Theoretical Studies Concerning the Optimization of Conjugated Molecules for Third-Order Nonlinear Optics

Ab initio theoretical reinvestigation of the ground and excited state properties of silylated coumarins: Good candidates for solid state dye lasers and dye-sensitized solar cells

We present ab initio theoretical calculations of various properties of the ground and excited states of basic coumarin (1) and its derivatives: 4-methylcoumarin (2), 7-aminocoumarin (3), 7-amino-4-methylcoumarin or coumarin 120 (4), 4-trifluoromethylcoumarin (5), 7-amino-4-trifluoromethylcoumarin or coumarin 151 (6), silylated coumarin 120 (7) and silylated coumarin 151 (8). We calculate the following: (i) ground and excited state dipole moments (ii) energies and locations of HOMOs and LUMOs (iii) SCF total energies of ground state (iv) excitation energies with oscillator strengths for first six excited states (v) C=O and C-N bond lengths in ground and excited states (vi) ground state thermodynamic and electronic properties. The ground and excited state properties of coumarins 1-8 are obtained within the framework of density functional theory using B3LYP and long-range-corrected (LRC) ωB97X-D functionals with 6-31G(d,p) basis set. A detailed comparative analysis of different photo physical and electronic properties of silylated and unsilylated coumarins is made. On the basis of theoretical results we find many interesting features of silylation process and we can conclude that silylation will result in better long-term photo and thermodynamic stability compared to its unsilylated counterpart due to increase in the values of thermodynamic parameters like SCF total energy, G(0) and H(0), etc. Therefore, silylated molecules may become good candidates for solid state dye lasers and dye sensitized solar cells. In contrast, we find that both the functional B3LYP and LRC-ωB97X-D predict nearly the same results for electronic, thermodynamic and photo physical properties of studied coumarins 1-8 in their ground states but B3LYP hybrid functional severely overestimates excited state dipole moments, underestimates vertical excitation energies, oscillator strengths, C=O and C-N bond lengths of studied coumarins. On the basis of our theoretical results we conclude that LRC-ωB97X-D functional must be used for prediction of excited state properties of a molecule.

Correspondence between Gentile oscillators and N-annulenes

The cyclic hydrocarbon polyenes with the general formula CNHN are called N-annulenes. In this paper, we discover that Gentile oscillators and N-annulenes in the Hückel approximation have the same energy spectra determined by the contact points of a regular polygon inscribed on a circle. This correspondence is derived from the symmetry of a C(n + 1) rotational group and dihedral group DihN. On the basis of their energy spectra, we further demonstrate that these two kinds of systems have the same partition functions and, thus, the same thermodynamics properties. N-annulenes can, therefore, be viewed as the natural realization of Gentile systems.