Selenium-containing BODIPY dyes as photosensitizers for triplet-triplet annihilation upconversion

Article abstract of DOI:10.1039/c8tc00944a

In this study, two new selenium-containing BODIPY dyes were synthesized as photosensitizers for triplet-triplet annihilation upconversion (TTA-UC). Bis(phenylselanyl)-BODIPY 1 absorbed visible light at 518 nm (?_max = 7.84 × 10⁴ cm

Full text of DOI:10.1039/c8tc00944a

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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

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Selenium-containing BODIPY dyes as photosensitizers for triplet-triplet annihilation upconversion
a
a
b
b
a
Mika Nakashima, Keita Iizuka, Masanobu Karasawa, Kazuyuki Ishii and Yuji Kubo*
a
Department of Applied Chemistry for Environment, Graduate School of Urban Environmental Sciences, Tokyo
Metropolitan University, 1ꢀ1 Minamiꢀohsawa, Hachioji, Tokyo 192ꢀ0397, Japan. *Eꢀmail; yujik@tmu.ac.jp
b
Institute of Industrial Science, The University of Tokyo, 4ꢀ6ꢀ1 Komaba, Meguroꢀku, Tokyo, Japan
Abstract: In this study, two new seleniumꢀcontaining BODIPY dyes were synthesized as photosensitizers for
tripletꢀtriplet annihilation upconversion (TTAꢀUC). Bis(phenylselanyl)ꢀBODIPY 1 absorbed visible light at 518
4
−1
−1
nm (εmax = 7.84 × 10 cm M ) and exhibited fluorescence at 594 nm with a low fluorescence quantum yield
) of 0.03. The low Ф value was ascribed to heavy atom effects. When a TTAꢀUC system comprised of 1 and
triplet acceptor perylene was set up, a purplish white emission was observed with a significant antiꢀStokes shift of
7 nm and high UC quantum yield (Ф’UC) of 17% in CH Cl . Notably, the intramolecular charge transfer of 1
enabled us to tune the emission color of the TTAꢀUC system. On the other hand, the use of selenophene derivative
as a sensitizer instead of 1 led to a decrease in the UC efficiency. The relationship between structure and
(
Ф
F
F
7
2
2
2
sensitizer properties in the TTAꢀUC system is discussed herein.
Introduction
Tripletꢀtriplet annihilation upconversion (TTAꢀUC) has garnered considerable interest because of its ability to
1
ꢀ6
harvest photoexcitation energy. Its amenability with low power and incoherent photo excitation, such as by
7
, 8
sunlight, allows for practical applications in the fields of lighting and photovoltaic devices
as well as
9
ꢀ11
bioimaging.
Typical TTAꢀUC systems consist of a wellꢀtailored sensitizer (S) and emitter (annihilator) (E).
1
Under light illumination, the first singlet excited state of S is ( S*), which is followed by intersystem crossing
3
(
ISC) to the triplet state ( S*). In addition, tripletꢀtriplet energy transfer (TTET) then occurs between the sensitizer
3
and annihilator to enrich E* species. Subsequently, tripletꢀtriplet annihilation of two emitter triplets leads to the
1
production of one emitter in the singlet state ( E*). Given the synthetic diversity of organic dyes, new dyes
applicable to TTAꢀUC constitute an intriguing research target. In this context, we focused on organic sensitizers
12
due to their easier molecular design as compared to annihilators. Because efficient ISC is critical for sensitizers,
heavy atoms such as rare metals (Pt, Ru) and halogens (I, Br) have been employed in most sensitizers. Currently,
effective triplet formation based on efficient ISC and longꢀlived triplet excited states have been explored for new
13, 14
design concepts, such as spinꢀorbital charge transfer intersystem crossing
and radicalꢀenhanced intersystem
15
16
crossing in addition to thermally activated delayed fluorescence, leading to preparation of heavy metalꢀfree
sensitizers.
17ꢀ19
4
,4ꢀDifluoroꢀ4ꢀboraꢀ3a,4aꢀdiazaꢀsꢀindacene (BODIPY)ꢀbased dyes,
exhibit outstanding photophysical
properties such as high molar extinction coefficients (ε), fluorescence quantum yields and photostabilities, which
20ꢀ30
has led to their use in numerous applications.
sensitizer, versatile derivatization of the BODIPY chromophore enables one to tune the photophysical properties.
,6ꢀDiiodeꢀBODIPYs are wellꢀknown triplet photosensitizers, wherein the introduction of iodide directly onto the
Although BODIPY chromophore in itself is not a suitable triplet
31
2
21
dye skeleton results in a heavy atom effect and enhances the ISC process. Considerable efforts have been put
1

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3
2
33
forth to prepare various derivatives and congeners to date. Among them, I -BODIPY (Fig. 1) has been
2
3
4, 35
thoroughly investigated as a sensitizer for TTAꢀUC systems.
In this context, I -BODIPY covalently
2
3
6
connected by perylene was prepared although such a dyad showed no TTAꢀUC behavior. It has also been
3
7
pointed out that the introduction of iodide onto the chromophore leads to photoꢀinduced cytotoxity. Accordingly,
exploration of new BODIPYꢀbased triplet sensitizers is worthwhile because the facile modification of BODIPY
can facilitate the preparation of efficient sensitizers with strong light harvesting abilities. With this strategy in
mind, organoselenium compounds attracted our attentions. The relatively low toxicity has motivated chemists to
prepare a number of seleniumꢀcontaining compounds for application to electroconducting materials and
3
8
39
catalysts. Their pharmacological properties, coupled with the low oxidation potentials have been used to sense
4
0, 41
reactive oxygen species, as well as biologically important thiols.
In addition, redoxꢀresponsive probes based
Given the versatile derivatization of BODIPYs,
4
2, 43
on seleniumꢀcontaining fluorescent dyes have been reported.
the combination of the “heavy atom effect” of selenium and the photophysical properties of the BODIPY
chromophore may facilitate the production of an efficient photosensitizer for TTAꢀUC.
Fig. 1 Chemical structures of BODIPY dyes.
Here, bis(phenylselanyl)ꢀ and diselenophenoꢀBODIPY dyes 1 and 2 were synthesized and investigated as
4
4
sensitizers for UC systems with perylene as a triplet acceptor. Perylene was employed in this study due to the
high fluorescence quantum yield and lower triplet energy than the sensitizer as required for a triplet acceptor in
TTAꢀUC. Subsequently, when a TTAꢀUC system comprised of 1 and perylene was set up, an orangeꢀtoꢀpurplish
white UC emission was observed with a large antiꢀStokes shift of 77 nm and high UC quantum yield (Ф’UC) of
1
7% in CH Cl . To our knowledge, this is the first demonstration that a seleniumꢀcontaining πꢀconjugated system
2 2
can serve as a photosensitizer for applications in TTAꢀUC systems.
Experimental section
General
1
13
1
NMR spectra were taken by a Bruker Avance 500 ( H: 500 MHz, C: 126 MHz) spectrometer. In H and
1
3
C NMR measurements, chemical shifts (
atom bombardment (FAB) mass spectra were obtained on a JEOL JMSꢀ700 spectrometer where
ꢀnitrobenzylalcohol was used as a matrix. The absorption and fluorescence spectra were measured using
δ) are reported downfield from the initial standard Me Si. Fast
4
m
a Shimadzu UVꢀ3600 and a JASCO FPꢀ6500 spectrophotometers, respectively. Elemental analyses were
performed on an Exeter Analytical, Inc. CEꢀ440F Elemental Analyzer. Fluorescence lifetimes were
determined by Hamamatsu photonics QuantaurusꢀTau C11367ꢀ21.
2

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Materials
Unless otherwise indicated, the reagents used for the synthesis were commercially available and used as
4
5
supplied.
8ꢀMesitylꢀ1.3,5,7ꢀtetramethylꢀ4,4ꢀdifluoroꢀ4ꢀboraꢀ3a,4aꢀdiazaꢀsꢀindavene
3,
4
6
2
,6ꢀDiiodoꢀ4,4ꢀdifluoroꢀ1,3,5,7ꢀtetramethylꢀ8ꢀmesitylꢀ4ꢀboraꢀ3a,4aꢀdiazaꢀsꢀindacene
4
and
4
7
4
,4,5,5ꢀtetramethylꢀ2ꢀ(selenophenꢀ2ꢀyl)ꢀ1,3,2ꢀdioxaborolane were prepared according to previously
reported methods, respectively.
4
,4’-Difluoro-1,3,5,7-tetramethyl-8-mesityl -2,6-bis(phenylselanyl)-4-bora-3a,4a-diaza-s-indacene 1
To a solution of (0. 203 g, 0.554mmol) in dry CH Cl (21 mL) was added PhSeCl (0. 210 g, 1.10 mmol).
3
2
2
The reaction mixture was stirred at room temperature for 1.5 h, during which PhSeCl (0. 0209 g, 0.109
mmol) was added as a second portion. After removal of solvent, the residue was chromatographed on
silica gel (Wakogel Cꢀ300) using a gradient eluent benzene in hexane (20 − 50%) and reprecipitated with CH Cl /
2
2
1
MeOH system to afford 0.317 g of 1 as a red solid in 85% yield. H NMR (500 MHz, CDCl ) δ (ppm) : 7.09 –
3
1
3
7
1
.20 (m, 10H), 6.96 (s, 2H), 2.68 (s, 6H), 2.32 (s, 3H), 2.10 (s, 6H), 1.51 (s, 6H) ; C NMR (126 MHz) 159.7,
47.6, 142.8, 139.2, 134.6, 132.5, 131.1, 130.9, 129.4, 129.3, 128.6, 126.0, 118.3, 21.2, 19.60, 14.4, 13.7 ;
+
APCIꢀMS : m/z = 678 [M] elemental analysis calculate (%) for C H BF N Se : C 60.36, H 4.92 N 4.14, found
3
4
33
2
2
2
(
%) :C 60.09, H 4.85, N 4.08
4
,4’-Difluoro-1,3,5,7-tetramethyl-8- mesityl-2,6-di(selenophene-2-yl)-4-bora-3a,4a-diaza-s-indacene 2
To a solution of 4 (0.243 g, 0.393 mmol), 4,4,5,5ꢀtetramethylꢀ2ꢀ(selenophenꢀ2ꢀyl)ꢀ1,3,2ꢀdioxaborolane
0.253 g, 0.983 mmol), Pd(dppf)Cl (0.1 g, 0.1 mmol) and a small amount of Aliquat 336 in deaerated toluene
(
2
(
25 mL), was added 2M K CO aqueous solution (4 mL) under a N atmosphere. The mixture was stirred at 90 °C
2
3
2
for overnight. The resultant solution was poured into water and extracted with AcOEt. The organic phase was
dried over Na SO and filtrated. After removal solvent from the filtrate, the residue was chromatographed on
2
4
silica gel (Wakogel Cꢀ300) using benzene / hexane (1:1 v/v) as an eluent to afford 0.132 g of 2 as redꢀgreen solid
1
in 54% yield. H NMR (500 MHz, CDCl ) δ (ppm): 8.06 (2H, dd, J = 5.60 and 1.15 Hz), 7.31 (2H, dd, J = 5.65
3
Hz and 3.70 Hz), 7.02 (2H, dd, J = 3.65 and 1.05 Hz), 6.96 (2H, s), 2.61 (6H, s), 2.33 (3H, s), 2.16 (6H, s), 1.40
13
(
6H, s). C NMR (126 MHz, CDCl ) δ (ppm): 154.5, 142.6, 140.1, 139.5, 138.9, 134.9, 131.8, 131.2, 130.4,
3
+
1
30.1, 129.7, 129.2, 128.2, 21.2, 19.7, 13.6, 11.8. FABꢀMS: m/z = 626 [M] Elemental analysis: Calcd. for
C H BF N Se : C 57.72, H 4.68, N 4.49, found C 57.70, H 4.65, N 4.49.
30
29
2
2
2
The measurement of fluorescent quantum yield
4
8
F
The fluorescence quantum yields (Ф ) were calculated from eq. (1).
ꢆ
ꢊꢋ
ꢂ
ꢃ(ꢄ)ꢅꢄ
ꢉꢈ
ꢇ
ꢀ
= ꢀ ×
ꢆ
ꢇ
× ꢉ × ꢊ (1)
ꢁ
ꢋ
ꢂ
ꢃ
ꢈ
(ꢄ)ꢅꢄ
ꢈ
Where F(λ) and F (λ) describe the corrected fluorescence intensities of the compound and the reference,
R
respectively, and A and A describe the corresponding absorbance at the excitation wavelength. The reference
R
3

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4
9
used was Rhodamine B (Ф = 97% in EtOH).
R
X-ray crystallography for 1
A red prism crystal of 1 having approximate dimensions of 0.25 × 0.16 × 0.16 mm was mounted on a glass fiber.
All measurements were made on a Xꢀray diffractometer using multiꢀlayer mirror monochromated MoꢀKα
50
radiation (λ = 0.71075 Å). The structure was solved by direct methods (SHELXS2013) and expanded using
Fourier techniques. The nonꢀhydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the
riding model. The refinement was made by using a fullꢀmatrix leastꢀsquares technique (SHELXL2013).
Electrochemistry
−1
Cyclic voltammograms (CV) were recorded on a potentiostat operated at a scan rate of 50 mV s and room
temperature under a N2 atmosphere. The solvent was CH Cl2 containing 0.1 M tetrabutylammonium
2
+
hexafluorophosphate (TBAPF ) as the supporting electrolyte. The potentials were measured against Ag/Ag (0.01
6
+
M of AgNO ) as a reference electrode; ferrocene/ferrocenium (Fc/Fc ) was used as the internal standard and
3
measured to be 0.22 V under same conditions. The onset potentials were determined from the intersection of two
tangents drawn at the rising and background currents of the cyclic voltammogram.
Theoretical calculations
All geometries of the dyes at the ground state were fully optimized by means of the CAM B3LYP/6ꢀ31G(d,p)
level method. Density functional theory (DFT) calculations at the CAMꢀB3LYP/6ꢀ31G(d,p) level were performed
5
1
in the Gaussian 09 package. (Gaussian 09, Gaussian, Inc, Wallingford, CT, 2010) These molecular orbitals were
visualized using Gauss view 5.0.8 program. Based on the optimal structure timeꢀdependent DFT (TDꢀDFT) with
the PCM model was used to calculate the excitation energies of the lowest singlet S and triplet T states of
1
1
sensitizers and annihilator.
Nanosecond transient absorption measurements
Transient absorption measurements were performed at ambient temperature using a monochromator (JASCO
CTꢀ25CP) and a photomultiplier (Hamamatsu Photonics R928) with continuousꢀwave illumination from a Xe
5
2, 53
lamp (JASCO PSꢀX150B).
Time profiles of the photomultiplier signals were integrated using a digital
oscilloscope (IwatsuꢀLeCroy LT342). The timeꢀprofiles recorded pointꢀbyꢀpoint at 10 nm intervals were converted
to absorbance values, and then the transient spectra were obtained using a personal computer. The samples were
deaerated by freezeꢀpumpꢀthaw cycles before the measurements, and were excited at 532 nm using an Nd:YAG
laser (Spectra Physics INDI 40; 7 ns pulse width (FWHM)). The incident laser energy was attenuated to 0.3 mJ
per pulse (transient absorption spectrum) or 0.01 mJ per pulse (timeꢀprofile at 450 nm).
Triplet-triplet annihilation upconversion
The upconversion fluorescence spectra were measured by a modified a JASCO FPꢀ6500
spectrophotometer. A diode laser of 51 mW as maximum output power (RGB Lasersystems, 520 nm) was
4

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used as the excitation light in the TTAꢀUC system. The diameter of the laser spot was 1.1
.3 mm. The power of the laser beam was measured with a photopower meter (ADC, 8230E). The
TTAꢀUC quantum yields (ΦUC) were determined using the fluorescence of I2-BODIPY as the standard
×2.2 to 1.2×
4
3
4
(
Φref = 2.7% in MeCN), the following equation being employed:
2
ΦUC
=
Φref
(
Aref
/
AUC)(IUC
/
Iref) (ηUC
/
ηref) = 0.5Ф’UC
where
Φ
,
A
,
I
and
η
represent the quantum yield, absorbance, integrated fluorescence intensity and the
refractive index of the solvent. The CIE coordinates of the UC emission were derived from spectra
manager (FP6500 control driver). Although the maximum quantum yield (Ф ) of bimolecular TTAꢀUC
UC
process is 50%, related papers mostly adopt the value multiplied by 2 to set maximum quantum yield of 100%. In
16
this study, we have used Ф’UC (2 × ФUC).
Results and Discussions
Synthesis and Characteristics of 1 and 2
Bis(phenylselanyl)ꢀcontaining BODIPY 1 was synthesized through electrophilic substitution of 3 with
phenylselanyl chloride in 85% yield after column chromatography (Scheme 1). To prepare selenopheneꢀderivative
2
,
starting material
3
was subjected to iodination, followed by
a
Suzuki reaction with
4
,4,5,5ꢀtetramethylꢀ2ꢀ(selenophenꢀ2ꢀyl)ꢀ1,3,2ꢀdioxaborolane in the presence of Pd(dppf)Cl . The target
2
seleniumꢀcontaining BODIPYs
1
and 2 were fully assigned based on spectroscopic data.
PhSeCl
Se
Se
N
N
N
N
dry CH2Cl2
5%
B
3
B
1
8
F
F
F
F
1
2
) I2
) HIO3
EtOH
1%
R
R
N
N
9
B
F
F
O
Se
B
4
; R = I
O
Pd(dppf)Cl2
Aliquat 336
2; R =
Se
54%
Scheme 1 Synthesis of seleniumꢀcontaining BODIPY dyes 1 and 2.
A red prism crystal of 1 was successfully obtained for X ray diffraction analysis. The structure belonged to the
orthorhombic P2 2 2 group with four molecules in one unit cell (Fig. 2). As a typical feature, the phenylselenyl
1
1 1
groups at the 2 and 6 positions of the BODIPY core were tilted by 65 ° and 73 ° with respect to the core structure,
5

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as shown in Fig. 2b. Such torsion may be ascribed to intermolecular interactions in the packing structure (Fig. 2c).
Indeed, CHꢀπ interactions were observed between the phenylselanyl unit and mesityl group of the neighboring
molecule.
Fig. 2 Xꢀray crystal structure of 1 where thermal ellipsoids are drawn at the 50% probably level, showing front
view (a), side view (b) and packing structure (c).
2 2
Fig. 3 shows absorption and fluorescence spectra of a series of BODIPYs in CH Cl at 25 °C (data is summarized
in Table S1). SelanylꢀBODIPY 1 absorbs visible light at 518 nm, with a molar extinction coefficient (ε) of 7.84 ×
4
−1
−1
1
0 M cm . Although the λmax value is 16 nm larger than that of parent BODIPY 3, a hypthochromic shift was
observed as compared to iodideꢀappended BODIPY 4. On another front, the λmax value of selenophene derivative
was comparable to that of 4, and 2 exhibited the lowest ε value among the tested dyes. The full width half
maximum (FWHM) of 2 was 52 nm, whereas the FWHM of parent BODIPY 3 was 17 nm. Seleniumꢀcontaining
2
−
1
dyes 1 and 2 exhibited emission bands at 594 and 613 nm, respectively, and large Stokes shifts of about 2500 cm
as compared to 3 were observed. This suggested that the introduction of seleniumꢀcontaining substituents at the 2
and 6 positions of the BODIPY core resulted in pronounced effects on the optical properties. Particularly, larger
Stokes shift in 2 could be responsible for the remarkable geometry relaxation upon photoexcitation; notably, a
54
similar behavior was observed in 2,6ꢀbisthienyl BODIPY. Further, the fluorescence quantum yield (Ф
F
) of 1 was
0
.03, being oneꢀfifth as small as that of 2. Such a low value may be attributed to the efficient ISC between the
F
singlet and triplet state, which was comparable to that of 4 (Ф = 0.05). Considering the electron donor ability of
selenium, we measured the solventꢀdependency of the spectroscopic properties of 1 (Fig. S1 and Table S2). As
solvent polarity increased, the Ф value decreased, and was accompanied by a shift in the emission to a longer
wavelength, despite almost no change in the absorption band. This trend was rationalized by employing a
F
55, 56
LippertꢀMataga plot (Fig. S2),
and the change in the static dipole moment of 1 (ꢁꢁ) was calculated to be 11.2
D, indicating an intramolecular charge transfer (ICT) and a heavy atom effect due to selenium. As shown in Fig.
S3 and Table S2, similar trend was observed in 2 with a relative small ꢁꢁ value (6.74 D; Fig. S4).
6

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2 2
Fig. 3 Absorption (a) and fluorescence (b) spectra of dyes 1 − 4 (1.0 ꢂM) in CH Cl .
The electrochemical properties of the dyes were investigated using cyclic voltammetry (CV). The formal potential
+
Fc/Fc+
+
of F
c
/F
c
(E1/2
) was 0.220 V versus Ag/Ag . Reversible reduction waves were observed with halfꢀwave
red
red
potentials (E1/2 ) of −1.24 V for 1 and −1.41 V for 2, respectively (Fig. 4). Given that the E1/2 of 3 was −1.55V,
selenium incorporation onto BODIPY led to the stabilization of the LUMO energy level. On the other hand, a
ox
reversible oxidation wave was observed for 1 with an E1/2 of 1.01 V. When the measurements were calibrated
57
against ferrocene (−4.8 eV) as the standard, the HOMO energy level of 1 was estimated to be −5.59 eV. Taking
red
into account the LUMO energy level (−3.24 eV), which was estimated from the E1/2 value, the electrochemical
band gap was 2.35 eV (527 nm), which was almost similar to that of the experimental value (Fig. 3). In contrast, 2
showed an irreversible oxidation wave with an Epa of 0.96 V. The HOMO and LUMO of 2 were higher than those
of 1, possibly due to the electron donor ability of the selenophene moiety.
−
1
2 2 6
Fig. 4 Cyclic voltammetry of 1 − 3 in CH Cl solution containing 0.1 M TBAPF . The scan rate is 0.05 V s .
To obtain further insight into the triplet excited state of the sensitizers, timeꢀresolved fluorescence
measurements were carried out (Fig. S5). The fluorescence lifetime (τ ) of 1 was determined to be 0.285 ns in
F
1
CH
2
Cl
2
, which was significantly shorter than that of 2 (1.84 ns). The O
2
production quantum yields (Ф
ꢁ
) of the
58
dyes were also evaluated using 1,3ꢀdiphenyisobenzofuran (DPBF), a wellꢀknown singlet oxygen indicator. As
shown in Fig. S6, the absorption intensity of DPBF in CH Cl decreased with time, due to photooxygenation.
Notably, 1 showed a high Ф of 0.86, which was almost similar to that of I -BODIPY. Moreover, the Ф value of
was oneꢀthird that of 1. Assuming that the energy transfer efficiency could be 100% from the excited T
2
2
ꢁ
2
ꢁ
2
1
state of
3
the dye to O
2
, Ф
ꢁ
= ФISC (quantum yield of intersystem crossing). Combined with the fluorescence lifetime and
Ф
ISC values, the relevant parameters of the photophysical process were obtained using the following equations,
eqs. (1) – (3):
7

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ꢍ
τꢌ =
(1)
ꢎꢌ + ꢎ + ꢎꢏꢑꢐ
ꢏꢐ
ꢎꢌ
Φ =
ꢌ
(2)
ꢎꢌ + ꢎ + ꢎꢏꢑꢐ
ꢏꢐ
ꢎꢏꢑꢐ
Φꢏꢑꢐ = Φ =
ꢀ
(3)
ꢎꢌ + ꢎ + ꢎꢏꢑꢐ
ꢏꢐ
where k
F F
, kIC and kISC are the rate of fluorescence, internal conversion, and intersystem crossing, respectively. k ,
9
−1
k
IC and kISC are summarized in Table 1. The kISC value of 1 was calculated to be 3.02 × 10 s , which was 19 times
larger than that of 2, indicating that the singlet excited state of 1 was effectively converted into the corresponding
triplet state. Furthermore, the phosphorescence spectra of 1 and 2 were measured at 77 K; peaks at 746 and 756
59
nm were observed in toluene for 1 and 2, respectively (Fig. S7). The ꢁEST (= E
for 1 and 0.38 eV for 2. The slight difference in ꢁEST values indicates that the heavy atom effect of selenium
affected ФISC
S T
– E ) was calculated to be 0.43
.
Table 1. Photophysical properties of dyes 1 and 2 in CH
Cl
2 2
at 25 °C.
4
−1
−1
b
d
c
Dye
λ
abs
/
ε / 10 M cm
λ
F
Ф
F
τ
F
/ns
Ф
ꢁ
=
k
F
/
k
IC
−1
/
k
ISC
/
p
λ /nm
a
−1
−1
nm
/nm
ФISC
ns
0.105
0.0978 0.282
ns
ns
1
518
532
7.84
6.15
594
613
0.03
0.16
0.285
1.84
0.86
0.386
3.02
0.163
746
756
2
0.30
a
b
The excitation wavelengths of 1 and 2 were 490 nm, respectively. Singlet state lifetime, excited at 405 nm for 1
c
and 590 nm for 2, respectively. Phosphorescence maxima in toluene at 77 K, excited at 540 nm for 1 and 2,
d
respectively. Singlet oxygen quantum yield.
A better understanding of the photophysical properties of 1 allowed us to carry out density functional theory
60
(
DFT) and timeꢀdependent density functional theory (TDꢀDFT) analysis at the CAMꢀB3LYP/6ꢀ31G(d,p) level.
Structural optimizations revealed that the dihedral angles between phenylselanyl plane and BODIPY core were
3 ° and 71 °. The calculated absorption bands were located at 435 and 349 nm, and were assigned to the S → S1
6
0
and S → S transitions, respectively. Based on the oscillator strength, the main transition was assigned to the S
0
0
2
→
S1 transition where the HOMO → LUMO was the major configuration of the transition. The electronꢀdensity
distribution in the HOMO was spread over the BODIPY core and selanyl moieties, whereas the corresponding
surface plot of the LUMO spread over the BODIPY core. This represents a π−π* transition involving the ICT
from the phenylselanyl moiety to the BODIPY core, which was supported by the solvent polarityꢀdependent
photophysical properties. On another front, the S → S transition was assigned to the broad absorption band at
0
2
3
−1
−1
418 nm (
1
ε = 5.91×10 M cm ), as shown in Fig. 3 (vide supra). The T state of 1 was optimized where the
orientation of the phenylselanyl moieties was similar to that in the S state. The vertical S ꢀ T excitation was
0
0
1
calculated by the DFT/TDꢀDFT method. The molecular orbitals in the transition was shown in Fig. 5. The T state
1
8

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is localized on the BODIPY core and does not have CT character. Fig. 5 also shows the spin density surface at the
optimized triplet geometry, indicating that the T state was located on the BODIPY core. Such localization
strongly supports that the direct introduction of selenium onto the BODIPY core should be effective for the
production of the T state through an efficient ISC path. To verify the production of the triplet state upon
1
1
photoexcitation, nanosecond transient absorption measurements were carried out upon pulsed excitation at 532 nm.
For the measurements, molecular oxygen dissolved in the solution was carefully removed by freezeꢀpumpꢀthaw
cycles. And the pulsed laser energy was low enough to avoid T−T annihilation. As shown in Fig. 6a, a broad
transition absorption band at ca 450 nm was observed, being attributable to absorption of the T
decay profile at 450 nm allowed us to determine the lifetime (τ) of 1.4 ms. The T energy was evaluated to be 1.66
eV from the phosphorescence spectrum. We reasoned that tripletꢀtriplet energy transfer (TTET) between these
1
state of 1. The
1
4
4
dyes and perylene (1.53 eV) would be thermodynamically possible.
2 2
Fig. 5 A simplified Jablonski diagram illustrating the photophysical properties of 1 in CH Cl where selected
frontier orbitals in the singlet and triplet excited states are described. The excitations were calculated by
DFT/TDꢀDFT at the CAMꢀB3LYP/6ꢀ31G (d,p) level with Gaussian 09W. Phosphorescence spectrum was
measured at 77 K in toluene. IC and ISC denote internal conversion and intersystem crossing, respectively.
Fig. 6. (a) Nanosecond transient absorption spectra of 1 (10 µM) in deaerated toluene. (b) Decay profile of the
transient at 450 nm of 1 at room temperature.
TTA-UC study in solution
We investigated the TTAꢀUC process of 1 (10 µM) and perylene as an annihilator in deaerated toluene upon
photoꢀexcitation with a 524 nm laser (5 mW). Upconverted emission with λem values of 449 and 473 nm indicated
a significant antiꢀStokes shift of 75 nm in the UC process, which was consistent with the fluorescence spectrum of
perylene (Fig. 7a). The longer emission at 591 nm was ascribed to the fluorescence of 1. To explore the optimal
conditions for UC behavior, upconverted emissions were measured by varying concentration of perylene. Fig. 7b
9

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shows the relationship of Ф’UC vs. perylene concentration; the curve almost leveled off when more than 20 ꢂM
6
1
perylene was added. As a result, the biꢀmolecular system of 1 (10 ꢂM) and perylene (30 ꢂM) provided an
efficient Ф’UC of 12% in deaerated toluene under excitation with a 524 nm laser (5.0 mW). Compared to the Ф’UC
of 0.5% for the related system comprised of 2 and perylene (Fig. S8), the direct connection of selenium heavy
atom into BODIPY core was more effective for the upconverted behavior. Although dye 1 emitted orange light
−
2
under irradiation at 532 nm (100 mW cm ), the addition of perylene to the solution resulted in purplish white
light (Fig. 8), which deviated marginally from the ideal white light (0.33, 0.33) of the CIE chromaticity
coordinates. To determine if this behavior originated from the TTAꢀUC mechanism, we measured the excitation
powerꢀdependency on the upconverted emission in deaerated toluene. A linear relationship was obtained from the
3
, 62,
doubleꢀlogarithm plot of the UC emission intensity as a function of incident power density at 524 nm (Fig. 9).
63
A slope of 1.95 was obtained at a lower power range, and a slope of 1.11 was obtained at a higher power density
−
2
64,
(
≥ 116 mW cm ). As a result, the crossing point of the two fitting lines gave an excitation power threshold (Ith)
65
−2
of 116 mW cm , which represented the main parameter for the TTAꢀbased upconversion process at which half
of the triplet sensitizer was subjected to TTA. The intermolecular TTET process between the triplet excited states
of 1 and perylene is among the key parameters for TTAꢀUC, and was investigated by quenching titrations of the
phosphorescence of 1 upon adding increasing amounts of perylene (Fig. S9). Although the phosphorescence
intensity was decreased upon addition of perylene, our attempt to determine the SternꢀVolmer constant was
p
unsuccessful because of the low intensity. It has been reported that the Ф values of halogenated BODIPYs are in
−3 66
the order of 10 .
Fig. 7 (a) Change in upconverted fluorescence upon addition of different amounts of perylene in deaerated toluene.
(
b) Ф’UC value as a function of perylene concentration in deaerated toluene. Excited with a 524 nm laser (5 mW).
1] = 10 ꢂM.
[
Fig. 8 (a) Images of fluorescence of 1 (10 ꢂM) (A) and 1 (10 ꢂM) plus perylene (30 ꢂM) (B). (b) CIE
−
2
chromaticity coordinates for the emission behaviors of (A) and (B). λex = 532 nm (100 mW cm ).
10

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Fig. 9 Excitation power dependency of upconverted perylene emission with 1 as a sensitizer. (a)
Photoluminescence spectra. (b) UC emission intensity at 449 nm at different power intensities. [1] = 10 µM,
−
2
[
perylene] = 30 µM in degassed toluene upon irradiation with a 524 nm laser (5.24 ~ 2650 mW cm ).
Considering the ICT characteristics of dye 1, we measured the UC behavior in deaerated CH
Ф’UC of 17% was obtained, which was larger than that in deaerated toluene. As a control experiment, a
biꢀmolecular system composed of I -BODIPY (10 ꢂM) and perylene (30 ꢂM) was investigated under similar
conditions (Fig. S11). A Ф’UC of 16% was obtained, which was similar to that of 1 and perylene in deaerated
CH Cl . In addition, the ICT of sensitizer 1 enabled us to tune the emission color of the UC system by varying the
solvent. Upon replacing toluene with CH Cl , a purplish white emission corresponding to CIE coordinate (0.34,
.26) changed to that of (0.25, 0.18) (Fig. S12). Such solvent dependency on the upconversion efficiency may be
2 2
Cl (Fig. S10); a
2
2
2
2
2
0
due to the tripletꢀstate generation efficiency of 1.
Conclusion
In this work, seleniumꢀcontaining BODIPY dyes 1 and 2 were synthesized as photosensitizers for TTAꢀUC
systems. The introduction of a phenylselanide group at the 2 and 6 positions of the BODIPY core endowed the
dye with ICT characteristics. Although electronꢀdonor strength of selenophene unit in 2 is larger than that of
phenylselanyl unit in 1, the direct binding of selenium to the pyrrole carbon led to a heavy atom effect on the
photophysical properties. Significant upconverted fluorescence by perylene was observed with dye 1 upon
excitation with a 524 nm laser (5.0 mW). Subsequently, a significant antiꢀStokes shift of 77 nm with a Ф’UC value
2 2 2
of 17% was observed in CH Cl ; the quantum yield was comparable to that of I -BODIPY. Due to the π−π*
transition involving the ICT of 1, the emission color of the solution, arising from the fluorescence of sensitizer 1,
and the upconverted emission of perylene could be tuned by the solvent polarity. The excited state of 1 was
1
rationalized by timeꢀresolved fluorescence spectroscopic analysis, phosphorescence measurements, O
2
detection
and nanosecond transient absorption spectroscopy. The simple and straightforward synthesis of
seleniumꢀcontaining πꢀconjugated systems would be useful for the preparation of photosensitizers applicable to
NIR upconversion, which deserves further investigation.
Acknowledgments
This research was supported by The Iwatani Naoji Foundation’s research grant and JSPS KAKENHI Grant
Numbers 26620033 and 17H06375.
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14

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New 2,6-bis(phenylselanyl)-BODIPY 1 was synthesized and served as a triplet photosensitizer for triplet-triplet
annihilation upconversion (TTA-UC).