Excited State Behavior Research Articles

Effects of single atom doping on the ultrafast electron dynamics of M1Au24(SR)18 (M = Pd, Pt) nanoclusters.

Atomically precise, doped metal clusters are receiving wide research interest due to their synergistic properties dependent on the metal composition. To understand the electronic properties of doped clusters, it is highly desirable to probe the excited state behavior. Here, we report the ultrafast relaxation dynamics of doped M1@Au24(SR)18 (M = Pd, Pt; R = CH2CH2Ph) clusters using femtosecond visible and near infrared transient absorption spectroscopy. Three relaxation components are identified for both mono-doped clusters: (1) sub-picosecond relaxation within the M1Au12 core states; (2) core to shell relaxation in a few picoseconds; and (3) relaxation back to the ground state in more than one nanosecond. Despite similar relaxation pathways for the two doped nanoclusters, the coupling between the metal core and surface ligands is accelerated by over 30% in the case of the Pt dopant compared with the Pd dopant. Compared to Pd doping, the case of Pt doping leads to much more drastic changes in the steady state and transient absorption of the clusters, which indicates that the 5d orbitals of the Pt atom are more strongly mixed with Au 5d and 6s orbitals than the 4d orbitals of the Pd dopant. These results demonstrate that a single foreign atom can lead to entirely different excited state spectral features of the whole cluster compared to the parent Au25(SR)18 cluster. The detailed excited state dynamics of atomically precise Pd/Pt doped gold clusters help further understand their properties and benefit the development of energy-related applications.

Transient spectroscopic characterization of the ring‐opening reaction of tetrahydrochromeno[2,3‐dimethyl]indole

Tetrahydrochromeno is a structural variant of spiropyran that undergoes a reversible ring‐opening to generate a colored nitrophenolate intermediate. Earlier work confirmed this intermediate through trimethylsilyl cyanide trapping under continuous irradiation. We have performed transient absorption spectroscopy to further characterize the mechanism of the ring‐opening reaction. Excitation at 355 nm produced a transient species with an absorption maximum at 445 nm, which we assign to the nitrophenolate unit of the ring‐opened product. The transient absorption decays after ~970 ns with small optical density changes corresponding to a 0.15 quantum yield. Exposure to oxygen did not exhibit a significant deleterious effect on the photoisomerization of the chromeno dye. Time‐dependent density functional theory corroborated spectroscopic assignments of the starting chromeno and the putative ring‐opened intermediate. The excited state behavior of this system parallels the structurally similar oxazine system reported by Raymo and coworkers. The one significant difference is the longer lifetime of the photochemically generated intermediate from chromeno. Copyright © 2015 John Wiley & Sons, Ltd.

UV-induced DNA Damage: The Role of Electronic Excited States.

The knowledge of the fundamental processes induced by the direct absorption of UV radiation by DNA allows extrapolating conclusions drawn from in vitro studies to the in-vivo DNA photoreactivity. In this respect, the characterization of the DNA electronic excited states plays a key role. For a long time, the mechanisms of DNA lesion formation were discussed in terms of generic "singlet" and "triplet" excited state reactivity. However, since the beginning of the 21(st) century, both experimental and theoretical studies revealed the existence of "collective" excited states, i.e. excited states delocalized over at least two bases. Two limiting cases are distinguished: Frenkel excitons (delocalized ππ* states) and charge-transfer states in which positive and negative charges are located on different bases. The importance of collective excited states in photon absorption (in particular in the UVA spectral domain), the redistribution of the excitation energy within DNA, and the formation of dimeric pyrimidine photoproducts is discussed. The dependence of the behavior of the collective excited states on conformational motions of the nucleic acids is highlighted.

7-Hydroxyflavone Revisited. 2. Substitution Effect on Spectral and Acid-Base Properties in the Ground and Excited States.

Three derivatives of 7-hydroxy-2-phenyl-4H-chromen-4-one (7-hydroxyflavone), containing chloro, methoxy, and dimethylamino substituents at position 4', were synthesized and investigated from the view of their ground-state and electronically excited-state behavior. Spectral and fluorescent properties in a wide range of pH/H0, thermodynamics of the ground and S1 states, and kinetics of the excited-state deactivation of the compounds were investigated by means of steady-state electronic absorption, steady-state, and time-resolved fluorescent spectroscopies as well as by computational methods. The results are rationalized from the point of view of the substituent effect. In spite of a similar structure and the same acid-base centers, the compounds strongly differ in fluorescence characteristics as well as in the dependence of fluorescent properties on pH/H0 of the media. Various protolytic/tautomeric forms of the compounds investigated absorb light in the 300-500 nm range and fluoresce in the whole visible range of spectra. The electron-releasing substituents at position 4' of 7-hydroxyflavone immensely affect spectral properties as well as the excited-state thermodynamics and kinetics, whereas the electron-withdrawing ones cause minimal effect.

Computational Insights into Excited-State Proton-Transfer Reactions in Azo and Azomethine Dyes.

State of the art density functional theory approaches are employed to provide an accurate description of the photophysical properties of azodyes and Schiff bases displaying intramolecular hydrogen-bonding features. These compounds exist as tautomeric mixtures at the ground state and, in the case of Schiff bases, an excited-state intramolecular proton transfer (ESIPT) occurs upon excitation. The experimentally observed photophysical properties are discussed here in light of the theoretical findings. To rationalize the different experimentally observed radiative behavior of the azo and azomethine structures, a nonradiative decay pathway that is possibly active in such systems is determined. The characterization of this deactivation path, tested for two related compounds exhibiting different fluorescence quantum yields, enables us to disentangle the different and contrasting effects governing the excited-state behavior of these molecular systems.

Photochemistry within a water-soluble organic capsule.

Photochemistry along with life as we know it originated on earth billions of years ago. Supramolecular Photochemistry had its beginning when plants that sustain life began transforming water into oxygen by carrying out light initiated reactions within highly organized assemblies. Prompted by the efforts of J. Priestly (photosynthesis), F. Sestini, S. Cannizaro, and C. Liebermann (solid-state photochemistry of santonin, quinones, and cinnamic acid), orderly scientific investigations of the link between light absorption by matter and molecules and the chemical and physical consequences began in the mid-1700s. By 1970 when Molecular Photochemistry had matured, it was clear that controlling photochemical reactions by conventional methods of varying reaction parameters like temperature and pressure would be futile due to the photoreactions' very low activation energies and enthalpies. During the last 50 years, the excited state behavior of molecules has been successfully manipulated with the use of confining media and weak interactions between the medium and the reactant molecule. In this context, with our knowledge from experimentation with micelles, cyclodextrins (CD), cucurbitruils (CB), calixarenes (CA), Pd nanocage, crystals, and zeolites as media, we began about a decade ago to explore the use of a new water-soluble synthetic organic cavitand, octa acid (OA) as a reaction container. The uniqueness of OA as an organic cavitand lies in that two OA molecules form a closed hydrophobic capsule to encapsulate water-insoluble guest molecule(s). The ability to include a large number of guest molecules in OA has provided an opportunity to examine the excited state chemistry of organic molecules in a hydrophobic, confined environment. OA distinguishes itself from the well-known cavitands CD and CB by its active reaction cavity absorbing UV-radiation between 200 and 300 nm and serving as energy, electron, and hydrogen donor. The freedom of guest molecules in OA, between that in crystals and isotropic solution can be transformed into photoproducts selectivity. The results of our photochemical investigations elaborated in this Account demonstrate that OA with a medium sized cavity exerts better control on excited state processes than the more common and familiar organic hosts such as CD, CB, CA, and micelles. By examining the photochemistry of a number of molecules (olefins, carbonyls, aromatics and singlet oxygen) that undergo varied reactions (cleavage, cycloaddition, cis-trans isomerization, oxidation and cyclization) within OA capsule, we have demonstrated that the free space within the container, the capsule influenced conformation and preorientation of guest molecules, supramolecular steric control, and capsular dynamics contribute to the altered excited state behavior. In this Account, we have shown that photochemistry based on concepts of physical organic and supramolecular chemistry continues to be a discipline with unlimited potential. The future of supramolecular photochemistry lies in synthetic, materials, medicinal, and biological chemistries. Success in these areas depends on synthesizing well-designed water-soluble hosts that can emulate complex biological assemblies, organizing and examining the behavior of supramolecular assemblies on solid surfaces, rendering the photoreactions catalytic, and delivering encapsulated drugs in a targeted fashion.

Theoretical Study on the Excited Electronic States of CHCl: Application to Photodissociation at 193 nm.

We present herein a high-level ab initio study on the electronic excited states of CHCl using the internally contracted multireference configuration interaction method including Davidson correction (icMRCI+Q). A total of 13 electronic states with energy of up to 7 eV have been investigated. The vertical transition energies, oscillator strengths, electron configurations, and transitions of the electronic states of CHCl have been calculated at the icMRCI+Q/aug-cc-pv(5+d)Z level. The potential energy curves of the electronic states have been studied along the H-C-Cl angle, the C-H bond length, and the C-Cl bond length, respectively. Our theoretical study has provided comprehensive information for understanding the interaction and the behavior of the electronic excited states of CHCl. In particular, the excited state involved in the 193 nm photodissociation as well as the corresponding dissociation dynamics have been discussed on the basis of our calculation results. The present study should shed more light on the photochemistry of CHCl in the ultraviolet region.

Excited state behavior of benzoxazole derivatives in a confined environment afforded by a water soluble octaacid capsule

Abstract This feature article presents a brief description of the excited state intramolecular proton transfer mechanism (ESIPT), describes the solvent dependence of the ESIPT phenomenon and present the potential applications that can be envisaged for these fluorophores. Photophysics of three dye molecules exhibiting ESIPT were examined free in solution and within a water soluble supramolecular host, octaacid (OA) by steady-state and time resolved fluorescence emission techniques. Inclusion of the above dyes within OA was confirmed by 1H NMR spectra. The free and octaacid complexed dyes had electronic absorption in the UV region. Within the OA container the fluorophores showed fluorescence emission similar to that in benzene, dichloromethane and acetonitrile. Moreover, the observed large Stokes’ shift in the emission of the dyes when confined within OA capsule was unlike that in aqueous medium. The results suggest that a confined medium is a powerful tool to tailor the fluorescence emission of ESIPT compounds in aqueous media.

Understanding the Excited State Behavior of Cyclometalated Bis(tridentate)ruthenium(II) Complexes: A Combined Experimental and Theoretical Study.

The synthesis and characterization of the donor-acceptor substituted cyclometalated ruthenium(II) polypyridine complex isomers [Ru(dpb-NHCOMe)(tpy-COOEt)](PF6) 1(PF6) and [Ru(dpb-COOEt)(tpy-NHCOMe)](PF6) 2(PF6) (dpbH = 1,3-dipyridin-2-ylbenzene, tpy = 2,2';6,2"-terpyridine) with inverted functional group pattern are described. A combination of resonance Raman spectroscopic and computational techniques shows that all intense visible range absorption bands arise from mixed Ru → tpy/Ru → dpb metal-to-ligand charge transfer (MLCT) excitations. 2(PF6) is weakly phosphorescent at room temperature in fluid solution and strongly emissive at 77 K in solid butyronitrile matrix, which is typical for ruthenium(II) polypyridine complexes. Density functional theory calculations revealed that the weak emission of 2(PF6) arises from a (3)MLCT state that is efficiently thermally depopulated via metal-centered ((3)MC) excited states. The activation barrier for the deactivation process was estimated experimentally from variable-temperature emission spectroscopic measurements as 11 kJ mol(-1). In contrast, 1(PF6) is nonemissive at room temperature in fluid solution and at 77 K in solid butyronitrile matrix. Examination of the electronic excited states of 1(PF6) revealed a ligand-to-ligand charge-transfer ((3)LL'CT) as lowest-energy triplet state due to the very strong push-pull effect across the metal center. Because of the orthogonality of the participating ligands, emission from the (3)LL'CT is symmetry-forbidden. Hence, in this type of complex a stronger push-pull effect does not increase the phosphorescence quantum yields but completely quenches the emission.

Solvent Dependent Excited State Behaviors of Luminescent Gold(I)–Silver(I) Cluster with Hypercoordinated Carbon

Polynuclear Au(I) complexes continues to attract considerable attention because of their bright emissions in the visible wavelength, which hold promise in applications in luminescence, fluorescence sensing, and bioimaging. Despite various spectroscopic investigations on their steady state properties, detailed understanding of the origin of their emissions and excited state relaxations is still lacking. Here, we report femtosecond time-resolved transient absorption experiments combined with quantum chemical calculations on a brightly emissive [Au6Ag2(C)(dppy)6](BF4)4 cluster in different solvents. Global analysis on the transient absorption spectra based on a sequential model gives three spectral components: (1) excited state absorption (ESA) of 1MLCTAu state (τ = 1–3 ps); (2) ESA of 3MLCTAu state (τ = 11–40 ps), and (3) ESA of 3MLCTAg state (long-lived). By variation of the solvent’s polarity and hydrogen bonding ability, the relative population of the triplet MLCT states and the emission properties can b...

Multifaceted Excited State of CH3NH3PbI3. Charge Separation, Recombination, and Trapping.

A need to understand the excited-state behavior of organic-inorganic hybrid perovskites, such as CH3NH3PbI3, has arisen due to the rapid development of perovskite solar cells. The photoinduced processes leading to the efficient charge separation observed in these materials remain somewhat elusive. This Perspective presents an overview of the initial attempts to characterize the excited-state and charge recombination dynamics in the prototypical material CH3NH3PbI3. While much has been accomplished in designing high-efficiency solar cells, the multifaceted nature of the CH3NH3PbI3 excited state offers ample challenges for the photovoltaic community to better comprehend. Building on this foundation may enable us to tackle the stability concerns that have shadowed the rise of perovskite solar cells. Furthermore, a better understanding of the excited-state properties can provide insight into the specific properties that have thrust this material to the forefront of photovoltaic research.

Folding-induced modulation of excited-state dynamics in an oligophenylene-ethynylene-tethered spiral perylene bisimide aggregate.

The excited-state photophysical behavior of a spiral perylene bisimide (PBI) folda-octamer (F8) tethered to an oligophenylene-ethynylene scaffold is comprehensively investigated. Solvent-dependent UV/Vis and fluorescence studies reveal that the degree of folding in this foldamer is extremely sensitive to the solvent, thus giving rise to an extended conformation in CHCl(3) and a folded helical aggregate in methylcyclohexane (MCH). The exciton-deactivation dynamics are largely governed by the supramolecular structure of F8. Femtosecond transient absorption (TA) in the near-infrared region indicates a photoinduced electron-transfer process from the backbone to the PBI core in the extended conformation, whereas excitation power- and polarization-dependent TA measurements combined with computational modeling showed that excitation energy transfer between the unit PBI chromophores is the major deactivation pathway in the folded counterpart.

Solvent impact on the photophysical properties and excited state behavior of p-aminobenzoic acids

• The impact of solvent on photo-induced charge transfer is analysed. • The quantum yield of two derivatives is determined in different solvents. • Carbonyl stretching (IR) modes shift in protic solvents. • In methanol there is a thermodynamic barrier, but none in aprotic solvents. The photophysical properties of p -aminobenzoic acid and dimethyl- p -aminobenzoic acid are compared in a range of solvents. The electronic spectra are used to determine quantum efficiencies and evaluate excited state behavior. The latter system is found to undergo a charge transfer following photoexcitation and the thermodynamics for the process are assessed. The strength and type of solvent–solute interaction are important in determining the driving force for the charge transfer, with entropy being dominant in protic solvents and enthalpy in aprotic solvents. Vibrational spectroscopy and calculations provide further insight into the nature of the specific solvent–solute interactions, with shifts in the infrared spectrum observed due to hydrogen bonding interactions with the solvent.

Towards ratiometric sensing of amyloid fibrils in vitro.

The aggregation of amyloid-β peptide and its accumulation in the human brain has an important role in the etiology of Alzheimer's disease. Thioflavin T has been widely used as a fluorescent marker for these amyloid aggregates. Nevertheless, its complex photophysical behavior, with strong wavelength dependencies of all its fluorescence properties, requires searching for new fluorescent probes. The use of 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine (HPIP), which shows two emission bands and a rich excited-state behavior due to the existence of excited-state intramolecular processes of proton transfer and charge transfer, is proposed. These properties result in a high sensitivity of HPIP fluorescence to its microenvironment and cause a large differential fluorescence enhancement of the two bands upon binding to aggregates of the amyloid-β peptide. Based on this behavior, a very sensitive ratiometric method is established for the detection and quantification of amyloid fibrils, which can be combined with the monitoring of fluorescence anisotropy. The binding selectivity of HPIP is discussed on the basis of the apparent binding equilibrium constants of this probe to amyloid-β (1-42) fibrils and to the nonfibrillar protein bovine serum albumin. Finally, an exhaustive comparison between HPIP and thioflavin T is presented to discuss the sensitivity and specificity of these probes to amyloid aggregates and the significant advantages of the HPIP dye for quantitative determinations.

Triplet excited States as a source of relevant (bio)chemical information.

The properties of triplet excited states are markedly medium-dependent, which turns this species into valuable tools for investigating the microenvironments existing in protein binding pockets. Monitoring of the triplet excited state behavior of drugs within transport proteins (serum albumins and α1-acid glycoproteins) by laser flash photolysis constitutes a valuable source of information on the strength of interaction, conformational freedom and protection from oxygen or other external quenchers. With proteins, formation of spatially confined triplet excited states is favored over competitive processes affording ionic species. Remarkably, under aerobic atmosphere, the triplet decay of drug@protein complexes is dramatically longer than in bulk solution. This offers a convenient dynamic range for assignment of different triplet populations or for stereochemical discrimination. In this review, selected examples of the application of the laser flash photolysis technique are described, including drug distribution between the bulk solution and the protein cavities, or between two types of proteins, detection of drug-drug interactions inside proteins, and enzyme-like activity processes mediated by proteins. Finally, protein encapsulation can also modify the photoreactivity of the guest. This is illustrated by presenting an example of retarded photooxidation.

Luminescent copper(I) halide and pseudohalide phenanthroline complexes revisited: simple structures, complicated excited state behavior.

We have synthesized a series of luminescent trigonal [CuX(dtbphen)] (X = I (), Br (), Cl (), CN (), dtbphen = 2,9-di-tert-butylphenanthroline) and tetrahedral [Cu2(μ-I)2(L)2] (L = phenanthroline (), 2,9-dimethylphenanthroline ()) copper diimine complexes. Bearing in mind the chemical simplicity of this class of long-known Cu(i) phenanthroline compounds, it is surprising that they exhibit non-trivial photophysical properties, which have not been fully recognized. They display broad XMLCT absorption between ca. 450-600 nm, and the broad emission between ca. 550-850 nm in the solid state occurring with lifetimes on the μs timescale indicates phosphorescence, although the energetic overlap between excitation and emission suggests thermally activated delayed fluorescence (TADF) from S1. In line with the latter assumption, low temperature measurements of in the solid state show an energetic separation of emission and excitation. However, a counter-intuitive decrease of emission intensity and simultaneous increase of the emission lifetime at low temperatures are observed for , which indicates two triplet states also being involved. Our DFT and TD-DFT calculations show that emission from the lowest excited triplet state T1 is of (3)LMXCT nature, separated by only ca. 0.16 eV from S1. Low temperature photophysical measurements at 77 K in a glassy matrix of in 2-Me-THF and of in the solid state are in agreement with the theoretical results, revealing in addition that π-interactions in the solid state also greatly influence the photophysical properties, making a clear conclusion towards TADF ambiguous. This study suggests that other related simple and long-known Cu(i) systems may exhibit a similar, more complex excited state behavior than previously appreciated, involving several emitting states and important intermolecular interactions.

Surface enhanced Raman spectral studies of 2-bromo-1,4-naphthoquinone

Silver nanoparticles have been synthesized by a simple and inexpensive solution combustion method with urea as fuel. The structural and morphology of the silver nanoparticles were investigated through X-ray powder diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersion Spectra (EDS) techniques. Structural and morphological results confirmed the nanocrystalline nature of the silver nanoparticles. Density Functional Theory (DFT) calculations were also performed to study the ground and excited state behavior of 2-bromo-1,4-naphthoquinone (2-BrNQ) and 2-BrNQ on silver nanoparticles. Surface-Enhanced Raman Scattering (SERS) spectra of 2-BrNQ adsorbed on silver nanoparticles were investigated. The CO, CH in-plane bending and CBr stretching modes were enhanced in SERS spectrum with respect to normal Raman spectrum. The spectral analysis reveals that the 2-BrNQ adsorbed ‘stand-on’ orientation on the silver surface. Density Functional Theory (DFT) calculations are also performed to study the vibrational features of 2-BrNQ molecule and 2-BrNQ molecule on silver surface.

Electronic Interactions of Michler's Ketone with DNA Bases in Synthetic Hairpins

The mechanism and dynamics of photoinduced electron transfer in two families of DNA hairpins possessing Michler's ketone linkers have been investigated by means of steady state and time-resolved transient absorption and emission spectroscopies. The excited state behavior of the diol linker employed in hairpin synthesis is similar to that of Michler's ketone in methanol solution. Hairpins possessing only a Michler's ketone linker undergo fast singlet state charge separation and charge recombination with an adjacent purine base, attributed to well-stacked ground state conformations, and intersystem crossing to the triplet state, attributed to poorly stacked ground state conformations. The failure of the triplet to undergo electron transfer reactions on the 7 ns time scale of our measurements is attributed to the low triplet energy and reduction potential of the twisted triplet state. Hairpins possessing both a Michler's ketone linker and a perylenediimide base surrogate separated by four base pairs undergo photoinduced hole transport from the diimide to Michler's ketone upon excitation of the diimide. The efficiency of hole transport is dependent upon the sequence of the intervening purine bases.

Excited-state proton and charge transfer in protonated amino and methylated derivatives of 2-(2'-hydroxyphenyl)benzimidazole.

We studied the excited-state behavior of a family of mono- and diprotonated derivatives of 2-phenylbenzimidazole in different solvents, using steady-state and time-resolved fluorescence spectroscopy. The species investigated were 2-(4'-amino-2'-hydroxyphenyl)benzimidazole (1), the diethylamino analogue 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (2) and its N-methylated derivative 1-methyl-2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (3). The O-methoxy derivatives of 2 and 3 (2-OMe and 3-OMe), and the simpler models 2-phenylbenzimidazole (4) and its 4'-amino (5) and 4'-dimethylamino (6) derivatives were also studied. We found that the dications of 1, 2, and 3 (protonated at the benzimidazole N3 and at the amino group) were strong photoacids, which were deprotonated at the hydroxyl group upon excitation in aqueous solution (totally for 2 and 3) to give a tautomer of the ground-state monocation. In contrast, no photodissociation was observed for the monocations of these species. Instead, some of the monocations studied behaved as molecular rotors, for which electronic excitation led to a twisted intramolecular charge transfer (TICT) state. The monocations of 2, 3, 2-OMe, 3-OMe, and 6, protonated at the benzimidazole N3, experienced a polarity- and viscosity-dependent radiationless deactivation associated with a large-amplitude rotational motion. We propose that this process is connected to an intramolecular charge transfer from the dimethylaminophenyl or diethylaminophenyl moiety (donor) to the protonated benzimidazole group (acceptor) of the excited monocation, which yields a twisted charge-transfer species. No fluorescence from this species was detected except for 3 and 3-OMe in low-viscosity solvents.

Estimation of photo-degradation of dielectrics surrounding the narrow channel due to PD activity

Partial discharge (PD) taking place within narrow channels produces a large number of charged particles, causing degradation of the polymer. Additionally, PD pulses also produce UV photons due to decay of excited states (radiative states) in air. These may have enough energy to break C–C bonds and thereby add to the degradation of the dielectrics surrounding the narrow channel. In this paper, a radiation transport (RT) model has been developed and integrated with a Particle-in-Cell/Monte Carlo collision (PIC-MCC) model to study the behavior of excited (radiative) states of air within discharge in the narrow channel. The radiative state atoms are described by a fluid model combined with the Holstein–Biberman equation. This model has the ablation to follow the spatial evolution of the radiative excited-state density throughout narrow channel. The effect of applied electric field, narrow channel dimensions, gas pressure on the extent of degradation of dielectrics surrounding the narrow channel is studied.