Self-assembly of a luminescent zinc(II) complex: A supramolecular host-guest fluorescence signaling system for selective nitrobenzene inclusion

Article abstract of DOI:10.1021/ic060518p

A luminescent Zn(II) complex, [Zn(bpy)(aba)₂] (1) {bpy = 2,2′-bipyridyl and aba = 4-dimethylaminobenzoate} has been synthesized as a white solid. Complex 1 shows unusually high selectivity toward nitrobenzene in the presence of other organic guests in solution, as well as in the vapor phase, resulting in both a dramatic color change and a concomitant quenching of luminescence. When crystallized from nitrobenzene, 1 affords deep red crystals with the composition [Zn(bpy)(aba)₂]·C₆H ₅NO₂ (2) as a hydrogen-bonded channel structure via unusual intermolecular C-H...C(sp³) and H...H interactions. Inside the channels, nitrobenzene molecules form infinite polar linear tapes through strong C-H...O interactions in a head-to-tail fashion. The desorption and resorption of nitrobenzene can be achieved in a thermally reversible manner that can be monitored by X-ray powder diffraction patterns.

Full text of DOI:10.1021/ic060518p

Inorg. Chem. 2006, 45, 5257−5259
Self-Assembly of a Luminescent Zinc(II) Complex: a Supramolecular
Host Guest Fluorescence Signaling System for Selective Nitrobenzene
−
Inclusion
Sanjib Das and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology Kanpur, 208016, India
Received March 27, 2006
A luminescent Zn(II) complex, [Zn(bpy)(aba)2] (1)
{
bpy
)
2,2
′
-
nescent metal complexes are gaining significant attention as
sensors and probes in view of their applications in light
emitting diode3 (LED), biomedical analyses,4 fluorescence
imaging,5 and possibly, cancer phototherapy.6 In a recent
report, Wang and co-workers showed that the solid-state
fluorescence intensity of a Zn(II) complex could be partially
quenched upon benzene inclusion.7 Eisenberg et al. showed
that the linear chain of stacked Au(I) dimers become
luminescent upon exposure to vapors of aprotic solvents such
as acetone, CH3CN, CH2Cl2, or CHCl3 where as Balch et
al. have reported, solvent-stimulated luminescence of Au(I)
organometallic complexes is quite important from the
perspectives of sensor applications for the detection of
volatile organic compounds in environmental and public
safety control.8 In this communication, we report the
synthesis of a luminescent compound, [Zn(bpy)(aba)2] (1),
which shows remarkable selectivity toward nitrobenzene, in
solution as well as in vapor states, resulting in a sharp color
change with concomitant quenching of luminescence due to
supramolecular self-assembly in a thermally reversible man-
ner (Scheme 1). Nitrobenzene is highly toxic and causes
vomiting, skin and eye irritation, and headache. Continuous
exposure to this compound leads to liver damage and
anemia.9
bipyridyl and aba 4-dimethylaminobenzoate has been synthe-
)
}
sized as a white solid. Complex 1 shows unusually high selectivity
toward nitrobenzene in the presence of other organic guests in
solution, as well as in the vapor phase, resulting in both a dramatic
color change and a concomitant quenching of luminescence. When
crystallized from nitrobenzene, 1 affords deep red crystals with
the composition [Zn(bpy)(aba)2]
channel structure via unusual intermolecular C
‚‚‚H interactions. Inside the channels, nitrobenzene molecules
form infinite polar linear tapes through strong C ‚‚‚O interactions
in head-to-tail fashion. The desorption and resorption of
‚C6H5NO2 (2) as a hydrogen-bonded
−H
‚‚‚C(sp3) and
H
−H
a
nitrobenzene can be achieved in a thermally reversible manner
that can be monitored by X-ray powder diffraction patterns.
Supramolecular self-assembly of molecular solids is a
potentially important area of contemporary research for
designing a variety of functional materials with specific
properties1 such as optical nonlinearity, magnetism, conduc-
tivity, supramolecular storage of molecules, catalytic activity,
selective adsorption, and so on. In this context, self-assembly
of luminescent metal complexes triggered by specific organic
guest molecules to design supramolecular functional archi-
tectures can be very useful in emerging optoelectronic and
photonic technologies because analytical methods based on
the emission phenomena are among the most sensitive
available. Few reports are available on the guest-specific
solid-state fluorescence modulation and photochromism in
organic crystalline inclusion complexes.2 However, lumi-
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* To whom correspondence should be addressed. E-mail: pkb@iitk.ac.in.
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A. K.; Carrell, H. L.; Boese, R.; Zyss, J.; Nangia, A.; Desiraju, G. R.
J. Am. Chem. Soc. 1998, 120, 2563. (b) Coronado, E.; Gala´n-Mascaro´s,
J. R.; Go´mez-Garc´ıa, C. J.; Laukhin, V. Nature 2000, 408, 447. (c)
Akutsu, H.; Sato, A. A.; Turner, S. S.; Pevelen, D. L.; Day, P.;
Laukhin, V.; Klehe, A.-K.; Singleton, J.; Tocher, D. A.; Probert, M.
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10.1021/ic060518p CCC: $33.50
Published on Web 06/17/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 14, 2006 5257

COMMUNICATION
Scheme 1. Schematic Representation of Nitrobenzene Inclusion by
Compound 1
Figure 1. Packing diagram of 2 viewed down the crystallographic a axis
showing the H-bonded channels occupied by nitrobenzene tapes.
Complex 1 is synthesized10 by reacting Zn(OAc)2‚2H2O,
2,2′-bipyridyl, and 4-(dimethylamino)benzoic acid in 1:1:2
molar ratio at room temperature and crystallizes with a metal-
bound water molecule, [Zn(bpy)(aba)2(H2O)] (1′), from DMF
solvent. When 1 is dissolved with stirring in nitrobenzene
(∼1 × 10-1 M), a red solid is precipitated in about 10 min.
Elemental analysis gives the composition of the red solid as
[Zn(bpy)(aba)2]‚C6H5NO2 (2). The filtrate, on slow evapora-
tion at room temperature, affords deep red crystals after 1
week with the same composition as 2. The powder patterns
of 2 and the red solid are also identical.
X-ray crystallographic analyses shows that the structure11
of 2 consists of neutral [Zn(bpy)(aba)2] units where car-
boxylate groups are asymmetrically bound to the metal,
showing a large difference in the Zn-O bond distances (2.08,
2.18 Å) due to the mismatch between the size of the metal
ion and the bite angle of the carboxylate group. Each aba
unit in the complex is involved in an intricate array of strong
C-H‚‚‚O hydrogen bonding with its neighbor thus forming
a hydrogen-bonded polymeric chain along the crystal-
lographic a axis. The bipyridyl groups are oriented perpen-
dicular to this chain structure and are involved in π‚‚‚π
stacking interactions with each other at 3.937 Å besides
forming strong C-H‚‚‚O interactions with the neighboring
carboxylate O atoms. When viewed down the a axis, it looks
like a pillar with four extensions. This structure exhibits two
rare noncovalent interactions: One of them is the C-H‚‚‚
C(sp3) interactions between the methyl terminals of two pillar
structures10 showing a wide range of distances [2.729-3.565
Å] where the methyl group acts as a H-bond acceptor.12 The
second unusual feature is the existence of short intermo-
lecular H‚‚‚H contacts10 of 2.07 Å that are only slightly
greater than the shortest reported value (1.96 Å) to date.13
Figure 2. Perspective view of 2 showing π‚‚‚π interactions between the
aba units and nitrobenzene.
These intermolecular noncovalent interactions lead to a large
H-bonded channel structure along the crystallographic a axis.
The channels are not empty but are occupied by nitrobenzene
molecules that are bound to the [Zn(bpy)(aba)2] units via
C-H‚‚‚O and C-H‚‚‚π interactions with the aba moieties.
There are also considerable π‚‚‚π stacking interactions
between the aba units and nitrobenzene molecules with a
distance of 3.977 Å inside the channels (Figure 2). The
nitrobenzene molecules themselves form infinite polar linear
tapes through strong C-H‚‚‚O interactions in a head-to-tail
fashion along the crystallographic a axis.10 Two such tapes
are paired via C-H‚‚‚O interactions in an antiparallel
fashion. Thus, the stability of the solid-state structure of 2
is derived from the large number of noncovalent interactions
present.
In the solid state, 1 exhibits a single absorption band at
370 nm while 2 shows bands at 370 and 550 nm.10 The red
color of 2 is derived from the charge-transfer transition
between aromatic moieties of electron rich aba unit and
electron-poor nitrobenzene unit.14 Complex 1 exhibits a broad
intense emission at λmax ) 520 nm, while 2 is nonemissive.
The emission in 1 is due to a strong intraligand charge
transfer (ILCT) from the donor N,N-dimethylamino group
to the acceptor carboxylate end of the aba units.15 Existence
of strong C-H‚‚‚π [3.821, 3.649 Å], as well as π‚‚‚π [3.977
Å] stacking interactions involving nitrobenzene and aba
units, readily converts the emissive intramolecular charge
transfer (ICT) state to nonemissive ligand-to-ligand CT state.
Because of this, 2 does not show any emission behavior.
Fluorescence quenching by C-H‚‚‚π and π‚‚‚π stacking
interactions are well documented in the literature.16
(9) Cronin, M. T. D.; Gregory, B. W. Chem. Res. Toxicol. 1998, 11, 902.
(10) See Supporting Information.
(11) Crystal data for 2: deep red, rectangular, crystal dimensions 0.17 ×
0.15 × 0.13 mm3, monoclinic, P21/c, Z ) 4, a ) 8.180(5) Å, b )
27.217(5) Å, c ) 13.890(5) Å, â ) 95.04(5)°, V ) 3080(2) Å3, Fcald
) 1.45 g cm-3, T ) 100(2) K, µ ) 0.852 mm-1, θmax ) 28.32°, 7579
reflections collected of which 5496 were unique. R1 ) 0.0540, wR2
) 0.1200 with a GOF of 1.027 for I > 2σ(I); residual electron
density: 0.575 and -0.377 e Å-3
.
(12) (a) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int.
Ed. 2003, 42, 1210. (b) Steiner, T.; Desiraju, G. R. Chem. Commun.
1998, 891.
(14) (a) Allwood, B. L.; Spencer, N.; Zavaresh, H. S.; Stoddart, J. F.;
Williams, D. J. J. Chem. Soc. Chem. Commun. 1987, 1064. (b) Philp,
D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154.
(15) Zhang, C. H.; Chen, Z. B.; Jiang, Y. B. Spectrochim. Acta, Part A
2004, 60, 2729.
(13) Bombicz, P.; Czugler, M.; Tellgren, R.; Kalman, A. Angew. Chem.,
Int. Ed. 2003, 42, 1957.
5258 Inorganic Chemistry, Vol. 45, No. 14, 2006

COMMUNICATION
Compound 1 exhibits high selectivity toward nitrobenzene
over other organic aromatic guests such as benzene, toluene,
chlorobenzene, xylene, etc. or aliphatic guests, such as
alcohols, amines, etc. in solution. All efforts to crystallize 1
from different organic solvents other than nitrobenzene
remain unsuccessful. However, when a small amount of
nitrobenzene is added to these solutions, deep red crystals
of 2 are obtained after a few weeks in each case as the sole
product. To probe further the emission behavior of 1 toward
nitrobenzene vapor, a thin film is prepared by spin-coating
a CH2Cl2 solution of the complex onto a quartz plate. When
the plate-supported film is exposed to nitrobenzene vapor,
the white color changes to red within 10 min and the resulting
red film becomes nonemissive.
Figure 3. Schematic representation showing the change of fluorescence
quantum yield (ΦF - Φq) of compound 1 in dichloromethane (1 × 10-6
M) upon addition of the selected guest molecules with λex ) 307 nm. ΦF
and Φq are quantum yields of compound 1 in absence and presence of a
guest, respectively.
The spectral characteristics of 1 in the solution phase are
consistent with its solid-state behavior. In dichloromethane,
1 exhibits an absorption band at λmax ) 307 nm attributable
to an ILCT transition that is solvatochromic in nature.10 It
exhibits dual fluorescence in dichloromethane and can be
assigned15 as the locally excited (LE) and the ICT transition
from the donor N,N-dimethyl amino group to the acceptor
carboxylate end of the aba units, respectively. The ICT
emission, located at λmax ) 442 nm (λex ) 307 nm), is
solvatochromic in nature, while LE emission remains virtu-
ally unchanged with respect to solvent polarity. Upon gradual
addition of nitrobenzene to the dichloromethane solution of
1, the intensity of the emission bands decreases. The linear
Stern-Volmer response10 with nitrobenzene as quencher is
consistent with well-behaved fluorescence quenching sys-
tems.17 To demonstrate the selectivity of 1 toward nitroben-
zene, we have monitored the change in fluorescence quantum
yields in the presence of different guests structurally similar
or otherwise to nitrobenzene in dichloromehane solution.
Figure 3 clearly shows that compound 1 has a remarkably
high selectivity toward nitrobenzene in terms of change of
fluorescence quantum yield. The X-ray crystal structure of
1 clearly revels that the linear nitrobenzene tapes strongly
interact to each other, as well as with the aba units of 1 via
C-H‚‚‚π(aba unit) and π(nitrobenzene tape)‚‚‚π(aba unit)
interactions inside the H-bonded cavity that leads to fluo-
rescence quenching. Thus, we speculate that this may be the
possible reason in solution also. To support this fact, we
further carry out the effect of p-nitrotoluene in the fluores-
cence intensity of 1 in dichloromethane. Interestingly, we
do not find any significant lowering of quantum yield of 1.
This supports the fact that the methyl group in p-nitrotoluene
inhibits interaction in a head-to-tail fashion like nitrobenzene
to form linear tapes and hence cannot be incorporated inside
the open channels to interact with the chromophoric units
(aba moiety) of 1 to induce fluorescence quenching, although
a very precise nature of interactions between nitrobenzene
molecules and 1 in solution cannot be established at this time.
Thermogravimetric investigation10 of 2 clearly shows that
all nitrobenzene is completely lost at 148 °C. With the escape
of nitrobenzene, the solid-state structure of 2 breaks down
and the fluorescence is recovered completely. To show
thermal reversibility of nitrobenzene inclusion, we have
recorded X-ray powder diffraction (XRD) patterns of 1 in
solvated, desolvated, and resolvated conditions.10 It shows
that, after removal of nitrobenzene from the solvated
compound at 150 °C followed by resolvation, the initial
spectral pattern can be restored and the cycle of solvation
followed by desolvation can be repeated several times
without destroying the complex. Similar behavior is observed
in the case of a quartz-supported thin film of 1 upon exposure
to nitrobenzene vapor. Thus, the reversible solvation and
desolvation leads to reversible on/off switching of solid-state
luminescence (Scheme 1).
In conclusion, a luminescent Zn(II) complex is reported
that can selectively capture nitrobenzene in solution, as well
as detect nitrobenzene vapor through both sharp color change
and luminescence quenching in a thermally reversible
manner. The self-assembled structures exhibit a number of
unusual rare noncovalent interactions in addition to conven-
tional hydrogen bonding. Designing of supramolecular host-
guest fluorescence signaling systems for other toxic guests
is in progress in our laboratory.
Acknowledgment. Financial support received from the
Department of Science and Technology, New Delhi, India
(Grant No. SR/S5/NM-38/2003 to P.K.B.) is gratefully
acknowledged.
(16) (a) Bevers, S.; Schutte, S.; McLaughlin, L. W. J. Am. Chem. Soc.
2000, 122, 5905. (b) Shirai, K.; Matsuoka, M.; Fukunishi, K. Dyes
Pigm. 1999, 42, 95. (c) Balon, M.; Guardado, P.; Munoz, M. A.;
Carmono, C. Biospectroscopy 1998, 4, 185. (d) Hinzmann, J. S.;
McKenna, R. L.; Pierson, T. S.; Han, F.; Kezdy, F. J.; Epps, D. E.
Chem. Phys. Lipids 1992, 62, 123.
Supporting Information Available: X-ray crystallographic data
for 1′ and 2; TGA; fluorescence/UV-vis spectra; XRD pattern;
and synthetic details of 1 and 2. This material is available free of
charge via the Internet at http://pubs.acs.org.
(17) Mojtaba, S.; Javad, C. M. J. Photochem. Photobiol. A 2003, 155, 69.
IC060518P
Inorganic Chemistry, Vol. 45, No. 14, 2006 5259