Theoretical study of nonlinear optical properties of cobalt bis (dicarbollide) derivatives: the effect of substituents

Abstract

interesting feature of the boron-based cages, due to high electronic and thermal stability, delocalized structures and easy functionalization, is the potential application in nonlinear optics (NLO) [3,5,21]. Generally, molecules with NLO applications are systems with acceptor and donor electron groups in its extremities (push-pull type), high values of first-order hyperpolarizability (β) and permanent dipole moment (d). The second-order responses can be enhanced by increasing the electronic asymmetry or the conjugation length between the extremity groups [22,23]. Besides, the structure must be non-centrosymmetric, and the molecules must present absorption at wavelength greater than 400 nm for second harmonic generation (SHG), to avoid re-absorption of the converted light [24]. The energy (E) of a molecular system under an external electric field F can be written as Eq. (1), where E(0) is the energy of the system in the absence of an external field, and F i are the components of the applied field [25]:The dipole moment (d), the linear polarizability (α), and the first-and second-order hyperpolarizabilities (β and γ) play a significant role mainly in the study of NLO properties [24,26,27].The NLO behavior of icosahedral carboranes has been investigated for more than 20 years [14]. Studies in this area involving carboranes include synthesis of novel compounds [14,18] and NLO properties' calculations [5,8,[15][16][17]21,28]. For example, a new class of dodecarboranes ([B 12 H 11 -C 2 B 10 H 11 ] 2− ) with potential application as NLO materials was proposed through ab initio calculations (1)In the present work, molecular first-order hyperpolarizability (β tot ) and dipole moment (d) are obtained at B3LYP/6-31G(d,p) level of theory by coupled perturbed Hartree-Fock method within the static approach. The investigated molecules are a series of substituted cobalt bis (dicarbollide) derivatives: Hydrogens bonded to the two carbon atoms are replaced by acceptor and donor electron substituents. Correlations between the Hammett electronic parameters of the substituents and the molecular properties are tested. Among them, the named push-pull compounds produced the largest calculated values of β tot and d. The UV-Vis spectra are reported for all studied compounds.