Emission Polarization and Circular Dichroism of Hexahelicene

Measurements of the relative polarization of electronic transitions and their circular dichroism are reported for hexahelicene (phenanthro[3,4-c]phenanthrene). A new assignment scheme for the electronic spectrum is proposed in the light of these measurements. The longest wavelength 1Lb (375–425 mμ) transition shows evidence of mixed polarization in the fluorescence which would arise from vibronic coupling in the emission process. Its circular dichroism, which may contain ``forbidden'' character, is negative for the dextrorotatory molecule. The 1La (325–375 mμ) transition has a strong positive circular dichroism which determines the optical activity at visible wavelengths. The success of previous chirality calculations for hexahelicene hinges on a satisfactory treatment of this transition and the rather complex detailed polarization of its excited state. Discrepancies between predicted optical activity by free-electron and by Hückel MO calculations are noted. The 1Bb (280–325 mμ) transition has a weak circular dichroism whose sign is difficult to determine from experiment; calculations predict that it should have the same sign as 1La. What appears to be the 1Ba (or 1Cb) (230–280 mμ) transition has a stronger negative circular dichroism. Because of the out-of-plane distortion of the molecule, relative transition polarizations somewhat peculiar for polynuclear aromatics are expected. These account for certain data in the emission polarization, phosphorescence lifetime, and phosphorescence—fluorescence relative quantum yield.