Exploring 2-D σ-σ* Conjugation in Cyclic Polysiloxane Copolymers

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, Vy₄D₄ (Vy = vinyl consistent with previous reports). Thus, 1:1 Vy₄D₄: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 F₄TCNQ. As with traditional conjugated polymers, formation of F₄TCNQ^-. radical anion indicates electron transfer only seen when conjugation exists. In the current studies, F₄TCNQ^-. forms for both undecorated and decorated copolymers indicating conjugation and σ-σ* transitions. Most important, decorated Vy₄D₄ or [Vy₄D₄]Ar'_2-5 systems not part of a copolymer also form the radical anion F₄TCNQ^-. 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 {[Vy₄D₄]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?