Solvent relaxation effects on the luminescence properties of 7-hydroxyflavone: steady state polarization and time-resolved emission studies (original) (raw)
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Luminescence behaviour of 7-hydroxyflavone: temperature-dependent effects
Journal of Photochemistry and Photobiology A: Chemistry, 1989
On excitation at 365 nm, 7-hydroxyflavone (7HF) emits a blue fluorescence (X,,, = 450 nm) in ethanol-methanol (1: 1 by volume) glass at 77 K. This fluorescence shifts to longer wavelengths, leading to a green emission 0 max = 537 nm) at room temperature (298 K). The blue and green fluorescence is due to selective excitation of the same ground state species, namely the conjugate anion of 7HF (7HFA), which is present at low concentration in neutral alcohol solution. A large change in the excited state dipole moment of 7HFA compared with the ground state takes place; this has been calculated to be 13.7 * 1 D using the solvatochromic shift method. The large blue shift of the steady state emission maximum on lowering the temperature from 298 to 77 K is rationalized in terms of solvent relaxation around the excited fluorophore.
Time-resolved fluorescence relaxation of 3-methyllumiflavin in polar solution
Journal of Fluorescence, 1995
We have studied the fluorescent properties of a well-defined model flavin compound (3-methyllurniflavin) in a relatively polar solvent like propylene glycol or ethanol. Inhomogeneous spectral broadening effects were directly time-resolved by detection at the extreme blue and red edges of the fluorescence band of 3-methyllurniflavin using excitation in the main absorption band. At the high-energy side of the emission band a rapid decay component (tens of picoseconds) was resolved indicative for the disappearance of the initially prepared, nonequilibrium state with a characteristic dipolar relaxation time. At the low-energy side the rise of a solvent relaxed fluorescent species could be time-resolved. The wavelength-dependent effects on the dipolar relaxation were abolished when excitation was at the low-energy side of the absorption band. The experimental decays of the flavin "solvate" at different energies of fluorescence and excitation are presented as they represent an easy diagnosis for energy dependent solvation dynamics. Wavelength dependent rotation of 3-methyllumiflavin, examined by fluorescence anisotropy decay, turned out to be absent for 3-methyllurniflavin in propylene glycol between 263 and 293 K, probably because of the small change in dipole moment upon flavin excitation.
The Journal of Physical Chemistry B, 2019
Based on the unusually high fluorescence anisotropy (FA) of 3-hydroxyflavone (3HF) in water medium in contrast to the very low FA of its methoxy counterpart (3MF), our proposition invoked formation of intermolecular hydrogen bonded cage-like probe−solvent cluster of 3HF in water. In the present work ab-initio DFT-based quantum chemical calculations have been exploited to provide a foundation for our interpretation. Ground state optimization of 3HF with varying number of water molecules leads to the formation of cage-like or loop-like probe−water cluster. Our calculations reveal that the structures with 4-5 water molecules are stabilized to the maximum extent. Classical molecular dynamics (MD) simulations reveal that the rotational dynamics of 3HF is much slower in water compared to that in alkane medium which also goes in favor of the probe−solvent cluster formation in water medium. Apart from the theoretical studies, an indirect experimental approach has been adopted to substantiate formation of the probe−water cluster. The atypical observation of reduced FA of 3HF entrapped in micelles relative to that of the fluorophore in water, implies disruption of the probe−water cluster with the addition of micelles, corroborating our original proposition of formation of intermolecularly hydrogen bonded 3HF−water cluster.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2019
In this work the photophysical properties of 7-hydroxyflavanone in acetonitrile, ethanol and methanol have been studied. The quantum chemical calculations as well as the experimental measurements have been performed. The absorption and emission spectra have been obtained for 7-hydroxyflavanone dissolved in acetonitrile, ethanol and methanol. Emission spectrum of 7-hydroxyflavanone in studied solvents is characterized by one band with maximum at about 360 nm in the case of acetonitrile and 370 nm in the case of ethanol and methanol solvent. Moreover, it was found that normalized fluorescence spectra do not show significant dependence on excitation wavelength. It can be interpreted as evidence that just fluorophores of one species cause emission. Additionally, the molar extinction coefficients have been determined for 7-hydroxyflavanone. It is equal to about 11.6 × 10 3 M −1 •cm −1 at 270 nm, about 11.2 × 10 3 M −1 •cm −1 at 277 nm and about 12.0 × 10 3 M −1 •cm −1 at 275 nm for 7-hydroxyflavanone dissolved in acetonitrile, ethanol and methanol, respectively. Moreover, the total electronic energies and dipole moments have been found using by quantum chemical calculations. The dipole moment of 7-hydroxyflavanone neutral form in the ground state equals about 2.72 D, 2.70 D and 2.71 D for the case of acetonitrile, ethanol and methanol solvents, respectively. The dipole moment in the singlet-excited state equals about 4.24 D, 4.21 D and 4.23 D for the case of acetonitrile, ethanol and methanol solvents, respectively.
Elimination of the Hydrogen Bonding Effect on the Solvatochromism of 3-Hydroxyflavones
The Journal of Physical Chemistry A, 2003
We address the important, frequently discussed, and still unresolved question of the role of solute-solvent hydrogen bonding in modulating the excited-state intramolecular proton transfer (ESIPT) reaction in 3-hydroxyflavone (3HF) derivatives, which provides a dramatic variation of the relative intensities of normal (N*) and phototautomer (T*) emissive species. We synthesized a new 3HF derivative, 5,6-benzo-4′diethylamino-3-hydroxyflavone (BFE), in which the additional benzene ring protects the 4-carbonyl from H-bonding with the protic solvents but allows the intramolecular bond with the 3-hydroxyl group, which is the pathway of ESIPT, to be maintained. The absorption and fluorescence properties of BFE and its parent analogue 4′-diethylamino-3-hydroxyflavone (FE) were studied in a set of 20 representative solvents. In aprotic media, these dyes in absorption and fluorescence spectra show similar solvatochromism, whereas in protic solvents dramatic differences are observed, which demonstrate that for BFE the effects of intermolecular H-bonding are eliminated. The elimination of the specific interaction of BFE with protic media in the ground state is shown by thin-layer chromatography. The most dramatic differences in spectroscopic properties between BFE and FE are observed for the intensity ratio of the two emission bands, I N* /I T* . Whereas for FE a linear correlation of log(I N* /I T* ) with the solvent polarity function f( ) exists only in aprotic media and in protic solvents strong systematic deviations are observed, for BFE this linear correlation extends to all of the studied solvents. Therefore, we suggest that the protic solvents modulate ESIPT in 3-hydroxyflavone derivatives exclusively by proton-donor intermolecular H-bonding with 4-carbonyl, which occurs with the preservation of the intramolecular H-bond. Our results show how molecular design allows the elimination of the strong intermolecular H-bonding perturbation of the solvent polarity-dependent ESIPT reaction, which provides the means for constructing fluorescent probes with strong selectivity to universal solvent effects.
Journal of the American Chemical Society, 1987
Solvent dipolar reorientations following excitation of DANCA in glycerol at various temperatures were investigated by using multifrequency-phase fluorometry. The time evolution of the emission spectrum was determined by performing lifetime measurements at a number of wavelengths across the emission band and then reconstructing the spectrum at selected times after excitation. At low temperatures, a large displacement of the time-resolved spectrum was apparent. The spectral center of gravity was observed to move faster at short times and then to exhibit a long decay. The spectral width was also observed to increase at short times and then to decrease at much longer times. Several models have been considered to account for the major features of our experimental results. A two-state model agreed well with the data only in the high-temperature regime; at lower temperatures, the data were not consistent with the characteristic features of the two-state model. In the high-temperature regime, we obtained a reorientation rate for the glycerol molecules using the two-state model. Other models which consider a continuous translation of the emission spectra as a function of time also failed to account for the experimental results. Instead, a good qualitative agreement was found with the model proposed by Weber in which the charge separation of the excited fluorophore is followed by reorientation of the solvent molecules around the individual monopoles.
The Journal of Physical Chemistry A, 2003
Picosecond time-resolved fluorescence spectroscopy has been applied to the studies of excited-state intramolecular proton transfer (ESIPT) dynamics in two 4′-(dialkylamino)-3-hydroxyflavone derivatives (unsubstituted and substituted at the 6-position) in ethyl acetate and dichloromethane. In all the studied cases, the fluorescence decay kinetics of both short-wavelength normal (N*) and long-wavelength tautomer (T*) bands can be characterized by the same two lifetime components, which are constant over the all wavelength range of the emission. In the meantime, the preexponential factor of the short-lifetime component changes its sign, being positive for the N* and negative for the T* emission band. Moreover, the two preexponential factors of the T* emission decay are the same in magnitude but opposite in sign. These features are characteristic of a fast reversible two-state ESIPT reaction. Reconstruction of time-resolved spectra allows observing the evolution of these spectra with the appearance, rapid growth, and stabilization (in less than 200 ps) of the relative intensities of the two emission bands. A detailed kinetic model was applied for the analysis of these data, which involved the determination of radiative and nonradiative decay rate constants of both N* and T* forms and of forward and reverse rate constants for transitions between them. We show that ESIPT reaction in the studied conditions occurs on the scale of tens of picoseconds and thus is uncoupled with dielectric relaxations in the solvent occurring at subpicosecond times. Moreover, the radiative and nonradiative deactivation processes were found to be much slower than the ESIPT reaction, suggesting that the relative intensities of the two emission bands are mainly governed by the ESIPT equilibrium. Therefore, both electrochromic and solvatochromic effects on the relative intensities of the two emission bands in 4′-(dialkylamino)-3-hydroxyflavones result from the shifts in the ESIPT equilibrium.
H-bond sensing with 3-hydroxyflavones: steady-state and time-resolved fluorescence studies
Optical Sensing, 2004
The spectroscopic behavior of the 4′-dialkylamino-3-hydroxyflavones in protic environments is very unusual. Previous studies showed that in contrast to other solvatochromic dyes containing carbonyl group (coumarins, Nile Red, PRODAN, etc.), their Stokes shift does not increase on the formation of intermolecular H-bonds with protic solvents. The present steady-state and time-resolved studies show that the ground-state equilibrium between the H-bonded and non-H-bonded forms of this derivative in mixed solvents is not changed significantly when the dye is excited to the normal (N*) excited state. New H-bonds do not form, but those already existing in the ground state can disrupt on a slow time scale. This last process is probably coupled with the slow excited-state intramolecular proton transfer (ESIPT) reaction of the H-bonded form of the dye. Therefore, the fluorescence spectra of the dye provide a measure of the ground state distribution between its H-bonded and non-H-bonded forms, which in turn reflects the H-bonding potential of the environment. Due to this feature, this dye can serve not only as a calibrator of solvent properties but also as a unique sensor of H-bonding potential in unknown media. This sensing can be provided by the relative intensities of the two separated emission bands in the fluorescence spectra.
Computational Study on the Anomalous Fluorescence Behavior of Isoflavones
The Journal of Physical Chemistry A, 2011
Isoflavones are known to show fluorescence with intensities that depend strongly on the solvent properties and exhibit Stokes' shifts as large as 1.4 eV. While some of this behavior can be explained by (excited state) deprotonation, this mechanism does not apply for all isoflavones. The aim of this study is to computationally and experimentally investigate the reasons for this anomalous behavior of neutral isoflavones, taking the daidzein molecule as a model compound. We find that the absence in fluorescence in aprotic solvents and the weak fluorescence in protic solvents can be explained by a change of order of the lowest singlet states in which a fluorescent charge-transfer state lies below the nonfluorescent locally excited state in water but not in acetonitrile. The large Stokes' shift is partly due to a significant rotation among the chromone-phenyl bond in the excited state.
Influence of solvents and temperature on the photophysical characteristics of flavanones
Indian Journal of Pure Applied Physics, 2003
The absorption and fluorescence emission spectra of substituted flavanones viz., Il avanone (I) , 6-methyl Il avanone (II) and 6-methyl-4'-methoxy fl avanone, have been reported in va riou s po lar and non-polar organic solvents. The va lu es o f exci ted state dipole moments of th ese Il avano nes have been calcu lated, using solvatochro mi c dat a. The values of excited state dipole moments for all these mol ecul es investi gated, are hi g her than their co rres po ndin g va lu es in the groll nd state. It is also observed th at, the excited stat e dipol e moment of 6-methyl-4'-methoxy fl avanone> 6-meth yl fl av anone > Il avanone. Further. th e valu es o f percentage po larizati on of these molecules in various solvents of different pol arit y ha ve been ob tain ed and interp reted in terms of Perrin's theory as we ll as dipo le-dipo le interaction. Al so , th e effect o f temperaturc on iluo rescence int ensit y in water and a non-polar so lvent have been observed. The variations in th e tluoresce nce inte nsity have been di scussed in terms of the positions of I(n n*) and \ 11 n*) energy levels of th e mol ecul es studied .