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

The surprising optical properties of the non--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 microstructure and solid-state optical properties of PPM and its derivative poly(2,4,6-trimethylphenylene methylene) (PTMPM). 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, while, simultaneously, a preferentially parallel orientation of the transition dipole moment. PL spectroscopy under applied hydrostatic pressure yields a nearly 4-fold increase in PL intensity at 8 GPa, together with a surprising absence of excimer emission. These characteristics, being highly atypical of conventional -conjugated polymers, highlight the different origin of the optical properties of poly(phenylene methylene)s as well as unique opportunities for applications.The authors thank Drs Nathan Cheetham (Imperial College London) and Johannes Gierschner (IMDEA Nanociencia) for stimulating discussions. A.P. is indebted to the Great European Lockdown (GEL) network for enabling Prof. Paul Smith (ETH Zürich) to comprehensively review this manuscript on one of his lesser favored topics, for which he is also thanked sincerely. The Spanish Ministerio de Ciencia, Innovación y Universidades is gratefully acknowledged for its support through grant no. SEV-2015-0496 in the framework of the Spanish Severo Ochoa Centre of Excellence Program and through grant PGC2018-095411-B-100 (RAINBOW). We also thank the Catalan agency AGAUR for grant 2017-SGR-00488. AFL acknowledges a FPI fellowship (BES-2016-076913) from the Spanish Ministerio cofinanced by the European Social Fund and the Ph.D. program in Materials Science from Universitat Autònoma de Barcelona in which he is enrolled. Financial support is also acknowledged from the European Research Council through project ERC CoG648901.Peer reviewe