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 prep. a series of N-t-BOC-protected D-p-A oligopyrroles. After purifn., 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 obsd. for the four oligomers. The charge-transfer band around lmax = 365 nm shows a small, unexpected, hypsochromic shift, while the p-p* band around lmax = 285 nm shows a small, expected, bathochromic shift upon elongation of the mol. Their nonlinear optical properties, however, show a surprising proceeding; going from the D-p-A oligomer with one pyrrole unit to that with three pyrrole units, the hyperpolarizability, as measured by hyper-Rayleigh scattering, increases addnl. with the no. of pyrrole units within the oligomer, up to 277 * 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-p-A oligomer with a bithienyl spacer inserted between a 2-(4-nitrophenyl)-5-pyrrolyl and a 2-(4-methoxyphenyl)-5-pyrrolyl group is prepd. analogously. This mol. shows only one combined absorption at lmax = 378 nm for both the charge transfer and the p-p* band, while the hyperpolarizability is as high as 440 * 10-30 esu. These data, showing a very favorable transparency-hyperpolarizability tradeoff, are explained in terms of the contribution of two transitions that are superimpose