Ewa Krystkowiak

Influence of Hydrogen Bonds and Nonspecific Interactions on the Spectral and Photophysical Properties of the Excited Singlet States of 4-Aminophthalimide in Amine Solution

The hydrogen-bond and nonspecific interaction energies for 4-aminophthalimide (4-AP), often used as a probe, in the ground electronic and excited singlet states are determined using ab initio computational methods. It is shown that the 4-AP molecule can form three relatively strong hydrogen bonds with trimethylamine (TMA) and triethylamine (TEA), which leads to the formation of S(0)-complexes between the solute and solvent molecules. Only two of the hydrogen bonds with the amine group of 4-AP change significantly their energies upon excitation and deactivation. The theoretical results are necessary to explain the spectral and unusual photophysical properties of 4-AP in amine solutions.

Kinetics of fluorescence quenching of S2-thioketones studied by the Smoluchowski-Collins-Kimball model: Standard systems

The fluorescence quenching of S2-xanthione and S2-deuterated xanthione by 3,3-diethylpentane, cyclohexane, and deuterated cyclohexane in perfluoro-1,3-dimethylcyclohexane has been studied by means of a picosecond time–resolved, single–photon counting technique. The donor fluorescence decay in the presence of the quencher for six analyzed systems could satisfactorily be fit using the Smoluchowski–Collins–Kimball function. The molecular parameters: R (the sum of the molecular radii), D (the sum of diffusion coefficients), and the specific rate constant of the process (κ) for all electronically excited donor–quencher pairs have been determined. The values of molecular parameter D, obtained from fluorescence quenching studies, are significantly lower than the sum of the macroscopic measured diffusion coefficients values. These results points to substantial differences in the rate of translational diffusion established in the molecular and macroscopic scale. The systems studied are proposed as model systems fo...

Examination of the viability of the Smoluchowski–Collins–Kimball model in fluorescence quenching of S2-xanthione

The fluorescence quenching of S2-xanthione by five different hydrocarbon quenchers in perfluoro-1,3-dimethylcyclohexane has been studied assuming the Smoluchowski–Collins–Kimball (SCK) model. The fluorescence decay curves were measured by means of a picosecond time-resolved, single-photon counting technique. The model used has been shown to satisfactorily describe the process of S2-thioketones fluorescence quenching for six donor–quencher–solvent systems best fitting the model assumption [Krystkowiak and Maciejewski, J. Chem. Phys. 117, 2246 (2002)]. The systems studied were characterized by a short lifetime of an excited donor (equal to 152±2 ps in perfluoro-1,3-dimethylcyclohexane), the quenching process mechanism demanding a direct contact of reacting molecules and at the end the inert solvent. The influence of the shape, size, reactivity anisotropy of quencher molecules on the possibility of the SCK model application, and the values of molecular parameters: R (the sum of the molecular radii), D (the s...

Deactivation of 6-Aminocoumarin Intramolecular Charge Transfer Excited State through Hydrogen Bonding

This paper presents results of the spectral (absorption and emission) and photophysical study of 6-aminocoumarin (6AC) in various aprotic hydrogen-bond forming solvents. It was established that solvent polarity as well as hydrogen-bonding ability influence solute properties. The hydrogen-bonding interactions between S1-electronic excited solute and solvent molecules were found to facilitate the nonradiative deactivation processes. The energy-gap dependence on radiationless deactivation in aprotic solvents was found to be similar to that in protic solvents.

Rapid fluorescence quenching of S2-xanthione by 3,3-diethylpentane in perfluorohydrocarbons.

Rapid fluorescence quenching of S2-xanthione by 3,3-diethylpentane has been studied in three perfluorohydrocarbons of different viscosities. The donor fluorescence decay in the presence of a quencher was fitted using the Smoluchowski-Collins-Kimball (SCK) model. The molecular parameters, R (the sum of the molecular radii), D (the sum of diffusion coefficients), and the specific rate constant of the process, kappa, were determined. The values of parameter D for all systems studied differed from the sum of the macroscopic diffusion coefficients Dx measured independently. This behavior is explained by the dependence of the molecular diffusion coefficient (as determined from the SCK model) on the distance traveled by the donor and quencher molecules during the excited donor lifetime.

Spectral and Photophysical Behavior of Cytisine in n-Hexane. Experimental Evidence for the S1(n,π*) → S0 Fluorescence

Spectral and photophysical properties of (-)-cytisine (the compound used as a smoking cessation aid and a potential drug in Alzheimers and Parkinsons diseases) were investigated. The two conformers of cytisine, whose presence in the S0 state has been earlier proved by the NMR and IR methods as well as in theoretical calculation, in nonpolar n-hexane show a rarely observed prompt fluorescence from the S1(n,π*) excited state. This observation is unambiguously evidenced by very small radiative rate constants of these two emitting conformers, kF = 7.4 × 105 and 3.0 × 105 s-1. Their lifetimes in the S1(n,π*) state are relatively long, τS1 = 1.9 and 6.7 ns; therefore, their fluorescence quantum yield is relatively high ϕF ∼ 10-3. The long-wavelength band in the cytisine absorption originates from the excitation to the S2(π,π*) state, while the S1(n,π*) state is not observed in this spectrum. Thus, the excited state S2(π,π*) is manifested only in the absorption spectrum, while the excited state S1(n,π*) is seen only in the fluorescence spectrum, so cytisine in n-hexane is characterized by close lying (n,π*) and (π,π*) excited singlet states.

Spectral and photophysical properties of cytisine in acetonitrile – Theory and experiment

Spectral and photophysical properties of (-)-cytisine that is used as a smoking cessation aid, and which derivatives are promising tools in a treatment of neurological diseases, were investigated in acetonitrile, non-specifically interacting solvent with a polarity similar to water. The two chair conformers of cytisine were found the most stable in the ground state S0 and the lowest excited singlet state S1(π,π*), wherein axial one was characterized by a significantly larger abundance, fluorescence lifetime 0.15 ns and fluorescence quantum yield 0.008. The S1(π,π*) excited state of both cytisine conformers deactivated almost exclusively via internal conversion.