Pure Organic Persistent Room‐Temperature Phosphorescence at both Crystalline and Amorphous States

An Organophosphorescence Probe with Ultralong Lifetime and Intrinsic Tissue Selectivity for Specific Tumor Imaging and Guided Tumor Surgery

Organic phosphorescent materials are excellent candidates for use in tumor imaging. However, a systematic comparison of the effects of the intensity, lifetime, and wavelength of phosphorescent emissions on bioimaging performance has not yet been undertaken. In addition, there have been few reports on organic phosphorescent materials that specifically distinguish tumors from normal tissues. This study addresses these gaps and reveals that longer lifetimes effectively increase the signal intensity, whereas longer wavelengths enhance the penetration depth. Conversely, a strong emission intensity with a short lifetime does not necessarily yield robust imaging signals. Building upon these findings, an organo-phosphorescent material with a lifetime of 0.94 s was designed for tumor imaging. Remarkably, the phosphorescent signals of various organic nanoparticles are nearly extinguished in blood-rich organs because of the quenching effect of iron ions. Moreover, for the first time, we demonstrated that iron ions universally quench the phosphorescence of organic room-temperature phosphorescent materials, which is an inherent property of such substances. Leveraging this property, both the normal liver and hepatitis tissues exhibit negligible phosphorescent signals, whereas liver tumors display intense phosphorescence. Therefore, phosphorescent materials, unlike chemiluminescent or fluorescent materials, can exploit this unique inherent property to selectively distinguish liver tumor tissues from normal tissues without additional modifications or treatments.

Time-dependent and clustering-induced phosphorescence, mechanochromism, structural-function relationships, and advanced information encryption based on isomeric effects and host-guest doping

To explore the intrinsic mechanism of pure organic room temperature and clustering-induced phosphorescence and investigate mechanochromism and structural-function relationships, here, 4-(2-(9H-carbazol-9-yl)phenyl)-2-amino-6-methoxypyridine-3,5-dicarbonitrile (Lo-CzAD), 4-(3-(9H-carbazol-9-yl)phenyl)-2-amino-6-methoxypyridine-3,5-dicarbonitrile (Lm-CzAD), and 4-(4-(9H-carbazol-9-yl)phenyl)-2-amino-6-methoxypyridine-3,5-dicarbonitrile (Lp-CzAD) were designed and synthesized by choosing self-made carbazole and 3, 5-dicyanopyridine (DCP) unit as electron acceptor and electron donor in sequence. Compared with crystals Lm-CzAD and Lp-CzAD, crystal Lo-CzAD shows better room temperature phosphorescence (RTP) performance, with RTP lifetimes of 187.16 ms, as well as afterglows 1s, which are attributed to twisted carbazole unit and donor-acceptor (D-A) molecular conformation, big crystal density and spin orbit coupling constant ξ (S1 → T1 and S1 → T2), as well as intermolecular H type stacking and small ξ (S0 → T1). By choosing urea and PPh3 as host materials and tuning doping ratio, four doping systems were successfully constructed, significantly improving RTP performance of Lo-CzAD and Lp-CzAD, as well as showing different fluorescence and RTP. The lifetimes and afterglows of pure organic Urea/Lo-CzAD and Urea/Lp-CzAD systems are up to 478.42 ms, 5 s, 261.66 ms and 4.5 s in turn. Moreover, Lo-CzAD and Lp-CzAD show time-dependent RTP in doping systems due to monomer and aggregate dispersion, as well as clustering-induced phosphorescence. Based on the different luminescent properties, multiple information encryptions were successfully constructed.

Crucial Role of Defect States in the Ultralong Phosphorescence of Organic Molecular Crystals

Ultralong organic phosphorescence (UOP) in pure organic molecular crystals has attracted a lot of interest recently. There is much debate on the emission mechanism of this UOP. Two recent experimental works published in Nat. Photonics 2019, 13, 406-411 and Nat. Mater. 2021, 20, 175-180 attribute UOP in the 2,4,6-trimethoxy-1,3,5-triazine (TMOT) crystals and the carbazole crystals to H-aggregation of the TMOT molecules or the formation of charge-transfer excitons between the carbazole and impurity molecules. Our first-principles many-body Green's function theory calculations show that the lowest triplet states of these two crystals are in fact the localized defect states originating from the twisted TMOT molecules and the impurities, respectively. Energies of the H-aggregation-induced exciton and the charge-transfer exciton are too high to account for UOP. UOP should be mainly due to the little orbital overlap between the localized defect state and the delocalized band edges of the crystal. Strong intermolecular interactions suppress nonradiative decay of the triplet exciton localized on the defect.

Mapping the Regioisomeric Space and Visible Color Range of Purely Organic Dual Emitters with Ultralong Phosphorescence Components: From Violet to Red Towards Pure White Light

We mapped the entire visible range of the electromagnetic spectrum and achieved white light emission (CIE: 0.31, 0.34) by combining the intrinsic ns-fluorescence with ultralong ms-phosphorescence from purely organic dual emitters. We realized small molecular materials showing high photoluminescence quantum yields (ΦL ) in the solid state at room temperature, achieved by active exploration of the regioisomeric substitution space. Chromophore stacking-supported stabilization of triplet excitons with assistance from enhanced intersystem crossing channels in the crystalline state played the primary role for the ultra-long phosphorescence. This strategy covers the entire visible spectrum, based on organic phosphorescent emitters with versatile regioisomeric substitution patterns, and provides a single molecular source of white light with long lifetime (up to 163.5 ms) for the phosphorescent component, and high overall photoluminescence quantum yields (up to ΦL =20 %).

Room Temperature Phosphorescence vs Triplet-Triplet Annihilation in N-Substituted Acridone Solids

Organic room temperature phosphorescent (ORTP) compounds have recently emerged as a promising class of emissive materials with a multitude of potential applications. However, the number of building blocks that give rise to efficient ORTP materials is still limited, and the rules for engineering phosphorescent properties in organic solids are not well understood. Here, we report ORTP in a series of N-substituted acridone derivatives with electron-donating, electron-withdrawing, and sterically bulky substituents. X-ray crystallography shows that the solid-state packing varies progressively between coparallel and antiparallel π-stacking and separated π-dimers, depending on the size of the substituent. The detailed photophysical studies supported by DFT calculations reveal complex dynamics of singlet and triplet excited states, depending on the electronic effects of substituents and solid-state packing. The programmable molecular packing provides a lever to control the triplet-triplet annihilation that is manifested as delayed fluorescence in acridone derivatives with continuous (both parallel and antiparallel) π-stacking.

Carbazole isomers induce ultralong organic phosphorescence

Commercial carbazole has been widely used to synthesize organic functional materials that have led to recent breakthroughs in ultralong organic phosphorescence¹, thermally activated delayed fluorescence^2,3, organic luminescent radicals⁴ and organic semiconductor lasers⁵. However, the impact of low-concentration isomeric impurities present within commercial batches on the properties of the synthesized molecules requires further analysis. Here, we have synthesized highly pure carbazole and observed that its fluorescence is blueshifted by 54 nm with respect to commercial samples and its room-temperature ultralong phosphorescence almost disappears⁶. We discover that such differences are due to the presence of a carbazole isomeric impurity in commercial carbazole sources, with concentrations <0.5 mol%. Ten representative carbazole derivatives synthesized from the highly pure carbazole failed to show the ultralong phosphorescence reported in the literature^1,7-15. However, the phosphorescence was recovered by adding 0.1 mol% isomers, which act as charge traps. Investigating the role of the isomers may therefore provide alternative insights into the mechanisms behind ultralong organic phosphorescence^1,6-18.

Revealing Topological Influence of Phenylenediamine Unit on Physicochemical Properties of Donor-Acceptor-Donor-Acceptor Thermally Activated Delayed Fluorescent Macrocycles

A new thermally activated delayed fluorescence (TADF)-displaying macrocyclic compound m-1 comprising of two electron-donors (N,N'-diphenyl-m-phenylenediamine) and two electron-acceptors (dibenzo[a,j]phenazine) has been synthesized. The macrocycle developed herein is regarded as a regioisomer of the previously reported TADF macrocycle p-1, which has two N,N'-diphenyl-p-phenylenediamines as the donors. To understand the influence of the topology of the phenylenediamine donors on physicochemical properties of TADF-active macrocycles, herein the molecular structure in the single crystals, photophysical properties, electrochemical behavior, and TADF properties of m-1 have been investigated compared with those of p-1. The substitution of p-phenylene donor with m-phenylene donor led to distinct positive solvatoluminochromism over the full visible-color range, unique oxidative electropolymerization, and slightly lower contribution of TADF, due to the lower CT character in the excited states.

Mechanoresponsive turn-on phosphorescence by a desymmetrization approach

The room-temperature phosphorescence (RTP) of metal-free organic crystals is normally quenched by mechanical stimulation. Herein, we demonstrate the opposite mechanoresponse of turn-on RTP. A desymmetrization of a C₂-symmetric 1,2-diketone creates space for molecular motion in the crystal, quenching the RTP from the crystal while maintaining that from the amorphous solid.

Manipulating the Ultralong Organic Phosphorescence of Small Molecular Crystals

Ultralong organic phosphorescence (UOP) of metal-free organic materials has received considerable attention recently owing to their long-lived emission lifetimes, and the fact that they present an attractive alternative to persistent luminescence in inorganic phosphors. Enormous research effort has been devoted on improving UOP performance in metal-free organic phosphors by promoting the intersystem crossing (ISC) process and suppressing the non-radiative decay of triplet state excitons. This minireview summarizes the recent advances in the rational approaches for manipulating the UOP properties of small molecular crystals, such as phosphorescence lifetime, efficiency, and emission colors. Finally, the present challenges and future development of this field are proposed. This review will provide a guideline to rationally design more advanced metal-free organic phosphorescence materials for potential applications.

Organic Room-Temperature Phosphorescence with Strong Circularly Polarized Luminescence Based on Paracyclophanes

Pure organic materials with intrinsic room-temperature phosphorescence typically rely on heavy atoms or heteroatoms. Two different strategies towards constructing organic room-temperature phosphorescence (RTP) species based upon the through-space charge transfer (TSCT) unit of [2.2]paracyclophane (PCP) were demonstrated. Materials with bromine atoms, PCP-BrCz and PPCP-BrCz, exhibit RTP lifetime of around 100 ms. Modulating the PCP core with non-halogen-containing electron-withdrawing units, PCP-TNTCz and PCP-PyCNCz, successfully elongate the RTP lifetime to 313.59 and 528.00 ms, respectively, the afterglow of which is visible for several seconds under ambient conditions. The PCP-TNTCz and PCP-PyCNCz enantiomers display excellent circular polarized luminescence with dissymmetry factors as high as -1.2×10^-2 in toluene solutions, and decent RTP lifetime of around 300 ms for PCP-TNTCz enantiomers in crystalline state.

A turn-on phosphorescent sensor of Pb2+ in water by the formation of a coordination polymer

Persulfurated asterisks functionalized with six carboxylic groups form a strongly green phosphorescent coordination polymer upon addition of Pb²⁺ ions in aqueous solution. The self-assembly process is selective and reversible, enabling Pb²⁺ sensing with a detection limit of 6.0 × 10⁻⁷ M.

Proton transfer triggered proton transfer: a self-assisted twin excited state intramolecular proton transfer

The double excited state intramolecular proton transfer (ESIPT) of 3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazole (bis-HPTA) has been investigated and found to undergo a new type of proton transfer.