On the effect of a single solvent molecule on the charge-transfer band of a donor-acceptor anion

Many biochromophore anions located within protein pockets display charge-transfer (CT) transitions that are perturbed by the nearby environment, such as water or amino acid residues. These anions often contain the phenolate moiety as the electron donor and an acceptor group that couples to the donor via a π-conjugated system. Here we show using action spectroscopy that single molecules of water, methanol, and acetonitrile cause blue shifts in the electronic transition energy of the bare m-nitrophenolate anion by 0.22, 0.22, and 0.12 eV, respectively (uncertainty of 0.05 eV). These shifts are similar to CC2-predicted ones and are in accordance with the weaker binding to the phenolate end of the ion by acetonitrile in comparison with water and methanol. The nitro acceptor group is almost decoupled from the phenolate donor, and this ion therefore represents a good model for CT excitations of an anion. We found that the shift caused by one acetonitrile molecule is almost half of that experienced in bulk acetonitrile solution, clearly emphasizing the important role played by the microenvironment. In protic solvents, the shifts are larger because of hydrogen bonds to the phenolate oxygen. Finally, but not least, we provide experimental data that serve to benchmark calculations of excited states of ion-solvent complexes.