Electronic properties of heterocyclic aromatic hydroxyl rigid-rod polymers
Introduction
Most organic polymers, owing to their long and flexible chain structures, assume random intermolecular orientations in the amorphous state and in solution. However, there is a class of organic polymers, which have more rigid molecular backbones, are called rigid-rod polymers. These rigid-rod polymers tend to aggregate and orient themselves into much more ordered solutions, films and fibers than typical flexible polymers [1], [2], [3], [4], [7]. Among these rigid-rod polymers, heat-treated polymers like poly{2,6-diimidazo[4,5-b:4′5′-e] pyridinylene-1,4(2,5-dihydroxy)phenylene} (PIPD) even form a monoclinic three-dimensional crystalline structure [3]. Because of rigidity, these polymers display superior mechanical tenacity and thermo-oxidative stability. The basic rigid-rod system comprises the heterocyclic aromatic polymers of poly(p-phenylenebenzazoles) (PBX) as illustrated in Fig. 1 [4], [6], [7]. PBX constitutes a class of polymers that have a para-catenated backbone yielding to a rod like configuration. The only conformational flexibility is provided by the rotation of bonds between alternating phenylene and heterocyclic groups [4], [7]. Furthermore, the PBX backbones all have alternating single and double bonds leading to fully conjugated polymers. Because of the conjugated backbone and the collinear and coplanar configuration unique to PBX polymers, investigations are currently centered on the optoelectronic properties of these rigid polymers, mainly for applications as light emitting diodes (LEDs).
The substitution by heteroatoms O, S, and N in PBO (X=O), PBT (X=S), and PBI (X=NH) as shown in Fig. 1 usually give rise to a nonbonding lone-pair electron state (the n state) lying between the bonding, π, and anti-bonding, π∗, states. Most applications of the organic compounds to absorption spectroscopy are based on transition from the n or π to the π∗ excited state [8]. These substitutions also cause shifts in the wavelength of the absorption maxima and corresponding changes in the fluorescence peaks. The substitution of oxygen or nitrogen in OH-PBI(N) for the CH group in the benzene ring has also been known to result in a bathochromic shift, i.e. the shift of the fluorescence band toward longer wavelength, and higher band intensity. The addition of the hydroxyl may improve the compression performance and the coplanarity of the molecule. In this study the electronic structure calculations are performed to understand the effects of the substitution of heteroatoms and the addition of the hydroxyl on the light absorption properties of the rigid-rod polymers.
Section snippets
Experimental determination of the local order
Since PBT, OH-PBT, OH-PBI and OH-PBI(N) rigid-rod polymers tend to aggregate and orient themselves into much more ordered structures than typical flexible polymers, X-ray diffraction (XRD) measurements can be used to elucidate the local order of these polymers. XRD analyses of the freestanding films of these polymers were carried out at room temperature using a Siemens® D5000 diffractometer with a θ–2θ gonimeter. The 2θ angle region was between 1 and 41° in steps of 0.05° and 3 s per step
The molecular structures
The structural formula units of PBT, OH-PBT, OH-PBI and OH-PBI(N) are shown in Fig. 1. Fig. 3 shows sketches of the polymer systems, in which a, b, and c periodicities are indicated. The coordinates of the atoms in a structural formula unit are given in Table 2. The coordinates for PBT were obtained by the linked-atom least squares (LALS) method [5] by constraining the bond lengths and bond angles to the values shown in Table V of Ref. [6]. For OH-PBT, the H end of the hydroxyl radical was
Calculation method
Hageman et al. [9] have used the solid-state calculation method based on the Bloch states of three-dimensional crystalline system to study the electronic structure of heat-treated PIPD. The PIPD system was found to have a monoclinic crystalline structure based on XRD measurement [3]. The present XRD analyses stated previously show that PBT, OH-PBT, OH-PBI and OH-PBI(N) have a local structural order. However, the structural order is not macroscopically long ranged because the features in the
Experimental excitation energies
The ultraviolet–visible (UV–Vis) absorption measurements were performed for the freestanding films of PBT, OH-PBT, OH-PBI and OH-PBI(N) polymers to reveal their excitation energies. The Hitachi® 3500 spectrophotometer was used to obtain UV–Vis transmission spectra [14]. The measurements were performed normal to the freestanding films in ambient conditions covering an energy range from 1.55 to 6.7 eV at a scanning rate of 0.5 eV/min using a photomultiplier as the detector. The transmission value
Calculated electronic structures
The calculated energy bands of PBT, OH-PBT, OH-PBI, and OH-PBI(N) polymers along five directions in the first Brillouin zone are shown in Fig. 4(a)–(d). In these figures, the zero energy is chosen to be the Fermi level, EF, which is the highest occupied level. In these figures Γ, M, X, N, and Y represent the Brillouin zone center, ((π/a),(π/b),0), ((π/a),0,0), ((π/a),(π/b),(π/c)), and, (0,(π/b),0) points, respectively. Along ΓM, ΓN, and ΓY directions, the large dispersion of energy bands shows
Conclusion
The good agreement between the UV–Vis absorption spectra and our calculated excitation energies show that the solid-state calculation method is applicable to the polymer systems. Our band structure results show significant inter-molecular pz-orbital coupling perpendicular to the molecular plane and insignificant inter-molecular coupling parallel with the molecular plane. N and C atoms are found to contribute dominantly to the leading features in the UV–Vis absorption spectra of these polymers.
Acknowledgements
C.C Wu and S.J. Bai wish to thank the National Science Council of R.O.C. for their support (Contract No. NSC-92-ET-7-110-004-ET).
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