Homoconjugation in Poly(Phenylene Methylene) s: A Case Study of Non-π-Conjugated Polymers with Unexpected Fluorescent Properties

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, pi-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: pi-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. (C) 2017 Wiley Periodicals, Inc.