Charge stabilization in a closely spaced ferrocene-boron dipyrrin-fullerene triad

Article abstract of DOI:10.1039/c000565g

New molecular triads composed of closely spaced ferrocene-boron dipyrrin-fullerene, 1 and triphenylamine-boron dipyrrin-fullerene, 2 are synthesized, and photoinduced electron transfer leading to charge stabilization is demonstrated using a femtosecond transient spectroscopic technique.

Full text of DOI:10.1039/c000565g

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Charge stabilization in a closely spaced ferrocene–boron
dipyrrin–fullerene triadw
Channa A. Wijesinghe,a Mohamed E. El-Khouly,b James D. Blakemore,a Melvin E. Zandler,a
Shunichi Fukuzumi*bc and Francis D’Souza*a
Received (in Cambridge, UK) 11th January 2010, Accepted 16th February 2010
First published as an Advance Article on the web 24th March 2010
DOI: 10.1039/c000565g
New molecular triads composed of closely spaced ferrocene–
boron dipyrrin–fullerene, 1 and triphenylamine–boron dipyrrin–
fullerene, 2 are synthesized, and photoinduced electron transfer
leading to charge stabilization is demonstrated using a femto-
second transient spectroscopic technique.
Photoinduced electron transfer is one of the most fundamental
and extensively investigated reactions.1,2 Its importance in
photosynthesis and in the development of energy harvesting
devices3 has inspired the design and study of several elegant
model systems that made possible to probe the often complex
multistep electron-transfer processes.1–3 The boron dipyrrin,
also known as BODIPY, has frequently been used as an
energy absorbing and transferring antenna molecule in the
photosynthetic antenna-reaction center mimic as well as
a photosensitizer in chemosensor and fluorescent tags.4,5
BODIPY can also acts as both an electron donor and an
acceptor.6 However, BODIPY has yet to be utilized to
construct photosynthetic reaction center models that undergo
multi-step photoinduced electron-transfer processes.
Chart 1 Structures of the ferrocene–boron dipyrrin–fullerene, 1 and
triphenylamine–boron dipyrrin–fullerene 2 triads investigated in the
present study. Compounds 1a, 2a and 1b are control compounds.
N-methylglycine in toluene8 (see ESIw for synthetic and
experimental details). The newly synthesized compounds were
fully characterized by 1H and 13C NMR, mass and other
spectroscopic methods (see Fig. S1 and S2, ESI,wfor 1H
NMR spectra).
We report herein two new type of triads in which the
BODIPY is either directly linked or having a smaller spacer
group with electron donor and acceptor. The structures of
these triads are shown in Chart 1. The first triad, 1 is
composed of ferrocene–boron dipyrrin–fullerene while the
second one, 2 is made out of triphenylamine–boron dipyrrin–
fullerene. As demonstrated here, femtosecond transient
absorption studies reveal occurrence of sequential electron
transfer in triad 1 ultimately resulting in charge stabilization.
The syntheses of the triads, 1 and 2 are accomplished, first,
by synthesizing meso-phenylferrocene or meso-triphenylamine
substituted difluoroboron dipyrrin dyads. The difluoroboron
dipyrrin was subsequently converted into dioxyboron dipyrrin
derivatives 1a and 1b by treating with dihydroxy benzaldehyde
in the presence of AlCl3 in dry CH2Cl2.7 Finally, the electron
acceptor was appended by reacting with fullerene and
Fig. 1(a) and (b) show the optical absorption spectra of the
triads 1 and 2 along with their dyad analogs, 1a and 2a having
no fullerene entity. The ferrocene entity in 1a shows a band at
326 nm while the triphenylamine entity in 2a revealed an
absorption band around 311 nm. The boron dipyrrin entity
in both dyads revealed aborption maximum at 504 nm. Upon
appending fullerene to form the triads, the band maxima
a Department of Chemistry, Wichita State University, 1845,
Fairmount, Wichita, KS 67260, USA.
E-mail: Francis.DSouza@Wichita.edu; Tel: 316-978-7380
b Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871, Japan. E-mail: Fukuzumi@chem.eng.osaka.u.ac.jp;
Tel: +81-6-6879-7368
c Department of Bioinspired Science, Ewha Womans University,
120-750, Seoul, Korea
w Electronic supplementary information (ESI) available: Synthetic
and experimental details, femtosecond and nanosecond transient
spectra of the dyads, 1a and 1b, nanosecond transient spectra of the
triads, 1 and 2 in PhCN. See DOI: 10.1039/c000565g
Fig. 1 Optical absorption and fluorescence emission of 1 and 2 and
the reference compounds in PhCN.
ꢀc
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corresponding to the boron dipyrrin revealed a small red shift
of 5 nm, suggesting intramolecular interactions. The 433 nm
sharp band of fulleropyrrolidine was also evident for both
triads. The dyads, 1a and 2a, revealed emission of the
boron dipyrrin entity at 519 nm, however, with substantially
diminished intensity as compared with the control compound,
1c suggesting occurrence of intramolecular events in the dyads.
Interestingly, the fluorescence emission of boron dipyrrin in
the triads was virtually absent (o1%) indicating additional
photochemical events.
oxidation around 0.74 V vs. Fc/Fc+. Free-energy calculations
for different charge-separated (CS) states were performed
according to Weller approach.10 Such calculations revealed
energy levels for Fc –BDPꢁÀ–C60 and Fc+–BDP–C60 À to be
+
ꢁ
1.60 and 1.10 eV, respectively. Similar caÀlculations for 2
yielded energy levels for TPA–BDPꢁ+–C60 to be 1.77 eV
ꢁ
higher than that of TPA–BDP–3C60* (1.53 eV) (see Fig. S5
and S8 in ESIw for energy level diagrams). That is, occurrence
of sequential electron transfer resulting in distant CS state in
the case of 1 and population of low lying 3C60* in the case of 2
was visualized from these studies.
The structure of the triads were visualized by performing
computational calculations at the B3LYP/6-31G* level.9 Both
triads revealed stable structures on the Born–Oppenheimer
potential energy surface (Fig. 2). For 1, the center-to-center
distances between ferrocene iron to the boron atom, and
boron to center of fullerene were found to be 10.28 and
10.36 A, respectively. Similarly, for 2, the center-to-center
distances between triphenylamine nitrogen to boron atom of
boron dipyrrin, and boron to center of fullerene were found to
be 8.77 and 10.23 A, respectively. Additionally, the molecular
size, measured between the farthest C-atoms of the triads, were
found to be 25.5 and 24.9 A, respectively, for 1 and 2
indicating smaller size for the present triads compared to most
of the triads with fullerene as electron acceptor in literature.1,2
Differential pulse voltammetry (DPV) experiments (Fig. 3)
were performed to evaluate the redox potentials. The site of
electron transfer corresponding to different entities was
arrived by performing experiments involving the control
compounds, the dyads and the monomers. For both 1 and 2,
the first reduction corresponding to fullerene was located at
À1.03 V vs. Fc/Fc+. Further reductions at À1.43, À1.61 and
À2.05 V were also observed corresponding to the reductions of
fullerene and boron dipyrrin entities. The oxidation corres-
ponding to ferrocene in 1 was located at 0.05 V while
that corresponding to triphenylamine in 2 was located at
0.61 V vs. Fc/Fc+. Boron dipyrrin in 1 and 2, revealed
Femtosecond and nanosecond transient spectral measure-
ments were performed to identify the electron-transfer
products, mechanistic details of charge migration, and kinetics
of CS and CR processes. The femtosecond measurements of
dyad 1a in PhCN by using 460 nm radiation, which selectively
excites the BDP entity, showed absorption band at 590
corresponding to the boron dipyrrin radical anion (Fig. 4).11
The ferrocenium cation could not be observed due to its
extremely low extinction coefficient (e). These observations
suggest occurrence of electron transfer from the Fc to the
+
attached BDP forming Fcꢁ –BDPꢁÀ. Additionally, the nano-
second transient spectra of dyad 1a in deaerated PhCN did not
show the absorption banÀds of triplet BDP. Because the energy
+
level of the Fcꢁ –BDPꢁ (1.60 eV) is comparable to that of
the triplet BDP but substantially lower than the anticipated
low-lying triplet Fc (see Fig. S3 in ESIw for energy diagram),
one could expect the CR to the triplet states of Fc entity.12
Upon photoirradiation of the BDP entity of the triad 1 in
deaerated PhCN, the characteristic absorption bands of the
À
BDPꢁ were observed at early time scale (B1 ps) (FigÀ. 4).
At longer time scale, the band corresponding to BDPꢁ at
590 nm decreased in intensity with concurrent build-up of the
À
absorption band of CÀ60 at 1000 nm13 suggesting electron-
ꢁ
shift from the BDPꢁ to the attached C60 entity forming
À
Fcꢁ+–BDP–C60 as final CS state (see Fig. S4 in ESIw for
ꢁ
decayÀcurves). The rate constant of the charge-shift from
BDPꢁ to C60 was determined to be 3.8 Â 1011
s
À1. By
following the decay of the Fcꢁ+–BDP–C60 in PhCN, the
CR rate constant was determined to be 2.4 Â 109 sÀ1 from
which a lifetime of 416 ps for the radical ion-Àpair was derived.
À
ꢁ
The finding that the tRIP of Fc+–BDP–C60 is much longer
ꢁ
+
than that of Fcꢁ –BDPꢁÀ (kCR = 5.8 Â 1010
s
À1; tRIP = 17 ps)
reveals the effect of the electron shift mechanism in prolonging
the tRIP in the triad 1. The photochemical events are
summarised as in Fig. S5 (ESIw). Excitation of the central
boron dipyrrin of the triad 1 results in abstraction of an
Fig. 2 Space filling models of the B3LYP/6-31G* optimized structures
of (a) 1 and (b) 2 triads.
Fig. 3 Differential pulse voltammograms (DPV) of (i) 1 and (ii) 2 in
PhCN, 0.1 M (TBA)ClO4. Scan rate = 5 mV sÀ1, pulse width = 0.25 s,
pulse height = 0.025 V.
Fig. 4 Femtosecond spectra of Fc–BDP (left) and Fc–BDP–C60
(right) in deaerated PhCN. lex = 460 nm.
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In summary, charge stabilization in the donor–acceptor1–
acceptor2 type molecular triad composed of closely spaced
ferrocene–boron dipyrrin-fullerene, but not in triphenylamine-
boron dipyrrin–fullerene is reported. Excitation of the central
boron dipyrrin of 1 results in abstraction of an electron from
the ferrocene entity to produce boron dipyrrin anion radical
which subsequently undergoes electron shift to produce
fullerene anion radical. As a result of relatively distant positioning
of the cation and anion radicals charge stabilization is
accomplished in the ferrocene–boron dipyrrin–fullerene triad
in spite of closely disposed donor and acceptor entities.
We are thankful to N. K. Subbaiyan for help in recording DPV
experiments. This work was financially supported by the National
Science Foundation (Grant Nos. 0804015 and EPS-0903806)
and matching support from the State of Kansas through
Kansas Technology Enterprise Corporation, Grant-in-Aid (No.
20108010), 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.
Fig. 5 Femtosecond (left) and nanosecond (right) transient absorption
spectra of TPA–BDP in deaerated PhCN.
electron from the ferrocene entity to produce boron dipyrrin
anion radical which subsequently undergoes electron shift to
produce fullerene anion radical. As
a
result of
relatively distant positioning of the cation and anion radicals
charge stabilization is accomplished in the ferrocene–boron
dipyrrin–fullerene triad in spite of closely disposed donor and
acceptor entities.
The femtosecond measurements of the dyad, 2a by using
460 nm light in PhCN showed absorption bands at 590 and
690 nm corresponding to the boron dipyrrin radical anion and
TPA radical cation, respectively (Fig. 5).11 This observation
suggests the electron transfer from the TPA to the attached
Notes and references
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P. Comte, T. Torres, J. M. J. Frechet, M. K. Nazeeruddin,
+
BDP formingÀ TPAꢁ –BDPꢁÀ. Interestingly, the kCR of
TPAꢁ –BDPꢁ was found to be 6.1 Â 108 sÀ1, from which
+
t
RIP was evaluated as 1.6 ns. The complementary nanosecond
transient measurements with 510 nm laser excitation revealed
absorption bands at 430 and 600 nm, in addition to the
bleaching at 510 nm. These long-lived absorption bands are
attributed to the triplet state of BDP. Because the formation of
the triplet BDP is not efficient in the case of the reference 1b,
the formation of triplet BDP may occur via CR of the radical-
ion-pair of 2a by taking into account the energy level of
+
À
3TPA*–BDP (1.58 eV) is lower than that of TPAꢁ –BDPꢁ
M. Gratzel and M. D. McGehee, Nat. Photonics, 2009, 3, 667;
¨
(2.00 eV) (see Fig. S6 in ESIw).
Photoirradiation of the triad 2 in a deaerated PhCN
(d) T. M. Figueira-Duarte, A. Gegout and J.-F. Nierengarten,
Chem. Commun., 2007, 109.
À
ꢁ
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solution revealed the characteristic absorption band of C60
in the NIR region with a maximum at 1000 nm (see Fig. S7 in
+
ESIw).2,13 The absorption band of TPAꢁ waÀs not observed,
ꢁ
suggesting that the formation of the C60
results from
electron transfer from the singlet excited BDP, as electron
donor, to the C60. Such electron transfer is exothermic
(ÀDGCR = 1.77 eV and ÀDGCS = 0.45 eV). The kCR value
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of the TPA–BDPꢁ+–C60 À was determined to be 2.0 Â 109 sÀ1
,
ꢁ
from which the tRIP was evaluated as 500 ps. The finding that
the tRIP of triad 2 is shorter than that of dyad 2a suggests the
absence of the electron-shift and/or hole-migration processes
between the TPA, BDP and C60 entities of 2, by taking intÀo
consideration the energy level of TPA–BDPꢁ+–C60
(1.77 eV) is comparable to that of TPAꢁ+–BDP–C60
ꢁ
ꢁ
9 Gaussian 03, Gaussian Inc., Pittsburgh, PA, 2003.
À
10 ÀDGCR = e(Eox – Ered) + DGS; ÀDGCS = DE00 À (ÀDGCR),
where DE00 and DGS are the energy of 1BDP* and static energy,
respectively.
11 Electronic Absorption Spectra of Radical Ions, ed. T. Shida,
Elsevier, Amsterdam, 1988.
12 T. M. Figueira-Duarte, Y. Rio, A. Listorti, B. Delavaux-Nicot,
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13 Y. Araki, Y. Ysumura and O. Ito, J. Phys. Chem. B, 2005, 109, 9843.
14 C. S. Foote, in Physics and Chemistry of the Fullerenes, ed. K.
Prassides, Kluwer Academic Publishers, Amsterdam, 1994, p. 79.
(1.64 eV) (see Fig. S8 in ESIw). At longer time scale (1800 ps),
the spectrum revealed the characteristic absorption of the
triplet C60 suggesting that the radical-ion pair decays through
CR to populate the triplet C60. This was also the conclusion
based on nanosecond transient absorption measurements
(see Fig. S9 in ESIw). The spectra in this time scale lacked
bands of the radical ion-pairs instead an absorption band at
3
700 nm corresponding to the C60* was observed.14
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 3301–3303 | 3303