FACTORING THROUGH-SPACE AND THROUGH-BOND CONTRIBUTIONS TO RATES OF PHOTOINDUCED ELECTRON-TRANSFER IN DONOR-SPACER-ACCEPTOR MOLECULES

Contributions from direct orbital overlap (through-space interactions) and superexchange (through-bond interactions) to the electronic coupl ing matrix elements for photoinduced charge separation and recombinati on in a series of linked donor-spacer-acceptor molecules were studied. The molecules consisted of a 4-piperidinyl-naphthalene-1,8-dicarboxim ide (ANI) electron donor and a N-(n-octyl) pyromellitimide (PI) electr on acceptor attached to the 1,5- and 1,8-positions of either anthracen e (ANC) or dibenzobicyclo (2.2.2) octatriene (DBO) spacers. For the 1, 8-disubstituted compounds, ANI and PI are held approximately cofacial with a center-to-center distance of 5.3 Angstrom, whereas for the 1,5- disubstituted compounds, the center-to-center distance increases to 13 .5 Angstrom. The through-bond interaction was investigated by replacin g the ANC spacer with DBO. The charge separation and recombination rea ctions were examined in both toluene and tetrahydrofuran (THF). The re sults show that for the 1,8-disubstituted DBO system in both toluene a nd THF, charge separation and recombination is dominated by through-sp ace interactions. For the 1,8-disubstituted ANC system in both toluene and THF, charge separation occurs by means of a direct, through-space interaction, while charge recombination occurs through the intermedia cy of the ANI-ANC(+)-PI- ion pair in toluene and directly from ANI(+)- ANC-PI- to ground state in THF. For the 1,5-disubstituted molecules, p ossessing either the ANC or the DBO spacers, electron transfer from (1 )ANI to PI was not kinetically competitive with the decay of (1*)ANI to ground state in either toluene or THF.