The design of Optical Limiters Based on the Formation of Bipolaron-Like Dications
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NLO response as potential RSA sensor and eye protectors. For the past several years, various research groups have studied the formation and optical characteristics of bipolaronic 2 species in extended in-systems, either by chemical or electrochemical oxidation.' "1 Chemically generated bipolarons typically exhibit much higher absorption cross sections than the neutral precursors, and are quite stable. In addition, bipolaron-like dications can be chemically generated in small polymer oligomers in solution, or in oligomeric repeat units in formal copolymers." The absorption characteristics are directly related to the oligomer conjugation length and chromophore identity, and can be manipulated by the addition of mesomerically interactive electron-donating or -withdrawing groups. Thus the bipolaron absorption characteristics can be designed and fine-tuned within a specific region of the visible and NIR spectra. SYNTHESIS OF COPOLYMERS INCORPORATING DITHIENYLPOLYENE REPEAT UNITS Photogeneration of bipolarons has been studied extensively in arylene vinylene polymers,' and these charge states can have lifetimes approaching 0.1 sec. 6 Spangler and coworkers' 7 2 ' have demonstrated that very stable bipolaronic-like dications can be formed from dithienylpolyenes in solution, and that dithienylpolyene repeat units can be incorporated in various copolymer formulation. 2 Oxidative doping of these copolymers show that each and every polyene repeat unit can be converted to the dication form. The synthesis of a typical copolymer series in which the polyene conjugation length can be systematically varied is illustrated in Scheme 1. The copolymers can be oxidized in solution, or as spin-cast thin films. The absorption characteristics of both neutral and oxidized copolymers are illustrated in Figures 1 and 2 for the dithienyl pentaene, hexaene and heptaenes, and are also listed in Table 1 compared to a series of bis-(butylthio) dithienylpolyene model compounds. As can readily be seen, controlling the polyene conjugation length allows the bipolaron absorption maxima to be varied over several hundred nm.
439. 466 452 481 460. 491 476. 511 485
4 5 6 7 8
x 4 5 6 7 8
X,,
t _- * (nm) 439 451 466 476 488
660 694 752 739. 796 794 864 835, 922
Xm,, BP (nm) 657 705 754 801 849
Bu
Bu
Bu
Bu
Me Jn
Bus
(CH=CH ),Z
/
S
Bu
Bu
Bu
Bu
SBu
S
'Underlined peaks represent peaks of maximum absorption.
Table 1 Bipolaron formation in dithienylpolyene oligomers and copolymers
60
R
R
R -H or Bu
r 3r
-CHO S
R
R
/
- 1.2.,3 ...
Br
(CH=CH).CHO
or itL iii
R
R
R
R
\/ Br
p-3-8
(CH=CH)P
Br
I 1.4 eq. tert-BuLi/THF q - I, 2 ...
2. Cl-(R 2 Si)---CI R
R
R
R Me
S (CH=CH )P
1=- 1. C
Sir
-•CH2P Bu, Bgr'/ N aO Et /D MF
2. H30+ /THF Repeat •
4-
i - Bu3PCH 2CH=CHCH 2PBu 3. 2C1 /NaOEt/EtOH or DMF iii
Bu3 PCH 2(CH=CH) 2CH 2PBu 3. 2Br" / NaOEt / EtOH or DM F
Scheme 1 Synthesis of copolymers incorporating dithienylpolyene repeat units
61
I SbCl
5 / CH 2C1
--t
+l-t
+
I
I
I+
+
+ -+
+1-
+ -- +
neutral dithienyl
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