Two-dimensional electronic conjugation: Cooperative folding and fluorescence switching

Article abstract of DOI:10.1021/ja063413u

Conformational switching of tris(N-salicylideneamine)-derived C₃-symmetric molecules could be driven by organizing and disrupting mutually reinforcing hydrogen bonding networks. The emission properties associated with these shape-adaptive const

Full text of DOI:10.1021/ja063413u

Published on Web 08/19/2006
Two-Dimensional Electronic Conjugation: Cooperative Folding and
Fluorescence Switching
Xuan Jiang, John C. Bollinger, and Dongwhan Lee*
Department of Chemistry, Indiana UniVersity, 800 East Kirkwood AVenue, Bloomington, Indiana 47405
Received May 16, 2006; E-mail: dongwhan@indiana.edu
Scheme 2. Synthesis of C3-Symmetric Switch, Schematically
Represented as C, and Its Derivatives
Mechanical control of electronic properties has proven to be an
effective signal transduction strategy in molecular-level devices.1
Linearly π-conjugated “molecular wire” sensors represent one such
example, in which binding-induced conformational restriction can
significantly modify their emission or conductive properties.2 Within
this context, two-dimensional (2-D) π-conjugations should further
benefit from the additional dimensionality of the space sampled
by electrons or excited states.3 The expected decrease in the
HOMO-LUMO energy gap would be an additional bonus for their
practical applications in sensing and switching with lower energy
excitation.
Conceptually intuitive and operationally simple as it might
appear, however, the design and implementation of viable switching
mechanisms for a planar conjugated system is a formidable synthetic
task. As shown in Scheme 1, simple twisting of one bond can
effectively change the overall conjugation length of a one-
dimensional (1-D) system (A), whereas simultaneous rotations of
at least three bonds are required to maximize shrinkage and
expansion of the conjugation area of a 2-D system (B). In this
communication, we describe our innovations to meet this challenge
by correlating motions occurring at multiple nonproximate locations
within dynamic conjugated molecules.
We have recently shown that bulky m-terphenyl groups can be
placed around a tris(N-salicylideneamine) core to furnish shape-
adaptive biconcave structures.4 Within such sterically congested
scaffolds, van der Waals contacts facilitate unidirectional tilting of
symmetrically disposed aromatic groups. A related structural
prototype, represented as C in Scheme 2, was expected to serve as
an ideal testbed for the conformational switching depicted in
Scheme 1. A convergent synthetic route (Scheme 2) was imple-
mented to furnish compounds 1-5 in efficient two-step sequences
from readily available starting materials.5
Concentration independence of the significantly downfield-shifted
O-H proton resonance of 1 indicated the presence of strong
intramolecular hydrogen bonds, which was unambiguously con-
firmed by X-ray crystallography.
As shown in Figure 1, the planar {C6O3(CHNH)3} core of 1 is
surrounded by three aryl groups that are related by crystallographic
3-fold symmetry. The tertiary alcohol groups extending from each
of the six ethynyl units engage in O-H‚‚‚O hydrogen bonding
interactions that bring pairs of neighboring “wing tips” to close
proximity to define a propeller-like arrangement. These polar
interactions extend further to form three Oalcohol-H‚‚‚Oalcohol
-
H‚‚‚Oketone‚‚‚H-Nenamine networks, which converge at the center of
the molecule (dotted lines, Figure 1a). Overall, nine X-H‚‚‚Y (X
) O, N; Y ) O) bonds effectively flatten and rigidify the entire
molecule 1 (Figure 1b).
In CHCl3 at 25 °C, 1 displays intense (ꢀ ∼ 7.6 × 104 M-1 cm-1
)
At 25 °C, the 1H NMR spectrum of 1 in CDCl3 displayed a sharp
O-H resonance at 5.90 ppm, which remained invariant within the
concentration range of 2.0-75 mM. The corresponding O-H signal
of 4, however, gradually shifted from 2.62 to 3.12 ppm when the
concentration was increased from 12 to 210 mM (Figure S1).
absorptions at λmax ) 415 and 432 nm, which are significantly red-
shifted compared with that of its model compound 4 (λmax ) 350
nm) or its analogue 5 (λmax ) 360 nm) having smaller [π,π]/[n,π]-
Scheme 1. Switching in Linear (A and A′) and Planar (B and B′)
Conjugated Systems. Curved Arrows Indicate Rotation about
Bonds between Adjacent Repeating Units Represented as Disks
Figure 1. X-ray structure of 1: (a) ORTEP diagram with thermal ellipsoids
at 50% probability, in which carbon atoms of the gem-dimethyl groups
and all the hydrogen atoms except O-H and N-H have been omitted for
clarity; (b) space-filling model, where N is blue and O is red. Selected
interatomic distances (Å): O1A‚‚‚N5A, 2.634; O1A‚‚‚O11, 2.765;
O11‚‚‚O22A, 2.809. The {C6O3(CHNH)3} core is disordered over two
positions,5 for which only one model is shown.
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10.1021/ja063413u CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Conformational Switching
with Me3SiCl, added to scavenge fluoride ion, restored >95% of
the original fluorescence intensity,11 thus convincingly demonstrat-
ing the reversible nature of this sensing event. Other halide ions,
n
added as Bu4NX (X ) Cl-, Br-, or I-; 60 equiv), had no effects
on the emission spectra of 2. Under similar conditions, 1 did not
respond to F- (Figure S4), presumably due to limited access to the
sterically more shielded O-H‚‚‚O units.
In summary, fast and reversible conformational switching of a
2-D conjugated system was achieved by manipulating a mutually
reinforcing hydrogen bonding network. Similarly to naturally
occurring assemblies that fold cooperatively,12 individually weak
but collectively strong noncovalent interactions could further be
reinforced by their symmetric arrangement within a flexible
structural scaffold. Folding and unfolding motions of our synthetic
constructs are tightly coupled to changes in their emission proper-
ties, the technological implications of which are currently being
explored.
Figure 2. Emission spectra of (a) 1, 2, 3, and 5 in CHCl3 normalized with
absorption at 340 nm; (b) 1 in CHCl3-DMSO with increasing volume
fraction of DMSO (0, 20, 40, 60, and 96%, v/v); (c) 1 in CHCl3 measured
as a function of temperature from -5 to +55 °C with an increment of 10
°C between traces; (d) 2 ()1.3 µM) in CH2Cl2 (i) before and (ii) after
n
addition of Bu4NF (60 equiv), and (iii) after treatment of (ii) with Me3-
SiCl (7.8 mM), λexc ) 340 nm, T ) 25 °C.
conjugation areas (Figure S2). Upon excitation at 340 nm, 1 displays
blue emission at λmax,em ) 454 nm with ΦF ) 5.3%. The Stokes
shift (∆λ ) 22 nm) of 1 is significantly smaller than that (∆λ )
73 nm) of 5 with λmax,em ) 433 nm, presumably due to the
mechanically interlocked hydrogen bonding network which sup-
presses structural reorganization in the excited states.6 This
structure-property relationship was explored with a set of com-
pounds that share a common tris(N-salicylideneaniline) skeleton
but differ in the peripheral functionalities.
Acknowledgment. This work was supported by Indiana Uni-
versity, the National Science Foundation (CAREER CHE 0547251),
and the American Chemical Society Petroleum Research Fund
(42791-G3).
Supporting Information Available: Experimental details (PDF)
and crystallographic data (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
As shown in Figure 2a, removal of the methyl groups encapsu-
lating the Oalcohol-H‚‚‚Oalcohol-H‚‚‚Oketone linkages (1 f 2) has
negligible influence on the emission efficiency, whereas complete
elimination of this polar network to “loosen” the structure (1 or 2
f 3 or 5) quenches the fluorescence. Interaction of 1 with externally
added hydrogen bonding solvent, such as DMSO, elicited similar
effects (Figure 2b). In contrast, the emission spectrum of 3 lacking
this peripheral polar network remained essentially unchanged in
response to DMSO. From these observations emerges a simple
binary switching model shown in Scheme 3.
References
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Solution dynamics relating D and D′ (Scheme 3) was initially
1
hinted by variable-temperature (VT) H NMR spectroscopy of 1
(5) See Supporting Information.
in CDCl3, in which the O-H proton resonance reversibly shifted
as a function of temperature (Figure S3).7,8 As the temperature was
raised from -45 to 55 °C, the O-H proton signal of 1 systemati-
cally moved upfield from 6.39 to 5.66 ppm in response to the
increasing solution population of non-hydrogen-bonded D′. This
interpretation is corroborated by the VT fluorescence spectra of 1
in CHCl3 (Figure 2c), in which loss of hydrogen bonding at higher
temperatures facilitates relaxation of the excited states through
internal torsional motions within D′.6
(6) Valeur, B. Molecular Fluorescence: Principles and Applications; Wiley-
VCH: Weinheim, Germany, 2002.
(7) This spectral behavior could best be explained by invoking conformational
distribution that is established rapidly on the NMR time scale to provide
a single chemical shift reflecting weight-averaged contributions of
hydrogen-bonded (D) and non-hydrogen-bonded (D′) conformers.8
(8) (a) Connors, K. A. Binding Constants: The Measurement of Molecular
Complex Stability; Wiley: New York, 1987. (b) Gellman, S. H.; Adams,
B. R.; Dado, G. P. J. Am. Chem. Soc. 1990, 112, 460-461. (c) Maslak,
V.; Yan, Z.; Xia, S.; Gallucci, J.; Hadad, C. M.; Badjic, J. D. J. Am.
Chem. Soc. 2006, 128, 5887-5894.
(9) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486-516.
(10) This process is accompanied by the disappearance of the O-H resonance
(5.03 ppm) as well as the collapse of the methylene doublet (J ) 5.6 Hz,
coupled to O-H) to a broad singlet at 4.52 ppm, which reflects dynamic
exchange process through interaction between O-H and F-.
The reversible conformational switching between D and D′
prompted us to drive this process through molecular recognition
events. A chemical system was desired that can effectively compete
against the intramolecular hydrogen bonding network. The strong
hydrogen bonding acceptor ability of the fluoride ion became
particularly attractive in this context.9 At 25 °C, addition of
nBu4NF (60 equiv) to a CH2Cl2 solution of 2 immediately quenched
the emission (Figure 2d).10 Subsequent treatment of this mixture
(11) The formation of Me3SiF as the reaction product was confirmed by both
19F NMR (δ ) -158.67 ppm vs CF3CO2H) and HR-MS (m/z ) 92.0456).
(12) For a recent example of peptide complexes that fold cooperatively through
a C3-symmetric hydrogen bonding network, see: Tatko, C. D.; Nanda,
V.; Lear, J. D.; DeGrado, W. F. J. Am. Chem. Soc. 2006, 128, 4170-4171.
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