Conformational Analysis of the Electron-Transfer Kinetics across Oligoproline Peptides Using N,N-Dimethyl-1,4-benzenediamine Donors and Pyrene-1-sulfonyl Acceptors†

Photoinduced intramolecular charge separation across proline-bridged donor−acceptor complexes of the type Pyr-(Pro)n−DMPD (where Pyr = pyrene-1-sulfonyl and DMPD = N,N-dimethyl-1,4-phenylenediamine) was studied. The steady-state emission spectrum for n = 0, 1, 2, 3 showed an increase in emission intensity with the number of proline residues. Time-dependent emission measured by streak camera showed increasing emission signal amplitude with increasing n, along with a decrease in decay rate. In all these studies, Pyr-Pro was used as a control complex for the decay of the excited pyrene acceptor moiety without the donor DMPD. Detailed photon counting experiments carried out in DMF/water, DMF, and toluene showed single-exponential kinetics for n = 0, 1 and multiexponential kinetics for n = 2, 3. Rate constants observed in DMF are for n = 0, k = ∼5 × 1010 s-1; n = 1, k = 9.70 × 108 s-1; n = 2, k = 35.9 × 108 s-1 (70%) and 5.58 × 108 s-1 (30%); and n = 3, k = 16.6 × 108 s-1 (55%) and 3.87 × 108 s-1 (45%). These ...