Self-assembled polycarbazole microspheres as single-component, white-colour resonant photoemitters

Abstract

Self-assembled polycarbazole (PCz) microspheres exhibit whispering gallery mode photoluminescence (PL), where resonant PL lines appear at the whole visible spectral range. The ultra-wide-range PL results from a partial oxidation of PCz upon strong photoexcitation. The single component micro-photoemitters, preparable using a simple and inexpensive process, are applicable as full-colour, narrow-width light emission sources.

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White-colour photoemitters with high brightness and low fabrication cost are required for a practical use of light sources.¹ A sufficient condition to display white-colour photoluminescence (PL) is that the PL band covers a wide wavelength range over the visible region (400–800 nm) like sunlight. However, for organic molecules and polymers, the PL line-width is generally in the wavelength range of ca. 50–100 nm. Therefore, binary (complementary colour) or ternary (red, green and blue colour) fluorophores are utilized for realizing white-colour luminescence.¹ Therein, a delicate optimization of the partial donor-to-acceptor energy transfer by changing the mixing ratio and the mixed state of the fluorophores is requisite. Such fine optimization of the mixing is not necessary if a single component material exhibits white PL.²

Optical microcavities play important roles for sharpening and amplifying emission.³ Lasers are widely utilized as highly monochromatic, directional, and coherent light sources. Recently, white-colour laser is demonstrated from monolithic inorganic crystals.⁴ Nevertheless, the fabrication process is rather complicated with a difficulty in controlling the devices with micrometer-scale accuracy. Therefore, multi-mode microresonators with a sharp emission lines in a wide spectral range will be useful not only for high brightness white-colour light sources but also for multi-colour sharp emitters by sorting the desired emission lines.

In this communication, we report on white-colour whispering gallery mode (WGM) photoemitters from single-component polycarbazole (PCz) microspheres. A simple self-assembly process prepares the micrometer-scale polymeric resonators. Upon weak photoexcitation with stationary light, the PCz microspheres show blue PL. In contrast, strong photoexcitation by a focused laser beam to a single microsphere results in sharp and periodic PL lines from whole wavelength range of the visible region (400–800 nm). The single component white-colour resonant micro-photoemitters are beneficial for low-cost, multi-colour and high brightness light sources.

The PCzs we used in this study are poly(2,7-carbazole) derivatives P1 and P2 (Fig. 1a), which are synthesized by Yamamoto coupling reaction from the corresponding 2,7-dibromocarbazole precursors (see ESI† and ref. 5). The wavelengths of the photoabsorption maxima (λ_abs) of P1 and P2 in CHCl₃ are 391 and 383 nm, respectively, while those of the PL maxima (λ_em) in CHCl₃ are 420 and 418 nm with PL quantum yield (ϕ_PL) of 0.80 and 0.78, respectively (Table S1, ESI†).

Fig. 1 (a) Molecular structures of poly(2,7-carbazole) P1 and P2. (b and c) SEM micrographs of self-assembled microspheres of P1 (b) and P2 (c). Insets show histograms of d of the microspheres.

Self-assembly of P1 and P2 was carried out by vapour diffusion method.⁶ Typically, a 5 mL vial containing 1 mL of CHCl₃ solution of the polymer (0.5 mg mL⁻¹) was placed in a 50 mL vial containing 5 mL of MeOH. The outside vial was capped and then allowed to stand for 3 days at 25 °C. The MeOH vapour was slowly diffused into the CHCl₃ solutions, resulting in a white suspension. Scanning electron microscopy (SEM) micrographs of air-dried suspensions of the precipitates displayed the formation of microspheres (Fig. 1b and c). The average diameters (d_av) of the microspheres of P1 and P2 are 2.88 and 2.11 μm with the standard deviations (σ) of 0.99 and 0.82 μm, respectively. We recently reported that π-conjugated polymers with amorphous aggregation tend to form spherical assembly when a polar nonsolvent is slowly diffused into the solution of the polymers.⁶ The branched alkyl chain or alkoxyphenyl substituent at 9-position of the carbazole (Cz) moiety in P1 and P2, as well as the octyloxy group at the 4-position, possibly results in a steric hindrance, which disturbs the interchain π-stacking of the Cz moiety,⁵ leading to the spherical aggregation. In fact, PCz P3 without octyloxy group and P4 and P5 with branched alkyl chain and alkoxyphenyl substituents at 9-position, separated by one methylene group, hardly formed well-defined microspheres under the identical self-assembling conditions but only gave irregular or harsh aggregates (Fig. S1, ESI†). There are several methods to prepare microspheres from conjugated polymers such as self-organized precipitation (SORP) method, emulsion polymerization method, and so forth.⁷ However, the vapor diffusion method has a high advantage to obtain well-defined, high-sphericity microspheres quantitatively.⁶ Below 80 °C, the microspheres maintain their shapes, while further heating results in a melting of the microspheres above 95 °C with a slight PL color change (Fig. S2, ESI†).

Cast films of the microspheres of P1 and P2, upon photoexcitation with stationary light (λ_ex = 380 nm) displayed PL with λ_em of 445 and 447 nm, respectively (Fig. 2a, black), which are slightly red-shifted in comparison with the PL spectra in CHCl₃ (Table S1, ESI†). The ϕ_PL values for P1 and P2 in the solid state are 0.21 and 0.16, respectively. In contrast with the photoexcitation with stationary light, focused laser excitation (λ_ex = 405 nm) to a single microsphere of P1 and P2 results in sharp and periodic PL lines at the whole visible spectral range (Fig. 2a and b, red). The sharp and periodic PL lines are attributed to WGMs, where PL, generated inside the microsphere, is confined and resonates through propagation at the circumference.^8,9 The average refractive indices (η) of P1 and P2 at the wavelength range of 500–700 nm are 1.55 and 1.58, respectively (Fig. S3, ESI†), which are high enough to confine PL via total internal reflection (TIR) at the polymer/air interface. As the diameter (d) of the microspheres increased, the intervals of the PL lines become narrow due to the increase of the optical path length (Fig. 2c and d). Using the η and d values, each PL line is assigned as transverse electric (TE) and magnetic (TM) modes of WGM (Fig. S4, ESI†).⁸ As d increased from 2.9 to 5.0 μm, the Q-factor enhanced from 290 to 520 due to the increase of the efficiency of TIR by the decrease of the curvature (Fig. S5, ESI†).¹⁰

Fig. 2 (a and b) PL spectra of microspheres of P1 (a) and P2 (b). Black spectra show PL of a thin film of the microspheres upon excitation with stationary light (λ_ex = 380 nm), while red spectra shows PL from a single microsphere upon focused laser excitation (λ_ex = 405 nm). (c and d) PL spectra of a single microsphere of P1 (c) and P2 (d) with different d. (e) CIE coordinates of P1 (circles) and P2 (squares) upon excitation with stationary light (black) and focused laser (red) to microspheres (open symbols) and cast films from solution (closed symbols).

Similar to the microspheres, spin-cast films from solutions of these polymers displayed large differences in PL spectra between weak and strong excitation. Upon weak excitation with stationary light, spin-cast films of P1 and P2 from CHCl₃ solutions show PL with λ_em of 427 and 430 nm, respectively (Fig. 3a and b, black, and Table S1, ESI†). In contrast, PL spectra of these films, upon focused laser excitation, displayed broad PL bands at 500–700 nm, in addition to the PL band at around 450 nm (Fig. 3a and b, red). The Commission Internationale de L'éclairage (CIE) coordinates of the spectra are plotted in Fig. 2e. For weak excitations, the CIE coordinates of the films of P1 (circles) and P2 (squares), prepared by spin-cast from their solutions, are (0.17, 0.11) and (0.20, 0.19), respectively (black, filled). The thin films of the microspheres of P1 and P2, with weak excitation, still show the CIE coordinates in the blue region of (0.19, 0.16) and (0.20, 0.20), respectively (black, open). Contrastively, upon strong excitation, CIE coordinates of the spin-cast films of P1 and P2 shift to the white region of (0.31, 0.38) and (0.33, 0.38), respectively (red, closed). The points further shift to (0.40, 0.42) and (0.40, 0.43) for a single microsphere of P1 and P2, respectively (red, open).

Fig. 3 (a and b) PL spectra of thin films of P1 (a) and P2 (b), prepared by drop-cast from CHCl₃ solutions, upon excitation with stationary light (black, λ_ex = 380 nm) and focused laser excitation (red, λ_ex = 405 nm). (c) Fluorescence decay profiles of a cast film of microspheres of P1 at 450 (blue), 550 (green), and 650 nm (red). Inset shows decay profiles with the shorter time range (<3 ns). (d) ESR spectra of P1 in dark (black, dotted), upon laser irradiation in N₂ (black, solid) and in air (red), and after laser irradiation (green). λ_ex = 355 nm. (e) FT-IR spectra of a cast film of P1 before (black) and after (red) laser irradiation (λ_ex = 355 nm) in air. (f) Schematic representation of the possible structure of the partly oxidized PCz (top) and its electronic state (bottom). CB; conduction band, VB; valence band.

The PL lifetime of the microspheres show clear differences depending on the PL wavelength (Fig. 3c). Upon pulsed laser excitation (λ_ex = 377 nm) of a thin film of microspheres of P1, average PL lifetime (τ_av) at 450 nm is 0.191 ns. In contrast, τ_av at λ = 550 and 650 nm are 1.23 and 2.25 ns, which are roughly one order of magnitude longer than that at 450 nm (Table S2, ESI†). Similar tendency was observed for a thin film of microspheres of P2 and spin-cast films of P1 and P2 from their solutions (Fig. S6 and Table S2, ESI†). The results indicate the presence of the low energy-lying chromophore that has narrower energy gap than that of the original PCz. It is known that poly(N-vinylcarbazole) shows PL with long lifetime (∼20 ns) at room temperature, originating from the excimer state.¹¹ However, poly(2,7-carbazole) has been reported to show pure blue PL without excimer emission.¹² Accordingly, it is plausible that PL at the long wavelength region observed from P1 and P2 is not derived from the excimer state.

Electron spin resonance (ESR) spectra displayed the irreversible spectral change upon strong photoexcitation. Before laser irradiation, a thin film of P1 showed weak ESR signal (Fig. 3d, black dotted) with g-factor of 2.0048, which possibly originates from the naturally oxidized species. The intensity of the ESR signal was enhanced upon laser irradiation in N₂ atmosphere (Fig. 3d, black), while the g-factor was almost intact (2.0047). The shape of the ESR signal is analogous with that of P1 upon exposure to an iodine vapour (g = 2.0048, Fig. S7, ESI†), indicating that laser irradiation in N₂ results in a generation of cation radical species steadily in the π-conjugate system.¹³ On the other hand, under laser irradiation in ambient atmosphere, ESR signal was much enhanced, accompanying a shift of the spectrum to the lower magnetic field side with the g-factor of 2.0061 (Fig. 3d, red). After the laser irradiation, the ESR spectrum hardly recovered to the initial state (Fig. 3d, green). PL spectrum of the thin film of P1, after the laser irradiation, involved broad PL band at the longer wavelength region (Fig. S8a, ESI†), indicating that irreversible change occurs in the polymers by the laser irradiation.¹³

The PL spectral change was examined upon continuous laser irradiation. At the initial excitation, the WGM peaks covered whole visible spectral range. Further irradiation gradually suppressed the PL intensity, and after 2 min of irradiation, the intensity becomes less than one sixth of the initial PL intensity (Fig. S9a and c, ESI†). However, the WGM spectral shape does not change by laser irradiation (Fig. S9b, ESI†), indicating that the spherical shape does not become deformed.

Fourier-transform infrared (FT-IR) spectrum of P1, after laser irradiation, showed a broad absorption band centered at 1720 cm⁻¹ (Fig. 3e). This band possibly derives from the stretching vibration of a carbonyl group.‡ Because the 3-position of the carbazole moiety in P1 is the most electron-rich part¹⁴ with an influence of the electron-pushing alkoxy group at the 4-position,⁴ the 3-position is possibly photooxidized (possible structure is drawn in Fig. 3f).§ As a result of the electron-withdrawing carbonyl group, π-system at the oxidized moiety is stabilized to some extent, which induces a narrow band gap region in the polymer main chain (Fig. 3f bottom), analogous to donor–acceptor polymers.¹⁵

Finally, by virtue of the wide-range resonant PL over visible region, PL lines of any colours can be picked out from a single microsphere. Using band-pass filters centered at 470, 520, 570, 620, and 670 nm (full-widths at the half maximum of all the filters: 10 nm), single or double resonant PL lines of blue, green, yellow, red, and deep red can be sorted out, respectively, while a use of long-pass filter of >750 nm can extract resonant PL lines at the near infrared region (Fig. 4). Such full-colour selectivity of resonant PL from a single-component microsphere is valuable for application to a light source with multi-colour compatibility.

Fig. 4 PL spectra of a single microsphere of P1, upon focused laser excitation (λ_ex = 405 nm), without an optical filter (black), with band-pass filters centered at 470 (blue), 520 (green), 570 (orange), 620 (pale-red) and 670 nm (deep red) and with a long-pass filter (>750 nm, brown).

In summary, white-colour resonant photoluminescence was achieved from self-assembled microspheres composed of single-component polycarbazoles. The extremely wide photoluminescence window results from the photoinduced partial oxidation of the polymers by a strong photoexcitation. Generation of the electron-withdrawing carbonyl group results in narrow band gap spots in the polymer. The high brightness resonant photoemitter is valuable for micrometer-scale, white-colour light source and will further be applied as full-colour light sources by sorting the necessary emission lines.

Acknowledgements

The authors acknowledge Mr Kosuke Shibasaki in University of Tsukuba for synthesis of PCz P3–P5. This work was supported by KAKENHI (25708020, 15K13812, 15H00860, 15H00986, 16H02081) from JSPS/MEXT Japan and Asahi Glass Foundation.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Materials and measurements, simulation, SEM, PL, ellipsometry, PL decay, ESR. See DOI: 10.1039/c6ra10662e

‡ Judging from the FT-IR spectrum of poly(3,6-Cz) in ref. 16, the photocrosslink of the interpolymer Cz moiety is not plausible.

§ In case of P3 without alkoxy group, λ_em shifted to 500 nm after laser irradiation with the PL color change from blue to green, not to white (Fig. S8b, ESI†).

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