Close proximity dibenzo[a,c]phenazine-fullerene dyad: Synthesis and photoinduced singlet energy transfer

Article abstract of DOI:10.1002/ejoc.201000136

A dibenzo[a, c]phenazine-fullerene (DBPZ-C₆₀) dyad in which two chromophores are linked in close proximity to each other has been synthesized and studied in detail by optical spectroscopy to explore a new energy donor-acceptor system. The dyad

Full text of DOI:10.1002/ejoc.201000136

FULL PAPER
DOI: 10.1002/ejoc.201000136
Close Proximity Dibenzo[a,c]phenazine–Fullerene Dyad: Synthesis and
Photoinduced Singlet Energy Transfer
Rajeev K. Dubey,*[a] Tatu Kumpulainen,[a] Alexander Efimov,[a] Nikolai V. Tkachenko,[a] and
Helge Lemmetyinen[a]
Keywords: Fullerenes / Energy transfer / Energy conversion / Donor-acceptor systems / Photochemistry
A
dibenzo[a,c]phenazine–fullerene (DBPZ-C60
)
dyad in
tremely fast (ken = 5ϫ1011 s–1) intramolecular photoinduced
singlet–singlet energy-transfer process from singlet excited
state of the DBPZ moiety to fullerene. In both polar and non-
polar environment transduction of singlet excited state en-
ergy governs the excited state deactivation, but the effi-
ciency and rate of energy transfer were found to be higher in
nonpolar solvents in comparison to polar. The DBPZ singlet
excited state decays within 2 and 4.7 ps in toluene and
benzonitrile, respectively, via singlet–singlet energy transfer
to produce a fullerene singlet excited state which decays
which two chromophores are linked in close proximity to
each other has been synthesized and studied in detail by op-
tical spectroscopy to explore a new energy donor–acceptor
system. The dyad was prepared by Prato reaction between
11-formyldibenzo[a,c]phenazine and fullerene. 3,5-Di-tert-
butylbenzyl group was introduced onto the fulleropyrrolidine
unit to achieve adequate solubility of the dyad. A thorough
study of the photophysical properties of the dyad and rel-
evant reference compounds, performed by means of steady
state and time resolved spectroscopic measurements, has re- with a life time of 1.5 ns to give a very long-lived fullerene
vealed the presence of highly efficient (ca. 98%) and ex-
triplet state as final populated excited state.
for the development of the new molecular photovoltaic de-
vices.
Introduction
Besides many favorable properties, the low molar absorp-
tion coefficient of fullerene in the visible region limits its
utility as promising n-type material in many applications
like bulk heterojunction type polymer/fullerene solar cells.[7]
The attachment of a molecule which can efficiently and
rapidly transfer harvested light energy to fullerene without
diminishing its electron-acceptor ability may provide a rea-
sonable solution to this problem. Moreover, in order to per-
form the specific role of harvesting and transferring of light
energy to fullerene, the light-harvesting molecule must have
high enough oxidation potential to prevent any electron
transfer to fullerene. At the same time it is also important
that the molecule has higher first reduction potential than
fullerene so that only the fullerene moiety could act as un-
ambiguous electron acceptor when the system is incorpo-
rated in a bulk heterojunction device with the electron-do-
nor material.
A literature survey shows that a large number of fuller-
ene-donor systems have been reported in which photoin-
duced energy and electron-transfer processes either compete
with each other or take place selectively depending on the
polarity of the media.[8] However, there are very few exam-
ples which exhibit exclusive transfer of energy from the
light-harvesting antenna to the fullerene unit in both polar
and nonpolar environment.[9] In our quest for finding new
light-harvesting C60-based systems in which the light-
harvesting antenna not only satisfies the electrochemical
Many intensive efforts have been carried out over the
past few decades to construct photosynthetic models which
can mimic natural photosynthesis involving photoinduced
electron and energy transfer as fundamental processes.[1]
Fullerene (C60) has emerged as one of the most promising
energy and electron acceptor due to its fascinating physical
and chemical properties such as easy functionalization,[2]
excellent electron accumulation ability[3] and ability to act
as an energy trap.[4] Therefore a wide range of fullerene-
based molecular dyads and more complex systems that ab-
sorb visible light and undergo directional electron and/or
energy transfer have been synthesized and investigated ex-
tensively during the past two decades to attain a better un-
derstanding of mechanisms that convert sunlight into chem-
ical energy.[5] These systems have also been used to con-
struct devices of future interest like organic solar cells.[6]
Therefore a better knowledge of electronic interactions of
fullerene with various molecules that can function as ef-
ficient electron and/or energy donors is essential; not only
to enhance our understanding of natural systems but also
[a] Department of Chemistry and Bioengineering, Tampere
University of Technology,
P. O. Box 541, 33101 Tampere, Finland
Fax: +358-3-31152108
E-mail: rajeev.dubey@tut.fi
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000136.
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Close-Proximity Dibenzo[a,c]phenazine–Fullerene Dyad
Scheme 1. Synthesis of DBPZ-C60 dyad 6 and ref-C60 (7).
requirements but also performs efficient and fast transfer ous organic solvents such as toluene, chloroform, dichloro-
of energy to the fullerene, we have designed a novel energy methane, THF and benzonitrile, which was essential for the
donor–acceptor system involving dibenzo[a,c]phenazine as characterization and detailed spectroscopic and electro-
energy donor. The rationale behind the selection of DBPZ chemical studies of the dyad. Previously we had also syn-
as energy donor is based on its interesting spectroscopic thesized the dyad 6a using sarcosine (N-methylglycine) in
and electrochemical properties which came into light very place of N-(3,5-di-tert-butylbenzyl)glycine, but the resulting
recently.[10] In addition, this compound has some synthetic dyad had very low solubility in common organic solvents.
importance too, as its corresponding phenazine derivative, The poor solubility led to the low yield (17%) of the reac-
namely dipyrido[a,c]phenazine, can also be synthesized and
functionalized following the same synthetic procedure and
may be utilized for the supramolecular attachment with
various metals.[11]
In this paper we describe the results of an exploratory
study on the synthesis and on the electrochemical and pho-
tophysical properties of a novel close proximity DBPZ-C60
dyad along with parallel studies on relevant model com-
pounds. The electrochemical and photophysical properties
of these compounds have been undertaken thoroughly by
Scheme 2. Synthesis of N-(3,5-di-tert-butylbenzyl)glycine (2).
means of differential pulse voltammetry, steady state UV/
Vis absorption, steady state and time resolved fluorescence,
flash photolysis and pump-probe measurements to shed
some light on the photoinduced processes taking place be-
tween the two covalently linked moieties.
Results and Discussion
Synthesis and Characterization
The syntheses of the dyad and reference compounds are
summarized in Schemes 1, 2, and 3. 1,3-Dipolar cycload-
dition reaction between 11-formyldibenzo[a,c]phenazine (5)
and fullerene in the presence of N-(3,5-di-tert-butylbenzyl)-
glycine (2) led to the target DBPZ-C60 dyad 6.[12] Introduc-
tion of 3,5-di-tert-butylbenzyl group on the pyrrolidine ring
was necessary to enhance the solubility of the dyad in vari- Scheme 3. Synthesis of 11-formyldibenzo[a,c]phenazine (5).
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R. K. Dubey et al.
FULL PAPER
tion and also caused trouble during purification and char- in toluene. Oxidation of 11-bromomethyldibenzo[a,c]phen-
acterization of the dyad. To the best of our knowledge, this azine (4) by NaIO4/DMF yielded 5 in good yield (60%).[16]
is the first time when N-(3,5-di-tert-butylbenzyl)glycine was All the synthesized compounds were unambiguously char-
1
synthesized and successfully employed in the Prato reac- acterized by spectroscopic analysis including H NMR and
tion. Earlier longer alkyl chains (e.g. dodecyl, octadecyl) ESI-TOF mass spectra (see Exp. Section).
were used to increase the solubility of fulleropyrrolidine de-
Since dibenzo[a,c]phenazine can be easily replaced by di-
rivatives.[13] A similar procedure was applied to synthesize pyrido[a,c]phenazine in the dyad following exactly the same
ref-C60 (7) by using paraformaldehyde instead of 11-formyl- synthetic procedure, this synthetic work openes access to a
dibenzo[a,c]phenazine.
novel fullerene-containing ligand. The ability of resulting
N-(3,5-Di-tert-butylbenzyl)glycine tert-butyl ester (1) was dyad to coordinate with various transition metals may be
synthesized in good yield (66%) by the reaction of 3,5-di- utilized for the synthesis of fullerene-containing supra-
tert-butylbenzaldehyde with glycine tert-butyl ester hydro- molecular systems.[17,11] This will be the focus area of our
chloride and sodium cyanoborohydride in ethanol at room future research.
temperature. Subsequently N-(3,5-di-tert-butylbenzyl)gly-
cine (2) was synthesized in 68% yield by deprotection of
carboxylic group by the treatment with TFA in dichloro-
1H NMR Spectra
methane.[14]
1
11-Formyldibenzo[a,c]phenazine (5) was synthesized in
The H NMR spectrum (Figure 1) of the dyad displays
three steps. First, 9,10-phenanthrenequinone and 3,4-di- some characteristic features supporting the assumption of
aminotoluene were condensed in ethanol (in presence of the close proximity of the two acting moieties in the dyad.
concentrated aqueous HCl) to give 11-methyldibenzo[a,c]- Large downfield shifts were observed for the H9, H10 and
phenazine (3). By using ethanol/HCl combination, we man- H12 protons of the DBPZ moiety of the dyad with respect
aged to improve the yield up to 83.2% which is far better to the corresponding protons in ref-DBPZ. The H12 proton
than 64% obtained earlier by condensing the reactants in is the one which is most affected by the presence of the
toluene.[10b] 11-Bromomethyldibenzo[a,c]phenazine (4) was fullerene unit. In addition to the large downfield shift, its
obtained in 75% yield by benzylic bromination of 11- sharp singlet appears as broad signal in the dyad. The
methyldibenzo[a,c]phenazine (3) with N-bromosuccin- doublet of H1 and H8 protons also turned into doublet of
imide.[15] We always obtained around 10% of dibrominated triplet. These changes are attributed to the deshielding ef-
compound and some unreacted starting material which fect resulting from C60 π-electrons. The methyne proton
were difficult to separate by chromatography because of (H17) of the pyrrolidine ring also exhibits a downfield shift
their almost same mobility. So we separated them out from with respect to the corresponding methyne proton of very
the desired product 4 on the basis of their higher solubility similar fulleropyrrolidine derivative N-benzyl-2-(meth-
1
Figure 1. H NMR spectra of the dyad and ref-DBPZ in CDCl3.
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Close-Proximity Dibenzo[a,c]phenazine–Fullerene Dyad
oxycarbonyl)pyrrolidine–C60 (reported earlier[12]) because the DBPZ moiety has a much higher first reduction poten-
of the deshielding effect of DBPZ moiety. All these changes tial than fullerene. The large difference in the first reduction
may be taken as indication of the close proximity of the two potentials of two electroactive chromophores [∆E =
moieties and ground state interaction between them.[18]
E1red(C60) – E1red(DBPZ) = 86 mV] ensures that the two
moieties will perform their well-defined roles, i.e. DBPZ will
act as light-harvesting antenna whereas fullerene will accept
the electrons when this dyad is mixed with p-type materials
in bulk-heterojunction-based solar cells.
Electrochemistry
The electrochemical properties of the DBPZ-C60 dyad
and relevant reference compounds were investigated by
differential pulse voltammetry in benzonitrile containing
0.1  tetrabutylammonium tetrafluoroborate as supporting
electrolyte. The differential pulse voltammogram of dyad is
shown in Figure 2 and the obtained redox potentials are
shown in Table 1.
Steady-State Absorption Studies
The absorption spectra of the dyad and the model com-
pounds, ref-DBPZ and ref-C60 in toluene are shown in Fig-
ure 3. ref-DBPZ strongly absorbs at wavelengths lower than
420 nm and exhibits two absorption maxima at 375 and
396 nm corresponding to n–π* and π–π* transitions, respec-
tively. In the dyad these absorption bands appear at 380
and 403 nm and are shifted to longer wavelengths by 5 and
7 nm, respectively, indicating the substantial ground-state
electronic interactions between DBPZ and fullerene moie-
ties.
Figure 2. Differential pulse voltammogram of the dyad in PhCN,
vs. Ag/AgCl reference electrode. In measurement with ferrocene in
the solution, the oxidation peak of ferrocene was observed at
0.46 V.
Table 1. Redox potential values (voltage measured vs. Ag/AgCl) of
DBPZ-C60 dyad and reference compounds obtained by DPV.
Figure 3. Steady-state absorption spectra of the dyad, ref-DBPZ
and ref-C60 in toluene. The concentrations were maintained at
41 µ.
red
red
red
red
ox
E1
E2
E3
E4
–1.59
E1
dyad
ref-C60
ref-DBPZ
–0.55
–0.58
–1.41
–0.98
–0.99
–1.41
–1.57
+1.47
+1.44
n.o.[a]
Although, an additional 1–2 nm red shift was observed
in the absorption maxima of DBPZ moiety of the dyad and
of ref-DBPZ in more polar solvents including chloroform
and benzonitrile, but the extent of ground state interaction
between the two moieties found to be almost same as in
toluene. These results are in accordance with the NMR and
DPV studies which also indicate for the ground state inter-
action between the two moieties of the dyad. A sharp band
at 432 nm, characteristic of the fulleropyrrolidine provides
a way for the selective excitation of the fullerene moiety of
the dyad since DBPZ moiety has no absorption at wave-
lengths higher than 420 nm. Similarly at the wavelength
403 nm, where the DBPZ moiety absorbs strongly, fullerene
has a low absorption (ε ≈ 4ϫ103 –1 cm–1).
[a] Not observed.
The dyad shows four one-electron reduction processes,
which occur at –0.55, –0.98, –1.41 and –1.59 V vs. Ag/AgCl.
Comparison with the reduction potentials of ref-C60 (–0.58,
–0.99 and –1.57 V) and ref-DBPZ (–1.41 V) reveals that the
first, second and fourth reduction process corresponds to
the mono-, di-, and tri-anion of fullerene and the third one
to the DBPZ reduction. The only oxidation potential for
the dyad was observed at 1.47 V and corresponds to the
oxidation of the fullerene moiety. The influence of the
DBPZ moiety on the fullerene in the dyad is clearly visible
by the shift in the oxidation and first reduction potential
of the fullerene moiety in the positive direction by 30 mV,
compared to that of ref-C60.
Steady-State Emission Studies
The dyad has two noticeable features; firstly, the DBPZ
moiety of the dyad does not display an oxidation potential
The fluorescence emission spectra of the dyad and ref-
(not observed earlier also[10b]) in the scanning range of the DBPZ in toluene obtained with excitation at 378 nm are
measurement, i.e. 1.7 to +1.7 V, which reflects its incapa- shown in Figure 4. Optical densities of both solutions were
bility to donate electrons to the fullerene moiety. Secondly, identical at the excitation wavelength; ref-DBPZ shows an
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R. K. Dubey et al.
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emission maximum at 422 nm. In the case of the dyad,
emission of DBPZ moiety is efficiently quenched (98%) in
toluene.
Figure 5. Sensitized emission of fullerene moiety of the dyad with
respect to ref-C60 obtained from excitation of equimolar toluene
solutions at 403 nm.
Time-Resolved Fluorescence Studies
Figure 4. Steady-state fluorescence spectra of the dyad and the ref-
DBPZ compound in toluene at the excitation wavelength (λ =
378 nm).
Time-resolved fluorescence studies were carried out by
time correlated single photon counting (TCSPC) method.
For TCSPC measurements the solutions of dyad and refer-
ence compounds in toluene and benzonitrile were excited at
404 nm and emission time profiles of DBPZ and fullerene
moieties were collected at 450 and 704 nm, respectively. For
the DBPZ moiety of the dyad, the singlet-state life time of
12 ps was observed, whereas, for the reference compound,
ref-DBPZ, life time of 97 ps was observed. It must be noted
that lifetime of ca. 12 ps obtained is only an upper limit of
the actual lifetime due to the time resolution limitation of
the instrument i.e. 60 ps. On the other hand, singlet-state
life time of the fullerene moiety of the dyad (1.46 ns) was
found to be almost same as that for ref-C60 (1.50 ns), see
Figure 6.
The same experiments were carried out in chloroform
and also in benzonitrile. The dyad showed efficient quench-
ing (97%) of DBPZ emission with respect to ref-DBPZ in
moderately polar chloroform, but in polar benzonitrile rela-
tively less efficient quenching (89%) was observed. The de-
crease in the fluorescence quantum yield of DBPZ moiety
in the dyad in comparison to ref-DBPZ also resemble with
the quenching ratios (Table 2).
There could be two possible mechanisms for the ob-
served efficient quenching of the DBPZ emission; electron
and/or energy transfer to the fullerene unit. The involve-
ment of the electron transfer process, however, can be ruled
out on electrochemical grounds. Therefore we presume that
quenching of the DBPZ singlet excited state is solely due to
singlet–singlet energy transfer to fullerene, whose first ex-
cited singlet state lies at lower energy. To verify the energy
transfer in the dyad, emission spectra of equimolar solu-
tions of dyad and ref-C60 were measured with irradiation
(λ = 403 nm) where the DBPZ moiety of the dyad absorbs
predominately (the dyad has a four times higher absorbance
than ref-C60). We observed an almost four times enhance-
ment of the fullerene emission in the dyad with respect to
the ref-C60 (Figure 5). The observed sensitized emission of
fullerene moiety of the dyad confirms singlet–singlet energy
transfer from DBPZ to fullerene. Nearly the same results
were obtained in more polar solvents like chloroform and
benzonitrile.
Figure 6. Fluorescence decay time profiles for fullerene moiety of
the dyad and ref-C60 at 704 nm, in toluene, λex = 404 nm.
Table 2. Selected photophysical data of dyad, ref-DBPZ and ref-C60.
DBPZ
Φfl
Fulleropyrrolidine
[a]
[c]
Compound
Dyad
Solvent
Quenching
τ [ps][b]
τ [ns][b]
ken
toluene
PhCN
toluene
PhCN
toluene
PhCN
98%
89%
4ϫ10–4
12
21
97
105
1.46
1.50
5.0ϫ1011 s–1
8ϫ10–4
2.1ϫ1011 s–1
ref-DBPZ
ref-C60
7.3ϫ10–3
1.3ϫ10–2
1.50
1.47
[a] Fluorescence quantum yields were determined using anthracene as a standard (Φfl = 0.27 in ethanol). [b] Fluorescence singlet life
times measured by TCSPC. [c] Rates of singlet–singlet energy transfer; calculated from the lifetimes data obtained from picosecond
transient absorption experiments at room temperature.
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Close-Proximity Dibenzo[a,c]phenazine–Fullerene Dyad
In benzonitrile similar results were obtained, the ref- red to the fullerene singlet excited state, which undergoes
DBPZ and ref-C60 had singlet life times of 105 ps and intersystem crossing to give the fullerene triplet as the final
1.47 ns, respectively. In the dyad, singlet life times of DBPZ populated state in high yield. The triplet character of this
and fullerene moieties were found to be 21 ps and 1.50 ns, species is verified by its ability to sensitize singlet-oxygen
respectively. Thus, the singlet state of DBPZ moiety is formation.
strongly quenched, whereas singlet state of the fullerene
moiety remains unaffected.
Similar experiments were also carried out in benzonitrile
and the life times of the triplet transients of dyad, ref-C60
and ref-DBPZ were found to be 43, 38 and 67 µs, respec-
tively. The dyad has, also in benzonitrile, the triplet tran-
sient life time close to the model compound, ref-C60, and
amount of the absorption below 600 nm is lesser than ref-
DBPZ indicating that the negligible amount of DBPZ trip-
let contributes to the transient absorption of the dyad. In
addition, in both toluene and benzonitrile, no triplet–triplet
energy transfer was observed between fullerene triplet state
and DBPZ triplet state.
Microsecond Transient Absorption Studies
In both steady-state and time-resolved emission measure-
ments, carried out in polar and nonpolar environment,
quenching of the DBPZ excited singlet state was observed,
whereas, no quenching was observed for the fullerene moi-
ety. In order to investigate, the fate of excited singlet states
of DBPZ and fullerene moieties, transient absorption stud-
ies were carried out in the microsecond time scale. Nitro-
gen-purged equimolar toluene solutions of dyad, ref-DBPZ
and ref-C60 were excited at 401 nm, where DBPZ moiety
absorbs strongly, but the fullerene absorption is moderate.
Thus at that excitation wavelength the absorbances of the
dyad and ref-DBPZ solutions were identical, whereas ab-
sorbance of ref-C60 solution was one third of these. The
transient absorption spectra of the three compounds are
shown in Figure 7.
Picosecond Transient Absorption Studies
The transient pump-probe method was used to shed light
on the nature of the short living products and to investigate
the formation of charge separation state. For these measure-
ments the dyad, ref-C60 and ref-DBPZ were excited at wave-
length 404 nm at which DBPZ moiety of the dyad has maxi-
mum absorption. The decay component spectra of dyad in
toluene are shown in Figure 8.
Figure 7. Transient absorption spectra of the dyad, ref-DBPZ and
ref-C60 in toluene obtained with the excitation at 401 nm. The con-
centrations of the solutions were maintained at 41 µ.
Figure 8. Decay component spectra of the dyad in toluene (λex
404 nm).
=
The ref-DBPZ has an absorption band at 480 nm which
The very long living component, with a life timeϾϾ3 ns
is attributed to its triplet excited state. It has no absorption and a positive amplitude at 700 nm, clearly corresponds to
at wavelengths higher than 600 nm. The dyad and ref-C60 the triplet state of the fullerene which was also observed in
show absorption bands at 700 nm, which is the characteris- flash photolysis measurements. The short living compo-
tic band of the fullerene triplet state.[19] The life times of nent, with a life time of roughly 1350 ps and a negative
the triplet transients of dyad, ref-C60 and ref-DBPZ were amplitude at 700 nm, corresponds the decay of the singlet
found to be 37, 31 and 81 µs, respectively. The dyad has a excited state of fullerene. The fastest component with a life
triplet-transient life time close to the ref-C60 and exhibits a time of 2 ps can be attributed to the energy transfer from
diminished absorption below 600 nm. These observations the singlet excited state of DBPZ moiety to the singlet ex-
clearly suggest that there is significant decrease in the trip- cited state of fullerene. In benzonitrile the life time of the
let-state absorption of DBPZ moiety of the dyad which is component corresponding to the singlet–singlet energy
also in accordance with the fact that DBPZ singlet state is transfer was found to be 4.7 ps, which clearly indicates that
quenched by singlet–singlet energy transfer to fullerene, and the rate of energy transfer is lower in benzonitrile than in
essentially no DBPZ triplet state is therefore formed by in- toluene.
tersystem crossing. The sensitized triplet-state absorption
As far as charge separation is concerned, no spectral
observed for the fullerene moiety of the dyad indicates that evidences for the fullerene radical anion (which should
the harvested light energy by the DBPZ antenna is transfer- exhibit characteristic transient absorption band around
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R. K. Dubey et al.
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dehyde was synthesized according to the literature procedure.[20]
The solvents were of HPLC grade and purchased from VWR and
used without further purification. The sorbents for column
chromatography (Silica 60, Silica 100), TLC plates (silica gel 60
F254 on aluminium sheets) were purchased from Merck.
1000 nm[8d]), DBPZ radical cation or CS state were found
either in toluene or in benzonitrile. These results confirm
that the energy transfer from the DBPZ moiety to the ful-
lerene unit is the only photoinduced process taking place in
the dyad in both polar and nonpolar environment.
The proton NMR spectra were recorded with Varian Mercury
300 MHz spectrometer in CDCl3 at room temperature. All chemi-
cal shifts are quoted relative to TMS (δ = 0.0 ppm); δ values are
given in ppm and J values in Hz. Melting points were measured
on Stuart SMP 10 apparatus. Mass spectra were measured with
Waters LCT Premier XE ESI-TOF benchtop mass spectrometer.
Differential pulse voltammetry was performed with Faraday MP,
Obbligato Objectives Inc., in a three-electrode single-compartment
cell consisting of a platinum-in-glass working electrode, Ag/AgCl
as reference electrode, and a graphite rod as counter electrode. Dur-
ing the measurements, the values of pulse height, pulse width and
step voltage were set to 20 mV, 20 ms, and 2.5 mV, respectively.
Benzonitrile containing 0.1  tetrabutylammonium tetrafluoro-
borate was used as solvent. The measurements were done under a
continuous flow of nitrogen. A Fc/Fc+ couple was used as an in-
ternal standard. The measurements were carried out in both direc-
tions: towards the positive and negative potential. The reduction
and oxidation potentials were calculated as an average of the two
scans.
Conclusions
We have designed, synthesized and investigated a novel
energy donor-acceptor system in which two chromophores,
namely dibenzo[a,c]phenazine and fullerene, are linked at
close proximity. In both polar and nonpolar media, the har-
vested light energy by the DBPZ antenna is transferred very
efficiently to the fullerene unit on a picosecond time scale.
Subsequently the fullerene singlet excited state decays on a
nanosecond time scale to give the fullerene triplet state as
final populated excited state. We presume that such a high
rate of singlet–singlet energy transfer is a consequence of
the close proximity of the two acting moieties. The main
evidence in support of the energy transfer hypothesis comes
from the steady state emission studies in which sensitized
emission of the fullerene moiety was observed upon exciting
DBPZ moiety of the dyad and further confirmed by the
TCSPC measurement which does not show any quenching
of the fullerene singlet-state life time (Figure 9).
If not otherwise indicated, all the spectroscopic measurements were
carried out at room temperature. The absorption spectra were re-
corded with Shimadzu UV-2501PC spectrophotometer and the
fluorescence spectra using a Fluorolog-3 (SPEX Inc.) fluorimeter.
The emission spectra were corrected using a correction function
supplied by the manufacturer. Fluorescence quantum yields were
determined relative to anthracene in ethanol (Φ = 0.27).[21]
Fluorescence decays of the samples on nanosecond and sub-nano-
second time scales were measured using a time-correlated single
photon counting (TCSPC) system (PicoQuant GmbH) consisting
of PicoHarp 300 controller and PDL 800-B driver. The samples
were excited with the pulsed diode laser head LDH-P-C-405B at
404 nm and fluorescence decays were measured at the wavelength
of emission maximum. The signals were detected with a micro
channel plate photomultiplier tube (Hamamatsu R2809U). The
time resolution of the TCSPC measurements was about 60–70 ps
(FWHM of the instrument response function).
The flash-photolysis method was used to study time-resolved ab-
sorption on the microsecond time-scale. The instrument has been
described elsewhere.[22] The samples were deoxygenated by nitrogen
bubbling for 20 min before measurements. Pump-probe technique
for time-resolved absorption was used to detect the fast processes
with a time resolution shorter than 0.2 ps. The instrument and the
used data analysis procedure have been described earlier.[5c,23]
Figure 9. Energy-level diagram showing the processes that take
place in DBPZ-C60 dyad upon excitation of DBPZ moiety in tolu-
ene at room temperature.
It has to be emphasized that no spectral evidences for
the electron transfer were found, neither in polar nor in
nonpolar solvents. Due to the combination of the efficient
and fast energy transfer and favorable electrochemical prop-
erties, the synthesized dyad could be a good candidate for
future photovoltaic applications with well-defined roles of
both acting chromophores; DBPZ acting as a light-harvest-
ing antenna and fullerene acting as an electron acceptor in
the photoactive layer.
N-(3,5-Di-tert-butylbenzyl)glycine tert-Butyl Ester (1): To a solution
of 3,5-di-tert-butylbenzaldehyde (100 mg, 0.46 mmol) in ethanol
(15 mL), glycine tert-butyl ester hydrochloride (155 mg, 0.92 mmol)
and sodium cyanoborohydride (500 mg, 7.95 mmol) were added
and suspension was stirred at room temperature for 3 d. Then the
solvent was evaporated under reduced pressure and the residue dis-
solved in diethyl ether (100 mL) and washed with water
(3ϫ100 mL). The organic layer was collected, dried with sodium
sulfate and the solvents evaporated. The crude product was purified
by column chromatography using CHCl3 as eluent to yield the de-
sired product (100 mg, 65.7%) as a colorless oil. 1H NMR
(300 MHz, CDCl3): δ = 1.34 (s, 18 H, PhtBu), 1.50 (s, 9 H, CO-
OtBu), 1.94 (s, 1 H, N-H), 3.35 (s, 2 H, CH2COOtBu), 3.79 (s, 2
Experimental Section
General Remarks: All reagents for syntheses were purchased from
Sigma–Aldrich Co. and used as received. 3,5-Di-tert-butylbenzal-
3434
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 3428–3436

Close-Proximity Dibenzo[a,c]phenazine–Fullerene Dyad
H, PhCH2), 7.18 (d, J = 1.8 Hz, 2 H, phenyl-H), 7.34 (t, J = 1.9 Hz,
1 H, phenyl-H) ppm. MS (ESI-TOF): m/z = 356.2555 [M + Na]+.
m/z = 309.0334 [M +
H]+. UV/Vis (CHCl3): λmax/nm (ε/
Lmol–1 cm–1) = 406 (16600), 385 (16500). Fluorescence (CHCl3):
λmax = 468 nm.
N-(3,5-Di-tert-butylbenzyl)glycine (2): Ester 1 (100 mg) was dis-
solved in dichloromethane (2 mL) and subsequently 2,2,2-trifluoro-
acetic acid (2 mL) was added. The reaction mixture was stirred
for 4 h at room temperature. Then dichloromethane and TFA were
removed under reduced pressure. The residue was dissolved in chlo-
roform (50 mL) and washed with water (3ϫ50 mL). The organic
layer was collected, dried with sodium sulfate and evaporated to
yield a solid residue. The crude product was washed with hexane
Dibenzo[a,c]phenazine–Fullerene Dyad (6): To a solution of fuller-
ene (35 mg, 0.048 mmol) in toluene (100 mL), 5 (75 mg, 0.24 mmol)
and 2 (26 mg, 0.093 mmol) were added. The reaction mixture was
refluxed for 48 h under argon atmosphere, allowed to cool down
and evaporated under reduced pressure. The crude product was
purified by column chromatography eluting with 2:1 hexane/chlo-
roform to afford the dyad (15 mg, 25%). 1H NMR (300 MHz,
CDCl3): δ = 1.43 (s, 18 H, tBu), 3.90 (d, J = 13.5 Hz, 1 H, H13),
4.32 (d, J = 9.6 Hz, 1 H, H15), 4.63 (d, J = 13.5 Hz, 1 H, H14), 5.05
(d, J = 9.6 Hz, 1 H, H16), 5.54 (s, 1 H, H17), 7.47 (t, J = 1.8 Hz, 1
H, phenyl-H), 7.56 (d, J = 1.8 Hz, 2 H, phenyl-H), 7.83–7.68 (m,
5 H, H2, H3, H6, H7, H10), 8.41 (d, J = 8.7 Hz, 1 H, H9), 8.54 (d,
J = 8.1 Hz, 2 H, H4, H5), 8.76 (br. s, 1 H, H12), 9.39 (dt, J = 1.8,
J = 7.7 Hz, 2 H, H1, H8) ppm. MS (ESI-TOF): m/z = 1245.3136
[M + H]+.
1
and dried to yield the product (57 mg, 68%) as a white solid. H
NMR (300 MHz, CDCl3): δ = 1.27 (s, 18 H, tBu), 3.61 (s, 2 H,
CH2COOH), 4.15 (s, 2 H, PhCH2), 6.20 (br. s, 1 H, N-H), 7.21 (d,
J = 1.7 Hz, 2 H, phenyl-H), 7.44 (t, J = 1.7 Hz, 1 H, phenyl-H)
ppm. MS (ESI-TOF): m/z = 300.1938 [M + Na]+.
11-Methyldibenzo[a,c]phenazine (3): 9,10-Phenanthrenequinone
(2.00 g, 9.60 mmol) and 3,4-diaminotoluene (1.17 g, 9.60 mmol)
were placed into a round-bottomed flask (500 mL) equipped with a
reflux condenser. Subsequently ethanol (200 mL) and concentrated
aqueous HCl (10 mL) were added and reaction mixture was heated
to reflux. After a short period of time the product began to precipi-
tate from the solution. The progress of the reaction was monitored
by TLC and after 4 h the reaction mixture was allowed to cool
to room temperature; the precipitate was filtered off and washed
thoroughly with ethanol. The crude product was recrystallized
from CHCl3/ethanol and dried to afford the product (2.35 g, 83.2%
yield) as yellow needles; m.p. 222 °C (ref.[24] 220–221 °C). 1H NMR
(300 MHz, CDCl3): δ = 2.66 (s, 3 H, CH3), 7.66 (dd, J = 1.8, J =
8.7 Hz, 1 H, H10), 7.85–7.69 (m, 4 H, H2, H3, H6, H7), 8.06 (s, 1
H, H12), 8.18 (d, J = 8.7 Hz, 1 H, H9), 8.53 (d, J = 8.0 Hz, 2 H,
H4, H5), 9.36 (d, J = 7.5 Hz, 2 H, H1, H8) ppm. MS (ESI-TOF):
ref-C60 (7): To a solution of fullerene (26.4 mg, 0.037 mmol) in tol-
uene (70 mL), paraformaldehyde (5.5 mg, 0.18 mmol) and
2
(20.5 mg, 0.073 mmol) were added. The reaction mixture was re-
fluxed for 18 h under argon atmosphere and then allowed to cool
down. The solvent was evaporated under reduced pressure and the
crude product was purified by column chromatography eluting with
2:1 hexane/chloroform to afford 7 (12.8 mg, 36%). 1H NMR
(300 MHz, CDCl3): δ = 1.40 (s, 18 H, tBu), 4.34 (s, 2 H, PhCH2),
4.47 (s, 4 H, pyrrolidine-H), 7.44 (t, J = 1.9 Hz, 1 H, phenyl-H),
7.58 (d, J = 1.8 Hz, 2 H, phenyl-H) ppm. MS (ESI-TOF): m/z =
966.2309 [M + H]+.
Supporting Information (see also the footnote on the first page of
1
this article): H NMR spectra of all synthesized compounds.
m/z
= 295.1241 [M +
H]+. UV/Vis (CHCl3): λmax/nm (ε/
Lmol–1 cm–1) = 396 (22500), 376 (17000). Fluorescence (CHCl3):
λmax = 408 nm.
Acknowledgments
11-(Bromomethyl)dibenzo[a,c]phenazine (4): A mixture of 3 (1.08 g,
3.68 mmol), NBS (0.79 g, 4.42 mmol) and benzoyl peroxide (50 mg)
in CCl4 (300 mL) was refluxed for 48 h. The reaction mixture was
cooled, kept overnight at room temperature and the precipitate of
succinimide and unreacted NBS was filtered off. The filtrate was
evaporated under reduced pressure and the solid residue was dis-
solved in chloroform. The solution was washed with water, dried
with sodium sulfate and the solvents evaporated. The crude prod-
uct was dissolved in 30 mL of toluene (slight heating was required)
and kept overnight at room temperature to get the precipitate of 4
The authors acknowledge financial support from the Academy of
Finland.
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4
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Eur. J. Org. Chem. 2010, 3428–3436
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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Received: February 1, 2010
Published Online: May 6, 2010
3436
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Eur. J. Org. Chem. 2010, 3428–3436