Ultrafast Photoinduced Electron Transfer and Charge Stabilization in Donor-Acceptor Dyads Capable of Harvesting Near-Infrared Light

Article abstract of DOI:10.1002/chem.201500728

To harvest energy from the near-infrared (near-IR) and infrared (IR) regions of the electromagnetic spectrum, which constitutes nearly 70'% of the solar radiation, there is a great demand for near-IR and IR light-absorbing sensitizers that are capable of

Full text of DOI:10.1002/chem.201500728

DOI: 10.1002/chem.201500728
Full Paper
&
Near Infrared Sensitizers
Ultrafast Photoinduced Electron Transfer and Charge Stabilization
in Donor–Acceptor Dyads Capable of Harvesting Near-Infrared
Light
Venugopal Bandi, Habtom B. Gobeze, and Francis D’Souza*[a]
Abstract: To harvest energy from the near-infrared (near-IR)
and infrared (IR) regions of the electromagnetic spectrum,
which constitutes nearly 70% of the solar radiation, there is
a great demand for near-IR and IR light-absorbing sensitizers
that are capable of undergoing ultrafast photoinduced elec-
tron transfer when connected to a suitable electron accept-
or. Towards achieving this goal, in the present study, we
report multistep syntheses of dyads derived from structurally
modified BF2-chelated azadipyrromethene (ADP; to extend
absorption and emission into the near-IR region) and fuller-
ene as electron-donor and electron-acceptor entities, respec-
tively. The newly synthesized dyads were fully characterized
based on optical absorbance, fluorescence, geometry opti-
mization, and electrochemical studies. The established
energy level diagram revealed the possibility of electron
transfer either from the singlet excited near-IR sensitizer or
singlet excited fullerene. Femtosecond and nanosecond
transient absorption studies were performed to gather evi-
dence of excited state electron transfer and to evaluate the
kinetics of charge separation and charge recombination pro-
cesses. These studies revealed the occurrence of ultrafast
photoinduced electron transfer leading to charge stabiliza-
tion in the dyads, and populating the triplet states of ADP,
benzanulated-ADP and benzanulated thiophene-ADP in the
respective dyads, and triplet state of C60 in the case of BF2-
chelated dipyrromethene derived dyad during charge re-
combination. The present findings reveal that these sensitiz-
ers are suitable for harvesting light energy from the near-IR
region of the solar spectrum and for building fast-respond-
ing optoelectronic devices operating under near-IR radiation
input.
Introduction
However, the majority of such donor–acceptor systems have
utilized sensitizers that exhibit absorption and emission in the
UV/Vis region of the electromagnetic spectrum, which limits
their potential of harvesting energy from the near-IR and IR re-
gions.[1–6,8] Only a handful of near-IR sensitizers, such as p-ex-
panded phthalocyanine derivatives, squaraine compounds, and
thieno-pyrrole-fused BODIPY (=4,4-difluoro-4-bora-3a,4a-diaza-
s-indacenes)[9] have been used successfully. This limitation is
due to the tedious multistep synthesis, low reaction yields, re-
active intermediates, poor solubility, poor photostability, and
limited scope of functionalization of these near-IR sensitizers.
BF2-chelated dipyrromethene (BODIPY or BDP) and its azadi-
pyrromethene analogue (azaBODIPY or ADP) are fluorophores
that are well-known for their synthetic advantages, tunable ab-
sorption and emission with high extinction coefficients, and for
their ability to modulate redox properties.[10] Accordingly, pris-
tine forms of both BDP and ADP have been employed in build-
ing photosynthetic antenna-reaction center model com-
pounds.[11–15] Advantageously, chemical modification of ring pe-
riphery with extended p-conjugation or appropriate substitu-
ents modulates the HOMO-LUMO levels of these compounds,
extending their absorption and emission well into the near-IR
portion while retaining their robust physicochemical proper-
ties,[16] thus making them ideal candidates to build donor–ac-
ceptor systems that are capable of harvesting near-IR light.
This has been verified in the present study, first, by synthesiz-
In the field of solar energy harvesting, photoinduced electron
transfer[1] plays a very important role in establishing the useful-
ness of the newly constructed donor–acceptor systems for
building devices for light-into-electricity and light-into-fuel
conversion applications[2–5] as well as optoelectronic devices
for pertinent applications.[6] In the construction of such donor–
acceptor architectures, a biomimetic approach of mimicking
natural photosynthesis is often targeted.[2–5] Starting from
simple donor–acceptor dyads and moving to more complex
multi-modular systems, a number of constructs have been suc-
cessfully designed and studied in this regard. It has been possi-
ble to generate the highly desired long-lived charge-separated
states, similar to that found in natural photosynthetic sys-
tems,[7] by engineering the energy levels of these systems.
[a] V. Bandi, H. B. Gobeze, Prof. Dr. F. D’Souza
Department of Chemistry, University of North Texas
1155 Union Circle, #305070, Denton, TX 76203-5017 (USA)
E-mail: Francis.DSouza@UNT.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500728. These include all the manuscript
figures in color, DPVs of near-IR probes, femtosecond transient absorption
spectra of BDP, ADP, BDP-C60 and ADP-C60 in toluene and benzonitrile, spec-
tra of chemically oxidized BADP and TADP in benzonitrile, fluorescence
1
decay curves of the probes, MS (ESI) of newly synthesized dyads, H and
13C NMR spectra of newly synthesized compounds.
Chem. Eur. J. 2015, 21, 11483 – 11494
11483
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
also shown. The highest Stokes
shift of 695 cmÀ1 was observed
for BDP, whereas that the lowest
was for TADP (240 cmÀ1), among
the series of employed sensitiz-
ers. Furthermore, the ability of
the newly synthesized BADP-C60
and TADP-C60 dyads to undergo
photoinduced electron transfer
leading to charge separated
states has been investigated
using femtosecond transient ab-
sorption spectroscopy.
Figure 1. Structures of the BDP-C60, ADP-C60, BADP-C60, and TADP-C60 dyads, investigated in the present study.
ing p-extended benzanulated-ADP (BADP) and benzanulated Results and Discussion
thiophene-ADP (TADP) moieties,[16] and, second, by synthesiz-
Synthesis of BADP-C60 and TADP-C60 dyads
ing dyads with BDP, ADP, BADP, and TADP moieties covalently
linked to the well-known electron-acceptor fullerene
(Figure 1).[17]
The methodology adopted for the synthesis of the dyads is il-
lustrated in Scheme 1; the details are given in the experimental
section. Briefly, first, benzo-fused dipyrromethene, having
either phenyl or thiophene substituents at the peripheral posi-
tions, were synthesized by reacting dicyanobenzene with the
corresponding Grignard reagent (phenyl magnesium bromide
or 2-methylthiophene magnesium bromide) in diethyl ether at
À208C followed by heating the reactant mixture to reflux in
formamide.16a The precipitate formed upon cooling the reac-
tion mixture was treated with boron trifluoride in the presence
of diisopropylethylamine in dichloroethane. The BADP and
TADP moieties obtained after purification by column chroma-
tography were treated with 3,4-dihydroxybenzaldehyde in the
presence of AlCl3 followed by purification of the
formyl functionalized BADP and TADP derivatives. Fi-
nally, these aldehydes were reacted with fullerene in
the presence of sarcosine (N-methyl glycine) in anhy-
drous toluene according to Prato’s method[18] of full-
eropyrrolidine synthesis. The purity of the dyads
were checked by thin-layer chromatography and the
dyads were characterized by 1H and 13C NMR, ESI-
mass, optical spectral and electrochemical methods.
The pure products were stored in the dark and fresh
solutions were prepared for photochemical measure-
ments.
As shown in Figure 2, both BADP and TADP sensitizers ex-
hibit absorption and emission well into the near-IR region
compared with those of traditionally used BDP and ADP sensi-
tizers. That is, BADP revealed absorption and emission maxima
at 718 and 751 nm, respectively, while these peaks for TADP
are at 774 and 830 nm, respectively. The TADP peaks also re-
vealed substantial broadening, likely due to extended p-conju-
gation involving peripheral substituents, and absorbance cov-
ering a wavelength region of 900 nm. For comparison, BDP ab-
sorption and emission peak maxima at 500 and 520 nm and
that of ADP at 656 and 691 nm, respectively, in benzonitrile are
Optical absorption and fluorescence studies
The absorption spectrum of the dyads in benzonitrile
normalized to the most intense peak is shown in
Figure 3. Attachment of the fullerene through the
central boron (replacing F with O) had a minimum
effect on the absorbance behavior, expect for TADP-
C60, for which the TADP peak was red-shifted and ap-
peared at 828 nm. A sharp peak at 432 nm, character-
istic of fulleropyrrolidine (indicated with an asterisk in
Figure 3) was also observed. These results indicate
Figure 2. Normalized (to the most intense peak maxima) absorption (solid line) and emis-
sion (dashed line) spectra of the investigated sensitizers in benzonitrile. The structure of
the corresponding sensitizer is shown at the top.
some ground-state interactions between fullerene
and modified BODIPY moieties (referred as donor
Chem. Eur. J. 2015, 21, 11483 – 11494
www.chemeurj.org
11484
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Scheme 1. Synthetic methodology developed for BADP-C60 and TADP-C60 dyads in the present study.
Computational and electrochemical studies
Geometry optimization and electronic structure elucidation
was performed at the B3LYP/6-31G* level using the Gaussi-
an 03 program[19,20] on the new dyads. The optimized struc-
tures along with the first HOMO and LUMO of the new BADP-
C60 and TADP-C60 along with BDP-C60 and ADP-C60 dyads, for
comparison purposes, are shown in Figure 4. The structures
were fully optimized on the Born–Oppenheimer potential
energy surface. In all the dyads, the donor moiety was found
to be almost flat, with a center-to-center distance between the
boron atom and the center of fullerene of approximately 10 Š,
irrespective of the nature of the modifications. The tetrahedral
boron meant that the peripheral substituents on BADP and
TADP caused no steric constraints on fulleropyrrolidine. Except
for ADP-C60, the LUMO of other three dyads was on fulleropyr-
rolidine, while for ADP-C60, LUMO+1 was on fulleropyrrolidine.
The HOMO of both BADP-C60 and TADP-C60 were fully localized
on BADP and TADP moieties, while for BDP-C60 and ADP-C60,
the contributions were also on the bridging catechol segment.
This could be a consequence of the moderate level of the MO
calculations performed here.[15i] Nevertheless, these studies
Figure 3. Absorption spectrum of the dyads and fulleropyrrolidine in benzo-
nitrile (normalized to the intense peak maxima).
hereafter) in the dyads. The small spectral shift suggest mini-
mal perturbation of the electronic structure of the donor upon
functionalization through the central boron atom. Predictably,
fluorescence of the donor moieties (see Figure 2 for the spec-
tra) and 2-tolyl fulleropyrrolidine (in the 720 nm region) in the
dyads was found to be fully quenched, indicating the occur-
rence of excited-state events in the dyads. The absorption
spectrum of control compound, 2-phenyl fulleropyrrolidine is
also shown in Figure 3. It is important to note that the donor
moieties and fulleropyrrolidine both have appreciable absorb-
ance at 400 nm, which is the excitation wavelength of the fem-
tosecond transient spectrometer used in the present study. It
may be mentioned here that no spectral evidence for energy
reveal the formation of BADP +-C60 and TADP +-C60 upon ex-
C
CÀ
C
CÀ
citation of the dyads.
By using differential pulse voltammetry (DPV), the redox
properties of the near-IR sensitizers and the dyads were inves-
tigated in benzonitrile containing 0.1m (TBA)ClO4. The DPV
plots of the near-IR sensitizers are shown in Figure S1, and the
data are provided in Table 1. For the investigated series of
near-IR sensitizers, the oxidation potential followed the trend:
BDPꢀADP> BADP> TADP, while the reduction potentials fol-
lowed the trend: BDP> BADP> TADP> ADP. As pointed out
earlier,[15c] ADP reduction was easier than reduction of the full-
eropyrrolidine, where as that for TADP the reduction over-
lapped with that of fulleropyrrolidine. The electrochemical
HOMO–LUMO gap (potential difference between the first oxi-
dation and first reduction) followed the trend BDP (2.36 V)>
ADP (1.56 V)> BADP (1.44 V)> TADP (1.27 V) and were gener-
1
transfer from 1BADP* (1TADP*) to fullerene or from C60* to
BADP (or TADP) from steady-state fluorescence measurements
was obtained.
Chem. Eur. J. 2015, 21, 11483 – 11494
www.chemeurj.org
11485
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
electronic configurational inter-
actions in the transition, espe-
cially for ADP structural ana-
logues.[15j]
The DPVs of the dyads in ben-
zonitrile are shown in Figure 5.
To a large extent, the peaks
were a superposition of the indi-
vidual entities, revealing the ab-
sence of significant interactions
in the ground state between the
entities of the dyad. The redox
data for the dyads are given in
Table 1. By using the redox, com-
putational, and spectral data,
free-energy change for charge
separation (DGCS) and charge
recombination (DGCR) from the
singlet excited donor and fuller-
ene were calculated according
Figure 4. B3LYP/6-31G* optimized structures of the investigated dyads. The HOMO (top) and LUMO (bottom) for
to
Rehm–Weller’s
approach
each dyad is also shown along with the optimized structure.
(Table 1).[21] The DGCR (=ERIP,
energy of radical ion-pair) fol-
lowed the trend: BDP> ADP>
BADP> TADP, indicating lower energy storage capability of
Table 1. Redox potentials, free-energy change for charge separation
(DGCS) and charge recombination (DGCR) of the investigated dyads in ben-
zonitrile. Redox potentials of the sensitizers are also given for compari-
son.
near-IR sensitizer derived donors. It is also important to note
that electron transfer is feasible from both singlet excited sen-
+
C
sitizer and singlet excited fulleropyrrolidine to produce donor
CÀ
-C60 charge-separated states. Such calculations also reveal
[a]
[b,c]
[b,d]
CÀ
C
+
Compd
1st Ox. 1st Red. 2nd Red. ÀDGCR
ÀDGCS
ÀDGCS
that the formation of donor -C60 charge-separated states are
energetically not possible from either singlet excited donor or
fulleropyrrolidine in the present series of dyads.
BDP
ADP
BADP
TADP
Toluyl-C60 0.95
BDP-C60
ADP-C60
BADP-C60 0.41
TADP-C60 0.28
0.73
0.76
0.40
0.28
À1.63
À0.79
À1.04
À0.99
À1.02
À1.02
À0.78
À1.01
À0.99
–
–
–
–
–
–
–
–
–
–
–
–
–
À1.54
À1.75
À1.74
À1.43
À1.45
À1.02
À1.05
À1.02
The energy level diagram showing different photochemical
events originating from the singlet excited state of the probes
(sensitizer (donor) as well as fulleropyrrolidine) is shown in
Figure 6.[22] The energy of charge-separated states generally
follow the trend of singlet excited state energy of the donor,
except for ADP-C60. Apart from charge separation, singlet-sin-
glet excited energy transfer is also a possibility; however, as
pointed out earlier, because of the weak absorbance of fullero-
pyrrolidine in the emission wavelengths of the near-IR sensitiz-
ers, such a process could be considered to be a minor one.[24]
In fact, steady-state fluorescence measurements revealed no
indication of this event among the series of dyads. The radical
ion pairs could relax to the ground state directly; alternatively,
they could populate the triplet excited state of the probes (for
azaBODIPY derivatives) or fulleropyrrolidine (in the case of
BDP-C60 dyads) prior to returning to the ground state. To inves-
tigate these possibilities, femtosecond and nanosecond transi-
ent absorption studies were performed in nonpolar toluene
and polar benzonitrile, as discussed below.
–
–
–
0.73
0.77
1.69
1.73
1.36
1.21
0.73
0.11
0.32
0.31
0.06
0.02
0.38
0.51
[a] ÀDGCR =e(EoxÀEred)ÀDGS.
[b] ÀDGCS =E0,0À(ÀDGCR),
in
which
hꢀ
ꢁ
ꢀ
ꢁ
i
e2
1
1
1
1
CC eR
ÀDGS ¼
=
=
þ =
D
=
eR
À R
, where DGS is the solva-
4pe0
2R
2R
þ
À
tion energy, Eox is the first oxidation potential of the dyad, Ered is the first
reduction potential of the dyad, E0,0 is the energy associated with the 0,0
optical transition (midpoint of absorption and emission corresponding to
0,0 transition), being 2.43 eV for BDP, 1.84 eV for ADP, 1.69 eV for BADP,
1.55 eV for TADP, and 1.75 eV for fulleropyrrolidine. R+ and R refer to
À
radii of the cation and anion species, respectively. e0 and eR refer to
vacuum permittivity and dielectric constant of the employed solvent, re-
spectively. RCtÀCt is the center-to-center distance (distance between boron
and center of fullerene) being 10.3 Š for BDP, 10.0 Š for ADP, 9.78 Š for
BADP, and 9.66 Š for TADP. The calculated DGS was found to be 0.06 eV
for the dyads in benzonitrile. [c] From the singlet excited donor molecule.
[d] From the singlet excited fulleropyrrolidine
ally smaller than that calculated from spectral measurements
(midpoint of absorption and emission corresponding to the 0,0
transition, see Table 1 footnote for values). This difference was
0.07, 0.28, 0.25, and 0.28 V for BDP, ADP, BADP, and TADP, re-
spectively. These results indicate the probable existence of
Femtosecond transient absorption studies
The transient spectral features of pristine ADP and BDP, shown
in Figure S2, have been discussed earlier.[15,25] Briefly, both sen-
sitizers revealed almost instantaneous formation of the singlet-
Chem. Eur. J. 2015, 21, 11483 – 11494
www.chemeurj.org
11486
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
and intersystem crossing. Assuming the latter two processes
are minor contributors, the inverse time constant could be at-
tributed to fluorescence. The time constant for the recovery of
the 505 nm band in toluene was 2500 ps, while that for the
508 nm band in benzonitrile it was 1896 ps, although the re-
covery lasted over the 3 ns monitoring time window of our in-
strument. These values compared with the fluorescence life-
times of 3.36 and 2.86 ns, respectively, in toluene and benzoni-
trile for BDP from the time-correlated single-photon counting
method (TSCPC; see Figure S5 for decay curves). Similarly, tran-
sient absorption spectra of ADP in toluene and benzonitrile re-
vealed the almost instantaneously formed singlet-excited state
features with peak maxima at 515 nm and 845 nm range, and
minima at 658 nm in toluene, and maxima at 544 and 845 nm
range, and minima at 664 nm in benzonitrile, the opposite of
the ground-state absorption (including contributions from
stimulated emission). Extending the monitoring wavelength
window into the near-IR region also revealed a new band in
the 1260 nm region, corresponding to the singlet–singlet tran-
sition. The time constants for the recovery of the 658 nm peak
in toluene was 3204 ps, while that for the 660 nm peak in ben-
zonitrile it was 1825 ps, which are close to their lifetimes of
1.78 and 1.64 ns, respectively, in toluene and benzonitrile de-
termined by using the TCSPC method.
Figure 5. Differential pulse voltammograms of the investigated dyads in
benzonitrile, 0.10m (nBu4N)ClO4. Scan rate=5 mVsÀ1, pulse width=0.25 s,
pulse height=0.025 V. The asterisk in the second panel indicates oxidation
process of ferrocene used as an internal standard. The vertical dashed lines
show fullerene reductions.
The femtosecond transient spectra of the near-IR
probes, BADP and TADP, in toluene and benzonitrile
are shown in Figure 7. For BADP, the almost instantly
formed singlet 1BADP* revealed positive peaks at
585, 1267(sh), and 1490 nm, and depleted peaks at
716 and 821 nm in toluene (Figure 7a). The origin of
the 831 nm peak is not fully apparent at this point. In
benzonitrile these peaks were red-shifted by another
5–8 nm. The 716 nm peak corresponded to that of
ground-state bleaching and stimulated emission of
the probe. Given the similarity to the ADP near-IR
band, the 1490 nm peak of BADP has been ascribed
to a singlet–singlet transition. This peak decayed
with a time constant of 1768 ps in toluene and
1684 ps in benzonitrile, which are close to the life-
times of 1.91 and 1.89 ns, in toluene and benzonitrile,
respectively, determined by using the TCSPC method.
The transient spectral features of TADP in toluene
and benzonitrile are shown in Figure 6c and d, re-
spectively. Positive peaks at 440, 625 and approxi-
mately 1400 nm, and a negative peak at approxi-
Figure 6. Energy-level diagram showing different photochemical events in the investigat-
mately 830 nm were observed, while that in benzoni-
trile these peaks were located at 455, 620 and ap-
proximately 1330 nm, and the negative peak was ob-
served at 842 nm. The near-IR peaks decayed with
a time constant of 614 ps in toluene and 608 ps in
benzonitrile, which is faster than relaxation of BADP
ed near-IR sensitizer derived donor-fullerene dyads. Solid arrow indicates the most likely
3
process; dashed arrow indicates the less likely process. BDP* energy level (ET =1.66 eV)
3
was taken from Ref. [24]; for C60* (ET =1.55 eV) the energy level was taken from Ref. [25].
3
The broken energy level of D* refers to an estimated value.
excited state features. In case of BDP, the transient absorption
spectra were dominated by pronounced bleaching in the
500 nm range, which was due to depletion of the singlet
ground state along with contributions from stimulated emis-
sion. The inverse time constant of this peak represents the
sum of rate constants for fluorescence, internal conversion,
singlet excited state. Singlet excited lifetimes of TADP from the
TCSPC method in toluene and benzonitrile were found to be
1.06 and 1.02 ns, respectively (see Figure S5 in the Supporting
Information for decay curves).
The femtosecond transient spectra of BDP-C60 and ADP-C60
in toluene and benzonitrile are shown in Figure S3 in the Sup-
Chem. Eur. J. 2015, 21, 11483 – 11494
www.chemeurj.org
11487
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 7. Differential absorption spectra obtained upon femtosecond flash photolysis (400 nm) of BADP in argon saturated (a) toluene (b) benzonitrile, and
TADP in (c) toluene and (d) benzonitrile. Figure inset shows the time profile of the 595, 602, 650, and 625 nm peaks of the respective probes.
porting Information. In agreement with our earlier report,[25]
charge separation in BDP-C60 was evident with the fast decay
of the 1BDP* peaks, accompanied by a new peak in the
sured kCS were an order of magnitude higher than that ob-
served for BDP-C60 (Table 2). Additionally, the charge recombi-
nation involved populating the 3ADP* instead of C60, as re-
3
CÀ
1020 nm range corresponding to the formation of C60 (Fig-
vealed by the absence of C60* in the 700 nm range and an in-
3
ure S3a and b). At the excitation wavelength of 400 nm, forma-
creased absorbance of ADP* in the 440 nm range.[15a]
1
tion of C60* was also revealed, as seen by a broad peak in the
To spectrally identify the oxidation products of BADP-C60
and TADP-C60 during photochemical charge separation, these
1
900–950 nm region. The formation of C60* could be due to
either direct excitation or ultrafast energy transfer
due to closely associated donor–acceptor entities
(Dexter type excitation transfer). The rate of charge
Table 2. Rates of charge separation, kCS and charge recombination, kCR for the investi-
gated dyads in toluene in benzonitrile.
separation, kCS, and charge recombination, kCR, evalu-
CÀ
ated by monitoring the rise and decay of the C60
[a]
Compd
BDP-C60
Solvent
kCS [sÀ1
]
FCS
kCR [sÀ1
]
tRIP [ps]
Ref.
signal (see Figure S3a and b inset for decay time pro-
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
7.5”1010
4.9”1010
1.0”1012
1.0”1012
5.5”1010
21”1010
0.7”1010
1.2”1010
0.993
0.994
0.992
0.993
0.98
0.985
0.42
0.80
0.7”109
3.2”109
3.5”109
5.0”109
1.2”109
11”109
0.4”109
0.7”109
1351
312
285
200
833
90
this work
[24]
this work
[15c]
this work
this work
this work
this work
CÀ
files) are given in Table 2. The decay of the C60 peak
representing charge recombination was accompanied
by a new peak at 710 nm, corresponding to the for-
ADP-C60
BADP-C60
TADP-C60
3
mation of C60*. These results indicate that the
3
charge-separated state populates the C60* prior to
2500
1428
returning to the ground state. As shown in Fig-
ure S3c and d, ADP-C60 in toluene and benzonitrile
also revealed charge separation,[15c] in which the mea-
benzonitrile
[a] FCS =[(1/tf)dyadÀ(1/tf)ref]/(1/tf)dyad. See Ref. [26] for details.
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2015, 21, 11483 – 11494
www.chemeurj.org
11488

Full Paper
CÀ
dyads were chemically oxidized by using nitrosonium tetra-
fluoroborate as an oxidizing agent. As shown in Figure S4a,
chemical oxidation of BADP in benzonitrile revealed dimin-
ished intensity of the BADP peak at 716 nm with no character-
istic new bands, perhaps due to low molar extinction coeffi-
C60 signal were found to be 5.5”1010 and 1.2”109 sÀ1, re-
spectively. The kCS determined by monitoring the decay of
BADP singlet peak at 1490 nm was found to be 4.2”1010 sÀ1
,
CÀ
which is close to that obtained from the growth of the C60
peak. The quantum yields, FCS, obtained from kCS values by
using the standard procedure[25] were found to be 0.98 in tolu-
ene and 0.985 in benzonitrile, respectively. The decay of the
radical ion peaks was accompanied by a new peak in the 490–
530 nm range, which has been tentatively assigned to the for-
mation of 3BADP* (see above). Interestingly, no peak at
cient of BADP +. On the contrary, TADP revealed a new peak at
C
+
C
535 nm corresponding to the formation of TADP
.
The femtosecond transient spectra of BADP-C60 at the indi-
cated delay times in toluene and benzonitrile are shown in
Figure 8. As anticipated from steady-state fluorescence studies,
the transient peaks of the singlet excited state deactivated
much more rapidly with the appearance of a new transient sig-
3
710 nm, corresponding to C60*, was observed, indicating that
the charge-separated state does not populate this state prior
to returning to the ground state, as envisioned from the
energy level diagram in Figure 6.
CÀ
nature band at 1020 nm due to C60 . A clear peak correspond-
+
C
ing to BADP was not observed, perhaps due to a low extinc-
tion coefficient of that species. The singlet–singlet peak of
BADP located at 1490 nm also revealed rapid decay. The kCS
and kCR measured by monitoring the growth and decay of the
Femtosecond transient spectra of TADP-C60 in the investigat-
ed solvents are shown in Figure 9. A negative peak at 830 nm,
having contribution from both the ground-state depletion and
Figure 8. Differential absorption spectra obtained upon femtosecond flash photolysis (400 nm) of BADP-C60 in argon saturated (a) toluene and (b) benzonitrile.
CÀ
Figure on the right hand panel shows the time profile of the C60 peak monitored at 1020 nm.
Chem. Eur. J. 2015, 21, 11483 – 11494
www.chemeurj.org
11489
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 9. Differential absorption spectra obtained upon femtosecond flash photolysis (400 nm) of TADP-C60 in argon saturated (a) toluene and (b) benzonitrile.
CÀ
Figure on the right hand panel shows the time profile of the C60 peak monitored at 1020 nm.
Nanosecond transient absorption studies
stimulated emission, was observed. Recovery of this peak is ac-
+
C
companied by new transient bands attributable to the TADP
CÀ
[25]
-C60 radical ion pair were observed in both solvents. That is,
As mentioned earlier, in the case of BDP-C60 and ADP-C60,[15c]
+
3
3
C
new peaks at 525 nm, corresponding to TADP (see Fig-
population of C60* and ADP*, respectively, by the charge-sep-
arated species prior to returning to the ground state were ob-
served. The femtosecond transient spectral studies discussed
in the previous section revealed populating 3BADP* and
3TADP* by the charge-separation states prior to returning to
the ground state. Nanosecond transient absorption spectra
were recorded to ascertain this return path of the charge-sepa-
rated species in the case of these dyads. It may be mentioned
here that pristine BADP and TADP revealed very weak, if any,
transient bands, leading to lower yields of triplet states, which
is a problem that is often encountered with BDP and ADP de-
rivatives. As shown in Figure 10a, nanosecond transient ab-
sorption spectra of BADP-C60 revealed a peak in the 525 nm
ure S4b for spectra of TDAP during chemical oxidation), and
CÀ
1020 nm, corresponding to C60 , were observed. As expected,
1
the near-IR band of TADP* in the dyad decayed much more
rapidly than that in pristine TADP due to an electron-transfer
process. The measured kinetic values and quantum yields are
provided in Table 2. The decay of the radical ion peak was ac-
companied by a new peak at 690 nm in toluene and 695 nm
3
in benzonitrile (different from that of C60* at 710 nm with
a shoulder band at 825 nm). These spectral features have been
3
assigned to TADP* formation. That is, the TADP +-C60 radical
C
CÀ
3
ion pair populating the TADP* prior returning to the ground
state, as shown in Figure 6.
3
range, corresponding to the formation of BADP*-C60, whereas
in the 700 nm region no spectral signatures corresponding to
3C60* was observed. Similar spectral observations were also
Chem. Eur. J. 2015, 21, 11483 – 11494
www.chemeurj.org
11490
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 10. Nanosecond transient absorption spectra (lex =475 nm of 8 ns pulses) of (a) BADP-C60 and (b) TADP-C60 in Ar-saturated toluene. Time profile of
525 nm peak of BADP-C60 in toluene and benzonitrile are shown in figures (c) and (d), respectively. The time profile of the 680 nm peak of TADP-C60 in toluene
and benzonitrile are shown in figures (e) and (f), respectively.
made in benzonitrile. From the fitting of the transient band at
525 nm, the decay rate constants of the 3BADP* (kT) were
found to be 1.84”104 and 3.05”104 sÀ1 in toluene and benzo-
nitrile, respectively. Similarly, the nanosecond transient absorp-
tion spectra of TADP-C60 revealed a broad peak in the 680 nm
range both in toluene and benzonitrile, corresponding to the
used directly to construct wide-band light energy harvesting
devices and also fast-responding optoelectronic devices oper-
ating under near-IR light input.
Conclusion
3
formation of TADP*-C60 (Figure 10b). From the fitting of the
transient band at 680 nm, the decay rate constants of the
3TADP* (kT) were found to be 1.13”105 and 1.15”105 sÀ1 in tol-
uene and benzonitrile, respectively.
New donor–acceptor dyads utilizing near-IR sensitizers capable
of harvesting energy from the near-IR region of the electro-
magnetic spectrum have been synthesized by using a multistep
procedure and characterized by using a variety of physico-
chemical techniques. The energy level diagram established by
using the gathered optical, computational, and electrochemical
data revealed the possibility of electron transfer from the sin-
glet excited near-IR sensitizer with possible contributions from
the singlet excited fullerene. Accordingly, femtosecond transi-
ent absorption studies performed in nonpolar toluene and
polar benzonitrile revealed evidence of charge separation in
these dyads. The measured rates were found to be very fast
for the forward reaction and relatively slow for the reverse re-
action, implying charge stabilization. Nanosecond transient
spectral studies revealed population of the triplet states of
ADP, BADP, and TADP in the respective dyads, and the triplet
state of C60 in the case of BDP derived dyad. These findings
suggest that the near-IR sensitizers developed here are suitable
for harvesting light energy from the near-IR region of the solar
An examination of kinetic data for the studied dyads reveal
the following: 1) Both kCS and kCR values are higher in polar
benzonitrile than those in nonpolar toluene, a trend that gen-
erally agrees with the solvation effects on electron transfer
rates.[1–2] 2) The kCS values are an order of magnitude higher
than kCR, a trend that is often observed for fullerene-based
donor–acceptor dyads, implying that kCS falls in the normal
region of Marcus parabola, whereas kCR lies in the inverted
region.[27] 3) Neither kCS nor kCR follow a trend predicted accord-
ing to DGCS and DGCR from Table 1, suggesting other factors,
especially reorganization energies, might be different due to
different geometries of the modified donor moieties utilized in
the present study. 4) The magnitude of charge stabilization,
given as tRIP, suggests better charge stabilization in the TADP-
C60 dyad. These observations point out that the near-IR sensi-
tizer based donor–acceptor dyads investigated here could be
Chem. Eur. J. 2015, 21, 11483 – 11494
www.chemeurj.org
11491
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
lidine-H); 13C NMR (500 MHz, CDCl3 and CS2): d=174, 148, 146–
144(m), 144–142(m), 140,140–198(m), 136, 131, 129, 128, 127, 124,
122, 70, 69, 57, 34, 32, 31, 30–29 (m), 28–27 (m), 27–26 (m), 25, 23,
19, 14 ppm; MS (ESI, +, MeOH): m/z calcd for C95H29BN4O2:
1331.20; found: 804.5 [C65H10N, fulleropyrrolidine fragment], 454.3
[C29H21BN3O2, BADP fragment].
spectrum and for relevant technological developments. Further
studies along these lines are in progress.
Experimental Section
Chemicals: All the reagents were obtained from Aldrich Chemicals
(Milwaukee, WI), and the bulk solvents utilized in the syntheses
were obtained from Fischer Chemicals (Plano, TX). Tetra-n-butylam-
monium perchlorate [(nBu4N)ClO4] used in electrochemical studies
was obtained from Fluka Chemicals (Ronkonkoma, NY). The synthe-
ses of ADP-C60 and BDP-C60 were performed according to our earli-
er procedure.[15c,25]
Synthesis of TADP 2b: Compound 2a (500 mg) was dissolved in
anhydrous dichloroethane (80 mL) and stirred for 15 min under ni-
trogen. Diisopropylethylamine (1 mL, 6.05 mmol) was added and
the mixture was stirred for 1 h followed by addition of boron tri-
fluoride diethyl etherate (1 mL, 8.1 mmol). The mixture was heated
to reflux under N2 for 4 h, then the mixture was cooled to RT, dilut-
ed with CH2Cl2, and washed with water. The organic layer was sep-
arated, dried over Na2SO4, and evaporated to dryness. The residue
was purified by column chromatography on silica gel (CH2Cl2/
hexane, 1:1) to give 2b as a dark-green solid. Yield: 150 mg (27%);
1H NMR (400 MHz, CDCl3): d=8.18–8.00 (m, 6H; Ar-H), 7.54–7.50 (t,
J=16 Hz, 2H; Ar-H), 7.36–7.30 (t, J=24 Hz, 2H; thiophene-H), 6.94
(d, J=5.9 Hz, 2H; thiophene-H), 2.58 ppm (s, 6H; methyl-H).
Synthesis of BADP-C60 and TADP-C60 dyads: Scheme 1 outlines
the general procedure developed for syntheses of these dyads.
Synthesis of 1a and 2a:[16a] In a 250 mL round-bottomed flask,
1,2-dicyanobenzene (5 g, 0.39 mol) and diethyl ether (50 mL) were
added. Grignard reagent (1.1 equiv, RMgBr, R=phenyl, 3m or 2-
methylthiophene, 2m) was added and the mixture was stirred for
30 min at À208C under nitrogen. The reaction mixture was stirred
for another 3 h at RT. Solvent was evaporated and the reside was
heated to reflux with 300 mL formamide (100 mL each for every
30 min) for 3 h. During the reflux, the compound settled as a shiny
precipitate. After cooling the reaction mixture, the compound was
extracted by filtration and washed with a mixture of methanol/
water (2:1). The crude product was used for the next steps without
further purification.
Synthesis of BADP 1b:[16a] Compound 1a (500 mg, 1.258 mmmol)
was dissolved in anhydrous dichloroethane (80 mL) and stirred for
15 min under nitrogen. Diisopropylethylamine (0.8 mL, 6.29 mmol)
was then added and the mixture was stirred for 1 h followed by
addition of boron trifluoride diethyl etherate (0.7 mL, 6.29 mmol).
The mixture was heated to reflux under N2 for 4 h, then cooled to
RT, diluted with CH2Cl2, and washed with water. The organic layer
was separated, dried over Na2SO4, and evaporated to dryness. The
residue was purified by column chromatography on silica gel
(CH2Cl2/hexane, 1:1) to give 1b as a dark-green solid. Yield:
250 mg (45%); 1H NMR (400 MHz, CDCl3): d=8.10 (d, J=13.6 Hz,
2H; Ar-H), 7.90 (d, J=12 Hz, 2H; Ar-H), 7.77 (m, 4H; Ar-H), 7.50 (m,
8H; Ar-H), 7.30 ppm (d, 2H; Ar-H); MS (ESI, À, MeOH): m/z calcd
for C28H18BF2N3: 446.15; found: 445.15.
Synthesis of TADP-aldehyde 2c: Compound 2b (100 mg,
0.205 mmol) was dissolved in anhydrous CH2Cl2 (30 mL) and the
solution was stirred under nitrogen for 15 min. AlCl3 (137 mg,
1.027 mmol) was added and the solution was stirred for 30 min
before the addition of 3,4-dihydroxybenzaldehyde (142 mg,
1.028 mmol). The mixture was stirred for 30 min then flushed
through deactivated basic alumina as a filter column (CH2Cl2 as
eluent). The crude product was further purified by column chroma-
tography (silica; CH2Cl2) to give the product 2c. Yield: 50 mg
(42%); 1H NMR (400 MHz, CDCl3): d=9.50 (s, 1H; aldehyde-H),
8.30–8.14 (m, 6H; Ar-H), 7.56 (m, 2H; Ar-H), 7.35 (t, J=3.5 Hz, 2H;
Ar-H), 7.10 (d, J=11.8 Hz, 2H; dioxyaryl-H), 6.95 (d, J=1.4 Hz, 2H;
thiophene-H), 6.80 (d, J=3.8 Hz, 1H; dioxyaryl-H), 2.68 ppm (s, 6H;
methyl-H).
Synthesis of TADP-C60 2: To a solution of C60 (60 mg, 0.674 mmol)
in anhydrous toluene (120 mL), sarcosine (60 mg, 0.674 mmol) and
2c (75 mg, 0.128 mmol) were added. The solution was heated to
reflux for 24 h and the solvent was removed under vacuum. The
residue was purified by column chromatography (silica; CH2Cl2/tol-
1
uene 1:3) to give 2. Yield: 30 mg (18%); H NMR (400 MHz, CDCl3):
d=8.28 (d, J=12 Hz, 2H; Ar-H), 7.88 (d, J=12 Hz, 2H; Ar-H), 7.72–
7.64 (m, 4H; Ar-H), 7.54–7.48 (m, 5H; thiophene-H, dioxyaryl-H),
7.10 (d, J=19 Hz, 2H; thiophene-H), 4.55 (s, 1H; fulleropyrrolidine-
H), 4.42 (d, J=4 Hz, 1H; fulleropyrrolidine-H), 3.80 (s, 1H; fullero-
pyrrolidine-H), 2.80 (s, 3H; fulleropyrrolidine-H), 2.56 ppm (s, 6H;
methyl-H); MS (ESI, +, MeOH): m/z calcd for C95H27BN4O2S2:
1331.20; found: 1151.8 [M-thiophene units], 467.1 [TADP].
Synthesis of BADP-aldehyde 1c: Compound 1b (136 mg,
0.304 mmol) was dissolved in anhydrous CH2Cl2 (30 mL) and stirred
under nitrogen for 15 min. AlCl3 (203 mg, 1.522 mmol) was then
added and the solution was stirred for 30 min before the addition
of 3,4-dihydroxybenzaldehyde (210 mg, 1.520 mmol). The mixture
was stirred for 30 min and the reaction mixture was flushed
through deactivated basic alumina column with CH2Cl2 as eluent.
The crude product was further purified by column chromatography
Spectral measurements: The UV/Vis and near-IR spectral measure-
ments were carried out with a Shimadzu 2550 UV/Vis spectropho-
tometer or a Jasco V-670 spectrophotometer. The steady-state fluo-
rescence emission was monitored with a Varian (Cary Eclipse) fluo-
rescence spectrophotometer or a Horiba Jobin Yvon Nanolog spec-
trofluorimeter equipped with PMT (for UV/Vis) and InGaAs (for
near-IR) detectors. The fluorescence lifetimes were measured with
the Time Correlated Single Photon Counting (TCSPC) option with
nano-LED excitation sources on the Nanolog. A right angle detec-
tion method was used for both steady-state and time-resolved
emission measurements at RT. All the solutions were purged prior
1
(silica; CH2Cl2/hexanes, 1:1) to give 1c. Yield: 80 mg (48%); H NMR
(400 MHz, CDCl3): d=9.56 (s, 1H; aldehyde-H), 8.10 (d, J=11.2 Hz,
2H; Ar-H), 7.60–7.56 (m, 4H; Ar-H), 7.52–7.48 (m, 8H; Ar-H), 7.32
(d, J=8 Hz, 2H; Ar-H), 7.0–6.9 ppm (m, 5H; Ar-H, dioxyaryl-H).
Synthesis of BADP-C60 1: To
a solution of C60 (103 mg,
0.143 mmol) in anhydrous toluene (100 mL), sarcosine (22 mg,
0.246 mmol) and 1c (26 mg, 0.047 mmol) were added. The solution
was heated to reflux for 24 h and the solvent was removed under
vacuum. The residue was purified by column chromatography
(silica; ethyl acetate/CH2Cl2, 1:4) to give 1. Yield: 15 mg (24%);
1H NMR (400 MHz, CDCl3 and CS2): d=8.12–8.06 (m, 5H; Ar-H),
7.52–7.46 (m, 7H; Ar-H), 7.34–7.28 (m, 9H; Ar-H, dioxyaryl-H), 4.75
(d, J=4 Hz, 1H; fulleropyrrolidine-H), 4.55 (s, 1H; fulleropyrrolidine-
H), 4.05 (d, 1H; fulleropyrrolidine-H), 2.55 ppm (s, 3H; fulleropyrro-
1
to spectral measurements by using nitrogen gas. H NMR studies
were carried out with a Bruker 400 MHz spectrometer. Tetramethyl-
silane (TMS) was used as internal standard.
The computational calculations were performed by B3LYP methods
with the GAUSSIAN 03 software package.[19] The HOMO and LUMO
orbitals were generated with the GaussView program.
Chem. Eur. J. 2015, 21, 11483 – 11494
www.chemeurj.org
11492
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[2] a) M. R. Wasielewski, Chem. Rev. 1992, 92, 435; b) G. J. Meyer, Prog.
Inorg. Chem. 1997, 44, 167; c) P. Qu, G. J. Meyer in Electron Transfer in
Chemistry, Vol. 4 (Ed.: V. Balzani), 2001, p.353; d) W. Leibl, P. Mathis, Elec-
tron Transfer in Photosynthesis. Series on Photoconversion of Solar Energy
2004, 2, 117; e) V. Balzani, A. Credi, M. Venturi, ChemSusChem 2008, 1,
26; f) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2001, 34, 40;
g) J. L. Sessler, C. M. Lawrence, J. Jayawickramarajah, Chem. Soc. Rev.
2007, 36, 314; h) S. Fukuzumi, K. Ohkubo, J. Mater. Chem. 2012, 22,
4575; i) Photochemical Conversion and Storage of Solar Energy (Ed.: J. S.
Connolly), Academic Press, New York, 1981.
[3] a) D. M. Guldi, G. M. A. Rahman, F. Zerbetto, M. Prato, Acc. Chem. Res.
2005, 38, 871; b) L. Sµnchez, N. Martín, D. M. Guldi, Angew. Chem. Int.
Ed. 2005, 44, 5374; Angew. Chem. 2005, 117, 5508; c) S. Fukuzumi, Phys.
Chem. Chem. Phys. 2008, 10, 2283; d) M. E. El-Khouly, O. Ito, P. M. Smith,
F. D’Souza, J. Photochem. Photobiol. C 2004, 5, 79; e) M. R. Wasielewski,
Acc. Chem. Res. 2009, 42, 1910; f) D. I. Schuster, K. Li, D. M. Guldi, C. R.
Chemie 2006, 9, 892; g) S. Fukuzumi, K. Ohkubo, F. D’Souza, J. L. Sessler,
Chem. Commun. 2012, 48, 9801.
[4] a) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2009, 42, 1890; b) G.
Bottari, G. de La Torre, D. M. Guldi, T. Torres, Chem. Rev. 2010, 110, 6768;
c) D. M. Guldi, R. D. Costa, J. Phys. Chem. Lett. 2013, 4, 1489; d) A.
Satake, Y. Kobuke, Org. Biomol. Chem. 2007, 5, 1679; e) H. Imahori, T.
Umeyama, K. Kei, T. Yuta, Chem. Commun. 2012, 48, 4032; f) F. D’Souza,
O. Ito, Chem. Soc. Rev. 2012, 41, 86; g) S. Fukuzumi, Org. Biomol. Chem.
2003, 1, 609; h) N. V. Tkachenko, H. Lemmetyinen in Handbook of
Carbon Nanomaterials (Eds.: F. D’Souza, K. Kadish), World Scientific Pub-
lications, Singapore, 2011, Chapter 13, pp. 405–440; i) G. de La Torre, P.
Vazquez, F. Agullo-Lopez, T. Torres, Chem. Rev. 2004, 104, 3723.
[5] a) T. Umeyama, H. Imahori, Energy Environ. Sci. 2008, 1, 120; b) N. S.
Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15729; c) P. V.
Kamat, J. Phys. Chem. C 2007, 111, 2834; d) T. Hasobe, Phys. Chem.
Chem. Phys. 2010, 12, 44; e) V. Balzani, A. Credi, M. Venturi in Organic
Nanostructures (Eds.: J. L. Atwood, J. W. Steed), Wiley-VCH, Weinheim;
2008, pp. 1–31; f) Energy Harvesting Materials (Ed.: D. L. Andrews),
World Scientific, Singapore, 2005; g) J. Barber, Chem. Soc. Rev. 2009, 38,
185.
[6] a) Introduction of Molecular Electronics (Eds.: M. C. Petty, M. R. Bryce, D.
Bloor), Oxford University Press, New York, 1995; b) A. Aviram, B. Davis,
M. Kemp, V. Mujica, A. Roitberg, S. Yaliraki, Ann. NY Acad. Sci. 1998, 852,
22; c) Molecular Switches (Ed.: B. L. Feringa), Wiley-VCH, Weinheim,
2001; d) D. Gust, T. A. Moore, A. L. Moore, Chem. Commun. 2006, 11,
1169; e) V. Balzani, A. Credi, M. Venturi, in Organic Nanostructures (Eds.:
J. L. Atwood, J. W. Steed), Wiley-VCH, Weinheim, 2008, pp. 1–31; f) J. M.
Tour, Molecular Electronics; Commercial Insights, Chemistry, Devices, Ar-
chitectures and Programming, World Scientific, River Edge, 2003; g) J. S.
Lindsey, D. F. Bocian, Acc. Chem. Res. 2011, 44, 638; h) F. D’Souza, O. Ito,
Chem. Commun. 2009, 4913.
Electrochemistry: Differential pulse voltammetry was recorded
with a Princeton Applied Research potentiostat/galvanostat Model
263A using a three-electrode system. A platinum button electrode
was used as the working electrode, a platinum wire served as the
counter electrode, and an Ag/AgCl electrode was used as the refer-
ence electrode. Ferrocene/ferrocenium redox couple was used as
an internal standard. All the solutions were purged prior to electro-
chemical and spectral measurements with nitrogen gas.
Femtosecond laser flash photolysis: Femtosecond transient ab-
sorption spectroscopy experiments were performed with an Ultra-
fast Femtosecond Laser Source (Libra) by Coherent incorporating
diode-pumped, mode locked Ti:Sapphire laser (Vitesse) and diode-
pumped intra cavity doubled Nd:YLF laser (Evolution) to generate
a compressed laser output of 1.45 W. For optical detection,
a Helios transient absorption spectrometer coupled with femtosec-
ond harmonics generator, both provided by Ultrafast Systems LLC,
was used. The source for the pump and probe pulses were derived
from the fundamental output of Libra (compressed output 1.45 W,
pulse width 100 fs) at a repetition rate of 1 kHz. 95% of the funda-
mental output of the laser was introduced into harmonic genera-
tor, which produces second and third harmonics of 400 and
267 nm besides the fundamental 800 nm for excitation, while the
rest of the output was used for generation of white light continu-
um. In the present study, the second harmonic 400 nm excitation
pump was used in all the experiments. Kinetic traces at appropri-
ate wavelengths were assembled from the time-resolved spectral
data. Data analysis was performed with Surface Xplorer software
supplied by Ultrafast Systems. All measurements were conducted
in degassed solutions at 298 K.
Nanosecond laser flash photolysis: The studied compounds were
excited with a Opolette HE 355 LD pumped by a high energy
Nd:YAG laser with second and third harmonics OPO (tuning range
410–2200 nm, pulse repetition rate 20 Hz, pulse length 7 ns) with
the powers of 1.0 to 3 mJ per pulse. The transient absorption
measurements were performed with a Proteus UV/Vis-NIR flash
photolysis spectrometer (Ultrafast Systems, Sarasota, FL) with a fi-
beroptic delivered white probe light and either a fast rise Si photo-
diode detector covering the 200–1000 nm range or a InGaAs pho-
todiode detector covering 900–1600 nm range. The output from
the photodiodes and a photomultiplier tube was recorded with
a digitizing Tektronix oscilloscope. Data analysis was performed
with Surface Xplorer software supplied by Ultrafast Systems.
[7] a) The Photosynthetic Reaction Center (Eds.: J. Deisenhofer, J. R. Norris),
Academic Press, New York, 1993; b) Molecular Mechanism of Photosyn-
thesis (Ed.: R. E. Blankenship), Blackwell Science, Oxford, 2002.
[8] a) H. Imahori, D. M. Guldi, K. Tamaki, Y. Yoshida, C. Luo, Y. Sakata, S. Fu-
kuzumi, J. Am. Chem. Soc. 2001, 123, 6617; b) H. Imahori, K. Tamaki,
D. M. Guldi, C. Luo, M. Fujitsuka, O. Ito, Y. Sakata, S. Fukuzumi, J. Am.
Chem. Soc. 2001, 123, 2607; c) F. D’Souza, E. Maligaspe, K. Ohkubo, M. E.
Zandler, N. K. Subbaiyan, S. Fukuzumi, J. Am. Chem. Soc. 2009, 131,
8787; d) M. E. El-Khouly, C. A. Wijesinghe, V. N. Nesterov, M. E. Zandler, S.
Fukuzumi, F. D’Souza, Chem. Eur. J. 2012, 18, 13844; e) G. N. Lim, E. Mali-
gaspe, M. E. Zandler, F. D’Souza, Chem. Eur. J. 2014, 20, 17089; f) C. B.
KC, G. N. Lim, F. D’Souza, Nanoscale 2015, 7, 6813; g) C. B. KC, G. N. Lim,
F. D’Souza, Angew. Chem. Int. Ed. 2015, 54, 5088; Angew. Chem. 2015,
127, 5177.
[9] a) A. Dualeh, J. H. Delcamp, M. K. Nazeeruddin, M. Grätzel, Appl. Phys.
Lett. 2012, 100, 173512; b) E. Arunkumar, A. Ajayaghosh, Chem.
Commun. 2005, 5, 599; c) V. Bandi, S. K. Das, S. G. Awuah, Y. You, F.
D’Souza, J. Am. Chem. Soc. 2014, 136, 7571.
[10] a) A. Treibs, F. H. Kreuzer, Justus Liebigs Ann. Chem. 1968, 718, 208; b) A.
Loudet, K. Burgess, Chem. Rev. 2007, 107, 4891; c) N. Boens, V. Leen, W.
Dehaen, Chem. Soc. Rev. 2012, 41, 1130; d) M. Tasior, D. F. O’Shea, Bio-
conjugate Chem. 2010, 21, 1130; e) M. Grossi, A. Palma, S. O. McDonnell,
M. J. Hall, D. K. Rai, J. Muldoon, D. F. O’Shea, J. Org. Chem. 2012, 77,
9304.
Acknowledgements
This work was supported by the National Science Foundation
(Grant No. 1401188 to F.D.).
Keywords: donor–acceptor systems
energy conversion · fullerenes · sensitizers
· electron transfer ·
[1] a) H. Taube, Electron Transfer Reactions of Complex Ions in Solution, Aca-
demic Press, New York, 1970; b) J. R. Bolton, N. Mataga, G. McLendon,
Advances in Chemical Series 1991, 228, 1; c) Electron Transfer in Chemis-
try and Biology: An Introduction to the Theory (Eds.: A. M. Kuznetsov, J.
Ulstrup), Wiley, New York, 1998; d) Photoinduced Electron Transfer,
Vols. A–D (Eds.: M. A. Fox, M. Chanon), Elsevier, Amsterdam, 1988;
e) Topics in Current Chemistry: Photoinduced Electron Transfer, Vol. 156–
157 (Ed.: J. Mattay), Springer, Berlin, 1990; f) Electron Transfer: from Iso-
lated Molecules to Biomolecules (Eds.: J. Jortner, M. Bixson), parts 1 and
2, John Wiley, New York, 1999, p.107; g) Electron Transfer in Chemistry,
Vols. 1–4 (Ed.: V. Balzani), Wiley-VCH, Weinheim, 2001–2002.
Chem. Eur. J. 2015, 21, 11483 – 11494
www.chemeurj.org
11493
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[11] a) G. Ulrich, R. Ziessel, A. Harriman, Angew. Chem. Int. Ed. 2008, 47, 1184;
Angew. Chem. 2008, 120, 1202; b) R. Ziessel, A. Harriman, Chem.
Commun. 2011, 47, 611; c) M. E. El-Khouly, S. Fukuzumi, F. D’Souza,
ChemPhysChem 2014, 15, 30.
[12] a) F. Li, S. I. Yang, T. Ciringh, J. Seth, C. H. Martin III, D. L. Singh, D. Kim,
R. R. Birge, D. F. Bocian, D. Holten, J. S. Lindsey, J. Am. Chem. Soc. 1998,
120, 10001; b) C. B. KC, G. N. Lim, V. N. Nesterov, P. A. Karr, F. D’Souza,
Chem. Eur. J. 2014, 20, 17100 and references cited therein.
[13] a) S. O. McDonnell, M. J. Hall, L. T. Allen, A. Byrne, W. M. Gallagher, D. F.
O’Shea, J. Am. Chem. Soc. 2005, 127, 16360; b) K. Flavin, K. Lawrence, J.
Bartelmess, M. Tasior, C. Navio, C. Bittencourt, D. F. O’Shea, D. M. Guldi,
S. Giordani, ACS Nano 2011, 5, 1198.
[14] a) W.-J. Shi, R. Menting, E. A. Ermilov, P. C. Lo, B. Roeder, D. K. P. Ng,
Chem. Commun. 2013, 49, 5277; b) W.-J. Shi, M. E. El-Khouly, K. Ohkubo,
S. Fukuzumi, D. K. P. Ng, Chem. Eur. J. 2013, 19, 11332; c) M. Yuan, X.
Yin, H. Zheng, C. Ouyang, Z. Zuo, H. Liu, Y. Li, Chem. Asian J. 2009, 4,
707; d) P. A. Bouit, K. Kamada, P. Feneyrou, G. Berginc, L. Toupet, O.
Maury, C. Andraud, Adv. Mater. 2009, 21, 1151; e) S. Y. Leblebici, L.
Catane, D. E. Barclay, T. Olson, T. L. Chen, B. Ma, ACS Appl. Mater. Interfa-
ces 2011, 3, 4469; f) K. Flavin, I. Kopfa, J. Murtagh, M. Grossi, D. F.
O’Shea, S. Giordani, Supramol. Chem. 2012, 24, 23.
[15] a) A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan, M. E. Zandler, M. Supur, S.
Fukuzumi, F. D’Souza, J. Phys. Chem. A 2011, 115, 9810; b) M. E. El-
Khouly, A. N. Amin, M. E. Zandler, S. Fukuzumi, F. D’Souza, Chem. Eur. J.
2012, 18, 5239; c) A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan, M. E. Zan-
dler, S. Fukuzumi, F. D’Souza, Chem. Commun. 2012, 48, 206; d) F.
D’Souza, A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan, M. E. Zandler, S.
Fukuzumi, J. Am. Chem. Soc. 2012, 134, 654; e) V. Bandi, K. Ohkubo, S.
Fukuzumi, F. D’Souza, Chem. Commun. 2013, 49, 2867; f) V. Bandi, M. E.
El-Khouly, V. N. Nesterov, P. A. Karr, S. Fukuzumi, F. D’Souza, J. Phys.
Chem. C 2013, 117, 5638; g) V. Bandi, M. E. El-Khouly, K. Ohkubo, V. N.
Nesterov, M. E. Zandler, S. Fukuzumi, F. D’Souza, Chem. Eur. J. 2013, 19,
7221; h) V. Bandi, H. B. Gobeze, P. A. Karr, F. D’Souza, J. Phys. Chem. C
2014, 118, 18969; i) V. Bandi, H. B. Gobeze, V. N. Nesterov, P. A. Karr, F.
D’Souza, Phys. Chem. Chem. Phys. 2014, 16, 25537; j) V. Bandi, F. P.
D’Souza, H. B. Gobeze, F. D’Souza, Chem. Eur. J. 2014, 20, 1; k) V. Bandi,
H. B. Gobeze, V. Lakshmi, M. Ravikanth, J. Phys. Chem. C 2015, 119, 8095.
[16] a) R. Gresser, M. Hummert, H. Hartmann, K. Leo, M. Riede, Chem. Eur. J.
2011, 17, 2939; b) H. Lu, S. Shimizu, J. Mack, Z. Shen, N. Kobayashi,
Chem. Asian J. 2011, 6, 1026; c) X. Zhang, H. Yu, Y. Xiao, J. Org. Chem.
2012, 77, 669.
1594; Angew. Chem. 1997, 109, 1666; c) Q. Xie, E. Perez-Cordero, L.
Echegoyen, J. Am. Chem. Soc. 1992, 114, 3978.
[18] M. Maggini, G. Scorrano, M. Prato, J. Am. Chem. Soc. 1993, 115, 9798.
[19] Gaussian 03, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Strat-
mann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin,
M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Men-
nucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y.
Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashen-
ko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith,
M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challa-
combe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres,
M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc., Pittsburgh
PA, 2003.
[20] For a general review on DFT applications of porphyrin-fullerene systems
see: M. E. Zandler, F. D’Souza, C. R. Chemie 2006, 9, 960.
[21] a) D. Rehm, A. Weller, Isr. J. Chem. 1970, 8, 259; b) N. Mataga, H. Miyasa-
ka, in Electron Transfer (Eds.: J. Jortner, M. Bixon), John Wiley & Sons,
New York, 1999, Part 2, pp. 431–496.
[22] Phosphorescence spectrum of ADP, BADP, and TADP in the presence of
CH3I did not show any peaks in the range of 700–1200 nm, which sug-
gests that the energy triplet levels lie below 1.0 eV. For the phosphores-
cence spectrum of BDP, the value reported for halogenated BDP was
employed, see Ref. [23].
[23] X.-F. Zhang, X. Yang, K. Niu, H. Geng, J. Photochem. Photobiol. A 2014,
285, 16.
[24] Principles of Fluorescence Spectroscopy, 3rd ed. (Ed.: J. R. Lakowicz),
Springer, Singapore, 2006.
[25] a) C. A. Wijesinghe, M. E. El-Khouly, J. D. Blakemore, M. E. Zandler, K. Fu-
kuzumi, F. D’Souza, Chem. Commun. 2010, 46, 3301; b) C. A. Wijesinghe,
M. E. El-Khouly, N. K. Subbaiyan, M. Supur, M. E. Zandler, K. Ohkubo, S.
Fukuzumi, F. D’Souza, Chem. Eur. J. 2011, 17, 3147.
[26] F. D’Souza, R. Chitta, K. Ohkubo, M. Tasior, N. K. Subbaiyan, M. E. Zandler,
M. K. Rogachi, D. T. Gryko, S. Fukuzumi, J. Am. Chem. Soc. 2008, 130,
14263.
[27] R. A. Marcus, N. Sutin, Biochimica. Biophys. Acta 1985, 811, 265.
Received: February 21, 2015
[17] a) H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, R. E. Smalley, Nature
1985, 318, 162; b) R. E. Smalley, Angew. Chem. Int. Ed. Engl. 1997, 36,
Published online on June 30, 2015
Chem. Eur. J. 2015, 21, 11483 – 11494
www.chemeurj.org
11494
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim