Hydrogen bond formation between 4-(dimethylamino)pyridine and aliphatic alcohols.

The photophysics of 4-(dimethylamino)pyridine (DMAP) has been investigated in different solvents in the presence of aliphatic and fluorinated aliphatic alcohols, respectively. For most systems, consecutive two-step hydrogen-bonded complex formation is observed in the presence of alcohols. Equilibrium constants are determined from UV spectroscopic results for the formation of singly and doubly complexed species. The resolved absorption and fluorescence spectra for the singly and doubly complexed DMAP are derived by means of the equilibrium constants. Exceptionally large hydrogen bond basicity values are found for the ground and singlet excited DMAP molecules. In n-hexane, as a consequence of complex formation, the intramolecular charge transfer (ICT) emission becomes dominant over of the locally excited fluorescence; the fluorescence and triplet yields increase considerably with complexation. In polar solvents, both the fluorescence and triplet yields of the complex are much smaller than that of the uncomplexed DMAP. The dipole moments derived for the singly complexed species from the Lippert-Mataga analysis are much larger than those of the uncomplexed molecules. However, for the relaxed ICT excited-state one obtains different dipole moments in apolar and polar solvents. This may be explained by a conformational change of the molecule in the ICT excited state from planar geometry in apolar solvent to the perpendicular structure (characterized with bigger dipole moment) in polar solvent.

Kinetics and mechanism of the reactions of CH3CO and CH3C(O)CH2 radicals with O2. Low-pressure discharge flow experiments and quantum chemical computations.

The reactions CH(3)CO + O(2)--> products (1), CH(3)CO + O(2)--> OH +other products (1b) and CH(3)C(O)CH(2) + O(2)--> products (2) have been studied in isothermal discharge flow reactors with laser induced fluorescence monitoring of OH and CH(3)C(O)CH(2) radicals. The experiments have been performed at overall pressures between 1.33 and 10.91 mbar of helium and 298 +/- 1 K reaction temperature. OH formation has been found to be the dominant reaction channel for CH(3)CO + O(2): the branching ratio, Gamma(1b) = k(1b)/k(1), is close to unity at around 1 mbar, but decreases rapidly with increasing pressure. The rate constant of the overall reaction, k(2), has been found to be pressure dependent: the fall-off behaviour has been analysed in comparison with reported data. Electronic structure calculations have confirmed that at room temperature the reaction of CH(3)C(O)CH(2) with O(2) is essentially a recombination-type process. At high temperatures, the further reactions of the acetonyl-peroxyl adduct may yield OH radicals, but the most probable channel seems to be the O(2)-catalysed keto-enol transformation of acetonyl. Implications of the results for atmospheric modelling studies have been discussed.

Hydrogen-bond formation between isoindolo[2,1-a]indol-6-one and aliphatic alcohols in n-hexane.

The spectroscopic, kinetic, and equilibrium properties of isoindolo[2,1-a]indol-6-one (I) were studied in n-hexane in the presence and absence of alcohols (X). Hydrogen-bonded-complex formation was found to occur between the alcohol and the ground state as well as the excited state of the I molecule. The spectra of I and its singly complexed derivative (IX) are similar; however, that of IX is red shifted. The extent of red shift increases with the hydrogen-bonding ability of the alcohol. Equilibrium constant measurements were made to determine the hydrogen-bond basicity (beta(2)(H)) for I and the singlet excited (1)I. The beta(2)(H) value for (1)I is found to be about twice that of the ground-state I. Time-resolved fluorescence decay measurements indicate that the reaction of singlet excited I with fluorinated alcohols is diffusion controlled, while the rate of complexation with nonfluorinated (weaker hydrogen bonding) aliphatic alcohols depends on the Gibbs energy change in the complexation reaction. The quantitative correlation between the rate coefficient of complexation of (1)I with alcohols and the Gibbs energy change in the complexation process allowed us to estimate the rate coefficient for the complexation of the ground-state I with alcohols. The formation of the singlet excited hydrogen-bonded complex is irreversible; (1)IX disappears in a first order and an alcohol induced second order reaction. The first order decay is predominantly due to internal conversion to the ground state, the rate of which depends on the ionization energy of the complexing alcohol.

Pressure dependence of the dual luminescence of twisting molecules. The case of substituted 2,3-naphthalimides.

The effect of high pressure (up to 5 kbar) has been studied for triacetin solutions of 2-phenyl-2,3-dihydro-1H-benzo[f]isoindole-1,3dione 1 (N-phenyl-2,3-naphthalimide) and its 3-fluorophenyl- (2), 4-carbethoxyphenyl(4) and 4-methoxyphenyl (5) derivatives which all show dual fluorescence. When the N-phenyl group is unsubstituted (compound 1) or substituted with electron-attracting groups (2 and 4), the increase of pressure over the solution decreases slightly the emission at the long-wavelengths (LW) and increases dramatically the intensity of the short-wavelength (SW) fluorescence. Plotting the logarithm of the SW/LW fluorescence quantum yield ratio for compounds, 1,2 and 4 versus the logarithm of the viscosity of the medium shows a substantial increase of this ratio which corresponds mainly to the increase of the SW emission intensity, the effect on the LW emission being only moderate. As the pressure is increased, the rotation of the N-phenyl group of compound 1 is progressively hindered and the prevailing emission comes from a state which has the same geometry as the ground state (in which the planes of the two moieties of the molecule form an angle close to 60 degree). The effect is different when an electron-donating methoxy group is attached in the para position to the N-phenyl ring, compound 5, as mainly the LW fluorescence intensity increases with pressure. For this molecule which has an electron-donating p-substituent on the N-phenyl ring, the two moieties of the ground state molecule have a more planar geometry (43 degree angle) and the LW fluorescence appears to originate from an intramolecular charge transfer state the fluorescence of which increases with pressure. A three-level reaction scheme is proposed to account for the observed kinetics. In all cases, the viscosity of the medium is found to be the main factor which induces the changes in the fluorescence spectra, and the deceleration of the non-radiative deactivation from the SW* excited state is responsible for these modifications whether a reversible process between the two emitting SW* and LW* states is observed (as for compounds 2 and 4) or not (as for compound 1).

Dual luminescence properties of differently benzo-fused N-phenylphenanthridinones.

The photophysics of some newly prepared N-arylphenanthridinone derivatives have been investigated. It has been demonstrated how the luminescence properties are influenced by the size of the aromatic ring system. It has been shown that the replacement of the phenyl group in N-phenyphenanthridinone (PP) by an alpha-naphthyl or beta-naphthyl group (alphaNP and betaNP, respectively), influences the fluorescence spectra very differently. For alphaNP, the long-wavelength (LW) emission, which is well observable in case of PP, disappears, while for betaNP, the intensity of LW emission increases compared to the short-wavelength (SW) fluorescence. The rotation of the alpha-naphthyl group to the coplanar geometry, which is a requirement of the formation of the LW state, is strongly hindered, resulting in the lack of LW emission. In respect of steric hindrance, the beta-napthyl group is similar to phenyl, however, it decreases the energy of the LW state more as a consequence of its better electron donating character and the more extended conjugation of the coplanar system. This causes the increase of the LW/SW fluorescence ratio. The benzo-fusing on the phenanthridinone moiety results in a 6-7 kcal mol(-1) decrease in the SW singlet energy, however, surprisingly the LW state energy also decreases in almost the same manner. The phenomenon shows that the entire benzo-phenanthridinone group is strongly involved in both transitions. As a consequence, the benzo-fused N-aryl derivatives also show dual luminescence. The dipole moments of the LW state of betaNP and betaNBiP (6-naphthalen-2-yl-6H-benzo[i]phenanthridin-5-one) proved to be bigger by 30 and 50%, than that of the SW state, respectively. MO calculation indicates that in the SW --> LW reaction not only the size but the direction of the excited state dipole also changes significantly. In apolar solvents, the dominant deactivation process of the examined molecules is intersystem crossing. In polar solvents, where the LW emission energy is smaller, internal conversion becomes more significant than the other processes, resulting in low fluorescence yield.