Circularly Polarized Light Enhancement by Helical Polysilane Aggregates Suspension in Organic Optofluids

Circularly polarized (CP) light may play key roles in the migration and delocalization of photoexcited energy in optically active macroscopic aggregates of chiral chlorophylls surrounded by an aqueous fluid in the chloroplasts under incoherent unpolarized sunlight. Learning from the chiral fluid biosystem, we designed artificial polymer aggregates of three highly luminescent helical polysilanes, 1-S, 2-S, and 2-R (Chart 1). Under specific conditions (molecular weights and good-and-poor solvent ratio), 1-S aggregates with similar to 5 mu m in organic fluid generated an efficient circularly polarized luminescence (CPL) with g(CPL), = -0.7 at 330 nm while retaining a high quantum efficiency (Phi(PL)) similar to 53% at room temperature under incoherent unpolarized photoexcitation at 290 nm. This huge g(CPL) value was the consequence of the intense bisignate circularly dichroism (CD) signals (g(CD) = -0.35 at 325 nm and +0.31 at 313 nm) due to coupled oscillators with electric-dipole-allowed-transition origin. Also, 2-S and 2-R aggregates gave almost identical intense CD and CPL amplitudes of 1-S. The most critical factors for the CD/CPL enhancements were the molecular weights of 1-S, 2-S, and 2-R and a refractive index of good/poor cosolvents. The former was connected to a long persistence length of similar to 70 nm, characteristic of rod-like helical polysilanes. The latter was due to an efficient photoexcited energy confinement effect of slow CP-light in the aggregate.