Substitution pattern dependent behavior of the singlet excited states in symmetrically fluorinated biphenyls

Biphenyls are important basic chromophore systems that offer a possibility to study the effects of chemical substitution on the lower-lying excited states without complications from photoisomerization or other side processes. For several symmetric biphenyls, pristine biphenyl (bP0), 4,4'-difluorobiphenyl (bP2), 2,3,5,6,2',3',5',6'-octafluorobiphenyl (bP8), and perfluorobiphenyl (bP10), we report stationary and ultrafast solution-phase spectra rationalized with the aid of computations by means of the XMCQDPT2 multi-configuration perturbation theory and TDDFT. Polyfluorination tends to broaden the gap between the nearly degenerate S₁ + S₂ pair of states and the S₃ state in bP8 and bP10, yet relaxation from any sheet of the S₁-S₃ manifold leads through a system of state crossings to the same stationary points in S₁. Unlike bP0 and bP2 where the relaxed excited state is planar and non-polar, excited bP8 and bP10 exhibit sudden polarization to give a symmetry-lowered excited state via pseudo-Jahn-Teller interactions involving S₁ and S₂. Of particular interest is excited bP10 which reveals both sudden polarization and loss of planarity of one phenyl ring. We also demonstrate the unsatisfactory performance of the TDDFT methodology as applied to the biphenyls.