Figure 5. a) Normalized PL spectra of MPCbZ during the hand-actuated grinding process. b) Powder X-ray diffraction (PXRD) patterns and time-resolved spectra c) of Re-phase in the pristine state and after slight and full grinding. d) The absorption spectra of MPCbZ in the Rephase and full-grinding state.

Excitation-dependent emission (Ex-de) materials have been of considerable academic interest and have potential applications in real life. Such multicolour luminescence is a characteristic exception to the ubiquitously accepted Kasha's rule. This phenomenon has been increasingly presented in some studies on different luminescence systems; however, a systematic overview of the mechanisms underlying this phenomenon is currently absent. Herein, we resolve this issue by classifying multicolour luminescence from single chromophores and dual/ternary chromophores, as well as multiple emitting species. The underlying processes are described based on electronic and/or geometrical conditions under which the phenomenon occurs. Before we present it in categories, related photophysical and photochemical foundations are introduced. This systematic overview will provide a clear approach to designing multicolour luminescence materials for special applications.

Organic single crystals (OSCs) with excellent flexibility and unique optical properties are of great importance due to their broad applicability in optical/ optoelectronic devices and sensors. Nevertheless, fabricating flexible OSCs with room-temperature phosphorescence (RTP) remains a great challenge. Herein, we propose a host-guest doping strategy to achieve both RTP and flexibility of OSCs. The single-stranded crystal is highly bendable upon external force application and can immediately return to its original straight shape after removal of the stress, impressively emitting bright deep-red phosphorescence. The theoretical and experimental results demonstrate that the bright RTP arises from Förster resonance energy transfer (FRET) from the triphenylene molecules to the dopants. This strategy is both conceptually and synthetically simple and offers a universal approach for the preparation of flexible OSCs with RTP.

Organic materials with excitation wavelength‐dependent (Ex‐de) emission are highly attractive for anticounterfeiting, optoelectronics and bioassay applications; however, the realization of Ex‐de fluorescence, independent of aggregation states, remains a challenge. We herein report a photoinduced electron transfer (PeT) strategy to design Ex‐de fluorescence materials by manipulating the relaxation pathways of multiple excited states. As expected, the o‐carborane dyad presents a clear Ex‐de fluorescence colour in the aggregated states, resulting from the tunable relative intensity of the dual‐fluorescence spectra. Taking TP[1]B as an example, the amorphous powders emitted bright blue‐violet, white and yellow colours under 390 nm, 365 nm and 254 nm UV illumination, respectively. Importantly, multicolour, flexible and transparent films as well as an anticounterfeiting application using this o‐carborane dyad are demonstrated.

Achieving blueshifted and enhanced emission from purely organic luminophors remains a major challenge. Herein, we report a tetracoordinate boron complex with polymorphism (Y-phase and O-phase)dependent luminescence. Impressively, the Y-phase crystals exhibited rare piezochromic luminescent properties such as pressure-induced blue-shifted and enhanced emission. The results of theoretical and experimental tests demonstrated that this phenomenon results from the cooperative effect between the restriction of intramolecular motions and alterable charge transfer (CT) behavior during compression. This work provides an ideal model to investigate the optoelectronic properties of boron complexes. Importantly, manipulating the CT behavior through the electrophilicity of boron atoms may become a new principle for the design of piezochromic materials with blueshifted and enhanced emission.