Effect of Conformational States on the Photophysical Properties of Di-para-biphenyl-benzothiadiazole and a Dimethyl Derivative of It

The authors discuss results from studying the photophysical properties of linear molecules based on 2,1,3-benzothiadiazole, specifically 4,7-di([para-biphenyl]-4-yl)benzothiadiazole (Ph-Ph-BTD) and 4,7-bis(2,5-dimethyl-[1,1'-biphenyl]-4-yl)benzothiadiazole (Ph-Xy-BTD). The synthesis of a new phenylxylene derivative of benzothiadiazole—Ph-Xy-BTD—is described. The thermal stability of the Ph-Ph-BTD and Ph-Xy-BTD compounds is studied. Their parameters of melting and solubility are determined, and the absorption spectra of solutions in n-hexane and THF are recorded. It is shown that having pendant methyl substituents in Ph-Xy-BTD lowers its melting point, increases its solubility, reduces the probability of a transition between the ground and excited state orbitals, and raises the energy of transition. The effect solvation has on the spectra, fluorescence quantum yield, and lifetime of the fluorescence of compounds in THF and n-hexane is studied. Static dipole moments in the ground (HOMO) and excited states (LUMO) of Ph-Ph-BTD and Ph-Xy-BTD molecules are determined. It is shown that the conformation of the excited state (LUMO) changes during the relaxation of the solvate and becomes more planar than that of the ground state (HOMO). A relationship is revealed between the conformation of the excited state of the studied compounds and the polarity of the solvent.