Excited-state double proton transfer in 3-formyl-7-azaindole: Role of the n pi* state in proton-transfer dynamics

3-Formyl-7-azaindole (3FAI) and its derivatives have been synthesized to st udy the role of the n pi* state in the excited-state double proton transfer (ESDPT) reaction. In 3FAI monomer as well as its associated hydrogen-bonde d complexes the lowest excited singlet state has been concluded to be in th e 1n pi* configuration. The association constants incorporating the hydroge n bonding formation were determined to be 1.9 x 10(4) (313 K), 2.2 x 10(4) (298 K) and 1.8 x 10(5) M-1 (298 K) for 3FAI dimer, 3FAI/azacyclohexanone a nd 3FAI/acetic acid, respectively, in cyclohexane. In alcohols, the rate of solvent (e.g., methanol) diffusional migration, forming a "correct" precur sor for ESDPT, is concluded to be much slower than the rate of S-pi pi* --> S-n pi* internal conversion which has been deduced to be 4.37 x 10(12) s(- 1). ESDPT is prohibited in the S-n pi* state of which the relaxation dynami cs are dominated by the rate of S-n pi* --> T-pi pi* intersystem crossing. In contrast, for 3FAI dimer or SFAI/acetic acid complex possessing intact d ual hydrogen bonds the intrinsic ESDPT is competitive with the rate of S-pi pi* --> S-n pi* internal conversion, resulting in a prominent imine-like t automer emission. The results provide the first model among 7AI analogues i n which the fast rate of S-pi pi* --> S-n pi* internal conversion serves as an internal clock to examine the mechanism of guest molecules (including t he bulk alcohols) assisted ESDPT.