Obtaining Enhanced Circular Dichroism in [4]Heterohelicenium Analogues

Electronic circular dichroism in the three helicenium analogues dimethoxyquinacridinium (DMQA(+)), dimethoxychromenoacridinium (DMCA(+)), and dimethoxychromenoxanthenium (DMCX(+)) were investigated in vacuum with time-dependent density functional theory methods using the CAM-B3LYP functional with the basis set 6-311++G**. The systems were thoroughly studied by designing derivatives with several different electron-donating and -withdrawing substituents while at the same time keeping the net charge of the molecule either positive, neutral, or negative. Fifty-six derivatives were examined, and we identify a superior substitution pattern that is found to be independent of the bridging atoms and gives a rotational strength close to 90 × 10(-40) esu(2) cm(2) for DMQA, DMCA, and DMCX. The optimal system shows promising applications as a chromophore because it has a highly allowed primary electronic transition with an angle between the electronic and magnetic transition dipole moments close to 50° and its chiroptical response is thereby limited only by the magnetic transition.