Photoinduced electron transfer between acenaphthylene and 1,4-benzoquinones. Formation of dimers of acenaphthylene and 1 ∶ 1-adducts and effect of excitation mode on reactivity of the charge-transfer complexes

Abstract

Photochemical reactions of acenaphthylene (ACN) with 1,4-benzoquinones (BQs) of varying reduction potentials in solution have been investigated in order to determine final products and quantum yields of the reactions and to get an insight into the factors which govern their reactivities. The products were the two isomeric dimers of ACN and three types of 1 ∶ 1-adducts (cyclobutanes, furans, and oxetanes) between ACN and BQs. However, the product distribution varied widely with the substitution on the BQ skeleton. Two modes of excitation, that is, selective excitation of the charge transfer (CT) complex (the CT mode; typical wavelength: 546.1 nm) and direct excitation of ACN or BQs (the direct mode; typical wavelength: 435.8 nm), essentially gave similar product distributions when the reaction took place through both modes. However, the direct mode showed higher quantum yields for the reactions, Φ_435.8, than the CT mode, Φ_546.1. Moreover, Φ_546.1 tended to increase with increase of free energy gap, −ΔG_BET, between the ground state CT complexes and the resulting radical ion pair (RIP). These observations can be rationalized by a mechanism involving a distinctive RIP as an intermediate generated by photoinduced electron transfer in each mode. Thus, the solvent-separated radical ion pair (SSIP) produced in the direct mode excitation will undergo dissociation to the free radical ions (FRIs), ACN⁺˙ and BQ⁻˙, affording final products competing with backward electron transfer (BET). In contrast, the contact radical ion pair (CIP) produced in the CT mode excitation much more rapidly deactivates to the ground state than dissociates to FRIs via SSIP due to faster BET, whose rate depends on −ΔG_BET. The 1 ∶ 1-adducts can be formed from either a cage reaction inside the RIP (CIP or SSIP) or reaction between FRIs, whereas the dimers of ACN should be formed from reaction of ACN⁺˙ with the ground state ACN.