Time-evolved, far-red, circularly polarised luminescent polymer aggregates endowed with sacrificial helical Si–Si bond polymers

Abstract

An enantiomeric pair of non-charged, helical dialkylpolysilanes (PSi-R and PSi-S) were endowed with circularly polarised luminescence (CPL) at 700 nm and circular dichroism (CD) at 660 nm when non-helical poly{[dioctylfluorene]-alt-[bis(thiophenyl)-benzothiazole]} (PF8DBT) formed a hetero-aggregate with PSi-R and PSi-S in a 1-to-1 ratio as repeating units. The initial ultraweak CPL (g_em) and CD (g_abs) amplitudes of the as-prepared aggregates were magnified up to −0.019 at 706 nm (induced by PSi-S) and +0.017 at 695 nm (induced by PSi-R) after prolonged ageing at ambient temperature for 24 h. These CPL amplitudes were maintained after the selective photoscissoring of PSi-S and PSi-R at 313 nm for 60 s. A time-dependent growth behaviour in the hetero-aggregate size (initially ≈400 nm; reached ≈2000 nm in one day) was observed and characterised using dynamic light scattering (DLS). To investigate the origin of the time-evolution characteristics, the energy potential, dipole moments and CD/UV-Vis-near-infrared (NIR) spectra were simulated upon altering the dihedral angle sets of several model oligomers of PF8DBT using density functional theory (DFT)/time-dependent DFT (TD-DFT) (B3LYP/6-31G(d,p)). An additional Zerner's Intermediate Neglect of Differential Overlap (ZINDO) calculation qualitatively established that the two hypothetical, chirally dipole–dipole-cancelled, π–π-stacked, slipped dimers cause red shifts in the CD and UV-Vis-NIR spectra, which were associated with an enhanced |g_abs|, ≈1 × 10⁻³ at 715 nm and 0.5 × 10⁻³ at 660 nm, compared to the corresponding monomeric value (|g_abs| ≈ 0.1 × 10⁻³ at 620 nm) obtained using TD-DFT.