500-Fold Amplification of Small Molecule Circularly Polarised Luminescence through Circularly Polarised FRET

Abstract

Strongly dissymmetric circularly polarised (CP) luminescence from small organic molecules could transform a range of technologies, such as display devices. However, highly dissymmetric emission is usually not possible with small organic molecules, which typically give dissymmetric factors of photoluminescence (g_PL ) less than 10^-2 . Here we describe an almost 10³ -fold chiroptical amplification of a π-extended superhelicene when embedded in an achiral conjugated polymer matrix. This combination increases the |g_PL | of the superhelicene from approximately 3×10^-4 in solution to 0.15 in a blend film in the solid-state. We propose that the amplification arises not simply through a chiral environment effect, but instead due to electrodynamic coupling between the electric and magnetic transition dipoles of the polymer donor and superhelicene acceptor, and subsequent CP Förster resonance energy transfer. We show that this amplification effect holds across several achiral polymer hosts and thus represents a simple and versatile approach to enhance the g-factors of small organic molecules.

Keywords: FRET; chirality; circular dichroism; helical structures; polymers.

Figure 1

A cartoon to indicate the similarities and differences between our a) prior and b) current work. We previously demonstrated that enantiopure aza[6]helicene can be used to induce a chiroptical response in achiral polymer systems, resulting in CP emission from the polymer. Here, CP FRET amplifies the otherwise weak intrinsic response of the small molecule (g _PL: 0.0003) and result in strong CP emission from the helicene (g _PL: 0.15).

Figure 2

Normalised absorption and photoluminescence (λ_ex: 385 nm) spectra of thin films of the donor (PFO, thickness, t=140 nm) and acceptor (oxa[7]H, t=90 nm) systems used in this work. The overlap between the donor emission and acceptor absorption is highlighted in turquoise.

Figure 3

a) Absorbance and b,c) circular dichroism spectra of as‐cast (black, red) and annealed (160 °C, 10 minutes in an N₂ glovebox) (green, blue) PFO:oxa[7]H thin films (t=135 nm).

Figure 4

a) As‐cast (red) and annealed (160 °C, 10 minutes in an N₂ glovebox) (blue) PL spectra (λ_ex: 385 nm) for PFO:oxa[7]H blends (t=135 nm), b) LH and RH PL spectra for PFO:oxa[7]H and the c) CP PL and d) associated g _PL spectra of annealed PFO:oxa[7]H [P] (green) and [M] (blue) thin films.

Figure 5

a) Absorbance, b) circular dichroism, c) photoluminescence and d) photoluminescence dissymmetry of neat and annealed (T=140 °C, 10 minutes) F8BT:oxa[7]H (t=131 nm, λ_ex: 475 nm). Further systems are considered in the Supporting Information.