Effect of molecular conformation on the efficiency of the spin orbital charge recombination-induced intersystem crossing in bianthryls

Article abstract of DOI:10.1016/j.dyepig.2020.109121

Two bianthryl dyads, with the two units connected at either the 2- or 9- positions of the anthryl moiety, were studied to establish the relationship between orientation of the anthryl moieties and the electronic coupling and intersystem crossing (ISC) efficiency. The anthryl moieties in the two dyads adopt a close-to-orthogonal geometry, with dihedral angles of 90° (9,9′-bianthryl) and 106° (2,9′-bianthryl) at the ground state, respectively. The charge transfer (CT) emission efficiency and the fluorescence lifetimes are clearly dependent on the electronic coupling between the two anthryls, and stronger coupling lead to higher fluorescence quantum yields (34% vs. 9%) and shorter luminescence lifetimes (13.9 ns vs. 38.6 ns). The bianthryl with more orthogonal geometry shows higher singlet oxygen quantum yields Φ_Δ (9,9′-bianthryl, Φ_Δ = 53%) than 2,9′-bianthryl (Φ_Δ = 32%). Moreover, highly solvent polarity-dependent fluorescence emission and Φ_Δ were observed for the dyads (Φ_Δ = 22–53%), which is different from the trend of the monomer anthracene, thus we propose the spin-orbit charge transfer ISC (SOCT-ISC) is responsible for the triplet state productions of the dyads. Interestingly, we found that inducing a heavy atom (Br) does not increase the ISC yield of anthracene. ISC in bianthryls was also confirmed with nanosecond transient absorption spectroscopy, the featured T₁→T_n absorption at ca. 433 nm was observed, and the triplet state lifetime are long (9,9′-bianthryl, τ_T = 353 μs; 2,9′-bianthryl, τ_T = 493 μs, in acetonitrile).

Full text of DOI:10.1016/j.dyepig.2020.109121

Dyes and Pigments 187 (2021) 109121
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Effect of molecular conformation on the efficiency of the spin orbital charge
recombination-induced intersystem crossing in bianthryls
a
,
b
c
b
a,
b
b
Xiaoyu Zhao , Andrey A. Sukhanov , Kepeng Chen , Xinyu Geng , Yu Dong ,
c
a
,
b
,
*
a,**
Violeta K. Voronkova , Jianzhang Zhao
, Lang Liu
a
Key Laboratory of Energy Materials Chemistry, Institute of Applied Chemistry, College of Chemistry, Xinjiang University, Urumqi, 830046, PR China
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 2 Ling Gong Road, Dalian, 116024, PR China
Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of Russian Academy of Sciences, Kazan, 420029, Russia
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two bianthryl dyads, with the two units connected at either the 2- or 9- positions of the anthryl moiety, were
studied to establish the relationship between orientation of the anthryl moieties and the electronic coupling and
intersystem crossing (ISC) efficiency. The anthryl moieties in the two dyads adopt a close-to-orthogonal geom-
Bianthryl
Charge transfer
Intersystem crossing
Spin-orbit coupling
Triplet state
◦
′
◦
′
etry, with dihedral angles of 90 (9,9 -bianthryl) and 106 (2,9 -bianthryl) at the ground state, respectively. The
charge transfer (CT) emission efficiency and the fluorescence lifetimes are clearly dependent on the electronic
coupling between the two anthryls, and stronger coupling lead to higher fluorescence quantum yields (34% vs.
9
%) and shorter luminescence lifetimes (13.9 ns vs. 38.6 ns). The bianthryl with more orthogonal geometry
′
′
shows higher singlet oxygen quantum yields Φ
Δ
(9,9 -bianthryl, Φ
Δ
= 53%) than 2,9 -bianthryl (Φ
Δ
= 32%).
Δ Δ
Moreover, highly solvent polarity-dependent fluorescence emission and Φ were observed for the dyads (Φ =
2
2–53%), which is different from the trend of the monomer anthracene, thus we propose the spin-orbit charge
transfer ISC (SOCT-ISC) is responsible for the triplet state productions of the dyads. Interestingly, we found that
inducing a heavy atom (Br) does not increase the ISC yield of anthracene. ISC in bianthryls was also confirmed
with nanosecond transient absorption spectroscopy, the featured T
1
→T
n
absorption at ca. 433 nm was observed,
′
′
and the triplet state lifetime are long (9,9 -bianthryl,
τ
T
= 353
μ
s; 2,9 -bianthryl,
τ
T
= 493
μ
s, in acetonitrile).
1
. Introduction
photochemistry, it is elusive to achieve efficient ISC in heavy atom-free
triplet PSs. Some methods have been developed to enhance the ISC in
Triplet photosensitizers (PSs) are versatile compounds, which have
heavy atom-free compounds, for instance the compounds with n-
* transitions (El Sayed Rule) [29], exciton coupling [30,31], energy
matching S /T states [10,17], and molecules containing electron spin
π ↔
been used for photocatalysis [1–4], photovoltaics [5,6], photodynamic
therapy [5,7–11], and tripletꢀ triplet annihilation (TTA)-induced
photon upconversion [10,12–16]. Triplet PSs should show strong ab-
sorption of excitation light and efficient ISC [10]. ISC is an electron spin
forbidden process, thus magnetic torque is required for the electron spin
rephrasing or flip. The straight forward method to enhance the ISC is
using the heavy atom effect [10,17–22]. Triplet PSs contain Br, I, Pt, Ir,
Ru atoms belong to this category [23–28]. However, the heavy atom can
shorten the triplet state lifetime, induces toxicity, and in some cases the
ISC is inefficient even with heavy atoms directly attached to the
π-π
1
n
converter [32,33]. However, normally these compounds are with syn-
thetically demanding molecular structures. Therefore, it is highly
desired to develop efficient triplet PSs based on simple molecular
structures.
Concerning this aspect, charge recombination (CR)-induced ISC in
electron donor/acceptor dyads, which has been known for decades, is in
particular of interest [34–38]. Initially, these dyads were used to mimic
natural photosynthesis centers to attain long-lived CT states [35,37,39,
40]. Given the chromophore (either the electron donor or acceptor) is
with lower localized triplet state energy level than the CT state, then the
π
-conjugation core of the chromophore. Thus, heavy atom-free triplet
PSs are of particular interest. However, from point of view
a
*
Corresponding author. Key Laboratory of Energy Materials Chemistry, Institute of Applied Chemistry, College of Chemistry, Xinjiang University, Urumqi, 830046,
PR China.
** Corresponding author.
E-mail addresses: zhaojzh@dlut.edu.cn (J. Zhao), liulang@xju.edu.cn (L. Liu).
https://doi.org/10.1016/j.dyepig.2020.109121
Received 19 October 2020; Received in revised form 21 December 2020; Accepted 22 December 2020
Available online 28 December 2020
0
143-7208/© 2020 Elsevier Ltd. All rights reserved.

X. Zhao et al.
Dyes and Pigments 187 (2021) 109121
CR may induce ISC and the located excited (LE) triplet state will be
populated. However, the triplet state yields (ISC quantum yields) of
these molecular systems were normally not studied. High ISC yield is not
expected for these dyads because of the radical pair ISC (RP ISC)
mechanism of these molecular systems. RP ISC is achieved with the
hyperfine coupling interaction, which is known to occur with slow ki-
netics [34,41]. On the other hand, the separation of the electron donor
and acceptor in these conventional dyads is large, in order to reduce the
electronic coupling between the donor and acceptor (to prolong the CT
state lifetime), thus with the resulted extremely small electron exchange
energy (J), the RP ISC becomes feasible [42–44]. However, the large
separation of the electron donor and acceptor makes the synthesis of the
compounds difficult. Unfortunately, shortening the linker will increase
the electronic coupling between the electron donor and acceptor, which
makes the electron exchange energy large, and the RP ISC is inhibited.
Therefore, electron donor/acceptor dyads with more simple molecular
structure and fast ISC kinetics are desired.
conventional charge transfer state, rather, it is a charge resonance state
without any transfer of one electron from one anthryl subunit to another
[61]. In some cases the triplet state of 9,9′-BA was studied with nano-
second transient absorption (ns TA), but the wavelength range moni-
tored for the TA spectrum is limited, and the triplet state lifetime was not
reported [59]. Some BA analogues were also studied for their charge
transfer [69–71], but the molecular structural profile is still the 9,
′
9 -biaryl, molecular structures with other geometry were not thoroughly
studied. It has been shown that the electronic coupling and geometry
play important role in determination of the photophysical property of
the electron donor/acceptor dyads [67].
In order to address the above challenges, herein we studied the
photophysical properties of two BAs, one is the 9,9′-BA and another is
the 2,9′-BA (Scheme 1), the study is focused on the ISC and the triplet
state. The two BAs show different geometry, 9,9′-BA shows orthogonal
geometry, whereas 2,9′-BA is expected to show more coplanar geome-
try. As such the electronic coupling magnitude between the two anthryl
units will be different, the photophysical properties of the two BAs,
especially the ISC, will be different as well. However, to the best of our
knowledge, such comparison was not performed. 2,9′-BA was reported
recently as a fluorescent emitter for TTA upconversion [65], but its ISC
and triplet state property was not studied. The two BAs were studied
with steady state and time-resolved spectroscopies and Density Func-
tional Theory (DFT) computations.
Recently, compact electron donor/acceptor dyads were reported to
show efficient ISC [45–49]. In these dyads containing short linker be-
tween the electron donor and acceptor, the donor and acceptor adopt
orthogonal geometry [46,48], thus the CR is accompanied with molec-
ular orbital angular momentum change, which compensate the electron
spin angular momentum change of the ISC, as a result, the CR in these
dyads may enhance ISC because the angular momentum conservation is
satisfied in the CR/ISC. One advantage of these dyads is the simple
molecular structure, the donor and acceptor can be connected by a
single bond, given the orthogonal geometry is achieved with molecular
conformation restriction [45,46,48,50–53]. Moreover, due to the strong
electronic coupling between the donor and acceptor, the electron
transfer (both the photo-induced charge separation (CS) and the CR) is
fast, thus a high ISC can be expected. To date Bodipy [51], perylene
2. Results and discussion
2.1. Design and synthesis of the compounds
9,9′-BA was known to adopt an orthogonal geometry [72,73], and
TICT state was proposed for this BA, especially in polar solvent. In order
to probe the relationship between the molecular geometry and the
photophysical properties, especially the SOCT-ISC efficiency, 2,9′-BA
was prepared (Scheme 1). During the preparation of this manuscript, 2,
[
54], perylenemonoimide [55] have been used as the visible
light-harvesting chromophore, and anthryl [56], phenothiazine [52]
and phenoxiazine [57] have been used as the electron donors for con-
struction of compact electron donor/acceptor dyads showing efficient
ISC. These electron donor/acceptor dyads share a common feature, i.e.
the electron donor/acceptor are with drastically different structure, and
normally with different redox potentials. One of the disadvantages of
this molecular structural profile is the low CT state energy level, as a
result, the energy level of the triplet state, which is formed by CR, is
intrinsically low (it should be lower than the CT state) [37]. Triplet state
with higher energy level is beneficial for the applications in electron
transfer and energy transfer. Thus, a molecular structural profile for
compact electron donor/acceptor dyads showing higher CT state energy
levels are desired.
9′-BA
was
reported
as
a
fluorescent
emitter
for
triplet-triplet-annihilation upconversion (TTA-UC), but the ISC and the
triplet state property was not studied [65]. The synthesis of 9,9′-BA is
based on reductive coupling of 9-anthraquinone using zinc. The syn-
thesis of 2,9′-BA is based on the Suzuki-Miyama crossing coupling re-
1
action. The molecular structures were fully characterized by H NMR,
1
3
C NMR and high resolution mass spectra (see ESI).
2.2. UV–Visible absorption and fluorescence emission spectra
The UV–Vis absorption of the compounds was studied (Fig. 1). The
main absorption bands of BA derivatives are in the range of 300–425
nm, which are the structured absorption bands of the anthryl moiety.
There is no obvious CT absorption band, indicating weak coupling be-
tween the two anthryl moieties at ground state, especially for 9,9′-BA
[59]. However, for 2,9′-BA, the dihedral angle between two anthryl
′
Concerning this aspect, the 9,9 -bianthryl (9,9′-BA) attracted our
attention. 9,9′-BA was intensively studied for its symmetry breaking
charge transfer (SBCT) [58–62], as well as the LE and the CT dual
emission [59]. Triplet state formation was observed [59]. The bianthryl
(
BA) analogues were also used for organic light emitting diodes [63],
◦
especially for the reverse ISC from higher triplet state (T state) to the
5
moieties is 106 (see Fig. 6), thus the electronic coupling is expected to
◦
singlet state to harvest the triplet excitons [64]. Recently, 9,9′-BA was
used for emission color-tunable tripletꢀ triplet annihilation upconver-
sion [65]. The solvent polarity dependency of the fluorescence emission
of 9,9′-BA was studied previously, it was proposed that the emissive
state of 9,9′-BA is a twisted intramolecular charge transfer (TICT) state,
especially in polar solvents [66]. The charge recombination relaxation
of the CT state leads to triplet state [59]. Based on the orthogonal ge-
ometry, we propose the ISC mechanism in BA should be the SOCT-ISC.
Triplet PSs with the SOCT-ISC mechanism, and contain two identical
chromophores, i.e. showing SBCT, was rarely reported previously [67,
be stronger than that in 9,9′-BA (dihedral angle: 90 , see Fig. 6). As a
result, a broader absorption band was observed for 2,9′-BA (full width at
ꢀ 1
half maximum (FWHM): 3950 cm ) than that of in 9,9′-BA (FWHM:
ꢀ 1
2110 cm ), which is probably due to the geometry distribution at the
ground state and the stronger coupling between the two anthryl units.
This is in agreement with the rotation potential energy studies (see later
section).
Fluorescence emissions of the BA compounds were studied (Fig. 2).
The fluorescence band of An is with significant vibrational progression,
and the intensity and position are almost solvent polarity-independent
(see ESI, Fig. S7a). However, the fluorescence bands of 9,9′-BA and
2,9′-BA are structureless and broad, especially in polar solvents (Fig. 2),
and with increasing of the solvent polarity, the maximal emission
wavelength are redshifted. In non-polar solvents, the emissive state is
basically a LE state without significant CT feature, which was confirmed
6
8].
Although the BAs have been studied for a long time, however their
ISC mechanisms and the photophysical processes are still in controversy
to some extent. For instance, some studies suggest that the emissive state
of 9,9′-BA in polar solvent such as acetonitrile (ACN), is not a
2

X. Zhao et al.
Dyes and Pigments 187 (2021) 109121
◦
Scheme 1. Reagents and conditions: (a) zinc powder, acetic acid, HCl, refluxed at 110 C for 13 h under N
2
, yield: 10%; (b) 2-anthraceneboronic acid, K
2
CO
3
, Pd
◦
(
PPh
3
)
4
, toluene/ethanol/water, refluxed at 100 C for 8 h, yield: 77%.
9
,9′-BA than that of 2,9′-BA [59].
To study the effect of the magnitude of electronic coupling on the CT
fluorescence profile, the electronic coupling matrix element between the
ground state and CT state were calculated as using the following equa-
tion (1) [35,79].
[
]
1
2
5
ꢀ
)
1.4 × 10 k
ꢀ
r
V
DA cm 1
=
(eq. 1)
3
2
n R
ν
m ax
c
max (cm 1) is position of the CT fluorescence maximum, k
ꢀ
In eq. (1),
ν
r
ꢀ
1
(
s
) is the radiative rate constant, n is the solvent refractive index, and
(Å) is the distance between the center of electron donor and acceptor,
(9,9′-BA) = 4.35 Å and R
2,9′-BA) = 6.60 Å. The results are presented in Table 1. The VDA values
indicate the electronic coupling between the ground state and CT state
35], and the order of the electronic coupling extent for BA derivatives is
R
c
which was optimized by DFT calculations. R
c
c
(
[
2
,9′-BA > 9,9′-BA under low polarity solvents. But in highly polar
solvent (i.e. ACN), electronic coupling extent is 9,9′-BA > 2,9′-BA,
which manifests the stronger coupling in 9,9′-BA than in 2,9′-BA be-
1
tween CT (after relaxation) and the ground state.
ꢀ
5
Fig. 1. UV–Vis absorption spectra of the compounds. c = 1.0 × 10 M in
For electron donor/acceptor dyads with charge transfer emissive
state, fluorescence is originated from either the LE state in non-polar
solvents, or CT state in polar solvents. The former is featured with a
structured emission band at shorter wavelength range, whereas the
latter gives an emission band without fine structure, and in relatively
red-shifted spectral region [75,80]. For 9,9′-BA, the fluorescence
emission profile changes in different solvents in both aspects of emission
intensity and emission wavelengths, which is more significant than that
of 2,9′-BA. This result indicates that the excited dipole moment changes
of 9,9′-BA is larger than that of 2,9′-BA. In order to quantitatively study
the dipole moment of the emissive singlet excited states of the two
dyads, the Stokes shift variation of the compounds in solvents with
different polarity was analyzed with Lippert-Mataga equation (Fig. 2d
and eq. (2)-(3) [75,81].
◦
dichloromethane, 20 C.
with femtosecond transient absorption spectroscopy [59]. However, it
was proposed that even in non-polar solvent such as heptane, there is
equilibrium between the LE state and a partially CT (PCT) state, based
on time-resolved near IR absorption spectroscopy [74]. Thus the broad
emission band of 9,9′-BA in n-hexane can be rationalized as the sum of a
LE and PCT emission bands. In polar solvents, the emission intensity of
BAs become weaker, which is the typical feature of the emissive excited
state with significant charge transfer character [74,75]. It was also
proposed that the broad emission bands are composed of two over-
lapping bands (LE emission and CT emission) [59]. In polar solvents, the
PCT transfers to a fully charge separate state [74]. Based on femtosecond
transient absorption spectroscopy, the charge separation in 9,9′-BA was
determined as ca. 20 ps in alcohols [76]. In N,N-Dimethylformamide
ꢀ
)
2
(
ꢁ
ꢀ
)
2
μe
ꢀ
μf
0
0
hc
ν
a
ꢀ
ν
f
= hc
ν
ꢀ
ν
ꢀ
f(
ε
, n)
(eq. 2)
a
f
3
a
(
DMF), the CS time constant was reported as 1.54 ps [65]. For 9,9′-BA,
this spectral behavior was proposed as twisted ICT state [59,66,72,77,
2
ε
ꢀ 1
n ꢀ 1
1
3
′
7
8]. Similar results were also observed for other 9,9 -biaryl compounds
f(ε, n) =
ꢀ
, a = (3M/4N
π
d)
(eq. 3)
2
2ε
+ 1 2n + 1
[
69].
For 2,9′-BA, interestingly, the change of the fluorescence intensity
where f is the orientational polarizability of solvents,
state dipole moment,
solvent cavity (Onsager) radius, derived from the Avogadro number (N),
μ
e
is the excited-
with solvent polarity is less significant, which is in agreement with the
recent study [65]. We propose that the CT state is with less extent of
charge separation in 2,9′-BA, as compared in that of 9,9′-BA. The more
significant solvent polarity-dependency of the emission of the 9,9′-BA
μ
g
is the ground-state dipole moment, a is the
3
molecular weight (M), and density (d = 1.0 g/cm ),
ε
and n are the
solvent dielectric and the solvent refractive index, respectively. The
slopes of Lippert-Mataga plots indicate that the excited dipole moment
1
should be attributed to the more significant CT feature of the S state of
3

X. Zhao et al.
Dyes and Pigments 187 (2021) 109121
Fig. 2. Fluorescence emission spectra of (a) 9,9′-BA and (b) 2,9′-BA in different solvents. (c) Fluorescence emission spectra of 9,9′-BA, 2,9′-BA, An and Br-An in
ACN, (d) Lippert–Mataga regressions of 9,9′-BA and 2,9′-BA. Optically matched solutions were used for all the measurements, (λex = 350 nm, A = 0.093), 20 ◦
C
(
slight variation of the concentration is necessary to obtain optically matched solutions).
Table 1
Photophysical parameters of the compounds.
a
b
c
d
e
f
g
h
i
solvent
λ
abs
ε
λ
em
τ
F
Φ
F
k
r
k
nr
V
DA
k
j
9
,9′-BA
,9′-BA
TOL
DCM
ACN
TOL
DCM
ACN
TOL
DCM
ACN
TOL
392
391
389
367
366
364
359
358
356
372
2.4
2.3
2.4
1.6
1.6
1.5
0.7
0.7
0.6
0.9
418
445
462
425
448
459
403
402
399
398
7.0 /10.7
60
41
9.0
77
76
34
30
16
23
7.0
8.6
2.4
0.5
16
5.7
3.4
4.6
4.9
4.3
8.3
21
0.10
0.21
0.35
0.20
0.33
0.33
k
j
j
17.2 /26.7
k
19.7 /38.6
k
j
2
4.7 /6.1
k
j
5.6 /9.9
14
k
j
8.0 /13.9
4.3
9.1
7.3
5.9
23
k
j
l
An
3.3 /4.0
ꢀ
k
j
l
2.2 /2.3
38
ꢀ
k
j
l
3.9 /5.0
0.3(98.7%),
20
ꢀ
l
Br-An
310
323
490
ꢀ
k
4
.2 (1.3%)
l
DCM
ACN
371
369
0.9
0.8
418
416
0.3(99.8%),
3.0
2.0
10
10
ꢀ
k
4
.7 (1.2%)
l
0.2(99.2%),
ꢀ
k
3
.9 (0.8%)
a
ꢀ 1
E
T
(30) values of the solvents, in kcal mol . TOL (33.9), DCM (40.7) and ACN (45.6), respectively.
b
c
d
e
f
ꢀ 5
c = 1.0 × 10 M, in nm.
4
ꢀ 1
ꢀ 1
cm .
Molar extinction coefficient at the absorption maxima.
In nm, λex = 350 nm.
ε: 10
M
ꢀ 5
Fluorescence lifetimes (λex = 340 nm), in ns, c = 1.0 × 10 M.
Fluorescence quantum yields with anthracene as standard (Φ = 27% in ethanol) in aerated, λex = 350 nm, in %.
F
g
h
i
7
ꢀ 1
Radiative decay rate constant, derived from fluorescence data k
r
= Φ
ꢀ 1
, in aerated solution.
F
/
τ
F
, in 10
s
, in aerated solution.
7
Nonradiative rate constant knr = (1ꢀ Φ
F F
)/τ , in 10 s
ꢀ 1
The electronic coupling matrix element between the CT state and ground state, in cm
.
j
Measured in deaerated solutions.
k
l
Measured in oxygen-saturated solutions.
Not applicable.
4

X. Zhao et al.
Dyes and Pigments 187 (2021) 109121
of 9,9′-BA (7.12 D) is larger than that of 2,9′-BA (6.97 D), based on the
ground state dipole moments of 9,9′-BA (0 D) and 2,9′-BA (0.36 D),
large spatial overlap of the HOMO and LUMO orbitals (Fig. 8). Thus the
forbidden feature of the CT excited state is less in 2,9′-BA [17].
′
′
obtained with the optimized ground state (S
0
) geometry. Previously it
Recently, the fluorescence intensity of 10,10 -diphenyl-9,9 -bian-
thracene was studied, on the reverse intersystem crossing (RISC) process
in organic light emitting diodes (OLEDs) [64]. It was proposed that the
was proposed the excited state dipole moment of 9,9′-BA in polar sol-
vent is up to 20 D [71].
For further study of charge transfer in BAs, we estimated the degree
different fluorescence intensity under different atmosphere (i.e. N
and O ) is due to the exciton at the S state on the orthogonal bian-
thracene can be transferred to T and finally quenched by triplet oxygen
[64]. However, we measured fluorescence intensity of 9,9′-BA and 2,
9′-BA under N , air and O atmosphere in TOL and ACN (Fig. S8). It was
found that the I
/I100 values of 2,9′-BA are 2.90 and 5.15 in TOL and
ACN, respectively (where I stands for the fluorescence intensity in inert
atmosphere (i.e. N ) and I100 stands for the fluorescence intensity in neat
2
, air
of electron transfer, using Δ
dipole moments and the excited state dipole moments) and R
centroid-to distance between the electron donor and acceptor) [82]. For
,9′-BA, R = 4.35 Å and Δ = 7.12 D, a transfer of 0.87 electron
happened from the one anthryl to another anthryl part. However, R (2,
′-BA) = 6.60 Å, Δ = 6.61 D and 0.56 electron transferred in 2,9′-BA,
which indicated the more significant degree of electron transfer in 9,
′-BA.
The fluorescence lifetimes of the compounds were studied (Fig. 3),
μ
(the different between the ground state
2
1
c
(the
1
9
c
μ
2
2
c
0
9
μ
0
2
9
O
2
). While those of 9,9′-BA are 3.75 and 13.25, which are highly solvent
dependent and it is more significant for 9,9′-BA than 2,9′-BA. Thus, we
concluded that fluorescence intensity change is more likely to be caused
by the difference of fluorescence lifetimes under different atmosphere.
with the time-correlated single photon counting (TCSPC) technique.
Both dyads show normal fluorescence lifetimes in non-polar solvents
such as n-hexane (n-HEX, ca. 6 or 7 ns). In more polar solvents, for in-
stances dichloromethane (DCM) and ACN, however, the fluorescence
lifetime prolonged significantly, for example, 26.7 ns in DCM and 38.6
ns in ACN were observed for 9,9′-BA, respectively. The result is very
close to the previously reported lifetime for 9,9′-BA in ACN (35.26 ns)
2
Longer fluorescence will cause more significant quenching by O .
The photophysical properties of the compounds were summarized
(Table 1). For 9,9′-BA, the fluorescence property is more dependent on
the solvent polarity as compared to 2,9′-BA, in aspects of emission
wavelengths and fluorescence quantum yields. For instance, the
maximal emission wavelength of 9,9′-BA is red-shifted from 418 nm in
TOL to 462 nm in ACN, at the same time, the fluorescence quantum
yields decreased from 60% to 9%. In comparison, 2,9′-BA shows emis-
sion red-shift from 425 nm in TOL to 459 nm in ACN, accordingly the
fluorescence quantum yields decreased from 77% to 34%. These results
demonstrated the effect of the molecular geometry and the electronic
coupling magnitude on the photophysical properties of the dyads.
[
59]. Previously it was reported the fluorescence decay of 9,9′-BA in
ꢀ
1
heptane (E
T
(30) = 31.1 kcal mol ) is (13 ± 1) ns, both the LE and the
PCT state decay with the same lifetime [74]. In our case we observed a
ꢀ 1
lifetime of 7.2 ns in n-HEX (E
T
(30) = 31.0 kcal mol ).
The fluorescence lifetimes of 2,9′-BA show a similar trend, but to a
lesser extent. For instance, the fluorescence lifetime is 6.2 ns in non-
polar solvents, n-HEX. However, in more polar solvents, the fluores-
cence lifetime has no more significant extension like 9,9′-BA, for
example, 8.4 ns in DCM and 13.9 ns in ACN were observed, respectively.
Recently the fluorescence of 2,9′-BA was reported as 6.07 ns in toluene
Notably the fluorescence lifetime of 9-bromoAn (Br-An) (
τ = 0.2 ns)
F
is very short as compared to that of An (
τ
F
= 2.2–3.9 ns) and BAs (
τ =
F
4.7–19.7 ns in aerated solution) (Table 1). For instance, the fluorescence
lifetime of Br-An in ACN is ca. 200 ps, very close to the previously re-
ported 160 ps, determined with streak camera [83]. The sum of the
fluorescence quantum yield and the singlet oxygen quantum yield is
much smaller than unit, every different from that of the BAs [59]. Thus
we propose there exist a non-radiative decay channel for Br-An, for
instance the loose belt effect of the C–Br bond [17].
(
TOL) and 11.81 ns in DMF [65]. Similar to that of 9,9′-BA, we propose
the prolonged fluorescence lifetimes of the dyads in polar solvents are
due to forbidden feature of the radiative CT state. For 9,9′-BA, the ge-
◦
ometry of the S
1
state is more orthogonal in polar solvents (ϕ = 67 ,
based on the optimized excited state S
→S more forbidden, thus the emission intensity decreased but the
fluorescence lifetime is prolonged. It was proposed that the S state is
1
geometry in ACN), makes the
S
1
0
1
As an approximation of the ISC ability of the compounds, the singlet
distorted from the Franck-Condon geometry, and the torsion between
oxygen quantum yields (Φ
(Table 2). For An, it is known the ISC is via the S
Δ
) of the compounds were studied in detail
channel, both
◦
the two anthryl units is 70 [59,76].
1
→T
2
For 2,9′-BA, the prolonging of the lifetime in polar solvent is less
states share similar energy levels, which facilitate the ISC by enhance the
significant, which is in agreement with the geometry of the dyad (less
coupling matrix elements and the density of states [84–86]. Interest-
ingly, for Br-An, we did not observe any significant increase of the Φ
Δ
◦
orthogonal geometry, ϕ = 132 , i.e. the two anthryl units are more
coplanar than that of 9,9′-BA), electronic coupling is stronger due to the
values in the presence of heavy atom Br, but the fluorescence quantum
Fig. 3. Fluorescence decay curves of (a) 9,9′-BA and (b) 2,9′-BA in different deaerated solvents. λex = 340 nm (EPL picosecond pulsed laser; TCSPC detection mode).
For 9,9′-BA, the monitored emission wavelengths are 412 nm (n-HEX), 446 nm (DCM) and 467 nm (ACN), respectively. For 2,9′-BA, the monitored emission
wavelengths are 416 nm (n-HEX), 448 nm (DCM) and 459 nm (ACN), respectively. c = 1.0 × 10ꢀ M, 20 C.
5
◦
5

X. Zhao et al.
Dyes and Pigments 187 (2021) 109121
Table 2
Table 3
Electrochemical redox potentials of the compounds
a
a
Singlet oxygen quantum yields (Φ
Δ
, in %) in different solvents
.
.
b
c
n-HEX
TOL
THF
DCM
ACN
MeOH
E(ox)/V
E(red)/V
E
(HOMO)/eV
E(LUMO)/eV
9
2
,9′-BA
,9′-BA
21
21
20
25
22
15
52
35
24
18
28
31
27
12
65
42
53
32
51
55
26
20
18
27
9,9′-BA
2,9′-BA
An
+0.88
+0.92
+0.90
ꢀ 2.30
ꢀ 2.40
ꢀ 2.40
ꢀ 5.66
ꢀ 5.70
ꢀ 5.68
ꢀ 2.48
ꢀ 2.38
ꢀ 2.38
An
Br-An
a
2
Cyclic voltammetry measurements in N -saturated ACN containing a 0.10 M
a
ꢀ 1
E
T
(30) values of the solvents, in kcal mol . n-HEX (31.0), TOL (33.9), THF
4 6
BuN [PF ] supporting electrolyte. Redox potentials of the compounds were
(
37.4), DCM (40.7), ACN (45.6) and MeOH (55.4), respectively. Singlet oxygen
quantum yield with [Ru(bpy) ][PF as standard (Φ = 57% in DCM), in %, λex
350 nm.
determined with ferrocene (Fc) as the internal standard (0 V). The counter
electrode is the Pt electrode, the working electrode is the glassy carbon elec-
trode, and the Ag/AgNO3 couple is the reference electrode.
3
6
]
2
Δ
=
b
Calculated by E(HOMO) = ꢀ 4.78 + (E(OX)(Fc) – E(OX)).
c
Calculated by E(LUMO) = ꢀ 4.78 + (E(OX)(Fc) – E(RED)).
yields decreased sharply as compared to that of An (Table 1). For both
BAs, the Φ values are solvent polarity-dependent. For 9,9′-BA, the
highest Φ value was observed in ACN (53%), in other solvent, the Φ
values are ca. 21–27%. We noted these results are different from the
previous report on the solvent-dependency of the ISC quantum yield
Δ
Δ
Δ
E
CS = e[EOX ꢀ ERED] + ΔG
S
(eq. 6)
in which ΔG
S
is the static Coulombic energy, which is described by eq.
(
based on ns TA method, the triplet state quantum yields were deter-
(
6). In eq. (4)ꢀ 6, e is the electronic charge, EOX is the half-wave potential
mined with relative method) [59]. But we believed that the singlet ox-
for one-electron oxidation of the electron-donor unit (for irreversible
oxidation, the anodic peak potential was used instead of the half-wave
potential), and ERED is the half-wave potential for one-electron reduc-
tion of the electron-acceptor unit, E00 is the energy level approximated
with the crossing point of the normalized UV–Vis absorption and fluo-
ygen photosensitizing method is more convincing. For 2,9′-BA, the Φ
Δ
value is the highest in ACN (32%), in other solvents the Φ
Δ
values are
generally lower than that of 9,9′-BA.
rescence emission spectra of anthracene (LE state), in n-HEX solution,
is the static dielectric constant of the solvent, R is the center-to-center
separation distance between the two anthracene moieties, determined
by DFT optimization of the geometry, R (2,9′-
(9,9′-BA) = 4.35 Å, R
BA) = 6.60 Å, R is the radius of the electron donor, R is the radius of
the electron acceptor, REF is the static dielectric constant of the solvent
used for the electrochemical studies, is permittivity of free space. The
solvents used in the calculation of free energy of the electron transfer is
TOL (
ε
S
2
.3. Redox properties: cyclic voltammogram of the compounds
C
In order to study the CS process, Gibbs free energy changes(ΔGCS) of
electron transfer as well as the energy level of the charge separated state
C
C
D
A
(
E
CS) in BA derivatives, the electrochemical properties of 9,9′-BA and
,9′-BA were studied with cyclic voltammograms (CV, Fig. 4).
For An, an irreversible oxidation wave at +0.90 V and a reversible
ε
2
ε
0
reduction wave at ꢀ 2.40 V were observed, respectively (Fig. 4c). For
ε
S
= 2.38), DCM (
ε
S
= 8.93) and ACN (
ε = 37.5).
S
9
,9′-BA, a reversible oxidation wave at +0.88 V and.
The values of ΔGCS indicated that the PET is thermodynamically
allowed for all compounds listed in Table 4, and the driving forces for
PET were larger in polar solvents (such as ACN) than in nonpolar sol-
vents (such as TOL), which corresponds to the fluorescence changes. The
A reversible reduction wave at ꢀ 2.30 V were observed (Fig. 4a),
which are different from An. However, for 2,9′-BA (Fig. 4b), an oxida-
tion wave at +0.92 V and a quasi-reversible reduction wave at ꢀ 2.40 V
(
Table 3) were observed. The electrochemical properties of 9,9′-BA was
energy levels of CT states are 3.14 eV (in DCM) and 3.09 eV (in ACN) for
reported and the first reduction wave of 9,9′-BA was E(red) = ꢀ 2.13 V
vs Ag/AgCl) [87]. The bianthryl derivative (9,10-diphenylanthracene)
3
9
,9′-BA (Table 4), which lie higher than An energy (1.78 eV). For 2,9′-
(
BA, the energy level of CT state is 3.26 eV in ACN. Moreover, the Gibbs
free energy changes were calculated by eq. (4) (with the crossing point
of the normalized UV–Vis absorption and fluorescence emission spectra
of anthracene as the E00 values). The ΔGCS of 9,9′-BA are ꢀ 0.13 eV (in
DCM) and ꢀ 0.18 eV (in ACN), and for 2,9′-BA is ꢀ 0.01 eV (in ACN),
respectively, which indicates that the CS process driven is exoergic for
was also reported at the same experiment conditions and showed
reversible oxidation wave at +0.87 V and a reversible reduction wave at
+
ꢀ
2.26 V(vs Fc/Fc ) [88]. Thus, the redox potentials of BA dyads
determined herein are reasonable.
The Gibbs free energy change (ΔGCS) of the electron-transfer process
was calculated using the Rehm-Weller equation (eq. (4)ꢀ 6) [36,89],
9
,9′-BA and 2,9′-BA in polar solvent, but endergonic for 2,9′-BA in
ΔGCS
=
​
e[EOX ꢀ ERED] ꢀ E
0
+ ΔG
S
(eq. 4)
solvents with moderate polarity (i.e. in DCM).
0
ꢂ
ꢃꢂ 1
ꢃ
e2
πε
e2
πε
1
D
ΔG
S
= ꢀ
ꢀ
+ R1
ꢀ ε1
(eq. 5)
4
S
ε
0
R
c
8
0
R
A
ε
REF
S
Fig. 4. Cyclic voltammograms of (a) 9,9′-BA, (b) 2,9′-BA and (c) An in deaerated ACN containing 0.10 M Bu
4
N[PF
6
] as the supporting electrolyte and Ag/AgNO
3
as
the reference electrode. Scan rate: 100 mV/s. Ferrocene (Fc) was used as the internal reference, c = 5.0 × 10ꢀ M, 20 C.
4
◦
6

X. Zhao et al.
Dyes and Pigments 187 (2021) 109121
Table 4
a
Gibbs free energy changes (ΔGCS) for the intramolecular photo-induced electron transfer and CT state energy levels are presented
.
ΔGCS (eV)
E
CS (eV)
b
c
d
e
e
f
g
g
d
e
e
f
f
g
g
9
,9′-BA
,9′-BA
0.19
0.06
0.68
ꢀ 0.13
ꢀ 0.18
ꢀ 0.01
3.46
4.32
3.33
3.95
3.14
3.41
3.09
3.26
d
f
d
2
1.05
0.14
a
b
c
The redox potentials are approximated based on the redox potential measured in ACN.
00 = 3.27 eV.
E
E00 = 3.27 eV. The energy levels of E00 are approximated with the crossing point of the normalized UV–Vis absorption and fluorescence emission spectra of
anthracene, in n-HEX solution.
d
In n-HEX.
e
In TOL.
f
In DCM.
g
In ACN.
ꢀ 5
Fig. 5. Nanosecond time-resolved transient absorption spectra of (a) 9,9′-BA, (b) 2,9′-BA and (c) An (λex = 355 nm). c = 2 × 10 M in deaerated ACN. Decay traces
values in (d), (e) and (f) are the intrinsic lifetimes
of (d) 9,9′-BA at 433 nm and (e) 2,9′-BA at 434 nm and (f) An at 427 nm (λex = 355 nm) in deaerated ACN. The
τ
T
obtained by fitting the decay traces with the kinetic model by taking into account the TTA quenching effect (see ESI for details). The residues of the fittings are
also shown.
2
.4. Nanosecond transient absorption spectroscopy: the triplet excited
lifetimes, the effect of the TTA self-quenching was not considered, thus
for the compounds showing efficient ISC and long triplet state lifetimes,
the reported lifetimes are shorter than the intrinsic triplet state lifetime
state property of the compounds
In order to study the triplet excited state of the compounds, the
nanosecond transient absorption spectra measurements were studied
[90]. Previously the triplet state lifetime of An was reported as ca. 100
[91]. It should be noted that the triplet state lifetimes of An varied
significantly in different reports, in the range of 160–2900 s [92]. The
efficient ISC (approximated with the singlet oxygen quantum yield, Φ
μ
s
(
Fig. 5). Upon pulsed laser excitation, a structured excited-state ab-
sorption (ESA) band centered 427 nm for An was observed (Fig. 5c),
which is the typical absorption feature of the T state of An [88]. With
TDDFT computation, we assign this absorption band to T transition
→T
see ESI, Table S5 and Fig. S12 for orbitals involved in the transitions.
The energy gap between T and T is 2.90 eV, based on the optimized T
state geometry). The intrinsic triplet state lifetime was determined as ca.
57 s (in ACN. Fitting of the decay traces with a kinetic model with the
tripletꢀ triplet annihilation quenching effect considered) [90]. The
μ
Δ
1
= 51%, in ACN) of An is attributed to the closely lying S
1 2
/T states,
1
5
which share similar energies [84,88,93,94].
(
The ns TA spectrum of the BA shows a prominent ESA band centered
at ca. 433 nm (Fig. 5a and b), which is similar but broader than that of
1
5
1
An. From the molecular frontier orbital, T
1
state of 9,9′-BA is delo-
2
μ
calized in the whole molecular. Furthermore, an ESA band at 1321 nm (f
= 0.169, f is the Oscillator strengths) was predicted through TDDFT
calculation, which is approximate to a literature report of an absorption
band at 1250 nm of the CT state (in ACN), characterized by nanosecond
ꢀ 5
apparent triplet state lifetime is 59
μs (c = 2.0 × 10 M, in deaerated
ACN). It should be noted that for most of the reported triplet state
7

X. Zhao et al.
Dyes and Pigments 187 (2021) 109121
Fig. 6. Optimized ground state (S
0
state) geometries of (a) 9,9′-BA and (b) 2,9′-BA. The dihedral angles between two anthryl moieties are also presented. Spin
density surfaces of the triplet excited state of (c) 9,9′-BA and (d) 2,9′-BA at the optimized triplet state geometry (Isovalue = 0.001). Calculated at B3LYP/6-31G (d)
level with Gaussian 09W.
time-resolved near-IR spectroscopy [74]. These results imply that the
annihilation self-quenching effect considered) [56,57]. The ESA bands
triplet state of 9,9′-BA might be a special CT state with charge resonance
of 2,9′-BA was calculated, absorption bands centered at 445 nm and
character in polar solvents and different from the CT feature of the S
1
397 nm were predicted, which are due to the T
1
→T10 (f = 0.386) and
state [61,66,74]. The ns TA spectra of 9,9′-BA were studied previously
59], but not all the visible spectral range was monitored, and the triplet
state lifetime was not reported. The intrinsic triplet state lifetime of 9,
s in ACN (fitting based on the TTA model.
T
1
→T24 (f = 0.129) transitions (see ESI, Table S4). These predictions are
in good agreement with the experimental observations. The resulting
configurations of T were listed in supporting information (see ESI,
→ T
Table S2, Table S4, Table S6 for 9,9′-BA, 2,9′-BA and An, respectively)
And the comparison between the T absorption bands based on
[
1
n
9
′-BA was determined as 353 μ
Fig. 5d). The ESA bands of 9,9′-BA was calculated with TDDFT method,
1
→ T
n
absorption bands at 307 nm, 358 nm and 452 nm were predicted in polar
TDDFT and experimental spectra were showed in supporting informa-
solvent (i.e. ACN), which are due to the T
.042) and T
1
→T30 (f = 0.119), T
1
→T22 (f =
tion (Fig. S12).To our knowledge, Castellano’s group [95] and Beames’s
0
1
→T14 (f = 0.164) transitions between delocalized orbitals,
group [96] also calculate the T
1
→T
n
absorption with same method,
respectively (see ESI, Table S2).
which are similar to experimental results.
Recently 2,9′-BA was reported as a fluorescence emitter for TTA-UC,
but the triplet state and the ISC property were not reported [65]. For 2,
2
.5. DFT computations
9
′-BA, there is an ESA band centered as ca. 380 nm, which is in super-
imposition with the ground state blenching (GSB) (Fig. 5b). A prominent
ESA band centered at ca. 433 nm was also observed, which is similar to
that of 9,9′-BA (Fig. 5a). From the molecular frontier orbital, the tran-
The geometries of the bianthryl at the ground state were optimized
(
Fig. 6a and b). The dihedral angle between the two anthryl moieties in
,9′-BA and 2,9′-BA are 90◦ and 106 , respectively. This orthogonal
◦
9
sition of T
1
state in 2,9′-BA belongs to HOMO → LUMO, which is the
geometry of 9,9′-BA will make the two anthryl units electronically
decoupled at ground state, which is in agreement with the UV–Vis ab-
sorption, in which no significant CT bands were observed (Fig. 1).
This is also in agreement with the long fluorescence lifetimes of 9,9′-
BA in polar solvent (38.6 ns in ACN), because the TICT state is weakly
emissive [75]. In comparison, 2,9′-BA is with more coplanar geometry,
feature of CT properties. Moreover, an ESA band at 940 nm (f = 0.041)
was predicted through TDDFT calculation for 2,9′-BA. Therefore, we
propose the triplet state of 2,9′-BA in polar solvent is also special CT
state similar to that of 9,9′-BA. The triplet state lifetimes of 2,9′-BA
were determined as 493
μs (based on a kinetic model with tripletꢀ triplet
8

X. Zhao et al.
Dyes and Pigments 187 (2021) 109121
◦
thus the electronic coupling between the two anthryl units is stronger,
the ICT state is more emissive, demonstrated by the higher fluorescence
quantum yields and the shorter fluorescence lifetimes (Table 1).
The spin density surfaces of the triplet states of the dyads were
computed (Fig. 6c and d), and it was found that the triplet state wave
function is distributed over the whole molecule, i.e. both anthryl units,
especially for 9,9′-BA. This is similar to the charge resonance mecha-
nism proposed for the CT state of 9,9′-BA, confirmed with time-resolved
near-IR transient absorption spectroscopy of the 1CT state [61]. Inter-
estingly, an absorption band at 1321 nm (f = 0.169) was also predicted
in triplet state absorption for 9,9′-BA, based on TDDFT calculation,
which is consistent with CT absorption bands centered at 1250 nm re-
ported previously [61]. However, no similar triplet state absorption
anthryl units is 70 [59,76].
The frontier molecular orbitals of the BAs were studied for the
ground state (S state) and the S
0
1
state (Fig. 8). For 9,9′-BA, the HOMO
and LUMO are fully delocalized on the two anthryl units, which is
similar to the previous results obtained with semiempirical calculation
[59]. For the optimized S
LUMO are not equally distributed on the two anthryl units. It was pro-
posed that the S state is distorted from the Franck-Condon geometry,
and the torsion between the two anthryl units is 70 [59,76]. This is in
agreement with the CT feature of the S state, especially in polar sol-
vents. The MOs of the ground state and the S state infer that the CT is a
1
state geometry, however, the HOMO and
1
◦
1
1
two-step process, i.e. firstly population of the LE Franck-Condon state,
and then the CT state was formed, in other words, the CT state is not
directly populated by photoexcitation. Based on femtosecond transient
absorption spectroscopy, the charge separation in 9,9′-BA was deter-
mined as ca. 20 ps in alcohols [76], although different results were
observed (1.54 ps in DMF) [65]. With time-resolved near-infrared ab-
sorption spectroscopy, the CS time constant was determined as ca. 0.3 ps
(in heptane) [99]. The orthogonal LE state is unable to undergo CT,
torsion is required for the FC state before the occurring of the CT [71].
For 2,9′-BA, however, the HOMO orbitals of the two anthryl units
are not degenerated, the HOMO localized on the x-anthryl is with higher
energy (ꢀ 5.13 eV) than that on the y-anthryl unit (ꢀ 5.32 eV). Inter-
estingly, the LUMO localized on the x-anthryl is with lower energy
(ꢀ 1.76 eV) than the LUMO of the y-anthryl unit (ꢀ 1.58 eV), where x-
anthryl stands for the part of 9-poisition substituted anthryl, and y-
anthryl stands for the part of 2-poisition substituted anthryl. The mo-
lecular orbitals of the ground state of 2,9′-BA indicate that CT absorp-
tion is possible, which is unlikely for 9,9′-BA. The UV–Vis absorption of
3
band were predicted for An, thus we supposed that CT state have also
the charge resonance character. We studied the time-resolved electron
paramagnetic resonance (TREPR) spectra of 9,9’-BA, but no signal was
observed. The degenerate triplet state on the two units in 9,9′-BA may
be responsible for the fast SLR of the triplet state. For 2,9′-BA, the triplet
state is more localized on one anthryl unit and the energy level differ-
1 2
ence is 0.5 eV between T and T . The anion and cation spin densities of
the two compounds are also calculated, and the spin density surfaces are
distributed on the whole molecule (see ESI, Fig. S11). These results are
in agreement with the previously reported results that the CT state of the
9
,9′-BA is not the conventional CT state [61].
In order to investigate the conformational constraints of the dyads
for the torsion of the two anthryl units, the potential energy curves (PEC)
of the rotation about the linker bonds were constructed (Fig. 7). For 9,9′-
BA, the PEC is steeper than that of 2,9′-BA due to the large steric hin-
drance, as a result of the H atoms at peri-positions (i.e. the 1- and 8-
position of the anthryl moiety) of the two anthryl units in 9,9′-BA. The
geometries accessible with the thermal energy at room temperature are
2,9′-BA was calculated with TDDFT method, transitions from S
422 nm (f = 0.051) and 400 nm from S → S (f = 0.035) were predicted,
which are assigned to the CT absorption, due to their smaller f value than
LE absorption (385 nm), S (f = 0.127) (see ESI, Table S3).
→ S
The photophysical properties of the compounds are summarized in
Scheme 2. For An, it is known that the ISC is via the energy matching S
states (see ESI, Scheme S1) [88,93,94]. Experiments show it is also
solvent-dependent (Table 1), which can be attributed to the variation of
the S /T state energy level by different solvents [100].
0
1
→ S at
0
2
◦
◦
mainly with dihedral angles in the range of 79 –100 for 9,9′-BA,
◦
◦
however, it is 58 –123 for 2,9′-BA (Fig. 7b), which indicates that 9,9′-
BA prefers to take the orthogonal geometry. This situation is similar to
the geometry isomers of the phenothiazine-anthracene dyads, i.e. the
substitution at the 9-position is beneficial for conformation restriction
0
3
1
/
T
2
0
than that at 2-position of the anthryl moiety [56]. The optimized S state
geometries show that the dihedral angle between the two anthryl moi-
1
2
◦
eties in 9,9′-BA is 90 , which is in agreement with literature reports [97,
For the BAs, the scenario is more complicated, due to the formation
of CT states. Steady state and time-resolved spectroscopies have
demonstrated the formation of the CT state, especially in polar solvents
for 9,9′-BA. Thus we expected contribution from the SOCT to the ISC, as
well as the conventional SO-ISC for the anthryl, which is well known
[101]. Note the SOCT-ISC efficiency is dependent on solvent polarity
because the CT state energy levels, thus the matching of the CT state and
◦
9
8]. In addition, the dihedral angle of 2,9′-BA at ground state is 106 .
However, molecules at the excited states (i.e. S
on optimized excited state geometry) are more coplanar than ground
state, for instance, the dihedral angles of 9,9′-BA at S state and T state
are 67 and 119 , whereas they are 132 and 123 for 2,9′-BA. It was
reported that the dihedral angle at S
state of 9,9′-BA between the two
1 1
state and T state, based
1
1
◦
◦
◦
◦
1
Fig. 7. B3LYP/6-31G (d) calculated potential energy curves of the rotation around the linker bond of 9,9′-BA and 2,9′-BA ground states. ϕ is the dihedral angle
between two anthracene units at ground state. (b) Magnified low energy range of the potential energy surfaces near the minima. The thermal energy at room
temperature (ERT = 0.026 eV) is indicated by the dashed line.
9

X. Zhao et al.
Dyes and Pigments 187 (2021) 109121
Fig. 8. The ground state (S
0
) and the excited state (S
1
) frontier molecular orbitals of 9,9′-BA and 2,9′-BA calculated by DFT (B3LYP/6-31G (d)) (Isovalue = 0.03).
The energy levels of the orbits are presented (in eV).
the T
trend of the Φ
BAs and An. For instance, the Φ
2%, 15% and 52%, respectively. For both BAs, the highest Φ
n
state energy levels, are dependent on solvent polarity. Thus, the
of the compounds in different solvents varied for the two
of 9,9′-BA, 2,9′-BA and An in TOL are
were
It should be noted the striking feature of the SBCT molecular systems
is the high CT state energy levels [37]. For instance, we reported
phenothiazine-anthracene compact electron donor/acceptor dyads
showing SOCT-ISC previously [56], the CT states are with energy levels
of 1.9–2.4 eV (with changing of the solvents from ACN to TOL). In
comparison, the 9,9′-BA shows CT energy levels in the range of 3.1–3.3
eV (solvents ranging from ACN to TOL). Therefore, we propose that the
electron donor/acceptor dyads with two identical chromophores
showing the symmetry breaking spin orbit charge transfer induced ISC
will be helpful for design of triplet PSs showing high triplet state energy
levels.
Δ
Δ
2
Δ
observed in ACN, but for An, the highest singlet oxygen quantum yield
was observed in DCM. We propose the reason of the low ISC yield in 2,
1
ꢀ •
+•
9
′-BA is not only slow [An -An ]→T
4
transition (both of
ꢀ • +•
4
An -An ] and T possess the charge-transfer character), but also the
1
[
effect of the molecular geometry, in which electron donor and acceptor
adopt a more coplanar configuration. Based on SOCT-ISC mechanism,
the orthogonal configuration between electron donor and acceptor will
enhance ISC, due to the total angular momentum of the molecular sys-
tem is conserved. However, the dihedral angle of 2,9′-BA at ground state
3. Conclusion
◦
is 106 , which is with more coplanar geometry, compared to 9,9′-BA
◦
(
90 ). Thus the more coplanar geometry in 2,9′-BA will be lead to lower
In summary, we studied the relationship between the molecular
geometry, and the electronic coupling between the two anthryl units
′
ISC yield than that of 9,9 -BA. TDDFT computation showed that after
′
′
derivatization, the relative S
that of An. For instance, there are degenerated T
with 2.93 eV energy level, which is much lower than S
In addition, degenerated T , T and degenerated T , T
calized. This is different from An, for which the S /T
1
/T
n
energy levels changes as compared to
/T
states for 9,9′-BA,
state (3.27 eV).
between the 9,9 -bianthryl and 2,9 -bianthryl, and the spin orbit charge
transfer efficiency (SOCT-ISC). We found that the effect of the molecular
geometry on the photophysical properties, i.e. lower fluorescence
quantum yields, more significant solvent polarity-dependency and
3
4
1
3
4
5
6
are both delo-
states are with
′
′
1
2
longer fluorescence lifetime and higher SOCT-ISC efficiency for 9,9 -
energy levels of 3.27 eV and 3.23 eV, respectively. In n-HEX, the CS can
be inhibited, but still moderate Φ were observed for 9,9′-BA (21%) and
,9′-BA (21%), indicating ISC mechanisms other than SOCT-ISC exit for
bianthryl, which is with orthogonal geometry, in comparison with 2,9 -
Δ
bianthryl which is with more coplanar geometry. Efficient ISC in BAs
2
was confirmed with nanosecond transient absorption spectroscopy, and
8
ꢀ 1
′
′
s, 2,9 -
both BAs. The ISC rate constant of An is (3 ± 0.6) × 10
temperature [102], whereas the dynamics of TICT states is 3.2 ps for 2,
′-BA and 1.54 ps for 9,9′-BA in DMF, respectively [65]. Thus,
SOCT-ISC should be faster than SO ISC in solvents with high polarity.
s
at room
the triplet state lifetime are long (9,9 -bianthryl,
bianthryl, s in ACN).
= 493
τ
T
= 353
μ
τ
T
μ
9
1
0

X. Zhao et al.
Dyes and Pigments 187 (2021) 109121
Scheme 2. Simplified Jablonski Diagram Illustrating the Photophysical Processes Involved in (a) 9,9′-BA and (b) 2,9′-BA. The energy levels of the excited state are
derived from the spectroscopic and electrochemical study data. and the energy levels of the triplet states are from TDDFT//B3LYP/6-31G (d) with Gaussian 09W.
4
. Experimental section
column chromatography. UV–Vis absorption spectra were taken on a
UV-2550 UV–vis spectrophotometer (Shimadzu Ltd, Japan). Fluores-
cence spectra were obtained on a RF-5301PC spectrofluorometer (Shi-
madzu Ltd, Japan). Fluorescence lifetimes were measured on an OB920
(Edinburgh Instruments Ltd, UK) luminescence lifetime spectrometer
(Time correlated single photon counting, TCSPC detection mode).
4
.1. Materials and equipment
All the chemicals used for synthesis are analytically pure and were
◦
used as received. Petroleum ether (boiling point: 60–90 C) was used for
1
1

X. Zhao et al.
Dyes and Pigments 187 (2021) 109121
′
4
.2. Synthesis of 9,9 -BA
calculated by TDDFT//B3LYP/6-31G (d), which is similar to the calcu-
lation of normal UV–Vis spectra. All these calculations were performed
with Gaussian 09W [104].
9
,9′-BA was synthesized with a modified literature method [103].
Acetic acid (19 mL) was added in the mixture of anthraquinone (784.5
mg, 3.75 mmol) and zinc powder (1.96 g, 30 mmol). Under N atmo-
2
CRediT authorship contribution statement
sphere, 37% HCl solution (4.7 mL) was added slowly with dropping
◦
funnel at 50 C. The reaction mixture was refluxed for 13 h. Then the
Xiaoyu Zhao: synthesized the compounds, performed spectral
measurement and prepared part of the manuscript. Andrey A. Sukha-
nov: data analysis. Kepeng Chen: parts of synthesis and spectral data
analysis. Xinyu Geng: parts of synthesis and spectral data analysis. Yu
Dong: parts of synthesis and spectral data analysis. Violeta K. Vor-
onkova: parts of spectral data analysis. Jianzhang Zhao: conceived the
project, analyzed the data and wrote the manuscript. Lang Liu: parts of
spectral data analysis.
heating source bath was removed and the mixture was cooled to room
temperature (RT). After the mixture was filtered, the filter residue was
washed with methanol, and then was re-dissolved in chloroform (4 mL).
Then the mixture was added to ethanol (50 mL). The precipitate was
filtered to give a crude product. The crude product was further purified
with column chromatography (silica gel, 2–4% CH
ether, v/v) to give a beige solid (69 mg, yield: 10%). M.p.: > 250 C. H
2 2
Cl in petroleum
◦
1
NMR (400 MHz, CDCl
3
): δ = 8.69 (s, 2H), 8.16 (d, J = 8.5 Hz, 4H),
+
7
.47–7.43 (m, 4H), 7.17–7.07 (m, 8H). HRMS (EI): Calcd ([C28
H
18] ),
m/z = 354.1410; found, m/z = 354.1409.
Declaration of competing interest
′
4
.3. Synthesis of 2,9 -BA
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
A solution of 9-bromoanthracene (128 mg, 0.5 mmol) in TOL (10
mL) was mixed with a solution of 2-anthraceneboronic acid (105 mg,
.47 mmol) in ethanol (6 mL). Then an aqueous solution of K CO (2.1
mL, 2 M) was added. The suspension was bubbled with nitrogen for 30
0
2
3
Acknowledgement
min and Pd(PPh
3
)
4
(14 mg, 0.012 mmol) was then added. Kept in
J. Z. thanks the NSFC (U2001222, 21673031, 21761142005, and
◦
darkness, the reaction mixture was stirred at 100 C under N
2
atmo-
2
1911530095), the State Key Laboratory of Fine Chemicals
sphere for 8 h. After completion of the reaction, the reaction mixture was
cooled to RT. Then water (30 mL) was added. The mixture was extracted
(
ZYTS201901), Xinjiang University and the Department of Education of
the Xinjiang Uyghur Autonomous Region (TianShan Chair Professor) for
financial support.
with CH
2
Cl
2
(3 × 50 mL). The combined organic layers were dried over
SO and filtered. Then the solvent was evaporated under
anhydrous Na
2
4
reduced pressure to obtain a crude solid. The crude product was further
purified with column chromatography (silica gel, petroleum ether) to
Appendix A. Supplementary data
give a pale yellow solid (128 mg, yield: 77%). M.p.: 230–233 ◦C. 1
H
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.109121.
NMR (400 MHz, CDCl
3
): δ = 8.59 (d, J = 13.9 Hz, 2H), 8.48 (s, 1H), 8.22
(
d, J = 8.5 Hz, 1H), 8.10–8.03 (m, 5H), 7.77 (d, J = 8.7 Hz, 2H),
1
3
7
1
1
.56–7.46 (m, 5H), 7.36–7.32 (m, 2H). C NMR (126 MHz, CDCl ): δ =
3
36.8, 135.7, 132.1, 132.0, 131.5, 131.4, 131.0, 130.3, 130.2, 129.3,
28.4, 128.2, 128.1, 126.8, 126.7, 126.4, 126.2, 125.6, 125.5, 125.2.
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