Photoinduced charge transfer in a conformational switching chlorin dimer-azafulleroid in polar and nonpolar media

Article abstract of DOI:10.1002/chem.201404786

In the present study, a biomimetic reaction center model, that is, a molecular triad consisting of a chlorin dimer and an azafulleroid, is synthesized and its photophysical properties are studied in comparison with the corresponding molecular dyad, which consists only of a chlorin monomer and an azafulleroid. As evidenced by ¹H NMR, UV/Vis, and fluorescence spectroscopy, the chlorin dimer-azafulleroid folds in nonpolar media into a C₂-symmetric geometry through hydrogen bonding, resulting in appreciable electronic interactions between the chlorins, whereas in polar media the two chlorins diverge from contact. Femtosecond transient absorption spectroscopy studies reveal longer charge-separated states for the chlorin dimer-azafulleroid; 1.6 ns in toluene, compared with the lifetime of ≈0.9 ns for the corresponding chlorin monomer-azafulleroid in toluene. In polar media, for example, benzonitrile, similar charge-separated states are observed, but the lifetimes are inevitably shorter: 65 and 73 ps for the dimeric and monomeric chlorin-azafulleroids, respectively. Nanosecond transient absorption and singlet oxygen phosphorescence studies corroborate that in toluene, the charge-separated state decays indirectly via the triplet excited state to the ground state, whereas in benzonitrile, direct recombination to the ground state is observed. Complementary DFT studies suggest two energy-minima conformations, that is, a folded chlorin dimer-azafulleroid, which is present in nonpolar media, and another conformation in polar media, in which the two hydrophobic chlorins wrap the azafulleroid. Inspection of the frontier molecular orbitals shows that in the folded conformation, the HOMO on each chlorin is equivalent and is shared owing to partial π-π overlap, resulting in delocalization of the conjugated π electrons, whereas the wrapped conformation lacks this stabilization. As such, the longer charge-separated lifetime for the dimer is rationalized by both the electron donor-acceptor separation distance and the stabilization of the radical cation through delocalization. The chlorin folding seems to change the photophysical properties in a manner similar to that observed in the chlorophyll dimer in natural photosynthetic reaction centers.

Full text of DOI:10.1002/chem.201404786

DOI: 10.1002/chem.201404786
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&
Biomimetics |Hot Paper|
Photoinduced Charge Transfer in a Conformational Switching
Chlorin Dimer–Azafulleroid in Polar and Nonpolar Media
[a]
[b]
[b]
[a]
[a]
Taru Nikkonen, Marꢀa Moreno Oliva, Axel Kahnt,* Mikko Muuronen, Juho Helaja,*
[b]
and Dirk M. Guldi*
Abstract: In the present study, a biomimetic reaction center
model, that is, a molecular triad consisting of a chlorin dimer
and an azafulleroid, is synthesized and its photophysical
properties are studied in comparison with the corresponding
molecular dyad, which consists only of a chlorin monomer
studies corroborate that in toluene, the charge-separated
state decays indirectly via the triplet excited state to the
ground state, whereas in benzonitrile, direct recombination
to the ground state is observed. Complementary DFT studies
suggest two energy-minima conformations, that is, a folded
chlorin dimer–azafulleroid, which is present in nonpolar
media, and another conformation in polar media, in which
the two hydrophobic chlorins wrap the azafulleroid. Inspec-
tion of the frontier molecular orbitals shows that in the
folded conformation, the HOMO on each chlorin is equiva-
lent and is shared owing to partial p–p overlap, resulting in
delocalization of the conjugated p electrons, whereas the
wrapped conformation lacks this stabilization. As such, the
longer charge-separated lifetime for the dimer is rationalized
by both the electron donor–acceptor separation distance
and the stabilization of the radical cation through delocaliza-
tion. The chlorin folding seems to change the photophysical
properties in a manner similar to that observed in the
chlorophyll dimer in natural photosynthetic reaction centers.
1
and an azafulleroid. As evidenced by H NMR, UV/Vis, and
fluorescence spectroscopy, the chlorin dimer–azafulleroid
folds in nonpolar media into a C -symmetric geometry
2
through hydrogen bonding, resulting in appreciable elec-
tronic interactions between the chlorins, whereas in polar
media the two chlorins diverge from contact. Femtosecond
transient absorption spectroscopy studies reveal longer
charge-separated states for the chlorin dimer–azafulleroid;
ꢀ
1.6 ns in toluene, compared with the lifetime of ꢀ0.9 ns
for the corresponding chlorin monomer–azafulleroid in
toluene. In polar media, for example, benzonitrile, similar
charge-separated states are observed, but the lifetimes are
inevitably shorter: 65 and 73 ps for the dimeric and
monomeric chlorin–azafulleroids, respectively. Nanosecond
transient absorption and singlet oxygen phosphorescence
Introduction
unit, it may undergo light absorption, excitation energy
[1]
transfer, and/or charge separation.
In the photosynthetic systems of green plants and algae,
chlorophylls (Chls) and bacteriochlorophylls (BChls) function in
variable roles to convert light energy efficiently into chemical
energy. Depending on the environment and the distance of
a (B)Chl from the neighboring chlorophyll of one and the same
To adapt to divergent environments, nature has developed
different assemblies of photosynthetic systems to produce
charges from light with the utmost efficiency. In particular,
light-harvesting antennae assemblies, responsible for excitation
energy transfer, have broad structural diversity. In contrast, the
study of primary electron-transfer events in various photosyn-
thetic reaction centers has revealed similarities in terms of the
initial charge separation and the subsequent charge shift in
those systems that utilize a dimeric Chl as a primary electron
+
[
a] T. Nikkonen, M. Muuronen, Dr. J. Helaja
Department of Chemistry
Laboratory of Organic Chemistry
University of Helsinki
P. O. Box 55,00014,University of Helsinki (Finland)
E-mail: juho.helaja@helsinki.fi
[2]
donor. This dimer is commonly known as a Chl special pair
SP). Characteristically, it exhibits a C symmetry consisting of
(
2
two intermolecularly assembled Chls, which are in close
proximity with a partial p–p overlap. Unambiguously, such an
assembly enables strong interactions between the HOMOs,
+
[
b] Dr. M. M. Oliva, Dr. A. Kahnt, Prof. Dr. D. M. Guldi
Department of Chemistry and Pharmacy & Interdisciplinary Center for Mo-
lecular Materials (ICMM)
[
3]
Friedrich-Alexander-Universitꢀt Erlangen-Nꢁrnberg
Egerlandstraße 3, 91058 Erlangen (Germany)
E-mail: axel.kahnt@fau.de
creating a shared “supermolecular” HOMO for the SP. On one
hand, this feature renders the SP prone to fast and efficient
photoinduced oxidation, namely, charge separation. On the
other hand, the reverse pathway, that is, charge recombination,
inevitably slows down and does not occur in the natural envi-
ronment. In short, direct photoexcitation or excitation energy
dirk.guldi@fau.de
+
[
] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404786.
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transfer from the antennae leads to excitation and subsequent
+
oxidation to afford SP , which possesses an equally delocal-
ized radical cation. Subsequently, the electron is transferred to
electron acceptors of the electron-transfer cycle. In this cycle,
+
electrons flow, and the positive charge at SP is balanced by
an electron. The structure of the natural reaction center of
Rhodopseudomonas viridis utilizing dimeric BChl as a primary
[
4]
electron donor is shown in Scheme 1.
Scheme 1. The pigment organization in the photosynthetic reaction center
of Rhodopseudomonas viridis, in which the dark gray BChl dimer forms the
[
4]
“
special pair”.
Several groups have been inspired to design and probe
artificial mimicries of photosynthetic reaction centers. The
common feature for most of the corresponding systems is that
[
5]
[6]
monomeric electron donors such as chlorins, porphyrins, or
[
6a–c,7]
phthalocyanines
have been attached either covalently or
noncovalently to electron acceptors such as fullerenes, carbon
nanotubes, and so on. Models with biomimetic SPs are rare,
[
8]
and are limited to a few studies. In these charge-transfer
[
8b–k]
models, the SP is often constructed from porphyrins
or
[
8a]
Scheme 2. Chlorin dimer 1, chlorin dimer–azafulleroid 2, chlorin monomer 3,
chlorin monomer–azafulleroid 4, and benzyl azafulleroid 5.
phthalocyanines. Despite the fact that a number of reaction
[
9]
centers mimicking chlorin dimers have been reported, only
a handful of self-assembling chlorin dimers combined with
[
10]
electron acceptors have been described. The self-assembly
strategy in biomimetic reaction center SP dimers is often
based on the tendency of Chls to self-aggregate in the
As the dimer, we selected a Chl dimer with a m-xylene linkage,
1, which, according to our previous studies, features the opti-
mal linker length with a five-carbon backbone and a rigid
structure to ease the stacking of the two chlorins. In addition
to the chlorin dimer 1 and chlorin dimer–azafulleroid 2, the
corresponding chlorin monomers 3 and 4 and the benzyl
azafulleroid 5 were synthesized as references.
[
9,10]
presence of external bridges such as water or alcohol.
In our previous study, we demonstrated that amide-linked
[11]
pyro-pheophorbide a dimers are susceptible to folding.
These dimers, featuring linkages of suitable lengths, fold in
nonpolar solvents through intramolecular hydrogen bonding
into C symmetry. Evidence for this has come from absorption,
2
fluorescence, IR, CD, and NMR spectroscopies as well as DFT Results and Discussion
studies. Here, the two amide protons in the linkage between
Synthesis
1
the two Chls act as hydrogen-bond donors, whereas the 13
carbonyls of the two Chls are hydrogen-bond acceptors. Nota-
bly, in the stacked assembly, the chlorins give rise to interpla-
nar orbital interactions similar to what is known from the pho-
tosynthetic reaction center SP. In the current work, we have
utilized this folding strategy to construct an electron donor–ac-
ceptor conjugate biomimetically by attaching an azafulleroid
covalently to a stacked dimer to afford a triad, 2 (Scheme 2).
The synthesis of the studied molecules is presented in
Schemes 3 and 4. Acid-catalyzed esterification of commercially
available benzene-1,3,5-tricarboxylic acid 6 in methanol fol-
lowed by lithium aluminum hydride reduction gave triol 7 in
[12]
excellent yields. Two out of the three benzyl alcohol groups
of 7 were brominated with CBr /PPh in acetonitrile to obtain
4
3
the known (3,5-bis(bromomethyl)phenyl)methanol 8 in 29%
Chem. Eur. J. 2015, 21, 590 – 600
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nucleus in C60 (see magnified image in Figure S32, Supporting
Information). The chemical shift of the bridging carbon atoms
2
(
d =144.17 ppm) is in the sp region of the spectrum, and cor-
C
[15,21,22]
responds to the [5,6]-open adduct of the cycloaddition.
13
Furthermore, the absence of C NMR signals in the range
3
d =60–80 ppm indicates [5,6]-open adducts lacking any sp -
C
[15]
hybridized carbons in C .
60
Monomeric pyro-pheophorbide a (3) and the corresponding
pyro-pheophorbide a with an open [5,6]-bridged azafulleroid
(
4) were synthesized in a similar fashion to dimers 1 and 2.
The starting 3-(aminomethyl)benzyl alcohol 14 was prepared
[
23]
from 3-cyanobenzoic acid by reduction with borane. Penta-
fluorophenol-ester-activated pyro-pheophorbide a 10 was al-
lowed to react with amines 14/15 to obtain 16/3 (Scheme 4).
Scheme 3. Synthesis of 1 and 2 (TEA=triethylamine).
[
13]
yield.
Both bromines were substituted with azides by
heating 8 with sodium azide in acetone and reduced by Pd/C-
catalyzed hydrogenation in MeOH to afford diamine 9 in
quantitative yields. Chl a was extracted from algae, Spirulina
pacifica, and modified to give pentafluorophenol-ester-activ-
[11]
ated pyro-pheophorbide a (10), as described previously. Mol-
ecule 10 was allowed to react with diamines 9/11 to afford
[
11]
amide-linked pyro-pheophorbide a dimers 12/1. The benzyl-
alcohol-functionalized dimer 12 was treated with mesyl
[14]
chloride by using a slightly modified literature procedure.
The obtained mesyl derivative was relatively unstable and was
allowed to react immediately with sodium azide in DMF to
afford azide 13.
Scheme 4. Synthesis of 3 and 4 (Ms=methanesulfonyl, ODCB=o-dichloro-
The 1,3-dipolar cycloaddition (1,3-DC) of azide to the [6,6]
benzene).
bonds of C60 followed by thermal extrusion of N from the tria-
2
zoline intermediate commonly leads to a mixture of two imino
adducts, that is, open [5,6]-bridged azafulleroids and closed
Benzyl alcohol 16 was transformed into the corresponding
benzyl azide 17 through the same procedure as described
above for 12 to obtain 13. Diverging from the synthesis of the
chlorin dimer–azafulleroid, the reaction between the benzyl-
azide-functionalized chlorin monomer 17 and C60 did not
afford chlorin azafulleroid 4 at 1008C in toluene. The structure
[
14–19]
[
6,6]-bridged aziridinofullerenes.
The ratio of these two
[
15]
species depends on the nature of the substituent. The [6,6]-
triazoline intermediates lose N immediately upon heating at
2
[
14,16,20]
temperatures slightly higher than 608C,
so this was not
1
13
isolated or chemically characterized. Thermal reaction of 13
with 1.5 equivalents of C60 was performed in toluene at 1008C
under argon. After stirring of the reaction mixture for 20 h, the
solvent was evaporated and the crude material was purified by
flash chromatography on silica gel (dichloromethane/methanol
of the product was clarified by H, C, COSY, NOESY, HSQC,
and HMBC spectroscopy (Figures S16–S27, Supporting Informa-
1
tion). The H NMR spectrum of the product showed two sets of
chlorin protons with a 1:1 measured integral ratio (Figure S22).
Characteristically, in the HMBC spectrum, methylene protons
(1’’) of the two chlorins at d=4.11 and 3.76 ppm were both
1
00:1!10:1, gradient) to afford unreacted C60 and [5,6]-open
1 13
azafulleroid 2 as a main product. This was assigned by H, C,
COSY, NOESY, HSQC, and HMBC spectra (Figures S1–S12,
Supporting Information). The HMBC spectrum of 2 displayed
couplings between methylene (1’’) protons at d=4.25 ppm
coupled to a carbon atom resonating at d =161.48 ppm (Fig-
C
ures S23, S27), which confirms the formation of bisazafulleroid
18. The corresponding low-field chemical shift has been re-
ported for a carbon atom adjoined to two nitrogen atoms in
13
[20,24]
and C resonance at d =144.17 ppm arising from the carbon
bisazafulleroids (diazabishomofullerenes),
which, at times,
C
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1
Figure 1. H NMR spectra of different pyro-pheophorbide a derivatives (measured in CDCl
3
): a) chlorin monomer 3, b) chlorin monomer–azafulleroid 4, c) chlor-
in dimer 1, and d) chlorin dimer–azafulleroid 2.
are obtained in reactions of azides with C , whereas the
the two chlorins diverge from contact. During the titration
experiments, chlorin dimer 1 remained soluble, but the hydro-
phobic chlorin dimer–azafulleroid 2 started to precipitate
gradually.
60
double bonds adjacent to the nitrogen bridge in azafulleroids
[24,25]
are prone to regioselective formation of bisazafulleroids.
Bisazafulleroid 18 was transformed into 4 upon heating at
[
24]
1408C overnight.
The open [5,6]-bridged attachment was
Comparing the structures of 1–4, one can observe that mon-
omeric chlorins 3 and 4 also contain equivalent amide and
ketone groups that are capable of forming hydrogen bonds
through intermolecular interactions with neighboring Chls. Fur-
thermore, 2 and 4 feature an extra nitrogen lone pair located
at the azafulleroid, which is a potential hydrogen-bond accept-
confirmed by HMBC, which showed coupling between
2
methylene (1’’) protons at d=4.41 ppm and sp carbon at
dC =145.36 ppm (Figures S28–S31 and S33, Supporting
Information).
Benzyl azafulleroid (5) was prepared successfully by heating
benzyl azide and C60 in o-dichlorobenzene at 1608C overnight,
1
or. Analysis of the H NMR spectra of 4 revealed that proton
[
20,21,24]
1
2
1
4
as described previously.
signals H-8 , -8 , -10, -12 , -17 , and -1’’ appeared in “unusual”
spectral regions as the sample concentration was increased.
Molecule 4 was diluted through stepwise addition of CDCl ,
and the H NMR spectra were taken after each dilution step
3
Study of intra- and intermolecular interactions
1
In our previous work, we observed that 1 and related chlorin
(Figure S36, Supporting Information). Upon dilution, protons
1
2
1
dimers fold into
a
C2-symmetric geometry in nonpolar
H-8 , -8 , -10, -12 , and -1’’ were downfield shifted, whereas the
[
11]
4
media. The folding was driven by two intramolecular hydro-
gen bonds between the amide as a hydrogen-bond donor and
carbonyl as a hydrogen-bond acceptor (Scheme 2). The related
amide proton (17 ) was upfield shifted. In other words, the
“usual” chemical shifts of monomeric nonaggregated Chls
were finally obtained upon dilution. This observation implies
that in concentrated media, 4 forms aggregates or at least
dimers, in which two or more chlorin monomer–azafulleroids
are intermolecularly locked. This interaction is probably driven
by hydrogen bonds between the amide(s) and nitrogen lone
pair(s) of azafulleroid in combination with aromatic p–p inter-
actions between Chls and C . However, these interactions are
1
H NMR studies indicated notable shieldings for the protons
that were exposed to the ring current of the neighboring mac-
1
rocycle. The strongest shielded protons were H-10, H-12 , and
2
H-13 , whereas clear deshieldings were observed for the amide
protons owing to the hydrogen-bonding interactions. Compar-
1
ison of the H NMR spectra of 1–4 (Figure 1a–d) reveals upfield
60
1
2
shifts for the H-10, H-12 , and H-13 protons in Chl dimer 1 (c)
and Chl dimer–azafulleroid 2 (d), whereas downfield shifts are
measured for the amide protons. This indicates that hydrogen-
bond-driven folding occurs in the chlorin dimer in nonpolar
environments.
too weak to be prevailing under the diluted photophysical
measurement conditions (vide infra), showing that the
aggregate$monomer equilibrium is clearly on the side of the
monomer.
The corresponding spectral analysis of 1–3 did not indicate
any intermolecular interactions under the same experimental
conditions. Notably, 2 contains the same hydrogen-bond
donors and acceptors as 4, but in the former case, the
formation of intramolecular hydrogen bonds to give the C2
symmetry is entropically more favorable than intermolecular
aggregation.
In order to break the hydrogen bonds and the folded dimer,
both chlorin dimer 1 and chlorin dimer–azafulleroid 2 were ti-
trated with [D ]acetonitrile in CDCl (Figure S34 and S35, Sup-
3
3
1
porting Information). Under H NMR monitoring, protons H-10
1
4
and H-12 are downfield shifted and amide protons (17 ) up-
field shifted during the titration. This is in accordance with the
unfolding event, in which the hydrogen bonds are broken and
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Photophysical studies
zonitrile, the corresponding absorptions are maximized at 329,
418, 512, 542, 613, and 671 nm (Figure S40, Supporting Infor-
The absorption spectrum of chlorin monomer 3 in toluene
gives rise to a strong absorption at around 415 nm, flanked by
minor absorptions at 319, 510, 538, 613, and 671 nm. Here, the
mation). Similarly to what is seen for 4, the absorption spectra
of the electron donor–acceptor conjugate 2 in toluene and
benzonitrile are best described as the superimpositions of the
chlorin dimer absorption of 1 and that of benzyl azafulleroid 5.
Notably, the observed dependence between the solvent
polarity and absorption spectra in any of the chlorin-dimer-
containing systems 1 and 2 is similar to that observed
previously in similar systems, and stems from intramolecular
hydrogen bonding and p–p interactions between partly
overlapping chlorins in nonpolar solvents.
415 nm band is attributed to the Soret-band absorption,
whereas the bands between 500 and 700 nm are part of the
Q-band absorptions. In contrast, benzyl azafulleroid 5 shows
absorptions in the UV and visible regions of the solar spec-
trum, maximizing at 334 nm. Molecule 4 (Figure 2) exhibits ab-
[11]
Insights into electron donor–acceptor interactions have
come from fluorescence assays. Chlorin monomer 3 exhibits
strong fluorescence in the range 620–800 nm, with a fluores-
cence maximum at 676 nm (Figure 4), a fluorescence quantum
Figure 2. Absorption spectra of 4 (thin solid line) and references 3 (bold
solid line) and 5 (dashed line) in toluene.
sorptions at 340, 416, 511, 540, 613, and 672 nm. As such, the
absorption spectrum of 4 is best described as the superimposi-
tion of the UV/Vis spectra of reference molecules 3 and 5. No-
tably, the solvent polarity has hardly any effect on the spectral
features of the absorptions, as the maxima of 3 and 4 retain
their positions in polar solvents such as benzonitrile. In particu-
Figure 4. Fluorescence spectra of 4 (thin solid line) and reference 3 (bold
solid line) in toluene upon photoexcitation at 403 nm.
lar, the Soret and Q band maxima of 3 appear at 418 and
yield of 0.2 in toluene, and a fluorescence lifetime of 6.7 ns
(Figure S41, Supporting Information). In the case of 4, the gen-
eral shape of the fluorescence spectrum is practically identical
to that observed for 3 (Figure 4). However, the chlorin fluores-
cence quantum yield is quenched, with values ranging from
0.029 in toluene to 0.034 in benzonitrile (Table 1).
y
671 nm, whereas those of 4 are seen at 419 and 672 nm
(Figures S37 and S38, Supporting Information).
Turning to chlorin dimer 1, absorptions in toluene are ob-
served at 327, 403, 513, 544, 620, and 679 nm (Figure 3). How-
ever, unlike monomer 3, appreciable changes are discernable
upon dissolution in benzonitrile. To this end, the absorption
Regarding the chlorin dimer reference, 1 exhibits strong
fluorescence in the range 600–800 nm, with a fluorescence
maximum at 684 nm, a fluorescence quantum yield of 0.27 in
toluene, and a fluorescence lifetime of 6.0 ns (Figure S41, Sup-
porting Information). Molecule 2 shows, in general, the same
chlorin-dimer-centered fluorescence (Figure 5), but the fluores-
cence quantum yields are quenched with values ranging from
0.075 in toluene to 0.083 in benzonitrile (Table 1).
Our observation, namely that the fluorescence in 2 and 4 is
nearly quantitatively quenched, points to an additional decay
mechanism, that is, electron or energy transfer, of the chlorin
or chlorin-dimer-centered singlet excited states. Again,
evidence for intramolecular hydrogen bonding is obtained
from the fluorescence spectra as the maxima of chlorin dimers
Figure 3. Absorption spectra of 2 (thin solid line) and 1 (bold solid line) in
toluene.
1
and 2 appear in toluene at longer wavelengths than those of
chlorin monomers 3 and 4. In polar benzonitrile, 1, 2, 3, and 4
feature fluorescence maxima at 676 nm.
spectrum reveals maxima at 324, 418, 511, 540, 614, and
6
71 nm (Figure S39, Supporting Information). For chlorin
dimer–azafulleroid 2, absorptions are observed in toluene at
34, 407, 514, 545, 618, and 679 nm (Figure 3), whereas in ben-
Transient absorption spectroscopy based on femtosecond
and nanosecond pump–probe experiments shed light on the
notion of an electron-transfer deactivation. For 3, the singlet
3
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acteristics, we derived lifetimes between 907ꢁ20 ps (o-xylene)
and 21ꢁ1 ps (DMF) (Table 1). As far as the product of charge
recombination is concerned, transient absorption measure-
ments based on nanosecond laser photolysis (Figure S45, Sup-
porting Information) and singlet oxygen phosphorescence
studies (Table 1) corroborate that the triplet excited state
evolves in nonpolar solvents. In contrast, deactivation to the
ground state is the main channel in polar solvents.
Concerning chlorin dimer 1, the singlet first excited state is
formed immediately after the laser pulse, showing a transient
absorption with maxima at 490, 535, 595, 635, and 1100 nm as
well as minima at 520 and 550 nm (Figure S42, Supporting In-
formation). With a lifetime of 6.0 ns, this transient ab-
sorption transforms into the triplet manifold featur-
ing maxima at 300, 490, and 600 nm as well as
minima at 400 and 680 nm (Figure S46, Supporting
Information).
Figure 5. Fluorescence spectra of 2 (thin solid line) and reference 1 (bold
solid line) in toluene upon photoexcitation at 403 nm.
Table 1. Summary of the photophysical properties.
Fluorescence
First excited Singlet oxygen Charge-separated
quantum yield singlet state quantum yield state lifetime [ps]
lifetime
Similarly to 1, 2 features the chlorin-dimer-based
singlet excited state. The latter decays rapidly and is
replaced by a new set of transient absorptions, in-
cluding maxima at 465, 530, 595, 635, 790, and
[
a]
1
in toluene
0.27
6.0ꢁ0.6 ns
0.70
–
[
b]
2
2
2
2
2
2
in o-xylene
in toluene
in anisole
in THF
in benzonitrile 0.066
in DMF
0.070
0.095
0.083
0.072
74ꢁ9.0 ps
12ꢁ1.0 ps
1622ꢁ87
1606ꢁ46
283ꢁ15
68ꢁ3
0.99
[
1050 nm (Figure 7). The transient absorption maxi-
b]
2.7ꢁ1.6 ps
2.0ꢁ1.0 ps
3.5ꢁ1.0 ps
0.8ꢁ0.8 ps
[
b]
mum around 1050 nm is attributed to the absorption
of the azafulleroid radical anion, whereas the remain-
ing transient absorptions are attributed to the chlorin
0.23
[
73ꢁ4
b]
0.063
21ꢁ1
[5c]
dimer radical cation. Our assumption regarding the
features of the chlorin dimer radical cation was fur-
ther corroborated by pulse radiolysis assays (Fig-
ure S48, Supporting Information). As such, the transi-
ent absorption of the radiolysis-induced oxidation of
[
a]
3
in toluene
0.20
6.7ꢁ0.7 ns
0.76
–
[
b]
[b]
4
4
4
4
4
4
in o-xylene
in toluene
in anisole
in THF
in benzonitrile 0.034
in DMF 0.038
111 ꢁ10 ps
103ꢁ7 ps
37ꢁ3 ps
6ꢁ1 ps
907ꢁ20
830ꢁ17
301ꢁ9
66ꢁ4
0.029
0.031
0.050
0.74
[
b]
[
b]
1
matches well the transient absorption of the chlor-
1ꢁ1 ps
0.19
65ꢁ3
[
b]
0.6ꢁ0.3 ps
21ꢁ1
in dimer radical cation formed upon photoinduced
oxidation in 2. Particularly diagnostic for the chlorin
dimer radical cation is the transient absorption maxi-
mizing around 790 nm, where neither the singlet ex-
cited nor triplet excited state of the chlorin dimer
shows strong transient absorption bands.
[
a] Determined from the fluorescence lifetime measurements. [b] Not determined.
excited state is formed immediately after the laser pulse, re-
vealing a broad transient absorption with maxima at 490, 530,
The transient absorption changes caused by the charge-sep-
arated state were taken to determine the underlying lifetimes,
which range from 1622ꢁ87 ps (o-xylene) to 21ꢁ1 ps (DMF;
Table 1).
5
80, 625, and 1100 nm, accompanied by minima at 515, 540,
and 620 nm mirroring the ground-state absorption (Figure S43,
Supporting Information). This transient absorption decays with
a lifetime of 6.8 ns into the triplet manifold with transient
maxima at 320, 460, and 560 nm and transient minima at 400
and 660 nm (Figure S44, Supporting Information).
Singlet oxygen phosphorescence (Table 1) and nanosecond
transient absorption measurements (Figure S47, Supporting In-
formation) were employed to clarify the mechanism of charge
recombination. In polar solvents, the charge-separated state
reinstates directly the ground state, whereas in nonpolar sol-
vents, the chlorin-dimer-centered triplet excited state evolves
with maxima at 300, 420, 500, and 600 nm as well as minima
at 400, 440, and 680 nm. Please note, however, that we have
no particular evidence for the formation of any azafulleroid
triplet excited state with its characteristic absorptions at 450
and 710 nm (Figure S49, Supporting Information).
In 4, the same transient absorption features were observed.
For example, immediately after the laser pulse, the chlorin-
based singlet excited state is seen, which decays rapidly
(
Figure 6). The decay of the singlet excited state gives rise to
a new set of transient absorption maxima at 460, 530, 585,
7
5
4
90, and 1050 nm as well as transient absorption minima at
10 and 540 nm (Figure 6). The transient absorption maxima at
60, 530, 585, and 790 nm are attributed to the chlorin radical
cation, whereas the transient absorption at 1050 nm matches
the absorption of the azafulleroid radical anion. In other
words, the transient absorption spectrum provides evidence
for the successful formation of a charge-separated state. By fol-
lowing the time evolution of the charge-separated state char-
Electrochemical studies
We performed differential pulse voltammetry (DPV) experi-
ments in benzonitrile to gather information about any of the
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benzyl azafulleroid 5. The latter shows only three reductions at
ꢂ0.33, ꢂ0.76, and ꢂ1.25 V. On one hand, 1 and 3 display one
oxidation at +0.94 and +0.89 V, respectively, and two reduc-
tions, at ꢂ0.99 and ꢂ1.36 V in 1, and at ꢂ1.06 and ꢂ1.31 V in
3
. On the other hand, in 2, chlorin-centered oxidation appears
at +0.98 V with reductions at ꢂ0.99 and ꢂ1.36 V, whereas the
azafulleroid-centered reductions appear at ꢂ0.33, ꢂ0.78, and
ꢂ1.18 V. In 4, Chl-centered oxidation appears at +1.03 V and
Chl-centered reductions at ꢂ0.95 and ꢂ1.27 V. In addition, aza-
fulleroid-centered reductions are observed at ꢂ0.33, ꢂ0.66,
and ꢂ1.13 V. Calculations of the differences between the
one-electron oxidation and one-electron reduction lead to
estimations of the HOMO–LUMO gaps of 1.93, 1.31, 1.95, and
1.36 V for 1, 2, 3, and 4, respectively.
Theoretical calculations
In order to elucidate the origin of the charge-separated state
lifetimes, we performed computational studies at the DFT level
[26]
using TPSS-D3/def2-SVP.
In our previous supramolecular
chlorin-C60 study we showed that the chosen computational
method and level provide geometries and MOs that were
Figure 6. Top: Femtosecond transient absorption spectra of 4 in argon-
saturated toluene; 1 ps (bold line), 10 ps (thin line), 100 ps (bold dashed
line), 1000 ps (thin dashed line), and 5500 ps (dotted line) after excitation at
[5k]
appropriate for rationalizing the experimental phenomena.
Specifically, the chlorin-C60 center-to-center distances as well as
the solvent polarity were found to be crucial for rationalizing
the charge recombination dynamics.
675 nm. Bottom: Corresponding absorption time profiles at 590 nm (filled
circles) and 1050 nm (open triangles).
In the present study, the charge-separated state lifetime of
the chlorin monomer–azafulleroid 4 and chlorin dimer–azaful-
leroid 2 are affected significantly by the solvent polarity. In
general, longer charge-separated state lifetimes are observed
in less polar media (Table 1). As such, 2 and 4 exhibit similarly
short lifetimes in polar solvents, whereas in nonpolar media
their lifetimes differ significantly, with the lifetime of 2 nearly
twice that observed for 4. These observations contrast with
the usual media effect, namely, that polar solvents stabilize
charged molecular species, and, in turn, lead to longer charge-
separated state lifetimes. Therefore, we assume that the
folding and the specific molecular conformations play an
important role in the charge separation events. With respect to
the NMR studies performed with 2, in nonpolar media, the hy-
drogen-bonding interactions stabilize the folded chlorin dimer
conformation A, whereas an increase in solvent polarity forces
the dimer moiety into open conformations.
In the quest for energy minima conformations, two extreme
geometries were found (Figures 8 and S50): A, in which the
chlorins are folded by the hydrogen bonds, and B, wherein the
chlorins wrap an azafulleroid. Computational free-energy
values, DG (298.15 K), indicated that, upon application of
either toluene or benzonitrile implicit solvent models, the
wrapped conformation B would be more stable than the
Figure 7. Top: Femtosecond transient absorption spectra of 2 in argon-
saturated toluene; 1 ps (bold line), 10 ps (thin line), 100 ps (bold dashed
line), 1000 ps (thin dashed line), and 5500 ps (dotted line) after excitation at
ꢂ1
675 nm. Bottom: Corresponding absorption time profiles at 590 nm (filled
folded one by 12 or 13 kcalmol , respectively (Table S2). In
circles) and 1050 nm (open triangles).
general, the incomplete computational accuracy in the DG pre-
dictions, especially in toluene, probably arises because no real
solvent molecules or dynamics were included, and solute–sol-
vent coordinative and especially p–p-stacking interactions are
missing. As a result, the chlorin–azafulleroid p–p-stacking inter-
actions are overestimated if the competing solvent interactions
+
redox reactions occurring. All values are relative to a Fc/Fc in-
ternal reference. As such, all compounds show amphoteric
redox behavior with oxidative and reductive processes
(Table S1, Supporting Information), with the exception of
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ture A. This may be the most critical parameter for explaining
the differences in the observed lifetimes, as the charge-sepa-
rated state lifetime of 4 is increased by a factor of approxi-
mately 40 upon going from polar to nonpolar solvents. Still,
this comparison excludes the effect of hydrogen bonds, which
is seen to increase the charge-separated state lifetime by
a factor of two.
Overall, the effect of hydrogen bonds on the charge-separat-
ed state lifetime is seen from a comparison between 2 and 4
in two different nonpolar solvents, that is, toluene and o-
xylene. With increasing polarity, the hydrogen bonds in the
dimer structure open up, leading to a charge-separated state
lifetime similar to that seen in the monomer. The rapid de-
crease in strongly polar solvents is explained by the favoring
of the wrapped conformer B, which features short center-to-
center distances, and, hence, destabilized charge-separated
states.
Finally, the structures of 1 and 2 and their folded geometries
bear little resemblance to the pairs in photosynthetic reaction
centers such as Rhodopseudomonas viridis (Scheme 1). First of
all, in the current model the chlorins are metal-free, whereas in
natural systems, Mg is coordinated. Secondly, the folded con-
formations have partial p–p overlap between C rings, whereas
the natural systems exhibit precise ring overlapping in ring A
Figure 8. TPSS-D3/def2-SVP geometry-optimized conformers and frontier
MOs for chlorin dimer–azafulleroid 2 in solution. HOMO (light) and LUMO
(dark) MOs for A: chlorin-folded conformation (D=0.95 eV) and for B: C60
-
(Schemes 1 and 2). Moreover, the Q transition dipole moment
y
wrapped conformation (D=1.12 eV), calculated in toluene and benzonitrile,
respectively. Isosurface value 0.01.
vectors are oriented at a 358 angle, whereas in the natural sys-
tems, the optimal angle of the Q vectors is 1808. Also, upon
y
[9a–e,10]
comparison of our models with classic SP models,
the
features mentioned above are, in most cases, more accurately
imitated in the latter ones. However, from a functional point of
view, the charge-separated state lifetime in 2 mimics qualita-
tively that of the original SP. Furthermore, the strength of our
are not implemented. Nevertheless, despite the lack of direct
experimental evidence, the wrapped conformation is the most
likely geometry in benzonitrile. An additional notion for this
hypothesis comes from the fact that various porphyrin dimers
models lies in its simplicity, that is, the C -symmetry dimer fold-
2
[
27]
have been reported to host C . Furthermore, the interaction
ing is based purely on intramolecular processes, without the
60
in the wrapped conformation is considered to be hydrophobic.
In other words, it is more stabilized in polar solvents, corre-
sponding to the DDG(B-A) values between the studied
solvents and experimental data. We consider that despite the
deficiency in the DG values, the geometries are still relevant
for describing possible extreme conformations.
need for external bridging units such as H O, and, thus, is en-
2
tropically favored. The chlorin complexes are also chemically
stable, and in nonpolar media, the folded geometry is highly
populated even at room temperature.
Conclusions
Inspection of the frontier orbitals (Figure 8) reveals that in
the A and B conformers, the HOMO is located on the electron-
donating chlorin, whereas the LUMO is centered on the elec-
tron-accepting azafulleroid. Notably, in the folded structure A,
the HOMO is delocalized evenly and also partly shared be-
tween chlorins, which leads to stabilization of the radical
cation and, in turn, to an increase in the charge-separated
state lifetime. In contrast, in the wrapped conformation B pres-
ent in polar solvents, the radical cation cannot be stabilized by
the delocalization. Nevertheless, according to the Marcus
theory of electron transfer, the charge-separated-state lifetime
To summarize, we have described the synthesis and characteri-
zation of chlorin monomer–azafulleroid 4 and chlorin dimer–
azafulleroid 2. Their photophysical properties were studied
with particular emphasis on solvent-induced conformational
changes. To this end, in nonpolar media, 2 presents a folded
conformation, as confirmed by UV/Vis, fluorescence, and NMR
spectroscopies. The folding is unequivocally favored entropical-
ly, because the m-xylene linkage brings the chlorins into close
proximity. Here, intramolecular folding occurs without the
need for crosslinking bridges, which are frequently found in
biomimetic SP dimers.
[
28]
depends strongly on the solvent polarity (that is, as the sol-
vent polarity decreases, the charge-recombination dynamics
are pushed deeper into the Marcus inverted region) and on
Advanced photophysical techniques such as transient ab-
sorption spectroscopy provided unambiguous evidence for the
successful formation of the charge-separated state, which
decays more slowly in 2 than in the corresponding reference
[29]
the distance between the electron donor and acceptor. In
the present study, the wrapped conformer B displays a signifi-
cantly shorter center-to-center distance than the folded struc-
4, regardless of solvent polarity. Moreover, 2 exhibits C sym-
2
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metry in nonpolar media, which leads to a long-lived charge-
separated state, whereas in polar media, the charge-separated
state has a shorter lifetime.
Nanosecond transient absorption laser photolysis measurements
were performed with the output from an optical parametric oscilla-
tor (OPO, Rainbow VIR, Opotek/Quantel, output: 420 nm, 10 mJ/
pulse) pumped by the third harmonic (355 nm) of a Nd/YAG laser
Complementary DFT calculations corroborated the existence
of two different energy minimum conformations, namely
a folded one in nonpolar media and a wrapped one in polar
media.
(
(
Brilliant, Quantel). The optical detection was based on a pulsed
pulser MSP 05, Mꢃller Elektronik-Optik) xenon lamp (XBO 450,
Osram), a monochromator (Spectra Pro 2300i, Acton Research),
a R928 photomultiplier tube (Hamamatsu Photonics), or a fast
InGaAs photodiode (Nano 5, Coherent) with 300 MHz amplification,
and a 1 GHz digital oscilloscope (WavePro7100, LeCroy).
As was pursued in the synthetic and conformational design,
the folded conformer of 2 features functional similarities with
the SP found in photosynthetic reaction centers, even though
it is structurally far from the complexity of natural systems. In
this context, our next challenge is to develop a folding strat-
egy that addresses, for instance, the use of synthetic tools to
adjust the geometry, to incorporate metal ligands, and to use
auxiliary chlorins to facilitate and mediate charge transfer.
Pulse radiolysis
The samples were dissolved in CHCl , saturated with O , and irradi-
3
2
ated with high-energy electron pulses (1 MeV, 15 ns duration) by
a pulse-transformer-type electron accelerator (Elit, Institute of Nu-
clear Physics, Novosibirsk, Russia). The dose delivered per pulse
was measured by electron dosimetry. Doses of 100 Gy/pulse were
employed. Optical detection of the transients was performed with
a detection system consisting of a pulsed (pulser MSP 05, Mꢃller
Elektronik Optik) xenon lamp (XBO 450, Osram), a SpectraPro 500
monochromator (Acton Research Corporation), a R9220 photo-
Experimental Section
The synthesis and characterization of all compounds are described
in detail in the Supporting Information. The NMR spectra including
multiplier (Hamamatsu Photonics), and
a 500 MHz digitizing
1
13
H, C, COSY, NOESY, HSQC, and HMBC are presented in
oscilloscope (TDS 640, Tektronix). A more detailed description of
Figures S1–S36 in the Supporting Information.
[33]
the overall setup is reported elsewhere.
Absorption and emission spectroscopy
Electrochemical studies
Steady-state UV/Vis absorption spectra were measured on
Cary5000 (Varian) and PerkinElmer Lambda 2 two-beam spectro-
photometers. Steady-state fluorescence spectra were taken from
samples with a FluoroMax3 spectrometer (Horiba Jobin Yvon). The
experiments were performed at room temperature. Fluorescence
Electrochemical experiments were performed with a Metrohm FRA
2 mAutolab Type III potentiostat, in benzonitrile containing 0.1m
TBAPF as the supporting electrolyte. A single-compartment, three-
6
electrode cell configuration was used in this work. A glassy carbon
electrode (3 mm diameter) was used as the working electrode, a Pt
wire as the counter electrode, and a Ag wire as the reference elec-
[30,31]
quantum yields were determined by the comparative method
+
using H TPP and ZnTPP as references with fluorescence quantum
trode. All potentials were corrected against the Fc/Fc internal
2
yields of 0.11 and 0.03, respectively, in toluene. Singlet oxygen
phosphorescence measurements were conducted using a Fluorolog
reference.
3
1
(Horiba Jobin Yvon) equipped with an IGA Symphony (512ꢂ1ꢂ
mm) detector (Horiba Jobin Yvon). The singlet oxygen phosphor-
Computational methods
escence quantum yields were determined by the comparative
Full computational details, including Cartesian coordinates of all
studied structures, are given in the Supporting Information.
method, employing H TPP and ZnTPP as references with singlet
2
[32]
oxygen quantum yields of 0.66 and 0.72, respectively, in toluene.
Acknowledgements
Time-resolved fluorescence studies
Fluorescence lifetimes were determined through the time-correlat-
ed single photon counting (TCSPC) technique using a Fluorolog 3
Dr. Petri Heinonen is gratefully accredited for the HRMS meas-
urements. M.Sc. Valtteri Mꢄkelꢄ and Dr. Sami Heikkinen are ac-
knowledged for their assistance in the NMR measurements.
Funding was received from the Academy of Finland (project
No 129062). M.Sc. Taru Nikkonen acknowledges the Marie
Curie Initial Training Network FUNMOLS, the Graduate School
of Organic Chemistry and Chemical Biology (GSOCCB), and the
Finnish Cultural Foundation for financial support. Dr. Marꢀa
Moreno Oliva acknowledges financial support from the Marie
Curie COFUND programme “U-Mobility”, cofinanced by Univer-
sidad de Malaga and the European Community’s Seventh
Framework Programme under Grant Agreement No. 246550.
Dr. Axel Kahnt gratefully acknowledges funding from the Deut-
sche Forschungsgemeinschaft (DFG) through grant KA 3491/2-
(
Horiba Jobin Yvon). The sample was excited with a NanoLED-405
instrument (403 nm), and the signal was detected by a Hamamatsu
MCP photomultiplier (type R3809U-50). The time profiles were
recorded at 680 nm.
Transient absorption spectroscopy
Transient absorption measurements based on femtosecond laser
photolysis were performed with output from a Ti/sapphire laser
system (CPA2110, Clark-MXR Inc.): 775 nm, 1 kHz, and 150 fs FWHM
pulses. The excitation wavelength was generated either by second
harmonic generation (387 nm) or by using a non-colinear optical
parametric amplifier (NOPA Pulse, Clark MXR) (675 nm). For both
excitation wavelengths, pulse widths of <150 fs and energies of
1. Funding from the Bayerische Staatsregierung as part of the
2
00 nJ/pulse (387 nm) and 120 nJ/pulse (675 nm) were selected.
The transient absorption detection was performed with a transient
absorption pump/probe system (TAPPS, Ultrafast Systems).
“Solar Technologies go Hybrid” initiative is gratefully acknowl-
edged. We would like to thank Prof. Abel and his group from
Chem. Eur. J. 2015, 21, 590 – 600
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Leipzig University, Leipzig, Germany, for support during the
pulse radiolysis measurements. The Finnish National Centre for
Scientific Computing (CSC) is recognized for computational
resources.
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