Syntheses, structures, and photoisomerization of (E)- and (Z)-2-tert-butyl-9-(2,2,2-triphenylethylidene)fluorene

"Sterically geared" 9-(2,2,2-triphenylethylidene)fluorene (1) is of potential interest as a photoactive moiety in molecular devices, and the 2-tert-butyl derivative (6) has been synthesized to investigate photoisomerization. E and Z stereoisomers of 6 were separated and identified by X-ray crystallography. The tert-butyl group does not introduce additional strain, and its close proximity to the trityl group in the Z isomer suggests an attractive van der Waals interaction. The UV spectra of (E)-6 and (Z)-6 are nearly identical, showing absorption bands that are similar to those of fluorene occurring at wavelengths longer than 240 nm. Photoisomerization of 6 was investigated at 266, 280 and 320 nm. Solutions initially containing only (E)-6 or (Z)-6 were irradiated with pulsed laser light, monitoring isomerization by 1H NMR spectroscopy. Negligible photodecomposition was observed when the solutions were agitated by N2 ebullition. Experimental data were fitted to theoretical curves to obtain photoisomerization quantum yields (phi(ZE) and phi(EZ)) ranging from 0.04 to 0.09. This first photoisomerization study of a dibenzofulvene reveals significant quantum yields, despite theoretical prediction of inefficient or negligible isomerization of the parent hydrocarbon, fulvene. Thermal isomerization of 6 at 270 degrees C (t(1/2) = 120 min) was also followed by 1H NMR spectroscopy, resulting in an estimated activation energy (deltaG(double dagger)) of 43 kcal/mol.