Synthesis and characterizations of benzothiadiazole-based fluorophores as potential wavelength-shifting materials

Abstract

► The benzothiadiazole-based fluorophores were synthesized as wavelength-shifting materials for improving the efficiency of man-made solar cells. ► They show specific spectral properties with absorbance from UV to 600 nm and strong emission around 700 nm. ► Their large Stokes shifts were due to the strong intramolecular charge transfer effect during excitation. ► More planar geometry in their excitation states gave them higher quantum yields. The synthesized benzothiadiazole-based series fluorophores as potential wavelength-shifting materials exhibit large Stokes shifts (>160 nm) with multiple broad absorbance bands from UV region to 600 nm and a strong fluorescence peak around 700 nm (in CHCl 3 ). Intramolecular charge transfer (ICT) characters of the synthesized compounds are examined using UV–vis and photoluminescence solvatochromic shift measurements. Among the synthesized compounds, the fluorophores with asymmetrical structures exhibit larger Stokes shifts than those with symmetrical structures due to large dipole moment changes upon excitation. The fluorophores with electron-donating methoxyl groups attached to the triphenylamine donors are found to have strong ICT properties. Photophysical experimental results are supported by theoretical calculations using Density Function Theory (DFT) and Time Dependent Density Function Theory (TD-DFT) methods. Calculated frontier molecular orbitals (MOs) of ground states on these fluorophores showed an increase in ICT character up to 50% from HOMO to LUMO. Geometric optimization calculations of the excited state reveal that these fluorophores show a more planar structure for the excited state than the ground state, which allows more π–π* overlap and leads to larger Stokes shifts and higher quantum yields.