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Abstract
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.
Highlights
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 20–30 cmÀ1 splitting of the –CRN bands found here in HFP is very modest compared the 130 cmÀ1 band splitting observed with another A–D–A type quadupolar molecule in the same solvent.[17,18]. These results reveal that the Time-resolved IR (TRIR) and electronic transient absorption (TA) spectra report on the symmetry of the electronic distribution relatively to different molecular planes of the dye
In BCN, the Laporte-forbidden transitions become visible in the electronic TA spectra upon solvent relaxation, indicating an electronic structure practically identical to that observed with D3 in polar solvents
Summary
Control on the extent of ES-SB can be achieved by tuning the properties of the local environment of the molecule with solvents of different polarity and H-bonding ability,[16,17,18,19,20,25,41] as well as the strength of D and A subunits.[30] The latter property is directly linked to the change of quadrupole moment upon excitation, which is an important tuning parameter for ES-SB. The electronic coupling between the D–A branches is a crucial factor for the extent of asymmetry.[35] In strongly coupled systems, full localisation of the excitation on one branch is not possible even in the most polar environments.[18] From a material point of view, ES-SB can be 15150 | Phys. A strong dependence of the extent of ES-SB on the branch length is observed and is discussed in terms of inter-branch coupling and solvation energy
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