Does diphenylacetylene (tolan) fluoresce from its second excited singlet state? Semiempirical MO calculations and fluorescence quantum yield measurements

It is confirmed by measurements of fluorescence spectra and quantum yields that the fluorescence in tolan originates from the same state that causes the absorption band at lowest energy. The temperature dependence of the fluorescence quantum yield shows that this state is thermally deactivated with an activation energy of E(A) = 14.0 kJ/mol. Geometry optimizations of the states S-0, S-1, and T-1 of tolan with the semiempirical AM1 method lead to planar structures with D-2h symmetry. Potential energy curves along the triple-bond stretching coordinate have been calculated for several low-lying excited states with a combination of the AM1 and the INDO/S methods. It is found that for large triple-bond lengths, the 1(1)A(u)-state with sigma pi* character becomes the lowest excited singlet state. It is proposed that thermal deactivation of S-1(1(1)B(1u) leads to this state. Nonvertical excitation of 1(1)A(u) could explain the weak lines found in supersonic jet experiments below the onset of the 1(1)A(g) --> 1(1)B(1u) transition.