A novel BF2-chelated azadipyrromethene-fullerene dyad: Synthesis, electrochemistry and photodynamics

Article abstract of DOI:10.1039/c1cc16071k

The synthesis, structure, electrochemistry and photodynamics of a BF ₂-chelated azadipyrromethene-fullerene dyad are reported in comparison with BF₂-chelated azadipyrromethene without fullerene. The attachment of fullerene resulted i

Full text of DOI:10.1039/c1cc16071k

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COMMUNICATION
A novel BF2-chelated azadipyrromethene–fullerene dyad: synthesis,
electrochemistry and photodynamicswz
Anu N. Amin,a Mohamed E. El-Khouly,c Navaneetha K. Subbaiyan,b Melvin E. Zandler,a
Shunichi Fukuzumi*cd and Francis D’Souza*ab
Received 29th September 2011, Accepted 3rd November 2011
DOI: 10.1039/c1cc16071k
The synthesis, structure, electrochemistry and photodynamics of
a BF2-chelated azadipyrromethene–fullerene dyad are reported
in comparison with BF2-chelated azadipyrromethene without
fullerene. The attachment of fullerene resulted in efficient
generation of the triplet excited state of the azadipyrromethene
via photoinduced electron transfer.
Study of photoinduced energy and electron transfer in novel
donor–acceptor dyads and polyads (having multiple number
of donor and acceptor entities) is of paramount importance
due to their significance in mimicking the natural photosynthesis,
and also for technological advances in solar energy conversion, a
promising solution for global energy demands and environmental
issues.1–3 Consequently, various donor–acceptor architectures
have been constructed and light induced electron transfer
events have been probed. In most cases, tetrapyrroles such
as porphyrin and phthalocyanine have been commonly employed
as donors due to their close resemblance to the natural chlorophyll
pigment.1–3 In a few selected cases, donors other than tetra-
pyrroles but structurally related have been employed and novel
features that are otherwise not attainable using normal tetra-
pyrroles have been accomplished.4,5 Thus, exploration of novel
donors with relatively different spectral and electrochemical prop-
erties is highly attractive since the constructed new dyads could
exhibit novel photochemical properties useful for energy harvest-
ing and optoelectronic applications.
Difluoroboron chelated dipyrromethene, 1 (also known as
BODIPY or BDP), a bidentate Lewis base, is one of the com-
monly used fluorophores in developing sensors and tags to detect
biomolecules owing to its superior fluorescent properties that
include lifetimes of B4 ns and quantum yields exceeding 80%.6
BDP has also been used in building photosynthetic antenna and
reaction center mimics.1–3 Interestingly, BF2-chelated azadipyrro-
methene, 2 (abbreviated as ADP), a structural analog of BDP, has
recently gained much attention for its photochemical properties.7
ADP absorbs near 300 and 600 nm regions with very high molar
absorptivities, that is, at spectral regions of solar energy capture
and photodynamic therapy interests. Like BDP, ADP is also
fluorescent, however with desirable emission in the red region
(660–700 nm) with quantum yields exceeding 40%. However,
usage of ADP in building donor–acceptor dyads has been limited
due to the associated synthetic challenges.8 In the present study,
we report a novel dyad featuring ADP covalently linked to the
well-known electron acceptor, [60]fullerene, (abbreviated as
ADP–C60) and report photoinduced electron transfer originating
in this dyad.
a Department of Chemistry, Wichita State University, Wichita,
KS 67260, USA
b Department of Chemistry, University of North Texas, 1155 Union
Circle, #305070, Denton, TX 76203, USA.
E-mail: Francis.DSouza@unt.edu; Tel: +1-940-369-8832
c Department of Material and Life Science, Graduate School of
Engineering, Osaka University, ALCA, JST, Suita, Osaka 565-0871,
Japan. E-mail: Fukuzumi@chem.eng.osaka-u.ac.jp;
Tel: +81-6-6879-7368
d Department of Bioinspired Science, Ewha Woman’s University,
Seoul, 120-750, Korea
w This article is part of the ChemComm ‘Artificial photosynthesis’ web
themed issue.
z Electronic supplementary information (ESI) available: Synthetic and
experimental details along with mass and NMR spectra, femtosecond
and nanosecond transient spectra of the control ADP compound in
PhCN. See DOI: 10.1039/c1cc16071k
The synthesis of the ADP–C60 dyad involved
a
multi-step procedure. First, the BF2 chelate of (3,5-
diphenyl-1H-pyrrol-2-yl)(3,5-diphenylpyrrol-2-ylidene)-amine was
c
206 Chem. Commun., 2012, 48, 206–208
This journal is The Royal Society of Chemistry 2012

Fig. 2 (a) Space filling model and (b) molecular electrostatic
potential map of the ADP–C60 dyad calculated at the B3LYP/
6-31G* level. The frontier HOMO ꢁ 1, HOMO, LUMO and LUMO + 1
of the dyad are also shown.
Fig. 1 (a) Optical absorbance and (b) fluorescence emission spectra
of (i) ADP–C60 dyad and (ii) ADP-control in benzonitrile.
synthesized by literature methods.7a This compound was further
reacted with 3,4-dihydroxybenzaldehyde in the presence of AlCl3 to
obtain the formyl phenyl dioxyboron derivative. Finally, the
electron acceptor was appended by reacting it with fullerene and
N-methylglycine in toluene (see ESIz for synthetic and experi-
mental details along with a scheme).9 The newly synthesized
compounds were fully characterized by NMR, mass and other
spectroscopic methods.
Fig. 1a and b show the optical absorption and emission
spectra of the dyad and the control ADP having no fullerene
entity in benzonitrile. The ADP peaks in the control compound
were located at 313, 482 and 632 nm, respectively. Upon
appending fullerene these bands were shifted to 312, 479 and
640 nm, respectively. The band at 312 nm was much higher in
intensity due to overlap of the fullerene absorption band.
A sharp peak at 434 nm characteristic of fulleropyrrolidine was
also observed for the dyad. The 8 nm red shift of the 632 nm
peak of ADP can be attributed to the electronic interactions
between the donor and acceptor entities in the dyad. As shown in
Fig. 1b, the ADP control exhibited an emission band at 682 nm,
over 150 nm red shifted to the structural analog, BDP. However,
in the ADP–C60 dyad, intensity of this band was found to be
quenched over 98% indicating very efficient photochemical
events. Energy transfer as a quenching mechanism could be
ruled out due to the absence of spectral overlap between ADP
emission and C60 absorption. The quenching of 1ADP*-emission
in the ADP–C60 dyad suggests the formation of either
ADP ꢁ–C60 + or ADP +–C60 charge-separation products.
The structure of the dyad was visualized by performing com-
putational calculations at the B3LYP/6-31G* level.10 Fig. 2a and b
show optimized structure on the Born–Oppenheimer potential
energy surface and the molecular electrostatic potential map of the
dyad. The center-to-center distance between the boron atom and
the center of the fullerene was found to be B10 A, indicating close
positioning of the donor and acceptor entities. The HOMOs were
found to be on the ADP part of the dyad, while the LUMO
and LUMO + 1 were on the ADP and the fullerene entities,
respectively. The gas phase HOMO–LUMO gap was found to
be 1.84 eV, smaller than that reported for porphyrin–fullerene
dyads.2,3 The presence of LUMO on the ADP suggests that it is
electron deficient. To confirm this computational prediction,
electrochemical studies were performed to evaluate the redox
potentials of the ADP and C60 entities of the dyad.
Fig. 3 Cyclic voltammograms of (a) ADP–C60 dyad and (b) ADP-control
compound in benzonitrile, 0.1 M (TBA)ClO4. Scan rate = 100 mV sꢁ1
.
The peak at 0.0 V in (a) represents ferrocene oxidation used as an internal
standard.
The cyclic voltammogram of the dyad revealed reduction
peaks at E1/2 = ꢁ0.81, ꢁ1.06 and ꢁ1.49 V vs. Fc/Fc+, while
irreversible oxidation peaks at Epa = 0.76 and 0.94 V vs. Fc/Fc+
(Fig. 3a) were also observed. By comparison with the ADP
control (Fig. 3b), the first reversible reduction has been attributed
to the reduction of ADP macrocycle, while the second reduction
to the reduction of fullerene spheroid. The third reduction wave
is an overlap of the second reduction of ADP and fullerene
entities. Importantly, the easy reduction of ADP compared to
fullerene is evident from these studies. Based on the redox
ꢀ
ꢀ
ꢀ
ꢀꢁ
ꢀ
ꢀ
+
potential measurements, the formation of ADP ꢁ–C60 (B2 eV)
is excluded because of its higher energy compared with the
1
1ADP* (1.91 eV) and C60* (1.75 eV) excited states.11
The photodynamics of ADP–C60 dyad via the singlet excited
ADP was examined by femtosecond transient measurements
by using 480 nm excitation, as shown in Fig. 4. The transient
spectrum of the ADP control following excitation with 130 fs
flash at 460 nm in benzonitrile showed the absorption of the
singlet ADP in the visible region with a maximum at 520 nm
and a decay rate constant of 7.8 ꢂ 108 sꢁ1 (see ESIz for the
spectra). In contrast, the transient spectrum of ADP–C60 dyad
exhꢀibited the NIR absorption at 1000 nm, which is diagnostic of
C60 ꢁ, with a rate of formation of 1.0 ꢂ 1012
s
ꢁ1, followed by a
decay rate constant of 5.0 ꢂ 109 sꢁ1 (lifetime: 200 ps). This
indicates that ultrafast electron transfer from 1ADP* to C60
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 206–208 207

absorption studies. Excitation of the ADP entity of the dyad
resulted in the electron transfer from the singlet excited ADP
ꢀ
to the attached C60 forming ADPꢀ+–C60 ꢁ. Production of the
triplet ADP state via charge recombination of ADP–C60 is the
likely pathway, taking into consideration that the energy of
ꢀ
ADPꢀ+–C60 ꢁ is significantly higher than the triplet ADP. The
present study opens up a venue for utilization of difluoroborane
chelated azadipyrromethene in light energy harvesting donor–
acceptor systems.
Support by the National Science Foundation (Grant No. CHE-
0804015 and CHE-1110942 to FD), Grant-in-Aid (No. 20108010),
and the Global COE (center of excellence) program of Osaka
University from MEXT, Japan, and NRF/MEST of Korea
through WCU (R31-2008-000-10010-0) and GRL (2010-00353)
programs is acknowledged.
Fig. 4 Femtosecond transient absorption spectra of the ADP–C60
dyad in deaerated benzonitrile. lex = 480 nm.
Notes and references
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transfer (0.07 eV) and desirable short distance between the ADP
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ꢀ
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state with a rate of 1.2 ꢂ 104 sꢁ1, slightly faster than the ADP
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Fig. 5 Nanosecond transient absorption spectra of the ADP–C60
dyad in deaerated benzonitrile. lex = 460 nm.
c
208 Chem. Commun., 2012, 48, 206–208
This journal is The Royal Society of Chemistry 2012