Long-Lived Charge-Transfer State Induced by Spin-Orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in a Compact Spiro Electron Donor/Acceptor Dyad

Article abstract of DOI:10.1002/anie.202003560

We prepared conceptually novel, fully rigid, spiro compact electron donor (Rhodamine B, lactam form, RB)/acceptor (naphthalimide; NI) orthogonal dyad to attain the long-lived triplet charge-transfer (³CT) state, based on the electron spin control using spin-orbit charge transfer intersystem crossing (SOCT-ISC). Transient absorption (TA) spectra indicate the first charge separation (CS) takes place within 2.5 ps, subsequent SOCT-ISC takes 8 ns to produce the ³NI* state. Then the slow secondary CS (125 ns) gives the long-lived ³CT state (0.94 μs in deaerated n-hexane) with high energy level (ca. 2.12 eV). The cascade photophysical processes of the dyad upon photoexcitation are summarized as ¹NI*→¹CT→³NI*→³CT. With time-resolved electron paramagnetic resonance (TREPR) spectra, an EEEAAA electron-spin polarization pattern was observed for the naphthalimide-localized triplet state. Our spiro compact dyad structure and the electron spin-control approach is different to previous methods for which invoking transition-metal coordination or chromophores with intrinsic ISC ability is mandatory.

Full text of DOI:10.1002/anie.202003560

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
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Accepted Article
Title: Long-Lived Charge Transfer State Induced by Spin-Orbit Charge
Transfer Intersystem Crossing (SOCT-ISC) in a Compact Spiro
Electron Donor/Acceptor Dyad
Authors: Dongyi Liu, Ahmed M. El-Zohry, Maria Taddei, Clemens
Matt, Laura Bussotti, Zhijia Wang, Jianzhang Zhao, Omar F.
Mohammed, Mariangela Di Donato, and Stefan Weber
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202003560
Link to VoR: https://doi.org/10.1002/anie.202003560

Angewandte Chemie International Edition
10.1002/anie.202003560
RESEARCH ARTICLE
Long-Lived Charge Transfer State Induced by Spin-Orbit Charge
Transfer Intersystem Crossing (SOCTISC) in a Compact Spiro
Electron Donor/Acceptor Dyad
[a]
[b]
[c]
[d]
[c]
[a]
Dongyi Liu, Ahmed M. El-Zohry, Maria Taddei, Clemens Matt, Laura Bussotti, Zhijia Wang,
[a]
[b]
[c],[e]
and Stefan Weber*[d]
Jianzhang Zhao,* Omar F. Mohammed,* Mariangela Di Donato*
Abstract: We prepared conceptually novel, fully rigid, spiro compact
electron-transfer processes takes place in the reaction center
upon photoexcitation of the light-harvesting antenna, leading to
the formation of a final CT state at ca. 0.5 eV, that lives up to a
few seconds.[5a,7,8] Molecular systems that exhibit such long-lived
CT states are of fundamental importance for artificial
electron donor (Rhodamine B, lactam form)/acceptor (naphthalimide)
3
orthogonal dyad to attain the long-lived triplet charge transfer ( CT)
state, based on the electron spin control using spin-orbit charge
transfer intersystem crossing (SOCTISC). Transient absorption (TA)
spectra indicate the first charge separation (CS) takes place within 2.5
photosynthesis, photovoltaics, molecular electronics/photonics
3
[913]
ps, subsequent SOCTISC takes 8 ns to produce the NI* state. Then
and photochemistry studies.
Controlling electron-transfer is
3
[1416]
the slow secondary CS (125 ns) gives the long-lived CT state (0.94
also pivotal for fluorescent molecular sensors
and charge
17]
s in deaerated n-hexane) with high energy level (ca. 2.12 eV). The
recombination (CR) induced persistent luminescence.[ Various
donor(D)/acceptor(A) polyads mimicking natural photosynthesis
have been reported in the past years to generate long-lived CT
states.[18,19] To this scope, it is necessary to attain large electronic
coupling and efficient CS between adjacent electron D and A in
each step, and weak electronic coupling between primary
electron D and terminal electron A (a large distance between the
two units is required) to retard CR process. However, very often
stepwise electron transfer produces CT states with rather low
cascade photophysical processes of the dyad upon photoexcitation
1
1
3
3
are summarized as NI* CT NI* CT. With time-resolved
electron paramagnetic resonance (TREPR) spectra, an EEEAAA
electron-spin polarization pattern was observed for the naphthalimide-
localized triplet state. Our spiro compact dyad structure and the
electron spin-control approach is different as compared to previous
methods for which invoking transition-metal coordination or
chromophores with intrinsic ISC ability is mandatory. This new method
3
of accessing long-lived CT states is useful for artificial photosynthesis, energy level.
photovoltaics, photocatalysis and fundamental photochemistry
studies.
The non-adiabatic CT rate kCT depends on GCT, and kCT
should decrease as the driving force (GCT) increases in the so-
called Marcus inverted region, i.e. higher CT state energy level
usually leads to long-lived CT states.[
3,5a,20]
However, it is not
always reliable to obtain a long-lived 1CT state based on the
Marcus inverted region effect with the classic Marcus equations,
because the electron transfer (or CR) rate constants may be much
larger than those theoretically predicted in the Marcus inverted
Introduction
Mimicking the natural photosynthetic reaction centers with the
aim to attain a CT state with high energy level and long lifetime
has attracted much attention.[16] In photosynthesis, a cascade of
[5a]
region, due to the quantum tunneling effect.
1
3
The lifetime of a CT state is usually shorter than that of a CT
state because 1CTS is a spin-allowed transition. 3CT state
0
usually longer ifetime thatn 1CT state, enabling the so-called
electron spin control effect. Radical pair intersystem crossing (RP
[
a]
b]
D. Liu, Dr. Z. Wang and Prof. J. Zhao
State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology, 2 Ling Gong Road,
Dalian 116024, China.
ISC) mechanism can be used as an electron spin control method,
in which the probability of 1CT3CT transition increases, thus
prolonging the lifetime of the CT states. RP ISC accounts on
hyperfine interactions (HFI) to facilitate ISC in electron D/A
dyads/polyads with an extremely small electronic coupling (< 0.1
E-mail: zhaojzh@dlut.edu.cn (J. Z.)
[
Dr. A.M. El-Zohry and Prof. O.F. Mohammed
Division of Physical Sciences and Engineering, King Abdullah
University of Science and Technology (KAUST), Thuwal 23955-
1
cm , so that rigid and long bridges are required to connect
electron D and A).[
5a,21]
However, a small energy gap between CT
1
6900, Kingdom of Saudi Arabia.
E-mail: omar.abdelsaboor@kaust.edu.sa (O. F. M.)
3
3
1
and CT states may lead to backward electron transfer CT CT,
and the 1CT CT transition (several nanoseconds) is usually
3
[c]
[d]
[e]
M. Taddei, L. Bussotti and Prof. M. Di Donato
LENS (European Laboratory for Non-Linear Spectroscopy), via N.
Carrara 1, 50019 Sesto Fiorentino (FI), Italy.
1
[5a]
slower than CTS
0
decay. These issues cause short-lived
CT states.
E-mail: didonato@lens.unifi.it (M. D. D.)
A possible alternative to attain long-lived CT states is to use the
electron spin control approach to initiate the CS with a triplet state
precursor.[2224] One possibility is to use a transition-metal
C. Matt and Prof. S. Weber
Institute of Physical Chemistry, Albert-Ludwigs-Universität Freiburg,
Albertstr. 21, 79104 Freiburg, Germany.
coordination framework to attach the electron D and A because
the coordination center has usually a ultrafast ISC, through a
E-mail: Stefan.Weber@physchem.uni-freiburg.de (S. W.)
ICCOM-CNR, via Madonna del Piano 10, 50019 Sesto Fiorentino
1
3
3
[22,24]
MLCT MLCT CT process.
For instance, a long-lived
(FI), Italy.
3
CT state (ca. 1 s) was observed in a N^N Pt (II) bis(acetylide)
Supporting information for this article is given via a link at the end of
the document.
complex and the formation of a spin-correlated radical pair (SCRP)
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Angewandte Chemie International Edition
10.1002/anie.202003560
RESEARCH ARTICLE
was confirmed by TREPR.[22] These molecular systems are
synthetically demanding, because the -conjugation frameworks
of electron D and A must be isolated from the coordination center,
to reduce the electronic coupling between electron D and A and
avoid the shortening of the 3CT state lifetime (heavy-atom
effect).[5a,6,22] Another strategy is based on the intrinsic ISC ability
of the electron D or A (e.g. anthraquinone (Aq)-D linked systems)
The use of a short rigid linker will reduce the reorganization
energy  of electron transfer, which is beneficial for fast CS and
for attaining long-lived CT state.[
5a,7]
However, this also makes the
electron exchange energy (J) large, ingreasing the 1CT - 3CT
3
1
energy gap (2J), and inhibiting CT CT process (by HFI). For
conventional electron D/A dyads, the very small J value makes
the 3CT/1CT energy levels very similar, promoting the reverse
1
3
3
[5b,25]
3
1
3
[29]
to establish LE LE CT, without any heavy atoms.
In a
CT CS which drains CT.
In our method, SOCTISC is
3
1
3
reported cyclopeptide-derived spiro electron D/A system, the CT
lifetime is 3.35 s in THF.[5b] The molecular structure is quite
complicated and this method is restricted to a limited number of
chromophores exhibiting intrinsic ISC with high yield. Thus, it is
still a major challenge to construct simple, compact electron D/A
supposed to render CT NI* process fast, and the close
proximity of NI and the RB units to increase the J value, thus
suppressing the undesired backward CT NI* CT processes.
3
3
1
3
compact dyads to access long-lived CT states.
Herein, we report a new electron spin control approach, based
on the spin orbit charge transfer ISC (SOCTISC) mechanism, to
3
obtain
temperature),
a
long-lived
CT state (0.94 s, at room
[
22,2628]
using a novel, simple, fully rigid, spiro
electron D (lactam form Rhodamine B, RB)/A (naphthalimide, NI)
compact dyad to induce orthogonal orientation and a fully rigid
connection between RB and NI units. In our case, SOCTISC will
3
1
3
3
generate NI* state ( CT NI* transition). NI* is the precursor to
3
form a long-lived CT state via charge separation, achieving
electron spin control. As demonstrated by steady-state and time-
resolved optical and magnetic resonance spectroscopies, the
proposed photophysical processes of RB-NI dyad are
1
1
3
3
NI* CT NI* CT. Our strategy will be useful for designing
3
simple, compact electron D/A dyads showing long-lived CT state
with high energy level.
Results and Discussion
Molecular Structure Design Rationale
3
[6]
Inspired by high energy LE state of NI (ca. 2.20 eV), we
3
selected NI moiety as electron A for formation of a low-lying CT
Figure 1. (a) Molecular structures of rhodamine-naphthalimide derivatives and
reference compounds. ORTEP view of the molecular structures determined with
Single crystal X-ray diffraction of the compounds of (b) RB-NI and (c) RB-NI-N;
hydrogen atoms and solvents are omitted for clarity. The xanthene and NI
moieties are highlighted in pink and yellow color, respectively. Thermal
ellipsoids are at 50% probability.
state. Spiro rhodamine with high energy excited states will not be
involved in dyad’s photophysical processes. The strutrues of the
dyads and reference compounds are shown in Figure 1a. The
highly constrained conformation of RB-NI will reduce -
conjugation between the N atom and NI moiety (the -conjugation
frameworks are separated by three -bonds), which is crucial for
1
NI moiety to maintain its high T energy and strong electron
Single crystals of RB-NI and RB-NI-N were obtained by slow
evaporation of the solutions of the dyads (DCM/n-hexane for RB-
withdrawing ability. Spiro rhodamine is linked with the NI moiety
at the amide-position in the reference dyad RB-NI-N, note for the
dyad RB-NI the link is at the 4-amino position of the NI moiety.
For RB-NI-N, less conformational restriction is expected (refer to
the potential energy curves in Supporting Information, Figure
S14f). Hence, -conjugation between the N atom and the NI’s -
framework is significant for RB-NI-N, which will decrease the
energy level of 3LE below that of the CT state. The reduced
electron withdrawal ability of the NI moiety in RB-NI-N will
promote the energy level of the CT state, as compared to that of
RB-NI (Supporting Information, Table S5). Recently the
NI, CHCl
dyads were thus characterized by single-crystal X-ray diffraction
XRD; Figure 1, Supporting Information, Table S1 and S2). RB-
3
/n-hexane for RB-NI-N). The molecular structures of the
(
NI crystallized in the triclinic crystal system in the Pī space group
with two molecules in a unit cell and RB-NI-N crystallized in the
monoclinic crystal system in the P2
angle between xanthene and NI moieties is 70.8 in RB-NI and
6.7 in RB-NI-N (Supporting Information, Figure S13b), the
1
/n space group. The dihedral
6
former being more perpendicular than predicted by density
functional theory (DFT) at the B3LYP/631G(d) level (60.4 for
aggregation induced luminescence property of RB-NI was
reported, but the charge separation was not studied.[28b]
RB-NI, 67.6 for RB-NI-N, Supporting Information, Figure S14a
30]
and
b).[
These
geometries
are
beneficial
for
SOCTISC.[
26,27,3134]
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Angewandte Chemie International Edition
10.1002/anie.202003560
RESEARCH ARTICLE
Table 1. Photophysical Parameters of the Compounds.
[
a]
 [b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
Compounds

abs (nm)

F
(nm)


(%)

F
(%)

F
(ns)

P
(nm)

P
(ms)

T
[
[
[
l]
[l]
[l]
[l]
[l]
[l]
NI-NH
2
390
408
308
1.03
1.06
500
480
410
545
578
67.2
67. 5
1.2
8.6
7.5
4.7
l]
l]
NI-NH
RB-Ph
RB-NI
490
20.3
[l]
[l]
1.35
319/342
427
1.99/1.44
1.15
29.8
24.5
1.2
15.7
18.4
556/604
606
20.0
36.3
125 ns [j] / 935 ns [k]
50.5 s
RB-NI-N
10.7
[
a] UVvis absorption maxima in n-hexane, c = 1.0 105 M, 25 C. [b] Molar absorption coefficient at absorption maxima, : 104 M1 cm1. [c] Maximal fluorescence
emission wavelength in n-hexane, absorbance A = 0.113, 25 C. [d] Singlet oxygen quantum yield in n-hexane, Ru(bpy) [PF was used as standard ( = 57% in
3
]
6 2

[
35a]
ACN).
1
[e] Absolute photoluminescence quantum yield in n-hexane, determination error: 0.1%, A = 0.10, 25 C. [f] Luminescence lifetime in n-hexane, c = 1.0 

5
−5
0
M, ex  340 nm. [g] Maximal phosphorescence wavelength in n-hexane/iodoethane (4:1, v/v), c = 5.0 × 10 M, 77 K. [h] Phosphorescence lifetime in n-
−5
3
hexane/iodoethane (4:1, v/v), c = 5.0 × 10 M, 77 K. [i] Triplet state and CT state lifetimes, determined with nanosecond transient absorption spectroscopy. In n-
hexane. [j] Growth kinetics of the transient at 425 nm in the nanosecond transient absorption spectra. [k] Decay kinetics of the transient at 425 nm in the nanosecond
transient absorption spectra. [l] Not observed.
0
0
0
0
0
0
.25
.20
.15
.10
.05
.00
2.5
2.0
1.5
1.0
reduces the -conjugation of the N atom with NI moiety in RB-
NI.
Compared to reference compounds, the fluoresence of NI
moiety in RB-NI is strongly quenched (Figure 2b and Table 1).
The residual fluorescence of NI moiety in RB-NI is centered at
a
[28a]
b
NI-NH
2
NI-NH2
NI-NH
RB-Ph
RB-NI
NI-NH
RB-Ph
RB-NI
RB-NI-N
RB-NI-N
400 nm (Figure 2b and 2c), which is shorter in wavelength and
0
.5
.0
much weaker than that of NI-NH . This blue-shifted fluorescence
2
emission is an indication of the limited -conjugation between the
amino N atom and NI moiety. We propose that quenching of the
NI fluorescence in RB-NI is due to photo-induced electron transfer
between RB and NI moieties, which is supported by
cyclicvoltammetry studies (Supporting Information, Figure S16,
Table S5) and the analysis of frontier molecular orbitals of RB-NI
0
3
00 350 400 450 500 550
400 500 600 700 800
Wavelength / nm
Wavelength / nm
1
.5
1.2
c
NI-NH RB-NI
d
NI-NH2
RB-Ph
NI-NH
RB-NI
RB-NI-N
0.9
1
0
0
.0
.5
.0
RB-NI-N

(
Supporting Information, Figure S14c). The weak broad CT
emission band of RB-NI and RB-NI-N are at around 545 nm and
78 nm (Figure 2c), respectively. The photophysical parameters
of the dyads are listed in Table 1. The fluorescence of RB-NI-N is
stronger ( = 10.7%) than that of RB-NI ( = 1.2%), but it is
much weaker than that of NI-NH ( = 67.5%). This result
0.6
0.3
0.0
5
F
F
4
00 500 600 700 800
500
600
700
F
Wavelength / nm
Wavelength / nm
indicates that electron transfer is also possible for RB-NI-N.
Temperature-dependent luminescence of RB-NI were measured,
however, thermal activated delayed fluorescence did not occur
Figure 2. (a) UVvis absorption spectra of the compounds in n-hexane, c = 1.0

5

10 M. (b) Fluorescence emission spectra (optically-matched solutions were
used, A 0.113, ex 320 nm) and (c) the corresponding normalized
fluorescence emission spectra of the compounds in n-hexane, 20 °C. The
=

(
Supporting Information, Figure S15).
asterisk in (c) represents the Raman scattering peak of the solvent at 354 nm.
Phosphorescence spectra of the dyads at 77 K were
(
d) Normalized phosphorescence spectra of the compounds RB-NI (ex  340
measured (Figure 2d, iodoethane was added to induce external
heavy atom effect, which will be helpful on enhance the
phosphorescence). The phosphorescence lifetimes are
nm), NI-NH (ex  340 nm) and RB-NI-N (ex  425 nm) at 77 K, in mixed solvents
of n-hexane/iodoethane (4:1, v/v), c = 5.0 × 10 M.
−5
3
presented in Table 1. The energy levels of the NI* state of RB-NI
and RB-NI-N can be approximated as 2.21 eV and 2.10 eV,
respectively. The energy of the CT state of the dyads can be
evaluated based on electrochemical data (Supporting Information,
Table S5).
UVVis absorption and Luminescence Spectra
The steady-state UVvis absorption spectra of the compounds
were studied (Figure 2a). For RB-NI with its nearly orthogonal
conformation, we expect weak electronic coupling between
electron D and A. The absorption band of RB-NI (342 nm) is blue-
Femtosecond, Subnanosecond and Nanosecond Transient
Absorption
Spectra:
Charge
Separation,
Charge
2
shifted as compared to that of NI-NH (390 nm) and RB-NI-N (427
Recombination and Intersystem Crossing
nm). We attribute this blue shift to the electron-deficient nature of
the N atom at 4-position of NI moiety, which is connected to the
carbonyl group (amide). The molecular conformation also plays a
role in this respect, because constrained molecular geometry
To verify the photophysical processes involved in the dyads
upon pulsed laser excitation, we measured transient absorption
(TA) spectra of RB-NI, RB-NI-N and reference compounds.
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Angewandte Chemie International Edition
10.1002/anie.202003560
RESEARCH ARTICLE
0
0
0
.15
.12
.09
0.15
0.12
0.09
0.06
0.03
0.00

0
0
0
0
0
0
.15 NI
a
b
c
1
2
.5 ps ( NI*)
.12

425 nm
RB
1
1
1 ns ( CT)
NI*
 1.5 ns
.09
0
.5 ps
.
..
540 nm
.06
1.5 ns
0.06
.03
0.03
.00
0.00
500
600
700
400
500
600
700
0.1
1
10
100 1000
Wavelengthnm
Wavelength / nm
Time / ps
0
0
.12

0.04
.03
NI
d
e
f
0.04
0.03
0.02
0.01
0.00
.09
1
140 ns
1
1
1
3.1 ns ( CT)
1
4
0.6 ns
3.4 ns
3
0
95.5 ns ( NI*)
0.06
.03
3
98.0 ns ( CT)
0.02
0.01


0
RB

229 ns

8 ns
0.00
0.03
3
NI*
0.00
-
4
00 500 600 700 800 900
0
100 200 300 400
400 500 600 700 800 900
Wavelength / nm
Wavelength / nm
Time / ns
0
.25
.20

NI


NI
0
0
0
0
0
0
.25
.20
.15
.10
.05
.00
0.25
.20
g
h
i
0
0
3
06.0 ns
..
0.31 s

 125 ns
0.15
0.10
0.05
0.00
-0.05
0.15
.
...
0.0 ns
4.26 s
0.10
0.05
0.00
 935 ns
3
NI*
-
0.05
4
00 500 600 700 800
Wavelength / nm
400 500 600 700 800
Wavelength / nm
0
2
4
6
8
10
Time / s
Figure 3. Transient spectra of RB-NI. (a) Femtosecond transient absorption spectra, color code goes from red to blue covering the time interval from 0.5 ps to1.5
ns, (b) EADS obtained from global analysis and (c) decay kinetics at 425 nm and 540 nm; spectra were recorded up to 1.5 ns, ex  400 nm, in deaerated toluene.
(d) Sub-nanosecond transient absorption spectra, (e) decay trace at 434 nm and (f) species-associated difference spectra (SADS) obtained from target analysis.

ex  350 nm, in toluene. (g) Nanosecond transient absorption spectra and (h) the subsequent decay of the ESA band at 425 nm at longer delay times.(i) Evolution
kinetics at 425 nm (derived from g and h); in deaerated n-hexane, excited with nanosecond pulsed laser, ex  350 nm, c = 4.0  10 M. 20 °C.
5
Femtosecond TA (fs TA) spectra of NI-Br in toluene shows that
ISC occurs within 30 ps (Supporting Information, Figures S19),
similarly to what reported for non-brominated NI, with ISC time
concomitantly (Supporting Information, Figure S24c). These
spectral changes are attributed to the intermolecular electron
transfer, with TEA as sacrificial electron D and NI-Br as electron
[35b]

constants of 10  20 ps.
the NI state and the NI radical anion (NI ), the nanosecond TA
In order to obtain the absorption of
A, which generates NI radical anion (NI ). The apparently longer
3

CS and longer lifetime of NI absorption for NI-Br/TEA mixture
are reasonable, considering that an intermolecular diffusion-
controlled electron transfer occurred.
(
ns TA) spectra of NI-Br in the absence and presence of sacrificial
electron donor trimethylamine (TEA) were recorded (Supporting
Information, Figure S24). In the absence of TEA, two excited state
absorption (ESA) bands at 380 nm and in the range of 400600
For RB-NI, the evolution of fs TA spectra is presented in
Figure 3a. With global analysis,[36] the evolution-associated
difference spectra (EADS) were obtained (Figure 3b; there are
3
nm are observed for NI-Br, assigned to NI state. The triplet state
1
1
lifetime in deaerated solution is 30.0 s (Supporting Information,
only two species, NI* and CT states; the comparison of the fit
obtained using two-component and three-component models for
the TA data of the RB-NI in toluene are shown in Supporting
Information, Figure S20, demonstrating that a third component
3
Figure S24b). In the presence of TEA, the ESA bands of the NI
state at 485 nm and 380 nm decay faster ( = 6.3 s, Supporting
Information, Figure S24d). A new ESA band at 422 nm grows
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3
significantly improves the fit). Upon pulsed laser excitation at 400
nm, a broad excited state absorption (ESA) band in the range of
band in the range of 480–650 nm which is attributed to NI* state
of RB-NI (Figure 3g). Thus the formation of the final transient
species of RB-NI is driven by the 3NI precursor ( NI* CT
transition) and it is long-lived (0.94 s in deaerated n-hexane,
Figure 3i). Its lifetime was ca. 0.16 s in aerated n-hexane
(Supporting Information, Figure S22c) and 0.62 s in deaerated
toluene (Supporting Information, Figure S23c). In other polar
solvents, the lifetime is further shortened (Supporting Information,
3
3
6
00750 nm is visible, similar to that of NI-NH
2
(Supporting
1
Information, Figures S18a), and assigned to NI* state. Within 2.5
ps (Figure 3b), we observe the growth of two sharp peaks at 425
nm and 540 nm. The decay kinetics at 425 nm and 540 nm (Figure
3c) are similar and do not show complete recovery on the
nanosecond timescale, thus implying the possibility of slow ISC;
a more accurate estimate of the last time constant (>1.5 ns in
Figure 3b) is presented in Figure 3d). Based on
spectroelectrochemical data (Supporting Information, Figure S17)
and ns TA spectra of NI absorption (Supporting Information,
3
Table S6). No signal was observed in CH CN, in agreement with
fs TA spectra (Supporting Information, Figures S21). Similarly, in
a reported cyclopeptide derived electron D/A dyad, the CT state
lifetime shows strong solvent polarity dependency.[22] The
decreased CT state lifetime was attributed to the energy gap law,[6]
because the CT state is better stabilized in more polar media.
Conversely, the 3LE state is usually not sensitive to solvent
polarity, and the effect of the electron spin inhibition on the decay


Figure S24c), the ESA band at 425 nm can be assigned to NI


absorption and the RB radical cation (RB ) is at around 540 nm.
To better understand the photophysics of RB-NI, fs TA
measurements were repeated in acetonitrile (Supporting
Information, Figures S21). CS in this polar solvent is extremely
fast, occurring within less than 1 ps (Supporting Information,
Figures S21b). CR is also fast, occurring in less than 100 ps, thus
implying that the triplet state is not populated.
kinetics of the T
1
state is more significant than the energy gap law.
3
Thus, the final transient species observed for RB-NI is a CT state,
3
not a LE state.
To exclude the possibility of intermolecular electron transfer,
which is a diffusion-controlled process possibly resulting in slow
highly viscous solvent,
polydimethylsiloxane (PDMS, viscosity  = 0.48  0.029 Pas,
Sub-ns TA spectra for RB-NI were recorded (Figure 3d) to
evaluate the kinetics of ISC process. A target analysis with a
CS/CR kinetics, we used
a
1
3
model of reversible excited-state processes related to CT NI*
3
3
[36]
4
and NI* CT was applied (Figure 3f). However, there are no
much higher than n-hexane,  = 3.3  10 Pas) to study the slow
3
3
evidences for the reverse processes, e.g. CT NI*, via target
analysis. The initial spectrum shows a sharp ESA band at 425 nm,
a moderate band at around 540 nm and a broad, weak ESA band
CS process. In this case, a similar kinetic is observed for the
growth of NI absorption at 425 nm (101 ns). The decay of the
band peaked at 425 nm was ca. 0.94 s (Supporting Information,
Figure S25c), similar to that in deaerated n-hexane (Figure 3i).
Thus, the slow CS and CR kinetics in deaerated n-hexane are not
due to intermolecular electron transfer. Weak electronic coupling
between the electron D and A, and a small driving force may result
in slow CS and CR.[5a,6,37] The slow CS of RB-NI (125 ns in
deaerated n-hexane, Figure 3i) suggests a weak electronic
coupling between the electron D and A. The small CS driving
force ( NI/ CT states energy gap) may also contribute to the slow
CS.[6] Recently, Zn porphyrin unit was connected with rectilinear
rigid oligo-p-xylene bridge with fullerene (C60) unit, it was found
the efficiency and the kinetics of CS/CR are highly dependent on
the electronic coupling magnitude between the donor and
acceptor (CT state lifetime: 0.56 s).[37b]
1
at 700900 nm, attributed to CT state (Figure 3d and 3f, it is very
similar to that obtained on fs TA spectra). The decay at around
3
4
25 nm is accompanied with the formation of NI* state, indicated
by the appearance of two ESA bands at 447 nm and 570 nm (red
line with circle symbols in Figure 3f). The blue line with triangle
symbols in Figure 3f shows a sharp ESA band at around 425 nm
and a broad negative band at around 600 nm that can be
assigned to stimulated CT emission. The decay kinetic at 434 nm
3
3
(
Figure 3e) shows that the 1CT state is formed rapidly upon
3
photoexcitation, then the formation of NI* state occurs via
SOCTISC with timescale ca. 8 ns ( CT NI*).
1
3
The ns TA spectra of RB-NI recorded upon pulsed laser
excitation at 350 nm were measured. We observed the growth of
an ESA band at 425 nm, assigned to the NI absorption (Figure
3
g),[6,5b,22] concurrently with the decrease of a broad weak ESA
0
0
0
0
.06
.04
.02
.00
0.04
a
0.04
0.02
b
c
1
CT 1.5 ns
...
0.03
0.5 ps
1
0
.00
0.02
-0.04
400
0.02
0.01
NI*
501 nm
1
2
1
>
1.5 ps ( NI*)
-
1
.2 ns ( CT)
3
560 nm
10
1.5 ns ( NI*)
-
0.02
0.00
4
00
500
600
700
500
600
700
0.1
1
100 1000
Wavelength / nm
Wavelength / nm
Time / ps
Figure 4. (a) Femtosecond transient absorption spectra of RB-NI-N, color code goes from red to blue covering the time interval from 0.5 ps to1.5 ns, (b) EADS of
RB-NI-N obtained from global analysis and (c) decay kinetics at 501 nm and 560 nm; in deaerated toluene, 20 C.
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In the fs TA spectra of RB-NI-N, we observe a broad ESA
band with two peaks at about 450530 nm and 530750 nm
Z' Y X
X'Y'
Z
(
black line in Figure 4b), and a ground state bleaching (GSB) band
centered at 430 nm. The ESA band intensified substantially within
0 ps, undergoing a change in shape, thus showing a single broad
2
50 ns
NI-Br
RB-NI
2
280 K
A
E
peak at around 550 nm (Figure 4b). The trace at 560 nm decays
faster than that at 501 nm (Figure 4c), indicating the conversion
5
2
00 ns
30 K
1
from the initially excited singlet ( NI*) state towards a CT state.
500 ns
180 K
The NI band is invisible in this case and it may be obscured by
RB-NI-N
the GSB band. Within 1 ns, the ESA band undergoes further
evolution, decreasing in intensity and changing again to a double-
peak structure (Figure 4b). On the same timescale the negative
peak also increases, possibly due to the recovery of NI signal at
a similar spectral position but with opposite sign. The spectrum of
the long-lived component (Figure 4b, blue line with triangular
marks) is assigned to the triplet state of the NI moiety. From global
analysis we can thus estimate that the initial charge separation
500 ns
80 K
a
2
b
50 300 350 400 450
200 250 300 350 400 450 500
B / mT
B / mT
Figure 6. (a) TREPR spectra recorded at 80 K in frozen toluene/MeTHF solution,
after pulsed laser excitation (355 nm for NI-Br and RB-NI, and 427 nm for RB-
NI-N; the laser pulse length ∼35 ns, 1.5 mJ per pulse; integration time window
is 500  700 ns after the laser pulse). The red curves are simulations of the
experimental TREPR spectra (black curves). (b) TREPR spectra of RB-NI in the
liquid crystal E7 (the glass transition temperature is 210 K) recorded at different
temperatures after pulsed laser excitation (355 nm, ∼35 ns, 1.5 mJ per pulse).
occurs in RB-NI-N within about 20 ps and the timescale of the
charge recombination induced ISC is about 1 ns.[36]
The triplet state formation of RB-NI-N were also studied with
ns TA spectra (Figure 5). A broad ESA band in the range of
4
50800 nm (Figure 5a) is observed, similar to fs TA data of RB-
assumption of a disk-like spin-density distribution, that is typically
applied for planar aromatic systems.[42] In Figure 6a, the signs of
the ZFS parameters are as follows: D > 0 and E > 0 (all simulation
parameters are compiled in Supporting Information, Table S7).
The ZFS parameters D and E of RB-NI triplet are very similar to
NI-N (Figure 4). The transient species of RB-NI-N shows a lifetime
of 50.5 s in deaerated n-hexane. The lifetime is reduced to 0.68

Importantly, the transient lifetime of RB-NI-N shows no strong
solvent polarity dependency. These observations confirm the
s in aerated n-hexane (Supporting Information, Figure S26).
3
3
those of NI state in NI-Br, indicating the triplet state is localized
transient signal of RB-NI-N is a LE state, which is different from
on NI moiety of RB-NI. The ESP patterns of RB-NI and RB-NI-N
are EEEAAA (where A denotes enhanced absorption and E
denotes emission), the same as that of NI-Br (SO-ISC). The
simulations show that the triplet states in NI-Br and RB-NI have
RB-NI.
a
0.06
b
the
p
and
p
sublevels overpopulated; for RB-NI-N, the
x
y
3
0
.05
.00
NI*
populations p
overpopulated (Supporting Information, Table S7).
x
and p are similar, but the p sublevel is slightly
y
x
0.04
0.02
0.00
0
 = 50.5 s
The centroids of xanthene and NI planes of RB-NI and RB-NI-
N assume distances of 5.104 Å and 5.566 Å (XRD data),
respectively. Thus, spin-spin exchange interactions between the
radical anion and the cation of RB-NI and RB-NI-N will be
strong.[6,22] Only the signal of NI state was detected in RB-NI and
1
88.5 s
-
0.05
0.10
...
0
.0 s
3
-
4
00 500 600 700 800
0
100
200
300
3
RB-NI-N samples recorded at 80 K. The lack of any CT state
Wavelength / nm
Time / s
signal of RB-NI under the conditions used for the TREPR
experiments may be due to the increased energy level of CT state
in frozen solution at 80 K, and it is consistent with the ns TA
spectra results measured at same condition (Supporting
Information, Figure S28).[25] In the frozen solution at low
temperature, the solvent molecules cannot reorient to stabilize a
changed electron-density distribution of the molecule (lack of
effective solvation). As a result, the energy level of the CT state is
Figure 5. (a) Nanosecond transient absorption spectra of RB-NI-N and (b)
decay trace of RB-NI-N at 485 nm. Excited with nanosecond laser at 427 nm, c
5.0  10 M in deaerated n-hexane. 20 °C.
5
=
Pulsed Laser Excited Electron Paramagnetic Resonance
Spectra (TREPR): Electron Spin Polarization Dynamics
expected to increase as compared to that at room temperature
TREPR is a powerful tool for studying the spin multiplicity of the
CT states, as well as 3LE states;[3840] misinterpretation of data
from transient optical spectroscopy on these states may thus be
avoided. The CT and the LE can be easily distinguished by the
zero-field splitting (ZFS) parameters, D and E, as well as the
specific electron spin polarization (ESP) pattern.[26,41]
To study the CT state and to confirm the ISC mechanism, the
TREPR spectra of triplet states of NI-Br, RB-NI and RB-NI-N
recorded in frozen solution at 80 K are presented in Figure 6a.
Spectral simulations of the TREPR data revealed the ZFS
3
(
note: in toluene solution at room temperature, the CT state lies
3
only slightly lower than the LE state, by ca. 0.1 eV. Scheme 1).
Thus, the energy of the CT state may be higher than that of the
3
3
3
3
3
LE state in frozen solution, and consequently, the CT state was
not detected by TREPR (Figure 6a). We exclude the RP ISC
mechanism from our TREPR spectral data, because the specific
ESP pattern of a triplet state formed via the RP ISC is AEEAAE
or EAAEEA, was not observed.
The liquid crystal E7 was used as a solvent for RB-NI to
measure TREPR data at more elevated temperature (Figure 6b,
a nematic phase is formed above 210 K). The TREPR signals
parameters, D and E, as well as the zero-field populations, p
1
,
p
2
and p . The latter can be assigned to p , p and p under the
3
x
y
z
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
from RB-NI in liquid crystal nematic phase are very noisy, even if Conclusion
measured at low temperature. Therefore, the ZFS parameters
can be given only with relatively large uncertainties. At 80 K and
80 K, rather large D values around 100 mT are obtained that
resemble the ones observed for NI-Br. Hence, under these
conditions, the triplet state is rather confined and likely localized
on the NI moiety of RB-NI. As in toluene/MeTHF mixture, the
In summary, we proposed
donor/acceptor compact dyad structural profile to access the
long-lived triplet charge transfer state ( CT), using a rhodamine
a
novel spiro electron
1
3
unit as electron donor (lactam form. RB) and a naphthalimide (NI)
unit as electron acceptor. The long-lived CT state is based on
electron spin control, i.e. to initiate the final charge separation (CS)
with a triplet precursor, which is achieved with the spin orbit
charge transfer intersystem crossing (SOCT-ISC), without
invoking of any heavy atom effect or a chromophore with intrinsic
ISC ability. The novel spiro linker is rigid and short, thus a well-
defined close-to-orthogonal geometry is attained in the dyad
(confirmed with single crystal X-ray diffraction molecular structure
determination), beneficial for the SOCT-ISC process. We
x
population p is very high. Above the glass transition temperature
in the nematic phase, at 230 and 280 K, a triplet species with a
much smaller D value ca. 65 mT was observed, comparable to
that of RB-NI-N. This is indicative for stronger spatial
delocalization of the triplet wavefunction. Tentatively we assign
this triplet species to the rhodamine (RB) moiety of RB-NI-N. As
x y
in the latter molecule, the population p and p are comparable in
size: p  p . Within the glassy phase of E7, at 80 and 180 K, an
x
y
1
1
enhanced absorption feature shows up at around 300 mT, that is
outside the simulation of a spin-polarized triplet species (Figure
observed fast NI* CT CS process (2.5 ps), and the subsequent
3
charge recombination (CR. i.e. SOCT-ISC) to give the NI state,
i.e. 1CT  NI* (8 ns). Due to the small driving force for the
secondary CS with 3NI* state as the precursor, as well as the
weak electronic coupling between the donor and acceptor, we
3
6b, lower two traces). This resonance could in principle arise from
a CT state, however, due to the poor signal-to-noise ratio, we
refrain from a definite assignment.
3
The energy diagrams of RB-NI and RB-NI-N are presented in
Scheme 1. For RB-NI, the CS (2.5 ps) process with 1NI* as
precursor is exoergic (GCS = 0.85 eV, Supporting Information,
observed a slow secondary CS (125 ns) with NI as the precursor,
i.e. 3NI* CT. Control experiments confirmed this is a slow
intramolecular CS process, not an intermolecular process. The
final CT state is long-lived (0.94 s) and exhibits a high energy
level (ca. 2.12 eV) in fluid solution at room temperature. These
cascade photophysical processes are summarized as
3
1
Table S5), which leads to the formation of CT state. The
3
subsequent SOCTISC (8 ns) produces the NI* state. Then a
secondary, slow CS (125 ns in deaerated n-hexane) occurs with
3
3
1
1
3
3
NI* state as precursor. The final low-lying state is a CT state
NI* CT NI* CT. Note the electron spin control for
1
3
(
0.94 s, in deaerated n-hexane). The energy level of CT of RB-
accessing long-lived CT state is realized. Time-resolved electron
paramagnetic resonance (TREPR) spectra indicated 3LE state,
with electron spin polarization (ESP) of EEEAAA, via SOCTISC
mechanism. Radical pair ISC (RP ISC) is excluded based the
ESP pattern. To the best of our knowledge, it is the first report that
a long-lived CT state is generated in a compact electron
donor/acceptor dyad via an electron spin-control strategy
implemented by SOCTISC, instead of the previously reported
method based on transition metal complexes or chromophores
showing intrinsic ISC ability, thus some of the limitations of the
conventional methods for accessing long-lived CT state are
addressed. Our results present a new method to design simple,
compact electron donor/acceptor dyads to access long-lived CT
states with a high energy level, which is important for artificial
photosynthesis, photovoltaics, photocatalysis and as well as for
fundamental photochemistry studies.
NI in n-hexane is ca. 2.24 eV, which is much higher than that
occurring in the natural photosynthetic centers (ca. 0.5 eV).[7] The
CT state energy of RB-NI is high enough to drive catalytic
processes.[ Note although the CT and LE states of RB-NI
2a]
3
3
3
3
share similar energy levels, we didn’t find any CT LE process
in the global analysis of the transient absorption data. This result
may be due to the slow kinetics as a result of the endoergic
feature. For RB-NI-N (Supporting Information, Scheme S2), CS
(
20 ps) occurs upon photoexcitation, then the triplet state of NI
moiety is produced by SOCTISC (1 ns), however no further CS
in the triplet state was observed, so that the low-lying state is the
3
NI* (114.2 s in deaerated toluene), not a CT state.
Acknowledgements
J.Z. thanks the NSFC (21673031, 21761142005, 21911530095
and 21421005), the State Key Laboratory of Fine Chemicals
(
ZYTS201901) and the Fundamental Research Funds for the
Central Universities (DUT19TD28) for financial support. S.W.
thanks the SIBW/DFG for financing EPR instrumentation that is
operated within the MagRes Center of the University of Freiburg.
We also thank Prof. J. M. Verhoeven at University of Amsterdam
for helpful discussions.
Scheme 1. Simplified Jablonski diagram illustrating the photophysical
processes involved in RB-NI. CT energy levels were calculated based on the
1
electrochemical data and TDDFT computations. TDDFT calculations were
performed at the B3LYP/631G(d) level by using Gaussian 09W. The triplet
excited state energy levels of RB-NI were from the phosphorescence at 77 K.
Conflict of interest
The authors declare no conflict of interest.
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RESEARCH ARTICLE
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This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.202003560
RESEARCH ARTICLE
Entry for the Table of Contents
RESEARCH ARTICLE
Text for Table of Contents
Dongyi Liu, Ahmed M. El-Zohry, Maria
Taddei, Clemens Matt, Laura Bussotti,
Zhijia Wang, Jianzhang Zhao,* Omar F.
Mohammed,* Mariangela Di Donato*
and Stefan Weber*
We prepared a rigid, simple, spiro
compact electron D/A dyad showing a
3
long-lived CT state (0.94 s) with a
high energy level (ca. 2.12 eV), based
on a new electron spin-control method
Page No. – Page No.
1
1
using SOCTISC ( NI* CT 
3
3
Long-Lived Charge Transfer State
Induced by Spin-Orbit Charge
Transfer Intersystem Crossing
NI* CT), without invoking of heavy
atoms or chromophores with intrinsic
ISC ability.
(
SOCTISC) in a Compact Spiro
Electron Donor/Acceptor Dyad
This article is protected by copyright. All rights reserved.