Theoretical study on some carbohelicenes dyes with tunable emission wavelength: Optical properties, vibronic effect, quantum yield

This work presents a theoretical study on a series of full-color-tunable wavelength carbohelicene dyes to understand what controls the color of fluorescence and quantum efficiency at the quantum level. The tunable wavelength of the experimental spectra and substituent effect on the spectral properties have been reproduced and analyzed. The results show that the introduction of NMes2 and (or BMe2) enlarges the conjugated length, stabilizes the LUMO and lowers the HOMO-LUMO gap which finally tunes the spectral wavelength. Though the pure electronic calculations reproduce well the relative position of different spectra, vibronic effect is further considered to make up the limitation of pure electronic spectra in reproducing the experimental fine structures. To further reveal the features of excited states, the radiative rate (kr) and non-radiative rates (kIC and kISC) are computed to predict the quantum yield (QY). The calculated QY agree well with experimental ones. The results show that the introduction of BMes2 (7B-HC) increases kr and reduces kISC which finally leads to the increase of QY. We also show that the competition between kr and kISC is the dominant reason for the quenched QY of molecule 7B-HC by 35%. The large SOC and small energy gap between S1 and the lowest six triplet excited states are highlighted for the highly accelerated ISC processes. The nice agreement with the experiment indicates that the QY calculation method can be provided as an efficient approach for the prediction of promising fluorescent dyes.