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

Abstract

The emissio.n spectrum o.fpo.lycrystalline [22]paracyclo.phane shows a resolved vibronic structure ~ith a 241 cm-1 pro.gressio.n at He temperatures. The dependence orthe energy or this mode upon selective deuteration in co.mbinatio.n with results from FIR and Raman spectra could be used to identify the mode as a torsional dimer vibration_ The emission spectra could be simulated assuming a linear co.upling o.fthe to.rsio.nal mode to. the electronic transitions with co.upling strengths o.f S = 10 (fluorescence) and S = I3 (phosphorescence). This co.rrespo.nds to. an· equilibrium displacement o.fthe benzene rings under electronic excitatio.n by a torsional angie of 10.6 0 (Sl) and 12..1 0 (Tl)' in addition to the small torsion in the ground state So by about 3°.

Electronic spectra and triplet state properties of superphane

“…The fluorescence and phosphorescence O,Obands of 1 in a crystalline sample are shifted to the red by 93 and 151 cm-' with respect to the corresponding O&bands of [2.2]paracyclophane (3) [13]. This can be qualitatively understood as originating from the stronger transanular interaction (TAI) and the equally stronger grout-bond interaction (TBI) in the case of 1 although an even larger red shift might have been expected.…”

“…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.…”

“…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].…”

UV/Vis Spectra of Cyclophanes

“…However, early models that were based solely on the p electron system and neglected the r core were unsatisfactory [28]. The anomalous band shape has been explained by the assumption of large structural rearrangements on excitation or emission [30][31][32]. It yielded, even on a qualitative level, a satisfactory explanation of the electronic spectra, the ESR spectrum and the photochemical behavior of such compounds.…”

“…Cram et al [9] have studied the UV spectra of [m.n]paracyclophanes in which m and n were varied stepwise, one carbon at a time from m = n = 2 (14) to m = 5, n = 6 (27,17,(28)(29)(30)(31)(32)(33)(34)(35). Some spectra are shown in Fig.…”