Environmental Effects on the Electronic Spectra of Electrostatically Assembled Layers of Conjugated Polymers
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nucleic acids to a surface [11] and nylon-like stable film over a wide pH range through chemically induced amide formation [12]. On the basis of the previous information, it is conceivable that induced optical properties of conjugated polymers in the programmed electrostatic assembly can be achieved by the modification of environment (of the refractive indices of polycations) or by post-assembly processing. However, to our knowledge, there has been no systematic attempt to modify optical properties of programmed assemblies of conjugated polymers by post-assembly-processing. EXPERIMENTAL HP4BCMU (Mn = 7x105 g/mol) [13], poly(4-vinylpyridine) quaternized 90% with dimethylsulfate (Mn = 5x105 g/mol) [14], poly[2-(3-thienyl)ethanol n-butoxycarbonylmethylurethane] (PURET), and 60% HT-HPURET (Mn = 1.7x104 g/mol) [15] were prepared according to the literature. Hydrolyzed 88% polyvinylalcohol (PVA, Mn = 7.8x104 g/mol) and poly(4-vinylpyridine) were purchased from Polysciences Inc and PDADMAC (Mn = 1-2x 105 g/mol) from Aldrich. The structures of these polymers are shown in figure 1. HP4BCMU (10-4 M at pH 11) was deposited on glass slides up to 16 bilayers with PDADMAC or QPVP (10-3 M at pH 11), with each alternating for 10 min. dipping time. HPURET in 10-3 M aqueous solution at pH 4 was deposited on glass slides alternately with 2x10-3 M PDADMAC at pH 11 (dipping time was 10 min. each). OH n H N O
N CH3
Cl
HP4BCMU __ Mn = 7x10 5 g/mol
O
CH3
O
PDADMAC __ Mn = 1-2x10 5 g/mol
O
n H
OH N
O CH2
CH
O
n O H
CH3
N
CH3 SO 4
H O
N O
QPVP __ Mn = 5x10 4 g/mol
O S
n
Figure 1. Polyelectrolytes
60% HT-HPURET __ Mn = 1.7x10 4 g/mol
RESULTS AND DISCUSSION HP4BCMU in 10-4 M aqueous solution at pH 11 was processed as described in experimental, by programmed electrostatic assembly (PEA) on glass slides up to 16 bilayers with PDADMAC or QPVP in10-3 M aqueous solution at pH 11, with each alternating 10 minutes dipping time. The inset diagram in figure 2 shows the linear increase of optical density as bilayers are built up to a total of 16. The spectrum shifted to longer wavelength as the bilayers built up, possibly due to reorganized structure. The red shift of the higher energy peak on visible absorption spectra was from 476 nm (bottom line, 2 bilayers) to 499 nm (top line, 16 bilayers) and the increase of optical density of lower energy peak was larger than that of higher energy peak (figure 2a). It is well known that the chromophore of PDA is sensitive to the refractive index of the solvent [4]. But, there is no significant difference on overall visible absorption spectra of between HP4BCMU-PEA with PDADMAC and that with QPVP, even though the refractive index of 4-vinylpyridine is 1.5500 and that of 50% wt. PDADMAC in water 1.4370. The peak positions of the visible absorption spectra of HP4BCMU-PEA (16 bilayers) were 499 nm and 518 nm and the energy difference of two absorption peaks was 735 cm-1 (solid line in figure 2b). Absorbance at 490 nm
0.16 0.14
0.1
0.15 0.0
0.10
0 2 4 6 8 10 12 14 16
Number of Bilayers
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