Theoretical design of molecular photo- and acido-triggered non-linear optical switches

Chemical Physics Letters 427 (2006) 153–158 www.elsevier.com/locate/cplett

Theoretical design of molecular photo- and acido-triggered non-linear optical switches Fabien Manc¸ois a,b, Vincent Rodriguez a, Jean-Luc Pozzo b, Benoıˆt Champagne c, Fre´de´ric Castet a,* a

Laboratoire de Physico-Chimie Mole´culaire – UMR 5803 CNRS – Universite´ Bordeaux I, Cours de la Libe´ration, 351, F-33405 Talence Cedex, France b Chimie Supramole´culaire, Biomime´tisme et Nanoscience – UMR 5802 CNRS – Cours de la Libe´ration, 351, F-33405 Talence Cedex, France c Laboratoire de Chimie The´orique Applique´e, Faculte´s Universitaires Notre-Dame de la Paix, rue de Bruxelles, 61, B-5000 Namur, Belgium Received 29 April 2006 Available online 13 June 2006

Abstract This theoretical Letter addresses the non-linear optical properties of a series of substituted dimethylaminophenylethylenyl-indolino[2,1-b]oxazolidine derivatives, which were recently shown to behave as multiaddressable NLO switches. The influence of the chemical nature of the substituent on the static and dynamic first hyperpolarizability, as well as of its grafting location, is investigated by using various semi-empirical and high level ab initio schemes. It is found that a para-substitution on the indolinic skeleton with a [(N,N-1,3dimethylpyrimidine-2,4,6-trione)-5-yl]methylidenyl group should increase by 75% the efficiency of the switches.  2006 Elsevier B.V. All rights reserved.

1. Introduction The design of molecular devices with switchable non-linear optical (NLO) properties is of crucial interest in the development of novel materials for optical telecommunication, optical information processing or data storage. Molecular NLO switches are also involved in authentication systems, optical power-limiting substances, photoresponsive materials, as well as biosensors [1]. Among systems of interest to display substantial variations of the NLO responses, organic photochromic compounds have motivated numerous studies [2,3]. These systems are characterized by their ability to alternate under light irradiation between two thermodynamically stable chemical forms having different absorption spectra. However, the progress in miniaturizing the components of electronics down to the molecular level embodies the integration of several switchable functions and/or the integration of multi-addressable functions into a single mole*

Corresponding author. Fax: +33 5 40 00 66 45. E-mail address: [email protected] (F. Castet).

0009-2614/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.06.011

cule. Developments in molecular switching technology thus imply the design of molecular systems with tunable properties revealed using more than one stimulus. In this context, organic compounds both displaying photo- and acido-chromic properties are promising candidates. In such systems, the commutation between species having different absorption spectra may be indifferently induced by electromagnetic irradiation or by pH variations, leading to zwitterionic or charged species, respectively. The rational design of multi-addressable NLO-phores is necessary interdisciplinary and combines (i) the synthesis of new compounds, (ii) the characterization of their secondorder NLO properties for which hyper-Rayleigh scattering (HRS) is a much appropriate method since it can be used for both neutral and charged species [4,5], and (iii) quantum chemical calculations. The latter encompass the determination of the NLO responses together with the physical description of the molecular NLO phenomena in terms of structural and electronic parameters. Structure–property relationships deduced from theoretical simulations constitute in turn a guide driving back the synthesis of optimal systems.

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tinuity of our previous experimental and theoretical works. The influence of the chemical nature of the group on the first hyperpolarizability, as well as of its position R1–R4 on the indolinic skeleton (Scheme 1), is investigated by using various semi-empirical and ab initio quantum chemical schemes.

Our strategy in the design of multi-addressable NLOphores presenting a large NLO contrast is based on the use of cyanine dye derivatives. Recently, we reported the synthesis and the NLO properties of a series of photoand acido-tunable molecular switches, combining the indolino[2,1-b]oxazolidine core with various styrylic residues [6]. The photochromic process implies the breaking of a r-bond on the oxazolidine moiety, leading to the formation of a stable zwitterionic species. The geometrical relaxation induced by the oxazolinic ring opening leads to an improved electron conjugation/delocalization along the molecule, responsible for the different optical signatures of the two photochromic forms. These drastic changes in the optical properties due to the photostimulated ringopening are also revealed on pH variation. In this case, the protonation of the oxazoline ring leads to the formation of a colored protonated open form (POF), which can be reversibly reverted upon base addition. Moreover, the absorption spectra arising from UV irradiation and upon acidic addition are superimposable, indicating that photo-induced and acido-generated colored forms adopt similar geometries. All the prepared indolino-oxazolidine derivatives were shown to display remarkable contrasts in their NLO response along the reversible transformations. Within the series, the structure involving the strong electron donating N,N-dimethylamino group on the aryl moieties (Scheme 1) exhibits the strongest enhancement of its first hyperpolarizability between the open and closed forms. However, theoretical calculations revealed that the efficiency of the light-induced charge transfer between the acceptor and donor parts of these chromophores, and hence the NLO contrast of the switches, was limited by the fact that, in the open forms, the conjugation does not extend to the oxazolidine moiety. A first strategy to extend the conjugation path was to replace the indolinic unit by a benzimidazolic [7] or a benzothiazolic [8] unit, in order to increase the number of delocalizable electrons. Moreover, an increase of the efficiency of the push–pull system might also be achieved by grafting electron attracting substitutents onto the indolinic residue. Then, this theoretical Letter addresses the NLO properties of a series of substituted dimethylaminophenylethylenyl-indolino[2,1-b]oxazolidines in their protonated open forms and is thus the natural con-

2. Computational methods The molecular structures were obtained at the B3LYP/ 6-31G* level. The static and dynamic hyperpolarizability tensors were calculated using the time-dependent Hartree–Fock (TDHF) method, which consists in expanding the matrices of the TDHF equation in Taylor series of the perturbation (static and dynamic electric fields) and in solving it order by order [9,10]. In its static limit, the TDHF method is also referred to as the coupled-perturbed Hartree–Fock (CPHF) method. TDHF calculations were performed at the ab initio level using the 6-31G, 6-31G* and 6-311G* basis sets, as well as in combination with semiempirical AM1 [11], PM3 [12], and PM5 [13] parametrizations, which were all shown to qualitatively reproduce the results obtained with more sophisticated but much more time consuming correlated ab initio methods [6,7]. To size up the magnitude of electron correlation effects, static first hyperpolarizabilities were also calculated at the second-order Møller–Plesset (MP2) level within the finite field (FF) procedure [14]. In this scheme, a perturbation potential, ~ l~ E, is added to the Hamiltonian and the wavefunctions and energies are obtained as in the field-free case. The elements of the hyperpolarizability tensors are then evaluated from numerical differentiations of the energies obtained for electric fields of different magnitudes and directions. A Romberg procedure was applied here to improve the accuracy of the numerical derivatives [15,16]. Frequency dispersion effects were then estimated by adopting the percentage or multiplicative correction scheme [17– 19]: bMP2 ð2x; x; xÞ  bMP2 ð0; 0; 0Þ 

R4 R3

hυ or H

hυ or OH– N

R2 R1

NMe2

+

R3 O

ð1Þ

which has been shown to be suitable for different systems including push–pull polyenes. A wavelength of 1064 nm was adopted in all dynamic TDHF calculations.

NMe2 R4

bTDHF ð2x; x; xÞ bCPHF ð0; 0; 0Þ

x

N+ R2 R1

z

CF

OF O– (H+)

y

Scheme 1. Photochromic/acidochromic equilibrium for the 10-[2-(4-dimethylaminophenyl)ethenyl]-9,9-dimethylindolino[2,1-b]oxazolidine (R1 = R2 = R3 = R4 = H). The cartesian frame used in the calculations is also displayed.

F. Manc¸ois et al. / Chemical Physics Letters 427 (2006) 153–158

In addition to the longitudinal component bzzz of the hyperpolarizability tensor (see Scheme 1 for the definition of the longitudinal axis z), one focuses on the squareroot of the hyper-Rayleigh scattering intensity for plane-polarized incident light observed perpendicularly to the propagation plane, which enables a direct comparison with experimental data: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi bHRS ð2x; x; xÞ ¼ fhb2ZZZ i þ hb2XZZ ig ð2Þ The associated depolarization ratio is given by: DR ¼

hb2ZZZ i hb2XZZ i

ð3Þ

Full expressions for hb2ZZZ i and hb2XZZ i without assuming Kleinman’s conditions are given in Ref. [20] and correspond to an orientational average of the b-tensor components. All semiempirical calculations have been performed using the MOPAC2000 [21] code, while GAUSSIAN03 [22] was used for the ab initio part. All first hyperpolarizability values are consistent with convention B of Ref. [23]. 3. Molecular structures Starting from the indolino-oxazolidine derivative reported in Scheme 1, the effect of the addition of standard chemical substituents acting as electron attractors onto the indolinic skeleton has been investigated in terms of the molecular NLO response. First, a nitro group has been added on the different R1–R4 positions of the oxazolidine, in order to determine the optimal location for the acceptor group. Then, the effects of the acceptor strength have been investigated by replacing the nitro group by a tricyanoethylenyl or a [(N,N-1,3-dimethylpyrimidine-2,4,6-trione)5-yl]methylidenyl substituent (Scheme 2), referred to as A and B in the following for simplicity, respectively. These substituents are known to be very efficient for push–pull NLO molecules [24]. Only monosubstitutions have been considered here, in regard with the feasability of the synthesis.

N

Et C

O C

C

N

O

N

C

N Et

C N

(A)

O

(B)

Scheme 2. Chemical substituents used as electron attractors. Tricyanoethylenyl (A) and [(N,N-1,3-dimethylpyrimidine-2,4,6-trione)-5-yl]methylidenyl (B).

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The molecular structure of the non-substituted derivative have been described in Ref. [6]. DFT calculations show that the nature and the position of the electron-withdrawing group do not affect significantly the structural parameters of the rest of the molecule. In all structures, the dihedral angle between the indoline and the dimethylaminophenyl mean planes does not exceed 10, maximizing the electron conjugation between the donor group and the indolinic residue. Moreover, the impact of the substituent on the bond length alternation along the conjugated path is negligible, the bond lengths in the substituted compounds ˚ with respect to the reference differing by less than 0.02 A structure without any substituents. 4. Non-linear optical properties The static longitudinal first hyperpolarizability (bzzz), HRS first hyperpolarizability (bHRS), and depolarization ratio (DP) evaluated using different levels of theory are given in Tables 1 and 2. Dynamic values associated to second harmonic generation (SHG) are reported in Table 3. At the CPHF/6-31G level, the presence of a nitro group on the oxazolidine moiety leads to an increase of the static longitudinal NLO response by 7–19% compared to the non-substituted compound. The largest NLO enhancement is obtained when the nitro group is located in para with respect to the indolinic nitrogen (R3). Relative values obtained using the polarized 6-31G* and 6-311G* basis sets are very similar. Moreover, for the para-substituted compound, calculations performed with the 6-31+G* basis set provide bHRS values (2714 a.u.) in between the 6-31G and 6-31G* results. This evidence rather small basis set effects. At the semi-empirical AM1 and PM3 levels, the NLO enhancement is not significant, whatever the location of the nitro group on the oxazolidine skeleton. However, CPHF/PM5 calculations lead to qualitatively similar conclusions to the ab initio CPHF ones and predict the largest increase of bzzz (+9%) in the case of a para-substitution. Moreover, this compound is also the most stable among the nitro-substituted structures, in agreement with conventional resonance arguments. In what concerns the eventual synthesis of the compounds, these results confirm that the para location is the most reactive. These calculations indicate that the para-substitution is the most efficient to enhance the NLO responses in these molecular switches, and that the attracting character of a single nitro group is not sufficient to cause a significant enhancement of the first hyperpolarizability. Calculations using the stronger electron-withdrawing substituents A and B (Scheme 1) in para position were then performed using the same quantum chemical schemes. For both substituents, CPHF calculations within the three different basis sets predict a substantial increase of the static longitudinal first hyperpolarizability (+46–52% for A and +46–48% for B). All semi-empirical schemes also indicate a significant enhancement of the NLO response (13–23% for A and 14–25% for B).

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Table 1 Static longitudinal first hyperpolarizability (bzzz), HRS first hyperpolarizability (bHRS), and depolarization ratio (DR) evaluated using different ab initio schemes Structure

CPHF/6-31G

CPHF/6-31G*

CPHF/6-311G*

MP2/6-31G

MP2/6-31G*

R1 = R2 = H R3 = R4 = H

bzzz bHRS DR

5845 (1.000) 2379 (1.000) 4.30

5385 (1.000) 2192 (1.000) 4.30

5292 (1.000) 2155 (1.000) 4.28

11 983 (1.000) 4896 (1.000) 4.70

10 070 (1.000) 4112 (1.000) 4.68

R2 = R3 = R4 = H R1 = NO2

bzzz bHRS DR

6233 (1.066) 2580 (1.084) 4.29

5701 (1.059) 2358 (1.076) 4.26

5605 (1.059) 2321 (1.077) 4.25

– – –

– – –

R1 = R3 = R4 = H R2 = NO2

bzzz bHRS DR

6350 (1.086) 2588 (1.089) 4.37

5808 (1.079) 2367 (1.080) 4.35

5694 (1.076) 2320 (1.077) 4.33

– – –

– – –

R1 = R2 = R4 = H R3 = NO2

bzzz bHRS DR bzzz bHRS DR

6955 2879 4.30 6301 2593 4.41

6318 2614 4.29 5790 2383 4.40

6213 2570 4.26 5691 2340 4.38

12 049 (1.005) 4955 (1.012) 4.61 – – –

10 222 (1.015) 4208 (1.023) 4.59 – – –

R1 = R2 = R4 = H R3 = A

bzzz bHRS DR

8560 (1.464) 3603 (1.515) 4.36

8126 (1.509) 3431 (1.565) 4.36

8047 (1.521) 3301 (1.532) 4.31

15 711 (1.311) 6552 (1.338) 4.70

14 012 (1.391) 5876 (1.429) 4.68

R1 = R2 = R4 = H R3 = B

bzzz bHRS DR

8646 (1.479) 3658 (1.538) 4.61

7841 (1.456) 3320 (1.515) 4.59

7745 (1.464) 3283 (1.523) 4.58

19 290 (1.610) 8058 (1.646) 4.90

15 955 (1.584) 6679 (1.624) 4.89

R1 = R2 = R3 = H R4 = NO2

(1.190) (1.210) (1.078) (1.090)

(1.173) (1.193) (1.075) (1.087)

(1.174) (1.193) (1.076) (1.086)

All values are given in atomic units (1 a.u. of first b = 3.2063 · 1053 C3 m3 J2 = 8.641 · 1033 esu). Values relative to the non-substituted compound are given in parentheses.

Table 2 Static longitudinal first hyperpolarizability (bzzz), HRS first hyperpolarizability (bHRS), and depolarization ratio (DR) evaluated using different semiempirical schemes Structure

CPHF/AM1

CPHF/PM3

CPHF/PM5

R1 = R2 = H R3 = R4 = H

bzzz bHRS DR

14 591 (1.000) 5929 (1.000) 4.54

17 454 (1.000) 7107 (1.000) 4.61

16 006 (1.000) 6507 (1.000) 4.56

R2 = R3 = R4 = H R1 = NO2

bzzz bHRS DR

13 775 (0.944) 5644 (0.952) 4.37

15 940 (0.913) 6534 (0.919) 4.41

15 556 (0.972) 6371 (0.979) 4.44

R1 = R3 = R4 = H R2 = NO2

bzzz bHRS DR

14 456 (0.995) 5878 (0.991) 4.47

17 163 (0.983) 6985 (0.983) 4.55

16 473 (1.029) 6704 (1.030) 4.53

R1 = R2 = R4 = H R3 = NO2

bzzz bHRS DR

14 718 (1.009) 6021 (1.016) 4.39

17 609 (1.009) 7194 (1.012) 4.45

17 471 (1.091) 7188 (1.105) 4.45

R1 = R2 = R3 = H R4 = NO2

bzzz bHRS DR

14 519 (0.991) 5947 (1.003) 4.52

17 268 (0.989) 7067 (0.994) 4.58

16 254 (1.016) 6673 (1.026) 4.57

R1 = R2 = R4 = H R3 = A

bzzz bHRS DR

16 458 (1.128) 6761 (1.140) 4.47

19 924 (1.142) 8163 (1.149) 4.54

19 692 (1.230) 8120 (1.248) 4.51

R1 = R2 = R4 = H R3 = B

bzzz bHRS DR

19 568 (1.341) 8058 (1.359) 4.66

23 340 (1.337) 9603 (1.351) 4.74

24 361 (1.522) 10 078 (1.549) 4.69

All values are given in atomic units. Values relative to the non-substituted compound are given in parentheses.

As already observed in other push–pull chromophores [25–28], the inclusion of electron correlation effects at the MP2 level leads to an increase of the NLO response with

respect to CPHF calculations. For R3 = NO2, the static longitudinal hyperpolarizability is multiplied by a factor of 1.73 and 1.62 when using the 6-31G or 6-31G* basis

F. Manc¸ois et al. / Chemical Physics Letters 427 (2006) 153–158 Table 3 Dynamic (SHG, k = 1064 nm) longitudinal first hyperpolarizability (bzzz), HRS first hyperpolarizability (bHRS), and depolarization ratio (DR) evaluated using different ab initio schemes for the para-substituted compounds (R1 = R2 = R4 = H) R3 = NO2

R3 = A

R3 = B

HF/6-31G bzzz 11 800 (1.000) bHRS 4793 (1.000) DR 4.61

R3 = H

14 490 (1.228) 5959 (1.243) 4.62

18 640 (1.580) 7779 (1.623) 4.65

19 348 (1.640) 8093 (1.689) 4.81

HF/6-31G* bzzz 10 926 (1.000) bHRS 4437 (1.000) DR 4.61

13 186 (1.207) 5420 (1.222) 4.61

17 928 (1.641) 7503 (1.691) 4.65

17 571 (1.608) 7355 (1.658) 4.80

Eq. (1)/6-31G 24 192 (1.000) bzzz bHRS 9864 (1.000)

25 103 (1.038) 10 256 (1.040)

34 212 (1.414) 14 146 (1.434)

43 167 (1.784) 17 828 (1.807)

Eq (1)/6-31G* bzzz 20 432 (1.000) bHRS 8323 (1.000)

21 334 (1.044) 8725 (1.048)

30 914 (1.513) 12 850 (1.544)

35 754 (1.750) 14 796 (1.778)

All values are given in atomic units. Values relative to the non-substituted compound are given in parentheses.

set, respectively. In the same basis set order, the increase amounts to 84 and 72% when R3 = A and to 123 and 103% when R3 = B. DR also increases and gets closer to 5, the value that corresponds to an ideal rodlike dipolar compound. Besides, when comparing the non-substituted structure to the substituted ones, MP2 calculations predict a more pronounced enhancement of bzzz than the CPHF calculations for R3 = B, while the opposite is found for R3 = A. Similarly, enhanced differences between the secondorder NLO responses are observed when accounting for frequency dispersion, whereas the hierarchy between the different compounds is preserved. Finally, using the percentage correction scheme (Eq. (1)) with static MP2 values together with the 6-31G* basis set (which corresponds to the most elaborated level of theory considered here), the para-substitution with R3 = NO2, R3 = A and R3 = B predicts an enhancement of the longitudinal first hyperpolarizability of 4%, 51% and 75%, respectively. 5. Conclusions The impact of the addition of an electron withdrawing substituent on the static and dynamic non-linear optical properties of a series of dimethylaminophenyl-indolino[2,1-b]oxazolidine derivatives, which were recently shown to behave as photo and acido-addressable NLO switches, has been investigated using various quantum chemical schemes. Our computational study of nitro compounds clearly demonstrates that the para-substituted derivative will constitute the most promising NLO switches. Stronger electrowithdrawing substituents such as tricyanoethylenyl or [(N,N-1,3-dimethylpyrimidine2,4,6-trione)-5-yl]methylidenyl were also tested. The latter

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is found to cause the largest NLO enhancement, increasing the first hyperpolarizability by 75%. Acknowledgements B.C. thanks the Belgian National Fund for Scientific Research (FNRS) for his research director position. This work has benefited from a scientific cooperation established and supported by the Centre National de la Recherche Scientifique (CNRS), the FNRS, and the Commissariat Ge´ne´ral aux Relations Internationales (CGRI) de la Communaute´ Wallonie-Bruxelles. The calculations were performed thanks to computing time made available by the SiMoA (Simulation et Mode´lisation en Aquitaine, France), the intensive calculation pole ‘M3PEC-MESOCENTRE’ of the University Bordeaux I, as well as by the Interuniversity Scientific Computing Facility (ISCF), installed at the Faculte´s Universitaires Notre-Dame de la Paix (Namur, Belgium), for which the authors gratefully acknowledge the financial support of the FNRS-FRFC and the ‘Loterie Nationale’ for the Convention No. 2.4578.02, and of the FUNDP. References [1] See for example B.L. Ferringa, Molecular Switches, Wiley–VCH, Weinheim, 2001. [2] Y. Atassi, J.A. Delaire, K. Nakatani, J. Phys. Chem. Rev. 99 (1995) 16320. [3] V. Barachevsky, G. Chudinova, Mol. Sci. Eng. C 8–9 (1999) 73. [4] K. Clays, A. Persoons, L. De Maeyer, Adv. Chem. Phys. 85 (1994) 455. [5] E. Hendrickx, K. Clays, A. Persoons, Acc. Chem. Res. 31 (1998) 675. [6] L. Sanguinet, J.L. Pozzo, V. Rodriguez, F. Adamietz, F. Castet, L. Ducasse, B. Champagne, J. Phys. Chem. B 109 (2005) 11139. [7] L. Sanguinet, J.-L. Pozzo, M. Guillaume, B. Champagne, F. Castet, L. Ducasse, E. Maury, J. Soulie´, F. Manc¸ois, F. Adamietz, V. Rodriguez, J. Phys. Chem. B 110 (2006) 10672. [8] L. Sanguinet, PhD Thesis, University of Bordeaux I, France, 2003. [9] H. Sekino, R.J. Bartlett, J. Chem. Phys. 85 (1986) 976. [10] S.P. Karna, M. Dupuis, J. Comput. Chem. 12 (1991) 487. [11] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [12] J.J.P. Stewart, J. Comput. Chem. 10 (1989) 209. [13] J.J.P. Stewart, J. Molec. Mod. 10 (2004) 6. [14] H.D. Cohen, C.C.J. Roothaan, J. Chem. Phys. 43 (1965) S34. [15] P.J. Davis, P. Rabinowitz, in: Numerical Integration, Blaisdel Publishing Company, London, 1967, p. 166. [16] See also for an example of the Romberg scheme in the case of push– pull p-conjugated systems B. Champagne, B. Kirtman, in: H.S. Nalwa (Ed.), Handbook of Advanced Electronic and Photonic Materials and Devices, Nonlinear Optical Materials, vol. 9, Academic Press, San Diego, 2001, p. 63 (Chapter 2). [17] M.J. Rice, N. Handy, Int. J. Quantum Chem. 43 (1992) 91. [18] H. Sekino, R.J. Bartlett, Chem. Phys. Lett. 234 (1995) 87. [19] D. Jacquemin, B. Champagne, C. Ha¨ttig, Chem. Phys. Lett. 319 (2000) 327. [20] R. Bersohn, Y.H. Pao, H.L. Frisch, J. Chem. Phys. 45 (1966) 3184. [21] MOPAC2000,  Fujitsu Limited, 1999; J.J.P. Stewart, Quantum Chemistry Program 7 Exchange, n. 455.

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[22] M.J. Frisch et al., GAUSSIAN 03, Revision C.02, Gaussian Inc., Wallingford CT, 2004. [23] A. Willetts, J.E. Rice, D.A. Burland, D.P. Shelton, J. Chem. Phys. 97 (1992) 7590. [24] K.D. Singer, S.F. Hubbard, A. Shober, L.M. Hayden, K. Johnson, in: M.G. Kusik, C.W. Dirk (Eds.), Characterization Techniques and Tabulations for Organic Non-linear Optical Materials, Marcel Dekker, New York, 1998, p. 311.

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