Quantum-Chemical Study of Proton Transfer and Photoreactions of Quinolone Derivatives

The possible routes of deactivation of electronically excited states of 4-methyl-7-hydroxy-2-quinolone and its protolytic forms were analyzed. The relative contribution of radiative and nonradiative deactivation channels of electronic excitation energy was determined, and the rate constants of photophysical processes (internal conversion and intersystem crossing) occurring upon light absorption by these forms were estimated.     

Quenching and blinking of fluorescence of a single dye molecule bound to gold nanoparticles

A fluorescein derivative (SAMSA) bound to gold nanoparticles of different diameters is investigated by time-resolved fluorescence at the single molecule level in a wide dynamic range, from nanosecond to second time scale. The significant decrease of both SAMSA excited state lifetime and fluorescence quantum yield observed upon binding to gold nanoparticles can be essentially traced back to an increase of the nonradiative deactivation rate, probably due to energy transfer, that depends on the nanoparticle size. A slow single molecule fluorescence blinking, in the ms time scale, has a marked dependence on the excitation intensity both under single and under two photon excitation. The blinking dynamics is limited by a low probability nonlinear excitation to a high energy state from which a transition to a dark state occurs. The results point out a strong coupling between the vibro-electronic configuration of the dye and the plasmonic features of the metal nanoparticles that provide dye radiationless deactivation channels on a wide dynamic range.     

Violation of Vavilov’s law upon excitation of luminescence of uranyl solutions at the short-wavelength absorption band (200–240 nm)

It was found that in the UV spectral region (200–240 nm) where intense absorption bands of the UO ₂ ²⁺ ion are located, excitation of its luminescence in solutions is not observed, thereby contradicting Vavilov’s law about independence of luminescence quantum yields from the excitation light wavelength. The violation of Vavilov’s law is explained in terms of nonradiative deactivation processes as the result of photoinduced electron transfer to the uranyl ion with its reduction to the pentavalent state and the subsequent disproportionation reaction to form uranium(IV). The presence of uranium(IV) ions during UV irradiation of uranyl solutions was proved by the chemiluminescent method.     

Spectroscopic and photophysical properties of hexanuclear rhenium(III) chalcogenide clusters

The electronic, vibrational, and excited-state properties of hexanuclear rhenium(III) chalcogenide clusters based on the [Re(6)(mu(3)-Q)(8)](2+) (Q = S, Se) core have been investigated by spectroscopic and theoretical methods. Ultraviolet or visible excitation of [Re(6)Q(8)](2+) clusters produces luminescence with ranges in maxima of 12 500-15 100 cm(-)(1), emission quantum yields of 1-24%, and emission lifetimes of 2.6-22.4 microseconds. Nonradiative decay rate constants and the luminescence maxima follow the trend predicted by the energy gap law (EGL). Examination of 24 clusters in solution and 14 in the solid phase establish that exocluster ligands engender the observed EGL behavior; clusters with oxygen- or nitrogen-based apical ligands achieve maximal quantum yields and the longest lifetimes. The excited-state decay mechanism was investigated by applying nonradiative decay models to temperature-dependent emission experiments. Solid-state Raman spectra were recorded to identify vibrational contributions to excited-state deactivation; spectral assignments were enabled by normal coordinate analysis afforded from Hartree-Fock and DFT calculations. Excited-state decay is interpreted with a model where normal modes largely centered on the [Re(6)Q(8)](2+) core induce nonradiative relaxation. Hartree-Fock and DFT calculations of the electronic structure of the hexarhenium family of compounds support such a model. These experimental and theoretical studies of [Re(6)Q(8)](2+) luminescence provide a framework for elaborating a variety of luminescence-based applications of the largest series of isoelectronic clusters yet discovered.     

Excitation-energy dependence of the phosphorescence quantum yields of pyridinecarboxaldehyde vapors

Emission and excitation spectra of 3- and 4-pyridinecarboxaldehyde vapors have been measured at different pressures down to 10(-2)Torr. The phosphorescence quantum yield measured at low pressure as a function of excitation energy is nearly constant in the range of excitation energy corresponding to the S1(n, pi*) state, but it decreases abruptly at the S2(pi, pi*) threshold. The onset of the abrupt decrease of the yield corresponds to the location of the S2 absorption origin of each molecule, indicating that the nonradiative pathway depends on the type of the excited singlet state to which the molecule is initially excited. The relaxation processes are discussed based on the pressure and excitation-energy dependence of the phosphorescence quantum yield.     

Proton-transfer photoreactions in hydrogen-bonded complexes and the formation of ion pairs by aromatic hydroxy compounds in the presence of crown ethers

This study is concerned with proton-transfer photoreactions in hydrogen-bonded complexes and the formation of ionized forms of naphthols in the ground state, in the presence of crown ethers. It has been found that a loss in electronic excitation energy may occur during the proton transfer in the hydrogen bond. The efficiencies of the observed radiationless deactivation processes aie controlled by the nature of the excited proton donor.     

Photophysical and electrochemical properties of 1,3-bis(2-pyridylimino)isoindolate platinum(II) derivatives

A series of twelve platinum(II) complexes of the form (N^N^N)PtX have been synthesized and characterized where N^N^N is 1,3-bis(2-pyridylimino)isoindolate ligands (BPI) or BPI ligands whose aryl moieties are substituted with tert-butyl, nitro, alkoxy, iodo or chloro groups, and X is a chloride, fluoride, cyano, acetate, phenyl or 4-(dimethylamino)phenyl ligand. All complexes display at least one irreversible oxidation and two reversible reduction waves at potentials dependent on the position and the electron donating or withdrawing nature of both X and the substituted N^N^N ligand. Broad room temperature phosphorescence ranging in energy from 594 to 680 nm was observed from the complexes, with quantum efficiencies ranging from 0.01 to 0.05. The efficiency of emission is dictated largely by nonradiative processes since the rate constants for nonradiative deactivation [(1.1-100) × 10(5) s(-1)] show greater variation than those for radiative decay [(0.57-4.0) × 0(4) s(-1)]. Nonradiative deactivation for compounds with X = Cl follow the energy gap law, i.e. the nonradiative rate constants increase exponentially with decreasing emission energy. Deactivation of the excited state appears to be strongly influenced by a non-planar distortion of the BPI ligand.     

Time-dependent density functional theory excited state nonadiabatic dynamics combined with quantum mechanical/molecular mechanical approach: photodynamics of indole in water

We present a combination of time-dependent density functional theory with the quantum mechanical/molecular mechanical approach which can be applied to study nonadiabatic dynamical processes in molecular systems interacting with the environment. Our method is illustrated on the example of ultrafast excited state dynamics of indole in water. We compare the mechanisms of nonradiative relaxation and the electronic state lifetimes for isolated indole, indole in a sphere of classical water, and indole + 3H(2)O embedded in a classical water sphere. In the case of isolated indole, the initial excitation to the S(2) electronic state is followed by an ultrafast internal conversion to the S(1) state with a time constant of 17 fs. The S(1) state is long living (>30 ps) and deactivates to the ground state along the N-H stretching coordinate. This deactivation mechanism remains unchanged for indole in a classical water sphere. However, the lifetimes of the S(2) and S(1) electronic states are extended. The inclusion of three explicit water molecules opens a new relaxation channel which involves the electron transfer to the solvent, leading eventually to the formation of a solvated electron. The relaxation to the ground state takes place on a time scale of 60 fs and contributes to the lowering of the fluorescence quantum yield. Our simulations demonstrate the importance of including explicit water molecules in the theoretical treatment of solvated systems.     

Excitation energy migration processes in cyclic porphyrin arrays probed by single molecule spectroscopy

By using single molecule fluorescence spectroscopy we have investigated the excitation energy migration processes occurring in a series of cyclic porphyrin arrays bearing a close proximity in overall architectures to the LH2 complexes in purple bacterial photosynthetic systems. We have revealed that the conformational heterogeneity induced by the structural flexibility in large cyclic porphyrin arrays, which provides the nonradiative deactivation channels as an energy sink or trap, reduces significantly the energy migration efficiency. Our study provides detailed information on the energy migration efficiency of the artificial light-harvesting arrays at the single molecule level, which will be a guideline for future applications in single molecular photonic devices in the solid state.     

Influence of diffusion on nonradiative energy transfer between FMN molecules in aqueous solutions

Concentration dependence of photoluminescence quantum yield of FMN aqueous solutions (66mM potassium phosphate buffer, pH 7.0) is investigated over the concentration range from 6.31x10(-5) M to 1.8x10(-2) M at temperatures 298.2 and 323.9K. Experimental data are compared with those obtained theoretically based on two different models of excitation energy transfer and migration in the system of FMN monomers and dimers. The first model does not take the material diffusion into account [Acta Phys. Acad. Sci. Hung. 30 (1971) 145] and the second model is based on the second-order transfer rates which are diffusion dependent [Chem. Phys. Lett. 41 (1976) 139; J. Lumin. 27 (1982) 441]. The comparison shows that the process of material diffusion cannot be neglected in the solutions studied as the relative contribution of the diffusion accelerated nonradiative energy transfer to the total drop of the quantum yield can be even higher then 70%. It is also shown, that in order to obtain a good agreement of the experimental and theoretical data it is necessary to introduce into the theory an additional channel of deactivation for the excitation energy. It is proposed that this additional channel can be partial degradation of excitation energy during its migration between the monomers.     

桥环菁染料II:1-(N-烷基-4-吡啶基)-3-(N-烷基-4-吡啶亚基)-4,5-二取代-1,4-环戊二烯菁染料的合成和质子化及荧光性质研究

本文报道带有疏水长链的桥环菁染料的合成。并研究这类菁染料的质子化过程和平衡常数、取代效应, 以及胶束、糖淀粉和环糊精对染料的可见光谱及质子化过程的影响, 本文也考察了桥环菁染料的荧光光谱, 这些研究在理论上和实际应用有一定意义。     

Ultrafast excited-state dynamics of DNA fluorescent intercalators: new insight into the fluorescence enhancement mechanism

The excited-state dynamics of the DNA bisintercalator YOYO-1 and of two derivatives has been investigated using ultrafast fluorescence up-conversion and time-correlated single photon counting. The free dyes in water exist in two forms: nonaggregated dyes and intramolecular H-type aggregates, the latter form being only very weakly fluorescent because of excitonic interaction. The excited-state dynamics of the nonaggregated dyes is dominated by a nonradiative decay with a time constant of the order of 5 ps associated with large amplitude motion around the monomethine bridge of the cyanine chromophores. The strong fluorescence enhancement observed upon binding of the dyes to DNA is due to both the inhibition of this nonradiative deactivation of the nonaggregated dyes and the dissociation of the aggregates and thus to the disruption of the excitonic interaction. However, the interaction between the two chromophoric moieties in DNA is sufficient to enable ultrafast hopping of the excitation energy as revealed by the decay of the fluorescence anisotropy. Finally, these dyes act as solvation probes since a dynamic fluorescence Stokes shift was observed both in bulk water and in DNA. Very similar time scales were found in bulk water and in DNA.     

Nonradiative energy transfer as a method for investigation of self-assembling nanostructures of lanthanide complexes in solutions

Works concerning the application of nonradiative transfer of electronic excitation energy to investigation into nanostructures of lanthanide complexes in aqueous solutions are surveyed. The effect of the formation of nanosized structures on the quenching of energy donors Ln(III) ions by acceptor ions in concentrated chloride solutions of structuring ions (Li(I), Ca(II)) was discussed. The columinescence phenomenon observed in aqueous solutions of lanthanide chelates was considered. It was shown that the enhancement of luminescence Eu(III) and Tb(III) complexes in water in the presence of excess β-diketones with an admixture of other Ln(III) ions, primarily Gd(III), (columinescence) is due to sensitization via energy transfer over triplet levels of the ligands in the nanostructures formed under these conditions and to the weakening of deactivation of excited luminescent ions by the formation of nanostructures. The influence of the solution preparation procedure on the formation of nanostructures of chelates with different lanthanide ions was revealed, which manifest itself as a variation in the enhancement and quenching of luminescence in the presence of ions from the cerium and yttrium subgroups. Possible applications of the columinescence phenomenon to chemical and medical analysis are briefly discussed.     

Intramolecular Phototransfer of Protons and the Quenching of Fluorescence of Derivatives of 2-(Coumarinyl-3)-5-(ortho-hydroxyphenyl)-1,3,4-oxadiazole

The spectral-luminescence properties of ortho-hydroxy derivatives of 2-(coumarinyl-3)-5-phenyl-1,3,4-oxadiazole have been studied. It is shown that the basic reason for the decreased quantum yield of emission for the compounds studied is the high-speed phototransfer of a proton (estimated as 109 s^-1). Fluorescence of the products of this reaction (phototautomers) was not observed. It was confirmed by quantum-chemical calculations that the increase in efficiency of nonradiative dissipation of the electron excitation energy in phototautomeric forms of ortho-hydroxycoumarinyloxadiazoles is explained by an increase in intramolecular donor-acceptor interaction on introduction of the coumarin unit into the molecule. As a result of the high efficiency of nonradiative deactivation of the excited state, the ortho-hydroxyderivatives studied have promise as UV photostabilizers in polymeric materials.     

WAVELENGTH-DEPENDENT QUANTUM YIELD FOR Z →E ISOMERIZATION OF BILIRUBIN COMPLEXED WITH HUMAN SERUM ALBUMIN

The quantum yield, phi ZE, for configurational photoisomerization (4Z,15Z----4Z,15E) of bilirubin bound non-covalently to human serum albumin was determined (at 23 +/- 2 degrees C) by laser excitation and chromatographic analysis of products. Values obtained for photoexcitation at 465 nm were about one-half those previously reported. The quantum yield was dependent on excitation wavelength, decreasing from a value of 0.109 +/- 0.010 for excitation at 457.9 nm to a value of 0.054 +/- 0.005 for excitation at 514.5 nm. The wavelength dependence is consistent with rapid transfer of excitation energy between the two non-identical pyrromethenone chromophores of bilirubin in the singlet excited state. Since the quantum yields for photoisomerization and luminescence of bilirubin bound to serum albumin at room temperature are both low, internal conversion processes, rather than Z----E configurational isomerizations, are probably the major pathways for deactivation of photo-excited bilirubin.     

Photodissociation of molecules of benzene and some of its derivatives in the liquid phase

The effect of intramolecular interactions on photodissociation has been studied. The quantum yields have been determined for the primary free radicals of benzene, its polymethyl-substituted derivatives Ph-(CH₃), and its monosubstituted Ph-(CH₂)-X derivatives, where X is a substituent containing a heteroatom. The conclusion made previously on the importance of singlet-singlet deactivation in nonradiative transitions of an excited molecule has been confirmed. The compounds Ph-(CH₂)-X have different photolytic stability on excitation by different normal vibrations within the limits of a single electron absorption band, A comparison of the spectral data with the photodissociation data for Ph-(CH₂)n-X enable us to propose that, depending on the nature of the intramolecular interactions, a transfer of both electron and vibrational excitation energy may take place from the ring to the substituent. This process is exceeded in the latter case by the conversion of electron excitation to vibrational excitation.We thank Professor Kh. S. Bagdasar'yan for his interest in the investigation and for discussion of the results.     

Photophysics of alpha,omega-diphenyloctatetraene in the vapor phase

Fluorescence and fluorescence excitation spectra of diphenyloctatetraene vapor have been measured at different temperatures from 98 to 136 degrees C and at different buffer gas pressures from 0 to 300 Torr. The fluorescence quantum yields were determined as functions of the excitation energy and buffer gas pressure. It is shown that diphenyloctatetraene vapor exhibits weak fluorescence from the S2 (1(1)Bu) state in addition to the fluorescence from the S1 (2(1)Ag) state. The quantum yield of the S1 fluorescence is shown to decrease with decreasing pressure and with increasing excitation energy. The electronic relaxation processes of diphenyloctatetraene vapor are discussed based on the pressure and excitation-energy dependence of the fluorescence quantum yield.     

Ultrafast nonradiative decay of electronically excited States of malachite green: ab initio calculations

We have investigated the nonradiative deactivation process of malachite green in the singlet excited states, S(1) and S(2), by high-level ab initio quantum chemical calculations using the CASPT2//CASCF approach. The deactivation pathways connecting the Franck-Condon region and conical intersection regions are identified. The initial population in the S(1) state is on a flat surface and the relaxation involves a rotation of phenyl rings, which leads the molecule to reach the conical intersection between the S(1) and S(0) states, where it efficiently decays back to the ground state. There exists a small barrier connecting the Franck-Condon and conical intersection regions on the S(1) potential energy surface. The decay mechanism from the S(2) state also involves the twisting motion of phenyl rings. In contrast to the excitation to the S(1) state, the initial population is on a downhill ramp potential and the barrierless relaxation through the rotation of substituted phenyl rings is expected. During the course of relaxation, the molecule switches to the S(1) state at the conical intersection between S(2) and S(1), and then it decays back to the ground state through the intersection between S(1) and S(0). In relaxation from both S(1) and S(2), large distortion of phenyl rings is required for the ultrafast nonradiative decay to the ground state.     

Optical and photophysical properties of indolocarbazole derivatives

We present a study of the optical and photophysical properties of five ladder indolo[3,2-b]carbazoles, namely, M1, M2, M3, M4, and M5. The ground-state optimized structures were obtained by B3LYP/6-31G* density functional theory (DFT) calculations, whereas the optimization (relaxation) of the first singlet excited electronic state (S1) was performed using the restricted configuration interaction (singles) (RCIS/6-31G*) approach. The excitation to the S1 state does not cause important changes in the geometrical parameters of the compounds, as corroborated by the small Stokes shifts. The excitation and emission energies have been obtained by employing the time-dependent density functional theory (TDDFT). For all the compounds, excitation to the S1 state is weakly allowed, whereas the S2 <-- S0 electronic transition of each oligomer possesses a much larger oscillator strength. The absorption and fluorescence spectra of the compounds have been recorded in chloroform. A reasonable agreement is obtained between TDDFT vertical transition energies and the (0,0) absorption and fluorescence bands. On one hand, the pattern of the aliphatic side chains does not affect the absorption and fluorescence maxima of the compounds. On the other hand, the replacement of aliphatic chains by phenyl or thiophene rings induces hypsochromic shifts in the absorption and fluorescence spectra. Finally, the fluorescence quantum yield and lifetime of the compounds in chloroform have been obtained. From these data, the radiative and nonradiative rate constants of the deactivation of the S1 state have been determined.     

Photophysical properties of BODIPY-derived hydroxyaryl fluorescent pH probes in solution.

The photophysical properties of five fluorescent pH probes derived from 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene with phenolic or naphtholic subunits at position 8 and with substituents having different electron driving forces at positions 3 and 5 have been investigated in several organic solvents, by means of absorption, steady-state, and time-resolved fluorimetry. For each compound, the fluorescence quantum yield and lifetime are lower in solvents with higher polarity, owing to an increase in the rate of nonradiative deactivation. The rate constants for radiative deactivation, k(f), are nearly constant for all dyes in all solvents studied [k(f)=(1.7+/-0.2)x10(8) s(-1)]. In aqueous solution, these probes undergo a reversible protonation-deprotonation in the near-neutral to basic pH range, producing intensity increases with lower pH. The pK(a) values of the indicators are between 7.5 and 9.3, depending on the substitution pattern on positions 3, 5, and 8. The difference between the absorption and excitation spectra as a function of pH is indicative of the presence of two species in aqueous solution: the phenol- or naphthol-based indicator and its conjugate base.