The electrochemical properties of the squaraine dyes (SQ1 & SQ2) were investigated. The effects of the chemical structures of two SQ1&SQ2 dyes were selected in relation to the variable property of -COOH substituent for SQ1 compared to SQ2 which is without substituent and determine their potential energy levels in ground and excited states HOMO and LUMO. The HOMO energy levels of the SQ1 & SQ2 dyes are -5.19 eV and -5.16 eV. The LUMO energy levels are calculated to be -3.08 eV and -3.1eV

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... vely. The ground state geometry has been computed by applying density functional theory (DFT). The excitation energy was calculated by using time-dependent (DFT-TD) at DFT/B3LYP/6-31G** level of theory. The -COOH substituent affect the potential values, but the effect is small. The Ip range from -4.901 to -6.072 eV for SQ1 and -4.971 to -6.362 eV for SQ2 showed that the -COOH acid substituent exert a small effect on the frontier orbital energy level. The relationship between Ea and Ip was evaluated as a good slope 0.99 between HOMO and LUMO for SQ1& SQ2, this implies the feasibility and validity of the theoretical estimation and the experimental determination.