Sensitized triplet-triplet annihilation upconversion in water and its application to photochemical transformations

Abstract

Sensitized triplet-triplet annihilation (TTA) is a promising mechanism for solar energy conversion, but so far its application has been practically completely limited to organic solvents and self-assembled or solid state systems. Combining water-soluble ruthenium complex-pyrene dyads with particularly long excited-state lifetimes as sensitizers and highly fluorescent commercial anthracenes as acceptors/annihilators, we were able to achieve green-to-violet upconversion with unprecedented quantum yields in pure water. Compared to the only known system exploiting sensitized TTA in homogeneous aqueous solution, we improve the overall photon upconversion efficiency by a full order of magnitude and present the very first example for a chemical transformation on a laboratory scale via upconversion in water. Specifically, we found that a thermodynamically challenging carbon-chlorine bond activation can be driven by green photons from an inexpensive continuous wave light source in the presence of dissolved oxygen. Our study is thus potentially relevant in the context of cleaning water from halogenated (toxic) contaminants and for sustainable photochemistry in the most environmentally friendly solvent.

Scheme 1. (Left) Molecular structures and abbreviations of the water-soluble sensitizers (upper row) and acceptors/annihilators (lower row) investigated in this work. (Right) Qualitative energy diagram illustrating upconversion through sensitized TTA using a Ru-based sensitizer (whose MLCT-excited state is in equilibrium with the pyrene (Py) triplet in the dyads19) and an anthracene derivative (An) as the acceptor/annihilator. Excited state energies were taken from ref. 20. For further explanations, see the main text.

Fig. 1. Calibrated UV-Vis absorption (solid lines) and normalized luminescence (dotted lines) spectra (upon excitation at 500 nm and 330 nm for the complexes and the pyrene reference, respectively) of reference compounds (upper panel) and metal complex–arene dyads (lower panel) in homogeneous aqueous solution.

Fig. 2. Excited-state properties of the sensitizers in Ar-saturated aqueous solution (Rubpy, 30 μM, panels a and d; RuPy1, 22 μM, panels b and e; RuPy2, 28 μM, panels c and f) upon excitation with green (532 nm) laser pulses of ca. 10 ns duration. Panels a–c contain transient absorption spectra (immediately following excitation with 45 mJ pulses and time-integrated over 200 ns). Arrows indicate the detection wavelengths for the kinetic measurements (with laser pulse energies of 16 mJ) shown in panels d–f. For further explanations, see text.

Fig. 3. Ground-state and singlet-excited state properties of the acceptors/annihilators, which were investigated in this study (the same colour code for the compounds as in Scheme 1). Panels a–c, calibrated UV-Vis absorption (solid lines) and normalized luminescence (dotted lines) spectra. Panels d–f, unquenched excited-singlet state lifetimes measured at the respective emission maxima (λ_ex = 375 nm). The instrument response function (IRF) is included in panel d. For details, see text and page S2 of the ESI.

Fig. 4. Triplet–triplet energy transfer from ³Rubpy (orange) to the anthracene derivatives MAMA (violet), APA (black) and ACA (gray) in deoxygenated aqueous solutions upon excitation of the sensitizer (30 μM) with 532 nm laser pulses (37 mJ). Main plot, transient absorption spectra after sensitizer excited-state decay with MAMA (0.82 mM), APA (0.28 mM), or ACA (1 mM) (recorded by time integration over a detection period of 200 ns with a time delay of 3 μs). For clarity, the spectra with APA and ACA have been multiplied by 1/2 and 1/5, respectively. Inset, comparative kinetic traces for the system Rubpy/MAMA (0.54 mM) at the maxima of Rubpy bleaching (455 nm) and ³MAMA absorption (424 nm). For further details, see text.

Fig. 5. Photon upconversion in deoxygenated water using a pulsed laser (panels a and b) or a cw light source (panel c) for excitation at 532 nm. (a) Main plot, MAMA (0.25 mM) triplet formation and decay with the three different sensitizers. For all experiments, initial absorbance (0.022 per cm) of the respective sensitizer at 532 nm and laser intensity (33 mJ) were identical. Inset, time integrated (over 400 μs starting with the laser pulse) upconversion emission under the conditions of the main plot. (b) Delayed luminescence spectra employing 30 μM Rubpy and 1 mM of the respective acceptor/annihilator. The laser energies were adjusted such that the initial concentrations of anthracene triplets were identical in all cases. The removal of remaining Rubpy phosphorescence from the spectrum was carried out as in Fig. S5 (ESI†). (c) Power dependences of the upconversion emission for the systems RuPy2 (25 μM)/MAMA (0.25 mM) (violet) and Rubpy (27 μM)/ACA (2 mM) (gray, spectrum only shown at the highest laser power used).

Scheme 2. Reductive monodechlorination of trichloroacetate (TCA^–) in air-saturated aqueous solution by singlet-excited APA generated through sensitized TTA using green light. For details, see the text and Section 7 of the ESI.