Synthesis and Photophysical Properties of Some Rigidized Hepta‐ and Nonamethine Mono‐ and Bis(merocyanines): Ring‐Opening of Quaternized 2‐Methylbenzothiazole

We have developed a simple and efficient method for the synthesis of rigidized mono- and bis-heptamethine merocyanines that contain one methine unit incorporated into a cyclopentene framework. The synthetic approach is based on C1/C5 regioselective nucleophilic addition of mono- and diketone lithium enolates to readily available pentamethine cyanines with inherent rigidity followed by a Hofmann elimination reaction. The new dyes react easily with heterocyclic benzothiazolium compounds containing an activated methyl group. Depending on the reaction conditions one or two reactive centres can be attacked, thus giving access to new merocyanine derivatives with a lengthened polymethine chain and/or ramified by a second heterocyclic unit. X-ray crystallographic analysis reveals that the second heterocycle is incorporated via a sulfur bridge as a result of the opening of the benzothiazole ring. All new compounds were fully characterized by 1D and 2D NMR spectroscopic techniques (1H, 13C, DEPT, COSY, NOESY, HSQC with and without decoupling, HMBC). Their photophysical properties have been studied and correlated with solvent polarities. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)