Achieving high circularly polarized luminescence with push–pull helicenic systems: from rationalized design to top-emission CP-OLED applications

While the development of chiral molecules displaying circularly polarized luminescence (CPL) has received considerable attention, the corresponding CPL intensity, g_lum, hardly exceeds 10⁻² at the molecular level owing to the difficulty in optimizing the key parameters governing such a luminescence process. To address this challenge, we report here the synthesis and chiroptical properties of a new family of π-helical push–pull systems based on carbo6]helicene, where the latter acts as either a chiral electron acceptor or a donor unit. This comprehensive experimental and theoretical investigation shows that the magnitude and relative orientation of the electric (μ_e) and magnetic (μ_m) dipole transition moments can be tuned efficiently with regard to the molecular chiroptical properties, which results in high g_lum values, i.e. up to 3–4 × 10⁻². Our investigations revealed that the optimized mutual orientation of the electric and magnetic dipoles in the excited state is a crucial parameter to achieve intense helicene-mediated exciton coupling, which is a major contributor to the obtained strong CPL. Finally, top-emission CP-OLEDs were fabricated through vapor deposition, which afforded a promising g_El of around 8 × 10⁻³. These results bring about further molecular design guidelines to reach high CPL intensity and offer new insights into the development of innovative CP-OLED architectures.

A CPL intensity of up to 3 × 10⁻² is achieved in π-extended 6-helicene derivatives, owing to an intense helicene-mediated exciton coupling. Corresponding top-emission CP-OLEDs afforded a promising g_El of around 8 × 10⁻³.

The design of chiral emitters displaying intense circularly polarized luminescence (CPL) has attracted significant interest, thanks to the potential of CP light in a diverse range of applications going from chiroptoelectronics (organic light-emitting diodes (OLEDs), optical information processing, etc.) to bio-imaging and chiral sensing.¹ Recently, designing OLEDs with CP electroluminescence (CP-OLEDs) has emerged as an interesting approach to improve high-resolution display performance. Namely, using unpolarised OLEDs, up to 50% of the emitted light can be lost due to the use of antiglare polarized filters.² In CP-OLEDs, the electro-generated light can pass these filters with less attenuation owing to its circular polarization and thus lead to an increase of the image brightness with lower power consumption.³ To develop CP-OLED devices, the main approach relies on the doping of the device''s emitting layer by a CPL emitter, which should ensure simultaneously high exciton conversion and a high degree of circular polarization. The harvesting of both singlet and triplet excitons has been successfully addressed using either chiral phosphorescent materials or thermally activated delayed fluorescence (CP-TADF) emitters with device efficiencies of up to 32%.⁴ However, the intensity of circularly polarized electroluminescence (CPEL), evaluated by the corresponding dissymmetry factor g_El, remains inefficient and typically falls within the range of 10⁻³ with limited examples reaching g_El > 10⁻² based on polymeric materials and lanthanide complexes.⁵ For CP-OLEDs using a molecular chiral emissive dopant, g_El, defined as the ratio between the intensity difference of left- and right-CPEL, and the total generated electroluminescence, 2(El_L − El_R)/(El_L + El_R), can be generally related to the luminescence dissymmetry factor g_lum measured in diluted solution.² Accordingly, it is of crucial importance to design luminescent molecules with high g_lum values,^3,28a–d,29 in order to reach strong CP electro-luminescence when going to practical devices. However, structural and electronic factors that govern the CPL of chiral compounds are still poorly understood even if a few studies have recently tried to rationalize and establish molecular guidelines to obtain high g_lum values.⁶Our team has contributed to the research in this area by developing extended π-helical molecular architectures resulting from the association of carbo6]helicene and achiral dyes,⁷ which afforded enhanced chiroptical properties, with notably a g_lum up to 10⁻², owing to an uncommon chiral exciton coupling process mediated by the chiral helicenic unit.⁸ In addition, we also described an unusual solvent effect on the intensity of CPL of π-helical push–pull helicene–naphthalimide derivatives,^7b which showed a decrease of g_lum from 10⁻² to 10⁻³ upon increasing the polarity of solvent.^7b This solvatochromism effect was shown to be related to a symmetry breaking of the chiral excited state before emission,⁹ which modifies the relative intensity of the magnetic (μ_m) and electric (μ_e) dipole transition moments, and the angle, θ, between them (Fig. 1), ultimately impacting g_lum. The latter is well approximated as 4|m|cos θ/(|μ|) for an electric dipole-allowed transition.¹⁰Open in a separate windowFig. 1Chemical structures of “push–pull” 2,15-diethynylhexahelicene-based emitters with their polarized luminescence characteristics including their calculated electric and magnetic transition dipole moments and the angle between them corresponding to the S₁ → S₀ transition.While these results highlight interesting aspects regarding the key parameters influencing the CPL of organic emitters, this type of “helical push–pull design” remains limited to only one example, which render the systematic rationalization of these findings difficult. Accordingly, we decided to develop a complete family of new chiral push–pull compounds to explore the structural and electronic impact of the grafted substituents on the helical π-conjugated system. In addition, we went a step further and incorporated the designed chiral emitter into proof-of-concept CP-OLEDs using a top-emission architecture,¹¹ which remains scarcely explored for CP-light generation despite its considerable potential for micro-display applications. To the best of our knowledge, only one example of such type of electroluminescent device has been reported, using a CP-TADF emitter, affording a modest g_El of 10⁻³.^11aHerein, we report the synthesis and chiroptical properties of a new family of π-helical push–pull systems based on chiral carbo6]helicene, functionalized by either electron donor or acceptor units. Interestingly, the chiral π-conjugated system of the helicene may act as either an electron acceptor or a donor, depending on the nature of the attached substituents, thereby impacting the chiroptical properties, notably the resulting CPL. By optimizing the chiral exciton coupling process through the modulation of the magnitude and relative orientation of the electric (μ) and magnetic (m) dipoles, the chiroptical properties of classical carbo6]helicene-based emitters can be dramatically enhanced and reach high g_lum values at the molecular level, i.e. up to 3–4 × 10⁻². Experimental and theoretical investigations revealed that the mutual orientation of the electric and magnetic dipoles in the excited-state is a crucial parameter and is optimal when the substituents attached to the helicene core possess a rather weak electron withdrawing or donating ability. Finally, proof of concept top-emission CP-OLEDs were fabricated through vapor deposition of π-helical push–pull derivatives and afforded a g_El of around 8 × 10⁻³, which represents a significant improvement for the polarization of electroluminescence emitted using this device architecture.