Solvent-Free Synthesis Using Nb2O5 and a Theoretical-Experimental Study of Solvent Effect in New Rhodamine Dyes

Rhodamine dyes have several applications based on properties, such as high molar absorptivity, high fluorescence quantum yield, photostability, absorption and emission wavelengths in the visible region. In this paper, we describe the solvent-free synthesis of rhodamines between m-aminophenol derivatives (3-dimethylamino and 3-diethylaminophenol) and phthalic anhydride derivatives (with and without halogens in the benzene ring) using Nb2O5 as catalyst. The compounds were produced in high yields (79 to 90%) within one hour of reaction time and with catalyst recovery. Additionally, a solvatochromic study of rhodamine derivatives was described. These compounds have absorption and emission bands at wavelengths in the visible region and a high quantum yield that potentially makes them good dyes for use in electronic devices (e. g. lasers, OLEDs). The DFT calculations provided a basis for understanding the effect of the solvent on the structural forms of rhodamine and corroborated with the experimental data, where the zwitterionic form is predominant in polar solvents, while in apolar solvents the lactonic form is the most stable.