Star-shaped D-π-A conjugated molecules: Synthesis and broad absorption bands

Article abstract of DOI:10.1021/ol802845w

Two donor-acceptor hybrid star-shaped D-π-A molecules were facilely developed. The absorption spectra of TrTD2A and TrT2DA, which almost covered the whole visible range, were tuned by changing the ratio of donor and acceptor groups. However, the PL quantu

Full text of DOI:10.1021/ol802845w

ORGANIC
LETTERS
2009
Vol. 11, No. 4
863-866
Star-Shaped D-π-A Conjugated
Molecules: Synthesis and Broad
Absorption Bands
Jin-Liang Wang, Zheng-Ming Tang, Qi Xiao, Yuguo Ma,* and Jian Pei*
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratories of Bioorganic
Chemistry and Molecular Engineering and of Polymer Chemistry and Physics of Ministry of
Education, College of Chemistry, Peking UniVersity, Beijing 100871, China
jianpei@pku.edu.cn; ygma@pku.edu.cn
Received December 9, 2008
ABSTRACT
Two donor-acceptor hybrid star-shaped D-π-A molecules were facilely developed. The absorption spectra of TrTD2A and TrT2DA, which
almost covered the whole visible range, were tuned by changing the ratio of donor and acceptor groups. However, the PL quantum effieciencies
of TrTD2A and TrT2DA in solutions were dramatically reduced after the introduction of benzothiadiazole unit as acceptor chromophore.
π-Conjugated small molecules have attracted considerable
attention as active materials in solution-processable solar cells
due to their high purity and reproducible properties.1 To
improve the power conversion efficiencies of solar cells, it
is of critical importance to develop new organic small
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10.1021/ol802845w CCC: $40.75
Published on Web 01/22/2009
2009 American Chemical Society

molecules with a higher absorption coefficient and broader
absorption features.2 Herein, we present a series of soluble
star-shaped molecules with triphenylamine unit as donor,
functionalized benzothiadiazole moiety as acceptor, and
truxene as the core. The aim of systematic structural
variations on these molecules is to provide insight into
understanding the relationship between the structures and
properties and to modulate their absorption bands to cover
the whole visible region.3 Such structural modulation might
make our star-shaped D-π-A molecules as good candidates
for solution-processable solar cells.
Scheme 2. Synthesis of Partially Functionalized Truxenes
Scheme 1 illustrates the synthetic approaches to acceptor
segment and core chromophore. 4,7-Bis(2-thienyl)-2,1,3-
Scheme 1. Synthesis of Core and Acceptor Chromophores
is the AB2 building block, which bears two or one active
aromatic aldehyde groups with other branches functionalized
by triphenylamine donor units. A Wittig-Horner coupling
reaction between diethyl 4-(diphenylamino)benzyl phospho-
nate9 and trialdhehyde 5 obtained from a Vilsmeier reaction
of 4 afforded dialdehyde 6 (30% yield) and monoaldehyde
7 (19% yield).10 Reduction of 6 with NaBH4 gave alcohol 8
in 96% yield, which was treated in triethyl phosphate with
iodine and DBU to afford phosphonate 9 in 80% yield.11
Through the same procedures, 11 was obtained from 7 in
54% overall yield.
benzothiadiazole4 was facilely converted to monobrominate
1 with N-bromosuccinimide (NBS) in 57% yield.5 A Suzuki
coupling reaction between sodium 4-hexyl-2-thienylboronate6
and 1 followed by a Vilsmeier reaction7 afforded 3 in 84%
overall yield. Core 4 was prepared by a Suzuki coupling
reaction between tribromohexahexyltruxene and sodium
4-hexyl-2-thienylboronate6 in 89% yield.8
Scheme 3 illustrates the synthetic approach to our target
molecules. The Wittig-Horner coupling between aldehyde
and phosphonate afforded TrT2DA (66%), TrTD2A (63%),
and TrT3D (81%), respectively. All new compounds showed
good solubility in common organic solvents at room tem-
perature. Their structures and purity were fully characterized
Scheme 2 outlines the synthesis of some precursors. The
key component for the synthesis of TrTD2A and TrT2DA
1
and verified by H and 13C NMR, elemental analysis, and
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4410.
MALDI-TOF MS or ESI MS (see the Supporting Informa-
tion). The E-configured double bonds of our new compounds
were confirmed from the values of the coupling constant (J)
between vinyl protons (ca. 16 Hz).
The photophysical properties of these D-π-A star-shaped
molecules were investigated both in dilute solutions and in
thin films. Table 1 summarizes their photophysical data in
solutions and in thin films. As shown in Figure 1, TrT3D
showed an absorption band corresponding to a π-π*
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864
Org. Lett., Vol. 11, No. 4, 2009

Scheme 3. Synthesis of TrT2DA, TrTD2A, and TrT3D
Figure 1. Absorption and PL spectra of TrT3D, TrT2DA, and
TrTD2A in cyclohexane solution (10-6 M). PL spectra were
recorded at different excitation wavelengths shown in parentheses.
Emission spectrum of TrT3D is on a reduced scale (1%).
in these compounds.12 The molar extinction coefficient of
this absorption peak in longer wavelength increased about 2
times from TrT2DA (0.61 × 105 M-1 cm-1) to TrTD2A
(1.3 × 105 M-1 cm-1). Figure 1 also shows the photolumi-
nescent (PL) spectra of these star-shaped molecules in
cyclohexane solutions. A maximum emission peak at 484
nm was observed from TrT3D. Under excitation at 425 nm,
for TrT2DA, two emission bands at ca. 510 and 619 nm
were observed; however, for TrTD2A, the emission band
at ca. 484 nm was dramatically quenched, and the emission
band comes almost exclusively from the acceptor units (623
nm). This result indicated a highly efficient intramolecular
energy-transfer process from the donor branch to the acceptor
branch. For TrT2DA, residual fluorescence in the short
wavelength region was clearly observed due to its relatively
high content of donor groups compared with that of TrTD2A
and the high quantum efficiency of TrT3D. Under excitation
at 542 nm, only one emission band was observed both from
the PL spectra of TrT2DA and TrTD2A. At the same
concentration, the emission intensities under excitation at 425
nm were ca. 1.5 times for TrTD2A and 3 times for TrT2DA
higher than those under excitation at 542 nm, respectively,
which indicated an effecive antenna effect in our molecules.13
Moreover, the fluorescence quantum yields (ΦPL) of TrT3D
in dilute cyclohexane solution were measured to be 71%;
however, after introduction of the acceptor unit, ΦPL was
dramatically decreased to 1.5% for TrT2DA and 1% for
TrTD2A.14 The excitation wavelength for the quantum yield
measurements was 542 nm for TrTD2A and TrT2DA and
425 nm for TrT3D, respectively.
transition with λmax at 425 nm. TrT2DA and TrTD2A
exhibited similar absorption features, and two bands with
λmax at 425 and 542 nm were observed after the introduction
of acceptor groups. The absorption band at 542 nm was
assigned to those of benzothiadiazole unit as acceptor branch
The absorption spectra of our D-π-A star-shaped molecules
were nearly independent of solvent polarity. However, their
emission spectra exhibited distinct solvent dependence. Less
pronounced vibronic structure and larger Stokes shift were
observed with the increase of the solvent polarity (see the
Supporting Information). For example, the maximum emis-
sion peak of TrTD2A changed from ca. 623 nm in
cyclohexane to ca. 662 nm in DMF. Moreover, the emission
Table 1. Photophysical Properties of Star-Shaped Molecules in
Solutions and in Thin Films
λmax
λmax emisa λmax absb λmax emisb
compd absa (nm) (log ε)
(nm)
(nm)
(nm)
ΦPL (%)
TrT3D
425 (5.34)
425 (5.31)
542 (4.78)
427 (5.28)
542 (5.12)
484
619
433
433
550
433
560
531
684
71c
TrT2DA
1.5d
TrTD2A
623
700
1d
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a In cyclohexane solution (10-6 M). b In thin films. c In cyclohexane
solution and coumarin 152 (ΦPL ) 0.21 in ethanol) as the standard. d In
cyclohexane solution and rhodamine B (ΦPL ) 0.65 in ethanol) as the
standard.
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Org. Lett., Vol. 11, No. 4, 2009
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above 600 nm was obviously quenched with increase of the
polarity of the solvents, which indicated that such D-π-A
star-shaped molecules possessed more polar character at the
excited states than at the ground state.
Figure 2 shows the absorption and PL spectra of these
three molecules in thin films. The thin films used for
parison with TrTD2A (0.78 eV), TrT2DA showed a higher
oxidation potential onset (0.86 eV), which indicated that the
oxidative potential enhanced with the increase of the con-
tent of the acceptor unit in these molecules, which caused
an obvious change in HOMO and LUMO energy level of
the corresponding molecules. The oxidation potential onset
for TrTD2A was closed to that of 4,7-bis(5-(5-hexylth-
iophen-2-yl)thiophene-2-yl)benzo[1.2.5]thiadiazole (about
0.80 eV);16 however, it was reduced relative to that of
TrT3D (1.12 eV), which was because that TrTD2A and
TrT2DA had the longer effictive conjugation length com-
pared with Tr3D. The calculated HOMO and LUMO level
were -5.52/-3.27 eV for TrT3D, -5.26/-3.46 eV for
TrT2DA, and -5.18/-3.48 eV for TrTD2A.17 Molecular
modeling was employed to study the electronic properties
of these molecules. The conformation of these molecules is
shown in Figure S10 of the Supporting Information.
Figure 2. Absorption and PL spectra of star-shaped molecules in
In conclusion, we have developed a series of new solution-
processable star-shaped D-π-A organic molecules via facile
synthetic approaches in good yields. Their absorption and
PL features are significantly modulated by changing the
content of the acceptor in our molecules. TrTD2A and
TrT2DA in thin films show a broader and stronger absorp-
tion band in the range of 250 to 700 nm, which covers the
whole visible region. These results indicated that the two
acceptor branches of TrTD2A are conductive to increase
the intermolecular interaction and light-harvesting ability.18
These results show that TrTD2A and TrT2DA are good
candidates for solution-processable organic solar cells. The
photoinduced electron-transfer studies with regard to another
electron acceptors such as C60 and further experiments to
explore their applications in organic solar cells are in progress
in our laboratory.
thin films. PL spectra recorded under excitation at the absorption
λmax wavelength at room temperature.
absorption and emission measurements were obtained by
spin-coating toluene solutions (ca. 10 mg/mL) of our star-
shaped molecules onto quartz plates at 1000 rpm. All
compounds exhibited excellent film-forming properties. The
absorption of three molecules in thin film showed almost
the same features as those in dilution solution, although their
absorption λmax slightly red-shifted. The absorption features
of TrT2DA and TrTD2A covered almost the whole visible
region from 250 to 700 nm. As shown in Figure 2, TrT3D
showed strong emission with the maximum λmax at 531 nm;
however, the emission of TrT2DA and TrTD2A in thin film
was almost totally quenched when excited either at shorter
(433 nm) or at longer wavelength (550 nm). These results
indicated that a highly efficient intra- and intermolecular
energy transfer in thin films occurred from the donor branch
to the acceptors, and the strong fluorescence quenching was
attributed to the poor PL quantum yield of TrT2DA and
TrTD2A and strong intermolecular interaction in thin films.15
Such photophysical property is beneficial for active materials
in solar cells.
Acknowledgment. This work was supported by the Major
State Basic Research Development Program (Nos. 2006-
CB921602 and 2007CB808000) from the Ministry of Science
and Technology and National Natural Science Foundation
of China.
Supporting Information Available: Experimental pro-
1
cedures and H and 13C NMR and MS data for all new
The energy bandgaps of our star-shaped molecules in thin
films were determined from the onset of their absorption
spectra to be 2.25 eV for TrT3D, 1.80 eV for TrT2DA,
and 1.70 eV for TrTD2A. The cyclic voltammetry of TrT3D
and TrT2DA shows irreversible oxidation peaks at 1.67 and
1.25 V vs Ag/AgCl. TrTD2A exhibits good reversible
oxidation waves peaking at 1.06 V vs Ag/AgCl. In com-
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL802845W
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866
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