Photochemical, photophysical and redox properties of novel fulgimide derivatives with attached 2,2′-bipyridine (bpy) and [M(bpy)3]2+ (M = Ru and Os) moieties

Abstract

Fulgimides monosubstituted with [M(bpy)(3)](2+) (M = Ru, Os; bpy = 2,2'-bipyridine) chromophore units and with a single bpy group were synthesized and investigated as components of conceivable dinuclear photochromic switches of luminescence. The E-, Z- and closed-ring (C) photoisomer forms of the bpy-bound fulgimide were successfully separated by semi-preparative HPLC. The same procedure failed, however, in the case of the [M(bpy)(3)](2+)-substituted fulgimides. Energy transfer from the excited photochromic unit to the metal-bpy centre competes with the fulgimide cyclization, reducing the photocyclization quantum yields by approximately one order of magnitude compared to the non-complexed fulgimide-bpy ligand (phi(EC) = 0.17, phi(EZ) = 0.071, phi(ZE) = 0.15 at lambda(exc) = 334 nm). The cycloreversion of the fulgimide-bpy ligand is less efficient (phi(CE) = 0.047 at lambda(exc) = 520 nm). The intensity of the (3)MLCT-based luminescence of the metal-bpy chromophore (in MeCN, phi(deaer) = 6.6 x 10(-2) and tau(deaer) = 1.09 micros for Ru; phi(deaer) = 6.7 x 10(-3) and tau(deaer) = 62 ns for Os) is not affected by the fulgimide photoconversion. These results and supporting spectro-electrochemical data reveal that the lowest triplet excited states of the photochromic fulgimide moiety in all its E-, Z- and closed-ring forms lie above the lowest (3)MLCT levels of the attached ruthenium and osmium chromophores. The actual components are therefore unlikely to form a triad acting as functional switch of energy transfer from [Ru(bpy)(3)](2+) to [Os(bpy)(3)](2+) through the photochromic fulgimide bridge.

Photochromic Transitional Metal Complexes for Photosensitization

“…Similarly, the significant decrease of photocyclization quantum yields of the fulgimide-containing photochromic ligand in the tris(bipyridyl)osmium(II) and tris(bipyridyl)ruthenium(II) complexes (Figure 2.7c) due to intramolecular quenching of the reactive 3 LC state by the lower-lying 3 MLCT excited state have also been proposed and supported by transient absorption spectroscopic studies [20].…”

“…However, the photochromism of the spirooxazine-containing bipyridine ligands could not be sensitized by the 3 MLCT excited state of the rhenium(I) complexes as the 3 MLCT state to the 3 LC state triplet-triplet energy transfer was disfavored by the lower-lying 3 MLCT state (166-188 kJ mol −1 , estimated from the phosphorescence of these complexes) compared to the 3 LC state of the spirooxazine (210-225 kJ mol −1 ) [20]. On the contrary, the lower-lying 3 MLCT excited states of these complexes have been shown to lead to quenching of the triplet photochromic pathway (Figure 2.7b), which resulted in lower quantum efficiency for the photochromic ring-opening reaction [9,20]. The quenching of the reactive 3 LC state by the 3 MLCT state was supported by the photochromic ring-opening reaction quantum yields, which have been found to be in line with the energies of the MLCT excited states…”

“…(b) Qualitative energy state diagram for the quenching of the photochromic reaction with the lower lying MLCT excited state. (From[20]. Reproduced with the permission from Wiley-VCH.…”

Observation of cascade f → d → f energy transfer in sensitizing near-infrared (NIR) lanthanide complexes containing the Ru(ii) polypyridine metalloligand

“…2b, Table 1), originating from the 3 MLCT state of the metalloligand with the lifetime and the absolute quantum yield of 469 ns and 5.1% at room temperature (r.t.), respectively, which are comparable to other [Ru(bipy) 3 ] 2+ -based chromophores. [26][27][28] After coordination with Eu/Tb 3+ ions, the resulting d-f heterometallic Eu-Ru and Tb-Ru complexes do not emit the characteristic ff luminescence of Ln 3+ ions, but still show a broad red emission similar to that of pure L Ru . However, the emission is significantly blue-shifted to B630 nm, owing to the changes in the metalloligand structure brought about by the Eu/Tb 3+ coordination.…”