Fluorescent 'twist-on' sensing by induced-fit anion stabilisation of a planar chromophore

Article abstract of DOI:10.1002/chem.200903293

Twist-and-shout: An anion receptor showing a turn-on fluorescence response to chloride ions due to an induced conformational change upon anion binding is reported (see picture). (Graph Presented)

Full text of DOI:10.1002/chem.200903293

DOI: 10.1002/chem.200903293
Fluorescent ꢀTwist-onꢁ Sensing by Induced-Fit Anion Stabilisation of a Planar
Chromophore
Adam N. Swinburne,[a] Martin J. Paterson,[b] Andrew Beeby,*[a] and
Jonathan W. Steed*[a]
Rigidly preorganised receptors for anionic, cationic and
molecular guests exhibit high binding constants and high se-
lectivity. Such characteristics are desirable in the design of
high-specificity molecular sensors. However, because there
is no change in conformation between free and bound host,
any change in observable characteristics such as fluores-
cence emission must result from the electronic effects of the
bound guest.[1] In contrast flexible systems allow the possi-
bility of induced fit[2] sensing, in which the binding event
brings sensing moieties into close mutual proximity. For ex-
ample binding-induced conformational change can cause
close mutual proximity of fluorophores resulting in excimer
formation[3] or may change the degree of twist in a conjugat-
ed aromatic system, altering its photophysical properties.[4,5]
Similarly, conformational changes may result in changes to
the electrochemical properties of redox-active sensing
units.[6] However, flexible receptors are often less selective
and exhibit lower overall affinities compared to their more
rigid analogues.[7] With careful molecular design it should be
possible to reduce the number of conformational degrees of
freedom of a receptor such that binding of a specific anion
results in a well-defined induced-fit process involving only a
single bond rotation, thus combining the benefits of rigid
preorganisation and induced-fit sensing. We now report the
realisation of this concept in a neutral, rigid dialkyne based
anion receptor.[8]
Both compounds exhibit low quantum yield of fluorescence
!
1
due to the presence of a non-emissive A1u state (a py*
px
1
transition), which lies close in energy to the emissive B1u
!
state (px*
px transition). Excited state population transfer
1
1
to the non-emissive A1u state from the emissive B1u state
results in low emission and is an activated process.[9] By cou-
pling the diarylalkyne unit to ortho anion binding substitu-
ents as in 1, we obtain rigid bisACTHNUGTRNEUNG(urea)-based molecular
ꢀtweezersꢁ,[10] which in the case of 1 have an anion binding
site of suitable size that is almost exactly complementary to
1
chloride. The accessibility of the ꢀdarkꢁ A1u state depends
significantly on the angle of rotation about the alkyne
unit.[11] If anion binding can be used to induce a planar con-
formation in the molecule this should disfavour the reloca-
tion of excitation from the x to the y plane and hence ꢀturn
onꢁ the emission.
Compound 1 was prepared by an Eglington reaction using
copper(II) acetate as catalyst.[12] Two control compounds 2
and 3 were also prepared for comparison using Sonogashira
cross-coupling of 2-iodoaniline with 2-ethynylaniline and
phenyl acetylene respectively, followed by reaction with p-
tolyl isocyanate (see Scheme 1 and the Supporting Informa-
tion).[13]
DFT calculations (B3LYP/6-311GACTHNUTRGNEUNG(d,p)) on 1 show that a
twisted (and hence non-emissive) geometry, in which the
urea groups are well-separated, is more stable than an
eclipsed structure by approximately 3 kJmolꢀ1 with an ex-
The diphenylacetylene unit and its longer congener diphe-
nylbutadiyne have interesting photophysical properties.
[a] A. N. Swinburne, Dr. A. Beeby, Prof. J. W. Steed
Department of Chemistry, Durham University
South Road, Durham, DH1 3LE (UK)
Fax : (+44)191-384-4737
E-mail: jon.steed@durham.ac.uk
[b] Dr. M. J. Paterson
Department of Chemistry
School of Engineering and Physical Sciences
Heriot-Watt University, Edinburgh, EH14 4AS (UK)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903293.
Scheme 1. Fluorescent receptor 1 and model compounds 2 and 3.
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COMMUNICATION
+
ceptionally low rotational barrier between the two conform-
ers (determined by DFT calculations and consistent with re-
lated systems; Figure 1a).[14] Interestingly the calculations
Compound 1 binds chloride (as the NBu4 salt) in CHCl3/
DMSO (70:30) solution in a 1:1 host anion ratio with log
K11 =2.557(5) (determined by H NMR spectroscopic titra-
1
tion, Table 1). The 1:1 stoichiometry was confirmed by Job
Table 1. Anion binding constants for receptors 1–3 in CHCl3/DMSO
(70:30) solution.
Anion
Receptor, logb
1
2
3
Clꢀ
2.557(5)
1.56(1)
<1
logb11 4.01(6)
logb12 5.58(6)
3.61(1)
1.831(7)
<1
<1
logb11 4.09(6)
logb12 6.58(5)
2.993(6)
<1
<1
<1
Brꢀ
Iꢀ
Fꢀ
MeCO2
logb11 2.45(3)
logb21 5.21(9)
logb11 2.41(4)
logb21 5.11(9)
<1
ꢀ
ꢀ
NO3
H2PO4
HSO4
<1
4.29(5)
1.61(1)
<1
3.29(1)
1.2888(1)
ꢀ
logb11 2.01(2)
logb21 4.60(4)<1
Figure 1. a) DFT-optimised geometry of free 1; b) DFT-optimised geome-
try of 1·Clꢀ.
ꢀ
also suggest a degree of preorganisation arising from intra-
molecular CH···O and NH···pACTHNUTRGNEUGN(alkyne) interactions that
plot and can be readily explained by the binding of the
anion in between the two urea functionalities as shown in
the DFT optimised model (Figure 1b). The fact that K11 for
1 is significantly greater than for compound 2 and control
compound 3 highlights the high degree of complementarity
favour the positioning of the urea NH groups over the cen-
tral alkyne unit. This conformational feature is also ob-
served in the X-ray crystal structure of the DMSO disolvate
of the receptor (Figure 2). The X-ray structure shows a
planar transoid geometry with NH···O hydrogen bonding in-
teractions to the DMSO guest molecules which are there-
fore positioned adjacent to the alkyne moieties.
between the bisACTHNUTRGNEUNG
(urea) binding site and Clꢀ. The affinity for
the larger bromide anion is very significantly less than that
for chloride, highlighting the high degree of size selectivity,
while iodide is not bound at all.
The fluorescent emission of 1 in CHCl3/DMSO (95:5) is
very low with a quantum yield of 0.0063 (see supplementary
information for methods). This value is consistent with that
observed for diphenylbutadiyne. Addition of chloride (as
the tetrabutyl ammonium salt) to 2 results in only a small
change in the emission intensity (enhancement by a factor
of 2), however, chloride binding by 1 gives a dramatic in-
crease in fluorescence quantum yield to 0.033 (a five-fold
enhancement). In contrast, binding of other anions, often
with stronger binding than chloride, does not result in the
same degree of fluorescence enhancement, Figure 3.
The addition of fluoride and acetate to 1 results in a ini-
tial increase in fluorescence emission. However further addi-
tion of anion reduces the emission to almost the initial in-
tensity. 1H NMR spectroscopic titrations suggest that for
fluoride a 1:1 and a 1:2 host:guest stoichiometry is present
and is consistent with the 1:1 species causing planarisation
of 1 and a turn-on response. As a second fluoride is bound,
it can be imagined a twisted conformation similar to the
free host is obtained and the emission is therefore reduced.
In the case of acetate, 1:1 host:guest binding is observed via
1H NMR spectroscopic titrations and Job plot, however the
less competative environment used in the fluorescence study
may lead to a 1:2 host:guest stoichiometry being observed
analagous to fluoride. Further investigation into this behav-
iour is currently in progress.
A competition study was undertaken in which acetate (as
the TBA salt) was titrated into a solution of 1 with 50 equiv-
Figure 2. X-ray crystal structure of 1·2DMSO (ellipsoids 50%).[15]
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A. Beeby, J. W. Steed et al.
Figure 3. Fluorescence spectroscopic titration of receptor 1 with various anions in CHCl3/DMSO (95/5) solution (1ꢃ10ꢀ5 mol dmꢀ3, lex =340 nm, lem
=
395 nm).
alents of TBA-Cl present. Upon addition of acetate, the in-
tensity of emission showed a decrease in intensity, consistant
with the host preferentially binding to acetate over chloride
and forming a complex with low emission (see supplementa-
ry information).
tries. For example at the trans conformation the bright S1
!
state is only 0.03 eV lower than the dark py*
px state, Fig-
ure 4a. This excited surface crossing is further confirmed by
complete active-space self-consistent field (CASSCF) calcu-
!
lations on diphenylbutadiyne, which show that the px*
and py*
px,
!
The intensity of emission in tethered DPA derivatives has
been shown to increase as the conformation is restricted to
a more planar geometry.[11c] This change is due to the in-
px electronic states form the components of a de-
generate state at fully twisted (D2d) geometries. Thus, there
exists a Jahn–Teller conical intersection (that facilitates very
efficient radiationless population transfer between such
states. Thus, in 1 excitation drives the system towards a
region of surface crossing and the optically dark state be-
comes populated. In contrast, in 1·Clꢀ, the binding-enforced
planarity of the chloride complex means the two states are
~0.5 eV apart and population transfer cannot readily occur,
Figure 4b.
In conclusion, a carefully designed conformational fluo-
rescent switch anion receptor has been realized, which
allows an induced fit approach to binding. The addition of
chloride results in the planarisation of the chromophore and
hence an increase in the energy of the non-emissive dark
state. This results in a significant increase in the fluorescence
emission from the receptor.
1
creased activation energy of the A1u-1B1u internal conver-
sion for the planar conformation. This increased activation
energy results in a larger kf (rate of fluorescence) value ob-
served for conformationally restricted derivatives and a
lower knr (rate of non-radiative excitation transfer).
The emission from compound 1 shows a biexponential
decay (lifetimes of 1.97 and 0.49 ns), possibly suggesting
non-exponential or complex emissive decay processes. Both
of these values reduce upon addition of chloride (cf. 1.01
and 0.15 ns). Given the increase in quantum yield of fluores-
cence upon addition of chloride, this suggests an increase in
kf and a decrease in knr, consistent with the planarisation of
the chromophore and an increase in the activation energy of
internal conversion.
In order to further understand the origins of this behav-
iour a series of time dependent DFT calculations were un-
dertaken using the recently developed Coulomb attenuated
extension of the B3LYP functional, CAM-B3LYP.[16] The 6-
311GACHTUNGTRENNUNG(d,p) basis was used on all atoms for 1, and augmented
Acknowledgements
for 1·Clꢀ with the 6-311+G(d) on Cl, and an additional dif-
We thank Durham University Doctoral Fellowship for funding and the
Heriot-Watt University Information and Computing Services for use of
the Heriot-Watt high performance cluster service.
fuse set of s, p and d functions added to each peripheral hy-
!
drogen atom. In the case of free 1, the S1
S0 transition is
!
strongly allowed (a px*
px transition) and would appear
to be responsible for the observed emission in the free host.
!
There is a low-lying accessible dark state (py*
crosses with the populated px*
px) which
Keywords: anion binding
supramolecular chemistry · urea
·
luminescence
·
sensors
·
!
px state at twisted geome-
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Chem. Eur. J. 2010, 16, 2714 – 2718

ꢀTwist-onꢁ Sensing of Anions
COMMUNICATION
Figure 4. Principal components of relevant
low-lying electronic states giving rise to
the fluorescence (S1) and non-radiative
decay (S2) as obtained from TD-DFT
computations (f oscillator strength of
transition, ci coefficent of orbital transi-
tion in response eigenvector) for (a) free
host in cisoid geometry and (b) optimised
1·Clꢀ complex. (c) unrelaxed scan across
the alkyne dihedral angle for the bright
and dark states.
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A. Beeby, J. W. Steed et al.
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[15] Crystal data for Compound 1: C36H38N4O4S2, M=654.82, yellow
plate, 0.15ꢃ0.10ꢃ0.02 mm3, monoclinic, space group P21/c (No. 14),
a=15.6775(11) ꢅ, b=6.1785(5) ꢅ, c=17.4171(12) ꢅ, b=96.985(2)8,
V=1674.6(2) ꢅ3, Z=2, 1cald =1.299 gcmꢀ3, F000 =692, SMART 6 K,
MoKa radiation, l=0.71073 ꢅ, T=120(2) K, 2qmax =50.08, 19711
reflections collected, 2948 unique (Rint =0.1772). Final GooF=0.986,
R1=0.0605, wR2=0.1350, R indices based on 1483 reflections with
I>2s(I) (refinement on F2), 217 parameters, 0 restraints. Lp and ab-
sorption corrections applied, m=0.204 mmꢀ1
.
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Received: December 1, 2009
Published online: February 3, 2010
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Chem. Eur. J. 2010, 16, 2714 – 2718