Excited State Relaxation Research Articles

Sulfur oxidation states manipulate excited state electronic configurations for constructing highly efficient organic type I photosensitizers.

The multiple relaxation processes of excited states are a bridge connecting molecular structures and properties, providing enormous application potential for organic luminogens. However, a systematic understanding and manipulation of the relationship between the molecular structure, excited state relaxation processes, and properties of organic luminogens is still lacking. Herein, we report a strategy for manipulating excited state electronic configurations through the regulation of the sulfur oxidation state to construct eminent organic type I PSs. Combined with the experimental results and theoretical calculations, we have successfully revealed the decisive role of high sulfur oxidation states in promoting ROS production capacity. Impressively, a higher sulfur oxidation state can reduce the singlet-triplet energy gap (ΔE ST), increase the matching degree of transition configurations, promote the changes of the excited state electronic configurations, and boost the effective ISC proportion by enhancing intramolecular interactions. Therefore, DBTS2O with the highest sulfur oxidation state exhibits the strongest type I ROS generation ability. Additionally, guided by our strategy, a water-soluble PS (2OA) is designed and synthesized, showing selective imaging capacity and photokilling ability against Gram-positive bacteria. This study broadens the horizons for both molecular design and mechanism study of high-performance organic type I PSs.

Unravelling photoisomerization dynamics in a metastable-state photoacid.

Direct access to trans-cis photoisomerization in a metastable state photoacid (mPAH) remains challenging owing to the presence of competing excited-state relaxation pathways and multiple transient isomers with overlapping spectra. Here, we reveal the photoisomerization dynamics in an indazole mPAH using time-resolved fluorescence (TRF) spectroscopy by exploiting a unique property of this mPAH having fluorescence only from the trans isomer. The combination of these experimental results with time-dependent density function theory (TDDFT) calculations enables us to gain mechanistic insight into this key dynamical process.

Nonadiabatic molecular dynamics under adiabatic representation

<sec>In this paper, we develop a nonadiabatic molecular dynamics method based on Su-Schriffer-Heeger (SSH) Hamiltonian, and this method is widely used to study the photoexcitation dynamics and polaron motion in conjugated polymers. However, in this method, the time-dependent Schrödinger equation has so far been solved in a diabatic representation, also known as site representation. In order to provide a deeper insight into the nonadiabatic molecular dynamics method, we solve the time-dependent Schrödinger equation in an adiabatic representation. The new method can directly provide the important information about the strength of nonadiabatic couplings between different molecular orbitals in the excited-state relaxation process, helping us to predict the electron and energy transfer within or between polymer chains.</sec><sec>Solving the time-dependent Schrödinger equation in an adiabatic representation is much more complicated, it is mainly because we need to calculate the nonadiabatic couplings between different molecular orbitals. In this paper, the detailed formula derivation and actual calculation process of the nonadiabatic molecular dynamics method in an adiabatic representation are given. Using this new method, we simulate three photoexcitation processes in a conjugated polymer chain, HOMO→LUMO, HOMO–1→LUMO+1 and HOMO–2→LUMO+2. We analyze in detail the time evolutions of lattice configuration for these three photoexcitation processes, and compare these results with those obtained by diabatic representation (site representation) showing that the results obtained from these two representations are consistent with each other.</sec>

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.

An innovative probe based on aluminum ion-triggered aggregation induced emission effect for ratiometric fluorescence visual detection of fluoroquinolone

Based on the aggregation-induced emission effect triggered by Al3+, a novel probe was constructed for the ratiometric fluorescence visualization detection of fluoroquinolones (FQs) residues. In this probe, Al3+ cased the glutathione-modified copper nanoclusters (GSH-CuNCs) to aggregate by the interaction with surface carboxyl groups, restricting the intramolecular vibration and rotation of GSH-CuNCs and reducing the non-radiative relaxation of excited states. Significant enhancement of fluorescence and quantum yield were achieved. In the presence of FQs, Al3+/GSH-CuNCs coordinated with FQs to form a polymer that inhibited intramolecular proton transfer in the excited state of FQs, thereby enhancing the intrinsic fluorescence of FQs, while the fluorescence intensity of Al3+/GSH-CuNCs was not disturbed. The probe solution changed from red to blue (or green) under UV lamp as the concentration of FQs increased, providing the possibility for semi-quantitative visual analysis. It also showed sensitive spectral responses to FQs with detection limits of 7.9 (ciprofloxacin), 6.8 (gatifloxacin), and 9.5 nM (enrofloxacin), respectively. The application potential in the detection of FQs residues in poultry eggs was verified by real sample test. Additionally, the probe has been extended to the development of fluorescent test paper and the smartphone detection platform, contributing to rapid, portable, visual quantitative analysis of FQs.

New Twist on the Light-Switch Effect: Controlling the Fate of Excited States with pH in a 4-Hydroxy-thiazol-extended Ruthenium(II) Dppz Complex.

We present a pH-dependent study of the excited state dynamics of a novel Ru complex bearing a 4-hydroxy thiazol-substituted dppz (dipyridophenazine) ligand (RuTzOH) and its deprotonated form (RuTzO-). We combine steady-state and time-resolved absorption and emission spectroscopy with electrochemical investigations to characterize the excited state relaxation, which upon photoexcitation at 400 nm is determined by a multitude of initially populated MLCT states for both complexes. Subsequently, for RuTzOH, two long-lived excited states are populated, leading to dual emission from the complexes, a feature that vanishes upon deprotonation. Upon deprotonation, the electron density on the dppz moiety increases significantly, leading to rapid energy populating ligand-centered states and thus deactivating the initially excited MLCT states.

Theoretical insights into the photophysics of an unnatural base Z: A MS-CASPT2 investigation.

We have employed the highly accurate multistate complete active space second-order perturbation theory (MS-CASPT2) method to investigate the photoinduced excited state relaxation properties of one unnatural base, namely Z. Upon excitation to the S2 state of Z, the internal conversion to the S1 state would be dominant. From the S1 state, two intersystem crossing paths leading to the T2 and T1 states and one internal conversion path to the S0 state are possible. However, considering the large barrier to access the S1 /S0 conical intersection and the strong spin-orbit coupling between S1 and T2 states (>40 cm-1 ), the intersystem crossing to the triplet manifolds is predicted to be more preferred. Arriving at the T2 state, the internal conversion to the T1 state and the intersystem crossing back to the S1 state are both possible considering the S1 /T2 /T1 three-state intersection near the T2 minimum. Upon arrival at the T1 state, the deactivation to S0 can be efficient after overcoming a small barrier to access T1 /S0 crossing point, where the spin-orbit coupling (SOC) is as large as 39.7 cm-1 . Our present work not only provides in-depth insights into the photoinduced process of unnatural base Z, but can also help the future design of novel unnatural bases with better photostability.

Photophysics of Fe-Fe hydrogenase mimic complexes for hydrogen evolution

The photophysical and electrochemical properties of the diiron hydrogenase mimic complex, [Fe2(dpet)(CO)6] (dpet = 1,2-diphenylethene-1,2-dithiol) have been studied using a combination of spectroscopic and electrochemical techniques including cyclic voltammetry, infrared spectroelectrochemistry (IR-SEC), ultrafast transient absorption (TA) spectroscopy and time-resolved infrared spectroscopy (TRIR), with the data interpretation aided by density functional theory (DFT). The complex is a viable catalyst for electrochemical hydrogen evolution, as shown by its electrochemical reversibility and significant increase in the catalytic current upon addition of trifluoroacetic acid (TFA). IR-SEC was used to identify the first and second reduction products, where formation of a bridging μ-CO species was observed. The ultrafast spectroscopy data show that CO loss is only a minor excited state relaxation pathway, unlike other previously reported [FeFe] catalysts, in agreement with steady-state photolysis. The excited state lifetime obtained for this complex (135 ps in DCM and 237 ps in MeCN) is insufficient for [Fe2(dpet)(CO)6] to function as a photocatalyst on its own. However, the photostability of the complex opens up an opportunity for its future use in conjunction with a photosensitiser in photochemical hydrogen evolution.

Manipulating Lone-Pair-Driven Luminescence in 0D Tin Halides by Pressure-Tuned Stereochemical Activity from Static to Dynamic.

The excellent luminescence properties and structural dynamics driven by the stereoactivity of the lone pair in a variety of low-dimensional ns2 metal halides have attracted growing investigations for optoelectronic applications. However, the structural and photophysical aspects of the excited state associated with the lone pair expression are currently open questions. Herein, zero-dimensional Sn-based halides with static stereoactive 5 s2 lone pairs are selected as a model system to understand the correlations between the distinctive lone pair expression and the excited-state structural relaxation and charge carrier dynamics by continuous lattice manipulation. Lattice compression drives 5 s2 lone pair active switching and self-trapped exciton (STE) redistribution by suppressing excited-state structural deformation of the isolated SnBr4 2- units. Our results demonstrate that the static expression of the 5 s2 lone pair results in a red broadband triplet STE emission with a large Stokes shift, while its dynamic expression creates a sky-blue narrowband emission dominated by the radiative recombination of singlet STEs. Our findings and the photophysical mechanism proposed highlight the stereochemical effects of lone pair expression in controlling light emission properties and offer constructive guidelines for tuning the optoelectronic properties in diverse ns2 metal halides.

Wavelength dependent excited state dynamics observed in canonical pyrimidine nucleosides

Epidemiological evidence indicates that damage to DNA/RNA initialized by ultraviolet (UV) radiation is associated with skin cancer. Wavelength dependence of DNA photodamage was proposed as early as 1990s and demonstrated later on. Unraveling the photo-activated dynamics involved in related reactions is essential. However, studies aimed at uncovering the wavelength dependent excited state dynamics in canonical pyrimidine nucleosides have not received enough attention. In this work, excitation wavelength dependent excited state dynamics of 2′-deoxy-thymidine (dThd) and oxy-uridine (Urd) are investigated in acetonitrile solutions by femtosecond broadband transient absorption spectroscopy. Varying the excitation wavelength leads to a significant difference in the branching of the excited state population at the Franck-Condon (FC) region, resulting higher fluorescence quantum yield with 285 nm pump but higher triplet state quantum yield under 267 nm excitation. Based on our results, a vibronic coupling regulated excited state relaxation mechanism is proposed. This mechanism information is important for understanding the formation of harmful photoproducts for DNA/RNA with different wavelength UV excitations.

Reveal Ultrafast Electron Relaxation across Sub-bands of Tellurium by Time- and Energy-Resolved Photoemission Microscopy.

Exploring ultrafast carrier dynamics is crucial for the materials' fundamental properties and device design. In this work, we employ time- and energy-resolved photoemission electron microscopy with tunable pump wavelengths from visible to near-infrared to reveal the ultrafast carrier dynamics of the elemental semiconductor tellurium. We find that two discrete sub-bands around the Γ point of the conduction band are involved in excited-state electron ultrafast relaxation and reveal that hot electrons first go through ultrafast intra sub-band cooling on a time scale of about 0.3 ps and then transfer from the higher sub-band to the lower one on a time scale of approximately 1 ps. Additionally, theoretical calculations reveal that the lower one has flat-band characteristics, possessing a large density of states and a long electron lifetime. Our work demonstrates that TR- and ER-PEEM with broad tunable pump wavelengths are powerful techniques in revealing the details of ultrafast carrier dynamics in time and energy domains.

Nickel(II) Analogues of Phosphorescent Platinum(II) Complexes with Picosecond Excited-State Decay.

Square-planar Ni(II) complexes are interesting as cheaper and more sustainable alternatives to Pt(II) luminophores widely used in lighting and photocatalysis. We investigated the excited-state behavior of two Ni(II) complexes, which are isostructural with two luminescent Pt(II) complexes. The initially excited singlet metal-to-ligand charge transfer (1MLCT) excited states in the Ni(II) complexes decay to metal-centered (3MC) excited states within less than 1 picosecond, followed by non-radiative relaxation of the 3MC states to the electronic ground state within 9 - 21 ps. This contrasts with the population of an emissive triplet ligand-centered (3LC) excited state upon excitation of the Pt(II) analogues. Structural distortions of the Ni(II) complexes are responsible for this discrepant behavior and lead to dark 3MC states far lower in energy than the luminescent 3LC states of Pt(II) compounds. Our findings suggest that if these structural distortions could be restricted by more rigid coordination environments and stronger ligand fields, the excited-state relaxation in four-coordinate Ni(II) complexes could be decelerated such that luminescent 3LC or 3MLCT excited states become accessible. These insights are relevant to make Ni(II) fit for photophysical and photochemical applications that relied on Pt(II) until now.

Parameter estimation in ultrafast spectroscopy using probability theory.

Ultrafast spectroscopy is a powerful technique that utilizes short pulses on the femtosecond time scale to generate and probe coherent responses in molecular systems. While the specific ultrafast methodologies vary, the most common data analysis tools rely on discrete Fourier transformation for recovering coherences that report on electronic or vibrational states and multi-exponential fitting for probing population dynamics, such as excited-state relaxation. These analysis tools are widely used due to their perceived reliability in estimating frequencies and decay rates. Here, we demonstrate that such "black box" methods for parameter estimation often lead to inaccurate results even in the absence of noise. To address this issue, we propose an alternative approach based on Bayes probability theory that simultaneously accounts for both population and coherence contributions to the signal. This Bayesian inference method offers accurate parameter estimations across a broad range of experimental conditions, including scenarios with high noise and data truncation. In contrast to traditional methods, Bayesian inference incorporates prior information about the measured signal and noise, leading to improved accuracy. Moreover, it provides estimator error bounds, enabling a systematic statistical framework for interpreting confidence in the results. By employing Bayesian inference, all parameters of a realistic model system may be accurately recovered, even in extremely challenging scenarios where Fourier and multi-exponential fitting methods fail. This approach offers a more reliable and comprehensive analysis tool for time-resolved coherent spectroscopy, enhancing our understanding of molecular systems and enabling a better interpretation of experimental data.

Tandem High-Pressure Crystallography-Optical Spectroscopy Unpacks Noncovalent Interactions of Piezochromic Fluorescent Molecular Rotors.

To develop luminescent molecular materials with predictable and stimuli-responsive emission, it is necessary to correlate changes in their geometries, packing structures, and noncovalent interactions with the associated changes in their optical properties. Here, we demonstrate that high-pressure single-crystal X-ray diffraction can be combined with high-pressure UV-visible absorption and fluorescence emission spectroscopies to elucidate how subtle changes in structure influence optical outputs. A piezochromic aggregation-induced emitter, sym-heptaphenylcycloheptatriene (Ph7C7H), displays bathochromic shifts in its absorption and emission spectra at high pressure. Parallel X-ray measurements identify the pressure-induced changes in specific phenyl-phenyl interactions responsible for the piezochromism. Pairs of phenyl rings from neighboring molecules approach the geometry of a stable benzene dimer, while conformational changes alter intramolecular phenyl-phenyl interactions correlated with a relaxed excited state. This tandem crystallographic and spectroscopic analysis provides insights into how subtle structural changes relate to the photophysical properties of Ph7C7H and could be applied to a library of similar compounds to provide general structure-property relationships in fluorescent organic molecules with rotor-like geometries.

CASPT2//CASSCF studies on mechanistic photophysics of 3-hydroxyflavone

3-Hydroxyflavone (3-HF) as the backbone of flavonols has been extensively studied on the dual fluorescence and excited-state intramolecular proton transfer (ESIPT) after photoexcitation. However, its detailed excited-state relaxation mechanism is still unclear. Herein, the minimum-energy structures, conical intersections and singlet–triplet crossing points of 3-HF in the 1ππ∗, 1nπ∗, 3ππ∗, 3nπ∗, and S0 states in the gas phase were first optimized by the CASPT2//CASSCF method. Then, the minimum-energy proton transfer (MEPT) and excited-state decay (ESD) pathways were calculated as well. Based on the calculation results, we proposed several possible ESD pathways from the initially populated bright S2(ππ*) state. The major channel is singlet-involved and stretch-torsion coupled ESIPT pathway. 3-HF in normal (N) form first undergoes efficient 1ππ* ESIPT with a small barrier of 2.2 kcal/mol to yield the 1ππ* proton-transferred tautomer (T). Subsequently, the T tautomer approaches the nearby accessible S1/S0 conical intersection and then occurs internal conversion (IC) to fall back to the S0 state. Finally, the S0 species quickly transfers into the N conformer via the reverse ground state intramolecular proton transfer (GSIPT) to realize the ground-state recovery. In addition, the minor ESD pathways are involved less efficient intersystem crossing (ISC) processes. The present work could provide insights into understanding the photophysics of flavonols and their derivatives.

The performance of approximate equation of motion coupled cluster for valence and core states of heavy element systems

The equation of motion coupled cluster singles and doubles model (EOM-CCSD) is an accurate, black-box correlated electronic structure approach to investigate electronically excited states and electron attachment or detachment processes. It has also served as a basis for developing less computationally expensive approximate models such as partitioned EOM-CCSD (P-EOM-CCSD), the second-order many-body perturbation theory EOM (EOM-MBPT(2)) and their combination (P-EOM-MBPT(2)) [S. Gwaltney et al., Chem. Phys. Lett. 248, 189–198 (1996)]. In this work we outline an implementation of these approximations for four-component based Hamiltonians and investigate their accuracy relative to EOM-CCSD for valence excitations, valence and core ionisations and electron attachment, and this for a number of systems of atmospheric or astrophysical interest containing elements across the periodic table. We have found that across the different systems and electronic states of different nature considered, partition EOM-CCSD yields results with the largest deviations from the reference, whereas second-order based approaches tend show a generally better agreement with EOM-CCSD. We trace this behaviour to the imbalance brought about by the removal of excited state relaxation in the partition approaches, with respect to degree of electron correlation recovered.

Disclosing Early Excited State Relaxation Events in Prototypical Linear Carbon Chains.

One-dimensional (1D) linear nanostructures comprising sp-hybridized carbon atoms, as derivatives of the prototypical allotrope known as carbyne, are predicted to possess outstanding mechanical, thermal, and electronic properties. Despite recent advances in their synthesis, their chemical and physical properties are still poorly understood. Here, we investigate the photophysics of a prototypical polyyne (i.e., 1D chain with alternating single and triple carbon bonds) as the simplest model of finite carbon wire and as a prototype of sp-carbon-based chains. We perform transient absorption experiments with high temporal resolution (<30 fs) on monodispersed hydrogen-capped hexayne H─(C≡C)6─H synthesized by laser ablation in liquid. With the support of computational studies based on ground state density functional theory (DFT) and excited state time-dependent (TD)-DFT calculations, we provide a comprehensive description of the excited state relaxation processes at early times following photoexcitation. We show that the internal conversion from a bright high-energy singlet excited state to a low-lying singlet dark state is ultrafast and takes place with a 200 fs time constant, followed by thermalization on the picosecond time scale and decay of the low-energy singlet state with hundreds of picoseconds time constant. We also show that the time scale of these processes does not depend on the end groups capping the sp-carbon chain. The understanding of the primary photoinduced events in polyynes is of key importance both for fundamental knowledge and for potential optoelectronic and light-harvesting applications of low-dimensional nanostructured carbon-based materials.

Understanding the Excited-State Relaxation Mechanisms of Xanthophyll Lutein by Multi-configurational Electronic Structure Calculations.

The contradictory behaviors in light harvesting and non-photochemical quenching make xanthophyll lutein the most attractive functional molecule in photosynthesis. Despite several theoretical simulations on the spectral properties and excited-state dynamics, the atomic-level photophysical mechanisms need to be further studied and established, especially for an accurate description of geometric and electronic structures of conical intersections for the lowest several electronic states of lutein. In the present work, semiempirical OM2/MRCI and multi-configurational restricted active space self-consistent field methods were performed to optimize the minima and conical intersections in and between the 1Ag-, 2Ag-, 1Bu+, and 1Bu- states. Meanwhile, the relative energies were refined by MS-CASPT2(10,8)/6-31G*, which can reproduce correct electronic state properties as those in the spectroscopic experiments. Based on the above calculation results, we proposed a possible excited-state relaxation mechanism for lutein from its initially populated 1Bu+ state. Once excited to the optically bright 1Bu+ state, the system will propagate along the key reaction coordinate, i.e., the stretching vibration of the conjugated carbon chain. During this period of time, the 1Bu- state will participate in and forms a resonance state between the 1Bu- and 1Bu+ states. Later, the system will rapidly hop to the 2Ag- state via the 1Bu+/2Ag- conical intersection. Finally, the lutein molecule will survive in the 2Ag- state for a relatively long time before it internally converts to the ground state directly or via a twisted S1/S0 conical intersection. Notably, though the photophysical picture may be very different in solvents and proteins, the current theoretical study proposed a promising calculation protocol and also provided many valuable mechanistic insights for lutein and similar carotenoids.

Quasi-classical simulations of resonance Raman spectra based on path integral linearization.

Based on a linearization approximation coupled with path integral formalism, we propose a method derived from the propagation of quasi-classical trajectories to simulate resonance Raman spectra. This method is based on ground state sampling followed by an ensemble of trajectories on the mean surface between the ground and excited states. The method was tested on three models and compared to a quantum mechanics solution based on a sum-over-states approach: harmonic and anharmonic oscillators and the HOCl molecule (hypochlorous acid). The method proposed is able to correctly characterize resonance Raman scattering and enhancement, including the description of overtones and combination bands. The absorption spectrum is obtained at the same time, and the vibrational fine structure can be reproduced for long excited state relaxation times. The method can also be applied to dissociating excited states (as is the case for HOCl).

Ultrafast dynamics and relaxation pathways in Eu(III) complexes with fluorinated β–diketonate ligands

A series of novel β-diketonate Eu3+ complexes with minor variations in the chemical structure of the diketone ligand were systematically studied. Six Eu3+ coordination compounds based on β-diketones, bearing linear perfluorinated substituents of various length (C1, C2, C3 and C6) and fixed pyrazole moiety were obtained. In addition, two water molecules, 1,10-phenanthroline or 4,7-diphenyl-1,10-phenanthroline as an ancillary ligand in compounds with fixed β-diketone ligand were used. Their photophysical properties were investigated by UV–Vis absorption and time-resolved femtosecond transient (fs-TA) absorption spectroscopy techniques. In this study, we investigate the impact of ancillary ligands and the fluorinated chain length of β-diketone ligands on the ultrafast relaxation processes that occur in the ligand environment. Relying on the ultrafast transient absorption measurements, it was demonstrated that the extension of fluorinated chains of β-diketone ligand leads to decrease of first excited singlet and triplet states relaxation rates. The compound with excessively long chain C6F13 exhibits the slowest relaxation of the triplet state T1.Besides, variation in the ancillary ligand alters the electronic energy transfer pathways in the whole complex. It was established, that introducing 4,7-diphenyl-1,10-phenanthroline (bath) instead of 1,10-phenanthroline (phen) dramatically changing the electronic energy transfer dynamics in the whole coordination compound. On the basis of the fs-TA measurements we conclude the bath ligand act as acceptor of electronic energy of diketone ligand in the excited state.