Synthesis, characterization, and nonlinear optical (NLO) properties of truxene-cored diphenylamine derivatives

Article abstract of DOI:10.1016/j.saa.2017.05.030

Three star-shaped compounds based on a truxene core (FS11, FS12 and FS13) were prepared. The truxene core is incorporating with asymmetric diphenylamines, including one phenyl of diphenylamine substituted by methoxy group and the other phenyl substituted by tolyl, fluorophenyl and phenylethynyl for FS11, FS12 and FS13, respectively. Their one-photon, two-photon absorption, geometric structures, electrochemical behavior and thermal properties were investigated. The absorption maxima of charge transfer band for FS11, FS12 and FS13 are 375?nm, 373?nm and 383?nm, and the corresponding molar extinction coefficients of FS11, FS12 and FS13 is 79,950?M^??1?cm^??1, 67,220?M^??1?cm^??1 and 108,780?M^??1?cm^??1. The “pull-push” structure promotes charge transfer between asymmetric diphenylamine branches and the truxene core. Their two-photon absorbtion property is measured by two-photon induced fluorescence. The maximum two-photon cross-sections values of FS11, FS12 and FS13 are excited at 750?nm, which are 260 GM, 204GM and 367 GM, respectively.

Full text of DOI:10.1016/j.saa.2017.05.030

Accepted Manuscript
Synthesis, characterization, and nonlinear optical (NLO)
properties of truxene-cored diphenylamine derivatives
Fusheng Li, Baodong Zhao, Yu Chen, Yufei Zhang, Tao Wang,
Song Xue
PII:
DOI:
Reference:
S1386-1425(17)30406-7
doi: 10.1016/j.saa.2017.05.030
SAA 15172
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
10 February 2017
11 May 2017
14 May 2017
Please cite this article as: Fusheng Li, Baodong Zhao, Yu Chen, Yufei Zhang, Tao Wang,
Song Xue , Synthesis, characterization, and nonlinear optical (NLO) properties of truxene-
cored diphenylamine derivatives, Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy (2017), doi: 10.1016/j.saa.2017.05.030
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ACCEPTED MANUSCRIPT
Synthesis, characterization, and nonlinear optical (NLO)
properties of truxene-cored diphenylamine derivatives
Fusheng Li, a Baodong Zhao, b Yu Chen, a,* Yufei Zhang, a Tao Wang, b Song Xue a
a Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion,
School of Chemistry & Chemical Engineering, Tianjin University of Technology,
Tianjin 300384, P. R. China;
b State Key Laboratory of Chemical Resource Engineering, College of Science,
Beijing University of Chemical Technology, Beijing, 100029, People’s Republic of
China.
*Corresponding author: E-mail address: cy26tj@163.com; Fax: +86-22-60214252.
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Abstract:
Three star-shaped compounds based on a truxene core (FS11, FS12 and FS13)
were prepared. The truxene core is incorporating with asymmetric diphenylamines,
including one phenyl of diphenylamine substituted by methoxy group and the other
phenyl substituted by tolyl, fluorophenyl and phenylethynyl for FS11, FS12 and FS13,
respectively. Their one-photon, two-photon absorption, geometric structures,
electrochemical behavior and thermal properties were investigated. The absorption
maxima of charge transfer band for FS11, FS12 and FS13 are 375 nm, 373 nm and
383 nm, and the corresponding molar extinction coefficients of FS11, FS12 and FS13
is 79950 M-1cm-1, 67220 M-1cm-1 and 108780 M-1cm-1. The “pull-push” structure
promotes charge transfer between asymmetric diphenylamine branches and the
truxene core. Their two-photon absorbtion property is measured by two-photon
induced fluorescence. The maximum two-photon cross-sections values of FS11, FS12
and FS13 are excited at 750 nm, which are 260 GM, 204GM and 367 GM,
respectively.
Keyword: Truxene; diphenylamine; asymmetric; two-photon absorption; two-photon
induced fluorescence
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1. Introduction
Nonlinear optical (NLO) materials with strong two-photon absorption (2PA) have
attracted a great deal of attention in recent years [1-4]. Organic molecules and
polymers have been developed [5-8], and their NLO and electronic properties have
been investigated. A lot of work has been devoted in enhancing the 2PA property by
strategy of molecular design. The 2PA behavior can be influenced by parameters such
as molecular symmetry, molecular weight, and solubility [9]. Star-shaped organic
molecules possessing extended π-conjugated construction exhibit excellent
one-photon absorption, NLO and fluorescence emission properties, and thus they are
widely used in organic light-emitting diodes [10], two-photon photodynamic therapy
[11] and 3D optical data storage [12]. Star-shaped organic molecules consist of a
central core and branches. Multi-branched π-conjugation structures enhance NLO
response of star-shaped molecules, benefiting from enlarged electronic delocalization
and increased intramolecular charge transfer [13, 14]. The introduction of electron
donor and electron acceptor as end groups onto the branches symmetrically or
unsymmetrically can further extend the electron-donating or electron-accepting
strength and the conjugation length of star-shaped molecules, leading to increase 2PA
cross-section (σ2)[14]. The flexibility on structure modification and facile synthesis
provide a good basis for star-shaped organic π-conjugation molecules to be used in
NLO devices.
Truxene
(10,
15-dihydro-5H-diindeno[1,2-a;1’,2’-c]fluorene)
is
a
C3-sythmmetric organic molecule, and it possesses satisfactory solubility, rigid
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2-dimensinal structure, good thermal stability and ease to be modified [15, 16].
Truxene-cored materials have been widely used in organic light-emitting devices [17,
18], three-photon absorption [19], dye-sensitized solar cell [20] and perovskite solar
cells [21, 22], two-photon absorption [23-26]. The good solubility of truxene-cored
materials can be realized by attaching long alkyl chains on C9, C12 and C15 positions
of truxene, as well as the restraint of molecule aggregation. Different branched
moieties can be introduced onto the C2, C7, and C12 positions of the truxene core
[23]. Chan and co-workers have studied 2PA properties of truxene-containing
platinum (II) alkynyl complexes [23]. In their report, one complex incorporated with
electron-deficient oxadiazole moiety gave the greatest σ2 values of 51 GM at 720 nm,
which create a large change in the dipole moment within the molecules. Lin and
co-workers have reported a series of two-photon-active truxene-cored derivatives with
varied number and size of functionalized pyrazine units [24]. The overall coplanarity
between the central core and the “fusing” peripheral branches is improved, yielding a
very active 2PA response up to 5100 GM at 730 nm. Yuan and co-workers have
prepared three thiophenevinyl substituted one-, two-, and three-branched truxene
π-conjugated compounds [25]. The σ2 of three-branched compound (235 GM) is
several times that of the two-branched compound (76 GM) and one-branched
compound (27 GM). In addition, Zheng and co-workers have reported highly soluble
π-conjugated turuxene-containing dendritic chromophores with large two-photon
absorption in the range of 92.8-1412 GM [26]. In their research, diphenylamine was
chosen as an electron donor and butyl groups were introduced for increasing
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chromophore solubility in organic solvents. The dendritic or the three-branched
chromophore with diphenylamine has both an extended π-conjugated system and an
increased intramolecular charge redistribution compared to the one-branched
chromophore. These results indicate that the star-shaped molecules can make a great
contribution for obtaining strong 2PA absorption, and truxene is a promising building
block for the construction of NLO materials. The three-branched truxene compounds
can show a large π-conjugation length, and the introduction of appropriate electron
donors or accepters onto branched chromophore can enhance 2PA capability.
In this paper, three truxene-cored compounds (FS11, FS12, and FS13) were
synthesized by a facile route. The structures of FS11, FS12, and FS13 are shown in
Fig.1. The whole molecule based on a truxene core is symmetric with a
three-branched structure. The diphenylamine branch incorporating with the truxene
core is asymmetric. For the studied compounds, one phenyl of diphenylamine is
substituted by methoxy group, and the other phenyl of diphenylamine is substituted
by tolyl for FS11, fluorophenyl for FS12 and phenylethynyl for FS13, respectively.
The electron-donating -OMe on tolyl, the electron accepting –F on fluorophenyl and
the extended π-conjugation -C≡C- on phenylethynyl possibly have a great influence
on their optical properties. Therefore, their one-photon and two-photon photophysical
properties, geometric structure, energy levels and thermodynamic characters have
been investigated. The results indicate these truxene-cored diphenylamine derivatives
have a great potential in optical materials.
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O
N
N
O
N
O
F
FS11
O
O
N
N
N
N
O
O
F
N
N
O
O
F
FS12
FS13
Fig. 1. The structures of FS11, FS12 and FS13.
2. Experimental
2.1. Materials and instruments
The solvents and chemicals used in this work were analytical grade and used
without further purification and purchased from Energy Chemical (China).The
reagents for reactions and spectral measurements were dried and distilled before use.
1H NMR and 13C NMR spectra were recorded on a Bruker AM-300 or AM-400
o
spectrometer at 25 C using DMSO-d6 as solvent. The reported chemical shifts were
against TMS. High resolution mass spectra were obtained with a Micromass
GCT-TOF mass spectrometer. The melting point was taken on a RY-1 thermometer
and the temperatures were uncorrected.
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2.2. Spectral, electrochemical and thermal stability measurements
The absorption spectra of FS11, FS12, and FS13 in solution were measured by
SHIMADZU UV-2600 spectrophotometer. Fluorescence measurements were carried
out with a HITACHI F-4500 fluorescence spectrophotometer. The absolute
fluorescence quantum yield was directly measured by a FLS980 fluorescence
spectrometer at room temperature [27]. The corresponding concentration of FS11,
FS12, and FS13 is 5×10-7 mol L-1 in dichloromethane, and Rhodamine B is used as a
reference. Cyclic voltammetry (CV) measurements were performed on a Zennium
electrochemical workstation (ZAHNER, Germany) using a three-electrode system.
Tetrabutylammoniun hexafluorophosphate (n-Bu4NPF6) of 0.1 M was added into a
dichloromethane solution (DCM) as electrolyte. The thermal stability was determined
by thermogravimetry (TG) and differential scanning calorimetry (DSC). TG analyses
were performed on TG 209 F3 Tarsus (NETZSCH, Germany) at a heating rate of 10
oC/min under a nitrogen atmosphere. DSC analyses were conducted on DSC 200 F3
o
Maia (NETZSCH, Germany) at a heating rate of 10 C/min under a nitrogen
atmosphere.
2.3. Synthesis
Compounds 1–3 and 4a-c were synthesized according to literature methods
reported [22, 28-33] and the synthesis procedure of FS11, FS12 and FS13 is shown in
Scheme 1.
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Br
C3H7
C3H7
C3H7
C3H7
C3H7
Bromopropane， n-BuLi
THF,0℃,12h
Br2
C3H7
C3H7
C3H7
DCM， 避光， RT， 12h
C3H7
Br
C3H7
C3H7
C3H7
Br
3
2
1
O
OH
B
OH
N
H
I
Br
5a
O
NH2
Pd（ PPh3） 4， K2CO3， （ CH2OCH3）
N2,reflux， 24h
2
OH
B
OH
F
F
R
O
Br
Pd2(dba)3, P(t-Bu)3, t-BuONa,
Tol, reflux, 12h
4a-c
N
I
Br
H
Pd（ AcO） 2， P(t-Bu） 3， CuI， NEt3
THF， N2，RT， 1h
5b
O
N
H
R
5c
O
N
C3H7
3
C3H7
F
CH3
C3H7
O
C3H7
Pd2(dba)3， X-phos， t-BuOK，
toluene, refluxing， 12h
R2＝
R1＝
R3＝
R
N
C3H7
C3H7
N
FS12
FS13
FS11
R
O
FS11-13
Scheme 1. The synthetic route of FS11, FS12, and FS13.
2.3.1. General procedure for the synthesis of compounds 5a, 5b and 5c
To a 100 mL two neck round-bottom flask were added compound 4c (500.0 mg,
1.0eq), p-anisidine (287.2 mg, 1.2eq), Pd2(dba)3 (35.6 mg, 0.02eq), t-BuONa (280.1
mg, 1.5eq), P(t-Bu)3 (7.9 mg, 0.02eq) and 10 mL anhydrous toluene. The reaction
mixture was refluxed 12 h under argon. After cooling down to room temperature, the
mixture was exacted with ethyl acetate and washed with water. The organic layer was
dried over anhydrous Na2SO4, filtered, and concentrated in vacuum to give the crude
product, which was purified by column chromatograph packed with silica gel using
petroleum ether (PE)-ethyl acetate (EA) (100:1) as the eluent to afford pure
compound 5c.
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1
5a: gray soild (421.2 mg, 72%), H NMR (400 MHz, DMSO) δ 7.99 (s, 1H),
7.48 (d, J = 2.7 Hz, 2H), 7.46 (d, J = 3.2 Hz, 2H), 7.21 (d, J = 7.9 Hz, 2H), 7.08 (d, J
= 8.9 Hz, 2H), 6.98 (d, J = 8.7 Hz, 2H), 6.89 (d, J = 8.9 Hz, 2H), 3.72 (s, 3H), 2.31 (s,
3H).
1
5b: yellow soild (367.7 mg, 63%), H NMR (400 MHz, DMSO) δ 8.01 (s, 1H),
7.61 (dd, J = 8.7, 5.5 Hz, 2H), 7.47 (d, J = 8.6 Hz, 2H), 7.22 (t, J = 8.8 Hz, 2H), 7.09
(d, J = 8.9 Hz, 2H), 6.99 (d, J = 8.6 Hz, 2H), 6.90 (d, J = 8.9 Hz, 2H), 3.73 (s, 3H).
1
5c: pale green soild (372.3 mg, 64%), H NMR (400 MHz, DMSO) δ 8.24 (s,
1H), 7.49 (d, J = 6.4 Hz, 2H), 7.38 (t, J = 7.7 Hz, 3H), 7.34 (d, J = 8.6 Hz, 2H), 7.10
(d, J = 8.9 Hz, 2H), 6.89 (dd, J = 9.7, 10.3 Hz, 4H).
2.3.2 General procedure for the synthesis of FS11, FS12 and FS13
To a 100 mL two neck round-bottom flask were added compound 5c (593.0 mg,
3.3eq), 3 (500.0 mg, 1.0eq), Pd2(dba)3 (55.0 mg, 0.1eq), t-BuOK (403.8 mg, 6.0eq),
X-phos (57.3 mg, 0.2eq) and 10 mL anhydrous toluene. The reaction mixture was
refluxed 12 h under argon. After cooling down to room temperature, the mixture was
exacted with ethyl acetate and washed with water. The organic layer was dried over
anhydrous Na2SO4, filtered, and concentrated in vacuum to give the crude product,
which was purified by column chromatograph packed with silica gel using PE-EA
(40:1) as the eluent to afford pure compound FS13.
1
FS11: Yellow powder (72% yield). M.p:178-180 oC. H NMR (400 MHz, DMSO) δ
8.10 (d, J = 8.2 Hz, 3H), 7.56 (d, J = 8.5 Hz,6H), 7.53 (d, J = 8.2 Hz, 6H), 7.24 (d, J =
8.0 Hz, 6H), 7.20 (s, 3H), 7.14 (d, J = 8.8 Hz, 6H), 7.06 (d, J = 8.6 Hz, 6H), 6.98 (d, J
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= 8.7 Hz, 6H), 6.95 (d, J = 8.4 Hz, 3H), 3.78 (s, 9H), 2.78–2.70 (m, 6H), 2.33 (s, 9H),
1.91–1.78 (m, 6H), 0.64–0.43 (m, 30H). 13C NMR (100 MHz, CDCl3) δ 156.66,
155.07, 148.34, 145.72, 143.44, 140.16, 137.85, 135.36, 131.46, 127.68, 123,76,
121.58, 121.34, 117.39, 115.28, 114.93, 89.90, 88.49, 55.52, 39.18, 17.45, 14.60.
HRMS (ESI) calcd for C105H105N3O3 (M+H+):1457.8268, found: 1457.8223.
1
FS12: Yellow powder (74% yield). M.p:167-169 oC. H NMR (400 MHz, DMSO) δ
8.09 (d, J = 8.6 Hz, 3H), 7.65 (dd, J = 8.7, 5.5 Hz, 6H), 7.54 (d, J = 8.6 Hz, 6H), 7.24
(t, J = 8.8 Hz, 6H), 7.19 (s, 3H), 7.12 (d, J = 8.8 Hz, 6H), 7.04 (d, J = 8.6 Hz, 6H),
6.97 (d, J = 9.0 Hz, 6H), 6.93 (d, J = 7.3 Hz, 3H), 3.76 (s, 9H), 2.79–2.65 (m, 6H),
1.90–1.76 (m, 6H), 0.63–0.41 (m, 30H). 13C NMR (100 MHz, CDCl3) δ 163.37,
160.92, 156.36, 155.05, 147.67, 146.19, 143.17, 140.70, 136.93, 134.91, 133.28,
128.04, 127.38, 125.25, 122.72, 120.98, 116.88, 115.68, 115.46, 114.85, 55.53, 39.21,
17.46, 14.63. HRMS (ESI) calcd for C102H96F3N3O3 (M+H+):1469.7475, found:
1469.7469.
FS13: Fluorescence green powder (71% yield). M.p:177-179 oC. 1H NMR (400 MHz,
DMSO) δ 8.15 (d, J = 8.0 Hz, 3H), 7.50 (d, J = 5.3 Hz, 6H), 7.40 (d, J = 6.9 Hz, 15H),
7.28 (s, 3H), 7.16 (d, J = 8.7 Hz, 6H), 7.00 (d, J = 8.7 Hz, 9H), 6.92 (d, J = 8.6 Hz,
6H), 3.78 (s, 9H), 2.81–2.69 (m, 6H), 1.95–1.82 (m, 6H), 0.66–0.40 (m, 30H). 13C
NMR (100 MHz, CDCl3) δ 156.25, 155.03, 147.40, 146.28, 143.10, 140.84, 137.95,
136.38, 134.80, 134.32, 129.47, 127.31, 125.21, 122,82, 120.86, 116.78, 114.81,
55.53, 55.48, 39.20, 21.07, 17.45, 14.63. HRMS (ESI) calcd for C108H99N3O3
(M+H+):1487.7728, found: 1487.7753.
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1
13
The H NMR and C NMR of compounds FS11, FS12 and FS13 are given in
the supplementary materials.
2.4. Two-photon absorption measurements
The σ2 values of the three compounds have been measured with the
two-photon-induced fluorescence (2PEF) method by using the femto-second laser
pulses as described [34]. The 2PEF spectra were recorded on SD2000 spectrometer
(Ocean Optical) with excitation with a femtosecond laser (Tsunami, Spectra-Physics).
This laser provided pulses of 100-fs of duration at a repetition rate of 80 MHz and
was tunable in the wavelength range of 720-880 nm. The laser beam was focused into
a quartz cell of 1 cm path length by using a 5 cm focal-length lens. To calculate the
2PEF cross sections, Rhodamine B in methanol solution (1×10-6 mol L-1) was utilized
as reference for the calculation. For all the products, an acetonitrile solution (1×10-5
mol L-1) was prepared to calculate the 2PEF cross sections. The fluorescence quantum
yields of FS11, FS12, FS13 and Rhodamine B are 0.2369, 0.2323, 0.3562
and
0.3977, respectively. All the samples and refernce were tested under the same
experimental conditions.
The 2PEF cross sections were then calculated by the equation (1):
refcref nref
cnFref
F
TPEF

ref
(1)
where c and Φ were the concentration and the fluorescence quantum yield of the
samples (FS11, FS12 and FS13) and reference (Rhodamine B), and n was refractive
index of the solvents. F was the integral of the TPEF spectrum. The σref value of
reference was taken from the literature [35].
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3. Results and discussion
The synthetic procedure of FS11, FS12 and FS13 was showed in Scheme 1.
2,7,12-tribromo-5,5’,10,10’,15,15’-hexapropyltruxene (compound 3) was obtained by
compound 1 reacted with Bromopropane, n-BuLi and a bromination reaction. Then,
compound 5 was prepared by compound 4 reacting with p-methoxyaniline via a
Buchwald-Hartwig reaction. Finally, the target compound FS11 was synthesized by
Buchwald-Hartwig reaction of 3 with 5. FS12 and FS13 were synthesized by similar
procedures. All the intermediates and target dyes were purified and identified by
NMR spectra. The UV-vis absorption and fluoresce emission spectra, and
electrochemical data of FS11, FS12 and FS13 were also measured, respectively.
3.1. One-photon absorption and emission properties
The one-photon absorption and fluorescence emission properties of FS11, FS12
and FS13 in dichloromethane are shown in Fig. 2, and the corresponding
spectroscopic data are summarized in Table 1. Three compounds have a strong
absorption at the region of 250-400 nm, and the maximum absorption wavelengths
(λmax) of FS11, FS12 and FS13 are 375 nm, 373 nm and 383 nm, respectively. The
absorption band at 260-280 nm can be attributed to π-π* transitions, while the band at
350-380 nm is mainly corresponding to the intramolecular charge transfer (ICT) from
the diphenylamine to the truxene core [36]. A red-shifted absorption band of FS13 is
observed, due to the introduction of phenylethynyl extended π-conjugation length
compared to FS11 and FS12. The emission spectra of FS11, FS12 and FS13 are also
measured. The maximum emission wavelengths of FS11, FS12 and FS13 are 424 nm,
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425 nm, 461 nm, respectively. The optical band gap (E0-0) of the three compounds
was calculated based on their spectroscopic data [37]. The E0-0 values of FS11, FS12
and FS13 are calculated to be 3.092 eV, 3.119 eV and 2.964 eV, respectively. Among
the studied compounds, FS13 shows the lowest energy gap, due to its extended
π-conjugation structure. In contrast, FS12 with the electron accepting –F on the
diphenylamine branches show a higher HOMO-LUMO gap than FS11 and FS13.
1.0
FS11
FS12
FS13
0.8
0.6
0.4
0.2
0.0
300
400
500
600
Wavelength / nm
Fig. 2. Normalized absorption and fluorescence emission spectra of FS11, FS12 and
FS13 in dichloromethane with the concentration of 10-5 mol L-1.
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Table 1. The spectroscopic and electrochemical data of FS11, FS12 and FS13.
a
d
λmax
εa/104
M-1cm-1
E0–0
LUMO
/V
vs NHE f
b
b
c
Eox
Ered
/V
Eox,1/2
/V
HOMO /V
vs NHE e
Compound
/nm
/eV
/V
375
373
7.995
6.722
3.092
3.119
FS11
FS12
FS13
1.103
1.123
1.158
0.614
0.645
0.698
0.859
0.884
0.928
0.969
0.994
1.038
-2.123
-2.125
-1.926
383
10.878
2.964
a The absorption spectra were measured in dichloromethane.
b Eox and Ered were calibrated from cyclic voltammograms.
c Eox,1/2 = (Eox + Ered)/2
d
E0–0 values were estimated from the intersections of normalized absorption and
emission spectra in dichloromethane, λint:E0–0= 1240/λint.
e
HOMO (vs NHE) were measured in CH3CN and calibrated with ferrocene (0.63 V
vs NHE).
f LUMO (vs NHE) were estimated from calculated from LUMO=HOMO-E0-0.
12
10
FS13
8
FS11
6
FS12
4
2
0
300
400
500
Wavelength / nm
Fig. 3. UV-vis absorption spectra of FS11, FS12 and FS13 in dichloromethane.
The molar extinction coefficients (ε) changes of FS11, FS12 and FS13 in
dichloromethane with the concentration 10-5 mol L-1 are shown in Fig. 3. The molar
extinction coefficients of the three truxene-cored compounds follow the order of
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FS13 > FS11 > FS12, and FS13 (108780 M-1cm-1) has a much higher ε value at
300-400 nm than that of FS11 (79950 M-1cm-1) and FS12 (67220 M-1cm-1). Besides,
UV-vis absorption spectra of FS11, FS12 and FS13 with the concentration of 10-5mol
L-1 in different organic solvents are shown in Fig. S1 (Supporting Information). FS11,
FS12 and FS13 are insensitive to the solvent polarity, indicating that the charge
transfer do not happen in ground-states of the studied compounds [24, 38]. The
fluorescence spectra of compounds FS11, FS12 and FS13 in different organic solvents
have also been investigated, as shown in Fig.S2 (Supporting Information). Fig.S2
shows the fluorescence spectra of three compounds are sensitive to solvent polarity.
The maximum emission wavelengths in different organic solvents shows a red shift in
the order of Toluene<THF<Acetone<CH2Cl2. This result indicates that the charge
transfer of the studied compounds may occur in excited state.
3.2. Quantum chemical calculation, electrochemical and thermal properties
To gain insight into the electronic properties and geometries of FS11, FS12 and
FS13, quantum chemical calculations on based on DFT at the B3LYP/6-31G level
were carried out. Their geometric structures were optimized. All structural
optimization and energy calculations were performed using the GAUSSIAN 09
program. The optimized structures and the frontier orbitals of FS11, FS12 and FS13
are shown in Fig. 4. The highest occupied molecular orbital (HOMO) of FS11, FS12
and FS13 almost delocalize over the whole molecule, while the lowest unoccupied
molecular orbital (LUMO) is mainly transferred to the truxene core and the longer
functional phenylamine chain. The representative dihedral angles and bond lengths of
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FS11, FS12 and FS13 are presented in Fig. S3 (Supporting Information) and Table S1.
The dihedral angle between the truxene core and the diphenylamine is reduced in the
order of FS11 (141.361) > FS12 (140.629) > FS13 (139.381). FS13 has the
smallest dihedral angle between the truxene core and the diphenylamine, and it
exhibits a rigid three-dimensional structure with phenylethynyl. The dihedral angle
between the phenyl ring of phenylamine and phenylethynyl in FS13 is 174.066. The
dihedral angle between the phenyl ring of phenylamine and tolyl in FS11 is -147.094,
and that between the phenyl ring of phenylamine and fluorophenyl in FS12 is
-145.755. The bond lengths between the truxene core and the diphenylamine is
increased in the order of FS11 (1.426 Å) < FS12 (1.427 Å) < FS13 (1.429 Å). The
result indicates that FS13 exhibits a longer conjunction length than FS11 and FS12.
To further evaluate the potential applicability in electronic devices, the
electrochemical properties of the three truxene-based compounds were studied by
cyclic voltammetry, and their CV diagrams are shown in Fig. 5. The corresponding
electrochemical data are listed Table 1. All the compounds display a reversible
oxidation-reduction behavior. The HOMO levels of FS11, FS12 and FS13
corresponding to the oxidation potentials (Eox) was 0.969 eV, 0.994 eV and 1.038 eV,
and the LUMO level of FS11, FS12 and FS13 was -2.123 eV, -2.125 eV and -1.926
eV, respectively. This result suggests the different substituted groups on the
diphenylamine have an effect on their redox ability, and the oxidation potentials
follow the order of FS13 > FS12 > FS11. It can be concluded that substantial
electronic communication exists between the diphenylamine donor and the truxene
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core in the studied molecules.
Fig. 4. The frontier molecular orbital distributions of FS11, FS12 and FS13 optimized
by DFT calculations at the B3LYP/6-31G level.
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0.0014
FS11
FS12
FS13
Ferrocene
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
-0.0002
-0.0004
-0.0006
-0.0008
0.0
0.3
0.6
0.9
1.2
1.5
Potential / V (vs Ag/Ag+)
Fig. 5. Cyclic voltagrams of FS11, FS12 and FS13 in dichloromethane with ferrocene
as a reference at a scan speed of 25 mV/s.
The thermal stability of FS11, FS12 and FS13 has been analyzed by differential
scanning calorimetry (DSC) and thermogravimetry (TG). The corresponding results
of DSC and TG were plotted in Fig. 6, respectively. In Fig. 6 (a), the glass transition
o
o
temperature (Tg) of compounds FS11-13 are 153 C, 153 oC and 154 C, respectively.
The Tg values of the three compounds are high, suggesting that a good morphological
stability owing to the inflexible structure of the truxene core. In Fig. 6 (b), TG curves
o
shows compounds FS11-13 have a high decomposition temperature (Td) at 422 C,
o
o
440 C and 423 C, respectively. It is also found that the introduction of fluorine
group obviously enhance the thermostability properties of FS12 [39].
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(a)
FS11
FS12
FS13
153℃
153℃
154℃
50
100
150
200
Temperature(℃)
100
80
60
40
20
0
(b)
423 ℃
FS11
FS12
FS13
440 ℃
422 ℃
200
300
400
500
600
Temperature / ℃
Fig. 6. (a) DSC and (b) TG curves of compounds FS11-13.
3.3. Two-photon induced fluorescence properties
The 2PA properties of FS11, FS12 and FS13 have been studied by using
two-photon induced fluorescence (2PEF) spectra. The three compounds exhibit
obvious 2PEF, and their σ2 values with the excited wavelengths ranging from 720 nm
to 820 nm per 10 nm were depicted in Fig.7 and Table S2. The maximum σ2 values
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(σ2max) for FS11, FS12 and FS13 are exhibited at the excited wavelength of 750 nm,
which are 260 GM, 204GM and 367 GM, respectively. FS13 also yields a σ2max of 36
GM and 15 GM at 840 nm and 860 nm, respectively. The result is consistent with the
linear absorptions of these compounds. According to the references [23-26], the
truxene-cored compounds can give a very good 2PA response from 27GM to 5100
GM. In our studied compounds, the asymmetric diphenylamines substituted on the
truxene central core enhance the overall molecular rigidity, and the “pull-push” effect
of the electron-withdrawing and electron-donating substituents can increase charge
transfer between the asymmetric diphenylamine branch and the truxene core. FS11,
FS12 and FS13 yield a good 2PA property.
400
FS11
350
300
250
200
150
100
50
FS12
FS13
0
720
740
760
780
800
820
840
860
Wavelength / nm
Fig. 7. Two-photon absorption cross sections of FS11, FS12 and FS13 in acetonitrile.
4. Conclusions
In this paper, three-branched compounds based on a truxene core and asymmetric
diphenylamines were prepared (FS11, FS12 and FS13). The asymmetric
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diphenylamine includes one phenyl of diphenylamine substituted by methoxy group
and the other phenyl substituted by tolyl, fluorophenyl and phenylethynyl for FS11,
FS12 and FS13, respectively. The introduction of electron donor (-OMe), electron
acceptor (-F) and π-bridge (-C≡C-) has an positive effect on their charge transfer
properties. The one-photon absorption maxima of FS11, FS12 and FS13 (charge
transfer band) are 375 nm, 373 nm and 383 nm, respectively. The corresponding
molar extinction coefficients of the three truxene-cored compounds follow the order
of FS13 (108780 M-1cm-1) > FS11 (79950 M-1cm-1) > FS12 (67220 M-1cm-1). FS13
has an extended a longer π-conjugation length than FS11 and FS12, which is in
consistent with the lowest energy gap among three compounds according to CV
curves. The geometric structures optimized by DFT theoretical calculation indicate
that these star-shaped truxene derivatives exhibit a rigid spatial structure. DSC and
TG results show that they exhibit high Tg (> 150 oC) and Td (> 420 oC) temperatures.
The 2PA cross section maxima are observed at the excited wavelength of 750 nm
from the 2PEF measurement results, which are 260 GM, 204GM and 367 GM for
FS11, FS12 and FS13, respectively. Therefore, the studied truxene-diphenylamine
compounds have a great potential in the optoelectronics fields.
Acknowledgement
This work was supported by the National Natural Science Foundation of China
(21506164, 21376179).
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Graphical abstract
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Highlights
* Three truxene-cored compounds with asymmetric diphenylamines were prepared.
* The “pull-push” structure promotes charge transfer.
* All three compounds exhibit strong two-photon excited fluorescence.
29