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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. [ABSTRACT FROM AUTHOR]

Mechanoresponsive luminescent (MRL) materials have become a high-profile research system in the fields of force sensing, anti-counterfeiting, and optical information storage because of their obvious variety on luminescence intensity or luminescence color under mechanical force. In this work, nitro-triphenylamine (TPA-NO2) crystal was selected to conduct in situ high pressure experiments based on diamond anvil cell combined with theory calculation, and the piezochromism of TPA-NO2 crystal was discussed in detail. During the compression process, the TPA-NO2 crystal showed the piezochromic behavior and a redshift of the emission peak. The mechanical response behavior of TPA-NO2 provides insight into the study of multicolor properties from a structural perspective, offering a clean means of using pressure to modulate different colors and contributing to the exploration of high-potency MRL materials. [ABSTRACT FROM AUTHOR]

Excimer materials are frequently applied to organic chemistry, optoelectronic devices and stimulus response. Specific research on the entire process of excimer generation in solid materials, as well as the intricate mechanism of piezochromic luminescence based on excimer, are still absent and need to be examined in-depth through comprehensive experiments. Therefore, we designed an anthracene-based molecule CZPAN , which adopts no overlap area between anthracene molecules in its crystal at ambient conditions, to investigate the piezochromic luminescence behavior and expect the anthracene group to form excimers under high-pressure stimulation. As pressure increases, CZPAN gradually changes from blue monomer luminescence to green excimer luminescence accompanied with the considerably red-shifted emission spectra, and exhibits a discontinuity of fluorescence quenching during the formation of excimers (in the pressure range of 5.3–6.6 GPa). Moreover, the pressure relief process is fully reversible. This study presents abundant piezochromic luminescence of CZPAN and proves that the formation of excimers requires a certain overlap area between molecules. Pursuing the appropriate strength of π–π interactions will thus be the focus of future research on excimer materials of high luminescence efficiency, so that excimers can be better applied in sensors and imaging devices. [Display omitted] • There are no overlap areas between anthracene molecules in the designed crystal at ambient conditions. • High pressure induces the formation of excimers. • The designed crystal shows abundant phenomena of piezochromic luminescence during compression. • High luminescence efficiency of excimers requires the appropriate strength of π–π interactions. [ABSTRACT FROM AUTHOR]

Noncovalent dimerization usually can induce distinct luminescence properties from those of monomer, but it is quite difficult to achieve in solid state, especially in films. Two acridine (AC) derivatives pTPA-AC and mTPA-AC were designed and synthesized with the chemical modification of AC by electron-donating triphenylamine (TPA) substituent at para - and meta -positions of TPA unit, respectively. Their crystals were resolved with antiparallel π–π dimeric AC stacking, demonstrating green fluorescence. Their doped films in a polymer matrix exhibited a distinct emission color change from blue to green with increasing doping percentage of pTPA-AC and mTPA-AC, indicating a facile noncovalent dimerization of AC in neat films. Upon protonation, a high-contrast acid-responsive luminescence switching was observed in these films with increasing doping ratio, which possesses not only a significant color change but also an on-off switch characteristic. This work provides a new molecular design strategy to develop the dimer materials of organic heterocycles, facilitating the antiparallel π–π dimer stacking in the film for both novel functions and practical applications. [Display omitted] • The crystals of two acridine derivatives both presented antiparallel π–π dimeric acridine stacking mode. • With increasing doping ratio in polymer films, a transformation from monomer to acridine dimer was achieved, inducing rich emission colors. • Upon protonation, both monomer- and dimer-based films displayed high-contrast luminescence switching. [ABSTRACT FROM AUTHOR]