Ab initio (CASPT2) excited state calculations, including circular dichroism, of helically twisted cyanine dyes

Ab initio calculations at the CASSCF/CASPT2 level were performed on helically twisted mono-, tri-, and pentamethine cyanine dyes in the all-Z-configurations. Excitation energies and oscillator and rotatory strengths were calculated for the five lowest energy singlet states. Both the long wavelength methine band and the cis-band could be identified unambiguously from their configurational parentage. The calculated state energies are within 0.09 eV of the experimental value for the methine band and within 0.16 eV for the cis-band. The calculated rotatory strengths of the methine band shows sign inversion as the length of the chromophore increases: negative for the short monomethine, strongly positive for the pentamethine. The trimethine presents a borderline case: the measured rotatory strength is almost nil, the calculated one depends on the geometry. There is good agreement between rotatory strengths calculated in the velocity and in the length formalism.