The optical activity of beta,gamma-enones in ground and excited states using circular dichroism and circularly polarized luminescence

The circularly polarized luminescence (CPL) and electronic circular dichroism (CD) spectroscopic parameters corresponding to the n <- pi* and n -> pi* transitions, respectively, have been calculated for selected beta,gamma-enones using density functional theory. For the smallest beta,gamma-enone, (1R,4R)-bicyclo[2.2.1]hept-5-en-2-one (norbornenone), coupled-cluster calculations have also been carried out. The excited-state potential energy surface for three of the five enones studied reveals two minima with different C = O center dot center dot center dot C = C dihedral angles, and with rotatory strengths of opposite sign. The relative energies of the minima determine the sign of the CPL intensity, which may be the same or opposite as in the CD spectrum, in agreement with experimental data. The results obtained in this first computational study of CPL demonstrate its usefulness as an indicator of excited-state structures of chiral species.