L. Groenendaal et al., SYNTHESIS AND (NON)LINEAR OPTICAL-PROPERTIES OF A SERIES OF DONOR-OLIGOPYRROLE-ACCEPTOR MOLECULES, Chemistry of materials, 10(1), 1998, pp. 226-234
The Pd-catalyzed cross-coupling reaction involving organostannanes (St
ille reaction) is applied to prepare a series of N-t-BOC-protected D-p
i-A oligopyrroles. After purification, oligomers with one to four pyrr
ole 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, unexp
ected, 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 surp
rising proceeding; going from the D-pi-A oligomer with one pyrrole uni
t to that with three pyrrole units, the hyperpolarizability, as measur
ed by hyper-Rayleigh scattering, increases additionally with the numbe
r of pyrrole units within the oligomer, up to 277 x 10(-30) esu in cas
e of the trimer. On the basis of the assumption that both transitions
contribute to the hyperpolarizability, a better conjugated D-pi-A olig
omer with a bithienyl spacer inserted between a 2-(4-nitrophenyl)-5-py
rrolyl and a 2-(4-methoxyphenyl)-5-pyrrolyl group is prepared analogou
sly. This molecule shows only one combined absorption at lambda(max) =
378 nm for both the charge transfer and the pi-pi band, while the hy
perpolarizability is as high as 440 x 10(-30) esu. These data, showing
a very favorable transparency-hyperpolarizability tradeoff, are expla
ined in terms of the contribution of two transitions that are superimp
osed.