The amide bridge photocleavage in ruthenium bichromophoric complex

Abstract

Photoinduced electron transfer between [Ru(bpy)₂mbpy–pyr]²⁺ complex, where mbpy = 4-methyl-4′-carbonyl-2,2′-bipyridine and pyr = 1-aminopyrene, and N,N-dimethylaniline (DMA) gives rise to an irreversible process which ultimately leads to the cleavage of the amide bond that links the pyrene to the ruthenium diimine complex. The photochemical reaction under stationary irradiation was monitored by emission and IR spectroscopic techniques as well as by HPLC chromatographic methods. The results suggest that the amide bond fragmentation occurs after the initial electron transfer process, involving the ³MLCT state of the Ru complex and DMA moiety that results in the formation of [Ru(bpy)₂mbpy]²⁺ complex and pyrene-1-isocyanate primary photoproducts. This last photoproduct suffers hydrolysis in aqueous medium regenerating the 1-aminopyrene ligand and carbon dioxide.

Introduction

One of the most active topics of investigation in photochemistry over the last two decades has been photoinduced electron transfer within model compounds [1], [2]. The design of luminescent polypyridine–ruthenium(II) complexes is a subject of great interest in light-harverting units for elaboration of multi-component supramolecular systems endowed with photo- and electroactive properties [3], [4]. There are a large number of transition metal complexes which exhibit metal-to-ligand charge transfer (MLCT) excited states involving diimine ligands [5], [6]. This is in part due to the fact that diimine complexes of the metals like Re(I), Ru(II) and Os(II) serve as effective sensitizers for photochemical reactions involving net electron transfer [5], [6], [7], [8]. The properties of the excited states of these complexes can be varied systematically by introducing a range of appropriate ligands. For instance, several compounds have been prepared with amide bridges. These have allowed studies concerning peptides and protein modification studies by using metal moieties as chromophores such as polypyridyl ruthenium complexes [9], [10], [11], [12], [13]. Recently, we reported the synthesis and the photophysical study of the complex [Ru(bpy)₂mbpy–pyr]²⁺, illustrated in Fig. 1, where mbpy = 4-methyl-4′-carbonyl-2,2′-bipyridine and pyr = 1-aminopyrene [14]. This system shows a low efficiency of energy transfer due to small difference in triplet energy between donor and acceptor species and weak electronic coupling and unfavorable Franck–Condon factors, despite the short distance separation of donor and acceptor species by an amide bridge. However, the close lying triplet excited state of donor and acceptor results in a long lived component due to reversible energy transfer between the two chromophores.

Ru(II) diimine complex covalently linked to pyrene by an alkyl bridge was reported in the early 1990s by Ford and Rodgers [15]. The photoexcitation of the MLCT transition of this complex was followed by rapid relaxation to an equilibrium mixture of the two states and the transient absorption spectra revealed bands assigned to the ³MLCT state of the Ru(II) complex and the ³(π–π*) state of pyrene. Similar behavior was reported by Sasse and co-workers in the study of a related ruthenium complex with a shorter alkyl tether [16], [17]. More recently, the Schmehl and co-workers [18] prepared a complex having pyrene covalently attached to 2,2′-bipyridine through a single C–C bond.

Very often, quenching by electron transfer in solution leads initially to the formation of a complex between the excited-state and the ground-state quencher. This complex we call an exciplex [19]. Exciplexes are distinct intermediates in their own right and possess unique properties. The most conspicuous property of exciplexes is their luminescence, which almost always is at longer wavelengths (lower energy) than the luminescence of the excited state. It is well known that pyrene singlet excited state is quenched by N,N-dimethylaniline (DMA) in solution by an exciplex mechanism [20]. Also, [Ru(bpy)₃]²⁺ in the ³MLCT state has a high afinity for electron transfer with DMA or other electron donors. In this work, the luminescence quenching and reactivity of [Ru(bpy)₂mbpy–pyr]²⁺ complex with DMA was investigated in tetrahydrofuran (THF) as well as in micellar aqueous environment formed by sodium dodecyl sulfate (SDS). We report here the photochemistry of the bichromophoric compound [Ru(bpy)₂mbpy–pyr]²⁺ studied by steady state and time resolved luminescence techniques combined with standard HPLC and IR analysis of products.