Spectroscopy of Nitrophenolates in Vacuo: Effect of Spacer,Configuration, and Microsolvation on the Charge-Transfer Excitation Energy

Accounts Of Chemical Research 47, 1417 - 1425 (2014)

Spectroscopy of Nitrophenolates in Vacuo: Effect of Spacer,Configuration, and Microsolvation on the Charge-Transfer Excitation Energy

Steen Brøndsted Nielsen,Mogens Brøndsted Nielsen, Angel Rubio

In a charge-transfer (CT) transition, electron density moves from one end of the molecule (donor) to the other end (acceptor). This type of transition is of paramount importance in nature, for example, in photosynthesis, and it governs the excitation of several protein biochromophores and luminophores such as the oxyluciferin anion that accounts for light emission from fireflies. Both transition energy and oscillator strength are linked to the coupling between the donor and acceptor groups: The weaker the coupling, the smaller the excitation energy. But a weak coupling necessarily also causes a low oscillator strength possibly preventing direct excitation (basically zero probability in the noncoupling case). The coupling is determined by the actual spacer between the two groups, and whether the spacer acts as an insulator or a conductor. However, it can be difficult or even impossible to distinguish the effect of the spacer from that of local solvent molecules that often cause large solvent shifts due to different ground-state and excited-state stabilization. This calls for gas-phase spectroscopy experiments where absorption by the isolated molecule is identified to unequivocally establish the intrinsic molecular properties with no perturbations from a microenvironment. From such insight, the effect of a protein microenvironment on the CT excited state can be deduced.

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http://dx.doi.org/10.1021/ar500025h
Notes
We thank present and previous group members who have contributed to the results, in particular, Drs. M. Wanko Universidad del País Vasco), M.-B.S. Kirketerp (AU), K.Støchkel (AU), M.Å. Petersen, M.A. Christensen (University of Copenhagen), H. Zettergren (Stockholm University), and J. Houmøller (AU).

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