Control of On-Off or Off-On fluorescent and optical [Cu²⁺] and [Hg²⁺] responses via formal Me/H substitution in fully characterized thienyl "scorpionate"-like BODIPY systems

One 8-phenyl and two 8-mesityl-substituted "scorpionate"-like BODIPY-type species of the formula [3,4,4-tris(5-R-(2-thienyl))-8-(2,4,6- R′-phenyl)-4-bora-3a,4a-diaza-s-indacene (R = H, R′ = H, 3a; R, = H, R′ = Me, 2a; R, = Me, R′ = Me, 2b)] have been synthesized and fully characterized. Importantly, differences in their solution (MeCN) optical Cu²⁺ and Hg²⁺ probing capacity via SSS-chelation were investigated. Compounds 2a-3a were prepared from the requisite 8-substituted BODIPY complexes. They were characterized first by complete ¹H, ¹¹B and ¹³C NMR spectroscopic assignments (CD ₃Cl or CD₃C(O)CD₃); the molecular structures of 2a and 3a were determined by X-ray crystallography. Compounds 2a-3a were studied by UV-vis and fluorescence spectroscopy [Φ_F = 0.27 ± 0.013 (2a); 0.024 ± 0.0016 (2b); 0.0034 ± 0.00047 (3a)]. Importantly, low [Cu²⁺] with 3a (<3.0 × 10^-5 M) gave rise to an increase of fluorescence intensity (off-on; 6.3-fold), whereas with 2a it decreased (on-off). When [Hg²⁺] (<3.0 × 10 ^-5 M) was added to 2b, the λ_em,max value increased (off-on; 3.2-fold), and for 2a, it decreased (on-off). The association constant (K_a) for Hg²⁺•2a was determined to be 3120 ± 307 M^-1. An approximate stoichiometric 1:1 binding determined by Job plot analysis is in line with successful DFT modeling of SSS-Cu²⁺ binding for this system type. ¹H NMR spectroscopy also revealed tentative sets of product complex peaks. These simple differences caused by formal ligand Me-group incorporation are the first for any related fluorophores, to the best of our knowledge.