Photochemical Upconversion in Water Using Cu(I) MLCT Excited States: Role of Energy Shuttling at the Micellar/Water Interface

Abstract

Photochemical upconversion (UC) through triplettriplet annihilation (TTA), which employs a visible absorbing triplet photosensitizer and an annihilator, is a process that generates a high energy photon from two lower energy photons. TTA-UC has been largely developed in pure organic solvents and solid-state polymeric constructs while featuring near exclusive use of rare and expensive metals within the photosensitizer. In this current investigation, we demonstrate that TTA-UC from the long lifetime earthabundant photosensitizer [Cu(dsbtmp)2](PF)6 (dsbtmp = 2,9di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline), abbreviated as Cu-PS, functions in water through encapsulation within a cationic-based assembly. Cetyltrimethylammonium bromide (CTAB) was the surfactant of choice as it electrostatically binds the negatively charged water-soluble 10-phenylanthracene-9-carboxylate (PAC) acceptor/annihilator and ultimately facilitated energy transfer across the interface. Efficient and diffusion limited triplettriplet energy transfer (TTET) from Cu-PS to the PAC acceptor was achieved in this aqueous assembly. Unfortunately, the hindered mobility of the PAC moieties ultimately hampered the annihilation process and this was reflected in attenuated TTA rates and efficiencies. The combined experimental data illustrated that the water-soluble PAC acceptor was able to vectorially deliver the excited state energy stored in Cu-PS across the interface into the bulk aqueous solution by engaging in excited state electron transfer with methyl viologen acceptors. These results are important for remotely operating photoredox reactions in water while rendering a photosensitizer spatially isolated in the hydrophobic core of a micelle.

Harnessing Intermolecular Interactions to Promote Long-Lived Photoinduced Charge Separation from Copper Phenanthroline Chromophores

“…For applications where a purely aqueous environment is the optimal and most environmentally conscious choice, this limitation forces researchers to work in water/organic mixtures or seek other means of circumventing the issue such as encapsulating the chromophores within micelles. 46 Given that a 2,9-diphenyl phenanthroline ligand with sulfonates at the meta positions of the phenyl groups has been reported to form water-soluble copper(I) complexes, 47,48 we anticipated that the sulfonated copper phenanthrolines explored here might also be compatible with purely aqueous environments. Indeed, 1-SO 3 with four sulfonate groups dissolves readily in water to very high concentrations (at least 50 mM); 2-SO 3 can also be dissolved in water, but only up to 0.5 mM.…”

“…Most Cu­(I)­bis­(phenanthroline)­s, especially those where the phenanthroline backbones are functionalized with aryl substituents or heavily alkylated, will not dissolve in pure water. For applications where a purely aqueous environment is the optimal and most environmentally conscious choice, this limitation forces researchers to work in water/organic mixtures or seek other means of circumventing the issue such as encapsulating the chromophores within micelles . Given that a 2,9-diphenyl phenanthroline ligand with sulfonates at the meta positions of the phenyl groups has been reported to form water-soluble copper­(I) complexes, , we anticipated that the sulfonated copper phenanthrolines explored here might also be compatible with purely aqueous environments.…”

Micellar Effects and their Relevance in Photochemistry and Photocatalysis

“…One of the challenges within photoredox catalysis is back electron transfer, which has been elegantly solved by employing micellar conditions, which facilitates a charge-based phase separation between the aqueous phase and the micellar interior. [31,38,47] For this, the charges of the headgroup of the micellar amphiphile as well as the molecules participating in the electron transfer and back electron transfer processes are key for the reaction design. [46] In a reaction design using a negatively charged headgroups the following trends can be observed with differently charged photocatalysts: a positively charged photocatalyst shows a strong association with the negatively charged headgroups, which results in fast forward and back electron transfer to the substrate due to inhibited diffusion of the photocatalyst (Figure 5a); in the case of a highly water-soluble neutral photocatalyst a fast electron transfer to an acceptor and reduced back electron transfer, due to the photocatalysts ability to more easily diffuse away from the micelles surface, is observed (Figure 5b); when a negatively Mayo.…”

Red Light-Based Dual Photoredox Strategy Resembling the Z-Scheme of Natural Photosynthesis

“…Many tetrahedral complexes of this type have long been known but received much attention for photoredox catalysis only recently. 10 , 66 − 74 Most of them absorb predominantly in the blue or green, 72 , 75 − 81 whereas the [Cu(dap) 2 ] + compound (dap = 2,9-dianisyl-1,10-phenanthroline, Figure 2 f) stands out in its capacity to absorb up to ca. 650 nm.…”

“…Blue or green light-absorbing photocatalysts are widespread, − whereas alternatives that feature sizeable extinction coefficients in the red spectral range are less common. ,− ,− , Osmium polypyridyls are a well-known option, ,− but in the spirit of our research program geared at the development of new photocatalysts based on Earth-abundant transition metals, − Cu I complexes attracted our attention. Many tetrahedral complexes of this type have long been known but received much attention for photoredox catalysis only recently. ,− Most of them absorb predominantly in the blue or green, ,− whereas the [Cu­(dap) 2 ] + compound (dap = 2,9-dianisyl-1,10-phenanthroline, Figure f) stands out in its capacity to absorb up to ca. 650 nm. ,− With its photoactive excited state storing 2.05 eV and an excited-state oxidation potential of −1.4 V vs SCE, ,, [Cu­(dap) 2 ] + looked like an attractive alternative to precious Os II polypyridyls and was therefore chosen as the primary photocatalyst (Figure b).…”