Theoretical Investigation on the ESIPT Process and Detection Mechanism for Dual-Proton Type Fluorescent Probe

Abstract

Recently, a new fluorescent probeAE-Phoswas reported to detect the activity of alkaline phosphatases (ALP) in different living cell lines. Here, we present an in-depth computational analysis of the mechanism and source of the fluorescence of the AE-Phos probe. There is an intermediate product (AE-OH-Phos) in the experiment as well as a different configuration of products that may emit fluorescence. It is essential to investigate the origin of fluorescence and the detection mechanism of the probe, which could help us eliminate the interference of other substances (including an intermediate product and possible isomers) on fluorescence during the experiment. According to the change of geometric parameters and Infrared spectra, we deduce that the dual intramolecular hydrogen bonds of salicylaldehyde azine (SA) were enhanced at the excited state, while AE-OH-Phos was attenuated. Considering the complex ESIPT behavior of the dual proton-type probe, the potential energy surfaces were further discussed. It can be concluded that the single proton transfer structure of SA (SA-SPT) is the most stable form. Both the concerted double proton transfer process and stepwise single proton transfer process of SA were forbidden. The fluorescence for SA was 438 nm, while that of SA-SPT was 521 nm, which agrees with the experimentally measured fluorescence wavelength (536 nm). The conclusion that single proton transfer occurs in SA is once again verified. In addition, the distribution of electron-hole and relative index was analyzed to investigate the intrinsic mechanism for the fluorescence quenching of the probe and the intermediate product. The identification of the origin of fluorescence sheds light on the design and use of dual-proton type fluorescent probes in the future.

Coherent Vibrational Spectrum via Time-Resolved Fluorescence for Molecular Dynamics and Identification of Emitting Species–Application to Excited-State Intramolecular Proton Transfer

“…In the S 1 state, the di-keto isomer is higher by 4690 cm –1 than the enol isomer and 5490 cm –1 than the keto isomer (we call the mono-keto isomer simply the keto isomer). Therefore, the keto isomer should be the dominant product as reported previously, , while keto and di-keto isomers are supposed to emit at similar wavelengths. The enol isomer in the ground state may have trans or gauche conformations concerning the central N–N bond.…”

“…SAA also shows aggregation-induced emission, which, in conjunction with the ESIPT, has been exploited for fluorescence turn-on probe and cellular imaging. , It was reported that photoexcitation produces an excited cis-keto isomer, which returns to the ground state or undergoes twisting motion to form a trans-keto isomer. ,, The ESIPT time constants of salicylaldehyde (SA) and SAA were estimated to be faster than 50 fs (80 fs in ref ) by Ziolek et al, , whereas it was estimated to be 200–300 fs by Mitra and Tamai . SAA was suggested to undergo single proton transfer, although it has two equivalent channels of ESIPT, as shown in Scheme . , Molecules with two possible ESIPT sites that may allow excited-state intramolecular double proton transfer (ESIDPT) are interesting, but the mechanism of the double proton transfer, which can be concerted or stepwise, has been poorly investigated. − Determination of accurate reaction rate and recording of the NWPs of the product state(s), which could be the enol, mono-keto, and di-keto isomers, are essential to investigate the molecular dynamics of the ESIPT and ESIDPT. In particular, identification of the product state is critical, and CVSF should be a unique tool because a vibrational spectrum descended from the emission spectrum can be recorded exclusively.…”

Photophysical Properties of (E)-1-(4-(Diethyla-mino)-2-hydroxybenzylidene)-4,4-dimethylthiosemicarbazide Compound and Its Triple Fluorescence Emission Mechanism: A Theoretical Perspective

“…It was well-known that the charge redistribution phenomenon after molecular excitation could directly reflect the excited-state behavior of molecules. 50 Therefore, the transition properties are displayed in Table 5.…”

Excited‐State Proton Transfer in Luminescent Dyes: From Theoretical Insight to Experimental Evidence

“…[58] The analysis of Non-Covalent Interactions (NCI) allows a further refinement of the description. [59] For this, the Reduced Density Gradient (RDG) methodology is popular, [60] as the visual inspection of the scatter plots directly gives an indication of the interactions strength. [61] Additionally, the tautomerization energy gap in the ground and excited states is a useful information for distinguishing ground-state (intramolecular) proton transfer (GSIPT) from ESIPT, and to explain differences in the photophysical properties of closely related dyes.…”

“…The analysis of Non‐Covalent Interactions (NCI) allows a further refinement of the description [59] . For this, the Reduced Density Gradient (RDG) methodology is popular, [60] as the visual inspection of the scatter plots directly gives an indication of the interactions strength [61] …”