'Lock and key' control of optical properties in a push-pull system

Article abstract of DOI:10.1039/b718015b

We report the modulation of the absorbance of a flavin push-pull derivative through specific recognition by a complementary diamidopyridine (DAP), shifting the flavin intramolecular charge transfer band by ～30 nm. The Royal Society of Chemistry.

Full text of DOI:10.1039/b718015b

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‘Lock and key’ control of optical properties in a push–pull systemw
Brian J. Jordan,a Michael A. Pollier,a Yuval Ofir,a Steven Joubanian,a
Jonathan G. Mehtala,a Carsten Sinkel,b Stuart T. Caldwell,b Andrew Kennedy,b
Gouher Rabani,b Graeme Cooke*b and Vincent M. Rotello*a
Received (in Austin, TX, USA) 25th November 2007, Accepted 11th January 2008
First published as an Advance Article on the web 1st February 2008
DOI: 10.1039/b718015b
We report the modulation of the absorbance of a flavin push–pull
derivative through specific recognition by a complementary
diamidopyridine (DAP), shifting the flavin intramolecular
charge transfer band by B30 nm.
The influence of molecular recognition on the optical pro-
perties of ABFL was determined via UV-Vis spectroscopy.
Toluene was used as a solvent to promote specific three-point
hydrogen bonding between ABFL and complementary diami-
dopyridine (DAP). Aliquots of DAP (stock solution 1.5 mM)12
were titrated into a constant host solution of ABFL (15 mM)
and the resulting spectra were measured at each addition
(Fig. 2). The spectra were corrected after each interval to
eliminate any overlapping absorbance from DAP.
Incorporating molecular recognition into donor–p–acceptor
1
(D–p–A)
molecules offers a potential strategy to create
responsive push–pull materials for applications in optical data
storage,2 telecommunications,3 nonlinear optics,4 and optical
switching devices (including sensors).5 Push–pull materials
combine electron donors (D) and acceptors (A) through a
conjugated bond network to produce a D–p–A chemical motif
with intrinsic properties: low-lying charge-transfer excited
states, hyperpolarizability, solvatochromism, and second-
order optical nonlinearities.6 Molecular recognition utilizes
noncovalent interactions, such as hydrogen bonding, p-stack-
ing, and electrostatics, to selectively assemble molecules into a
particular orientation with its complementary counterparts.
Coupling molecular recognition directly to push–pull materi-
als provides an opportunity to build synthetic host–guest
systems with tunable optical and electro-optical properties.
Current D–p–A systems do not utilize the advantages asso-
ciated with molecular recognition, such as reversibility, specifi-
city and directionality. Research has mainly focused on
colorimetric responses of D–p–A systems in regards to subtle
changes in pH or solvent polarity.7 For example, solvatochromic
dyes such as Reichardt’s Dye8 and Methyl Red9 act as environ-
mental indicators as they change color in different solvents
primarily due to dipole–dipole and intermolecular solute–sol-
vent interactions. The interactions remain relatively non-specific
and function only as a response to the surrounding environment.
Our approach builds upon previous push–pull azobenzene
derivatives, such as 4-nitro-40-(dimethylamino)azobenzene,
that demonstrate short fluorescent lifetimes (pico- and fem-
to-second) and large solvatochromic charge transfer bands
around B400 nm.10 Direct conjugation to a flavin ring system
creates an azobenzene flavin D–p–A push–pull derivative
8-[[p-[bis(ethyl)amino]phenyl]azo]isobutylflavin (ABFL) that
contains molecular recognition capabilities (specifically
three-point hydrogen bonding to the imido moiety) (Fig. 1).11
ABFL initially displayed a strong absorption at lmax B552
nm in the visible spectrum providing a vivid purple color
commonly associated with azobenzene dye derivatives.13 The
strong absorption was assigned to an intramolecular charge
transfer (ICT) and not an intermolecular charge transfer since
dilution studies exhibited a linear correlation with absorbance
intensity (see ESIw). As compared to similar push–pull azoben-
zene derivatives, ABFL exhibited a moderate bathochromic shift
in ICT primarily due to the extended conjugation of ABFL. The
weaker p–p* transition originally at lmax B335 nm was attrib-
uted to the combination of flavin14 and azobenzene15 p–p*
transitions.
Upon addition of DAP, a significant shift in the ICT and a
noticeable color change (purple to blue) was observed for
ABFL. There was a B30 nm bathochromic shift (from 552 to
584 nm) for the ICT band and a slight increase in absorbance
intensity. Furthermore, the weaker p–p* transition at 335 nm
decreased while the peak at 374 nm increased upon addition of
DAP (Fig. 2(b)). The resulting isosbestic points at 324, 346 and
424 nm implied that continuous additions of DAP shifted the
equilibrium from an unbound ABFL + DAP to a bound
a Department of Chemistry, University of Massachusetts at Amherst,
Amherst, MA 01003, USA. E-mail: rotello@chem.umass.edu
b WestCHEM, Department of Chemistry, Joseph Black Building,
University of Glasgow, Glasgow, UK G12 8QQ
w Electronic supplementary information (ESI) available: Synthetic
procedures and characterization, Kamlet–Taft parameters, and addi-
tional UV-Vis spectroscopy. See DOI: 10.1039/b718015b
Fig. 1 Representative ABFL D–p–A chemical structure and its
molecular recognition capability via three-point hydrogen bonding.
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Fig. 3 Variable temperature UV-Vis for ABFL (a, b) and MABFL
(c, d) in toluene. Curves show entire spectrum (a) and (c) and
expanded regions in the range 320–430 nm (b) and (d).
Fig. 2 UV-Vis titrations of DAP into ABFL (a, b) and MABFL (c, d)
performed at 25 1C in toluene. Curves show entire spectrum (a and c)
and expanded regions in the range 320–430 nm (b and d).
[ABFL:DAP] state.16 The binding constant (Ka) for [ABFL:-
DAP] complex was calculated to be Ka = B1100 Mꢁ1 (see
ESIw) which was significantly larger than previous reported
flavin host–guest complexes.17
the ABFL ‘‘push–pull’’ character. ABFL was comprised of a
neutral resonance form (I) that resembled a bond-alternate
polyene structure19 and a zwitterionic resonance form (II) that
exhibited a large charge separation on the order of B15 A
(Scheme 1). The combination of these resonance contributions
provided a strong ‘‘push–pull’’ electronic character for ABFL.
UV-Vis spectra of ABFL were recorded in eleven solvents
with various polarities. Extra care was taken to compare all
solvent samples at a constant ABFL concentration. Both ICT
and p–p* transitions demonstrated a strong solvent depen-
dence but did not exhibit a linear correlation to solvent
polarity (see ESIw). Less polar solvents displayed a peak
around lmax B335 nm and a prominent ICT at lmax
B544–556 nm. Halogenated solvents, as well as more polar
solvents, demonstrated various bathochromic shifts in ICT
and p–p* transitions, suggesting that an additional solute–sol-
vent interaction acted as a mechanism to stabilize ABFL.
Kamlet–Taft parameters20 in combination with linear free
energy relationships (LFER) have been previously used to
model solute–solvent interactions by describing solvents in
terms of hydrogen bond donation (a), hydrogen bond accep-
tance (b), and polarizability (p*). We applied this strategy to
reveal the extent of solute/solvent interaction for ABFL using
specific Kamlet–Taft parameters from a similar electronic
system (see ESIw).21 The contribution from each parameter
(a, b, p*) to influence the resultant ICT absorption value was
calculated as a coefficient in eqn (1).
A control MABFL (ABFL methylated at the N(3) position)
was used to probe the specific molecular recognition binding
process between ABFL and DAP. MABFL displayed similar
electronic behavior (ICT B552 nm) but remained relatively
unaffected after addition of DAP. The methyl group at the
N(3) position served to disrupt the three-point hydrogen bonding
between MABFL and DAP. Additionally, methylation of DAP
(see ESIw) provided a non-complementary hydrogen-bonding
derivative which was used to further confirm the specific three-
point hydrogen-bonding process. Both the ICT and p–p* transi-
tions remained unaltered upon addition of methylated DAP to
ABFL indicating that the shift in absorbance was a direct result of
the specific [ABFL:DAP] molecular recognition event.
The reversibility of the three-point hydrogen bonding be-
tween ABFL and DAP was examined via variable-temperature
UV-Vis spectroscopy. The resultant endpoints from the titra-
tions performed in toluene (15 mM ABFL with excess DAP)
were used to evaluate the temperature dependence upon
complex binding for both ABFL and MABFL (Fig. 3).
ABFL exhibited a B30 nm blue shift in the ICT and an
apparent color change (blue to purple) at 65 1C. A re-emer-
gence of psuedo-isosbestic points18 at B350 and B410 nm
suggested that heating the solution reduced the hydrogen
bonding for [ABFL:DAP] complex and shifted the dynamic
equilibrium towards the unbound ABFL + DAP state. Con-
trol MABFL demonstrated a slight blue shift in ICT (B8 nm)
but did not exhibit any psuedo-isosbestic points or distinct
color change in the presence of DAP. The results were con-
sistent with a molecular recognition three-point hydrogen-
bonding motif for [ABFL:DAP] complex.
In addition to the specific molecular recognition event for
ABFL, both flavin molecules ABFL and MABFL were likewise
solvatochromic. A comprehensive analysis of this solvatochro-
mic effect of ABFL was performed to evaluate the nature of
Scheme 1 Representative resonance structures of ABFL push–pull
system: neutral (I) and zwitterionic (II) resonance forms.
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Vmax = 18.824 ꢁ 2.318p* ꢁ 1.433a ꢁ 1.351b
(1)
The Vmax values for the predicted ICT absorption were
expressed in kilokaysers (1 kK = 10ꢁ4 nmꢁ1) for comparison
with previous solute probes using Kamlet–Taft parameters.
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meters (a, b, p*) had a significant impact on the ICT of ABFL
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hydrogen-bond donating solvents appeared to stabilize an
ABFL excited state consistent with a more zwitterionic form.
As a result, the ICT shifted to a longer wavelength for polar
and hydrogen-bond donating solvents. Less polar solvents and
hydrogen-bond accepting solvents destabilized ABFL shifting
the ICT to a shorter wavelength. Control MABFL exhibited
similar push–pull behavior in the UV-Vis spectra with respect
to the various solvents (see ESIw). This result indicated that
the effective hydrogen bonding between hydrogen-bond do-
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solvent loss which lead to a slight increase in absorbance intensity
for both ABFL and MABFL as a function of temperature.
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In conclusion, we have the ability to tune the electronics and
hence color of the ABFL ‘‘push–pull’’ system via molecular
recognition. The addition of complementary DAP shifts both
ICT and p–p* transitions (B30 nm) in toluene, changing the
color of the solution from purple to blue and establishing an
equilibrium between bound and unbound states. Stabilization
of ABFL via hydrogen bonding occurs specifically at the
negatively charged oxygen in the zwitterionic resonance form
as indicated by its solvatochromic response. The binding
process is reversible at elevated temperatures demonstrating
our control over the noncovalent interactions.
V. M. R. acknowledges NSF for CHE-0518487 and G. C.
gratefully acknowledges the EPSRC. B. J. J. thanks NSF
(IGERT, DUE-044852) and IBM for fellowships.
Notes and references
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