The Mechanistic Study On 2-(2-Hydroxyphenyl)-Benoxazole Derivatives Mediated By Solvents And Substitutes

Excited-state intramolecular proton transfer（ESIPT）reactions are characteristic of the avoiding self-absorption and large Stokes’shift,usually accompanying with double fluorescence emission,solvatochromism and sensitive to p H and etc.This is exactly the reason why the molecules with ESIPT nature are widely applied in the biological,materials and chemical fields.In this paper,we designed and synthesized three2-（2-hydroxyphenyl）benzoxazole derivatives（HBOgens）,2-（2-hydroxy-5-methylphenyl）benzoxazole（HBO-p CH₃）,2-（2-hydroxy-5-bromophenyl）benzoxazole（HBO-p Br）,and2-（2-hydroxy-5-nitrophenyl）benzoxazole（HBO-p NO₂）.By obtaining UV absorption and fluorescence spectra in different solvents,we investigated the mechanism of the ESIPT process mediated by solvent polarity and functional groups of HBOgens combining with density functional theory（DFT）and time-dependent density functional theory（TDDFT）calculation.The novel ideas are as follow:HBOgens is proved to be a typical photoacid.Fluorescence emission mediated by solvent polarity and substitutes has been successfully interpreted in HBOgens.The excited state proton transfer reaction process has been explained by constructing the proton transfer potential energy curves and aromatic calculations.The twisted intramolecular charge transfer（TICT）gives rise to the weakening and even quenching of fluorescence in HBOgens.The main results are included as follow:（1）The solvatochromic effect is found for three 2-（2-hydroxyphenyl）benzoxazole derivatives.Solvatochromism confirmed ESIPT reaction inhibited by solvent polarity and intermolecular hydrogen bonding.HBO-p CH₃exhibits a typical from single to dual fluorescence phenomenon in different solvents attributed to the ESIPT reaction.For HBO-p Br,strong large Stokes’shifts are observed in either polar or non-polar solvents due to the withdrawing electron group-Br.Whereas,a weak large Stokes’shift in polar solvents and a strong short Stokes’shift in non-polar solvents are observed in HBO-p NO₂.（2）The emission bands are dependent on the excited wavelengths.Both HBO-p CH₃ and HBO-p Br demonstrate the nature of the photoacid.The different acidic strength of photoacids contributes to the excited-wavelength-dependent emission due to the different polarity of solvent.While for HBO-p NO₂,it may be determined to the twisting of Nitro substitute.（3）TDDFT calculations revealed an emission-strengthening mechanism of ESIPT for both HBO-p CH₃ and HBO-p Br,coupled with aggregation-induced emission enhancement（AIEE）（in mixed water/methanol solvents）,which has been successfully explained by twisted intramolecular charge transfer and the restriction of intramolecular rotations（RIR）after the occurrence of ESIPT.（4）The calculated ground state and excited-state intramolecular proton transfer reaction potential energy curves indicate that the ESIPT process is differentially inhibited for HBO-p CH₃and HBO-p Br in polar solvents,and promoted in nonpolar solvents.The calculated aromaticity（nuclear independent chemical shift value,NICS＿zz（1））indicates that the aromaticity of HBOgens in the ground states reverse into antiaromaticity in the corresponding excited states,facilitating ESIPT,in agreement with the results of the reaction energy barriers.Our strategy provides a prospect for designing new“ESIPT+AIE”fluorescent materials and enhancing the emission intensity of ESIPT molecules,as well as activating the quenched fluorescence due to the electron-withdrawing-group（EWG）.