Intramolecular electron transfer in the excited state of a [(bpy)Re I (CO) 3 (L)] + complex (bpy ) 2,2′-bipyridine), in which L contains a pendant azacrown ether that acts as an electron donor (L ) N- [4-(4,7,10,13-tetraoxa-1-azacyclopentadecyl)benzoyl]-4-aminopyridine), has been studied directly using picosecond and nanosecond time-resolved UV-visible absorption spectroscopy. Picosecond studies show that the metal-to-ligand chargetransfer (MLCT) state produced on excitation, [(bpy •-)Re II (CO) 3 (L)] + , undergoes forward electron transfer with a rate constant of k FET ) 2.0 × 10 9 s -1 to generate a ligand-to-ligand charge-transfer (LLCT) state, [(bpy •-)Re I (CO) 3 (L •+ )] + , in which the metal has been reduced back to Re(I) and charge separation has been effected between the bipyridine and azacrown ligands. Nanosecond studies show that the LLCT state returns to the ground state by back electron transfer from the bipyridine to azacrown ligand, with a rate constant of k BET ) 5.3 × 10 7 s -1 . Studies of complexes in which the azacrown complex is protonated, or is absent, demonstrate that intramolecular electron transfer to form the LLCT state does not occur in these cases. Forward electron transfer in the azacrown complex takes place on the picosecond time scale: it is weakly exoergonic and occurs in the Marcus normal region, with electronic coupling between the azacrown ligand and the rhenium metal center of ca. 100 cm -1 . Back electron transfer takes place on the nanosecond time scale: it is strongly exoergonic and occurs in the Marcus inverted region, with much weaker electronic coupling between the bipyridine and azacrown ligands. The rapid formation of a long-lived charge-separated state indicates that this molecule has a suitable design for a photochemical device.

A [(bpy)Re(CO) 3 L] + complex (bpy ) 2,2′-bipyridine) in which L contains an azacrown ether Schanze, K. S. J. Am. Chem. Soc. 1991, 113, 6108) acts as a reversible light-controlled switch of alkali and alkaline earth metal cations bound to the azacrown, as observed directly by time-resolved UV-vis spectroscopy. Excitation to the metal-to-ligand charge-transfer (MLCT) state of the metal-complexed form, [(bpy)Re(CO) 3 L] + -M n+ , results in cation release on the nanosecond time scale for M n+ ) Li + , Na + , Ca 2+ , and Ba 2+ , with Li + and Na + being released more rapidly than Ca 2+ and Ba 2+ ; by contrast, Mg 2+ is not released. After decay to the ground state, [(bpy)Re(CO) 3 L] + recaptures metal cations on the microsecond time scale to restore the starting thermal equilibrium. A multistep rebinding mechanism is observed for Li + and Na + , in which the cation attaches initially to the azacrown nitrogen atom before binding to the equilibrium position within the azacrown ring. The excited states and other intermediates in the cation release-and-recapture cycle have been observed directly in real time, and their decay rate constants have been determined as a function of cation identity, enabling a generalized light-controlled cation-switching mechanism to be developed for this generic molecular design.

No abstract

Two [(bpy)Re(CO)3L]+ complexes (bpy = 2,2'-bipyridine), where L contains an aza-15-crown-5 ether which is linked to Re via an alkenyl- or alkynyl-pyridine spacer, have been synthesised along with model complexes. Solutions of the complexes in acetonitrile have been studied by UV-Vis absorption spectroscopy, and by 1D and 2D 1H NMR spectroscopy. Strong UV-Vis bands, assigned to intraligand charge-transfer transitions localised at the L ligands, blue shift on protonation of the azacrown nitrogen atom or on complexation of alkali-metal (Li+, Na+ and K+) or alkaline-earth metal (Mg2+, Ca2+ and Ba2+) cations to the azacrown; the magnitude of the blue shift is dependent on the cation, with protonation giving the largest shift of ca. 100 nm. Cation binding constants in the range of log K= 1-4 depend strongly on the identity of the metal cation. Protonation or cation complexation causes downfield shifts in the 1H NMR resonances from most of the azacrown and L ligand protons, and their magnitudes correlate with those of the blue shifts in the UV-Vis bands; shifts in the azacrown 1H NMR resonances report on how the different metal cations interact with the macrocycle. UV-Vis and 1H NMR spectra of the free L ligands enable the effect of the Re centre to be assessed. Together, the data indicate that the alkene spacer gives a more responsive sensor than the alkyne spacer by providing stronger electronic communication across the L ligand.