Unusual, Highly Efficient Fluorescence Emission Enhancement of

Note Cite This: Macromolecules XXXX, XXX, XXX−XXX

Unusual, Highly Efficient Fluorescence Emission Enhancement of Conjugated Polymers with an Intramolecular Stack Structure through Thermal Annealing at High Temperature Young-Jae Jin and Giseop Kwak* Department of Polymer Science & Engineering, Polymeric Nanomaterials Laboratory, School of Applied Chemical Engineering, Kyungpook National University, 1370 Sankyuk-dong, Buk-ku, Daegu 702-701, Korea S Supporting Information *

the polymer film. When contacted with various organic solvents such as alcohols and hydrocarbons, these polymer films swell and simultaneously exhibit a significant FL emission enhancement.24−27 This is because the solvent molecules diffuse into the solid films rapidly and relax the entangled and twisted polymer chains hydrodynamically. The swelling loosens the IaSS, leading to an increase in π*−π electronic transition energy (blue-shift) and degeneration of the static emission quenching site (FL emission enhancement). Thus, it is possible if the IaSS is loosened thermodynamically, FL emission enhancement may be achieved more easily simply by annealing the film. In this study, we examined changes in FL emission of the PDPA film through thermal annealing. The FL emission was remarkably increased by the thermal treatment, contrary to the behavior exhibited by conventional conjugated polymers. In particular, significant FL enhancement was achieved by heating at extremely high temperature. Annealing at 300 °C for only a few seconds produced a more twisted polymer chain accompanied by trans−cis isomerization and thermodynamic relaxation of the IaSS between the side phenyl rings. These processes occurred simultaneously and were responsible for the FL emission enhancement. The mechanism of the FL enhancement will be described in detail based on various spectroscopic and thermodynamic (thermogravimetric) analyses. The thermal annealing effect on the FL emission properties of PDPA derivatives in the solid state was first examined using a representative PDPA derivative, PDPA-C1 (chemical structure shown in Figure 1). Figure 2a shows the FL emission and ultraviolet−visible (UV−vis) absorption spectra of the PDPAC1 film before and after thermal annealing. Surprisingly, the FL intensity increased by approximately 2.6 times, and the FL band shifted to a shorter wavelength by 5 nm after the annealing at the extremely high temperature of 300 °C for only 10 s. The FL enhancement reached equilibrium within 10 s and was unchanged even with longer annealing times (Figure S1). This indicates that the FL enhancement can be sufficiently maximized at high temperature with a very short duration. A quite thick PDPA-C1 film that cast from the toluene solution also showed the same tendency in the FL change by annealing

ecently, π-conjugated polymers have been developed in the field of organic thin-film optoelectronic devices for use in electrodes, polymer light-emitting diodes, field-effect transistors, and solar cells.1−4 The chain conformation of the polymers in the solid state is an important factor in determining their electrical and photophysical properties. Therefore, device properties, such as electric conductivity, electroluminescence emission efficiency, charge mobility, and photoelectric conversion efficiency, are greatly affected by conformational and morphological changes in the polymer chains containing the active layer of the devices.5,6 These polymer assemblies are constructed based on strong intermolecular π−π interactions, and thermal annealing treatment is the most common method of lowering the entropy of the polymer chains to obtain a welldeveloped chain packing structure in the solid state.7−10 Thermal annealing is very efficient especially when applied to thermotropic liquid crystalline or crystalline conjugated polymers with a definite phase transition temperature. Although this method significantly improves the intermolecular charge transport ability, it is not favorable for achieving a high fluorescence (FL) emission efficiency in the solid state. This is because an intermolecular excimer is formed due to the face-toface interchain packing structure, which causes a lower electronic transition energy (red-shift), and a significant FL quenching occurs through a nonradiative emission decay process.11−16 As described above, conventional conjugated polymers have a highly stacked chain structure due to their intrinsic planar geometry and strong intermolecular interactions, resulting in relatively weak FL emission in the solid state. In contrast, poly(diphenylacetylene) (PDPA) derivatives are known to exhibit considerably stronger FL emission even in the solid state over a wide visible region with colors ranging from sky blue to greenish-yellow.17−19 This unusual, highly efficient FL emission arises from effective exciton confinement based on the intramolecular stack structure (IaSS) of the side phenyl rings of PDPA. The main chains of PDPA derivatives are highly twisted due to steric repulsion between the bulky side phenyl rings and are not coplanar with the phenyl rings. Thus, these polymers have a wormlike molecular chain structure with relatively weak intermolecular interactions and nonplanar geometry. As a result, intermolecular excimer emission hardly occurs even in the solid state, but intramolecular excimer emission due to the side phenyl ring stack structure becomes dominant.20−23 Since the PDPA derivatives have an intrinsically amorphous structure and extremely large free volume, fluids can diffuse very quickly into

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© XXXX American Chemical Society

Received: November 5, 2017 Revised: December 2, 2017

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disappears (Figure 2a). PDPA derivatives are known to have an isosbestic point near 380 nm where the main chain and the side phenyl rings intersect.17−19 Accordingly, the 420 nm absorption band with a wavelength above the isosbestic point likely arises from the main chain while the 370 nm shoulder band can be attributed to the side phenyl rings. This indicates the reduction of the conjugation length of the main chain and the relaxation of the IaSS of the side phenyl rings after annealing.19 Consequently, the FL enhancement can be attributed to a thermodynamically more twisted polymer chain. To investigate the FL enhancement of the PDPA-C1 film by thermal annealing treatment in more detail, other polymer derivatives (PDPA-C18, PNPA-C1, and PPHA in Figure 1) were examined for annealing-induced FL enhancement and compared to PDPA-C1. PDPA-C18 with a long alkyl side chain shows neither significant FL enhancement nor blue-shift, as was observed for PDPA-C1 (Figure 3a). Similarly, the UV−vis absorption spectrum does not show any significant change. This is likely due to the fact that the long alkyl chains serve as an internal plasticizer and already relax the IaSS sufficiently before annealing.17−19 On the other hand, PNPA-C1 with a naphthyl ring instead of the phenyl ring shows slightly more significant changes in the FL emission and UV−vis absorption spectra after annealing when compared to PDPA-C1 (Figure 3b). The FL emission band increases by a factor of 3.1 and shifts to a shorter wavelength by 9 nm, while the absorption band blue-shifts by 18 nm. This is probably because PNPA-C1 has a much greater intramolecular stacking cross section due to the presence of the naphthyl rings, and the conformational change by thermal annealing is more significant. As for PPHA where a hexyl group is substituted for the phenyl ring, no change is observed in the absorption spectra, and the FL emission decreases after annealing (Figure 3c). This is likely because PPHA does not have IaSS unlike the PDPA-C1 and

Figure 1. Chemical structures of the PDPA derivatives and the other disubstituted polyacetylenes used in this study.

(Figure S2). To investigate the effect of annealing temperature on the FL enhancement, polymer films were heat treated at different temperatures ranging from 50 to 300 °C with the annealing time fixed at 10 s. Figure 2b shows the FL emission spectra of the PDPA-C1 films after annealing at different temperatures. With higher temperature annealing, higher FL intensity was observed after the annealing process was completed. The FL changes can even be recognized by the naked eye (Figure 2c). As will be described in detail later, the tiny but distinct FL enhancement observed by heating even at the low temperature of 50 °C suggests that the FL change is not due to thermal decomposition and oxidation. Similar to the hydrodynamic FL emission behavior in the swollen state described previously, the FL enhancement and blue-shift of the PDPA-C1 film by the thermal treatment should be regarded as a photophysical phenomenon attributed to the relaxation of the IaSS of the side phenyl rings.20−23 In addition, the absorption band at 420 nm is shifted to shorter wavelength by about 10 nm after the annealing, and the shoulder band at 370 nm

Figure 2. (a) UV−vis absorption and FL emission spectra (excited at 420 nm) of the PDPA-C1 film (thickness ∼200 nm, spin-coated) before and after thermal annealing at 300 °C for 10 s. Variation of (b) FL emission spectra and (c) FL photographs after thermal annealing at different temperatures. B

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Figure 3. UV−vis absorption and FL emission spectra (excited at 420 nm) of (a) PDPA-C18, (b) PNPA-C1, and( c) PPHA films (thickness ∼200 nm, spin-coated) before and after thermal annealing at 250 °C (a, c) and at 300 °C (b) for 10 s. (d) FL photographs of PDPA-C18, PNPA-C1, and PPHA films before and after annealing.

Figure 4. (a) Isothermal TGA (at 300 °C) and (b) DSC thermogram (at rate of 10 °C min−1) of PDPA-C1, (c) FT-IR spectra, and (d) XRD patterns of PDPA-C1 before and after thermal annealing at 300 °C for 10 s.

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the PDPA-C1 film during the heating and cooling cycle over a temperature range from 20 to 100 °C. In the first cycle, the FL decreases upon heating and recovers via a different pathway upon cooling. This thermal hysteresis is also observed in the second cycle and shows a 10% increase in FL intensity after the third cycle. In terms of thermodynamics, this result strongly supports the notion that the polymer chain undergoes a certain conformational change; i.e., a transformation into a more twisted chain with loosened IaSS, as the molecular motion increases with heat. Owing to the limitations of our equipment, heating was permitted only up to 100 °C in this study. If the film can be heated to 300 °C, the thermal hysteresis is expected to be more significant even with a single heating and cooling cycle. Raman and electron spin resonance (ESR) spectroscopic analyses were performed to further investigate the conformational change considered to be the cause of the FL enhancement. Figure 6a shows the Raman spectra of the

PNPA-C1. In addition, PPHA is less resistant to heat and is more easily decomposed relative to the other polymer derivatives. Consistent with the spectral results, the FL change of PNPA-C1 was distinguished clearly by the naked eye but no change could be visually observed for the other two polymers (Figure 3d). Consequently, the IaSS of the side phenyl rings plays a critical role in the FL emission enhancement by thermal annealing. Figure 4a shows the isothermal thermogravimetric analysis (TGA) of PDPA-C1 during thermal annealing. No change in weight is observed during heating at 300 °C for 30 min, indicating a high resistance to heat. Also, the differential scanning calorimetry (DSC) thermogram does not show any thermodynamic phase transitions over a wide temperature range from room temperature to 300 °C (Figure 4b). The FTIR spectra are shown in Figure 4c. Absorption peaks appear from 3100 to 2850 cm−1 due to C−H stretching, 1250 cm−1 due to C−H bending, and 825 cm−1 due to Si−phenyl stretching before thermal annealing and is largely unchanged after the thermal treatment. These results suggest that PDPAC1 is very stable thermally and undergoes no chemical degradation, such as pyrolysis or oxidation, during thermal annealing. However, the thermal annealing effect induced a slight morphological change in the PDPA-C1. Figure 4d shows the X-ray diffraction (XRD) patterns before and after the annealing. A broad halo peak appears at a large angle of approximately 25° before and after annealing, indicating that the polymer is basically amorphous. A slight difference is seen in the smallangle region where a sharp peak appears corresponding to the intermolecular distance. The peak at 6.9° (12.8 Å) is shifted to a larger angle of 7.1° (12.5 Å) after annealing. This is likely because the polymer chain is more twisted due to the annealing as previously described for the UV−vis and FL spectra. PDPA derivatives are known to show significant exciton deconfinement upon heating relative to conventional conjugated polymers.28 This is due to the efficient vibrational relaxation of the main chain with increased heat, and as a result, the molecular perturbation may induce a local conformational change of the polymer chain. Therefore, if the FL change of PDPA-C1 is tracked in real time during the heating and cooling cycle, the conformational change may be estimated by the thermal hysteresis. Figure 5 shows the change in FL intensity of

Figure 6. (a) Raman and (b) ESR spectra of PDPA-C1 before and after thermal annealing at 300 °C for 10 s.

PDPA-C1 film before and after annealing. A broad peak at around 1448 cm−1 due to the trans CC bond disappears after annealing while two peaks at 1318 cm−1 corresponding to the cis C−C bond and at 1551 cm−1 corresponding to the cis C C bond increase after annealing. This indicates a trans−cis isomerization. From the ESR spectra, the radical concentration in a molecule can be determined from the peak intensity relative to the reference substance MnSO4, and the degree of electron spin localization of the radical can be estimated from the line width at maximum slope (ΔHmsl). According to previous studies on the isomerization of polyacetylene and

Figure 5. Changes in FL intensity of PDPA-C1 during heating and cooling cycles (excited at 420 nm, monitored at the FL maximum wavelength with a heating/cooling rate of 5 °C min−1). D

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substituted polyacetylenes during heating or pressing, ΔHmsl is closely related to the sequence of stereoisomers.29−35 As the trans sequence increases, the electron spin of the radical becomes delocalized. As a result, a wide range of defects are generated and less motional narrowing occurs, leading to a decrease in the ΔHmsl value. In contrast, as the cis sequence increases, the electron spin of the radical is localized, and the motional narrowing becomes more pronounced, increasing the value of ΔHmsl. Figure 6b shows the ESR spectra of the PDPAC1 film before and after thermal annealing. The peak intensity is reduced after thermal annealing, and the value of ΔHmsl is increased by 0.24 mT. Similar to the Raman results, this indicates that the proportion of cis sequence in the film is increased by thermal annealing. Consequently, the conformational change into a more twisted main chain is accompanied by trans−cis isomerization. In summary, we found that the PDPA-C1 film experiences very unusual, highly efficient FL emission enhancement through thermal annealing. The FL emission was significantly enhanced and shifted to shorter wavelength simply by heating at a temperature of 300 °C for only 10 s. This unique FL enhancement phenomenon was ascribed to the thermodynamic relaxation of the IaSS accompanied by the structural transformation into a more twisted chain and trans−cis isomerization through thermal annealing. This technique may be adapted to other conjugated polymers with IaSS. The results presented herein will be helpful for developing highly emissive conjugated polymers for further applications.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.K.). ORCID

Young-Jae Jin: 0000-0002-4670-4199 Giseop Kwak: 0000-0003-3111-0918 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program of National Research Foundation of Korea (NRF) grants funded by the Korea government (MEST) (2017R1A2B4007348 and 2017R1A6A3A11034225).

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REFERENCES

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Materials. The polymers used in this study were synthesized according to a method reported previously.36−38 The chemicals including solvents were purchased from Sigma-Aldrich and TCI Ltd. and used as received. Measurements. UV−vis absorption spectroscopy was performed on a JASCO V-650 spectrophotometer. The FL emission spectra were recorded on a JASCO FP-6500 spectrofluorometer equipped with a JASCO ETC-273 temperature controller. The FT-IR spectra were recorded on a JASCO FT/IR-4100 spectrometer equipped with a JASCO ATR (attenuated total reflectance) model PR0450-S. DSC (TA Instruments Q2000) was performed in a pure nitrogen atmosphere at heating and cooling rates of 10 °C min−1. TGA was conducted in a N2 atmosphere using a TA Instruments Q 600 thermal analyzer. XRD measurements were performed at room temperature using an X-ray diffractometer (PANalytical X̀ Pert PRO-MPD) at the Korea Basic Science Institute (Daegu). The samples were mounted directly into the diffractor. The experiment was performed using Cu Kα (1.54 Å) radiation operating at 40 kV and 25 mA. The photographs were taken using a digital camera (Sony Alpha 6000) equipped with a macro lens (Sony SEL30M35). The Raman spectra were recorded on a Thermo Almega X Raman spectrometer (Thermo Fisher Scientific Inc., USA) using an Ar+ 796 nm laser. The ESR spectra were recorded on an electron spin resonance spectrometer (Bruker, EMXplus-9.6/2.7)

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02338. Figures S1 and S2 (PDF) E

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