Solvent polarity effect on intramolecular electron transfer in a corrole–naphthalene bisimide dyad

Abstract

A dyad (C3–NI) based on corrole and naphthalene bisimide has been synthesized and its photoreactivity compared to that of the model component corrole (C3) and naphthalene bisimide (NI) in solvents of different polarity: toluene (TL) and dichloromethane (DCM). The major emitting species in NI solutions, in TL, is identified as a dimeric species (λ = 470 nm, τ = 2.3 ns) but traces of monomer can also be detected (λ ca. 390 nm, τ = 40 ps). In DCM the major emitting component is the monomer (λ = 383 nm, τ = 20 ps) but traces of different aggregates (λ = 540 and 570 nm, τ = 4.5 and 11 ns) are present. C3 has a fluorescence nearly unaffected by solvent polarity, with a maximum around 655 and a lifetime of 3.5 or 3.8 ns in DCM and TL, respectively. The dyad C3–NI does not appear to be affected by aggregation problems in any of the solvents. Excitation of the imide component in C3–NI (C3–¹NI) results in an energy transfer to corrole (¹C3–NI) with rate k = 2.0 × 10¹¹ s⁻¹ in both solvents. The latter state reacts further via a LUMO–LUMO electron transfer to the naphthalene bisimide yielding the charge separated state C3⁺–NI⁻ (k = 1.8 × 10⁹ s⁻¹ in TL and k = 3.7 × 10⁹ s⁻¹ in DCM). The same type of reactivity is displayed by direct excitation of the corrole moiety in the dyad to ¹C3–NI. C3⁺–NI⁻ decays with a rate comparable to that of its formation in DCM (k = 4.0 × 10⁹ s⁻¹in DCM), precluding its accumulation, whereas it decays with a slower rate in TL (k = 7.1 × 10⁸ s⁻¹). The charge separated state recombines to the ground singlet state; recombination to the triplet state of corrole (excited state at the lowest energy) is in fact excluded on the basis of the experimentally determined triplet yields. The failure of the commonly used methods in the calculation of CS energy levels in apolar solvents is confirmed.