Experimental and theoretical studies of a chiral azulenophane: synthesis, structure and circular dichroism spectra of 14,17-dimethyl[2](1,3)azuleno[2]paracyclophane

The synthesis of 14,17-dimethyl[2](1,3)azuleno[2]paracyclophane 2c was achi eved by dithia[3.3]phane cyclisation and photodesulfurization. The enantiom eric separation of the planar chiral (+/-2c) was performed by HPLC on cellu lose carbamate and the circular dichroism (CD) spectrum is reported. The re sults of theoretical density functional calculations combined with a random phase approximation (DFT/RPA) approach for the CD spectrum compare well wi th the experimental data in the recorded energy range (1.5-6.5 eV). A good agreement between the X-ray structure and the theoretically optimized groun d state geometry could only be achieved at the correlated ab initio MP2 lev el. The character of the excited states could be assigned and qualitatively analyzed with data obtained for azulene and dimethyl[7]paracyclophane as m odel chromophores. It is found that most of the seven experimentally resolv ed bands can be interpreted as perturbed locally excited transitions in the two chromophoric units. Charge transfer pi-->pi states between the benzene and azulene fragments are found at relatively low energies (>3.2 eV) but t hey have low rotatory strengths. Furthermore, the CD intensities of the loc ally excited benzene type pi-->pi(E-1u) states are amplified in the phane b y a factor of about ten due to the presence of the azulene chromophore. By comparison of theoretical and experimental CD data the absolute configurati on of 2c could be assigned as [CD(-)638, CD(+)218] = (S).