We report on a series of electron donor−acceptor (D-A) dyads that undergo singlet-initiated charge separation to produce a strongly spin coupled radical ion pair that subsequently undergoes charge recombination to produce a triplet state with unusual spin polarization. The molecules consist of either a 4-(N-piperidinyl)naphthalene-1,8-imide (6P) or 4-(N-pyrrolidinyl)naphthalene-1,8-imide (5P) donor and a 1,8:4,5-naphthalenediimide (NI) or pyromellitimide (PI) acceptor. Selective photoexcitation of D within D-−A produces the radical ion pair 1[D•+-A•-] quantitatively. This is followed by the formation of 3[D•+-A•-] via singlet−triplet mixing within the radical pair. Radical pair intersystem crossing (RP-ISC) leads to charge recombination to yield [D-3*A] or [3*D-A]. Time-resolved optical absorption and emission spectroscopy is coupled with EPR spectroscopy to characterize the mechanism of the nearly quantitative initial charge separation reaction and the subsequent radical ion pair recombination reaction leading to the unusually spin polarized triplet state. These radical pairs also possess charge transfer emission bands that aid in the data analysis. The small number of previously reported covalent donor−acceptor systems that yield a triplet state from radical ion pair recombination use multistep charge separation reactions to achieve a ≥20 Å spacing between the oxidized donor and reduced acceptor. These examples have small exchange couplings, J, within the radical pair, so that S-T0 mixing between the radical pair energy levels occurs. In the strongly coupled systems described here, we show that the triplet states are formed by means of both S-T0 and S-T- 1 mixing, producing novel spin-polarized EPR spectra characterized by anisotropic spin lattice relaxation.

We have synthesized a series of structurally related, covalently linked electron donor-acceptor triads having highly restricted conformations to study the effects of radical ion pair (RP) structure, energetics, and solvation on charge recombination. The chromophoric electron acceptor in these triads is a 4-aminonaphthalene-1,8-dicarboximide (6ANI), in which the 4-amine nitrogen atom is part of a piperazine ring. The second nitrogen atom of the piperazine ring is part of a para-substituted aniline donor, where the para substituents are X = H, OMe, and NMe(2). The imide group of 6ANI is linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) electron acceptor across a phenyl spacer in a meta relationship. The triads undergo two-step photoinduced electron transfer to yield their respective XAn(*)(+)-6ANI-Ph-NI(*)(-) RP states, which undergo radical pair intersystem crossing followed by charge recombination to yield (3)NI. Time-resolved electron paramagnetic resonance experiments on the spin-polarized RPs and triplet states carried out in toluene and in E-7, a mixture of nematic liquid crystals (LCs), show that for all three triads, the XAn(*)(+)-6ANI-Ph-NI(*)(-) RPs are correlated radical pairs and directly yield values of the spin-spin exchange interaction, J, and the dipolar interaction, D. The values of J are all about -1 mT and show that the LC environment most likely enforces the chair conformation at the piperazine ring, for which the RP distance is larger than that for the corresponding boat conformation. The values of D yield effective RP distances that agree well with those calculated earlier from the spin distributions of the radical ions. Within the LC, changing the temperature shows that the CR mechanism can be changed significantly as the energy levels of the RPs change relative to that of the recombination triplet.

Differences between mesophases and hysteresis effects can be examined by electron paramagnetic resonance (EPR) detection of light-induced transient triplets oriented in liquid crystals (LC). Time-resolved EPR spectroscopy (direct detection) has been applied to study the photoexcited triplet state dynamics of tetraphenylporphyrin (free-base) oriented in a multiphase LC, ZLI-1167, having a negative anisotropic susceptibility, Δχ<0. The feasibility of triplet detection over a temperature range of 100–370 K, enables one to monitor conspicuous changes of the triplet line shape associated with the different mesophases, i.e., cryst.↔(287 K) smec.↔(305 K) nem.↔(356 K) isot. Line shape analysis shows that all phases consist of domains having their directors spanned in a plane normal to the external magnetic field, as expected for Δχ<0. The triplet spectra in the fluid smectic phase depend on sample rotation, indicating the known rigidity of this phase. On the other hand, the spectra in the nematic phase are independent of the sample rotation but strongly depend on the temperature and sample alignment in the magnetic field. Two mechanisms of triplet dynamics are proposed; namely an inter- and intradomain triplet energy transfer between the porphyrin chromophores. In addition, the results suggest that, in a specific alignment procedure, the smectic character is maintained deep in the nematic and, to some extent, in the isotropic phases. Thus the EPR line shapes and temporal behavior of the nematic spectra, are associated with a phase memory effect (hysteresis), and the smectic phase persistence may indicate the existence of cybotactic groups within the nematic and isotropic phases.

We report the results of time-resolved electron paramagnetic resonance (TREPR) studies of photoinduced charge separation in a series of biomimetic supramolecular compounds dissolved in oriented liquid crystal solvents. The molecules contain a chlorophyll-like (zinc 9-desoxomethylpyropheophorbide a) electron donor, D (ZC), and two electron acceptors with different reduction potentials, i.e., pyromellitimide, A1 (PI), and 1,8:4,5-naphthalenediimide, A2 (NI). The compounds investigated are ZCPI, ZCNI, and ZCPINI, and they have small but well-defined differences of their ion-pair energies. Temperature-dependent TREPR studies on this series of compounds permit the determination of the radical pair energy levels as the solvent reorganization energy increases from the low-temperature crystalline phase, through the soft glass phase, to the nematic phase of the liquid crystal. As the temperature is increased, the radical pair with the lowest energy is the first to exhibit triplet-initiated charge separation as the solvent reorganization energy increases in the liquid crystal. The energy levels of the radical pairs and the solvent reorganization energy are determined by using the known singlet and triplet excited state energy levels of ZC, the electrochemically determined relative energies between the radical ion pairs in polar isotropic solvents, and the TREPR data. All these yield information about the ordering of the radical ion pair energy levels relative to the excited-state energy levels of ZC.