The synthesis of the chiral monomethine cyanine dyes 8 and starting wi
th the achiral indole 1 is described. Key intermediates are the new ch
iral Fischer bases R- and S-4, which were separated after diastereomer
ic salt formation. The conformation of the dyes 8 and 9 is twisted di-
Z, according to NOE and dynamic NMR spectroscopy. The free energy diff
erence between oppositely twisted conformations is ca. 2.5 kJ mol(-1).