Syntheses, electrochemistry, and photodynamics of ferrocene- azadipyrromethane donor-acceptor dyads and triads

Article abstract of DOI:10.1021/jp205236n

A near-IR-emitting sensitizer, boron-chelated tetraarylazadipyrromethane, has been utilized as an electron acceptor to synthesize a series of dyads and triads linked with a well-known electron donor, ferrocene. The structural integrity of the newly synthesized dyads and triads was established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a molecular clip-type structure for the triads wherein the donor and acceptor entities were separated by about 14 A. Differential pulse voltammetry combined with spectroelectrochemical studies have revealed the redox states and estimated the energies of the charge-separated states. Free-energy calculations revealed the charge separation from the covalently linked ferrocene to the singlet excited ADP to yield Fc⁺-ADP^?- to be energetically favorable. Consequently, the steady-state emission studies revealed quantitative quenching of the ADP fluorescence in all of the investigated dyads and triads. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of photoinduced electron transfer in these donor-acceptor systems by providing spectral proof for formation of ADP radical anion (ADP^?-) which exhibits a diagnostic absorption band in the near-IR region. The kinetics of charge separation and charge recombination measured by monitoring the rise and decay of the ADP ^?- band revealed ultrafast charge separation in these molecular systems. The charge-separation performance of the triads with two ferrocenes and a fluorophenyl-modified ADP macrocycle was found to be superior. Nanosecond transient absorption studies revealed the charge-recombination process to populate the triplet ADP as well as the ground state.

Full text of DOI:10.1021/jp205236n

ARTICLE
pubs.acs.org/JPCA
Syntheses, Electrochemistry, and Photodynamics of
FerroceneÀAzadipyrromethane DonorÀAcceptor Dyads and Triads
Anu N. Amin,† Mohamed E. El-Khouly,§ Navaneetha K. Subbaiyan,‡ Melvin E. Zandler,† Mustafa Supur,§
,†
Shunichi Fukuzumi,*,|| and Francis D’Souza*,†,‡
†Department of Chemistry, Wichita State University, Wichita, Kansas 67260-0051, United States
‡Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States
§Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan
Department of Bioinspired Science, Ewha Womans University, Seoul, 120-750, Korea
S Supporting Information
b
ABSTRACT: A near-IR-emitting sensitizer, boron-chelated tetraarylazadipyrromethane, has
been utilized as an electron acceptor to synthesize a series of dyads and triads linked with a well-
known electron donor, ferrocene. The structural integrity of the newly synthesized dyads and
triads was established by spectroscopic, electrochemical, and computational methods. The DFT
calculations revealed a ‘molecular clip’-type structure for the triads wherein the donor and
acceptor entities were separated by about 14 Å. Differential pulse voltammetry combined with
spectroelectrochemical studies have revealed the redox states and estimated the energies of the
charge-separated states. Free-energy calculations revealed the charge separation from the
covalently linked ferrocene to the singlet excited ADP to yield Fc+ÀADP•À to be energetically
favorable. Consequently, the steady-state emission studies revealed quantitative quenching of the
ADP fluorescence in all of the investigated dyads and triads. Femtosecond laser flash photolysis studies provided concrete evidence
for the occurrence of photoinduced electron transfer in these donorÀacceptor systems by providing spectral proof for formation of
ADP radical anion (ADP•À) which exhibits a diagnostic absorption band in the near-IR region. The kinetics of charge separation and
charge recombination measured by monitoring the rise and decay of the ADP•À band revealed ultrafast charge separation in these
molecular systems. The charge-separation performance of the triads with two ferrocenes and a fluorophenyl-modified ADP
macrocycle was found to be superior. Nanosecond transient absorption studies revealed the charge-recombination process to
populate the triplet ADP as well as the ground state.
involving development of sensors and photodynamic therapy
agents.20,21 In addition, as demonstrated here, the electron-transfer
dynamics can be readily monitored, because the one-electron-
reduced product exhibits a near-IR band beyond 800 nm, suffi-
ciently far from the spectral region of the tripletÀtriplet absorp-
tion of most of the sensitizers. However, their application in
light-energy-harvesting donorÀacceptor systems has yet to be
explored.22
We report herein the utility of ADP derivatives as a light-
harvesting unit as well as an electron acceptor unit in a series of
newly synthesized dyads and triads featuring ferrocene as an
electron donor (Scheme 1). In the first type, ADP is linked to one
or two ferrocene units (FcÀADP and Fc2ÀADP), while in the
second type, two 4-fluorobenzenes instead of the phenyl rings
are substituted to introduce additional electron deficiency of the
ADP macrocycle (FcÀADPF2 and Fc2ÀADPF2). Compounds
ADP, ADP(OH)2, and ADPF2(OH)2 were also synthesized as
control compounds. Efficient photoinduced electron transfer has
1. INTRODUCTION
Porphyrins and phthalocyanines have extensively been used as
light-harvesting and electron-transfer units in artificial photosyn-
thetic systems performing photoinduced energy and electron
transfer,1À6 because the photosynthetic reaction center includes
the porphyrin derivatives, i.e., bacteriochlorophylls, which act as
light-harvesting antenna and electron donors in the electron-
transfer cascade. On the other hand, boron dipyrromethene
(BODIPY) fluorophores, which are derived from 4,4-difluoro-
1,3,5,7-tetramethyl-4-bora-3a,4adiaza-s-indacene,7 have also been
widely employed as light-harvesting antenna and electron donor
or acceptor as well as laser dyes in a variety of applications,
because a large number of BODIPY derivatives are readily ob-
tained by modification at carbon positions at 1, 3, 5, 7, and 8,8,9
exhibiting attractive photophysical characteristics with strong
absorption bands in the visible region, which are comparable to
those of porphyrins.10À17
AzaÀBODIPY dyes (abbreviated as ADP), with a nitrogen in
position 8 instead of the carbon in C8 BODIPY, have attracted
increasing attention, because ADPs show high extinction coeffi-
cients (7À8 Â 105 MÀ1 cmÀ1), large fluorescence quantum
yields beyond 700 nm, and a less negative one-electron reduction
potential.18,19 Consequently, they have been used in applications
Received: June 3, 2011
Revised:
July 16, 2011
Published: July 27, 2011
r
2011 American Chemical Society
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Scheme 1. Structures of Newly Synthesized FerroceneÀADP Dyads and Triads in the Present Study
been witnessed in these donorÀacceptor dyads and triads by
femtosecond laser flash photolysis measurements.
reactions of 3a and 3b with diisopropylethylamine and boron
trifluoride diethyl etherate in dry CH2Cl2. Finally, the ferroceneÀ
ADP dyads and triads were obtained by reactions of 4a and 4b
with appropriate amounts of 4-ferrocenylbenzoic acid in the
presence of EDCl followed by chromatographic purification.
The structural integrity of the newly synthesized compounds was
established from H NMR, mass, and optical techniques (see
Figures S1ÀS7, Supporting Information).
2. RESULTS AND DISCUSSION
2.1. Syntheses of FerroceneÀADP Dyads and Triads. The
syntheses of the donorÀacceptor systems involved a multistep
approach as shown in Scheme 2, and the details are given in the
Experimental Section. Briefly, 1-(4-hydroxyphenyl)-3-phenyl-
propenone (1a) and 1-(4-hydroxyphenyl)-3-(4-fluorophenyl)-
propenone (1b) were synthesized from a reaction of the corres-
ponding benzaldehyde, 4-hydroxyacetophenone, and potassium
hydroxide. These compounds were subsequently reacted with
nitromethane and diethylamine in dry ethanol to obtain respec-
tive 1-(4-hydroxyphenyl)-4-nitro-3-phenylbutan-1-one (2a) and
1-(4-hydroxyphenyl)-4-nitro-3-(4-fluorophenyl)butan-1-one (2b).
The [5-(4-hydroxyphenyl)-3-phenyl-1H-pyrrol-2-yl]-[5-(4-hy-
droxyphenyl)-3-phenylpyrrol-2-ylidene]amine (3a) and [5-(4-
hydroxyphenyl)-3-(4-fluorophenyl)-1H-pyrrol-2-yl]-[5-(4-
hydroxyphenyl)-3-(4-fluorophenyl)pyrrol-2-ylidene]amine (3b)
were synthesized by reactions of 2a and 2b with ammonium
acetate in ethanol. Next, the BF2 chelate of [5-(4-hydroxyphenyl)-
3-phenyl-1H-pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-phenylpyrrol-
2-ylidene]amine (4a) and BF2 chelate of [5-(4-hydroxyphenyl)-
3-(4-fluorophenyl)-1H-pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-(4-
fluorophenyl)pyrrol-2-ylidene]amine (4b) were synthesized by
1
2.2. Steady-State Absorption and Fluorescence Measure-
ments. Figure 1a and 1b shows the absorption spectra of the
investigated compounds in benzonitrile. The most intense band
of ADP was located in the range 665À700 nm depending upon
the substituents on the aromatic rings. A less intense band in the
465À485 nm region was also observed. The near-IR band was
red shifted by about 150 nm compared to the BODIPY deri-
vatives,16,17 which suggests its usefulness in light-harvesting
system. Interestingly, the dihydroxy derivatives, 4a and 4b,
revealed significant bathochromic shifts, suggesting a significant
influence of the hydroxyl groups. However, the presence of
fluoro groups on the phenyl rings had virtually no influence on
the optical behavior. In the allowed wavelength range of the
benzonitrile solvent and due to the low molar extinction coeffi-
cient, the absorbance band corresponding to ferrocene was also
not observed.
As shown in Figure 1c and 1d, ADP revealed a single fluo-
rescence band at 682 nm while the dihydroxy derivatives (4a and
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Scheme 2. Synthetic Procedure Adapted for FerroceneÀADP Dyads and Triads
4b) revealed a red-shifted band located at 730 nm. The presence
of ferrocene entities for both dyads and triads quantitatively
quenched the fluorescence, suggesting the occurrence of photo-
chemical events from the singlet excited ADP. The fluorescence
quantum yields of FcÀADP (Φf = 5 Â 10À3) and Fc2ÀADP
(Φf = 2 Â 10À3) were found to be much smaller than that of ADP
reference (Φf = 0.13).
In the dyads FcÀADP and FcÀADPF2, the ferrocene oxida-
tion occurred at around 0.07 V vs Fc/Fc+, which is about 70À
80 mV anodically shifted as compared with pristine ferrocene
oxidation that could easily be attributed to chemical functiona-
lization of the ferrocene entity.24 The first reduction potential of
ADP macrocycle was located at À0.84 V vs Fc/Fc+; the fluorine
entities had no effect on the reduction potential of the dyad.
However, compared with pristine ADP reduction, these reduc-
tions were cathodically shifted by 50 mV. In the case of triads, the
current for ferrocene oxidation was twice as much as that of
ADP reduction, and the potentials for ferrocene oxidation were
close to that observed for the corresponding dyads, while the
reduction potentials were close to that of pristine ADP. These
results collectively suggest ferrocene is electron rich and ADP is
an electron-deficient entity of the dyad.25 Since there were no
appreciable changes in the oxidation potentials of the ferrocene
entity in the dyads and triads, the absence of inter- or intramole-
cular interactions could be envisioned. The energies of the charge-
separated states were calculated using the redox, geometric, and
optical data according to Weller’s approach,26 and the calculated
values are given in Table 1. By comparing these energy levels of
the charge-separated states with the energy levels of the excited
states, the driving forces of charge separation (ΔGSCS) were also
evaluated. Generation of Fc+ÀADP•À for the dyad and
Fc+FcÀADP•À for the triad was found to be exergonic via the
singlet excited state of ADP in benzonitrile.
2.3. Electrochemistry, Spectroelectrochemistry, and En-
ergy Levels. In order to establish the energy levels and spectral
characterization of electroreduced ADP, electrochemical studies
using differential pulse voltammetry and spectroelectrochemical
studies were performed. Figure 2 shows differential pulse vol-
tammograms (DPVs) of the dyads and triads along with the
control compounds in benzonitrile, 0.1 M (n-Bu4N)ClO4, while
the redox data are summarized in Table 1. ADP revealed both
one-electron reduction and one-electron oxidation processes
located at À0.79 and 0.80 V vs Fc/Fc+, respectively. The
first reduction was found to be cathodically shifted by nearly
200 mV compared with the well-known electron acceptor, C60.23
Introducing the hydroxyphenyl groups on ADP caused the
reduction harder by about 100 mV, while the oxidation process
was irreversible (from cyclic voltammetric experiments), how-
ever, with cathodically shifted potentials. In contrast, the added
fluorophenyl entities in ADPF2(OH)2 derivative had little or no
effect on the reduction potentials compared with the reduction
potentials of the corresponding ADP(OH)2.
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Figure 1. Absorbance spectra (a and b) and emission spectra (c and d) of the indicated compounds (8.0 μM) in benzonitrile (see Scheme 1 for
abbreviated structures). The compounds were excited at the corresponding near-IR peak maxima of the ADP macrocycle.
Facile reduction of ADP prompted us to perform spectro-
electrochemical studies to characterize the one-electron-reduced
product. Figure 3 shows the spectral changes recorded during
the course of the first reduction of ADP in benzonitrile, 0.1 M
(n-Bu4N)ClO4, in a thin-layer spectroelectrochemical cell. Dur-
ing the course of reduction, the peak located at 658 nm dimin-
ished in intensity with the concurrent appearance of new bands at
446 and 820 nm. This process was reversible, that is, switching
the potential to 0.0 V resulted in recovery of the entire spectrum
of the neutral ADP. The near-IR band at 820 nm ADP•À is
located sufficiently far from the spectral region of the triplet
absorption of the donor and acceptor, serving as a diagnostic
band to identity the electron-transfer products in photochemical
electron-transfer reactions, and follows the kinetics of charge
separation in the newly developed dyads.
Figure 4 shows the B3LYP/3-21G(*)-optimized structure and
molecular electrostatic potential map (MEP) of a representative
Fc2ÀADP triad.27,28 The structure was completely optimized on
a BornÀOppenheimer potential energy surface. The ADP seg-
ment was found to be flat, while the peripheral aromatic rings
were slightly tilted as evidenced by a dihedral angle of 22À29° to
the ADP ring atoms. The two ferrocene entities linked to ADP
formed a ‘molecular clip’-type structure in which the distance
between the boron of ADP to the iron of ferrocene was 14.1 Å
while the distance between the closest carbon of ADP to the
closet carbon of ferrocene was 12.1 . The FeÀFe distance be-
tween the two ferrocene entities was 15.0 Å, while the angle
between FeÀBÀFe was 63°. The MEP diagram also suggested
the electron-deficient nature of the ADP unit of the triad.
2.4. Photodynamics Studies. Femtosecond and nanosecond
laser flash photolysis techniques were used to investigate photo-
dynamics of the dyads and triads. Figures 5 and S8, Supporting
Information, show the transient spectral response for represen-
tative dyads and triad. In all of these systems, fast growth of bands
corresponding to ADP•À were observed in the 440 and 820 nm
region,22 confirming charge separation. The observed absorption
bands at 440 and 820 nm were assigned to ADP•À by comparison
with those of ADP•À produced either by spectroelectrochemistry
studies (Figure 3) or by the one-electron reduction of ADP with
tetrakis(dimethylamino)ethylene (Figure S9 in the Supporting
Information). The bands corresponding to Fc+ were not ob-
served due to its low molar extinction coefficient. The rate con-
stants of the charge separation (kSCS) and the charge recombina-
tion (kCR) were evaluated by monitoring the rise and decay of the
820 nm band, and such data are given in Table 2. The fast and
efficient charge separation in the investigated dyads (∼1012 sÀ1
)
suggests that the CS process is located at the top of the normal
region of the Marcus parabola.29 The kSCR values are found to be
considerably smaller than kSCS, suggesting that the CR processes
are located at the upward of the inverted region of the Marcus
parabola (ÀΔGCR = 0.98 eV).29À31 For a given series of triads
(fluorinated or nonfluorinated), the kS values were slightly
CS
larger for the triads. Between fluorinated and nonfluorinated
CS
ADPs, the kS values were slightly higher for the fluorinated
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Figure 2. Differential pulse voltammograms of the indicated compounds (see Scheme 1 for structures) in benzonitrile, 0.1 M (n-Bu4N)ClO4. Scan rate
= 5 mV/s, pulse width = 0.25 s, pulse height = 0.025 V. The asterisk in the top panel shows the oxidation process of ferrocene used as an internal standard.
Table 1. Electrochemical Redox Potentials (V vs Fc/Fc+),
Energy Levels of the Charge-Separated States (ΔGCR), and
Free-Energy Changes for Charge Separation (ΔGSCS) for the
FerroceneÀADP Dyads and Triads in Benzonitrile
compound Fc0/+/V ADP0/•À/V ADP0/•+/V ÀΔGCRa/eV ÀΔGS b/eV
CS
ADP
À0.79
À0.90
À0.89
À0.84
À0.78
À0.84
À0.76
0.80
0.42
0.38
0.52
0.77
0.55
0.81
ADP(OH)2
ADPF2(OH)2
FcÀADP
Fc2ÀADP
FcÀADPF2
Fc2ÀADPF2
0.08
0.07
0.07
0.08
0.98
0.91
0.97
0.91
0.87
0.94
0.88
0.94
ÀΔGCR = e(Eox À Ered) + ΔGS, where ΔGS = Àe2/(4πε0εRRCt-Ct) and
a
ε0 and εR refer to the vacuum permittivity and dielectric constants of
CS
benzonitrile. ÀΔGS = ΔE0À0 À (ÀΔGCR), where ΔE0À0 is the
b
energy of the lowest excited states of ADP being 1.85 eV in benzonitrile.
Figure 3. Spectral changes observed during the first reduction of ADP
in benzonitrile containing 0.10 M (n-Bu4N)ClO4 using a thin-layer
spectroelectrochemical cell. Potential applied was À0.80 V vs Fc/Fc+.
dyads and triads. Interestingly, the kCR values were larger for the
triads compared with the dyads. The figure of merit for charge
stabilization, kSCS/kCR, was also evaluated as given in Table 2. Such
data indicate moderate amounts of charge stabilization in these
systems and better values for the dyads compared to the triads.
Nanosecond transient absorption spectra were recorded to
probe the fate of charge-separated species. As shown in Figures 6
and S10, Supporting Information, nanosecond transient absorp-
tion spectra of pristine ADP, FcÀADP dyad, and Fc2ÀADP triad
revealed a broad band in the 400À550 nm region corresponding
to the triplet excited state of ADP (3ADP*). From these ob-
constants of the triplet ADP (kT) were found to be 1.5 Â 104,
3
2.0 Â 106, and 2.1 Â 107 sÀ1 for the 3ADP*, FcÀ ADP* dyad,
3
3
and Fc2À ADP* triad, respectively. The faster kT of FcÀ ADP*
3
3
and Fc2À ADP* compared to that of ADP* may arise from
quenching of the triplet ADP by the attached Fc entities. It is
obvious that quenching of the triplet ADP increases with
increasing the number of Fc entities.
As depicted in Figure 7, the charge-separation process
1
from the Fc moiety to ADP* takes place quite efficiently,
3
servations, one could see the enhancement of the ADP* for-
mation in the case of dyad and triad compared to pristine ADP.
From the fitting of the transient band at 440 nm, the decay rate
leading to formation of the charge-separated state (Fc+À
ADP•À) in benzonitrile. The formed charge-separated states
decayed with lifetimes of 10À27 ps to populate the ADP
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Figure 4. (a) B3LYP/3-21G(*)-optimized structure and (b) molecular electrostatic potential map of the Fc2ÀADP triad.
Figure 5. Femtosecond transient absorption spectral traces at different time intervals for FcÀADP (left) and Fc2ÀADP (right) in benzonitrile; λex
440 nm. The figure inset shows decay of the transient band corresponding to ADP radical anion.
=
Table 2. Charge-Separation (kSCS) and Charge-Recombina-
tion Rate Constants (kCR) and Singlet Excited States of ADP
for the FerroceneÀADP DonorÀAcceptor Systems in
Benzonitrile
triads whose structural integrity was established using spectro-
scopic, electrochemical, and computational methods. A ‘mole-
cular clip’-type structure for the triads was envisioned from
computational energy minimization calculations involving DFT
methods. The redox potentials and site of electron transfer were
established from electrochemical and spectroelectrochemical
studies. Free-energy calculations revealed charge separation from
the covalently linked ferrocene to the singlet excited ADP to yield
the charge-separated state (Fc+ÀADP•À) to be energetically
favorable. Steady-state emission and femtosecond laser flash
photolysis studies provided tangible evidence for the occurrence
of photoinduced electron transfer in these donorÀacceptor
systems by providing a spectral signature for ADP•À formation
in the near-IR region. The kinetics of the charge separation
and charge recombination evaluated from femtosecond laser
flash photolysis studies revealed fast and efficient charge
separation in these molecular systems. Nanosecond transient
absorption studies revealed the charge-recombination process
to populate the triplet ADP as well as the ADP ground state.
The present study successfully demonstrates utilization of a
near-IR-absorbing sensitizer, boron-chelated tetraarylazadi-
pyrromethane, as a suitable candidate to build new types of
compound
kSCS/sÀ1
kCR/sÀ1
kSCS/kCR
FcÀADP
1.1 Â 1012
1.1 Â 1012
1.7 Â 1012
2.3 Â 1012
6.8 Â 1010
1.1 Â 1011
6.1 Â 1010
1.4 Â 1011
16
10
27
17
Fc2ÀADP
FcÀADPF2
Fc2ÀADPF2
ground state as well as the low-lying ADP triplet state (<1 eV),32
which in turn was quenched by the attached Fc and decayed to its
ground state.
3. CONCLUSIONS
A series of ferroceneÀADP dyads and triads has been newly
synthesized to probe the electron-acceptor behavior of the near-
IR-emitting sensitizer, boron-chelated tetraarylazadipyrromethane
(ADP). The multistep synthesis yielded the desired dyads and
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Figure 6. Nanosecond transient absorption spectra of ADP (left) and FcÀADP (right) in benzonitrile; λex = 475 nm. The figure inset shows decay of
the 440 nm transient band corresponding to 3ADP*.
the product precipitated. Filtration of the reaction mixture gave
a pale white solid product (85% yield, 3.81 g). 1H NMR (300 MHz,
CDCl3): δ = 7.99À8.04 (m, 2H), 7.77À7.84 (m, 1H), 7.60À
7.68, (m, 2H), 7.5À7.58 (m, 1H), 7.39À7.45 (m, 3H), 6.9À6.97
(m, 2H) ppm.
Preparation of 1-(4-Hydroxyphenyl)-3-(4-fluorophenyl)-
propenone (1b). 4-Fluorobenzaldehyde (2.5 g, 2 Â 10À2 mol),
4-hydroxy acetophenone (2.74 g, 2 Â 10À2 mol), and potassium
hydroxide (0.03 g, 6 Â 10À4 mol) were dissolved in ethanol/
water (85:15 v/v, 100 mL) and stirred at room temperature for a
period of 24 h. The reaction mixture was allowed to cool in an
iceÀwater bath during which the product precipitated. Filtration
of the reaction mixture gave a yellow solid product (85%, yield,
4.14 g). 1H NMR (300 MHz, CDCl3): δ = 7.95À8.03 (m, 2H),
7.7À7.8 (m, 1H), 7.58À7.66, (m, 2H), 7.4À7.48 (m, 1H),
7.05À7.14 (m, 2H), 6.87À6.93 (m, 2H) ppm.
Preparation of 1-(4-Hydroxyphenyl)-4-nitro-3-phenylbu-
tan-1-one (2a). 1-(4-Hydroxyphenyl)-3-phenyl propenone (5.0 g,
2.2 Â 10À2 mol), nitromethane (13.61 g, 0.223 mol), and die-
thylamine (8.12 g, 0.111 mol) were dissolved in dry ethanol
(35 mL) and heated under reflux for 24 h. The solution was cooled
Figure 7. Energy-level diagram of photoinduced intramolecular events
of FcÀADP in benzonitrile.
and acidified with 1 M HCl to precipitate the compound (72%
1
yield, 4.58 g). H NMR (300 MHz, CDCl3): δ = 7.82 (d, J =
8.48 Hz, 2H), 7.2À7.34 (m, 5H), 6.8 (d, J = 8.5 Hz, 2H), 4.8À4.83
(m, 1H), 4.62À4.7 (m, 1H), 4.15À4.2 (m, 1H), 3.35À3.4
(m, 2H) ppm.
donorÀacceptor dyads and triads for light-energy-harvesting-
related applications. Further studies along this line are in
progress in our laboratories.
Preparation of 1-(4-Hydroxyphenyl)-4-nitro-3-(4-fluoro-
phenyl)butan-1-one (2b). 1-(4-Hydroxyphenyl)-3-(4-fluoro-
phenyl)propenone (5.0 g, 2.1 Â 10À2 mol), nitromethane
(12.64 g, 0.207 mol), and diethylamine (7.53 g, 0.103 mol) were
dissolved in dry ethanol (35 mL) and heated under reflux for 24
h. The solution was cooled and acidified with 1 M HCl to
4. EXPERIMENTAL SECTION
Chemicals. All reagents were from Aldrich Chemicals
(Milwaukee, WI), while the bulk solvents utilized in the syn-
theses were from Fischer Chemicals. Tetra-n-butylammonium
perchlorate, (n-Bu4N)ClO4, used in electrochemical studies was
from Fluka Chemicals. The initial synthesis of ADP macrocycle
was performed according to the procedure reported by O’Shea
and co-workers12 by needed modifications.
1
precipitate the compound (70% yield, 4.38 g). H NMR (300
MHz, CDCl3): δ = 7.80 (d, J = 8.4 Hz, 2H), 7.2 (m, 2H),
6.95À7.03 (m, 2H), 6.8 (d, J = 8.45 Hz, 2H), 4.75À4.82
(m, 1H), 4.6À4.67 (m, 1H), 4.14À4.21 (m, 1H), 3.3À3.38
(m, 2H) ppm.
Preparation of 1-(4-Hydroxyphenyl)-3-phenylpropenone
(1a). Benzaldehyde (2.1 g, 2 Â 10À2 mol), 4-hydroxy acetophenone
(2.69 g, 2 Â 10À2 mol), and potassium hydroxide (0.03 g, 6 Â
10À4 mol) were dissolved in ethanol/water (85:15 v/v, 100 mL)
and stirred at room temperature for a period of 24 h. The reaction
mixture was allowed to cool in an iceÀwater bath during which
Preparation of [5-(4-Hydroxyphenyl)-3-phenyl-1H-pyrrol-2-
yl]-[5-(4-hydroxyphenyl)-3-phenylpyrrol-2-ylidene]amine (3a).
1-(4-Hydroxyphenyl)-4-nitro-3-phenylbutan-1-one (5 g, 1.8 Â
10À2 mol), ammonium acetate (47.31 g, 0.61 mol), and ethanol
(125 mL) were heated under reflux for 24 h. During the course of the
reaction, the product precipitated as a blue-black solid. The reaction
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was allowed to cool to room temperature, and solid was filtered and
washed with ethanol to give the product (51% yield, 2.16 g). H
acidification of the salt gave 4-ferrocenyl benzoic acid as a red
solid (65% yield, 10.52 g). 1H NMR (300 MHz, CDCl3): δ = 7.9
(d, 2H), 7.2 (d, 2H), 4.7 (s, 2H), 4.3 (s, 2H), 4.02 (s, 5H).
Fc2ÀADP and FcÀADP Triad and Dyad (6 and 7).
4-Ferrocenylbenzoic acid (200 mg, 0.7 mmol) was dissolved in
20 mL of DMF, to which EDCI (125.35 mg, 0.7 mmol) was
added at 0 °C under N2, followed by addition of compound 4a
(115.38 mg, 0.2 mmol), after which the mixture was stirred for
24 h. The mixture was washed with water, the organic layer was
separated and dried over Na2SO4, and solvent was removed
under reduced pressure. The residue was purified by column
chromatography on silica gel with CH2Cl2:hexanes (1:1) to give
compound 6 (6%, yield, 15 mg). 1H NMR (400 MHz, CDCl3):
δ = 8.19À8.06 (m, 12H), 7.6 (d, J = 8.69 Hz, 4H), 7.53À7.43 (m,
6H), 7.4 (d, J = 8.94 Hz, 4H), 7.08 (s, 2H), 4.76 (t, 4H), 4.43 (t,
4H), 4.07 (s, 10H) ppm. MS (MALDI-TOF); C66H46BF2Fe2N3O4
calcd, 1105.22; found, 1105.22. Subsequent elution with CH2Cl2
1
NMR (300 MHz, CDCl3): δ = 8.04 (d, J = 6.62 Hz, 4H), 7.84 (d,
4H), 7.32À7.4 (m, 6H), 7.12 (s, 2H), 6.98 (d, J=8.69Hz, 4H) ppm.
Preparation of [5-(4-Hydroxyphenyl)-3-(4-fluorophenyl)-
1H-pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-(4-fluorophenyl) pyrrol-
2-ylidene]amine (3b). 1-(4-Hydroxyphenyl)-4-nitro-3-(4-fluor-
ophenyl)butan-1-one (5 g, 1.6 Â 10À2 mol), ammonium acetate
(44.50 g, 0.58 mol), and ethanol (125 mL) were heated under
reflux for 24 h. During the course of the reaction, the product
precipitated as a blue-black solid. The reaction was allowed to cool
to room temperature, and solid was filtered and washed with etha-
nol to give the product (51% yield, 2.18 g). 1H NMR (300 MHz,
CDCl3): δ = 7.96À8.0 (m, 4H), 7.76 (d, J = 8.74 Hz, 4H),
7.0À7.08 (m, 6H), 6.92 (d, J = 8.73 Hz, 4H) ppm.
Preparation of BF2 Chelate of [5-(4-Hydroxyphenyl)-3-
phenyl-1H-pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-phenylpyrrol-
2-ylidene]amine (4a). [5-(4-Hydroxyphenyl)-3-phenyl-1H-pyrrol-
2-yl]-[5-(4-hydroxy-phenyl)-3-phenylpyrrol-2-ylidene]amine (1 g,
2.07 mmol) was dissolved in dry CH2Cl2 (100 mL). Diisopropy-
lethylamine (2.71 g, 2.1 Â 10À2 mol) and boron trifluoride diethyl
etherate (4.16 g, 2.9 Â 10À2 mol) were added, and the mixture was
stirred at room temperature under N2 for 24 h. The mixture was
washed with water, and the organic layer was separated, dried over
Na2SO4, and evaporated to dryness. The residue was purified by
column chromatography on silica gel with CH2Cl2/ethyl acetate 4:1
to give metallic red solid (69% yield, 0.76 g). 1H NMR (300 MHz,
CDCl3): δ = 8.1À8.15 (m, 8H), 7.35À7.45 (m, 6H), 7.18 (s, 2H),
6.9À6.93(m, 4H) ppm. MS(MALDI-TOF);m/zC32H22BF2N3O2
calcd, 529.18; found, 529.39.
Preparation of BF2 Chelate of [5-(4-Hydroxyphenyl)-3-(4-
fluorophenyl)-1H-pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-(4-fluoro-
phenyl)pyrrol-2-ylidene]amine (4b). [5-(4-Hydroxyphenyl)-
3-(4-fluoropheny)-1H-pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-(4-
fluorophenyl)pyrrol-2-ylidene]amine (1 g, 1.93 mmol) was
dissolved in dry CH2Cl2 (100 mL). Diisopropylethylamine (2.46 g,
1.9 Â 10À2 mol) and boron trifluoride diethyl etherate (3.83 g,
2.7 Â 10À2 mol) were added, and the mixture was stirred at room
temperature under N2 for 24 h. The mixture was washed with
water, and the organic layer was separated, dried over Na2SO4,
and evaporated to dryness. The residue was purified by column
chromatography on silica gel with CH2Cl2/ethyl acetate 9:1 to
give metallic red solid (48% yield, 0.524 g). 1H NMR (300 MHz,
CDCl3): δ = 7.95À8.0 (m, 8H), 7.02À7.13 (m, 4H), 6.92
(s, 2H), 6.8À6.87 (m, 4H) ppm. MS(MALDI-TOF); m/z
C32H20BF4N3O2 calcd, 565.16; found, 565.45.
Preparation of 4-Ferrocenylbenzoic Acid (5). 4-Amino
benzoic acid (7.0 g, 50 mmol), 80 mL of water, and 12 mL of
concentrated hydrochloric acid was cooled to 0À5 °C in an ice
bath. To this 20 mL of an aqueous solution of sodium nitrite was
added dropwise with stirring; after further stirring it for 30 min
the solution was kept under 5 °C for subsequent use. Ferrocene
(10 g, 50 mmol) was dissolved in 100 mL of ethyl ether, 0.5 g of
hexadecyltrimethylammonium bromide was added, and the mix-
ture was cooled to 0À5 °C under stirring. To this the diazonium
salt solution was added dropwise with stirring. The solution was
then allowed to react for 2 h at room temperature. The ethyl
ether was then rotary evaporated to give red solid. The solid was
dissolved in 500 mL of water containing 5 g of NaOH at 90 °C
and filtered while it was hot. The solid recovered was unreacted
ferrocene. The filtrate was cooled, and the sodium salt of
4-ferrocenyl benzoic acid was precipitated. Filtration and
1
gave compound 7 (21%, yield 38 mg). H NMR (400 MHz,
CDCl3): δ = 8.17À8.03 (m, 10H), 7.52 (d, J = 8.6 Hz, 2H),
7.51À7.41 (m, 6H), 7.38 (d, J = 8.8 Hz, 2H), 7.1 (s, 1H), 6.97 (d
J = 8.85 Hz, 3H), 4.78À4.75 (m, 2H), 4.45À4.42 (m, 2H), 4.07
(s, 5H) ppm. MS(MALDI-TOF); m/z: C49H34BF2FeN3O3 calcd,
816.93; found, 817.20.
Fc2ÀADPF2 and FcÀADPF2 Triad and Dyad (8 and 9).
4-Ferrocenylbenzoic acid (200 mg, 0.7 mmol) was dissolved in
20 mL of DMF, to which EDCI (125.35 mg, 0.7 mmol) was
added at 0 °C under N2, followed by addition of compound 4b
(123.23 mg, 0.2 mmol), after which the mixture was stirred for
24 h. The mixture was washed with water, the organic layer was
separated and dried over Na2SO4, and solvent was removed
under reduced pressure. The residue was purified by column
chromatography on silica gel with CH2Cl2:hexanes (1:1) to give
compound 8 (4% yield, 11 mg). 1H NMR (400 MHz, CDCl3): δ
= 8.18À8.09 (m, 8H), 8.08À7.99 (m, 4H), 7.6 (d, J = 8.63, 4H),
7.4 (d, J = 8.86, 4H), 7.18 (m, 4H), 7.03 (s, 2H), 4.76 (t, 4H),
4.43 (t, 4H), 4.07 (s, 10H) ppm. MS (MALDI-TOF);
C66H44BF4Fe2N3O4 calcd, 1141.21; found, 1141.00. Subsequent
elution with CH2Cl2 yielded compound 9 (19% yield, 35 mg).
1H NMR (400 MHz, CDCl3): δ = 8.16À7.98 (m, 10H), 7.6 (d,
J = 8.61 Hz, 2H), 7.38 (d, J = 8.87 Hz, 2H), 7.20À7.13 (m, 4H),
7.05 (s, 1H), 6.99À6.93 (m, 3H), 4.79À4.74 (m, 2H),
4.46À4.42 (m, 2H), 4.07 (s, 5H) ppm. MS(MALDI-TOF);
m/z C49H32BF4FeN3O3 calcd, 853.18; found, 852.70.
Spectral Measurements. UVÀvis spectral measurements
were carried out with a Shimadzu model 2550 double mono-
chromator UVÀvisible spectrophotometer. The fluorescence
emission was monitored using a Varian Eclipse spectrometer.
A right angle detection method was used. 1H NMR studies were
carried out on a Varian 400 MHz spectrometer. Tetramethylsi-
lane (TMS) was used as an internal standard. Differential pulse
voltammograms were recorded on an EG&G PARSTAT elec-
trochemical analyzer using a three-electrode system. A platinum
button electrode was used as the working electrode. A platinum
wire served as the counter electrode, and a Ag/AgCl electrode
was used as the reference electrode. The ferrocene/ferrocenium
redox couple was used as an internal standard. A homemade
thin-layer spectroelectrochemical cell was used to record the
spectral characteristics of reduced ADP. All solutions were
purged prior to electrochemical and spectral measurements using
argon gas. Matrix-assisted laser desorption/ionization time-of-
flight mass spectra (MALDI-TOF) were measured on a Kratos
Compact MALDI I (Shimadzu) for metal complex in PhCN with
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dithranol used as a matrix. Computational calculations were
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continuous xenon lamp (150 W) and an InGaAs-PIN photo-
diode (Hamamatsu 2949) as a probe light and a detector,
respectively. The output from the photodiodes and a photo-
multiplier tube was recorded with a digitizing oscilloscope
(Tektronix, TDS3032, 300 MHz). Femtosecond transient ab-
sorption spectroscopy experiments were conducted using an
ultrafast source, Integra-C (Quantronix Corp.), an optical para-
metric amplifier, TOPAS (Light Conversion Ltd.), and a com-
mercially available optical detection system, Helios provided by
Ultrafast Systems LLC. The source for the pump and probe
pulses were derived from the fundamental output of Integra-C
(780 nm, 2 mJ/pulse and fwhm =130 fs) at a repetition rate of
1 kHz. Seventy five percent of the fundamental output of the
laser was introduced into TOPAS, which has optical frequency
mixers resulting in tunable range from 285 to 1660 nm, while the
rest of the output was used for white light generation. Typically,
2500 excitation pulses were averaged for 5 s to obtain the
transient spectrum at a set delay time. Kinetic traces at appro-
priate wavelengths were assembled from the time-resolved spec-
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spectra were recorded using fresh solutions in each laser excitation.
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’ ASSOCIATED CONTENT
S
Supporting Information. MALDIi-mass spectra of the
b
investigated compounds, transient absorption spectrum of ADP
in benzonitrile, and absorption spectra of ADP•À. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: Francis.DSouza@UNT.edu (F.D.); Fukuzumi@chem.
eng.osaka.u.ac.jp (S.F.).
’ ACKNOWLEDGMENT
This work was supported by the National Science Foundation
(Grant No. 0804015 to F.D.), Flossie West Foundation, a
Grant-in-Aid (Nos. 20108010 and 21750146), and the Global
COE (center of excellence) program “Global Education and
Research Center for Bio-Environmental Chemistry” of Osaka
University from Ministry of Education, Culture, Sports, Science
and Technology, Japan, KOSEF/MEST through WCU project
(R31-2008-000-10010-0) from Korea.
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