Spectral observation of conversion between ionized vs. non-ionized proton-coupled electron transfer interfaces

Article abstract of DOI:10.1039/b717747j

Two-point hydrogen bonding between acid and base functionalities provides a convenient method for the modular assembly of proton-coupled electron transfer (PCET) networks, especially when that interface comprises an amidinium and two-point anionic partner; a system is presented that permits the proton configuration within the interface to be determined when pK_a values of the conjugate acids are known. The Royal Society of Chemistry.

Full text of DOI:10.1039/b717747j

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Spectral observation of conversion between ionized vs. non-ionized
proton-coupled electron transfer interfacesw
Elizabeth R. Young, Joel Rosenthal and Daniel G. Nocera
Received (in Austin, TX, USA) 15th November 2007, Accepted 29th February 2008
First published as an Advance Article on the web 14th April 2008
DOI: 10.1039/b717747j
Two-point hydrogen bonding between acid and base functional-
ities provides a convenient method for the modular assembly of
proton-coupled electron transfer (PCET) networks, especially
when that interface comprises an amidinium and two-point
anionic partner; a system is presented that permits the proton
configuration within the interface to be determined when pKa
values of the conjugate acids are known.
Chart 1
Proton-coupled electron transfer (PCET) may be established
the series tetrahydrofuran (THF, e = 7.58), dichloromethane
(DCM, e = 8.93) and acetonitrile (ACN, e = 37.5).19
within donor–acceptor dyads assembled by a hydrogen
bonded interface.1,2 PCET networks assembled from Watson–
Crick base pairs3 and dicarboxylic acid dimers4 do not support
proton transfer within the interface thus limiting the extent to
which PCET may be investigated. For this reason, the asym-
metric proton interface of an amidinium bound to two-point,
anionic partners, especially carboxylate, has come to the
fore.5–10 The two tautomers shown in Chart 1 are possible:
an ionized amidinium–carboxylate or a non-ionized amidi-
ne–carboxylic acid, both of which can support proton transfer
along a PCET coordinate.
Interrogation of the proton configuration of the amidinium
interface was accomplished using purpurin 1, in which con-
jugation is maintained between the amidinium and porphyrin
via an unsaturated five-membered ring. We have shown that
the absorption spectrum of 1 is sensitive to its protonation
state.10 Reduction of the five-membered isocyclic ring of 1
yields the corresponding chlorin homologue. Unlike 1, the
absorption spectrum of the chlorin–amidinium is invariant
with pH (see Fig. S1, ESIw).
Fig. 1 shows the absorption spectrum of 1 before (1-amH+)
and after (1-am) addition of the base 4-dimethylaminopyridine
(DMAP). Deprotonation of the amidinium functionality in
ACN is indicated by a shift in the absorption maximum of the
Soret band from 428 to 423 nm (Fig. 1(c)). Similar spectral
trends are observed in THF and DCM (Fig. 1(a) and (b),
respectively). Absorption spectra (Fig. S2–S4, ESIw) demon-
strate analogous spectral shifts with each BM and exhibit well-
anchored isosbestic points during the titration of the BM with
1-amH+. A 1 : 1 binding motif is signified from a Benesi–
Hildebrand analysis of the titration data (1/DA vs. 1/[BM])
(Fig. S2–S4, ESIw).20 The binding constant, Kassoc, for 1:2 is
found to be B4.5 Â 104 MÀ1 and for 1:3 is B1.5 Â 102 MÀ1 in
ACN. The Kassoc for 4, 5 and 6 are all the same within error
The amidinium–carboxylate interface combines the dipole
of an electrostatic ion pair interaction, a two-point hydrogen
bond, and secondary electrostatic interactions to produce a
highly stable proton interface.11–15 These factors along with
the relative pKa values of amidinium and carboxylate, conspire
to determine the extent to which the interface configuration is
ionized.16 The electrostatic interactions within the interface
stabilize charge accumulation, leading in some cases to an
ionized interface even when a simple DpKa calculation predicts
formation of a non-ionized interface in a solution of a given
dielectric constant.10 We now show the DpKa value necessary
to achieve the ionized vs. the non-ionized interface by stepping
through a series of binding moieties (BMs) with different
acidity constants. The transition from a non-ionized to ionized
interface occurs as the acidity of the BM increases.
Efficient binding of an amidinium (amH+) to carboxylate and
sulfonate anions occurs in aprotic solvents of low dielectric
constant. A series of BMs of varying pKa values in ACN was
chosen as summarized in Table 1.17,18 The trend in the depen-
dence of the amH+:BMÀ association and interface configura-
tion on the dielectric constant of the solvent can be probed along
and are measured to be B9.9 Â 103 MÀ1
.
The endpoint spectrum of each titration reveals the config-
uration of the proton interface as indicated by the final
Department of Chemistry, 6-335, Massachusetts Institute of
Technology, 77 Massachusetts Ave., Cambridge, MA 02139-4307,
USA. E-mail: nocera@mit.edu; Fax: 01 617 253 7670; Tel: 01 617
253 5537
w Electronic supplementary information (ESI) available: Experimental
procedures, additional spectra and charts. See DOI: 10.1039/b717747j
Scheme 1
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Table 1 DGpK and interface configuration for 1:BM complexes
a
Ionized interface?
DGpK /eV
a
a
Binding moiety
pKa
(1:BM)b
THF DCM ACN
1
2
3
4
5
6
a
Purpurin-amH+
3,5-Dinitrobenzoate 17
9.55
0.44
À0.15
0.68
0.50
0.26
|
|
|
|
|
|
Phenylsulfonate
Acetate
Chloroacetate
Dichloroacetate
7
21
18
14
b
|
|
|
|
In CH3CN, from ref. 17. At 298 K, calculated from pKa values
Scheme 2 Square scheme representation of free energies involved in
determining the tautomeric form of the interface.
recorded in ACN, DGpK = ÀRT[pKa(BM)/pKa(1-am+)].
a
1-amÁ Á ÁHO2C complex (Scheme 2, inset). The driving force
for formation of amÁ Á ÁHO2C from the neutral analogues
(DG2) can be determined by the difference between the DG3
position of the Soret (S2 ’ S0 transition at B430 nm) or
Q-band (S1 ’ S0 transition at B650 nm) maximum. Convers-
ion of 1-amH+ to 1-am in the non-ionized interface is indicated
by a shift of the peak maxima to wavelengths nearly coincident
with that observed for 1-am upon deprotonation of 1-amH+
with DMAP. We interpret the small 1–2 nm shifts of the Soret
band of 1-amH+ in the presence of the BM to the establishment
of a non-ionized amidine–carboxylic acid interface.10
and DGpK as shown by the inset in Scheme 2,
a
DG2 = DG3 À DGpK
(1)
a
Accordingly, the stabilization imparted by proton transfer
within the interface (DGinterface) can be determined from the
free energies of the square scheme and the pKas of the
constituent moieties (DGpK ).
Fig. 1 (inset) shows the final Soret peak position of 1 in each
solvent for all BMs studied. The solid data points at each end
represent the peak position of 1-amH+ (highest wavelength) and
1-am (lowest wavelength). The dotted line in each plot shows an
approximate boundary for the transition between the ionized to
non-ionized interface as determined by the final Soret position,
and augmented by spectral analysis of the initial and final spectra
(see ESIw for details of spectral analysis). The unavailability of
additional BMs with precise pKa values limits the observation of
a clearly identifiable pKa transition between the two tautomers.
Experimental observation of the proton interface configura-
tion for each BM and determination of Kassoc for a given
interface enables the construction of the thermodynamic cycle
shown in Scheme 2. Since the interface under consideration is
formed from amH+ and ÀO2C as the reactants (Scheme 2 top,
left), either an amH+Á Á ÁÀO2C or an amÁ Á ÁHO2C interface is
produced. The driving force for formation of the former (DG1)
is measured directly from the measurement of Kassoc for the
ionized 1-amH+:ÀO2C complex, while the driving force
for the latter (DG3) is determined for the non-ionized
a
Although the binding of the various BMs of Table 1 to
1-amH+ was carried out in ACN, DCM and THF, the
thermodynamic description and stabilization energy of the
interface outlined above can only be rigorously determined
for ACN since pKa values for the BMs used in this study have
been determined in this solvent. Inspection of Fig. 1(c) reveals
that conversion between the tautomeric forms of the interface
occurs between BMs 4 and 5. Thus, Scheme 2 is analyzed for
these two BMs as they most closely define the razor’s edge
between formation of the ionized and neutral hydrogen bond-
ing interface. Accordingly, these two points allow for the
determination of the minimum interface stabilization neces-
sary to obtain an ionized interface as per eqn (2):
DGinterface = (DG2 + DGpK ) À DG1
(2)
a
The amÁ Á ÁHO2C tautomer is formed upon association of
1-amH+ with 4, allowing for the determination of DG2 via
eqn (1). DG1 is obtained from Kassoc(1:5), since association of
1-amH+ with 5 generates the ionized amH+:ÀO2C complex.
Fig. 1 UV-visible absorption spectra of 1-amH+, solid line; 1-am formed by deprotonated of 1-amH+ with DMAP, dotted line. (a) lmax(1-amH+) =
430 nm shifts to lmax(1-am) = 426 nm in THF, 2000 eq. DMAP, (b) lmax(1-amH+) = 436 nm shifts to lmax(1-am) = 426 nm in DCM, 550 eq. DMAP
and (c) lmax(1-amH+) = 428 nm shifts to lmax(1-am) = 423 nm in ACN, 950 eq. DMAP. Inset: Soret peak position of the purpurin vs. DGpK for each
a
of the associated 1:BM complexes. The highest and lowest peak maxima (solid circles) represent 1-amH+, solid line; 1-am, respectively.
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Table 2 Calculated free energy values for thermodynamic considerations for assignment of DGInterface in ACN
DG3/eVb
DGpK c/eV
DGinterface/eV
a
Binding moiety (BM)
pKa
DG1b/eV
DG2/eV
a
1
2
3
4
5
6
Purpurin-amH+
3,5-Dinitrobenzoate
Phenylsulfonate
Acetate
Chloroacetate
Dichloroacetate
9.55
17
7
21
18
14
—
—
—
—
0.44
—
—
—
—
—
À0.44
—
—
—
0.18
À0.28
À0.13
—
—
À0.24
0.15
À0.68
À0.50
À0.26
À0.24
—
—
À0.24
—
a
b
c
In CH3CN, from ref. 17. DG = ÀRT ln Kassoc(1:BM). At 298 K, calculated from pKa values recorded in ACN, DGpK = ÀRT[pKa(BM)/
a
pKa(1-amH+)].
That the ionized interface for 1:5 persists when the pKa
difference dictates formation of the amÁ Á ÁHO2C interface
reveals the importance of electrostatic contributions to the
interface configuration. The electrostatic stabilization of the
interface imparted by formation of the ionized amH+:ÀO2C
complex (DGinterface) can be determined using eqn (2); it is
minimally 0.18 eV in ACN. This electrostatic interaction
energy can be calculated for point charges, q1 and q2, accord-
ing to V = (q1q2)/(4per) where e is the dielectric constant of the
medium.21 An electrostatic stabilization energy of 0.10 eV is
calculated for the amidinium–carboxylate salt bridge at a
distance of r = 3.9 A in THF. This distance is measured
between the central carbon atoms of the amidinium and
carboxylate groups from the energy optimized structure.8
The experimental results reported here agree with a simple
ion solvation model. The discrepancy likely arises from the
approximation that the electrostatic charge within the inter-
face is confined to a point-dipole (Table 2).
the same D–A pair bearing different tautomeric forms of the
interface, illustrates the pronounced effect that proton position
exerts on photoinduced charge transfer.
Notes and references
1 C. J. Chang, J. D. Brown, M. C. Y. Chang, E. A. Baker and D. G.
Nocera, in Electron Transfer in Chemistry, ed. V. Balzani, Wiley-
VCH, Weinheim, Germany, 2001, vol. 3.2.4, pp. 409–461.
2 R. I. Cukier and D. G. Nocera, Annu. Rev. Phys. Chem., 1998, 49,
337–369.
3 J. L. Sessler, B. Wang, S. L. Springs and C. T. Brown, in
Comprehensive Supramolecular Chemistry, ed. Y. Murakami,
Pergamon Press, Oxford, 1996, vol. 4, pp. 311–336, and references
therein.
4 C. Turro, C. K. Chang, G. E. Leroi, R. I. Cukier and D. G.
´
Nocera, J. Am. Chem. Soc., 1992, 114, 4013–4015.
5 J. A. Roberts, J. P. Kirby and D. G. Nocera, J. Am. Chem. Soc.,
1995, 117, 8051–8052.
6 J. A. Roberts, J. P. Kirby, S. T. Wall and D. G. Nocera, Inorg.
Chim. Acta, 1997, 263, 395–405.
7 J. A. Roberts, J. P. Kirby and D. G. Nocera, J. Am. Chem. Soc.,
1997, 119, 9230–9236.
8 N. H. Damrauer, J. M. Hodgkiss, J. Rosenthal and D. G. Nocera,
J. Phys. Chem. B, 2004, 108, 6315–6321.
9 J. M. Hodgkiss, N. H. Damrauer, S. Presse, J. Rosenthal and D.
G. Nocera, J. Phys. Chem. B, 2006, 110, 18853–8.
10 J. Rosenthal, J. M. Hodgkiss, E. R. Young and D. G. Nocera, J.
Am. Chem. Soc., 2006, 128, 10474–10483.
11 J. Pranata, S. G. Wierschke and W. L. Jorgensen, J. Am. Chem.
Soc., 1991, 113, 2810–2819.
12 T. Uchimaru, J. Korchowiec, S. Tsuzuki, K. Matsumura and S.
Kawahara, Chem. Phys. Lett., 2000, 318, 203–209.
13 O. Lukin and J. Leszczynski, J. Phys. Chem. A, 2002, 106,
6775–6782.
14 O. Lukin and J. Leszczynski, J. Phys. Chem. A, 2003, 107,
9251–9252.
While the dearth of known pKa values in DCM and THF
preclude a similar thermodynamic analysis in those solvents, a
qualitative understanding of interface behaviour is inferred
from experimental results. In the dielectric environment of
DCM, the proton interface maintains the ionized configuration
at the same DpKa difference as ACN, however in the lower
dielectric environment of THF conversion to the ionized tauto-
mer shifts to lower DpKa. THF does not tolerate charge build-
up to the extent that ACN and DCM do, and thereby favours a
non-ionized interface configuration at a lower free energy.
The energetics of the amidinium–carboxylate interface are
essential to assigning a general interface stabilization energy
with the goal of accurately projecting the tautomeric state of the
proton interface to PCET systems. For example, the hydrogen
bonded donor–acceptor, system D–[H+]–A (D = amidinium
appended Zn(II)TMP, A = naphthalene diimide carboxylic
acid) reveals a PCET rate of 9.50 Â 108 sÀ1 with a kinetic
isotope effect (KIE) of 1.06 at room temperature.9 In this case,
the interface is the non-ionized amidine–carboxylic acid tauto-
mer. When this D–[H+]–A system is subtly altered to produce
an ionized amidinium–sulfonate interface (A = naphthalene
diimide sulfonic acid) of similar driving force, (Table 1, repre-
sented by the 1:3 interface), the change in interface configura-
tion has a clear effect on the PCET rate and solvent dependent
ET parameters (|V|, l, Eact).22 The PCET rate roughly doubles
to 19 Â 108 sÀ1 with a KIE of 1.17 and more strikingly ET
parameters show a clear solvent dependence through the io-
nized interface, which is absent in the first system. The enhance-
ment of PCET rate and solvent dependent ET parameters for
15 S. Schlund, C. Schmuck and B. Engels, J. Am. Chem. Soc., 2005,
127, 11115–11124.
16 J. M. Hodgkiss, J. Rosenthal and D. G. Nocera, in Handbook of
Hydrogen Transfer. Physical and Chemical Aspects of Hydrogen
Transfer, ed. J. T. Hynes, J. P. Klinman, H.-H. Limbach and R.
L. Schowen, Wiley-VCH, Weinheim, Germany, 2006, vol. 2.4.17,
pp. 503–562.
17 K. Izutsu, Acid–Base Dissociation Constants in Dipolar Aprotic
Solvents, Blackwell Scientific, Cambridge, USA, 1990.
18 The pKa range reported for each BM are as follows: acetate (4)
pKa = 20–22; chloroacetate (5) pKa = 15–19; and, dichloroace-
tate (6) pKa = 13–15. The pKa of phenylsulfonate (3) is estimated
to be 8 from vales for p-toluenesulfonic acid, pKa = 8 and
2.5-dichlorobenzenesulfonic acid, pKa = 6.2–6.4.
19 Y. Marcus, in Ion Solvation, Wiley, Chichester, 1985, pp.
136–137.
20 K. A. Connors, in Binding Constants: A Measurement of Mole-
cular Complex Stability, Wiley, New York, 1987.
21 Y. Marcus, in Ion Solvation, John Wiley, Chichester, 1985, p. 136.
22 E. R. Young, J. R. Rosenthal, J. M. Hodgkiss and D. G. Nocera,
manuscript in preparation.
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2324 | Chem. Commun., 2008, 2322–2324