Concerning the acidity and hydrogen bonding of hydroxyphenanthroperylene quinones like fringelite D, hypericin, and stentorin

The strongly enhanced acidity of the bay hydroxyl group as compared to the respective peri hydroxyl groups of fringelite D, hypericin, and stentorin could be rationalized on the basis of a vinylogous carboxylic acid and was nicely corroborated by semiempirical calculations of the AM1 type. Experimental data obtained from several independent experimental methods, like polarized absorption spectroscopy, hole burning, and isotope effects, as well as from semiempirical AMI and 6-31G level ab initio calculations conclusively pointed to dissymmetrical hydrogen bonding systems in both the peri and bay regions of the corresponding bay phenolate ions.