Excited-State Intramolecular Proton Transfer of Naphthalene-Fused 2-(2′-Hydroxyaryl)benzazole Family

Abstract

Intramolecularly hydrogen-bonded organic compounds often exhibit fluorescence emission at considerably longer wavelengths than typical fluorescence as a result of excited-state intramolecular proton transfer (ESIPT). The structure-property relationship of such ESIPT molecules, however, remains obscure. The present article reports the excited-state dynamics of a new family of ESIPT molecules, 2-(2'-hydroxynaphthyl)benzazoles 1-3, based on steady-state and time-resolved spectroscopy measurements. In comparison with the parent compound HBO, all three compounds 1-3 exhibited absorption bands at longer wavelengths and emitted fluorescence from the excited keto-tautomer K* at shorter wavelengths, indicating that the introduction of a naphthalene ring increases the energy gap between the ground and excited states for the keto-tautomer despite the expansion of the aromatic ring. Time-resolved fluorescence spectra revealed dual emission for compounds 1 and 3, consisting of two distinct fluorescence bands originating from K* and the excited rotamer E'*, whereas 2 exhibited fluorescence only from the K* state. In the transient absorption spectra, both the T-T absorption band and the ground state absorption band of the Z-keto tautomer were observed for 1, whereas only the T-T absorption band was observed for 2 and only the Z-keto tautomer band was observed for 3.

Effect of 3,5‐Disubstitution on the Optical Properties of Luminescent 2‐(2′‐Hydroxyphenyl)benzoxazoles and Their Borate Complexes

“…This is presumably due to the improved electronic delocalization brought by the two ethynyl fragments, which in return minimize nonradiative deactivations. Finally, the lifetimes of the K* emission for dyes 7 and 8 are in the nanosecond regime, which is consistent with the literature 19…”

Advanced Organic Optoelectronic Materials: Harnessing Excited‐State Intramolecular Proton Transfer (ESIPT) Process

“…However dual emission was observed even in non-polar solvents such as benzene in the case of 2-hydroxynaphthyl benzoxazole ( Figure 2 a, 1 ) and benzthiazole derivative (Figure 2 c, 3 ). [ 40 ] This observation can be rationalized by ascribing to the smaller relative energy difference between their two rotamers than that of phenyl derivatives, such as 2-(2′-hydroxyphenyl)benzoxazole (HBO) and 2-(2′-hydroxyphenyl)benzthiazole (HBT). Arai and coworkers have also found that the K * can be decayed without emission by undergoing intersystem crossing to the excited triplet state ( 3 K * ) and also the photochemical E-Z isomerization to give the groundstate Z-keto isomer (K z ) by transient absorption spectroscopy.…”

Synthesis, characterization and X-ray crystal structure of copper(I) complexes of the 2-(2-quinolyl)benzothiazole ligand. Electrochemical and antibacterial studies

“…The increasing applications of metal complexes of 2-substituted benzothiazoles, capable of forming multifunctional complexes, in the area of optoelectronics, the synthesis of fluorogenic enzyme substrates and fluorescent materials are also well recognized [25][26][27].…”