A circularly polarized (CP) white organic light-emitting diode (WOLED) based on a chiral organo-Sm³⁺ complex

Abstract

Using the designed chiral [Sm(tta)₃(D-phen)] as an emitter, the first example of a chiral organo-Ln³⁺-based CP-WOLED with both attractive white-light efficiencies (η^Max_EQE = 1.55% and η^Max_CE = 1.61 cd A⁻¹) and high dissymmetry factor (|g_EL|^Max = 0.011) is reported.

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Inspired by the direct generation of circularly polarized light (CPL), CP-OLEDs (organic light-emitting diodes)¹ capable of high-contrast 3D images and true backlight, are significantly superior to traditionally unpolarized OLEDs.² In this perspective, to realize efficient CP electroluminescence, concerted efforts have been devoted to the development of different chiroptical luminophores, such as chiral organic small-molecules,³ chiral conjugated polymers,⁴ chiral metal (transition⁵/lanthanide⁶) complexes as well as chiral thermally active delayed fluorescent (TADF) emitters.⁷ In consideration of a restriction of theoretical 25% internal quantum efficiency (η_IQE)^3,4 for chiral small-molecules and conjugated polymers with fluorescence, chiral metal complexes^5,6 and TADFs⁷ enable the harvesting of both ¹S and ¹T excitons, which makes the resulting phosphorescent CP-OLEDs with η^Max_IQE = 100% more appealing. Nonetheless, it remains a great challenge to achieve satisfactory polarization degree and high device efficiency, simultaneously. On the other hand, accompanying the attractive high-performance with monochrome (visible and even near-infrared (NIR)) features of the reported CP-OLEDs,¹ the realization of reliable panchromatic or white CP-OLEDs (CP-WOLEDs) is very rare and in its infancy.⁸ For instance, although the current efficiency (η^Max_C, 2.0–50.0 cd A⁻¹) of the reported CP-WOLEDs fabricated from chiral bis-benzoxanethone fluorescent emitters by Cheng et al.^8a or spiro-type TADFs by Zheng et al.^8b is high enough for portable full-colour 3D displays (η^Max_C > 0.5 cd A⁻¹),⁹ one issue of their inferior white-light quality (CIE coordinates 0.32–0.35, 0.45–0.46) still occurs, probably arising from the dichromatic and voltage-dependent white-light integrations.¹⁰ Additionally, their intrinsically low polarization degree is also difficult to solve.

As a matter of fact, CPL activity quantified by the dissymmetry factor g_lum (corresponding to photo-excited g_PL or electric-driven g_EL) via the following equation¹¹ originates from the intensity (I_L or I_R) difference of the emissive left and right CP lights. Meanwhile, from the viewpoint of quantum mechanics, g_lum can further be simplified to be relative to the electric (μ) and magnetic (m) transition dipole moments and the θ angle between.

For most chiral organic luminophores (small-molecules, TADFs and polymers, etc.) or chiral transition-metal complexes, their relatively small |g_PL| values in the 10⁻⁵∼10⁻⁴ range,¹² are attributed to the high μ and the negligible m sizes. In contrast, uniquely beneficial from the magnetically allowed while electrically forbidden f–f transitions of the Ln³⁺ ion, distinctively higher |g_PL| values (>10⁻²), especially for Eu³⁺, Tb³⁺ or Sm³⁺ ions, are found for their chiral organo-Ln³⁺ complexes.¹³ For example, chiral Cs[Eu((+/−)-hfbc)₄]¹⁴ (hfbc = 3-heptafluoro-butylyrylcamphorate) showed the highest |g_PL| of 1.38 among the reported CPL-active materials^12,13 so far, and by using them as dopants, smart examples of visible-monochromatic (Eu³⁺-centred colour-purity red-light) CP (|g_EL| = 0.15–1.00) devices^14,15 have been realized. However, arising from the (+/−)-hfbc-induced unsatisfactory photo-luminescence efficiency (Φ_PL, < 2%), the subsequent standstill is reflected in their very low maximal external quantum efficiency (η^Max_EQE, 4.2 × 10⁻³ − 0.05%) even with light out-coupling technology. Recently, through a modular design strategy to chiral [Eu(tta)₃(R/S-PyBox)] (Htta = thenoyltrifluoroacetone; PyBox = pyridine bis-oxazoline) complexes,¹⁶ the Htta-induced triplet state ¹T compatible with the first excited state (⁵D₀) of Eu³⁺ ions was demonstrated to give desirably improved (η^Max_EQE = 0.48% and |g_EL| = 0.51) Eu³⁺-red-light CP-OLEDs. However, along with no other visible-monochromatic CP-OLED reports from chiral organo-Tb³⁺/Sm³⁺ systems, the outward field of chiral organo-Ln³⁺-based CP-WOLEDs, to the best of our knowledge, is also not reported. Noticeably, contributed from the unique f–f transitions, the Eu³⁺/Tb³⁺/Sm³⁺-centred high-purity (red/green/orange) primary colour is relatively fixed, and receives much less interference from external stimulations (such as ligand-field, pH, temperature, concentration, photo-excitation or electric-driving, etc.).¹⁷ Therefore, despite great efforts towards white-light emitters¹⁸ with desirable g_PL sizes, chiral organo-Eu³⁺/Tb³⁺/Sm³⁺ complexes should offer a conceptual opportunity for CP-WOLEDs. Moreover, in comparison to the chiral non-Ln³⁺-dye counterparts,⁸ the salient advantages of their white-light stability and high CPL-activity could be expected during photo-excitation and/or electric-driving.

Encouraged by the progressive advance of organo-Eu³⁺/Tb³⁺/Sm³⁺-colour-primary OLEDs¹⁹ or white-light emitters,²⁰ it is of significance to explore the state-of-the-art chiroptical activity of chiral organo-Eu³⁺/Tb³⁺/Sm³⁺ complexes toward desirable CP-WOLEDs while not monochromatic CP-OLEDs.^14–16 Considering the simplification of white-light modulation²¹ from a dichromatic while not typical trichromatic (RGB) strategy, herein, one efficient chiral organo-Sm³⁺ complex [Sm(tta)₃(D-phen)] (see Scheme 1) is molecularly designed. The conception points include: (i) besides the lower triplet state (¹T) energy of Htta than that of D-phen, the compatibility between the ¹T of Htta and the first excited state (⁴G_5/2) of the Sm³⁺ ion should engender efficient and high-purity orange-light for [Sm(tta)₃(D-phen)]; (ii) motivated by the higher |g_lum| while shorter lifetime values of organo-Sm³⁺ complexes than those of the corresponding organo-Eu³⁺/Tb³⁺ counterparts in some cases,¹³ strong CPL-activity and relatively low efficiency-roll-off from [Sm(tta)₃(D-phen)] can be expected; (iii) most importantly, keeping Sm³⁺-centred stable and high-purity orange-light at hand and further using the popular blue-emitting PVK (poly(N-vinylcarbazole) as the colour-compensated host for low-cost solution-processed device fabrication, the targeted [Sm(tta)₃(D-phen)]-doped CP-WOLED ruled by the blue-orange dichromatic principle, could be realized. And thus, this present research, to the best of our knowledge, is the first example of chiral organo-Ln³⁺-based CP-WOLEDs.

Scheme 1 Structural scheme of the designed chiral organo-Sm³⁺ complex [Sm(tta)₃(D-phen)].

The chiral Phen (phenanthroline) derivative D-phen was synthesized in a higher yield (72% versus 65%) according to the improved synthetic procedure (Scheme S1, ESI†) for 2-L-Phen by the stronger Lewis-base KH instead of NaH as in the literature.²² Following the full characterization by EA, FT-IR and ¹H NMR (ESI†), its chiroptical activity was further checked to afford [α]²⁵_D = 56.0 ± 0.1°, which almost resembled that ([α]²⁵_D = 50°) of (+)-menthol, showing the retention of the original absolute configuration. Also as shown in Scheme S1 (ESI†), further through the self-assembly of the chiral ancillary D-phen with the complex precursor [Ln(tta)₃(H₂O)] (Ln = Sm or La),²³ the corresponding chiral organo-Ln(III) complex [Sm(tta)₃(D-phen)] or [La(tta)₃(D-phen)] was obtained as the iso-structural product, respectively. The two chiral complexes [Sm(tta)₃(D-phen)] and [La(tta)₃(D-phen)] soluble in common organic solvents except water, were well characterized by EA, FT-IR, ¹H NMR and ESI-MS (see ESI†). In particular, based on the ¹H NMR spectrum (Fig. 1) of anti-magnetic²⁴ counterpart [La(tta)₃(D-phen)], the signals (δ = 9.57–0.75 ppm) of both (tta)⁻ and D-phen proton resonances were identified, respectively, which together with a stipulated 3 : 1 molar ratio, well confirms its desirable binary tris-(β-diketonate)-La³⁺ component. Meanwhile, as compared with the free Htta (δ = 7.83–6.45 ppm), the high-field shifts of all the proton signals (δ = 7.54–6.14 ppm) to the coordinated (tta)⁻ ligands were observed. However, in contrast to the free D-phen (δ = 9.17–0.78 ppm), the aromatic proton resonances (δ = 9.57–6.93 ppm) of the coordinated D-phen within were significantly low-field shifted, which, also upon La³⁺ coordination, should be the reason for the broadened proton resonances (δ = 9.57–0.75 ppm) of [La(tta)₃(D-phen)]. The thermogravimetric analysis (TGA) result (see Fig. S1, ESI†) shows that [Sm(tta)₃(D-phen)] has similar thermal stability to [La(tta)₃(D-phen)], and the decomposition (5 wt% loss) temperature of about 300 °C is sufficient for the following device fabrication.

Fig. 1 ¹H NMR spectra of the free ligands Htta and D-phen, and their chiral complex [La(tta)₃(D-phen)] in CDCl₃ at room temperature, respectively.

The optical properties of the chiral complex [Sm(tta)₃(D-phen)] in solution were explored at room temperature, and the results are summarized in Fig. 2. In contrast to the UV-visible absorptions of the free ligands (229, 244, 271 and 314 nm for D-phen, 262 and 329 nm for Htta; Fig. S2, ESI†), the ligand-centred while significantly red-shifted (229, 246, 284 and 330 nm; see Fig. 2(a)) absorption bands of the chiral complex [Sm(tta)₃(D-phen)] are observed, due to the Sm³⁺-coordination. Upon photo-excitation, λ_ex within the 230–420 nm range just renders the Sm³⁺-centred emissions for the chiral complex [Sm(tta)₃(D-phen)], as shown in Fig. 2(b), where the splitting λ_em = 565, 595, 643 and 711 nm, corresponding to ⁴G_5/2 → ⁶H_J/2 (J = 5, 7, 9, 11) transitions of Sm³⁺ ion, are assigned, correspondingly. Noticeably, the highest intensity (λ_em = 643 nm) is at the hypersensitive ⁴G_5/2 → ⁶H_9/2 transition and the lowest one (λ_em= 711 nm) from the ⁴G_5/2 → ⁶H_11/2 transition, meaning that the central Sm³⁺ ion is located in a site without inversion symmetry.²⁵ Meanwhile, apart from the absence of the ligand-centred emissions, the integration of the two electric dipole (μ) transitions above and the other two ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_5/2 magnetic dipole (m) transitions (λ_em = 565 and 595 nm), engenders a bright colour-purity orange-light with the CIE (Commission International De L’Eclairage) coordinates of x = 0.604, y = 0.371. Moreover, its outstanding photoluminescence is further reflected from the attractive Φ_PL of ca. 10% in the solid-state, which is almost comparable to that²⁶ of non-chiral complex [Sm(tta)₃(Phen)]. Furthermore, the decay lifetime of the Sm³⁺-centred (λ_em = 643 nm) transition for the chiral complex [Sm(tta)₃(D-phen)] is 72 μs, distinctively shorter than those (10² μs grade) of typical tris-(β-diketonate)-Eu³⁺/Tb³⁺ complexes,²⁷ and thus, it renders an additional opportunity for relatively weak efficiency-roll-off during device application.

Fig. 2 (a) Normalized absorption of chiral [Sm(tta)₃(D-phen)] in solution and emission of PVK-PBD (65 : 30; weight ratio) in the solid-state (λ_ex = 273 nm); (b) emission, (c) CD or (d) CPL spectra of chiral [Sm(tta)₃(D-phen)] in solution at room temperature, respectively.

To deeply understand the photo-physical behaviour of the chiral complex [Sm(tta)₃(D-phen)], TD-DFT (time-dependent density functional theory) calculations of the anti-magnetic chiral complex [La(tta)₃(D-phen)] counterpart for simplification were carried out, and the details are summarized in Tables S1, S2 (ESI†) and Fig. 3. As shown in Fig. 3, similar domination from three (tta)⁻ ligands to each of the HOMOs (12.16/32.47/53.96% for the HOMO; 50.03/12.54/36.06% for the HOMO−1; 36.65/53.02/7.88% for the HOMO−2), and the most contribution (85.05%) also from three (tta)⁻ ligands in non-equivalent mode (33.60/45.47/5.96%) to the LUMO+1 is observed. However, a substantial proportion (12.67%) from the D-phen ligand to the LUMO+1 cannot be neglected, which is different from the minor ones to all the HOMOs. As to the LUMO or the LUMO+2, it is mostly localized at the chiral D-phen unit (94.64% versus 86.37%). Through the calculated HOMO → LUMO transition (388 nm) dominated for the S₀ → S₁ excitation, the experimentally determined low-energy (over 330 nm; also see Fig. 2(a)) should mainly be attributed to the ¹LLCT (LLCT = ligand-to-ligand charge transfer; (tta)⁻ to D-phen) transition. Accordingly, besides the HOMO–LUMO energy of 3.69 eV, a theoretical first excited state level (¹T; ³π–π*) of 2.453 eV (19802 cm⁻¹) can further be calculated. By checking the energy level match between the ³π–π* and the ⁴G_5/2 (17 064 cm⁻¹) of the Sm³⁺ ion, a suitable energy gap ΔE (2738 cm⁻¹) within the ideal 2500–4000 cm⁻¹ range according to the so-called Latva's empirical rule,²⁸ reasonably confirms the effective sensitization (see Fig. S3, ESI†) of Sm³⁺ ions. Therefore, not relevant to the chirality but beneficial to the strengthened absorption of D-phen, the efficient and colour-purity Sm³⁺-centred orange-light for its chiral complex [Sm(tta)₃(D-phen)] is understandable.

Fig. 3 The HOMO and LUMO patterns for the chiral complex [Sm(tta)(D-phen)] based on its optimized S₀ geometry, respectively.

The chiroptical properties including CD (circular dichroism; also see Fig. 2(c)) and CPL (see Fig. 2(d)) spectra of the chiral complex [Sm(tta)₃(D-phen)] in solution were studied. As shown in Fig. 2(c), the evident CD signals are exhibited, indicative of the chirality retention²⁹ upon complex formation. And in contrast to the Cotton-effect bands (Fig. S4 (ESI†); (+)-231, (−)-246, (+)-267 and (+)-312 nm) of D-phen, the significantly red-shifted Cotton-effect ones at (+)-245, (−)-291, (+)-327 and (−)-365 nm arising from the Sm³⁺-coordination are observed for the chiral complex [Sm(tta)₃(D-phen)]. In particular, the low-energy Cotton-effect absorption can be assigned to the exciton-coupled CT transition. Upon photo-excitation, as shown in Table S3 (ESI†) and Fig. 2(d), the typically Sm³⁺-centred CPL spectrum of the chiral complex [Sm(tta)₃(D-phen)] was exhibited, where based on the corresponding ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_5/2 magnetic dipole (m) allowed transitions, the largest g_PL of +0.009 is detected at λ_em = 560 nm ascribed to the ⁴G_5/2 → ⁶H_5/2 transition. It is worth noting that the D-phen-ancillary |g_PL|^Max value (0.009) is relatively lower than those (0.10–1.15; |g_PL|) of the previous chiral organo-Sm³⁺ complexes³⁰ with chirality from the N^O/O^O main ligands, whilst their ³π–π* energies are too high to effectively sensitize the Sm³⁺-centred orange-light. On the other hand, besides the top level of the |g_PL| = 0.009 among those of the reported chiral organo-Sm³⁺complexes with chirality from the ancillary ligands,³¹ the (tta)⁻-incorporation leads to the attractive Φ_PL (ca. 10%) for its high-purity orange-light. Importantly, the uniquely CPL-active property is unreachable from chiral non-organo-Ln³⁺ sources (|g_PL| values within the 10⁻⁵∼10⁻⁴-grade),³² despite the more efficient while non-high-purity orange-light.

Considering the strong CPL activity and high-purity orange-light arising from the chiral complex [Sm(tta)₃(D-phen)], it is of special interest for low-cost solution-processed CP-WOLEDs. Through its doping into the commercial electron-transporting PBD (2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4-oxadiazole) and the hole-transporting PVK mixture (5 : 30 : 65, weight ratio) as the EML (emitting layer), the colour-integration with blue-light (also see Fig. 2(a)) from the bipolar PVK-PBD host and the orange-light from [Sm(tta)₃(D-phen)] should be compensated forward to a desirable CP white-light. In particular, besides an effective Förster energy transfer³³ confirmed from the significant spectral overlap (also see Fig. 2(a)) between the emission of PVK-PBD and the CT absorption of [Sm(tta)₃(D-phen)], its experimentally obtained HOMO/LUMO energies (−5.40/−2.17 eV; CV as Fig. S5, ESI†) fall well within those (−5.80/−2.16 eV) of PVK-PBD, making PVK-PBD a suitable bipolar host. Profiting from a peculiar stepwise alignment of the HOMO/LUMO levels from the EML to TPBi (−6.20/−2.70 eV; further facilitating electron-transport) and to BCP (−6.70/−3.20 eV; hole-blocking), the carrier-balancing device with ITO/PEDOT:PSS (40 nm)/EML (80 nm)/TPBi (30 nm)/BCP (10 nm)/LiF (1 nm)/Al (100 nm) was configured (see Fig. 4(a and b)).

Fig. 4 (a) Device structure and energy level diagram; (b) molecular structures of PBD, BCP and TPBi; (c) electroluminescent spectra; (d) J–V and L–V curves; (e) η_EQE–L, η_P–L and η_C–L curves; (f) g_EL–λ_EL curve for the [Sm(tta)₃(D-phen)]-doped CP-WOLED, respectively.

As expected, upon illumination with the turn-on voltage (V_on at 1 cd m⁻²) of 7.0 V, the normalized electroluminescent spectra (Fig. 4(c)) in the applied bias voltage range of 7.0–13.0 V exhibit the simultaneous emissions of the host-based blue-light (λ_em = 424 nm) and the Sm³⁺-centred orange-light (⁴G_5/2 → ⁶H_J/2 (J = 5, 7, 9, 11) transitions). Noticeably, although the dichromatic colour-integration with CIE coordinates x = 0.268–0.297, y = 0.203–0.224 is highly dependent on the applied bias voltage (see Table S4, ESI†), all the emissive colours fall well within the desirable white-light regime. Moreover, all the warm-white-lights, endowing the CRIs (colour render indices) of 96–97 and the CCTs (correlated colour temperatures) of 2625–2631 K, mostly integrated from Sm³⁺-characteristic orange-light, are relatively stable. Among them, the minor colour-coordinate shifts (|Δx| = 0.029 and |Δy| = ≤ 0.021) with different blue-to-orange relative intensity ratios should probably be due to doping-induced phase-separation of the EML. As shown in Fig. 4(d and e), in contrast to the monotonous increase of the current density (J, mA cm⁻²) or the luminance (L, cd m⁻²) with increasing the applied bias voltage, all the efficiencies (η_C (current efficiency; cd A⁻¹)), η_P (power efficiency; lm W⁻¹) and η_EQE (external quantum efficiency)) increase instantly and then decrease steadily throughout the whole illumination with the η^Max_C = 1.61 cd A⁻¹, the η^Max_P = 0.59 lm W⁻¹ and the η^Max_EQE = 1.55% at 8.5 V (L = 3.86 cd m⁻²), respectively. Even at a practical luminance of 100 cd m⁻², the considerable efficiencies (η^Max_C = 1.09 cd A⁻¹, η^Max_P = 0.36 lm W⁻¹ and η^Max_EQE = 1.03%) can be maintained, from which, the efficiency-roll-offs of 32–39% are acceptable. Intriguingly, as shown in Table S5 (ESI†) and Fig. 4(f), the sizable dissymmetric factor |g_EL|^max of 0.011 at λ_em = 560 nm (⁴G_5/2 → ⁶H_5/2 transition of Sm³⁺ ion) is kept.

As compared with those (η^Max_C, 2.0–50.0 cd A⁻¹; |g_EL| ∼ 10⁻³) of the reported CP-WOLEDs from chiral bis-benzoxanethone^8a or spiro-type TADFs,^8b the η^Max_C = 1.61 cd A⁻¹ of the [Sm(tta)₃(D-phen)]-doped CP-WOLED is relatively lower while satisfactory enough to that (η^Max_C > 0.5 cd A⁻¹) for portable full-colour 3D displays.⁹ Undoubtedly, one of the merits for the [Sm(tta)₃(D-phen)]-doped CP-WOLED rests with the |g_EL| size increased by one order of magnitude. More importantly, another evident advantage lies in the high-quality and stable white-lights mainly integrated with the Sm³⁺-centred orange-light. On the other hand, the device performance of the [Sm(tta)₃(D-phen)]-doped CP-WOLED is also at the top-level among those (η^Max_C = 0.65–4.90 cd A⁻¹) of the previously reported WOLEDs³⁴ from non-chiral organo-Sm³⁺ dyes (Φ_PL= 5.6–8.1%). Saliently, the higher-efficiency (η^Max_C = 1.61 cd A⁻¹ and η^Max_EQE = 1.55%) should inherently benefit from the improved Φ_PL up to 10% from [Sm(tta)₃(D-phen)]. In the meantime, relying on the PBD/TPBi-facilitated electron-transport, more effective carrier confinement and recombination in the EML should be in a subordinate position. Furthermore, upon a stepwise alignment³⁵ of the HOMO/LUMO levels combined with the BCP-interface blocking, the desirable carrier balance might take into effect and especially benefit the relatively weak efficiency-roll-offs.

In conclusion, through the incorporation of the enantiopure D-phen as the ancillary, its chiral complex [Sm(tta)₃(D-phen)] displaying both efficient (Φ_PL = 10%) Sm³⁺-centred colour-purity orange-light and strong CPL activity (g_PL = 0.009; ⁴G_5/2 → ⁶H_5/2 transition) was molecularly designed. Moreover, by using the chiral complex [Sm(tta)₃(D-phen)] as the dopant, the resulting CP-WOLED (η^Max_CE = 1.61 cd A⁻¹) and |g_EL|^Max = 0.011) was successfully developed. Noticeably, this research work, as the first example of chiral organo-Ln³⁺-based CP-WOLEDs, suggests that chiral organo-Sm³⁺/Eu³⁺/Tb³⁺complexes should be promising candidates for portable 3D full-colour displays.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the NSFC (21373160, 21173165, 51873176, 22201229), the Young Elite Scientists Sponsorship Program by the Association of Shaanxi Science and Technology (20220405), the Hong Kong Research Grants Council (PolyU153058/19P), the CAS-Croucher Funding Scheme for Joint Laboratories (ZH4A), the Hong Kong Polytechnic University (1-ZE1C and YW4T), Research Institute for Smart Energy, and the Endowed Professorship in Energy from Ms Clarea Au (847S).

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Footnotes

† Electronic supplementary information (ESI) available: Starting materials and characterization; UV; PL. See DOI: https://doi.org/10.1039/d2tc04802g

‡ These authors contributed equally and should be considered co-first authors.

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