- Email: [email protected]
Third-order optical nonlinearities of near-infrared dyes
28 January 2000
Chemical Physics Letters 317 Ž2000. 9–12 www.elsevier.nlrlocatercplett
Third-order optical nonlinearities of near-infrared dyes Zhifei Dai b
a,)
, Xiuli Yue b, Bixian Peng a , Qiguang Yang c , Xuchun Liu c , Peixian Ye c
a Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101 China Basic Science and Information Engineering College, Beijing Forestry UniÕersity, Beijing 100083 China c Institute of Physics, Chinese Academy of Sciences, Beijing 100080 China
Received 30 August 1999
Abstract Near-infrared dyes – di- and tri-indocarbocyanine dyes and nickel dithiolenes – were synthesized and their third-order optical nonlinearities in solutions were investigated by using the degenerate four-wave mixing technique. Large third-order nonlinear optical susceptibilities with g values of up to 10y29 esu were observed. The g value of the indocarbocyanine dyes increases with increase of conjugation length of the polymethine chain. The central ring structure in the chromophore lowers the g value of the indocarbocyanine dyes. With change of the substituents on the benzene ring an increase in g value is observed for all the asymmetric nickel dithiolenes. The order of increase of nonlinear hyperpolarizability is as follows: H - Cl - CH 3 - C 2 H 5 - CHŽCH 3 . 2 . q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Third-order nonlinear optical ŽNLO. processes have attracted considerable interest recently because of their potential applications in optical phase conjugation, optical computing, dynamic holography and their power as a spectroscopic tool w1x. Organic materials are attractive because of their ultrafast, broad band responses and low absorption. However, the main problem with the materials studied, e.g. polydiacetylenes and main chain polymers w2,3x, has been the small nonlinear coefficients. Over the past ten years there has been increasing interest in the search for molecular materials exhibiting large third-linear optical effects. Conjugated organic molecules and polymers, which have large and fast ) Corresponding author. Fax: q81-0798-54-6397; e-mail: [email protected]
optical nonlinearities due to a delocalized p electron system, have potential applications in nonlinear optical devices w4x. Hermann and Ducuing measured the thirdharmonic generation of polyene and found a dramatic enhancement of third-order hyperpolarizability with increasing number of double bonds w5x. This enhancement was attributed to the delocalization of p electrons over the molecular chain. As reported for aromatic molecules, the optical nonlinearities are expected to increase by substituting electron donors through the electric-induced effects and the charge transfer effects w4x. An extensive series of studies have made for the NLO properties of square coplanar dithiolenes w6,7x. These compounds were identified for study due to the presence of an extensive delocalized p system within the molecule and the presence of a strong charge-transfer band in the near-IR, whose position
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 9 9 . 0 1 3 7 2 - X
Z. Dai et al.r Chemical Physics Letters 317 (2000) 9–12
10
In the present Letter, we synthesized asymmetric nickel dithiolenes, symmetric di- and tri-indocarbocyanine dyes, as reported by the authors in the literature w13,14x, the molecular structures of which are depicted in Fig. 1, and investigated their thirdorder optical nonlinearities by using degenerate four-wave mixing ŽDFWM.. It was the first time to demonstrate the substituent effect on the third-order hyperpolarizabilities of asymmetric nickel dithiolene complexes. 2. Experimental DFWM was used to evaluate g values of the dyes. The experimental setup was described in detail elsewhere w15x. As light source, we used a modelocked, Q-switched, amplified Nd:YAG laser operating at a repetition rate of 10 Hz, in which a single 8 ns pulse was selected from the pulse train at l s 1064 nm. It should be noted that thermal grating was avoided by the cross-polarization configuration and thus the obtained g was of electron origin. A sample dissolved in 1,2-dichloroethane at a concentration of ; 10y5 mol ly1 was placed in a 1 mm thick quartz cell. g values were obtained by the pump-beam intensity dependence on the phase-conjugated reflectivity by the comparison with the value for THF w x Ž3. s 2.85 = 10y13 esux. The third-order nonlinear susceptibility was calculated from
Fig. 1. Molecular structures of near-infrared dyes.
can be varied by changing the central metal and the substituents on the ligand. These studies have shown that at 1064 nm the compounds with an absorption band maximum around 800 nm exhibit high g values corresponding to low values of linear and nonlinear two-photon absorption. The fundamental studies on the relationship between molecular structures and third-order optical nonlinearities should be done in small molecules because sufficient molecular designs for third-order NLO materials have never been found yet w8x. Ikeda et al. w9x and Sastre et al. w10x studied third-order optical nonlinearities of asymmetric carbocyanine dyes and phthalocyanine dyes, respectively. Winter et al. w11x and Calvert w12x indicated that nickel dithiolene complexes showed great promise as third-order NLO materials. However, the relationship between their substituents and third-order nonlinearities has seldom been studied systematically.
Ž3. x sample
s
ž
2
n sample
/ž
n THF
Isample I THF
1r2
/
aL 1 y ey a L
Ž3. e a L r2x THF ,
Ž 1. where I is the pump beam intensity, a the linear absorption coefficient, n the linear refractive index, and L the sample length.
Table 1 Third-order hyperpolarizabilities at 1064 nm and visiblernear-infrared absorption maxima 1
lma x Žnm. ´ Ž10 5 My1 cmy1 . a Žat 1064 nm. Žcmy1 . g Ž10y3 0 esu. a
In ethanol.
2 a
642 1.68 0 0.018
3 a
738 1.75 0 0.176
a
768 2.50 0 0.107
4a
4b
4c
4d
4e
4f
855 0.302 0.013 14.20
861 0.354 0 8.34
856 0.368 0 19.24
870 0.386 0 25.06
868 0.333 0.002 25.79
868 0.293 0.0054 33.10
Z. Dai et al.r Chemical Physics Letters 317 (2000) 9–12
For molecular systems the parameter scale is the hyperpolarizability. The third-order hypepolarizability Žg . is related to x Ž3. by
x Ž3. Ž3. gsample s
T 4 N0
,
Ž 2.
where N0 is the number density of dye molecules, related to the concentration c by N0 s NA c with NA being Avogadro’s number. For the calculations the formula for spherical molecules T s Ž n2sample q 2.r3 was used, which is sufficiently accurate to describe the studied molecules within the experimental errors.
3. Results and discussion The obtained g values for the dyes and nickel dithiolenes are listed in Table 1 together with the visible absorption maximaŽ lmax . in 1,2-dichloroethane and ethanol solutions. The length of the polymethine chain is considered to be the prime factor which determines the cyanine dye’s absorption wavelength. Compared with dye 1, for the corporation of a –C5C group in the polymethine chain the wavelength of dye 2 is found to increase by a magnitude of ; 96 nm. However, as the length of the polymethine chain increases, the dye 2 become less stable. All the nickel dithiolenes exhibited a very strong absorption band in the region 835–877 nm, arising from a low-energy p–p ) transition. The position of the band varies with the substituents on the benzene ring or charge state of the dithiolene as a result of a complex interplay of the relative energies of the ligand p system. We reported in the literature w13x that a reasonable linear relationship exists between the l max values and the Hammett substituent constants w sp ŽX. q sp ŽH..r2x for asymmetric substituted nickel dithiolenes. These complexes show a pronounced hypsochromic effect relative to the corresponding symmetric nickel complexes. It is clear therefore that it is possible to tune the position of this absorption band to a desired wavelength in order to create the optimum trade-off between near resonance and low a , as required in NLO devices. For all these compounds there is a long, slowly decreasing tail on the low-energy side of the absorption band which extends into the near-infrared and results
11
in varying amounts of absorption at the excitation wavelength of 1064 nm. All these nickel dithiolenes except for the unsubstituted compound 4a, which was studied by Winter et al. w11x, show low or no absorption at 1064 nm and two-photon absorption does not seem to be significant. Thus population effects were rather weak and the nonlinear susceptibility was obtained from electron effects. Compared with the indodicarbocyanine 1, the g value of the indotricarbocyanine 2 has a drastic enhancement with increasing conjugation length. This result reveals that the effective delocalization of p-electrons enhances the their-order hyperpolarizabilities. Comparison of dyes 2 and 3 shows that the g value of the former was larger than that of dye 3. The central ring structure in the chromophore of dye 3 increases its chemical and light stability. However, this kind of rigid structure favors the formation of a local charge transfer in the central ring rather than along the main conjugated chain and lowers its g value due to a disruption of the p-electron system. Obviously, the nature of the substituents on the benzene ring is important for the nickel dithiolenes, as is illustrated in Table 1. Comparison of the chloro-substituted dithiolenes 4b and 4c shows that the asymmetric nickel dithiolene 4c enhances the molecular nonlinearity far better than the symmetric nickel dithiolene 4b. In addition, there is a substantially blue-shifted charge transfer band connected with the better charge-asymmetry in compound 4c. This result suggests an advantage of asymmetric structures for optical nonlinearities. With the change of the substituents on the benzene ring an increase in g value is observed for all asymmetric nickel dithiolenes, in contrast to the unsubstituted compound 4a, whether the substituent is electron-withdrawing or -donating, and the order of enhancement of nonlinear susceptibility is as follows: 4b - 4a - 4c - 4d - 4e - 4f In conclusion, the introduction of asymmetric structure to the nickel dithiolenes has enhanced the g value. The g value of the indocarbocyanine increases with increasing polymethine length and decreases due to the incorporation of the central ring into the polymethine chain. The nickel dithiolenes have good photochemical stability and overcome the risk of degradation due to long-term irradiation. In
12
Z. Dai et al.r Chemical Physics Letters 317 (2000) 9–12
contrast to the corresponding symmetric nickel dithiolenes, the asymmetric nickel dithiolenes have a lower melt point and better solubility in organic solvents w13x, and have great promise as third-order NLO materials. This family of materials has the drawback of bad processability, but it might be overcome by better synthetic chemistry in the future. A further improvement in their efficiency might come from tuning the structure to shift the absorption peak to optimize the balance of absorption and nonlinearity. These materials may be built into guest–host or side-chain polymer systems so that good films can be readily cast from solution. This would improve the processability for waveguide devices. References w1x Y.R. Shen, The Principles of Nonlinear Optics, Wiley, New York, 1984.
w2x G.M. Carter, Y.J. Chen, S.K. Tripathy, Opt. Eng. 24 Ž1985. 609. w3x D.N. Rao, J. Swiatkiewicz, P. Chopra, S.K. Ghoshal, Appl. Phys. Lett. 48 Ž1986. 1187. w4x D.S. Chemla, J. Zyss, Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press, New York, 1987. w5x J.P. Hermann, J. Ducuing, J. Appl. Phys. 45 Ž1997. 5100. w6x J. Zuo, T. Yao, X. You, J. Mater. Chem. 6 Ž1996. 1633. w7x C.S. Winter, C.A.S. Hill, A.E. Underhill, Appl. Phys. Lett. 58 Ž1991. 107. w8x S.T. Kowel, L. Ye, Y. Zhang, L.M. Hayden, Opt. Eng. 26 Ž1987. 107. w9x H. Ikeda, T. Sakai, K. Kawasaki, Chem. Lett. Ž1991. 1075. w10x A. Sastre, M.A. Diaz-Garcia, B. del Rey, C. Dhenaut et al., J. Phys. Chem. A 101 Ž1997. 9773. w11x C.S. Winter, S.N. Oliver, J.D. Rush, R.J. Manning, C. Hill, A. Underhill, Am. Chem. Soc. Symp. on Novel Nonlinear Optical Materials, 1990, Chap. 41, p. 617. w12x P. Calvert, Nature ŽLondon. 350 Ž1991. 114. w13x D. Zhifei, P. Bixian, Dyes Pigments 35 Ž1997. 23. w14x D. Zhifei, P. Bixian, Dyes Pigments 36 Ž1998. 169. w15x S. Jinhai, Y. Qiguang, Y. Peixian, Opt. Commun. 132 Ž1996. 311.
Chemical Physics Letters 317 Ž2000. 9–12 www.elsevier.nlrlocatercplett
Third-order optical nonlinearities of near-infrared dyes Zhifei Dai b
a,)
, Xiuli Yue b, Bixian Peng a , Qiguang Yang c , Xuchun Liu c , Peixian Ye c
a Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101 China Basic Science and Information Engineering College, Beijing Forestry UniÕersity, Beijing 100083 China c Institute of Physics, Chinese Academy of Sciences, Beijing 100080 China
Received 30 August 1999
Abstract Near-infrared dyes – di- and tri-indocarbocyanine dyes and nickel dithiolenes – were synthesized and their third-order optical nonlinearities in solutions were investigated by using the degenerate four-wave mixing technique. Large third-order nonlinear optical susceptibilities with g values of up to 10y29 esu were observed. The g value of the indocarbocyanine dyes increases with increase of conjugation length of the polymethine chain. The central ring structure in the chromophore lowers the g value of the indocarbocyanine dyes. With change of the substituents on the benzene ring an increase in g value is observed for all the asymmetric nickel dithiolenes. The order of increase of nonlinear hyperpolarizability is as follows: H - Cl - CH 3 - C 2 H 5 - CHŽCH 3 . 2 . q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Third-order nonlinear optical ŽNLO. processes have attracted considerable interest recently because of their potential applications in optical phase conjugation, optical computing, dynamic holography and their power as a spectroscopic tool w1x. Organic materials are attractive because of their ultrafast, broad band responses and low absorption. However, the main problem with the materials studied, e.g. polydiacetylenes and main chain polymers w2,3x, has been the small nonlinear coefficients. Over the past ten years there has been increasing interest in the search for molecular materials exhibiting large third-linear optical effects. Conjugated organic molecules and polymers, which have large and fast ) Corresponding author. Fax: q81-0798-54-6397; e-mail: [email protected]
optical nonlinearities due to a delocalized p electron system, have potential applications in nonlinear optical devices w4x. Hermann and Ducuing measured the thirdharmonic generation of polyene and found a dramatic enhancement of third-order hyperpolarizability with increasing number of double bonds w5x. This enhancement was attributed to the delocalization of p electrons over the molecular chain. As reported for aromatic molecules, the optical nonlinearities are expected to increase by substituting electron donors through the electric-induced effects and the charge transfer effects w4x. An extensive series of studies have made for the NLO properties of square coplanar dithiolenes w6,7x. These compounds were identified for study due to the presence of an extensive delocalized p system within the molecule and the presence of a strong charge-transfer band in the near-IR, whose position
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 9 9 . 0 1 3 7 2 - X
Z. Dai et al.r Chemical Physics Letters 317 (2000) 9–12
10
In the present Letter, we synthesized asymmetric nickel dithiolenes, symmetric di- and tri-indocarbocyanine dyes, as reported by the authors in the literature w13,14x, the molecular structures of which are depicted in Fig. 1, and investigated their thirdorder optical nonlinearities by using degenerate four-wave mixing ŽDFWM.. It was the first time to demonstrate the substituent effect on the third-order hyperpolarizabilities of asymmetric nickel dithiolene complexes. 2. Experimental DFWM was used to evaluate g values of the dyes. The experimental setup was described in detail elsewhere w15x. As light source, we used a modelocked, Q-switched, amplified Nd:YAG laser operating at a repetition rate of 10 Hz, in which a single 8 ns pulse was selected from the pulse train at l s 1064 nm. It should be noted that thermal grating was avoided by the cross-polarization configuration and thus the obtained g was of electron origin. A sample dissolved in 1,2-dichloroethane at a concentration of ; 10y5 mol ly1 was placed in a 1 mm thick quartz cell. g values were obtained by the pump-beam intensity dependence on the phase-conjugated reflectivity by the comparison with the value for THF w x Ž3. s 2.85 = 10y13 esux. The third-order nonlinear susceptibility was calculated from
Fig. 1. Molecular structures of near-infrared dyes.
can be varied by changing the central metal and the substituents on the ligand. These studies have shown that at 1064 nm the compounds with an absorption band maximum around 800 nm exhibit high g values corresponding to low values of linear and nonlinear two-photon absorption. The fundamental studies on the relationship between molecular structures and third-order optical nonlinearities should be done in small molecules because sufficient molecular designs for third-order NLO materials have never been found yet w8x. Ikeda et al. w9x and Sastre et al. w10x studied third-order optical nonlinearities of asymmetric carbocyanine dyes and phthalocyanine dyes, respectively. Winter et al. w11x and Calvert w12x indicated that nickel dithiolene complexes showed great promise as third-order NLO materials. However, the relationship between their substituents and third-order nonlinearities has seldom been studied systematically.
Ž3. x sample
s
ž
2
n sample
/ž
n THF
Isample I THF
1r2
/
aL 1 y ey a L
Ž3. e a L r2x THF ,
Ž 1. where I is the pump beam intensity, a the linear absorption coefficient, n the linear refractive index, and L the sample length.
Table 1 Third-order hyperpolarizabilities at 1064 nm and visiblernear-infrared absorption maxima 1
lma x Žnm. ´ Ž10 5 My1 cmy1 . a Žat 1064 nm. Žcmy1 . g Ž10y3 0 esu. a
In ethanol.
2 a
642 1.68 0 0.018
3 a
738 1.75 0 0.176
a
768 2.50 0 0.107
4a
4b
4c
4d
4e
4f
855 0.302 0.013 14.20
861 0.354 0 8.34
856 0.368 0 19.24
870 0.386 0 25.06
868 0.333 0.002 25.79
868 0.293 0.0054 33.10
Z. Dai et al.r Chemical Physics Letters 317 (2000) 9–12
For molecular systems the parameter scale is the hyperpolarizability. The third-order hypepolarizability Žg . is related to x Ž3. by
x Ž3. Ž3. gsample s
T 4 N0
,
Ž 2.
where N0 is the number density of dye molecules, related to the concentration c by N0 s NA c with NA being Avogadro’s number. For the calculations the formula for spherical molecules T s Ž n2sample q 2.r3 was used, which is sufficiently accurate to describe the studied molecules within the experimental errors.
3. Results and discussion The obtained g values for the dyes and nickel dithiolenes are listed in Table 1 together with the visible absorption maximaŽ lmax . in 1,2-dichloroethane and ethanol solutions. The length of the polymethine chain is considered to be the prime factor which determines the cyanine dye’s absorption wavelength. Compared with dye 1, for the corporation of a –C5C group in the polymethine chain the wavelength of dye 2 is found to increase by a magnitude of ; 96 nm. However, as the length of the polymethine chain increases, the dye 2 become less stable. All the nickel dithiolenes exhibited a very strong absorption band in the region 835–877 nm, arising from a low-energy p–p ) transition. The position of the band varies with the substituents on the benzene ring or charge state of the dithiolene as a result of a complex interplay of the relative energies of the ligand p system. We reported in the literature w13x that a reasonable linear relationship exists between the l max values and the Hammett substituent constants w sp ŽX. q sp ŽH..r2x for asymmetric substituted nickel dithiolenes. These complexes show a pronounced hypsochromic effect relative to the corresponding symmetric nickel complexes. It is clear therefore that it is possible to tune the position of this absorption band to a desired wavelength in order to create the optimum trade-off between near resonance and low a , as required in NLO devices. For all these compounds there is a long, slowly decreasing tail on the low-energy side of the absorption band which extends into the near-infrared and results
11
in varying amounts of absorption at the excitation wavelength of 1064 nm. All these nickel dithiolenes except for the unsubstituted compound 4a, which was studied by Winter et al. w11x, show low or no absorption at 1064 nm and two-photon absorption does not seem to be significant. Thus population effects were rather weak and the nonlinear susceptibility was obtained from electron effects. Compared with the indodicarbocyanine 1, the g value of the indotricarbocyanine 2 has a drastic enhancement with increasing conjugation length. This result reveals that the effective delocalization of p-electrons enhances the their-order hyperpolarizabilities. Comparison of dyes 2 and 3 shows that the g value of the former was larger than that of dye 3. The central ring structure in the chromophore of dye 3 increases its chemical and light stability. However, this kind of rigid structure favors the formation of a local charge transfer in the central ring rather than along the main conjugated chain and lowers its g value due to a disruption of the p-electron system. Obviously, the nature of the substituents on the benzene ring is important for the nickel dithiolenes, as is illustrated in Table 1. Comparison of the chloro-substituted dithiolenes 4b and 4c shows that the asymmetric nickel dithiolene 4c enhances the molecular nonlinearity far better than the symmetric nickel dithiolene 4b. In addition, there is a substantially blue-shifted charge transfer band connected with the better charge-asymmetry in compound 4c. This result suggests an advantage of asymmetric structures for optical nonlinearities. With the change of the substituents on the benzene ring an increase in g value is observed for all asymmetric nickel dithiolenes, in contrast to the unsubstituted compound 4a, whether the substituent is electron-withdrawing or -donating, and the order of enhancement of nonlinear susceptibility is as follows: 4b - 4a - 4c - 4d - 4e - 4f In conclusion, the introduction of asymmetric structure to the nickel dithiolenes has enhanced the g value. The g value of the indocarbocyanine increases with increasing polymethine length and decreases due to the incorporation of the central ring into the polymethine chain. The nickel dithiolenes have good photochemical stability and overcome the risk of degradation due to long-term irradiation. In
12
Z. Dai et al.r Chemical Physics Letters 317 (2000) 9–12
contrast to the corresponding symmetric nickel dithiolenes, the asymmetric nickel dithiolenes have a lower melt point and better solubility in organic solvents w13x, and have great promise as third-order NLO materials. This family of materials has the drawback of bad processability, but it might be overcome by better synthetic chemistry in the future. A further improvement in their efficiency might come from tuning the structure to shift the absorption peak to optimize the balance of absorption and nonlinearity. These materials may be built into guest–host or side-chain polymer systems so that good films can be readily cast from solution. This would improve the processability for waveguide devices. References w1x Y.R. Shen, The Principles of Nonlinear Optics, Wiley, New York, 1984.
w2x G.M. Carter, Y.J. Chen, S.K. Tripathy, Opt. Eng. 24 Ž1985. 609. w3x D.N. Rao, J. Swiatkiewicz, P. Chopra, S.K. Ghoshal, Appl. Phys. Lett. 48 Ž1986. 1187. w4x D.S. Chemla, J. Zyss, Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press, New York, 1987. w5x J.P. Hermann, J. Ducuing, J. Appl. Phys. 45 Ž1997. 5100. w6x J. Zuo, T. Yao, X. You, J. Mater. Chem. 6 Ž1996. 1633. w7x C.S. Winter, C.A.S. Hill, A.E. Underhill, Appl. Phys. Lett. 58 Ž1991. 107. w8x S.T. Kowel, L. Ye, Y. Zhang, L.M. Hayden, Opt. Eng. 26 Ž1987. 107. w9x H. Ikeda, T. Sakai, K. Kawasaki, Chem. Lett. Ž1991. 1075. w10x A. Sastre, M.A. Diaz-Garcia, B. del Rey, C. Dhenaut et al., J. Phys. Chem. A 101 Ž1997. 9773. w11x C.S. Winter, S.N. Oliver, J.D. Rush, R.J. Manning, C. Hill, A. Underhill, Am. Chem. Soc. Symp. on Novel Nonlinear Optical Materials, 1990, Chap. 41, p. 617. w12x P. Calvert, Nature ŽLondon. 350 Ž1991. 114. w13x D. Zhifei, P. Bixian, Dyes Pigments 35 Ž1997. 23. w14x D. Zhifei, P. Bixian, Dyes Pigments 36 Ž1998. 169. w15x S. Jinhai, Y. Qiguang, Y. Peixian, Opt. Commun. 132 Ž1996. 311.