On the contribution of f electrons to the quadratic hyperpolarizability: the case of lanthanide terpyridyl complexes.

Over the last decades, trivalent lanthanide ions (Ln3+) have gained much attention due to their peculiar luminescence, which opened the way to a broad range of applications, from medical diagnostic to lasers. Their impact on nonlinear optical (NLO) properties also attracted interest, especially in the framework of lanthanide complexes. Several experimental studies demonstrated that the quadratic hyperpolarizability varies with the number of 4f-electrons, with a stronger effect on dipolar than octupolar components. The main interpretation put forward to explain the observed trends relied on the polarizable character of the 4f-electrons. We report here a first step towards understanding the role of 4f-electrons in NLO responses, considering a series of dipolar terpyridyl-trinitro lanthanide complexes LLn(NO3)3 (Ln = Gd, Dy, Yb, Lu as well as La and Y; L = terpyridil-like ligand). Using DFT and TD-DFT we investigate their linear and non-linear optical properties. Consistently with earlier experimental findings, simulated UV-visible spectra show minor changes by varying Ln. The same holds for dipole moments and polarizabilities, whereas the nature of the lanthanide affects hyperpolarizabilities. It is shown that the observed changes are not a direct effect of the 4f-electrons that behave like core electrons.

Experimental and theoretical studies of quadrupolar oligothiophene-cored chromophores containing dimesitylboryl moieties as π-accepting end-groups: syntheses, structures, fluorescence, and one- and two-photon absorption.

Quadrupolar oligothiophene chromophores composed of four to five thiophene rings with two terminal (E)-dimesitylborylvinyl groups (4 V-5 V), and five thiophene rings with two terminal aryldimesitylboryl groups (5 B), as well as an analogue of 5 V with a central EDOT ring (5 VE), have been synthesized via Pd-catalyzed cross-coupling reactions in high yields (66-89%). Crystal structures of 4 V, 5 B, bithiophene 2 V, and five thiophene-derived intermediates are reported. Chromophores 4 V, 5 V, 5 B and 5 VE have photoluminescence quantum yields of 0.26-0.29, which are higher than those of the shorter analogues 1 V-3 V (0.01-0.20), and short fluorescence lifetimes (0.50-1.05 ns). Two-photon absorption (TPA) spectra have been measured for 2 V-5 V, 5 B and 5 VE in the range 750-920 nm. The measured TPA cross-sections for the series 2 V-5 V increase steadily with length up to a maximum of 1930 GM. We compare the TPA properties of 2 V-5 V with the related compounds 5 B and 5 VE, giving insight into the structure-property relationship for this class of chromophore. DFT and TD-DFT results, including calculated TPA spectra, complement the experimental findings and contribute to their interpretation. A comparison to other related thiophene and dimesitylboryl compounds indicates that our design strategy is promising for the synthesis of efficient dyes for two-photon-excited fluorescence applications.

Tunable double photochromism of a family of bis-DTE bipyridine ligands and their dipolar Zn complexes.

The photoinduced ring-closure/ring-opening reactions of a series of bis-dithienylethene derivatives, as free ligands and Zn(II)-complexes, are investigated by resorting to theoretical (time-dependent density functional theory) and kinetic analyses in solution. The originality of the system stems from the tunability of the photoreaction quantum yields and conversion yields as a function of the electronic structure. The latter could be varied by modifying the electron-donating character of the DTE-end substituents L(a-d) (o,o) (a, D = H; b, D = OMe; c, D = NMe(2); d, D = NBu(2)) and/or the Lewis character of the metal ion center L(a-d)ZnX(2) (o,o) (L(a-c), X = OAc; L(d), X = Cl). The orbital description of the doubly-open form (o,o) and half-closed form (o,c) predicts that double closure to the form (c,c) would occur using UV irradiation. Photokinetic studies on the complete series demonstrate that photocyclization proceeds following a sequential ring closure mechanism. They clearly point out distinct quantum yields for the first and second ring closures, the latter being characterized by a significantly lower value. Dramatic decrease in both the quantum yields of the ring-closure and ring-opening processes is demonstrated for the complex L(d)ZnCl(2) exhibiting the strongest charge-transfer character in the series investigated. These studies show that this series of DTE derivatives provides an efficient strategy to tune the photochromic properties through the combination of the electron-donor and electron-acceptor (D-A) groups.

Effects of the metal center and substituting groups on the linear and nonlinear optical properties of substituted styryl-bipyridine metal(II) dichloride complexes: DFT and TDDFT computational investigations and harmonic light scattering measurements.

UV-visible absorption spectroscopy and harmonic light scattering measurements coupled with density functional theory (DFT) calculations have been carried out for a series of 4,4'-bis(X-styryl)-2,2'-bipyridine M(II) dichloride complexes (M = Co, Ni, Cu, Zn; X = H, OMe, SMe, NMe(2), NEt(2), CN, NO(2)). The roles of the metal and the substituent X on their coordination geometries, absorption, and quadratic nonlinear optical properties have been investigated. We show that these complexes all exhibit a high-spin configuration and display a distorted tetrahedral metallic environment except the copper ones, which are distorted square-planar complexes. When X is a strong electron-donating group (X = NMe(2), NEt(2)), TDDFT calculations clearly demonstrate that, whereas the Zn complexes show an ILCT transition in the visible range, the Co, Ni, and Cu complexes exhibit additional MLCT and LLCT transitions. These latter transitions are vectorially opposed to the ILCT and could contribute to the decrease of the experimental quadratic hyperpolarizability beta values, in the order Zn > Ni approximately Cu > Co. The computation of the beta values using TDDFT for the whole series of the closed-shell Zn(II) complexes featuring different X substituents established that the NLO activity increases with the donating strength of X and more generally with the decrease of the HOMO-LUMO energy gap. When X is a strong withdrawing group, the drastic decrease of the NLO response is explained by the negligible participation of the HOMO-LUMO transitions.