Zinc(II) as a Versatile Template for Efficient Dipolar and Octupolar Second-Order Nonlinear Optical Molecular Materials §

This short review outlines the main results obtained in the field of molecular materials based on zinc coordination compounds for second-order nonlinear optics. It presents an overview of the main classes of second-order nonlinear optical (NLO) active complexes bearing monodentate, bidentate, tridentate, or tetradentate π-delocalized ligands such as substituted stilbazoles, bipyridines, phenanthrolines, terpyridines, and Schiff bases. Macrocyclic ligands such as porphyrins and phthalocyanines are not covered. This paper shows how coordination to the Zn(II) center of π-delocalized nitrogen donor ligands produces a significant enhancement of their quadratic hyperpolarizability. Dipolar complexes are mainly presented, but octupolar zinc complexes are also presented. The coverage is mainly focused on NLO properties that are measured at the molecular level, working in solution, by means of the electric field-induced second harmonic generation (EFISH) or the hyper-Rayleigh scattering (HRS) techniques.

Unexpected High Second-Order Nonlinear Optical Activity of Metal Complexes with Three-Branched Hexadentate 2,2'-Bipyridine Ligands

Two hexadentate bipyridine ligands and their Ru^II and Ni^II complexes were prepared. The helical alignment of the three electron-donor-π-bridge-electron-acceptor (d-π-A) single-strands with bundle architecture in cooperation with the metal center can strongly enhance the nonlinear optical (NLO) properties. The complexation of the novel cage-type hexadentate ligands with a paramagnetic Ni^II -core almost doubles the β_HRS values compared with the corresponding diamagnetic Ru^II complexes. The hyper-Rayleigh scattering (HRS) was performed with a highly sensitive setup for simultaneous discrimination between multi-photon fluorescence and the molecular first hyperpolarizability.

A time-dependent density functional theory investigation on the nature of the electronic transitions involved in the nonlinear optical response of [Ru(CF3CO2)3T] (T = 4′-(C6H4-p-NBu2)-2,2′:6′,2″-terpyridine)

We report a theoretical study based on density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on the nature and role of the absorption bands involved in the nonlinear optical response of the complexes [Ru(CF3CO2)3T] (T = T1, T2; T1 = 4'-(C6H4-p-NBu2)-2,2':6',2''-terpyridine, T2 = 4'-(C6H4-p-NMe2)-2,2':6',2''-terpyridine). Geometry optimizations, performed without any symmetry constraints, confirm a twisting of the -C6H4-p-NBu2 moiety with respect to the plane of the chelated terpyridine. Despite this lack of strong pi interaction, TDDFT excited states calculations of the electronic spectrum in solution provide evidence of a relevant role of the NBu2 donor group in the low-energy LMCT band at 911 nm. Calculations also show that the two bands at higher energy (508 and 455 nm) are not attributable only to LMCT and ILCT transitions but to a mixing of ILCT/MLCT and ILCT/pi-pi* transitions, respectively. The 911 nm LMCT band, appearing at lower wavelength of the second harmonic (670 nm) of the EFISH experiment, controls the negative value of the second-order NLO response. This is confirmed by our calculations of the static component beta0(zzz) of the quadratic hyperpolarizability tensor, showing a large positive value. In addition we have found that the increase of the dipole moment upon excitation occurs, in all the characterized transitions, along the dipole moment axis, thus explaining why the EFISH and solvatochromic experimental values of the quadratic hyperpolarizability agree as sign and value.

Syntheses, Crystal Structures, and NLO Properties of New Chiral Inorganic Chromophores for Second-Harmonic Generation

A new chiral ligand (H(2)L) based on the condensation of diaminocyclohexane and 4-(diethylamino)salicylaldehyde is reported along with its nickel(II) and manganese(III) complexes. These compounds all crystallize in noncentrosymmetric space groups. Crystal data are as follows. For the ligand (C(28)H(40)N(4)O(2)): monoclinic, I2, a = 15.487(2) Å, b = 8.1988(6) Å, c = 20.958(3) Å, beta = 95.92(1) degrees, Z = 4. For the Ni(II)L complex: monoclinic, P2(1), a = 8.613(2) Å, b = 21.714(4) Å, c = 15.870(4) Å, beta = 91.02(2) degrees, Z = 4. For the Mn(III)LCl complex: orthorhombic, P2(1)2(1)2(1), a = 7.233(2) Å, b = 15.159(4) Å, c = 25.591(7) Å, Z = 4. The manganese derivative exhibits an efficiency 8 times that of urea in second-harmonic generation at 1.9 &mgr;m. INDO/SCI-SOS quantum-chemical calculations predict a sizable molecular nonlinear response and an enhancement of the nonlinearity after metal complexation.