Using Spin Dynamics of Covalently Linked Radical Ion Pairs To Probe the Impact of Structural and Energetic Changes on Charge Recombination

Abstract

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.

EPR-detected photoinduced electron transfer in three structurally related molecular triads

“…The relatively long lifetime of the triplet-bom radical pair is therefore not due to any attenuation effect on the ET rates produced by the nematic potential [52,53] but to intrinsic properties of the electronic states involved in the ET process. The role played by the LC is that of tuning the reorganization and stabilization energies of the charge-separated states leading in this way to the activation of different photochemical pathways [54,55].…”

“…For all the molecular triads, recombination of the singlet-born radical pair in the nematic phase of E-7 is not forced in the solvent-controlled adiabatic limit, although many examples of the operating mechanism of solvent-controlled ET have been reported for biomimetic compounds in LC [40,52,54,56]. For weakly coupled donor-acceptor systems as the triads under investigation, as already demonstrated for the C-P-C60 triad bearing the tetraalkylporphy¡ bridge [18], the electronic coupling element VDA is notable to contrast the retardation effect produced by the nematic potential on the longitudinal relaxation time r L. The adiabacity factor ~c, which describes the transition from the nonadiabatic to the solvent-controlled adiabatic limit is a function of both V2A and r L [52]:…”

Screening Electronic Communication through ortho‐, meta‐ and para‐Substituted Linkers Separating Subphthalocyanines and C60

“…A reasonable interpretation for this finding is lent from the superexchange mechanism, [9] which leads to a stronger increase in coupling at the ortho and para positions relative to that at the meta connections. Leading examples include singlet-singlet and triplet-triplet energy transfer, [10] photoinduced charge separation and charge recombination, [11] charge transfer to electrodes, [12] or electronic communication in conjugated dendrimers [13] and intervalence metal complexes. [14] In addition, some of these studies reveal that meta linkers may provide enhanced electronic coupling in the excited state, operating as a gated wire in p-conjugated systems.…”

Influence of structural dynamics on charge recombination rates in photogenerated radical ion pairs: Evidence from EPR spectroscopy and computation

“…Throughout the temperature range (190-295 K) of liquid dichloromethane solutions, the reference compound D+'-C exhibits highly structured and symmetric EPR spectra centered at g = 2.0028 +_ 0.0001 [14] (Fig. la) as expected for an organic radical in an isotropic medium.…”

“…The covalent donor-chromophore dyad D-C has been studied in a variety of contexts [10][11][12][13][14][15][16][17][18], one of the most significant is its incorporation into the donorchromophore-acceptor triad D-C-A (Chart 1). Here, the donor D is p-methoxyaniline, the chromophore C is 4-(N-piperidinyl)-naphthalene-l,8-dicarboximide, and the acceptor A is naphthalene-l,8:4,5-bis(dicarboximide).…”