Investigation of heavy atom effect on the low power upconversion

Article abstract of DOI:10.14233/ajchem.2014.17335

Selective low energy excitation of palladium(II)tetraphenylporphyrin (PdTPP) and palladium(II)tetrabromophenylporphyrin (PdBrTPP) in the presence of 2-chloro-9,10-di p-tolylanthracene (DTACl), respectively, can yield easily visualized upconversion fluores

Full text of DOI:10.14233/ajchem.2014.17335

Asian Journal of Chemistry; Vol. 26, No. 6 (2014), 1721-1725
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.17335
Investigation of Heavy Atom Effect on the Low Power Upconversion†
*
*
BAO WANG, CHANGQING YE , ZUOQIN LIANG, YUYANG ZHOU, PING DING and XIAOMEI WANG
Jiangsu Key Laboratory for Environment Function Materials, College of Chemistry, Biology and Material Engineering, Suzhou University of
Science and Technology, Suzhou 215009, P.R. China
*
Corresponding author: E-mail: wangxiaomei@mail.usts.edu.cn; yechangqing@mail.usts.edu.cn
Published online: 10 March 2014;
AJC-14876
Selective low energy excitation of palladium(II)tetraphenylporphyrin (PdTPP) and palladium(II)tetrabromophenylporphyrin (PdBrTPP)
in the presence of 2-chloro-9,10-di p-tolylanthracene (DTACl), respectively, can yield easily visualized upconversion fluorescence. The
external and internal heavy atom effects were firstly found to significantly increase upconversion efficiency (Φuc) of emitter (DTACl)
2
doped with PdTPP or PdBrTPP as much as 10 % under excitation of low power density at 60 mW/cm .
Keywords: Low-power upconversion, Triplet-triplet annihilation, Two-photon process, Photoelectrochemistry.
Based on this idea, we studied two different Pd(II)pho-
phyrins by peripheral substitution to examine the influence of
heavy atom effect upon upconversion. Extensionally, the
solvents with heavy atom such as bromobenzene (PhBr) is
also used to examine the intermolecular heavy atom effect on
the upconversion. The results obtained have shown that the
substitution of Pd(II)tetrabromophenylporphyrin (PdBrTPP)
for Pd(II)tetraphenylporphyrin (PdTPP) leads to 10 % enhan-
cement of upconversion efficiency, due to effectively increasing
INTRODUCTION
Upconversion, that is, the processes of short-wavelength
radiation from long-wavelength light sources, has been inten-
sively studied due to its potential application in fields such as
1
photovoltaics , photocatalysis , biological imaging and sensing
2-4
5
6,7
and photodynamic therapy of cancer . A few techniques for
upconversion have been well established, such as the two
photon absorption. Two-photon absorption (TPA) is a nonlinear
optical process of a molecule from the ground state to the
excited singlet by simultaneous absorption of two photons,
p
phosphorescence lifetime (t ) and phosphorescence quantum
yield (Φp). For example, the upconversion values of bimolecular
composition in DMF were obtained in the order of PdBrTPP/
DTACl (16.88 %) PdTPP/DTACl (10.33 %) under low-powered
8,9
which needs to get across intra-molecular virtual state . How-
ever, it has always been associated with the use of very high
2
intensity light excitation on the order of MW-GM/cm that
2
density (532 nm, 60 mW/cm ). On replacing internal heavy
10-12
has led to the serious application limitation . In comparison
to two-photon absorption upconversion (TPA-UC), a new
upconversion technology that is triplet-triplet annihilation
upconversion (TTA-UC) has been developed. Triplet-triplet
annihilation upconversion requires a triplet sensitizer and an
acceptor to accomplish the cascade processes of light-harves-
ting, triple-triplet energy transfer (TTT), TTA and upcon-
verted fluorescence emission. It can be obtained at the very
atom effect with external heavy atom effect (i.e., solvent heavy
atom effect), the upconversion values of bimolecular compo-
sition in bromobenzene (PhBr) are in the order of PdBrTPP/
DTACl (27.21 %), PdTPP/DTACl (20.04 %). To the best of
our knowledge, the current study represents the first example
of the external heavy atom effect as a simple approach to
increase the upconversion efficiency.
2
EXPERIMENTAL
low intensity of excitation source (as low as 1 W/cm ), which
is of importance for the future applications. Over the past few
years, tremendous advances have been made on the
development of various sensitizers and acceptor to achieve
Synthesis of Pd(II) tetra-(4-phenyl)-porphine (PdTPP):
2.5 g benzaldehyde was added to 45 mL dried propionic acid
acid. After complete addition, they were reuxed at 140 ºC.
3 mL newly distilled pyrrole which was dissolved in 45 mL
1
3-16
higher upconversion efficiency
.
†Presented at The 7th International Conference on Multi-functional Materials and Applications, held on 22-24 November 2013, Anhui
University of Science & Technology, Huainan, Anhui Province, P.R. China

1
722 Wang et al.
Asian J. Chem.
dried propionic acid was spotted with slowly. After refluxing
for 45 min, the turbid solution changed from white to purplish
red, the solution is cooled to room temperature and filtered
and the filter cake is washed thoroughly with methanol. Tetra-
anhydrous magnesium sulfate. Removing the solvent afforded
a yellow oil, which was used without further purification.
2-Chloro-9,10-di p-tolyl-9,10-dihydroanthracene-9,10-
diol (4.05 g, 9.0 mmol), sodium phosphinate monohydrate
(16.6 g, 157 mmol) and potassium iodide (15.8 g, 95.0 mmol)
in acetic acid (40 mL) were reuxed at 120 ºC for 3 h. After the
mixture was cooled, water was added to it. The crude solid
product was filtered and washed with water. The crude product
(
4-phenyl)-porphine and palladium dichloride were mixed in
benzonitrile (20 mL), the mixture was refluxed under the
nitrogen atmosphere for 3 h.After extracting, the crude product
was purified through flash column chromatography using
petroleum/dichloromethane (v/v = 3:1) as eluent to give a purple
1
solid (Scheme-I). Yield (2.11 g, 47.3 %). H NMR (CDCl
was recrystallized using acetic acid to give a light yellow solid.
1
Yield (3.27 g, 50.6 %). H NMR (CDCl
3
,
3
, 300 MHz, ppm) δ:
2.35 (s, 6H, CH ), 7.20-7.72 (d, 15H,Ar-H). Elemental analysis:
3
1
00 MHz, ppm) δ: H NMR (400 MHz, TMS): δ 8.83 (s, 8H,
3
thiophene-H),8.19, 8.21(d, 8H, benzene-H), 7.76,7.78(d, 12H,
benzene-H).
Synthesis of Pd(II) tetra-(4-bromophenyl)-porphine
calcd. (%) for C28
H
21Cl: C, 85.59; H, 5.39; Cl, 9.02. Found
(%): C, 85.62; H, 5.52; Cl, 8.86.
Spectral characterization and triplet-triplet annihi-
lation upconversion measurements: Solvent DMF and
(PdBrTPP): A procedure analogous to the preparation of
PdTPP was used, but instead of starting from 4-bromobenzal-
1
bromobenzene was purified before used. The H NMR spectra
dehyde (1.84 g, 10.0 mmol). PdTPP was obtained as a purple
1
solid (Scheme-II). Yield (1.32 g, 48.9 %). H NMR (CDCl
was recorded at 25 ºC using a BrukerAvance 300 MHz spectro-
meter. Elemental analyses were performed on an Elementar
Vario EL-III instrument. Linear absorption measurements of
dilute solution (8 mM) have been measured with Hitachi
U-3500 recording spectrophotometer from quartz cuvettes of
1 cm path. Steady-state emission and time-resolved decay
curves were measured on an Edinburgh FLS 920 fluoropho-
tometer equipped with time-correlated single-photon counting
(TCSPC) card. With the aid of nF 900 software, phosphore-
3
,
1
00 MHz, ppm) δ: H NMR (400 MHz, TMS): 8.83 (s, 8H,
3
thiophene-H),8.04, 8.06 (d, 8H, benzene-H), 7.91,7.93(d, 8H,
benzene-H).
Synthesis of 2-chloro-9,10-di p-tolylanthracene
(
DTACl): Under N
mL, 62.0 mmol) was added with dropwise to a mixture of
-bromo-4-methylbenzene (5.50 mL, 41.0 mmol) and 40 mL
dried THF. After complete addition, stirring is continued for
.5 h. At about 40-50 ºC, a solution of 2-chloroanthraquinone
5 g, 30.8 mmol) in 60 mL THF was injected through a syringe.
2
atmosphere at -78 ºC, 1.6 M t-BuLi (25.0
1
p
scence lifetime (τ ) for sensitizers were measured under detection
0
of nF-lamps; and then obtained by reconvolution fit analysis
of the decay profiles. In the fluorescence and phosphorescence
decay profiles, the monoexponential fit gives acceptable
(
The turbid solution changed from white to wine red. The mixture
was allowed to warm to room temperature and maintained for
2
2
statistics parameters of χ < 1.1, where χ is the “reduced chi-
square”. The phosphorescence lifetime (τ ) measurements were
done at the room temperature under nitrogen atmosphere.
24 h. The reaction mixture was added to a saturated aqueous
p
solution of NH Cl (120 mL). The organic solvent was evaporated
4
and the suspension was extracted with ethyl acetate. The
organic extracts were washed by water and brine and dried by
Diode pumped solid state laser (emission wavelength: 532
-2
nm, 60 mW cm ) was used as the excitation source for the
Scheme-I: Synthesis route of Pd(II) tetra-(4-phenyl)-porphine (PdTPP) and Pd(II) tetra-(4-bromophenyl)-porphine (PdBrTPP)
OH
t-BuLi
2 2 2
NaH PO ·H O
Br
+
O
O
THF
KI HAc
Cl
Cl
HO
DTACl
Scheme-II: Synthesis route of 2-chloro-9,10-di p-tolylanthracene (DTACl)

Vol. 26, No. 6 (2014)
Investigation of Heavy Atom Effect on the Low Power Upconversion 1723
upconversion. The laser power was measured with photodiode
detector. For the upconversion experiments, the mixed solution
1.0
(a)
2
of the sensitizer and emitter was degassed for 10 min with N .
Then the solution was excited with laser. The upconverted
fluorescence was observed with PR655 SpectraScan colori-
meter.
0.8
PdTPP
PdBrTPP
DTACl
0.6
0.4
0.2
0
The upconversion efficiency (UC) of DTACl in presence
of different sensitizers was obtained relative to rhodamine 6G
in ethanol using 532 nm excitation according to the literature.
2

A  F  η 
r
s
s
ΦUC = 2Φ 
 

r

 

A
F
η

s  r  r 
350
400
450
500
550
600
where the subscript ‘‘s’’and ‘‘r’’represent sample and reference
respectively (here rhodamine 6G in ethanol at concentration
of 0.5 mM was used as reference and DTACl as the sample).
Wavelength (nm)
r
Φ is the fluorescence quantum yield of rhodamine 6G
1
0
0
0
0
.0
.8
.6
.4
.2
0
(
b)
17,18
(0.88) in ethanol using 532 nm excitation. F is the integrated
emission of DTACl and rhodamine 6G under the excitation of
PdTPP
PdBrTPP
DTACl
532 nm. A is the absorbance of both DTACl and rhodamine
6G at 532 nm, while η is the refractive index of solvent used.
The factor 2 accounts for the fact that two absorbed photons
are required to produce one upconverted photon.
RESULTS AND DISCUSSION
Absorption and emission properties: To investigate their
photophysical properties, the absorption and fluorescence
spectra of PdTPP (8.0 µM), PdTPP (8.0 µM) and DTACl
-6
400
450
500
550
600
650
700
750
(
10 M) were measured in DMF solution, as shown in Fig. 1a.
Two Pd(II)porphyrins show the Q-band in the range of 500-
40 nm that can overlap with the diode pumped solid state
Wavelength (nm)
Fig. 1. Normalized absorption (a) and emission (b) spectra of PdTPP,
PdBrTPP and DTACl (DMF solvent, 8 µM, 1 µM)
5
laser (532 nm). At the same time, the acceptor DTACl shows
the characteristic vibronic pattern of the isolated anthracene
group at 359, 380 and 400 nm in DMF solution. The molar
extinction coefficient (ε) at 532 nm is in the order of PdTPP
2.8
DMF
PhBr
4
-1
-1
4
-1
-1
2.4
(
1.5 × 10 M cm ) > PdBrTPP (0.42 × 10 M cm ) in DMF.
2.05
Interestingly, the solvent heavy atom effect can be significantly
1.78
2
.0
.6
increasing the ε values at 532 nm. As can be seen in Fig. 2,
1.50
4
-1
-1
sensitizer PdBrTPP (ε 1.78 × 10 M cm ) exhibit 4-5-fold
increase in PhBr with respect to those in DMF, showing very
high light harvesting (ε) to the excitation source in the solvent
with heavy atom effect.
1
1.2
0
.8
.4
0
0.42
Their emission bands are centered at 441 nm for DTACl
(
Fig. 1b). Meanwhile, emission spectra of Pd(II)porphyrins
in DMF shows that there are dual fluorescence (locating at
60 and 610 nm) and dual phosphorescence (locating at 650
and 720 nm) under the excitation of Q-band wavelength at
room temperature and N atmosphere. It is observed that the
0
5
PdTPP
PdBrTPP
Fig. 2. Molar extinction coefficient in different solvents of sensitizers (the
concentrations are at 8 µM)
2
peripheral substitution from H to Br, the phosphorescence
spectra are enhanced in the order of PdBrTPP > PdTPP. All of
these have proven that Pd(II)tetraphenylphophyrins by
peripheral substitution with bromine atom (internal heavy atom
effect) and in the solvent with heavy atom (external heavy
atom effect) exhibit significant improvement in phosphore-
scence behaviors. The emission quantum yield (Φp), lifetime
TABLE-1
ABSORPTION AND FLUORESCENCE
PROPERTIES OF SENSITIZER PALLADIUM(II)
TETRAPHENYLPORPHYRINS IN DMF (8.0 µM)
*
-1
-1
fluo
λmax (nm)
Sensitizer Soret-band Q-band ε (M cm )
4
PdTPP
416 nm
417 nm
522 nm
523 nm
1.5 × 10
564, 609
561, 607
(
τp) are shown in Tables 1 and 2, respectively.
Triplet-triplet annihilated upconversion: Under the
3
PdBrTPP
*
4.2 × 10
ε is the molar absorbing coefficient at 532 nm; the emission spectra
atmosphere under the excitation
-2
excitation of 532 nm at 60 mW cm , the blue upconversion
can radiate from the bichromophore solution mixed emitter
obtained at room temperature and N
of Q-band.
2

1
724 Wang et al.
Asian J. Chem.
TABLE-2
TRIPLET PROPERTIES AND DYNAMICS DATA OF SENSITIZERS (8.0 µM) IN DIFFERENT
SOLVENTS UNDER ROOM TEMPERATURE AND NITROGEN ATMOSPHERE
PdTPP
PdBrTPP
Sen.
phos
λmax (nm)
phos
λmax (nm)
Φ (%)
p
τp (µs)
9.92
Φ (%)
p
τp (µs)
10.95
15.52
In DMF
650,725
0.45
0.99
673,697
0.96
1.25
In PhBr
696,773
b
10.88
690,767
d
a
c
Fluorescence quantum yields, Upconversion efficiency, Stern-Volmer quenching constants, Edelayed fluorescence.
3
6.00×10
4
1
.6×10
(a)
3
3.2
.6
(b)
3
3.6
4.50×10
4
3.2
2.8
2.4
1
.2×10
2
2
.8
.4
3
2.0
3.00×1
3
2.0
1.6
1.2
0.8
0.4
0.2
8.0×1
1
1
.6
.2
3
0.8
0.4
1
.50×10
3
4
.0×10
0.2
0
0
4
00 450 500 550 600 650 700 750
Wavelength (nm)
4
00 450 500 550 600 650 700 750
Wavelength (nm)
4
1.6×10
3
4
.50×10
(c)
(d)
3.6
3.2
2.8
3
3
2
2
2
1
1
.6
.2
.8
.4
.0
.6
.2
4
1.2×10
3
3
.00×1
2.4
2.0
1.6
1.2
0.8
0.4
0.2
3
8
.0×1
3
1
.50×10
0.8
0.4
3
4
.0×10
0.2
0
0
4
00 450 500 550 600 650 700 750
Wavelength (nm)
4
00 450 500 550 600 650 700 750
Wavelength (nm)
Fig. 3. Dependence of the upconversion intensity on concentration of PdTPP/DTACl in DMF (a), PdTPP/DTACl in PhBr (b), PdBrTPP/DTACl in DMF (c)
-2
and PdBrTPP/DTACl in PhBr (d) in degassed DMF and PhBr at fixed concentration of sensitizer (8 mM) upon excitation at 532 nm (60 mW cm )
and sensitizer, which shows strongly dependent on the concen-
30
25
20
tration of the emitter. Thus, the relationship between upconver-
sion intensity and the concentration of emitter was investigated
PdBrTPP/PhBr
(Fig. 3). With the concentration of emitter DTACl increasing
PdTPP/PhBr
from 0.2-3.6 mM, the corresponding upconversion intensity
enhances rapidly at the beginning and then gradually stops to
increase, showing 15.5-folds increasing (Fig. 3a).
PdBrTPP/DMF
Accordingly, the upconversion efficiency (ΦUC) of different
dual chromophores are calculated and presented in Fig. 4. For
example, when the concentration of DTACl was increased to
15
PdTPP/DMF
1
0
5
0
3.6 mM, the ΦUC of PdTPP/DTACl in DMF reached its maximum
value as high as 10.33 %. Continuing to increase the concen-
tration, the quantum yield started to remain constant. This is
because that the increase of DTACl concentration can make
the encounter of the sensitizer and the emitter more likely to
enhance the yields of the energy transfer process. However,
when the concentration of DTACl exceeds the threshold value,
the upconversion fluorescence can be suppressed due to the
concentration quenching effect. The similar results for others
were also obtained for their maximum upconversion effici-
encies are at 16.88 % (PdBrTPP/DTACl, DMF).As anticipated,
sensitizer PdBrTPP is more effective to DTACl upconversion.
0
1
2
3
3
Concentration of DTACl (mmol/dm )
Fig. 4. Dependence of the upconversion efficiency (ΦUC) on concentration
of emitter at fixed concentration of sensitizer (8 mM) upon excitation
-2
at 532 nm (60 mW cm )
Since PdBrTPP possesses peripheral heavy atom subs-
titution, phosphorescence dynamics measurements (Fig. 2 and

Vol. 26, No. 6 (2014)
Investigation of Heavy Atom Effect on the Low Power Upconversion 1725
Table-2) have shown that PdBrTPP possesses the longest
ACKNOWLEDGEMENTS
p
phosphorescence lifetime (τ 10.95 µs) and largest phosphore-
p
scence quantum yield (Φ 0.96 %). The most interesting thing
The authors are grateful to the National Natural Science
Grant No. 51273141, 51303122), the Natural Science
(
is that the external heavy atom effect (solvent effect) can play
much more important role than the internal heavy atom effect
in enhancement upconverted efficiency. As seen in Table-3,
the ΦUC values of DTACl/sensitizer compositions are increased
to more than 27.21 % in the solvent of PhBr. With respect to
composition DTACl/PdTPP in DMF and DTACl/PdBrTPP
presents about 10 % enhancements in ΦUC values in DMF
and PhBr, respectively. Evidently, the influence of external
heavy atom effect (solvent heavy atom effect) on ΦUC efficiency
is larger than that of internal heavy atom effect (peripheral
substitution by heavy atom) on ΦUC efficiency. All sensitizers
possess both increasing phosphorescence lifetime and
increasing quantum yield, accompanied by decreasing non-
phosphorescence decay rate constant, which are in good
agreement with the internal heavy atom effect on the dynamics
behaviors.
Foundation of Jiangsu Province (BK20130262), the Natural
Science Foundation of Jiangsu Province Education Committee
(
11KJA430003), Project of Person with Ability of Jiangsu
Province (2010-xcl-015), the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD),
Project of Science and Technology of Suzhou (SYG201204)
for financial supports and the Excellent Innovation Team in
Science and Technology of Jiangsu Universities.
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