Nonradiative Decay Processes Research Articles

Correction to "Comprehensive Investigation into Luminescent Properties of Ir(III) Complexes: An Integrated Computational Study of Radiative and Nonradiative Decay Processes".

A comprehensive and concrete exploration into the deactivation mechanisms of luminescent materials is imperative, with the improvement of simulating and computing technology. In this study, an inte...

Suppressed Triplet Exciton Diffusion Due to Small Orbital Overlap as a Key Design Factor for Ultralong-Lived Room-Temperature Phosphorescence in Molecular Crystals.

Persistent room-temperature phosphorescence (RTP) under ambient conditions is attracting attention due to its strong potential for applications in bioimaging, sensing, or optical recording. Molecular packing leading to a rigid crystalline structure that minimizes nonradiative pathways from triplet state is often investigated for efficient RTP. However, for complex conjugated systems a key strategy to suppress the nonradiative deactivation is not found yet. Here, the origin of small rates of a nonradiative decay process from triplet states of conjugated molecular crystals showing RTP is reported. Optical microscopy analysis showed that, despite a favorable molecular stacking, an aromatic crystal with strong RTP is characterized by small diffusion length and small values of the diffusion coefficient of triplet excitons. Quantum chemical calculations reveal a large overlap between the lowest unoccupied molecular orbitals but very small overlap between the highest occupied molecular orbitals (HOMOs). Inefficient electron exchange caused by the small overlap of HOMOs prevents triplet excitons from diffusing over long distances and consequently from quenching at defect sites inside the crystal or at the crystal surface. These results will allow design of comprehensive molecular structures to obtain molecular solids with more efficient RTP.

Small substituent groups as geometric controllers for tridentate platinum(ii) complexes to effectively suppress non-radiative decay processes.

For phosphorescent emitters, the rigidity of the geometry is a crucial indicator, which can directly determine the non-radiative decay rate. In this article, density functional theory (DFT) calculations were performed to investigate the influence of the small substituent groups on the rigidities of tridentate Pt(ii) complexes in detail. The calculated results indicate that the small substituent groups can serve as geometric controllers to suppress the structural distortion on going from the ground state (S0) to the lowest-lying triplet excited state (T1) (Jahn-Teller distortion). For instance, when electron-donating substituent groups, including -NH2, -N(CH3)2 and -OCH3, were employed, the rigidities of the corresponding Pt(ii) complexes can be effectively enhanced because the highest occupied molecular orbital (HOMO)-HOMO-1 energy gaps could be increased. Different from the electron-donating substituent groups, electron-withdrawing substituent groups, i.e., -NO2 and -COCH3, can cause a negligible change in HOMO and HOMO-1 energies during the S0 → T1 transition process, and therefore, for Pt-NO2 and Pt-COCH3, no Jahn-Teller distortion occurs. According to the calculated results, the rigidities of tridentate Pt(ii) complexes could be raised via tuning the energies of the frontier molecular orbital (FMO) with the help of small substituent groups.

Electrocatalytic glycerol oxidation enabled by surface plasmon polariton-induced hot carriers in Kretschmann configuration.

Plasmonic hot carrier generation has attracted increasing attention due to its ability to convert light to electrical energy. The generation of plasmon-induced hot carriers can be achieved via Landau damping in the non-radiative decay process of the plasmonic excitation energy. Localized surface plasmons (LSPs) undergo both radiative and non-radiative decays, while surface plasmon polaritons (SPPs) dissipate only via the non-radiative decay. Thus, it is a challenging task to exploit the surface plasmon polaritons for the efficient generation of hot carriers and their applications. In this study, a model hot-carrier-mediated electrocatalytic conversion system was demonstrated using an Au thin film in Kretschmann configuration, which is the representative platform to excite SPPs. AgPt-decorated Au nanobipyramids (AuNBPs) were designed and introduced onto the Au film, creating hot-spots to revolutionize the thin film-based photon-to-carrier conversion efficiency. The glycerol electro-oxidation reaction enabled by such SPP-induced hot carriers was evaluated and exhibited a photon-to-hot carrier conversion efficiency of 2.4 × 10-3%, which is ∼2.5 times enhanced as compared to the efficiency based on the neat Au film.

Two coumarin-based turn-on fluorescent probes based on for hypochlorous acid detection and imaging in living cells.

This work, two turn-on fluorescent probes (3-acetyl-2H-chromen-2-one (ACO) & (1E)-1-(1-(2-oxo-2H-chromen-3-yl)ethylidene)thiosemicarbazide (CETC)) based on coumarin have been designed and synthesized, which could selectively and sensitively recognize ClO- with fast response time. ACO &CETC were almost non fluorescent possibly due to both the lacton form of coumarin and unbridged CN bonds which can undergo a nonradiative decay process in the excited state. Upon the addition of ClO-, ACO &CETC were oxidized to ring - opened by cleavage the CO and CN and the fluorescence intensity were increased considerably. Fluorescence titration experiments showed that the detection limit ACO &CETC is as low as 22 nm and 51 nm respectively. In particular, some relevant reactive species, including OH, 1O2, H2O2, KO2, some anions and cations cannot be interference with the test. In live cell experiments, ACO &CETC were successfully applied to image exogenous ClO- in HepG2 cells. Therefore, ACO &CETC not only could image ClO- in living cells but also proved that CO and CN can be cleavage by ClO-.

NO2-BDC as sensitizer and photoluminescence properties of [(La)(NO2-BDC)1.5(H2O)4] and [(LaxLny)(NO2-BDC)1.5(H2O)4] (Ln = Eu, Tb)

Sensitization function of 2-nitro-1,4-benzenedicarboxylate (NO2-BDC2−) in the isostructural [(La)(NO2-BDC)1.5(H2O)4] (1), [(La0.92Eu0.08)(NO2-BDC)1.5(H2O)4] (2) and [(La0.92Tb0.08)(NO2-BDC)1.5(H2O)4] (3) have been investigated. Based on the characteristic emissions of 5D0-7Fj (EuIII; J = 1, 2, 3, 4) and 5D4-7Fj (TbIII; J = 6, 5, 4, 3), the effective sensitization of EuIII (2) and TbIII (3) is demonstrated. The triplet state energy of NO2-BDC2- has been estimated based on photoluminescence of 1. Temperature dependent photoluminescence of 2 (EuIII; 5D0-7F2) and 3 (TbIII; 5D4-7F5) has also been studied in term of intensity, revealing the significant effect of thermal activated non-radiative decay process.

A yellow carbon dots-based phosphor with high efficiency for white light-emitting devices

The yellow-emitting carbon dots (CDs) were successfully synthesized by a simple one-step hydrothermal route using o-phenylenediamine (o-PD) as the starting material. The emssion peak of CDs shifted from 535 to 563 nm and the corresponding photoluminescence quantum yield (PLQY) enhanced from 10.4% to 23.1% when the reaction solvent of ethanol was replaced by deionized water and reaction temperature was increased by 20 °C. After CDs were effectively and uniformly attached to starch particles by hydrogen bonding and physical adsorption, a new type of environmentally friendly and yellow CDs-based phosphors has been developed. Effective dispersion of small CDs on the surface of large starch particles can lengthen the distance among CDs, as a result, the non-radiative decay process and photoluminescence quenching were steadily hindered. Highly efficient solid-state CDs phosphors with a PLQY of 66.9% were acquired. Combing with the excellent thermal stability and photostability, a daylight white LED based on a single color converter of yellow CDs phosphors was fabricated with a CRI of 83, luminous efficiency of 30.54 lm/W, and CIE coordinates of (0.3429, 0.2817). Suggesting that as-prepared CDs-based phosphors have a great potential in phosphor-based light-emitting devices.

Recent Advances in Polymer‐Based Metal‐Free Room‐Temperature Phosphorescent Materials

AbstractPolymer‐based materials have evoked increasing attention in the research field of room‐temperature phosphorescence (RTP) because they can not only possess fantastic properties such as good flexibility, easy processing, low cost, high thermal stability and so forth, but also serve as rigid matrices to suppress the nonradiative decay process and thus contribute to phosphorescence emission at room temperature. This Review outlines the development of RTP in organic polymer‐based materials from the perspective of nondoped and doped polymer systems, containing the detailed photophysical properties, luminescent mechanisms and their applications. The conclusion and outlook are presented to point out the advances and challenges for further study of polymer‐based metal‐free RTP materials.

Asymmetric substitution changes the UV-induced nonradiative decay pathway and the spectra behaviors of β-diketones.

Asymmetric substitution has not been termed as an essential factor in studying photo-induced ultrafast dynamics of molecular system. Asymmetric 4-hydroxybut-3-en-2-one (HEO), together with symmetric malonaldehyde (MA) and acetylacetone (AA), have been provided as target sample to study the nonradiative decay (ND) processes of β-diketones. An effective ND pathway of the three molecules is presented that their excited second (S2) states transfer to first (S1) state by nonadiabatic surface hopping, and then transfer to triplet (T1) state by crossing minimum energy crossing point (MECP), after which decay to ground (S0) state through MECP. More importantly, the asymmetric substitution of HEO induces the proton transfer in the S1 state and generates a proton-transferred conformer with lowest energy, which does not occur for MA and AA. This change exploits a new ND pathway that the S1 state decays to the proton transferred T1 state and then undergoes reverse proton transfer to S0 state through the MECPs between the three states. The two pathways of HEO give detailed energy and geometric information on surface hopping of S2/S1 and MECPs of S1/T1/S0, and interpret the reason of the ND pathway while not spectra emission. This result is significantly different from the previous reported ND pathway of photoisomerization or conical intersection between different states. This work shows that asymmetric substitution changes the molecular structure and then changes their spectra behaviors.

Emission Tuning of Heteroleptic Arylborane-Ruthenium(II) Complexes by Ancillary Ligands: Observation of Strickler-Berg-Type Relation.

Novel heteroleptic arylborane-ruthenium(II) complexes having a series of ancillary ligands L' ([Ru(B2bpy)L'2]2+) in CH3CN showed low-energy/intense metal-to-ligand charge transfer (MLCT)-type absorption and intense/long-lived emission compared to the reference complexes. The spectroscopic and photophysical properties of [Ru(B2bpy)L'2]2+ were shown to be manipulated synthetically by the electron-donating ability of the ancillary ligand(s). The intense and long-lived emission observed for [Ru(B2bpy)L'2]2+ in CH3CN at 298 K is responsible for the accelerated radiative and decelerated nonradiative decay processes, which are controllable through the electronic structures of the ancillary ligand(s). On the basis of the present systematic study, furthermore, we succeeded in demonstrating the Strickler-Berg-type relation between the molar absorption coefficients of the MLCT bands and the radiative rate constants of the complexes.

DFT/TDDFT insight into the impact of ring size of the NHC chelating unit of high effective phosphorescent Platinum (II) complexes

Uncovering the photodeactivation mechanisms of unique N‐heterocyclic carbene (NHC)‐based transition metal complexes is favorable for designing more high‐efficiency phosphorescent materials. In this work, four bidentate platinum (II) complexes with NHC‐chelate are investigated by the density functional theory (DFT) and time‐dependent density functional theory (TDDFT) to probe into how the ring size of NHC‐chelate unit influences on electronic structures and the phosphorescent properties. To illustrate the photodeactivation mechanisms clearly, three significant photodeactivation processes (radiative decay process, temperature‐independent and temperature‐dependent nonradiative decay processes) were taken into consideration. We stated that radiative decay rate constants kr slightly increased with declined number of NHC‐chelate ring, owing to the gradually larger SOC matrix elements between the T1 state and Sn states. Combining the temperature‐independent with temperature‐dependent nonradiative decay processes, the nonradiative decay rate knr is Pt‐4 (five‐membered) < Pt‐3 (six‐membered) < Pt‐2 (seven‐membered) < Pt‐1 (eight‐membered). The calculated results testify that the decrease of size of the NHC chelating unit is a reliable insurance to improve the quantum yield. The designed complex Pt‐4 with five‐membered NHC‐ring can serve as a highly efficient phosphorescent material in the future. The results indicated controlling the ring size of NHC‐chelate is a feasible method to tune phosphorescence properties of Pt (II) complexes.

Influence of restricted rotation of small-sized substituent on phosphorescence efficiency for Pt(II) complexes: A theoretical investigation

Abstract Herein, the radiative and non-radiative decay processes of six carborane-based Pt(II) complexes were explored with the help of the density functional theory and time-dependent density functional theory. In order to interpret the influence of non-metal heavy atoms on the phosphorescent quantum yields, the determined factors, i.e., the ZFS (zero-point filed splitting) parameters, radiative decay rate constants and photodeactivation mechanisms, were unveiled. The results indicate that the spin-orbital coupling effects can be slightly regulated via introducing the non-metal heavy atoms into the carborane. Also, the non-metal heavy atoms substituents could cause slightly decrease of the energy barriers between the 3ES and 3MC excited states, and thus, for Pt-6, a fast non-radiative decay process may occur. Moreover, compared with the Pt-1, the rotation of methyl in Pt-2 ∼ 5, can be effectively restricted with the aid of hydrogen bonding and halogen bonding, which is beneficial for obtaining small non-radiative decay rate constants. Accordingly, it provides a strategy for realizing the restriction of small group rotation to achieve the high-efficient phosphorescent emitters.

Theoretical study on the vibrationally resolved spectra and quantum yield of blue phosphorescent iridium(III) complexes with 2-(4-fluoro-3-(trifluoromethyl)-phenyl)pyridine as the cyclometalated ligand

Abstract In this paper, the phosphorescent properties of six Ir(III) complexes with 2-(4-fluoro-3-(trifluoromethyl)-phenyl)pyridine (4 F-3CF3ppyH) as the cyclometalated ligand (two experimentally reported and four novelly designed) are investigated by density functional theory (DFT) and quadratic response (QR) time-dependent density functional theory (TDDFT) including spin−orbit coupling (SOC). The vibrationally resolved phosphorescence spectra were calculated to elucidate the experimental spectra. The strongest emission peak is mainly contributed by 0-0 transition. The strong shoulder peak can be ascribed as benzene ring and pyridine ring skeleton fundamental vibration transitions coupled with C-H wagging vibration transitions. The wide and weak emission band arises from the combination frequency. It was found that all of the six compounds could emit blue phosphorescence. To obtain the quantum yields, the radiative rate constant is quantitatively determined, while the nonradiative decay rate is qualitatively evaluated both by temperature-independent and temperature-dependent nonradiative decay processes. It shows that the quantum yields of all the six compounds are very high, which are evaluated to exceed 0.85. As a result, all of them are expected to be potential blue-emitting Ir(III) complexes.

A Simple Strategy to Construct Amorphous Metal-Free Room Temperature Phosphorescent and Multi-Color Materials.

It is urgent to develop a universal strategy for producing small organic molecules exhibiting efficient room temperature phosphorescence (RTP). An important contribution made by Ma and Tian et al. provides a general and applicable strategy to obtain amorphous organic molecules with efficient RTP emission by simply modifying phosphors onto β-cyclodextrin (β-CD). The nonradiative decay processes and oxygen quenching have been significantly suppressed by the strong intermolecular hydrogen bonding between β-CD derivatives. Furthermore, the cavity of β-CDs endows them with the ability to incorporate the adamantane moiety of the fluorescent guest molecules to construct a supramolecular system which exhibits excellent RTP-fluorescence dual emission properties and multicolor emission by altering the host to guest ratio and the excitation wavelength.

Comprehensive Investigation into Luminescent Properties of Ir(III) Complexes: An Integrated Computational Study of Radiative and Nonradiative Decay Processes.

A comprehensive and concrete exploration into the deactivation mechanisms of luminescent materials is imperative, with the improvement of simulating and computing technology. In this study, an integrated calculation scheme is employed on five Ir(III) complexes for thorough investigation of their photophysical properties, including radiative ( kr) and nonradiative ( knr) decay rates. As a most famous Ir(III) complex with superior quantum efficiency, fac-Ir(ppy)3 herein serves as a reference relative to the other four β-diketonate complexes. Both temperature-independent and temperature-dependent knr are taken into account quantitatively for the first time, to unearth the role of different ancillary ligands in the determination of luminescent properties. Since the validated calculations of kr for the five complexes are of the same order of magnitude, the nonemissive peculiarity of 4 is caused by large knr. The newly designed compound 5, which simply has two more -CH2 groups than 4 in the ancillary ligand, further manifests that the reason for large knr in molecule 4 should be attributed to the ligand resonance caused by great π conjugation.

Vancomycin Determination by Disrupting Electron-Transfer in a Fluorescence Turn-On Squaraine-Anthraquinone Triad.

A highly sensitive and selective probe for Vancomycin (Van) in aqueous and serum samples is developed in this study. The probe is based on a triad consisting of a near-infrared squaraine dye (Seta-640) conjugated to two anthraquinone molecules via Lys-d-Ala-d-Ala peptides. In the absence of Van, the close proximity and efficient electron-transfer from the excited Seta-640 dye to anthraquinone result in significant fluorescence quenching of the dye ("off"-state). When Van is added, the antibiotic molecules bind with high affinity to the -d-Ala-d-Ala ligands in a 2:1 stoichiometry (Van:triad), resulting in fluorescence recovery that is as high as 30 times ("on"-state). Even though bound Van enhances the fluorescence by reducing the rate of (intrinsic) polarity-induced nonradiative decay process, this effect plays only a minor role. Instead, the main reason behind the observed fluorescence recovery after drug binding is the effective inhibition of electron-transfer; plausibly arising from a steric-induced lengthening of the spatial separation between electron donor and acceptor. The probe has detection limits of 7.0 and 96.9 nM in buffer and human serum, respectively, operates in the clinically relevant range, is insensitive to Van crystalline degradation product (CDP-1), and is easy to operate by using a commonly available fluorescence spectrometer.

Multifunctional Dithiadiazolyl Radicals: Fluorescence, Electroluminescence, and Photoconducting Behavior in Pyren-1′-yl-dithiadiazolyl

The pyren-1'-yl-functionalized dithiadiazolyl (DTDA) radical, C16H9CNSSN (1), is monomeric in solution and exhibits fluorescence in the deep-blue region of the visible spectrum (440 nm) upon excitation at 241 nm. The salt [1][GaCl4] exhibits similar emission, reflecting the largely spectator nature of the radical in the fluorescence process, although the presence of the radical leads to a modest quenching of emission (ΦF = 98% for 1+ and 50% for 1) through enhancement of non-radiative decay processes. Time-dependent density functional theory studies on 1 coupled with the similar emission profiles of both 1+ and 1 are consistent with the initial excitation being of predominantly pyrene π-π* character. Spectroscopic studies indicate stabilization of the excited state in polar media, with the fluorescence lifetime for 1 (τ = 5 ns) indicative of a short-lived excited state. Comparative studies between the energies of the frontier orbitals of pyren-1'-yl nitronyl nitroxide (2, which is not fluorescent) and 1 reveal that the energy mismatch and poor spatial overlap between the DTDA radical SOMO and the pyrene π manifold in 1 efficiently inhibit the non-radiative electron-electron exchange relaxation pathway previously described for 2. Solid-state films of both 1 and [1][GaCl4] exhibit broad emission bands at 509 and 545 nm, respectively. Incorporation of 1 within a host matrix for OLED fabrication revealed electroluminescence, with CIE coordinates of (0.205, 0.280) corresponding to a sky-blue emission. The brightness of the device reached 1934 cd/m2 at an applied voltage of 16 V. The crystal structure of 1 reveals a distorted π-stacked motif with almost regular distances between the pyrene rings but alternating long-short contacts between DTDA radicals. Solid state measurements on a thin film of 1 reveal emission occurs at shorter wavelengths (375 nm) whereas conductivity measurements on a single crystal of 1 show a photoconducting response at longer wavelength excitation (455 nm).

Ultrafast carrier dynamics and third-order nonlinear optical properties of AgInS2/ZnS nanocrystals

Broad photoluminescence (PL) emission, a large Stokes shift and extremely long-lived radiative lifetimes are the characteristics of ternary I–III–VI semiconductor nanocrystals (NCs), such as CuInS2 and AgInS2. However, the lack of understanding regarding the intriguing PL mechanisms and photo-carrier dynamics limits their further applications. Here, AgInS2 and AgInS2/ZnS NCs were chemically synthesized and their carrier dynamics were studied by time-resolved PL spectroscopy. The results demonstrated that the surface defect state, which contributed dominantly to the non-radiative decay processes, was effectively passivated through ZnS alloying. Femtosecond transient absorption spectroscopy was also used to investigate the carrier dynamics, revealing the electron storage at the surface state and donor state. Furthermore, the two photon absorption properties of AgInS2 and AgInS2/ZnS NCs were measured using an open-aperture Z-scan technique. The improved third-order nonlinear susceptibility of AgInS2 through ZnS alloying demonstrates potential application in two photon PL biological imaging.

Efficient amplified spontaneous emission based on π-conjugated fluorophore-cored molecules studied by density functional theory

Abstract The π-conjugated molecules, anthracene, pyrene and perylene with stepwise enhanced conjugation, have become a key ingredient in the design of new organic semiconductors for light emission and energy harvesting applications. Here, new biphenyl dissubstituted anthracene, pyrene and perylene derivatives are synthesized and investigated as blue-emitting materials. The designed molecules exhibited typical amplified spontaneous emission (ASE) behavior in solid thin films for the compounds dispersed in the inert polystyrene host at a rather large (5 wt%) chromophore concentration. Theoretical analysis assists in understanding the emission properties and optical gain performance. The calculations reveal enhanced transition density and increased transition dipole moment accompanying by stepwise enhanced molecular conjugation of fluorophore-cores. In addition, enhanced fluorescence quantum yield, significant shortened fluorescence lifetimes and increased radiative decay rate for designed compounds promote the occurrence of ASE behavior compared with anthracene, pyrene and perylene. As well, the vibration analysis show that the intramolecular vibrational modes of researched molecules possessed small Huang-Rhys factors, which thus bring in effective competition between radiative and non-radiative decay process in favor of optical gain. The experimental and theoretical provide evidence that the reported molecules are promising candidates for organic laser applications.

Modeling the Modulation of Emission Behavior in E/Z Isomers of Dipyrrolyldiphenylethene: From Molecules to Nanoaggregates

The dipyrrolyldiphenylethene (DPYDPE) molecule shows a switching between aggregation-induced-emission (AIE) and crystallization-induced emission (CIE) upon modification of the stereochemistry of the molecule. Herein, we propose a theoretical study based on molecular dynamics, time-dependent (TD-) density functional theory (DFT), and QM/QM′ calculations to investigate the structural and optical properties of the E and Z isomers in three different phases (solution, crystal, and aggregate). By computing the Huang–Rhys factors and the reorganization energies, we demonstrate that the fluorescence quenching observed in solution for both isomers is due to a nonradiative decay process involving low-frequency vibrational modes assigned to scissoring motions. In the crystal and in the aggregates, the effects of steric hindrance strongly modify the topology of the potential energy surface of the first excited state, and this results in a restriction of the vibrational modes involved in the energy dissipation. The mo...