Efficient Intersystem Crossing in the Tr?ger's Base Derived From 4-Amino-1,8-naphthalimide and Application as a Potent Photodynamic Therapy Reagent

Article abstract of DOI:10.1002/chem.201905248

Intersystem crossing (ISC) was observed for naphthalimide (NI)-derived Tr?ger's base, and the ISC was confirmed to occur by a spin-orbital charge-transfer (SOCT) mechanism. Conventional electron donor/acceptor dyads showing SOCT-ISC have semirigid linkers. In contrast, the linker between the two chromophores in Tr?ger's base is rigid and torsion is completely inhibited, which is beneficial for efficient SOCT-ISC. Femtosecond transient absorption (TA) spectra demonstrated charge-separation and charge-recombination-induced ISC processes. Nanosecond TA spectroscopy confirmed the ISC, and the triplet state is long-lived (46 μs, room temperature). The ISC quantum yield is dependent on solvent polarity (8–41 %). The triplet state was studied by pulsed-laser-excited time-resolved EPR spectroscopy, and both the NI-localized triplet state and triplet charge-transfer state were observed, which is in good agreement with the spin-density analysis. The Tr?ger's base was confirmed to be a potent photodynamic therapy reagent with HeLa cells (EC₅₀=5.0 nm).

Full text of DOI:10.1002/chem.201905248

A Journal of
Accepted Article
Title: Efficient Intersystem Crossing in Tröger's Base Derived From
4-Amino-1,8-Naphthalimide and Application as Potent PDT
Reagent
Authors: Yingjie Zhao, Kepeng Chen, Elif Akhuseyin Yildiz, Shujing Li,
Yuqi Hou, Xue Zhang, Zhijia Wang, Jianzhang Zhao, Antonio
Barbon, Halime Gul Yaglioglu, and Huijian Wu
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201905248
Link to VoR: http://dx.doi.org/10.1002/chem.201905248
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10.1002/chem.201905248
Chemistry - A European Journal
RESEARCH ARTICLE
Efficient Intersystem Crossing in Tröger’s Base Derived From 4-
Amino-1,8-Naphthalimide and Application as Potent PDT Reagent
Yingjie Zhao,[a],§ Kepeng Chen,[a],§ Elif Akhuseyin Yildiz,[b] Shujing Li,[c] Yuqi Hou,[a] Xue Zhang,[a] Zhijia
Wang,[a] Jianzhang Zhao,*[a] Antonio Barbon,*[d] Halime Gul Yaglioglu*[b] and Huijian Wu*[c]
crossing (ISC), long-lived triplet state, appropriate redox
potentials, and robust photo- and chemostability.[12] Efficient ISC
is one of the crucial characters of the triplet PSs. However, ISC is
electron spin-forbidden process, therefore, some driving force
must be available to enhance this forbidden process.[13] The most
straightforward method to enhance the ISC in organic compounds
is the heavy atom effect. Attaching a heavy atom, for instance,
iodine, bromine, or the transition metal atoms (Pt, Ir, Ru, Os, etc)
to a chromophore, will enhance the ISC.[13] However, the heavy
atom-containing compounds are normally toxic or the synthesis is
costly (e.g. using precious metals such as Pt, Ir, Ru, etc).[12]
Therefore, it is important to devise novel heavy atom-free
molecular structural motifs to enhance the ISC.
Driving forces other than heavy atom effect for enhancing ISC
has been reported. For instance, molecules with n-*-*
transition are usually known to have efficient ISC (El-Sayed
rule).[13] Exciton coupling may also enhance ISC, this mechanism
is based on the energy level splitting and the matching of the S1/Tn
states.[14] Charge recombination (CR) induced ISC was also
reported for the electron donor/acceptor dyads, based on the
radical-pair ISC (RP-ISC) mechanism.[15,16] Recently the electron
spin converter strategy was also reported.[17,18] Radical enhanced
ISC based on electron spin exchange interaction was also
studied.[19,20] However, various drawbacks exist for these
approaches, e.g., the synthesis of the molecules is demanding, or
ISC kinetics is slow and the ISC efficiency is unsatisfactory.
Abstract: Intersystem crossing (ISC) was observed for
naphthalimide-derived Tröger’s base, and the ISC is confirmed as
spin orbital charge transfer ISC (SOCT-ISC) mechanism.
Conventional electron donor/acceptor dyads showing SOCT-ISC are
with semi-rigid linkers. In contrast, the linker between the two
chromophores in Tröger’s base is rigid and torsion is completely
inhibited, which is beneficial for efficient SOCT-ISC. Femtosecond (fs)
transient absorption (TA) spectra demonstrated the charge separation
and the charge recombination-induced ISC processes. Nanosecond
TA spectroscopy confirmed the ISC and the triplet state is long-lived
(46 s, room temperature). The ISC quantum yield is solvent polarity-
dependent (8% ~ 41%). The triplet state were studied with pulsed
laser excited time-resolved electron paramagnetic resonance
3
(TREPR) spectroscopy, both NI localized triplet state and CT state
were observed, which is in good agreement with the spin density
analysis. The Tröger’s base was confirmed as a potent photodynamic
therapy (PDT) reagent with HeLa cells (EC50 = 5.0 nM).
Introduction
Triplet photosensitizers (PSs) have attracted much attention in
recent years,[1,2] for not only fundamental photochemistry
studies,[3] but also for applications in photocatalytic synthetic
organic
reactions,[47]
photodynamic
therapy
(PDT),[2]
photovoltaics,[8] and more recently for the triplet-triplet-
annihilation upconversion.[911] Triplet PSs are expected to show
strong absorption of UV/Vis excitation light, efficient intersystem
Recently, the spin orbital charge transfer ISC (SOCT-ISC) in
compact electron donor/acceptor dyads has attracted much
attention.[2125] In these dyads the linker between the electron
donor and acceptor is short, it can be a CC or CN single bond,
which is different from the conventional electron donor/acceptor
dyads showing RP-ISC, in which a long and rigid linker between
electron donor and acceptor has to be used to reduce the electron
exchange values (the exchange interaction, J, should be much
smaller than 1 cm1), thus to make the RP-ISC possible.[16]
Moreover, in the SOCT-ISC dyads, the electron donor/acceptor
adopt orthogonal geometry, so that the electron transfer is
accompanied with molecular orbital angular momentum change,
which compensates the electron spin angular momentum change
of the ISC process. As a result, the ISC can be enhanced by
electron transfer between the perpendicularly orientated electron
donor/acceptors.
[a]
Y. Zhao, K. Chen, Y. Hou, X. Zhang, Dr. Z. Wang 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. Yildiz, Prof. G. Yaglioglu
Department of Engineering Physics, Faculty of Engineering, Ankara
University, 06100 Beşevler, Ankara, Turkey.
E-mail: yoglu@eng.ankara.edu.tr (G. Y.)
S. Li and Prof. H. Wu
School of Bioengineering & Key Laboratory of Protein Modification
and Disease, Liaoning Province, Dalian University of Technology,
Dalian, China.
E-mail: wuhj@dlut.edu.cn (H. Wu)
[d]
Dr. A. Barbon
One critical issue for the molecular design of the SOCT-ISC
triplet PS is implementation of the molecular geometry restriction
to attain the orthogonally aligned electron donor and acceptor.
Previously, in a julolidine-anthracene dyads, methyl groups were
introduced to the ortho-position of the linkers to enhance the
molecular conformation restriction.[21] The 1,7-methyl groups on
Dipartimento di Scienze Chimiche, Università degli Studi di Padova,
Via Marzolo 1, 35131 Padova, Italy.
E-mail: antonio.barbon@unipd.it
§
Y. Z. and K. C. contribute equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201905248
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RESEARCH ARTICLE
the Boron dipyrromethene (Bodipy) moiety play the similar role in
the Bodipy-derived electron donor/acceptor dyads showing
SOCT-ISC.[22,23,26] We found that the presence of peri-H (C) of
electron donor phenothiazine is helpful for conformation
to study the ISC mechanism and the electron spin polarization
(ESP) of the triplet state in detail. PDT effect of NI-TB was studied
as well. These results are helpful for development of novel
molecular motifs for attaining efficient ISC in heavy atom-free
restriction.[24,27,28] However, in all these molecular structural motifs, organic compounds as well as for application of these novel triplet
PSs in photocatalysis and photon upconversion, etc.
the rotation about the CC or the CN linker is not fully restricted,
and various conformations are still accessible with the thermal
energy. Deviation from the orthogonal geometry may be
detrimental to the SOCT-ISC efficiency. As such, fully rigid linker
is desired for linking the electron donor/acceptor in these dyads,
but this kind of linker is difficult to attain from a point of view of
synthesis.
Concerning this aspect, we noted the potential of the unique
molecular structure of Tröger’s base to enhance the SOCT-ISC.
Tröger’s bases, which have been known for decades, are cleft-
shaped molecules containing a diazocine ring to connect two
chromophores.[29] Tröger’s bases have played a considerable role
in supramolecular chemistry, in the areas of molecular recognition,
molecular separation, and photoswitches,[3033] or as hole
transport materials for OLED.[34] The most intriguing feature of
these molecules to us, is the fully rigid, and the almost
perpendicular geometry of the two aromatic moieties (90  104 ).
These features are beneficial for the purpose of attaining SOCT-
ISC. However, the ISC of Tröger’s bases was not reported. In
2005, Lewis et al. reported the naphthalimide (NI)-derived
Tröger’s base.[35] The red-shifted and quenched fluorescence in
polar solvent were noted, but the non-radiative decay mechanism
in polar solvents was not studied in detail.[35] Gunnlaugsson et al.
Scheme 1. Synthesis of NI-TB. (a) Na2Cr2O72H2O, AcOH, reflux, 5 h, under
N2, yield: 57%; (b) n-C4H9NH2, EtOH, reflux, 2h, under N2, yield: 56%; (c)
SnCl22H2O, Con. HCl, EtOH, RT, 15 min, yield: 85%; (d) paraformaldehyde,
CF3COOH, RT, 9 h, under N2, yield: 73%; (e) similar to step (b), yield: 91%; (f)
n-C4H9NH2, CH3OCH2CH2OH, reflux, 8 h, under N2, yield 63%.
Results and Discussions
made
a detail study on the luminescence imaging and
photocleavage of DNA and sensing of explosives with the
Conventional Tröger’s bases are normally with substituted
benzene ring as the chromophore, which show absorption in UV
spectral region.[29] Clearly this is a disadvantage for most of the
applications, which normally require absorption in visible spectral
region. We considered the NI-derived Tröger’s base as a
candidate for triplet PSs based on SOCT-ISC,[31,3538] in which the
NI moiety show absorption in visible region. Thus we studied the
possible ISC of the NI-derived Tröger’s base. The preparation of
the NI-TB (Scheme 1) is based on the reported method,[37] and
the molecular structure of the target compounds was fully verified
with 1H NMR and HR MS spectra (see Experimental Section).
[3638]
naphthalimide (NI)-derived Tröger’s base.
Although it is
known that the photocleavage of DNA is usually due to generation
of reactive oxygen species (ROS) by the intercalator, for instance,
singlet oxyen (1O2) or superoxide radical anion (O2),[39,40] yet the
ISC of Tröger’s bases was not reported. Recently a NI-derived
Tröger’s base–Ru(II)–curcumin organometallic conjugate was
demonstrated to show cytotoxicity against cervical cancer cells,
but the phototoxicity and the ISC of the base of itself were not
studied.[41]
Based on these previous studies on Tröger’s bases, i.e. the
quenching of the fluorescence in polar solvents and the
orthogonal geometry, we suppose that SOCT-ISC may occur in
Tröger’s bases, via the symmetry-breaking CT (SBCT), i.e. one
NI moiety acts as the electron donor and another NI moiety in the
Tröger’s base acts as electron acceptor.[42]
The UVVis absorption spectra of some of the compounds
presented in Scheme 1 were studied (Figure 1). NI-Br shows a
structured absorption spectrum, with vibronic bands centered at
NI-TB
NI-NH
NI-Br
Herein we propose to use the rigid, almost orthogonal
geometry of the two subunits in Tröger’s base to implement the
SOCT-ISC. Due to its excellent photophysical property and
stability, we selected the NI moiety as the visible light-harvesting
chromophore to prepare the Tröger’s base. We studied the
photophysical properties of the NI-TB with various steady state
and time-resolved transient spectroscopies. Photo-induced
electron transfer was confirmed with femtosecond transient
absorption spectroscopy (fs TA). The formation of the triplet
excited state was confirmed with nanosecond transient
absorption (ns TA) spectroscopy. Time-resolved electron
paramagnetic resonance (TREPR) spectroscopy was also used
0.2
0.1
0.0
300
400
500
600
Wavelength / nm
Figure 1. UVVis absorption spectra of NI-NH, NI-TB and NI-Br. c = 1.0 × 10−5
M in n-hexane. 25 °C.
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RESEARCH ARTICLE
1x106
1x106
8x105
6x105
4x105
2x105
0
9000
8000
7000
6000
5000
4000
3000
NI-TB
NI-NH
Linear Fit
a
b
c
8x105
6x105
HEX
TOL
DCM
THF
EtOAc
MeCN
HEX
TOL
DCM
THF
EtOAc
MeCN
Slope: 12185
 = 6081
R2 = 0.81
4x105
2x105
0
Slope: 2526
 = 2011
R2 = 0.62
400
500
600
700
800
500
600
700
800
0.0
0.1
f(,n)
0.2
0.3
Wavelength / nm
Wavelength / nm
Figure 2. Fluorescence emission spectra of (a) NI-TB in different solvents (λex = 345 nm, A = 0.137) and (b) NI-NH in different solvents (λex = 400 nm, A
= 0.132); Optically matched solutions were used (the concentration is varied slightly to attain the same absorbance at the excitation wavelength). (c)
Lippert-Mataga plots for NI-TB (the coefficient of determination R2 = 0.81, the estimated standard deviations  = 6081) and NI-NH (R2 = 0.62,  =2011 for
the fitting); f (, n) = [(1)/(2+1)  0.5(n21)/(2n2+1)]. The solid lines are the fitting results. The solvents used are: n-hexane, toluene, toluene : DCM
(4/1, v/v), THF, ethyl acetate, MeCN, CHCl3 and acetone, in the order of increasing f values. 25 °C.
324 nm, 338 nm and 354 nm, respectively. With amino group
attached on the 4-position of the NI moiety, NI-NH shows broad,
strongly red-shifted and structureless absorption band centred at
408 nm. It indicates the lone pair electrons of the amino group are
in -conjugation with the NI unit, and the electronic push-pull
interaction (i.e. intramolecular charge transfer, ICT) between the
amino group and NI unit is strong. However, for NI-TB, a broad
band at 370 nm was observed. The main absorption band at 370
nm is blue-shifted as compared to NI-NH (408 nm). The
absorption feature of NI-TB studied herein is in agreement with
those of the NI-based Tröger’s base reported previously.[31,37]
polarity-dependency of the fluorescence of NI-TB should be
weaker. Based on these results, we propose there is CT between
the two NI units in NI-TB upon photoexcitation. Previously it was
shown that the linker of the two chromophore in Tröger’s base
molecule, i.e. methano-1,5-diazocine ring, is efficient for shift of
the electron/hole in the molecules.[47]
To further investigate the excited state dipole moment
changes of NI-TB and NI-NH upon photoexcitation, the effect of
the solvent polarity on the CT emission band were studied in detail,
and the data were analyzed with the Lippert-Mataga plots (Figure
2c).[46] Linear plots observed for the two compounds indicated the
general solvent effect.[48] The dipole moments of the S1 state of
two compounds are obtained as e (NI-TB) = 21.0 D, e (NI-NH) =
9.1 D, based on the ground state dipole moment g (NI-TB) = 4.8
D and g (NI-NH) = 6.0 D, approximated from the ground state
geometry optimization by DFT calculation. The charge separation
distance of two compounds are approximated as d(NI-TB) = 4.3
Å and d(NI-NH) = 1.9 Å by the equation  = q  d, assuming one
unit of charge is separated completely in the molecules. Note the
optimized ground state geometry indicates that the centroid-to-
centroid distance of the two NI units in NI-TB is ca. 7.3 Å; the
distance between the bridging methylene carbon to the NI center
is ca. 4.8 Å. The charge separation distance of NI-TB is about
twice than that of NI-NH, which rationalize the larger dipole
moment changes upon photoexcitation. Thus we propose that
symmetry breaking CT occurs in NI-TB,[46] which is different from
the normal ICT in NI-NH (N atom in amino group as the electron
donor).[49,50] Previously the fluorescence of a carboxylate ester-
attached benzene derived Tröger’s base was studied with the
Lippert-Mataga plot, and it was found that the dipole moment
change upon excitation is up to 14.79 D.[44]
The red-shifted broad absorption of NI-TB as compared to NI-
Br is not resulted from the exciton coupling between the two NI
units.[43] The red-shifted and broadened absorption band is
attributed to the ICT effect in NI-TB, because the amino groups
are not fully decoupled from the NI moiety, which is similar to NI-
NH, although in minor extent. This is also in agreement with the
results obtained for the phenyl derived Tröger’s base.[44]
It is interesting that the absorption of NI-TB is blue-shifted as
compared to NI-NH, which indicates that the lone pair of the N
atom in the bridge of NI-TB do not fully participate in the -
conjugation system as compared to NI-NH, we assign this limited
-conjugation to the conformation restriction in NI-TB by the rigid
diazocine linker between the two NI units.[45] By changing the
polarity of the solvent, bathochromic effect for the absorption
bands of NI-TB and NI-NH was observed (Supporting Information,
Figure S7).
The fluorescence emission of NI-TB and NI-NH were studied
(Figure 2). For NI-TB, a broad fluorescence emission band was
observed in most of the solvents used. The emission band is red-
shifted, the intensity is reduced with increasing solvents polarity.
These behaviour is typical for emission from a charge transfer
state.[46] However, for NI-NH, with the solvent polarity increased,
the emission band is only slightly red-shifted, and no substantial
change was observed for the emission intensity (Figure 2b),
indicating that the ICT effect in the two compounds is different.
This is interesting, because the UVVis absorption studies show
that the -conjugation of the amino N atom is less efficient in NI-
TB than that in NI-NH at ground state (Figure 1), thus the solvent
The fluorescence lifetime of NI-TB decreases in more polar
solvents, i.e., from n-hexane (8.0 ns) to MeCN (1.6 ns, Supporting
Information, Figure S8a). One reason is the energy gap law, i.e.
the smaller energy gap of S1/S0 in polar solvent makes the non-
radiative decay of S1 state prevail.[46] However, for NI-NH, the
fluorescence emission lifetime prolongs with increasing of the
solvent polarity (Figure S8b), e.g. the fluorescence lifetime
increased from 7.7 ns (in hexane) to 12.8 ns (in MeCN). These
results indicated that the bright state of NI-NH is with twisted
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intramolecular charge transfer (TICT) character,[45] especially in
polar solvents. It also indicates the solvent polarity-dependency
of the CT fluorescence bands in NI-TB and NI-NH is different, i.e.,
the ICT effect of two compounds is different. The photophysical
parameters of the compounds are summarized in Table 1 (refer
to Supporting Information, Table S1 for more information on the
fluorescence lifetimes).
was determined as 0.3 ps, and the charge recombination takes
ca. 3 ns to give the NI localized triplet state.
The ISC of the compounds was studied with pulsed laser
excited time-resolved electron paramagnetic resonance (TREPR)
spectroscopy (Figure 4). TREPR is an unique and powerful
spectroscopy to detect spin unpaired electrons and to study the
triplet state and electron spin polarization (ESP) dynamics.[51] It
has been used for study the ISC mechanisms,[21,52] as well as the
confinement of the triplet state wave function.[53,54]
Table 1. Photophysical properties of the compounds.
[e]
[f]
Solvent
[a]
abs
 [c]
em
ΦF
F
 [g]
[b]
[d]
NI-TB
HEX
369
384
382
1.78
1.85
1.77
431
500
563
0.84
0.28
0.02
8.0
6.5
21(1CT)
4.8(S0)
0 μs
22 μs
…
3NI*
0.03
0.02
0.01
0.00
-0.01
b
a
0.06
0.04
0.02
0.00
DCM
MeCN
578 μs
1.6
 = 46 s
(98%)
13.2
(2%)
NI-NH
HEX
408
427
433
1.06
1.13
1.19
476
502
522
0.61
0.52
0.58
7.7
9.1(1CT)
6.0(S0)
400
500
600
700
0
100
200
300
Wavelength / nm
Time / s
DCM
MeCN
11.2
12.8
Figure 3. Nanosecond transient absorption spectra of NI-TB. (a) the spectra
in absence of TEA and (b) corresponding decay trace at 480 nm. c [NI-TB] =
5.0 ×105 M in deaerated DCM. Excitation was realized with a nanosecond
pulsed laser at 355 nm. 25 C.
[a] HEX, DCM and MeCN stand for n-hexane, dichloromethane and
acetonitrile, respectively. The ET(30) values are n-hexane (31.0), DCM (40.7)
and MeCN (45.6). [b] The maximum absorption wavelength, in nm. [c]The molar
extinction coefficient at the absorption maxima, in 104 M1 cm1. [d] The
maximum fluorescence emission wavelength, in nm. [e] Fluorescence quantum
It is known that NI moiety itself shows efficient ISC (T
=
92%).[55] Thus we need to clarify that the ISC of NI-TB is not due
to this intrinsic ISC of NI moiety. TREPR spectroscopy is able to
discriminate paramagnetic states produced by different
mechanisms. Mostly, triplet states and radicals are observed with
TREPR spectroscopy. Here we observed triplet states, with
different magnitude of the interaction between the electrons. For
formation of triplet states, the spin-orbit coupling ISC (SO-ISC),
the radical pair ISC (RP-ISC), and the SOCT-ISC mechanisms,[16]
are able to produce a specific selective population rates of the
sublevels of the T1 state, either for the zero-field splitting (ZFS)
states or the high-field Zeeman sublevels, thus specific ESP
pattern may develop for different ISC mechanisms. As such the
ESP of the TREPR spectrum of the triplet states give information
on the triplet state formation mechanism. On the other hand, the
ZFS parameters D and E values obtained from the simulation of
yield, with anthracene as the standard (ΦF
fluorescence lifetime, in ns. [g] The dipole moment of the states.
= 0.30 in ethanol). [f] The
The singlet oxygen quantum yields () of NI-TB is solvent-
dependent (Supporting Information, Table S2). Highest value was
observed in DCM ( = 41%), moderate values were observed in
toluene ( = 21%) and THF ( = 22%),  is much smaller in
other solvents. In comparison, NI-NH shows negligible  values.
The triplet state formation was studied by nanosecond
transient absorption (ns TA) spectra of NI-TB (Figure 3). In the ns
TA transient absorption spectra, the typical absorption band of
3NI* state at 480 nm was observed and the triplet state lifetime
was determined as 46 s (in deaerated DCM). In aerated solution,
the lifetime was quenched to 0.7 s (Supporting Information,
Figure S12d). Due to the ground state bleaching band of NI-TB at
around 370 nm, which overlaps with the ESA band in this range,
the typical ESA band of the triplet state of NI moiety at 390 nm
was not observed for NI-TB, but a new ESA band at 600 nm was
observed for NI-TB as compared to NI-Br (Supporting Information,
Figure S11a). The charge separation (CS) upon photoexcitation
was confirmed with femtosecond transient absorption
spectroscopy (Supporting Information, Figure S17a), indicated by
the absorption band at 435 nm (NI radical anion absorption, NI.
It should be pointed out that the absorption of the cation may be
much weaker, or overlapped with the absorption of anion,
therefore no distinct cation absorption was observed in fs TA
spectra), subsequently, fs TA spectra show that a species with
ESA at 480 nm was produced, which is assigned to the NI triplet
state. With global fitting of the fs TA data, the CS time constant
3
the TREPR spectra are useful for discrimination of the CT and
the 3LE states,[51,5660] i.e. the confinement of the triplet state wave
functions.
The TR-EPR spectra of NI-Br and of NI-TB were studied at
80 K, in frozen solution (Figure 4). For NI-Br, a TREPR spectrum
with ESP pattern of eee/aaa was observed (Figure 4a). The
TREPR spectrum of NI-Br is composed of one species, and a
prevalent population of the X state (the Y state is also populated.
Supporting Information, Table S4). The ESP is that derived from
SO-ISC. The theoretical computations on NI-Br with the wave
function obtained by a HOMO-LUMO excitation (localized on the
NI plane) reproduce the experimental ZFS parameters very well
(Supporting Information, Table S4). For NI-TB, more complicated
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NI-TB
Z
X'Y'
Z'
Y
X
NI-TB
2 species simulation
3 species simulation
a
Simulation
Species 1
Species 2
Species 3
c
b
A
A
A
*
*
*
E
A
E
*
E
NI-Br
Simulation
E
2000
3000
4000
5000
2000
3000
4000
5000
2000
3000
4000
5000
Magnetic Field / G
Magnetic field / G
Magnetic field / G
Figure 4. The TR-EPR spectra of NI-Br and NI-TB obtained at about 1 s after a laser flash of 3  105 M solution in toluene/MeTHF (3:1, v/v) at 80 K. (a) NI-Br
upon photoexcited at 355 nm (3.5 mJ per pulse) and (b) NI-TB irradiated at 355 nm (3 mJ per pulse). The red lines are the simulation curves whose fitting parameter
are presented in Table 4. For NI-TB, the blue curve is the simulation with three species, and the red curve is the simulation with two species A+B). (c) The simulation
with three species curves of (b) and each species were presented, respectively.
TREPR spectrum was obtained (Figure 4b). Satisfactory
simulation can be obtained with the sum of three species; by
comparison with the calculation conducted for NI-NMe
(Supporting Information, Table S4 as a NI-TB monomer, D = 895
G), we assign species A (D = 795 G) as a triplet state localized on
one of the two NI units (likely, the 100 G D-reduction is due to a
perturbation from the other -system, or spreading of the spin
density to the bridge amino moiety). With respect to NI-Br, the
population of the substrates are rather different, with the X-state
the most populated one in NI-Br, and no population along Z
direction (PX : PY : PZ = 1 : 0.5 : 0), whereas for NI-TB, the Z-state
is the most populated one (PX : PY : PZ = 0.65 : 0 : 1.Supporting
Information, Table S4). This observation supports our hypothesis
that the population of the triplet state in NI-TB is not via a regular
SO-ISC, which shows a population mainly of the X-state (the
same holds for the NDI molecule). The RP-ISC mechanism can
be excluded as well, which selectively populates the high-field
Zeeman states and produces (for D > 0) a typical aee/aae
pattern.[55]
might be related to the ESA band at 600 nm in the ns TA spectrum.
The Z-values indicates it is similar to a distribution of naphthalene,
thus further localized. DFT calculation have proven a localization
for the T4 state. T4 is energetically accessible from the S1 state,
therefore, a possibility is that the observed triplet is a T4 state. At
low temperature the CR, or even the internal conversion of multi-
chromophore compounds can be slowed down.[60] No quintet
state spectrum was observed, indicating that the triplet state is
confined on one NI unit in NI-TB (the two NI units are not in triplet
excited state simultaneously).
The dihedral angle between the two NI units in NI-TB was
found to be 80 in the optimized ground state geometry (Figure
5a), which is similar to the X-ray crystal structure results reported
previously (89.17).[37] Thus in principle the SOCT-ISC can be
enhanced with this geometry. The NCH2N angle in the bridge
part was found to be 111.16  and the bond length of the NC
bond in the bridge was 1.463 Å in the optimized ground state
geometry, which are very close to those determined with the X-
ray diffraction of single crystal (111.84 and 1.463 Å,
respectively).[37] It indicates the basis sets and the function
(B3LYP/6-31G (d)) used herein in our study for the theoretical
computation are convincing.
We also observed a state with smaller ZFS parameters
(species B) for NI-TB, e.g. with a substantial smaller D-value (ca.
260 Gauss smaller than species A), which is compatible with the
formation of a CT state, characterized by a well distributed wave
function for both electrons, having more significant localization on
the NI unit, the other on the bridging nitrogen atoms, proposed by
the spin density surface of the radical anion and cations of NI-TB,
Figure 5). We calculated the average distance between the two
electrons under a point-dipole approximation, assuming that this
state originates from a CT state, thus with the distribution function
of the two electrons spatially separated. In this case we obtained
an average distance of 0.38 nm for the two spin unpaired
electrons, which is in good agreement of the bridging N
atomcentroid C atom of the NI core distance (0.38 nm. Based on
optimized ground state geometry). Involvement of the bridge
amino moiety in this triplet state was also supported by the
calculation of the D values of the CT state, the magnitude of the
calculated D values indicate that the species B is neither a triplet
state localized on one NI unit, nor fully delocalized CT state on
two NI units (see Supporting Information, Table S4). We are not
clear about the origin of the third species (C) at the moment. It
Figure 5. (a) Optimized ground state geometry of NI-TB showing the dihedral
angle between two moieties (80 ); (b) Isosurface of spin density of NI-TB at the
optimized triplet state (T1); Isosurface of spin density of (c) NI-TB radical anion
at the optimized doublet state (D0) geometry and (d) NI-TB radical cation at the
optimized doublet state (D0) geometry. Isovalues = 0.0004. The calculations
were performed at the (U)B3LYP/6-31G (d) level using Gaussian 09W.
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10.1002/chem.201905248
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RESEARCH ARTICLE
The triplet state spin density of NI-TB is mostly localized on
one of the NI moiety with a little delocalization on the bridge
(Figure 5b). It is in agreement with the ns TA spectra of NI-TB
(Figure 3a), for which the absorption of the NI triplet state is
different from that of NI-Br. The spin density of the triplet state of
Tröger’s base also spread to the bridge in the Tröger’s base. This
slightly delocalized 3LE state was also confirmed by the TREPR
spectrum of NI-TB (species A, with D and E values of 795 and 10
G, respectively. Note the D parameters is smaller than that of NI-
Br, D = 915 G). It should be noted that two degenerated triplet
states were observed with TDDFT computations, localized on the
two NI units, respectively. This is reasonable since the two units
are essentially the same.
Information, Figure S24). Then CS occurs (time constants are 0.3
or 0.53 ps, in DCM or ACN, respectively). The amino units and
the NI unit act as electron donor (Figure 5). The large dipole
moment of the S1 state of NI-TB also infer that there is charge
separation between the two NI units within the Tröger's base, thus
we propose a SBCT occurs for NI-TB. Then a slow CR (ca. 3.3
ns in DCM) produces the 3NI* state.
Applications of novel heavy atom-free PSs, for instance, the
radical-containing triplet PSs, orthogonal Bodipy dimers in PDT
have attracted much attention.[53,61] Herein we demonstrated that
Tröger’s base NI-TB shows efficient SOCT-ISC ( = 41% in
DCM). As such we explored its potential application in PDT
towards cancer cells (Figure 6). Confocal microscopy was
conducted after HeLa cells were incubated with Tröger’s base (NI-
TB) (0.11 µM) for 24 hours (Supporting Information, Figure S21),
which shows that the NI-TB can be uptakes by the HeLa cells.
Then the cell viabilities with and without photoirradiation were
determined using a standard MTT assay (Figure 6). Even at very
low concentrations of Tröger’s base, a significant decrease of the
cell viability was observed with a remarkable the lowest half
concentration of maximal inhibitory (EC50) value (5.0 nM). The
photocytotoxicity of the Tröger’s base was demonstrated. In
The spin density of NI-TB radical anion and cation were also
investigated (Figure 5c and 5d). For radical anion, most of spin
density is localized on two NI unit with a little part distributed on
the N atom of the bridge. It indicates the electron affinity ability of
NI unit. However, for radical cation, the spin density is delocalized
on the molecule with significant distribution on the bridge part.
This computation is in agreement with a previously reported
triarylamine-derived Tröger’s base, for which the spin of the
radical cation is symmetrically delocalized between the two
arylamine moieties (confirmed with EPR spectra).[47] Moreover,
the bridge nitrogen atoms contribute significantly to the spin
density. This theoretical prediction was also confirmed by the
observation of the 3CT state with TREPR (Figure 4b and
Supporting Information, Table S4). Moreover, either the CT with
the bridge amino moiety as electron donor, or the CT between the
two NI moieties, constitutes a SOCT feature because the
molecular orbitals involved in these CT processes are non-
coplanar.
140
No Light
EC50 = 5.0 nM
With Light
120
100
80
60
40
20
0
139 305
29 63
13
0
1.2 2.7 5.9
Concentration (nM)
Figure 6. MTT assay of NI-TB with different concentrations. Comparison of the
cell viability of HeLa cells pre-treated with increasing doses of NI-TB with (365
nm, 0.7 mW/cm2, 30 min, i.e. the irradiation dose is 1.26 J/ cm2) and without
light irradiation. 20 C.
comparison, no significant change was observed when the cells
were kept in the dark, in the presence of the same concentration
of the Tröger’s base (Black bars). Thus, our studies demonstrated
a Tröger’s base as a novel efficient singlet oxygen PS with a
potential applications in photodynamic therapy. In comparison,
the heavy atom-free Bodipy dimer triplet PSs show EC50 of 50
nM.[53] Concerning the EC50 value, NI-TB is also more potent than
the iodinated cyanine triplet PSs (EC50 = 25 M),[62] or brominated
Bodipy sensitizers.[63] The potent PDT activity of NI-TB may be
attributed to its tight binding ability with DNA.[32-34] Preparing of
Tröger’s base showing red and near IR light absorption is in
progress in our laboratory.
Scheme 2. Simplified Jablonski diagram illustrating the photophysical
processes of NI-TB. Note: The CT excited states denote the (NINI) excited
state. The energy levels of the 1CT excited state were approximated from the
blue edge of the fluorescence emission peak in different solvents. The energy
level of Tn excited states was obtained from the TDDFT calculation based on
the optimized geometry of the ground state, the assignment of the 3CT or 3LE
state of the Tn state is based on NTO orbital analysis. Some Tn states, include
T1 are degenerated, but not shown in the Scheme. The calculations were
performed at B3LYP/6-31G (d) level using Gaussian 09W.
The photophysical processes of Tröger's base NI-TB are
summarized in Scheme 2. Upon photoexcitation, the S1 state is
populated (note this is a mixed 1NI/1CT state, based on the
UVVis absorption, and the TDDFT computations, Supporting
Conclusion
In conclusion, we observed efficient intersystem crossing (ISC)
for a Tröger’s base, which is derived from naphthalimide (NI)
chromophore, and the unique structure of Tröger’s base is
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10.1002/chem.201905248
Chemistry - A European Journal
RESEARCH ARTICLE
region. The ZFS principal directions and principal values were calculated
proposed as a new molecular structural platform for design heavy
atom-free triplet photosensitizers. Solvent polarity-dependent
singlet oxygen (1O2) photosensitizing was observed, with
quantum yields () of 8%  41%. With steady state and
femtosecond/nanosecond transient absorption spectroscopies,
as well as pulsed laser excited time-resolved electron
from
a simple HOMO-LUMO excitation under the point dipole
approximation,[6466] where the molecular orbital wave functions were
computed by the AM1 method, as implemented in the MOPAC code.[67]
Acknowledgements
paramagnetic
resonance
(TREPR)
spectroscopy,
we
demonstrated that the ISC mechanism is the spin orbital charge
transfer intersystem crossing (SOCT-ISC). Conventional electron
donor/acceptor dyads showing SOCT-ISC are with semi-rigid
linker between the electron donor and acceptor units, very often
torsion (rotation) about the linker is not fully inhibited, which is
detrimental to the SOCT-ISC efficiency; moreover, the electron
donor and acceptor moieties are usually different. Different from
those conventional compact donor/acceptor dyads, two same
chromophores are used in Tröger’s base (one is electron donor
and another as electron acceptor), and the linker between the two
chromophores in Tröger’s base is fully rigid, torsion about the
linker is completely inhibited. This conformation restriction is
beneficial for attaining efficient SOCT-ISC. UVVis absorption
spectroscopy shows a very weak -conjugation between the N
atom of the amino moiety with NI unit in NI-TB, due to the
molecular conformation restriction. Fluorescence study shows
that a highly polar emissive charge transfer state (CT) was formed,
formed via the symmetry-breaking charge transfer. Nanosecond
transient absorption spectra indicated the formation of triplet state
(T1 state lifetime is up to 46 s). Both localized 3NI state and
delocalized 3CT state were observed in TREPR spectra. The 3NI
state of the Tröger’s base shows different sublevel population
rates (px : py : pz = 0.65 : 0 : 1) as compared to that of 4-
bromonaphthalimide (px : py : pz = 1 : 0.5 : 0), for which the ISC is
via the normal spin-orbit coupling (SO-ISC). Photodynamic
therapy studies with HeLa cells was carried out, high phototoxicity
was observed for the Tröger’s base (EC50: 5.0 nM). Our results
show that the Tröger’s bases can be developed as a novel
molecular structural platform for heavy atom-free triplet
photosensitizers showing efficient ISC.
J. Z. thanks the NSFC (21673031, 21761142005, 21911530095
and 21421005), the Fundamental Research Funds for the Central
Universities (DUT2019TA06) and State Key Laboratory of Fine
Chemicals (ZYTS201901) for financial support. The support from
Dipartimento di Scienze Chimiche, Università degli Studi di
Padova for academic visit of J. Z. is also gratefully acknowledged.
Keywords: Intersystem Crossing • Naphthalimide • Triplet state
• Tröger’s base • Electron Spin Polarization • Photodynamic
therapy
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Entry for the Table of Contents
RESEARCH ARTICLE
Tröger’s Base as potent triplet
photosensitizer! We discovered
efficient intersystem crossing in
Tröger’s base, a novel molecular
structural motif for attaining efficient
spin orbital charge transfer ISC
(SOCT-ISC). The fully rigid orthogonal
structure is beneficial for efficient
SOCT-ISC ( = 41%). The ISC
mechanism was confirmed with time-
resolved electron paramagnetic
resonance spectroscopy (TREPR).
Yingjie Zhao, Kepeng Chen, Elif
Akhuseyin Yildiz, Shujing Li, Yuqi Hou,
Xue Zhang, Zhijia Wang, Jianzhang
Zhao,* Antonio Barbon,* Halime Gul
Yaglioglu* and Huijian Wu*
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Efficient Intersystem Crossing in
Tröger’s Base Derived From 4-Amino-
1,8-Naphthalimide and Application as
Potent PDT Reagent
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