In this paper, the theoretical analysis on the geometries and electronic properties for some N-heterocyclic compounds: N1,N1,N3,N3-tetrakis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)propane-1,3-diamine (Nh1), N1,N1,N2,N2- tetrakis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)benzene-1,2-diamine (Nh2), N1,N1,N5,N5-tetrakis((3,5-dimethyl-1Hpyrazol- 1-yl)methyl)naphthalene-1,5-diamine (Nh3) and N,N,N',N'-Tetrakis-(3,5-dimethyl-pyrazol-1-ylmethyl)-9Hfluorene- 2,7-diamine (Nh4). These new four organic donor-π-acceptor dyes (D-π-A), used for dye-sensitized solar cells (DSSC), were studied by DFT and TDDFT approaches to shed light on how the π-conjugation order influence the performance of the dyes. The electron acceptor group was 3,5-dimethylpyrazole for all dyes whereas the electron-donor unit varied and the influence was investigated. The theoretical results have shown that TDDFT calculations using the Coulomb attenuating method CAM-B3LYP were reasonably capable of predicting the excitation energies, the absorption and the emission spectra of the molecules. The HOMO and LUMO energy levels of these dyes can ensure a positive effect on the process of electron injection and dye regeneration. The trend of the calculated ΔEg gaps nicely compares with the spectral data. Key parameters for these compounds in close connection with the (Jsc), (LHE), (ΔGinject), (λtotal) and (Voc) were discussed and presented. The calculated results of these dyes reveal that the Nh3 can be used as a potential sensitizer for TiO2 nanocrystalline solar cells due to its best electronic and optical properties and good photovoltaic parameters.
