Measurements of the absorption, polarized fluorescence, and magnetic circular dichroism of 1,6-methano [ lOIannulene, 1,6:8,annulene, and 1,6:8,17:10,15-trismethano[ 18lannulene along with semiempirical *-electron and all-valence-electron calculations on these and related annulenes are reported. It is concluded that (i) transannular interaction in the methano-bridged annulenes is strong, so that these homoaromatic molecules can be viewed as examples of "arrested transition states" along electrocyclic reaction paths; (ii) in addition to the anticipated Lb, La, Bb, and B, transitions originating in the conjugated (4N + 2)-electron perimeter, at least one further low-energy transition appears to be present but its nature is not well understood; (iii) the magnetic circular dichroism of the bridged annulenes is in line with the expectation based on the general theory for cyclic (4N + 2)-electron systems and the absolute signs agree with the relative size of orbital energy differences obtained from CNDO/S calculations.
IntroductionThe stable 1,6-methano[ l01annulene (1) has been known for some time.2 Recent syntheses of 1,6:8,13-bismethano[ 141-annulene3 (2) and 1,6:8,17:10,1 Strismethano[ 18]annulene4 (3) offer a unique opportunity to investigate the electronic properties of a homologous series of bridged annulenes. Earlier studies in annulene spectro~copy,~-~ as well as previous measurements on a homologous series of tetra-tert-butylbisdehydroannulenes,I0 suggest very strongly that expectations based on the simple perimeter model" will be fulfilled also for the series 1-3, and that the four excited states resulting from electron promotions from the two highest occupied MOs to the lowest two free MOs will dominate the low-energy region in the absorption spectra (weak