Phosphorescence Quantum Yield Research Articles

Improving triplet excited-state absorption and lifetime of cationic iridium(III) complexes by extending π-conjugation of the 2-(2-quinolinyl)quinoxaline ligand

The synthesis and photophysical properties (UV−vis absorption, emission, and transient absorption) of four cationic Ir(III) complexes (C^N)2Ir(R-quqo)+ (HC^N = 1-phenylisoquinoline (piq) and 1,2-diphenylpyreno[4,5-d]imidazole (dppi), quqo = 2-(2-quinolinyl)quinoxaline, R = H or fluorenyl) are reported. The UV–vis absorption and emission were simulated by time-dependent density functional theory (TDDFT). Influences of extending π-conjugation of the C^N ligand and the diimine ligand on the singlet and triplet excited-state absorption and lifetimes of these complexes were explored. All complexes exhibited intense ligand-localized 1π,π transitions, broad and structureless metal-to-ligand charge transfer (1MLCT) / ligand-to-ligand charge transfer (1LLCT) transitions, and very weak spin-forbidden 3MLCT/3LLCT/3π,π transitions in their UV–vis absorption spectra. The two complexes that bear fluorenyl-substituted quqo ligands (Ir-3 and Ir-4) also possessed a broad intraligand charge transfer (1ILCT) / 1π,π band at 430–550 nm. The predominant 3ILCT/3π,π characters of the triplet excited states of Ir-3 and Ir-4 improved their phosphorescent emission quantum yields and prolonged their triplet lifetimes compared to the weaker and short-lived emission of Ir-1 and Ir-2. In contrast to the very weak nanosecond transient absorption (TA) of Ir-1 and Ir-2, Ir-3 and Ir-4 possessed much stronger TA signals at 520−800 nm upon nanosecond laser excitation. These complexes exhibited moderate to strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses, with the RSA trend following Ir-1 >Ir-2 ≈ Ir-3 >Ir-4. Considering the long triplet excited-state lifetimes and broadband TA, complexes Ir-3 and Ir-4 could be potential broadband RSA materials.

Iridium Corroles Exhibit Weak Near-Infrared Phosphorescence but Efficiently Sensitize Singlet Oxygen Formation

Six-coordinate iridium(III) triarylcorrole derivatives, Ir[TpXPC)]L2, where TpXPC = tris(para-X-phenyl)corrole (X = CF3, H, Me, and OCH3) and L = pyridine (py), trimethylamine (tma), isoquinoline (isoq), 4-dimethylaminopyridine (dmap), and 4-picolinic acid (4pa), have been examined, with a view to identifying axial ligands most conducive to near-infrared phosphorescence. Disappointingly, the phosphorescence quantum yield invariably turned out to be very low, about 0.02 – 0.04% at ambient temperature, with about a two-fold increase at 77 K. Phosphorescence decay times were found to be around ~5 µs at 295 K and ~10 µs at 77 K. Fortunately, two of the Ir[TpCF3PC)]L2 derivatives, which were tested for their ability to sensitize singlet oxygen formation, were found to do so efficiently with quantum yields Φ(1O2) = 0.71 and 0.38 for L = py and 4pa, respectively. Iridium corroles thus may hold promise as photosensitizers in photodynamic therapy (PDT). The possibility of varying the axial ligand and of attaching biotargeting groups at the axial positions makes iridium corroles particularly exciting as PDT drug candidates.

Amphiphilic Rhenium-Oxo Corroles as a New Class of Sensitizers for Photodynamic Therapy.

A set of rhenium(V)-oxo meso-triarylcorroles bearing ester and carboxylic acid functionalities were synthesized with a view to determining their potential for photodynamic therapy. Toward this end, we measured their near-IR phosphorescence and their ability to sensitize singlet oxygen formation. The two esters studied, ReVO 5,10,15-tris(meta-carbomethoxyphenyl)corrole and ReVO 5,10,15-tris(para-carbomethoxyphenyl)corrole, were found to exhibit phosphorescence quantum yields of around 1% and fairly long phosphorescence lifetimes of about 60 μs in toluene. The corresponding carboxylic acids, which were examined in ethanolic/aqueous media, in contrast, showed much lower phosphorescence quantum yields on the order of 0.01% and somewhat shorter phosphorescent lifetimes. The quantum yields for singlet oxygen formation, on the other hand, turned out to be equally high (0.72 ± 0.02) for the esters and corresponding carboxylic acids. For the two carboxylic acids, we also carried out photocytotoxicity measurements on rat bladder cancer cells (AY27) and human colon carcinoma cells (WiDr). Cell viability measurements (MTT assays) indicated 50% cell death (LD50) for AY27 cells upon 5 min of blue light exposure with the meta carboxylic acid and upon 7 min of exposure with the para carboxylic acid; complete cell death resulted after 20 min for both compounds. The WiDr cells proved less sensitive, and LD50 values were reached after 8 and 12 min illumination with the meta and para carboxylic acids, respectively.

A strategy for monitoring oxygen concentration, oxygen consumption, and generation of singlet oxygen using a phosphorescent photosensitizer

Monitoring oxygen concentration ([O2]), oxygen consumption, and generation of singlet oxygen (1O2) is critical in photodynamic therapy (PDT) used for the precise treatment of cancer. To this end, a strategy based on a phosphorescent photosensitizer, gadolinium-hematoporphyrin monomethyl ether (Gd-HMME), was developed for monitoring [O2] in this study. Diphenylisobenzofuran (DPBF) was used to consume 1O2 produced by the Gd-HMME-mediated PDT. A sealed system comprising Gd-HMME and DPBF under irradiation was designed to simulate PDT when tissue oxygen was insufficient. Upon irradiation by a 532-nm laser, Gd-HMME phosphorescence initially increased rapidly, reached a maximum, and remained unchanged thereafter, because oxygen in the system was exhausted. The increase in phosphorescence increased with increasing fluence rates, suggesting that the change in phosphorescence was related to oxygen consumption. The depletion of DPBF in the system indicated the generation of 1O2 and a reaction between DPBF and 1O2. The phosphorescence decreased rapidly when the system was re-exposed to air. In contrast, the change in the phosphorescence intensity in an open cuvette was minimal and could be neglected. Thus, [O2] and generation of 1O2 in PDT could be monitored using the phosphorescence intensity of the photosensitizer, based on the dependence of phosphorescence and singlet oxygen quantum yield on [O2].

Long‐Lived Circularly Polarized Phosphorescence in Helicene‐NHC Rhenium(I) Complexes: The Influence of Helicene, Halogen, and Stereochemistry on Emission Properties

AbstractThe first enantiopure chiral‐at‐rhenium complexes of the form fac‐ReX(CO)3(:C^N) have been prepared, where :C^N is a helicene‐N‐heterocyclic carbene (NHC) ligand and X=Cl or I. These have complexes show strong changes in the emission characteristics, notably strongly enhanced phosphorescence lifetimes (reaching 0.7 ms) and increased circularly polarized emission (CPL) activity, as compared to their parent chiral models lacking the helicene unit. The halogen along with its position within the dissymmetric stereochemical environment strongly affect the photophysics of the complexes, particularly the phosphorescence quantum yield and lifetime. These results give fresh insight into fine tuning of photophysical and chiroptical properties of Re‐NHC systems.

Long-Lived Circularly Polarized Phosphorescence in Helicene-NHC Rhenium(I) Complexes: The Influence of Helicene, Halogen, and Stereochemistry on Emission Properties.

The first enantiopure chiral-at-rhenium complexes of the form fac-ReX(CO)3 (:C^N) have been prepared, where :C^N is a helicene-N-heterocyclic carbene (NHC) ligand and X=Cl or I. These have complexes show strong changes in the emission characteristics, notably strongly enhanced phosphorescence lifetimes (reaching 0.7 ms) and increased circularly polarized emission (CPL) activity, as compared to their parent chiral models lacking the helicene unit. The halogen along with its position within the dissymmetric stereochemical environment strongly affect the photophysics of the complexes, particularly the phosphorescence quantum yield and lifetime. These results give fresh insight into fine tuning of photophysical and chiroptical properties of Re-NHC systems.

Blue luminescent silver(I) complexes constructed by 2-diphenylphosphinopyridine and dicyanamide or tricyanomethanide

Two new silver(I) complexes 2-diphenylphosphinopyridine (dppy), {Ag2(dppy)2[N(CN)2]}2[μ-N(CN)2]2 (1) and {Ag2(dppy)2[μ-C(CN)3]2}n (2) linked by dicyanamide (N(CN)2−) or tricyanomethanide (C(CN)3−) were synthesized and characterized by X-ray crystallography. Complex 1 forms a tetranuclear structure with two binucleate moieties [Ag2(dppy)2]+ linked by two dicyanamide ligands. Interestingly, when bridging ligand is changed to tricyanomethanide, complex 2 displays one-dimensional infinite chain structure. Both complexes exhibit strong blue emission with phosphorescent quantum yields of 13.4% for 1 and 39.2% for 2 in solid state.

Color-tunable ultralong organic room temperature phosphorescence from a multicomponent copolymer

Functional materials displaying tunable emission and long-lived luminescence have recently emerged as a powerful tool for applications in information encryption, organic electronics and bioelectronics. Herein, we present a design strategy to achieve color-tunable ultralong organic room temperature phosphorescence (UOP) in polymers through radical multicomponent cross-linked copolymerization. Our experiments reveal that by changing the excitation wavelength from 254 to 370 nm, these polymers display multicolor luminescence spanning from blue to yellow with a long-lived lifetime of 1.2 s and a maximum phosphorescence quantum yield of 37.5% under ambient conditions. Moreover, we explore the application of these polymers in multilevel information encryption based on the color-tunable UOP property. This strategy paves the way for the development of multicolor bio-labels and smart luminescent materials with long-lived emission at room temperature.

Люминесценция Pd и Pt бензогидропорфиразинов в ближнем ИК диапазоне

Phosphorescence in the near-IR range under stationary lamp excitation was studied of three palladium and three platinum complexes of benzocondensed hydroporphyrazine phthalocyanine analogues in which molecule one or two isoindole fragments are replaced by a hydrogenated pyrrole ring, giving macrocycles of photosynthetic pigments analogues of chlorin, bacteriochlorin and isobacteriochlorin types. For the first time, the phosphorescence spectra of trans-dibenzotetrahydroporphyrazines (for both platinum and palladium) have been registered, which detection failed during long time due to the record removal of 0-0 bands from the visible area (1.67 and 1.52 µm for Pd and Pt), and also because of the small quantum yield of phosphorescence. The fluorescence of these complexes was also detected.in the near-IR range. On the basis of comparison of experimental results with own and literature data for palladium complexes of a number of tetrapyrroles is shown the approximate additivity of the influence of structural factors — hydrogenation of pyrrole rings, aza- and benzo substitution — on the energy of the lower triplet state T1. The information obtained is essential for targeted creation of emitters in the near-IR range (1.0-1.7 µm).

Near-Infrared-Excitable Organic Ultralong Phosphorescence through Multiphoton Absorption.

Organic ultralong room-temperature phosphorescence (OURTP) with a long-lived triplet excited state up to several seconds has triggered widespread research interests, but most OURTP materials are excited by only ultraviolet (UV) or blue light owing to their unique stabilized triplet- and solid-state emission feature. Here, we demonstrate that near-infrared- (NIR-) excitable OURTP molecules can be rationally designed by implanting intra/intermolecular charge transfer (CT) characteristics into H-aggregation to stimulate the efficient nonlinear multiphoton absorption (MPA). The resultant upconverted MPA-OURTP show ultralong lifetimes over 0.42 s and a phosphorescence quantum yield of ~37% under both UV and NIR light irradiation. Empowered by the extraordinary MPA-OURTP, novel applications including two-photon bioimaging, visual laser power detection and excitation, and lifetime multiplexing encryption devices were successfully realized. These discoveries illustrate not only a delicate design map for the construction of NIR-excitable OURTP materials but also insightful guidance for exploring OURTP-based nonlinear optoelectronic properties and applications.

X-ray excited ultralong room-temperature phosphorescence for organic afterglow scintillators.

We present X-ray-excitable organic ultralong room temperature phosphorescence (X-OURTP) for afterglow scintillators through implanting lone-pair electron involved n-π* transitions and efficient charge transfer characters into H-aggregations of organic crystals. The thus designed X-OURTP molecule exhibits ultralong lifetimes up to 790 ms, high phosphorescence quantum yields of ∼8%, and excellent scintillation stability. These findings will pave the way to the design of organic afterglow scintillators, simulating extensive investigations on the extraordinary X-OURTP.

Roles of Ancillary Chelates and Overall Charges of Bis-tridentate Ir(III) Phosphors for OLED Applications.

A series of charge-neutral bis-tridentate Ir(III) complexes (1, 3, and 4) were prepared via employing three distinctive tridentate prochelates, that is, (pzptBphFO)H2, [(phpyim)H2·(PF6)], and [(pimb)H3·(PF6)2], which possess one dianionic pzptBphFO, together with a second monoanionic tridentate chelate, namely, (pzptBphFO)H, phpyim, and pimb, respectively. Moreover, a homoleptic, charge-neutral complex 2 was obtained by methylation of chelating (pzptBphFO)H of 1 in basic media, while closely related cationic complexes 5-7 were obtained by further methylation of the remaining pyrazolate unit of previously mentioned neutral complexes 2-4, followed by anion metatheses. All of these Ir(III) metal complexes showed a broadened emission profile with an onset at ∼450 nm, a result of an enlarged ligand-centered ππ* transition gap, but with distinct efficiencies ranging from 0.8% to nearly unity. Comprehensive spectroscopic and computational approaches were executed, providing a correlation for the emission efficiencies versus energy gaps and between the metal-to-ligand charge transfer/ππ* emitting excited state and upper-lying metal-centered dd quenching state. Furthermore, Ir(III) complexes 3 and 4 were selected as dopant emitters in the fabrication of sky-blue phosphorescent organic light-emitting diodes, affording maximum external quantum efficiencies of 16.7 and 14.6% with CIEx,y coordinates of (0.214, 0.454) and (0.191, 0.404) at a current density of 102 cd/m2, respectively. Hence, this research highlights an inherent character of bis-tridentate Ir(III) complexes in achieving high phosphorescence quantum yield at the molecular level.

Crystal Engineering of Room Temperature Phosphorescence in Organic Solids.

We report a series of highly emissive azatriangulenetrione (TANGO) solids in which the luminescent properties are controlled by engineering the molecular packing by adjusting the steric size of substituents. The co-alignment of "phosphorogenic" carbonyl groups within the π-stacks results in an almost pure triplet emission in HTANGO, TCTANGO, TBTANGO and TITANGO, while their rotation by ≈60° in the sterically hindered tBuTANGO leads to an almost pure singlet emission. Despite strong π-interactions, aggregation-induced quenching and triplet-triplet annihilation are avoided in HTANGO and TCTANGO which display efficient phosphorescence in the solid state. To our knowledge, HTANGO with the solid-state phosphorescence quantum yield of 42 % at room temperature is the most efficient phosphor composed of the 1st /2nd raw elements only.

Crystal Engineering of Room Temperature Phosphorescence in Organic Solids

AbstractWe report a series of highly emissive azatriangulenetrione (TANGO) solids in which the luminescent properties are controlled by engineering the molecular packing by adjusting the steric size of substituents. The co‐alignment of “phosphorogenic” carbonyl groups within the π‐stacks results in an almost pure triplet emission in HTANGO, TCTANGO, TBTANGO and TITANGO, while their rotation by ≈60° in the sterically hindered tBuTANGO leads to an almost pure singlet emission. Despite strong π‐interactions, aggregation‐induced quenching and triplet–triplet annihilation are avoided in HTANGO and TCTANGO which display efficient phosphorescence in the solid state. To our knowledge, HTANGO with the solid‐state phosphorescence quantum yield of 42 % at room temperature is the most efficient phosphor composed of the 1st/2nd raw elements only.

Aqueous phase and amorphous state room temperature phosphorescence from a small aromatic carbonyl derivative

Harvesting triplet excitons via room temperature phosphorescence (RTP) in solution or amorphous state from purely organic chromophores is a formidable challenge. Supramolecular hybrid co-assembly between a brominated aromatic carbonyl derivative (BrPhS) and laponite clay (LP) particles is shown to result in remarkable triplet stabilization to result room temperature phosphorescence (RTP) in aqueous solution as well as in amorphous thin films. This remarkable feature is realized by means of highly organized, rigid molecular network of the dye molecules on the inorganic scaffold which reduces the vibrational dissipations as well as limits the oxygen diffusion to facilitate the triplet harvesting under ambient conditions. The water soluble phosphor, BrPhS is also shown to be an excellent triplet emitter in other amorphous polymer matrices like polyvinyl alcohol (PVA) and sodium polystyrene sulphonate (PSS) with phosphorescence quantum yield over 30% in air.

Room-Temperature Phosphorescence from a Series of 3-Pyridylcarbazole Derivatives.

Exploration of pure metal-free organic molecules that exhibit strong room-temperature phosphorescence (RTP) is an emerging research topic. In this regard, unveiling the design principles for an efficient RTP molecule is an essential, but challenging, task. A small molecule is an ideal platform to precisely understand the fundamental role of each functional component because the parent molecule can be easily derivatized. Here, the RTP behaviors of a series of 3-pyridylcarbazole derivatives are presented. Experimental studies in combination with theoretical calculations reveal the crucial role of the n orbital on the central pyridine ring in the dramatic enhancement of the intersystem crossing between the charge-transfer-excited singlet state and the locally excited triplet states. Single-crystal X-ray crystallographic studies apparently indicate that both the pyridine ring and fluorine atom contribute to the enhancement of the RTP because of the restricted motion owing to weak C-H⋅⋅⋅N and H⋅⋅⋅F hydrogen-bonding interactions. The single crystal of the fluorine-substituted derivative shows an ultra-long phosphorescent lifetime (τP ) of 1.1 s and a phosphorescence quantum yield (ΦP ) of 1.2 %, whereas the bromine-substituted derivative exhibits τP of 0.15 s with a ΦP of 7.9 %. We believe that this work provides a fundamental and universal guideline for the generation of pure organic molecules exhibiting strong RTP.

Aggregation-Induced Dual-Phosphorescence from Organic Molecules for Nondoped Light-Emitting Diodes.

Aggregation-induced emission (AIE) is a beneficial strategy for generating highly effective solid-state molecular luminescence without suffering losses in quantum yield. However, the majority of reported AIE-active molecules exhibit only strong fluorescence, which is not ideal for electrical excitation in organic light-emitting diodes (OLEDs). By introducing various substituent groups onto the biscarbazole compound, a series of molecular materials with aggregation-induced phosphorescence (AIP) is designed, which exhibits two distinctly different phosphorescence bands and an absolute solid-state room-temperature phosphorescence quantum yield up to 64%. Taking advantage of the AIE feature, the AIP molecules are fabricated into OLEDs as a homogeneous light-emitting layer, which allows for relatively small efficiency roll-off and shows an external electroluminescence quantum yield of up to 5.8%, more than the theoretical limit for purely fluorescent OLED devices. The design showcases a promising strategy for the production of cost-effective and highly efficient OLED technology.

Highly Efficient Organic Room-Temperature Phosphorescent Luminophores through Tuning Triplet States and Spin-Orbit Coupling with Incorporation of a Secondary Group.

Achieving efficient ultralong purely organic phosphorescent luminophores is still a big challenge due to the slow intersystem crossing (ISC) process. Herein, we present a facile molecular design strategy that incorporates a secondary group (Br atom or methoxy group) into o-BrCz that can significantly enhance the ISC rate constant (kISC) and achieve high phosphorescence quantum yields (ΦP). As a result, DBrCz and MeBrCz achieved a profound increase of kISC ≈ 108 s-1 and obtained excellent ΦP values up to 24.53 and 27.81% in solid powder, respectively. Given the highly efficient ΦP and proper τp, DBrCz and MeBrCz are applied to alternating current (AC) light-emitting diodes (LEDs), achieving a white LED with CIE coordinates (0.28, 0.29) and a CRI over 90. As a proof of concept, we demonstrate its compensation effect on the dark duration of AC-LED with a reduced percent flicker of 78%. This result extends a new potential application for RTP luminophores in the lighting field.

Manipulating the Stacking of Triplet Chromophores in the Crystal Form for Ultralong Organic Phosphorescence.

Provided here is evidence showing that the stacking between triplet chromophores plays a critical role in ultralong organic phosphorescence (UOP) generation within a crystal. By varying the structure of a functional unit, and different on-off UOP behavior was observed for each structure. Remarkably, 24CPhCz, having the strongest intermolecular interaction between carbazole units exhibited the most impressive UOP with a long lifetime of 1.06 s and a phosphorescence quantum yield of 2.5 %. 34CPhCz showed dual-emission UOP and thermally activated delayed fluorescence (TADF) with a moderately decreased phosphorescence lifetime of 770 ms, while 35CPhCz only displayed TADF owing to the absence of strong electronic coupling between triplet chromophores. This study provides an explanation for UOP generation in crystal and new guidelines for obtaining UOP materials.

Enhancement of the Photofunction of Phosphorescent Pt(II) Cyclometalated Complexes Driven by Substituents: Solid-State Luminescence and Circularly Polarized Luminescence.

Ligand functionalization is an attractive strategy for enhancing the performance of metal-based phosphorescent emitters. Here, we report the synthesis and characterization of cyclometalated Pt(II) complexes Pt3 and Pt4 containing organosilyl-substituted (2-(2-thienyl)pyridine) ligands and compare their properties with those of Pt1 (no substituent) and Pt2 (organocarbon substituent). The photophysical characteristics of these molecules, including their absorption and phosphorescence spectra, phosphorescence quantum yield and lifetime, were investigated. The molecular structures were revealed by X-ray diffraction analysis. Under UV light irradiation, Pt2-Pt4 emitted intense orange phosphorescence in the solid state because of the bulkiness of their side chains (up to ΦP: 0.49). Optically pure (-)-(S)Si-Pt4 and (+)-(R)Si-Pt4 were prepared using the optically active ligands (+)-L4 and (-)-L4, respectively. The chiroptical properties of (+)-(R)Si-Pt4, which has an asymmetric silicon atom, were investigated. Circular dichroism and circularly polarized luminescence measurements showed that these structural motifs are suitable for applications in chiroptical phosphorescent materials.