HELICALLY TWISTED CHIRAL CYANINE DYES - INFLUENCE OF CHROMOPHORE LENGTH ON OBSERVED AND CALCULATED ROTATORY STRENGTHS

Synthesis, chiroptical properties, and quantum-mechanical calculations of the monomethine dye 3 and of the trimethine dyes 7 and 11 are repo rted. In 3 and 11, the chromophore is forced into a twisted all-Z-conf ormation by steric interaction of the end groups in the former and the presence of a t-butyl group in the mesoposition of the latter, which is manifest in the UV/Vis spectra not only in the reduced intensity of the longest wavelength absorption, but also in the occurrence, at sho rter wavelength, of a ''cis-peak.'' Chiral substitution of tile end gr oups serves as chiral anchor to discriminate between otherwise enantio meric forms and makes them amenable to chiroptical investigation. The results are in agreement with theoretically calculated chiroptical dat a based on helically twisted cyanine chromophores. They support the co ntention that not only the sense of the helix, but also its length det ermines the sign of the associated Cotton effect. Chirality 9:243-249, 1997. (C) 1997 Wiley-Liss, Inc.