Thieno-pyrrole-fused 4,4-difluoro-4-bora-3a,4a-diaza- s -indacene-fullerene dyads: Utilization of near-infrared sensitizers for ultrafast charge separation in donor-acceptor systems

Article abstract of DOI:10.1021/ja503015f

Donor-acceptor dyads featuring near-IR sensitizers derived from thieno-pyrrole-fused BODIPY (abbreviated as SBDPiR) and fullerene, C ₆₀ have been newly synthesized and characterized. Occurrence of ultrafast photoinduced electron transfer (PET)

Full text of DOI:10.1021/ja503015f

Communication
pubs.acs.org/JACS
Thieno-Pyrrole-Fused 4,4-Difluoro-4-bora-3a,4a-diaza‑s‑indacene−
Fullerene Dyads: Utilization of Near-Infrared Sensitizers for Ultrafast
Charge Separation in Donor−Acceptor Systems
Venugopal Bandi,† Sushanta K. Das,† Samuel G. Awuah,‡ Youngjae You,*,‡ and Francis D’Souza*,†
†Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States
‡Department of Pharmaceutical Sciences, University of Oklahoma Health Sciences Center, 1110 N. Stonewall Avenue, Oklahoma
City, Oklahoma 73117, United States
S
* Supporting Information
BODIPY to construct novel donor−acceptor dyads involving
ABSTRACT: Donor−acceptor dyads featuring near-IR
sensitizers derived from thieno-pyrrole-fused BODIPY
(abbreviated as SBDPiR) and fullerene, C60 have been
newly synthesized and characterized. Occurrence of
ultrafast photoinduced electron transfer (PET) leading to
the formation of charge-separated state in these dyads,
capable of harvesting light energy from the near-IR region,
is established from femtosecond transient absorption
studies.
fullerene, C60, as an electron acceptor. As shown here, the
newly constructed dyads undergo ultrafast PET leading into
charge separation.
The structures of the near-IR sensitizer probes along with the
dyads are shown in Chart 1, while the synthetic details are given
Chart 1. Structure of the Newly Synthesized SBDPiR-C60
a
Dyads (SBDPiR = Thieno-Pyrrole-Fused BODIPY)
he majority of the photosensitizers used in building
Tdonor−acceptor hybrids for light-to-electricity and light-
to-fuel conversion, so-called artificial photosynthesis, either lack
absorption or reveal inefficient electron transfer efficiency in the
red and the near-infrared (near-IR) regions of the electro-
magnetic spectrum1−11 (except for a few π-expanded
phthalocyanine derivatives8−10 and squaraine com-
pounds10−13), which represents more than 70% of the solar
spectrum. The primary reason for this limitation is in molecular
engineering of such sensitizers in regulating their HOMO−
LUMO gap such that they can harvest most of the photons
from the near-IR regions, including part of the red region.
Primary criterion includes maintaining the LUMO of the
sensitizer above the LUMO level of the acceptor or the
acceptor HOMO level above the donor HOMO level to furnish
thermodynamically efficient excited state electron transfer.10
Additionally, near-IR sensitizer development often requires
tedious multistep synthesis, low reaction yields, reactive
intermediates, poor solubility, poor photostability, and limited
functionalization options.
The 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) is
a very well-known molecule for its synthetic advantages in
addition to its photostability, sharp absorption and emission,
high molar absorptivity properties, and facile tuning of excited
states.14 Chemical modifications to the dipyrromethene core as
well as functionalization to the boron center and the meso
positions make BODIPYs adaptable for numerous applications,
including protein-labeling fluorophores,15 photosensitizers for
photodynamic therapy (PDT),16,17 and development of
artificial photosynthetic model compounds.18−21 In the present
study, we have utilized the BODIPY molecular framework to
develop near-IR absorbing and emitting thieno-pyrrole-fused
a
The abbreviations 690, 731, and 840 refer to the absorption
wavelength maxima of the near-IR sensitizers.
in the Supporting Information. The synthesis involved (i)
building a reactive brominated BODIPY electrophile having
absorption in the visible region (635 nm) as an intermediate,20
(ii) functionalization of the reactive BODIPY intermediate via
transition metal-catalyzed coupling and nucleophilic substitu-
tion reactions,22 (iii) converting the BF2-chelated macrocycle to
formyl phenyl dioxyboron derivative by reacting it with 3,4-
dihydroxybenzaldehyde in the presence of AlCl3,23,24 and (iv)
covalent linking of fullerene by reacting the formyl derivatives
with fullerene and N-methylglycine (Scheme 1).25 The newly
synthesized dyads were fully characterized by spectral, mass,
and electrochemical methods (see Figures S1−S5 in the
Supporting Information).
Received: March 25, 2014
© XXXX American Chemical Society
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Communication
benzonitrile (Figure 1b). Similarly, SBDPiR 731 revealed a
band at 770 nm with a shoulder band at 829 nm.
Scheme 1. Synthetic Scheme of SBDPiR-Fullerene Dyads
The fluorescence lifetime of the SBDPiR 690 and SBDPiR
731 sensitizers evaluated from time-correlated singlet photon
counting was found to be 1.55 ( 0.02) ns and 3.22 ( 0.02) ns,
respectively. However, no emission for SBDPiR 840 was
observed due to the presence of two N,N-dimethylaminophenyl
entities which would promote intramolecular electron transfer
(vide infra). Notably, upon appending fullerene to form the
donor−acceptor dyads, quantitative quenching of fluorescence
in all three dyads was witnessed, indicating the occurrence of
photoinduced intramolecular events (Figure 1b). Energy
transfer as a quenching mechanism was ruled out due to the
absence of spectral overlap between SBDPiR emission and C60
absorption.
In order to visualize the geometry and the electronic
structures of the dyads, ground state optimization of these
dyads at the B3LYP/3-21G(*) level with Gaussian 0326 were
performed. As shown in Figure 2 (panels a−c), all of the dyads
As shown in Figure 1a, the combination of thieno-pyrrole
fusion and substituents on the 2-position of thieno ring
Figure 2. B3LYP/3-21G(*) optimized structures of (a) SBDPiR 690-
C60, (b) SBDPiR 731-C60, and (c) SBDPiR 840-C60. (d) HOMO and
LUMO of SBDPiR 840-C60.
revealed stable structures on the Born−Oppenheimer potential
energy surface with no steric constraints between the donor
and acceptor entities. The center-to-center distance between
the boron atom and the center of the fullerene was found to be
∼9.9 Å, indicating close proximity of the donor and acceptor
entities. As shown in Figure 2d, in the optimized structure of
the SBDPiR 840-C60 dyad, the HOMO was located on the
SBDPiR 840 entity while the LUMO was located on fullerene
entity. Interestingly, for SBDPiR 690-C60 and SBDPiR 731-C60
dyads, the LUMO was on the SBDPiR macrocycle, while the
LUMO+1 was on the fullerene entity and the HOMO on the
SBDPiR entities (see Figure S6). The presence of LUMO on
SBDPiR is suggestive of these macrocycles being electron
deficient; electrochemical studies were performed on these
dyads to confirm this.
Figure 1. (a) Optical absorbance, normalized to the intense visible
band, (b) fluorescence emission spectra of precursor, thieno-pyrrole-
fused BODIPYs and the dyads formed by covalent attached of
fullerene in benzonitrile. The compounds were excited at the
respective peak maxima. The 875−1000 nm range was expanded
(10×) for clarity.
extended the wavelength of absorption of BODIPY well into
the near-IR region. Further, appending C60 through the boron
center caused an additional red shift of 5−12 nm. The
characteristic sharp peak of fulleropyrrolidine at 430 nm in all
of the dyads was also observed. The fluorescence emission
spectrum of SBDPiR 690 revealed an emission peak at 722 nm
with a shoulder band at 795 nm when excited at 690 nm in
Figure 3 shows DPVs of the investigated dyads while Figure
S7 of the Supporting Information shows DPV of pristine
SBDPiR and fulleropyrrolidine under similar solution con-
ditions for comparison. The site of electron transfer was arrived
B
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Communication
The transient absorption spectral changes were distinctly
different for the dyads; that is, spectral features corresponding
to the occurrence of ultrafast charge separation were observed.
Figure 4 shows the spectral changes recorded at different time
Figure 3. Differential pulse voltammograms (DPV) of SBDPiR 690-
C60, SBDPiR 731-C60, and SBDPiR 840-C60 dyads in benzonitrile, 0.1
M (n-Bu4N)ClO4. Scan rate = 5 mV/s, pulse width = 0.25 s, and pulse
height = 0.025 V. The blue dotted line shows fullerene reductions,
while the red dotted arrow shows the corresponding SBDPiR’s first
oxidation and first reduction peaks.
by comparing the redox potentials of the control compounds.
In the dyads, the first two one-electron reductions correspond-
ing to fullerene were located at −1.02 and −1.44 V versus Fc/
Fc+, not significantly different from the control compound, 2-
phenylfulleropyrrolidine.27 The first reduction of the SBDPiR
entity in the dyads was located at −0.68, −0.75, and −0.87 V,
respectively, for SBDPiR 690-C60, SBDPiR 731-C60, and
SBDPiR 840-C60 dyads, while the corresponding first oxidation
was located at 0.77, 0.60, and 0.11 V versus Fc/Fc+,
respectively. The gradual narrowing of the HOMO−LUMO
gap of the SBDPiR dyes across the series with facile oxidation
was evident from these studies.
Figure 4. Femtosecond transient spectra at the indicated time intervals
of SBDPiR 690-C60, SBDPiR 731-C60, and SBDPiR 840-C60 dyads in
benzonitrile. The samples were excited using 400 nm 100 fs laser
pulses. The decay profile of the C60•− at 1020 nm is shown in the right
panels for a while for (b and c); the decay profile of SBDPiR•+ at 917
and 1130 nm is shown.
The energetics for charge separation were calculated using
the redox and optical data according to Rehm−Weller’s
approach,28 and the calculated values were found to be
exergonic for PET via the singlet excited state of SBDPiR
intervals. Transient bands to the formation of SBDPiR•+ in the
near-IR region and around 1020 nm corresponding to the
leading to the formation of SBDPiR•+-C60 charge-separated
formation C60 were observed. In order to identify the cation
•−
•−
states.29 That is, the magnitudes of −ΔGS were found to be
radical of the SBDPiR, the probes were chemically oxidized, as
shown in Figure S11 of the Supporting Information. In the case
of SBDPiR 690, the main band corresponding to SBDPiR•+
appeared at 756 nm, while this band for SBDPiR 731 and
SBDPiR 840 was located at 810 and 1024 nm, respectively. As
shown in Figure 4a for SBDPiR 690-C60 dyad, bands in the
850−960 nm region corresponding to SBDPiR•+ and at 1020
CS
−0.11, −0.12, and −0.47 eV, respectively, for the SBDPiR 690-
C60, SBDPiR 731-C60, and SBDPiR 840-C60 dyads. Such
calculations also ruled out the possibility of SBDPiR•‑-C60
•+
formation because of their higher-energy states. An energy level
diagram showing the main photochemical events is shown in
Figure S8 in the Supporting Information.
•−
Further, femtosecond and nanosecond transient absorption
spectral studies were performed to gather evidence for the
occurrence of PET and to secure kinetic information on charge
separation and charge recombination. For this, first, transient
spectra of the SBDPiR probes in benzonitrile were recorded. As
shown in Figure S9 in the Supporting Information, immediately
after excitation with 100 fs flash at 400 nm in benzonitrile, the
spectra showed the absorption of the singlet SBDPiR in the
form of bleaching at 708 nm for SBDPiR 690, and at 732 and
758 nm for SBDPiR 731. The decay of the singlet excited states
was accompanied by populating the triplet excited states (see
Figure S10 of the Supporting Information for nanosecond
transient spectra of SBDPiR derivatives). However, for SBDPiR
840 having electron donating N,N-dimethylamino groups, the
spectral features were that of fast charge separation.
nm corresponding to C60 were observed, establishing charge
separation in the dyad. The kinetics of charge separation, kCS
(estimated from the rise time constant) and charge
recombination, kCR calculated from the decay profile of the
•−
C60 band (Figure 4a, right panel) were found to be 1.10 ×
1011 and 7.66 × 108 s−1, respectively. For SBDPiR 731-C60
dyad, the cation peak expected at 810 nm overlapped with the
•−
overtone band of the excitation band, while the C60 band
appeared as a shoulder to the 925 nm transient band (Figure
4b). The kCS and kCR calculated from the decay profile of the
anion radical (Figure 4b, right panel) were found to be 1.12 ×
1011 and 9.78 × 108 s−1, respectively. In the case of SBDPiR
840-C60, both cation and anion bands having the same
absorption maxima, appeared at 1020 nm (Figure 4c). For
calculating the kCS and kCR values, the time profile of this band
C
dx.doi.org/10.1021/ja503015f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(12) Dualeh, A.; Delcamp, J. H.; Nazeeruddin, M. K.; Gratzel, M.
̈
was utilized. The measured kCS and kCR were found to be 7.14
× 1010 and 2.93 × 109 s−1, respectively. The estimated errors in
the kinetic values are not more than 10%. The charge
recombination proceeded in populating the triplet state of the
sensitizers prior to returning to the ground state, as witnessed
from the developing triplet features at higher timescales in the
femtosecond transient spectra in Figure 4.
In summary, the present work reports the synthesis and
characterization of three new donor−acceptor dyads derived
from thieno-pyrrole-fused BODIPY and fullerene. The SBDPiR
macrocycle in these dyads acted as near-IR sensitizers and
revealed ultrafast photoinduced electron transfer (PET) to the
covalently linked fullerene as revealed by femtosecond transient
absorption studies. The charge recombination was 1−2 orders
of magnitude slower, a predicted trend for fullerenes due to
their low reorganization energy demand in electron transfer
reactions.30 The present class of dyads is important not only for
harvesting light energy from the near-IR region but also in
building optoelectronic devices operating under near-IR light as
the excitation source. Further studies along this line are in
progress in our laboratories.
Appl. Phys. Lett. 2012, 100, 173512−173514.
(13) Arunkumar, E.; Ajayaghosh, A. Chem. Commun. 2005, 5, 599−
601.
(14) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891−4932.
(15) Umezawa, K.; Nakamura, H.; Makino, D.; Citterio, D.; Suzumi,
K. J. Am. Chem. Soc. 2008, 130, 1550−1551.
(16) Kamkaew, A.; Lim, S. H.; Bee, H. B.; Kiew, L. Y.; Chung, L. Y.;
Burgess, K. Chem. Soc. Rev. 2013, 42, 77−88.
(17) Awuah, S. G.; You, Y. RSC Adv. 2012, 2, 11169−11183.
(18) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008,
47, 1184−1201.
(19) El-Khouly, M. E.; Fukuzumi, S.; D’Souza, F. ChemPhysChem
2014, 15, 30−47.
(20) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki,
I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100−110.
(21) Engelhardt, V.; Khuri, S.; Fleischhauer, J.; Garcia-Iglesias, M.;
Gonzalez-Rodriguez, D.; Bottari, G.; Torres, T.; Guldi, D. M.; Faust,
R. Chem. Sci. 2013, 4, 3888−3892.
(22) Awuah, S. G.; Das, S. K.; D’Souza, F.; You, Y. Chem.Asian J.
2013, 8, 3123−3132.
(23) Amin, A. N.; El-Khouly, M. E.; Subbaiyan, N. K.; Zandler, M. E.;
Fukuzumi, S.; D’Souza, F. Chem. Commun. 2012, 48, 206−208.
(24) Bandi, V.; El-Khouly, M. E.; Ohkubo, K.; Nesterov, V. N.;
Zandler, M. E.; Fukuzumi, S.; D’Souza, F. J. Phys. Chem. C 2014, 118,
2321−2332.
ASSOCIATED CONTENT
■
S
* Supporting Information
(25) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993,
115, 9798−9799.
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M. et al.
Gaussian 03; Gaussian, Inc.: Pittsburgh, PA, 2003.
Synthetic, experimental, and complete citation details of ref 26;
Maldi-mass and IR; HOMO and LUMO orbitals of dyads;
DPV of pristine SBDPiR and fulleropyrrolidine; an energy level
diagram showing photochemical events; and spectra of
femtosecond and nanosecond transient and chemically oxidized
SBDPiR probes. This material is available free of charge via the
Internet at http://pubs.acs.org.
(27) Smith, P. M.; McCarty, A. L.; Nguyen, N. Y.; Zandler, M. E.;
D’Souza, F. Chem. Commun. 2003, 1754−1755.
(28) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 7, 259−271.
(29) −ΔGSCS = ΔE0−0 − e(Eox − Ered) − Esol, ΔE0−0 is the energy of
the lowest excited state of SBDPiR being 1.76, 1.65 and 1.46 eV
(estimated) for SBDPiR 690, SBDPiR 731, and SBDPiR 840,
respectively. Symbols Eox and Ered correspond to the first oxidation
potential of SBDPiR derivatives and the first reduction potential of
C60, respectively. Symbol Esol is electrostatic energy derived from the
distance between the electron donor and acceptor using the
continuum model.
AUTHOR INFORMATION
■
Corresponding Author
E-mail: youngjae-you@ouhsc.edu; Francis.DSouza@UNT.edu
Notes
The authors declare no competing financial interest.
(30) Imahori, H.; Hagiwara, K.; Akiyama, T.; Aoki, M.; Taniguchi, S.;
Okada, T.; Shirakawa, M.; Sakata, Y. Chem. Phys. Lett. 1996, 263, 545−
550.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(Grant 1110942 to F.D.) and OCAST (HR11-58 to Y.Y.).
REFERENCES
■
(1) Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.;
Elsevier: Amsterdam, 1988; Part A−D.
(2) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, 2001, Vols. 1−5.
(3) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2009, 42,
1890−1898.
(4) Fukuzumi, S.; Kojima, T. J. Mater. Chem. 2008, 18, 1427−1439.
(5) Fukuzumi, S.; Ohkubo, K.; D’Souza, F.; Sessler, J. L. Chem.
Commun. 2012, 48, 9785−9940.
(6) Wasielewski, M. R. Acc. Chem. Res. 2009, 42, 1910−1921.
(7) Sanchez, L.; Martín, N.; Guldi, D. M. Angew. Chem., Int. Ed. 2005,
44, 5374−5382.
(8) Bottari, G.; de la Torre, G.; Guldi, D. M.; Torres, T. Chem. Rev.
2010, 110, 6768−6816.
(9) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem.
Photobiol. C 2004, 5, 79−104.
(10) D’Souza, F.; Ito, O. Chem. Soc. Rev. 2012, 41, 86−96.
(11) Tkachenko, N. V.; Lemmetyinen, H. In Handbook of Carbon
Nanomaterials, D’Souza, F.; Kadish, K. M., Eds.; World Scientific
Publications: Singapore, 2011; Chapter 13, pp 405−440.
D
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