Influences of polarizability effect of alkyl group and homoring competition effect of substituents on the fluorescence emission spectra of salen‐type Schiff bases

Abstract

AbstractTo explore the effect of substituent attached to aromatic ring and the length of diamine chain in salen‐type Schiff base molecules on the fluorescence emission spectra (FLem), the FLem of 63 diamine‐salicylaldehyde condensates in an ethanol solution were obtained. The influence of substituent electronic effects and chain lengths on the wavenumber (νem) of fluorescence emission wavelengths (λem) was subsequently studied. The results show that the length of the diamine chain in the target compound has an effect on the νem, which can be quantified by the alkyl polarizability effect index (PEI), where compounds with alkyl of higher PEI have lower νem. Second, the λem of salen‐type Schiff bases can be quantified by an equation of the seven parameters, PEI, σ(X)─OH, σ(X)─CH═N, , , and , form where the stability and predictive ability of the obtained equation was confirmed by leave‐one‐out cross validation. Lastly, it was observed that the influence on the λem of substituent X attached to the aromatic ring is related to its position relative to OH or CH═N groups as reference, whose Hammett constants (σ(X)─OH, σ(X)─CH═N) and the excited‐state substituent constants ( , ) of the substituents X with varying locating methods have different effects on the λem; and that there is a “homoring competition effect” of substituent.