Triplet–Triplet Annihilation Upconversion Circularly Polarized Luminescence That Originates From Achiral and Racemic Luminophores Encapsulated in Chiral Silica

ABSTRACT

Circularly polarized luminescence (CPL) has emerged as a key optical property with chiral‐photonic and optoelectronic applications. However, conventional CPL systems rely on synthetically demanding chiral luminophores that emit at fixed wavelengths, which limits tunability and efficiency. In this study, we demonstrate that achiral or racemic luminophores co‐encapsulated with photosensitizers in the helical nanocavities of chiral silica exhibit upconversion CPL (UC‐CPL). The chiral silica, which is derived from polymethylvinylsiloxane comprising polyhedral oligomeric silsesquioxane decorated with enantiomeric N‐(tert‐butoxycarbonyl)cysteine methyl ester moieties (PMVS‐POSS‐Cys), has a helical structure with a preferred‐handedness that facilitates efficient triplet–triplet energy transfer (TTET) and triplet–triplet annihilation (TTA). The resulting materials exhibit distinct CPL and UC‐CPL signals when excited at 365 and 532 nm, respectively, despite the achiral nature of the luminophore. This study unprecedentedly demonstrates UC‐CPL from achiral emitters within chiral silica matrices, which is achieved through chiral induction in the solid‐state helical silica frameworks. The strategy described herein provides a general and versatile platform for developing energy‐efficient, wavelength‐tunable chiral‐photonic materials without the need for elaborate chiral syntheses.

We show upconversion‐circularly polarized luminescence (UC‐CPL) from achiral or racemic luminophores co‐encapsulated with photosensitizers in the helical nanocavities of chiral silica from PMVS‐POSS‐Cys. The preferred‐handed helical framework allows triplet–triplet energy transfer and annihilation, with CPL and UC‐CPL at 365 and 532 nm excitation. This is a versatile strategy for wavelength‐tunable, energy‐efficient CPL without chiral emitters.

1. Introduction

Circularly polarized luminescence (CPL) has attracted considerable attention for use in a wide range of applications, including three‐dimensional optical displays, chiral optoelectronic devices, and sensors [1, 2]. CPL is typically generated when a chiral chromophore is excited by UV light. However, the CPL‐emission wavelength is longer than the excitation wavelength, which leads to low energy efficiency as a major limitation. The use of triplet–triplet annihilation upconversion (TTA‐UC) has emerged as a promising strategy for addressing this issue by generating upconversion CPL (UC‐CPL) [3]. To date, UC‐CPL has only been observed for chiral luminophores in organic solvents or liquid crystals in the presence of photosensitizers. The UC‐CPL dissymmetry factor (g_lum) is typically of the order of 10⁻³ in an organic solvent, whereas higher values of between 10⁻² and 10⁻¹ have been observed for liquid‐crystal systems [4, 5, 6, 7, 8]. Nevertheless, critical challenges remain, including chiral‐luminophore synthesis that often involves tedious multistep reactions, and the single‐wavelength emission of the luminophore, which necessitates the use of new compounds to produce different CPL colors. Moreover, there are few examples of achiral luminophores producing UC‐CPL [9].

We addressed these limitations by focusing on helically structured chiral silica [10, 11], which is transparent in the UV‐to‐visible region; its helical nanocavities are also capable of encapsulating achiral molecules, leading to specific optical properties [12, 13]. We previously prepared chiral silica from polymers containing polyhedral oligomeric silsesquioxane (POSS) or polysiloxane derivatives as templates [14, 15, 16]. Furthermore, we demonstrated that chiral silica encapsulates pyranine and solvents, leading to CPL emissions that exhibit solvatochromic behavior [17]. While triplet–triplet energy transfer (TTET) is essential for achieving UC‐CPL in chiral silica, to the best of our knowledge, energy transfer within chiral silica has not been reported. The key question here is whether TTET can occur along the helical nanocavities of chiral silica, as shown in Figure 1. Accordingly, we hypothesized that a luminophore and a photosensitizer co‐encapsulated within chiral silica exhibit UC‐CPL even when achiral or racemic luminophores are used. This strategy realizes UC‐CPL via a mechanism wherein energy is transferred along the helical nanocavities in the silica.

FIGURE 1.

Schematic showing TTET along a helical nanocavity in chiral silica. D and A denote “donor” and “acceptor,” respectively.

2. Results and Discussion

We previously synthesized a POSS‐containing polymer (PMAPOSS) via living anionic polymerization [14, 15]. While PMAPOSS served as a template for chiral silica, propagating the living anionic polymerization process was significantly hindered by the substantial steric bulk of the POSS moieties in the methacrylate functional groups. Accordingly, we used a novel post‐functionalization approach to introduce POSS molecules and overcome this limitation. Figure 2a shows the strategy used to synthesize the POSS‐containing polymer, which we refer to as “PMVS‐POSS‐Cys”. Polymethylvinylsiloxane (PMVS) was prepared as previously described [16]. The number average molecular weight (M _n) and polydispersity index (PDI) of the PMVS were 40,000 and 1.21, respectively, as determined by SEC. Chlorodimethylsilane was introduced to the PMVS via hydrosilylation promoted by Karstedt's catalyst to afford PMVS‐SiCl. VinylPOSS‐OH, which was prepared according to literature [18], was then reacted with PMVS‐SiCl to afford PMVS‐VinylPOSS (see Supporting Information for details). Enantiomeric N‐(tert‐butoxycarbonyl)cysteine methyl ester (Cys) was subsequently introduced using a thiol‐ene reaction, which led to the formation of PMVS‐POSS‐Cys. Next, both PMVS‐POSS‐Cys enantiomers were calcined at 600°C, and the resulting residues were characterized by vibrational circular dichroism (VCD) spectroscopy and transmission electron microscope (TEM). The VCD spectrum displayed in Figure 2b shows a band at 1240–1000 cm⁻¹ corresponding to asymmetric Si–O–Si stretching vibrations, and a lack of other characteristic peaks, consistent with the formation of silica. Notably, the Si–O–Si band exhibited a split‐type Cotton effect, with the spectra of the two enantiomeric samples exhibiting a mirror‐image relationship. Clear helical structures are also evident in the TEM image shown in Figure 2c,d. These findings clearly demonstrate that chiral silica with a preferred chiral helical structure is formed from enantiomeric PMVS‐POSS‐Cys.

FIGURE 2.

(a) Strategy for the synthesis of PMVS‐POSS‐Cys. (b) VCD spectra of chiral silica prepared by calcining PMVS‐POSS‐(L)‐Cys (red) and PMVS‐POSS‐(D)‐Cys (blue). TEM images of chiral silica prepared from (c) PMVS‐POSS‐(L)‐Cys and (d) PMVS‐POSS‐(D)‐Cys.

The photosensitizer and luminophore should be encapsulated within the helical nanocavities of the chiral silica to achieve UC‐CPL. Alignment along these cavities facilitates efficient energy transfer, leading to excited states with induced chirality. A racemic mixture of (R)‐(−)‐ and (S)‐(+)‐2,2,2‐trifluoro‐1‐(9‐anthryl)ethanol ((R)‐ and (S)‐TFAE) was prepared by mixing the enantiomers in a 1:1 molar ratio, after which the rac‐TFAE and platinum octaethylporphyrin (PtOEP) were dissolved in 4‐cyano‐4′‐pentylbiphenyl (5CB) in a 2.70:0.01:97.29 (w/w/w) ratio. Chiral silica (5 mg) was then immersed in the mixture.

The chiral silica obtained in this manner is referred to as “chiral silica⊃TFAE+PtOEP;” Figure 3 shows a photographic image of this material when excited by 532 nm light, the photoluminescence (PL) spectra, and the relationship between PL intensity at 532 nm and excitation power. Chiral silica⊃TFAE+PtOEP exhibited a UC blue emission between 400 and 500 nm at a position similar to that observed for TFAE, and no UC emission was observed in the absence of PtOEP (Figure S13a). The double‐logarithmic plot of UC emission intensity as a function of excitation intensity, shown in Figure 3c, reveals two distinct slopes (2.0 and 1.1) at approximately I _th = 190 mW cm⁻²; this behavior is characteristic of a TTA process, thereby confirming that the UC emission from the chiral silica originates from TTA [19, 20].

FIGURE 3.

(a) Photographic image of chiral silica⊃TFAE+PtOEP exited at 532 nm. (b) Excitation‐power‐dependent PL spectra acquired when excited at 532 nm. (c) Double‐logarithmic plot of UC emission intensity as a function of excitation intensity.

Figure 4a–d display CPL and UC‐CPL spectra acquired when excited at 365 and 532 nm, respectively. The black trace corresponds to a mixture of TFAE, PtOEP, and 5CB without chiral silica. Although both PL and UC emissions were observed, the mixture exhibited no CPL signal when excited at either 365 or 532 nm. By contrast, silica⊃TFAE+PtOEP clearly exhibited distinct CPL and UC‐CPL signals. Together, these findings suggest that TTET occurs within the helical nanocavities of the chiral silica, resulting in UC‐CPL. CPL and UC‐CPL g_lum values of |8.9 × 10⁻³| and |2.0 × 10⁻²|, respectively, were recorded; thus, UC‐CPL is associated with a g_lum value that is approximately twice that observed for CPL. The ECD spectra of silica⊃TFAE+PtOEP (Figure S8a) show Cotton effects with mirror‐image signals observed, confirming induced chirality arising from the confined chiral environment rather than intrinsic molecular chirality. These results collectively support that TTET and UC emission occur within the chiral silica nanocavities.

FIGURE 4.

(a) CPL and (b) corresponding g_lum spectra arising from down‐conversion with excitation at 365 nm. (c) UC‐CPL and d) corresponding g_lum spectra with excitation at 532 nm, presented together with the corresponding PL intensity profiles, measured for silica⊃TFAE+PtOEP.

Diphenyl anthracene (DPA) was used instead of racemic TFAE to evaluate whether an achiral luminophore can be accommodated within the chiral silica matrix and if TTET occurs in its helical nanocavity. Chiral silica⊃DPA+PtOEP exhibited a clear UC emission when excited at 532 nm (Figure S14); its ECD spectra also exhibited mirror‐image Cotton effects (Figure S8b). Figure 5a–d show CPL and UC‐CPL spectra of chiral silica⊃DPA+PtOEP, which display distinct signals. UC‐CPL is associated with a g_lum value of |1.5 × 10⁻²|, whereas a significantly lower value of |5.2 × 10⁻³| was recorded for down‐conversion CPL. The g_lum value can be rationalized using Equation (1),

$$g_{\mathrm{lum}}=\frac{4{\mu }_{n0}·m_{0n}}{{\left|{\mu }_{n0}\right|}^2+{\left|m_{0n}\right|}^2}$$ (1)

where μ _n0 and m _0n are the electric and magnetic transition dipole moments between the emitting excited state (n) and the ground state (0), respectively; μ _n0 is much larger than m _0n in a typical CPL process, which results in a small g_lum value.

FIGURE 5.

(a) CPL and (b) corresponding g_lum spectra arising from down‐conversion with excitation at 365 nm. (c) UC‐CPL and (d) corresponding g_lum spectra with excitation at 532 nm, presented with the corresponding PL intensity profiles, measured for silica⊃DPA+PtOEP.

By contrast, the TTA‐UC process involves nonradiative transitions, including TTET and TTA, which effectively reduce μ _n0. This reduction in μ _n0 may lead to an apparently amplified g_lum [3].

Finally, TTET quantum efficiency (Φ _TTET) was estimated from phosphorescence lifetime measurements, yielding values of 63% for TFAE and 40% for DPA (see Supporting Information). The UC quantum efficiency (Φ _UC) was also evaluated and was comparable to that of the corresponding bulk mixtures (7% for TFAE and 11% for DPA). These results suggest that efficient triplet energy transfer and upconversion processes are maintained within the chiral silica nanocavity.

This study is the first example of TTET in chiral silica and the first observation of UC‐CPL within this chiral silica matrix.

3. Conclusion

We demonstrated TTET in chiral silica as well as UC‐CPL within this chiral silica matrix unprecedentedly. We realized UC‐CPL by co‐encapsulating luminophores and photosensitizers within the helical nanocavities of chiral silica. The UC‐CPL g_lum values were higher than those of conventional CPL, confirming that efficient energy transfer and triplet–triplet‐annihilation processes occur along the helical nanocavities. This approach provides a versatile platform for tuning the CPL properties of chiral‐photonic materials through simple encapsulation and without the need for complex molecular syntheses. Our strategy provides new pathways for the development of energy‐efficient chiral optoelectronic systems and next‐generation circularly polarized light sources.

Author Contributions

Shunsuke Morii: investigation, validation, formal analysis. Himawari Kunitake: investigation, validation, formal analysis. Yasumasa Miyoshi: investigation, validation, formal analysis. Shinichi Kusaka: investigation, validation, formal analysis. Towa Shinoda: investigation, validation, formal analysis. Kai Terami: investigation, validation, formal analysis. Yu‐Hsuan Chan: investigation, validation, formal analysis. Yi‐Ting Lin: investigation, validation, formal analysis. Hao‐Cheng Yu: investigation, validation, formal analysis. Syuji Fujii: investigation. Yoshinobu Nakamura: investigation. Yoshiro Kaneko: investigation. Teruaki Hayakawa: investigation. Tatsuo Nakagawa: investigation. Hiroaki Hanada: investigation. Ming‐Chia Li: writing – review and editing. Tomoyasu Hirai: writing – review and editing, writing – original draft, conceptualization, funding acquisition, project administration, supervision.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Preparation of the chiral silica; characterization data; accommodation properties; details of UC emission‐intensity studies; Figures S1–S15 (PDF). Supporting File: anie72847‐sup‐0001‐SuppMat.docx

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.