Theoretical insight on the saturated stimulated emission intensity of a squaraine dye for STED nanoscopy

Stimulated emission depletion nanoscopy (STED) is increasingly applied for the insights into the ultra-structures of organelles in live cells because of the bypassing of the Abbe's optical diffraction limit. Theoretically, with the increase of excitation and depletion laser power, the imaging resolution can be accordingly enhanced and even close to the infinity. Unfortunately, powerful laser illuminations usually produce severe phototoxicity and photo bleaching, which will lead to more extra-interference with biological events in live cells and accelerate the decomposition of the fluorescent probes. In view of the trade-off of cell viability and imaging resolution, excellent probes with superior photophysical properties are great in demand. For a qualified STED probes, the saturated stimulated emission intensity (I-sat) is considered as a key evaluating factor. According to the formula, I(sat )is inversely proportional to the stimulated emission cross section (sigma(sti)) of the fluorescent probe. However, the relationship between the sigma(sti) and chemical structure of the STED probe remain to be unclear. In this work, we explore the influence factors by theoretical calculations on a squaraine dye (MitoEsq-635) and a commercial dye (Atto647N). The results indicate that the increase of transition dipole moment (mu) are beneficial for the increase of sigma(sti), thereafter reducing I-sat. Furthermore, we firstly proposed that stimulated emission depletion was qualitatively interpreted by the investigation on the potential energy surfaces of ground states (S-0) and the first excited states (S-1) of the dyes.