Related Experiment Video
Updated: Sep 1, 2025

Design, Synthesis, and Photochemical Properties of Clickable Caged Compounds
Published on: October 15, 2019
Excited state deactivation mechanisms in Shikonin rationalized from its naphthoquinone parent structures
Catarina M Pinto1, João Pina1, Estefanía Delgado-Pinar1
1Department of Chemistry, University of Coimbra, CQC-IMS, Rua Larga, 3004-535 Coimbra, Portugal. sseixas@ci.uc.pt.
Abstract:
Shikonin, a naphthoquinone dye, is a molecule of colour of natural origin, whose peculiar properties have not yet been fully rationalized. Its core structure consists of a di-hydroxy-naphthoquinone with an additional non-aromatic hydroxy group. From a comprehensive study involving fast spectroscopic techniques (fs-TA and fs-UC) and TDDFT electronic structure calculations on shikonin (Shk) and its derivatives 5-hydroxy-1,4-naphthoquinone (5HNQ), 5,8-diacetoxy-1,4-naphthoquinone (DiAc), 5,8-dihidroxy-1,4-naphthoquinone (DHNQ) and acetylshikonin, AcShk, it is shown that intramolecular excited state proton transfer (ESIPT) is present and is determinant in the deactivation of the hydroxy containing molecules. This is mirrored by the dominance of the internal conversion deactivation channel. In Shk, the non-aromatic hydroxy group determines the preferred conformer in both the ground- and excited-state, as reflected in the doubling of the fluorescence quantum yield value of this molecule relative to DHNQ. From fs-UC, a kinetic isotopic effect of 1.7 was obtained for DHNQ.
Related Concept Videos
Deactivation Processes: Jablonski Diagram
Oxidation of Phenols to Quinones
o-hydroxy phenols are oxidized to o-quinones and p-hydroxy phenols to p-quinones. Such redox reactions involve the transfer of two electrons and two protons. The reversible redox...
meta-Directing Deactivators: –NO2, –CN, –CHO, –⁠CO2R, –COR, –CO2H
ortho–para-Directing Deactivators: Halogens
Radical Reactivity: Steric Effects
Along with electronic...
Aldehydes and Ketones with HCN: Cyanohydrin Formation Mechanism
![Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions](/_next/image?url=https%3A%2F%2Fcloudfront.jove.com%2FCDNSource%2Fteasers%2F60786.jpg&w=3840&q=50)
