Substituenteneffekte im spektroskopischen Verhalten von Maleinsäurederivaten und 4-Cyclopenten-1.3-dionen / Substituent Effects on the Spectroscopic Behavior of Maleic Acid Derivatives and 4-Cyclopentene-1,3-diones

The lowest excited states of maleic acid- and cyclopentendionderivatives have been investigated by UV-absorption and phosphorescence emission spectroscopy. While the first excited singlet-state S is of (nπ*)-type in all compounds, the lowest triplet-state T was found to be a mixture of (nπ*) and (ππ*)-configurations. For the emitting derivatives (Ζ ≠ Η) phosphorescence-lifetimes and -quantum yields as well as the probability for intersystem crossing have been determined. Considering the inductive effects of all substituents on the S (nπ*) -energy, Coulombpotentials for sulfur and iodine in terms of Huckel-parameters: h = 0,2 and hj = —0,5 have been derived. The results of Hückel-MO-calculations can be correlated to the measured values of all compounds with the exception of the iodine containing derivatives.