- Email: [email protected]
First-order hyperpolarizabilities of octupolar aromatic molecules: symmetrically substituted triazines
29 September1995
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical PhysicsLetters244 (1995) 153-156
First-order hyperpolarizabilities of octupolar aromatic molecules: symmetrically substituted triazines Paresh Chandra Ray, Puspendu Kumar Das Department of lnorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
Received28 April 1995; in final form 27 July 1995
Abstract
The first hyperpolarizabilities of some symmetrically substituted triazines have been measured and compared with those of the corresponding symmetrically substituted benzenes. The octupolar triazines have higher quadratic polarizabilities than the corresponding octupolar benzenes. The triazine ring seems to be a better central acceptor than the benzene ring, but if it acts as a donor as in sym-triphenyl triazine, the nonlinearity improves further.
1. Introduction
The second-order nonlinear optical (NLO) properties of dipolar molecules are a widely investigated area of research. Experiments have been carried out [1-3] to develop a better understanding of the relationship between molecular structure and NLO properties and to provide guidelines for the design and synthesis of new types of molecules with enhanced /3. Although there are many examples of dipolar molecules exhibiting extremely large hyperpolarizabilities, some major limitations persist in utilizing them for NLO. For example, more often than not dipolar interactions force the molecules to pack in a centrosymmetric lattice [4] or lead to undesirable aggregation. Octupolar molecules which have been recognized only recently as potentially useful for nonlinear applications are devoid of the above-mentioned disadvantages. Due to the absence of ground state dipole moment, these molecules [5-7] cannot be oriented by applying an electric field. Thus the well known
electric field induced second harmonic generation (EFISHG) [8] and poling techniques [9] are not applicable to octupolar molecules for measuring the microscopic and macroscopic second-order polarizabilities, respectively. As a consequence, octupolar molecules have not yet been investigated thoroughly for efficient second harmonic generation. Ledoux et al. [10] have reported that triaminotrinitrobenzene which is octupolar, shows three times higher SHG efficiency than urea in powder form. The hyperRayleigh scattering (HRS) technique [11-15] provides a means of measuring /3 in these nonpolar molecules. Zyss et al. [16] and Verbiest et al. [17,18] have reported large hyperpolarizabilities in some octupolar molecules and sometimes even more than in the corresponding dipolar molecules. In this Letter, we report a systematic study of the second-order NLO properties of a series of octupolar substituted triazines and the corresponding substituted benzenes. Theoretical estimates of /3 by AM1 as well as ab initio methods are also compared with the experimental results.
0009-2614/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved SSDI 0009-2614(95)00913-2
154
P.C. Ray, P.K. Das / Chemical Physics Letters 244 (1995) 153-156 200
2. Theoretical treatment The treatment of octupolar molecules for quadratic NLO has been developed and discussed by Zyss and others [5-7] previously. In short, any tensor T (") of arbitrary rank n can be decomposed in a sum of irreducible tensorial components TJ,m (") and can be expressed as
"~160
/
J
T(")= ~ 2El Tj!",,, ) j=0
m=-j
(1)
It turns out from Eq. (1) that /3 (or X (2)) will only have two irreducible components of order 1 (vector or dipolar part) and 3 (septor or octupolar part),
/3 =/3j=~ +/3j=3.
(2)
Each component has 2j + 1 independent coefficients. Zyss [6] has shown that since octupolar molecules belong to an octupolar space group ( j = 3), all dipolar quantities (j = 1) vanish, and only octupolar contributions to /3 remain. Purely octupolar groups are, for example, the orthorombic D2, the hexagonal D3h and the cubic T symmetry groups. In the HRS measurements the averaged (/3 2> value of the rank 6 tensor 13 ×/3 will depend on the symmetry of the molecule, whereas the polarization of the fundamental laser beam and the presence or absence of an analyzer at the second harmonic wavelength will determine the /3 ×/3 coefficients to be averaged. A general method for obtaining the terms has been proposed by Cyvin et al. [19] and its application to nonlinear octupoles in solution described by Zyss et al. [5-7]. Application of this method in the case of unpolarized observations for molecules with D3h symmetry gives <
2 /3D 3h> -82T/3}yy. =
(3)
The /3yyycomponentsof the first hyperpolarizability tensor of all the substituted triazines and benzenes in benzene, chloroform, methanol and water (for only compounds 2, 3, 6 and 8 which are soluble) were obtained from the measured average /3. At low solute concentrations, assumingthat the solvent concentration does not change significantly, 12,~/12o = G is a linear function of the solute concentration (Fig. l). From the slope and intercept of this
0
I 8
I
1 12 Im
I
I
16
20
Nsolute(10molecutes/c.c.) Fig. 1. 12o,/I~ versus number density plot of compound 4 at room temperature in methanol.
plot, for the solute is obtained with the solvent value as a reference.
3. Experimental All the compounds (Fig. 2) were purchased from Aldrich, USA and the solvents were obtained commercially and purified using standard procedures. The apparatus for the /3 measurements has been described in detail elsewhere [20]. In brief, the fundamental of a Q-switched Nd:YAG laser (Spectra
Y
Y
Y~/
Y
Compound
Y
Compound
Y
!
Cl
8
Cl
2
OH
?
OH
3
OMe
8
OMe
4
SH
9
Me
5
Ph
I0
Ph
Fig. 2. Structure of compounds 1-10.
P.C. Ray, P.K. Das / Chemical Physics Letters 244 (1995) 153-156 Table 1 Calculated and measured first-order hyperpolarizabilities (103o esu) of compounds 1-10 in methanol Compound fl?M1 flgb initio flsabinitio ( ~)HRS [~yyy 1
2 3 4 5 6 7 8 9 10
3.2 4.6 5.8 4.9 6.1 2.0 2.1 2.9 1.4 3.2
2.9 5.2 6.2 5.4 6.4 1.9 2.1 2.8 1.5 3.4
3,7 5.6 6.9 5.9 7.0 2.2 2.4 3.5 1.8 4.2
7.8 9.0 13.0 11.3 16.0 3,1 3,6 5,8 2,2 7.1
12.6 14.6 21.1 18.3 25.9 5.0 5.8 9.4 3.6 11.5
Physics, 8 ns) is focused onto a glass cell containing the solute in solution. The second harmonic scattered light is collected through an efficient condenser system on the photocathode of a UV-VIS photomultiplier tube. A 3 nm bandwidth interference filter at 532 nm is used to isolate the green light. All data are collected after extensive signal-averaging at laser powers (~< 12 mJ/pulse) well below the threshold for stimulated Raman and Brillouin scattering, selffocusing or self-defocusing and dielectric breakdown.
4. Results and discussion The experimental /3 values are listed in Table 1. All triazine derivatives exhibit higher nonlinearity than the corresponding benzene analogs. The N atoms in the triazine ring each have a lone pair of electrons on the o'-plane which makes the central core more polarizable. In the presence of donors, the center acts as an acceptor as in benzene. However, the last example from the triazine derivatives, namely symtriphenyl triazine, has been predicted [6] to have optimized octupolar /3 components. Here we verify that the quadratic polarizability of this molecule is indeed high. In this example the triazine core probably acts as a donor and interacts with the surrounding acceptor (phenyl) groups in a manner similar to that in the tricyanomethanide anion. However, the first hyperpolarizability of the latter is shown to be smaller ((7 _+ 1.5) X 10 -30 e s u ) [17] than the former.
155
Sym-triphenyl benzene has a fl value comparable to that of the tricyanomethanide anion but approximately half of that of sym-triphenyl triazine. In order to calculate the gas phase fl, the geometry of each molecule was fully optimized by the Hartree-Fock semiempirical Austin model 1 (AM1) [21] parameterization. The molecular first-order hyperpolarizability was then computed using the finite field method [22]. Utilizing the same AM1 optimized geometry we have also obtained the static fl at the Hartree-Fock ab initio level using a 6-31G * basis set in the GAUSSIAN 92 set of programs. The /3 values calculated by the two different methods are also listed in Table 1. The agreement between theoretical and experimental 13 is excellent. However, the calculated gas-phase /3 values are about = 50% smaller than the measured /3 values in methanol. This is, perhaps, due to the neglect of effects related to frequency dispersion, electron correlation, etc. We have also measured /3 in other solvents e.g. benzene, chloroform and water (data not shown) and conclude that there does not exist any significant solvent dependence of the hyperpolarizability in these molecules. This is also reflected in the negligible difference between the calculated /3 values by two methods, one that does not include solvent effects and another which treats the molecules as being embedded in a dielectric continuum [23] (Table 1). In conclusion, we have demonstrated in this Letter that octupolar triazine derivatives have, in general, higher second-order nonlinearity than the corresponding octupolar symmetrically substituted benzene compounds. Their first hyperpolarizabilities are comparable to those of the dipolar molecules. In concurrence with earlier works on octupolar nonlinear optics, we believe that these molecules will soon become a new family of second-order NLO materials.
Acknowledgement We thank J. Chandrasekhar for critical reading of this manuscript. The laser used in this work was purchased with a generous grant from the Department of Science and Technology, Government of India.
156
P.C. Ray, P.K. Das / Chemical Physics Letters 244 (1995) 153-156
References [1] P.N. Prasad and D.J. Williams, Introduction to nonlinear optical effects in molecules and polymers (Wiley, New York, 1991). [2] D. Chemla and J. Zyss, Nonlinear optical properties of organic materials and crystals, Vols. 1, 2 (Academic Press, New York, 1987). [3] S.R. Marder, J.E. Sohn and G.D. Stucky, eds, Materials for nonlinera optics: chemical perspectives, ACS symp. Set. No. 455 (Am. Chem. Soc., Wasington, 1991). [4] J. Zyss, and G. Tsoucaris, Mol. Cryst. Liquid Cryst. 137 (1986) 303, [5] J. Zyss, Nonlinear Opt. 1 (1991) 3. [6] J. Zyss, J. Chem. Phys. 98 (1993) 6583. [7] V. Joffie, D. Yaron, R.J. Silbey and J. Zyss, J. Chem. Phys. 97 (1992) 5607. [8] B.F. Levine and C.J. Bethea, Appl. Phys. Letters 24 (1974) 445. [9] D.M. Burland, R.D. Miller and C.A. Walsh, Chem. Rev. 94 (1994) 31. [10] I. Ledoux, J. Zyss, J. Siegel, J. Brienne and J.M. Lehn, Chem. Phys. Letters 172 (1990) 440.
[11] R.W. Terhune, P.D. Maker and C.M. Savage, Phys. Rev. Letters 17 (1965) 1647. [12] P.D. Maker, Phys. Rev. A 1 (1970) 923. [13] K. Clays and A. Persoons, Phys. Rev. Letters 66 (1991) 2980. [14] K. Clays, A. Persoons and L. DeMayer, Advan. Chem. Phys. 85 (1994) 455. [15] K. Clays and A. Persoons, Rev. Sci. Instr. 63 (1992) 3285. [16] J. Zyss, C. Dhenaut, T. Chauvan and I. Ledoux, Chem. Phys. Letters 206 (1993) 409. [17] T. Verbiest, K. Clays, A. Persoons, F. Meyers and J.L. Bredas, Opt. Letters 18 (1993) 525. [18] T. Verbiest, K. Clays, C. Samyn, J. Wolff, D. Reinhoudt and A. Persoons, J. Am. Chem Soc. 116 (1994) 9320. [19] S.G. Cyvin, J.E. Rauch and J.C. Dccius, J. Chem. Phys. 43 (1965) 4083. [20] K. Mohanalingam, P.C. Ray and P.K. Das, submitted for publication. [21] J.J.P. Stewart, J. Comput. Chem. 10 (1989) 209. [22] H.A. Kurtz, J.J.P. Stewart and K.M. Dieter, J. Comput. Chem. 11 (1990) 82. [23] L. Onsager, J. Am. Chem. Soc. 58 (1936) 1486.
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical PhysicsLetters244 (1995) 153-156
First-order hyperpolarizabilities of octupolar aromatic molecules: symmetrically substituted triazines Paresh Chandra Ray, Puspendu Kumar Das Department of lnorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
Received28 April 1995; in final form 27 July 1995
Abstract
The first hyperpolarizabilities of some symmetrically substituted triazines have been measured and compared with those of the corresponding symmetrically substituted benzenes. The octupolar triazines have higher quadratic polarizabilities than the corresponding octupolar benzenes. The triazine ring seems to be a better central acceptor than the benzene ring, but if it acts as a donor as in sym-triphenyl triazine, the nonlinearity improves further.
1. Introduction
The second-order nonlinear optical (NLO) properties of dipolar molecules are a widely investigated area of research. Experiments have been carried out [1-3] to develop a better understanding of the relationship between molecular structure and NLO properties and to provide guidelines for the design and synthesis of new types of molecules with enhanced /3. Although there are many examples of dipolar molecules exhibiting extremely large hyperpolarizabilities, some major limitations persist in utilizing them for NLO. For example, more often than not dipolar interactions force the molecules to pack in a centrosymmetric lattice [4] or lead to undesirable aggregation. Octupolar molecules which have been recognized only recently as potentially useful for nonlinear applications are devoid of the above-mentioned disadvantages. Due to the absence of ground state dipole moment, these molecules [5-7] cannot be oriented by applying an electric field. Thus the well known
electric field induced second harmonic generation (EFISHG) [8] and poling techniques [9] are not applicable to octupolar molecules for measuring the microscopic and macroscopic second-order polarizabilities, respectively. As a consequence, octupolar molecules have not yet been investigated thoroughly for efficient second harmonic generation. Ledoux et al. [10] have reported that triaminotrinitrobenzene which is octupolar, shows three times higher SHG efficiency than urea in powder form. The hyperRayleigh scattering (HRS) technique [11-15] provides a means of measuring /3 in these nonpolar molecules. Zyss et al. [16] and Verbiest et al. [17,18] have reported large hyperpolarizabilities in some octupolar molecules and sometimes even more than in the corresponding dipolar molecules. In this Letter, we report a systematic study of the second-order NLO properties of a series of octupolar substituted triazines and the corresponding substituted benzenes. Theoretical estimates of /3 by AM1 as well as ab initio methods are also compared with the experimental results.
0009-2614/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved SSDI 0009-2614(95)00913-2
154
P.C. Ray, P.K. Das / Chemical Physics Letters 244 (1995) 153-156 200
2. Theoretical treatment The treatment of octupolar molecules for quadratic NLO has been developed and discussed by Zyss and others [5-7] previously. In short, any tensor T (") of arbitrary rank n can be decomposed in a sum of irreducible tensorial components TJ,m (") and can be expressed as
"~160
/
J
T(")= ~ 2El Tj!",,, ) j=0
m=-j
(1)
It turns out from Eq. (1) that /3 (or X (2)) will only have two irreducible components of order 1 (vector or dipolar part) and 3 (septor or octupolar part),
/3 =/3j=~ +/3j=3.
(2)
Each component has 2j + 1 independent coefficients. Zyss [6] has shown that since octupolar molecules belong to an octupolar space group ( j = 3), all dipolar quantities (j = 1) vanish, and only octupolar contributions to /3 remain. Purely octupolar groups are, for example, the orthorombic D2, the hexagonal D3h and the cubic T symmetry groups. In the HRS measurements the averaged (/3 2> value of the rank 6 tensor 13 ×/3 will depend on the symmetry of the molecule, whereas the polarization of the fundamental laser beam and the presence or absence of an analyzer at the second harmonic wavelength will determine the /3 ×/3 coefficients to be averaged. A general method for obtaining the terms has been proposed by Cyvin et al. [19] and its application to nonlinear octupoles in solution described by Zyss et al. [5-7]. Application of this method in the case of unpolarized observations for molecules with D3h symmetry gives <
2 /3D 3h> -82T/3}yy. =
(3)
The /3yyycomponentsof the first hyperpolarizability tensor of all the substituted triazines and benzenes in benzene, chloroform, methanol and water (for only compounds 2, 3, 6 and 8 which are soluble) were obtained from the measured average /3. At low solute concentrations, assumingthat the solvent concentration does not change significantly, 12,~/12o = G is a linear function of the solute concentration (Fig. l). From the slope and intercept of this
0
I 8
I
1 12 Im
I
I
16
20
Nsolute(10molecutes/c.c.) Fig. 1. 12o,/I~ versus number density plot of compound 4 at room temperature in methanol.
plot, for the solute is obtained with the solvent value as a reference.
3. Experimental All the compounds (Fig. 2) were purchased from Aldrich, USA and the solvents were obtained commercially and purified using standard procedures. The apparatus for the /3 measurements has been described in detail elsewhere [20]. In brief, the fundamental of a Q-switched Nd:YAG laser (Spectra
Y
Y
Y~/
Y
Compound
Y
Compound
Y
!
Cl
8
Cl
2
OH
?
OH
3
OMe
8
OMe
4
SH
9
Me
5
Ph
I0
Ph
Fig. 2. Structure of compounds 1-10.
P.C. Ray, P.K. Das / Chemical Physics Letters 244 (1995) 153-156 Table 1 Calculated and measured first-order hyperpolarizabilities (103o esu) of compounds 1-10 in methanol Compound fl?M1 flgb initio flsabinitio ( ~)HRS [~yyy 1
2 3 4 5 6 7 8 9 10
3.2 4.6 5.8 4.9 6.1 2.0 2.1 2.9 1.4 3.2
2.9 5.2 6.2 5.4 6.4 1.9 2.1 2.8 1.5 3.4
3,7 5.6 6.9 5.9 7.0 2.2 2.4 3.5 1.8 4.2
7.8 9.0 13.0 11.3 16.0 3,1 3,6 5,8 2,2 7.1
12.6 14.6 21.1 18.3 25.9 5.0 5.8 9.4 3.6 11.5
Physics, 8 ns) is focused onto a glass cell containing the solute in solution. The second harmonic scattered light is collected through an efficient condenser system on the photocathode of a UV-VIS photomultiplier tube. A 3 nm bandwidth interference filter at 532 nm is used to isolate the green light. All data are collected after extensive signal-averaging at laser powers (~< 12 mJ/pulse) well below the threshold for stimulated Raman and Brillouin scattering, selffocusing or self-defocusing and dielectric breakdown.
4. Results and discussion The experimental /3 values are listed in Table 1. All triazine derivatives exhibit higher nonlinearity than the corresponding benzene analogs. The N atoms in the triazine ring each have a lone pair of electrons on the o'-plane which makes the central core more polarizable. In the presence of donors, the center acts as an acceptor as in benzene. However, the last example from the triazine derivatives, namely symtriphenyl triazine, has been predicted [6] to have optimized octupolar /3 components. Here we verify that the quadratic polarizability of this molecule is indeed high. In this example the triazine core probably acts as a donor and interacts with the surrounding acceptor (phenyl) groups in a manner similar to that in the tricyanomethanide anion. However, the first hyperpolarizability of the latter is shown to be smaller ((7 _+ 1.5) X 10 -30 e s u ) [17] than the former.
155
Sym-triphenyl benzene has a fl value comparable to that of the tricyanomethanide anion but approximately half of that of sym-triphenyl triazine. In order to calculate the gas phase fl, the geometry of each molecule was fully optimized by the Hartree-Fock semiempirical Austin model 1 (AM1) [21] parameterization. The molecular first-order hyperpolarizability was then computed using the finite field method [22]. Utilizing the same AM1 optimized geometry we have also obtained the static fl at the Hartree-Fock ab initio level using a 6-31G * basis set in the GAUSSIAN 92 set of programs. The /3 values calculated by the two different methods are also listed in Table 1. The agreement between theoretical and experimental 13 is excellent. However, the calculated gas-phase /3 values are about = 50% smaller than the measured /3 values in methanol. This is, perhaps, due to the neglect of effects related to frequency dispersion, electron correlation, etc. We have also measured /3 in other solvents e.g. benzene, chloroform and water (data not shown) and conclude that there does not exist any significant solvent dependence of the hyperpolarizability in these molecules. This is also reflected in the negligible difference between the calculated /3 values by two methods, one that does not include solvent effects and another which treats the molecules as being embedded in a dielectric continuum [23] (Table 1). In conclusion, we have demonstrated in this Letter that octupolar triazine derivatives have, in general, higher second-order nonlinearity than the corresponding octupolar symmetrically substituted benzene compounds. Their first hyperpolarizabilities are comparable to those of the dipolar molecules. In concurrence with earlier works on octupolar nonlinear optics, we believe that these molecules will soon become a new family of second-order NLO materials.
Acknowledgement We thank J. Chandrasekhar for critical reading of this manuscript. The laser used in this work was purchased with a generous grant from the Department of Science and Technology, Government of India.
156
P.C. Ray, P.K. Das / Chemical Physics Letters 244 (1995) 153-156
References [1] P.N. Prasad and D.J. Williams, Introduction to nonlinear optical effects in molecules and polymers (Wiley, New York, 1991). [2] D. Chemla and J. Zyss, Nonlinear optical properties of organic materials and crystals, Vols. 1, 2 (Academic Press, New York, 1987). [3] S.R. Marder, J.E. Sohn and G.D. Stucky, eds, Materials for nonlinera optics: chemical perspectives, ACS symp. Set. No. 455 (Am. Chem. Soc., Wasington, 1991). [4] J. Zyss, and G. Tsoucaris, Mol. Cryst. Liquid Cryst. 137 (1986) 303, [5] J. Zyss, Nonlinear Opt. 1 (1991) 3. [6] J. Zyss, J. Chem. Phys. 98 (1993) 6583. [7] V. Joffie, D. Yaron, R.J. Silbey and J. Zyss, J. Chem. Phys. 97 (1992) 5607. [8] B.F. Levine and C.J. Bethea, Appl. Phys. Letters 24 (1974) 445. [9] D.M. Burland, R.D. Miller and C.A. Walsh, Chem. Rev. 94 (1994) 31. [10] I. Ledoux, J. Zyss, J. Siegel, J. Brienne and J.M. Lehn, Chem. Phys. Letters 172 (1990) 440.
[11] R.W. Terhune, P.D. Maker and C.M. Savage, Phys. Rev. Letters 17 (1965) 1647. [12] P.D. Maker, Phys. Rev. A 1 (1970) 923. [13] K. Clays and A. Persoons, Phys. Rev. Letters 66 (1991) 2980. [14] K. Clays, A. Persoons and L. DeMayer, Advan. Chem. Phys. 85 (1994) 455. [15] K. Clays and A. Persoons, Rev. Sci. Instr. 63 (1992) 3285. [16] J. Zyss, C. Dhenaut, T. Chauvan and I. Ledoux, Chem. Phys. Letters 206 (1993) 409. [17] T. Verbiest, K. Clays, A. Persoons, F. Meyers and J.L. Bredas, Opt. Letters 18 (1993) 525. [18] T. Verbiest, K. Clays, C. Samyn, J. Wolff, D. Reinhoudt and A. Persoons, J. Am. Chem Soc. 116 (1994) 9320. [19] S.G. Cyvin, J.E. Rauch and J.C. Dccius, J. Chem. Phys. 43 (1965) 4083. [20] K. Mohanalingam, P.C. Ray and P.K. Das, submitted for publication. [21] J.J.P. Stewart, J. Comput. Chem. 10 (1989) 209. [22] H.A. Kurtz, J.J.P. Stewart and K.M. Dieter, J. Comput. Chem. 11 (1990) 82. [23] L. Onsager, J. Am. Chem. Soc. 58 (1936) 1486.