Photoaddition of two guanine bases to single Ru-TAP complexes. Computational studies and ultrafast spectroscopies to elucidate the pH dependence of primary processes.
The covalent photoadduct (PA) between [Ru(TAP)3](2+) (TAP = 1,4,5,8-tetraazaphenanthrene) and guanosine monophosphate (GMP) opened the way to interesting photobiological applications. In this context, the PA's capability upon illumination to give rise to the addition of a second guanine base is especially interesting. The origins of these intriguing properties are for the first time thoroughly investigated by an experimental and theoretical approach. The PA's spectroscopic and redox data combined with TDDFT results corroborated with resonance Raman data show that the properties of this PA (pKa around 7) depend on the solution pH. Theoretical results indicate that the acid form PA.H(+) when excited should relax to MLCT (metal-to-ligand charge transfer) excited states, in contrast to the basic form PA whose excited state should have LLCT/ILCT (ligand-to-ligand charge transfer/intra ligand charge transfer) characteristics. Ultrafast excitation of PA.H(+) at pH 5.9 produces continuous dynamic processes in a few hundred picoseconds involving coupled proton-electron transfers responsible for luminescence quenching. Long-lived species of a few microseconds capable of reacting with GMP are produced at that pH, in agreement with the formation of covalent addition of a second GMP to PA, as shown by mass spectrometry results. In contrast, at pH 8 (mainly nonprotonated PA), other ultrafast transient species are detected and no GMP biadduct is formed in the presence of GMP. This pH dependence of photoreaction can be rationalized with the different nature of the excited states, thus at pH 8, unreactive LLCT/ILCT states and at pH 5.9 reactive MLCT states.
§Chimie Organique et Photochimie, Université Libre de Bruxelles, CP 160/08, 50 Avenue Franklin Roosevelt, B-1050 Brussels, Belgium.
Lionel
L
Marcélis
†Laboratoire de Spectrochimie Infrarouge et Raman UMR 8516, CNRS-Université Lille 1 Sciences et Technologies, 59655 Villeneuve d'Ascq Cedex, France.
Mateusz
M
Rebarz
‡Laboratory for Chemistry of Novel Materials, UMons, 20 Place du Parc, B-7000 Mons, Belgium.
Vincent
V
Lemaur
@Molecular Imaging and Photonics, KULeuven, Celestijnenlaan 200f, B-3001 Leuven, Belgium.
Eduard
E
Fron
∥Organic Synthesis and Mass Spectrometry Laboratory, University of Mons, UMons, 23 Place du Parc, B-7000 Mons, Belgium.
Julien
J
De Winter
§Chimie Organique et Photochimie, Université Libre de Bruxelles, CP 160/08, 50 Avenue Franklin Roosevelt, B-1050 Brussels, Belgium.
Cécile
C
Moucheron
∥Organic Synthesis and Mass Spectrometry Laboratory, University of Mons, UMons, 23 Place du Parc, B-7000 Mons, Belgium.
Pascal
P
Gerbaux
‡Laboratory for Chemistry of Novel Materials, UMons, 20 Place du Parc, B-7000 Mons, Belgium.
David
D
Beljonne
†Laboratoire de Spectrochimie Infrarouge et Raman UMR 8516, CNRS-Université Lille 1 Sciences et Technologies, 59655 Villeneuve d'Ascq Cedex, France.
Michel
M
Sliwa
§Chimie Organique et Photochimie, Université Libre de Bruxelles, CP 160/08, 50 Avenue Franklin Roosevelt, B-1050 Brussels, Belgium.
Andrée
A
Kirsch-De Mesmaeker
D001372:Aza Compounds / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D003198:Computer Simulation; D004583:Electrons; D006147:Guanine / Q000737:chemistry*; D006157:Guanosine Monophosphate / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008956:Models, Chemical; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010616:Phenanthrenes / Q000737:chemistry*; D055668:Photochemical Processes; D017785:Photons; D011789:Quantum Theory; D013057:Spectrum Analysis
D001372:Aza Compounds; D010616:Phenanthrenes; D006147:Guanine; D006157:Guanosine Monophosphate