Excited State Deactivation Mechanism Research Articles

Impact of the Peripheral Ligand Layer on the Excited-State Deactivation Mechanism of Au38S2(S-Adm)20 and Au30(S-Adm)18 (S-Adm = Adamantanethiolate) Clusters.

Gold nanoclusters are ideal fluorescent labels for biological imaging, disease diagnosis, and treatment. Understanding the origin of the photoluminescence phenomenon in ligand-protected gold nanoclusters is crucial for both basic science and practical applications. In this study, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed to study the mechanism of excited state deactivation of Au38S2(S-Adm)20 and Au30(S-Adm)18 (S-Adm = adamantanethiolate) clusters, which have similar sizes and compositions. The computational results indicate that the differences in structural symmetry and peripheral ligand layer lead to quite different excited state deactivation mechanisms and excited state lifetimes in Au38S2(S-Adm)20 and Au30(S-Adm)18. Specifically, the μ3-S atoms and bridging thiolate (SR) in the ligand layer of Au38S2(S-Adm)20 significantly suppress the structural relaxation of ligand motifs, resulting in a prolonged excited state lifetime and higher quantum yield. For the Au30(S-Adm)18, due to the symmetry forbidden and large structural relaxation of the ligand shell, a rapid nonradiative transition process resulted. This study provides new insights into how the photoluminescence of ligand-protected gold nanoclusters is influenced by their structure and symmetry.

Unravelling the Competition between Internal Conversion and Intersystem Crossing in Twisted molecule 9‐Phenylacridine by Femtosecond Time‐resolved Spectroscopy

Abstract9‐Phenylacridine (9‐PA) is an important acridine‐based medicine that has been proven to possess significant anticancer activity and can be used as a photodynamic therapy (PDT) agent. Meanwhile, the possible twisting of the C−C single bond at the C9 position after photo‐excitation makes it a potential probe responsive to changes in the viscosity of living cells. However, the photophysical properties of 9‐PA is poorly understood. In this study, we utilized femtosecond time‐resolved spectroscopy combined with quantum chemical calculation methods to investigate the excited state dynamics of 9‐PA in solutions with different viscosities. Notably, we demonstrated that the viscosity could strongly influence the deactivation pathway of the initially populated S1 (ππ*) state of 9‐PA. In low‐viscosity solutions, the single bond at the C9 could twist after photo‐excitation, leading to a conformation that shows efficient intersystem crossing. However, such process is suppressed in high‐viscosity solutions, resulting a ~2.5 times higher internal conversion (IC) yield. A full picture of the excited state deactivation mechanism of 9‐PA is proposed.

Excited-state deactivation mechanisms of VIO system based on the non-adiabatic dynamics simulations

UV-resistant substances violacein have attracted much attention for the antioxidant properties such as inhabiting extreme environments. Herein, semi-empirical non-adiabatic dynamics simulations were employed to explore the S1 excited-state photochemical properties of violacein. After the system is excited to the Franck-Condon region, it relaxed to the S1 minimum. Subsequently, the single bond between 5-hydroxyindole and 2-pyrrolidone and the double bond between 2-pyrrolidone and oxindole were both twisted during deactivation to the conical intersection, and the 2-pyrrolidone ring was distorted with pyramidalization of a carbon atom. After passing through the conical intersection, the system returns to the ground state fastly. In non-adiabatic dynamics, it was found that 88.5 % of trajectories successfully returned to the ground state within 1 ps of the simulation time. In conclusion, we propose an excited-state deactivation mechanism for violacein system in vacuum, which will provide detailed mechanistic insights for similar light-shielding substances.

Intrinsic Photostability in Dithiolonaphthalimide Achieved by Disulfide Bond-Induced Excited-State Quenching.

Disulfide bridges common in proteins show excellent photostability achieved by ultrafast internal conversion and maintain the stability of the tertiary structure. When disulfide bonds exist in aromatic compounds, the rigid chemical structure may affect the cleavage and reforming dynamics of disulfide bonds. In this work, a model compound with a disulfide five-membered-ring structure, 4,5-dithiolo-N-(2,6-dimethylphenyl)-1,8-naphthalimide (DTDPNI), is selected to elaborate the effect of disulfide modification on the excited-state deactivation mechanism. Quantum chemical calculations show that the S-S stretching leads to a dramatic decrease in the energy gap between the S1 and S0 states, similar to the situation in 1,2-dithiane. Due to the efficient nonradiative process, the excited-state lifetime of DTDPNI resolved by ultrafast spectroscopy is determined to be ∼20 ps. It is found that the excellent photostability is achieved by ultrafast excited-state quenching induced by the S-S stretching, rather than the cleavage of the disulfide bond; even the disulfide bridge is in a very rigid aromatic molecular system.

Multiconfigurational Calculations and Photodynamics Describe Norbornadiene Photochemistry.

Storing solar energy is a vital component of using renewable energy sources to meet the growing demands of the global energy economy. Molecular solar thermal (MOST) energy storage is a promising means to store solar energy with on-demand energy release. The light-induced isomerization reaction of norbornadiene (NBD) to quadricyclane (QC) is of great interest because of the generally high energy storage density (0.97 MJ kg-1) and long thermal reversion lifetime (t1/2,300K = 8346 years). However, the mechanistic details of the ultrafast excited-state [2 + 2]-cycloaddition are largely unknown due to the limitations of experimental techniques in resolving accurate excited-state molecular structures. We now present a full computational study on the excited-state deactivation mechanism of NBD and its dimethyl dicyano derivative (DMDCNBD) in the gas phase. Our multiconfigurational calculations and nonadiabatic molecular dynamics simulations have enumerated the possible pathways with 557 S2 trajectories of NBD for 500 fs and 492 S1 trajectories of DMDCNBD for 800 fs. The simulations predicted the S2 and S1 lifetimes of NBD (62 and 221 fs, respectively) and the S1 lifetime of DMDCNBD (190 fs). The predicted quantum yields of QC and DCQC are 10 and 43%, respectively. Our simulations also show the mechanisms of forming other possible reaction products and their quantum yields.

Enabling Peculiar Photophysics and Mechanochromic Luminescence by Introducing Bromine in Push–Pull Pyridine Derivatives

Pyridine-based derivatives Py-1, Py-2, and Py-3 were synthesized by the Suzuki cross-coupling reaction, and the effect of different substituents at the ortho position of the pyridine ring was investigated. In the molecular framework of push–pull compounds, electron acceptor (A) pyridine was substituted with an electron donor (D) dimethoxy triphenylamine unit. The effect of passing from a dipolar (D-A, Py-1) to a quadrupolar (D-A-D, Py-3) structure as well as the impact of functionalizing the pyridine with a bromine in the dipolar structure (D-A-Br, Py-2) was investigated. All the pyridine derivatives were found to be highly emissive both in solution and in the solid state, with fluorescence properties markedly sensitive to the environment and responsive to external stimuli. The spectral and photophysical properties in solution were investigated through conventional steady-state and advanced time-resolved spectroscopies with nanosecond and femtosecond temporal resolution. The expected efficient intersystem crossing for the heavy-atom-containing molecule was surprisingly not highlighted by our experiments, but an increased push–pull character was enabled by functionalizing the pyridine acceptor with the bromine. The ultrafast spectroscopic study revealed a peculiar excited state deactivation mechanism for the Br derivative among the others, involving photoinduced intramolecular charge transfer accompanied by twisting of the molecular structure (population of a TICT excited state). Similarly, a significant color change in the solid-state emission was observed for Py-2 upon mechanical grinding, suggesting a mechanochromic luminescence behavior in this case. Our findings suggest introducing heavy atoms in organic fluorophores as a new design strategy to obtain mechanoluminescent materials.

Room-Temperature Phosphorescence and Cellular Phototoxicity Activated by Triplet Dynamics in Aggregates of Push-Pull Phenothiazine-Based Isomers.

In this study, we report a comprehensive time-resolved spectroscopic investigation of the excited-state deactivation mechanism in three push-pull isomers characterized by a phenothiazine electron donor, a benzothiazole electron acceptor, and a phenyl π-bridge where the connection is realized at the relative ortho, meta, and para positions. Spin-orbit charge-transfer-induced intersystem crossing takes place with high yield in these all-organic donor-acceptor compounds, leading also to efficient production of singlet oxygen. Our spectroscopic results give clear evidence of room-temperature phosphorescence not only in solid-state host-guest matrices but also in highly biocompatible aggregates of these isomers produced in water dispersions, as rarely reported in the literature. Moreover, aggregates of the isomers could be internalized by lung cancer and melanoma cells and display bright luminescence without any dark cytotoxic effect. On the other hand, the isomers showed significant cellular phototoxicity against the tumor cells due to light-induced reactive oxygen species generation. Our findings strongly suggest that nanoaggregates of the investigated isomers are promising candidates for imaging-guided photodynamic therapy.

The investigation of the ultrafast excited state deactivation mechanisms for coumarin 307 in different solvents.

The intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) processes of coumarin 307 (C307) in different solvents were investigated by performing steady-state/time-resolved transient absorption spectroscopic and steady-state photoluminescence spectroscopic characterizations in combination with time-dependent density functional theoretical calculation (TDDFT). The study unveiled the remarkable influence of solvent polarity and the strength of intermolecular hydrogen bonds formed between the solutes and solvents on the relaxation dynamics of the electronic excited state. Significantly, the emergence of the TICT state was observed in polar solvents, specifically dimethylformamide (DMF) and dimethyl sulfoxidemethanol (DMSO), owing to their inherent polarity as well as the enhanced intermolecular hydrogen bonding interactions. Interestingly, even in a weak polar solvent such as methanol (MeOH), the TICT state was also observed due to the intensified hydrogen bonding effects. Conversely, nonpolar solvents, exemplified by 1,4-dioxane (Diox), resulted in the stabilization of the ICT state due to the augmented N-H⋯O hydrogen bonding interactions. The experimental findings were corroborated by the computational calculations, thus ensuring the reliability of the conclusions drawn. Furthermore, schematic diagrams were presented to illustrate the mechanisms underlying the excited-state deactivation. Overall, this investigation contributes valuable mechanistic insights and provides a comprehensive understanding of the photochemical and photophysical properties exhibited by coumarin dyes.

Excited-State Dynamics of Proflavine after Intercalation into DNA Duplex

Proflavine is an acridine derivative which was discovered as one of the earliest antibacterial agents, and it has been proven to have potential application to fields such as chemotherapy, photobiology and solar-energy conversion. In particular, it is well known that proflavine can bind to DNA with different modes, and this may open addition photochemical-reaction channels in DNA. Herein, the excited-state dynamics of proflavine after intercalation into DNA duplex is studied using femtosecond time-resolved spectroscopy, and compared with that in solution. It is demonstrated that both fluorescence and the triplet excited-state generation of proflavine were quenched after intercalation into DNA, due to ultrafast non-radiative channels. A static-quenching mechanism was identified for the proflavine-DNA complex, in line with the spectroscopy data, and the excited-state deactivation mechanism was proposed.

Direct Observation of Excitation Wavelength-Dependent Ultrafast Intersystem Crossing in Cytosine Nucleoside Solution.

A triplet excited state can lead to different DNA photolesions, especially in cytosine and its nucleoside/nucleotide as they are hotspots for DNA mutations. However, the triplet state generation mechanism is in controversy, and experimental evidence of ultrafast intersystem crossing (ISC) has not been registered in these molecules. In this work, ultrafast ISC is directly observed in 2'-deoxycytidine (dCyd) solution by using femtosecond transient absorption spectroscopy. Surprisingly, we demonstrate that ISC in dCyd is sensitive to the excitation wavelength, and a spin-vibronic ISC mechanism is proposed. This finding is the last piece of the dCyd excited-state deactivation mechanism puzzle and sets the base for further investigation of triplet state-involved photophysics and photochemistry in dCyd-containing DNA.

Ultrafast Intersystem Crossing Dynamics of 6-Selenoguanine in Water.

Rationalizing the photochemistry of nucleobases where an oxygen is replaced by a heavier atom is essential for applications that exploit near-unity triplet quantum yields. Herein, we report on the ultrafast excited-state deactivation mechanism of 6-selenoguanine (6SeGua) in water by combining nonadiabatic trajectory surface-hopping dynamics with an electrostatic embedding quantum mechanics/molecular mechanics (QM/MM) scheme. We find that the predominant relaxation mechanism after irradiation starts on the bright singlet S2 state that converts internally to the dark S1 state, from which the population is transferred to the triplet T2 state via intersystem crossing and finally to the lowest T1 state. This S2 → S1 → T2 → T1 deactivation pathway is similar to that observed for the lighter 6-thioguanine (6tGua) analogue, but counterintuitively, the T1 lifetime of the heavier 6SeGua is shorter than that of 6tGua. This fact is explained by the smaller activation barrier to reach the T1/S0 crossing point and the larger spin–orbit couplings of 6SeGua compared to 6tGua. From the dynamical simulations, we also calculate transient absorption spectra (TAS), which provide two time constants (τ1 = 131 fs and τ2 = 191 fs) that are in excellent agreement with the experimentally reported value (τexp = 130 ± 50 fs) (Farrel et al. J. Am. Chem. Soc.2018, 140, 11214). Intersystem crossing itself is calculated to occur with a time scale of 452 ± 38 fs, highlighting that the TAS is the result of a complex average of signals coming from different nonradiative processes and not intersystem crossing alone.

Excited state deactivation mechanisms in Shikonin rationalized from its naphthoquinone parent structures.

Shikonin, a naphthoquinone dye, is a molecule of colour of natural origin, whose peculiar properties have not yet been fully rationalized. Its core structure consists of a di-hydroxy-naphthoquinone with an additional non-aromatic hydroxy group. From a comprehensive study involving fast spectroscopic techniques (fs-TA and fs-UC) and TDDFT electronic structure calculations on shikonin (Shk) and its derivatives 5-hydroxy-1,4-naphthoquinone (5HNQ), 5,8-diacetoxy-1,4-naphthoquinone (DiAc), 5,8-dihidroxy-1,4-naphthoquinone (DHNQ) and acetylshikonin, AcShk, it is shown that intramolecular excited state proton transfer (ESIPT) is present and is determinant in the deactivation of the hydroxy containing molecules. This is mirrored by the dominance of the internal conversion deactivation channel. In Shk, the non-aromatic hydroxy group determines the preferred conformer in both the ground- and excited-state, as reflected in the doubling of the fluorescence quantum yield value of this molecule relative to DHNQ. From fs-UC, a kinetic isotopic effect of 1.7 was obtained for DHNQ.

Excited state deactivation mechanisms of protonated adenine: a theoretical study.

The quantum chemical computational method and Born-Oppenheimer (BO) dynamics simulation were employed to investigate the non-radiative relaxation mechanism of protonated 9H- and 7H-adenine (AH+). We located three conical intersections (CIs) between the first 1ππ* excite state and the S0 ground state potential energy surfaces for the two most stable protonated isomers of adenine. It was predicted that the barrier-free potential energy profile along the out-of-plane deformation coordinates of the six-member ring plays the most prominent role in the deactivation of the excited AH+ from 1ππ* to the ground state via ultrafast internal conversions. This ring deformation was predicted to provide a common deactivation pathway in protonated DNA/RNA bases, describing their high level of photostability, and corresponding neutral homologues.

Direct observation of singlet ππ∗ and nπ* equilibrium state in 2-amino-1,3,5-triazine solution

1,3,5-Triazine molecules represent a class of molecules that may have been prebiotic information carriers in a primordial soup in early Earth and their excited state dynamics has received attention in recent years. In our previous study, one component with lifetime longer than 100 ps was discovered in 2-amino-1,3,5-trainzine (2-AT), but its nature has not been revealed. In this study, excited state dynamics of 2-AT is studied in different solvents by using femtosecond time-resolved transient absorption and fluorescence up-conversion spectroscopy. Interestingly, an equilibrium state consisting of the bright ππ∗ and dark nπ∗ states in 2-AT is directly observed in aqueous solution and its dynamics is solvent sensitive. The whole picture of the excited state deactivation mechanism of 2-AT is proposed based on our spectroscopy results.

Picosecond excited-state lifetimes of protonated indazole and benzimidazole: The role of the N-N bond.

Certain chemical groups give rise to characteristic excited-state deactivation mechanisms. Here, we target the role of a protonated N-N chemical group in the excited-state deactivation of protonated indazole by comparison to its isomer that lacks this group, protonated benzimidazole. Gas-phase protonated indazole and protonated benzimidazole ions are investigated at room temperature using picosecond laser pump-probe photodissociation experiments in a linear ion-trap. Excited state lifetimes are measured across a range of pump energies (4.0-5.4eV). The 1ππ* lifetimes of protonated indazole range from 390 ± 70ps using 4.0eV pump energy to ≤18ps using 4.6eV pump energy. The 1ππ* lifetimes of protonated benzimidazole are systematically longer, ranging from 3700 ± 1100ps at 4.6eV pump energy to 400 ± 200ps at 5.4eV. Based on these experimental results and accompanying quantum chemical calculations and potential energy surfaces, the shorter lifetimes of protonated indazole are attributed to πσ* state mediated elongation of the protonated N-N bond.

Intramolecular charge transfer dynamics in the excited states of diphenylamine substituted 1,3,4-oxadiazole derivatives

The excited state dynamics processes of two diphenylamine substituted symmetric 1,3,4-oxadiazole derivatives in different solvents were studied through femtosecond transient absorption spectroscopy. It was revealed that in cyclohexane, the locally excited (LE) state relaxes to the intramolecular charge transfer (ICT) state within 2 ps timescale, then decays to the ground state. In tetrahydrofuran and acetonitrile, the ICT state can be stabilized via solvation, so besides the LE → ICT conversion within 1 ps, another ICT → solvent stabilized ICT (SSICT) conversion in the 1–200 ps time scale could also be observed, and eventually decay to the ground state. Whereas, ICT → SSICT process in acetonitrile is slower than that in tetrahydrofuran, which leads to radiationless deactivation dominates the ICT state deactivation process and low fluorescence quantum efficiency. These results offer a guidance to understand the relationship of low fluorescence quantum efficiency and excited state deactivation mechanism of organic π-conjugated molecules, which would be very helpful for designing new advanced opto-electronic materials.

Theoretical insights on the excited-state-deactivation mechanisms of protonated thymine and cytosine.

Ab initio and surface-hopping nonadiabatic dynamics simulation methods were employed to investigate relaxation mechanisms in protonated thymine (TH+) and cytosine (CH+). A few conical intersections were located between 1ππ* and S0 states for each system with the CASSCF (8,8) theoretical model and relevant contributions to the deactivation mechanism of titled systems were addressed by the determination of potential energy profiles at the CASPT2 (12,10) theoretical level. It was revealed that the relaxation of the 1ππ* state of the most stable conformer of both systems to the ground state is mostly governed by the accessible S1/S0 conical intersection resulting from the barrier-free out-of-plane deformation. Interestingly, it was exhibited that the ring puckering coordinate driven from the C6 position of the heterocycle ring in TH+ and CH+ plays the most prominent role in the deactivation mechanism of considered systems. Our ab initio results are also supported by excited-state nonadiabatic dynamics simulations based on ADC(2), describing the ultrashort S1 lifetime of TH+/CH+ by analyzing trajectories leading excited systems to the ground. It was confirmed that the excited-state population mostly relaxes to the ground via the ring puckering coordinate from the C6 moiety. Overall, the theoretical results of this study shed light on the deactivation mechanism of protonated DNA bases.

New insight into the ultrafast excited state deactivation mechanism of guanosine in the gas phase

The deactivation process of guanosine molecule on the first singlet excited state in the gas phase has been explored through the density functional theory (DFT/TDDFT) and the complete active space self-consistent field (CASSCF) methods. To unravel the decay mechanism of this system, the topography of excited potential energy surfaces relevant to the deactivation process were mapped. Two deactivation decay channels characterized by the excited state intramolecular proton transfer process and the puckering motion of ring were explored, and the latter one was proposed to play significant roles in the deactivation process of guanosine for the barrierless energy profile. However, the probability for the excited state intramolecular proton transfer process pathway is lower because of the existence of an energy barrier. This conclusion was further confirmed by the two minimum energy conical intersections (MECIs) located between S1 and S0 states, characterized by the ESIPT process and pyramidalization of the C2 atom, respectively. In addition, by comparing the decay mechanism with the previously reported adenosine and pyrimidine nucleosides, it was found that two competitive decay pathways coexist in the purine nucleosides, which is different from the pyrimidine nucleosides. As the next step in the bottom-up investigation of the DNA photostability properties, the new mechanism of the guanosine in gas phase will be conducive for more comprehensive understanding of the photostability of DNA and RNA molecules and provide theoretical guidance for the investigation of the different decay mechanism of the purine nucleosides and pyrimidine nucleosides.

Excited State Deactivation Mechanism in Protonated Uracil: New Insights from Theoretical Studies.

We have conducted here a theoretical exploration, discussing the distinct excited state lifetimes reported experimentally for the two lowest lying protonated isomers of uracil. In this regard, the first-principal computational levels as well as the nonadiabatic surface hopping dynamics have been employed. It has been revealed that relaxation of the 1ππ* state of enol-enol form (EE+) to the ground is barrier-free via out-of-plane coordinates, resulting in an ultrashort S1 lifetime of this species. For the second most stable isomer (EK+), however, a significant barrier predicted in the CASPT2 S1 potential energy profile along the twisting coordinate has been proposed to explain the relevant long lifetime reported experimentally.

The role played by solvent polarity in regulating the competitive mechanism between ESIPT and TICT of coumarin (E-8-((4-dimethylamino-phenylimino)-methyl)-7-hydroxy-4-methyl-2H-chromen-2-one).

Excited-state intramolecular proton transfer (ESIPT) and twist intramolecular charge transfer (TICT) are the two most fundamental dynamic processes, ubiquitous in biological and chemical reactions. The excited-state properties of (E-8-((4-dimethylamino-phenylimino)-methyl)-7-hydroxy-4-methyl-2H-chromen-2-one (CDPA) in various solvents with different polarities were investigated by using steady-state and femtosecond transient absorption spectroscopy combined with DFT/TDDFT calculations. The results demonstrated that CDPA exhibited low fluorescence in polar acetonitrile (ACN) due to ESIPT but high fluorescence in nonpolar n-Hexane was attributed to intramolecular rotation blocking ESIPT. TDDFT calculations confirmed that the dramatic phenyl group torsional of CDPA in Hexane, whereas a near planar conformation in ACN solvent. The ESIPT barrier decreases regularly with the increase of solvent polarity from n-Hexane, tetrahydrofuran to ACN solvent. These results demonstrated that the ESIPT and TICT processes of CDPA are competitive mechanisms. Our work revealed the effect solvent polarity on the emission behavior and excited-state deactivation mechanism of CDPA, which could help to design and develop new polarity probe in the microenvironments.