Excited states of methano-bridged [10]-, [14]-, and [18]annulenes. Evidence for strong transannular interaction, and relation to homoaromaticity

Abstract

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

Bis(trifluoromethyl)sulfene: generation and cycloaddition reactions

“…Decoupling experiments were required to determine the 'H chemical shifts and coupling constants unambiguously. The relatively large 15 Hz coupling constant for the vinyl protons H6,H7 in the adducts is consistent with trans but not cis vinyl proton stereochemistry (3Jtrans = 12-18 Hz, 3Jcir [6][7][8][9][10][11][12]. This immediately rules out the [4 + 21 cycloadducts 22 and the [2 + 21 cycloadducts 23.…”

Emerging Prospects for Unusual Aromaticity in Organic Electronic Materials: The Case for Methano[10]annulene

“…Dewey et al studied the photophysics of methano-bridged annulenes in order to learn more about their transannular interactions in the excited state. [56] These insights are particularly important given the prospects for homoaromaticity within the annulene core as well as the importance of the underlying σ-framework as a factor for aromatic delocalization. [57] Efforts such as the examples above provide a strong foundation for the development of new π-conjugated materials whose performance depends upon the formation and mobility of quasi-particle excitons or radical ion charge carriers.…”

1,6-Methano[10]annulene has a Delocalized Structure in S0 and S1: High-Resolution Spectroscopy in a Supersonic Jet This work was supported by the Fonds der Chemischen Industrie.

“…Hence additional lines may appear in the spectrum which gain their intensity by Herzberg–Teller coupling with higher electronic states. That such vibrationally induced transitions contribute to the spectrum has been concluded already from the fact that the degree of polarization of the transition varies over the bands 7. The low resolution of the spectra in solution and organic glasses at low temperatures prevents separation of the vibrational transitions from each other.…”