Bodipy-Anthracene Dyads as Triplet Photosensitizers: Effect of Chromophore Orientation on Triplet-State Formation Efficiency and Application in Triplet-Triplet Annihilation Upconversion

Article abstract of DOI:10.1021/acs.orglett.7b02047

Bodipy-anthracene dyads with two chromophores assuming orthogonal geometry to enhance the spin-orbital charge-transfer intersystem crossing (SOCT-ISC) were prepared. The photosensitizers show strong absorption of visible light, efficient triplet-state formation (quantum yield 90%), and a long-lived triplet state (85 μs). The dipole moment orientation exerts significant effect on the ISC efficiency. It is also the first time that photosensitizers based on SOCT-ISC were used for triplet-triplet annihilation upconversion. The upconversion quantum yield is up to 15.8%.

Full text of DOI:10.1021/acs.orglett.7b02047

Letter
pubs.acs.org/OrgLett
Bodipy−Anthracene Dyads as Triplet Photosensitizers: Effect of
Chromophore Orientation on Triplet-State Formation Efficiency and
Application in Triplet−Triplet Annihilation Upconversion
Zhijia Wang and Jianzhang 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, P. R. China
S
* Supporting Information
ABSTRACT: Bodipy−anthracene dyads with two chromophores assum-
ing orthogonal geometry to enhance the spin-orbital charge-transfer
intersystem crossing (SOCT-ISC) were prepared. The photosensitizers
show strong absorption of visible light, efficient triplet-state formation
(quantum yield 90%), and a long-lived triplet state (85 μs). The dipole
moment orientation exerts significant effect on the ISC efficiency. It is
also the first time that photosensitizers based on SOCT-ISC were used for
triplet−triplet annihilation upconversion. The upconversion quantum
yield is up to 15.8%.
ighly efficient triplet photosensitizers (PSs) are important
acceptor into the molecule will offer additional functionality;
for instance, the triplet PSs can be responsive to external redox
stimuli, which is important for biological systems.12 However,
some critical problems still remain in this new area: the
molecular structure−ISC efficiency relationship is unclear, the
molecular structures that have been investigated are very
limited, and the chromophores studied until now are limited to
those showing weak absorption of visible light. Moreover, no
application has been demonstrated for this ISC strategy.
To address the above challenges, herein we designed
Bodipy−anthracene dyads to probe the relationship between
the molecular structure, mainly the chromophore orientation,
and the ISC efficiency (Figure 1). This relationship is crucial to
elucidate the SOCT-ISC mechanism and it is useful for future
molecular structural design.
Owing to the versatile derivatization of Bodipy chromophore,
we prepared new dyads BDP-AN-3 and BDP-AN-4. The
dipole moment orientation of the anthracene and Bodipy of
BDP-AN-3 is different from that of BDP-AN-1 and BDP-AN-
2, although the energy-minimized geometry is orthogonal for
all three dyads (the dihedral angle between Bodipy and
anthracene are 90°, 90°, and 93° for BDP-AN-1, BDP-AN-2,
and BDP-AN-3, respectively). Conversely, BDP-AN-4 has a
52° dihedral angle between Bodipy and the anthracene moiety.
The restriction on the rotation of the anthranyl moiety is on
the order of BDP-AN-3 > BDP-An-1 ≈ BDP-An-2 > BDP-
AN-4 due to the presence of the methyl groups.
H
for photocatalysis,1 photodynamic therapy (PDT),2 and
more recently, triplet−triplet annihilation upconversion (TTA
UC).3 Efficient intersystem crossing (ISC) is pivotal for triplet
PSs.4 Normally a heavy atom effect is used to enhance the ISC.4
As such, most typical triplet PSs contain Pt, Ru, Ir, I, or Br, etc.5
Heavy atom-free triplet PSs have been rarely reported but are
highly desired in terms of cost efficiency, toxicity, and
fundamental photochemistry.4,6 However, it is still a major
challenge to develop efficient heavy atom-free triplet PSs.
Some strategies have been developed to enhance the ISC of
heavy atom-free compounds, for instance, exciton coupling,7
double excited state,8 spin converter,9a thermally activated
reverse ISC,9b−d and singlet fission.10 Although these
approaches are promising, drawbacks do exist. For instance,
for most of these methods, a synthetically demanding
bichromophore molecular structure is required.7−10 New
methods to enhance ISC in organic chromophores with simple
molecular structures are desired. Recently, it was shown that
spin−orbital charge transfer (SOCT) is effective to enhance the
ISC in organic chromophores.10,11 For instance, julolidine−
anthracene dyads show ISC ability.10 Recently Bodipy-
anthracene dyads were shown to show ISC.11 The key
molecular structural design strategy is the orthogonal
orientation between the electron donor and the electron
acceptor, so that the change of molecular orbital angular
momentum can compensate for the change of electron-spin
angular momentum during ISC. As a result, the ISC can be
enhanced because the conservation of the angular momentum
is achieved.
The UV−vis absorption of the compounds was studied
(Figure 2). The absorption profiles are the sum of the Bodipy
monomer and anthracene; thus, the electronic coupling
SOCT-ISC is particularly interesting in terms of ISC since
only one major chromophore is required and a structurally
simple electron donor or acceptor can be used, thus making the
synthesis easier. Moreover, introducing an electron donor or
Received: July 5, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02047
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Organic Letters
Letter
Figure 3. Nanosecond transient absorption spectra of BDP-AN-1: (a)
BDP-AN-1 upon ns pulsed laser excitation (λex = 502 nm) and (b)
decay trace of BDP-AN-1 at 505 nm. c = 1.0 × 10−5 M in deaerated
dichloromethane, 20 °C.
Figure 1. Molecular structures of BDP-AN-1, BDP-AN-2, BDP-AN-3,
and BDP-AN-4 and reference compounds BDP and IBDP.
The results show that the triplet state quantum yields are
highly dependent on the chromophore orientations, as well as
the dipole moment orientation, in the dyads (Table 1). This is
a new finding.11 For instance, with the anthracene moiety
attached at the meso-position (i.e., BDP-AN-1 and BDP-AN-2),
the triplet states yields are much higher (up to 96%) than the
dyads with the anthracene moiety attached at the 2-position of
the Bodipy core (BDP-AN-3, <31%). Note in both cases that
the orientation of the chromophores is “orthogonal”, but the
dipole moments alignments are different. For BDP-AN-2, the
dipole moment vectors of anthracene part and the Bodipy part
are antiparallel, while for BDP-AN-3 they are approximately
orthogonal (Figure S25).
This result indicates that the previously proposed concept of
orthogonal geometry for the chromophores in the dyads is
insufficient to attain high ISC yield. An additional description of
the geometry of the components of the PSs should be used, i.e.,
vectorial dipole moment orientations of the chromophores. We
tentatively propose that the parallel/antiparallel dipole moment
orientation of the CT donor and acceptor maybe more
beneficial to SOCT-ISC rather than orthogonal dipole moment
alignment. Moreover, the restriction of the rotation of the
anthracene moiety also exerts a non-negligible effect on the ISC
yields, as such BDP-AN-3 shows a higher triplet-state quantum
yield (16-31%) than BDP-AN-4 (6−21%). BDP-AN-4 has
larger rotational freedom, and the dihedral angle of the donor
and acceptor (52°) deviates greatly from 90°, which is
detrimental to the SOCT-ISC. Moreover, compared with
BDP-AN-1 and BDP-AN-2, the red-shifted absorption and
emission wavelength of BDP-AN-3 and BDP-AN-4 indicated a
relatively larger π-conjugation (thus a relatively stronger
coupling) between bodipy and anthracene units, which also
contributes to a less efficient SOCT-ISC for BDP-AN-3 and
BDP-AN-4.
We found that the triplet quantum yields of the dyads are
highly dependent on the solvent polarity. In nonpolar solvents
such as toluene, the triplet-state quantum yields are very low,
whereas in high polar solvents (such as DCM) the triplet-state
quantum yields become much higher. This is in agreement with
the CT character of the ISC pathway. This charge-transfer-
induced ISC mechanism was illustrated with an energy diagram
(Scheme 1).15 The energy level of the CSS is highly dependent
on the solvent polarity, and generally, the energy level becomes
lower in polar solvents. It should be pointed out that the CSS
energy level in polar solvents should be higher than the T1 state
of the chromophore; otherwise, the charge recombination
(CR) will not give triplet state or the triplet state will be
quenched.15
Figure 2. Normalized UV−vis absorption and fluorescence emission
spectra of BDP-AN-1 in (a) n-hexane and (b) acetonitrile at 20 °C.
between the anthracene and the Bodipy moiety is very weak at
the ground state. A lack of charge-transfer band in the UV−vis
absorption spectra is evidence of the decoupling of the
anthracene and the Bodipy moieties. This is reasonable since
the anthracene and the Bodipy moiety take the most stable
orthogonal geometry at the ground state.
The fluorescence emission spectrum in hexane (Figure 2a) is
the mirror of the absorption spectrum. However, a new red-
shifted emission band at 600−800 nm was observed in
acetonitrile (Figure 2b), which is attributed to the charge-
separated state (CSS) emission.11 Similar results were observed
for BDP-AN-2, BDP-AN-3, and BDP-AN-4 (Figure S13) The
fluorescence intensity was significantly quenched in polar
solvents (Figure S12). Charge separation and emission from
charge separated state is favored in polar solvents due to the
larger driving force for charge transfer (CT) and the stabilized
CSS.13 Observation of the CSS emission is an indication for
possible SOCT-ISC.
In order to confirm the production of the triplet state upon
photoexcitation, the nanosecond transient absorption spectra
(ns TA) of the compounds were studied (for BDP-AN-1, see
Figure 3). Upon nanosecond pulsed laser excitation, an excited-
state absorption (ESA) band at 420 nm was observed, as well as
a weak absorption band in 560 nm −630 nm. This is the typical
transient absorption feature of the triplet excited state of
Bodipy.14 A triplet-state lifetime of 85 μs was observed. The
triplet state quantum yield was determined as 90% (Table 1).
Similar results were observed for other dyads (Figures S14−
S16). Using a singlet-state depletion method, the triplet-state
quantum yields (ΦT) of the dyads in different solvent were
determined (Table 1).
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DOI: 10.1021/acs.orglett.7b02047
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Table 1. Photophysical Properties of the Compounds
a
b
c
d
e
f
g
h
solvent
toluene
λabs
ε
λem
521
ΦF
0.81
τF (ns)
τT (μs)
ΦΔ
ΦT
ΦUC (%)
BDP-AN-1
BDP-AN-2
BDP-AN-3
BDP-AN-4
508/373
506/373
503/373
508/366
506/366
502/366
516/367
513/367
510/367
520/364
517/364
513/364
503
9.0/1.7
8.6/1.7
8.0/1.7
9.9/1.5
9.7/1.5
8.7/1.5
8.0/1.0
7.8/1.0
7.5/1.0
8.7/1.2
8.9/1.2
8.4/1.2
8.8
5.8
317
85
0.10
0.95
0.84
0.04
0.82
0.86
0.20
0.24
0.11
0.11
0.13
0.05
l
0.06
0.90
0.92
0.03
0.80
0.96
0.31
0.20
0.16
0.17
0.21
0.06
l
i
j
j
dichloromethane
acetonitrile
toluene
518/595
512/641
521
0.01/0.02
0.002/0.007
0.84
5.0 /4.8
15.8
3.2
i
4.2 /2.9
68
5.5
5.5
345
82
dichloromethane
acetonitrile
toluene
518
0.14
10.1
3.9
i
j
j
j
510/634
556
0.01/0.01
0.39
5.0 /3.0
78
4.2
3.0
127
118
137
102
116
125
l
5.9
dichloromethane
acetonitrile
toluene
584
0.10
5.6
i
524/627
568
0.01/0.02
0.42
2.6 /1.3
4.1
3.3
2.5
2.1
dichloromethane
acetonitrile
575
0.20
i
587
0.04
1.8 /1.3
k
k
BDP
toluene
515
0.90
3.4
l
a
b
c
In toluene (1.0 × 10−5 M). Molar absorption coefficient (104 M−1 cm−1). Fluorescence quantum yield, BDP as standard (ΦL = 0.9 in toluene).
d
e
f
Luminescence lifetimes. Triplet-state lifetimes. Measured by ns TA in deaerated solutions. Singlet oxygen quantum yield (ΦΔ), diiodobodipy as a
g
h
standard (ΦΔ = 0.87 in DCM). Triplet state quantum yield, measured with ns TA, with IBDP as the standard (ΦT= 0.88 in toluene). TTA UC
quantum yield, with IBDP as the standard (ΦF= 0.027 in acetonitrile). Upconversion quantum yield of BDP-AN-1/BDP-AN-2 and BDP-AN-3/
i
j
k
l
BDP-AN-4 were measured at 50 mW cm−2 and 100 mW cm−2, respectively. At short wavelength. At long wavelength. Literature values. Not
applicable.
BDP-AN-1). Perylene was used as the triplet acceptor, and a
510 nm continuous wave (cw) laser was used for excitation.
BDP-AN-1 alone gave a yellowish emission due to the residue
fluorescence of the Bodipy unit. Upon addition of perylene, the
blue emission of perylene was observed, which is the
upconverted fluorescence of perylene. The upconversion
quantum yield with BDP-AN-1 as the triplet PS was
determined as 15.8%, which is competitive with the conven-
tional heavy atom PSs (IBDP, 15.1% measured under the same
conditions, Figure S22). This study presents a new opportunity
to tune the TTA UC with external stimuli,16 such as by
monitoring the in vivo redox stress.12
Scheme 1. Simplified Jablonski Diagram Illustrating the
Photophysical Process in BDP-AN-1
In conclusion, we prepared a series of Bodipy−anthracene
dyads with the two components assuming orthogonal geometry
to enhance the spin−orbital charge-transfer intersystem
crossing. As such, heavy-atom-free efficient triplet photo-
sensitizers were obtained. The PSs show strong absorption of
visible light, efficient triplet-state formation (quantum yield,
90%), and long triplet-state lifetimes (85 μs). Moreover, we
found that the dipole moment orientation of the anthracene
moiety (electron donor) exerts a significant effect on ISC
efficiency; thus, the previously suggested orthogonal geometry
of the chromophore is not exclusively sufficient to achieve
efficient SOCT-ISC. The SOCT-ISC triplet PSs are employed
in triplet−triplet annihilation upconversion for the first time,
and the upconversion quantum yield is up to 15.8%. Our
finding may be useful for future molecular structural design of
new efficient triplet PSs, photocatalysis, external stimulate-
responsive PDT, TTA upconversion and for fundamental
photochemistry.
Most of the triplet PSs for TTA UC are based on the heavy
atom effect to enhance the ISC.3 Use of heavy-atom-free triplet
PSs with a SOCT−ISC mechanism in TTA upconversion has
never been reported. However, Bodipy−anthracene dyads,
especially BDP-AN-1 and BDP-AN-2, show a strong
absorption of visible light (ε = 8.6 × 104 M−1 cm−1) and an
efficient ISC (ΦT = 90%) and long-lived triplet state (85 μs),
and TTA UC was carried out with these new PSs (Figure 4, for
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b02047.
Figure 4. (a) Upconversions with BDP-AN-1 as triplet PS and
perylene as triplet acceptor. Excited with 510 nm cw-laser (50 mW
cm−2). c[perylene] = 3.0 × 10−5 M, in deaerated dichloromethane, 20
°C. Inset: Photographs of BDP-AN-1 alone and the upconversion. (b)
CIE diagram.
Experimental procedures and characterization data
(PDF)
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DOI: 10.1021/acs.orglett.7b02047
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Organic Letters
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zhaojzh@dlut.edu.cn.
ORCID
Zhijia Wang: 0000-0002-8309-0050
Jianzhang Zhao: 0000-0002-5405-6398
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSFC (21473020, 21673031, and 21421005) for
financial support.
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