Abnormal effect of hydroxyl on the longest wavelength maximum in ultraviolet absorption spectra for bis-aryl Schiff bases

Two sets of bis-aryl Schiff bases that contain 4(or 4')-OH and 2(or 2')-OH were synthesized. The first set consists of 4-HOArCH=NArY and XArCH=NArOH-4', and the second set consists of 2-HOArCH=NArY and XArCH=NArOH-2'. Their ultraviolet absorption spectra were measured and investigated. A very interesting phenomenon was observed by analyzing their wave number lambda(max) (cm(-1)) of longest wavelength maximum lambda(max) (nm) of ultraviolet. Compared with the change regularity of the lambda(max) of XArCH=NArY (where the X and Y excluded OH), the 4-position hydroxyl (4-OH) and 2-position hydroxyl (2-OH) have abnormal performance. The details are the following: the 4-OH contributes an additional red shift to the lambda(max) of XArCH=NArOH-4' (lambda(max) increase), whereas the 2-OH contributes an additional blue shift to the lambda(max) of XArCH=NArOH-2' (lambda(max) decrease). In addition, there are ortho steric effects of all 2-OH and 2'-OH on the (lambda max) for 2-HOArCH=NArY and XArCH=NArOH-2, and the ortho steric effect contributes a red shift to their lambda(max). These experimental facts can provide an important theoretical reference for us using aryl Schiff base compounds as optical materials and performing the molecular design.