Ultralong room temperature phosphorescence via the charge transfer-separation-recombination mechanism based on organic small molecule doping strategy

Ultra-long room temperature phosphorescence (URTP) has been increasingly recognized in pure organic luminophor in recent years. Through a simpler molecular design and charge separation-recombination pathway, organic luminophor can achieve even better URTP properties. In this work, we achieved URTP in a system of host-guest doped benzophenone derivatives whose phosphorescence is visible to the naked eye. The differences in the wavelength lifetimes of luminescent emission correspond to different photophysical mechanisms. Through a combination of theoretical calculations and experiments, the host acts as a powerful substrate that restricts the motion of the guest and inhibits the non-radiative transitions of the guest, accompanied by a charge transfer separation-recombination process between the host and the guest, resulting in an URTP phenomenon. Transient absorption results demonstrate the existence of a charge-separated state. The design strategy via charge separation is generic and easy to implement, providing a direction for the future design of doped URTP.     

Tunable Phosphorescence/Fluorescence Dual Emissions of Organic Isoquinoline-Benzophenone Doped Systems by Alkoxy Engineering

Dual/multi-component organic doped systems with room-temperature phosphorescence (RTP) properties have been developed. However, the unknown luminescence mechanism still greatly limits the development of the doped materials. Herein, a new doped system exhibiting phosphorescence/fluorescence dual emission (Φ_phos=4–24 % and τ_phos=101–343 ms) is successfully constructed through prediction and design. A series of isoquinoline derivatives with different alkoxy chains were selected as the guests. Benzophenone was chosen as the host owing to the characteristics of low melting point and good crystallinity. The alkoxy chain lengths of the guests are first reported to be used to control the fluorescence and phosphorescence intensities of the doped materials, which results in different prompt emission colors. Additionally, the doped ratio of the guest and host can also control the luminous intensities of the materials. In particular, the doped materials still exhibit phosphorescent properties even if the ratio of the guest/host is as low as 1:100 000.     

Wide-Range Color-Tunable Organic Phosphorescence Materials for Printable and Writable Security Inks

Organic materials with long-lived, color-tunable phosphorescence are potentially useful for optical recording, anti-counterfeiting, and bioimaging. Herein, we develop a series of novel host–guest organic phosphors allowing dynamic color tuning from the cyan (502 nm) to orange red (608 nm). Guest materials are employed to tune the phosphorescent color, while the host materials interact with the guest to activate the phosphorescence emission. These organic phosphors have an ultra-long lifetime of 0.7 s and a maximum phosphorescence efficiency of 18.2 %. Although color-tunable inks have already been developed using visible dyes, solution-processed security inks that are temperature dependent and display time-resolved printed images are unprecedented. This strategy can provide a crucial step towards the next-generation of security technologies for information handling.     

Delayed fluorescence in perhydrotriphenylene-oligothiophene inclusion compounds: evidence for molecular oxygen-related excited States

Delayed emission from α-terthiophene (3T) and α-quinquethiophene (5T) in a perhydrotriphenylene (PHTP) host is investigated. Delayed fluorescence for the lowest singlet excited state of 3T and 5T is detected at both low (80 K) and room temperatures. In addition, at low temperature, phosphorescence from 3T is observed with a lifetime of ～100 μs. Comparison of the dependence of delayed fluorescence and phosphorescence on excitation intensity and time shows that delayed fluorescence does not originate from triplet--triplet annihilation. A dependence of the delayed fluorescence on atmospheric pressure indicates that it originates, at least partially, from complexes of photoexcited oligothiophene and molecular oxygen O(2).     

Long-Persistent Circularly Polarized Luminescence from a Host-Guest System Regulated by the Multiple Roles of a Gold(I)-Carbene Motif

The promotion of intersystem crossing (ISC) is critical for achieving a high-efficiency long-persistent luminescence (LPL) from organic materials. However, the use of a transition-metal complex for LPL materials has not been explored because it can also shorten the emission lifetime by accelerating the phosphorescence decay. Here, we report a new class of LPL materials by doping a monovalent Au-carbene complex into a boron-embedded molecular host. The donor-acceptor systems exhibit photoluminescence with both high efficiencies (>57 %) and long lifetimes (ca. 40 ms) at room temperature. It is revealed that the Au atom promotes the population of low-lying triplet excited states of the host aggregate (T₁*) which can be converted into the charge-transfer (CT) state, thereby resulting in afterglow luminescence. Moreover, the use of a chirality unit on the guest molecule results in the LPL being circularly polarized. This work illustrates that transition-metal complexes can be used for developing organic afterglow systems by exquisite control over the excited state mechanism.     

Color-Tunable Dual-Mode Organic Afterglow from Classical Aggregation-Caused Quenching Compounds for White-Light-Manipulated Anti-Counterfeiting

Color-tunable dual-mode organic afterglow excited by ultraviolet (UV) and white light was achieved from classical aggregation-caused quenching compounds for the first time. Specifically, two luminescent systems, which could produce significant organic afterglow composed of persistent thermally activated delayed fluorescence and ultralong organic phosphorescence under ambient conditions, were constructed by doping fluorescein sodium and calcein sodium into aluminum sulfate. Their lifetimes surpassed 600 ms, and the dopant concentrations were as low as 5×10⁻⁶ wt %. Moreover, the persistent luminescence colors of the materials could be tuned from blue to green and then to yellow by simply varying the concentrations of guest compounds or the temperature in the range of 260–340 K. Inspired by these exciting results, the afterglow materials were used for UV- and white-light-manipulated anti-counterfeiting and preparation of elastomers with different colors of persistent luminescence.     

Wide‐Range Color‐Tunable Organic Phosphorescence Materials for Printable and Writable Security Inks

Organic materials with long‐lived, color‐tunable phosphorescence are potentially useful for optical recording, anti‐counterfeiting, and bioimaging. Herein, we develop a series of novel host–guest organic phosphors allowing dynamic color tuning from the cyan (502 nm) to orange red (608 nm). Guest materials are employed to tune the phosphorescent color, while the host materials interact with the guest to activate the phosphorescence emission. These organic phosphors have an ultra‐long lifetime of 0.7 s and a maximum phosphorescence efficiency of 18.2 %. Although color‐tunable inks have already been developed using visible dyes, solution‐processed security inks that are temperature dependent and display time‐resolved printed images are unprecedented. This strategy can provide a crucial step towards the next‐generation of security technologies for information handling.     

Restricted Conformational Flexibility of a Triphenylamine Derivative on the Formation of Host–Guest Complexes with Various Macrocyclic Hosts

Herein, we report the host–guest‐type complex formation between the host molecules cucurbit[7]uril (CB[7]), β‐cyclodextrin (β‐CD), and dibenzo[24]crown‐8 ether (DB24C8) and a newly synthesized triphenylamine (TPA) derivative 1 X₃ as the guest component. The host–guest complex formation was studied in detail by using ¹H NMR, 2D NOESY, UV/Vis fluorescence, and time‐resolved emission spectroscopy. The chloride salt of the TPA derivative was used for recognition studies with CB[7] and β‐CD in an aqueous medium. The restricted internal rotation of the guest molecule on complex formation with either of these two host molecules was reflected in the enhancement of the emission quantum yield and the average excited‐state lifetime for the triphenylamine‐based excited states. Studies with DB24C8 as the host molecule were performed in dichloromethane, a medium that maximizes the noncovalent interaction between the host and guest fragments. The Förster resonance energy transfer (FRET) process involving DB24C8 and 1 (PF₆)₃, as the donor and acceptor fragments, respectively, was established by electrochemical, steady‐state emission, and time‐correlated single‐photon counting studies.     

Lifetime determination of fluorescence and phosphorescence of a series of oligofluorenes

The photoluminescence (PL) properties of oligofluorenes with 2-ethylhexyl group in 9, 9' position in solution and as thin films were investigated by time-resolved techniques at both room temperature and 77 K. The fluorescence lifetimes of the oligomers decrease with chain length. The lifetimes tau follow the relation tau=386+808(1/n) (ps) where n is the number of fluorene units in the oligomer. Concentration and laser excitation energy dependences of PL spectra of the oligofluorenes are also given. Phosphorescence was observed for oligofluorenes in the frozen matrix of MTHF at 77 K. The lifetime of phosphorescence increases with increasing molecular length. Similar emission bands were observed for oligofluorenes with a central ketogroup. A lifetime analysis clearly reveals that the "green emission" of the oligomers free of ketogroups results from a phosphorescence with lifetime tau of 3 ms while the green emission from the keto-oligomer is a fluorescence from a charge transfer pi-pi* level of tau=8 ns.     

Sensitized phosphorescence of benzil-doped ladder-type methyl-poly(para-phenylene)

The delayed luminescence and phosphorescence of ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with benzil at a concentration of 20% by weight has been measured. The introduction of benzil leads to a dramatic reduction of the polymer singlet emission. At the same time, a new band with maximum at 611 nm appears, corresponding to the phosphorescence of MeLPPP. The phosphorescence decay on the short time scale is close to an exponential law with a time decay of 15 ms. This indicates that benzil can efficiently sensitize the phosphorescence of the polymer. In addition, a broad and featureless emission is observed in the delayed luminescence spectra of benzil-doped MeLPPP, which is attributed to an exciplex formed between the polymer host and the dopant. We further observe that the delayed fluorescence is enhanced by the addition of benzil. It is concluded that the delayed fluorescence of benzil-doped MeLPPP is mainly due to the annihilation of triplet excitons on the polymer. Finally, efficient triplet-triplet energy transfer from the benzil-doped polymer to the red-emitting phosphorescent dye Pt(II)octaethylporphyrin is established.     

Host–Guest Interactions of Phosphorescent Molecular Tweezers Based on an Alkynylplatinum(II) Terpyridine System with Polyaromatic Hydrocarbons

Host–guest interactions of a molecular tweezer complex 1 with various planar organic molecules including polyaromatic hydrocarbons (PAHs) were investigated by 1D and 2D ¹H NMR spectroscopy, UV/Vis absorption and emission titration studies. 2D and DOSY NMR spectroscopies support the sandwiched binding mode based on 1:1 host–guest interactions. The binding constants (K_S) of complex 1 for various PAHs were determined by NMR titration studies and the values were found to span up to an order of 10⁴ M ^?1 for coronene to no observable interaction for benzene, indicating that the π‐surface area is important for such host–guest interactions. The substituent effect on the host–guest interaction based on the guest series of 9‐substituted anthracenes was also studied. In general, a stronger interaction was observed for the anthracene guest with electron‐donating groups, although steric and π‐conjugation factors cannot be completely excluded. The photophysical responses of complex 1 upon addition of various PAHs were measured by UV/Vis and emission titration studies. The UV/Vis absorption spectra were found to show a drop in absorbance of the metal‐to‐ligand charge‐transfer (MLCT) and ligand‐to‐ligand charge‐transfer (LLCT) admixture band upon addition of various guest molecules to 1 , whereas the emission behavior was found to change differently depending on the guest molecules, showing emission enhancement and/or quenching. It was found that emission quenching occurred either via energy transfer or electron transfer pathway or both, while emission enhancement was caused by the increase in rigidity of complex 1 as a result of host–guest interaction.     

Cucurbit[8]uril-mediated phosphorescent supramolecular foldamer for antibiotics sensing in water and cells

A phosphorescent supramolecular foldamer is conveniently constructed by the 1:1 host–guest complexation with cucurbit[8]uril and 1,2-diaminocyclohexane-bridged 4-(4-bromophenyl)-pyridinium salt. The tightly compact host–guest complexation in molecular foldamer can greatly suppress the fluorescence emissive channel and promote the intersystem crossing from singlet to triplet states, thus leading to the green phosphorescence at ambient temperature in aqueous solution. More intriguingly, the phosphorescence emission shows very rapid and sensitive responsiveness to different antibiotics in both inanimate milieu and living cells. Remarkably, the limit of detection of such binary inclusion complex toward sulfamethazine can reach as low as 1.86 × 10^?7 mol/L. Thus, it is envisaged that this supramolecular nanoplatform featuring unique complexation-enhanced phosphorescence emission may hold great promise in sensing and detecting many other biological targets under physiological environment.     

Diboraanthracene-Doped Polymer Systems for Colour-Tuneable Room-Temperature Organic Afterglow

Organic ultralong room temperature phosphorescence (RTP), or organic afterglow, is a unique phenomenon, gaining widespread attention due to its far-reaching application potential and fundamental interest. Here, two laterally expanded 9,10-dimesityl-dihydro-9,10-diboraanthracene (DBA) derivatives are demonstrated as excellent afterglow materials for red and blue-green light emission, which is traced back to persistent thermally activated delayed fluorescence and RTP. The lateral substitution of polycyclic DBA scaffold, together with weak transversal electron-donating mesityl groups, ensures the optimal molecular properties for (reverse) intersystem crossing and long-lived triplet states in a rigid poly(methyl methacrylate) matrix. The achieved afterglow emission quantum yields of up to 3 % and 15 %, afterglow lifetimes up to 0.8 s and 3.2 s and afterglow durations up to 5 s and 25 s (for red and blue-green emitters, respectively) are attributed to the properties of single molecules.     

Efficient and organic host–guest room-temperature phosphorescence: tunable triplet–singlet crossing and theoretical calculations for molecular packing

Organic host–guest doped materials exhibiting the room temperature phosphorescence (RTP) phenomenon have attracted considerable attention. However, it is still challenging to investigate their corresponding luminescence mechanism, because for host–guest systems, it is very difficult to obtain single crystals compared to single-component or co-crystal component materials. Herein, we developed a series of organic doped materials with triphenylamine (TPA) as the host and TPA derivatives with different electron-donating groups as guests. The doped materials showed strong fluorescence, thermally activated delayed fluorescence (τ: 39–47 ms), and efficient room temperature phosphorescence (Φ_phos: 7.3–9.1%; τ: 170–262 ms). The intensity ratio between the delayed fluorescence and phosphorescence was tuned by the guest species and concentration. Molecular dynamics simulations were used to simulate the molecular conformation of guest molecules in the host matrix and the interaction between the host and guest molecules. Therefore, the photophysical properties were calculated using the QM/MM model. This work provides a new concept for the study of molecular packing of guest molecules in the host matrix.

Molecular dynamics simulations were used to simulate the molecular conformation and interaction between hosts and guests. This work provides a new concept for the study of molecular packing for the investigation of the luminescence mechanism.     

Enhanced Solution and Solid-State Emission and Tunable White-Light Emission Harvested by Supramolecular Approaches

Organic luminescent materials with high quantum yields and/or white-light-emitting properties in particular play a crucial role in labeling and optoelectronic devices. In this work we have synthesized a new 2,3,6,7-tetramethoxy-9,10-di-p-tolylanthracene-bridged pillar[5]arene dimer with persistent mazarine blue fluorescent emission and much higher quantum yields in both solution and the solid state in comparison with its corresponding emissive linker without pillarene units, which exhibits typical aggregation-caused quenching. According to the fluorescence data and single-crystal analyses, their contrasting fluorescent performances can be rationally ascribed to their different stacking structures and intermolecular interactions. Three fluorescent guests containing different chromophores and/or terminal binding sites have also been synthesized to interact with the pillar[5]arene dimer to construct supramolecular ensembles with highly controllable luminescence, taking advantage of the stimuli-responsive properties of the supramolecular host–guest interactions. Intriguingly, multicolor fluorescence, including white-light emission (0.31, 0.35), which is in high demand, has been achieved by tuning the molar ratio of the host and guest and/or by changing the solvent system. This strategy holds great potential for the design and development of fluorescent materials with high quantum yields, controllable emission wavelength, and good stimuli-responsiveness.     

Phosphorescence Energy Transfer: Ambient Afterglow Fluorescence from Water‐Processable and Purely Organic Dyes via Delayed Sensitization

Ambient afterglow luminescence from metal‐free organic chromophores would provide a promising alternative to the well‐explored inorganic phosphors. However, the realization of air‐stable and solution‐processable organic afterglow systems with long‐lived triplet or singlet states remains a formidable challenge. In the present study, a delayed sensitization of the singlet state of organic dyes via phosphorescence energy transfer from organic phosphors is proposed as an alternative strategy to realize “afterglow fluorescence”. This concept is demonstrated with a long‐lived phosphor as the energy donor and commercially available fluorescent dyes as the energy acceptor. Triplet‐to‐singlet Förster‐resonance energy‐transfer (TS‐FRET) between donor and acceptor chromophores, which are co‐organized in an amorphous polymer matrix, results in tuneable yellow and red afterglow from the fluorescent acceptors. Moreover, these afterglow fluorescent hybrids are highly solution‐processable and show excellent air‐stability with good quantum yields.     

Competition between Arene–Perfluoroarene and Charge‐Transfer Interactions in Organic Light‐Harvesting Systems

Two typical types of luminescent organic cocrystals comprising pyrene–octafluoronaphthalene (pyrene–OFN) and pyrene–1,2,4,5‐tetracyanobezene (pyrene–TCNB) were developed by a simple supramolecular assembly strategy. The cocrystals exhibit distinct optical properties because of their different intermolecular interaction modes; that is, arene–perfluoroarene (AP) and charge‐transfer (CT) interactions. Unexpectedly, a pyrene–TCNB system with strong CT interactions was incorporated into a pyrene–OFN host as a robust guest to generate white‐light emission (WLE). In the supramolecular cocrystal system, an efficient energy‐transfer process from pyrene–OFN to pyrene–TCNB occurred because of the well‐matched spectra of the constituents and a desirable energy donor/acceptor (D/A) distance. The present competitive intermolecular interaction strategy could be applied to the fabrication of more complicated organic light‐harvesting systems.     

Phosphorescence Energy Transfer: Ambient Afterglow Fluorescence from Water-Processable and Purely Organic Dyes via Delayed Sensitization

Ambient afterglow luminescence from metal-free organic chromophores would provide a promising alternative to the well-explored inorganic phosphors. However, the realization of air-stable and solution-processable organic afterglow systems with long-lived triplet or singlet states remains a formidable challenge. In the present study, a delayed sensitization of the singlet state of organic dyes via phosphorescence energy transfer from organic phosphors is proposed as an alternative strategy to realize “afterglow fluorescence”. This concept is demonstrated with a long-lived phosphor as the energy donor and commercially available fluorescent dyes as the energy acceptor. Triplet-to-singlet Förster-resonance energy-transfer (TS-FRET) between donor and acceptor chromophores, which are co-organized in an amorphous polymer matrix, results in tuneable yellow and red afterglow from the fluorescent acceptors. Moreover, these afterglow fluorescent hybrids are highly solution-processable and show excellent air-stability with good quantum yields.     

Donor–σ–Acceptor Motifs: Thermally Activated Delayed Fluorescence Emitters with Dual Upconversion

A family of organic emitters with a donor–σ–acceptor (D‐σ‐A) motif is presented. Owing to the weakly coupled D‐σ‐A intramolecular charge‐transfer state, a transition from the localized excited triplet state (³LE) and charge‐transfer triplet state (³CT) to the charge‐transfer singlet state (¹CT) occurred with a small activation energy and high photoluminescence quantum efficiency. Two thermally activated delayed fluorescence (TADF) components were identified, one of which has a very short lifetime of 200–400 ns and the other a longer TADF lifetime of the order of microseconds. In particular, the two D‐σ‐A materials presented strong blue emission with TADF properties in toluene. These results will shed light on the molecular design of new TADF emitters with short delayed lifetimes.     

Remarkable mechanochromism and force-induced thermally activated delayed fluorescence enhancement from white-light-emitting organic luminogens with aggregation-induced emission

The development of organic materials with white-light emission and thermally activated delayed fluorescence (TADF) properties in the solid state remain a challenge. Herein, a series of white-light-emitting organic luminogens have been developed and are found to show aggregation-induced delayed fluorescence (AIDF) characteristics. The AIDF emitters present dual-emission consisted of prompt fluorescence and TADF in the crystalline state. Their white-light emissions can be easily tuned by altering the chemical structure and connecting position of the heterocyclic aromatic substituent. Under the stimuli of mechanical force and solvent vapor, the compounds exhibit remarkable and reversible mechanochromism, in which their emission colors are switchable between white and yellow. Upon grinding, they also display linearly tunable luminescence colors, as well as force-induced TADF enhancement, which may be associated with the more compact molecular packing and the restriction of intramolecular motions. The results from time-resolved emission scanning and theoretical calculation suggest that the dual-emission of the AIDF luminogens likely results from the twisted intramolecular charge transfer transitions of the molecules, and the reversible mechanochromism properties probably stem from the interconversion of the quasi-axial and the quasi-equatorial conformations.