Photoluminescence of Aramid Nanofibers: The Benefits of Partially Disordered State.

Aramid nanofibers (ANFs) have emerged as promising polymeric building blocks for high-performance composites and porous solids due to their exceptional mechanical, electrical, and thermal properties. In contrast, their optical, and specifically, luminescent properties have remained unexplored because conventional aramid polymers are weak emitters. Here we demonstrate that ANFs can exhibit strong and tunable photoluminescence. ANFs prepared in DMSO-KOH initially exist as fully deprotonated nanofiber dispersions and are nonfluorescent. Gradual protonation induces intense emission accompanied by a blue shift of the photoluminescence maximum from 650 to 510 nm. Quantum-mechanical calculations reveal that protonation increases the HOMO-LUMO gap and converts the dominant relaxation pathway of the optically excited electronic states from the forbidden π* → n transition to allowed π* → π transition. Unlike macroscale fibers of aramid that are also protonated but virtually nonfluorescent, ANFs have a partially disordered nanofibrillar structure with much reduced size of crystalline domains, which also plays an important role in ANF photophysics. Misalignment of the macromolecules suppresses rapid nonradiative recombination via overlapping aromatic states in π - π stacks. Confocal microscopy shows that (1) each ANF fibril has strong luminescence and (2) macroscale ANF-based materials preserve nanoscale organization responsible for optical emission. These findings demonstrate that controlled molecular disorder impart functional optical benefits to ANFs, opening new opportunities in optoelectronics, sensing, and photonics.