Anion-dependent fluorescence in bis(anilinoethynyl)pyridine derivatives: Switchable ON-OFF and OFF-ON responses

Article abstract of DOI:10.1039/c1cc10947b

Urea and sulfonamide derivatives of 1 exhibit ON-OFF and OFF-ON switchable fluorescent and colorimetric responses upon protonation. The magnitude of the fluorescence event is dictated by the anion, resulting in a rare, fully organic "turn-on" fluorescent

Full text of DOI:10.1039/c1cc10947b

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COMMUNICATION
Anion-dependent fluorescence in bis(anilinoethynyl)pyridine derivatives:
switchable ON–OFF and OFF–ON responsesw
Calden N. Carroll, Brian A. Coombs, Sean P. McClintock, Charles A. Johnson II,
Orion B. Berryman, Darren W. Johnson* and Michael M. Haley*
Received 17th February 2011, Accepted 18th March 2011
DOI: 10.1039/c1cc10947b
Urea and sulfonamide derivatives of 1 exhibit ON–OFF and
OFF–ON switchable fluorescent and colorimetric responses
upon protonation. The magnitude of the fluorescence event is
dictated by the anion, resulting in a rare, fully organic ‘‘turn-on’’
fluorescent sensor for chloride.
Recently we reported the synthesis and halide binding
studies of the parent phenylurea-substituted receptor
2
(Fig. 1) with a series of tetrabutylammonium halides.7 During
the preparation and characterization of the trifluoroacetic acid
(TFA) salts of the protonated receptors, we found that the
fluorescence of the arylacetylene core 1 and phenylurea 2 were
both quenched upon protonation in CHCl3. This occurred
concurrently with a change in the solution from colorless to
yellow (Fig. 2). This change in solution color is seen in the
UV-visible spectrum as a charge transfer absorbance band
A great deal of effort has been devoted to tailoring photoactive
switches on the molecular level for use in applications from
biological imaging to molecular logic gates and photodynamic
therapy.1,2 In particular, advances in our understanding of
intermolecular luminescence processes have led to elegant
multicomponent supramolecular switches and probes with
nanomolar sensitivity for anions, which have been the subject
of many detailed reviews.3
Fluorescence as a sensing mechanism is desirable due to its
inherent sensitivity and relative ease of measurement with
common laboratory equipment. These sensors are often
modulated by pH, with the protonation or deprotonation of
incorporated heterocycles as
a
common fluorescence
ON–OFF, or more rarely OFF–ON, switching motif.4,5
Additionally, the tunable luminescence properties of simple
organic photoswitches have led to systems capable of responding
to multiple inputs or outputs, which has opened the way to
entirely organic logic gates.6 However, these structures are
commonly engineered to exploit a single, distinct fluorescence
phenomenon (e.g., polarity dependent emission shifting or
PET quenching upon binding a guest) or colorimetric changes
as their signaling events, and can thus be limited in the
flexibility of their application. Consequently, there persists
a need for selective, small-molecule organic systems with
discrete, easily tunable negative and positive fluorescent
responses. Herein we report the development of a compact
fluorescent scaffold easily derivatized to yield switchable
ON–OFF and OFF–ON responsive compounds.
Fig. 1 Structures of 2,6-bis(2-anilinoethynyl)pyridine receptor core 1
and urea (2,3a,b) and sulfonamide (4a,b) derivatives.
Department of Chemistry, 1253 University of Oregon, Eugene,
OR 97403-1253, USA. E-mail: haley@uoregon.edu,
dwj@uoregon.edu; Fax: (+1) 541-346-0487
w Electronic supplementary information (ESI) available: Experimental
details for 3a,b and 4a,b; UV-visible and excitation spectra of the HCl-
protonated receptors 1, 2, 3a,b; emission spectra of HBr-protonated
receptors 3a,b and 4a,b; complete ref. 12a. See DOI: 10.1039/
c1cc10947b
Fig. 2 UV-Vis spectra of 1 and 2 as both protonated and neutral
receptors ([Host] and [HostꢀH+] = 12 mM in CHCl3).
c
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Chem. Commun., 2011, 47, 5539–5541 5539

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Fig. 5 Colorless solutions of neutral compounds in CHCl3 turn
yellow upon protonation (top); fluorescence (excitation 365 nm) is
quenched in electron-rich systems 3a, 4a and turned ‘‘on’’ in electron
poor systems 3b, 4b (bottom).
Fig. 3 Normalized emission spectra of both neutral and protonated
electron-rich receptors ([Host] and [HostꢀH+] = 12 mM in CHCl3;
excitation 1: 360 nm, 2,3a: 343 nm; 4a: 338 nm).
and 4b were then treated with gaseous HCl, as Clꢁ is known to
ꢁ 10
from ca. 450–500 nm and is indicative of protonation at the
pyridine nitrogen, which has been noted in similar systems.8
Due to our interest in fluorescent anion sensors, we have
explored how donor/acceptor functionalization could influence
the binding strengths and optoelectronic responses of the core
dianiline 1. Consequently, we prepared hydrogen bonding
electron-rich and electron-poor ureas 3a,b and sulfonamides
4a,b (Fig. 1). We reasoned that electron deficient receptors
would possess a higher binding affinity for anionic guests and
thus expected changes in fluorescence upon protonation, much
like the parent compounds. In electron-rich systems 3a and 4a,
protonation with TFA gave yellow solutions, and the fluorescence
emission was indeed quenched (Fig. 3). In both these and the
parent systems, the residual emission peak bathochromically
shifted only when Clꢁ was present as the counterion (see ESI
for spectra of 1–3a, 4a with Clꢁ, as well as UV-visible and
excitation spectra). In the case of ureas 2ꢀTFA and 3aꢀTFA, the
fluorescence spectra showed a second weak, hypsochromically
shifted peak below 400 nm, but this additional feature
occurred only when CF3CO2ꢁ was present as the counterion.9
In contrast to electron-rich 3a and 4a, electron-poor analogues
3b and 4b were non-fluorescent in the freebase form.
Protonation with TFA also resulted in yellow solutions, but
significantly increased the fluorescence maxima at B515–555 nm
(Fig. 4). To examine the effect of the counterion, receptors 3b
bind much more strongly than CF3CO2
.
The resultant,
excimer-like fluorescence (Fig. 4, dash-dot lines) occurred at
the same wavelength as the residual fluorescence in the
quenched 2ꢀTFA, 2ꢀHCl, 3aꢀHCl and 4aꢀHCl receptors.11
These data were corroborated by the addition of Bu4NCl to
the TFA-protonated receptors, which produced the same
emission bands observed upon addition of gaseous HCl to
the neutral receptor (i.e., bathochromic shifts in the emission
bands).
Fig. 5 visually illustrates the very simple trends observed in
both the urea and sulfonamide derivatives of 1: colorless
solutions turn yellow upon exposure to acid regardless of
the proton source and arene substituent. Electron-donating
substituents on the pendant phenyl rings afford compounds
that are fluorescent (ON) in the neutral state and quenched
(OFF) when protonated. On the other hand, substitution with
electron-withdrawing groups furnishes a weakly fluorescent
freebase receptor (OFF) with greatly enhanced fluorescence
(ON) and equivalent red shifting in the emission spectra upon
exposure to acids with appropriately sized conjugate bases.z
These trends are also mirrored by the experimentally determined
quantum yields in Table 1. For example, receptor 3b
experiences a near full order of magnitude increase in its
OFF–ON fluorescence response when Clꢁ is the counterion
present, even when pre-protonated with TFA.
Frontier molecular orbital calculations (B3LYP/6-31G*
level of theory) provide insight into the mechanisms behind
the behavior of the freebase receptors.12 Compounds 3a and
4a have nearly equivalent lowest-unoccupied molecular orbital
(LUMO) maps, while the highest-occupied molecular orbital
(HOMO) maps are significantly different, although still
overlap with the LUMO on the tri-substituted arene rings
(Fig. 6). In both, excitation retains electron density near the
Table 1 Quantum yields of the freebase and protonated receptorsa
1
2
3a
3b
4a
4b
Freebase
TFA
HCl
2.26%
1.84%
1.65%
0.3%
0.04%
0.02%
8.3%
0.05%
0.01%
0.09%
2.16%
12.6%
3.04%
1.02%
0.69%
0.16%
0.36%
2.14%
Fig. 4 Emission spectra of electron-poor receptors both neutral and
protonated with TFA or HCl ([Host] and [HostꢀH+] = 12 mM in
CHCl3; excitation 3b: 360 nm, 4b: 365 nm).
a
Absolute photoluminescent quantum yields obtained with
Horiba–Jobin Yvon integrating sphere in O2-containing CHCl3.
a
c
5540 Chem. Commun., 2011, 47, 5539–5541
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Fig. 6 Calculated frontier molecular orbitals for neutral ureas 3a,b and sulfonamides 4a,b. Non-overlapping HOMO and LUMO orbitals in the
electron-acceptor substituted systems result in charge transfer quenching of the neutral receptors.
central alkynyl system, resulting in radiative de-excitation and
thus fluorescence. In 3b and 4b, however, a charge-transfer
fluorescence state is generated, resulting in quenched
fluorescence, similar to other known arylacetylene scaffolds.13
The fluorescence OFF–ON response seems to be intramolecularly
excimeric in nature, with this emission having an intriguing
dependence on counterion, which warrants further study.
In conclusion, we have disclosed the development of a
receptor scaffold with switchable ‘‘ON–OFF’’ or ‘‘OFF–ON’’
fluorescence based upon judicious choice of the pendant
phenyl functionalities. Remarkably, these results indicate that
this class of receptors can function as a positive response
(OFF–ON) fluorescent indicator for chloride ion, which
augurs well for applications in cellular ion staining applications
and molecular probe design.1 Additionally, the occurrence of
both a colorimetric and switchable fluorescent ON–OFF or
OFF–ON response in a single class of compounds is promising
for application in molecular logic systems, where they could
conceivably function as complex gates, e.g., INHIBIT or
enabled OR operators. Binding studies with simple biological
anions, epifluorescence studies and further modifications
of receptor structures are currently underway and will be
reported in due course.
1 For reviews of small molecule biological imaging see:
(a) K. Kikuchi, Chem. Soc. Rev., 2010, 39, 2048;
(b) C. N. Carroll, J. J. Naleway, M. M. Haley and
D. W. Johnson, Chem. Soc. Rev., 2010, 39, 3875.
2 For reviews of molecular logic see: (a) C. A. Mirkin and M. A. Ratner,
Molecular Electronics, Annual Reviews, Inc, 1992, vol. 43;
(b) B. L. Feringa, Molecular Switches, Wiley-VCH GmbH, Weinheim,
Germany, 2001; (c) A. G. Montalban, S. M. Baum, A. G. M. Barrett
and B. M. Hoffman, Dalton Trans., 2003, 2093; (d) O.-J. Norum,
P. K. Selbo, A. Weyergang, K.-E. Giercksky and K. Berg,
J. Photochem. Photobiol. B, 2009, 96, 83.
3 (a) Themed issue in supramolecular chemistry of anions and
references therein: P. A. Gale and T. Gunnlaugsson, Chem.
Soc. Rev., 2010, 39, 3595; (b) B. Valeur, Molecular Fluorescence,
Wiley-VCH Verlag GmbH, Weinheim, Germany, 2002, p. 273.
4 (a) L. Fabbrizzi, F. Gatti, P. Pallavicini and L. Parodi, New J.
Chem., 1998, 1403; (b) H. Shizuka and S. Tobita, J. Am. Chem.
Soc., 1982, 104, 6919; (c) J. H. Clements and S. E. Webber,
Macromolecules, 2004, 37, 1531; (d) A. P. de Silva, H. Q. Nimal
Gunaratne and C. P. McCoy, Chem. Commun., 1996, 2399.
5 (a) G. De Santis, L. Fabrizzi, M. Licchelli, A. Poggi and
A. Taglietti, Angew. Chem., Int. Ed. Engl., 1996, 35, 202;
(b) T. Gunnlaugsson, A. P. Davis, J. E. O’Brien and M. Glynn,
Org. Biomol. Chem., 2005, 3, 48; (c) S. E. Garcıa-Garrido,
C. Caltagirone, M. E. Light and P. A. Gale, Chem. Commun.,
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6 (a) S. J. Dickson, A. N. Swinburne, M. J. Paterson, G. O. Lloyd,
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(b) A. Roque, F. Pina, S. Alves, R. Ballardini, M. Maestri and
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This research was supported by the NIH (GM087398-02)
and the University of Oregon (UO). C.N.C., O.B.B. and
S.P.M. acknowledge the NSF for Integrative Graduate
Education and Research Traineeships (DGE-0549503).
C.A.J. thanks UO for a Doctoral Research Fellowship.
D.W.J. is a Cottrell Scholar of the Research Corporation for
Science Advancement.
7 C. N. Carroll, O. B. Berryman, C. A. Johnson II, L. N. Zakharov,
M. M. Haley and D. W. Johnson, Chem. Commun., 2009, 2520.
8 J. M. Heemstra and J. S. Moore, Org. Lett., 2004, 6, 659–662.
9 Protonation of the receptors with HBr resulted in mixed emission
bands at both B404 nm and B515 nm. See theꢁESI.
10 The binding preference for Clꢁ over CF3CO2 can be inferred
from the magnitude of the binding constant for Clꢁ with 2ꢀTFA
from ref. 7.
11 See ESI for spectra of these receptors (2, 3a, 4a) with gaseous HCl.
12 (a) M. J. Frisch, et al., Gaussian 03, Revision B.04, Gaussian, Inc,
Pittsburgh PA, 2003. Full reference in ESI; (b) Spartan 08,
Wavefunction, Inc., Irvine, CA, USA.
Notes and references
z The addition of HNO3 or camphorsulfonic acid results in the
protonation of the receptor by UV-Vis, but does not result in an increase
in fluorescence in receptors 3b or 4b. We tentatively attribute this to the
size difference of these anions compared to Clꢁ, which precludes the
formation of either the ‘‘U’’ conformation reported in ref. 7 or dimeric
species seen in O. B. Berryman, C. A. Johnson II, L. N. Zakharov,
M. M. Haley, and D. W. Johnson, Angew. Chem., Int. Ed., 2008, 47, 117.
13 (a) J. A. Marsden, J. J. Miller, L. D. Shirtcliff and M. M. Haley,
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11872; (c) E. L. Spitler, L. D. Shirtcliff and M. M. Haley, J. Org.
Chem., 2007, 72, 86; (d) A. J. Zucchero, P. L. McGrier and U. H.
F. Bunz, Acc. Chem. Res., 2010, 43, 397.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5539–5541 5541