Nonradiative Decay Rate Research Articles

Theoretical insights on the luminescent mechanism of an efficient aggregation‐induced nondoped delayed fluorescence emitter using QM/MM method

AbstractThermally activated delayed fluorescence (TADF) materials with an aggregation‐induced emission (AIE) mechanism have great potential for use in organic light‐emitting diodes (OLEDs). In this study, the luminescent mechanism of a novel efficient aggregation‐induced, nondoped, delayed fluorescence emitter 2‐(10H‐phenoxazin‐10‐yl)‐10‐phenyl‐10H‐phenothiazine‐5,5‐dioxide (PXZ2PTO) is theoretically studied based on first‐principles calculations. The polarizable continuum model and quantum mechanics and molecular mechanics method are used to simulate the surrounding environments for molecules in the toluene solution and the solid phase, respectively. Our results show that PXZ2PTO exhibits the superior AIE property. The nonradiative decay rate in the solid phase is two orders of magnitude lower than that in toluene, caused by the suppressed geometry changes during excitations and decay channels. Meanwhile, studies of intermolecular interactions using the dimer model indicate that PXZ2PTO possesses a special highly stereoscopic structure in the solid phase, and the electron acceptor group is almost perpendicular to the electron donor group, inducing a strong steric hindrance effect between adjacent molecules and leading to enhanced intramolecular interaction. Furthermore, the excited‐state dynamics studies show that the intersystem and reverse intersystem crossing rates of PXZ2PTO are comparable to the radiative decay rate, which indicates that PXZ2PTO is an excellent potential TADF emitter with full exciton usage. Our theoretical research gives a reasonable explanation of the experimental results and provides valuable information for designing highly efficient AIE‐TADF emitters in practical OLEDs.

Orange-red thermally activated delay fluorescence emitters based on asymmetric difluoroboron chelated enaminone: Impact of donor position on luminescent properties

Asymmetric enaminone difluoroboron complexes (BFs) exhibit highly solid emission and appropriated electron-accepting properties, whereas the position of the substituents has a significant influence on their optical properties due to the structural asymmetry. In this work, we employed BF as acceptor and 9,9-dimethylacridine (DMAC) as donor to construct two thermally activated delay fluorescence (TADF) molecules DMAC-BF and BF-DMAC, in which DMAC unit was set on the B-O and B-N sides of BF, respectively. DMAC-BF and BF-DMAC exhibited the broad orange-red emission bands in toluene, while they emitted yellow and red fluorescence in solid state, respectively. The photoluminescence quantum yield (PLQY) of DMAC-BF (52%) is higher than that of BF-DMAC (29%), owing to its lower nonradiative decay rate. Solution-processed organic light-emitting diode (OLED) devices based on both of DMAC-BF and BF-DMAC exhibited orange-red electroluminicence spectra. Owing to its higher photoluminescence quantum yield, DMAC-BF-based OLED exhibited better electroluminicence performance with a maximum current efficiency of 24.9 cd/A, a maximum power efficiency of 23.0 lm/W, and a maximum EQE of 11.3%.

Isotopic and temperature effects on the photoemission and predissociation rates of N2+(C2Σu+)

Radiative and nonradiative decay rates of the $C{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}^{\phantom{\rule{0.16em}{0ex}}+}$ electronic state of ${\mathrm{N}}_{2}^{\phantom{\rule{0.16em}{0ex}}+}$ are calculated as a function of the temperature. This makes it possible to study the effect of the temperature on the photoemission spectrum and on the ion yields $I$(${\mathrm{N}}^{+}$) and $I$(${\mathrm{N}}_{2}^{\phantom{\rule{0.16em}{0ex}}+}$). It is shown that the temperature has different effects on the isotopes $^{28}\mathrm{N}_{2}^{\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.28em}{0ex}}+}$, $^{29}\mathrm{N}_{2}^{\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.28em}{0ex}}+}$, and $^{30}\mathrm{N}_{2}^{\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.28em}{0ex}}+}$. Our results confirm that, regardless of the temperature, predissociation is the dominant relaxation mechanism for vibrational states ${\ensuremath{\nu}}^{\ensuremath{'}}\ensuremath{\geqslant}3$ of the $^{28}\mathrm{N}_{2}^{\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.28em}{0ex}}+}$ and $^{29}\mathrm{N}_{2}^{\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.28em}{0ex}}+}$ molecules.

Two-Photon Photoexcited Photodynamic Therapy with Water-Soluble Fullerenol Serving as the Highly Effective Two-Photon Photosensitizer Against Multidrug-Resistant Bacteria.

BackgroundMultidrug-resistant (MDR) bacterial strain is a serious medical problem. Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to many antibiotics and is often associated with several diseases such as arthritis, osteomyelitis, and endocarditis. The development of an alternative treatment for eliminating MDR bacteria such as MRSA has attracted a considerable amount of research attention. Moreover, the development of a material for highly efficient generation of reactive oxygen species (ROS) involving two-photon photodynamic therapy (PDT) is currently desirable.Materials and MethodsWe present an example demonstrating that the use of water-soluble C60(OH)30 fullerenol with a 0.89 singlet oxygen quantum yield serving as a photosensitizer in PDT has the superior ability in effectively generating ROS.ResultsIt has ultra-low energy (228.80 nJ pixel−1) and can perform 900 scans under two-photon excitation (TPE) in the near-infrared region (760 nm) to completely eliminate the MDR species. Furthermore, the favorable two-photon properties are absorption of approximately 760 nm in wavelength, absolute cross-section of approximately 1187.50 Göeppert–Mayer units, lifetime of 6.640 ns, ratio of radiative to nonradiative decay rates of approximately 0.053, and two-photon stability under TPE.ConclusionThis enabled water-soluble C60(OH)30 fullerenol to act as a promising two-photon photosensitizer proceeding with PDT to easily eliminate MDR species.

Influence of the ancillary ligands on the luminescence of platinum(II) complexes with a triazole-based tridentate C^N^N luminophore

The effect of different ancillary ligands and counterions in platinum(II) complexes has been investigated. Based on the previously reported tridentate C^N^N ligand precursor 2-(1-benzyl-1H-1,2,3-triazol-4-yl)-6-phenylpyridine (HL), the photophysical properties of complexes of the type [Pt(L)(X)]n+ have been varied by changing the fourth (monodentate) ligand (X) of the square-planar platinum(II) complexes. Different lifetimes and quantum yields were observed, depending on the identity of this ancillary ligand. The most favorable photophysical properties within this series of complexes were obtained for neutral complexes with the phenylacetylido ligand with a quantum yield of 35% and a lifetime of 2.22 µs, while for cationic complexes bearing nitrile, isonitrile and triphenylphosphane units gave comparable results with quantum yields ranging from 11% to 16% and lifetimes from 3.59 µs to 4.93 µs. Introducing a ferrocene moiety attached to an acetylido ligand, the complex became hardly emissive. The investigated counterions perchlorate, tetrafluoroborate and hexafluorophosphate of positively charged complexes regarding their photophysical properties were found to affect the non-radiative decay rates. To understand the minor effect observed for the emission maxima of the complexes, density functional theory (DFT) was applied. The experimental emission spectra of the complexes were reproduced by using simplified model systems. The distribution of the frontier orbitals used for the description of the emissive T1 state in its optimized geometry mainly involves the tridentate luminophore rather than the ancillary ligand. This explains why the emission is dominated by the pincer unit with perturbative participation of the metal center while excluding significant influence of the ancillary ligand.

STRATIFY: a comprehensive and versatile MATLAB code for a multilayered sphere

We present a computer code for calculating near- and far-field electromagnetic properties of multilayered spheres. STRATIFY is a one-of-a-kind open-source package that allows for efficient calculation of electromagnetic near-field, energy density, total electromagnetic energy, and radiative and non-radiative decay rates of a dipole emitter located in any (non-absorbing) shell (including a host medium), and fundamental cross-sections of a multilayered sphere, all within a single program. Because of its speed and broad applicability, our package is a valuable tool for analysis of numerous light scattering problems, including but not limited to fluorescence enhancement, upconversion, downconversion, second harmonic generation, and surface enhanced Raman spectroscopy. The software is available for download from GitLab as Code 1.

Density Functional Studies on Photophysical Properties of Boron-Pyridyl-Imino-Isoindoline Dyes: Effect of the Fusion.

In this work, to make out the aryl-fusion effect on the photophysical properties of boron-pyridyl-imino-isoindoline dyes, compounds 1–5 were theoretically studied through analyses of their geometric and electronic structures, optical properties, transport abilities, and radiative (kr) and non-radiative decay rate (knr) constants. The highest occupied molecular orbitals of aryl-fused compounds 2–5 are higher owing to the extended conjugation. Interestingly, aryl fusion in pyridyl increases the lowest unoccupied molecular orbital (LUMO) level, while isoindoline decreases the LUMO level; thus, 4 and 5 with aryl fusion both in pyridyl and isoindoline exhibit a similar LUMO to 1. Compounds 4 and 5 show relatively low ionization potentials and high electron affinities, suggesting a better ability to inject holes and electrons. Importantly, the aryl fusion is conducive to the decrease of kIC. The designed compound 5 exhibits a red-shifted emission maximum, low λh, and low kIC, which endow it with great potential for applications in organic electronics. Our investigation provides an in-depth understanding of the aryl-fusion effect on boron-pyridyl-imino-isoindoline dyes at molecular levels and demonstrates that it is achievable.

Determining the Correlation between Excited State Dynamics and Donor and Acceptor Structure in Nonfullerene Acceptors

The design of nonfullerene acceptors (NFAs), which comprise donor and acceptor moieties, has led to significant improvement in organic photovoltaic (OPV) device performance over the past few years. Although there have been many NFAs synthesized and device-optimized, there have been fewer studies on the excited-state photophysics of donor-acceptor-containing NFAs. Using a combination of femtosecond transient absorption spectroscopy and time-resolved photoluminescence (PL), we have investigated the excited-state photophysical processes in three donor-acceptor-containing NFAs, where the chemical structure of the NFAs are systematically modified to allow direct comparison (D-A1-A1-D, D-A1-D, and D-A1-A2, where D represents 9,9-di-n-propylfluorene, A1 represents benzothiadiazole, and A2 represents dicyanovinyl). A large triplet population was observed for the dimer NFA (D-A1-A1-D), whereas for D-A1-D or D-A1-A2, the nonradiative decay rate was significantly decreased, and higher PL quantum yields observed. Furthermore, a singlet exciton relaxation process between S1∗ and S1 on the picosecond timescale was observed for each of the NFAs, although the mechanism was not always the same. Torsional relaxation was found to dominate the picosecond singlet relaxation for D-A1-D and D-A1-A2 in solution and film, and for D-A1-A1-D in solution. In the film, the singlet excited state of D-A1-A1-D predominantly relaxed via resonant excitonic energy transfer. The observation of singlet relaxation on the tens of picosecond timescale also provides a plausible explanation to the reduced VOC loss commonly associated with NFA-based OPV devices.

Laser cooling of ytterbium-doped silica glass

Laser cooling of a solid is achieved when a coherent laser illuminates the material in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. Solid-state laser cooling has been successfully demonstrated in several materials, including rare-earth-doped crystals and glasses. Here we show the net cooling of high-purity Yb-doped silica glass samples that are fabricated with low impurities to reduce their parasitic background loss for fiber laser applications. The non-radiative decay rate of the excited state in Yb ions is very small in these glasses due to the low level of impurities, resulting in near-unity quantum efficiency. We report the measurement of the cooling efficiency as a function of the laser wavelength, from which the quantum efficiency of the Yb-doped silica is calculated.

Efficient structural modification of electron‐withdrawing substituents on Pt(II) complexes for red emitters: A theoretical study

In this study, the electronic structures and optical properties of a cyclometalated Pt(II) complex (M1) and a series of derivatives (M1–F, M1–CF3, and M1–CN) with electron‐withdrawing substituents (–F, –CF3, and –CN) at the carbazole moiety were theoretically investigated by density functional theory and time‐dependent density functional theory. The calculation results reveal that these Pt complexes display deep red phosphorescence emission above Λ = 640 nm. When the 3MLCT/π → π* to triplet metal‐centered 3MC/d–d state decay mechanism is taken into consideration, the nonradiative decay rate constant (knr) decreased in the order M1 > M1–CF3 > M1–F > M1–CN. The <T1|HSOC|Sm> and kr values of M1‐F are similar with those of M1, however the Knr rate ofM1‐F is larger than that of M1. M1–F is expected to have improved quantum yields. Moreover, through the analyses of the HOMO/LUMO level and triplet energy, it is found that the introduction of –F and –CN substituents in M1 results in efficient energy transfer from the host material 4,4′‐N,N′‐dicarbazole‐biphenyl to these complexes. In view of the electroluminescent applications in organic light‐emitting diodes, M1–F can serve as efficient deep‐red guest materials with improved electron injection and transport ability.

Solvatochromic and solid-state emissive azlactone-based AIEE-active organic dye: Synthesis, photophysical properties and color-conversion LED application

Here, we report one-pot synthesis of a new azlactone-based dye bearing isoxazole moiety as a donor group (i.e., Dye-Iso), which displays fluorescence emission both in solution and solid-state, and a systematic study of its photophysical properties. The analyses of steady-state photoluminescence (PL) spectroscopy measurements reveal that Dye-Iso exhibits positive solvatochromic behavior accompanied by the decreasing trend in PL quantum yield (QY), as the polarity of solvent increases. In addition, time-resolved fluorescence (TRF) spectroscopy shows that the PL lifetime is shortening and overall effective nonradiative decay rate becomes accelerated with increasing polarity of the medium, which explain well the decrease in PL QY. These findings strongly suggest that the dipole-dipole interactions between the dye molecule having intramolecular charge transfer (ICT) characteristic and solvent are responsible for the redshift in PL emission with the increase in solvent polarity. Furthermore, the electronically ground- and excited-state dipole moments were determined by using linear correlation methods having different solvent polarity functions. The results depict that the Dye-Iso has a larger excited-state dipole moment as against that in the ground-state, which reflects the ICT mechanism. The spectroscopic studies show that the Dye-Iso exhibits aggregation-induced emission enhancement (AIEE) under aggregate formation and is AIEE-active compound. The PL QYs of the Dye-Iso in aggregation- and solid-state were also found to be higher in comparison with those in solutions, which has been elucidated by the increase in the radiative decay rate due to restriction of intramolecular motions (RIM). Finally, we have demonstrated that the AIEE-active Dye-Iso can be used as a color-conversion material for solid-state lighting applications by employing that on a commercial near-ultraviolet LED.

Ancillary ligand increases the efficiency of heteroleptic Ir-based triplet emitters in OLED devices

The excellent contrast ratio, visibility, and advantages in producing thin and light displays let organic light emitting diodes change the paradigm of the display industry. To improve future display technologies, higher electroluminescence efficiency is needed. Herein, the detailed study of the non-radiative decay mechanism employing density functional theory calculations is carried out and a simple, general strategy for the design of the ancillary ligand is formulated. It is shown that steric bulk properly directed towards the phenylisoquinoline ligands can significantly reduce the non-radiative decay rate.

Nonradiative multi-phonon transition: state-of-the-artcalculations of the classic theory

The classic theory of multi-phonon transition was first developed by K. Huang in 1940s, which has been widely used since then to study many fundamental properties of semiconductors. Recently, Huang ’s classic theory was revisited and further developed with ab initio calculations. The previously ignored anharmonic effects have been considered and the calculated nonradiative decay rates for defects in GaN and SiC agree well with experiments.

Tetradentate Pt(II) Phosphors: A Computational Analysis of Nonradiative Decay Rates and Luminescence Efficiency

Cyclometalated tetradentate Pt(II) complexes are promising class of phosphors used in organic light-emitting diodes (OLEDs). We calculated decay rates and resulting photoluminescence quantum yield ...

Investigating the Redox Properties of Two-Dimensional MoS2 Using Photoluminescence Spectroelectrochemistry and Scanning Electrochemical Cell Microscopy.

Control over photophysical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) is the key to advance their applications in next-generation optoelectronics. Although chemical doping and surface modification with plasmonic metals have been reported to tune the photophysical and catalytic properties of 2D TMDs, there have been few reports of tuning optical properties using dynamic electrochemical control of electrode potential. Herein, we report (1) the photoluminescence (PL) enhancement and red-shift in the PL spectrum of 2D MoS2, synthesized by chemical vapor deposition and subsequent transfer onto an indium tin oxide electrode, upon electrochemical anodization and (2) spatial heterogeneities in its photoelectrochemical (PEC) activities. Spectroelectrochemistry shows that positive electrochemical bias causes an initial ten-fold increase in the PL intensity followed by a quick decrease in the enhancement. The PL enhancement and spectrum red-shift are associated with the decrease in nonradiative decay rates of excitons formed upon electrochemical anodization of 2D MoS2. Additionally, scanning electrochemical cell microscopy (SECCM) study shows that the 2D MoS2 crystal is spatially sensitive to PEC oxidation at positive potentials. SECCM also shows a photocurrent increase caused by spatially heterogeneous edge-type defect sites of the crystal.

Plasmonic Hot‐Electron‐Induced Control of Emission Intensity and Dynamics of Visible and Infrared Semiconductor Quantum Dots

AbstractPlasmonic hot‐electron‐assisted control of emission intensities and dynamics of CdSe/ZnS and infrared PbS quantum dots are studied. This is done by exploring the impact of Au/Si and Ag/Si Schottky junctions on the decay rates of such quantum dots when these junctions are placed in close vicinity of a Si/Al oxide charge barrier, forming metal‐oxide plasmonic metafilms. Such structures are used to investigate how metal‐dependent distributions of hot electrons and their capture via Schottky junctions can lead to suppression of the defect environments of quantum dots, offering a novel platform wherein off‐resonant (non‐Purcell) plasmonic processes are used to control exciton dynamics. These results show that Ag metafilms can enhance the emission of CdSe/ZnS quantum dots and elongate their lifetimes more than Au metafilms. This highlights the more efficient nature of Ag/Si Schottky junctions for hot electron excitation and capture. These results also show that such junctions can significantly suppress the nonradiative decay rates of PbS quantum dots at frequencies far from the plasmon resonances. These results demonstrate a field‐effect passivation of quantum dot defects via entrapment of hot electrons and control of emission intensities and dynamics of quantum dots via the nearly frequency‐independent electrostatic field of such electrons.

Tb3+/Eu3+ Complex-Doped Rigid Nanoparticles in Transparent Nanofibrous Membranes Exhibit High Quantum Yield Fluorescence.

In this study, transparent membranes containing luminescent Tb3+ and Eu3+ complex-doped silica nanoparticles were prepared via electrospinning. We prepared the electrospun fibrous membranes containing Tb(acac)3phen- (acac = acetylacetone, phen = 1,10-phenanthroline) and/or Eu(tta)3phen- (tta = 2-thenoyltrifluoroacetone) doped silica (M-Si-Tb3+ and M-Si-Eu3+) and studied their photoluminescence properties. The fibrous membranes containing the rare earth complexes were prepared by electrospinning. The surface morphology and thermal properties of the fibrous membrane were studied by atomic force microscopy (AFM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. Fluorescence spectroscopy was used to characterize the fluorescence properties of the membranes. During the electrospinning process, the PVDF transitions from the α phase to the β phase, which exhibits a more rigid structure. The introduction of rigid materials, like PVDF and silica, can improve the fluorescence properties of the hybrid materials by reducing the rate of nonradiative decay. So the emission spectra at 548 nm (Tb) and 612 nm (Eu) were enhanced, as compared to the emission from the pure complex. Furthermore, the fluorescence lifetimes ranged from 0.6 to 1.5 ms and the quantum yields ranged from 32% to 61%. The luminescent fibrous membranes have potential applications in the fields of display panels, innovative electronic and optoelectronic devices.

Planar AIEgens with Enhanced Solid‐State Luminescence and ROS Generation for Multidrug‐Resistant Bacteria Treatment

AbstractPlanar luminogens have encountered difficulties in overcoming intrinsic aggregation‐caused emission quenching by intermolecular π‐π stacking interactions. Although excited‐state double‐bond reorganization (ESDBR) can guide us on designing planar aggregation‐induced emission (AIE) luminogens (AIEgens), its mechanism has yet been elucidated. Major challenges in the field include methods to efficiently restrict ESDBR and enhance AIE performance without using bulky substituents (e.g., tetraphenylethylene and triphenylamine). In this study, we rationally developed fluoro‐substituent AIEgens with stronger intermolecular H‐bonding interaction for restricted molecular motions and increased crystal density, leading to decreased nonradiative decay rate by one order of magnitude. The adjusted ESDBR properties also show a corresponding response to variation in viscosity. Furthermore, their aggregation‐induced reactive oxygen species (ROS) generations have been discovered. The application of such planar AIEgen in treating multidrug‐resistant bacteria has been demonstrated in a mouse model. The relationship between ROS generation and distinct E/Z‐configurational stacking behaviors have been further understood, providing a design principle for synthesizing planar AIEgen‐based photosensitizers.

Planar AIEgens with Enhanced Solid-State Luminescence and ROS Generation for Multidrug-Resistant Bacteria Treatment.

Planar luminogens have encountered difficulties in overcoming intrinsic aggregation-caused emission quenching by intermolecular π-π stacking interactions. Although excited-state double-bond reorganization (ESDBR) can guide us on designing planar aggregation-induced emission (AIE) luminogens (AIEgens), its mechanism has yet been elucidated. Major challenges in the field include methods to efficiently restrict ESDBR and enhance AIE performance without using bulky substituents (e.g., tetraphenylethylene and triphenylamine). In this study, we rationally developed fluoro-substituent AIEgens with stronger intermolecular H-bonding interaction for restricted molecular motions and increased crystal density, leading to decreased nonradiative decay rate by one order of magnitude. The adjusted ESDBR properties also show a corresponding response to variation in viscosity. Furthermore, their aggregation-induced reactive oxygen species (ROS) generations have been discovered. The application of such planar AIEgen in treating multidrug-resistant bacteria has been demonstrated in a mouse model. The relationship between ROS generation and distinct E/Z-configurational stacking behaviors have been further understood, providing a design principle for synthesizing planar AIEgen-based photosensitizers.

Enhancement of the Luminescent Efficiency in Carbene-Au(I)-Aryl Complexes by the Restriction of Renner-Teller Distortion and Bond Rotation.

A series of (carbene)Au(I)(aryl) complexes are reported. The nature of the lowest excited state in these complexes changes character from metal-to-ligand charge transfer (MLCT) to interligand charge transfer (ICT) with increasing electron-donating strength of the aryl ligand. Complexes that have the MLCT lowest excited state undergo a Renner-Teller bending distortion upon excitation. Such a distortion leads to a large rate of nonradiative decay, on the order of 108 s-1. Renner-Teller-based nonradiative decay does not occur in chromophores with an ICT emissive state. Introducing a julolidine moiety and ortho-methyl substituents to the aryl group makes the molecule rigid and hinders the rotation along the Au-Caryl-coordinate bond. Consequently, the nonradiative decay rates of these ICT emitters are decreased and become lower than the radiative decay rate constants (kr = 105 s-1). Thus, high-luminescent efficiencies (ΦPL = 0.61 and 0.77) along with short lifetimes (τ < 2 μs) are obtained for yellow and green emitters, respectively. Thermally assisted delayed fluorescence behavior is observed, owing to the small exchange energy (ΔEST < 1600 cm-1) in these emitters.