C O M M U N I C A T I O N S
plex is responsible for the luminescence quenching of phenylene-
acetylene dendrimers with BINOL core by amino alcohols.14,15 It
is interesting to note that no enantioselectivity was observed for
the luminescence quenching of 4 by 1-amino-2-propanol, which
supports the involvement of amino groups in the formation of a
ground-state hydrogen-bonded complex and an excited-state proton-
transfer complex.16
In summary, a family of novel chiral molecular squares have
been readily assembled using enantiopure atropisomeric bipyridyl
bridging ligands and fac-Re(CO)3Cl corners. Metallocycle 4 exhibits
interesting enantioselective luminescence quenching by chiral amino
alcohols. Higher enantioselectivity of 4 versus L4 is probably a
consequence of a better-defined chiral environment in the metal-
locycle. Exploration of chiral metallocycles for applications in
asymmetric catalysis is currently underway.
Figure 1. Circular dichroism spectra of (S)-1-4 and (R)-4 in acetonitrile
at concentrations of 1.3-1.9 × 10-5 M.
Acknowledgment. We thank NSF (DMR-9875544) for financial
support, Mr. X. Linghu for experimental help, and Prof. H. Thorp
for helpful discussions. W. L. is an A.P. Sloan Fellow, a Beckman
Young Investigator, a Cottrell Scholar of Research Corp, and a
Camille Dreyfus Teacher-Scholar.
Supporting Information Available: Experimental procedures,
analytical data, one table, and 10 figures (PDF). This material is
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3
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We have studied the luminescence quenching of (R)- and (S)-4
in the presence of chiral amino alcohols. The luminescence signal
of enantiopure 4 at 412 nm can be quenched by both enantiomers
of 2-amino-1-propanol, but at significantly different rates. Figure
2 shows the Stern-Vo¨lmer plots of (R)-4 (2.2 × 10-6 M) in the
presence of (R)- and (S)-2-amino-1-propanol in THF.9 It is evident
from Figure 2 that luminescence quenching of chiral metallocycle
4 by 2-amino-1-propanol is enantioselective. For (R)-4, the Stern-
Vo¨lmer quenching constant Ksv is 7.35 M-1 in the presence of (S)-
2-amino-1-propanol, and 6.02 M-1 in the presence of (R)-2-amino-
1-propanol. (R)-4 has an enantioselectivity factor ksv(R - S)/ksv(R
- R) of 1.22 for luminescence quenching in favor of (S)-2-amino-
1-propanol. The opposite trend in enantioselectivity was observed
for the quenching of (S)-4 by 2-amino-1-propanol, which lends
further support to a chirality-based luminescence-quenching selec-
tivity. This magnitude of enantioselectivity for 4 is significantly
higher that of free ligand L4 (1.04), suggesting a better-defined
chiral environment conferred by metallocycle 4.14 Pu et al. has
proposed that the formation of a nonemissive hydrogen-bonded
complex and a poorly emissive excited-state proton-transfer com-
(8) Chiral dinuclear and trinuclear organozirconium cycles have recently been
synthesized diastereoselectively via zirconocene coupling. See: Shafer,
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(9) Enzyme-like enhanced stability of an epoxidation catalyst via encapsulation
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(10) The dominant peaks are due to [M - Cl]+ in their FAB-MS spectra.
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(12) Free ligand L4 emits at 401 nm with ∼1/3 of the intensity of 4.
(13) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer
Academic: Plenum: New York, 1999.
(14) The enantioselectivity factor is reported to be 1.03 for fluorescence
quenching of BINOL by amino alcohols. See: Pugh, V. J.; Hu, Q.-S.;
Pu, L. Angew. Chem., Int. Ed. 2000, 39, 3638-3641.
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66, 6136-6140.
(16) The greater distance of the amino group from the chiral center in 1-amino-
2-propanol is responsible for its lack of fluorescence-quenching
enantioselectivity.
JA0256257
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