Tuning photophysical properties of phosphorescent benzoporphyrin complexes via 1-step π-extension

Article abstract of DOI:10.1016/j.dyepig.2018.07.017

Complexes of benzoporphyrins are highly promising for application in optical oxygen sensors, as triplet sensitizers for generation of singlet oxygen and upconversion based on triplet-triplet annihilation (TTA). Particularly, they benefit from efficient ab

Full text of DOI:10.1016/j.dyepig.2018.07.017

DyesandPigments159(2018)610–618
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Tuning photophysical properties of phosphorescent benzoporphyrin
complexes via 1-step π-extension
Peter W. Zach1, Maximilian Maierhofer1, Sabrina D. Püschmann, Ingo Klimant,
Sergey M. Borisov∗
Institute of Analytical Chemistry and Food Chemistry, Graz University of Technology, Stremayrgasse 9, 8010, Graz, Austria
A R T I C L E I N F O
A B S T R A C T
Keywords:
Porphyrin
Phosphorescence
Platinum
Oxygen sensor
Upconversion
Complexes of benzoporphyrins are highly promising for application in optical oxygen sensors, as triplet sensi-
tizers for generation of singlet oxygen and upconversion based on triplet-triplet annihilation (TTA). Particularly,
they benefit from efficient absorption in red part of the spectrum and strong NIR phosphorescence. In this
contribution, we investigate different strategies of tuning the photophysical properties of these readily available
complexes via 1-step modification. The new π-extended derivatives are obtained via Friedel-Crafts acylation of
tetraphenyltetrabenzoporphyrin (TPTBP) complexes and by Suzuki or Sonogashira cross-coupling reaction of the
tetrabromo-substituted benzoporphyrins. The Soret and Q absorption bands of the tetra-substituted dyes shift
bathochromically by about 15 nm compared to the parent TPTBP compounds. The modified dyes retain their NIR
phosphorescent properties and feature slight improvement of the quantum yield. Application in optical oxygen
sensing materials and triplet-triplet annihilation-based upconversion systems is demonstrated.
1. Introduction
far the most promising way was the extension of the π-conjugated
system of the porphyrin core through fusion of various aromatic moi-
In the last couple of years, phosphorescent metal complexes ex-
citable in the red part of the spectrum with efficient emission in the
NIR-region received much attention due to their broad field of potential
applications, for instance in organic light emitting diodes (OLEDs) [1,2]
and optical sensors [3–5]. Compared to UV–Vis dyes, these complexes
are especially promising for sensing applications due to lower auto-
fluorescence and light scattering, and, because of the compatibility to
the NIR optical window (700–950 nm), enable measurements of oxygen
(and glucose via an oxygen transducer) in tissues as well as in living
organisms [6–9]. Due to efficient generation of singlet oxygen upon
quenching, application of such systems in photodynamic therapy is also
of high interest [10–12]. Such complexes were also intensively in-
vestigated as sensitizers in TTA-based upconversion systems for po-
tential application in photovoltaics [13,14], bioimaging [15] and
photocatalysis [16]. Red-light- and NIR-excitable sensitizers are espe-
cially attractive for the light conversion applications, since almost 50%
of the energy from the sun reaching the earth is NIR radiation [17] with
a maximum flux of sunlight at approximately 680 nm [18].
eties at the β-pyrrole positions leading to tetrabenzoporphyrins (TBP)
[19,20] tetranaphthaloporphyrins (TNP) [2,21–23] or even tetra-an-
throporphyrins (TAP) [24]. Significant bathochromic shifts of absorp-
tion as well as emission were also achieved via modification of ben-
zoporphyrins in an intramolecular Scholl reaction leading to bridged
benzoporphyrin derivatives [9]. All these complexes possess phos-
phorescence ranging from 770 to 1020 nm but the non-radiative decay
rate increases substantially with the decreasing T1 - S0 energy gap [17].
This indicates a limited potential for the extension of absorption and
emission to longer wavelengths. In fact, most NIR absorbing porphyr-
inoids are non-emissive [25–28]. For many practical applications, it is
therefore necessary to find a compromise between the bathochromic
shift of absorption and the phosphorescence quantum yield (Φ). An-
other very important parameter is the photostability of the dyes which
can be dramatically reduced by π-extension such as in case of naph-
thoporphyrins and their molecular hybrids with benzoporphyrins [29].
Although poor photostability and solubility of naphthoporphyrins can
be improved, a sophisticated multistep synthesis is necessary [21].
Therefore, investigation of alternative possibilities for adjusting the
spectral properties of the phosphorescent porphyrins via π-extension
Bathochromic shift of the absorption and the emission bands of
various porphyrinoids can be achieved in different ways. However, so
∗
Corresponding author.
E-mail address: sergey.borisov@tugraz.at (S.M. Borisov).
These authors contributed equally.
1
https://doi.org/10.1016/j.dyepig.2018.07.017
Received 5 June 2018; Received in revised form 10 July 2018; Accepted 10 July 2018
0143-7208/©2018ElsevierLtd.Allrightsreserved.

P.W. Zach et al.
DyesandPigments159(2018)610–618
remains of much interest.
CH:DCM, 3:1), yielding a dark green solid. Yield: 11 mg, 31%.
In this contribution we report the synthesis of new complexes pre-
pared via one step modification of existing benzoporhyrins. Platinum
(II) meso-tetra-(4-t-butyl)phenyl-tetra-bromobenzo-porphyrin (Pt-
TPTBPtBu4Br4) was modified via Suzuki or Sonogashira cross-coupling
1H NMR (300 MHz, CD2Cl2) δ 8.29–8.01 (m, 16H), 7.97–7.71 (m,
7H), 7.66–7.40 (m, 28H), 7.39–7.27 (m, 13H), 7.26–7.00 (m, 12H),
1.65 (t, J = 4.1 Hz, 9H), 1.53–1.45 (m, 18H), 1.35–1.25 (m, 9H).
MALDI-TOF: m/z [M+] C148H112N8Pt calcd: 2196.8689, found:
2196.8257.
reaction
and
platinum(II)
meso-tetra(4-fluorophenyl)tetra-
benzoporphyrin (Pt-TPTBPF) [30] with help of Friedel-Crafts acylation.
The resulting platinum(II) benzoporphyrin derivatives show a bath-
ochromic shift of approximately 15 nm in the absorption and therefore
become compatible with 632.8 nm line of He-Ne laser as well as 635 nm
laser diodes. Furthermore, the new dyes possess similarly good photo-
stability and solubility but higher phosphorescence quantum yields
compared to the benzoporphyrin precursors (except Pt-Ph-acetylene).
2.2.3. Pt-benzoyl-Cl
Pt-TPTBPF (20.0 mg, 18.5 μmol, 1.00 eq) was dissolved in 1,2-di-
chlorobenzene (8 mL) under argon atmosphere. 4-Chlorobenzoyl
chloride (0.12 mL, 0.93 mmol, 50.0 eq) and AlCl3 (40.0 mg, 0.30 mmol,
16.2 eq) were added to the solution. The reaction mixture was heated to
130 °C and stirred for 30 min. The reaction progress was monitored via
absorption spectroscopy (solvent mixture of CHCl3 and ethanol (EtOH)
10:1). After complete conversion the reaction mixture was cooled down
to RT and treated with EtOH:H2O (1:1 v/v, 40 mL) and stirred for
10 min to neutralize the excess of AlCl3. After addition of DCM, the
organic layer was washed with H2O and dried over Na2SO4 and the
solvent was removed under reduced pressure. The crude product was
purified via column chromatography (silica-gel, cond. toluene,
DCM:Tol, 7:1), yielding a dark green solid. Yield: 26.0 mg, 33%.
1H NMR (300 MHz, CD2Cl2) 8.33–8.18 (m, 2H), 8.18–8.07 (m, 3H),
8.06–7.92 (m, 2H), 7.80 (dt, J = 13.5, 7.5 Hz, 4H), 7.66 (d, J = 7.4 Hz,
2H), 7.53 (dd, J = 11.8, 5.4 Hz, 16H), 7.44–7.32 (m, 5H), 7.31–7.19
(m, 2H), 7.19–7.02 (m, 4H).
2. Experimental
2.1. Materials
1,2-dichlorobenzene, copper(I) iodide, potassium carbonate, phe-
nylacetylene and toluene were ordered from Sigma-Aldrich and acetyl
chloride from Fluka. 4-Chlorobenzoyl chloride was purchased from
ABCR and Silica-gel 60 and aluminum trichloride from Merck.
Polystyrene (PS; Mw = 260000 g mol−1) was obtained from Acros
Organics. Sodium sulfate, potassium carbonate and sodium chloride
were from VWR. Tetrakis (triphenylphosphine)palladium (0), 9-phe-
nylcarbazole-3-boronic acid and 9,9-dimethylfluoren-2-boronic acid
were purchased from TCI. All the solvents and triethylamine (TEA)
were from and Roth. Nitrogen, oxygen (both of 99.999% purity) and
test gas (2% O2 in nitrogen) were acquired from Air Liquide and Linde
Gas GmbH. Poly (ethyleneterephthalate) (PET) support Melinex 505
was purchased from Pütz (Taunusstein, Germany). Platinum(II) meso-
tetra-(4-t-butyl)phenyl-tetra-bromobenzo-porphyrin (Pt-TPTBPtBu4Br4)
(Fig. S3 and S46, ESI) was prepared via template condensation fol-
lowing the procedure reported earlier [31]. Synthesis of platinum(II)
meso-tetra(4-fluoro-phenyl)tetrabenzoporphyrin (Pt-TPTBPF) is de-
scribed elsewhere [30]. In all cases deionized water was used.
MALDI-TOF: m/z [M+
found: 1634.1671.
] C88H44Cl4F4N4O4Pt calcd: 1634.1699,
2.2.4. Pt-acetyl
This acylation was performed analogously to Pt-TPTBPF-benzoyl-Cl
but with acetyl chloride (66 μL, 0.93 mmol, 50.0 eq) instead of 4-
chlorobenzoyl chloride. The crude product was purified via column
chromatography (silica-gel, cond. toluene, DCM:Tol, 8:1), yielding a
dark green solid. Yield: 18.3 mg, 77%.
1H NMR (300 MHz, CD2Cl2) δ 8.37–8.09 (m, 8H), 8.07–7.43 (m,
16H), 7.28–7.03 (m, 4H), 2.43–2.17 (m, 12H).
MALDI-TOF: m/z [M+] C68H40F4N4O4Pt calcd: 1248.2655, found:
1248.2158.
2.2. Synthesis
2.2.1. Pt-fluorene
2.2.5. Pt-Ph-acetylene
Pt-TPTBPtBu4Br4 (20.0 mg, 12.9 μmol, 1.00 eq) was dissolved in
toluene (6.5 mL) under argon atmosphere. 9,9-Dimethylfluorene-2-
boronic acid (30.8 mg, 129 μmol, 10.0 eq) and K2CO3 as base (71.4 mg,
517 μmol, 40.0 eq) were added to the solution. The base was pre-dis-
solved in H2O (1.5 mL). Catalyst Pd(PPh3)4 (0.89 mg, 0.77 μmol, 0.06
eq) was added and the reaction mixture was heated to 100 °C and
stirred for 4 h. The reaction progress was monitored via absorption
spectroscopy (solvent: toluene). The reaction mixture was cooled down
to RT. After addition of DCM (dichloromethane), the organic layer was
washed with H2O and dried over Na2SO4 and the solvent was removed
under reduced pressure. The crude product was purified via column
chromatography (silica-gel, cond. cyclohexane (CH), CH:DCM),
yielding a dark green solid. Yield: 6 mg, 20%.
This synthesis was performed analogously to literature [32,33]. Pt-
TPTBPtBu4Br4 (50.0 mg, 32.3 μmol, 1.00 eq) and catalytic amount of
CuI (1.23 mg, 6.50 μmol, 0.20 eq) was dissolved in 1.5 mL abs. THF and
0.7 mL abs. triethylamine (TEA) in a Schlenk flask under Ar atmo-
sphere. Pd(PPh3)4 (3.73 mg, 3.23 μmol, 0.10 eq) and phenylacetylene
(33.0 mg, 323 μmol, 10.00 eq) were added to the solution. The solution
was stirred for 18 h at 75 °C. Conversion control of the reaction was
performed via TLC (silica-gel, CH:DCM 3:1). The green solution was
first washed 4 times with 10% CuSO4 to remove the excess of TEA. The
product was extracted with DCM, the organic layer was dried over
Na2SO4. The solvent was removed under reduced pressure. Further
purification was conducted via column chromatography (silica-gel,
cond. CH, CH:DCM 7:1) yielding fractions with different number of
substituents. The product containing fractions were determined via
absorption spectra. Yield: 2.0 mg, 4%.
1H NMR (300 MHz, CD2Cl2) δ 8.37–8.19 (m, 8H), 8.08–7.94 (m,
8H), 7.84–7.53 (m, 22H), 7.54–7.48 (m, 4H), 7.42–7.27 (m, 14H), 7.21
(d, J = 8.9 Hz, 2H), 7.10 (d, J = 8.6 Hz, 2H), 1.77–1.62 (m, 36H), 1.50
(s, 28H).
1H NMR (300 MHz, CD2Cl2) δ 8.16 (m, 8H), 7.98 (td, J = 20.4,
5.7 Hz, 8H), 7.49 (m, 5H), 7.37 (m, 10H), 7.32 (s, 1H), 7.29 (d,
J = 5.4 Hz, 2H), 7.25 (m, 5H), 7.22–7.19 (m, 1H), 7.16 (m, 7H),
7.10–7.03 (m, 2H), 1.68 (d, J = 8.1 Hz, 36H).
MALDI-TOF m/z [M+] C136H116N4Pt calcd: 2000.8879, found:
2000.8828.
MALDI-TOF: m/z [M+] C108H84N4Pt calcd: 1632.6372, found:
1632.6532.
2.2.2. Pt-carbazole
Pt-carbazole was synthesized analogously to Pt-fluorene but using
25.0 mg (16.2 μmol, 1.00 eq) of Pt-TPTBPtBu4Br4 dissolved in toluene
and tetrahydrofuran (THF) (6 + 3 mL) and 9-phenylcarbazole-3-
boronic acid (46.4 mg, 162 μmol, 10.0 eq) instead. The crude product
was purified via column chromatography (silica-gel, cond. CH,
2.3. Preparation of sensor films
The “cocktails” were prepared by dissolving the respective indicator
(0.5–1 wt% in respect to the polymer) and polystyrene (10 wt% in
611

P.W. Zach et al.
DyesandPigments159(2018)610–618
Scheme 1. Synthesis of the new platinum(II) benzoporphyrin derivatives via one step modification of porphyrin complexes available via template condensation.
respect to the solvent) in chloroform. Then the “cocktails” were knife-
coated on a dust-free PET support using a 25 mm-wide Gardner coating
knife (Pompano Beach, United States) with a wet thickness of 25 μm.
Finally, the sensor films were dried for 24 h at 60 °C to ensure complete
removal of solvent.
2.5. Photostability tests
The dye solutions in toluene (HPLC-grade) were filled in a screw-cap
cuvette and illuminated with a high-power LED array (λmax 635 nm,
www.LED-TECH.de) using a constant current LED driver from Dehner
(Model: LED-350MA35W-IP67; Oestrich-Winkel, Germany) at the 35 W
and 350 mA. The light of the LED-array was focused onto the glass
2.4. Measurements
cuvette utilizing
a
lens from Edmund optics. The photon flux
as de-
was > 20.000 μmol s-1 m-2 μA (irradiance 377 mW*cm−2
)
1H and 13C NMR were recorded on a 300 MHz instrument (Bruker
AVANCE III). In all spectra, the residual signal of the deuterated solvent
was used as an internal standard to reference the chemical shifts δ. Data
analysis was performed with the MestreNova NMR software. High re-
solution mass spectra were recorded using a Micromass TofSpec 2 E as
positive reflector on a Bruker Ultraflex Extreme MALDI-TOF/TOF
spectrometer. The mass spectra were analyzed with the FlexAnalysis
3.0 software (Bruker Daltonics).
termined with a Li-250 A light meter from Li-COR (Nebraska, USA). For
measurements at air saturation the cuvette was unsealed and shaken
after each irradiation interval to ensure air saturation in the sample.
2.6. Oxygen response in solution and polystyrene
The composition of the gas was adjusted utilizing a custom-build
gas-mixing device based on mass-flow controllers from MKS (Munich,
Germany) and Voegtlin (Aesch, Switzerland) by mixing nitrogen and
the test gas (2% O2 in nitrogen) for the solution studies and by mixing
compressed air, nitrogen and oxygen for the sensor characterization.
The temperatures during the characterization were controlled and kept
constant using a cryostat ThermoHaake DC50 (www.thermoscientific.
de/home).
The absorption spectra of the dyes were recorded on a CARY 50
conc UV–Vis spectrophotometer from Varian (Palo Alto, United States).
Molar absorption coefficients were determined in three independent
measurements. The luminescence spectra were recorded on
a
FluoroLog® 3 spectrofluorometer from Horiba Scientific equipped with
a NIR-sensitive R2658 photomultiplier from Hamamatsu. Quantum
yields were determined according to Crosby and Demas [34] using a
solution of dibutoxy-aza-BODIPY (BF2 chelate of [5-(4-buthoxyphenyl)-
3-phenyl-1Hpyrrol-2-yl][5-(4-buthoxyphenyl)-3-phenylpyrrol-2-yli-
dene]-amine) in chloroform as a reference (Φ = 36%) [35,36]. All dye
solutions were deoxygenated in a screw-cap cuvette (Hellma; Müllheim,
Germany) by bubbling argon through the solution for 10 min. Photo-
physical studies in solutions were performed for the dye concentrations
of between 2 and 4 × 10−6 mol*L−1. The phosphorescence decay times
of the complexes in solution and in sensing foils were obtained in fre-
quency domain using a Firesting oxygen meter from PyroScience (Aa-
chen, Germany) at a modulation frequency of 4 kHz.
2.7. Upconversion experiments
The upconversion experiments were conducted on FluoroLog®
3
spectrofluorometer from Horiba. For excitation of the sensitizer-anni-
hilator solutions in toluene a 450 W xenon lamp from the fluorometer
or a laser diode (635 nm, from www.roithner-laser.com.) were used.
Due to the high concentration of the sensitizer-annihilator solutions all
spectra were recorded in the front-face mode. The solutions were
deoxygenated in a screw-cap cuvette by bubbling argon through the
612

P.W. Zach et al.
DyesandPigments159(2018)610–618
solution for 10 min. Neutral density filters (50%, 25%, 10%, 5% of
transmission) were used to modulate the intensity of the excitation
light. The exact transmissions at the excitation wavelength were cal-
culated from absorption spectra of the filters. The quantum yields of the
upconverted fluorescence were roughly estimated by comparing the
emission of the annihilator and the emission of the sensitizer without
the annihilator. The photographic images of the upconverted fluores-
cence were acquired using 635, 650 and 675 nm laser diodes (Roithner)
with Canon 5D camera.
3. Results and discussion
3.1. Synthesis
The new complexes are prepared via one step modification of Pt(II)
meso-tetraaryltetrabenzoporphyrins conveniently accessible via a 3-
step synthesis. The template condensation of either phthalimides or
dicyanobenzenes with phenylacetic acid (Scheme 1) results in Zn(II)
benzoporphyrins with moderate yields (5–10%) which however can be
obtained on multi-gram scale from inexpensive chemicals. The Zn(II)
complexes are demetalated in acidic conditions and the Pt(II) dyes are
prepared via platination of the free-base porphyrin. It should be men-
tioned that only condensation using dicyanobenzenes results in analy-
tically pure benzoporphyrins, whereas synthesis of the dyes in the
“classical” template condensation is accompanied by formation of
benzyl-fused adducts and other by-products (porphyrins without 1 Br
atom; Fig. S46, ESI) which are virtually impossible to separate. These
by-products significantly diminish the yields of the aimed complexes in
subsequent π-extension reactions. Since 4-bromodicyanobenzene was
not commercially available and optimization of the yield was not the
aim of the current study, we used the route starting from 4-bro-
mophthalimide to obtain tetrabromo-substituted benzoporphyrins.
Friedel-Crafts acylation of Pt-TPTBPF with either 4-chlorobenzoyl
chloride or acetyl chloride in 1,2-dichlorobenzene in presence of AlCl3
results in formation of Pt-benzoyl-Cl or Pt-acetyl, respectively, in good
yields (30–70%) obtained as a mixture of isomers (Scheme 1).
Suzuki coupling reaction of Pt-TPTBPtBu4Br4 with the respective
boronic acids (9,9-dimethylfluorene-2-boronic acid and 9-phe-
nylcarbazole-3-boronic acid) led to formation of Pt-fluorene and Pt-
carbazole derivatives, respectively (Scheme 1). Sonogashira coupling of
the same porphyrin with phenylacetylene resulted in the formation of
Pt-Ph-acetylene (Scheme 1). Due to above-mentioned impurities in Pt-
TPTBPtBu4Br4 and incomplete substitution during the Suzuki and So-
nogashira reactions, the yields of the tetra-substituted products are very
low and chromatographic purification is laborious.
Fig. 1. Absorption (solid lines) and emission (dashed lines) spectra of new Pt(II)
complexes and precursors in toluene at 25 °C; anoxic conditions for emission
measurements.
and 632.8 nm line of the He-Ne laser. Importantly, all these sources
almost perfectly fit the maxima of the Soret and the Q-bands, which
together with very high molar absorption coefficients (exceeding
210.000 and 110.000 M−1cm−1 for the Soret and the Q-band
respectively) and high phosphorescence quantum yield (QY), Table 1,
ensures excellent brightness. This can be of high interest if the dyes are
used for imaging in cells and tissues. Notably, the position of the ab-
sorption bands of the new Pt(II) porphyrins is very similar to that of the
Pd(II) complex with TPTBPF, which phosphorescence quantum yield is,
however, significantly lower than for the Pt(II) complexes.
The new benzoporphyrin derivatives possess strong room-tempera-
ture phosphorescence in the NIR region in deoxygenated organic sol-
vents such as toluene. The bathochromic shift of the emission spectra is
smaller than the changes observed in the absorption spectrum, i.e. the
new dyes possess smaller S1-T1 energy gap compared to the parent
benzoporphyrins. This effect is particularly pronounced for the Pt-
benzoyl-Cl and Pt-acetyl complexes, which show the behaviour similar
to that for the previously reported benzoporphyrins bearing highly
electron-withdrawing sulfone groups in the same position [36]. Inter-
estingly, all substitutions, except for the introduction of a triple bond in
Pt-Ph-acetylene, favourably affect the phosphorescence quantum
yields, which increase by about 25% compared to Pt-TPTBPF. This is
again in good correlation to the properties of the sulfone-substituted
complexes [36]. On the other hand, bulky phenylcarbazole and
3.2. Photophysical properties
Fig. 1 shows absorption and emission spectra of the new platinum
(II) benzoporphyrins in toluene in comparison with their respective
benzoporphyrin precursors. π-extension via Suzuki and Sonogashira
coupling results in bathochromic shift of the Soret and Q bands by
about 15 nm (Table 1). Interestingly, the π-extended complexes of
fluorene, carbazole and Ph-acetylene have higher ratio of the molar
absorption coefficients for the Q to Soret bands compared to the pre-
cursor.
Very similar bathochromic shift is observed upon introduction of
acetyl and chlorobenzoyl groups in the same position. Interestingly, the
Q-bands for these dyes are broader than for the Pt-TPTBPF precursor
(FWHM of Pt-TPTBPF: 460 cm−1 at 615 nm; Pt-benzoyl-Cl: 666 cm−1 at
631 nm and Pt-acetyl: 634 cm−1 at 629 nm).
These subtle effects, however, have pronounced impact on potential
applications of the dyes in sensing and imaging. In fact, the newly
synthesized dyes are much better compatible with a number of light
sources used in compact instruments and in microscopy such as bright
blue 455-nm LED, 635-nm red laser diode, 455 nm line of the Ar laser
613

P.W. Zach et al.
DyesandPigments159(2018)610–618
Table 1
Photophysical properties of the new platinum(II) complexes and their precursors in anoxic toluene at 25 °C.
Complex
λ
maxabs (ε), nm (M−1cm−1
)
λ
maxem, nm
Stokes shift, cm−1
QY, Ф
τ, μs
Ksv, kPa−1
kq, Pa−1s−1
Pt-TPTBPF
Pt-TBrTtBuPP
Pt-fluorene
Pt-carbazole
Pt-benzoyl-Cl
Pt-acetyl
430 (212.000); 565 (16.300); 615 (146.000)
436 (238.000); 564 (16.100); 615 (137.500)
443 (274.000); 575 (22.000); 627 (171.000)
444 (240.000); 578 (21.400); 630 (171.700)
448 (218.000); 579 (17.400); 631 (121.000)
445 (216.000); 577 (16.000); 629 (114.000)
446 (285.000), 577 (22.500) 630 (169.000)
769
760
777
777
773
773
772
3260
3100
3080
3000
2910
2960
2920
0.24
0.17
0.28
0.29
0.29
0.31
0.13
53.0
32.6
45.5
44.0
47.4
49.8
45.0
4.58
3.78
2.68
2.56
2.75
3.19
2.34
87
116
59
58
58
64
52
Pt-Ph-acetylene
dimethylfluorene substituents may reduce radiativeless deactivation via
molecular vibrations. Obviously, binding of phenyl-acetylene groups
via triple bond, as it is the case for Pt-Ph-acetylene, results in similar
absorption characteristics compared to the other Pt(II) complexes, but
to a less pronounced bathochromic shift of the emission band and de-
creased quantum yields.
377 mW cm−2). Fig. 2 shows that the photostability of the new por-
phyrins is rather similar to that of palladium(II) tetra-
phenyltetrabenzoporphyrin (Pd-TPTBP), which was chosen as a re-
ference due to almost identical position of the Q-band corresponding to
the excitation wavelength. Electron-withdrawing acetyl and chlor-
obenzoyl substituents improve the photostability whereas electron-do-
nating phenylcarbazole and phenyl-acetylene slightly decrease it. Thus,
photooxidation appears to be the most likely photodegradation me-
chanism. Despite faster photobleaching of Pt-carbazole and Pt-Ph-
acetylene, even in this case the photostability is adequate for most
applications. For instance, in sensing applications not only the in-
tensities of the excitation light are much lower but also interrogation
times of 10–20 ms are sufficient for acquisition of a measurement point.
Apart from platinum(II) porphyrins, the respective palladium(II)
analogues are also known for their fairly strong fluorescence. Compared
to the Pt(II) porphyrins, the Pd(II) complexes generally feature bath-
ochromically shifted absorption and emission, significantly longer
phosphorescence decay times but lower quantum yields. Pd-benzoyl-Cl
was obtained analogously to Pt-benzoyl-Cl from palladium(II) tetra(4-
fluorophenyl)tetrabenzoporphyrin with good yields (Fig S1, ESI). Its
photophysical properties (Table S1, Fig. S4, ESI) follow the above ex-
pectations with absorption maxima at 461, 595 and 645 nm (molar
3.4. Luminescence quenching in solution and in polymeric matrix
absorption coefficients 283.000, 16.000 and 108.000 M−1cm−1
,
respectively), emission maximum at 799 nm, phosphorescence decay
time of 313 μs and quantum yield of 0.13 (all for anoxic toluene at
25 °C).
Because of the favorable photophysical properties the new dyes can
be promising as indicators for application in optical oxygen sensors.
First, phosphorescence quenching in solution was investigated (Fig. 3).
The quenching constants KSV obtained from the linear fit of the Stern-
Volmer plots are reported in Table 1. Interestingly, the bimolecular
quenching constants kq = KSV/τ0 were found to be lower than for the
parent compounds Pt-TPTBPF and Pt-TBrTtBuPP. According to Smo-
luchovski equation [37,38], the rate constant for the diffusion-limited
reaction kd is proportional to the sum of the diffusion coefficients of the
reactants and to the reaction distance which is approximated as the sum
of the radii for the reacting molecules. Since the diffusion coefficient for
oxygen is about 1 order of magnitude higher than for the metallopor-
phyrin [30], the kd is expected to be very similar for all the dyes.
Therefore, it can be speculated that the bulky substituents on periphery
of the dye protect the porphyrin core from collision with molecular
oxygen similar to porphyrin-based dendrimers. This results in smaller
kq values than can be expected from the radii and diffusion coefficients
3.3. Photostability
This property is of great importance especially if the dye-based
materials are exposed to high light densities or/and irradiated over a
longer period of time, conditions which are typical for many applica-
tions such as upconversion or microscopy. Photodecomposition in air-
saturated conditions might be particularly critical due to formation of
singlet oxygen. Moreover, Pt-benzoyl-Cl possesses the benzophenone
motive, which is a well-known photoinitiator. Photodegradation of the
new platinum(II) complexes dissolved in air saturated toluene was
tested upon continuous irradiation with a high power 635 nm LED array
(35 W, 350 mA, photon flux: > 20.000 μmol s-1 m-2; irradiance
Fig. 2. Photodegradation profiles for the new Pt(II) complexes and Pd-TPTBP as
reference in air-saturated toluene solution at 25 °C upon irradiation with a high
power 635 nm LED.
Fig. 3. Phosphorescence quenching by molecular oxygen for the Pt(II) com-
plexes dissolved in toluene (at 25 °C).
614

P.W. Zach et al.
DyesandPigments159(2018)610–618
Fig. 4. Stern-Volmer plots for the Pt(II) complexes embedded in polystyrene (at 25 °C).
of the reactants. On the other hand, the kq value for Pt-acetyl is closer to
the values obtained for the other π-extended dyes rather than to kq of
Pt-TPTBPF (Table 1). As was discussed previously [30], the bimolecular
compared to Pt-TPTBPF (Table 1) and shows about 30% higher kq.
In order to evaluate their sensing behaviour, the dyes were em-
bedded in polystyrene (0.5 % wt. of the indicator) serving as a model
matrix. Not surprisingly, the phosphorescence of all dyes is efficiently
quenched by oxygen in this polymer (Fig. 4).
The non-linear Stern-Volmer plots are typical for the oxygen sensors
based on polystyrene-immobilized (benzo)porphyrins. The plots can be
described by the so called “two-site model” [40] which assumes
quenching constant may also correlate to the energy of the triplet state
of the dye due to its proximity to the 1Σg
level of oxygen
+
(E = 13121 cm−1) [39]: i.e. the lower the triplet energy the lower the
kq. In fact, the triplet energy of the new π-extended dyes is lower than
for Pt-TPTBPF. Pt-TBrTtBuPP emits even at shorter wavelength
615

P.W. Zach et al.
DyesandPigments159(2018)610–618
Table 2
Fit parameters for the oxygen sensors based on the new benzoporphyrins embedded in polystyrene.
Complex
f
m
τ0/KSV1 at 5 °C, μs/kPa−1
τ0/KSV1 at 25 °C, μs/kPa−1
τ0/KSV1 at 45 °C, μs/kPa−1
dτ/dt, %/K at 25 °C
Pt-TPTBPF [41]
Pt-fluorene
Pt-carbazole
Pt-benzoyl-Cl
Pt-acetyl
0.85
0.81
0.78
0.80
0.84
0.83
0
–
52.6/0.218
48.1/0.246
46.4/0.182
50.6/0.220
51.21/0.245
45.0/0.281
–
0.07
0.107
0.103
0.075
0.072
0.147
48.7/0.208
47.0/0.160
51.3/0.184
52.1/0.206
45.9/0.235
47.5/0.271
45.6/0.201
49.5/0.255
50.23/0.292
44.0/0.309
0.065
0.074
0.089
0.092
0.105
Pt-Ph-acetylene
location of the indicator in two different microenvironments. The
equation also adequately fits the decay time plots for the average decay
times obtained in frequency domain:
τ0
τ
1
=
f
1 − f
+
1 + K
∗ p
1 + (K
∗ m) ∗ pO
SV1
SV1
2
(1)
O
2
where τ0 and τ represent the lifetimes of the indicator under anoxic
conditions and in presence of oxygen, while KSV1 and KSV1*m are the
Stern-Volmer constants for the two microenvironments, and f is the
fraction of the overall emission for the first site. The fit parameters at
different temperatures are summarized in Table 2.
It is evident that all the complexes have very similar behaviour in
respect to the quenching efficiency and the linearity of the Stern-
Volmer plots. In contrast to the solution, no evident correlation be-
tween the size of the substituent or energy of the triplet state and the
quenching efficiency is observed. However, because of much higher
complexity of the polymeric matrix compared to the solution some
minor trends may not be visible. In fact, due to different size and po-
larity of the substituents, the localization of the dyes in the polymer
may be not fully identical. Since different polymeric domains are
characterised by different oxygen permeabilities, the linearity of the
Stern-Volmer plots and the calculated quenching constants may be in-
directly influenced by the substituents.
In respect to thermal quenching of phosphorescence (determined
from the temperature dependency of the decay time under anoxic
conditions) all the dyes are again rather similar with slightly higher
temperature coefficients for Pt-benzoyl-Cl, Pt-acetyl and Pt-Ph-acet-
ylene. Moreover, these properties match very well those of polystyrene-
immobilized Pt-TPTBPF which was used as a reference material.
Although the dynamic range of the new sensors is from about 0.1 to
100 kPa, and thus is the same as for the parent compounds, it can be
extended into significantly lower concentration range by using poly-
mers with higher oxygen permeability (ethyl cellulose, organically-
modified silica etc.). Also, the sensitivity of the oxygen sensor based on
Pd-benzoyl-Cl embedded in polystyrene (Table S2, Fig. S5, ESI) is about
5-fold higher than for the sensor based on Pt-benzoyl-Cl (Ksv1 at 25 °C is
1.03 and 0.22 kPa−1, respectively) which is explained by significantly
longer phosphorescence decay time of the Pd(II) complex (346 μs at
25 °C).
Fig. 5. Left: Emission spectrum (λexc 635 nm) of the TTA system based on
5·10−5 M of Pt-benzoyl-Cl and 2.5·10−4 M of Solvent green 5 (structure shown)
in anoxic toluene. The insert shows the photographic image of the upconverted
fluorescence for the same system upon excitation of the solution with two laser
diodes.
conversion applications, the sensitizer should efficiently absorb light in
the red part of the electromagnetic spectrum due to the considerable
solar power and low photovoltaics efficiency in this region [44–47] and
transfer its energy to the annihilator. Not only benzoporphyrins [36]
but also their molecular hybrids with naphthalooporphyrins proved to
be viable sensitizers for TTA upconversion systems [44,45,47,48], but
the photostability of the latter compounds is poor [29]. Since new
benzoporphyrin derivatives benefit from higher quantum yields, bath-
ochromically shifted absorption as well as emission and photostability
similar to that of the parent benzoporphyrins, they are expected to be
promising sensitizers for upconversion of red light. As a proof of con-
cept, ability of the new dyes to act as sensitizers in TTA upconversion
was investigated. As can be seen (Fig. 5, Fig. S12-S15 ESI), bright up-
conversion from perylene dye Solvent Green 5 (2.5 × 10−4 M) is ob-
served upon excitation of the new Pt(II) complexes (5 × 10−5 M) with a
635 laser diode. A quadratic dependence of upconverted fluorescence
on the intensity of excitation light was observed upon excitation of the
dyes with a Xenon lamp (slope of the logarithmic plot approximately 2),
Fig. S6-S11 ESI. On the contrary, excitation with the 635 nm laser diode
revealed a linear region with a slope of unity demonstrating saturation
conditions (Fig. S12-17, ESI) [49]. This phenomenon is typical for
nonlinear processes like TTA based upconversion [42–44,50]. Since the
Q-bands of the Pd-benzoyl-Cl located at even longer wavelength com-
pared to the corresponding Pt(II) complex, this dye may even be more
promising sensitizer for the red part of the spectrum. Indeed (Fig. S10,
S11, S16-S18, Table S3 ESI), bright upconversion was observed with
two perylene dyes, Solvent Green 5 and Lumogen Orange even at
comparably long excitation wavelengths.
3.5. Dyes as sensitizers in TTA-based upconversion systems
As mentioned above, benzoporphyrins have already been success-
fully used as synthesizers in triplet-triplet-annihilation (TTA) based
upconversion systems. Briefly, after absorption of light by the sensitizer
and inter-system crossing, an annihilator in triplet excited state is
formed via triplet-triplet energy transfer from sensitizer to annihilator
(Scheme S1). Collision of two excited annihilator molecules results in
formation of one of them in the singlet excited state following by
fluorescence which is located at shorter wavelength compared to the
excitation light. The annihilator should feature very high fluorescence
quantum yields (close to unity), good solubility in organic solvents and
outstanding photostability [42,43]. In order to be useful for energy
Considering scientific progress in developing of nanoparticles fea-
turing TTA [15,51], the combination of the red-light-excitable sensiti-
zers with different annihilators may result in a new generation of
616

P.W. Zach et al.
DyesandPigments159(2018)610–618
imaging agents for in vivo measurements which possess much higher
brightness than the lanthanide nanoparticles [52,53].
catheter system for continuous glucose measurement and simultaneous insulin in-
fusion. Biosens Bioelectron 2015;64:102–10. https://doi.org/10.1016/j.bios.2014.
08.012.
[9] Zach PW, Hofmann OT, Klimant I, Borisov SM. NIR phosphorescent in-
tramolecularly bridged benzoporphyrins and their application in an oxygen-com-
pensated glucose optode. Anal Chem 2018;90:2741–8. https://doi.org/10.1021/
acs.analchem.7b04760.
[10] Pervaiz S, Olivo M. Art and science of photodynamic therapy. Clin Exp Pharmacol
Physiol 2006;33:551–6. https://doi.org/10.1111/j.1440-1681.2006.04406.x.
[11] Yuan A, Wu J, Tang X, Zhao L, Xu F, Hu Y. Application of near-infrared dyes for
tumor imaging, photothermal, and photodynamic therapies. J Pharmaceut Sci
2013;102:6–28. https://doi.org/10.1002/jps.23356.
[12] Qian J, Wang D, Cai F, Zhan Q, Wang Y, He S. Photosensitizer encapsulated or-
ganically modified silica nanoparticles for direct two-photon photodynamic therapy
and in Vivo functional imaging. Biomaterials 2012;33:4851–60. https://doi.org/10.
1016/j.biomaterials.2012.02.053.
[13] Schulze TF, Czolk J, Cheng Y-Y, Fückel B, MacQueen RW, Khoury T, et al. Efficiency
enhancement of organic and thin-film silicon solar cells with photochemical up-
conversion. J Phys Chem C 2012;116:22794–801. https://doi.org/10.1021/
jp309636m.
[14] Lissau JS, Gardner JM, Morandeira A. Photon upconversion on dye-sensitized na-
nostructured ZrO2 films. J Phys Chem C 2011;115:23226–32. https://doi.org/10.
1021/jp209774p.
[15] Liu Q, Yang T, Feng W, Li F. Blue-emissive upconversion nanoparticles for low-
power-excited bioimaging in vivo. J Am Chem Soc 2012;134:5390–7. https://doi.
org/10.1021/ja3003638.
[16] Khnayzer RS, Blumhoff J, Harrington JA, Haefele A, Deng F, Castellano FN.
Upconversion-powered photoelectrochemistry. Chem Commun 2011;48:209–11.
https://doi.org/10.1039/C1CC16015J.
[17] Xiang H, Cheng J, Ma X, Zhou X, Chruma JJ. Near-infrared phosphorescence:
materials and applications. Chem Soc Rev 2013;42:6128–85. https://doi.org/10.
1039/C3CS60029G.
4. Conclusions
We have demonstrated several different strategies for modification
of known benzoporphyrins complexes within one reaction step.
Modification via Friedel-Crafts acylation is particularly efficient and
can be performed with good yields both for Pt(II) and Pd(II) complexes.
The new platinum(II) complexes show a bathochromic shift in the ab-
sorption spectra, now enabling efficient excitation with 635 nm laser
diode/632.8 nm line of He-Ne laser (Q band) and with blue LEDs/
455 nm line of the Ar laser (Q-band). Modification with the respective
groups also results in increased quantum yields with the exception of
phenylacetylene-modified dye, whereas the decay time and photo-
stability are similar to those of the parent compounds. The new dyes
represent promising indicators for oxygen sensors particularly in re-
spect to in vivo applications, due to better light penetration and lower
light scattering at longer wavelengths. They may also be useful for
application in enzymatic glucose sensors relying on oxygen transducers.
The new complexes are also valuable sensitizers for TTA upconversion
systems and allow more efficient harvesting of red light than the parent
benzoporphyrin dyes. Even longer sensitization wavelengths can be
achieved if the central atom is substituted to palladium(II).
[18] Hou J, Park M-H, Zhang S, Yao Y, Chen L-M, Li J-H, et al. Bandgap and molecular
energy level control of conjugated polymer photovoltaic materials based on benzo
[1,2-b:4,5-b′]dithiophene. Macromolecules 2008;41:6012–8. https://doi.org/10.
1021/ma800820r.
[19] Hutter LH, Müller BJ, Koren K, Borisov SM, Klimant I. Robust optical oxygen sen-
sors based on polymer-bound NIR-emitting platinum(II)–benzoporphyrins. J Mater
Chem C 2014;2:7589–98. https://doi.org/10.1039/C4TC00983E.
[20] Carvalho CMB, Brocksom TJ, Oliveira KT de. Tetrabenzoporphyrins: synthetic de-
velopments and applications. Chem Soc Rev 2013;42:3302–17. https://doi.org/10.
1039/C3CS35500D.
Associated content
Synthesis, photophysical properties, optical oxygen sensor, TTA
upconversion, 1H and 13C NMR and mass spectra (PDF).
Acknowledgements
The support from Ing. Karin Bartl (ICTM, TU Graz) in the acquisition
of the MS-Spectra, Josef Lechner, in the synthesis, Marion Stampfer-
Maric in the oxygen calibration measurements and Matthias Schwar in
the luminescent quenching measurements is gratefully acknowledged.
The work was financially supported by the ERC Project “Oxygen”
(Grant Number 267233) and the European Union FP7 Project
“SenseOcean” (Grant Number 614141).
[21] Finikova OS, Cheprakov AV, Vinogradov SA. Synthesis and luminescence of soluble
meso-unsubstituted tetrabenzo- and tetranaphtho[2,3]porphyrins. J Org Chem
2005;70:9562–72. https://doi.org/10.1021/jo051580r.
[22] Finikova OS, Aleshchenkov SE, Briñas RP, Cheprakov AV, Carroll PJ, Vinogradov
SA. Synthesis of symmetrical Tetraaryltetranaphtho[2,3]porphyrins. J Org Chem
2005;70:4617–28. https://doi.org/10.1021/jo047741t.
[23] Finikova OS, Cheprakov AV, Carroll PJ, Vinogradov SA. Novel route to functiona-
lized tetraaryltetra[2,3]naphthaloporphyrins via oxidative aromatization. J Org
Chem 2003;68:7517–20. https://doi.org/10.1021/jo0347819.
[24] Yamada H, Kuzuhara D, Takahashi T, Shimizu Y, Uota K, Okujima T, et al. Synthesis
and characterization of tetraanthroporphyrins. Org Lett 2008;10:2947–50. https://
doi.org/10.1021/ol8008842.
Appendix A. Supplementary data
[25] Tanaka T, Osuka A. Conjugated porphyrin arrays: synthesis, properties and appli-
cations for functional materials. Chem Soc Rev 2015;44:943–69. https://doi.org/
10.1039/C3CS60443H.
[26] Diev VV, Schlenker CW, Hanson K, Zhong Q, Zimmerman JD, Forrest SR, et al.
Porphyrins fused with unactivated polycyclic aromatic hydrocarbons. J Org Chem
2011;77:143–59. https://doi.org/10.1021/jo201490y.
[27] Spence JD, Lash TD. Porphyrins with exocyclic rings. 14.1 synthesis of tetra-
acenaphthoporphyrins, a new family of highly conjugated porphyrins with record-
breaking long-wavelength electronic absorptions. J Org Chem 2000;65:1530–9.
https://doi.org/10.1021/jo991730w.
[28] Lewtak JP, Gryko DT. Synthesis of π-extended porphyrins via intramolecular oxi-
dative coupling. Chem Commun 2012;48:10069–86. https://doi.org/10.1039/
C2CC31279D.
[29] Niedermair F, Borisov SM, Zenkl G, Hofmann OT, Weber H, Saf R, et al. Tunable
phosphorescent NIR oxygen indicators based on mixed benzo- and naphthopor-
phyrin complexes. Inorg Chem 2010;49:9333–42. https://doi.org/10.1021/
ic100955z.
[30] Borisov SM, Nuss G, Haas W, Saf R, Schmuck M, Klimant I. New NIR-emitting
complexes of platinum(II) and palladium(II) with fluorinated benzoporphyrins. J
Photochem Photobiol -Chem 2009;201:128–35. https://doi.org/10.1016/j.
jphotochem.2008.10.003.
[31] Borisov SM, Klimant I. Efficient metallation in diphenylether – a convenient route
to luminescent platinum(II) complexes. Dyes Pigments 2009;83:312–6. https://doi.
org/10.1016/j.dyepig.2009.05.008.
[32] Hancock JM, Gifford AP, Zhu Y, Lou Y, Jenekhe SA. n-Type conjugated oligoqui-
noline and oligoquinoxaline with triphenylamine Endgroups: efficient ambipolar
light emitters for device applications. Chem Mater 2006;18:4924–32. https://doi.
org/10.1021/cm0613760.
Supplementary data related to this article can be found at https://
doi.org/10.1016/j.dyepig.2018.07.017.
References
[1] Borek C, Hanson K, Djurovich PI, Thompson ME, Aznavour K, Bau R, et al. Highly
efficient, near-infrared electrophosphorescence from a Pt–Metalloporphyrin com-
plex. Angew Chem Int Ed 2007;46:1109–12. https://doi.org/10.1002/anie.
200604240.
[2] Sommer JR, Farley RT, Graham KR, Yang Y, Reynolds JR, Xue J, et al. Efficient
near-infrared polymer and organic light-emitting diodes based on electropho-
sphorescence from (Tetraphenyltetranaphtho[2,3]porphyrin)platinum(II). ACS
Appl Mater Interfaces 2009;1:274–8. https://doi.org/10.1021/am800236x.
[3] Wang X, Wolfbeis OS. Optical methods for sensing and imaging oxygen: materials,
spectroscopies and applications. Chem Soc Rev 2014;43:3666–761. https://doi.org/
10.1039/C4CS00039K.
[4] Staudinger C, Borisov SM. Long-wavelength analyte-sensitive luminescent probes
and optical (bio)sensors. Methods Appl Fluoresc 2015;3:042005https://doi.org/10.
1088/2050-6120/3/4/042005.
[5] Napp J, Behnke T, Fischer L, Würth C, Wottawa M, Katschinski DM, et al. Targeted
luminescent near-infrared polymer-nanoprobes for in vivo imaging of tumor hy-
poxia. Anal Chem 2011;83:9039–46. https://doi.org/10.1021/ac201870b.
[6] Lebedev AY, Cheprakov AV, Sakadzić S, Boas DA, Wilson DF, Vinogradov SA.
Dendritic phosphorescent probes for oxygen imaging in biological systems. ACS
Appl Mater Interfaces 2009;1:1292–304. https://doi.org/10.1021/am9001698.
[7] Yaseen MA, Srinivasan VJ, Sakadžić S, Wu W, Ruvinskaya S, Vinogradov SA, et al.
Optical monitoring of oxygen tension in cortical microvessels with confocal mi-
croscopy. Optic Express 2009;17:22341–50.
[33] Hur JA, Bae SY, Kim KH, Lee TW, Cho MJ, Choi DH. Semiconducting 2,6,9,10-
Tetrakis(phenylethynyl)anthracene derivatives: effect of substitution positions on
molecular energies. Org Lett 2011;13:1948–51. https://doi.org/10.1021/
ol200299s.
[8] Nacht B, Larndorfer C, Sax S, Borisov SM, Hajnsek M, Sinner F, et al. Integrated
617

P.W. Zach et al.
DyesandPigments159(2018)610–618
[34] Crosby GA, Demas JN. Measurement of photoluminescence quantum yields. Rev J
Phys Chem 1971;75:991–1024. https://doi.org/10.1021/j100678a001.
[35] Loudet A, Bandichhor R, Wu L, Burgess K. Functionalized BF2 chelated azadi-
pyrromethene dyes. Tetrahedron 2008;64:3642–54. https://doi.org/10.1016/j.tet.
2008.01.117.
[36] Zach PW, Freunberger SA, Klimant I, Borisov SM. Electron-deficient near-infrared
Pt(II) and Pd(II) benzoporphyrins with dual phosphorescence and unusually effi-
cient thermally activated delayed fluorescence: first demonstration of simultaneous
oxygen and temperature sensing with a single emitter. ACS Appl Mater Interfaces
2017;9:38008–23. https://doi.org/10.1021/acsami.7b10669.
[37] Guthrie J, Cossar J, Klym A. Halogenation of acetone. A method for determining
pKas of ketones in aqueous solution, with an examination of the thermodynamics
and kinetics of alkaline halogenation and a discussion of the best value for the rate
constant for a diffusion-controlled reaction. Energetic requirements for a diffusion-
controlled reaction involving heavy-atom bond formation. J Am Chem Soc
1984:106. https://doi.org/10.1021/ja00317a030.
chromophores: next generation triplet acceptors/annihilators for low power up-
conversion schemes. J Am Chem Soc 2008;130:16164–5. https://doi.org/10.1021/
ja807056a.
[45] Monguzzi A, Borisov SM, Pedrini J, Klimant I, Salvalaggio M, Biagini P, et al.
Efficient broadband triplet–triplet annihilation-assisted photon upconversion at
subsolar irradiance in fully organic systems. Adv Funct Mater 2015;25:5617–24.
https://doi.org/10.1002/adfm.201502507.
[46] Kim J-H, Deng F, Castellano FN, Kim J-H. High efficiency low-power upconverting
soft materials. Chem Mater 2012;24:2250–2. https://doi.org/10.1021/cm3012414.
[47] Xu K, Zhao J, Cui X, Ma J. Photoswitching of triplet-triplet annihilation upcon-
version showing large emission shifts using a photochromic fluorescent dithieny-
lethene-Bodipy triad as a triplet acceptor/emitter. Chem Commun 2015;51:1803–6.
https://doi.org/10.1039/C4CC09202C.
[48] Kim J-H, Deng F, Castellano FN, Kim J-H. Red-to-Blue/Cyan/Green upconverting
microcapsules for aqueous- and dry-phase color tuning and magnetic sorting. ACS
Photonics 2014;1:382–8. https://doi.org/10.1021/ph500036m.
[38] Subczynski WK, Hyde JS. Diffusion of oxygen in water and hydrocarbons using an
electron spin resonance spin-label technique. Biophys J 1984;45:743–8.
[39] Kearns DR. Physical and chemical properties of singlet molecular oxygen. Chem Rev
1971;71:395–427. https://doi.org/10.1021/cr60272a004.
[49] Cheng YY, Khoury T, Clady RGCR, Tayebjee MJY, Ekins-Daukes NJ, Crossley MJ,
et al. On the efficiency limit of triplet–triplet annihilation for photochemical up-
conversion. Phys Chem Chem Phys 2009;12:66–71. https://doi.org/10.1039/
B913243K.
[40] Carraway ER, Demas JN, DeGraff BA, Bacon JR. Photophysics and photochemistry
of oxygen sensors based on luminescent transition-metal complexes. Anal Chem
1991;63:337–42. https://doi.org/10.1021/ac00004a007.
[41] Borisov SM, Nuss G, Klimant I. Red light-excitable oxygen sensing materials based
on platinum(II) and palladium(II) benzoporphyrins. Anal Chem 2008;80:9435–42.
https://doi.org/10.1021/ac801521v.
[42] Islangulov RR, Lott J, Weder C, Castellano FN. Noncoherent low-power upconver-
sion in solid polymer films. J Am Chem Soc 2007;129:12652–3. https://doi.org/10.
1021/ja075014k.
[43] Singh-Rachford TN, Lott J, Weder C, Castellano FN. Influence of temperature on
low-power upconversion in rubbery polymer blends. J Am Chem Soc
2009;131:12007–14. https://doi.org/10.1021/ja904696n.
[50] Borisov SM, Saf R, Fischer R, Klimant I. Synthesis and properties of new phos-
phorescent red light-excitable platinum(II) and palladium(II) complexes with Schiff
bases for oxygen sensing and triplet-triplet annihilation-based upconversion. Inorg
Chem 2013;52:1206–16. https://doi.org/10.1021/ic301440k.
[51] Wang W, Liu Q, Zhan C, Barhoumi A, Yang T, Wylie RG, et al. Efficient triplet–-
triplet annihilation-based upconversion for nanoparticle phototargeting. Nano Lett
2015;15:6332–8. https://doi.org/10.1021/acs.nanolett.5b01325.
[52] Seydack M. Nanoparticle labels in immunosensing using optical detection methods.
Biosens Bioelectron 2005;20:2454–69. https://doi.org/10.1016/j.bios.2004.11.
003.
[53] Gnach A, Bednarkiewicz A. Lanthanide-doped up-converting nanoparticles: merits
and challenges. Nano Today 2012;7:532–63. https://doi.org/10.1016/j.nantod.
2012.10.006.
[44] Singh-Rachford TN, Haefele A, Ziessel R, Castellano FN. Boron dipyrromethene
618