Nonradiative Decay Processes Research Articles

Electrostatic Interaction-Induced Room-Temperature Phosphorescence in Pure Organic Molecules from QM/MM Calculations.

Room temperature phosphorescence (RTP) from pure organic material is rare due to the low phosphorescence quantum efficiency. That is why the recent discovery of crystallization induced RTP for several organic molecules aroused strong interests. Through a combined quantum and molecular mechanics CASPT2/AMBER scheme taking terephthalic acid (TPA) as example, we found that electrostatic interaction not only can induce an enhanced radiative decay T1 → S0 through the dipole-allowed S1 intermediate state, but also can hinder the nonradiative decay process upon crystallization. From gas phase to crystal, the nature of S1 state is converted to (1)(π,π*) from (1)(n,π*) character, enhancing transition dipole moment and serving as an efficient intermediate radiative pathway for T1 → S0 transition, and eventually leading to a boosted RTP. The intermolecular packing also blocks the nonradiative decay channel of the high-frequency C═O stretching vibration with large vibronic coupling, rather than the conventional low-frequency aromatic rotation in crystal. This mechanism also holds for other organic compounds that contain both ketones and aromatic rings.

Solvent Polarity Effect on Nonradiative Decay Rate of Thioflavin T.

It has been established earlier that fluorescence quantum yield of thioflavin T (ThT)-a probe widely used for amyloid fibrils detection-is viscosity-dependent, and photophysical properties of ThT can be well-described by the fluorescent molecular rotor model, which associates twisted internal charge transfer (TICT) reaction with the main nonradiative decay process in the excited state of the dye. Solutions of ThT in a range of polar solvents were studied using steady-state fluorescence and sub-picosecond transient absorption spectroscopy methods, and we showed that solvent effect on nonradiative transition rate knr cannot be reduced to the dependence on viscosity only and that ∼3 times change of knr can be observed for ThT in aprotic solvents and water, which correlates with solvent polarity. Different behavior was observed in alcohol solutions, particularly in longer n-alcohols, where TICT rate was mainly determined by rotational diffusion of ThT fragments. Quantum-chemical calculations of S0 → S1 transition energy were performed to get insight of polar solvent contribution to the excited-state energy stabilization. Effect of polar solvent on electronic energy levels of ThT was simulated by applying homogeneous electric field according to the Onsager cavity model. Static solvent effect on the excited-state potential energy surface, where charge transfer reaction takes place, was not essential to account for experimentally observed TICT rate differences in water and aprotic solvents. From the other side, nonradiative decay rate of ThT in water, ethylene glycol, and aprotic solvents was found to follow dynamics of polar solvation knr ∼ τS(-1), which can explain dependence of the TICT rate on both polarity and viscosity of the solvents.

Stark Spectroscopy of Rubrene. II. Stark Fluorescence Spectroscopy and Fluorescence Quenching Induced by an External Electric Field.

We report Stark fluorescence spectroscopy investigation of rubrene dispersed in a poly(methyl methacrylate) film. The features of the fluorescence spectrum are analogous to those in solutions. In the Stark fluorescence spectrum, the decrease of the fluorescence quantum yield in the presence of an external electric field is observed. This result shows that the yield of nonradiative decay processes is increased by the application of an external electric field. It is known that the fluorescence quantum yield for rubrene, which is nearly unity at room temperature, depends on temperature, and a major nonradiative decay process in photoexcited rubrene is ascribed to a thermally activated intersystem crossing (ISC). Equations that express the field-induced fluorescence quenching in terms of the molecular parameters are derived from the ensemble average of electric field effects on the activation energy of the reaction rate constant in random orientation systems. The molecular parameters are then extracted from the observed data. It is inferred that the field-induced increase in the yield of other intramolecular and intermolecular photophysical processes in addition to the ISC should be taken into account.

Bond Fission and Non-Radiative Decay in Iridium(III) Complexes.

We investigate the role of metal-ligand bond fission in the nonradiative decay of excited states in iridium(III) complexes with applications in blue organic light-emitting diodes (OLEDs). We report density functional theory (DFT) calculations of the potential energy surfaces upon lengthening an iridium-nitrogen (Ir-N) bond. In all cases we find that for bond lengths comparable to those of the ground state the lowest energy state is a triplet with significant metal-to-ligand change transfer character ((3)MLCT). But, as the Ir-N bond is lengthened there is a sudden transition to a regime where the lowest excited state is a triplet with significant metal centered character ((3)MC). Time-dependent DFT relativistic calculations including spin-orbit coupling perturbatively show that the radiative decay rate from the (3)MC state is orders of magnitude slower than that from the (3)MLCT state. The calculated barrier height between the (3)MLCT and (3)MC regimes is clearly correlated with previously measured nonradiative decay rates, suggesting that thermal population of the (3)MC state is the dominant nonradiative decay process at ambient temperature. In particular, fluorination both drives the emission of these complexes to a deeper blue color and lowers the (3)MLCT-(3)MC barrier. If the Ir-N bond is shortened in the (3)MC state another N atom is pushed away from the Ir, resulting in the breaking of this bond, suggesting that once the Ir-N bond breaks the damage to the complex is permanent-this will have important implications for the lifetimes of devices using this type of complex as the active material. The consequences of these results for the design of more efficient blue phosphors for OLED applications are discussed.

Understanding of the Off-On Response Mechanism in Caged Fluorophores Based on Quantum and Statistical Mechanics.

For many years, numerous fluorescent probes have been synthesized and applied to visualize molecules and cells. The development of such probes has accelerated biological and medical investigations. As our interests have been focused on more complicated systems in recent years, the search for probes with sensitive environment off-on response becomes increasingly important. For the design of such sophisticated probes, theoretical analyses of the electronically excited state are inevitable. Especially, understanding of the nonradiative decay process is highly desirable, although this is a challenging task. In this study, we propose an approach to treat the solvent fluctuation based on the reference interaction site model. It was applied to selected bioimaging probes to understand the importance of solvent fluctuation for their off-on response. We revealed that the this switching process involves the nonradiative decay through the charge transfer state, where the solvent relaxation supported the transition between excited and charge transfer states. In addition, energetically favorable solvent relaxation paths were found due to the consideration of multiple solvent configurations. Our approach makes it possible to understand the nonradiative decay facilitated by a detailed analysis and enables the design of novel fluorescent switching probes considering the effect of solvent fluctuation.

Enhanced Fluorescence in Tetraylnitrilomethylidyne–Hexaphenyl Derivative-Functionalized Periodic Mesoporous Organosilicas for Sensitive Detection of Copper(II)

Highly fluorescent and copper(II)-responsive periodic mesoporous organosilicas (TH-PMOs) were successfully obtained by using a (tetraylnitrilomethylidyne–hexaphenyl (TH))-derived tetrasiloxane (TH-Si4) as the organosilica precursor. The TH unit was embedded within the framework of PMOs by four silyl groups without forming associated species, and high fluorescence quantum yields were achieved even for the PMOs prepared from 100% organosilane precursor. The optical studies indicated that the intramolecular rotation of TH was restricted by the framework of TH-PMOs, resulting in the decline of the nonradiative decay process and enhanced monomeric fluorescence emission. The unique structure of the TH groups not only assured their aggregation-induced enhanced emission (AIEE) characteristics but also provided potential coordinating sites for metal ions. Therefore, the enhanced fluorescence of PMOs showed a highly selective response to copper ions in aqueous solution with the detection sensitivity up to the 10–8 ...

Exploring the Mechanism of Ultrafast Intersystem Crossing in Rhenium(I) Carbonyl Bipyridine Halide Complexes: Key Vibrational Modes and Spin-Vibronic Quantum Dynamics.

The mechanism of ultrafast intersystem crossing in rhenium(I) carbonyl bipyridine halide complexes Re(X)(CO)3(bpy) (X = Cl, Br, I) is studied by exploring the structural deformations when going from Franck-Condon (FC) to critical geometries in the low-lying singlet and triplet excited states and by selecting the key vibrational modes. The luminescent decay observed in [Re(Br)(CO)3(bpy)] is investigated by means of wavepacket propagations based on the multiconfiguration time-dependent Hartree (MCTDH) method. The dominant coordinates underlying the nonradiative decay process are extracted from minima, minimum energy seam of crossing (MESX) and minimum energy conical intersection (MECI) geometries obtained by the seam model function (SMF)/single-component artificial force induced reaction (SC-AFIR) approach. By choosing the normal modes used in MCTDH from the MECI and MESX geometries, not only the degenerate energy points but also the low-energy-gap regions are included. For this purpose a careful vibrational analysis is performed at each critical geometry and analyzed under the light of the pertinent nonadiabatic coupling terms obtained from the linear vibronic coupling (LVC) model augmented by spin-orbit coupling (SOC) in the electronic diabatic representation.

Decay processes of the core-excited states 1s2p3 for Be-like systems with Z=8–54

On the basis of a fully relativistic multiconfiguration Dirac–Fock method, a systematic study has been carried out for the energies, radiative and non-radiative (Auger) decay processes of the core-excited states 1s2p3 for Be-like ions with Z=8–54. The energies and radiative transition rates are smoothly varying functions of Z. Auger rates do not change significantly with Z as the radiative rates do. Electron correlation effects on the energies and the radiative transition rates are investigated. Good agreement is found between our results and other theoretical data.

Sensitive and selective detection of adsorbed explosive molecules using opto-calorimetric infrared spectroscopy and micro-differential thermal analysis

Multi-modal explosive sensors based on microelectromechanical systems (MEMS) have been developed. Opto-calorimetric infrared (IR) spectroscopy, capable of obtaining molecular signatures of extremely small quantities of adsorbed explosive molecules, has been realized with the use of microheater/thermometer devices and a widely tunable quantum cascade laser. These devices respond to the heat generated by non-radiative decay process when the adsorbed explosive molecules are resonantly excited with IR light. Monitoring the variation in microthermometer signal as a function of illuminating IR wavelength corresponds to the conventional IR absorption spectrum of the adsorbed molecules. Moreover, the mass of the adsorbed molecules is determined by measuring the resonance frequency shift of the device for the quantitative analysis. In addition, micro-differential thermal analysis, which can be used to differentiate exothermic or endothermic reaction of heated molecules, has been performed with the same devices to provide additional orthogonal signal for trace explosive detection and sensor surface regeneration. We have demonstrated successful detection, differentiation, and quantification of trace amounts of explosive molecules (cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN)) and their mixtures using three orthogonal sensing signals.

Controlling S2 Population in Cyanine Dyes Using Shaped Femtosecond Pulses.

Fast population transfer from higher to lower excited states occurs via internal conversion (IC) and is the basis of Kasha's rule, which states that spontaneous emission takes place from the lowest excited state of the same multiplicity. Photonic control over IC is of interest because it would allow direct influence over intramolecular nonradiative decay processes occurring in condensed phase. Here we tracked the S2 and S1 fluorescence yield for different cyanine dyes in solution as a function of linear chirp. For the cyanine dyes with polar solvation response IR144 and meso-piperidine substituted IR806, increased S2 emission was observed when using transform limited pulses, whereas chirped pulses led to increased S1 emission. The nonpolar solvated cyanine IR806, on the other hand, did not show S2 emission. A theoretical model, based on a nonperturbative solution of the equation of motion for the density matrix, is offered to explain and simulate the anomalous chirp dependence. Our findings, which depend on pulse properties beyond peak intensity, offer a photonic method to control S2 population thereby opening the door for the exploration of photochemical processes initiated from higher excited states.

Theoretical Insights into the Phosphorescence Quantum Yields of Cyclometalated (C∧C*) Platinum(II) NHC Complexes: π-Conjugation Controls the Radiative and Nonradiative Decay Processes

In this article, the radiative and nonradiative decay processes of four cyclometalated (C∧C*) platinum(II) N-heterocyclic carbene (NHC) complexes were unveiled via density functional theory and time-dependent density functional theory. In order to explore the influence of π-conjugation on quantum yields of (NHC)Pt(acac) (NHC═N-heterocyclic carbene, acac = acetylacetonate) complexes, the factors that determine the radiative process, including singlet–triplet splitting energies, transition dipole moments, and spin–orbit coupling (SOC) matrix elements between the lowest triplet states and singlet excited states were calculated. In addition, the SOC matrix elements between the lowest triplet state and the ground state as well as Huang–Rhys factors were also computed to describe the temperature-independent nonradiative decay processes. Also, the triplet potential energy surfaces were investigated to elucidate the temperature-dependent nonradiative decay processes. The results indicate that complex Pt-1 has hig...

Room‐Temperature Phosphorescence of Crystalline 1,4‐Bis(aroyl)‐2,5‐dibromobenzenes

AbstractThere is a growing interest in metal‐free organic luminophores that exhibit phosphorescence at room temperature. We report herein that 1,4‐bis(aroyl)‐2,5‐dibromobenzenes serve as a new class of such luminophores. The bis(aroyl)benzene derivatives were not emissive either in solution or in a doped polymer thin film. In contrast, crystals of bis(aroyl)benzenes showed photoluminescence under ambient conditions, which was subsequently confirmed as phosphorescence with a luminescence lifetime measurement. The emission color ranged from blue to green, depending on the nature of the aroyl groups, and the luminescent quantum yields were 0.05–0.18 at room temperature and 0.38–0.67 at 77 K. In each crystal, several intermolecular interactions such as C=O···H, Br···Br, C=O···Br, F···F, S···H, and MeO···H were observed, which contributed to the restriction of intramolecular motions, thereby diminishing the nonradiative decay process from the triplet excited states.

Exploring the Photodeactivation Pathways of Pt[O^N^C^N] Complexes: A Theoretical Perspective.

In this article, the influence of the tert-butyl unit on the photodeactivation pathways of Pt[O^N^C^N] (O^N^C^N=2-(4-(3,5-di-tert-butylphenyl)-6-(3-(pyridin-2-l)phenyl) pyridin-2-yl)phenolate) is investigated by DFT/TDDFT calculations. To further explore the factors that determine the radiative processes, the transition dipole moments of the singlet excited states, spin-orbit coupling (SOC) matrix elements, and energy gaps between the lowest triplet excited states and singlet excited states are calculated. As demonstrated by the results, compared with Pt-3, Pt-1 and Pt-2 have larger SOC matrix elements between the lowest triplet excited states and singlet excited states, an indicator that they have faster radiative decay processes. In addition, the SOC matrix elements between the lowest triplet excited states and ground states are also computed to elucidate the temperature-independent non-radiative decay processes. Moreover, the temperature-dependent non-radiative decay mechanisms are also explored via the potential energy profiles.

Double surface plasmon enhanced organic light-emitting diodes by gold nanoparticles and silver nanoclusters

The influence of gold nanoparticles (GNPs) and silver nanoclusters (SNCs) on the performance of organic light-emitting diodes is investigated in this study. The GNPs are doped into (poly (3, 4-ethylenedioxythiophene) poly (styrenesulfonate)) (PEDOT: PSS) and the SNCs are introduced between the electron-injection layer and cathode alumina. The power efficiency of the device, at the maximum luminance, with double surface plasmon resonance and buffer layer is about 2.15 times higher than that of the device without GNPs and SNCs because the absorption peaks of GNPs and SNCs are as good as the photoluminescence peak of the emission layer, resulting in strong surface plasmon resonance effect in the device. In addition, the buffer layer is inserted between PEDOT: PSS and the emitting layer in order to avoid that the nonradiative decay process of exciton is generated.

Methane sensing at room temperature using photothermal cantilever deflection spectroscopy

Detection of methane at room temperature using adsorption-based chemical sensing is a challenge since methane is inert and does not adsorb on surfaces. We demonstrate that nanomechanical photothermal spectroscopy is capable of detecting femtogram level physisorbed methane on a thermally sensitive microcantilever. Even though the residence time of methane is in sub nanoseconds, it is enough for a vibrationally excited methane molecule to transfer its energy to the cantilever through a non-radiative decay process. The variation of cantilever deflection as a function of excitation wavelength shows the characteristic vibrational spectrum of methane. Photothermal cantilever deflection spectroscopy, therefore, offers a powerful technique for chemical characterization of physisorbed molecules which are chemically inert.

Non-radiative deactivation in phenol-pyridine complex: theoretical study.

Minimum energy structures of the ground and lowest excited states of the phenol (PhOH)-pyridine (Py) hydrogen-bonded complex in the gas phase were determined by ab initio calculations. Photophysical and photochemical features of the complex under Cs symmetry (planar (Pl) and perpendicular (Pe) conformers) and without any symmetry constraints (unconstrained (Un) conformer) were studied with respect to nonradiative decay processes to the ground state. The mechanism involves internal conversion (IC) and intersystem crossing (ISC) along the O-H bond elongation coordinate, where a coupled electron/proton-transfer reaction plays a decisive role in the photophysics of this complex. For the Pl conformer, nonradiative decay proceeds from a locally excited (1)ππ*(LE) minimum over a conical intersection barrier (0.12 eV) to a charge-transfer (CT) minimum, which corresponds to a hydrogen-bonded PhO˙HPy˙ biradical. Near this second minimum, a barrierless conical intersection (1)A'(ππ*(CT))-S0 funnels the electron population from the CT to the ground S0 state, completing the nonradiative deactivation. Calculations performed for the Pe and Un conformers confirmed that the same radiationless mechanism proceeds with no (1)ππ*(LE)/(1)ππ*(CT) conical intersection near the Franck-Condon region. Furthermore, the population of the lowest triplet states via ISC and their contribution to the photophysics of PhOH-Py complex have been discussed. These findings appear to suggest that there is no single dominant path, but rather many distinct paths involving different quenching mechanisms.

Photophysical Properties of a Post-Self-Assembly Host/Guest Coordination Cage: Visible Light Driven Core-to-Cage Charge Transfer.

Supramolecular systems are capable of unique photophysical properties due to possible interactions between subcomponents, such as between an encapsulated molecule and its cage in a host/guest environment. Here, we report that the encapsulation of a chromophore by a metallacage dramatically enhances its photophysical properties. In the visible region, the encapsulated photosensitizer achieves a 6.5-fold enhancement to its absorptivity. The triplet lifetime of the encapsulated photosensitizer is three times longer than that of its free analogue. These enhancements are attributed to two key factors: (i) encapsulation-induced core-to-cage charge transfer (CCCT) generates new visible-light absorbing states, accounting for the enhanced absorption, and (ii) the microenvironment inside the metallacage inhibits nonradiative decay processes, resulting in prolonged triplet lifetime. The CCCT arises from the electrostatic interaction between the delocalized electrons of the guest coronene and the positive charge associated with the metallacage host. The work herein provides insight into the CCCT phenomenon.

Exciton diffusion, end quenching, and exciton-exciton annihilation in individual air-suspended carbon nanotubes

Luminescence properties of carbon nanotubes are strongly affected by exciton diffusion, which plays an important role in various nonradiative decay processes. Here we perform photoluminescence microscopy on hundreds of individual air-suspended carbon nanotubes to elucidate the interplay between exciton diffusion, end quenching, and exciton-exciton annihilation processes. A model derived from random-walk theory as well as Monte Carlo simulations are utilized to analyze nanotube length dependence and excitation power dependence of emission intensity. We have obtained the values of exciton diffusion length and absorption cross section for different chiralities, and diameter-dependent photoluminescence quantum yield have been observed. The simulations have also revealed the nature of a one-dimensional coalescence process, and an analytical expression for the power dependence of emission intensity is given.

Aggregation-Induced Fluorescence Emission Properties of Dicyanomethylene-1,4-dihydropyridine Derivatives

A series of dicyanomethylene-1,4-dihydropyridine (DCMP) derivatives with aggregation-induced emission (AIE) were designed and synthesized. These target compounds emit low fluorescence in THF solutions because of the free rotation of the phenyl and dihydropyridine rings about the axes of the olefinic double bonds and the resultant nonradiative decay process, but they exhibit strong fluorescence in the aggregate state because of the restriction of intramolecular rotation (RIR), as confirmed by solution thickening and cooling experiments. Compared with the almost-planar dicyanomethylene-4H-pyran (DCM) derivative DCM-1 with aggregation-caused quenching (ACQ), DCMP-5 with AIE takes a highly twisted conformation by replacing the oxygen atom of the DCM skeleton with an N-ethyl group, as revealed by crystallographic data and theoretical calculations. Depending on the different electron-donating substituents, the solids of these compounds emit yellow or orange fluorescence. Moreover, the cyano group endows DCMP-5 with a strong self-assembly ability under proper conditions, and the obtained regular microparticles emit bright orange fluorescence. These materials will broaden the new family of AIE-exhibiting fluorophores.

Enhanced Multiphoton Emission from CdTe/ZnS Quantum Dots Decorated on Single-Layer Graphene

Time-resolved single-molecule spectroscopy of individual colloidal CdTe/ZnS quantum dots (QDs) decorated on single-layer graphene (SLG) demonstrates significantly enhanced probability of multiphoton emission, accompanied by reduced fluorescence intensity and decreased lifetime. A rigorous analysis of time-resolved spectroscopy on individual QDs on SLG reveals a decrease in radiative decay rate and a significant enhancement (12.6 times) in nonradiative decay rate. The enhanced multiphoton emission of QDs on SLG is a consequence of the competition between the enhanced nonradiative decay and fast Auger process, as suggested by the dominant fast decay (<0.8 ns) of QDs on SLG. Our findings provide important information on the emission behavior of QDs coupled with carbon nanomaterials, such as graphene and carbon nanotubes, and will assist in the creation of effective single-photon or multiphoton sources and devices. Furthermore, this is also a possible pathway for suppressing the Auger recombination of QDs whi...