Exceptionally strong multiphoton-excited blue photoluminescence and lasing from ladder-type oligo(p -phenylene)s

Article abstract of DOI:10.1021/ja302085q

We report the synthesis and investigation of multiphoton absorption properties of a novel series of diphenylamino-end-capped ladder-type oligo(p-phenylene)s which exhibit greatly enhanced and efficient multiphoton (from two- to five-photon) upconverted blue photoluminescence with which the record-high intrinsic three-photon absorption cross-section of 4.56 × 10^-76 cm⁶ s² in the femtosecond regime has been obtained. Exceptionally efficient two- to five-photon-excited lasing in the blue region has also been demonstrated in which the highest two-photon-excited lasing efficiency of 0.34% has been achieved.

Full text of DOI:10.1021/ja302085q

Communication
pubs.acs.org/JACS
Exceptionally Strong Multiphoton-Excited Blue Photoluminescence
and Lasing from Ladder-Type Oligo(p-phenylene)s
Hai Hua Fan,†,⊥ Lei Guo,‡,⊥ King Fai Li,† Man Shing Wong,*,‡,§ and Kok Wai Cheah*,†
‡
†Department of Physics and Institute of Advanced Materials, Department of Chemistry and Institute of Advanced Materials, and
§Institute of Molecular Functional Materials, Hong Kong Baptist University, Hong Kong, China
S
* Supporting Information
ment in optical nonlinearity is limited by the non-coplanarity of
ABSTRACT: We report the synthesis and investigation of
the fluorenyl−fluorenyl backbone. On the other hand, the rigid
multiphoton absorption properties of a novel series of
diphenylamino-end-capped ladder-type oligo(p-
phenylene)s which exhibit greatly enhanced and efficient
multiphoton (from two- to five-photon) upconverted blue
photoluminescence with which the record-high intrinsic
three-photon absorption cross-section of 4.56 × 10−76 cm6
s2 in the femtosecond regime has been obtained.
Exceptionally efficient two- to five-photon-excited lasing
in the blue region has also been demonstrated in which the
highest two-photon-excited lasing efficiency of 0.34% has
been achieved.
and planar structure of ladder-type oligo(p-phenylene)s would
facilitate π-electron delocalization and thus improve multi-
photon absorption response. Such a structure motif would also
increase photoluminescence efficiency with enhanced thermal
and photochemical stability. In addition, it has been shown that
9,9-diphenyl substitution onto fluorene moiety is a very useful
strategy/tool to enhance the 2PA properties with excellent
nonlinearity−transparency trade-off, which is due to the
participation of the non-coplanar phenyl rings in the enhance-
ment of the electron delocalization.8 Nevertheless, there is only
limited study on exploring the ladder-type oligomers for
multiphoton absorption,3e and no attempts have been made for
multiphoton lasing applications. The potential of 9,9-diphenyl
substitution onto fluorene moiety for three-photon absorption
(3PA) properties still remains to be explored.
We report herein the design and synthesis of a novel series of
highly extended diphenylamino-end-capped ladder-type oligo-
(p-phenylene)s which exhibits greatly enhanced two-, three-,
four-, and five-photon absorption-induced blue-light photo-
luminescence. The 2PA and 3PA cross-sections are as high as
1045.6 GM and 45.63 × 10−77 cm6 s2, which is the highest
intrinsic 3PA cross-section obtained so far,7f respectively. In
addition, these oligo(p-phenylene)s exhibit exceptionally
efficient two-, three-, and four-photon-induced up-conversion
blue lasing, peaking at 460−482 nm, with record-high two-
photon and three-photon lasing efficiency of 0.336% and
0.0025%, respectively. Our findings open up a new vista to
develop highly efficient multiphoton-absorbing fluorophores for
practical applications, particularly the blue emissive molecules
for high-contrast imaging and new coherent light sources in the
short wavelength region.
ultiphoton absorption is a higher order nonlinear optical
M
process in which an atom or a molecule simultaneously
absorbs two or more photons with identical or different
energies to undergo an excitation. The multiphoton absorption
process has a transition probability that is proportional to In,
where I is the intensity of the input laser pulse and n is the
number of photons involved. The intensity dependence of the
multiphoton absorption process gives rise to a stronger spatial
confinement and hence a higher spatial resolution and contrast
in imaging.1 As a result, multiphoton absorption offers many
advantages for various emerging technological applications,
including three-dimensional optical data storage,2 optical
limiting,3 lithographic microfabrication,4 fluorescence excitation
microscopy and imaging,5 photodynamic therapy,6 and infrared
frequency up-conversion lasing.7 The use of multiphoton
absorption to produce smaller and cheaper visible laser sources
especially in the short wavelength region is one of intriguing
applications as compared to other coherent frequency
upconversion techniques, such as optical harmonic generation
or sum frequency mixing. Multiphoton upconversion lasing
offers capability of adopting waveguide and fiber configurations
and feasibility of using semiconductor lasers as pump sources,
and importantly, it does not require stringent phase-matching
requirements. The short lasing wavelengths will bring in new
merits and breakthroughs in various laser-based applications.
Although molecules/materials with large two-photon-absorp-
tion (2PA) cross-sections have been developed in the past
decades, most of them are not useful and efficient for
multiphoton-induced blue lasing.
Novel D-π-D ladder-type oligophenylenes, composed of
linear fused fluorenes, containing up to seven phenyl rings,
symmetrically end-capped with diphenylamino donors, namely
(L)-Ph(n)-NPh, where n = 3−7, have been designed,
synthesized, and investigated for highly efficient multiphoton
absorption and frequency-upconverted blue-lasing applications.
To circumvent the solubility and processability problem for the
extended ladder-type oligomers as well as to enhance the
electron delocalization for large nonlinear optical response,8 the
non-coplanar 4-decylphenyl substituents are tethered at the 9-
position of a fluorenyl unit in these ladder-type oligopheny-
Fluorene-based π-conjugated oligomers have recently been
shown to exhibit excellent multiphoton-excited blue photo-
luminescence and lasing properties; however, further enhance-
Received: March 2, 2012
Published: April 20, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
lenes, as shown in Figure 1. They were synthesized by means of
convergent approach using palladium-catalyzed Suzuki cou-
Figure 2. (a) Absorption and (b) emission spectra of (L)-Ph(n)-NPh,
where n = 3−7, measured in toluene.
structure. Such a rigid planar system would also facilitate p-
orbital overlap and electron delocalization through the entire
system and thus enhance the multiphoton absorption
responses.3e Upon excitation at the absorption maximum,
these oligo(p-phenylene)s show two resolved emission bands in
the range of 420−480 nm, with high fluorescence quantum
yields (Φ) greater than 85%. Consistently, the emission
wavelength maximum also shows bathochromic shifts with an
increase in the conjugation length. On the other hand, the
absorption and emission spectra show no significant red/blue
shifts in different solvents, e.g., CHCl3 and DMF, except
merging of the two emission bands in DMF, suggesting
nonpolar ground and excited states.
Interestingly, these ladder-type oligomers exhibit very strong
multiphotonincluding two-, three-, four-, and five-photon
upconverted photoluminescence (PL) when excited by near-
infrared femtosecond laser pulses. The multiphoton-excited PL
spectral characteristics are very similar to those of the
corresponding one-photon-excited counterparts (Figure S1),
indicating that their emission energy levels are likely to be
identical. Figure 3 shows the logarithmic plots of power
dependence of relative multiphoton-induced fluorescence
intensity of (L)-Ph(5)-NPh on pulse intensity using a
femtosecond laser as excitation source at 800, 1250, 1650,
and 2000 nm. The solid lines are the best-fit straight lines with
gradient n = 1.94, 3.08, 4.09, and 5.08. Hence, the square, cubic,
fourth-order, and fifth-order dependence of the fluorescence
intensity versus pumping power at 800, 1250, 1650, and 2000
nm, respectively, is clearly demonstrated. These corroborate
that the frequency-upconverted fluorescence is due to the two-,
three-, four-, and five-photon absorption, respectively.
The two-, three-, and four-photon excitation fluorescence
spectra of the (L)-Ph(n)-NPhs are shown in Figures 4. The
2PA cross-section, σ2, was measured by the two-photon-
induced fluorescence method in the range of 800−950 nm
using Rhodamine 6G as the reference standard.9 The results of
the maximum 2PA cross-sections for (L)-Ph(n)-NPhs are
tabulated in Table 1. The 2PA enhancement is found
throughout the full spectrum measured as the conjugation
length increases. In addition, very large σ2 up to 1045.6 GM at
880 nm was obtained for (L)-Ph(7)-NPh, highlighting the
promise of ladder-type oligo(p-phenylene)s for 2PA applica-
Figure 1. Molecular structures of (L)-Ph(n)-NPh, n = 3−7.
pling, Buchwald amination, and triflic acid-catalyzed intra-
molecular ring closure as key steps. The full synthetic details of
the entire homologous series can be found in the Supporting
Information. All the newly synthesized diphenylamino end-
capped ladder-type oligo(p-phenylene)s were fully character-
1
ized with H NMR, 13C NMR, HRMS, and elemental analysis
and found to be in good agreement with the proposed
structures (see Supporting Information).
All the ladder-type oligo(p-phenylene)s are highly soluble in
common organic solvents such as toluene, chloroform, and
DMF. For example, the solubility of (L)-Ph(7)-NPh in toluene
can be up to 10−1 M. The results of the linear optical properties
of (L)-Ph(n)-NPh measured in toluene are summarized in
Table 1 and S1. As seen in the absorption spectra shown in
Figure 2a, these oligomers exhibit similar absorption character-
istics, mainly composed of two absorption bands: the n→π*
transition of triarylamine moieties and the π→π* transition of
the oligomeric cores. In contrast to diphenylamino end-capped
oligofluorenes, the absorption wavelength maximum of these
ladder-type oligo(p-phenylene)s exhibits bathochromic shifts as
the conjugation length increases, indicating the enhancement of
electron delocalization along the π-conjugated core. These
ladder-type oligophenylenes also show very large molar
absorption coefficients up to 2.53 × 105 M−1 cm−1 for (L)-
Ph(7)-NPh, which is attributed to the rigid planar ladder
Table 1. Linear and Nonlinear Optical Properties of (L)-Ph(n)-NPh (n = 3−7) Measured in Toluene
a
b
c
d
e
n
λabs/nm
λ
em/nm
Φ
σ2 (λex/nm)
σ3 (λex/nm)
η2 (λex/nm)
η3 (λex/nm)
3
4
5
6
7
405
420
431
437
442
417, 443
431, 459
440, 469
446, 475
449, 478
0.85
0.86
0.84
0.92
0.90
59.6 (810)
220.9 (840)
570.6 (850)
782.9 (840)
2.02 (1230)
7.23 (1250)
no lasing
no lasing
0.132 (830)
0.163 (890)
0.253 (890)
0.336 (890)
1.01 (1220)
1.33 (1250)
2.10 (1220)
2.50 (1220)
22.41 (1300)
34.30 (1310)
45.63 (1330)
1045.6 (880)
a
b
c
d
e
Dissolved in toluene. 2PA cross-section (GM). 3PA cross-section (× 10−77 cm6 s2). 2PA lasing efficiency (%). 3PA lasing efficiency (× 10−3 %).
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Communication
Ph(n)-NPh, where n = 4−7, exhibit exceptionally strong
multiphoton-excited blue lasing. In addition, in sharp contrast
to this diphenylamino end-capped series, the un-end-
capped(L)-Ph(n) series, where n = 3−5, only exhibit
multiphoton-excited photoluminescence but not lasing phe-
nomenon. This signifies the importance of the extended π-
conjugation and diphenylamino end-caps for multiphoton-
excited lasing. Figure 5a shows the intense and highly
Figure 5. (a) Photo of highly directional, frequency-upconverted
stimulated (blue) emission from a (L)-Ph(4)-NPh solution pumped
with a focused 1.2 μm laser beam. (b) Plots of input power versus
output power of two-photon-pumped lasing process for (L)-Ph(n)-
NPhs excited at 890 nm.
Figure 3. Logarithmic plots of the power dependence of relative
multiphoton-induced fluorescence intensity of (L)-Ph(5)-NPh on
pulse intensity using (a) 800, (b) 1250, (c) 1650, and (d) 2000 nm
femtosecond laser as an exciting source. The solid lines are the best-fit
straight lines with gradient n = 1.94, 3.08, 4.09, and 5.08, respectively.
directional stimulated blue emission from the (L)-Ph(4)-NPh
solution upon pumped with laser pulses at 1200 nm. The plots
of input pumped power versus two-photon-excited lasing
output relationship for (L)-Ph(n)-NPh, n = 4−7, are shown in
Figure 5b, unambiguously showing evidence of lasing threshold
behavior. It is worth mentioning that the lasing threshold
decreases with an increase in the conjugation length of these
oligo(p-phenylene)s, demonstrating the advantage of using
higher homologues for lasing. Remarkably, the two-photon-
excited lasing efficiency enhances greatly with the conjugation
length, increasing up to 0.34% for (L)-Ph(7)-NPh, which is the
record high value achieved so far for the multiphoton-excited
blue lasing efficiency. The three-photon upconversion lasing
spectra of (L)-Ph(n)-NPh, n = 4−7, excited at 1.25 μm are
shown in Figure 6a. The optimized full width at half-maximum
Figure 4. (a) Two-, (b) three-, and (c) four-photon-excited spectra of
(L)-Ph(n)-NPh.
tions. The 3PA cross-sections, σ3, were determined by
comparing the fluorescence intensity with that of PhN-
OF(4)-TAZ-OF(4)-NPh in the range of 1100−1500 nm.7f
Consistently, the 3PA cross-sections increase with the
extension of conjugation length in this series. Remarkably,
the exceptionally large intrinsic σ3 of 4.56 × 10−76 cm6 s2 at
1330 nm for (L)-Ph(7)-NPh in the femtosecond regime was
obtained, affirming the importance of extended and planar
oligo(p-phenylene) core as well as 9,9-diaryl-substitution onto
fluorenyl moiety for large 3PA enhancement. The four-photon-
excited fluorescence intensities measured in the range of 1700−
1950 nm were also found to increase with extension of
conjugation length, further corroborating the merit of
extending the π-conjugated ladder-type system for multiphoton
absorption.
Figure 6. (a) Three-photon absorption upconverted lasing spectra of
(L)-Ph(n)-NPh (n = 4−7) at the concentration of 1.2 × 10−2 M. (b)
Plots of input power versus output power of three-photon pumped
lasing process for (L)-Ph(n)-NPh, where n = 4−7, excited at 1300,
1300, 1250, and 1230 nm, respectively.
(fwhm) of the three-photon lasing spectra narrowed sharply to
∼4 nm with a peak in the range of 460−482 nm. In addition,
the plots of input power versus output power of the three-
photon pumped lasing process for (L)-Ph(n)-NPh, n = 4−7,
are shown in Figure 6b. Compared to the slopes in Figure 5b,
the slopes in Figure 6b appear to be gentle, which is attributed
to the fact that 3PA is a much higher order nonlinear optical
process resulting in smaller gain. The three-photon-excited
lasing efficiency of (L)-Ph(n)-NPh, n = 4−7, also increases
Upon pumping with near-infrared femtosecond laser pulses,
the longer homologues of ladder-type oligo(p-phenylene)s (L)-
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Journal of the American Chemical Society
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ASSOCIATED CONTENT
* Supporting Information
■
S
Details of synthesis and characterization; multiphoton
absorption measurements and results. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
mswong@hkbu.edu.hk; kwcheah@hkbu.edu.hk
■
(8) Wu, P. L.; Xia, P. F.; Li, Z. H.; Feng, X. J.; Tam, H. L.; Li, K. F.;
Jiao, Y.; Wong, M. S.; Cheah, K. W. Chem. Commun. 2009, 5421.
Author Contributions
⊥H.H.F. and L.G. contributed equally to this work.
(9) Zojer, E.; Beljonne, D.; Pacher, P.; Bred
2004, 10, 2668.
́
as, J. L. Chem. Eur. J.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by GRF (HKBU 202408), Hong
Kong Research Grant Council, Faculty Research Grant (FRG2/
10-11/015), Hong Kong Baptist University and Institute of
Molecular Functional Materials which was supported by a grant
from the University Grants Committee, Areas of Excellence
Scheme (AoE/P-03/08).
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