Abstract: Tuning the pKa of Fluorescent Rhodamine pH Probes via Substituent Effects


My name is Sarah Stratton and I am a junior chemistry major. I have been working in Professor Harbron’s research lab since the fall of my freshman year, where I have been studying rhodamine dyes which fluoresce in acid. Specifically, we have been modifying the structure of these dyes to determine how the size and electronic properties of the molecule impact the pH at which the molecule fluoresces. This project has also been lovingly referred to as the “pink project” because the rhodamine derivatives I work with are clear in basic solutions and bright pink in acidic ones. Having stained various surfaces in the lab as well as some articles of clothing, I can personally attest to the tenacity and eye-popping color of these dyes!

One aspect of the project I’m particularly excited to be working on over the summer is crystal structures. Since the pH at which the rhodamine B and rhodamine 6G derivatives fluoresce depends on the attached aniline group, how this group interacts with the rest of the molecule is a major key to figuring out why it affects the rhodamine like it does. In order to gain a better understanding of the 3D structure of our dyes we have been attempting to growing crystals for some time now so that we could view them through the help of x-ray crystallography. X-ray beams are fired at crystal and diffract off. The angles of diffraction provide information on the electronic density of the molecule which can then be used to determine structure. We recently succeeded in growing crystals, having found a solvent which evaporates slowly enough to allow big crystals to form. When a solvent evaporates too quickly, it typically leaves behind powder or needle like crystals which are too small for experimentation. Growing crystals takes a while, as our crystals took almost six months to grow. Currently we have more solutions prepared which will hopefully grow crystals we can study over the summer for a better understanding of the structure of our dyes and why they act like they do. Once we better understand what kind impact the attached groups have on the pKa of the rhodamine dyes we can craft fluorescent pH probes for very specific pH ranges. This technique would be very useful for studying pH changes in biological systems. I’m looking forward to developing a more in-depth understanding of my project and possibly even moving on to connected rhodamine studies over the summer. I hope you will enjoy hearing about my findings!