Theoretical insights into the effect of pyridine position and furan number on ESIPT and ICT of cyanine derivatives

Abstract

Cyanine derivatives with the characteristics of excited state intramolecular proton transfer (ESIPT) are fluorescent probes, which have near infrared (NIR) luminescence properties. In this work, the effects of pyridine position and furan number on ESIPT process, the intramolecular charge transfer (ICT) degree and photophysical properties of Cyanine-1, Cyanine-2 and Cyanine-3 are systematically calculated. The electronic spectra reveal that the introduction of one furan and para-pyridine gives a greater redshift in the fluorescence spectra. Therefore, a new probe (Cyanine-4) with two furan group and para-pyridine is designed in our work to investigate the effects of furan number on ESIPT process, ICT degree and to better meet the needs of NIR imaging. The electronic spectra show that para-pyridine and the incremental furan number lead to redshifts in the fluorescence spectrum and larger Stokes shifts. The potential barriers confirm that the ESIPT process is less likely to occur as furan number increases. The analyses of electronic excitation character confirm that as furan number increases, the ICT degree becomes larger which leads a larger red-shifted spectrum. However, pyridine position has a small effect on ESIPT process and ICT degree. Therefore, furan number and pyridine position can regulate the ESIPT process, the ICT degree and the luminescence performance. This work will provide ideas for experimental synthesis of new cyanine probes.