Benzothiazole derived NLO-phores: EFISH Measurements and Computational Study

Donor – acceptor substituted p -conjugated organic molecules are potential candidates for non-linear optic materials. Owing to their potential application in electro-optic devices there is a growing need in the design and synthesis of such NLO-phores [1]. Recently we have synthesised a number of biologically active 2-styryl benzothiazole derivatives [2] which we anticipate to be possible parent structures for useful NLO-phores. Although considerable progress has been made in understanding the factors responsible for high molecular nonlinearities [1] there is still an element of trial and error in development of electro-optic materials. Computational methods have proven to be very useful for a more rational design of organic chromophores with high quadratic molecular hyperpolarizabilities. In the following we want to present the results of a combined experimental electric field induced second harmonic generation (EFISH measurements [3]) and theoretical (semi-empirical ZINDO SOS [4]) study on some selected benzothiazole containing NLO-phores (Scheme 1).

Scheme 1

 
 
 
 

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RESULTS

Experimentally, by the EFISH measurement, the quantity µ.ß is obtained (equ. 1): (equ. 1)

where ß_i (i = x, y, z) is defined as:

equ. 2

and µ_i is a vector component of the dipole moment. Measured and calculated µ.ß – values for compounds 1 – 4 are summarised (together with l_max data) in Table 1. The strong charge transfer character of the longest wavelength electronic absorption band - which according to the two-state model of molecular hyperpolarizabilities - should have a profound influence on ß, is evident from the two orbitals (HOMO and LUMO) involved in these transitions ( Figure 1).

Given the rather promising results (high µ.ß – values and good to excellent agreement between experimental and calculated data) we wondered whether structural modifications of these molecules could further enhance molecular hyperpolarizabilities. Owing to the above mentioned charge trnafer nature, increasing the acceptor and/or donor strength should be a good choice.
A selection of such variations is shown in Scheme 2 and the results of the ZINDO SOS calculations are given in Table 2. Not unexpectedly, di-and, especially, tricyanovinyl groups are favourable acceptor groups in terms of high optic nonlinearities (1 vs. 5 or 6 ). Replacing the p-aminoaryl by 5-aminohetaryl moieties ( 1 vs. 7 or 8 ) also leads to a predicted increase of µ.ß – values. Most interestingly, however, reversal of the donor/acceptor substitution pattern (5 vs 9 and 6 vs 10 ) should have a rather pronounced effect.

Scheme 2

 
 
 
 

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CONCLUSIONS

Guided by the results of the semi-empirical calculations as well as EFISH measurements, possible structural modifications of 2-styryl benzothiazole derivatives for obtaining NLO-phores with high quadratic molecular hyperpolarizabilities can be established. For instance, structures 7 – 10 are predicted to be worthwhile targets for synthesis.
 

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REFERENCES

[1] A. P. Alivisatos, P. F. Barbara, A. W. Castleman, J. Chang, D. A. Dixon, M. L. Klein, G. L. McLendon, J. S. Miller, M. A. Ratner, P. J. Rossky, S. I. Stupp and M. E. Thompson, Adv. Mater., 1998, 10, 1297; L. Dalton, Advances in Polymer Science, 2002, 158, 1; S. R. Marder, B. Kippelen, A. K. Y. Jen and N. Peyghammbarian, Nature, 1997, 388, 845; C. Bosshard, M.-S. Wong, F. Pan, R. Spreiter, S. Follonier, U. Meier and P. Gunter, NATO ASI Series, Series 3: High Technology, 1997, 24, 279; J. Zyss, B. Dick, G. Stegeman and R. Twieg, Chem. Phys., 1999, 245, 1; S. R. Marder and J. W. Perry, Adv. Mat., 1993, 5, 804; D. R. Kanis, M. A. Ratner and T. J. Marks, Chem. Rev., 1994, 94, 195; A. K. Y. Jen, T.-A. Chen, V. P. Rao, Y. Cai, Y.-J. Liu and L. R. Dalton, Advances in Nonlinear Optics, 1997, 4, 237.

[2] P. Zahradnik and R. Buffa, Molecules, 2002, 7, 534 ;R. Buffa, P. Zahradnik and P. Foltinova, Coll. Czech. Chem. Comm., 2002, 67, 1820.

[3] B. F. Levine and C. G. Bethea, J. Chem. Phys., 1975, 63, 2666; K. D. Singer and A. F. Garito, J. Chem. Phys., 1981, 75,3572; R. Dworczak and D. Kieslinger, Phys. Chem. Chem. Phys., 2000, 2, 5057.

[4] J. Ridley and M. C. Zerner, Theor. Chim. Acta., 1973, 32, 111; M. C. Zerner, ZINDO, A Comprehensive Semiempirical Quantum Chemistry Package, Quantum Theory Project, Gainesville, Florida, USA, 1993; V. J. Docherty, D. Pugh and J. O. Morley, J. Chem. Soc., Faraday Trans. 2, 1985, 81, 1179.

[5] R. D. Miller, V. Y. Lee and C. R. Moylan, Chem.Mat., 1994, 6, 1023.
 

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Table 1   Comparison of calculated values with experiment (l_max in nm; mb in 10^-48 esu)

Cmp.	solvent	lmax	lmax exp	w[eV]	m.b	m.b
1	dioxane	427	441	1.17	1847	not enough sample
2	dioxane	385	369	1.17	706	775
3a	chloroform	453	458	0.65	bvec= 67	bvec=78.8
4	dioxane	406	416	1.17	1066	1216

^aExperimental value from ref [5]

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Table 2   ZINDO SOS calculated absorption maxima and hyperpolarizabilities for compounds 5 - 10
(l_max in nm; mb in 10^-48 esu)

	gas phase				dioxane
compd	lmax / nm	ßvec0.00	ßvec0.65	ßvec1.17	lmax / nm	ßvec0.00	ßvec0.65	ßvec1.17
5	384	24	30	65	410	37	47	110
6	396	32	43	101	450	46	63	170
7	412	35	47	123	458	46	64	193
8	420	37	50	139	462	49	68	216
9	422	51	72	223	515	68	98	354
10	454	74	110	464	602	100	154	925

 

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Figure 1 HOMO (left) and LUMO (right) of compound 1 (B3LYP/6-31G*)