Interplay between singlet and triplet excited states in a conformationally locked donor-acceptor dyad

Article abstract of DOI:10.1039/c5dt03784k

The synthesis and photophysical characterization of a palladium(ii) porphyrin - anthracene dyad bridged via short and conformationally rigid bicyclo[2.2.2]octadiene spacer were achieved. A spectroscopic investigation of the prepared molecule in solution has been undertaken to study electronic energy transfer in excited singlet and triplet states between the anthracene and porphyrin units. By using steady-state and time-resolved photoluminescence spectroscopy it was shown that excitation of the singlet excited state of the anthracene leads to energy transfer to the lower-lying singlet state of porphyrin. Alternatively, excitation of the porphyrin followed by intersystem crossing to the triplet state leads to very fast energy transfer to the triplet state of anthracene. The rate of this energy transfer has been determined by transient absorption spectroscopy. Comparative studies of the dynamics of triplet excited states of the dyad and reference palladium octaethylporphyrin (PdOEP) have been performed.

Full text of DOI:10.1039/c5dt03784k

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Interplay Between Singlet and Triplet Excited States in a
Conformationally Locked Donor-Acceptor Dyad
Mikhail A. Filatov,a,b*† Fabian Etzold,a Dominik Gehrig,a Frédéric Laquai,a,c Dmitri Busko,a Katharina
Landfester,a and Stanislav Baluscheva,d,e
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The synthesis and photophysical characterization of a palladium (II) porphyrin – anthracene dyad bridged via short and
conformationally rigid bicyclo[2.2.2]octadiene were achieved. A spectroscopic investigation of the prepared molecule in
solution has been undertaken to study electronic energy transfer of excited singlet and triplet states between the
anthracene and porphyrin units. By using steady-state and time-resolved photoluminescence spectroscopy it was shown
that excitation of the singlet excited state of the anthracene leads to energy transfer to the lower-lying singlet state of
porphyrin. Alternatively, excitation of the porphyrin followed by intersystem crossing to the triplet state leads to very fast
energy transfer to the triplet state of anthracene. The rate of this energy transfer has been determined by transient
absorption spectroscopy. Comparative studies of the dynamics of triplet excited states of the dyad and reference
palladium octaethylporphyrin (PdOEP) have been performed.
www.rsc.org/
applications based on triplet–triplet annihilation photon
upconversion (TTA-UC).8
Introduction
In the TTA-UC process, D-b-A triplet sensitizers promise to
provide a new generation of molecular photonic devices.
Chromophores producing long-lived triplet excited states
under excitation with light have attracted much attention
during the past decade due to great promise for application in
diverse areas of research, particularly aimed at sunlight
harvesting.1 Chemical modification is a key tool for the control
of excited-state properties, and consequently emission and
energy transfer parameters (efficiency and rates) of the
chromophore.2 Particularly, introduction of intramolecular
excited state donor-acceptor couplings prolongs the triplet
Efficient implementation of the process requires two
components in a mixture: a sensitizer (transition metal
complex) and an emitter (aromatic hydrocarbon or BODIPY),
which generate higher energy photons through a sequence of
energy transfer processes.9 The development of a single
molecular system combining the properties of sensitizer and
emitter and capable of anti-Stokes emission is of fundamental
importance and is desired for sensing applications. Attempts
to incorporate 9,10-diphenylanthracene (DPA) into ruthenium
(III) complex structures provided a delayed fluorescence (TTA-
based upconversion emission) of covalently bound anthracene,
however with low efficiency compared to the mixture.10 The
key factors governing energy transfer properties in such dyads
are to be elucidated in order to develop efficient single
molecular upconversion systems.
lifetime by enabling
a reversible intramolecular exciton
transfer, the so called ‘‘energy reservoir’’ effect.3 Such
modifications allow for improving the efficiencies of
corresponding photonic applications.4 Phosphorescent donor-
bridge-acceptor (D-b-A) dyes are important materials for a
number of optoelectronic,5 sensing,6 bioimaging7 and
A
number
of
studies
carried
out
with
covalently/noncovalently linked porphyrin-porphyrin11 and
porphyrin-nonporphyrinic chromophore12 assemblies have
dealt with the effect of D-b-A distance on the rates of the
triplet-triplet energy transfer (TTET) process, however,
relatively little attention has been paid to the effects of mutual
orientation between donor and acceptor.13 Giribabu and co-
workers demonstrated striking orientation dependence of the
energy transfer in a porphyrin-based D-b-A system, in which
the donor subunits are linked both at the axial and peripheral
a.Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz,
Germany.
b.Institute of Polymers, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., block
103-A, BG - 1113 Sofia, Bulgaria.
c. Physical Sciences and Engineering Division (PSE), Material Science and
Engineering (MSE), Solar and Photovoltaics Engineering Research Center (SPERC),
King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900,
Kingdom of Saudi Arabia.
d.Optics and Spectroscopy Department, Faculty of Physics, Sofia University ‘‘St.
Kliment Ochridski’’, 5 James Bourchier, 1164 Sofia, Bulgaria.
e. Freiburg Institute for Advanced Studies (FRIAS), Albert-Ludwigs-Universität
Freiburg, Albertstraße 19, D-79104 Freiburg, Germany.
† Current address: Trinity Biomedical Science Institute, Trinity College Dublin, 152-
160 Pearse Street, Dublin 2, Ireland
E-mail: filatovm@tcd.ie
Electronic Supplementary Information (ESI) available: Optical spectra, NMR and
mass-spectra of newly synthesized compounds. See DOI: 10.1039/x0xx00000x
sites
of
the
porphyrin
macrocycle
(acceptor).
Photoluminescence studies revealed that energy transfer (ET)
from the porphyrin donor to the acceptor subunits bound at
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aromatic systems by a rigid bridges based on polycyclic
hydrocarbon scaffolds has been shown to provide well-defined
distances and orientations of the photoactive fragments.22
Based on these findings, we designed a porphyrin D-d-A dyad
surrounded by four anthracene moieties connected via
bicyclo[2.2.2]octene (Fig. 1B) and studied its optical properties
and excited state dynamics.
the peripheral positions is efficient, while no ET takes place to
the acceptors bound at the axial positions.14
Another fundamental question arising from these studies is
whether the electronic excitation leading to a long-lived triplet
state is accompanied by
a change in the molecular
conformation, especially for porphyrins, known to possess
various macrocycle distortion modes.15 Little is known about
photophysical behaviour of conformationally rigid D-b-A
porphyrin systems, specifically regarding the triplet state
dynamics and phosphorescence energies. Although porphyrin
D-b-A systems have been extensively studied,16 no examples of
such system, which can be regarded as “rigid” meaning that no
conformational changes in the excited state can take place, are
known. In this work we studied a D-b-A system, capable of
TTET, in which conformational changes are excluded due to
the specific arrangements of donor and acceptor moieties
along a bridging bicyclic subunit.
Results and discussion
Synthesis
The synthesis of the target molecule is shown in Scheme 1. Starting
quinizarine was converted to 1,4-anthraquinone 1 by the reduction
with NaBH4. Compound 1 was further subjected to Diels-Alder
reaction with 1,3-cyclohexadiene in boiling chloroform which
delivered product 2. It should be noted, that in the case of toluene
as a solvent, heating of the reaction mixture above 100 °C resulted
in the formation of tetracene-5,12-dione as a by-product via
thermal retro-Diels-Alder reaction.
Acenes are known to possess low solubility, requiring the
introduction of alkyl, alkoxy or alkoxycarbonyl substituents.
Moreover, introduction of such substituents is known to affect their
excited states energies.23 The latter is of particular importance with
respect to photophysical properties of a designed system, since an
overlap of donor and acceptor triplet manifolds is to be achieved.
Thus, we treated compound
2 with a base (DBU), to form
deprotonated hydroquinone, which was then reacted with MeI to
provide the 1,4-dimethoxyanthracene derivative 3 in good overall
yield. Anthracene 3 was subjected into a reaction with PhSCl that
resulted in the corresponding chlorosulfide, which was further
oxidized to give chlorosulfone 4. Subsequent HCl elimination under
DBU treatment yielded the corresponding allylic sulfone 5, which
was introduced into Barton-Zard reaction with isocyanacetate
leading to pyrrole 6. Its reduction by LiAlH4 delivered pyrrole
derivative
7
which was introduced into acid-catalyzed
tetramerization24 without isolation to give corresponding porphyrin
8 in a good yield (25%).
Fig. 1. (A) Possible structures of porphyrin D-b-A system, (B)
porphyrin-anthracene dyad with bicyclo[2.2.2]octane spacer.
Finally, the free-base porphyrin 8 was subjected to palladium
(II) insertion reaction leading to the target complex 9. Careful
control over the reaction temperature was critically important to
prevent by-product formation. Palladium insertion into porphyrin
macrocycles usually requires prolonged reflux in high boiling point
solvents such as benzonitrile.25 In the case of porphyrin 9, heating
over 150°C was found to cause a retro-Diels-Alder reaction,26
leading to the corresponding tetracene-fused derivatives, as was
evidenced by mass-spectroscopy.
A molecular design concept of the target system is
illustrated in Fig. 1. The assembly of porphyrin D-b-A dyads has
commonly been based on the functionalization of meso-
positions of the macrocycle, since corresponding synthetic
methods are well developed. 17 However, meso-substitution is
known to cause non-planar distortion of the porphyrin
macrocycle, adopting saddle, waved, ruffled or domed
conformation.18
Systematic analysis of photophysical
properties of meso-substituted porphyrins reveals that: 1)
chromophores attached through meso-positions are in partial
conjugation to the porphyrin’s aromatic system and 2)
rotational motions along Cmeso-Caryl bonds are possible.19 It
results in lower quantum yields of emission and slows down
excited state energy transfer processes. On the other hand, it
was shown that β-substituted porphyrins with four
bicyclo[2.2.2]octene-fused frameworks adopt a slightly waved,
planar conformation.20 In all reported crystal structures of
these porphyrins, the porphyrin chromophore was almost flat
and showed only slight out-of-plane distortion.21 Separation of
In the course of isolation and purification of crude products 8
and
9 we noticed a formation of by-products upon their
illumination with light under air. This probably results from
porphyrin-sensitized singlet oxygen formation and its successive
addition to the anthracene moiety, similarly to previously reported
anthraporphyrins and anthracene-substituted porphyrins.27
However, protecting the corresponding solutions from light during
purification we succeeded to isolate products 8 and 9 in pure form
as evidenced by solution NMR and mass-spectroscopy data (see
Supporting Information).
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Scheme 1. Synthesis of target molecule.
It
should
be noted, that the obtained product 9 obviously consists of a
mixture of stereoisomers, resulting from the fact that the attached
anthracene fragment is not symmetrical with the respect to the
porphyrin mean plane. However, taking into account that the
geometry of the photoactive fragment (porphyrin-bridge-
anthracene) is identical for each of the possible isomers, their
photophysical properties are unlikely to show individual features.
UV/Vis and photoluminescence spectra of
8 and 9 are
presented in Fig. 2. Similarity of the absorption features, i.e.
maxima and vibrational structure, to those of octaethylporphyrin
(OEP) and its Pd(II) complex (see Fig. S20 and S21 for comparison)
confirms the absence of conjugation between porphyrin and
anthracene. The only difference observed is a slight bandwidth
broadening due to increased molecular size. Absorption of
anthracene subunits is seen in the 300-400 nm region.
Optical properties
Upon excitation at the wavelength corresponding to the
Absorption and emission spectra of compound 3 (Fig. 2A) generally porphyrin Q-band absorption, compound 8 exhibits fluorescence
resembled those of reference compound, namely 9,10- consisting of two bands with maxima at 620 and 687 nm, typical of
a
diphenylanthracene, DPA, in its maxima, band structure and free base porphyrins, and virtually identical regarding peak maxima
lifetimes. On the other hand, a considerably lower fluorescence and vibronic structure to that obtained with the same excitation
quantum yield was observed (0.51). In order to obtain its wavelength for OEP base. Fluorescence quantum yields were
phosphorescence spectrum, we performed a photoluminescence obtained as 0.17 and 0.13 for 8 and OEP, respectively (compared to
measurement at 77 K in frozen methyl-cyclohexane solution. In this TPP free-base as a reference). When excitation was done at 336
case a red-shifted emission with a considerably longer lifetime nm, in the region of the anthracene chromophore absorption,
(~300 ns) than the fluorescence (~14.9 ns) was observed (see Fig. emission spectra showed predominantly emission from the
S18 and S19). However, a comparison with literature data shows, porphyrin (Fig. S20). An excitation spectrum registered at different
that the observed emission does not correspond to the anthracene emission wavelengths (Fig. S20) closely resembles the absorption
triplet state, but rather to the emission of excimers.28
spectrum of 8.
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Fig. 2. Absorption (black) and emission (red) spectra of anthracene 3 (A), porphyrins 8 (B) and 9 (C). (D) Comparison of
porphyrin 9 (3x10-5 M) emission under inert athmosphere (black) and under air (red).
Emission spectra of 9 solution (5 x 10-6 M), prepared in glove- the total excited state population. Between 850-1000 nm the
box conditions (1 ppm O2), differ to those of the parent PdOEP (Fig. photoinduced absorption (PAS1) of the singlets shows another
S21) and consist of four bands at 597, 648, 689 and 724 nm. Upon prominent peak, which consequently can be used to monitor the
selective excitation in the region of anthracene absorption, similar dynamics of the excited singlet state population. After a few ps
emission spectra were observed. Excitation spectra measured at all both the SE and the PAS1 of the singlets have vanished and another
emission maxima are similar to the absorption spectrum of 9 (Fig. broad PA has appeared with a distinct peak between 740-790nm.
S21). When the sample was left open to air, the emission beyond As this feature stayed constant throughout the time window of the
670 nm was almost completely quenched (Fig. 2D), while the peaks TA experiment (4 ns), we assigned it to long-lived porphyrin T1
at 597 and 648 nm were still observed, although with rather low states (PT1). Thus the initially generated porphyrin singlet excitons
intensity. Lifetimes of the emission at 597 and 648 nm were undergo intersystem crossing to form porphyrin triplet states.
measured to be 602 and 648 μs, respectively (Fig. S23).
Global fitting of the dataset with a single exponential function
described the decay of the singlet excited states and concomitant
rise of the triplet state population very well, allowing us to extract a
rate constant for the intersystem crossing, as shown in the bottom
of Fig. 3. The kinetics were extracted at the indicated spectral
regions after the initial photogeneration of the singlet excitons
(faster than the response time of the setup). The solid lines
Excited state dynamics in PdOEP and porphyrin 9
In order to further clarify the excited state dynamics, porphyrin 9
was studied by ps-µs broadband transient absorption spectroscopy
(TA); measurements on the pristine PdOEP were also performed
and compared to those of 9.
represent a monoexponential fit with a rate constant of kISC
=
1/(7.97) ps-1 obtained from a global fit of the whole data set. The
correlated decrease of porphyrin singlet PAS1 and increase of
porphyrin triplet PAT1 allows to assign this rate constant to the ISC
process. The obtained individual component spectra for the two
states are shown in the SI.
The measurements conducted on pristine PdOEP were done in
solution at a concentration of 10-4 M. After excitation into the Soret
band at 400 nm, porphyrin singlet states are generated on a time
scale faster than the TA instrument response. The respective sub-ps
transient absorption spectrum is shown in Fig. 3. The prominent
peaks between 500-550 nm are assigned to the bleaching of the
ground state, as they perfectly well match the Q-band ground state
absorption spectrum. The positive feature at 610 nm results from
stimulated emission (SE) of the porphyrin S1 states (PS1) and can be
used as a measure for the contribution of singlet excited states to
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10
8
state, which is subsequently transferred to the anthracene, causing
the signal to decay up to 100 ps. The remaining weak PA then
stayed constant in signal height over the measured time frame. Due
to its long-lived character and different shape compared to the
triplet-induced absorption of the porphyrin, we assigned this
feature to triplet states on anthracene. The global fit yielded an ISC
GSB
0-1ps
2-4ps
10-20ps
1000-2000ps
6
4
2
rate of kISC = 1/8.97 ps-1 and a triplet energy transfer rate of ktrans
=
SES1
0
1/14.1 ps-1. The spectra obtained for the porphyrin singlet and
triplet state from the global fit are in good agreement with those
obtained from the pristine porphyrin and thus further validate our
data analysis (Figures S26 and S27).
PAS1
-2
-4
PAT1
500 600 700 800 900 1000
λ
/ nm
0
GSB
0.10
0.08
0.06
0.04
0.02
0.00
-0.02
-0.04
0-1ps
2-4ps
10-20ps
100-200ps
1000-2000ps
-2
-4
-6
PAT1 (765 nm)
PAS1 (920 nm)
-8
SES1
-10
-12
-14
-16
PAS1
PAT1
500 600 700 800 900 1000
/ nm
0.1
1
10
time / ps
100
1000
λ
-1
-2
Fig. 3. Transient absorption spectra of PdOEP after excitation at 400
nm and kinetics of the singlet and triplet population.
-3
-4
-5
-6
With the knowledge of the singlet and triplet state transient
absorption spectra, we interpreted the transient absorption data of
compound 9 obtained in solution at a concentration of 10-4 M. Fig. 4
shows the respective transient absorption spectra.
-7
-8
@ 600nm
@ 765 nm
-9
-10
At early times, precisely 0-4 ps, porphyrin singlets can be
identified by their characteristic SE peak around 600 nm and the
photoinduced absorption feature at 850-950 nm. Both features
disappear on the same time scale as observed for the PdOEP
indicating triplet state formation on the porphyrin. However, the
triplet signatures quickly decayed in contrast to the pristine PdOEP,
and a very weak residual signal remained. The fast decay of the
triplet signatures indicates efficient triplet energy transfer to the
anthracene, which we modeled by a sequential model including two
processes that follow single exponential dynamics, of which one
describes the ISC and the other the triplet energy transfer from the
porphyrin core to the adjacent anthracene.
0.1
1
10
100
1000
time / ps
Fig. 4. Transient absorption spectra of compound 9 and kinetics
observed at the indicated wavelength regions.
Excited state energy transfer processes in porphyrin 9
The energetic overlap of the porphyrin and anthracene triplet
manifolds in molecule 9 was confirmed by measuring of the
emission of 3 in a mixture with PdOEP (20:1) in toluene. Upon
excitation at 542 nm (500 μWcm-2), blue fluorescence (457 nm) was
observed due to the TTA-UC process (Fig. 5). The efficiency of the
process was found to be 7.5%, comparable with those of the PdOEP
– DPA (12.5%).
Fig. 4 displays the kinetics in the region of SE of the porphyrin
singlets (600 nm) and the PAT1 peak region of the porphyrin triplets
(765 nm) together with the fit to the experimental data. Due to the
Thus, intersystem crossing giving the triplet state localized on
superposition of the individual photoinduced absorption signatures, the porphyrin subunit (1) is followed by excited state triplet transfer
an initial signal rise can be observed, corresponding to the ISC to the anthracene, as was shown by TA data (2):
process from porphyrin singlets to porphyrin triplets. After the ISC
is completed the triplet population stays constant for more than 2
1Por-b-Ant
3Por-b-Ant
3Por-b-Ant
Por-b-3Ant
(1)
(2)
ns in the pristine PdOEP, while in compound 9 triplet energy
transfer to the anthracene takes place and rapidly reduces the
porphyrin triplet population. The rise of the signals on the sub-10ps
time scale corresponds to the population of the porphyrin triplet
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2,0x104
6x104
4x104
2x104
10-6
M
M
O
O
N
N
N
N
1,5x104
1,0x104
5,0x103
0,0
Pd
0
1x105
8x104
6x104
4x104
2x104
10-5
Excitation
542 nm
0
3x104
10-4
M
2x104
2x104
1x104
400
500
600
700
800
Wavelength, nm
Fig. 5. Upconversion spectrum of PdOEP (10-5 M) and 5,12-
dimethoxy-1,4-dihydro-1,4-ethanotetracene (5x10-4 M) in air-free
toluene.
5x103
0
600
650
700
750
800
850
Wavelength, nm
On the other hand, steady-state emission spectra and
corresponding excitation spectra of
8 and 9 clearly indicate
Fig. 6. Emission spectra of porphyrin 9 at different concentrations in
intramolecular excited state singlet energy transfer (3) from
anthracene to the porphyrin core, since excitation in the region of
anthracene absorption leads to the emission corresponding to the
porphyrin chromophore. This is consistent with the excited-state
behaviour of the previously reported porphyrin-anthracene dyads.29
Although direct excitation of both anthracene and porphyrin
chromophores can’t be excluded due to the overlap of the
absorption, emission of the PdOEP – anthracene 3 mixture (1:4
ratio), coming predominantly from anthracene (Fig. S23) upon
excitation in the same region, confirms energy transfer in dyad 9.
toluene. Excitation wavelength – 542 nm.
To prove the formation of porphyrin singlet states we
performed long delay transient absorption measurement, i.e. at ns-
µs timescale. TA spectra obtained after 1 ns are in agreement with
the proposed process. After 1 µs the emergence of a clear positive
signal between 550-600 nm (Fig. S28) is seen. Due to its spectral
position this signal can only be assigned to the porphyrin’s singlet
state. The lifetime of this signal is in the range of hundreds of
microseconds (Fig. S29), which is in line with the observation of
long-lived photoluminescence from compound 9 in the same
spectral region and supports our assignment. However, it remains
unclear why the singlet states do not undergo rapid intersystem
crossing to the triplet state as observed upon direct photoexcitation
of the porphyrin’s singlet state.
Por-b-1Ant
(3)
1Por-b-Ant
Comparison of the emission spectra of palladium porphyrin 9
with the reported fluorescence and phosphorescence spectra of
anthracene 3 and other anthracenes (9,10-dimethoxy or 9,10-
diphenylanthracene) shows that there is no contribution from the
anthracene subunits to the observed emission comprised by a
series of bands at 597, 648, 689 and 724 nm. Analytical data,
suggesting reasonable purity in conjunction with the good
agreement of excitation and absorption spectra allow to rule out
that some of these bands are resulting from impurities.
Alternatively, the fact that the emission peaks at 597 and 648 nm
were observed in oxygen-saturated conditions, when the
phosphorescence is completely quenched, allows to assign this
emission as a fluorescence from the singlet state of the porphyrin,30
which is normally very weak for Pd and Pt porphyrins due to the
high intersystem crossing efficiency. 31
Intermolecular energy transfer processes, such as TTET, are
concentration dependent. Indeed, registering the emission of 9 at
different concentrations shows changes in the delayed fluorescence
intensity, which almost completely disappears upon strong dilution.
At 10-6 M concentration emission spectrum of 9 becomes nearly
identical to those of PdOEP (Fig. 6), due to a loss of intermolecular
TTA process efficiency.
Influence of the concentration on the emission spectra, i.e.
intense delayed fluorescence at high concentration, evidences that
intermolecular energy transfer between the molecules takes place
in solution. Taking into previous findings on triplet excited state
relaxation channels,33 this confirms that triplet-triplet annihilation
between two anthracene localized triplet states takes place, which
results in the formation of a singlet excited state of anthracene:
3
We conclude the observed spectrum of
9 represents a
combination of the emission from singlet and triplet excited states.
Intensive singlet emission in degassed solution of 9 may result
either from triplet-triplet annihilation or inefficient intersystem
crossing.32 The latter can be however excluded in the case of
molecule 9 based on the transient absorption studies results. We
assigned the emission at 597 nm as a delayed fluorescence, while
Por-b-3Ant +
Por-b- Ant
Por-b-1Ant Por-b-Ant (4)
+
Such process is also consistent with the reported photophysical
behavior of 9,10-diphenylanthracene appended ruthenium (III)
complex structures, which undergo TTA process in the excited
state.10
the emission at 648 nm as
a superposition of the delayed
fluorescence and phosphorescence. Indeed, similarly long-lived
singlet emission was observed for PdOEP, although with much
lower intensity compared to its phosphorescence.33
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As shown in the steady-state emission experiments, singlet 1:5 ratio (see Fig. S24 for comparison with PdOEP). Moreover, even
energy transfer from the anthracene to the porphyrin is very in 1:1 ratio weak upconversion signal was still detected (Fig. S25).
efficient. Thus TTA process (4) should be followed by the formation This suggests that dimethoxyanthracene antennas are enhancing
of porphyrin S1 states via intermolecular energy transfer process the rate of TTET or triplet-triplet annihilation as well, or both rates.
(2). Although the intersystem crossing is very fast, emission of the However, this is beyond the scope of the present manuscript and
porphyrin singlet is observed due to recombination to the ground will be reported in a separate study.
state. ns-µs TA experiments on compound 9 further corroborate
porphyrin singlet excited state formation.
1,4x104
The observed processes are summarized in Fig. 7:
O
O
1,2x104
1,0x104
8,0x103
6,0x103
4,0x103
2,0x103
0,0
O
O
O
O
O
O
N
N
N
N
Pd
3Por-b-Ant
O
O
1Por-b-Ant
Por-b-Ant
Por-b-3Ant
Por-b-1Ant
Excitation
542 nm
1Por-b-Ant
400
500
600
700
800
1(Por-b-*Ant)
SET
1(*Por-b-Ant)
TTA
Wavelength, nm
1(*Por-b-Ant)
ISC
3(*Por-b-Ant)
3(Por-b-*Ant)
E
TTET
DF
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
hν
P
intermolecular
Fig. 7. Transitions among excited states of 9 and qualitative
Jablonski diagram. hv – photon excitation, ISC – intersystem
crossing,
transfer, TTA – triplet-triplet annihilation, SET – singlet energy
transfer, DF – delayed fluorescence.
P – phosphorescence, TTET – triplet-triplet energy
0.0
0.5
1.0
1.5
2.0
An additional point is noteworthy: in the spectra of compound 9
no upconverted luminescence coming from the anthracene
subunits was observed. This is in contrast with previous results
reported for Ru(III) cyclometallated complexes, surrounded by
diphenylanthracenes.10 A possible explanation could be that it is the
short distance between donor and acceptor subunits in 9, which
promotes fast energy back transfer in the singlet excited state.
2
Excitation intensity, W/cm
Fig. 8. Upconversion luminescence of the mixture of 9 and 3 and
quantum yield dependence on the excitation intensity (for the
mixture of 6 x 10-4 M 3 and 3 x 10-5 M 9 respectively). Solvent:
toluene. Samples were excited at λ = 542 nm (10 mWcm-2).
Excited state behavior of 9 is intriguing from the viewpoint of
chromophores design for photonic applications. Delayed
fluorescence has been demonstrated as a tool for a real-time
It was of particular interest to investigate whether molecule 9
can act as a triplet sensitizer for TTA-UC process in spite of the
observed interplay between the singlet and triplet states. Another
important question was if the intramolecular TTET from porphyrin
to anthracene subunit, forming anthracene localized triplet state,
can facilitate TTA-UC in a two-component system containing “free”
anthracene 3 in the solution (emitter). Fig. 8 shows upconversion
spectra for the samples containing 9 (3·10-5 M) as a sensitizer and 5,
10 or 20 equivalents of 3 as an emitter. The spectra were registered
at different concentrations of “free” anthracene 3 while keeping
the sensitizer concentration constant. Efficiency of the process
reaches 3.8 % at excitation intensities > 1.5 W/cm2. However, in
contrast to previously studied upconversion systems, which require
excessive amounts of emitter with respect to amount of sensitizer
used (usually 1 : 20-50), this couple shows strong emission even in
temperature probe with ratiometric response for
a broad
temperature interval.34 The main limitation of this approach is the
relatively weak intensity of the delayed fluorescence of the
metallated macrocycles, compared to phosphorescence signal. To
increase the intensity of delayed fluorescence, encapsulation into
polymer films (e.g. polystyrene), was applied.33 Although steric
effects of the triplet-triplet annihilation process have not been
adequately studied so far, experimental observations suggest that
the TTA efficiency is critically dependent on the mutual orientation
of two annihilating triplet excited state molecules. In particular,
simulations showed that the magnetic field effect on TTA is very
sensitive to mutual chromophore alignment, and the direction of
the effect is consistent with a local ordering of excited state
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Journal Name
molecules.35 The fact that molecule 9 possesses strong delayed mounted on a mechanical delay stage. Only reflective optics were
fluorescence in solution, may imply involvement of the factor of used to guide the probe beam to the sample to minimize chirp. The
mutual orientation between annihilating excited states in the excitation pulse was chopped at 500 Hz, while the white light pulses
intermolecular TTA process. For further investigation of these steric were dispersed onto a linear photodiode array which was read out
factors, more dyad molecules with distinct geometries are to be at 1 kHz. Adjacent diode readings corresponding to the transmission
prepared and investigated.
of the sample after an excitation pulse and without an excitation
pulse were used to calculate ΔT/T.
Conclusions
Preparation and characterization of the compounds
In conclusion, we have synthesized and characterized a
conformationally rigid porphyrin-anthracene dyad that exhibits
strong delayed fluorescence and phosphorescence in solution.
Using broadband transient absorption spectroscopy, we have
elucidated the formation of anthracene-localized triplet states
via intramolecular energy transfer from the porphyrin core
with an inverse rate constant of 14 ps and its further
transformation into the porphyrin singlet state at µs –
timescale. Excited state dynamics along with other
spectroscopy data suggest that the porphyrin delayed
fluorescence arises as a result of intermolecular triplet-triplet
annihilation process. Observed properties of the prepared
molecule demonstrate an example of singlet and triplet
excitation management in donor-acceptor dyad and are of
interest with respect to photon energy conversion systems.
1,4,4a,12a-Tetrahydro-1,4-ethanotetracene-5,12-dione (2).
A
mixture of anthracene-1,4-dione (12 g, 58 mmol), 1,3-
1
cyclohexadiene (9.28 g, 118 mmol) and chloroform (20 mL) was
placed into a high-pressure glass tube and heated at 80 °C for 48 h.
After cooling to room temperature the reaction mixture was
evaporated in vacuum. Crude product was recrystallized from
ethanol. 2: 13.4 g (80%), yellow crystals, m.p. 195-196 °C. 1H NMR
(250 MHz, CD2Cl2) δ 8.54 (s, 2H), 8.15 – 7.96 (m, 2H), 7.75 – 7.57 (m,
2H), 6.22 – 6.02 (m, 2H), 3.42 – 3.21 (m, 4H), 1.92 – 1.71 (m, 2H),
1.45 – 1.31 (m, 2H). 13C NMR (75 MHz, CD2Cl2) δ 198.3, 135.6, 134.3,
132.2, 130.3, 129.7, 128.9, 51.4, 36.6, 25.4. Anal. Calcd for C20H16O2:
C, 83.31; H, 5.59. Found: C, 83.18; H, 5.68.
5,12-Dimethoxy-1,4-dihydro-1,4-ethanotetracene
(3).
A
mixture of 2 (12 g, 42 mmol), methyliodide (17.9 g, 126 mmol),
anhydrous K2CO3 (11.6 g, 84 mmol), and acetone (150 mL) was
refluxed for 30 h. The resulting mixture was filtered, concentrated
by evaporation, and the residue was recrystallized from ethanol. 3:
11.7 g (88%), pale yellow crystals, m.p. 174-175 °C. 1H NMR (250
MHz, CD2Cl2) δ 8.61 (s, 2H), 8.04 (dd, J = 6.4, 3.2 Hz, 2H), 7.46 (dd, J
= 6.5, 3.2 Hz, 2H), 6.61 (dt, J = 7.8, 3.9 Hz, 2H), 4.44 (d, J = 1.3 Hz,
2H), 4.01 (s, 6H), 1.64 (dt, J = 29.2, 14.0 Hz, 4H). 13C NMR (75 MHz,
CD2Cl2) δ 145.3, 135.6, 132.7, 131.8, 128.8, 127.1, 125.7, 121.2,
63.1, 34.1, 26.0. UV/Vis (toluene) λmax (log ε): 391 (3.67), 371 (3.77),
352 (3.62), 337 (3.35). Anal. Calcd for C22H20O2: C, 83.51; H, 6.37.
Found: C, 83.59; H, 6.61.
Experimental
Equipments and methods
Anthracene-1,4-dione was prepared according to published
procedure. 36 NMR spectra were recorded with the solvent proton
or carbon signal as an internal standard. Emission quantum yields of
the porphyrin compounds were measured relative to the
fluorescence of free-base tetraphenylporphyrin (φf = 0.11)37 in
deoxygenated toluene. For quantum yields measurements, sample
concentrations were chosen to obtain an absorbance of 0.03 - 0.07
at the excitation wavelength, at least three measurements were
performed for each sample. Emission of the diluted (1·10-6 M)
rubrene solution in toluene under 542 nm excitation was used as a
reference for UC quantum yield calculations. UC measurements
were performed using previously described set-up.38 All the
measurements were performed using deoxygenated samples
prepared in glovebox (1 ppm oxygen).
2-Chloro-5,12-dimethoxy-3-(phenylsulfonyl)-1,2,3,4-
tetrahydro-1,4-ethanotetracene (4). Synthesis was performed
following
a previously published general procedure for the
preparation of α-chlorosulfones.39 Thiophenol (3.52 g, 32 mmol)
was added dropwise to a suspension of N-chlorosuccinimide (4.27
g, 32 mmol) in CH2Cl2 (50 mL). The mixture was stirred for 1 h, then
the precipitate of succinimide formed was filtered off. The orange
filtrate was added dropwise to a stirred solution of 3 (10 g, 32
mmol) in CH2Cl2 (100 mL) at 0 °C. The mixture was stirred at room
temperature for 2 h, and then evaporated in vacuum. The residue
was dissolved in THF-methanol (1:1) mixture (300 mL), and a
suspension of Oxone (39.3 g, 64 mmol) in water (100 mL) was
added under vigorous stirring. The mixture was stirred at room
temperature for 3 days, diluted with water (200 mL), and extracted
with CH2Cl2. The combined organic layers were dried with Na2SO4
and evaporated to dryness. Solid residue was recrystallized from
ethanol. 4: 13.41 g (85%), yellow semisolid froth. 1H NMR (250 MHz,
CD2Cl2) δ 8.69 (d, J = 5.0 Hz, 2H), 8.08 (dt, J = 6.0, 3.5 Hz, 2H), 7.93 –
7.80 (m, 2H), 7.61 – 7.41 (m, 5H), 4.36 – 4.29 (m, 1H), 4.23 (dd, J =
4.8, 2.9 Hz, 1H), 4.10 (s, 3H), 3.99 (s, 3H), 3.82 (dd, J = 5.3, 2.9 Hz,
Transient absorption measurements were performed with a
home-built pump-probe setup. For measurements in the time range
of 1-3 ns with a resolution of ~100 fs the output of a commercial
titanium:sapphire amplifier (3.5 mJ, 1 kHz, 100 fs) was split with one
portion used to generate a 400 nm excitation pulse (second
harmonic generation in a BBO crystal cut at theta = 29.2°) and
another used to generate a white light probe using a home-built
two-stage broadband (480-850 nm) non-collinear optical
parametric amplifier for white light generation and amplification in
the visible spectral range. The variable delay of up to ~3 ns between
pump and probe was introduced by a broadband retroreflector
8 | J. Name., 2012, 00, 1-3
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1H), 3.64 (dd, J = 5.6, 1.8 Hz, 1H), 2.43 (m, 1H), 2.13 – 2.00 (m, 1H), stirred for 18 h and DDQ (0.53 g, 2.34 mmol) was added. All the
1.79 – 1.59 (m, 1H), 1.56 – 1.43 (m, 1H). 13C NMR (63 MHz, CD2Cl2) δ following manipulations were performed either in the dark or in
147.8, 147.3, 138.3, 134.5, 132.1, 131.9, 129.6, 129.6, 128.9, 128.9, vessels covered by aluminum foil. After the additional stirring for 18
127.5, 127.4, 127.2, 126.3, 126.3, 126.0, 122.0, 121.8, 72.8, 63.6, h, the solution was washed with saturated NaHCO3 solution, water
63.4, 57.0, 37.0, 29.3, 27.3, 18.7. Anal. Calcd for C28H25ClO4S: C, and brine, dried over Na2SO4, and the solvent was removed in
68.21; H, 5.11. Found: C, 68.63; H, 5.55.
vacuum. The residue was purified by silica gel column
chromatography (CH2Cl2). 8: 214 mg (25%), brown powder, m.p. >
1
5,12-Dimethoxy-2-(phenylsulfonyl)-1,4-dihydro-1,4-
300 °C. H NMR (500 MHz, CD2Cl2) δ 10.89 – 10.79 (m, 4H), 8.85 –
ethanotetracene (5). Synthesis was performed following a modified
literature procedure.40 To a solution of chlorosulfone 4 (12 g, 24
8.74 (m, 8H), 8.17 – 8.04 (m, 8H), 7.56 – 7.44 (m, 8H), 6.86 (d, J =
8.8 Hz, 8H), 4.65 – 4.33 (m, 24H), 2.76 – 2.44 (m, 16H). 13C NMR (75
MHz, CD2Cl2 – trifluoroacetic acid) δ 148.9, 147.7, 147.6, 138.0,
132.4, 132.3, 131.0, 130.7, 129.0, 129.0, 128.9, 127.6, 127.3, 126.4,
126.3, 126.0, 122.1, 64.1, 34.6, 30.7, 28.1. UV/Vis (toluene) λmax (log
ε): 394 (5.3), 497 (4.37), 528 (4.1), 566 (3.96), 619 (3.45). HRMS
(ESI-TOF): m/z found 1463.5874, calcd for [M+] C100H78N4O8
1463.5853.
mmol) in CH2Cl2 (100 mL) was added
a solution of 1,8-
diazabicycloundec-7-ene (3.83 g, 25 mmol) CH2Cl2 (10 mL),
dropwise over a period of 10 min at 0 °C. The mixture was stirred
for 1 h at room temperature, washed with water, dried with
Na2SO4, and evaporated in vacuum. Solid residue was recrystallized
1
from ethanol. 5: 9.97 g (91%), yellow semisolid froth. H NMR (250
MHz, CD2Cl2) δ 8.58 (d, J = 9.7 Hz, 2H), 8.14 – 7.98 (m, 2H), 7.96 –
7.85 (m, 2H), 7.64 – 7.42 (m, 6H), 4.82 – 4.62 (m, 2H), 4.01 (s, 3H),
Porphyrin 9. All the following manipulations were performed
3.83 (s, 3H), 1.92 – 1.24 (m, 4H). 13C NMR (63 MHz, CD2Cl2) δ 147.8, either in the dark or in vessels covered by aluminum foil. A mixture
146.4, 146.1, 144.9, 140.3, 134.0, 132.0, 131.9, 130.6, 129.9, 129.9, of porphyrin 8 (0.1 g, 0.068 mmol), bis(benzonitrile)palladium (II)
129.3, 129.2, 128.8, 128.5, 128.5, 127.1, 127.0, 126.1, 121.7, 121.5, chloride (0.052 g, 0.136 mmol), and benzonitrile (5 mL) was heated
63.5, 63.2, 35.7, 34.7, 26.6, 25.9. Anal. Calcd for C28H24O4S: C, 73.66; at 120 °C for 8 h under Ar atmosphere. Then the reaction mixture
H, 5.30. Found: C, 73.26; H, 5.68.
was evaporated to dryness in vacuum, dissolved in CH2Cl2 (10 ml)
and filtered through a layer of silica (eluent CH2Cl2). Filtrate was
evaporation in vacuum. 9: 91 mg (85 %), purple powder, m.p. > 300
°C. 1H NMR (250 MHz, d8-toluene) δ 11.04 – 10.74 (m, 4H), 8.86 (d, J
= 10.9 Hz, 8H), 8.01 – 7.73 (m, 8H), 7.30 – 7.15 (m, 8H), 6.69 (dd, J =
20.9, 13.2 Hz, 8H), 4.37 – 3.95 (m, 24H), 2.68 – 2.15 (m, 16H). 13C
NMR was not registered because of insufficient solubility. UV/Vis
(toluene) λmax (log ε): 397 (5.3), 512 (4.26), 542 (4.68). HRMS (ESI-
TOF): m/z found 1567.4713, calcd for [M+] C100H76N4O8Pd
1567.4731.
Ethyl
5,12-Dimethoxy-4,13-dihydro-2H-4,13-ethano-
anthra[2,3-f]isoindole-1-carboxylate (6). Barton−Zard synthesis of
pyrrole 6 was performed following a previously published general
procedure.40 A solution of ethyl isocyanoacetate (2.03 g, 18 mmol)
in THF (10 mL) was added dropwise over a period of 10 min to a
stirred suspension of potassium tert-butoxide (2.13 g, 19 mmol) at 0
°C. The resulting mixture was stirred for 30 min, and then a solution
of sulfone 5 (8 g, 17.5 mmol) in THF (20 mL) was added dropwise
over a period of 10 min. The reaction mixture was stirred at room
temperature for 4 h, then evaporated in vacuum, dissolved in
CH2Cl2 (50 mL), washed with water, dried with Na2SO4, and Acknowledgements
evaporated in vacuum. Solid residue was recrystallized from
M. Filatov acknowledges the European Commission for Marie
1
ethanol. 6: 6.14 g (82%), white powder, m.p. 204 - 205 °C. H NMR
Curie Individual Fellowship (CONSORT, grant No. 655142). S.
Baluschev acknowledges the financial support from the
Bulgarian National Science Fund (DFNI E 02/11 - SunStore-
project “Molecular solar thermal systems, enhanced by
annihilation upconversion”) and from the FCFP FRIAS COFUND
Fellowship Programme (FP7-MCA-609305). F. Laquai thanks
the Max Planck Society for funding the Max Planck Research
Group. F. Etzold acknowledges financial support in the
framework of the SPP1355 “Elementary Processes in Organic
Photovoltaics”. D.W. Gehrig thanks the Fonds der Chemischen
Industrie (FCI) for a Kekulé scholarship.
(250 MHz, CD2Cl2) δ 8.76 (s, 1H), 8.62 (d, J = 5.4 Hz, 2H), 8.03 (dt, J =
9.4, 3.7 Hz, 2H), 7.52 – 7.40 (m, 2H), 6.82 – 6.75 (m, 1H), 5.33 – 5.29
(m, 1H), 4.84 (d, J = 2.0 Hz, 1H), 4.44 – 4.28 (m, 2H), 4.05 (s, 3H),
4.02 (s, 3H), 1.90 – 1.76 (m, 4H), 1.44 (t, J = 7.1 Hz, 3H). 13C NMR (63
MHz, CD2Cl2) δ 207.0, 161.9, 146.1, 145.6, 134.7, 133.0, 132.3,
131.7, 131.8, 130.0, 128.8, 128.8, 127.2, 127.1, 125.9, 125.8, 121.4,
121.3, 115.7, 114.7, 63.2, 63.0, 60.7, 31.8, 31.4, 31.1, 28.0, 27.3,
14.9. Anal. Calcd for C27H25NO4: C, 75.86; H, 5.89. Found: C, 76.12;
H, 6.14.
Porphyrin 8, free base. Pyrrole 6 (1 g, 2.34 mmol) was dissolved
in 50 ml of dry THF under Ar atmosphere, and cooled to 0 ºC. Then
LiAlH4 (0.89 g, 23 mmol) was added in portions and the stirring was
continued 2 h. Water (5 mL) was added to the reaction mixture and
the precipitate was filtered off using celite. The filtrate was diluted
with water and extracted with CH2Cl2. The organic layer was
washed with water and brine, dried over Na2SO4, and the solvent
was removed in vacuum. The residue was dried in vacuum, and
then dissolved in 300 ml of dry CHCl3. After the addition of p-
toluenesulfonic acid (0.01 g, 58 μmol), the reaction mixture was
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J. Name., 2013, 00, 1-3 | 9
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