Solvent Effects upon the Electronic Absorption and Fluorescence Spectra of Pteridines and Riboflavin and Their Ground and First Excited Singlet State Dipole Moments

The electronic absorption. fluorescence excitation, and fluorescence emission spectra of a series of pteridines (lumazine, xanthopterin, isoxanthopterin, biopterin) and riboflavin (vitamin B2) were measured at room temperature (298 K) in a number of solvents covering a wide range of polarities (dioxane, ethyl ether, chloroform, ethyl acetate, 1-butanol. 2-propanol, ethanol, methanol, dimethylformamide, acetonitrile, and dimethyl sulfoxide). The effects of the solvent upon the spectral properties are discussed. Experimental groundstate dipole moments were measured for selected compounds and were used in combination with the spectral data to evaluate their first excited singlet-state dipole moments by means of the solvatochromic shift method (Bakhshiev's and Kawski-Chamma-Viallet's equations based on the variation of the Stokes shift with the solvent dielectric constant-refractive index term). The theoretical ground and excited singlet-state dipole moments for all pteridines and riboflavin were calculated as a vector sum of the π-component (obtained by the PPP method) and the a-component (obtained from a-bond moments). A second set of theoretical values was obtained by using the CNDO/2method. A good agreement was observed between the experimental and the theoretical values. Excited singlet-state dipole moments are higher than the ground state values by 1 to 6 Debye units for all the pteridines under study with the exception of xanthopterin.