Thermal Truncation of Heptamethine Cyanine Dyes

Cyanine dyes are a class of organic, usually cationic molecules containing two nitrogen centers linked through conjugated polymethine chains. Unlike phototruncation, the thermal truncation (chain-shortening) reaction is a phenomenon that has rarely been described for these important fluorophores. Here, we present a systematic investigation of the truncation of heptamethine cyanines (Cy7) to pentamethine (Cy5) and trimethine (Cy3) cyanines via homogeneous, acid-base catalyzed nucleophilic exchange reactions. We demonstrate how different substituents at the C3′ and C4′ positions of the chain and dif-ferent heterocyclic end groups, the presence of different bases, nucleophiles and oxygen, solvent properties, and tempera-ture affect the truncation process. The mechanism of chain shortening, studied by various analytical and spectroscopic techniques, was verified by extensive ab initio calculation, demonstrating the need to model catalytic reactions by highly correlated wavefunction-based methods. We show that entropic effects control the course of this process. The study provides a critical insight into the reactivity of the polyene chains of cyanines and offers new approaches to the synthesis of meso-substituted symmetrical and unsymmetrical pentamethine cyanines from Cy7 derivatives.