Excited-state proton coupled charge transfer modulated by molecular structure and media polarization

A combined theoretical and experimental investigation on the solvatochromism of ESIPT3-(1,3-benzothiazol-2-yl)-2-hydroxynaphthalene-1-carbaldehyde

Excited state intramolecular proton transfer inspired 3-(1,3-benzothiazol-2-yl)-2-hydroxynaphthalene-1-carbaldehyde, showing solid state fluorescence has been synthesized. Existence of excited state intramolecular proton transfer process between carbonyl group and phenolic OH group has been theoretically predicted using computational method. Density functional theory and time dependent density functional theory computations have been used to investigate structural parameters and understand the photophysical properties of the synthesized carbaldehyde. The photophysical properties of carbaldehyde were evaluated using UV-Visible and fluorescence spectroscopy and are found to be very sensitive to the microenvironment such as solvent polarity and pH. The experimental absorption-emission a results are correlated with the computed values. The increase in the dipole moment of A2-Keto(*) than A2-Enol(*) suggested the presence of keto form in the excited state and which is responsible for the single fluorescence emission with a large Stokes shift in all solvents.

Branching Ratio between Proton Transfer and Electron Transfer Channels of a Bidirectional Proton-Coupled Electron Transfer

Rigorous quantum dynamical study of concerted proton-coupled electron transfer (PCET) on the time scale of a few femtoseconds (fs) has been rarely reported. Herein, a time-dependent quantum wavepacket propagation method was applied to the dynamics of the charge-transfer excited electronic state of FHCl corresponding to F(+)HCl(-). The dynamics corresponds to a bidirectional PCET with two dissociation channels: the electron transfer (ET, generating FH+Cl) and proton transfer (PT, generating F+HCl) paths. The calculated branching ratio (Cl/F), 0.78, implies a surprising fact: PT prevails over ET. A detailed analysis of the proton movement and electron readjustment suggests that the proton movement starts ∼3 fs earlier than the electron movement, and the electron readjustment is triggered by the initial movement of the proton. The branching ratio drastically inverts to 1.24 because of a reduced nonadiabatic effect in the isotope-substituted system, FDCl.

Investigation of excited-state intramolecular proton transfer coupled charge transfer reaction of paeonol

An excited-state intramolecular proton transfer (ESIPT) coupled charge transfer reaction of paeonol was investigated both experimentally and theoretically. The ESIPT reaction of paeonol was predicted based on the large Stokes shift, which is observed in steady-state absorption and fluorescence spectra in an ethanol solution. The steady-state spectra in some solutions, such as methanol, ethanol, propanol, dichloromethane, and n-hexane, illustrate that the ESIPT reaction of paeonol has no dependence on the solvent properties. Therefore, the excited-state intermolecular proton transfer cannot be generated in protic solvents. Using the density functional theory and time-dependent density functional theory methods, we make a subsequent theoretical calculation that indicates that the ESIPT reaction of paeonol occurs through the intramolecular hydrogen bond O−H···O=C. The excited-state potential energy curve of paeonol indicates that the ESIPT reaction is a barrierless process, and the fluorescence emission of paeonol at 493 nm in the ethanol solution was assigned to the keto isomer fluorescence. Additionally, we also found an intramolecular charge transfer in the excited state by analysing the frontier molecular orbitals of paeonol.

Intermolecular proton shuttling in excited state proton transfer reactions: insights from theory

The mechanism of intermolecular proton shuttling involved in a prototypical excited state proton transfer reaction is disclosed using DFT and TD-DFT.

Fluorescent probes for lipid rafts: from model membranes to living cells

Membrane microdomains (rafts) remain one of the controversial issues in biophysics. Fluorescent molecular probes, which make these lipid nanostructures visible through optical techniques, are one of the tools currently used to study lipid rafts. The most common are lipophilic fluorescent probes that partition specifically into liquid ordered or liquid disordered phase. Their partition depends on the lipid composition of a given phase, which complicates their use in cellular membranes. A second class of probes is based on environment-sensitive dyes, which partition into both phases, but stain them by different fluorescence color, intensity, or lifetime. These probes can directly address the properties of each separate phase, but their cellular applications are still limited. The present review focuses on summarizing the current state in the field of developing and applying fluorescent molecular probes to study lipid rafts. We highlight an urgent need to develop new probes, specifically adapted for cell plasma membranes and compatible with modern fluorescence microscopy techniques to push the understanding of membrane microdomains forward.

Realizing molecular pixel system for full-color fluorescence reproduction: RGB-emitting molecular mixture free from energy transfer crosstalk

A full-color molecular pixel system is realized for the first time using simple mixtures composed of RGB-emitting excited-state intramolecular proton transfer (ESIPT) dyes, each of which has delicately tailored Stokes shift and independent emission capability completely free from energy transfer crosstalk between them. It is demonstrated that the whole range of emission colors enclosed within the RGB color triangle on the CIE 1931 diagram is predictable and conveniently reproducible from the RGB molecular pixels not only in the solution but also in the polymer film. It must be noted that mixing ratios to reproduce the desired color coordinates can be precisely calculated on the basis of additive color theory according to their molecular pixel behavior.

Direct observation of ground-state lactam-lactim tautomerization using temperature-jump transient 2D IR spectroscopy

We provide a systematic characterization of the nanosecond ground-state lactam-lactim tautomerization of pyridone derivatives in aqueous solution under ambient conditions using temperature-jump transient 2D IR spectroscopy. Although electronic excited-state tautomerization has been widely studied, experimental work on the ground electronic state, most relevant to chemistry and biology, is lacking. Using 2D IR spectroscopy, lactam and lactim tautomers of 6-chloro-2-pyridone and 2-chloro-4-pyridone are unambiguously identified by their unique cross-peak patterns. Monitoring the correlated exponential relaxation of these signals in response to a laser temperature jump provides a direct measurement of the nanosecond tautomerization kinetics. By studying the temperature, concentration, solvent, and pH dependence, we extract a thermodynamic and kinetic characterization and conclude that the tautomerization proceeds through a two-state concerted mechanism. We find that the intramolecular proton transfer is mediated by bridging water molecules and the reaction barrier is dictated by the release of a proton from pyridone, as would be expected for an efficient Grothuss-type proton transfer mechanism.