Excited-State Hydrogen Bonding Dynamics of Hydrogen-Bonded Clusters Formed by of Coumarin Derivatives in Aqueous Solution: A Time-Dependent Density Functional Theory Study

The time-dependent density functional theory and the density functional theory are used to investigate the nature of hydrogen bonds formed by the derivative of the coumarin (TFKC) and the water molecules. The ground-state geometry optimizations, electronic excited energies and corresponding oscillation strengths for the TFKC monomer, the hydrogen-bonded TFKC–Water (HBA) dimer, TFKC–Water (HBB) dimer and TFKC–2Water complex are calculated. We find that, upon photoexcitation, the weaker hydrogen bond in the ground state will be affected by the relatively large impact for TFKC in the water. For better understanding the properties of the hydrogen bonds in the excited states, the frontier molecular orbitals of the S0 and S1 states are shown, and we find the obvious electron density transitions form the water molecules to the TFKC monomer. The electron transfer is expected to be the reason the hydrogen bond dynamics happens.     

Competing Excited-State Hydrogen and Proton-Transfer Processes in 6-Azaindole-S3,4 and 2,6-Diazaindole-S3,4 Clusters (S=H2O,NH3)

Excited state hydrogen (ESHT) and proton (ESPT) transfer reaction pathways in the three and four solvent clusters of 6-azaindole (6AI-S_3,4) and 2,6-diazaindole (26DAI-S_3,4)(S=H₂O, NH₃) were computationally investigated to understand the fate of photo-excited biomolecules. The ESHT energy barriers in (H₂O)₃ complexes (39.6–41.3 kJmol⁻¹) were decreased in (H₂O)₄ complexes (23.1–20.2 kJmol⁻¹). Lengthening the solvent chain lowered the barrier because of the relaxed transition states geometries with reduced angular strains. Replacing the water molecule with ammonia drastically decreased the energy barriers to 21.4–21.3 kJmol⁻¹ in (NH₃)₃ complexes and 8.1–9.5 kJ mol⁻¹ in (NH₃)₄ complexes. The transition states were identified as H_a atom attached to the first solvent molecule. The formation of stronger hydrogen bonds in (NH₃)_3,4 complexes resulted in facile ESHT reaction than that in the (H₂O)_3,4 complexes. The ESPT energy barriers in 6AI-S_3,4 and 26DAI-S_3,4 were found to range between 40–73 kJmol⁻¹. The above values were significantly higher than that of the ESHT processes and hence are considered as a minor channel in the process. The effect of N(2) insertion was explored for the very first time in the isolated solvent clusters using local vibrational mode analysis. In DAI-S₄, the higher K_a(H_a⋯S_a) values depicted the increased photoacidity of the N(1)-H_a group which may facilitate the hydrogen transfer reaction. However, the increased N(6)⋯H_b bond length elevated the reaction barriers. Therefore, in the ESHT reaction channel, the co-existence of two competing factors led to a marginal/no change in the overall energy barriers due to the N(2) insertion. In the ESPT reaction pathway, the energy barriers showed notable increase upon N(2) insertion because of the increased N(6)⋯H_b bond length.     

Excited-State Raman Spectroscopy of Inorganic Compounds

Abstract— Raman spectra of inorganic complexes in excited electronic states are discussed. A brief overview of the field of transient Raman spectroscopy and experimental considerations are presented. Two examples from the author's laboratory are used to illustrate the type of information that can be obtained. The first example, an excited-state Raman spectroscopic study of K₃[Mn(CN)₅NO], is chosen because it illustrates the connections between excited-state molecular structure and vibrational properties. The pump pulse causes a change from a linear sp-hybridized NO containing a triple bond to a bent sp²-hybridized NO containing a double bond. Both the NO stretch and normal modes involving other ligands are measured and interpreted. The second example is chosen to illustrate the vibrational consequences of photoinduced electron transfer. The Raman spectra of W(CO)₄(diimine) complexes (diimine = 2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine, and isopropyl-pyridine-2-carbaldehyde imine) in the lowest tungsten to diimine charge transfer excited state are discussed. The excited-state peaks are assigned to ligand ring deformation modes and to carbonyl stretching modes. The totally symmetric cis -carbonyl stretching mode in the charge transfer excited state is about 50 cm' higher in energy than that of the molecule in the ground electronic state. The increase is interpreted in terms of loss of metal-car-bonyl back-bonding in the charge transfer excited state. Finally, a summary of the field's strengths and difficulties and a brief discussion of the future perspectives are presented.     

Two-Photon Absorption and Excited-State Refractions in Anthraquinone Dyes

We examine the nonlinear optical properties of solutions of 9,10-anthraquinone, 1,4-dihydroxy-anthraquinone, 5,8-dichloro-1,4-dihydroxy-anthraquinone and 1,4-bisethylamino-anthraquinone by means of the intensity-dependent transmission and the Z-scan method with 532 nm 8 ns pulses. The results demonstrated that the 9,10-anthraquinone displayed a two-photon absorption (TPA) character. The molecular TPA cross section of this compound is estimated as σ₂ = 7.22 × 10^?19 cm⁴/GW or σ₂ = 26.96 × 10^?48 cm⁴/photon/s, respectively, and an essential state of ‘m¹Ag’, which should be located near the 2ω virtual transition state, is predicted. On the other hand, the effective excited-state nonlinear refractive coefficients n₂^eff of these anthraquinones are found to be ～10^?10 esu, which are comparable with those highly conjugated planar molecules of tetrabenzporphyrins and phthalocyanines.     

间位基团激发态取代基常数的扩展及应用

合成了七个系列含间位取代基X的二苯乙烯m-XArCH=CHArY-p(简称m-XSBY-p),其中X为NO_2、I、CHCH_2、Ph、Et、NMe_2和CCH。在无水乙醇中测定它们的紫外(UV)吸收光谱,得到紫外吸收最大波长λ_(max)(nm)。对λ_(max)的波数ν_(max)(cm~(-1))进行定量相关,采用曲线拟合方法,得到上述7个间位基团的激发态取代基常数σ_(CC(m))~(ex)。将对位基团和间位基团的σ_(CC)~(ex)与Hammett常数σ进行对比,表明σx_(CC)~(ex)与σ分别表达取代基不同的电子效应。另外,合成了含上述间位基团的二芳基希夫碱(10个)和二苯乙烯(14个),用所得σ_(CC(m))~(ex)预测它们的λ_(max),并用实验测定它们的λ_(max),结果表明预测值与实验值相吻合,验证了所得σ_(CC(m))~(ex)常数的可靠性。收集了225个化合物(涉及二取代苯及二苯乙烯)的ν_(max),建立了一个统一的定量方程来表达这些化合物ν_(max)的变化规律。     

Excited-State Symmetry Breaking in an Aza-Nanographene Dye

The photophysics of a structurally unique aza-analogue of polycyclic aromatic hydrocarbons characterized by 12 conjugated rings and a curved architecture was studied in detail. The combined experimental and computational investigation reveals that the lowest excited state has charge-transfer character, in spite of the absence of any peripheral electron-withdrawing groups. The exceptionally electron-rich core comprised of two fused pyrrole rings is responsible for it. The observed strong solvatofluorochromism is related to symmetry breaking occurring in the emitting excited state, leading to a significant dipole moment (13.5 D) in the relaxed excited state. The anomalously small fluorescence anisotropy of this molecule, which is qualitatively different from what is observed in standard quadrupolar dyes, is explained as due to the presence of excited states that are close in energy but have different polarization directions.     

染料敏化太阳电池中染料的激发态超快动力学

正超快时间分辨光谱(ultrfast time-resolved spectroscopy)在飞秒甚至皮秒至纳秒时间尺度通过光谱技术探究超快的物质运动和变化,可用于研究激发态、过渡态的的瞬时结构变化和能量变化,可以获得化学反应的实时物理图象。该技术诞生于1987年,并于1999年获得诺贝尔化学奖~(1,2),     

Excited-State Intramolecular Proton Transfer: A Short Introductory Review

In this short review, we attempt to unfold various aspects of excited-state intramolecular proton transfer (ESIPT) from the studies that are available up to date. Since Weller’s discovery of ESIPT in salicylic acid (SA) and its derivative methyl salicylate (MS), numerous studies have emerged on the topic and it has become an attractive field of research because of its manifold applications. Here, we discuss some critical aspects of ESIPT and tautomerization from the mechanistic viewpoint. We address excitation wavelength dependence, anti-Kasha ESIPT, fast and slow ESIPT, reversibility and irreversibility of ESIPT, hydrogen bonding and geometrical factors, excited-state double proton transfer (ESDPT), concerted and stepwise ESDPT.     

Total Synthesis of Farnesin through an Excited-State Nazarov Reaction

The asymmetric total synthesis of farnesin, a rearranged ent-kaurenoid, was achieved through a convergent approach involving photo-Nazarov and intramolecular aldol cyclizations to build the syn-syn-syn hydrofluorenol ABC ring system and bicyclo[3.2.1]octane CD ring system in the first application of a UV-light-induced excited-state Nazarov cyclization of a non-aromatic dicyclic divinyl ketone in a total synthesis. Unlike the conventional acid-promoted ground-state Nazarov reaction, the excited-state Nazarov reaction enables stereospecific formation of the highly strained syn-syn-syn-fused hydrofluorenone scaffold through a disrotatory cyclization.     

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.     

Nonradiative Excited-State Decay via Conical Intersection in Graphene Nanostructures

Chemical groups are known to tune the luminescent efficiencies of graphene-related nanomaterials, but some species, including the epoxide group (−COC−), are suspected to act as emission-quenching sites. Herein, by performing nonadiabatic excited-state dynamics simulations, we reveal a fast (within 300 fs) nonradiative excited-state decay of a graphene epoxide nanostructure from the lowest excited singlet (S₁) state to the ground (S₀) state via a conical intersection (CI), at which the energy difference between the S₁ and S₀ states is approximately zero. This CI is induced after breaking one C−O bond at the −COC− moiety during excited-state structural relaxation. This study ascertains the role of epoxide groups in inducing the nonradiative recombination of the excited electron-hole, providing important insights into the CI-promoted nonradiative de-excitations and the luminescence tuning of relevant materials. In addition, it shows the feasibility of utilizing nonadiabatic excited-state dynamics simulations to investigate the photophysical processes of the excited states of graphene nanomaterials.     

Single-Benzene Dual-Emitters Harness Excited-State Antiaromaticity for White Light Generation and Fluorescence Imaging

Molecular emitters simultaneously generating light at different wavelengths have wide applications. With a small molecule, however, it is challenging to realize two independent radiative pathways. We invented the first examples of dual-emissive single-benzene fluorophores (SBFs). Two emissive tautomers are generated by synthetic modulation of the hydrogen bond acidity, which opens up pathways for excited-state proton transfer. White light is produced by a delicate balance between the energy and intensity of the emission from each tautomer. We show that the excited-state antiaromaticity of the benzene core itself dictates the proton movements driving the tautomer equilibrium. Using this simple benzene platform, a fluorinated SBF was synthesized with a record high solubility in perfluorocarbon solvents. White light-emitting devices and multicolor imaging of perfluorocarbon nanodroplets in live cells demonstrate the practical utility of these molecules.     

Molecular Dynamics Simulation of the Excited-State Dynamics of Bacteriorhodopsin

Abstract— The excited-state dynamics of bacteriorhodopsin was studied by molecular dynamics simulation. For the purpose of suppressing large displacement of amino acid residues on the surface of bacteriorhodopsin, positional restraints were imposed on these residues. A new method was developed to investigate the movement of amino acid residues upon photoexcitation and their role on the ultrafast photoisomerization of the chromophore. The structural change of bacteriorhodopsin was then traced up to 200 fs, i.e. until the formation of the intermediate I. We found that when all the conjugated bonds of the chromophore were allowed to twist freely in the excited state, many bonds including the C13=C14 bond twist in large scale within 100 fs. When only the C13=C14 bond and the single bonds were allowed to twist freely, the twisting took place at most 20° within 200 fs. From these results, it is claimed that a special potential surface is provided for the C13=C14 bond twisting by the protein environment in the course of the isomerization reaction, giving rise to the specific, ultrafast photoisomerization of bacteriorhodopsin. As a trace of such a mechanism, we observed that several functionally important residues incuding Asp85, Asp212 and Tyr185 responded quickly to the photoexcitation of the chromophore.     

Bis(tridentate) Ruthenium-Terpyridine Complexes Featuring Microsecond Excited-State Lifetimes

A series of heteroleptic bis(tridentate) ruthenium(II) complexes, each bearing a substituted 2,2':6',2″-terpyridine (terpy) ligand, is characterized by room temperature microsecond excited-state lifetimes. This observation is a consequence of the strongly σ-donating and weakly π-accepting tridentate carbene ligand, 2',6'-bis(1-mesityl-3-methyl-1,2,3-triazol-4-yl-5-idene)pyridine (C(∧)N(∧)C), adjacent to the terpy maintaining a large separation between the ligand field and metal-to-ligand charge transfer (MLCT) states while also preserving a large (3)MLCT energy. The observed lifetimes are the highest documented lifetimes for unimolecular ruthenium(II) complexes and are four orders in magnitude higher than that associated with [Ru(terpy)(2)](2+).     

GPU-Accelerated Large-Scale Excited-State Simulation Based on Divide-and-Conquer Time-Dependent Density-Functional Tight-Binding

The present study implemented the divide-and-conquer time-dependent density-functional tight-binding (DC-TDDFTB) code on a graphical processing unit (GPU). The DC method, which is a linear-scaling scheme, divides a total system into several fragments. By separately solving local equations in individual fragments, the DC method could reduce slow central processing unit (CPU)-GPU memory access, as well as computational cost, and avoid shortfalls of GPU memory. Numerical applications confirmed that the present code on GPU significantly accelerated the TDDFTB calculations, while maintaining accuracy. Furthermore, the DC-TDDFTB simulation of 2-acetylindan-1,3-dione displays excited-state intramolecular proton transfer and provides reasonable absorption and fluorescence energies with the corresponding experimental values. © 2019 Wiley Periodicals, Inc.     

Fluorescence Lifetimes of NIR-Emitting Molecules with Excited-State Intramolecular Proton Transfer

Molecular probes based on the excited-state intramolecular proton-transfer (ESIPT) mechanism have emerged to be attractive candidates for various applications. Although the steady-state fluorescence mechanisms of these ESIPT-based probes have been reported extensively, less information is available about the fluorescence lifetime characteristics of newly developed NIR-emitting dyes. In this study, four NIR-emitting ESIPT dyes with different cyanine terminal groups were investigated to evaluate their fluorescence lifetime characteristics in a polar aprotic solvent such as CH₂Cl₂. By using the time-correlated single-photon counting (TCSPC) method, these ESIPT-based dyes revealed a two-component exponential decay (τ₁ and τ₂) in about 2–4 nanoseconds (ns). These two components could be related to the excited keto tautomers. With the aid of model compounds (5 and 6) and low-temperature fluorescence spectroscopy (at −189 ℃), this study identified the intramolecular charge transfer (ICT) as one of the major factors that influenced the τ values. The results of this study also revealed that both fluorescence lifetimes and fractional contributions of each component were significantly affected by the probe structures.     

Excited-State Double Proton Transfer in 1-Azacarbazole Hydrogen Bonded Complexes

Excited-state double proton transfer (ESDPT) has been studied in a variety of 1-azacarbazole (1AC) hosted hydrogen-bonded complexes. In 1 AC/carboxylic acids hydrogen bonded complexes, large association constants of > 1.0 × 10⁴ M^?1 are measured in the ground state and the rate of ESDPT is » 5.0 × 10⁹ s^?1, resulting in a dominant proton-transfer tautomer emission. In several 1 AC/lactam hydrogen bonded complexes, however, spectral and dynamic results show the existence of a fast excited-state equilibrium between normal and proton-transfer tautomer states. The result can be tentatively rationalized by a non-catalytic ESDPT mechanism incorporating tautomerization energy of the guest molecule.