Enhancement of phosphorescence and unimolecular behavior in the solid state by perfect insulation of platinum-acetylide polymers

Article abstract of DOI:10.1021/ja508636z

Controlling the thermal fluctuations and molecular environment of a phosphorescent polymer backbone is vital to enhancing its phosphorescence intensity in the solid state. Here, we demonstrate enhanced phosphorescence control through a systematic investig

Full text of DOI:10.1021/ja508636z

Communication
pubs.acs.org/JACS
Enhancement of Phosphorescence and Unimolecular Behavior in the
Solid State by Perfect Insulation of Platinum−Acetylide Polymers
Hiroshi Masai,† Jun Terao,*,† Satoshi Makuta,‡ Yasuhiro Tachibana,*,‡ Tetsuaki Fujihara,†
and Yasushi Tsuji†
†Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto, Japan
‡School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Bundoora, Australia
S
* Supporting Information
intrinsic disorder in the solid state, which is due to their high
ABSTRACT: Controlling the thermal fluctuations and
molecular environment of a phosphorescent polymer
backbone is vital to enhancing its phosphorescence
intensity in the solid state. Here, we demonstrate
enhanced phosphorescence control through a systematic
investigation of cyclodextrin-based insulated platinum−
acetylide polymers with well-defined coverage areas.
Modification of the coverage areas revealed two unpreced-
ented effects of macrocyclic insulation on phosphor-
escence behavior. First, the insulation of particular areas
suppresses the thermal relaxation processes of the triplet
species because of the restriction of structural fluctuations.
Cyclic insulation fixes a polymer chain and concomitantly
enhances the phosphorescence intensity in both the
solution and solid states. Second, complete three-dimen-
sional insulation protects the polymer from interactions
with other platinum and acetylide units, and even oxygen
molecules. Notably, these polymers display identical
phosphorescence behaviors in both the solution and
solid states, essentially achieving “unimolecular phos-
phorescence.”
degrees of freedom, makes it difficult to organize a well-ordered
structure, such as in the crystalline solid state. For this reason,
applications of phosphorescent polymer materials have been
limited to highly diluted systems, which are usually formed by
doping a polymer matrix or through copolymerization with a low
mixing ratio.9 Thus, to achieve high-density phosphorescence in
polymer solid materials, two contradictory requirements must be
simultaneously satisfied: (i) a higher concentration for brighter
emission and (ii) a lower concentration to inhibit molecular
interactions. Nevertheless, because of their potential application
in solid-state phosphorescent polymer devices, the development
of such materials has been eagerly pursued; the insights obtained
would contribute to progress in the field of device science.
In 2005, Schanze et al. reported a Pt−acetylide polymer in
which bulky iptycene side chains were used to suppress molecular
interactions and enhance phosphorescence in the solid state.10
This concept motivated us to establish a general insulation
strategy for a phosphorescent polymer, to optimize the emission
behavior in the solid state. Herein, we systematically investigate
insulated solid-state phosphorescent polymers. By using macro-
cycles for higher-order inhibition, only the desired area of a
polymer chain can be covered, without any interstitial space, to
achieve higher-density phosphorescence. During our study on
the synthesis of a 3-D insulated conjugated polymer, poly-
(phenylene ethynylene) (polyPE), covered with permethylated
α-cyclodextrin (PM α-CD) macrocycles,11 we developed
omnidirectionally insulated metallopolymers composed of Pt−
acetylide and oligoPE as the polymer backbone.12 In this study,
we systematically designed and synthesized various omnidirec-
tionally insulated polymers with different coverage ratios and
areas. Two unprecedented cyclic insulation effects on the
phosphorescence behavior of the metallopolymers will be
described: (i) the cyclic insulation enhances phosphorescence;
and (ii) identical phosphorescence behaviors are observed in the
high-density solid state as well as in dilute solution.
ransition-metal complexes have gained importance as
T
components for light-emitting diodes (LEDs)1 and in
optical sensing2 and bioimaging3 devices because of their highly
effective and long-wavelength phosphorescence at room temper-
ature. In the solid state, they are expected to produce higher-
density phosphorescent materials, which will display brighter
emissions-per-unit-volume than their dilute counterparts.
However, their desirable phosphorescence in the solid state
decreases or changes drastically4 as a result of their indiscriminate
molecular interactions with adjacent molecules.5 Therefore, they
exhibit high performance only under highly dilute conditions
(such as in a matrix with a very low doping concentration).
Several examples of exceptional solid-state phosphorescence
have been achieved through the adequate control of molecular
arrangements or the inhibition of undesirable molecular
interactions in crystalline systems.6,7 However, the simple
concentration of diluted phosphorescent materials cannot
increase their emission densities but, rather, can lead to
unpredictable changes in emission behavior.8
Monomer 1 was synthesized in three steps from the PM α-CD
monotosylate (Scheme 1).11a Then, 1 was converted to insulated
complex 2 through hydrophilic−hydrophobic interactions in
MeOH/H2O (2:1). Under the same solvent conditions, the
intermediate pseudorotaxane structure of 2 was fixed by
expanding the oligoPE unit through Sonogashira coupling with
As practical device materials, polymers are of interest owing to
their high processability and thermodynamic stability as
compared with low-molecular-weight materials. However, their
Received: August 21, 2014
Published: October 2, 2014
© 2014 American Chemical Society
14714
dx.doi.org/10.1021/ja508636z | J. Am. Chem. Soc. 2014, 136, 14714−14717

Journal of the American Chemical Society
Communication
Scheme 1. Synthetic Route for Polymers 4−6
Figure 2. (a) Emission spectra of polymer solids of 4 (red), 5 (blue), and
6 (green) under degassed conditions (excitation: 405 nm). Emission
behavior of Pt−acetylide polymer solids of 4−6 under (b) N2 and (c) air
(excitation: 380−425 nm).
phosphorescence and a 2-fold-higher quantum yield than its
uninsulated counterpart 4 (5: 5.4% vs 4: 2.3%), indicating that
the cyclic insulation for the Pt−acetylide backbones is essential
for the notable enhancements in 5. The cyclic insulation of 5
provides high rotational and bending barriers to the polymer
backbone, which prohibits structural fluctuations under ambient
conditions.17 It is generally difficult to inhibit the molecular
fluctuations of benzene rings adjacent to oligoPE units with
conventional protectants such as dendrons and macrocycles
(Figure S5). In contrast, our [1]rotaxane structures successfully
fix the oligoPE units in 5 by insulation and linkages. Such 3-D
restraint for 5 efficiently enhances its phosphorescence through
the peculiar effects of linked rotaxane structures.
In order to assess the potential for protection against oxygen, a
typical phosphorescence quencher, we measured the transient
emission decays of dilute solutions of 4 and 5 (Figure S6a). Under
deoxygenated conditions, the transient emission decay fittings of
4 and 5 in Figure S6a show the same lifetime (30 μs). In contrast,
under an air atmosphere, the lifetime of 5 (10 μs) is ∼15 times
longer than that of 4 (0.65 μs). This difference clearly indicates
that the insulation of 5 extends the phosphorescence lifetime by
inhibiting interactions with oxygen. These results demonstrate
the isolation of insulated polymer 5 from the external
environment because of its 3-D covering.18
We next investigated the effect of insulation in the solid state.
Triplet-based phosphorescence is subject to quenching via
molecular interactions in the solid state, since the triplet-state
lifetime is generally longer than that of the singlet state.19,20
Amorphous films were prepared by evaporating a polymer
solution in CHCl3 on a glass substrate without annealing. Figure
2a shows the photoluminescence spectra of polymer solids 4−6
under degassed conditions.21 As expected, insulated polymer 5
displays a strong orange emission. In contrast, the corresponding
uninsulated polymer 4, which exhibited phosphorescence in
dilute solution, shows very weak phosphorescence in the solid
state, despite its bulky PM α-CD side chains which cover the
polymer backbone. These results indicate that the insulation in
polymer 5 prevents interactions with adjacent molecules, even in
the solid state, achieving high-density phosphorescence. Because
5 also tolerated oxygen, as described above, it even exhibits
phosphorescence in the solid state under an air atmosphere
(Figures 2c and S3). In contrast, despite its insulation, 6 does not
show phosphorescence in the solid state, but only fluorescence.
We postulate that the triplet-state quenching observed in the
polymer 6 solid is caused by intermolecular interactions, similar
to the case of 4, because 6 also displayed phosphorescence in
dilute solution.
Figure 1. (a) Normalized emission spectra (excitation: 405 nm) of 4
(red), 5 (blue), and 6 (green) in CHCl3 under N2 atmosphere. (b)
Normalized transient emission spectra of 5 in CHCl3 under N2
atmosphere at 1 ns and 10 μs after excitation at 405 nm.
tert-butyldimethylsilyl (TBS)-protected p-iodophenylacetylene
in the presence of tris(4,6-dimethyl-3-sulfanatophenyl)phos-
phine trisodium salt (TXPTS) as a water-soluble phosphine
ligand, followed by deprotection of the TBS groups to form the
fixed insulated conjugated monomer 3.11c The uninsulated Pt−
acetylide polymer 4 was synthesized by Cu-catalyzed trans-
metalation of 1 with trans-PtCl2(PEt3)2 in a hydrophobic solvent
(piperidine).12 The corresponding insulated polymer 5 was
obtained from the same monomers in the hydrophilic solvent,
MeOH/H2O (2:1), from insulated complex 2. In this study, to
examine the effects of the cyclic insulation area and PE units,
insulated polymer 6 was synthesized from 3 bearing different
oligoPE units.13,14
We first compared the phosphorescence emission spectra of
polymers 4−6 in degassed dilute CHCl3 (Figure 1a).15 5 shows
emission maxima at 433 and 585 nm. The transient emission
spectra obtained at 1 ns and 10 μs (Figure 1b) indicate that the
shorter-wavelength emission can be attributed to fluorescence. In
contrast, the long-wavelength emission accompanying the clear
vibration band can be identified as phosphorescence.10,16
Similarly, 4 shows fluorescence at 438 nm and phosphorescence
at 608 nm in CHCl3. The emissions of insulated polymers 5 and 6
are blue-shifted compared with those of the uninsulated polymer
4 because of the fixation of the twisted conformation of the
oligoPE units by encapsulation, which shortens the effective
conjugated length of the oligoPE units of 5 and 6.11a,17 The
phosphorescence intensity of 6 is considerably lower than those
of 4 and 5 owing to a decrease in spin−orbit coupling with the
decreasing content of Pt-acetylide units per polymer segment
(Figure S4). Interestingly, insulated polymer 5 displays stronger
14715
dx.doi.org/10.1021/ja508636z | J. Am. Chem. Soc. 2014, 136, 14714−14717

Journal of the American Chemical Society
Communication
Figure 4. (a) Emission spectra of polymer solids 8 (gray) and 10 (dark
pink) under degassed conditions (excitation: 405 nm). (b) Emission
behavior of polymer solids 8 and 10 under N2 (excitation: 380−425 nm).
Figure 3. Comparison of the potential for shuttling between (a)
conventional and (b) our linked rotaxanes. (c) Illustration of the
coverage ratios and areas of insulation for polymers 5, 6, 8, and 10. Red
lines show the repeating units of the polymers.
5 showed both phosphorescence and fluorescence, whereas 6
showed only fluorescence, although they are both insulated
polymers. To investigate the intermolecular interactions in detail,
we focused on the three structural differences between these two
insulated polymers: (i) 5 has a higher coverage ratio over its
backbone than 6; (ii) the Pt complex area in 5 is covered but that
in6 isnot; and(iii) the oligoPE unitin 5 isshorter thanthat in 6.22
Our synthetic method for insulated metallopolymers bearing the
poly[1]rotaxane structure allows the synthesis of individual well-
defined polymers insulated in targeted areas at desired coverage
ratios with the PE units, since the linkages between the
conjugated axles and macrocycles of the rotaxanes prevent the
shuttling of cycles that occurs in conventional rotaxanes (Figure
3a and 3b). To investigate the relationship between phosphor-
escence and the insulated sites in the oligoPE units, we
synthesized 8 (Scheme S5), having different coverage positions
than 6. Polymer 8 contains oligoPE units of the same length as in
6, and the same number of PM α-CDs in its repeating units. Yet,
in 8, the areas near platinum are covered, whereas, in 6, the
centers of the PE units are covered, albeit in the same ratio
(Figure 3c). Polymer 8 was synthesized by the sequential self-
inclusion of 7 followed by Cu-catalyzed transmetalation with the
Pt complex in a hydrophilic solvent.14
In the solid state, 8 exhibited only fluorescence emission,
whereas it displayed phosphorescence in CHCl3, similarly to 6.
This indicates the failure of 8 to inhibit intermolecular
interactions (Figure 4).21 This result shows that solid-state
phosphorescence is not affected by the location of the insulation
in the main chain but, rather, by the other two factors: the
coverage ratio or conjugation length. A completely insulated
polymer, 10, bearing expanded oligoPE units covered by four PM
α-CDs, was synthesized from monomer 9.14 10 displayed
phosphorescence emission in the solid state (Figure 4). These
results suggest that the triplet-state quenching of 6 and 8 was not
related to the longer conjugation lengths compared with that of 5
and demonstrate that a high coverage ratio of the polymer
backbone is essential to obtain high phosphorescence intensity in
the solid state and that the phosphorescence of 4, 6, and 8 is
quenched in the solid state because of their low coverage ratios.
To rationalize the origin of quenching by interactions, we
performed density functional theory (DFT) calculations for the
partial structure 11 of 6 and 8 (Figure 5a). The singly occupied
molecular orbital (SOMO) had predominantly lowest unoccu-
pied molecular orbital (LUMO) characteristics, fully spread over
Figure 5. (a) Optimized molecular structures and frontier orbitals of
partial structure 11, calculated using DFT. Normalized emission spectra
of (b)dilute CHCl3 solutions of 6 (green) and 8 (gray), and (c) insulated
polymer 5 in dilute CHCl3 solution (dashed) and in the solid state
(solid), under degassed conditions.
the oligoPE, and the highest occupied molecular orbital
(HOMO) was delocalized across the main axle. Phosphorescence
in Pt−acetylide polymers with oligoPEs is known to be
dominated by π−π* transitions in the PE units,23 which is also
supported by the clear vibration bands in the emission spectra, as
mentioned. The π-orbitals in the steady and excited states of
polymers 6 and 8 in the solid state can be exposed to adjacent
polymers, which results in multimeric species and destabilizes
their excited forms. In contrast, in fully insulated 5 and 10, the π-
orbitals are protected, disrupting interactions with nearby
polymers in the solid state. Moreover, the excited molecular
orbitals of 11, especially the SOMO, are focused at the center of
the PE unit rather than near the Pt complexes. The location of the
orbital distribution would strongly affect the phosphorescence
intensity of the partially insulated polymers 6 and 8, which differ
only in their coverage areas. The cyclic insulation of 6 on the PE
centers results in stronger phosphorescence in dilute CHCl3
solution than that of 8 near the Pt complexes (Figure 5b).21 This
difference in the insulation area is probably derived from the
cyclic restraint of the thermal relaxation process. As mentioned
above, the fixation of molecular motions and rotations would
protect the triplet species. The π−π*-transition-based phosphor-
escence of 6 was efficiently stabilized by covering the center of the
π-conjugation. An enhancement in the phosphorescence from 6
was also observed in the solid state compared with that in 8
(Figure S11).
Finally, to quantitatively evaluate the insulating effect of 5, we
compared the emission wavelengths and quantum yields between
a dilute solution and the solid state. Almost identical emission
spectra were obtained under deoxygenated conditions in solution
(λmax = 585 nm) and the solid state (λmax = 581 nm) (Figure 5c).
14716
dx.doi.org/10.1021/ja508636z | J. Am. Chem. Soc. 2014, 136, 14714−14717

Journal of the American Chemical Society
Communication
(5) Triplet−triplet annihilations are proposed as thermal relaxation
processes of excited species; Wagner, P. J.; Hammond, G. S. Adv.
Photochem., Vol. 5; Noyes, W. A., Jr.; Hammond, G. S.; Pitts, J. N., Jr.,
Eds.; Interscience Publishers: New York, 1968; p 21.
(6) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Soc. Rev. 2011, 40, 5361.
(7) (a) Komiya, N.; Okada, M.; Fukumoto, K.; Jomori, D.; Naota, T. J.
Am. Chem. Soc. 2011, 133, 6493. (b) Bolton, O.; Lee, K.; Kim, H.-J.; Lin,
K. Y.; Kim, J. Nat. Chem. 2011, 3, 205.
(8) Rare dendric protections have contributed to the same emission
behaviors in the solution and solid: (a) Lo, S.-C.; Harding, R. E.; Shipley,
C. P.; Stevenson, S. G.; Burn, P. L.; Samuel, I. D. W. J. Am. Chem. Soc.
2009, 131, 16681. Most dendritic examples change the wavelength and/
or quantum yield of phosphorescence according to their concentration:
(b) Lo, S.-C.; Male, N. A. H.; Markham, J. P. J.; Magennis, S. W.; Burn, P.
L.; Salata, O. V.; Sammuel, I. D. W. Adv. Mater. 2002, 14, 975. (c) Kim, J.
J.; You, Y.; Park, Y.-S.; Kim, J.-J.; Park, S. Y. J. Mater. Chem. 2009, 19,
8347. (d) Tang, M.-C.; Tsang, D. P.-K.; Chan, M. M.-Y.; Wong, K. M.-
C.; Yam, V. W.-W. Angew. Chem., Int. Ed. 2013, 52, 446.
(9) Gong, S.; Yang, C.; Qin, J. Chem. Soc. Rev. 2012, 41, 4797.
(10) Zhao, X.; Cardolaccia, T.; Farley, R. T.; Abboud, K. A.; Schanze, K.
S. Inorg. Chem. 2005, 44, 2619.
(11) (a) Terao, J.; Tsuda, S.; Tanaka, Y.; Okoshi, K.; Fujihara, T.; Tsuji,
Y.; Kambe, N. J. Am. Chem. Soc. 2009, 131, 16004. (b) Terao, J.;
Wadahama, A.; Matono, A.; Tada, T.; Watanabe, S.; Seki, S.; Fujihara, T.;
Tsuji, Y. Nat. Commun. 2013, 4, 1691. (c) Terao, J.; Homma, K.;
Konoshima, Y.; Imoto, R.; Masai, H.; Matsuda, W.; Seki, S.; Fujihara, T.;
Tsuji, Y. Chem. Commun. 2014, 50, 658. (d) Recent reviews: Terao, J.
Polym. Chem. 2011, 2, 2444.
Likewise, the quantum yields were nearly identical in solution
(5.4%) and the solid state (4.4%). This is the first demonstration
of nearly equal phosphorescence emission behaviors in dilute-
solution and high-density-solid states. These results suggest that
the complete insulation of phosphorescent polymers realizes
“unimolecular phosphorescence,” even in the solid state.
In conclusion, higher-order inclusion effects were observed on
the phosphorescence behavior of Pt−acetylide polymers fully
covered with PM α-CDs. To our knowledge, this is an
unprecedented example which integrates the features of
phosphorescence and rotaxane structure.24 Systematic syntheses
of targeted-coverage polymers distinguished two cyclic insulation
effects that stabilized the intermediate triplet species, which are
sensitive to interactions with neighboring molecules. First, the
targeted insulation for π-conjugated areas efficiently enhanced
the phosphorescence intensity in both the solution and solid
states owing to the restriction of structural fluctuations. Second,
complete 3-D insulation generated almost identical phosphor-
escence emission behaviors in solid systems as in dilute solutions
because of protection from all interactions. Moreover, such
insulation also led to oxygen tolerance: phosphorescence was
observed under air in the solid state. This is the first example of
the unimolecular phosphorescence of a polymer material in the
high-density solid state. These results, derived from the linked
rotaxane structures, indicate that even triplet species can be
enhanced and stabilized by the appropriate molecular design and
can guide the development of solid-state molecular devices.
(12) Terao, J.; Masai, H.; Fujihara, T.; Tsuji, Y. Chem. Lett. 2012, 41,
652.
(13) The 31P NMR spectra of all polymers indicating that the polymers
have a trans-Pt(PEt3)2 coordination geometry: Ohshiro, N.; Takei, F.;
Onitsuka, K.; Takahashi, S. J. Organomet. Chem. 1998, 569, 195.
(14) The chemical shifts of the PEt3 protons in the 1H NMR spectra of
polymers 5, 8, and 10 in CDCl3 (1.99−2.01 ppm) were clearly shifted
upfield compared with those for 4 and 6 (2.17 ppm), since the former
protons existed near the Me groups of the outer-ring lip of PM α-CD.
(15) (a) Wong, W.-Y.; Liu, L.; Poon, S.-Y.; Choi, K.-H.; Cheah, K.-W.;
Shi, J.-X. Macromolecules 2004, 37, 4496. (b) Haskins-Glusac, K.; Pinto,
M. R.; Tan, C.; Schanze, K. S. J. Am. Chem. Soc. 2004, 126, 14964.
(16) Wittmann, H. F.; Friend, R. H.; Khan, M. S.; Lewis, J. J. Chem. Phys.
1994, 101, 2693.
ASSOCIATED CONTENT
* Supporting Information
Detailed synthetic procedures, chromatogram, and spectral data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
terao@scl.kyoto-u.ac.jp
yasuhiro.tachibana@rmit.edu.au
Notes
■
(17) Kiguchi, M.; Nakashima, S.; Tada, T.; Watanabe, S.; Tsuda, S.;
Tsuji, Y.; Terao, J. Small 2012, 8, 726.
The authors declare no competing financial interest.
(18) The lifetimes of triplet species had a large difference between 5 (30
μs) and 4 (10 μs) in a nitrogen atmosphere, suggesting that the
insulation of 5 also prohibited triplet quenching processes other than
that by oxygen, possibly such as molecular motion (Figure S6b).
(19) The singlet-based fluorescence of conjugated molecules did not
decay dramatically even in the solid state with a certain level of
insulation: Terao, J.; Ikai, K.; Kambe, N.; Seki, S.; Saeki, A.; Ohkoshi, K.;
Fujihara, T.; Tsuji, Y. Chem. Commun. 2011, 47, 6816.
ACKNOWLEDGMENTS
■
This research was supported by the Funding Program for JSPS
Research Fellow and Grant-in-Aid for Scientific Research on
Innovative Areas (“Molecular Architectonics” and “Soft Molec-
ular Systems”) from MEXT, Japan.
(20) Many solid-state fluorescent polymers bearing protection have
been reported: (a) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120,
11864. (b) Sato, T.; Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1999, 121,
10658. (c) Pan, C.; Sugiyasu, K.; Wakayama, Y.; Sato, A.; Takeuchi, M.
Angew. Chem., Int. Ed. 2013, 52, 10775.
(21) The spectra were corrected to the same number of photons
absorbed at the excitation wavelength.
(22) (a) Wilson, J. S.; Chawdhury, N.; Al-Mandhary, M. R. A.; Younus,
REFERENCES
■
(1) (a) Costa, R. D.; Ortí, E.; Bolink, H. J.; Monti, F.; Accorsi, G.;
Armaroli, N. Angew. Chem., Int. Ed. 2012, 51, 8178. (b) Evans, R. C.;
Douglas, P.; Winscom, C. J. Coord. Chem. Rev. 2006, 250, 2093. (c) Xiao,
L.; Chen, Z.; Qu, B.; Luo, J.; Kong, S.; Gong, Q.; Kido, J. Adv. Mater.
2011, 23, 926.
(2) (a) Ma, D.-L.; Ma, V. P.-Y.; Chan, D. S.-H.; Leung, K.-H.; He, H.-Z.;
Leung, C.-H. Coord. Chem. Rev. 2012, 256, 3087. (b) Liu, Z.; He, W.;
Guo, Z. Chem. Soc. Rev. 2013, 42, 1568.
(3) (a) Mou, X.; Wu, Y.; Liu, S.; Shi, M.; Liu, X.; Wang, C.; Sun, S.;
Zhao, Q.; Zhou, X.; Huang, W. J. Mater. Chem. 2011, 21, 13951.
(b) Baggaley, E.; Weinstein, J. A.; Williams, J. A. G. Coord. Chem. Rev.
2012, 256, 1762.
M.; Khan, M. S.; Raithby, P. R.; Kohler, A.; Friend, R. H. J. Am. Chem. Soc.
̈
2001, 123, 9412. (b) Silverman, E. E.; Cardolaccia, T.; Zhao, X.; Kim, K.-
K.; Haskins-Glusac, K.; Schanze, K. S. Coord. Chem. Rev. 2005, 249, 1491.
(23) Kohler, A.; Wilson, J. S.; Friend, R. H.; Al-Suti, M. K.; Khan, M. S.;
̈
Gerhard, A.; Bassler, H. J. Chem. Phys. 2002, 116, 9457.
̈
(24) (a) Liang, A.-H.; Zhang, K.; Zhang, J.; Huang, F.; Zhu, X.-H.; Cao,
Y. Chem. Mater. 2013, 25, 1013. (b) Cao, J.; Ma, X.; Min, M.; Cao, T.;
Wu, S.; Tian, H. Chem. Commun. 2014, 50, 3224.
(4) (a) Zhu, Y.-C.; Zhou, L.; Li, H.-Y.; Xu, Q.-L.; Teng, M.-Y.; Zheng,
Y.-X.; Zuo, J.-L.; Zhang, H.-J.; You, X.-Z. Adv. Mater. 2011, 23, 4041.
(b) Julia,
Herrero, P. Chem. Sci. 2014, 5, 1875.
́ ́ ́ ́
F.; Bautista, D.; Fernandez-Hernandez, J. M.; Gonzalez-
14717
dx.doi.org/10.1021/ja508636z | J. Am. Chem. Soc. 2014, 136, 14714−14717