Push-pull 1,8-naphthalic anhydride with multiple triphenylamine groups as electron donor

Article abstract of DOI:10.1016/j.molstruc.2013.10.004

In this paper, the push-pull molecules consisting of different number of triphenylamino groups and 1,8-naphthalic anhydride ring were designed and synthesized. The UV-vis absorption and emission spectra of these compounds were recorded. Along with the inc

Full text of DOI:10.1016/j.molstruc.2013.10.004

Journal of Molecular Structure 1056–1057 (2014) 339–346
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
‘‘Push–pull’’ 1,8-naphthalic anhydride with multiple triphenylamine
groups as electron donor
⇑
⇑
Limin Wang, Yan Shi, Yingyuan Zhao, Heyuan Liu, Xiyou Li , Ming Bai
Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Department of Chemistry, Marine College, Shandong University, Jinan 250100, China
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
Three new naphthalic anhydride compounds with linear triphenylamino oligomer were prepared.
The enhanced electron donating ability of the donor leads to excited state changes from ICT state to electron transfer.
Theoretical calculation reveals that a higher ICT states may responsible for the weak emission of 3.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 August 2013
Received in revised form 2 October 2013
Accepted 3 October 2013
Available online 12 October 2013
In this paper, the ‘‘push–pull’’ molecules consisting of different number of triphenylamino groups and
1,8-naphthalic anhydride ring were designed and synthesized. The UV–vis absorption and emission spec-
tra of these compounds were recorded. Along with the increase on the number of the electron donating
triphenylamino groups, both the absorption and emission bands show significant red shift. More impor-
tantly, the fluorescence quantum yields drop sharply along with the increase on the number of triphe-
nylamino groups. The molecular structure, the frontier molecular orbital energies and the energy gaps
between the highest occupied molecular orbital (HOMO) and the lowest un-occupied molecular orbital
(LUMO) were calculated with DFT method. The calculated results indicate that the connection of more
electron donating triphenylamino groups in molecule caused a change for the first excited state from
an intramolecular charge transfer (ICT) state to an intramolecular electron transfer state (ET). This change
on the first excited state has led to the fluorescence quenching.
Keywords:
Push–pull
Absorption spectra
Fluorescence spectra
Triphenylamine
Naphthalic anhydride
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
emission bands centered at longer wavelength. The ICT transition
is highly solvent dependent, and therefore, their photophysical
Due to the unique photophysical properties, 1,8-naphthalimide
compounds have found application in various areas of chemistry
[1–3]. Moreover, their absorption and emission spectra can be eas-
ily tuned through careful structural modification on either the aro-
matic ‘naphthalene’ moiety itself, or at the nitrogen of the imide
site [4]. Consequently, the 1,8-naphthalimide compounds has been
extensively used as strongly absorbing and colorful dyes [5], build-
ing block for artificial light harvesting arrays [6], and fluorescent
chemical probes [7–9] for the sensing of biologically relevant cat-
ions and anions [5,10,13].
The photophysical properties of 1,8-naphthalimide compounds
are governed by the nature of the substituent. Connection of elec-
tron donating groups at C-4 position of the naphthalic ring give a
‘‘push–pull’’ electronic configuration and generate an intramolecu-
lar charge transfer (ICT) excited state [11–13]. This ICT character
leads to a large excited-state dipole and broad absorption and
properties, such as the kmax of absorption and emission spectra,
the fluorescence quantum yield as well as the fluorescence life-
time, are all dramatically affected by the property of solvents
[11,4]. As the most important precursor for 1,8-naphthalimide
compounds, 1,8-naphthalic anhydride compound have very similar
properties with the corresponding imide compounds. They have
the same ‘‘push–pull’’ electronic configuration and similar ICT ex-
cited states.
Triphenylamino group is well known for its good electron
donating abilities [14]. It has been introduced to the C-4 position
of naphthalic ring by Jiang and co-workers for the first time [15].
The electron donating nature of triphenylamino group had induced
significant red-shift on the maximum of both absorption and emis-
sion. In the present work, we introduced linearly arranged triphe-
nylamino groups as electron donor at the C-4 position of
naphthalic ring (1–4 in Scheme 1), which we believe have stronger
electron donating ability. The object of this research is to reveal the
effects of enforced electron donating ability of the substituents at
⇑
Corresponding authors. Tel.: +86 531 88369877.
E-mail address: xiyouli@sdu.edu.cn (X. Li).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.10.004

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L. Wang et al. / Journal of Molecular Structure 1056–1057 (2014) 339–346
Scheme 1. Molecular structures and synthesis of the title compounds.
C-4 position on the photophysical properties of naphthalic
anhydride.
The scan rate was 20 and 10 mV sꢁ1 for cyclic voltammetry (CV)
and differential pulse voltammetry (DPV), respectively.
2.2. The details of quantum chemical calculation
2. Experimental
The hybrid density function B3LYP (Becke–Lee–Young–Parr
composite of exchange-correction functional) method [16–18]
and the standard 6-31G(d) basis set [19] were used for both struc-
ture optimization and the property calculations, which has been
proved to be more accurate in simulating vibrational spectra com-
pared with the Hartree–Fock method [20]. No imaginary frequency
is predicted, indicating that the optimized structures are true en-
ergy-minimums. Based on the energy-minimized structure gener-
ated in the last step, charge population calculations were carried
out with a full natural bond orbital analysis (NBO) population
method [21] and the molecular orbital distribution are discussed
according to the NBO results. All the calculations were performed
using the Gaussian 03 program [22] in the IBM P690 system at
the Shandong Province High Performance Computing Centre.
2.1. General methods
1H NMR and 13C NMR spectra were recorded on a Bruker
DPX300 spectrometer (300 MHz) in CDCl3 with TMS as the internal
standard (chemical shifts are given as d in parts per million). ESI-
TOF mass spectra were taken on a Q-TOF6510 Angilent instrument.
Maldi-TOF mass spectra were taken on a Bruker/ultra flex instru-
ment. The elemental analyses were performed on an Elementar
Vario Micro. Electronic absorption spectra were recorded on a Shi-
madzu UV-2450 spectrophotometer. Steady state fluorescence
spectra were recorded on a K2 system of ISS. Electrochemical mea-
surements were carried out under nitrogen atmosphere on an elec-
trochemical working station. The cell comprised inlets for a glassy
carbon disk working electrode of 2.0 mm in diameter and a silver-
wire counter electrode. The reference electrode was Ag/Ag+, which
was connected to the solution by a Luggin capillary whose tip was
placed close to the working electrode. It was corrected for junction
potentials by being referenced internally to the ferrocenium/ferro-
cene (Fe+/Fe) couple [E1/2(Fe+/Fe) = 501 mV vs. SCE]. Typically, a
0.1 mol dmꢁ3 solution of [Bu4N][ClO4] in dichloromethane contain-
ing 0.5 mmol dmꢁ3 of sample was purged with nitrogen for 10 min,
then the voltammograms were recorded at ambient temperature.
2.3. Materials and synthesis
N,N0-diphenyl-p-phenylenediamine and diphenylamine were
purchased from commercial source and used as received without
further purification. Solvents were of analytical grades and were
purified by standard methods. The synthetic procedures of the tar-
get compounds are outlined in Scheme 1.

L. Wang et al. / Journal of Molecular Structure 1056–1057 (2014) 339–346
341
2.3.1. 4-Dimethylamino-1,8-naphthalic anhydride (1) [23]
2.3.5. N-(4-iodophenyl)-N-phenyl-benzenamine (6)
4-Dimethylamino-1,8-naphthalic anhydride was prepared fol-
lowing the literature methods with a yield of 80% [23]. Mp. 208–
210 °C; 1H NMR (300 MHz, CDCl3, 25 °C, TMS): d 8.59 (d, 1H),
8.51–8.46 (m, 2H), 7.69 (t, 1H), 7.10 (d, 1H), 3.18 (s, 6H); MS(ESI):
m/z, calculated for C14H11O3N: 241.25, found: 242.19 [M + H+].
To a mixture of 1,4-diiodobenzene (396 mg, 1.2 mmol), diphe-
nylamine (169 mg, 1 mmol), CuI (19 mg, 0.1 mmol), phenanthro-
line (28.2 mg, 0.12 mmol) and potassium hydroxide (224 mg,
4 mol), dry toluene (20 mL) was added under the protection of
nitrogen. Then mixture was heated to 120 °C and kept at this tem-
perature for about 12 h. After then, the reaction was quenched by
the addition of water (30 ml). The reaction mixture was extracted
with CH2Cl2 (30 ml) for three times and the combined organic ex-
tracts were dried over anhydrous magnesium sulfate for overnight.
Then the organic solvent was evaporated under reduced pressure.
The residue was purified by column chromatography on silica gel
using petroleum ether as eluent. N-(4-iodophenyl)-N-pheny-ben-
zenamine (6) was obtained as a white powder (315.4 mg, 85%).
Mp > 300 °C; 1H NMR (300 MHz, CDCl3): 7.58 (d, 2H), 7.28 (m,
4H), 7.08 (m, 6H), 6.83 (d, 2H). MS (ESI): m/z, calculated for
2.3.2. 4-Diphenylamino-1,8-naphthalene anhydride (2)
To a three-necked flask, 4-bromo-1,8-naphthalic anhydride
(276 mg,
1 mmol),
diphenylamine
(254 mg,
1.5 mmol),
[Pd(dppf)Cl2] (3.65 mg, 0.0050 mmol) and cesium carbonate
(488 mg, 1.5 mmol) were added. The flask was purged with nitro-
gen gas for several times and then 1,4-diethylene dioxide (3 ml)
was added via a syringe [29]. The mixture was heated to 95 °C un-
der the protection of nitrogen flow and kept at this temperature for
about 3 h. After cooled to room temperature, a mixture of water
(3 ml) and ethyl acetate (20 ml) was added to the flask, the organic
layer was then separated from the aqueous layer in a separation
funnel and washed successively with water and brine solution.
The organic layer was dried over anhydrous magnesium sulfate
for overnight. After filtration, the solvents were removed under re-
duced pressure. The residue was purified by column chromatogra-
phy on silica gel with chloroform/petroleum (v/v = 2/1) as eluent.
Compound 2 was collected as orange powder (310.2 mg, 85%).
Mp > 300 °C; 1H NMR (300 MHz, CDCl3, 25 °C, TMS): 8.55 (d, 1H),
8.48 (d, 1H), 8.23 (d, 1H), 7.54 (t, 1H), 7.38 (d, 1H), 7.35 (m, 4H),
7.15 (m, 2H), 7.06 (m, 4H). 13C NMR (75 MHz, in CDCl3): 161.00,
160.21, 152.46, 148.15, 134.33, 133.36, 132.82, 132.71, 129.81,
127.59, 126.40, 124.84, 124.47, 124.38, 119.39. MS (ESI): m/z, cal-
C
18H14IN: 371.21, found: 372.02 [M + H]+.
2.3.6. N-(4-(N0,N0-diphenyl)amino)phenyl-N-(4-(N000-phenyl)amino)
phenyl-N-phenyl-amine (7)
A
mixture of N,N0-diphenyl-p-phenylenediamine (260 mg,
1 mmol), N-(4-iodophenyl)-N-phenyl-benzenamine (6445 mg,
1.2 mmol), [Pd(dppf)Cl2] (3.65 mg, 0.005 mmol), sodium tert-
butoxide (144 mg, 1.5 mmol) and toluene (10 mL) was stirred at
112 °C for 5 h under the protection of nitrogen. After cooled to
room temperature, a mixture of water (3 ml) and ethyl acetate
(20 ml) was added to the reaction mixture. The organic layer was
separated from the aqueous layer and washed successively with
water and brine solution, and dried over anhydrous magnesium
sulfate for overnight. Then the solvents were evaporated under re-
duced pressure. The residue was purified by column chromatogra-
phy on silica gel with chloroform/petroleum (v/v = 3:1) as eluent.
N-(4-(diphenylamino)phenyl)-N,N0-diphenyl-1,4-diaminobenzene
(7) was collected as dark green powder (251.5 mg, 50%).
Mp > 300 °C; 1H NMR (400 MHz, CDCl3): 7.45 (d, 4H), 7.28 (m,
8H), 7.13 (m, 12H), 7.01 (t, 4H). MS (MALDI-TOF): m/z, calculated
for C36H29N3: 504.24, found: 503.64 [M+].
culated for C24H15O3N: 365.11, Found: 365.38 [M+]. IR (KBr, cmꢁ1
)
1687 (mas C@O), 1652 (ms C@O).
2.3.3. N,N-diphenyl-N0-phenyl-1,4-diaminobenzene (5)
A
mixture of N,N0-diphenyl-p-phenylenediamine (260 mg,
1 mmol), [Pd(dppf)Cl2] (3.65 mg, 0.05 mmol), sodium tert-butoxide
(144 mg, 1.5 mmol), toluene (10 ml) was stirred at 112 °C for 5 min
under the protection of nitrogen, iodobenzene (1.2 mmol) was
then added to the flask via a syringe. Then the reaction mixture
was kept at this temperature for 5 h. After cooled to room temper-
ature, water (3 ml) and ethyl acetate (20 ml) was added to the mix-
ture. The organic layer was separated from the aqueous layer
through a separation funnel, and washed successively with water
and brine solution. After dried over anhydrous magnesium sulfate
for overnight, the solvents were evaporated under reduced pres-
sure. The residue was purified by column chromatography on silica
gel with chloroform/petroleum (v/v = 2:1) as eluent. Compound 5
was collected as green powder (210.4 mg, 65%). MS (ESI): m/z, cal-
culated for C24H20N2: 336.43, found: 336.11 [M+]. Elemental anal-
ysis (%), calculated for C24H20N2: C 78.60, H 5.94, N 8.32; found: C
78.47, H 6.12, N 8.45.
2.3.7. 4-(N-(4-(N0,N0-diphenyl)amino)phenyl-N-(4-(N000-phenyl)amino)-
phenyl-N-phenyl)amine-1,8-naphthalic anhydride (4)
Following similar procedures as that of compound 2, except
with N-(4-(N0,N0-diphenyl)amino)phenyl-N-(4-(N000-phenyl)ami-
no)phenyl-N-phenyl-amine (7) instead of diphenylamine, com-
pound 4 was obtained as red dark solid (210.1 mg, 30%). 1H NMR
(400 MHz, CDCl3): 8.54 (m, 2H), 8.27 (d, 1H), 7.54 (t, 1H), 7.34–
7.35 (b, 10H), 6.99–7.11 (br, 19H). 13C NMR (75 MHz, in CDCl3):
161.085, 160.225, 148.201, 134.367, 133.326, 132.853, 130.898,
129.735, 129.351, 128.840, 126.153, 125.685, 125.212, 124.141,
123.849, 123.135, 139.397. MS (MALDI-TOF): m/z, calculated for
C
48H33O3N3: 699.79, Found: 700.26 [M + H+]. Elemental analysis
(%) calculated for C48H33O3N3: C 82.31, H 4.72, N 6.00; found: C
81.71, H 5.43, N 4.63. IR (KBr, cmꢁ1) 1687 (mas C@O), 1650 (ms
C@O).
2.3.4. 4-(N-phenyl-N-(4-(N0,N0-diphenyl)amino)phenyl)amino-1,8-
naphthalene anhydride (3)
3. Results and discussion
Following the similar procedures as that of compound 2, except
with N,N-diphenyl-N0-phenyl-1,4-diaminobenzene (5) instead of
3.1. Synthesis
diphenylamine, compound
3
was obtained as red solid
(266.5 mg, 50%). Mp > 300 °C; 1H NMR (300 MHz, CDCl3, 25 °C,
TMS): 8.56–8.47 (m, 2H), 8.27 (d, 1H), 7.56–7.50 (t, 1H), 7.40 (d,
1H), 7.31–7.24 (m, 7H), 7.11–6.91 (d, 12H). MS (MALDI-TOF): m/
z, calculated for C36H24O3N2: 532.18, found: 533.19 [M + H]+. Ele-
mental analysis (%) calculated for C36H24O3N2: C 81.17, H 4.51, N
5.26; found: C 80.63, H 4.72, N 4.72. IR (KBr, cmꢁ1) 1690 (mas
C@O), 1662 (ms C@O).
The introduction of dimethylamine at the C-4 position of 1,
8-naphthalic anhydride has been achieved by heating N,N-di-
methyl-2-nitrile-ethylamine in iso-propanol following the litera-
ture methods [23], which leads to the formation of compound 1.
4-Bromo-1,8-naphthalic anhydride reacts with diphenylamine in
the presence of palladium-catalyst to generate compound 2. Under
similar conditions, compound 3 is obtained by the reaction of

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L. Wang et al. / Journal of Molecular Structure 1056–1057 (2014) 339–346
4-bromo-1,8-naphthalic anhydride with N,N-diphenyl-N0-phenyl-
1,4-diaminobenzene (5) while compound 4 was obtained by the
reaction of 4-bromo-1,8-naphthalic anhydride with N-(4-(N0,N0-di-
phenyl)amino)phenyl-N-(4-(N000-phenyl)amino)phenyl-N-phenyl-
amine (6). The important precursor 5 and 7 were prepared by the
palladium-catalyzed reaction of corresponding amines with iodo-
benzene or N-(4-iodophenyl)-N-phenyl-benzenamine (6). Com-
pound 5 gave correct mass spectrum and reasonable elemental
analysis results, but failed to give a meaningful 1H NMR. All the sig-
nals in 1H NMR spectra are significantly broad and the integrations
are unreliable. Because we synthesized compound 3 in a good yield
with this product as precursor, we can conclude that the structure
of compound 5 is correct.
Table 1
Parameters of absorption and fluorescence spectra of compounds 1–4 in
dichloromethane.
Compounds
k, nm (
e
/103 L molꢁ1 cmꢁ1
)
U
(%)a
1
2
3
4
ꢂ280 (29), 425 (14.7)
280 (33), 463 (14.4)
312 (49.4), 494 (13.4)
321 (69.2), 503 (13.6)
100
5.7
1.0
0
a
Relative fluorescence quantum yields of these compounds (2–4: kex = 440 nm,
1: kex = 380 nm) calculated with compound 1 as reference.
300–350 nm. But this absorption band became more significant
in the absorption spectra of compound 3 due to the doubled num-
ber of triphenylamino groups in this molecule. As expected, this
absorption band became more significant in the absorption spectra
of compound 4 due to the even more triphenylamino groups. More
interestingly, the peak of this absorption band appears at about
321 nm, which red shifted for about 9 nm in relative to that of
compound 3. This red-shift suggests the presence of efficient inter-
actions between the triphenylamino groups within both com-
pounds 3 and 4. This has also been proved by the theoretical
calculation as mentioned below.
The absorption spectra of these compounds in different solvents
with different polarities are also recorded and the resulted spectra
are shown in Supporting Information, Fig. S7. The polarity of the
solvent does not show large effects on the absorption spectrum of
these compounds on both absorption intensity and absorption
wavelength. Only very small red shift on the maximal absorption
band of these compounds in polar solvents in relative to that in
non-polar solvent can be identified from the spectra. This is because
the stabilization of polar solvents to the polar ICT excited states has
decreased the energy gap between the ground states and excited
states, and consequently leads to the small red-shift on the maxi-
mal absorption peak. Because the absorption position determined
predominately by the ground states as Frank Condon principle sug-
gested, the solvation of the polar solvents to the ICT excited states
does not show large effects on the absorption spectra.
3.2. Absorption spectra
The absorption spectra of these four compounds in dichloro-
methane at the concentration of 10ꢁ5 mol/L are shown in Fig. 1.
The corresponding spectrum parameters are summarized in
Table 1. As indicated in the absorption spectra, compound 1 shows
one absorption peak at 425 nm, which can be assigned to the ICT
band [24]. Another band in the UV region, which might be ascribed
to the absorption of
p–
p⁄ transition of naphthalic ring, cannot be
completely shown duo to the limit of instrument. Other three com-
pounds show similar absorption spectra with one ICT band in the
visible region and another band around 300 nm in the UV region.
The absorption band around 300 nm can be assigned to the absorp-
tion of triphenylamino groups of compounds 2, 3 and 4 [25]. The
spectral parameters of compound 2 as shown in Table 1 revealed
that the replacement of dimethylamino group with diphenylamino
group at C-4 position of naphthalic ring leads to a 38 nm red-shift
on the ICT absorption band. This can be attributed to the stronger
electron donating ability of diphenylamino group in relative to that
of dimethylamino group. When the number of triphenylamino
groups in compounds 3 and 4 increased to 2 and 3 respectively,
the ICT band further red shifted for about 31 and 9 nm respectively.
This result suggests that the increasing on the number of triphe-
nylamino groups has indeed enhanced the electron donating abil-
ity of the ‘‘push’’ end of the ‘‘push–pull’’ molecules.
3.3. Fluorescence spectra
The absorption of triphenylamine subunits at about 300 nm of
compounds 2, 3, and 4 show some differences. The absorption of
the triphenylamine subunit in compound 2 is presented as a small
shoulder of the absorption of naphthalic ring in the range of
The fluorescence spectra of compounds 1–3 are shown in Fig. 2.
When these compounds were excited by light (380 nm for 1,
440 nm for 2 and 3), the ICT emission band was observed. The
Fig. 1. UV–vis absorption of compound
compound 2 (red line with open squares), compound 3 (green line with open
1
(black line with closed squares),
Fig. 2. Normalized fluorescence spectra of compound 1 (black line with closed
squares) compound 2 (red line with open squares) compound 3 (green line with
open circles) in dichloromethane at concentration 10ꢁ5 mol/L (2–3: kex = 440 nm, 1:
kex = 380 nm). (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
circles) and compound
4 (blue line with closed circles) in dichloromethane
(10ꢁ5 mol/L). (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)

L. Wang et al. / Journal of Molecular Structure 1056–1057 (2014) 339–346
343
Table 2
ICT emission band of compounds 1 and 2 appeared at 526 nm and
Half-wave redox potentialsa (vs. SCE) of compounds 2–4 in dried dichloromethane.
633 nm respectively, which is in accordance with the results of
absorption spectra. But the ICT emission for compound 3 appeared
at 560 nm, which is unexpected because the ICT emission should
further red shifted in relative to that of compound 2. At the very
beginning, we suspected that this might be the emission from
some impurities, but the excitation spectra as well as the elemen-
tal analysis, HPLC analysis reveal the high purity of this compound,
therefore, we attribute this emission to a high excited state emis-
sion, which will be further discussed lately. Compound 4 did not
show any identical ICT emission in the fluorescence spectra. The
ICT nature of the fluorescence of compounds 1–2 is further proved
by their solvent dependent fluorescence spectra, Fig. S8 in Support-
ing Information. It can be seen that the maximal emission position
shifts to red in polar solvent, which is accompanied with distinc-
tive decrease on the fluorescence quantum yield. This can be
attributed to the solvation of the polar solvents to the polar ICT ex-
cited states, which has reduce the energy of excited states signifi-
cantly, and then caused the remarkable red-shift of the maximal
emission peak.
The fluorescence quantum yields of these four compounds in
dichloromethane are measured with compound 1 as reference.
The results are summarized in Table 1. It can be found that the
fluorescence quantum yield of compound 2 is much smaller than
that of compound 1, which can be attributed to the stronger elec-
tron donating ability of diphenylamine than that of dimethylamine
[26–29]. The fluorescence quantum yield of compounds 3 and 4 are
extremely small, which suggests again that the connection of tri-
phenylamino groups in compounds 3 and 4 has indeed enhanced
the electron donating abilities of the group at the C-4 position. This
is in line with the results of absorption spectra.
Eh1=2
b
Compounds Ox3
Ox2
Ox1
Red1
HOMO
(eV)
LUMO
(eV)
2
3
4
0.93 ꢁ1.35 2.28
0.87 0.51 ꢁ1.33 1.84
1.12 0.61 0.30 ꢁ1.26 1.56
ꢁ5.33
ꢁ4.91
ꢁ4.70
ꢁ3.05
ꢁ3.07
ꢁ3.14
a
Values obtained by DPV in dry CH2Cl2 with 0.1 M TBAP as the supporting
electrolyte and Fc/Fc+ as internal standard.
Eh1=2 ¼ Eox1 ꢁ Ered1
.
b
compounds, the redox potentials were measured by DPV experi-
ments. The results are summarized in Table 2. Compound 1 cannot
give any reliable reduction or oxidation signals in both DPV and CV
experiments in the electrochemical window of the solvent. The
first oxidation of compound 2 happens at 0.93 V and a reduction
at ꢁ1.35 V. After the introduction of another triphenylamino
group, compound 3 shows its first oxidation at 0.51 V, which is
much smaller than that of compound 2. This means the first oxida-
tion of compound 3 happens on the triphenylamino group. Besides
the first oxidation, the second oxidation is observed at 0.87 V for
compound 3. The first oxidation potential of compound 4 is further
reduced to 0.30 V. Moreover, other two oxidation process are ob-
served at 0.61 and 1.12 V, which correspond to the second and
third oxidation, respectively. The difference of the oxidation poten-
tials of compounds 2–4 suggests that the first oxidation happens
on the triphenylamino groups. More triphenylamino groups are
introduced, the easier these molecules are oxidized. Along with
the increase on the number of triphenylamino groups, the reduc-
tion potentials of compounds 2–4 decrease following the order of
2 > 3 > 4. But the magnitude of the reduction potential decrease
is much smaller than that of the first oxidation potentials. This re-
sult suggests that the reduction happened probably at the naph-
thalic ring. Because the naphthalic ring is far away from the
triphenylamino groups, the effect of the triphenylamino groups
on the reduction potential is limited. The HOMO and LUMO energy
3.4. The electrochemical properties
To investigate the effects of the electron donating groups at the
C-4 position on the oxidation and reduction properties of these
Fig. 3. Labels of the atoms in the molecular structure (A) during calculation and the minimized molecular structure of compounds 1–4 (B–E).

344
L. Wang et al. / Journal of Molecular Structure 1056–1057 (2014) 339–346
levels are also estimated from the oxidation potentials, the results
are summarized in Table 2. It can be found that the energy of
HOMO increases significantly from compounds 2 to 4, which indi-
cates that the introduction of electron donating groups varies the
HOMO energy efficiently. However, the energy decrease of LUMO
is very small, suggesting no large effect of the triphenylamino
groups on LUMO energy. Due to the significant increase on the en-
ergy of HOMO along with the increase on the number of triphe-
1.443 Å in compounds 2 and 3 and 1.398 and 1.444 Å in compound
4. The bond length decrease of compounds 2–4 can be ascribed to
the large steric hindrance introduced by the triphenylamino
groups at the C-4 position. The bonds length of C4–N, N–R1 and
N–R2 are also strongly affected by the substituents. Similar results
can be found from the comparison of the bond length of C4–N, N–
R1 and N–R2 in compounds 1–4.
nylamino groups from compounds
between HOMO and LUMO decreases remarkably, and conse-
quently the maximal absorption peak shifts to red dramatically.
2 to 4, the energy gap
3.5.2. Calculated frontier molecular orbital
The distribution of frontier molecular orbitals as well as their
energy levels is shown in Fig. 4. It can be found that the energy of
the LUMOs is not significantly affected by the triphenylamino
groups at C-4 position. The energies of LUMOs of compounds
1–4 are similar. But the energy of the HOMO increases dramati-
cally along with the increase on the number of triphenylamino
groups at C-4 position. The HOMO energy of compound 1 is
ꢁ5.98 eV, which is significantly smaller than that of compound
4 (ꢁ4.86 eV). The result of this increase on the HOMO energy
from compounds 1 to 4 is the decrease on the energy gap be-
tween HOMO and LUMO. The HOMO–LUMO energy gap decrease
following the order of 1 > 2 > 3 > 4, this is in line with the red
shift on the ICT band in the UV–vis absorption spectra as shown
in Fig. 1. This also suggests that the ICT transition is dominated
by the transition from HOMO to LUMO in these compounds. It
is notable that the calculated energies of HOMO and LUMO as
shown Fig. 4are a little bit different from those measured by elec-
trochemical method as shown in Table 2. This is because the
quantum chemical calculation is performed on the molecules in
vacuum while the electrochemical measurement is carried out
on the molecules in solution. Nevertheless, the energies of HOMO
and LUMO resulted from both quantum chemical calculation and
electrochemical experiments change in the same trend along with
3.5. Theoretical calculation
3.5.1. Minimized molecular structure
Using b3lyp/6-31g(d) method, the minimized structure of com-
pounds 1–4 were optimized, Fig. 3. It is worth noting that similar
calculating method has been used to calculate similar naphthalic
anhydride compounds [24]. The resulted geometry and structure
parameters are proved to be reliable by the crystal structure.
Therefore, we choose the similar calculating method for our com-
pounds. The calculated geometric parameters, including bond
length and torsion angle are listed in Supporting Information. It
can be seen that, naphthalic ring has a planar structure with a
twisting angle of ꢂ90° from the plane of directly connected amino
group. The bonds which close to the C-4 positions of the naphthalic
ring, including C4–N, N–R1, N–R2, C3–C4, C4–C5 are all get shorter
a little bit when the side groups are introduced at C-4 positions, so
as the angle of C4–N–R1, C4–N–R2. The bonds C3–C4 and C4–C5
were mostly impacted probably because of their adjacency with
the substituents. The bond length of C3–C4 and C4–C5 in com-
pound 1 is 1.403 and 1.450 Å, which decreased to 1.396 and
Fig. 4. Electronic density contours and the energy level of the frontier molecular orbitals for compounds 1–4.

L. Wang et al. / Journal of Molecular Structure 1056–1057 (2014) 339–346
345
the increase on the number of triphenylamino groups, which
indicates further that the results of quantum chemical calculation
are reliable.
mino groups at the ‘‘push’’ end due to the enhanced electron
donating ability. The fluorescence quantum yields of compounds
3 and 4 are extremely small, which is because the process in-
duced by photo excitation is no more ICT, but electron transfer
from triphenylamines to naphthalic anhydride instead. More
The distributions of the frontier molecular orbital on the
molecular backbone illustrate visually the push–pull effects in
these molecules. In the molecule of 1, the HOMO distributes
throughout the whole molecule, but with about 41.7% locates
on the dimethylamino group. Meaning while, the LUMO spreads
also over the molecule, with a contribution from dimethylamino
group of only 7.9%. Transition from HOMO to LUMO under pho-
toexcitation leads to charge redistribution over the whole mole-
cule. A partial charge will move from dimethylamino group to
the anhydride group along with this HOMO to LUMO transition.
In compound 2, the HOMO distributes mainly on the diphenyl-
amine and the part of naphthalic ring close to the diphenyl-
amine group. The contribution of the diphenylamine to the
HOMO is about 71.1%. But the LUMO of compound 2 locates
mainly on the naphthalic ring and the anhydride group. This
indicates that the transition from HOMO to LUMO in compound
2 will leads to more significant charge transfer from electron
donating diphenylamine group to naphthalic anhydride group
than that happens in compound 1. In compounds 3 and 4, the
HOMOs distribute almost 100% on the triphenylamino groups
while the LUMOs are restricted on the naphthalic anhydride ring.
Therefore, transition from HOMO to LUMO in these two com-
pounds leads to an intramolecular electron transfer from the
side group to the naphthalic anhydride ring instead of the ICT
in compounds 1 and 2. This is why the fluorescence of ICT, cor-
responding to HOMO–LUMO transition, in compounds 3 and 4
were completely quenched.
interestingly, compound
3 show a very weak emission at
560 nm after excitation at 440 nm. This weak emission might
be assigned to a special ICT, which involve a high energy transi-
tion, such as the transition from HOMO-1 to LUMO. The results
of this research revealed that for a ‘‘push–pull’’ molecule, strong
electron donating ability at the ‘‘push’’ end could change the
ICT into electron transfer and lead to a completely fluorescence
quenching.
Acknowledgements
We thank the Natural Science Foundation of China (Grand Nos.
21073112 and 21173136), the National Basic Research Program of
China (973 Program: 2012CB93280) and Natural Science founda-
tion of Shandong Province (ZR2010EZ007) for the financial
support.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2013.
10.004.
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4. Conclusion
In this paper we have designed and synthesized a series of
‘‘push–pull’’ naphthalic anhydride compounds with a linear tri-
phenylamine oligomer of different length as electron donor.
UV–vis absorption, emission spectra and theoretical calculation
were revealed that with the increase on the number of triphe-
nylamino groups at the ‘‘push’’ end, the red-shift on the absorp-
tion maximum and the energy gap between HOMO and LUMO
decrease gradually. The decrease on the HOMO/LUMO energy
gap is primarily caused by the increase on the energy level of
HOMO. The fluorescence quantum yields of these compounds de-
crease also along with the increase on the number of triphenyla-

346
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