RESUMEN

A coumarin analogue, 8-methoxy-4-methyl-2H-benzo[g]chromen-2-one (MMBC), is almost non-fluorescent in non-polar media, whereas it exhibits dramatically enhanced fluorescence in polar protic solvents. This study investigates the mechanistic features of the significant solvent effects on the fluorescence properties of MMBC and a related compound, 4-methyl-2H-benzo[g]chromen-2-one (MBC), by time-resolved fluorescence and photoacoustic measurements and by theoretical calculations. Time-resolved photoacoustic measurements reveal that the extremely fast non-radiative processes of MBC and MMBC in non-polar solvents can be attributed predominantly to internal conversion. The internal conversion rates of MBC and MMBC are remarkably reduced in a rigid matrix of 3-methylpentane at 77 K, suggesting that internal conversion is a thermally activated process. The photophysical properties of MBC and MMBC examined in selected solvents with different polarities and hydrogen-bond donating abilities show that the internal conversion rate is greatly reduced by hydrogen-bonding interactions with protic solvents. Furthermore, remarkable fluorescence enhancement is observed by adding a small amount of trifluoroethanol to n-hexane solutions of MBC and MMBC, indicating that internal conversion is suppressed by formation of hydrogen-bonded complexes with protic solvents. In light of theoretical considerations based on time-dependent density functional theory (TD-DFT) and INDO/S-CI calculations, the occurrence of fast internal conversion in MBC and MMBC can be explained in terms of the proximity effect, i.e., pseudo Jahn-Teller coupling between energetically close S(1) and S(2) states.

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RESUMEN

We reevaluate the absolute fluorescence and phosphorescence quantum yields of standard solutions by using a novel instrument developed for measuring the absolute emission quantum yields of solutions. The instrument consists of an integrating sphere equipped with a monochromatized Xe arc lamp as the light source and a multichannel spectrometer. By using a back-thinned CCD (BT-CCD) as the detector, the sensitivity for spectral detection in both the short and long wavelength regions is greatly improved compared with that of an optical detection system that uses a conventional photodetector. Using this instrument, we reevaluate the absolute fluorescence quantum yields (Phi(f)) of some commonly used fluorescence standard solutions by taking into account the effect of reabsorption/reemission. The value of Phi(f) for 5 x 10(-3) M quinine bisulfate in 1 N H(2)SO(4) is measured to be 0.52, which is in good agreement with the value (0.508) obtained by Melhuish by using a modified Vavilov method. In contrast, the value of Phi(f) for 1.0 x 10(-5) M quinine bisulfate in 1 N H(2)SO(4), which is one of the most commonly used standards in quantum yield measurements based on the relative method, is measured to be 0.60. This value is significantly larger than Melhuish's value (0.546), which was estimated by extrapolating the value of Phi(f) for 5 x 10(-3) M quinine bisulfate solution to infinite dilution using the self-quenching constant. The fluorescence quantum yield of 9,10-diphenylanthracene in cyclohexane is measured to be 0.97. This system can also be used to determine the phosphorescence quantum yields (Phi(p)) of metal complexes that emit phosphorescence in the near-infrared region: the values of Phi(p) for [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine) are estimated to be 0.063 in water and 0.095 in acetonitrile under deaerated conditions at 298 K, while that in aerated water, which is frequently used as a luminescent reference in biological studies, is reevaluated to be 0.040.

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RESUMEN

The unusual fluorescence properties of 8-methoxy-4-methyl-2H-benzo[g]chromen-2-one (1) are described. The fluorophore 1 is almost nonfluorescent in aprotic solvent (e.g., fluorescence quantum yield Phi(f) < 0.0003 in n-hexane), whereas it strongly fluoresces at long wavelengths (>450 nm) in protic solvent (e.g., Phi(f) = 0.21 in methanol). The fluorophore 1 also shows good applicability in developing a new fluorogenic (fluorescent "off-on") sensor. [structure: see text]

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A dissociation mechanism of the S-S bond in the alpha,alpha'-dinaphthyl disulfide radical anion (NpSSNp*-) in organic solution was investigated on the basis of transient absorption measurements and DFT calculations. NpSSNp*- generated during gamma-radiolysis of NpSSNp in MTHF at 77 K showed the absorption band at 430 nm, which shifted to 560 nm with an increase of the ambient temperature up to room temperature. With the aid of DFT calculations at the B3LYP/6-31G(d) level, the shift of the absorption band was interpreted in terms of molecular conformational changes of NpSSNp*- due to the elongation of the S-S bond. It was observed that NpSSNp*- dissociates into naphthylthiyl radical and thionaphtholate anion in organic solution with a first-order rate constant in the magnitude of 10(6) s-1. From Arrhenius plots of the decay rate constants of NpSSNp*- in a temperature range of 160-293 K, an activation energy for the S-S bond cleavage in NpSSNp*- in solution was determined along with a frequency factor. Based on the state energies of NpSSNp*- calculated at the B3LYP/6-31G(d) level, a Morse-like energy potential for the S-S bond cleavage of NpSSNp*- is depicted as a function of the S-S bond distance.

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The proton transfer to solvent in the excited state of protonated diaminonaphthalenes, 1,5-diaminonaphthalene (1,5-DAN) and 1,8-diaminonaphthalene (1,8-DAN), in aqueous solution, has been investigated by picosecond time-resolved fluorescence measurements. The deprotonation rate constants of the dications of 1,8-DAN and 1,5-DAN in the excited state to produce the corresponding monocations are determined to be 1.3 x 10(10) and 5.6 x 10(9) s(-1), respectively, from dynamic analyses of their fluorescence time profiles. The much larger proton-dissociation rates compared with that of 1-aminonaphthalene (0.6 x 10(9) s(-1)) can be attributed to an electron-withdrawing effect due to the ammonium group at the 5- or 8-position in the naphthalene ring. The remarkably large proton-dissociation rate in 1,8-DAN can be ascribed to its larger reaction exergonicity which results from the electrostatic repulsion between the two ammonium groups in the reactant (the dication state) and the stabilization of the monocation state due to hydrogen bonding interactions between the NH3+ and NH2 moieties. The difference in their acidities in the excited state is discussed in terms of the reaction free energy and the proton affinities are evaluated from ab initio MO calculations.

RESUMEN

Many fluorescent reagents with a benzofurazan (2,1,3-benzoxadiazole) skeleton have been developed and widely used in bio-analyses. In this study, we try to elucidate the fluorescence on-off switching mechanism of three fluorogenic reagents and their derivatives. Ten 4,7-disubstituted benzofurazans were used for this purpose and the measurements of their fluorescence, phosphorescence, photolysis, and time-resolved thermal lensing signal in acetonitrile were obtained in order to understand the relaxation processes of these compounds. These results indicate that the competition of fluorescence with a fast intersystem crossing or fast photoreaction plays a key role in the fluorescence on-off switching. Semi-empirical molecular orbital calculations show that the existence of the triplet n pi* state is responsible for the fast intersystem crossing while the proximity of the reactive second single pi pi* state to the first singlet pi pi* state contributes to the fast photoreaction in the excited states.