Widening or Lengthening? Enhancing the First Hyperpolarizability of Tubiform Multilithium Salts

Very recently, tubiform multilithium salts have been investigated as new candidates for high-performance nonlinear optical (NLO) materials because of their large static first hyperpolarizability (beta(0)) (J. Phys. Chem. C 2009, 113, 4984-4986). The interesting question of how to further enhance the beta(0) value of the multilithium salt, by widening or lengthening the tubiform cyclacene, is studied in this work. On one hand, the effect of widening on the beta(0) value of tubiform multilithium salts is investigated using Li-n-[n]cyclacene (n = 5-8). We found that the beta(0) value (15497 au) of Li-8-[8]cyclacene is 3 times larger than the 5028 au value of the Li-5-[5]cyclacene, with the total number of carbon atoms increasing from 20 to 32. On the other hand, lengthening Li-5-[5]cyclacene to form Li-5-CNT (5,0) results in the dramatically enhanced beta(0) value of 146642 au, which is 29 times larger than the value for the Li-5-[5]cyclacene, even though the total number of carbon atoms is increased only from 20 to 30. Further, the second harmonic generation (SHG) beta(-2 omega;omega,omega) and the electro-optical Pockels effect (EOPE) beta(-omega;omega,0) increase with an increase in frequency (omega) from 0.0000 to 0.0200 au. Because cyclacenes have been proposed to be super-short single-walled carbon nanotubes, it is our expectation that this work could provide more useful information for the development of nonlinear optical nanomaterials.