Optical and electrochemical properties of hydrogen-bonded phenol-pyrrolidino[60]fullerenes

Article abstract of DOI:10.1039/c2pp05351a

We report the photophysical and electrochemical properties of phenol-pyrrolidino[60]fullerenes 1 and 2, in which the phenol hydroxyl group is ortho and para to the pyrrolidino group, respectively, as well as those of a phenyl-pyrrolidino[60]fullerene model compound, 3. For the ortho analog 1, the presence of an intramolecular hydrogen bond is supported by ¹H NMR and FTIR characterization. The redox potential of the phenoxyl radical-phenol couple in this architecture is 240 mV lower than that observed in the associated para compound 2. Further, the C₆₀ excited-state lifetime of the hydrogen-bonded compound 1 in benzonitrile is 260 ps, while the corresponding lifetime for 2 is identical to that of the model compound 3 at 1.34 ns. Addition of excess organic acid to a benzonitrile solution of 1 gives rise to a new species, 4, with an excited-state lifetime of 1.40 ns. In nonpolar aprotic solvents such as toluene, all three compounds have a C₆₀ excited-state lifetime of a??1 ns. These results suggest that the presence of an intramolecular H-bond in 1 poises the potential of phenoxyl radical-phenol redox couple at a value that it is thermodynamically capable of reducing the photoexcited fullerene. This is not the case for the para analog 2 nor is it the case for the protonated species 4. This work illustrates that in addition to being used as light activated electron acceptors, pyrrolidino fullerenes are also capable of acting as built-in proton-accepting units that influence the potential of an attached donor when organized in an appropriate molecular design.

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PAPER
Optical and electrochemical properties of hydrogen-bonded
phenol-pyrrolidino[60]fullerenes†
Gary F. Moore,‡ Jackson D. Megiatto Jr, Michael Hambourger,§ Miguel Gervaldo,¶ Gerdenis Kodis,
Thomas A. Moore,* Devens Gust* and Ana L. Moore*
Received 19th October 2011, Accepted 2nd January 2012
DOI: 10.1039/c2pp05351a
We report the photophysical and electrochemical properties of phenol–pyrrolidino[60]fullerenes 1 and 2,
in which the phenol hydroxyl group is ortho and para to the pyrrolidino group, respectively, as well as
those of a phenyl–pyrrolidino[60]fullerene model compound, 3. For the ortho analog 1, the presence of
an intramolecular hydrogen bond is supported by 1H NMR and FTIR characterization. The redox
potential of the phenoxyl radical–phenol couple in this architecture is 240 mV lower than that observed in
the associated para compound 2. Further, the C60 excited-state lifetime of the hydrogen-bonded
compound 1 in benzonitrile is 260 ps, while the corresponding lifetime for 2 is identical to that of the
model compound 3 at 1.34 ns. Addition of excess organic acid to a benzonitrile solution of 1 gives rise to
a new species, 4, with an excited-state lifetime of 1.40 ns. In nonpolar aprotic solvents such as toluene, all
three compounds have a C60 excited-state lifetime of ∼1 ns. These results suggest that the presence of
an intramolecular H-bond in 1 poises the potential of phenoxyl radical–phenol redox couple at a value
that it is thermodynamically capable of reducing the photoexcited fullerene. This is not the case for the
para analog 2 nor is it the case for the protonated species 4. This work illustrates that in addition to being
used as light activated electron acceptors, pyrrolidino fullerenes are also capable of acting as built-in
proton-accepting units that influence the potential of an attached donor when organized in an appropriate
molecular design.
coupling of the proton and electron transfer chemistry is a
crucial feature of the photosynthetic process and is imperative
1. Introduction
Proton coupled electron transfer (PCET) is a fundamental aspect
of many biological electron transfer reactions.1–13 In the water-
oxidizing enzyme Photosystem II (PSII)14 a tyrosine residue
(TyrZ), hydrogen bonded to a nearby histidine moiety (His190),
forms a redox-active pair that mediates electron transfer between
the primary chlorophyll donor (P680) and the Mn4CaO5 oxygen-
for achieving the appropriate redox leveling necessary to drive
the multi-electron catalytic processes with photogenerated oxi-
dizing equivalents.
Aspects of PCET may also prove essential for developing
efficient interfaces between light absorbers and redox-active–cat-
alytic components in artificial photosynthesis. Likewise, serious
efforts have been devoted to understanding the kinetic details
and mechanistic implications of PCET reactions in simplified
model systems.15–21 A typical molecular design of a model
system consists of an intra- or inter-molecular hydrogen bond
between a phenolic proton and the lone pair electrons of an
organic base. Pyridine, imidazole, and amines have been investi-
gated as proton acceptors in both electrochemically- and light-
driven redox reactions. Despite this progress, experimental and
theoretical data remain limited.
evolving complex (OEC). Photoexcitation of PSII produces the
+
radical cation P680˙+. In turn, P680
˙
is reduced by the TyrZ–
His190 pair. In this process, formation of the tyrosyl radical is
coupled to release of the phenolic proton to an adjacent base
(most likely the His190 imidazole residue). The neutral tyrosyl
radical (TyrZ˙) is then reduced by the OEC, resulting in acti-
vation of this complex for water-splitting reactions.1,3 The
Center for Bioenergy and Photosynthesis, Department of Chemistry and
Biochemistry, Arizona State University, Tempe, Arizona 85287-1604,
USA. E-mail: gust@asu.edu, tmoore@asu.edu, amoore@asu.edu
†This article is published as part of a themed issue in honour of
Professor Kurt Schaffner on the occasion of his 80th birthday.
‡Current address: Joint Center for Artificial Photosynthesis, Lawrence
Berkeley National Laboratory, Berkeley CA 94720, USA.
§Current address: A. R. Smith Department of Chemistry, Appalachian
State University, Boone, NC 28608, USA.
¶Current address: Departamento de Química Facultad de Ciencias
Exactas, Físico-Químicas y Naturales Universidad Nacional de Río
Cuarto, Río Cuarto, Argentina, 5800.
We have previously reported21a a photochemically active
model of the TyrZ–His190-chlorophyll complex of PSII. In
this system, a hydrogen-bonded phenol–benzimidazole pair
(PhOH–Bi) is covalently attached to a bis(pentafluorophenyl)–
porphyrin (PF10) that is adsorbed at the surface of titanium
dioxide nanoparticles (TiO2). Light excitation of the porphyrin
triggers a sequence of electron and proton transfer reactions that
ultimately yields the PhO˙–BiH+–PF10–TiO2˙− state, which is
characterized by a neutral phenoxyl radical, a protonated
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with tetramethylsilane as an internal reference using a Wilmad
528-PP 5 mm NMR tube. In some cases, carbon disulfide was
included in the NMR solvent to enhance solubility. Mass spectra
were obtained with a matrix-assisted laser desorption–ionization
time-of-flight spectrometer (MALDI-TOF), using (1E,3E)-1,4-
diphenylbuta-1,3-diene (DPB), cyano-4-hydroxycinnamic acid
(CCA) or terthiophene as a matrix. The reported mass is of the
most abundant isotopic ratio observed. To facilitate comparison,
calculated values of the expected most abundant isotopic ratio
are listed after the experimental result. Steady-state UV-Vis
absorption spectra were measured on a Shimadzu UV-3101PC
UV-Vis-NIR spectrometer. FTIR spectra were measured on a
Thermo Scientific Nicolet 380 FT-IR spectrometer with an atte-
nuated total reflectance head by evaporating a dichloromethane
solution of the sample onto the sample compartment base plate.
Steady-state fluorescence spectra were measured using a Photon
Technology International MP-1 spectrometer and corrected for
detection system response. Excitation was provided by a 75 W
xenon arc lamp and single grating monochromator. Fluorescence
was detected 90° to the excitation beam via a single grating
monochromator and an R928 photomultiplier tube having S-20
spectral response and operating in the single-photon-counting
mode. Time-resolved fluorescence decay measurements were
performed by the time-correlated single-photon-counting
method. The excitation source was a mode-locked Ti:sapphire
laser (Spectra Physics, Millennia-pumped Tsunami) with a 130
fs pulse duration operating at 80 MHz. The laser output was sent
through a frequency doubler and pulse selector (Spectra Physics
Model 3980) to obtain 370–450 nm pulses at 4 MHz. Fluor-
escence emissions were detected at the magic angle using a
double grating monochromator (Jobin Yvon Gemini-180) and
Fig. 1 Phenol–pyrrolidino[60]fullerene compounds investigated in the
present work.
benzimidazole moiety, and an electron delocalized across the
TiO2 conduction band. Acid–base titration experiments21b indi-
cate that an ionizable proton controls the electrochemical poten-
tial of the phenolic species, while maintaining the chemical
reversibility of the PhOH–Bi component.
In this contribution, we extend our investigation of photoin-
duced PCET reactions in artificial systems to include a molecular
architecture in which the phenol moiety is covalently attached to
a pyrrolidino[60]fullerene (Fig. 1). In 1 (PhOH–PyrC60), the
phenolic unit is positioned to form an intramolecular hydrogen
bond with the basic nitrogen lone pair electrons of the pendant
pyrrolidino[60]fullerene. Light excitation of the fullerene triggers
a PCET reaction, presumably forming a charge-separated radical
pair composed of a neutral phenoxyl radical and a zwitterionic
pyrrolidino[60]fullerene group (PhO˙–PyrH+C60˙−). The behav-
ior of the hydrogen-bonded construct 1 is compared to that of
analogs 2, 3, and 4 in which the intramolecular hydrogen bond
is either absent or disrupted.
a
microchannel plate photomultiplier tube (Hamamatsu
R3809U-50). The instrument response function was 35–55 ps.
The spectrometer was controlled by software written in the
LabView programming language22 and data acquisition was
done using
a single-photon-counting card (Becker-Hickl,
SPC-830). Data analysis was carried out using locally written
software (ASUFIT) developed in a MATLAB environment
(Mathworks Inc.). Data were fitted as a sum of exponential
decays, which were reconvoluted with the appropriate instrument
response function. Goodness of fit was established by examin-
ation of residuals and the reduced χ2 value, which was <1.20 for
all kinetics reported. The data were obtained in benzonitrile
unless otherwise specified. When noted, the spectra have been
normalized to facilitate comparison of the data. In situ generation
of the protonated form of the phenol–pyrrolidinofullerene was
achieved by the addition of trifluoroacetic acid (TFA). For acid–
base titrations, dilution series of TFA in benzonitrile were used.
Cyclic voltammetry was performed with a CHI 650C poten-
tiostat (CH Instruments) using a glassy carbon (3 mm diameter)
or platinum (1.6 mm diameter) disc working electrode, a plati-
num gauze counter electrode, and a silver wire pseudoreference
electrode in a conventional three-electrode cell. Benzonitrile, dis-
tilled from phosphorus pentoxide, was used as the solvent for
electrochemical measurements. The supporting electrolyte was
0.10 M tetrabutylammonium hexafluorophosphate. The solution
was deoxygenated by bubbling with argon. Experiments in
acidified benzonitrile were performed by adding trifluoroacetic
acid (from stock solutions in the same solvent) to a final
2
Experimental section
2.1 Materials
Trifluoroacetic acid, 2,4-di-tert-butylphenol and N-methylglycine
were purchased from Alfa Aesar and Acros. Aldehyde precursors
3,5-di-tert-butyl-4-hydroxybenzaldehyde and 3,5-di-tert-butyl-
benzaldehyde were purchased from Aldrich. All chemicals were
used without further purification. Solvents were obtained from
EM Science. Toluene was distilled over CaH2, and dichloro-
methane was distilled over potassium carbonate. All solvents
were stored over the appropriate molecular sieves prior to use.
Thin layer chromatography (TLC) was performed with silica gel-
coated glass plates from Analtech. Column chromatography was
carried out using Silicycle silica gel 60 with 230–400 mesh.
2.2 Spectroscopic measurements
1
The H-NMR spectra were recorded on a Varian spectrometer at
400 MHz. NMR samples were prepared in deuteriochloroform
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concentration as high as 0.5 M. The working electrode was
cleaned between experiments by polishing with alumina (50 nm
dia.) slurry, followed by solvent rinses. The concentration of
the electroactive compound was maintained between 4 × 10−4
and 8 × 10−4 M. The potential of the pseudoreference electrode
was determined using the ferrocenium–ferrocene redox couple
as an internal standard (with Em taken as 0.45 V vs. SCE in ben-
terthiophene as matrix): m/z 981.9 (M)+ (100%), calculated
981.21 for C77H27NO. UV-Vis (CH2Cl2): 256, 309, 430, 702 nm.
N-Methyl-3,4-[60]-fullero-2-(3′,5′-di-tert-butyl-4′-hydroxyphe-
nyl)pyrrolidine 2. Following the general procedure, the title
compound was isolated as a brown solid in 55% yield (0.115 g).
1H NMR (400 MHz, CS2/CDCl3, δ ppm): 7.50 (brs, 2H, ArH);
5.09 (s, 1H, NCH); 4.93 (d, J = 9.3 Hz, 1H, NCH2); 4.81 (s, 1H,
OH); 4.23 (d, J = 9.3 Hz, 1H, NCH2); 2.82 (s, 3H, NCH3); 1.37
(s, 18H, But). MALDI-TOF (positive mode, terthiophene as
matrix): m/z 981.20 (M)+ (100%), calculated 981.21 for
C77H27NO. UV-Vis (CH2Cl2): 257, 307, 431, 706 nm.
zonitrile). The voltammograms were recorded at 100 mV s−1
.
Cyclic voltammograms obtained at a glassy carbon working
electrode have had the baseline subtracted, using a baseline
recorded under comparable conditions in the absence of the elec-
troactive species.
N-Methyl-3,4-[60]-fullero-2-(3′,5′-di-tert-butylphenyl)pyrroli-
dine 3. Following the general procedure, the title compound was
isolated as a brown solid in 58% yield (0.115 g). 1H NMR
(400 MHz, CS2/CDCl3, δ ppm): 7.56 (brs, 2H, ArH); 7.24 (d,
J = 1.6 Hz, 1H, ArH); 4.95 (d, J = 9.3 Hz, 1H, NCH2); 4.89
(s, 1H, NCH); 4.26 (d, J = 9.3 Hz, 1H, NCH2); 2.84 (s, 3H,
NCH3); 1.26 (s, 18H, (CH3)3). MALDI-TOF (positive mode,
terthiophene as matrix): m/z 965.20 (M)+ (100%), calculated
965.21 for C77H27N. UV-Vis (CH2Cl2): 256, 307, 431, 702 nm.
2.3 Synthesis
3,5-Di-tert-butyl-2-hydroxybenzaldehyde.23
A
solution of
commercially available 2,4-di-tert-butylphenol (4.12 g, 20 mmol,
1 equiv.), hexamethylenetetramine (5.66 g, 40 mmol, 2 equiv.)
and trifluoroacetic acid (20 mL) was heated at reflux for 6 h. The
reaction was quenched while hot with a 33% (v/v) aqueous
H2SO4 solution (20 mL) and the resulting mixture was allowed
to cool to room temperature with stirring. The crude product was
extracted with diethyl ether (3 × 50 mL), and the extract was
neutralized with a saturated aqueous solution of sodium bicar-
bonate (2 × 100 mL) and finally washed with water (3 ×
100 mL). The organic phase was dried over sodium sulfate,
filtered through paper and concentrated under reduced pressure.
Final purification was achieved by column chromatography
(SiO2), using hexanes–EtOAc (9 : 1, v/v) as eluent to afford the
3
Results
Molecular structures of the phenol–pyrrolidinofullerenes and the
model phenyl–pyrrolidinofullerene studied in this work are pre-
sented in Fig. 1. All compounds were synthesized from commer-
cially available materials, using the Duff formylation protocol23
to prepare the appropriate benzaldehyde precursors, followed by
the Prato reaction using N-methylglycine (sarcosine) in toluene
at reflux24–26 (for details, see Experimental section).
1
title compound as a colorless solid in 35% yield (1.64 g). H
NMR (400 MHz, CDCl3, δ ppm): 11.80 (s, 1H, OH); 9.88 (s,
1H, CHO); 7.82 (d, J = 1.7 Hz, 1H, ArH); 7.58 (d, J = 1.7 Hz,
1H, ArH); 1.45 (s, 18H, But). 13C NMR (400 MHz, CDCl3, δ
ppm): 191.5; 154.8; 138.4; 138.1; 131.2; 128.3; 123.9; 34.5;
31.2; 31.0. MALDI-TOF (positive mode, cyano-4-hydroxy-
cinnamic acid as matrix): m/z 235.04 (M + H)+ (100%), calcu-
lated 234.20 for C15H22O2.
The presence of an intramolecular hydrogen bond in 1 is sup-
ported by 1H NMR spectroscopy (Fig. 2), with the OH resonance
appearing at δ = 11.33 ppm in a 1 : 1 (v/v) chloroform-d–carbon
disulfide mixture. By comparison, the OH resonance of 2 appears
at δ = 4.81 ppm in the same solvent mixture. The resonance of the
methyl group directly attached to the nitrogen in the pyrrolidine
ring in 1 (δ = 3.03 ppm) is shifted downfield by Δδ = 0.21 ppm
compared to that of 2 (δ = 2.82 ppm). This is ascribed to the
deshielding effect induced by the hydrogen-bonded phenolic
proton, which decreases the electron density at the nitrogen atom.
In the solid state, the presence of an intramolecular hydrogen
bond is supported by the absence of the characteristic free OH
vibration (ν = 3640 cm−1) in the FTIR spectrum of 1, which is
clearly observed as a sharp band in the para-substituted com-
pound 2 (Fig. 3). Instead, a broad band centered at ν ∼
3100 cm−1 is observed in the case of 1. Congruently, neither of
these features is observed in the phenyl derivative, 3, which
lacks the phenolic functional group.
The absorption spectra of 1 and 2 are dominated by features
associated with the fullerene27,28 and are very similar to that of
model compound 3, which lacks the phenolic group (Fig. 4).
Visible excitation (400 nm) of compound 3 in dichloromethane
results in an emission spectrum with maxima at 716 nm and
791 nm (Fig. 4). Similar emission wavelengths are observed in 2.
However, the fullerene singlet-excited-state lifetime of 1 (τ =
0.26 ns, 97%) in benzonitrile, is significantly shorter than that
observed in 2 and 3 (in both cases τ = 1.34 ns) in the same
General method for synthesis of pyrrolidino[60]fullerenes 1, 2
and 3.24,25 [60]Fullerene (0.308 g, 0.42 mmol, 2 equiv.),
N-methylglycine (0.192 g, 2.13 mmol, 10 equiv.) and the suit-
able benzaldehyde (0.21 mmol, 1 equiv.) in toluene were heated
at reflux under a nitrogen atmosphere for 24 h. The solvent was
evaporated at reduced pressure and the residue was suspended in
30% carbon disulfide in toluene. The suspension was filtered
through a pad of silica and the filtrate was evaporated at reduced
pressure. The crude product was purified by column chromato-
graphy on silica using a gradient mixture of carbon disulfide,
toluene, and hexanes (from 1 : 1 : 4 to 1 : 1 : 1, v/v) as the eluent.
The product was recrystallized from carbon disulfide–methanol
to afford the final pyrrolidino[60]fullerene as a solid brown
powder.
N-Methyl-3,4-[60]-fullero-2-(3′,5′-di-tert-butyl-2′-hydroxyphe-
nyl)pyrrolidino 1. Following the general procedure, the title
compound was isolated as a brown solid in 57% yield (0.12 g).
1H NMR (400 MHz, CS2/CDCl3, δ ppm): 11.33 (s, 1H, OH);
7.13 (brm, 2H, ArH + ArH); 5.07 (s, 1H, NCH); 5.05 (d, J = 10
Hz, 1H, NCH2); 4.27 (d, J = 10 Hz, 1H, NCH2); 3.03 (s, 3H,
NCH3); 1.31 (s, 18H, But). MALDI-TOF (positive mode,
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Fig. 2 1H NMR spectra (400 MHz, 25 °C) of compounds 1, 2 and 3 in a chloroform-d–carbon disulfide mixture (1 : 1, v/v). The downfield shifts
observed for the phenolic proton and the methyl group resonances in 1 compared to 2 are attributed to an intramolecular hydrogen bond between the
phenol and the pyrrolidine moieties. Peaks marked with an asterisk are due to residual chloroform.
solvent (Fig. 5).28 In toluene, a non-polar aprotic solvent that
destabilizes charge-separated species,29 the singlet-excited-state
lifetime of 1 (τ = 1.01 ns) is similar to that of 2 and 3. The
addition of excess TFA to a benzonitrile solution of 1 presumably
gives rise to the protonated form, 4. Fitting of the fluorescence
decay from the acidified solution reveals a bi-exponential decay
with a major component of 1.40 ns (60%) attributed to 4 and a
minor component of 0.26 ns (40%) assigned to residual 1. Proto-
nation of the basic pyrrolidino nitrogen presumably disrupts the
intramolecular hydrogen bond, leading to the change in lifetime
of the fullerene excited-singlet state. Neutralization of the acidic
solution with base (tetramethylammonium hydroxide) restores the
0.26 ns component; showing that loss of the short-lived com-
ponent upon acid titration is not a result of decomposition.
Electrochemical studies (vs. SCE) of 1 and 2 reveal two
chemically reversible reductions in the potential range investi-
gated (Em = −0.58 V and −0.98 V for 1 and Em = −0.58 V and
−0.99 V for 2), characteristic of mono-pyrrolidino-functionalized
Fig. 3 FTIR spectra of compounds 1 (blue dash), 2 (red dash dot) and
3 (black).
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Fig. 4 (a) Normalized steady-state absorption of 1 (blue dash), 2 (red dash dot) and 3 (black); (b) normalized steady-state absorption (black) and
emission (black dash) of 3 in dichloromethane at room temperature.
C60 (Fig. 6 and Table 1).26,27,30 An irreversible oxidation,
assigned to the phenoxyl radical–phenol couple, was also
observed in these compounds. For 1, the first oxidation is 0.24 V
lower than that observed in 2 (Ep = +1.17 V for 1, and Ep =
+1.41 V for 2). Electrochemical measurements also show that
the addition of sufficient trifluoroacetic acid (up to 425 mM) to a
benzonitrile solution of 1 anodically shifts the phenoxyl–phenol
couple to a range outside of the observable electrochemical
window. The addition of TFA to a solution of 1 presumably
shifts the acid–base equilibrium to favor protonation of the basic
nitrogen, disrupting the associated hydrogen bond and thus alter-
ing the energetics of the phenolic donor. Protonation also affects
the potential of the fullerene acceptor.31 In 425 mM TFA in
benzonitrile, the potential of the C60–C60− couple is −0.46 V.
Fig. 5 Time-resolved fluorescence single-photon counting (SPC)
measurements of compound 1 in benzonitrile (blue circles), compound 2
in benzonitrile (red circles), compound 1 in toluene (blue triangles) and
4
Discussion
compound 1 in benzonitrile acidified with TFA to form compound 4
(green triangles). Kinetic fitting of the data (solid lines) shows that in
benzonitrile the fullerene singlet-excited state of 1 (τ = 0.26 ns, 97%) is
significantly shorter-lived than that of the para analog 2 (τ = 1.34 ns) or
the protonated form of 1.
1
In the H NMR spectra, the chemical shift of the OH resonance
yields information regarding the local electronic environment
including the relative strength of hydrogen bonds. A stronger
hydrogen bond is typically associated with a larger downfield
Fig. 6 Cyclic voltammograms of 1 (blue dash), 2 (red dash dot) and 3 (black) recorded in a 0.1 M TBAPF6 benzonitrile solution at a scan rate of
100 mV s−1, with a platinum disk working electrode at room temperature at potentials negative (a) and positive (b) of the SCE reference potential.
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Table 1 Redox potentials (V vs. SCE) for oxidation (niE) and
reduction (nIE) of compounds 1–3, as determined by cyclic voltammetry.
features observed in 1 and 2 suggest that phenolic proton is ulti-
mately transferred to the bulk.
Despite the relatively weak pyrrolidinofullerene base, in the
presence of the intramolecular hydrogen bond the fullerene
excited-singlet state (E0–0 ∼ 1.76 eV)26,28 is thermodynamically
In all cases the electrolyte was 0.1
M tetrabutylammonium
hexafluorophosphate in benzonitrile, and the scan rate was 100 mV s−1
Compound
IIE
IE
iE
iiE
iiiE
capable of photo-oxidizing the attached phenol in 1 (ΔGET
−40 meV) (Fig. 7). This is not the case for 2 (ΔGET
≈
≈
1
2
3
−0.98a
−0.99a
−0.97a
−0.58a
−0.58a
−0.57a
1.17b
1.41b
1.57b
1.59b
1.40b
1.70b
1.72b
1.72b
+200 meV), which lacks the built in hydrogen bond. Similarly,
for the acidified form of 1 (compound 4) the fullerene excited-
singlet state is incapable of oxidizing the phenol moiety. The C60
excited-state lifetimes are consistent with these redox thermo-
dynamics. The excited-state lifetime of 1 in benzonitrile (τ =
0.26 ns) is significantly shorter than that observed in 2 and 3
(τ = 1.34 ns for both) in the same solvent, and the quenching is
ascribed to photoinduced electron transfer to the fullerene to
yield a transient charge-separated state. This conclusion is sup-
ported by the fact that the excited-state lifetime of 1 is restored
to a more typical value in toluene (Fig. 5), a less polar solvent
that destabilizes charge-separated species, thereby reducing the
driving force for and rate of photoinduced electron transfer.29
Furthermore, the excited-state lifetime quenching is eliminated
when the hydrogen bond is disrupted by the addition of an
exogenous acid (TFA) to yield 4 (Fig. 5). The relative stability of
the fullerene unit in this architecture allows for investigation of
these compounds under acidic conditions. Similar studies were
precluded when using our previously reported porphyrin based
photochemical system,21b due to protonation of the pyrrole nitro-
gens on the porphyrin macrocycle. These results indicate that in
addition to the ionizable phenolic proton, the protonation state of
the pendant base can also influence the redox properties of a
photochemically active redox pair.
a E = E1/2; reversible or quasi-reversible. b E = Epeak; irreversible.
shift.16a For compound 1, the OH resonance is observed at
11.33 ppm, consistent with the formation of the predicted hydro-
gen bond. For comparison, the phenolic proton resonance of
compound 2 appears at a value that is typical of the OH func-
tional group in
a
non-hydrogen bonding environment
(4.81 ppm). However, the resonance of the OH group in 1 is not
as far downfield as has been observed in other systems where a
phenol is hydrogen bonded to an adjacent imidazole (δ >
13 ppm)15b,16,21 or pyridine (δ > 14 ppm)16,20 unit, suggesting a
relatively weaker hydrogen bond in the present system.
In many cases, the relative strength of a hydrogen bond is cor-
related with the pKa of the protonated associated base.16 For 1,
the electron-withdrawing and steric effects of the attached fuller-
ene significantly decrease the effective pKa of the pyrrolidinium
ring.26–28,30 Accordingly, the potential of the phenoxyl radical/
phenol couple of 1 (Ep = +1.17 V vs. SCE) is significantly
higher than those reported for similar phenolic species featuring
an intramolecular hydrogen bond to a covalently-attached pyrro-
lidine ring lacking the C60 functionality (Em = +0.65 V vs.
SCE).17b However, the potential of the phenoxyl radical/phenol
couple associated with 1 is still considerably lower (ΔE = 0.24
V) than that observed in the para analog 2 (Ep = +1.41 V vs.
SCE, Fig. 7). This difference in potential can be partially
explained in terms of the stabilizing energy of the intramolecular
hydrogen bond in 1. However, the irreversible electrochemical
5
Conclusion
Our optical and electrochemical investigations show that in
addition to using the pyrrolidinofullerene as an electron acceptor,
Fig. 7 Energy level diagram including the E1/2 of the C60/C60− and *C60/C60− processes (black), Ep for phenoxyl/phenol couple of 1 (blue), and the
effect of acidification (which shifts the phenoxyl/phenol couple outside of the electrochemical window) (green), as well as Ep for the phenoxyl/phenol
couple of 2 (red).
Photochem. Photobiol. Sci.
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M. Abrahamsson, D. LeGourriérec, Y. Frapart, A. Magnuson, P.
H. Kenéz, P. Brandt, A. Tran, L. Hammarström, S. Styring and
B. Åkermark, Hydrogen-bond promoted intramolecular electron transfer
to photogenerated Ru(III): A functional mimic of TyrosineZ and Histidine
190 in Photosystem II, J. Am. Chem. Soc., 1999, 121, 6834–6842.
16 (a) T. F. Markle, I. J. Rhile, A. G. Dipasquale and J. M. Mayer, Probing
concerted proton–electron transfer in phenol–imidazoles, Proc. Natl.
Acad. Sci. U. S. A., 2008, 105, 8185–8190; (b) T. F. Markle and J.
M. Mayer, Concerted proton–electron transfer in pyridylphenols: The
importance of the hydrogen bond, Angew. Chem., Int. Ed., 2008, 47,
738–740; (c) I. J. Rhile, T. F. Markle, H. Nagao, A. G. Dipasquale,
O. P. Lam, M. A. Lockwood, K. Rotter and J. M. Mayer, Concerted
proton−electron transfer in the oxidation of hydrogen-bonded phenols,
J. Am. Chem. Soc., 2006, 128, 6075–6088; (d) I. J. Rhile and J.
M. Mayer, One-electron oxidation of a hydrogen-bonded phenol occurs
by concerted proton-coupled electron transfer, J. Am. Chem. Soc., 2004,
126, 12718–12719.
17 (a) C. Costentin, M. Robert and J.-M. Savéant, Adiabatic and non-adia-
batic concerted proton−electron transfers. Temperature effects in the oxi-
dation of intramolecularly hydrogen-bonded phenols, J. Am. Chem. Soc.,
2007, 129, 9953–9963; (b) C. Costentin, M. Robert and J.-M. Savéant,
Electrochemical and homogeneous proton-coupled electron transfers:
Concerted pathways in the one-electron oxidation of a phenol coupled
with an intramolecular amine-driven proton transfer, J. Am. Chem. Soc.,
2006, 128, 4552–4553.
the pyrrolidino nitrogen can be used as a built in proton-
accepting unit that modulates the energetics of the attached
donor. The system mimics certain aspects of PCET reactions in
biology and thus provides a model for better understanding the
role of this class of reactions in forming efficient interfaces
between light absorbing reaction centers and multi-electron cata-
lytic components.
Acknowledgements
This work was supported as part of the Center for Bio-Inspired
Solar Fuel Production, an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences under Award Number
DE-SC0001016.
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