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Letter
Exciton Localization on Ru-based Photosensitizers Induced by Binding to Lipid Membranes Pedro A. Sánchez-Murcia, Juan José Nogueira, and Leticia Gonzalez J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03357 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018
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The Journal of Physical Chemistry Letters
Exciton Localization on Ru-based Photosensitizers Induced by Binding to Lipid Membranes Pedro A. Sánchez-Murcia,* Juan José Nogueira and Leticia González*
Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Str. 17 A-1090 Vienna, Austria
Corresponding Author *
[email protected] *
[email protected] ACS Paragon Plus Environment
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ABSTRACT The characterization of electronic properties of metal complexes embedded in membrane
environments
is
of
paramount
importance
to
develop
efficient
photosensitizers in optogenetic applications. Molecular dynamics and QM/MM simulations together with quantitative wavefunction analysis reveal a directional electronic redistribution of the exciton formed upon excitation of [Ru(bpy)2(bpyC17)]2+ when going from water to a lipid bilayer, despite the media neither influences the metal-to-ligand charge-transfer character nor the excitation energy of the absorption spectra. When the photosensitizer is embedded into the DOPC lipid membrane, exciton population is mainly located in the bypyridyl sites proximal to the positively charged surface of the bilayer due to electrostatic interactions. This behavior shows that the electronic structure of metal complexes can be controlled through the binding to external species, underscoring the crucial role of the environment in directing the electronic flow upon excitation and thus helping rational tuning of optogenetic agents.
TOC GRAPHICS
KEYWORDS. Exciton localization, Photosensitizers, Lipid membranes, Optogenetics.
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The Journal of Physical Chemistry Letters
Ru(II)-based complexes are powerful photosensitizers with attractive biological and medical applications,1-3 including optogenetic cellular control. Recently, it has been shown that the complex [Ru(bpy)2(bpy-C17)]2+ (Figure 1A) is able to insert into cell membranes and induce alterations in the cell electrical activity4 upon illumination with light. Given the potential of optogenetic reagents, the importance of understanding the behavior of generated excited species on lipid membranes cannot be understated. However, an unequivocal electronic characterization of metal complexes excited states is a very challenging task, and even more in a biological environment. The excited electron can acquired different degrees of delocalization (also termed the exciton size) that do not only depend on the chemical structure of the complex5-6 but also on the time scale of the experimental measurement.7-8 The need of considering vibrational motion and environmental effects in the characterization of the electronic structure of the chromophore poses a severe challenge to theory.9-11 This complexity is very well represented by the archetype photosensitizer [Ru(bpy)3]2+:12 After light absorption, the complex populates a singlet metal-to-ligand charge-transfer state (1MLCT), which efficiently goes to a triplet 3MLCT state by intersystem crossing in a ultrafast manner (~25 fs).8, 13-14 The long lived 3MLCT state (~600-800 ns) is well characterized as it has been found that the excited electron localizes on only one of the bipyridine units (Figure 1B);7,
15
in contrast, different degrees of delocalization have been reported for the
initially populated 1MLCT state,7-8, 12, 14, 16-17 likely due to its very short lifetime (
