| Abstract
| - Coordination to Zn(II) of 5-X-1,10-phenanthrolines (X = electron donor) produces a significant enhancement of their quadratic hyperpolarizability, in contrast to the case for coordination to Cd(II). In Zn(II) and Pt(II) complexes carrying two trans-4-(dimethylamino)-4‘-stilbazoles in a tetrahedral or planar coordination, coordination induces a limited increase of the quadratic hyperpolarizability along the ILCT transition, in agreement with the limited increase of EFISH β1.34, its component along the dipole moment. Some Zn(II) complexes with phenanthrolines show significant μβ0 values.
- Coordination to a Zn(II) center of chelating π-delocalized nitrogen donor ligands such as5-X-1,10-phenanthrolines (X = OMe, NMe2, trans-CHCHC6H4-4‘-NMe2, trans,trans-(CHCH)2C6H4-4‘-NMe2) produces a significant enhancement of their quadratic hyperpolarizabilitymeasured either by the EFISH technique (β1.34) or by absorption and emission solvatochromicinvestigations (βCT) working with an incident wavelength of 1.34 μm. However, theenhancement factor by metal coordination of the quadratic hyperpolarizability of planar5-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 andrigid by coordination. Coordination of 5-(trans-CHCHC6H4-4‘-NMe2)-1,10-phenanthrolineto the softer Cd(CH3CO2)2 does not produce an enhancement of the quadratic hyperpolarizability. This is due to a significant decrease of Δμeg (difference of the dipole moment in theexcited and ground states of the intraligand charge transfer (ILCT)) upon coordination tothe Cd(II) center. Strangely enough, the second-order NLO response of Zn(II) and Pt(II)complexes carrying two 4-trans-NC5H4CHCHC6H4-4‘-NMe2 stilbazole ligands in a tetrahedral or planar coordination indicates a significant increase, upon coordination, of thequadratic hyperpolarizability βCT, along the ILCT transition of the ligand, but an irrelevantincrease of β1.34, which is the vectorial component of the tensor β along the molecular dipolemoment axis, although in the free ligand both βCT and β1.34 are comparable. The lack ofcoincidence of the molecular dipole moment axis with the direction of the ILCT transition ofthe ligand, a major origin of the second-order NLO response, could explain why βCT and β1.34values are not comparable. In fact, a detailed solvatochromic analysis shows that the increaseof βCT 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.
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