Fluoreszenz / Intersystem Crossing / Temperaturabhangigkeit / SubstituenteneinfluJThe energies of the excited states which are involved in the deactivation, the quantum yields of fluorescence and intersystem crossing, and the fluorescence decay times were determined for acridizinium and seven substituted acridizinium derivatives. Taking into account a contribution of temperature independent intersystem crossing the temperature dependence of the fluorescence quantum yield can be described by means of the Arrhenius equation. The thermally activated isc-process in acridizinium (kizc = 6 . lo8 s -I) which determines the radiationless deactivation, is equivalent to the S, ('B2J --t T3 ('B3,) transition observed in anthracene. The level inversion of the energetic sequence of S, and T2 which is known from the exocyclic substitution of anthracene, takes place during the endocyclic azinium-substitution, too. -Depending on their nature and position, additional exocyclic substituents will influence the activation energy, resulting in the case of heavy atoms in a drastic acceleration of ki:c or the rate constant for the temperature independent isc-process (I&).
Deactivation Behaviour of Arenes and Heteroarenes. XXXII. The Influence of Endo‐ and Exocyclic Substitution on the Reversible Dimerization Behaviour of Anthracenes
The kinetic and thermodynamic values of the reversible photodimerization of 17 endo‐ and exocyclic substituted anthracenes are presented and discussed on the basis of the potential correlation diagram. The deactivation kinetics of the partial steps of photodimerization (excimer formation, transition into the pericyclic state, dimer formation) is influenced in dependence on the type and the place of substitution. From the determined partial values the interpretation of the integral behaviour is possible.
A relationship between the enthalpy of dimer formation, thermal stability and quantum yield of dimer formation is obtained. The sum of the quantum yields of forward and back reaction is 1 with the exception of derivatives substituted by heavy atoms and in the case of fluorescent excimers, respectively. With that the model of the common pericyclic transition state is confirmed.
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