Organic Nanocrystals with Bright Red Persistent Room‐Temperature Phosphorescence for Biological Applications

Persistent room‐temperature phosphorescence (RTP) in pure organic materials has attracted great attention because of their unique optical properties. The design of organic materials with bright red persistent RTP remains challenging. Herein, we report a new design strategy for realizing high brightness and long lifetime of red‐emissive RTP molecules, which is based on introducing an alkoxy spacer between the hybrid units in the molecule. The spacer offers easy Br−H bond formation during crystallization, which also facilitates intermolecular electron coupling to favor persistent RTP. As the majority of RTP compounds have to be confined in a rigid environment to quench nonradiative relaxation pathways for bright phosphorescence emission, nanocrystallization is used to not only rigidify the molecules but also offer the desirable size and water‐dispersity for biomedical applications.     

Cyclic Triimidazole Derivatives: Intriguing Examples of Multiple Emissions and Ultralong Phosphorescence at Room Temperature

The performance of solid luminogens depends on both their inherent electronic properties and their packing status. Intermolecular interactions have been exploited to achieve persistent room‐temperature phosphorescence (RTP) from organic molecules. However, the design of organic materials with bright RTP and the rationalization of the role of interchromophoric electronic coupling remain challenging tasks. Cyclic triimidazole has been shown to be a promising scaffold for such purposes owing to its crystallization‐induced room‐temperature ultralong phosphorescence (RTUP), which has been associated with H‐aggregation. Herein, we report three triimidazole derivatives as significant examples of multifaceted emission. In particular, dual fluorescence, RTUP, and phosphorescence from the molecular and supramolecular units were observed. H‐aggregation is responsible for the red RTUP, and Br substituents favor yellow molecular phosphorescence while halogen‐bonded Br⋅⋅⋅Br tetrameric units are involved in the blue‐green phosphorescence.     

Persistent Room Temperature Phosphorescence from Triarylboranes: A Combined Experimental and Theoretical Study

Achieving highly efficient phosphorescence in purely organic luminophors at room temperature remains a major challenge due to slow intersystem crossing (ISC) rates in combination with effective non‐radiative processes in those systems. Most room temperature phosphorescent (RTP) organic materials have O‐ or N‐lone pairs leading to low lying (n, π*) and (π, π*) excited states which accelerate k_isc through El‐Sayed's rule. Herein, we report the first persistent RTP with lifetimes up to 0.5 s from simple triarylboranes which have no lone pairs. RTP is only observed in the crystalline state and in highly doped PMMA films which are indicative of aggregation induced emission (AIE). Detailed crystal structure analysis suggested that intermolecular interactions are important for efficient RTP. Furthermore, photophysical studies of the isolated molecules in a frozen glass, in combination with DFT/MRCI calculations, show that (σ, B p)→(π, B p) transitions accelerate the ISC process. This work provides a new approach for the design of RTP materials without (n, π*) transitions.     

Mechanoluminescence or Room‐Temperature Phosphorescence: Molecular Packing‐Dependent Emission Response

Mechanoluminescence (ML) and room‐temperature photophosphorescence (RTP) were achieved in polymorphisms of a triphenylamine derivative with ortho‐substitution. This molecular packing‐dependent emission afforded crucial information to deeply understand the intrinsic mechanism of different emission forms and the possible packing–function relationship. With the incorporation of solid‐state ¹³C NMR spectra of single crystals, as well as the analysis of crystal structures, the preferred packing modes for ML and/or RTP were investigated in detail, which can guide the reasonable design of organic molecules with special light‐emission properties.     

Achieving room temperature phosphorescence from organic small molecules on amino acid skeleton

Room temperature phosphorescence (RTP) generated by small molecules has attracted great attention due to their unique potentials for biosensor, bioimaging and security protection. While, the design of RTP materials is extremely challenging for organic small molecules in non-crystalline solid state. Herein, we report a new strategy for achieving non-crystalline organic small molecules with RTP emission by modifying different phosphors onto diphenylalanine or phenylalanine derivatives. Benefiting from the skeletal structure of the amino acid derivatives, there are intermolecular hydrogen bond formation and rigidification effect, thereby minimizing the intermolecular motions and enhancing their RTP performance     

Achieving room temperature phosphorescence from organic small molecules on amino acid skeleton

Room temperature phosphorescence (RTP) generated by small molecules has attracted great attention due to their unique potentials for biosensor, bioimaging and security protection. While, the design of RTP materials is extremely challenging for organic small molecules in non-crystalline solid state. Herein, we report a new strategy for achieving non-crystalline organic small molecules with RTP emission by modifying different phosphors onto diphenylalanine or phenylalanine derivatives. Benefiting from the skeletal structure of the amino acid derivatives, there are intermolecular hydrogen bond formation and rigidification effect, thereby minimizing the intermolecular motions and enhancing their RTP performance     

Induction of Strong Long‐Lived Room‐Temperature Phosphorescence of N‐Phenyl‐2‐naphthylamine Molecules by Confinement in a Crystalline Dibromobiphenyl Matrix

The design and preparation of metal‐free organic materials that exhibit room‐temperature phosphorescence (RTP) is a very attractive topic owing to potential applications in organic optoelectronic devices. Herein, we present a facile approach to efficient and long‐lived organic RTP involving the doping of N‐phenylnaphthalen‐2‐amine (PNA) or its derivatives into a crystalline 4,4′‐dibromobiphenyl (DBBP) matrix. The resulting materials showed strong and persistent RTP emission with a quantum efficiency of approximately 20 % and a lifetime of a few to more than 100 milliseconds. Bright white dual emission containing blue fluorescence and yellowish‐green RTP from the PNA‐doped DBBP crystals was also confirmed by Commission Internationale de l'Eclairage (CIE) coordinates of (x=0.29–0.31, y=0.38–0.41).     

An Unexpected Chromophore–Solvent Reaction Leads to Bicomponent Aggregation‐Induced Phosphorescence

Organic luminogens with persistent room‐temperature phosphorescence (RTP) have found a wide range of applications. However, many RTP luminogens are prone to severe quenching in the crystalline state. Herein, we report a strategy to construct a donor‐sp³‐acceptor type luminogen that exhibits aggregation‐induced emission (AIE) while the donor‐sp²‐acceptor counterpart structure exhibits a non‐emissive solid state. Unexpectedly, it was discovered that a trace amount (0.01 %) of the structurally similar derivative, produced by a side reaction with the DMF solvent, could induce strong RTP with an absolute RTP yield up to 25.4 % and a lifetime of 48 ms, although the substance does not show RTP by itself. Single‐crystal XRD‐based calculations suggest that n–σ* orbital interactions as a result of structural similarity may be responsible for the strong RTP in the bicomponent system. This study provides a new insight into the design of multi‐component, solid‐state RTP materials from organic molecular systems.     

Polymorphic Pure Organic Luminogens with Through‐Space Conjugation and Persistent Room‐Temperature Phosphorescence

Pure organic luminogens with persistent room‐temperature phosphorescence (p‐RTP) have attracted increasing attention owing to their vital significance and potential applications in security inks, bioimaging, and photodynamic therapy. Previously reported p‐RTP luminogens normally possessed through‐bond conjugation. In this work, we report a pure organic luminogen, AN‐MA, the Diels–Alder cycloaddition adduct of anthracene (AN) and maleic anhydride (MA), which possesses isolated phenyl groups and an anhydride moiety. AN‐MA exhibits aggregation‐enhanced emission (AEE) characteristics with efficiency of approximately 2 % and up to 8.5 % in solution and crystals, respectively. Two polymorphs of AN‐MA were readily obtained that were able to generate UV emission from individual phenyl rings together with bright blue emission owing to the effective through‐space conjugation. Moreover, p‐RTP with a lifetime of up to approximately 1.6 s was obtained in the crystals. These results not only reveal a new system with both fluorescence and RTP dual emission but also suggest an alternative through‐space conjugation strategy towards pure organic p‐RTP luminogens with tunable emissions.     

Clustering-Triggered Efficient Room-Temperature Phosphorescence from Nonconventional Luminophores

Pure organic room-temperature phosphorescence (RTP) and luminescence from nonconventional luminophores have gained increasing attention. However, it remains challenging to achieve efficient RTP from unorthodox luminophores, on account of the unsophisticated understanding of the emission mechanism. Herein, we propose a strategy to realize efficient RTP in nonconventional luminophores through incorporation of lone pairs together with clustering and effective electronic interactions. The former promotes spin-orbit coupling and boosts the consequent intersystem crossing, whereas the latter narrows energy gaps and stabilizes the triplets, thus synergistically affording remarkable RTP. Experimental and theoretical results of urea and its derivatives verify the design rationale. Remarkably, RTP from thiourea solids with unprecedentedly high efficiency of up to 24.5 % is obtained. Further control experiments testify the crucial role of through-space delocalization on the emission. These results will spur the future fabrication of nonconventional phosphors and advance the understanding of the underlying emission mechanism.     

Visible‐Light‐Excited Room‐Temperature Phosphorescence in Water by Cucurbit[8]uril‐Mediated Supramolecular Assembly

Solid‐state materials with efficient room‐temperature phosphorescence (RTP) emissions have found widespread applications in materials science, while liquid or solution‐phase pure organic RTP emission systems has been rarely reported, because of the nonradiative decay and quenchers from the liquid medium. Reported here is the first example of visible‐light‐excited pure organic RTP in aqueous solution by using a supramolecular host‐guest assembly strategy. The unique cucurbit[8]uril‐mediated quaternary stacking structure allows tunable photoluminescence and visible‐light excitation, enabling the fabrication of multicolor hydrogels and cell imaging. The present assembly‐induced emission approach, as a proof of concept, contributes to the construction of novel metal‐free RTP systems with tunable photoluminescence in aqueous solution, providing broad opportunities for further applications in biological imaging, detection, optical sensors, and so forth.     

An Unexpected Chromophore–Solvent Reaction Leads to Bicomponent Aggregation-Induced Phosphorescence

Organic luminogens with persistent room-temperature phosphorescence (RTP) have found a wide range of applications. However, many RTP luminogens are prone to severe quenching in the crystalline state. Herein, we report a strategy to construct a donor-sp³-acceptor type luminogen that exhibits aggregation-induced emission (AIE) while the donor-sp²-acceptor counterpart structure exhibits a non-emissive solid state. Unexpectedly, it was discovered that a trace amount (0.01 %) of the structurally similar derivative, produced by a side reaction with the DMF solvent, could induce strong RTP with an absolute RTP yield up to 25.4 % and a lifetime of 48 ms, although the substance does not show RTP by itself. Single-crystal XRD-based calculations suggest that n–σ* orbital interactions as a result of structural similarity may be responsible for the strong RTP in the bicomponent system. This study provides a new insight into the design of multi-component, solid-state RTP materials from organic molecular systems.     

Crystal‐State Photochromism and Dual‐Mode Mechanochromism of an Organic Molecule with Fluorescence,Room‐Temperature Phosphorescence,and Delayed Fluorescence

A D‐A‐D′ type pure organic molecule, named ODFRCZ, has unique triple‐emission character covering fluorescence, phosphorescence, and delayed fluorescence (DF). The phosphorescence of ODFRCZ has a rather long lifetime of about 350 ms at room temperature. One dimer of ODFRCZ with enhanced parallel molecular packing acts more effectively to prompt ISC processes, which further generates room‐temperature phosphorescence (RTP), owing to the larger transition dipole moment and closer energy level between S₁ and T_n. ODFRCZ is a rare example of an organic RTP molecule that shows dual‐stimuli responsiveness of dual‐mode mechanochromism (fluorescence red‐shift and RTP/DF on‐off switch) and reversible crystal‐state photochromism. This work may broaden the knowledge for stimuli‐responsive RTP organic molecules and lay the foundation for their wide‐scale applications.     

Crystal‐State Photochromism and Dual‐Mode Mechanochromism of an Organic Molecule with Fluorescence,Room‐Temperature Phosphorescence,and Delayed Fluorescence

A D‐A‐D′ type pure organic molecule, named ODFRCZ, has unique triple‐emission character covering fluorescence, phosphorescence, and delayed fluorescence (DF). The phosphorescence of ODFRCZ has a rather long lifetime of about 350 ms at room temperature. One dimer of ODFRCZ with enhanced parallel molecular packing acts more effectively to prompt ISC processes, which further generates room‐temperature phosphorescence (RTP), owing to the larger transition dipole moment and closer energy level between S₁ and T_n. ODFRCZ is a rare example of an organic RTP molecule that shows dual‐stimuli responsiveness of dual‐mode mechanochromism (fluorescence red‐shift and RTP/DF on‐off switch) and reversible crystal‐state photochromism. This work may broaden the knowledge for stimuli‐responsive RTP organic molecules and lay the foundation for their wide‐scale applications.     

Long‐Lived Room‐Temperature Phosphorescence for Visual and Quantitative Detection of Oxygen

An unconventional organic molecule (TBBU) showing obvious long‐lived room temperature phosphorescence (RTP) is reported. X‐ray single crystal analysis demonstrates that TBBU molecules are packed in a unique fashion with side‐by‐side arranged intermolecular aromatic rings, which is entirely different from the RTP molecules reported to date. Theoretical calculations verify that the extraordinary intermolecular interaction between neighboring molecules plays an important role in RTP of TBBU crystals. More importantly, the polymer film doped with TBBU inherits its distinctive RTP property, which is highly sensitive to oxygen. The color of the doped film changes and its RTP lifetime drops abruptly through a dynamic collisional quenching mechanism with increasing oxygen fraction, enabling visual and quantitative detection of oxygen. Through analyzing the grayscale of the phosphorescence images, a facile method is developed for rapid, visual, and quantitative detection of oxygen in the air.     

Two‐Coordinate Copper(I)/NHC Complexes: Dual Emission Properties and Ultralong Room‐Temperature Phosphorescence

As a kind of photoluminescent material, Cu^I complexes have many advantages such as adjustable emission, variable structures, and low cost, attracting attention in many fields. In this work, two novel two‐coordinate Cu^I‐N‐heterocyclic carbene complexes were synthesized, and they exhibit unique dual emission properties, fluorescence and phosphorescence. The crystal structure, packing mode, and photophysical properties under different conditions were systematically studied, proving the emissive mechanism to be the locally excited state of the carbazole group. Based on this mechanism, ultralong room‐temperature phosphorescence (RTP) with a lifetime of 140 ms is achieved by selective deuteration of the carbazole group. These results deepen the understanding of the luminescence mechanism and design strategy for two‐coordinate Cu^I complexes, and prove their potential in applications as ultralong RTP materials.     

9,9‐Dimethylxanthene Derivatives with Room‐Temperature Phosphorescence: Substituent Effects and Emissive Properties

Room‐temperature phosphorescence (RTP) emitters with ultralong lifetimes are emerging as attractive targets because of their potential applications in bioimaging, security, and other areas. But their development is limited by ambiguous mechanisms and poor understanding of the correlation of the molecular structure and RTP properties. Herein, different substituents on the 9,9‐dimethylxanthene core (XCO) result in compounds with RTP lifetimes ranging from 52 to 601 ms, which are tunable by intermolecular interactions and molecular configurations. XCO‐PiCl shows the most persistent RTP because of its reduced steric bulk and multiple sites of the 1‐chloro‐2‐methylpropan‐2‐yl (PiCl) moiety for forming intermolecular interactions in the aggregated state. The substituent effects reported provide an efficient molecular design of organic RTP materials and establishes relationships among molecular structures, intermolecular interactions, and RTP properties.     

Self‐Stabilized Amorphous Organic Materials with Room‐Temperature Phosphorescence

The stability of pure organic room‐temperature phosphorescent (RTP) materials in air has been a research hotspot in recent years. Without crystallization or encapsulation, a new strategy was proposed to obtain self‐stabilized organic RTP materials, based on a complete ionization of a photo‐induced charge separation system. The ionization of aromatic phenol 4‐carbazolyl salicylaldehyde (CSA) formed a stable H‐bonding anion–cation radical structure and led to the completely amorphous CSA‐I film. Phosphorescent lifetimes as long as 0.14 s at room temperature and with direct exposure to air were observed. The emission intensity was also increased by 21.5‐fold. Such an amorphous RTP material reconciled the contradiction between phosphorescence stability and vapor permeability and has been successfully utilized for peroxide vapor detection.     

Room temperature phosphorescence from organic luminogens in“non-crystalline” state

ABSTRACT

The field of room temperature phosphorescence (RTP) from purely organic materials has made rapid strides in recent years primarily due to its tremendous promise in the areas of photovoltaics, photocatalysis, bioimaging, sensing, etc. Although, the RTP properties, at one time, were considered to be exclusive features of organometallic and inorganic phosphors, a great progress in the molecular design coupled with a much better understanding of the triplet state stabilisation has led to the creation of a plethora of organic RTP materials in the current decade. In this focussed review, a special category of organic luminogens which, rather remarkably, exhibit efficient RTP emission in amorphous or fluidic state is discussed. A few selected examples of such ‘non-crystalline’ organic RTP luminogens are highlighted with an emphasis on the basic design principles and the strategies to increment the phosphorescence efficiency.     

Carbon Dots with Dual‐Emissive,Robust, and Aggregation‐Induced Room‐Temperature Phosphorescence Characteristics

Carbon dots (CDs) with dual‐emissive, robust, and aggregation‐induced RTP characteristics are reported for the first time. The TA‐CDs are prepared via hydrothermal treatment of trimellitic acid and exhibit unique white prompt and yellow RTP emissions in solid state under UV excitation (365 nm) on and off, respectively. The yellow RTP emission of TA‐CDs powder should be resulted from the formation of a new excited triplet state due to their aggregation, and the white prompt emission is due to their blue fluorescence and yellow RTP dual‐emissive nature. The RTP emission of TA‐CDs powder was highly stable under grinding, which is very rare amongst traditional pure organic RTP materials. To employ the unique characteristics of TA‐CDs, advanced anti‐counterfeiting and information encryption methodologies (water‐stimuli‐response producing RTP) were preliminarily investigated.