Cu2+ colorimetric sensing and fluorescence enhancement and Hg2+ fluorescence diminution in "Scorpionate"-like tetrathienyl-substituted boron-dipyrrins

Abstract

Four novel tetrathienyl-substituted boron-dipyrrin-type (BODIPY-type) complexes, 3-(R')-4+di(R')-8-R-4-bora3a,4a-diaza-s-indacene (4a, R = 2-T, R' = 2-T; 4b, R = 3-T, R' = 2-T; 5a, R = 2-T, R' = 3-T; 5b, R = 3-T, R' = 3-T; T = thienyl) have been prepared and fully characterized to explore patterns of stoichiometric Mn+ recognition in solution. Treatment of the respective parent BF2 dipyrrin with 2- or 3-thienyllithium gave the unexpected asymmetric tetrathienyl-substituted products in 8.5-35% yield. Compounds 4a and 4b bear a neutral "scorpionate"like [SSS] tridentate binding pocket. Extensive NMR and UV-vis spectroscopic studies were performed on 4a-5b; 5a, 4b, and 5b were structurally characterized. The Phi(F) values for 4a-5b all decrease compared to the BF2-containing parent molecules (0.00058, 0.012, 0.00090, and 0.0051, respectively), with lambda(abs,max) values (is an element of, M-1 cm(-1)) of 563 (44 000), 553 (29 000), 539 (33 000), and 531 (44 000) nm, respectively, and Stokes' shifts of 25-36 nm. Upon treatment with metal ion (Ca2+, CS+, Mn2+, CO2+, Cu2+, Ag+, Zn2+, Cd2+, Hg2+, Pb2+) perchlorate salts, the solution of 4b undergoes rapid pink-to-clear switch-off behavior upon Cu2+ addition (10 mu M scale) with smaller effects seen for 4a. Further, there were 2- to 19-fold Cu2+ fluorescence enhancements for these ligands. Cu2+ and Hg2+-L (L = 4a-5b) binding was modeled, and response patterns for Mn+-L 1:1 molar solutions upon Cu2+ addition were measured. Upon treatment with Hg2+, all ligand solutions show a significant fluorescence decrease accompanied by minor absorption increases. The UV-vis spectroscopic detection limit for Cu2+ and Hg2+ is similar to 270 ppb and similar to 1.7 ppm, respectively; the naked eye detection limit for Cu2+ with 4b (1.0 x 10(-5) M) is similar to 23 mu M. DFT calculations gave HOMO-LUMO gaps of 478 (4a), 462 (4b), 448 (5a), and 442 nm (5b). Molecular orbital diagrams for 4a-5b revealed that the HOMO and LUMO electron density is distributed onto the 3-position-thienyl group and to a lesser degree the B(thienyl)(2) moiety.