Release of CO and Production of <sup>1</sup>O<sub>2</sub> from a Mn-BODIPY Photoactivated CO Releasing Molecule with Visible Light
Shabnam Pordel, Rachael N. Pickens, Jessica K. White
Abstract
Carbon monoxide (CO) is shown to enhance the sensitivity of cancer cells to generate reactive oxygen species such as singlet oxygen (1O2) from chemotherapeutics and reduce the drug resistance. Herein, we introduced two Mn-based photoactivated CO releasing molecules conjugated with an emissive BODIPY (BDP) moiety with a general formula of Mn(CO)3(bpy-R-BDP)Br (R = H or I), labeled as Mn-bpy-H-BDP and Mn-bpy-I-BDP. While both complexes release CO with visible light, Mn-bpy-I-BDP releases CO and produces 1O2 from a single molecule. In addition to 1O2 generation, iodination of BDP red shifts the absorption of Mn-bpy-I-BDP further into the visible region and significantly increases the dark stability of the complex. Cyclic voltammetry and density functional theory (DFT) calculations suggest a Mn-based highest occupied molecular orbital (HOMO) and a BDP-based lowest unoccupied molecular orbital (LUMO) in these complexes. However, time-dependent DFT calculations suggest that the HOMO – 2(πBDP) → LUMO(πBDP*) is the main optical transition upon excitation with visible light in both complexes. The presence of a phosphorescence peak from the triplet excited state of bpy-I-BDP in both the free ligand and its Mn complex at 77 K was used as additional evidence for 1O2 production. We also probed formation of photo-intermediates and products during photolysis with Fourier-transform infrared, 1H NMR, emission, and absorption experiments and DFT calculations.