Third-Order Nonlinear Optical Properties of Donor−Acceptor Organometallic Compounds in Films and Solution

Abstract

The third-order nonlinear optical susceptibilities, χ(3)(−3ω; ω, ω, ω), of push−pull organobimetallic Mo/Fe compounds of the type [Mo(TpAn)(NO)(Cl){(HN-π-bridge)(4-Fc)}] {TpAn = hydrotris(3-p-methoxyphenylpyrazol-1-yl)borate; Fc= (η5-C5H4)Fe(η5-C5H5); bridge = C6H4 (1), C6H3(3-R)-4-[NN−C6H3(3-R‘)] [R= R‘= H (2); R= CH3, R‘= H (3); R= R‘= CH3 (4)], C6H4-4-[CHCH−C6H4] (7)} (“straight” compounds) and [Mo(TpAn)(NO)(Cl){HNC6H3(3-CH3)[−NN−C6H3(5-CH3)-2-{(η5-C5H4)Fe(η5-C5H5)}]-4}] (5) (“bent” compound) have been measured in both chloroform solutions (at λ = 1907 nm) and dispersed into spin-coated poly(methyl methacrylate) films (at λ = 1907 nm and λ = 1064 nm). For comparison, THG measurements were carried out on films containing two related organometallic compounds, [4-O2NC6H3(2-CH3)NNC6H3(5‘-CH3)-2‘-{(η5-C5H4)Fe(η5-C5H5)}] (6) (“bent” derivative) and [4-O2NC6H4CHCHC6H4-4‘-{(η5-C5H4)Fe(η5-C5H5)}] (8) (“straight” derivative), where the nitro group has been substituted for the [Mo(TpAn)(NO)(Cl)] one as the acceptor moiety. Some trends for the role of conjugation length, substituents in the bridge, and molecular characteristics (“straight” or “bent”) have been identified. From the solution data the real and imaginary components of the molecular cubic hyperpolarizability γ(3ω; ω, ω, ω) have been determined for 1−5 and 8. The real cubic hyperpolarizability γr is negative for the compounds 1−5, whereas it becomes positive for the compound 8. Highest values |γ| ≅ 5 × 10-33 esu have been obtained for “straight” aza-bridged derivatives 2−4 having the [Mo(TpAn)(NO)(Cl)] fragment as acceptor group. They are comparable to those reported for highly conjugated planar phthalocyanines and other related compounds. The susceptibilities χ(3)(3ω; ω, ω, ω) measured at λ = 1907 nm for the spin-coated films show similar trends as those derived for the hyperpolarizabilities. For the “straight” organobimetallic compounds 1−4 and 7 the χ(3)(3ω; ω, ω, ω) values at λ = 1907 nm are higher than those measured at λ = 1064 nm. On the other hand, for the “bent” organobimetallic compound 5 and the two nitro derivatives 6 and 8 having the acceptor NO2 group, the wavelength behavior is just opposite.