AM1 semiempirical calculated potential energy surfaces for the isomerization of symmetrical carbocyanines

Semiemipirical ground state (S0) and first excited singlet state (S1) potential surfaces were calculated for the isomerization of symmetrical carbocyanines such as 3,3ʹdiethylthiadicarbocyanine iodide (DTDCI), 1,1ʹ-diethyl-2,2ʹ-dicarbocyanine iodide (DDI), 1,1ʹ-diethyl-2,2ʹ-carbocyanine iodide (DCI), and cryptocyanine (1,1ʹ-diethyl-4,4ʹcarbocyanine) iodide (CCI), as a function of the twist angle around a carbon¯carbon bond of the polymethine chain. It was found that the isomerization potential surface depends on the polymethine chain length and its position on the quinoline moiety. The results of this study also indicate that steric hindrance associated with the rotation around a carbon¯carbon bond plays an important role in the isomerization dynamics of cyanines, which correlates with the photoisomer lifetime that was measured from picosecond time-resolved fluorescence spectra.