Phosphorescence Process Research Articles

Photoluminescence Properties of Gold(I)-N-Heterocyclic Carbene Complexes

Gold(I)-N-heterocyclic carbenes (NHCs) complexes have undergone rapid development and developed as useful tools for the design of various scaffolds in diverse applications such as biological, catalytic and photophysical applications. This review covers photoluminescence transition properties of gold complexes containing NHC ligands, which give resulted in high energy emissions with desired blue colour needs for new optical devices, bioimaging and organic light emitting diodes (OLEDs). In addition, the great versatility in the structural morphology of the complexes with aurophilic interactions, provides numerous possibilities in the synthesis of desired luminescence complexes with lifetimes corresponding to a fluorescence or phosphorescence process that depends on the origin of the emissions. Herein, we present an overview on structural and electronic properties of NHC ligand, metal-NHC complexes, gold(I)-NHC complexes and photoluminescence properties reported from various research works.

Phosphine Oxide‐Containing Gold(III) Complexes with Tunable Emission Color and Thermally Enhanced Luminescence Behavior

AbstractA series of phosphine oxide‐containing gold(III) complexes with tunable emission colors spanning from sky‐blue to near‐infrared region is reported. This is accomplished by the switching of the excited state characters from intraligand to ligand‐to‐ligand charge transfer through the replacement of the auxiliary ligand from aryl to nitrogen‐based ligands. In addition to high photoluminescence quantum yields in both solution and solid‐state thin films, these complexes exhibit large radiative decay rate constants of the order of 106 s−1, much larger than those commonly found for other gold(III) complexes. The origin of such enhanced performance is believed to be arising from the occurrence of both thermally activated delayed fluorescence and thermally stimulated delayed phosphorescence processes within the emitters. This is probed by ns‐ and fs‐transient absorption spectroscopy, time‐resolved, and temperature‐dependent emission spectroscopy. In particular, the direct observation of the upconversion processes and the determination of the activation barriers are achieved in the variable‐temperature fs‐transient absorption spectroscopic studies. Solution‐processed organic light‐emitting devices with satisfactory external quantum efficiencies of up to 15.2% are achieved, which could be ascribed to the presence of thermally activated delayed fluorescence and/or thermally stimulated delayed phosphorescence processes.

Tunable Self-Referenced Molecular Thermometers via Manipulation of Dual Emission in Platinum(II) Pyridinedipyrrolide Complexes.

Optical temperature sensors based on self-referenced readout schemes such as the emission ratio and the decay time are crucial for a wide range of applications, with the former often preferred due to simplicity of instrumentation. This work describes a new group of dually emitting dyes, platinum(II) pincer complexes, that can be used directly for ratiometric temperature sensing without an additional reference material. They consist of Pt(II) metal center surrounded by a pyridinedipyrrolide ligand (PDP) and a terminal ligand (benzonitrile, pyridine, 1-butylimidazol or carbon monoxide). Upon excitation with blue light, these complexes exhibit green to orange emission, with quantum yields in anoxic toluene at 25 °C ranging from 13% to 86% and decay times spanning from 8.5 to 97 μs. The emission is attributed to simultaneous thermally activated delayed fluorescence (TADF) and phosphorescence processes on the basis of photophysical investigations and DFT calculations. Rather uniquely, simple manipulations in substituents of the PDP ligand and alteration of the terminal ligand allow fine-tuning of the ratio between TADF and phosphorescence from almost 100% TADF emission (Pt(MesPDPC6F5(BN)) to over 80% of phosphorescence (Pt(PhPDPPh(BuIm)). Apart from ratiometric capabilities, the complexes also are useful as decay time-based temperature indicators with temperature coefficients exceeding 1.5% K-1 in most cases. Immobilization of the dyes into oxygen-impermeable polyacrylonitrile produces temperature sensing materials that can be read out with an ordinary RGB camera or a smartphone. In addition, Pt(PhPDPPh)Py can be incorporated into biocompatible RL100 nanoparticles suitable for cellular nanothermometry, as we demonstrate with temperature measurements in multicellular colon cancer spheroids.

Tailoring Fluorescence-Phosphorescence Emission with a Single Nanocavity.

Realizing the dual emission of fluorescence-phosphorescence in a single system is an extremely important topic in the fields of biological imaging, sensing, and information encryption. However, the phosphorescence process is usually in an inherently "dark state" at room temperature due to the involvement of spin-forbidden transition and the rapid non-radiative decay rate of the triplet state. In this work, we achieved luminescent harvesting of the dark phosphorescence processes by coupling singlet-triplet molecular emitters with a rationally designed plasmonic cavity. The achieved Purcell enhancement effect of over 1000-fold allows for overcoming the triplet forbidden transitions, enabling radiation enhancement with selectable emission wavelengths. Spectral results and theoretical simulations indicate that the fluorescence-phosphorescence peak position can be intelligently tailored in a broad range of wavelengths, from visible to near-infrared. Our study sheds new light on plasmonic tailoring of molecular emission behavior, which is crucial for advancing research on plasmon-tailored fluorescence-phosphorescence spectroscopy in optoelectronics and biomedicine.

Modulating the Room Temperature Phosphorescence by Tweaking SOC and P = X Interactions (X = O, S, and Se) in Phosphoramides: Magnetic Circularly Polarized Luminescence from Achiral Phosphoramides

AbstractHerein, the synthesis, structure, room temperature phosphorescence, and magnetic chiroptical properties of phosphoramides 1‐6 with donor–acceptor architecture are reported.5). The electronic interactions between donor phenothiazine (PTZ) and acceptor (C6H5)n‐P = X (n = 1, 2; X = O, S, and Se)moieties in 1–6 are modulated by varying the number of donor units. The Lewis acidity of the phosphorus center is controlled by the nature of heteroatoms and the number of phenyl moieties attached to it. All compounds exhibit phosphorescence in the solid state under ambient conditions with a photoluminescence lifetime in the millisecond range (≈8–40 ms). The energy of the phosphorescence bands and their excited state lifetimes are governed by the P=X units. The phosphorescence features observed for 1–6 are in complete contrast to the observations noted for (PTZ)3P = X (X = O, S, and Se), where the phosphorescence process is completely controlled by the PTZ moiety, and there is a negligible contribution from the P = X moiety. Thus, the present and earlier studies together reveal that the steric crowding and the number of C6H5 moieties around the P = X unit play a crucial role in controlling the electronic coupling between the donor and acceptor moieties in phosphoramides containing non‐planar donor moieties. Furthermore, for the first time, the magnetic chiroptical properties of achiral phosphoramides 1–6, exhibiting a luminescence dissymmetry factor (gMCPL) in the order of 10−3 (≈4.0 × 10−3–7.4 × 10−3) are demonstrated.

Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States.

The design and regulation of multiple room-temperature phosphorescence (RTP) processes are formidably challenging due to the restrictions imposed by Kasha's rule. Here, we report a general design principle for materials that show multiple RTP processes, which is informed by our study of four compounds where there is modulation of the linker hybridization between donor (D) and acceptor (A) groups. Theoretical modeling and photophysical experiments demonstrate that multiple RTP processes can be achieved in sp3 C-linked D-A compounds due to the arrest of intramolecular electronic communication between two triplet states (T1H and T1L) localized on the donor and acceptor or between two triplet states, one localized on the donor and one delocalized across aggregated acceptors. However, for the sp2 C-linked D-A counterparts, RTP from one locally excited T1 state is observed because of enhanced excitonic coupling between the two triplet states of molecular subunits. Single-crystal and reduced density gradient analyses reveal the influence of molecular packing on the coincident phosphorescence processes and the origin of the observed aggregate phosphorescence. These findings provide insights into higher-lying triplet excited-state dynamics and into a fundamental design principle for designing compounds that show multiple RTP.

Intrinsic Characterization Method on the Heavy Atom Effect of Metalloporphyrins.

The presence of room temperature phosphorescence emission in metalloporphyrin, via the transition from the excited triplet state (T1) to the ground state (S0), relies on the chelated heavy metal ions, which is known as the heavy atom effect (HAE). Despite the HAE being a reliable method to tune the phosphorescence process widely, the realization of the HAE nature is a tough task as the induced phosphorescence process is sensitive to not only the specie of bonded heavy atoms but also chemical environments such as the oxygen quenching and solvent effect. In this study, we have aimed at a quantitative determination of the intrinsic phosphorescent transition rate (kP) in metalloporphyrin gadolinium-labeled hematoporphyrin monomethyl ether (Gd-HMME). After the theoretical analysis based on the rate equation model to remove the nonintrinsic contribution and the experimental results of phosphorescence, the kP is calculated to be ∼2.4 × 10-4 μs-1. This study enables us to approach the intrinsic energy characteristic of metalloporphyrins; moreover, our work provides an effective pathway for the further optimization of the varied functional metalloporphyrin.

Amplified Spontaneous Emission from Organic Phosphorescence Emitters.

Organic gain materials (OGMs) currently used in organic lasers and optical amplifiers are focused on singlet-fluorescence materials, while triplet-phosphorescence-based OGMs have hardly been developed yet. Herein, we report a novel pure organic phosphorescence gain molecule (SBP) for optical amplification by stimulated emission from triplet states. The introduction of the benzophenone carbonyl group and sulfur atoms increases the spin orbit coupling constant of SBP, which accelerates the intersystem crossing (ISC) and phosphorescence processes. Experimental and theoretical results verify that the formation of the H-type dimer aggregate decreases the fluorescence radiation rate while accelerating the ISC rate, also enhancing the phosphorescence emission of SBP. Doping SBP molecules into a PMMA matrix can stabilize triplet excitons, yielding the maximum phosphorescence quantum yield of 18.9%. We realized triplet phosphorescent amplified spontaneous emission (ASE) at 557 nm from 30.0 wt % SBP@PMMA samples. Our results provide a novel strategy to develop triplet phosphor OGMs.

Front Cover: Origin of the Aggregation‐Induced Phosphorescence of Platinum(II) Complexes: The Role of Metal–Metal Interactions on Emission Decay in the Crystalline State (Chem. Asian J. 20/2021)

The Front Cover, ”Shachihoko” (Protectors of Nagoya Castle in Japan), represents two potential energy surfaces of the excited triplet state of Pt(II) complexes in the crystalline state with and without metal-metal interaction, which are crucial for the expression of phosphorescent aggregation-induced emission (AIE). This work describes, for the first time, phosphorescent deactivation processes of four class of AIE-active Pt(II) complexes in the crystalline state based on experimental and theoretical investigations. Furthermore, the role of metal-metal interaction in AIE has been clarified by comparing two computational models. More information can be found in the Full Paper by Ryo Inoue, Takeshi Naota, and Masahiro Ehara.

Origin of the Aggregation-Induced Phosphorescence of Platinum(II) Complexes: The Role of Metal-Metal Interactions on Emission Decay in the Crystalline State.

Discerning the origins of the phosphorescent aggregation-induced emission (AIE) from Pt(II) complexes is crucial for developing the broader range of photo-functional materials. Over the past few decades, several mechanisms of phosphorescent AIE have been proposed, however, not have been directly elucidated. Herein, we describe phosphorescence and deactivation processes of four class of AIE active Pt(II) complexes in the crystalline state based on experimental and theoretical investigation. These complexes show metal-to-ligand and/or metal-metal-to-ligand charge transfer emission in crystalline state with different heat resistance against thermal emission quenching. The calculated energy profiles including the minimum energy crossing point between S0 and T1 states were consistent with the heat resistant properties, which provided the mechanism for AIE expression. Furthermore, we have clarified the role of metal-metal interaction in AIE by comparing two computational models.

Emission color management of dual emitting organic light-emitting diodes by selective switching of phosphorescence through host engineering

A device approach to manage the emission color of the organic light-emitting diodes (OLEDs) which show two emission processes of thermally activated delayed fluorescence (TADF) and phosphorescence was developed by engineering the host of the emitter. The emitting material was phenyl(10-phenyl-10H-phenoselenazin-3-yl)methanone (PSeBz) which emitted at 496 nm by TADF process and at 559 nm by phosphorescence process. The phosphorescence of the PSeBz emitter was selectively switched by the host because the triplet excitons can be intentionally quenched by the low triplet energy host. Therefore, the color coordinate of the OLEDs could be tuned from (0.35, 0.52) to (0.41, 0.52) by just changing the host of the PSeBz emitter.

DFT vs. TDDFT vs. TDA to simulate phosphorescence spectra of Pt- and Ir-based complexes.

This paper presents a thorough quantum investigation of the optical properties of twelve transition metal complexes using state of the art (TD)DFT computations. The studied molecules are two Pt-based and ten Ir-based complexes. Geometrical parameters, absorption and emission spectra are directly compared to available experimental data. Phosphorescence spectra have been computed within the Adiabatic Hessian (AH) method which takes into account mode mixing and a proper description of both ground and excited states potential energy surfaces (frequency calculations). For each compound, three methods have been considered to obtain the relaxed triplet excited state supposedly involved in the phosphorescence process, i.e. unrestricted DFT, TDDFT and its Tamm-Dancoff approximation (TDA). In overall, unrestricted DFT and TDA overperform TDDFT for the investigated complexes especially when an Ir centre is present. The AH model demonstrates its good capability to reproduce accurately phosphorescence spectra. Finally, simulation and experimental data are represented over a CIE chromaticity horseshoe.

Modulation of the Structure and Photoluminescence of Bismuth(III) Chloride Hybrids by Altering the Ionic-Liquid Cations.

Two bismuth(III) halides hybrids with room-temperature phosphorescence (RTP), namely, [BPy]2[Bi2Cl8(bpym)] (1, BPy = N-butylpyridinium) and [EPy]2[Bi2Cl8(bpym)] (2, EPy = N-ethylpyridinium), were synthesized and characterized. Structural comparison reveals that 1 and 2 possess similar anionic zigzaglike chain of [Bi2Cl8(bpym)]n2n-; however, different packing modes of anion/cations and thus different weak interactions. Interestingly, the utilization of pyridinium cations with different length of alkyl chain could tune the RTP behaviors efficiently. The RTP quantum yield (QY) is increased more than 5-fold from 1 to 2 probably due to more rigid structure of 2 arising from the additional H-bond and anion-π interactions, as confirmed by Hirshfeld surfaces analyses and PLATON calculations. Moreover, additional π-π interactions in 1 could stabilize the triplet excitons, leading to an average lifetime of 1 (11.36 ms at 77 K and 1.407 ms at 298 K) being higher than 2 (0.3618 ms at 77 K and 0.07511 ms at 298 K). Density functional theory (DFT) calculations confirm that inorganic moiety to organic ligand charge-transfer (IOCT) is involved in the phosphorescence process. The present work provides a new sight into the design of RTP metal halides through studying the structure-RTP relationship.

Singlet–Triplet Transition Rate Enhancement inside Hyperbolic Metamaterials

AbstractThe spontaneous emission process is known to be largely affected by the surrounding electromagnetic environment of emitters, which manifests itself via the Purcell enhancement of decay rates. This phenomenon has been extensively investigated in the case of dipolar transitions in quantum systems, commonly delivering fast decay rates in comparison to forbidden transitions such as high‐order multipolar transitions or spin‐forbidden, singlet–triplet phosphorescence processes. Here, a decay rate enhancement of almost 2750‐fold is demonstrated for a ruthenium‐based phosphorescent emitter located inside a plasmonic hyperbolic metamaterial. The standard electromagnetic local density of states description, typically employed for the Purcell factor analysis of dipolar transitions, is unable to account for a photoluminescence enhancement of this magnitude, which is attributed to the interplay between the local density of states and strongly inhomogeneous electromagnetic fields inside the metamaterial. The large available range of spontaneous emission lifetimes reported here enables application of phosphorescent emitters in novel, fast, and efficient light‐emitting sources, beneficial for optical communications, quantum information processing, spectroscopy, or bio‐imaging.

Photosensitized Retinal Isomerization in Rhodopsin Mediated by a Triplet State

AbstractDirect 11‐cis to all‐trans retinal photoisomerization within rhodopsin is well known to be the initial chemical reaction triggering the process of vision in mammalians, such as bovine. Nevertheless, deep‐sea fish are known to use chlorophyll derivatives as photosensitizers in order to see deep‐red light, at wavelengths where retinal does not absorb. Also, some photodynamic therapy treatments were shown to enhance the vision of patients in dim light conditions. Energy transfer from the photosensitizer to rhodopsin was therefore proposed as a mechanism to populate the triplet state of the retinal chromophore. Herein, by means of hybrid quantum mechanics‐coupled‐molecular mechanics modeling techniques, we give insights into the possible energy mechanism and describe the retinal isomerization mediated by the lowest‐lying triplet state. Especially, we show how a few kcal/mol energy barrier separates a T1 minimum from a S0/T1 intersection region, hence proposing an equilibrium between phosphorescence and isomerization processes. Moreover, the eventual self‐production of singlet oxygen, constituting a potential danger for the integrity of rhodopsin, is discussed.

Silica nanoparticles doped with a benzo[e]indolium-tethered iridium(III) complex for reversible detection of HSO3− and Hg2+/Cu2+ in water

A reusable colorimetric and phosphorescent probe for continuous detection of bisulfite and Hg2+/Cu2+ in water has been rationally designed by silica nanoparticles doped with a new iridium(III) complex. In the presence of HSO3−, it demonstrated intensively enhanced phosphorescence as well as instantaneous solution color change from dark-green to light-yellow. However, it is interesting that the enhanced phosphorescence caused by bisulfite can be subsequently quenched by Hg2+ or Cu2+, and then be re-enhanced by the further addition of bisulfite. Moreover, a good linearity was observed between phosphorescent intensities of the probe and concentrations of HSO3−, Hg2+ and Cu2+ with satisfactory detection limits of 0.80, 0.77 and 0.85 μM, respectively. It should be noted that this off-on-off phosphorescent process can be circled for several times by adding bisulfite and Hg2+ or Cu2+ alternately. As a result, the newly designed probe can be repeatedly used for detection of HSO3−, Hg2+ and Cu2+ at low detection limits, which would be more efficient and less expensive than a one-to-one analysis method.

Dinuclear Design of a Pt(II) Complex Affording Highly Efficient Red Emission: Photophysical Properties and Application in Solution-Processible OLEDs.

The light-emitting efficiency of luminescent materials is invariably compromised on moving to the red and near-infrared regions of the spectrum due to the transfer of electronic excited-state energy into vibrations. We describe how this undesirable "energy gap law" can be sidestepped for phosphorescent organometallic emitters through the design of a molecular emitter that incorporates two platinum(II) centers. The dinuclear cyclometallated complex of a substituted 4,6-bis(2-thienyl)pyrimidine emits very brightly in the red region of the spectrum (λmax = 610 nm, Φ = 0.85 in deoxygenated CH2Cl2 at 300 K). The lowest-energy absorption band is extraordinarily intense for a cyclometallated metal complex: at λ = 500 nm, ε = 53 800 M-1 cm-1. The very high efficiency of emission achieved can be traced to an unusually high rate constant for the T1 → S0 phosphorescence process, allowing it to compete effectively with nonradiative vibrational decay. The high radiative rate constant correlates with an unusually large zero-field splitting of the triplet state, which is estimated to be 40 cm-1 by means of variable-temperature time-resolved spectroscopy over the range 1.7 < T < 120 K. The compound has been successfully tested as a red phosphor in an organic light-emitting diode prepared by solution processing. The results highlight a potentially attractive way to develop highly efficient red and NIR-emitting devices through the use of multinuclear complexes.

QM/MM studies on luminescence mechanism of dinuclear copper iodide complexes with thermally activated delayed fluorescence.

The QM/MM method is employed to investigate the photophysical mechanism of two dinuclear copper iodide complexes with thermally activated delayed fluorescence (TADF). The S1–T1 energy differences (ΔEST) in these two complexes are small enough so that repopulating the S1 state from T1 becomes energetically allowed. Both forward and reverse intersystem crossing (ISC and rISC) processes are much faster than the corresponding radiative fluorescence and phosphorescence processes [kISC (108 s−1) > kFr (106 s−1), krISC (105 s−1) > kPr (103 s−1)]. The faster rISC process than the phosphorescence emission enables TADF. Moreover, the diphosphine ligands are found to play an important role in regulating the electronic structures and thereto the radiative and nonradiative rate constants. The present work rationalizes experimental phenomena and helps understand the intrinsic luminescence properties. The obtained insights could be useful for tuning the luminescence performance of dicopper-based luminescence materials.

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.

Electronic structure, photoluminescence and phosphorescence properties in Sr2ScGaO5:Sm3+

Novel reddish long lasting phosphorescence material Sr2ScGaO5:Sm3+ was designed and prepared by a solid state reaction. The results of morphology showed that the irregular microparticles were formed by the agglomeration of uniform submicron particles with the size of about 0.5–1 μm. The electronic structure with an indirect bandgap was evaluated in detail by the density functional theory calculations and the diffuse reflectance spectrum for the first time. By the two different methods, the bandgap energy of Sr2ScGaO5 was determined to be about 4.24 and 4.75 eV, respectively. The emission of Sm3+ ion at 604 nm is strongest in the photoluminescence process, while in the phosphorescence process the emission at 619 nm is dominant. According to the analysis on the optical properties, the interesting phenomenon about the difference between the photoluminescence and phosphorescence was revealed and explained by the fact that the conduction band played an important role in the phosphorescence process. Additionally, the result showed that the material can be stimulated effectively in very short time and would have great potential in the applications such as AC-light emitting diodes.