Journal Article asymmetric engineering benzo[1,2-b:4, 5-b′]difuran charge dynamics covalent organic frameworks deep-blue emission deep‐blue emission exciton‐phonon coupling interfacial engineering light-emitting diodes light‐emitting diodes mechanical exfoliation mixed-halide perovskites organic solar cells permittivity photocatalytic overall water splitting pure‐halide quantum dots reduced‐dimensional perovskites spectral stability ultrathin structures