Visible light-harvesting perylenebisimide-fullerene (C60) dyads with bidirectional "ping-pong" energy transfer as triplet photosensitizers for photooxidation of 1,5-dihydroxynaphthalene

Article abstract of DOI:10.1039/c2cc30345k

Visible light-harvesting perylenebisimide (PBI)-C₆₀ dyads were prepared as organic triplet photosensitizers for photooxidation of 1,5-dihydroxynaphthalene and the efficiency of the dyads is 6-fold of the conventional Ir(iii) complex triplet pho

Full text of DOI:10.1039/c2cc30345k

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COMMUNICATION
Visible light-harvesting perylenebisimide–fullerene (C60) dyads with
bidirectional ‘‘ping-pong’’ energy transfer as triplet photosensitizers
for photooxidation of 1,5-dihydroxynaphthalenew
Yifan Liu and Jianzhang Zhao*
Received 16th January 2012, Accepted 20th February 2012
DOI: 10.1039/c2cc30345k
Visible light-harvesting perylenebisimide (PBI)–C60 dyads were
prepared as organic triplet photosensitizers for photooxidation
of 1,5-dihydroxynaphthalene and the efficiency of the dyads is
6-fold of the conventional Ir(III) complex triplet photosensitizer.
Photosensitized reactions, such as photocatalytic hydrogen
(H2) production, photooxidation and photodynamic therapy
(PDT), have attracted much attention.1–3 Concerning these
applications, design of efficient triplet photosensitizers showing
strong absorption of visible light and long-lived triplet excited
states is crucial.4,5 Transition metal complexes are usually used as
triplet photosensitizers due to their efficient intersystem crossing
(ISC), with which the triplet excited states of the chromophores
are populated upon photoexcitation.6 For example, cyclometalated
Ir(III) complexes have been used as a singlet oxygen (1O2)
photosensitizer for photooxidation.2 However, these conven-
tional transition metal complex photosensitizers suffer from
high cost, weak absorption of visible light, etc.2
Fig. 1 Visible light-harvesting triplet photosensitizers PBI-C60 and
Thus, similar to the development of dye-sensitized solar cells
NPBI-C60. The light-harvesting antenna precursors PBI-Ph and
(DSCs), organic triplet photosensitizers with strong absorption
of visible light are highly desired to replace the transition metal
complex triplet photosensitizers.5 However, it is a challenge to
design organic triplet photosensitizers because it is difficult to
populate the triplet excited states of organic chromophores
upon photoexcitation. In this case the heavy atom effect is
very often indispensable.5,7 Although triplet photosensitizers
without any heavy atoms do exist, their ISC property is
almost unpredictable.6a Thus design of heavy atoms-free
organic triplet sensitizers with predetermined ISC capability
will be the next milestone for the development of organic
triplet photosensitizers.
NPBI-Ph.
fullerene have been rarely applied in a photophysical process.8
Fullerene is known for its weak absorption in the visible range.8
Thus, C60 itself is not an ideal triplet photosensitizer. Herein we
prepared visible light-harvesting C60–chromophore dyads as
organic triplet photosensitizers for photooxidation (PBI-C60
and NPBI-C60, Fig. 1, PBI = perylenebisimide, the visible
light-harvesting antenna).9 In order to access absorption in
the red range, we prepared dyad NPBI-C60 (Fig. 1), which
absorbs at 634 nm. Remarkably improved photooxidation of
1,5-dihydroxyl naphthalene (DHN) was found with these dyads
as triplet photosensitizer, compared to the Ir(III) complex
sensitizer Ir(ppy)2(Phen)[PF6] (Ir-1, see ESIw).
Concerning this challenge, the efficient ISC capability of
fullerene (e.g. C60, triplet excited state quantum yield >95%)
is particularly interesting.8 Although C60 has been studied for
more than a quarter of a century, the triplet manifolds of
C60 shows very weak absorption at ca. 700 nm,8,10–12 thus
we propose that visible light-harvesting chromophores with an
appropriate S1 state energy level can be attached to C60 and
the singlet energy transfer (ET) from the light-harvesting
antenna to C60 will occur. Then the singlet excited state, and
in turn the T1 excited state of C60 will be populated via ISC.8
Following this line, we designed dyads PBI-C60 and NPBI-C60
(Fig. 1).13 The visible-light harvesting antenna, perylenebisimide,
was prepared with 2-ethylhexylamine. Prato reaction was used
State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology, Dalian 116024,
P. R. China. E-mail: zhaojzh@dlut.edu.cn;
Web: http://finechem.dlut.edu.cn/photochem;
Fax: +86 (0) 411 8498 6236; Tel: +86 (0) 411 8498 6236
w Electronic supplementary information (ESI) available: Synthesis,
structural characterization and DFT calculation details. See DOI:
10.1039/c2cc30345k
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3751–3753 3751

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Fig. 2 UV-vis absorption spectra of (a) PBI-C60, PBI-Ph and C60
(b) NPBI-C60, NPBI-Ph and C60. In toluene, c = 1.0 Â 10À5 mol LÀ1
20 1C.
,
Fig. 4 (a) Nanosecond time-resolved transient difference absorption
,
spectra of PBI-C60 upon pulsed laser excitation (lex = 532 nm).
Triplet excited state life (tT) is 105.9 ms (see ESIw). In deaerated toluene.
c = 1.0 Â 10À5 mol LÀ1; 20 1C. (b) Spin density of PBI-C60. Calculated
at B3LYP/6-31G(d) level with Gaussian 09W.
to link the PBI unit to C60 and the dihydro-fullerenes were
obtained (Fig. 1 and see ESIw for details).14
The UV-vis absorption of dyads was studied (Fig. 2). C60
shows strong absorption at 335 nm (e = 64 400 MÀ1 cmÀ1),
whereas the absorption in the visible range is very weak.
For PBI-Ph, the absorption maximum is at 535 nm
(e = 42 000 MÀ1 cmÀ1). The absorption of PBI-C60 is super-
imposable on the sum of the absorption spectra of C60 and
PBI. Thus the electronic interaction of C60 and PBI moieties in
PBI-C60 at the ground state is weak. Notably the absorption
of PBI-C60 in the visible range is greatly enhanced vs. C60
(Fig. 2a). Previously a dyad similar to PBI-C60 was reported
(the main difference is the alkyl chains) but only electrochemical, no
photophysical properties were studied in detail.9a NPBI-C60 shows
This result is useful for design of effective visible-light harvesting
C60 dyads.
The excited state of PBI-C60 was studied with nanosecond
time-resolved transient difference absorption spectroscopy
(Fig. 4a). Upon pulsed laser excitation, bleaching at 540 nm
was found. Concurrently, transient absorption at 400–650 nm
was observed, which is characteristic for the PBI moiety,13,15
indicating that the T1 state of the dyad is localized exclusively
on the PBI moiety, not on the C60 unit. The population of the
PBI-localized T1 state is supported by the lack of characteristic
transient absorption of the triplet state of C60 at 720 nm. The
T1 excited state energy level of PBI (1.2 eV) is much lower than
that of C60 (1.50 eV).13 The lifetime of the T1 state of PBI-C60
is 105.9 ms, much longer than that of a recently reported
C60–chromophore dyad (o40 ms).9b The lifetime is also much
longer than the intrinsic T1 state lifetime of C60 (40 ms).8a
With the fluorescence data, we conclude that singlet ET
from PBI to C60 is efficient. Since the triplet excited state is
localized on the PBI moiety, instead of C60, thus we propose
that the backward triplet ET from C60 to the PBI moiety
occurs. Overall, a ‘‘ping-pong’’ ET process (singlet energy
transfer from PBI to C60 and then the backward triplet excited
state energy transfer from C60 to PBI) occurs for the dyads
upon photoexcitation.16 It should be pointed out that the intra-
molecular charge transfer from the antenna to C60 cannot be
completely excluded, especially for NPBI-C60. The triplet excited
state can also be populated by the charge recombination.12,13
The population of the PBI-localized triplet excited state is
confirmed by the spin density surface of the dyad PBI-C60
(Fig. 4b), which is exclusively localized on PBI, which is in full
agreement with the transient absorption spectroscopy (Fig. 4a).
Similar transient absorption and spin density profiles were
found for NPBI-C60 (see ESIw).
red-shifted absorption (labs = 634 nm, e = 25 400 MÀ1 cmÀ1
)
(Fig. 2b).
To confirm the ET between the antenna (PBI) and the
energy acceptor (C60) in dyads, photoluminescence was studied
(Fig. 3). PBI-Ph emits strongly at 575 nm (FF = 36.0%). For
PBI-C60, however, the emission of the PBI-Ph moiety was
quenched (FF = 0.7%). Interestingly, weak emission at 707 nm
was observed for PBI-C60, which is attributed to the C60 unit in
PBI-C60. Excitation of C60 alone at 490 nm did not produce this
emission band, which confirms the intramolecular ET between
PBI and C60 in PBI-C60. The ET efficiency is estimated as 98%
based on the FF of PBI-Ph and PBI-C60.13 The efficient ET of
PBI-C60 is similar to a dyad based on C60 and PBI with a
flexible linker.13
The intramolecular ET of the red-absorbing NPBI-C60 was
also studied (Fig. 3b). Much lower ET efficiency was observed
(ca. 62%) compared to PBI-C60, probably due to the inappropriate
S1 energy level of NPBI-Ph, which is close to the S1 energy
level of C60, thus the small ET driving force leads to inefficient ET.
To the best of our knowledge, light-harvesting C60-dyads
have been rarely used for triplet photosensitizing.9b,c Recently
fluorene–C60 dyads were used for 1O2 sensitizing, but the
absorption is slightly blue-shifted vs. PBI-C60 and NPBI-C60.9b
1
Previously O2 photosensitizing ability of dihydrofullerenes was
studied, but those derivatives show absorption in the UV range.9b
Herein the visible light-harvesting C60 dyads were used as an
1O2 sensitizer for photooxidation of 1,5-dihydroxyl naphthalene
(DHN) (Fig. 5).2 Previously Ir(III) complexes were used for the
same purpose, but Ir(III) complexes show weak absorption of
visible light, which is a disadvantage.
Fig. 3 Emission of (a) PBI-C60, PBI-Ph and C60, lex = 490 nm;
(b) NPBI-C60, NPBI-Ph and C60, lex = 633 nm. c = 1.0 Â 10À5 mol LÀ1
in toluene, 20 1C.
c
3752 Chem. Commun., 2012, 48, 3751–3753
This journal is The Royal Society of Chemistry 2012

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With nanosecond time-resolved transient absorption and spin
density analyses, we confirmed that the triplet excited state of the
dyads is exclusively localized on the PBI unit. Thus ‘‘ping-pong’’
ET from PBI to C60 and in turn backward from C60 to
PBI produces the PBI localized T1 state. The dyads were used
as a singlet oxygen (1O2) photosensitizer for photooxidation of
1,5-dihydroxy-naphthalene. The photooxidation with C60 dyads
is more efficient than with the conventional Ir(III) complex
photosensitizer. Our result is useful for design of universal visible
light-harvesting organic triplet photosensitizers and for the
applications of these dyads in photocatalysis, photooxidation,
photodynamic therapy (PDT), etc.
Fig. 5 Absorption spectral change for the photooxidation of DHN
using (a) PBI-C60. (b) Plots of ln(A/A0) vs. irradiation time (t) for the
photooxidation of DHN using different sensitizers. c [sensitizers] =
We thank the NSFC (20972024 and 21073028), Royal
Society (China–UK Cost-Share program, 21011130154) and
Ministry of Education (NCET-08-0077) for financial support.
2.0
Â
10À5 mol LÀ1
,
c
[DHN]
=
2.0
Â
10À4 mol LÀ1
.
In CH2Cl2–MeOH (9/1, v/v), 20 1C.
3
Upon irradiation of PBI-C60 in aerated solution, O2 was
Notes and references
1
sensitized to O2 by the dyad, DHN was then oxidized by the
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301 nm decreased and the absorption of the oxidation product
juglone at 427 nm developed (Fig. 5a).2
The photooxidation reaction rate constants (kobs) with
PBI-C60 (3.0 Â 10À2 minÀ1) is much larger than that with
Ir-1 (4.5 Â 10À3 minÀ1), and C60 (9.7 Â 10À3 minÀ1) (Fig. 5b
and Table 1). We attribute the enhanced photosensitizing
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were used for sensitizing reactive oxygen species, but the
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a better photosensitizing property than the typical organic
triplet sensitizer methylene blue (MB) and its efficiency is
comparable to that of TPP (Fig. 5b). No photobleaching
was observed for the sensitizers (see ESIw for details).
¨
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¨
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efficient intramolecular energy transfer (ET) were prepared.
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Table 1 Photophysical parameters of the compounds and the model
triplet photosensitizer Ir-1, 5,10,15,20-tetraphenylporphyrin (TPP)
and methylene blue (MB)a
c
e
labs
eb
lem FF (%) td/ms
kobs
PBI-C60
PBI-Ph
NPBI-C60 286, 634 9.39, 2.54 735 0.5
327, 537 5.98, 4.37 572 0.7
535 4.20 575 36.0
105.9 30.0
f
f
—
12.6
—
—
22
—
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´
f
f
J. Bourdelande and J. Font, Phys. Chem. Chem. Phys., 2001, 3, 1638.
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2009, 15, 7382.
13 C. Hofmann, S. Lindner, M. Ruppert, A. Hirsch, S. Haque,
NPBI-Ph
C60
Ir-1
629
335
371
419
533
2.51
6.44
0.70
32.1
2.0
728 1.3
706 o0.1%
40.0
0.77
82.5
9.7
4.5
43.2
23.1
584 0.5
f
TPP
MB
650
654
—
—
f
f
—
a
b
In toluene. c = 1.0 Â 10À5 mol dmÀ3
.
Molar extinction coefficient at
c
. Fluorescence quantum yields
M. Thelakkat and J. Kohler, J. Phys. Chem. B, 2010, 114, 9148.
¨
the absorption maxima. e: 104/MÀ1 cmÀ1
in toluene, with 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-
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d
4H-pyran (DCM) as the standard (FF = 0.1 in CH2Cl2). Triplet
lifetimes, measured by transient absorption. Pseudo-first-order rate
constant. ln(Ct/C0) = Àkobst. In 10À3 minÀ1
e
f
.
Not determined.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3751–3753 3753