Synthesis, electrochemistry, and photophysics of a family of phlorin macrocycles that display cooperative fluoride binding

Article abstract of DOI:10.1021/ja401391z

A homologous set of 5,5-dimethylphlorin macrocycles in which the identity of one aryl ring is systematically varied has been prepared. These derivatives contain ancillary pentafluorophenyl (3H(Phl^F)), mesityl (3H(Phl ^Mes)), 2,6-bisme

Full text of DOI:10.1021/ja401391z

Article
pubs.acs.org/JACS
Synthesis, Electrochemistry, and Photophysics of a Family of Phlorin
Macrocycles That Display Cooperative Fluoride Binding
Allen J. Pistner,† Daniel A. Lutterman,‡ Michael J. Ghidiu,† Ying-Zhong Ma,‡ and Joel Rosenthal*,†
†Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
‡Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
S
* Supporting Information
ABSTRACT: A homologous set of 5,5-dimethylphlorin macro-
cycles in which the identity of one aryl ring is systematically varied
has been prepared. These derivatives contain ancillary pentafluor-
ophenyl (3H(PhlF)), mesityl (3H(PhlMes)), 2,6-bismethoxyphenyl
(3H(PhlOMe)), 4-nitrophenyl (3H(PhlNO )), or 4-tert-butylcarbox-
2
yphenyl (3H(PhlCO tBu)) groups at the 15-meso-position. These
2
porphyrinoids were prepared in good yields (35−50%) and display
unusual multielectron redox and photochemical properties. Each
phlorin can be oxidized up to three times at modest potentials and
can be reduced twice. The electron-donating and electron-releasing
properties of the ancillary aryl substituent attenuate the potentials of
these redox events; phlorins containing electron-donating aryl
groups are easier to oxidize and harder to reduce, while the opposite
trend is observed for phlorins containing electron-withdrawing functionalities. Phlorin substitution also has a pronounced effect
on the observed photophysics, as introduction of electron-releasing aryl groups on the periphery of the macrocycle is manifest in
larger emission quantum yields and longer fluorescence lifetimes. Each phlorin displays an intriguing supramolecular chemistry
and can bind 2 equiv of fluoride. This binding is allosteric in nature, and the strength of halide binding correlates with the ability
of the phlorin to stabilize the buildup of charge. Moreover, fluoride binding to generate complexes of the form 3H(PhlR)·2F−
modulates the redox potentials of the parent phlorin. As such, titration of phlorin with a source of fluoride represents a facile
method to tune the ability of this class of porphyrinoid to absorb light and engage in redox chemistry.
displayed light conversion efficiencies below 7%.14,15 These
low efficiencies are partly due to the fact that simple porphyrin
INTRODUCTION
■
Porphyrin is a ubiquitous and versatile protein cofactor that
derivatives display relatively narrow absorbances in the Soret
plays many roles in biology.1 The heme cofactor, which is the
(375−425 nm) and Q-band (500−600 nm) regions, which
iron complex of protoporphyrin IX, is the most common form
of porphyrin found in nature. By control of the axial
limits light capture at longer wavelengths (650−900 nm).
Nature confronts this issue by using carotenoids to harvest
coordination of the iron atom, as well as hydrogen-bonding
photons on the low energy end of the visible region.16−18
and other noncovalent interactions with the porphyrin
Furthermore, recent work has shown that designer porphyrins
periphery, protein environments successfully tune the elec-
tronic/redox properties of the cofactor.2 This electronic
incorporating ancillary electron donors and extended π-
conjugation display much broader absorption profiles upon
tailoring allows enzymatic systems to effect a wide range of
incorporation into DSC devices, giving rise to solar conversion
redox processes such as single-electron transfer by cyto-
efficiencies in excess of 12%.19
chromes,3 dioxygen coordination and transport by globins,4
O−O bond activation and substrate oxidation by cytochrome
P-450,5 and peroxide/superoxide removal by peroxidases and
catalases.6
In addition to making use of tetrapyrroles as platforms for
important catalytic transformations, nature also relies on
porphyrin-based chromophores for solar light harvesting.
Plants7,8 and cyanobacteria9 both make use of chlorophylls
and other natural porphyrinoids to absorb sunlight in the UV−
vis region. These biological systems have driven the widespread
study of photoinduced electron transfer by synthetic porphyrin
constructs10,11 and inspired the use of porphyrin chromophores
in DSC assemblies.12,13 Traditionally, such systems have
Despite their ubiquitous role in biology and chemistry,
porphyrins only represent a subset of tetrapyrrole macrocycles
that display rich photophysical properties. Phthalocyanines and
corroles are two non-natural porphyrinoids that have been used
for light harvesting applications. Additional tetrapyrrole macro-
cycles include porphyrinogens and other porphyrin derivatives
with sp3 hybridized meso-carbons.20−23 Unlike conventional
porphyrinoids, these compounds display an intriguing multi-
electron redox chemistry due to the sp3 hybridized meso-
Received: February 10, 2013
Published: April 17, 2013
© 2013 American Chemical Society
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carbons; however, the lack of π-conjugation between pyrrole
units compromises the spectroscopic properties of these
macrocycles in comparison to the rich photophysics typical of
porphyrins.24
We recently detailed the phlorin macrocycle, in which a
single sp3 hybridized carbon is introduced at one of the meso-
positions.25 This structure engenders multielectron redox
properties and intriguing photophysical characteristics, which
distinguish it from more common tetrapyrrole architectures
such as porphyrins and corroles (Chart 1).25 To this point,
RESULTS AND DISCUSSION
■
The family of phlorin derivatives under consideration was
prepared by adapting a modular synthetic methodology
(Scheme 1).25,28 This route begins with the preparation of a
diacylated dipyrromethane (1) via the deprotonation of 5,5-
dimethyldipyrromethane with EtMgBr, followed by addition of
pentafluorophenylbenzoyl chloride. Reduction of 1 with
NaBH4 in THF/MeOH (3:1) generated the corresponding
dicarbinol, which was condensed with various 5-aryldipyrro-
methane derivatives (2−6)29,30 in the presence of TFA to yield
the corresponding phlorin frameworks, following oxidation
with DDQ and purification by flash chromatography. The
modular nature of this synthetic method provided phlorin
derivatives with an ancillary pentafluorophenyl (3H(PhlF)),
mesityl (3H(PhlMes)), 2,6-bismethoxyphenyl (3H(PhlOMe)), 4-
Chart 1
nitrophenyl (3H(PhlNO )), or 4-tert-butylcarboxyphenyl (3H-
2
(PhlCO tBu)) substituent at the meso-position across from the sp3
2
hybridized center of the phlorin core in 35−50% yield. These
yields are relatively high compared to those of typical porphyrin
syntheses and scale up well to roughly 0.5 g. We note that
synthesis of the family of porphyrinoids shown in Scheme 1
nearly triples the library of known 5,5-dimethylphlorin
derivatives that have been reported to date.
With each of the phlorins of Scheme 1 in hand, we sought to
characterize the photophysical and redox properties of these
compounds. Solutions of each of these macrocycles are an
intense emerald green color and display broadly absorbing
bands across the visible region from 300 to 750 nm (Figure 1).
Variation of the substituents on the aryl group at the 15-
position of the phlorin ring does not drastically attenuate the
shape of the corresponding absorption profiles. Subtle changes
in the extinction coefficients for both the Soret and Q-band
regions are observed, with the more electron rich phlorins (i.e.,
3H(PhlMes) and 3H(PhlOMe)) displaying more strongly
absorbing bands compared to 3H(PhlF).
however, only a handful of 5,5-dimethylphlorins have appeared
in the literature25−27 and the general redox and photophysical
properties of such systems remain largely unknown. In order to
gain a better appreciation for the manner in which synthetic
alterations attenuate the electronic structure of the phlorin
construct, we have undertaken the preparation and physical
characterization of a family of phlorin homologues. This work
demonstrates that the multielectron redox and photochemical
properties are inherent to the phlorin core and that these
properties can be tuned by variation of the ancillary aryl groups
appended to the phlorin framework. We have also uncovered
that these macrocycles display an unusual supramolecular
chemistry with fluoride anions and that cooperative binding of
fluoride to the phlorin core significantly perturbs the electronic
structure of the chromophore as judged by UV−vis absorption
and electrochemistry measurements.
Similar trends are observed for the emission profiles for each
of the phlorins studied. Each of the porphyrinoids produces a
broad emission profile upon excitation into the Q-band region,
Scheme 1. Synthesis of a Family of Phlorins with Varying Substitutents at the 15-meso-Position
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Figure 1. UV−vis absorption spectra recorded for each of the phlorins
studied in CH2Cl2.
Figure 2. Cyclic voltammograms recorded for a family of 5,5-
dimethylphlorin derivatives (1.0 mM) at a scan rate of 50 mV/s in
CH2Cl2 containing TBAPF6 with an internal decamethylferrocene
standard (FC*).
with fluorescence maxima centered between 720 and 732 nm
(Figure S1 in Supporting Information). Table 1 highlights the
photoluminescence data obtained for these compounds,
including the quantum yields (ΦFl) of emission, which range
from 5.3 × 10−4 for the most electron deficient phlorin
(3H(PhlF)) to 17.6 × 10−4 for the methoxy substituted
macrocycle (3HPhlOMe). In keeping with these low fluores-
cence quantum yields, the emission lifetimes (τFl) observed for
each of these systems are short compared to those of typical
freebase porphyrin derivatives;31−33 the phlorin lifetimes range
from ∼37 ps for 3H(PhlF) to ∼101 ps for 3H(PhlOMe). The
the aryl ring at the 15-meso-position. DPV experiments (Figure
S2) reveal that the reduction potentials for the phlorins studied
in this work increase to more negative potentials along the
series 3H(PhlF) (E1/2 = −0.93 V) ∼ 3H(PhlNO ) (E1/2 = −0.94
2
V) < 3H(PhlCO tBu) (E1/2 = −0.99 V) < 3H(PhlMes) (E1/2
=
2
−1.05 V) ∼ 3H(PhlOMe) (E1/2 = −1.07 V) versus Ag/AgCl.
nitro (3H(PhlNO )), mesityl (3H(PhlMes)), and tert-butyl ester
The reduction waves for the nitro-substituted phlorin (3H-
2
(3H(PhlCO tBu)) phlorin derivatives display fluorescence life-
(PhlNO )) are significantly attenuated compared to the other
2
2
times of 37, 58, and 66 ps, respectively. These represent the
first measurements of excited state lifetimes of 5,5-dimethyl-
phlorin frameworks. As shown in Table 1, the observed trend in
τFl values for each of the phlorins parallels that observed for
ΦFl, with more electron rich phlorin constructs displaying
longer-lived excited states and larger emission quantum yields.
Each of the phlorin architectures of Scheme 1 also displays
redox properties that are attenuated by the aryl substituent at
the 15-meso-position. Cyclic voltammetry experiments were
carried out for 1.0 mM solutions of each of these porphyrinoids
in CH2Cl2 containing 0.1 M TBAPF6 and an internal
decamethylferrocene standard. The resulting CV traces are
reproduced in Figure 2. Each phlorin displays reduction waves
at roughly −1.0 and −1.5 V versus Ag/AgCl. Both these redox
events are largely irreversible. This is especially apparent for the
first reduction, for which cathodic and anodic peak potentials
are separated by approximately 0.8 V for each of the phlorins
other than 3H(PhlF). Furthermore, these reduction potentials
are perturbed by incorporation of electron releasing groups on
phlorins studied. This difference is likely due to redox events
involving reduction of the nitro functionality, as has been
observed for analogous porphyrin derivatives.34
Each of the phlorin macrocycles also displays three quasi-
reversible oxidations between 0.6 and 1.25 V. These redox
potentials are also influenced by the electronic nature of the
aryl substituent at the phlorin 15-meso-position, as summarized
in Table 1. Inclusion of electron donating groups shifts each of
the three oxidations to significantly less positive potentials
compared to those observed for 3H(PhlF). For example, both
3H(PhlOMe) and 3H(PhlMes) display similar potentials for each
of the oxidation events, which occur at approximately 0.6, 0.9,
and 1.1 V versus Ag/AgCl. The first oxidation potentials
observed for these porphyrinoids are especially low and are
close to that of ferrocene,35 which is a quintessential
organometallic electron donor. Moreover, the first oxidation
potentials of these species are nearly a full volt lower than those
observed for analogous fluorinated porphyrin derivatives,25,36
which further highlights the distinctive electronic nature of the
Table 1. Redox and Photophysical Data Recorded for Homologous Phlorin Derivatives
a
b
b
redox properties
absorption
emission
Soret λmax, nm
Q band λmax, nm
λem
,
τFl,
ps
Eox(1), V Eox(2), V Eox(3), V Ered(1), V Ered(2), V E0−0, V
(ε, cm−1 M−1
)
(ε, cm−1 M−1
)
nm
ΦFl
3H(PhlF)
3H(PhlNO
0.75
0.65
0.63
0.60
0.61
0.99
0.93
0.89
0.87
0.87
1.26
1.16
1.12
1.14
1.10
−0.93
−0.94
−0.99
−1.05
−1.07
−1.43
−1.53
−1.49
−1.56
−1.58
1.68
1.73
1.62
1.65
1.68
435 (29300)
445 (30800)
435 (45100)
420 (36400)
415 (38400)
655 (16000)
670 (21200)
665 (24700)
669 (24200)
665 (24900)
722
732
731
721
720
37
37
5.34 × 10−4
6.35 × 10−4
9.96 × 10−4
9.65 × 10−4
17.6 × 10−4
2
)
3H(PhlCO tBu
3H(PhlMes
3H(PhlOMe
)
66
2
)
58
)
101
a
b
Redox potentials are referenced to Ag/AgCl. All spectroscopic data were recorded for phlorin samples in deoxygenated CH2Cl2 solution.
Fluorescence data were recorded by exciting solutions of each of the phlorins at λ = 650 nm. Time-resolved fluorescence data were recorded by
exciting the solution samples of each of the phlorins at λ = 650 nm and monitoring the emission at λ = 750 nm. The lifetime of the dominant decay
component with relative amplitude >97% is shown for each phlorin derivative.
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phlorin framework. The electron-withdrawing aryl groups of
For each titration, well-anchored isosbestic points are
maintained during the course of the fluoride binding experi-
ment, which would suggest a clean conversion of freebase
phlorin (3H(PhlR)) to a 1:1 fluoride adduct, without the
formation of any intermediary species. Job analysis of the
fluoride binding for each of the phlorins indicates that binding
is not 1:1, however. Each Job plot (Figure S4) maximizes at a
phlorin molar ratio of ∼0.3 to 0.35, indicating that the phlorins
are capable of binding 2 equiv of fluoride to generate
3H(PhlR)·2F−. Although this binding stoichiometry is un-
common for simple tetrapyrrole platforms, such behavior has
been observed for a phlorin construct containing a dangling
pyrrole moiety at the sp3 center44 and for 3H(PhlF) in THF.25
We note that unlike other protonated porphyrinoids that can
bind larger halogens such as chloride,45,46 none of the phlorins
of Scheme 1 bind halogens other than fluoride, as judged by
UV−vis spectroscopy.
3H(PhlCO tBu) and 3H(PhlNO ) increase the observed potentials
of the oxidation waves by 10−15 mV to values between those
of the most electron deficient (3H(PhlF)) and rich (3H-
(PhlOMe)) architectures (Table 1).
2
2
The relatively low first oxidation potentials observed for each
member of the phlorin family of Scheme 1 give rise to
compressed electrochemical HOMO−LUMO gaps compared
to typical fluorinated porphyrins.25 This contraction is
consistent with the significant absorption bands displayed by
the phlorins at energies lower than 600 nm (vide supra). As
displayed in Table 1, HOMO−LUMO gaps (E0−0) for most of
the phlorins range from 1.65 to 1.75 eV; however, 3H-
(PhlCO tBu) has a notably small HOMO−LUMO gap of ∼1.62
2
eV and is the smallest of all the phlorin constructs we have
studied to date. Each of these E0−0 values is significantly lower
than those observed for typical fluorinated porphyrin (∼2.4 V)
and corrole frameworks (∼1.85 V).25
A Hill analysis of the titration data for 3H(PhlMes) (Figure 3,
inset) supports the 2:1 binding model. This analysis produces a
well-fit linear regression and yields a cooperativity constant of
β2 = 4.5 × 108 M−2 for formation of ternary complex
3H(PhlMes)·2F−. The magnitude of this β2 value attests to the
strength of the fluoride binding interaction. Similar to previous
studies in THF,25 Hill analysis of fluoride binding to
3H(PhlMes) does not allow for the individual equilibrium
constants (K1 and K2) for formation of 3H(PhlMes)·F− and
3H(PhlMes)·2F− to be extracted from the titration data but
reveals the extent to which allosteric effects drive fluoride
binding. It is clear, however, that the equilibrium constant for
binding of the second fluoride ion (K2) to generate
3H(PhlMes)·2F− is much larger than that for the first binding
event (K1) and suggests that a destabilizing conformational
change to the phlorin core may occur upon binding of the first
fluoride to produce 3H(PhlR)·F−. The cooperative nature of
fluoride binding precludes the build-up of a singly fluoride
bound species in solution and gives rise to the isosbestic
crossings observed for the UV−vis titration. The fact that there
is no observable singly fluoride bound intermediate (3H-
(PhlMes)·F−) is consistent with this binding model.
The phlorin supports a notable supramolecular chemistry via
formation of hydrogen bonds between the pyrrole N−H units
and fluoride anions. Similar chemistry has been documented for
calipyrroles37−39 and other polypyrrolic platforms.40−43 Slow
titration of anhydrous CH2Cl2 solutions (10 μM) of each of the
systems shown in Scheme 1 with n-tetrabutylammonium
fluoride (TBAF) results in a change in color from emerald
green to light brown. This colorimetric transformation can be
closely monitored by UV−vis spectroscopy. Indeed, slow
addition of TBAF aliquots is manifest in shifts in both the
visible and near-IR regions of the phlorin absorption spectrum.
As shown in Figure 3, fluoride binding to 3H(PhlMes) is
3H(PhlR) + F− ⇄ 3H(PhlR)·F−
(1)
3H(PhlR)·F− + F− ⇄ 3H(PhlR)·2F−
(2)
Hill analyses for the fluoride titrations recorded for each of
the other four phlorins (Figure S3) have also been carried out.
The resulting Hill plots are reproduced in Figure S5, and the
corresponding binding parameters are presented in Table 2.
Each of the systems studied displays β2 values that are greater
than 108 M−2, establishing that each of the phlorins is capable
of strongly coordinating 2 equiv of fluoride to generate
3H(PhlR)·2F−. There exists a general trend that shows that
cooperative fluoride binding is strongest for the more electron
Figure 3. Changes in the UV−vis absorption profile of 3H(PhlMes
)
upon titration with TBAF. Inset: Hill plot confirming the allosteric
formation of 3H(PhlMes)·2F−.
Table 2. Thermodynamic Parameters for Binding of
Fluoride to Freebase Phlorins
accompanied by a large increase in absorptivity in the Soret
region at ∼420 nm, as well as a dramatic shift of the Q-band
region from ∼670 to 805 nm. Although TBAF can be basic, 1H
and 19F NMR studies conducted for 3H(PhlF) confirm that the
observed UV−vis spectral changes are consisitent with
hydrogen-bonding of fluoride to the phlorin N−H groups
and not deprotonation of the macrocycle.25 UV−vis titration
data for each of the phlorins of Scheme 1 are qualitatively
similar to data presented for 3H(PhlMes) and are reproduced in
the Supporting Information (Figure S3).
Hill constant
ΔEox(1),
ΔEred(1),
β2, M−2
(η)
mV
mV
3H(PhlF)
3H(PhlNO
1.6 × 1015
2.5 × 1012
2.4 × 1010
4.5 × 108
6.3 × 109
8.6
7.4
5.1
5.0
5.4
210
540
470
400
410
450
420
320
280
260
2
)
3H(PhlCO tBu
3H(PhlMes
3H(PhlOMe
)
2
)
)
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Figure 4. Correlation between phlorin oxidation potential and (a) Hill coefficient (η) and (b) strength of fluoride binding (β2).
deficient phlorin frameworks, including 3H(PhlF) and 3H-
(PhlNO ). By contrast, the phlorin macrocycles containing
2
electron-releasing groups at the 15-meso-position such as
3H(PhlMes) and 3H(PhlOMe) are weaker fluoride binders.
This trend can be rationalized in a general sense, as a hard
fluoride anion is expected to be more strongly attracted to
hydrogen-bond donors with greater electropositive character.
The Hill coefficient (η) for binding of fluoride to 3H(PhlMes
)
was also determined. The size of this parameter reflects the
extent of cooperativity between supramolecular binding
events.47 The Hill coefficient (η) for binding of fluoride to
3H(PhlMes) was determined to be 5.0, which confirms that a
large and positive allosteric interaction is at play for this system.
Hill coefficients for the other phlorins studied range from 5.4
for 3H(PhlOMe) to 8.6 for 3H(PhlF). The trend along the series
of phlorins tracks that of β2 values, with the more electron
deficient phlorin macrocycles displaying larger Hill coefficients.
The correlation between η and the electronic structure of the
phlorin is illustrated by the plot in Figure 4. A linear relation
exists between the oxidation potentials of the phlorins and the
extent to which fluoride binding is cooperative. The less easily
oxidized phlorins (i.e., those containing electron-withdrawing
aryl groups at the 15-meso-position) display larger Hill
coefficients for fluoride binding (Figure 4a). A similar
relationship is evident upon comparing the log(β2) values for
3H(PhlR)·2F− formation with the oxidation potentials of the
parent phlorins (Figure 4b). This second correlation
demonstrates that in addition to the level of cooperativity,
the strength of the fluoride binding interaction can also be
directly linked to phlorin electronic structure.
The extent to which binding of fluoride to the phlorin N−H
residues perturbs the macrocycle’s electronic properties is
evident from UV−vis spectroscopy (vide supra). Formation of
the fluoride bound adducts 3H(PhlR)·2F− also attenuates the
porphyrinoids’ redox properties (Table 2). As shown in Figure
5a, fluoride binding is manifest in large changes in the
potentials at which 3H(PhlMes)·2F− is oxidized, as judged by
DPV experiments. For instance, the first oxidation potential
(Eox(1)) of this system shifts to less positive potentials by
roughly 400 mV (Table 2). Similar results have been reported
for fluoride binding to oxoporphyrinogen complexes in
dichlorobenzene.48 The second and third oxidation potentials
also shift to less positive potentials by roughly 320 and 450 mV,
respectively. Fluoride binding attenuates the potentials at which
the other four phlorins are oxidized to a similar extent (Table
Figure 5. DPV traces recorded for 3H(PhlMes) (orange) and
3H(PhlMes)·2F− (brown) in CH2Cl2 containing 0.1 M TBAPF6 with
an internal decamethylferrocene standard (FC*). DPV scans were
recorded (a) to positive and (b) to negative potentials versus Ag/
AgCl.
S1), as observed variations in Eox(1) range from 210 to 540 mV
(Table 2).
Association of 2 equiv of fluoride to 3H(PhlMes) also pushes
the first reduction potential (Ered(1)) of the resultant
3H(PhlMes)·2F− complex to more negative potentials (Figure
5b) by 280 mV. Similar behavior is observed for each of the
other phlorins, where fluoride binding induces changes in
Ered(1) that range from 260 to 450 mV (Table 2).
SUMMARY AND CONCLUSIONS
■
The phlorin macrocycle has historically been an under-
developed tetrapyrrole framework. Experiments have demon-
strated that this porphyrinoid displays unique multielectron
redox properties and a broad absorption profile that is well
suited for solar harvesting applications.25 In this work, we have
roughly tripled the library of known 5,5-dimethylphlorin
constructs by preparing a family of phlorins in which the aryl
substituent at the 15-meso-position was systematically varied.
All phlorins studied display a multielectron redox chemistry;
each system can be oxidized three times at modest potentials
and also displays two irreversible reduction waves. The
electron-donating and withdrawing properties of the ancillary
aryl substituent on the phlorin backbone attenuate the
potentials of these redox events and provide a handle to tune
the electrochemical properties of the porphyrinoid framework.
Phlorin substitution also has a pronounced effect on the
observed photophysics, as introduction of electron-releasing
aryl groups on the periphery of the macrocycle is manifest in
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larger emission quantum yields and longer fluorescence
lifetimes.
REFERENCES
■
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All of the phlorins also display an intriguing supramolecular
chemistry with fluoride anions. Each phlorin can bind 2 equiv
of this halide in highly cooperative fashion. The strength and
cooperative nature of fluoride binding to generate 3H-
(PhlR)·2F− correlate with the ability of the phlorin to stabilize
the build-up of charge. Fluoride binding is manifest in dramatic
changes to the phlorin electronic structure, as demonstrated by
both UV−vis spectroscopy and electrochemistry experiments.
Formation of 3H(PhlR)·2F− is accompanied by increases in
absorptivity and the appearance of strongly absorbing bands
toward the near-IR region (750−900 nm). Fluoride binding
also perturbs the ability of the phlorin to serve as an electron
donor and/or acceptor, as each of the 3H(PhlR)·2F− complexes
is much more easily oxidized than the parent phlorins. Titration
of phlorin with a source of fluoride represents a facile method
to tune the ability of this porphyrinoid to absorb light and
engage in redox chemistry. When coupled with the ability to
modulate the phlorin’s redox and photophysical properties via
synthetic alteration of the macrocycle’s periphery, this general
framework provides a versatile platform with potential
applications in the fields of photocatalysis, electrocatalysis,
anion sensing, and solar-light capture. The modular synthesis of
the phlorin framework, coupled with the flexibility offered by its
supramolecular chemistry with fluoride, distinguishes this
porphyrinoid from more commonly studied porphyrin and
corrole macrocycles. As such, the phlorin is an intriguing
candidate for incorporation into energy-storing schemes and
colorimetric/electrochemical platforms for fluoride detection.
Future efforts in our laboratory will focus on the use of phlorin
frameworks for such endeavors.
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ASSOCIATED CONTENT
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* Supporting Information
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AUTHOR INFORMATION
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Corresponding Author
joelr@udel.edu
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank Sean Herron (University of Delaware) for assistance
with UV−vis fluoride titration experiments. Research reported
in this publication was supported by an Institutional Develop-
ment Award (IDeA) from the National Institute of General
Medical Sciences of the National Institutes of Health under
Grant P20GM103541. J.R. also thanks the University of
Delaware Research Foundation and the Donors of the
American Chemical Society’s Petroleum Research Fund for
financial support. D.A.L. and Y.-Z.M. were sponsored by the
Division of Chemical Sciences, Geosciences, and Biosciences,
Office of Basic Energy Sciences, U.S. Department of Energy.
NMR and other data were acquired at University of Delaware
using instrumentation obtained with assistance from the NSF
and NIH (Grants NSF-MIR 0421224, NSF-CRIF MU CHE-
0840401 and CHE-0541775, and NIH P20 RR017716).
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