Alkyl-substituted spiropyrans: electronic effects, model compounds and synthesis of aliphatic main-chain copolymers

• Published in: Polymer chemistry 2017-09, Vol.8 (35), p.5407-5414
• Main Authors: Schmidt, Simon B., Kempe, Fabian, Brügner, Oliver, Walter, Michael, Sommer, Michael
• Format: Article
• Language: English
• ISSN: 1759-9954; 1759-9962
• DOI: 10.1039/C7PY00987A

The isomerisation behaviour of spiropyrans (SP) depends considerably on their substituents. This work investigates the synthesis, isomerisation behaviour and polymer chemistry of alkylated spiropyrans. While several Kumada- and Suzuki-based protocols failed to alkylate dibromo-spiropyran, classical 9-BBN chemistry is shown to proceed with high yield suitable also for polycondensation. Alkyl substituents strongly influence the acid-induced SP-merocyanine (MC)-protonated MC (MCH + ) equilibrium and render the MCH + form most stable. In agreement with measured isomerisation rates, density functional theory calculations corroborate the enhanced stabilisation of ethyl-substituted MCH + as compared to phenyl-substituted MCH + . Aliphatic main chain spiropyran copolymers are synthesised from in situ generated bis-9-BBN monomers and dibromo-spiropyrans. The empirical optimisation of stoichiometry yields a molecular weight M w of ∼15 kg mol −1 after purification. This approach to alkylated spiropyrans opens new opportunities to fine-tune the isomerisation behaviour of small molecules and polymeric SP derivatives for potential use as sensors and smart materials.