Absorption spectra of thioindigo dyes in benzene and chloroform

Abstract

The spectral absorpt ion curves of 10 purifi ed t hioindigo d yes, co nt aini ng mostly m ethy l a nd ethoxyl groups or halogens a s subst ituent s, in ben ze ne and chloroform solut ion were determined over t he ultraviolet a nd v isible part s of t he spectrum. The existence of a n equilibrium betwee n ci s and tran s isomers was indica ted b y t he st ud y of t he effect of li ght o n t he absorpt ion spectra. So me of t h e res ult s of t his in vestigation have bee n published. 2 The present paper s uppleme nts t he earlier paper and con tain s t he spectral a bsorption curves t hat have not previously bee n publish ed, along wit h t he d ata on t he purifi cat io n of t he t hree d yes t hat had not bee n reported t here. In general , t he first (lo ng-wave) a bsorption band of t he cis-form of ea ch dye occurs a t a shor te r (by approx imately 60 mIL) wa vele ngth t ha n t hat of t he correspondin g tran s isomer. The spectra a re displaced s li ght ly towa rd lo nger wavelengths in chloroform solut ions t ha n in ben ze ne. Substit uent s in t he fiv e a nd seven positio ns cause a bath ochromi c shift in t he spectra of both isomers of ea ch dye in eaeh solvent wi t hout a ffe cting t he shape of th e spectra l a bsorpt ion cur ves. The i n t rodu ction of s ubst it ue nt s in t he fo ur or six posit ions res ul ts in a cha nge in t he shape of the a bsorpt ion curves a nd a s hift of t he a bso rpt ion t o s hor t er ,yavelengt hs.