Research Article published by Scientific Research Publishing Inc.The dye sensitized solar cells (DSSCs) have been extensively studied due to their low production cost and simple fabrication process. Dye co-sensitization broadens the absorption spectrum of the sensitizer; thus enhances light harvesting efficiency; and contributes to the improvement of the DSSCs overall efficiency. In this study we performed theoretical design of complex molecule (C45H32N2O4) through combination (esterification reaction) of the natural dye lawsone and synthetic metal free indoline dye D131. The excitation energies, vibration spectra, molecular structures, electronic absorption spectra and electron transitions in individual dyes and complex molecule were investigated using density functional theory (DFT) and time dependent density functional theory (TD-DFT) B3LYP5 methods, with 3-21G, 6-31G and 6-31G(d,p) basis sets. The UV-Vis absorption spectra of the individual dyes and their mixture in chloroform solution were measured using spectrophotometer. For the complex formation reaction, enthalpy, entropy and Gibbs free energy were calculated and the results indicated the reaction was endothermic and non-spontaneous. Electron density distribution of the frontier and adjacent molecular orbitals and energy levels alignment were used for analysis of the electronic spectra and mechanism of transitions. The results indicated that the designed complex molecule satisfied the requirements for good photosensitizer of DSSCs.