Energy transfer in new D-π-A conjugated dendrimers: Their synthesis and photophysical properties

Article abstract of DOI:10.1021/ol801671w

(Chemical Equation Presented) A series of new D-π-A conjugated dendrimers based on benzothiadiazole and triphenylamine were developed via facile synthetic approaches. By changing the types of bridges between the different functional moieties of these dendrimers, their photophysical properties, especially the intramolecular energy transfer process, were effectively modulated.

Full text of DOI:10.1021/ol801671w

ORGANIC
LETTERS
2008
Vol. 10, No. 19
4271-4274
Energy Transfer in New D-π-A
Conjugated Dendrimers: Their Synthesis
and Photophysical Properties
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 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 July 23, 2008
ABSTRACT
A series of new D-π-A conjugated dendrimers based on benzothiadiazole and triphenylamine were developed via facile synthetic approaches.
By changing the types of bridges between the different functional moieties of these dendrimers, their photophysical properties, especially the
intramolecular energy transfer process, were effectively modulated.
π-Conjugated dendrimers integrating different functional groups
represent a unique class of shaped-persistent macromolecules
and show great application potential for the artificial light-
harvesting system.1 In our previous contributions, we reported
a family of new gradient conjugated dendrimers, in which the
photophysical properties, especially the intramolecular energy
transfer efficiency, were modulated by the dendrimers’ genera-
tion.2 Herein, we develop a series of new D-π-A dendrimers
composed of four triphenylamine moieties as the donor groups
share one benzothiadiazole chromophore in the middle as the
acceptor, through various conjugated spacers. The properties
of donor-acceptor substituted conjugated dendrimers have been
tuned by changing the structures of the donor, acceptor, and
π-conjugated moieties.3,4 In our system, different conjugated
bridges between the core and the periphery are integrated,
which results in different effective conjugated lengths and
electronic communication between the rim and the core. The
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10.1021/ol801671w CCC: $40.75
Published on Web 09/09/2008
2008 American Chemical Society

aim of these systematic structural variations was to provide
insight into the relationship between the effective conjugation
length, the photophysical properties, and the intramolecular
energy transfer process, which is conductive for the design
of new optoelectronic materials in nonlinear optical devices
or solar cells.
was subjected to a Suzuki reaction with 2 catalyzed by
Pd(PPh3)4 to afford 4 in 84% yield. Treating 4 with NIS afforded
5 in 91% yield. SBTTrTPA was obtained through the 4-fold
Suzuki coupling between 5 and the monoboronic pinacol ester
of triphenylamine in 74% yield.7
Scheme 2 illustrates the synthetic approaches to DBTTrTPA.
Compound 7 with all E-configured double bonds was prepared
Scheme 1 illustrates the synthetic approaches to SBTTrTPA.
Lithiation of 15 followed by reaction with 2-isopropoxy-4,4,5,5-
Scheme 2. Synthesis of DBTTrTPA
Scheme 1. Synthesis of SBTTrTPA
tetramethyl-1,3,2-dioxaborolane gave monoboronic ester 2 in
36% yield, while unreacted 1 was recovered. The diiodinated
product 3 was obtained through the lithiation of 4,7-bis(2-
thienyl)-2,1,3-benzothiadiazole with LDA at -78 °C followed
by the addition of elementary iodine in 86% yield.6 Diiode 3
via Wittig-Horner reaction between 65 and diethyl 4-(diphe-
nylamino)benzylphosphonate8 using t-BuOK in 81% yield. 7
was converted to the monoaldehyde 8 using n-BuLi and
anhydrous DMF in 83% yield. 9 was obtained by reduction of
4,7-bis(5-formylthiophen-2-yl)-2,1,3-benzothiadiazole9 with
NaBH4 in 88% yield and then converted to diphosphonates 10
in 41% yield by treating 9 in triethylphosphate with iodine and
DBU at 125 °C.10 DBTTrTPA was obtained through Wittig-
Horner reaction between 7 and 10 in 51% yield.
Scheme 3 illustrates the synthetic approaches to TBTTrTPA.
Compound 112 reacted with 2-methylbut-3-yn-2-ol under
standard Sonogashira reaction conditions to give 12 in 28%
isolated yield, and unreacted 11 was recovered.11 Another
Sonogashira reaction between 12 and 4-ethynyl-N,N-dipheny-
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Scheme 3. Synthesis of TBTTrTPA
Figure 1. Absorption and PL spectra of three dendrimers in
cyclohexane solution (10-6 M). PL spectra recorded at different
excitation wavelengths shown in every parenthesis.
SBTTrTPA, the insertion of vinylene units results in a red-
shift of ca. 20 nm and a higher value of molar extinction
coefficient, beneficial to the light-harvesting process.1
laniline12 afforded 13 in 82% yield. Removal of the protection
group of 13 to give 14 was performed in refluxing toluene in
76% yield. TBTTrTPA was obtained through the Sonogashira
reaction between 3 and 14 in 67% yield.
Table 1. Photophysical Properties of Dendrimers and Model
Compounds in Solutions and in Thin Films
All new compounds were readily soluble in common
organic solvents such as toluene, THF, and DMF. Their
structures and purity were fully characterized and verified
by 1H and 13C NMR, elemental analysis, as well as MALDI-
TOF MS (see the Supporting Information). The stereochem-
istries of all double bonds of DBTTrTPA and 7 were
confirmed from the values of the coupling constant (J)
between vinyl protons (ca. 16 Hz).
λmax
λmax
λmax
λmax
compd
abs.a nm (log ε) emis.a nm abs.b nm emis.b nm ΦPL
%
SBTTrTPA
396(5.42)
535(4.70)
421(5.60)
552(4.90)
391(5.48)
512(4.71)
344(5.28)
535(4.70)
421(5.30)
614
628
592
614
468
405
552
426
567
397
521
345
553
426
658
670
628
660
512
8c
3c
DBTTrTPA
TBTTrTPA
16c
9c
4
7
64d
The photophysical properties of these dendrimers and
model compounds 4 and 7 were investigated in dilute
solutions and in thin films. Figure 1 shows their absorption
and photoluminescent (PL) spectra in cyclohexane solution.
Table 1 summarizes their photophysical data in both solutions
and in thin films. All molecules show two distinct absorption
bands. Absorption peaks were at 396 and 535 nm for
SBTTrTPA, 421 and 552 nm for DBTTrTPA, and 391 and
512 nm for TBTTrTPA. By comparing the maximum
absorption peak of 7 (421 nm) and 4 (344 nm, 535 nm), we
assigned the absorption band in the shorter wavelength region
to the four end-capping triphenylamine units in these
dendrimers. Another absorption peak for SBTTrTPA, DBT-
TrTPA, and TBTTrTPA was assigned to the π-π*
absorption band of the benzothiadiazole core. Among three
dendrimers, DBTTrTPA exhibited the longest effective
conjugation length along the molecule. In comparison with
a In cyclohexane solution (10-6 M). b In thin films. c In cyclohexane
solution and rhodamine B (ΦPL ) 0.65 in ethanol) as the standard. d In
cyclohexane solution and Coumarin 152 (ΦPL ) 0.21 in ethanol) as the
standard.
The maximum emission wavelengths λmax in cyclohexane
solutions for SBTTrTPA to TBTTrTPA were 614, 628, and
592 nm, respectively. Red shifts of the fluorescence spectra were
observed for SBTTrTPA and DBTTrTPA compared with
TBTTrTPA, confirming the more effective conjugation through
single bond and double bond bridges. Compound 4 exhibited
an emission feature similar to that of SBTTrTPA (see Figure
S1, Supporting Information). The fluorescence quantum yields
(ΦPL) of these dendrimers in dilute cyclohexane solution were
measured to be 8% for SBTTrTPA, 3% for DBTTrTPA, and
16% for TBTTrTPA (Rhodamine B as the standard, ΦPL
)
0.65 in ethanol).13 Although DBTTrTPA has the longest
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Org. Lett., Vol. 10, No. 19, 2008
4273

effective conjugation length, its fluorescence quantum yield was
dramatically lower than others, which might be related to the
nonluminescent property of oligothienylenevinylenes.14 More-
over, the maximum emission peaks were excitation wavelength
independent. For SBTTrTPA and TBTTrTPA, when excited
at 394 or 391 nm (the maximum absorption peaks of the
periphery group), the emission came almost exclusively from
the core units (see Figure 1). The residual fluorescence from
the periphery group in the range from 400 to 550 nm became
very weak, which indicated a highly efficient intramolecular
energy transfer process from the outside to the core. In addition,
the emission intensities excited at the periphery group were
several times higher than those when excited at maximum
absorption of the core units, showing clearly an antenna effect.15
When DBTTrTPA was excited at 421 nm, residual fluores-
cence in the short wavelength region was clearly observed. The
energy transfer efficiencies were calculated to be 74% for
SBTTrTPA, 82% for DBTTrTPA, and 79% for TBTTrTPA,
respectively.3c Note that DBTTrTPA, despite the obvious
residual fluorescence in the short wavelength region, has the
largest energy transfer efficiency. These seemingly conflicting
results be caused from the very high PL quantum yield (64%)
of the donor unit and the comparatively low PL quantum yield
of the acceptor unit (3%) in this molecule.
To gain further insight into the photophysical process within
these dendrimers, we investigated their absorption and emission
behaviors in different solvents. The absorption spectra are nearly
independent of solvent polarity in our D-π-A dendrimers, except
for a slight red shift in methanol due to aggregation. This result
indicated a negligible intramolecular interaction between donor
and acceptor chromophores in the ground states.3g 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. As shown
in Figure 2 and Table S1 (Supporting Information), the
successively decreased fluorescence intensity. These results
suggest that the excited states of our molecules possess more
polar character than the ground state. Meanwhile, the residual
emission exhibits less sensitive solvent dependence than those
of the red-emitting region, which means that the energy transfer
efficiency was also affected by the solvent polarity.
The absorption and PL spectra of these three dendrimers
in thin films were also measured and shown in Figure S19
(Supporting Information). The thin films used for absorption
and emission measurements were obtained by spin coating
toluene solutions (ca. 10 mg/mL) onto quartz plates at 1000
rpm. All compounds exhibited excellent film forming
properties. Absorption peaks were observed for SBTTrTPA,
DBTTrTPA, and TBTTrTPA at 552 (405), 567 (426), and
521 nm (397 nm), respectively. The absorption spectra of
molecules in thin films retain most of the spectral features
in solution and exhibit a red shift compared with those
obtained from cyclohexane solution. The PL spectra of these
molecules in thin film became very broad, and maximum
peaks were obviously red-shifted compared with those in
cyclohexane solutions, similar to those in DMF solution. For
example, red shifts of 44 nm for SBTTrTPA and 36 nm for
TBTTrTPA were observed, respectively, indicating the
formation of excimer in the solid state. Moreover, DBT-
TrTPA exhibits nearly nonluminescent property in thin film.
The strong fluorescence quenching can be attributed to its
lowest PL quantum yield in solution and additional inter-
molecular interaction in thin films.16
In conclusion, we have developed a series of new π-conju-
gated dendrimers based on benzothiadiazole as the core, truxene
derivatives as the conjugated bridge, and triphenylamine as the
end-group through various cross-coupling reactions in good
yields. Their UV-vis absorption and PL properties are signifi-
cantly affected by the electronic nature of the π-conjugated
bridge, which provides an effective tool to fine-tune their
functional properties. In addition, solvent polarity also proves
to be an effective tool to modulate the energy transfer efficiency
in these dendrimers. All dendrimers show excellent light-
harvesting ability. Further experiments to explore their nonlinear
optical properties and applications in organic solar cells are in
progress in our laboratory.
Acknowledgment. This work was supported by the Major
StateBasicResearchDevelopmentProgram(Nos.2006CB921602
and 2007CB808000) from the Ministry of Science and Tech-
nology and National Natural Science Foundation of China.
Figure 2. Emission spectra of DBTTrTPA in various solvents at
10-6 M. Emission spectra were obtained upon excitation at 421
nm (left) or 552 nm (right).
Supporting Information Available: Experimental pro-
cedures, 1H and 13C NMR, and MS data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
maximum emission peak of DBTTrTPA changed from ca. 628
nm in cyclohexane to ca. 672 nm in methanol along with a
OL801671W
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