Homoconjugated Poly(Phenylene Methylene)s—A Case Study of Light Emission Enabled by Through-Space Conjugation

Following the commercial success and ubiquitous application of organic light-emitting diodes (OLEDs), on-going research into light-emitting (macro-)molecules seeks to uncover and advance further functionalities these materials, motivated by emerging application concepts in optoelectronics, fluorescent bioprobes, molecular imaging, coatings, and numerous others. In the established paradigm, the design of next-generation fluorophores is generally restricted to π-conjugated aromatic (macro-)molecules. Recent years, however, are witnessing a growing research interest in light-emitting molecules that are not based on extended conjugated π-systems.[1–3] This largely unexplored class of materials promises to overcome some of the principal limitations of conventional materials related to their cytotoxiticity, high intramolecular rigidity, non-biocompatibility and limited solubility. Key for progress, therefore, is exploring and understanding the nuances of ‘through-space conjugation’ (alternatively termed ‘homoconjugation’) that enables luminescence in the visible spectral range.<br/>Poly(phenylene methylene) (PPM) has recently emerged as a promising multifunctional polymer with a unique range of physico-chemical properties, including thermal stability, resistance to oxidizing agents, excellent barrier properties, and high hydrophobicity.[4,5] Arguably, given the presence of an electronically insulating methylene group separating adjacent phenylene groups, its most remarkable characteristics are related to photoluminescence (PL). Specifically, PPM exhibits photoluminescence in the 400–600 nm range, solid-state PL quantum yield &gt;41% and an unusually long solid-state PL lifetime of &gt;8 ns.[4]<br/>In this contribution we present a detailed spectroscopic study of poly(phenylene methylene) and its derivative poly(2,4,6-trimethylphenylene methylene) in the form of isotropic films and oriented melt-drawn fibers, and provide compelling evidence for the occurrence of homoconjugation. In particular, polarized Raman and PL spectroscopy of melt-drawn fibers reveal a preferentially <i>perpendicular</i> orientation of the phenylene rings relative to the fiber axis and, simultaneously, a preferentially <i>parallel</i> orientation of the transition dipole moment. Moreover, PL spectroscopy under hydrostatic pressures up to 8 GPa reveals a 4-fold increase in PL intensity and an apparent absence of excimer emission, both of which are highly atypical of π-conjugated emitters.[6]<br/>Coupled with straightforward synthesis for yielding high-molar-mass products and excellent processability into films, foams and microparticles, PPM holds promise for light-emitting applications due to it reduced susceptibility to the usual non-radiative decay pathways. Given the generality of underlying principles, we hope that this work will stimulate further research into non-conventional light-emitting materials with hitherto unexplored optoelectronic properties.<br/>[1] H. Zhang <i>et al.</i>, <i>J. Am. Chem. Soc</i>. <b>2017</b>, <i>139</i>, 16264; [2] D. P. Chatterjee <i>et al</i>., <i>ACS Omega</i> <b>2020</b>, <i>5</i>, 30747; [3] L. Liao <i>et al</i>., <i>Mater. Horiz</i>. <b>2020</b>, <i>7</i>, 1605; [4] A. Braendle <i>et al</i>., <i>J. Polym. </i><i>Sci., Part B: Polym. Phys</i>. <b>2020</b>, <i>55</i>, 707; [5] M. F. D’Elia <i>et al.</i>, <i>Coatings</i> <b>2018</b>, <i>8</i>, 274; [6] A. Perevedentsev <i>et al</i>., <i>Macromolecules</i> <b>2020</b>, <i>53</i>, 7519.