Shape-persistent binuclear cyclometalated platinum(II) luminophores: Pushing π-mediated excimeric fluid emissions into the NIR region and ion-induced perturbations

Article abstract of DOI:10.1002/chem.200902328

The synthesis and spectroscopic properties of new binuclear cyclometalated 6-aryl-2, 2'-bipyridine (C,N,N) platinum(II) complexes tethered by a xanthene backbone has been reported. The rigid bridging unit is chosen to preclude intramolecular metal-metal interactions and hence MMLCT states, while allowing π-stacking interactions within the (Pt-C, N, N) moiety, which has pushed the associated excimeric-type emission into the near-infrared region. The emission features and particularly the relative intensities of the high- and low-energy emissions for desirable electronic and recognition/signaling, are independent of complex concentration. These emissions are susceptible to perturbations in different solvents and by selected metal ions. The NIR emissions are desirable reporting tools for biomolecular probes and chemosensing applications.

Full text of DOI:10.1002/chem.200902328

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DOI: 10.1002/chem.200902328
Shape-Persistent Binuclear Cyclometalated Platinum(II) Luminophores:
Pushing p-Mediated Excimeric Fluid Emissions into the NIR Region and
Ion-Induced Perturbations
Zhengqing Guo and Michael C. W. Chan*[a]
In recent years, researchers have judiciously investigated
molecular phenomena, such as restricted rotation, crowded
conformation, rigidization, and supramolecular attraction
(e.g. p-stacking) in crowded aromatic frameworks in search
of desirable electronic,[1] recognition/signaling,[2] mechani-
cal[3] and catalytic properties.[4] Although fluorescent ver-
sions have appeared,[5] the incorporation of luminescent
metal–organic and phosphorescent components have gener-
ally been neglected. We are currently engaged in the crea-
tion of congested molecular scaffolds[6] derived from envi-
ronmentally sensitive platinum(II) luminophores, which may
offer possibilities in controllable conformational changes, in
the reporting of molecular-level events, and give unusual
photophysical and sensing characteristics. In this respect, lu-
minescent square-planar oligopyridyl PtII complexes and
multinuclear assemblies have been extensively studied over
the past two decades, owing to their propensity to engage in
interplanar stacking and metal–metal interactions and for
their diverse excited-state properties. Namely, their “open”
geometry facilitates the observation of low-energy fluid
emissions in the lmax =600–700 nm region that are ascribed
to metal–metal-to-ligand charge transfer (MMLCT; ds*!
p*) transitions,[7–9] but this assignment is complicated[8,10] be-
cause pp* excimeric emissions occur in a similar spectral
region (lmax =600–650 nm; often from 77 K glassy solu-
tion),[11] hence it is difficult to distinguish between these two
states.
(C,N,N) platinum(II) complexes tethered by a xanthene
backbone. The xanthene bridging unit has been utilized by
Tanaka to study electronic interactions in mixed Ru-/Pt-ter-
pyridine derivatives,[12] and very recently by Williams to link
together two [2,6-di(2-pyridyl)ACTHUNTGRNEUNG(s-aryl)]PtCl moieties to
afford intense deep-red excimeric emission.[13] In this work,
the rigid bridging unit is chosen to preclude intramolecular
metal–metal interactions and hence MMLCT states, while
allowing p-stacking interactions within the [Pt-C,N,N]2
moiety, and this has pushed the associated excimeric-type
emission into the near-infrared region. In addition, interest-
ing perturbations of the excimeric emission (and by correla-
tion the weak p-stacking interactions) in different solvents
and in response to metal ions have been observed.
The conformationally rigid binuclear complexes 1 and 2
(Figure 1) are prepared by modified Krçhnke-type reactions
by using the appropriate xanthene-4,5-bisACHTNUTRGNEUNG(aldehyde) precur-
sor to give the corresponding ligands, followed by treatment
with K2PtCl4 in refluxing glacial acetic acid (see the Sup-
Herein, we describe the synthesis and spectroscopic prop-
erties of new binuclear cyclometalated 6-aryl-2,2’-bipyridine
[a] Z. Guo, Dr. M. C. W. Chan
Department of Biology and Chemistry
City University of Hong Kong
Tat Chee Avenue, Kowloon, Hong Kong (China)
Fax : (+852)27887406
E-mail: mcwchan@cityu.edu.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902328 and contains the ex-
perimental details and characterization data; photophysical data and
spectra; a color version of Figure 4; molecular modeling for 2.
Figure 1. The binuclear structures of 1 and 2 alongside the mononuclear
congener 3.
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porting Information for details of synthetic route). Function-
alization of the cyclometalated rings in 2 with ethoxy groups
is intended to modulate the mechanical (i.e. rotational) and
photophysical characteristics of the Pt-C,N,N units, as well
as improve solubility.
emission of 2 at lmax =594 nm in CH2Cl2 displays minimal
solvatochromism and identical emission energy and lifetime
3
to that of 3, indicating a ILCT excited state[15,16] although
3
some MLCT character cannot be ruled out. Like 1, the ad-
ditional broad, low-energy emission for 2 at lmax ꢀ720 nm
alters in intensity against the high-energy band in different
solvents: the lmax ꢀ720 nm band is negligible in DMSO (like
3 in CH2Cl2), weak in toluene and THF, and intensifies from
CH2Cl2 to MeOH and becomes the dominant feature in
CH3CN at the expense of the high-energy band (Figure 3).
The photophysical properties of 1–3 have been investigat-
ed in detail.[14] The intense absorption bands at l<400 nm
for 1 and 2 (slightly red-shifted for the latter) are attributed
to intraligand ACHTUNGTRENNUNG
1(pp*) transitions. For 1, the moderately in-
tense absorptions in the visible region at lmax =424 and
443 nm are very similar in shape and energy to known mon-
onuclear congeners,[8,9] while displaying comparable negative
solvatochromic behavior,[14] and are therefore ascribed as
d(Pt)!p*
ACHTUNGTRENNUNG
(bpy) 1MLCT. No absorption tailing is apparent
below l=500 nm, in contrast to that observed for binuclear
analogues supported by short diphosphine bridges (e.g.
dppm),[8] thus signifying the expected absence of ground
state Pt···Pt interactions. The visible absorption of 2 at
lmax =444 nm is less structured than 1, but comparable in
shape to the mononuclear congener 3, and is assigned to an
admixture of p
A
(bpy) (1ILCT/1MLCT)
ACHTUNGTRENNUNG
like that recently proposed for an alkoxy congener of 3.[15]
Using the rationale in a previous report,[8b] the corrected ab-
sorption difference spectrum between 2 and 3 contains a dis-
cernible band with a maximum at l=451 nm (eꢀ2000
dm3 molꢁ1 cmꢁ1; Figure 2), which is tentatively attributed to
weak ground-state intramolecular p–p interaction. A similar
band at lmax =458 nm is apparent in the corrected absorp-
tion difference spectrum between 1 and the non-ethoxy con-
gener of 3.[14]
Figure 3. Emission spectra for 2 (10ꢁ5 molLꢁ1) in different solvents (nor-
malized with respect to high-energy band; lex =430 nm) at 298 K (inset:
calculated offset p-stacked interactions in 2).
Interestingly, these emission features and particularly the
relative intensities of the high- and low-energy emissions for
both 1 and 2 are independent of complex concentration
(down to 10ꢁ6 molLꢁ1, thus discounting intermolecular ag-
gregation) and excitation wavelength (to reduce possibility
of contribution from highly emissive impurities).[14] Further-
more, the same excitation spectra are recorded for the high-
and low-energy emissions from 1 and 2 respectively, which
closely resemble their UV absorptions, especially for the
lowest-energy band at lmax ꢀ440 nm. The broad nature
(fwhmꢀ3400 cmꢁ1 in CH3CN for 2) and low F (10ꢁ4) of the
low-energy band are consistent with excimer-type emis-
AHCTUNGTRENNUNG
sion.[11a,17] In this connection, an energy-minimized (Gaus-
sian) calculated structure of 2 (Figure 3) has predicted weak
p-stacked interplanar contacts within the (Pt-C,N,N)2 moiety
(plus a Pt···Pt separation of 5.63 ꢁ).[14] Since Pt···Pt interac-
tions can be disregarded, the low-energy emission is thus as-
signed to offset p-stacked interactions (i.e. “slipped” intra-
molecular interplanar contacts such as p–p and d(Pt)–p(aryl
and py) within the (Pt-C,N,N)2 moiety).[18] Owing to the
rigid nature of the xanthene backbone, the p-stacking inter-
actions may be very weak in the ground state (as tentatively
indicated in Figure 2), but become significantly more favora-
ble in the excited state, especially for 2 in solvents such as
CH3CN (Figure 3), and therefore the term excimer rather
than dimer emission is preferred. The relationship between
the unusual emission changes in Figure 3 and solvent polari-
Figure 2. UV/vis absorption spectra of 2 and 3 in CH2Cl2 at 298 K. Inset:
Plot of De versus wavelength for 2 and 3 (e values multiplied by 2) in
CH2Cl2.
Complexes 1 and 2 are luminescent in solution at room
temperature. For 1, in addition to the solvatochromic
3MLCT emission at lmax =562 nm in CH2Cl2, a weaker low-
energy shoulder is evident at lmax ꢀ750 nm. Upon further in-
vestigation in CH2Cl2/CH3OH (1:1), the low-energy feature
3
intriguingly grows (versus the MLCT emission) into a dis-
tinct peak at lmax ꢀ730 nm. Compared to 1, the red-shifted
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Chem. Eur. J. 2009, 15, 12585 – 12588

Binuclear Cyclometalated Platinum Luminophores
COMMUNICATION
ty is tentative, and is presumably related to the capacity of
the solvent to favor or compete with p-stacking interactions,
leading to intramolecular aggregation and deaggregation re-
spectively.
The intramolecular p-stacked excimeric emission for 1
and 2, at lmax ꢀ720–730 nm respectively in CH3CN at room
temperature, is of interest because it approaches the NIR
region and is uncharacteristically red-shifted, considering
the absence of MMLCT character and in comparison with
previously reported pp* excimeric emissions at lmax =600–
or p-cation interactions[19] at the [Pt-C,N,N] units, resulting
in disruption of the weak p-stacking contacts, whereas the
observed enhancement with Hg2+ may arise from an in-
crease in the rigidity[20] of the [Pt-C,N,N]2 moiety, as indicat-
1
ed by H NMR studies.[7f,14] Nevertheless, the detailed mech-
anism for the ion-induced luminescent perturbations and the
role of the ethoxy groups are topics under investigation.
In summary, we have described rigidly tethered binuclear
PtII luminophores that display excimeric-type fluid emissions
at lmax ꢀ720–730 nm without ambiguity from MMLCT
states. Intriguingly, these emissions are susceptible to pertur-
bations in different solvents and by selected metal ions. The
associated intramolecular p-stacking interaction within the
[Pt-C,N,N]2 cavity is observed to push the excimeric emis-
sion down into the near-IR region. NIR emissions are desir-
able reporting tools for biomolecular probes and chemo-
sensing applications, and our future work will aim to exploit
the integration of weak interplanar interactions and sensi-
tive luminophores in these systems.
650 nm[8,11] and fluid MMLCT ds*!p* emissions at lmax
=
600–700 nm.[7–9,11c,d] To rationalize the red-shifted excimeric
emissions for 1 and 2 relative to that reported by Williams[13]
for the related [N,C,N] congener (lmax =690 nm in CH2Cl2),
we note that the p-stacking interactions in 1 and 2 would in-
volve mixed p(py)–pACTHNUTRGNEU(GN aryl) and may be different.
The fragile p-stacked interactions giving rise to the pro-
posed low-energy excimeric emission for 2 is anticipated to
be sensitive to mechanical perturbations (e.g. axial twisting)
at the molecular level, and the intrinsic luminescent signal-
ing element is attractive. For example, the ethoxy groups
may impede rotation of the [Pt-C,N,N] units as well as assist
or mediate host–guest interactions. Preliminary studies on
the luminescent response for 2 in the presence of various
metal ions have been undertaken (Figure 4). While minimal
Acknowledgements
The work described in this paper was fully supported by grants from the
Research Grants Council of the Hong Kong Special Administrative
Region, China (CityU 100408) and City University of Hong Kong
(7002278). We are grateful to Wai-Kit Cheng and Wah-Leung Tong for
technical assistance.
Keywords: excimeric emission
·
platinum
· restricted
conformation · rigidization · pi interactions
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Received: August 24, 2009
Published online: October 21, 2009
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Chem. Eur. J. 2009, 15, 12585 – 12588