For two photochromic isopropyl-thiophenefulgides a complete photokinetic analysis is given. For one compound (Sb), the partial quantum yield for the coloming process (&c= 54%) is the highest value found for thiophenefulgides. EZ-isomerisations do not play an important role. The colouringquantum yield is temperature independent down to 20 K, whereas bleaching requires little thermal activation. In case of an adamantylidene substituted isopropyl-thiophenefulgide (Sc), the bleaching efficiency (@cs= 3 1%) is very high. Upon heating of the cyclic products, thermal bleaching occurs with an activation energy of 1 eV.
IntmductionArylfulgides are capable of two different types of reversible photoreactions: EZ-isomerisations and ring closure reactions [ l-3,8,9,1 1,15-17 1. Whereas the former ones are accompanied by only a slight colour change, the colour is altered dramatically during the latter ones. Therefore, the reversible cyclisation reactions are referred to as photochromism of fulgides. In most cases, ring closure and ring opening reactions follow the Woodward-Hoffmann rules [ l-9,13 1. Heller and his group [3][4][5][6] found that alkyl-substituted heterocyclic fulgides show high reversibility ("fatigue resistance") along with thermal stability.As far as any practical application of photochromic materials is concerned, high efficiencies of the photochromic processes are desirable. Some years ago, predictions (based on semiempirical PPP-calculations) were made how to optimize the quantum yield for the colouring process [ 191. According to these results, bulky substituents R' (increasing sterical hindrance and constraints of the exocyclic double bond) should increase the colouring quantum yield and lower the efficiencies for EZ-isomerisations and radiationless deactivations. On the other hand, it has been shown [ 3,9] that bleaching efficiency of some furanfulgides is enhanced by introduction of the adamantylidene group R". (see scheme 1).Applying these concepts, isopropyl-and adamantylidene substituted thiophenefulgides were synthesized. We report here a complete photokinetic analysis of the different isomerisations of these compounds. Furthermore, we studied the temperature dependence of the photochromic behaviour including thermal bleaching.
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