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 BF3 fragments (acting as acceptor groups). The second-order NLO response was assessed by the EFISH technique working with a nonresonant incident wavelength of 1.907 mum and by solvatochromic studies beta(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 except-ion 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)(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 --> pi*L) transition, originating from the W(CO)(5) moiety, dominates the second-order response of these push-pull heteronuclear dinieric; chromophores.