Photophysical properties and protein binding studies of piperazine-substituted anthracene-BODIPY dyads for antimicrobial photodynamic therapy

<p dir="ltr">This work presents the synthesis, characterisation, photophysical properties, time-resolved spectroscopic behaviour, and biological evaluation of two structurally distinct heavy-atom-free BODIPY-anthracene dyads (<b>BDP-1</b>) and the newly designed 2,6-bis[1-(tert-butyl) 4-(prop-2-yn-1-yl) piperazine-1,4-dicarboxylate] BODIPY-anthracene (<b>BDP-2)</b>, incorporating 2,6-alkynyl-piperazine substituents for potential application in antimicrobial photodynamic therapy. <b>BDP-1 </b>exhibits absorption and emission maxima at 507 nm and 516 nm, respectively, with a Stokes shift of 344 cm^-1 in dichloromethane (DCM), characteristic of unsubstituted BODIPYs. In contrast, <b>BDP-2</b> undergoes a red-shift in the absorption maximum to 552 nm (Stokes shift of 633 cm^-1), which is attributed to the extended conjugation from the introduction of the alkyne groups. Time-resolved infrared spectroscopy confirmed efficient spin-orbit charge transfer intersystem crossing, and nanosecond transient absorption studies confirmed the formation of a long-lived triplet state for <b>BDP-2</b> (up to 138 µs in MeCN). A binding constant (K_b) of 9.6 × 10⁴ M^-1 was obtained for <b>BDP-2</b> when titrated with bovine serum albumin (BSA), which is higher than comparable BODIPY derivatives.<b> BDP-2</b> displayed improved hemocompatibility compared to <b>BDP-1</b> (<5% haemolysis of human erythrocytes up to 200 μg·mL^-1). Antimicrobial activity of <b>BDP-1</b> and <b>BDP-2 </b>was most potent when irradiated at 370 nm compared to the other wavelengths employed. However, <b>BDP-2 </b>did not retain the potent (6 log) and rapid (within 15 min) eradication of <i>Staphylococcus aureus</i> achieved by <b>BDP-1</b> under irradiation at 370 nm. These findings demonstrate the rational design of <b>BDP-2</b> as a biocompatible, and heavy-atom-free BODIPY offering promise for targeted antimicrobial photodynamic therapeutic applications.</p>