3c
40
20
0
0
50
100
150
200
250
300
350
Input Energy ( J) Fig. 5. A comparison of optical limiting results among 3a, 3b, 3c and C60 in toluene solutions with 70% linear transmittance at 532 nm.
0.0
0
50
100
150
200
250
300
350
Input Energy( J) Fig. 6. Nonlinear optical transmission of 3a, 3b, 3c, and C60 at 532 nm in Geometry (i) (pulse duration: 30 ns, repetition rate: 1 Hz) (linear transmission, 70% at 532 nm).
nonlinear transmission plots of compound 3a–3e in Geometry (i)). The OL behaviour (in Geometry (i)) in 3a–3e was compared by measuring the change in transmittance at 350 J. The transmittance is reduced from 70% to 7%, 6%, 8%, 12%, and 13% for 3a, 3b, 3c, 3d, and 3e respectively. In case of C60 the transmission is reduced from 70% to 10%. Therefore 3a–3c show superior performance than the benchmark optical limiting material fullerene C60 . The nonlinear transmission behaviour of 3a–3e in Geometry (ii), i.e. collection of the transmitted beam without any aperture was used to estimate ex / 0 ratio. Although exact calculation of ratio requires rate equation analysis considering all the states in the energy level, but due to unavailability of parameters such as relaxation times of different states, a simplified two state model is considered for estimation of the ratio. Initially, all the population in this model was considered in the ground state, hence transmission was exp(−N 0 L), where N is the population density, 0 is the absorption cross-section of the ground state, and L is the thickness of the medium. On excitation with laser pulses, due to faster intersystem crossing rate population gets transferred to triplet state within the pulse duration. The decrease in the transmission is mainly due to increased absorption by the triplet state. In this case transmission can be considered exp(−N ex L), where ex is the absorption cross-section of the excited state. Nonlinear transmission behaviour in Geometries (i) and (ii) is not very different; hence essentially the main mechanism responsible for observation of OL behaviour is RSA. Relatively superior behaviour in Geometry (i) can be assigned to contribution from thermal nonlinearity and scattering. The calculated ( ex / 0 ) ratio in Geometry (ii) i.e. without aperture is shown in Table 1. The table indicates that 3a, 3b, and 3c have ( ex / 0 ) ratio 7.08, 6.45 and 6.19 whereas, 3d and 3e, 2.94, and 4.51 respectively. The compounds 3a–3c are having low ground state absorption cross-section ( 0 ), whereas 3d and 3e are having high 0 (Table 1), which results in superior value of ( ex / 0 ) ratio for 3a–3c and inferior for 3d and 3e. The value of ( ex / 0 ) ratio in 3a–3e is also consistent with the UV–vis absorption spectra in Fig. 3. The UV–vis absorption spectrum clearly indicates that 3d and 3e are having larger linear absorption than 3a–3c at 532 nm. The compounds with low ground state absorption cross-section at the excitation wavelength show better OL behaviour. Since the linear transmission at 532 nm wavelength is same for all the samples in the same path length cell. Sample with low ground state absorption cross-section would have larger number density of the molecules to have a fixed linear transmission hence, larger
P. Gautam et al. / Journal of Photochemistry and Photobiology A: Chemistry 239 (2012) 24–27
27
Table 1 Comparison of the optical limiting performance of 3a–3e. Compound
3a 3b 3c 3d 3e C60
Limiting plateau (J)
20 15 25 35 40 30
Initial transmission
0.7 0.7 0.7 0.7 0.7 0.7
Final transmission at 350 J energy
Geometry (i) (through 90% transmission aperture)
Geometry (ii) (without aperture)
0.07 0.06 0.08 0.12 0.13 0.10
0.08 0.10 0.11 0.35 0.20 0.14
probability to get in to the excited state. Such behaviour is desirable for device applications. This finding further supports that the main mechanism responsible for the observation of OL behaviour is RSA. Therefore the OL response in 3a–3e is primarily due to the stronger excited triplet state absorption than the ground state absorption. The compounds 3a–3e show no sign of fluorescence, which supports the argument that these compounds exhibits strong triplet state absorption, and weak ground state absorption [21]. 4. Conclusions In summary we have explored meso-tetraferrocenyl porphyrin and its metal derivatives as optical limiter. The OL behaviour of compounds 3a–3e was compared with standard optical limiter fullerene C60 . The insertion of the closed shell metal Zn in the meso-tetraferrocenyl porphyrin results in better performance due to heavy metal effect. 3a–3c show better OL performance than C60 and rest of the compounds 3d and 3e are comparable. RSA phenomena are mainly responsible for observation of OL behaviour in these compounds. The meso-tetraferrocenyl porphyrin and its metal derivatives can be used as optical limiter for class 3B lasers. Although the protection level is not sufficient according to DIN EN 60825-1 standards but due to easy synthesis and tunability of structures, they are worth to be explored for their possibility to serve as an optical limiter. The detailed characterization of NLO properties of these materials is currently on going in our group. Acknowledgement R.M. Thanks CSIR, New Delhi for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jphotochem.2012.04.020
Absorption cross-section (×10−17 cm2 ) 0
Calculated ratio ( ex / 0 ) without aperture
8.5 9.5 10.9 13.9 16.1 0.12
7.08 6.45 6.19 2.94 4.51 5.51
References [1] G.D.L. Torre, P. Vazquez, F.A. Lopez, T. Torres, Chemical Reviews 104 (2004) 3723. [2] P.N. Prasad, D.J. Williams, Introduction to Non-linear Optical Effects in Molecules and Polymers, Wiley, New York, 1991. [3] M. Calvete, G.Y. Yang, M. Hanack, Synthetic Metals 141 (2004) 231. [4] R.R. Dasari, M.M. Sartin, M. Cozzuol, S. Barlow, J.W. Perry, S.R. Marder, Chemical Communications 47 (2011) 4547. [5] J.W. Perry, K. Mansour, I.Y.S. Lee, X.L. Wu, P.V. Bedworth, C.T. Chen, D. Ng, S.R. Marder, P. Miles, T. Wada, M. Tian, H. Sasabe, Science 273 (1996) 1533. [6] E.G.A. Notaras, M. Fazekas, J.J. Doyle, W.J. Balu, M.O. Senge, Chemical Communications (2007) 2166. [7] R.B. Martin, H. Li, L. Gu, S. Kumar, C.M. Sanders, Y. Sun, Optical Materials 27 (2005) 1340. [8] Z. Liu, Y. Zhu, Y. Zhu, J. Tian, J. Zheng, Journal of Physical Chemistry B 111 (2007) 14136. [9] S. Fu, X. Zhu, G. Zhou, W. Wong, C. Ye, W. Wong, Z. Li, European Journal of Inorganic Chemistry (2007) 2004. [10] P.P. Kiran, N.K.M.N. Srinivas, D.R. Reddy, B.G. Maiya, A. Dharmadhikari, A.S. Sandhu, G.R. Kumar, D.N. Rao, Optics Communications 202 (2002) 347. [11] W. Su, T.M. Cooper, Chemistry of Materials 10 (1998) 1212. [12] K. McEwan, K. Lewis, G.Y. Yang, L.L. Chng, Y.W. Lee, W.P. Lau, K.S. Lai, Advanced Functional Materials 13 (2003) 863. [13] C. Bucher, C.H. Devillers, J.C. Moutet, G. Royal, E. Saint-Aman, Coordination Chemistry Reviews 253 (2009) 21. [14] R. Misra, R. Kumar, T.K. Chandrashekar, A. Nag, D. Goswami, Organic Letters 8 (2006) 629. [15] Q. Zheng, G.S. He, C. Lu, P.N. Prasad, Journal of Materials Chemistry 15 (2005) 3488. [16] F. Yang, X.L. Xu, Y.H. Gong, W.W. Qiu, Z.R. Sun, J.W. Zhou, P. Audebert, J. Tang, Tetrahedron 63 (2007) 9188. [17] S.J. Narayanan, S. Venkatraman, S.R. Dey, B. Sridevi, V.R.G. Anand, T.K. Chandrashekar, Synlett 12 (2000) 1834. [18] V.N. Nemykin, P. Galloni, B. Floris, C.D. Barrett, R.G. Hadt, R.I. Subbotin, A.G. Marrani, R. Zanoni, N.M. Loim, Dalton Transactions 129 (2008) 4233. [19] V.N. Nemykin, C.D. Barrett, R.G. Hadt, R.I. Subbotin, A.Y. Maximov, E.V. Polshin, A.Y. Koposov, Dalton Transactions (2007) 3378. [20] R. Misra, R. Kumar, T.K. Chandrashekar, C.H. Suresh, A. Nag, D. Goswami, Journal of the American Chemical Society 128 (2006) 16083. [21] V.A. Nadtochenko, N.N. Denisov, V.Y. Gak, N.V. Abramova, N.M. Loim, Russian Chemical Bulletin 148 (1999) 1900.
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Short note
Optical limiting performance of meso-tetraferrocenyl porphyrin and its metal derivatives Prabhat Gautam a , Bhausaheb Dhokale a , Vijay Shukla b , Chandra Pal Singh b , Kushvinder Singh Bindra c , Rajneesh Misra a,∗ a
Department of Chemistry, Indian Institute of Technology Indore, MP 452017, India Laser Physics Applications Section, Raja Ramanna Centre for Advanced Technology, Indore, MP 452013, India c Solid State Laser Division, Raja Ramanna Centre for Advanced Technology, Indore, MP 452013, India b
a r t i c l e
i n f o
Article history: Received 14 March 2012 Received in revised form 23 April 2012 Accepted 26 April 2012 Available online 4 May 2012 Keywords: NLO Reverse saturable absorption Porphyrin Ferrocene
a b s t r a c t meso-Tetraferrocenyl porphyrin and its metal derivatives were found to be excellent optical limiters towards second harmonic of Q-switched Nd:YAG nanosecond laser. meso-Tetraferrocenyl porphyrin 3a, its zinc 3b, and copper 3c derivatives exhibited superior optical limiting performance than the benchmark fullerene C60 , and rest of the metal derivatives 3d–3e are comparable. Reverse saturable absorption phenomena is mainly responsible for the optical limiting behaviour in these compounds. The ratio ex / 0 was estimated from the nonlinear transmission characteristics of 3a–3e. The ratio ex / 0 more than 7 was obtained in case of 3a, which makes them attractive candidates as optical limiting material. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In order to protect human eyes and sensitive optical systems from the lasers, the intensity of incoming laser must be reduced. Optical limiters are the materials, which reduce the intensity of hostile lasers by its instantaneous filtering action. The incident laser light alters the absorptive and refractive properties of the material (optical limiting material) in such a manner that the resulting transmitted intensity is drastically reduced. Optical limiting materials based on reverse saturable absorption (RSA) are very useful for this purpose [1]. Recently several materials and device configurations have been proposed, and developed to meet this challenge. Among new materials, organic and organometallic compounds with nonlinear optical (NLO) properties have emerged as promising candidates [2]. These materials include porphyrins, phthalocyanines, fullerenes, and organometallic compounds. Among them, porphyrins and phthalocyanines are of interest because their NLO properties can be tuned by suitable structural modifications [3,4]. The optical limiting (OL) behaviour of phthalocyanines is widely studied, and they have shown some promising results [5]. On the other hand few reports are available on the OL behaviour of porphyrins [6–8].
∗ Corresponding author. Tel.: +91 7312438710; fax: +91 7312361482. E-mail addresses: [email protected] (C.P. Singh), [email protected] (R. Misra). 1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2012.04.020
Porphyrins and metalloporphyrins are effective optical limiters, because their ground state absorption is confined to narrow regions, which allows high transmission between Soret and Q bands. They exhibit large excited state cross-section ex and long triplet excited state life time. These properties of porphyrin make it material of choice for OL effect. The literature reveals that peripheral substitution on the meso or ˇ-positions and central atom influences the OL behaviour of the porphyrins [9–12]. Organometallic compounds are the other promising group of materials that have recently been studied for optical limiting effect [13]. Many organometallic compounds exhibit RSA under condition of intense optical excitation. The RSA in these materials, which is a consequence of the excited state absorption cross-section being larger than the ground state, can occur either in the singlet or triplet state. The RSA process in porphyrins, organometallics, and other dyes is often explained by simplified five level model (Fig. 1). Ferrocene is the most widely studied organometallic compound. The ferrocene derivatives are thermally and photochemically stable, and show large NLO effect [14]. To the best of our knowledge, there are only few reports which deal with the OL behaviour of ferrocenyl derivatives. One manuscript published by Paras N. Prasad group reported three-photon absorption based OL behaviour of ferrocenyl derivatives 1a and 1b (Chart 1) using subpicosecond IR laser pulse [15]. Moreover Tang group explored that molecules with more ferrocenyl groups 2a and 2b are better optical limiting materials (Chart 1). They reported two-photon absorption based OL behaviour of ferrocenyl derivative using nanosecond (ns) pulsed
P. Gautam et al. / Journal of Photochemistry and Photobiology A: Chemistry 239 (2012) 24–27
S2
25
T2 S
KISC
S1 E
T1 0
KS0 KT0
S0 Fig. 2. meso-Tetraferrocenyl porphyrin and its metal complexes 3a–3e. Fig. 1. The five-level reverse saturable absorption model for nonlinear absorptive optical limiting.
laser [16]. From these literature reports, it is evident that, ferrocenyl derivatives show good OL behaviour, and as the number of ferrocenyl group in the molecule increases the OL performance also increases. Encouraged by these literature reports, we were interested to incorporate four ferrocenyl moieties in the molecule and to explore its OL behaviour. The smart way to incorporate the four ferrocenyl moieties into the molecular system was to substitute the ferrocenyl group at the meso-positions of the porphyrin. Therefore we designed meso-tetraferrocenyl porphyrin, 3a. We were also interested to see the effect of metallation on the OL behaviour of meso-tetraferrocenyl porphyrin, 3b–3e. meso-Tetraferrocenyl porphyrin and its metal complexes 3a–3e (Fig. 2) were synthesised by reported procedure [17–19]. The ferrocenyl group act as an electron donor, as it pumps the electron density towards the porphyrin ring [20]. The meso-tetraferrocenyl porphyrin shows no sign of fluorescence. It has very low quantum
yield of singlet and triplet states, due to fast back electron transfer in the excited state [21]. meso-Tetraferrocenyl porphyrin and its metal derivatives 3a–3e are stable in air, and with laser light under experimental conditions. The electronic absorption spectrum of 3a–3e is shown in Fig. 3, which consists of Soret and Q bands. The Soret band in meso-tetraferrocenyl porphyrins 3a–3e is red shifted as compared to meso-tetraphenyl porphyrin, which shows former is more conjugated than the latter. The metal derivatives of meso-tetraferrocenyl porphyrin 3b–3e exhibit Soret band between 420 and 435 nm, with a shoulder around 490 nm and the Q band between 660 and 680 nm, with a shoulder at higher energy. 2. Experimental method The experimental set-up used is shown in Fig. 4. The optical limiting experiments were carried out with frequency doubled Q-switched Nd:YAG laser at 532 nm wavelength, delivering 30 nanosecond (FWHM) laser pulses at 1 Hz repetition rate. Beam was focused using a 20 cm focal length lens on a 5 mm path length cell containing the sample. Incident and transmitted energies were measured with calibrated photodiodes in two geometries. Geometry (i), for optical limiting measurements, in which transmitted beam was collected through a 90% transmission aperture placed in front of a photodiode in the far field. In this geometry decrease
3a 3b 3c 3d 3e
Absorbance
1
0 400
500
600
700
Wavelength (nm)
Chart 1.
Fig. 3. Normalized UV–vis absorption spectra of 3a–3e in chloroform at room temperature.
26
P. Gautam et al. / Journal of Photochemistry and Photobiology A: Chemistry 239 (2012) 24–27
1.0
3b C60
Fig. 4. Layout of the experimental setup used for the transmission measurements. L: lens; F: neutral density filters; BS: beam splitter; A: aperture; D: photo diode; and S: sample.
Transmission
0.8
3a 3c
0.6
0.4
0.2 in the transmission would account for absorption, defocusing, and scattering originating from absorption induced thermal effects. Geometry (ii), in which transmitted light was measured simultaneously by putting a beam splitter near to the sample to exclude defocusing and small angle scattering in the forward direction. The input energy was varied with neutral density filters. Linear transmission of all the samples was kept 70% at excitation wavelength. Samples including fullerene C60 were dissolved in toluene to exclude the contribution of relative thermal effect in the optical limiting behaviour arising due to temperature dependent refractive index variation of the solvent. 3. Results and discussion Fig. 5 shows the OL response of 3a, 3b, and 3c along with C60 (see ESI for the OL response of 3a–3e). In all the plots, a linear increase in output intensity is followed by a decrease in the slope, and finally the emergence of flat region at high input intensities. This is a typical OL behaviour. For 3a, the deviation from linearity occurs at 20 J, whereas, for 3b, 3c, 3d, and 3e it occurs at 15 J, 25 J, 35 J, and 40 J respectively. Therefore 3a–3c show a better limiting action, which is superior to 3d and 3e in terms of onset of OL and flattening of output at high input intensities. The OL behaviour and its efficiency in 3a–3e were estimated by measuring the transmittance change on going from 0 to 350 J input energy. The data of final transmission in Geometry (i), which is through 90% transmission aperture (the optical limiting effect) and Geometry (ii), which is without aperture (the RSA effect) at 350 J energy are shown in Table 1. In both the geometries there is a drastic reduction in the transmittance of meso-tetraferrocenyl porphyrins and its metal derivatives 3a–3e, Fig. 6 (see ESI for the 100
Output Energy ( J)
80
70 % LT
60
3b 70% linear transmission C60 3a 3c
40
20
0
0
50
100
150
200
250
300
350
Input Energy ( J) Fig. 5. A comparison of optical limiting results among 3a, 3b, 3c and C60 in toluene solutions with 70% linear transmittance at 532 nm.
0.0
0
50
100
150
200
250
300
350
Input Energy( J) Fig. 6. Nonlinear optical transmission of 3a, 3b, 3c, and C60 at 532 nm in Geometry (i) (pulse duration: 30 ns, repetition rate: 1 Hz) (linear transmission, 70% at 532 nm).
nonlinear transmission plots of compound 3a–3e in Geometry (i)). The OL behaviour (in Geometry (i)) in 3a–3e was compared by measuring the change in transmittance at 350 J. The transmittance is reduced from 70% to 7%, 6%, 8%, 12%, and 13% for 3a, 3b, 3c, 3d, and 3e respectively. In case of C60 the transmission is reduced from 70% to 10%. Therefore 3a–3c show superior performance than the benchmark optical limiting material fullerene C60 . The nonlinear transmission behaviour of 3a–3e in Geometry (ii), i.e. collection of the transmitted beam without any aperture was used to estimate ex / 0 ratio. Although exact calculation of ratio requires rate equation analysis considering all the states in the energy level, but due to unavailability of parameters such as relaxation times of different states, a simplified two state model is considered for estimation of the ratio. Initially, all the population in this model was considered in the ground state, hence transmission was exp(−N 0 L), where N is the population density, 0 is the absorption cross-section of the ground state, and L is the thickness of the medium. On excitation with laser pulses, due to faster intersystem crossing rate population gets transferred to triplet state within the pulse duration. The decrease in the transmission is mainly due to increased absorption by the triplet state. In this case transmission can be considered exp(−N ex L), where ex is the absorption cross-section of the excited state. Nonlinear transmission behaviour in Geometries (i) and (ii) is not very different; hence essentially the main mechanism responsible for observation of OL behaviour is RSA. Relatively superior behaviour in Geometry (i) can be assigned to contribution from thermal nonlinearity and scattering. The calculated ( ex / 0 ) ratio in Geometry (ii) i.e. without aperture is shown in Table 1. The table indicates that 3a, 3b, and 3c have ( ex / 0 ) ratio 7.08, 6.45 and 6.19 whereas, 3d and 3e, 2.94, and 4.51 respectively. The compounds 3a–3c are having low ground state absorption cross-section ( 0 ), whereas 3d and 3e are having high 0 (Table 1), which results in superior value of ( ex / 0 ) ratio for 3a–3c and inferior for 3d and 3e. The value of ( ex / 0 ) ratio in 3a–3e is also consistent with the UV–vis absorption spectra in Fig. 3. The UV–vis absorption spectrum clearly indicates that 3d and 3e are having larger linear absorption than 3a–3c at 532 nm. The compounds with low ground state absorption cross-section at the excitation wavelength show better OL behaviour. Since the linear transmission at 532 nm wavelength is same for all the samples in the same path length cell. Sample with low ground state absorption cross-section would have larger number density of the molecules to have a fixed linear transmission hence, larger
P. Gautam et al. / Journal of Photochemistry and Photobiology A: Chemistry 239 (2012) 24–27
27
Table 1 Comparison of the optical limiting performance of 3a–3e. Compound
3a 3b 3c 3d 3e C60
Limiting plateau (J)
20 15 25 35 40 30
Initial transmission
0.7 0.7 0.7 0.7 0.7 0.7
Final transmission at 350 J energy
Geometry (i) (through 90% transmission aperture)
Geometry (ii) (without aperture)
0.07 0.06 0.08 0.12 0.13 0.10
0.08 0.10 0.11 0.35 0.20 0.14
probability to get in to the excited state. Such behaviour is desirable for device applications. This finding further supports that the main mechanism responsible for the observation of OL behaviour is RSA. Therefore the OL response in 3a–3e is primarily due to the stronger excited triplet state absorption than the ground state absorption. The compounds 3a–3e show no sign of fluorescence, which supports the argument that these compounds exhibits strong triplet state absorption, and weak ground state absorption [21]. 4. Conclusions In summary we have explored meso-tetraferrocenyl porphyrin and its metal derivatives as optical limiter. The OL behaviour of compounds 3a–3e was compared with standard optical limiter fullerene C60 . The insertion of the closed shell metal Zn in the meso-tetraferrocenyl porphyrin results in better performance due to heavy metal effect. 3a–3c show better OL performance than C60 and rest of the compounds 3d and 3e are comparable. RSA phenomena are mainly responsible for observation of OL behaviour in these compounds. The meso-tetraferrocenyl porphyrin and its metal derivatives can be used as optical limiter for class 3B lasers. Although the protection level is not sufficient according to DIN EN 60825-1 standards but due to easy synthesis and tunability of structures, they are worth to be explored for their possibility to serve as an optical limiter. The detailed characterization of NLO properties of these materials is currently on going in our group. Acknowledgement R.M. Thanks CSIR, New Delhi for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jphotochem.2012.04.020
Absorption cross-section (×10−17 cm2 ) 0
Calculated ratio ( ex / 0 ) without aperture
8.5 9.5 10.9 13.9 16.1 0.12
7.08 6.45 6.19 2.94 4.51 5.51
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