Release of CO and Production of ¹O₂ from a Mn-BODIPY Photoactivated CO Releasing Molecule with Visible Light

Abstract

Carbon monoxide (CO) is shown to enhance the sensitivity of cancer cells to generate reactive oxygen species such as singlet oxygen ( 1 O 2 ) 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 1 O 2 from a single molecule. In addition to 1 O 2 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(π BDP ) → LUMO(π BDP * ) 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 1 O 2 production. We also probed formation of photo-intermediates and products during photolysis with Fourier-transform infrared, 1 H NMR, emission, and absorption experiments and DFT calculations.

Highly Efficient Singlet Oxygen Generation by BODIPY–Ruthenium(II) Complexes for Promoting Neurite Outgrowth and Suppressing Tau Protein Aggregation

“…51 The complexes exhibit similar absorption bands to the free ligand at 250−330, 330−430, and 450−540 nm, while the metal-to-ligand charge-transfer (MLCT) bands associated with Ru(II) in the visible region are completely hidden by the intense absorption bands of BODIPY. The ligand transitions at 255−310 nm, which are π → π* transitions, are significantly red-shifted to 270−320 nm on coordination, as has also been observed in Mn complexes, 42 and may be related to a charge-separated (CS) state between the π and π*. 52 Such charge-separated states (CS) are typical features of multicomponent molecules, 52 and they emerge from the BODIPY-based 1 π−π* level by oxidative electron transfer to the Ru metal center.…”

“…The calculated triplet state at 609 nm (T1) was found to have the same orbital components as S2, i.e., HOMO – 4/5 → LUMO, and hence can be assigned as a 3 ππ* transition of the BODIPY subunits. Although the calculated triplet state is slightly overestimated at 2.03 eV, BODIPY-centered triplet state fluorescence spectra have been observed in previous studies on BODIPY-conjugated ruthenium, iridium, and manganese complexes. ,, In fact, all these complexes were observed to possess relatively high BODIPY-centered triplet excited states capable of producing singlet oxygen, which has a triplet–singlet gap of 0.98 eV. , …”

“…Despite coordination with the highly luminescent organic dye, quantum yields are exceedingly low for the complexes. This can be attributed to internal conversion, which transfers electrons from the metal-based 3 MLCT to the BODIPY-centered 3 BODIPY state. ,, On the other hand, quenching of the BODIPY fluorescence is correlated to the charge-separated state (CS); the thermodynamically allowed process provides electron transfer from 1 BODIPY to the lower CS level, resulting in fluorescent quenching by the heavy metal effect . On account of the relatively low fluorescence, the emission spectra and lifetimes for the complexes were measured in degassed solutions.…”

“…Although the calculated triplet state is slightly overestimated at 2.03 eV, BODIPY-centered triplet state fluorescence spectra have been observed in previous studies on BODIPY-conjugated ruthenium, iridium, and manganese complexes. 42,52,54 In fact, all these complexes were observed to possess relatively high BODIPY-centered triplet excited states capable of producing singlet oxygen, which has a triplet−singlet gap of 0.98 eV. 43,44 Phototoxicity Studies and Time-Dependent Raman Spectroscopy.…”

“…Ru­(II) metal complexes are especially suitable for medical applications due to their long-lived excited state, large Stoke shift, and especially their light-switching properties . Conjugation of an organic photosensitizer dye not only enhances light-harvesting in the visible region, but it allows for excited energy decay from the higher-lying ruthenium triplet state to the lower-lying BODIPY triplet state, and subsequently, the formation of singlet oxygen. − …”