Nonradiative Relaxation Processes Research Articles

Phonon-Driven Femtosecond Dynamics of Excitons in Crystalline Pentacene from First Principles.

Nonradiative exciton relaxation processes are critical for energy transduction and transport in optoelectronic materials, but how these processes are connected to the underlying crystal structure and the associated electron, exciton, and phonon band structures, as well as the interactions of all these particles, is challenging to understand. Here, we present a first-principles study of exciton-phonon relaxation pathways in pentacene, a paradigmatic molecular crystal and optoelectronic semiconductor. We compute the momentum- and band-resolved exciton-phonon interactions, and use them to analyze key scattering channels. We find that both exciton intraband scattering and interband scattering to parity-forbidden dark states occur on the same ∼100 fs timescale as a direct consequence of the longitudinal-transverse splitting of the bright exciton band. Consequently, exciton-phonon scattering exists as a dominant nonradiative relaxation channel in pentacene. We further show how the propagation of an exciton wave packet is connected with crystal anisotropy, which gives rise to the longitudinal-transverse exciton splitting and concomitant anisotropic exciton and phonon dispersions. Our results provide a framework for understanding the role of exciton-phonon interactions in exciton nonradiative lifetimes in molecular crystals and beyond.

Water/Light Multiregulated Supramolecular Polypseudorotaxane Gel with Switchable Room-Temperature Phosphorescence.

Water/light regulated room-temperature phosphorescence (RTP) of polypseudorotaxane supramolecular gel is constructed by threading the poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) chain with the bromoaromatic aldehyde into mono-(6-ethylenediamine-6-deoxygenated)-β-cyclodextrin (ECD) cavities and further assembling with negatively charged Laponite XLG (CNS) and diarylethene derivative (DAE) through electrostatic interaction. This hydrogel exhibits significant blue fluorescence emission; instead, after lyophilization to xerogel, the system exhibits both blue fluorescence and yellow RTP based on the rigid network structure of the xerogel, which restricts the vibration of the phosphor and suppresses the nonradiative relaxation process. Interestingly, the addition of excess ECDs to the gel system can enhance the RTP emission. Furthermore, the reversible luminescence behavior can be adjusted by the photoresponsive isomerism of DAE and humidity. This polypseudorotaxane supramolecular gel system provides a novel strategy for constructing tunable RTP materials.

Photophysical analysis of dual fluorescence and phosphorescence emissions observed for semi-aliphatic polyimides at lower temperatures.

The photoluminescence (PL) properties of four types of blue fluorescent semi-aliphatic polyimides (PIs) derived from aromatic dianhydrides (ODPA, BPDA, HQDEA, and BPADA) and an alicyclic diamine (DCHM) were investigated at temperatures ranging from room temperature (RT, 298 K) to 30 K to analyse the origins of their non-radiative relaxation (NR) processes. These PIs exhibited significant increases in fluorescence (FL) intensity and lifetimes when lowering the temperature, stabilising below 100 K. The PIs containing ether (-O-) linkages showed a shoulder peak at around 500 nm below 150 K, which is attributable to phosphorescence (PH). These results show that the NR deactivation at RT includes three processes: intersystem crossing (ISC) from the excited singlet (S1) to the triplet (T1) state, temperature-dependent NR from the S1 state, which becomes suppressed below around 100 K, and temperature-independent NR. Based on the analyses of the temperature dependences, polymer structures, and quantum chemical analysis of molecular orbitals, we contemplate that the temperature-dependent NR is attributable to the excitation quenching by defect states mediated by excitation migration, and the temperature-independent NR may be caused by the deactivation of the excited state induced by molecular vibrations.

Molecular structures in the non-adiabatic relaxaiton processes of excited states of pentafluoropyridine

In this work, the molecular structure and energy of some critical points in nonradiative relaxation process of the excited state of pentafluoropyridine are studied through quantum chemistry calculation. The structures and the vertical excitation energies and adiabatic excitation energies of the ground state and two lowest exited states are calculated. The geometry of the ground state is a planar structure with C<sub>2v</sub> symmetry, while the geometries of the two lowest excited states are half-boat structures with out-of-plane distortions. Furthermore, the topology structures and energy of the conical intersections of S<sub>2</sub>/S<sub>1</sub>, S<sub>1</sub>/S<sub>0</sub> and S<sub>2</sub>/S<sub>0</sub> are determined. The topology structures of the conical intersections S<sub>2</sub>/S<sub>1</sub>, S<sub>1</sub>/S<sub>0</sub> and S<sub>2</sub>/S<sub>0</sub> in the branching space are all peaked with asymmetric structures, and are determined to be structure of boat, half-boat, and chair, respectively. Their corresponding energy values are estimated at 6.39, 5.16 and 8.51 eV, respectively. The results show that the primary non-adiabatic relaxation pathway is the wavepacket of the S<sub>2</sub> state rapidly evolving into the S<sub>1</sub> state via the S<sub>2</sub>/S<sub>1</sub>, and then directly relaxing to the ground state via the S<sub>1</sub>/S<sub>0</sub>. In addition, the probability of directly relaxing to the ground state through S<sub>2</sub>/S<sub>0</sub> is smaller.

Luminescence and Excited-State Reactivity in a Heteroleptic Tricyanido Fe(III) Complex.

Harnessing sunlight via photosensitizing molecules is key for novel optical applications and solar-to-chemical energy conversion. Exploiting abundant metals such as iron is attractive but becomes challenging due to typically fast nonradiative relaxation processes. In this work, we report on the luminescence and excited-state reactivity of the heteroleptic [FeIII(pzTp)(CN)3]- complex (pzTp = tetrakis(pyrazolyl)borate), which incorporates a σ-donating trispyrazolyl chelate ligand and three monodentate σ-donating and π-accepting cyanide ligands. Contrary to the nonemissive [Fe(CN)6]3-, a broad emission band centered at 600 nm at room temperature has been recorded for the heteroleptic analogue attributed to the radiative deactivation from a 2LMCT excited state with a luminescence quantum yield of 0.02% and a lifetime of 80 ps in chloroform at room temperature. Bimolecular reactivity of the 2LMCT excited state was successfully applied to different alcohol photo-oxidation, identifying a cyanide-H bonding as a key reaction intermediate. Finally, this research demonstrated the exciting potential of [Fe(pzTp)(CN)3]- as a photo-oxidant, paving the way for further exploration and development of emissive Fe-based photosensitizers competent for photochemical transformations.

Study on temperature-dependent thermal population and non-radiative relaxation process of the triplet states in Platinum(Ⅱ) octaethylporphyrin

The present study focused on examining the thermal population and non-radiative relaxation process by leveraging the unique characteristics of the triplet states of Platinum (Ⅱ) octaethylporphyrin (PtOEP). The investigation of the thermal population process involved the utilization of the luminescence exhibited by the PtOEP triplet state in an oxygen-free environment. Time-resolved spectroscopy was employed to investigate the non-radiative relaxation process occurring on the higher energy levels of the triplet states in the presence of oxygen. The findings indicate that both thermal population and non-radiative relaxation process follow the Boltzmann distribution law. Furthermore, the energy differences obtained from the fitting procedure closely aligned with the energy differences calculated based on the emission spectrum. This observation implies that the underlying reason for the congruence between these two processes is their inherent reversibility.

Temperature Dependence of Mn4+ Emission Intensity and Lifetime in TriGain® Phosphor (K2SiF6:Mn4+)

The temperature dependence of the Mn4+ photoluminescence and lifetime has been measured in the red emitting K2SiF6:Mn4+ phosphor that is sold under the trade name, TriGain®. In the range 12−450 K, the 2E → 4A2 emission intensity increases considerably while the lifetime decreases. Above 450 K, rapid decrease in the intensity and lifetime is observed. The results are interpreted in terms of dynamically induced increases in the emission intensity and decay rate coupled with an Arrhenius-type thermal quenching. The parameters of our fitting to physical models are compared with those reported on laboratory synthesized K2SiF6:Mn4+ phosphor. This comparative study sheds light on factors responsible for non-radiative relaxation processes, which is fundamental to the understanding of phosphor quantum efficiency and performance. The temperature dependence of the Stokes to anti-Stokes vibronic line intensity ratios is also measured and analyzed. Due to low thermal conductivity of K2SiF6, it was necessary to develop protocols to ensure that the surface temperature of the phosphor was close to that of the cold finger in the closed cycle He cryostat.

Near-Infrared II Hemicyanine Dye with Large Stokes Shift Designed by TICT Regulation for Boosting Imaging-Guided Photothermal Therapy.

The serious threat that cancer poses to human health highlights the significance of early detection and effective treatment. The integration of fluorescence diagnosis and photothermal therapy in NIR-II has gained attention due to its high sensitivity, fast response, and noninvasiveness. Fluorescence, produced by the radiative relaxation process of electrons in a molecule, and photothermal, generated by the nonradiative relaxation process of electrons in a molecule, are competing photophysical processes. Hence, it is a challenge for the molecule to balance between the properties of fluorescence and photothermal. In this study, a NIR-II hemicyanine with TICT character is designed to obtain molecules with both better fluorescence and photothermal properties, utilizing positively charged pyridine salt and triphenylamine as electron acceptor and donor, respectively, and oxole as the conjugated π-bridge. HCY-995, one of the synthesized compounds, has a quantum yield of 0.09%, photothermal conversion efficiency of 54.90%, and a significant Stoke shift of 232nm, which makes it appropriate for the integration of photothermal therapy and high-resolution imaging. This study provides new insights into the development of NIR-II molecules with fluorescent and photothermal integrated properties.

Theoretical spectroscopy for unraveling the intensity mechanism of the optical and photoluminescent spectra of chiral Re(I) transition metal complexes.

In this work, we present a computational study that is able to predict the optical absorption and photoluminescent properties of the chiral Re(I) family of complexes [fac-ReX(CO)3L], where X is either Cl or I and L is N-heterocyclic carbene extended with π-conjugated [5]-helicenic unit. The computational strategy is based on carefully calibrated time dependent density functional theory calculations and operates in conjunction with an excited state dynamics approach to treat in addition to absorption (ABS) and photoluminescence (PL), electronic circular dichroism (ECD), and circularly polarized luminescence (CPL) spectroscopies, respectively. The employed computational approach provides, an addition, access to the computation of phosphorescence rates in terms of radiative and non-radiative relaxation processes. The chosen molecules consist of representative examples of non-helicenic (NHC) and helicenic diastereomers. The agreement between theoretical and experimental spectra, including absorption (ABS, ECD) and emission (PL, CPL), is excellent, validating a quantitative interpretation of the spectral features on the basis of natural transition orbitals and TheoDore analyses. It is demonstrated that across the set of studied Re(I) diastereomers, the emission process in the case of NHC diastereomers is metal to ligand charge transfer in nature and is dominated by the easy-axis anisotropy of the emissive excited multiplet. On the contrary, in the cases of the helicenic diastereomers, the emission process is intra ligand charge transfer in nature and is dominated by the respective easy-plane anisotropy of the emissive excited multiplet. This affects remarkably the photoluminescent properties of the molecules in terms of PL and CPL spectral band shapes, spin-vibronic coupling, relaxation times, and the respective quantum yields. Spin-vibronic coupling effects are investigated at the level of the state-average complete active space self-consistent field in conjunction with quasi-degenerate second order perturbation theory. It is in fact demonstrated that a spin-vibronic coupling mechanism controls the observed photophysics of this class of Re(I) complexes.

Photoluminescence enhancement of ZrO2:Dy3+ Nanocrystals adding Li+ ions with controlled N2 atmosphere synthetized by coprecipitation method

A series of Dy3+ (0–1 mol%) and Li+ (0.05–1.5 mol%) co-doped zirconium oxide (ZrO2) nanopowders are prepared via coprecipitation, followed by a hydrothermal treatment in nitrogen atmosphere. It was found an optimal concentration of Dy-doped ZrO2 (0.5 mol %) with high emission, showing peaks at 490 nm (blue) and 577 nm (yellow) corresponding to 4F9/2 → 6H15/2, and 4F9/2 → 6H13/2 transitions, respectively with excitation at 350 nm wavelength. Furthermore, the samples of ZrO2:Dy3+ annealing with a controlled N2 showed an increase of 73 and 83% in blue and yellow bands due to decrease of hydroxyl groups (–OH), according to infrared (IR) spectroscopy, which helps to reduce the non-radiative relaxation process. In addition, incorporation Li+ ions up to 0.25 mol% into ZrO2:Dy3+ nanopowders exhibited a further enhancement emission intensity of both bands, owing to this monovalent cation act as coactivator and charge compensator. On the other hand, the color coordinates (x, y) and correlated color temperature (CCT) results suggests that the synthesized nanophosphor can be used as a yellow-orange component for the fabrication of near UV-based w-LEDs.

Design of Near-Infrared-Triggered Metallo-Photosensitizers via a Self-Assembly-Induced Vibronic Decoupling Strategy.

Development of highly efficient near-infrared (NIR)-excited photosensitizers is hampered by the fast nonradiative vibrational relaxation process regulated by the energy gap law. Here, from the fundamental perspective we propose that the intermolecular coupling of well-designed photosensitizers has the potential to facilitate exciton delocalization and hence reduce the exciton-vibration coupling, thereby boosting their phototherapeutic efficacy via inhibition of the vibrational relaxation pathway. Such conceived NIR-excited metallo-photosensitizers (IrHA1 and IrHA2) were prepared and studied for experimental validation. The resulting iridium complexes exhibited a little singlet oxygen (1O2) production in the monomeric state, but produced substantially enhanced 1O2 generation efficiency via benefiting from the exciton-vibration decoupling in the self-assembly state. Particularly, IrHA2 exhibits an unprecedented high 1O2 quantum yield of 54.9% (FDA-approved NIR dye indocyanine green: ΦΔ = 0.2%) under 808 nm laser irradiation with negligible heat generation, probably attributed to the suppression of vibronic couplings from the stretching mode of the acceptor ligand. In phototherapy, IrHA2-NPs with high biocompatibility and low dark toxicity can induce substantial tumor regression with 92.9% tumor volume reduction in vivo. This self-assembly-induced vibronic decoupling strategy would offer an effective approach to the design of high-performance NIR-excited photosensitizers.

Ratiometric nanothermometer CaSc2O4: Nd3+ operating in biological window for deep-tissue photothermal therapy

Accurate and real-time temperature detection is an urgent requirement in photothermal therapy (PTT). In this work, the CaSc2O4: Nd3+ nanorod with an average length of 150 nm has been determined to be an outstanding optical thermometer based on the fluorescence intensity ratio (FIR) of thermally coupled Stark sublevels of Nd3+: 4F3/2 → 4I11/2 transition, of which the excitation and emission wavelength are located in the first and second biological window respectively, resulting in an 8 mm penetration depth in the biological tissues. The maximal absolute sensitivity and relative sensitivity as well as the minimal temperature resolution of the present sample is 0.0008 K-1, 0.0018 K-1 and 0.19 K respectively, which is superior to the vast majority of the same type thermometers. Besides that, the nanorods show a 7 K temperature increment after 180 s’ 808 nm-laser radiation, revealing its photothermal conversion capacity thanks to the considerable nonradiative relaxation processes among the metastable energy levels of Nd3+. These results indicate that Nd3+ single-doped CaSc2O4 nanorods can be utilized as the optical thermometer in the deep-tissue PTT process for real-time thermometry along with the function of light-to-heat conversion.

Switching between Fluorescence and Room Temperature Phosphorescence in Carbon Dots: Key Role of Heteroatom Functionalities

Herein, we have synthesized three different types of carbon dots, i.e., (a) CDs from citric acid, (b) nitrogen atom functionalized CDs from citric acid and ammonia, and (c) phosphorus atom functionalized CDs from citric acid and sodium dihydrogen orthophosphate through a simple bottom-up carbonization technique. Detailed morphological and elemental features are investigated by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy study, and it is further correlated to the ongoing photophysical properties. To investigate the specific role of the heteroatom functionalities on the room temperature phosphorescence, we have incorporated all these three types of CDs in a boric acid matrix to diminish the flexibility of surface functional groups and decrease the nonradiative relaxation processes. Various heteroatom functionalities play a very specific role to tune the afterglow properties by altering the energy gap (ΔEST) between the lowest excited singlet (S1) and triplet state (T1) and the spin–orbit coupling constant which eventually control the radiative recombination from the triplet state. Finally, a switchable fluorescence and room temperature phosphorescence have been observed depending on the specific heteroatom functionalities. A detailed temperature-dependent study has been performed to investigate the tunability between prompt fluorescence and phosphorescence properties. This is further correlated to the conversion of phosphorescence with thermally activated delayed fluorescence (TADF). Computational studies based on time-dependent density functional theory (TD-DFT) have been performed by using optimized model systems in connection to the elemental study, which nicely support our experimental findings.

Non-adiabatic dynamics simulations of the S1 excited-state relaxation of diacetyl phenylenediamine.

The small molecule built around the benzene ring, diacetyl phenylenediamine (DAPA), has attracted much attention due to its synthesis accessibility, large Stokes shift, etc. However, its meta structure m-DAPA does not fluoresce. In a previous investigation, it was found that such a property is due to the fact that it undergoes an energy-reasonable double proton transfer conical intersection during the deactivation of the S1 excited-state, then returns to the ground state by a nonradiative relaxation process eventually. However, our static electronic structure calculations and non-adiabatic dynamics analysis results indicate that only one reasonable non-adiabatic deactivation channel exists: after being excited to the S1 state, m-DAPA undergoes an ultrafast and barrierless ESIPT process and reaches the single-proton-transfer conical intersection. Subsequently, the system either returns to the keto-form S0 state minimum with proton reversion or returns to the single-proton-transfer S0 minimum after undergoing a slight twist of the acetyl group. The dynamics results show that the S1 excited-state lifetime of m-DAPA is 139 fs. In other words, we propose an efficient single-proton-transfer non-adiabatic deactivation channel of m-DAPA that is different from previous work, which can provide important mechanistic information of similar fluorescent materials.

Upconversion luminescence and effect of pump power on optical thermometry of Yb3+/Er3+ co-doped YOF self-crystallization glass ceramics

The optical thermometry based on the fluorescence intensity ratio (FIR) method overcomes many disadvantages of traditional contact temperature measurement, and appropriate host materials can improve the temperature sensing sensitivity. Herein, Yb3+/Er3+ co-doped YOF glass ceramics (GC) were prepared, and the luminescence and temperature sensing properties were researched under 980 nm excitation with different pump power. The FIR of thermal couple energy level (TCEL) and non-thermal couple energy level (NTCEL) was used to evaluate the temperature sensing sensitivity. The higher pump power results in a decrease in relative sensitivity (SR) due to the thermal effect and increased rate of energy transfer and non-radiative relaxation process between dopants and host. The maximum relative sensitivity of this sample reaches 1.59 × 10−2 K−1 at 310 K with a pump power of 0.2 W, which shows that this GC is a prospective application in optical thermometry.

Size-controlled synthesis and red up-converted luminescence of CeO2:Yb3+/Er3+ nanospheres

In this work, cubic-phased CeO2:18 mol%Yb3+/2 mol%Er3+ nanospheres were prepared by coprecipitation method, in which the size of nanosphere was precisely controlled by regulating the amount of co-precipitator urea. The morphology evolution of CeO2:18%Yb3+/2%Er3+ samples that vary from nano-sphere to nano-flower with extending the reaction time was elaborately investigated via increasing the amount of urea and increasing the reaction temperature. The influence of non-radiative relaxation (NR) processes and surface quenching on up-conversion emission output of CeO2:18%Yb3+/2%Er3+ was investigated through varying the sizes and environmental temperatures. Tissue imaging experiment demonstrates that CeO2:18%Yb3+/2%Er3+ nanospheres have the potential to act as luminescent imaging reagents in tissue imaging.

Simulations of nonradiative processes in semiconductor nanocrystals

The description of carrier dynamics in spatially confined semiconductor nanocrystals (NCs), which have enhanced electron-hole and exciton-phonon interactions, is a great challenge for modern computational science. These NCs typically contain thousands of atoms and tens of thousands of valence electrons with discrete spectra at low excitation energies, similar to atoms and molecules, that converge to the continuum bulk limit at higher energies. Computational methods developed for molecules are limited to very small nanoclusters, and methods for bulk systems with periodic boundary conditions are not suitable due to the lack of translational symmetry in NCs. This perspective focuses on our recent efforts in developing a unified atomistic model based on the semiempirical pseudopotential approach, which is parameterized by first-principle calculations and validated against experimental measurements, to describe two of the main nonradiative relaxation processes of quantum confined excitons: exciton cooling and Auger recombination. We focus on the description of both electron-hole and exciton-phonon interactions in our approach and discuss the role of size, shape, and interfacing on the electronic properties and dynamics for II-VI and III-V semiconductor NCs.

Modulation of Thermally Stable Photoluminescence in Cr3+-Based Near-Infrared Phosphors

Broadband near-infrared (NIR) light sources based on phosphor-converted light-emitting diodes (pc-LEDs) are desirable for various photonics applications, while developing thermally stable NIR phosphors remains a great challenge. Increasing the temperature accelerates the severe nonradiative relaxation process gorverned by the intrinsic energy gap law, which further suspends the efficient low-energy emission of Cr3+ emitters in the inorganic lattice. To address this rule, several state-of-the-art strategies have been put forward in this perspective to modulate the critical law from the viewpoints of (1) crystal structure design, (2) defect engineering, (3) strengthened rigidity, and (4) energy transfer. This perspective suggests avenues for exploring novel broadband NIR phosphors with high thermal stability and will also stimulate further studies on NIR spectroscopy for high-power applications.

Synthesis and macrocyclization-induced emission enhancement of benzothiadiazole-based macrocycle

We presented an effective and universal strategy for the improvement of luminophore’s solid-state emission, i.e., macrocyclization-induced emission enhancement (MIEE), by linking luminophores through C(sp3) bridges to give a macrocycle. Benzothiadiazole-based macrocycle (BT-LC) has been synthesized by a one-step condensation of the monomer 4,7-bis(2,4-dimethoxyphenyl)−2,1,3-benzothiadiazole (BT-M) with paraformaldehyde, catalyzed by Lewis acid. In comparison with the monomer, macrocycle BT-LC produces much more intense fluorescence in the solid state (ΦPL = 99%) and exhibits better device performance in the application of OLEDs. Single-crystal analysis and theoretical simulations reveal that the monomer can return to the ground state through a minimum energy crossing point (MECPS1/S0), resulting in the decrease of fluorescence efficiency. For the macrocycle, its inherent structural rigidity prohibits this non-radiative relaxation process and promotes the radiative relaxation, therefore emitting intense fluorescence. More significantly, MIEE strategy has good universality that several macrocycles with different luminophores also display emission improvement.

An Algorithmic Approach to Compute the Effect of Non-Radiative Relaxation Processes in Photoacoustic Spectroscopy

Successful transfer of photoacoustic gas sensors from laboratory to real-life applications requires knowledge about potential cross-sensitivities towards environmental and gas matrix changes. Multi-dimensional calibration in case of cross-sensitivities can become very complex or even unfeasible. To address this challenge, we present a novel algorithm to compute the collision based non-radiative efficiency and phase lag of energy relaxation on a molecular level (CoNRad) for photoacoustic signal calculation. This algorithmic approach allows to calculate the entire relaxation cascade of arbitrarily complex systems, yielding a theoretical photoacoustic signal. In this work the influence of varying bulk compositions, i.e. nitrogen (N2), oxygen (O2) and water (H2O) on the photoacoustic signal during methane (CH4) detection is demonstrated. The applicability of the algorithm to other photoacoustic setups is shown exemplary by applying it to the relaxational system investigated in [1]. Hayden et al. examined the effect of water on photoacoustic carbon monoxide (CO) detection.