Ionization potential, charge dependence

Charge-Transfer Compounds. Similat to iodine and chlorine, bromine can form charge-transfer complexes with organic molecules that can serve as Lewis bases. The frequency of the iatense uv charge-transfer adsorption band is dependent on the ionization potential of the donor solvent molecule. Electronic charge can be transferred from a TT-electron system as ia the case of aromatic compounds or from lone-pairs of electrons as ia ethers and amines. 

Hyun et al. [345] prepared PbS Q-dots in a suspension and tethered them to Ti02 nanoparticles with a bifunctional thiol-carboxyl linker molecule. Strong size dependence due to quantum confinement was inferred from cyclic voltammetry measurements, for the electron affinity and ionization potential of the attached Q-dots. On the basis of the measured energy levels, the authors claimed that pho-toexcited electrons should transfer efficiently from PbS into T1O2 only for dot diameters below 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of the PbS/Ti02 assembly were consistent with electron transfer from small Q-dots. The measured charge transfer time was surprisingly slow ( 100 ns). Implications of this fact for future photovoltaics were discussed, while initial results from as-fabricated sensitized solar cells were presented. 

A crude justification for such a study can be found in the work of Widing and Levitt [167], who described numerous correlations between adiabatic ionization potentials and Taft s inductive substituent constants (an indirect way of correlating IPs with charges) and in a note by Streitwieser [168], who justified to some extent a dependence of IPs on local charge densities. 

Several attempts were made, including attempts to correlate the IPs with the net charge of the electron-richest C atom of each molecule or with a combination of the electron-richest carbon-hydrogen pair of each molecule, all of which failed in giving any monotonic dependence of the IPs on these quantities. The only solution is that resulting from comparisons of ionization potentials with the electron-richest pair of bonded C atoms in each molecule. This correlation turns out to be linear (Fig. 7.1), although such a linearity had not been postulated a priori. The least-square... 

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