Investigation of the excited triplet states of [2.2]- and [3.3]paracyclophane

Abstract

We have studied the excited triplet states of [2.21-and [3.31 paracycIophane by emission spectroscopy and by ODMR and have compared the results with the corresponding monomer para xylene. The most striking result is that both phanes exhibit two sets of zero field splitting parameters indicating the existence of two nearly degenerate low lying excited triplet states. Furthermore, we fmd in distinction to expection a stronger reduction of the zero field splitting parameters as compared to the monomers for [3.3 J paracyclophane than for [2.2 Jparacyclophane. The causes of these surprising experimental findings are discussed.

Influence of an identified dimer vibration on the emission spectrum of [2,2]paracyclophane

“…P "]paracyclophane was crystallized from solution [13]. The isotopically dilTerent molecules investigated are given in fig.…”

Electronic spectra and triplet state properties of superphane

“…Table 1 O&bands in cm-' of the fluorescence and phosphorescence of 2) the relatively weak fluorescence is broad band and more or less structureless while the more intense phosphorescence shows again vibronic structure. The detectable O&bands and the resolved vibronic structures are in contrast to the usually obsenred broad band and structureless emissions of [2.2]-and [3.3lparacyclophane [12], but can be explained by a much weaker el~tron-phonon coupling @PC) for 1 and 2 due to the much more rigid frame caused by the six respectively five methylene bridges.…”

“…While in [2.2]-and [3.3lparacyclophane the strong coupling of several low-fr~uency dimer modes to the electronic transitions (strong EPC) with coupling strengths S= 11-13 [13] leads to a relatively large equilibrium shift of the nuclear positions under electronic excitation and therefore to the structureless and broad band emissions [12,13], here in 1 and 2 the EPC coupling strength is much weaker (S = 1 for the 1480 cm-" mode and S = 2-3 for the other observed modes; see At the moment none of the observed lowfrequency modes of 1 (table 2) can be identified as a special dimer mode (for instance breathing or torsional mode) -as it was possible in the case of [2.2jparacyclophane 3 ]13]. This is due to the fact that superphane (1) has Db, symmetry in the ground state, and therefore neither A,, nor A,, modes are IR or Raman active.…”

“…In addition to the transanular interaction between the a-systems of the two aromatic six-rings in cyclophane, the through-bond interaction via the connecting methylene bridges is also important for the electronic structure of the ground and excited states [12,. Only the combined consideration of transanular interaction and through-bond interaction accounts satisfactorily for the observed ionization potentials [14] and for the longest wavelength absorption in the UV spectra in the series of [2,] (i) Both phanes 1 and 2 show with 262.4 pm for 1 [17] and with 270 pm (mean value) for 2 [18] the smallest transanular distance which can be realized in a hydrocarbon phane.…”

“…Secondly, the much more rigid molecular frame of 1 and 2 should result in a much weaker electron-phonon coupling (EPC). Since the EPC is responsible for the unstructured broad band emission spectra [12,13], we expect for 1 and 2 a better resolution of the vibronic structure. (iii) The superphane 1 possesses a sixfold symmetry axis and hence the HOMO and LUMO of both subunits are degenerate [14].…”