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-1 <- S(0)excitation, Delta Q, influences the propensity of a dye to undergo ES-SB using a series of molecules with aA-pi-D-pi-Amotif whereDis the exceptionally electron-rich pyrrolo[3,2-b]pyrrole andAare accepting groups. Tuning of Delta Qis achieved by appending a secondary acceptor group,A', on both sides of theDcore and ES-SB is monitored using a combination of time-resolved IR and broadband fluorescence spectroscopy. The results reveal a clear correlation between Delta Qand the tendency to undergo ES-SB. WhenAis a stronger acceptor thanA', ES-SB occurs already in non-dipolar but quadrupolar solvents. WhenAandA'are identical, ES-SB is only partial even in highly dipolar solvents. WhenAis a weaker acceptor thanA', the orientation of Delta Qchanges, ES-SB is observed in dipolar solvents only and involves major redistribution of the excitation over theD-pi-AandD-A'branches of the dye.