Anthracene-Naphthalenediimide Compact Electron Donor/Acceptor Dyads: Electronic Coupling, Electron Transfer, and Intersystem Crossing

Article abstract of DOI:10.1021/acs.jpca.8b11828

We attached different electron donors of phenyl, anthryl, and alkylamino moieties, to electron acceptor naphthalenediimide (NDI) to construct compact electron donor/acceptor dyads. The purpose is to study the effect of electron coupling (the magnitude is the matrix element, V_DA) on the photophysical properties of UV-vis absorption, fluorescence emission, especially spin-orbit charge transfer intersystem crossing. We found that the magnitude of V_DA depends on the electron donating strength of the aryl moieties (V_DA = 0.22-0.55 eV), as well as the molecular conformation, based on steady state and time-resolved transient absorption spectroscopies. We also found that electron coupling does not show the add-up (or synergetic) effect. Solvent polarity-dependent intersystem crossing (ISC) was observed for the dyad/triads and singlet oxygen quantum yield decreases in polar solvents. Femtosecond transient absorption results indicate that the charge separation (CS) for 9-An-NDI-NH occurs on time scale of 0.83 ps (in toluene) or 0.71 ps (in acetonitrile). The charge recombination (CR) process (50 ps in toluene) produces triplet state with ω_ISC = 19%. The triplet state lifetime is up to 22 μs. This result indicates that orthogonal geometry for a compact electron donor/acceptor does not lead to efficient ISC via CR. Other factors such as the energy gap between the CS state and triplet state also determine the ISC efficiency.

Full text of DOI:10.1021/acs.jpca.8b11828

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A: Kinetics, Dynamics, Photochemistry, and Excited States
Anthracene-Naphthalenediimide Compact Electron Donor/Acceptor
Dyads: Electronic Coupling, Electron Transfer and Intersystem Crossing
Kepeng Chen, Jianzhang Zhao, Xiaoxin Li, and Gagik G. Gurzadyan
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b11828 • Publication Date (Web): 12 Mar 2019
Downloaded from http://pubs.acs.org on March 15, 2019
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Anthracene-Naphthalenediimide Compact Electron
Donor/Acceptor Dyads: Electronic Coupling,
Electron Transfer and Intersystem Crossing
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Kepeng Chen,a Jianzhang Zhao,*,a,b Xiaoxin Lic and Gagik G. Gurzadyan*,c
a State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University
of Technology, E-208 West Campus, 2 Ling-Gong Road, Dalian 116024, P. R. China. E-mail:
zhaojzh@dlut.edu.cn
b School of Chemistry and Chemical Engineering and Key Laboratory of Energy Materials
Chemistry, Ministry of Education, Institute of Applied Chemistry, Xinjiang University, Urumqi
830046, Xinjiang, China.
c Institute of Artificial Photosynthesis, State Key Laboratory of Fine Chemicals, Dalian
University of Technology, West Campus, 2 Ling-Gong Road, Dalian 116024, P. R. China. E-
mail (G.G.G.): gurzadyan@dlut.edu.cn
RECEIVED DATE (automatically inserted by publisher)
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Abstract: We attached different electron donors of phenyl, anthryl and alkylamino moiety, to
electron acceptor naphthalenediimide (NDI) to construct compact electron donor/acceptor dyads.
The purpose is to study the effect of electron coupling (the magnitude is the matrix element, VDA)
on the photophysical properties of UVvis absorption, fluorescence emission, especially spin-
orbit charge transfer intersystem crossing. We found that the magnitude of VDA depends on the
electron donating strength of the aryl moieties (VDA = 0.22 eV ~ 0.55 eV), as well as the
molecular conformation, based on steady state and time-resolved transient absorption
spectroscopies. We also found that electron coupling does not show the add-up (or synergetic)
effect. Solvent polarity-dependent intersystem crossing (ISC) was observed for the dyad/triads
and singlet oxygen quantum yield decreases in polar solvents. Femtosecond transient absorption
results indicate that the charge separation (CS) for 9-An-NDI-NH occurs on time scale of 0.83
ps (in toluene) or 0.71 ps (in acetonitrile). The charge recombination (CR) process (50 ps in
toluene) produces triplet state with ISC = 19%. This result indicates that orthogonal geometry
for a compact electron donor/acceptor does not lead to efficient ISC via CR. Other factors such
as the energy gap between the CS state and triplet state also determine the ISC efficiency.
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Tuning the photophysical properties of organic chromophores by molecular structure
modification is pivotal for designing of functional organic materials,1 for instance the
photosensitizers for photovoltaics,2,3 artificial photosynthesis,4,5 photodynamic therapy (PDT),6 9

novel biosensors,10 12 and triplet-triplet-annihilation upconversion (TTA UC), etc.13 21
Concerning this aspect, two popular approaches are available for tuning the photophysical
properties of organic chromophores, one is introducing electron donating or withdrawal groups


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to the -conjugation framework.22,23 In this case the absorption and fluorescence emission
feature of a chromophore can be changed substantially.24 Another approach is to connect
different chromophores together, with a non-conjugation linkage (dyad, triads, etc.). In this case
the two components connected are in supramolecular feature, the two components retain their
own property at the ground state. With this method, light-harvesting molecular polyads were
developed.25,26 In general, there exist strong electronic coupling (the magnitude is electronic
coupling matrix elements, VDA) between the different units for the first method.1 However, for
the second approach, the geometry or mutual orientation and the distance between the two units
can affect the interaction, and normally the interaction between the two units is weak.4,5
Actually, another method exists for tuning the photophysical properties of the organic
chromophores. i.e. to construct a molecular system, in which there is weak to moderate
electronic couplings between the electron donor and acceptors.1,4,5,27 This method is unique in the
sense that new absorption bands can be resulted,1 for instance, the charge transfer (CT)
absorption band. This feature is different from the fully coupled systems, which usually give a
new narrow absorption band, but with the disappearance of the original absorption bands of the
respective chromophores. More importantly, this method offers the possibility to tune the
intersystem crossing (ISC) property of the heavy atom-free chromophores, by employing the ISC
channel of the charge recombination (CR), or by the hyperfine coupling interaction (HFI), i.e. the
radical-pair ISC (RP-ISC).4,5,27 However, the CR induced triplet formation attracted much less
attention until now.
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Concerning ISC, compact electron donor/acceptor dyads, i.e. the electron donor and acceptor
are connected with short linker or directly linked together, are with large J values, thus the HFI is
non-sufficient to ensure efficient ISC, usually the ISC of such compact dyads is non-efficient.4,5
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Recently it was found the spin-orbit charge transfer intersystem crossing (SOCT-ISC) occurs for
electron donor/acceptor systems with strong coupling, given the electron donor and acceptor are
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in orthogonal geometry (in other words,  planes are in perpendicular geometry).28 34 As a result,

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the molecular angular momentum changes generated during CR can compensate the electron
spin angular momentum changes for ISC, thus the CR is accompanied with electron spin flip, i.e.
the ISC is greatly enhanced. This is in particular interest for preparation of heavy atom-free
organic triplet photosensitizers, and the molecular structure can be greatly simplified, as
compared to other heavy atom-free triplet photosensitizers, such as those based on symmetry
breaking charge transfer dyads,35,36 exciton coupling,37,38 or those based on electron spin
converters.39 42 However, the examples for SOCT-ISC are still limited, and the chromophores

used for SOCT-ISC are limited to pyrene,28 acridine,29 and anthracene.30 Recently Bodipy unit
was used for SOCT-ISC studies.31 34,43 However, It is clear that more molecular structures need

to be studied for elucidation of the molecular structure-ISC property for the SOCT-ISC
molecular systems.
Concerning these aspects, naphthalenediimide (NDI) is interesting for its excellent photo- and
chemo- stability, feasible derivatization chemistry, and interesting photophysical properties.44 46

Previously NDI was used for molecular dyads/triads with very weak electronic coupling,27 but it
was not used for compact electron donor/acceptor dyads, and it was not used for study of SOCT-
ISC.28 34 Herein NDI was selected as a light absorption chromophore (also the electron acceptor)

to construct compact electron donor/acceptor dyads. NDI is a strong electron acceptor,47 50 which

favors electron transfer. Different aryl moieties, such as phenyl, anthryl and arylamine, are used
as electron donor.51 In the case of anthryl electron donor, different conformation was achieved by
connection at either the 9- or 2- position of the anthryl moiety, as such the VDA was varied
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because the dihedral angel between the two parts are different (Scheme 1).30 The electron
coupling between the electron donors and the NDI moiety, as well as its effect on the UVvis
absorption and emission properties, were studied with various steady state and time-resolved
transient spectroscopies. SOCT was observed for the days attached with anthryl moiety and the
SOCT-ISC efficiency is highly dependent on the solvent polarity. We found that the orthogonal
geometry is not the only requirement for efficient ISC. The results may be useful for study of the
ISC via CR, a ubiquitous process for organic photovoltaics and artificial photosynthesis.
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2.1. General Methods. All the chemicals used in synthesis are analytical pure and were used
as received. Fluorescence quantum yields were measured in toluene. For NDI, anthracene (F =
27% in ethanol) was used as reference, for the other compounds, diiodobodipy (F = 2.7% in
CH3CN) was used as reference. The structures of diiodoBodipy is shown in Scheme 1. The
fluorescence lifetimes of the compounds were measured with OB920 luminescence lifetime
spectrometer (with TCSPC detection mode. Edinburgh Instrument Ltd., UK). For NDI, the
fluorescence lifetimes were measured with 340 nm EPL picosecond pulsed laser (pulse width:
800 ps, maximum average power: 40 W). For Ph-NDI-NH, Br-NDI-NH, 2-An-NDI-NH and
9-An-NDI-NH, the luminescence lifetime were measured with 445 nm EPL picosecond pulsed
laser (pulse width: 70 ps, maximum average power: 5 mW). Fluorescence spectra were recorded
with a RF 5301PC spectrofluorometer (Shimadzu, Japan).
2.2. Synthesis of NDI. 1,4,5,8-naphthalenetetracarboxylic dianhydride (NDA) (670 mg, 2.5
mmol) was added in freshly distilled DMF (5 mL) at room temperature, then 2-ethylhexylamine
(970 mg, 7.5 mmol) was added to the above slurry. The slurry was heated at 140 °C for
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overnight under nitrogen (N2) atmosphere. This mixture was then cooled to room temperature
and poured into cold water (100 mL). The precipitate was collected with filtration and washed
with water. After drying under vacuum, the solid was further purified by column
chromatography (silica gel; DCM/petroleum ether, 1:1, v/v) to give the product as a white solid.
Yield: 545 mg, 42.0%. 1H NMR (CDCl3, 400 MHz): 8.76 (s, 4H), 4.20–4.10 (m, 4H), 1.97–1.91
(m, 2H), 1.38–1.25 (m, 16H), 0.96–0.88 (m, 12H). TOF–HRMS ([C30H38N2O4]+): calcd m/z =
490.2832, found m/z = 490.2821.
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2.3. Synthesis of 2Br-NDA. A mixture of 1,4,5,8-naphthalenetetracarboxylic dianhydride
(NDA) (5.00 g, 18.64 mmol) and oleum (20% SO3, 75 mL) was stirred at room temperature.
Iodine (60 mg, 0.24 mmol) and bromine (5.96 g, 37.3 mmol) were then added to the above
mixture. The resulting mixture was heated at 50 °C for 48 h. After cooling to room temperature,
the mixture was poured into cold water (100 mL). The precipitate was collected with filtration
and washed with water. After drying under vacuum, 6.19 g yellow solid was obtained. This
crude product was used for the next step of the synthesis without further purification.
2.4. Synthesis of 2Br-NDI. Under N2 atmosphere, the mixture of 2Br-NDA (1.06 g, 2.5
mmol), 2-ethylhexylamine (968 mg, 7.5 mmol) and acetic acid (25 mL) was stirred at 120 °C for
2 h. After cooling to room temperature, the mixture was poured into cold water (100 mL). The
precipitate was collected with filtration and washed with water (3  50 mL) and methanol (3 
50 mL). After drying under vacuum, the reddish solid was purified by column chromatography
(silica gel; CH2Cl2/petroleum ether, 1:1, v/v). A yellow solid was obtained. Yield: 524 mg,
1
31.0%. H NMR (CDCl3, 400 MHz): 9.00 (s, 2H), 4.15 (t, J = 5.9 Hz, 4H), 1.94 (s, 2H),
1.391.29 (m, 16H), 0.950.88 (d, 12H). TOF–HRMS ([C30H36N2O4Br2]+): calcd m/z =
646.1042, found m/z = 646.1030.
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2.5. Synthesis of Bis-Ph-NDI. Under N2 atmosphere, a solution of sodium carbonate (25 mg,
0.24 mmol) in distilled water (1 mL) was added to a solution of phenylboronic acid (43 mg, 0.36
mmol), 2Br-NDI (52 mg, 0.08 mmol) in 1,4-dioxane (3 mL) and then tetrakis(triphenyl-
phosphane)palladium (1 mg, 0.0008 mmol) was added after the above solution was degassed
with N2 for 30 min. The mixture was stirred at reflux for 14 h under N2. After cooling to room
temperature, a solution of sodium sulfite (1.0 g) in water (10 mL) was added to the reaction
mixture. The mixture was extracted by DCM and dried over anhydrous Na2SO4. The solvents
were removed under reduced pressure and the residue was purified by chromatography (silica gel;
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DCM/petroleum ether, 1:1, v/v). A yellow solid was obtained. Yield: 5 mg, 10.0%. H NMR
(CDCl3, 400 MHz): 8.66 (s, 2H), 7.547.48 (m, 6H), 7.427.40 (m, 4H), 4.03 (d, J = 8 Hz, 4H),
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1.911.85 (m, 2 H), 1.35–1.26 (m, 16H), 0.97–0.86 (m, 12H). C NMR (100 MHz, CDCl3):
162.9, 162.6, 147.7, 140.5, 135.9, 128.4, 128.3, 128.1, 127.2, 125.6, 123.0, 44.4, 37.7, 30.6, 28.5,
23.9, 23.1, 14.1, 10.6. TOF–HRMS ([C42H46N2O4]+): calcd m/z = 642.3458, found m/z =
642.3434.
2.6. Synthesis of Br-NDI-NH. Under N2 atmosphere, a mixture of 2Br-NDI (488 mg, 0.75
mmol), 2-ethylhexylamine (284 mg, 2.2 mmol), and 2-methoxyethanol (3 mL) was stirred at
120 °C for 6 h. After cooling to room temperature, water was added to the mixture and then the
mixture was extracted with DCM. After removing the solvent under reduced pressure, the crude
product was purified by column chromatography (silica gel; DCM/petroleum ether, 1:1, v/v). A
red solid was obtained. Yield: 246 mg, 47.0%. 1H NMR (CDCl3, 400 MHz): 10.19 (t, J = 5.4 Hz,
1H), 8.87 (s, 1H), 8.30 (s, 1H), 4.18–4.10 (m, 4H), 3.50 (t, J = 5.8 Hz, 2H), 1.96–1.90 (m, 2H),
1.801.77 (m, 1H), 1.57–1.29 (m, 24H), 1.01–0.87 (m, 18H). TOF–HRMS ([C38H54N3O4Br]+):
calcd m/z = 695.3298, found m/z = 695.3317.
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2.7. Synthesis of 9-An-NDI-NH. 9-An-NDI-NH was prepared with a method similar to that
used for Bis-Ph-NDI, except the molar ratio of the 9-anthraceneboronic acid and Br-NDI-NH is
2:1. A red solid was obtained. Yield: 8 mg, 15.0%. 1H NMR (CDCl3, 400 MHz): 10.29 (t, J = 5.2
Hz, 1H), 8.55 (d, J = 9.9 Hz, 2H), 8.39 (s, 1 H), 8.08 (d, J = 8.4 Hz, 2H), 7.43 (t, J = 8.5 Hz, 2H),
7.33 (d, J = 9.5 Hz, 2H), 7.23 (d, J = 9.2, 2H), 4.20–4.10 (m, 2H), 3.80 (d, J = 7.4 Hz, 2H), 3.58
(t, J = 5.9 Hz, 2H), 2.02–1.94 (m, 1H), 1.85–1.82 (m, 1H), 1.64–1.30 (m, 18H), 1.11–1.01 (m,
9H), 0.97–0.86 (m, 10H), 0.74–0.66 (m, 6H). 13C NMR (CDCl3, 100 MHz): 166.8, 163.2, 162.2,
152.5, 137.8, 136.7, 135.6, 134.1, 133.5, 131.5, 131.4, 129.4, 128.8, 128.3, 127.2, 126.8, 125.7,
125.6, 125.5, 125.0, 123.2, 120.4, 120.3, 100.1, 46.5, 44.0, 43.9, 39.4, 37.8, 37.5, 31.2, 30.7, 30.3,
28.9, 28.6, 28.4, 24.6, 24.0, 23.8, 23.1, 23.0, 22.9, 14.1, 14.0, 14.0, 11.0, 10.7, 10.5. TOF–HRMS
([C52H63N3O4]+): calcd m/z = 793.4819, found m/z = 793.4837.
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2.8. Synthesis of Bis-2-An-NDI. Bis-2-An-NDI was prepared with a method similar to that
used for Bis-Ph-NDI but the molar ratio of the 2-anthraceneboronic acid and 2Br-NDI is 4:1. A
1
dark blue solid was obtained. Yield: 16 mg, 30.0%. H NMR (CDCl3, 400 MHz): 8.86 (s, 2H),
8.50 (s, 4H), 8.13 (s, 2H), 8.09–8.04 (m, 6H), 7.53–7.51 (m, 4H), 7.45 (d, J = 1.6 Hz, 1H), 7.44
(d, J = 1.6 Hz, 1H), 4.11–4.03 (m, 4H), 1.94–1.88 (m, 2H), 1.37–1.30 (m, 12H), 0.91–0.84 (m,
16H). 13C NMR (CDCl3, 100 MHz): 163.0, 162.7, 147.5, 137.8, 136.3, 132.3, 132.1, 131.5,
130.9, 128.3, 127.7, 127.3, 127.1, 126.6, 126.4, 125.8, 125.7, 125.6, 123.0, 44.5, 37.7, 30.6, 29.7,
28.5, 23.9, 23.1, 14.1, 10.6. TOF–HRMS ([C58H54N2O4]+): calcd m/z = 842.4084, found m/z =
842.4077.
2.9. Synthesis of Ph-NDI-NH. Ph-NDI-NH was prepared with a method similar to that used
for Bis-Ph-NDI, except the molar ratio of the phenylboronic acid and Br-NDI-NH is 3:1. A
1
yellow solid was obtained. Yield: 29 mg, 60.0%. H NMR (CDCl3, 400 MHz): 10.19 (t, J = 5.2
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Hz, 1H), 8.51 (s, 1H), 8.31 (s, 1H), 7.48–7.40 (m, 3H), 7.35 (m, 2H), 4.20–4.10 (m, 2H), 4.06–
3.97 (m, 2H), 3.52 (t, J = 5.8 Hz, 2H), 2.00–1.77 (m, 3H), 1.58–1.52 (m, 2H), 1.51–1.46 (m, 2H),
1.40–1.21 (m, 20H), 1.00 (t, J = 7.4 Hz, 3H), 0.94–0.84 (m, 15H). 13C NMR (CDCl3, 100 MHz):
166.6, 163.3, 163.1, 152.2, 141.5, 141.2, 135.5, 128.4, 128.3, 128.1, 127.6, 123.0, 122.5, 120.3,
120.2, 99.9, 46.4, 44.3, 43.9, 39.4, 37.8, 37.7, 31.2, 30.7, 30.6, 28.8, 28.8, 28.6, 28.5, 24.5, 24.0,
23.9, 23.1, 23.0, 14.1, 14.1, 14.0, 11.0, 10.7, 10.6. TOF–HRMS ([C44H59N3O4]+): calcd m/z =
693.4506, found m/z = 693.4522.
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2.10. Synthesis of 2-An-NDI-NH. 2-An-NDI-NH was prepared with a method similar to that
used for Bis-Ph-NDI, except the reaction ratio of the 2-anthraceneboronic acid and Br-NDI-NH
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is 2:1 and finally obtained as a red solid. Yield: 36 mg, 65.0 %. H NMR (CDCl3, 400 MHz):
10.21 (t, J = 7.0 Hz, 1H), 8.67 (s, 1H), 8.44 (s, 2H), 8.32 (s, 1H), 8.03 (d, J = 8.9 Hz, 2H), 8.00
(d, J = 6.4 Hz, 2H), 7.50–7.46 (m, 2 H), 7.38 (dd, J1 = 1.6, J2 = 8.7 Hz, 1H), 4.23–4.11 (m, 2H),
4.05–4.00 (m, 2H), 3.53 (t, J = 5.8 Hz, 2H), 1.99–1.93 (m, 1H), 1.89 (t, J = 5.8 Hz, 1H), 1.85–
1.78 (m, 1H), 1.59–1.29 (m, 24H), 1.01 (t, J = 8.5 Hz, 3H), 0.96–0.92 (m, 6H), 0.91–0.84 (m,
9H). 13C NMR (CDCl3, 100 MHz): 166.6, 163.3, 163.1, 152.2, 141.3, 138.6, 135.9, 132.0, 132.0,
131.7, 130.8, 129.1, 128.2, 127.3, 127.2, 126.8, 126.7, 126.2, 125.4, 123.0, 122.7, 120.5, 120.3,
99.9, 46.5, 44.4, 43.9, 39.4, 37.8, 37.8, 31.2, 30.7, 30.6, 29.7, 28.8, 28.6, 28.6, 24.5, 24.1, 23.9,
23.1, 23.0, 14.1, 14.1, 14.0, 11.0, 10.7, 10.6. TOF–HRMS ([C52H63N3O4]+): calcd m/z =
793.4819, found m/z = 793.4830.
2.11. Singlet Oxygen Quantum Yield (). 1,3-Diphenylisobenzofuran (DPBF) was used as
1O2 scavenger. The 1O2 production was monitored by following the absorbance of DPBF at 414
nm. To determine the singlet oxygen quantum yield (), a relative method was used according
to Eq. (1):
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(Eq. 1)
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where ‘sam’ and ‘std’ represent the sample and the standard. , A, m and  represent the singlet
oxygen quantum yield, the absorbance at excitation wavelength, the slope of the absorbance of
DPBF changing over time, and the refractive index of the solvent used for measurement,
respectively. Optically matched solutions were used (the solutions of the sample and the standard
give same absorbance at the excitation wavelength). Singlet oxygen quantum yields () were
measured in different solvents. For NDI, Bis-Ph-NDI, Bis-2-An-NDI, the excitation is at 358
nm (S0S1 band, A = 0.215). The monochromatic light source includes a xenon lamp and a
monochromator. Phenazine was used as standard ( = 0.83 in benzene) for NDI and anthracene
was used as standard ( = 0.70 in methanol) for other compounds; For Ph-NDI-NH, Br-NDI-
NH, 2-An-NDI-NH and 9-An-NDI-NH, the excitation is at 496 nm (S01CT band, A = 0.223)
and 2,6-diiodo-bodipy was used as standard ( = 0.85 in toluene).
2.12. Triplet State Quantum Yield (T). To determine the triplet state quantum yield with
nanosecond transient absorption spectroscopy, a ground state bleaching method was used and the
T was calculated according to:
(Eq. 2)
In the above equation, ‘sam’ and ‘std’ represent the sample and the standard. ,  and A
respectively represent the triplet quantum yield, the molar absorption coefficient at steady state
and the change of optical density (absorbance) of bleaching band in nanosecond transient
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absorption spectroscopy. Optically matched solutions were used (the solutions of the sample and
the standard give same absorbance at the excitation wavelength). Diiodo-bodipy in toluene was
used as standard (T = 88%).
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2.13. Nanosecond Transient Absorption Spectroscopy. The nanosecond transient absorption
spectra were studied with LP980-K Laser Flash Photolysis Spectrometer (Kinetic mode,
Edinburgh Instruments, UK). The analogue signal was digitized with a Tektronix TDS 3012C
oscilloscope. The samples were bubbled with N2 for 15 min and excited with a nanosecond
pulsed laser (OpoletteTM, the wavelength is tunable in the range of 210 − 2400 nm. OPOTEK,
USA).
2.14. Femtosecond Transient Absorption and Fluorescence Up-conversion Spectroscopy.
The femtosecond transient absorption spectra were measured by a pump-probe setup described in
detail previously.52 Briefly, the amplified pulse is with wavelength of 800 nm and duration of 35
fs, repetition rate 1 kHz and average power 4 W (Spitfire Ace, Spectra-Physics). The output
beam was split into two parts, 90% was converted into UV-VIS-IR in the range of 240  2400
nm radiation using an optical parametric amplifier (Topas, Light Conversion) and used as pump
beam. The left 10% output beam was used to generate a white light continuum (WLC) in a 3 mm
thickness rotated CaF2 plate, and used as a probe beam. The weak probe beam passed through a
variable delay line (up to 6 ns) and its absorbance change in the presence and in the absence of
the pump beam is measured. The polarizations between pump and probe beams were set at magic
angle 54.7 in order to avoid rotational depolarization effects. The entire setup was controlled by
a PC with the help of LabView software (National Instruments). Global target analyses were
carried out using Glotaran software.53
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The fluorescence lifetimes were recorded with a time-resolved fluorescence upconversion
spectrometer (Newport) in combination with a mode-locked Ti-sapphire laser (Mai Tai DeepSee,
Spectra-Physics). Briefly, the femtosecond laser system generated light pulses at 800 nm of
duration 150 fs at a repetition rate 80 MHz and average power 2.9 W. The frequency of the laser
pulse was doubled with a BBO crystal and used for excitation (400 nm). The residual 800 nm
fundamental pulse was served as a gate beam. All measurements were performed at room
temperature under aerated conditions.
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2.15. Theoretical Computations. The spin density surfaces of the compounds were calculated
based on the DFT//B3LYP/6-311G(d, p)-optimized ground state geometries. The Gaussian 09W
program package was used for the calculations.54
3.1. Design and Synthesis of the Compounds. Firstly we designed Bis-Ph-NDI and Bis-2-
An-NDI (scheme 1), in which phenyl and anthryl moieties were used as electron donors. Anthryl
moiety (HOMO energy level is 5.47 eV) is a stronger electron donor than the phenyl moiety
(HOMO energy level is 6.69 eV), which were obtained by DFT calculation. Thus we expect
more significant electronic coupling between the door and acceptor in Bis-2-An-NDI than Bis-
Ph-NDI. We also prepared mono-amino substituted derivatives with extra Br, phenyl, 9-anthryl
and 2-anthryl moieties attached (Scheme 1). The coupling of the amino moiety or the phenyl,
anthryl moiety with the NDI moiety was compared. The aim of preparation of 9-An-NDI-NH
and 2-An-NDI-NH is to study the effect of anthryl substitution on the photophysical properties
of the two compounds. The attempt to prepare an analogue of Bis-2-An-NDI with connection of
9-position of the anthryl was unsuccessful.
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The synthesis is mainly based on Pd(0) catalyzed Suzuki-Miyaura cross coupling reaction. The
compounds were obtained with satisfying yields. The molecular structures are fully verified with
1H NMR, 13C NMR and HRMS (see Experiment Section).
Scheme 1. Synthesis of NDI Derivatives a
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a Key words: (a) 2-ethylhexylamine, distilled DMF, N2, 140 °C for overnight, yield: 42.0%; (b)
Br2, oleum (20% SO3), 50 °C, 48 h, yield: 78.0%; (c) 2-ethylhexylamine, 120 °C, N2, 4 h, yield:
31.0%; (d) (f) (g) (h) (i) Arylboronic acid, Pd(PPh3)4, Na2CO3, 1,4-dioxane/H2O, N2, 85 °C for
overnight, yield are 10.0 %, 15.0%, 30.0%, 60.0% and 65.0%, respectively; (e) 2-
ethylhexylamine, 1,2-dimethoxyethane, 120°C, N2, 6 h, yield: 47.0%.
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3.2. UVVis Absorption and Fluorescence Emission Spectra. The UVVis absorption
spectra of the parent compounds NDI, Bis-Ph-NDI and Bis-2-An-NDI were compared (Figure
1a). NDI gives structured absorption band in the range of 350  400 nm. For Bis-Ph-NDI, an
extra broad shoulder absorption at 421 nm was observed. This band is assigned to charge transfer
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(CT) absorption (S0  CT transition). For Bis-2-An-NDI, a distinct broad absorption band at
524 nm was observed. Moreover, the absorption profiles in the < 400 nm UV region are similar
for the three compounds. The broad band at 421 nm and 524 nm are assigned as the CT
absorption bands.1 Based on the observations, we conclude that both the Bis-Ph-NDI and Bis-2-
An-NDI show strong electronic coupling between the electron donor and acceptor. It should be
pointed that the CT absorption band should not be sensitive to solvent polarity. CT state is
formed via the electronic coupling between the donor and acceptor, S01CT is an allowed
transition, and it is manifested by the CT absorption band in the UVVis absorption spectrum.
This is different from the charge separated state formed after photoexcitation. In this case the
Figure 1. UVvis absorption spectra of (a) NDI, Bis-Ph-NDI and Bis-2-An-NDI; (b) Br-NDI-
5
NH, Ph-NDI-NH, 2-An-NDI-NH and 9-An-NDI-NH; c = 1.0  10 M in toluene. 20 C.
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direct S01CT is forbidden, and these is no such CT absorption band (this is the case for the
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photo-induced electron transfer systems).
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To quantitatively compare the probability of the S0  CT transition of the molecules, the
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transition dipole moments (Mabs) of S0  1CT transition were calculated using the equation 3:1
(Eq. 3)
~
1
1
is the molar absorption coefficient in the wavenumber scale, in M cm ;
is the
~




where
abs
1
1
1
maximum of the S0  CT transition band, in cm ; NA is the Avogadro constant, in mol ;
h
1
[erg s] is the Plank constant, in erg s; is the speed of light, in cm s and is the refractive
c
n
index of the solvent respectively. The Mabs of Bis-Ph-NDI and Bis-2-An-NDI were calculated as
2.38 Debye and 2.91 Debye, respectively (Table 1), indicating that both phenyl and anthryl
moiety favor the S0  1CT transition. However, the Mabs of Ph-NDI-NH, 2-An-NDI-NH and 9-
An-NDI-NH is little smaller than Br-NDI-NH, indicating that both phenyl and anthryl moiety
1
do not favor S0  CT transition as compared with amino moiety in Br-NDI-NH, thus the
amino-containing compounds give more significant CT absorption band.
Ganguly et al. used Hush equation 4 to calculate the electronic coupling matrix element (VDA)
in neutral donor/acceptor systems.55 From the charge transfer absorption band in these NDI
derivatives (Figure 1a), the electronic coupling matrix element (VDA) were calculated according
to eq. 4 and the data are presented in table 1.
(Eq. 4)
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CT
max
where R is the separation between the center of electron donor and acceptor, in Å;

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CT
1
1
molar absorption coefficient at the maximum of the CT absorption band, in [M cm ];

is
max
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9
CT
1
the absorption maximum of the CT absorption band in wavenumber scale, in cm ;

is the
1/2
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full width of the band at the half maximum, in cm . The distance between the center of the
donor and acceptor, which determined by DFT optimization of the geometry, was used as upper
limit distance. Herein, RBr-NDI-NH = 3.92 Å, R9-An-NDI-NH = 5.59 Å, R2-An-NDI-NH = 7.59 Å, RBis-Ph-NDI
= 5.48 Å and RBis-2-An-NDI = 7.59 Å are used to estimated VDA.
The electronic coupling matrix elements (VDA) of Bis-Ph-NDI and Bis-2-An-NDI were
calculated as 0.34 eV and 0.22 eV, respectively (Table 1). The VDA of Bis-2-An-NDI is smaller
than Bis-Ph-NDI, even though Bis-2-An-NDI has a larger value of the transition dipole
moments (Mabs). The main reason is that the smaller size of benzene ring decreases donor’s
distance, which finally reduces the separation between the center of electron donor and acceptor
in Bis-Ph-NDI (RBis-Ph-NDI = 5.48 Å and RBis-2-An-NDI = 7.59 Å).
Most amino-substituted NDI compounds have two absorption bands. The one bellowed 400
nm is assigned to the unsubstituted naphthalenebisimides and another broad long-wavelength
band in visible region is assigned to charge transfer transition, which was well demonstrated by
Würthner and coworkers.56,57 This two absorption bands were labeled with S0S2 and S0S1
transition, respectively. Herein, amino-substituted NDI derivatives with extra electron donor,
such as phenyl, anthryl, were studied.
The UVVis absorption of mono amino-substituted NDI derivatives was presented in Figure
1b. These compounds share a common significant CT band at ca. 570 nm. The results are
interesting, since the presence of anthryl moiety in 2-An-NDI-NH and 9-An-NDI-NH does not
increase the CT band magnitude, although it exerts significant effect on the UVVis absorption
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of Bis-2-An- NDI (Figure 1a). Based on the CT absorption bands, the VDA values of the
complexes ware calculated (Table 1). The result in Figure 1b show that with the mono-amino
substitution on the NDI moiety, the anthryl moiety exerts minor effect on the magnitude of the
CT band. In other words, there is no add-up effect (or synergic effect) for the electronic coupling
between the electron donor and acceptor, given the two electron donors in the triad are with
different electron donating ability. In comparison of the electron donating ability, the HOMO of
anthracene is with energy level of 5.47 eV, the HOMO energy level of the amine moiety is
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6.25 eV. Hence the anthracene moiety should be the main electron donor, but have no add-up
effect (or synergic effect) for the electronic coupling between the electron donor and acceptor.
It should be pointed out that for the Bodipy-anthracene dyads,31,32 the electronic coupling
between the electron donor and acceptor is weak, thus there is no CT absorption band (i.e. there
is no adiabatic CT). But upon photoexcitation of the Bodipy-anthracene dyad, there will be non-
adiabatic CT, i.e. the PET process.
The fluorescence emission property of the compounds was studied (Figure 2). NDI is known
to show very weak fluorescence due to efficient ISC.58 However, much stronger fluorescence
was observed in toluene. One possible reason is the formation of exciplexes of NDI with toluene,
which was proved by monitoring the emission of the compound in n-hexane in presence of
different equivalent of toluene.
The emission of these proposed exciplexes was also observed in o-xylene and p-xylene
(Supporting information, Figure S24). Bis-Ph-NDI and Bis-2-An-NDI do not show similar band
as that of NDI (Figure 2a). Previously the exciplex or CT complex emission of NDI with toluene
or xylene were observed with coordination framework materials, but not in solution phase.59,60
The amino-containing derivatives such as Br-NDI-NH and Ph-NDI-NH show intense emission
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at ca. 570 nm.61 Other amino-substituted NDI derivatives such as 2-An-NDI-NH and 9-An-NDI-
NH give emission at similar wavelength (Figure 2b), but the emission intensity is weaker. The
emission is structureless and is assigned to CT band.
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Figure 2. Fluorescence emission spectra of (a) NDI; Bis-Ph-NDI and Bis-2-An-NDI (A =
0.126), ex = 340 nm, the asterisk “” designates the Raman scattering peak of the solvent
molecules; (b) Br-NDI-NH, Ph-NDI-NH, 2-An-NDI-NH and 9-An-NDI-NH (A = 0.035), ex =
470 nm. Optically matched solutions were used. In toluene. 20 C.
The amino-substituted NDI derivatives shows red-shifted emission in polar solvents
(Supporting information, Figure S25a). For Br-NDI-NH, the fluorescence intensity does not
vary significantly in different solvents (Supporting information, Figure S25b), which is not the
typical feature of the molecules with photo-induced electron transfer (PET). For Ph-NDI-NH
and 2-An-NDI-NH (Figure 3a and 3b), however, a clear trend of fluorescence quenching was
observed in polar solvents. This quenching effect in polar solvents for Ph-NDI-NH is attributed
to charge-transfer character of S1 state and for 2-An-NDI-NH the electron transfer from the
anthryl moiety to the NDI moiety.62 In comparison, no clear quenching effect was observed for
9-An-NDI-NH (Figure 3c).
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Figure 3. Fluorescence emission spectra of (a) Ph-NDI-NH, (b) 2-An-NDI-NH and (c) 9-An-
NDI-NH in different solvents; A = 0.035, ex = 470 nm; Optically matched solutions were used.
20 C.
The quenched fluorescence of Br-NDI-NH (F = 23%) as compared to Ph-NDI-NH (F =
84%) can be rationalized by the heavy atom effect. It should be noted the fluorescence quantum
yield of 2-An-NDI-NH (F = 5.7%) and 9-An-NDI-NH (F = 6.0%) are much lower than Ph-
NDI-NH, the PET (anthryl moiety as the electron donor) is a possible reason for the fluorescence
quenching.51 The results infer that although the coupling of the anthryl and NDI is not
manifested in the UVvis absorption spectra, the coupling indeed exerts significant effect on the
excited state property, such as the fluorescence. It is known that the two coupling for the S0 
1CT (FC) and the for 1CT*  S0 can be different.4,5
The singlet oxygen quantum yields () of the compounds were determined (Table 2), as a
preliminary evolution of the triplet state production ability of the compounds. For Bis-Ph-NDI
( = 0%) and Bis-2-An-NDI ( = 9.0%), the quantum yields are much lower than NDI ( =
17%). These results indicate that CR in the dyes, both Bis-Ph-NDI and Bis-2-An-NDI, does not
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produce triplet state efficiently. Thus the SOCT-ISC is not significant. Similar results were
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NH, even the anthryl moiety and NDI moiety with orthogonal conformation (Figure 9), the
triplet quantum yield (T = 19%) is not high.
Table 1. The Absorption Properties, the Ground State Dipole Moments (g), Transition
Dipole Moments for the CT Absorption Bands (Mabs) and the Electronic Coupling Matrix
Elements (VDA) of the Compounds.
c
λabs a ( b)
g / D Mabs d /D VDA e /eV
363 (1.3), 381 (1.6) f /421 (0.7) g
360 (2.4), 380 (2.2) f /533 (0.6) g
0
0
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2.91
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2.94
0.34
0.22
0.55
0.40
0.25
0.36
Bis-Ph-NDI
Bis-2-An-NDI
Br-NDI-NH
Ph-NDI-NH
2-An-NDI-NH
9-An-NDI-NH
352 (0.80), 371 (0.90), 531(1.3) g 5.03
351 (0.7), 371 (0.7) f /533 (1.2) g
351 (1.2), 371 (1.1) f /533 (1.1) g 2.90
2.83
371 (2.4), 390 (2.2) f /523 (1.1) g
3.05
5
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1
1 c
a c = 1.0 × 10 M, in nm. Molar absorption coefficient. , in 104 M cm . The dipole
moment of optimized ground state geometry at the B3LYP/6-31G(d)/level using Gaussian 09W,
in Debye. The transition dipole moment of S0  CT transition, in Debye. The electronic
d
1
e
f
1
g
coupling element matrix for electron transfer, in eV. Absorption of S0  LE band.
Absorption of S0  1CT band.
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Table 2. Photophysical Properties of the Compounds
abs/nm a ( b)
em/nm F/ns c F(%) d T(s) (%) e
T(%) f
363 (1.5), 381 (1.6) g 469
0.65
0.24
440
17 a, 58 h 86 i
0 a ,h, i
 j
 j
NDI
10
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363 (1.3), 381 (1.6) g
 j
 j
 j
 j
Bis-Ph-NDI
/421 (0.7) k
360 (2.4), 380 (2.2) g
/533 (0.6) k
352 (0.8), 371 (0.9) g 573
/531(1.3) k
9.0 a
0 h, i
73 a 84 h
62 i
20 a 16 h
13 i
19 a 6.5 h
3.8 i
13 a 5.0 h
2.8 i
 j
 j
 j
 l
74
42
42
19
1.6
62
Bis-2-An-NDI
Br-NDI-NH
Ph-NDI-NH
2-An-NDI-NH
9-An-NDI-NH
3.5
10.4
9.1
9.6
23
84
5.7
6.0
351 (0.7), 371 (0.7) g 573
161
18
k
/533 (1.2)
351 (1.2), 371 (1.1) g 574
k
/533 (1.1)
371 (2.4), 390 (2.2) g 568
22
k
/523 (1.1)
a In toluene, c =1.0 × 10−5 M. b Molar absorption coefficient.  = 104 M−1 cm−1. c Fluorescence
lifetimes, biexponential fitting in toluene. d Fluorescence quantum yields. For NDI, anthracene in
ethanol (F = 27%) as reference. For other compounds, diiodo-bodipy in CH3CN (F = 2.7%) as
reference. Quantum yield of singlet oxygen (1O2). The triplet quantum yields were obtained
e
f
g
and 2,6-diio-dobodipy in toluene (T = 88%) as standard. ex = 532 nm in toluene Absorption
of S0  1LE band.h In DCM. i In CH3CN. j Not applicable. k Absorption of S0  1CT band. l Not
determined.
3.3. Electrochemical Characterization and the Gibbs Free Energy Changes (ΔGCS) of the
Photo-induced Electron Transfer (PET) of the Dyads. The electrochemical properties of the
NDI derivatives and reference compound anthracene were studied with cyclic voltammetry
(Figure 4).45
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Figure 4. Cyclic voltammogram of the compounds. (a) anthracene (An), NDI and Ferrocene
(Fc); (b) Bis-Ph-NDI, Bis-2-An-NDI and Fc; (c) Ph-NDI-NH, Br-NDI-NH, 2-An-NDI-NH, 9-
An-NDI-NH and Fc. In deaerated DCM solutions containing 0.10 M Bu4NPF6 as supporting
electrolyte and with Ag/AgNO3 reference electrode. Fc was used as the internal reference, scan
rates: 50 mV/s. 20 °C.
For NDI, an irreversible oxidation wave at +1.35 V was observed, as well as two reversible
reduction waves at 1.09 V and 1.52 eV were observed (Figure 4a). One oxidation wave at
+1.11 V for anthracene (An) was observed (Figure 4a). For 9-An-NDI-NH, the first oxidation
wave at +0.90 eV assigned to anthracene moiety and the first reduction wave at 1.32 eV
assigned to NDI moiety were observed (Figure 4c), indicating that anthracene moiety acts as
electron donor and the NDI unit as the electron acceptor in the PET process. Similar results were
observed for NDI derivatives such as Bis-2-An-NDI and 2-An-NDI-NH which are attached
with anthracene moiety (Figure 4b and 4c). As for other NDI compounds attached with bromo-
or phenyl moiety, both an oxidation wave and the first reduction wave were according to NDI
moiety. The electrochemical parameters of the compounds are presented in Table 3.
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Table 3. Redox Potentials of the Compounds a
5
6
7
8
9
Bis-Ph- Bis-2-
Ph-NDI- Br-NDI- 2-An-
9-An-
An
NDI
NDI
An-NDI NH
NH
NDI-NH NDI-NH
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E(ox) (V) 1.11
1.35
1.09
1.52
1.35
1.13
1.40
0.94
1.08
1.49
1.37
1.35
1.77
1.35
1.26
1.66
0.93
1.33
1.75
0.90
1.32
1.77
E(red) (V)

a
Cyclic voltammetry in N2 saturated DCM containing a 0.10 M Bu4N[PF6] supporting
electrolyte; Counter electrode is Pt electrode; working electrode is glassy carbon electrode;
Ag/AgNO3 couple as the reference electrode.
The free energy changes of charge separation (ΔGCS) can be calculated with the Weller
equation (eqs 5 and 6).63 66 In the equations ΔGS is the static Coulombic energy, e = electronic

charge, EOX = half-wave potential for one-electron oxidation of the electron-donor unit, ERED
=
half-wave potential for one-electron reduction of the electron-acceptor unit, E00 = energy level
approximated with normalized UVvis absorption spectra and Fluorescence emission spectra, εS
= static dielectric constant of the solvent, RCC = center-to-center separation distance between the
electron donor (Anthracene) and electron acceptor (NDI), determined by DFT optimization of
the geometry, RD is the radius of the electron donor, RA is the radius of the electron acceptor, εREF
is the static dielectric constant of the solvent used for the electrochemical studies, and ε0 is the
permittivity of free space. The solvents used in the calculation of free energy of the electron
transfer are toluene (εS = 2.38), CH2Cl2 (εS = 8.93), MeOH (εS = 33.7) and acetonitrile (εS = 37.5).
The free energy differences for the charge-separation (ECS) were calculated with eqs 5 and 6. The
data are collected in Table 4.
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(Eq. 5)
(Eq. 6)
(Eq. 7)
(Eq. 8)
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Table 4. Data of Gibbs Free Energy Changes of Charge Separation (ΔGCS) and Energy
Level of Charge Transfer States (ECTS) of the Compounds in Different Solvents
ECTS (eV)
GCS (eV)
Tol
DCM MeOH ACN
Tol
DCM MeOH ACN
Bis-2-An-NDI a
+2.36 +1.81 +1.67
+2.59 +2.05 +1.91
+2.25 +1.93 +1.84
+1.66
+1.90
+1.84
0.05 0.60 0.74
0.75
0.37
0.40
2-An-NDI-NH b +0.32
0.22 0.36
0.31 0.40
9-An-NDI-NH c +0.01
E00 = 2.41 eV. b E00 = 2.27 eV. c E00 = 2.24 eV. E00 (E00 = 1240/) is the singlet state of NDI
derivatives,  is CT band absorption for Bis-2-An-NDI, but for 2-An-NDI-NH and 9-An-NDI-
NH,  is the wavelength of the crossing point of normalized UVvis absorption spectra and
Fluorescence emission spectra. “Tol” stands for toluene and “ACN” stands for CH3CN.
a
It is clear that the CS is more favorable in polar solvents. PET of all three compounds is
thermodynamically allowed in polar solvents, which was demonstrated by calculation of the
Gibbs free energy changes using the Weller equation. PET is possible for 2-An-NDI-NH and 9-
An-NDI-NH even in the non-polar solvent toluene. This result is in agreement with the
fluorescence emission study of the compounds, showing a fluorescence quenching even in
toluene in comparison with the reference compound Ph-NDI-NH, so determination error may
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exist for the free energy changes of the charge separation. The energy level of the charge transfer
state (CTS) was also calculated (Table 4). It was found that the CTS energy levels decrease in
polar solvents as compared with that in non-polar solvents.
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3.4. Nanosecond Transient Absorption Spectroscopy: The Triplet Excited States
Properties of the Compounds. We studied the triplet states of the compounds by use of
nanosecond transient absorption (ns TA) technique (Figure 5). For Bis-2-An-NDI, 532 nm
radiation was used to selectively excite the CT absorption band. Excited state absorption (ESA)
in the range of the 400700 nm was observed. This ESA band is superimposed with the ground
state bleaching band at 500 nm. The profile is similar to the T1 state absorption of NDI
moiety.61,67,68 The missing of the ground state bleaching band is due to the overlap with the ESA
1
band, as well as the weak steady state absorption of Bis-2-An-NDI at 524 nm ( = 6900 M
1
cm ) (Figure 5a).
Figure 5. Nanosecond transient absorption spectra of (a) Bis-2-An-NDI (ex = 532 nm), (b)
5
Decay trace of Bis-2-An-NDI at 440 nm. c = 4.0  10 M in deaerated toluene. 25 C.
The lifetime of triplet state was determined as 1.6 s (Figure 5b). This lifetime is much shorter
than the previously reported triplet state of amino-substituted NDI chromophore (30 s),61,67,68 as
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well as that of Br-NDI-NH (62 s). However, it is close to the T1 state lifetime of the
unsubstituted NDI (2.5 s in CH3CN).58 Moreover, the transient signal was totally quenched in
aerated solution, indicating that the transient observed is due to the triplet excited state
(Supporting information, Figure S33).
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It is known that unsubstituted NDI shows efficient ISC via the S1 (-)  T4 (n-)
transition.58 However, we carried out the excitation at 532 nm for Bis-2-An-NDI, which is
selectively excited into the CT band, yet T1 state was observed. This result indicates the ISC is
1
from the CT state, not the SO-ISC of An or NDI moieties, and the ISC mechanism should be
SOCT-ISC.28 34 Other ISC channels, such as HFI, or mixing of the S1/T0 or S1/T sates, are

1
unlikely due to the large J values of this compact electron donor/acceptor dyad.
The ns TA spectra of the amino-NDI compounds were also recorded (Figure 6 and Supporting
information, Figure S2932). For Br-NDI-NH, the typical T1 absorption feature of the NDI
moiety was observed upon photoexcitation (Figure 6a). The ground state bleaching band is
overlapped with the ESA band in the range of 400 – 700 nm. Similar results were observed for
the non-brominated NDI derivatives (Supporting information, Figure S3032). The triplet state
lifetimes observed for the amino-NDI compounds are in the range of 18 – 161 s (Table 2).
These values (T = 18 s for 2-An-NDI-NH or 22 s for 9-An-NDI-NH) are close to the
previously reported T1 state lifetimes of amino-NDI moieties (32 s).61 These results show that
the T1 state of the dyads is the NDI-localized LE state, not the 3CT state.5 Previously an electron
donor/acceptor triad based on NDI was shown to be with RP-ISC, and the electron exchange was
studied with time-resolved EPR spectra, but the triplet state was not studied in detail with
nanosecond transient absorption spectroscopy.27 The triplet state of Bodipy was produced in C60-
Bodipy dyads by charge recombination in non-polar solvents, such as toluene, but not in polar
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solvent of acetonitrile.63,64 Dyads Bis-2-An-NDI and Bis-Ph-NDI show similar solvent-
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dependent ISC.
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Figure 6. Nanosecond transient absorption spectra of (a) Br-NDI-NH and (c) 9-An-NDI-NH
with different delay times, Decay trace of (b) Br-NDI-NH at 440 nm, (d) 9-An-NDI-NH at 440
5
nm. ex = 532 nm, c = 4.0  10 M in deaerated toluene, 25 C.
3.5. Femtosecond Transient Absorption Spectroscopy. The ISC of NDI and Br-NDI-NH
has already been reported,53 however, the excited-state dynamics of anthracene substituted NDI
derivatives was rarely studied. In this donor-acceptor system, anthracene moiety acts as electron
donor and NDI moiety as electron acceptor. Hence, the forming triplet state may undergo the
charge separation and charge recombination pathway, known as SOCT-ISC. To study the
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kinetics of the above two processes, the S0S1 band at 340 nm instead of S01CT was excited
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Figure 7. (a) Femtosecond transient absorption spectra of 9-An-NDI-NH in toluene, (b)
Corresponding species-associated difference spectra (SADS) obtained from target analysis and
(c), (d) Kinetic traces at selected wavelengths, λex = 340 nm, 20 °C.
Upon excitation at 340 nm in toluene, 9-An-NDI-NH exhibits a broad ESA band in the range
350  750 nm, which is attributed to S1  Sn absorption, with a “valley” between 500 and 525
nm as a result of overlap with ground state bleaching (GSB) band. The ESA band at 450 nm
decays with  = 820 fs (Figure 7c). Meanwhile, one broad band between 350 to 530 nm centered
at 460 nm and another broad band centered at 600 nm (the ESA band of NDI) develop with the
same time scale (Figure 7a), indicating the formation of a CS state.69 71 The biexponential decay

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of ESA at 710 nm of An was obtained, though this band was buried by the ESA of NDI
(Figure 7d).72 Subsequently, on an 50 ps time scale, both of two positive features decay and
show blue-shift, indicating the occurrence of charge recombination (CR). Both of 420 and 580
nm bands can be assigned to triplet state absorption since they did not decay totally on the time
scale of our measurements (1 ns).
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We have made the target analysis of our transient absorption spectra (Glotaran software) and
assumed that three states, the S1 state, the CSS due to NDIand An, T1 state of 9-An-NDI-NH,
are involved. We have applied the sequential model, thus the species-associated decay spectra
(SADS) of each state and the rate constants are obtained. In CH3CN, the decay of S1 state (710 fs)
is slightly faster than that in toluene (850 fs), indicating faster electron transfer in CH3CN
(Supporting information, Figure S34). The rate constant of CR in CH3CN is 18 ps, which is also
faster than that in toluene (50 ps). Considering the close proximity of CSS in toluene (2.25 eV)
and T4 state (2.18 eV). Thus the CR in toluene is probably in Marcus inverted region.4,5,73
(Scheme 2).
The rate constants of CS and CR of 2-An-NDI are 0.22 ps and 17 ps, respectively (Supporting
Information, Figure S35). The rate constants of CS and CR of Bis-2-An-NDI are also presented
(Supporting Information, Figure S36).
3.6. Fluorescence Up-conversion Spectroscopy. In order to further study the decay of the CT
state, femtosecond fluorescence spectroscopy was applied (Figure 8 and Figure S37 from
Supporting information) show the femtosecond fluorescence data for 9-An-NDI-NH at ex = 400
nm and em = 550  610 nm. Two decay time components are obtained.
The time scale of the first decay component is 0.80  3.4 ps, which is attributed to vibrational
relaxation. The amplitude of this time component becomes smaller at longer emission
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