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.

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 dipyrrins2a–2c. These were subsequently treated with F3B?OEt2to afford the respectiveN,N′-difluoroboryl-5-(thienyl)dipyrrin compounds,3a–3c; the latter two may serve as polymer precursors. Significant fluorescence signal enhancement is achieved with3a–3cover2a–2c. Orange fluorescence is observed for3awhereas its isomer3bgives green fluorescence (365 nm). An X-ray diffraction study of3awas performed (orthorhombic,P212121,a=10.684(10) ?,b=15.208(13) ?,c=7.256(6) ?,V=1179.0(17) ?3,Z=4,R1=0.0428,wR2=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 of2ademonstrated that the [N–H⋯N] interaction is favored byca.10 kcal/mol, whereas [N–H⋯S] bonding is sterically unattainable. [ABSTRACT FROM AUTHOR]