π–π overlap and second hyperpolarizability

By taking quantum chemical calculations on various isomers of higher fullerenes and some π–π conjugated compounds under different conformations, this Letter demonstrates that the π–π overlap plays a crucial role in the second hyperpolarizabilities of 1-D and 3-D π-conjugated systems. Namely less π–π overlap is responsible for the higher barrier of the π-conjugated electron flow and thus the lowering of the second hyperpolarizability. An exponential law is for the first time obtained by fitting the computed data to depict quantitatively the relationship between the π–π overlap and the second hyperpolarizability.