Organometallic complexes for nonlinear optics .3. Molecular quadratic hyperpolarizabilities of ene-, imine-, and azo-linked ruthenium sigma-acetylides: X-ray crystal structure of Ru((E)-4,4'-C CC6H4CH=CHC6H4NO2)(PPh(3))(2)(eta-C5H5)

The series of complexes Ru((E)-4,4'-C=CC(6)H(4)X=CHC6H4NO2)PR(3))(2)(eta-C5H5) (X = CH, R = Ph, 11a; X = CH, R = Me, 11b; X = N, R = Ph, 12a; X = N, R = Me, 12b) has been synthesized by reaction of RuCl(PR(3))(2)(eta-C5H5) with (E)-4,4'-HC=CC(6)H(4)X=CHC6H4NO2 and deprotonation of the intermediate vinylidene complex. Complex 11a has been structurally characterized; it is the first example of a donor-acceptor organometallic ''extended'' chromophore bearing the prototypical acceptor -NO2 to be crystallographically studied. Molecular quadratic hyperpolarizabilities at 1.9 mu m were evaluated computationally for the complexes above and imine- and ate-linked analogues by employing ZINDO with crystallographically obtained atomic coordinates. The results are consistent with a substantial increase in quadratic nonlinearity for (i) chain lengthening of the organometallic chromophore (replacing 4-C=CC6H4NO2 by (E)-4,4'-C=CC6H4CH=CHC6H4NO2) and (ii) an azo linkage compared with an ene linkage (replacing (E)-4,4'-C=CC6H4CH=CHC6H4NO2 by (E)-4,4'C=CC6H4N=NC6H4NO2). Little variation in computed response was found upon substituting an imine linkage for an ene Linkage in the organometallic chromophore (replacing (E)-4,4'-C=CC6H4CH=CHC6H4NO2 by (E)-4,4'-C=CC6H4N=CHC6H4NO2 or (E)-4,4'-C=CC6H4-CH=NC6H4NO2). Molecular quadratic optical nonlinearities were determined experimentally for 11a, 12a, and Ru(C=CC6H4NO2-4 (PR(3))(2)(eta-C5H5) (R = Ph, Me) by electric-field-induced second-harmonic generation(EFISH; 11a and 12a only) and hyper-Rayleigh scattering (HRS) techniques, EFISH-derived mu beta(1064) values for 11a (9700 x 10(-48) cm(5) esu(-1)) and 12a (5800 x 10(-48) cm(5) esu(-1)) are large compared to those for other organometallic complexes. Resonance-enhanced quadratic nonlinearities at 1.06 mu m from HRS are large (1455 x 10(-30) cm(5) esu(-1), 11a; 840 x 10(-30) cm(5) esu(-1), 12a). Two-level-corrected values confirm a substantial increase in quadratic nonlinearity for chain lengthening but suggest a significant decrease in nonlinearity on replacing an ene linkage by an imine linkage; the latter is contrary to the ZINDO result, and the reasons for this are discussed.