Photoacid for extremely long-lived and reversible pH-jumps

Article abstract of DOI:10.1021/ja901930c

The ability to change the acidity of an aqueous solution within the time of a short laser pulse and use the resulting low pH to drive acid-catalyzed reactions in the irradiated volume opens news perspectives for the spatial and temporal control of a varie

Full text of DOI:10.1021/ja901930c

Published on Web 06/11/2009
Photoacid for Extremely Long-Lived and Reversible
pH-Jumps
Rui M. D. Nunes, Marta Pineiro,* and Luis G. Arnaut*
Chemistry Department, UniVersity of Coimbra, 3000 Coimbra, Portugal
Received March 18, 2009; E-mail: mpineiro@qui.uc.pt
Abstract: The ability to change the acidity of an aqueous solution within the time of a short laser pulse
and use the resulting low pH to drive acid-catalyzed reactions in the irradiated volume opens news
perspectives for the spatial and temporal control of a variety of processes. Persistent and reversible
acidification of an aqueous solution is achieved with a new molecule, 1-(2-nitroethyl)-2-naphthol, that
combines the fast photoacid properties of an aromatic alcohol with the slow proton transfer rates of a
nitroalkane. The protons released in a few nanoseconds by pulsed laser excitation of the photoacid last for
nearly one second in aqueous solutions. We show that acid-base equilibria with other species is established
at the lower pHs of the irradiated volume, that the process is reversible and that it can be maintained
under continuous irradiation.
1. Introduction
than this to ensure that the nanometric features are controlled
by the kinetics of the acid-catalyzed reaction with the substrate.
Furthermore, it is practical to focus a laser beam to a diameter
below 1 µm, which is comparable to the sizes of cellular
organelles of most Eukaryotic animal cells. However, these
potential applications require the stabilization of the pH drop
in the irradiated volume within a nanosecond laser pulse, the
maintenance of the acidity for orders of magnitude longer time
scales and the absence of contamination by side products. The
Aromatic alcohols, such as naphthols, become strong acids
when they absorb light.1 Their enhanced acidity in the excited
state is reasonably well predicted by the Fo¨rster equation
*
pKa ) pKa - (νHA - νA-)/2.3RT
(1)
-
where hνΗΑ(A is the energy of the 0-0 electronic transition
)
for the conjugated acid (base). In addition to a decrease in 6-8
pKa units upon electronic excitation, such photoacids also
transfer very rapidly a proton to a nearby water molecule and
are capable of transforming a neutral aqueous solution into an
acidic one within nanoseconds.
ideal photoacid should have a pKa > 8 and a low pK* (<2),
a
deprotonate in a few nanoseconds in aqueous solutions upon
electronic excitation and reversibly reprotonate very slowly. A
strong, persistent and reversible photoacid would meet the
specifications for the applications mentioned above.
The acidification of a solution with a photoacid and a short
laser pulse can be used to perturb rapid acid-base equilibria
and to initiate acid-catalyzed reactions in less than 10 ns, which
are of interest to uncover mechanisms of acid-base catalysis,2
study the initial steps of protein folding3 or phototrigger
liposomal drug delivery.4 Moreover, the spatial control of pH
in the nanoscale can be exploited for photolithography,5 and
for mechanistic studies in confined (subcellular,6 nanostructured)
volumes. Indeed, the features obtained by photolithography in
the semiconductor industry now approach 100 nm and even
considering the abnormally large value for the proton diffu-
sion coefficient in water, D ) 9.3 nm2/ns at 300 K,7 the
translation distance Rt ) ꢀ(2Dt) ) 100 nm is only accessed in
550 ns. The duration of the pH jump must be sufficiently longer
The fast production of an acidic solution using a short laser
pulse attracted much interest over the last 30 years. This was
the original aim of the pH jump experiment, which employed
naphthol derivatives.8 However, the conjugated bases of these
photoacids rapidly return to the electronic ground state, where
they reprotonate and neutralize the solutions in hundreds of
nanoseconds.9 The bimolecular reprotonation rate increases with
the concentration of the conjugated base and limits naphthols
to the generation of small and short-lived pH jumps. Much larger
pH jumps have been achieved with the introduction of “super”
photoacids, with negative pK*s,10,11 but this did not solve the
a
problem of the fast reprotonation of the conjugated base.
Alternatively, longer-lived pH jumps are produced when a
proton is released from a photolabile “caging” group.12,13
(1) Arnaut, L. G.; Formosinho, S. J. J. Photochem. Photobiol. A: Chem.
1993, 75, 1–20.
(8) Gutman, M.; Huppert, D. J. Biochem. Biophys. Methods 1979, 1, 9–
19.
(2) Politi, M. J.; Fendler, J. H. J. Am. Chem. Soc. 1984, 26, 5–273.
(3) Abbruzzetti, S.; Crema, E.; Masino, L.; Vecli, A.; Viappiani, C.; Small,
J. R.; Libertini, L. J.; Small, E. W. Biophys. J. 2000, 78, 405–415.
(4) Shum, P.; Kim, J.-M.; Thompson, D. H. AdV. Drug DeliVery ReV.
2001, 53.
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Chem. Soc. 1983, 105, 2210–2216.
(10) Tolbert, L. M.; Haubrich, J. E. J. Am. Chem. Soc. 1994, 116, 10593–
10600.
(5) Wallraff, G. M.; Hinsberg, W. D. Chem. ReV. 1999, 99, 1801–1821.
(6) Fraser, J. A.; Middlebrook, C. E.; Usher-Smith, J. A.; Schwiening,
C. J.; Huang, C. L. H. J. Physiol. 2005, 563, 745–764.
(7) Agmon, N. Isr. J. Chem. 1999, 39, 493–502.
(11) Tolbert, L. M.; Solntsev, K. M. Acc. Chem. Res. 2002, 35, 19–27.
(12) Bonetti, G.; Vecli, A.; Viappiani, C. Chem. Phys. Lett. 1997, 269,
268–273.
(13) Barth, A.; Corrie, J. E. T. Biophys. J. 2002, 83, 2864–2871.
9
9456 J. AM. CHEM. SOC. 2009, 131, 9456–9462
10.1021/ja901930c CCC: $40.75  2009 American Chemical Society

Photoacid for Long-Lived and Reversible pH-Jumps
A R T I C L E S
∆V0 ) -RT(2.303pKHA + ln(pA/qA) - 2.303pKHB
-
Photoactivation of such compounds initiates a series of reactions
where one of the intermediates is a strong acid that deprotonates
and rapidly converts into a photolysis product with low pKa.14
With such caged proton compounds, the size of the pH jump is
limited by the pKa of the photolysis product and the irrevers-
ibility is a source of contamination.
In(pB/qB)) - ZHA + ZHB (4)
pKAH and pKBH are the thermodynamic acidity constants of
AH and BH, pA (pB) is the number of equivalent protons in AH
(BH), qA (qB) is the number of equivalent basic sites in AH
(BH), and ZAH (ZBH) is the zero-point energy of the AH (BH)
bond. The presence of hydrogen-bonded intermediates is
included in this reaction path using the Lippincott-Schroeder
potential.22 Zero-point energies are added to the classical
reaction coordinate of ISM and the resulting vibrationally
adiabatic path is employed to calculate rate constants using the
semiclassical transition state theory. Intramolecular proton
transfers are treated in the same way, except that the entropy
lost in attaining the six-membered ring may be taken from that
The advances in the pH jump technique have been insufficient
to promote it as a general tool to perturb acid-base equilibria
or to induce acid-catalyzed reactions. Further progress requires
a reversible photoacid capable of acidifying persistently its
aqueous environment when irradiated with a single laser pulse.
In this work we develop the concept of persistent and reversible
photoacids using Formosinho’s Interacting-State Model
(ISM)15-17 and its applications to proton transfers in solution
and in enzymes.16,18 ISM is employed to design the structure
of a photoacid capable of very fast excited-state deprotonation
and very slow ground-state reprotonation. It is shown that this
can be achieved with naphthol derivatives provided that
intramolecular proton transfer from an activated carbon acid is
made competitive with the excited-state decay of the naphtho-
late. ISM calculations show that one nitro group is sufficient
for this purpose if the acidic proton closes a 6-membered ring
with the oxygen atom of the naphtholate. These predictions
motivated the synthesis of 1-(2-nitroethyl)-2-naphthol (NO2nH).
We show that aqueous solutions of this molecule remain acidic
under continuous irradiation, reversibly retuning to neutrality
in the dark. On-off light cycles generate acid-neutral pH cycles,
each lasting for approximately one second. The reversibility of
the system supports the operation of a molecular proton pump
driven by light. Using intense and focused laser beams, it
becomes possible to control acidity in the nanoscale within
nanoseconds.
of cyclohexane, ∆qS ) -18.8 cal mol-1 K-1 23 and included in
,
the pre-exponential factor. Details are given in the Supporting
Information. The calculations were run in the freely available
Internet application,16b uploading the input files presented in
the Supporting Information. The output files are also presented
in the Supporting Information.
Figure 1 presents the rates of the proton transfer cycle
calculated with ISM based on known pKa values of analogous
compounds, pKa(2-naphthol) ) 9.4510 and pKa(2-nitro-ethyl)-
benzene ) 8.78 in water and 9.82 in 50% MeOH/H2O,24
respectively. ISM predicts that intramolecular quenching of the
electronically excited naphtholate ion by an acidic proton of a
nitroalkane moiety is competitive with its nanosecond lifetime.
For example, the anion derived from 1-propyl-2-naphthol has
an excited state decay with a major component ca. 5 × 107
s-1,25 whereas the calculated intramolecular proton transfer rate
constant from the acidic carbon to RO- is kipt ) 2.6 × 108 s-1
.
Thus, we expect that 80% of the naphtholate fluorescence is
quenched by nonadiabatic intramolecular proton transfer to yield
a ground-state carbanion located at the R position to the nitro
group. This carbanion is trapped in a deep potential well, with
a barrier of 7 kcal/mol preventing its protonation by the oxonium
ion and a barrier of 14 kcal/mol in the way of the ground-state
intramolecular proton transfer from the naphthol moiety.
The intramolecular proton transfer neutralizes the naphtholate
but generates a carbanion next to a nitro group, which is
recognized as the best carbanion-stabilizing group in organic
chemistry.26 ISM calculated rate constants for the protonation
of this carbanion and for the deprotonation of the corresponding
2. Theoretical Design
ISM has been described in detail in various recent publica-
tions,16,17,19 including applications to proton transfer reactions
in solution16,20 and in enzymes.18,21 Briefly, the classical reaction
path of ISM for the general proton transfer reaction
B- + HA f {B...H...A}-q f BH + A-
(2)
is a linear interpolation between the Morse curves of HA and
HB along the reaction coordinate
nitroalkane are kp ) 5 × 104 and kd ) 8 × 10-7 M-1 s-1
,
Vcl(n) ) (1 - n)VHA + nVHB + n∆V0
(3)
respectively, but the deprotonation in water is more conveniently
expressed as a first-order rate with kd[H2O] ≈ 4 × 10-5 s-1
.
The protonation of this carbanion is 106 times slower than that
of ground-state 2-naphtholate, which is controlled by diffusion,
and is expected to lead to pH jumps lasting one million times
longer than those produced by naphthols.
where n is the HB bond order and the classical reaction energy
is
(14) Laimgruber, S.; Schreier, W. J.; Schrader, T.; Koller, F.; Zinth, W.;
Gilch, P. Angew. Chem., Int. Ed. 2005, 44, 7901–7904.
(15) Arnaut, L. G.; Pais, A. A. C. C.; Formosinho, S. J.; Barroso, M. J. Am.
Chem. Soc. 2003, 125, 5236–5246.
The quantitative treatment of the carbanion reprotonation
kinetics in a pH jump experiment must take into consideration
that the return of the solution to neutrality is governed by the
re-establishment of the equilibrium
(16) (a) Barroso, M.; Arnaut, L. G.; Formosinho, S. J. J. Phys. Chem. A
2007, 111, 591–602. (b) http://www.ism.qui.uc.pt:8180/ism/.
(17) Arnaut, L. G.; Formosinho, S. J. Chem.sEur. J. 2008, 14, 6578–
6587.
(22) Barroso, M.; Arnaut, L. G.; Formosinho, S. J. ChemPhysChem 2005,
6, 363–371.
(18) Barroso, M.; Arnaut, L. G.; Formosinho, S. J. J. Phys. Org. Chem.
2009, 22, 254–263.
(23) Lowry, T. H.; Richardson, K. S., Mechanism and Theory in Organic
Chemistry, 3rd ed.; Harper & Row Publishers: New York, 1987.
(24) Bordwell, F. G.; Bartmess, J. E. J. Org. Chem. 1978, 43, 3101–3107.
(25) Tolbert, L. M.; Harvey, L. C.; Lum, R. C. J. Phys. Chem. 1993, 97,
13335–13340.
(19) Arnaut, L. G.; Formosinho, S. J. Chem.sEur. J. 2007, 13, 8018–
8028.
(20) Arnaut, L. G.; Formosinho, S. J.; Barroso, M. J. Mol. Struct. 2006,
786, 207–214.
(21) Barroso, M.; Arnaut, L. G.; Formosinho, S. J. J. Phys. Org. Chem.
2008, 21, 659–665.
(26) Backstrom, N.; Burton, N. A.; Turega, S.; Watt, C. I. F. J. Phys. Org.
Chem. 2008, 21, 603–613.
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A R T I C L E S
Nunes et al.
H+ + NO2n- h NO2nH
(5)
Potassium ferrioxalate actinometry was made according to standard
procedures.27,28
1-(2-Nitro-vinyl)-naphthalen-2-ol. 2-Hydroxynaphthalene-1-
carbaldehyde (500 mg, 5.8 mmol), nitromethane (0.31 mL, 5.8
mmol) and L-proline (125 mg, 25%, w/w) were dissolved in 20
mL of methanol and heated at 50 °C. The reaction was followed
by TLC until the reagents were undetectable. The reaction mixture
was cooled to room temperature and the solvent evaporated. The
reaction crude was dissolved in a mixture ether:dichlorometh-
ane (1:5, v/v) and warmed at 40 °C in a water bath until complete
solubilization. Cooling to room temperature, 1-(2-nitro-vinyl)-
naphthalen-2-ol precipitates as a solid in 17% yield.
Under typical experimental conditions, [NO2nH]0 ≈ 10-4 M,
pKa ≈ 9, pH 7, the relaxation time calculated with (6) is 2
seconds. The intramolecular carbanion protonation channel
shown in Figure 1 also contributes to the relaxation time at
millimolar photoacid concentrations, reducing it to ca. 1 s.
1
τ )
(6)
kp([NO2n-]eq + [H+]eq) + kd[H2O]
1-(2-Nitro-ethyl)-naphthalen-2-ol (NO2nH). 1-(2-Nitro-vinyl)-
naphthalen-2-ol (250 mg, 1.2 mmol) was dissolved in 10 mL of a
mixture of THF/methanol (1:10, v/v). Sodium borohydride was
added, in portions to the stirring solution until the reaction mixture
turned colorless. After quenched with water, the reaction mixture
was extracted using dichloromethane/NaCl(aq) at low temperature.
After evaporation the dry product was obtained in 71% yield. In
this work we employ the more trivial name 1-(2-nitroethyl)-2-
In summary, the ISM design of a persistent and reversible
photoacid indicates that 1-(2-nitroethyl)-2-naphthol should
deprotonate in the excited state within a nanosecond laser pulse
to yield a carbanion competitively with a ground-state naph-
tholate, and that the carbanion may relax in one second back to
neutrality.
1
3. Experimental Procedures
naphthol for this molecule. H NMR (400 MHz, CDCl3) δ, ppm:
7.89(d, 1H, J ) 8.6 Hz), 7.79(d, 1H, J ) 8.1 Hz), 7.70(d, 1H, J )
8.8 Hz), 7.54(t, 1H, J ) 7.6 Hz), 7.37(t, 1H, J ) 7.5 Hz), 7.04(d,
1H, J ) 8.8 Hz), 4.67(t, 1H, J ) 7.6 Hz), 3.81(t, 1H, J ) 7.6 Hz).
13C NMR (400 MHz, CDCl3) δ, ppm: 151.4, 132.9, 129.5, 129.4,
129.0, 127.4, 123.6, 121.8, 117.6, 113.6, 74.1, 19.7. MS (MALDI-
TOF), m/z: 217 (M+).
Materials. The reactants 2-hydroxynaphthalene-1-carbadehyde,
nitromethane, L-proline, NaBH4 and o-nitrobenzaldehyde were
1
purchased from Aldrich. H and 13C NMR spectra were recorded
in CDCl3 solutions on Bruker Avance III de 400 MHz spectrometer.
Methods. Absorption and luminescence spectra were recorded
with Shimadzu UV-2100 and SPEX Fluoromax 3.22 spectropho-
tometers, respectively. Fluorescence lifetimes were measured with
a previously described home-built time-correlated single-photon
counting (SPC) apparatus using IBH nanoLEDs (281 nm) as
excitation sources, Philips XP2020Q photomultiplier with excitation
and emission wavelengths selected with Jobin-Ivon H20 mono-
chromators, and a Canberra Instruments time-to-amplitude converter
and multichannel analyzer.29 Transient triplet-triplet absorption
was obtained with an Applied Photophysics LKS.60 flash photolysis
spectrometer with the R928 photomultiplier from Hamamatsu for
detection and the HP Infinium (500 MHz, 1 GSa/s) or Tektronix
DPO 7254 (2.5 GHz, 40 GSa/s) oscilloscopes. Excitation employed
the fourth harmonic of a Spectra-Physics Quanta Ray GCR 130
Nd:YAG laser (5-6 ns pulse width). The energy per pulse was
typically below 5 mJ at 266 nm.
4. Results and Discussion
The synthesis of 1-(2-nitroethyl)-2-naphthol, NO2nH, was
achieved through a Henry reaction using L-proline as catalyst,30
and subsequent reduction with methanolic sodium borohy-
dride.31 The negative influence of the hydroxyl group at the
ortho position of the aldehyde in the Henry reaction was avoided
using 2-benzyloxy-naphthalene-1-carbaldehyde. In fact, the
corresponding condensation product, 2-benzyloxy-1-(2-nitro-
vinyl)-naphthalene, was obtained in 63% yield. The use of the
aldehyde with the unprotected hydroxyl group in the condensa-
tion opens new routes for other reduction strategies for the
carbon-carbon double bond, and gives us access to the desired
product, although compromising the overall yield. Figure 2
shows the NMR spectrum of the molecule, with the triplet
(27) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. 1956, A235, 518.
(28) Arnaut, L. G.; Formosinho, S. J.; Burrows, H. D. Chemical Kinetics;
Elsevier: Amsterdam, 2007; p 549.
(29) Melo, J. S.; Fernandes, P. F. J. Mol. Sruct. 2001, 69, 565.
(30) Ku¨rti, L.; Czako´, B. Strategic applications of named reactions in
organic synthesis; Elsevier Academic Press: London, 2005.
(31) Varma, R. S.; Kabalka, G. W. Synth. Commun. 1985, 15, 151.
Figure 1. Energy landscape of NO2nH according to ISM. It was assumed
that pK*(naphthol) ) 2, pKa(naphthol) ) 9.4 and pKa(nitroalkane) ) 9 [see
a
Supporting Information for the details of ISM calculations].
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Photoacid for Long-Lived and Reversible pH-Jumps
A R T I C L E S
Figure 2. NMR spectra of NO2nH. (Inset) Alkyl region of the NMR
spectra before and after 1 min irradiation with a low-pressure halogen
lamp at 254 nm.
corresponding to benzylic protons centered at 3.80, and the
triplet at 4.67 assigned to the protons of the carbon R to the
nitro group.
Figure 3A presents the absorption spectra of NO2nH at
different pH values. NO2nH is mostly present in the neutral
form at pH e 7. Two ionization constants were measured, with
pKas 9.7 and 11.6, corresponding to the loss of one and two
protons, respectively, in 2% MeOH/H2O. In view of the
expected similarity between the pKas of naphthol and 2-phenyl-
1-nitroethane, the lower pKa corresponds to an anion exchanging
a proton between the naptholate and the carbanion. The second
ionization of this species requires a high pH. Figure 3B shows
that the ratio of fluorescence intensities *RO-/*ROH of NO2nH
at pH 7.2 is only 0.75, much lower than the value of 2 measured
for 1-propyl-2-naphthol.25 Assuming that the radiative and
nonradiative decay rates of 1-propyl-2-naphthol and NO2nH
are identical, and that the decay rates in the corresponding anions
differ from each other only because of the presence of the
nonadiabatic intramolecular proton transfer rate constant (kipt)
in NO2nH, we can write
k1F + k1nr + kipt
2
0.75
)
(7)
k1F + k1nr
and conclude that 63% of the *RO- fluorescence quenching is
due to the acidic proton of the nitroalkane moiety.
The enhanced quenching of *RO- fluorescence in NO2nH
versus 1-propyl-2-naphthol, analyzed above, indicates that the
+
efficiency of intramolecular proton transfer is φH ≈ 0.63. The
proton comes from the acidic carbon of the nitroalkane moiety,
and this efficiency is also the efficiency of carbanion formation.
In order to obtain the quantum yield of carbanion formation,
we also need to know the *RO- formation quantum yield in
NO2nH. At pH 11.9 (only anionic species present) *RO- must
be formed with a unit quantum yield. As shown in Figure 3B,
the emission intensity of the *RO- band at 420 nm decreases
by a factor of 2.1 as the pH decreases from 11.9 to 7.2 (only
neutral species). At pH 7.2 *RO- is formed via NO2nH
excitation and deprotonation, and we can assign its fluorescence
intensity decrease to the formation of a smaller quantity of *RO-
Figure 3. Spectroscopic characterization of NO2nH; (A) Absorption, (B)
Fluorescence and (C) fluorescence-excitation spectra at the indicated pH
values in 2% MeOH/H2O. The insets in (A) and (B) report pKa and pK*
a
measurements, respectively.
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A R T I C L E S
Nunes et al.
due to the presence of decay mechanisms in *ROH that are
competitive with proton transfer to the solvent. Following this
reasoning, the decrease in fluorescence intensity gives the *RO-
-
formation quantum yield Φ*RO ≈ 0.48. The carbanion formation
-
+
+
-
quantum yield is then ΦC ) ΦH ) φH Φ*RO ≈ 0.3. With such
a high quantum yield, even under very mild experimental
conditions (laser energy of 3 mJ/pulse at 266 nm, 5-6 ns laser
pulse width, photoacid absorbance A266 ) 0.4, irradiated volume
of 0.1 mL) an aqueous solution initially at pH 7 should acidify
to pH 5 within the laser pulse and return to neutrality in one
second.
The predominant species in the 8-11 pH range is a
monoanion, with the proton located either in the oxygen atom
of the naphthol or in the carbon atom at the R position to the
nitro group. Carbanions of nitroalkanes may delocalize part of
the charge to the adjacent nitro group and resemble a nitronate
anion.32 This is an intrinsic property of nitroalkanes and a
integral part of their characteristic proton transfer reactions, but
only the C-protonation drives the system to its minimum energy
and is the reaction coordinate of interest to this work. In view
of the calculated intramolecular proton transfer rate and of the
similarity between the pKas of naphthol and 2-phenyl-1-
nitroethane, both the naphtholate and the carbanion forms are
present in equilibrium in the ground state. The absorption spectra
at pH 10.2 and 11.9 reflect the change from a mono to a dianion,
but do not inform on the spectral properties on the two
monoanions. Fluorescence excitation spectra of Figure 3C
suggest that the band near 350 nm corresponds to the naphtho-
late form of the anion.
Other mechanisms may be invoked for the quenching of
*RO- fluorescence in NO2nH. However, we can exclude
solvation effects, because the presence of hydroxyl groups in
the alkyl chain increases the intensity of the anion emission,25
whereas we observe a decrease in fluorescence intensity in the
presence of the nitro group. The fluorescence decays obtained
at the emission maxima of *ROH (360 nm) and *RO- (430
nm), measured by single-photon counting and shown in Figure
4, are not amenable to a simple interpretation because geminate
recombination leads to nonexponential behavior.33 Nevertheless,
the decay of *RO- gives a good fit to an exponential decay
with a time constant k*RO- ) 1.4 × 108 s-1 which, after
subtraction of the decay constant measured for the anion of
1-propyl-2-naphthol,25 gives kipt ≈ 108 s-1 in 2% MeOH:H2O.
This is in excellent agreement with the rate constant kipt ) 2.6
× 108 s-1 anticipated with ISM.
Figure 4. Single photon counting of 1-(2-nitroethyl)-2-naphthol. (a) At
pH 1.0. (B) At pH 6.4. (C) pH 12.2.
experimental data as shown in Figure 5B. The value obtained,
τ ≈ 0.6 s, is in remarkable agreement with the relaxation time
predicted by theoretical modeling of this system, 1 s. We can
classify this pH jump as “persistent” in the perspective of all
the processes that occur in millisecond and submillisecond
regimes and that cannot be studied by rapid-mixing techniques.
Definitive evidence for the mechanism of Figure 1 can be
obtained with photolysis in 2% MeOD/D2O, which must lead
to the incorporation of deuterium in NO2nH. The inset in Figure
2A shows the NMR spectra of NO2nH collected before and
after 1 min photolysis in 2% MeOD/D2O with a mercury lamp
at 254 nm. The compound was extracted with CDCl3 and the
NMR spectra were registered in this solvent. The triplet assigned
to the protons next to the nitro group decreases in intensity, as
expected from the protonation of the carbanion by D3O+. At
the same time, the benzylic protons become a doublet because
the coupling is now with only one proton. Potassium ferrioxa-
late actinometry reveals that 9 × 10-5 moles of photons were
absorbed and 9 × 10-7 moles of photoacid were present. The
Direct evidence for the formation of a long-lived monoanion
upon electronic excitation of NO2nH, is presented in Figure 5.
Excitation at 266 nm of 0.1 mM of NO2nH in 2% MeOH/H2O
initially at pH 7 produces a transient absorption spectrum with
bands at 315 and 355 nm with similar decays. The spectra were
collected at sufficiently long times to ensure that any naphtholate
formed directly from its excited state is already reprotonated
back to the neutral form of NO2nH. Considering that the rate
of formation of the naphtholate from the carbanion is much
slower than the rate of protonation of the naphtholate, we assign
the spectra in Figure 5A to the carbanion. The reestablishment
of Equilibrium (5) is characterized by a relaxation time, defined
as the time at which the distance from equilibrium is 1/e of the
initial distance. The relaxation time can be extracted from the
(32) Erden, I.; Keefe, J. R.; Xu, F.-P.; Zheng, J.-B. J. Am. Chem. Soc.
1993, 115, 9834–9835.
(33) Agmon, N.; Huppert, D.; Masad, A.; Pines, E. J. Phys. Chem. 1991,
95, 10407–10413.
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Photoacid for Long-Lived and Reversible pH-Jumps
A R T I C L E S
Figure 5. Flash photolysis of [NO2nH] ) 4.3 × 10-4 M in 2% MeOH:
H2O. (A) Transient absorption spectra. (B) Decay at 360 nm.
Figure 6. Transient bromocresol green absorption at 616 nm following
flash photolysis of [NO2nH] ) 2 × 10-4 M in 2% MeOH/H2O. (A)
Bleaching and recovery of bromocresol green absorption with estimate of
the relaxation time τ. (B) Dependence of the maximum photobleaching of
bromocresol green on the laser intensity; the slight offset of the laser energy
is probably due to some 355 nm stray light reaching the energy meter. No
photobleaching is observed in the absence of NO2nH.
excess of protons released should lead to 100% deuteration of
NO2nH. The NMR spectra show that this extensive deuteration
is not accompanied by significant degradation. Moreover,
detailed analysis of the aromatic region of the NMR spectra
reveals that incorporation of deuterium in the R position of the
nitro group is not accompanied by changes in the aromatic
protons. Thus, we conclude that our persistent photoacid is also
reversible.
The data above proves the success of the design of persistent
and reversible photoacids that employ competitive intramolecu-
lar proton transfer from a carbon acid. The amplitude of the
pH jump in such systems is limited by the acidity of the
coefficient is ꢀ616 ) 39550 M-1 s-1. Figure 6A shows that the
absorbance change produced by a 3 mJ laser pulse 2 mm in
diameter in a 2% MeOH/H2O solution with initial indicator
concentration [In2-] ) 9.1 × 10-6 M at pH 7, is ∆A616 ) -2.4
× 10-3. This absorbance change corresponds to ∆[In2-] ) 3 ×
10-7 M and a jump to pH 6.3. This is a conservative estimate
given the geometric mismatch between the analyzing light beam
and the laser flash. A more realistic estimate is that the lowest
pH attained with this 3 mJ laser pulse is between the theoretical
estimate of pH 5 and the experimental evidence for pH 6. More
importantly, as shown in Figure 6B, is that the magnitude of
the pH jump is directly proportional to the laser energy. With
photoacid (pK* ) 1.5 from fluorescence titration), by its
a
solubility and by the intensity of the laser pulse. The color
change of pH indicators is a very convenient tool to assess the
amplitude of the pH jump and the availability of the excess
protons to participate in chemical reactions. We employed
bromocresol green (pKa ) 4.9) and observed the bleaching and
recovery of its basic form (In2-) at 616 nm, where the absorption
9
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A R T I C L E S
Nunes et al.
of pH jumps lasting for seconds when the initial pH is close to
7. However, in our experiments with pH 5.2, adjusted with HCl,
the pH jump produced with NBA is quenched in milliseconds
presumably as a result of reactions of the NBA product.
Terazima observed a similar effect and concluded that experi-
mental results utilizing caged compounds must be carefully
analyzed in view of the possible reactions of NBA photoprod-
ucts, namely with Cl-.36
5. Conclusions
The design and synthesis of a persistent and reversible
photoacid was motivated by theoretical modeling, and is another
illustration of scientific advances driven by theory. The synthesis
of 1-(2-nitroethyl)-2-naphthol was achieved by a simple two
step procedure, that allows the isolation of the desired product
without requiring chromatographic separations and is a valid
starting point for scale-up procedures.
In aqueous solutions, deprotonation of 1-(2-nitroethtyl)-2-
naphthol is essentially completed in 10 ns, and leads to a
carbanion located at the R position to the nitro group with a ca.
30% efficiency. The relaxation time for the reprotonation of
this carbanion, and the consequent return to neutrality, is close
to 1 s. Thus, from 10 ns to 0.1 s, the irradiated volume remains
acidic and at a nearly constant pH. This offers a simple method
to study pH-dependent processes that are faster than the time
resolution of rapid-mixing techniques.
Figure 7. Transient bleaching of bromocresol green absorption at 616 nm
following flash photolysis of [NO2nH] ) 2 × 10-4 M or of [NBA] ) 4 ×
10-3 M in 2% MeOH:H2O. The initial pH was 5.2 and the indicator
concentration was 1.2 × 10-5 M for NO2nH and 2.5 × 10-6 M for NBA
experiments.
intense laser pulses, the proton concentration should approach
the solubility limit of NO2nH.
The flash photolysis data was collected as an average of 5
laser pulses with laser repetition rates below 0.2 Hz. Each laser
shot produces a similar transient spectrum, and the baseline fully
recovers before the next shot. The pH jumps corresponding to
the on-off light cycles are highly reproducible. Pure samples
can be subject to hundreds of laser pulses without appreciable
spectral changes. NO2nH is the first example of a persistent
and reversible photoacid and proves the feasibility of this
concept.
The acid-base recombination rate constant of bromocresol
34
green is kon ) 5.8 × 1010 M-1 s-1
,
H+ + In2- h HIn-
(8)
and the ionization rate constant is koff ) 6.8 × 105 s-1. Both
these rates are much faster than the corresponding ones for
Equilibrium (5), which means that the acid and basic forms of
bromocresol green are in equilibrium at the medium pH all along
the recovery of the system to neutrality. Thus, the rate-
determining factor for the recovery of bromocresol green is the
bimolecular reprotonation of the carbanion, and the relaxation
time of [In2-] should be the same as that of the carbanion. Figure
6A shows that the relaxation time of bromocresol green is τ )
1 s, which is the same within experimental error as that of the
carbanion in Figure 5B. This experiment shows that the protons
generated in the irradiated volume establish new equilibria in
the submillisecond time scale and that pH-dependent equilibrium
concentrations in that regime can be followed directly.
Finally, it must be emphasized that these photoacids offer a
local control of acidity and that while diffusing a few microme-
ters from the irradiated volume, the carbanion is neutralized.
Acknowledgment. This work was supported by FCT and
FEDER (project no. POCI/QUI/55505/2004). R.M.D.N. thanks FCT
for the grant SFRH/BD/24005/2005. We thank the Nuclear
Magnetic Resonance Laboratory of the Coimbra Chemistry Centre
for obtaining the NMR data.
Supporting Information Available: 1H NMR and 13C NMR.
Input and output files for ISM calculations through the Internet,
together with the explanation of the terms employed. This
material is available free of charge via the Internet at http://
pubs.acs.org.
The breakthrough in the pH jump technique made possible
by NO2nH can be best appreciated comparing its persistent and
reversible pH jump with the pH jump produced by the most
common caged proton compound, o-nitrobenzaldehyde
(NBA),35 under similar conditions, Figure 7. NBA is capable
JA901930C
(35) Viappiani, C.; Bonetti, G.; Carcelli, M.; Ferrari, F.; Sternieri, A. ReV.
Sci. Instrum. 1998, 69, 270–276.
(34) Warrick Jr., P.; Auborn, J. J.; Eyring, E. M. J. Phys. Chem. 1972, 76,
1184–1191.
(36) Choi, J.; Terazima, M. J. Phys. Chem. B 2003, 107, 9552–9557.
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