Artist’s Dye Fading: Summary

This summer has seen a lot of frustration but also a lot of progress. The main focus of my work has been trying to figure out photo-physical behavior of purpurin, purpurin lake, and alizarin in different conditions in order to fill in the blanks from the previously accomplished single molecule studies. This new data allows us to more accurately interpret the single molecule data and gives us parameters for the Monte Carlo simulations. The single molecule data only tells us the relative behavior of the molecules by showing us a portion of the on and off lifetimes and the functions they are characterized by. In order for this data to have a physical interpretation we need to understand what sort of photo physical processes are happening which is where this summer’s research comes into play.

So far we’ve gained fairly strong evidence that alizarin and purpurin lake undergo excited state intramolecular proton transfer (ESIPT) based on the existence of two emissive states and their behavior in various conditions. For example, the relative emission of the ESIPT state (in the dyes that undergo it) goes down as more polar solvents are introduced. This is likely because the stronger solvent interaction blocks ESIPT.¬†Also, the wavelength of the ESIPT state blue shifts with increasing solvent polarity. This lines up with calculations of the energies of the ESIPT and ground state of the alizarin furthering the ESIPT argument. We also see an excitation wavelength effect with the ESIPT peak increasing in relative intensity with decreased excitation wavelength (increased energy), further proving ESIPT by showing us there is an energy barrier to this emissive state.¬†Using both the qualitative and quantitative data we estimated the physical rate constants for some of the excited state processes. This helps us fill in the blanks when it comes to the monte carlo simulation– leaving less guesswork and expediting the process.