Mono(η⁵-cyclopentadienyl)metal(II) Complexes with Thienyl Acetylide Chromophores: Synthesis, Electrochemical Studies, and First Hyperpolarizabilities

Abstract

A series of mono(η5-cyclopentadienyl)metal(II) complexes with nitro-substituted thienyl acetylide ligands of general formula [M(η5-C5H5)(L)(CC{C4H2S} n NO2)] (M = Fe, L = κ2-DPPE, n = 1, 2; M = Ru, L= κ2-DPPE, 2 PPh3, n = 1, 2; M = Ni, L = PPh3, n = 1, 2) has been synthesized and fully characterized by NMR, FT-IR, and UV–Vis spectroscopy. The electrochemical behavior of the complexes was explored by cyclic voltammetry. Quadratic hyperpolarizabilities (β) of the complexes have been determined by hyper-Rayleigh scattering (HRS) measurements at 1500 nm. The effect of donor abilities of different organometallic fragments on the quadratic hyperpolarizabilities was studied and correlated with spectroscopic and electrochemical data. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were employed to get a better understanding of the second-order nonlinear optical properties in these complexes. In this series, the complexity of the push–pull systems is revealed; even so, several trends in the second-order hyperpolarizability can still be recognized. In particular, the overall data seem to indicate that the existence of other electronic transitions in addition to the main MLCT clearly controls the effectiveness of the organometallic donor ability on the second-order NLO properties of these push–pull systems.

Large Hyperpolarizabilities at Telecommunication-Relevant Wavelengths in Donor–Acceptor–Donor Nonlinear Optical Chromophores

“…While wavelengths near 1500 nm define a critically important telecommunications spectral region, no molecular chromophore has thus far been delineated that exhibits a dynamic hyperpolarizability measured at a 1.5 μm incident irradiation wavelength that exceeds 700 × 10 –30 esu; 27,61,65,74−81 this work provides new insights into the design of octopolar NLO chromophores, and demonstrates the utility of MPZnM compositions for realizing substantial β HRS values at long wavelengths.…”

Computational studies of the nonlinear optical properties of organometallic complexes

“…In recent years, with the rapid growth of computer power as well as efficient algorithms, NLO calculations have been performed using ab initio methods coupled with derivative methods (discussed later), in particular density functional theory (DFT) [81][82][83][84][85][86]. A detailed discussion of quantum chemical NLO calculations with reference to the metal alkynyl complexes is given in a later section.…”

“…Further, it cannot be applied in the resonant regime, where the effects of damping and vibronic structure of the excited states should also be taken into account [92][93][94]. The adequacy of the two-level model of Oudar and Chemla is uncertain for numerous NLO chromophores [29,84,[94][95][96][97], so two-level corrected data (β0) should be used with great care.…”

“…Reliable quantum chemical calculations can be very useful for this purpose as they allow one to not only rationalize the experimental observations, but also to study the structure-β relationships that are inaccessible by experimental methods, thereby guiding experimentalists in designing chromophores with optimum NLO-activity. The use of quantum chemical methods in the calculation of linear optical and quadratic nonlinear optical properties of organometallic complexes, particularly of metal alkynyl complexes, has been an active field of research [73,[75][76][77][78][80][81][82]84,90,91,221,[224][225][226][227][228][229][230].…”

“…For thiophene-based alkynyl complexes, e.g. 25a and 25b, both HRS (at 1500 nm) and two-level corrected data in Table 7 show that the β response of the Ru complex is better than that of its Fe homologue, although the calculated response for the model complexes (with H atoms substituting for phenyl groups in the dppe ligand) was less profound [84]. Overall, although there is some ambiguity for β values of the Fe(II) and Ru(II) homologues, the Os(II) centre is demonstrably superior to the Fe and Ru centres with the same coordination.…”