A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO-PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO-PDI show that enhanced intersystem crossing occurs with tau = 45 +/- 1 ps, resulting in formation of TEMPO-(3*)PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO-(1*)PDI --> TEMPO(+*)-PDI(-*) occurs with tau = 1.2 +/- 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO-(3*)PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized (3*)PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO-(3*)PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to (3*)PDI having a spin polarization pattern characteristic of overpopulation of its T(0) sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of (3*)PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism, which requires that the radical and attached triplet are in the weak exchange regime.

A series of donor-bridge-acceptor (D-B-A) triads have been synthesized in which the donor, 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An), and the acceptor, naphthalene-1,8:4,5-bis(dicarboximide) (NI), are linked by p-oligophenylene (Ph(n)) bridging units (n = 1-5). Photoexcitation of DMJ-An produces DMJ(+*)-An(-*) quantitatively, so that An(-*) acts as a high potential electron donor, which rapidly transfers an electron to NI yielding a long-lived spin-coherent radical ion pair (DMJ(+*)-An-Ph(n)-NI(-*)). The charge transfer properties of 1-5 have been studied using transient absorption spectroscopy, magnetic field effects (MFEs) on radical pair and triplet yields, and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The charge separation (CS) and recombination (CR) reactions exhibit exponential distance dependencies with damping coefficients of beta = 0.35 A(-1) and 0.34 A(-1), respectively. Based on these data, a change in mechanism from superexchange to hopping was not observed for either process in this system. However, the CR reaction is spin-selective and produces the singlet ground state and both (3*)An and (3*)NI. A kinetic analysis of the MFE data shows that superexchange dominates both pathways with beta = 0.48 A(-1) for the singlet CR pathway and beta = 0.35 A(-1) for the triplet CR pathway. MFEs and TREPR experiments were used to measure the spin-spin exchange interaction, 2J, which is directly related to the electronic coupling matrix element for CR, V(CR)(2). The magnitude of 2J also shows an exponential distance dependence with a damping coefficient alpha = 0.36 A(-1), which agrees with the beta values obtained from the distance dependence for triplet CR. These results were analyzed in terms of the bridge molecular orbitals that participate in the charge transport mechanism.

A series of donor-chromophore-acceptor-stable radical (D-C-A-R(•)) molecules having well-defined molecular structures were synthesized to study the factors affecting electron spin polarization transfer from the photogenerated D(+•)-C-A(-•) spin-correlated radical pair (RP) to the stable radical R(•). Theory suggests that the magnitude of this transfer depends on the spin-spin exchange interaction (2JDA) of D(+•)-C-A(-•). Yet, the generality of this prediction has never been demonstrated. In the D-C-A-R(•) molecules described herein, D is 4-methoxyaniline (MeOAn), 2,3-dihydro-1,4-benzodioxin-6-amine (DioxAn), or benzobisdioxole aniline (BDXAn), C is 4-aminonaphthalene-1,8-dicarboximide, and A is naphthalene-1,8:4,5-bis(dicarboximide) (1A,B-3A,B) or pyromellitimide (4A,B-6A,B). The terminal imide of the acceptors is functionalized with either a hydrocarbon (1A-6A) or a 2,2,6,6-tetramethyl-1-piperidinyloxyl radical (R(•)) (1B-6B). Photoexcitation of C with 416-nm laser pulses results in two-step charge separation to yield D(+•)-C-A(-•)-(R(•)). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy using continuous-wave (CW) microwaves at both 295 and 85 K and pulsed microwaves at 85 K (electron spin-echo detection) was used to probe the initial formation of the spin-polarized RP and the subsequent polarization of the attached R(•) radical. The TREPR spectra show that |2JDA| for D(+•)-C-A(-•) decreases in the order MeOAn(+•) > DioxAn(+•) > BDXAn(+•) as a result of their spin density distributions, whereas the spin-spin dipolar interaction (dDA) remains nearly constant. Given this systematic variation in |2JDA|, electron spin-echo-detected EPR spectra of 1B-6B at 85 K show that the magnitude of the spin polarization transferred from the RP to R(•) depends on |2JDA|.

A covalent, fixed-distance donor-bridge-acceptor (D-B-A) molecule was synthesized that upon photoexcitation undergoes ultrafast charge separation to yield a radical ion pair (RP) in which the spin-spin exchange interaction (2J) between the two radicals is sufficiently large to result in preferential RP intersystem crossing to the highest-energy RP eigenstate (T(+1)) at the 350 mT magnetic field characteristic of X-band (9.5 GHz) EPR spectroscopy. This behavior is unprecedented in covalent D-B-A molecules, and is evidenced by the time-resolved EPR (TREPR) spectrum at X-band of (3*)D-B-A derived from RP recombination, which shows all six canonical EPR transitions polarized in emission (e,e,e,e,e,e). In contrast, when the RP is photogenerated in a 3400 mT magnetic field, the TREPR triplet spectrum at W-band (94 GHz) of (3*)D-B-A displays the (a,e,e,a,a,e) polarization pattern characteristic of a weakly coupled RP precursor, similar to that observed in photosynthetic reaction center proteins, and indicates a switch to selective population of the lower-energy T(0) eigenstate.

The spin mixing process of the radical pair in the sodium dodecyl sulfate (SDS) micelle is studied by using a novel technique nanosecond pulsed magnetic field effect on transient absorption. We have developed the equipment for a nanosecond pulsed magnetic field and observed its effect on the radical pair reaction. A decrease of the free radical yield by a reversely directed pulsed magnetic field that cancels static field is observed, and the dependence on its magnitude, which is called pulsed MARY (magnetic field effect on reaction yield) spectra, is studied. The observed spectra reflect the spin mixing in 50-200 ns and show clear time evolution. Theoretical simulation of pulsed MARY spectra based on a single site modified Liouville equation indicates that the fast spin dephasing processes induced by the modulation of electron-electron spin interaction by molecular reencounter affect to the coherent spin mixing by a hyperfine interaction in a low magnetic field.