Synthesis and Characterization of New Polyaryleneethynylenes based on 3-Hexyl Multisubstituted Oligothiophene Blocks
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RIMENTAL SECTION The experimental details on the monomer, model-compound and polymer preparation together with the instruments used will be provided elsewhere [8]. For the sake of clarity molecule numbering scheme will be the same used in ref [8]. RESULTS AND DISCUSSIONS The preparation of terminal acetylene derivatives is based on the key methodology pictured in the Scheme 1 for the preparation of 5,5’’-dibromo ethynyl-3,3’’-dihexylα-terthiophene(10): a Pd-catalized coupling [9], followed by hydrolisys at room temperature [10]. This Heck-Sonogashira procedure was applied to prepare both all the monomers and model compounds. Me Me Br
S
S
Si
C
CH
Me
Br
S C 6 H 13
C
C
S
S
C
S C 6 H 13
C 6 H 13
Toluen e
E t3N ;
C
KOH (a q)
S
S
C
C
Si M e Me
Me
P d(P h 3P ) 4 ; C uI ;
3
HC
Me
Me M e Si
C 6H 13
E tO H ; THF
9
CH
S C 6 H 13
C6 H 13
10
SCHEME 1
In Table 1 the prepared compounds together with their optical features are reported. All the pure monomers with bromine and acetylene groups were molecularly characterized by Mass, FTIR and NMR, spectroscopies. As shown in Table 1 the monomers and the model compounds were also optically characterized by absorption and emission spectroscopies. Oligomers 8, 9 and 10 in solution show a large Stokes shift (energy difference between absorption and emission maxima) indicating that a high torsional relaxation occurs in the excited state. This effect, joined with the high inter-system crossing rate towards triplet states, is responsible for the quite low PL QY of thiophene based molecules[11]. The Q.Y. values (9-12%) measured for molecules 8-10 are similar to that of diformyl terthiophene [12], and higher than that of terthiophene (6%) [13], in agreement with the increased conjugation length of these molecules with respect to T3. The conjugation length of molecule 8 is strongly increased by adding anthracene (compare 8-11 in Table 1), although PL is only due to emission from the lower-energy moiety. Differently from molecule 11, the series of molecules 12, 13 and 14, shows optical spectra strongly resembling those of the anthracene moiety (see Fig. 1), but red-shifted of 0.35, 0.39, and 0.48 eV, respectively, indicating an increase in conjugation length in the series. The PL QY values of molecules 12,13, and 14 slightly decrease as the conjugation length increases but they are much higher than those of the molecules containing thiophene units. The polymerization procedure for the preparation of all the polymers are quite similar to those used for monomer and models. The polymers, prepared using mostly dibromide oligothiophenes, dibromoanthracene and diethynyl compound of benzene, anthracene and dihexylsubstituted terthiophene, are listed in Fig.2. The materials were
2
Table 1. Compounds with Acetylene Groups Absorption PL λmax (nm)
Compounds
λmax (nm)
PL QY
THF
%
357
422; 440; 473
9.1
379
456; 484
12.0
369
444; 471
11.8
449
561
28
397;
397; 423; 449
69
414;
438; 444; 468
54
401;
400; 424; 452
55
CHCl3
360
Me S
S
C C
Si
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