Dipole Moments of 4?-Aminoflavonol Fluorescent Probes in Different Solvents

Ultrafast Ground-State Intramolecular Proton Transfer in Diethylaminohydroxyflavone Resolved with Pump-Dump-Probe Spectroscopy

4'- N, N-Diethylamino-3-hydroxyflavone (DEAHF), due to excited-state intramolecular proton transfer (ESIPT) reaction, exhibits two solvent-dependent emission bands. Because of the slow formation and fast decay of the ground-state tautomer, its population does not accumulate enough for its detection during the normal photocycle. As a result, the details of the ground-state intramolecular proton-transfer (GSIPT) reaction have remained unknown. The present work uses femtosecond pump-dump-probe spectroscopy to prepare the short-lived ground-state tautomer and track this GSIPT process in solution. By simultaneously measuring femtosecond pump-probe and pump-dump-probe spectra, ultrafast kinetics of the ESIPT and GSIPT reactions are obtained. The GSIPT reaction is shown to be a solvent-dependent irreversible two-state process in two solvents, with estimated time constants of 1.7 ps in toluene and 10 ps in the more polar tetrahydrofuran. These results are of great value in both fully describing the photocycle of this four-level proton transfer molecule and for providing a deeper understanding of dynamical solvent effects on tautomerization.

Solvation Dynamics and Proton Transfer in Diethylaminohydroxyflavone

4'-N,N-Diethylamino-3-hydroxyflavone (DEAHF) exhibits dual fluorescence in most solvents as a result of a rapid excited-state intramolecular proton transfer reaction. The high sensitivity of its dual emission to solvent polarity and hydrogen bonding make DEAHF of interest as a ratiometric fluorescence sensor. In addition, prior work has suggested that the rate of this proton transfer should depend on solvent relaxation in an unusual manner. It has been proposed that rapid solvation of the initially excited reactant should retard reaction. The present work tests this idea by using femtosecond Kerr-gated emission spectroscopy to measure the reaction kinetics of DEAHF in mixtures of propylene carbonate (PC) + acetonitrile (ACN). This mixture was chosen to maintain constant solvent polarity and thereby constant reaction energies while varying solvation times ∼10-fold with composition. The reaction kinetics observed in these mixtures are multiexponential, consisting of resolvable components of ∼2 and ∼30 ps and a small fraction of reaction faster than detectable by the 400 fs resolution of the experiment. Average reaction times increase by approximately a factor of 2 as a function of ACN mole fraction, primarily as a result of changes to the slower kinetic component. This trend is opposite to the composition dependence of solvation times, thereby supporting the unusual role of polar solvation dynamics in this proton transfer. In n-alkane solvents, where electrostatic coupling is minimized, frictional properties of the solvent do not influence reaction rates.

Indole-Based NLOphoric Donor-π-Acceptor Styryl Dyes: Synthesis, Spectral Properties and Computational Studies

Donor-π- acceptor styryl chromophores based on indole core were synthesized by Knoevenagel condensation of N-ethyl indole-3-carbaldehyde and different active methylene compounds. The absorption and emission properties of these dyes in different solvents were studied. The dyes displayed a broad absorption maximum in the UV and visible region between 397 and 469 nm with FWHM, 50 to 75 nm. Due to the extended π - conjugated systems this styryl chromophores shows strong intramolecular charge transfer characteristics. The dyes showed solid state emission and emission in solid state was red shifted as compared to their emission in less polar solvents. Density Functional Theory [B3LYP/6-311 + G(d)] computations were used to correlate the structural, molecular, electronic and photo physical parameters of styryl dye with experimental study. Synthesized dyes were confirmed by using FT-IR, ¹H NMR, ¹³C NMR and HRMS spectral analysis.

Unusually slow intramolecular proton transfer dynamics of 4′-N,N-dimethylamino-3-hydroxyflavone in high n-alcohols: involvement of solvent relaxation

The S₁ energy surface of DMA3HF as a function of solvation and ESIPT coordinates, viewed down the energy axis as a 2-dimensional projection.

N-2-Aryl-1,2,3-Triazoles: A Novel Class of Blue Emitting Fluorophores-Synthesis, Photophysical Properties Study and DFT Computations

Novel fluorescent 2-[4-(4,5-diphenyl-1H-imidazol-2-yl) phenyl]-2H-naphtho [1,2-d] [1,2,3] triazolyl derivatives were synthesized from 4-(4,5-diphenyl-1H-imidazol-2-yl) aniline and substituted naphthalen-2-amine. The photophysical properties of the three new fluorophores were evaluated in acetonitrile, methanol, dimethylsulfoxide and N,N-dimethylformamide solvents and were compared with the reported analogs. The compounds show the absorption in the ultraviolet region and the emission in the blue region. The thermal stabilities of these compounds was evaluated by thermogravimetric analysis. The solvatochromism data are used for ground and excited state dipole moment determination of the synthesized triazoles using Bakhshiev and Bilot-Kawski correlations. The experimental absorption and emission were compared with the theoretical data obtained by DFT and TD-DFT computations and they are well in agreement with each other.

Proton Transfer Dynamics of 4'-N,N-Dimethylamino-3-hydroxyflavone Observed in Hydrogen-Bonding Solvents and Aqueous Micelles

Photophysical studies on the 4'-N,N-dimethylamino-3-hydroxyflavone fluorophore were performed in hydrogen-bonding solvents. Both in hydrogen-bonding acids and bases, clear evidence of excited state intramolecular proton transfer (ESIPT) emerged from steady-state and time-resolved spectroscopies. The same was also observed for the fluorophores residing in the hydrophilic shell region of aqueous micelles, where they come into close contact with water molecules at the micelle-water interface. Slow ∼100 ps ESIPT time-constants were determined in these systems that correlated well with solvation dynamics. The slow ESIPT time-constants are attributed to activated barrier crossing from the solvent-relaxed enol form to tautomer form in the excited state energy surface of the flavone. In contrast to the barrier-less ESIPT occurring in early (<1 ps) time-scales, this activated proton-transfer event necessarily requires extensive reorganization of flavone···solvent intermolecular hydrogen bonds, a process heavily modulated by the relatively slower dynamics of solvent relaxation around the excited fluorophore.