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Investigation of picosecond optical nonlinearity in porphyrin metal complexes derivatives
3 March 2000
Chemical Physics Letters 318 Ž2000. 511–516 www.elsevier.nlrlocatercplett
Investigation of picosecond optical nonlinearity in porphyrin metal complexes derivatives A.G. Bezerra Jr. 1, I.E. Borissevitch 2 , R.E. de Araujo, A.S.L. Gomes ) , Cid B. de Araujo ´ Departamento de Fısica, UniÕersidade Federal de Pernambuco, 50670-901 Recife, PE, Brazil ´ Received 6 September 1999; in final form 30 November 1999
Abstract Porphyrin metal complex derivatives were experimentally studied using the Z-scan technique and the optical Kerr gate method with picosecond pulses at 532 nm. The sign and magnitude of the nonlinear refraction and nonlinear absorption were determined for a negatively charged meso-tetrakisŽ p-sulfonatophenyl.porphyrin ŽTPPS 4 . and a positively charged mesotetrakisŽ4-N-methyl-pyridiniumyl.porphyrin ŽTMPyP. in their free-base forms and their FeŽIII. and MnŽIII. complexes. The results were phenomenologically understood and the origin of the nonlinearity was inferred. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Porphyrins and porphyrin derivatives, which are two-dimensional molecules characterized by highly developed p-conjugation, present nonlinear optical properties such as the intensity-dependent refraction index andror nonlinear absorption whose magnitude and time response depend upon the dominant physical process as well as the environment w1–8x. On the
) Corresponding author. Fax: q55-81-2710359; e-mail: [email protected] 1 Present address: Departamento de Fısica, Centro Federal de ´ Educac¸ao do Parana, ˜ Tecnologica ´ ´ CEFET-PR, 80230-901 Curitiba, PR, Brazil. 2 Present address: Departamento de Fısica e Matematica, Facul´ ´ dade de Filosofia, Ciencias e Letras de Ribeirao ˆ ˜ Preto, Universidade Sao ˜ Paulo, 14040-901, Ribeirao ˜ Preto, SP, Brazil.
other hand, porphyrins and porphyrin-like compounds have long been identified as important components of living nature. For example, the porphyrin derivative chlorophyll is the main molecule in the process of photosynthesis and porphyrin-like compounds are active centers of the cytochromes, essential in the oxidation chain. Porphyrins are also widely used as photosensitizers in photodynamic therapy of cancer w5x. Water-soluble porphyrins in their free-base form may exist in aqueous solutions in protonated or nonprotonated state while their metal complexes, with FeŽIII., for example, can bind other particles, such as OHy, H 2 O andror form physical aggregates and chemically bound m-oxo-dimers. These interactions can change significantly the electronic structure of the porphyrins and the studies of their nonlinear optical properties should give valuable information about these changes.
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 0 7 6 - 2
A.G. Bezerra Jr. et al.r Chemical Physics Letters 318 (2000) 511–516
512
2. Experimental In the present Letter, we report on the measurements of the nonlinear refractive index Žor Kerr coefficient., n 2 , which is proportional to the real part of the third-order nonlinear susceptibility, x Ž3. and of a 2 , the nonlinear absorption coefficient, which is proportional to the imaginary part of x Ž3. of two water-soluble porphyrins ŽPPh.: a negatively charged meso-tetrakisŽ p-sulfonatophenyl.porphyrin ŽTPPS 4 . and a positively charged meso-tetrakisŽ4-N-methylpyridiniumyl. porphyrin ŽTMPyP. in their free-base forms, and their FeŽIII. and MnŽIII. complexes. For these measurements, the Z-scan technique w9,10x with picosecond pulses was employed. To fully characterize the origin of the nonlinearity of these materials in the picosecond regime, we also performed a time-resolved experiment using the optical Kerr gate ŽOKG. technique w11x. From the data, we could understand the origin of the nonlinearity as well as infer conformational changes in the porphyrin derivatives studied. Fig. 1 shows the structures of the studied PPh complexes. TPPS 4 , TMPyP and their FeŽIII. and MnŽIII. complexes were obtained from Midcentury Chemicals ŽUSA. and used as purchased. The PPh were dissolved in Milli-Q quality water and the PPh concentration was controlled spectrophotometrically. Fig. 2 shows typical linear absorption spectra for three of the samples studied, measured on a commercial spectrophotometer. They are typical of porphyrin derivatives, with the main absorption peak due to the Soret band between 420 and 470 nm, depending upon pH. The linear absorption coefficient at the pump wavelength was kept at ; 2 cmy1 for all the samples adjusting the PPh concentration. It is clear, from Fig. 2, that there is no absorption peak at 532
Fig. 1. The chemical structure of the porphyrin complexes.
Fig. 2. Linear absorption spectra of: Ža. FeŽIII.TPPS 4 , pH 9.0; Žb. TPPS 4 , pH 4.0; and Žc. MnŽIII.TMPyP, pH 7.0. The linear absorption coefficient was kept at ; 2 cmy1 .
nm. The pH changes were produced by the addition of HCl or NaOH stock solutions. The light source used was the second harmonic of a cw pumped Q-switched and mode-locked Nd:YAG laser delivering pulses of 70 ps ŽFWHM. at 532 nm. A single pulse at a 10 Hz repetition rate was selected using a ‘pulse picker’ and all experiments were made in a 2 mm glass cuvette at room temperature.
3. Results Fig. 3 shows a typical result of the Z-scan measurements with small aperture in front of the detector ŽFig. 3a. and with the aperture fully open ŽFig. 3b.. The nonlinear refractive index signature observed is characteristic of a self-defocusing medium whereas the open-aperture measurements demonstrate a small nonlinear absorption which is due to an excited state absorption process. The results of all the samples studied are summarized in Table 1 and an analysis of the data is very clarifying. First of all, we note that the magnitude of the nonlinear refractive index may reach the same order of magnitude as that of CS 2 Ž n 2 s 3.3 = 10y1 4 cm2rW w9,10x., a standard nonlinear optical material and its symmetrical shape shows that there is no significant nonlinear absorp-
A.G. Bezerra Jr. et al.r Chemical Physics Letters 318 (2000) 511–516
Fig. 3. Ža. Small-aperture Z-scan trace of PPh complex Žsample P4. and Žb. open-aperture Z-scan measurement of sample P5.
tion due to resonant one-photon mechanism. Since the aim of the present work is to exploit the optical methods to characterize the physical mechanisms
513
and structural changes in the molecule upon environment changes, the magnitude of the nonlinearity is not an issue. Hence, the values presented for some samples are given only to indicate the detection limit of our apparatus. The values of n 2 and a 2 shown in Table 1, depend on the porphyrin structure and the pH value. The measurable values of n 2 were observed only for the metal complexes FeŽIII.TMPyP Žsamples P2 and P3., MnŽIII.TMPyP Žsample P4. and MnŽIII.TPPS 4 Žsample P9.. For the free-base PPh forms Žsamples P1, P5, P6. the nonlinear effects in the refractive index were below our detection limit, as well as for FeŽIII.TPPS 4 porphyrin Žsamples P7 and P8.. The dependence of n 2 with pH is clearly seen for samples P2 and P3, although it could not be measured in samples P7 and P8. To qualitatively understand the observed features, we should have in mind that one of the important characteristics of a molecule which determines the third-order optical nonlinearity is its dipole moment w12x, which in turn is determined by the space symmetry of the molecule. The free-base form of the studied porphyrins was completely symmetric in the plane of the p-conjugated ring, and cannot produce a dipole moment in this plane. However, it is possible to have a dipole moment in the direction perpendicular to this plane if the central metal atom or protons come out-of-plane, which is the case for the metaloporphyrins. Out-of-plane metal central atoms in porphyrins are known to happen and their characterization is currently under investigation w13–15x. The central nitrogen atoms of porphyrins in the metal-free form Žsamples P1, P5, P6. can be proto-
Table 1 Measured nonlinear refractive index, n 2 , and the nonlinear absorption coefficient, a 2 , for the porphyrin complex studied See text for identification of samples P1–P9 Porphyrin
R
X
pH
n2 Ž10y14 cm2rW.
a2 ŽcmrGW.
P1 P2 P3 P4 P5 P6 P7 P8 P9
I I I I II II II II II
2Hq FeŽIII. FeŽIII. MnŽIII. 2Hq 2Hq FeŽIII. FeŽIII. MnŽIII.
7 4 9 7 4 7 4 9 7
- y0.1 y0.6 - y0.1 y5.4 - y0.1 - y0.1 - y0.1 - y0.1 y0.3
1.2 - 0.1 - 0.1 - 0.1 0.7 0.6l - 0.1 - 0.1 0.2
514
A.G. Bezerra Jr. et al.r Chemical Physics Letters 318 (2000) 511–516
nated, depending on the porphyrin structure and pH. This protonation can be observed through changes in the linear absorption spectrum. In either case Žprotonated or nonprotonated., the structure of the free-base porphyrins are symmetric with respect to the ring plane and thus they have no dipole moment perpendicular to the ring plane. This explains why the n 2 values for these porphyrins are negligible independently of the state of protonation. In the MnŽIII.PPh complexes the metal atom is located out of the ring plane w13,14x and this produces a dipole moment perpendicular to the ring plane. Moreover, in air-saturated solution the binding of the oxygen molecule to the MnŽIII. atom increases the distance between the plane of the PPh ring and MnŽIII., increasing the dipole moment. The electrostatic repulsion between positively charged substituents and the metal atom in MnŽIII.TMPyP molecules should increase the distance of MnŽIII. from the ring plane, increasing the dipole moment even more. On the other hand, in MnŽIII.TPPS 4 the attraction between the negatively charged substituents and MnŽIII. should decrease this value. This explains the data obtained for samples P4 and P9, with sample P4 being, as expected, the one that presents the higher value of n 2 . FeŽIII.PPh complexes in aqueous solution are present as several species in equilibrium, depending on pH w16–18x, and may form m-oxo-dimers, as well. It was shown in Ref. w19x that at pH F 4.0 these porphyrins exist in a symmetric monomeric form H 2 OFeŽIII.PPhOH 2 4 . Similar to MnŽIII.PPh the interaction between positively charged FeŽIII. and substituents affects the metal atom distance from the ring plane, increasing it for FeŽIII.TMPyP and decreasing for FeŽIII.TPPS 4 . Moreover, FeŽIII.TMPyP should exist in this pH range in a nonsymmetric form H 2 OFeŽIII.PPh4 characterized by higher FeŽIII. distance from the ring plane and thus higher permanent dipole moment perpendicular to the ring plane. This explains a higher n 2 value for sample P2 as compared to that for P7. At pH G 7.8 FeŽIII.PPh form m-oxo-dimers w19x which are symmetric in all directions. Therefore the n 2 values for FeŽIII.PPh at pH 9.0 ŽP3 and P8. are small. A more detailed study, using the Z-scan technique, as a function of pH value for the FeŽIII. complexes has been carried out in our laboratory and will be reported elsewhere w20x.
The values of the nonlinear absorption coefficients are small Žbelow our detection limit. for the metal PPh complexes and slightly higher for their free bases, where nonlinear refraction is negligible. As mentioned above, the nonlinear absorption is due to the singlet andror triplet excited state absorption and therefore increases with the increase of lifetimes and quantum yields of these states w2,4x. The presence of a paramagnetic atom reduces these characteristics due to the intramolecular paramagnetic quenching and hence decreases the probability of nonlinear absorption of the porphyrin metal complexes as compared with their free-base forms. In order to confirm the above analysis of the nonlinear refractive measurements in relation to a dipole moment, a time-resolved experiment was performed to compare the time response of the nonlinearity of a metal complex and a nonmetal one. As the presence of the metal in the central atom induces it to be out of the plane of the PPh ring, it should lead to a reorientational nonlinearity possibly dominating the overall nonlinearity, which can have contributions from other origins, even electronic. Therefore, an OKG experiment can reveal the time response of the nonlinearity. It is known that reorientational nonlinearities are characterized by time decays of several tens of picoseconds w12x. The OKG was setup using the light beam at 532 nm with 70 ps single pulses, which was split into two, a strong and a weak beam. The weak beam was passed through crossed polarizers, whereas the strong beam was set at 458 with respect to the weak beam input polarization. A slow detector connected to a boxcar obtained the data as a function of time delay of the strong pulse. The system was calibrated using a sample of semiconductor doped glass, whose time response at 532 nm is - 10 ps w21x. For the free-base form of PPh, a symmetric curve was observed with the 50 ps pulsewidth limited by the laser coherence time. Fig. 4a and b shows the measured Kerr signal as a function of delay for the PPh without the metal central atom and with MnŽIII. as the central atom in the ring, respectively. It can be clearly seen that the nonmetal PPh Kerr curve is symmetric, with the decay Žand rise. time of ; 18 ps, whilst the MnŽIII. porphyrin derivative presents a fast rise time of ; 12 ps and a two-component decay time, a fast one with ; 12 ps and a slow one with 90 ps. The fast
A.G. Bezerra Jr. et al.r Chemical Physics Letters 318 (2000) 511–516
515
original disordered configuration and that takes the reorientational time of ; 90 ps, as measured.
4. Conclusions In summary, we have experimentally studied the refractive and absorptive nonlinearities in porphyrin derivatives in the picosecond regime using visible Ž532 nm. excitation. Using the Z-scan technique, we obtained the sign and magnitude of the nonlinearities, and the data analysis could be performed in the light of structural changes in the porphyrin molecule taking into account the central atom substituent as well as the pH of the solution. Optical Kerr gate measurements complemented the analysis and led us to the conclusion that, in the metal complexes, the metal atom is located out of the plane and gives rise to a strong reorientational contribution to the optical nonlinearity.
Acknowledgements
Fig. 4. Optical Kerr gate signal as a function of delay time for: Ža. PPh without central metal atom and Žb. PPh with Mn as the central atom in the ring plane. The lines are fits to the decay curves.
The authors wish to acknowledge financial support to this work by the Brazilian Agencies FINEP, PRONEXrMCT Program, CNPq and FACEPE.
References decay time can be attributed to several mechanisms, including electronic or nuclear contributions. The shortening of the fast component Žfrom ; 18 to ; 12 ps. is not presently understood. A laser source delivering a shorter pulse is required to repeat the experiment and clarify this part of the decay time. However, the slow decay time of 90 ps is definitively characteristic of a reorientational nonlinearity, not limited by the laser pulse width or coherence time. As pointed out before, this should be expected – and therefore confirmed by our data – since the metal central atom comes out of the plane and, in the presence of an aligning electromagnetic field Žstrong beam., the dipole moments are aligned in the field direction. When the time delay is such that there is no longer interaction, the dipoles relax back to their
w1x S. Guha, K. Kang, P. Porter, J.F. Roach, D.E. Remy, F.J. Aranda, D.V.G.L.N. Rao, Opt. Lett. 17 Ž1992. 264. w2x K. Dou, X. Sun, X. Wang, R. Parkhill, Y. Guo, E.T. Knobbe, Solid State Commun. 107 Ž1998. 101. w3x S.R. Mishra, H.S. Rawat, M. Laghat, Opt. Commun. 147 Ž1998. 328. w4x K. Kandasamy, P.N. Puntambekar, B.P. Singh, S.J. Shetty, T.S. Srivastava, J. Nonlinear Opt. Phys. Mater. 6 Ž1997. 361. w5x R. Bonnett, Chem. Soc. Rev. 24 Ž1995. 19. w6x W. Sun, C.C. Byeon, C.M. Lawson, G.M. Gray, D. Wang, Appl. Phys. Lett. 74 Ž1999. 3254. w7x R.A. Norwood, J.R. Sounik, Appl. Phys. Lett. 60 Ž1992. 295. w8x D.V.G.L.N. Rao, F.J. Aranda, J.F. Roach, D.E. Remy, Appl. Phys. Lett. 58 Ž1991. 1241. w9x M. Sheik-Bahae, A.A. Said, E.W. Van Stryland, Opt. Lett. 14 Ž1989. 955. w10x M. Sheik-Bahae, A.A. Said, T.-H. Wei, D.J. Hagan, E.W. Van Stryland, IEEE J. Quantum Electron. 26 Ž1990. 760.
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w11x P.P. Ho, in: R.R. Alfano ŽEd.., Semiconductors Probed by Ultrafast Laser Spectroscopy, Vol. II, Academic Press, London, 1994, p. 1. w12x P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, Wiley, New York, 1991. w13x D. Dolphin ŽEd.., The Porphyrins, Vols. I–V, Academic Press, New York, 1978. w14x Z. Gryczynski, R. Paolesse, K.M. Smith, E. Bucci, Chem. Phys. Lett. 69 Ž1997. 71. w15x V. Knyushko, E. Zenkevich, E. Sagun, A. Shulga, S. Bachilo, Chem. Phys. Lett. 69 Ž1997. 71.
w16x S.B. Brown, M. Shillocock, P. Jones, Biochem. J. 153 Ž1976. 279. ¨ Almarsson, T.C. Bruice, J. Am. Chem. Soc. 117 Ž1995. w17x O. 4533. w18x P.K. Shantha, A.L. Verma, Inorg. Chem. 35 Ž1996. 2723. w19x V.E. Yushmanov, H. Imasato, T.T. Tominaga, M. Tabak, Inorg. Biochem. 61 Ž1996. 233. w20x A.G. Bezerra, Jr., I.E. Borissevitch, A.S.L. Gomes, C.B. de Araujo, ´ Opt. Lett. Ž1999., submitted. w21x L.H. Acioli, A.S.L. Gomes, C.B. de Araujo, ´ Electron. Lett. 25 Ž1989. 120.
Chemical Physics Letters 318 Ž2000. 511–516 www.elsevier.nlrlocatercplett
Investigation of picosecond optical nonlinearity in porphyrin metal complexes derivatives A.G. Bezerra Jr. 1, I.E. Borissevitch 2 , R.E. de Araujo, A.S.L. Gomes ) , Cid B. de Araujo ´ Departamento de Fısica, UniÕersidade Federal de Pernambuco, 50670-901 Recife, PE, Brazil ´ Received 6 September 1999; in final form 30 November 1999
Abstract Porphyrin metal complex derivatives were experimentally studied using the Z-scan technique and the optical Kerr gate method with picosecond pulses at 532 nm. The sign and magnitude of the nonlinear refraction and nonlinear absorption were determined for a negatively charged meso-tetrakisŽ p-sulfonatophenyl.porphyrin ŽTPPS 4 . and a positively charged mesotetrakisŽ4-N-methyl-pyridiniumyl.porphyrin ŽTMPyP. in their free-base forms and their FeŽIII. and MnŽIII. complexes. The results were phenomenologically understood and the origin of the nonlinearity was inferred. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Porphyrins and porphyrin derivatives, which are two-dimensional molecules characterized by highly developed p-conjugation, present nonlinear optical properties such as the intensity-dependent refraction index andror nonlinear absorption whose magnitude and time response depend upon the dominant physical process as well as the environment w1–8x. On the
) Corresponding author. Fax: q55-81-2710359; e-mail: [email protected] 1 Present address: Departamento de Fısica, Centro Federal de ´ Educac¸ao do Parana, ˜ Tecnologica ´ ´ CEFET-PR, 80230-901 Curitiba, PR, Brazil. 2 Present address: Departamento de Fısica e Matematica, Facul´ ´ dade de Filosofia, Ciencias e Letras de Ribeirao ˆ ˜ Preto, Universidade Sao ˜ Paulo, 14040-901, Ribeirao ˜ Preto, SP, Brazil.
other hand, porphyrins and porphyrin-like compounds have long been identified as important components of living nature. For example, the porphyrin derivative chlorophyll is the main molecule in the process of photosynthesis and porphyrin-like compounds are active centers of the cytochromes, essential in the oxidation chain. Porphyrins are also widely used as photosensitizers in photodynamic therapy of cancer w5x. Water-soluble porphyrins in their free-base form may exist in aqueous solutions in protonated or nonprotonated state while their metal complexes, with FeŽIII., for example, can bind other particles, such as OHy, H 2 O andror form physical aggregates and chemically bound m-oxo-dimers. These interactions can change significantly the electronic structure of the porphyrins and the studies of their nonlinear optical properties should give valuable information about these changes.
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 0 7 6 - 2
A.G. Bezerra Jr. et al.r Chemical Physics Letters 318 (2000) 511–516
512
2. Experimental In the present Letter, we report on the measurements of the nonlinear refractive index Žor Kerr coefficient., n 2 , which is proportional to the real part of the third-order nonlinear susceptibility, x Ž3. and of a 2 , the nonlinear absorption coefficient, which is proportional to the imaginary part of x Ž3. of two water-soluble porphyrins ŽPPh.: a negatively charged meso-tetrakisŽ p-sulfonatophenyl.porphyrin ŽTPPS 4 . and a positively charged meso-tetrakisŽ4-N-methylpyridiniumyl. porphyrin ŽTMPyP. in their free-base forms, and their FeŽIII. and MnŽIII. complexes. For these measurements, the Z-scan technique w9,10x with picosecond pulses was employed. To fully characterize the origin of the nonlinearity of these materials in the picosecond regime, we also performed a time-resolved experiment using the optical Kerr gate ŽOKG. technique w11x. From the data, we could understand the origin of the nonlinearity as well as infer conformational changes in the porphyrin derivatives studied. Fig. 1 shows the structures of the studied PPh complexes. TPPS 4 , TMPyP and their FeŽIII. and MnŽIII. complexes were obtained from Midcentury Chemicals ŽUSA. and used as purchased. The PPh were dissolved in Milli-Q quality water and the PPh concentration was controlled spectrophotometrically. Fig. 2 shows typical linear absorption spectra for three of the samples studied, measured on a commercial spectrophotometer. They are typical of porphyrin derivatives, with the main absorption peak due to the Soret band between 420 and 470 nm, depending upon pH. The linear absorption coefficient at the pump wavelength was kept at ; 2 cmy1 for all the samples adjusting the PPh concentration. It is clear, from Fig. 2, that there is no absorption peak at 532
Fig. 1. The chemical structure of the porphyrin complexes.
Fig. 2. Linear absorption spectra of: Ža. FeŽIII.TPPS 4 , pH 9.0; Žb. TPPS 4 , pH 4.0; and Žc. MnŽIII.TMPyP, pH 7.0. The linear absorption coefficient was kept at ; 2 cmy1 .
nm. The pH changes were produced by the addition of HCl or NaOH stock solutions. The light source used was the second harmonic of a cw pumped Q-switched and mode-locked Nd:YAG laser delivering pulses of 70 ps ŽFWHM. at 532 nm. A single pulse at a 10 Hz repetition rate was selected using a ‘pulse picker’ and all experiments were made in a 2 mm glass cuvette at room temperature.
3. Results Fig. 3 shows a typical result of the Z-scan measurements with small aperture in front of the detector ŽFig. 3a. and with the aperture fully open ŽFig. 3b.. The nonlinear refractive index signature observed is characteristic of a self-defocusing medium whereas the open-aperture measurements demonstrate a small nonlinear absorption which is due to an excited state absorption process. The results of all the samples studied are summarized in Table 1 and an analysis of the data is very clarifying. First of all, we note that the magnitude of the nonlinear refractive index may reach the same order of magnitude as that of CS 2 Ž n 2 s 3.3 = 10y1 4 cm2rW w9,10x., a standard nonlinear optical material and its symmetrical shape shows that there is no significant nonlinear absorp-
A.G. Bezerra Jr. et al.r Chemical Physics Letters 318 (2000) 511–516
Fig. 3. Ža. Small-aperture Z-scan trace of PPh complex Žsample P4. and Žb. open-aperture Z-scan measurement of sample P5.
tion due to resonant one-photon mechanism. Since the aim of the present work is to exploit the optical methods to characterize the physical mechanisms
513
and structural changes in the molecule upon environment changes, the magnitude of the nonlinearity is not an issue. Hence, the values presented for some samples are given only to indicate the detection limit of our apparatus. The values of n 2 and a 2 shown in Table 1, depend on the porphyrin structure and the pH value. The measurable values of n 2 were observed only for the metal complexes FeŽIII.TMPyP Žsamples P2 and P3., MnŽIII.TMPyP Žsample P4. and MnŽIII.TPPS 4 Žsample P9.. For the free-base PPh forms Žsamples P1, P5, P6. the nonlinear effects in the refractive index were below our detection limit, as well as for FeŽIII.TPPS 4 porphyrin Žsamples P7 and P8.. The dependence of n 2 with pH is clearly seen for samples P2 and P3, although it could not be measured in samples P7 and P8. To qualitatively understand the observed features, we should have in mind that one of the important characteristics of a molecule which determines the third-order optical nonlinearity is its dipole moment w12x, which in turn is determined by the space symmetry of the molecule. The free-base form of the studied porphyrins was completely symmetric in the plane of the p-conjugated ring, and cannot produce a dipole moment in this plane. However, it is possible to have a dipole moment in the direction perpendicular to this plane if the central metal atom or protons come out-of-plane, which is the case for the metaloporphyrins. Out-of-plane metal central atoms in porphyrins are known to happen and their characterization is currently under investigation w13–15x. The central nitrogen atoms of porphyrins in the metal-free form Žsamples P1, P5, P6. can be proto-
Table 1 Measured nonlinear refractive index, n 2 , and the nonlinear absorption coefficient, a 2 , for the porphyrin complex studied See text for identification of samples P1–P9 Porphyrin
R
X
pH
n2 Ž10y14 cm2rW.
a2 ŽcmrGW.
P1 P2 P3 P4 P5 P6 P7 P8 P9
I I I I II II II II II
2Hq FeŽIII. FeŽIII. MnŽIII. 2Hq 2Hq FeŽIII. FeŽIII. MnŽIII.
7 4 9 7 4 7 4 9 7
- y0.1 y0.6 - y0.1 y5.4 - y0.1 - y0.1 - y0.1 - y0.1 y0.3
1.2 - 0.1 - 0.1 - 0.1 0.7 0.6l - 0.1 - 0.1 0.2
514
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nated, depending on the porphyrin structure and pH. This protonation can be observed through changes in the linear absorption spectrum. In either case Žprotonated or nonprotonated., the structure of the free-base porphyrins are symmetric with respect to the ring plane and thus they have no dipole moment perpendicular to the ring plane. This explains why the n 2 values for these porphyrins are negligible independently of the state of protonation. In the MnŽIII.PPh complexes the metal atom is located out of the ring plane w13,14x and this produces a dipole moment perpendicular to the ring plane. Moreover, in air-saturated solution the binding of the oxygen molecule to the MnŽIII. atom increases the distance between the plane of the PPh ring and MnŽIII., increasing the dipole moment. The electrostatic repulsion between positively charged substituents and the metal atom in MnŽIII.TMPyP molecules should increase the distance of MnŽIII. from the ring plane, increasing the dipole moment even more. On the other hand, in MnŽIII.TPPS 4 the attraction between the negatively charged substituents and MnŽIII. should decrease this value. This explains the data obtained for samples P4 and P9, with sample P4 being, as expected, the one that presents the higher value of n 2 . FeŽIII.PPh complexes in aqueous solution are present as several species in equilibrium, depending on pH w16–18x, and may form m-oxo-dimers, as well. It was shown in Ref. w19x that at pH F 4.0 these porphyrins exist in a symmetric monomeric form H 2 OFeŽIII.PPhOH 2 4 . Similar to MnŽIII.PPh the interaction between positively charged FeŽIII. and substituents affects the metal atom distance from the ring plane, increasing it for FeŽIII.TMPyP and decreasing for FeŽIII.TPPS 4 . Moreover, FeŽIII.TMPyP should exist in this pH range in a nonsymmetric form H 2 OFeŽIII.PPh4 characterized by higher FeŽIII. distance from the ring plane and thus higher permanent dipole moment perpendicular to the ring plane. This explains a higher n 2 value for sample P2 as compared to that for P7. At pH G 7.8 FeŽIII.PPh form m-oxo-dimers w19x which are symmetric in all directions. Therefore the n 2 values for FeŽIII.PPh at pH 9.0 ŽP3 and P8. are small. A more detailed study, using the Z-scan technique, as a function of pH value for the FeŽIII. complexes has been carried out in our laboratory and will be reported elsewhere w20x.
The values of the nonlinear absorption coefficients are small Žbelow our detection limit. for the metal PPh complexes and slightly higher for their free bases, where nonlinear refraction is negligible. As mentioned above, the nonlinear absorption is due to the singlet andror triplet excited state absorption and therefore increases with the increase of lifetimes and quantum yields of these states w2,4x. The presence of a paramagnetic atom reduces these characteristics due to the intramolecular paramagnetic quenching and hence decreases the probability of nonlinear absorption of the porphyrin metal complexes as compared with their free-base forms. In order to confirm the above analysis of the nonlinear refractive measurements in relation to a dipole moment, a time-resolved experiment was performed to compare the time response of the nonlinearity of a metal complex and a nonmetal one. As the presence of the metal in the central atom induces it to be out of the plane of the PPh ring, it should lead to a reorientational nonlinearity possibly dominating the overall nonlinearity, which can have contributions from other origins, even electronic. Therefore, an OKG experiment can reveal the time response of the nonlinearity. It is known that reorientational nonlinearities are characterized by time decays of several tens of picoseconds w12x. The OKG was setup using the light beam at 532 nm with 70 ps single pulses, which was split into two, a strong and a weak beam. The weak beam was passed through crossed polarizers, whereas the strong beam was set at 458 with respect to the weak beam input polarization. A slow detector connected to a boxcar obtained the data as a function of time delay of the strong pulse. The system was calibrated using a sample of semiconductor doped glass, whose time response at 532 nm is - 10 ps w21x. For the free-base form of PPh, a symmetric curve was observed with the 50 ps pulsewidth limited by the laser coherence time. Fig. 4a and b shows the measured Kerr signal as a function of delay for the PPh without the metal central atom and with MnŽIII. as the central atom in the ring, respectively. It can be clearly seen that the nonmetal PPh Kerr curve is symmetric, with the decay Žand rise. time of ; 18 ps, whilst the MnŽIII. porphyrin derivative presents a fast rise time of ; 12 ps and a two-component decay time, a fast one with ; 12 ps and a slow one with 90 ps. The fast
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original disordered configuration and that takes the reorientational time of ; 90 ps, as measured.
4. Conclusions In summary, we have experimentally studied the refractive and absorptive nonlinearities in porphyrin derivatives in the picosecond regime using visible Ž532 nm. excitation. Using the Z-scan technique, we obtained the sign and magnitude of the nonlinearities, and the data analysis could be performed in the light of structural changes in the porphyrin molecule taking into account the central atom substituent as well as the pH of the solution. Optical Kerr gate measurements complemented the analysis and led us to the conclusion that, in the metal complexes, the metal atom is located out of the plane and gives rise to a strong reorientational contribution to the optical nonlinearity.
Acknowledgements
Fig. 4. Optical Kerr gate signal as a function of delay time for: Ža. PPh without central metal atom and Žb. PPh with Mn as the central atom in the ring plane. The lines are fits to the decay curves.
The authors wish to acknowledge financial support to this work by the Brazilian Agencies FINEP, PRONEXrMCT Program, CNPq and FACEPE.
References decay time can be attributed to several mechanisms, including electronic or nuclear contributions. The shortening of the fast component Žfrom ; 18 to ; 12 ps. is not presently understood. A laser source delivering a shorter pulse is required to repeat the experiment and clarify this part of the decay time. However, the slow decay time of 90 ps is definitively characteristic of a reorientational nonlinearity, not limited by the laser pulse width or coherence time. As pointed out before, this should be expected – and therefore confirmed by our data – since the metal central atom comes out of the plane and, in the presence of an aligning electromagnetic field Žstrong beam., the dipole moments are aligned in the field direction. When the time delay is such that there is no longer interaction, the dipoles relax back to their
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