Star-shaped ladder-type ter(p-phenylene)s for efficient multiphoton absorption

Article abstract of DOI:10.1039/c3cc40952j

Star-shaped ladder-type ter(p-phenylene)s exhibit remarkably efficient multiphoton absorption properties with 2PA cross-section up to 2579 GM at 700 nm and 3PA cross-section up to 3.35 × 10^-76 cm⁶ s ² in the femtosecond regime for a blue-emissive molecule despite having such a short π-conjugated framework.

Full text of DOI:10.1039/c3cc40952j

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Star-shaped ladder-type ter(p-phenylene)s for efficient
multiphoton absorption†
Lei Guo,a King Fai Li,b Man Shing Wong*a and Kok Wai Cheah*b
Cite this: Chem. Commun., 2013,
49, 3597
Received 4th February 2013,
Accepted 11th March 2013
DOI: 10.1039/c3cc40952j
www.rsc.org/chemcomm
Star-shaped ladder-type ter(p-phenylene)s exhibit remarkably effi- and low-cost high-energy coherent light sources. Nevertheless, the
cient multiphoton absorption properties with 2PA cross-section introduction of multi-dimensional p-conjugated systems provides
up to 2579 GM at 700 nm and 3PA cross-section up to 3.35 Â an alternative means to increase the 2PA cross-sections due to the
10À76 cm6 s2 in the femtosecond regime for a blue-emissive mole- excitonic coupling among the p-conjugated moieties.8 On the
cule despite having such a short p-conjugated framework.
other hand, the structure–property relationships for three-photon
absorption properties and the effect of the multi-dimensional
The development of organic materials that exhibit large multi- structure on the 3PA cross-sections are far less studied. It has
photon absorption (MPA) properties has drawn considerable recently been shown that besides enhanced photoluminescence
interest in the past decades because of the utilization of the efficiency and improved photochemical stability, the rigid and
long-wavelength/low-energy excitation source and the unique planar ladder-type oligophenylene backbone is superior to oligo-
characteristics of the intensity-dependent multiphoton absorption fluorene counterparts in enhancing multiphoton absorption prop-
process, which offers many advantages for various technological erties of a linear p-conjugated system.9
applications including three-dimensional optical data storage,1
To continue our ongoing effort on investigating the struc-
two-photon optical power limiting,2 two-photon excited fluores- ture–property relationships for efficient multiphoton absorp-
cence (TPEF) microscopy,3 photodynamic therapy,4 two-photon tion responses and exploring structural factors that can achieve
microfabrication,5 and frequency-upconversion lasing.6 To be excellent nonlinearity–transparency trade-off for multiphoton
useful and efficient for MPA applications, molecules/materials absorption, we report herein synthesis and an investigation of
exhibiting large MPA cross-sections such as two-photon absorp- multiphoton absorption properties of novel star-shaped ladder-
tion (2PA) cross-sections (s2) or three-photon absorption (3PA) type ter(p-phenylene)s, composed of an amine-based central
cross-sections (s3) are essential. Besides, other desirable physical core, three fused ladder-type ter(p-phenylene) arms decorated
and chemical properties of a molecule or material such as a high with non-coplanar alkylphenyl rings and electron-donating
fluorescence quantum yield, good solubility and processibility, as diarylamine-based or electron-accepting triazole-based end-
well as high photochemical and thermal stability would play a caps. Despite such a short p-conjugated length being employed,
crucial role in the realization of practical MPA applications.
these multi-branched ladder-type oligomers have been found to
The strategies for the design of one-dimensional p-conjugated exhibit remarkably large MPA properties with 3PA cross-section
chromophores that exhibit strong two-photon absorption (2PA) (s3) up to 3.35 Â 10À76 cm6 s2 in the femtosecond regime as well
responses have been successfully developed and established in as excellent nonlinearity–transparency trade-off with 2PA cross-
the past decade.7 Most of these structure modifications/guidelines section (s2) up to 2579 GM emitting at 445 nm which is the
often cause a red shift of the 2PA spectrum due to the enhanced highest s2 reported so far for a blue emissive molecule.10
intramolecular charge transfer. Thus, blue-emissive molecules
The molecular structures of star-shaped ladder-type oligo-
with large 2PA cross-sections are relatively limited although they mers built onto the framework of triarylamine endcapped with
are potentially useful for 2PA induced fluorescence microscopy various electron-donating or withdrawing groups, namely
N(TL)-Ph(3)-TAZ, N(TL)-Ph(3)-CBZ, and N(TL)-Ph(3)-NPh are
a Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist
University, Kowloon Tong, Hong Kong SAR, China. E-mail: mswong@hkbu.edu.hk;
Fax: +852 34117348; Tel: +852 34117069
b Department of Physics and Institute of Advanced Materials, Hong Kong Baptist
University, Kowloon Tong, Hong Kong SAR, China. E-mail: kwcheah@hkbu.edu.hk;
Fax: +852 34115813; Tel: +852 34117029
shown in Fig. 1. By adapting the convergent approach, novel
star-shaped ladder-type ter(p-phenylene)s bearing either amine-
based or triazole-based endcaps have been successfully synthe-
sized via palladium catalyzed Suzuki-cross coupling, Buchwald
amination, and acid catalyzed intramolecular ring-closure as
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc40952j key steps. The synthetic routes for the star-shaped ladder-type
c
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Fig. 2 (a) UV-Vis absorption and (b) fluorescence spectra of star-shaped oligo-
mers in toluene with a linear counterpart, (L)-Ph(3)-NPh, for comparison.
Fig. 1 Molecular structures of star-shaped ladder-type ter(p-phenylene)s
N(TL)-Ph(3)-TAZ, N(TL)-Ph(3)-CBZ, and N(TL)-Ph(3)-NPh, respectively.
excited state (Table S1, ESI†). As compared with the linear
quadrupolar counterparts, (L)-Ph(3)-NPh,9 the star-shaped
N(TL)-Ph(3)-NPh shows much enhanced absorption properties
including the red-shift and broadening of spectra as well as
increased molar absorptivity, which is attributed to the strong
coupling of the proximate, rigid, ladder-type ter(p-phenylene)
backbones leading to the enhanced electron delocalization
throughout the entire system. This also results in B26–33 nm
red-shift of the emission maxima for these star-shaped oligo-
mers as compared to that of the linear ter(p-phenylene)s.
To gain an insight into the influence of the star-shaped
structure motif on nonlinear optical responses, the MPA prop-
erties of the newly synthesized star-shaped ter(p-phenylene)s
were investigated by using a femtosecond pulsed laser as an
excitation source. These star-shaped oligomers exhibit extre-
mely strong multiphoton induced blue photoluminescence (PL)
upon excitation at 700, 1270, and 1800 nm corresponding to the
two-, three-, and four-photon absorption, respectively. The MPA
excited PL spectral characteristics were almost identical to
those of the corresponding one-photon excited one in Fig. S3
(ESI†) suggesting that their emission states are the same.
Fig. 3 shows the logarithmic plots of power dependence of
upconversion fluorescence intensity of the star-shaped oligomers
ter(p-phenylene)s are outlined in Scheme S1 (see ESI†). All the
newly synthesized star-shaped ladder-type ter(p-phenylene)s
1
were fully characterized using H NMR, 13C NMR, HRMS, and
elemental analysis and found to be in good agreement with
their structures (see ESI†). With the use of fused aromatics such
as p-conjugated backbones, all the star-shaped ladder-type
oligomers are highly thermally stable with thermal decomposi-
tion temperatures of over 400 1C (see Table S1, ESI†).
All the star-shaped ladder-type ter(p-phenylene)s are highly
soluble in most of the common organic solvents but poorly
soluble in highly polar DMF. The summary of the linear and
nonlinear optical properties is provided in Table 1 and the linear
optical spectra in toluene are shown in Fig. 2. These star-shaped
oligomers exhibit mainly two absorption bands. The strong
absorption band spanning from 380 to 440 nm with overlapped
vibronic structures corresponds to the p - p* transition of the
rigid p-conjugated ter(p-phenylene) backbones. The weaker
absorption at around 300 to 325 nm is due to the n - p* trans-
ition of the triarylamino moiety. Upon excitation of these oligo-
mers at around 400 nm, a very strong blue-light emission with a
fluorescence quantum yield over 84% peaking at 443–450 nm is
observed. In contrast to the absorption spectra, these star-
shaped oligomers show a notable solvatochromic effect in the
emission spectra in which the emission peaks exhibit a red shift
in polar solvents particularly for N(TL)-Ph(3)-TAZ (Dlem = 43 nm
in DMF), suggesting dipolar or charge-transfer character in the
Table 1 Summary of photophysical properties of star-shaped ladder-type ter(p-
phenylene)s, N(TL)-Ph(3)-NPh (I), N(TL)-Ph(3)-TAZ (II) and N(TL)-Ph(3)-CBZ (III)
a
a,b
lmax
/
lem
nm
/
2PA
3PA
f
a
e
g
h
nm (e)
FFL lmaxe/nm s2
s2,M lmaxg/nm s3 s3,M
(I) 429 (1.74) 443
(II) 429 (1.34) 450
(III) 423 (1.33) 443
0.86c 445
0.84c 453
0.96d 443
2579 0.66 444
1620 0.39 450
1887 0.49 442
2.45 6.3
3.35 8.0
2.23 5.8
a
b
Measured in toluene, e  105 MÀ1 cmÀ1
.
Excited at B400 nm.
c
Average of two independent measurements using Norharman (F330–390
=
0.58) as a standard. d Average of two independent measurements using
Coumarin 6 (F420 = 0.78) as a standard. 2PA cross-section (GM) deter-
e
mined by a two-photon-induced fluorescence method using 700 nm femto-
Fig. 3 The logarithmic plots of the power dependence of relative multiphoton
induced fluorescence intensity of the star-shaped oligomers, (a) N(TL)-Ph(3)-NPh,
(b) N(TL)-Ph(3)-CBZ, and (c) N(TL)-Ph(3)-TAZ, as a function of pump power at an
excitation wavelength of 700 nm, 1270 nm and 1800 nm, respectively. The red
solid lines are the best-fit straight lines with a gradient, n indicates the two-photon,
three-photon or four-photon nature of the absorptions.
f
second laser pulses in toluene at a concentration of B10À5 M. s2,M
=
g
2PA cross-section scaled by molecular weight (GM/g). 3PA cross-section
(Â 10À76 cm6 s2) determined by comparing the fluorescence intensity with
that of PhNOF(4)-TAZ-OF(4)-NPh using 1270 nm femtosecond laser pulses
in toluene at a concentration of B10À5 M. h
by molecular weight (Â 10À80 cm6 s2 gÀ1).
s3,M = 3PA cross-section scaled
c
3598 Chem. Commun., 2013, 49, 3597--3599
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ladder-type ter(p-phenylene) backbone and the importance of
the star-shaped structure motif to induce the strong intra-
chromophoric coupling for the large 3PA enhancement. The
four-photon excited fluorescence intensities were also mea-
sured in the range of 1650–2000 nm. Similar to 3PA responses,
the triazole-endcapped N(TL)-Ph(3)-TAZ shows the strongest
four-photon absorption among the star-shaped ladder-type
ter(p-phenylene)s.
In conclusion, we have synthesized a novel series of triar-
ylamine-based star-shaped ladder-type ter(p-phenylene)s with
various electron-donating or electron-withdrawing endcaps,
which exhibit a high fluorescence quantum yield and remark-
ably large multiphoton absorption properties. Although these
star-shaped oligomers only consist of short p-conjugated
ter(p-phenylene) units, they have been found to show extremely
large 3PA cross-section (smax) up to 3.35 Â 10À76 cm6 s2 from
N(TL)-Ph(3)-TAZ which is comparable to the highest intrinsic s3
values reported previously. In addition, N(TL)-Ph(3)-NPh
exhibits excellent nonlinearity–transparency trade-off with a
2PA cross-section (s2) of 2579 GM emitting at 445 nm which
is the highest s2 for a blue emissive molecule reported so far.
Our findings demonstrate that the star-shaped structural motif
combined with the rigid, planar ladder-type oligophenylene
backbone is highly efficient in enhancing multiphoton
absorption, in particular 3PA properties offering an excellent
nonlinearity–transparency trade-off.
Fig. 4 (a) 2PA, (b) 3PA, and (c) 4PA excited spectra of the star-shaped ladder-
type ter(p-phenylene)s measured in toluene by femtosecond laser pulses.
as a function of the input pump pulse power using a femto-
second laser as the excitation source at 700, 1270, and 1800 nm.
The solid lines are the best fitting straight lines with gradients
n = 1.96, 2.79, and 4.09 for N(TL)-Ph(3)-NPh; 2.00, 2.95, and 3.94
for N(TL)-Ph(3)-CBZ; and 1.98, 3.07, and 3.78 for N(TL)-Ph(3)-TAZ,
respectively. Therefore, the square, cubic, and fourth-order
dependence of the fluorescence intensity against input pump
power is experimentally shown providing direct evidence of the
MPA excitation process for the upconversion fluorescence.
The conventional two-photon induced fluorescence method
was used to determine the TPA cross-sections and two-photon
excited spectra of these ladder-type oligomers in the range of
600–1000 nm using a femtosecond pulsed laser. Fig. 4a
shows the 2PA excited spectra of the star-shaped ladder-type
ter(p-phenylene)s by comparing with rhodamine 6G, in which
their maximum 2PA cross-section (smax) appears at around
700 nm with smax up to 2579 GM for N(TL)-Ph(3)-NPh, which
is the highest value reported so far for a blue-light emissive
molecule. Furthermore, the 2PA cross-sections of these star-
shaped oligomers are more than one order of magnitude higher
This work was supported by GRF (HKBU 203212), Hong
Kong Research Grant Council, and Faculty Research Grant
(FRG2/10-11/015), Hong Kong Baptist University.
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=
127 GM at 700 nm), highlighting the advantage of using the
multi-dimensional structure motif together with the rigid,
planar ladder-type ter(p-phenylene) backbone to enhance the
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that of PhNOF(4)-TAZ-OF(4)-NPh at 1270 nm with s3 = 2.72 Â
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c
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Chem. Commun., 2013, 49, 3597--3599 3599