Singlet energy transfer and photoinduced electron transfer in star-shaped naphthalimide derivatives based on triphenylamine

Article abstract of DOI:10.1246/bcsj.78.1362

A series of star-shaped naphthalimide derivatives (TPA-NP) were synthesized with a triphenylamine (TPA) core and chromophores at the rim. Their steady-state absorption, fluorescence, and transient fluorescence spectra were investigated, indicative of severely fluorescent quenching with regard to the corresponding 1,8-naphthalimide (NP) parent chromophores. Excitation into either the triphenylamine or naphthalimide absorption bands results in the occurrence of singlet energy transfer and photoinduced electron transfer (PIET) effectively. Single photon-counting experiments show a fast charge separation and a thermally activated back reaction. As a consequence, the fluorescent lifetime of the naphthalimide moiety in the system of TPA-NP is reduced by more than an order of magnitude, which mainly arises from the intramolecular PIET process of TPA and naphthalimide. The kinetics and PIET mechanism involving TPA-NP were elucidated.

Full text of DOI:10.1246/bcsj.78.1362

1362 Bull. Chem. Soc. Jpn., 78, 1362–1367 (2005)
Ó 2005 The Chemical Society of Japan
Singlet Energy Transfer and Photoinduced Electron Transfer in
Star-Shaped Naphthalimide Derivatives Based on Triphenylamine
ꢀ
Weihong Zhu, Yisheng Xu, Yan Zhang, Jianping Shen, and He Tian
Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology,
Shanghai 200237, P. R. China
Received January 13, 2005; E-mail: tianhe@ecust.edu.cn
A series of star-shaped naphthalimide derivatives (TPA-NP) were synthesized with a triphenylamine (TPA) core
and chromophores at the rim. Their steady-state absorption, fluorescence, and transient fluorescence spectra were inves-
tigated, indicative of severely fluorescent quenching with regard to the corresponding 1,8-naphthalimide (NP) parent
chromophores. Excitation into either the triphenylamine or naphthalimide absorption bands results in the occurrence
of singlet energy transfer and photoinduced electron transfer (PIET) effectively. Single photon-counting experiments
show a fast charge separation and a thermally activated back reaction. As a consequence, the fluorescent lifetime of
the naphthalimide moiety in the system of TPA-NP is reduced by more than an order of magnitude, which mainly arises
from the intramolecular PIET process of TPA and naphthalimide. The kinetics and PIET mechanism involving TPA-NP
were elucidated.
Recently, two concepts toward amorphousness and bipolar-
ity have received growing attention for the design of organic
electroluminescent (EL) materials. Amorphous materials1 can
form stable glasses with high glass transition temperatures
(Tg), which possess excellent processability, transparency,
isotropic properties, and most important the absence of grain
boundaries. Amorphous thin films with high Tg in OLEDs
are less vulnerable to Joule heating, which accelerates the for-
mation of crystalline boundaries. Also, EL materials for use in
the emitting layer should desirably possess a bipolar charac-
ter,2 i.e., both electron-donating and electron-withdrawing
properties, to permit the formation of both stable cation and
anion radicals. Of particular interest are emitting materials that
can form homogeneous thin films with morphological and
thermal stability, thus meeting the energy-level matching for
charge–carrier injection and acceptance of both holes and elec-
trons.3
However, the intra-molecular photo-induced electron trans-
fer (PIET) on the luminescence efficiency in bipolar emitters
and various donor–acceptor covalently linked systems has
not drawn much attention.4,5 Electron transfer (ET) is the most
elementary and ubiquitous of all chemical reactions, and plays
a crucial role in many essential photophysic processes.6 PIET
can take place from polymers with electron-donor abilities,
such as polysilane, polygermane, poly(vinylcarbazole), and
polythiophene, to the triplet states of fullerenes in polar sol-
vents.7 Such electron transfer from their lowest excited singlet
or triplet states always reduces the electron acceptor, and even
produces charge separation, thus resulting in severe quenching
in luminance.8 An enhanced understanding of the PIET effect
on the multi-component luminescent properties is significant
for developing novel highly efficient chromophores. Consider-
ing the high electron-donating and low oxidation potential
properties of triphenylamine, PIET in the luminescent materi-
als containing triphenylamine unit might be very possible to
take place in the solid state or solution.9 To further explore
these effects, here we consider the intra-molecular photo-in-
duced energy- and electron-transfer behavior in star-shaped
naphthalimide derivatives based on triphenylamine (TPA-NP
shown in Scheme 1). Our results suggest that when incorporat-
ing a triphenylamine unit to the chromophore for designing
bipolar or star-shaped emitters, the fluorescence quenching in-
duced by possible through-bond PIET should be desirably con-
sidered. In contrast to the corresponding 1,8-naphthalimide pa-
rent chromophores, there exists severely fluorescent quenching
in the system of TPA-NP due to effective singlet energy trans-
fer and PIET. The fluorescent lifetime of the naphthalimide
moiety in the system of TPA-NP was reduced by more than
one order of magnitude. We also considered the PIET mecha-
NO
Cl
NO
2
2
NO
N
NO
2
ClHH
N
N
NH HCl
2
2
2
ii
i
+
NH
2
NO
NH HCl
2
2
R
N
N
O
O
iii
C H
2
5
TPA-NP1
R:
=
=
N
N
O
N
O
O
TPA-NP2
TPA-NP3
O
O
=
N
N
N
N
O
O
R
R
TPA-NP
NP
Scheme 1. Synthetic routes of target compounds TPA-NP,
and chemical structure of reference compound N-ethyl-
4-piperidin-1-yl-1,8-naphthalimide (NP). Reaction condi-
tions: i. DMF, K2CO3, Cu, reflux for 48 h; ii. SnCl2/
HCl (3:1), reflux for 5 h; iii. Corresponding naphthalic an-
hydride, 2-mehoxyethanol and piperidine, reflux for 16 h.
Published on the web July 1, 2005; DOI 10.1246/bcsj.78.1362

W. Zhu et al.
Bull. Chem. Soc. Jpn., 78, No. 7 (2005) 1363
nism involving TPA-NP by thermodynamic and transient fluo-
rescent analysis. To the best of our knowledge, the model of
TPA-NP is the first example, in which PIET occurred from
the imide side of naphthalimide to the naphthalene ring.
5
4
3
2
1
0
TPA-NP1
TPA-NP2
TPA-NP3
Experimental
General. The melting points were measured on a X4 micro-
1
melting point apparatus. H NMR and 13C NMR spectra were re-
corded on a Bruker AM-500 spectrometer. Mass spectra were de-
termined by VG12-250 mass spectroscopy. Absorption and fluo-
rescence spectra were recorded on a Varian Cary 500 and a Varian
Cary Eclipse, respectively. The fluorescence lifetimes were de-
tected by an Edinburgh FL900 single-photon counting system.
Cyclic voltammetry (CV) were carried out on a CHI 800 elec-
trochemical instrument with a platinum electrode using milli-mo-
lar solutions in CH2Cl2 containing supporting electrolyte tetra-
butylammonium perchloride (0.1 mol Lꢁ1) in a cell with three
electrodes and a potentiostat assembly. The potentials were mea-
sured at a scan rate of 0.1 V/s, a Ag/AgCl electrode was used as a
reference electrode. Tri(4-amino)phenylamine hydrochloride was
synthesized according to a previous reference.10
Synthesis of TPA-NP1. A mixture of tri(4-amino)phenyl-
amine hydrochloride (175 mg, 0.44 mmol), 4-piperidin-1-yl-1,8-
naphthalic anhydride (390 mg, 1.39 mmol), piperidine (1.0 mL),
and 2-methoxyethanol (17 mL) was refluxed for 16 h under an ar-
gon atmosphere. After being cooled and filtrated, the filtrated cake
was recrystallized from 2-methoxyethanol to give a bright yellow
solid in 60% yield. mp > 300 ꢂC. 1H NMR (500 MHz, CDCl3): ꢀ
(ppm) 1.67 (m, 6H, CH2), 1.85 (m, 12H, CH2), 3.19 (s, 12H,
CH2), 7.14 (d, J ¼ 7:9 Hz, 3H, naphthalene-H), 7.16 (d, J ¼ 8:6
Hz, 6H, benzene-H), 7.37 (d, J ¼ 8:6 Hz, 6H, benzene-H), 7.64
(t, J ¼ 7:9 Hz, 3H, naphthalene-H), 8.39 (d, J ¼ 8:4 Hz, 3H,
naphthalene-H), 8.48 (d, J ¼ 8:1 Hz, 3H, naphthalene-H), 8.55
(d, J ¼ 7:2 Hz, 3H, naphthalene-H). 13C NMR (125 MHz, CDCl3):
ꢀ (ppm) 24.36, 26.24, 54.55, 114.79, 116.05, 123.35, 124.79,
125.42, 126.38, 129.57, 130.33, 130.88, 131.41, 133.05, 147.19,
157.53, 164.34, 164.85. FAB-MS: m=z [Mþ] 1082. Other two com-
pounds were synthesized by a similar method.
TPA-NP2 was prepared from 4-morpholin-4-yl-1,8-naphthalic
anhydride in 46% yield. 1H NMR (500 MHz, CDCl3): ꢀ (ppm)
3.31 (m, 12H, CH2), 4.04 (m, 12H, CH2), 7.22 (d, J ¼ 8:2 Hz,
3H, naphthalene-H), 7.26 (d, J ¼ 8:6 Hz, 6H, benzene-H), 7.44
(d, J ¼ 8:6 Hz, 6H, benzene-H), 7.75 (t, J ¼ 7:9 Hz, 3H, naphtha-
lene-H), 8.47 (d, J ¼ 8:4 Hz, 3H, naphthalene-H), 8.58 (d, J ¼ 8:0
Hz, 3H, naphthalene-H), 8.64 (d, J ¼ 7:2 Hz, 3H, naphthalene-H).
13C NMR (125 MHz, CDCl3): ꢀ (ppm) 30.44, 53.45, 66.99,
115.06, 117.30, 123.55, 124.83, 125.94, 126.25, 129.57, 130.20,
130.27, 130.36, 131.59, 132.94, 147.24, 155.87, 164.21, 164.68.
FAB-MS: m=z 1087 [Mþ].
TPA-NP3 was prepared from 4-dimethylamino-1,8-naphthalic
anhydride and tri(4-amino)phenylamine hydrochloride. 1H NMR
(500 MHz, CDCl3): ꢀ (ppm) 3.14 (s, 18H, –N(CH3)2), 7.16 (d,
J ¼ 8:2 Hz, 3H, naphthalene-H), 7.23 (d, J ¼ 8:4 Hz, 6H, ben-
zene-H), 7.37 (d, J ¼ 8:5 Hz, 6H, benzene-H), 7.70 (t, J ¼ 7:9
Hz, 3H, naphthalene-H), 8.50 (d, J ¼ 8:5 Hz, 3H, naphthalene-
H), 8.54 (d, J ¼ 8:2 Hz, 3H, naphthalene-H), 8.63 (d, J ¼ 7:2
Hz, 3H, naphthalene-H). MS (ESI): m=z 983.1 ½Mþ þ Naꢃ.
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 1. UV–vis absorption spectra of TPA-NP in CH2Cl2.
shown in Scheme 1) were synthesized, which are based on
the electron-donating triphenylamine (TPA) core and the elec-
tron-withdrawing naphthalimide (NP) chromophores at the
rim. Naphthalimides due to their high fluorescence yield, good
stability, and high electron affinity (about 3.1 eV) have been
utilized as electron-transporting electroluminescent emitters.11
Due to the extremely easy oxidation of the intermediate
tri(4-aminophenyl)amine, we chose its hydrochloride to direct-
ly react with 4-substituted naphthalic anhydride in 2-methoxy-
ethanol using excess secondary amine piperidine (Scheme 1).
Here, piperidine plays double roles, i.e. as an imidization
catalyst and an acid-trap for liberating HCl. We found that
the imidization could achieve a satisfactory yield with the base
catalysis of piperidine.12 All molecular structures were con-
1
firmed by H NMR, 13C NMR, and mass spectra.
Steady-State Absorption and Fluorescence Properties.
Figure 1 shows UV–vis absorption spectra of star-shaped com-
pounds, TPA-NP. A band peak at about 310 nm for these com-
pounds is attributed to the absorption transition of the triphen-
ylamine unit, while the absorption peaks characterized for
the 1,8-naphthalimide moieties of TPA-NP1, TPA-NP2, and
TPA-NP3 are located at 412, 396, and 420 nm, respectively
(Table 1). The absorption peak variation of the naphthalimide
moiety is due to a different substituent group effect at the 4-
position of the 1,8-naphthalimide moiety.
Star-shaped compounds have an ideal architecture to study
the interaction between the chromophores. To study this, we
prepared a simple mixture solution of triphenylamine (TPA)
and N-ethyl-4-piperidin-1-yl-1,8-naphthalimide (NP) as a ref-
erence (molar ratio = 1:3). As shown in the steady-state emis-
sion spectra of a simple mixture of TPA and NP (shown in
Fig. 2), two obvious emission peaks at 361 and 515 nm corre-
spond to emission of the triphenylamine and the naphthalimide
units, respectively. The relative ratio of the integrated lumines-
cence intensities of these solutions are listed in Table 1. If the
fluorescence intensity of the NP unit for the reference system
excited at 310 nm (TPA absorption maximum) is arbitrary
defined as one unit, the relative fluorescence quantum yields
of the 1,8-naphthalimide moiety under the same condition
for TPA-NP1, TPA-NP2, and TPA-NP3 are 0.060, 0.018,
and 0.117, respectively. Similarly, when directly excited at
the corresponding absorption peak of the 1,8-naphthalimide
moiety, TPA-NP1, TPA-NP2, and TPA-NP3 had relative fluo-
rescence quantum yields of 0.052, 0.041, and 0.157, respec-
Results and Discussion
Design and Synthesis of Star-Shaped Compounds TPA-
NP. A series of star-shaped bipolar derivatives (TPA-NP

1364 Bull. Chem. Soc. Jpn., 78, No. 7 (2005)
Luminescence Quenching in Star-Shaped Naphthalimide Derivatives
Table 1. Absorption and Fluorescence Data (1 ꢄ 10ꢁ6 mol Lꢁ1), Oxidation Potentials and Calculated Energies of Electron Transfer for
TPA-NP and Reference Compounds (TPA and NP) in CH2Cl2
aÞ
bÞ
cÞ
dÞ
Eox
Compds
ꢁmaxabs/nm
ꢁmaxflu/nm ꢂTPA
ꢂNP1
ꢂNP2
ꢃ/ns
HOMO
/eV
ꢀGet
/kcal molꢁ1
(" ꢄ 10ꢁ4
M
ꢁ1 cmꢁ1
)
/V
TPA-NP1
TPA-NP2
TPA-NP3
310 (4.22)
412 (3.98)
310 (4.29)
396 (4.46)
304 (4.71)
420 (4.19)
300 (7.20)
518
516
508
0.337
0.060
0.018
0.117
0.052
0.041
0.157
0.095 (56.9%)
1.220 (43.1%)
0.033 (59.1%)
2.800 (40.9%)
0.112 (40%)
0.655 (60%)
1.19
1.15
1.08
5.47
5.43
5.36
ꢁ5:4
ꢁ4:5
ꢁ2:4
0.244
0.302
1.000
TPA
NP
361
515
0.98
1.12
1.000
1.000
9.320 (100%)
5.50
a) ꢂTPA is the relative fluorescence quantum yields of triphenylamine unit excited at 310 nm. The fluorescence intensity of reference
compound TPA is arbitrary defined as one unit. b) ꢂNP1 is the relative fluorescence quantum yields of 1,8-naphthalimide moiety ex-
cited at 310 nm. The fluorescence intensity of reference compound NP is arbitrary defined as one unit. c) ꢂNP2 is the relative fluo-
rescence quantum yields of 1,8-naphthalimide moiety excited at the absorption peak of 1,8-naphthalimide moiety. The fluorescence
intensity of reference compound NP is arbitrary defined as one unit. d) The oxidation potential of reference ferrocene (Fc) vs refer-
ence electrode Ag/AgCl is 0.52 V.
1.0
0.8
0.6
0.4
0.2
0.0
100
80
60
40
20
0
Absorption TPA-NP1
Emission TPA
TPA-NP1
TPA-NP2
TPA-NP3
TPA+NP
300
350
400
450
500
550
Wavelength (nm)
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. Absorption spectra of TPA-NP1 in CH2Cl2 (1:0 ꢄ
10ꢁ5 mol Lꢁ1), and fluorescence spectra of TPA excited
at 310 nm in CH2Cl2 (1:0 ꢄ 10ꢁ5 mol Lꢁ1).
Fig. 2. Fluorescence spectra in CH2Cl2 excited at 310 nm:
TPA-NP (1 ꢄ 10ꢁ6 mol Lꢁ1), and the reference compound
mixture of triphenylamine (TPA) and N-ethyl-4-piperidin-
1-yl-1,8-naphthalimide (NP) (molar ratio = 1:3, TPA:
1:0 ꢄ 10ꢁ6 mol Lꢁ1, NP: 3:0 ꢄ 10ꢁ6 mol Lꢁ1).
fer) from the triphenylamine unit to the 1,8-naphthalimide
moiety occurred in the TPA-NP system.13 It has an efficient
spectral overlap and an energy transfer that results in signifi-
cantly fluorescent quenching of the triphenylamine unit in
TPA-NP. With respect to triphenylamine, the fluorescence in-
tensities of the triphenylamine unit in the system of TPA-NP1,
TPA-NP2, and TPA-NP3 are 0.337, 0.244, and 0.302, respec-
tively (Fig. 2). However, an apparent paradox arises for the tri-
phenylamine units in these star-shaped systems of TPA-NP,
where such efficient singlet energy transfer from TPA to NP
doesn’t increase the fluorescence efficiency of the acceptor
naphthalimide moiety. In other words, another nonradiative re-
laxation process might compete with the contribution from the
singlet energy transfer, and then results in a quenching effect
on the luminescence of naphthalimide unit. As a matter of fact,
naphthalimide derivatives have been utilized as electron-trans-
porting electroluminescent emitters with a high electron affin-
ity (about 3.1 eV).11 Therefore, the occurrence of PIET is very
reasonable when incorporating some low oxidation potential
units, such as triphenylamine, to the naphthalimide moiety.9
The strongly fluorescent quenching of the naphthalimide moi-
ety in the TPA-NP systems most likely results from the PIET
between the triphenylamine (donor) and the naphthalimide
tively. This demonstrates the existence of strong fluorescence
quenching for the naphthalimide moiety bonded to a triphenyl-
amine unit. Especially for TPA-NP1 and TPA-NP2, the emis-
sion intensities of the naphthalimide moiety are reduced by
more than two orders of magnitude, and the quenching effi-
ciency reaches as high as 95% in both cases of direct excitation
(400 nm) and indirect excitation (310 nm).
Obviously, the intramolecular interaction between the two
chromophores for TPA-NP is much stronger than the intermo-
lecular interaction in the system of a simple mixture. This phe-
nomenon is further supported by the fluorescence dependence
on the solution concentration. The fluorescence from the sim-
ple mixture is much more significantly dependent on the con-
centration than that from TPA-NP. Indeed, when diluted for
the simple mixture, almost no fluorescent quenching of these
two chromophores is observed.
As can be seen in Fig. 3, there exists an overlap between the
fluorescence of triphenylamine (peak at 361 nm) and the ab-
sorption of the 1,8-naphthalimide (peak at 410 nm). Thus,
the singlet–singlet energy transfer (Forster-type energy trans-
¨

W. Zhu et al.
Bull. Chem. Soc. Jpn., 78, No. 7 (2005) 1365
6
4
5
4
TPA-NP1
3
TPA
2
2
1
0
0
-1
-2
-2
-0.4
0.0
0.4
0.8
1.2
1.6
-0.5
0.0
0.5
1.0
1.5
2.0
Potential (V)
Potential (V)
6
4
8
6
TPA-NP2
TPA-NP3
4
2
2
0
0
-2
-4
-6
-2
-4
-0.5
0.0
0.5
1.0
1.5
2.0
-0.4
0.0
0.4
0.8
1.2
1.6
Potential (V)
Potential (V)
1.5
NP
1.0
0.5
0.0
-0.5
0.0
0.5
1.0
1.5
Potential (V)
Fig. 4. Cyclic voltammograms of TPA, TPA-NP1, TPA-NP2, TPA-NP3, and NP in CH2Cl2.
units (acceptor), which is supported by the following electro-
chemistry and transient fluorescence study.
PIET process. The ꢀGet values were calculated by the follow-
ing Rehm–Weller equation:14
Electrochemical Properties and Thermodynamics. Cy-
clic voltammetry (CV) measurements were carried out in a
conventional three-electrode cell using millimolar solutions
in CH2Cl2 containing 0.1 mol Lꢁ1 of the support electrolyte,
tetrabutylammonium perchloride. Figure 4 shows cyclic vol-
tammograms of TPA, TPA-NP1, TPA-NP2, and TPA-NP3 in
CH2Cl2. The corresponding oxidation potentials are listed in
Table 1. It should be noted that for TPA-NP compounds, the
predominated oxidative sweep of triphenylamine peaks at 1.2
V, and is overlapped with an irreversible peak of the naphthal-
imide unit located between 1.10 and 1.30 V. The HOMO
energy values for TPA-NP are also given in Table 1, which
were calculated using a value of ꢁ4:8 eV for ferrocene (Fc)
with respect to zero vacuum level. Also the optical edge of
the absorption spectra was utilized to derive the band gap,
and indirectly give the corresponding LUMO energy and
reduction potential values.
2
ꢀGet ¼ 23:06½EðDþ/DÞ ꢁ EðAꢁ/AÞ ꢁ ꢀE0;0 ꢁ e0 =ꢄ"ꢃ; ð1Þ
where E(Dþ/D) is the donor oxidation potential (TPA unit),
E(Aꢁ/A) is the acceptor reduction potential (1,8-naphthal-
imide moiety), ꢀE0;0 is the excitation energy of 1,8-naphthal-
2
imide moiety, and e0 =ꢄ" is the energy gained in bringing the
two radical ions; e0 is the charge of an electron, " is the static
dielectric constant of solvent, and ꢄ is the distance between the
donor and the acceptor. Molecular modeling shows that the
ꢁ
distance between the donor and the acceptor is 4.76 A. It
was calculated that e0 =ꢄ" is 0.024 eV in CH2Cl2; the result-
ing free energy values (ꢀGet) are listed in Table 1. These
data show that electron transfer from TPA to NP is strongly
favored.
Transient Fluorescence Spectroscopy and Electron
Transfer. To further explore the nature of intramolecular
luminescence quenching, the transient fluorescence of these
compounds was investigated (the data listed in Table 1). The
fluorescence decay of the naphthalimide moiety in the TPA-
NP system has two contribution components with different
lifetimes. As a case illustrated in Fig. 5, the fluorescence kinet-
ics of TPA-NP1 can be fit to a dual-exponential decay with
2
In the TPA-NP system, PIET from the triphenylamine unit
to the naphthalimide chromophore can be elucidated from
the viewpoint of thermodynamics. The measured oxidation po-
tentials and absorption spectral data show a quantitative esti-
mation of the thermodynamic driving force (ꢀGet) for the

1366 Bull. Chem. Soc. Jpn., 78, No. 7 (2005)
Luminescence Quenching in Star-Shaped Naphthalimide Derivatives
Fig. 5. Fluorescence decay profiles of naphthalimide moiety for TPA-NP1 (Left) and reference compound NP (Right) in CH2Cl2
(1:0 ꢄ 10ꢁ5 mol Lꢁ1, ꢁex ¼ 410 nm, ꢁem ¼ 513 nm).
calculated to 0.48 ns (9:32 ꢄ 0:052), which is consistent with
*TPA - NP
the determined average lifetime for TPA-NP1 (0.4 ns).
It should be pointed out that the occurrence of PIET from
TPA - *NP
TPA to naphthalimide in the TPA-NP system is from the imide
side of naphthalimide to the naphthalene unit, which is exactly
.
.
-
+
supplement to the previous report by de Silva.15 It was demon-
strated that the naphthalimide chromophore is only unidirec-
tional electron transfer, that is, only from the 4-position of
naphthalimide to the naphthalene unit.15 If considering that
the unidirectional ET for the naphthalimide is unambiguously
established, it’s hard to elucidate the phenomenon observed
here, i.e. strongly fluorescent quenching in the system of
TPA-NP. As discussed above, one reason to be explained is
based on the triphenylamine unit, which has a quite low oxida-
tion potential and a strong electron donating tendency. In TPA-
NP, the spacer between the imide side and the nitrogen is a
benzene bridge, not a single carbon bond. Two possibilities,
both the specifically low oxidation of TPA and the benzene
bridge, contribute to the occurrence of such a very rare oppo-
site PIET from the imide side to the naphthalene unit, which
was well elucidated in our thermodynamic and transient fluo-
rescence study. From this point of view, the model of TPA-NP
is an unprecedented example for studying the unexpected PIET
direction for the naphthalimide moiety.
TPA - *NP
kr
bet
TPA - NP
Fig. 6. Schematic diagram for energy transfer and electron
transfer in TPA-NP.
lifetimes of 95 ps (56.9%) and 1.220 ns (43.1%). In contrast,
the reference compound NP exhibits a single-exponential
decay with a lifetime of 9.320 ns (100%). In the system of
TPA-NP1, the short-lived component of 95 ps can be assigned
to PIET between donor triphenylamine and acceptor naphthal-
imide units (Fig. 6), which shows a fast charge separation and
a thermally activated back reaction. PIET plays a major role in
the fluorescence quenching of the naphthalimide moiety.
For an isolated chromophore without covalent bonding,
the luminescence yield, ꢂꢂ, for NP when excited at 410 nm is
given by
Conclusion
ꢂ
ꢂꢂ ¼ kr =ðkrꢂ þ knrꢂÞ;
ð2Þ
A novel series of bipolar star-shaped naphthalimide deriva-
tives (TPA-NP) were synthesized with triphenylamine as a
core and the chromophores at the rims. Upon combining
steady-state and transient fluorescence measurements, it is
shown that there exist two possible processes, or effects, on
ꢂ
ꢂ
where kr is radiative decay rate, and knr is the non-radiative
decay rate, both for an isolated molecule. When TPA is incor-
porated into NP, the yield, ꢂ, of the star-shaped compound
TPA-NP1 is, assuming that the quenching is mostly due to
photo-induced charge transfer (shown in Fig. 6), given by
the emission of the naphthalimide moiety: Forster singlet ener-
¨
gy transfer and photo-induced electron transfer (PIET). The
fluorescence decay of the naphthalimide moiety in the system
of TPA-NP exhibits two contribution components with differ-
ent lifetimes, in which the relatively short-lived fluorescent
lifetime corresponds to PIET from the triphenylamine unit to
the naphthalimide moiety. The existence of strong fluores-
cence quenching for the naphthalimide moiety bonded to the
triphenylamine unit was observed. Especially for TPA-NP1
and TPA-NP2, quenching of the naphthalimide moiety reached
as high as 95% in both cases of direct excitation (400 nm) and
indirect excitation (310 nm). Such a strongly fluorescent
ꢂ ¼ kr=ðkr þ knr þ kqÞ ¼ kr=ðkr þ knr þ ket þ kbetÞ; ð3Þ
where kr is the radiative decay rate, knr is the non-radiative de-
cay rate, and kq is the photo-induced electron transfer rate for
the TPA-NP1 system. If considering that the luminescence
quenching of the naphthalimide moiety in TPA-NP1 mostly re-
sults from photo-induced electron transfer, kr can be assumed
ꢂ
to be kr . As indicated in Table 1, the luminescence yield, ꢂ,
for TPA-NP1 is much smaller than ꢂꢂ (ꢂ ¼ 0:052ꢂꢂ). Since
the luminescence lifetime of NP shows that 1=ðkrꢂ þ knrꢂÞ is
about 9.32 ns in CH2Cl2, the lifetime data for TPA-NP1 is

W. Zhu et al.
Bull. Chem. Soc. Jpn., 78, No. 7 (2005) 1367
Yang, M. D’Iorio, and S. N. Wang, J. Mater. Chem., 12, 206
(2002). d) I. Y. Wu, J. T. Lin, Y. T. Tao, E. Balasubramaniam,
Y. Z. Su, and C. W. Ko, Chem. Mater., 13, 2626 (2001).
quench of the naphthalimide moiety in the TPA-NP systems
mostly results from PIET between the triphenylamine (donor)
and naphthalimide units (acceptor). It is believed that the two
possibilities, both the specifically low oxidation of TPA and
the benzene bridge, can be attributed to the occurrence of such
very rare opposite PIET from the imide side of naphthalimide
to the naphthalene unit. Our studied model suggests that the
luminescence quenching induced by possible through-bond
PIET should be desirably considered when incorporating a
triphenylamine unit to a chromophore for designing bipolar
or star-shaped emitters.
5
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This work was supported by NSFC/China (Project
No. 20106006), Education Committee of Shanghai and Scien-
tific Committee of Shanghai (04dz05102). W. H. Zhu thanks
the foundation for National excellent dissertation of P. R.
China (Project No. 200143).
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