On the interplay of solvent and conformational effects in simulated excited-state dynamics of a copper phenanthroline photosensitizer

Post-print (lokagerð höfundar)Copper(I) bis-phenanthroline complexes represent Earth-abundant alternatives to ruthenium-based sensitizers for solar energy conversion and photocatalysis. Improved understanding of the solvent- mediated excited-state structural dynamics can help optimize their photoconversion efficiency. Through direct dynamics simulations in acetonitrile and excited-state minimum energy path calculations in vacuum, we uncover the mechanism of the photoinduced flattening motion of the prototypical system [Cu(dmphen)2]+ (dmphen = 2,9-dimethyl-1,10-phenanthroline). We find that the ligand distortion is a two-step process in acetonitrile. The fast component (~110 fs) is due to spontaneous pseudo Jahn– Teller instability and is largely solvent independent, while the slow component (~1.2 ps) arises from the mutual interplay between solvent molecules closely approaching the metal center and rotation of the methyl substituents. These results shed new light on the influence of a donor solvent such as acetonitrile and methyl substituents on the flattening dynamics of [Cu(dmphen)2]+.The present work has received funding from the Icelandic Research Fund (grants number 196070-051 and 196279-051). The authors are grateful to Aleksei Ivanov for support with the NEB calculations and Ask H. Larsen for discussion about the implementation of the acetonitrile force field in ASE.Peer Reviewe

Ultrafast Structural Dynamics of Photo-Reactions Revealed by Model-Independent X-ray Cross-Correlation Analysis

We applied angular X-ray Cross-Correlation analysis (XCCA) to scattering images from a femtosecond resolution LCLS X-ray free-electron laser (XFEL) pump-probe experiment with solvated PtPOP ([Pt$_2$(P$_2$O$_5$H$_2$)$_4$]$^{4-}$) metal complex molecules. The molecules were pumped with linear polarized laser pulses creating an excited state population with a preferred orientational (alignment) direction. Two time scales of $1.9\pm1.5$ ps and $46\pm10$ ps were revealed by model-independent XCCA, associated with an internal structural changes and rotational dephasing, respectively. Our studies illustrate the potential of XCCA to reveal hidden structural information in a model independent analysis of time evolution of solvated metal complex molecules.Comment: 8 pages, 5 figures, 50 reference

The First Direct Detection of Kirkwood Transitions in Concentrated Aqueous Electrolytes using Small Angle X-ray Scattering

Ion-ion correlations, screening, and equilibrium bulk structure in various concentrated electrolytes are investigated using synchrotron small angle X-ray scattering (SAXS), theory, and molecular simulation. Utilizing SAXS measurements we provide estimates of the Kirkwood Transition (KT) for a variety of aqueous electrolytes (NaCl, CaCl$_2$, SrCl$_2$, and ErCl$_3$). The KT may be defined as the concentration above which the ion-ion correlations cease to decay exponentially with a single length scale given by the Debye length $\lambda_{\rm D}$ and develop an additional length scale, $d=2\pi/Q_0$ that reflects the formation of local domains of charge. Theoretical models of the KT have been known for decades for highly idealized models of electrolytes, but experimental verification of KT in real electrolytes has yet to be confirmed. Herein, we provide consistent theoretical and experimental estimates of both the inverse screening lengths $a_0$ and inverse domain size, $Q_0$ for the aforementioned electrolyte systems. Taken together, $a_0$ and $Q_0$ are known descriptors of the KT and provide a view into the complexity of ion-ion interaction beyond the well-accepted Debye-H\"{u}ckel limit. Our findings suggest a picture of interaction for real electrolytes that is more general than that found in idealized models that is manifest in the precise form of the non-local response function that we estimate through the interpretation of the experimental SAXS signal. Importantly, the additional complexity of describing ion-ion interaction of real electrolytes will implicate the short-range ion-ion interactions that can only be computed via molecular simulation and provide a quantitative approach to describe electrolyte phenomena beyond Debye-H\"{u}ckel theory.Comment: 3