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Acid–base properties of four 3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b; 4′,3′-e]-pyridine derivatives with different substituents in position 4, i.e. N,N-dimethylaminophenyl, phenyl, nitrophenyl, and methoxyphenyl (DMA-DMPP, H-DMPP, NO2-DMPP, and CH3O-DMPP) were investigated in nonaqueous solutions. Absorption and stationary fluorescence as well as time-resolved spectroscopy were applied as methods. The acidity of the amino group in DMA-DMPP which is preferentially protonated in the ground state, does practically not change in the first excited state. The second protonation step of DMA-DMPP and the protonation of the other compounds occurs at the heterocyclic subunit, in the ground state most probably on the pyridine nitrogen. The basicity of this nitrogen atom increases remarkably in the first excited state. Experimental results were compared with those of semiempirical calculations. The amino group as the preferential site of protonation before the pyridine nitrogen in the ground state was confirmed. According to quantum mechanical computations, one pyrazolo nitrogen is preferentially protonated in the first excited state. The protonation is accompanied by a geometrical relaxation leading to a planarized conformer with a lower total energy.

Fluorescence properties of 1-heptanoylpyrene in homogeneous solutions and incorporated into liposome membranes have been measured. The fluorescence intensity of the alkanoylpyrene is small in neat hydrocarbon solutions (Qf = 0.006) but increases with increasing solvent polarity. On the contrary, Qf of the 1-alkylpyrene is larger by a factor of 10 in a hydrocarbon solvent and decreases slightly in polar solvents. In alcohols the fluorescence yield of 1-heptanoylpyrene becomes considerably larger in comparison to a nonhydroxylic environment and nearly identical to that of heptylpyrene in the same solvent. The fluorescence spectra shift bathochromically in that polar environment. The fluorescence properties of 1-heptanoylpyrene incorporated in a liposome membrane indicate that the probe molecules are well embedded within the hydrophobic core of the bilayer. Semiempirical calculations were performed for 1-ethanoylpyrene and 1-ethylpyrene as model compounds in order to analyze the influence of solvent polarity and of hydrogen bonding on excited state energies and fluorescence properties of 1-acyl derivatives of pyrene. Hydrogen-bonding increases the energy gap between the lowest singlets, which are nearly degenerated, and the 3π*-n triplet and reduced consequently the intersystem crossing efficiency. But also a change in the sequence of the excited 1π*-π states is concluded from experimental data and should contribute to the increase of the fluorescence yield upon hydrogen bonding.

The photophysical properties of 4-(4′-N,N′-dimethylaminophenyl)-1,7-diphenyl-3,5-dimethyl-bis-pyrazolo-[3,4-b;4′,3′-e]pyridine (DMA-DMPP), highly emissive in non-polar solvents and composed from a dimethylanilino electron donor and a bulky electron acceptor, have been investigated. The position of the fluorescence maximum as well as the fluorescence quantum yield and the radiative decay rate are strongly solvent polarity dependent; the magnitude of this effect becomes dramatically larger as the solvent polarity exceeds that of 1-chlorobutane. In protic solvents dual fluorescence is observed. In accordance with theoretical calculations the results are explained in terms of a polarity induced change in the energetic ordering of two emissive states of largely different charge transfer character.

In addition to the yellow emission ascribed to the J aggregates of 1,1′-diethyl-2,2′-cyanine, this compound also exhibits a high-energy emission around 400 nm in dilute aqueous solutions (c

Steady-state and dynamic photophysical characteristics of a number of 4- and 5-amino-2-phenylindan-1,3-dione derivatives have been studied at 293 and 77 K in solvents of widely different polatiry and hydrogen-bonding ability. The observed changes of the fluorescence Franck-Condon transition energy, and of the fluorescence quantum yields and the singlet lifetimes, as well as the solvent deuterium-isotope effect, indicate the formation of intermolecular hydrogen bonds in the excited state with protic solvents for the 5-amino compounds at room temperature.

The photochemistry of some α-azohydroperoxides (phenyl(phenylazo)methyl hydroperoxide and its halogenated derivatives) was investigated in various solvents by microsecond and nanosecond flash photolysis methods and steady state product analysis. Photodecomposition of the molecule, demonstrated by the formation of benzaldehyde, is a major, but not the only, photochemical pathway. Investigation of the influence of solvent and substitution on the photoinduced transients provides strong evidence for alternative photoreactions. One of the species produced by flash photolysis was studied in detail and assigned to a transient zwitterion formed from the α-azohydroperoxide by proton transfer via an excited state pathway. Semiempirical calculations add further support to the proposed mechanism.

Exciplex formation between triethylamine and some alcohols in hexane solution has been followed by nanosecond time- and energy-resolved fluorescence spectroscopy. The experimental results have been discussed on the basis of a successive association model and rate constants have been obtained from an iterative adjustment of the model parameters to the measured concentration dependence of the decay times. Fluorescence quenching, increasing in efficiency with decreasing alcohol chain length, may be attributed to a charge transfer reaction of the amine—alcohol exciplexes with alcohol clusters formed in low concentration hydrocarbon solutions. The deuterium isotope effect observed in these systems can be explained by hydrogen migration in triethylamine—alcohol exciplexes.