Aggregation induced emission enhancement in ionic self-assembled aggregates of benzimidazolium based cyclophane and sodium dodecylbenzenesulfonate

Article abstract of DOI:10.1021/ol401452t

Cyclophane BIMCP-1 undergoes ionic self-assembly with surfactant sodium dodecylbenzenesulfonate (SDBS) at a concentration far below its CMC value to form aggregates with spherical morphology. The rotational restriction of rings in these aggregates facilitates 32-fold enhancement in emission intensity (AIEE) to allow fluorescence based determination of SDBS with 4 μM (1.4 ppm) as the lowest detection limit. Sodium salts of fatty acids and inorganic anions halides, CN^-, HSO₄^-, SO₄ ^2-, H₂PO₄^-, SCN^-, and NO₃^- do not interfere in the estimation of SDBS.

Full text of DOI:10.1021/ol401452t

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Aggregation Induced Emission Enhancement
in Ionic Self-Assembled Aggregates
of Benzimidazolium Based Cyclophane and
Sodium Dodecylbenzenesulfonate
Sandeep Kumar,† Prabhpreet Singh,† Aman Mahajan,‡ and Subodh Kumar*,†
Department of Chemistry, UGC Centre for Advanced Studies, Guru Nanak Dev
University, Amritsar 143 005, India, and Department of Physics, Guru Nanak Dev
University, Amritsar 143 005, India
subodh_gndu@yahoo.co.in
Received May 23, 2013
ABSTRACT
Cyclophane BIMCP-1 undergoes ionic self-assembly with surfactant sodium dodecylbenzenesulfonate (SDBS) at a concentration far below
its CMC value to form aggregates with spherical morphology. The rotational restriction of rings in these aggregates facilitates 32-fold
enhancement in emission intensity (AIEE) to allow fluorescence based determination of SDBS with 4 μM (1.4 ppm) as the lowest detection limit.
Sodium salts of fatty acids and inorganic anions halides, CNꢀ, HSO4ꢀ, SO42ꢀ, H2PO4ꢀ, SCNꢀ, and NO3ꢀ do not interfere in the estimation of SDBS.
In light of the inspirations which natural processes1 have
provided us, the substantial advances in the design
and fabrication of supramolecular assemblies have been
achieved through a bottom-up approach by elegantly
utilizing noncovalent interactions such as electrostatic,
hydrophobic, van der Waals and hydrogen bonding,
etc.2,3Among varied processes of self-assembly, the ionic
self-assembly, i.e. the coupling of two structurally different
building blocks through columbic interactions, has turned
into a powerful tool to create new nanostructured chemical
materials.3 Various combinations between peptides, poly-
electrolytes, surfactants, and extended rigid organic scaf-
folds could be employed for creating new materials via ionic
self-assembly.4 In such systems, though molecular recogni-
tion (space, size and complementarity) and columbic inter-
actions between two oppositely charged components play a
vital role in the initiation of ionic self-assembly process, the
cooperative binding mechanism relying on weak interac-
tions further propagates the final self-assembled structure.
Surfactants due to their amphiphilic nature are well-
known to produce supramolecular aggregates such as
† Department of Chemistry.
‡ Department of Physics.
(1) (a) Benyus, J. M. Biomimicry: Innovation Inspired by Nature;
Quill-William Morrow: New York, 1997. (b) Branden, C.-I.; Tooze, J.
Introduction to Protein Structure, 2nd ed.; Garland: New York, 1999. (c)
Zhang, S. Nat. Biotechnol. 2003, 21, 1171.
(2) (a) Hoeben, F. J. M.; Jonkheijm, P. L.; Meijer, E. W.; Schenning,
A. P. H. J. Chem. Rev. 2005, 105, 1491. (b) Wang, L.; Li, L. L.; Ma, H. L.;
Wang, H. Chin. Chem. Lett. 2013, 24, 351.
(4) (a) Cheng, Z. Y.; Ren, B. Y.; Chang, X. Y.; Liu, R.; Tong, Z. Chin.
Chem. Lett. 2012, 23, 619. (b) Ma, T. Y.; Li, H.; Deng, Q. F.; Liu, L.; Ren,
T. Z.; Yuan, Z. Y. Chem. Mater. 2012, 24, 2253. (c) Lv, C.; Xu, G.; Chen, X.
ꢀ
Chem. Lett. 2012, 41, 1201. (d) Percebom, A. M.; Janiak, J.; Schillen, K.;
(3) (a) Faul, C. F. J.; Antonietti, M. Adv. Mater. 2003, 15, 673. (b)
Greaves, T. L.; Drummond, C. J. Chem. Soc. Rev. 2008, 37, 1709. (c)
Rehm, T. H.; Schmuck, C. Chem. Soc. Rev. 2010, 39, 3597. (d) Thomas,
J. A. Dalton Trans. 2011, 40, 12005. (e) Greaves, T. L.; Drummond, C. J.
Chem. Soc. Rev. 2013, 42, 1096.
Piculell, L.; Loh, W. Soft Matter 2013, 9, 515. (e) Guo, Y. N.; Li, Y.; Zhi, B.;
Zhang, D.; Liu, Y.; Huo, Q. RSC Adv. 2012, 2, 5424. (f) Chen, L. G.;
Bermudez, H. Langmuir 2012, 28, 1157. (g) de-Rooy, S. L.; Das, S.; Li, M.;
El-Zahab, B.; Jordan, A.; Lodes, R.; Weber, A.; Chandler, L.; Baker, G. A.;
Warner, I. M. J. Phys. Chem. C 2012, 116, 8251.
r
10.1021/ol401452t
XXXX American Chemical Society

micelles and vesicles in aqueous solution.5 The coupling of
anionic surfactants and other sulfonates with cationic
motifs viz. ammonium, pyridinium, anilinium, etc. has
resulted in the formation of many ordered ionic self-
assemblies.6 As the self-assembly of two components is
highly dependent on the components’ structure, even a
small structural change can put the self-assembling process
into total disarray. So, the recognition of stimuli based on
the self-assembly process provides higher selectivity over
simple equilibrium based processes.7
Scheme 1. Synthesis of Cyclophane BIMCP- 1 and Its Acyclic
Analog 2
Molecular architectures possessing imidazolium and its
benzo derivatives as motifs have been used for the recogni-
tion of anions primarily through electrostatic interactions
between the positively charged imidazolium moiety and the
target anion.8 The imidazolium based cyclophanes, due to
their high degree of structural rigidity and predefined cavity
size, have further shown energetically favored encapsulation
of guest molecules including anions, cations, and neutral
molecules.9 The recognition process in such systems is usually
associated with the change in the fluorescence intensity and/
or shift in emission maxima due to various photophysical
processes taking place between two components. Recently,
Sessler et al. have reported that a large imidazolium based
tetracationic cyclophane can adjust its shape and conforma-
tion to accommodate anionic guests of different sizes and
charges within its central core and promotes the formation of
different macromolecular aggregates.10
value (1.5 mM) leads to the formation of the ionic self-
assembling process to provide aggregates with spherical
morphology of a 1ꢀ1.5 μm diameter, as observed from
SEM and confocal images of their thin films. This aggrega-
tion process is also associated with aggregation induced
emission enhancement (AIEE) to enable fluorescence
based determination of SDBS. The confocal images of
these films also confirm that the aggregation is associated
withanincreaseintheemission intensity. Interestingly, this
ionic self-assembling process is also initiated by sodium
dodecyl sulfate (SDS) but long chain fatty carboxylic acids
or lower homologues of SDS/SDBS do not induce such
assembly or cause any change in the fluorescence intensity of
BIMCP-1. This recognition of SDBS/SDS is based on both
columbic interactions of the binding site (SO3ꢀ/OSO3ꢀ) and
the hydrophobic effect of the long alkyl chain. The low
fluorescence enhancement in the case of acyclic analog 2
also points to the significance of the cyclic structure in
BIMCP-1 in the aggregation and recognition process.
The target benzimidazolium based cyclophane BIMCP-1
and its acyclic analog 2 were synthesized by alkylation of 1,4-
bis(benzimidazol-1-yl)benzene with 1,4-bis(bromomethyl)-
2,3,5,6- and 3-bromomethyl-1,2,4,5-tetramethylbenzene,
respectively (Scheme 1). All the compounds were thoroughly
characterized by the usual spectroscopic techniques viz. 1H
NMR, 13C NMR, HRMS, and CHN analysis (Supporting
Information Figure SI-1ꢀ6).
Herein, we report benzimidazolium based tetracationic
cyclophane BIMCP-1 which on addition ofSDBS (sodium
dodecylbenzenesulfonate, <25 μM) far below its CMC
(5) (a) Rosoff, M., Ed. Vesicles; Dekker: New York, 1996. (b) Carmona-
Ribeiro, A. M. Chem. Soc. Rev. 1992, 21, 209.
(6) (a) Cortez, M. L.; Pallarola, D.; Ceolın, M.; Azzaroni, O.;
Battaglini, F. Chem. Commun. 2012, 48, 10868. (b) Evans, R. C.;
Knaapila, M.; Willis-Fox, N.; Kraft, M.; Terry, A.; Burrows, H. D.;
Scherf, U. Langmuir 2012, 28, 12348. (c) Li, Y.; Zhang, D.; Gai, F.; Zhu,
X.; Guo, Y.; Ma, T.; Liu, Y.; Huo, Q. Chem. Commun. 2012, 48, 7946.
(d) Ahmed, R.; Priimagi, A.; Faul, C. F. J.; Manners, I. Adv. Mater.
2012, 24, 926. (e) Zhao, M.; Zhao, Y.; Zheng, L.; Dai, C. Chem.;Eur. J.
2013, 19, 1076. (f) Choudhury, S. D.; Bhasikuttan, A. C.; Pal, H.;
Mohanty, J. Langmuir 2011, 27, 12312.
(7) (a) Kumar, M.; Jonnalagadda, N.; George, S. J. Chem. Commun.
2012, 48, 10948. (b) Xin, Y.; Kong, X.; Zhang, X.; Lv, Z.; Du, X.
Langmuir 2012, 28, 11153. (c) Harada, A.; Kobayashi, R.; Takashima,
Y.; Hashidzume, A.; Yamaguchi, H. Nat. Chem. 2011, 3, 34. (d) Faul,
C. F. J.; Krattiger, P.; Smarsly, B. M.; Wennemers, H. J. Mater. Chem.
2008, 18, 2962.
To evaluate the aggregation behavior, the thin films of
BIMCP-1 (10 μM) and its 1:2 solutions with SDBS/SDS
(2 equiv) in aqueous solution (5% DMSO) were prepared
on the glass surface using the drop cast method. The respec-
tive films of BIMCP-1 alone do not show any morphology
under both confocal microscopy and SEM. The films
of BIMCP-1 and SDBS exhibit spherical structures with
diameters = 0.4ꢀ1.2 μm (Figure 1a). Enlargement under
SEM show these spheres to be formed by the aggregation
of fiberlike structures (Figure 4). Under confocal micro-
scopy, these films show circular aggregates 0.5ꢀ1.5 μm in
size (Figure 1c), which are in consonance with the SEM
image. AFM images of these films are also in consonance
with the formation spherical morphology (Figure SI-7).
Also under confocal, these spheres appear intense blue in
comparisontothe almostnonfluorescent film of BIMCP-1
alone (Figure SI-8). The intense blue fluorescence is in
consonance with 32-fold increase in fluorescence intensity
on addition of SDBS to the solution of BIMCP-1. In the
(8) (a) Xu, Z.; Chen, X.; Kim, H. N.; Yoon, J. Chem. Soc. Rev. 2010,
39, 127. (b) Yoon, J.; Kim, S. K.; Singh, N. J.; Kim, K. S. Chem. Soc.
Rev. 2006, 35, 355.
(9) (a) Serpell, C. J.; Cookson, J.; Thompson, A. L.; Beer, P. D.
Chem. Sci. 2011, 2, 494. (b) Shirinfar, B.; Ahmed, N.; Park, Y. S.; Cho,
G. S.; Youn, S.; Han, J. K.; Nam, H. G.; Kim, K. S. J. Am. Chem. Soc.
2013, 135, 90. (c) Ahmed, N.; Shirinfar, B.; Youn, I. S.; Bist, A.; Suresh,
V.; Kim, K. S. Chem. Commun. 2012, 48, 2662. (d) Guo, Z.; Song, N. R.;
Moon, J. H.; Kim, M.; Jun, E. J.; Choi, J.; Lee, J. Y.; Bielawski, C. W.;
Sessler, J. L.; Yoon, J. J. Am. Chem. Soc. 2012, 134, 17846. (e) Chun, Y.;
Singh, N. J.; Hwang, I. C.; Lee, J. W.; Yu, S. U.; Kim, K. S. Nat.
Commun. 2013, 4, 1797. (f) Chellappan, K.; Singh, N. J.; Hwang, I. C.;
Lee, J. W.; Kim, K. S. Angew. Chem., Int. Ed. 2005, 44, 2899.
(10) (a) Rambo, B. M.; Gong, H. Y.; Oh, M.; Sessler, J. L. Acc. Chem.
Res. 2012, 45, 1390. (b) Gong, H. Y.; Rambo, B. M.; Lynch, V. M.;
Keller, K. M.; Sessler, J. L. Chem.;Eur. J. 2012, 18, 7803. (c) Gong,
H. Y.; Rambo, B. M.; Nelson, C. A.; Cho, W.; Lynch, V. M.; Zhu, X.;
Oh, M.; Sessler, J. L. Dalton Trans. 2012, 41, 1134. (d) Gong, H. Y.;
Rambo, B. M.; Nelson, C. A.; Cho, W.; Lynch, V. M.; Zhu, X.; Oh, M.;
Sessler, J. L. Chem. Commun. 2011, 47, 5973. (e) Gong, H. Y.; Rambo,
B. M.; Karnas, E.; Lynch, V. M.; Keller, K. M.; Sessler, J. L. J. Am.
Chem. Soc. 2011, 133, 1526.
B
Org. Lett., Vol. XX, No. XX, XXXX

Figure 2. Plot of fluorescence intensity (I/Io) vs concentration of
SDBS and SDS for BIMCP-1 and 2 in water (5% DMSO) [Inset:
shows selectivity of BIMCP-1 with SDBS/SDS in comparison to
open analog 2].
Figure 1. SEM (aꢀb) and confocal (cꢀd) images, respectively, of
BIMCP-1 with different surfactants; (a,c) BIMCP-1 þ SDBS;
(b,d) BIMCP-1 þ SDS.
case of SDS, hemispheres 10ꢀ15 μm in size are formed by
aggregation of BIMCP-1 with SDS (Figure 1b, d) and
exhibit increased luminescence under confocal micro-
scopy. Dynamic light scattering experiments of BIMCP-
1/SDS and BIMCP-1/SDBS (1:2) complexes at 5 μM show
the formation of aggregates with 100ꢀ1000 nm sizes and
are in agreement with aggregation of BIMCP-1 with SDS
and SDBS under fluorescence experimental conditions
(Figure SI-9).
solution decreases ∼7-fold, which is recovered after cool-
ing the solution back to 25 °C (Figure SI-12). This could be
attributed to the occurrence of deaggregation and aggre-
gation processes respectively on increasing and decreas-
ing the temperature of the solution. Furthermore, the 1H
NMR spectrum of BIMCP-1 (2.5 mM), in CD3CN/D2O
(1:1), on addition of 2 equiv of SDBS exhibits an upfield
shift of the singlet due to p-phenylene protons by 0.27 ppm
1
(from δ 7.96 to 7.69) (Figure SI-13). Upon reverse H
The UVꢀvis spectra of BIMCP-1 and 2 in H2O (5%
DMSO) exhibit the absorption maxima at265 nm ((5 nm).
On excitation at 270 nm, the solutions of both BIMCP-1
and 2 exhibit two emission maxima at 360 and 385 nm and
display a linear increase in the fluorescence intensity be-
tween 1 and 50 μM and point to the absence of aggregation
or any intramolecular interaction in various components
of BIMCP-1 (Figure SI-10). The addition of SDBS and
SDS (50 μM) to the solution of BIMCP-1 (2 μM) in H2O
(5% DMSO) leads to respectively a 32- and 27-fold increase
in emission intensity at 360 and 380 nm. The acyclic analog 2
exhibits only ∼3-fold emission intensity enhancement on
addition of 50 μM SDS and SDBS and ∼7-fold emission in-
tensity enhancement at >400 μM SDBS and SDS (Figure 2).
Remarkably, the sodium salts of long chain carboxylates
such as laurate, myristate, palmitate, and stearate (100 μM
each) cause a <1-fold increase in the emission intensity of
BIMCP-1 and 2, whereas the addition of methyl sulfonate,
butyl sulfonate, octyl sulfonate, decyl sulfonate, octyl
sulfate, decyl sulfate, etc. or inorganic anions viz. halides,
H2PO4ꢀ, CNꢀ, SCNꢀ, HSO4ꢀ, SO42ꢀ (100 μM each), etc.
causes an insignificant change in both the absorption and
emission spectrum of BIMCP-1 and 2 (Figure SI-11).
Thus, BIMCP-1 shows remarkably high sensitivity toward
SDBS and SDS.
NMR titration, i.e. addition of BIMCP-1 to a 2.5 mM
solution of SDBS, the aryl protons of the SDBS molecule
at δ 7.73 and 7.31 were upfield shifted to δ 7.63 and 7.25
revealing aromatic πꢀπ interactions between BIMCP-1
and SDBS (Figure SI-14,15).
It could be attributed that the surfactant SDBS binds
strongly with BIMCP-1 through electrostatic interactions
between oppositely charged BIMCP-1 and a sulfonate
moiety of the surfactant along with some πꢀπ interactions
between the phenyl ring of the SDBS and phenylene core of
BIMCP-1 to produce ionic self-assembled aggregates. The
lifetime measurement of the excited state of BIMCP-1
in the presence of g2 equiv of SDBS or SDS shows an
increase in lifetime from 0.76 to 8.9 ns. The plot of the
concentration of the BIMCP-1/SDBS complex vs Kf and
Knr shows a respective increase and decrease in the values.
Therefore, both the increase in fluorescence of the BIMCP-1/
SDBS complex and decrease in the deactivation of the excited
state through nonradiative processes due to restricted rota-
tion of the aryl rings in these aggregates contribute to the
overall emission enhancement (Figure SI-16ꢀ19).
SDBS and SDS are widely used surfactants in domestic
and industrial formulations, and their presence in natural
waters arouses concern for human health and the global
ecosystem.11 Several techniques are currently used to
Temperature dependent fluorescence titrations reveal
that the formation of aggregates between BIMCP-1 and
surfactants, which are responsible for AIEE phenomena,
is fully reversible. On increasing the temperature of the
solution of BIMCP-1 (2 μM) and SDBS/SDS (50 μM)
from 25 to 85 °C, the fluorescence emission intensity of the
(11) (a) Chen, B.; Wang, S.; Zhang, Q.; Huang, Y. Analyst 2012, 137,
1232. (b) Perales, J. A.; Manzano, M. A.; Sales, D.; Quiroga, J. M. Bull.
Environ. Contam. Toxicol. 1999, 63, 94. (c) Palazzesi, F.; Calvaresi, M.;
Zerbetto, F. Soft Matter 2011, 7, 9148. (d) Brandt, K. K.; Hesselsøe, M.;
Roslev, P.; Henriksen, K.; Sørensen, J. Appl. Environ. Microbiol. 2001,
67, 2489. (e) Urakawa, H.; Inaba, K.; Tsuneda, S. FEMS Microbiol.
Lett. 2008, 282, 166.
Org. Lett., Vol. XX, No. XX, XXXX
C

Figure 3. Effect of gradual addition of SDBS on the emission
spectrum of BIMCP-1 (2 μM) in water (5% DMSO) [Inset:
Job’s plot of BIMCP-1 with SDBS shows 1:2 (BIMCP-1/SDBS)
stoichiometry].
Figure 4. Proposed model for the formation of spherical aggre-
gates. (a) BIMCP-1 þ SDBS; (b) part of Figure 4a after enlarg-
ing; (c) model for encapsulation of the anionic surfactant
showing electrostatic/πꢀπ interaction with hydrophobic tails
extending outside for stabilization; (d) lateral view of Figure 4b;
(e) top to bottom view of Figure 4b.
detect SDBS, SDS, and their homologues.12 However,
fluorescent probes are considered to be more sensitive
and specific, possessing potential in regards to portability
and real time monitoring with a fast response time.13 To
demonstratethe practicalutilityof the presentfluorescence
turn-on assay for the detection of surfactants, detailed
fluorescence titrations have been carried out. The gradual
addition of aliquots of SDBS to the solution of BIMCP-1
(2 μM) in water (5% DMSO) shows a gradual increase
in fluorescence intensity and achieves a plateau after addi-
tion of 50 μM (25 equiv) of SDBS (Figure 3). Similarly,
the addition of SDS to the solution of BIMCP-1 results
in a 27-fold increase in emission intensity at 50 μM and
then achieves a plateau (Figure SI-20). The lowest detec-
tion limit for both SDBS and SDS is 4.0 μM (1.4 ppm).
The nonlinear regression analysis of these titration data
using SPECFIT-32 shows the formation of 1:2 (BIMCP-1/
SDBS or BIMCP-1/SDS) stoichiometric complexes with
stability constant (log β) values of 9.11 ( 0.04 and 8.61 (
0.02, respectively. The Job plot of BIMCP-1 with both
SDBS and SDS shows an inflection point near 0.7 and
further confirms the formation of a 1:2 (BIMCP-1:SDBS/
SDS) stoichiometric complex (Figure 3, Inset, SI-21).
On the other hand, the acyclic analog 2 (5 μM, water
(5% DMSO) on addition of SDBS or SDS exhibits a much
smaller increase in the emission intensity at 352 and 385 nm
and achieves a plateau after addition of 400 μM of
SDBS/SDS. The spectral fitting of these titration data
shows the formation of a mixture of 1:1 and 1:2 stoi-
chiometric species (2:SDBS/SDS) with relatively lower
binding constant values than those observed for BIMCP-1
(Figure SI-22ꢀ23).
We propose that the mechanism for the observation of
these fine spherical morphologies for the BIMCP-1/SDBS
complex is related to micelle type aggregation (Figure 4).
In polar solvent (H2O), the complexes of benzimidazolium
moieties of BIMCP-1 with sulfonate groups are present on
the outside surface of the aggregates to form the hydro-
philic exterior, whereas the dodecyl chains (hydrophobic
tails) tend to gather in the core to stabilize the aggregates and
result in aggregation induced fluorescence enhancement.
The encapsulation of the anionic surfactant molecules
in the cyclophane cavity largely neutralizes the positive
charge of BIMCP-1, reduces the repulsive electrostatic
interactions among the molecules of BIMCP-1, and in
turn facilitates the aggregation process.
In conclusion, the cyclophane BIMCP-1 in the presence
of <25 equiv of (1ꢀ50 μM) SDBS/SDS undergoes aggre-
gation induced emission enhancement well below their
CMC values and allows fluorescence based determination
of SDS/SDBS. Confocal imaging and SEM studies con-
firm the formation of aggregates.
Acknowledgment. We thank DST, New Delhi for fi-
nancial assistance under the FIST program; CSIR, New
Delhi for a fellowship to Sandeep Kumar; and UGC, New
Delhi under the CAS program.
(12) (a) Hu, Y. Y.; He, Y. Z.; Qian, L. L. Anal. Chim. Acta 2005, 536,
251. (b) Villar, M.; Callejon, M.; Jimenez, J. C.; Alonso, E.; Guiraum, A.
Anal. Chim. Acta 2007, 599, 92. (c) Moldovan, Z.; Avram, V.; Marincas,
O.; Petrov, P.; Ternes, T. J. Chromatogr., A 2011, 1218, 343. (d) Akyuz,
M. Talanta 2007, 71, 471. (e) Kumar, S.; Arora, S.; Singh, P.; Kumar, S.
RSC Adv. 2012, 2, 9969.
Supporting Information Available. Experimental proce-
dures, additional fluorescence/UVꢀvis data, HRMS, and
1H NMR titrations are available. This material is available
free of charge via the Internet at http://pubs.acs.org.
(13) (a) Kikuchi, K. Chem. Soc. Rev. 2010, 39, 2048. (b) Yang, Y.; Zhao,
Q.; Feng, W.; Li, F. Chem. Rev. 2013, 113, 192. (c) Moragues, M.; Martınez-
ꢀ~
ꢀ
Manez, E. R.; Sancenon, F. Chem. Soc. Rev. 2011, 40, 2593. (d) Wu, J.; Liu,
W.; Ge, J.; Zhang, H.; Wang, P. Chem. Soc. Rev. 2011, 40, 3483. (e) Du, J.;
Hu, M.; Fan, J.; Peng, X. Chem. Soc. Rev. 2012, 41, 4511.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX