Synthesis and UV-visible properties of thienyl-substituted pyrrole azo dyes

The synthesis of thienylsubstituted pyrrole azo dyes and their UV-visible properties are described.


Introduction
2] They have great potential especially for use in optical communication, information processing, frequency doubling and integrated optics. 310] Employment of conjugated thiophene and pyrrole derivatives as donors combined with substituted acceptor groups are promising candidates among such D-A systems due to their numerous applications.[13][14][15] As part of our continuing interest in non-linear optical material [8][9][10]16 we report in this paper the

UV-visible study
Electronic absorption spectra of all push-pull compounds 3-6 show an intense lowest energy charge-transfer absorption band in the UV-visible region.The position of this band is strongly influenced by the structure of the compounds, for example by the type of substitution pattern in the donor and the acceptor moieties.Communication between the electron donating and accepting termini can be evaluated by comparing the λ max values.
The influence of the strength of the acceptor group is demonstrated by comparison of the absorption maxima of compounds 3a and 6a as the longest wavelength transition is shifted from 469. 5 ). Figure 2 demonstrates the influence of the acceptor group on absorption maxima for the 1aryl-2-(thieno-2-yl)-5-phenylazopyrrole series.This effect has been attributed to the stabilization of LUMO by the electron-withdrawing groups. 19The influence of the substituent on the nitrogen atom of the pyrrole ring is demonstrated by comparison of the absorption maxima of compounds 5a and 5b as the longest wavelength transition is shifted from 486.0 nm in 1-(n-propyl)-  Compounds 3-6 were completely characterized by elemental analysis and/or HRMS, 1 H NMR and 13 C NMR spectroscopy and UV-visible and IR spectroscopy.
The examination of the solvatochromic, the electrochemical and the non-linear optical properties of the new push-pull systems 3-6 will be investigated in the future.

Conclusions
In summary, we have achieved the first synthesis of a series of thienyl-substituted pyrrole azo dyes 3-6, and have measured their UV-visible absorptions.By comparing the derivatives synthesized, it can be seen that the withdrawing group on the phenylazo moiety and the type of substituent on the nitrogen atom of the pyrrole ring has significant influence on the UV-visible properties.
In agreement with previous findings 11,13,15 the new compounds prepared can be applied for the manufacture of new materials with strong non-linear optical (NLO) properties.

ii) Coupling reaction with thienyl pyrroles 1a-b
The diazonium salt solution previously prepared (4.0 mmol) was added drop wise to the solution of thienyl pyrroles 1 (4.0 mmol) in acetonitrile (50 ml) and some drops of acetic acid.The combined solution was maintained at 0 ºC for 2 h with stirring.After this time the resulting mixture was diluted with petrol ether (20 ml) and water (40 ml) and the product formed was isolated by filtration.The organic layer was diluted with chloroform, washed with water and dried with anhydrous synthesis and the UV-visible study of new 1-alkyl(aryl)-2-(thieno-2-yl)-5-phenylazopyrrole derivatives 3-6 which have the 4-CO 2 Me, 4-CN, 4-NO 2 and 2,4-[NO 2 ] 2 groups as the electronwithdrawing groups on the phenylazo moiety and the conjugated 1-alkyl(aryl)-2-(thieno-2yl)pyrrole, as strong π-electron donor moieties.

Figure 3 Figure 2 -Figure 3 -
Figure 2 -UV-visible spectra of compounds 3a and 6a recorded in ethanol, demonstrating the electron-withdrawing effect of the acceptor group on the absorption maxima.