Diarylamine-Fused Subporphyrins: Proof of Twisted Intramolecular Charge Transfer (TICT) Mechanism

Diarylamine-fused subporphyrins 1a and 1b were synthesized from beta,beta-diiodo-meso-chloro subporphyrin 2 through a one-pot procedure involving nucleophilic aromatic substitution and S(RN)1-type intramolecular fusion reaction as the first example of meso-nitrogen-embedded subporphyrin. While non-fused counterparts3b and 4b display effective fluorescence quenching due to twisted intramolecular charge transfer (TICT) in CH2Cl2 at room temperature, 1b emits fluorescence with Phi(F)=0.18 under the same conditions, because conformational twisting is not allowed due to the fused structure. In addition, the oxidative titration with tris(4-bromophenyl)ammoniumyl hexachloroantimonate gave cation radical 7 for 1a and cation radical and dication for 1b. Actually, cation radical 7 was isolated through separation over silica gel column but was found to slowly decompose in concentrated solutions.