PET-based bisBODIPY photosensitizers for highly efficient excited triplet state and singlet oxygen generation: Tuning photosensitizing ability by dihedral angles

Article abstract of DOI:10.1039/c7cp02645e

Herein, four covalent BODIPY heterodimers that differ by dihedral angles were shown to be highly efficient excited triplet state (T₁) photosensitizers (PSs) for singlet oxygen formation with a quantum yield (Φ_Δ) of up to 0.94 as comp

Full text of DOI:10.1039/c7cp02645e

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PET-based BisBODIPY photosensitizers for Highly efficient excited
triplet state and singlet oxygen generation: Tuning
photosensitizing ability by the dihedral angles
Received 00th January 20xx,
Accepted 00th January 20xx
Xian-Fu Zhang,a,b,* Xudong Yanga, Baomin Xuc
DOI: 10.1039/x0xx00000x
Four covalent BODIPY hetero-dimers which differ by dihedral angles were shown to be highly efficient excited triplet state
(T1) photosensitizers (PS) for singlet oxygen formation with quantum yield (Φꢀ) up to 0.94 in contrast to their respective
monomers which have only negligible Φꢀ of ca. 0.060. More interestingly, these PS generate T1 by charge recombination
mechanism rather than the traditional inter-system crossing. The photosensitizing ability of the dimers is easily tuned by
either the dihedral angle (between the two linked BODIPYs) or solvent polarity. Laser flash photolysis, time-resolved and
steady state fluorescence, quantum chemical calculation as well as thermodynamic analysis were employed to study the
associated photophysical process to reveal the T1 formation mechanism: photo-induced electron transfer (PET) followed
by charge recombination. Due to its heavy atom free, polarity selectivity, high efficiency and easy tunability, this PET based
PS and its mechanism are very useful in developing new PS for photodynamic therapy of tumor, photobiology and organic
photochemistry.
www.rsc.org/
and 2), ISC efficiency can be very significantly enhanced due to
heavy atom effect (HAE).
Introduction
However, Akkaya and us showed recently that the covalent
The development of excited triplet state photosensitizing
materials has been an active research field,1,2 since the excited
triplet states of organic molecules play crucial roles in many
applications, such as electroluminescence of OLED,3-9
photodynamic therapy (PDT) via sensitization of singlet oxygen
1O2(1ꢀg),10-12 phosphorescent bioimaging or molecular
sensing,9,13 photoinitiated polymerization, and photocatalytic
organic reactions.14-16 An ideal triplet photosensitizer (PS) must
have high triplet state formation quantum yield and a long
excited triplet state lifetime.1 The excited triplet state (T1) of
an organic aromatic molecule is usually originated from the
inter-system crossing (ISC) of its excited singlet state S1 in
BODIPY dimers can efficiently generate singlet oxygen,11,19,20
while Zhao and us reported that BODIPY dimers can yield long-
lived T1 production with good quantum yield,11, 19-22 although
their corresponding molecular monomers are non photoactive
in the T1 and singlet oxygen formation, unless iodo- or bromo-
atoms are introduced to enable HAE as evidenced in our
previous laser flash photolysis (LFP) study.18,21 These results
indicate that the BODIPY dimers can act as novel
photosensitizers with desirable unique properties, such as
halogen and heavy metal atom free, which reduces health risk
in their applications. Nevertheless, some very important issues
for the novel PSs have not been addressed, including: 1) the
general mechanism of T1 generation, 2) the factors that control
the T1 formation efficiency of the dimers, and 3) the methods
to tune the efficiency of the novel PSs.
which one electron changes its spin direction (S1→T1). To
obtain an efficient triplet PS, four strategies are often used in
designing its molecular structures: 1) bromo- or iodo-heavy
atom substitution,17,18 2) using
coordination center, 3) using n-
a
large metal ion as
* transition (carbonyl
Taking account of these considerations, we designed and
synthesized BODIPY dimers d1 to d4 (Figure 1) for two
purposes: 1) to elucidate how dihedral angles in a bisBODIPY
affect triplet state formation and if it can be used to tune T1
formation efficiency, 2) to reveal the mechanism and unique
features for the efficient formation of excited triplet state for
this method. Based on the experimental results we propose a
PET-based T1 generation mechanism for a dimer D containing
monomer M1 and M2. For this type of PET-based
photosensitizer, we show that Φꢀ can be tuned from 0.24 to
0.94 by changing the dihedral angle of the dimers.
π
conjugation with a core
PS1 onto a PS2 molecule (3PS1-PS2
π
-system), 4) linking a spin converter
PS1-3PS2). For process 1)
→
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2H), 7.66 (ddt, J = 23.8, 14.9, 7.4 Hz, 5H), 7.21 (s, 1H), 7.17 (t, J
= 4.6 Hz, 3H), 6.76 (d, J = 3.9 Hz, 1H), 6.63-6.53 (m, 2H) ppm.
Experimental
General
13C NMR (151 MHz, Chloroform-d)
δ 148.76, 148.59, 143.61,
Reagents and solvents for synthesis were used as received
from commercial suppliers unless noted otherwise. All solvents
for spectrum measurements and 1,3-diphenylisobenzofuran
(DPBF) photosensitized oxidation are dried and re-distilled
according to standard procedures. All reactions were
performed in oven-dried or flame-dried glassware unless
otherwise stated, and were monitored by TLC using 0.25 mm
142.34, 136.67, 134.83, 134.69, 133.97, 133.01, 131.61, 130.57,
130.51, 130.07, 128.93, 125.33, 121.19, 118.43 ppm. IR
(KBr)/cm-1: 733, 982, 1074, 1121, 1198, 1402, 1514, 1560
(ν BODIPY ring). UV/vis (DCM) λ
max(lgε/M-1cm-1)/nm: 360(4.43),
506(4.96). HRMS (APCI) Calcd. For C24H16B2F4N4 [M F]+ calcd.
−
439.1513, found 439.1510.
1
silica gel plates with UV indicator (60F-254). H and 13C NMR
Synthesis of BODIPY-dimer d2
are obtained on
a
Bruker Top Spin 600 MHz NMR
) are
The synthesis and purification procedure are the same as that
of d1. BODIPY 2b (62 mg, 0.18 mmol), pyrrole (2 ml, 28 mmol),
spectrometer at room temperature. Chemical shifts (
δ
given in ppm relative to CDCl3 (7.26 ppm for 1H and 77 ppm for
13C) or to internal TMS (0 ppm for 1H). High-resolution mass
spectra were obtained using APCI-TOF in positive mode on a
Thermo Fisher MS spectrometer. IR spectra were recorded at
room temperature on a Shimadzu FTIR-8900 spectrometer.
UV-visible spectra were recorded on an Angilent 8454
spectrophotometer using 1 cm matched quartz cuvettes.
Fluorescence spectra were acquired on a FLS 920 instrument
of Edinburgh Instruments Ltd. Transient Absorption spectra
were recorded with an LP920 laser flash photolysis system of
Edinburgh Instruments Ltd.
DDQ (60 mg, 0.24 mmol), dark red solid, yield 11% (10 mg), mp
1
220-223 oC; H NMR (600 MHz, Chloroform-d)
δ 7.89 (s, 2H),
7.59-7.50 (m, 3H), 7.41-7.33 (m, 2H), 6.88 (d, J = 4.2 Hz, 2H),
6.52 (dd, J = 4.2, 1.8 Hz, 2H), 6.15 (s, 1H), 2.65 (s, 3H), 2.50 (s,
3H), 1.45 (s, 3H), 1.29 (s, 3H) ppm; 13C NMR (151 MHz,
Chloroform-d)
δ 159.86, 151.78, 146.31, 144.15, 143.23,
142.71, 140.08, 139.77, 135.58, 134.39, 133.23, 130.89, 130.73,
129.49, 129.45, 127.69, 124.34, 123.18, 118.84, 118.56, 118.53,
15.01, 14.74, 13.62, 13.27 ppm. IR (KBr)/cm-1: 721, 976, 1074,
1114, 1191, 1263, 1409, 1508, 1541 (
ν
BODIPY ring); 1309,
max(lgε/M-1cm-
1473,2852, 2924 CH3); UV/vis (DCM) λ
(
ν
The detailed procedures for singlet oxygen detection by
NIR spectra and DPBF as chemical trapping agent,
photophysics and photochemistry measurements are provided
in electronic supporting information (ESI), which is basically
similar to our previous report.20, 21, 23
1)/nm:355(4.13),520(4.92); HRMS (APCI positive): m/z calcd.
for C28H25B2F4N4 [M+H]+ calcd. 515.2201, found 515.2191;
C28H24B2F3N4 [M-F]+ 495.2139, found 495.2138.
Synthesis of BODIPY-dimer d3
The preparation procedure is the same as that for the dimer
d1, BODIPY 2a (47 mg, 0.16 mmol), 2,4-dimethyl-pyrrole (2 ml,
29 mmol), DDQ (65 mg, 0.29 mmol), red solid in 24% yield (21
Synthesis and Characterizations of Compounds
BODIPY monomer m1, m2, m3, m4, 2a, 2b were synthesized
according to the procedure in our previous report.19 The dimer
d1 to d4 were prepared by the procedure shown in Scheme 1.
Full details for the synthesis, NMR and MS spectra are given in
ESI.
mg). 1H NMR (600 MHz, Chloroform-d)
7.81 (s, 1H), 7.63 7.47 (m, 3H), 7.28 (s, 3H), 7.10 (d, J = 4.4 Hz,
δ 8.10 (d, J = 8.4 Hz, 1H),
−
1H), 6.87 (d, J = 8.4 Hz, 1H), 6.68 (d, J = 4.5 Hz, 1H), 6.02 (d, J =
8.7 Hz, 1H), 1.60 (s, 6H), 1.28 (s, 6H) ppm; 13C NMR (151 MHz,
Chloroform-d)
δ 155.87, 146.84, 142.64, 140.96, 140.07,
Synthesis of BODIPY-dimer d1
136.25, 135.70, 134.56, 133.56, 133.44, 133.27, 131.71, 131.37,
130.48, 129.33, 128.76, 121.51, 120.04, 15.29, 14.66 ppm. IR
(KBr)/cm-1: 723, 983, 1074, 1161, 1188, 1402, 1465, 1514,
To a mixture of 2a (47 mg, 0.16 mmol) and pyrrole (2 ml, 28
mmol) was added trifluoroacetic acid (0.021 mL, 0.27 mmol)
under argon. The reaction mixture was stirred at room
temperature for 30 min, quenched by adding 30 mL aqueous
solution of NaOH (0.2 mol L-1), extracted with CH2Cl2 and dried
over anhydrous Na2SO4. Organic layers were combined and
evaporated under vacuum. The residue was purified through
1539 (
(DCM)
ν BODIPY ring); 1305, 1476, 2854, 2924 (ν CH3). UV/vis
λ
max(lgε/M-1cm-1)/nm:357(4.18),508(4.90). HRMS (APCI
positive): C28H24B2F3N4[M-F]+, calcd. 495.2139, found 495.2138;
C28H25B2F4N4 [M+H]+ calcd. 515.2201, found 515.2191.
column
chromatography
(silica,
CH2Cl2)
afforded
Synthesis of BODIPY-dimer d4
dipyrromethane intermediate, which was dissolved in 20 ml
CH2Cl2 and directly used for the subsequent oxidation with
DDQ (66 mg, 0.29 mmol) (1 h at room temperature). The
resultant mixture was further treated with triethylamine (1 ml,
7.2 mmol) for 20 min, and complexed with boron trifluoride
etherate (3 ml, 23.9 mmol) for 2 hrs at room temperature.
Solvent was removed under vacuum and the reside was
purified through column chromatograph (silica, hexane/ethyl
The synthesis and purification procedure are the same as that
for the dimer d1, BODIPY 2b (62 mg, 0.18 mmol), 2,4-dimethyl-
pyrrole (0.2 ml, 1.9 mmol), DDQ (60 mg, 0.24 mmol), red solid
in 9% yield (9 mg). IR (KBr)/cm-1: 723, 979, 1070, 1157, 1193,
1406, 1512, 1544 (
CH3); UV/vis (DCM)
ν
λ
BODIPY ring); 1309, 1465, 2852, 2924 (
max(lgε/M-1cm-1)/nm: 369(3.89),507(4.94).
7.53 (h, J = 3.6 Hz, 3H),
ν
1H NMR (600 MHz, Chloroform-d)
δ
7.31 (dd, J = 7.1, 2.2 Hz, 2H), 6.10 (s, 1H), 6.01 (s, 2H), 2.62 (s,
acetate = 3/1, v/v) to afford d1 as a red solid in 31% yield (22
1
mg). H NMR (600 MHz, Chloroform-d)
3H), 2.55 (s, 6H), 2.44 (s, 3H), 1.73 (s, 6H), 1.44 (s, 3H), 1.22 (s,
3H) ppm; 13C NMR (151 MHz, Chloroform-d)
δ 8.20 (s, 2H), 7.93 (s,
δ 158.66, 155.69,
2 | J. Name., 2012, 00, 1-3
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150.49, 145.55, 142.45, 138.30, 134.48, 133.65, 132.42, 131.82, PSs, and the corresponding Φꢀ value can be as high as 0.94
130.93, 129.42, 127.72, 125.60, 122.49, 121.22, 14.67, 14.00, which is comparable to the HAE of bromine or iodine
12.76, 12.12 ppm. HRMS (APCI): m/z C32H33B2F4N4 [M+H]+ substitution. Furthermore, Φꢀ values of the dimers can be
calcd. 571.2827, found 571.2821; C32H32B2F3N4 [M-F]+ calcd. tuned by adding more methyls on the
π-skeleton in the
551.2765, found 551.2764.
presence of the meso-phenyl, different from adding more
halogen atom Br or I for HAE method. One obvious advantage
of the current method is that the obtained PS is halogen free.
Obviously, the four dimers show very different capability to
Results and discussion
yield singlet oxygen (d1<d3<d4<d2) although their core dimer
The four BODIPY dimers were synthesized by the procedures
shown in Scheme 1, HRMS, NMR, IR, and UV-Vis data of the
purified samples are well consistent with their structures. The
molecular structure and geometry for the dimers are shown in
Figure 1. The bisBODIPYs belong to the J-type dimers according
to molecular exciton theory, since all dimers are formed by
head-to-head linking of two planar π−systems.24 The
calculated dihedral angles of the dimers are 89.5o (d4) > 88.3o
structure is the same. Three questions arise: i) what makes the
dimer d1 to d4 be good photosensitizers although their
monomers m1 to m4 are not? ii) Why does the
photosensitizing capability ranks in the order d1<d3<d4<d2? iii)
Does exciton coupling or singlet fission play roles? A detailed
photophysical study was carried out to reveal the mechanism.
(
d3) > 62.6o (d2) > 42.9o (d1).
Detection of Triplet excited state by laser flash photolysis
To be certain that d1 to d4 are really good 1O2 photosensitizers
Singlet oxygen (1ꢀg) detection by NIR emission and trapping agent
(PSs), one needs to show that the BODIPY dimers can indeed
→ →
1PS 3PS) efficiently, and
1
We then recorded NIR luminescence of singlet oxygen O2(1ꢀg
)
generate excited triplet state T1 (PS
then T1 can effectively transfer energy to molecular oxygen to
to examine the photosensitizing ability of the four BODIPY
dimers. With excitation at 505 nm, strong NIR luminescence
band at 1270 nm was observed for the four dimers in air
saturated solutions, and the emission decay showed a lifetime
of 61 ms, consistent with the known lifetime 59 ms of singlet
oxygen in CCl4 (Figure 3).25 The 1270 nm band was absent
when the solution was argon saturated or a dimer compound
was not added into the solution, which clearly shows that the
1
form O2 (3PS+O2 PS+1O2). We used laser flash photolysis to
→
identify T1 generated by the BODIPY dimers. Figure 4 shows
the strong and well resolved transient absorption spectra of
the dimer d2 and d4 in argon-saturated solution, in which the
425 nm band is assigned to triplet-triplet (T1-Tn) absorption as
described below. BODIPY monomers m1 to m3, on the other
hand, did not yield detectable transient absorption signal
1
1270 nm band is due to O2(1ꢀg) and the presence of singlet
within ns to ꢁs range in any solvent, i.e. the formation yield of
excited triplet state for the monomers is too low to be
recorded, i.e. ISC from S1 to T1 of the monomers is not efficient.
Dimer d2 and d4 show similar TA spectra. For example,
both of them showed a positive peak at 425 nm, and a
negative peak with the minimum at 510 nm (Figure 4). The
shape and position of the spectra are very similar to the
reported T1-Tn absorption spectra of the bromo-BODIPY dyes
oxygen is due to the photosensitization of the dimers.
DPBF was used as the chemical substrate to test the
photosensitizing ability of the bisBODIPYs. The decrease of
absorption at 410 nm (Figure 3) showed rapid DPBF
decomposition, but it occurred only in the co-presence of a
BODIPY dimer, light irradiation (505 nm absorbed by the
dimers) and molecular oxygen. On the other hand, DPBF
absorption showed no change if anyone of the BODIPY dimer,
oxygen, or the light was absent. Figure 3 also shows that no
bleaching of the BODIPY dimers occurred during the light
irradiation period, which means that they are involved in only
physical but not chemical processes, i.e. the dimers only act as
photosensitizers within this irradiation time period.
in our previous report (in which T1 is generated by internal HAE
of Br atoms).20,
21
This similarity suggests that the positive
signal of the dimer d2 and d4 is also due to T1-Tn absorption. In
addition, the following spectral behavior is also typical of the
T1-Tn absorption. The minimum of the negative absorption
exhibited peaks matching the maximum of the corresponding
ground state absorption. With the decrease of the positive
signal (T1 decay), the negative signal increases (S0 formation),
and the positive bands are separated from the ground state
bleaching with well defined isosbestic points. The presence of
isosbestic points indicates only two states are involved in the
Based on the DPBF oxidation data, the formation quantum
yield of singlet oxygen (Φ
0.94, 0.81, 0.39 and 0.24 for d2
ꢀ) in toluene was determined to be
d4 d3 and d1, respectively
,
,
(Table 1). Clearly all four dimers d1 to d4 can generate singlet
oxygen in good yield. dimer d2 and d4, in particular, show very
high Φꢀ values of 0.94 and 0.81 respectively, making them
excellent singlet oxygen PSs. In contrast, all the monomers
show negligible ability to produce singlet oxygen (Φꢀ is 0.024,
transformation (T1→S0).
The decay curve at 425 nm can be well fit by the mono
exponential function (Figure 4 right), indicating only one
transient species (triplet state T1) is present in the solutions.
0.065, and 0.083 for m1, m3, and m4, respectively). Since
DPBF is a specific chemical trapper of singlet oxygen (1ꢀg), we
can conclude that the BODIPY dimer d2 and d4 are excellent
singlet oxygen PSs.
The triplet lifetime (
s for d2, which are sufficiently long for photosensitizing the
production of singlet oxygen.
In air saturated solution, the τT value is shortened
dramatically. For d4 τT was decreased from 28 s to 0.31 s,
τT) is computed to be 28 ꢁs for d4 and 23
ꢁ
These results show that the covalent dimerization method
can be used to design and synthesize excellent singlet oxygen
,
ꢁ
ꢁ
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but TA spectra still showed the same shape and position. The BODIPY units are forced to adopt perpendicular orientation
triplet quenching rate constant by oxygen can be then due to steric congestion, their electronic clouds have the least
evaluated to be 1.6×
109 M-1 s-1, which is close to the diffusion overlapping so that the overall energy is the lowest.
rate constant. This effective oxygen quenching also suggests Nevertheless, d4 is highly more photoactive than d1 and d3 in
that the positive bands are indeed due to T1-Tn triplet producing T1 and singlet oxygen as shown in the previous
absorption. Since TA spectra were not changed during O2 section, which strongly suggests that exciton coupling plays no
quenching, the T1 quenching by oxygen must be a physical important role in forming T1 and singlet oxygen.
process, i.e. T1 + O2 →
The triplet quantum yield (
S0 + 1O2.
Dimer d1, on the other hand, exhibits quite different
Φ
T) was also measured for absorption spectrum from its monomer m1 (Figure 5). A new
different dimers. ΦT value is close to the Φꢀ value for each red-shifted band occurs at 526 nm (suggesting that d1 is
indeed of J-type dimer), together with a weak shoulder at
dimer (Table 1), suggesting that 1O2 (1ꢀg) is generated by T1.
Dimers d1 and d3 showed similar TA spectra (see around 500 nm, this 500 nm band is located at the same
supported information) to that of the dimer d2 and d4, so is position as that of m1 unit.
the T1 decay and oxygen quenching behavior, this suggests
that all BODIPY dimers can generate T1 state and singlet the strong overlapping of electronic clouds between m1 and
oxygen efficiently upon light absorption, while all the m2 units in d1 (since the optimal dihedral angle between
monomers do not show significant photosensitizing ability.
two BODIPY units in d1 is 42.9o) which causes strong J-type
The presence of the red-shifted band at 526 nm is due to
θ
Nevertheless, the LFP study did not tell why T1 is produced exciton coupling. Due to the absence of any methyl hindrance,
efficiently from the dimers and why the four dimers show in solution state each BODIPY fragment of dimer d1 can rotate
different triplet formation quantum yields. To solve the in a large freedom with
puzzles we measured the singlet excited-state (S1) properties is the state at which the energy is the lowest), the energy at
θ
(dihedral angle) from 0 to 180o (42.9o
of the dimers in different solvents, since S1 is usually the different
θ
was computed and shown in Figure 6. With the
precursor of T1.
change of
θ
in d1, there are two most stable conformations at
θ
1=132o and
θ θ
3=42.9o respectively, and 2=90o is a third
Correlation of photoactivity with exciton coupling and molecular
structure revealed by ground state UV-Vis absorption spectra
conformation for the conversion from θ θ
1=132o to 3=42.9o (or
the reverse), therefore the following thermal equilibrium
exists:
The much higher photosensitizing ability of a dimer (M1-M2)
than the monomers is certainly due to the intramolecular
interaction of the two BODIPY moieties. However, this
interaction could be of either ground state or excited state.
We then examined the intramolecular ground state
interaction and its relation with the enhanced photosensitizing
ability. The ground state interaction between two
chromophores in a dimer, also called exciton coupling, is
reflected in the UV-vis absorption spectra according to
molecular exciton theory (MET).24 MET predicts that a J-type
dimer is photoactive and exhibits red-shifted band maximum,
while an H-type dimer is non photoactive and shows blue-
shifted band. All the dimers in this study are photoactive in
producing singlet oxygen and excited triplet state, since they
are all J-type dimers. A perfect J-type dimer has a dihedral
angle of 0o, so d1 has the least deviation from the perfect J-
dimer while d4 has the largest deviation from it based on their
dihedral angle values. To reveal the intensity of exciton
coupling in the dimers, the UV-Vis absorption spectra of the
BODIPY dimers are compared with that of the corresponding
monomers, as shown in Figure 5.
M1-M2(
In other words, dimer d1 mainly exists in the three
conformations, and those with M1-M2 (
~90o) are responsible
θ
~132o)
≒M1-M2(
θ
~90o)
≒
M1-M2(
θ
~42.9o).
(1)
θ
for the band ca. 500 nm, since the two monomer moieties in
this conformer are perpendicular and show little exciton
coupling. This explains the observed band at ca. 500 nm, which
is the same as that of monomer m1 (e.g. M1-M2 + h
[M2] or [M1]-[M2]*). In fact the two stable states will show
similar exciton coupling, since
1=132o (i.e. 1=-48o) and
3=42.9o lead to the very similar orientation between two
BODIPY units. Dimer d4, however, has only one minimum
energy point at (Figure 6).
1=89.5o with the change of
ν→[M1]*-
θ
θ
θ
θ
θ
The normalized absorption spectra of d1 to d4 are
compared in Figure 7, which reveals the difference in exciton
coupling for different dimers. Dimer d2 displays a new red-
shifted shoulder band (Figure 7), but its relative intensity is
lower than that of dimer d1, indicating a weaker exciton
coupling due to the dihedral angle
θ
of 62.6o (>42.9o of d1 and
deviated more from 0o). Dimer d3 shows an even weaker band
at 520-550 nm (slightly broad and higher than that of d4 at ca.
550 nm, Figure 7) suggesting an even weaker exciton coupling
than that in d2, because of a dihedral angle of 88.3o, close to
90o.
Four monomers m1, m2, m3 and m4 show very similar
absorption spectra in the visible region, since they have the
same BODIPY chromophore. Dimer d4 shows almost the same
absorption spectra as the monomer m4 (Figure 5), indicating
The relative height at 550 nm reflects the extent of exciton
coupling in the dimers, and the order is d4
order of exciton coupling intensity is consistent with the order
of the calculated dihedral angle in the dimers: 89.5o
d4) <
88.3o (d3) < 62.6o (d2) < 42.9o (d1). Figure 7 reveals that the
dihedral angles lead to large difference of exciton coupling in
the four dimers.
<d3<<d2<d1. This
that the two monomer fragments in d4 (m3 and m4) have
negligible exciton coupling and thus are essentially two
independent units. This result is consistent with the computed
molecular geometry of d4 in which two units are perpendicular
each other. Each BODIPY moiety in d4 has four methyls, two
(
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Compare the intensity of exciton coupling with the dimers, and a further support for PET is that the increase in
efficiency of singlet oxygen (or excited triplet state) formation solvent polarity led to the stronger quenching.
of the dimers (d1
<
d3
<
d4
<
d2), we conclude that the exciton
Solvent polarity has a dramatic effect on the relative
coupling is not responsible for the formation of excited triplet intensity and band shape of the dual fluorescence of the
state (and singlet oxygen), since d4 has very high Φꢀ and ΦT dimers (Figure 9). Although dimer d4 in toluene shows weak
values but negligible exciton coupling, while d1 shows the ICT emission (its band is red-shifted and only slightly broader
strongest exciton coupling but the smallest Φꢀ and ΦT values. than its monomer, Figure 8), it exhibits strong ICT emission in
So what else is the reason that makes the dimers efficiently more polar solvents, such as DCM and CH3CN (Figure 9). In
generate T1 and singlet oxygen?
more polar DCM the rate constant of PET in d4 is increased
•−
Singlet fission (S1+S0→
formation, since Es (energy of S1 state, 2.4 eV) < 2ET (energy of charge separation state is also more stabilized in more polar
2T1) is not possibly involved in the T1 and more M1•+-M2 are formed. On the other hand, the
T1 state, 1.62 eV), according to ET values measured in this DCM than that in toluene which makes ICT emission has more
report using the method in our previous report.23
chance.
Also noticed is that the solvent polarity effect is not
PET or ICT revealed by fluorescence spectra
monotonic, i.e. every dimer has an optimal solvent which
makes ICT emission most significant relative to its LE band. Let
R be the ratio of the ICT emission intensity over that of its LE
emission. For d4, we can see that R(n-hexane) < R(toluene) <
R(DCM) > R(CH3CN) > R(ethanol). It is DCM for d4, toluene for
d2, benzene for d1, which makes ICT state the most emissive.
What does this mean for the unusual solvent polarity effect on
The intramolecular excited state interaction in a dimer can
be responsible for the efficient generation of T1 and singlet
oxygen. The excited state interaction and its relation with ΦT
and Φꢀ values were revealed by steady state and time
resolved fluorescence methods.
Both fluorescence and T1 formation of an aromatic
compound are from its S1 state, so fluorescence competes with
T1 formation, therefore the emission quantum yield and
lifetime of fluorescence is related to T1 formation efficiency. In
particular, by comparing the fluorescence of a BODIPY dimer
(spectral shape, quantum yield and lifetime) with that of the
BODIPY monomer we can gain insights on which photophysical
processes from S1 are present in the BODIPY dimer but absent
in the monomer, through which we can reveal the mechanism
of T1 formation for the BODIPY dimers.
ICT emission? ICT emission is from [M1
[M1]•+-[M2]•−
M1-M2 + ICT emission.
]
+-[M2] − state, i.e.
→
(4)
ICT emission intensity is proportional to the steady state
concentration of [M1]•+-[M2]•− under steady state Xe lamp
irradiation. Increase in solvent polarity promotes the forward
PET process which lead to the formation of more ICT state, but
it also enhances the CR (charge recombination), i.e. backward
ET process which consumes ICT state and reduces its
concentration:[M1]•+-[M2]•−
→M1-M2+heat. The former process
enhances ICT fluorescence but the latter reduces ICT emission,
the two contradicted processes cause the existence of an
optimal solvent polarity which makes ICT the most emissive.
Using toluene as the solvent, the fluorescence emission
spectrum of each dimer (d1 to d4) is displayed and compared
to that of the corresponding monomer in Figure 8. Each
monomer shows the typical fluorescence of the BODIPY type
at ca. 520 nm. However, the dimer d1, d2, and d3 exhibit PET for local excited states revealed by fluorescence quantum
obvious dual fluorescence, while d4 show significant dual yield and fluorescence lifetime
fluorescence when more polar solvents were used (Figure 9).
Fluorescence parameters for LE state are listed in Table 2
and Table S1 (supporting information), while that for ICT state
is summarized in Table S2 (supporting information). Dimer d4
for example, has the f value of 0.22 in toluene ( f is the value
The short-wavelength emission band is located at the same
position as that of the corresponding monomer unit, indicating
that it is due to the fluorescence from the local excited state of
,
Φ
Φ
the dimer (LE band, M1*-M2 or M1-M2*→M1-M2 + hν1 or hν2).
for LE band), it is significantly smaller than the Φf value of its
monomer component m4 and m3, which is 0.74 and 0.60
The second band of the dual emission is very broad,
structureless and largely red-shifted compared to that of the
monomers. This broad emission has the typical feature of the
fluorescence from an ICT (intra-molecular charge transfer)
state, suggesting the occurrence of following emissive process:
respectively. The τf of d4 LE band is also remarkably shortened
compared to that of its monomer m4 and m3 (Figure 10) in
DCM, in which m4 and m3 show a single fluorescence lifetime
of 5.18 and 4.28 ns, respectively, but d4 exhibits bi-
exponential decay with the lifetime of 0.35 and 2.77 ns. Based
[M1]•+-[M2]•−
→ M1-M2+hν''. An ICT state is formed by either
photo-induced electron transfer (PET) process (2) or charge
transfer absorption process (3), or both of them:
M1-M2 + hν → M1*-M2 or M1-M2*, then
on the change in Φf and τf, it is apparent that the emission
from any individual unit (such as m3 unit) in the dimer d
quenched by another linked unit (such as m4).
4 is
•−
•+-M2
.
(2)
PET
M1*-M2 or M1-M2*
M
→
1
Furthermore, this intramolecular fluorescence quenching is
strongly enhanced by the increase in solvent polarity, since Φf
of d4 LE band becomes smaller in more polar solvents (0.91,
0.22, 0.0052, 0.0018 in n-hexane, toluene, DCM, acetonitrile,
M
•+-M2•−
→
1
(3)
CTabsorption
M1-M2 + h
ν
PET process (2) can quench the fluorescence of M1* or M2*
while process (3) is not. As shown below, the presence of PET
is evidenced by the large decrease in Φf and τf values for the
respectively). On the other hand, the Φf value of monomer m3
or m4 is not significantly affected by the solvent polarity (for
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m3, it is 0.57, 0.60, 0.62, 0.63 for n-hexane, toluene, DCM, In which C1, C2, and C3 are constants, while kPET is the rate
acetonitrile respectively. for m4, it is 0.87, 0.89, and 0.88 for n- constant of PET, and kCR,ic is the rate constant of charge
hexane, DCM, and ethanol respectively).
The Φf and τf values of d2 d3, and d1 showed the similar releasing. We see that ΦT is dependent on both kPET and
behavior as that of d4 in respect to their monomers and kCR,ic. This expression explains why the dihedral angle and
solvent polarity. The Φf value of d2 in toluene is 0.0017, while solvent have effect on ΦT and ꢀ. Increase in the solvent
its monomer m2 and m3 showed the value of 0.80 and 0.60 polarity leads to the increase of both kPET and kCR,ic
respectively. The fluorescence decay of d2 LE band in toluene Mathematically, based on Eq. (7), increasing kPET favors higher
is bi-exponential with the lifetime of 2.79 ns (64%) and 3.87 ns T, while increasing kCR,ic lowers T. This opposite effect
(36%), the lifetime is shorter than 5.47 ns of m2 and 4.04 ns of explains why there is an optimal solvent which makes ΦT and
recombination of ICT state to the ground state with heat
,

Φ
.
Φ
Φ
m3. The Φf of d2 LE band is 0.86, 0.0017, and 0.0014 in n-
hexane, toluene, DCM, respectively), so the solvent polarity
effect is very significant.
Φ
ꢀ highest for each dimer.
Since the dihedral angle θD affects the electronic coupling
between M1 and M2 in a dimer M1-M2, so that the rate
These quenching results also suggest the presence of PET constant of PET is tuned by
process in the LE state of a dimer: [M1]*-[M2] or [M1]-[M2]*
θ
D. It has been reported that28
θ
[M1]•+-[M2]•−. PET process in donor-acceptor systems
involving BODIPYs was also reported by other researchers.26-27
(8)
kPET = k0 × e−
+ [sin2 ( D ) + cos2 θT ]× k0 × e−
β
iRe
βaRe
→
2
photo-induced energy transfer does not likely play in which θD is the dihedral angle between the centroids of the
important roles in the quenching since it is not solvent polarity planes of the donor and acceptor, θT is the twist angle
dependent. Since kPET (the rate constant of PET) becomes between the planes of donor and acceptor and Re is the edge
larger with the increase in solvent polarity, so a more polar to edge distance (in Angstroms). k0 is the rate for Re = 0
solvent enhances the fluorescence quenching. PET competes (~1×
1013 s-1 for a barrierless process), βi is related to the
with LE emission process and decreases the fluorescence angular independent rate (defined by the bridging medium
quantum yield and lifetime values according to the following and the solvent) and βa is the attenuation factor that is
equation for LE state.
f = 1/(kic+kf+kisc+kPET),
f = kf/(kic+kf+kisc+kPET) = kfτf
modulated by the twist and dihedral angles (both
in Angstroms-1). As shown in the literature,28 the optimal θD
(6) which makes kPET the largest is θD = 0.22Re×180ο 180ο
61ο 180ο. This calculated value is close to the dihedral angle
β values are
τ
Φ
(5)
.
+
≈
+
In which kic, kf, kisc is the rate constant of internal conversion, of d2 (62.2ο), which explains why d2 is the most efficient for
fluorescence, and intersystem crossing process for LE state, singlet oxygen formation.
respectively. Based on the fluorescence study, the following
mechanism is given in Figure 11. For a dimer D composed of PET indicated by quantum chemical calculation
monomer M1 and M2 units (D: M1-M2), the photo excitation of
After the optimization of molecular structures by
D can lead to two local excited states M1(S1)-M2 and M1-M2(S1).
Gaussian09, the HOMO, HOMO-1, and LUMO is obtained and
Local emission bands (hν1 and h
M2 or M1-M2(S1). PET occurs from LE state and leads to ICT
state: M1(S1)-M2 or M1-M2(S1) ICT emission is
M1•+-M2•−(S1)
due to the charge recombination (CR) of the ICT state: M1
M2•−(S1) 3. For ICT state M1•+-M2•−(S1), each
M1-M2(S0) + h
ν2) come from either M1(S1)-
presented in Figure 12. For example, the HOMO of d4 is
located on the m4 moiety, while the LUMO and HOMO-1 of d4
are located on the m3 moiety, which means that upon light
excitation one electron will be transferred from m4 unit to m3
moiety. This quantum chemical result fully supports that PET
occurs from m4 moiety to m3 unit in d4. So are the cases for
other BODIPY dimers, such as d3 shown in Figure 12.
→
.
•+
-
→
ν
unpaired electron is independently localized on M1 and M2
respectively, it is therefore not bound by another unpaired
electron, so that it easily undergoes spin conversion to form
triplet ICT state M1•+-M2•−(T1). The ICT state also has an energy
Thermodynamics of PET from electrochemistry study
higher than the local triplet state M1(T1)-M2 or M1-M2(T1), as
•+
Based on the fluorescence data, we have analyzed the
kinetic aspect of PET that causes T1 and 1O2 formation. A
thermodynamic analysis provides further evidence and insights
on the PET process. Taking dimer d4 as an example,
electrochemistry measurements were carried out in argon-
saturated DCM for monomer m3 and monomer m4 to
calculate the Gibbs free energy of photo-induced electron
transfer (ꢀGPET). The redox potentials from cyclic
voltammogram data are shown in Table 3.
shown in next section. After charge recombination from M1
-
M2•−(T1) to M1(T1)-M2 or M1-M2(T1), the local excited triplet
state M1(T1)-M2 or M1-M2(T1) is formed (Figure 11).
This mechanism predicts that solvent polarity has
substantial effect on the triplet formation and singlet oxygen
production, since the precursor of the local T1 state is a charge
separation state formed by PET. Both the formation and
•−
charge recombination of M1•+-M2 are solvent polarity
dependent, which makes T1 and singlet oxygen formation
affected by medium polarity.
ꢀGPET is calculated via eq 9, in which ED+• is the oxidation
/D
potential of the donor D, EA/A• is the reduction potential of
the acceptor A, ES is the excitation energy, and 0.06 is a
constant due to solvent polarity.
Based on the mechanism, we can easily derive the
expression of
ΦT as the following.
Φ
T = C1[(C2/kPET+1) (C3+kCR ic)]-1
(7)
6 | J. Name., 2012, 00, 1-3
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GPET = [(ED+•
View Article Online
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ARTICLE
ꢀ
−
EA/A
For dimer d4, suppose m3 unit is the donor, m4 is the acceptor, financial support from Hebei Provincial Hundred Talents Plan
then GPET=1.17-(-1.55)-2.43-0.06=0.23 eV, this positive value (Contract E2013100005), the Peacock Team Project funding
tells that m3 can not be a donor. Assuming m4 is the donor, from Shenzhen Science and Technology Innovation Committee
m3 is the acceptor, then GPET =0.74-(-1.26)-2.43-0.06 = -0.49 (Grant No. KQTD2015033110182370).
•
)
−
ES - 0.06]
(9)
There are no conflicts of interest to declare. We thank the
/D
ꢀ
ꢀ
eV, this negative value means that m4 is indeed the donor, m3
is the acceptor for the PET. This result is in agreement with the
prediction from quantum chemical calculation.
Notes and references
Based on the thermodynamic and kinetic studies,
combining the energy for triplet excited state we obtained
previously, we can give the energy level diagram for T1
formation, as shown in Figure 13. For dimer d4, the energy
level has the following relation:
1.
J. Zhao, W. Wu, J. Sun and S. Guo, Chem. Soc. Rev., 2013,
42, 5323-5351.
2.
J. Zhao, K. Xu, W. Yang, Z. Wang and F. Zhong, Chem. Soc.
Rev., 2015, 44, 8904-8939.
3.
4.
Y. Chi and P.-T. Chou, Chem. Soc. Rev., 2010, 39, 638-655.
H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck and T.
Fischer, Coord. Chem. Rev., 2011, 255, 2622-2652.
J. A. G. Williams, Top. Curr. Chem., 2007, 281, 205-268.
V. W.-W. Yam and E. C.-C. Cheng, Chem. Soc. Rev., 2008,
37, 1806-1813.
m4(S1)-m3 or m4
m3(T1) > m4 m3, which makes T1 formation possible.
- - > m4(T1)-m3 or m4-
m3(S1) > m4•+ m3•−
-
5.
6.
Solvent polarity effect on
Φꢀ and T values in different solvents are measured for d2
and d4, the results are summarized in Table 4. As shown in
Table 4, ꢀ value is dependent on the solvent polarity, but not
Φꢀ and ΦT
Φ
7.
S. C. F. Kui, F.-F. Hung, S.-L. Lai, M.-Y. Yuen, C.-C. Kwok, K.-
H. Low, S. S.-Y. Chui and C.-M. Che, Chem.–Eur. J., 2012,
18, 96-109.
Φ
a monotonic increase upon the increase in solvent polarity. It
is toluene for d2 and DCM for d4 which makes ΦT (and so
Φꢀ) the highest. This result is the same as that of solvent
polarity effect on ICT emissive intensity, also clearly indicating
that T1 state is originated from ICT state. The results clearly
supports the prediction based on the proposed mechanism.
8.
W.-Y. Wong and C.-L. Ho, Coord. Chem. Rev., 2009, 253,
1709-1758.
9.
Q. Zhao, F. Li and C. Huang, Chem. Soc. Rev., 2010, 39,
3007-3030.
10.
11.
A. Kamkaew, S. H. Lim, H. B. Lee, L. V. Kiew, L. Y. Chung
and K. Burgess, Chem. Soc. Rev., 2013, 42, 77-88.
Y. Cakmak, S. Kolemen, S. Duman, Y. Dede, Y. Dolen, B.
Conclusions
Kilic, Z. Kostereli, L. T. Yildirim, A. L. Dogan, D. Guc and E.
We have synthesized and characterized BODIPY dimers d1 to
d4, which have different dihedral angles between two BODIPY
units. We measured the triplet T1 and singlet oxygen formation
U. Akkaya, Angew. Chem., Int. Ed., 2011, 50, 11937-11941.
12.
13.
S. G. Awuah and Y. You, Rsc Adv., 2012, 2, 11169-11183.
E. Baggaley, J. A. Weinstein and J. A. G. Williams, Coord.
Chem. Rev., 2012, 256, 1762-1785.
properties of the dimers. Dimer d2, d4 can generate excited
triplet state and singlet oxygen with very high quantum yields
up to 0.94 and 0.81, respectively, making them the excellent
excited triplet photosensitizers, while their corresponding
14.
S. Maity, M. Zhu, R. S. Shinabery and N. Zheng, Angew.
Chem., Int. Ed., 2012, 51, 222-226.
monomer compounds show very small
Φ
ꢀ~0.060. This means 15.
M. Neumann, S. Fu¨ldner, B. Ko¨nig and K. Zeitler, Angew.
Chem., Int. Ed., 2011, 50, 951-954.
that the dimerization method can be used to design excellent
photosensitizers, which is comparable to the heavy atom
effect of iodine substitution. More importantly, T1 formation is
due to neither exciton coupling nor singlet fission but because
of charge recombination of charge separation state resulted
from PET. PET involvement is fully supported by studies on
fluorescence, electrochemistry and quantum chemical
calculation. We showed that the photosensitizing efficiency of
the PET-based photosensitizer can be tuned not only by the
dihedral angle but also by solvent polarity. The PET-based
triplet state formation provides a novel strategy for designing
efficient and activable photosensitizers which are halogen free
and medium polarity selective. This type of photosensitizers
may find important applications in photodynamic therapy of
tumor, photobiology and organic photochemistry.
16.
17.
18.
D. Ravelli, M. Fagnoni and A. Albini, Chem. Soc. Rev., 2013,
42, 97-113.
B. C. D. Simone, G. Mazzone, J. Pirillo, N. Russo and E.
Sicilia, Phys. Chem. Chem. Phys., 2017, 19, 2530-2536.
A. J. Sánchez-Arroyo, E. Palao, A. R. Agarrabeitia, M. J.
Ortiz and D. García-Fresnadillo, Phys. Chem. Chem. Phys.,
2017, 19, 69-72.
19.
20.
21.
22.
23.
W. Pang, X.-F. Zhang, J. Zhou, C. Yu, E. Hao and L. Jiao,
Chem. Commun., 2012, 48, 5437-5439
X.-F. Zhang and X. Yang, J. Phys. Chem. B, 2013, 117, 9050-
9055.
X.-F. Zhang and X. Yang, J. Phys. Chem. B, 2013, 117, 5533-
5539.
W. Wu, X. Cui and J. Zhao, Chem. Commun., 2013, 49
,
9009-9011.
Acknowledgements
X.-F. Zhang, X. Yang, K. Niu and H. Geng, J. Photochem.
Photobiol. A: Chemistry, 2014, 285, 16-20.
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DOI: 10.1039/C7CP02645E
ARTICLE
Journal Name
24.
M. Kasha, H. Rawls and M. Ashraf El-Bayoumi, Pure App.
Chem., 1965, 11, 371-392.
25.
26.
27.
28.
W. W. Grabo, Master of Science, University of Central
Florida, 2004, p. 19.
A. Prlj, A. Fabrizio and C. Corminboeuf, Phys. Chem. Chem.
Phys., 2016, 18, 32668-32672.
C. O. Obondi, G. N. Lim, P. A. Karr, V. N. Nesterov and F.
D'Souza, Phys. Chem. Chem. Phys., 2016, 18, 18187-18200.
R. M. Williams, Photochem. Photobiol. Sci., 2010, 9, 1018-
1026.
8 | J. Name., 2012, 00, 1-3
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Physical Chemistry Chemical Physics
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Table 1. Excited triplet state properties and singlet oxygen formation yield in toluene*
Compound
ΦT
τT (μs)/Ar
τT (μs)/Air
0.33
kq (109M-1S-1)
1.49
Φꢀ
λT-T (nm)
d1
d2
d3
d4
0.24
0.94
0.39
0.81
0.34
0.94
0.40
0.86
425
425
430
425
18
23
42
28
0.23
2.15
0.39
1.27
0.31
1.60
*the formation quantum yield of Excited triplet state or singlet oxygen (ΦT or Φꢀ) showed about 10% relative
errors, the lifetime of Excited triplet state (τT) showed about 5% relative errors, the T1-Tn absorption maximum
(λT-T) showed an ±2 nm error. The triplet quenching rate constant by oxygen (kq) showed about 10% relative
errors.
Table 2. Absorption and fluorescence properties in toluene*
Comp# λabs (nm) λem (nm)
Δλ (nm) Φf
τ1 (ns)
τ2 (ns) kf, 109s-1
m1
m2
m3
m4
d1
501
506
503
509
526
509
510
510
522
515
516
518
21
9
0.044 0.44(99%) 5.13
~
0.80 5.47
0.60 4.04
0.74 5.46
~
~
~
0.15
0.15
0.14
~
13
9
642, 558s 116
517, 606 99
727, 520w 217
529 19
0.013 2.20(84%) 3.59
0.0017 2.79(64%) 3.87
0.0010 1.32(68%) 4.90
0.22 0.49(3.4%) 5.96
d2
~
d3
~
d4
~
*
λabs: absorption maximum with error ±2 nm, λem :emission maximum with error ±2 nm, Δλ=λem -λabs
with error ±2 nm, Φf: fluorescence quantum yield of LE band with about 10% error, τ1: fluorescence
lifetime of emitting component 1 with error ±0.2 ns, τ2: fluorescence lifetime of emitting component
2 with error ±0.2 ns, ~: not available.

Physical Chemistry Chemical Physics
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Table 3. Electrochemical data** of the precursors of the BODIPY dimers in DCM
Eox (V)
1.17
Ered (V)
-1.26
HOMO (eV)
-5.57
LUMO (eV)
-3.14
Eg (eV)
2.43
E00
monomer m3
monomer m4
2.44
2.42
0.74
-1.55
-5.38
-2.42
2.48
** The energy of HOMO, LUMO and their energy gap are calculated by EHOMO = - (Eox+ 4.4 eV), ELUMO = - (Ered+ 4.4
eV), Eg = ELUMO - EHOMO, these values and E00 have estimated errors about ±0.02 eV. The oxidation and reduction
potential (Eox and Ered) have estimated errors about ±0.03 V.

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Table 4. Φꢀ values of d2 and d4 in different solvents*
Solvent
d4
d2
c-Hexane
n-Hexane
Tol
0.054
0.053
0.81
0.85
0.37
0.18
0.15
0.14
0.94
0.63
n. d.
0.025
DCM
Acetone
EtOH
*the formation yield of singlet oxygen (Φꢀ) showed about 15% relative errors. n. d. = not determined

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Scheme 1. Synthesis procedure of the BODIPY dimers

Page 13 of 26
Physical Chemistry Chemical Physics
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Figure 1. Top and Middle: the chemical structures of BODIPY monomers and dimers in this study.
Bottom: optimized dimer structures that show the change of the dihedral angle from d1 to d4
(left to right), H atoms are hidden for clear viewing. The calculated dihedral angle in the dimers:
89.5o (d4) > 88.3o (d3) > 62.6o (d2) > 42.9o (d1), every BODIPY chromophore in a monomer or
dimer is strictly planar. Structural optimization were carried out by Gaussian 09 on DFT
B3LYP/6-31G level with toluene as solvent (cpcm model).

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Figure 2. Mechanism of triplet state and singlet oxygen formation for a dimer D
containing monomer M1 and M2, in which the monomer M1 or M2 do not effectively
form T1 state upon photoexcitation. PET: photoinduced electron transfer, CR: charge
recombination. ICT: intramolecular charge transfer. Triplet state T1 is originated from
ICT state rather than S1 state.

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Figure 3. Top & Middle: Time-dependent absorption spectra for DPBF (40 µM) in air-saturated
toluene containing a BODIPY dimer (10 µM) as the photosensitizer with light irradiation at 505
nm. The decrease in DPBF absorbance (410 nm band) is due to the oxidized decomposition by
singlet oxygen. The BODIPY dimer spectra (505 nm band) showed no change with time evolution.
Bottom Right: The decay of [DPBF] is monoexponential: [DPBF]t=[DPBF]0exp(-kt), from which the
rate constant of first order kinetics (k) was obtained. Bottom left: NIR luminescence of singlet
oxygen 1O2(1∆g) in air saturated CCl4 solutions, and Bottom right: the emission decay at 1270 nm

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showed a lifetime of 61 ms. Both were measured with excitation at 505 nm (absorbance is 0.30
for each photosensitizer).

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Figure 4. Top Left: Transient absorption spectra of BODIPY dimer d2 in
argon-saturated toluene with 532 nm laser excitation. Top Right: the triplet state
decay and ground state recovery of BODIPY dimer d2 in argon-saturated toluene.
Bottom Left: Transient absorption spectra of BODIPY dimer d4 in argon-saturated
ethanol with 532 nm laser excitation. Bottom Right: the triplet decay and ground
state recovery of BODIPY dimer d4 in argon-saturated ethanol.

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Figure 5. The comparison of UV-Vis absorption spectra between a BODIPY dimer (d1,
d2, d3 and d4) and its corresponding monomer components. Solvent is toluene.

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Figure 6. The effect of dihedral angle θ on the molecular energy of dimer d1 and d4.
The calculations were carried out using density functional theory (DFT) method as
implemented in the Gaussian 09 package. The B3LYP exchange-correlation functional
was chosen together with a 6-31G(d) basis set for initial structural optimization.
Then only the dihedral angle θ was changed to a particular value to calculate the
single point energy at the θ value for the molecule.

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Figure 7. The comparison of UV-Vis absorption spectra between BODIPY dimers d1
to d4. Solvent: Toluene.

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Figure 8. Comparison of the fluorescence between BODIPY dimers and between a
dimer with its monomers in toluene. Ex. Wavelength is 470 nm. The emission is
normalized at the emission maximum.

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Figure 9. Solvent effect on the fluorescence spectral shape of BODIPY dimers. Ex.
Wavelength is 470 nm. All the emission spectra is normalized at the emission
maximum.

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Figure 10. Left: The fluorescence decay of d4, m3 and m4 in DCM with excitation at
509 nm (70 ps diode laser) and emission at 520 nm. Right: The fluorescence decay of
d2 in different solvents with excitation at 509 nm (70 ps diode laser).

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Figure 11. Photoinduced processes that leads to forming excited triplet state T1 in a
BODIPY dimer (D: M1-M2). M1, M2 represents a BODIPY unit. S0: ground st ate, S1:
lowest lying excited singlet state, T1: lowest lying excited triplet state, LE: local
excited state, PET: photoinduced electron transfer, ICT: intromolecular charge
transfer, CR: charge recombination, ISC: intersystem crossing. There are three
processes (4’), (5’), and (6) leading to fluorescence emission hν1, hν2, and hν3, hν1
and hν2 are from LE state while hν3 are from ICT state.

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Figure 12. (Top) Left to Right: The HOMO, LUMO, and HOMO-1 of d4 S0 state
obtained from the optimized structure. (Bottom) Left to Right: The HOMO, LUMO,
and HOMO-1 of d3 S0 state obtained from the optimized structure.

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Figure 13. The energy level and kinetic processes for T1 generation in dimer d4 upon
photo excitation.