PHOTOISOMERIZATION OF ARYLETHYLENES - EXPLORING THE SINGLET POTENTIAL-ENERGY SURFACE OF A PARTIALLY PLANAR, SPECIALLY STABILIZED COMPOUND

A uniquely substituted tetraarylethylene was prepared and studied spectroscopically and photochemically. 9-Fluorenylidenetribenzocycloheptatriene (1) has a singlet lifetime of 4.1 ns at room temperature and fluorescence and transient absorption spectra that are independent of temperature and solvent. The fluorescence emission is assigned to the vertical (Franck-Condon) state. The relatively long lifetime of this state is a consequence of the barrier to planarization of the tribenzosuberenyl group. There are no photochemical reactions of 1 in cyclohexane, acetonitrile, or methanol solution. From the results it is clear that planarization of the groups on both ethylenic carbon atoms is required to facilitate rotation about this double bond. Further, the perpendicular excited state even in this highly stabilized example appears to be zwitterionic.