Theoretical Investigations On PH Two-photon Fluorescent Probes

Intracellular pH plays an important role in many cellular events, such as cellgrowth, endocytosis, cell adhesion and other cellular processes. It have been reportedthat abnormal pH values in acidic organelles will result in some cellular dysfunction.There is already a wide development of fluorescent probes for metal cations,including alkalis metal cations, alkaline earth metal cations, Zn2+, heavy metal cationsand so on. However, the fluorescent probes for proton and neutral organic moleculeshave not been studied as much as these cations. Therefore, it is extremely desirable todesign a broad variety of pH probes in this acidic pH range.There are few theoretical investigations on two-photon fluorescent probes.Exactly theoretical investigations on these two-photon pH fluorescent probes have notbeen found. The main purpose of this paper is to validate the principles of these twotypical two-photon pH fluorescent probes specifically by theoretical methods anddesign some improved probes on the basis of these molecules which maybe useful forexperimenters. So some molecules with larger TPA cross section will be designed andinvestigated.The main researches are summarized as follows:The geometrical structure, electronic structure, OPA of these pH fluorescentprobes have been studied by DFT methods. TPA properties of these probes arecalculated by applying the ZINDO program compiled by our group. The calculatedresults show that the most distinct difference between the PCT probe and the PETprobe is changes in One-photon properties upon protonation. For the PCT probesPYMPON-X, the absorption spectra and emission spectra are largely red-shifted, aswell as an enhancement of the TPA cross section upon protonation. On the contrary,for the PET probes, there are no significant changes in band positions uponprotonation. For the PCT probes, in order to increase the TPA cross section ofPYMPON-1H+, it is advisable to only change the terminal methoxyl group into astronger electron-donating group, rather than increase the numbers of donor or extendthe conjugation length. Once the terminal methoxyl group has been changed into the dibutylamino group, the maxima cross section of PYMPON-3H+is nearly twofoldlarger than the referenced molecule PYMPON-1H+. With the significant red shifts inabsorption spectra and emission spectra, this molecule can serve as an efficienttwo-photon fluorescent probe for ratiometric pH measurements in aqueous solution.For the PET probes, the PET process has been specifically modeled by theoreticalmethods. The calculated results indicate that the maxima cross section of AH1andAH2will increase upon protonation.