Intramolecular Charge Transfer in 5-Methoxy-2-(2-pyridyl)thiazole-Derived Fluorescent Molecules with Different Acceptor or Donor Substituents

Abstract

On the basis of the rational derivation of 5-methoxy-2-(2-pyridyl)thiazole (2-MPT), we synthesized a new series of charge-transfer-based fluorescent molecules bearing different electronic donors or acceptors. The substituents range from strong electronic donors (e.g., amino and hydroxyl groups) to weak donating groups (e.g., proton and methyl groups) and electronic acceptors (e.g., pyridine ring). Through systematic investigation on the substituent-/polarity-dependent spectra (including room-temperature absorption, room-/low-temperature steady-state fluorescence spectra, and transient fluorescence lifetime characterization) and theoretical calculations, the emission properties of MPT derivatives are found to be governed by the rotation of the substituent around the triple bond axis, which produces distinct intramolecular charge transfer processes in either the twisted or planar excited states. The interconversion of excited-state geometry triggered by local interactions in polar solvents may produce a bathochromic shift of approximately 100 nm in fluorescence spectra. The solution state may also affect the ground- and excited-state conformation and hence results in the solvent-frozen-point sensitive fluorescence for some of the as-prepared molecules.