Abstract
The effects of the incorporated heteroatoms Si and S on tuning the optical properties of rhodamine-and fluorescein-based fluorescence probes is investigated using DFT and time-dependent DFT with four different functionals. As previously proposed, the large redshift (90 nm) produced by a Si atom in both the absorption and emission spectra can be attributed to the sigma*-pi* conjugation between the sigma* orbital of the Si atom and the pi* orbital of the adjacent carbon atoms. However, the presence of a Si atom does not alter the fluorescence quenching mechanism of the nonfluorescent forms of the investigated compounds. For the first time, these theoretical results indicate that the n orbital of the S atom plays an important role in determining the optical properties of the nonfluorescent form of rhodamine-based fluorescence probes. It alters the fluorescence quenching mechanism by lowering the energy of the dark n pi* state, which is due to breakage of the C10-S52 bond upon photoexcitation.