Sequence-defined donor-acceptor-donor oligo(para-phenylene ethynylene)s with emission across the visible spectrum

Donor-acceptor engineering is a widely employed strategy for π-conjugated molecules to modulate their electronic structure and photophysical behavior. Oligo(para-phenylene ethynylene)s (OPEs) provide a rigid and modular π-conjugated scaffold that is particularly well suited for the systematic incorporation of donor and acceptor units. In this work, a symmetric donor-acceptor-donor (D-A-D) sequence was adopted to tune the electronic structure of the OPE molecular rods. By varying the acceptor unit, two series of OPE emitters, OPE-x (3-mers) and OPE-x' (5-mers), were synthesized via Sonogashira cross-coupling reactions. With increasing acceptor strength, the emission wavelength could be tuned from the blue to the red spectral region for both applied conjugation lengths, while preserving high photoluminescence quantum yields (PLQY). By extending the degree of conjugation by a defined number of repeating units, a decrease was observed for both the Stokes shift and the emission full width at half maximum (FWHM) across the series. Further analysis aided by quantum-chemical calculations provides a molecular-level understanding of the observed photophysical trends.