High-efficiency fluorescent polyimides based onlocally excited triarylamine-containingdianhydride moieties†
Jia-Hao Wu and Guey-Sheng Liou*
Three novel high fluorescent polyimides (PIs) were readily synthesized from the polycondensation of tri-
arylamine-based tetracarboxylic dianhydride monomers with a commercially available aliphatic diamine
monomer. The photoluminescence (PL) intensity of the solid film and the nanofiber fabricated by solution
casting and electrospinning methods revealed high quantum yields of up to 32% and 35%, respectively.
Furthermore, in order to investigate the fluorescent transition mechanisms of the PIs, a series of
model compounds corresponding to the repeat units of PIs were also synthesized, and density functional
theory calculation results were also used to support the deduction. Moreover, the competition of
aggregation enhanced emission (AEE) and aggregation caused quenching (ACQ) effects was investigated
and demonstrated.
Introduction
Polymeric fluorescent materials have drawn significant atten-tion due to the advantages including structural flexibility, low-cost, and feasibility of large area fabrication by solution spin-coating or inkjet printing1 for their potential applications suchas in light emitting diodes,2 plastic lasers,3 fluorescentsensors,4 and optical wavelength converters.5 However, theemerged π-conjugated fluorescent polymers, such as polyfluor-ene6 derivatives, do not have sufficient thermal stability andglass transition temperatures to sustain high temperaturetreatment during device fabrication processes. Thus, themolecular design and synthesis of heat-resistant high-perform-ance polymers with high fluorescence intensity in the solidstate is beneficial and also crucial for advanced optoelectronicapplications.
Polyimides (PIs), such as Kapton and Upilex, are well-known high-performance polymers with excellent combinationof thermal and chemical stability, radiation resistance, andsuperior mechanical properties due to their rigid molecularstructures and strong intermolecular interactions.7 However,most of the aromatic PIs revealed too low photoluminescence
quantum yield (PLQY) to be used as fluorescent materials. Theoptical absorption and emission behavior of PIs have beenwidely investigated by using molecular orbital (MO) tran-sitions.8 The most critical issue is charge transfer (CT) tran-sition between electron-donating diamine and electron-accepting dianhydride moieties of PIs after UV-Vis irradiation,resulting in red-shifted and suppressed PL intensity. The otherone is locally excited (LE) transition that occurs between occu-pied and unoccupied MOs, and both orbitals are locatedaround the dianhydride moieties of PIs, thus could inducehigh PL intensity. Based on the above findings, PL efficiency ofPIs could be effectively improved by introducing alicyclic di-amines or dianhydrides into the PIs to reduce CT interactionsand enhance LE transitions.8a,9 Furthermore, most of theluminogens in the solid state generally lead to partial or evencomplete quenching of light emissions and reveal much lowerPLQY than in solutions called aggregation-caused quenching(ACQ) effect. Recently, a novel phenomenon of aggregation-induced emission (AIE) or aggregation-enhanced emission(AEE)10 which is exactly opposite to the ACQ effect paves a newstrategy for designing and synthesis of highly efficient lumino-gens in the aggregated state and are attributed to the restric-tion of intramolecular rotation in the condensed phase andalmost non-fluorescent in solution.
Triphenylamine (TPA) derivatives are well-known for theirelectrical and photophysical properties in many potentialareas, such as hole-transporters, light-emitters, electrochromicand memory devices.11 Recently, we developed and reportedTPA-based fluorescent polyimides by the excellent combi-nation of LE transition and AIE effect,12 and the demonstrated
AIE effect of TPA derivatives could be attributed to their pro-peller-like structure similar to other AIE-active luminogens.13
In order to gain more insight into the working principle ofAIE-active fluorescent PIs, a series of fluorescent PIs were pre-pared from triarylamine-based dianhydride monomers and ali-phatic diamine monomer bis(4-aminocyclohexyl)methane inthis study. In addition, a series of model compounds corres-ponding to the repeat units of PIs were also synthesized. TheUV-Vis absorption and PL spectra, MO transitions with densityfunctional theory calculations, and PL solvatochromism ofthese triarylamine-containing phthalimide model compoundswere extensively investigated. Furthermore, the competition ofAIE and ACQ effects in the PI system was also described.
Experimental sectionMaterials
Triarylamine-based dianhydride monomers, N,N-bis(3,4-dicar-boxyphenyl)aniline dianhydride (1), N,N-bis(3,4-dicarboxy-phenyl)-1-aminonaphthalene dianhydride (2), N,N-bis(3,4-dicarboxyphenyl)-1-aminopyrene dianhydride (3), and diaminemonomer, 4,4′-diamino-triphenylamine (1′) were preparedaccording to the previously reported procedures.12c Commer-cially available bis(4-aminocyclohexyl)methane was purified byrecrystallization from hexane. All other reagents were used asreceived from commercial sources.
Synthesis of model compounds
M-Ph was chosen as an example to illustrate the general syn-thetic route used to produce the model compounds. A 50 mLround-bottom flask equipped with a magnetic stirrer wascharged with 0.303 g (0.79 mmol) of dianhydride (1), 0.160 g(1.61 mmol) of cyclohexylamine, and 2.5 mL of acetic acid.The reaction mixture was heated with stirring at 140 °C for 6 h.The resulting reaction solution was poured into 30 mL of stir-ring methanol/water giving rise to a yellow precipitate that wascollected by filtration and dried.
The synthesis of Ph-DCHPI was used as an example to illus-trate the general synthetic route used to produce the PIs. Intoa 50 mL round-bottom flask were added 0.3850 g (1.00 mmol)of dianhydride (1), 0.210 g (1.00 mmol) of bis(4-aminocyclo-hexyl)methane, 0.24 ml isoquinoline, and 2.4 ml m-cresol. Thereaction mixture was stirred at 50 °C under a nitrogen atmos-phere for 18 h. Then, the reaction temperature was increasedto 200 °C for 15 h. Then, the mixture was cooled to room temp-erature, and the viscous polymer solution was then pouredslowly into 300 ml of stirred methanol giving rise to a yellowfibrous precipitate. Reprecipitation of the polymer by DMAc/methanol was carried out twice for further purification.
Fabrication of electrospun fibers
A polymer solution with the concentration of 25 wt% in CHCl3was used to produce the electrospun (ES) fiber. The ES fiberwas prepared using a single-capillary spinneret. First, the solu-tion was fed into the syringe pumps (KD Scientific model 100)connected to the metallic needle with the feed rate of 0.1 mLh−1. The metallic needle was attached to a high-voltage powersupply (YSTC), and a piece of aluminum foil was placed 6 cmbelow the tip of the needle to collect the nanofiber. The spin-ning voltage was set at 10 kV. All experiments were carried outat room temperature.
Fourier transform infrared (FT-IR) spectra were recorded on aPerkinElmer Spectrum 100 Model FT-IR spectrometer. Elemen-tal analyses were run in a Heraeus VarioEL-III CHNS elementalanalyzer. 1H and 13C NMR spectra were recorded on a BrukerDPX-400 MHz FT-NMR, and peak multiplicity was reported asfollows: s, singlet; d, doublet; t, triplet; m, multiplet. Theinherent viscosities were determined at 0.5 g dL−1 concen-tration using a Tamson TV-2000 viscometer at 30 °C. Thermo-gravimetric analysis (TGA) was conducted with a TAInstruments Q50 in flowing nitrogen or air (flow rate = 60 cm3
min−1) at a heating rate of 20 °C min−1. Thermal MechanicalAnalysis (TMA) was conducted with a TA instrument TMAQ400. The TMA experiments were conducted from 40 to 400 °Cat a scan rate of 10 °C min−1 with a film/fiber probe under anapplied constant load of 5 mN. Ultraviolet-visible (UV-Vis)spectra of the polymer films were recorded on a HitachiU-4100 spectrometer. Photoluminescence (PL) spectra wererecorded with a Fluorolog-3 spectrofluorometer. The relativePL quantum yield (ΦPL) of the samples in different solventswere measured by using quinine sulfate dissolved in 1 N sulfu-ric acid as a reference standard (ΦPL = 0.546), and the ΦPL ofpolymer thin films was determined by using a calibrated inte-grating sphere. All spectra were obtained by averaging fivescans. The morphology of the ES fiber was characterized byusing an optical microscope (HUVITZ Co., Ltd).
Quantum chemical calculation
The molecular simulation in this study was performed byusing the Gaussian 09 program package, which implementsanalytical gradients at the time-dependent density functionaltheory (TD-DFT). The DFT method at the Becker style three-parameter density functional theory using the Lee–Yang–Parrcorrelation functional level of theory (B3LYP) with the 6-31G(d) basis set was adopted for calculating electronic structuresand spectroscopic properties of the model compounds.
Results and discussionMaterial synthesis
The model compounds of M-Ph, M-Np, and M-Py were syn-thesized from the corresponding triarylamine-containing di-anhydrides (1–3) with two equivalents of cyclohexylamine(Scheme 1), and M-Ph′ was prepared from 4,4′-diamino-tri-phenylamine (1′) and 1,2-cyclohexanedicarboxylic anhydride.Elemental analysis, 1H NMR, 13C NMR, and FT-IR spectro-scopic techniques were used to identify the structures of thesemodel compounds as shown in Fig. S1 to S5.† The results ofthese spectroscopic analyses suggest that the target modelcompounds were prepared successfully.
The polyimides were synthesized by high-temperature solu-tion polycondensation (Scheme 2). These dianhydrides and bis(4-aminocyclohexyl)methane were polymerized in m-cresol at200 °C in the presence of isoquinoline as a catalyst. The result-ing viscous solutions then could be precipitated to obtain
tough fiber-like polymers when slowly pouring into methanol.The inherent viscosities and molecular weights of these poly-mers are summarized in Table S1.† All these high molecularweight polymers could afford transparent and tough films bysolution casting as shown in Fig. 1. 1H NMR and FT-IRspectroscopic techniques were used to identify structures ofthe obtained polyimides, and the results agree well with theproposed molecular structures (Fig. S6 and S7†).
Basic polymer properties
The solubility behavior of these resulting polymers was investi-gated at 5 mg ml−1 concentration, and the results are also
Scheme 1 Synthesis of imide model compounds.
Scheme 2 Synthesis of triarylamine-based polyimides.
Fig. 1 The photographs of polyimides with thickness around 30 μm. (a)Ph-DCHPI, (b) Np-DCHPI, and (c) Py-DCHPI.
listed in Table S2.† These semi-aromatic PIs were readilysoluble in less polar solvents like CHCl3 and THF and highpolar aprotic solvents such as NMP and DMAC. Thus, these PIshave potential to be used in spin-coating or inkjet-printing toafford high performance thin films for optoelectronic devices.
Thermal properties of the obtained PIs were examined byTGA and TMA, and the results are summarized in Table S3.†TGA curves of these resulting polymers shown in Fig. S8†exhibited high thermal stability with insignificant weight lossup to 430 °C both under nitrogen and air atmospheres. TypicalTMA curves of these resulting PIs are depicted in Fig. S9† andhand glass-transition temperatures (Tg) around 340–386 °Ccould be obtained, depending upon the stiffness and rigidityof the polymer chain, and the higher Tg of the PI Py-DCHPIcould be attributed to the rigidity and stronger π–π interactionsof pyrene moieties.
Optical properties
Optical behavior of the model compounds was investigated byUV-vis and PL spectroscopy depicted in Fig. 2, and the resultsare summarized in Table 1. These imide-containing modelcompounds of M-Ph, M-Np, and M-Py derived from triaryl-amine-based dianhydrides showed two absorptions at around222–272 and 378–395 nm, and exhibited PL emissionmaximum at 433–461 nm in cyclohexane solution (conc.:10 μM) with PLQY (ΦPL) ranging from 25.7% to 36.0%. While,M-Ph′ prepared from TPA-based diamine which is a structuralisomer of M-Ph revealed not only a lower PL quantum yield of4.3% (Fig. 3) compared with M-Ph (ΦPL = 36.0%) but also a
hypsochromic shift both in the absorptions at 208 and311 nm, and a PL emission peak at 366 nm. The imide-con-taining model compounds of M-Ph, M-Np, and M-Py withmore extended conjugation phthalimide structures have alower energy band gap and result in a bathochromic shift ofabsorption and emission in the UV-Vis and PL spectra, respect-ively. In order to gain more insight into the photophysical be-havior of the conjugated triarylamine-containing phthalimidesystem, TD-DFT calculations could be used to clarify thenature of electron transitions of the model compounds withdifferent triarylamine-based phthalimide moieties. Calculatedmolecular orbital diagrams of these model compounds aredepicted in Fig. 4, and the excitation wavelengths, the oscil-lator strengths ( f ), the contribution molecular orbitals (MOs),the assignments of electron transitions from ground state (S0)to excited states (Si), and the contributions of each transitionof model compounds are summarized in Table S4.† In thecase of M-Ph, M-Np, and M-Py, the MOs related to the lowestexcited transition from HOMO to LUMO delocalized over thetriarylamine-based phthalimide moieties derived from triaryl-amine-based dianhydrides, implying that the S0 to S1 could beattributed to LE (π–π*) transition. In addition, higher excitedtransition states such as LUMO+1, LUMO+2, and LUMO+3 of
Fig. 2 (a) UV-Vis and (b) PL spectra of model compounds in cyclo-hexane solution (10 μM).
Table 1 Optical properties of the imide model compounds
a They were excited at λabsmax for solution states. b The quantum yield wasmeasured by using quinine sulfate (dissolved in 1 N H2SO4 with aconcentration of 10 μM, assuming photoluminescence quantumefficiency of 0.546) as a standard at 25 °C.
Fig. 3 PL photographs of the model compound solutions M-Ph andM-Ph’ (concentration is 10 μM in cyclohexane) taken under illuminationat 365 nm UV light.
M-Ph, M-Np, and M-Py also revealed similar LE (π–π*) orbitaltransitions around the triarylamine-based phthalimide moie-ties. In the case of M-Ph′, the lowest excited transition fromHOMO to LUMO also exhibited LE (π−π*) transition, but
revealed CT (π–π*) transition from the HOMO of electron-donating diamine units to higher excited LUMO+1 andLUMO+2 states of the electron-accepting anhydride moieties.Therefore, the model compound M-Ph derived from TPA-baseddianhydride showed higher PL intensity than the isomericcompound M-Ph′ prepared from the TPA-based diamine.
In addition, all the model compounds of M-Ph, M-Np, andM-Py exhibited solvatochromic PL behavior in differentpolarity solvents (conc.: 10 μM) as shown in Fig. S12–S14,† andthe results are summarized in Tables S5–S7,† respectively. Thesolvatochromism could be attributed to the fast conversionprocess from the emissive local excited state to the low emis-sive state. These results clearly indicate that solvent polarityexerts little effect on absorption behavior, while the PL emis-sion behavior shows strong solvent polarity dependence,resulting in a broad emission band and remarkable bathochro-mic shift with increasing solvent polarity.
UV-vis absorption and PL emission behavior of the result-ing PIs were investigated and depicted in Fig. 5, and PL photo-graphs of these PIs in solution, film, and fiber states weretaken under illumination at 365 nm as shown in Fig. 6. ThesePIs exhibited maximum UV-vis absorption bands at around378–407 nm in NMP solution (conc.: 10 μM) due to the LE(π–π*) transitions of the triarylamine-based phthalimide lumi-nogens, and revealed yellowish-green PL emission with
Fig. 4 Calculated molecular orbitals of the model compounds (TD-DFTmethod at B3LYP/6-31G(d)).
Fig. 5 Absorption and photoluminescence (PL) spectra of the polyimide (a) Ph-, (b) Np-, and (c) Py-DCHPI in solution, film and fiber states. (d) OMimage of the Ph-DCHPI nanofiber.
maximum peaks around 519–549 nm and ΦPL ranging from0.3 to 5.2%. Interestingly, the resulted PIs revealed muchhigher ΦPL in the solid film than in the NMP solution withhypsochromic-shifted PL behavior as shown in Fig. 6, and ΦPL
of the Ph-DCHPI film could be greatly enhanced to 32.6%. Fur-thermore, the ES fibers were also fabricated to confirm theAEE phenomenon of high ΦPL in the solid state as shown inFig. 5d and 6. The highly orientated ES fiber of Ph-DCHPIexhibited not only a stronger PL intensity with enhanced ΦPL
up to 35.4% but also great bathochromic-shift onset absorp-tion when compared with the corresponding solid film (assummarized in Table 2). These luminogenic polymers withAIE and AEE characteristics could be attributed to the triaryl-amine-based phthalimide moieties. However, the ΦPL
increased only slightly from the solution to the film state forPy-DCHPI that maybe ascribed to the competition of ACQ(pyrene group) and AIE (triarylamine moiety) effects.
To demonstrate the competition of ACQ and AIE effects onPL behavior, the PI NMP solutions with different methanol
fractions were used to investigate the aggregation effect on PLintensity as depicted in Fig. 7, and the results are summarizedin Table S8.† By introducing a poor solvent of methanol intothe PI NMP solutions, the existing forms of luminogenic moi-eties could be tuned from solution to aggregated particles inthe mixture, resulting in changes in their PL behavior. Forinstance, Np-DCHPI in pure NMP exhibited weak yellowish-green PL emission with a maximum peak at 523 nm, while theemission was slightly blue-shifted and the intensity enhancedsimultaneously on increasing the methanol fraction to 30 vol%. The enhancement of PL intensity could be attributed to theAIE effect by the formation of molecular aggregation, and therestriction of luminogen intramolecular rotations results inhigher PL intensity. However, by further increasing the metha-nol fraction more than 50 vol%, the intermolecular π–π stack-ing enhanced with increasing the extent of aggregation thatinduced intermolecular energy transfer to decrease the PLintensity and also red-shift in the meantime.
Conclusions
Three high PL efficiency polyimides could be readily preparedvia one-step polycondensation from triarylamine-based tetra-carboxylic dianhydrides and aliphatic diamine, which can sup-press the formation of the CT complex. The resulting Ph-DCHPI exhibited high PLQY values of 32.6% and 35.4% in thesolid film and nanofiber states, respectively. Py-DCHPI con-taining the pyrene group could induce more ACQ than the AIEeffect, and reveal a lower PLQY of 5.9% in the film state. Inaddition, a series of model compounds corresponding to therepeat unit of the PIs were also synthesized, and density func-tional theory calculations were used to support the deduction.We demonstrate that the model compound M-Ph′ with centraltriarylamine diamine showed higher capability to form acharge transfer complex due to CT (π–π*) states, resulting inlower PL intensity, while, the other model compounds derivedfrom triarylamine-based dianhydrides exhibited high LE (π–π*)emission and stronger PL intensity. Moreover, the competitionof AIE and ACQ effects of these triarylamine-based PIs wasinvestigated and demonstrated. This comparative study of theisomeric triarylamine-based phthalimide structure effect on
Fig. 6 Photographs of polyimides in solution, film, and fiber stateswere taken under illumination of a 365 nm UV light.
a They were excited at λabsmax.b The quantum yield was measured by using quinine sulfate (dissolved in 1 N H2SO4 with a concentration of 10 μM,
assuming photoluminescence quantum efficiency of 0.546) as a standard at 25 °C. c PL quantum yields of polymer thin films were determinedusing a calibrated integrating sphere.
high LE transition and AIE-active behavior may provide someguidance for designing new high-efficiency PL polyimides.
Acknowledgements
The authors are grateful to the Ministry of Science and Techno-logy of Taiwan for the financial support.
Notes and references
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