Deactivation Of Excited State Research Articles

Ultrafast dynamics of electron excited states of phenylacetylene

Interaction of light with matter has always been important in the field of natural science. Particularly, the ultrafast radiationless relaxation induced by UV light of molecular electronic excited states accompanied by ultrafast energy transfer plays an important role in the natural photophysical, photochemical and biological reactions. Generally, the molecular electronic excited state can be deactivated through a variety of decay channels, including dissociation, isomerization, internal conversion, intersysterm crossing, vibrational energy redistribution, and autoionization. This complexity of relaxation channels brings about a wide variety of deactivation mechanisms. The ultrafast nonadibatic relaxation dynamics of the excited state of phenylacetylene is studied by using femtosecond time-resolved photoelectron imaging and femtosecond time-resolved mass spectrometry. The first excited state S<sub>2</sub> of phenylacetylene is excited by 235 nm pump light, and the excited state deactivation process is detected by 400 nm probe light. The time-dependent curves of parent ions include two exponential curves. One is the fast component with a time constant of 116 fs, and the other is the slow component with a time constant of 106 ps. The time-resolved photoelectron kinetic energy distribution is obtained from the time-resolved photoelectron images. Combined with the time-resolved photoelectron spectroscopy data, the fast component with a time constant of 116 fs is found to reflect the internal conversion process from S<sub>2</sub> state to S<sub>1</sub> state. The experimental results also show that S<sub>1</sub> state is arranged by internal conversion, and the inter system jump process to T<sub>1</sub> state is an important attenuation channel. This work provides a clearer physical picture for S<sub>1</sub> state nonadibatic relaxation dynamics of phenylacetylene.

A theoretical study on the excited-state deactivation paths for the A-5FU dimer.

The photostability of DNA plays a key role in the normal function of organisms. A-5FU is a base pair derivative of the A-T dimer where the methyl group is replaced by a F atom. Here, accurate static TDDFT calculations and non-adiabatic dynamic simulations are used to systematically investigate the excited-state decay paths of the A-5FU dimer related to the proton transfer and the out-of-plane twisting deformation motion of A and 5FU in the 1ππ* and 1nπ* states. CC2 is used to check the accuracy of the current TDDFT calculations. Our results show that the deformation of the C[double bond, length as m-dash]C or C[double bond, length as m-dash]N double bond in A and 5FU provides an efficient pathway for the depopulation of the lowest excited states, which can compete with the excited-state proton transfer paths in the dimer. This finding indicates that monomer-like decay paths could be important for the photostability of weakly hydrogen-bonded DNA base pairs and provide a new insight into the excited-state decay paths in base pairs and their analogues.

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.

Towards Iron(II) Complexes with Octahedral Geometry: Synthesis, Structure and Photophysical Properties.

The control of ligand-field splitting in iron (II) complexes is critical to slow down the metal-to-ligand charge transfer (MLCT)-excited states deactivation pathways. The gap between the metal-centered states is maximal when the coordination sphere of the complex approaches an ideal octahedral geometry. Two new iron(II) complexes (C1 and C2), prepared from pyridylNHC and pyridylquinoline type ligands, respectively, have a near-perfect octahedral coordination of the metal. The photophysics of the complexes have been further investigated by means of ultrafast spectroscopy and TD-DFT modeling. For C1, it is shown that—despite the geometrical improvement—the excited state deactivation is faster than for the parent pseudo-octahedral C0 complex. This unexpected result is due to the increased ligand flexibility in C1 that lowers the energetic barrier for the relaxation of 3MLCT into the 3MC state. For C2, the effect of the increased ligand field is not strong enough to close the prominent deactivation channel into the metal-centered quintet state, as for other Fe-polypyridine complexes.

Triphenylamine/Tetracyanobutadiene-Based π-Conjugated Push-Pull Molecules End-Capped with Arene Platforms: Synthesis, Photophysics, and Photovoltaic Response.

π-Conjugated push-pull molecules based on triphenylamine and 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) have been functionalized with different terminal arene units. In solution, these highly TCBD-twisted systems showed a strong internal charge transfer band in the visible spectrum and no detectable photoluminescence (PL). Photophysical and theoretical investigations revealed very short singlet excited state deactivation time of ≈10 ps resulting from significant conformational changes of the TCBD-arene moiety upon photoexcitation, opening a pathway for non-radiative decay. The PL was recovered in vacuum-processed films or when the molecules were dispersed in a PMMA matrix leading to a significant increase of the excited state deactivation time. As shown by cyclic voltammetry, these molecules can act as electron donors compared to C60 . Hence, vacuum-processed planar heterojunction organic solar cells were fabricated leading to a maximum power conversion efficiency of ca. 1.9 % which decreases with the increase of the arene size.

Luminescence properties of 5-Bromoindole in PVA films at room temperature: Direct triplet state excitation

We studied spectral properties of 5-Bromoindole (5-BrI) in poly (vinyl alcohol) (PVA) film. The fluorescence of 5-BrI in PVA films is similar to indole in PVA film but strongly quenched compared to unsubstituted indole. The quantum yield of 5-BrI is only 0.0034 comparing to 0.26 of indole. In contrast, phosphorescence of 5-BrI in PVA is comparable to indole phosphorescence in magnitude and lifetime but its emission shifted towards longer wavelengths by 50 nm for a maximum at 530 nm. The phosphorescence of 5-BrI in PVA can be effectively excited at longer wavelengths (450–500 nm). The maximum of phosphorescence excitation spectrum of the 5-BrI in PVA is at about 480 nm. This direct excitation to the triplet state results in high positive phosphorescence anisotropy of 0.2. Although the S0 → T1 absorption transition is only about 800 fold weaker from the main 280 nm UV absorption band yielding measurable trace of absorption for 5-BrI in PVA film in the visible/blue range. We believe this is a first report of singlet-triplet absorption (S0 → T1) at room temperature, not involving external heavy atoms or oxygen presence. The comparison of phosphorescence of 5-BrI in PVA film to Rhodamine 6G in ethanol fluorescence allows us to estimate the phosphorescence quantum yield of 5-BrI in PVA to be ~0.02. We measured independently fluorescence and phosphorescence lifetimes and quantum yields as well as phosphorescence excitation spectrum across all wavelengths from UV to the visible range. These measurements were used to determine rate constants for all processes involved the in 5-BrI in PVA excited state deactivation.

Fluorescence of curcumin in alkaline dimethyl sulfoxide and the effects of alkali metal cations on it

Earlier, we reported our finding of the fluorescence emission derived from the triple-deprotonated form of curcumin (Cur3–) in dimethyl sulfoxide (DMSO) in the presence of NaOH. Herein, we disclose the results of the studies on the luminescent properties of Cur3– in the presence of LiOH, NaOH and KOH, which are indicative of the cation-dependent behavior of the Cur3– fluorescence excitation and the emission spectra. The fluorescence quantum yield in the presence of LiOH and NaOH significantly increases compared to that of neutral curcumin. Studying the fluorescence polarization properties revealed that the observed increase in the quantum yield is accounted for by the decreased rate of the nonradiative excited-state deactivation. We assume that the origin of the cation-dependent fluorescence properties refers to the formation of coordination bonds between the alkali metal cations and the oxygen atoms of the β-diketone (keto-enol) group of curcumin, which is substantiated by the experimental results and supported by the semiempirical modeling.

Phenanthroline chromophore as efficient antenna for Tb3+ green luminescence: A theoretical study

The photophysical processes, from excitation in the UV range, energy migration pathways to luminescence of Tb(phen)2(NO3)3 complex in the Vis region have been studied by theoretical approaches and spectroscopic measurements. The UV–Vis absorption, diffuse reflectance, excitation and luminescence spectra indicated that the ligand 1,10-phenanthroline (phen) was a suitable antenna chromophore for Tb3+ luminescence at excitation of wavelength range 350–352 nm, corresponding to the S1 state of phen. The energy level diagram and character of the singlet and triplet excited states of the ligand-chromophore and Tb(phen)2(NO3)3 in gas phase and polar solution were obtained by the DFT/TDDFT/ωB97xD/B1 method and multireference ab-initio calculations with perturbative spin-orbit coupling corrections. It was also established that the vibro-electronic contribution to the excitation energy is important for correct reproduction and interpretation of the experimental absorption and emission spectra of Tb(phen)2(NO3)3. The rate constants for competitive singlet-triplet intersystem crossing relaxation transitions, as well as radiative and non-radiative deactivation of the ligand-centered excited states were qualitatively estimated and the fastest electronic relaxation mechanism was revealed. The excitation channel S0→S1→T2→T1→5D4 was predicted as the most probable one for effective phen→Tb3+ energy transfer. The sensitization results in a high luminescent quantum yield of 13%, which suggests energy transfer rates, sufficiently higher than the rates of radiative and non-radiative depopulation of the T1 state.

Al3+ regulated competition between TICT and ESIPT of a chemosensor

Twisted intramolecular charge transfer (TICT) and excited state intramolecular proton transfer (ESIPT) are of fundamental importance during the excited state dynamics of dyes and fluorescent sensors. Co-existence of the two processes usually complicates the whole photophysical process of dyes and leave the fluorescence detection mechanism of sensors uncovered. In this contribution, competition between the two processes are observed during the excited state relaxation of a fluorescence sensor 1-[(2-Amino-phenylimino)-methyl]-naphthalen-2-ol (short for L). Al3+ is proved to be capable of regulating this competition during the excited state deactivation process. With the addition of Al3+, the original barrier for the ESIPT process is removed and ESIPT fluorescence peak of the sensor is significantly enhanced. TICT process, which is originally barrier-less, is prohibited by the strong coordination between Al3+ and the sensor. The regulation of TICT and ESIPT process to achieve Al3+ detection is interesting and intrinsically different from the previously proposed sensing mechanism. The newly proposed mechanism is quite rare in the field of metal ion detection and may inspire scientists to design sensors from a different perspective.

Critical Role of Protons for Emission Quenching of Indoline Dyes in Solution and on Semiconductor Surfaces.

By combining time-correlated single photon counting (TCSPC) measurements, density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations, we herein investigate the role of protons, in solutions and on semiconductor surfaces, for the emission quenching of indoline dyes. We show that the rhodanine acceptor moieties, and in particular the carbonyl oxygens, undergo protonation, leading to nonradiative excited-state deactivation. The presence of the carboxylic acid anchoring group, close to the rhodanine moiety, further facilitates the emission quenching, by establishing stable H-bond complexes with carboxylic acid quenchers, with high association constants, in both ground and excited states. This complexation favors the proton transfer process, at a low quencher concentration, in two ways: bringing close to the rhodanine unit the quencher and assisting the proton release from the acid by a partial-concerted proton donation from the close-by carboxylic group to the deprotonated acid. Esterification of the carboxylic group, indeed, inhibits the ground-state complex formation with carboxylic acids and thus the quenching at a low quencher concentration. However, the rhodanine moiety in the ester form can still be the source of emission quenching through dynamic quenching mechanism with higher concentrations of protic solvents or carboxylic acids. Investigating this quenching process on mesoporous ZrO2, for solar cell applications, also reveals the sensitivity of the adsorbed excited rhodanine dyes toward adsorbed protons on surfaces. This has been confirmed by using an organic base to remove surface protons and utilizing cynao-acrylic dye as a reference dye. Our study highlights the impact of selecting such acceptor group in the structural design of organic dyes for solar cell applications and the overlooked role of protons to quench the excited state for such chemical structures.

Structure-relation properties of N-substituted phenothiazines in solution and solid state: Photophysical, photostability and aggregation-induced emission studies

Five N-substituted phenothiazines (with different alkyl and aryl groups) were investigated aiming to rationalize the influence of the substitution in the photooxidation processes and in the aggregation induced emission (AIE) properties of these compounds. A comprehensive spectroscopic and photophysical investigation in solution (diluted in solutions of methylcyclohexane, MCH, tetrahydrofuran, THF, and in THF: water mixtures aiming to promote aggregation) and in the solid state (powder and films), with the characterization of all the excited state deactivation routes with the determination of quantum yields (fluorescence, phosphorescence and singlet oxygen sensitization), lifetimes and rate constants, was performed. Room temperature phosphorescence in the solid state was observed for the methyl and bi-phenyl derivatives. Stability upon irradiation of the compounds was further investigated and the biphenyl derivative was found stable in contrast with phenothiazine and the other alkyl (methyl and hexyl) derivatives. These were further rationalized with TD-DFT calculations. These findings present relevant information to the efficient design of photo-stable phenothiazine derivatives in light emitting applications.

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.

Binol diuryl dipyrene fluorescent probe: Dual detection of silver and carbonate ions and its bioimaging applications

Binol-pyrene conjugate (BPC), as fluorescent probe was synthesized by multi-step synthetic strategies. BPC acts as a single sensor for dual analyte detection by a turn “on-off” mechanism for Ag+ and CO32− ions. In the emission spectra, the monomer peaks of BPC were drastically quenched by the addition of Ag+ and CO32− ions, due to the reverse photo-induced electron transfer (PET) process, excited-state deactivation via vibration and rotation of binaphthyl ring and the heavy atom effect responsible for the spin forbidden process. The probe BPC is highly selective and sensitive to Ag+ and CO32− ions even in the presence of various interfering cations and anions at the physiological pH. The similar sensing strategy of BPC for Ag+ and CO32− ions were further applied into the living cells, and their limit of detection (LOD) downs to 1.2 × 10-7 M and 14 × 10-6 M, respectively, which are close to the World Health Organization (WHO) drinking water standards. Furthermore, the binding interaction between the probe BPC with Ag+ and CO32− ions and dynamic mode of quenching are evidenced by Stern-Volmer plots, quantum yields, NMR and DFT analysis.

Ultrafast Dynamics of the Transoid-cis Isomer Formed in Photochromic Reaction from 3H-Naphthopyran.

Recent efforts in designing new 3H-naphthopyran derivatives have been focused on efficient coloration process with a short fading time of the colored transoid-cis TC isomer. It is desirable to avoid photoisomerization of TC leading to transoid-trans TT isomers in the photoreaction. Long lifetime of TT can hamper fast applications such as dynamic holographic materials and molecular actuators, the residual color is one of the serious issues for photochromic lenses. Herein we characterize the photophysical and photochemical channels of TC excited state deactivation competing with the unwanted TC→TT isomerization process. Transient absorption spectroscopy reveals a very short lifetime of the singlet excited TC (≈0.8 ps) and its deactivation channels as S1 →S0 internal conversion (major), intersystem crossing S1 →T1 , pyran ring formation, photoenolization and TC→TT isomerization. Computations support the S1 →S0 and T1 →S0 channels as responsible for photostabilization of the TC form.

Bimolecular fluorescence quenching of benzoxazole/benzothiazole-based functional dyes

Four dyes that belong to the group of hemicyanines and possess benzoxazole and benzothiazole moieties are synthesised and described. The spectroscopic properties of these compounds in 1-methyl-2-pyrrolidinone (MP) were studied. Synthesised dyes absorb in the UV–Vis region. The emission spectra are broad with a maximum located at approximately 500–520 nm. The deactivation of excited states of the synthesised molecules by various quenchers is also presented. The fluorescence quenching parameters were calculated using the Stern-Volmer equation. It was found, that the fluorescence quenching of the excited dye by quencher molecules occurs. The fluorescence quenching rates (kq) are about 1010 M−1 × s−1.

Energy transfer in polymeric film: Selective photoluminescent approach applied in durable optical sensing of flumequine in pharmaceutical and veterinary formulations

This work reports for energy transfer from the antibiotic flumequine (FLQ) to Tb3+ ions in homogenous solution in comparison to heterogeneous polymeric matrix. Accordingly, antenna sensitization of Tb3+ PL is recorded and successfully augmented to the optical sensation of FLQ. The polymeric host “PMMA” raises positive impacts to the antenna sensitization due to its co-sensitization of Tb3+ ions, isolating the probing system from occasional quenchers and delimiting the nonradiative deactivation of excited state. Thereby, the energy transfer is markedly improved and the Tb3+ PL intensity is increased by ≈2.5 times over the working in DMF homogenous solutions where LOD and LOQ reveals estimates of 3.0 × 10−9 versus 8.5 × 10−9 mol L−1 and 1.25 × 10−8 versus 2.82 × 10−8 mol L−1, respectively. FTIR and SEM analysis confirm the FLQ chelation to surfacial Tb3+ ions impregnated in PMMA. The method applicability was validated via the assaying of FLQ analyte in veterinary medicines and spiked poultry serum samples. Closeness of the %recovery to 100% with appropriate %RSD indicates fairly good accuracy of the proposed sensor. Furthermore, the improved photostability and good mechanical strength of the Tb3+/PMMA probing system enables the sensation of FLQ within a facile, high selectivity, sensitivity and durability over analysis using homogenous solutions of probe.

Photoprotective mechanisms in the core LH1 antenna pigment-protein complex from the purple photosynthetic bacterium, Rhodospirillum rubrum

Time-resolved absorption spectra from the femtoscecond to sub-millisecond time regimes were recorded in order to eluciate the photoprotective mechanisms of carotenoid (spirilloxanthin) in core LH1 light-harvesting pigment-protein complexes from a purple photosynthetic bacterium, Rhodospirillum (Rsp.) rubrum. Carotenoids can prevent the production of singlet oxygen by rapidly quenching the Bchl a triplet state. Through this triplet-triplet energy-transfer reaction the carotenoid is believed to protect the biological system from exposure to excess light. We set out to investigate this paricular function in the purple bacterium, Rsp. rubrum strain S1. We also investigated Rsp. rubrum strain G9+, which is the carotenoidless mutant of Rsp. rubrum, to make sure that the carotenoid is truly playing this important role. As a result, we could demonstrate that strain S1 realizes nearly 100% excitation energy transfer from triplet Bchl a to carotenoid. In addition, we found another major excited state deactivation channel of carotenoids, which takes away excess energy directly from the singlet excited state of Bchl a and so prevents the production of triplet excited Bchl a.

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.

Direct Observation of Different One- and Two-Photon Fluorescent States in a Pyrrolo[3,2-b]pyrrole Fluorophore.

Two-photon fluorophores are frequently employed to obtain superior spatial resolution in optical microscopy applications. To guide the rational design of these molecules, a detailed understanding of their excited-state deactivation pathways after two-photon excitation is beneficial, especially to assess the often-assumed presumption that the one- and two-photon excited-state dynamics are similar after excitation. Here, we showcase the breakdown of this assumption for one- and two-photon excitation of a centrosymmetric pyrrolo[3,2-b]pyrrole chromophore by combining time-resolved fluorescence and broadband femtosecond transient absorption spectroscopy. Compared to one-photon excitation, where radiative decay dominates the photodynamics, two-photon excitation leads to dynamics arising from increased nonradiative decay pathways. These different photodynamics are manifest to different quantum yields, thus highlighting the types of time-resolved studies described here to be valuable guideposts in the design of two-photon fluorophores for imaging applications.

Elucidating the excited-state dynamics behavior in near-infrared Bodipy dye and aggregates toward biophotonics

Photo-induced excited-state dynamics within organic materials fundamentally determine their photophysical properties for various applications, and thus understanding the primary excited-state dynamics behavior is of fundamental and practical significance. Until recently, the excited-state dynamics of organic materials towards biophotonics have been rarely studied, although numerous endeavors have been devoted to the design of organic materials for biophotonics. Herein, various spectroscopy technologies including femtosecond transient absorption (fs-TA) spectroscopy clearly reveal a totally different excited state dynamics behavior within Bodipy monomer (2B-BDP dye) and aggregates (2B-BDP NPs), indicating strongly morphology dependent character. 2B-BDP dye undergoes an ultrafast conversion from S1 to intramolecular charge transfer (ICT) state for subsequent photoluminescence (PL) and nonradiative (NR) decay, while 2B-BDP NPs show an accelerated excited-state deactivation mainly through S1→S0 NR decay. The potential bioapplications based on the corresponding excited state dynamics behavior are discussed together with experimental results. Interestingly, the accelerated NR decay in 2B-BDP NPs does not yield a stronger photoacoustic (PA) signal than that in 2B-BDP dye, which violates the conventional wisdom that faster NR decay would benefit the photothermal effects for better photoacoustic imaging (PAI). These insightful and fundamental observations of the excited-state dynamics may contribute an alternative approach at the molecular level towards the future design of functional Bodipy-based organic molecules with desirable performances.