Tactfully regulating the ESIPT mechanism in the Cyanine-n probe molecules by electron withdrawing substitutions

In this study, we analyzed the dynamics of excited hydrogen bonds and the ESIPT process in Cyanine-n. According to the assessment of hydrogen bond energy and the redshift in the infrared (IR) vibration spectra, we propose that during the photoexcitation, the hydrogen bond interaction of Cyanine-n is enhanced, and Cyanine-9 exhibiting the strongest intramolecular hydrogen bond (IHB) strength and a higher likelihood to initiate the ESIPT process. The redistribution of excited state charges validates strong intramolecular charge transfer (ICT) characteristics in Cyanine-n, which acts as a driving force for the ESIPT process. Notably, the evidence from the scanned potential energy curves (PECs) and the intrinsic reaction coordinate (IRC) collectively confirms the easy order of the ESIPT process follows: Cyanine-9 > Cyanine-8 > Cyanine-7. Additionally, the simulated electronic spectra clearly elucidate how electron-withdrawing group substitution affects fluorescence emission mechanisms. In particular, our analysis suggests that Cyanine-9 is most likely to undergo the ESIPT process. From this paper, the novel dynamics in the Cyanine-n system not only comprehensively set forth, but the optimal substitution form is identified. We hope our work can offer new insights into the design and synthesis of bioimaging and environmental science applications.