The spectral full-width at half maximum (FWHM) of organic emitters is determined by the structural changes between the ground state (S 0 ) and the excited state (S 1 ) ( Figure 1 ). Generally, molecules with rigid planar structures favor narrowband emission. However, intense π − π stacking in aggregates produces excimers, which can cause spectral broadening and reducing the color purity of materials. Although utilizing host-guest doping systems with ultra-low concentrations (≤ 2 wt%) can preserve the narrowband spectral characteristics of the single-molecule state as much as possible, this strategy also raises the precision requirements of the vapor deposition process. This raises a fundamental question: Whether the inhibition of aggregation can be shifted into the use of aggregation to achieve a new photophysical phenomenon where the spectra become narrower as the molecules aggregate?
In a new paper published in Light: Science & Applications , a team of scientists, led by Professor Zhiming Wang from State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology and co-workers achieved the construction of emitters from wideband emission to narrowband emission by gradually eliminating steric hindrance across a series of isomers featuring a central 8 π −electron structure ( Figure 1 ). Among these isomers, the 8 π −electron center plays as key role in tuning the spectral shape by regulating the molecular aromaticity and intermolecular stacking modes. In dilute solution, via the synergy of “ strain-release-driven planarization in the ground state” and “aromaticity-driven planarity in the excited state”, structural differences between S 0 and S 1 are effectively minimized, that a new class of narrowband emission materials is developed. The FWHM of PDBP- b,i reaches 35 nm in solution.
In aggregates, PDBP- b,i molecules adopt a cross-dipole stacking mode that suppresses intermolecular π−π interactions while introducing abundant hydrogen-bond interactions ( Figure 2 ). This stacking mode effectively restricts molecular vibration and suppresses exciton-vibration coupling, giving rise to the AIUNE behavior of PDBP- b,i aggregates, with an ultra-narrow FWHM of merely 13 nm. Comparative analyses of Huang-Rhys factors and reorganization energies confirms that the AIUNE mechanism originates from aggregation-induced restriction of high-frequency vibrations ( Figure 2 ).
Electroluminescence performances of PDBP- b,i based OLED devices are summarized in Figure 3 . For 10 wt%-doped OLEDs, the maximum emission peak locates at 432 nm with an ultranarrow FWHM of 13 nm and CIE chromaticity coordinates of (0.156, 0.045), satisfying the BT. 2020 blue standard. In particular, the FWHMs of these devices remain constant at 13 nm as the doping concentration increases from 10 wt% to 30 wt%, which is comparable to the smallest FWHM value for reported MR-based OLEDs, demonstrating the superiority of the molecular design with an AIUNE property.
Light: Science & Applications
Display and organic LEDs