Change of Quadrupole Moment upon Excitation and Symmetry Breaking in Multibranched Donor-Acceptor Dyes.

Upon photoexcitation, a majority of quadrupolar dyes, developed for large two-photon absorption, undergo excited-state symmetry breaking (ES-SB) and behave as dipolar molecules. We investigate how the change of quadrupole moment upon S ←S excitation, ΔQ, influences the propensity of a dye to undergo ES-SB using a series of molecules with a A-π-D-π-A motif where D is the exceptionally electron-rich pyrrolo[3,2-b]pyrrole and A are accepting groups. Tuning of ΔQ is achieved by appending a secondary acceptor group, A', on both sides of the D core and ES-SB is monitored using a combination of time-resolved IR and broadband fluorescence spectroscopy. The results reveal a clear correlation between ΔQ and the tendency to undergo ES-SB. When A is a stronger acceptor than A', ES-SB occurs already in non-dipolar but quadrupolar solvents. When A and A' are identical, ES-SB is only partial even in highly dipolar solvents. When A is a weaker acceptor than A', the orientation of ΔQ changes, ES-SB is observed in dipolar solvents only and involves major redistribution of the excitation over the D-π-A and D-A' branches of the dye.