FRET-mediated near infrared whispering gallery modes: studies on the relevance of intracavity energy transfer with Q-factors

Abstract

Near infrared (NIR) optical microsphere resonators are prepared by coassembly of energy-donating and accepting conjugated polymers. In the microspheres, fluorescence resonance energy transfer occurs, leading to sharp and periodic photoluminescence from whispering gallery modes in the NIR region with Q-factors as high as 600.

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Near infrared (NIR) emitting materials represent powerful tools for biosensing and light sources for bioimaging due to the high transparency of biological organisms in this spectral window.¹ Therein, NIR luminescent colloidal probes with sharp emission lines are often utilized. Recently, polymer whispering gallery mode (WGM) microresonators with narrow luminescence and laser emission have been reported.² Furthermore, π-conjugated organic and polymer microparticles, in some cases doped with rare-earth metals, were shown to exhibit resonant emission and lasing in the visible and NIR region.^2,3 Such organic microresonators find applications for barcoding in live cells, where lasing with a unique spectral profile makes it possible to distinguish each cell.⁴ However, NIR emitters often suffer from low luminescence efficiency because of the small energy gap between the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO, respectively), which causes non-radiative thermal deactivation.⁵ Furthermore, aggregation of molecules often causes serious quenching of the NIR emission. The absence of high-performance organic and polymeric lasers in the NIR spectral regime hinders the development of new biomedical imaging probes as well as NIR communication sources. Advancing polymeric microsphere lasers into the NIR spectral region would open up numerous applications in biological systems.

Fluorescence resonance energy transfer (FRET) is an energy transfer mechanism that yields down-converted photoluminescence (PL) with high efficiency.⁶ Because the effective distance of FRET is typically within 10 nm,⁶ molecules that interchange energy have to be blended homogeneously without macroscopic phase separation.⁷ Recently, we reported color conversion of WGM PL in microsphere resonators consisting of two blended π-conjugated polymers. By utilizing FRET, effective red-shifted WGM PL was achieved in the microsphere resonators.⁸

In this communication, we realize FRET in blended conjugated polymer microspheres that leads to NIR emission from a WGM cavity. The polymers we used in this study, named P1 and P2, act as an energy donor and acceptor, respectively. P2 displays PL in the red-to-NIR region in solution but hardly fluoresces in the solid state because of the aggregation quenching. By contrast, microspheres formed by coassembly of P1 and a small fraction of P2 exhibit clear WGM PL in the NIR spectral region, resulting from efficient intrasphere FRET from P1 to P2. The utilization of FRET inside a microresonator is useful to efficiently extract NIR light with narrow line width.

The π-conjugated polymers, P1 (poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(5-octylthieno[3,4-c]pyrrole-4,6-dione-1,3-diyl)] with number-average molecular weight (M_n) of 24 kg mol⁻¹, Fig. 1a) and P2 (poly[(5-(2,4,6-triisopropylphenyl)thieno[3,4-c]phosphole-4,6-dione)-alt-(4,4-bis(2-ethylhexyl)-silolo-[3,2-b:4,5-b′]dithiophene)], M_n = 6.5 kg mol⁻¹, Fig. 1b) were synthesized according to reported procedures.⁹ The HOMO and LUMO energy levels are −6.08 and −3.58 eV for P1 and −5.44 and −3.83 eV for P2, respectively, with respect to the vacuum level (Fig. 1c). As shown in the photoabsorption and PL spectra of P1 and P2 in CHCl₃, the PL band of P1 largely overlaps with the absorption band of P2, indicating that efficient FRET from P1 to P2 is possible (Fig. 1d). Indeed, in cast films prepared by spin coating of P1 and P2 in CHCl₃ solution, PL from P1 is mostly quenched even when the weight fraction of P2 (f_P2) is only 0.02 (Fig. S1, ESI†).

Fig. 1 Chemical structures of P1 (a) and P2 (b). (c) HOMO and LUMO energy levels of P1 and P2 with respect to the vacuum level. (d) Photoabsorption (broken lines) and PL (solid lines) spectra of P1 (green) and P2 (red) in CHCl₃.

Self-assembly of P1 and P2 and coassembly of their blends into WGM microsphere resonators were carried out by slow diffusion of MeOH vapour into their CHCl₃ solutions (see the Experimental section).^2,10 The total initial concentration of polymers P1 and/or P2 was set at 0.5 mg mL⁻¹. After three days of vapour diffusion at 25 °C, the solution changed to a suspension, and the polymers were precipitated. Scanning electron microscopy (SEM) shows that P1 (f_P2 = 0) forms well-defined microspheres with a typical diameter of 5 μm (Fig. 2a),^2a,8,11 while P2 (f_P2 = 1) gave irregular aggregates (Fig. 2f). For the P1/P2 blend, well-defined microspheres with smooth surface morphology could be obtained within a f_P2 range of 0.01–0.05 (Fig. 2b and c). However, in the case of the coassembly with f_P2 greater than 0.06, the surface morphology of the resulting spheres deteriorated (Fig. 2d and e). Polymer blends tend to phase separate due to their small mixing entropy.⁷ In the case of f_P2 ≥ 0.06, microspheres with rough surfaces are obtained with characteristic features inherent to phase segregation. On the other hand, in the case of f_P2 ≤ 0.05, P1 and P2 are well mixed, and segregation of P2 is suppressed due to its small fraction. In fact, energy dispersive X-ray spectrometry (EDS) mapping of the microsphere with f_P2 = 0.04 shows that signals of Si and P from P2 are homogeneously distributed throughout the entire microsphere. This result indicates that P2 is well dispersed in a single microsphere (Fig. 2g–j). The signals of each element are much clearer for microspheres with f_P2 = 0.07, yet it is hard to recognize the phase separation of P1 and P2, because the resolution of the apparatus is not sufficient (Fig. S3, ESI†).

Fig. 2 (a–f) SEM micrographs of the self-assembled precipitates of P1 and P2 with f_P2 = 0 (a), 0.03 (b), 0.04 (c), 0.07 (d), 0.1 (e), and 1 (f). Insets show fluorescent micrographs of the precipitates. Image size: 10 × 10 μm. λ_ex = 400–440 nm. (g–j) EDS mapping images of the self-assembled microsphere with f_P2 = 0.04 on an aluminum stage. (g) C Kα₁ and Kα₂, (h) Si Kα₁, (i) P Kα₁, and (j) S Kα₁.

The change of PL quantum yield (ϕ_PL) with f_P2 further indicates that P2 is well dispersed in the P1 matrix. Upon excitation at 470 nm, ϕ_PL of the microspheres decreases monotonically with increasing f_P2 (Fig. 3a). However, the fraction of the PL in the red and NIR region (λ = 650–950 nm) increases with increasing f_P2, and at f_P2 ≥ 0.03, the PL spectrum is the same as for pure P2 (Fig. 3b, closed squares). These results show that efficient energy transfer takes place from P1 to P2. When f_P2 is greater than 0.07, ϕ_PL decreases largely, indicating that the aggregation of P2 causes concentration quenching of the NIR PL from P2. The excitation spectrum of a cast film of microspheres (f_P2 = 0.05) shows that PL at 650 nm is mostly generated by photoabsorption of P1, further supporting that efficient energy transfer from P1 to P2 occurs inside the microspheres (Fig. S2, ESI†).

Fig. 3 (a) Bar graph of ϕ_PL (λ_PL = 500–900 nm) and its red-to-NIR fraction (λ_PL = 650–900 nm, red) versus f_P2 of cast films of the self-assembled precipitates (microspheres for f_P2 = 0–0.1 and irregular aggregates for f_P2 = 1). λ_ex = 470 nm. (b) Plots of the fraction of NIR PL to PL in the entire wavelength region versus f_P2 for cast films of the microspheres (closed squares) and a single microsphere (open squares). (c) PL spectra of a single microsphere of P1 and P1/P2 blends with f_P2 = 0–0.2 upon focused laser irradiation at 470 nm. (d) PL decay profiles at λ_PL = 540 nm (±2.5 nm range) of a single microsphere with f_P2 = 0 (black), 0.01 (red), 0.02 (orange), 0.04 (green), and 0.07 (blue). λ_ex = 470 nm. Insets show plots of τ_1/2versus f_P2 at λ_PL = 540 nm (±2.5 nm range). The plots are the average value with the measurements of 5–7 isolated microspheres. (e) Plot of γ versus f_P2.

To quantify the energy transfer, we measure PL spectra of individual microspheres (λ_ex = 470 nm, see the Experimental section and Fig. S4, ESI†). For microspheres with f_P2 = 0–0.2, characteristic spectral modulation from the WGM cavity can be observed, indicating that the generated PL is confined inside of the microspheres and self-interferes (Fig. 3c).^2,8,12 As f_P2 increases, the spectral series of WGM modes shift from the visible to the NIR region, and at f_P2 = 0.04, WGM PL is observed mostly in the NIR region at 700–900 nm (Fig. 3c, red). According to the area intensity, more than 80% of the PL originates from P2 in the microsphere with f_P2 of 0.04 (Fig. 3b, open squares). At f_P2 greater than 0.07, the NIR PL slightly decreases and the PL in the visible region reappears (Fig. 3c), possibly caused by the meso- and macroscopic phase segregation of P1 and P2 in the microsphere (Fig. 2d and e).

We performed time-resolved PL spectroscopy, which further supports the intrasphere energy transfer. As f_P2 increases from 0 to 0.04, the PL half lifetime (τ_1/2) at 540 nm (PL from P1) monotonically decreases from 0.327 to 0.172 ns (Fig. 3d), while τ_1/2 at 675–730 nm (PL from P2) is almost constant at around 0.6 ns (Fig. S5, ESI†). Further increase of f_P2 to 0.07 results in an increase of τ_1/2 at 540 nm to 0.315 ns.

The energy transfer efficiency (γ) in the single microsphere is evaluated with eqn (1),

γ = 1 − τ_B/τ_D (1)

where τ_D and τ_B represent the PL half lifetime of the energy donating polymer (λ_PL = 540 nm) in the microspheres of P1 and the P1/P2 blend, respectively.^6b We find that γ increases from 0.276 to 0.473 with increasing f_P2 from 0.01 to 0.04 (Fig. 3e). However, further increase of f_P2 to 0.07 greatly suppresses γ. The phase separation of P1 and P2 in the microsphere starts to occur at this blending fraction, which also reduces the donor–acceptor interface area in the microsphere.

We further carry on to determine the performance of the WGM resonator by evaluating the Q-factor. The average Q-factor is defined as the peak wavelength divided by the full-width at the half maximum of the observed WGM PL peaks (Q_av). For the microspheres with f_P2 ≤ 0.07 and d ∼ 5 μm, Q_av is around 400 (Fig. 4a). With increasing f_P2 ≥ 0.1, Q_av decreases greatly to ∼100, which results from the increased surface roughness that causes scattering losses of the confined PL. It is noteworthy that the Q-factor shows wavelength dependency. The WGM PL lines of the microspheres with f_P2 = 0 display Q-factors around 500 (Fig. 4b and Fig. S6, ESI† green).¹³ By contrast, WGMs of the microspheres with f_P2 = 0.01 and 0.02 exhibit Q-factors around 200 in the visible range (600–750 nm) and as large as 600 in the NIR region (>800 nm, Fig. 4b and Fig. S6, ESI† red and blue). The Q-factor is described as follows:

Q⁻¹ = Q_rad⁻¹ + Q_scat⁻¹ + Q_abs⁻¹ (2)

where Q_rad⁻¹ is the leakage loss for the internal reflection, Q_scat⁻¹ is the scattering loss by the surface roughness, and Q_abs⁻¹ is the absorption loss by the resonator medium.¹³Q_abs⁻¹ is shown as

Q_abs⁻¹ = αλ/2nπ (3)

where α is the absorption coefficient and n is the refractive index of the medium.¹⁴Q_abs⁻¹ increases when λ overlaps with the absorption wavelength, thereby resulting in the decrease of Q. As shown in Fig. 4b, the increase of the Q-factor is observed at wavelengths higher than 750 nm. This wavelength coincides with the absorption edge of the doped P2 (Fig. 4b, blue and red). Using eqn (3), we simulate the wavelength dependency of Q_abs, and the tendency of the experimental plot in Fig. 4b matches well with the simulated plots (Fig. S7, ESI†).

Fig. 4 (a) Plot of the average Q-factor versus f_P2. (b) Plots of Q-factors of each WGM PL line for microspheres with f_P2 = 0 (circle), 0.01 (square), and 0.02 (triangle). The red-colored area indicates the photoabsorption spectrum of a CHCl₃ solution of P2.

On the other hand, Q_rad⁻¹ and Q_scat⁻¹ of the compared microspheres are assumed not differ greatly, because microspheres with identical diameters (∼5 μm) and similar degrees of surface roughness are applied here. Q_rad is indeed size dependent,^14b which is clearly visible when plotting Q versus the particle diameter (Fig. S8, ESI†). As a result of the reabsorption loss, WGM lines in the visible range display smaller Q values (∼200) than those in the NIR range, where they reach as high as 600. This value is of similar level as previously reported NIR WGMs of self-assembled organic resonators.³

Conclusions

We fabricate whispering gallery mode (WGM) microresonators by self-assembly of an energy donating conjugated polymer doped with a small portion of energy accepting, NIR-emitting polymer. Due to efficient energy transfer (FRET) from the donor to acceptor polymers inside the microspheres, WGM in the NIR region appears upon photoexcitation at the perimeter of the microsphere. The observed Q-factor is as high as 600 in the NIR region, at which the absorption loss by the energy-accepting polymer is almost dismissed. The NIR WGM resonators will be valuable for applications in sensing and imaging in biological systems.¹⁵

Experimental section

Self-assembly of P1 and P2 and coassembly of their blends

Typically, a 5 mL vial containing a CHCl₃ solution of P1, P2 or their blends with a total concentration and amount of 0.5 mg mL⁻¹ and 2 mL, respectively, 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 vapour of the nonsolvent was slowly diffused into the solution, resulting in precipitation of the polymers through the supersaturated state.

μ-PL measurements

μ-PL measurements were carried out using a μ-PL measurement system (Fig. S4, ESI†).⁸ An optical microscope was used with a long-distance 100× objective (NA = 0.8) to identify suitable particles and determine their diameters. For measurements, a WITec μ-PL system was used with a model Alpha 300S microscope combined with a Princeton Instruments model Action SP2300 monochromator (grating: 300 grooves mm⁻¹) and an Andor iDus model DU-401A BR-DD-352 CCD camera cooled to −60 °C. The PL spectrum was measured at a resolution of 0.27 nm. The edge of a single sphere was photoexcited at 25 °C under ambient conditions by a diode pulsed laser (a PicoQuant model LDH-D-C-470B with a PDL 828 ‘Sepia II’ driver) with the wavelength, power, integration time, frequency, pulse duration and spot size of 470 nm, 1.5 μW, 1 s, 2.5 MHz, 70 ps, and ∼1 μm, respectively. Time-resolved PL decay experiments on individual isolated microspheres were carried out on the samples sealed under nitrogen in a glove box using a PicoHarp 300 TCSPC module from PicoQuant hooked to a Leica TCS SP8 microscope equipped with a pulsed White Light Laser.

Author contribution

YY and YT designed experiment, KH, YT, SM, JK, TK synthesized the polymers, OO and SK conducted self-assembly, OO, AM, TD, SI, TN, AK, FD, RF conducted μ-PL and time-resolved PL measurements, and YY and OO prepared the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge Prof. Tatsuya Nabeshima, Masaki Yamamura, and Takashi Nakamura of University of Tsukuba for PL quantum yield experiments. This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “π-System Figuration” (JP17H05141, JP17H05142, JP17H05155), Scientific Research (A) (JP16H02081), and Joint International Research (JP15KK0182) from Japan Society for the Promotion of Science (JSPS), University of Tsukuba Pre-strategic initiative “Ensemble of light with matters and life”, Asahi Glass Foundation, and the Deutsche Forschungsgemeinschaft DFG (KU 2738/3-2).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qm00498b

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