Synthesis of novel sulfonyl-substituted pyrrole chromophores for second-order nonlinear optics

Article abstract of DOI:10.1016/S0040-4039(00)02232-2

The first synthesis of sulfonyl-substituted pyrrole chromophores 1-4 and their UV-vis absorptions, second-order nonlinear optical properties and thermal stability are described and compared with those of the benzene, thiophene and furan analogues 5-9.

Full text of DOI:10.1016/S0040-4039(00)02232-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1309–1311
Synthesis of novel sulfonyl-substituted pyrrole chromophores for
second-order nonlinear optics
Shang-Shing P. Chou* and Yu-Hsin Yeh
Department of Chemistry, Fu Jen Catholic University, Taipei, Taiwan 242, Republic of China
Received 30 October 2000; revised 24 November 2000; accepted 1 December 2000
Abstract—The first synthesis of sulfonyl-substituted pyrrole chromophores 1–4 and their UV–vis absorptions, second-order
nonlinear optical properties and thermal stability are described and compared with those of the benzene, thiophene and furan
analogues 5–9. © 2001 Elsevier Science Ltd. All rights reserved.
The design and synthesis of organic chromophores as
nonlinear optical (NLO) materials have attracted much
attention in recent years.1,2 They have great potential
especially for use in optical communication, informa-
tion processing, frequency doubling and integrated
optics.3 Organic NLO materials have many advantages
over inorganic materials, such as large nonlinear optical
coefficients, greater ease for synthetic design, easy
preparation and lower cost.4,5 It has been revealed that
the product of dipole moment and molecular hyperpo-
larizabilities (vi) of heterocyclic chromophores are
often higher than their benzene analogues.6 We have
also demonstrated that sulfonyl-substituted thiophene7
and furan chromophores8 have many favorable features
as NLO materials. Most recently, we have reported the
first synthesis of some pyrrole-containing chro-
mophores with a p-bridge of CꢀN.9 However, these
CꢀN compounds are rather prone to hydrolytic decom-
position. Thus, we want to improve the stability of
these compounds by replacing with a CꢀC p-bridge.
stability of pyrrole chromophores 1–4 bearing the p-
bridge of CꢀC which have the sulfonyl group as the
electron-withdrawing group on the pyrrole ring and the
amino group as the electron-donating group to increase
the polarization of the chromophore. Although the
sulfone group is not a particularly strong acceptor
substituent, it is synthetically more flexible and has
broader transparency range in the visible spectrum.10
From our previous experiences,9 we have substituted a
methyl group on the pyrrole nitrogen in order to
increase the thermal stability of these compounds. We
will also compare various properties of pyrroles 1–4
with those of benzene, thiophene and furan compounds
5–9.7–9
The synthesis of these sulfonyl-substituted pyrrole chro-
mophores 1–4 is shown in Scheme 1. Reduction of
aldehydes 12 and 159 with sodium borohydride gave the
alcohols 13 and 16, respectively. Treatment of these
alcohols with triphenylphosphine hydrobromide gave
the phosphonium salts 14 and 17.11 Deprotonation of
14 with n-BuLi followed by reaction with aldehydes 15
gave only the cis products 1a and 1b, which are
In this paper we describe the first synthesis, UV–vis
absorptions, second-order NLO properties and thermal
Keywords: pyrroles; heterocyclic compounds; sulfones; nonlinear optics.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02232-2

1310
S.-S. P. Chou, Y.-H. Yeh / Tetrahedron Letters 42 (2001) 1309–1311
expected from nonstabilized ylides.12 On the other hand,
the Wittig reaction of the stabilized ylide derived from
phosphonium salts 17 with aldehydes 12, 18 and 19 gave
predominantly the trans products 2, 3 and 4, respec-
tively.13 The cis compounds 1 can also be converted to
the more stable trans products 2 by treatment with a
catalytic amount of iodine in refluxing toluene.
have slightly longer umax values (2–10 nm) and thus
lower charge-transfer energies than the methyl sulfones
1b–4b. Polar solvents cause a red shift of the umax values
of compounds 1–4 more significantly for the phenyl
sulfones than the methyl sulfones. It is important to note
that the umax values of the heterocyclic chromophores
follow the order thiophene>furan>pyrrole; for example,
5a, 403 nm; 6a, 385 nm; 2a, 366 nm; 7a, 450 nm; 8a, 435
nm; 3a, 370 nm. The umax values of the pyrrole com-
pounds show that trans->cis-CꢀC (e.g. 2a, 366 nm; 1a,
347 nm; 2b, 364 nm; 1b, 337 nm) and CꢀN>CꢀC (e.g. 9a,
394 nm; 2a, 366 nm; 9b, 387 nm; 2b, 364 nm).
The UV–vis absorptions of compounds 1–4 show that
the umax values (in dioxane) are all below 370 nm, and
have virtually no absorption above 480 nm, as judged by
their ucutoff values (Table 1). The phenyl sulfones 1a–4a
Scheme 1. Reagents and conditions: (i) NaBH4 (0.6 equiv.), methanol, 60°C, 2.5 h (for 13), or 0°C, 0.5 h (for 16a), or 0°C, 2.5 h
(for 16b); (ii) PPh3·HBr (1 equiv.), CH2Cl2, reflux, 4 h (for 14 and 17a), or CH2Cl2/CH3CN, reflux, 4 h (for 17b); (iii) (a) 1.6 M
n-BuLi (1.2 equiv.), Et2O, rt, 20 min; (b) 15 (0.25 equiv.), CH2Cl2, 50°C, 2.5 h; (iv) (a) 1.6 M n-BuLi (1.2 equiv.), THF, rt, 10
min, 50°C, 10 min; (b) 12, 18 or 19 (1.2 equiv.), CH2Cl2, reflux, 2.5 h; (v) cat. I2, toluene, reflux, 20 h.
Table 1. UV–vis absorptions veg, vg(ve−vg), vgi, vgi0 values for PNA and compounds 1–9
vg(ve−vg) (Debye2)
vgic (10−48 esu2 · cm)
vgi0 (10−48 esu2 · cm)
Td (°C)
a
b
d
Compound
umax (nm)
ucutoff (nm)
veg (Debye)
1a
1b
2a
2b
3a
3b
4a
4b
347
337
366
364
370
368
354
351
403
393
385
450
435
394
387
352
426
406
421
412
480
469
449
466
465
453
448
518
507
470
470
–
5.6
5.6
4.3
4.6
5.8
4.6
3.6
4.4
–
69
47
65
43
82
57
59
30
–
195
118
130
98
314
135
74
100
63
60
46
143
62
316
278
351
277
282
210
253
192
311
288
–
278
–
300
252
–
37
27
53
5ae
5be
6af
7af
8af
9ag
9bg
PNA
590
488
243
1364
395
119
94
219
195
101
312
108
46
–
–
6.0
7.8
5.7
6.5
6.3
4.7
51
66
46
55
51
54
38
55
110
a Measured in dioxane.
b The wavelength where the absorbance is 5% of that at the umax
.
c Estimated for a fundamental wavelength of 1064 nm.
d Heating rate of 10°C/min, heating range of 50–500°C.
e Ref. 7.
f Ref. 8.
g Ref. 9.

S.-S. P. Chou, Y.-H. Yeh / Tetrahedron Letters 42 (2001) 1309–1311
1311
considerably improved by replacing the p-bridge of
CꢀN with CꢀC. By comparing the thiophene, furan and
pyrrole analogues it can be seen that the heterocyclic
moiety has significant influence on the UV–vis absorp-
tions, NLO properties and thermal stability.
Acknowledgements
Figure 1. X-Ray crystal structure of 2a.
Financial support of this work by the National Science
Council of the Republic of China is gratefully acknowl-
edged (NSC 89-2113-M-030-003). We also thank Dr.
C.-T. Chen for his assistance in using the solvato-
chromic method.
References
1. Chemla, D. S.; Zyss, J. Nonlinear Optical Properties of
Organic Molecules and Crystals; Academic Press:
Orlando, USA, 1987; Vols. 1 and 2.
Figure 2. Packing drawing of compound 2a.
2. Prasad, P. N.; Zhang, Y.; Gao, X.; Pan, H. Chem. Mater.
1995, 7, 816–821.
The molecular hyperpolarizabilities (vgi) of chro-
mophores 1–4 were estimated by solvatochromism.14
The vgi values at zero-frequency (vgi0) were obtained
using the two-level model.15 The vgi0 for compounds
1–4 are 0.5–2.6 times that of p-nitroaniline (PNA). By
comparing the vgi0 values of 2a, 5a, 6a; 3a, 7a, 8a and
4a, 9a, 10a, it can be seen that thiophene>furan>
pyrrole, which is in agreement with the theoretical
calculations.16 Comparison of the vgi0 values of 1–9
reveals that replacing the methyl sulfone group with the
phenyl sulfone group increases their second-order non-
linearities. Pyrrole compounds with a p-bridge of CꢀC
have higher vgi0 values than the CꢀN analogues (2a/
9a=1.3, 2b/9b=1.2).
3. Prasad, P. N.; Williams, D. J. Introduction to Nonlinear
Optical Effects in Molecules and Polymers; Wiley: New
York, 1991; pp. 132–174.
4. Drost, K. J.; Jen, A. K.-Y.; Rao, V. P. Chemtech 1995,
25(9), 16–25 and references cited therein.
5. Marder, S. R.; Perry, J. W. Science 1994, 263, 1706–1707.
6. (a) Rao, V. P.; Jen, A. K.-Y.; Wong, K. Y.; Drost, K. J.
Tetrahedron Lett. 1993, 34, 1747–1750; (b) Jen, A. K.-Y.;
Rao, V. P.; Wong, K. Y.; Drost, K. J. J. Chem. Soc.,
Chem. Commun. 1993, 1118–1120.
7. (a) Chou, S. S. P.; Sun, D. J.; Lin, H. C.; Yang, P. K. J.
Chem. Soc., Chem. Commun. 1996, 1045–1046; (b) Chou,
S. S. P.; Sun, D. J.; Hung, J. Y.; Yang, P. K.; Lin, H. C.
Tetrahedron Lett. 1996, 37, 7279–7282.
8. Chou, S. S. P.; Shen, C. H. Tetrahedron Lett. 1997, 38,
6407–6410.
9. Chou, S. S. P.; Hsu, G. T.; Lin, H. C. Tetrahedron Lett.
1999, 40, 2157–2160.
10. Ulman, A.; Willand, C. S.; Kohler, W.; Robbello, D. S.;
Williams, D. J.; Handley, L. J. Am. Chem. Soc. 1990,
112, 7083–7090.
11. Zhang, J.-X.; Dubois, P.; Jerome, R. Synth. Commun.
1996, 26, 3091–3095.
12. Maryanoff, B. E.; Reitz, A. B. Chem. Rev. 1989, 89,
863–927.
13. Effenberger, F.; Schlosser, H. Synthesis 1990, 11, 1085–
1094.
The decomposition temperatures (Td) of 1–4 were mea-
sured by thermogravimetric analysis (TGA) at a heat-
ing rate of 10°C min−1. It can be seen that the
(phenyl)sulfonyl-substituted pyrrole chromophores
have higher Td than the (methyl)sulfonyl derivatives.
The Td shows a general trend of thiophene:pyrrole
(N-methylated)>furan. The Td of compound 2a (351°C)
is among the highest we have observed for these hetero-
cyclic compounds. Furthermore, within the pyrrole
series, the Td follows the order trans>cis and CꢀC>
CꢀN.
We have also obtained the X-ray crystal structure of 2a
(Fig. 1)17 which shows the trans configuration about the
CꢀC bond and a dihedral angle of 4.53° between the
benzene and the pyrrole ring. Compound 2a has a
noncentrosymmetric alignment with a P212121 space
group (Fig. 2).
14. (a) Paley, M. S.; Harris, J. M. J. Org. Chem. 1989, 54,
3774–3778; (b) For the determination of veg and vg(ve−
vg), see footnote 9 of Ref. 8.
15. Oudar, J. L.; Chemla, D. S. J. Chem. Phys. 1977, 66,
2664–2668.
16. Varanasi, P. R.; Jen, A. K.-Y.; Chandrasekhar, J.; Nam-
boothiri, I. N. N.; Rathna, A. J. Am. Chem. Soc. 1996,
118, 12443–12448.
In summary, we have achieved the first synthesis of a
series of sulfonyl-substituted pyrrole chromophores 1–4
and have measured their UV–vis absorptions, second-
order nonlinearities and decomposition temperatures.
17. Crystal data for C21H22N2O2S (2a): fw 366.47,
,
orthorhombic, space group P212121, a=10.07(17) A, b=
3
,
,
,
15.597(3) A, c=23.952(5) A, V=3763.0(12) A , Z=8,
The vgi0 and Td values of the pyrrole compou
.
nds are
D
calcd=1.294 g/cm3, orange crystal (0.60×0.50×0.45 mm).