Photophysical Properties of Ru(II) Complexes

A. Masschelein, L. Jacquet, A. Kirsch-De Mesmaeker and J. Nasielski, Inorg. Chem., 1990, 29, 855-860

click on the absorption spectrum or the emission spectrum to go to the corresponding part or just scroll down the page

• LC (Ligand Centered) Transitions : These bands centered on the ligands appear in the U.V. region and are characterised by intense absorption.
• MC (Metal Centered) Transitions : These "dd" bands represent a forbidden transition and show weak absorption in the U.V.
• MLCT (Metal to Ligand Charge Transfer) Transitions : These transitions of lowest excited energy between the metal centered dp ground state and ligand p* states are observed in the visible.

The choice of the ligands allows to tune the MLCT absorption. Indeed p-acceptors ligands will present a low lying p* orbital and in the same time stabilise the dp orbital centered on the metal by retro-coordination. This allows also to tune the redox properties as the lowest lying p* orbital of the ligands will be involved in the reduction processes and the dp orbital centered on the metal will be involved in the oxidation processes.

After inter-system crossing from the 1MLCT singlet excited state to the 3MLCT triplet state, several deactivations pathways can be followed :

• Radiationless deactivation (knr)
• InterSystem Crossing to a 3MC triplet excited state (kMC). This processes is at the origin of the photolability and photoracemisation of Ru(II) complexes. It depends both on the temperature and on the energy gap between the 3MLCT and 3MC excited states.
• Radiative deactivation which leads to the luminescence of the complexes (hn).

In the Presence of DNA

The interaction of the complexes with DNA produce usually an increase of the emission intensity :

• Radiationless deactivation (knr) decreases as the luminophore is protected from aqueous deactivation in the hydrophobic DNA environment.
• InterSystem Crossing to the 3MC triplet excited state (kMC) decreaeses also as the complex is emprisonned within the double helix, increasing thereby the rigidity of the complex.
• Quenching by O2 (kqO2) decreases also as the oxygen remains in the bulk solution.

J.P. Lecomte, A. Kirsch-De Mesmaeker and G. Orellana, I. Phys. Chem., 1994, 98, 5382-5388

However, in some cases, when photreactions occurs between the complex and the DNA bases (photo-induced electron transfer), the luminescence is quenched in the presence of DNA as it is explain in the following part of the introduction "Photoreactions"

Redox properties

• In the fundamental state,

oxidation = abstraction of an electron from the HOMO = dp orbital centered on the metal.

reduction = addition of an electron on the LUMO = p* orbital centered on the ligand.

• In the excited state,

If the orbitals involved in the redox and in the spectroscopic processes (spectroelectochemical correlation) the redox potential in the excited state can be expressed as

Pred* = Pred + DE00

Pox* = Pox - DE00

where DE00 corresponds to the energy of the 0-0 transition from the 3MLCT excited state.

From these relation, one can see that the redox properties are easily tuned by the choice of the ligands considerring their p-acceptor strengths. This is of particular importance as one of our objective is to design photo-reactive complexes toward DNA (introduction page).