Time-resolved fluorescence spectroscopy of helically distorted aromatic systems

The fluorescence deactivation of several lactones and phenanthrene derivatives has been investigated by stationary and time-resolved fluorescence spectroscopy. Due to steric repulsions, these molecules are helically twisted around a central axis. Surprisingly, they show a negative temperature dependence of the natural fluorescence constant κfo = Φfκf. Considering both electronic and structural contributions we propose that the strain within the fluorescing molecules can be qualitatively described using the observed temperature dependence of κfo. The larger this negative temperature dependence, the larger is the internal motion barrier in the molecules.