Quantum chemical calculation and analysis of third-order nonlinear polarizabilities for linear conjugated π-electron molecules

Abstract

The nonresonant π-electronic component of ω(-3ω; ω, ω, ω), ω π, is calculated and analyzed for two important types of linear conjugated π-electron molecules: linear cyanine cations and linear polyenes. The specific linear cyanine cations are all- trans, linear symmetric cyanine and streptocyanine cations with no double-single bond-lenght alternation, and cyanine cations with asymmetric geometries resulting from the artificial imposition of double-single bond-length alternation. The linear polyenes of interest are all- trans linear polyenes with double-single bond-length alternation, and those with geometries resulting from the artificial imposition of no double-single bond-length alternation. Bond-length alternation is found to dramatically affect ω π for the linear cyanines: the ω π's for the symmetric cyanines are calculated to be negative; the ω π's for the assymmetric cyanines change from negative to positive with increasing chain length. The ω π's for the linear polyenes are always positive regardless of the extent of bond-length alternation; the |ω π|'s for the linear polyenes increase with decreasing bond-lenght alternation. The |ω π|'s for the symmetric linear cyanines increase more rapidly with the number of π-electrons than the |ω π|'s for the linear polyenes: |ω π(symmetric cyanines)|∝ N 8 π e and |ω π (linear polyenes)|∝ N 4 π− e , where N π− e = 4,6,8,10,12. The linear cyanine cations comprise a very promising class of nonlinear optical, π-electron molecules that merit further experimental study.