A Unified Model to Explain the Large Chiroptical Effects in Polymer Systems Through Natural Optical Activity

Abstract

<div><div><div><p>Polymer thin films that emit and absorb circularly polarised light have been demonstrated with the promise of achieving important technological advances; from efficient, high-performance displays, to 3D imaging and all-organic spintronic devices. However, the origin of the large chiroptical effects in such films has, until now, remained elusive. We investigate the emergence of such phenomena in achiral polymers blended with a chiral small-molecule additive (1-aza[6]helicene) and intrinsically chiral-sidechain polymers using a combination of spectroscopic methods and structural probes. We show that – under conditions relevant for device fabrication – the large chiroptical effects are caused by coupling of electric and magnetic transition dipole moments (natural optical activity), not structural chirality as previously assumed, and may occur because of local order in a cylinder blue phase-type organisation. This disruptive mechanistic insight into chiral polymer thin films will offer new approaches towards chiroptical materials development after almost three decades of research in this area.</p></div></div></div>

Highly Selective High‐Speed Circularly Polarized Photodiodes Based on π‐Conjugated Polymers

“…[23][24][25] We have since postulated that the origins of these chiroptical phenomena lie in the formation of a weakly ordered double-twist cylinder blue phase, where the aza [6]H serves to template the polymers into twisted fibrils with strong coupling between electric and magnetic transition dipole moments. [26][27][28] Here we report the realization of highly selective CP OPDs based on a simple, planar heterojunction architecture comprising a poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2):aza [6]H blend electron donor layer and a C 60 electron acceptor layer. To the best of our knowledge, these devices represent the highest photocurrent dissymmetry ever reported for a CP OPD (|g ph | = 0.72 at zero bias), along with fast response times (t rise/fall ≈ 7 µs) that are three orders of magnitude faster than those reported for all other CP photodetecting devices.…”

Highly Selective Ultrafast Circularly Polarized Photodiodes Based on π-Conjugated Polymers

“…We previously showed that when considering thick films (tD >150 nm) the true gabs of our annealed blend materials does not vary with thickness. [26] The 6 same does not hold true for the thin films (tD <150 nm) evaluated here, which we attribute to the strong optical interference of forward and backward traversing waves caused by multiple reflections at the substrate-film and other neighboring layers' interfaces, typical of optically thin films. [28] The CD (Figure 1b), as well as the apparent |gabs| (Figure S4), increases with increasing tD and are equal-andopposite for [M]-and [P]-aza [6]H blends.…”

“…[28] The CD (Figure 1b), as well as the apparent |gabs| (Figure S4), increases with increasing tD and are equal-andopposite for [M]-and [P]-aza [6]H blends. [26] Irrespective of the polarization of the excitation, the EQE decreases as tD increases (Figure 1c). As can be expected from the increasing gabs, |gph| values corresponding to the spectral region of first CD Cotton band (~480 nm) increase with increasing tD, from ~0.15 at tD = 81 nm to ~0.41 for tD = 110 nm (shown in Figure 1d case of an [M]-aza [6]H-doped CP OPD).…”

“…The use of thermal annealing to induce a giant chiroptical response in F8T2:aza [6]H blends has already been evaluated by our group, and, unless stated otherwise, we followed the optimized protocol (140 °C for 10 minutes in a N2 filled glovebox) for all experiments. [26] The naming convention for LH−CPL and RH−CPL is illustrated in the SI (Figure S1). To ensure that the chiral phase is not impacted by the subsequent deposition of the C60 layer, we compared the CD spectra of donor-only thin films (F8T2:aza[6]H) to those obtained for the donor-acceptor (D-A) bilayer heterojunction (F8T2:aza [6]H-C60), and find no evidence of the thermally evaporated C60 layer disrupting the formation of the chiral phase (Figure S2).…”

“…[23][24][25] We have since postulated that the origins of these chiroptical phenomena lie in the formation of a weakly ordered double-twist cylinder blue phase, where the aza [6]H serves to template the polymers into twisted fibrils with strong magneto-electric coupling. [26] Here we report the realization of highly selective CP OPDs based on a simple, planar heterojunction architecture comprising a poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2):aza [6]H blend electron donor layer and a C60 electron acceptor layer. To the best of our knowledge, these devices represent the highest photocurrent dissymmetry ever reported for a CP OPD (|gph|= 0.72 at zero bias), along with ultrafast response times (trise ≈ 50 ns; tfall ≈ 270 ns) that are four orders of magnitude faster than those reported for all other CP photodetecting devices.…”