Static hyperpolarizability of the van der Waals complex CH 4 N 2

A Computational Study of the Interaction and Polarization Effects of Complexes Involving Molecular Graphene and C60 or a Nucleobases

A systematic analysis of the molecular structure, energetics, electronic (hyper)polarizabilities and their interaction-induced counterparts of C60 with a series of molecular graphene (MG) models, CmHn, where m = 24, 84, 114, 222, 366, 546 and n = 12, 24, 30, 42, 54, 66, was performed. All the reported data were computed by employing density functional theory and a series of basis sets. The main goal of the study is to investigate how alteration of the size of the MG model affects the strength of the interaction, charge rearrangement, and polarization and interaction-induced polarization of the complex, C60-MG. A Hirshfeld-based scheme has been employed in order to provide information on the intrinsic polarizability density representations of the reported complexes. It was found that the interaction energy increases approaching a limit of -26.98 kcal/mol for m = 366 and 546; the polarizability and second hyperpolarizability increase with increasing the size of MG. An opposite trend was observed for the dipole moment. Interestingly, the variation of the first hyperpolarizability is relatively small with m. Since polarizability is a key factor for the stability of molecular graphene with nucleobases (NB), a study of the magnitude of the interaction-induced polarizability of C84H24-NB complexes is also reported, aiming to reveal changes of its magnitude with the type of NB. The binding strength of C84H24-NB complexes is also computed and found to be in agreement with available theoretical and experimental data. The interaction involved in C60 B12N12H24-NB complexes has also been considered, featuring the effect of contamination on the binding strength between MG and NBs.

SCC-DFTB calculation of the static first hyperpolarizability: from gas phase molecules to functionalized surfaces

The performance of the self-consistent charge density functional tight binding (SCC-DFTB) method for calculating the first hyperpolarizability of π-conjugated compounds has been assessed with respect to results obtained with high-level ab initio methods and density functional theory (DFT). The SCC-DFTB method performs similarly or better than DFT with the PBE XC functional. Thus, if for small π-conjugated linkers SCC-DFTB can reproduce trends, for longer chains the first hyperpolarizabilities are overestimated. In the case of push-pull thiophenes, the β values are strongly overestimated, as it is also the case with the B3LYP and PBE XC functionals. On the other hand, the SCC-DFTB method closely reproduces the evolution of β in p-disubstituted benzenes as a function of the donor and acceptor groups, as estimated at the MP2 level. The reliability of SCC-DFTB to determine the bond length alternation and the dihedral angles between the aromatic rings has also been tackled, demonstrating that both are underestimated. Overall, the SCC-DFTB calculations are of the same quality as those performed with the conventional PBE XC functional on which the method was parameterized but the SCC-DFTB calculations are computationally very little demanding, and it can therefore be adopted for very large systems for screening nonlinear optical materials as well as for assessing structure-property relationships. This is illustrated with an application on the first hyperpolarizability of an indolino-oxazolidine molecular switch grafted on a SiO2 surface. This has enabled to pinpoint (i) the effect of the surface on the donor/acceptor character of the linking substituent, (ii) the impact of molecular orientation, (iii) the role of a spacer between the π-conjugated switch and the surface, (iv) the global effect of the surface on the β contrast, and also (v) the fact that the molecular switches can maintain this contrast when adsorbed.