Tuning the SOCT-ISC of bodipy based photosentizers by introducing different electron donating groups and its application in triplet-triplet-annihilation upconversion

Article abstract of DOI:10.1016/j.dyepig.2019.108003

To study the effect of electron-donating ability on spin orbit charge transfer intersystem crossing (SOCT-ISC) efficiency, a series of bodipy derivatives (BDP-O, BDP-2-MN, BDP-2-EN) linked with electron-donating groups with different electron-donor abilities were synthesized. For BDP-2-MN and BDP-2-EN, both compounds show a sharp charge transfer emission band at 630 nm, and the local excited (LE) emission in both compounds were completely quenched due to photo-induced electron transfer (PET). By femtosecond transient absorption measurement, the triplet state population in both compounds was confirmed to occur through charge recombination (1996 ps ~2307 ps) process after the photo-induced electron transfer upon photoexcitation of bodipy moiety. A long-lived triplet excited state was observed for both compounds (lifetimes: 37 ~ 42 μs) via nanosecond transient absorption. The BDP-2-MN and BDP-2-EN can act as the triplet photosensitizers without heavy elements for the application of triplet-triplet-annihilation upconversion (TTA-UC, Φ_UC can reach 4.88%). In case of BDP-O molecule, no triplet signal was observed for this compound because of poor electronic coupling between donor and acceptor. Therefore, the triplet excited state properties of bodipy chromophore was successfully regulated by adjusting electronic strength of donor moiety and implement these photosensitizers in the application of TTA-UC.

Full text of DOI:10.1016/j.dyepig.2019.108003

Journal Pre-proof
Tuning the SOCT-ISC of bodipy based photosentizers by introducing different
electron donating groups and its application in triplet-triplet-annihilation upconversion
Hui Liang, Shanshan Sun, Mahmood Zafar, Zhanxiang Yuan, Yuye Dong, Shaomin
Ji, Yanping Huo, Ming-De Li, Jianzhang Zhao
PII:
S0143-7208(19)31953-9
DOI:
https://doi.org/10.1016/j.dyepig.2019.108003
DYPI 108003
Reference:
To appear in: Dyes and Pigments
Received Date: 17 August 2019
Revised Date: 16 October 2019
Accepted Date: 27 October 2019
Please cite this article as: Liang H, Sun S, Zafar M, Yuan Z, Dong Y, Ji S, Huo Y, Li M-D, Zhao J,
Tuning the SOCT-ISC of bodipy based photosentizers by introducing different electron donating groups
and its application in triplet-triplet-annihilation upconversion, Dyes and Pigments (2019), doi: https://
doi.org/10.1016/j.dyepig.2019.108003.
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Graphical abstract

Tuning the SOCT-ISC of Bodipy based Photosentizers by
Introducing Different Electron Donating Groups and Its
Application in Triplet-Triplet-Annihilation Upconversion
Hui Lianga, Shanshan Sunb, Mahmood Zafara, Zhanxiang Yuana, Yuye Donga,
Shaomin Jia, *, Yanping Huoa, *, Ming-De Lib, * and Jianzhang Zhaoc, *
a Light Industry and Chemical Engineering College, Guangdong University of Technology
，
510006, China
b Department of Chemistry and Key Laboratory for Preparation and Application of Ordered
Structural Materials of Guangdong Province, Shantou University, Guangdong, 515063, PR China
c State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of
Technology, Dalian 116024
* Corresponding authors.
*Emails: smji@gdut.edu.cn (S. Ji); yphuo@gdut.edu.cn (Y. Huo); mdli@stu.edu.cn (M. -D.
Li); zhaojzh@dlut.edu.cn (J. Zhao)
Abstract:
To study the effect of electron-donating ability on spin orbit charge transfer intersystem
crossing (SOCT-ISC) efficiency, a series of bodipy derivatives (BDP-O, BDP-2-MN, BDP-2-EN)
linked with electron-donating groups with different electron-donor abilities were synthesized. For
BDP-2-MN and BDP-2-EN, both compounds show a sharp charge transfer emission band at 630
nm, and the local excited (LE) emission in both compounds were completely quenched due to
photo-induced electron transfer (PET). By femtosecond transient absorption measurement, the
triplet state population in both compounds was confirmed to occur through charge recombination
(1996 ps ~ 2307 ps) process after the photo-induced electron transfer upon photoexcitation of
bodipy moiety. A long-lived triplet excited state was observed for both compounds (lifetimes:
37-42 µs) via nanosecond transient absorption. The BDP-2-MN and BDP-2-EN can act as the
triplet photosensitizers without heavy elements for the application of triplet-triplet-annihilation
1

upconversion (TTA-UC,
ΦUC can reach 4.88%). In case of BDP-O molecule, no triplet signal was
observed for this compound because of poor electronic coupling between donor and acceptor.
Therefore, the triplet excited state properties of bodipy chromophore was successfully regulated
by adjusting electronic strength of donor moiety and implement these photosensitizers in the
application of TTA-UC.
Key Words: triplet photosensitizer, SOCT-ISC, TTA-UC, photophysical properties
1 Introduction
Triplet photosensitizers are important and they are widely used in photodynamic therapy (PDT)
[1-4], photocatalysis [5,6], hydrogen production [7] and triplet triplet annihilation-upconversion
(TTA-UC) [8-11]. Intersystem crossing (ISC) is one of the most critical properties of triplet
photosensitizers. To improve the efficiency of ISC, the properties of triplet excited state of
photosensitizers can be controlled by modifying the molecular structure, which is pivotal to
photosensitizers. Conventionally, different approaches were utilized to improve ISC efficiency,
such as heavy atom effect (I, Br) [17-21] and transition metal complexes (Pt, Ir, Ru) [12-16]. But
high cost of these metals and dark toxicity of heavy atoms based photosensitizers limit their
applications. Recently exciton coupling, using spin convertor and symmetry breaking charge
transfer methods were utilized to develop the triplet photosensitizers, but all these approaches is
still facing some problems, such as the choice of donor–acceptor dyads and control of the dihedral
angle between donor and acceptor. Up to now, to design heavy atom free triplet photosensitizers
with efficient ISC ability is a great challenge.
Recently, a new approach, i.e. the spin orbit charge transfer ISC (SOCT-ISC) was introduced to
improve ISC efficiency by devising donor-acceptor based compact systems [22-24]. In this
approach the electron donor and acceptor chromophores must be linked directly or by short linker
to achieve an orthogonal orientation of the conjugations planes of the electron donor and acceptor
moieties in the dyad molecule. Upon photoexcitation of acceptor or donor moiety in these
donor-acceptor systems, the charge transfer taking place forms a radical anion center and a cation
center. Charge separated state (CSS) is established afterwards the charge recombination (CR)
process happens which facilitate the ISC process due to angular momentum change. In principle,
CR process can populate triplet state by two ways, first by radical-pair ISC or through SOCT-ISC.
2

If the system is compact and donor and acceptor moieties are orthogonal, then SOCT-ISC is the
main mechanism. In this approach, CR process will produce magnetic torque by the change of the
molecular orbit angular momentum, by which ISC process is facilitated.
SOCT-ISC process occurs in picosecond time scale and has many advantages over conventional
spin orbit intersystem crossing (SO-ISC) process. As the molecular structure is compact, synthesis
is easy and no heavy elements are required. Recently few examples of SOCT-ISC based triplet
photosensitizers have been reported using pyrene, PBI and anthracene moieties [24], but the
photosensitizers with strong absorption in the visible light range are rarely reported.
Bodipy chromophore has strong absorption in the visible region and is of much interest.
Recently few attempts have been made to populate the triplet state of bodipy chromophore
through CR process by attaching different electron donor groups with its core, such as
phenothiazine [22] or anthracene [25-27]. As a strong electron donor, the dimethylamine group is
often introduced into the photosensitizer to achieve the effect of regulating the production of the
triplet state by change pH, but the mechanism is not clear [28-34].
SOCT-ISC process is controlled by many factors, and the role of electronic strength of donor
moiety on ISC ability is still ambiguous. This is a key issue for molecular design. To solve above
problems, herein, different electron donating groups was introduced on the bodipy chromophore to
investigate effect of donor electronic strength on SOCT-ISC process. Efficient SOCT-ISC was
observed in some compounds although all compounds show the CT (Charge Transfer) process. By
changing the electronic donating strength and solvent polarity, the CSS energy levels was changed,
which finally affects the ISC process. The electronic coupling between donor and acceptor
moieties and its effect on triplet generation of bodipy chromophore were deeply studied. The
SOCT-ISC mechanism was uncovered by using the femtosecond and nanosecond transient
absorption (fs-TA, ns-TA) measurements. Theoretical studies were also performed, which supports
the proposed ISC mechanism. In addition, the BDP-O molecule does not have a SO-CT process
due to weak electron donating ability. Finally, these SOCT-ISC based photosensitizers were
applied to the TTA-UC and high upconversion quantum yield was observed.
2 Experimental sections
2.1. Reagents and apparatus
3

All chemicals and reagents were purchased from Energy Chemical Reagent Co., Ltd. (China),
1
and Organic solvent was purchased from Tianjin Damao Company (China). HNMR, 13CNMR,
were performed at Bruker dmx NMR spectrometer (400 MHz). High resolution mass spectrometry
using Thermo Scientifc Q Exactive spectometer.
N
N
O
R
N
N
F
N
F
N
F
N
F
N
F
N
F
(1)
B
B
CHO
B
BDP-O
BDP-2-MN
BDP-2-EN
Scheme 1. Synthesis of the BDP-O, BDP-2-MN, BDP-2-EN. Reagents and conditions: (1)
trifluoracetic acid, DDQ, Et3N, BF3.OEt2, DCM, RT.
2.2 General procedure for the synthesis of BDP-O, BDP-2-MN, BDP-2-EN
Compound 4-butoxybenzaldehyde 4-Hydroxybenzaldehyde (2.44 g, 20 mmol) was added into
a solution of K2CO3 (4.15 g, 30 mmol) in DMF (50 mL). The reaction mixture was stirred at
153 °C for 1 h. After cooling to 35 °C, n-butyl bromide (2.72 g, 20 mmol) was added and the
reaction mixture was further stirred for 10 h at 153 °C. After cooling to room temperature (RT),
the solvent was removed under reduced pressure, water was added and the mixture was extracted
with dichloromethane. The combined organic layer was dried over anhydrous Na2SO4. The crude
product was purified with column chromatography (silica gel, ethyl acetate as eluent), yellow oil
1
(3.04 g) was obtained. Yield: 85.4%. HNMR (400 MHz, CDCl3) δ 9.88 (s, 1H), 7.82 (d, J = 8.0
Hz, 2H), 6.99 (d, J = 8.0Hz, 2H), 4.05 (t, J = 8.0 Hz, 2H), 1.88 – 1.72 (m, 2H), 1.61 – 1.44 (m,
2H), 0.99 (t, J = 8.0 Hz, 3H).
Compound BDP-O Under Ar atmosphere, pyrrole (1.18 mL, 16.60 mmol) and trifluoracetic
acid (0.15 mL) were added into a solution of compound 1 (1.50 g, 8.43 mmol) in CH2Cl2 (150
mL), then the mixture was stirred for 12 h at RT. After that, DDQ (1.90 g, 8.43 mmol) was added
into the solution, the reaction medium was further stirred for 1h. Then Et3N (11.60 mL) and
BF3 OEt2 (12.00 ml) were added dropwise into the reaction mixture at 0 °C. The solution was
4

stirred overnight. The solvent was removed under reduced pressure, extracted with CH2Cl2,
washed several times with water, and the crude oil was purified by column chromatography (silica
1
gel, CH2Cl2 / Petroleum = 1:1)；An orange solid (203 mg) was obtained. Yield: 7.6%. HNMR
(400 MHz, DMSO) δ 8.09 (s, 2H), 7.64 (d, J = 8.0 Hz, 2H), 7.16 (d, J = 8.0 Hz, 2H), 7.07 (d, J =
4.0 Hz, 2H), 6.69 (d, J = 4.0 Hz, 2H), 4.10 (t, J = 8.0 Hz, 2H), 1.81 – 1.70 (m, 2H), 1.53 – 1.43
(m, 2H), 0.96 (t, J = 8.0 Hz, 3H). 13CNMR (100 MHz, CDCl3): δ (ppm) = 12.81, 18.22, 30.17,
67.01, 113.50, 117.18, 125.06, 130.33, 131.81, 142.28, 160.75. ESI HRMS: m/z calcd for
(C19H19BF2N2NaO) 363.1456；found：m/z [M+Na]+ 363.1451
For BDP-2-MN and BDP-2-EN, the synthesis method is the same as that used for BDP-O.
BDP-2-MN: 1H NMR (400 MHz, CDCl3) δ 7.86 (s, 2H), 7.54 (d, J = 8.7 Hz, 2H), 7.04 (d, J = 3.3
Hz, 2H), 6.78 (d, J = 8.7 Hz, 2H), 6.54 (s, 2H), 3.10 (s, 6H). 13C NMR (100 MHz, CDCl3) δ
152.61, 148.46, 141.65, 134.46, 133.15, 130.77, 121.72, 117.59, 111.47, 40.14, 29.72, ESI HRMS:
m/z calcd for (C17H16BF2N3Na) 334.1303, found: m/z [M+Na] +:334.1295.
1
BDP-2-EN: HNMR (400 MHz, CDCl3) δ 7.86 (s, 2H), 7.55 (d, J = 8.9 Hz, 2H), 7.06 (s, 2H),
6.76 (d, J = 8.8 Hz, 2H), 6.53 (s, 2H), 3.55 – 3.39 (m, 4H), 1.28 (s, 6H).13 CNMR (100 MHz,
CDCl3) δ 150.44, 148.46, 141.33, 133.62, 130.59, 121.10, 117.41, 110.96, 44.62, 29.72, 12.59,
ESI HRMS: m/z calcd for (C19H20BF2N3Na) 362.1616; found: [M+Na]+: 362.1605。
2.3 UV-visible absorption and fluorescence spectroscopy measurement
The UV-Vis absorption spectrum of samples was measured with a UV-Vis spectrometer
(Shimadzu UV-2700). The steady-fluorescence emission measurements were performed by the
fluorometer (IDR-MICRO-532-2 Ocean Optics. Inc USA). The time-resolved fluorescence
spectrum was measured by using 441 nm picosecond pulsed laser at Edinburgh FLS980
instrument.
2.4 Femtosecond Transient Absorption Spectroscopy
The fs-TA spectra were measured using a femtosecond regenerative amplified Ti: sapphire laser
system, as described elsewhere [35]. The laser probe pulse was produced by utilizing 12.5% of the
amplified 800 nm laser pulses to generate a white-light continuum (320−680 nm) by a CaF2
crystal. Then, this probe beam was splitted into two parts before traversing the sample. One probe
beam passed through the sample, while the other probe laser beam passed through the reference
5

spectrometer to monitor the fluctuations of the probe beam intensity. The sample was excited by a
485 nm pulsed laser. The sample solutions were excited in a 2 mm path-length cuvette with a
stirring bar. The absorption intensity at 485 nm of sample is ca. 1 when UV-vis absorption spectra
were measured in a 2 mm path-length cuvette.
2.5 Nanosecond Transient Absorption Spectroscopy
The ns-TA is performed with LP980-KS Laser Flash Photolysis Spectrometer (Kinetic/Spectral
Mode, Edinburgh Instruments, U.K.). The spectral signal is formed by oscilloscope (Tektronix
MDO3022) conversion. The excitation source is OPO laser (Continuum Surelite OPO Plus),
which was pumped by 355 nm third harmonic of Nd : YAG laser (Continuum, Model Surelite SL
I-10, Q-switched, 10 Hz). All the sample solutions were bubbled with Ar for 15 min.
2.6 DFT Calculations
The optimization of all molecular geometry is obtained by DFT calculation. The energy of the
excited state and the distribution map of the electron cloud are calculated by TD-DFT on the basis
of the optimized structure. The basis set used in the calculation is 6-31G (d). All these calculations
were performed with Gaussian 09w [22-24].
2.7 Triplet-triplet annihilation-upconversion (TTA-UC)
TTA-UC is performed by keeping the photosensitizer concentration constant (10-5 M) and
varying the concentrations of triplet acceptor to achieve high upconversion quantum yield. The
samples were bubbled with argon for 15 minutes prior to measurements. Upconversion was
performed with 532 nm CW laser (Changchun Radium Optoelectronics Technology Co., Ltd.).
Upconversion quantum yield and Stern-Volmer quenching constants are calculated using
following equations [37,38].
ꢌꢅꢎꢎꢇꢊꢋꢌꢏ
Equation 1
Equation 2
Equation 3
ꢆꢇꢈꢉ
ꢍꢇꢊꢋ
Φꢀꢁ = 2 • Φꢂꢃꢄ ꢅ
ꢌ ꢅ
ꢍꢇꢈꢉ
ꢆꢇꢊꢋ
ꢇꢈꢉ
ꢐꢑ
= 1 + ꢒꢓꢔ[ꢕꢖꢗꢘꢙℎꢗꢚ]
ꢐ
ꢒꢛ = ꢜ
ꢝꢞ
ꢐꢑ
BDP-2-MN fluorescence quantum yield (ΦF = 7.41%) is used as reference. A is absorbance, I is
integrated photoluminescence intensity, η is refractive index. τ0 is triplet lifetime without receptor
molecules, τ is triplet lifetime in the presence of different concentrations of acceptor molecules.
6

2.8 Singlet O2
The singlet oxygen of the photosensitizer was measured using 520 nm excitation lamp source
using 1,3-Diphenylisobenzofuran (DPBF) as 1O2 scavenger. The 1O2 production was monitored by
following the absorbance of DPBF at 414 nm. To determine the singlet oxygen quantum yield
(Φ∆), a relative method was used according to the following equation 4. 2, 6-diiodobodipy was
used as standard (Φstd = 0.87 in dichloromethane).
ꢏ
1 − 10ꢢꢆ
1 − 10ꢢꢆ
kꢂꢟꢠ
kꢂꢃꢄ
ꢇꢈꢉ
η
Φꢂꢟꢠ = Φꢂꢃꢄ
ꢡ
ꢣ ꢤ
ꢥ ꢡ ꢂꢟꢠꢣ
ꢦꢧꢨꢩꢪꢫꢬꢭ ꢮ
ꢇꢊꢋ
ηꢂꢃꢄ
Φ is singlet oxygen quantum yield of reference. A is absorbance, η is refractive index. K is
slope.
3 Result and Discussion
3.1 UV-visible absorption spectra and fluorescence spectra
80000
0.8
a
b
BDP-O
BDP-2-MN
BDP-2-EN
BDP-O
BDP-2-MN
BDP-2-EN
60000
40000
20000
0
0.6
0.4
0.2
0.0
500
600
700
800
300
400
500
600
Wavelength (nm)
Wavelength ( nm )
Fig 1. (a) UV-visible absorption spectra (c = 1 × 10-5 M) and (b) fluorescence emission spectra of
BDP-O, BDP-2-MN and BDP-2-EN (λex = 485 nm. A = 0.15). In toluene, 25 °C.
The UV-Vis absorption and fluorescence emission spectra of compounds were studied (Fig 1).
In toluene, all compounds show a sharp absorption peak centered around 496 nm which is
characteristic LE absorption of the boron-dipyrromethene (BD) chromophore [39]. For the
BDP-O molecule, there is a distinct absorption peak at around 387 nm, and its absorption is
stronger relative to BDP-2-MN, BDP-2-EN (Fig 1a). It is caused by the partial electron transfer
of boron-dipyrromethene to the benzene ring. The electron transfer process can be observed by
theoretical calculation (Fig 8). For the BDP-2-MN and BDP-2-EN molecules, their absorption
spectra became wider near 496 nm relative to the BDP-O molecule, and a new peak appeared at
7

around 530 nm. The molar extinction coefficient of BDP-2-MN (74200 M cm-1) is much higher
than that of BDP-2-EN (23000 M cm-1) (Table 2), The N, N-Diethylaniline moiety has a stronger
electron coupling ability as compared to the N, N-Dimethylaniline. Therefore, the electron transfer
of the S0 state to the 1CT state is significant. (BDP-2-MN = 0.60 ps, BDP-2-EN = 0.52 ps). From
their fluorescence emission spectra, the BDP-O molecule only has an emission peak in the LE
state, while the BDP-2-MN molecule and the BDP-2-EN molecule is completely quenched due to
the action of PET, only CSS emission.
150000
80000
b
30000
20000
10000
0
c
a
ACN
CHCl3
ACN
CHCl3
60000
40000
20000
0
100000
50000
0
ACN
CHCl3
DCM
DIOX
HEX
TOL
DCM
DIOX
HEX
TOL
DCM
DIOX
HEX
TOL
500 600 700 800 900
Wavelength(nm)
500
600
700
800
500 550 600 650 700
Wavelength(nm)
Wavelength(nm)
Fig 2. Fluorescence emission spectra of (a) BDP-O and (b) BDP-2-MN (c) BDP-2-EN in
different solvents, optically matched solutions were used (A = 0.15). λex = 485 nm. 20 °C.
Photoinduced CS (charge separation) and CR were explored by studying their absorption and
emission in different solvents. With increasing the solvent polarity, the absorption of the CT state
of BDP-2-MN and BDP-2-EN molecules became prominent, while the BDP-O molecule showed
no significant change (Fig S11). In the emission study, it was observed that in n-hexane, the
1
BDP-O molecule only shows a LE emission band at 513 nm. BDP-2-MN has dual emission
peaks of 513 nm (1LE) and 542 nm (CSS). BDP-2-EN can only observe emission at 529 nm
1
(CSS). In the polar solvent, the position of the LE state emission peak of the BDP-O molecule
1
did not change significantly, while the LE state emission of the BDP-2-MN and BDP-2-EN
molecules completely disappeared, and the charge separated state emission gradually shifted red
(Fig 2), This is due to the difference in charge separation state levels in different polar solvents.
By monitoring the fluorescence decay trace at 630 nm, bi-exponential decay was observed, one for
the 1LE to S0 state and one for the CSS to S0 state (Fig 3).
8

10000
1000
100
10
c
a
BDP-2-EN
b
BDP-O
BDP-2-MN
10000
1000
100
10
10000
1000
100
10
1.1ns ( 95%)
1.34ns ( 91%)
21.49ns ( 9%)
0.87 ns ( 95%)
17.03 ns ( 5%)
18.2ns ( 5%)
1
1
1
0
50
100
Time (ns)
150
200
0
50
100
150
200
0
50
100
150
200
Time (ns)
Time (ns)
Fig 3. The fluorescence lifetimes of the molecules in toluene. (a) BDP-O was measured at 518 nm;
(b) BDP-2-MN and (c) BDP-2-EN were measured at 628 nm and 639 nm. The excitation source
used is 441.6 nm. c = 1× 10-5 M.
In order to confirm that the BDP-2-MN and BDP-2-EN molecules do have a process of ground
state charge transfer and excited state charge separation, the transition dipole moment (Mabs) of
their UV-visible absorption, the electron coupling constant (VDA) between donor and acceptor, and
transition dipole moment of the state emission were calculated (Mflu. In toluene), according to
equation 5, equation 6, and equation 7. The Mabs of BDP-2-MN (2.17 D) value was smaller than
BDP-2-EN (2.49D) (Table 1), which is consistent with the UV-visible absorption spectrum. Their
Mflu and VDA values are 0.98 D, 0.95 D and 0.071 eV and 0.068 eV. The values of BDP-2-MN are
slightly higher than BDP-2-EN.
ꢳ ꢴꢵ ꢶꢷ
ꢸπꢹꢺꢻ
ꢼꢽ
ꢏ
|
|
ꣃ ꣄
ν
ꢯꢰꢱꢲ
=
∫
ꢱꢰꢾꣂ Ɛ
d
ν
Equation 5
Equation 6
ꢾ
νꢿꣀꣁ
ꣅꢯ꣆꣇ꣅꢏ =
ꢳꢼ꣈꣉
꣊꣋π꣌ ꣍ꢾ
ν
꣎꣏꣐ꢹ
ꢶ/ꢏ
꣛
ꢶ.꣋×ꢶꢷ꣕꣈꣉
ꣃ
ꢢꢶ꣄
꣑꣒꣓ cm
= ꣔ꢾꢹ꣖
Equation 7
꣘ν꣙ꢿ꣚
꣗
In equation 5, h, c, n and NA represent Planck's constant, speed of light, solvent refractive index
and Avogadro's constant. νabs is maximum absorption wavelength of the S0→CT1 state.
ν
is
molar extinction coefficient corresponding to absorption wavelength.
In equation 6 and equation 7, kr represent the radiative rate constant of the emission of the CSS
band, fl and max are the fluorescence maximum in the wave number scale and maximum emission
ν
ν
wavelength, Rc is the separation between the center of electron donor and acceptor.
9

Table 1：Dipole Moments and Electronic Coupling Matrix Elements of the Compounds
a
b
c
Mabs
Mflu
VDA
2.11
2.49
0.98
0.95
0.071
0.068
BDP-2-MN
BDP-2-EN
a
b
The transition dipole moment of S0 →1CT transition, in Debay (D). The transition dipole
c
moment of CSS→S0 transition in toluene, in Debye. The electronic coupling matrix element
between CSS and ground state calculated by fluorescence, in eV.
The fluorescence excitation spectrum (Fig S12) of the three molecules is compared with the
UV-visible absorption spectrum. It could be finding two points. First, the intensity of the peak near
387 nm becomes weaker with the enhancement of electron donating ability. Second, for the
excitation spectra of BDP-2-MN and BDP-2-MN, there is a small shoulder peak at around 530
nm (1CT), and there are also excitation peaks in the 1LE state. These show that there is a transition
between 1LE→S0 and CSS→S0. In other words, both molecules produce CSS.
Table 2. Photophysical Properties of Compounds.
e
a
d
Solvent
Tol
Kr
λabs
ε b
λem
518
515
516
628
680
670
639
680
670
ΦF c (%)
11.09
6.93
τF /(ns)
387/501
384/499
1.1/5.1
1.1/4.9
1.2/5.1
7.4/1.5
6.0/1.5
6.2/2.0
2.3/0.9
1.8/0.7
1.6/0.8
0.8/17.00
0.60/17.10
0.80/18.00
1.10/18.20
0.60/16.00
0.94/16.00
1.30/21.40
0.70/15.60
0.80/17.10
1.3/0.06
1.7/0.06
1.2/0.06
0.9/0.05
1.7/0.06
1.1/0.06
0.7/0.05
1.3/0.06
1.2/0.06
BDP-O
Diox
CHCl3 406/499
3.67
Tol
498/528
498/529
7.41
BDP-2-MN
BDP-2-EN
Diox
0.07
CHCl3 496/531
0.21
Tol
496/534
493/533
8.79
Diox
0.09
CHCl3 490/538
0.17
a In toluene (1.0 × 10−5 M). In nm. b Molar absorption Coefficient (104 M-1 cm-1). c Fluorescence
quantum yield. d Luminescent lifetimes (in maximum emission wavelength), In ns. e Radiation
transition rate constant ( 109 s-1).
3.2 Femtosecond transient absorption spectroscopy
Through the previous UV-Vis absorption spectroscopy and fluorescence emission
spectroscopy, it could be found that BDP-2MN and BDP-2-EN have strong electron coupling
ability and obvious CSS emission, and their emission peaks in toluene are at 630 nm. The energy
10

level of the CSS is higher than the triplet energy level of the compounds. Therefore, fs-TA
experiments were carried out to explore the mechanism of CR induced ISC. From fs-TA spectra
(Fig 4a), there is a significant increase in absorption was observed (at 750 fs scale) from 315 nm
to 410 nm. This is mainly due to the electronic transitions of the S0→S1 (S1BD*).
b
10
5
S1BD*
0.08
0.00
a
S1BD*
N,N-Dimethylaniline•+
ꢀ
0
BD+•
ꢀ
-5
-0.08
-0.16
-0.24
0.76 ps
1.32 ps
3.7 ps
11.3 ps
94.4 ps
-10
-15
-20
-25
56.2 fs
262 fs
706 fs
300
400
500 550 600 650
300
400
500 550 600 650
Wavelength (nm)
Wavelength (nm)
6
5
4
3
2
1
0
S1BD*
Kinetic at 410 nm
Fit
c
T1BD*
0.06
0.00
d
N,N-Dimethylaniline•+
τ1=0.60 ps
3
2
1
0
BD+•
ꢀ
T1BD*
94.4 ps
294 ps
618 ps
1090 ps
2220 ps
4000 ps
7040 ps
τ2=3.92 ps
τ3=1996 ps
-0.06
-0.12
-0.18
-0.24
0
100
200
300
400
500
300
400
500 550 600 650
0
1500 3000 4500 6000 7500
Wavelength (nm)
Delay time / ps
Fig 4. Fs-TA spectra of BDP-2-MN, (a), (b), (c) are representative for testing in different time
scale. (d) The decay trace at 410 nm with fitting lines. Conditions: λex = 485 nm, 25°C, in toluene.
Fig 4b shows that the absorption peak at 315 nm ~ 410 nm (S1BD*) gradually decreased as
delay time increased, while a new absorption peak appeared at ca. 550 nm (N,
N-Dimethylaniline + and BD+ ), and its absorption intensity gradually increases. The observation
of N, N-Dimethylaniline + at 550 nm demonstrates that this step can be assigned to the charge
separation. However, when the time interval is after 96.6 ps (Fig 4c), the absorption peak near
410 nm (T1BD*) and 640 nm (T1BD*) gradually increases while the absorption peak near 550 nm
(N, N-Dimethylaniline +and BD+ ) gradually decreases. This indicates that the BD radical further
undergoes charge rearrangement and finally forms the triplet state of BDP-2-MN. It is well
known that 410 nm and 640 nm are the triplet absorption peaks of BDP molecules. Ns-TA
11

quenching experiments also demonstrates that the transient absorption at 410 nm and 640 nm is
attributed to the triplet state of BDP-2-MN owing to these two absorption peaks can be quenched
by oxygen. To trace the dynamics of BDP-2-MN and BDP-2-EN, the time dependent data plot at
410 nm were analyzed, three exponential functions were used to fit the kinetics at 410 nm, the
time constants are 0.6 ps, 3.92 ps, 1996 ps for BDP-2-MN and 0.52 ps, 4.61 ps, 2307 ps for
BDP-2-EN. Based on the above explanation, 0.6 ps and 0.52 ps is associated with the excitation
from S0 to S1, 3.92 ps and 4.61 ps is attributed by the charge separation process, 1.996 ns and
2.307 ns is accounted by the charge rearrangement and ISC process. By comparison the lifetimes
of two compounds, it reveals that the lifetimes charge separation and charge rearrangement &
ISC of BDP-2-MN is faster than that of BDP-2-EN.
3.3 Nanosecond Transient Absorption Spectroscopy
b
0.05
0.00
a
0.02
0.00
-0.05
-0.10
-0.15
-0.20
75 µs
…
…
…
0 µs
-0.02
-0.04
-0.06
70
…
…
…
µ
s
0
µ
s
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig 5. Nanosecond transient absorption spectra of (a) BDP-2-MN and (b) BDP-2-EN. The
excitation is at 485 nm. c = 1.0×10-5 M in toluene. 25°C.
To study the properties of the triplet state, nanosecond transient absorption spectra were used to
study the generation of the transient species upon photoexcitation. No triplet signal was observed
for BDP-O, indicated a low intersystem crossing yield. For BDP-2-MN and BDP-2-EN in the
toluene solution, we can clearly observe excited state absorption peak of the excited state at ca.
410 nm, upon pulsed laser excitation at 485 nm, a ground state bleaching band at 496 nm was also
observed. The ESA bands in the ranges of 350 nm ~ 440 nm and 580 nm ~ 700 nm are the
absorption of the T1→Tn transitions of the molecules (Fig 5). Under the same conditions, the
triplet signal of BDP-2-MN is much stronger than BDP-2-EN, indicating efficient ISC for
12

BDP-2-MN.
Since the triplet signal can be clearly observed in BDP-2-MN and BDP-2-EN, the triplet
lifetime in different solvents were studied (Fig 6), the two molecules' triplet lifetimes are similar.
For BDP-2-MN, the lifetime in chloroform, toluene, and 1,4-dioxane is 37 µs, 42 µs, and 100 µs,
respectively. We propose that the difference of triplet lifetimes are mainly due to the viscosity of
the solvent. The viscosity of chloroform and toluene are similar, so the lifetime is close, while the
viscosity of 1,4-dioxane is larger, so the lifetime is longer. Similar situations have been reported in
related literature [39,40].
1.2
1.2
a
b
0.9
0.6
0.3
0.0
-0.3
Tol
CHCl
CHCl
3
3
0.8
0.4
Diox
Tol
Diox
0.0
-0.4
4.0x105
8.0x105
2.0x105
4.0x105
0.0
0.0
Time (ns)
Time (ns)
Fig 6. The decay traces of the BDP-2-MN and BDP-2-EN in different solvents, λex = 485 nm,
25°C.
3.4 Density functional theory (DFT) calculation
Based on density functional theory calculation, the dihedral angle between the donor and
acceptor of the optimized geometry of the three compounds are 51°, 48° and 48°, respectively (Fig
S20). We also predict the presence of molecules at room temperature by calculation of the
potential energy level curve of the molecule, the thermal energy at room temperature (kBT) is
0.026 eV. Thus, for BDP-O, the dihedral angle between the receptors can exist between 71° − 90°,
while for BDP-2-MN and BDP-2-EN, the dihedral angles are in the range of 40° − 60° and 116° −
138°, respectively. Through the above calculations, we can find the optimal structures of
BDP-2-MN and BDP-2-EN are at the lowest energy level, and the intramolecular benzene ring is
easy to rotate. This illustrates the possibility that the molecular BDP-2-MN and BDP-2-EN
molecules have intramolecular rotation to generate a CSS. They are verified to be efficient for
13

conformational restriction, i.e., for SOCT-ISC purpose.
0.5
0.5
0.4
0.3
0.2
0.1
0.0
BDP-O
BDP-2-MN
BDP-2-EN
b
a
0.8
0.6
0.4
0.2
0.0
c
0.4
0.3
0.2
0.1
0.0
0
40
80 120 160
0
40
80 120 160
0
40
80 120 160
Dihedral angle ( ° )
Dihedral angle ( ° )
Dihedral angle ( ° )
Fig 7. Ground state potential energy curves of the (a) BDP-O, (b) BDP-2-MN, and (c) BDP-2-EN,
calculated by DFT//B3LYP/6-31G (d) level with Gaussian 09W. The dihedral angle refers to the
phenyl moiety with respect to the BODIPY scaffold.
LUMO+1
LUMO
HOMO
HOMO-1
Fig 8. Selected frontier molecular orbitals of BDP-O, BDP-2-MN, and BDP-2-EN calculated by
DFT (B3LYP/6 - 31G (d)). The energy levels of the orbits are presented (in eV).
From the calculation of the frontier orbital orbits. We find for the BDP-2-MN and BDP-2-EN
molecules, the electronic cloud distributions of HOMO and LUMO are obvious orbital overlaps in
the fluoroboron dipyrrole moiety and benzene ring, so they have electron coupling ability, and the
HOMO orbitals is mostly localized on the N, N-dimethylaniline or N, N-Diethylaniline moieties;
therefore, the transition of HOMO → LUMO is in CT feature. This is consistent with the UV-Vis
absorption spectrum (Fig 1) and the fluorescence spectrum. In addition, it is found that they have
the absorption of S0 → S2, which is mainly caused by the transition of HOMO-1 orbital to LUMO
14

orbital, which is consistent with the absorption of LE state in the ultraviolet absorption spectrum.
3.5 Triplet-triplet energy transfer and triplet- triplet annihilation upconversion system.
Nanosecond and the femtosecond transient absorption spectral studies show that both
BDP-2-MN and BDP-2-EN can produce triplet state upon photoexcitation, and the TD-DFT
calculation indicates the molecular triplet state energy level is 1.63 eV. Therefore the TTA-UC was
studied with perylene (Py) as acceptor (triplet state energy level: 1.53 eV) and BDP-2-MN or
BDP-2-EN as photosensitizer. The TTA-UC emission of BDP-2-MN and BDP-2-EN with
perylene as emitter were investigated in toluene after deaeration. Under excitation at 532 nm, the
mixed solution clearly displayed upconverted emissions in the blue range (Fig 9a). Upconversion
quantum yield (ΦUC) up to 4.88% was observed for BDP-2-EN. Considering the intensive
absorption at 532 nm of BDP-2-MN (ε is up to 2-fold of BDP-2-EN), the intensity of the
upconverted emission is significant even with a ΦUC value of 4.36%.
6x104
a
b
BDP-2-MN
BDP-2-MN+Py
BDP-2-EN
BDP-2-EN+Py
5x104
4x104
3x104
2x104
6
4
2
0
BDP-2-MN
BDP-2-EN
1x104
0
0
5
10
15
20
400
600
800
1000
Quncher ( 10 -6 M )
Wavelength(nm)
Fig 9. (a) Upconverted luminescence spectra of the mixture of the photosensitizers (10-5 M) and
perylene upon excitation with a CW 532 nm laser. In toluene. (b) Stern-Volmer plots generated
from triplet state lifetime-quenching of BDP-2-MN and BDP-2-EN (λex = 532 nm); perylene was
used as a quencher. In toluene.
The quenching of the triplet state of the BDP-2-MN and BDP-2-EN by perylene in toluene
was studied by ns-TA spectroscopy. Stern−Volmer plots were constructed based on the quenched
triplet state lifetimes (Fig 9b), and the quenching constants (KSV) of BDP-2-EN (KSV = 3.30 × 105
M-1) was a little larger than BDP-2-MN (KSV = 2.90 × 105 M-1). This is attributed to the efficient
15

ISC of BDP-2-EN.
The photophysical and TTA-UC processes of BDP-2-MN are shown in Fig 10. First, sensitizer
molecules BDP-2-MN absorb light, the singlet CSS is produced by CS from the 1BDP* state. The
CSS of BDP-2-MN in toluene (1.96 eV) is close to the T1 state (1.63 eV). Thus, the triplet excited
state T1 is formed through SOCT-ISC from the singlet CSS. This is followed by triplet−triplet
energy transfer (TTET) from the sensitizer to an emitter (perylene). Two sensitized emitter triplets
T1 then diffuse and collide to undergo TTA, and the consequently formed excited singlet S1
produces upconverted delayed fluorescence. The photophysical and TTA-UC processes of
BDP-2-EN is similar with BDP-2-MN.
S1
S1
2.33 ev
CS
CSS
TTA
3.92 ps
CR
T1
TTET
2.00 ns
1.96 ev
632 nm
T1
445nm
1.63ev
532 nm
1.53ev
S0
Photosensitizer
Acceptor
Fig 10. Simplified jablonski diagram illustrating the photophysical and TTA-UC processes of
BDP-2-MN.
3.6 Applied to sensitized singlet oxygen
Table 3. Triplet State Properties of Organic Triplet Photosensitizers and Reference Compounds
b
c
d
e
τT/(µs) a
Κsv
Κq
ΦUC
Φ∆
Solvent
(105 M-1)
(108 M-1s-1)
BDP-2-MN
BDP-2-EN
Tol
37
118
42
2.90
0.70
4.36%
21.0
5.4
g
Diox
Tol
5.30
0.50
−
3.30
0.80
4.88%
32.0
12.0
g
Diox
98
6.13
0.63
−
a Molecules triplet lifetime, b Stern-Volmer Quenching Constant (KSV), c Bimolecular Quenching
Constants (
Κ
q), d Upconversion fluorescence quantum yield. e Singlet oxygen Quantum yield (Φ∆)
with di-iodobodipy as a standard (Φ∆ = 0.87, in toluene).
The singlet oxygen quantum yields (Φ∆) of the compounds were determined (Table 3), as a
16

preliminary evolution of the triplet state production ability of the compounds. It is well known that
triplet photosensitizers can be used to photodynamic therapy. For BDP-2-MN (Φ∆ = 0.21) and
BDP-2-EN (Φ∆ = 0.32) in toluene (Fig S19), the quantum yields are relatively high. These results
indicate that the SOCT-ISC is relatively significant for BDP-2-MN and BDP-2-EN.
4 Summary
In this paper, a series of triplet photosensitizers without heavy elements (BDP-O, BDP-2-MN,
BDP-2-EN) were synthesized. The electron donors of butoxy, N, N-dimethylaniline and N,
N-dimethylaniline were attached to BODIPY moiety to study the effect of electron-donating
ability on SOCT-ISC efficiency. N, N-dimethylaniline and N, N-dimethylaniline are stronger
electron donating moiety than butoxy for BODIPY show more significant SOCT-ISC for
BDP-2-MN, BDP-2-EN. CT absorption and fluorescence bands were observed for the
BDP-2-MN, BDP-2-EN. The fluorescence of the BODIPY moiety was significantly quenched,
and efficient ISC was confirmed with nanosecond transient absorption spectroscopy. The
SOCT-ISC mechanism was confirmed by femtosecond transient absorption spectroscopy.
BDP-2-MN and BDP-2-EN molecules are successfully used for triplet-triplet-annihilation
upconversion with perylene as the triplet acceptor, the upconversion efficiency are 4.36% and
4.88%, respectively. The research has certain reference value for the design of triplet
photosensitizers.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (61671162，
21975055，21975053) and Technology Plan of Guangdong Province (2016A010103031). This
work is also supported by Guangdong Province Universities and Colleges Young Pearl River
Scholar Funded Scheme (2016).
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21

Highlights
Two triplet photosensitizers BDP-2-MN and BDP-2-EN based BODIPY with
efficient SOCT-ISC were synthesized and used on TTA-UC
Femtosecond transient absorption and nanosecond transient absorption were
used to confirm the SOCT-ISC mechanism. The effect of electron-donating ability on
SOCT-ISC was studied

Conflict of Interest Statement
We declare that we do not have any commercial or associative
interest that represents a conflict of interest in connection with the work
submitted.