Directional Self-Sorting with Cucurbit[8]uril Controlled by Allosteric π–π and Metal–Metal Interactions

Article abstract of DOI:10.1002/chem.201800856

To maximize Coulombic interactions, cucurbit[8]uril (CB[8]) typically forms ternary complexes that distribute the positive charges of the pair of guests (if any) over both carbonylated portals of the macrocycle. We present here the first exception to this recognition pattern. Platinum(II) acetylides flanked by 4′-substituted terpyridyl ligands (tpy) form 2:1 complexes with CB[8] in an exclusively stacked head-to-head orientation in a water/acetonitrile mixture. The host encapsulates the pair of tpy substituents, and both positive Pt centers sit on top of each other at the same CB[8] rim, leaving the other rim free of any interaction with the guests. This dramatic charge imbalance between the CB[8] rims would be electrostatically penalizing, were it not for allosteric π–π interactions between the stacked tpy ligands, and possible metal-metal interactions between both Pt centers. When both tpy and acetylides are substituted with aryl units, the metal-ligand complexes form 2:2 assemblies with CB[8] in aqueous medium, and the directionality of the assembly (head-to-head or head-to-tail) can be controlled, both kinetically and thermodynamically.

Full text of DOI:10.1002/chem.201800856

DOI: 10.1002/chem.201800856
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Macrocycles |Hot Paper|
Directional Self-Sorting with Cucurbit[8]uril Controlled by
Allosteric p–p and Metal–Metal Interactions
Kondalarao Kotturi and Eric Masson*[a]
Abstract: To maximize Coulombic interactions, cucurbit[8]ur-
il (CB[8]) typically forms ternary complexes that distribute
the positive charges of the pair of guests (if any) over both
carbonylated portals of the macrocycle. We present here the
first exception to this recognition pattern. Platinum(II) acety-
lides flanked by 4’-substituted terpyridyl ligands (tpy) form
2:1 complexes with CB[8] in an exclusively stacked head-to-
head orientation in a water/acetonitrile mixture. The host
encapsulates the pair of tpy substituents, and both positive
Pt centers sit on top of each other at the same CB[8] rim,
leaving the other rim free of any interaction with the guests.
This dramatic charge imbalance between the CB[8] rims
would be electrostatically penalizing, were it not for alloste-
ric p–p interactions between the stacked tpy ligands, and
possible metal-metal interactions between both Pt centers.
When both tpy and acetylides are substituted with aryl
units, the metal-ligand complexes form 2:2 assemblies with
CB[8] in aqueous medium, and the directionality of the as-
sembly (head-to-head or head-to-tail) can be controlled,
both kinetically and thermodynamically.
Introduction
Cucurbit[8]uril (CB[8]) has been used since 2000[1] to dynami-
cally connect two units via ternary complex formation. A well-
known and early[2] example is the formation of a ternary com-
plex between CB[8], electron-deficient methylviologen and
electron-rich 2,6-dihydroxynaphthalene and 1,4-dihydroxyben-
zene, which is accompanied by a new absorption band at ap-
proximately 580 nm due to charge transfer between both
guests. This recognition pattern has been applied on numer-
ous occasions to the design of stimulus-responsive materi-
als.[3–12]
CB[8] typically assembles with guest pairs in a conformation
that distributes positive charges, if present, over both portals
of the macrocycle. For example, two berberine guests (1) un-
dergo concomitant encapsulation into CB[8], with each isoqui-
nolinium nitrogen interacting with one CB[8] carbonylated rim.
A similar behavior has been observed on multiple occasions
with benzyl,[13] naphthen-2-ylmethyl[13–15] and anthracen-2-yl-
methyl[16,17] units bearing positive substituents (see Figure 1).
CB[8] also distributes positive charges evenly when forming
homoternary complexes with phenylalanine residues, either as
part of a small peptide[18,19] or as the N-terminal residue of a
protein.[4,19–21] We recently showed that CB[8] can connect
metal-terpyridine complexes into dynamic oligomers upon ter-
Figure 1. Structures of berberine (1), dynamic oligomers (2·CB[8]·3·CB[8])n,
electron-rich diarylviologens 4a and azobenzene 4b.
nary complex formation with their 4’-substituents (see
Figure 1).[22] FeII and IrIII complexes 2 and 3 could thus be as-
sembled into an alternate (FeꢀIr-)n sequence, due to favorable
quadrupole-quadrupole interactions between the 4-tolyl and
tetrafluorophenyl substituents of the terpyridyl ligands. Here
again, on average, every positive metal center interacts with
one CB[8] portal, and the ligands act as areas of low dielectric
that promote the interaction.
[a] K. Kotturi, Prof. E. Masson
Department of Chemistry and Biochemistry
Ohio University
181 Clippinger Hall, Athens, Ohio 45701 (USA)
E-mail: masson@ohio.edu
In 2010, Kim and co-workers showed a first exception to this
recognition pattern in the binary complex of CB[8] and the
1,12-dodecane diammonium cation.[23] Both ammonium heads
Supporting information and the ORCID identification numbers for the au-
thors of this article can be found under:
https://doi.org/10.1002/chem.201800856.
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were found to sit at the same CB[8] portal, in solution and in
the solid state. In the latter case, a water molecule separated
the cationic heads at the portal. The authors concluded that
enhanced interactions between the U-shaped guest and the
inner wall of CB[8], as well as limited exposure of the alkyl
chain to water, advantageously compensate conformational
strain and Coulombic repulsion between the ammonium
groups.[23] Very recently, Scherman and co-workers showed
that electron-rich diarylviologens 4a (X=H, CH3, OCH3, SCH3,
and N(CH3)2, NHCOCH3) form discrete 2:2 complexes with CB[8]
instead of the expected dynamic oligomers (see Figure 1).[24]
The balance of positive charges between the carbonylated
rims is again drastically uneven, with two pyridinium units in-
teracting with one pair of portals, and none with the other
pair. The authors justify the 2:2 binding motif by proposing
(1) favorable Coulombic interactions between the electron-rich
aryl substituents of one guest with the electron-deficient pyri-
dinium unit of the other, and (2) a reduced electrostatic repul-
sion between both positive methylviologen groups as the pos-
itive charges are weakened by the neighboring electron-rich
units. A similar 2:2 complexation had been unequivocally iden-
tified a year earlier by the same group with CB[8] and azoben-
zene 4b.[25] In this study, we show that both guests do not
even need to interact favorably inside the cavity of CB[8] or
benefit from charge annihilation to adopt this 2:2 arrange-
ment, and that allosteric p–p and possibly metal-metal interac-
tions between two positive guests outside the macrocycle can
be significant enough to promote its formation. More surpris-
ingly, we also show that these interactions promote the forma-
tion of ternary CB[8]/guest2 complexes with the positive cen-
ters of both guests residing at the same CB[8] portal. To the
best of our knowledge, the orientation of the guests, as well
as the charge imbalance between both CB[8] rims, are unpre-
cedented in ternary CB[n] complexes.
Results and Discussion
The guests presented in this study are square-planar d8 PtII ter-
pyridyl (tpy) acetylides 5–7 (see Figure 2). Most bear potentially
CB[n]-binding units at the 4’-position of the tpy ligand and at
the acetylide unit (see supporting information section for de-
tails on their preparation). CB[8] was initially expected to as-
semble complexes 6 and 7 into dynamic oligomers (6·CB[8])n
and (7·CB[8])n upon encapsulation of both binding sites, simi-
larly to FeII and IrIII complexes 2 and 3. However, the sharpness
Figure 2. Complexes 5–7. ONIOM-optimized structures (wB97x-D/def2-
1
SVP:PM6) of assemblies (6b2·CB[8]2)HT and (6b2·CB[8]2)HH. H–1H NOESY spec-
trum of complex (6b2·CB[8]2)HT in a deuterium oxide buffer (5.0 mm Na2B4O7
and 50 mm H3BO3); key correlations are highlighted with green rectangles.
Chemical shifts in ppm.
1
of their H nuclear magnetic resonance (NMR) signals and the
inability for complexes 5b and 5c to undergo encapsulation,
made us suspect a radically different arrangement.
6a, 6b and 7b, the CB[8] methylene signals present two pairs
of doublets (5.88, 5.79, 4.18 and 4.12 ppm in the case of com-
plex 6b, see Figure 2), a characteristic feature for CB[n] hosts
with non-equivalent rims. A dynamic oligomer of the type
(6·CB[8])n or (7·CB[8])n would have resulted in a wide broaden-
While PtII complexes 6 and 7b were poorly soluble in deute-
rium oxide (<0.10 mm), addition of CB[8] aliquots allowed
their gradual dissolution, which was completed after addition
of 1.0 equivalent CB[8] at 1.0 mm. 1H NMR experiments re-
vealed that the composition of the assemblies in solution re-
mains steady, regardless of CB[8] concentration, even when
the latter is added in excess. In the case of complexes 6a, 6b,
6c and 7b, the Pt/CB[8] ratio after encapsulation is unequivo-
cally 1:1, and in the case of complex 6d, 2:1. For Pt acetylides
1
ing of all H NMR signals,[22] and formation of a binary complex
is implausible, as neither the geometry nor the size of the 4’-
aryl substituents of the terpyridyl ligands are suitable for CB[8]
binding as sole guests. We thus considered the formation of
2:2 complexes of the type 62·CB[8]2. This quaternary assembly
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was first confirmed by diffusion-ordered NMR spectroscopy
(DOSY). Logarithms of diffusion coefficients for complex-
es 62·CB[8]2 (6a–6c) and 7b2·CB[8]2 all ranged between ꢀ9.76
and ꢀ9.70. A similar diffusion coefficient (log D=ꢀ9.69) was
measured for ternary complex 6d·CB[7]2, which occupies a
slightly smaller volume, but retains the same general shape.
An analogue of FeII complex 2, with a tolyl group attached to
the 4’-position of the tpy unit instead of a 2-naphthyl unit, also
returned a diffusion constant logD of ꢀ9.72 after encapsula-
tion of both tolyl substituents with CB[7].[22] To the contrary,
dynamic oligomers (2·CB[8])n and (3·CB[8])n (n=11 and 15, re-
spectively) had average logD coefficients of ꢀ10.15 and
ꢀ10.28.[22]
Two arrangements are possible for 2:2 complexes: “head-to-
head” (HH), with CB[8] macrocycles forming two different
homo-ternary complexes with a pair of tpy 4’-substituents and
a pair of acetylides, or “head-to-tail” (HT), with CB[8] forming
two identical hetero-ternary complexes with the tpy substitu-
ents and the acetylides (see Figure 2). The CB[8] methylene sig-
nals, which reveal two non-equivalent CB[8] rims, would only
support a HT conformation, as four non-equivalent rims would
be expected in a HH arrangement. The HT conformation was
confirmed by two-dimensional nuclear Overhauser effect spec-
troscopy (NOESY). Correlations between the para-methyl
groups of the 4’-tolyl tpy substituents, the para-methyl groups
of the tolylacetylide units and the meta-hydrogens of each
unit were observed in complex (6b2·CB[8]2)HT (see signals la-
beled 7, 8, 10 and 11 in Figure 2). For a better depiction of the
assembly, we optimized complexes (6b2·CB[8]2)HH and
(6b2·CB[8]2)HT using the ONIOM two-layer model,[26,27] with the
high layer being Pt complexes optimized using density func-
tional theory, the wB97x-D functional and def2-SVP basis sets,
and the low layer being the whole 2:2 assembly treated with
the semi-empirical PM6 method (see Figure 2).
Figure 3. 1H–1H NOESY spectrum of a 59:41 mixture of complexes
(6c2·CB[8]2)HH and (6c2·CB[8]2)HT in a deuterium oxide buffer. Key correlations
are highlighted in green. Chemical shifts in ppm.
Figure 4. 1H NMR spectrum of complex (6d2·CB[8])HH. Signals labeled “*”
refer to excess CB[8] present in the mixture.
plex. The imbalance seems to be driven by allosteric p–p
stacking between the extensively planar complexes, and even
metal-metal interactions. Such interactions have been identi-
fied on numerous occasions in PtII terpyridyl complexes, both
in the solid state and in solution.[28–37] Square-planar d8 metals
bear filled dz2 orbitals, which can overlap into ds and ds*
bonding and antibonding molecular orbitals with the dz2 orbi-
tal of a stacked neighbor.[38–40] The strength of the isolated Ptꢀ
Pt interaction is significant, and reaches 4.2 kcalmolꢀ1 in the
The case of complex 6c is not as straightforward, and two-
dimensional NMR experiments are consistent with a 59:41 mix-
ture of assemblies (6c2·CB[8]2)HH and (6c2·CB[8]2)HT that ex-
1
change slowly on the NMR time scale. H-1H NOESY correla-
tions between the nuclei of the 4’-tpy substituent and of the
biaryl unit (see cross-peaks 8HT-13HT, 7HT-10HT, 7HT-9HT, 6HT-9HT, 6HT-
10HT, 5HT-12HT and 5HT-11HT in Figure 3) are evidence for some HT
arrangement. The lack of correlation between hydrogens 8HH
and 13HH is consistent with concomitant HH orientation (see
insert in Figure 3). Correlation spectroscopy experiments
(COSY; see Supporting Information) allowed the unequivocal
assignment of all signals in both HH and HT arrangements.
Complex 6d (R=tolyl, R’=adamantyl) forms a unique 2:1
assembly in the presence of CB[8]. As two adamantyl units are
too bulky to undergo ternary complex formation inside CB[8],
only two arrangements can be considered. Both are HH assem-
blies with CB[8] encapsulating two 4’-tolyl terpyridyl substitu-
ents, one with both platinum centers near the same rim, and
the other with a platinum center at each rim. Yet the two non-
equivalent CB[8] portals can only support the first scenario
(see Figure 4). To the best of our knowledge, this represents
the first case of a severely unbalanced charge distribution be-
tween both CB[8] portals (+2 and 0) in a 2:1 guest/CB com-
2+
case of the [Pt(tpy)Cl]2 dimer (unsubstituted tpy).[41] Reper-
cussions of such dz2 ꢀ dz2 interactions on the photophysics of
our PtII complexes are discussed below.
To test the strength of the extended p–p (and PtꢀPt) inter-
actions in assembly 6d2·CB[8], we combined it with a solution
of homoternary complex 82.CB[8], and monitored the scram-
bling process, if any. Remarkably, even after heating to 408C
for 20 h, self-sorting remained exclusively narcissistic,[42,43] that
is, only homoternary complexes 6d2·CB[8] and 82·CB[8] were
detected in solution, without any trace of heteroternary com-
plex 6d·8·CB[8]. p–p (and Pt-Pt) interactions are thus strong
enough to compensate favorable quadrupole-quadrupole in-
teractions between tolyl and 3,5-difluorophenyl units inside
CB[8].
In a second attempt at testing the limits of the p–p/Pt-Pt in-
teractions in ternary complex 6d2·CB[8], we combined it with
2.0 equiv CB[7]. As CB[7] does show significant affinity for ada-
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mantyl units,[44,45] but cannot encapsulate any of the two ada-
mantyl substituents in ternary complex 6d2·CB[8] due to their
proximity, only three scenarios are possible: (1) CB[7] remains a
free host in solution; (2) CB[7] competes advantageously with
CB[8], ejects it, and forms complex 6d·CB[7]2; or (3) CB[7] dis-
rupts the dimer and converts the assembly into unstacked
[5]pseudorotaxane CB[7]·6d·CB[8]·6d·CB[7]. 1H NMR experi-
ments showed unambiguously that the third option is pre-
ferred;
a
75:18:7
mixture
of
complexes
CB[7]·6d·CB[8]·6d·CB[7], 6d·CB[7]2 and 6d2·CB[8] was obtained
(see Figure 5).
Figure 6. 1H NMR spectra of (a) complex (6b2·CB[8]2)HT, (b) a 31:69 mixture of
complexes (6b2·CB[8]2)HH and (6b2·CB[8]2)HT after a 30 day heating period at
408C, (c) a 59:41 kinetic mixture of complexes (6c2·CB[8]2)HH and
(6c2·CB[8]2)HT, (d) complex (6c2·CB[8]2)HT obtained after heating the mixture
of HH and HT assemblies for 30 h at 408C.
er, one can certainly try to decipher the mechanism of the as-
sembly process. Assuming that free, monomeric platinum ace-
tylides are present in solution before addition of CB[8] (an hy-
pothesis that we will refine later), the first encapsulation event
with a non-negligible activation barrier is the formation of a
1:1 inclusion complex between one of the two binding sites
and CB[8] (see assembly labeled “1:1” in Figure 7). The second
Figure 5. 1H NMR spectra of (a) complex (6d2·CB[8])HH, (b) assembly
CB[7]·6d·CB[8]·6d·CB[7], and (c) complex 6d·CB[7]2. Blue and green dots in-
dicate residual signals of complexes 6d·CB[7]2 and (6d2·CB[8])HH, respectively,
in spectrum b.
Figure 7. Proposed mechanism for the formation and switching of HH and
HT quaternary assemblies.
As mentioned above, HH and HT assemblies exchange
slowly on the NMR time scale. However, we questioned wheth-
er the exchange could also be slow on the experiment time
scale (i.e. within hours instead of ms). In other terms, are these
assemblies just kinetically favored, or has the system reached
thermodynamic equilibrium? To answer this question, the 2:2
complexes were heated to 408C, and the ratios of HH vs. HT
orientations were monitored by 1H NMR spectroscopy as a
function of time. Complexes (6a2·CB[8]2)HT and (6b2·CB[8]2)HT
both saw their partial conversion to HH assemblies, up to 17
and 31%, respectively, once equilibrium was reached after 6
and 30 days at 408C (see Figure 6, spectra a and b, and
Figure 2 for numbering). Surprisingly, the 59:41 mixture of as-
semblies (6c2·CB[8]2)HH and (6c2·CB[8]2)HT underwent quantita-
tive conversion to the HT arrangement in 30 h at 408C (see
spectra c and d in Figure 6). Finally, assembly (7b2·CB[8]2)HT
consistently retained its HT orientation.
Pt complex guest may then (1) ingress the 1:1 complex to
form a ternary complex (labeled “2:1” in Figure 7; rate con-
stants kHH and kHT), or (2) enter a free CB[8] unit via one of its
binding sites (rate constants k’HH and k’HT; see complex labeled
“(1:1)2”). The last step is either the dimerization of the (1:1)2
complex or the capping of the 2:1 complex with a free CB[8],
both affording the desired 2:2 assembly.
The sequence from free ditopic guests and CB[8] hosts to
the 2:2 complexes is cascade-like, with each intermediate as-
sembly being more stable than its precursor, and with low acti-
vation barriers connecting each step. Under kinetic control, the
HH vs. HT selectivity is thus governed by the rates of 2:1 or
(1:1)2 complex formation from the 1:1 assembly (rate constants
kHH, k’HH, kHT and k’HT, see Figure 7). Under thermodynamic con-
Rationalizing the stabilities would be highly speculative,
except perhaps for assembly (7b2·CB[8]2)HT, where quadrupole-
quadrupole interactions between 4-tolyl and 3,5-difluorophen-
yl units are arguably more favorable than the geometric mean
of the corresponding interactions in the homodimers. Howev-
trol, the stabilities of HH and HT 2:2 complexes determine
their ratios in solution.
To test the validity of the proposed mechanism, we mea-
sured apparent rate constants, and extracted free energies of
activation for the partial conversion of complexes (6a2·CB[8]2)HT
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and (6b2·CB[8]2)HT into their HH analogues. The apparent rate
constants are 3.2(ꢁ0.5)ꢁ10ꢀ6 and 7.1(ꢁ0.2)ꢁ10ꢀ7 sꢀ1, respec-
tively (see Supporting Information for details). Blending the
HT!HH conversion into a single step, and applying the Eyring
equation, returns free activation energies of 26.2(ꢁ0.1) and
27.2(ꢁ0.1) kcalmolꢀ1. Using emission titration experiments
(and a series of approximations and limitations, see Supporting
Information), we determined a binding constant of 1.6(ꢁ0.1)ꢁ
1013 mꢀ3 for two complexes 6a forming a 2:2 assembly with
two CB[8] hosts (i.e. an 18 kcalmolꢀ1 free binding energy). As
alluded above, at least 4–5 kcalmolꢀ1 must be added to ac-
count for favorable p–p/Pt-Pt interations between both guests,
bringing the total stabilization upon double CB[8] encapsula-
tion of guests 6a to at least 22–23 kcalmolꢀ1. These interac-
tions are likely significant, as the emission titration could also
be fitted reasonably well with a 2:1 binding model that uses
dimer 6a2 as the “free guest” (see Supporting Information).
Biczꢂk and co-workers determined that the rate constant for
berberine (1) ingression into free CB[8] is 6.4ꢁ107 mꢀ1 sꢀ1, and
5.0ꢁ106 mꢀ1 sꢀ1 for ingression into binary complex 1·CB[8]; the
corresponding free energies of activation are thus 6.8 and
8.3 kcalmolꢀ1, respectively. Assuming that complex 6a suffers
from the same penalty upon entering CB[8] (approximately
7 kcalmolꢀ1), the free energy of activation for the HT!HH pro-
cess should thus be lower than 29–30 kcalmolꢀ1 unless the
conversion proceeds through a fully dissociative mechanism.
As the barrier we measured is indeed lower (26.2 kcalmolꢀ1),
we propose the following mechanism for the HT!HH conver-
sion: complex (2:2)HT proceeds energetically uphill to complex
(1:1)HT; the free guest, which may form an exclusion complex
with the CB[8] rim, would then flip towards assembly (1:1)HH
(see Figure 7), before undergoing reassembly to the (2:2)HH
complex. The free energy of activation for the overall process
is lower than 29–30 kcalmolꢀ1, as one CB[8]-binding unit re-
mains encapsulated throughout the process.
Figure 8. 1H NMR spectra of (a) quaternary assembly (6b2·CB[8]2)HT in D2O
buffer (0.50 mL); (b) a mixture of assemblies (6b2·CB[8]2)HT and (6b2·CB[8])HH
upon addition of CD3CN (80 mL); (c) ternary complex (6b2·CB[8])HH obtained
after further addition of CD3CN (total volume 0.20 mL).
switch is accompanied by dramatic downfield shifts for the
methyl and meta-hydrogens at the tolylacetylide moiety (0.95
and 1.21 ppm, respectively), due to their release from CB[8].
Adding an organic co-solvent to an aqueous solution of a
CB[n]/guest complex has been shown to lower the interaction
between the CB[n] and their guests,[47] as (1) the suboptimal ar-
rangement of “high-energy” water inside the cavity compared
to bulk water, whose release is the major driving force for
guest encapsulation, is not as stark when the bulk is contami-
nated with a co-solvent; and (2) free guests may be better sol-
vated in the presence of a co-solvent than in pure water. Both
tpy substituents and acetylides are thus expected to display re-
duced binding affinities towards CB[8] in the presence of a co-
solvent, but the loss of affinity is clearly greater at the acety-
lides. Again, the remarkable stacked arrangement, with short
PtꢀPt distances, a biased charge distribution between both
CB[8] rims, and extensive p–p interactions between both
guests is quantitatively preferred over a typical ternary com-
plex assembly with two equivalent CB[8] rims.
We finally note that a different mechanism for the kinetically
controlled self-sorting process can be envisioned. As the solu-
bility of all Pt acetylides is very poor in water, and extensive p–
p and PtꢀPt interactions promote the formation of stacked ag-
gregates in the solid state, one could argue that the rate-limit-
ing step of the assembly process is the detachment of a
stacked pair of Pt complexes from the aggregate, followed by
rapid CB[8] encapsulation. Complex 6a crystallizes in a HT ar-
rangement,[46] in agreement with the structure of the kinetical-
ly favored quaternary assembly in solution. The arrangement
could not be confirmed for the other complexes, and the
59:41 kinetic mixture of (6c2·CB[8]2)HH and (6c2·CB[8]2)HT assem-
blies either invalidates this mechanism or suggests a poorly or-
ganized aggregate.
We also note that CB[7] binds Pt acetylides 6 and 7b
(except complex 6c, which remains aggregated in aqueous so-
lution even after heating and sonication). Remarkably, CB[7] ex-
clusively encapsulates the 4’-substituent of the tpy ligand even
in the presence of an excess amount of the macrocycle, except
in complex 6d; the latter undergoes concomitant encapsula-
tion at both binding sites, due to the excellent shape comple-
mentary between the adamantyl group and the cavity of CB[7]
(see Figure 5). We attribute this selectivity to the more positive
electrostatic potential surrounding the terpyridyl ligand com-
pared to the acetylide. Also, the acetylide moiety may retain
some carboanionic character at the terminal alkynyl carbon,
which may then hamper ligand/CB[7] interactions. The CB[7]
and CB[8] preference for the tpy binding site in water and
water/acetonitrile mixtures, respectively, may suggest that the
The self-sorting properties of the Pt acetylides and CB[8] do
not stop here, however. We found that adding aliquots of
[D3]acetonitrile to deuterium oxide solutions of assemblies
62·CB[8]2 (6a–6c) and 7b2·CB[8]2 triggers the ejection of one
CB[8] macrocycle and the reshuffling of the guests into 2:1 HH
complexes (62·CB[8])HH and (7b2·CB[8])HH. The 2:1 assemblies
are quantitatively formed after addition of 40% (v/v)
[D3]acetonitrile (see Figure 8). In the case of complex 6b, the
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favored complexation pathway towards 2:2 assemblies under
kinetically controlled conditions is the formation of 1:1 com-
plexes between the tpy substituent and CB[8] (rate constant
k’HT, see Figure 7), followed by exclusion complex (1:1)2 and
(2:2)HT assembly. In the case of Pt complex 6c, extensive p–p
stacking caused by the longer biphenyl substituent may pro-
mote competitive 2:1 HH complex formation (rate constant
kHH, see Figure 7). The other two pathways (kHT and k’HH) are
certainly less likely.
As studied in detail by Castellano,[48–52] Yam[29–37,53–58] and
Che,[59–61] among others, Pt acetylides have a rich photochemis-
try. Absorption bands at l<400 nm are intraligand p!p*
transitions, and bands at lower energies correspond to
1[dp(Pt)!p*(tpy)] metal-to-ligand charge transfers (1MLCT) and
1
1
a combination of overlapping MLCT and [p(CꢂCR)!p*(tpy)]
ligand-to-ligand (1LLCT) transitions. In the solid state or in dinu-
clear complexes, short PtꢀPt distances (<3.5 ꢃ,[40] and as short
as 2.8 ꢃ[62]) destabilize the filled dz2 orbitals, resulting in lower
energy 1[ds*!p*(tpy)] metal-metal-to-ligand charge transfer
transitions (1MMLCT). Emissions typically originate from the
3[p(CꢂCR)!p*(tpy)] and 3[dp(Pt)!p*(tpy)] transitions. Exten-
sive stacking may also result in emission at lower energies
(typically >800 nm) corresponding to 3MMLCT excited
states.[52,56,57,62–71]
Figure 9. UV/Vis spectra of (a) free guest 6a (black line), complex 6a·CB[7]
(blue line), assembly (6a2·CB[8]2)HT (green line), a 17:83 mixture of assemblies
(6a2·CB[8]2)HH and (6a2·CB[8]2)HT (dashed green line), complex (6a2·CB[8])HH
(dotted green line; 5:2 aqueous borate buffer/acetonitrile solvent mixture);
(b) free guest 6d (black line), ternary complex 6d·CB[7]2 (blue line), ternary
complex (6d2·CB[8])HH (dotted green line). Emission spectra of (c) free guest
6a (black line), complex 6a·CB[7] (blue line), assembly (6a2·CB[8]2)HT (green
line); (d) free guest 6d (black line), ternary complex 6d·CB[7]2 (blue line), ter-
nary complex (6d2·CB[8])HH (dotted green line). Excitation wavelengths are
chosen to match absorption maxima. The solvent is an aqueous borate
buffer unless noted otherwise; Pt concentration for both absorption and
emission experiments is 50 mm.
The impact of CB[n] encapsulation on transition energies
varies greatly, with bathochromic shifts being typically more
common than hypsochromic ones.[72–77] Shifts result from a
combination of competitive factors: (1) as shown by Nau and
co-workers in the case of 2,3-diazabicyclo-[2.2.2]oct-2-ene and
its n!p* transitions,[78] a decrease in the environment polariza-
bility from the solvent to the cavity of CB[7] causes hypsochro-
mic shifts;[79] (2) as shown by Biczꢂk and co-workers in the case
of berberine (1),[77] a decrease in the orientation polarizability
parameter of the environment (that has both a polarity and a
polarizability term) causes bathochromic shifts from water to
CB[7]. One should also note that while the CB[7] cavity has
low polarities and polarizabilities, Kaifer and co-workers recent-
ly showed that the polarity of the environment surrounding
the carbonyl rims of CB[7] is in fact more polar than water;[80]
the impact of this specific environment on the optical proper-
ties of chromophores is not known.
tions, possibly with a 1LLCT component.[81] 1:2 Complexation
with CB[7] at both its tpy and acetylide units again triggers a
blueshift and the emergence of a single band at 424 nm (spec-
trum in blue). A very clear hypsochromic shift (16 nm) is also
observed for the intra-ligand p!p* transitions. Surprisingly,
the 2:1 HH complex (6d2·CB[8])HH shows comparable bands in
addition to significant hyperchromicity in the 350–420 nm
range (dotted green spectrum). In the whole series, bathochro-
mic shifts are only observed with 2:2 assemblies, while red-
shifts are obtained in all other configurations. Intriguingly, par-
tial conversion of assembly (6a2·CB[8]2)HT to its HH analogue
(HH/HT ratio 17:83) alters those bands significantly, with an en-
hanced absorption between 380 and 440 nm, and a very sig-
nificant hypochromicity at the 470 nm band (see solid and
dashed green spectra in Figure 9a). The pattern may in fact be
caused by an hypsochromic shift of the lowest energy transi-
tion, from 473 nm to approximately 450 nm. We cannot justify
this trend at the moment.
Pt complex 6a presents its lowest energy transition at
1
441 nm, likely as a mixture of MLCT/1LLCT (see spectrum in
black in Figure 9a). As mentioned above, addition of CB[7] trig-
gers the formation of binary complex 6a·CB[7], with CB[7] in-
teracting exclusively with the 4’-tolyl substituent of the tpy
ligand. This complexation is accompanied by a significant hyp-
sochromic shift (33 nm; spectrum in blue). To the contrary,
complexation with CB[8], which affords complex (6a2·CB[8]2)HT,
triggers a bathochromic shift (32 nm; solid green spectrum).
Complex (6a2·CB[8])HH only sees a 7 nm hypsochromic shift
compared to the free guest (dotted green spectrum in Fig-
ure 9a). Similar effects are observed with Pt complex 6b. In
the case of complex 6d, a pair of bands is observed at 448
and 468 nm (see spectrum in black in Figure 9b). Those have
been reported on several occasions for Pt acetylides, and may
As far as emissions are concerned, blueshifts, reduced Stokes
shifts and fluorescence enhancements are frequently observed
upon guest encapsulation by CB[n], as the polarity and polariz-
ability of the macrocycles are low, and encapsulation limits
nonradiative decay from the excited state through solvent re-
laxation.[82] To the best of our knowledge, we report here for
1
1
correspond to [dxz(Pt)!p*(tpy)] and [dyz(Pt)!p*(tpy)] transi-
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the first time the effect of CB[n] encapsulation on the phos-
phorescence of a chromophore. Pt acetylide 6a shows very
weak emission at 693 nm upon excitation at 460 nm (see Fig-
ure 9c). CB[7] complexation increases phosphorescence by 24-
arrangements, with one Pt guest leaving the CB[8] hosts to
allow the reshuffling (see Figure 7). Adding acetonitrile to the
aqueous solution of the quaternary assemblies (either as pure
HT conformers or a mixture of HH and HT assemblies) triggers
the ejection of one CB[8] host, and the reshuffling into ternary
complexes in an exclusive HH arrangement, with the remain-
ing CB[8] macrocycle forming a homoternary complex with the
pair of tpy substituents. A similar 2:1 HH arrangement is ob-
tained with complex 6d, even in the absence of organic co-sol-
vent, as the cavity of CB[8] is too small to form ternary com-
plexes with one of the guests being an adamantyl unit. To the
best of our knowledge, this specific host-guest arrangement,
with both positive centers sitting at the same CB[8] rim, and
leaving the other rim free of any interaction with guests, is un-
precedented in ternary CB[n] complexes. In all cases we sur-
veyed, charged substituents are distributed between both
CB[8] rims as evenly as possible. While Scherman’s guests 4a[24]
and our Pt complexes 6a–6c and 7b form quaternary com-
plexes with uneven charge distributions at the CB[8] rims, one
can argue that stacked 2:2 assemblies are typically more stable
than unstacked ones (see Figure 10a), as long as encapsulating
fold and triggers an extremely large blueshift (131 nm; lmax
=
562 nm). CB[8] complexation towards assembly (6a2·CB[8]2)HT
enhances phosphorescence by a factor of 240, and causes a
86 nm blueshift (lmax =607 nm), relative to the free metal-
ligand complex (see Figure 9c). Such shifts are unprecedented
in the literature; to the best of our knowledge, the longest
shift measured until now was 51 nm, for the fluorescence of
berberine (1) and CB[7].[77] In the case of Pt acetylide 6d, com-
plexation by CB[7] and CB[8] towards assemblies 6d·CB[7]2 and
6d2·CB[8] also results in very large blueshifts (from 673 nm to
559 and 568 nm, respectively, see Figure 9d). In this case,
phosphorescence enhancement is greater in the presence of
CB[7] (27-fold, similar to that of complex 6a·CB[7]); upon ter-
nary complex formation with CB[8], phosphorescence is only
increased by a factor of 6.4. Both p–p stacking and double
CB[8] encapsulation are thus needed to maximize emissions.
We finally note that despite short PtꢀPt distances (3.94 and
3.89 ꢃ in ONIOM-optimized assemblies (6b2·CB[8]2)HT and
(6b2·CB[8]2)HH; see Figure 2), we cannot unequivocally identify
MMLCT transitions in both absorption and emission spectra.
Yam and co-workers[54] showed that two Pt acetylide-function-
alized calixarenes 9a and 9b undergo extensive p–p and Pt-Pt
stacking in the solid state, with extremely short PtꢀPt distances
at least in the case of complex 9b (3.27 ꢃ, shorter than the
sum of both approximate van der Waals radii, 3.4 ꢃ). Com-
plex 9a could not be crystallized, however it was found to be
emissive above 820 nm as a solid, at both room temperature
and 77 K. Based on ample literature precedent,[83–85] the au-
Figure 10. Equilibria between stacked and unstacked (a) 2:2 assemblies, and
(b) 2:1 assemblies.
3
thors assigned this emission to MMLCT transitions caused by
the proximity of both Pt centers. In agreement with our results,
MMLCT transitions were not observed in acetonitrile solution,
where complexes 9a and 9b emitted between 640 and
the free moiety of one of the ditopic guests (labeled G in Fig-
ure 10a) into the water-filled cavity of the binary complex (la-
beled H) is more favorable than switching from an even to an
uneven distribution of charges. An interaction between a posi-
tive group and a CB[n] rim corresponds to an approximate
4 kcalmolꢀ1 gain in total free energy (at least in the case of
CB[7]),[45,87] and the free binding energy of a guest such as ber-
berine (1) into binary complex 1·CB[8] is approximately 8–
9 kcalmolꢀ1.[73] Therefore, a stacked 2:2 assembly will remain
favorable over an unstacked arrangement even if a CB[8] rim
can only stabilize one positive charge, as long as the repulsion
between both charged units does not exceed 4–5 kcalmolꢀ1. A
2:2 assembly will also be entropically favored over a dynamic
n:n oligomer, unless obvious steric hindrance prevents the for-
mation of the quaternary assembly (like in the case of oligo-
mers (2·CB[8])n and (3·CB[8])n, for example).[22]
The formation of ternary HH assemblies is much more sur-
prising (see Figure 10b). In this case, the allosteric interaction
between both guests (i.e. outside CB[8]) must be favorable to
compensate for (1) the loss of proper charge/rim interaction
(ꢃ4 kcalmolꢀ1), and (2) Coulombic repulsion between both
positive charges, even as those are weakened by their interac-
tions with the CB[8] rim[87] and by solvation. We have shown
3
740 nm; the origin of the emission was attributed to MLCT/
3LLCT combinations, as discussed above. Similarly to com-
plex 9b that does not present any MMLCT emission even in
the solid state, our results further underline that MMLCT transi-
tions are not a de facto consequence of p–p and PtꢀPt interac-
tions in stacked Pt acetylides, and are highly geometry and dis-
tance dependent.[86]
Conclusions
We showed that PtII acetylides 6 and 7 engage in multiple self-
sorting behaviors depending on the nature of the imposed ex-
ternal stimuli. Pt complexes 6a, 6b and 7b, which carry aryl
substituents at both tpy and acetylide ligands, form exclusive
quaternary HT assemblies under kinetic control (i.e. at room
temperature), but complexes 6a and 6b partially reshuffle to
HH assemblies at higher temperature under equilibrium condi-
tions. Complex 6c adopts both HH and HT arrangements
under kinetic control, but resolves into a pure HT orientation
under thermodynamic control. We propose a partially dissocia-
tive mechanism for the conversion between both HH and HT
Chem. Eur. J. 2018, 24, 1 – 10
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We are grateful to the National Science Foundation (grant
CHE-1507321), the American Chemical Society Petroleum Re-
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from the Condensed Matter and Surface Science Program
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Manuscript received: February 19, 2018
Accepted manuscript online: March 30, 2018
Version of record online: && &&, 0000
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Chem. Eur. J. 2018, 24, 1 – 10
www.chemeurj.org
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
A novel recognition pattern between a
pair of positively charged guests and
cucurbit[8]uril is presented.
&
Macrocycles
K. Kotturi, E. Masson*
&& – &&
Directional Self-Sorting with
Cucurbit[8]uril Controlled by Allosteric
p–p and Metal–Metal Interactions
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&
Chem. Eur. J. 2018, 24, 1 – 10
www.chemeurj.org
10
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!