Interplays between Functional Groups and Substitution Sites Modulate the Photophysics of the Bithiophenes

Bithiophene has an electron-rich conjugated ring, enabling highly tunable photophysical properties for the design of novel organic light-emitting materials. Extensive research was focused on the functionalization of α-site-connected bithiophene, while recent work reported the synthesis of β-bithiophene, substantially enlarging the chemical space for bithiophene design. However, the design rule for modulating the physical properties of β-bithiophene has remained unexplored. We performed comprehensive quantum chemical calculations to investigate how functional groups and substituent sites control the absorption and emission wavelengths of β-bithiophene. Our results show that the functional groups lead to red-shifts of the wavelengths by extending the electron delocalization, while the substitution sites have fewer effects on the wavelengths. The absorption and emission calculation for trithiophene and tetrathiophene suggest that the photophysical properties of thiophene polymer are controlled by the short thiophene chains, underscoring the significance of the rational design of β-bithiophene derivatives.