Direct measurement of singlet-triplet splitting within rodlike photogenerated radical ion pairs using magnetic field effects: Estimation of the electronic coupling for charge recombination

Determining the electronic coupling matrix element, V, for an electron transfer reaction is challenging both experimentally and theoretically. The magnitude of the singlet-triplet splitting (spin-spin exchange interaction), 2J, within a radical ion pair (RP) is directly related to the sum of the squares of the matrix elements that couple the RP state to the ground state and to other energetically proximate excited and ionic states. Each term in this sum is weighted by the reciprocal of the energy gap between the RP state and the particular state to which it is coupled. We present here a series of intramolecular triads with linear, rodlike structures that undergo very efficient two-step electron transfer following direct excitation of a 4-(N-piperidinyl)naphthalene-1,8-dicarboximide (6ANI) chromophore. Attachment of a p-methoxyaniline (MeOAn) donor by means of the piperazine bridge and naphthalene-1,8:4,5-bis(dicarboximide) (NI) or pyromellitimide (PI) acceptors, either directly or through a 2,5-dimethylphenyl (Me2Ph) spacer to 6ANI results in the triads MeOAn-6ANI-NI, MeOAn-6ANI-Pl, MeOAn-6ANI-Me2Ph-NI, and MeOAn-6ANI-Me2Ph-PI. The two-step charge separation from the lowest excited singlet state of 6ANI yields singlet radical ion pairs in which the charges are separated by 14 to 19 Angstrom and whose lifetimes range from about 15 to 200 ns. These lifetimes are long enough such that radical pair intersystem crossing occurs to form the triplet radical ion pair, which then recombines to form the ground state and a neutral excited triplet state, which is localized either on 6ANI or NI. The yield of this locally excited triplet state, monitored by nanosecond transient absorption as a function of applied magnetic field strength, exhibits distinct resonances that directly yield 2J. The value of 2J is used to estimate V-CR for charge recombination of the radical ion pair. These measurements provide a highly sensitive method of determining the dependence of the electronic coupling on the structure of the radical ion pair.