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 X 10(-4) 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 degrees and its chiroptical response is thereby limited only by the magnetic transition.