Photosensitized generation of singlet oxygen from rhenium(I) complexes: A review

Abstract

I examine the photophysical properties for a number of thirty-eight Re(I) complexes in dilute solutions along with efficiencies of singlet oxygen production. Due to the structural differences around the Re(CO)3 core, the examples found in the literature of these efficient singlet oxygen photosensitizers can be divided into five categories: (I) tetracarbonyl Re(I) complexes bearing benzo[h]quinoline (bzq) and 1,10-phenanthroline (phen) ligands. (II) Re(CO)3 complexes of general formula LS-CO2-Re(CO)3(bpy) (LS = 2-pyrazine, 2-naphthalene, 9-anthracene, 1-pyrene, 2-anthraquinone) and XRe(CO)3L (X = CF3SO3, pyridine, Cl, norharmane; L = 2,2́-bipyridine (bpy), phen, Bathocuproinedisulfonate or dipyrido[3,2-a:2′,3′-c]phenazine (dppz)). (III) BrRe(CO)3L complexes where L is a pyridazine derivative. (IV) XRe(CO)3L complexes with X = P(Ph)3 and L = dipyrrinato derivative. (V) ClRe(CO)3L complexes where L = bpy derivatized with boron-dipyrromethene (BODIPY). Using these Re(I) complexes as photosensitizers, 1O2 (1Δg) can be efficiently produced either from non-emissive or from the highly emissive complexes. Depending on the nature of the Re(I) complex, the excited state responsible for the generation of singlet oxygen, via energy transfer to molecular oxygen, is either 3MLCT or a 3LC state. With LS-CO2-Re(CO)3(bpy) complexes, 1O2 (1Δg) is produced from oxygen quenching of 3LC states of anthracene and pyrene with high quantum yields (ΦΔ between 0.8 and 1.0). XRe(CO)3L complexes generate 1O2 (1Δg) mainly by oxygen quenching of their 3MLCT luminescence with an efficiency of 1O2 (1Δg) formation close to unity. Bimolecular rate constants for the quenching of the XRe(CO)3L complexes emission by molecular oxygen range between 1 and 2.5 × 109 M−1s−1. Complexes in (III) show ΦΔ values in dichloromethane which lie between 0.03 and 0.2, i.e. they are considerably lower than those of complexes in (I) and (II). In a series of rhenium dipyrrinates comprising a variety of aromatic substituents the phosphorescence lifetimes of the complexes are varied by the substituents in the micro-second order, and these long-lived triplet rhenium dipyrrinates are capable of sensitizing for efficient singlet oxygen generation (ΦΔ: 0.75–0.99). On the other hand, the trends observed across a series of rhenium complexes including the BODIPY ligand in their structure with respect to emission and 1O2 sensitization properties are rationalized in terms of the varied distal separation between the metal center and BODIPY groups in each system. It is suggested that the placement of the BODIPY group at the shortest possible distance to the metal center might be responsible, due to the enhanced ISC through a spin-orbit coupling, of the enhancement of ΦΔ.