Theoretical Spectroscopy and Photodynamics of a Ruthenium Nitrosyl Complex

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Leon Freitag and Leticia González *

Institut für theoretische Chemie, Universität Wien, Währinger Straße 17, 1090 Vienna, Austria

Inorg. Chem., Article ASAP

DOI: 10.1021/ic500283y

Publication Date (Web): April 21, 2014

Copyright © 2014 American Chemical Society

*E-mail: leticia.gonzalez@univie.ac.at.

Synopsis

How is nitric oxide (NO) released in photoactive transition-metal nitrosyl complexes? Electronic structure calculations and nonadiabatic molecular dynamics are employed to investigate the NO photodissociation mechanism of [Ru(PaPy3)(NO)]2+. Excited states are calculated with time-dependent density functional theory and multiconfigurational CASSCF/CASPT2 spin-corrected methods. Molecular dynamics show that intersystem crossing is ultrafast (10 fs) and that internal conversion takes place in less than 100 fs. Both processes and the competing NO dissociation are accompanied by bending of the NO ligand.

Abstract

Photoactive transition-metal nitrosyl complexes are particularly interesting as potential drugs that deliver nitric oxide (NO) upon UV-light irradiation to be used, e.g., in photodynamic therapy. It is well-recognized that quantum-chemical calculations can guide the rational design and synthesis of molecules with specific functions. In this contribution, it is shown how electronic structure calculations and dynamical simulations can provide a unique insight into the photodissociation mechanism of NO. Exemplarily, [Ru(PaPy3)(NO)]2+ is investigated in detail, as a prototype of a particularly promising class of photoactive metal nitrosyl complexes. The ability of time-dependent density functional theory (TD-DFT) to obtain reliable excited-state energies compared with more sophisticated multiconfigurational spin-corrected calculations is evaluated. Moreover, a TD-DFT-based trajectory surface-hopping molecular dynamics study is employed to reveal the details of the radiationless decay of the molecule via internal conversion and intersystem crossing. Calculations show that the ground state of [Ru(PaPy3)(NO)]2+ includes a significant admixture of the RuIII(NO)0 electronic configuration, in contrast to the previously postulated RuII(NO)+ structure of similar metal nitrosyls. Moreover, the lowest singlet and triplet excited states populate the antibonding metal d → πNO* orbitals, favoring NO dissociation. Molecular dynamics show that intersystem crossing is ultrafast (<10 fs) and dissociation is initiated in less than 50 fs. The competing relaxation to the lowest S1 singlet state takes place in less than 100 fs and thus competes with NO dissociation, which mostly takes place in the higher-lying excited triplet states. All of these processes are accompanied by bending of the NO ligand, which is not confined to any particular state.