Organometallic Complexes for Nonlinear Optics. 3.1Molecular Quadratic Hyperpolarizabilities of Ene-, Imine-, and Azo-Linked Ruthenium σ-Acetylides: X-ray Crystal Structure of Ru((E)-4,4‘-C⋮CC6H4CHCHC6H4NO2)(PPh3)2(η-C5H5)

Abstract

The series of complexes Ru((E)-4,4‘-C⋮CC6H4XCHC6H4NO2)(PR3)2(η-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(PR3)2(η-C5H5) with (E)-4,4‘-HC⋮CC6H4XCHC6H4NO2 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 μm were evaluated computationally for the complexes above and imine- and azo-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⋮CC6H4CHCHC6H4NO2) and (ii) an azo linkage compared with an ene linkage (replacing (E)-4,4‘-C⋮CC6H4CHCHC6H4NO2 by (E)-4,4‘-C⋮CC6H4NNC6H4NO2). 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⋮CC6H4CHCHC6H4NO2 by (E)-4,4‘-C⋮CC6H4NCHC6H4NO2 or (E)-4,4‘-C⋮CC6H4CHNC6H4NO2). Molecular quadratic optical nonlinearities were determined experimentally for 11a, 12a, and Ru(C⋮CC6H4NO2-4)(PR3)2(η-C5H5) (R = Ph, Me) by electric-field-induced second-harmonic generation (EFISH; 11a and 12a only) and hyper-Rayleigh scattering (HRS) techniques. EFISH-derived μβ1064 values for 11a (9700 × 10-48 cm5 esu-1) and 12a (5800 × 10-48 cm5 esu-1) are large compared to those for other organometallic complexes. Resonance-enhanced quadratic nonlinearities at 1.06 μm from HRS are large (1455 × 10-30 cm5 esu-1, 11a; 840 × 10-30 cm5 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.