White-Light Emission and Tunable Luminescence Colors of Polyimide Copolymers Based on FRET and Room-Temperature Phosphorescence

Abstract

Thermally stable copolyimide (CoPI) films exhibiting high optical transparency and room-temperature phosphorescence (RTP) were prepared by copolymerizing fluorescent dianhydride and brominated phosphorescent dianhydride with an alicyclic diamine. The CoPI films underwent a 5 wt % degradation at a temperature higher than 349 °C and exhibited dual fluorescent and phosphorescent emissions owing to their efficient Förster resonance energy transfer from the fluorescent to phosphorescent dianhydride moieties in the main chains, followed by an intersystem crossing from the singlet to triplet state of the latter moiety atoms. The CoPIs displayed bright RTP under a vacuum with various colors produced when adjusting the copolymerization ratio. CoPI with 5 mol % phosphorescent moiety (CoPI-05) emitted white light with high optical transparency owing to the suppression of the PI chain aggregation that causes a yellowish coloration. The copolymerization of fluorescent and phosphorescent PI moieties can control the photoluminescent properties of PI films and is applicable to color-tunable solid-state emitters, ratiometric oxygen sensors, and solar-spectrum converters.

1. Introduction

Photoluminescent organic polymers, including polyfluorene, poly(3-alkylthiophene), and polyphenylenevinylene derivatives, have elicited significant interest owing to their potential application in color-tunable solid-state emitters for organic light-emitting diodes (OLEDs), ratiometric oxygen sensors, luminescence solar concentrators, photovoltaic devices, and flexible organic phototransistors.¹⁻¹¹ Organic polymeric materials possess excellent advantages, including low cost, low weight, flexibility, and superior processability when applied to solid thin films, but they lack adequate thermal, chemical, and/or environmental stability, as well as light resistance, owing to their π-conjugated sequences. Thermal, environmental, and mechanical stabilities are strongly desired for their application to luminescence solar-spectrum converters, which convert short-wavelength ultraviolet (UV) or violet-blue light into green, yellow, red, or near-infrared (IR) light with longer wavelengths.¹²⁻¹⁴

Polyimides (PIs) are a class of superengineering plastics and are widely known for their high thermal, environmental, and radiation stabilities, originating from their rigid repeating unit structures and strong intermolecular interactions. As a result, PIs have been applied in numerous fields, including the microelectronic, photonic, electronic, and aerospace industries. Owing to their photoluminescence (PL) properties and excellent performance, PI films have recently attracted significant interest as novel, thermally stable, and luminescent materials.^15,16

The PL quantum yield (Φ) of wholly aromatic PIs synthesized from aromatic dianhydrides and diamines is generally very low (Φ ≪ 0.01) owing to strong intra- and intermolecular charge-transfer (CT) interactions between the electron-donating diamine and the electron-accepting dianhydride moieties.^16,17

In relation to this property, the authors previously reported that a series of semialiphatic PI films exhibit significantly enhanced fluorescence emissions within the visible region owing to the suppression of the CT interactions through the introduction of alicyclic diamines.¹⁵ However, these fluorescent PIs exhibit relatively small Stokes shifts, namely, an energy gap between the excitation (absorption) and emission wavelengths; consequently, a notable enhancement of the Stokes shifts is needed for photoluminescent polymers applied to solar-spectrum converters. We also reported that PI films containing −OH groups at the pyromellitic dianhydride moiety in the main chain, or phthalimide termini, exhibit a large Stokes-shifted fluorescence originating from the excited state through an intramolecular proton transfer (ESIPT).¹⁸⁻²⁰

Room-temperature phosphorescence (RTP) in organic molecules has recently attracted significant attention owing to the resulting extremely large Stokes shifts and ultralong luminescence lifetime.^21,22 In general, phosphorescence is rarely observed at room temperature in air because it is easily deactivated by local molecular motion and energy transfer to oxygen during the ultralong lifetime of the excited triplet state.

There are two major strategies used to obtain RTP materials. One is to enhance the intersystem crossing (ISC) efficiency by introducing heavy atoms such as heavy halogens or metallic atoms and/or phenyl–carbonyl groups into the molecular structure. Such heavy atoms effectively enhance the ISC because of their large spin–orbit coupling (SOC), which is termed the heavy-atom effect.²³ Meanwhile, the incorporation of carbonyl groups facilitates ISC through the transitions allowed from ¹(n → π*) to ³(π → π*).²⁴ Another way is to reduce nonradiative processes in an excited state by suppressing the local molecular motion, either by cooling to extremely low temperatures or by applying high pressure. For this reason, most RTP materials have been studied under cryogenic conditions below the temperature of liquid nitrogen in transparent rigid matrices such as crystalline compounds or in host–guest systems.^25,26 However, such systems are impractical for use in solar-spectrum converters.

We previously reported the development of novel PIs exhibiting RTP by introducing heavy halogens (bromine, iodine) into the main chain structure.^27,28 These PIs exhibit reddish to bright green RTP with extremely large Stokes shifts of 10 000–12 200 cm^–1 through the irradiation of UV light, whereas PI films displaying RTP derived from 1,4-dibromo- and 1,4-diiodopyromellitic dianhydrides present intense yellowish colors owing to the absorption bands appearing within the visible region.²⁷ These absorption bands originate from their aggregation structures in solid films because their model compounds exhibiting bright RTP are completely colorless in a solution under visible light.

The copolymerization of two or more dianhydrides and diamines is a powerful and versatile way to control the physical properties of PIs. It has been reported that good solubility,²⁹ enhanced gas transport and separation properties,³⁰ low coefficients of thermal expansion,³¹ and other positive performance characteristics^32,33 have been achieved through PI copolymerization. In this study, we describe the design and synthesis of new copolyimides (CoPIs) combining fluorescent and phosphorescent dianhydrides in the development of a series of highly transparent PI films exhibiting RTP with large Stokes shifts and a tunable PL color.

An energy transfer generally occurs during an excited state between two chromophores when they are mixed and energetically resonated. The Föster-type resonance energy transfer (FRET) process, which is dependent on the size of the overlap between the emission spectra of the energy donor and the absorption spectra of the energy acceptor, has been widely used as a fluorescent probe for detecting changes in pH³⁴ and temperature,³⁵ as well as the presence of specific substances.³⁶ Furthermore, FRET has been applied toward the development of white-light-emitting materials.^37,38

Single-molecule white-light-emitting materials have been developed using dual-emission mechanisms such as a monomer/excimer complex,^39,40 ESIPT,^41,42 thermally activated delayed fluorescence,⁴³ and phosphorescence.⁴⁴ These materials have demonstrated good stability, high reproducibility, and can be fabricated via a simple process, compared to white-light-emitting materials obtained through the mixing of plural luminophores, which emit different colors. However, the development of white-light-emitting materials through FRET is difficult because precise control of the combination between the energy donor and energy acceptor is required.

In this study, we designed and synthesized a series of CoPIs exhibiting high thermal stability, high optical transparency, and phosphorescence emissions by copolymerizing 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 1,4-dibromopyromellitic dianhydride (DBrPMDA) as dianhydrides with 4,4′-diaminocyclohexylmethane (DCHM) as a diamine together with a model compound derived from DBrPMDA (see Schemes 1–3). In addition, we attempted to control the PL color of the CoPIs, while maintaining high quantum yields, by adjusting the copolymerization ratio (see Scheme 4).

Scheme 1. Synthesis Route of Brominated Pyromellitic Dianhydride (DBrPMDA).

Scheme 3. Synthesis Scheme for DBr-PI.

Scheme 4. Synthesis Scheme of Copolyimide.

2. Results and Discussion

2.1. Optical Properties of DBr-MC Imide Compound

Syntheses of DBrPMDA dianhydride and imide model compound (DBr-MC) are shown in Schemes 1 and 2.

Scheme 2. Synthesis Route of Imide Model Compound (DBr-MC).

Figures 1a and S1 show the UV–vis absorption and PL emission spectra for DBr-MC imide compound dissolved in CHCl₃. The DBr-MC solution exhibits sharp and strong absorption bands at 240 and 420 nm, respectively, as well as a weak broad band at 520 nm, and sharp and narrow emission bands at approximately 459 and 640 nm. These peak intensities proportionally increase with an increase in the concentration of DBr-MC in the solution (Figure S1). According to time-dependent density-functional theory (TD-DFT) calculations, the absorption bands at 240 and 420 nm are mainly attributable to the π → π* transitions of HOMO – 1 → LUMO and HOMO – 1 → LUMO + 1, respectively (Figures S2 and S3). Each of these π-orbitals is located on the DBr dianhydride moiety, and the nonbonding HOMO does not contribute to any transitions appearing within the UV and visible regions. The weak absorption band at approximately 520 nm may have been caused by the aggregated molecules of DBr-MC in the solution because according to the TD-DFT result, an absorption band is not expected to appear at wavelengths longer than 400 nm.

Figure 1.

(a) UV–vis absorption (solid line)/emission (dotted line) spectra of a model compound DBr-MC in CHCl₃. The excitation wavelength for the emission spectrum is 420 nm. (b) Excitation (solid line)/emission (dotted line) spectra of solid DBr-MC. The excitation wavelength for the emission spectrum is 369 nm, and the emission wavelength for the excitation spectrum is 609 nm.

By contrast, the emission peaks can be attributable to the fluorescence and phosphorescence of DBr-MC owing to their small and large Stokes shifts (2023 and 8185 cm^–1), respectively.²⁷ In general, the phosphorescence lifetime (on the order of micro- or milliseconds) is much longer than the fluorescence lifetime (order of pico- or nanoseconds).^21,22 However, as shown in Figure S4, the luminescence lifetimes of the DBr-MC solution at two peaks (440 and 640 nm) are estimated to be on the order of nanoseconds, i.e., 1.35 and 7.53 ns, respectively. The phosphorescence lifetime in the solution is frequently shortened by molecular motion and energy transfer to oxygen. Because the luminescence lifetime of the DBr-MC solution at 640 nm does not change after bubbling with nitrogen, these results suggest that vigorous molecular motions in the solution destabilize the excited triplet state and induce a nonradiative process.¹⁹ By contrast, when excited at 521 nm, which is close to the weak absorption band at approximately 520 nm, a small Stokes-shifted emission can be observed at approximately 618 nm (Figure S5). This peak is attributable to the fluorescence from the aggregated forms and can be seen even at a low concentration (5 × 10^–6 M) (Figure S6a). Figure S6b shows the dependence of the fluorescence, phosphorescence, and aggregation fluorescence intensities on the concentration of DBr-MC in CHCl₃, in which the luminescence intensities at 5 × 10^–6 M are considered as unity. All luminescence intensities increase proportionally at the same rate as the increase in concentration, which suggests that a significant intermolecular interaction exists in the aggregated forms even at 5 × 10^–6 M.

Figure 1b shows the excitation and emission spectra of DBr-MC as a powdery solid at room temperature, where a broad excitation band can be observed between 320 and 560 nm and only a large Stokes-shifted emission band is evident at approximately 610 nm. When excited at 555 nm, a small Stokes-shifted emission can be seen at 609 nm (Figure S7). Considering that the luminescence lifetime at 610 nm is estimated to be 5.64 ns (Figure S8a), and a weak fluorescence appears at approximately 610 nm in a solution when excited at 521 nm, this peak is attributable to the fluorescence occurring from the aggregated forms, which indicates that DBr-MC forms a significant amount of aggregates in a solid. The highly planar structure of the DBr dianhydride moiety and the electron-withdrawing nature of the bromine atoms may promote aggregate formation.²⁷ In addition, the PL quantum yield of DBr-MC in a solid (Φ = 0.13) is lower than that of DBr-MC in a solution (Φ = 0.41) (Figure 1), which is due to the aggregation-induced quenching of the PL through an intermolecular energy transfer. To examine the nature of the phosphorescence, the phosphorescence lifetime was measured at room temperature. As shown in Figure S8b, the lifetime of DBr-MC in a solid is estimated to be 1.32 ms, which indicates that DBr-MC shows a weak RTP. In the time-resolved spectra of DBr-MC in a solid (Figure S11b), the luminescence peak at 610 nm was not shifted during the duration of the measurement, which supports the idea that the RTP intensity is much lower than that of the fluorescence from the aggregated forms.

2.2. Optical Properties of Homopolyimide (DBr-PI) Film

A synthesis of homopolyimide derived from DBrPMDA dianhydride and DCHM diamine (DBr-PI) is shown in Scheme 3, and the thermogravimetric analysis (TGA) curve of a DBr-PI film is shown in Figure S9. The thermal decomposition temperature at 5% weight loss (T_d⁵) is 328 °C for the DBr-PI film, which indicates that this film has adequate thermal stability.

Figure 2a shows the UV–vis absorption spectrum of the DBr-PI thin film. Unlike in the case of DBr-MC, the absorption peaks attributable to isolated molecular chains cannot be clearly seen for the DBr-PI film, although an intense broad absorption band appears at 450–550 nm. Note that this band does not appear in the DBr-MC solution (Figure 1a), which indicates that the absorption peaks at 400 and 500 nm are also attributable to the isolated PI chains and densely packed aggregated PI chains, respectively.

Figure 2.

DBr-PI film: (a) UV–vis absorption spectrum and (b) excitation/emission spectra. The excitation wavelength for the emission spectrum is 397 nm, and the emission wavelength for the excitation spectrum is 600 nm.

Figure 2b shows the excitation/emission spectra for a DBr-PI thin film. DBr-PI shows excitation bands at approximately 400 and 500 nm, which are attributable to isolated PI chains and densely packed aggregated forms, respectively. When excited at 400 nm, which corresponds to the excitation of the isolated repeating units, a large Stokes-shifted broad emission can be observed at approximately 600 nm. By contrast, when excited at 510 nm, which corresponds to the excitation of the aggregate forms, the emission band observed at 580 nm is attributable to fluorescence from the aggregates owing to the relatively small Stokes shift (ν = 2366 cm^–1) (Figure S10).

To examine the origins of these emission peaks, the phosphorescence spectra and phosphorescence lifetimes are separately measured under a vacuum. For the phosphorescence spectra, longer-lifetime PL emissions ranging from 1 to 24 ms after excitation were selectively detected by blocking the shorter-lifetime emissions using an optical chopper at a frequency of 40 Hz.

Figure 3a shows the phosphorescence spectra of a DBr-PI film. Under atmospheric conditions, DBr-PI demonstrates no emissions across the entire wavelength range. By contrast, DBr-PI shows an apparent emission peak at 630 nm under a vacuum, which does not coincide with the peak positions observed in air (Figure 2b). As indicated in Figure 3b, the lifetime of the DBr-PI film measured under atmospheric and vacuum conditions is estimated to be on the order of 0.71 and 1.51 ms, respectively, which strongly suggests that the emission peak at 630 nm is attributable to RTP.

Figure 3.

DBr-PI film: (a) phosphorescence spectra (excitation wavelength: λ_ex = 345 nm) and (b) phosphorescence decay curves under atmospheric and vacuum conditions (λ_ex = 340 nm, emission wavelength: λ_em = 630 nm).

Although an RTP peak cannot be observed in the phosphorescence spectra, DBr-PI exhibits extremely weak RTP under atmospheric conditions. Thus, the emission peak at 600 nm observable in the excitation/emission spectra (Figure 2b) consists of RTP emitted from the isolated PI chains at 630 nm, overlapping with the fluorescence of the aggregated forms at 580 nm. The time-resolved spectra of the DBr-PI film support the view (Figure S11a) that the luminescence peak is red-shifted from 570 to 630 nm during the measurement time. These results clearly indicate that an efficient excitation energy transfer, or FRET, occurs from the isolated PI chains to the aggregated PI chains in competition with the transfer from the ISC to the excited triplet state.

A similar phenomenon can also be observed for the newly designed PI, which emits prominent reddish-orange fluorescence through the ESIPT occurring at the hydrogen-bonded moiety.^19,20 We demonstrated that FRET from the enol form to the aggregate form is in ongoing competition with the ESIPT process to reach the keto form during the early stage upon excitation of the enol form. A schematic energy-state diagram and the photophysical processes for the excited states of a DBr-PI film are shown in Figure 4.

Figure 4.

Schematic energy-state diagram and illustration of the photophysical processes occurring during DBr-PI film excited states.

2.3. Optical Properties of Copolyimide (CoPI) Film

Synthesis and the TGA curves of the CoPI film are shown in Scheme 4 and Figure S12, respectively. The thermal decomposition temperatures at 5% weight loss (T_d⁵) were estimated to be 349, 369, 366, and 374 °C for the CoPI-01, CoPI-05, CoPI-10, and CoPI-20 films, respectively, which indicates that these CoPIs possess sufficiently high thermal stability.

Figure 5 shows the UV–vis absorption spectra for thin films of BP-PI and DBr-PI and for CoPI containing the 5 mol % DBr moiety (CoPI-05). BP-PI shows an absorption band at shorter than 370 nm, whereas DBr-PI shows absorption bands at 420 and 500 nm. By contrast, CoPI-05 shows three absorption bands at 370, 400, and 500 nm, which are readily attributable to the absorptions of the BP-PI moiety, the isolated chain of the DBr-PI moiety, and the aggregated forms of the DBr-PI moiety, respectively. Note that the absorbance at 500 nm (Abs₅₀₀) of the CoPI-05 (0.0406) film was much smaller than that of DBr-PI (1.0911) owing to suppression of the aggregation through copolymerization.

Figure 5.

UV–vis absorption spectra for BP-PI, DBr-PI, and CoPI-05 thin films.

We estimated the relative fractions of the aggregated forms in reference to DBr-PI, which was taken as unity. The relative fraction of the aggregate of CoPI-05 was estimated to be 3.7%, which is smaller than the molar content of the DBr-PI moiety (5%). This result indicates that the optical transparency of CoPI films can be significantly enhanced by copolymerization with a highly transparent PI. Figure 6a shows the excitation/emission spectra for BP-PI, DBr-PI, and CoPI-05 thin films measured at room temperature, and Table 1 summarizes the excitation and emission wavelengths (λ_ex, λ_em), Stokes shifts (ν), and photoluminescence quantum yield (Φ) of the BP-PI, DBr-PI, and CoPI-05 films.

Figure 6.

(a) Excitation/emission spectra of BP-PI, DBr-PI, and CoPI-05 thin films. The excitation wavelengths for the BP-PI, DBr-PI, and CoPI-05 emission spectra (λ_ex) are 360, 390, and 350 nm, respectively, and the emission wavelengths for the excitation spectra (λ_em) are 411, 600, and 550 nm, respectively. (b) Excitation (λ_em = 550 nm) and emission (λ_ex = 495 nm) spectra for CoPI-05 thin films.

Table 1. Photoluminescence (PL) Properties, Excitation and Emission Wavelengths (λ_ex, λ_em), Stokes Shifts (ν), and Quantum Yields (Φ) of BP-PI, DBr-PI, and CoPI-05 Thin Films.

	λexa/nm	λemb/nm	ν /cm–1	Φ
BP-PI	362	411	3293	0.14
DBr-PI	397	600	8522	0.02
		640	9563
CoPI-05	346	418	4978	0.05
		549	10686
	495	550	1979

^aExcitation wavelength for measuring emission spectra.

^bMonitoring wavelength of emission for excitation spectra.

When the BP-PI moiety is excited at 346 nm, CoPI-05 exhibits two emission peaks at λ_em = 418 and 549 nm. The former is readily attributable to fluorescence from the BP-PI moiety because it coincides with the BP-PI thin film fluorescent wavelength (411 nm). By contrast, the latter peak is attributable to fluorescence from the aggregated forms of the DBr-PI moiety because when the aggregates are excited at 495 nm, a small Stokes-shifted emission (ν = 1979 cm^–1) can be observed at the same wavelength of 550 nm (Figure 6b). These results indicate that an efficient FRET process occurs from the excited state of the BP-PI moiety to the aggregated DBr-PI chains either directly or through the isolated DBr-PI chains.

It should be noted that CoPI-05 does not exhibit phosphorescence from the DBr-PI moiety in the steady-state spectra, although it does exhibit fluorescence from the aggregates. To clarify this phenomenon, the phosphorescence lifetime was measured under a vacuum. Figure 7a shows the phosphorescence spectra of the CoPI-05 film. Under atmospheric conditions, CoPI-05 shows no phosphorescence peak across the entire range, whereas under a vacuum, a broad and apparent emission peak can be observed at 630 nm, which coincides well with the RTP in the phosphorescence spectra of DBr-PI (∼630 nm) (Figure 3a). As shown in Figure 7b, the phosphorescence lifetimes for a CoPI-05 film under atmospheric and vacuum conditions are estimated to be on the order of 1.2 and 2.9 ms, respectively. These results indicate that an efficient FRET occurs, from the excited state of the BP-PI moiety to the isolated DBr-PI chains, followed by an ISC, from the singlet to the triplet state of the DBr-PI moiety, owing to the heavy-atom effects of the bromines. The presence of an efficient FRET mechanism is supported by the fact that the isolated DBr-PI (397 nm) absorption wavelength coincides well with the BP-PI fluorescence wavelength (411 nm) (Figure S13). A schematic energy-state diagram and the photophysical processes in CoPI film excited states are shown in Figure 8.

Figure 7.

CoPI-05 film: (a) phosphorescence spectra (λ_ex = 360 nm) and (b) phosphorescence decay curves under atmospheric and vacuum conditions (λ_ex = 340 nm, λ_em = 630 nm).

Figure 8.

Schematic energy-state diagram and photophysical processes during excited states of CoPI films.

2.4. Control of Luminescence Colors

For the CoPI films, it was confirmed that an efficient FRET occurs from the BP-PI moiety to the isolated DBr-PI moiety; hence, we attempted to control the luminescence colors of the CoPI film by adjusting the copolymerization ratio between the two dianhydrides. Figure 9 shows the UV–vis absorption spectra for CoPI films, and Table 2 summarizes their absorbances at 500 nm (Abs₅₀₀) and the relative fractions of the aggregated forms (see above). The CoPIs showed three absorption bands at shorter than 370, 400, and 500 nm, which are attributable to those of the BP-PI moiety and the isolated chains and aggregated forms of the DBr-PI moiety, respectively. As shown in Figure 9, although the CoPI films maintain a high degree of transparency, even when the DBr-PI moiety ratio reaches up to 20 mol %, they show pale to bright orange colors, originating from the absorption band of the aggregated forms with the increased DBr-PI moiety.

Figure 9.

UV–vis absorption spectra of CoPI and BP-PI films.

Table 2. Relative Fractions of DBr-PI and CoPI Films.

	Abs500	relative fraction/%
DBr-PI	1.0911	100
CoPI-20	0.1814	17
CoPI-10	0.1296	12
CoPI-05	0.0406	3.7
CoPI-01	0.0030	0.27

Figure 10 shows the PL spectra and CIE coordinates⁴⁵ of CoPI films when irradiated using a hand-held UV lamp (254–365 nm). In addition, the photophysical properties of the CoPIs and BP-PI are summarized in Table 3, and Figures S14,7b, and S15 show the fluorescence and phosphorescence decay curves for the CoPI-01, CoPI-05, CoPI-10, and CoPI-20 films. Based on the average fluorescence lifetime (τ_FL), as evaluated from Figure S14, the FRET efficiency (E_FRET) was estimated using eq 1,(46,47) where τ_DA is the fluorescence lifetime of the BP-PI moiety in the CoPIs and τ_D is the lifetime of the BP-PI fluorescence of the BP-PI film.

1

Figure 10.

Emission spectra and CIE coordinates for CoPI films. The CIE coordinates of the CoPI-01, CoPI-05, CoPI-10, and CoPI-20 films are (0.189, 0.133), (0.284, 0.301), (0.420, 0.420), and (0.466, 0.467), respectively.

Table 3. Photophysical Properties of CoPIs and BP-PI^a.

	λem/nm	Φ	τFL/ns	τPH/ms (atmospheric)	τPH/ms (vacuum)	EFRET/%
CoPI-20	409	0.001	0.16	0.42	1.38	94
	576	0.042
CoPI-10	421	0.0024	0.51	1.29	3.62	82
	576	0.041
CoPI-05	418	0.010	1.05	1.13	3.48	63
	549	0.038
CoPI-01	417	0.036	2.05	0.90	3.74	29
	540	0.023
BP-PI	411	0.14	2.87

^aExcitation wavelength (λ_em), fluorescence quantum yield (Φ), fluorescence lifetime (τ_FL), phosphorescence lifetime (τ_PH), and FRET efficiency (E_FRET).

Figure S16 shows the phosphorescence spectra of the CoPI-01, CoPI-10, and CoPI-20 films under atmospheric and vacuum conditions. No emission bands can be seen across the entire wavelength range in air, but an apparent emission peak appears at approximately 630 nm under vacuum conditions, which indicates that an efficient ISC occurs from the singlet to the triplet states of the DBr-PI moiety, as with the CoPI-05 film. As Table 3 indicates, the phosphorescence lifetime (τ_PH) of CoPI-20 is significantly shorter than that of the other PI films. This result suggests that the triplet excitons are rapidly quenched through the energy transfer among the intra- or intermolecular chains, which is promoted by an increase in the aggregated forms with an increase in the copolymerization ratio of the DBr-PI moiety.

As the copolymerization ratio of the DBr-PI moiety in the CoPIs increases from 1 to 20%, E_FRET drastically increases from 29 to 94% because, as the average distance (r) between the energy donor (BP-PI moiety) and the energy acceptor (DBr-PI moiety) decreases, the FRET rate constant (k_FRET), which is inversely proportional to r⁶, significantly increases. The relative intensities of the BP-PI and DBr-PI moiety emission peaks also dramatically change, as shown in Figure 10. When the luminescence colors of the CoPIs are evaluated based on the CIE coordinates, CoPI-01, CoPI-05, CoPI-10, and CoPI-20 show different colors, ranging from blue, white, and yellow to orange, respectively. Note that the colors are tunable and that a white color emission is successfully obtained when the DBr-PI moiety content is adjusted to only 5%.

These results indicate that the introduction of an efficient FRET mechanism into the CoPIs, derived from the copolymerization of fluorescent and brominated RTP dianhydride with an alicyclic diamine, is a versatile and effective way to control the luminescence colors because the spectral shapes of the PL strongly depend on the copolymerization ratio.

3. Conclusions

To develop and analyze the photophysical processes of highly transparent and RTP PI films, a novel PI (DBr-PI) with two Br atoms at the pyromellitic moiety was synthesized. Although the DBr-PI thin film showed phosphorescence properties at room temperature, a strong absorption band within the visible region was also observed, similar to the RTP PIs reported in our previous study. To overcome this problem, we synthesized new CoPIs and investigated their optical absorption and photoluminescent properties. The new CoPIs display a high transparency even when the DBr-PI moiety ratio reached up to 20%, owing to a suppression of the aggregation formation causing film coloration. In the luminescence spectra, the CoPIs show two emission peaks, which are attributable to the fluorescence of the BP-PI moiety and luminescence of the DBr-PI moiety. In addition, the CoPIs demonstrate RTP under a vacuum, which occurs owing to the efficient FRET mechanism from the excited singlet state of the BP-PI moiety to the excited singlet state of DBr-PI, followed by ISC from the singlet to triplet state of the DBr-PI moiety, owing to the heavy-atom effect of the bromines. The CoPIs exhibit clearly different photoluminescent colors, which can change from blue, white, and yellow to orange when adjusting the copolymerization ratio. In conclusion, by utilizing the efficient FRET mechanism effect in the CoPIs, we successfully developed thermally stable and highly transparent RTP PIs with controlled optical properties, such as tunable luminescent colors, using a copolymerization technique. Such CoPIs are promising for light-emitting materials applicable to color-tunable solid-state emitters, ratiometric oxygen sensors, and solar-spectrum converters.

4. Experimental Section

4.1. Materials

In this study, BPDA, provided by Ube Industries, Ltd., was dried at 270 °C for 5 h under reduced pressure. Durene (1), purchased from Tokyo Kasei Co., Ltd, was used as received. Cyclohexylamine, purchased from Kanto Chemical Co., Inc, was purified through distillation under reduced pressure. In addition, DCHM, purchased from Tokyo Kasei Co., Ltd, was purified through recrystallization from n-hexane and subsequent sublimation under reduced pressure. Finally, N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA, ≥99%) and N,N-dimethylacetamide (DMAc, anhydrous) were purchased from Sigma-Aldrich and used as received.

4.2. Synthesis and Film Preparation

4.2.1. Brominated Pyromellitic Acid

The synthesis of brominated pyromellitic dianhydride (DBrPMDA) is shown in Scheme 1. Durene (1) (10 g, 74.5 mmol) and iodine (0.4 g, 1.58 mmol) were dissolved in dichloromethane (60 mL), following which bromine (10 mL) dissolved in dichloromethane (40 mL) was slowly added to the solution, which was then refluxed at 40 °C for 2 h. After the solution was cooled to room temperature, an aqueous sodium hydroxide solution (5 N, 100 mL) was added and stirred for 20 min. When the solution had cooled to 4 °C, the precipitate was filtered and then dried at 70 °C for 3 h under a vacuum; compound 2 was then obtained (19.39 g, 89% yield). For ¹H NMR (400 MHz, DMSO-d₆, ppm), δ = 2.50 ppm (s, 12H).

Compound 2 (5.0 g, 17.1 mmol) was dissolved in pyridine (250 mL), following which water (50 mL) and potassium permanganate (7.5 g, 47.5 mmol) were added to the solution, which was refluxed at 100 °C. Every 2 h, 7.5 g of potassium permanganate was added, and this procedure was repeated five times. After the fifth addition, the solution was refluxed for an additional 20 h, cooled to room temperature, and then filtered to remove any deposited manganese dioxide. The products contained in the filter were recovered by washing with a calcium hydroxide solution. The filtrate was concentrated using a rotary evaporator and dissolved in water (50 mL). Potassium permanganate (15 g, 95.0 mmol) was added to this solution and refluxed at 100 °C for 20 h. After 20 h, 2-propanol was added to react with the residual potassium permanganate. The solution was then cooled to room temperature and filtered to remove the deposited manganese dioxide. The filtrate was concentrated with a rotary evaporator and acidified with concentrated aqueous hydrochloric acid (HCl) to pH 1. The precipitate was then filtered and dried at 95 °C for 3 h under a vacuum. The white powder was purified through recrystallization from an aqueous HCl solution, and compound 3 was obtained (2.34 g, 5.68 mmol, 33% yield). For ¹H NMR (400 MHz, DMSO-d₆, ppm), no ¹H NMR signal was detected because compound 3 only has active hydrogen atoms.

4.2.2. Brominated Pyromellitic Dianhydride (DBrPMDA)

Compound 3 (0.43 g, 1.04 mmol) was heated and maintained at 180 °C for 4 h under reduced pressure for dehydration, yielding a bright reddish powder of DBrPMDA (DBr) (0.34 g, 0.90 mmol, 87% yield).

4.2.3. Brominated Imide Model Compound (DBr-MC)

The synthetic scheme for an imide model compound (DBr-MC) is shown in Scheme 2. A precursor of the imide compound was prepared using an in situ silylation method. Next, 6,7-cyclohexylamine (1.06 g, 10.7 mmol) and BSTFA (2.87 g, 11.2 mmol) were stirred in DMAc (5.3 mL) for 30 min (solution I), and DBr (2.0 g, 5.32 mmol) was stirred into DMAc (10.6 mL) for 5 min (solution II). Solution I was mixed with solution II and stirred overnight in an ice bath. Xylene (10 mL) was then added, and the mixture was refluxed at 150 °C for 6 h under a N₂ flow. After cooling to room temperature, the red solid reprecipitated by excess water was filtered and then dried at 90 °C under a vacuum. A red powder of DBr-MC was then obtained (100 mg, 0.19 mmol, 3.6% yield) through recrystallization from 1,4-dioxane (50 mL).

4.2.4. Preparation of Homopolyimide Film

The PI (DBr-PI) synthesis is shown in Scheme 3. A PI precursor, poly(amic acid) silyl ester, was also prepared using an in situ silylation method. DCHM (0.15 g, 0.71 mmol) and BSTFA (0.19 g, 0.74 mmol) were stirred into DMAc (2.3 mL) for 30 min. The reason for adopting the in situ silylation method with BSTFA is to increase the solubility of the precursor in DMAc. The solubility of poly(amic acid)s derived from DBrPMDA could be lower than those from other dinahydrides due to the preferred formation of aggregates of DBrPMDA moiety. Compound DBr (0.27 g, 0.71 mmol) was added to the solution and stirred for 2 days. The resulting viscous orange solution was spin-coated onto a fused silica (amorphous SiO₂) substrate, followed by soft-baking at 70 °C for 50 min and a subsequent one-step thermal imidization until reaching the final curing conditions of 220 °C at a heating rate of 3 °C/min under a N₂ flow. After maintaining the coating at 220 °C for 1.5 h, the DBr-PI film was cooled to room temperature.

In a similar manner, reactions of BPDA (0.21 g, 0.71 mmol) mixed with DCHM (0.15 g, 0.71 mmol) and BSTFA (0.19 g, 0.74 mmol) in DMAc (2 mL) yielded a BP-PI film.

4.2.5. Preparation of Copolyimide Films (CoPIs)

The CoPI synthesis scheme is shown in Scheme 4. The PI precursor, a poly(amic acid) silyl ester, was prepared using the in situ silylation method. DCHM (0.03 g, 0.14 mmol) and BSTFA (0.0378 g, 0.15 mmol) were stirred into DMAc (0.4 mL) for 30 min. Compound DBr (0.027 g, 0.07 mmol) was added to the solution, and after stirring for 5 min, BPDA (0.4025 g, 1.37 mmol) and a solution containing the residual DCHM (0.27 g, 1.28 mmol), BSTFA (0.34 g, 1.32 mmol), and DMAc (3.6 mL) were added and stirred for 2 days. The resulting yellow viscous solution was spin-coated onto a fused silica substrate, followed by soft-baking at 70 °C for 50 min and a subsequent one-step thermal imidization procedure; the final curing conditions were 220 °C for 1.5 h under a N₂ flow. Heating from 70 to 220 °C was conducted at a rate of 3.0 °C/min, and the CoPI-05 film was cooled to room temperature.

CoPI-01, CoPI-10, and CoPI-20 films were prepared in the same manner using different DBr molar ratios. The thickness of the CoPI films was controlled to 5 μm by adjusting the spin-coating rate.

4.3. Measurements

4.3.1. UV–Vis Absorption and Excitation/Emission Spectra

The concentrations of the model compounds in chloroform (CHCl₃) were set within the 10^–5–10^–4 M range. The solvent, CHCl₃ (99.9%, Kanto Chemical Co. Inc., fluorescence grade), was used without further purification. Ultraviolet–visible (UV–vis) absorption and PL excitation/emission spectra were measured separately at room temperature using a JASCO V-760 spectrophotometer (JASCO Co., Tokyo Japan) and a Hitachi F-7100 fluorescence spectrometer (Hitachi High-Technologies Co., Tokyo, Japan) equipped with an R928 photomultiplier tube (Hamamatsu photonics Co., Japan), respectively. The front-face method was adopted for the film samples to reduce the self-absorption of the emitted luminescence.

The phosphorescence spectra were measured using the same fluorescence spectrometer in the phosphorescence mode, in which a quasi-excitation pulse was generated from a continuous-wave (CW) light source using an optical chopper. The excitation pulse width (duration) was 2 ms, and the repetition rate was 40 Hz (one excitation pulse was followed by a 25 ms interval). After the excitation beam was blocked by the optical chopper, the shutter of the detector was opened after a duration of 1 ms and phosphorescence signals were collected for 23 ms.

PL quantum yields were measured using a calibrated integrating sphere (C9920, Hamamatsu) connected to a multichannel analyzer (C7473, Hamamatsu) using an optical fiber link.

4.3.2. Time-Resolved Luminescence Measurements

Fluorescence lifetime measurements with a lower time resolution of 1 ns were conducted using a fluorescence lifetime measurement system (Quantaurus-Tau, C11367–03, Hamamatsu Photonics, Japan) at room temperature. The decay component was recorded using excitation by applying a flashing light-emitting diode (LED) light at a wavelength of 340 nm. Fluorescence decay curves were accumulated until the peak intensity reached 1000. The phosphorescence lifetimes were measured using a xenon flash lamp unit (C11567-02, Hamamatsu). The decay component was recorded under excitation using a band pass filter (340 ± 10 nm), and phosphorescence decay curves were accumulated for 5 min. The emission decay was well fitted using one to three exponential functions. The average lifetime was calculated as ⟨τ⟩ = ∑A_i τ_i²/∑A_i τ_i, where A_i is the pre-exponential factor for lifetime τ_i.

4.3.3. Other Measurements

The ¹H NMR spectra were measured using a JEOL AL-400 spectrometer operating at a ¹H resonance frequency of 400 MHz. The chemical shifts were calibrated in ppm (δ_H) using tetramethylsilane (TMS) as the standard (0 ppm). Thermogravimetric analysis (TGA) was conducted using a Shimadzu TGA-50 analyzer with a heating rate of 5 °C/min under a N₂ flow.

4.4. Quantum Chemical Calculations

Density-functional theory (DFT) calculations were conducted using Gaussian-16 software (RevA.03),⁴⁸ as described in our previous studies.^49,50 The structure of a model compound (DBr-MC) was optimized at the B3LYP/6-311G(d,p) level, followed by calculations of one-electron transitions at the B3LYP/6-311++G(d,p) level using the time-dependent DFT (TD-DFT) method. Each calculated transition was replaced by a Gaussian broadening function with a width of 0.12 eV, producing the shapes of the experimental spectra. The calculated absorbance was represented by the oscillator strengths divided by the van der Waals volumes of the molecules. The van der Waals volumes were calculated from the optimized geometries using Slonimski’s method,⁵¹ in which the van der Waals radii of atoms reported by Bondi⁵² were applied.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c01949.

• Photoluminescence decay curves of DBr-MC in a CHCl₃ solution and in a solid state; photoluminescence emission spectra of DBr-MC in a CHCl₃ solution and in a solid state; TGA curves of DBr-PI and CoPI films; phosphorescence spectra of CoPI films when excited at 360 nm; phosphorescence decay curves of CoPI films under atmospheric and vacuum conditions (PDF)

The authors declare no competing financial interest.