Poly(phenylene methylene) (PPM) exhibits pronounced blue fluorescence in solutions as well as in the solid state despite its non-p-conjugated nature. Optical spectroscopy was used to explore the characteristics and the physical origin of its unexpected optical properties, namely absorption in the 350-450 nm and photoluminescence in the 400-600 nm spectral regions. It is shown that PPM possesses two discrete optically active species, and a relatively long photoluminescence lifetime (>8 ns) in the solid-state. Given the evidence reported herein, p-stacking and aggregation/crystallization, as well as the formation of anthracene-related impurities, are excluded as the probable origins of the optical properties. Instead there is sufficient evidence that PPM supports homoconjugation, that is: p-orbital overlap across adjacent repeat units enabled by particular chain conformation(s), which is confirmed by DFT calculations. Furthermore, poly(2-methylphenylene methylene) and poly(2,4,6-trimethylphenylene methylene) -two derivatives of PPM -were synthesized and found to exhibit comparable spectroscopic properties, confirming the generality of the findings reported for PPM. Cyclic voltammetry measurements revealed the HOMO-LUMO gap to be 3.2-3.3 eV for all three polymers. This study illustrates a new approach to the design of light-emitting polymers possessing hitherto unknown optical properties.
Metamaterials are a promising new class of materials, in which sub-wavelength physical structures, rather than variations in chemical composition, can be used to modify the nature of their interaction with electromagnetic radiation. Here we show that a metamaterials approach, using a discrete physical geometry (conformation) of the segments of a polymer chain as the vector for a substantial refractive index change, can be used to enable visible wavelength, conjugated polymer photonic elements. In particular, we demonstrate that a novel form of dip-pen nanolithography provides an effective means to pattern the so-called β-phase conformation in poly(9,9-dioctylfluorene) thin films. This can be done on length scales ≤500 nm, as required to fabricate a variety of such elements, two of which are theoretically modelled using complex photonic dispersion calculations.
Two doping mechanisms are known for the well-studied materials poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), namely, integer charge transfer (ICT) and charge transfer complex (CTC) formation. Yet, there is poor understanding of the effect of doping mechanism on thermal stability and the thermoelectric properties. In this work, we present a method to finely adjust the ICT to CTC ratio. Using it, we characterize electrical and thermal conductivities as well as the Seebeck coefficient and the long-term stability under thermal stress of P3HT and PBTTT of different ICT/CTC ratios. We establish that doping through the CTC results in more stable, yet lower conductivity samples compared to ICT doped films. Importantly, moderate CTC fractions of ∼33% are found to improve the long-term stability without a significant sacrifice in electrical conductivity. Through visible and IR spectroscopies, polarized optical microscopy, and grazing-incidence wide-angle X-ray scattering, we find that the CTC dopant molecule access sites within the polymer network are less prone to dedoping upon thermal exposure.
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