Application of PET deprotection for orthogonal photocontrol of aqueous solution viscosity

Article abstract of DOI:10.1039/c0cc02203a

Photorelease and photoisomerization of trans-cinnamic acid in aqueous CTAB solutions induces a bulk solution viscosity increase and decrease, respectively, triggered by orthogonal irradiation wavelengths.

Full text of DOI:10.1039/c0cc02203a

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Application of PET deprotection for orthogonal photocontrol of aqueous
solution viscosityw
J. Brian Borak,a Hee-Young Lee,b Srinivasa R. Raghavanb and Daniel E. Falvey*a
Received 29th June 2010, Accepted 24th September 2010
DOI: 10.1039/c0cc02203a
Photorelease and photoisomerization of trans-cinnamic acid
in aqueous CTAB solutions induces a bulk solution viscosity
increase and decrease, respectively, triggered by orthogonal
irradiation wavelengths.
photoisomerization of trans ortho-methoxycinnamic acid
(OMCA) in aqueous solutions of the cationic surfactant cetyl
trimethylammonium bromide (CTAB).13 Aqueous solutions
of CTAB are well known to form wormlike micelles upon
addition of certain aromatic carboxylates and sulfonates.14–16
In the presence of trans-OMCA, CTAB forms long, entangled
wormlike micelles, resulting in very high solution viscosities.
Upon UV-induced trans to cis photoisomerization, the geometry
of OMCA is altered,17 and in turn, the cis-OMCA molecules
tend to desorb from the micelles. This leads to shorter micelles
and thus to lower solution viscosities. A key goal in the
development of these photorheological (PR) fluids18 is to
achieve a high degree of viscosity reversibility for applications
such as microfluidic flow-control valves.19–21 While, in principle,
it should be possible to reverse the above changes, the similarities
in absorption properties between trans- and cis-OMCA
preclude reversal once a photostationary state is reached.
Since OMCA and several other organic additives contain
carboxylate functionalities that are directly involved with the
viscosity transition, we envisioned using the NAP group to
protect this site and thus enable a transition from low to high
viscosity upon photorelease that can precede the UV light
induced viscosity decrease. We report herein the development
of such a system which allows for modulation of solution
viscosity in two photochemically orthogonal steps.
Photorelease of functional molecules has found broad utility
in materials and biological sciences. Photolabile protecting
groups (PPGs) are frequently the active component in these
systems and have been developed for lithographic applications1,2
as well as the release of ‘‘caged’’ biologically significant
compounds (e.g., caged ATP, glutamate, etc.).3,4 Commonly
studied PPGs include derivatives of the nitrobenzyl, benzoin,
phenacyl, and coumarin families.5,6 Our efforts have focused
on utilizing sensitized photoinduced electron transfer (PET) to
release a PPG that responds to one electron reduction7
(Scheme 1A). The N-alkylpicolinium (NAP) group has been
the primary PPG that we have used to protect carboxylates,
phosphates, and amino acids.8–10 Release of protected substrates
using sensitizers absorbing UV or visible light irradiation has
been demonstrated. Furthermore, we have developed several
aqueous compatible systems utilizing a large excess of an
inexpensive electron donor along with a sub-stoichiometric
amount of visible light absorbing sensitizers (mediated deprotec-
tion, Scheme 1B).8,11,12 An electron is shuttled between the
donor and the NAP group by the sensitizer (mediator) to
release the substrate in these systems.
In designing this system, we chose to protect the parent
trans-cinnamic acid compound (tCA) rather than OMCA due
to the ease of synthesis of the corresponding NAP–ester
(NAP–tCA). NAP–tCA was prepared as the bromide salt
due to solubility difficulties with other counterions. We sought
to use a visible light absorbing sensitizer for the photorelease
step to avoid absorption by or energy transfer to the cinnamic
acid moiety. Tris(bipyridyl)ruthenium(II) (Rubpy) was chosen
for this purpose due to its strong visible absorption band and
aqueous compatibility. To complete the mediated electron
transfer scheme, a good electron donor must be included in
the system to photogenerate Rubpy+1, a suitable reductant for
NAP–tCA (Scheme 2). In the current system, ascorbic acid
(ASC) serves as this source. Photorelease of tCA in aqueous
CTAB solutions using visible light was expected to induce
wormlike micelle formation and thereby a high solution
viscosity. Subsequent UV-induced photoisomerization of
tCA would reduce the micellar length and thereby cause a
drop in viscosity (Scheme 3).
Having developed these useful systems, we sought to demon-
strate their applicability to produce higher order responses
upon photolysis. Our interest was drawn toward systems
developed to modulate solution viscosity through the
Scheme 1 Direct (A) versus mediated (B) photoinduced electron
transfer photorelease.
a Department of Chemistry and Biochemistry, University of Maryland,
College Park, MD 20742, USA. E-mail: falvey@umd.edu
b Department of Chemical and Biomolecular Engineering,
University of Maryland, College Park, MD 20742, USA.
E-mail: sraghava@eng.umd.edu
w Electronic supplementary information (ESI) available: Synthetic
and photolysis procedures, control rheology and photolysis, molecular
modeling, mechanistic studies by laser flash photolysis. See DOI:
10.1039/c0cc02203a
Scheme 2 Deprotection of NAP–tCA by mediated electron transfer.
Chem. Commun., 2010, 46, 8983–8985 8983
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Scheme 3 Photorelease and isomerization of trans-cinnamic acid.
Solutions were prepared such that photolysis would generate
sufficient tCA to maximize solution viscosity. Samples typi-
cally contained 2 mM Rubpy, 30 mM ASC, 30 mM NAP–tCA
and 20 mM tCA along with 50 mM of CTAB. Among these
components, NAP–tCA and Rubpy had negligible effect on
the viscosity; however, ASC reduced the viscosity at high
concentrations and therefore its concentration was kept low.
The initial solution (Fig. 1A) was a freely-flowing, water-
like liquid. Upon irradiation with broad-band visible light
(360–800 nm) for 15–60 min, the sample became noticeably
viscous (Fig. 1B). Analysis of the irradiated sample by steady-
shear rheology (Fig. 2) reveals a substantial (ca. 105) increase
in viscosity at low shear rates. The sample also displays
the strong shear-thinning behavior expected of viscoelastic
wormlike micellar fluids18 in contrast to the Newtonian response
of the initial solution. Subsequent irradiation with broad-band
UV/VIS light (250–800 nm) for 2 h results in an appreciable
drop in viscosity, which is evident visually (Fig. 1C) and
confirmed by the rheological data (Fig. 2). Note that the drop
in viscosity is by ca. 102.5 at low shear rates and some shear
thinning still occurs. This indicates that some moderate-sized
micelles are still present.
Fig. 3 1H NMR spectra of 10 mM CTAB solutions in D2O with
added tCA (concentrations as indicated) demonstrating binding of
tCA to the micelles. The top-most spectrum contains only tCA for
comparison.
analysis confirm release of tCA upon visible light photolysis
(Table S1, ESIw). (2) Addition of tCA to CTAB solutions
causes viscosity increases similar to those generated from
visible light photolysis of the NAP–ester (see ESIw). (3)
Finally, addition of tCA to solutions of CTAB in D2O causes
a dramatic broadening and upfield shift in the tCA aromatic
and vinylic signals in the 1H NMR spectra (Fig. 3). The NMR
data are consistent with the binding of tCA to CTAB micelles,
in accordance with similar studies22 with other aromatic
additives. Specifically, an upfield shift of the aromatic protons
is observed at 1 mM tCA. Additional shifts and broadening of
these peaks at higher concentrations reflect both binding as
well as a conversion of spherical CTAB micelles into long
entangled wormlike micelles (Fig. 4). A more detailed analysis
of these effects will be reported subsequently. As the carboxylate
of tCA binds to the cationic headgroup of CTAB, the effective
charge is reduced (smaller orange circles) thereby allowing
tighter packing of the headgroups and a cylindrical (wormlike)
micelle geometry.
That tCA production is responsible for the viscosity increase
is supported by several observations. (1) HPLC and NMR
Photoreversal of the viscosity increase with UV light is
correlated to the conversion of tCA to its cis isomer (cCA),
as confirmed by HPLC and NMR. cCA has a weaker affinity
for CTAB micelles than tCA, leading to its unbinding from the
1
micelles. This is shown by H NMR (Fig. 5): the broad and
1
upfield-shifted aromatic H NMR signals for tCA gradually
Fig. 1 Photographs of aqueous solutions of CTAB (50 mM), tCA
(30 mM), Rubpy (2 mM), and ASC (30 mM) before photolysis (A), after
1 h visible light photolysis (B), and after 2 h UV light photolysis (C).
return to their unbound downfield values with increasing UV
irradiation time. Simultaneously, signals for cCA grow in as
the tCA signals diminish. The signals also become sharper as
the additives are now in free solution. Unbinding of cCA can
be rationalized by comparing the preferred conformations of
tCA and cCA. For cCA, the carboxylate group is not coplanar
with the aromatic ring, an assertion that is supported by
density functional theory calculations (B3LYP/6-31+G(d,p),
Fig. S7, ESIw). In contrast, the carboxylate and aromatic
moieties of tCA prefer to adopt a coplanar geometry, an
important characteristic for binding of aromatic carboxylates
to cationic micelles.17 The unbinding of cCA restores much of
the positive charge to CTAB headgroups and the increase in
effective headgroup size leads to shorter cylinders or spheres
(Fig. 4). Thus, the rheological changes can be understood as
originating from these molecular-scale differences. Note that,
due to the significant overlap of the absorption spectra for
each isomer, complete conversion from trans to cis is not
possible with our apparatus once a photostationary state is
Fig. 2 Steady-shear rheology of the starting solution, after visible
irradiation (VIS), and after UV irradiation (UV).13
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Fig. 4 Changes in micelle structure and corresponding higher-order behavior induced by binding and unbinding of tCA to CTAB.
Notes and references
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In summary, we have applied the NAP photorelease system
to control aqueous solution viscosity with two photochemical
steps. Large changes in viscosity (105 fold increase, 102.5 fold
decrease) have been observed, and each solution state remains
stable for several days. By using visible light to initiate
photorelease and UV light to control photoisomerzation,
the two viscosity transitions are conveniently orthogonal to
one another. Additionally, NMR data have been obtained on
a cinnamate-based PR fluid for the first time which validates
the viscosity transition mechanism proposed in other cinnamate-
based PR fluids. This system may prove useful in specific PR
fluid applications and demonstrates the broader utility
of the NAP photorelease system for biological or materials
applications.
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