Excited State Processes in 1-Deazariboflavin Studied by Ultrafast Fluorescence Kinetics (original) (raw)
Related papers
Photo-dynamics of roseoflavin and riboflavin in aqueous and organic solvents
Chemical Physics, 2009
a b s t r a c t and riboflavin in aqueous and organic solvents are studied by optical absorption spectroscopy, fluorescence spectroscopy, and fluorescence decay kinetics. Solvent polarity dependent absorption shifts are observed. The fluorescence quantum yields are solvent dependent. For roseoflavin the fluorescence decay shows a bi-exponential dependence (ps to sub-ps time constant, and 100 ps to a few ns time constant). The roseoflavin photo-dynamics is explained in terms of fast intra-molecular charge transfer (diabatic electron transfer) from the dimethylamino electron donor group to the pteridin carbonyl electron acceptor followed by intra-molecular charge recombination. The fast fluorescence component is due to direct locally-excited-state emission, and the slow fluorescence component is due to delayed locally-excited-state emission and charge transfer state emission. The fluorescence decay of riboflavin is mono-exponential. The S 1 -state potential energy surface is determined by vibronic relaxation and solvation dynamics due to excited-state dipole moment changes (adiabatic optical electron transfer).
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.
ChemPhysChem, 2009
Substitution on the parent fluorenone is expected to affect the hydrogen-bonding properties and hence the reactivities of its derivatives very significantly. Therefore, the photophysical properties and solvent interactions of the amino-and dimethylamino-substituted fluorenones have been the subject of several recent publications. These studies, which used steady-state and time-resolved fluorescence spectroscopic techniques to obtain an insight into the major factors governing the nonradiative deactivation process in the excited state, revealed dramatic changes in these properties depending on the nature and position of the substituent. However, because of limitation of the time resolution of the fluorescence spectrometer (the instrument response function was reported to be about 20 ps) used in these studies, ultrafast processes with lifetimes shorter than 20 ps could not be resolved and many of the lifetimes reported were obviously overestimated, particularly those determined in alcoholic solvents. For example, the fluorescence lifetimes determined by analyzing the single exponential decay of 1-(N,N-dimethylamino)-fluoren-9-one (1DMAF) in acetonitrile and ethanol (both were reported to have a value of 20 ps) were assigned to the locally excited singlet, S 1 (LE), state, which was assumed to undergo a twisting process to form a "twisted intramolecular charge-transfer" (TICT) state, on the basis of the dependence of its lifetime on viscosity. The S 1 A C H T U N G T R E N N U N G (TICT) state has been described to be nonfluorescent, because of the prevalence of a very fast nonradiative relaxation process due to the very low energy gap between the excited and ground states. The present work, however, which uses the visible pump-probe absorption spectroscopic technique with about 100 fs time resolution, resolves the LEto-TICT conversion process and describes the microscopic dynamics of the relaxation processes undergone by the excited states of 1DMAF to provide the true lifetimes of these two states.
Ultrafast photophysical processes in electronically excited flavin and beta-carotene
2016
Excited-state protonation of riboflavin in the oxidized form is studied in water. In the -1 < pH < 2 range, neutral and N(1)-protonated riboflavin coexist in the electronic ground state. Transient absorption shows that the protonated form converts to the ground state in <40 fs after optical excitation. Broadband fluorescence upconversion is therefore used to monitor solvation and protonation of the neutral species in the excited singlet state exclusively. A weak fluorescence band around 660 nm is assigned to the product of protonation at N(5). Its radiative rate and quantum yield relative to neutral riboflavin are estimated. Protonation rates agree with proton diffusion times for H concentrations below 5 M but increase at higher acidities, where the average proton distance is below the diameter of the riboflavin molecule.
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 A, 2008
Femtosecond dynamics of riboflavin, the parent chromophore of biological blue-light receptors, was measured by broadband transient absorption and stationary optical spectroscopy in polar solution. Rich photochemistry is behind the small spectral changes observed: (i) loss of oscillator strength around time zero, (ii) sub-picosecond (ps) spectral relaxation of stimulated emission (SE), and (iii) coherent vibrational motion along a′ (in-) and a″ (out-of-plane) modes. Loss of oscillator strength is deduced from the differences in the time-zero spectra obtained in water and DMSO, with stationary spectroscopy and fluorescence decay measurements providing additional support. The spectral difference develops faster than the time resolution (20 fs) and is explained by formation of a superposition state between the optically active (1 ππ*) S 1 and closely lying dark (1 nπ*) states via vibronic coupling. Subsequent spectral relaxation involves decay of weak SE in the blue, 490 nm, together with rise and red shift of SE at 550 nm. The process is controlled by solvation (characteristic times 0.6 and 0.8 ps in water and DMSO, respectively). Coherent oscillations for a′ and a″ modes show up in different regions of the SE band. a″ modes emerge in the blue edge of the SE and dephase faster than solvation. In turn, a′ oscillations are found in the SE maximum and dephase on the solvation timescale. The spectral distribution of coherent oscillations according to mode symmetry is used to assign the blue edge of the SE band to a 1 nπ*-like state (A″), whereas the optically active 1 ππ* (A′) state emits around the SE maximum. The following model comes out: optical excitation occurs to the Franck-Condon ππ* state, a ππ*-nπ* superposition state is formed on an ultrafast timescale, vibrational coherence is transferred from a′ to a″ modes by ππ*-nπ* vibronic coupling, and subsequent solvation dynamics alters the ππ*/nπ* population ratio.
Analysis of excited-state processes by phase-modulation fluorescence spectroscopy
Biophysical Chemistry, 1982
Fluorescence phase shift and demodulation methods were used in the analysis of excited-state reactions and to investigate solvent relaxation around fluorophores in viscous solvents. The chosen samples illustrate the results expected for fluorophores bound to biological macromolecules. These moderately simple samples served to test the theoretical predictions described in the preceding paper (J.R. Lakowicz and A.B. Balter, Biophys. Chem. 16 (1982) 99.) and to illustrate the characteristic features of phase-modulation data expected from samples which display time-dependent spectral shifts. The excited-stale protonation of acridine and exciplex formation between anthracene and diethylaniline provided examples of one-step reactions in which the lifetimes of the initially excited and the reacted species were independent of emission wavelength. Using these samples we demonstrated the following: (I) Wavelength-dependent phase shift and demodulation values can be used to prove the occurrence of an excited-state process. Proof is obtained by observation of phase angles (φ) larger than 90° or by measurement of ratios of m/cos φ > 1, where m is the modulation of the emission relative to that of the excitation. (2) For a two-state process the individual emission spectra of each state can be calculated from the wavelength-dependent phase and modulation data. (3) The phase difference or demodulation factor between the initially excited and the reacted states reveals directly the fluorescence lifetime of the product of the reaction. (4) Phase-sensitive detection of fluorescence can be used to prove that the lifetimes of both the initially excited and the reacted states are independent of emission wavelength. (5) If steady-state spectra of the individual species are known, then phase-sensitive emission spectra can be used to measure the lifetimes of the individual components irrespective of the extent of spectral overtap. (6) Spectral regions of constant lifetime can be identified by the ratios of phase-sensitive emission spectra. In addition, we examined 6-propionyl-2-dimethylaminonaphthalene(PRODAN) and N-acetyl-l-tryptophanamide (NATA) in viscous solvents where the solvent relaxation times were comparable to the fluorescence lifetimes. Using PRODAN in n-butanol we used m/cos φ measurements, relative to the blue edge of the emission, to demonstrate that solvent relaxation requires more than a single step. For NATA in propylene glycol we used phase-sensitive detection of fluorescence to directly record the emission spectra of the initially excited and the solvent relaxed states. These measurements can be easily extended to fluorophores which are bound to proteins and membranes and are likely to be useful in studies of the dynamic properties of biopolymers.
Amitriptyline (AMI) and nortriptyline (NT) hydrochlorides were studied by 266 nm laser transient absorption spectroscopy and quantum theoretical calculations. Both drugs photoionize through a biphotonic mechanism producing a radical cation and the solvated electron. A triplet excited state in a twisted conformation around the exocyclic bond is proposed as the intermediate state in the photoionization process. The solvated electron reacts with the ground state drug molecules with rate constants of 6.5 and 5.5 × 10 9 M −1 s −1 to form electron adducts, that absorb in the same wavelength region as the radical cation. Photosensitization experiments using thioxanthone triplet state as the sensitizer demonstrated that AMI or NT quenches this state by a mechanism that depends on the protonation of the amino group in the alkylamine side chain. The protonated species favors energy transfer, while the unprotonated species produces the tricyclic antidepressive radical cation of these drugs and the thioxanthone ketyl radical. These results follow the Rehm-Weller equation for an electron transfer mechanism. Quantum theoretical calculations indicate that ground and excited singlet states photophysical properties of these molecules are determined by the 1,2-diphenylethane system with little participation of the exocyclic double bond. The presence of these primary radicals could explain the reported Type I photodamaging effects for these drugs.