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

Abstract

The synthesis of 14,17-dimethyl[2](1,3)azuleno[2]paracyclophane 2c was achieved by dithia[3.3]phane cyclisation and photodesulfurization. The enantiomeric separation of the planar chiral (±2c) was performed by HPLC on cellulose carbamate and the circular dichroism (CD) spectrum is reported. The results of theoretical density functional calculations combined with a random phase approximation (DFT/RPA) approach for the CD spectrum compare well with 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 ground state geometry could only be achieved at the correlated ab initio MP2 level. The character of the excited states could be assigned and qualitatively analyzed with data obtained for azulene and dimethyl[7]paracyclophane as model chromophores. It is found that most of the seven experimentally resolved bands can be interpreted as perturbed locally excited transitions in the two chromophoric units. Charge transfer π→π states between the benzene and azulene fragments are found at relatively low energies (>3.2 eV) but they have low rotatory strengths. Furthermore, the CD intensities of the locally excited benzene type π→π(E_1u) states are amplified in the phane by a factor of about ten due to the presence of the azulene chromophore. By comparison of theoretical and experimental CD data the absolute configuration of 2c could be assigned as [CD(–)638, CD(+)218] = (S ).

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