Modification of fluorescent photoinduced electron transfer (PET) sensors/switches to produce molecular photo-ionic triode action

Abstract

The fluorophore-spacer1-receptor1-spacer2-receptor2 system (where receptor2 alone is photoredox-inactive) shows ionically tunable proton-induced fluorescence off-on switching, which is reminiscent of thermionic triode behavior. This also represents a new extension to modular switch systems based on photoinduced electron transfer (PET) towards the emulation of analogue electronic devices.

Keywords: electron transfer; fluorescence; molecular devices; molecular switches; triodes.

Figure 1

a) A fluorophore for photon transactions and two receptors for ion binding are the three crucial components of the molecular AND logic gate, where the two spacers serve as connectors. b) In a similar vein, the three crucial components of the molecular photo-ionic triode consist of a fluorophore and a principal receptor₁ alongside an auxiliary receptor₂. The latter endows the system with a way of tuning the input/output (I/O) characteristic curve. c) The three crucial components of the vacuum thermionic triode consist of a filament, plate, and an interspersed grid. This set-up also produces a tunable I/O characteristic.

Figure 2

a) Fluorescence emission spectra for 10⁻⁵ m 3 in methanol/water (1:1, v/v) with 10⁻⁴ m morpholinopropylsulfonic acid in the presence of 0.3 m Me₄NCl, when excited at 369 nm. pH adjustments were performed with Me₄NOH and HCl. The pH values in order of decreasing fluorescence intensity are: 6.4, 7.6, 8.2, 8.5, 8.7, 8.9, 9.5, 9.8, and 10.3. It is notable that all the spectral features except the quantum yield are essentially independent of the pH value, as expected for fluorescent PET sensors containing fluorophores with ππ* excited states.[34] Similar spectra are found when Me₄NCl is replaced by other salts (see below). b) Fluorescence quantum yield (ϕ_F)-pH profiles for 3 in the presence of various chloride salts. The concentrations of monovalent cation salts and divalent cation salts were chosen to minimize ionic strength changes. The salt concentrations were chosen to allow for as much as possible of 3 to be bound to the cation through the crown ether, while respecting solubility limits. Such a choice is enabled by data tables of cation/crown ether binding constants.[46] Studies at lower salt concentrations were not conducted since those would require dissection of the ϕ_F-pH profiles into metal-free and metal-bound components, with large attendant uncertainties. The cations employed are: 0.3 m Me₄N⁺ (filled squares), 0.3 m Na⁺ (open diamonds), 0.3 m K⁺ (filled triangles), 0.1 m Ca²⁺ (filled diamonds), 0.1 m Sr²⁺ (open circles), and 0.1 m Ba²⁺ (open triangles). The full lines are calculated according to Equation (1), by employing the experimentally determined parameters pK_a, ϕ_Fmax, and ϕ_Fmin from Table 1.