Synthesis and optical properties of (A)n-π-(Ph)3N type dyes

Article abstract of DOI:10.1080/15421400902946327

Two new (A)_n-π-(Ph)₃N type dyes were designed, synthesized and characterized. The optical properties and structures of them were studied, including UV-visible spectroscopy, photo-luminescence spectroscopy. The geometric structures of

Full text of DOI:10.1080/15421400902946327

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Synthesis and Optical
Properties of (A)n-π-(Ph)3N
Type Dyes
Xiaochuan Li a b , Sung-Hoon Kim c , Jun Kun d
Young-A Son a
&
a School of Chemical and Biological Engineering ,
Chungnam National University , Daejeon, S. Korea
b College of Chemistry and Environmental Science ,
Henan Normal University , Xinxiang, Henan, China
c Kyungpook National University , Department of
Textile System Engineering , Daegu, S. Korea
d Chemical Biotechnology Research Center, Korea
Research Institute of Chemical Technology ,
Daejeon, S. Korea
Published online: 25 Jun 2009.
To cite this article: Xiaochuan Li , Sung-Hoon Kim , Jun Kun & Young-A Son (2009)
Synthesis and Optical Properties of (A)n-π-(Ph)3N Type Dyes, Molecular Crystals and
Liquid Crystals, 504:1, 164-172, DOI: 10.1080/15421400902946327
To link to this article: http://dx.doi.org/10.1080/15421400902946327
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Mol. Cryst. Liq. Cryst., Vol. 504, pp. 164–172, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902946327
Synthesis and Optical Properties of (A)n-p-(Ph)3N
Type Dyes
Xiaochuan Li1,2, Sung-Hoon Kim3, Jun Kun4,
and Young-A Son1
1School of Chemical and Biological Engineering, Chungnam National
University, Daejeon, S. Korea
2College of Chemistry and Environmental Science, Henan Normal
University, Xinxiang, Henan, China
3Kyungpook National University, Department of Textile System
Engineering, Daegu, S. Korea
4Chemical Biotechnology Research Center, Korea Research Institute
of Chemical Technology, Daejeon, S. Korea
Two new (A)n-p-(Ph)3N type dyes were designed, synthesized and characterized.
The optical properties and structures of them were studied, including UV-visible
spectroscopy, photo-luminescence spectroscopy. The geometric structures of them
were calculated theoretically based on the DFT level using BLYP=GGA basis set,
the HOMO=LOMO levels were also measured and calculated.
Keywords: geometric structures; HOMO=LUMO; optical properties; synthesize
INTRODUCTION
Small organic functional molecules have attracted more and more
attention as active materials in organic solar cells (OSCs) [1,2] and
organic light-emitting diodes (OLEDs) [3] due to their high purity
and good reproducible optoelectronic properties. Recently considerable
research efforts have led to major progress in the design and synthesis
This research was supported by the Program for the Training of Graduate Students
in Regional Innovation which was conducted by the Ministry of Commerce Industry
and Energy of the Korean Government.
Address correspondence to Young-A Son, School of Chemical and Biological
Engineering, Chungnam National University, Daejeon 305-764, S. Korea. E-mail:
yason@cnu.ac.kr
164

(A)n-p-(Ph)3N Type Dyes
165
of new small organic functional molecules [4–7]. The properties of the
devices, such as power conversion efficiency, photostability, electron
mobility, and so on, are closely related to the chemical structure of
the molecules used as the active layer of the device. Thus, it is vital
to understand the relationship between the structure design and prop-
erties of small organic functional molecules which plays key roles for
obtaining higher efficiency.
Herein, we report two new cap-shaped or half cap-shaped organic
molecules (as shown in Scheme 1) with (A)n-p-D structured branches
based on triphenylamine (TPA) as core and donor units (D), 1-dicyano-
viylindane as acceptor units, and the ethylene double bonds as
p-connections between them. The design of the molecule structure
was from the following considerations: (1) the TPA-based cap-shaped
molecules possess three-dimensional spatial structure with good solu-
tion processibility and have been used in electroluminescence and
photovoltaics [8,9]; (2) the D-p-A structure is in the molecule to reduce
the bandgap of the material and make its absorption spectrum reach
to the solar spectrum; (3) TPA core are for improving its solubility
and enhancing its hole mobility because TPA is a typical hole trans-
porting group [10].
SCHEME 1 Synthetic routes for D1 and D2.

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X. Li et al.
EXPERIMENTAL
Materials
Solvents used in photochemical measurements were spectroscopic
1
grade and were purified by distillation before use. H-NMR spectra
were recorded on a NMR spectrometer JEOL-AL400 operating at
400 MHz. Chemical shifts were referenced to internal Me4Si (TMS).
Mass spectra were measured with a mass spectrometer LC-MS
(Waters QTof). Absorption spectra measured with Agilent 8453 spec-
trophotometer. Fluorescence spectra measured on a SHMADZU
RF-5301PC Fluorescence spectrophotometer.
Electrochemical Measurements
Cyclic-voltammetric measurements were carried out in a conventional
three-electrode cell using a Pt button working electrode of 2 mm
diameter, a platinum-wire counter electrode, and a Ag=AgCl reference
electrode on a computer-controlled VersaStat 3 model V3 at room tem-
perature. A 0.1 M solution of tetrabutylammonium hexafluoropho-
sphate (TBAPF6) was used as supporting electrolyte and the scan
rate was set to 100 mV=s. The energy levels were calculated using
the ferrocene (EFOC) value of ꢀ4.8 eV as the standard. In this report,
we use the ionization potential converted from oxidation potentials
for comparison of the energies of the highest occupied molecular
orbital (HOMO).
Preparation and Characterization
As presented in scheme 1, the two target compounds were synthesized
by conventional condensation. 1-dicyanovinlyindane was synthesized
by literature method [11].
Dye 1 (D1)
261 mg (1.45 mmol) 1-dicyanovinlyindane and 199 mg (0.66 mmol)
4,40-diformyltriphenylamine were dissolved in 15 mL toluene. Several
drops of piperidine were added. The mixture was heated to reflux for
12 h. Then the mixture was cooled and the solvent was evaporated
in vacuum. The residue was purified by column chromatography
(dichloromethane) to give pure D1 210 mg (51%).
1H NMR (CDCl3): 8.61 (d, 2H); 8.29 (s, 1H); 7.61-7.42 (m, 14H);
7.22-7.17 (m, 5H); 4.03 (s, 4H). MS m=z: 625 (Mþ). Anal. Calcd. for
C44H27N5: C, 84.46; H, 4.35; N, 11.19; Found: C, 84.50; H, 4.39; N,
11.15.

(A)n-p-(Ph)3N Type Dyes
167
Dye 2 (D2)
216 mg (1.20 mmol) 1-dicyanovinlyindane and 120mg (0.36 mmol)
tris-(4-formylphenyl)amine were dissolved in 15 mL toluene. Several
drops of piperidine were added. The mixture was heated to reflux for
24 h (the reaction time was not optimized). Then the mixture was cooled
and the solvent was evaporated in vacuum. The residue was purified by
column chromatography (dichloromethane) to give pure D2 130 mg
(44%).
1H NMR (CDCl3): 8.62 (d, 3H); 8.30 (s, 3H); 7.61-7.46 (m, 16H);
7.31-7.23 (m, 5H); 4.06 (s, 6H). MS m=z: 815 (Mþ). Anal. Calcd. for
C57H33N7: C, 83.91; H, 4.08; N, 12.02; Found: C, 83.87; H, 4.11; N, 12.19.
RESULTS AND DISCUSSION
Both D1 and D2 are soluble in common organic solvents, such as
dichloromethane, toluene, THF and so on. The absorption spectrum
of D1 and D2 solution in several typical organic solvents, as shown
in Figure 1, displays two peaks in the UV-visible region. The absorp-
tion spectrum of D1 solution in dichloromethane displays two peaks
in the UV-visible region. The UV absorption band with a maximum
at 310 nm corresponds to the p–pꢁ absorption of the molecule, and
the visible absorption band with a peak at 540 nm could be assigned
to the intramolecular charge transfer (ICT) transition between the tri-
phenylamine (TPA) moiety and 1-dicyanovinlyindane unit. The D2
solution in dichloromethane shows UV absorption with a maximum
at 320 nm and a shoulder at 360 nm, and the ICT band peak at
540 nm. Similar results were also found in other organic solvents.
FIGURE 1 Absorption spectra of D1 (a) and D2 (b) in different solvents with
1 ꢂ 10ꢀ5 M.

168
X. Li et al.
Obviously, the ICT band of D1 and D2 is red-shifted 40 nm with the
decreasing of polarity of the solvent (Fig. 1).
The emission spectra of D1 and D2 were measured in several
different solvents. The two compounds display a strong emission
solvatochromism that is reflected by a large bathochromic shift of their
fluorescence emission maxima with increasing solvent polarity. This
solvatochromic behavior, which results from the stabilization of the
highly polar emitting state by polar solvents, is typical for compounds
presenting an internal charge transfer upon excitation and has been
fully documented for numerous fluorophores containing donor-acceptor
units. All the two dyes exhibited positive solvatochromism with the
increasing solvent polarity, Figure 2. The emission spectrum of D1
was observed with a peak around 405 nm in CH2Cl2, which was excited
at 315 nm. A broad and red-shifted emission (110 nm) of D1 was
observed (kmax ¼ 515 nm) in CH3CN. In case of D2, 55 nm red-shift
was found, which is smaller than that of D1. These results demonstrate
that the three-branched star-burst compound D2 has smaller push-pull
effect than that of the V-shaped molecule D1. These results further
indicate that the push-pull effect was also influenced by the shape of
molecules.
According to previous theoretical results [12,13], ICT is the major
mechanism responsible for the solvent-reliant emission properties.
However, the emissions of D1 and D2 at the ICT band are very
weak. The relatively strong emissions of D1 and D2 are measured
on irradiation at 315 nm which obtained by corresponding excitation
spectra. Interestingly, the two compounds having the same kind of
acceptor-spacer-donor units, display large solvent-reliant properties.
Compared with D2, D1 has much longer emissions, much larger
Stokes shifts, and much broader emission bands, which indicate more
effective ICT processes. Photo-excitation of a ‘‘push-pull’’ p electron
system will generate an electric field with the positive pole at the
position of the electron donor and the negative pole at the position of
the electron acceptor [14]. In D1 or D2, their positive poles are located
at the triphenylamine nitrogen atom, but their negative poles are
affected greatly by geometric structure and solvents.
In order to determine the HOMO and LUMO energy levels of D1
and D2, electrochemical cyclic voltammetry was carried out on Pt but-
ton electrode. Figure 3 shows the cyclic voltammetry. It can be seen
that there are reversible oxidation peaks in the positive potential
ox
region with the onset oxidation potential (E
Þ at 0.68 V (D1) and
onset
0.29 V (D2) vs. Ag=Agþ, and an irreversible reduction peak in the
red
negative potential region with peak reduction potential (E
) at
onset
ꢀ0.80 V (D1) and ꢀ0.86 V (D2) vs. Ag=Agþ. The HOMO and LUMO

(A)n-p-(Ph)3N Type Dyes
169
FIGURE 2 Emission spectra of D1 (a) and D2 (b) excited at 315nm in different
solvents with 1ꢂ 10ꢀ5 M. The quantum yield were estimated to be 0.05 (CH2Cl2),
0.10 (toluene), 0.30 (THF), 0.26 (CH3CN) for D1 and 0.003 (CH2Cl2), 0.25
(toluene), 0.27 (THF), 0.16 (CH3CN) for D2 with quininesulfate (U ¼ 0.55 in
50 mM H2SO4 solution) as a standard.
levels were calculated to be ꢀ5.39 eV and ꢀ3.91 eV respectively for D1,
and ꢀ5.00 eV and ꢀ3.85 eV respectively for D2. It showed the LUMO
values of D1 and D2 were considerably lower than the most popular
electron-transport material Alq3 (ꢀ3.0 eV) [15].
The geometry and electronic properties of D1 and D2 were inves-
tigated by means of theoretical calculation with the Material Studio
program package at hybrid density functional theory (DFT) level.
Becke’s gradient corrected functional (BLYP) with a GGA basis
was used for full geometry optimization. Figure 4 presents the

170
X. Li et al.
FIGURE 3 Cyclic voltammograms of D1 and D2 measured in CH2Cl2 solution
containing 0.1 M TBPAF6 at 20ꢀC.
geometric structure of D1 and D2. For the geometry observed for D1
and D2, steric hindrance between the (A-p-core) arms grafted on the
central nitrogen atom generated the propeller shape of molecule.
This geometry could be beneficial for good solution process-ability
and to forming amorphous films. The HOMO of D1 is mainly distrib-
uted on the central TPA moiety with some extending to the 1-dicya-
novinlyindane moieties (Fig. 5). The LUMO is localized mainly on
one of 1-dicyanovinlyindane electron-withdrawing groups. The orbi-
tal contribution of D2 is the same as that of D1. According to the
orbital level analysis, the band gaps were calculated to be 1.52 eV
for D1 and 1.22 eV for D2, which agrees well with that obtained from
CV and absorption edges.

(A)n-p-(Ph)3N Type Dyes
171
FIGURE 4 Optimized structure of D1 (left) and D2 (right).
FIGURE 5 The HOMO and LUMO of D1 (left) and D2 (right) that are
involved in the lowest-energy transition.

172
X. Li et al.
CONCLUSIONS
We designed and synthesized V-shaped and star-shaped molecules
with (A)n-p-(Ph)3N structure. The two compounds exhibit good stabi-
lity and solubility in common organic solvents. Both D1 and D2 show
broader and stronger absorption in the visible region covering the
region of 400–600 nm. A positive solvatochromism in the emission
spectra of D1 and D2 was observed. These properties qualify them
as multifunctional fluorophores that can be employed in light-emitting
or sensing applications.
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