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 hydrogenbonded complexes the lowest excited singlet state has been concluded to be in the 1 nπ* configuration. The association constants incorporating the hydrogen bonding formation were determined to be 1.9 × 10 4 (313 K), 2.2 × 10 4 (298 K) and 1.8 × 10 5 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 ππ* f S nπ* internal conversion which has been deduced to be 4.37 × 10 12 s -1 . ESDPT is prohibited in the S nπ* state of which the relaxation dynamics are dominated by the rate of S nπ* f T ππ* 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 ππ* f S nπ* 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 ππ* f S nπ* internal conversion serves as an internal clock to examine the mechanism of guest molecules (including the bulk alcohols) assisted ESDPT.
The carboxylic acid catalyzed excited-state amino-imino tautomerism for β-carboline (β-CB) and its analogues has been investigated. Thermodynamics and microsolvation (i.e., stoichiometry of the complex formation) of various β-CB/acetic acid complexes in nonpolar solvents have been studied by means of absorption and emission titration experiments. Supplementary support of the stoichiometric ratio and structure for the hydrogenbonding formation was provided by molecular design and syntheses of various β-CB analogues incorporating either only one hydrogen bonding site or dual hydrogen bonding sites where interplay between two sites are sterically prohibited. The results in combination with time-resolved measurements and theoretical approaches suggest the 1:2 β-CB/acetic acid complex with a structure of triple hydrogen bonding formation to be responsible for the excited-state proton-transfer tautomerism in cyclohexane. The proton transfer time is beyond the response limit of the detecting system of 15 ps, indicating that only a negligibly small geometry adjustment is required for the guest molecule (i.e., acetic acid) to a correct geometry, i.e., a hydrogen-bond relay configuration, for the triple proton transfer to proceed. In comparison, for the 1:1 β-CB/acetic acid non-hydrogen-bond relayed complexes, amino-imino tautomerism is prohibited during the excited-state lifetime, giving rise to a normal Stokes shifted emission. The results provide detailed ground-state thermodynamics of β-CB HB complexes as well as the dynamics of proton-transfer tautomerism mediated by the hydrogen-bonding structures.
The low-lying triplet states of 10-hydroxybenzo[h]quinoline (HBQ) and its halogenated derivatives 7,9-diiodo-10-hydroxybenzo[h]-quinoline (DIHBQ) in an excited-state intramolecular proton-transfer (ESIPT) process have been investigated. For DIHBQ, which is enhanced by the intramolecular heavy atom effect, the protontransfer tautomer (i.e., the keto form) phosphorescence that is maximized at 735 nm (τ p ) 1.75 µs) was resolved in a 77 K methylcyclohexane (MCH) glass. Further transient absorption and 1 O 2 sensitization experiments allow us to deduce the population yield and radiative decay rate of the keto triplet state to be 0.85 and 8.8 s -1 , respectively. Upon increasing the excitation intensity, photolysis reactions were observed for both HBQ and DIHBQ in the MCH glass. The reversibility of the photolysis reaction throughout a thawing and freezing cycle led us to conclude that the rupture of an intramolecular hydrogen bond through the excessive energy dissipated does occur. The product exhibits a non-hydrogen-bonding type of enol emission that would otherwise be inaccessible in the hydrogen-bonded enol form because of the ultrafast ESIPT. Accordingly, relative energy levels in different spin manifolds are established during a proton-transfer cycle. The ketofenol reverse proton transfer in the lowest triplet manifold was estimated to be endergonic by ∼7.42 kcal/mol.
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