Bromo-Heptahelicene-Bis-Thiadiazole: Photophysics, Chiroptics, and Excited-State Dynamics

The synthesis of a [7]helicene bromide derivative with two fused 2,1,3-thiadiazole heterocycles (TD[7]Br) and a comprehensive study of its photophysical and chiroptical characteristics are presented along with a comparison with 9-bromo-carbo[7]helicene ([7]Br) and 2,15-dibromo-carbo[6]helicene ([6]Br). The integration of a bromine heavy atom onto the helicene backbone facilitates efficient singlet-to-triplet conversion allowing to investigate the resulting fluorescence and phosphorescence properties. The steady-state chiroptical features of the systems are demonstrated through electronic circular dichroism and circularly polarized luminescence. Interestingly, a fluorescence quantum yield of 14% is obtained, a 17-fold increase compared to the corresponding bromo-heptacarbohelicene, and phosphorescence dissymmetry factors reach ±1.2 × 10^-2 at 580 nm at low temperature. Finally, the exploration of various excited states generated during the excitation process and their dynamics is delved into by employing nonpolarized transient absorption and emission spectroscopies, thus, highlighting the fruitful combination of heavy-atom effect and charge transfer. The experimental results are understood through time-dependent density functional theory computations.