Excited-State Double Proton Transfer in 3-Formyl-7-azaindole:  Role of the nπ<SUP>*</SUP><SUP></SUP> State in Proton-Transfer Dynamics

3-Formyl-7-azaindole (3FAI) and its derivatives have been synthesized to study the role of the nπ* state in the excited-state double proton transfer (ESDPT) reaction. In 3FAI monomer as well as its associated hydrogen-bonded complexes the lowest excited singlet state has been concluded to be in the ¹nπ* configuration. The association constants incorporating the hydrogen bonding formation were determined to be 1.9 × 10⁴ (313 K), 2.2 × 10⁴ (298 K) and 1.8 × 10⁵ M^-1 (298 K) for 3FAI dimer, 3FAI/azacyclohexanone and 3FAI/acetic acid, respectively, in cyclohexane. In alcohols, the rate of solvent (e.g., methanol) diffusional migration, forming a “correct” precursor for ESDPT, is concluded to be much slower than the rate of S<INF>ππ</INF><INF>*</INF> → S<INF>n</INF><INF>π</INF><INF>*</INF> internal conversion which has been deduced to be 4.37 × 10¹² s^-1. ESDPT is prohibited in the S<INF>n</INF><INF>π</INF><INF>*</INF> state of which the relaxation dynamics are dominated by the rate of S<INF>n</INF><INF>π</INF><INF>*</INF> → T<INF>ππ</INF><INF>*</INF> intersystem crossing. In contrast, for 3FAI dimer or 3FAI/acetic acid complex possessing intact dual hydrogen bonds the intrinsic ESDPT is competitive with the rate of S<INF>ππ</INF><INF>*</INF> → S<INF>n</INF><INF>π</INF><INF>*</INF> internal conversion, resulting in a prominent imine-like tautomer emission. The results provide the first model among 7AI analogues in which the fast rate of S<INF>ππ</INF><INF>*</INF> → S<INF>n</INF><INF>π</INF><INF>*</INF> internal conversion serves as an internal clock to examine the mechanism of guest molecules (including the bulk alcohols) assisted ESDPT.