Schiff bases with various donor–acceptor substituents and regulating groups as non-linear optical materials: ab initio quantum mechanical calculations

Journal of Molecular Structure: THEOCHEM 712 (2004) 117–122 www.elsevier.com/locate/theochem

Schiff bases with various donor–acceptor substituents and regulating groups as non-linear optical materials: ab initio quantum mechanical calculations ¨ nverb, Ayhan Elmalic,* Aslı Karakas¸a, Hu¨seyin U a

Department of Physics, Faculty of Arts and Sciences, Selc¸uk University, 42049 Campus, Konya, Turkey b Department of Physics, Faculty of Sciences, Ankara University, 06100 Tandog˘an, Ankara, Turkey c Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Tandog˘an, Ankara, Turkey Received 1 June 2004; accepted 1 October 2004 Available online 25 November 2004

Abstract The electric dipole moments (m), first (b) and second (g) hyperpolarizabilities of a series of Schiff bases with different donor–acceptor substituents and regulating groups (R) were calculated using ab initio quantum mechanical methods. The hyperpolarizabilities obtained have been found to be rather adequate for assessing connectivities between the electronegativities of substituents and regulating groups and nonlinear optical properties. q 2004 Elsevier B.V. All rights reserved. Keywords: Non-linear optics; Electric dipole moment; Hyperpolarizability; Ab initio calculation; Time-dependent Hartree–Fock

1. Introduction Comprehensive treatments of the physics of non-linear optics (NLO) originating from electron–photon interactions can be found elsewhere [1–5]. Large optical non-linearity could be seen in organic conjugated molecules having an electron acceptor group at one end and a donor group at the opposite end [6]. For the prototypical electron donor– acceptor molecule, the molecule would exhibit an asymmetric polarization response to a symmetric electric field at intense field strengths since electron intensity is more easily displaced toward an acceptor substituent than toward a substituent donor. Davydov and co-workers [7] concluded that dipolar aromatic molecules possessing an electron donor group and an electron acceptor group contribute to large optical non-linearity arising from the intramolecular charge transfer between the two groups of opposite nature. It is also well known that the first and second hyperpolarizabilities in p-electron systems with donor–acceptor groups

* Corresponding author. Tel.: C90 312 212 67 20x1142. E-mail address: [email protected] (A. Elmali). 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.10.010

increase with increasing donor and acceptor strengths. Organic molecules with p-electron delocalization are currently of wide interest as NLO materials with potential applications in optical switches and other NLO devices [8]. In spite of the rapid development of highly advanced experimental techniques on NLO, the theoretical understanding of the NLO properties show that the responses of the molecule to the external application of an electric field are also much important. It is known that there are several well-established computational procedures that include correlation at various levels of rigor and used for computation of NLO properties. Because the time-dependent Hartree–Fock (TDHF) is rather useful among the various computational procedures given the details in Ref. [9], to calculate the NLO properties of the compounds studied here we have preferred using the TDHF procedure. In this paper, we aimed to evaluate the electric dipole moment, first and second hyperpolarizabilities of various Schiff bases shown in Figs. 1 and 2 based on ab initio calculations using TDHF method at the 3-21CG** basis set level. The purpose of this work is also to assess the effects of the electronegativities of different substituents and regulating groups connected to these Schiff bases on the first and

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Fig. 1. Molecular structures of various Schiff bases with different regulating groups (R).

the second hyperpolarizabilities of various Schiff bases. It is shown that the order of electronegativities for the various Schiff bases studied in this paper is almost similar to that of b and g.

2. Theory and computational strategy This paper involves determining the minimum energy conformation of all the compounds given in this study and the calculation of the first and the second hyperpolarizabilities for all these systems. In this study molecular orbital calculations were carried out on different structures given in Figs. 1 and 2 using the 3-21CG** basis set of the GAMESS program [10] with full optimization of all bond lengths, angles and torsion angles. The optimized structures were then used to calculate the electric dipole moments and hyperpolarizabilities of these compounds with 3-21CG** polarized and diffused basis set which appeared to be a good compromise between accuracy and calculation duration. Both the addition of d polarization functions on the carbon and nitrogen atoms and the addition of p functions on hydrogen atoms or diffuse functions are critical in order to have a precise estimation of the first and the second hyperpolarizability values. The ab initio optimized geometries of the crystal structure of the investigated compounds correspond to slightly non-planar conformation having 6.388 dihedral angle between the benzaldehyde plane and

the aniline plane with different donor–acceptor/regulating groups. For calculation of the hyperpolarizabilities of the compounds studied, by ab initio, we used the TDHF procedure of the GAMESS program. The electric dipole moments m of all the compounds shown in Figs. 1 and 2 were automatically calculated by the TDHF procedure. BSHG and GEFISH groups in the TDHF procedure of GAMESS program carried out the the calculations at 1.17 eV of the first hyperpolarizability b for second harmonic generation (SHG) and the second hyperpolarizability g for electric-field induced Second-Harmonic generation (EFISHG), respectively. Our calculations are performed on a PC with an Intel Pentium IV operator, 512 MB RAM memory and 1.7 GHz frequency using Linux PC GAMESS version running under Linux 7.3 environment. Many types of hyperpolarizabilities have been discussed in the literature denoted as b-V(b vector) which is the vector part of the hyperpolarizability and g(g average). The b-V [11] and g [12] could be calculated using the following expressions, respectively, qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi b-V Z b2x C b2y C b2z (1) where bi (iZx, y, z) is given by, X ðbijj C bjij C bjji Þ: bi Z ð1=3Þ jZx;y;z

Fig. 2. Molecular structures of various Schiff bases with different donor–acceptor substituents.

(2)

A. Karakas¸ et al. / Journal of Molecular Structure: THEOCHEM 712 (2004) 117–122

g Z ð1=5Þ½gxxxx C gyyyy C gzzzz C 2ðgxxyy C gxxzz C gyyzz Þ: (3) In this study, to calculate all the electric dipole moments and all the hyperpolarizabilities, the origin of the Cartesian coordinate system (x, y, z)Z(0,0,0) has been chosen at own center of mass of each Schiff base in Figs. 1 and 2.

3. Results and discussion In the present paper the electric dipole moments m, first b and second g hyperpolarizabilities for four different donor– acceptor substituents and regulating groups R of various Schiff bases which were given in Figs. 1 and 2 were calculated by ab initio quantum mechanical methods with 3-21CG** polarized and diffused basis set. The –HCaN– of the amine linkage acts as an electron donating group, while the –OH group acts as an electron withdrawing group in the benzylanil moiety. In general, one moiety of the Schiff base has a hyperpolarizable group, and the other moiety of the compound has a configuration which ensures that the resultant compound crystallizes in a non-centrosymmetric configuration. In the Schiff bases, the valence orbitals of the nitrogen are the hybridized sp2, with the orbitals of the nonbonding pair of electrons being coplanar with the bonding orbital [13]. The change in donor–acceptor strength and, thereby, in electron density, affects the dipole moment, oscillator strength and transition energy [14]. When the Coulomb energies for donor and acceptor groups are equal, the molecular asymmetry is zero, the change in dipole moment is also zero, and the charge distribution is

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symmetrical, and, hence, no SHG are observed. As the Coulombic energy increases, the induced asymmetry in the electron population increases and the bridge-mediated donor–acceptor orbital mixing decreases and hence, might lose charge transfer character in molecules such as anils with weak donors and acceptors. When electronic polarization leads to an increased quinoidal character in a resonance structure, it will result in a net loss of resonance stabilization energy because of the Coulombic force along the charge separation. As a result, the effective stability of the donor–acceptor pair connected by an aromatic bridge will be less than when they are attached to a degenerate p-electron system [15]. All the compounds investigated have the donor–acceptor group arranged in the p–p 0 (p, p 0 Zpara) position, indicating that the transfer of electrons follows the interaction between the donor and acceptor groups. For different regulating groups at the meta position (Fig. 1), the first hyperpolarizability b decreases in the order –FO–IO–ClO–Br and the second hyperpolarizability g decreases in the order –FO–IO–BrO–Cl (in Table 1) with b-V and g, respectively, written in bold letters. For halogen substituents (–F, –Cl, –Br, –I), the inductive effects may be predominant as compared to resonance effects. Also, the effect of polarization, size of electron density distribution and configuration are so balanced that the system is alligned for maximum charge transfer. The different groups at meta to Schiff base nitrogen affect the basicity of its lone-pair electrons to a much smaller extent than the acidity of the –OH group. The electronegativity scale is important in providing a qualitative statement of NLO properties of

Table 1 All b and g components calculated ab initio using 3-21CG** basis set for the molecules I, II, III and IV b(K2u; u, u) Components for uZ1.17 eV I (10

K30

bxxx byxx bzxx bxxy byxy bzxy bxxz byxz bzxz bxyy byyy bzyy bxyz byyz bzyz bxzz byzz bzzz bx by bz b-V

esu)

2102.37 276.058 9.744 K824.67 K91.953 34.163 1111.145 128.77 K64.68 345.47 79.99 K30.138 K435.38 K52.498 45.74 582.74 45.895 K69.02 6922.049 K995.9 1889.83 7244.17

II (10

K30

esu)

K259.33 K69.44 K3.0026 K58.53 K72.057 1.109 3.275 1.132 0.338 K126.47 K94.864 4.363 1.354 K0.218 K0.611 2.432 0.167 K0.0448 K1047.28 K472.15 7.34 1148.8

III (10

g(K2u; u, u, 0) Components for uZ1.17 eV esu)

K30

K70.776 40.264 15.198 K48.626 K7.925 68.633 1.834 5.219 K12.60 109.54 K58.36 K203.75 15.006 2.312 K39.83 1.535 0.641 K5.33 K142.31 K311.08 K196.25 394.39

IV (10

esu)

I (10K36esu)

II (10K36esu)

III (10K36esu)

IV (10K36esu)

gxxxx gyyyy gzzzz gxyxy gxzxz gyxyx gyzyz gzxzx gzyzy gxxyy gxxzz gyyxx gyyzz gzzxx gzzyy

726064.94 12432.52 K250.07 K161754.9 3058.28 K50266.39 K383.12 K38235.86 10196.2 100918.6 149461.45 83512.15 19497.23 K44.61 K117.15

3602.83 K678.54 K0.2348 K542.9 13.337 K821.63 3.608 5.812 K4.874 K643.88 K2.335 K1016.56 3.392 34.42 1.872

97.867 2850.8 33.865 1233.98 22.348 K265.42 K14.836 286.93 706.37 K4345.7 K32.38 K1009.07 K9.055 K64.51 K20.53

12627.6 K86044.54 309.488 K67250.85 K1315.91 4391.296 K3039.34 K1468.27 K606.856 30104.02 72695.34 K17435.04 K155026.8 K19.68 K89.842

g

139546.55

K30

K50.29 K43.88 K29.336 K219.84 K178.78 K14.588 401.539 205.689 48.202 K391.152 K498.787 79.56 657.06 593.41 K33.619 K1147.008 K691.38 K46.658 K1950.197 K2738.56 1900.154 3861.81

297.05

493.67

K28511.61

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Table 2 The electric dipole moments m and components of m calculated ab initio using 3-21CG** basis set for the molecules I, II, III and IV

of electronegativities, i.e. –FO–ClO–BrO–I, b-V value for compound IV with the Iodine regulator is much greater than the values for compounds with the Chlorine and Brom regulators, i.e. –FO–IO–ClO–Br. Moreover, in Table 2, it is seen that the electric dipole moment for compound III with the Brom regulator is found to be much higher than that for compound II with the Chlorine regulator; thus g value is found to be higher than that for the compound II with the Chlorine regulator, i.e. –FO–IO–BrO–Cl. It has been seen that the organic conjugated molecules in Fig. 2 have an electron acceptor group (–NO2, –NO, –CN, –CHO) at one end and a donor group (–F) at the opposite end. The electron withdrawing power of the acceptor group of the molecular system will have an effect on the polarization of the molecular structure. For the molecules with different donor–acceptor groups given in Fig. 2 ab initio calculated the first hyperpolarizability b decreases in the order –CNO–CHOO–NOO–NO2 and the second hyperpolarizability g decreases in the order –CHOO –NOO–NO2O–CN in Table 3 with b-V and g, respectively, written bold letters. The magnitude of optical nonlinearities depends on the strength of donor–acceptor groups and the best combination of donor–acceptor groups provides a great enhancement. The b values are quite large for VII and VIII compounds with cyano and aldehyde groups in Fig. 2. The electronegativity of the acceptor groups decreases in the order –CNO–CHOO–NOO–NO2. This order of electronegativities is in complete agreement with the order of the calculated first hyperpolarizabilities b in Table 3 with b-V written in bold letters. The ab initio calculated electric dipole moments of the molecules in

Dipole moment [m (Debye)] components mx my mz m

I

II

III

IV

18.238688 K17.635652 K0.633122 25.378472

K13.716395 12.704320 K1.527867 18.758294

8.166306 K22.325230 K0.679217 23.781627

K8.235457 K23.649775 K0.663248 25.051437

elements. Across any given period, the effective nuclear charge increases and the atomic volume decreases both of which contribute to the tendency of an atom to attract electrons and leads to increasing electronegativities. Through a family of halogens, the atomic volume increases as more shells are added. At the same time, there is a corresponding decrease in the atoms ability to attract the electrons in a bond, so electronegativities decrease with increasing atomic weight [16]. The electronegativity of the halogenic regulating groups decreases in the order –FO–ClO–BrO–I and this leads to altering electron density on the nitrogen orbital. The molecule with the substituent Iodine regulator which has the most weight, has been found to have much greater dipole moment than with the substituents Brom and Chlorine, and found to have very low dipole moment with the Fluorine regulator in Table 2 with m written in bold letters. As a result, the greater the electric dipole moment for I the greater its hyperpolarizabilities values than the values in Table 1 for Br and Cl with b-V and g written in bold letters. Hence, while the iodine regulator is found in the last position according to the order

Table 3 All b and g components calculated ab initio using 3-21CG** basis set for the molecules V, VI, VII and VIII b(K2u; u, u) components for uZ1.17 eV V(10

K30

bxxx byxx bzxx bxxy byxy bzxy bxxz byxz bzxz bxyy byyy bzyy bxyz byyz bzyz bxzz byzz bzzz bx by bz b-V

esu)

30.58 K12.53 0.581 3.632 K53.479 0.4822 K6.8805 1.1195 K0.7786 22.199 12.5824 K4.81 K0.7508 4.3972 1.626 K5.1985 13.867 K0.765 0.2502 49.595 K11.498 50.91

VI(10

esu)

K30

1.528 K11.195 6.225 K27.369 K45.277 15.104 8.449 K5.627 K5.556 K3.963 K46.687 12.266 K11.963 K3.307 0.888 31.44 7.996 K0.197 K69.602 K196.225 28.184 210.103

VII(10

g(K2u; u, u, 0) components for uZ1.17 Ev esu)

K30

K12.44 K12.14 K1.471 K26.68 K46.31 3.936 2.355 1.389 K2.288 K8.982 K57.834 5.989 K2.418 0.803 K0.250 11.121 7.578 0.0969 K132.39 K231.94 11.126 267.30

VIII(10

esu)

K30

K19.531 10.13 25.523 K35.515 K23.57 K2.079 4.736 0.802 45.78 K23.62 K69.745 3.986 K0.619 1.819 7.967 3.280 4.978 K3.720 K34.50 K249.22 31.459 253.55

gxxxx gyyyy gzzzz gxyxy gxzxz gyxyx gyzyz gzxzx gzyzy gxxyy gxxzz gyyxx gyyzz gzzxx gzzyy

g

V(10K36esu)

VI(10K36esu)

VII(10K36esu)

VIII(10K36esu)

573.13 462.09 3.811 1.904 K34.563 263.342 K27.586 237.40 K16.319 K242.534 231.698 87.26 K152.59 5.109 8.116

898.62 918.41 0.942 990.97 K50.167 520.71 K44.344 K74.423 K137.405 324.94 861.18 1841.83 143.29 6.768 K45.30

219.39 165.188 0.563 191.74 K20.716 4.718 K40.019 K28.473 K120.69 134.73 120.206 K167.516 44.77 K2.114 K36.85

603.64 414.86 1.531 188.8 K5.689 K86.239 K41.223 7856.26 13.961 370.71 K72.176 K25.784 26.157 K70.10 K74.588

260.17

733.159

81.45

1271.073

A. Karakas¸ et al. / Journal of Molecular Structure: THEOCHEM 712 (2004) 117–122 Table 4 The electric dipole moments m and components of m calculated ab initio using 3-21CG** basis set for the molecules V, VI, VII and VIII Dipole moment [ m (Debye)] components mx my mz m

V

VI

VII

VIII

11.017502 K14.232034 K0.616670 18.008787

K3.307138 K7.544175 0.315422 8.243254

K3.779205 K7.218597 0.374815 8.156655

K5.924792 K7.100717 0.506678 9.261753

Fig. 2 are given in Table 4. It is seen that the electric dipole moments for the compound VI with –NO substituent and the compound VIII with –CHO substituent have been obtained to be very similar (see Table 4), and thus it is found that there is no difference between the order of electronegativities and the order of g values for both substituents, – NO and –CHO, i.e. –CHOO–NO. The compound VII with –CN acceptor group with m written in bold letters has the smallest electric dipole moment. This result has yielded the smallest second hyperpolarizability g for the compound VII with –CN group. There are many examples of Schiff bases with donor– acceptor groups. Nicoud and Twieg [17] reported a Schiff base compound having an electron acceptor group –NO2 at one end and a donor group –NH2 at the opposite end. It is seen that this compound is similar to compound V in Fig. 2 and –NH2 donor group has been substituted instead of –F atom substituted in compound V. The ground state electric dipole moment and the first hyperpolarizability values of the compound studied in Ref. [17] are as 13.3 D and 77! 10K30 esu, respectively. These m and b values, respectively, have been found to be smaller and higher than those calculated in this study (see Tables 3 and 4 for compound V). Because the –NH2 donor group is stronger than –F atom, it is found that the b value in Ref. [17] is greater than that in compound V (Fig. 2). When a halogenic substituent –F atom instead of –CH3 group is connected, we investigated whether this compound has a higher b-V and g, and thus a good NLO character or not. It is shown that the compound V having –F substituent instead of –CH3 group in Refs. [18,19] has rather high b-V and g values (Table 3). Gotoh et al. [20] obtained 35 times larger powder SHG efficiency than that of urea for compound II in Fig. 1. It is shown that the compound II has got high b-V and g values calculated ab initio (Table 1) and a high powder SHG efficiency measured experimentally by Gotoh et al. [20]. It is important to stress that in our investigations for b and g, we do not take into account the effect of the field on the nuclear positions, i.e. we evaluate only the electronic component of b and g. The vibrational contributions which can be important according to the NLO process are left for further investigations. The frequency (u) of the applied field (in eV) used in the calculation should be large enough to avoid numerical imprecision but small enough to avoid the higher order contaminations that are not removed by various iterations. In addition, too large frequency of the applied

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field can lead to SCF convergence problems. In practice, we have chosen uZ1.17 eV. Such tightened thresholds and well-chosen electric field amplitudes are necessary to obtain almost precise hyperpolarizability values. Moreover it is seen that most scientists studying NLO properties experimentally have been performing the measurements at this frequency value uZ1.17 eV (1.064 mm wavelength) for many years. One often meets with this frequency of the applied field in the studies—both the theoretically used ab initio and semiempirical techniques [21–29] and the experimental techniques such as SHG, EFISHG, etc. [18, 20,30–37]—on NLO properties. b and g of Schiff bases studied are particularly sensitive to the electronegativities of substituents and regulating groups connected to these Schiff bases. It is necessary to give some theoretical prediction and calculations in advance on their structure/properties and to help experimental experts in designing and synthesizing their desired compounds. In our theoretical calculations, TDHF method has been found to have on the one hand, a high efficiency and appropriateness between, this level of theory employed and the accuracy of the calculations performed here and, on the other hand, the computational capabilities. Considerable progress has been achieved with this paper toward understanding the nature of non-linear optical phenomenon of the Schiff base compounds studied here and ab initio theory has played a significant role in this progress.

References [1] R.W. Boyd, Nonlinear Optics, Academic Press, New York, 1992. [2] Y.R. Shen, The Principles of Nonlinear Optics, Wiley, New York, 1984. [3] N. Bloembergen, W.A. Benjamin, Nonlinear Optics 1965 New York. [4] J.G. Bergman, S.K. Kurtz, Mater. Sci. Eng. 5 (1969) 235. [5] P.A. Franken, J.F. Ward, Rev. Mod. Phys. 35 (1963) 23. [6] H.S. Nalwa, S. Miyata, Nonlinear Optics of Organic Molecules and Polymers, CRC Press, New York, 1997. [7] B.L. Davydov, L.D. Derkacheva, V.V. Dunina, M.E. Zhabotinskii, V.K. Zolin, L.G. Kreneva, M.A. Samokhina, JEPT Lett. 12 (1970) 16. [8] M. Lanata, C. Bertarelli, M.C. Gallazzi, A. Bianco, M. Del Zoppo, G. Zerbi, Synth. Metals 138 (2003) 357. [9] D.R. Kanis, M.A. Ratner, T.J. Marks, Chem. Rev. 94 (1994) 195. [10] Intelx86 (win 32, Linux, OS/2, DOS) version. PC GAMESS version 6.2, build number 2068. This version of GAMESS is described in: M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen, S. Koseki, N. Matsunaga, K.A. Nguyen, S.J. Su, T.L. Windus, M. Dupuis, J.A. Montgomery, J. Comput. Chem. 14 (1993) 1347. [11] P.P. Korambath, S.P. Karna, J. Phys. Chem. A 104 (2000) 4801. [12] (a) M.P. Bogaard, B. Orr, J. Phys. Chem., Ser. Two. (b) A.D. Buckingham (Ed.), MTP International Review of Science vol. 2, Butterworths, London, 1975, p. 149. [13] K. Bhat, K.J. Chang, M.D. Aggarwal, W.S. Wang, B.G. Penn, D.O. Frazier, Mater. Chem. Phys. 44 (1996) 261. [14] S.R. Marder, D.N. Beratan, B.G. Tiemann, L.T. Cheng, W. Tam, in: R.A. Hahn, D. Bloor (Eds.), Organic Materials for Non-linear Optics II, The Royal Society of Chemistry, Cambridge, 1991. [15] A.W. Baker, T. Shulgin, J. Am. Chem. Soc. 81 (1959) 1523.

122

A. Karakas¸ et al. / Journal of Molecular Structure: THEOCHEM 712 (2004) 117–122

[16] L.W. Fine, H. Beall, Chemistry for Engineers and Scientists, Saunders College Publishing, USA, 1990. [17] J.F. Nicoud, R.J. Twieg, in: D.S. Chemla, J. Zyss (Eds.), Nonlinear Optical Properties of Organic Molecules and Crystals vol. 1, Academic, New York, 1987, p. 227. chap. II-3. [18] R.T. Bailey, G. Bourhill, F.R. Cruickshank, D. Pugh, J.N. Sherwood, G.S. Simpson, J. Appl. Phys. 73 (1993) 1591. [19] P.J. Halfpenny, E.E.A. Shepherd, J.N. Sherwood, G.S. Simpson, SPIE Proc. 2025 (1993) 171. [20] T. Gotoh, T. Tsunekawa, T. Kondoh, S. Fukuda, H. Mataki, M. Iwamoto, Y. Maeda, Proceedings of the First International Workshop on Crystal Growth of Organic Materials 1989 p. 234. [21] J.O. Morley, J. Phys. Chem. 99 (1995) 1923. [22] S. Allen, T.D. McLean, P.F. Gordon, B.D. Bothwell, M.B. Hursthouse, S.A. Karaulov, J. Appl. Phys. 64 (1988) 2583. [23] J.O. Morley, V.J. Docherty, D. Pugh, J. Chem. Soc. Perkin Trans. II (1987) 1351. [24] J.O. Morley, J. Chem. Soc. Perkin Trans. 2 (1995) 177.

[25] C.R. Moylan, J. Phys. Chem. 98 (1994) 13513. [26] M. Dory, J-M. Andre, J. Delhalle, J.O. Morley, J. Chem. Soc. Faraday Trans. 90 (1994) 2319. [27] I.D.L. Albert, D. Pugh, J.O. Morley, J. Phys. Chem. 99 (1995) 8024. [28] J.O. Morley, J. Chem. Soc. Perkin Trans. 2 (1994) 1211. [29] J.O. Morley, J. Phys. Chem. 98 (1994) 11818. [30] R.A. Huijts, G.L.J. Hesselink, Chem. Phys. Lett. 156 (1989) 209. [31] B.L. Davydov, L.G. Korenneva, E.A. Lavrovsky, J. Chem. Phys. 66 (1977) 3806. [32] S.N. Oliver, P. Pantelis, P.L. Dunn, Appl. Phys. Lett. 56 (1990) 307. [33] C.V. Francis, G.V.D. Tiers, Chem. Mater. 4 (1992) 353. [34] S.R. Marder, J.W. Perry, W.P. Schaefer, B.G. Tiemann, P.C. Groves, K.J. Perry, SPIE Proc. 1147 (1989) 108. [35] J. Zyss, I. Ledoux, M. Bertault, E. Toupet, Chem. Phys. 150 (1991) 125. [36] C.R. Moylan, R.D. Miller, R.J. Twieg, K.M. Betterton, V.Y. Lee, T.J. Matray, C. Nguyen, Chem. Mater. 5 (1993) 1499. [37] W. Tam, B. Guerin, J.C. Calabrese, S.H. Stevenson, Chem. Phys. Lett. 154 (1989) 93.