Electronic transition moment directions and tautomerization of 9,10,19,20-tetra-n-propylporphycene

Linear dichroism, fluorescence polarization and quantum-mechanical studies of 9,10,19,20-tetra-n-propylporphycene led to the determination of the absolute directions of transition moments for six electronic transitions. The analysis of the transition moment directions reveals a double minimum character of the potential of motion of the inner protons in the ground and the first excited singlet electronic states. The trans tautomers are the dominant form both in the S0 and in S1 states, but a minor contribution from the cis tautomers cannot be excluded, in particular in S1.