Molecular Phosphorescence Research Articles

Efficient and Stable NIR‐II Phosphorescence of Metallophilic Molecular Oligomers for In Vivo Single‐Cell Tracking and Time‐Resolved Imaging

AbstractMolecular phosphorescence in the second near‐infrared window (NIR‐II, 1000–1700 nm) holds promise for deep‐tissue optical imaging with high contrast by overcoming background fluorescence interference. However, achieving bright and stable NIR‐II molecular phosphorescence suitable for biological applications remains a formidable challenge. Herein, we report a new series of symmetric isocyanorhodium(I) complexes that could form oligomers and exhibit bright, long‐lived (7–8 μs) phosphorescence in aqueous solution via metallophilic interaction. Ligand substituents with enhanced dispersion attraction and electron‐donating properties were explored to extend excitation/emission wavelengths and enhanced stability. Further binding the oligomers with fetal bovine serum (FBS) resulted in NIR‐II molecular phosphorescence with high quantum yields (up to 3.93 %) and long‐term stability in biological environments, enabling in vivo tracking of single‐macrophage dynamics and high‐contrast time‐resolved imaging. These results pave the way for the development of highly‐efficient NIR‐II molecular phosphorescence for biomedical applications.

Efficient and Stable NIR-II Phosphorescence of Metallophilic Molecular Oligomers for In Vivo Single-Cell Tracking and Time-Resolved Imaging.

Molecular phosphorescence in the second near-infrared window (NIR-II, 1000-1700 nm) holds promise for deep-tissue optical imaging with high contrast by overcoming background fluorescence interference. However, achieving bright and stable NIR-II molecular phosphorescence suitable for biological applications remains a formidable challenge. Herein, we report a new series of symmetric isocyanorhodium(I) complexes that could form oligomers and exhibit bright, long-lived (7-8 μs) phosphorescence in aqueous solution via metallophilic interaction. Ligand substituents with enhanced dispersion attraction and electron-donating properties were explored to extend excitation/emission wavelengths and enhanced stability. Further binding the oligomers with fetal bovine serum (FBS) resulted in NIR-II molecular phosphorescence with high quantum yields (up to 3.93 %) and long-term stability in biological environments, enabling in vivo tracking of single-macrophage dynamics and high-contrast time-resolved imaging. These results pave the way for the development of highly-efficient NIR-II molecular phosphorescence for biomedical applications.

Molecular phosphorescence enhancement by the plasmon field of metal nanoparticles.

A theoretical model is proposed that allows the estimation of the quantum yield of phosphorescence of dye molecules in the vicinity of plasmonic nanoparticles. For this purpose, the rate constants of the radiative and nonradiative intramolecular transitions for rhodamine 123 (Rh123) and brominated rhodamine (Rh123-2Br) dyes have been calculated. The plasmon effect of Ag nanoparticles on various types of luminescence processes has been studied both theoretically and experimentally. We show that in the presence of a plasmonic nanoparticle, the efficiency of the immediate and delayed fluorescence increases significantly. The phosphorescence rate of the rhodamine dyes also increases near plasmonic nanoparticles. The long-lived luminescence i.e., delayed fluorescence and phosphorescence is more enhanced for Rh123-2Br than for Rh123. The largest phosphorescence quantum yield is obtained when the dye molecule is at a distance of 4-6 nm from the nanoparticle surface. Our results can be used in the design of plasmon-enhancing nanostructures for light-emitting media, organic light-emitting diodes, photovoltaic devices, and catalysts for activation of molecular oxygen.

Hydrogen-bond organized 2D metal–organic microsheets: direct ultralong phosphorescence and color-tunable optical waveguides

Ultralong phosphorescent materials have numerous applications across biological imaging, light-emitting devices, X-ray detection and anti-counterfeiting. Triplet-state molecular phosphorescence typically accompanies the singlet-state fluorescence during photoluminescence, and it is still difficult to achieve direct triplet photoemission as ultralong room temperature phosphorescence (RTP). Here, we have designed Zn-IMDC (IMDC, 4,5-imidazoledicarboxylic acid) and Cd-IMDC, two-dimensional (2D) hydrogen-bond organized metal–organic crystalline microsheets that exhibit rarely direct ultralong RTP upon UV excitation, benefiting from the appropriate heavy-atom effect and multiple triplet energy levels. The excitation-dependent and thermally stimulated ultralong phosphorescence endow the metal–organic systems great opportunities for information safety application and temperature-gated afterglow emission. The well-defined 2D microsheets present color-tunable and anisotropic optical waveguides under different excitation and temperature conditions, providing an effective way to obtain intelligent RTP-based photonic systems at the micro- and nano-scales.

Crystal-state quad-mode triplet emissions of D-A-A’-D type phosphors with AIEE and visible-light-excited persistent phosphorescence

Herein, we succeed in finding crystal-state quad-mode triplet emissions through the concept of integration of thermally activated exciton release from T1* and introducing intramolecular electron coupling. Two new D-A-A′-D type carbazol-nicotinamide isomers (P1 and P2) with the pyridine ring as the core are synthesized. The crowded D-A-A′-D structure makes P1 and P2 in a highly twisted state, which is conducive to lowering the ΔEST and ETD. Excitingly, P1 and P2 achieve the crystal-state quad-mode triplet emissions, including ultralong room temperature phosphorescence from T1*, TADP from T1via thermally activated exciton release (TAER), the original molecular phosphorescence from T1 and TADF from S1via TAER and reverse intersystem crossing (RISC). Especially, the long-lived emissions of P1 have a rather high quantum yield of 34.3%. The intramolecular electron coupling is observed in the single crystal, verifying the original T1 phosphorescence. The ΔEST and ETD of the crystals can be calculated experimentally to be 0.19/0.18 eV and 0.19/0.20 eV, respectively, for P1 and P2. And the TAER and RISC process is validated by temperature-dependent experiment. Besides, the P2 crystals manifest fascianting visible-light-excited persistent phosphorescence and P1 shows remarkable AIEE property. To the best of our knowledge, it is the first time to find crystal-state quad-mode triplet emissions at room temperature experimentally. This study is a breakthrough in the development of multi-mode phosphorescence mechanism.

Wide range zero-thermal-quenching ultralong phosphorescence from zero-dimensional metal halide hybrids

Materials with ultralong phosphorescence have wide-ranging application prospects in biological imaging, light-emitting devices, and anti-counterfeiting. Usually, molecular phosphorescence is significantly quenched with increasing temperature, rendering it difficult to achieve high-efficiency and ultralong room temperature phosphorescence. Herein, we spearhead this challenging effort to design thermal-quenching resistant phosphorescent materials based on an effective intermediate energy buffer and energy transfer route. Co-crystallized assembly of zero-dimensional metal halide organic-inorganic hybrids enables ultralong room temperature phosphorescence of (Ph4P)2Cd2Br6 that maintains luminescent stability across a wide temperature range from 100 to 320 K (ΔT = 220 °C) with the room temperature phosphorescence quantum yield of 62.79% and lifetime of 37.85 ms, which exceeds those of other state-of-the-art systems. Therefore, this work not only describes a design for thermal-quenching-resistant luminescent materials with high efficiency, but also demonstrates an effective way to obtain intelligent systems with long-lasting room temperature phosphorescence for optical storage and logic compilation applications.

Intrinsic and Extrinsic Heavy-Atom Effects on the Multifaceted Emissive Behavior of Cyclic Triimidazole.

Considering that heavy halogen atoms can be used to tune the emissive properties of organic luminogens, the understanding of their role in photophysics is fundamental for materials engineering. Here, the extrinsic and intrinsic heavy-atom effects on the photophysics of organic crystals were separately evaluated by comparing cyclic triimidazole (TT) with its monoiodo derivative (TTI) and its co-crystal with diiodotetrafluorobenzene (TTCo). Crystals of TT showed room-temperature ultralong phosphorescence (RTUP) originated from H-aggregation. TTI and TTCo displayed two additional long-lived components, the origin of which is elucidated through single-crystal X-ray and DFT/TDDFT studies. The results highlight the different effects of the I atom on the three phosphorescent emissions. Intrinsic heavy-atom effects play a major role on molecular phosphorescence, which is displayed at room temperature only for TTI. The H-aggregate RTUP and the I⋅⋅⋅N XB-induced (XB=halogen bond) phosphorescence on the other side depend only on packing features.

Aggregation-Induced Enhancement of Molecular Phosphorescence Lifetime: A First-Principle Study

Pure organic phosphorescent molecules are promising compounds for applications of phosphorescence, yet their utilization is restricted because of inefficient intersystem crossing (ISC) between sing...

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

AbstractThe 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.

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.

Luminescent Eu3+–dibenzoylmethanate complex with sulfoxide ligand as sensitizer applied to organic light- emitting diodes

In this work the synthesis, characterization and photoluminescent and electroluminescent behavior of the [Eu(DBM)₃(PTSO)₂] complex have been investigated. The emission spectrum of this Eu³⁺–β– diketonate complex show characteristics narrow bands arising from the ⁵D₀→⁷Fȷ (J = 0–4) transitions, which are split according to the selection rule for Cs, Cn or Cnv site symmetries. Triple and doublelayer organic light-emitting diodes (OLEDs) using NPB as hole transporting layer, Alq₃ as electron transporting layer and [Eu(DBM)₃(PTSO)₂] complex as emitting and electron transporting layer were grown and characterized. We found the presence of two concurrent mechanisms for the electroluminescent behavior: (1) the electroluminescence (EL) from the exciton-ligand-Eu³+ ion energy transfer, responsible for the EL narrow bands; and (2) the molecular electrophosphorescence with an initial energy transfer exciton-ligand and posterior intersystem crossing from the excited singlet S₁ to the triplet T state followed by molecular phosphorescence (T→S₀). The electroluminescent behavior of [Eu(DBM)₃(PTSO)₂] complex suggest that this complex is promising to applied at Light Conversion Molecular Devices (LCMDs) such as in OLED devices.

Aggregation-induced intersystem crossing: a novel strategy for efficient molecular phosphorescence.

"Aggregation-caused quenching" (ACQ) and "aggregation-induced emission" (AIE) are two well-known mechanisms for polymer luminescence. Here we proposed an alternative mechanism termed "aggregation-induced intersystem crossing" (AI-ISC). By aggregating certain fluorescent dye molecules, one can improve the energy matches between excited singlet and triplet states so as to promote the intersystem crossing (ISC) rate, and consequently prolong the lifetime of excited electrons by steering them into triplet states. First-principles calculations suggested that the enhanced ISC rate could substantially promote molecular phosphorescence in aggregated systems of originally fluorescent dye molecules, as later validated by experimental measurement. Meanwhile, the emission spectra experience a red shift along with the aggregation, providing a convenient knob to tune the phosphorescence wavelength. The proposed AI-ISC mechanism may open up a new design approach for the emerging luminescent material applications.

Формування електролюмінесценції в системі електрод–молекула–електрод

The kinetics of electroluminescence formation in an electrode–molecule–electrode system has been considered in the framework of the HOMO–LUMO model. The appearance of electroluminescence in a charge-neutral molecule is shown to be driven by the hopping and tunneling mechanisms of electron transfer between the electrodes. The corresponding electron transmission is found to occur with the participation of both real and virtual states of the charged molecule. Conditions for the electron transmission through the molecule to occur by the hopping and tunneling mechanisms in the resonant regime are determined. In this case, the molecular electrofluorescence and molecular phosphorescence, as well as the current through the molecule, achieve their maximum values.

Luminescence Switching of a Cyclometalated Iridium(III) Complex through a Redox‐active Tetrathiafulvalene‐based Ligand

OFF-ON-OFF: By reversible modulation of the oxidation states of the synthesised IrIII complex seen here, distinct electron-transfer efficiencies are exhibited and, remarkably, affect the properties of the IrIII core in the excited state. Thereby, a redox-controlled “OFF-ON-OFF” molecular phosphorescence switch is realized.

The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N′-dicarbazolyl-1,1′-biphenyl

In this paper we report the results of optical characterization of 4,4-N,N(')-dicarbazolyl-1,1(')-biphenyl (CBP), known as a host material for phosphorescent light emitting devices. Using absorption, steady state, and time-resolved spectroscopy, we explore the singlet and triplet states in solid and solution samples of CBP. In solutions we observe two distinct short-lived states with well-resolved emission originating from individual molecule singlet states (at 365 and 380 nm) and "quenching" low energy (LE) states (at 404 and 424 nm). The latter are seen only in saturated solutions and solid samples. Both of those species have different lifetimes. After UV exposure of very concentrated degassed solution the intensities of the LE bands starts to decrease. The longer the solution is exposed to UV, the less emission is seen at 404 and 424 nm, until it is totally gone. The spectrum of the highly concentrated solution is then the same as the spectrum of dilute solution, i.e., only emission at 365 and 380 nm is present. An increase in intensities of the singlet emission peaks correlates with an increase in UV exposure time. Similar behavior is observed in evaporated CBP film. We propose that this behavior is due to chemical instability of the weak N-C bonding of carbazolyl moiety-this creates new degradational species over time which dissociate after exposure to UV. We believe this to be the reason for variation in CBP fluorescence and delayed fluorescence spectra recorded by various research groups. Further, we detected two types of very long-lived states. One of these states (higher energy) is ascribed to molecular phosphorescence emission, the other to emission from low energy triplet trap states which we relate to degradational species. We propose that triplets are more easily caught by these latter sites when their hopping rate increases, and they emit inefficiently from these lower energy sites.

The Evolution of Organometallic Complexes in Organic Light-Emitting Devices

This article is an edited transcript of the MRS Medal presentation given by Mark Thompson (University of Southern California) on November 28, 2006, at the Materials Research Society Fall Meeting in Boston. Thompson was awarded the Medal for the “development of highly efficient heavy-metal phosphor complexes.” The MRS Medal recognizes a specific outstanding recent discovery or advancement which is expected to have a major impact on the progress of any materials-related field.Successful research efforts have led to improvements in the internal efficiencies of organic light-emitting devices (OLEDs) from 25% to 100%. The electroluminescence process in OLEDs involves the formation of both singlet and triplet excitons, formed in a ratio of 1:3. There is a drive to improve spin statistics by developing compounds in which triplet excitons, in addition to singlet excitons, can be used efficiently. Success with the incorporation of heavy-metal–based phosphors into OLEDs, in which the strong spin-orbit coupling of the metal atom allows for efficient molecular phosphorescence from triplet excitons, resulted in the identification and synthesis of an iridium complex, fac-tris-phenylpyridine iridium, with internal efficiencies of 100%. This, in turn, has led to the synthesis of more than 100 iridium- and platinum-based compounds, which have become the most efficient light-emitting compounds yet discovered. Intellectual property from Thompson's research in this field has led to more than 50 U.S. patents and substantial entrepreneurial investment toward commercial applications and devices.

Phosphorescence as a probe of exciton formation and energy transfer in organic light emitting diodes

The development of highly efficient phosphorescent molecules has approximately quadrupled the quantum efficiency of organic light emitting devices (OLEDs). By harnessing triplet as well as singlet excitons, efficient molecular phosphorescence has also enabled novel studies of exciton physics in organic semiconductors. In this review, we will summarize recent progress in understanding exciton formation and energy transfer using phosphorescent molecular probes. Particular emphasis is given to two topics of current interest: energy transfer in blue phosphorescent OLEDs, and quantifying the formation ratio of singlet to triplet excitons in small-molecular weight materials and polymers. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular phosphorescence enhancement via tunneling through proton-transfer potentials

A set of four substituted aminosalicylates, whose respective structures permit normal fluorescence, twisted intramolecular charge-transfer fluorescence, and proton-transfer fluorescence, in various combinations, were studied at 77 K to observe the effect on intersystem crossing. It is shown that the molecular structures which are capable of intramolecular proton transfer exhibit greatly enhanced normal molecule phosphorescence. The excitation mechanism involves proton-tunneling S{sub 1}{yields}S{sub 1}{prime}(PT) after primary excitation, enhanced intersystem crossing from the tautomer excited state S{sub 1}{prime}(PT){yields}T{sub 1}{prime}(PT), followed by reverse proton-transfer tunneling via triplet potential T{sub 1}{prime}(PT){yields}T{sub 1}. The competitive singlet-state excitation processes are delineated by picosecond transient absorption spectrometry. The mechanism described requires the specific electronic state energy ordering S{sub O} < S{sub O}{prime} < T{sub 1} < T{sub 1}{prime} < S{sub 1}{prime} < S{sub 1} {hor ellipsis} where the primes represent the states of the tautomer form of the molecule.

Photoionization and triplet—triplet absorption of 4-pyrrolidinopyridine

A variable-temperature flash photolysis study of 4-pyrrolidinopyridine coupled with phosphorescence and photolysis experiments at 77 K provides evidence that photoionization is a primary process involving an upper triplet state which reverses upon warming. Triplet—triplet absorption was observed at 77 K in ether—isopentane—ethanol with a wavelength maximum of 510 nm and a triplet state lifetime of 1.1 s and was confirmed by comparison with the molecular phosphorescence which has the same lifetime and a wavelength maximum of 380 nm. A second emission in the vicinity of 500 nm observed at 173 K in isopropyl alcohol, which was shorter lived than the molecular phosphorescence, is tentatively assigned as emission from the molecular cation. Evidence for cation emission and recombination with an electron was also obtained for 4- N-dimethylaminopyridine. The broadening of the UV absorption band at 257 nm for 4-pyrrolidinopyridine upon cooling in alcoholic and acidic media is attributed to the presence of two or more conformers or to complexation via hydrogen bonding or protonation with the solvent.

Photoionization and triplet—triplet absorption of 4-pyrrolidinopyridine

A variable-temperature flash photolysis study of 4-pyrrolidinopyridine coupled with phosphorescence and photolysis experiments at 77 K provides evidence that photoionization is a primary process involving an upper triplet state which reverses upon warming. Triplet—triplet absorption was observed at 77 K in ether—isopentane—ethanol with a wavelength maximum of 510 nm and a triplet state lifetime of 1.1 s and was confirmed by comparison with the molecular phosphorescence which has the same lifetime and a wavelength maximum of 380 nm. A second emission in the vicinity of 500 nm observed at 173 K in isopropyl alcohol, which was shorter lived than the molecular phosphorescence, is tentatively assigned as emission from the molecular cation. Evidence for cation emission and recombination with an electron was also obtained for 4- N-dimethylaminopyridine. The broadening of the UV absorption band at 257 nm for 4-pyrrolidinopyridine upon cooling in alcoholic and acidic media is attributed to the presence of two or more conformers or to complexation via hydrogen bonding or protonation with the solvent.