Excited State Relaxation Research Articles

Unidirectional Photoisomerization of the Green Fluorescent Protein Chromophore in a Reversibly Photoswitchable Fluorescent Protein rsKiiro: Insights from Quantum Mechanics/Molecular Mechanics Simulations.

In this study, a quantum mechanics/molecular mechanics (QM/MM) framework combined with the CASPT2//CASSCF approach was used to investigate the excited-state decay and isomerization of the rsKiiro green fluorescent protein (GFP) from its neutral "OFF" trans state. Upon irradiation at 400 nm, the trans conformation is initially excited to the bright S1 state. A rapid decay of the excited state then occurs and ultimately leads the molecule to the ground state. Notably, the clockwise and counterclockwise rotations of the C8C9C11N12 [or C5C8C9C11] dihedral angle are asymmetric or unidirectional, with only one direction of rotation effectively driving the excited-state relaxation. This process is shaped by hydrogen-bonding networks and steric constraints within the protein. In addition, trans-cis isomerization may not occur directly in the S1 state because the energy of the S1 cis minimum is relatively higher than that of the S1 trans minimum. Instead, the S1 cis minimum may be generated through the reabsorption of light near 400 nm, as the vertical excitation energy of the S0 cis minimum is close to that of the S0 trans minimum. This work provides important insights into the early photodynamics of rsKiiro GFP and aids in the design of novel GFP-like fluorescent proteins.

Ultrafast Intersystem Crossing in Naturally Occurring Plant Pigments 5-Hydroxyflavones under Direct UV Excitation.

Flavonoids, a group of natural pigments, have attracted notable attention for their intrinsic fluorescent bioactive properties and potential therapeutic implications. Recent studies have suggested that the photoexcitation of specific flavonoids can also lead to the formation of triplet states, thereby potentially enhancing their applications in photoactivated antioxidant mechanisms. However, the crucial mechanism details about triplet state formation are still poorly understood. In this Letter, the ultrafast excited state relaxation mechanism for a series of 5-hydroxyflavone derivatives was studied by femtosecond time-resolved spectroscopy combined with quantum chemical calculations. Our results reveal the ultrafast ISC (kISC ≈ 1011 s-1) channel, which is sensitive to molecular structure and solvent environment, in 5-hydroxyflavones for the first time. Notably, the triplet excited state quantum yield of 4',7-dimethoxy-5-hydroxyflavone can reach up to 8% in acetonitrile solution. These results are essential for understanding the triplet state generation mechanism in 5-hydroxyflavone derivatives and could help the further development of 5-hydroxyflavone scaffold antioxidant agents.

Through-Space Charge Transfer Dynamic Mechanism in V-Shaped Flexible Carbazole Aromatic Imides Dyads.

Cofacial electron donor-acceptor dyads exhibiting through-space charge-transfer (TSCT) characteristics are widely employed in the development of optoelectronic functional materials. The flexible molecular frameworks between the electron donor and acceptor components allow dynamic modulation of electronic coupling, influenced by excited-state structural relaxation or intermolecular interactions, thereby affecting the charge-transfer (CT) dynamics and the emission properties of TSCT states. In this work, we examine the TSCT dynamic processes of two electron donor-acceptor dyads, CzPhNI and CzPhPI formed by ortho-substitution of phenyl linkage with V-shaped flexible TSCT structures using carbazole as donor and naphthalimide or phthalimide as acceptor. A pseudo-cofacial TSCT conformation formed in the excited state effectively shortens the donor-acceptor distance and enhances CT coupling. Femtosecond spectroscopy reveals an ultrafast TSCT kinetics in the (200 fs)-1 timescale. Moreover, the intermolecular interaction-induced D-A stacking further strengthens the electronic coupling, accelerating the TSCT reaction rate from (2.07 ps)-1 to (174 fs)-1 as solution concentrations increases from 10-5 to 10-3 M. The results obtained in this work offers valuable physical insights into the TSCT dynamic mechanism, potentially explaining the widely observed emission enhancement in weakly-coupled, flexible TSCT emitters transitioning from solution to aggregation states.

Impact of terminal group on temperature-dependent excited state relaxation in cationic dyes

Impact of terminal group on temperature-dependent excited state relaxation in cationic dyes

Ultrafast processes in Fe<sup>ɪɪɪ</sup> complexes with malic and succinic acids in aqueous solutions

Femtosecond pump-probe spectroscopy is used to reveal photophysical processes in Fe<sup>ɪɪɪ</sup> complexes with two dicarboxylic acids (malic and succinic) in aqueous solutions. A model of primary photoprocesses is proposed that includes ultrafast vibrational cooling, solvent-mediated Franck-Condon excited-state relaxation and subsequent internal conversion to the ground state, accompanied by the formation of a long-lived Fe<sup>ɪɪ</sup> radical complex.

Double configuration interaction singles: Scalable and size-intensive approach for orbital relaxation in excited states and bond-dissociation

We present a novel theoretical scheme for orbital relaxation in configuration interaction singles (CIS) based on a perturbative treatment of its electronic Hessian, whose analytical derivation is also established in this work. The proposed method, which can be interpreted as a “CIS-then-CIS” scheme, variationally accounts for orbital relaxation in excited states, thus significantly reducing the overestimation of charge-transfer excitation energies commonly associated with standard CIS. In addition, by incorporating de-excitation effects from CIS, we demonstrate that our approach effectively describes single bond dissociation. Notably, all these improvements are achieved at a mean-field cost, with the pre-factor further reduced with the efficient algorithm introduced here, while preserving the size-intensive property of CIS.

Changes in the Excited State Relaxation Pathway of Ruthenium(II) Bipyridine Complexes with Substituted Imidazo[4,5-f]-1,10-phenanthroline Ligands upon Additional Coordination of the Second Metal Cation

Changes in the Excited State Relaxation Pathway of Ruthenium(II) Bipyridine Complexes with Substituted Imidazo[4,5-f]-1,10-phenanthroline Ligands upon Additional Coordination of the Second Metal Cation

Enabling Ultrafast Intramolecular Singlet Fission in Perylene Diimide Tetramer with Saddle-Shaped Linker.

Intramolecular singlet fission (SF) in multichromophore systems is of high interest for photovoltaic application. As an attractive candidate for SF-based devices, enabling efficient SF in covalent oligomers of perylene diimide (PDI) still remains challenging. In this work, inter-PDI SF with τSF = ∼150 ps and ∼150% triplet yield in a covalent tetramer COTh-FPDI was facilitated by employing a saddle-shaped cyclooctatetrathiophene (COTh) core and fused linking with PDIs. In comparison, ultrafast symmetry-breaking charge separation (τCS = ∼100 fs) was observed for the tetramer COTh-αPDI with flexible linking. Taking advantage of rigid linking and minimized excited-state structural relaxation, unique inter-PDI geometry in COTh-FPDI can be fully defined by the topological characteristic of COTh, which plays a key role for inter-PDI electronic coupling required by SF. Our work provides a new strategy for enabling intramolecular SF by predefining interchromophore geometry by a rigid structure, which might be inspired for future designing of multichromophore SF systems.

Encapsulation-induced hypsochromic shift of emission properties from a cationic Ir(III) complex in a hydrogen-bonded organic cage: A theoretical study.

Encapsulation of coordination complexes within the confined spaces of self-assembled hosts is an effective method for creating supramolecular assemblies with distinct chemical and physical properties. Recent studies with calix-resorcin[4]arene hydrogen-bonded hexameric capsules revealed that encapsulated metal complexes exhibit enhanced and blue-shifted photoluminescence compared to their unencapsulated forms. The photophysical change has been hypothetically attributed to encapsulation-induced confinement, which isolates the metal complex from the solvent, suppressing stabilization of the excited state of the guest by solvent reorganization and structural relaxation, and altering the local environment, such as solvent polarity and viscosity, around the guest. In this study, density-functional theory calculations were conducted to explore how encapsulation affects the photophysical properties of a cationic iridium complex within a hydrogen-bonded hexameric capsule. The encapsulation-induced emission shift was analyzed by separating it into three factors: suppression of solvent reorganization, suppression of structural relaxation of the complex, and electronic interactions between the complex and the capsule. The findings indicate that the photoluminescence modulation is driven by the electronic interaction between the host and guest, which affects the energy levels of the molecular orbitals involved in the T1 excited state and the suppression of excited-state structural relaxation of the Ir complex due to the presence of the host. This study advances our understanding of the photophysical dynamics of coordination complexes within the confined spaces of hexameric capsules, providing a valuable approach for tuning the excited state properties of guest molecules.

Increased Formation of Trions and Charged Biexcitons by Above-Gap Excitation in Single-layer WSe2.

Two-dimensional semiconductors exhibit pronounced many-body effects and intense optical responses due to strong Coulombic interactions. Consequently, subtle differences in photoexcitation conditions can strongly influence how the material dissipates energy during thermalization. Here, using multiple excitation spectroscopies, we show that a distinct thermalization pathway emerges at elevated excitation energies, enhancing the formation of trions and charged biexcitons in single-layer WSe2 by up to 2× and 5× , respectively. Power- and temperature-dependent measurements lend insights into the origin of the enhancement. These observations underscore the complexity of excited state relaxation in monolayer semiconductors, provide insights for the continued development of carrier thermalization models, and highlight the potential to precisely control excitonic yields and probe nonequilibrium dynamics in 2D semiconductors.

Excited state relaxation mechanisms of paracetamol and acetanilide.

The photochemical pathways of acetanilide and paracetamol were investigated using the XMS-CASPT2 quantum chemical method and the cc-pVDZ (correlation consistent polarized valence double- ) basis set. In both compounds, the bright state is the second excited state, designated as a La) state. Through a detailed exploration of the potential energy profile and the conical intersection structure between the La) and ground states, we gained a better understanding of how cleavage might occur in both molecules upon photoexcitation. Other potential relaxation mechanisms, including crossings with the dark and La) states, are also discussed in detail.

When a Twist Makes a Difference: Exploring PCET and ESIPT on a Nonplanar Hydrogen-Bonded Donor-Acceptor System.

Bioinspired benzimidazole-phenol constructs with an intramolecular hydrogen bond connecting the phenol and the benzimidazole have been synthesized to study both proton-coupled electron transfer (PCET) and excited-state intramolecular proton transfer (ESIPT) processes. Strategic incorporation of a methyl group disrupts the coplanarity between the aromatic units, causing a pronounced twist, weakening the intramolecular hydrogen bond, decreasing the phenol redox potential, reducing the chemical reversibility, and quenching the fluorescence emission. Infrared spectroelectrochemistry and transient absorption spectroscopy confirm the formation of the oxidized product upon PCET and probe excited-state relaxation mechanisms, respectively. Density functional theory calculations of redox potentials corroborate the experimental findings. Additionally, time-dependent density functional theory calculations uncover the fluorescence quenching mechanism, showing that the nonradiative twisted intramolecular charge transfer state responsible for fluorescence quenching is more energetically favorable in the methyl-substituted system. Incorporating groups causing steric hindrance expands the design of biomimetic systems capable of performing both PCET and ESIPT.

Excited-State Decay and Photolysis of O-Nitrophenol before Proton Transfer. I: A Theoretical Investigation in the Microsolvated Atmospheric Environment.

As a potential source of the hydroxyl (OH) radical and nitrous acid (HONO), photolysis of o-nitrophenol (ONP) is of significant interest in both experimental and theoretical studies. In the atmospheric environment, the number of water molecules surrounding ONP changes with the humidity of the air, leading to an anisotropic chemical environment. This may have an impact on the photodynamics of ONP and provide a mechanism that differs from previously reported ones in the gas phase or in solution. Herein, the high-level MS-CASPT2//CASSCF method was performed to elucidate the excited-state decay and the generation of the OH radical for ONP before proton transfer in the microsolvated surrounding. We found that the varying number of water molecules affects the ground-state structures and alters the energy levels of nπ* and ππ* at the Franck-Condon (FC) region. Nevertheless, this is not the case for the excited-state minima, which exhibit very similar adiabatic excitation properties. In addition, the presence of water molecules also significantly influences the intersection structures since hydrogen bonds will hinder or alleviate the rotation or pyramidalization of the nitro (NO2) group. This will, in turn, change the excited-state relaxation mechanism of ONP. Finally, we speculated that the OH radical might be formed in the hot ground state of ONP in the microsolvated surrounding after exploring all possible electronic states.

Ultrafast Charge Separation Driven by Solvation-Coupled Intramolecular Torsion.

Photoinduced intramolecular charge separation in a pyrene- and triarylamine-based donor-acceptor dyad was studied by polarization-dependent femtosecond time-resolved transient absorption (TA) spectroscopy in polar solvents. Photoexcitation forms an excited state with charge transfer (CT) character due to the intrinsic electronic coupling between the triarylamine and pyrene groups, resulting in ultrafast charge separation (CS) in polar solvents. TA measurements reveal a correlation between the rate of CS and solvation dynamics, which implies that solvation is involved in the CS reaction. In addition, polarization-dependent TA spectroscopy was devoted to tracking the ultrafast anisotropy evolution of the cationic absorption band, which is attributed to intramolecular torsional motion and is proposed to be coupled to diffusive orientational solvent modes. The results therefore reveal that the evolution of the CT state in the condensed phase is driven by solvation-coupled excited-state structural relaxation. In other words, intramolecular torsional motion is directly confirmed to be involved in the reaction coordinate of the CS reaction in a strongly coupled donor-acceptor dyad.

Visualizing Thermally Activated Conical Intersections Governing Non-Radiative Triplet Decay in a Ni(II) Porphyrin-Nanographene Conjugate with Variable Temperature Transient Absorption Spectroscopy.

Metalloporphyrins based on open-shell transition metals, such as Ni(II), exhibit typically fast excited-state relaxation. In this work, we shed light into the nonradiative relaxation mechanism in a nanographene-Ni(II) porphyrin conjugate. Variable temperature transient absorption and global fit analysis are combined to produce a picture of the relaxation pathways. At room temperature, photoexcitation of the lowest π-π* transition is followed by vibrational cooling in 1.6 ps, setting a short 20 ps temporal window wherein a small fraction of relaxed singlets radiatively decay to the ground state before intersystem crossing proceeds. Following intersystem crossing, triplets relax rapidly to the ground state (S0) in a few tens of picoseconds. By performing measurements at low temperature, we provide evidence for a competition between two terminal relaxation pathways from the lowest (metal-centered) triplet to the ground state: a slow ground state relaxation process proceeding in time scales beyond 1.6 ns and a faster pathway dictated by a sloped conical intersection, which is thermally accessible at room temperature from the triplet state. The overall triplet decay at a given temperature is dictated by the interplay of these two contributions. This observation bears significance in understanding the underlying fast relaxation processes in Ni-based molecules and related transition metal complexes, opening avenues for potential applications for energy harvesting and optoelectronics.

The phonon-modulated Jahn–Teller distortion of the nitrogen vacancy center in diamond

The negatively charged nitrogen vacancy (NV) center in diamond is an optically accessible material defect with a unique combination of spin and optical properties that has attracted interest in quantum-information sciences and as a design candidate for nanoscale quantum sensors. Here, we present time-resolved nonlinear optical spectroscopy measurements, conducted with ultrabroadband laser pulses, that reveal strong modulation of the excited-state by the longitudinal optical (LO) phonon of the diamond lattice. The LO phonon and its overtones geometrically distort neighboring NV centers, driving long lived (3.5 ps) excited state relaxation of coupled NV centers after the initial excitation and ultrafast (<150 fs) decay of the Jahn–Teller distortion. These observations elevate the LO phonon to an important tuning mode of the Jahn–Teller conical intersection and help resolve previous spectroscopy experiments that noted longer-lived excited-state dynamics.

Development of artificial photosystems based on designed proteins for mechanistic insights into photosynthesis.

This review aims to provide an overview of the progress in protein-based artificial photosystem design and their potential to uncover the underlying principles governing light-harvesting in photosynthesis. While significant advances have been made in this area, a gap persists in reviewing these advances. This review provides a perspective of the field, pinpointing knowledge gaps and unresolved challenges that warrant further inquiry. In particular, it delves into the key considerations when designing photosystems based on the chromophore and protein scaffold characteristics, presents the established strategies for artificial photosystems engineering with their advantages and disadvantages, and underscores the recent breakthroughs in understanding the molecular mechanisms governing light-harvesting, charge separation, and the role of the protein motions in the chromophore's excited state relaxation. By disseminating this knowledge, this article provides a foundational resource for defining the field of bio-hybrid photosystems and aims to inspire the continued exploration of artificial photosystems using protein design.

Charge dynamics in the 2D/3D semiconductor heterostructure WSe2/GaAs

Understanding the relaxation and recombination processes of excited states in two-dimensional (2D)/three-dimensional (3D) semiconductor heterojunctions is essential for developing efficient optical and (opto)electronic devices, which integrate van der Waals 2D materials with more conventional 3D ones. In this work, we unveil the carrier dynamics and charge transfer in a monolayer of WSe2 on a GaAs substrate. We use time-resolved differential reflectivity to study the charge relaxation processes involved in the junction and how they change when compared to an electrically decoupled heterostructure, WSe2/hBN/GaAs. We observe that the monolayer in direct contact with the GaAs substrate presents longer optically excited carrier lifetimes (3.5 ns) when compared with the hBN-isolated region (1 ns), consistent with a strong reduction of radiative decay and a fast charge transfer of a single polarity. Through low-temperature measurements, we find evidence of a type-II band alignment for this heterostructure with an exciton dissociation that accumulates electrons in GaAs and holes in WSe2. The type-II band alignment and fast photoexcited carrier dissociation shown here indicate that WSe2/GaAs is a promising junction for advanced photovoltaic and other optoelectronic devices, making use of the best properties of van der Waals (2D) and conventional (3D) semiconductors.

Subpicosecond Dynamics of Rydberg Excitons Produced from Ultraviolet Excitation of Neutral Cuprite (Cu2O)n Clusters, n < 13.

The ultrafast dynamics of subnanometer neutral cuprite clusters (Cu2O)n, n < 13, are examined with pump probe spectroscopy. Upon absorption of an ultraviolet (400 nm) photon, all clusters exhibit a subpicosecond lifetime that we attribute to carrier recombination. Density functional theory (DFT) shows a change in the structural motif between small planar clusters and three-dimensional structures at n = 4. This transition is accompanied by a change in the excited state relaxation behavior, marking the onset for which lifetimes increase gradually with size. Time-dependent DFT calculations show that the excited state lifetimes align with calculated topological parameters and charge carrier delocalization associated with the formation of Rydberg excitons. Terminal Cu atoms are found to be important for the production of Rydberg excitons at the lowest optically allowed excited state. Upon excitation, the electron resides on terminal Cu atoms and the hole becomes delocalized across the remainder of the cluster.

Regulating the photoluminescence of aluminium complexes from non-luminescence to room-temperature phosphorescence by tuning the metal substituents

Although luminescent aluminum compounds have been utilized for emitting and electron transporting layers in organic light-emitting diodes, most of them often exhibit not phosphorescence but fluorescence with lower photoluminescent quantum yields in the aggregated state than those in the amorphous state due to concentration quenching. Here we show the synthesis and optical properties of β-diketiminate aluminum complexes, such as crystallization-induced emission (CIE) and room-temperature phosphorescence (RTP), and the substituent effects of the central element. The dihaloaluminum complexes were found to exhibit the CIE property, especially RTP from the diiodo complex, while the dialkyl ones showed almost no emission in both solution and solid states. Theoretical calculations suggested that undesired structural relaxation in the singlet excited state of dialkyl complexes should be suppressed by introducing electronegative halogens instead of alkyl groups. Our findings could provide a molecular design not only for obtaining luminescent complexes but also for achieving triplet-harvesting materials.