Third-order nonlinear optical properties of donor-acceptor organometallic compounds in films and solution

The third-order nonlinear optical susceptibilities, chi((3))(-3 omega; omeg a, omega, omega), of push-pull organobimetallic Mo/Fe compounds of the type [Mo(Tp(An))(NO)(Cl){(HN-pi-bridge)(4-Fc)}] {Tp(An) = hydrotris(3-p-methoxy phenylpyrazol-1 -yl)borate; Fc= (eta(5)-C5H4)Fe(eta(5)-C5H5); bridge = C6H4 (1), C6H3(3-R)-4-[N=N-C6H3(3-R')] [R= R'= H (2); R= CH3, R'= H (3); R= R'= CH3 (4)], C6H4-4-[CH=CH-C6H4] (7)} ("straight" compounds) and [Mo(Tp(An))( NO)(Cl){HNC6H3(3-CH3)[-N=N-C6H3(5-CH3) -2-{(eta(5)-C5H5)}]-4}](5) ("bent" c ompound) have been measured in both chloroform solutions (at lambda = 1907 nm) and dispersed into spin-coated poly(methyl methacrylate) films (at lamb da = 1907 nm and lambda = 1064 nm). For comparison, THC measurements were c arried out on films containing two related organometallic compounds, [4-O2N C6H3(2-CH3)N=NC6H3(5'-CH3)-2'-{(eta(5)-C5H4)Fe(eta(5)-C5H5)}](6) ("bent" de rivative) and [4-O2NC6H4CH=CHC6H4-4'-{(eta(5)-C5H4)Fe(eta(5)-C5H5)}] (8) (" Straight" derivative), where the nitro group has been substituted for the [ Mo(Tp(An))(NO)(Cl)] one as the acceptor moiety. Some trends for the role of conjugation length, substituents in the bridge, and molecular characterist ics ("straight" or "bent") have been identified. From the solution data the real and imaginary components of the molecular cubic hyperpolarizability g amma(3 omega; omega, omega, omega) have been determined for 1-5 and 8. The real cubic hyperpolarizability gamma(r) is negative for the compounds 1-5, whereas it becomes positive for the compound 8. Highest values \y\ congruen t to 5 x 10(-33) esu have been obtained for "straight" aza-bridged derivati ves 2-4 having the [Mo(Tp(An))(NO)(Cl)] fragment as acceptor group. They ar e comparable to those reported for highly conjugated planar phthalocyanines and other related compounds. The susceptibilities chi((3))(3 omega; omega, omega, omega) measured at lambda = 1907 nm for the spin-coated films show similar trends as those derived for the hyperpolarizabilities. For the "str aight" organobimetallic compounds 1-4 and 7 the chi((3))(3 omega; omega, om ega, omega) values at;l = 1907 nm are higher than those measured at a = 106 4 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 wavelengt h behavior is just opposite.