Cu2+ Colorimetric Sensing and Fluorescence Enhancement and Hg2+ Fluorescence Diminution in “Scorpionate”-like Tetrathienyl-Substituted Boron−Dipyrrins

• Published in: Inorganic chemistry 2007-12, Vol.46 (25), p.10564-10577
• Main Authors: Choi, Shin Hei, Pang, Keliang, Kim, Kibong, Churchill, David G
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic701101j

Four novel tetrathienyl-substituted boron−dipyrrin-type (BODIPY-type) complexes, 3-(R‘)-4,4-di(R‘)-8-R-4-bora-3a,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 M n + 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 Φ 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 λabs,max values (ε, 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 μM 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 M n +−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 ∼270 ppb and ∼1.7 ppm, respectively; the naked eye detection limit for Cu2+ with 4b (1.0 × 10-5 M) is ∼23 μM. 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.