Exploring 2‐D σ−σ* Conjugation in Cyclic Polysiloxane Copolymers

ABSTRACT

Here, we aim to clearly define phenomena previously termed unconventional conjugation involving conjugation across Si─O─Si bonds. The goal is to define operative mechanisms and provide general routes to easily accessible Si─O─Si containing copolymers using commercially available [vinylMeSiO] ₄ , Vy ₄ D ₄ (Vy = vinyl consistent with previous reports). Thus, 1:1 Vy ₄ D ₄ :Ar copolymers synthesized using Heck catalytic cross coupling (DPs 5–20) exhibit absorption λ _max typical of aromatic groups with weak, red‐shifted shoulders now recognized as σ‐σ* transitions. Emission λ _max redshift 40–80 nm from model compounds. Substituent dependent Ar’ substitution at remaining ring vinyls modulates emission photophysics. Thiophene co‐monomers in the main chain or as “decorated” substituents present the most red‐shifted, lowest‐energy excited states. In undecorated main chain copolymers, σ‐σ* conjugation appears operative. In decorated systems, three mechanisms are possible. Conjugation via the ring to the main chain, possible Förster‐type energy transfer involving decorated substituents or both. Excited state lifetime emission studies reveal two different emitting states, suggesting both mechanisms operate. Further proof is obtained on mixing copolymers with F ₄ TCNQ. As with traditional conjugated polymers, formation of F ₄ TCNQ ^−. radical anion indicates electron transfer only seen when conjugation exists. In the current studies, F ₄ TCNQ ^−. forms for both undecorated and decorated copolymers indicating conjugation and σ‐σ* transitions. Most important, decorated Vy ₄ D ₄ or [Vy ₄ D ₄ ]Ar’ _2‐5 systems not part of a copolymer also form the radical anion F ₄ TCNQ ^−. implying that even simple, substituted cyclomers can exhibit σ‐σ* conjugation, suggesting multiple families of easily accessible σ‐σ* conjugated systems. Further proof comes from theoretical modeling, indicating the cyclomer flattens significantly in the excited state. Thus, decorated {[Vy ₄ D ₄ ]Ar’ _2‐4 }‐Ar‐ copolymers offer what can be termed 2‐D conjugation. One question remains: Do the conjugated cyclomers exhibit “ring current” in the excited state via σ‐σ* conjugation?