Long-lived Room-temperature Phosphorescence Research Articles

The phosphorescence property of carbon dots presenting as powder, embedded in filter paper and dispersed in solid solution

Room temperature phosphorescence (RTP) has distinctive advantages over the fluorescence counterparts, such as larger Stokes shifts and longer lifetimes. It is challenging to design and prepare RTP carbon dots (CDs) because many of them need appropriate solid matrix or introducing heavy atoms for promoting intersystem crossing and suppressing vibrational dissipation between singlet and triplet states. In this paper, long-lived RTP carbon dots (EDA-CDs) were synthesized by microwave-assisted pyrolysis of ethylenediamine (EDA) in phosphoric acid solution. Interestingly, at room temperature, bright phosphorescence was observed for EDA-CDs embedded in filter paper and presenting as solid powder, and could last for 7 s to naked eyes, with the phosphorescence lifetime being up to 693.67 and 887.55 ms, respectively. At low temperature of 123 K, the phosphorescence lifetime of EDA-CDs in solid solution can reach the highest value of 1538 ms, while the phosphorescence lifetime of EDA-CDs presenting as solid powder is 1287.54 ms. It can be deduced that robust bridge-like hydrogen-bonded networks formed by highly ordered hydration water molecules around EDA-CDs in solid solution at low temperature are beneficial for rigidifying the surface groups of EDA-CDs and thus the longer phosphorescence lifetime. Finally, RTP EDA-CDs had shown a great potential to be as advanced security ink for information encryption and anti-counterfeiting.

Hydrogen Bonding-Induced Morphology Dependence of Long-Lived Organic Room-Temperature Phosphorescence: A Computational Study.

Organic room-temperature phosphorescence (RTP) is generally only exhibited in aggregate with strong dependence on morphology, which is highly sensitive to the intermolecular hydrogen bonding interaction. Here, 4,4'-bis(9H-carbazol-9-yl)methanone (Cz2BP), emitting RTP in a cocrystal consisting of chloroform but not in the amorphous nor in the crystal phase, was investigated to disclose the morphology dependence through molecular dynamics simulations and first-principles calculations. We find that the strong intermolecular C═O···H-C hydrogen bonds between Cz2BP and chloroform in cocrystals decrease the nonradiative decay rate of T1 → S0 by 3-6 orders of magnitude due to the vibronic decoupling effect on the C═O stretching motion and the increase of (π,π*) composition in the T1 state. The former is responsible for high efficiency and the latter for long-lived RTP with a calculated lifetime of 208 ms (exp. 353 ms). Nevertheless, the weak hydrogen bonds cannot cause any appreciable RTP in amorphous and crystal phases. This novel understanding opens a way to design organic RTP materials.

Revealing Insight into Long-Lived Room-Temperature Phosphorescence of Host-Guest Systems.

The control of the emission properties of doping materials through molecular design makes organic materials potentially promising candidates for many optoelectronic applications and devices. However, organic doping systems with high quantum yields and persistent luminescence processes have rarely been reported, and their luminescence mechanisms are still not well established. Here we developed a series of purely organic heavy-atom-free doping systems. The guest molecules can dope either donor or acceptor matrixes, both leading to an enhanced fluorescence (Φ = 63-76%) and room-temperature phosphorescence (Φ = 7.6-14.5%, τ = 119-317 ms) under ambient conditions. XRD measurements and density functional calculations results indicated ultralong phosphorescence was determined by both the cocrystalline state and the energy levels between the host and guest materials. The doping materials are fairly stable to light, heat, and humidity. This work may provide unique insight for designing doping systems and expanding the scope of organic phosphorescence applications.

Enabling long-lived organic room temperature phosphorescence in polymers by subunit interlocking

Long-lived room temperature phosphorescence (LRTP) is an attractive optical phenomenon in organic electronics and photonics. Despite the rapid advance, it is still a formidable challenge to explore a universal approach to obtain LRTP in amorphous polymers. Based on the traditional polyethylene derivatives, we herein present a facile and concise chemical strategy to achieve ultralong phosphorescence in polymers by ionic bonding cross-linking. Impressively, a record LRTP lifetime of up to 2.1 s in amorphous polymers under ambient conditions is set up. Moreover, multicolor long-lived phosphorescent emission can be procured by tuning the excitation wavelength in single-component polymer materials. These results outline a fundamental principle for the construction of polymer materials with LRTP, endowing traditional polymers with fresh features for potential 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.

Efficient and Long-Lived Room-Temperature Organic Phosphorescence: Theoretical Descriptors for Molecular Designs.

Room-temperature phosphorescence (RTP) with long afterglow from pure organic materials has attracted great attention for its potential applications in biological imaging, digital encryption, optoelectronic devices, and so on. Organic materials have been long considered to be nonphosphorescent owing to their weak molecular spin-orbit coupling and high sensitivity to temperature. However, recently, some purely organic compounds have demonstrated highly efficient RTP with long afterglow upon aggregation, while others fail. Namely, it remains a challenge to expound on the underlying mechanisms. In this study, we present the molecular descriptors to characterize the phosphorescence efficiency and lifetime. For a prototypical RTP system consisting of a carbonyl group and π-conjugated segments, the excited states can be regarded as an admixture of n → π* (with portion α) and π → π* (portion β). Starting from the phosphorescent process and El-Sayed rule, we deduced that (i) the intersystem crossing (ISC) rate of S1 → T n is mostly governed by the modification of the product of α and β and (ii) the ISC rate of T1 → S0 is determined by the β value of T1. Thus, the descriptors (γ = α × β, β) can be employed to describe the RTP character of organic molecules. From hybrid quantum mechanics and molecular mechanics (QM/MM) calculations, we illustrated the relationships among the descriptors (γ, β), phosphorescence efficiency and lifetime, and spin-orbit coupling constants. We stressed that the large γ and β values are favorable for the strong and long-lived RTP in organic materials. Experiments have reported confirmations of these molecular design rules.

Benzoperylene probe/ZnS quantum dots nanocomposite for the ratiometric detection and intracellular imaging of highly reactive oxygen species

A ratiometric nanosensor for the selective detection and intracellular imaging of highly reactive oxygen species (hROS) is developed. Benzoperylene probe and Mn-ZnS quantum dots based nanoparticles were fabricated through cost effective and simple procedures. The nanoparticles exhibit distinct excimer emission and long-lived room-temperature phosphorescence (RTP) emission at the same time under single wavelength excitation. The nanosensor shows highly sensitive and selective ratiometric response towards hROS over other ROS. The dual emission property also endows the nanoparticles the capability for sensitive live cell imaging of hROS, and the potential for real time monitoring of intracellular hROS.

Aggregation-Induced Emission with Long-Lived Room-Temperature Phosphorescence from Methylene-Linked Organic Donor-Acceptor Structures.

Aggregation-caused quenching (ACQ), where excited-state and/or ground-state electronic structures are altered to exhibit an increased proclivity for non-radiative decay for the aggregates, is largely responsible for the lack of fluorescence and phosphorescence in molecular solids in general. Here we show that ACQ could be effectively circumvented by constructing an aromatic system with a methylene-linker, where the system exhibits typical aggregation-induced emission (AIE) with long-lived room-temperature phosphorescence, since the tetrahedral structure in the solid state may significantly reduce strong intermolecular interactions contributing to ACQ.

Induction of long-lived room temperature phosphorescence of carbon dots by water in hydrogen-bonded matrices

Phosphorescence shows great potential for application in bioimaging and ion detection because of its long-lived luminescence and high signal-to-noise ratio, but establishing phosphorescence emission in aqueous environments remains a challenge. Herein, we present a general design strategy that effectively promotes phosphorescence by utilising water molecules to construct hydrogen-bonded networks between carbon dots (CDs) and cyanuric acid (CA). Interestingly, water molecules not only cause no phosphorescence quenching but also greatly enhance the phosphorescence emission. This enhancement behaviour can be explained by the fact that the highly ordered bound water on the CA particle surface can construct robust bridge-like hydrogen-bonded networks between the CDs and CA, which not only effectively rigidifies the C=O bonds of the CDs but also greatly enhances the rigidity of the entire system. In addition, the CD-CA suspension exhibits a high phosphorescence lifetime (687 ms) and is successfully applied in ion detection based on its visible phosphorescence.

Ordered assembly of hybrid room-temperature phosphorescence thin films showing polarized emission and the sensing of VOCs.

Benefitting from the self-organization of organic phosphors into 2D galleries of clay nanosheets, we have developed a method for the ordered assembly of long-lived room-temperature phosphorescence (RTP) thin films through a layer-by-layer (LBL) process. The clay-based thin films present polarized RTP and serve as sensors for VOCs.

CNDs@zeolite: new room-temperature phosphorescent materials derived by pyrolysis of organo-templated zeolites

CNDs@zeolite composites with long-lived room-temperature phosphorescence have been prepared by pyrolysis of organic templates confined in zeolites.

Ultralong Persistent Room Temperature Phosphorescence of Metal Coordination Polymers Exhibiting Reversible pH-Responsive Emission.

Ultra-long-persistent room temperature phosphorescence (RTP) materials have attracted much attention and present various applications in illumination, displays, and the bioimaging field; however, the persistent RTP is generally from the inorganic phosphor materials to date. Herein, we show that the metal coordination polymers (CPs) could be new types of emerging long-lived RTP materials for potential sensor applications. First, two kinds of Cd-based CPs, Cd(m-BDC)(H2O) (1) and Cd(m-BDC)(BIM) (2) (m-BDC = 1,3-benzenedicarboxylic acid; BIM = benzimidazole), were obtained through a hydrothermal process, and the samples were found to exhibit two-dimensional layered structures, which are stabilized by interlayer C-H···π interaction and π···π interaction, respectively. The CPs show unexpected second-time-scale ultra-long-persistent RTP after the removal of UV excitation, and this persistent emission can be detected easily on a time scale of 0-10 s. The CPs also feature a tunable luminescence decay lifetime by adjusting their coordination situation and packing fashion of ligands. Theoretical calculation further indicates that the introduction of the second ligand could highly influence the electronic structure and intermolecular electron transfer toward tailoring the RTP of the CP materials. Moreover, CP 2 exhibits well-defined pH- and temperature-dependent phosphorescence responses. Therefore, this work provides a facile way to develop new type of CPs with steady-state and dynamic tuning of the RTP properties from both experimental and theoretical perspectives, which have potential applications in the areas of displays, pH/temperature sensors, and phosphorescence logic gates. On account of suitable incorporation of inorganic and organic building blocks, it can be expected that the ultra-long-persistent RTP CPs can be extended to other similar systems due to the highly tunable structures and facile synthesis routes.

Phenylacetylide ligand mediated tuning of visible-light absorption, room temperature phosphorescence lifetime and triplet–triplet annihilation based up-conversion of a diimine Pt(II) bisacetylide complex

We addressed the impact of coumarin ligands linkage on the photophysics of a series of diimine Pt(II) bisacetylide complexes. The long-lived room temperature phosphorescence of the coumarin unit was observed (λem = 636 nm, ΦP = 0.2%, τT = 20.15 μs) within the complex Pt(dbbpy)(CC-phenylene-coumarin)2, where dbbpy stands for 4,4′-di(tert-butyl)-2,2′-bipyridine and CC-phenylene-coumarin is 6-(4′-ethynylphenyl)-coumarin. Comparison with the alternative complex Pt(dbbpy)(CC-coumarin)2 which has ɛ = 32,300 M−1 cm−1 at 404 nm, where CC-coumarin stands for 6-ethynylcoumarin, revealed that Pt(dbbpy)(CC-phenylene-coumarin)2 also showed strong absorption in the visible range (ɛ = 71,900 M−1 cm−1 at 407 nm). The emissive triplet excited state of Pt(dbbpy)(CC-phenylene-coumarin)2 was characterized as 3IL and the extension of the π-conjugation system inside the coumarin ligand was found account for elongation of the lifetime of triplet excited states. Inspired by its unusual phosphorescence behavior, Pt(dbbpy)(CC-phenylene-coumarin)2 was used as the sensitizer for the triplet–triplet annihilation based upconversion and the observed upconversion quantum yield was up to 19.7%.

Inclusion complexes naphthalene-γ-cyclodextrin-adamantane and naphthalene-γ-cyclodextrin-o-carborane: the structure and luminescence properties

The luminescence properties of inclusion complexes of naphthalene-d8 with γ-cyclodextrin (γ-CD) in the presence of adamantane or o-carborane added as third parties were studied in aqueous solutions. It was found that the structure of the cage compound added to the aqueous solution of the naphthalene-d8@γ-CD complex completely determines the luminescence type of the ternary complex. For instance, the intensity of excimer fluorescence (EF) band increases considerably at the expense of reduction of the intensity of monomer fluorescence (MF) band on adding adamantane. On the contrary, adding o-carborane causes a decrease in the intensity of the EF band of naphthalene-d8 and simultaneous appearance of MF in addition to long-lived room-temperature phosphorescence (RTP) whose lifetime increases from 1.5 s to 9.1 s after deoxygenation of the solution. Structural differences between the complexes affecting their behavior under the action of the third parties were explained using the results of semiempirical quantum chemical calculations.

Tuning the luminescence lifetimes of ruthenium(ii) polypyridine complexes and its application in luminescent oxygen sensing

Ru(Phen)(bpy)2 (1) and its new derivatives (2–5) with pyrenyl or ethynylated pyrene and phenyl units appended to the 3-position of the phenanthroline (Phen) ligand were prepared and these complexes generate long-lived room temperature phosphorescence in the red and near IR range (600–800 nm). The photophysical properties of these complexes were investigated by UV-Vis absorption, luminescence emission, transient absorption spectra and DFT/TDDFT calculations. We found the luminescence lifetime (τ)can be drastically extended by ligand modification (increased up to 140-fold), e.g. τ = 58.4 μs for complex 3 (with pyrenyl ethynylene appendents) was found, compared to τ = 0.4 μs for the reference complex 1. Ethynylated phenyl appendents alter the τ also (complex 2, τ = 2.4 μs). With pyrenyl appendents (4 and 5), lifetimes of 2.5 μs and 9.2 μs were observed. We proposed three different mechanisms for the lifetime extension of 2, 3, 4 and 5. For 2, the stabilization of the 3MLCT state by π-conjugation is responsible for the extension of the lifetime. For 3, the emissive state was assigned as an intra-ligand (IL) long-lived 3π–π* state (3IL/3LLCT, intraligand or ligand-to-ligand charge transfer), whereas a C–C single bond linker results in a triplet state equilibrium between 3MLCT state and the pyrene localized 3π–π* triplet state (3IL, e.g.4 and 5). DFT/TDDFT calculations support the assignment of the emissive states. The effects of the lifetime extension on the oxygen sensing properties of these complexes were studied in both solution and polymer films. With tuning the emissive states, and thus extension of the luminescence lifetimes, the luminescent O2 sensing sensitivity of the complexes can be improved by ca. 77-fold in solution (I0/I100 = 1438 for complex 3, vs. I0/I100 = 18.5 for complex 1). In IMPES-C polymer films, the apparent quenching constant KSVapp is improved by 150-fold from 0.0023 Torr−1 (complex 1) to 0.35 Torr−1 (complex 3). The KSVapp value of complex 3 is even higher than that of PtOEP under similar conditions (0.15 Torr−1).

Boron Polylactide Nanoparticles Exhibiting Fluorescence and Phosphorescence in Aqueous Medium

Difluoroboron dibenzoylmethane-polylactide, BF(2)dbmPLA, a biocompatible polymer-luminophore conjugate was fabricated as nanoparticles. Spherical particles <100 nm in size were generated via nanoprecipitation. Intense blue fluorescence, two-photon absorption, and long-lived room temperature phosphorescence (RTP) are retained in aqueous suspension. The nanoparticles were internalized by cells and visualized by fluorescence microscopy. Luminescent boron biomaterials show potential for imaging and sensing.

Sensitization of Lanthanide Luminescence in Heterotrinuclear PtLn2(Ln = Eu, Nd, Yb) Complexes with Terpyridyl-Functionalized Alkynyl by Energy Transfer from a Platinum(II) Alkynyl Chromophore

The preparation and characterization of the mononuclear complexes cis-Pt(dppe)(C⋮CPhtpy)2 (1) and cis-Pt(dppp)(C⋮CPhtpy)2 (5) (dppe = 1,2-bis(diphenylphosphino)ethane, dppp = 1,3-bis(diphenylphosphino)propane, HC⋮CPhtpy = 4‘-(4-ethynylphenyl)-2,2‘:6‘,2‘ ‘-terpyridine) and the corresponding PtLn2 (Ln = Eu, Nd, Yb) heterotrinuclear complexes by incorporating the precursor 1 or 5 with Ln(hfac)3(H2O)2 are described. 1 and 5 exhibit intense, long-lived room-temperature phosphorescence, originating probably from an admixture of 3ILCT (π → π*(C⋮Cphtpy)) and 3MLCT (d(Pt) → π*(C⋮CPhtpy)) triplet states, which qualifies them as favorable energy donors to facilitate Pt → Ln energy transfer in the corresponding PtLn2 heterotrinuclear complexes. As anticipated, sensitized lanthanide luminescence in PtLn2 heterotrinuclear complexes is attained through efficient energy transfer from PtII alkynyl chromophore based 3MLCT and 3ILCT triplet states.

Long-lived phosphorescence of arenes in complexes with cyclodextrins 2. Room-temperature phosphorescence of ternary complexes of naphthalene and phenanthrene with β-cyclodextrin and adamantane derivatives in the presence of oxygen

Long-lived room-temperature phosphorescence (RTP) of arene—β-cyclodextrin (β-CD)— cage hydrocarbon complexes in the presence of oxygen was studied. Naphthalene-d8, phenanthrene, and fluorene were used as arenes and adamantane, 1,3-dimethyladamantane, diamantane, and diadamantyl were used as the cage hydrocarbons (according to PM3 quantum chemical calculations, the use of these compounds might cause the appearance of long-lived RTP). The RTP lifetime of the naphthalene-d8—β-CD—diadamantyl complex is 11.9 s at 20 °C.

Room Temperature Phosphorescence from Ruthenium(II) Complexes Bearing Conjugated Pyrenylethynylene Subunits

We describe the synthesis, electrochemistry, and photophysical properties of several Ru(II) complexes bearing different numbers of pyrenylethynylene substituents in either the 5- or 5,5'-positions of 2,2'-bipyridine, along with the appropriate Ru(II) model complexes bearing either bromo- or ethynyltoluene functionalities. In addition, we prepared and studied the photophysical behavior of the diimine ligands 5-pyrenylethynylene-2,2'-bipyridine and 5,5'-dipyrenylethynylene-2,2'-bipyridine. Static and dynamic absorption and luminescence measurements reveal the nature of the lowest excited states in each molecule. All model Ru(II) complexes are photoluminescent at room temperature and exhibit excited-state behavior consistent with metal-to-ligand charge transfer (MLCT) characteristics. In the three Ru(II) molecules bearing multiple pyrenylethynylene substituents, there is clear evidence that the lowest excited state is triplet intraligand (3IL)-based, yielding long-lived room temperature phosphorescence in the red and near IR. This phosphorescence emanates from either 5-pyrenylethynylene-2,2'-bipyridine or 5,5'-dipyrenylethynylene-2,2'-bipyridine, depending upon the composition of the coordination compound. In the former case, the excited-state absorption difference spectra that were measured for the free ligand are easily superimposed with those obtained for the metal complexes coordinated to either one or two of these species. The latter instance is slightly complicated since coordination of the 5,5'-ligand to the Ru(II) center planarizes the diimine structure, leading to an extended conjugation on the long axis with a concomitant red shift of the singlet pi-pi absorption transitions and the observed room temperature phosphorescence. As a result, transient absorption measurements obtained using free 5,5'-dipyrenylethynylene-2,2'-bipyridine show a marked blue shift relative to its Ru(II) complex, and this extended pi-conjugation effect was confirmed by coordinating this ligand to Zn(II) at room temperature. In essence, all three pyrenylethynylene-containing Ru(II) complexes are unique in this genre of chromophores since the lowest excited state is 3IL-based at room temperature and at 77 K, and there is no compelling evidence of interacting or equilibrated excited states.

Long-lived room temperature phosphorescence of a naphthalene-β-cyclodextrin-adamantane complex in the presence of oxygen

Naphthalene-d8—β-cyclodextrin—adamantane triple complexes were prepared in an aqueous solution at room temperature. Irradiation (λ = 285 nm) of the solution in the presence of molecular oxygen results in the long-lived (τ = 10.3 s) room temperature phosphorescence (RTP). The removal of oxygen from the solution increases the RTP intensity and phosphorescence lifetime by 1.5 times. The RTP spectrum contains a well-resolved vibrational structure, whose bands are assigned to full symmetric vibrations of naphthalene, their overtones, and the combination tones of full symmetric vibrations. The quantum-chemical calculation of the triple complex structure confirms that both naphthalene and adamantane can simultaneously be included into the β-cyclodextrin cavity and suggests that the role of the latter as the third component is the more efficient shielding of naphthalene from the oxygen effect due to both the formation of three-component complexes and their aggregation to form submicronic particles.