Synthesis and Optical Properties of Excited-State Intramolecular Proton Transfer Active π‑Conjugated Benzimidazole Compounds: Influence of Structural Rigidification by Ring Fusion

Two
excited-state intramolecular proton transfer (ESIPT) active
benzimidazole derivatives (1 and 2) were
synthesized by acid-catalyzed intramolecular cyclization. The steady-state
fluorescence spectrum in THF revealed that ring-fused derivative 1 exhibits a dual emission, namely, the major emission was
from the K* (keto) form (ESIPT emission) at 515 nm with a large Stokes
shift of 11 100 cm–1 and the minor emission
was from the E* (enol) form at below 400 nm. In contrast, the normal
emission from the E* form was dominant and the fluorescence quantum
yield was very low (Φ ∼ 0.002) for nonfused derivative 2. The time-resolved fluorescence spectroscopy of 1 suggested that ESIPT effectively occurs due to the restricted conformational
transition to the S1–TICT state, and
the averaged radiative and nonradiative decay rate constants were
estimated as ⟨kf⟩ = 0.15
ns–1 and ⟨knr⟩ = 0.60 ns–1, respectively. The fluorescence
emission of 1 was influenced by the measurement conditions,
such as solvent polarity and basicity, as well as the presence of
Lewis base. The ESIPT process and solvatochromic behavior were nicely
reproduced by the DFT/TDDFT calculation using the PCM model. In the
single-crystal fluorescent spectra, the ESIPT emissions were exclusively
observed for both fused and nonfused compounds as a result of hydrogen-bonding
interactions.