Photochemical generation of thiophene analogs of 9-fluorenyl cations

The 9-fluorenyl cation is a member of the 4N Huckel antiaromatic series of intermediates, first observed by time-resolved spectroscopy on UV photo-excitation of 9-fluorenol. Thiophene analogs of 9-fluorenol in which one of the annelated benzene rings is replaced by a thiophene were prepared in three regioisomeric forms, and subjected to preparative and laser flash photolysis. Photoproduct studies in methanol indicated products derived from the corresponding fluorenyl cations and radical intermediates. Time-resolved spectroscopy showed transients which were assigned to the corresponding cations as evident from methanol quenching. The lifetimes and methanol quenching rates of these transients were compared with those of parent fluorenyl cation, and were found to be about two orders of magnitude more stable than the latter. On the other hand thermodynamic stabilities obtained from theoretical calculations based on isodesmic reactions of the open form cations, and NICS indicate that two of these ions (2 and 4) are less stable than 9-fluorenyl cation, whereas the "side" isomer 3 is slightly more stable. Copyright (C) 2010 John Wiley & Sons, Ltd.