Electron-transfer absorption of sterically bulky donor–acceptor pairs: electron donor–acceptor complexes or random pairs?

Electron-transfer absorption of substituted benzene donors with tetracyanoethene (TCNE) is studied in dichloromethane and at several temperatures. For quantitative comparison of the intensities of electron-transfer absorbance, A of different electron donor/acceptor (D/A) pairs at various free donor and acceptor molar concentrations (CD and CA) an absorption coefficient κ=A/(CDCA) is introduced. The results indicate that κ values of sterically bulky alkyl substituted D/A pairs are several orders of magnitude lower than those of the corresponding sterically unhindered pairs with similar structures and electron transfer properties. For a series of comparable sterically bulky and unhindered donors, when strong donors are compared to weak donors, a more than 10-fold increase in κ values is observed for the sterically unhindered donors, while the change is less than two-fold for sterically bulky donors. This supports the view that the electron transfer absorption of the sterically bulky D/A pairs originates from unassociated random pairs. Further evidence comes from the fact that κ values of sterically bulky D/A pairs show very weak or no temperature dependence. By fitting the absorption spectra of the CT bands of the electron D/A complexes formed between TCNE and sterically unhindered donors, electronic coupling elements and reorganization parameters are obtained. Based on these parameters, the electron transfer absorption spectra of the random pairs discussed earlier are simulated and compared with experiments.