Adaptive photoluminescence through a bioinspired antioxidative mechanism

Abstract

Transition metal complexes are archetypal luminescent probes that are widely used for various applications ranging from optoelectronics to biomedicine. However, they face significant challenges such as photobleaching and photooxidative stress, which limit their performance. Herein, we introduce a photosystem-inspired concept based on the use of a vitamin (ascorbic acid, Asc-Ac) to adaptively suppress photobleaching of molecular luminophores. As a proof-of-concept compound, we have selected a new bis-cyclometalated Pt(II) complex (Pt-tBu) and investigated its adaptive photoluminescence resulting from singlet dioxygen (¹O₂) photoproduction in the presence of Asc-Ac. Interestingly, the excited state quenching and subsequent photobleaching of Pt-tBu in aerated solutions is suppressed by addition of Asc-Ac, which scavenges the ¹O₂ photosensitized by Pt-tBu upon irradiation and results in an adaptive oxygen depletion with enhancement of luminescence. The adaptation is resilient for successive irradiation cycles with oxygen replenishment, until peroxidation overshooting leads to the degradation of Pt-tBu by formation of a dark Pt(iv) species. The complexity-related adaptation with initial overperformance (luminescence boost) relies on the external energy input and cascaded feedback loops, thus biomimicking inflammation, as the repeated exposure to a stressor leads to a final breakdown. Our antioxidative protection mechanism against photobleaching can be successfully extended to multiple coordination compounds (e.g., Ir(iii), Ru(ii) and Re(i) complexes), thus demonstrating its generality. Our findings broaden the scope of molecular adaptation and pave the way for enhancing the stability of molecular luminophores for multiple applications.

Fig. 1. Structural formula of Pt-tBu and schematic representation of its photoluminescence changes in response to light upon deoxygenation with Ar followed by equilibration with air, in the presence and absence of Asc-Ac. Feedback loops and light-driven equilibrium displacement enables the adaptation to the stressor (³O₂) through ¹O₂ scavenging.

Fig. 2. (a) UV-vis absorption spectra of Pt-tBu in air-equilibrated DMF with (blue) and without (black) Asc-Ac, as well as of an Ar-purged (red) solution untreated (solid) and directly after irradiation (120 s) at 365 nm (dashed). (b) Exemplary photoluminescence spectra (Pt-tBu 10 μM and Asc-Ac 5 mM) before and after prolonged irradiation in a photoreactor. Inset: pictures of the cuvette before and after the irradiation process in a photoreactor (λ_ex = 365 nm). (c) Plot of I/I_maxvs. irradiation time at different concentrations of Pt-tBu and Asc-Ac at room temperature in DMF.

Fig. 3. (a) Immediate (blue) and delayed (red) luminescence lifetime measurement (monitored at 514 nm) of the mixture (Pt-tBu 25 μM and Asc-Ac 10 mM with an air headspace) after the irradiation process in the photoreactor; comparison with Ar-purged mixtures before (black) and after irradiation (green). Inset: picture of the air-equilibrated cuvette shortly after irradiation under UV light (λ_ex = 365 nm). (b) Plot of the emission intensity at 514 nm vs. the number of irradiation cycles at different concentrations of Pt-tBu and Asc-Ac (with air headspace) at room temperature in DMF. Each cycle consists of irradiation in the photoreactor (λ_ex = 365 nm) and subsequent re-equilibration with the headspace by shaking the cuvette. The system shows a resistance against stressor replenishment but collapses after multiple re-equilibrations by the formed products. (c) Schematic illustration of an irradiation cycle under UV light (λ_ex = 365 nm).

Fig. 4. Structural formulae of fac-tris-(2-phenylpyridine)iridium(iii) (a), tricarbonyl(cyanido)(1,10-phenanthroline)rhenium(i) (b), [PtCl(L_Th)] (c) and tris(2,2′-bipyridyl)ruthenium(ii) dichloride hexahydrate (d). Shown are the corresponding pictures of the cuvettes with each complex (25 μM) and Asc-Ac (10 mM) in DMF before and after the irradiation in a photoreactor (as observed under UV light, λ_ex = 365 nm). Pictures on the right demonstrate the diffusion of fresh oxygen from the headspace after irradiation.