Efficient photorelease of carbon monoxide from a luminescent tricarbonyl rhenium(I) complex incorporating pyridyl-1,2,4-triazole and phosphine ligands

Abstract

Precise control over the production of carbon monoxide (CO) is essential to exploit the therapeutic potential of this molecule. The development of photoactive CO-releasing molecules (PhotoCORMs) is therefore a promising route for future clinical applications. Herein, a tricarbonyl-rhenium(i) complex (1-TPP), which incorporates a phosphine moiety as ancilliary ligand for boosting the photochemical reactivity, and a pyridyltriazole bidentate ligand with appended 2-phenylbenzoxazole moiety for the purpose of photoluminescence, was synthesized and characterized from a chemical and crystallographic point of view. Upon irradiation in the near-UV range, complex 1-TPP underwent fast photoreaction, which was monitored through changes of the UV-vis absorption and phosphorescence spectra. The photoproducts (i.e. the dicarbonyl solvento complex 2 and one CO molecule) were identified using FTIR, 1H NMR and HRMS. The results were interpreted on the basis of DFT/TD-DFT calculations. The effective photochemical release of CO associated with clear optical variations (the emitted light passed from green to orange-red) could make 1-TPP the prototype of new photochemically-active agents, potentially useful for integration in photoCORM materials.

Fig. 1

Molecular view of the solvate of complex 1-TTP. Hydrogen atoms have been deleted for the sake of clarity.

Fig. 2

Unit cell of 1-TTP. Triflate anions and solvent molecules have been deleted for the sake of clarity.

Fig. 3

Monitoring by UV-vis absorption spectroscopy of the photochemical reaction of 1-TPP in undegassed acetonitrile solution (4.17 × 10⁻⁵ M) under irradiation at 350 nm over 3 min. One measurement was collected every 10 s for 120 s, then one measurement was made at 180 s. Initial spectrum in black dashed line. The arrows indicate the evolution of the spectra. Inset: plot of ln(a-x) vs time, with a being the initial concentration of 1-TPP and x the concentration of formed photoproduct.

Fig. 4

Monitoring by fluorescence spectroscopy of the photochemical reaction of 1-TPP in undegassed acetonitrile solution (2.1 × 10⁻⁵ M) under irradiation at 350 nm. Scans were obtained every 15 s of irradiation. Initial spectrum (t = 0 s) is represented with the black dashed line. λ_ex = 370 nm. Inset: photographs of concentrated solutions (~2.3 × 10⁻³ M) illuminated by a UV lamp (365 nm) before (a) and after complete photolysis (b).

Fig. 5

a) Emission spectrum of complex 1-TPP in the solid state (pristine powder). λ_ex = 380 nm. b) Powder sample observed under a hand-held UV lamp (365 nm), protected from light (left) and illuminated for 15 min by the same UV lamp (right). c) Optical microscopy images of the microcrystalline powder dispersed in water, taken every 15 s. The formation and merging of bubbles due to photolysis by the microscope beam (435-450 nm) are indicated by white and red arrows, respectively.

Fig. 6

Evolution of the ¹H NMR spectra of 1-TPP in CD₃CN (~2.3 × 10⁻³ M) throughout photolysis at 350 nm in situ in the NMR tube. From bottom to top: t = 0, 3.5, 10 and 19 min irradiation. The dotted lines indicate the centres of multiplets and the arrows indicate the sense of variation of the chemical shifts during photolysis. See Fig. S17† for proton numbering and more details concerning H25.

Fig. 7

Evolution of the FTIR spectrum of 1-TPP in acetonitrile (9.9 × 10⁻⁴ M) throughout photolysis at 300 nm. From bottom to top: t = 0, 3, 8 and 13 min irradiation. The arrows indicate the sense of variations of the peak intensities during photolysis.

Fig. 8

Isodensity plots of selected frontier molecular orbitals involved in the first electronic transitions for the cation of complex 1-TPP (left) and cation 2(iso1) (right) in acetonitrile, according to TD-DFT calculations at the PBE1PBE/LANL2DZ/6-31+G** level of theory. Orbitals mainly centred on PBO (red line), Re and CO (blue line), and pyta (green line).

Scheme 1.

Synthesis route to complex 1-TPP.