Sulfur rich electron donors-formation of singlet versus triplet radical ion pair states featuring different lifetimes in the same conjugate

Article abstract of DOI:10.1039/C6SC03207A

An unprecedented family of novel electron-donor acceptor conjugates based on fullerenes as electron acceptors, on one hand, and triphenyl amines as electron donors, on the other hand, have been synthesized and characterized in a variety of solvents using

Full text of DOI:10.1039/C6SC03207A

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Sulfur Rich Electron Donors – Formation of Singlet versus Triplet
Radical Ion Pair States Featuring Different Lifetimes in the Same
Conjugate
Received 00th January 20xx,
Accepted 00th January 20xx
‡
a
‡b
a
a
*b
*a
DOI: 10.1039/x0xx00000x
Avishek Saha, Muqing Chen, Marcus Lederer, Axel Kahnt, Xing Lu, and Dirk M. Guldi
www.rsc.org/
An unprecedented family of novel electron-donor acceptor conjugates based on fullerenes as electron acceptors, on one
hand, and triphenyl amines as electron donors on the other hand, have been synthesized and characterized in a variety of
solvents using steady state absorption/emission as well as transient absorption spectroscopy. Unprecedented in terms of
outcome of radical ion pair formation, that is, singlet versus triplet excited state. This was corroborated by femto- /
nanosecond pump probe experiments and by molecular orbital calculations. Not only has the donor strength of the
triphenylamines been systematically altered by appending one or two sulfur rich dithiafulvenes, but the presence of the
latter changed the nature of radical ion pair state. Importantly, depending on the excitation wavelength, that is, either
where the fullerenes or where the triphenylamines absorb, short-lived or long-lived radical ion pair states, respectively,
are formed. The short-lived component with a lifetime as short as 6 ps has singlet character and stems from a fullerene
singlet excited state precursor. In contrast, the long-lived component has a lifetime of up to 130 ns in THF, has triplet
character, and evolves from a triplet excited state precursor. Key in forming more than three orders of magnitude longer
lived radical ion pair states is the presence of sulfur atoms, which enhance spin-orbit coupling and, in turn, intersystem
crossing. Independent confirmation for the singlet versus triplet character came from temperature dependent
measurements with focus on the radical ion pair state lifetimes. Here, activation barriers of 2.4 and 10.0 kJ / mol for the
singlet
and
triplet
radical
ion
pair
state,
respectively,
were
established.
In the context of electron acceptors, fullerenes were extensively
studied in electron donor–acceptor systems. This is mainly due to
their unique π-electronic nature, their marked excited-state
electronic properties, and their low reorganization energy in
Introduction
Natural photosynthesis is one of the essential processes that runs
1
organic life on earth. It operates based on the mutual interplay
2
-8
electron transfer reactions. A myriad of possible electron donors
have been investigated as counterpart to the electron accepting
fullerene ranging from organic entities like, for example,
between light harvesting, energy transfer, electron transfer, and
catalysis. Inspired by this, a plethora of covalent and non-covalent
electron donor-acceptor systems, in which electron donating and
electron accepting building blocks are integrated, have been
synthesized and investigated. For instance, the most commonly
employed electron donors range from chlorophylls and carotenoids
to porphyrins / phthalocyanines and triphenylamines. As far as
6, 9-16
tetrathiafulvalenes, to complexes based on porphyrins etc.
What renders the resulting electron donor-acceptor systems most
appealing is the possibility to adjust their structure and, in turn, to
fine-tune their physicochemical features such as quantum yields
1
7, 18
and lifetimes of charge separation.
electron
acceptors
are
concerned,
perylenebisdiimides,
napthalenediimides, endohedral fullerenes, and empty fullerenes Among the electron donors, triphenylamines and their derivatives
should be mentioned. In the resulting systems, particular focus has stand out owing to their good charge transport ability. As a matter
been placed on fundamental aspects of electron transfer.
of fact, they were widely used as hole injectors or as active
1
9-23
materials in organic electronic devices.
Photophysical assays with a system, where C60 is covalently linked
to a TPA, revealed that the photo-induced charge separation
•+
•+
process predominantly produces TPA -C60 radical ion pair state in
1
6
polar solvents via
C60* excitation. Later, in the analogues TPA-
3
Sc N@C80, in which TPA is connected via the nitrogen, a significant
improved thermal stability resulted in longer lived radical ion pair
states, when compared to the corresponding 2-substituted
2
4
conjugate. This led us to consider a different strategy to alter the
lifetime of the radical ion pair state, namely TPA modification. For
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example, dithiafulvenes, derivatives of tetrathiafulvalene, are
Journal Name
2
5, 26
known for their high electron density and hole mobility.
As
such, they bear great potential for modifying TPA and, in turn, for
extending the lifetime of the radical ion pair state
Like in photosynthesis, the lifetime of the radical ion pair state is
crucial to link energy conversion to an efficient energy storage. To
this end, spatially separating electron donors and acceptors is one
promising approach, while electronically decoupling them is yet
2
7-30
another one.
Products of both approaches are long-lived radical
ion pair states of singlet or triplet nature. A less frequently
explored approach implies the conversion of the singlet radical ion
3
1, 32
pair state character to the corresponding triplet manifold.
Due
to the needed spin-conversion, which proceeds via hyperfine
induced singlet-triplet mixing, starting with a triplet excited state
precursor in electron donor-acceptor systems enables the efficient
formation of long-lived, triplet radical ion pairs. Next to transition
metals, whose internal heavy-atom effects facilitate the rapid
formation of triplet excited state precursors, second-order vibronic
spin–orbit coupling in sulfur-containing building blocks provides
3
3, 34
similar means.
To the best of our knowledge, formation of
singlet versus triplet radical ion pairs in, however, the same
electron donor-acceptor conjugate is unprecedented. Especially,
the simple variation of the excitation wavelength has not been
shown to be linked to such a different electron transfer outcome.
In this regard, we have synthesized two novel TPA-C60 electron
donor-acceptor conjugates
4 and 5 bearing one or two
dithiafulvenes by means of 1,3-dipolar cycloaddition of azomethine
ylides – Scheme 1. We have investigated their photophysical
properties with
a particular focus on photoinduced electron
transfer processes, which resulted in radical ion pair states followed
by deactivation to the ground state. Importantly, our investigations
have enabled for the first time the selective and direct formation of
either singlet or triplet radical ion pair states in the same electron
donor-acceptor conjugates. To this end, photoexcitation of either
the electron accepting fullerenes or the electron donating
triphenylamines / dithiafulvenes provide the means for activating
different charge separation and recombination pathways. As such,
we believe that our work will trend-set the field of electron donor-
acceptor design by means of adapting “triplet” precursors.
Scheme 1 Synthesis of 4 and 5 as well as reference 6.
Results and discussion
Turning to 3, absorptions are seen in the UV region of the solar
spectrum with a maximum at 317 nm accompanied by a shoulder-
like maximum at around 360 nm. For 5, optical absorptions are
located within the same region of the optical spectrum as observed
for 3, with maxima at 338, 369, and 395 nm. Similar to 4, the
absorption of 5 appears as the superimposition of the absorption
spectra of 3 and 6 including an additional red shift by about 40 nm
of the absorption characteristics located at 400 nm – Figures 2 and
S2.
Ground state characterization
In the absorption spectrum of TPA-dithiafulvene 2, strong
absorptions in the near-UV region of the optical spectrum in the
form of a maximum at 315 nm and a shoulder around 370 nm are
noted. N-methylfulleropyrollidine 6 gives rise to absorptions in the
near-UV and blue region of the optical spectrum with a dominating
maximum at 325 nm. Regarding TPA-dithiafulvene-C60 4, the
ground state absorption is best described as the superimposition of
the absorptions of 2 and 7. In addition, a weak absorption tail that
extends into the violet / blue region of the solar spectrum emerges.
This is explained by the enhanced conjugated π-system in 4 – see
Figure 1.
2
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Figure 1. Absorption spectra of 2 (black), 6 (green), and 4 (red) in toluene. The blue line
depicts the sum of the absorption spectra of 2 and 6.
Pulse radiolysis
•+
•+
In order to characterize the absorption spectra of 2 and 3 , pulse
radiolysis experiments under oxidizing condition were performed. 2
and 3 were dissolved in 1-Chlorobutane (BuCl), saturated with N
2
and irradiated with short high energy electron pulses. These
•+
conditions lead to the formation of BuCl , which is known as a
3
5, 36
•+
strong oxidizing agent.
The ability of BuCl oxidizing aromatic
amines via free electron transfer is well established in the Figure 3. Optimized structures and electron densities of LUMOs and HOMOs of (a) 4
3
7, 38
and (b) 5 calculated at B3LYP/3-21G level of theory.
literature.
TPA - described in the literature forming 2 and 3 respectively.
The pulse radiolysis spectrum of solutions of 2 in BuCl shows after
So it is safe to assume that 2 and 3 are oxidized like
•+
•+
37
Molecular Orbital Calculations
•
+
the electron pulse the typical transient absorption of BuCl with a
characteristic transient absorption maximum around 520 nm (not
Next, we performed computational studies to shed light onto the
geometrical and electronic features of 4, 5, and C60-TPA lacking
dithiafulvenes. All of the structures were optimized at the B3LYP/3-
3
5, 36, 39
shown).
This transient absorption decays rapidly giving rise to
2
1G level using Gaussian 09 program package. Figure 3 shows the
spatial electron densities of the highest occupied molecular orbital
HOMO) and lowest unoccupied molecular orbital (LUMO) for 4, 5,
a new set of transient absorptions, one covering the visible part of
the optical spectrum and one in the NIR - see Figure S10. These
(
transient absorption bands are attributed to the transient and C60-TPA. First of all, the electron density of the HOMO in C60
-
TPA was found to be located on the aromatic amine moiety, while
the LUMO is localized on the C60 spheroid. These preliminary
calculations suggest that the anion radical will be localized on the
•
+
absorption of 2 .
•+
Regarding the reaction of 3 with BuCl , again directly after the
electron pulse the characteristic transient absorption features of
•+
C60 spheroid, while the radical cation will be localized on the
the BuCl were observed, which rapidly decay, giving rise to new
transient absorption bands maximizing at 580 nm and 1100 nm -
aromatic amine moiety in case of a charge separated state, which is
6
in line with the work of O. Ito et al. Secondly, for 4 and 5, the
•+
see Figure S6 - belonging to the transient absorption of 3 .
corresponding LUMOs are mainly localized on the fullerene,
while the spatial electron densities of the HOMO are extended
to the side chains, which bear the dithiafulvene ring. In
summary, functionalization of the TPA moiety tunes energies of
HOMOs (better electron donating ability) while energies of
LUMOs remain essentially unchanged – see Figure 3.
Excited State Characterization
First insight into possible electron donor acceptor interactions
came from fluorescence assays. 2, for example, exhibits a strong
emission between 400 and 650 nm with a maximum at 435 nm –
Figure S3 in the supporting information – and an emission quantum
yield of 0.007 in toluene. When turning to 4, the TPA-dithiafulvene
centered emission is completely quenched in toluene and in THF.
For TPA-bis-dithiafulvene 3, the strong emission between 400 and
5
00 nm maximizes at 425 nm – Figure S3. A value of 0.015 was
Figure 2. Absorption spectrum of 5 in toluene.
obtained for the emission quantum yield in toluene. For 5, the TPA-
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bis-dithiafulvene based emission is fully quenched in toluene and
THF.
Figure 4. (a) Transient absorption spectra of 5 in argon saturated THF upon fs-laser photolysis (387 nm, 400 nJ / pulse) with time delays of 2 ps (black), 10 ps (red), 100 ps (green),
000 ps (blue), and 5000 ps (cyan) after the laser pulse. (b) Corresponding time absorption profile (black) and exponential decay fit (red) at 590 nm. (c) Nanosecond transient
absorption spectra of 5 in argon saturated THF upon laser photolysis (387 nm, 400 nJ / pulse) with time delays of 5 ns (black), 25 ns (red), 50 ns (green), 100 ns (blue) and 500 ns
cyan) after the laser pulse. (d) Corresponding time absorption profile (black) and exponential decay fit (red) at 590 nm.
1
(
As a complement, we investigated the C60 based fluorescence. quenching of the C60 fluorescence in 4 / THF, in 5 / toluene, and in 5
1
*
.
Here, in order to excite C60 exclusively, 470 nm was selected as / THF indicates an additional decay process starting from C60
excitation wavelength. N-methylfulleropyrollidine 6 was used as
Transient absorption spectroscopy based on femtosecond and
nanosecond laser photolysis was employed to shed light onto the
additional decay processes. In the case of 2 and 3, the singlet
excited states are formed immediately after laser excitation. Both
show strong transient absorptions throughout the visible and NIR
region of the optical spectrum with maxima at 615 for 2 or 612 nm
for 3 – Figure S5. The latter decay with a lifetime of ~2.5 ps and
transform into transient absorptions maximizing at either 565 nm
reference component with a reported fluorescence quantum yield
-4
40
of 6.0 x 10 in toluene. 4, 5, and 6 all show typical N-
4
0
methylfulleropyrollidine centered emissions with maxima located
at 710 ± 1 nm – Figure S4. In toluene, only 5 showed a significant
quenching of the C60 fluorescence with a quantum yield of 3.2 ± 0.5
-5
x 10 . Turning to THF, the fluorescence of 4 is quenched by an
-5
order of magnitude (5.3 ± 0.5 x 10 ). In 5, the fluorescence was
quantitatively quenched in THF.
(
2) or 590 nm (3). A likely rationale infers formation of the
In summary, the complete quenching of the TPA-dithiafulvene corresponding triplet manifold by means rapid intersystem crossing
and TPA-bis-dithiafulvene fluorescence in 4 and 5 relative to due to the presence of the sulfur atoms. In both cases, that is, for 2
references 2 and 3, respectively, points to an additional decay and 3, these transient absorptions decay with lifetimes of 370 and
mechanism – energy transfer and / or electron transfer. Likewise, 480 ps, respectively, to repopulate the ground state.
4
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Turning to the electron donor-acceptor conjugate 5 upon 387 nm verification of the triplet spin multiplicity is the quenching of the
photoexcitation with femtosecond laser pulses in THF, two parent triplet excited state or the triplet radical ion pair state with
4
1
transient absorption bands appear 3 ps after the laser pulse. For molecular oxygen. In the current case, this is, however, unfeasible.
5, the maxima occur around 590 and 1010 nm – see Figures 4a and The triplet quenching by oxygen is based on a bimolecular energy
4b. Notably, the latter absorption is diagnostic for the one electron transfer and the lifetimes observed for our compounds are too
reduced C60 radical anion, whereas the former matches well the short to observe reliable and interpretable oxygen effects.
transient absorption fingerprint of the one electron oxidized TPA-
A different approach to verify our hypothesis is based on taking
bis-dithiafulvene radical cation. The latter was determined via
advantage of the different absorption cross sections. Considering,
pulse radiolytic oxidation – Figure S6. From this assignment we
for example, the absorption spectra of TPA-bis-dithiafulvene 3 –
•
+
conclude the successful formation of the TPA-bis-dithiafulvene -
Figure 2 – and C60 reference 7 – Figure 1, we performed
femtosecond transient absorption experiments by exciting solutions
of 5 at 568 nm. Only contributions from the C60 singlet excited state
should be discernable, due to the lack of TPA-bis-dithiafulvene
absorption. This is, indeed, the case. Importantly, exciting at 568
nm shows transient absorptions exclusively attributed to short lived
radical ion pair state – Figure 5 – which is formed with 1.8 ps.
•-
C60 radical ion pair state in conjugate 5. Taking a closer look at the
•
+
•-
decay of the TPA-bis-dithiafulvene -C60 radical ion pair state in
THF the transient absorption features decay biphasic. In particular,
a short-lived component was found along with a long-lived
component. For the short lived component a lifetime of 6 ps has
been derived while the long lived component decays on a timescale
beyond the detection limit of our femtosecond laser photolysis
setup of 5500 ps – see Figures 4.
As
a complement, we performed femtosecond transient
absorption measurements following 420 nm fs-laser excitation. The
latter generates both C60 singlet excited state and TPA-bis-
dithiafulvene singlet excited state similar to those experiments, in
which 387 nm excitation was employed. Notably, the TPA-bis-
dithiafulvene singlet excited state transforms rapidly via
intersystem crossing into the corresponding triplet manifold. Like in
the 387 nm excitation experiments, biphasic decay kinetics are
observed. In particular, a short-lived 6 ps component accompanies
a long-lived 85 ns component – Figure S8. At the 420 nm excitation,
the relative C60 to TPA-bis-dithiafulvene absorption cross section
increases and, in turn, contributions from the C60 singlet excited
state dominate over those of the TPA-bis-dithiafulvene triplet
excited state. In line with the latter, the relative ratio between the
In order to reveal the identity and the lifetime of the long lived
transient, the same experiments were repeated with a nanosecond
laser photolysis set up. Upon photoexcitation at 387 nm, the same
transient absorption appeared as observed in the femtosecond
laser photolysis experiments – vide supra. In other words, maxima
at 590 and 1010 nm confirm the formation of the TPA-bis-
dithiafulvene -C60 radical ion pair state. Next to seeing the short
lived component with its 6 ps lifetime, the long lived component
was seen to decay with a lifetime of 85 ns – Figures 4c and 4d. In
Figure S7, normalized transient absorption spectra from
femtosecond and nanosecond laser pulse measurements are shown
to demonstrate the resemblance of transients across the different
timescales. Any notable differences, as they are noted in the near-
infrared parts of the transient absorption spectra are based on the
different white light sources used for the femto- and nanosecond
laser photolysis set ups.
•+
•-
short- and long-lived radical ion pair states intensified
.
Our findings that the radical ion pair state shows two lifetimes
and that a 387 nm excitation excites TPA-bis-dithiafulvene as well
as C60 – Figures 1 and 2 – leads us to hypothesize that two radical
ion pair states are formed. One of them is born upon exciting C60
,
while the other stems from exciting TPA-bis-dithiafulvene. The
different radical ion pair state lifetimes are only reasonable when
different spin multiplicities, that is, singlet versus triplet, are
considered. In other words, we postulate the competition of a
singlet versus triplet radical ion pair state.
It is well documented that photoexcitation of C60 leads to its
singlet first excited state, for which a lifetime of 1.4 ns is found
4
2
unless it is quenched. Please note that the initially populated C60
singlet excited state with its characteristic absorption maximum at
around 900 nm is invisible due to the rapid occurring charge
separation. Considering that the spin polarization remains constant
during the electron transfer it is reasonable to postulate the
formation of a short lived, singlet radical ion pair state, originating
from C60 singlet excited state. This leaves the long lived radical ion
pair state with a triplet electron spin multiplicity. The only feasible
pathway in electron donor-acceptor conjugate 5 towards a triplet
radical ion pair state is a triplet excited state precursor. Here, the
strong spin orbit coupling induced by sulfur atoms comes into play
as it facilitates in 2 and 3 the intersystem crossing from the TPA-bis-
dithiafulvene and TPA-dithiafulvene singlet first excited states to
4
3
the corresponding triplet excited states.
The formation of radical ion pair states with different spin
4
4
multiplicities has been reported by Fukuzumi et al. A common
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Figure 6. Plot of rate constant for the charge recombination of the singlet radical ion
-
1
pair state vs T for 5 in 2-methyl THF upon photoexcitation at 387 nm. Linear fit is
shown in red. Inset: Corresponding time-absorption profiles at 600 nm obtained at 300
K (black), 270 K (red), 240 K (green), 210 K (blue), and 180 K (cyan).
Figure 5. (a) Transient absorption spectra of 5 in argon saturated THF upon fs-laser
photolysis (568 nm, 400 nJ / pulse) with time delays 2 ps (black), 5 ps (red), 10 ps
(
green), 25 ps (blue), and 100 ps (cyan) after the laser pulse. (b) Corresponding time
absorption profile (black) and exponential decay fit (red) at 590 nm.
Figure 7. Plot of the rate constant for the charge recombination of the triplet radical
Additionally, the finding of two different radical ion pair states –
-1
ion pair state vs T for 5 in 2-methyl THF upon photoexcitation at 387 nm. Linear fit is
one with singlet and one with triplet spin multiplicity – was shown as in red. Inset: Corresponding time-absorption profiles at 600 nm obtained at
confirmed in temperature dependent laser photolysis transient 300 K (black), 270 K (red), 240 K (green), 210 K (blue) and 180 K (cyan).
absorption measurements with, for example, 5. When reducing the
temperature from 300 to 180 K in 2-methyl-THF the lifetime of the
radical ion pair state with singlet spin multiplicity increases from 10
ps (300 K) to 19 ps (180 K) – Figure 6. From the fit to the Arrhenius
C60- *TPASS
1
2.5 ps
C60-3*TPASS
equation
a
rather small activation barrier for the charge
1*
C60-TPASS
recombination of the singlet radical ion pair state of 2.4 kJ / mol
was obtained. When turning to the temperature dependency of
the triplet radical ion pair state of 5 in 2-methyl THF the lifetime
increases from 92 ns (300 K) to 1090 ns (180 K) – Figure 7. From an
Arrhenius analysis, a fairly substantial activation barrier of 10.0 kJ /
mol was derived for the charge recombination of the triplet radical
ion pair state.
1.8 ps
1[
60•--TPASS•+
> 6 ps
3[C60•--TPASS•+
]
C
]
Both activation barriers, that is, 2.4 kJ / mol and 10.0 kJ / mol for
the charge recombination of the singlet and triplet radical ion pair
states, respectively, are well in line with our expectations. In 5, the
activation barrier for the recombination of the singlet radical ion
pair state is comparable to RT at room temperature and, in turn,
negligible.
6
ps
85 ns
Figure 8. Simplified energy diagrams for 5 in THF illustrating on the left and on the right
the different deactivation pathways upon photoexcitation of TPA-bis-dithiafulvene
TPASS) at 387, 420, and 568 nm and C60 at 387 and 420 nm, respectively. The energy
(
of the radical ion pair state energy was determined as 1.25 eV for 5 from the one
+
electron reduction of C60 as -1.19 V versus Fc/Fc and the one electron oxidation of
+
TPASS as +0.05 V versus Fc/Fc and as 1.25 eV for 4 from the one electron reduction of
+
C
60 as -1.14 V versus Fc/Fc and the one electron oxidation of TPASS as +0.11 V versus
+
Fc/Fc .
6
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In stark contrast, the activation energy for the triplet radical ion lifetime of up to 130 ns in THF, has triplet spin multiplicity, and
pair state is substantially higher due to the presence of parallel evolves from triplet excited state precursor. Both are
electron spins. The latter requires spin flipping as the rate limiting energetically nearly degenerate, but the exact determination
a
step in the repopulation of the ground state.
requires application of a varying external field. Key in forming more
than three orders of magnitude longer lived triplet radical ion pair
states is the presence of sulfur atoms, which enhance spin-orbit
coupling and, in turn, render intersystem crossing of TPA-
dithiafulvene / TPA-bis-dithiafulvene in the excited state fast with a
lifetime of about 2.5 ps. The change in spin multiplicity of the
radical ion pair state is likely to be slower. At room temperature,
upper and lower limits are given by the radical ion pair states of 6
ps and 130 ns, respectively. Notably, the interconversion between
the singlet and triplet radical ion pair states is characterized by
magnetic field strength dependence. The physical process, which
Taking the aforementioned into concert, it is safe to conclude
that formation of a singlet radical ion pair state results from C60
photoexcitation in 5, while a triplet radical ion pair state evolves
from TPA-bis-dithiafulvene photoexcitation – Figure 8.
Turning to the electron donor-acceptor system 4 featuring only
one TPA-dithiafulvene upon laser excitation with 387 nm
femtosecond laser pulses evidence for the short lived and the long
lived radical ion pair state is noted. In particular, the transient
absorption bands maximize at 585 and 1010 nm – Figures S7 and
S11. On one hand, the feature at 1010 nm is a perfect match of the
1
7, 45
allows for such detailed analysis, involves Zeeman splitting of the T
0
one electron reduced C60 radical anion.
state by the magnetic field. It can be modulated by the strength of
the magnetic field, which is a path that we are currently exploring in
our laboratory. Regardless of the respective spin multiplicity both
radical ion pair states decay directly to the ground state without
forming a triplet excited state. In other words, one state decays in a
spin allowed and fast manner, while the other one decays spin
forbidden and slow. Independent confirmation for the singlet
versus triplet character came from temperature dependent
measurements with focus on the radical ion pair state lifetimes.
Here, activation barriers of 2.4 and 10.0 kJ / mol for the singlet and
triplet radical ion pair state, respectively, were established. Our
results document the incentives of a “triplet” precursor and, as
such, will pave the way towards new electron donor-acceptor
materials for photovoltaics and photocatalysis.
On the other hand, the transient absorption in the visible range
of the solar spectrum is in sound agreement with the transient
absorption of the one electron oxidized TPA-dithiafulvene radical
cation obtained in our pulse radiolysis study – Figures S9 and S10.
Our finding that the transient absorption matches the transient
absorption of the C60 radical anion and the TPA-dithiafulvene radical
•
+
cation strongly suggests the formation of the TPA-dithiafulvene -
•-
C
60 radical ion pair state. Like in the case of 5, the radical ion pair
state of 4 decays in 387 nm excitation experiments with two
lifetimes, namely 6 ps and 130 ns. One is due to singlet spin
multiplicity and one is due to triplet spin multiplicity. In the
presence of air the lifetime of the 130 ns is reduced to 31 ns from
which we conclude an activation controlled deactivation of the
radical ion pair state.
Experimental Section
General Methods:
Considering similar absorption spectra for 4 and 5, 387 nm
1
excitation leads to the formation of the C60 singlet excited state as All reactions were performed under an argon atmosphere. The H
1
3
well as to the generation of the TPA-dithiafulvene singlet excited and C NMR chemical shifts are given relative to tetramethylsilane
state. The latter, however, rapidly transforms via intersystem (TMS). All chemicals were purchased and used as received unless
crossing to the TPA-dithiafulvene triplet excited state. Again, we
postulate that from the C60 singlet excited state the short lived,
singlet radical ion pair state evolves, while the TPA-dithiafulvene
triplet excited state is the precursor to the long lived, triplet radical
ion pair state. Support for this notion came from femtosecond
transient absorption measurements exciting at 568 nm. Similar to
what was described for 5, the 568 nm excitation results exclusively
in the C60 singlet excited state and, in turn, the singlet radical ion
otherwise noted. Triphenylamine (TPA) was purchased from
Aldrich Chemical Co. 4,5-bis(hexylthio)-1,3-dithiole-2-thione was
46
prepared according to the reported procedures.
Synthesis:
The details of synthesis of 1a, 1b, 2, 3, 4 and 5 as well as their
characterizations are provided in the supporting information.
•
+
•-
Photophysical studies:
pair state formation. Importantly, the TPA-dithiafulvene -C60
transient absorptions decay with a 6 ps component. Here, no long
lived transient absorption remains on the time scale beyond 10 ps –
Figure S12.
Steady-state UV/Vis absorption spectra were measured on
PerkinElmer Lambda 35 and PerkinElmer Lambda 2 two-beam
spectrophotometers. Steady-state emission spectra were recorded
with a FluoroMax P spectrometer (Horiba Jobin Yvon). The
experiments were performed at room temperature. Emission
quantum yields for the TPA-derivative based emission were
determined by the comparative method using 9, 10-di-
phenylantracene as references with a fluorescence quantum yield
Conclusions
In summary, we have observed fast, photoinduced charge
separation in a set of two novel electron donor-acceptor conjugates
based on fullerenes and sulfur-containing triphenylamines.
Importantly, depending on the excitation wavelength, that is, either
47
of 0.9 in cyclohexane.
where the fullerenes or where the triphenylamines absorb, for the Femtosecond transient absorption laser photolysis measurements
•
+
•-
first time short-lived or long-lived TPA-dithiafulvene -C
/ TPA- were performed with an output from a Ti/sapphire laser system
6
0
•+
•-
bis-dithiafulvene -C60 radical ion pair states, respectively, are
formed in the same conjugate. On one hand, the short-lived
component with a lifetime as short as 6 ps has singlet spin
multiplicity and stems from a fullerene singlet excited state
precursor. On the other hand, the long-lived component has a
(
CPA2110, Clark-MXR Inc.): 775 nm, 1 kHz, and 150 fs FWHM pulses.
The excitation wavelength was either generated by second
harmonic generation (387 nm) or using a NOPA (NOPA Plus – Clark
MXR) (420 and 568 nm). For all excitation wavelengths, pulse
widths of <150 fs and energies of 400 nJ / pulse were selected. The
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transient absorption measurements were performed with an
HELIOS (Ultrafast Systems LLC) transient absorption spectrometer.
Notes and references
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Nanosecond transient absorption measurements were performed
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LLC). The pump pulses were generated from the amplified
Ti:Sapphire laser system described above. The probe pulse (2 kHz,
9
8
3
.
,
5
0
.5 ns pulse width), which was generated in a 1064 nm pumped
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5
.
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photonic fiber, synchronized with the femtosecond amplifiers.
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Conventional nanosecond laser flash photolysis transient
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–
OptikElektronik Müller) Xenon lamp (XBO 450, Osram), a
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,
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9
.
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Pulse radiolysis:
The samples were dissolved in 1-chlorobutane, saturated with N
2
and irradiated with high energy electron pulses (1 MeV, 15 ns
duration) by a pulse transformer type electron accelerator (Elit -
Institute of Nuclear Physics, Novosibirsk, Russia). The dose
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1
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48
delivered per pulse was measured by electron dosimetry. Doses of
00 Gy were employed. The optical detection of the transients was
N. Martin, Energy Environ. Sci., 2011, 4, 679-684.
1
2. J. Lehl, M. Vartanian, M. Holler, J.-F. Nierengarten, B. Delavaux-
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MSP 05 - Müller Elektronik Optik) Xenon lamp (XBO 450, Osram), a
SpectraPro 500 monochromator (Acton Research Corporation), a R
1
9220 photomultiplier (Hamamatsu Photonics), and a 500 MHz
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2
4
9
Gaussian 09 program package.
,
Electrochemical measurements:
3
III (METROHM) potentiostat, in a conventional three-electrode
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Journal, 2003,
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supporting electrolyte. All potentials were referenced to
+
19. Y. Shirota, J. Mater. Chem., 2005, 15, 75-93.
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0. L. Liu, H. Li, J. Bian, J. Qian, Y. Wei, J. Li and W. Tian, New J.
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Financial support from Bavarian “Solar Technologies go Hybrid” –
initiative (SolTech) and from the German Science Foundation
through SFB 953, is gratefully acknowledged. AK gratefully
acknowledges funding from the Deutsche Forschungsgemeinschaft
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