Directed assembly of chiral organometallic squares that exhibit dual luminescence

Article abstract of DOI:10.1039/b307727f

Chiral molecular squares based on the Pt-alkynyl linkage were synthesized via stepwise directed assembly, and exhibit interesting dual luminescence at room temperature which is potentially exploitable for chiral sensory applications.

Full text of DOI:10.1039/b307727f

Directed assembly of chiral organometallic squares that exhibit dual
luminescence†
Suk Joong Lee,a Charles R. Luman,b Felix N. Castellanob and Wenbin Lin*a
a Department of Chemistry, CB#3290, University of North Carolina, Chapel Hill, NC 27599, USA.
E-mail: wlin@unc.edu
b Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University,
Bowling Green, Ohio 43403, USA
Received (in Columbia, MO, USA) 7th July 2003, Accepted 15th July 2003
First published as an Advance Article on the web 29th July 2003
Chiral molecular squares based on the Pt–alkynyl linkage
were synthesized via stepwise directed assembly, and exhibit
interesting dual luminescence at room temperature which is
potentially exploitable for chiral sensory applications.
With the intermediates Pt-2L1a, Pt-2L2a, and Pt-2L3a in
hand, chiral molecular squares 1–3 were prepared in 34–46%
yields by treating them with 1 equiv of cis-PtCl2(PEt3)2 in the
presence of CuCl catalyst and diethylamine at low tem-
1
peratures. 1–3 have been characterized by H, 13C{1H}, and
Inorganic and organometallic macrocycles have been ex-
tensively pursued owing to their relative ease of synthesis and
potential applications in host–guest chemistry, catalysis, and
sensing.1 The presence of metallo-corners in such cyclic
structures can also impart novel properties that are unique to the
metals.2 In contrast to extensive applications of directed
synthesis in the preparation of organic cyclic rigid structures,3
metallocycles have been exclusively synthesized via self-
assembly processes.4 Although intrinsically more efficient,
such self-assembling synthetic methodologies often favor small
macrocycles from entropic considerations and prohibit the
assembly of topologically and functionally interesting larger
metallocycles. Alternative directed assembly techniques are
needed in order to overcome the entropic disadvantage of larger
metallocycles. The relative lability of metal–ligand ligation
used for the construction of most metallocycles however
prevents the synthesis and isolation of requisite larger building
units and is thus incompatible with directed synthetic method-
ologies.
31P{1H} NMR, IR, UV-Vis, fluorescence, and CD spectros-
copy, as well as FAB and MALDI-TOF mass spectrometry.
NMR spectra of 1–3 showed a single ligand environment,
suggesting the formation of cyclic species. The 31P{1H} NMR
spectra of 1–3 exhibit a sharp singlet at ~ 3.60 ppm, with
accompanying 195Pt satellites (JPt–P = ~ 2262 Hz).7 A single set
We recently showed that chiral triangles were exclusively
obtained from self-assembly between BINOL-derived 4,4A-
bis(alkynyl) linear bridging ligands and cis-PtCl2(PEt3)2 angu-
lar building units.5 We believe that molecular triangles were
obtained owing to their entropic advantage although their
corresponding molecular squares should be favored from
enthalpic considerations. Herein we wish to report the develop-
ment of directed assembly techniques for the synthesis of chiral
molecular squares 1–3 that exhibit interesting dual lumines-
cence at room temperature.6
Scheme 1
Chiral molecular squares 1–3 were synthesized via stepwise
directed assembly according to Scheme 1. Mono-protected
enantiopure bis(alkynes) L1–L3 were synthesized in ~ 50%
yield by treatment of their corresponding bis(alkyne)s with 1
equiv of n-BuLi followed by Me3SiBr at 278 °C. Treatment of
ligands L1–L3 with 0.5 equiv of cis-PtCl2(PEt3)2 in the presence
of CuCl catalyst in diethylamine at r.t. gave the Pt-containing
intermediates, Pt-2L1, Pt-2L2, and Pt-2L3, in relatively high
yield (74–89%), which were further deprotected by K2CO3 to
afford the Pt-containing intermediates with terminal alkynes Pt-
2L1a, Pt-2L2a, and Pt-2L3a. All these intermediates were
characterized by 1H, 13C{1H}, and 31P{1H} NMR and IR
spectroscopy and FAB mass spectrometry. For example, the 1H
NMR spectra of L1, P-2L1, and Pt-2L1a all exhibit two sets of
signals for the aromatic protons of the naphthyl rings, consistent
with the chemical inequivalency of their two naphthyl rings
(Fig. 1). L1 and Pt-2L1a show their characteristic n(C·C–H)
stretches at ~ 3290 cm21 which is as expected absent in Pt-
2L1.
† Electronic supplementary information (ESI) available: experimental
procedures and nine figures. See http://www.rsc.org/suppdata/cc/b3/
b307727f/
Fig. 1 1H NMR spectra of L1, Pt-2L1, Pt-2L1a, and 1.
2124
CHEM. COMMUN., 2003, 2124–2125
This journal is © The Royal Society of Chemistry 2003

Table 1 Excitation and emission data for 1–3 in CH2Cl2 at r.t.a
of signals was observed for both bis(alkynyl) bridging ligands
and triethylphosphine groups in the 1H and 13C{1H} NMR
spectra of 1–3. FAB and MALDI-TOF MS data showed the
presence of molecular ions due to tetranuclear species for 1–3,
and thus unambiguously established their cyclic tetrameric
nature. The formulations of 1–3 are also supported by
microanalysis results. The IR spectra of metallocycles 1–3 show
the presence of acetylenic n(C·C) stretches at ~ 2100 cm21 and
c
Complex lmax/nm lf(max)/nmb lp(max)/nm Ffc
Fp
tp/ms
1
2
3
373
372
373
424
424
426
582
582
584
0.018
0.019
0.017
0.038
0.019
0.021
92
98
93
a Emission maxima measured with 375 nm excitation. b The TCSPC (Time-
Correlated Single Photon Counting) histogram of the singlet emission was
completely superimposible with the instrument response, therefore we
estimate ts < 400 ps with our current instrumentation. c Fluorescence
quantum yields measured relative to quinine sulfate in 0.1 M H2SO4 (Ff =
0.577) (±10%), while phosphorescence quantum yield measured relative to
[Ru(bpy)3]2+ in H2O (Fp = 0.042) (±10%).
the absence of the n(C·C–H) stretches at ~ 3290 cm21
.
The electronic spectra of 1–3 are similar to those of the
corresponding chiral molecular triangles 4.5 While the naphthyl
p ? p* transition at ~ 240 nm remains unshifted upon the
formation of 1–3 from L1–3, the acetylenic p ? p* transitions
at ~ 307 and ~ 360 nm have red-shifted (by ~ 15 nm) and
increased in intensity, presumably a result of the mixing of Pt p-
orbitals into the acetylenic p ? p* bands.8 A new band also
appeared at ~ 202 nm, assignable to the cis-Pt(PEt3)2 moie-
ties.
CD spectra of metallocycles 1–3 (Fig. 2) exhibit three major
bands similar to those of L1–L3, but with red-shifts and much
higher intensities. The enhanced CD signals for 1–3 are
consistent with the presence of multiple ligands in each
metallocycle, while the similarity between the CD spectra of
1–3 and L1–3 indicates that no chiral amplification has occurred
during the self-assembly of enantiopure metallocycles. Similar
to the chiral molecular triangles, a new intense CD band
appeared at ~ 202 nm for 1–3, attributable to the propeller-type
arrangement of cis-Pt(PEt3)2 moieties which is steered by chiral
binaphthyl moieties.9
425 nm. The relatively small Stokes’ shifts are characteristic of
singlet fluorescence emanating from the BINOL-derived 4,4A-
bis(alkynyl) ligands. The subnanosecond fluorescence decay
times (tf < 400 ps) and low quantum yields (Ff ~ 0.02) suggest
rapid and efficient singlet-to-triplet intersystem crossing un-
doubtedly assisted by the presence of Pt(II) heavy atoms. Upon
saturating the CH2Cl2 solutions with argon at r.t., vibronically
structured phosphorescence with a maximum at 582 nm
becomes apparent (Fig. 3). The relatively long lifetimes (tp
~ 100 ms for 1–3) are indicative of a spin-forbidden triplet
parentage.10,11 The phosphorescence bands of 1–3 are assigned
to the radiative decay of triplet intraligand states residing on the
BINOL-derived 4,4A-bis(alkynyl) ligands. The phosphores-
cence quantum yields displayed by 1–3 are similar in magnitude
to known linear Pt(II)–acetylide oligomers.10
In summary, we have successfully synthesized chiral molec-
ular squares via stepwise directed assembly processes, which
will prove very useful for the construction of topologically and
functionally interesting larger metallocycles. The present chiral
Pt–alkynyl molecular squares also exhibit interesting dual
luminescence at room temperature, and are thus potentially
exploitable as chiral sensory materials.
Compounds 1–3 exhibit dual luminescence in CH2Cl2 at r.t.
(Fig. 3). As shown in Table 1, 1–3 each display an emission near
W.L. acknowledges financial support from NSF (CHE-
0208930), ACS, Alfred P. Sloan Foundation, Arnold and Mabel
Beckman Foundation, Research Corp, Henry and Camille
Dreyfus Foundation. F.N.C. acknowledges support from NSF
(CHE-0134782) and ACS.
Notes and references
1 (a) P. J. Stang and B. Olenyuk, Acc. Chem. Res., 1997, 30, 502; (b) M.
Fujita, Chem. Soc. Rev., 1998, 27, 417–425; (c) B. J. Holliday and C. A.
Mirkin, Angew. Chem., Int. Ed., 2001, 40, 2022–2043; (d) P. H. Dinolfo
and J. T. Hupp, Chem. Mater., 2001, 13, 3113.
2 (a) S. J. Lee and W. Lin, J. Am. Chem. Soc., 2002, 124, 4554; (b) M. L.
Merlau, M. del Pilar Mejia, S. T. Nguyen and J. T. Hupp, Angew. Chem.,
Int. Ed., 2001, 113, 4369.
Fig. 2 CD spectra of 1–3.
3 (a) D. Zhao and J. S. Moore, Chem. Commun., 2003, 807; (b) U. H. F.
Bunz, Y. Rubin and Y. Tobe, Chem. Soc. Rev., 1999, 28, 107.
4 S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000, 100,
853.
5 S. J. Lee, A. Hu and W. Lin, J. Am. Chem. Soc., 2002, 124, 12948.
6 Achiral Pt–alkynyl molecular squares were previously reported. See: (a)
S. M. ALQaisi, K. J. Galat, M. Chai, D. G. Ray III, P. L. Rinaldi, C. A.
Tessier and C. A. Youngs, J. Am. Chem. Soc., 1998, 120, 12149; (b) K.
Onitsuka, S. Yamamoto and S. Takahashi, Angew. Chem., Int. Ed.,
1999, 38, 174.
7 C. Müller, J. A. Whiteford and P. J. Stang, J. Am. Chem. Soc., 1998, 210,
9827.
8 V. W.-W. Yam, Acc. Chem. Res., 2002, 35, 555.
9 H. Jiang, A. Hu and W. Lin, Chem. Commun., 2003, 96.
10 (a) Y. Liu, S. Jiang, K. Glusac, D. H. Powell, D. F. Anderson and K. S.
Schanze, J. Am. Chem. Soc., 2002, 124, 12412; (b) N. Hagihara, K.
Sonogashira and S. Takahashi, Adv. Polym. Sci., 1981, 41, 150.
11 I. E. Pomestchenko, C. R. Luman, M. Hissler, R. Ziessel and F. N.
Castellano, Inorg. Chem., 2003, 42, 1394.
Fig. 3 Excitation and emission spectra of 1 in CH2Cl2.
CHEM. COMMUN., 2003, 2124–2125
2125