Novel high-efficiency PL polyimide nanofiber containing aggregation-induced emission (AIE)-active cyanotriphenylamine luminogen

Article abstract of DOI:10.1039/c2cc37857d

Two aggregation-induced emission (AIE)-active high-performance polymers and their electrospun (ES) nanofibers were synthesized and prepared. Strong luminescence is induced when they are aggregated in poor solvents or fabricated into solid films and this i

Full text of DOI:10.1039/c2cc37857d

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Novel high-efficiency PL polyimide nanofiber
containing aggregation-induced emission
(AIE)-active cyanotriphenylamine luminogen†
Cite this: Chem. Commun., 2013,
49, 630
Received 30th October 2012,
Accepted 21st November 2012
Hung-Ju Yen, Chih-Jung Chen and Guey-Sheng Liou*
DOI: 10.1039/c2cc37857d
www.rsc.org/chemcomm
Two aggregation-induced emission (AIE)-active high-performance
In addition, TPA derivatives also are well known as their
polymers and their electrospun (ES) nanofibers were synthesized photo- and electroactive properties show potential for opto-
and prepared. Strong luminescence is induced when they are electronic applications, such as photoconductors,8a hole-
aggregated in poor solvents or fabricated into solid films and this transporters,8b light-emitters,8c electrochromic,8d–f and memory
is further enhanced when they are prepared as ES nanofibers, devices.8g–i Although TPA itself is not AIE active, its propeller-
demonstrating their AIE feature and great potential for opto- like structure and electron-donating characteristics make it a
electronic applications.
useful building block for the construction of AIE luminogens,
and the emission colors can be tuned through judicious
Recent studies have shown that heteroatom-containing lumino- combinations of the TPA units with various electron-accepting
gens, such as cyano substituents and triphenylamine (TPA) moieties. This is demonstrated by AIE luminogens containing
derivatives, possess excellent light emission attributes. While donor–acceptor pairs, which can emit green or red light due to
these luminogens are almost non-fluorescent in solution they the different extents of push–pull interactions.9
fluoresce strongly in the aggregated state, which is attributed to
Recently, electrospinning (ES) has emerged as a new techni-
the restriction of intramolecular rotation in the condensed que to produce various functional nanofibers because of the
phase.1 Such a novel phenomenon, named ‘‘aggregation-induced advantages of low cost, flexible operation conditions, and an
emission (AIE)’’, is exactly the opposite of the aggregation- efficient continuous production process. The ES procedure
caused quenching (ACQ)2 effect observed in most conventional probably results in smaller phase separated domains than
chromophores,3 and paves a new way in the design and synthesis those of the spin-coated films.10 Furthermore, the resulting
of efficient solid-state emitters.4
nanofibers may produce a more extended chain conformation
Because of its structural simplicity and high molar polariza- along the fiber axis, thus enhancing the orientation and align-
tion, the cyano (CN) group has been frequently utilized as ment in comparison with the spin-coated films. As reported in
a functional unit in the design of advanced optoelectronic the literature, ES fibers showed different photoluminescence
materials.5 As a result, a great number of AIE luminogens (PL) characteristics to the corresponding thin films with much
containing cyano groups have been developed due to the internal higher PL efficiency.11
steric hindrance arising from the cyano group.6 In dilute
Based on this concept, our molecular design strategy is
solution, the twisted geometry allows intramolecular rotation, based on the embedding of a CN-TPA luminogen into high-
which annihilates its excitons non-radiatively. In crystals, such performance polymers, which have been widely used in gas
molecular motions are restricted. The C–HÁ Á ÁN hydrogen bonds separation membranes, composites, fuel cells, optical, electro-
formed between the cyano groups of one molecule and the chromic, and polymer memory applications due to their excellent
hydrogen atoms of an adjacent molecule thus rigidify the thermal stability, high mechanical strength, low flammability,
molecules and intensify their light emission.7
good chemical and radiation resistance, and good electronic
properties.12 In this communication, we therefore synthesized
two high-performance polymers from CN-TPA containing diamine
monomer (Scheme 1).
The model compounds M-PI and M-PA were readily syn-
thesized by condensation and the detailed synthetic routes and
characterization are described in the ESI† (Chart 1, ESI†).
The polyimide CN-PI and polyamide CN-PA were prepared by
a one-pot high-temperature solution polycondensation method13
Functional Polymeric Materials Laboratory, Institute of Polymer Science and
Engineering, National Taiwan University, 1 Roosevelt Road, 4th Sec., Taipei 10617,
Taiwan. E-mail: gsliou@ntu.edu.tw; Fax: +886-2-33665237; Tel: +886-2-33665315
† Electronic supplementary information (ESI) available: Experimental section.
Tables: inherent viscosity, molecular weights, solubility behavior, thermal, and
photophysical properties. Figures: NMR, IR, and PL behavior of model
compounds, NMR, IR, TGA, and TMA traces of polymers. See DOI: 10.1039/
c2cc37857d
c
630 Chem. Commun., 2013, 49, 630--632
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Scheme 1 Synthesis of CN-TPA based high-performance polymers. The
photographs show the appearance of the flexible films (thickness: B60 mm).
Fig. 1 Normalized absorbance and photoluminescence (PL) spectra of polyimide
CN-PI (left) and polyamide CN-PA (right) in solution, solid film, and ES fiber
states. Photographs were taken under illumination of a 365 nm UV light.
and phosphorylation polycondensation,14 respectively (Scheme 1).
The detailed synthetic procedures and basic properties are
described in the ESI.†
bathochromic shift with an increase of the solvent polarity.
The optical properties of the polymers were investigated and For the amide-type model compound M-PA, the emission color
summarized in Table 1. These soluble polymers exhibited changes from violet-blue in cyclohexane (CH) (lem = 415 nm) to
maximum UV-vis absorption bands at around 310–313 and yellowish-green in DMSO (lem = 508 nm).
313–316 nm in N-methyl-2-pyrrolidinone (NMP) solutions and
It is reasonable that the model compounds showed higher
solid film states, respectively, due to the p–p* transitions of the PL quantum yields as compared with the corresponding poly-
aromatic TPA chromophores. Fig. 1 depicts the UV-vis absorp- mers (Table 1). The solvatochromism could be attributed to the
tion and PL behavior of these resulting CN-TPA-based poly- fast interconversion process from the emissive local excited
mers. In the case of NMP solutions (conc.: 10À5 mol LÀ1), state to the low emissive state. In addition, they exhibit much
polyimide CN-PI and polyamide CN-PA exhibited blue and stronger emission in nonpolar solvents or in the solid state due
green PL emission with a PL quantum yield of 34 and 14% at to the restricted molecular transitions.6,15
the maximum peaks of 443 and 497 nm, respectively. In the
To further confirm the AIE attribution, PL spectra of the
solid film state, the obviously high PL emission could be CN-PA NMP–water diluted mixtures were studied with different
observed (as shown in Fig. S6, ESI†), and the polymers both water fractions, and the results are summarized in Table S6
showed a hypsochromic shift in PL emission relative to those of (ESI†). Since the polyamide was insoluble in water and soluble
the solutions. Moreover, the PL quantum yields of the films are in NMP, increasing the water fraction in the mixed solvent
as high as 65% and 46% for CN-PI and CN-PA, respectively, could change their existing forms from a solution in pure NMP
which are greatly enhanced compared with those of the to aggregated luminogen particles in the mixtures, which will
solution state. The phenomenon could be attributed to the result in changes in their UV and PL spectra. The PL spectra
solvatochromism and AIE effect resulting from the introduc- and images of 10 mM of CN-PA in NMP–water mixtures with
tion of the CN-TPA luminogens.
different water contents are shown in Fig. 2. Polyamide CN-PA
In order to demonstrate the solvatochromic behavior, the in pure NMP exhibited green PL emission with a maximum
optical absorption and PL of model compounds M-PI and M-PA peak at 497 nm, while the PL emission was blue-shifted and the
were investigated in solvents (conc.: 10À5 mol LÀ1) with different intensity enhanced with increasing water fraction.
polarity. Fig. S7 and S8 (ESI†) exhibit the normalized PL spectra
These results demonstrate that the CN-TPA unit is indeed
of M-PA and M-PI in various solvents, together with fluores- AIE-active.
cence images of their dilute solutions, respectively, and the
FE-SEM images of these two ES fibers having smooth fiber-
absorption and PL emission data are summarized in Tables S4 like structure without bead formation are shown in Fig. 3. The
and S5 (ESI†). These results clearly indicate the PL emission absorption spectra of the ES fibers showed a bathochromic shift
spectra show strong solvent-polarity dependence, revealing a to their solution and film states, suggesting the higher orienta-
dominant broad emission band that undergoes remarkable tion and alignment of the polymer chains in the fiber state.
Table 1 Optical properties of polymers
NMP (10 mM) solution (nm)
Solid film (nm)
ES fiber (nm)
PL (%)b
labs
lem
FPL (%)c
labs
lem
FPL (%)c
a
a
a
labs
lem
F
Index
CN-PI
CN-PA
313
310
443
497
34
14
316
313
440
472
65
46
350
360
446
475
70
57
a
b
They were excited at labs for solid and solution states. The quantum yield was measured by using quinine sulfate (dissolved in 1 N H2SO4
c
with a concentration of 10 mM, assuming photoluminescence quantum efficiency of 0.546) as a standard at 24–25 1C. PL quantum yields in the
solid state were determined using a calibrated integrating sphere.
c
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The authors gratefully acknowledge the National Science
Council of Republic of China for financial support.
Notes and references
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Fig. 3 SEM images of ES nanofibers of CN-PI (left) and CN-PA (right).
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