Luminescent metallogels of alkynylrhenium(I) tricarbonyl diimine complexes

Article abstract of DOI:10.1021/om800610x

A series of alkynylrhenium(I) tricarbonyl diimine complexes has been synthesized and shown to display rich thermotropic gelation behavior.

Full text of DOI:10.1021/om800610x

Copyright 2008
Volume 27, Number 18, September 22, 2008
American Chemical Society
Communications
Luminescent Metallogels of Alkynylrhenium(I) Tricarbonyl Diimine
Complexes
Siu-Tung Lam, Guoxin Wang, and Vivian Wing-Wah Yam*
Centre for Carbon-Rich Molecular and Metal-Based Materials Research and Department of Chemistry,
The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China
ReceiVed June 30, 2008
interactions.3 A key to the molecular design of a low-molecular-
Summary: A series of alkynylrhenium(I) tricarbonyl diimine
complexes has been synthesized and shown to display rich
thermotropic gelation behaVior.
mass organic gelator (LMOG) is the control of the above
noncovalent molecular forces. While there has been a growing
interest in the study of organogels, corresponding studies in
metallogels based on metal complexes are still relatively rare
and are rather unexplored, despite numerous works on metal
complexes with rich spectroscopic properties being known.4 A
particular class of transition-metal complexes that has aroused
immense interest is the luminescent rhenium(I) tricarbonyl
diimine complexes with their rich and promising photophysical
With the growing interest in the search for new advanced
materials that show smart responsive behavior, a variety of
investigations into organogels have emerged.1,2 These kinds of
materials have been found to show potential applications in
materials science,2a-c drug delivery,2d,e and the construction of
supramolecular nanostructures.2f,g The formation of gels in
organic solvents by small organic molecules poses interesting
challenges in the field of molecular recognition and self-
assembly, requiring a stabilizing intermolecular interaction, and
represents a balance between the tendency of the molecules to
dissolve or to aggregate in organic solvents. The intermolecular
interactions that cause the gelator molecules to self-assemble
are usually noncovalent bonds, commonly van der Waals forces,
hydrogen bonding, electrostatic attractions, and π-π stacking
(3) (a) An, B. K.; Lee, D. S.; Lee, J. S.; Park, Y. S.; Song, H. S.; Park,
S. Y. J. Am. Chem. Soc. 2004, 126, 10232. (b) Ziessel, R.; Pickaert, G.;
Camerel, F.; Donnio, B.; Guillon, D.; Cesario, M.; Prange´, T. J. Am. Chem.
Soc. 2004, 126, 12403. (c) Sugiyasu, K.; Fujita, N.; Takeuchi, M.; Yamada,
S.; Shinkai, S. Org. Biomol. Chem. 2003, 1, 895. (d) Yu, H.; Kawanishi,
H.; Koshima, H. J. Photochem. Photobiol. A 2006, 178, 62. (e) van Esch,
J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263.
(4) (a) Tam, A. Y.-Y.; Wong, K. M.-C.; Wang, G.-X.; Yam, V.W.-W.
Chem. Commun. 2007, 2028. (b) Lu, W.; Law, Y.-C.; Han, J.; Chui, S. S.-
Y.; Ma, D.-L.; Zhu, N; Che, C.-M. Chem. Asian J. 2008, 3, 59. (c)
Shirakawa, M.; Fujita, N.; Tani, T.; Kaneko, K.; Shinkai, S. Chem. Commun.
2005, 4149. (d) Kishimura, A.; Yamashita, T.; Aida, T. J. Am. Chem. Soc.
2005, 127, 179. (e) Camerel, F.; Bonardi, L.; Schmutz, M.; Ziessel, R. J. Am.
Chem. Soc. 2006, 128, 4548. (f) Bremi, J.; Brovelli, D.; Caseri, W.; Ha¨hner,
G.; Smith, P.; Tervoort, T. Chem. Mater. 1999, 11, 977. (g) Weng, W.;
Beck, J. B.; Jamieson, A. M.; Rowan, S. J. J. Am. Chem. Soc. 2006, 128,
11663. (h) Camerel, F.; Ziessel, R.; Donnio, B.; Bourgogne, C.; Guillon,
D.; Schmutz, M.; Iacovita, C.; Bucher, J. P. Angew. Chem., Int. Ed. 2007,
46, 2659. (i) Naota, T.; Koori, H. J. Am. Chem. Soc. 2005, 127, 9324. (j)
Liu, Q.-T.; Wang, Y.-L.; Li, W.; Wu, L.-X. Langmuir 2007, 23, 8217. (k)
Cardolaccia, T.; Li, Y.; Schanze, K. S. J. Am. Chem. Soc. 2008, 130, 2535.
(l) Kawano, S.-i.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2004, 126, 8592.
* To whom correspondence should be addressed. E-mail: wwyam@
hku.hk. Fax: +(852) 2857-1586. Tel: +(852) 2859-2153.
(1) (a) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133. (b) Llusar,
M.; Sanchez, C. Chem. Mater. 2008, 20, 782. (c) Sangeetha, N. M.; Maitra,
U. Chem. Soc. ReV. 2005, 10, 821.
(2) (a) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Chem. Soc.
ReV. 2008, 1, 109. (b) Xia, Y.; Yang, P. AdV. Mater. 2003, 15, 351. (c)
Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B. T.; Gates, B.; Yinn, Y.;
Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (d) Vintiloiu, A.; Leroux,
J.-C. J. Controlled Release 2008, 125, 179. (e) Shaikh, I. M.; Jadhav, K. R.;
Gide, P. S.; Kadam, V. J.; Pisal, S. S. Curr. Drug DeliVery 2006, 3, 417.
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10.1021/om800610x CCC: $40.75
2008 American Chemical Society
Publication on Web 08/19/2008

4546 Organometallics, Vol. 27, No. 18, 2008
Communications
Chart 1
properties.5,6 The metal-to-ligand charge-transfer (MLCT) ex-
cited states of these luminescent organometallic systems are well
documented to show large responsive changes to changes in
the environment.5,6 The rich photophysical properties of the
MLCT excited states of these metal complex systems are
believed to serve as versatile spectroscopic reporters and probes
for changes in the environment. Moreover, the fine balance and
the interplay of the noncovalent interactions in these gelators
can be readily modified by the judicious choice and rational
design of the coordinating ligands in the organometallic system,
which may hopefully give rise to a fine tuning of the gelation
properties of the LMOG. Herein, we report the incorporation
of these rhenium(I) tricarbonyl diimine chromophores into
organogel systems to form luminescent metallogels.
Complex 1 was synthesized by refluxing a mixture of
[Re(tBu2bpy)(CO)3Br], AgOTf, and 3,4,5-tris(dodecyloxy)-
phenylacetylene (L1)4a,7 in THF in the presence of Et3N.
Complexes 2 and 3 were similarly prepared by reaction with
the corresponding rhenium(I) tricarbonyl diimine complex
precursors. Complexes 4-6 were also similarly prepared using
N-(4-ethynylphenyl)-3,4,5-tris(octadecyloxy)benzamide (L2)4a
in place of L1 (Chart 1).
of the diimine and alkynyl ligands. In addition, low-energy
absorptions at ca. 400-450 nm with extinction coefficients on
the order of 103 dm3 mol-1 cm-1 were observed, assigned to
the [dπ(Re) f π*(diimine)] metal-to-ligand charge transfer
(MLCT) transition, probably mixed with some [π(Ct CR) f
π*(diimine)] alkynyl-to-diimine ligand-to-ligand charge transfer
(LLCT) character.6c,e,g,h
Upon excitation at λ g350 nm, the complexes displayed a
moderately intense emission band in THF solution at 580-650
nm. The long-lived emission with lifetimes in the submicro-
second range, typical of rhenium(I) tricarbonyl diimine com-
plexes, was suggestive of a triplet parentage. With reference to
previous spectroscopic studies on other related rhenium(I)
alkynyl complexes,6c,e,g,h the luminescence was assigned as
derived from excited states of a 3MLCT [dπ(Re) f π*(diimine)]
origin, with some mixing of 3LLCT [π(Ct CR) f π*(diimine)]
character.
The complexes were tested for their gelation properties in
various organic solvents by the “stable to inversion of a test
tube” method. All the complexes formed an opaque but stable
The electronic absorption spectra of complexes 1-6 show
intense high-energy absorption bands at ca. 270-360 nm,
ascribed to an admixture of intraligand (IL) π f π* transitions
(5) (a) Wrighton, M. S.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998.
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Yam, V.W.-W. J. Am. Chem. Soc. 2007, 129, 6058. (c) Lam, S. C.-F.;
Yam, V. W.-W.; Wong, K. M.-C.; Cheng, E. C.-C.; Zhu, N. Organometallics
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2004, 126, 12734. (e) Chong, S. H.-F.; Lam, S. C.-F.; Yam, V. W.-W.;
Zhu, N.; Cheung, K.-K.; Fathallah, S.; Costuas, K.; Halet, J.-F. Organo-
metallics 2004, 23, 4924. (f) Ko, C.-C.; Wu, L.-X.; Wong, K. M.-C.; Zhu,
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Figure 1. Representative images of (a) the gel formed from 1 in
hexane (10 wt %) and (b) the gel formed from 4 in DMSO (9 wt
%). The sol-gel transition is found to be thermally reversible, and
the gel is stable toward inversion of a test tube for more than 1
week.
(7) (a) Wu, J.-S.; Watson, M. D.; Zhang, L.; Wang, Z.-H.; Mu¨llen, K.
J. Am. Chem. Soc. 2004, 126, 177. (b) Pieterse, K.; Lauritsen, A.; Schenning,
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5597.

Communications
Organometallics, Vol. 27, No. 18, 2008 4547
Table 1. Summary of Gelation Properties in Various Solventsa
solvent
1
2
3
4
5
6
DMSO
hexane
acetone
G (7)
G (10)
S
G (8)
P
P
G (10)
P
P
G (9)
I
G (12)
G (10)
I
P
G (12)
I
I
a Legend: G ) opaque gel; I ) insoluble; P ) precipitation; S )
soluble. The values given in parentheses are the minimum concentrations
(mg mL–1) needed to achieve gelation (critical gelation concentration) at
25 °C.
Figure 3. (a) UV-vis absorption spectra of the DMSO gel of 1 at
25 °C (s) and the DMSO sol at 75 °C (- - -). (b) UV-vis absorption
spectra of the hexane gel of 1 at 25 °C (s) and the hexane sol at
55 °C (- - -). (c) Corrected emission spectra of the DMSO gel of 1
at 25 °C (s) and the DMSO sol at 75 °C (- - -).
solvent for the neutral alkynylrhenium(I) complexes, a shift of
the MLCT energy to the red has been observed upon gelation.
It is likely that aggregation occurs in DMSO gel via π-π
stacking interactions between the aromatic moieties that give
rise to the observed MLCT changes. A similar phenomenon
has also been observed in other organogel systems in DMSO,9
attributed to J-aggregate formation.10
Figure 2. Representative SEM image of xerogel formed from 1 in
DMSO (7 wt %).
Upon excitation at the absorption maxima or at the wave-
length with the same absorbance of the gel and sol forms, the
DMSO metallogels emit strongly at 580-650 nm; upon an
increase in the temperature, their emission intensity dropped
and was almost “turned off” in the sol form (Figure 3c). Such
luminescence enhancement in the gel state is in line with the
increased rigidity of the media in the gel state, which reduces
molecular vibrations and motions and hence slows down the
nonradiative deactivation pathways. Similar phenomena have
also been reported in other luminescent organogels and
metallogels.4,9,11
In conclusion, an alkynylrhenium(I) tricarbonyl diimine
system was found to form metallogels, which showed a
detectable color and emission changes during the sol-gel
transition. This property can serve as a probe for microenvi-
ronmental changes. The fine balance and the interplay of the
yellow to orange metallogel with a low critical gelation
concentration (cgc) (7 mg mL-1) in DMSO, as shown in Figure
1. In addition, complexes 1 and 4 with Bu2bpy moieties were
t
found to be more soluble than the other complexes with other
diimine ligands, and they showed gelation properties in n-hexane
and acetone, respectively, while the other complexes gave only
precipitation or insoluble suspensions in these solvents (Table
1). This demonstrates the importance of the design of the
coordinating ligands in governing the interplay and the fine
balance of noncovalent interactions in the formation of gels.
Scanning electron microscopy (SEM) images were recorded
to study the morphology of the DMSO xerogel (air-dried gel)
formed by the complexes. Gel samples of 1 in DMSO (7 wt
%) were slowly evaporated to dryness, and the resultant xerogels
were examined. A representative SEM image is shown in Figure
2. A dense network of continuous fibrous structures, which is
characteristic of LMOGs, was observed for all samples. The
approximate diameter of the fibers was 200 nm.
A temperature-dependent UV-vis absorption study was
performed in the temperature range 25-75 °C. The UV-vis
absorption spectra of the gel and sol forms of 1 are shown in
Figure 3a and b. Upon an increase in the temperature, the MLCT
absorption band at ca. 400-450 nm gradually dropped in
absorbance, as the opaque gel became a clear sol in hexane
and DMSO. Concomitant with this drop in intensity, a red shift
of the MLCT band was observed on going from the gel to the
sol form of 1 in hexane. In contrast, a blue shift of the MLCT
band was observed on going from the gel to the sol form in
DMSO for all the complexes.
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The shift of the MLCT absorption band of 1 to higher energy
in the gel state relative to that in the sol form in hexane can be
ascribed to rigidochromism resulting from the transition of
dynamic nanostructures of the fluid sol form to its solidified
gel form. Such a rigidochromic effect is commonly observed
in the MLCT excited state of related ruthenium(II) and rhe-
nium(I) polypyridyl systems.8 Conversely, in DMSO, which is
a more polar solvent than hexane and presumably a poorer

4548 Organometallics, Vol. 27, No. 18, 2008
Communications
noncovalent interactions in these gelators can be readily
modified by the judicious choice and rational design of the
coordinating ligands in the organometallic system.
graduate studentship and G.X.W. the receipt of a University
Postdoctoral Fellowship, both from The University of Hong
Kong. We also thank the Electron Microscope Unit at The
University of Hong Kong for helpful technical assistance.
Acknowledgment. V.W.-W.Y. acknowledges the receipt
of the Distinguished Research Achievement Award from The
University of Hong Kong. We also acknowledge support
from the University Development Fund of The University
of Hong Kong. The work described in this paper has been
supported by a Central Allocation Vote (CAV) Grant from
the Research Grants Council of Hong Kong Special Admin-
istrative Region, People’s Republic of China (Project No.
HKU 2/05C). S.-T.L. acknowledges the receipt of a post-
Supporting Information Available: Text, tables, and figures
giving details of the syntheses, characterization data, photophysical
data for the newly synthesized complexes, and UV-vis absorption
and emission spectra of the gel and sol forms of complexes 1-6.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM800610X