Electronic spectra of 7,14‐dithioketo‐5,7,12,14‐tetrahydroquinolino‐[2,3‐b]‐acridine in solution and in the solid state

Abstract

Abstract7,14‐Dithioketo‐5,7,12,14‐tetrahydroquinolino‐[2,3‐b]‐acridine (DTQ) is a thionated derivative of quinacridone (QA) that is known as a red pigment. Thionation of QA brings about an intense near‐IR absorption in the solid state which is of practical importance for GaAsAl‐laser printers and optical information storage systems. The electronic properties of DTQ have been investigated in solution and in the solid state in order to characterize the near‐IR absorption from the standpoint of intermolecular hydrogen bonding. The electronic state of the DTQ‐chromophore is found to be most sensitively affected by an environment of proton acceptors which interact with the NH group of the chromophore. In solution, the hydrogen bonds between the NH group and the O atom of a solvent cause a bathochromic shift by 5 – 15 nm relative to the absorption spectrum in dioxane. This is interpreted as being due to electron transfer from the proton to the nitrogen atom, thereby increasing the electron density in the chromophore. A similar increase in electron density is also operative in the solid state through the intermolecular hydrogen bonds between the NH group of one DTQ and the S atom of the neighboring molecule. This is mainly responsible for the bathochromic displacement of DTQ in going from solution to the solid state. The near‐IR absorption is, however, not explicable in terms of the hydrogen bonding interaction alone. Another intermolecular interaction, such as π‐π interactions along the stacking axis, appears to be operative that might lead to a near‐IR absorption.