Zn(II)-Induced Ground-State π-Deconjugation and Excited-State Electron Transfer in N,N-Bis(2-pyridyl)amino-Substituted Arenes

Article abstract of DOI:10.1021/jo049902z

The synthesis and X-ray crystal structures of two N,N-bis(2-pyridyl)amino (dpa)-substituted aromatic systems (Ar-dpa) 1 (Ar = 4,4′-disubstituted trans-stilbene) and 2 (Ar = 1,4-disubstituted benzene) and their ZnCl ₂ complexes (1/ZnCl₂ and 2/ZnCl₂) are reported. The fluoroionophoric behavior of 1-2 in response to Zn(II) in acetonitrile also has been investigated. In addition, compound 3DPA has been prepared and served as a π-deconjugated model for 1DPA. The observed crystal structures for 1/ZnCl₂ and 2/ZnCl₂ could be divided into two distinct types, the planar and the twisted forms, depending on the aryl-dpa (C_ph-NC₃) dihedral angle. The twisted form is more favorable for these complexes unless the arene has a strong " push-pull" character. Nonetheless, the degree of π-conjugation between the N-pyridyl and the N-aryl group is reduced in both complex forms when compared with the free ligands. Such a Zn(II)-induced π-deconjugation not only directly affects the internal charge transfer (ICT) fluorescence of the dpa-substituted stilbenes but also facilitates the occurrence of photoinduced electron transfer (PET) from the stilbene donor to the dpa/Zn(II) acceptor. The PET process is particularly important in accounting for the observed Zn-(II)-induced fluorescence quenching for 1DPA as well as 3DPA.

Full text of DOI:10.1021/jo049902z

Zn (II)-In d u ced Gr ou n d -Sta te π-Decon ju ga tion a n d Excited -Sta te
Electr on Tr a n sfer in N,N-Bis(2-p yr id yl)a m in o-Su bstitu ted Ar en es
J ye-Shane Yang,*,† Yan-Duo Lin,† Yu-Hsi Lin,† and Fen-Ling Liao‡
Department of Chemistry, National Central University, Chung-Li, Taiwan 32054, and
Center for Nano Science and Technology, UST, and Instrumentation Center,
National Tsing Hua University, Hsinchu, Taiwan 30043
jsyang@cc.ncu.edu.tw
Received J anuary 16, 2004
The synthesis and X-ray crystal structures of two N,N-bis(2-pyridyl)amino (dpa)-substituted
aromatic systems (Ar-dpa) 1 (Ar ) 4,4′-disubstituted trans-stilbene) and 2 (Ar ) 1,4-disubstituted
benzene) and their ZnCl2 complexes (1/ZnCl2 and 2/ZnCl2) are reported. The fluoroionophoric
behavior of 1-2 in response to Zn(II) in acetonitrile also has been investigated. In addition,
compound 3DP A has been prepared and served as a π-deconjugated model for 1DP A. The observed
crystal structures for 1/ZnCl2 and 2/ZnCl2 could be divided into two distinct types, the planar and
the twisted forms, depending on the aryl-dpa (Cph-NC3) dihedral angle. The twisted form is more
favorable for these complexes unless the arene has a strong “push-pull” character. Nonetheless,
the degree of π-conjugation between the N-pyridyl and the N-aryl group is reduced in both complex
forms when compared with the free ligands. Such a Zn(II)-induced π-deconjugation not only directly
affects the internal charge transfer (ICT) fluorescence of the dpa-substituted stilbenes but also
facilitates the occurrence of photoinduced electron transfer (PET) from the stilbene donor to the
dpa/Zn(II) acceptor. The PET process is particularly important in accounting for the observed Zn-
(II)-induced fluorescence quenching for 1DP A as well as 3DP A.
In tr od u ction
a typical PET-based fluorescent chemosensor displays
either an “on-off” or an “off-on” fluorescence intensity
change in response to analytes.1 In contrast, for directly
linked and π-conjugated D-A systems, the phenomenon
of internal charge transfer (ICT) from D to A dominates
the electronic absorption and emission properties. As a
result of molecular recognitions, not only the intensity
but also the position of the fluorescence maximum of an
ICT-based sensory molecule could be modulated.1 Since
the electronic (redox) character of an ionophoric D or A
can be readily modified upon ion binding, both the PET
and the ICT signaling mechanisms are particularly
powerful in the construction of fluorescent ion sensors
(i.e., fluoroionophores).2
The ground-state conformation of D-A systems is
known to play a critical role in determining the excited-
state behavior and the resulting fluorescence proper-
ties.3,4 For instance, D-A conjugated molecules such as
arylamines tend to relax toward a more planar structure
in the excited state due to the enhanced electron delo-
calization interactions, whereas for a largely pretwisted
counterpart the twisted angle could be retained or even
increased, leading to a twisted ICT (TICT) state with
distinct dipole and emission transition moments.3 Ac-
cordingly, structural perturbations, in addition to elec-
trostatic perturbations, as a result of ion recognitions
The electron donor (D)-acceptor (A) interactions that
are associated with light emitting or quenching have
demonstrated particular utility in the design of molecule-
based fluorescent sensors.1,2 When the D and A groups
are separated by short aliphatic (σ) spacers such as one
or two methylene groups, the process of photoinduced
electron transfer (PET) in polar solvents generally leads
to a loose and nonemissive radical ion pair D+•A-•. Thus,
† National Central University and Center for Nano Science and
Technology, UST.
‡ Instrumentation Center, National Tsing Hua University
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McCoy, C. P.; Rademacher, J . T.; Rice, T. E. Chem. Rev. 1997, 97,
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10.1021/jo049902z CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/16/2004
J . Org. Chem. 2004, 69, 3517-3525
3517

Yang et al.
should be taken into account for the observed fluoroiono-
phoric properties of PET and ICT probes.5 Indeed, a
twisting of the C-N bond and/or an increase of the degree
of pyramidalization of the azacrown nitrogen (D) upon
cation binding have been observed for monoazacrown-
derived ICT fluoroionophores,6-8 and in some cases the
degree of nitrogen pyramidalization correlates well with
the chromoionophoric shift.6
We recently reported that trans-4-(N,N-bis(2-pyridyl)-
amino)stilbene (1H), an “uncrowned” ICT fluoroiono-
phore, undergoes a twisting of the Cph-N bond between
the stilbene (A) and the dipyridylamino (dpa, D) moieties
upon the binding of transition metal ions by the dpa
group (Figure 1a).9 Such a cation-induced D-A decon-
jugation process is in accord with the blue-shifted ab-
sorption maximum and the weakening of the ICT fluo-
rescence band (415 nm) of 1H in acetonitrile, particularly
in the cases of closed-shell metal ions such as Zn(II)
(Figure 1b). While the structures and luminescence
properties of many other dpa-substituted arenes
(Ar-dpa) and their Zn(II) complexes have been re-
ported,10-13 the origins of the Zn(II)-complexation effect
on the ground-state Ar-dpa conformation and the excited-
state D-A interactions are still poorly understood. To
gain insights into these two issues, we have synthesized
and investigated compounds 1-3. The X-ray crystal
structures of compound series 1 and 2 and their com-
plexes 1/ZnCl2 and 2/ZnCl2 reported herein indicate that
the electronic nature of the aryl group plays an important
role in determining the conformation of the complex,
where the Cph-N twisted form is more favorable unless
the arene possesses a sufficiently strong “push-pull”
character. On the basis of the fluorescence behavior of
1DP A and its D-A non-π-conjugated analogue 3DP A,
we have shown that the dpa/Zn(II) group could act as an
electron acceptor. As a result of Zn(II) complexation, the
stilbene D-A systems (1) become π-deconjugated and the
PET process is “turned on”, leading to an “on-off”
fluorescence response.
Resu lts a n d Discu ssion
Syn th esis. The synthesis of 4,4′-disubstituted trans-
stilbene 1DMA has been recently reported,5 and that of
1CN and 1DP A is outlined in Scheme 1. The conditions
F IGURE 1. (a) Crystal structures of 1H and 1H/ZnCl2 and
(b) absorption and fluorescence titration spectra of 1H (10 µM)
with Zn(II) (0-5 equiv) in acetonitrile (excitation at 330 nm).
(5) Yang, J .-S.; Hwang, C.-Y.; Hsieh, C.-C.; Chiou, S.-Y. J . Org.
Chem. 2004, 69, 719-726.
(6) J onker, S. A.; Van Dijk, S. I.; Goubitz, K.; Reiss, C. A.;
Schuddeboom, W.; Verhoeven, J . W. Mol. Cryst. Liq. Cryst. 1990, 183,
273-282.
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J . L.; Reck, G.; Radeglia, R.; Resch-Genger, U. J . Phys. Chem. A 2000,
104, 3087-3109.
described by Lee and Marvel14 were adopted for the
Wittig reaction to construct the stilbene backbone bearing
a nitro group and a cyano (4CN) or bromo group (4Br )
at the para positions. A reduction of the nitro group in 4
by tin(II) chloride15 produced compound 5, which was
followed by a Pd-catalyzed amination reaction16 to form
compounds 1CN and 1Br . Compound 1Br was then
converted to 1DP A by reacting with diphenylamine
under a similar Pd-catalyzed amination condition.
In principle, the series of compound 2 could be pre-
pared by a double N-pyridylation of the corresponding
(8) Witulski, B.; Weber, M.; Bergstra¨sser, U.; Desvergne, J .-P.;
Bassani, D. M.; Bouas-Laurent, H. Org. Lett. 2001, 3, 1467-1470.
(9) Yang, J .-S.; Lin, Y.-H.; Yang, C.-S. Org. Lett. 2002, 4, 777-780.
(10) Yang, W.; Schmider, H.; Wu, Q.; Zhang, Y.-S.; Wang, S. Inorg.
Chem. 2000, 39, 2397-2404.
(11) Wang, S. Coord. Chem. Rev. 2001, 215, 79-98.
(12) (a) Pang, J .; Marcotte, E. J .-P.; Seward, C.; Brown, R. S.; Wang,
S. Angew. Chem., Int. Ed. 2001, 40, 4042-4045. (b) Seward, C.; Pang,
J .; Wang, S. Eur. J . Inorg. Chem. 2002, 1390-1399. (c) Kang, Y.;
Seward, C.; Song, D.; Wang, S. Inorg. Chem. 2003, 42, 2789-2797.
(13) (a) Pang, J .; Freiberg, S.; Yang, X.-P.; D’Iorio, M.; Wang, S. J .
Mater. Chem. 2002, 12, 206-212. (b) J ia, W.-L.; Song, D.; Wang, S. J .
Org. Chem. 2003, 68, 701-705. (c) Liu, Q.-D.; J ia, W.-L.; Wu, G.; Wang,
S. Organometallics 2003, 22, 3781-3791. (d) Du, P.; Li, C.; Li, S.; Zhu,
W.; Tian, H. Synth. Met. 2003, 137, 1131-1132.
(14) Lee, B. H.; Marvel, C. S. J . Polym. Sci. Chem. Ed. 1982, 20,
393-399.
(15) Bellamy, F. D.; Ou, K. Tetrahedron Lett. 1984, 25, 839-842.
(16) (a) Wolfe, J . P.; Wagaw, S.; Marcoux, J .-F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805-818. (b) Hartwig, J . F. Angew. Chem., Int.
Ed. 1998, 37, 2046-2067.
3518 J . Org. Chem., Vol. 69, No. 10, 2004

N,N-Bis(2-pyridyl)amino-Substituted Arenes
SCHEME 1
is electron withdrawing.18 Such a feature of 2-aminopy-
ridines might in part account for the poor Pd-catalyzed
amination reactions of 8CN and 8NO with 2-bromopy-
ridine. Nonetheless, compound 2NO could be prepared
by using the Ullmann reaction, where Cu(II) and KOH
are used as catalysts.10
SCHEME 2
Compound 3DP A was prepared according to Scheme
3. The starting material 6 was synthesized in a way
similar to that for 4 (Scheme 1). The Pd-catalyzed
amination reaction between diphenylamine and 6 af-
forded 7. A reduction of the ester group in 7 by LAH
generated the alcohol 3OH, which was then converted
to 3P I by the Mitsunobu reaction.19 The treatment of 3P I
with hydrazine20 afforded the primary amine 3NH. The
final step in the 3DP A synthesis was accomplished by
the Pd-catalyzed amination reaction between 3NH and
2-bromopyridine.
commercially available 4-substitued anilines with 2-bor-
mopyridine under the above-mentioned Pd-catalyzed
amination condition (Scheme 2). While this is true in the
formation of compounds 2H, 2CN, and 2OM, this method
could only afford the monosubstituted adduct 8NO when
4-nitroaniline was used. It should be also noted that the
yield of 2CN is low (31%) in comparison to those of 2H
(91%) and 2OM (85%) and, in fact, the monoadduct 8CN
was isolated as the major product (43%) in the N-
pyridylation of 4-cyanoaniline. Apparently, the efficiency
of the second N-pyridylation reaction is strongly affected
by electron-withdrawing substituents, and it appears that
the stronger electron withdrawing the substituent, the
less efficient the reaction. These observations are remi-
niscent of the failure of the Pd-catalyzed amination
reaction between 4-bromostilbene and bis(2-pyridyl)-
amine for the synthesis of 1H,9 because both the 2-pyridyl
and 4-nitrophenyl groups are “π-electron-withdrawing”
(π-deficient) arenes.17 It has been recently reported that
2-aminopyridines could tautomerize and form stable
hydrogen-bonded dimers of the tautomer (an imine of
2-pyridone) when the group attached to the amino moiety
The complexation reactions of dpa-substituted arenes
1 and 2 with ZnCl2 were carried out in refluxed aceto-
nitrile/methanol mixed solvents and resulted in 1/ZnCl2
and 2/ZnCl2, respectively, in a 1:1 ratio. All the complexes
are air stable.
Molecu la r Str u ctu r e. Single crystals of ligands 1 and
2 and their complexes of ZnCl2 (1/ZnCl2 and 2/ZnCl2)
suitable for X-ray crystallographic studies were grown
by slow evaporation of solvent under ambient conditions.
The X-ray crystal structures and the corresponding
crystallographic details of 1CN, 1CN/ZnCl2, 1DMA,
1DMA/ZnCl2, 2H , 2H /ZnCl2, 2CN, 2CN/ZnCl2, 2OM,
2OM/ZnCl2, and 2NO/ZnCl2 are supplied in the Support-
ing Information.21 It should be noted that there are two
crystallographically independent conformers in the asym-
metric unit for free ligands 1H, 1DMA, 2H, and 2OM
SCHEME 3
J . Org. Chem, Vol. 69, No. 10, 2004 3519

Yang et al.
F IGURE 2. Schematic representation of the Zn(II)-induced Cph-N bond twisting and planarization processes in dpa-substituted
arenes. Symbols ø0, øp, and øt are the dihedral angles between the aryl (phenyl) plane and the NC3 plane for the free ligand and
the planar and the twisted forms of its ZnCl2 complex, respectively.
TABLE 1. Selected Str u ctu r a l Da ta , In clu d in g th e Su m
of Bon d An gles (θ) a r ou n d th e Am in o Nitr ogen Atom ,
Dih ed r a l An gles (øp h a n d øp y), a n d C-N Bon d Len gth s
(d p h a n d d p y) in th e Tr ia r yla m in o Moiety for Com p ou n d s
1 a n d 2 a n d Th eir Zn Cl2 Com p lexesa
counterclockwise, accounts for the primary difference
between the two crystallographically independent con-
formers for 1H, 1DMA, 2H, and 2OM. It is noted that
the dihedral angle (øph) between the central NC3 plane
and the N-phenyl plane is smaller for 2CN, but the
average of its two dihedral angles (øpy) between the NC3
plane and the N-pyridyl rings is relatively larger. In
addition, the øph value in one of the two conformers of
2H is 77.8°, significantly larger than that in the other
conformer (58.7°) and those in the other free ligands
(39.0-64.2°). The relative Cph-N bond length (dph) and
the average of the two Cpy-N bond lengths (dpy) appear
to parallel the relative øph and øpy values (Table 1). As
judged by the dihedral angle (R) between the two phenyl
rings, the stilbene moiety is more planar in 1CN (6°) than
that in 1H (21-35°) and 1DMA (13-30°). Regarding the
structures of the complexes, the geometry of the Zn(II)
ion in all cases is a distorted tetrahedron with N-Zn-N
bond angles (87.9-90.9°) and Zn-N bond lengths (2.016-
2.066 Å) similar to those in the previously reported
Ar-dpa/ZnCl2 complexes.10-12 When compared with the
corresponding free ligands, the stilbene moiety in 1H/
ZnCl2 (R ) 34°) and 1CN/ZnCl2 (R ) 3°) is not much
perturbed, but it becomes more planar in 1DMA/ZnCl2
(R ) 3°). More intriguing changes are in the triarylamino
moiety, where the øph value either increases or decreases
to a large extent, leading to two distinct geometries. As
depicted in Figure 2, an increase in øph results in a nearly
orthogonal conformation between the N-phenyl and the
dpa groups (the “twisted” form), as observed for all three
stilbene complexes and for 2H/ZnCl2 and 2OM/ZnCl2
(øph ) 74-88°). In contrast, the øph value is relatively
small in 2CN/ZnCl2 and 2NO/ZnCl2 (5-13°), and the
conformation is thus referred to as the “planar” form.
Since both the cyano and the nitro substituents are
strong electron-withdrawing groups, it appears that the
planar form is favorable only when the aryl group has a
strong “push-pull” character. It should be noted that the
øpy value is significantly increased with the decrease of
øph. As a result, no matter which form is adopted by the
complex, the degree of π-conjugation between the N-aryl
and the N-pyridyl groups is largely reduced on going from
the free ligands to the Zn(II) complexes (Figure 3).
compd
θ (deg) øph (deg) øpy (deg) dph (Å) dpy (Å)
1H
359.1
359.9
359.8
359.9
359.8
359.1
359.5
357.7
359.7
359.8
359.8
358.6
359.5
359.9
359.6
359.4
359.8
359.9
360.0
58.5
62.8
47.4
57.3
58.3
58.7
77.8
39.0
54.3
64.2
87.6
78.5
73.9
77.4
9.0
31.7
22.4
34.4
31.1
25.6
26.6
25.7
38.0
32.6
27.6
31.5
28.4
33.5
19.3
70.1
69.4
26.9
68.6
67.1
1.432
1.439
1.425
1.433
1.439
1.436
1.443
1.415
1.430
1.440
1.455
1.468
1.446
1.452
1.411
1.408
1.452
1.437
1.422
1.409
1.412
1.413
1.411
1.418
1.408
1.405
1.425
1.409
1.407
1.414
1.411
1.415
1.409
1.432
1.435
1.413
1.428
1.426
1CN
1DMA
2H
2CN
2OM
1H/ZnCl2
1CN/ZnCl2
1DMA/ZnCl2
2H/ZnCl2
2CN/ZnCl2
12.4
87.7
5.1
2OM/ZnCl2
2NO/ZnCl2
11.9
a
Two sets of data are provided for those having two independent
conformers in the crystals.
and for complexes 2CN/ZnCl2 and 2NO/ZnCl2, but only
one of the two conformers is shown for these cases. The
difference between the two conformers is small except
for the case of 2H (vide infra).
Selected structural data for 1 and 2 and their com-
plexes are given in Table 1. As evaluated by the sum of
bond angles (θ) about the amino nitrogen, the amino
nitrogen atom is essentially coplanar (NC3 plane) with
the three carbon atoms to which it attaches in all cases
(θ ∼ 360°). The three N-aryl rings (one N-phenyl and two
N-pyridyl rings) in free ligands 1 and 2 have a propeller
arrangement, which could be attributed to steric interac-
tions between them. A change in the tilted orientation
of such a propeller arrangement, either clockwise or
(17) (a) Abbotto, A.; Bradamante, S.; Pagani, G. A. J . Org. Chem.
1993, 58, 444-448. (b) Abbotto, A.; Bradamante, S.; Facchetti, A.;
Pagani, A. J . Org. Chem. 2002, 67, 5753-5772.
(18) Alkorta, I.; Elguero, J . J . Org. Chem. 2002, 67, 1515-1519.
(19) Mitsunobu, O. Synthesis 1981, 1-28.
An elegant balance between the steric and electronic
conjugation (delocalization) interactions in the triaryl-
amino moiety might account for the substituent effect on
the conformation of these complexes.22 For the free
ligands, a coplanar geometry of the triarylamino moiety
(20) Sheehan, J . C.; Bolhofer, W. A. J . Am. Chem. Soc. 1950, 72,
2786-2788.
(21) A good quality of single crystals of 2NO was not available, and
the quality of crystals of 2NO/ZnCl2 was less satisfactory due to the
presence of twinning. Nonetheless, a reasonable structure could be
derived from the X-ray diffraction data.
3520 J . Org. Chem., Vol. 69, No. 10, 2004

N,N-Bis(2-pyridyl)amino-Substituted Arenes
F IGURE 4. Two views of the X-ray crystal structure of
3-DP A.
The crystal structure of the arene-dpa non-π-conju-
gated derivative 3DP A was also determined (Figure 4).
The presence of a methylene group between the stilbene
and the dpa group does not affect the geometry of the
dpa group as compared with that in 1 and 2. The dpa
nitrogen atom is sp2-hybridized with a θ value of 359.9°,
and the values of øpy and dpy are 31.5° and 1.402 Å,
respectively. It is interesting to note that the dpa group
is nearly orthogonal (85°) to the adjacent phenyl ring of
stilbene. The nitrogen atom in the diphenylamino group
has as expected23 a trigonal planar geometry with θ )
359.4°. An attempt to grow single crystals for 3DP A/
ZnCl2 was unfortunately unsuccessful.
Electr on ic Sp ectr a a n d Zn (II)-In d u ced Sp ectr a l
Resp on ses. The absorption and fluorescence spectra of
the dpa-substituted stilbenes 1 in the absence and the
presence of Zn(II) in acetonitrile have been investigated.
As shown in Table 2, the large Stokes shifts in all four
free ligands, including the donor-donor-type stilbenes
1DMA and 1DP A, indicate the presence of strong ICT
upon electronic excitation.5 The strong fluorescence of 1H
is a result of the “amino conjugation effect” observed for
N-aryl-substituted trans-4-aminostilbenes.23 Compound
1DP A is also strongly fluorescent with a quantum
efficiency similar to those for 1H and trans-tetrameth-
yldiaminostilbene (ΦF ) 0.57 in MeCN).24 For compounds
1CN and 1DMA, their fluorescence quantum yields are
relatively lower, which could be attributed to the presence
of a strong electron-withdrawing cyano (1CN) or an
electron-donating dimethylamino (1DMA) group. A study
of substituent effect on the fluorescence quantum yield
of trans-4,4′-disubstituted stilbenes has been reported.25
Despite the different nature of these substituted stilbene
fluorophores, all four compounds display the same flu-
oroionophoric behavior in response to the presence of Zn-
(II) in acetonitrile: namely, most of the ICT fluorescence
is quenched upon the addition of 1 equiv of Zn(II), as
represented by the case of 1H (Figure 1b) and 1DP A
(Figure 5).26 It should be noted that the Zn(II)-induced
F IGURE 3. Two views of the crystal structures (space-filling
model) of (a) 2CN/ZnCl2 and (b) 2OM/ZnCl2 to show the
orientation of the N-aryl and N-pyridyl groups in the planar
and the twisted forms of complexes, respectively.
would have the maximum conjugation interaction, but
it would also encounter the most sever steric hindrance.
A compromise of these two effects thus leads to a
propeller arrangement of the three N-aryl groups. On the
basis of the relative values of the Cph-NC3 dihedral
angles (ø) and the C-N bond lengths (d) (Table 1), the
conjugation interaction of the amino nitrogen lone-pair
electrons with the N-aryl groups appears to be stronger
(i.e., smaller ø and d values) in the pyridylamino than
in the arylamino group in all free ligands except for those
having electron-withdrawing substituents in the aryl
group. The arylamino vs pyridylamino conjugation in-
teraction seems comparable in 1CN and 2CN. Upon
complexation with Zn(II), the propeller arrangement of
the triarylamino group is perturbed (Figure 2). In the
planar form of complexes, the conjugation interaction is
enhanced in the arylamino group but diminished in the
pyridylamino group. However, the reverse is true in the
twisted form. Given the fact that the binding of Zn(II)
by the dpa moiety will further enhance the charge-
delocalization interactions in the pyridylamino group, the
twisted form should be more favorable than the planar
form in terms of the conjugation effect. While this could
account for the observed twisted form for 1H/ZnCl2,
1DMA/ZnCl2, 2H/ZnCl2, and 2OM/ZnCl2, the choice of
the planar form by 2CN/ZnCl2 and 2NO/ZnCl2 indicates
that the electron-withdrawing CN and NO2 substituents
provide sufficient driving force for conjugation that the
aniline lone pair remains conjugated. Despite the pres-
ence of a CN substituent, 1CN/ZnCl2 adopts the twisted
form. This could be attributed to a relatively weak long-
rang conjugation interaction in the arylamino group due
to the larger separation of the D and A groups.
(23) (a) Yang, J .-S.; Chiou, S.-Y.; Liau, K.-L. J . Am. Chem. Soc. 2002,
124, 2518-2527. (b) Yang, J .-S.; Wang, C.-M.; Hwang, C.-Y.; Liau, K.-
L.; Chiou, S.-Y. Photochem. Photobiol. Sci. 2003, 2, 1225-1231.
(24) Le´tard, J .-F.; Lapouyade, R.; Rettig, W. J . Am. Chem. Soc. 1993,
115, 2441-2447.
(25) Papper, V.; Pines, D.; Likhtenshtein, G.; Pines, E. J . Photochem.
Photobiol. A: Chem. 1997, 111, 87-96.
(22) Other noncovalent interactions such as Cl‚‚‚H-C (in the twisted
form) or Cl‚‚‚π (in the planar form) should be weak due to the lack of
close contact.
J . Org. Chem, Vol. 69, No. 10, 2004 3521

Yang et al.
F IGURE 6. Tha AM1-calculated energies (eV) for the HOMO
and LUMO of DPhAS, dpaH, dpaH/ZnCl2, and trans-stilbene
in the gas phase.
cence quenching should result from the perturbation of
the electronic and structural properties of 1 by Zn(II).
The Zn(II)-induced fluorescence quenching for 1H has
been correlated to the deconjugation of the stilbene group
from the dpa group (Figure 1), which interrupts the ICT
process and the N-pyridyl conjugation effect.9,23 According
to the above structural analysis for 1/ZnCl2 and 2/ZnCl2,
a π-deconjugation of the N-pyridyl group from the stil-
bene fluorophore appears to be inevitable when these
dpa-substituted stilbenes bind Zn(II) (Figure 3). However,
such a π-deconjugation process alone seems not sufficient
to account for the fluorescence-quenching responses of
these stilbene derivatives to Zn(II), particularly in the
case of 1DP A (Figure 5). If the conformational effect
alone was in operation, the fluorescence spectrum of
1DP A would not be much affected by the presence of Zn-
(II), because the fragment of N,N-diphenylaminostilbene
(DPhAS) is known to be strongly fluorescent with a
quantum yield and fluorescence maximum similar to
those of the 1DP A molecule (Table 2).23 Apparently, new
pathways of nonradiative decay have been triggered
when these dpa-substituted stilbenes interact with Zn(II).
F IGURE 5. Absorption and fluorescence titration spectra of
1DP A (10 µM) with Zn(II) (0-5 equiv) in acetonitrile (excita-
tion at 375 nm).
TABLE 2. Ma xim a of UV-Vis Absor p tion (λa bs) a n d
Cor r ected F lu or escen ce (λfl), F lu or escen ce-Ba n d
Ha lf-Wid th (∆ν1/2), Stok es Sh ift (∆νst), a n d F lu or escen ce
Qu a n tu m Yield s (Φf) of Com p ou n d s 1-3, 1/Zn (II)-3/
Zn (II),a a n d DP h AS in Aceton itr ile a t Room Tem p er a tu r e
(ca . 298 K)
b
compd
λabs (nm) λfl (nm) ∆ν1/2 (cm-1
)
∆νst (cm-1
)
Φf
1H
1H/Zn(II)
1CN
1CN/Zn(II)
1DMA
1DMA/Zn(II) 368
1DP A
1DP A/Zn(II) 376, 293
2H
2H/Zn(II)
2OM
332
313
360
321
415
415
492
487
454
461
465
472
385
387
440
457
398
400
3483
3614
3542
3644
3436
3737
3602
4428
3888
4081
4898
5780
4123
4682
6024
0.60
7453
5147
5161
7924
10830
6027
0.19
0.12
0.64
0.07
0.07
0.32
368, 338
375, 304
295, 273
321, 291
298, 272
322, 287
321
2OM/Zn(II)
2CN
2CN/Zn(II)
319, 282
378, 296
2NOc
2NO/Zn(II)c 322, 288
3DP A
3DP A/Zn(II) 368, 294
365, 299
464
466
463
360
3609
3822
3773
5846
0.68
DphAS
362
6026
3968
0.66
0.12
dpaH/Zn(II)d 315
a
b
Data determined with 1-3:Zn(II) ) 1:5. ∆νst ) νabs - νfl of
We thus ascribe the Zn(II)-induced fluorescence quench-
ing for 1DP A to a result of PET from the DPhAS moiety
to the dpa/Zn(II) moiety. This is supported by the AM1-
calculated energies of the frontier orbitals for DPhAS,
dpaH, and dpaH/ZnCl2 (Figure 6).28 The highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) of dpaH were calculated to lie
below and above the corresponding HOMO and LUMO
of DPhAS, respectively. Accordingly, the PET between
the locally excited DPhAS and the ground-state dpaH
would be an unfavorable endothermic process. However,
the energies of both the HOMO and LUMO of dpaH/
ZnCl2 are lower than the HOMO and LUMO of dpaH,
respectively, in analogy to the results for the other dpa
derivatives based on ab initio calculations.10 This thus
allows an exothermic PET process from the LUMO of
spectral maxima. c Fluorescence too weak to be determined. Data
d
from ref 29.
spectral changes for 1CN and 1DMA resemble those for
1H and 1DP A, respectively. When compared with the
absorption spectra of 1H, Zn(II) complexation has a much
smaller effect on the long-wavelength absorption maxi-
mum of 1DP A but spectral broadening on the red edge
is more prominent. A study of ion selectivity by the dpa
group based on the relative extent of fluorescence quench-
ing for 1H has been reported in a preliminary account of
this work.9 The choice of Zn(II) for the current mecha-
nistic study on the fluoroionophoric properties of 1 is
mainly due to the negligible external fluorescence-
quenching effect of Zn(II).27 Thus, the observed fluores-
(26) The previously observed9 weak fluorescence grown in the blue
region at ca. 344 nm accompany the quenching of ICT fluorescence by
(28) (a) MOPAC-AM1 calculations were performed on a personal
computer, using the algorithms supplied by the package of Quantum
CAChe Release 3.2, a product of Fujitsu Limited. (b) Dewar, M. J . S.;
Zoebisch, E. G.; Healy, E. F.; Stewart, J . J . P. J . Am. Chem. Soc. 1985,
107, 3902-3909.
Zn(II) in the case of 1H was no longer observed when
acetonitrile solution of Zn(ClO4)2‚6H2O was used.
a fresh
(27) Varnes, A. W.; Dodson, R. B.; Wehry, E. L. J . Am. Chem. Soc.
1972, 94, 946-950.
3522 J . Org. Chem., Vol. 69, No. 10, 2004

N,N-Bis(2-pyridyl)amino-Substituted Arenes
F IGURE 8. Absorption and normalized excitation (emission
at 460 nm) and emission (excitation at 300 nm) spectra of 2OM
(10 µM) in the absence (full line) and the presence (dot line)
of 5 equiv of Zn(II) (50 µM) in acetonitrile.
F IGURE 7. Absorption and fluorescence titration spectra of
3DP A (10 µM) with Zn(II) (0-5 equiv) in acetonitrile (excita-
tion at 365 nm).
DPhAS to the LUMO of dpaH/Zn(II) when the DPhAS is
electronically excited.
It is interesting to point out that the non-π-conjugated
dpa analogue, bis(2-pyridylmethyl)amine (di-2-picolyl-
amine, dpma), is also a good ionophore for Zn(II) and has
been used to form PET-based fluorescent probes.33 The
participation of the amino nitrogen of dpma in the
binding of Zn(II) accounts for their “off-on” fluorescence
response.
To gain experimental evidence for the occurrence of
PET in 1DP A/Zn(II), the fluoroionophoric behavior of the
corresponding non-π-conjugated D-A analogue 3DP A
has been investigated. The structure of 3DP A nicely
mimics the π-deconjugated conformation of 1DP A, due
to the presence of a methylene group between the DPhAS
and the dpa groups (Figure 4). In the absence of Zn(II),
3DP A is strongly fluorescent with a feature similar to
DPhAS (Table 2), indicating that the DPhAS fluorophore
has negligible interactions with the dpa group. Like the
case of 1DP A, the ICT fluorescence of 3DP A is largely
quenched upon the addition of Zn(II) (Figure 7). These
observations clearly demonstrate that the dpa/Zn(II)
group interacts with the excited DPhAS fluorophore and
then quenches its fluorescence. Fluorescence quenching
resulting form an energy transfer from the excited
DPhAS to the dpa/Zn(II) is unlikely in 1DP A/ZnCl2 and
3DP A/ZnCl2, because of the absence of spectral overlap
of the DPhAS emission and the dpaH/Zn(II) absorption
(Table 2).29 On the other hand, the occurrence of PET
from the DPhAS donor to the dpa/Zn(II) acceptor is
theoretically allowed (Figure 6), and an induction of PET
as a result of Zn(II) complexation has been previously
observed for 4-(dialkylamino)pyrimidines (4-DMAP).30
The presence of PET in directly linked but π-deconju-
gated (orthogonal) D-A systems has long been proposed
for some twisted systems related to N,N-dimethylami-
nobenzonitrile (e.g., TMABN).3 A recent example has
been provided by fluorescein derivatives.31 A combination
of ICT and PET signaling mechanisms for the formation
of fluoroionophores that possess both of the features of
sensitivity (PET) and spectral shift (ICT) has been
reported.32 Thus, the Zn(II)-induced intramolecular PET
is more likely the main origin responsible for the “on-
off” fluorescence response of the ICT-based probe 1DP A.
For comparison, the electronic spectra of compound
series 2 (10 µM) in the absence and the presence of Zn(II)
(50 µM) in acetonitrile have also been investigated. The
spectra of 2OM are shown in Figure 8 and the spectro-
scopic data of all four compounds are collected in Table
2. The fluorescence of 2NO is too weak to be determined,
presumably due to an efficient intersystem crossing
promoted by the nitro group.34
The shorter conjugation
length of 2H vs 1H and 2CN vs 1CN is consistent with
the blue-shifted absorption and fluorescence maxima. The
Stokes shift of compounds 2 in acetonitrile is also
significant; however, the substituent effect on the Stokes
shift shows an opposite trend in 2 vs 1, i.e., 2OM > 2H
> 2CN but 1DMA < 1H < 1CN. As a result, despite the
great similarity in absorption spectrum between 2OM
(32) (a) Rurack, K.; Resch-Genger, U.; Bricks, J . L.; Spieles, M.
Chem. Commun. 2000, 2103-2104. (b) Rurack, K.; Bricks, J . L.; Schulz,
B.; Maus, M.; Reck, G.; Resch-Genger, U. J . Phys. Chem. A 2000, 104,
6171-6188. (c) Rurack, K.; Danel, A.; Rotkiewicz, K.; Grabka, D.;
Spieles, M.; Rettig, W. Org. Lett. 2002, 4, 4647-4650.
(33) (a) Walkup, G. K.; Burdette, S. C.; Lippard, S. J .; Tsien, R. Y.
J . Am. Chem. Soc. 2000, 122, 5644-5645. (b) Hirano, T.; Kikuchi, K.;
Urano, Y.; Higuchi, T.; Nagano, T. J . Am. Chem. Soc. 2000, 122,
12399-12400. (c) Burdette, S. C.; Walkup, G. K.; Spingler, B.; Tsien,
R. Y.; Lippard, S. J . J . Am. Chem. Soc. 2001, 123, 7831-7841. (d) Ojida,
A.; Mito-oka, Y.; Inoue, M.; Hamachi, I. J . Am. Chem. Soc. 2002, 124,
6256-6258.
(29) (a) Ho, K.-Y.; Yu, W.-Y.; Cheung, K.-K.; Che, C.-M. Chem.
Commun. 1998, 2101-2102. (b) Ho, K.-Y.; Yu, W.-Y.; Cheung, K.-K.;
Che, C.-M. J . Chem. Soc., Dalton Trans. 1999, 1581-1586.
(30) Herbich, J .; Grabowski, Z. R.; Wo´jtowicz, H.; Golankiewicz, K.
J . Phys. Chem. 1989, 93, 3439-3444.
(31) Miura, T.; Urano, Y.; Tanaka, K.; Nagano, T.; Ohkubo, K.;
Fukuzumi, S. J . Am. Chem. Soc. 2003, 125, 8666-8671.
(34) Munkholm, C.; Parkinson, D.-R.; Walt, D. R. J . Am. Chem. Soc.
1990, 112, 2608-2612.
J . Org. Chem, Vol. 69, No. 10, 2004 3523

Yang et al.
and 2H, the fluorescence spectrum of 2OM is relatively
broader with a maximum (440 nm) at longer wavelength
than those of 1H (415 nm) and 2H (385 nm). These
features indicate that the excited states of 2 have a large
ICT character and the direction of transition moment in
2 is different from that in 1. In the presence of Zn(II),
the absorption spectra of 2 shift to either blue (2CN and
2NO) or red (2H and 2OM), resulting in similar absorp-
tion maxima for 2/Zn(II). Like the cases of 1, the ICT
fluorescence of 2H and 2CN is broadened and mostly
quenched (85-99%). The ICT fluorescence of 2OM is also
largely reduced (95%) by Zn(II), but the residual fluo-
rescence maximum is significantly red-shifted, from 440
to 457 nm. In views of the D-A pretwisted structures of
2/Zn(II), the formation of an weakly fluorescent TICT
state might account for these results.3
The frontier orbitals of trans-stilbene have also been
investigated and compared with those of dpaH/ZnCl2
(Figure 6). Like the case of DPhAS vs dpaH/ZnCl2, both
the AM1-calculated HOMO and LUMO of trans-stilbene
lie above those of dpa/ZnCl2, respectively, indicating that
the process of PET could also take place in 1H/ZnCl2.
Therefore, a full account of the mechanism of Zn(II)-
induced fluorescence quenching for stilbenes 1 should
include (1) the ground-state π-deconjugation between the
dpa and the stilbene groups that interrupts the ICT
process in the aminostilbene fluorophore and (2) the
occurrence of PET from the stilbene donor to the dpa/
Zn(II) acceptor that presumably results in a nonemissive
radical ion pair.
responses. The participation of the PET process in the
latter but not the former cases is apparently responsible.
Since a reduction of fluorescence quantum yield is often
observed for dpa-substituted arenes upon interacting
with Zn(II) or other metal ions,12,13 a similar PET process
might be operative in some of these cases. Both the
structural and electronic aspects elucidated herein should
shed light on the future design of new fluoroionophores
and light-emitting materials based on dpa-incorporated
aromatic systems.
Exp er im en ta l Section
Meth od s. The X-ray crystal structures were determined at
room temperature. Intensity data were collected in 1315
frames with increasing ω (0.3° per frame) and corrected for
Lp and absorption effects, using the SADABS program. The
structures were solved by direct methods. Structural param-
eters were refined based on F2. All calculations were performed
by using SHELXTL programs. All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were assigned ideal-
ized locations and given isotropic thermal parameters 1.2×
the thermal parameter of the carbon atoms (but 1.5× for the
hydrogens in methyl groups) to which they were attached.
UV-vis absorption spectra and corrected fluorescence spectra
were recorded at room temperature. A N2-bubbled solution of
anthracene (Φf ) 0.27 in hexane)36 was used as a standard
for the fluorescence quantum yield determinations of 1-3 in
acetonitrile under N2-bubbled conditions with solvent refrac-
tive index correction. The optical density of all solutions was
about 0.1 at the wavelength of excitation, and an error of (10%
is estimated for the fluorescence quantum yields. Fluorescence
sensing measurements were performed in 1 × 10-5 M aceto-
nitrile solutions in all cases. The fluorescence four-wall cuvette
was charged with 3 mL of 1-3 and a magnetic stir bar.
Aliquots of freshly prepared Zn(II) solution (0.01 M of Zn(ClO4)2‚
6H2O in acetonitrile) were added at the prescribed increments.
Solutions were allowed to equilibrate for 15 min before taking
the measurement. Experimentation with longer equilibration
times did not produce noticeable differences.
Con clu d in g Rem a r k s
This work has provided insights into the Zn(II)-
complexation effect on the structures and fluorescence
properties of the dpa-derived stilbene probes 1 based on
systematic studies of 1-3 in the absence and the presence
of Zn(II). The X-ray crystal structures of dpa-substituted
arenes 1 and 2 and their ZnCl2 complexes have shown
that Zn(II)-complexation results in either the “planar”
or the “twisted” structure with a preference for the latter,
presumably due to a better conjugation interaction in the
dpa/Zn(II) vs the arylamino group. When the aryl group
possesses a sufficiently strong push-pull character, the
planar form would become more favorable. However, no
matter which form is adopted, the two N-pyridyl groups
always become much less π-conjugated with the aryl
group on going from the free ligands to the complexes. It
should be noted that, unlike the cases of free ligands,
structural relaxation of the complexes toward a more
π-conjugated conformation in the excited state will be
small or negligible due to the rigid and pretwisted
structure of the dpa/Zn(II) group. While such a structural
perturbation by Zn(II) can directly interrupt the amino
conjugation effect23 and thus reduce the ICT fluorescence
efficiency of 1, the concomitant electronic perturbation
by triggering the PET process should be the main origin
for the observed fluorescence quenching for 1DP A. The
occurrence of intramolecular PET is in principle facili-
tated by the deconjugation process between the electron
donor and acceptor.3 When compared with the monoaza-
crown-derived stilbene probes,5,35 the dpa amino nitrogen
atom in 1 does not directly participate in the binding of
metal ions, but the latter cases show much larger spectral
Ma ter ia ls. Solvents for organic synthesis were reagent
grade or HPLC grade but all were HPLC grade for spectra
and quantum yield measurements. All other compounds were
purchased from
a commercial source and were used as
received. Compounds 1DMA,5 1H,9 2H,37 4Br ,38 4CN,39 5CN,39
5Br ,38 6,40 and 75 have been previously reported, and our
melting points and/or 1H NMR spectra conform to the litera-
ture values. The characterization data for the other compounds
are provided in the Supporting Information. Typical synthetic
procedures are as follows:
Gen er a l P r oced u ces of Wittig Rea ction for Su bsti-
tu ted Stilben es. A mixture of 5-25 mmol of phospohonium
halide salt and 4-substituted benzaldehyde (0.9-1.0 equiv) and
0.5 g of tetrabutylammonium iodide in 20-50 mL of methylene
chloride in a round-bottom flask was slowly added to 10-15
mL of a 50% (w/w) aqueous solution of potassium carbonate.
In some cases the solution turned pink during the addition of
base. The solution was then stirred at ambient temperature
overnight. The organic layer became transparent at the top
and the alkaline aqueous layer became turbid at the bottom
(35) Dumon, P.; J onusauskas, G.; Dupuy, F.; Pe´e, P.; Rullie`re, C.;
Le´tard, J .-F.; Lapouyade, R. J . Phys. Chem. 1994, 98, 10391-10396.
(36) Dawson, W. R.; Windsor, M. W. J . Phys. Chem. 1968, 72, 3251-
3260.
(37) Mann, F. G.; Watson, J . J . Org. Chem. 1948, 13, 502-529.
(38) Everard, K. B.; Kumar, L.; Sutton, L. E. J . Chem. Soc. 1951,
2807-2815.
(39) Hanna, P. E.; Gammans, R. E.; Sehon, R. D.; Lee, M.-K. J . Med.
Chem. 1980, 23, 1038-1044.
(40) Fuson, R. C.; Cooke, H. G., J r. J . Am. Chem. Soc. 1940, 62,
1180-1183.
3524 J . Org. Chem., Vol. 69, No. 10, 2004

N,N-Bis(2-pyridyl)amino-Substituted Arenes
of the flask. The two layers were separated and the aqueous
layer was extracted with methylene chloride three times. The
combined organic layer was washed with distilled water or
brine and then dried over anhydrous MgSO4. The solid residue
after evaporation of the solvent was chromatographied with
1:2 ethyl acetate:hexane as eluent to remove the side product
triphenylphosphine oxide. The resulting mixture of trans and
cis isomers was recrystallized in CHCl3/MeOH to afford the
trans isomer. The filtrate that contains mainly the cis isomer
was added with a catalytic amount of I2 for cis f trans
isomerization. The precipitate of trans isomer was filtered and
then recrystallized in CHCl3/MeOH. The yield was determined
based on the total amount of trans isomer obtained.
Gen er a l P r oced u r es for Nitr o f Am in o Tr a n sfor m a -
tion s. A heterogeneous mixture of 1-5 mmol of nitrostilbene
and 5 equiv of SnCl2‚2H2O in 15-40 mL of anhydrous ethanol
was heated at 70 °C under nitrogen for 18 h. The solution was
allowed to cool and then poured into 15-30 g of ice. The
solution was made slightly basic by addition of 5% NaHCO3 (aq)
followed by extraction with ethyl acetate. The organic layer
was washed with brine twice and then dried over magnesium
sulfate. Evaporation of the solvent provided the crude product
of aminostilbene. Further purification was performed by
column chromatography.
Gen er a l P r oced u r es for P d -Ca ta lyzed Am in a tion Re-
a ction s. As illustrated by the formation of compound 1Br ,
compound 5Br (0.20 g, 0.73 mmol), 2-bromopyridine (157 µL,
1.6 mmol), NaOBut (0.20 g, 2.04 mmol), DPPF (0.032 g, 0.058
mmol), and Pd2(dba)3 (0.022 g, 0.024 mmol) in 3 mL of
anhydrous toluene under argon weres heated at 100 °C for 24
h. The solution was cooled and then 25∼30 mL of CH2Cl2 was
added. The insoluble residue was filtered off and the filtrate
was concentrated under reduced pressure to afford the crude
product. Further purification was performed by column chro-
matography, using a mixed solvent ethyl acetate/hexane (1:2)
as the eluent to provide the light yellow solid with a yield of
57%.
Syn th esis of 1-(N,N-Bis(2-p yr id yl)a m in o)-4-n itr oben -
zen e (2NO). A mixture of 4-nitroaniline (1.38 g, 10 mmol),
2-bromopyridine (2.93 mL, 30 mmol), KOH (1.80 g, 32 mmol),
and CuSO4 (0.052 g, 0.032 mmol) in a 50-mL flask was heated
under argon at 180 °C for 16 h. After the reaction mixture
was cooled to ambient temperature, dichloromethane and
water were added to dissolve the solids. The organic layer was
dried over anhydrous MgSO4. The filtrate was concentrated
under reduced pressure. Column chromatography with ethyl
acetate/hexane (1:1) as eluent afforded 2NO (11% yield).
Syn th esis of tr a n s-4-(N,N-Diph en ylam in o)-4′-(h ydr oxy-
m eth yl)stilben e (3OH). To a solution of 7 (1.28 g, 3.16 mmol)
in 30 mL of THF at 0 °C was added slowly LAH (0.24 g, 6.32
mmol). The mixture was stirred for 2 h and then cold water
was added dropwise to quench any unreacted LAH. The THF
was removed under reduced pressure and the residue was then
dissolved in ethyl acetate and washed with brine. The organic
layer was dried over anhydrous MgSO4 and the filtrate was
concentrated under reduced pressure. Purification was per-
formed by column chromatography (ethyl acetate:hexane )
1:5) and followed by recrystallization in CH2Cl2/hexane (0.63
g, 91% yield).
Syn th esis of tr a n s-4-(N,N-Diph en ylam in o)-4′-(N-ph th al-
im id om eth yl)stilben e (3P I). A mixture of 3OH (0.28 g, 0.75
mmol), triphenylphosphine (0.30 g, 1.13 mmol), and phthal-
imide (0.17 g, 1.13 mmol) in 4.7 mL of THF was added
dropwise to the THF solution (0.52 mL) of diisopropyl azadi-
carboxylate (DIAD) (0.23 g, 1.14 mmol). The mixture was
stirred at room temperature for 16 h and the solvent was
removed under reduced pressure. Column chromatography
with ethyl acetate/hexane (1:6) afforded the desired product
(0.63 g, 78% yield).
Syn th esis of tr a n s-4-(Am in om eth yl)-4′-(N,N-d ip h en yl-
a m in o)stilben e (3NH). A mixture of 3P I (0.30 g, 0.59 mmol)
and hydrazine (2.0 mL, 5.92 mmol) in 98 mL of ethanol was
refluxed for 30 min. The resulting precipitate was filtered off
and the filtrated was concentrated under reduced pressure.
The crude product was recrystallized in CH2Cl2/hexane to
afford the desired product (0.28 g, yield 88%).
Typ ica l P r oced u r es for t h e P r ep a r a t ion of Zn Cl2
Com p lexes. A mixture of dpa-substituted arenes (0.05 g) and
ZnCl2 (1.5 equiv) in 10 mL of acetonitrile or methanol was
heated under argon at 90 °C for 2 h. The solution was cooled
and then the solvent was removed under reduced pressure.
The residue was washed with dichloromethane and distilled
water and then recrystallized from methanol/acetonitrile to
afford colorless crystals with a yield of 50%.
Ack n ow led gm en t. We thank the UST and the
National Science Council of Taiwan, ROC, for financial
support (NSC 91-2113-M-008-011 and NSC 92-2113-M-
008-006), Professor S.-L. Wang (NTHU) for assistance
in X-ray crystallography, and the reviewers for helpful
comments.
Su p p or tin g In for m a tion Ava ila ble: Crystal structures
and structure refinement data, Cartesian coordinates of AM1-
optimized structures, and detailed characterization data and
1H and 13C NMR spectra of new compounds; X-ray experimen-
tal details (CIF). This material is available free of charge via
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