Organometallic Counterparts of Push−Pull Aromatic Chromophores for Nonlinear Optics: Push−Pull Heteronuclear Bimetallic Complexes with Pyrazine andtrans-1,2-Bis(4-pyridyl)ethylene as Linkers

We report the synthesis, electronic properties, and second-order nonlinear optical (NLO) response of asymmetric heteronuclear push-pull bimetallic complexes with pyrazine (pyz) and trans-1,2-bis(4-pyridyl)ethylene (BPE) as polarizable linkers, connecting the "W(CO)5" fragment (acting as donor group) and "cis-Rh(CO)2Cl", "cis-Re(CO)4Cl", and BF3 fragments (acting as acceptor groups). The second-order NLO response was assessed by the EFISH technique (βvec), working with a nonresonant incident wavelength of 1.907 μm and by solvatochromic studies (βCT). These results are compared with the behavior of mononuclear [W(CO)5L] complexes (L = pyridine or torss-stilbazole para substituted with acceptor groups) and of classical structurally related organic push-pull aromatic chromophores. The second-order NLO response of asymmetric chromophores with BPE as linker is comparable, with the exception of the sign, to classical push-pull stilbenic compounds. However when pyrazine is the linker, the response can be higher (BF3 as acceptor group) or higher but with opposite sign ("cis-Rh(CO)2Cl" as acceptor group) than the related push-pull benzenic chromophores. Only with a pyrazine linker and "cis-Re(CO)4Cl" as acceptor does the second-order NLO response becomes irrelevant. The above results are interpreted assuming that the MLCT (W → π*L) transition, originating from the "W(CO)5" moiety, dominates the second-order response of these push-pull heteronuclear dimeric chromophores.