Molecular conformation engineering in central 8π-electron system toward unique aggregation-induced ultra-narrowband emission with a FWHM of 13 nm.

Organic emitters with narrowband emission characteristics demonstrate significant advantages and application potential in ultra-high-definition displays and multiplexed fluorescence imaging. However, the obstinate spectral broadening phenomenon upon aggregation limits the practical application of organic narrowband emissive materials. Here, a referable strategy on conformation engineering is proposed in a central 8π-electron system with narrowband emissions in both solution and aggregate. Owing to the synergistic effect of strain-release-driven conformational planarity in ground state and excited-state aromaticity-driven planarity, PDBP-b,i exhibits minimal structural change between the S0 and S1 states, resulting in narrowband emission in solution with a full width at half maximum (FWHM) of 35 nm/0.24 eV. While in aggregate, the cross dipole stacking mode further confines molecular conformation and restricts high-frequency vibration, additionally narrowing with a record-small FWHM of 13 nm/0.08 eV. The concept of aggregation-induced ultra-narrowband emission is proposed, which also offers advantages in electroluminescence to exhibit a small FWHM of 13 nm and remaining stable as the concentration increases. These results provide new insights and instructive guidance for the design of organic narrowband emitters.