Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers

Five chiral squaraine dimers were synthesized by fusingchiralindolenine semisquaraines with three different benzobisthiazole bridges.The thereby created squaraine dimers show a strong splitting of thelowest energy absorption bands caused by exciton coupling. The intensitiesof the two exciton transitions and the energetic splitting dependon the angle of the two squaraine moieties within the chromophoredimer. The electric circular dichroism spectra of the dimers showintense Cotton effects whose sign depends on the used squaraine chromophores.Sizable anisotropies g (abs) of up to 2.6x 10(-3) could be obtained. TD-DFT calculationswere used to partition the rotational strength into the three Rosenfeldterms where the electric-magnetic coupling turned out to be the dominantcontribution while the exciton chirality term is much smaller. Thisis because the chromophore dimers are essentially planar but the anglebetween the electric transition dipole moment of one squaraine andthe magnetic transition dipole moment of the other squaraine stronglydeviates from 90 & DEG;, which makes the dot product between the twomoment vectors and, thus, the rotational strength substantial.