Relaxation Channels Research Articles

Relaxation Channels of Two Types of Hot Carriers in Gold Nanostructures.

A fundamental understanding of hot carrier relaxation in metal nanostructures is essential for realizing their application potential in energy conversion and photocatalysis. Despite previous investigations of the relaxation of hot carriers generated by surface plasmon resonance (SPR) excitation and interband transitions (IBTs), the hot carrier relaxation lifetimes and their associated mechanisms remain unclear. Herein, we demonstrate two distinct hot carrier relaxation channels in gold plasmonic nanostructures. The experimental observations reveal that the hot carrier relaxation is faster following SPR excitation than that from IBTs in gold nanoparticles and nanorods. The experimental results and theoretical calculations indicate that the numerous plasmon-induced hot carriers undergo surface-mediated carrier-carrier scattering in large gold nanostructures, whereas almost all IBT-induced hot carriers experience bulk carrier-carrier scattering. These findings advance our understanding of hot carrier relaxation and contribute to a clearer microscopic description of scattering channels in plasmonic nanostructures.

Thermal Quenching of Intrinsic Photoluminescence in Amorphous and Monoclinic HfO2 Nanotubes.

Nanotubular hafnia arrays hold significant promise for advanced opto- and nanoelectronic applications. However, the known studies concern mostly the luminescent properties of doped HfO2-based nanostructures, while the optical properties of nominally pure hafnia with optically active centers of intrinsic origin are far from being sufficiently investigated. In this work, for the first time we have conducted research on the wide-range temperature effects in the photoluminescence processes of anion-defective hafnia nanotubes with an amorphous and monoclinic structure, synthesized by the electrochemical oxidation method. It is shown that the spectral parameters, such as the position of the maximum and half-width of the band, remain almost unchanged in the range of 7-296 K. The experimental data obtained for the photoluminescence temperature quenching are quantitatively analyzed under the assumption made for two independent channels of non-radiative relaxation of excitations with calculating the appropriate energies of activation barriers-9 and 39 meV for amorphous hafnia nanotubes, 15 and 141 meV for monoclinic ones. The similar temperature behavior of photoluminescence spectra indicates close values of short-range order parameters in the local atomic surrounding of the active emission centers in hafnium dioxide with amorphous and monoclinic structure. Anion vacancies VO- and VO2- appeared in the positions of three-coordinated oxygen and could be the main contributors to the spectral features of emission response and observed thermally stimulated processes. The recognized and clarified mechanisms occurring during thermal quenching of photoluminescence could be useful for the development of light-emitting devices and thermo-optical sensors with functional media based on oxygen-deficient hafnia nanotubes.

Multifunctional cyanoaryl porphyrazine pigments with push-pull structure of macrocycle framing: Photophysics and possible applications

We report the experimental study of fluorescence properties of three porphyrazine derivatives, distinct in the number of fused aromatic rings in aryl groups contained in the macrocycle frame and/or presence of Pd in the macrocycle, in solutions with biological molecules and in methanol-glycerol mixtures. Spectral and lifetime fluorescence analysis has revealed that all three pigments demonstrated two fluorescence transition bands from the first S1 and the second S2 electronic excited states to the ground state S0, and thus followed the anti-Kasha’s rule. Strong dependence of the quantum yield and lifetimes in the S1 → S0 fluorescence band on solution viscosity was observed. A significant increase of the fluorescence decay times and fluorescence intensity with albumin concentration and with presence of other biological molecules in solution was observed as well. A model of the low-laying energy states of porphyrazine derivatives has been developed for elucidation of the fluorescence properties observed, that took into account several radiative and non-radiative relaxation channels in molecular excited states. In particular, the model suggests that the first electronic excited state consists of planar and bent conformations separated by a potential energy barrier and shows a typical molecular-rotor behavior as a function of solution viscosity. Potential applications of the porphyrazine pigments as viscosity sensors and fluorescence probes were considered.

R‐NG●‐Regulated Electron‐Phonon Coupling to Enhance Hot‐Hole Injection for Inverted Perovskite Solar Cells

AbstractExtracting hot‐carriers before they relax back to the band edge will reduce thermal dissipation above the bandgap, which is one of the primary sources of efficiency loss in perovskite solar cells (PSCs). It requires slow cooling of hot‐carriers together with the efficient extraction of hot‐carriers. Here, a novel interface engineering is demonstrated by embedding radical‐containing nanographene (r‐NG●) between perovskite and poly[bis(4‐phenyl)(2,4‐dimethoxyphenyl)‐amine] (PTOAA) to harvest this excess energy. This strategy accelerates the extraction rate of the hot‐hole (3 times that of control at 400 K) by augmenting the relaxation channel, which is related to the “quasi‐SOMO” energy state of r‐NG●. Nonadiabatic molecular dynamics calculation further revealed that radical character promotes the rotation of the cation in perovskite, which contributes to generating strong nonadiabatic couplings and multiple phonon modes. In addition, the band bending effect of buried perovskite and improved conductivity of PTOAA also facilitate exciton dissociation and hole collection efficiency (97.5%). Consequently, the resultant r‐NG● decorated PSCs delivered an impressive efficiency of 24.20%, along with better thermal and humidity stability. Moreover, it can maintain 93% of its initial efficiency after 300 hours of illumination.

Effect of Microhydration in Tuning the Photophysical Behavior of a Luminescent DNA Probe Revealed by Non‐Adiabatic Dynamics

AbstractWe report non‐adiabatic dynamics, performed in the surface hopping formalism, of 2‐thienyl‐3‐hydroxychromone, an environment‐dependent luminescent organic DNA probe. In particular we have shown that the first shell solvent water molecules undergo a rather complex reorganization upon light excitation. This involves also the triggering of a water‐mediated proton transfer process which leads to the formation of the tautomeric structure. The presence of this solvent‐mediated transfer mechanism globally diminishes the intersystem crossing efficiency, and hence the population of the triplet state manifold, as compared to the non‐solvated systems. Our results also point out the non‐innocent role of solvent networks in tuning complex photophysical processes, while opening competitive relaxation channels.

Chirality-Dependent Dynamic Evolution for Trions in Monolayer WS2.

Monolayer transition metal dichalcogenides exhibit valley-dependent excitonic characters with a large binding energy, acting as the building block for future optoelectronic functionalities. Herein, combined with pump-probe ultrafast transient transmission spectroscopy and theoretical simulations, we reveal the chirality-dependent trion dynamics in h-BN encapsulated monolayer tungsten disulfide. By resonantly pumping trions in a single valley and monitoring their temporal evolution, we identify the temperature-dependent competition between two relaxation channels driven by chirality-dependent scattering processes. At room temperature, the phonon-assisted upconversion process predominates, converting excited trions to excitons within the same valley on a sub-picosecond (ps) time scale. As temperature decreases, this process becomes less efficient, while alternative channels, notably valley depolarization process for trions, assume importance, leading to an increase of trion density in the unpumped valley within a ps time scale. Our time-resolved valley-contrast results provide a comprehensive insight into trion dynamics in 2D materials, thereby advancing the development of novel valleytronic devices.

Mean-field analysis of the glassy dynamics of an elastoplastic model of super-cooled liquids

We present a mean-field theory of a coarse-grained model of a super-cooled liquid in which relaxation occurs via local plastic rearrangements. Local relaxation can be induced by thermal fluctuations or by the long-range elastic consequences of other rearrangements. We extract the temperature dependence of both the relaxation time and the length scale of dynamical correlations. We find two dynamical regimes. First, a regime in which the characteristic time and length scales diverge as a power law at a critical temperature T c . This regime is found by an approximation that neglects activated relaxation channels, which can be interpreted as akin to the one found by the mode-coupling transition of glasses. In reality, only a crossover takes place at T c . The residual plastic activity leads to a second regime characterised by an Arrhenius law below T c . In this case, we show that the length scale governing dynamical correlations diverges as a power law as , and is logarithmically related to the relaxation time.

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.

Photoionization of Nitrile-substituted Naphthalene and Benzene: Cation Spectroscopy, Photostability, and Implications for Photoelectric Gas Heating

We investigate the vacuum ultraviolet (VUV) photodynamics of gas phase 1- and 2-cyanonaphthalene and cyanobenzene, recently detected in the Taurus molecular cloud, by combining synchrotron radiation and a double imaging electron/ion coincidence setup. The high-resolution threshold photoelectron spectra (TPES) of all three molecules are obtained experimentally from which the adiabatic ionization energies are reported with very high accuracy, particularly for 2-cyanonaphthalene, for which no data exist at this level of precision. Theoretical calculations are performed to compare with the TPES for the ground electronic state of the cations. Furthermore, the different features observed in the extended TPES have been assigned to the different molecular orbitals with the help of the outer valence Green's function calculations. The present experiments also shed light on the kinetic energy distribution of the photoelectrons as a function of the incident photon energy, to describe their contribution to the photoelectric heating effect in the interstellar medium. In this context, we show how kinetic energy distributions can be obtained from our data for any given photon energy, such as the omnipresent Lyα line, or any given interstellar radiation field (ISRF). In addition, from the total ion yields, we estimate the photorates for a few ISRFs. Finally, we discuss the photodissociation of the two cyanonaphthalenes, quoting the activation energies of the dissociation channels with the help of Rice–Ramsperger–Kassel–Marcus modeling. It is observed that CN substitution does not cause any appreciable change to the VUV dissociative photoionization relaxation channel.

Ultrafast exciton trapping dynamics in oxygen-functionalized single-walled carbon nanotubes

Artificial zero-dimensional quantum defects within single-walled carbon nanotubes hold immense promise for diverse optoelectronic applications such as bioimaging, near-infrared light emission, nanolasing, and single-photon generation. This study delves into the temporal behavior of excitons within oxygen-functionalized single-walled carbon nanotubes, employing ultrafast transient absorption spectroscopy. Our investigation unveils the emergence of functionalization-induced excitonic states, distinguished by a long-lasting induced transmittance signal. Remarkably, even at low degree of functionalization, we observe a reduction in the lifetime of band-edge excitons. In contrast, the population dynamics of deep-band excitons exhibit resilience against low and moderate functionalization degrees, with a pronounced reduction manifesting only at high degrees of functionalization. Our findings align closely with a kinetic model that accounts for an additional relaxation channel prompted by functionalization. This empirical evidence provides a significant breakthrough, establishing that the formation of functionalization-induced excitonic states within oxygen-functionalized single-walled carbon nanotubes is a consequence of the diffusive trapping of free band edge excitons, while interaction of deep band excitons with oxygen defect sites rather yields in exciton recombination. These insights shed light on the intricate dynamics of excitons within single-walled carbon nanotube with artificially embedded quantum defects, advancing our understanding and potential applications in the realm of optoelectronics.

Molecular Engineering and Confinement Effect Powered Ultrabright Nanoparticles for Improving Sensitivity of Lateral Flow Immunoassay.

The application of traditional lateral flow immunoassay (LFIA)-based gold nanoparticles (AuNPs) to measure traces of target chemicals is usually challenging. In this study, we developed an integrated strategy based on molecular engineering and the spatial confinement of nanoparticles (NPs) to obtain ultrahigh quantum yields (QYs) of aggregation-induced emission (AIE) fluorescence NPs and employed them for the highly sensitive detection of T-2 toxin on the LFIA platform. Tetraethyl-4,4',4″,4‴-(ethene-1,1,2,2-tetrayl)tetrabenzoate (TCPEME), an AIE luminogen, was designed using molecular engineering to lower the energy gap, achieving higher QYs (26.26%) than previous AIEgens (13.02%). Subsequently, TCPEME-doped fluorescence NPs (TFNPs) achieved ultrahigh QYs, up to 84.55%, which were generated from the strong restriction of the NP state, efficiently suppressing nonradiative relaxation channels verified by ultrafast electron dynamics. On the LFIA platform, the sensitivity of the designed TFNP-based LFIA (TFNP-LFIA) was 10.4-fold and 4.3-fold more sensitive than that of the AuNP-LFIA and TPENP-LFIA for detecting the T-2 toxin, respectively. In addition, TFNP-LFIA was used for detecting T-2 toxin in samples and showed satisfactory recoveries (79.5 to 122.0%) with CV (1.49 to 11.75%), which implied excellent application potential for TFNP-LFIA. Overall, dual improvement of the molecule in fluorescence performance originating from the molecular engineering and spatial confinement of NPs could be an efficient tool for promoting the development of high-performance reporters in LFIA.

Universal relaxation of turbulent binary fluids

Turbulent relaxation is the process of turbulent systems reaching the state of equilibrium, starting upon quenching the turbulence forcing acting on them. Such relaxation in binary fluids is instrumental for both fundamental science understanding and industrial applications, although potential differences in the relaxation of bulk and interface are still to be identified. Using direct numerical simulations of Cahn-Hilliard-Navier-Stokes equations, here we show that the bulk and the interface relax towards different states. However, both the relaxation channels can be accounted for via a universal pathway based on the recently proposed principle of vanishing nonlinear transfers. We find that the bulk of each fluid relaxes differently from the turbulent relaxation of a single hydrodynamic fluid. At the same time, the interface relaxes towards a Helmholtz-like pressure-balanced state. The present methodology can be directly applied to predict the turbulent relaxed states in active binary mixtures as well as other complex fluid systems.

Large Magnetic Anisotropy in Mono- and Binuclear cobalt(II) Complexes: The Role of the Distortion of the Coordination Sphere in Validity of the Spin-Hamiltonian Formalism.

To get a better insight into understanding the factors affecting the enhancement of the magnetic anisotropy in single molecule (single ion) magnets, two cobalt(II) complexes based on a tridentate ligand 2,6-di(thiazol-2-yl)pyridine substituted at the 4-position with N-methyl-pyrrol-2-yl have been synthesized and studied by X-ray crystallography, AC and DC magnetic data, FIRMS and HFEPR spectra, and theoretical calculations. The change of the counteranion in starting Co(II) salts results in the formation of pentacoordinated mononuclear [Co(mpyr-dtpy)Cl2]·2MeCN (1) complex and binuclear [Co(mpyr-dtpy)2][Co(NCS)4] (2) compound. The observed marked distortion of trigonal bipyramid geometry in 1 and cationic octahedral and anionic tetrahedral units in 2 brings up a question about the validity of the spin-Hamiltonian formalism and the possibility of determining the value and sign of the zero-field splitting D parameter. Both complexes exhibit field-induced slow magnetic relaxation with two or three relaxation channels at BDC = 0.3 T. The high-frequency relaxation time in the reciprocal form τ(HF)-1 = CTn develops according to the Raman relaxation mechanism (for 2, n = 8.8) and the phonon-bottleneck-like mechanism (for 1, n = 2.3). The high-frequency relaxation time at T = 2.0 K and BDC = 0.30 T is τ(HF) = 96 and 47 μs for 1 and 2, respectively.

Excited-state dynamics of 3-hydroxychromone in gas phase.

In recent years, 3-hydroxychromone (3-HC) and its derivatives have attracted much interest for their applications as molecular photoswitches and fluorescent probes. A clear understanding of their excited-state dynamics is essential for their applications and further development of new functional 3-HC derivatives. However, the deactivation mechanism of the photoexcited 3-HC family is still puzzling as their spectral properties are sensitive to the surrounding medium and substituents. The excited-state relaxation channels of 3-HC have been a matter of intense debate. In the current work, we thoroughly investigated the excited-state decay process of the 3-HC system in the gas phase using high-level electronic structure calculations and on-the-fly excited-state dynamic simulations intending to provide insight into the intrinsic photochemical properties of the 3-HC system. A new deactivation mechanism is proposed in the gas phase, which is different from that in solvents. The excited-state intramolecular proton transfer (ESIPT) process that occurs in solutions is not preferred in the gas phase due to the existence of a sizable energy barrier (∼0.8 eV), and thus, no dual fluorescence is found. On the contrary, the non-radiative decay process is the dominant decay channel, which is driven by photoisomerization combined with ring-puckering and ring-opening processes. The results coincide with the observations of an experiment performed in a supersonic jet by Itoh (M. Itoh, Pure Appl. Chem., 1993, 65(8), 1629-1634). The current work indicates that the solution environment plays an important role in regulating the excited-state dynamic behaviour of the 3-HC system. This study thus provides theoretical guidance for the rational design and improvement of the photochemical properties of the 3-HC system and paves the way for further investigation into its photochemical properties in complex environments.

Effects of quantum noise on Shor’s algorithm

Shor’s quantum factoring algorithm (Shor’s algorithm) can solve factorization problem of large integers by using a fully-operational quantum computer with the complexity of polynomial-time level, thereby cracking a series of encryption algorithms (such as Rivest-Shamir-Adleman encryption algorithm, and Diffie-Hellman key exchange protocol) whose security is guaranteed by factorizing large integers, which is a difficult problem. We are currently in a noisy intermediate-scale quantum era, which means that we can only operate on quantum computers with a limited number of qubits and we have to take care of the effects of quantum noise. Quantum states on a quantum computer are prone to quantum noise caused by low-fidelity gates or interactions between qubits and the environment, which results in inaccurate measurements. We study the influence of quantum noise on Shor’s algorithm through 3 typical quantum noise channels: the depolarizing channel, the state preparation and measurement channel, and the thermal relaxation channel. We successfully simulate the factorization of the numbers 15, 21, and 35 into their corresponding prime factors by using the quantum circuit we have constructed on a classical computer. Then we simulate a running quantum circuit of Shor’s algorithm in a noisy environment with different level of noise for a certain type of noise channel and present numerical results. We can obtain precise measurements by calculating the state vector prior to measurement, instead of simulating and measuring expending much time, which contributes to higher efficiency. Each experiment is repeated 1000 times to reduce discrepancy. Our research indicates that Shor’s algorithm is easily affected by quantum noise. Successful rate of Shor’s algorithm decreases exponentially with the increase of noise level in the depolarizing channel, where the successful rate is an indicator we propose in this research to quantify the influence of noise on Shor’s algorithm, meanwhile the noise in the state preparation and measurement channel and the thermal relaxation channel can linearly affect the successful rate of Shor’s algorithm. There are <inline-formula><tex-math id="M1">\begin{document}$O(n^4) $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231414_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231414_M1.png"/></alternatives></inline-formula> quantum gates in the circuit, each of which is disrupted by noise in depolarizing channel during running the circuit, meanwhile there are only <i>O</i>(<i>n</i>) interruptions caused by noise in state preparation and measurement channel since we repeat the measurements only <i>O</i>(<i>n</i>) times in the circuit where <i>n</i> is the number of bits of the integer about to be factored. Linear relationship in thermal relaxation channel is mainly due to the large gap between quantum gate time and relaxation time even if each gate in the circuit is disrupted by noise in thermal relaxation channel such as depolarizing channel. The present research results can be used for correcting the subsequent errors, improving Shor’s algorithm, and providing guidance for the fidelity required in engineering implementation of Shor’s algorithm.

Theoretical insight into photodeactivation mechanisms of adenine-uracil and adenine-thymine nucleobase pairs.

In this work, several plausible intra- and intermolecular photoinduced processes of the Watson-Crick base pairs of adenine with uracil (A-U) or thymine (A-T) according to the results of spin component scaling variant of algebraic diagrammatic construction up to the second order [SCS-ADC(2)] calculations are discussed. Although widely explored, these systems lack complete characterization of possible intramolecular relaxation channels perturbed by intermolecular interactions. In particular, we address the still open debate on photodeactivation via purine-ring puckering at the C2 or C6-atom position of adenine. We also show that the presence of low-lying, long-lived 1nπ* states can be a significant factor in hindering relaxation via an electron-driven proton transfer process, as the population of these states can lead to an efficient intersystem crossing to a triplet manifold, the estimated rate of which is 1.6 × 1010 s-1 which exceeds the corresponding internal conversion to the ground state by an order of magnitude. Additionally, the SCS variant of the ADC(2) method is shown to provide a more balanced description of valence and charge-transfer excited states.

Tailoring Broadband Nonlinear Optical Characteristics and Ultrafast Photocarrier Dynamics of Bi2O2S Nanosheets by Defect Engineering.

Low-dimensional bismuth oxychalcogenides have shown promising potential in optoelectronics due to their high stability, photoresponse, and carrier mobility. However, the relevant studies on deep understanding for Bi2O2S is quite limited. Here, comprehensive experimental and computational investigations are conducted in the regulated band structure, nonlinear optical (NLO) characteristics, and carrier dynamics of Bi2O2S nanosheets via defect engineering, taking O vacancy (OV) and substitutional Se doping as examples. As the OV continuously increased to ≈35%, the optical bandgaps progressively narrow from ≈1.21 to ≈0.81eV and NLO wavelengths are extended to near-infrared regions with enhanced saturable absorption. Simultaneously, the relaxation processes are effectively accelerated from tens of picoseconds to several picoseconds, as the generated defect energy levels can serve as both additional absorption cross-sections and fast relaxation channels supported by theoretical calculations. Furthermore, substitutional Se doping in Bi2O2S nanosheets also modulate their optical properties with the similar trends. As a proof-of-concept, passively mode-locked pulsed lasers in the ≈1.0µm based on the defect-rich samples (≈35% OV and ≈50% Se-doping) exhibit excellent performance. This work deepens the insight of defect functions on optical properties of Bi2O2S nanosheets and provides new avenues for designing advanced photonic devices based on low-dimensional bismuth oxychalcogenides.

Temporal evolution of laser-induced ionization and recombination processes in argon-helium mixture

Preparation of metastable atoms (1s5) through laser-induced preionization holds the potential to mitigate the electromagnetic interference (EMI) issues associated with the large volume, atmospheric pressure discharge of traditional optically pumped rare-gas metastable laser (OPRGL). In this work, we conducted experimental investigations into the temporal evolution of the Ar 763.5 nm (2p6→1s5) spectral line in Ar-He mixture. These experiments unveiled the intricate interaction mechanism involving the laser, Ar atoms, He atoms, and free electrons within the laser-induced plasma. Our findings highlight the dual contributions of the multiphoton ionization and the inverse bremsstrahlung process to the initial plasma formation. Notably, the time-resolved atomic emission spectrum at 763.5 nm reveals two distinct regimes, namely Regime1 and Regime2. Regime1 primarily arises from the “excitation + radiation + collisional relaxation” process, wherein excited states Ar atoms, populated via multiphoton excitation and electron impact excitation, accumulate on the 2p6 level. Conversely, Regime2 is predominantly a result of the “ion-electron recombination” process. In this regime, highly excited states Ar atoms are generated through the recombination of ion and electron, subsequently populating the 2p6 level through a combination of radiation and collisional relaxation channels. The differences in the temporal evolution between 763.5 nm and 811.5 nm spectral lines can be attributed to the distinct radiation and collisional relaxation channels in the two aforementioned processes.

Biaxial strain tuning of exciton energy and polarization in monolayer WS2

We perform micro-photoluminescence and Raman experiments to examine the impact of biaxial tensile strain on the optical properties of WS2 monolayers. A strong shift on the order of −130 meV per % of strain is observed in the neutral exciton emission at room temperature. Under near-resonant excitation, we measure a monotonic decrease in the circular polarization degree under the applied strain. We experimentally separate the effect of the strain-induced energy detuning and evaluate the pure effect coming from the biaxial strain. The analysis shows that the suppression of the circular polarization degree under the biaxial strain is related to an interplay of energy and polarization relaxation channels as well as to variations in the exciton oscillator strength affecting the long-range exchange interaction.

Ultrafast many-body bright-dark exciton transition in anatase TiO2.

The momentum-forbidden dark excitons can have a pivotal role in quantum information processing, Bose-Einstein condensation, and light-energy harvesting. Anatase TiO2 with an indirect band gap is a prototypical platform to study bright to momentum-forbidden dark exciton transition. Here, we examine, by GW plus the real-time Bethe-Salpeter equation combined with the nonadiabatic molecular dynamics (GW + rtBSE-NAMD), the many-body transition that occurs within 100 fs from the optically excited bright to the strongly bound momentum-forbidden dark excitons in anatase TiO2. Comparing with the single-particle picture in which the exciton transition is considered to occur through electron-phonon scattering, within the GW + rtBSE-NAMD framework, the many-body electron-hole Coulomb interaction activates additional exciton relaxation channels to notably accelerate the exciton transition in competition with other radiative and nonradiative processes. The existence of dark excitons and ultrafast bright-dark exciton transitions sheds insights into applications of anatase TiO2 in optoelectronic devices and light-energy harvesting as well as the formation process of dark excitons in semiconductors.