Photochromism of o-Nitrophenyl-Substituted Oxazabicycles

Abstract

The o-nitrophenyl-substituted oxazabicycle 1 was prepared via a one-pot tandem multicomponent reaction. Upon exposure to UV light in CH2Cl2, it turned purple and reverted back to its original color while being heated. The in situ trapping of the photogenerated product with trimethylsilyl cyanide yielded the adduct 4, which confirmed the formation of the ring-opened zwitterionic species 2. The EPR spectroscopic data, PBN spin-trapping experiments, and spectroelectrochemical studies of the model compound 7 provided evidence to support that the photochromism of 1 involved the zwitterionic biradical intermediate 5, generated by photoinduced intramolecular electron transfer from the amine moiety to the nearby o-nitrophenyl group. Further, two photogenerated side products responsible for the photofatigue of 1 were isolated and characterized, and a possible mechanism for their formation was proposed.

Revisiting the non-fluorescence of nitroaromatics: presumption versus reality

“…When a molecule contains an electron donor (D) and an electron acceptor (A) that are weakly coupled with each other, an intramolecular PCT may occur after the photoexcitation and the relaxation of the Franck–Condon, FC, to a locally excited, LE, state. 130,131 When the donor–acceptor electronic coupling is strong, a direct photoexcitation to the CT state can become possible. That is, in addition to the absorption leading to the LE state, which is not truly susceptible to solvent polarity, a bathochromically shifted CT band, originating from the S 0 → CT transition and manifesting pronounced solvatochromism, appears in the spectra.…”

Multicomponent synthesis of pyrano[2,3-c]coumarins

“… 11 Thus, the search for a cost-effective and simple protocol for the synthesis of pyrano[3,2- c ]coumarins remains desirable. In our continuing effort to develop new MCRs for the preparation of the coumarin-based heterocycles 12 to investigate their potential biological or functional properties, here we report the efficient synthesis of pyrano[3,2- c ]coumarins via a base-catalyzed, pseudo four-component reaction of 4-hydroxycoumarin, acetone, and amine, as well as a base-catalyzed, pseudo four-component reaction of 4-hydroxycoumarin and acetone. A plausible mechanism is also proposed on the basis of the experimental results.…”

One-Pot Construction of 1-Phenylchromeno[3,4-b]pyrrol-4(3H)-one: Application to Total Synthesis of Ningalin B and a Pyrrolocoumarin-Based Electrochromic Switch

“…With the increase of exposure time (0–60 s) to an oxidation potential of 0.55 V, a new broad absorption band with the peak wavelength around 515 nm gradually emerged, along with the appearance of three isosbestic points at 254, 290, and 383 nm. We speculate that the colorless solution changes to red may be attributed to the formation of an amine cation radical on the coumarin moiety . Conversely, applying a reduction potential of 0.20 V (vs Ag/AgCl) to the oxidized 24 resulted in the gradual decrease of the absorbance at 515 nm (Figure , bottom).…”