Homoconjugation in Light-Emitting Poly(phenylene methylene)s: Origin and Pressure-Enhanced Photoluminescence

The surprising optical properties of the non-π-conjugated polymer poly­(phenylene methylene) (PPM) and its derivativesthat is, absorption in the 350–450 nm and photoluminescence (PL) in the 400–600 nm spectral regionshave been attributed to chromophores formed by homoconjugation along the polymer chain. The enabling role of homoconjugation, however, was hitherto ascertained primarily by excluding alternative origins of luminescence. The present study offers direct evidence for homoconjugation by employing optical and vibrational spectroscopy to investigate the interplay between the microstructure and solid-state optical properties of PPM and its derivative poly­(2,4,6-trimethylphenylene methylene). In particular, polarized Raman and PL spectroscopy of melt-drawn fibers reveal a preferentially perpendicular orientation of the phenylene rings relative to the fiber axis and, simultaneously, a preferentially parallel orientation of the transition dipole moment. PL spectroscopy under applied hydrostatic pressure yields a nearly fourfold increase in PL intensity at 8 GPa, together with a surprising absence of excimer emission. These characteristics, being highly atypical of conventional π-conjugated polymers, highlight the different origin of the optical properties of PPMs and unique opportunities for applications.