Intersystem crossing: via charge recombination in a perylene-naphthalimide compact electron donor/acceptor dyad

Article abstract of DOI:10.1039/d0tc00017e

In order to study the relationship between the molecular structures of compact electron donor/acceptor dyads and the spin-orbit charge transfer intersystem crossing (SOCT-ISC) efficiency, we prepared a perylene (Pery)-naphthalimide (NI) dyad, in which the Pery unit is the electron donor and the NI unit is the electron acceptor, where the two units adopt an almost orthogonal geometry. The photophysical properties of the dyad were studied with steady state and femtosecond/nanosecond transient absorption (fs TA and ns TA) spectroscopies, time resolved electron paramagnetic resonance (TREPR) spectroscopy and DFT computations. A very weak charge transfer (CT) absorption band was observed, but the fluorescence of the Pery unit is strongly quenched. Upon selective excitation into the NI unit, the fast intramolecular CS process (10 ps) occurs, followed by ISC and only the 3Pery? state is observed; whereas upon selective photoexcitation into the perylene unit, an ultrafast charge separation (0.66 ps) is observed, followed by SOCT-ISC (8 ns) to populate the 3Pery? state. Moreover, the perylene radical cation is also observed on the ns timescale, presumably formed by intermolecular charge transfer. The lifetime of the 3Pery triplet state was determined to be τT = 214 μs with ns TA spectroscopy. The singlet oxygen quantum yield was measured as ΦΔ = 80%, although the potential energy curve of the torsion between the donor and acceptor is shallow. The SOCT-ISC mechanism was confirmed by TREPR spectroscopy. The dyad was used as a triplet photosensitizer for the generation of delayed fluorescence (luminescence lifetime τDF = 57.3 μs). This journal is

Full text of DOI:10.1039/d0tc00017e

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Weber, J. Mater. Chem. C, 2020, DOI: 10.1039/D0TC00017E.
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JPolueransael odfoMnaotteraiadljsuCsht emmairsgtriynsC
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DOI: 10.1039/D0TC00017E
Journal of Materials Chemistry C
Intersystem Crossing via Charge Recombination in a Perylene-
Naphthalimide Compact Electron Donor/Acceptor Dyad
Muhammad Imran,a‡ Ahmed M. El-Zohry,b‡ Clemens Matt,c‡ Maria Taddei,d Sandra Doria,d Laura
Bussotti,d Paolo Foggi,d,f Jianzhang Zhao,a,* Mariangela Di Donato,d,e,* Omar F. Mohammed b,* and
Stefan Weber c,*
Received 00th January 2020,
In order to study the relationship between the molecular structures of compact electron donor/acceptor dyads and the spin
orbit charge transfer intersystem crossing (SOCT-ISC) efficiency, we prepared a perylene (Pery)-naphthalimide (NI) dyad, in
which the Pery unit is the electron donor and the NI unit is the electron acceptor, where the two units adopt an almost
orthogonal geometry. The photophysical properties of the dyad were studied with steady state and
femtosecond/nanosecond transient absorption (fs TA and ns TA) spectroscopies, time resolved electron paramagnetic
resonance (TREPR) spectroscopy and DFT comptuations. A very weak charge transfer (CT) absorption band was observed,
but the fluorescence of the Pery unit is strongly quenched. Upon selective excitation into the NI unit, fast intramolecular CS
process (10 ps) occurs, followed by ISC and only the 3Pery* state is observed; whereas upon selective photoexcitation into
the perylene unit, an ultrafast charge separation (0.66 ps) is observed, followed by SOCT-ISC (8 ns) to populate the 3Pery*
state. Moreover, the prylene radical cation is also observed on the ns timescale, presumably formed by intermolecular
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
3
charge transfer. The lifetime of the Pery triplet state was estimated T = 214 s with ns TA spectroscopy. Singlet oxygen
quantum yield was measured as  = 80%, although the potential energy curve of the torsion between the donor and
acceptor is shallow. The SOCT-ISC mechanism was confirmed by TREPR spectroscopy. The dyad was used as triplet
photosensitizer for generation of delayed fluorescence (luminiscence lifetime DF = 57.3 s).
drawbacks, such as complicated synthesis routes, costly
preparation and toxicity. Therefore, it is highly desired to
develop novel heavy atom-free molecules with efficient ISC.
Introduction
The formation of long-lived triplet excited states is of
fundamental importance for
photosynthesis,1,2
photocatalysis,3 photovoltaics,7 photodynamic therapy10
Other than heavy atom effect for enhancing ISC, a few other
approaches have been reported. For instance, exciton

6

9

13
coupling,28 radical pair ISC (RP-ISC) or charge recombination

and triplet-triplet annihilation (TTA) upconversion.14
An
22
30
(CR)-induced ISC,29,
electron spin converter31 and radical
efficient triplet photosensitizer (PSs) should possess several
photophysical properties such as strong light absorption,
efficient intersystem crossing (ISC) and long triplet lifetime.23
The most straightforward approach to enhance ISC in organic
compounds is the heavy atom effect (based on I, Br, Pt, Ir, Os
enhanced ISC (REISC).32 Furthermore, molecules with n
* 
* transition are also known to have efficient ISC production
(the EI-Sayed rule).33 The main challenge of all these approaches
is the difficult synthesis and the difficulty in predicting the ISC
ability.

27
atoms, etc).23 However, this method suffers from some
RP-ISC is known for weakly coupled electron donor-acceptor
systems. In this case, photoexcitation of either the electron
donor or the acceptor induces an electron transfer from the
donor to acceptor moiety with the formation of charge
separated states (CSS).29, 34, 35 The triplet formation involves the
a State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, 2 Ling Gong Rd., Dalian 116024, China. E-mail:
zhaojzh@dlut.edu.cn
b KAUST Solar Center, Division of Physical Sciences and Engineering, King Abdullah
University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of
Saudi Arabia. E-mail: omar.abdelsaboor@kaust.edu.sa
c Institute of Physical Chemistry, Albert-Ludwigs-Universität Freiburg, Albertstr.
21, 79104 Freiburg, Germany. E-mail: stefan.weber@physchem.uni-freiburg.de
d LENS (European Laboratory for Non-Linear Spectroscopy), via N. Carrara 1,
50019 Sesto Fiorentino, FI, Italy
ISC among CS states 1CS 3CS promoted by hyperfine coupling

interactions (HFI), followed by the rapid decay of the 3CS to the
3LE state. However, the ISC efficiency of these systems is usually
low, this is a major disadvantage of this approach. Moreover,
the synthesis of these dyads is difficult, thus it is highly desired
to develop a simple molecular structure profile for electron
donor/acceptor dyads to attain efficient ISC.
e ICCOM-CNR, via Madonna del Piano 10, I-50019 Sesto Fiorentino (FI), Italy
E-mail: didonato@lens.unifi.it
f Dipartimento di Chimica, Università di Perugia, via Elce di Sotto 8, I-06123
Perugia, Italy
‡ M. I., A. M. E. and C. M. contributed equally to this work.
Electronic Supplementary Information (ESI†) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Recently, the production of triplet states through so-called
spin orbit charge transfer ISC (SOCT-ISC) mechanism in compact
electron donor/acceptor systems has attained much
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Scheme 1 Synthesis of the electron donor/acceptor dyad Pery-NI and the reference compounds. (a) NBS, chloroform, stirred at 0 C for 4 h, under N2, yield: 84%. (b)
Bis(pinacolato)diboron, Pd(dppf)Cl2CH2Cl2, CH3COOK, dry 1,4-dioxane, stirred at 80 C for 15 h, under N2, yield: 62%. (c) n-butylamine, EtOH, reflux at 78 C for 5 h, under N2, yield:
51%. (d) Compound 4, K2CO3, Pd(PPh3)4, DMF, stirred at 80 C for 4 h, under N2, yield: 51%. (e) K2CO3, Pd(PPh3)4, EtOH/Tol/H2O (2:2:1, v/v), stirred at 70 C for 9 h, under N2, yield:
57%. (f) Similar to step e, yield: 70%.

41
consideration.36 In these systems, the linker between the evidence is required to evaluate the limitation on the role that
electron donor and acceptor is short, and the two moieties are a chromophore can play in the electron donor/acceptor dyads.
preferably arranged in an orthogonal geometry with each other
In order to address the above challenges, we prepared a
(different from the conventional donor/acceptor dyads with compact electron donor/acceptor dyad, with perylene as
long and rigid linkers showing RP-ISC). For these compact dyads, electron donor/visible light-harvesting chromophore and
the RP-ISC mechanism is inhibited due to large electron naphthalimide (NI) as the electron acceptor (Scheme 1). In this
exchange (J values). In SOCT-ISC, electron transfer between system, the SOCT-ISC can be efficient, given the geometry of the
perpendicularly oriented donor/acceptor units is accompanied electron donor/acceptor is nearly orthogonal. We studied the
by a change in the molecular orbital angular momentum, which photophysics of the dyad using steady state and femtosecond,
generates a magnetic torque able to flip the electron spin. As a subnanosecond and nanosecond time-resolved transient
3
result, the change in molecular orbital angular momentum spectroscopies. Efficient formation of LE state of the perylene
compensates the change in the electron spin angular moiety is confirmed. We found that the photophysics of the


39, 41 43
momentum, facilitating the ISC process.36
dyad is excitation wavelength-dependent. Upon selective
Although the occurrence of SOCT-ISC has been demonstrated photoexcitation into the NI unit, only the 3Pery* state is
in several compact electron donor/acceptor dyads, still much populated. On the other hand, photoexcitation into the
room is left to fully understand this mechanism. Up to now, only perylene unit promotes the formation of perylene radical cation
a limited number of visible-light-harvesting chromophores were via intermolecular charge separation, as well as the 3Pery* state.
used for SOCT-ISC, such as anthracene,37 Bodipy40a and Time-resolved electron paramagnetic resonance (TREPR)
perylene.44 Different from Bodipy chromophores,45 we show spectroscopy was used to study the electron spin polarization
that perylene is unique concerning SOCT-ISC dyads, because it (ESP) of the triplet states. We confirmed the SOCT-ISC
can be used as either the electron donor or acceptor to achieve mechanism showing that the observed ESP and population ratio
SOCT-ISC.44,46 Different electron donors may offer different of sublevels of the triplet state (TX, TY, TZ) are different from
driving force for CS, and the geometric restriction may increase those of SO-ISC mechanism. Additionally, delayed fluorescence
the ISC efficiency. It was previously proposed that in case of was observed with the dyad as triplet PSs and perylenebisimide
Bodipy, SOCT-ISC works for dyads with Bodipy as electron (PBI) and Solvent Green 5 (SG-5) as triplet acceptors/emitters.
acceptor, but not for dyads with Bodipy as electron donor.45
However, it is still in doubt whether or not this rule is applicable
to other chromophores. Recently, we have investigated the
Results and discussion
Design rationales of the Pery-NI compact electron
donor/acceptor dyad
SOCT-ISC in orthogonal perylene-phenothiazine dyads with
NI is a typical electron acceptor, and it has been used widely in
perylene unit as electron acceptor, which showed excellent
the conventional electron donor/acceptor dyads where the
triplet formation.46 In perylene-Bodipy dyads, the perylene unit

donor and acceptor had large separation.47 49 On the other
is the electron donor, and efficient SOCT-ISC was observed.44 In
a NI-PTZ dyad, we observed long-lived 3CT state.40b More
2 | J. Mater. Chem. C, 2020, 00, 1-9
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DOI: 10.1039/D0TC00017E
c
0.36
0.24
0.12
0.00
b
a
Pery
Pery-Ph
Pery-NI
NI-Ph
Pery
Pery-Ph
Pery-NI
Pery-NI
NI-Ph
50
25
0
400
400 500 600 700
Wavelength / nm
500
600
700
300
400
500
600
Wavelength / nm
Wavelength / nm
Fig. 1 (a) UV−Vis absorpꢀon spectra of the compounds in DCM, c = 1.0 × 10−5 M, 20 C. (b) Fluorescence emission spectra of Pery, Pery-Ph and Pery-NI in DCM; (optically-matched
solutions were used. ex = 391 nm, A = 0.133). (c) Fluorescence emission spectra of Pery-NI and NI-Ph in DCM; (optically-matched solutions were used. ex = 323 nm, A = 0.093); c =
ca. 1.0 × 10−5 M, 20 C.

hand, arenes can be used as electron donors.50 52 We prepared result as an uphill energy process upon excitation of perylene,
Pery-NI compact electron donor/acceptor dyad (Scheme 1). The so that other reasons should be considered to explain the
mutual orientation of the two chromophores is expected to be observed fluorescence quenching of the Pery-NI upon selective
orthogonal, due to the conformation restrictions. The dyad was excitation of the perylene unit. One possibility is photo-induced
prepared by direct connection of the perylene unit and the NI electron transfer to form the charge separated state (CSS)
moiety with the Suzuki-Miyaura cross coupling reaction. The Pery+NI.46 It is noteworthy to mention here, in the electron
yield was satisfactory, and the molecular structure of the transfer process, perylene moiety acts as electron donor and NI
1
product was fully verified with H NMR, 13C NMR and HRMS. is electron acceptor (see next section).
Pery-Ph and NI-Ph were prepared as reference compounds for
the photophysical studies.
The fluorescence of Pery-NI is also quenched upon excitation
of the NI moiety at 323 nm (Fig. 1c. Note the perylene
fluorescence was also quenched). In this case, the lack of NI
fluorescence could be attributed to several processes, such as
ISC, FRET to the perylene moiety,55 or formation of a CSS.
Among these possibilities FRET from NI (energy donor) to
perylene (energy acceptor), should certainly be considered,
since, according to the estimation of the Förster radius (R0) of
the dyad (see ESI†) this process is expected to be very efficient
(FRET efficiency is estimated 96 % and Förster radius is 6.668 Å).
UV−vis absorption and fluorescence emission spectra
The UV−vis absorption spectra of the compounds are reported
in (Fig. 1a). Perylene shows the characteristic absorption band
at 437 nm with a significant vibrational progression. For Pery-
Ph, a slightly red-shifted absorption band is observed, while for
Pery-NI, a tail at the red-end of the absorption spectrum is
observed, which is assigned as a weak CT absorption band,
indicating a limited electronic coupling due to freedom of the
The backward FRET (PeryNI) is not possible due to the large
torsion in Pery-NI 26
. Furthermore, inhomogeneous broadening
S1 states energy difference of perylene and NI moieties, as
mention above. However, the lack of perylene fluorescence for
Pery-NI upon selective photoexcitation of perylene unit, which
should act as acceptor in the FRET process, implies that also
other processes should take place. A possible reason for
fluorescence quenching is again photoinduced electron transfer,
which could occur also in case of NI excitation.
due to the presence of slightly different conformations of the
dyad in the solution, as a result of the partial rotation around
the C−C single bond connecting the two moieties, could also
contribute to broaden the absorption spectrum. In the UV
region, the absorption band at 340 nm is attributed to the NI
moiety, as demonstrated by the UV−vis absorption of NI-Ph
This result also indicates that the -conjugation between the NI
and the perylene unit is limited in Pery-NI 53, 54
.

To further investigate the photophysics of our systems, we
analyzed the solvent polarity dependence of the fluorescence
.
The fluorescence spectra of the compounds measured in (Fig. 2). For Pery-NI, the fluorescence emission intensity upon
dichloromethane (DCM) are reported in Fig. 1b and c. The NI excitation is highly dependent on the solvent polarity (Fig. 2a).
spectra shown in Fig. 1b are registered upon excitation at 391 The emission intensity in toluene is only half of that in n-hexane
nm, where perylene absorbs. In this case, a fluorescence (note optically matched solutions were used, thus the
emission band centered at 443 nm, with significant vibrational fluorescence emission intensity can be compared directly) and
progression, was observed for both perylene (fluorescence it is quenched in solvents with medium polarity, for instance
quantum yield F = 94%. Table 1) and Pery-Ph. For Pery-NI
,
DCM. In acetonitrile (ACN), the fluorescence is almost
however, the fluorescence of the both the NI and the perylene completely quenched. These results imply that FRET and CS are
units is almost completely quenched. Only a very weak broad competitive processes, with the first prevailing in non-polar
emission band centered at about 650 nm is observed, which is media, and the latter becoming predominant in polar solvents.
assigned as a CT emissive band.
Since the perylene S1 state (2.79 eV) is lower than the S1 state
of NI (3.64 eV), Förster Resonance Energy Transfer (FRET) would
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DOI: 10.1039/D0TC00017E
36
a
HEX
TOL
DCM
ACN
MeOH
b
HEX
TOL
DCM
ACN
MeOH
HEX
TOL
DCM
ACN
MeOH
60
40
20
0
24
12
0
400
500
600
700
400
500
600
700
400
500
600
700
Wavelength / nm
Wavelength / nm
Wavelength / nm
3.6
3.3
3.0
2.7
2.4
2.1
Excited state dipole moment
16.9 Debye
HEX
HEX
d
e
f
1.2
0.8
0.4
0.0
1.2
0.8
0.4
0.0
Ether
THF
EA
DCM
ACN
Ether
THF
EA
27 nm
120 nm
DCM
Pery-NI
NI-Ph
29.3 Debye
400 500 600 700 800
Wavelength / nm
350 400 450 500 550
Wavelength / nm
0.07 0.14 0.21 0.28 0.35 0.42
F ()- 0.5  F(n)
Fig. 2 Fluorescence emission of the compounds in different solvents. (a) Pery-NI (λex = 326 nm); (b) NI-Ph (λex = 323 nm). (c) Pery-Ph (λex = 390 nm); Optically matched solutions were
used for each panel. c = ca. 1.0 × 10−5 M. 20 C. Solvatochromic fluorescence spectra of (d) Pery-NI and (e) NI-Ph in solvents with the increase of ET(30) values. (f) Lippert-Mataga
plots based on solvatochromic shift of the energy related to the fluorescence maxima in different solvents (respective solvents are mentioned in each panel).
For NI-Ph, interestingly, the fluorescence emission band is slightly
red-shifted in polar solvents, and the emission intensity is increased
(Fig. 2b). This intensified fluorescence in polar solvents is anomalous.
It is known that NI has ISC ability, due to the n-*-* transition.56
The solvent polarity may change the energy level of the n-* state,
thus the relative energy level of the states. As a result, the
fluorescence of NI-Ph will change together with the ISC efficiency in
different solvents.56 The detailed photophysical parameters of the where hc푣ꢀ
CT
ꢁꢂꢃꢄ
is the energy related to the position of the CT
compounds are presented in Table 1.
fluorescence maxima; 휇ꢅ⃗ꢆ and 휇ꢅ⃗ꢇ are the dipole moments of the
For Pery-Ph, the fluorescence emission profile hardly changed in molecules in the excited and ground state, respectively, a0 is the
different solvents (Fig. 2c), which is in stark contrast to that of Pery- effective radius of the Onsager’s cavity, and η are the static
NI. This result indicates that the dipole moment of the emissive dielectric constant and the refractive index of the solvent
(bright) singlet excited state of Pery-Ph is small. In order to quantify respectively. Linear plots were obtained for the two compounds,
the change in dipole moment of the emissive singlet excited states from which it is retrieved that the dipole moment of the
of the compounds upon photoexcitation, the effect of the solvent emissive singlet excited state of Pery-NI (29.3
 2 D) is much
polarity on the emission band were studied in detail and analyzed larger than that of NI-Ph (16.9 2 D), while the ground state

with the Lippert-Mataga plots (Fig. 2d, e, f). The excited state dipole dipole moment (approximated from the optimized ground state
moment (e) of Pery-NI and NI-Ph were determined by the geometry by DFT calculations) is almost similar for both
fluorescence solvatochromic shift method using the equation 1 and compounds, being 5.6 D and 5.4 D for Pery-NI and NI-Ph
2.37, 57
respectively, which justifies the different behavior of their
emission profiles.
Table 1. Photophysical parameters of the compounds a
c
d
e
f
g
h
i
Compounds
λabs
[nm]
437
445
347
 b
λF
F
F
kr
knr
T
DF

1

1
[nm]
443
461
420
606
[%]
94
92
17 j
5.6
[ns]
4.6
3.9
0.52k
4.0
[105 s ]
 n
23.5
32.6k
1.4
[105 s ]
 n
[s]
 n
76  5l
181  5k
214  5
[s]
 n
Pery
Pery-Ph
NI-Ph
3.59
3.17
1.67
2.05
1.60k
23.6
 n
 m
57.3
Pery-NI
445/340
3.58/1.47
a In dichloromethane (1.0 × 10−5 M). b Molar absorption coefficient; in 104 M−1 cm−1 c Fluorescence maxima. d Fluorescence quantum yields, with perylene as the standard
.
(F = 94 %, in n-hexane), in DCM. e Fluorescence lifetimes, c = 1.0 × 10−5 M, note: the IRF of the fluorescence lifetime spectrometer is ca. 100 ps, therefore fast radiative
decay may not be detected. f Radiative decay rate constant, derived from fluorescence data, kr = /. g Non-radiative decay rate constant, derived from fluorescence
data knr = (1)/. h Triplet state lifetime, determined with nanosecond transient absorption. I Delayed fluorescence lifetime. j Fluorescence quantum yields, with
anthracene (F = 30 %, ethanol) as the standard, in n-hexane. k In n-hexane. l by intermolecular TTET. m Not measured. n Not applicable.
4 | J. Mater. Chem. C, 2020, 00, 1-9
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Fc+ / Fc
c
a
Fc+ / Fc
b
Fc+ / Fc
1
0
-1
1
0
-1
-2
1
0
-1
-2
Potential / V
Potential / V
Potential / V
Fig. 3 Cyclic voltammogram of the compounds. (a) Pery-NI, (b) NI-Ph and (c) Pery-Ph. Ferrocene (Fc) was used as internal reference. In deaerated DCM containing 0.10 M Bu4N[PF6]
as supporting electrolytes, Ag/AgNO3 as reference electrode. Scan rates: 50 mV/s. c = 1.0 × 10−3 M. 20 C.
Electrochemical study: Gibbs free energy change of the charge
separation and the energy levels of the CS states.
CS for Pery-NI is allowed in polar solvents, see Table 2, and ΔG0
CS
values in different solvents are as follows, +0.06 eV (in toluene),
0.33 (in DCM) and 0.59 (in ACN). Moreover, the CS state energy
level is solvent polarity dependent. The same trend was observed for
perylene-phenothiazine46 and Julolidine-anthracene dyads.37
The redox properties of the compounds were studied (Fig. 3). Pery-
Ph shows a reversible oxidation wave at +0.54 V, while no reduction
wave was observed (Fig. 3c). For NI-Ph, a quasi-reversible reduction
wave was observed at 1.83 V, no oxidation wave was observed in
the potential range used (Fig. 3b)
Femtosecond transient absorption spectroscopy
For Pery-NI, a reversible oxidation wave at +0.64 V and a quasi-
reversible reduction wave at 1.82 V were observed (Fig. 3a), which The dynamic evolution of the excited states in Pery-NI has been
are assigned to the oxidation of the perylene unit and the reduction studied by femtosecond transient absorption spectroscopy with
of the NI moiety, respectively.
400 nm excitation, where absorption is mostly due to the
perylene moiety. The transient spectra were acquired in DCM
Table 2. Electrochemical redox potentials of compounds and Gibbs free energy changes with a time interval spanning up to 1.5 ns and analysed using
(ΔG0CS) for the intramolecular electron transfer (with perylene unit as electron donor and singular value decomposition (SVD) and global fitting to obtain
NI unit as electron acceptor) in Pery-NI. The CS state energy levels (ECS) are also the Evolution Associated Difference Spectra (EADS). To have a
presented a
reference, we also measured the ultrafast transient absorption
spectra of Pery-Ph (Fig. S18, ESI†). For this compound, an ESA
Compounds
E(OX) (V)
E(RED) (V)
ΔG0CS (eV)
ECS (eV)
band centred at 708 nm (S1Sn transitions) and GSB centred at
450 nm were observed. Moreover, a stimulated emission (SE)
band was observed between 480 nm to 535 nm. All these bands
are typical signatures of the singlet excited state of perylene
and no obvious ISC was observed for this compound.
For Pery-NI, a GSB band at 445 nm and an ESA extending up
to 700 nm, previously assigned as an S1Sn absorption of the
Pery
+0.55
+0.54
b
b
b
b
b
b
0.06c /
0.33d /
0.59e
b
b
b
2.65c /
2.25d /
2.14e
Pery-Ph
NI-Ph
 1.83
 1.82
Pery-NI
+0.64
perylene,46 were initially observed (Fig. 4a). A band centred at
553 nm rapidly appeared, similar to that observed in the ns-TA
spectra, where it has been assigned to the perylene radical
cation.50 Previously, it was reported that the NI radical anion
a
Cyclic voltammetry in N2-saturated DCM containing a 0.10 M Bu4NPF6. Pt
electrode is counter electrode and glassy carbon electrode is working electrode,
Ag/AgNO3 couple as the reference electrode. b No oxidation/reduction potential
observed or no ΔG0CS and ECSS values were calculated. c Values of ΔG0CS and ECSS are
determined in toluene. d Values of ΔG0CS and ECSS in DCM. e Values of ΔG0CS and ECS
are determined in ACN. E00 = 2.59 eV. E00 (E00 = 1240/) is the singlet state energy
of Pery-NI, is the wavelength of the crossing point of normalized UV−Vis
absorption spectra and fluorescence emission spectra.

1
shows absorption at 420 nm (

= 23500 M1 cm ) and 491 nm

1
(
= 3000 M1 cm ).58 In this case NI anion bands are not clearly
visible since they may overlap with negative GSB bands. Global
analysis allows to extract the timescale of the spectral evolution.
The first EADS (Fig. 4b, black line) shows the occurrence of
partial ultrafast CS, with the appearance of the GSB and a
positive band at 570 nm, which can be assigned to the perylene
cation, besides a broad tail extended up to 750 nm, due to
residual absorption of the neutral perylene excited state.
Thus, for Pery-NI upon photoexcitation, the perylene unit will act
as electron donor and the NI as electron acceptor. The Gibbs free
energy changes of the photo-induced electron transfer were
calculated, further details are given in ESI†. The data show that the
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Journal of Materials Chemistry C
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DOI: 10.1039/D0TC00017E
Pery+
0.02
0.01
0.00
-0.01
0.04
0.02
0.00
-0.02
0.04
a
c
b
0.02
0.00
1Pery*
490 nm in TOL
550 nm in DCM
550 nm in ACN
0.66 ps
13.5 ps
>1.5 ns
-0.02
400
GSB
0
300 600 900 1200
Time / ps
400
500
600
700
500
600
700
Wavelength / nm
Wavelength / nm
Fig. 4 (a) Femtosecond transient absorption spectra of Pery-NI at 0.1 ps to 1.5 ns time range, in DCM with the excitation at 400 nm. (b) Evolution Associated Difference Spectra
(EADS) obtained from global analysis. (c) Kinetic traces at the maxima of the perylene cation band at 550 nm in DCM and ACN; and ESA of triplet perylene at 490 nm in toluene. Black
lines represent the fitting obtained from global analysis.
In the following evolution, occurring in 0.66 ps, the positive
band due to the perylene cation increases in intensity and blue Subnanosecond transient absorption spectroscopy
shifts to 553 nm, while the ESA band of the perylene singlet
In order to reveal more details of the photophysical properties
excited state at 700 nm decays completely. The perylene cation
of the compounds, subnanosecond transient absorption
band slightly blue shifts on a timescale of 13.5 ps (red to blue
spectroscopy upon selective photoexcitation was performed.
evolution in Fig. 4b), due to vibrational cooling. The transient
Firstly, compound
sharp ESA band centered at about 702 nm (S1
2
(3-bromoperylene) was investigated. A
Sn transitions)
spectra show limited evolution on the following time interval
(1.5 ns), indicating that the charge separated state has a
lifetime > 1.5 ns so that triplet production is not observed during
this short delay time. This result confirms that, the 3Pery* state
is populated after CS process.

and stimulated emission at about 478 nm were observed. The
ESA band decayed within 10 ns and a new band at about 500
nm appeared, which is the ESA band of the triplet state of
perylene (T1
Tn transitions) populated by heavy atom effect.
In order to analyse the solvent dependence of CS and triplet
production, toluene and acetonitrile were also used as solvents.
In toluene, a stimulated emission band at 530 nm appears,
whose position match that of the dyad fluorescence observed
in this solvent, and is due to the stimulated emission of perylene,
which is directly excited at 400 nm. The SE band is
superimposed to the positive signal of the perylene cation
which starts to rise in about 6 ps (Fig. S19, ESI†). This result
shows that, charge separation partially occurs also in this non
polar solvent, although with slow kinetics and lower yield, as
demonstrated by the persistence of the ESA band assigned to
the neutral perylene singlet excited state up to the nanosecond
timescale. In ACN, the appearance of an intense positive band
at 550 nm, due to the formation of the perylene cation occurs
in less than 1 ps, indicating fast CS (Fig. S20, ESI†), while the
triplet state production was not observed. In this case, the
transient signal recovers within about 650 ps, due to CR (Fig.
S20, ESI†). The occurrence of fast CR in acetonitrile agrees with
the lack of triplet formation observed on the nanosecond-
microsecond timescale in this solvent. Selected kinetic traces
are reported in the Fig. 4c for three solvents.
The triplet state lifetime is ca. 500 ns (In deaerated solution. Fig.
S22, ESI†).
0 ns
500 nm
a
b
0.01 ns
550 nm
20
30
15
0
0.04 ns
0.22 ns
0.52 ns
Fitting
Pery+
= 300ns
3Pery*
10
0
= 8ns
 = 230ns (3%)
400
500
600
700
0
100
400 800
Wavelength / nm
Time / ns
12
8
0 ns
550 nm
500 nm
Fitting
c
d
20 ns
30 ns
520 ns
10
5
Pery+
3Pery*
= 370 ns
4
0
0
= 10 ns
-4
Finally, we also studied the dynamic of NI-Ph (Fig. S21, ESI†).
In this case, a broad ESA band with a dip at about 550 nm, due
to superimposed stimulated emission was observed. The
transient signal showed no significant evolution up to the
500
600
700
0
100 400 800
Time / ns
Wavelength / nm
Fig. 5 (a) Subnanosecond transient absorption spectra (extracted at different time delays)
of Pery-NI after 430 nm excitation, (b) the respective kinetics decay trace at 500 and 550
nm. Note: The sharp signal centred at 433 nm in (a) is due to of laser scattering; (c)
Subnanosecond transient absorption spectra (extracted at different time delays) of Pery-
NI after 340 nm excitation, (d) the respective kinetics decay trace at 500 and 550 nm.
The measurements were performed in the presence of oxygen in DCM.
nanosecond timescale, and the limited decay of the kineti
traces indicated a long living excited state.
c
6 | J. Mater. Chem. C, 2020, 00, 1-9
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ARTICLE
Pery-NI was excited at 430 nm (perylene absorption band), state absorption of NI moiety. During the later time delays these
View Article Online
showing an ultrafast CS process (i.e. <10 ps), as also confirmed signals becomes negative, this couldDObIe: 10d.1u0e39/tDo0TC0r0o0o17mE
in fs-TA spectra. The formation of perylene radical cation is temperature phosphorescence of this compound.49 The decay
confirmed, indicated by the absorption band centered at 550 kinetics of transient species were monitored at 435 nm and 500
nm (Fig. 5a).50 Moreover, a band at about 500 nm was observed nm to investigate the ISC process. ISC takes ca. 10 ns then the
to rise, which is attributed to the ESA of perylene-localized triplet state decays to ground state at long delay time. Notable
triplet state,58 in good agreement with our ns-TA observation here, the measurements were carried out in toluene.
(Fig. 6). As mentioned in fs-TA section, the NI radical anion
bands should appear at 420 and 490 nm.58 In sub-nanosecond Singlet oxygen photosensitizing and nanosecond transient
3
spectra they may overlap with ESA of Pery* or be covered by absorption spectroscopy: observation of the triplet state
laser scattering at 433 nm (Fig. 5a). The decay kinetics of the
3
bands assigned to Pery* and the perylene radical cation are We have shown that the fluorescence of the perylene moiety in the
presented in Fig. 5b. The ISC rate constant is extracted from the dyad Pery-NI is significantly quenched, especially in polar solvents
growth of triplet state signal at 500 nm, and determined as 8 ns. which can be due to excited state depopulation to the S0 state
The triplet state at 500 nm decays in ca. 300 ns in the presence directly or to the formation of a triplet state, via the SOCT-ISC

37, 60 63
of O2.
mechanism.36,
Firstly we studied the singlet oxygen (1O2)
Exciting the Pery-NI at 340 nm would preferentially excite the photosensitizing ability of the compounds and the singlet oxygen
NI part (Fig. 5c). In this case, fast FRET towards the perylene is quantum yields () are summarized in Table 3.
expected to occur. Within the time resolution of our sub-
Pery-NI shows the highest  in DCM (80%), in agreement with its
nanosecond measurement we nevertheless observe the band
low fluorescence quantum yield in this solvent, but the value is much

at 550 nm due to Pery+ , and, within 20 ns, the rise of the ESA
1
smaller in toluene ( = 11%). In n-hexane and acetonitrile, no O2
band at 500 nm assigned to the localized triplet state of
production was observed. Thus, the 1O2 production, which is

perylene. The observation of the Pery+ band agrees with the
correlated to the triplet state quantum yield of Pery-NI, is highly
fluorescence measurements reported in the previous section,
dependent on the solvent polarity, which is a characteristic of SOCT-
showing the quenching of perylene fluorescence in slightly
1
ISC. For Pery-Ph, only a weak O2 production was observed in DCM
polar and polar solvents, and suggests that FRET can occur on a
fast time scale, rapidly followed by charge separation. In ns-TA,
with this excitation wavelength, we only observed the triplet
(7%), and no singlet oxygen production was observed in other
solvents. For NI-Ph, the highest singlet oxygen production was
observed in n-hexane (72%), and  values are smaller in other
state of perylene, but the radical cation band may be very weak
solvents.
(Fig. 9a), rising within our instrument response function (IRF is
Table 3. Singlet oxygen quantum yields (. In %) of the compounds
in different solvents a
ca. 10 ns for the nanosecond transient absorption spectrometer,
LP980).
In sub-nanosecond measurements with NI excitation, the
band at 550 nm is due to a CS state because the perylene moiety
cannot be excited at this excitation wavelength. The lifetime of
this short-lived CS species is ca. 10 ns (in oxygen), which
supports the excitation dependent photophysics of Pery-NI
observed in nanosecond transient absorption spectra (Fig. 6 and
9). Indeed, CS occurs within 10 ps (time resolution of sub-
nanosecond measurements), and it should be in competition
with FRET to perylene moiety.
Compounds
Pery-NI b
NI-Ph c
HEX
d
TOL
DCM
ACN
d
11  0.3
36  2
d
80  3
25  2
7  3
72  3
d
21  0.8
d
Pery-Ph b
a The ET (30) values of the solvents are HEX (31.0), TOL (33.9), DCM (40.7) and ACN
(45.6). b Singlet oxygen quantum yield () with Ru(bpy)3[PF6]2 as standard (Δ =
0.57 in DCM) in different solvents. Singlet oxygen quantum yield () with
anthracene as standard (Δ = 0.70 in MeOH) in different solvents. d Not observed.
c
For NI-Ph, upon 350 nm laser excitation, a sharp ESA band at
about 500 nm appeared (Fig. S23, ESI†), which became broader
at later times by spanning up to 700 nm, this is typical triplet
0.15
c
3Pery*
b
a
0.03
0.02
0.01
0.00
0.0 s
11.0 s
...
0.12
0.10
0.08
1089.0 s

= 752 s

= 214 s
Pery+
553 nm
493 nm
0.05
0.00
0.04
0.00
GSB
-0.04
0
500
Time /s
1000
1500
0
1000 2000 3000
400 500 600 700 800
Wavelength / nm
Time / s
Fig. 6 Nanosecond time-resolved transient absorption spectra of (a) Pery-NI, (b) decay trace at 493 nm, c = 5.0 × 10−6 M; (c) decay trace at 553 nm. c = 1.0 × 10−5 M.ex = 445 nm. In
deaerated DCM. 20 C.
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Journal of Materials Chemistry C
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DOI: 10.1039/D0TC00017E
Previously we demonstrated that the perylene-phenothiazine
dyads, in which the perylene moiety is an electron acceptor, shows
efficient ISC.46 Thus, we conclude that the perylene chromophore
shows efficient SOCT-ISC in dyads with perylene unit as either
electron donor or acceptor.
a
b
0 s
0.018
0.012
0.006
0.000
Pery+
0.02
0.01
76.4s
...
2223.2 s

= 689s
553 nm
0.00
This behavior is different from that observed for Bodipy containing
dyads, for which only those with Bodipy as electron acceptor were
found to show efficient SOCT-ISC.45 Our results show that there is no
general limitation for a chromophore to play a specific role as
electron donor or acceptor in the dyad to achieve efficient SOCT-ISC.
-0.01
-0.02
GSB
400 500 600 700
Wavelength / nm
0
1000 2000 3000
Time /s
It is known that the ISC of NI moiety is due to S1

T4, a *n-
Fig. 7 Nanosecond time-resolved transient absorption spectra of (a) Pery-NI, (b) decay
trace at 553 nm.ex = 445 nm. c = 5.0 × 10−6 M. In aerated DCM. 20 C.
* transition.59 Since the n* state energy level is more
sensitive to the solvent polarity than the * state, the energy
matching of the n* and* states of NI-Ph is solvent
polarity-dependent, and the ISC efficiency of NI-Ph can be
solvent polarity-dependent. For Pery-NI, however, the ISC is not
due to the NI moiety, because we were selectively exciting the
perylene. In order to unambiguously confirm the formation of
triplet excited state upon photoexcitation, nanosecond
transient absorption spectra of Pery-NI were recorded (Fig. 6a).
Indeed, in aerated solution, the strong triplet state signal at 493
nm was quenched, but the perylene cation absorption at 553
nm was persistent (Fig. 7a), and showed a lifetime of 689
which is very close to that determined in deaerated solution
(752 s. Fig. 6c).
s,

The nanosecond transient absorption spectra of Pery-NI were
also studied in solvents with different polarity, toluene and ACN
(see ESI†, Fig. S24 and S25,). The perylene localized triplet state
was observed in both the solvents, while no ESA band centered
at 553 nm was observed. In ACN the signals were very weak due
to the low ISC ability.
The promptly observed ESA band in the range of 400
nm is the typical feature of the triplet state of perylene (T1

550
Tn

transitions).58 However, we found that at longer delay time, this
band almost completely recovers, while a positive absorption
band centered at 553 nm is persistent. It was previously
reported that the NI radical anion shows absorption at 420 nm
To further analyze the above observation, we did some
control experiments with pristine perylene in different polarity
solvents (Toluene, DCM and ACN) under aerated and deaerated
conditions. In DCM, a transient band centered at 545 nm was
observed (Fig. 8), which is similar to that seen for Pery-NI under
deaerated and aerated conditions (results under aerated
condition are given in Fig. S26, ESI†). Similar results were
observed in ACN, while in case of toluene no transient signal
were detected. Previously, the simultaneous formation of
perylene radical cation and anion was observed upon flash
excitation of perylene in polar solutions.68 Thus, we conclude
that the observed band at 553 nm in Fig. 6 and 7 could be


1
1
(
= 23500 M1 cm ) and 491 nm (

= 3000 M1 cm ).64 On the
other hand, the perylene radical cation absorption was reported

to be centered at 550 nm (
= 27000 M1 cm ).50 Based on the
1
TA spectra (Fig. 6a), we propose both the triplet state of
perylene (3Pery*) and its radical cation are observed in the
transient spectra, thus assigning the long-lived band at 553 nm

to the perylene radical cation (Pery+ ).
In order to obtain the decay kinetics of the perylene triplet
state and the cation band, the decay traces at 493 nm and 553
nm were monitored (Fig. 6b and c). The perylene triplet state
lifetime was found 214
s, while the observed lifetime for

assigned to Pery+ formed in a similar fashion upon ns pulsed
cation band at 553 nm is 752

s. Note there is no growth phase
excitation.
detected for the transient at 553 nm (IRF of the nanosecond
transient absorption spectrometer is ca. 10 ns). According to the
In order to confirm the electron transfer process in Pery-NI is
either intermolecular or intramolecular process, we used two

decay traces of the 3Pery* and the Pery+ , we propose that the
3Pery* state is not the precursor of the perylene cation because
we did not observe any growth of the cation band and
interconversion among the two, i.e. they are formed separately,
and simultaneously (with time constant <10 ns).65
0.02
Pery+
a
b
0 s
0.02
0.01
0.00
37.8s
...


= 157 s 16.57%
3Pery*
0.01
0.00
1
2
= 1180 s 83.43%
1860.6 s
To confirm the identity of the transient species giving
absorption at 553 nm, we carried out the nanosecond transient
absorption measurements also in aerated solution (Fig. 7). In
this case the triplet state will be substantially quenched, due to
its sensitivity to O2.66 In some cases the quenching of the anion
can be also observed,67 however, the cation absorption should
not be quenched by O2.
GSB
-0.01
400 500 600 700
Wavelength / nm
0
2000 4000 6000 8000
Time / s
Fig. 8 Nanosecond time-resolved transient absorption spectra of (a) Perylene (b) decay
trace at 545 nm.ex = 430 nm. c = 1.0 × 10−5 M. In deaerated DCM. 20 C.
8 | J. Mater. Chem. C, 2020, 00, 1-9
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different viscous solvents with different polarities, glycerin Time-resolved electron paramagnetic resonance (TREPR)
View Article Online
DOI: 10.1039/D0TC00017E
triacetate (triacetin, viscosity
triethylene glycol ( = 47.8 c.p. ET (30) = 52.8). In these solvents,
compared with the results in DCM ( = 0.41 c.p. ET (30) = 40.7),
only the perylene triplet state was observed. From these
measurements we conclude that the formation of perylene
cation in Pery-NI is due to an intermolecular electron transfer
process on the ns timescale, while it is an intramolecular
process on a faster timescale, preceding ISC. (Fig. S27, ESI†).
 = 23 c.p. ET (30) = 40.4) and spectroscopy

To gain further insight into the ISC mechanism of the Pery-NI
,

time-resolved electron paramagnetic resonance (TREPR)
spectra were recorded (Fig. 10). TRPER spectroscopy is an
important tool to study the triplet excited states, as well as the
ISC mechanisms.29,37,46 Moreover, on the basis of zero field
splitting parameters (ZFS), i.e. the D and E values, it is possible
to differentiate the LE and CS states. To confirm the ISC
mechanism of the dyad, the TREPR measurements were carried
out in frozen solutions or liquid crystal matrix at different
temperatures varying from 80 K to 280 K (Fig. 10).
3
3
Interestingly, by excitation into the NI unit in Pery-NI
(
ex
=
3
355 nm), only the Pery* state was observed with nanosecond
transient absorption (Fig. 9a and b). Thus, the photophysics is
excitation wavelength-dependent, and most likely there is a
competition between ISC, FRET, and CS processes. NI is known
for its intrinsic ISC ability (for unsubstituted NI, ISC takes 10-20
ps).59
Firstly, compound 2 (3-bromoperylene) was studied (Fig. 10a).
The observed electron spin polarization (ESP) pattern is (e, e, e,
a, a, a), simulation indicates that the population of the triplet
zero-field states TZ, TY and TX with relative rates of p1 / p2 / p3 =
0.000 : 0.400 : 0.600, the ESP is characteristic for the triplet state
formed via SO-ISC mechanism.46 The |D| and |E| parameters
were determined as 1560 MHz and 105 MHz, respectively. The
TREPR spectra of the dyad Pery-NI with excitation at 445 nm,
measured in frozen solutions are presented in Fig. 10a. The ESP
of the TREPR spectrum of Pery-NI is (e, e, a, e, a, a), which is
The nanosecond transient absorption spectra of NI-Ph were
also studied (Fig. 9c and d). Two ESA bands in the range of 400
– 650 nm were observed, which are typical features of the NI
triplet state.69 This result indicates that the triplet state of Pery-
NI is always localized on the perylene moiety, which is
determined by the relative triplet energy levels of perylene
(1.53 eV),70, 71 and NI (2.20 eV) moieties.72, 73
different from that of compound 2.
The ZFS parameter |D| value of Pery-NI was determined as
1652 MHz, only slightly changed in comparison with compound
(1560 MHz). This result indicates that the triplet state of the
0.03
3Pery*
a
0.024
0.016
0.008
0.000
b
2
0.0 s
37.8 s
...
0.02
0.01
0.00
dyad is confined on the perylene moiety to large extent.
Furthermore, the |D| value of Pery-NI in solution and in crystal
matrix even at different temperatures varied slightly, which
confirmed the triplet state confinement. The triplet state spin
density surface supports this assignment (Fig. S31, ESI†). The
TREPR spectra of the triplet species were found almost similar
in different solvents like toluene, or mixed solution of
toluene/MeTHF, (3:1, v/v).
1861.8 s

= 237s
496 nm
GSB
400 500 600 700
Wavelength / nm
0
400 800 1200 1600
Time /s
0.04
0.02
0.00
-0.02
0.04
0.03
0.02
0.01
0.00
c
3NI*
d
0.0 s
36.6 s
...
Pery-NI
a
Comp 2
b
1063.6 s
280 K
TOL

= 181 s
500 nm
250 K
A
E
A
E
230 K
210 K
180 K
80 K
Pery-NI
Pery-NI
TOL/MeTHF
TOL
400
500
600
700
0
400 800 1200
Time /s
Wavelength / nm
Fig. 9 (a) Nanosecond time-resolved transient absorption spectra of Pery-NI, (b) the
decay trace at 496 nm. ex = 355 nm. c = 5.0 × 10−6 M. In deaerated DCM; (c) Nanosecond
time-resolved transient absorption spectra of NI-Ph, (d) decay trace at 500 nm, ex = 355
nm. c = 5.0 × 10−6 M. In deaerated n-hexane. 20 C.
YX Z
XY
Y Z
X
Z
Z XY
280 320 360 400 440
B0 / mT
280 320 360 400
B0 / mT
For Pery-Ph, no apparent triplet excited state was observed
upon photoexcitation, although a band at about 575 nm was
observed in polar solvents (Fig. S28, ESI†) Therefore, we used
intermolecular triplet photosensitizing method to produce the
triplet state by triplet-triplet energy transfer (TTET) from 2,6-
diiodobodipy (Scheme 1) to Pery-Ph. Triplet production was
Fig. 10 (a) TREPR spectra of compound 2 in toluene, Pery-NI in toluene and in
toluene/MeTHF (3:1, v/v) mixed solution at 80 K. (b) TREPR spectra of Pery-NI in liquid
crystal matrix (E7) at different temperatures (80 K  280 K); following pulsed laser
excitation at 445 nm, 10 mJ per pulse. Red lines are computer simulation and black
curves are experimental TREPR spectra. The simulated parameters are given in Table 4.
A stands for enhanced absorptive positive signal amplitudes, E stands for emissive
negative amplitude polarization of the respective transition.
observed (Fig. S29, ESI†) with a lifetime of 76 s.
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Journal of Materials Chemistry C
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DOI: 10.1039/D0TC00017E
Table 4. Zero-field splitting parameters D and E, and related triplet-state populations sublevels p1, p2, p3 obtained from the simulation of
the TREPR spectra in different mediums and at different temperatures
Compound
Medium
T
|D|
|E|
(MHz)
105
60
E / D
p1
p2
p3
Order
(K)
80
(MHz)
1560
1652
1666
1653
1639
1637
1747
1670
1668
Comp. 2
Pery-NI
TOL
0.067
0.036
0.037
0.037
0.039
0.039
0.018
0.011
0.022
0.000
0.000
0.141
0.068
0.011
0.000
0.000
0.000
0.000
0.400
0.036
0.000
0.000
0.000
0.123
0.319
0.397
0.224
0.600
0.964
0.859
0.932
0.989
0.877
0.681
0.603
0.776


TOL
80
TOL/MeTHF
E7 (matrix)
E7 (matrix)
E7 (matrix)
E7 (matrix)
E7 (matrix)
E7 (matrix)
80
62

80
61


180
210
230
250
280
64
63
0.02
31
5.34
5.60
4.39
19
37
For the dyad Pery-NI in liquid crystal matrix (E7), within the
temperature range from 80 K to 180 K an ESP phase pattern (e,
e, a, e, a, a) was observed, while over the transition
temperature of the liquid crystal E7 (210.6 K) the ESP pattern
changed to (e, e, e, a, a, a). We propose that the change in the
polarization of ESP pattern is due to some change in the
orientation of the molecules above the glass transition
temperature of E7, not to an altered ISC mechanism. The TREPR
spectra of Pery-NI at different temperatures (80 K to 280 K)
measured in liquid crystal matrix (E7), with complete simulation
parameters are presented in Fig. 10b and Table 4. TREPR spectra
obtained after different sample orientation are supplied in Fig.
S30 and Table S1, ESI†. The RP-ISC mechanism can be excluded
because no (a, e, e, a, a, e) or its inverse (e, a, a, e, e, a) ESP
pattern was observed.37 We assume that the ISC mechanism in
the dyad Pery-NI is SOCT-ISC, the absence of any heavy atom in
Pery-NI and the solvent polarity dependent ISC efficiency
support this conclusion
Fig. 11 The dihedral angles (in degree) of compounds, based on optimized ground state
geometries. Calculated at B3LYP/631G(d) level using Gaussian 09W.
DFT computations
The ground state geometries of the compounds were optimized with
DFT computations (Fig. 11). For Pery-NI, the dihedral angle between
the NI and the perylene moieties is 72.5. Although it is not a perfect
orthogonal geometry, still we observed a very high singlet oxygen
quantum yield in DCM (80%. Table 3).
The optimized ground state geometry of the dyad is not
sufficient for acquiring the panoramic view of the energy
landscape for the torsion between the electron donor/acceptor.
Therefore, the potential energy curve of the torsion of the C
C
linker between the NI and the perylene units was constructed
Although the orthogonal geometry will give the smallest steric (Fig. S31, ESI†) to evaluate the geometries accessible at room
hindrance, -conjugation will decrease the electronic energy. temperature. The PES in the range of dihedral angle 60  120
This trend is significant with the reference compounds NI-Ph is rather flat, indicating that the geometries accessible at room
and Pery-Ph, for which the dihedral angles between the phenyl temperature are those within the range of ca. 54 123 . This
and the aryl moieties were found as ca. 54 range is larger than that recently reported for perylene-
Previously we studied the compact electron donor/acceptor phenothiazine dyads showing efficient SOCT-ISC (79 103
).46





.


dyads with perylene as electron acceptor and phenothiazine as This finding further shows that different factors may affect the
the electron donor.46 The dihedral angle between the electron SOCT-ISC significantly.
donor and acceptor for that dyad is 90

, however, the singlet
The frontier molecular orbitals of the dyad and the reference
oxygen quantum yield is 60%. For the present dyad Pery-NI
,
Pery-Ph are shown in Fig. 12. For Pery-NI, the HOMO is
although the dihedral angle is deviated from orthogonality, the exclusively localized on the perylene moiety, and the LUMO is
singlet oxygen quantum yield is higher (80%). These results exclusively localized on the NI moiety. The S0 S1 transition
LUMO orbitals, thus charge

indicate that factors other than the orthogonal geometry also mainly involves the HOMO
contribute to the SOCT-ISC efficiency.

10 | J. Mater. Chem. C, 2020, 00, 1-9
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ARTICLE
View Article Online
DOI: 10.1039/D0TC00017E
Scheme 2 Simplified Jablonski diagram indicating the photophysical pathways of Pery-
NI. The energy levels of the excited singlet states are derived from spectroscopic data,

singlet charge separated states 1[Pery+ -NI
] energy levels in toluene (TOL),
dichloromethane (DCM) and acetonitrile (ACN) are obtained from the average of
absorption and emission maxima. The triplet states energy levels are estimated by DFT
calculations at B3LYP/6-31G (d) level using Gaussian 09W.
Fig. 12 HOMO/LUMO energy levels of Pery-NI and Pery-Ph. Calculated at the (B3LYP/6-
31G (d)) level with Gaussian 09W.
emission of PBI centered at 580 nm, which is long-lived (57.3 s.
separation is expected upon photoexcitation. Similarly, the
HOMO1 and LUMO+1 are also localized molecular orbitals.
Fig. 13a). This emission matches to the prompt fluorescence of
PBI (Fig. S34a, ESI†). In order to monitor the kinetics of the TTA
delayed fluorescence, the transient fluorescence signal at 580
nm was analyzed (Fig. 13c). Upon laser excitation, the delayed
The fully confined molecular orbitals may be characteristics of
electron donor/acceptor dyads showing efficient SOCT-ISC, this
feature was found for Bodipy-phenothiazine dyads,40 Bodipy-
anthracene dyads,38, 39 and anthracene-phenothiazine dyads.74,
fluorescence raised in 5.2
s and decayed with a time constant
of 57.3 s. The initial growth phase is due to the TTET between

75
Fully confined frontier molecular orbitals are beneficial for
Pery-NI and PBI via Dexter mechanism, causing the
accumulation of the PBI triplet state. After reaching the
maximal concentration, the decay of the PBI triplet occurs, with
the conservation of the angular momentum of ISC. These
factors may be useful for future design of the electron
donor/acceptor dyads showing efficient SOCT-ISC.
a time constant of 57.3 s, due to the TTA process, by which the
The frontier molecular orbitals of Pery-Ph are more delocalized.
The confinement of the triplet state wavefunction of the
compounds was also studied (Fig. S32, ESI†). For Pery-NI, the
triplet state is localized on the perylene unit to large extent, but
leakage of the spin to NI moiety is also clear. For NI-Ph and Pery-
Ph (reference compounds) the spin density is delocalized on NI
and perylene moieties along with phenyl ring respectively.
Based on the above studies, we propose the energy diagram
and the photophysical processes of the dyad Pery-NI upon
photoexcitation (Scheme 2). With excitation into the perylene
part, fast CS will occur (0.66 ps), followed by SOCT-ISC, and the
perylene triplet state is populated in 8 ns. Upon excitation into
the NI unit, CS occurs within 10 ps probably in competition with
FRET and 3Pery* is populated in 10 ns.
emissive singlet state is produced with a probability of 11.1%
(lower limit), according to the spin statistic law.15 Based on the
intrinsic triplet state lifetimes and the quenched triplet state
lifetimes of Pery-NI, the TTET efficiency was calculated as 86%.
The delayed fluorescence was confirmed by excitation of
sample solution in aerated conditions; in this case only a prompt
fluorescence/laser scattering was observed, but no long-lived
fluorescence emission was observed (note: with 445 nm
excitation, triplet energy acceptor (PBI) can also be excited).
Similar results were observed with Solvent Green 5 (SG5) as
acceptor (Fig. S35, ESI†).
Conclusions
In summary, a compact electron donor/acceptor dyad is
prepared, in which the perylene (Pery, electron donor) and
naphthalimide (NI, electron acceptor) are connected with a
Triplet-Triplet Energy transfer (TTET)/ Triplet-Triplet Annihilation
(TTA) induced delayed fluorescence
To show the possible application of the compact electron
donor/acceptor dyad as triplet PSs, Pery-NI was used for
generation of TTET/TTA-induced delayed fluorescence, with
perylenebisimide (PBI) as the energy acceptor/emitter (Fig. 13).
Upon nanosecond pulsed laser excitation, we observed the
single C
orthogonal (dihedral angle between the Pery and NI moiety is
72.5 ), efficient spin-orbit charge transfer intersystem crossing
C bond. The geometry of the dyad is close to

(SOCT-ISC) was observed (singlet oxygen quantum yield 
=
80%). The photophysics of the dyad is excitation
This journal is © The Royal Society of Chemistry 2020
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Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/D0TC00017E
0.020
0.015
0.010
0.005
0.000
0.03
800
700
600
500
400
a
In N2
0 s
6 s
...
288 s
294 s
b
c
In Air
*
0.02
0.01
0.00
1= 5.2 s
2= 57.3 s
400 500 600 700 800
Wavelength / nm
0
200
Time / s
400
600
0
50
100
Time / s
150
200
Fig. 13 (a) Delayed fluorescence spectra of the mixture of Pery-NI with PBI. (b) contour map of the mixture Pery-NI with PBI (c) decay trace of the fluorescence at 580 nm in aerated
and deaerated solution. c = [Pery-NI] = 2.0 × 10−5 M and [PBI] = 4.0 × 10−5 M, in DCM. ex = 445 nm. The asterisks in (c) indicated the scattered laser.
Hz), 7.94 (d, 1H, J = 8.0 Hz), 7.85 (d, 1H, J = 8.0 Hz), 7.77 (dd, 2H,
wavelength-dependent. Upon excitation into the perylene unit,
J = 8.0 Hz), 7.63
1H, J = 8.0 Hz), 4.28 (t, 2H, J = 8.0 Hz), 1.84



7.50 (m, 4H), 7.35 (dd, 1H, J = 8.0 Hz), 7.18 (d,
1.77 (m, 2H), 1.53
1.50 (m, 2H), 1.04 (t, 3H, J = 8.0 Hz). 13C NMR (CDCl3, 125 MHz):
164.29, 164.13, 145.43, 135.81, 134.69, 133.66, 132.84,
ultrafast charge separation (CS) was observed (0.66 ps), followed by
SOCT-ISC (8 ns), giving rise to the 3Pery state (T = 214 s). In this case
a long-lived perylene radical cation was also observed (752 s). On
the other hand, upon selective photoexcitation into the NI unit, only
the 3Pery state was observed with nanosecond transient absorption
spectroscopy. Studies in viscous solvents, such as triethylene glycol,
suggested that the formation of the persistent perylene radical
cation is an intermolecular process. Pulsed laser excited time-
resolved electron paramagnetic resonance spectroscopy (TREPR)
confirmed the SOCT-ISC mechanism, since the observed ESP pattern
is different from that typical for SO-ISC. DFT computations showed
that the potential energy curve for the torsion of the linker between
the donor and acceptor is shallow, yet efficient SOCT-ISC occurs.
With the dyad as triplet photosensitizers, delayed fluorescence of
different acceptors was observed (luminescence lifetime up to
57.3s). These results are useful for future design of electron
donor/acceptor dyads showing efficient triplet state, as well as for

132.11, 131.66, 131.30, 131.21, 130.98, 130.85, 130.77, 128.89,
128.82, 128.62, 128.50, 128.42, 128.22, 127.07, 126.96, 126.74,
126.69, 125.80, 122.98, 122.35, 120.76, 120.63, 120.53, 119.61,
40.32, 30.28, 20.44, 13.89. Calcd ([C36H25NO2]+), m/z = 503.1885;
found, m/z = 503.1883.
Cyclic voltammetry
The cyclic voltammetry curves were recorded by CH instrument
(CHI610D electrochemical workstation, Inc., Shanghai China). In
a nitrogen purged saturated ACN solution containing 0.10 M
Bu4NPF6 as a supporting electrolyte; counter electrode is
platinum electrode; glassy carbon electrode is working
electrode; and the Ag/AgNO3 (0.1 M in ACN) couple as the
reference electrode. DCM was used as the solvent and
ferrocene was used as an internal reference. The experiments
design of the heavy atom-free triplet photosensitizers.
Experimental section
are carried out at 20 C.
General information
Determination of singlet oxygen quantum yield
All the chemicals used in the synthesis are analytically pure and
The singlet oxygen quantum yields of the compounds were
were used as received. Solvents were dried prior to use. UV
absorption spectra were taken on UV-2550 UV


Vis
Vis
measured with Ru(bpy)3[PF6]2 as standard (Δ = 0.57 in DCM).
1
1,3-Diphenylisobenzofuran (DPBF) was used as O2 scavenging
spectrophotometer (Shimadzu Ltd., Japan). Fluorescence
spectra were recorded on RF-5301PC spectrofluorometer
(Shimadzu Ltd., Japan).
agent. The absorbance of DBBF was adjusted to around 1.0 in
the air saturated dichloromethane (DCM) solvent. After that,
compound was added to the solution of DPBF, and the
compound’s absorbance was adjusted around 0.2−0.3. A
monochromatic light was used to irradiate the solution in the
cuvette for 30 seconds at the wavelength of peak absorption.
Then the absorbance at 414 nm was measured after each
irradiation and took 10 readings continuously. The slope of the
plots of the absorbance of DPBF at 414 nm vs irradiation time
for each compound was calculated in every solvent. Singlet
oxygen quantum yields were calculated by using the following
equation:
Synthesis of compound Pery-NI: Under N2 atmosphere,
compound 3 (132.3 mg, 0.35 mmol) and compound 4 (116.2 mg,
0.35 mmol) were mixed in DMF (10 mL) and water (0.5 mL) and
stirred. After that, K2CO3 (145.1 mg, 3 mol %) and Pd(PPh3)4
(20.2 mg, 5 mol %) was added. The reaction mixture was stirred
at 80 C for 4 h. After completion of the reaction, the reaction
mixture was cooled to room temperature, DCM (50 mL) and
water (5 mL) were added. The organic layer was washed with
brine (3  50 mL) and dried over anhydrous Na2SO4. The solvent
was removed under reduced pressure and the residue was
purified by column chromatography (silica gel, DCM/n-hexane
= 2:1, v/v) to give reddish orange solid (90 mg, yield: 51%). Mp:
1
283.0

284.0

C. H NMR (CDCl3, 400 MHz):

8.77 (d, 1H, J =
8.0 Hz), 8.66 (d, 1H, J = 8.0 Hz), 8.36 (q, 2H), 8.26 (dd, 2H, J = 8.0
12 | J. Mater. Chem. C, 2020, 00, 1-9
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Journal of Materials Chemistry C
ARTICLE
Where ‘sam’ and ‘std’ designate the compounds and standard The TREPR experiments were performed in frozen solutions at
View Article Online
(Ru(bpy)3[PF6]2) respectively, A stands for absorbance, m is the 80 K, additionally in liquid crystal E7 orienDteOdI: i1n0.a10m39a/gDn0eTCti0c0f0ie17ldE
slope of the curves of absorbance of DPBF at 414 nm verses the at different temperatures. All samples were excited at 445 nm
irradiation time, η is the refractive index of the solvents.
using a Nd:YAG laser GCR 190-10 (Spectra Physics, Santa Clara,
CA). The laser repetition rate was set to 10 Hz (pulse duration:
10 ns) at an energy of 1 mJ per pulse. A Bruker ESP-300 (Bruker,
Rheinstetten, Germany) spectrometer was used in combination
with a Bruker ER046 MRT (Bruker, Rheinstetten, Germany)
Nanosecond transient absorption spectroscopy
The nanosecond transient absorption spectra were measured
on LP980 laser photolysis spectrometer (Edinburgh Instrument,
UK) and recorded on a Tektronix TDS 3012B oscilloscope. The
sample solutions were excited with a nanosecond pulsed laser
(OpoletteTM 355II+UV nanosecond pulsed laser, the wavelength
is tunable in the range of 200 − 2200 nm. Typical laser energy is
5 mJ per pulse. OPOTEK, USA). The lifetime values (by
monitoring the decay trace of the transients) were obtained
with the L900 software. To monitor the complete decay of long-
lived species, the CW-mode of LP980 spectrometer was used.
The samples were purged with N2 for 10 − 15 minutes before
measurements.
microwave bridge.
A special (Ilmasil PS, Qsil GmbH,
Langewiesen, Germany) quartz tubes were used for
measurements having 3 mm inner diameter, and irradiated in a
dielectric-ring resonator ER 4118-MD5 (Bruker, Rheinstetten,
Germany). The resonator was immersed into a CF935 (Oxford
Instruments, Oxford, UK) helium-gas flow cryostat and the
temperature stabilizer by a temperature controller ITC503
(Oxford Instruments, Oxford, UK). The microwave frequency
was read by a frequency counter 5352B (Hewlett-Packard,
Böblingen, Germany). Signal acquisition was performed with a
transient recorder 9354A (LeCroy, Chestnut Ridge, NY) at an
overall detection bandwidth of 25 MHz. Distortions of the signal
baseline introduced by the laser pulse were corrected by
subtracting a signal collected at an off-resonant magnetic-field
position.
Subnanosecond transient absorption spectroscopy
The detail information of the setup was described previously.76,
77
It consists of white-light continuum probe pulses generated
in a 200
femtosecond pulses (370

m CaF2 nonlinear crystal, and spectrally tunable pump
550 nm; a few J pulse energy) were


generated in an optical parametric amplifier (Coherent). The DFT computations
pump and probe pulses were overlapped spatially and
Density functional theory was used to optimized the molecular
temporally into 1 mm thick cuvette cell containing the sample.
Change in transient absorption was monitored by focusing the
transmitted probe light through the solution onto a broadband
UV-Vis-near-IR detector.
structures of the synthesized molecules by using B3LYP 6-31G
(d) functional basis set. The excitation energy and energy gaps
between S0 state and excited triplet states of the compounds
were approximated based on the optimal ground-state
geometry. All these calculations were performed with Gaussian
09W.79, 80
Femtosecond transient absorption spectroscopy
Transient absorption spectra (fs-TA) were acquired on a system
based on Ti:sapphire regenerative amplifier (BMI Alpha 1000)
pumped by a Ti:sapphire oscillator (Spectra Physics Tsunami)
with 100 fs time resolution. The system produces 100-fs pulses
at 785 nm, with a repetition rate of 1 kHz and average power of
450-500 mW. Excitation pulses at 400 nm were obtained by
second-harmonic generation of the fundamental laser output
through a BBO crystal. The probe pulses were generated by
focusing a small portion of the fundamental laser output
radiation on a 3 mm thick calcium fluoride window. The pump
beam polarization has been set to the magic angle with respect
Acknowledgements
J. Z. thanks the NSFC (21673031, 217611442005, 21911530095
and 21421005), the Fundamental Research Funds for the
Central Universities (DUT19TD28) and State Key Laboratory of
Fine Chemicals (ZYTS201901) for financial support. We thank
Prof. Jan W. Verhoeven at University of Amsterdam for helpful
discussion.
to the probe beam by rotating a /2 plate. Pump-probe delays
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Table of Contents Entry:
Intersystem Crossing via Charge Recombination in a Perylene-Naphthalimide
Compact Electron Donor/Acceptor Dyad
Muhammad Imran,a‡ Ahmed M. El-Zohry,b‡ Clemens Matt,c‡ Maria Taddei,d Sandra Doria,d Laura Bussotti,d Paolo
Foggi,d,f Jianzhang Zhao,a,* Mariangela Di Donato,d,e,* Omar F. Mohammed b,* and Stefan Weber c,*
O
N
O
Br
e-
280 320 360 400
Magnetic Field / mT
Efficient triplet state production (singlet oxygen quantum yield: 80%) by SOCT-ISC mechanism was
observed for perylene-naphthalimide compact electron donor/acceptor dyad.