Thermally Stable Poled Polymers: Highly Efficient Heteroaromatic Chromophores in High Temperature Polyimides
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413 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
requires the evaluation of the ground state dipole moment (ji). The EFISH data obtained (in 1,4-dioxane solvent) for compounds 1 - 18 are presented in Tables 1 - 4. EFFECT OF THIOPHENE AS CONJUGATION MOIETY The dependence of molecular nonlinearity (0gji)on the effectiveness of the conjugating moieties (benzene vs. thiophene) was reported in our earlier communications 1012. A comparison of the pj values of benzenoid stilbene compounds 1, 2, and 5 with the corresponding thiophene analogs 2, 4, and 6 shows a large enhancement (1.8 to 2.5 x) due to the thiophene (Table 1). Furthermore, the comparison of their electronic absorption data reveals that the replacement of a benzene ring with a thiophene ring causes a dramatic red shift (- 100 nm). Our previous study of the dependence of NLO properties on a series of thiophene substituted stilbene compounds with different molecular lengths also revealed no clear sign of saturation of f0ji with length even up to four rings systems. 2 These observations clearly indicate that the thiophene moiety introduces more delocalized pielectron system in donor-acceptor compounds compared to benzenoid moieties. Table 1 Compound
N0N2
1 2
N
3
NN
4._ £
N-
'
__N-(r3----\••
p3it (10-48 esu
in dioxane
at 1.907 1t)
424
580
516
1040
CN CN
468
1100
•-CN CN CN C-N
584
2600
594
2700
NO2
S •-'
Xmax (nm)
CN
S2N/i• ,.•,•°.-&%718 N"•'S)••" 6
6900
K*L'O
EFFECT OF CONJUGATION LENGTH Comparison of the Pgjvalues obtained for the compounds 7, 9 and 11 with compounds 8, 10 and 12 reveals a significant enhancement of molecular nonlinearity (a factor of 1.5 - 2) due to the increase of effective conjugation length by one olefin moiety (Table 2). This result corresponds very well with literature reported values for donor-
414
acceptor substituted polyenes. However, the effectiveness for the olefin moiety to enhance molecular nonlinearity is also dependent on the combined effects of donor-acceptor strength. Table 2 clearly shows that the ratio of enhancement for fjti values decreases from 2 to 1.5 as the acceptor strength increases from dicyanovinyl to tricyanovinyl. Table 2
EFFECT OF ACCEPTOR STRENGTH The dependence of molecular nonlinearity on the acceptor strength of a series of stilbene-like compounds is listed in Table 3. The electronic absorption data of these compounds clearly show the difference of charge-transfer properties as a function of acceptor strength. In the comparison of dicyanovinyl substituted compound 7 and tricyanovinyl substituted compound 11, a dramatic red shift (- 120 nm) of the absorption peak and a large fij enhancement (- a factor of 5) clearly indicate that the tricyanovinyl group is a more effective electron-acceptor 14. The dipole moments of compounds 7 & 11 were not measured. However, the reported values for single ring compounds N,N-dialkylamino4-dicyanovinyl benzene and N,N--dialkylamino-4- tricyanovinyl benzene are quite similar. Thus, one may infer that the observed e
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