Ion-gated synthetic photosystems

Article abstract of DOI:10.1021/ja501389g

Herein, molecular strings of ions built along charge-transporting channels are shown to dramatically increase photocurrents and enable charge transport over long distances, thus confirming the existence and significance of ion-gated photosystems. For their synthesis, ordered and oriented stacks of naphthalenediimides were grown on indium tin oxide by ring-opening disulfide-exchange polymerization. To these charge-transporting channels, coaxial strings of anions or cations-fixed, mobile, complete, partial, pure, or mixed-were added by orthogonal hydrazone exchange. The presence of partially protonated carboxylates was found to most significantly increase activity, implying that they both attract holes and repel electrons, that is, facilitate photoinduced charge separation and hinder charge recombination at the same time. As a result of this quite remarkable situation, photocurrents increased rather than decreased with increasing charge stabilization on their stepping stones. The presence of mobile anions facilitated long-distance charge transport through thick films. Turned off by inhibited anion mobility, that is, proton hopping, hole/proton antiport is identified to account for long-distance charge transport in ion-gated photosystems.

Full text of DOI:10.1021/ja501389g

Communication
pubs.acs.org/JACS
Ion-Gated Synthetic Photosystems
Naomi Sakai, Pierre Charbonnaz, Sandra Ward, and Stefan Matile*
Department of Organic Chemistry, University of Geneva, CH-1211 Geneva, Switzerland
S
* Supporting Information
through π-stacked organic semiconductors using photocurrent
generation as a readout.
To construct desired photosystems with ionizable groups
attached along charge-transporting pathways, self-organizing
surface-initiated polymerization with templated stack exchange
(SOSIP-TSE) was the ideal synthetic approach (Scheme 1).5 In
ABSTRACT: Herein, molecular strings of ions built along
charge-transporting channels are shown to dramatically
increase photocurrents and enable charge transport over
long distances, thus confirming the existence and
significance of ion-gated photosystems. For their synthesis,
ordered and oriented stacks of naphthalenediimides were
grown on indium tin oxide by ring-opening disulfide-
exchange polymerization. To these charge-transporting
channels, coaxial strings of anions or cationsfixed,
mobile, complete, partial, pure, or mixedwere added
by orthogonal hydrazone exchange. The presence of
partially protonated carboxylates was found to most
significantly increase activity, implying that they both
attract holes and repel electrons, that is, facilitate
photoinduced charge separation and hinder charge
recombination at the same time. As a result of this quite
remarkable situation, photocurrents increased rather than
decreased with increasing charge stabilization on their
“stepping stones.” The presence of mobile anions
facilitated long-distance charge transport through thick
films. Turned off by inhibited anion mobility, that is,
proton hopping, hole/proton antiport is identified to
account for long-distance charge transport in ion-gated
photosystems.
Scheme 1. a. (1) NH2OH, (2) 2, 3, or 4, AcOH, (3) Tris
buffer pH 7.4
he efficient generation and transport of charges are
1
Tfundamental in material sciences as well as in biology. In
biological systems, charges usually migrate over long distance
via successive hopping from one site to another.1a−c These
hopping sites consist of relatively easily oxidizable residues/
bases, charges on which are further stabilized by simultaneous
proton transfer in proton-coupled electron transfer2 or by the
reorganization of polar surrounding molecules including ions.3
Although numerous model studies established the significance
of proton-coupled electron transfer in biological charge
transport, the importance of counterions attracted much less
attention.3 DNA is the privileged scaffold to study environ-
mental effects on charge transport, and indeed, the critical
importance of backbone phosphates was found for efficient
electron transport over long distances.1a,b,3a Ions attached next
to electroactive moieties are also known to influence their redox
properties through charge repulsion or attraction3c,d and, thus,
the rate of charge separation and recombination.3e,f We were
particularly intrigued by the possibility to modulate the rate of
long-distance charge transport by incorporating ion-gated redox
gradients4 in the charge-conducting pathway. Herein, we report
the influence of surrounding counterions on charge transport
brief, SOSIP of propagators containing two protected
hydrazides is initiated by thiolate groups in initiators that are
covalently bound to a conductive indium tin oxide (ITO)
surface. Molecular recognition between the propagator and the
initiator on the ITO surface aligns the central aromatic units
before ring-opening disulfide-exchange polymerization takes
place and, thus, produces oriented π-stack architectures as
shown in 1.6 Although the compatibility with the other
aromatic units is proven,5b,7 current studies are mostly
conducted with best established electron-deficient alkoxy-
substituted naphthalenediimides (NDIs) as the central
charge-transporting unit.8 Their ability to undergo symmetry-
breaking charge separation and to generate photocurrent has
been already demonstrated.9,10 Mild, efficient, and tolerant to
the size and nature of stack exchangers, TSE seems well suited
for the controlled incorporation of ions into the photosystems.
Thus, aldehydes with ionizable groups 2−4 were synthesized as
Received: February 10, 2014
Published: April 1, 2014
© 2014 American Chemical Society
5575
dx.doi.org/10.1021/ja501389g | J. Am. Chem. Soc. 2014, 136, 5575−5578

Journal of the American Chemical Society
Communication
stack exchangers from amino-acid derivatives by standard
peptide synthesis methodology (Supporting Information (SI)
Scheme S1). Then, starting from the already reported
photosystem 1,5a removal of the benzaldehyde protecting
group, hydrazone bond formation with aldehyde 2 containing a
glutamate residue, followed by an equilibration in neutral buffer
resulted in the desired oriented π-stacked architecture 5 with
negative charges attached along the conductive pathways.
Interestingly, >5 h of equilibration at 40 °C was necessary to
achieve stable photocurrents (see below). Control experiments
with pH-sensitive NDI stacks11 confirmed that strongly
decelerated acid−base chemistry accounts for this exceptionally
slow response, presumably because of hindered diffusion of
buffer within the complex surface architectures (not shown).
Slow acid−base chemistry has been observed before in organic
materials, although not to this extent.12 The photosystems with
positive (6) or neutral charges (7) were prepared similarly
using lysine or acetyl-lysine containing aldehyde 3 or 4,
respectively. According to absorption spectroscopy, hydrazone
exchange reactions proceeded in high yield (>86%) in all the
photosystems, and the obtained hydrazones were stable in
neutral buffer at 40 °C for >3 days (SI Figure S1a). AFM
images demonstrated that the highly ordered structures
characteristic of SOSIP architectures5 remain intact under
these conditions (SI Figure S1c).
Photocurrent generation was examined using the photo-
system as working electrode, a Pt wire as counter electrode and
an Ag/AgCl reference electrode, all immersed in a buffer
containing a mobile hole carrier, triethanol amine (TEOA). As
expected, pH dependence of photocurrent generation demon-
strated significant difference among the photosystems with
different charges (SI Figure S3a). In general, photocurrent
decreased upon lowering of pH because of the decreasing
concentration of active neutral TEOA in this pH range. Thus,
the neutral photosystem 7 was used as a reference to reveal true
pH dependence of photoactivity (Figure 1b). Compared with
the almost unresponsive 6, the clearly different, bell-shaped pH
dependence observed for photosystem 5 suggested that partial
deprotonation promotes photocurrent generation (Figure 1b,
● vs ○). The presence of protonated and deprotonated
carboxylic acids at pH ∼ 7.5 seems reasonable considering that
because of intramolecular charge repulsion, complete deproto-
nation of polyglutamic acid occurs at pH ∼ 9, about four units
above the intrinsic pKa of the carboxylic acids.13 Significantly
higher photocurrent generation at pH ∼ 7.5 by photosystem 5
compared with 6 or 7 could then be rationalized by facilitated
photoinduced charge separation between NDIs with and
without anion. Namely, as nearby carboxylates raise HOMO
and LUMO energy levels of NDIs by about 50 mV,3c charge
separation between them would be energetically favorable
(Figure 1c, HOMO = highest occupied molecular orbital,
LUMO = lowest unoccupied molecular orbital). Moreover, the
charge-separated state would be stabilized not only by the
Coulombic attraction between holes and carboxylate anions but
also by the repulsion between the electrons and anions.
Figure 1. (a) Photocurrent I generated by anionic (5, red solid line),
cationic (6, blue dashed line), and neutral photosystems (7, gray
dotted line) in response to irradiation with a solar simulator (power,
28 mW cm−2; 50 mM TEOA, 0.2 M NamHnPO4 buffer, pH 7.7 at 0 V
vs Ag/AgCl). (b) pH dependence of photocurrent I at saturation by 5
( ) and 6 ( ) relative to 7 (I0) (irradiation with solar simulator, P =
28 mW cm−2; 50 mM TEOA, 0.2 M sodium phosphate buffer, pH
6.2−8.8 at 0 V vs Ag/AgCl). (c) Anionic photosystem 5 at near
neutral pH, with expected HOMO (bold) and LUMO (dashed)
energy levels of NDIs with/without ions (in electronvolts against a
vacuum, normalized for −5.1 eV for Fc+/Fc).
●
○
pH 7.5 (I/I0 ∼ 7, Figure 1b), the hole-stabilizing counterions
move together with their holes by relaying protons in the other
direction along the coaxial string of partially protonated
carboxylic acids. Although similar mechanisms could apply to
the cationic photosystem 6, the observed much lower
photocurrent suggested that in this system, the stabilization
of electrons is less important than the stabilization of holes.
To gain further insight on ion gating, the temperature
dependence of photocurrent generation was investigated. As
implied from the biphasic kinetics (Figure 1a), photocurrent
generation by the anionic photosystem 5 increased sharply as
the temperature of the buffer was raised (Figure 2a, ●). The
temperature dependence was much weaker with cationic or
Photosystems 5′ containing both anions and neutral groups
like photosystem 5 around pH 7.5 was prepared by conducting
mixed TSE with 2 and 4. Unlike in photosystem 5 around pH
7.5, anions in this system cannot migrate together with charges.
Measured at full deprotonation at pH 9, photosystem 5′
generated only about 50% more photocurrent compared to
photosystem 7 (I/I0 ∼ 1.5). This result indicated that for the
generation of the large photocurrents by photosystem 5 around
Figure 2. (a) Temperature dependence of the initial photocurrent by
⧫
■
●
○
5 ( ), 6 ( ), 7 ( ), and 1 ( ) in response to irradiation with a solar
simulator (power, 75 mW cm−2; 50 mM TEOA, 0.2 M sodium
phosphate buffer, pH 7.7 at 0 V vs Ag/AgCl at 20−46 °C) fitted to the
Arrhenius equation. (b) Dependence of photocurrent generation on
■
●
○
the thickness of films for 5 ( ), 6 ( ), and 7 ( ). A480 = 0.1
corresponds roughly to 30 nm film thickness,9 curves are added as
visual guides.
5576
dx.doi.org/10.1021/ja501389g | J. Am. Chem. Soc. 2014, 136, 5575−5578

Journal of the American Chemical Society
Communication
neutral photosystems 6 and 7 (Figure 2a, ○ and ■). By
applying the Arrhenius equation (k = A exp(−Ea/RT)) to the
photocurrent-temperature dependence, the activation energy Ea
could be estimated (Figure 2a). The obtained values (i.e., Ea
(5) = 0.40 eV > Ea (7) = 0.33 eV > Ea (1) = 0.25 eV > Ea (6) =
0.19 eV) were in the range of organic semiconductors,14
DNA,15 or peptides16 but larger than those found with some
highly conductive organic materials.17a,18
The effect of ions was further examined using unsubstituted
NDI-perylenediimide (PDI) heterojunction photosystems 10−
12 (Figure 4).5d Although both NDIs and PDIs are n-
According to a multistep hopping mechanism, charge
transport occurs via thermal activation of charge carriers
residing mostly on “stepping stones” or “relays”.1,17 Highest Ea
found for the anionic photosystem 5, thus, implied that hole
transport is rate limiting, which is reasonable because NDIs are
well-known n-semiconductors.8,19 The found correlation
between high photocurrent and high Ea demonstrated that
stabilization of the charge-separated state, and thus, deceler-
ation of charge recombination determines photocurrent
generation.20 Consistent with the ability of “mobile” counter-
ions to assist charge transport over long distances by moving
together with the holes they stabilize and protect, only the
anionic photosystem 5 generated more photocurrent when the
thickness of the film was increased (Figure 2b, ● vs ○ and ■).
These results suggested that the stepwise installation of
anionic and cationic groups along NDI π-stacks would result in
charge-transporting pathways with redox gradients. We have
previously shown that redox gradients improve the overall
performance of synthetic photosystems.4,5a,c In those systems,
we installed redox gradients using different chromophores.
Here, we were interested in the possibility to build gradients by
directionally attaching ions along single-component charge-
transporting pathways. As in previous examples, partial removal
and exchange of stack exchangers proceeded in high yield to
give the desired gradient systems 8 and 9 (SI Figure S1b).
However, contrary to our expectations, the photosystem 8 with
a favorable redox gradient generated less photocurrent than 9
(Figure 3, ● vs ⧫). The overall larger contribution of ions near
the electrode surface supported the critical importance of
stabilizing holes that are generated near the electrode and far
from the hole acceptor in solution.
Figure 4. Structures of double-channel photosystems 10−12 with/
without appended ions. Inset: Temperature dependence of photo-
⧫
■
●
current generation by 10 ( ), 11 ( ), and 12 ( ).
semiconductors, surprisingly large photocurrents generated by
dyad 12 suggested efficient electron transfer from PDIs to
NDIs. The ability of PDIs to act as a hole transporter has been
demonstrated theoretically21 and experimentally.22 According
to the above interpretations, ions are expected to affect systems
with efficient charge separation in the same way but less
significantly. Using the “triple-channel” strategy,5d ionic
moieties were first attached to PDIs via oxime bonds.
Subsequent stack exchange with the obtained conjugates gave
the NDI-PDI double-channel photosystems with ions attached
along the PDI channels (SI Figure S2).
In the cationic photosystem 10, guanidinium instead of
ammonium groups were used to prevent the intramolecular
imine formation, which was found to hamper efficient stack
exchange. Unlike in the case of single-channel photosystems,
almost the same photocurrent was generated by anionic
photosystem 11 and the neutral control 12 over the pH
range tested (SI Figure S3). These results were in agreement
with above conclusion that partial charges can facilitate
symmetry-breaking charge separation between identical chro-
mophores, but that such ion gating should become less
important with efficient donor−acceptor dyads. Correspond-
ingly weaker effects of ions were found in the temperature
dependence of photocurrent generation (Figure 4). However,
the observed trends for the activation energies were as with the
single-channel systems 5−7 (Figure 2a), that is, Ea (11) = 0.31
eV > Ea (12) = 0.29 eV > Ea (10) = 0.20 eV (Figure 4).
Although linear correlations were found between photocurrent
and thickness of films with both 11 and 12, slightly higher Ea
supported the potential of anionic photosystem 11 to transport
charges over longer distances (SI Figure S3).
In summary, we have shown that photocurrent generation
can be greatly influenced by ions attached along the charge
transporting pathways. Anions were found to improve the
photocurrent generation by stabilizing holes and preventing
charge recombination at the same time. Most importantly,
“mobile” anions are demonstrated to mediate hole transfer over
long distances by a coupled proton antiport along a coaxial
string of mixed carboxylic acids and carboxylates, whereas
Figure 3. (a) Temperature-dependent photocurrent generation by
gradient photosystems 8 and 9. From lower to higher photocurrent,
measured at about 25, 35, and 45 °C. For other conditions, see Figure
⧫
2. (b) Dependence of the initial photocurrent by photosystem 8 ( )
●
and 9 ( ) to the temperature.
5577
dx.doi.org/10.1021/ja501389g | J. Am. Chem. Soc. 2014, 136, 5575−5578

Journal of the American Chemical Society
Communication
(4) Bhosale, R.; Sakai, N.; Mísek, J.; Matile, S. Chem. Soc. Rev. 2010,
39, 138−149.
“fixed” counterions fail to do so. Depending on the nature of
the charge-transporting channel, similar mechanisms will apply
also to electrons and cations.
(5) (a) Sakai, N.; Matile, S. J. Am. Chem. Soc. 2011, 133, 18542−
18545. (b) Areephong, J.; Orentas, E.; Sakai, N.; Matile, S. Chem.
Commun. 2012, 48, 10618−10620. (c) Sforazzini, G.; Turdean, R.;
Sakai, N.; Matile, S. Chem. Sci. 2013, 4, 1847−1851. (d) Sforazzini, G.;
Sakai, N.; Orentas, E.; Bolag, A.; Matile, S. J. Am. Chem. Soc. 2013, 135,
12082−12090.
Considering that long-distance charge-transfer is a funda-
mental process in the materials sciences as well as in
biology,1−3,23 the lessons learned in this study should be of
high general significance. They might help to better understand
biological charge transfer involved in respiration, photosyn-
thesis or enzymatic transformations (ribonucleotide reductase,
photolyase, etc).1−3,23 Conductive pilin filaments, for example,
which are responsible to electronically connect bacteria with
ferric oxide minerals, are composed of helix bundles with
clusters of charged and aromatic amino acids.23 Transcribed to
the materials science, we hope that these results will encourage
the strategic positioning of “fixed” or “mobile” counterions for
the design and development of future advanced organic
electronics, for example, solar cells, field-effect transistors,
light emitting diodes, nanowires, and so forth.
(6) (a) Bang, E.-K.; Gasparini, G.; Molinard, G.; Roux, A.; Sakai, N.;
Matile, S. J. Am. Chem. Soc. 2013, 135, 2088−2091. (b) Orentas, E.;
Lista, M.; Lin, N.-T.; Sakai, N.; Matile, S. Nat. Chem. 2012, 4, 746−
750. (c) Bang, E.-K.; Lista, M.; Sforazzini, G.; Sakai, N.; Matile, S.
Chem. Sci. 2012, 3, 1752−1763.
(7) Charbonnaz, P.; Sakai, N.; Matile, S. Chem. Sci. 2012, 3, 1492−
1496.
(8) Sakai, N.; Mareda, J.; Vauthey, E.; Matile, S. Chem. Commun.
2010, 46, 4225−4237.
(9) Sakai, N.; Lista, M.; Kel, O.; Sakurai, S.-I.; Emery, D.; Mareda, J.;
Vauthey, E.; Matile, S. J. Am. Chem. Soc. 2011, 133, 15224−15227.
(10) Vauthey, E. ChemPhysChem 2012, 13, 2001−2011.
(11) Fin, A.; Petkova, I.; Alonso Doval, D.; Sakai, N.; Vauthey, E.;
Matile, S. Org. Biomol. Chem. 2011, 9, 8246−8252.
(12) Hinsberg, W.; Houle, F. A.; Lee, S. W.; Ito, H.; Kanazawa, K.
Macromolecules 2005, 38, 1882−1898.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures. This material is available free
of charge via the Internet at http://pubs.acs.org.
(13) (a) Myer, Y. P. Macromolecules 1969, 2, 624−628.
(b) Zimmermann, R.; Kratzmuller, T.; Erickson, D.; Li, D.; Braun,
̈
H.-G.; Werner, C. Langmuir 2004, 20, 2369−2374. (c) Baumeister, B.;
Som, A.; Das, G.; Sakai, N.; Vilbois, F.; Gerard, D.; Shahi, S. P.; Matile,
S. Helv. Chim. Acta 2002, 85, 2740−2753.
AUTHOR INFORMATION
Corresponding Author
Stefan.Matile@unige.ch
■
(14) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.;
Silbey, R.; Bredas, J.-L. Chem. Rev. 2007, 107, 926−952.
(15) Wasielewski, M. R.; Conron, S. M. M.; Thazhathveetil, A. K.;
Burin, A. L.; Lewis, F. D. J. Am. Chem. Soc. 2010, 132, 14388−14390.
(16) Arikuma, Y.; Nakayama, H.; Morita, T.; Kimura, S. Angew.
Chem., Int. Ed. 2010, 49, 1800−1804.
(17) (a) Kim, B. J.; Yu, H.; Oh, J. H.; Kang, M. S.; Cho, J. H. J. Phys.
Chem. C 2013, 117, 10743−10749. (b) Podzorov, V.; Menard, E.;
Borissov, A.; Kiryukhin, V.; Rogers, J. A.; Gershenson, M. E. Phys. Rev.
Lett. 2004, 93, 086602.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NMR and the Sciences Mass Spectrometry
(SMS) platforms for services, and the University of Geneva, the
European Research Council (ERC Advanced Investigator), the
National Centre of Competence in Research (NCCR) in
Chemical Biology, and the Swiss NSF for financial support.
(18) (a) Fabiano, S.; Yoshida, H.; Chen, Z.; Facchetti, A.; Loi, M. A.
ACS Appl. Mater. Interfaces 2013, 5, 4417−4422. (b) Riedel, I.; Parisi,
J.; Dyakonov, V.; Lutsen, L.; Vanderzande, D.; Hummelen, J. C. Adv.
Funct. Mater. 2004, 14, 38−44.
REFERENCES
■
(19) Katz, H.; Lovinger, A.; Johnson, J.; Kloc, C.; Siegrist, T.; Li, W.;
Lin, Y.; Dodabalapur, A. Nature 2000, 404, 478−481.
(1) (a) Genereux, J. C.; Barton, J. K. Chem. Rev. 2010, 110, 1642−
1662. (b) Giese, B. Acc. Chem. Res. 2000, 33, 631−636. (c) Cordes,
M.; Giese, B. Chem. Soc. Rev. 2009, 38, 892−901. (d) Beaujuge, P. M.;
(20) (a) van der Holst, J. J. M.; van Oost, F. W. A.; Coehoorn, R.;
Bobbert, P. A. Phys. Rev. B 2009, 80, 235202. (b) Pivrikas, A.; Sariciftci,
́
Frechet, J. M. J. J. Am. Chem. Soc. 2011, 133, 20009−20029.
̈
̌
N. S.; Juska, G.; Osterbacka, R. Prog. Photovolt: Res. Appl. 2007, 15,
(e) Morisue, M.; Yamatsu, S.; Haruta, N.; Kobuke, Y. Chem.Eur. J.
677−696. (c) Vijila, C.; Singh, S. P.; Williams, E.; Sonar, P.; Pivrikas,
A.; Philippa, B.; White, R.; Kumar, E. N.; Sandhya, S. G.; Gorelik, S.;
Hobley, J.; Furube, A.; Matsuzaki, H.; Katoh, R. J. Appl. Phys. 2013,
114, 184503.
2005, 11, 5563−5574.
(2) (a) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.;
Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.;
Meyer, T. J. Chem. Rev. 2012, 112, 4016−4093. (b) Dempsey, J. L.;
Winkler, J. R.; Gray, H. B. Chem. Rev. 2010, 110, 7024−7039.
(c) Reece, S. Y.; Hodgkiss, J. M.; Stubbe, J.; Nocera, D. G. Philos.
Trans. R. Soc., B 2006, 361, 1351−1364. (d) Wenger, O. S. Acc. Chem.
Res. 2013, 46, 1517−1526.
(3) (a) Barnett, R. N.; Cleveland, C. L.; Joy, A.; Landman, U.;
Schuster, G. B. Science 2001, 294, 567−571. (b) Marcus, R. A. J. Phys.
Chem. B 1998, 102, 10071−10077. (c) Abraham, B.; McMasters, S.;
Mullan, M. A.; Kelly, L. A. J. Am. Chem. Soc. 2004, 126, 4293−4300.
(d) Rosokha, S. V.; Sun, D.; Fisher, J.; Kochi, J. K. ChemPhysChem
(21) (a) Delgado, M. C. R.; Kim, E.-G.; da Silva Filho, D. A.; Bredas,
J.-L. J. Am. Chem. Soc. 2010, 132, 3375−3387. (b) Vura-Weis, J.;
Ratner, M. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2010, 132, 1738−
1739.
(22) Singh, T. B.; Erten, S.; Guenes, S.; Zafer, C.; Turkmen, G.;
Kuban, B.; Teoman, Y.; Sariciftci, N. S.; Icli, S. Org. Electron. 2006, 7,
480−489.
(23) Giese, B.; Eckhardt, S. Chimia 2013, 67, 200−203.
2008, 9, 2406−2413. (e) Gao, J.; Muller, P.; Wang, M.; Eckhardt, S.;
̈
Lauz, M.; Fromm, K. M.; Giese, B. Angew. Chem., Int. Ed. 2011, 50,
1926−1930. (f) Yasutomi, S.; Morita, T.; Kimura, S. J. Am. Chem. Soc.
2005, 127, 14564−14565. (g) Fukuzumi, S.; Ohkubo, K.; D’Souza, F.;
Sessler, J. L. Chem. Commun. 2012, 48, 9801−9815. (h) Park, J. S.;
Karnas, E.; Ohkubo, K.; Chen, P.; Kadish, K. M.; Fukuzumi, S.;
Bielawski, C. W.; Hudnall, T. W.; Lynch, V. M.; Sessler, J. L. Science
2010, 329, 1324−1327.
5578
dx.doi.org/10.1021/ja501389g | J. Am. Chem. Soc. 2014, 136, 5575−5578