Chain Versus Ring: Characterization of a Meta-Phenylene Ladder Polymer and Its Octameric Macrocycle

A methyl-substituted ladder meta-phenylene macrocycle (MeLMMP) and its corresponding ladder polymer (MeLPMP), a meta-analogue of the well-known ladder-type poly(para-phenylene), LPPP, were synthesized and comprehensively investigated. Both compounds exhibit limited conjugation due to the meta-linked phenylene units, resulting in absorption and emission features shaped by cross-conjugation. Despite having a longer chain, MeLPMP and MeLMMP exhibit nearly identical electronic and photophysical behavior, suggesting that the number of repeat units has minimal influence. MeLPMP exhibits enhanced vibronic resolution compared to MeLMMP due to a broadening of the optical bands of the macrocycle caused by the presence of a mixture of stereoisomers formed during the non-stereoselective ladderization step. A small amount of fluorenone-type keto defects produces a weak emission near 500 nm, more evident in the macrocycle, and introduces radiationless decay pathways that compete with fluorescence. This decay pathway, together with the weak π,π* conjugation in these angular compounds, lowers the fluorescence quantum yield when compared with the linear MeLPPP, while promoting singlet-triplet conversion and phosphorescence. The data indicate the presence of three chromophoric populations: pristine oligo(meta-phenylene) units; units quenched by nearby keto defects; and intramolecular charge-transfer complexes (ICTCs) formed between oligo(meta-phenylene) units and keto defects. These findings clarify the photophysics of meta-phenylene ladder systems, showing that the MeLMMP macrocycle can act as a structural and electronic model for related ladder polymers.