Picosecond Time-Resolved Fluorescence Studies Are Consistent with Reversible Excited-State Intramolecular Proton Transfer in 4‘-(Dialkylamino)-3-hydroxyflavones

Abstract

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.

Polarity Assessment of Thermoresponsive Poly(NIPAM-co-NtBA) Copolymer Films Using Fluorescence Methods

“…However, these particular probes cannot be used for quantitative polarity measurements of hydrophilic polymers (using simple emission parameters such as log(I N* /I T* )), in a manner analogous to solvents because of heterogeneity in the ground state hydrogen-bonding and the non-reversible nature of the ESIPT process, which are evidenced by excitation wavelength dependence and a difference in the fluorescence lifetimes of the N* and T* bands [7,45]. Further studies are underway to explore how the gross polarity of the heterogeneous microenvironment of hydrophilic thermoresponsive copolymer films might be better measured using 3-hydroxyflavone fluorophores.…”

“…There are, however, very significant differences in lifetime between the N* and T* emission bands. In aprotic solvents, such as ethyl acetate and dichloromethane, the lifetimes for the N* and T* bands are equal, which indicates that ESIPT is a fast, reversible two-state process [45]. In this copolymer system this is not the case and the significant lifetime difference indicate a non-reversible ESIPT system.…”

Beyond annexin V: fluorescence response of cellular membranes to apoptosis

“…Since the dynamic equilibrium between these two forms is established prior to emission (Shynkar et al 2003), the factors changing the fluorescence lifetime (temperature, dynamic quenchers) do not change the ratiometric response Tomin et al 2007). Both these bands are strongly Stokes-shifted, so the ''blue'' excitation (with the maximum at about 420 nm) allows avoiding lightscattering artefacts.…”

Dramatic Pressure-Dependent Quenching Effects in Supercritical CO2 Assessed by the Fluorescence of 4‘-Dimethylamino-3-hydroxyflavone. Thermodynamic versus Kinetics Control of Excited-State Intramolecular Proton Transfer

“…The unexpected increase of I N* /I T* at lower pressures, where the polarity of scCO 2 should be lower, cannot be assigned to H-bonding of CO 2 molecules with the dye, because at lower densities this interaction should be even lower (thus contributing to the opposite, decrease in I N* /I T* ). Additionally, it cannot be H-bonding between DMA3HF molecules, because the I N* /I T* ratio is independent of the 18 and triangles 19 ) and DEA3HF (squares) 14 as a function of solvent polarity function ET (30) 31 concentration of DMA3HF. These data show that changes in the PT barrier, namely due to intermolecular hydrogen bonding, are not responsible for the decrease of the tautomer fluorescence at low pressures.…”

“…It was recently shown that their excited states have nanosecond lifetimes and undergo reversible ESIPT, 18,19 as the fluorescence decay rates are much slower than the ESIPT rates. Under reversible conditions, the I N* /I T* ratio is governed by the ESIPT equilibrium.…”