Bodipy-Phenylethynyl Anthracene Dyad: Spin-Orbit Charge Transfer Intersystem Crossing and Triplet Excited-State Equilibrium

Article abstract of DOI:10.1016/j.jphotochem.2020.112573

Spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC) is a promising method to design heavy atom-free triplet photosensitizers (PSs). Herein, a new organic triplet PS, BDP-An (Bodipy-phenylethynyl anthracene dyad) has been synthesized and studied. In polar solvents, charge transfer (CT) emission band was observed, and the singlet oxygen quantum yield (Φ_Δ) is up to 95%. From femtosecond transient absorption (fs TA) spectra, SOCT-ISC mechanism was verified, the charge separation (CS) time takes1.6 ps, the lifetime of charge recombination (CR) is 3.8 ns, moreover the triplet state of phenylethynyl anthracene was also observed. In nanosecond transient absorption (ns TA) spectra, long-lived triplet states (τ_T =108 μs) were observed, which are delocalized on both parts of the dyad, i.e. there is a triplet excited-state equilibrium. This is the first report on the triplet excited-state equilibrium observed in an electron donor/acceptor dyad showing SOCT-ISC. With BDP-An as the triplet donor and perylene as the triplet acceptor, triplet-triplet annihilation upconversion (TTA UC) was performed, the upconversion quantum yield was up to 18.9%, and the lifetime of TTA-based delayed fluorescence was determined as 70.8 μs.

Full text of DOI:10.1016/j.jphotochem.2020.112573

Journal Pre-proof
Bodipy-Phenylethynyl Anthracene Dyad: Spin-Orbit Charge Transfer
Intersystem Crossing and Triplet Excited-State Equilibrium
Yanming Lei, Kepeng Chen, Geliang Tang, Jianzhang Zhao, Gagik
G. Gurzadyan
PII:
S1010-6030(20)30372-5
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112573
JPC 112573
Reference:
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Journal of Photochemistry & Photobiology, A: Chemistry
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Accepted Date:
9 March 2020
6 April 2020
13 April 2020
Please cite this article as: Lei Y, Chen K, Tang G, Zhao J, Gurzadyan GG,
Bodipy-Phenylethynyl Anthracene Dyad: Spin-Orbit Charge Transfer Intersystem Crossing
and Triplet Excited-State Equilibrium, Journal of Photochemistry and amp; Photobiology, A:
Chemistry (2020), doi: https://doi.org/10.1016/j.jphotochem.2020.112573
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Bodipy-Phenylethynyl Anthracene Dyad: Spin-Orbit
Charge Transfer Intersystem Crossing and Triplet Excited-
State Equilibrium
a
a
a
a
b
Yanming Lei, Kepeng Chen, Geliang Tang, Jianzhang Zhao * and Gagik G. Gurzadyan *
a State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University
of Technology, 2 Ling Gong Rd., Dalian 116024, P.R. China. Email: zhaojzh@dlut.edu.cn (J.Z.)
b Institute of Artificial Photosynthesis, State Key Laboratory of Fine Chemicals, Dalian
University of Technology, 2 Ling Gong Rd., Dalian 116024, P. R. China. E-mail:
gurzadyan@dlut.edu.cn (G.G.G.)
٭To whom Correspondence should be addressed.
1

Graphical Abstract:
Highlights




Novel orthogonal Bodipy-phenylethynyl anthracene dyad was prepared.
Triple excited-state equilibrium in a photosensitizer showing SOCT-ISC.
Efficient intersystem crossing was observed (

= 95%) without heavy atom effect.
Efficient triplet-triplet annihilation upconversion was observed (UC = 18.9%).
2

Abstract
Spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC) is a promising method to
design heavy atom-free triplet photosensitizers (PSs). Herein, a new organic triplet PS, BDP-
An (Bodipy-phenylethynyl anthracene dyad) has been synthesized and studied. In polar
solvents, charge transfer (CT) emission band was observed, and the singlet oxygen quantum

yield ( ) is up to 95%. From femtosecond transient absorption (fs TA) spectra, SOCT-ISC
mechanism was verified, the charge separation (CS) time takes1.6 ps, the lifetime of charge
recombination (CR) is 3.8 ns, moreover the triplet state of phenylethynyl anthracene was also
observed. In nanosecond transient absorption (ns TA) spectra, long-lived triplet states (
T
=
1
08 s) were observed, which are delocalized on both parts of the dyad, i.e. there is a triplet
excited-state equilibrium. This is the first report on the triplet excited-state equilibrium
observed in an electron donor/acceptor dyad showing SOCT-ISC. With BDP-An as the triplet
donor and perylene as the triplet acceptor, triplet-triplet annihilation upconversion (TTA UC)
was approved, the upconversion quantum yield was up to 18.9%, and the lifetime of TTA-
based delayed fluorescence was determined as 70.8 s.
Keywords: Anthracene; Bodipy, Intersystem crossing, triplet state equilibrium, TTA
upconversion
3

1
Introduction
Triplet photosensitizers (PSs) have attracted more and more attention because of wide
applications including photodynamic therapy (PDT) [14], photo-catalytic reaction [59],
photovoltaics [1013], photocatalytic water splitting [1416] and triplet-triplet annihilation
upconversion (TTA UC) [1719]. Actually the transition from singlet excited state to triplet
state is spin-forbidden, thus the method to enhance the intersystem crossing (ISC) has been an
important research topic for triplet PSs. A few common methods are available to achieve
efficient ISC. The heavy atom effect is a typical one, which can be achieved by introducing
heavy atoms, such as Pt, Ru, Ir, I and Br, to organic chromophores [2,2026]. However, the
heavy atom effect is not always effective, especially for large chromophores [25]. Moreover,
the triplet excited states of most heavy atom substituted PSs are short-lived due to the heavy
atom effect [27,28], which limits their application. Furthermore, dark toxicity of heavy atoms
is also a drawback in areas, such as PDT [1,29,30]. Another method to enhance ISC is
establishing an -*  n-* transition [3134]. Nevertheless, triplet PSs in this category are
often limited in applications due to the weak absorption in visible spectral range.
Recently spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC) has been
employed to develop new triplet PSs. The electron donor/acceptor dyads have been reported to
exhibit ISC [3537]. Given the electron donor and acceptor adopt orthogonal geometry,
through photoinduced electron transfer (PeT), charge-transfer (CT) states are formed, then
charge recombination (CR) leads to molecular orbital angular momentum changes, which
compensates the spin angular momentum change of ISC, therefore ISC is enhanced, namely
4

the mechanism of SOCT-ISC [35,38]. Based on SOCT-ISC, new triplet PSs can be designed
by covalently linking different electron donor (or acceptor) with a chromophore. More
significantly, the photophysical properties can be readily changed by using different electron
donor/acceptor and chromophores.
A series of heavy atom-free Bodipy-anthracene dyads showing high ISC efficiency have
been reported in recent years [3942]. Triplet states distributed on Bodipy subunit were
observed through nanosecond transient absorption (ns TA) spectra, and the dyads were applied
to TTA UC and PDT. Meanwhile, according to the previous report by our group [43], the
extension of conjugation framework by CC bond at 9,10-position can inhibit ISC and lower
the triplet energy levels of anthracene efficiently.
Herein, we modified anthracene moiety of Bodipy-anthracene dyad with the phenylethynyl
group (BDP-An, Scheme 1), in order to adjust the energy levels and the triplet state quantum
yield. Photophysical properties of BDP-An were studied with steady-state and time-resolved
transient spectroscopies, electrochemistry as well as theoretical calculations. To verify SOCT-
ISC mechanism and obtain the quantitative analysis of the photophysical processes, ns TA
5

Scheme 1. Synthesis of the compounds
(
a) Br
THF, Et
RT overnight under N
Phenylboronic acid, Pd(PPh
CH Cl , 60 C for 1 h under N
refluxed for 18 h under N , yield: 65%.
2
, CH
N, refluxed at 70 C for 4 h under N
; then DDQ, N,N-diisopropylethylamine, BF
, K CO , toluene, ethanol, refluxed for 24 h under N
, yield: 92%. (f) Phenylacetylene, Pd(PPh , CuI, THF, diisopropylamine,
2
Cl
2
, stirred at RT for 8 h under N
2
, yield: 53%. (b) Phenylacetylene, PdCl
, yield: 92%. (c) 2,4-Dimethylpyrrole, TFA, DCM, stirred at
Et O, respectively, yield: 18%. (d)
, yield: 86%. (e) NBS,
2 3 2 2 3
(PPh ) , CuI, Na CO ,
3
2
2
3
2
)
3 4
2
3
2
2
2
2
3 4
)
2
spectra of BDP-An were studied, which showed a triplet excited-state equilibrium. To our best
knowledge, this is the first report on the triplet excited-state equilibrium in an electron
donor/acceptor dyad showing SOCT-ISC mechanism. With BDP-An as the triplet donor and
perylene as the triplet acceptor, the first TTA UC with a triplet excited-state equilibrium PS
based on SOCT-ISC was carried out, the upconversion quantum yield was up to 18.9%, and
6

the lifetime of TTA-based delayed fluorescence was determined as 70.8 s, which may be
useful for applications such as time-resolved luminescence imaging studies [44,45].
2
2
Experimental section
.1 General methods
All chemicals were analytically pure and used as received from suppliers. NMR spectra were
1
13
recorded by Bruker spectrometers (400 MHz for H NMR, 125 MHz for C NMR) with CDCl
3
as solvent and TMS as standard at 0.00 ppm. UVVis absorption spectra were recorded with
Agilent 8453 spectrophotometer (Agilent Ltd., USA). Emission spectra were recorded with
RF-5301PC spetrofluorophotometer (Shimadzu Ltd., Japan). Fluorescence quantum yields of
the compounds were measured with C13534-11 UVNIR Absolute Photoluminescence
quantum yield spectrometer (Hamamatsu Ltd., Japan). Luminescence lifetimes were measured
with OB920 fluorescence lifetime instrument (Edinburgh Instrument Ltd., U.K.).
2
.2 Synthesis of BDP-An
Compound 3 (200 mg, 0.65 mmol), which was synthesized according to the literature method
46,47], and 2,4-dimethylpyrrole (124 mg, 1.3 mmol) was dissolved in anhydrous
[
dichloromethane (DCM, 150 mL) under nitrogen atmosphere. Then one drop of trifluoroacetic
acid (TFA) was added, and the solution was stirred at room temperature overnight. Next
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 148 mg, 0.65 mmol) was added to the solution
and the stirring at room temperature was continued for 30 minutes. At 0 C, N,N-
diisopropylethylamine (DIEA, 4 mL) was added to the solution with syringe and the mixture
7

was stirred for 30 minutes at room temperature. After that 4 mL of boron trifluoride etherate
BF OEt ) was added at 0 C, and the solution was kept stirring for 2 hours. The reaction
mixture was washed with water, the organic layer was dried over NaSO , and the solvent was
(
3
2
4
evaporated under reduced pressure. The crude product was purified with column
chromatography (silica gel, DCM/hexane = 1:1 as eluent, v/v) to give an orange solid BDP-
1
An (61.6 mg, 18%) [48]. H NMR (400 MHz, CDCl
3
): δ = 8.72 (d, J = 8.7 Hz, 2H), 7.96 (d, J
=
8.7 Hz, 2H), 7.80 (dd, J = 7.9, 1.6 Hz, 2H), 7.66–7.59 (m, 2H), 7.53–7.44 (m, 5H), 5.91 (s,
1
3
2
1
1
H), 2.63 (s, 6H), 0.71 (s, 6H). C NMR (125 MHz, CDCl ) δ = 156.03, 142.93, 138.44,
3
32.36, 132.17, 131.70, 129.49, 129.43, 128.87, 128.64, 127.27, 126.98, 125.65, 123.24,
+
21.30, 119.35, 102.05, 85.95, 29.70, 14.74, 13.53. ESI-TOF-MS ([C35
H27BF
2
N
2
+ H] ): calcd
m/z 525.2314, found m/z 525.2314.
2
.3 Synthesis of Ph-An
1
Ph-An was synthesized according to the literature methods [49,50]. H NMR (400 MHz,
CDCl ):  = 8.75 (d, J = 8.4 Hz, 2H), 7.81 (d, J = 8.0 Hz, 2H), 7.68 (d, J = 8.4 Hz, 2H),
.627.55 (m, 6H), 7.477.38 (m, 6H).
3
7
2
.4 Singlet oxygen sensitization

The singlet oxygen quantum yield ( ) of the compounds were measured with
diphenylisobenzofuran (DPBF) as singlet oxygen scavenger. By following the change of
1
absorbance (max = 414 nm) of DPBF upon photo-irradiation of the mixture, the O
2
production
8
(
1)

was monitored. The following equation, eqn(1) was used to calculate the singlet oxygen
quantum yield.
− 10ꢀ퐴
ꢁ
ꢂꢃ
1
푚sam sam
)(
)2
sam = std(
)(
− 10ꢀ퐴
푚std std
ꢁꢄꢅ
1
In the equation, “std” and “sam” stand for standard and sample, respectively. , A,  and
m stand for refractive index of the solvents, absorbance at excitation wavelength, singlet
oxygen quantum yield and slope of the absorbance of DPBF changing with time. For
2
+
measurement of 
57% in DCM, ex = 440 nm) were used as standards. Optically matched solutions of samples
and standards were used for measument.

, 2,6-diiodobodipy (

= 87% in DCM, ex = 510 nm) and [Ru(bpy)
3
] (

=
2
.5 Cyclic voltammetry
The electrochemical curves were recorded by a three-electrodes electrolytic cell on the
CHI610D electrochemical workstation (CH Instruments, Inc., USA). The voltammograms
were recorded at a rate of 100 mV/s in N
2
saturated dichloromethane, with ferrocene (Fc) as
N[PF ], 0.1 M) as
the internal reference and tetrabutylammonium hexafluorophosphate (Bu
4
6
the supporting electrolyte. Glassy carbon electrode was used as the working electrode, and
platinum electrode as the counter electrode.
2
.6 Nanosecond transient absorption spectroscopy
The ns TA spectra of the compounds were recorded in N saturated solvents on a LP980 laser
2
flash photolysis spectrometer (Edinburgh Instruments Ltd. U.K.). The samples were excited
with a nanosecond pulsed laser the wavelength is tunable in the range of 2002200 nm, the
9

typical laser power is 5 mJ per pulse (OpoletteTM. 355II+UV nanosecond pulsed laser,
OPOTEK, USA). The signals were digitized on a Tektronix TDS 3012B oscilloscope, and
L900 software was used for processing the data. A collinear configuration of the pump and the
probe beams was used in the measurement to enhance the signal-to-noise ratio.
2
.7 Femtosecond transient absorption spectroscopy
The femtosecond transient absorption (fs TA) spectra were measured by a homemade pump−
probe setup in combination with a mode-locked Ti-sapphire amplified laser system (Spitfire
Ace, Spectra-Physics). Briefly, the system was amplified with wavelength 800 nm of duration
3
5 fs, repletion rate 1 kHz, and average power 4 W. The output beam was then split into two
parts; 90% of the beam after frequency conversion in optical parametric oscillator (Topas,
Light Conversion) into UV−VIS−IR in the range of 240−2400 nm was used as pump beam.
The remaining 10% output beam was used to generate a white light continuum (WLC) in a 3
mm thickness rotated CaF plate and serves as a probe beam. The probe beam passed through
2
a variable delay line (up to 6 ns), and its absorbance change in the presence and in the absence
of the pump beam is measured. The magic angle between pump and probe beam was set at
5
4.7° in order to avoid rotational depolarization effects. The entire setup was controlled by a
PC with the help of LabView software (National Instruments). All measurements were
performed at room temperature under aerated conditions. Global target analyses were carried
out with the software Glotaran [51].
3
Results and discussion
1
0

3
.1 Design and synthesis of the compounds
According to the previous reports on Bodipy-anthracene dyads [39,42], the compounds with
anthracene attached at the meso-position of Bodipy showed higher ISC efficiency and triplet
quantum yields. Therefore, we attach the phenylethynyl group to anthracene moiety and
synthesized the target molecule BDP-An (Scheme 1) [4648]. In order to study the
photophysical properties of BDP-An, three compounds BDP, Ph-An [43, 49, 50] and Ph-Br-
An were synthesized as reference. The corresponding reactions were with good yields and the
molecular structures of the compounds were fully characterized (See ESI†, Fig. S1S5).
3
.2 UVVis absorption and fluorescence spectroscopy
The UVVis absorption spectra of the compounds are presented in Fig. 1a. Compared with
reference compounds, the absorption profiles of BDP-An are the sum of the spectra of
phenylethynyl anthracene moiety (structured absorption bands in the range of 350440 nm)
Fig. 1 (a) UV−Vis absorption spectra of BDP-An, BDP, Ph-An and Ph-Br-An, c = 1.0  10
M in acetonitrile, 20 °C. (b) Optimized ground state geometry and dihedral angle of BDP-An
calculated with DFT at B3LYP/6-31G(d) level with Gaussian 09W.
5
1
1

and Bodipy moiety (sharp peak at 510 nm), and no absorption of charge-transfer (CT) state can
be observed, which indicates negligible interaction of two chromophores in the ground state
[37,52,53]. The density functional theory (DFT) calculation optimized geometry indicates an
orthogonal conformation with a dihedral angle of 89.7 (Fig. 1b) between two subunits [54],
thus conjugation between the Bodipy moiety and phenylethynyl anthracene moiety was
inhibited in BDP-An [55].
Fig. 2 (a) The emission spectra for BDP-An in different solvents, λex = 400 nm, A400 = 0.140,
2
0 °C. (b) The emission spectra for BDP-An in polar solvents with high sensitivity of the
spectrophotometer, λex = 400 nm, A400 nm = 0.100, 20 °C.
The fluorescence emission spectra of the compounds in different solvents were studied
(
Fig. 2). The emission band at 520 nm of BDP-An is attributed to the localized excited state
(1
LE state) emission band of the Bodipy chromophore (Fig. 2a) compared with that of BDP
See ESI†, Fig. S7). By selective excitation of the phenylethynyl anthracene moiety, there was
no corresponding luminescence band (See ESI†, Fig. S8) observed, which indicates high-
(
1
2

efficiency Förster resonance energy transfer (FRET, See ESI†, Fig. S10) or charge separation
CS). The emission spectra of BDP-An were studied in different solvents. The fluorescence
(
intensity of BDP-An decreased as the solvent polarity increased, and the fluorescence was
almost completely quenched in polar solvents such as dichloromethane and acetonitrile, which
indicates the photo-induced CS [56,57]. By comparison, the fluorescence intensity of reference
compounds did not change obviously as the solvent polarity varied (See ESI†, Fig. S9).
By means of increasing the sensitivity of the spectrophotometer, the fluorescence emission
1
of BDP-An in polar solvents was studied (Fig. 2b). In addition to the LE state emission band
at 520 nm, a broad band ranging from 540 nm to 800 nm was observed, which is assigned to
1
CT emission band ( CT  S
0
) [40,58]. Therefore, in polar solvents, the interaction between
two subunits becomes more significant upon excitation.
Fig. 3 Fluorescence lifetime decay curves of BDP-An in (a) hexane at 525 nm and (b)

5
acetonitrile at 525 nm and 630 nm, λex = 405 nm, c = 1.0 × 10 M, 20 °C.
1
3

Table 1. Photophysical properties of the compounds
a
b
c
/ ns d
6.8
e
f
Solvent
HEX
DCM
ACN
HEX
DCM
ACN
HEX
DCM
ACN

abs


em

F

F

T
/ s



h
104 k
m
BDP-An
401 / 505 1.7 / 6.8 i 516
0.99
404 / 507 1.7 / 6.2 i 520 / 604 0.05
h
5.3 / 5.3 78 / 77 l
k
0.87
402 / 504 1.7 / 5.5 i 525 / 650 0.02
h
3.3 / 2.9 35 / 108 l 0.95
k
n
n
n
o
o
o
n
n
n
o
o
o
BDP
501
501
497
402
406
404
405
409
406
8.9
8.4
8.2
1.6
1.6
1.6
0.8
0.8
0.8
510
511
507
437
444
440
436
442
439
0.57
0.63
0.55
0.77
0.82
0.76
0.50
0.73
0.59
3.1
3.6
3.5
3.9
4.6
4.6
2.4
3.6
3.0












Ph-An
Ph-Br-An HEX
29
35
28
0.17
0.19
DCM
ACN
0.23
a
5
b
Maximal UVVis absorption wavelength of each band in different solvents (c = 1.0  10 M). Molar
absorption coefficient, : 10 M1 cm ). Fluorescence quantum yields. Luminescence lifetimes. Triplet
4
–1
c
d
e
excited state lifetimes. Measured by nanosecond transient absorption spectroscopy in deaerated solutions. f
Singlet oxygen quantum yields, with 2,6-diiodoBodipy as the standard (
upconversion quantum yields, with 2,6-diiodoBodipy as the luminescence quantum yield standard (
.027 in acetonitrile). Upconversion quantum yields of BDP-An were measured at excitation laser power
= 0.87 in acetonitrile). g TTA

F
=
0

2
h
i
density of 50 mW cm . Absorption band of the phenylethynyl anthracene moiety. Absorption band of the
k
l
m
n
bodipy moiety. Monitored by the band at 480 nm. Monitored at 510 nm. Not observed. Not applicable.
o
Not determined.
1
4

The fluorescence lifetimes of the compounds were studied. In different solvents, there was
no significant change of the fluorescence lifetimes of the reference compounds (Table 1).
1
Nevertheless, the lifetime of LE state of BDP-An was shortened from 6.8 ns in hexane to 3.3
1
ns in acetonitrile (Fig. 3). The fluorescence lifetime of LE state is affected by two possible
photophysical processes: FRET (Förster resonance energy transfer) and CS. In polar solvents,
CS occurs so that the fluorescence is quenched, and the lifetime is shortened. The
photophysical parameters of the compounds are summarized in Table 1.
3
.3 Electrochemical studies: cyclic voltammetry and the Gibbs free energy changes of the
photoinduced charge separation (GCS
)
Fig. 4 (a) Cyclic voltammograms of BDP-An and BDP; (b) Cyclic voltammograms of BDP-
An, Ph-An and Ph-Br-An. Ferrocene (Fc) was used as internal reference (E1/2 = + 0.64 V,
+
Fc /Fc). In deaerated CH
2
Cl
as the supporting electrolyte, Ag/AgNO
mV/s, 20 C.
2
solutions containing 1.0 mM PSs with the ferrocene, 0.10 M
Bu
4
NPF
6
3
as the reference electrode. Scan rates: 100
1
5

The electrochemical properties of the compounds were measured with cyclic voltammetry (Fig.
) [5961]. All compounds showed both oxidation and reduction waves, and corresponding
4
data was listed in Table 2. For BDP, a reversible oxidation wave at +0.75 V was observed, as
well as a reversible reduction wave at 1.71 V. For Ph-An, the oxidation and reduction waves
were at +0.75 V and 2.17 V, respectively. For Ph-Br-An, the oxidation wave was at +0.91 V,
and two reduction waves were at 2.00 V and 2.22 V. As for BDP-An, the first reversible
reduction wave at 1.63 V was related to the Bodipy moiety, thus the reversible oxidation wave
at +0.75 V should be related to the anthracene moiety, indicating that Bodipy moiety acts as
electron acceptor and the phenylethynyl anthracene moiety acts as electron donor. Therefore,
the charge separated state (CSS) of BDP-An should be composed of Bodipy radical anion and
phenylethynyl anthracene radical cation.
Table 2. Electrochemical Redox Potentials of the compounds a
Compound
BDP-An
BDP
Oxidation(V)
0.81
Reduction(V)
1.63; 2.05
1.71
0.75
Ph-An
0.75
2.17
Ph-Br-An
0.91
2.00; 2.22
a
2 2 4 6
Cyclic voltammetry in nitrogen saturated CH Cl containing a 0.10 M Bu NPF supporting electrolyte, Pt
3
electrode as the counter electrode, glassy carbon electrode as the working electrode, Ag/AgNO couple as
the reference electrode, c = 1.0 mM, 20 C.
The Gibbs free energy changes for the charge transfer processes of BDP-An were
calculated with the Weller equation eqn (2), eqn(3) and eqn(4) [59,62].
1
6

푒ꢆ
푒ꢆ
ꢉ
ꢉ
ꢉ
ꢉ
(
(
2)
3)
퐺S = − 4
− 8 ( + )(
− 휀ꢇ)
휀 휀 푅
ꢇ ꢈ CC
휀ꢈ
푅
D
푅
A
휀REF
퐺ꢊꢋ = −(퐺 + 퐸 )
ꢊS ꢌꢌ
ꢌ
퐺
= ꢍ[퐸OX − 퐸ꢋꢎꢏ] − 퐸 + 퐺S
(
4)
ꢊS
ꢌꢌ
In the equations, G is the static Coulombic energy, e is the electronic charge, EOX is the
S
half-wave potential for one-electron oxidation of the electron-donor unit, ERED is the half-wave
potential for one-electron reduction of the electron-acceptor unit, E00 is the energy level
approximated with normalized UVVis absorption and fluorescence emission spectra, and 
S
is the static dielectric constant of the solvent. RCC is the centre-to-centre separation distance
between the electron donor (Phenylethynyl anthracene) and acceptor (Bodipy), determined by
DFT optimization of the geometry. R
the electron acceptor, REF is the static dielectric constant of the solvent used for the
electrochemical studies, and  is the permittivity of free space. The solvents used in the
calculation of free energy of the electron transfer are hexane ( = 1.88), toluene ( = 2.38),
tetrahydrofuran ( = 7.58), dichloromethane ( = 8.93) and acetonitrile ( = 37.5).
D
is the radius of the electron donor, R
A
is the radius of
0
S
S
S
S
S
According to the calculation results of BDP-An (Table 3), in nonpolar solvents, such as n-
hexane and toluene, CS is thermodynamically forbidden. While as solvent polarity increased,
CS becomes thermodynamic allowed and the driving forces for CS become larger, as well as
the energy level of charge separated state decreased, which is in agreement with the
fluorescence emission studies (Fig. 2a).
1
7

Table 3. The Driving Forces of Charge Separation (GCS), Static Coulombic Energy
(
G
S
), and the Charge-separated States Energy Level (ECS)
BDP-An
HEX a
0.11
TOL
0.05
0.02
2.46
THF
0.18
0.20
2.24
DCM
0.19
0.21
2.22
ACN
0.28
0.28
2.16


E
G
CS [eV] b
G
S
[eV]
0.10
CS [eV]
2.54
a
HEX, TOL, THF, DCM and ACN stand for n-hexane, toluene, tetrahydrofuran, dichloromethane and
acetonitrile, respectively. b Singlet excited state, E00 is approximated with the crossing point of UVVis
absorption and fluorescence emission after normalization at the singlet excited state.
Moreover, although the ground state redox potentials of Bodipy and phenylethynyl
anthracene moieties are similar, the femtosecond transient absorption spectra (see later section)
show that the excited Bodipy unit is the electron acceptor and the phenylethynyl anthracene
unit is the electron donor.
3
.4 Femtosecond transient absorption spectra
To reveal the photophysical processes and excited state dynamics of BDP-An after excitation,
femtosecond transient absorption spectra of the compounds were measured (Fig. 5). According
to the above results, possible photophysical processes of BDP-An in polar solvents are
presented with the following equations (5), (6) and (7) [42]. Briefly, upon fs pulsed laser
1
8

Fig. 5 (a) Femtosecond transient absorption spectra of BDP-An upon fs pulsed laser excitation.
b) Decay traces of BDP-An. (c) Evolution-associated difference spectrum (EADS) obtained
(
by global analysis of the transient data. ex = 500 nm in acetonitrile, 20 C. * The time window
of the fs TA measurements is 5.0 ns.
excitation, singlet excited state of Bodipy is generated, followed by PET, which results in
formation of CSS, which is composed of Bodipy radical anion and anthracene radical cation.
*
CSS undergoes CR and the triplet state of Bodipy (T
1
[BDP-An ]) or phenylethynyl anthracene
*
(
T
1
[BDP -An]) is produced.
∗
ꢐ (BꢑP − ꢒn) + h  ꢐ (BꢑP − ꢒn)
(5)
(6)
(7)
ꢌ
ꢉ
∗


ꢐ (BꢑP − ꢒn)  ꢓT(BꢑP −ꢒn )
ꢉ



∗
∗
ꢓT(BꢑP −ꢒn )  T (BꢑP − ꢒn) or T (BꢑP − ꢒn )
ꢉ
ꢉ
Fig. 5a shows the femtosecond transient absorption spectra of BDP-An in acetonitrile.
Upon photoexcitation, the ground state bleaching (GSB) bands at 504 nm (Refer to Fig. 1a)
were observed. At 570 nm, excited state absorption (ESA) band is due to Bodipy radical anion:
it develops within 50 ps, indicating CS process [42, 63, 64]. Meanwhile, another ESA band
1
9

centred at 640 nm developed, which is attributed to the phenylethynyl anthracene radical cation.
The above processes correspond to the eqn(4) and eqn(5).
Subsequently, both ESA bands decayed within 3.8 ns, and a broad triplet ESA band centred
at 490 nm (T
1
 T transition) developed. This ESA band didn’t decay within the time scale
n
of the measurements (5 ns), indicating SOCT-ISC process. All processes were further
convinced by global analysis with the software GLOTARAN [51], the CS time constant is 1.6
ps, and the ISC time constant is 3.8 ns (Fig. 5c). Note the final species is with infinite lifetime
for the fs TA measurements, and the spectrum is in good agreement with the nanosecond
transient absorption studies (see next section). We also performed fs TA study with selective
excitation into the anthryl unit, fast FRET was observed, the time is shorter than the IRF
(Supporting Information, Figure S12).
3
.5 Nanosecond transient absorption spectra
To further confirm the production of the triplet states of the dyad upon photoexcitation, ns TA
spectra were measured. The ns TA spectra of BDP-An and Ph-Br-An in acetonitrile are
presented in Fig. 6. For compound Ph-Br-An (Fig. 6c), two positive bands centred at 315 and
4
90 nm were observed, which are assigned to the T
phenylethynyl anthracene moiety. Note that the ground state bleaching band in the range of
50 nm450 nm overlaps with the excited state absorption [6567]. The transient signal at 490
1
 T
n
excited state absorption of the
3
nm decays within 28 s, while the lifetime was greatly decreased to 247 ns (See ESI†, Fig.
S19) in aerated solution which indicates that the positive peaks can be assigned to the triplet
*
state absorption band (T
1
[Ph-Br-An ]). In different solvents there was no great change of the
2
0

singlet oxygen quantum yield of Ph-Br-An (Table 1), as well as the signal intensities of
transient absorption (OD, See ESI†, Fig. S17, S18), thus the solvent polarity does not have a
significant effect on the triplet state quantum yield of Ph-Br-An.
Fig. 6 (a) Nanosecond transient absorption spectra of BDP-An upon ns pulsed laser excitation

5
and (b) decay trace at 510 nm in deaerated acetonitrile, ex = 425 nm, c = 1.0  10 M, 20 C.
(c) Nanosecond transient absorption spectra of Ph-Br-An upon ns pulsed laser excitation (c =

5
5
5
4
.0  10 M) and (d) decay trace at 490 nm in deaerated acetonitrile (c = 2.0  10 M), ex
=
25 nm, 20 C.
2
1

For compound BDP-An (Fig. 6a), the transient absorption signals differ strongly in various
solvents. In nonpolar solvents such as hexane (See ESI†, Fig. S13), the signal intensity is very
weak. As the solvent polarity increased, the signal intensity was greatly enhanced, which is in
agreement with the singlet oxygen quantum yields (Table 1) and SOCT-ISC mechanism. In
polar solvents such as acetonitrile, upon nanosecond pulsed laser excitation, ground state
bleaching bands at 400 and 510 nm were observed, corresponding to the phenylethynyl
anthracene and Bodipy subunits, respectively (Fig. 6a). Meanwhile, two positive peaks centred
at 315 and 490 nm can be attributed to the signal of the triplet state of phenylethynyl anthracene
*
(
T
1
[BDP-An ]), similar to that of Ph-Br-An. Interestingly, a ground state bleaching band
centred at 510 nm was observed, which is related to the Bodipy moiety. Thus the triplet state
of Bodipy moiety is also populated, supported by the weak and broad bands in the range of
5
40700 nm [39, 42, 68].
This result indicates triplet excited-state equilibrium of BDP-An. To the best of our
knowledge, this is the first report on observation of the triplet excited-state equilibrium based
on SOCT-ISC mechanism. Considering the fs TA result didn’t show significant triplet-triplet
energy transfer (TTET), thus we propose that the time constant of TTET is between the time
scale of ns TA and fs TA measurements, i.e. 10100 ns. For the previous reported Bodipy-
anthracene dyads [39, 42], the compounds with anthryl unit attached at the meso-position of
Bodipy showed efficient ISC and high triplet quantum yields. However, no triplet excited-state
equilibrium was observed for the previous Bodipy-anthracene dyads. The reason was that the
triplet energy level of anthryl subunit (1.84 eV) is much higher than the triplet energy level of
2
2

Bodipy subunits (1.46 eV). In our case, however, the anthryl unit is -conjugated the
phenylethynyl moiety, the triplet energy level of anthracene subunit deceased to 1.50 eV, which
makes the triplet excited-state equilibrium possible because the Bodipy unit is with similar
triplet state energy level (1.46 eV). We noted the triplet state energy level for the 2,6-diethyl
substituted Bodipy was reported as 1.61 eV, based on the phosphorescence in a Ru(II)
complexes [69], however, the Bodipy unit in BDP-An is with different structure, therefore we
estimated the triplet state energy level of the current Bodipy unit with TDDFT computations.
It should be noted that the triplet state lifetime of Ph-Br-An may be reduced by the heavy atom
of bromine whereas the triplet state of the dyad may be shortened by TTA self-quenching effect
[41,43,70].
The decay trace at 410 nm and 480 nm (these are the excited state absorption bands of the
phenylethynyl Anthracene triplet state) in ns transient absorption spectroscopy both showed
mono-exponential decay (see Supporting Information, Figure S13), no growth phase was
observed, which indicated that the triplet state of the phenylethynyl anthracene unit is not
populated by intermolecular triplet state energy transfer, instead, it is due to the intramolecular
triplet state energy transfer [71].
Based on the above results and time-dependent density functional theory (TD-DFT)
calculations, the photophysical processes of BDP-An were illustrated with the simplified
Jablonski diagram (Scheme 2). Upon excitation of the phenylethynyl anthracene moiety, the
singlet excited state of Bodipy moiety is produced through effective FRET, followed with the
1
LE state emission of Bodipy chromophore in nonpolar solvents, or CS ( = 1.6 ps) in polar
2
3

solvents. CSS is composed of Bodipy radical anion and phenylethynyl anthracene radical
cation, which formation is supported by the observation of CT emission band in the
fluorescence spectra, as well as the radical ions in fs TA spectra. The triplet states localized on
3
*
the phenylethynyl anthracene moieties ( [BDP-An ]) are generated through ISC enhanced by
CR ( = 3.8 ns), then TTET occurs between two subunits, followed by the triplet states localized
3
*
on the Bodipy moieties ( [BDP -An]) generated, i.e. the triplet excited-state equilibrium.
Scheme 2 Simplified Jablonski Diagram Illustrating the Photophysical Process of BDP-
An a
a
The energy levels of the triplet states of BDP-An are obtained by time-dependent density functional theory
calculations. The energy levels of the CSS are obtained from the electrochemical studies. The energy levels
of the other excited states are derived from the above spectral data in acetonitrile. The lifetimes of the excited
states are obtained with fs TA spectra data.
3
.6 TTA upconversion and delayed fluorescence
With BDP-An as triplet donor and Perylene as triplet acceptor, TTA upconversion experiment
was carried out excited by a 510 nm continuous wave (cw) laser in dichloromethane (Fig. 7)
and acetonitrile (See ESI†, Fig. S20).
2
4

b
Fig. 7 (a) Upconversion with BDP-An as triplet donor and perylene as triplet acceptor, excited

2
5
5
with 510 nm cw-laser (50 mWcm ), [BDP-An] = 1.0  10 M, [perylene] = 1.7  10 M, in
dearated dichloromethane, 20 C. (b) Photographs of the emission of BDP-An alone and
upconversion with perylene as triplet acceptor. (c) CIE diagram.
1
BDP-An alone gives a yellow emission which is the mixed emission of LE and CT states.
Upon addition of perylene, a new strong emission band in the range of 430490 nm was
observed, with the significant vibrational progression of perylene (Fig. 7a). The color change
of luminescence in solution was obvious to the naked eyes, and the upconverted blue emission
of perylene was observed, thus the upconversion of the mixture was confirmed (Fig. 7b). The
CIE coordinates of the emission of BDP-An and the upconversion are presented in Fig. 7c.
The upconversion quantum yield was determined as 18.9% in dichloromethane and 10.5% in
acetonitrile. This was the first TTA upconversion achieved with a triplet excited-state
equilibrium PS based on SOCT-ISC.
2
5

Fig. 8 (a) Delayed fluorescence spectrum of the mixture of BDP-An with Perylene, (b) decay

5
5
trace of the fluorescence at 465 nm. [BDP-An] = 1.0  10 M, [Perylene] = 7.0  10 M in
acetonitrile, ex = 510 nm. The spike in the delayed fluorescence traces is the scattered laser,
2
0 C.
The kinetics of the delayed fluorescence of TTA upconversion was studied with ns TA
spectrometer in the emission mode (Fig. 8). The decay trace was fitted with double exponential
decay model, which was composed of a rise phase with a lifetime constant of 3.8 s and a
decay component with the lifetime of 70.8 s. The rise is due to the formation of the singlet
excited state of perylene by TTET and TTA. The decay components indicate that the delayed
fluorescence lifetime was 70.8 s, which is due to the consumption of the triplet perylene by
TTA. Such long-lived delayed fluorescence is crucial for applications such as time-resolved
luminescence imaging studies [41,44,45,72].
4
Conclusion
2
6

In summary, we prepared an electron donor/acceptor dyad with phenylethynylanthracene and
Bodipy (BDP-An), with the aim to adjust the triplet state energy levels and the triplet quantum
yield. BDP-An and the reference compounds were synthesized and characterized. In the
ground state, the coupling between two chromophores of BDP-An is weak according to the
absorption spectra and optimized ground state conformation. In nonpolar solvents, the
1
fluorescence quantum yield (
F
) of LE state emission of the Bodipy moiety was up to 99%.
However, in polar solvents, the fluorescence was significantly quenched and the CT band was
observed. Furthermore, the singlet oxygen quantum yield ( ) was up to 95%, which indicates

that CS becomes thermodynamically allowed. The electrochemical studies were in agreement
with the above results, and revealed that CSS is composed of Bodipy radical anion and
phenylethynyl anthracene radical cation. In femtosecond transient absorption spectra, upon fs
pulsed laser excitation, CSS was generated, with that CR occurred and the triplet excited state
of phenylethynyl anthracene was observed. Time constants of CS and CR were determined as
1
.6 ps and 3.8 ns, respectively. In nanosecond transient absortion spectra, long-lived triplet
states of Bodipy ( = 108 s) and phenylethynyl anthracene ( = 35 s) were observed
T
T
simultaneously, which is the first report on triplet excited-state equilibrium of triplet PSs based
on SOCT-ISC. With perylene as the triplet acceptor, BDP-An was applied to TTA UC, the
quantum yield was up to 18.9% in dichloromethane, and the lifetime of delayed fluorescence
was 70.8 s, which is also the first report on TTA UC achieved with a triplet excited-state
energy equilibrium PS based on SOCT-ISC. These results are important for in-depth
understanding CR in organic molecules, designing new triplet PSs as well as adjusting the
triplet properties of PSs.
2
7

Conflicts of interest
There are no conflicts to declare.
Acknowledgement
J. Z. thanks the NSFC (21673031, 21761142005 and 21911530095) and the State Key
Laboratory of Fine Chemicals (ZYTS201901) for financial support. G.G.G. acknowledges the
basic research funding from Dalian University of Technology (DUT18GJ205).
Notes and references
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9