Visible light-harvesting naphthalenediimide (NDI)-C60 dyads as heavy-Atom-free organic triplet photosensitizers for triplet-triplet annihilation based upconversion

Article abstract of DOI:10.1016/j.dyepig.2012.09.008

Two naphthalenediimide (NDI)-C₆₀ dyads that show strong absorption of visible light were prepared as heavy atom-free organic triplet photosensitizers. Alkylamino substituted NDI was used as the light-harvesting antennas and the C₆₀ unit was used as spin converter for the intersystem crossing (ISC) from singlet excited state to triplet excited state. Upon visible light photoexcitation, triplet excited states of the dyads are populated (lifetime is up to 90.1 μs). Nanosecond time-resolved transient difference absorption spectroscopy indicated that the triplet excited state of the dyads is localized on either the NDI or the C₆₀ unit. As a proof of concept, the dyads are used as heavy-Atom-free organic triplet photosensitizers for triplet-triplet annihilation based upconversion. C ₆₀-organic chromophore dyads can be used as a general molecular structural motif of heavy-Atom-free organic triplet photosensitizers, for which the photophysical properties can be readily changed by using different light-harvesting antennas, and the ISC property of these dyads is predictable.

Full text of DOI:10.1016/j.dyepig.2012.09.008

Dyes and Pigments 96 (2013) 449e458
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Visible light-harvesting naphthalenediimide (NDI)-C60 dyads as heavy-atom-free
organic triplet photosensitizers for tripletetriplet annihilation based
upconversion
*
Song Guo, Jifu Sun, Lihua Ma, Wenqin You, Pei Yang, Jianzhang Zhao
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling-Gong Road, Dalian 116024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two naphthalenediimide (NDI)-C60 dyads that show strong absorption of visible light were prepared as
heavy atom-free organic triplet photosensitizers. Alkylamino substituted NDI was used as the light-
harvesting antennas and the C60 unit was used as spin converter for the intersystem crossing (ISC)
from singlet excited state to triplet excited state. Upon visible light photoexcitation, triplet excited states
Received 6 August 2012
Received in revised form
13 September 2012
Accepted 14 September 2012
Available online 24 September 2012
of the dyads are populated (lifetime is up to 90.1 ms). Nanosecond time-resolved transient difference
absorption spectroscopy indicated that the triplet excited state of the dyads is localized on either the NDI
or the C60 unit. As a proof of concept, the dyads are used as heavy-atom-free organic triplet photosen-
sitizers for tripletetriplet annihilation based upconversion. C60-organic chromophore dyads can be used
as a general molecular structural motif of heavy-atom-free organic triplet photosensitizers, for which the
photophysical properties can be readily changed by using different light-harvesting antennas, and the ISC
property of these dyads is predictable.
Keywords:
Fullerene
Naphthalenediimide
Photosensitizer
Triplet state
Triplet photosensitizer
Upconversion
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
substituents, and at the same time, to ensure efficient ISC [5a,7,8].
Another kind of triplet photosensitizer is the organic compounds
Triplet photosensitizers are widely used in photocatalysis [1],
photoirradiation initialized reaction [2], photodynamic therapy [3],
and more recently tripletetriplet annihilation (TTA) upconversion
[4]. Triplet photosensitizers can be divided into a few categories.
The first is transition metal complex photosensitizer. The
intersystem crossing (ISC) of these compounds is usually efficient,
due to the heavy atom effect of the Pt(II), Pd(II), Ir(III) or Ru(II)
atoms in the complexes [5]. However, these complexes are not cost-
efficient and their synthesis and purification is usually difficult.
Furthermore, the molecular structure of these complexes cannot be
readily changed to optimize the photophysical properties [5,6]. The
second kind of triplet photosensitizers are the organic compounds
which contain heavy atoms such as bromine or iodine [3d,4f,7]. For
example, iodo- or bromo boron-dipyrromethene (bodipy) and
bromo-naphthalenediimide (NDI) have been used as triplet
photosensitizers for sensitizing of singlet oxygen (1O2) or TTA
upconversion [3d,7]. However, it is not always feasible for the
organic chromophores to be derivatized with iodo- or bromo
that contain no heavy atoms, for example, 2,3-butanedione [9],
porphyrins, perylenebisimide derivatives, etc [5a,10]. However, the
disadvantage of these organic triplet photosensitizers is their
elusive photophysical property, e.g., subtle alternation of the
molecular structures may eliminate the ISC property completely,
and it is impossible to predict whether or not a derivative of these
compounds will undergo ISC upon photoexcitation.
As a result, the challenge facing the photochemical community is
how to design a heavy-atom-free organic triplet photosensitizer
with predictable ISC. Furthermore, the molecular structures of these
heavy atom-free triplet photosensitizers should be readily deriva-
tizable, and at the same time, with the ISC be persistent upon
derivatization. Concerning this aspect, we noticed the ISC property
of fullerene C60 is inparticular interesting [11]. It was known that C60
undergoes efficient ISC upon photoexcitation, and the fluorescence
quantumyield of C60 is very low (
is not an ideal triplet photosensitizer due to its very weak absorption
in visible range (
< 100 Mꢀ1 cmꢀ1 in the range beyond 400 nm)
FF < 0.1%) [11a]. However, C60 itself
3
[11e13]. The maximal absorption of C60 is located at 337 nm ( is ca.
3
52,350 Mꢀ1 cmꢀ1). Light-harvesting organic chromophores with
appropriate singlet state energy levels can be attached to C60, so that
singlet energy transfer from antenna to C60 moiety can occur [13].
* Corresponding author. Tel./fax: þ86 411 3960 8007.
E-mail address: zhaojzh@dlut.edu.cn (J. Zhao).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.09.008

450
S. Guo et al. / Dyes and Pigments 96 (2013) 449e458
Thereafter the C60 moiety will undergo ISC and the triplet excited
state will be populated. Therefore, we envisage that C60 unit can be
used as spin converter for ISC and these visible light-harvesting
C60-organic chromophore dyads can be used as heavy-atom-free
organic triplet photosensitizers in photocatalysis, photodynamic
therapy (PDT) and TTA upconversion.
excited state manifold was rarely been studied [7c,13d,15]. Its
photophysical properties, such as the absorption wavelength, can
be readily changed by modification [14c,16]. We prepared two
different NDI moieties with mono and bis-amino substituents, thus
the two NDI antenna of the dyads (Scheme 1) give different
absorption wavelength [7c]. Both antennas give fluorescence in the
range 500e700 nm, indicated the S1 state levels, thus the intra-
molecular energy transfer from the NDI moieties to C60 moiety is
possible [11b,13c,17]. The photophysical properties of the dyads
were studied with steady state and time-resolved spectroscopy. It
was proved that the triplet state of the dyads was populated upon
photoexcitation. As a proof of concept, the dyads were used as
organic triplet photosensitizers for TTA upconversions [4].
Although the C60-organic chromophores have been studied for
the basic photophysical properties [13], very few application of
these dyads as triplet photosensitizers have been reported [11]. On
the other hand, heavy atom-containing triplet photosensitizers
have been widely used for TTA upconversion [4], but heavy atom-
free organic triplet photosensitizers that show strong absorption
of visible light were not reported for TTA upconversion [4ae4c].
Although 2,3-butadione was used as triplet photosensitizer for TTA
upconversion, its absorption is in UV-blue range and the derivati-
zation cannot be readily accomplished without compromising the
ISC ability of the chromophore [9]. Recently we reported a visible
light-harvesting C60-chromophore dyads as triplet photosensitizers
for TTA upconversion, with the Bodipy as light harvesting antenna,
but the diversity of the antenna structure needs to be extended
[11de11f]. The information obtained with different C60-chromo-
phore dyads will be useful for future design of these novel heavy
atom-free triplet photosensitizers.
2. Experimental section
2.1. General information
NMR spectra were taken on a 400 MHz Varian Unity Inova
spectrophotometer. Mass spectra were recorded with a Q-TOF
Micro MS spectrometer. UVevis spectra were taken on an HP8453
UVevisible spectrophotometer. Fluorescence spectra were recor-
ded on a Shimadzu RF5301 spectrofluorometer and a Sanco 970
CRT spectrofluorometer. Luminescence quantum yields were
measured with 4,4-Difluoro-1,3,5,7-tetramethyl-8-phenyl-4-bora-
3a,4a-diaza-s-indacene (bodipy) as the standard (FF ¼ 72% in THF).
Luminescence lifetimes were measured on OB920 luminescence
In order to address the above challenges, herein we prepared
new C60-naphthalenediimide (NDI) dyads and these dyads were
used as organic triplet photosensitizers for TTA upconversion. NDI
has been widely used as fluorophore [14], but its role in triplet
R
R
O
O
O
O
O
O
O
O
O
O
O
O
N
O
O
O
N
O
H
N
Br
Br
g
a
b
c
R'
Br
Br
Br
O
O
N
R
O
N
R
O
4
1
2
3
R
R
N
R
N
O
O
N
O
O
O
O
O
O
O
O
O
N
h
H
N
d
CHO
e
R'
4
R'
N
R'
N
H
R
H
N
H
N
R
O
N
N
R
R
L-1
C-1
5
R
N
O
O
O
O
H
N
R
N
O
O
O
O
O
O
f
R
O
O
R
H
N
H
N
O
O
O
R
N
R
O
O
N
O
R'
H
OH
N
R
O
C-2
L-2
I
I
N
N
N
N
F
B
B
Cl
F
F
F
1CBPEA
BDP-2I
Scheme 1. Preparation of the C60-NDI dyads C-1 and C-2. The triplet acceptor 1CBPEA and quantumyield standard BDP-2I were also presented. (a) DBI, oleum (20% SO3), 40 ꢁC, 5 h; yield:
83.2%; (b) 2-ethylhexylamine, 120 ꢁC, under N2, 2 h; yield: 33.4%; (c) DGA, 2-methoxyethanol, 120 ꢁC, 8 h, 71.0%; (d) 2-ethylhexylamine, 2-methoxyethanol, 120 ꢁC, 7 h, 52.1%; (e) ethyl
malonyl chloride, pyridine, CH2Cl2, 8 h, 92.2%; (f) C60, I2, DBU, toluene, 8 h, 62.1%; (g) 4-formylphenylboronic acid, Pd(PPh3)4, NaCO3, 94%; (h) C60, sarcosine, toluene, reflux, 20 h, 58.8%.

S. Guo et al. / Dyes and Pigments 96 (2013) 449e458
451
lifetime spectrometer (Edinburgh, UK). Compound 2 was prepared
according to literature method [18].
methoxyethanol under reduced pressure, the residue was purified
with column chromatography (silica gel, dichloromethane/
methanol ¼ 100:1, v/v) to give 358.0 mg of a red solid. Yield: 71.0%.
M.p. 112.2e114.1 ꢁC. 1H NMR (400 MHz, CDCl3/CD3OD)
d 10.35 (s,
2.2. TTA upconversion
1H), 8.78 (d, 1H, J ¼ 3.2 Hz), 8.34 (d,1H, J ¼ 3.2 Hz), 4.03e4.16 (m,
4H), 3.90 (t, 2H, J ¼ 4.8 Hz), 3.80 (d, 4H, J ¼ 4.8 Hz), 3.70 (t, 2H,
J ¼ 4.8 Hz), 1.87e1.93 (m, 2H), 1.25e1.38 (m, 16H), 0.93 (t, 6H,
J ¼ 7.2 Hz), 0.88 (t, 6H, J ¼ 7.2 Hz). HR-MALDI-MS: Calc
(C34H46N3O6Br þ Naþ) m/z ¼ 694.2468, found, m/z ¼ 694.2462.
Diode pumped solid state laser (continuous wave) was used for
the upconversion. The samples were purged with N2 or Ar for
15 min before measurement. The upconversion quantum yields
were determined with 2,6-diiodo-4,4-difluoro-1,3,5,7-tetramethyl-
8-phenyl-4-bora-3a,4a-diaza-s-indacene as the quantum yield
standards (FF ¼ 2.70% in CH3CN) [7b]. The upconversion quantum
yields were calculated with modified equation (Eq. (1)), where
Funk, Funk, and Iunk represent the quantum yield, absorbance
correction factor (F ¼ 1e10ꢀO.D.), integrated photoluminescence
intensity of the samples. hunk stands for the refractive index of
solvents [4a]. The photography of the upconversion was taken with
Samsung NV 5 digital camera.
2.6. N,N0-Di-(2-ethylhexan-1-amine)-2-(diethyleneglycolamino)-
6-(2-ethylhexan-1-amine)naphthalene-1,4,5,8-tetracarboxy
dianhydride (5)
The mixture of compound 4 (100.0 mg, 0.15 mmol), 2-(2-
aminoethoxy)ethanol (125.0 mg, 1.2 mmol), and methoxyethanol
(20 mL) was stirred at 120 ꢁC for 8 h. After removal of methox-
yethanol in vacuum, the residue was purified with column chro-
matography (silica gel, dichloromethane/methanol ¼ 100:1, v/v) to
give 56.3 mg of a blue solid. Yield: 52.1%. M.p. 189.7e191.1 ꢁC. 1H
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Fstd
Funk
Iunk
Istd
hunk
hstd
Funk ¼ 2Fstd
(1)
NMR (400 MHz, CDCl3)
d 9.63 (s, 1H), 9.44 (s, 1H), 8.20 (s, 1H), 8.12
The density functional theory (DFT) calculations were used for
optimization of the singlet states and triplet states of the antennas.
All the calculations were performed with Gaussian 09W [19].
(s, 1H), 4.05e4.16 (m, 4H), 3.86 (t, 2H, J ¼ 24.2 Hz), 3.80 (t, 2H,
J ¼ 24.2 Hz), 3.68e3.73 (m, 4H), 3.42e3.39 (m, 2H), 2.87 (s, 1H),
1.88e1.92 (m, 2H),1.74e1.80 (m, 1H),1.25e1.55 (m, 24H), 0.88e1.00
(m, 18H). HR-MALDI-MS: Calc (C40H61N4O5 þ Hþ) m/z ¼ 677.4672,
found, m/z ¼ 677.4642.
2.3. Delayed fluorescence of the TTA upconversion
The delayed fluorescence of the upconversion was measured
with nanosecond pulsed laser OpoletteÔ 355II UV nanosecond
pulsed laser, typical pulse length: 7 ns. Pulse repetition: 20 Hz. Peak
optical parametric oscillator (OPO) energy: 4 mJ. Wavelength is
tunable in the range of 210e355 nm and 410e2200 nm (OPOTEK,
USA). The laser is synchronized to FLS 920 spectrofluorometer
(Edinburgh, UK). The pulsed laser is sufficient to sensitize the TTA
upconversion. The decay kinetics of the upconverted fluorescence
(delayed fluorescence) was monitored with FLS920 spectrofluo-
rometer (synchronized to the OPO nanosecond pulsed laser). The
prompt fluorescence lifetime of the triplet acceptor 1-chloro-bis-
phenylethynylanthracene (1CBPEA) was measured with EPL pico-
second pulsed laser (445 nm) which is synchronized to the FLS 920
spectrofluorometer.
2.7. L-1 [20]
A mixture of aqueous 0.24 M Na2CO3 (1 mL) and dioxane(3 mL)
was degassed by purging with Ar, and into it the 4-
formylphenylboronic acid (36.0 mg, 0.24 mmol), 4 (83.1 mg,
0.12 mmol) and 2 mol% of Pd(PPh3)4 (1.3 mg) were added. The
mixture was heated in darkness under Ar atmosphere for 12 h. The
reaction was then quenched with the addition of 10% aqueous HCl.
The organic layer was extracted with dichloromethane, washed
with water and brine before being dried over MgSO4. The
solvent was removed under reduced pressure. The crude product
was purified by column chromatography (silica gel, DCM/
hexane ¼ 2/1, v/v) to afford 77.8 mg of red solid. Yield: 93.1%. M.p.
105.4e106.3 ꢁC. 1H NMR (400 MHz, CDCl3)
d 10.45 (s, 1H), 10.11
(s, 1H), 8.45 (s, 1H), 8.43 (s, 1H), 7.99 (s, 1H), 7.97 (s, 1H), 7.53 (s,
1H), 7.51 (s, 1H), 4.07e4.18 (m, 2H), 3.98e4.00 (m, 2H), 3.89e3.91
(m, 2H), 3.81e3.86 (m, 4H), 3.70 (t, 2H, J ¼ 24.1 Hz), 2,80 (s,
1H), 1.89e1.95 (m, 1H), 1.81e1.85 (m, 1H), 1.25e1.37 (m, 16H),
0.84e0.94 (m, 12H). HR-MALDI-MS: Calc (C41H52N3O7) m/
z ¼ 698.3085, found, m/z ¼ 698.3839.
2.4. N,N0-Di-(2-ethylhexan-1-amine)-2,6-dibromonaphthalene-
1,4,5,8-tetracarboxy-dianhydride (3) [18]
A mixture of compound 2 (1.06 g, 2.5 mmol), 2-ethylhexylamine
(0.97 g, 7.5 mmol), and acetic acid (25 mL) was stirred at 120 ꢁC for
2 h under N2 atmosphere. After cooled to room temperature, the
mixture was poured into 100 mL cold water. The solid was collected
with filtration, and was washed with water. After drying under
vacuum, the resulting reddish solid was purified by column chro-
2.8. L-2 [21]
matography (silica gel, CH2Cl2/petroleum ether
¼
1:1, v/v).
Compound 5 (50.4 mg, 0.07 mmol) was dissolved in 20 ml
Compound 3 was obtained as yellow crystalline solid (530 mg).
CH2Cl2 and pyridine (7
atmosphere. The mixture was cooled on an ice bath and methyl
malonyl chloride (9 L, 0.09 mmol) in 10 ml CH2Cl2 was added
ml, 0.85 mmol) was added under Ar
Yield 33.4%. M.p. 245.2e248.6 ꢁC. 1H NMR (400 MHz, CDCl3/CD3OD)
d
8.99 (s, 2H), 4.11e4.20 (m, 4H), 1.93e1.96 (m, 2H), 1.25e1.43 (m,
m
16H), 0.94 (t, 6H, J ¼ 7.2 Hz), 0.89 (t, 6H, J ¼ 7.2 Hz). HR-MALDI-MS:
dropwise. The mixture was stirred for 12 h at room temperature.
Compound 6 was purified by column chromatography (silica gel,
CH2Cl2/ethyl acetate ¼ 20:1, v/v) to afford 52.7 mg of blue solid
Calc (C30H36N2O4Br2) m/z ¼ 646.1042, found, m/z ¼ 646.1032.
2.5. N,N0-Di-(2-ethylhexan-1-amine)-2-(diethyleneglycolamino)-
(yield: 92.2%). M.p. 94.8e96.2 ꢁC. 1H NMR (400 MHz, CDCl3)
d 9.48
6-bromonaphthalene-1,4,5,8-tetra-carboxydianhydride (4)
(s, 2H), 8.18 (s, 2H), 4.36 (s, 2H), 4.10e4.22 (m, 6H), 3.84 (t, 2H,
J ¼ 24.3 Hz), 3.78 (t, 2H, J ¼ 24.3 Hz), 3.71e3.73 (m, 2H), 3.41e3.44
(m, 4H), 1.90e1.97 (m, 2H), 1.75e1.80 (m, 1H), 1.25e1.57 (m, 30H),
0.87e0.99 (m, 18H). HR-MALDI-MS: Calc (C47H70N4O9Na) m/
z ¼ 857.5041, found, m/z ¼ 857.5121.
The mixture of compound 3 (500.0 mg, 0.75 mmol), 2-
ethylhexylamine (51.6 mg, 0.4 mmol), and methoxyethanol
(5 mL) was stirred at 120 ꢁC for
6 h. After removal of

452
S. Guo et al. / Dyes and Pigments 96 (2013) 449e458
2.9. C-1
either the NDI moiety or C60 unit, dependent on the relative energy
levels of the T1 excited states of the two moieties [13a,13b,23b]. We
prepared two different NDI moieties as the light harvesting
antennas (compounds L-1 and L-2, Scheme 1), one is with mono
amino substituent (L-1), and another is with bisamino substitutions
(L-2). These light harvesting ligands give absorption at 526 nm (L-
1) and 612 nm (L-2), respectively. Recently we used Bodipy to
construct C60 dyads [11de11f], which contain rigid linker between
the antenna and C60 moiety. Herein flexible linker was used.
The synthesis is with 1,4,5,8-naphthalenetetracarboxylic acid
dianhydride as the starting material (Scheme 1) [14ae14d]. The
bromination was carried out with the dibromoisocyanuric acid.
With two bromo substituents at 2,6-position were introduced, then
the monoamino substitution was carried out. Suzuki coupling
reaction between the resulted 2-amino-6-bromo imide and the 4-
formylbenzene boronic acid leads to intermediate 7. Bingle reac-
tion was used to produce the dyads C-1 (Scheme 1). In order to
prepare the bisamino substituted NDI-C60 dyad, a slightly different
method was used. The 2-amino-6-bromo imide 4 was treated with
4-formylphenylboronic acid to give intermediate compound L-1.
Bingle reaction of compound 6 with C60 gives the dyad C-2. All the
compounds were obtained in moderate to satisfying yields.
Under N2 atmosphere, compound 7 (35.0 mg, 0.05 mmol) and
sarcosine (24.0 mg, 0.27 mmol) were added to the solution of C60
(36.0 mg, 0.05 mmol) in toluene (40 mL). The mixture was heated
to 113 ꢁC, and was refluxed for 20 h. The reaction mixture was
cooled to ambient temperature. The solvent was then removed
under reduced pressure. The residue was purified by column
chromatography (silica gel, dichloromethane/methanol ¼ 40:1, v/
v). Red solid was obtained. 42.5 mg, yield: 58.8%. M.p. 247.6e
249.8 ꢁC. 1H NMR (400 M Hz, CDCl3)
d 10.39 (s, 1H), 8.51 (s, 1H),
8.40 (s, 1H), 7.89 (s, 2H), 7.41 (s, 1H), 7.39 (s, 1H), 5.02e5.05 (m, 2H),
4.31e4.34 (m,1H), 4.06e4.15 (m, 2H), 3.78e3.96 (m, 8H), 3.67e3.69
(m, 2H), 2.94 (s, 3H), 2.75 (s, 1H), 1.90e1.93 (m, 1H), 1.78e1.83 (m,
1H), 1.26e1.40 (m, 16H), 0.85e0.93 (m, 12H). HR-MALDI-MS: Calc
(C103H56N4O6) m/z ¼ 1444.4200, found, m/z ¼ 1444.4177.
2.10. C-2
Iodine (10.2 mg，0.04 mmol) were added to a solution of C60
(26.4 mg，0.04 mmol) in toluene (30 mL). The malonate 6 solution
in toluene was added and the mixture was degassed with Ar for
30 min. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) (15
mL,
0.06 mmol) in toluene (20 mL) was added dropwise within 1 h and
the reaction mixture was stirred over night. C-2 was purified with
column chromatography (silica gel, DCM) to yield 35.6 mg of blue
3.2. Steady state UVevis absorption and fluorescence spectra
The UVevis absorption spectra of the dyads and the light har-
vesting antennas were studied (Fig. 1). C60 gives very weak
absorption in the visible range [11a,13a,13b]. The maximal
absorption of C60 is located at ca. 336 nm (Fig. 1a). The light-
harvesting antenna L-2 shows absorption at 612 nm
solid, 62.1%. M.p. 138.2e140.1 ꢁC. 1H NMR (400 MHz, CDCl3)
d 9.52
(s, 1H), 9.47 (s, 1H), 8.14 (s, 1H), 8.12 (s, 1H), 4.73 (t, 2H, J ¼ 24.2 Hz),
4.54e4.59 (m, 2H), 4.03e4.15 (m, 4H), 3.94e3.96 (m, 2H), 3.85e
3.86 (m, 2H), 3.69 (t, 2H, J ¼ 24.2 Hz), 3.40e3.43 (m, 2H), 1.89e
1.94 (m, 2H), 1.74e1.80 (m, 1H), 1.26e1.37 (m, 24H), 0.87e0.97
(m, 18H). HR-MALDI-MS: Calc (C107H68N4O9) m/z ¼ 1552.5055,
found, m/z ¼ 1552.4986.
(
3
¼ 20,500 Mꢀ1 cmꢀ1), whereas the monoamino-substituted light-
harvesting antenna L-1 gives absorption at blue-shifted range
(526 nm,
3
¼ 12,700 Mꢀ1 cmꢀ1).
Both the C60 dyads C-1 and C-2 give strong absorption, which
are similar to the light harvesting antennas. Furthermore, the
absorption profile of the dyads in the visible range is roughly the
sum of absorption of C60 and the antennas (Fig. 1). Thus, the elec-
tronic interaction between the C60 and the light harvesting
antennas in the dyad is weak at the ground state [13a,13b]. It should
be noted that the tunability of the absorption wavelength of dyads
C-1 and C-2 are more significant that the Bodipy based C60 dyads
used for TTA upconversion [11de11f].
3. Results and discussion
3.1. Design and synthesis of C60-NDI dyads
The design of the dyads is based on the rationales that the
energy level of S1 state of light-harvesting antenna (NDI moieties)
must be higher than the C60 moiety (1.72 eV) [13c,17a,17b,22]. Thus
the intramolecular energy transfer from NDI antenna to C60 moiety
is allowed. Furthermore, it was known that the absorption of NDI
can be readily changed by derivatization [14a,14d]. Therefore NDI
was selected as the light-harvesting antenna. It should be pointed
out that the triplet excited state of the dyads can be located on
The emission of the dyads and the light-harvesting antennas
were also studied (Fig. 2). It was known that the fluorescence
quantum yields of C60 is very low (FF < 0.1%), due to its efficient ISC
[11a,13b]. Experimentally, very weak fluorescence emission was
a
b
0.6
C-1
C-2
L-2
0.8
L-1
C
0.4
0.6
C
60
60
0.4
0.2
0.0
0.2
0.0
400
500
600
700
800
400
Wavelength / nm
300
500
600
700
800
Wavelength / nm
Fig. 1. UVevis absorption spectra of (a) C-1, L-1 and C60. (b) C-2, L-2 and C60. c ¼ 1.0 ꢂ 10ꢀ5 M in toluene. 20 ꢁC.

S. Guo et al. / Dyes and Pigments 96 (2013) 449e458
453
As a proof of this intramolecular energy transfer, enhanced fluo-
rescence emission of C60 moiety was observed for C-1 at 708 nm, in
comparison to C60 itself (inset of Fig. 2a) [13a,13b]. Similar intra-
molecular energy transfer was found for Bodipy derived C60 dyads
[11de11f]. The intramolecular energy transfer is probably not Föster
resonance energy transfer (RET) mechanism, because the spectral
overlap between the emission of the antenna and the absorption of
the energy acceptor C60 is neglectable. We propose that the energy
transfer process can be considered as internal conversion [5a].
Photo-induced electron transfer (charge separation) may be also
responsible for the quenching of the antenna emission [13a,13b].
a
800
600
400
200
0
1000
800
600
400
200
0
708 nm
C-1
I
C60
X 20
L-1
600
λ / nm
700
800
3.3. Nanosecond time-resolved transient difference absorption
spectra
C-1
C60
Based on the relative energy levels of the singlet excited states of
antenna and C60 unit, we envisaged that photo-induced intramolecular
energy transfer from the antenna to C60 moiety will firstly produce the
singlet excited states of C60 moiety [13a,13b]. In turn the triplet excited
states of C60 will be populated, due to the intrinsic ISC property of C60
[11a,13ae13c]. In order to prove the population of triplet excited states
of C-1 and C-2, the nanosecond time-resolved transient difference
absorption spectra of the dyads were studied (Fig. 3).
The time-resolved transient absorption of C-1 was studied
(Fig. 3a). Upon pulsed laser excitation, no bleaching was observed at
the ground state absorption of antenna at 526 nm. Instead, the
characteristic transient absorption band of the triplet excited state
of C60 at 710 nm was observed (Fig. 3a) [13a]. Thus we propose the
T1 excited state of C-1 is localized on C60, and the light-harvesting
antenna is not involved in the T1 excited state of C-1. As a proof,
550
600
650
700
750
800
Wavelength / nm
b
500
400
300
800
600
400
200
0
C-2
200
100
0
C60
600
700
800
L-2
Wavelength / nm
C60
the lifetime of the dyad was determined as 37.2
the intrinsic triplet state lifetime of C60 (40.6
m
s, which is close to
C-2
m
s Table 1) [11a].
Upon pulsed laser excitation of C-2 at 532 nm, bleaching of the
absorption of ground state of antenna L-2 at 612 nm was observed,
as well as the bleaching of C60 at 350 nm (Fig. 3c). Furthermore, the
characteristic absorption of the triplet excited state of the C60
moiety at 707 nm was also observed [13a]. Recently we prepared
Bodipy-C60 dyads for TTA upconversion, but the triplet excited
states were found to be localized on C60 units [11de11f]. The
location of the triplet excited state in dyads may have effect on the
properties such as the tripletetriplet energy transfer efficiency.
Based on these results, we propose that for C-2, both the triplet
excited states of C60 moiety and the NDI moiety were populated, in
other words, the triplet excited states of L-2 and C60 moieties are in
equilibrium. The calculated T1 state energy level of the antenna L-2
is at 1.57 eV (See Supporting Information, Table S1), which is very
close to the T1 state energy level of the C60 moiety (1.51 eV). The
600
650
700
750
800
850
Wavelength / nm
Fig. 2. The fluorescence spectra of the C60 dyads. (a) The emission spectra of C-1, L-1
and C60. Inset is the magnified emission spectra of C-1 and C60
lex ¼ 526 nm. (b) The
.
emission spectra of C-2, L-2 and C60. Inset is the magnified emission spectra of C-2 and
C60
.
lex ¼ 591 nm. c ¼ 1.0 ꢂ 10ꢀ5 M in toluene, 20 ꢁC.
observed for C60 (Fig. 2). In comparison, the light harvesting
antenna L-1 and L-2 give intense fluorescence emission at 561 nm
and 634 nm, with fluorescence quantum yields (FF) of 85.7% and
44.7%, respectively (Table 1).
Interestingly, the C60 dyads C-1 and C-2 give much weaker
fluorescence emission than the light-harvesting antennas (Fig. 2).
Since the emission of the antenna is in blue-shifted range compared
to that of the unsubstituted C60 (ca. 720 nm, can be approximated as
the S1 state energy level of C60) [11a,13b], we propose the quenched
emission of antenna in the dyads is due to the photo-induced
intramolecular energy transfer from the antenna to the C60 moiety.
triplet state lifetime of C-2 was determined as 90.1 ms, much longer
than the T1 state lifetime of C60 itself, indicates that the triplet
excited state of C-2 is drastically different from that of C-1. The
triplet state lifetime of C-2 is also much longer than that of the
Bodipy-C60 dyad reported recently [11de11f]. This result indicated
that the localization of the triplet excited state of dyad has signif-
icant effect on the photophysical properties of dyad. The lifetime of
Table 1
Photophysical parameters of the C60 dyads and the components.a
the transient of C-2 was reduced to 0.5 ms in aerated solution, which
a
b
confirms that the transients are mainly due to the triplet excited
state, not charge-separated state [13a,13b]. The photophysical
properties of the dyads and the light harvesting antenna are
summarized in Table 1.
labs
3
lem
FF (%)c
s
C60
L-1
L-2
C-1
C-2
336
526
612
526
612
6.19
1.27
2.05
1.43
2.08
e
e
46.0 m
sd
561
636
555/710
634
85.7%c
44.7%c
<0.1%
<0.1%
10.5 nse
10.4 nse
37.2
90.1
m
m
sd
sd
3.4. Visible light-harvesting C60 dyads as organic triplet
photosensitizers for tripletetriplet annihilation based upconversion
In toluene (1.0 ꢂ 10ꢀ5 M).
a
b
c
Molar extinction coefficient at the absorption maxima.
With Bodipy as the standard (FF ¼ 72% in THF).
3 .
: 104 Mꢀ1 cmꢀ1
d
e
TTA upconversion has attracted much attention, due to the
advantages of its low excitation power requirement, strong
Triplet state lifetimes, measured by transient absorptions.
Fluorescence lifetimes.

454
S. Guo et al. / Dyes and Pigments 96 (2013) 449e458
b
a
0.09
0.06
0.03
0.00
0.10
0 μs
0.05
0.00
14.9 μs
...
313.8 μs
0
100
t / μ
200
100
300
s
0.01
0.00
-0.01
400
500
600
700
800
0
50
t / μ
150
Wavelength / nm
s
d
c
0.08
0.04
0.00
0.10
0.05
0.00
-0.05
0 μs
24.7 μs
....
716.3 μs
0
0
250
t /μs
500
750
0.01
0.00
400
500
600
700
800
-0.01
150
300
450
Wavelength / nm
t / μs
Fig. 3. Nanosecond time-resolved transient difference absorption spectra of compound C-1 and C-2. Transient difference absorption spectra with different delay times: (a) C-1, (c)
C-2. Decay trace of the transient of (b) C-1 at 700 nm and (d) C-2 at 610 nm. Excited with 532 nm nanosecond pulsed laser. c ¼ 2.0 ꢂ 10ꢀ5 M in deaerated toluene, 20 ꢁC.
absorption of the excitation light, and the high upconversion
quantum yields [4ae4c,4e,4g,24]. Other upconversion methods,
such as those with rare earth metal materials [25], show relatively
weak absorption of excitation light. For the two-photon absorption
dyes [26], however, coherent excitation with much higher power
density is required. TTA upconversion was reported by Parker et al.
half a century ago [27]. But those organic triplet sensitizers show
weak absorption in visible range and non-efficient ISC, thus the
upconversion capability is limited. Currently the triplet photosen-
sitizers for TTA upconversion are limited to the transition metal
complex photosensitizers, such as the Pt(II), Ir(III) and Ru(II)
complexes [4ae4c]. However, these metal complex photosensi-
tizers are not cost-efficient. Therefore, similar to the development
of the dye-sensitized solar cells (DSCs), to replace the transition
metal complex photosensitizers with organic photosensitizers will
be important for the development of TTA upconversion [4b].
From a photochemistry point of view, it is very difficult to
develop heavy atom-free (such as Pt, Ir, Ru, Or Br, I, etc.) organic
triplet photosensitizers because without the heavy-atom effect, the
ISC property of an organic chromophore is unpredictable. Deriva-
tization of a known heavy atom-free organic triplet photosensitizer
is not a reliable method to prepare new triplet photosensitizers. For
example, subtle derivatization of a triplet photosensitizer may
eliminate the ISC property, this is true for anthracene. Anthracene
shows ISC efficiency (FISC) of 72%, but for 9,10-diphenylanthracene,
the ISC efficiency is decreased significantly (FISC < 5%) [5a,28].
Thus, it is highly desired to develop a general molecular structural
motif for heavy-atom-free organic triplet photosensitizers that
show the predictable ISC property, and more importantly, the
molecular structure motif should be readily derivatizable. To the
best of our knowledge, no such general molecular structural motif
for heavy-atom-free organic triplet photosensitizers have been
reported [4ae4c,5a].
As discussed above, the C60-NDI dyads are ideal triplet photo-
sensitizers for TTA upconversion. As a proof of concept, the dyads C-
1 and C-2 were used as triplet photosensitizers for TTA upconver-
sion (Fig. 4). Upon 589 nm laser excitation (continuous wave), C-2
gives emission at 634 nm, which is due to the residual emission of
the antenna. C-1 did not show any emission under the same
experimental conditions, due to its blue-shifted absorption.
In the presence of triplet acceptor of 1-chloro-9,10-
bis(phenylethynyl)anthracene (1CBPEA, Scheme 1), green emis-
sion in the range of 470ꢀ600 nm was observed with C-2 as the
triplet photosensitizer (Fig. 4b). Excitation of the photosensitizer C-
2 or 1CBPEA alone did not produce this emission band. Thus, the
emission band in the 470e600 nm range is due to the TTA
upconversion. C-1 did not give TTA upconversion under the same
experimental conditions, due to the unmatched excitation and the
absorption wavelength (Fig. 4b). With 532 nm laser excitation, at
which C-1 shows stronger absorption, TTA upconversion was

S. Guo et al. / Dyes and Pigments 96 (2013) 449e458
455
a
500
400
300
200
100
0
500
b
400
300
C-2
600
C-2
200
C-1
100
C-1
C60
700
C60
0
500
800
500
600
700
800
Wavelength / nm
Wavelength / nm
Fig. 4. TTA upconversions with triplet photosensitizers C-1, C-2 and C60. (a) Fluorescence emission of the photosensitizers (dyads) alone. (b) Emission of dyads in the presence of
triplet acceptor 1CBPEA. Excited with 589 nm laser (5.2 mW, power density 70 mW cmꢀ2). c [dyads] ¼ 3.0 ꢂ 10ꢀ5 M. c [1CBPEA] ¼ 6.7 ꢂ 10ꢀ4 M. In deaerated toluene. 20 ꢁC.
Table 2
coefficient of the triplet photosensitizer) of C60 dyads are much
Triplet excited state lifetimes (sT), SterneVolmer quenching constant (KSV) and
bimolecular quenching constants (kq) of the C60-NDI dyads photosensitizers by
triplet acceptor (1CBPEA). In deaerated toluene solution 20 ꢁC.
higher than C60 alone. Previously a ferrocene-NDI-C60 triad was
reported [13d], but the application of the triplet excited state was
not studied [13d]. Furthermore, the NDI moiety in the reported
ferrrocene-NDI-C60 triad was without any amino substitution on
sT
/
m
sa
Ksv
kq
FUC (%)d
he
sDF/m
sf
b
c
C-1
C-2
C60
37.2
90.1
40.6
e
e
0.46g
0.33
62.6
47.4
0.5
16.5
32.3
43.9
the pecore of the chromophore, thus the absorption of the triad is
25.7
1.01
2.9
0.25
in UV range [13d]. It should be pointed out that compared to
a single-chromophore triplet photosensitizer, the potential draw-
back of the approach based on dyad is the loss of energy due to
multiple energy transfer process. We noticed that the upconversion
quantum yield of the present C60 dyads is lower than that of the
C60-Bodipy dyads [11d]. The exact reason for the decreased
upconversion quantum yields is unknown, although the TTET effi-
ciency difference can be excluded (Table 2).
0.81g
a
Triplet state lifetime. Determined with the nanosecond time-resolved transient
absorption spectroscopy.
b
Quenching constants. 103 Mꢀ1
.
c
d
e
f
Bimolecular quenching constants. kq ¼ Ksv/s .
T. In 109 Mꢀ1 sꢀ1
Upconversion quantum yield.
h
¼
3
ꢂ
FUC.
Lifetime of the delayed fluorescence (upconverted fluorescence).
g
Perylene as acceptor.
The excitation power-dependency of the upconverted emission
intensity was studied (see Supporting Information Figure S22 and
Figure S23). Nonlinear quadratic dependence was observed for
the upconversion with C-1 and C-2 as the triplet photosensitizers,
which is in agreement with the typical TTA upconversion [4d].
Inordertounambiguouslyprovethatthegreenemissionobserved
forthedyad/triplet acceptormixedsolutions (Fig. 4b)isduetotheTTA
upconversion, the lifetime of the green emission in Fig. 4b was
measured. With C-2, the lifetime of the emission band at 500 nmwas
observed for C-1 (see Supporting Information). The upconversion
quantum yields (FUC) with the C60 dyads C-2 and C-1 were deter-
mined as 0.33% and 0.46%, respectively (Table 2). C60 itself is also
effective for TTA upconversion. These FUC values are lower than
that with visible light-harvesting transition metal complex triplet
photosensitizers. However, the FUC values are comparable to that
with the conventional transition metal complex triplet photosen-
sitizers [4h]. Considering the strong absorptions of C-1 and C-2 in
the visible range, the TTA upconversion with the C60 dyads are still
determined as 32.3 ms (Fig. 5a). The lifetime of the prompt fluores-
cence of 1-CBEPA, measured in a different experiment, was deter-
minedas4.3ns(Fig. 5b). Theexceptionallylong-livedluminescenceof
the dyad/1-CBPEA mixed solution unambiguously confirms TTA
significant, the upconversion capability (
h
¼
3 ꢂ
FUC, where is the
3
upconversion quantum yield and FUC is the molar extinction
b
a
τDF = 32.3 μs
τPF = 4.3 ns
1000
100
10
1000
100
10
40
60
80
100
200
300
400
Time / μs
Time / ns
Fig. 5. (a) Delayed fluorescence observed in the TTA upconversion with C-2 as triplet photosensitizer and 1CBPEA as the triplet acceptor. Excited at 589 nm (nanosecond pulsed OPO
laser synchronized with spectrofluorometer) and the decay was monitored at 500 nm. Under this circumstance C-2 is selectively excited and the emission at 500 nm is due to the
upconverted emission of 1CBPEA. (b) The decay of the prompt fluorescence of acceptor 1-CBPEA determined in a different experiment. Excited with picosecond 475 nm laser, the
decay of the emission was monitored at 500 nm. c[Sensitizers] ¼ 3.0 ꢂ 10ꢀ5 M. c[1CBPEA] ¼ 6.7 ꢂ 10ꢀ4 M. In deaerated toluene, 20 ꢁC.

456
S. Guo et al. / Dyes and Pigments 96 (2013) 449e458
photoexcited and the singlet excited state was populated. Then the
intramolecular energy transfer from L-2 moiety to the C60 unit
occurs, with which the singletexcitedstate of C60 unit was produced.
In turn the triplet excited state of C60 was populated via the intrinsic
ISCof C60 unit. It shouldbe pointed outthatthe tripletexcited stateof
the dyad does not necessarily localize on the C60 unit. A backward
triplet state energy transfer from C60 to antenna is also possible [13].
For example, the nanosecond time-resolved transient difference
absorption spectroscopy of the dyads (Fig. 3) shows that the triplet
excited state is localized on C60 for C-1, but the triplet excited of dyad
C-2 is localized on antenna L-2. This different localization of the
triplet excited is due to the different triplet excited state energy level
of the antennas relative to C60 unit. Triplet-triplet-energy-transfer
(TTET) between the photosensitizers and the triplet acceptor
produces the acceptor molecules at the triplet excited state. TTA of
the acceptor molecules at the triplet state produce the acceptor at
the singlet excited state and the radiative decay from the singlet
state to the ground state gives the upconverted fluorescence, i.e. the
delayed fluorescence of the triplet acceptor.
4. Conclusions
In conclusion, naphthalenediimide (NDI)-C60 dyads with strong
absorption of visible light were prepared. The UVevis absorption
wavelength of the dyads was readily tuned by using different
light-harvesting antennas in the dyads. C60 unit was used as the
spin converter. Upon visible light photoexcitation, intramolecular
energy transfer from the light-harvesting antenna to the C60 unit
occurs, in turn the triplet excited state of C60 was produced via the
intrinsic ISC property of C60. Nanosecond time-resolved transient
absorption spectroscopy indicated that the triplet excited state is
localized on either NDI antenna or C60 unit, dependent on the
energy levels of the triplet excited state of the antennas and the C60
unit. Tripletetriplet annihilation (TTA) upconversion was observed
Fig. 6. Upconversions with photosensitizers C-1, C-2 and C60. Photographs of the
emission of (a) photosensitizers alone and (c) the upconversion. b) CIE diagram of the
emission of sensitizers alone and (d) the upconversion. Excited with 589 nm laser
(5.2 mW, power density is 70 mW cmꢀ2). c[Photosensitizers] ¼ 3.0 ꢂ 10ꢀ5 M c
[1CBPEA] ¼ 6.7 ꢂ 10ꢀ4 M. In deaerated toluene, 20 ꢁC.
upconversion [4g,11de11f]. Similar long-lived delayed fluorescence
with C-1 and C60 as triplet photosensitizer were also observed,16.5
ms
and 43.9 s, respectively (Table 2 and see Supporting Information).
m
The delayed fluorescence lifetime with C-1 and C-2 as the triplet
photosensitizerare muchshorterthanthatwiththeBodipy-C60 dyads
as the triplet photosensitizer [11d].
The TTA upconversion is clearly visible to un-aided eyes (Fig. 6).
Orange color (in the web version) was observed for the emission of
the dyad alone, mainly due to the scattering laser (589 nm). In the
presence of triplet acceptor (1CBPEA), green color (in the web
version) was observed for the emission, which is in agreement with
the spectroscopy studies (Fig. 4).
with the dyads are triplet photosensitizer. We propose that the C60
-
organic chromophore dyads can be used as a general molecular
structure motif for heavy atom-free organic triplet photosensi-
tizers, for which the structure of the light-harvesting antennas can
be readily changed, so that the absorption wavelength can be
tuned. These molecular structure variable organic triplet photo-
sensitizers can be used to replace the currently used precious metal
complex triplet photosensitizers or the heavy atom-containing
(such as Br, I) organic triplet photosensitizers in areas such as
photocatalysis, photooxidation, photodynamic therapy or TTA
upconversion.
The photophysical processes with C60 dyad as the triplet photo-
sensitizers for TTA upconversion are summarized in Scheme 2
[4a,4b,4fe4h]. Firstly the NDI antenna in the dyad C-2 was
Acknowledgments
We thank the NSFC (20972024, 21073028 and 21273028), the
Royal Society (UK) and NSFC (China-UK Cost-Share Science
Networks, 21011130154), the Fundamental Research Funds for the
Central Universities (DUT10ZD212), Ministry of Education (NCET-
08-0077) and Dalian University of Technology for financial support.
Appendix A. Supplementary data
Scheme 2. Jablonski diagram of photophysics of the C60-NDI dyads and the application
of dyads as triplet photosensitizers for triplet-triplet-annihilation (TTA) upconversion
(the triplet states of the sensitizers and acceptors are non-emissive). Exemplified with
C-2 as the triplet photosensitizer. The effect of the light-harvesting ability and the
lifetimes of the sensitizers on the efficiency of TTA upconversion are shown (please note
that the vibration energy levels of each electronic state are omitted for clarity). E is
energy. GS is ground state (S0). 1(*NDI-C60) is singlet excited state localized on NDI, and
1(NDI-*C60) is singlet excited state localized on fullerene. IC is inner conversion. ISC is
intersystem crossing. 3(NDI-*C60) is the triplet excited state localized on fullerene. TTET
is tripletetriplet energy transfer. 31CBPEA* is the triplet excited state of 1CBPEA. TTA is
tripletetriplet annihilation. 11CBPEA* is the singlet excited state of 1-CBPEA. The emis-
sion band observed in the TTA experiment is the 11-CBPEA* emission (fluorescence).
Supplementary data related to this article can be found in the
online version at http://dx.doi.org/10.1016/j.dyepig.2012.09.008.
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