Organometallic Counterparts of Push−Pull Aromatic Chromophores for Nonlinear Optics:  Push−Pull Heteronuclear Bimetallic Complexes with Pyrazine and trans -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) 2 Cl”, “ cis -Re(CO) 4 Cl”, and BF 3 fragments (acting as acceptor groups). The second-order NLO response was assessed by the EFISH technique (β vec ), working with a nonresonant incident wavelength of 1.907 μm and by solvatochromic studies (β CT ). These results are compared with the behavior of mononuclear [W(CO) 5 L] complexes (L = pyridine or trans -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 (BF 3 as acceptor group) or higher but with opposite sign (“ cis -Rh(CO) 2 Cl” as acceptor group) than the related push−pull benzenic chromophores. Only with a pyrazine linker and “ cis -Re(CO) 4 Cl” 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.