Soluble and stable alternating main-chain merocyanine copolymers through quantitative spiropyran–merocyanine conversion

Abstract

A combined experimental and theoretical study on the synthesis and solution isomerization behavior of main chain copolymers with multiple spiropyran incorporation is presented. A series of alternating copolymers P(SP-alt-C_x) of spiropyran (SP) and flexible linkers (C_x, x = 6,8,10) is synthesized by Suzuki polycondensation (SPC). Careful ¹H NMR polymer end group analysis is carried out to reveal termination reactions that limit molecular weight. Although methylene indoline and salicyl aldehyde end groups are found arising from SP cleavage during polymerization, acceptable M_n,SEC up to 34 kg mol⁻¹ is achieved. P(SP-alt-C_x) can be transformed into the corresponding protonated form of the red alternating merocyanine (MC) polymer, P(MCH⁺-alt-C_x), in quantitative yield by direct acidification or pulsed ultrasound. The origin of the latter method lies in the sonochemical degradation of chloroform, which provides a continuous source of hydrochloric acid. Deprotonation of P(MCH⁺-alt-C_x) occurs upon the addition of a base resulting in the blue form P(MC-alt-Cx), as characterized by UV-vis spectroscopy and confirmed by density functional theory (DFT) calculations of model compounds. DFT further reveals P(MCH⁺-alt-C_x) to be the thermodynamically most stable form. It is proposed that the para-linkage to the phenyl-based comonomer C_x decreases acidity of protonated merocyanine and hence increases stability of P(MCH⁺-alt-C_x), which poses a marked difference compared to the commonly employed 6-nitro-substituted analogs.