X-ray diffraction, DFT, and spectroscopic study of N,N′-difluoroboryl-5-(2-thienyl)dipyrrin and fluorescence studies of related dipyrromethanes, dipyrrins and BF2-dipyrrins and DFT conformational study of 5-(2-thienyl)dipyrrin

Abstract

To explore the characteristics of potential fluorescent probes that could be used to screen prospective HDS catalysts, new 5-thienyl dipyrromethene derivatives have been prepared. 5-(2-thienyl)- (1a), 5-(3-thienyl)- (1b), and 5-(3-(2,5-dibromothienyl))-dipyrromethane (1c), were oxidized with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) separately to give the corresponding dipyrrins 2a–2c. These were subsequently treated with F3B·OEt2 to afford the respective N,N′-difluoroboryl-5-(thienyl)dipyrrin compounds, 3a–3c; the latter two may serve as polymer precursors. Significant fluorescence signal enhancement is achieved with 3a–3c over 2a–2c. Orange fluorescence is observed for 3a whereas its isomer 3b gives green fluorescence (365 nm). An X-ray diffraction study of 3a was performed (orthorhombic, P212121, a=10.684(10) A, b=15.208(13) A, c=7.256(6) A, V=1179.0(17) A3, Z=4, R 1=0.0428, wR 2=0.0686) and its geometry was studied by way of DFT (Gaussian 03; B3LYP/6-31G*) to give a HOMO/LUMO energy level difference of 414 nm, and a 2-thienyl group rotational barrier of ∼6 kcal/mol, compared to that of ∼12 kcal/mol for the phenyl derivative. Theoretical modeling of 2a demonstrated that the [N–H⋯N] interaction is favored by ca. 10 kcal/mol, whereas [N–H⋯S] bonding is sterically unattainable.