Synthesis and (non)linear optical properties of a series of donor-oligopyrrole-acceptor molecules

The Pd-catalyzed cross-coupling reaction involving organostannanes (Stille reaction) is applied to prepare a series of N-t-BOC-protected D-pi-A oligopyrroles. After purification, oligomers with one to four pyrrole units inserted between a 4-nitrophenyl and a 4-methoxyphenyl group are isolated in yields between 20 and 47%. Only minor differences in the linear optical properties are observed for the four oligomers. The charge-transfer band around lambda(max) = 365 nm shows a small, unexpected, hypsochromic shift, while the pi-pi* band around lambda(max) = 285 nm shows a small, expected, bathochromic shift upon elongation of the molecule. Their nonlinear optical properties, however, show a surprising proceeding; going from the D-pi-A oligomer with one pyrrole unit to that with three pyrrole units, the hyperpolarizability, as measured by hyper-Rayleigh scattering, increases additionally with the number of pyrrole units within the oligomer, up to 277 x 10(-30) esu in case of the trimer. On the basis of the assumption that both transitions contribute to the hyperpolarizability, a better conjugated D-pi-A oligomer with a bithienyl spacer inserted between a 2-(4-nitrophenyl)-5-pyrrolyl and a 2-(4-methoxyphenyl)-5-pyrrolyl group is prepared analogously. This molecule shows only one combined absorption at lambda(max) = 378 nm for both the charge transfer and the pi-pi* band, while the hyperpolarizability is as high as 440 x 10(-30) esu. These data, showing a very favorable transparency-hyperpolarizability tradeoff, are explained in terms of the contribution of two transitions that are superimposed.