Micelle formation and red-light fluorescence emission of benzothiadiazole–triphenylamine amphiphilic molecules in water/methanol solutions explored with synchrotron small-angle X-ray scattering

Fluorescent organic dyes that emit red and infrared light, ranging from 600 to 1000 nm in wavelength, have quite a wide range of applications as biomarkers because light of these wavelengths is highly transmissible through organs and cells. One way to create such red-light fluorescence is to use the interaction between electron–donor and electron−acceptor moieties, which in most cases are covalently bonded nearby each other.¹ Benzothiadiazole is a strong electron-accepting moiety because the thiadiazole withdraws electrons from the adjacent aromatic ring and has the ability to strongly fluoresce.² Recently, Ishi-i et al.^{3, 4} reported that when benzothiadiazole is combined with electron-donating triphenylamines, an intense red light is emitted owing to the electron donor–acceptor interaction. Interestingly, the intermolecular donor–acceptor conjugates do not suffer from concentration quenching; thus, even aggregates or crystals of these conjugates fluoresce.

The fluorescence intensity is mainly determined by competition between the lifetime of the excited state and the rate at which the excited state is deactivated by non-radiative processes. However, the luminous efficiency of longer-wavelength emissions decreases in polar aqueous media because the highly polarized excited state arising from the donor–acceptor interaction increases the formation of a non-radiative deactivation channel. In other words, the emission of the donor–acceptor system is sensitive to the surrounding environment such as the polarity and the hydrogen bonding ability of a solvent.^{5, 6, 7, 8, 9, 10} In a polar aqueous environment, the donor–acceptor dye tends to adopt a highly polarized excited state such as the charge transfer state,^{11, 12} from which the formation of the non-radiative deactivation channel is accelerated, leading to strong fluorescence quenching. Therefore, in polar solutions, the emission is quenched, which reduces fluorescence efficiency. The extent of fluorescence quenching correlates with the increase in polarity, and this process facilitates the transition from a locally excited state to a highly polarized excited state such as an intramolecular charge transfer state. The polarized excited state is stabilized by the solvation provided by polar solvent molecules that increases the non-radiative deactivation.

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