Excited-state symmetry breaking is investigated in a series of symmetric 9,10-dicyanoanthracenes linked to electron-donating groups on the 2 and 6 positions via different spacers, allowing for a tuning of the length of the donor-acceptor branches. The excited-state properties of these compounds are compared with their dipolar single-branch analogues. The changes in electronic structure upon their optical excitation are monitored by transient electronic spectroscopy in the visible and near-infrared regions as well as by transient vibrational spectroscopy in the mid-infrared. Our results reveal that, with the shortest branches, electronic excitation remains distributed almost symmetrically over the molecule even in polar environments. Upon increasing the donor–acceptor distance, excitation becomes unevenly distributed and, with the longest one, it fully localises on one branch in polar solvents. The influence of the branch length on the propensity of quadrupolar dyes to undergo excited-state symmetry breaking is rationalised in terms of the balance between interbranch coupling and solvation energy.

Excited-state symmetry breaking (ES-SB) is common to a large number of multibranched electron donor-acceptor (DA) molecules in polar environments. During this process, the electronic excitation, originally evenly distributed over the molecule, localizes, at least partially, on one branch. Due to the absence of an unambiguous spectroscopic signature in the UV-vis region, electronic transient absorption (TA) has not been the method of choice for real-time observation of this phenomenon. Herein, we demonstrate that the Laporte rule, which states that one-photon transitions conserving parity are forbidden in centrosymmetric molecules, provides such clear signature of ES-SB in electronic TA spectroscopy. Using a dicyanoanthracene-based D-A-D dye, we show that transitions from the S1 state of this molecule, which are initially Laporte forbidden, become allowed upon ES-SB. This leads to the rise of new TA bands, whose intensity provides a direct measure of the extent of asymmetry in the excited state.