Related Experiment Video
Updated: Jun 21, 2026

Synthesis of pH Dependent Pyrazole, Imidazole, and Isoindolone Dipyrrinone Fluorophores using a Claisen-Schmidt Condensation Approach
Published on: June 10, 2021
Early events associated with the excited state proton transfer in 2-(2'-pyridyl)benzimidazole
Tarak Nath Burai1, Tushar Kanti Mukherjee, Priyanka Lahiri
1Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
Abstract:
2-(2(')-Pyridyl)benzimidazole (2PBI) undergoes excited state proton transfer (ESPT) in acidic solutions, leading to a tautomer emission at 460 nm. This photoprocess has been studied using ultrafast fluorescence spectroscopic techniques in acidic neat aqueous solutions, in viscous mixtures of glycerol with water, as well as in sucrose solutions. The tautomer is found to be stabilized in the more viscous medium, leading to a greater relative quantum yield as well as lifetime. The long rise time in tautomer emission is not affected by viscosity though. Rather, it appears to have the same value as the long component of the decay of the cationic excited state (C(*)). In addition to the subnanosecond lifetime reported earlier, C(*) is found to exhibit a decay time of 2 ps. This is assigned to its protonation to form the nonfluorescent dication in its excited state (D(*)) considering the ground and excited state pK(a) values reported earlier. An additional rising component of 100 ps is observed in the region of C(*) emission. This is likely to arise from a structural change or charge redistribution in C(*) immediately after its creation and before the phototautomerization.
Related Concept Videos
Photochemical Electrocyclic Reactions: Stereochemistry
Selection Rules: Photochemical Activation
NMR Spectroscopy of Benzene Derivatives
¹H NMR: Complex Splitting
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds
Thermal and Photochemical Electrocyclic Reactions: Overview
Basicity of Heterocyclic Aromatic Amines

![[(DPEPhos)(bcp)Cu]PF6: A General and Broadly Applicable Copper-Based Photoredox Catalyst](/_next/image?url=https%3A%2F%2Fcloudfront.jove.com%2FCDNSource%2Fteasers%2F59739.jpg&w=3840&q=50)