Effect of the position of substitution on the electronic properties of nitrophenyl derivatives of fulleropyrrolidines: Fundamental understanding toward raising LUMO energy of fullerene electron-acceptor

A series of substituted para -, meta - and ortho -nitrophenyl derivatives of fulleropyrrolidine were synthesized to investigate the effects of the position of substitution on electronic properties by using steady-state absorption and fluorescence spectra, combined with DFT calculations. The results confirmed that the position of substitution has little effect on absorption and fluorescence spectra, whereas a significant effect was observed on their LUMO energy levels. The theoretical calculations revealed that the LUMO energy of the ortho -nitrophenyl substituted derivative was increased 0.1 eV above those of para - and meta -substitution. The prominent effect of ortho -substitution was attributed to the through-space orbital interaction between spatially closed electron-withdrawing nitro group and fullerene cage. These findings could provide fundamental insights in raising LUMO levels of C 60 -based electron acceptor materials and an alternative strategy to increase open circuit voltage V oc in polymer solar cells.