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Molecular conformation is closely related to its luminescence process, and pressure is one of the effective tools to regulate molecular conformation. In this letter, we studied the evolution of salicylic acid crystal fluorescence under high pressure and obtained enhanced emission during the compression. At 0–4 GPa, the rotamerization process led to the energy dissipation, which resulted in the fluorescence quenching. After 4 GPa, the rotamerization process completed. The excited-state intramolecular proton transfer (ESIPT) process of SA was inhibited, and due to the strengthened intermolecular hydrogen bond, the non-radiative decay reduced, which resulted in emission enhancement. This work reveals the impact of hydrogen bond and molecular conformation on luminescence and ideas for the development of new small-molecule luminescent materials are furnished. [Display omitted] • Molecular isomerization can be induced by high pressure. • Pressure can control the excited-state intramolecular proton transfer process. • The pressure-induced emission enhancement can be achieved by restrict chemical bond vibrations. [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]

In this work, we designed and synthesized a deep-red to NIR emissive donor-acceptor (D-A) material NZBPXZ. The non-doped OLED of NZBPXZ demonstrates NIR electro-luminescence (EL) at 744 nm with a maximum external quantum efficiency (EQEmax) of 0.96% with an exciton utilize efficiency (EUE) of 90%. Additionally, the crystalline of NZBPXZ underwent a large red-shift of 127 nm from 670 nm in visible light region to approximately 797 nm (with a PLQY of 7.7%) in NIR region upon mechanical grinding. This work not only enables efficient NIR electroluminescence in OLEDs, but also gives a new functional route for construction deep-red to NIR pure organic mechanochromic materials. [Display omitted] • A deep-red to NIR emissive donor-acceptor (D-A) material NZBPXZ were designed and synthesized. • This molecular achieve efficient mechanochromic- and electro-fluorescence from deep red to near-infrared region. • The reason that NZBPXZ possesses largely red-shifted emission was further explored. [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]

In this work, two novel D-π-A compounds (PXZ-FR-DRZ and CZ-FR-DRZ) are designed and synthesized, aiming at the excited-state regulation from thermally-activated delayed fluorescence (TADF) to hybridized local and charge-transfer (HLCT) emission. This change of excited-state nature can be ascribed to the weakened electron-donating ability of carbazole relative to phenoxazine, together with the decreased twisting angle between carbazole and fluorine units. As a result of twisted charge-transfer (CT) excited state, the doped organic light-emitting diode (OLED) of PXZ-FR-DRZ demonstrates a blue-green TADF electroluminescence (EL) with a maximum external quantum efficiency (EQE max) of 11.5%, while its non-doped device shows a much lower EQE max. Due to HLCT state of CZ-FR-DRZ , both its doped and non-doped OLEDs achieve the deep-blue non-delayed EL with the satisfied external quantum efficiency (EQE), especially for the excellent efficiency roll-off. Overall, this excited-state transformation between TADF and HLCT allows us to harvest the better comprehensive performance of OLED emitters for the practical applications, which provides a feasible solution for the development of OLED industry. Two different donors (phenoxazine and carbazole) are designed to match the same acceptor of 2,4-diphenyl-1,3,5-triazine (DRZ) through a π-bridged linkage of 9,9-dimethyl-9H-fluorene (FR), resulting in two novel D-π-A molecules of PXZ-FR-DRZ and CZ-FR-DRZ , respectively. The PXZ-FR-DRZ realizes a blue-green thermally-activated delayed fluorescence (TADF) with high external quantum efficiency (EQE) in the doped organic light-emitting diodes (OLEDs) due to the twisted charge-transfer (CT) excited state, whereas the CZ-FR-DRZ achieves a deep-blue non-doped OLED with the better comprehensive performance as a result of the hybridized local and charge-transfer (HLCT) state. [Display omitted] • Two D-π-A materials CZ-FR-DRZ and PXZ-FR-DRZ are synthesized with HLCT and TADF properties respectively. • The TADF PXZ-FR-DRZ exhibits higher EQE than CZ-FR-DRZ. • As an HLCT material, CZ-FR-DRZ displays high performance in non-doped OLED. • The conversion between HLCT and TADF is investigated by changing different donors. [ABSTRACT FROM AUTHOR]

High performance blue emitters with nice color purity are highly desired in the commercialization of organic light-emitting diodes (OLEDs). Herein, two blue-emissive hybridized local and charge-transfer (HLCT) phenanthroimidazole (PPI) derivatives were designed and synthesized. Comparing to the 9,9-dimethyl-9,10-dihydroacridine (DMAC) substituted DPCN , the 10H-spiro[acridine-9,9′-fluorene] (SAF) substituted SPCN demonstrates higher photoluminescence quantum yield (PLQY) and improved radiative transition rate (k r). With the same device structure, despite that SPCN and DPCN show similar electroluminescence (EL) spectra, the EL efficiency of SPCN is about 1.5 times that of DPCN. Especially the non-doped OLED based on SPCN , the maximum external quantum efficiency (EQE) is 7.1%, which is a competitive result among the conventional blue-emissive fluorescent emitters. More importantly, this work provides a general strategy to achieve high performance blue/deep-blue OLED by rational donor optimization. In this work, we report two blue-emissive hybridized local and charge-transfer (HLCT) materials (DPCN and SPCN) with different donor substitution. Comparing to the 9,9-dimethyl-9,10-dihydroacridine (DMAC) substituted DPCN , the 10H-spiro[acridine-9,9′-fluorene] (SAF) substituted SPCN displays an improved EL efficiency and blue purity, which provides an ideal molecular design strategy for efficient blue/deep-blue EL materials. [Display omitted] • Two blue-emissive HLCT materials with different donor substitution were designed and synthesized. • The relationship of structure-physical property as well as OLED performance are comprehensively explored. • Comparing to DPCN, the SAF substituted SPCN achieved a better OLED performance. [ABSTRACT FROM AUTHOR]