Organic triplet sensitizer library derived from a single chromophore (BODIPY) with long-lived triplet excited state for triplet-triplet annihilation based upconversion

Article abstract of DOI:10.1021/jo200990y

Triplet-triplet annihilation (TTA) based upconversions are attractive as a result of their readily tunable excitation/emission wavelength, low excitation power density, and high upconversion quantum yield. For TTA upconversion, triplet sensitizers and acceptors are combined to harvest the irradiation energy and to acquire emission at higher energy through triplet-triplet energy transfer (TTET) and TTA processes. Currently the triplet sensitizers are limited to the phosphorescent transition metal complexes, for which the tuning of UV-vis absorption and T₁ excited state energy level is difficult. Herein for the first time we proposed a library of organic triplet sensitizers based on a single chromophore of boron-dipyrromethene (BODIPY). The organic sensitizers show intense UV-vis absorptions at 510-629 nm (ε up to 180,000 M^-1 cm^-1). Long-lived triplet excited state (τ_T up to 66.3 μs) is populated upon excitation of the sensitizers, proved by nanosecond time-resolved transient difference absorption spectra and DFT calculations. With perylene or 1-chloro-9,10-bis(phenylethynyl) anthracene (1CBPEA) as the triplet acceptors, significant upconversion (Φ_UC up to 6.1%) was observed for solution samples and polymer films, and the anti-Stokes shift was up to 0.56 eV. Our results pave the way for the design of organic triplet sensitizers and their applications in photovoltaics and upconversions, etc.

Full text of DOI:10.1021/jo200990y

ARTICLE
pubs.acs.org/joc
Organic Triplet Sensitizer Library Derived from a Single Chromophore
(BODIPY) with Long-Lived Triplet Excited State for TripletꢀTriplet
Annihilation Based Upconversion
Wanhua Wu, Huimin Guo, Wenting Wu, Shaomin Ji, and Jianzhang Zhao*
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 Western Campus,
2 Ling-Gong Rd., Dalian 116024, China
S Supporting Information
b
ABSTRACT: Tripletꢀtriplet annihilation (TTA) based up-
conversions are attractive as a result of their readily tunable
excitation/emission wavelength, low excitation power density,
and high upconversion quantum yield. For TTA upconversion,
triplet sensitizers and acceptors are combined to harvest the
irradiation energy and to acquire emission at higher energy
through tripletꢀtriplet energy transfer (TTET) and TTA pro-
cesses. Currently the triplet sensitizers are limited to the phosphorescent transition metal complexes, for which the tuning of UVꢀvis
absorption and T1 excited state energy level is difficult. Herein for the first time we proposed a library of organic triplet sensitizers
based on a single chromophore of boron-dipyrromethene (BODIPY). The organic sensitizers show intense UVꢀvis absorptions at
510ꢀ629 nm (ε up to 180,000 Mꢀ1 cmꢀ1). Long-lived triplet excited state (τT up to 66.3 μs) is populated upon excitation of the
sensitizers, proved by nanosecond time-resolved transient difference absorption spectra and DFT calculations. With perylene or
1-chloro-9,10-bis(phenylethynyl)anthracene (1CBPEA) as the triplet acceptors, significant upconversion (ΦUC up to 6.1%) was
observed for solution samples and polymer films, and the anti-Stokes shift was up to 0.56 eV. Our results pave the way for the design
of organic triplet sensitizers and their applications in photovoltaics and upconversions, etc.
produced by annihilation of two acceptor molecules at triplet
excited states, and finally the fluorescence of acceptor will be ob-
served (Scheme 2). For this upconversion approach, the excitation/
emission wavelength can be readily tuned simply by independent
selection of triplet sensitizers/acceptors (but at least with the
energylevels of the T1 states of the sensitizer and acceptor matched,
with the former appropriately higher than the latter).8,9 This
method is particularly promising because no coherent excitation
1. INTRODUCTION
Upconversion (UC), i.e., gain of emission, or more generally,
population of excited states at higher energy upon photoexcita-
tion at lower energy, has attracted much attention due to its
potential applications in photocatalysis, photovoltaics (e.g., dye-
sensitized solar cells), nonlinear optics, and molecular probes.1ꢀ3
A few techniques are available for upconversion, e.g., by using
two-photon absorption (TPA) dyes,4,5 nonlinear optical crystals,
such as potassium dihydrogen phosphate (KDP),6 etc. However,
these techniques suffer from some fundamental drawbacks. For
example, coherent photoexcitation with very high power den-
sity is required for TPA upconversion, usually up to MW cmꢀ2
(106 W cmꢀ2); note that terrestrial solar radiation is only
0.10 W cmꢀ2 (AM1.5G), thus ruling out any applications of
TPA upconversion with solar light as the excitation source.7
Furthermore, it is difficult to tailor the molecular structures of the
TPA dyes to optimize the upconversion wavelengths (energy
levels) and at the same time maintain high TPA cross sections.
Recently a new method for upconversion emerged, i.e., the
tripletꢀtriplet annihilation (TTA) (Scheme 2), with which the
above limitations can be addressed. Triplet sensitizer and accep-
tor are the two components of TTA upconversion. The excita-
tion energy is harvested by triplet sensitizer, and the energy is
transferred to triplet acceptor via tripletꢀtriplet energy transfer
(TTET). For detail photophysics, see the Jablonski diagram in
Scheme 2.8ꢀ10 The singlet excited state of the acceptor will be
is required, the excitation power can be down to a few mW cmꢀ2
,
and thus it is possible to use solar light as an excitation source.8ꢀ11
Currently the triplet sensitizers for TTA upconversion are
limited to transition metal complexes, e.g., Ru(II) polyimine
complexes10a,12 or Pt(II)/Pd(II) porphyrin complexes.1,13,14
Recently we proved that the long-lived 3IL excited state is more
efficient than the normal 3MLCT excited state to sensitize TTA
upconversion.12b A heavy atom is necessary for S1 f T1 inter-
system cross (ISC) (direct S0 f T1 transition is forbidden).15,16
Unfortunately, transition metal complex sensitizers are usually
toxic and not cost-efficient. Furthermore, the significant chal-
lenge for the current sensitizers is the difficulty to modify the
molecular structures to optimize the photophysical properties for
upconversion purpose, such as excitation wavelength (absorp-
tion) and T1 state energy levels. Actually, the currently available
Received: May 16, 2011
Published: July 25, 2011
r
2011 American Chemical Society
7056
dx.doi.org/10.1021/jo200990y J. Org. Chem. 2011, 76, 7056–7064
|

The Journal of Organic Chemistry
ARTICLE
transition metal complex sensitizers are limited to off-the-shelf
compounds and have been almost used up. It is urgent to develop
alternative, more flexible molecular design rationales with which
the related photophysical properties can be readily optimized by
chemical modification.
cover the yellow region (529ꢀ557 nm). The absorption of B-6
was extended to orange (576 nm) and shows an exceptionally
high ε value (180,000 Mꢀ1 cmꢀ1), indicating efficient light-
harvesting. Interestingly B-7 shows red-shifted absorption (ε =
89,500 Mꢀ1 cmꢀ1 at 618 nm) compared with that of B-6. This is
the first report of a single family of organic triplet sensitizers that
cover such a wide range of absorption wavelength.
The fluorescences of the sensitizers are at 532ꢀ706 nm
(Figure 1b), but the fluorescence quantum yields (ΦF) de-
creased with iodo substitution, for example, B-1 and B-2 show
ΦF of 0.036 and 0.027, respectively, vs ΦF = 0.712 for B-0
(Table 1). Decreased ΦF is an indication of ISC, a prerequisite
for the population of the T1 state of the organic sensitizers upon
photoexcitation. No phosphorescences were observed for these
sensitizers.
The emission spectra of the BODIPY derivatives at 77 K were
studied (see Supporting Information). Weak emission bands in
the range of 750ꢀ800 nm were found, which are attributed to the
phosphorescence of the chromophores. This assignment is sup-
ported by the calculated energy level of the T1 excited state of the
BODIPY chromophores (Table 1). For example, emission bands
at 752 and 785 nm were observed for B-2 (Figure S72 in the
Supporting Information), which are close to the calculated T1
state energy level of 826 nm (Table 1).
2.3. Nanosecond Time-Resolved Transient Difference Ab-
sorption Spectra. Population of the triplet excited states of these
organic sensitizers was proved by nanosecond time-resolved
transient absorption spectroscopy (Figure 2). With pulsed excita-
tion at 532 nm, significant bleaching at 520 nm was observed for
B-2, as well as transient absorptions at 420 nm and 550ꢀ700 nm,
which are characteristic for BODIPYs.23,24 The transients are due
to the absorption of the T1 state (i.e., T1 f T2, T1 f T3, etc.)
because formation of other species, such as a charge separated
state, is unlikely.23 This assignment is supported by DFT cal-
culations (Supporting Information). The lifetime of the T1 state
of B-2 (τT) is 57.1 μs, which is much longer than that observed
with an Ir(III) complex that contains BODIPY subunit (τT =
25 μs).24 For B-1, τT is even longer (66.3 μs). To the best of our
knowledge, these are the longest rt triplet excited state lifetimes
reported for BODIPY. Although the population of the triplet
excited state of BODIPY has been observed with metal com-
plexes,23ꢀ25 this is the first time that the triplet excited state of
BODIPY (metal-free) was observed with organic compounds.17,19
Transients were also observed for B-1 and B-3ꢀB-7 (Supporting
Information).
Concerning this aspect, similar to the scenario of dye-
sensitized solar cells (DSCs), we propose that organic triplet
sensitizers will be the next generation sensitizers for TTA UC
because they are more cost-efficient, environmentally benign,
and most importantly easy to be derivatized to optimize the
absorption wavelength and the T1 state energy levels; all of these
photophysical properties are crucial for TTA upconversion.
However, design of organic sensitizers for TTA based UC is
more challenging because the triplet excited state of the sensiti-
zers must be populated (metal-free) upon excitation, instead of
the singlet excited state for the organic sensitizers of DSCs.7
To the best of knowledge, however, no attempts have been
made to develop organic triplet sensitizers with readily derivatizable
molecular structures for TTA upconversion.10c Herein we report
the first attempt to prepare a series of organic triplet sensitizers, de-
rived from a single chromophore (boron-dipyrromethene, BODIPY).
The absorptions of the sensitizers were readily tuned from green
(510 nm) to red (629 nm) (ε up to 180,000 Mꢀ1 cmꢀ1). We
observed very long room temperature (rt) triplet excited state
lifetime (τT is up to 66.3 μs) for the organic sensitizers. Significant
upconversions were observed with the organic sensitizers in both
solution and polymer films, with upconversion quantum yields
(ΦUC) up to 6.1%.
2. RESULTS AND DISCUSSION
2.1. Design of the BODIPY-Based Organic Triplet Sensiti-
zers. Our design of organic triplet sensitizers is inspired by the
iodo-BODIPYs that show decreased fluorescence quantum yields
(ΦF) compared with the parent BODIPY, an indication of inter-
system crossing (ISC).17,18 BODIPY is particularly attractive
because of their intense absorption of visible light, excellent
photostability, and weak nonradiative decay of the excited state
(high fluorescence quantum yields). Furthermore, the rich chem-
istry of BODIPY makes it possible to modify the molecular struc-
ture, and thus the energy levels of the S1 and T1 excited state can
be fine-tuned.19,20
First we designed the mono- and bis-iodo-BODIPYs B-1 and
B-2 (Scheme 1). The iodine atom is attached to the core of
fluorophore, not the peripheral phenyl ring (B-8) (in this case
iodo is not close to the π-conjugation core).21 To shift the ab-
sorption to the red end, we designed B-3 with an extended
π-conjugation framework.22a B-3 shows an absorption band at
629 nm, but our DFT calculation predicts a T1 state of 1075 nm
(1.15 eV) for B-3, too low for the available triplet acceptors, and
thus, aided with DFT calculations, we designed B-4, B-5, B-6,
and B-7, which show T1 energy levels much higher than that of B-
3 (Table 1). These ethynylated BODIPY derivatives were pre-
pared by Sonogashira coupling reactions with satisfying yields.
2.2. UVꢀvis Absorption and Fluorescence Spectra of the
Sensitizers. UVꢀvis absorptions of the sensitizers cover a wide
range from 510 to 629 nm (Figure 1). It should be pointed out
that the absorption is intense, with molar extinction coeffi-
cients (ε) generally larger than 80,000 Mꢀ1 cmꢀ1. B-2 shows
red-shifted absorption (529 nm) compared with that of B-1
(510 nm). For B-3, absorption at 629 nm was observed. Interest-
ingly the absorptions of ethynylated sensitizers (B-4 and B-5)
2.4. DFT Calculations. The energy levels of the T1 states of
sensitizers are not readily available because these sensitizers are
not phosphorescent at either room temperature or 77 K (see
Supporting Information for the emission at 77 K). Thus we
used density functional theory (DFT) and time-dependent DFT
(TDDFT) calculations to predict the energy levels of the T1
excited state. The energy levels of the T1 states of the sensitizers
are crucial for the TTA upconversion because an appropriate
triplet acceptor can only be selected with the energy level of
sensitizer known; the energy level of the acceptor has to be lower
than the energy level of the triplet sensitizer.
For B-1 and B-7, the T1 energy levels were calculated as 1.51
and 1.43 eV, respectively (Table 1). For B-0, the T1 energy
is 1.52 eV (816 nm), which is close to the experimental value
(1.61 eV, 770 nm), thus validating our theoretical approach of
prediction of T1 state energy levels. The selected frontier molec-
ular orbitals (MOs) are presented in Figure 3 (for the MOs of
7057
dx.doi.org/10.1021/jo200990y |J. Org. Chem. 2011, 76, 7056–7064

The Journal of Organic Chemistry
ARTICLE
Scheme 1. Synthesis of the Organic Triplet Sensitizer Library (B-1ꢀB-7)a
a The triplet acceptor perylene and 1-chloro-bis-phenyl ethynylanthracene (1CBPEA) were presented. The molecular structures of the reported
compound B-0, B-2 and B-8 are included (B-0 and B-8 were not used as sensitizers). Reagents and conditions: 1) Trimethylsilylacetylene,
Pd(PPh3)2Cl2, PPh3, CuI, NEt3, reflux, 8 h; 2) NaOH(aq), THF/MeOH; 3) B-1, Pd(PPh3)2Cl2, PPh3, CuI, NEt3, reflux, 8 h; 4) 4-(dimethylamino)-
benzaldehyde, Piperidine, reflux; 5) Trimethylsilylacetylene, Pd(PPh3)2Cl2, PPh3, CuI, THF/NEt3, r. t.; 6) 9-butyl-3-ethynylcarbazole, Pd(PPh3)2Cl2,
PPh3, CuI, THF/NEt3, rt.
other sensitizers, see Supporting Information). From the results
of B-3, B-5, B-6, and B-7, the HOMO is distributed on the whole
π-conjugated framework, but the LUMO is more localized
(Figure 3). The red-shifted absorption of the sensitizers with
larger π-conjugation framework can be rationalized by these MOs.
However, we noted that the BODIPY sensitizers with larger
π-conjugation framework, such as B-5, B-6 and B-7, show T1
state energy levels similar to that of the parent compound. In
order to investigate the origin of the apparent unexpected high
T1 state energy level of the sensitizers, the spin density of the
sensitizers was investigated by optimization of the triplet state of
the sensitizers. The spin density distribution of the selected
sensitizers is presented in Figure 4.
core of the BODIPY; the iodine is far away from the spin density
surface (Figure 4). Clearly for the sensitizers we developed the
heavy atom iodine is attached to the BODIPY core directly and
the iodine is very close to the spin density distribution (Figure 4).
Furthermore, we found that the spin density (namely, the T1
excited state) of the B-5, B-6, and B-7 is more localized, i.e., it is
not distributed on the entire π-conjugation framework. Thus we
expected the T1 state energy level for these sensitizers would be
similar to that of the parent compound B-0 (800ꢀ900 nm). For
B-3, however, the spin density is more delocalized and is spread
over the whole π-conjugation framework, and thus the unique
low T1 state energy level of B-3 (1075 nm) can be rationalized by
its spin density distribution. The theoretical rationalization of
the T1 state energy level is helpful for design of organic triplet
sensitizers. Usually with red-shift of the UVꢀvis absorption, the
T1 state energy level will decrease, which will frustrate the TTA
upconversion. Sensitizers with red-shifted absorption but high T1
Previously B-8 was reported to show significant fluorescence
(ΦF = 0.69), and thus B-8 was used as triplet acceptor, not triplet
sensitizer.21 We attribute the nonefficient ISC of B-8 to the
distance between the heavy atom iodine and the fluorophore
7058
dx.doi.org/10.1021/jo200990y |J. Org. Chem. 2011, 76, 7056–7064

The Journal of Organic Chemistry
ARTICLE
Table 1. Photophysical Parameters of the Organic Triplet
Sensitizers
The green-absorbing sensitizers B-1, B-2, and B-4 give similar
ΦF, but the emission intensity varies drastically upon laser
excitation at 532 nm (Figure 5a), which is due to the different
ε values of the sensitizers at the excitation wavelength (Figure 1).
The emissions of the triplet sensitizers in the presence of triplet
acceptor (perylene) were studied (Figure 5b). Besides the prompt
fluorescence emission of the sensitizers beyond 500 nm, the up-
converted blueemission of perylene was observed (440ꢀ500 nm).
Excitation of the sensitizers or perylene alone does not produce
this emission band. B-2 and B-4 show similar upconversion. For
B-1, however, the upconversion is weak. Upconversion quantum
yield (ΦUC) up to 6.1% was observed for B-4. Considering the
intensive absorption of this sensitizer (ε is up to 5- or 10-fold of the
normaltransitionmetalcomplex sensitizers),10c the intensity of the
upconverted emission is significant even with a ΦUC value of 6.1%.
Interestingly, the fluorescence emission of the triplet sen-
sitizers was not quenched at all in the presence of perylene
(Figure 5b). This is reasonable since the triplet excited state of
the sensitizer, not the singlet excited state, is involved in TTET
(Scheme 2, Jablonski diagram).10d Thus, dual emissions were
observed for the triplet sensitizers in the presence of triplet
acceptor, i.e., the upconverted fluorescence of the triplet acceptor
and the prompt fluorescence of the triplet sensitizers.
ΦF
(%)d
τF
τT
ΔE,
λabs
B-0 503
εb
8.20
λem
(ns)e (μs)f T1 ꢀ S0 ΦISC
a
g
i
515 71.2 ( 0.4 3.86 0.02
532 3.6 ( 0.3 0.16 66.3
552 2.7 ( 0.3 0.13 57.1
706 9.5 ( 0.1 1.40 4.0
563 7.8 ( 0.4 0.42 57.2
631 4.6 ( 0.2 0.37 54.6
623 10.5 ( 0.2 0.42 26.9
816h 0.288
B-1 510
B-2 529
B-3c 629
B-4 539
B-5c 557
B-6c 576
8.80
8.90
7.28
7.54
5.94
18.0
819 0.964
826 0.973
1075 0.905
829 0.922
878 0.954
893 0.895
865 0.907
B-7c 575/618 9.09/8.95 646 9.3 ( 0.2 0.57 47.0
a In CH3CN (1.0 ꢁ 10ꢀ5 M). In nm. b Molar extinction coefficient. In
104 Mꢀ1 cmꢀ1. c In toluene. d Fluorescence quantum yields. In CH3CN.
e Fluorescence lifetimes. Triplet state lifetimes, measured by transient
f
absorption. 1.5 ꢁ 10ꢀ5 M in CH3CN. g The calculated energy gap between
S0 and T1 state. In nm. h Experimentalvalue:770 nm. i Intersystemcrossing
efficiency, approximated by the Ermolev’s rule. ΦISC = 1 ꢀ ΦF.22b,c
The potential of our organic triplet sensitizer approach is
demonstrated by tailoring the molecular structure to readily
switch the absorption from green to red. B-3, B-5, B-6, and B-7
give absorption at 629, 557, 576, and 618 nm, respectively
(Figure 1). 1CBPEA was used as triplet acceptor (Scheme 1)
(T1 = 1.20 eV, 1036 nm). B-3 shows intensive red-absorption,
but no upconversion was observed, probably due to its low T1
energy level. B-7 shows significant upconversion of 1CBPEA
(Figure 6), due to its intense absorption. Furthermore, DFT
calculations predict that B-7 gives a T1 energy level much higher
than that of B-3 (Table 1). The results demonstrated that
these organic sensitizers can be used as promising prototypes
for energy level tunable triplet sensitizers for TTA upconversion
or other photophysical processes.
Figure 1. UVꢀvis absorption (a) and fluorescent emission (b) spectra
of B-1, B-2, and B-4 (in CH3CN) and B-3, B-5, B-6, and B-7 (in
toluene); 1.0 ꢁ 10ꢀ5 M, 20 °C.
Previously 2,3-butanedione was used as organic sensitizer for
max
TTA upconversion, but it was excited in blue region (λabs
=
417 nm), and the absorptions are extremely weak in the visible
region (e.g., ε = 47 Mꢀ1 cmꢀ1 at 490 nm).26 Furthermore, the
ΦUC is very low (0.5%). It should be pointed out that the
absolute upconversion intensity is dependent on the absorption
property of the triplet sensitizers. 2,4,5,7-Tetraiodo-6-hydroxy-3-
fluorone (TIHF) was also used as organic triplet sensitizer,27 but
the τT of TIHF (25.0 μs) is much shorter than those of our triplet
sensitizers (τT is up to 66.3 μs, Table 1), and a much smaller ΦUC
was observed(0.6%) for TIHF.27 Moreover, it isdifficult to modify
the molecular structure of TIHF to optimize the absorption
wavelength at which the TTA upconversion can be performed.
Interestingly, we observed a linear relationship between the
upconverted fluorescence intensity and the excitation power
(Figure 7). This is unusual since TTA based UC normally shows
a quadratic relationship.10d Previously a linear relationship was
reported for upconversion with Pt(II)octaethylporphyrin/DPA,
and it was proposed to be due to the highly populated triplet
excited state of the sensitizers and efficient TTET process.28
Herein our new BODIPY organic sensitizers ensure efficient up-
conversion, i.e., intensive absorption (large ε) and efficient pro-
duction (small ΦF) of the long-lived triplet excited state (large
τT). Upconversion with a linear relationship is more energy-
efficient than with a quadratic relationship.
Figure 2. Nanosecond time-resolved transient absorption difference
spectra of B-2 in deaerated CH3CN after pulsed laser excitation (λex
=
532 nm). Inset: decay trace of B-2 at 525 nm; 1.5 ꢁ 10ꢀ5 M; 20 °C.
state energy level are beneficial for TTA upconversion. It is clear
that our theoretical approach will be useful in the screening
potential organic triplet sensitizers for TTA upconversions.
2.5. TTA Upconversion in Solution and Polymer Films with
BODIPYs as the Triplet Sensitizers. The detail photophysics of
TTA upconversion are presented in Scheme 2.8ꢀ10 The critical
photophysical properties of the sensitizer for TTA upconversion
include the UVꢀvis absorption, the ISC, and the lifetime of the
T1 state of the sensitizers. Our new BODIPY sensitizers fulfill
these requirements; for example, the sensitizers show intense
absorption in the visible range and long-lived T1 excited state.
7059
dx.doi.org/10.1021/jo200990y |J. Org. Chem. 2011, 76, 7056–7064

The Journal of Organic Chemistry
ARTICLE
Figure 3. Selected frontier molecular orbitals involved in the vertical excitation and the triplet excited state (T1) of the sensitizers. The results of B-8 are
presented for comparison. The calculations are based on the optimized ground state geometry (S0 state) at the B3LYP/6-31G(d)/ level using
Gaussian 09W.
Figure 5. Upconversions with sensitizers B-1, B-2, and B-4. (a) Fluo-
rescence emission of the sensitizers and perylene alone. (b) Emission of
the sensitizers in the presence of perylene. Excited with 532 nm laser (5 mW,
power density is 70 mW cmꢀ2). [Sensitizer] = 1.0 ꢁ 10ꢀ5 M; [perylene] =
1.1 ꢁ 10ꢀ4 M; in deaerated CH3CN, 20 °C.
Figure 4. Isosurfaces of spin density of BODIPY sensitizers and the
model compound B-8 at the optimized triplet state geometries (isovalue
= 0.0004). Calculation was performed at B3LYP/6-31G(d) level with
Gaussian 09W.
Scheme 2. Jablonski Diagram of TripletꢀTriplet Annihila-
tion (TTA) Upconversion with Iodo-BODIPYs as Triplet
Sensitizersa
Figure 6. Upconversion with the red-absorbing organic triplet sensiti-
zers B-5, B-6, and B-7 with 1CBPEA as acceptor (excited with 635 nm
laser, 40 mW, power density is 141 mW cmꢀ2). Note the residual
fluorescence of B-7 is omitted. In deaerated toluene. [Sensitizer] = 1.0 ꢁ
10ꢀ5 M; [1CBPEA] = 2.0 ꢁ 10ꢀ5 M; 20 °C.
B-4 and the upconverted fluorescence of perylene were observed
simultaneously (under air condition). Note that the relative
intensity of the upconverted fluorescence vs the emission of
B-4 is more significant than that in solution (Figure 8a). Similar
results were observed for B-7 (Figure 8b) and other sensitizers
(Supporting Information). For B-7, the prompt fluorescence was
a The triplet states of sensitizers and acceptors are non-emissive. TTET
stands for tripletꢀtriplet energy transfer. Exemplified by B-2.
omitted. We did not measure the upconversion quantum yields
We performed the upconversion in polymer films (with PEG-
1500 as the matrix) (Figure 8).10d For B-4, the fluorescence of
in polymer films, but it was noted that the upconversion emis-
sion intensity is comparable to the fluorescence emission of the
7060
dx.doi.org/10.1021/jo200990y |J. Org. Chem. 2011, 76, 7056–7064

The Journal of Organic Chemistry
ARTICLE
Figure 7. (a) Excitation power dependency of the upconverted per-
ylene emission with B-2 as sensitizer (λex = 532 nm) in CH3CN. (b) The
normalized integrated emission intensity plotted as a function of
normalized incident light power. The minimal and the maximal excita-
tion power densities are 14.2 and 237.8 mW cmꢀ2, respectively.
[Sensitizer] = 1.0 ꢁ 10ꢀ5 M; [perylene] = 1.1 ꢁ 10ꢀ4 M; 20 °C.
Figure 9. SternꢀVolmer plots generated from triplet excited state
lifetime (τT) quenching of compounds B-1, B-2, and B-4 measured as
a function of perylene concentration in MeCN. The quenching of B-5,
B-6, and B-7 were measured as a function of 1CBPEA concentration in
toluene. Measured with the nanosecond time-resolved transient absorp-
tion. The concentration of the sensitizers was fixed at 1.0 ꢁ 10ꢀ5 M;
20 °C. (b) Nanosecond time-resolved transient absorption difference
spectra of B-2 with 2.0 ꢁ 10ꢀ5 M of perylene added, in deaerated CH3CN
after pulsed laser excitation (λex = 532 nm), 1.0 ꢁ 10ꢀ5 M, 20 °C.
Table 2. Triplet Excited State Lifetimes (ττ), SternꢀVolmer
Quenching Constant (KSV), and Bimolecular Quenching
Constants (kq) of the BODIPY Sensitizers in Deaerated
CH3CN Solution at 20 °C
τT (μs)
Ksv (103 Mꢀ1
)
kq (109 Mꢀ1 sꢀ1
)
ΦUC (%)
B-1
B-2
B-4
B-5
B-6
B-7
66.3
57.1
57.2
54.6
26.9
47.0
1060.9
657.7
810.1
244.5
132.7
212.9
16.0
11.5
14.2
4.5
2.4 ( 0.0
5.4 ( 0.2
6.1 ( 0.0
0.4 ( 0.0
1.2 ( 0.1
1.7 ( 0.1
Figure 8. (a) Upconversion of B-4/perylene in PEG1500 polymer films
under air condition. For film 1, B-4 and perylene were mixed. Sensitizer/
acceptor/polymer = 33 μg/176 μg/165 μg. For film 2, only perylene was
used. λex = 532 nm, 5 mW, 20 °C. (b) Upconversion of B-7/1CBPEA in
PEG1500 polymer films under air condition. For film 1, B-7 and 1CBPEA
were mixed. Sensitizer/acceptor/polymer = 55 μg/288 μg/165 μg.
For film 2, only 1CBPEA was used. Note the prompt fluorescence of
the sensitizers is omitted in (b). λex = 635 nm, 40 mW, 20 °C.
5.0
3.8
fluorescence of the sensitizer and the upconverted emission of
perylene were simultaneously detected (Figure 5b), the emission
color changed to blue-green, blue-white, and white for B-1, B-2
and B-4, respectively. The CIE coordinates (x, y) of the
upconversion of B-4/perylene (0.36, 0.41) are close to the
optimal white color (0.33, 0.33). We believe the color can be
fine-tuned by optimization of the sensitizer/acceptor molar ratio.
Previously white emission was observed with TIHF/DPA UC,
but with much lower ΦUC (0.6%).27 With our iodo-BODIPY
organic sensitizers the white light emission was achieved with
much higher upconversion quantum yield (ΦUC = 6.1%). White
emission is significant because these materials can be used for
energy-efficient illumination devices.
2.6. Conclusion. In summery, for the first time a series of
organic triplet sensitizers (metal free) derived from a single
chromophore (BODIPY) for tripletꢀtriplet annihilation upcon-
version were devised, for which the absorption can be readily
optimized by chemical modification of the sensitizer molecular
structures. The organic sensitizers show UVꢀvis absorption
ranging from green (510 nm) to red (629 nm) (ε is up to
180000 Mꢀ1 cmꢀ1) and tunable T1 excited state energy levels.
Population of the long-lived triplet excited state was observed for
the organic sensitizers upon photoexcitation (τT is up to 66.3 μs).
DFT calculations were used to rationalize the UVꢀvis absorption
and the T1 excitedstate energy levels ofthe sensitizers. The organic
triplet sensitizers were used for tripletꢀtriplet annihilation (TTA)
based upconversion. Significant upconversion was observed in
sensitizers, for which the quantum yields are known (Table 1).
Upconversion in polymer films under air condition is significant
because it paves the way for practical applications.10d
Besides the light-harvesting (ε) and ISC of the organic triplet
sensitizers, TTET is another critical step involved in the cascade
photophysical process of TTA UC.8,9 TTET can be quantita-
tively studied by the quenching of triplet excited states of the
sensitizers with perylene. Herein it was monitored by the
decrease of τT in the presence of perylene (studied by transient
absorption, Figure 9) since the T1 states of the organic sensitizers
are nonemissive. The SternꢀVolmer quenching constants (KSV)
(Table 2) are ca. 100- to 200-fold of the reported values with
transition metal complex as sensitizers,10a,b,11a,29,30 and the
bimolecular quenching constants (kq) are among the highest
values ever reported.11a,b,29 This is attributed to the long-lived T1
state and probably the small size of the sensitizers, with which
longer diffusion distance is expected for the excited sensitizer to
encounter with the acceptors.9 Efficient TTET will improve the
upconversion efficiency. The TTET was also proved by the produc-
tion of the perylene T1 state, which shows transient absorption at
495 nm (Figure 9b; note the setting of the intrument does not allow
the full decay of the triplet excited state of perylene).
The upconversions are visible with the unaided eye (Figure 10).
Sensitizers give green (B-1 and B-2) or yellow (B-4) emission
upon excitation at 532 nm. With addition of perylene, the
7061
dx.doi.org/10.1021/jo200990y |J. Org. Chem. 2011, 76, 7056–7064

The Journal of Organic Chemistry
ARTICLE
(200 mg, 0.62 mmol) in anhydrous CH2Cl2 (25 mL) was added excess
NIS (558 mg, 2.48 mmol). The mixture was stirred at room temperature
for about 30 min (monitored by TLC until complete consumption of the
starting material). The reaction mixture was then concentrated under
vacuum, and the crude product was purified by silica gel column chro-
matography (hexane/CH2Cl2, 2:1, v/v). The red band was collected,
and the solvent was removed under reduced pressure to give the product
as red solid. Yield: 300.0 mg, 84.0%. 1H NMR (400 MHz, CDCl3): δ
7.54ꢀ7.51 (m, 3H), 7.26ꢀ7.24 (m, 2H), 2.65 (s, 6H), 1.38 (s, 6H). 13C
NMR (100 MHz, CDCl3): δ 156.9, 145.5, 141.5, 134.4, 129.7, 129.6,
127.9, 85.8, 17.1, 16.2. MALDI-HRMS: calcd ([C19H17BF2I2N2]ꢀ)
m/z = 575.9542, found m/z = 575.9528.
3.4. Synthesis of B-3. Under nitrogen atmosphere compound B-2
(100.0 mg, 0.174 mmol), p-N,N-dimethylamino-benzaldehyde (26.0
mg, 0.174 mmol), piperidine (0.1 mL), and a small amount of molecular
sieves were suspended in dry toluene (20 mL). The mixture was heated
to 70 °C and was kept at this temperature for about 6 h. After completion
of the reaction, the majority of the solvent was evaporated under reduced
pressure, and the crude product was subjected to column chromatog-
raphy (silica gel, hexane/CH2Cl2 = 1:1) to give the product as blue solid.
Yield: 40.0 mg, 32.5%. 1H NMR (400 MHz, CDCl3): δ 8.21 (d, 1H, J =
17.6 Hz), 7.59ꢀ7.50 (m, 6H), 7.28ꢀ7.27 (m, 2H), 6.81 (m, 2H), 3.06
(s, 6H), 2.68 (s, 3H), 1.44 (s, 3H), 1.38 (s, 3H). 13C NMR (100 MHz,
CDCl3): δ 154.9, 151.8, 151.3, 146.6, 143.3, 140.5, 138.8, 135.3, 132.5,
131.4, 129.5, 129.4, 128.3, 124.9, 114.0, 112.2, 85.2, 82.7, 40.3, 29.7, 17.6,
16.8, 16.1. MALDI-HRMS: calcd ([C28H26BF2I2N3]+) m/z = 707.0277,
found m/z = 707.0286.
3.5. Synthesis of B-4. Under argon atmosphere, 2,6-diiodo-1,3,
5,7-tetramethyl-8- phenyl-4,4-difluoroboradiazaindacene (B-2) (150.0
mg, 0.26 mmol), Pd(PPh3)2Cl2 (9.1 mg, 0.013 mmol), PPh3 (6.8 mg,
0.026 mmol), and CuI (5.2 mg, 0.026 mmol) were dissolved in
triethylamine (2 mL) and THF (5 mL). After stirring, trimethylsilyla-
cetylene (25.5 mg, 0.26 mmol) was added via syringe. The solution was
stirred at rt overnight. The solvent was removed under reduced pressure,
and the crude product was purified by column chromatography (silica
gel, with CH2Cl2 as the eluent); a dark red solid was obtained. Yield: 62.2
mg, 43.8%. 1H NMR (400 MHz, CDCl3): δ 7.51 (m, 3H), 7.26ꢀ7.24
(m, 2H), 2.65 (s, 3H), 2.64 (s, 3H), 1.44 (s, 3H), 1.39 (s, 3H). 13C NMR
(100 MHz, CDCl3): δ 158.8, 156.5, 145.3, 144.8, 142.0, 134.4, 131.8,
130.3, 129.5, 129.3, 129.3, 127.6, 116.4, 101.7, 96.9, 85.4, 16.8, 15.9, 13.6,
13.4, 0.9. MALDI-HRMS: calcd ([C24H26BF2IN2Si]+) m/z = 546.0971,
found m/z = 546.0938.
Figure 10. (a) Photographs of the emission of sensitizers alone and
(b) the upconversion. (c) CIE diagram of the emission of sensitizers
alone and (d) in the presence of perylene (upconversion). λex =532 nm
(5 mW); 20 °C.
solution as well as in polymer films, with quantum yield (ΦUC) up
to 6.1%. White light emission was observed with one of the up-
conversions. We believe our work will inspire the transition from
metal complex sensitizers to neat organic triplet sensitizers in the
research areas of TTA upconversion. This new methodology will
greatly increase the availability of the triplet sensitizers that can be
used for tripletꢀtriplet annihilation based upconversion or, more
generally, any other photophysical processes sensitized with triplet
excited states.
3. EXPERIMENTAL SECTION
3.6. Synthesis of B-5. B-5 was obtained following procedure
similar to that of B-4, except 9-butyl-3-ethynylcarbazole (64.0 mg, 0.26
mmol) was used instead of ethynyltrimethylsilane. Yield: 30.0 mg,16.6%.
1H NMR (400 MHz, CDCl3): δ 8.21(s, 1H), 8.08 (d, 1H, J = 7.8 Hz),
7.56ꢀ7.53 (m, 4H), 7.48 (t, 1H, J = 7.7 Hz), 7.42 (d, 1H, J = 8.1 Hz),
7.35 (d, 1H, J = 8.5 Hz), 7.31ꢀ7.28 (m, 2H), 7.25 (d, 1H, J = 7.4 Hz),
4.30 (t, 2H, J = 7.1 Hz), 2.77 (s, 3H), 2.67 (s, 3H), 1.89ꢀ1.81 (m, 2H),
1.56 (s, 3H), 1.41 (s, 3H), 1.40ꢀ1.34 (m, 2H), 0.94 (t, 3H, J = 7.3 Hz).
13C NMR (100 MHz, CDCl3): δ 159.2, 155.9, 144.3, 144.3, 141.7,
140.9, 140.1, 134.8, 131.7, 131.0, 129.4, 129.1, 127.9, 126.1, 123.7, 122.9,
122.4, 120.5, 119.4, 117.4, 113.3, 109.0, 108.7, 98.2, 85.0, 79.4, 43.0, 31.1,
20.5, 16.8, 15.9, 13.8, 13.5. MALDI-HRMS: calcd ([C37H33BF2IN3]+)
m/z = 695.1780, found m/z = 695.1746.
3.7. Synthesis of a1. Pd(PPh3)2Cl2 (29.5 mg, 0.042 mmol), PPh3
(22.0 mg, 0.085 mmol), and CuI (17.0 mg, 0.085 mmol) were added to a
solution of B-1 (200.0 mg, 0.44 mmol) in a mixed solvent of (i-Pr)2NH
(5 mL) and THF (10 mL) that had been deaerated with argon.
Trimethylsilylacetylene (65.0 mg, 0.67 mmol) was added via syringe.
The mixture was then heated to 60 °C for 6 h. The solvent was removed
under reduced pressure, water was added, and the mixture was extracted
with dichloromethane (DCM, 4 ꢁ 20 mL). The combined organic layer
was dried over anhydrous Na2SO4. After removal of the solvent, the
3.1. Analytical Measurements. Fluorescence lifetimes were
measured with a Fluoromax-4 spectrofluorometer (Horiba Jobin Yvon).
Luminescence quantum yields of the sensitizers were measured with
quinine sulfate as the standard (ΦF = 0.547 in 0.05 M sulfuric acid). All
data were measured three times independently.
3.2. Synthesis of 2-Iodo-1,3,5,7-tetramethyl-8-phenyl-4,4-
difluoroboradiazaindacene (B-1). At 10ꢀ15 °C, N-iodo-succini-
mid (NIS) (140.0 mg, 0.62 mmol) in anhydrous CH2Cl2 (10 mL) was
added dropwise into a solution of B-0 (200 mg, 0.62 mmol) in CH2Cl2
(50 mL) within ca. 1 h. After the addition, the reaction mixture was
allowed to stir at room temperature for 1 h. The reaction mixture was
then concentrated under reduced pressure, and the crude product was
purified by silica gel column chromatography (hexane/CH2Cl2, 2:1,
v/v). The second band was collected to give the product as a red solid.
1
Yield: 191.7 mg, 68.7%. H NMR (400 MHz, CDCl3): δ 7.51ꢀ7.48
(m, 3H), 7.27ꢀ7.25 (m, 2H), 6.04 (s, 1H), 2.63 (s, 3H), 2.57 (s, 3H),
1.38 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 157.9, 154.7, 145.3,
143.4, 141.7, 135.0, 132.0, 131.1, 129.8, 129.5, 129.4, 128.0, 122.5, 84.4,
16.8, 16.0, 14.9, 14.7. MALDI-HRMS: calcd ([C19H18BF2IN2]+) m/z =
450.0576, found m/z = 450.0535.
3.3. Synthesis of 2,6-Diiodo-1,3,5,7-tetramethyl-8-phen-
yl-4,4-difluoroboradiazaindacene (B-2).31 To a solution of B-0
7062
dx.doi.org/10.1021/jo200990y |J. Org. Chem. 2011, 76, 7056–7064

The Journal of Organic Chemistry
ARTICLE
crude product was purified with column chromatography (silica gel,
DCM/hexane = 1:1), and a red solid was obtained. NaOH (20% in
water, 0.5 mL) was added to a solution of the above trimethylsilane
protected intermediate (100 mg, 0.24 mmol) in THF (4 mL) and
MeOH (4 mL), and the solution was stirred at room temperature under
argon for 10 min. DCM (100 mL) and water (50 mL) were added. The
organic layer was separated, and the aqueous layer was extracted with
DCM (3 ꢁ 15 mL). The combined organic layers were washed with
brine (200 mL), dried over anhydrous MgSO4, and filtered, and then the
solvent was removed. The residue was purified by passing through a
silica plug using DCM as eluent to give a red solid. Yield: 70.0 mg, 30.0%.
1H NMR (400 MHz, CDCl3): δ 7.52ꢀ7.49 (m, 3H), 7.28ꢀ7.25
(m, 2H), 6.04 (s, 1H), 3.28 (s, 1H), 2.64 (s, 3H), 2.57 (s, 3H), 1.44
(s, 3H), 1.39 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 158.3, 156.7,
145.2, 143.8, 142.5, 135.4, 134.7, 132.8, 130.0, 129.4, 128.0, 125.2, 122.5,
114.1, 83.5, 76.6, 14.9, 14.7, 13.5, 13.1.
3.8. Synthesis of a2 and a3. Under argon atmosphere, a1 (54.2
mg, 0.16 mmol) and B-1 (70.0 mg, 0.16 mmol) were dissolved in the
mixed solvent of triethylamine (2 mL) and THF (5 mL). Then
Pd(PPh3)2Cl2 (11.0 mg, 0.016 mmol), PPh3 (8.0 mg, 0.032 mmol),
and CuI (6.2 mg, 0.032 mmol) were added, and the mixture was refluxed
for about 4 h. The reaction mixture was cooled to rt, and the solvent was
removed under reduced pressure. Water was added, and the mixture was
extracted with DCM. The combined organic layer was dried over
anhydrous Na2SO4. After removal of the solvent the crude product
was purified with column chromatography (silica gel, DCM/hexane =
1:1 as eluent), and the first band was collected to give a purple solid
a2 (27.0 mg, 0.039 mmol, 49.1%). 1H NMR (400 MHz, CDCl3):
δ 7.51ꢀ7.49 (m, 6H), 7.26ꢀ7.24 (m, 4H), 6.05 (s, 2H), 2.65 (s, 6H),
2.57 (s, 6H), 1.44 (s, 6H), 1.39 (s, 6H). 13C NMR (100 MHz, CDCl3):
δ 158.6, 157.6, 145.4, 144.0, 142.2, 134.5, 132.9, 129.9, 127.8, 122.5, 113.7,
80.2, 75.5, 14.8, 14.6, 13.6, 13.2. MALDI-HRMS: calcd ([C42H36B2F4N4]+)
m/z = 694.3062, found m/z = 694.3033. The second band was collected
as a dark green solid a3 (15.0 mg, 0.022 mmol, 13.8%). 1H NMR (400
MHz, CDCl3): δ 7.59ꢀ7.50 (m, 6H), 7.33ꢀ7.26 (m, 4H), 6.02 (s, 2H),
2.63 (s, 6H), 2.57 (s, 6H), 1.44 (s, 6H), 1.39 (s, 6H). 13C NMR (100
MHz, CDCl3): δ 157.4, 156.2, 144.5, 141.9,134.7, 132.4, 132.0, 1331.7,
130.3, 129.2, 128.5, 127.9, 122.0, 120.3, 119.9, 115.7, 88.4, 88.2, 14.7,
14.5, 13.5, 13.1. MALDI-HRMS: calcd ([C40H36B2F4N4]+) m/z =
670.3062, found m/z = 670.3041.
flash photolysis experiments were deaerated with argon for ca. 15 min
before measurement, and the gas flow was maintained during the
measurement.
3.12. TripletꢀTriplet Annihilation Upconversions. A diode
pumped solid state (DPSS) laser (532 and 635 nm) was used for the
upconversions. The diameter of the 532 nm laser spot was ca. 3 mm, and
for the 635 nm laser, it is ca. 6 mm. The power of the laser beam was
measured with a VLP-2000 pyroelectric power meter. The samples were
purged with N2 or Ar for at least 15 min before measurement (note the
upconversion can be significantly quenched by oxygen). For the upcon-
version experiments, the mixed solution of the BODIPYs (triplet sensitizer)
and perylene or 1-chloro-bis-phenylethynylanthracene (1CBPEA)
(triplet acceptor) was degassed for at least 15 min with N2 or Ar. Then
the solution was excited with laser. The upconverted fluorescence of
perylene or 1CBPEA was observed with spectrofluorometer. In order to
suppress the laser scattering, a black box with a small hole on it was put
behind the fluorescent cell to trap the laser beam (the small hole as the
entrance of the laser into the black box). The polymer used for the
upconversion was PEG-1500 (molecular weight 1500). The film was
obtained by casting a 1.5 mL solution of PEG1500 (11% in CH2Cl2), to
which 60 μL of sensitizer solution (1.0 ꢁ 10ꢀ3 M) and 0.7 mL of
perylene solution (1.0 ꢁ 10ꢀ3 M) had been added or only 0.7 mL of
perylene solution (1.0 ꢁ 10ꢀ3 M) had been added, on a glass disk. Then
after evaporation of the solvents, the films were studied under air.
The upconversion quantum yields (ΦUC) of B-1, B-2, and B-4 were
determined with the prompt fluorescence of the sensitizer as the inner
standard; for example, ΦUC of B-2 was determined by comparing the
upconverted fluorescence and its prompt fluorescence. For B-5, B-6, and
B-7, ΦUC was determined with the prompt fluorescence of B-5 as the
standard. The upconversion quantum yields were calculated with the
following equation, where ΦUC, Aunk, Iunk, and ηunk represent the quan-
tum yield, absorbance, integrated photoluminescence intensity, and the
refractive index of the solvents (eq 1). The equation is multiplied by a
factor of 2 in order to make the maximum quantum yield to be unity.10c
All these data were independently measured three times (with different
solutions samples).
ꢀ
ꢁꢀ ꢁꢀ
ꢁ
2
Astd
Iunk
ηunk
ηstd
ΦUC ¼ 2Φstd
ð1Þ
Aunk
Istd
3.9. Synthesis of B-6. B-6 was obtained following a procedure
similar to that of B-2, except an intermediate product a2 (27.0 mg,
0.0403 mmol) was used instead of B-0; 25.7 mg of a purple solid was
obtained. Yield: 67.3%. 1H NMR (400 MHz, CDCl3): δ 7.53ꢀ7.51 (m,
6H), 7.26ꢀ7.23 (m, 4H), 2.66 (s, 12H), 1.44 (s, 6H), 1.40 (s, 6H). 13C
NMR (100 MHz, CDCl3): δ 159.3, 157.7, 145.9, 145.8, 142.1, 134.3,
132.3, 130.2, 129.6, 129.5, 127.7, 114.8, 86.2, 80.5, 75.4, 17.0, 16.1, 13.8,
13.5. MALDI-HRMS: calcd ([C42H34B2F4N4I2]+) m/z = 946.0995,
found m/z = 946.1037.
3.10. Synthesis of B-7. B-7 was obtained following a similar
procedure outlined above for B-2, except an intermediate product a3
(10.0 mg, 0.015 mmol) was used instead of B-0; 7.7 mg of a blue solid
was obtained, Yield: 55.7%. 1H NMR (400 MHz, CDCl3): δ 7.54ꢀ7.50
(m, 6H), 7.26ꢀ7.21 (m, 4H), 2.65 (s, 6H),2.64(s, 6H), 1.44 (s, 6H),
1.40 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 158.2, 156.7, 144.9,
143.8, 141.8, 134.6, 131.9, 130.7, 129.4, 127.8, 116.6, 88.7, 85., 16.9, 16.0,
13.7, 13.4. MALDI-HRMS: calcd ([C40H34B2F4N4I2]+) m/z = 922.0995,
found, m/z = 922.0933.
For the measurement of the TTET efficiency, i.e., the SternꢀVolmer
quenching constants, the concentration of the sensitizer was fixed at
1.0 ꢁ 10ꢀ5 M, and the lifetime of the sensitizer was measured with in-
creasing perylene concentration in the solution.
The CIE coordinates (x, y) of the emission of the sensitizers alone
and the emission of the upconversion were derived from the emission
spectra with CIE Color Matching Linear Algebra software.
3.13. DFT Calculations. The density functional theory (DFT)
calculations were used for optimization of the ground state geometries of
both singlet states and triplet states. The energy level of the T1 state
(energy gap between S0 state and T1 state) were calculated with time-
dependent DFT (TDDFT), based on the optimized singlet ground state
geometries (S0 state). TDDFT calculations were also used for the pre-
diction of the UVꢀvis absorption of the organic triplet sensitizers at triplet
state, and in our case it is the transient absorption of the organic triplet
sensitizers after laser flash (the pulsed excitation of the organic triplet
sensitizer solution). Please note that the bleaching bands in the time-
resolved transient absorption spectra cannot be predicted by the TDDFT
calculations. All the calculations were performed with Gaussian 09W.32
3.11. Nanosecond Time-Resolved Transient Difference
Absorption Spectra. The nanosecond time-resolved transient ab-
sorption spectra were detected by Edinburgh analytical instruments (LP
920, Edinburgh Instruments, U.K.) and recorded on a Tektronix TDS
3012B oscilloscope. The lifetime values (by monitoring the decay trace
of the transients) were obtained with the LP900 software. All samples in
’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis of B-0 and the struc-
b
ture characterization data of the organic triplet sensitizers, details
7063
dx.doi.org/10.1021/jo200990y |J. Org. Chem. 2011, 76, 7056–7064

The Journal of Organic Chemistry
ARTICLE
of upconversion, transient absorption, calculation information,
77 K emission spectra, and photostability of the sensitizers and
photophysics of the acceptors. This material is available free of
charge via the Internet at http://pubs.acs.org.
(13) Singh-Rachford, T. N.; Castellano, F. N. Inorg. Chem. 2009,
48, 2541–2548.
(14) Singh-Rachford, T. N.; Castellano, F. N. J. Phys. Chem. A 2008,
112, 3550–3556.
(15) Wolfbeis, O.; Narayanaswamy, R. Optical Sensors: Industrial,
Environmental and Diagnostic Applications; Springer-Verlag: Berlin,
Heidelberg, 2004.
’ AUTHOR INFORMATION
(16) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.;
Kluwer Academic: New York, 1999.
(17) Yogo, T.; Urano, Y.; Ishitsuka, Y.; Maniwa, F.; Nagano, T. J. Am.
Chem. Soc. 2005, 127, 12162–12163.
Corresponding Author
*E-mail: zhaojzh@dlut.edu.cn.
(18) Gorman, A.; Killoran, J.; O’Shea, C.; Kenna, T.; Gallagher,
W. M.; O’Shea, D. F. J. Am. Chem. Soc. 2004, 126, 10619–10631.
(19) (a) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed.
2008, 47, 1184–1201. (b) Zhang, X.; Xiao, Y.; Qian, X. Angew. Chem.,
Int. Ed. 2008, 47, 8025–8029. (c) Peng, X.; Du, J.; Fan, J.; Wang, J.;
Wu, Y.; Zhao, J.; Sun, S.; Xu., T. J. Am. Chem. Soc. 2007, 129, 1500–1501.
(d) Ziessel, R.; Harriman, A. Chem. Commun. 2011, 47, 611–631.
(e) Guliyev, R.; Coskun, A.; Akkaya., E. U. J. Am. Chem. Soc. 2009,
131, 9007–9013.
’ ACKNOWLEDGMENT
We thank the NSFC (20972024 and 21073028), Fundamen-
tal Research Funds for the Central Universities (DUT10ZD212
and DUT11LK19), Royal Society (U.K.) and NSFC (China-
U.K. Cost-Share program/21011130154), Ministry of Education
(SRF for ROCS, SRFDP-200801410004 and NCET-08-0077),
State Key Laboratory of Fine Chemicals (KF0802), and the
Education Department of Liaoning Province (2009T015) for
financial support.
(20) Lee, J. S.; Kang, N. Y.; Kim, Y. K.; Samanta, A.; Feng, S.; Kim,
H. K.; Vendrell, M.; Park, J. H.; Chang, Y. T. J. Am. Chem. Soc. 2009,
131, 10077–10082.
(21) Singh-Rachford, T. N.; Haefele, A.; Ziessel, R.; Castellano, F. N.
J. Am. Chem. Soc. 2008, 130, 16164–16165.
’ REFERENCES
(22) (a) Baruah, M.; Qin, W.; Flors, C.; Hofkens, J.; Vallꢀee, R. A. L.;
Beljonne, D.; der Auweraer, M. V.; De Borggraeve, W. M.; Boens, N.
J. Phys. Chem. A 2006, 110, 5998–6009.(b) Turro, N. J.; Ramamurthy,
V.; Scaiano, J. C. Principles of Molecular Photochemistry: An Introduction;
University Science Books: Sausalito, CA, 2009. (c) Immitzer, B.; Falk, H.
Monatsh. Chem. 2000, 131, 1167–1171.
(23) Galletta, M.; Campagna, S.; Quesada, M.; Ulrich, G.; Ziessel, R.
Chem. Commun. 2005, 4222–4224.
(24) Rachford, A. A.; Ziessel, R.; Bura, T.; Retailleau, P.; Castellano,
F. N. Inorg. Chem. 2010, 49, 3730–3736.
(25) (a) Galletta, M.; Puntoriero, F.; Campagna, S.; Chiorboli, C.;
Quesada, M.; Goeb, S.; Ziessel, R. J. Phys. Chem. A 2006,
110, 4348–4358.
(26) Singh-Rachford, T. N.; Castellano, F. N. J. Phys. Chem. A 2009,
113, 5912–5917.
(27) Chen, H. C.; Hung, C. Y.; Wang, K. H.; Chen, H. L.; Fann,
W. S.; Chien, F. C.; Chen, P.; Chow, T. J.; Hsu, C. P.; Sun, S. S. Chem.
Commun. 2009, 4064–4066.
(28) Cheng, Y. Y.; Khoury, T.; Clady, R. G. C. R.; Tayebjee, M. J. Y.;
Ekins-Daukes, N. J.; Crossleya, M. J.; Schmidt, T. W. Phys. Chem. Chem.
Phys. 2010, 12, 66–71.
(1) Baluschev, S.; Yakutkin, V.; Miteva, T.; Avlasevich, Y.; Chernov,
S.; Aleshchenkov, S.; Nelles, G.; Cheprakov, A.; Yasuda, A.; M€ullen, K.;
Wegner, G. Angew. Chem., Int. Ed. 2007, 46, 7693–7696.
(2) Warther, D.; Bolze, F.; Lꢀeonard, J.; Gug, S.; Specht, A.; Puliti, D.;
Sun, X. H.; Kessler, P.; Lutz, Y.; Vonesch, J. L.; Winsor, B.; Nicoud, J. F.;
Goeldner, M. J. Am. Chem. Soc. 2010, 132, 2585–2590.
(3) Zhang, P.; Wang, M.; Dong, J.; Li, X.; Wang, F.; Wu, L.; Sun, L.
J. Phys. Chem. C 2010, 114, 15868–15874.
(4) Kim, H. M.; Cho, B. R. Acc. Chem. Res. 2009, 42, 863–872.
(5) Kim, M. K.; Lim, C. S.; Hong, J. T.; Han, J. H.; Jang, H. Y.; Kim,
H. M.; Cho, B. R. Angew. Chem., Int. Ed. 2010, 49, 364–367.
(6) Kushwaha, S. K.; Shakir, M.; Maurya, K. K.; Shah, A. L.; Wahab,
M. A.; Bhagavannarayana, G. J. Appl. Phys. 2010, 108, 033506/1–033506/7.
(7) Tian, H.; Jiang, X.; Yu, Z.; Kloo, L.; Hagfeldt, A.; Sun, L. Angew.
Chem. 2010, 122, 7486–7489; Angew. Chem., Int. Ed. 201049, 7328ꢀ
7331.
(8) Matsunami, M.; Chainani, A.; Taguchi, M.; Eguchi, R.; Ishida, Y.;
Takata, Y.; Okamura, H.; Nanba, T.; Yabashi, M.; Tamasaku, K.;
Nishino, Y.; Ishikawa, T.; Senba, Y.; Ohashi, H.; Tsujii, N.; Ochiai, A.;
Shin, S. Phys. Rev. B 2008, 78, 195112/1–195112/5.
(9) (a) Parker, C. A.; hatchard, C. G.; Joyce, T. A. Nature 1965,
205, 1282–1284. (b) Monguzzi, A.; Tubino, R.; Meinardi, F. Phys. Rev. B
2008, 77, 155122/1–155122/4.
(10) (a) Islangulov, R. R.; Kozlov, D. V.; Castellano, F. N. Chem.
Commun. 2005, 3776–3778. (b) Singh-Rachford, T. N.; Islangulov,
R. R.; Castellano, F. N. J. Phys. Chem. A 2008, 112, 3906–3910. (c)
Singh- Rachford, T. N.; Castellano, F. N. Coord. Chem. Rev. 2010,
254, 2560–2573. (d) Islangulov, R. R.; Lott, J.; Weder, C.; Castellano.,
F. N. J. Am. Chem. Soc. 2007, 129, 12652–12653.
(29) Du, P.; Eisenberg, R. Chem. Sci. 2010, 1, 502–506.
(30) Zhao, W.; Castellano, F. N. J. Phys. Chem. A 2006, 110,
11440–11445.
(31) Zhang, D.; Wang, Y.; Xiao, Y.; Qian, S.; Qian, X. Tetrahedron
2009, 65, 8099–8013.
(32) Frisch, M. J. et al. Gaussian 09 Revision A.1; Gaussian Inc.:
Wallingford, CT, 2009.
(11) (a) Singh-Rachford, T. N.; Castellano, F. N. J. Phys. Chem. Lett.
2010, 1, 195–200. (b) Cheng, Y. Y.; F€uckel, B.; Khoury, T.; Clady, R. G.
C. R.; Tayebjee, M. J. Y.; Ekins-Daukes, N. J.; Crossley, M. J.; Schmidt,
T. W. J. Phys. Chem. Lett. 2010, 1, 1795–1799.
(12) (a) Kozlov, D. V.; Castellano, F. N. Chem. Commun.
2004, 2860–2861. (b) Ji, S.; Wu, W.; Wu, W.; Guo, H.; Zhao, J. Angew.
Chem., Int. Ed. 2011, 50, 1626–1629.(c) Ji, S.; Guo, H.; Wu, W.; Wu, W.
Zhao, J. Angew. Chem., Int. Ed. 2011; Epub ahead of print, DOI:
10.1002/anie.201008134. (d) Wu, W.; Wu, W.; Ji, S.; Guo, H.; Zhao,
J. Dalton Trans. 2011, 40, 5953–5963.(d) Liu, Y.; Wu, W.; Zhang, X.;
Guo, H.; Zhao, J. Dalton Trans. 2011; Epub ahead of print, DOI:
10.1039/c1dt10679a. (e) Sun, H.; Guo, H.; Wu, W.; Liu, X.; Zhao, J.
Dalton Trans. 2011, 40, 7834ꢀ7841. (f) Sun, J.; Wu, W.; Guo, H.; Zhao,
J. Eur. J. Inorg. Chem. 2011, 3165ꢀ3173.
7064
dx.doi.org/10.1021/jo200990y |J. Org. Chem. 2011, 76, 7056–7064