Solvation dynamics: improved reproduction of the time-dependent Stokes shift with polarizable empirical force field chromophore models

The inclusion of explicit polarization in molecular dynamics simulation has gained increasing interest during the last several years. An understudied area is the role of polarizability in computer simulations of solvation dynamics around chromophores, particularly for the large solutes used in experimental studies. In this work, we present fully polarizable ground and excited state force fields for the common fluorophores N-methyl-6-oxyquinolium betaine and Coumarin 153. While analyzing the solvation responses in water, methanol, and the highly viscous ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate we found that the inclusion of solute polarizability considerably increases the agreement of the obtained Stokes shift relaxation functions with experimental data. Solute polarizability slows down the inertial solvation response in the femtosecond time regime and enables the chromophore to adapt its dipole moment to the environment. Furthermore, the developed chromophore force field reproduces the solute dipole moments in both the electronic ground and excited state in environments ranging from gas phase to highly polar media correctly. Based on these studies it is anticipated that polarizable models of chromophores will lead to an improved understanding of the relationship of their environment to their spectroscopic properties.

Spectral and physicochemical properties of difluoroboranyls containing N,N-dimethylamino group studied by solvatochromic methods

The solvatochromism of the dyes was analyzed based on the four-parameter scale including: polarizability (SP), dipolarity (SdP), acidity (SA) and basicity (SB) parameters by method proposed by Catalán. The change of solvent to more polar caused the red shift of absorption and fluorescence band position. The frequency shifts manifest the change in the dipole moment upon excitation. The ground-state dipole moment of the difluoroboranyls was estimated based on changes in molecular polarization with temperature. Moreover, the Stokes shifts were used to calculate the excited state dipole moments of the dyes. For the calculation, the ground-state dipole moments and Onsager cavity radius were also determined theoretically using density functional theory (DFT). The experimentally determined excited-state dipole moments for the compounds are higher than the corresponding ground-state values. The increase in the dipole moment is described in terms of the nature of the excited state.

Local environment inside a novel aromatic micelle investigated by steady-state and femtosecond fluorescence spectroscopy of an encapsulated solvatochromic probe

Steady-state and time-resolved spectroscopy of a recently-developed aromatic micelle reveals its high-polarity environment, ultrafast response, dynamic encapsulation of the guest.

Two-photon voltmeter for measuring a molecular electric field

We present a new approach for determining the strength of the dipolar solute-induced reaction field, along with the ground- and excited-state electrostatic dipole moments and polarizability of a solvated chromophore, using exclusively one-photon and two-photon absorption measurements. We verify the approach on two benchmark chromophores N,N-dimethyl-6-propionyl-2-naphthylamine (prodan) and coumarin 153 (C153) in a series of toluene/dimethyl sulfoxide (DMSO) mixtures and find that the experimental values show good quantitative agreement with literature and our quantum-chemical calculations. Our results indicate that the reaction field varies in a surprisingly broad range, 0-10(7)  V cm(-1) , and that at close proximity, on the order of the chromophore radius, the effective dielectric constant of the solute-solvent system displays a unique functional dependence on the bulk dielectric constant, offering new insight into the close-range molecular interaction.

Chemical sensing of neurotransmitters

This review focuses on the chemosensors for neurotransmitters published for the last 12 years, covering biogenic amines (dopamine, epinephrine, norepinephrine, serotonin, histamine and acetylcholine), amino acids (glutamate, aspartate, GABA, glycine and tyrosine), and adenosine.

Quantitative treatment of the effect of solvent on the electronic absorption and fluorescence spectra of substituted coumarins: Evaluation of the first excited singlet-state dipole moments

The electronic absorption and fluorescence spectra of coumarin and 11 substituted coumarins were measured in several solvents (dioxane, ethyl ether, ethyl acetate, ethanol, dimethylformamide, acetonitrile, and dimethyl sulfoxide). Ground-state dipole moments were determined in dioxane at 298 K. The results were used to obtain the first excited singlet-state dipole moments of the coumarins under study by the solvatochromic shift method (Bakhshiev, Kawski-Chamma-Viallet, McRae, and Suppan correlations). Also, the ground- and the first excited singlet-state dipole moments were calculated using a combination of the PPP method (π-contribution) and the vector sum of the σ-bond and group moments (σ-contribution). In general, the first excited singlet-state dipole moments of the coumarins are noticeably higher than the corresponding ground-state values, indicating a substantial redistribution of theπ-electron densities resulting in a more polar excited state. There is a reasonably good agreement between the calculated and the experimental dipole moments.

Surface-assisted transient displacement charge technique. I. Photoinduced charge transfer in self-assembled monolayers

Surface-assisted photoinduced transient displacement charge (SPTDC) technique was developed in order to study light-induced charge transfer in surface-bound molecules and applied to investigation of self-assembled monolayers of 7-diethylaminocoumarin and 2,4-dinitrophenylamine. The dipole moment change measured by SPTDC correlates reasonably well with that measured in solution by standard PTDC technique and with semiempirical calculations. Shortening of the excited-state lifetime of surface-immobilized coumarin due to stimulated emission was observed in both fluorescence and dipole measurements. The dipole signal decline in low-polarity solvents indicates the importance of dipole-dipole interaction that causes reorientation of molecules upon photoexcitation.

Relaxation Dynamics and Transient Behavior of Small Arenethiol Passivated Gold Nanoparticles

Novel gold nanoparticles, passivated by monolayers of benzenethiol, biphenylthiol, and similar derivatives, have been synthesized and characterized using UV/vis, NMR, and Fourier transform infrared (FTIR) spectroscopies. The nanoparticle sizes have been evaluated using transmission electron microscopy and UV/vis spectroscopy; they show diameters between 2.1 and 4.7 nm, depending on the method of synthesis and the monolayer protecting group. Femtosecond transient absorption measurements show that the nanoparticles possess optical properties on the boundary between molecular and nanoparticle behavior. The smaller systems based on benzenethiol exhibit long-lived excited states with lifetimes on the order of a few nanoseconds, resembling those of small gold molecular type clusters. The larger nanoparticles protected with biphenylthiol and benzylthiol groups relax much more rapidly on a picosecond time scale, similarly to related citrate stabilized systems reported in the literature.