Metallocyclic receptors with Re(I)/Os(II)-based moieties: molecular photophysics and selective molecular sensing.

ABSTRACT

New metallocyclic Re(I) and Os(II) complexes with polyphosphane/polyyne spacers, including dimers [(Re(CO)3Cl(C2nP2))2] (n = 1, 1; 2, 2) and tetramers [(Re(CO)3Cl(C2nP2))4] (n = 1, 3; 2, 4, C2P2 = Ph2P-C...C-PPh2, C4P2 = Ph2P-C...C-C...C-PPh2), as well as the mixed-metal [(Re(CO)3Cl)2(Os(bpy)2)2(C2P2)4](PF6)4 (6, bpy = 2,2'-bipyridine) and its precursor [Os(bpy)2(C2P2)2](PF6)2 (5) have been synthesized. Characterization has been carried out using 31P(1H) NMR, FAB/MS, ESI/MS, IR spectroscopy, elemental analysis (EA), and X-ray single crystal structure determination. These new metallocyclic complexes are found to be emissive, with a characteristic ReI-based emission at 505-525 nm (lifetimes of 3.4-6.8 ns) and an Os(II)-based emission at 600-605 nm (lifetimes of 650-675 ns). High quantum yields of 0.25 and 0.17 were observed for 5 and 6, which were representative of the few most emissive species reported with Os(II) centers. Efficient energy transfer from the Re(I) donor to the Os(II) acceptor was also found. In addition, a host-guest study was performed using emissive metallocycle 6, and host-guest binding constants of 775M(-1), 1580M(-1), and 1680M(-1) were obtained for the guests anisole, 1,4-dimethoxybenzene, and 1,3,5-trimethoxybenzene, respectively. The correlation between the guest molecule size, cavity dimension, and the host-guest binding constant is discussed. Furthermore, the relationship between the pi-acceptor ability of the nonchromophoric phosphanes, the energy gap between the ground and excited state, and the nonradiative decay rate constant (knr) is also explored.