Effect of the coordination to M(II) metal centers (M = Zn, Cd, Pt) on the quadratic hyperpolarizability of various substituted 5-X-1,10-phenanthrolines (X = donor group) and of trans-4-(dimethylamino)-4′-stilbazole

Coordination to a Zn(II) center of chelating pi-delocalized nitrogen donor ligands such as 5-X-1,10-phenanthrolines (X = OMe, NMe2, trans-CH=CHC6H4-4'-NMe2, trans, trans-(CH=CH)(2)C6H4-4'-NMe2) produces a significant enhancement of their quadratic hyperpolarizability measured either by the EFISH technique (beta(1.34)) or by absorption and emission solvatochromic investigations (beta(CT)) working with an incident wavelength of 1.34 mum. However, the enhancement factor by metal coordination of the quadratic hyperpolarizability of planar 5-X-1,10-phenanthrolines is lower than that of the nonplanar and flexible ligand 4-(p-(dibutylamino)styryl)-4'-methyl-2,2'-bipyridine because this latter ligand becomes planar and rigid by coordination. Coordination of 5-(trans-CH=CHC6H4-4'-NMe2)-1,10-phenanthroline to the softer Cd(CH3CO2)(2) does not produce an enhancement of the quadratic hyperpolarizability. This is due to a significant decrease of Deltamu(eg) (difference of the dipole moment in the excited and ground states of the intraligand charge transfer (ILCT)) upon coordination to the Cd(II) center. Strangely enough, the second-order NLO response of Zn(II) and Pt(II) complexes carrying two 4-trans-NC5H4CH=CHC6H4-4'-NMe2 stilbazole ligands in a tetrahedral or planar coordination indicates a significant increase, upon coordination, of the quadratic hyperpolarizability beta(CT), along the ILCT transition of the ligand, but an irrelevant increase of beta(1.34), which is the vectorial component of the tensor beta along the molecular dipole moment axis, although in the free ligand both beta(CT) and beta(1.34) are comparable. The lack of coincidence of the molecular dipole moment axis with the direction of the ILCT transition of the ligand, a major origin of the second-order NLO response, could explain why beta(CT) and beta(1.34) values are not comparable. In fact, a detailed solvatochromic analysis shows that the increase of beta(CT) upon coordination to Zn(II) or Pt(II) of the single stilbazole ligand is not significant, in agreement with EFISH measurements and the irrelevant red shift of the ILCT transition.