The planarization of side chain in carbazole sensitizer and its effect on optical, electrochemical, and interfacial charge transfer properties

The energy level is the critical property of the organic dye, determining the light-harvesting region of the dye, as well as the thermodynamic possibilities and the efficiencies of multiple interfacial charge transfer processes in DSSCs. In our previous work, the planarization of D-pi-A skeleton can efficiently and selectively optimize the energy levels, leading to the improvement of the light-harvesting capability and the interfacial charge transfer processes. However, whether the planarization of side chain will also produce similarly superior influence is still unclear. Thus, by the planarization of side chain, two novel carbazole isomers are developed and evaluated, while the planarization reaction is also investigated by nuclear magnetic resonance spectroscopy and theoretical calculation. With the planarization of side chain in donor part, the HOMO levels of two dyes are slightly lifted while their LUMO levels remain at the same level, leading to the slightly expansion of their light-harvesting region. However, the interfacial charge transfer processes cannot be efficiently improved by this planarization due to its irregular and negligible effect, which is far different from the influence caused by the planarization of D-pi-A skeleton. Consequently, our finding demonstrates that although the planarization of side chain in donor part has irregular influence on interfacial charge transfer processes, it still can selectively uplift the HOMO level and further improve the light-harvesting capability, which can be applied as the useful complement to the planarization of D-pi-A skeleton for the development of organic dye in the future.