Influence of the substitution pattern on exciton localisation in centrosymmetric quadrupolar dyes

Localisation of the electronic excitation via excited-state symmetry breaking (ESSB) is a characteristic property of many quadrupolar dyes in polar environments, and was shown to depend on the nature of the electron donor (D) and acceptor (A) subunits and their separation distance. Here, we compare the excited-state properties of two centrosymmetic D-π-A-π-D dyes with a dipyrrolonaphthyridinedione (DPND) acceptor and N,N-dimethylaniline donors, which only differ by the position of the -π-D arms on the DPND core. Time-resolved IR spectroscopy reveals that ESSB and hence exciton trapping on single A-π-D branch is facilitated with the donor arms in 1 and 7 positions on the DPND core compared to 3 and 9 positions, given that it occurs in a medium polar solvent for the former and only in highly polar media for the latter. This a priori unexpected difference is explained by the significantly larger dipole moment generated upon ESSB for the 1,7 isomer, as reflected by its much larger fluorescence solvatochromism compared to the other isomer. In this respect, the DPND core with its C_2h symmetry allows for a much finer tuning of the excited-state properties of quadrupolar dyes than most previously used D or A cores of higher symmetry.