Sensitizing dyes ionization potentials

One method used to position the energy levels of the dyes relative to those of the silver halide depends on the measurement of ionization potentials of the dyes adsorbed by a substrate. Nelson (251,252) measured ionization potentials of several sensitizing dyes coated on a variety of substrates ... 

The efficiency and energetics of spectral sensitization by most of the dyes could be correlated with the e v levels of the dyes. He calculated these levels from the ER values, using the linear relation between ER and Nelson s ionization potentials extrapolated to cover all of the dyes. However, quantitative calculations of c)>r on the assumption of direct electron transfer from dye to silver halide predicted values that were much smaller than the experimental ones for the group of dyes with ER... 

The family of photoreducible dyes (e.g, acridinium, xanthene, thiazinium among other classes of dyes) produce excited states of essentially quinoidal structures which can act as efficient acceptors of electrons. Amines [59], sulfur compounds, especially sulfmate salts [60], heterocycles of low ionization potential [61], alkylcarboxylates and stable enolate anions [62], and several classes of organo-metallic compounds, notably allylic and benzylic organostannanes [63], represent classes of compounds which have proved efficacious as coinitiators in electron transfer sensitization with these dyes. Electron transfer with the organometallics was unambiguously established in a series of model studies involving electron acceptors of the anthracene class [64],... 

Sensitization by charge transfer requires that the LUMO of the diacetylene monomer be below the excited singlet level of the dye. The latter is at Is — Eg where I, and Eji are ionization potential and energy of the first singlet state of the dye, respectively. On the basis of polarographic data I, — E j —2.7. .. —3.3 eV is estimated for cyanine dyes The lowest unoccupied orbital of a previously neutral diacetylene... 

The selection of the best analytical line has not been discussed explicitly and deserves attention. Because LEI is at least a two-step process involving laser excitation and thermal ionization steps, many transitions may be preferred for LEI which are not usable by purely optical methods. In other words, excited states which have low fractional populations may produce good LEI sensitivity due to proximity of the laser-populated state to the ionization potential. This makes the choice of the most sensitive LEI lines more complicated but it introduces an important practical advantage for dye laser spectrometry. 

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