Long-Lived Triplet Excited State Accessed with Spin–Orbit Charge Transfer Intersystem Crossing in Red Light-Absorbing Phenoxazine-Styryl BODIPY Electron Donor/Acceptor Dyads

Article abstract of DOI:10.1002/cphc.202000300

Orthogonal phenoxazine-styryl BODIPY compact electron donor/acceptor dyads were prepared as heavy atom-free triplet photosensitizers (PSs) with strong red light absorption (?=1.33×10⁵ M^?1 cm^?1 at 630 nm), whereas the previously reported triplet photosensitizers based on the spin-orbit charge transfer intersystem crossing (SOCT-ISC) mechanism show absorption in a shorter wavelength range (<500 nm). More importantly, a long-lived triplet state (τ_T=333 μs) was observed for the new dyads. In comparison, the triplet state lifetime of the same chromophore accessed with the conventional heavy atom effect (HAE) is much shorter (τ_T=1.8 μs). Long triplet state lifetime is beneficial to enhance electron or energy transfer, the primary photophysical processes in the application of triplet PSs. Our approach is based on SOCT-ISC, without invoking of the HAE, which may shorten the triplet state lifetime. We used bisstyrylBodipy both as the electron acceptor and the visible light-harvesting chromophore, which shows red-light absorption. Femtosecond transient absorption spectra indicated the charge separation (109 ps) and SOCT-ISC (charge recombination, CR; 2.3 ns) for BDP-1. ISC efficiency of BDP-1 was determined as Φ_T=25 % (in toluene). The dyad BDP-3 was used as triplet PS for triplet-triplet annihilation upconversion (upconversion quantum yield Φ_UC=1.5 %; anti-Stokes shift is 5900 cm^?1).

Full text of DOI:10.1002/cphc.202000300

Accepted Article
Title: Long-Lived Triplet Excited State Accessed with Spin-Orbit
Charge Transfer Intersystem Crossing in Red Light-Absorbing
Phenoxazine-StyrylBodipy Electron Donor/Acceptor Dyads
Authors: Yu Dong, Ayhan Elmali, Jianzhang Zhao, Bernhard Dick, and
Ahmet Karatay
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPhysChem 10.1002/cphc.202000300
Link to VoR: https://doi.org/10.1002/cphc.202000300
01/2020

10.1002/cphc.202000300
ChemPhysChem
ARTICLE
Long-Lived Triplet Excited State Accessed with Spin-Orbit
Charge Transfer Intersystem Crossing in Red Light-Absorbing
Phenoxazine-StyrylBodipy Electron Donor/Acceptor Dyads
Yu Dong,[a] Ayhan Elmali,[b] Jianzhang Zhao,*[a] Bernhard Dick*[c] and Ahmet Karatay*[b]
Orthogonal phenoxazine-styrylBodipy compact electron donor/acceptor dyads were prepared as heavy atom-free triplet photosensitizers (PSs)
with strong red light absorption ( = 1.33  105 M1 cm1 at 630 nm), whereas the previously reported triplet photosensitizers based on the
SOCT-ISC mechanism show absorption in a shorter wavelength range (<500 nm). More importantly, long-lived triplet state (T = 333 s) was
observed for the new dyads. In comparison, the triplet state lifetime of the same chromophore accessed with the conventional heavy atom effect
(HAE) is much shorter (T = 1.8 s). Long triplet state lifetime is beneficial to enhance the electron transfer or energy transfer, the primary
photophysical processes in the application of triplet PSs. Our approach is based on spin-orbit charge transfer intersystem crossing (SOCT-ISC),
without invoking of the HAE, which may shorten the triplet state lifetime. We used bisstyrylBodipy both as the electron acceptor and the visible
light-harvesting chromophore, which shows red-light absorption. Femtosecond transient absorption spectra indicated the charge separation
(109 ps) and SOCT-ISC (charge recombination, CR; 2.3 ns) for BDP-1. ISC efficiency of BDP-1 was determined as T = 25% (in toluene). The
dyad BDP-3 was used as triplet PS for triplet-triplet annihilation upconversion (upconversion quantum yield UC = 1.5%; anti-Stokes shift is
5900 cm1).
1. Introduction
Charge recombination (CR)-induced ISC was known for
electron donor/acceptor dyads with a long and rigid linker
between donor and acceptor.[8] A large distance, thus a weak
electronic coupling and a small electron exchange energy (J), is
indispensable for the radical pair ISC (RP ISC) mechanism in
these conventional electron donor/acceptor dyads. This type of
ISC is based on the hyperfine interaction enhanced 1CT3CT
Triplet photosensitizers (PSs) are compounds showing
intersystem crossing (ISC) to populate triplet excited state upon
photoexcitation. Much attention has been paid to the design of
new triplet PSs and for their applications in photo-redox catalytic
organic reactions,[1] photodynamic therapy (PDT),[2] H2 production
by photocatalytic water splitting,[3] triplet-triplet annihilation
upconversion (TTA-UC),[4] and photovoltaics.[5] The triplet state
production is via ISC, a spin forbidden non-radiative electronic
transition. A traditional strategy of enhancing ISC is to introduce
transition metal atoms such as Ru, Ir, Pt or other heavy atoms,
such as I or Br.[6] However, drawbacks of this heavy atom effect
(HAE) are the high cost of the synthesis, and the toxicity of the
compounds. Moreover, the triplet state lifetime of these PSs is
shortened since the HAE enhance not only the ISC of S1Tn, but
also the T1S0 ISC process.[7]
3
process (CT: charge transfer), followed by CTlocally excited
triplet state (3LE) internal conversion (given the purpose is to
access the 3LE, not the long-lived CT state). However, these
conventional electron donor/acceptor dyads are difficult to
prepare, and generally the molecular structures are not optimized
for triplet PS preparation, and the visible light-harvesting ability is
poor. When the linker between the electron donor and acceptor
was reduced in length, for instance by a direct link between donor
and acceptor in compact dyads, the electronic coupling and the
electron exchange energy increase, and RP ISC is inhibited.[9]
Recently, efficient ISC was observed for the CR in some
compact electron donor/acceptor dyads via the so-called spin
orbit charge transfer intersystem crossing (SOCT-ISC).[10] The
ISC process requires conservation of the total angular momentum,
i.e. the sum of orbital angular momentum and spin angular
momentum.[10] Given the electron donor and acceptor moieties in
a dyad adopt orthogonal orientation, the change of molecular
orbital angular momentum of the CR will offset the change of
electron spin angular momentum of ISC.[10b] Therefore,
conservation of angular momentum is satisfied in the ISC of an
orthogonal electron donor-acceptor dyad. Hence the SOCT-ISC
is efficient in orthogonal electron donor/acceptor dyad. It should
be pointed out the some electron donor/acceptor dyads undergo
twisted intramolecular charge transfer (TICT) may also show the
charge recombination induced ISC.
[a]
Y. Dong and Prof. J. Zhao
State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology, E-208 West
Campus, 2 Ling Gong Road, Dalian 116024, China.
E-mail: zhaojzh@dlut.edu.cn (J. Z.)
[b]
[c]
Dr. A. Karatay and Dr. A. Elmali
Department of Engineering Physics, Faculty of Engineering,
Ankara University, 06100 Beşevler, Ankara, Turkey.
E-mail: Ahmet.Karatay@eng.ankara.edu.tr (A. K.)
Prof. B. Dick
Lehrstuhl für Physikalische Chemie, Institut für Physikalische und
Theoretische Chemie, Universität Regensburg, Universitätsstr. 31,
D-93053 Regensburg, Germany.
E-mail:Bernhard.Dick@chemie.uni-regensburg.de
Advantages of these novel compact electron donor/acceptor
dyads are their simple molecular structure and long triplet state
Supporting information for this article is given via a link at the end
of the document.
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10.1002/cphc.202000300
ChemPhysChem
ARTICLE
lifetimes. These features are important for applications of triplet
PSs in photocatalysis and PDT. Several chromophores have
been used for preparation of electron donor/acceptor compact
Fc/Fc), PXZ has a more planar -conjugated structure and
different redox properties (oxidation potential EOX = +0.36 V vs.
Fc/Fc).[22] It may provide different solvent polarity-dependency
for the SOCT-ISC. To obtain more PSs with red light-absorption,
large -conjugated carbazole moiety is attached at the 2,6-
positions of Bodipy (BDP-3, Scheme 1), which may change the
triplet state lifetime or triplet state energy, and finally the ISC
efficiency.
The photophysical properties of the dyads were studied by
steady-state and time-resolved transient spectroscopies. The CS
and CR were studied with femtosecond transient absorption
spectra, and the triplet state spectra and lifetimes was studied
with nanosecond transient absorption spectra. The new heavy
atom-free triplet PSs were used for TTA-UC, and larger anti-
Stokes shift was achieved (0.68 eV) than the recently reported
PXZ-Bodipy dyads showing green light absorption (the anti-
Stokes shift is ca. 0.36 eV).[22]
dyads
showing
SOCT-ISC,
perylene,[12]
such
as
Bodipy,[10c,13]
acridinium,[10a]
and
anthracene,[10b]
perylenemonoimide/perylenediimide,[14] etc. However, triplet PSs
based on SOCT-ISC showing red light absorption were rarely
reported.[15] In some cases, long-lived 3CT state was observed for
the compact dyad.[16] On the other hand, although Bodipy-derived
triplet PSs showing red light absorption have been reported, for
instance
the
2,6-diiodostyrylBodipy,[17]
and
the
2,6-
diiodoazaBodipy,[18] the triplet state lifetimes of these red light-
absorbing triplet PSs are short (~1.8 s), which is a clear
disadvantage for the applications in PDT,[7b] TTA-UC,[4d,19] or
photocatalysis, etc.[20] In these applications, the intermolecular
electron transfer or triplet energy transfer efficiency increases with
longer triplet state lifetime of the PSs.
Inspired by the previous results, herein we selected
styrylBodipy as the electron acceptor and red-light-absorbing
chromophore, and phenoxazine (PXZ) as the electron donor, in
order to design compact, orthogonal electron donor/acceptor
dyads as novel heavy atom-free triplet PSs showing red light-
absorption and long-lived triplet states (Scheme 1). PXZ has been
used in thermally activated delayed fluorescence materials
(TADF),[21] and photovoltaics.[1a] Compared with the previously
used phenothiazine (PTZ. oxidation potential EOX = +0.21 V vs.
2. Results and Discussion
2.1. Molecular Structure Designing Rationales
Phenoxazine (PXZ) is used as electron donor. StyrylBodipy is
selected as electron acceptor and red-light-absorbing
Scheme 1. Synthesis of the Compounds. (a) n-C4H9Br, DMF, KOH, N2, stirred at RT for 2 h, yield: 60%; (b) POCl3, DMF, N2, 90 °C, 2 h, yield: 65%; (c) 2,4-
dimethylpyrrole, TFA, DDQ, TEA, BF3Et2O, DCM, RT, N2, yield: 6%; (d) aryl aldehyde, p-toluenesulfonic acid, piperidine, 15 min, yield: 42% for R = H and 13% for
R = CN; (e) NIS, DCM, RT for 2 h, yield: 89%; (f) KI, KIO3, acetic acid, 85 C, 10 min, 39%; (g) n-C4H9Br, DMSO, NaH, N2, stirred at RT for 2 h, yield: 90%; (h)
PdCl2(PPh3)2, PPh3, CuI, TEA, TMSA, N2, 80 C, 6 h; then stirred at RT over night, yield: 70%; (i) PdCl2(PPh3)2, PPh3, CuI, TEA and THF, N2, 65 C, 2 h, yield: 60%.
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10.1002/cphc.202000300
ChemPhysChem
ARTICLE
electronic ground state. A similar result was observed for BDP-2,
the absorption is similar as that of Styryl-BDP-CN. However, the
absorption band of BDP-2 is slightly red-shifted compared to
BDP-1. The UVVis absorption of BDP-3 and BDP-4 were also
studied (Figure 2b). For BDP-3, a broad absorption band centered
at 600 nm was observed, which shows the same absorption
profile as the reference compound BDP-4. We conclude that the
electronic coupling between donor and acceptor is also weak at
the ground state of BDP-3. These features are similar to those
observed for the BDP-PXZ dyads.[22]
Figure 1. Single-crystal structure of BDP-1 with 50% thermal ellipsoids.
Hydrogen atoms are omitted for clarity.
chromophore. The cyano groups attached to the styrylBodipy
moiety in BDP-2 will enhance the electron withdrawing ability,
which may produce a more efficient charge transfer. The PXZ
moiety was attached at the 8-position (meso- position) of
styrylBodipy. Therefore, the steric hindrance imposed by 1,7-
methyl groups on the styrylBodipy will restrict the rotation of PXZ,
thus the dyads will adopt a perpendicular orientation between
donor and acceptor. StyrylBodipy shows strong red light-
harvesting ability. The -conjugated structure of BDP-3 may lead
to different triplet state properties, such as triplet state lifetime and
triplet state energy. All molecular structures were fully
characterized (see Experimental section and Supporting
Information).
A single crystal of BDP-1 was obtained by slow diffusion
between n-hexane and DCM. The molecular structure determined
by single crystal X-ray diffraction of BDP-1 is presented in Figure
1. The dihedral angle between the electron donor (PXZ) and the
acceptor (styrylBodipy) is 71.4, which is slightly different from the
result of DFT calculation (89.7), see later section. It also shows
less orthogonality compared with the reported PTZ-styrylBodipy
dyad (81.9).[15] The possible reason is that the better planarity
of PXZ compared to PTZ weakens the conformational restriction
between the PXZ and styrylBodipy moieties. The deviation from
coplanar geometry of the styryl moieties is 10.9 and 28.4 due to
the - stacking in the single crystal. Because the DFT
optimization of the ground state geometry indicated planar
geometry. The styrylBodipy moiety shows partly propeller
structure in crystal phase. The structure of the styrylBodipy moiety
of BDP-1 is more twisted than the reported PTZ-styrylBodipy
molecule (10.9 and 28.4 vs 4.7 and 1.2).[15]
Figure 2. UVVis absorption spectra of compounds (a) BDP-1, BDP-2, Styryl-
BDP and Styryl-BDP-CN; (b) BDP-3 and BDP-4. c =1.0  105 M in toluene. 20
C.
Fluorescence spectra in different solvents were studied
(Figure 3). For BDP-1 (Figure 3a), structured emission bands
centred at 630 nm and 685 nm were observed, which is similar to
the unsubstituted compound Styryl-BDP (Supporting Information,
Figure S10b). The fluorescence quantum yield (F) was
determined as 73% in n-hexane (Table 1), which is similar to
Styryl-BDP (F = 77%, Table 1). The results indicate that CT is
inefficient in a non-polar solvent. However, the fluorescence is
significantly quenched in more polar solvents. For instance, the
F in toluene is almost half of that in n-hexane and it further
decreased to 0.2% in acetonitrile (ACN) (Table 1). We attributed
the fluorescence quenching to the electron transfer and formation
of CT state, and the CT state is a dark state. Similar results were
obtained for the dyad BDP-2 (Figure 3b), but quenching of the
fluorescence is more significant in polar solvents. For instance,
the fluorescence quantum yield is 35.4% in n-hexane but it
decreases to 0.5% in toluene (Table 1). In toluene, a broad and
red-shifted emission band centred at 768 nm was observed, along
with the LE emission band at 651 nm (Figure 4a). However, the
emission at longer wavelength was quenched in polar solvent
(Supporting Information, Figure S10b). Considering the relative
fluorescence emission intensity and the quantum yields, the CT
state of BDP-2 is in principle a dark state.
In toluene, the fluorescence is quenched both for BDP-1 and
BDP-2 compared with the reference compounds StyrylBDP and
StyrylBDP-CN. Furthermore, it is further quenched for BDP-2
compared with BDP-1 in toluene (Figure 3a). We attribute the
significant quenching of the fluorescence of BDP-2 as compared
to the BDP-1 to the electron-withdrawing CN groups, and the
more significant CT in BDP-2. For BDP-3, the emission was
2.2. UVVis Absorption and Fluorescence Emission Spectra
The UVVis absorption spectra of the compounds were studied
(Figure 2). The dyad BDP-1 shows similar absorption compared
with the reference Styryl-BDP in the region of 550700 nm, which
indicates negligible electronic coupling between the electron
donor (PXZ) and the electron acceptor (styrylBodipy) at the
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10.1002/cphc.202000300
ChemPhysChem
ARTICLE
Figure 3. Fluorescence emission spectra of the compounds in different solvents. (a) BDP-1; (b) BDP-2 and (c) BDP-3. Optically matched solutions were used, i.e.
all the sample solution show the same absorbance at the excitation wavelength (A = 0.195), ex = 570 nm, 20 C.
Table 1. The Photophysical Properties of Compounds.
[d]
[e]
[f]
[g]
[h]
[i]
Solvent[a]
abs[b] ([c]
)
F
F
F

T
T
BDP-1
HEX
TOL
ACN
HEX
TOL
ACN
HEX
TOL
ACN
HEX
TOL
ACN
HEX
TOL
ACN
HEX
TOL
ACN
620 (1.40)
630 (1.33)
619 (1.23)
629 (0.57)
639 (0.54)
628 (0.53)
597 (0.67)
600 (0.60)
590 (0.60)
596 (0.66)
597 (0.57)
587 (0.60)
619 (1.15)
628 (1.06)
617 (1.04)
627 (0.78)
636 (0.97)
627 (0.96)
629
641
631
640
651/768[j]
625
633
647
629
5.4
4.9
2.9
4.3
2.6/2.3[j]
1.5
3.0
2.4
1.4
0.732
0.393
0.002
0.354
0.005/0.018[j]
0.003
0.484
0.299
0.003
0.462
0.360
0.022
0.768
0.768
0.763
[l]
[l]
[l]
[l]
0.23
[l]
333.2[k]
[l]
0.25
[l]
BDP-2
0.05
0.18
[l]
[l]
[l]
382.2[k]
[l]
0.22
[l]
BDP-3
0.04
0.12
[l]
394.4[k]
392.7[k]
[l]
0.04
0.13
[l]
BDP-4
630
641
631
630
641
631
[l]
2.9
1.9
0.07
5.2
4,7
5.2
[l]
0.03
0.04
[l]
278.8[k]
244.0
[l]
0.05
0.04
[l]
Styryl-BDP
[l]
[l]
[l]
[l]
[l]
[l]
[l]
[l]
[l]
Styryl-
BDP-CN
[l]
[l]
[l]
649
639
4.2
4.4
0.652
0.612
[l]
[l]
[l]
[l]
[l]
[l]
[a] ET(30) values are HEX (31.0), TOL (33.9) and ACN (45.6), in kcal mol1. [b] c = 1.0 × 105 M, in nm. [c] Molar absorption coefficient.  = 105 M1 cm1. [d]
Fluorescence wavelength, in nm. [e] Fluorescence lifetime, ex = 635 nm, in ns, c = 1.0 × 105 M. [f] Absolute fluorescence quantum yield. [g] Singlet oxygen quantum
yield, methylene blue (MB) as standard ( = 0.57 in DCM). [h]Triplet state lifetime, in s. [i] Triplet quantum yield, methylene blue (MB) as standard (T = 0.50 in
methanol). [j] The transition of 1CT S0. [k] Intrinsic triplet state lifetimes. Obtained by fitting of the experimental curves based on the kinetic model with triplet-triplet-
annihilation self-quenching effect considered.[22] [l] Not observed.
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10.1002/cphc.202000300
ChemPhysChem
ARTICLE
3.5 ns (40%). The fluorescence lifetime of BDP-1 and BDP-2 are
4.9 ns and 2.6 ns in toluene, respectively, which indicates that the
CS of BDP-2 is more efficient than that of BDP-1. We propose the
bi-exponential decay is attributed to the existence of an excited
state of the emissive state with a dark state (CT state), or electron
transfer probability for the molecules at the 1LE state, and the
probability is less than unity.[6b] For BDP-3, a similar conclusion
can be obtained (Supporting Information, Figure S12a). The
fluorescence in n-hexane is mono-exponential with a lifetime of
3.0 ns. However, in ACN it turned to a bi-exponential decay with
an average lifetime of 1.4 ns (with a short component of 0.10 ns
(70%) and a longer component of 4.4 ns (30%)), indicating the
fluorescence quenched further in polar solvents.
Figure 4. Comparison of the fluorescence emission spectra of the compounds
in toluene (a) BDP-1, BDP-2, Styryl-BDP and Styryl-BDP-CN; (b) BDP-3 and
BDP-4. Optically matched solutions were used (A = 0.195), ex = 570 nm, 20 C.
2.3. Electrochemical Studies
The electrochemical properties of the dyads were studied by
cyclic voltammetry (Figure 6, Table 2, Supporting Information,
Figure S22 and Table S4). For BDP-1, the reversible oxidation
wave at 0.32 V (vs Fc/Fc) is attributed to the PXZ moiety,
another quasi-reversible oxidation wave at 0.55 V is attributed to
the styrylBodipy moiety, as is the quasi-reversible reduction wave
at 1.44 V. Therefore, PXZ is more likely the electron donor and
styrylBodipy serves as electron acceptor. The reduction wave of
BDP-2 was observed at 1.26 V, indicating the stronger electron
accepting ability of the styrylBodipy moiety with cyano groups
attached, as compared with styrylBodipy. Similar results were
obtained for BDP-3. Only one reversible oxidation wave at 0.34
V and one quasi-reversible reduction wave at 1.52 V were
observed, indicating that PXZ serves as electron donor and the
Bodipy moiety serves as electron acceptor. Compared with the
previously reported BDP-PXZ dyads, the oxidation potential of
PXZ is similar (0.32 V vs 0.36 V, vs Fc/Fc).[6b] The reduction
potentials of the styrylBodipy moieties of BDP-1 (1.44 V) and
BDP-2 (1.26 V) moves anodically than that of the BDP-PXZ dyad
(1.65 V, vs Fc/Fc), indicating the styrylBodipy moieties are
stronger electron acceptors than the parent Bodipy.
Figure 5. Fluorescence decay trances of the compounds in different solvents.
(a) BDP-1 at 650 nm and (b) BDP-2 at 650 nm. ex = 635 nm, c =1.0  105 M.
20 C. The IRF curves of the spectrometer are also presented.
quenched obviously in polar solvents, indicating an efficient CT.
Compared with the reference compound BDP-4, the emission
was slightly quenched in non-polar solvents such as n-hexane
and toluene (Figure 4b and Table 1). However, the emission was
quenched further in polar solvents, for instance, F is 0.001 and
0.105 in DCM for BDP-3 and BDP-4, respectively (Supporting
Information, Table S2).
The Weller equations (Supporting Information, eqs S1S4)
were used to determine the energy of the charge separated state
(CSS) and the driving forces of intramolecular charge separation
The fluorescence decay traces were studied using the time-
correlated single-photon counting (TCSPC) detection method
(Figure
5 and Supporting Information, Figure S12). The
fluorescence lifetimes of BDP-1 show solvent polarity
dependency (Figure 4a). In non-polar solvents (n-hexane and
toluene), the decay trace is mono-exponential. However, it shows
a biexponential decay in polar solvents, such as ACN. The
average lifetimes decrease from 5.4 ns (in n-hexane), to 2.9 ns (a
short component of 0.06 ns with a population ratio of 30% and a
longer component of 4.2 ns, in 70%) in ACN, meanwhile the
fluorescence quantum yield decreased by 366-fold (Figure 3a and
Table 1). Similar results are observed for BDP-2 (Figure 5b and
Table 1). The decay kinetics of BDP-2 shows a sharper decrease
along with increasing solvent polarity (Figure 5b). The lifetimes
are also shorter than BDP-1 in the same solvent (Table 1). For
instance, the lifetime of BDP-2 shows a bi-exponential decay with
a short component of 0.12 ns (60%) and a longer component of
Figure 6. Cyclic voltammogram of BDP-1, BDP-2 and BDP-3. Ferrocene (Fc)
was used as internal reference. In deaerated DCM containing 0.10 M Bu4N[PF6]
as supporting electrolyte. Scan rates: 100 mV/s. c = 1.0 × 103 M, 20 C.
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10.1002/cphc.202000300
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ARTICLE
Table 2. Redox Potentials, Driving Forces of Charge Separation (GCS), Charge Recombination (GCR) and the Energy of the CSS of the Compounds in Different
Solvents.[a]
ΔGCS (eV)[f]
ECSS (eV)
DCM
E(ox)[e] / V
E(red)[e] / V
1.44
HEX
0.09
TOL
DCM
ACN
HEX
2.05
TOL
1.93
ACN
1.51
BDP-1[b]
BDP-2[c]
BDP-3[d]
+0.55
+0.32
+0.70
+0.33
+0.34
1.60
1.45
1.62
0.03
0.13
0.29
0.36
0.47
0.36
0.44
1.93
1.73
1.80
1.70
1.35
1.59
1.26
0.004
0.25
0.57
1.52
0.38
[a] Cyclic voltammetry in N2 saturated DCM containing a 0.10 M Bu4NPF6 supporting electrolyte; Pt electrode was used as counter electrode; working electrode is
glassy carbon electrode; Ag/AgCl couple as the reference electrode. E00 is the energy difference between the potential minima, approximated with the crossing
point of UV−Vis absorption and fluorescence emission spectra after normalization. [b] E00 = 1.96 eV. [c] E00 = 1.92 eV. [d] E00 = 1.98 eV. [e] The value was obtained
by setting the oxidation potential of Fc/Fc as 0. [f] The redox potentials are approximated based on the redox potential measured in DCM.
(GCS).[8c] The results are summarized in Table 2 (The detailed
information please refer to Supporting Information).
the GCS values are all more negative.[22] For instance, the GCS
values are 0.40 eV and 0.74 eV in toluene and ACN,
respectively. For BDP-1, the GCS is 0.44 eV in ACN, indicating
the CS driving force is weaker for BDP-1. It may reduce the CS
and SOCT-ISC efficiency.
The calculation of GCS indicates that charge separation is
thermodynamically forbidden in n-hexane due to the positive
GCS value (0.09 eV) for BDP-1, which agrees with the
unquenched fluorescence of styrylBodipy moiety in n-hexane (F
= 73.2% in Table 1). However, thermodynamically allowed charge
separation is achieved in other more polar solvents according to
the negative GCS values of BDP-1, which also agrees with
fluorescence quenching of the styrylBodipy moiety (Table 1). For
BDP-2 and BDP-3, the GCS values are all negative, indicating
charge separation is thermodynamically allowed in both nonpolar
and polar solvents. For the previously reported BDP-PXZ dyads,
2.4. Femtosecond Transient Absorption Spectroscopy
Femtosecond transient absorption spectra (fs TA) were measured
in order to reveal the excited state dynamics of the dyads. For
BDP-1, the triplet state signal was observed in toluene (Figure 7),
but no triplet state signal was observed in ACN. Hence we
assume that the CS and CR are both much faster in ACN. In
Figure 7. Femtosecond transient absorption spectra of BDP-1. (a) Transient absorption spectra in toluene. ex = 630 nm, c = 1.0  105 M, (b) Species-associated
difference spectra (SADS) and (c) Decay traces at selected wavelengths. (d) Transient absorption spectra of BDP-1 in ACN (ex = 625 nm, c = 1  105 M), (e)
Species-associated difference spectra (SADS) and (f) Decay traces at selected wavelengths. SADS were obtained by global fitting in sequential model. 20 C.
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10.1002/cphc.202000300
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toluene, the intense GSB band at 630 nm was immediately
generated upon excitation (Figure 7a). An ESA band in the range
of 420550 nm increased in intensity in less than 1 ps. This ESA
band is attributed to the S1Sn transition, and the increasing
process may be due to vibrational relaxation. The negative band
at 700 nm is assigned to stimulated emission (SE) of the localized
singlet state (1StyrylBDP). However, no obvious absorption band
of the PXZ radical cation (around 540 nm, Supporting Information,
Figure S23b)[23] and the styrylBodipy radical anion (around 540
nm, Supporting Information, Figure S23a) were observed.
One possible reason is that the CS efficiency is not very high
(F = 0.39 in toluene). Furthermore, the radical anion absorption
of styrylBDP in the range of 650680 nm (Supporting Information,
Figure S23a and S23b, obtained by spectroelectrochemistry),
overlaps with ESA signals of the singlet state of StyrylBDP.
Moreover, the signal of the radical cation of PXZ also overlaps
with the singlet state signal of StyrylBDP in the range of 530550
nm. Thus, we can’t distinguish the characteristic 1StyrylBDP and
the 1CSS states unambiguously. The fs TA spectra of the
reference compound StyrylBDP were also recorded in toluene
(Supporting Information, Figure S21). No fast decay of the GSB
or the SE bands were observed, indicating that no CS exist in
StyrylBDP. Therefore, the CS is most likely responsible for the
fast decay of BDP-1 in toluene.
Species-associated difference spectra (SADS) obtained by
global fitting were used to analyse the photophysical processes
(Figure 7b). The species with the shortest lifetime (0.12 ps) is
assigned to the unrelaxed S1 state. The species with a lifetime of
109 ps displays the characteristic styrylBodipy ESA signals and is
assigned the relaxed S1 state, indicating that CS process takes
109 ps. Subsequently, the slow CR takes 2.3 ns, and a long-lived
species with infinite lifetime (on the time scale of the fs-TA
experiment) is obtained, which is attributed to the triplet state.
The final species is assigned as the T1 state on account of
showing a weak ESA band in the range 650750 nm (T1Tn
absorption) and a GSB band at 630 nm, which is in agreement
with ns TA data (Figure 8a). Therefore, we conclude that the triplet
state is generated by the CR and the SOCT-ISC (CR) takes 2.3
ns. In ACN, no triplet state signal was observed (Figure 7d). The
ESA band located in the range of 440570 nm is attributed to the
S1Sn absorption. Due to the higher CS efficiency, absorption
bands of the styrylBDP radical anion and the PXZ radical cation
were more obvious. The absorption bands centered at 670 nm
and 545 nm were attributed to StyrylBDP and PXZ,
respectively, which is in agreement with the results of
spectroelectrochemical studies (Supporting Information, Figure
S23a and S23b). Based on the SADS, we determined the time
constants of the CS and CR as 0.79 ps and 3.5 ps, respectively
(Figure 7e), which are faster than the CS (109 ps) and CR (2.2
ns) in toluene.
(Supporting Information, Figure S23c). Finally, along with the
decreasing of the absorption band centered at 670 nm, the ESA
signal in the range of 650750 nm become stronger, which is
characteristic for the T1Tn absorption of BDP-2, corroborated by
the ns TA spectra (Supporting Information, Figure S13a).
Therefore, we conclude that ISC occurs via CR. Based on the
results from SADS (Supporting Information, Figure S19b), we
determine that CS takes place in 1.2 ps, which is much faster than
in BDP-1 (109 ps in toluene). Following the CS, slow CR (SOCT-
ISC) continuing for 1.6 ns leads to the generation of the triplet
state of BDP-2. In the polar solvent ACN, results similar to those
of BDP-1 were obtained (Supporting Information, Figure S19e).
Faster CS (0.3 ps) and CR (1.6 ps) are observed. No triplet state
formation was observed, which agrees with the lack of singlet
oxygen photosensitizing of the dyad BDP-2 (Table 1).
Different results were obtained for BDP-3 (Supporting
Information, Figure S19). Upon excitation, the GSB band was
observed in the range of 550750 nm. It is broader than the UV-
Vis absorption, which is attributed to the overlap of the GSB and
the SE band. According to spectroelectrochemical results
(Supporting Information, Figure S23), we determine the
absorption of the radical anion and the radical cation centered at
580 nm and 540 nm, respectively. Based on the SADS analysis
and the fast decay of ESA at 480 nm, we determine that CS takes
3.2 ps in toluene. The lifetime of the 1CT state in toluene is ca. 4.0
ns. No triplet state formation was observed within the time
resolution of our instrument setup.
2.5. Nanosecond Transient Absorption Spectroscopy: Triplet
State Properties
Nanosecond transient absorption spectra were used to study the
triplet state production of the dyads (Figure 8 and Supporting
Information, Figure S13). For BDP-1, two negative peaks
centered at 340 nm and 630 nm were observed upon pulsed laser
excitation. In agreement with the UV-Vis absorption spectra
(Figure 2a), these bands are assigned as ground state bleaching
(GSB) bands. Moreover, several excited state absorption (ESA)
bands centred at 380 nm, 500 nm and 700 nm, respectively, were
observed (Figure 8a). These features are typical of the
styrylBodipy triplet state transient absorption spectra.[24]
Therefore, we conclude that the triplet state is localized on the
styrylBodipy moiety, and it is not a CT state. Similar results were
obtained for BDP-2 (In toluene, Supporting Information, Figure
S13a). The GSB and ESA bands of BDP-2 are both slightly red
shifted compared with BDP-1 due to the extension of -
conjugation structure. Recently with a PTZ-naphthalimide dyad,
we observed a CT state.[16] Note the ESA bands overlap with the
GSB bands.
The apparent triplet state lifetimes (T) were determined as
270.0 s for BDP-1 and 242.7 s for BDP-2, respectively, at a
specific concentration. However, triplet-triplet annihilation (TTA)
will quench the triplet state and shorten the triplet state lifetime,
especially for the compounds showing strong absorption at the
excitation wavelength, high ISC efficiency and long-lived triplet
state. Therefore, the intrinsic triplet state lifetime was determined
by fitting the decay traces at two different concentrations with a
For BDP-2, the fluorescence was quenched significantly in
toluene (F = 0.005 vs. 0.393 of BDP-1). In toluene, a GSB band
centered at 640 nm and a strong ESA band centered at 468 nm
(Supporting Information, Figure S19a) were observed, which are
assigned to the S1 state. Subsequently, an absorption band at 670
nm intensified along with the decreasing ESA band at 469 nm,
which is attributed to the formation of [StyrylBDP-CN]
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10.1002/cphc.202000300
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the triplet state is also localized on the Bodipy moiety. The intrinsic
triplet state lifetime was determined as 392.7 s in toluene (Table
1).
Theoretical computations were performed to study the ESA
bands of T1Tn transitions (Supporting Information, Figure S18).
For BDP-1, the ESA bands centered at 700 nm, 500 nm and 375
nm are attributed to the T1T4, T1T10 and T1T22 transitions,
respectively (Supporting Information, Figure S18a). Similar
results were obtained for BDP-2. The ESA band at 735 nm is
attributed to the T1T4 transition and the bands at 500 nm and
395 nm were assigned to the T1T10 and T1T22 transitions,
respectively (Supporting Information, Figure S18b). The results of
the calculations for BDP-3 deviated from the experimental results
(Supporting Information, Figure S18c). However, strong overlap
between ESA and GSB bands can shift the apparent band
maxima considerably. The ESA band at around 750 nm is
assigned to the T1T10 transition, the other two bands at around
460 nm and 380 nm were assigned to the T1T12 and T1T26
transitions, respectively.
Triplet-triplet energy transfer (TTET) was used to determine
the triplet state energy of the dyads. BDP-1 and BDP-2 were used
as triplet energy donors. Rubrene (ET1 = 1.14 eV)[26] and 1-chloro-
9,10-bis(phenylethynyl)anthracene (CBPEA. ET1 = 1.20 eV)[4c]
were selected as triplet energy acceptor. The triplet state lifetime
of BDP-1 wasn’t quenched (260.2 s) in the presence of 4 eq.
CBPEA compared with that in the absence of CBPEA (272.3 s)
(Supporting Information, Figure S14b and S14d), which indicates
the T1 state energy of BDP-1 is lower than that of CBPEA. In
contrast, the triplet state lifetime of BDP-1 was significantly
quenched in the presence of 4 eq. rubrene (37.0 s), indicating
that the T1 state energy of BDP-1 is higher than 1.14 eV.
Therefore, we estimate that the T1 state energy of BDP-1 is in the
range of 1.14  1.20 eV. A similar result was obtained for BDP-2
(Supporting Information, Figure S15), the T1 state energy of BDP-
2 is in the range 1.14 eV ~ 1.20 eV. The results are also similar to
those obtained by DFT calculation. The calculated T1 state
energies are 1.06 eV and 1.03 eV for BDP-1 and BDP-2,
respectively.
Due to the different -conjugated structure of BDP-3
compared with styrylBodipy (Scheme 1), we assume the T1 state
energy of BDP-3 is different from BDP-1 and BDP-2. We selected
Bodipy (ET1 = 1.69 eV)[27] and 9,10-diphenylanthracene (DPA)
(ET1 = 1.77 eV)[28] as triplet energy acceptors and BDP-3 as the
energy donor. The triplet state lifetime of BDP-3 decreased from
402.0 s to 339.1 s in the presence of Bodipy (Supporting
Information, Figure S16). Meanwhile, a new GSB band appeared
at around 500 nm, which is assigned to the GSB band of Bodipy.
The decay at 500 nm is composed of two components, the
increasing component is attributed TTET between BDP-3 and
Bodipy. These results demonstrate that the T1 state energy of
BDP-3 is higher than 1.65 eV. On the contrary, no reduction of
the triplet state lifetime of BDP-3 was observed when DPA was
used as the energy acceptor (Supporting Information, Figure S17).
Therefore, we conclude that the T1 state energy of BDP-3 is in the
range of 1.651.77 eV.
Figure 8. Nanosecond transient absorption spectra of compounds in deaerated
toluene. (a) Transient absorption spectra of BDP-1 at different delay time. (b)
the decay trace at 630 nm, ex = 620 nm, c = 5.0  106 M. (c) Transient
absorption spectra of BDP-3 and (d) the decay trace at 600 nm, ex = 590 nm,
c = 1.0  105 M. 20 C. The intrinsic lifetime was obtained by fitting of the decay
curves with a kinetic model that takes into account of the triplet-triplet-
annihilation effect.[22]
kinetic model including TTA self-quenching.[25] The intrinsic triplet
state lifetime of BDP-1 obtained with this kinetic model is T
=
333.2 s, which is much longer than the apparent triplet state
lifetime T = 270.0 s, indicating TTA quenching. For BDP-2, the
intrinsic triplet state lifetime (T = 382.2 s) is also longer than the
experimental values (242.7 s). Notably the triplet state lifetime of
BDP-1 is prolonged 185-fold (T = 333.2 s) as compared to the
triplet state of the same parent chromophore, i.e. bisstyrylBodipy,
but accessed by the HAE in 2,6-diiodostyrylBodipy (1.8 s).[17]
These results demonstrated one of the advantage of using the
compact electron donor/acceptor dyads as heavy atom-free triplet
PSs, i.e. the triplet state lifetime becomes much longer than that
accessed with the conventional HAE.[15] Long-lived triplet state
lifetime are important for photocatalysis, PDT and TTA-UC.
The ISC efficiency depends on the solvent polarity (Table 1).
For instance, the triplet state quantum yield (T) for BDP-1 is 25%
in toluene (Table 1). However, in other solvents, no triplet state
formation was observed. Similar results were observed for BDP-
2. Therefore, we conclude that the ISC mechanism is based on
charge recombination for BDP-1 and BDP-2. Due to the short
distance between electron donor/acceptor and strong electron
coupling, SOCT-ISC is the most likely mechanism, instead of RP
ISC. The poor SOCT-ISC in polar solvent may be due to the fast
CR to the ground state (S0 state), because the CR occurs
normally in the Marcus inverted region, i.e. the lower CT state
energy in polar solvent will accelerate CR to the S0 state, thus
inhibiting SOCT-ISC.[6b]
For BDP-3, the characteristic GSB and ESA signals are
similar as the reference compound BDP-4 (Figure 8c and
Supporting Information, Figure S13c), which demonstrates that
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2.6. DFT Computations
The ground-state geometries of the dyads were optimized (Figure
9). The relative orientations between electron donor (PXZ) and
acceptor (Bodipy chromophore) are all nearly orthogonal for all
the dyads, which should be beneficial for SOCT-ISC. For instance,
the dihedral angle between PXZ and styrylBodipy in BDP-1 is
89.7, very close to orthogonal geometry. The -conjugated
structure of styrylBodipy moiety of BDP-1 shows minor distortion
(by 1.8 and 1.7 at the two arms, respectively) (Figure 9a).
These distortions are smaller than those observed in the single
crystal structure (10.9 and 28.4), the discrepancy may be due
to the packing effect in the single crystal. The previously reported
SOCT-ISC dyad BDP-PXZ has a similar dihedral angle between
PXZ and Bodipy as found for BDP-1 (85.6 vs 89.7).[22]
For BDP-2, the relative orientations between the PXZ and the
styrylBodipy moieties are also orthogonal, and the distortions of
the styryl moieties are 1.6 and 1.5. However, for BDP-3, the
Bodipy chromophore moiety has better planarity (Supporting
Information, Figure S25). The potential energy surfaces (PES) of
the dyads against the torsional angles between electron donor
and acceptor were also constructed (Supporting Information,
Figure S26). For the three dyads, the thermally accessible
dihedral angles between the electron donor and acceptor are all
in the range of 65113. The range is similar (ca. 70110) for
the reported PTZ-StyrylBodipy dyads.[15]
The frontier molecular orbitals of the dyads are presented in
Figure 10. For BDP-1, the lowest unoccupied molecular orbital
(LUMO) is confined to the styrylBodipy moiety, and the highest
occupied molecular orbital (HOMO) is localized on the PXZ
moiety, indicating that electron transfer is possible. Slight
delocalization was observed. For BDP-2, the HOMO and LUMO
are exclusively localized on the PXZ and styrylBodipy moieties,
respectively. For BDP-3, a similar result was observed. The MOs
demonstrate that the attachment of electron withdrawing groups
may alter the HOMO and LUMO energies. Compared with BDP-
1, the HOMO energy of BDP-2 decreases from 4.90 eV to 5.02
eV and the LUMO energy shows a similar change (from 2.69 eV
to 3.09 eV). The lack of overlap of the MO leads to more
significant fluorescence quenching in BDP-2.
Figure 10. Selected frontier molecular orbitals and energies (eV) of the
compounds were calculated by DFT at the B3LYP/6-31G(d) level with Gaussian
09W, based on the optimized ground state structures. Isovalue = 0.02.
The triplet state spin density surfaces were studied at the
optimized triplet state geometries (Figure 11). The spin unpaired
electrons are localized on the styrylBodipy moiety for BDP-1 and
BDP-2, which agrees with nanosecond transient absorption
spectra (Figure 8). The spin density surfaces of the radical anion
and the radical cation of BDP-1 were also studied (Figure 12).
The spin density of the radical anion is entirely restricted to the
styrylBodipy moiety. On the contrary, the spin density surface of
the radical cation is completely localized on the PXZ moiety.
Figure 11. Isosurfaces of spin density of (a) BDP-1 and (b) BDP-2 at the
optimized triplet state geometries. Calculation was performed at the UB3LYP/6-
31G(d) level with Gaussian 09W. Isovalue = 0.0004.
Figure 9. Optimized ground state geometry and selected dihedral angles of
BDP-1 and BDP-2 calculated by DFT at the B3LYP/6-31G(d) level with
Gaussian 09W.
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The power dependence of the upconversion emission
intensity was measured (Supporting Information, Figure S31).
The integrated upconversion emission intensity increased almost
linearly rather than quadratically with the excitation laser power,
indicating efficient TTET and TTA for the TTA-UC system.
Finally, the intermolecular TTET was studied by Stern-Volmer
analysis of the quenching by monitoring the triplet state lifetime of
BDP-3 (Supporting Information, Figure S32 and Table S5). The
Stern−Volmer quenching constant (KSV = 1.74  106 M1 ) and
quenching efficiency (fQ = 40.0 %), indicating that intermolecular
TTET is efficient.
The photophysical processes of BDP-1 are summarized in
Scheme 2. Upon excitation at 630 nm, the dyad is excited to the
S1 state (LE state localized on styrylBodipy moiety). The following
CS leads to generation of a CS state, especially in polar solvents.
GCS is positive and therefore CS is inhibited in n-hexane,
resulting in unquenched fluorescence of the styrylBodipy moiety
(Table 1). In toluene, the fluorescence of the LE state is quenched
by CS (rate constant 109 ps), subsequent CR leads to population
of T1 (3LE state). According to DFT calculations, SOCT-ISC to
yield T3 is also possible because the two states have similar
energies,[10b] Ultrafast T3T1 internal conversion will nevertheless
populate T1. The CR (ISC) rate constant was determined as 2.3
ns by femtosecond transient absorption spectra. The triplet
Figure 12. Isosurfaces of spin density of BDP-1 at the optimized (a) radical
anion and (b) radical cation geometries. Isovalue = 0.0004. The calculation was
performed at the UB3LYP/6-31G(d) level with Gaussian 09W.
These results further imply that PXZ serves as electron donor and
styrylBodipy as the electron acceptor. Similar results were
obtained for BDP-2 and BDP-3 (Supporting Information, Figure
S27).
2.7. Application of the Dyads in TTA Upconversion
Recently, the application of heavy atom free triplet PSs on triplet-
triplet annihilation upconversion (TTA-UC) has attracted particular
interest.[29] Traditional heavy atom-free triplet PSs with absorption
in the red range show low triplet energies, such as methylene blue
(1.44 eV)[30] and 2,6-diiodostyrylBodipy (ca. 1.13 eV).[31] Due to
the low triplet energy of BDP-1 (1.14~1.20 eV) and BDP-2
(1.14~1.20 eV), the dyad BDP-3 was selected as the triplet PS of
TTA-UC. The dyad BDP-3 has high triplet energy (1.651.77 eV)
and long intrinsic triplet state lifetime (392.7 s in toluene), which
is ideal for application for TTA-UC. TTA-UC was studied with
BDP-3 as PS and perylene as annihilator (Figure 13). Upon 589
nm cw laser excitation, only fluorescence of BDP-3 was observed
in absence of annihilator. A new emission peak appear in the
region of 420  520 nm after addition of 4.0 eq. annihilator, which
is attributed to the upconverted fluorescence of perylene (Figure
13a). The anti-Stokes shift of this TTA-UC system is 5905 cm1,
and the upconversion quantum yield (UC) is 1.5% (in toluene).
The anti-Stokes shift of BDP-3 is comparable to the previously
reported SOCT-ISC PSs applied in TTA-UC (ca. 32765900
cm1).[13a,22,32]
1
quantum yield T is 25 % in toluene. The energies of CSS in
more polar solvents are much lower (1.60 eV in DCM and 1.51 eV
in ACN, respectively), which may facilitate the CR back to the
ground state, because the CR is normally in Marcus inverted
region of electron transfer.[6b] For instance, CR is much faster in
ACN (3.5 ps) than in toluene (2.3 ns). Therefore, no triplet state
signal was observed in DCM and ACN.
Similar photophysical processes are summarized for BDP-2
(Supporting Information, Figure S33). CS occurs within 3.4 ps.
The triplet state is confined to the styrylBodipy moiety. It is
produced by SOCT-ISC, with time constant of 1.4 ns (in toluene),
and triplet state quantum yield T = 22% (in toluene). For the dyad
BDP-3, fast CS (3.2 ps) was observed, as well as a slow CR (4.0
ns). However, no triplet state signal was observed in femtosecond
transient absorption spectra, due to the slow CR (SOCT-ISC)
kinetics.
Figure 13. TTA upconversion with BDP-3 as the triplet PS and perylene as the acceptor, ex = 589 nm. UC = 1.5 %. (a) Upconversion emission spectra. (b) CIE
diagram and (c) photographs of BDP-3 alone and the upconversion. All spectra were measured upon excitation of the solution with the same 589 nm continuous
wave laser (power density: 125 mW/cm2). Diagram (b) and (c) were measured with band-pass filter (transparent in the range 380560 nm). c (BDP-3) = 1.0 × 10−5
M, c (perylene) = 4.0 × 10−5 M, in toluene, 20 °C.
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emission spectra were recorded on a OB920 luminescence lifetime
spectrometer (Edinburgh Instrument Ltd, UK).
4.2. Synthesis of BDP-1
Compound 3 (50 mg, 0.1 mmol), benzaldehyde (42.4 mg, 0.4 mmol), p-
toluenesulfonic acid (PTSA) (15 mg, 0.08 mmol) and piperidine (0.5 ml)
were dissolved in dry toluene (5 mL) and heated to 140 C, then water
generated during the reaction was removed from reaction mixture along
with removal of toluene by distillation. The crude product was dissolved in
DCM and washed with water. The organic layer was dried over anhydrous
Na2SO4 and the solvent was removed with a rotary evaporator under
reduced pressure. The crude product was purified by column
chromatography (silica gel, PE/DCM = 1/1, v/v) to obtain BDP-1 as dark
blue solid (28 mg, yield: 42%). M.p.  250 C. 1H NMR (400 MHz, CDCl3):
δ = 7.777.73 (dd, 2H), 7.86 (d, 4H, J = 8.0 Hz), 7.41 (t, 3H, J = 6.0 Hz),
7.357.29 (m, 3H), 7.25 (s, 1H), 2.55 (t, 2H, J = 6.0 Hz), 6.85 (t, 1H, J =
8.0 Hz), 6.726.65 (m, 5H), 6.576.53 (m, 3H), 3.55 (t, 2H, J = 8.0 Hz),
1.77 (s, 6H), 1.731.68 (m, 2H), 1.531.47 (m, 2H), 1.06 (t, 3H, J = 8.0
Hz). MALDITOFHRMS: m/z [M] Calcd for C43H38BF2N3O: m/z =
661.3076, found: m/z = 661.3055.
Scheme 2. Photophysical processes of BDP-1. Energy of 1CSS were obtained
by the electrochemical characterization; Triplet state energies were obtained
from TD-DFT calculation at the B3LYP/6-31G(d) level with Gaussian 09W.
Triplet state lifetime is the intrinsic lifetime, obtained by fitting of the decay traces
with a kinetic model with the TTA effect considered.[22,25]
4.3. Synthesis of BDP-2.
Compound BDP-2 was prepared by a similar method used for BDP-1.
Dark blue solid was obtained (4.5 mg, yield: 13%). M.p.  250 C. 1H NMR
(400 MHz, DMSO-d6): δ = 8.38 (s, 2H), 8.23 (d, 2H, J = 8.0 Hz), 7.927.61
(m, 13H), 7.07 (s, 2H), 6.876.68 (m, 6H), 3.63 (s, 2H), 1.75 (s, 6H),
1.631.55 (m, 2H), 1.471.42 (m, 2H), 0.97 (t, 3H, J = 8.0 Hz).
MALDITOFHRMS: m/z [M] Calcd for C45H36BF2N5O: m/z = 711.2981,
found: m/z = 711.2998.
3. Conclusions
In summary, we prepared a series of phenoxazine-styrylBodipy
compact electron donor/acceptor dyads as novel heavy atom-free
triplet photosensitizers (PSs), based on the newly developed
spin-orbit charge transfer intersystem crossing (SOCT-ISC)
strategy. The striking property of the new triplet PSs is the strong
absorption of red light ( = 1.33  105 M1 cm1 at 630 nm), and
the long triplet state lifetime (T = 333 s), prolonged by a factor
of 180 compared to the triplet state of the same styrylBodipy
chromophore but accessed by the conventional heavy atom effect
(T = 1.8 s). Femtosecond transient absorption spectra show that
the charge separation and SOCT-ISC (charge recombination)
take 109 ps and 2.3 ns, respectively. The triplet state energies of
the dyads are 1.651.77 eV, determined by triplet energy transfer
studies. The dyad was used as triplet PSs for triplet-triplet
4.4. Synthesis of BDP-3.
Under N2 atmosphere, compound 4 (30 mg, 0.04 mmol) and 7 (80 mg,
0.32 mmol) was dissolved in mixed solvent TEA/THF (10 mL, 1:1, v/v).
Then PdCl2(PPh3)2 (7 mg), PPh3 (5 mg) and CuI (3.8 mg) were added and
the reaction was heated to 60 C for 3 h. The product was washed with
water after cooling to room temperature. The organic layer was collected
and dried over anhydrous Na2SO4. The crude product was purified by
column chromatography (silica gel, PE/DCM = 2/1, v/v) to obtain BDP-3
as dark violet solid (24 mg, yield: 60%). M.p.  250 C. 1H NMR (400 MHz,
DMSO-d6): δ = 8.38 (s, 2H), 8.23 (d, 2H, J = 8.0 Hz), 7.667.58 (m, 6H),
7.517.48 (m, 2H), 7.257.22 (m, 2H), 6.92 (s, 3H), 6.796.71 (m, 4H),
4.42 (t, 4H, J = 8.0 Hz), 3.65 (t, 2H, J = 6.0 Hz), 2.69 (s, 6H), 1.86 (s, 6H),
1.76 (t, 4H, J = 6.0 Hz), 1.62 (t, 2H, J = 6.0 Hz), 1.501.44 (m, 2H),
1.331.27 (m, 4H), 0.99 (t, 3H, J = 6.0 Hz), 0.88 (t, 6H, J = 8.0 Hz).
MALDITOFHRMS: m/z [M] Calcd for C65H60BF2N5O: m/z = 975.4859,
found: m/z = 975.4841.
annihilation upconversion (upconversion quantum yield T
=
1.5% in toluene; anti-Stokes shift is 5905 cm1). Our results are
useful for the design of novel heavy atom-free triplet PSs showing
red light-absorption and more importantly long-lived triplet states.
These triplet PSs are useful for photo-redox catalytic organic
reaction, photodynamic therapy, and triplet-triplet-annihilation
photon upconversion.
4.5. Femtosecond Transient Absorption Spectra
The femtosecond transient absorption spectra were performed on a
Ti:sapphire laser amplifier-optical parametric amplifier system with 52 fs
pulse duration and 1 kHz repetition rate (Spectra-Physics, Spitfire Pro XP,
TOPAS) and a commercial setup of an ultrafast transient absorption
spectrometer (Spectra Physics, Helios). The excitation wavelength was
determined by steady UVVis absorption spectra. Perpendicular angle
between the probe and the pump beam polarization direction was used.
The Surface Xplorer and Glotaran software were used for processing the
experimental data after chirp correction.
4. Experimental Section
4.1. Materials and Equipment
All compounds used for synthesis are analytically pure. Solvents for
synthesis were freshly dried before using. 1H and 13C NMR spectra were
recorded on Bruker 400/500 MHz spectrometers. HRMS (high resolution
mass spectra) were recorded with MALDI-TOF-MS and ESI-MS
spectrometers. Fluorescence were recorded on a FS5 spectrofluorometer
(Edinburgh Instrument Ltd, UK). UVVis spectra were recorded on a
8453A UVVis spectrophotometer (Agilent Ltd, USA). The time-resolved
4.6. Nanosecond Transient Absorption Spectra
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10.1002/cphc.202000300
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ARTICLE
Nanosecond transient absorption spectra were recorded on a LP980 laser
flash photolysis spectrometer (Edinburgh Instruments, Ltd UK). All the
sample solutions are deaerated with N2 for ca. 15 min before measuring.
Keywords: Bodipy • charge transfer • intersystem crossing •
photosensitizers • triplet state
The samples were excited with a nanosecond pulsed laser (OpoletteTM
,
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4.7. Single Crystal X-Ray Crystallography
X-ray diffraction data were collected on a Bruker SMART APEX-II CCD
diffractometer (Mο K radiation,  = 0.71073 Å)) at 200 K using the SMART
and SAINT programs. The structure was solved by direct method of
SHELXTL-97 and refined by full-matrix least-squares using the SHELXL-
2014 program on a PC. H atoms were generated geometrically. Detailed
crystallographic data and structure refinement parameters are available in
the Supporting Information, Table S1. The crystallographic data for the
structure can be obtained from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk with CCDC number: 1990853.
4.8. Triplet State Quantum Yield (T).
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


εstd ΔΑsam
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




Φsam  Φstd
(5)

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


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Acknowledgements
J.Z. thanks the NSFC (21673031, 21761142005 and
21911530095) and the State Key Laboratory of Fine Chemicals
(ZYTS201901) for financial support. B.D. acknowledges Dalian
University of Technology for the Haitian Professorship support.
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Entry for the Table of Contents
ARTICLE
Long-lived triplet state (T = 333 s)
was attained via the SOCT-ISC
mechanism for red light-absorbing
ompact, orthogonal electron
Yu Dong,[a] Ayhan Elmali,[b] Jianzhang
Zhao,*[a] Bernhard Dick,*[c] and Ahmet
Karatay*[b]
Page No. – Page No.
donor/acceptor dyads with phenoxazine
as electron donor and bisstyrylBodipy
chromophore as electron acceptor. The
triplet state lifetime is prolonged by 185-
fold than the triplet state of the same
chromophore accessed with the
traditional heavy atom effect (T = 1.8
s).
((Insert TOC Graphic here))
Long-Lived Triplet Excited State
Accessed with Spin-Orbit Charge
Transfer Intersystem Crossing in Red
Light-Absorbing Phenoxazine-
StyrylBodipy Electron
Donor/Acceptor Dyads
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