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 dipyrrins 2a-2c. These were subsequently treated with F3B center dot 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, P2(1)2(1)2(1), a=10.684(10) angstrom, b=15.208(13) angstrom, c=7.256(6) angstrom, V=1179.0(17) angstrom(3), 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 similar to 6 kcal/mol, compared to that of similar to 12 kcal/mol for the phenyl derivative. Theoretical modeling of 2a demonstrated that the [N-H center dot center dot center dot N] interaction is favored by ca. 10 kcal/mol, whereas [N-H center dot center dot center dot S] bonding is sterically unattainable.