Nonlinear optical properties of pyridinium-betaines of squaric acid: Experimental and theoretical study

The linear and nonlinear optical properties of six pyridinium-betaines of squaric acid were characterized by means of electro-optical absorption measurements (EOAM) in dioxane solution. All chromophores studied exhibit intense absorption bands in the visible region within 372-441 nm, accompanied with decrease in dipole moment upon excitation. The static hyperpolarizabilities of the NLO-phores studied depend strongly on the substituent in the pyridinium ring and increase significantly going from donor to acceptor substituent. Specifically, the noncentrosymmetrically crystallizing 4-benzoyl compound showed the highest static hyperpolarizability. These empirical trends were supported and verified by quantum-chemical calculations (DFT and ab initio RHF). Bulk crystal properties such as decomposition points within the high temperature range 480-665 K and improved thermal stability in melt demonstrate the advantage of these materials for nonlinear optical applications.