INVESTIGATIONS ON THE FLUORESCENCE CONCENTRATION QUENCHING OF FLAVOMONONUCLEOTIDE IN GLYCERINE-WATER SOLUTIONS

We measured concentration changes of the fluorescence quantum yield eta/eta 0 of flavomononucleotide (FMN) in glycerine-water solutions of various viscosities. The results obtained show that FMN dimers are traps for the exciting light energy as well as the energy transferred non-radiatively. Very good agreement between the experimental and theoretical eta/eta 0 results was obtained. It has been shown that in the investigated solutions non-radiative energy transfer from monomers to dimers takes place, preceded by the migration of energy. It has been stated that no monomer quenching occurs in these solutions. The contribution of the individual fluorescence concentration quenching mechanisms has been determined.     

Pyrromethene dyes (BODIPY) can form ground state homo and hetero dimers: photophysics and spectral properties

Homo and hetero dimerisation of two spectroscopically different BODIPY chromophores was studied, namely, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene and its 5-styryl-derivative. These exhibit very similar absorption and fluorescence spectral shape, but are mutually shifted by ca. 70 nm. For this reason the former and the latter are referred to as the green and red BODIPY, which here are denoted gB and rB, respectively. Various spectroscopic properties of the rB in different common solvents were determined. The calculated and experimental fluorescence quantum yield is found to be close to 100%, the fluorescence relaxation has a single exponential decay with a lifetime of about 4.5 ns, and the F?rster radius for donor-donor energy migration is 67+/-1A. The dimerisation in different solvents was examined by using custom synthesised; mono and bis BODIPY-labelled forms of 1,2-cis-diaminocyclohexane. It is shown that gB and rB can form ground state homo- as well as hetero dimers. The dimers are non-fluorescent, compatible with H-dimers and may act as excitation traps or as acceptors to the corresponding excited monomers.     

Photonics of dimers of cyanine dyes

The results of study on the properties of dimers of thiamonomethine-and thiatrimethinecyanines (thiacarbocyanines) in the ground and electronically excited states in aqueous solutions are presented. Dimers of cyanine dyes have the sandwiched structure with near-parallel alignment of the polymethine chains of the monomers in the dimer. The formation of dimers is manifested by two absorption bands of different intensities due to splitting of the S* level of the monomers upon their resonance interaction. Dimers of thiacarbocyanines are characterized by a low fluorescence quantum yield φ_fl as compared to monomers; however, φ_fl of dimers of thiamonomethinecyanines are markedly higher than that of monomers. Dimers of cyanine dyes are also characterized by a relatively high quantum yield of intersystem crossing to the triplet state. In the triplet-triplet absorption spectra, two bands of different intensities are revealed, which are due to the splitting of the higher triplet level of the monomers that form the dimer. In the presence of electron donors (ascorbic acid, hydroquinone) and/or acceptors (p-benzoquinone, p-nitroacetophenone, methylviologen), the triplet state of dimers is quenched as a result of electron transfer yielding radical products. Dimers in the triplet state can serve as photosensitizers of redox reactions.     

Forward and reverse excitation energy transport in concentrated two-component systems

The experimental and numerical study of the forward and reverse energy transport in concentrated two-component solutions is presented. Experimental results are compared with those of diagrammatic and hopping theories. It is shown that the effect of direct acceptor excitation by the light beam is important in the presence of the reverse transfer and it must be taken into account to describe correctly the experimental results within the framework of any theoretical model. This effect leads in particular to the decrease in the donor emission anisotropy and to the increase in its quantum yield. Our analysis indicates the advantage of the diagrammatic model over the hopping approach.     

Exciton coherence lifetimes from electronic structure

We model the coherent energy transfer of an electronic excitation within covalently linked aromatic homodimers from first-principles. Our results shed light on whether commonly used models of the bath calculated via detailed electronic structure calculations can reproduce the key dynamics. For the systems we model, the time scales of coherent transport are experimentally known from time-dependent polarization anisotropy measurements, and so we can directly assess whether current techniques are predictive for modeling coherent transport. The coupling of the electronic degrees of freedom to the nuclear degrees of freedom is calculated from first-principles rather than assumed, and the fluorescence anisotropy decay is directly reproduced. Surprisingly, we find that although time-dependent density functional theory absolute energies are routinely in error by orders of magnitude more than the coupling energy between monomers, the coherent transport properties of these dimers can be semi-quantitatively reproduced from these calculations. Future directions which must be pursued to yield predictive and reliable models of coherent transport are suggested.     

Theoretical studies on excited state proton transfer tautomerism reaction and spectroscopic properties of 8-hydroxyquinoline monomers and dimers

By means of ab initio HF methods, the ground state structures of 8-hydroxyquinoline (8-HQ) monomers and dimers were optimized using the 6-311+g* and 6-31G basis sets, respectively. The lowest singlet excited states of 8-HQ monomers and dimers have been studied by the single-excitation configuration interaction (CIS) approach at the same level. In the studies of the potential energy surface, it was found that all the stable configurations corresponded to enol form. The UV-vis and fluorescence spectra of 8-HQ monomers and dimers under a solvent effect condition were also calculated using the TD-B3LYP/6-31+G* method based on the HF- and CIS-optimized geometries. The computed absorption and fluorescence spectral characteristics for monomers and dimers were in good agreement with previously reported experimental values. The results also show that 8-HQ has very poor fluorescence in solvents.     

Photophysical processes in dimers of thiacarbocyanine cations

The results of quantum-chemical calculations of absorption, fluorescence, and induced T-T absorption spectra and the rate constants of photophysical processes for the monomer and dimers of the 3,3′-diethylthiacarbocyanine cation are presented. It has been shown that the formation of the dimers leads to a substantial reduction in the efficiency of the radiative pathways and enhancement of the efficiency of the non-radiative pathways of deactivation of the fluorescent state, thereby resulting in a decrease in the quantum yield of fluorescence. The fluorescence quantum yield depends in this case on the relative orientation of the molecules in the dimer with respect to each other. The calculation results satisfactorily describe experimental data on the spectral and luminescent properties of the dimers of the 3,3′-diethylthiacarbocyanine cation and their photophysical processes.     

6-甲基-4-羟基嘧啶单体及二聚体激发态质子转移异构化反应及光谱的理论研究

采用ab initio HF理论的组态相关CIS方法和连续溶剂模型PCM, 分别在6-311+G*和6-31G水平上研究了6-甲基-4-羟基嘧啶单体及二聚体激发态质子转移的异构化反应; 对其反应势能面的研究发现, 单体基态和激发态的异构化反应一起可以形成四能级的分子电子体系, 而二聚体的却不能, 由此解释了单体和二聚体的紫外吸收光谱和荧光发射光谱均对应于酮式构型的原因. 利用混合含时密度泛函TD/MPW1PW91理论方法在溶剂存在下计算了标题物质的紫外吸收光谱和荧光发射光谱.     

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.     

6-甲基-4-羟基嘧啶单体及二聚体质子转移过程的理论研究

应用密度泛函理论的B3LYP/6-311+G(d)方法研究了6-甲基-4-羟基嘧啶单体及二聚体质子转移的异构化反应.对反应势能面的研究发现,该化含物可能存在9种单体异构体,对其最稳定的单体构型进行分析.各单体间异构化反应的过渡态共有9种,反应的活化能最小为22.06 kJ/mol,最大为356.55 kJ/mol,最可能的反应路径在室温下即可进行. 研究了2种二聚体及其异构化反应的过渡态,发现二聚体均比其对应的单体稳定,而且质子转移所需要的活化能仅为20.13 kJ/mol,比单体低很多. 氢键在这种变化中起了主要作用,由单体和二聚体的总能量计算了氢键的键能.     

Steps to demarcate the effects of chromophore aggregation and planarization in poly(phenyleneethynylene)s. 2. The photophysics of 1,4-diethynyl-2-fluorobenzene in solution and in crystals

Crystals of 1,4-bis(2-hydroxy-2-methyl-3-butynyl)-2-fluorobenzene 4 have a rich packing structure with four distinct molecules in the unit cell. A complex hydrogen bonding network results in the formation of cofacial trimers, cofacial dimers, and monomers within the same unit cell. Given a remarkable opportunity to investigate the effect of aggregation on the photophysics of 1,4-diethynylbenzenes, we analyzed the absorption, diffuse reflectance, and emission spectra of compound 4 in solutions and in crystals. Diffuse reflectance and fluorescence excitation revealed a red-shifted absorption that is absent in dilute solution but becomes observable at high concentrations and low temperatures. The fluorescence emission in the solid state is dual with components assigned to monomers and aggregates. The excitation and emission assigned to the monomer are nearly identical in crystals and dilute solutions. The absorption and emission bands assigned to aggregates are broad and red-shifted by 60--80 nm. As expected for a sample with absorbers and emitters with different energies and incomplete equilibration, efficient monomer-to-aggregate energy transfer was observed by a proper selection of excitation wavelengths. The fluorescence quantum yield of 4 in solution is relatively low (Phi(F) = 0.15) and the singlet lifetime short (tau(F) = 3.8 ns). A lower limit for the triplet yield of Phi(T) = 0.64 was determined indirectly in solution by (1)O(2) sensitization, and a relatively strong and long-lived phosphorescence was observed in low-temperature glasses and in crystals at 77 K.     

Effect of Energy Transfer on UV Spectra of “H-shaped” Azobenzene Derivatives

A series of “H-shaped” organic dimers (azobenzene derivatives) exhibit linear absorption red shift compared with their corresponding monomers experimentally. Dipolar interaction model is not appropriate for the azobenzene derivatives due to the small distance between two “D-π-A” chains. Energy transfer model is suggested for explanation of the absorption red shift. Two necessary conditions for energy transfer were verified. In addition, bi-exponential florescence-delay behavior of the dimer as well as Bella's quantum chemistry calculation shows evidence of energy transfer.     

Effect of halogenated compounds on the photophysics of C70 and a monoadduct of C70: Some implications on optical limiting behaviour

The fluorescence spectra, quantum yields and lifetimes of C₇₀ and a pseudo-dihydro derivative (C₇₀R) have been measured in a wide range of solvents at room temperature. This information is important for the development of reverse saturable absorbers. Phosphorescence spectra and phosphorescence lifetimes were also measured at low temperature. The fluorescence is subject to quenching by halogenated compounds. The efficiency of quenching follows the order I>Br≫Cl. The nature of the quenching is shown to vary, with chlorinated compounds exhibiting static quenching of fullerene fluorescence, owing to nonfluorescent complex formation, whilst those compounds containing bromine and iodine exhibit dynamic quenching due to the external heavy-atom effect, that increases the intersystem crossing rate constant in the fluorophore–perturber complex. This constant is evaluated by an original method from the bimolecular quenching rate constants. The phosphorescence quantum yield of both fullerenes at 77 K slightly increases in the presence of iodobenzene, in spite of a strong decrease in phosphorescence lifetime. The marked increase of the intersystem crossing rate constant in concentrated solutions owing to the external heavy-atom effect is of interest for the application of fullerenes as fast-responding optical limiters (reverse saturable absorbers) of intense laser pulses, even in cases where the triplet quantum yield is of the order of unity.     

CARBOXYLATED ZINC-PHTHALOCYANINE, INFLUENCE OF DIMERIZATION ON THE SPECTROSCOPIC PROPERTIES. AN ABSORPTION, EMISSION, AND THERMAL LENSING STUDY

Monomer and dimer absorption and emission spectra, and dimerization constants are reported for the diamide of the zinc-tetracarboxyphthalocyanine in pure dimethylformamide (DMF) and in H₂O/DMF mixtures at room temperature. The dimerization constant increases steadily with the water content. The monomer absorption Q-band is insensitive to the solvent composition, whereas dimer spectra show great variations with the water content. Stationary emission measurements show that fluorescence originates exclusively from the monomers. The fluorescence spectrum as well as its absolute fluorescence quantum yield, measured by steady-state thermal lensing, are also insensitive to the solvent composition. The thermal lensing method is discussed for the case of two absorbing species in equilibrium.     

Resolution of the excitation-emission spectra of FMN in rigid poly(vinyl alcohol) matrices

The excitation-emission spectra of flavin mononucleotide (FMN) were measured in rigid PVA films for concentrations ranging from 6.92 x 10(-4)M to 1.03 M. The theoretical three-linear decomposition of the excitation-emission spectra indicated the presence of two absorption and emission centers corresponding to FMN monomer and dimer, respectively. The component of the fluorescence profile corresponding to the FMN monomer has a large negative part which is the mirror image of the emission band profile of the dimer. The elimination of this part by taking a linear combination of the emission components of the monomer and of the dimer resulted in emission spectrum, which is in a very good agreement with the monomer spectrum measured directly. The appearance of a negative part of the monomer emission profile obtained by trilinear decomposition of the emission-absorption spectra of FMN can be explained in terms of the non-radiative reverse energy transfer from the FMN dimers to the FMN monomers. The presented results confirm that the FMN molecules in rigid PVA form dimers but not higher order aggregates. Moreover, they enable to obtain fluorescence spectra of dimers and suggest that FMN dimers may take part in the process of non-radiative energy transfer occurring in photoreception phenomena.     

Photochemical and photosensitizing properties of monomeric and dimeric Sn(IV)-protoporphyrin

Sn(IV)-protoporphyrin IX (Sn-Pp) in aqueous media exists as a mixture of monomeric and dimeric species, which can be readily distinguished on the basis of their absorption maxima at around 410 and 386 nm respectively. Sn-Pp dimers prevail as the pH is decreased and are characterized by a lower fluorescence quantum yield, a larger tendency to undergo photobleaching and a reduced photosensitizing efficiency compared with the Sn-Pp monomer. The photosensitizing action of Sn-Pp appears to involve the intermediacy of singlet oxygen (1O2) as shown by photo-oxidation studies with N-acetyl-tryptophanamide in light and deuterated water solutions. Using 1,3-diphenyl-isobenzofuran as a substrate, the quantum yield of 1O2 generation by monomeric Sn-Pp was found to be about 0.6.     

Time-resolved study of thymine dimer formation

The formation of thymine dimers in the single-stranded oligonucleotide, (dT)20, is studied at room temperature by laser flash photolysis using 266 nm excitation. It is shown that the (6-4) adduct is formed within 4 ms via a reactive intermediate. The formation of cyclobutane dimers is faster than 200 ns. The overall quantum yield for the (6-4) formation is (3.7 +/- 0.3) x 10-3, and that of the cyclobutane dimers is (2.8 +/- 0.2) x 10-2. No triplet absorption is detected, showing that either the intersystem crossing yield decreases by 1 order of magnitude upon oligomerization (<1.4 x 10-3) or the triplet state reacts with unit efficiency in less than 200 ns to yield cyclobutane dimers.     

Free-probe fluorescence of light-up probes.

The fluorescence enhancement of light-up probes (thiazole orange (TO) conjugated peptide nucleic acids (PNAs)) upon hybridization to target nucleic acid depends on the probe sequence, mainly due to large variations in free-probe fluorescence. Here we study three probes where the fluorescence in free state varies more than 50-fold. We find that this variation is due to a fraction that has TO intramolecularly "back-bound" to the PNA bases. The intramolecular affinity constant for this unimolecular interaction was determined by temperature titrations using absorption spectroscopy, and the fluorescence quantum yields of the probes in back-bound conformation were calculated. The molar ratio of probes in back-bound conformation was 0.70-0.96 at 30 degrees C and 0.40-0.73 at 60 degrees C, and the fluorescence quantum yield in back-bound conformation varied between 0.0020 and 0.077 at 30 degrees C, and 0.00065-0.029 at 60 degrees C. These data show that the variation in free-probe fluorescence depends mainly on the fluorescence quantum yield of the probe in back-bound conformation and to a much lesser extent on the tendency of the probe to adopt the back-bound conformation. With increasing temperature the free-probe fluorescence decreases owing to both reduced degree of back-binding and a decrease of the fluorescence quantum yield in back-bound conformation.     

Synthesis and optical limiting properties of axially bridged phthalocyanines: [(tBu4PcGa)2O] and [(tBu4PcIn)2O

Highly soluble [(tBu(4)PcM)(2)O] phthalocyanine dimers (M=Ga(III) (3), In(III) (4)) were prepared by the reaction of [tBu(4)PcMCl] (M=Ga(III) (1), In(III) (2)) with excess of concentrated H(2)SO(4) at -20 degrees C. The Mbond;Obond;M linkages in 3 and 4 are not stable against concentrated H(2)SO(4) at room temperature, 6 n HCl at reflux, or during isolation under column chromatographic conditions (e.g. silica gel/toluene). The stability of 3 in solution is considerably higher than that of 4. The mu-oxo-bridged phthalocyanine dimers 3 and 4 have a more intense photoluminescence emission in the red region than the monomers 1 and 2. The gallium phthalocyanines 1 and 3 have fluorescence lifetimes of a few nanoseconds, while those of the indium phthalocyanines 2 and 4 last for only several hundred picoseconds. Comparison of the fluorescence lifetimes of monomers 1 and 3 with dimers 2 and 4, reveals that the dimers have longer lifetimes of the excited singlet states. The transient absorption spectrum is similar for all of the compounds, and the transient absorption band at about 520 nm, observed by nanosecond laser irradiation, can be assigned to the transition from the lowest triplet excited state to the upper triplet excited states (T-T absorption). The magnitude of the optical limiting exhibited by 1, 2, 3, and 4 in toluene at 532 nm laser pulse irradiation is in the order: 4>3>2>1 [corrected]. The values of the imaginary third-order nonlinear susceptibility Im[chi((3))] of the above compounds at 532 nm in toluene are also reported. These results demonstrate that these compounds are candidates for optical limiting applications.