Bipolaron Formation and Nonlinear Optical Properties in Bis-Thienyl Polyenes
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snR
n=3-10
SbCi8'CH21A RS
ZCH-+CH=CH+-.,
CH=(Y
SR
n =3-8
655 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
However, even with the solubility enhancement achieved through the use of R = n-decyl, we could not obtain solutions with high enough concentration for DFWM studies. Therefore, we have recently completed two synthetic series wherein butyl groups were incorporated in the 3 and 4 positions of each ring. The synthetic details have been reported elsewhere 6,7 , however the absorption spectra of the neutral and oxidized species are shown in Table I. Table I. Absorption Spectra of Bis-Thienyl Polyenes and Polyene Bipolarons Bu
Bu X
Bu - CH =CH~nA
x
n
Xnmax neutral (nm-a
H H H H H H H H
3
422, 399, 380 443, 418, 396 462, 435,412
BuS
3 4 5
BuS
BuS BuS BuS BuS
4 5
6 7 8 9 10
6 7
8
Bu
480, 450, 426 496, 464, 439 510, 477, 450 521, 489, 461 534, 499, 471
__max bipolaron (nm)a
M5, 593 i6, 600 715, 679 809, 719 849, 790 914, 855 971, 892 1022, 950 643
424 439 451 466 476 488
657 705 754 801 849
a 10-5 M solutions in CH 2C12; underlined peak equals peak of maximum absorption RESULTS AND DISCUSSION The question of length dependence of y has been debated for several years, since the original proposal that x( 3) is proportional to the reciprocal of the band gap raised to the sixth power8. Therefore, the most direct method of increasing -yis simply to increase the length of the pi-conjugation sequence. However, various workers have shown that -y/N will eventually level off with increasing N9-10 . For example, Prasad has demonstrated that for an oligomer series related to polythiophene, XP levels off at the hexamer level, whereas in poly(p-phenylene) oligomers, X(3) levels off at terphenyl (N = 3)11-13. Thus it might be better to consider the concept of effective conjugation length which may be less than the overall oligomer length due to such things as
conformational twisting out of conjugation. The question of charge state stabilization in small oligomers has been addressed by our group1-5 and others 14 . Although much of this work
656
originally centered around the question of charge delocalization in connection with the increased conductivity achieved upon chemical or electrochemical doping, more recently the question of charge state enhancement of NLO response has received increasing attention. DeMelo and Silbey have postulated that enhancement of y may be possible by incorporating delocalized charge states in the pi-conjugation sequence of electroactive polymers such as
polyacetylene l 5 ,16. We have recently shown that oxidative doping of bis-anthracenyl polyenes shows dramatic enhancement of X . Since increasing the conjugation length and solubility of the bis-thienyl polyene series is synthetically more readily achievable, we have chosen to examine the third order NLO properties of this series in more detail. Preliminary X(3) evaluation of the neutral and oxidatively doped polyenes were carried out at 532 and 1064 nm by degenerate four wave mixing in conventiona
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