Dual emission caused by ring inversion isomerization of a 4-methyl-2-pyridyl-pyrimidine copper(I) complex

Article abstract of DOI:10.1021/ja103718e

We developed a new convertible copper(I) complex using 2-pyridyl-4- methylpyrimidine and diphosphine as ligands. This complex exhibited mechanical bistability based on the inversion motion of the pyrimidine ring, leading to dual luminescence behavior. The inversion dynamics was strongly dependent on temperature and solvent. Variable-temperature ¹H NMR spectra revealed that the two isomers interconverted in solution via ring inversion, and the motion was frozen below 200 K. The complex exhibited characteristic CT absorption and emission bands in solution. Emission lifetime measurements demonstrated that the emission could be deconvoluted into two components. The fast and slow components were assigned to the two isomers, the excited states of which were characterized by different structural relaxation process and/or additional solvent coordination properties. The emission properties of the two isomers differed not only in lifetime and wavelength but also in heat sensitivity. The molar ratio of the two isomers varied with the polarity of the solvent via electrostatic interactions with the counteranion. The rate of inversion was affected by solvent, suggesting that inversion was promoted by solvent coordination.

Full text of DOI:10.1021/ja103718e

Published on Web 06/28/2010
Dual Emission Caused by Ring Inversion Isomerization of a
4-Methyl-2-pyridyl-pyrimidine Copper(I) Complex
Michihiro Nishikawa, Kuniharu Nomoto, Shoko Kume,* Keiichi Inoue,† Makoto Sakai,† Masaaki Fujii,†
and Hiroshi Nishihara*
Department of Chemistry, Graduate School of Science, UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
113-0033, Japan
Received May 7, 2010; E-mail: nisihara@chem.s.u-tokyo.ac.jp
Abstract: We developed a new convertible copper(I) complex
using 2-pyridyl-4-methylpyrimidine and diphosphine as ligands.
This complex exhibited mechanical bistability based on the
inversion motion of the pyrimidine ring, leading to dual lumines-
cence behavior. The inversion dynamics was strongly dependent
on temperature and solvent. Variable-temperature 1H NMR
spectra revealed that the two isomers interconverted in solution
via ring inversion, and the motion was frozen below 200 K. The
complex exhibited characteristic CT absorption and emission
bands in solution. Emission lifetime measurements demonstrated
that the emission could be deconvoluted into two components.
The fast and slow components were assigned to the two isomers,
the excited states of which were characterized by different
structural relaxation process and/or additional solvent coordination
properties. The emission properties of the two isomers differed
not only in lifetime and wavelength but also in heat sensitivity.
The molar ratio of the two isomers varied with the polarity of the
solvent via electrostatic interactions with the counteranion. The
rate of inversion was affected by solvent, suggesting that inversion
was promoted by solvent coordination.
Figure 1. Ring inversion equilibrium between the i- and o-isomers of 1+.
(a) An ORTEP view of 1•BF4•CHCl3, showing 50% probability displace-
ment ellipsoids. Hydrogen atoms, a counteranion, and a solvent molecule
1
are omitted for clarity. (b) H NMR spectra of 1+ in acetone-d6 at several
temperatures in the methyl group region.
The photoprocesses of transition metal complexes are of interest
for their potential use in dye-sensitized solar cells,1 light-emitting
devices,2 and photocatalysts3 due to a combination of high thermal
stability, reversible redox activity, intense visible absorption, and
the formation of a long-lived charge transfer (CT) excited state.
Recently, photofunctional molecules have been synthesized using
simple metal complexes that exhibit dual phosphorescence as a
result of the presence of two independent excited states.4 Unfor-
tunately, methods for selectively preparing one of the two states
have thus far been limited. The state selection requires a system
characterized by well-defined bistability via a reversible chemical
process.
We have recently described novel molecular mechanical systems
that operate as electron gates by exploiting the inversion motion
of pyrimidine on copper to give inner (i-) or outer (o-) isomers in
which a substituent on the pyrimidine is, respectively, proximal or
distal to the copper center.5 In the present work, we developed a
new copper(I) complex using 4-methyl-2-(2′-pyridyl)pyrimidine
(Mepypm) and bis[2-(diphenylphosphino)phenyl]ether (DPEphos)
as ligands. This complex exhibited geometric bistability based on
inversion of the pyrimidine, which led to dual luminescence
behavior. The dynamics of inversion were sensitive to temperature
and solvent, suggesting that the excitation process could be tuned
using the inversion process.
[Cu(Mepypm)(DPEphos)]BF4 (1•BF4) was newly synthesized
by a reaction of [Cu(MeCN)4]BF4 with Mepypm and DPEphos in
dichloromethane according to procedures described in the literature6
(for synthetic details, see the Supporting Information).
X-ray structural analysis of 1•BF4 revealed that the methyl group
in Mepypm was oriented away from the metal center, and no
disorder was found in the coordination mode (Figures 1a and S1).
This result suggested that all 1•BF4 species exist as the o-isomer
in a single crystal, in contrast with the existence of the i-isomer in
a single crystal of the previous complex formed from Mepypm
and a phenanthroline derivative.5 The bulky diphosphine appeared
to repel the methyl group of the i-isomer away from the metal
center.
1
The H NMR peaks of 1•BF4 in acetone-d6 at 243 K could be
clearly assigned to either the i- or o-isomer without evidence for
other coordination species (Figures 1b and S2). The chemical shifts
of these peaks were affected by the shielding effects of the copper
and the phenyl group of DPEphos, and the major and minor peaks
were assigned, respectively, to o- and i-isomers.5 These signals
broadened upon heating, indicating that the i- and o-isomers
interconverted in solution by ring inversion, which occurred on the
1
time scale of the H NMR experiment (Figures 1b and S3).
The rates of interconversion were sufficiently high that the system
reached equilibrium. The equilibrium constant was determined from
the integral of the signals as a function of temperature. The molar
† Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Na-
gatsuta, Midori-ku, Yokohama 226-8503, Japan.
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C O M M U N I C A T I O N S
Figure 3. van’t Hoff plots (a) and Arrhenius plots (b) for the i- f
o-inversion process of 1+ in CDCl3 (green), acetone-d6 (blue), and CD3CN
(red).
enhanced emission of i-isomer was derived from thermal activation
between close levels in fast equilibrium, which is discussed as 1CT
6,7
3
and low-lying CT excited states.
We conclude that the photoprocesses of the two isomers are
different in the identity of the excited state. Control over the
inversion dynamics is important for the development of a photo-
electron transfer system composed of this complex family.
As for the dynamics of inversion, a lower population of the
o-isomer was found in the polar solvent (Figures 3a, S9, and S10).
The molar ratio of the o-isomer at 253 K was 74% in CDCl3, 68%
in acetone-d6, and 65% in CD3CN. This trend was observed across
the full temperature range tested, and the equilibrium constant was
almost temperature-invariant. Because the i- and o-isomers were
similar with respect to the coordination skeleton, the polarity
differences between the complex cations of the two isomers were
expected to be small. Therefore, the isomer ratio was influenced
by the electrostatic effects of the solvated contact ion pair. In less
polar solvents, the i-isomer was destabilized by the approach of
the counteranion to the copper center due to steric effects of the
proximal methyl group. These results suggest that the ratio of the
inversion isomers could be tuned by electrostatic interactions.
The kinetics of the inversion motion was evaluated by simulating
the temperature-dependent 1H NMR spectra in CDCl3, acetone-d6,
and CD3CN (Figures 3b, S11, and S12). The rate constant for the
i- f o- inversion, k, at 293 K was 20 s-1 in CDCl3, 70 s-1 in
acetone-d6, and 120 s-1 in CD3CN. The k value at room temperature
in CD3CN was 1 order of magnitude larger than that in CDCl3,
suggesting that a coordinated solvent molecule promoted ring
inversion by assisting dissociation of the pyrimidine N atoms from
the copper center. The k value varied from 10-6 s-1 (frozen motion)
to 103 s-1 as the temperature and solvent were varied.
Figure 2. Photochemical properties of 1•BF4. (a) UV-vis absorption
spectrum (dotted line) and the steady-state emission spectra using 400 nm
excitation (solid line) of 1+ in acetone at room temperature. (b) Experimental
630 nm emission decay of 1+ in acetone at room temperature (red) and
203 K (purple) excited at 425 nm. Time-resolved emission spectra of 1+ in
acetone at several temperatures for the slower (τ ) 40 ns) component (c)
and the faster (τ ) 2 ns) component (d).
ratio of the i- and o-isomers in acetone remained 3:7 over the
temperature range from 203 to 293 K (Figure S4).
The UV-vis absorption spectrum of 1+ in acetone showed the
characteristic CT absorption bond (λabs ) 378 nm, ꢀ ) 2.7 × 103
M-1 cm-1) of the [Cu(diimine)(diphosphine)]+ family (Figure 2a).
The absence of an absorption around 450 nm and the independence
of the absorption profile on the concentration of 1+ suggested that
the formation of other species, such as [Cu(Mepypm)2]+, was
negligible.6 The steady-state emission spectrum of 1+ in acetone
(Figure 2a) exhibited emission at λem ) 635 nm from the CT state.6
These results indicate that the wavelengths of maximum absorption
and the emission energies of the i- and o-isomers were comparable.
Emission lifetime measurements of 1+ revealed that the decay
curve could be deconvoluted into two components (Figures 2b and
S5). The [Cu(diimine)(diphosphine)]+ family is known to show a
single decay.6,7 Therefore, the two components could be reasonably
assigned to the i- and o-isomers. The faster (τ ) 2 ns) and the
slower (τ ) 40 ns) components were attributed to emission from
the o- and i-isomers, respectively (Figure S5), because introduction
of a bulky substituent into the coordination sphere is known to
elongate the lifetime of the excited state of copper(I) complexes
by inhibiting structural relaxation and/or preventing additional
solvent coordination.6,7 This assignment was further supported by
the similarities between the emission lifetimes of the o-isomer and
[Cu(bpy)(DPEphos)]+ (bpy )2,2′-bipyridine), which does not
contain bulky groups near the metal center (Figure S6). The
emission spectra of two isomers were deconvoluted using time-
dependent spectral measurements. The faster component was
slightly red-shifted relative to the slower component, implying that
the long-lived excited state of the i-isomer has the higher energy
(Figure S7).
In conclusion, a novel two-state conformational isomer system
with an excitation process that was sensitive to the ligand ring
inversion on the copper(I) center was synthesized. The inversion
dynamics largely depended on the temperature and solvent,
suggesting that it may be possible to control the ring inversion
motions. This strategy may be useful in the construction of a single
molecular system for light-energy processing.
Acknowledgment. This work was supported by Grants-in-Aid
from MEXT of Japan (20750044, 20245013, and 21108002, area
2107) and the Global COE Program for Chemistry Innovation.
The differences between emission behaviors of these isomers
were clearly reflected in the heat sensitivity. The emission decay
profile showed that the relative intensity of the slower component
decreased upon cooling and became almost negligible at 203 K
(Figure S8). The lifetimes of the two components were almost
temperature-independent, as demonstrated by the best fits of the
decay profiles at each temperature (203-293 K). The deconvoluted
emission spectra of the slower component were blue-shifted and
showed an increase in the emission intensity upon heating (Figure
2c). The emission spectra of the faster component were relatively
insensitive to temperature (Figure 2d). We deduce that the thermally
Supporting Information Available: Materials and methods, crystal
structure data (CIF), and spectral data. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737–740. (b) Hagfeldt, A.;
Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269–277. (c) Ardo, S.; Meyer, G. J.
Chem. Soc. ReV. 2009, 38, 115–164. (d) Hamann, T. W.; Jensen, R. A.;
Martinson, A. B. F.; Ryswyk, H. V.; Hupp, J. T. Energy EnViron. Sci. 2008,
1, 66–78.
(2) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151–154. (b) Baldo,
9
9580 J. AM. CHEM. SOC. VOL. 132, NO. 28, 2010

C O M M U N I C A T I O N S
M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403, 750–753. (c)
Evans, R. C.; Douglas, P.; Winscom, C. J. Coord. Chem. ReV. 2006, 250,
2093–2126. (d) Qin, T.; Ding, J.; Wang, L.; Baumgarten, M.; Zhou, G.;
Mu¨llen, K. J. Am. Chem. Soc. 2009, 131, 14329–14336.
(5) (a) Nomoto, K.; Kume, S.; Nishihara, H. J. Am. Chem. Soc. 2009, 131, 3830–
3831. (b) Kume, S.; Nomoto, K.; Kusamoto, T.; Nishihara, H. J. Am. Chem.
Soc. 2009, 131, 14198–14199.
(6) (a) Cuttell, D. G.; Kuang, S. M.; Fanwick, P. E.; McMillin, D. R.; Walton,
R. A. J. Am. Chem. Soc. 2002, 124, 6–7. (b) Kuang, S. M.; Cuttell, D. G.;
McMillin, D. R.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. 2002, 41,
3313–3322.
(7) (a) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Top. Curr. Chem.
2007, 280, 69–115. (b) Vorontsov, I. I.; Graber, T.; Kovalevsky, A. Y.;
Novozhilova, I. V.; Gembicky, M.; Chen, Y.-S.; Coppens, P. J. Am. Chem.
Soc. 2009, 131, 6566–6573. (c) Siddique, Z. A.; Yamamoto, Y.; Ohno, T.;
Nozaki, K. Inorg. Chem. 2003, 42, 6366–6378.
(3) (a) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163–170. (b) Concepcion, J. J.;
Jurss, J. W.; Brennaman, M. K.; Hoertz, P. G.; Patrocinio, A. O. T.;
Murakami Iha, N. Y.; Templeton, J. L.; Meyer, T. J. Acc. Chem. Res. 2009,
42, 1954–1965. (c) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A.
2006, 103, 15729–15735. (d) Lazarides, T.; McCormick, T.; Du, P.; Luo,
G.; Lindley, B.; Eisenberg, R. J. Am. Chem. Soc. 2009, 131, 9192–9194.
(4) (a) Lo, K. K. W.; Zhang, K. Y.; Leung, S. K.; Tang, M. C. Angew. Chem.,
Int. Ed. 2008, 47, 2213–2216. (b) Glazer, E. C.; Magde, D.; Tor, Y. J. Am.
Chem. Soc. 2007, 129, 8544–8551. (c) DeAmond, M. K.; Carlin, C. M.
Coord. Chem. ReV. 1981, 36, 325–355.
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