Tuning optical properties of phenanthroline derivatives through varying excitation wavelength and pH values

Organic emissive materials equipped with tuneable optical properties have gained serious attention. Herein, two phenanthroline derivatives (ImPhTz and ImPhOz) have been prepared to possess an imidazole-thiadiazole/oxadiazole core and their tuneable emission behaviors have been investigated. As a result, ImPhTz is described with emission wavelengths spanning the visible region ( λ em = 481–611 nm) with different excitation wavelengths. The absorption (359 nm) and emission (478 nm) wavelengths of ImPhTz were red-shifted 52 nm and 93 nm when the pH value increased from 1.70 to 13.29, respectively. It is worth noting that the fluorescence quantum yields of ImPhTz did not reduce with the increase in pH values. Thus, multicolor photoluminescence has been achieved based on a single organic chromophore. Comparatively, ImPhOz was less sensitive to environmental stimulus due to a lower degree of conjugation. The emission wavelength of ImPhOz is red-shifted 82 nm and 69 nm when the excitation wavelengths and pH values increased, respectively. Moreover, the maximum two photon absorption cross-section value of ImPhTz (190 GM) is also higher than ImPhOz (27 GM). In other words, the optical properties of molecules equipped with thiadiazole may be more sensitive to the environment than those equipped with oxadiazole.