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

: Schmidt, S.B.; Kempe, F.; Brügner, O.; Walter, M.; Sommer, M.


Polymer chemistry 8 (2017), Nr.35, S.5407-5414
ISSN: 1759-9954
ISSN: 1759-9962
Deutsche Forschungsgemeinschaft DFG
SO 1213 7-1
Deutsche Forschungsgemeinschaft DFG
WA 1687/9
Fraunhofer IWM ()
ChainsDensity; functional theory; Isomerization; polymer

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 Mw 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.