Crowded bis-(M-salphen) [M = Pt(II), Zn(II)] coordination architectures: Luminescent properties and ion-selective responses

Article abstract of DOI:10.1021/ic400692x

For binuclear luminescent host systems, cooperativity between metal-organic moieties becomes feasible with regards to photophysical properties and sensing behavior. A new class of conformationally rigid binuclear platinum(II) and zinc(II) complexes bearing tetradentate aromatic Schiff base (salphen) ligands with limited rotational freedom has been prepared and characterized, and the molecular structure of a (Pt-salphen)₂ derivative has been determined by X-ray crystallography. Their UV-vis absorption and emission properties have been investigated and are tentatively ascribed to different excited states depending on the metal and the extent of intramolecular π-stacking interactions. Colorimetric and phosphorescent responses by the bis-Pt(II) complexes in the presence of selected metal ions have been observed. The nature of the host-guest interactions has been examined by quantitative binding studies, mass spectrometry and DFT calculations, and through comparisons with control complexes.

Full text of DOI:10.1021/ic400692x

Article
pubs.acs.org/IC
Crowded Bis-(M-salphen) [M = Pt(II), Zn(II)] Coordination
Architectures: Luminescent Properties and Ion-Selective Responses
Wah-Leung Tong, Shek-Man Yiu, and Michael C. W. Chan*
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
S
* Supporting Information
ABSTRACT: For binuclear luminescent host systems, cooperativity
between metal−organic moieties becomes feasible with regards to
photophysical properties and sensing behavior. A new class of
conformationally rigid binuclear platinum(II) and zinc(II) complexes
bearing tetradentate aromatic Schiff base (salphen) ligands with limited
rotational freedom has been prepared and characterized, and the
molecular structure of a (Pt-salphen)2 derivative has been determined
by X-ray crystallography. Their UV−vis absorption and emission
properties have been investigated and are tentatively ascribed to
different excited states depending on the metal and the extent of
intramolecular π-stacking interactions. Colorimetric and phosphorescent responses by the bis-Pt(II) complexes in the presence of
selected metal ions have been observed. The nature of the host−guest interactions has been examined by quantitative binding
studies, mass spectrometry and DFT calculations, and through comparisons with control complexes.
Chart 1. Previously Developed Shape-Persistent Mono- and
Binuclear Pt(II) Host Complexes
INTRODUCTION
■
Researchers have judiciously examined molecular phenomena
such as restricted rotation, rigidization, and supramolecular
attraction in crowded and shape-persistent aromatic frameworks
in search of desirable recognition and signaling,1 electronic,2 and
catalytic properties.3 While reports on fluorescent systems have
appeared,4 the incorporation of luminescent metal−organic and
phosphorescent components, which can confer attractive
characteristics such as well-defined coordination geometries
and visible-light signaling, have generally been overlooked. The
emergence of metal-Schiff base complexes as important
coordination and supramolecular motifs stems from their
structural diversity coupled with functionality.5 Considerable
efforts have been devoted to the construction of Schiff base host
architectures,6 while their ion binding/transport,7 chiral
recognition,8 and materials9 applications, plus their aggregation
into supramolecular assemblies and nanostructures, have been
investigated.10,11
We are engaged in the development of shape-persistent
phosphorescent host structures12 and materials13 containing
environmentally responsive Pt(II) reporting units. For example,
a new class of congested mononuclear Pt(II) ditopic frameworks
was designed and synthesized, and investigations into their
luminescent responses to amino acids revealed the ability to
differentiate aminothiols, as well as preferential binding of the
more sterically hindered cysteine over homocysteine to host M1
(Chart 1).14 For binuclear host complexes, cooperativity
between the metal−organic moieties becomes feasible with
regards to photophysical and sensing characteristics. We recently
described a series of cofacial (Pt-salphen)2 (H2salphen = N,N′-
bis(salicylidene)-1,2-phenylenediamine) host assemblies anch-
ored at each diimine phenyl ring to a rigid backbone component
(xanthene (B1 in Chart 1), biphenylene, or dibenzofuran), and
their colorimetric and luminescent (in some case selective)
responses to metal ions.15 The employment of square planar
Pt(II) luminophores can engender attractive properties such as
tunable excited states that are highly sensitive to the micro-
environment,16 and Pt-salphen derivatives have been reported to
exhibit high quantum efficiencies under ambient conditions.17
Importantly, these frameworks were designed so that variations
in molecular conformations are effectively limited to axial
rotations of the Pt-salphen units. Nevertheless, such rotations
can alter the intermetallic separations and possible intra-
molecular π-stacking interactions, as well as the distances and
geometry between the pairs of O(salphen) donor atoms and
hence the dimensions of the potential O4-binding pocket.
In this work, our aims are (a) to rearrange the host architecture
by anchoring the Pt-salphen moieties at a phenoxy ring, to
modify rotational flexibility and hence the extent of π-stacking
Received: March 20, 2013
© XXXX American Chemical Society
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Scheme 1. Synthesis of Binuclear Complexes 1−4 and Structures of Control Complexes 5−10
interactions and relative geometry of the cofacial luminophore
units (and the resultant binding pocket), and (b) to probe the
impact of these structural changes upon Mn+-binding character-
istics. The luminescent and colorimetric responses of these di-
Pt(II) frameworks, and their zinc(II) precursors, to metal ions
have been investigated. In addition, their binding behavior with
regards to selected metal ions has been examined by ESI−MS
and DFT calculations, and comparatively studies with control
complexes (bearing t-butyl substituents ortho to the O(salphen)
atoms, and mononuclear analogues) have been performed, in
order to rationalize the photophysical changes and offer insight
toward a viable binding mechanism.
and 7) and Pt (6 and 8) derivatives, which serve as control
compounds for comparative studies, were similarly synthesized
using the Zn-templated and transmetalation methods, respec-
tively. The parent Zn- (9)21 and Pt-salphen (10)17 complexes
were prepared according to literature reports. Complexes 1−8
were characterized by NMR spectroscopy, ESI−MS, and
elemental analysis. Studies by ESI−MS revealed parent peak
clusters with m/z values that correspond closely to the respective
calculated isotopic patterns (Supporting Information).
Single crystals of 4, 7, and 8 were obtained by slow evaporation
of CH2Cl2/CH3CN, CH2Cl2, and ethyl acetate solutions,
respectively, and their molecular structures have been
determined by X-ray crystallography (Supporting Information).
Regarding the binuclear complex 4 (Figure 1), each platinum
atom displays square planar geometry with minimal deviation of
the salphen rings from the respective Pt(N2O2) planes. Mean
Pt−N and Pt−O distances of 1.957 and 1.990 Å, respectively, are
observed, which resemble those previously reported for Pt Schiff
base complexes.17 The two Pt-salphen moieties are linked to the
xanthene bridge in a syn cofacial fashion, with dihedral angles
(aryl-to-aryl) of 52.7° and 53.2° to the xanthene unit. The
intramolecular interplanar separation (defined as mean separa-
tion between Pt and adjacent N2O2 plane) of 3.67 Å, plus the
non-coplanarity of the N2O2 plane (angle = 12.6°), signify the
absence of intramolecular π−π interactions. This can be
attributed to the steric impact of the t-butyl groups, although
crystal packing effects should not be disregarded. Intramolecular
Pt···Pt interactions22 are also absent, as revealed by the Pt1···Pt2
distance of 4.855(2) Å. Such repulsive effects are avoided
intermolecularly by a head-to-tail packing arrangement to afford
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. The binu-
clear complexes 1−4 (Scheme 1) were prepared from the 4,5-
dibromoxanthene precursor by substitution with bis-boronic acid
(L1), followed by Pd-catalyzed coupling with 5-bromosalicyl-
aldehyde at elevated temperature to give the ligand precursor L2.
Next, the condensation between L2 and the “semi-Schiff base”
(L3 or L4) was attempted, but isolation and purification of the
desired bis-salphen ligand, which would allow direct metalation
reactions, proved problematic. Instead, a Zn-templated, one-pot
procedure18 was adopted to afford the binuclear (Zn-salphen)2
complexes 1 and 3, which subsequently served as synthetic
precursors to the Pt2 complexes 2 and 4 through transmetalation
reactions;19,20 PtCl2 was preferred to K2PtCl4 as the metal source
because the former is known for faster reactivity. The presence of
tert-butyl substituents in 3 and 4 confers enhanced solubility in
organic solvents compared with 1 and 2. The mononuclear Zn (5
B
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Figure 1. Molecular structure of 4. Top: Perspective view (30% probability ellipsoids). Selected bond lengths (Å) and angles (deg): Pt1−O1 1.983(2),
Pt1−O2 2.000(2), Pt1−N1 1.966(3), Pt1−N2 1.950(3), Pt2−O4 1.981(2), Pt2−O3 1.996(2), Pt2−N4 1.962(3), Pt2−N3 1.952(3), Pt1···Pt2
4.855(2); O1−Pt1−O2 85.52(10), N1−Pt1−O1 95.28(11), N2−Pt1−N1 83.36(12), N1−Pt1−O2 179.13(9). Bottom: Packing diagram (tBu groups
on backbone omitted for clarity) showing intermolecular π−π separation.
intermolecular π-stacking interactions16a,23 (interplanar separa-
tion = 3.38 Å between parallel N2O2 planes; bottom of Figure 1).
Like 4, the molecular structure of the mononuclear congener 8
displays a head-to-tail crystal packing array, as well as interplanar
separations of around 3.3 Å between parallel N2O2 planes that are
indicative of intermolecular π-stacking (Supporting Informa-
tion). The square planar geometry at the Pt atom is
unremarkable in 8, and deviation of the salphen rings from the
Pt(N2O2) plane is only slight.
Complex 7 was recrystallized from CH2Cl2 to give a dimeric
structure (Figure 2), in which an O(salphen) atom in one
molecule acts as a bridge and coordinates to the Lewis acidic Zn
center of the adjacent molecule (Zn1−O1i 2.064(1) Å) to give a
Zn2O2 parallelogram, with O1−Zn1−O1i and Zn1−O1−Zn1i
angles of 82.94(4)° and 97.11(4)° respectively. The reliance of
molecular structure upon the solvent used for recrystallization
has been reported for Zn-Schiff base complexes.11,24 Within each
monomeric unit, a distorted tetragonal pyramidal geometry25 is
evident for the Zn atom, and the Zn−O bond for the nonbridging
O atom (Zn1−O2 1.944(1) Å) is noticeably shorter than that for
the bridging O atom (Zn1−O1 2.042(1) Å). An antiparallel
arrangement is adopted, so as to minimize repulsion between the
t-butyl substituents. The dimeric molecular structure of 7
emphasizes the coordinating ability of the O(salphen) moieties
in such complexes. These structural observations may be useful
for rationalizing the experimental findings (including lumines-
cent behavior) in this work.
Absorption and Emission Spectroscopy. The photo-
physical properties of the Pt (Figure 3 and Table 1) and Zn
(Figure 4 and Supporting Information) complexes have been
studied by UV−vis absorption and emission spectroscopy
(photophysical data for 921 and 1017c have been reported
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Figure 2. Perspective view of 7 (30% probability ellipsoids). Selected bond lengths (Å) and angles (deg): Zn1−Zn1i 3.0776(4), Zn1−O1i 2.064(1),
Zn1−O1 2.042(1), Zn1−O2 1.944(1), Zn1−N1 2.083(1), Zn1−N2 2.070(1); Zn1−O1−Zn1i 97.11(4), O1−Zn1−O1i 82.89(4), O1−Zn1−N1
87.29(5), O1−Zn1−N2 165.75(5), O2−Zn1−N1 147.27(5), N2−Zn1−N1 78.73(5).
Figure 3. Normalized emission spectra (inset: low-energy UV−vis
Figure 4. Normalized emission spectra (inset: low-energy UV−vis
absorption bands) of Pt-salphen complexes in CHCl3 (10−5 M) at 298
K.
absorption bands) of Zn-salphen complexes in CH3CN (10−5 M) at 298
K.
Table 1. Photophysical Data for (Pt-salphen)2 Complexes 2 and 4 (and Mononuclear Controls 6 and 8, Respectively)
fluid: λmax/nm (τ/μs); Φ
298 K 77 K
solid: λmax/nm (τ/μs)
298 K 77 K
650 (0.056)
a
c
compd
λmax/nm (ε/dm3 mol−1 cm−1
)
λex/nm
a
b
2
319 (55 490), 359 (47 150), 477 (7400), 521
(9830), 546 (8830)
521
635 (2.1); 0.044 [630 ]
590, 630 (max, 12), 683
665 (max, 1.5), 724
658 (max, 4.7), 723
a
4
6
8
321 (59 100), 370 (49 000), 485 (11 280), 530
(10 960), 557 (10 190)
530
515
532
610, 649 (max, 3.6), 701;
0.086 [603, 643 (max),
626 (max, 9.4), 687
648 (max, 0.16), 703
659 (0.096)
b
699]
a
291 (46 070), 366 (39 910), 385 (47 070), 470
(8890), 515 (7880), 546 (9210)
590, 631 (max, 4.5), 683;
0.17 [578, 625 (max),
602 (max, 11), 660
613 (max, 9.0), 674
676 (max, 2.4), 730
653 (max, 4.1), 713
b
675]
a
292 (39 310), 371 (34 330), 389 (39 070), 483
(8560), 535 (7030), 562 (7800)
612, 652 (max, 3.3), 703;
656 (max, 0.66), 705
0.11
a
b
c
n
In CHCl3. In CHCl3/CH3CN (1/1). In BuCN.
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previously). The UV−vis absorption spectra of the Pt complexes
contain intense bands at λmax < 390 nm (ε > 2.5 × 104 dm3 mol−1
cm−1) that are attributable to intraligand 1(π−π*) transitions of
the salphen ligands. The less intense absorption bands in the
visible region (inset of Figure 3) are assigned to O(p)/Pt(d)→
π*(diimine) transitions.17,26 When compared with the parent
salphen complex (10), the Pt derivatives in this work display red-
shifted absorptions at λmax 470−557 nm (ε = (7−11) × 103 dm3
mol−1 cm−1), which can be ascribed to the electronic effects of
the phenoxy substituents (aryl, tBu).
The Pt complexes in this study display red emission (λmax
631−652 nm) in CHCl3 at 298 K, which are assigned to mixed
triplet O(p)/Pt(d)→π*(diimine) excited states (Figure 3). In
general, the fluid emission undergoes a blue-shift in more polar
medium as a consequence of the polar nature of the electron-rich
oxygen donors in the ground state.17,26 For example, the
emission maximum for 2 at 637 nm in toluene is blue-shifted to
631 and 625 nm in CH2Cl2 and DMSO, respectively (Supporting
Information). Like the lowest-energy absorptions, the emissions
of 2, 4, 6, and 8 are red-shifted from that of the parent Pt-salphen
(10; λmax 617 nm). Hence, the phenyl/xanthene substituents on
the phenoxy group in 2 and 6 (λmax 634 and 631 nm respectively)
apparently destabilize the HOMO, while the additional red-shift
for 4 and 8 (λmax 649 and 652 nm, respectively) can similarly be
attributed to further HOMO destabilization due to the electron-
donating effect of the t-butyl groups.
display structureless emission in CH3CN at 298 K (Figure 4),
which are assigned to 1IL excited states with fluorescence
lifetimes in the subnanosecond regime. Complexes 1, 3, 5, and 7
emit at λmax 530−538 nm, which is red-shifted from the parent
Zn-salphen 9 due to greater π-conjugation and substitution of the
respective salphen ligands. The concentration dependence of the
emission of 7 was investigated in CHCl3 (Supporting
Information); the peak maximum appears at 545 nm at
concentrations up to 10−4 M, but undergoes a slight red-shift
to 550 nm at 1 × 10−3 M. This behavior brings to mind the
dimeric crystal structure of 7 and may be indicative of
luminophore aggregation at relatively low concentrations in
CHCl3 (indeed, a peak cluster corresponding to the calculated
isotopic pattern for [(7)2 + H]+ was detected by ESI−MS;
Supporting Information). The solid-state emissions of the Zn
complexes are structureless at 298 and 77 K.
Photophysical Responses to Metal Ions: Investigation
of Binding Process and Comparative Studies. The binding
behavior of the binuclear complexes in the presence of metal ions
(in the form of perchlorate salts) has been investigated by
spectrophotometric titrations. For the (Zn-salphen)2 complex 1,
varying emission responses without selectivity were obtained
(Supporting Information: enhancement for Mg2+, Ca2+, Cd2+
(partial for La3+); minor quenching for Cu2+, Hg2+, Pb2+;
minimal or no response for monovalent metals and Zn2+).
Bearing in mind the established tendency of Zn-salphen
complexes to undergo transmetalation processes,6e,19,20 as
observed in this work, and the reported facile formation of
Mg(II), Ca(II), and Cd(II) Schiff base derivatives under ambient
conditions,28 the likelihood that 1 engages in alternative
reactions29 (other than binding) with multivalent cations
therefore renders such Zn complexes unsuitable for further
photophysical studies.
Although the emission maxima for 2 and 6 are similar, 2
displays a broader, less structured emission (full width at half-
maximum = 1940 cm−1 (υ604 − υ684), compared with 1320 cm−1
(υ608 − υ661) for 6), which is concentration-independent and
tentatively ascribed to weak π-stacking interactions within the
cofacial (Pt-salphen)2 moiety. Such intramolecular interactions
are known to cause self-quenching,1f,12a,22a and this is reflected by
the lower luminescence quantum yield for 2 (Φ = 0.044 in
Interesting photophysical responses were observed for the Pt2
host 2 in the presence of various metal ions (1.0 equiv) in
CH3CN/CHCl3 (1/1). Upon addition of Pb2+, minor emission
enhancement was apparent and accompanied by a significant
blue-shift (λmax 630 to 605 nm). For Hg2+ and Cu2+, the emission
was quenched with minor blue-shifts in peak maximum, while
minimal or no responses were detected for Mg2+, Ca2+, Zn2+,
Cd2+, La3+, and all monovalent metals ions (Figure 5).
Competition experiments to study the ratiometric response of
2 to Pb2+ ions were conducted, revealing a degree of selectivity
t
CHCl3, compared with 0.17 for 6). On the contrary, the Bu-
substituted derivatives 4 and 8 show very similar emission
profiles (in terms of energy and band shape) and only a minor
difference in quantum yield, indicating that intramolecular π-
interactions are absent presumably due to the repulsive effects of
the tBu groups. This conclusion is consistent with the observed
large intramolecular separation and non-coplanarity of the Pt-
salphen units in the crystal structure of 4.
The 77 K glassy emissions of Pt complexes in nBuCN are blue-
shifted (due to the Franck−Condon effect) and more structured
compared with their respective solution emissions at 298 K, and
excimeric emissions are not observed at a concentration of 10−5
M. The glassy emissions of 2 and 4 are red-shifted and broader
compared with their respective mononuclear analogues 6 and 8
(Supporting Information), which may be tentatively attributed to
a contraction of the rigid xanthene backbone possibly bringing
the two tethered Pt-salphen moieties into closer proximity. The
solid-state emissions are generally red-shifted and become less
structured than their fluid emissions, presumably because of
enhanced intra- (for 2 and 4) and/or intermolecular π-stacking
interactions.
The UV−vis absorption spectra of the Zn complexes in
CH3CN show intense bands at λ < 350 nm (ε > 4 × 104 dm3
mol−1 cm−1) that are attributed to intraligand 1(π−π*)
transitions, while the low-energy absorptions at λmax 375−500
nm (ε = (1−4) × 104 dm3 mol−1 cm−1; inset of Figure 4) are
tentatively assigned to O(p)→π*(imine) intraligand charge
transfer (ILCT) transitions, in accordance with previous studies
of Zn-Schiff base complexes.27 The Zn complexes in this work
Figure 5. Change in emission intensity at 605 and 630 nm for 2 (2.0 ×
10−5 M; λex 501 nm) upon addition of Mn+ ions (1.0 equiv) in CH3CN/
CHCl3 (1/1).
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toward Pb2+ with partial interference from Cu2+ and Hg2+
(Supporting Information).
Pb2+-binding by 2 was obtained (Supporting Information).
Moreover, the binding constant30 was calculated from the
emission intensity at 605 nm as a function of [Pb2+], and the log
K was determined to be 6.30 0.35 in CH3CN/CHCl3 (1/1); a
comparison can be made to the closely related xanthene-linked
(Pt-salphen)2 complex B1 (Chart 1) in CH3CN/CH2Cl2 (1/1;
log K = 6.63 0.06).15 Quantitative spectroscopic titrations of 2
with Pb2+ in coordinating THF were performed to evaluate the
relevance of the proposed binding by O(salphen) moieties
(Supporting Information). Indeed, more subtle UV−vis and
emission changes were detected upon addition of 5.0 equiv of
Pb2+, and a noticeably smaller binding constant (log K = 3.12
0.27) was determined in THF than CH3CN/CH2Cl2. These
results signify that the THF solvent can interfere with the Pb2+-
binding process, although there are many contributing factors
(such as solvation effects). The binding process for 2 was further
investigated using ESI mass spectrometry. A major signal in the
ESI mass spectrum of 2 with 1.0 equiv of Pb(ClO4)2 in CH3CN/
CHCl3, using zero declustering potential, was a cluster at m/z
772.1 with peak separations of 0.5 amu, which is in excellent
agreement with the calculated isotopic pattern for the [2 + Pb]2+
(1:1) species.31 Furthermore, a cluster at m/z 1643.4
corresponding closely to [2 + Pb + ClO4]+ with 1 amu
separations was also prominent (Figure 7), implying that when
Quantitative titrations have been performed to probe the
spectroscopic response of 2 toward Pb2+ (top of Figure 6). The
Figure 6. Quantitative emission (inset: expanded UV−vis) titrations of
t
2 (top) and Bu-substituted analogue 4 (bottom; both 2.0 × 10−5 M)
upon addition of Pb2+ in aerated CH3CN/CHCl3 (1/1).
UV−vis spectral changes became saturated after addition of ca. 1
equiv of Pb2+; the absorption band at λmax 516 nm was blue-
shifted to λmax 463 nm and a well-defined isosbestic point at 501
nm was observed, which was used as the excitation wavelength
(λex) for emission studies. In the emission titration, incremental
changes were detected for up to 1.0 equiv of Pb2+, and this was
followed by minor changes until saturation occurred for 1.5
equiv; hence, the structureless red emission at λmax 630 nm was
blue-shifted to λmax 605 nm with partial enhancement (aerated
quantum yield increased from 4.9 × 10−3 to 6.8 × 10−3), and the
Figure 7. ESI−MS of [2 + Pb + ClO4]+ in CH3CN/CHCl3 (1/1) using
zero declustering potential (top: calculated isotopic distributions).
bound by 2, the Pb2+ ion may also be coordinated by perchlorate.
At this juncture, consideration for the related crystal structures of
[{Ni(salen)}2·Ba(ClO4)2(thf)] (featuring two chelating per-
chlorate groups at Ba2+ plus O4-coordination by two
mononuclear Ni-salen moieties)32 and (NBu4)[{(bzq)Pt(C
CC6H4-4-CF3)2}2·Pb(ClO4)] (bzq = 7,8-benzoquinoline; the
Pb2+ ion is coordinated by a bidentate perchlorate group, plus
four acetylide moieties of two [(bzq)Pt(CCAr)2] units in a
[η1(CCAr)]4 binding mode),33 plus the structurally charac-
terized multidentate O-coordination of M(OAc)2 (M = Zn, Ni)
by the O(salphen) and O-alkyl moieties of alkoxy-substituted M-
salphen complexes,20 is warranted. In contrast, signals
aerated emission lifetime became longer (τ630 = 0.13 μs; τ605
=
0.30 μs). Intriguingly, the resultant emission maximum of 605
nm is higher in energy than that (625 nm) for the mononuclear
congener 6 (see discussion below).
Studies have been undertaken to gain insight into the binding
mechanism for 2. A Job’s plot indicating 1:1 stoichiometry for
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Figure 8. Perspective views of energy-minimized (Gaussian) calculated structure of 2 (top) and [2 + Pb + ClO4]+ (bottom). Color legend: gray = C,
blue = N, red = O, orange = Pt, green = Cl, and purple = Pb.
corresponding to analogous host−guest species were not
observed for 2 with Hg(ClO4)2 (1.0 equiv) under identical
conditions.
slightly (by 0.010 and 0.034 Å, respectively). The Pb−O
distances (2.469−2.510 Å) are comparable to literature reports
of Pb−O bonds,34,35 and the mean Pb···Pt distance is 3.397 Å.
These results suggest that guest-induced conformational changes
(i.e., axial rotation) for the Pt-salphen moieties in 2 can engender
a viable O4-binding site for a perchlorate-bound Pb2+ ion.
Comparisons with control complexes and further photo-
physical studies have been undertaken to rationalize the spectral
changes and binding mechanism for 2. Upon addition of Pb2+,
the emission λmax of 2 in CHCl3/CH3CN (1/1) blue-shifts to 605
nm, which is substantially higher in energy than that for the
mononuclear congener 6 (625 nm). In addition to the
consequence of breaking the weak intramolecular π-interactions
(to afford monomer-like emission), we ascribe the “extended”
blue-shift to the destabilization of the O(p)→π*(diimine)
transition arising from stabilization of salphen O(p) orbitals
upon Pb2+-coordination, in a manner akin to that predicted by
DFT calculations. Notably, this is entirely consistent with the
observed blue-shift for the lowest-energy absorption band of 2
with increasing [Pb2+] (top of Figure 6).
Repeated attempts to grow crystals of 2 (including in the
presence of various ratios of Pb(ClO4)2) for structural
elucidation have been unsuccessful. The structures of 2 and the
[2 + Pb + ClO4]+ species observed by ESI−MS have been
optimized by density functional theory (DFT) calculations. The
energy-minimized calculated structures were each confirmed to
be a minimum from vibrational frequency calculations (Figure 8;
see the Supporting Information for selected calculated bond
lengths). The two Pt-salphen planes in 2 are arranged in a cofacial
manner with dihedral angles (aryl-to-aryl) to the xanthene
backbone of ca. 45°. The interplanar separation [mean
Pt···(N2O2 plane) distance] of 3.59 Å and the angle of 2.1°
between the N2O2 planes indicate that weak π−π interactions are
plausible, which is consistent with the emissive properties, while
the contrast with the structural parameters of 4 (without
intramolecular π-interactions; corresponding separation and
angle of 3.67 Å and 12.6°, respectively) is clear. In the calculated
structure of [2 + Pb + ClO4]+, the Pb2+ ion is located in the center
of the O4-binding pocket and is bound by a bidentate perchlorate
group. Starting from the conformation of 2, the Pt-salphen planes
evidently undergo axial rotation with respect to the xanthene
unit, and the angle between the N2O2 planes increases to 61.7°,
thus ensuring that any weak π-interaction is broken. The Pb2+-
bound Cl−O bonds (mean 1.822 Å) are longer than the
nonchelating Cl−O bonds (mean 1.730 Å), while the mean Pt−
O (2.042 Å) and C−O (1.373 Å) bond lengths are elongated
A comparison between 2 and its tBu-substituted analogue 4 is
informative. Significantly, the addition of Pb2+ to 4 caused minor
quenching of the relatively structured emission at λmax 643 nm in
CH3CN/CHCl3 (1/1) without any shift in peak maximum
(bottom of Figure 6). The absence of intramolecular π-
interactions in 4 (as evident from the crystal structure and
t
emissive properties) is attributed to the influence of the Bu
groups, and it is highly plausible that these bulky substituents
adjacent to the O(salphen) moieties could sterically hinder the
G
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formation of the O4-cavity and possibly block the binding of
guests (notwithstanding their electronic effects). The lack of a
blue-shift for the UV−vis and emission bands of 4 with Pb2+ is
consistent with this and implies that coordination by the
O(salphen) groups does not occur. Regarding the emission
quenching, it is revealing to note that the responses of 4 and the
mononuclear controls 6 and 10 are very similar (Supporting
Information). Taken together, these observations signify that
quenching via metallophilic or spin−orbit interactions36 is the
conventional luminescent response of mononuclear Pt-salphen
to Pb2+, and tentatively suggest that the binding of Pb2+ by 2 takes
place inside the (Pt-salphen)2 cavity to produce emission
enhancement, while such binding behavior is prevented by the
tBu groups in 4.
The following points should be emphasized when attempting
to rationalize the observed photophysical changes: (a) a blue-
shifted emission may be caused by (i) destabilization of the
O(p)→π*(diimine) transition upon Pb2+-binding, and/or (ii)
disruption of weak π−π interactions within the (Pt-salphen)2
unit; (b) although Pb2+ ions can invariably quench the emission
of Pt-salphen luminophores, the observation of enhanced
emission intensity upon Pb2+-binding may result from (i)
reduction in intramolecular self-quenching due to weaker or
cleaved π−π interactions, and/or (ii) rigidization of the host
framework. We therefore propose that, as shown by DFT
calculations, the binding of Pb2+ within the O4-cavity of 2
necessitates axial rotation of the cofacial Pt-salphen moieties and
disruption of the weak intramolecular π−π interactions that give
rise to the emission at λmax 630 nm in CHCl3/CH3CN;
consequently, the reduction in self-quenching and increased
host rigidity would counterbalance the expected Pb2+-mediated
quenching to yield enhanced (or undiminished) emission at λmax
605 nm.
CONCLUSION
■
The Mn+-binding properties of shape-persistent (M-salphen)2
(M = Pt, Zn) complexes, which may be considered as
phosphorescent relatives of calix[4]arene, have been inves-
tigated. Differences in fluid photophysical properties are
attributed to the combined effects of the metal centers,
substituents on the salphen moieties, and to some extent
intramolecular π-stacking interactions. The cation-induced
photophysical responses of the Pt2 coordination frameworks
may be ascribed to host−guest interactions, the nature of which
is determined by the propensity of the xanthene-appended Pt-
salphen units to undergo axial rotation to create an O4-cavity that
is capable of (and indeed complementary for) binding selected
guests, in this case Pb2+. Evidence in support of the binding
mechanism has been gathered from ESI−MS experiments, DFT
calculations, and quantitative spectrophotometric titrations, plus
comparative studies with control complexes. The present
binuclear system is structurally versatile, and astute modifications
may lead to novel properties and applications.
EXPERIMENTAL SECTION
General Considerations. Solvents for syntheses (analytical grade)
were used without further purification, and all metalation reactions were
performed under a nitrogen atmosphere. Solvents for photophysical
■
1
measurements were purified according to conventional methods. H
NMR spectra were obtained on Bruker DRX 300 and 400 FT-NMR
spectrometers (ppm) using Me4Si as internal standard. ESI mass spectra
were measured on a Perkin-Elmer SCIEX API 365 mass spectrometer.
Elemental analyses were performed on a Vario EL elemental analyzer
(Elementar Analysensysteme GmbH). Synthetic procedures for the
ligand precursors and mononuclear complexes are given in the
Supporting Information.
UV−vis absorption spectra were obtained on an Agilent 8453 diode
array spectrophotometer. Steady-state emission spectra were recorded
on a SPEX FluoroLog 3-TCSPC spectrophotometer equipped with a
Hamamatsu R928 PMT detector, and emission lifetime measurements
were conducted using NanoLed sources in the fast MCS mode and
checked using the TCSPC mode. Sample and standard solutions were
degassed with at least three freeze−pump−thaw cycles. Low-temper-
ature (77 K) emission spectra for glasses and solid-state samples were
recorded in 5 mm diameter quartz tubes, which were placed in a liquid
nitrogen Dewar equipped with quartz windows. The emission quantum
yield was measured by using [Ru(bpy)3](PF6)2 in degassed acetonitrile
as the standard (Φr = 0.062) and calculated by: Φs = Φr(Br/Bs)(ns/
nr)2(Ds/Dr), where the subscripts s and r refer to sample and reference
standard solution, respectively, n is the refractive index of the solvents, D
is the integrated intensity, and Φ is the luminescence quantum yield.
The quantity B is calculated by the equation B = 1 − 10−AL, where A is
the absorbance at the excitation wavelength and L is the optical path
length. Errors for λ ( 1 nm), τ ( 10%), and Φ ( 10%) are estimated.
Solutions of Pt(II) (CHCl3/CH3CN, 1/1) and Zn(II) (CH3CN)
complexes, and metal perchlorate salts (CH3CN), were prepared in
solvents of spectroscopic grade. Absorption and emission titrations were
carried out in a quartz cuvette by addition of small volumes of metal ion
solutions (5 × 10−3 M) to the Pt(II) or Zn(II) complex.
Crystals data were collected on an Oxford Diffraction Gemini S Ultra
X-ray single-crystal diffractometer using graphite-monochromated Cu
Kα radiation (λ = 1.5418 Å). The structures were solved by direct
methods and refined using the SHELXL-97 program on a PC.38 For 4,
electron density attributable to additional solvent molecules in the unit
cell was removed using the PLATON (SQUEEZE) program.39 DFT
calculations on molecular structures were performed at the B3LYP level
with the CEP-31G basis set using the Gaussian 09 program package.40 In
each case, the energy-minimized calculated structure was confirmed to
be a minimum from vibrational frequency calculations.
The observed selectivity of 2 for Pb2+ is tentatively ascribed to
the nature of the O4-binding pocket, which apparently exhibits
good size-complementarity for the Pb2+ ion (radius = 1.19 Å),37
as indicated by DFT. Although size-match should be an
important factor, selectivity is also determined by a variety of
additional factors including solvent effects and energetics of
solvation, chelate ring size, and “hard/soft” complementarity.
The emission responses of 2 and 4 to Hg2+ and Cu2+,
respectively, have also been compared (Supporting Informa-
tion). Incremental quenching (due to spin−orbit interactions
and paramagnetic nature, respectively)36 is observed in both
cases, but while the peak maximum for 4 remains constant, blue-
shifted emissions are apparent for 2 upon addition of Hg2+ and
Cu2+ (from λmax 630 to 617 and 620 nm, respectively, for up to 2
equiv; compared with λmax 605 nm with enhancement for Pb2+).
The less blue-shifted emissions for 2 are tentatively ascribed to
limited Hg2+/Cu2+···O(salphen) interactions leading to destabi-
lization of the O(p)→π*(diimine) transition, but the Hg2+ and
Cu2+ ions (radius = 1.02 and 0.73 Å, respectively)37 evidently do
not match the characteristics and dimensions of the O4-cavity (as
indicated by ESI−MS), and hence the weak intramolecular π-
interactions and associated self-quenching can persist. In
t
contrast, the Bu substituents in 4 would impede coordination
by the O(salphen) groups, and the quenched emission is
therefore not blue-shifted. Overall, the evidence in this work
suggests that a shape-persistent (Pt-salphen)2 framework bearing
a suitable binding cavity can afford a blue-shifted and enhanced
emission response for selected ions (Pb2+).
Synthesis. Complex 1. Compound L2 (0.200 g, 0.355 mmol in
powder form) and triethylamine (1 mL) were added to a stirred solution
H
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Article
of L3 (0.151 g, 0.711 mmol) and zinc acetate dihydrate (0.172 g, 0.782
mmol) in MeOH (10 mL) at room temperature. The reaction mixture
became an orange suspension after stirring at room temperature for 30
min under air, and a yellow suspension was formed after 20 h. The crude
product was filtered and copiously washed with MeOH, and
recrystallization by slow evaporation of THF afforded 1 (0.230 g,
60%) as a bright yellow solid. Anal. Calcd for C63H54N4O5Zn2·(H2O)6
(1186.04): C, 63.80; H, 5.61; N, 4.72. Found: C, 63.68; H, 5.22; N, 4.51.
1H NMR (400 MHz, d6-DMSO): δ 8.84 (s, 2H, H13), δ 8.77 (s, 2H, H8),
7.77−7.73 (m, 4H, H9, H12), 7.50 (d, J = 2.2 Hz, 2H, H7), 7.43 (d, J = 2.2
Hz, 2H, H3), 7.42 (dd, J = 8.9, 2.3 Hz, 2H, H5), 7.31 (dd, J = 7.9, 1.4 Hz,
2H, H14), 7.25−7.18 (m, 6H, H4, H10, H11), 7.16−7.12 (m, 2H, H16),
6.54 (d, J = 8.5 Hz, 2H, H17), 6.52−6.49 (m, 2H, H15), 6.45 (d, J = 8.8
Hz, 2H, H6), 1.71 (s, 6H, H1), 1.36 (s, 18H, H2). ESI−MS (+ve mode):
m/z 1079 [M + H]+.
125.5, 124.8, 121.5, 121.3, 119.3, 118.9, 116.7, 116.2, 35.8, 35.5, 34.9,
34.1, 32.0, 31.8, 31.5, 29.6. ESI−MS (+ve mode): m/z 1304 [M + H]+.
Complex 4. A solution of PtCl2 (0.17 g, 0.64 mmol) in DMSO (20
mL) was added to a solution of 3 (0.38 g, 0.29 mmol) in DMSO (30
mL). The reaction mixture was stirred at 90 °C for 14 d. The resultant
mixture was diluted with CH2Cl2 and washed with H2O. The organic
layer was separated, dried over MgSO4, and filtered. The filtrate was
concentrated in vacuo, and the residual was purified by column
chromatography (SiO2, 50% CH2Cl2 in hexane) and washed with
MeOH to give 4 (0.40 g, 88%) as a red solid. Anal. Calcd for
C79H86N4O5Pt2·(H2O) (1579.72): C, 60.06; H, 5.61; N, 3.55. Found: C,
60.02; H, 5.43; N, 3.31. 1H NMR (400 MHz, CD2Cl2): δ 8.32 (s, 2H,
H13), 7.83−7.78 (m, 4H, H5, H7), 7.71 (s, 2H, H8), 7.60 (d, J = 7.8 Hz,
2H, H12), 7.55 (d, J = 2.3 Hz, 2H, H16), 7.52 (d, J = 8.1 Hz, 2H, H9), 7.46
(d, J = 2.2 Hz, 2H, H3), 7.34 (d, J = 2.2 Hz, 2H, H4), 7.27−7.18 (m, 4H,
H10, H11), 6.93 (d, J = 8.8 Hz, 2H, H6), 6.91 (d, J = 2.3 Hz, 2H, H14), 1.79
Complex 2. A solution of PtCl2 (67.4 mg, 0.253 mmol) in DMSO (5
mL) was added to a yellow solution of 1 (0.130 g, 0.121 mmol) in THF/
DMSO (1:2, 15 mL). The reaction mixture was stirred at 90 °C for 7 d to
give a dark red suspension. After cooling to room temperature, the
suspension was diluted with CH2Cl2 (100 mL), washed with water (4 ×
100 mL), and dried over MgSO4. The resultant solution was filtered and
concentrated to dryness. The crude product was purified by column
chromatography (Al2O3, gradient elution from 50% CH2Cl2 in hexane
to THF), then washed with diethyl ether, and finally precipitated from
hot CHCl3 to give 2 (83.0 mg, 51%) as a red solid. Anal. Calcd for
C63H54N4O5Pt2·(H2O)4 (1409.34): C, 53.69; H, 4.43; N, 3.98. Found:
C, 53.32; H, 4.01; N, 3.72. 1H NMR (400 MHz, d6-DMSO): δ 8.70 (s,
2H, H13), 8.22−8.20 (m, 4H, H8, H12), 7.91−7.89 (m, 2H, H9), 7.76 (d, J
= 2.3 Hz, 2H, H7), 7.72 (dd, J = 8.8, 2.4 Hz, 2H, H5), 7.50 (d, J = 2.3 Hz,
2H, H3), 7.40−7.34 (m, 6H, H10, H11, H16), 7.28 (d, J = 2.3 Hz, 2H, H4),
7.07−7.05 (m, 2H, H14), 6.75 (d, J = 8.4 Hz, 2H, H17), 6.66 (d, J = 8.8
Hz, 2H, H6), 6.41 (m, 2H, H15), 1.77 (s, 6H, H1), 1.37 (s, 18H, H2).
ESI−MS (+ve mode): m/z 1359 [M + Na]+.
(s, 6H, H1), 1.41 (s, 18H, H2), 1.36 (s, 18H, H17), 1.29 (s, 18H, H15). 13
C
NMR (101 MHz, CD2Cl2): 164.8, 164.0, 149.0, 148.2, 146.0, 145.9,
145.3, 145.2, 141.1, 137.3, 137.1, 136.2, 130.9, 129.9, 127.9, 127.8, 127.8,
126.6, 125.9, 125.3, 122.7, 122.1, 121.8, 121.2, 115.9, 115.7, 36.1, 35.2,
34.9, 34.2, 33.5, 31.7, 31.4, 30.3. ESI−MS (+ve mode): m/z 1585 [M +
Na]+.
ASSOCIATED CONTENT
* Supporting Information
■
S
Listings of experimental details and characterization data; crystal
data and refinement details for 4, 7, and 8; molecular structure of
8; additional photophysical data, spectra, and responses and
curve fittings for substrate binding; and details of DFT
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
Complex 3. Compound L2 (30 mg, 0.053 mmol in powder form) and
triethylamine (2 mL) were added to a stirred solution of L4 (35 mg, 0.11
mmol) and zinc acetate dihydrate (27 mg, 0.12 mmol) in MeOH (20
mL) at room temperature. The reaction mixture became an orange
suspension after stirring at room temperature for 30 min under air, and
was allowed to stir at reflux for 20 h. The crude product was filtered and
copiously washed with cooled MeOH. Precipitation from CH2Cl2/
hexane (1:9) afforded 3 (50 mg, 72%) as a bright yellow solid. Anal.
Calcd for C79H86N4O5Zn2·(CH2Cl2)5 (1727.03): C, 58.42; H, 5.60; N,
3.24. Found: C, 58.86; H, 5.89; N, 2.85. 1H NMR (400 MHz, CD2Cl2):
δ 8.73 (s, 2H, H13), 8.45 (s, 2H, H8), 7.68 (d, J = 7.3 Hz, 2H, H12), 7.62
(d, J = 7.2 Hz, 2H, H9), 7.47−7.45 (m, 2H, H11), 7.43 (d, J = 2.1 Hz, 2H,
H3), 7.41−7.38 (m, 2H, H10), 7.37 (d, J = 2.7 Hz, 2H, H16), 7.22 (d, J =
2.2 Hz, 2H, H4), 7.19 (dd, J = 8.5, 2.4 Hz, 2H, H5), 7.09 (d, J = 2.8 Hz,
2H, H14), 7.08 (d, J = 2.4 Hz, 2H, H7), 6.48 (d, J = 8.5 Hz, 2H, H6), 1.77
AUTHOR INFORMATION
Corresponding Author
*E-mail: mcwchan@cityu.edu.hk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was fully supported by a grant from the Research
Grants Council of the Hong Kong SAR, China (CityU 100212).
We are grateful to the reviewers for helpful comments.
REFERENCES
■
(s, 6H, H1), 1.37 (s, 18H, H2), 1.34 (s, 18H, H17), 1.30 (s, 18H, H15). 13
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