Rotational isomerization of (E)-(2-anthryl)ethenes.: A consideration -: Why are the s-cis rotamers more stable than the s-trans rotamers in the excited state and less stable in the ground state?

Rotational isomerism between s-cis and s-trans rotamers of (E)-1-(2-anthryl)-2-phenylethene (E-2APE) and (E)1-(2-anthryl)-3,3-dimethyl-1-butene (E-2ADB) were investigated by comparing the absorption, emission and transient absorption spectra with those of the model compounds. The s-trans isomer is more stable than the s-cis rotamer in the ground state; however, the s-cis isomer is more stable than the s-trans rotamer in the excited state (the lowest singlet and triplet excited state). In the triplet excited state, s-trans-->s-cis one-way rotational isomerization is observed with activation energies of 30 and 18 kJ mol(-1) for E-2APE and E-2ADB, respectively. Explanations of why the s-cis rotamer is more stable than the s-trans rotamer in the excited state and less stable in the ground state are proposed using the HOMO and LUMO coefficients estimated by a MOPAC93 (Pha3) calculation.