ABSTRACT
In Ziessel and coworkers’ design of 64 and 65, quinones within the annu lus of the calixarene receptor accept electrons from a flexibly tethered metal polypyridyl complex [397,398]. Phosphorescence from the [Ru(bpy)2+] and /ac-[ReCl(CO)3bpy] reporter sites is weak and lifetimes of the MLCT triplet states are considerably shorter (64: t = 6 ns, <t>e = 4 x 10-4 ; 65: x = 5 ns, c|)e = 4 x 10-4 ) than those of the corresponding phenolic control compounds (66: x = 1100 ns, <J)e = 8.5 x 10-2 ; 67: x = 45 ns, 4>e = 3 x 10-3 ). The quench
ing of the MLCT luminescence was ascribed to intramolecular charge transfer from the MLCT excited state of the pendant metal complex to the quinone subunit of the calix[4]diquinone. The luminescence properties of 64 and 65 in acetonitrile are markedly affected by the presence of cations, which have no affect on the photophysical properties of the control compounds 66 and 67. De tailed NMR studies, augmented by molecular dynamics simulation, indicate that the four oxygens of the calixdiquinone and the two nitrogens of the dangling 2,2'-bipyridine hold a metal cation at the lower rim of the receptor. Association constants determined from luminescence titration experiments monotonically de crease with increasing charge of the metal cation, ranging from 200 to 400 M -1 for monocations (Li+ , Na+ , K+ ), 10 to 20 M -1 for dications (Ca2+, Ba2+, Sr2+, Cd2+), and 0.5 to 2.0 M _1 for trications (La3+, Gd3+). The association of metal ions to the lower rim renders the quinones more easily reduced. For instance, the quinone reduction potentials of the Ba2+ complexes of 64 and 65
are shifted more negative by 200 and 600 mV, respectively. This shift in poten tial increases the driving force for electron transfer; forward and back electron transfer rate constants of kp = 1.1 x 1010 s-1 and kB = 5.7 x 109 s-1 were determined using picosecond transient absorption spectroscopy. These enhanced electron transfer rates result in significant luminescence quenching upon metal ion association with 65. The excited state lifetime of 65 is reduced to 85 ps and the luminescence quantum yield is < 10-4 . Conversely, addition of Ba2+ to 64 causes an increase in luminescence intensity to values that are comparable to the emission observed from the control phenol complex, 6 6 . In this case, a concomi tant decrease in the electron transfer rate is observed (k£T = 4.4 x 105 s-1 ). The increase in driving force brought about by cation association with the quinone of 64 is offset by an electrostatically driven conformational change between the associated cation and charged pendant metal complex. The bound cation repels the positively charged metal complex, forcing the receptor into a cone conformation; MD simulations suggest that the edge-to-edge distance between reactants in the fully extended conformation is 5 A. In this elongated conforma tion, orbital overlap between reactants is curtailed and the electron transfer from the photoexcited complex to the quinone is constrained by a small Hda* The drop in k^r for the metal ion complexes of 64 varies logically with the charge of the bound metal. Monocations give a 30-fold decrease in the electron trans fer rate whereas a 300-fold decrease is observed for the electron transfer rate constant of dications. The average decrease of the photoinduced intramolecular electron transfer rate constant of the Ln3+ complexes is 1700-fold. A similar trend is observed in the electron transfer rate constants for the calix[4]diquinone receptor appended with two [RuCbpy)2-1-] metal complexes, although the overall luminescence enhancements for 6 8 are not as pronounced as those observed
for 64 [398]. Extremely small stability constants of the metal cation complexes (K = 21, 4, and 0.3 M -1 for K+ , Ba2+, and La3+, respectively) presumably result from the 4 + total charge of the two metal complexes, which electrostat ically hinders the strong association of metal cations to the lower rim of the calix[4]diquinone receptor.
