Proton gated emission

Proton Gated Emission. Table I provides a partial listing of various PS surface species which have been suggested to be either directly or indirectly related to the luminophore. The pKa provided in this Table is gleamed from the silica gel literature.(22,2i)... 

Our 11 [45] builds on the known ability of protonated pyridine units to quench the emission from fluorophores in their neighborhood [46]. The electron deficiency of such units makes them good acceptors in PET processes. Compound 11 possesses a 2,2 -bipyridyl unit which can be a receptor for H+, but it can also bind Zn + [47]. In either case the 2,2 -bipyridyl unit is rendered more electron-deficient. Hence, the fluorescence of the anthracene unit is quenched upon input of either ion in sufficiently high concentrations. The requirement for nonselectivity of the ion-induced luminescence response is not as stringent as for the OR gate described above. 

Fig. 7. A three-input INHIBIT gate exemplified by the tetraanion 54 and /1-cyclodextrin (j8-CD). a With neither protons nor /1-CD present in the solution, phosphorescent output is low, because of both PET from the tertiary amine, and through intermolecular triplet-triplet collisions of the bromonaphthalene phosphor, b Addition of calcium ions leads to a reduction in the PET-based quenching of the phosphorescence - however, intermolecular collisions still lead to a low emission, c Shielding of the phosphor with /l-CD reduces intermolecular triplet annihilations, but quenching still occurs via PET. d Only with both Ca2+ and /1-CD present does the solution phosphoresce, e In any combination of Ca2+ and /1-CD, the solution will yield a low output in the presence of molecular oxygen (the INHIBIT stimulus), as a consequence of triplet-triplet collisions...

The array of structurally related lanthanide complexes has been shown to exhibit well-defined luminescence responses to variations in pH, p02, pX and pOH. Judicious choice of the excitation wavelength allows the N-alkylated and N-protonated complexes to be selectively excited, while the imposition of a time-delay in luminescence observation can gate out the emission from the shorter-lived components. This degree of control over the nature of the input and output signals,... 

Scheme in illustrates a possible model for the luminescence behavior of p- PS consistent with the observed quenching phenomena. The luminescence emission is gated by a surface hole trap which is in communication with the valence band and has a pH dependent energy. This surface site is hypothesized to provide a site of non-radiative recombination. The deprotonated state must exist in the bandgap so that recombination of photoexcited electrons can bypass the radiative band-to-band transition, while the protonated state is expected to lie below the valence band edge. 

Pyranine (PY) is a large aromatic molecule (Scheme 4) which shows excited-state intermolecular proton-transfer reaction with water molecules [12]. Several studies have been reported on the intermolecular proton transfer from PY to water. The interaction at the excited-state results in an excited-state proton-transfer (ESPT) reaction producing an anionic form, which emits a greenish-yellow fluorescence band. Here, we will consider only those of CD complexes using ultrafast spectroscopy [13]. In presence of CD, a 1 1 complex is formed and the normal emission of PY in water due to the enol form (E, 440 nm) increases, while that due to the anionic (A, 550 nm) structure decreases (Scheme 4). The change clearly shows the effect of CD on the emission behaviour. Gating the emission of E (440 nm) and of A (550 nm), the fs study reported a 0.8 ps component assigned to solvation of locally excited (LE) enol prior to proton transfer, and a 2-3 ps component attributed to solvent-assisted interconversion of LE to... 

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