Rational design of a phthalocyanine-perylenediimide dyad with a long-lived charge-separated state

Article abstract of DOI:10.1039/c2cc31087b

A new ZnPc-PDI dyad presenting for the first time a charge-separated state lower in energy than the triplet excited state of the ZnPc and PDI has been synthesized. The rational design implies the substitution of the ZnPc with phenoxy groups and the bay substitution of the PDI with sulfonyl substituents. The lifetime of the charge-separated state was 72 μs. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc31087b

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COMMUNICATION
Rational design of a phthalocyanine–perylenediimide dyad with a
long-lived charge-separated statewz
Vicente M. Blas-Ferrando,a Javier Ortiz,a Latifa Bouissane,a Kei Ohkubo,b
a
a
Shunichi Fukuzumi,*bc Fernando Fernandez-Lazaro and Angela Sastre-Santos*
´
´
´
Received 14th February 2012, Accepted 2nd May 2012
DOI: 10.1039/c2cc31087b
A new ZnPc–PDI dyad presenting for the first time a charge-
separated state lower in energy than the triplet excited state of
the ZnPc and PDI has been synthesized. The rational design
implies the substitution of the ZnPc with phenoxy groups and the
bay substitution of the PDI with sulfonyl substituents. The
lifetime of the charge-separated state was 72 ls.
which preclude the generation of persistent charge-separated
states. Typically, the latter display lifetimes in the picoseconds9
or nanoseconds10 range, yielding as the final long-lived transient
species the triplet excited state of the perylene moiety. Only in the
presence of Mg2+ ions we have been able to generate charge-
separated systems with an energy content lower than that of the
triplet excited states, thus detecting the charged transient species
with lifetimes in the range of hundreds of microseconds (240 ms
for the dyad and 480 ms for the pentad).9b,c,10c,11 Consequently,
the synthesis of a Pc–PDI system yielding a long-lived photo-
induced charge-separated state in the absence of any Lewis acid
had remained till now as a defiant objective, which has been
overcome with the results that we present in this communication.
The target molecule, ZnPc–PDI dyad 1 (Chart 1), is
composed of an electron rich phthalocyanine subunit bearing 6
electron-donating phenoxy groups linked to a strongly electron-
demanding perylenediimide substituted at the bay positions with
two electron-accepting sulfone groups. This compound was
prepared by reaction of the aminoethoxyphthalocyanine 2 with
the perylene monoimide monoanhydride (PMI) 3 (Scheme 1).
The former was prepared by statistical condensation between
diphenoxy- and Boc-protected aminoethoxy- phthalonitriles and
Boc deprotection with TFA. The latter was obtained starting
from 1,7-dibromoperylenedianhydride in a five-step procedure
which included formation of the bisimide, bromine displacement
by p-tolylthiol, formation of the bisanhydride, monoamidation
and oxidation to the disulfone (see ESIz).
Photoinduced electron transfer from an electron-donor to an
electron-acceptor is a primary event taking place in the
naturally occurring photosynthetic process, but also in the man-
made solar cells.1 There has been intensive research in the
synthesis and photophysical study of donor–acceptor arrays
using different coupling partners to extract conclusions for
applications in artificial photosynthetic systems and organic
photovoltaics.2 Among the donors, phthalocyanines (Pcs),3
the synthetic tetraaza-analogues of porphyrins, have been
extensively used because of their outstanding stability (thermal
and chemical), their intense absorption in the red part of the
visible spectrum and the multiple possibilities of chemical
modification (metal center, peripheral and non-peripheral posi-
tions, even axial positions depending on the metal) to allow the
fine tuning of their physical and physicochemical properties, and
hence the better performances of the displays.4 On the other
hand, perylenediimides (PDI)5 have also been exploited in the
preparation of devices6 and systems for biological applications7
taking advantage of their stability, the intense light absorption
and the remarkable electron accepting properties, which can be
modulated as a function of the substitution pattern. The arrays
formed by combining both moieties benefit from several advan-
tages, e.g., high absorption coefficients almost all over the visible
and a positive driving force for photoinduced electron transfer.8
However, these systems have low-lying triplet excited states
From the very beginning, 1 displayed remarkable characteristics;
hence 2 and 3 in solution have a greenish and an orange colour,
respectively, while 1 has a purple one. Moreover, the UV-vis
a
´
´
´
´
Division de Quımica Organica Instituto de Bioingenierıa, Universidad
´
Miguel Hernandez, Elche 03202, Spain. E-mail: asastre@umh.es
b Department of Material and Life Science, Graduate School of
Engineering, Osaka University, ALCA, Japan Science and
Technology Agency (JST), Suita, Osaka 565-0871, Japan
c Department of Bioinspired Science, Ewha Womans University, Seoul,
120-750, Korea
w This article is part of the ChemComm ‘Porphyrins and phthalocyanines’
web themed issue.
z Electronic supplementary information (ESI) available: Synthesis and
characterization of the new compounds, and photophysical data. See
DOI: 10.1039/c2cc31087b
Chart Chemical structure of ZnPc–PDI dyad 1.
Chem. Commun., 2012, 48, 6241–6243 6241
c
This journal is The Royal Society of Chemistry 2012

below the energy of the excited triplet states of the ZnPc (1.18 eV)
and PDI (1.07 eV), thus pointing out to the real possibility of
obtaining the long time desired long-lived charge-separated state
without back electron transfer to the triplet excited states.
ZnPc (2) revealed fluorescence bands located at 690 and 760 nm
in PhCN (Fig. S19, ESIz). For ZnPc–PDI, these emission bands
were found to be fully quenched over 97% (Fig. S20, ESIz).
Femtosecond laser flash photolysis of a deaerated PhCN solution
of compound 1 with a 410 nm laser pulse (fwhm = 130 fs)
afforded the characteristic transient absorption bands corre-
sponding to the singlet excited state of the ZnPc moiety
(1ZnPc*) at 1 ps,10d which has a lifetime of 29 ps to produce
the triplet excited state (3ZnPc*) via the fast intersystem
crossing (Fig. 3). The shorter lifetime of 1ZnPc* in the
ZnPc–PDI dyad 1 than in ZnPc 2 (t = 3.1 ns; Fig. S20, ESIz)
may be ascribed to the occurrence of electron transfer from
1ZnPc* to PDI to produce the singlet radical ion pair, followed
by the faster back electron transfer to the ground state.12 The
strong bleaching at 680 nm is ascribed to the absorption and
fluorescence of the ZnPc subunit. Because the energy of
3ZnPc* is higher than that of 3PDI*, energy transfer from
3ZnPc* to PDI occurred to produce 3PDI*, which does not decay
in the time scale (3 ns) in the femtosecond laser flash photolysis
measurements. The same experiment performed in toluene
yielded the same transient spectrum, although the decay of the
absorption at 730 nm had to be fitted by a biexponential function.
On the other hand, nanosecond experiments lead to a
completely different scenario, allowing the unambiguous
identification of the triplet charge separated state, produced
by electron transfer from ZnPc to 3PDI*. As shown in Fig. 4a,
upon a 355 nm nanosecond laser pulse excitation of a deaerated
Scheme 1 Synthesis of the ZnPc–PDI (1) from ZnPc (2) and PMI (3).
spectrum of 1 shows a small increase in the absorption of the Q
band compared with that of the precursor Pc 2. Also, there is a
red-shift in the absorption corresponding to the perylene moiety
(450–575 nm), when compared with that of PMI 3 (Fig. 1). DFT
theoretical calculations using the B3LYP/6-31G(d) set located
the HOMO in the phthalocyanine fragment, and the LUMO
in the perylene moiety, as expected (see Fig. S19 in ESIz). No
interaction among the subunits could be predicted, due to the
absence of any orbital overlap.
Cyclic voltammetric studies were performed on a THF
solution of ZnPc–PDI dyad 1 containing 0.10 M of TBAPF6
as a supporting electrolyte, which showed a reduction peak at
ꢀ0.34 V vs. SCE corresponding to the reduction of the
perylene moiety (Fig. 2). On the other hand, the oxidation
of the phthalocyanine occurred at 0.61 V vs. SCE. These
values are extremely important because they indicated that
the charge-separated state has an energy of 0.95 eV, which is
Fig. 1 UV-vis spectra of ZnPc–PDI dyad 1 (purple line) and reference
compounds, ZnPc 2 (green line) and PMI 3 (red line) in THF as a solvent.
Fig. 3 (a) Femtosecond transient absorption spectra of ZnPc–PDI
dyad 1 in deaerated PhCN at 298 K after 410 nm laser excitation;
(b) time profile of absorbance at 730 nm. Gray line: biexponential fitting.
Fig. 2 Cyclic voltammograms (100 mV sꢀ1) of 0.5 mM ZnPc–PDI
dyad 1 (purple line), and reference compounds, ZnPc 2 (green line) and
PMI 3 (red line) in THF containing 0.10 M of TBAPF6 as a
supporting electrolyte.
Fig. 4 (a) Nanosecond transient absorption spectra of ZnPc–PDI
dyad 1 in deaerated PhCN at 298 K after 355 nm laser excitation; time
profiles of absorbance at (b) 520 nm and (c) 850 nm. Gray lines: single-
exponential fitting.
c
6242 Chem. Commun., 2012, 48, 6241–6243
This journal is The Royal Society of Chemistry 2012

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Scheme 2 Energy diagram of photoinduced charge separation in
ZnPc–PDI dyad 1.
PhCN solution of ZnPc–PDI dyad 1, the characteristic finger-
prints of the PDI radical anion, i.e., transient absorption bands
+
at 800, 850 and 920 nm and ZnPcꢁ at 520 nm (see Fig. S21,
ESIz),9b,c were observed, although other bands were not seen
because of the bleaching due to the PDI moiety. The decays of
+
ꢀ
absorbances at 520 nm for ZnPcꢁ and at 850 nm for PDIꢁ
obeyed first-order kinetics to afford the same lifetime of 72 ms
(Fig. 4b and c).12 Such a long-lived charge-separated state
was also observed by excitation of the PDI moiety at 530 nm
(see Fig. S22, ESIz).13 The energy diagram for the photo-
induced events is shown in Scheme 2.
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We have designed and synthesized a new ZnPc–PDI dyad
that for the first time presents a charge-separated state lower in
energy than the triplet excited state of the ZnPc and PDI. The
rational design implies the substitution of the ZnPc with phenoxy
groups and the bay substitution of the PDI with sulfonyl
substituents. Moreover, nanosecond laser flash photolysis
confirms the existence of the longest charge-separated state
described so far, without the addition of external components,
for phthalocyanine–perylenediimide arrays, 72 ms. This kind of
ZnPc–PDI dyad is a promising candidate for application in
organic photovoltaics. Furthermore, the concept of lowering
the energy of the charge-separated state below that of the
triplet excited state, as described here, is simple and should be
applicable to other organic donor–acceptor systems. Further
work is currently underway to expand this concept and synthesize
other Pc–PDI arrays and their use in organic photovoltaics.
This work was partially supported by Grants Consolider-Ingenio
2010 project HOPE CSD2007-00007, CTQ2010-20349, CTQ2011-
26455 from MICINN and FEDER, and by Grants-in-Aid
(No. 23750014 and 20108010) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, KOSEF/MEST
through WCU project (R31-2008-000-10010-0), Korea.
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13 Selective excitation of the PDI moiety at 530 nm would result
in energy transfer from 1PDI* to ZnPc in competition with
electron transfer from ZnPc to 1PDI* to form the charge-separated
state.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6241–6243 6243