The effect of silyl and phenyl functional group end caps on the nonlinear optical properties of polyynes: A long‐range corrected density functional theory study

Abstract

AbstractThe polarizabilities, α, second‐hyperpolarizabilities, γ, and γ scaling factors, c, of polyynes [i‐(Pr)3 Si–(CC)n–Si i‐(Pr)3, H–(CC)n–Si i‐(Pr)3, H–(CC)n‐Ph, and Ph–(CC)n–Ph: n = 1 to 8] end‐capped with triisopropylsilane (i‐(Pr)3 Si–; TIPS) and phenyl groups were calculated using HF and DFT using the recently developed LC‐BOP and LCgau‐BOP functionals. Comparison with the α and γ values of the uncapped polyynes shows that the TIPS and phenyl end caps do not increase the α and γ values by a simple additive constant, but instead seem to expand the effective conjugation length (n) of the polyyne backbone. Using a newly‐proposed power‐law function, γ = a + b(n + Δn)c, we found that a single phenyl or TIPS end cap has the effect of increasing n by approximately Δn = 1.35 or 0.83, respectively. In the case of the phenyl end caps, we confirm that this function gives a better fit to the data than the function γ = a + bnc, even with constant b and c values, determined from a fit to the uncapped polyyne γ values (γ = bnc). In the case of the TIPS end caps, the fit to γ = a + b(n + Δn)c gives a smaller Δn, but the assumption of constant b and c is less accurate. We also compared our c values to experimental data using the commonly used function γ = a + bnc. Our results are consistent with the experimental c values, in that the c values of Ph–(CC)n–Ph are higher than those of i‐(Pr)3 Si–(CC)n–Si i‐(Pr)3. Finally, using the power‐law, γ = a + b(n + Δn)c, we predict the, as yet undetermined, experimental c value of uncapped polyynes using the experimental γ values of Ph–(CC)n–Ph (Δn = 2.7), to be ∼4.88. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009