Natural optical activity as the origin of the large chiroptical properties in π-conjugated polymer thin films

Abstract

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 magneto-electric coupling (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.

CD Imaging at High Spatial Resolution at Diamond B23 Beamline: Evolution and Applications

“…The MMP was calibrated using aqueous solution of 1S (+) 10-Camphorsulfonic acid (5mg/ml) in a 1cm pathlength cuvette to give a positive CD intensity at 290.5nm of about 1500 mdeg. The other polarization elements of the differential Mueller Matrix were calibrated using a thin film of an optoelectronic material (F8BT:aza [P]) characterised by Mueller Matrix Ellpsometry using a Woollam RC2 (model DI) instrument and reported in Wade et al (2020). In Wade at al.…”

“…The other polarization elements of the differential Mueller Matrix were calibrated using a thin film of an optoelectronic material (F8BT:aza [P]) characterised by Mueller Matrix Ellpsometry using a Woollam RC2 (model DI) instrument and reported in Wade et al (2020). In Wade at al. (2020), the homogeneity of the supramolecular structure of the thin film of F8BT:aza [P] was assessed at 50 micron spatial resolution by CDi using the B23 vertical chamber of Figure 2I as the size of the collimated light beam of the Woollam RC2 ellipsometer was about 3-4 mm in diameter.…”

On the factors influencing the chiroptical response of conjugated polymer thin films

“…These are in reasonable agreement with the lowest absorption bands, 375 nm and 475 nm, measured for PFO and F8BT, respectively. 20 Fig. 2a shows the g-factor (calculated using eqn (1)) of the S 1 state of isolated PFO and F8BT oligomers as a function of n with a fixed y = 401, close to the optimised twist angle for F8BT 9 and the glassy-phase PFO.…”

“…11 It has been proposed that this strong CP luminescence occurs due to structural chirality. 8,12,19 However, recent works 9,20 have indicated that long-range structural chirality only dominates the chiroptical response when an alignment layer is used to template molecular packing of polymer chains at or beyond the mesoscale. Highly dissymmetric emission from non-aligned thin polymer films (i.e., those where the film thickness is considerably less than those at which this structural chiroptical phenomena manifest) suggests that intrinsic CP emission may play a significant role.…”

“…Highly dissymmetric emission from non-aligned thin polymer films (i.e., those where the film thickness is considerably less than those at which this structural chiroptical phenomena manifest) suggests that intrinsic CP emission may play a significant role. 20,21 To explore the origins of the strong chiroptical response of polymer-based systems, we turn to time-dependent density functional theory (TDDFT), within the approximation of the CAM-B3LYP exchange and correlation functional 22 (see ESI †). We consider the influence of four variables on the g-factor; chemical composition, the intramolecular twist angle (y) between adjacent repeat units, the number of monomers (n) a single oligomer chain is made of and the role of interchain (intermolecular) coupling.…”