A new approach for the photosynthetic antenna-reaction center complex with a model organized around an s-triazine linker

Article abstract of DOI:10.1002/chem.201302632

Two new artificial mimics of the photosynthetic antenna-reaction center complex have been designed and synthesized (BDP-H₂P-C₆₀ and BDP-ZnP-C₆₀). The resulting electron-donor/acceptor conjugates contain a porphyrin (either in its free-base form (H₂P) or as Zn-metalated complex (ZnP)), a boron dipyrrin (BDP), and a fulleropyrrolidine possessing, as substituent of the pyrrolidine nitrogen, an ethylene glycol chain terminating in an amino group C₆₀-X-NH₂ (X=spacer). In both cases, the three different components were connected by s-triazine through stepwise substitution reactions of cyanuric chloride. In addition to the facile synthesis, the star-type arrangement of the three photo- and redox-active components around the central s-triazine unit permits direct interaction between one another, in contrast to reported examples in which the three components are arranged in a linear fashion. The energy- and electron-transfer properties of the resulting electron-donor/acceptor conjugates were investigated by using UV/Vis absorption and emission spectroscopy, cyclic voltammetry, and femtosecond transient absorption spectroscopy. Comparison of the absorption spectra and cyclic voltammograms of BDP-H₂P-C₆₀ and BDP-ZnP-C ₆₀ with those of BDP-H₂P, BDP-ZnP and BDP-C₆₀, which were used as references, showed that the spectroscopic and electrochemical properties of the individual constituents are basically retained, although some appreciable shifts in terms of absorption indicate some interactions in the ground state. Fluorescence lifetime measurements and transient absorption experiments helped to elucidate the antenna function of BDP, which upon selective excitation undergoes a rapid and efficient energy transfer from BDP to H₂P or ZnP. This is then followed by an electron transfer to C ₆₀, yielding the formation of the singlet charge-separated states, namely BDP-H₂P^.+-C₆₀^.- and BDP-ZnP^.+-C₆₀^.-. As such, the sequence of energy transfer and electron transfer in the present models mimics the events of natural photosynthesis.

Full text of DOI:10.1002/chem.201302632

DOI: 10.1002/chem.201302632
Full Paper
&
Photosynthesis
A New Approach for the Photosynthetic Antenna–Reaction Center
Complex with a Model Organized Around an s-Triazine Linker
Susanne Kuhri,[a] Georgios Charalambidis,[b] Panagiotis A. Angaridis,[b] Theodore Lazarides,[c]
Georgia Pagona,[d] Nikos Tagmatarchis,*[d] Athanassios G. Coutsolelos,*[b] and Dirk M. Guldi*[a]
Abstract: Two new artificial mimics of the photosynthetic
antenna-reaction center complex have been designed and
synthesized (BDP-H2P-C60 and BDP-ZnP-C60). The resulting
emission spectroscopy, cyclic voltammetry, and femtosecond
transient absorption spectroscopy. Comparison of the ab-
sorption spectra and cyclic voltammograms of BDP-H2P-C60
and BDP-ZnP-C60 with those of BDP-H2P, BDP-ZnP and BDP-
C60, which were used as references, showed that the spectro-
scopic and electrochemical properties of the individual con-
stituents are basically retained, although some appreciable
shifts in terms of absorption indicate some interactions in
the ground state. Fluorescence lifetime measurements and
transient absorption experiments helped to elucidate the an-
tenna function of BDP, which upon selective excitation un-
dergoes a rapid and efficient energy transfer from BDP to
H2P or ZnP. This is then followed by an electron transfer to
C60, yielding the formation of the singlet charge-separated
electron-donor/acceptor conjugates contain
a porphyrin
(either in its free-base form (H2P) or as Zn-metalated com-
plex (ZnP)), a boron dipyrrin (BDP), and a fulleropyrrolidine
possessing, as substituent of the pyrrolidine nitrogen, an
ethylene glycol chain terminating in an amino group C60-X-
NH2 (X=spacer). In both cases, the three different compo-
nents were connected by s-triazine through stepwise substi-
tution reactions of cyanuric chloride. In addition to the facile
synthesis, the star-type arrangement of the three photo- and
redox-active components around the central s-triazine unit
permits direct interaction between one another, in contrast
to reported examples in which the three components are ar-
ranged in a linear fashion. The energy- and electron-transfer
properties of the resulting electron-donor/acceptor conju-
gates were investigated by using UV/Vis absorption and
states, namely BDP-H2P +-C60 and BDP-ZnP +-C60 . As such,
the sequence of energy transfer and electron transfer in the
present models mimics the events of natural photosynthesis.
C
CÀ
C
CÀ
Introduction
of mimicking photosynthesis to convert, for example, sunlight
into energy, but also with respect to the construction of pho-
tonics, photocatalytic, and optoelectronic devices.[2] The com-
plexity of artificial photosynthesis is the integration of light-
harvesting, energy transfer, charge transfer, charge transport,
and catalysis in a single system.[1a–c,3]
Mimicking the photochemical process of natural photosynthe-
sis by means of bioinspired synthetic models has been an area
of great interest during the last decades.[1] The development of
such molecular systems is important not only from a standpoint
As in natural photosynthesis, porphyrin-based reactions are
key steps in artificial photosynthesis. Porphyrins and related
tetrapyrrolic systems have widely been investigated as light-
capturing antenna systems and/or as electron donors.[4–7] Nota-
bly, porphyrins feature rather moderate absorption cross sec-
tion in the 430 to 500 nm region, and they allocate redox-
active excited states.[8]
[a] S. Kuhri, Prof. Dr. D. M. Guldi
Department of Chemistry and Pharmacy
Interdisciplinary Center for Molecular Materials (ICMM)
Friedrich-Alexander-Universitaet Erlangen-Nuernberg
Egerlandstr. 3, 91058 Erlangen (Germany)
E-mail: dirk.guldi@chemie.uni-erlangen.de
[b] Dr. G. Charalambidis, Prof. P. A. Angaridis, Prof. Dr. A. G. Coutsolelos
Department of Chemistry, University of Crete
Fullerenes, which are well-known for their remarkable elec-
tron-acceptor properties, evolved as an interesting class of
functional materials in the context of mimicking photosynthe-
sis.[9] In particular, they feature low reduction potentials and
small reorganization energies in charge-transfer reactions.[10–13]
From these features, fast charge-separation and slow charge-
recombination processes lead to long-lived charge-separated
states.[14] The use of either noncovalent interactions, such as
metal-to-ligand coordination or electrostatic interactions, or
covalent bonds, provides easy means for constructing new
electron-donor/acceptor ensembles and/or conjugates.[1f,7,15–17]
Voutes Campus, PO Box 2208, 71003 Heraklion, Crete (Greece)
E-mail: coutsole@chemistry.uoc.gr
[c] Prof. T. Lazarides
Department of Chemistry, University of Ioannina
Dourouti Campus, 45110 Ioannina (Greece)
[d] Dr. G. Pagona, Dr. N. Tagmatarchis
Theoretical and Physical Chemistry Institute
National Hellenic Research Foundation
48 Vassileos Constantinou Avenue, Athens 11635 (Greece)
E-mail: tagmatar@eie.gr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201302632.
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As a matter of fact, a wide variety of reaction center mimics
based on fullerenes and porphyrins have been devised, synthe-
sized, and examined.[7,18–22]
porphyrin energy transfer. As a matter of fact, BDP increases
the light-harvesting potential of the porphyrin chromophore.
In the present work, we have further exploited these conju-
gates as precursors in reactions with an amino-functionalized
electron-accepting fulleropyrrolidine possessing, as a substitu-
ent of the pyrrolidine nitrogen, an ethylene glycol chain termi-
nating in an amino group C60-X-NH2 (X=spacer; Scheme 1).
The new electron-donor/acceptor conjugates obtained in this
way, namely BDP-H2P-C60 and BDP-ZnP-C60, feature three differ-
ent components linked through s-triazine, and they were ex-
amined as artificial photosynthetic models in mimicking the
energy- and electron-transfer processes of natural photosyn-
thesis. The strategy followed in the synthesis of BDP-H2P-C60
and BDP-ZnP-C60 represents a new and facile approach for the
preparation of covalently linked multichromophoric systems
with a star-type arrangement of three photo- and redox-active
moieties around a central bridging unit. With such an assem-
bly, the mutual structural arrangement of the chromophores is
achieved, resulting in a triangular conformation, in which each
chromophore interacts directly with the other two. It is be-
lieved that the latter would enhance electronic communication
between the photoexcited electron donors and the fullerene
derivative acceptor.
Furthermore, boron dipyrrins (BDP) are also of particular in-
terest as light-harvesters.[23] BDPs are well-known for their high
extinction coefficients, high fluorescence quantum yields, long-
lived excited states, suitable excited-state energies, excellent
photostability, and stability in many solvents.[24] Remarkable is
also the ease of modification of the BDP core, which assists in
tailoring the absorption and emission properties throughout
the 450–680 nm range and, in turn, renders them complemen-
tary energy donors.[15e,25,26] In fact, several studies have demon-
strated energy-transfer interactions between BDP, porphyrins,
perylenediimides, and subphthalocyanines.[14,15b,e] Due to the
good spectral overlap between fluorescence of BDP and ab-
sorption of porphyrins, an efficient transduction of singlet ex-
cited-state energy takes place.[15b,13c] Notably, only a few re-
ports on the covalent linkage of BDP to a porphyrin and to
a fullerene exist.[13c,14,15b,c,e,f] Among the latter, the one by Fuku-
zumi and co-workers, stands out due to its uniqueness.[27]
We recently described the facile synthesis of two BDP–por-
phyrin conjugates, that is, BDP-H2P and BDP-ZnP, bridged by
an s-triazine unit.[28] Fluorescence and transient absorption ex-
periments showed that BDP excitation results in rapid BDP to
Scheme 1. Synthesis of BDP-H2-C60, BDP-ZnP-C60, and BDP-C60, and the structure of the reference compounds H2P-NHCl and ZnP-NHCl.
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Experimental Section
Results and Discussion
Photophysical measurements
Synthesis and characterization
All measurements were performed in solvents of spectroscopic
grade at room temperature in a 10ꢁ10 mm quartz glass cuvette.
Cyanuric chloride is a very useful synthon, in view of the fact
that the temperature-dependent stepwise substitution of its
three chlorine atoms allows the sequential introduction of up
to three different nucleophiles on the s-triazine unit. As
a result, it provides an easy synthetic route to molecular archi-
tectures with different chromophores in good yields, avoiding
multi-step and expensive preparation and purification meth-
ods. As shown in Scheme 1, BDP-H2P-C60 and BDP-ZnP-C60
were prepared starting from BDP-H2P and BDP-ZnP, respective-
ly.[28] Nucleophilic substitution of the chlorine atoms of cyan-
uric chloride of BDP-H2P-C60 and BDP-ZnP-C60 by the terminal
amino group of C60-X-NH2, in THF heated at reflux and in the
presence of N,N-diisopropylethylamine (DIPEA), produced the
desired electron-donor/acceptor conjugates. BDP-C60 was syn-
thesized in a similar way by treatment of BDP-NHCl with C60-X-
NH2 and used as reference.[28] All compounds were fully charac-
terized by 1H NMR spectroscopy and MALDI-TOF mass spec-
trometry.
UV/Vis absorption spectra were recorded on
a PerkinElmer
Lambda2 UV/Vis two-beam spectrophotometer with a slit width of
2 nm and a scan rate of 240 nmminÀ1. Emission spectra were re-
corded by using a Horiba Jobin Yvon FluoroMax-3 spectrometer
with a slit width of 2 nm for excitation and emission and an inte-
gration time of 0.5 s in a wavelength range of 300–1000 nm. Quan-
tum yields were determined from corrected emission spectra fol-
lowing the standard methods[29] using meso-tetraphenyl porphyrin
(H2P; F=0.11 in toluene)[30] and Zn-metalated meso-tetraphenyl
porphyrin (ZnP; F=0.03 in toluene)[30] as standards. Emission life-
times were determined by using time-correlated single-photon
counting (TCSPC) on a Horiba Jobin Yvon FluoroLog3 emission
spectrometer with a Hamamatsu MCP photomultiplier (R3809U-
58). For excitation, a laser diode (NanoLED-405L, 403 nm, pulse
width ꢀ200 ps, maximum of repetition rate 100 kHz) was used.
Femtosecond transient absorption studies were performed with
420 and 490 nm laser pulses (1 kHz, 150 fs pulse width) from an
amplified Ti:Sapphire laser system (Clark-MXR, Inc. CPA 2101), the
laser energies were 120 and 200 nJ, respectively. Transient spectra
were recorded with an analyzing solution in a 2 mm quartz glass
cuvette.
Electrochemical data were obtained by cyclic voltammetry and
square-wave voltammetry measurements, by using a conventional
single-compartment three-electrode cell arrangement in combina-
tion with a potentiostat “AUTOLAB, eco chemie”. Platinum and
silver wires were used as the auxiliary and reference electrodes, re-
spectively, whereas the working electrode was a glassy carbon
electrode. Measurements were carried out in saturated toluene/
acetonitrile (4:1, v/v) solutions, at room temperature, under argon.
Tetrabutylammonium hexafluorophosphate (TBAPF6 =0.1m) was
used as the supporting electrolyte. All potentials are referenced to
the ferrocene/ferrocenium couple (Fc/Fc+ =0.33 V).
Physicochemical studies
Electrochemistry
The electrochemical behavior of BDP-H2P-C60, BDP-ZnP-C60, and
the corresponding references was investigated by cyclic and
square-wave voltammetry. All redox potentials, which are re-
ported versus the ferrocene/ferrocenium (Fc/Fc+) couple, are
summarized in Table 1. The cyclic voltammograms of BDP-H2P-
C60 and BDP-ZnP-C60 (Figure S1 in the Supporting Information)
reveal reversible oxidations occurring at 0.6 V for ZnP and at
0.8 V for H2P as well as at approximately 1.0 V for BDP. No C60-
centered oxidations were observed within the recorded poten-
tial window. Additionally, reversible processes corresponding
to reductions were observed. In particular, C60-centered reduc-
tions occur at À0.8 and À1.8 V, whereas BDP reductions are
observed at À1.1 and À1.4 V. Reductions of ZnP and H2P occur
at À1.77 and À1.79 V, respectively. It should be noted that in
BDP-H2P-C60 and BDP-ZnP-C60 the reductions of H2P/ZnP, and
of C60 overlap.[32]
Materials
H2P-NHCl,[28] ZnP-NHCl,[28] BDP-H2P,[28] BDP-ZnP,[28] BDP-NHCl,[28] and
C60-X-NH2[31] were prepared according to published procedures. Tet-
rahydrofuran was freshly distilled from Na/benzophenone. All
other chemicals and solvents were purchased from commercial
sources and used as received.
NMR Spectra
1H NMR spectra were recorded on Bruker AVANCE III-
500 MHz and Bruker DPX-300 MHz spectrometers using
solutions in deuterated solvents and the solvent peak
was chosen as the internal standard.
Table 1. Redox potentials of BDP-NHCl, BDP-C60, BDP-H2P, BDP-H2P-C60, BDP-ZnP, and
BDP-ZnP-C60 versus Fc/Fc+ (0.33 V) in toluene/acetonitrile (4:1, v/v) with tetrabutylam-
monium hexafluorophosphate 0.1m as supporting electrolyte.
E1/2
Oxidation
Reduction
+
+
C
C
CÀ
CÀ
CÀ
CÀ
CÀ
BDP/BPD P/P
C60/C60 BDP/BDP BDP/BDP C60/C60 P/P
Mass spectroscopy
BDP-NHCl
BDP-C60
BDP-H2P
BDP-H2P-C60 1.054
BDP-ZnP 0.99
BDP-ZnP-C60 0.955
1.015
1.005
1.045
À1.098
À1.128
À1.115
À1.46
À1.45
À1.38
À1.4
À0.81
À1.775
À1.79
High-resolution mass spectra were obtained on a Bruker
UltrafleXtreme matrix-assisted laser desorption ionization
time-of-flight (MALDI-TOF) spectrometer using trans-2-[3-
(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononi-
trile (DCTB) as matrix.
0.855
À1.77
0.83 À0.845 À1.085
0.605
À1.08
À1.395
À1.775
0.595 À0.805 À1.115
À1.775
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of the BDP fluorescence when
excited at 495 nm. The BDP fluo-
rescence lifetime in BPD-C60 is
much shorter (0.21 ns) than that
of BDP-NHCl (3.2 ns). This is
taken as evidence of electronic
communication, either in the
form of energy or electron trans-
fer, between the light-harvesting
BDP and the electron-accepting
C60 in the absence of either H2P
or ZnP. Exciting BDP-H2P-C60
(Figure 2, middle panel) at
495 nm evokes again a rather
strong quenching of the BDP
fluorescence, namely 98.1%. Si-
multaneously, the H2P related
fluorescence evolves with 1%,
Table 2.
Summary of spectroscopic data of BDP-NHCl, BDP-C60, H2P-NHCl, BDP-H2P, BDP-H2P-C60, ZnP-NHCl, BDP-ZnP,
and BDP-ZnP-C60 measured in pure toluene.
Absorption
lmax [nm]
Fluorescence
lmax
f
f
t [ns]
(e [mÀ1 cmÀ1])
[nm]
(lex=495 nm)
(lex=418 nm)
BDP
P
P
BDP
P
BDP-NHCl
BDP-C60
H2P-NHCl
BDP-H2P
BDP-H2P-C60
ZnP-NHCl
BDP-ZnP
504 (75620)
504 (75580)
420 (221786)
420 (473980), 504 (98679) 514, 651, 718 0.01
422 (251268), 504 (74222) 514, 653, 718 0.011
423 (185072)
423 (460960), 504 (81767) 514, 596, 647 0.005
426 (278221), 504 (76451) 514, 598, 649 0.011
514
514
651, 718
0.58
0.018
3.25
0.21
0.11
0.11
0.107
0.01
0.04
0.04
0.003
0.127
0.019
0.04
0.04
0.006
0.08[a] 9.26
0.08[a] 9.63
596, 647
0.11
0.12
1.98
0.76
BDP-ZnP-C60
[a] These values reflect the upper limits imposed by the time resolution of the laser diodes that have been em-
ployed to photoexcite the samples.
Absorption spectra
which is in line with an earlier report[28] on an efficient energy-
transfer from the singlet excited-state of BDP to H2P in its
ground state. This is confirmed by means of an excitation spec-
trum of the evolving H2P fluorescence, which shows features
of H2P and BDP absorption (Figure S2 in the Supporting Infor-
mation).[28] Similarly, in BDP-H2P the BDP fluorescence is
quenched by 98.3% and the H2P fluorescence is activated with
15.5%. The H2P fluorescence is notably quenched in BDP-H2P-
C60 relative to BDP-H2P. Such quenching indicates deactivation
of H2P in the presence of C60, probably through electron trans-
fer. All of the aforementioned effects are even more pro-
nounced when comparing BDP-ZnP with BDP-ZnP-C60 upon
495 nm excitation (Figure 2, bottom). The BDP fluorescence
quenching in BDP-ZnP-C60 is 98.1% compared with BDP-NHCl,
whereas BDP-ZnP gives rise to 99.1% quenching of the
514 nm fluorescence. In BDP-ZnP-C60, ZnP fluorescence
evolved, which was at the expense of BDP fluorescence
quenching (as it was also observed in case of BDP-ZnP). Never-
theless, the presence of C60 causes appreciable quenching and
a 2 to 3 nm redshift in the fluorescence pattern. Besides the
BDP, ZnP, and H2P quenching, no additional features are noted.
In additional experiments all of the H2P- and ZnP-containing
systems were excited at 418 nm (Figure S3 in the Supporting
Information). Clearly, in both scenarios the overall fluorescence
intensity and, in turn, the corresponding quantum yields in-
crease when comparing, for example, H2P-NHCl/ZnP-NHCl with
BDP-H2P/BDP-ZnP and strongly decrease when comparing H2P-
NHCl/ZnP-NHCl and BDP-H2P/BDP-ZnP with BDP-H2P-C60/BDP-
ZnP-C60.
Absorption spectra of BDP-H2P-C60 and BDP-ZnP-C60 as well as
those of their references are shown in Figure 1 and a summary
of their absorption characteristics is given in Table 2. The ab-
sorption spectrum of BDP-NHCl compares well with what is
known from the literature, showing a dominant absorption
with a maximum at 504 nm and a weak absorption in the 300–
380 nm region (Figure 1, top).[28] Comparing the latter with
BDP-C60, again the characteristic feature at 504 nm is observed.
Additionally, a broad absorption originating from C60 is discern-
ible between 300 and 400 nm (Figure 1, top). Absorption spec-
tra of BDP-H2P-C60 and the corresponding references are
shown in Figure 1 (central panel). H2P-NHCl features the Soret
band at 420 nm as well as Q-bands between 500 and 700 nm.
For BDP-H2P, in addition to the latter absorptions, the broad
BDP absorption is observed at 504 nm; none of these peaks
are, however, shifted. In the BDP-H2P-C60 absorption spectrum
all of the features of the single constituents are identified, that
is, C60 between 300–380 nm, BDP at 504 nm, and H2P at
422 nm as well as between 500–700 nm. Noticeably, a redshift
of the Soret band to 422 nm is observed, which implies elec-
tronic communication between C60 and H2P. The absorption
spectrum of BDP-ZnP-C60 (Figure 1, bottom) also features a red-
shift of the Soret band from 423 to 426 nm, whereas all the re-
maining absorption characteristics of C60 (300–380 nm), BDP
(504 nm), and ZnP (423, 550–600 nm) remain the same. De-
pending on the strength of electronic interactions, additional
features in form of a charge transfer may arise between 800
and 900 nm, but no such features were observed.[9]
Subjecting BDP-ZnP-C60 as well as the respective BDP-ZnP
and ZnP-NHCl to strongly coordinating pyridine leads to
marked shifts (Figure 3).[33] We assume that the pyridine coordi-
nation cancels the through space electronic communication
with C60. On the contrary, the H2P analogues fail to reveal any
noticeable shifts in the presence of similar concentrations of
pyridine due to the lack of coordination (Figure S4 in the Sup-
porting Information).
Emission spectra
With respect to the steady-state and time-resolved fluores-
cence assays with BDP-H2P-C60 and BDP-ZnP-C60 and the corre-
sponding references, the fluorescence maxima, quantum
yields, and lifetimes are summarized in Table 2. In the top part
of Figure 2, the fluorescence spectrum of BDP-NHCl is shown,
which exhibits a maximum at 514 nm and a quantum yield of
58%. BDP-C60, on the other hand, reveals a 96.9% quenching
A closer inspection of the BDP fluorescence lifetimes further
corroborates the quenching seen in the steady state measure-
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Figure 1. Absorption spectra of, top: BDP-NHCl (black solid line) and BDP-C60
(black dashed line); middle: H2P-NHCl (black solid line), BDP-H2P (black
dashed line), and BDP-H2P-C60 (black dotted line), inset: a zoom-in to illus-
trate the red shift in the Soret band; bottom: ZnP-NHCl (black solid line),
BDP-ZnP (black dashed line), and BDP-ZnP-C60 (black dotted line), inset:
a zoom-in to illustrate the red shift of the Soret band, at room temperature
in toluene.
Figure 2. Fluorescence spectra (excited at 495 nm), top: BDP-NHCl (black
solid line) and BDP-C60 (gray solid line); middle: BDP-H2P (black dashed line)
and BDP-H2P-C60 (black dotted line); bottom: BDP-ZnP (black dashed line)
and BDP-ZnP-C60 (black dotted line) at room temperature with an optical
density (DOD)=0.1 in toluene.
1.98 ns in BDP-H2P and BDP-ZnP, respectively. Nevertheless, in
the presence of C60, in the case of BDP-ZnP-C60, even these are
quenched, as suggested by the lifetime of 0.76 ns. Obviously,
the flexible linkers modulate the through space interactions
between C60 and ZnP. Details about the fluorescence lifetimes
as well as time fluorescence profiles are summarized in the
Supporting Information (Figure S5 and Table S1 in the Support-
ing Information).
ments. For example, in BDP-H2P/BDP-H2P-C60 and BDP-ZnP/
BDP-ZnP-C60 the values that relate to BDP are 0.08 and 0.11 ns,
respectively. These results, compared with the intrinsic BDP-
NHCl fluorescence lifetime of 3.25 ns, support the suggestion
of a fast and efficient transduction of singlet excited-state
energy from photoexcited BDP to either H2P or ZnP. This is in
accordance with H2P or ZnP fluorescence lifetimes of 9.26 and
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Supporting Information). Concomitant with the latter decay,
we note the formation of a new transient. In the case of BDP-
H2P, the new transient features characteristic maxima at 500–
700 nm, which resemble those seen for H2P-NHCl. For BDP-ZnP,
the corresponding minima at 550 and 600 nm agree well with
the singlet excited-state absorptions of ZnP-NHCl. In other
words, in BDP-H2P and BDP-ZnP an intramolecular energy-
transfer transduces singlet excited-state energy from the initial-
ly excited BDP (2.43 eV) to either ZnP (2.08 eV) or H2P (1.9 eV).
As a matter of fact, the current finding compares well with
recent reports in the literature.[28]
Turning to BDP-H2P-C60, upon 490 nm excitation of the BDP
its singlet excited state is formed as evidenced from the occur-
rence of ground-state bleaching at 500 nm. In analogy to the
observation made with BDP-H2P (see above) an intramolecular
energy transfer sets in shortly after the formation of the BDP
singlet excited state. The H2P singlet excited maxima at 493,
538, 574, 624, and 685 nm and minima at 553, 595, and
654 nm grow in with 32 ps, which matches the decay of the
BDP singlet excited state (Figure 4). Instead of seeing the slow
Figure 3. Fluorescence spectra (excited at 495 nm) of BDP-ZnP (black solid
line) and BDP-ZnP-C60 (black dashed line) in comparison with BDP-ZnP (gray
solid line) and BDP-ZnP-C60 (gray dashed line) in the presence of pyridine.
Transient absorption spectroscopy
To shed more light on the mutual interactions in the excited
state, BDP-H2P-C60, BDP-ZnP-C60, and their references were
probed by using femtosecond transient absorption measure-
ments. Here, excitation of H2P/ZnP at 420 nm or excitation of
BDP at 490 nm were chosen to be studied.
Excitation of H2P-NHCl and ZnP-NHCl at 420 nm gives rise to
the direct formation of the singlet excited state with maxima
at 494, 535, 571, and 622 nm as well as minima at 515, 550,
592, and 648 nm for H2P-NHCl, and maxima at 531, 575, and
623 nm as well as minima at 552, 595, and 652 nm for ZnP-
NHCl. Due to intersystem crossing to the corresponding triplet
manifolds, the latter decay with lifetimes of 9.8 and 2.1 ns for
H2P-NHCl and ZnP-NHCl, respectively, featuring maxima at
780 nm (H2P-NHCl) as well as 840 nm (ZnP-NHCl).
Immediately after exciting BDP-NHCl at 490 nm, its singlet
excited-state features (i.e., 1 ps) appear in the form of a bleach-
ing at ꢁ500 nm (Figure S6 in the Supporting Information) fea-
turing a lifetime of 3.2 ns. When exciting BDP-C60, again the in-
stantaneous formation of the BDP singlet excited state evolves
ꢁ500 nm, followed by a 80 ps charge separation to afford
+
C
CÀ
BDP and C60 , with characteristic major fingerprints at 415,
455, 480, 501, 523, 620, 679, and 1024 nm, respectively, in line
with the spectroelectrochemical measurements (Figure S7 in
the Supporting Information) and recently published values.[34]
The metastable BDP +/C60 radical ion-pair state decays with
a lifetime of 1600 ps (Figure S8 in the Supporting Information),
to reinstate the singlet ground state. With the help of electro-
chemistry we have determined a thermodynamic driving force
for converting the BDP singlet excited state (2.43 eV) into the
C
CÀ
BDP +/C60 radical ion-pair state (1.81 eV) of 0.62 eV.
C
CÀ
Next, we turned to BDP-H2P and BDP-ZnP, which were excit-
ed at 490 nm. Initially, the spectral features of the BDP singlet
excited state (see above) are observed (i.e., 1 ps). The presence
of the covalently attached H2P/ZnP causes a significant short-
ening of the BDP singlet excited-state lifetimes, namely 32 ps
for BDP-H2P and 50 ps for BDP-ZnP (Figures S9 and S10 in the
Figure 4. Top: differential absorption spectra (visible and near-infrared) ob-
tained upon femtosecond flash photolysis (490 nm, 200 nJ) of BDP-H2P-C60
in argon-saturated toluene with several time delays between 0 and 8000 ps
at room temperature; inset: zoom-in to visualize the Q-band as well as the
formation of the radical ion-pair. Bottom: time–absorption profiles of the
spectra at 553 and 596 nm, monitoring the energy transfer (32 ps) as well as
at 1024 nm, monitoring the charge separation (30 ps).
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intersystem crossing, which transforms the singlet excited
state of H2P and ZnP with 9.8 and 2.1 ns, respectively, to the
corresponding long-lived triplet excited state, a rapid decay
takes place with lifetimes of 30 and 26 ps for H2P and ZnP, re-
spectively. However, this rapid decay may be assigned to
a direct electron-transfer from photoexcited BDP to C60. Nota-
bly, this component reveals a very low pre-exponential factor
and, in turn, we are inclined to neglect it in BDP-H2P-C60 al-
though it plays a major role in BDP-C60. The latter correlates
with an intramolecular charge separation to yield the one-elec-
tron-oxidized H2P and the one-electron-reduced C60 within
30 ps. Spectroscopic evidence for the earlier involves maxima
at 538, 574, 624, and 685 nm as well as minima at 553, 595,
and 654 nm, whereas for the latter, maxima at 504 and
1024 nm are observed. The H2P +/C60 radical ion-pair state is
metastable and is subject to charge recombination with a life-
time of 3000 ps. The thermodynamic driving force of 0.22 eV
C
CÀ
results from a H2P +/C60 radical ion-pair state of 1.67 eV and
a singlet excited state of H2P of 1.9 eV. Similarly, 490 nm excita-
tion of BDP-ZnP-C60 leads to a cascade of energy transfer,
charge-separation, and charge-recombination reactions
(Figure 5). The BDP-centered singlet excited state has a lifetime
of 35 ps and its features, that is, 500 nm ground-state bleach-
ing, convert during energy transfer into those of the ZnP sin-
glet excited state with maxima at 453, 581, and 617 nm as well
as minima at 553 and 593 nm. Once formed, the singlet excit-
ed state of ZnP is subject to a charge separation with a lifetime
C
CÀ
of 26 ps to yield the metastable ZnP +/C60 radical ion-pair
state. Again, the one-electron-reduced C60 was spectroscopical-
ly identified by its maximum at 1024 nm (see above), whereas
a maximum at 532 nm relates to that of the one-electron-oxi-
dized ZnP. The charge recombination, which reinstates the sin-
glet ground state, occurs with a lifetime of 900 ps. Here, the
thermodynamic driving force of charge separation (0.68 eV) is
larger than that found in the case of BDP-H2P-C60. For both
BDP-H2P-C60 and BDP-ZnP-C60 the initial transduction of singlet
excited-state energy is rate-determining with respect to the
cascade of energy- and electron transfer as inferred from the
lifetimes of singlet energy transfer in the reference compounds
BDP-H2P and BDP-ZnP (see above).
C
CÀ
Figure 5. Top: differential absorption spectra (visible and near-infrared) ob-
tained upon femtosecond flash photolysis (490 nm, 200 nJ) of BDP-ZnP-C60
in argon-saturated toluene with several time delays between 0 and 8000 ps
at room temperature; inset: zoom-in to visualize the Q-band as well as the
formation of the radical ion-pair. Bottom: time–absorption profiles of the
spectra at 553 and 594 nm, monitoring the energy transfer (32 ps) as well as
at 1024 nm, monitoring the charge separation (26 ps).
Conclusion
In complementary experiments involving 420 nm excitation
of BDP-H2P-C60 and BDP-ZnP-C60, only the spectral features of
H2P and ZnP, respectively, were observed. No appreciable BDP-
based features were seen at any time during the time evolu-
tion due to the dominating absorptions of H2P/ZnP. Instead,
absorption of H2P and ZnP at around 400 nm and between
500 and 700 nm, respectively, was observed (Figures S11 and
S12 in the Supporting Information). In line with the aforemen-
tioned experiments, in which the H2P and/or ZnP are excited,
The sequence of events that follow excitation of BDP-H2P-C60
and BDP-ZnP-C60 at 490 and 420 nm are sketched in the
energy diagrams of Figure 6.
Excitation of BDP-H2P-C60 and BDP-ZnP-C60 at 490 nm pro-
duces the first singlet excited state of BDP. A minor fraction
decays through fluorescence to the ground state, whereas the
major fraction (>99%) decays through photoinduced energy-
transfer to afford the first singlet excited states of either H2P or
ZnP with lifetimes of 32 and 35 ps, respectively. In the next
step, the H2P/ZnP singlet excited states decay in part radiative-
ly back to the ground state and in part by photoinduced elec-
tron-transfer, with lifetimes of 30 and 26 ps, to yield the BDP-
charge separation to afford the H2P +/C60 and ZnP +/C60 rad-
ical ion-pair states, respectively, and charge recombination to
recover the singlet ground states sets in. For BDP-H2P-C60, the
C
CÀ
C
CÀ
C
CÀ
lifetimes of the formation and the decay of H2P +/C60 are 5.7
and 3000 ps, respectively, whereas for BDP-ZnP-C60, ZnP +/C60
H2P +-C60 or BDP-ZnP +-C60 radical ion-pair states, respec-
C
CÀ
C
CÀ
C
CÀ
is formed with a lifetime of 9.2 ps and decays with a lifetime of
2600 ps.
tively. Finally, the latter decay back to the ground state with
lifetimes of 3000 ps for BDP-H2P +-C60 and 900 ps for BDP-
C
CÀ
ZnP +-C60
.
C
CÀ
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Excitation of BDP-H2P-C60 and BDP-ZnP-C60 at 420 nm leads
to the second singlet excited state of either H2P or ZnP, which
the Greek General Secretariat for Research and Technology and
the European Commission, through the European Fund for Re-
gional Development, NSRF 2007-2013 action “Development of
Research Centers -KRHPIS”, COST Action PERSPECT-H2O
CM1202, project “New Multifunctional Nanostructured Materi-
als and Devices - POLYNANO” to N.T. and the Bavarian initiative
“Solar Technologies go Hybrid” is acknowledged.
rapidly decays to the first porphyrin singlet excited state
+
C
through internal conversion. In the next step, the BDP-H2P
-C60 and BDP-ZnP +-C60 radical ion-pair states are formed
with lifetimes of 5.7 and 9.2 ps, respectively, which are shorter
than those observed upon 490 nm excitation. Please note that
in these particular cases the rate of electron transfer is not
governed by the rate of BDP to H2P/ZnP energy transfer. As in
the case of the 490 nm excitation, the BDP-H2P +-C60 and
BDP-ZnP +-C60 radical ion-pair states decay back to the
CÀ
C
CÀ
Keywords: electron transfer
·
energy transfer
·
cyclic
C
CÀ
voltammetry · photosynthesis · synthetic methods
C
CÀ
ground state with lifetimes of around 3000 ps.
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Please note: Minor changes have been made to this manuscript since its
publication in Chemistry—A European Journal Early View. The Editor.
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