relation between molecular structure and optical properties 4.1 Phenolphtalein is another acid/base indicator which can be colorless or pink, depending on the pH. Which of the two structures below is responsible for the pink coloration? Under which conditions of pH the molecule is pink? oto -OH OH 4.2 A series of thyacyanine dyes have been studied by UV/Visible spectroscopy (D.M. Sturmer, in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., 2000.) 3 The spectral characteristics are listed below: Molecule max (nm) 423 557 650 758 & (L.mol.cm') 8.5x10' 1.5x10 2.3x10 2.5x10 Assign to each molecule 1,2,3 and 4 its spectral characteristics.