3,7-Bis(trifluoromethyl)-2-methylpyrazolo[1,5-a]pyrimidines: Sequential synthesis via iodination (NIS)/trifluoromethylation (MFSDA) reactions and photoluminescent behavior

This paper presents the synthesis and photophysical characteristics of a new series of seven 5-alkyl(aryl/heteroaryl)-3,7-bis(trifluoromethyl)-2-methylpyrazolo[1,5-a]pyrimidines (3). These compounds yielded up to 85% through a sequential process involving iodination (N-bromo succinimide-NIS) followed by a trifluoromethylation (methyl fluorosulfonyldifluoroacetate [MFSDA]) reaction using 5-alkyl(aryl/heteroaryl)-7-(trifluoromethyl)-2-methylpyrazolo[1,5-a]pyrimidines (1) as a starting point. Additionally, seven novel iodinated heterocycles, specifically 5-alkyl(aryl/heteroaryl)-7-(trifluoromethyl)-3-iodo-2-methylpyrazolo[1,5-a]pyrimidines (2), were isolated as pure compounds with yields up to 90% through iodination reactions of pyrazolo[1,5-a]pyrimidines 1 using NIS. UV-Vis absorption, steady-state, and time-resolved fluorescence emission techniques revealed that compounds 3 generated emission peaks in the blue region (434–496 nm) with a Stokes shift (ΔλSS) ranging from 87–201 nm. However, these compounds displayed moderate to low fluorescence quantum yields (Φf = 0.01–0.49) in comparison to the standard compound 9,10-diphenylanthracene (Φf = 0.65) and lifetime values of 1.30–10.2 ns. Additionally, TD-DFT calculations were employed to enhance the interpretation of the studied molecules' photophysical properties.