Dynamics and mechanism of bridge-dependent charge separation in pyrenylurea-nitrobenzene pi-stacked protophanes

Herein are reported the synthesis, structure, and electronic properties of a series of tertiary di- and polyarylureas possessing pyrene and nitrobenzene end groups separated by a variable number of internal phenylenediamine bridging groups. These molecules adopt folded "protophane" structures in which the adjacent arenes are loosely pi-stacked. The behavior of both the pyrene and nitrobenzene singlet states has been investigated by means of femtosecond broadband pump-probe spectroscopy, and the transients have been assigned on the basis of comparison to reference molecules. Femtosecond time resolution permits direct observation of the fast internal conversion process for both the pyrene and nitrobenzene upper singlet states, as well as the intersystem crossing of nitrobenzene. The ultrafast (ca. 100 fs) charge separation of the donor-acceptor urea having no bridging group is attributed to an internal conversion process. The slower charge separation and charge recombination of the donor-acceptor urea having a single bridging group occur via a bridge-mediated superexchange process. Addition of a second bridging unit results in a role reversal for the pyrene singlet state, which now serves as an excited-state acceptor with the bridging units serving as the electron donors. The change in the directionality of electron transfer upon addition of a second bridging phenylenediamine is a consequence of a decrease in the bridge oxidation potential as well as a decrease in the rate constant for single-step superexchange electron transfer.