Supramolecular Ga4L612- cage photosensitizes 1,3-rearrangement of encapsulated guest via photoinduced electron transfer

Article abstract of DOI:10.1021/jacs.5b06317

The K₁₂Ga₄L₆ supramolecular cage is photoactive and enables an unprecedented photoreaction not observed in bulk solution. Ga₄L₆^12- cages photosensitize the 1,3-rearrangement of encapsulated cinnamylammonium cation guests from the linear isomer to the higher energy branched isomer when irradiated with UVA light. The rearrangement requires light and guest encapsulation to occur. The Ga₄L₆^12- cage-mediated reaction mechanism was investigated by UV/vis absorption, fluorescence, ultrafast transient absorption, and electrochemical experiments. The results support a photoinduced electron transfer mechanism for the 1,3-rearrangement, in which the Ga₄L₆^12- cage absorbs photons and transfers an electron to the encapsulated cinnamylammonium ion, which undergoes C-N bond cleavage, followed by back electron transfer to the cage and recombination of the guest fragments to form the higher energy isomer.

Full text of DOI:10.1021/jacs.5b06317

Communication
pubs.acs.org/JACS
Supramolecular Ga4L612− Cage Photosensitizes 1,3-Rearrangement of
Encapsulated Guest via Photoinduced Electron Transfer
Derek M. Dalton, Scott R. Ellis, Eva M. Nichols, Richard A. Mathies, F. Dean Toste,*
Robert G. Bergman,* and Kenneth N. Raymond*
Department of Chemistry, University of California, Berkeley, California 94720-1460, United States
Division of Chemical Sciences, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: The K12Ga4L6 supramolecular cage is
photoactive and enables an unprecedented photoreaction
12−
not observed in bulk solution. Ga4L6
cages photo-
sensitize the 1,3-rearrangement of encapsulated cinnamyl-
ammonium cation guests from the linear isomer to the
higher energy branched isomer when irradiated with UVA
light. The rearrangement requires light and guest
12−
encapsulation to occur. The Ga4L6
cage-mediated
reaction mechanism was investigated by UV/vis absorp-
tion, fluorescence, ultrafast transient absorption, and
electrochemical experiments. The results support a
photoinduced electron transfer mechanism for the 1,3-
12−
rearrangement, in which the Ga4L6
cage absorbs
photons and transfers an electron to the encapsulated
cinnamylammonium ion, which undergoes C−N bond
cleavage, followed by back electron transfer to the cage and
recombination of the guest fragments to form the higher
energy isomer.
Figure 1. K12Ga4L6 host 1 photosensitizes an allylic 1,3-rearrangement
of encapsulated cinnamylammonium guests.
ammonium ions 2 to form the thermodynamically disfavored
3-substituted ammonium ion 3. Chemical and photophysical
studies implicate a photoinduced electron transfer (PET)
mechanism for this process.
We envisioned that cationic allyldimethylcinnamyl-
ammonium ion 2a would be strongly encapsulated12 within
1, and 1 could photosensitize the [2+2] cyclization of 2a to
form a bicyclic cyclobutane product (5, eq 1) with UVA light,
hotosynthesis inspires the development of systems capable
P
of absorbing photons, transferring the harvested energy,
and storing it in chemical bonds.1 Artificial photosynthetic
systems pair light-harvesting molecules, commonly porphyrins,2
with energy acceptors such as quinones,3 carbon nanotubes,4
and fullerenes5 to investigate the intricacies of energy transfer.
Less explored are photosensitizing supramolecular nanovessels
(cyclodextrins,6 cavitands,7 tubular hosts8) that transfer
absorbed light energy to encapsulated guest acceptors and, in
rare cases, elicit chemical transformations. For example, Fujita
12+
and co-workers reported a cationic palladium M6L4 supra-
molecular assembly that participates in the photooxidation of
alkanes.9 In contrast to a polycationic photoreactive cage, a
polyanionic photosensitizing cage would have significantly
different photophysical properties and would preferentially
react with a different class of substrates. For this reason, we
12−
investigated the water-soluble, highly anionic, gallium M4L6
supramolecular assemblies10 that have polyaromatic bridging
ligands as potential photosensitizing agents. We hypothesized
that the redox-active catecholate ligands could be photoexcited
and donate excited-state energy to encapsulated guest
molecules to induce chemical reactivity (Figure 1). Herein,
the triplet-photosensitized process that occurs in the absence of
1.13 Addition of 2a (4.0 μmol, 6 mM) to a K3PO4-buffered (pD
8.0) aqueous solution of 1 (4.2 μmol) provides encapsulation
1
complex 2a⊂1 (⊂ denotes encapsulation) determined by H
NMR spectroscopy (see SI).14 Subsequent UVA irradiation of
12−
we report that Ga4L6 (L = diaminonaphthalene biscatecho-
lamide) 1 photosensitizes an unprecedented allylic 1,3-
Received: June 24, 2015
photorearrangement11 of encapsulated 1-cinnamylalkyl-
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b06317
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
2a⊂1 (14 h, 35 °C) provides not the expected encapsulated
cyclobutane complex 5⊂1 but instead a new product, assigned
Table 1. Yields of Products Formed in the 1,3-
Rearrangement of Variously Substituted
Cinnamylammonium Ions Determined by H NMR
Integration
1
1
by H NMR as encapsulated 3a, resulting from allylic 1,3-
rearrangement of the cinnamyl functional group (eq 2).
Independent synthesis of an authentic sample of 3a and
subsequent formation of encapsulation complex 3a⊂1 con-
firmed that the product resulting from UVA irradiation was
rearrangement product 3a, 4 kcal/mol higher in energy than
linear isomer 2a based on DFT calculations.
The photorearrangement can be extended to other
encapsulated cinnamylammonium ions, although competitive
cleavage to amines 4 is observed in some cases (see Table 1;
vide infra). In the case of trimethylammonium substrate 2b,
1
broad resonances in the H NMR spectrum of 2b⊂1 indicate
that 2b is in rapid equilibrium15 between the internal
encapsulation complex 2b⊂1 and an external association
complex.16 Due to evidence for rapid guest exchange, it was
important to assess whether the observed 1,3-rearrangement
occurred in bulk solution or within the confines of 1’s cavity.
UVA irradiation of 2b (without 1) in K3PO4-buffered D2O
solution (pD 8.0, 14 h, 30 °C) provided only starting material.
However, increasing the time of irradiation to 32 h showed 9%
conversion of the trans-2b isomer to cis-2b. In contrast, in the
presence of 1 and UVA light, 2b reacts completely after 14 h to
form rearrangement product 3b, trimethylamine 4b, and
cinnamyl alcohol.17 Further evidence that formation of the
1,3-rearrangement product requires encapsulation is provided
18
+
by a NEt4 -blocked host experiment. UVA irradiation of a
solution of NEt4⊂1 and 2b fails to provide rearrangement
product 3b after 12 h. A similar result is observed in an
attempted catalytic reaction, in which 1 equiv of 2b and 20 mol
% of 1 provide only 20 mol % of rearrangement product 3b,
suggesting that 3b⊂1 prevents catalyst turnover.
This suggests that the tertiary amine 4b is the result of a
photolytic process derived from the cinnamylammonium
substrate 2b as opposed to product 3b decomposition.
Heating 2b⊂1 at 50 °C for 12 h in the absence of light fails
to provide rearrangement product 3b or trimethylamine 4b and
cinnamyl alcohol, and only encapsulated starting material 2b⊂1
To assess whether 1 was acting simply as a reaction vessel for
the photo-rearrangement or also as a photosensitizing agent
transferring absorbed energy to encapsulated 2, we conducted
UV/vis absorption studies of 1 and 2b, respectively. The UV/
vis absorption spectrum of 1 in H2O shows absorption maxima
at 224 and 330 nm (Figure 2A). The molar absorptivity (ε) for
1 was determined to be (7.6 × 104) 0.3 M−1 cm−1 at 330 nm.
Importantly, the molar absorption coefficient of 1 is more than
4 orders of magnitude greater than that of 2b in the portion of
the spectrum where the UVA light source emits (315−400
nm). Thus, selective excitation of 1, and not 2b, is possible with
a UVA light source. Use of a UVA light source in the
photoreactions enables the unambiguous determination that 1
is a sensitizing agent in the photo-rearrangement of 2b. We
carried out quantum yield studies of this rearrangement and
found that the process occurs with a low quantum yield of
1
is observed via H NMR. These experiments provide evidence
that both UVA light and encapsulation in the cavity of 1 are
necessary for the formation of products 3b and 4b. Formation
of products 3b and 4b cannot be attributed to background
thermal reactions that occur outside the confines of the
supramolecular assembly.
Having established that the rearrangement is a photoinduced
process mediated by assembly 1, we carried out a further
exploration of the factors that influence the rearrangement:
cleavage ratio.19 We found that N-alkyl substitution affects the
ratio of rearrangement product 3 to tertiary amine 4 (Table 1).
All substrates that undergo the rearrangement have an internal
binding affinity of log(Kint) > 2, as determined by competition
14
+
studies with NMe4 . N-Ethyl 2c, N-propyl 2d, and N-allyl 2a
are optimal substrates for the rearrangement; in these cases, 3 is
formed in good yields, and tertiary amine side product is not
observed by 1H NMR. Substrates that are weakly encapsulated,
such as N-hexyl substrate 2g with log(Kint) < 2, do not form the
rearrangement product 3g, and only encapsulated tertiary
amine 4g is observed. These results suggest that weak binding
of the substrate correlates with weak binding of the fragments
formed upon photoreaction (see mechanistic discussion
below).
Φ
R‑2b = 0.010 0.007. Quantum yields were determined using
the Norrish type II cleavage of butyrophenone as an
actinometer.20
Photosensitization of the encapsulated guest by the supra-
molecular host could occur by two means: energy transfer or
electron transfer. For either Forster energy transfer (FRET) or
̈
Dexter energy transfer mechanisms to be operative, overlap of
the cinnamylammonium cation 2b absorption spectrum with
12−
the Ga4L6
fluorescence emission spectrum needs to be
present.21 However, an overlay of 2b absorption and Ga4L6
emission shows no overlap (Figure 2A), which means that a
mechanism of exciton energy transfer to 2b is not operative in
this system. This leaves a PET mechanism as the likely mode of
photosensitization.
12−
To test whether the trialkylamine products 4 were the result
of a decomposition pathway of rearrangement product 3,
encapsulated rearrangement product 3b⊂1 was irradiated with
UVA, but no decomposition was seen after 12 h of irradiation.
B
DOI: 10.1021/jacs.5b06317
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Journal of the American Chemical Society
Communication
transfer (eq 3).21 Gibbs energy of PET is approximated using
the standard reduction potentials of donor 1 (ED) and acceptor
*
°
+
°
−
ΔG° = ED /D − EA/A − ED − C
(3)
2b (EA) and the electronic excited-state energy of the donor
(ED*). Cyclic voltammetry was used to estimate that the ED of 1
is 0.73 V vs SHE (16.8 kcal/mol), and the EA of 2b is estimated
at −1.69 V vs SHE (−39.6 kcal/mol, see SI for details).24 The
electronic excitation energy (E*D) is estimated from the E0,0
transition at 391 nm, found at the intersection of the absorption
and emission spectra of 1 (Figure 2), to be 3.17 eV, or 73.1
kcal/mol. The Coulombic term, C, for solvent stabilization of
opposite charges in H2O is estimated to be 0.08 kcal/mol.21
With the above information, we find a negative free energy of
−17.2 kcal/mol for the transfer of an electron from excited
singlet 1 to the charge-transfer state of 2b⊂1. The negative free
energy value provides support for a PET mechanism.
We propose that photoexcited assembly 2⊂1* acts as a PET
agent in the 1,3-rearrangement mechanism. In this pathway, 1
absorbs incoming light to generate the excited charge-transfer
state 2⊂1*, in which an electron has been donated to the
encapsulated cinnamylammonium acceptor 2 (I, Scheme 1).
Figure 2. (A) UV/vis absorption spectra of K12Ga4L6, 1 (blue) and
cinnamylammonium 2b (black) at 4 × 10−6 M in H2O, 10 mM K3PO4,
pH 8.0, overlaid with fluorescence emission of 1 (gold). (B) Contour
plot of dispersed transient absorption of K12Ga4L6 in H2O from 200 fs
to 200 ps after 400 nm actinic excitation. (C) Band integral over each
10 nm range in the transient absorption and the global exponential fits
to the decay. (D) Comparison of transient absorption decay between
540 and 550 nm with and without 2b.
Scheme 1. Proposed Mechanism of K12Ga4L6-
Photosensitized 1,3-Shift of Cinnamylammonium Cations
Having identified 1 as the photosensitizing agent, we sought
to understand the role of the excited state of 1 in activating the
rearrangement of 2b through transient absorption spectroscopy
(see SI for details). Upon pulsed excitation with 400 nm light,
an excited-state absorption band appears with λmax = 540 nm, as
seen in the contour plot of the optical density change with
wavelength (Figure 2B) and in the wavelength-separated
absorption plot (Figure 2C). The excited-state absorption
band corresponds to a transition from the first excited singlet
state (S1) to a higher energy electronic state (Sn). The signals
were analyzed in the context of first-order kinetics with three
steps identified.
In the first resolvable step, the absorption blue-shifts (τ1 =
0.8 0.3 ps), corresponding to librational reorientation of the
water solvent shell.22 The next excited-state absorption decay
step (τ2 = 8.0 0.8 ps) likely corresponds to exciton migration
over the six ligands of tetrahedron 1. The highly symmetric
tetrahedral structure allows excitons to “hop” between the
Excited-state electron donation to cation 2 results in heterolytic
C−N cleavage (II), forming a tertiary amine and a geminal
radical ion pair (RIP). In this case, the RIP is the stabilized allyl
radical and a ligand-based radical cation moiety incorporated
into the multi-anionic cage (III). Back electron transfer from
either the allyl radical or the tertiary amine to the ligand-based
radical cation would form a stabilized allyl cation or tertiary
amine radical cation and reestablish the original charge on the
ligand (IV).25 The encapsulated tertiary amine recombines with
the allyl cation within the cavity to form the 3-substituted allyl
product (V) or, in some cases, competitively escapes the cavity
to give free amine. The cation either attacks one of the bridge
ligands or reacts with H2O to form cinnamyl alcohol.
orbitals of neighboring ligand residues through Forster-type
̈
dipole coupling.23 This interpretation is supported by the
depolarized fluorescence emission (see SI for details). In the
final step, the excited-state population S1 relaxes back to the
ground electronic state S0 (τ3 = 540 40 ps). In comparison
with “empty” 1, encapsulation complex 2b⊂1 shows that
interactions with 2b only weakly perturb the electronic
structure of 1. The greatest difference is apparent in the final
relaxation step out of S1. In the presence of 2b, the excited-state
population decays with a time constant of 190 60 ps, as
compared to 540 40 ps in the absence of 2b (Figure 2D).
Thus, we estimate that the excited-state electron transfer occurs
with a time constant of 290 150 ps and a quantum yield of
ΦET = 0.65 0.34.
In summary, we have found that K12Ga4L6 supramolecular
cages act as photosensitizers to transfer energy to encapsulated
guest molecules, inducing transformations not observed in bulk
solution. Encapsulation of the cinnamylammonium substrates is
required for the 1,3-rearrangement to occur. Energy transfer
was found to occur by a photoinduced electron transfer
mechanism. Photosensitization and 1,3-rearrangement via PET
12−
are new modes of action enabled by the cavity of the Ga4L6
Additional evidence for the PET mechanism is provided by
calculation of the Gibbs energy of photoinduced electron
cage.
C
DOI: 10.1021/jacs.5b06317
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Journal of the American Chemical Society
Communication
(16) Sgarlata, C.; Mugridge, J. S.; Pluth, M. D.; Tiedemann, B. E. F.;
Zito, V.; Arena, G.; Raymond, K. N. J. Am. Chem. Soc. 2010, 132,
1005−1009.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b06317.
■
S
(17) 1H NMR and mass spectral data indicate that a byproduct is the
reaction of the cinnamyl cation with one ligand of 1.
(18) Hart-Cooper, W. M. W.; Clary, K. N.; Toste, F. D.; Bergman, R.
G. R.; Raymond, K. N. K. J. Am. Chem. Soc. 2012, 134, 17873−17876.
(19) (a) Brown, C. J.; Bergman, R. G.; Raymond, K. N. J. Am. Chem.
Soc. 2009, 131, 17530−17531. (b) Hastings, C. J.; Fiedler, D.;
Bergman, R. G.; Raymond, K. N. J. Am. Chem. Soc. 2008, 130, 10977−
10983. (c) Fiedler, D.; Bergman, R. G.; Raymond, K. N. Angew. Chem.,
Int. Ed. 2004, 43, 6748−6751.
Experimental details and characterization data (PDF)
AUTHOR INFORMATION
Corresponding Authors
*fdtoste@berkeley.edu
*rbergman@berkeley.edu
*raymond@socrates.berkeley.edu
■
(20) Wagner, P. J.; Kemppainen, A. E. J. Am. Chem. Soc. 1972, 94
(21), 7495.
Notes
(21) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular
Photochemistry of Organic Molecules; University Science Books:
Sausalito, CA, 2010.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(22) Jarzeba, W.; Walker, G. C.; Johnson, A. E.; Kahlow, M. A.;
Barbara, P. F. J. Phys. Chem. 1988, 92, 7039−7041.
(23) (a) Langhals, H.; Esterbauer, A. J.; Walter, A.; Riedle, E.;
Pugliesi, I. J. Am. Chem. Soc. 2010, 132 (47), 16777−16782.
(b) Hwang, I.-W.; et al. J. Photochem. Photobiol., A 2006, 178 (2−3),
130−139.
(24) Irreversible electrochemical cycles are observed after multiple
scans for 1, dependent on scan rate, and after a single scan for 2b.
(25) We propose that the cinnamyl radical is the source of BET for
several reasons. We observe tertiary amine 4. We observe < 5%
product where the cinnamyl has reacted with one cage ligand. We do
not observe intramolecular cyclization onto a pendant alkene radical
cation trap. [ As an example, see: Newcomb, M.; Deeb, T. M. J. Am.
Chem. Soc. 1987, 109, 3163. ] We do not observe hemiaminal or
aldehyde products that would result from amine radical cation
decomposition. [ As an example, see: Shono, T.; et al. J. Am. Chem.
Soc. 1982, 104, 5753. ] Although we cannot rule out the formation of
an amine radical cation, at this time we have not found evidence to
support its formation.
■
This research was supported by the Director, Office of Science,
Office of Basic Energy Sciences, and the Division of Chemical
Sciences, Geosciences, and Biosciences of the U.S. Department
of Energy at LBNL (DE-AC02-05CH11231). E.M.N. gratefully
acknowledges support from the National Science Foundation
Graduate Research Fellowship Program (NSF GRFP). We
thank Dr. Heinz Frei, Dr. Daniel Dietze, and Rebecca Schafer
for helpful discussions.
̈
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DOI: 10.1021/jacs.5b06317
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