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The photochemically reversible trans-cis isomerisation of (E)-hex-3-ene-1,5-diynes (1,2-diethynylethenes, DEEs) and 3,4-diethynylhex-3-ene-1,5-diynes (tetraethynylethenes, TEEs) substituted with electrondonating (p-dialkylaminophenyl) and/or electron-accepting (p-nitrophenyl) groups has been examined. The type and degree of donor/acceptor (D/A) functionalisation has been found to drastically affect the partial quantum yields of isomerisation t→c and c→t . Total quantum yields in n-hexane vary from total ‫؍‬ 0.72 for a bis-acceptor substituted TEE to total ‫؍‬ 0.015 for a four-fold, bis-donor, bis-acceptor substituted TEE derivative. A strong relationship between total and solvent polarity as well as a strong dependence of t→c and c→t on the wavelength of excitation exc has been observed. The temperature dependence of the photoisomerisation has been investigated for a bis-acceptor-substituted DEE and shows no changes in total over the temperature range 6.5-65 ЊC. None of the compounds studied undergoes thermal isomerisation at 27 ЊC in n-hexane. Further analysis of these compounds by theoretical investigations at the semiempirical level of theory reveals a significant reduction of the bond order of the central olefinic double bond in the D-D, D-A and A-A TEEs upon electronic excitation, thus ultimately facilitating photoisomerisation.

Fluorescence emission and excitation spectra of tenylene in several n-alkane Shpol'skii matrixes have been studied at liquid helium temperatures. The presence of two dominant Shpol'skii sites has been detected in all matrixes; we tentatively attribute one of the sites to tenylene substitutionally entering the host lattice, and the other one to guest molecules situated in the region of defects. The experiments, supported by quantumchemical calculations, point to the existence of two stereoisomers of the solute. We found n-decane to form the best matrix for tenylene; the system seems to be suitable for single molecule spectroscopy studies.

In chemistry textbooks, the pK a,H2O value of water in the solvent water at 25 8C is sometimes given as 14.0, sometimes as 15.7. This is confusing. The particular chemical reaction considered is the one in which water as BrønstedÀLowry acid reacts with water as BrønstedÀLowry base in water as solvent to yield equal concentrations of hydrated oxonium and hydroxide ions, H 3 O þ (aq) and HO À (aq), respectively. This reaction is also known as the self-ionization of water for which the equilibrium constant is abbreviated as K w with its known value of 10 À14.0 at 25 8C, i.e., pK w (25 8C) ¼ 14.0. Identical values for pK a,H2O and pK w at a fixed temperature appear reasonable, since K a,H2O and K w refer to one and the same reaction. Therefore, reasons for the apparent disagreement between the thermodynamically correct pK a value for water (14.0 at 25 8C) and the value reported in most organic chemistry textbooks (15.7) should be discussed when teaching acidÀbase chemistry. There are good arguments for introducing, from the very beginning, the concepts of activity and thermodynamic standard states when teaching quantitative aspects of chemical equilibria. This also explains in a straightforward way why all thermodynamic equilibrium constants, including K w , are dimensionless, and why pK H3O þ(25 8C) ¼ 0.

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