Effect of a Single Water Molecule on the Electronic Absorption by o- and p-Nitrophenolate: A Shift to the Red or to the Blue?

Journal Of Physical Chemistry A 119, Issue 47, 11498 - 11503 (2015)

Effect of a Single Water Molecule on the Electronic Absorption by o- and p-Nitrophenolate: A Shift to the Red or to the Blue?

Jørgen Houmøller,Marius Wanko,Angel Rubio, Steen Brøndsted Nielsen

Many photoactive biomolecules are anions and exhibit * optical transitions but with a degree of charge transfer (CT) character determined by the local environment. The phenolate moiety is a common structural motif among biochromophores/luminophores, and nitrophenolates are good model systems as the nitro substituent allows for CT-like transitions. Here we report gas-phase absorption spectra of o- and p-nitrophenolate•H2O complexes to decipher the effect of just one H2O and compare them with ab initio calculations of vertical excitation energies. The experimental band maximum is at 3.01 and 3.00 eV for ortho and para and is redshifted by 0.13 and 0.10 eV relative to the bare ions. These shifts indicate that the transition has become more CT-like due to localization of negative charge on the phenolate oxygen, i.e., diminished delocalization of the negative excess charge. Still the transition bears less CT than that of m-nitrophenolate•H2O as this complex absorbs further to the red (2.56 eV). Our work emphasizes the importance of local perturbations: One water causes a larger shift than experienced in bulk for para and almost the full shift for ortho. Predicting microenvironmental effects in the boundary between CT and non-CT with high accuracy is nontrivial. However, in agreement with experiment our calculations show a competition between the effects of electronic delocalization and electrostatic interaction with the solvent molecule. As a result, the excitation energy of ortho and para is less sensitive to hydration than that of meta as donor and acceptor orbitals are only weakly coupled in meta.

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SBN acknowledges support from the Danish Council for Independent Research (grant no. 4181-00048B). AR and MW acknowledge financial support from the European Research Council Advanced Grant DYNamo (ERC-2010- AdG-267374), Spanish Grant (FIS2013-46159-C3-1-P), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13). Technical and human support provided by IZO-SGI (SGIker) of UPV/EHU.

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