A single crystal of sodium and potassium cinnamates was grown by slow evaporation of methanol solution at room temperature. The effect of metals sodium and potassium on the electronic structure of cinnamic acid was studied. In this research many analytical methods such as FTIR, UV, second harmonic generation (SHG) and TG–DTA were used: The spectroscopic studies lead to conclusions containing the distribution of the electronic charge in molecule, the delocalisation of π electrons and the reactivity of metal complexes. The SHG efficiency is more pronounced in the presence of sodium and potassium dopant in the growth medium. Incorporation of sodium and potassium increase the thermal stability ensuring the suitability of material for possible non-linear optical (NLO) application up to 180 °C.