Phenyleneanthracene derivatives as triplet energy acceptor/emitter in red light excitable triplet-triplet-annihilation upconversion

Article abstract of DOI:10.1016/j.dyepig.2016.09.057

A series of anthracene derivatives with 9,10-substituents were prepared as triplet acceptors/emitters for triplet-triplet-annihilation (TTA) upconversion. Different linkages of C[sbnd]C single bonds and C[tbnd]C triple bonds were used to tune the singlet and triplet state energy levels, which may enhance the TTA upconversion. The study of the photophysical properties of the compounds indicates that the C[sbnd]C linker does not alter the T₁ state energy level substantially, whereas the C[tbnd]C linker significantly reduced the T₁ state energy levels. With nanosecond transient absorption spectroscopy, the intermolecular triplet-triplet-energy-transfer (TTET) process was studied. The lack of the upconversion for some anthracene derivatives was attributed to the inappropriate T₁ energy levels thus the lack of TTET. On the other hand, different upconversion quantum yields were observed for some acceptors, although the TTET processes are similar. This result is due to the different TTA. These studies will be useful for future development of the TTA upconversion and for study of the triplet state properties of organic chromophores.

Full text of DOI:10.1016/j.dyepig.2016.09.057

Accepted Manuscript
Phenyleneanthracene derivatives as triplet energy acceptor/emitter in red light
excitable triplet-triplet-annihilation upconversion
Fangfang Zhong, Jianzhang Zhao
PII:
S0143-7208(16)30565-4
10.1016/j.dyepig.2016.09.057
DYPI 5509
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 26 August 2016
Revised Date: 26 September 2016
Accepted Date: 26 September 2016
Please cite this article as: Zhong F, Zhao J, Phenyleneanthracene derivatives as triplet energy acceptor/
emitter in red light excitable triplet-triplet-annihilation upconversion, Dyes and Pigments (2016), doi:
10.1016/j.dyepig.2016.09.057.
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Graphical abstract
Phenyleneanthracene Derivatives as Triplet Energy Acceptor/Emitter in Red
Light Excitable Triplet-Triplet-Annihilation Upconversion
Fangfang Zhong a and Jianzhang Zhao a,*
a State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208
West Campus, 2 Ling Gong Rd., Dalian 116024, P. R. China

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Phenyleneanthracene Derivatives as Triplet Energy
Acceptor/Emitter in Red Light Excitable Triplet-
Triplet-Annihilation Upconversion
Fangfang Zhong and Jianzhang Zhao*
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of
Technology, E-208 West Campus, 2 Ling Gong Rd., Dalian 116024, P. R. China
E-mail: zhaojzh@dlut.edu.cn http://finechem2.dlut.edu.cn/photochem
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Abstract: A series of anthracene derivatives with 9,10-substituents were prepared as triplet
acceptors/emitters for triplet-triplet-annihilation (TTA) upconversion. Different linkages of C−C
single bonds and C≡C triple bonds were used to tune the singlet and triplet state energy levels,
which may enhance the TTA upconversion. The study of the photophysical properties of the
compounds indicates that the C−C linker does not alter the T1 state energy level substantially,
whereas the C≡C linker significantly reduced the T1 state energy levels. With nanosecond
transient absorption spectroscopy, the intermolecular triplet-triplet-energy-transfer (TTET)
process was studied. The lack of the upconversion for some anthracene derivatives was attributed
to the inappropriate T1 energy levels thus the lack of TTET. On the other hand, different
upconversion quantum yields were observed for some acceptors, although the TTET processes
are similar. This result is due to the different TTA. These studies will be useful for future
development of the TTA upconversion and for study of the triplet state properties of organic
chromophores.
Keywords: Anthracene; Energy Transfer; Intersystem Crossing; Triplet state; Upconversion;
Triplet Acceptor
1. Introduction
In 1962, Parker and co-workers reported the sensitized anti-Stokes delayed fluorescence [1],
which is the first report of the triplet-triplet annihilation (TTA) upconversion. TTA upconversion
is interesting among the available upconversion methods, in terms of such as low power
excitation [2–5], strong visible light harvesting [6–8], and high upconversion quantum yield [9–
11]. As a result, TTA upconversion is becoming an important method to enhance the
photocatalysis [12–16], photovoltaics [11,17–19] and luminescence bioimaging [20,21].
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Triplet photosensitizers and triplet acceptors are vital for TTA upconversion, which are
responsible for harvesting of the photoexcitation energy and emission of the upconverted
fluorescence emission [22]. Much work has been done to develop triplet photosensitizers [23–
27], however, the investigation of triplet acceptors is limited [22,28,29]. The requirements for
triplet acceptor include at least two aspects, i.e. the energy levels with the relation of 2T1 > S1 and
high fluorescence quantum yield [30]. Previously, polycyclic aromatic hydrocarbons, such as
perylene [31,32], perylenebisimide (PBI) [30,33], and anthracene derivatives [10,16,21,32,34,35],
have been used as triplet emitters/acceptors. However, the triplet state energy of different
anthracene derivatives cannot be easily predicted based on the molecular structures, and
substituents may tune the energy levels substantially. For example, the T1 state of anthracene is
as high as 1.84 eV [36], while the T1 state of 1-chloro-9,10-bis(phenylethynyl)anthracene (A-5)
is only 1.20 eV [32]. Since the triplet state energy level is crucial for the TTA upconversion
quantum yield, it is fundamentally important to study the relationship between the energy levels
and the molecular structures of the anthracene-derived triplet acceptors.
The effect of substitutions on the fluorescence properties of anthracene derivatives have been
studied [37–40], but only little effort has been expended to tune the triplet state energy levels
with different substituents. Anthracene derivatives with phenyl and thienyl substituents at 9,10-
positions were reported [41]. The fluorescence quantum yields of the derivatives were greatly
reduced with attachment of an electron-donating moiety, such as a thienyl moiety, on the
anthracene chromophore, this property is unfavorable to the TTA upconversion [22,28,29].
Moreover, attachment of the aromatic moieties by C−C linker does not significantly reduce the
triplet state energy level, which actually exerts a limitation on the TTA upconversion system, that
is, only triplet photosensitizers with high T1 state energy level can be used. Thus the excitation
wavelength has to be short [41]. The advantage of the previously reported anthracene-derived
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triplet acceptors is that the prerequisite of 2ET1>ES1 can be well met, which is beneficial for TTA
upconversion [41].
In order to fine tune the triplet state energy levels of the anthracene-derived triplet acceptors
and the relation of 2ET1>ES1 energy levels, herein we prepared a series of anthracene derivatives
as triplet acceptors for TTA upconversion. The results indicated that the extension of the π-
conjugation framework by ethynyl bond (C≡C) can inhibit the ISC and lower the triplet state
energy level efficiently, while attachment of substituents by single bond (C−C) does not alter the
T1 state energy level substantially. The upconversion experiments revealed that bigger difference
between the triplet energy levels of the sensitizer and the acceptor makes triplet−triplet energy
transfer (TTET) more efficient, but this effect alone is not sufficient for increasing the TTA
upconversion quantum yield, the TTA process is another important factor dictating the
upconversion efficiency.
2. Results and Discussion
2.1. Design and Synthesis of the Compounds
The T1 state energy level of the anthracene is 1.84 eV [36]. The ISC efficiency (ΦISC) of
anthracene is 77 % [42]. The high T1 state energy level of anthracene chromophore limits its
application in TTA upconversion, because only photosensitizers with higher T1 state are suitable
to be used with anthracene, such as Ru(dmb)3 [dmb = 4,4'-dimethyl-2,2'-bipyridine] [2], PdOEP
[43], PtOEP [36,44], PdTTP complexes [45], thus the excitation wavelength has to be short.
Phenyl substitutions at the 9,10-position of anthracene significantly decrease the ISC efficiency
in 9,10-diphenylanthracene (DPA. ΦISC = 0.04) [46], but the T1 state energy level changes only
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slightly (1.77 eV) [36,46]. In stark contrast, the two phenylethynyl substitutions in 9,10-
bis(phenylethynyl)anthracene (A-4) obviously reduce the T1 state energy level (1.26 eV) and
suppresses the ISC [47], which is beneficial for TTA upconversion. Previously, A-4 [34,46–49],
and A-5 [30,32,50,51] were used as triplet acceptor/emitter for TTA upconversion. However,
fine tuning of the molecular structure of the anthracene derivatives to optimize the triplet state
energy levels have not been reported. Thus we prepared A-1, A-2 and A-3 as triplet
acceptor/emitter (Scheme 1). A few reference compounds A-3', A-4 and A-5 were used for study
of the photophysical properties (Scheme 1). The anthracene derivatives were prepared by
bromination, Suzuki coupling and Sonogashira coupling reactions (Scheme 1). We selected Pt-1
as the triplet photosensitizer, which shows strong absorption of red-light. Previously PBI was
used as triplet acceptor for Pt-1, and TTA upconversion was observed [4].
Scheme 1. Synthesis of the Acceptors and Molecular Structure of the Photosensitizer Pt-1. (a)
n-BuLi, I2, distilled THF, Ar, −78 °C, 4 h, 82.0%; (b) Pd(PPh3)4, CuI, toluene / (i-Pr)2NH, Ar,
55 °C, 24 h, 73.0%; (c) K2CO3, Pd(PPh3)4, Ar, reflux, 8 h, 68.3%; (d) Pd(PPh3)4,, CuI, NEt3, Ar,
reflux, 8 h, 74.8%; (e) Pd(PPh3)4, NEt3 / distilled THF, Ar, reflux, 8 h, 96.3%.
2.2. UV−vis Absorption and Fluorescence Emission Spectra.
The steady-state UV-vis absorption spectra of the compounds were studied (Fig. 1).
Substitutions with phenylethynyl moiety induce red-shifted of the absorption of the anthracene
derivatives. For example, A-3' shows absorption at 367 nm, but A-3 shows absorption at 399 nm.
A-4 and A-5 give the red-shifted absorption at 458 nm and 474 nm, respectively. All the
compounds show vibrational progression in the absorption band in visible region. These
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anthracene derivatives show red-shifted absorption as compared with the previously reported
9,10-substituted anthracene triplet acceptors [41]. This change may suggest that the triplet state
energy level will also decrease.
Fig. 1. The UV−vis absorption spectra of (a) A-1, A-2 and A-3, (b) A-4, A-5 and A-3'. c = 1.0
× 10−5 M in toluene. 20 °C.
Fig. 2. The normalized UV−vis absorption spectra and excitation spectra (determined with
emission wavelength at 520 nm) of the compounds: (a) A-1, (b) A-2, (c) A-3, and (d) A-3', c =
2.0 × 10−6 M in toluene at 20 °C.
The phenylethynyl substitutions on the 9,10-position of anthracene reduce the ISC of
anthracene chromophore, thus the fluorescence quantum yield of A-1, A-2 and A-3 are generally
high (Table 1). We attribute this change to the unmatched S1/T2 state energy level. In anthracene,
which shows ISC efficiency of 77 %, the energy gap between S1 and T2 state is small, thus ISC is
facilitated [52,53]. Recently it was shown that ISC from higher excited states Sn (n > 1) is
possible for other chromophores [54]. Since the ISC is known for anthracene, to study the
possible Sn→Tm ISC (n > 1) of the compounds, the fluorescence excitation spectra of the
derivatives were compared with the absorption spectra (Fig. 2) [54]. We found that upon
excitation in the UV region, i.e. by excitation into the S4 state, the fluorescence is weaker than
that upon photoexcitation into the S1 state, ISC may be the deactivation channel of the S4 state
[54]. Upon excitation at 286 nm, the singlet oxygen quantum yield (Φ∆) of A-1, A-2, and A-3
were determined as 0.47, 0.43 and 0.24, respectively (Table 1). With excitation into the S1 state
(ca. 420 nm), however, the singlet oxygen quantum yields are much lower. We propose this
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result may be due to the small S4/T9 energy gap (see later section), as a result the coupling of the
singlet and triplet states is strong, and thus ISC is enhanced. Therefore, derivatization of organic
chromophore may alter the match of the energy levels of the singlet excited states and triplet
excited states, which exert a significant effect on the fluorescence as well as ISC properties of the
compounds [55]. The comparison of the fluorescence excitation spectra and the UV-Vis
absorption of anthracene indicated that the ISC is more efficient upon promotion to high-lying
singlet excited states (see ESI †, Fig. S2).
Fig. 3. The emission spectra of (a) A-1 (λex = 400 nm, A = 0.14), (b) A-2 (λex = 440 nm, A =
0.23), (c) A-3 (λex = 400 nm, A = 0.14). Optically matched solutions were used, c = ca. 1.0 ×
10−5 M−1, 20 °C.
The fluorescence properties of the compounds were studied (Fig. 3). For A-1, the emission
band is with vibrational progress, which is similar to that of anthracene. This means the vibration
is strongly coupled to the electronic transition. Furthermore, the emission wavelength and the
emission intensity do not change in different solvents, i.e. the emission is independent of the
solvent polarity. Thus we propose there is no significant intramolecular charge transfer character,
and the dipole moment of the molecule does not change significantly upon excitation from the
ground state to the excited state. Similar results were observed for A-2 and A-3 (Fig. 3b and 3c)
and A-4 and A-5 (see ESI †, Fig. S3).
Fig. 4. The emission spectra of in optically matched solutions: (a) A-1, A-2 and A-3, (b) A-4,
A-5 and Py, λex = 400 nm, A400 nm = 0.14, in deaerated toluene, 20 °C.
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The fluorescence emission spectra of the compounds were compared (Fig. 4). With optically
matched solutions, we found that the fluorescence emission intensity of the compounds is
different (Table 1). The fluorescence quantum yields of the compounds are generally higher than
80%, which are ideal for TTA upconversion. On the other hand, the fluorescence emission
wavelengths vary significantly for the compounds. Generally, with more substituents on
anthracene chromophore, red-shifted fluorescence emission was observed. For example, the
fluorescence of A-3 is at 429 nm, but the fluorescence of A-1 is red-shifted to 444 nm.
Connection of substituents with ethynyl substituent induces more red-shift, indicated by A-2,
which shows emission at 454 nm. Moreover, we found that one atom substitution can induce
significant shifting of the fluorescence. For example, A-4 shows fluorescence emission at 476 nm.
But with a chloro-substituent, A-5 shows red-shifted fluorescence emission at 494 nm.
Table 1. Photophysical Properties of the Triplet Energy Acceptors
2.2 Electrochemical Studies: Cyclovoltametry
Since the anthracene derivatives were designed as triplet state acceptors, for which
intermolecular triplet-triplet-energy-transfer is involved. Electron exchange (Dexter mechanism)
is responsible for the TTET, thus the redox property of the triplet acceptors may exert influence
on the upconversion with these triplet photosensitizers. Thus the redox properties of the
anthracene derivatives were studied with cyclovoltammetry (Fig. 5).
Fig. 5. Cyclic voltammograms of (a) A-1, A-2 and A-3, (b) A-4 and A-5. Ferrocene (Fc) was
used as internal reference (E1/2 = + 0.17 V, Fc+/Fc). In deaerated CH2Cl2 solutions containing
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1.0 mM photosensitizers with the ferrocene, 0.10 M Bu4NPF6 as supporting electrolyte,
Ag/AgNO3 as reference electrode. Scan rates: 100 mV/s, 20 °C.
Table 2. Redox Potentials of the Compounds with Fc as the Internal Reference, for which E1/2
(Fc+/Fc) = + 0.17 V vs. Ag/AgNO3 Reference Electrode
Semi-reversible oxidation waves were observed for A-1 − A-3. But the reduction wave varied
for the three compounds. With more ethynyl substituents, the reduction wave moves anodically
by 0.3 V. Both A-1 and A-3 show similar reduction waves at −2.1 V. The oxidation waves of A-
4 and A-5 become irreversible. Semi-reversible reduction waves at −1.8 V and −1.7 V were
observed for A-4 and A-5.
2.3. Triplet-Triplet-Annihilation Upconversion
The anthracene derivatives were used as triplet acceptor/emitters for TTA upconversion (Fig.
6). A Bodipy-Pt(II) complex Pt-1 was used as the triplet photosensitizer [4]. The excitation is
with red laser at 635 nm. With perylene as triplet acceptor/emitter, weak upconversion was
observed in the wavelength range of 450 nm − 600 nm. With A-5, very weak upconversion was
observed, but A-4 gives much higher upconversion quantum yield (Fig. 6). A-1 and A-3 did not
show any upconversion capability. A-2 also gives strong upconversion.
Fig. 6. The upconversion emission spectra of the acceptors sensitized by Pt-1 (c = 1.0 × 10−5
M) in deaerated toluene, excited by a 635 nm laser (10 mW, power density is 35.4 mW cm−2),
at the optimized concentrations: c [sensitizer] = 1.0 × 10−5 M, c [A-1] = 1.5 × 10−4 M, c [A-2]
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= 1.0 × 10−4 M, c [A-3] = 1.5 × 10−4 M, c [A-4] = 2.0 × 10−5 M, c [A-5] = 2.0 × 10−5 M, c [Py]
= 1.5 × 10−4 M, 20 °C.
The upconversion quantum yields with A-2, A-4 and A-5 as triplet acceptors were determined
as 11.8%, 11.8% and 1.34%, respectively. Under the same experimental condition, the
upconversion with PBI was determined as 4.7%. Moreover, Although A-2 shows the same
upconversion quantum yield with that of A-4, but the anti-Stokes shift of the upconversion with
A-2 is more significant. For A-1 and A-3, no TTA upconversion was observed, because their T1
state energy levels are close to the triplet photosensitizer (see later section).
Table 3. Upconversion-Related Parameters of the Acceptors
2.4. Nanosecond Transient Absorption Spectra: Intermolecular TTET
The intermolecular triplet−triplet state energy transfer (TTET) between the photosensitizer Pt-
1 and the triplet acceptors, exemplified by A-2, was studied with the nanosecond transient
absorption spectra (Fig. 7). In the presence of A-2, the transient signal of Pt-1 was quenched, e.g.
indicated by the ground state bleaching band at 635 nm. At the same time, the characteristic
ground state bleaching band of the triplet energy acceptor A-2 at 450 nm developed (where A-2
gives steady state UV−Vis absorption. Fig. 1). These results indicate that the triplet state of A-2
was populated via the TTET process. It should be noted that the ground state absorption of A-2 is
much weaker than that of Pt-1, thus the ground state bleaching band of A-2 is much weaker than
that of Pt-1. The intermolecular TTET kinetics was followed by monitoring the development and
the decay of the transient at 450 nm, which indicates the production and the decay of the triplet
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excited state of A-2. Firstly there is a rising phase in the decay curve, with a rate constant of kTTET
= 4.5 × 104 s−1 (1 / k = 22 µs), then followed by a slow decaying phase with rate constant of kTTET
= 2.0 × 103 s−1 (1/k = 489 µs). The rising phase is the production of the triplet state of A-2 via the
intermolecular TTET, whereas the second phase is mainly the decay of the triplet state of A-2
(Fig. 7e). More significant intermolecular TTET was observed for triplet acceptor A-4 (Fig. 7f).
The photophysical parameters related to upconversion were summarized in Table 2.
Fig. 7. Intermolecular TTET: the nanosecond transient absorption spectra (a, b, c and d) and
the decay traces (e and f) of the mixture of Pt-1 and A-2 or A-4. The concentration of Pt-1
was fixed at 1.0×10−5 M, while the concentration of A-2 was varied at (a) 0 M, (b) 5.0 × 10−6
M, (c) 2.0 ×10−5 M and (d) 4.0 ×10−5 M. (e) 2.5 ×10−5 M of A-2, detected at 450 nm, (f) 3.5 ×
10−5 M of A-4, detected at 490 nm,
λex = 635 nm, in deaerated toluene, 20 °C.
In order to clarify the reason for different TTA upconversion of the triplet acceptors, the
quenching of the triplet state lifetime of the photosensitizer by the anthracene triplet acceptors
was quantitatively studied and the Stern-Volmer quenching plots were constructed (Fig. 8).
Fig. 8. Stern−Volmer plots of the quenching of the triplet state of complex Pt-1 (c = 1.0 ×
10−5 M), with A-2, A-4, A-5, or Py as the quenchers. In deaerated toluene, λex = 635 nm, 20
°C.
A-1 and A-3 did not show any quenching capability toward the triplet state of Pt-1, i.e. there is
no efficient TTET between Pt-1 and A-1, or A-3, thus no TTA upconversion was observed for
A-1 and A-3. For perylene, the Stern-Volmer constant is also small (KSV = 8.0 × 103 M−1), which
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is accounted for the relative poor TTA upconversion with perylene as the triplet acceptor (Fig. 6
and Table 3). On the other hand, large Stern-Volmer constants were observed for A-2, A-4 and
A-5, indicating that the TTET between the triplet photosensitizer and these acceptors are efficient.
However, efficient upconversion was observed for A-2 and A-4, weak upconversion was
observed for A-5. Thus, we propose the inefficient TTA is responsible for the low upconversion
quantum for A-5.
2.5. DFT Calculations: Excited State Energy Levels
No phosphorescence of the anthracene derivatives was observed at 77 K, thus DFT/TDDFT
computations were used for estimation of the triplet state energy levels. Firstly the ground state
geometry, frontier molecular orbitals and the electronic excited states were studied with DFT and
TDDFT calculations (Fig. 9). For A-1, we found that the phenyl moiety, connected with the
anthracene via C−C single bond, assumes a perpendicular geometry to the anthracene core. On
the other hand, the phenyl moiety connected with the anthracene moiety with the C≡C triple bond
is in a coplanar geometry with the anthracene core. As a result, we found that the HOMO and
LUMO are both spread on the co-planar part of A-1, the phenyl ring takes the orthogonal
geometry, and does not contribute to the HOMO and LUMO orbitals.
Fig. 9. Electron density maps of the frontier molecular orbitals of the acceptors based on the
optimized ground state geometry, calculated by DFT at the B3LYP/6-31G level with Gaussian
09W.
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We postulate that the attachment of the orthogonal phenyl ring does not change the T1 state
energy level significantly, which was confirmed by the later calculation on the T1 state energy
levels. For A-2, the HOMO and LUMO are distributed on the whole molecule. Similar results
were observed for A-3, A-4 and A-5. No significant charge transfer character was observed for
these compounds, based on the distribution of the frontier MOs, which is in agreement with the
experimental results that the fluorescence emission intensity and wavelength are independent on
the solvent polarity.
Fig. 10. Isosurfaces of spin density of the acceptors at the optimized triplet states, (a) A-1; (b)
A-2; (c) A-3; (d) A-4; (e) A-5, and (f) A-3', calculated by DFT at the B3LYP/6-31G level
with Gaussian 09W.
The spin density surfaces of the compounds were studied (Fig. 10). The spin density surfaces
of the triplet acceptors were spread on the whole molecules, which is different from that of the
singlet state (S0 or S1 state).
Table 4. Electronic Excitation Energies and Corresponding Oscillator Strengths (f), Main
Configurations and CI Coefficients of the Low-Lying Electronic Excited States of A-1
Fig. 11. The singlet and triplet energy levels of the triplet acceptors, based on the
DFT//B3LYP/6-31G−optimized ground state geometries. Calculated by TDDFT at the
B3LYP/6-31G level for the acceptors, and B3LYP/LANL2DZ level for the sensitizer with
Gaussian 09W.
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The calculated energy levels of the lowest singlet and triplet excited states were summarized in
Fig. 11. In general, A-1 and A-3 give higher T1 state energy levels than the other triplet acceptors,
indicating the TTET between A-1 or A-3 is less efficient than that of A-2, A-4 or A-5. This
postulation is in agreement with the ns TA studies, as well as the TTA upconversion. On the
other hand, A-4 and A-5 possess lower triplet state energy level, which is beneficial for TTET.
This postulation was confirmed by the higher KSV values of A-4 and A-5. However, for A-5, the
2ET1 is less than the ES1, thus the TTA is thermodynamically unfavorable. Experimentally very
weak upconversion was observed for A-5. Both A-2 and A-4 meet the prerequisites of
ET(sensitizer) > ES(acceptor) and 2ET1 ≥ ES1 for TTA upconversion, and they have the same high
upconversion quantum yield (ΦUC = 11.8%), however, ΦF and ΦTTET of A-2 are much lower than
A-4, thus ΦTTA is much higher for A-2. These results indicates that fine tune of the triplet and
singlet excited state of anthracene derivatives is important for these compounds to be used as
triplet acceptors for TTA upconversion.
3. Conclusions
In summary, a series of anthracene derivatives were prepared as triplet acceptors for triplet-
triplet-annihilation (TTA) upconversion. The aim is to optimize the upconversion quantum yields
by tuning the T1 and S1 state energy levels by modification of the molecular structures. Different
substituents and different linkers were used for preparation of the anthracene triplet acceptors.
The photophysical properties of the compounds were studied with steady-state and time-resolved
spectroscopies, electrochemical characterization and DFT calculations. We found that attachment
of substituents with C−C single linker does not alter the T1 state energy level substantially, but
the C≡C triplet bond linker significantly reduced the T1 state energy level, which is proved to be
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similar to the switching of S1 state energy level. With nanosecond transient absorption
spectroscopy, the intermolecular triplet-triplet-energy-transfer (TTET) was studied and the lack
of the upconversion for some anthracene derivatives was attributed to the lack of TTET, due to
the unmatched triplet state energy level as compared to the triplet photosensitizer used. On the
other hand, some anthracene derivatives did demonstrate the same TTET, but weak upconversion
was observed, for which the poor TTA is responsible. The triplet state energy levels were
calculated with TDDFT methods. These studies will be useful for future development of the TTA
upconversion and for study of the photochemistry of organic chromophores.
4. Experimental Section
4.1. General Information
All of the chemicals mentioned in the paper were of analytical purity and were used as received.
Solvents were dried by 4A molecular sieves before synthesis. Compounds 2, 3 [56], and A-4 [57],
and complex Pt-1 [4] were prepared exactly according to the reported procedures.
The NMR spectra were recorded by a Bruker spectrometer (400 MHz or 500 MHz for 1H NMR,
100 MHz for 13C NMR) with CDCl3 as solvent and TMS as standard at 0.00 ppm. High
resolution mass spectra (HRMS) were determined in GCT-MS or MALDI-TOF-MS system.
Emission spectra were recorded with a FS5 spectrofluorometer (Edinburgh Instrument Ltd., UK)
while the UV-vis absorption spectra were measured on UV2550 UV−vis spectrophotometer
(Shimadzu, Japan). The Fourier transform infrared (FT−IR) spectroscopy was recorded on a
Thermo Fisher Infrared Spectrometer (6700) by pelleting the sample with KBr powder.
4.2. Nanosecond Transient Absorption Spectroscopy
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All of the nanosecond transient absorption spectra were recorded in N2 saturated toluene, on a
LP920 laser flash photolysis spectrometer (Edinburgh Instruments, Livingston, UK), and the
signals were digitized on a Tektronix TDS 3012B oscilloscope.
4.3. Cyclic Voltammetry
The electrochemical curves were recorded by a three-electrodes electrolytic cell on the
CHI610D workstation (Shanghai, China). The measurements were performed at a 100 mV/s scan
rate at room temperature. Tetrabutylammonium hexafluorophosphate (Bu4N[PF6], 0.1 M) was
used as the supporting electrolyte, glassy carbon electrode as the working electrode, and platinum
electrode as the counter electrode. N2 saturated dichloromethane was used as the solvent with
ferrocene (Fc) for the internal reference.
4.4. Triplet-triplet annihilation upconversions
Diode pumped solid state (DPSS) continuous lasers with a 6 mm diameter (635 nm, Xi’an
Minghui Optoelectronic Technology Co., Ltd., China) was used for the TTA upconversion
experiments. The power of the laser was measured by a VLP-2000 pyroelectric meter. A N2
saturated mixed solution (with triplet photosensitizer and acceptor) was used and the
upconversion emission was recorded with a RF 5301PC spectrofluorometer. The upconversion
quantum yields (ΦUC) were determined by the reported method [30].
4.5. Singlet Oxygen Quantum Yield
The singlet oxygen (1O2) generation quantum yields (Φ∆) were measured and calculated by a
similar method to literature [58]. Naphthalene in hexane (Φ∆ = 0.50) was used as the standard
1
[59], 1,3-diphenylisobenzofuran (DPBF) as the O2 acceptor. Adjust the concentration of the
solutions, so that the absorbance of the solutions of standard and the sample at the intersection of
the absorption spectra reaches around 0.2 − 0.3 in hexane. The intersection of the absorption
16

ACCEPTED MANUSCRIPT
spectra was chosen as the excitation wavelength. The concentration of DPBF was set so that the
absorbance at 411 nm was around 1.0 in hexane in a 1 cm cuvette. A mixed solution of DPBF
and the sample or the standard was exposed to a monochromatic light at the excitation
wavelength for 2 min. The absorbance at 411 nm was recorded after each irradiation. A curve of
the absorbance against the increasing irradiation time was constructed. The slopes of curve
correlating the absorbance at 411 nm to irradiation time were calculated, and the singlet oxygen
quantum yields were calculated according to equation 1:
ꢃ ꢀꢁꢂꢃ ꢄ
ꢀ
ꢄ
ꢀ
ꢄ
∅Δ ꢁꢂꢃ = ∅Δ ꢁꢅꢆ ×
(1)
ꢃ ꢀꢁꢅꢆ ꢄ
where sam and std designate the sample and the standard, respectively.
absorbance difference in change at 411 nm with the irradiation time.
4.6. DFT Calculations
m is the slop of
The geometry optimization was calculated by using the B3LYP functional, 6-31G basis sets,
while the vertical excitation energy was calculated with the time-dependent DFT (TD-DFT)
method based on the singlet ground-state geometry. All the calculations were performed with
Gaussian 09W software (Gaussian, Inc.) [60].
4.7. Synthesis of A-1
An aqueous solution of K2CO3 (2 mL, 2 mol/L) was added to the solution of compound
3 (178
mg, 0.5 mmol) and phenylboronic acid (85 mg, 0.7 mmol) in mixed solvent of toluene (10 mL)
and ethanol (4 mL). After bubbling with argon for 15 min, Pd(PPh3)4 (7 mg, 0.06 mmol) was
added, and the mixture was refluxed for 8 h. Then the reaction mixture was extracted by
dichloromethane, and the organic layer was dried over anhydrous Na2SO4. The solution was
concentrated under reduced pressure to attain the crude product. Chromatography (silica gel,
17

ACCEPTED MANUSCRIPT
petroleum ether) afforded purified A-1 as light yellow solid (121 mg, 68.3%). M.p. 207.6
− 208.0
°
C. 1H NMR (400 MHz, CDCl3):
δ
= 8.74 (d,
7.37 (m, 6H) ppm. The H NMR data is in good
J = 8.4 Hz, 2H), 7.81 (d, J = 8.0 Hz, 2H), 7.68 (d, J
1
= 8.4 Hz, 2H), 7.61 7.54 (m, 6H), 7.47
−
−
agreement with literature [57]. GCT-HRMS: calcd ([C28H18]+)
m/z = 354.1409, found m/z =
354.1408.
4.8. Synthesis of A-2
Under Ar atmosphere, compound
CuI (5 mg, 0.025 mmol) were mixed in triethylamine (10 mL). After addition of
trimethylsilylacetylene (141 L, 1 mmol) via syringe, the mixture was refluxed for 8 h. After
3 (178 mg, 0.15 mmol), Pd(PPh3)4 (7 mg, 0.06 mmol) and
µ
extracted by dichloromethane, dried with anhydrous Na2SO4 and filtered, the organic layer was
concentrated by reduced pressure. The product was purified by column chromatography (silica
1
gel, petroleum ether) and collected as orange solid (140 mg, 74.8%). M.p. 118.5
−
119.7
°
C. H
NMR (400 MHz, CDCl3) δ = 8.69−8.58 (m, 4H), 7.64−7.60 (m, 4H), 7.48−7.42 (m, 3H), 0.43 (s,
13
9H) ppm. C NMR (100 MHz): 132.4, 132.0, 131.7, 128.7, 128.6, 127.3, 127.2, 126.9, 126.8,
123.4, 118.7, 118.2, 108.1, 102.4, 101.6, 86.4, 0.2 ppm. GCT-HRMS: calcd ([C27H22Si]+)
IR: 3402, 2922, 2136, 1623, 1383, 1246, 754, 692 cm−1.
m/z =
374.1491, found m/z = 374.1497. FT−
4.9. Synthesis of A-3
A-3 was synthesized by a similar method to the literature [39]. Under Ar atmosphere, 9-
bromoanthracene (500 mg, 1.94 mmol), phenylacetylene (427 L, 3.89 mmol) and Pd(PPh3)4
µ
(156 mg, 0.10 mmol) were dissolved in mixed solvent of triethylamine (2 mL) and distilled THF
(5 mL). The mixture was refluxed for 6 h. The solvent was removed under reduced pressure and
the product was purified by column chromatography (silica gel, petroleum ether). Yellow solid,
1
520 mg, 96.3%. M.p. 110.3
−
111.8
°
C. H NMR (400 MHz, CDCl3):
δ
= 8.67 (d,
J
= 8.5 Hz,
18

ACCEPTED MANUSCRIPT
2H), 8.44 (s, 1H), 8.03 (d,
J
= 8.6 Hz, 2H), 7.79−7.77 (m, 2H), 7.62−7.59 (m, 2H), 7.53−7.51 (m,
2H), 7.47
−
7.40 (m, 3H) ppm. GCT-HRMS: calcd ([C22H14]+)
m/
z
= 278.1096, found m/z =
278.1097.
Acknowledgement
We thank the NSFC (21273028, 21473020, 21673031 and 21421005), and Ministry of
Education (SRFDP-20120041130005), Program for Changjiang Scholars and Innovative
Research Team in University [IRT_13R06], and Dalian University of Technology
(DUT2016TB12) for financial support.
Supporting Information
1
The H, 13C NMR data, HRMS spectra, the photophysical spectra and DFT/TDDFT
calculations of the compounds are available in the Supporting Information.
Author Information
Corresponding Author
*E-mail: zhaojzh@dlut.edu.cn
Notes
The authors declare no competing financial interest.
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Table 1. Photophysical Properties of the Triplet Energy Acceptors
a
a
λabs
ε b
λem
τ c
ΦF / % d
76
A-1
A-2
A-3
A-3'
A-4
A-5
Py
407 / 430
427 / 454
399 / 421
367/ 387
439 / 458
450 / 474
412 / 439
1.66 / 1.57
2.00 / 2.33
1.42 / 1.30
1.97 / 1.84
2.32 / 2.45
0.35 / 0.39
2.71 / 3.61
444 / 469
454 / 484
429 / 455
398 / 419
476 / 508
494 / 525
444 / 473
3.85 ns
3.72 ns
4.22 ns
5.34 ns
3.05 ns
3.52 ns
3.87 ns
79
85
52 e
100
99
98
a In toluene (1.0 × 10-5 M) , 20 °C.
b Molar extinction coefficient at the absorption maxima.
ε
: 104 M-1 cm-1.
c Luminescence lifetimes, studied by time-correlated single photon counting (TCSPC).
d Fluorescence quantum yields in hexane, perylene (ΦF = 0.98 in hexane) as standard.
e Fluorescence quantum yield in ethanol, anthracene (ΦF = 0.30 in ethanol) as standard.
1