Polarized Helical Coumarins: [1,5] Sigmatropic Rearrangement and Excited-State Intramolecular Proton Transfer.

Łukasz Kielesiński,Olaf W Morawski,Cristina A Barboza,Daniel T Gryko

doi:10.1021/acs.joc.0c02978

Abstract

The tandem process of phenol addition to a cyclic α,β-unsaturated ester followed by intramolecular transesterification and [1,5] sigmatropic rearrangement affords a series of helical coumarins based upon a previously unknown 3-amino-7-hydroxybenzo[3,4]cyclohepta[1,2-c]chromen-6-one core. These novel polarized coumarins, possessing a β-ketoester moiety, have been employed to synthesize more rigid and helical coumarin–pyrazolones, which display green fluorescence. The enhanced emission of coumarin–pyrazolones in polar solvents depends on the nature of the S1 state. The coumarin–pyrazolones are predicted to have two vertical states close in energy: a weakly absorbing S1 (1LE) followed by a bright S2 state (1CT). In polar solvents, the 1CT can be stabilized below the 1LE and may become the fluorescent state. Solvatochromism of the fluorescence spectra confirms this theoretical prediction. The presence of an N—H···O=C intramolecular hydrogen bond in these coumarin–pyrazolone hybrids facilitates excited-state intramolecular proton transfer (ESIPT). This process leads to a barrierless conical intersection with the ground electronic state and opens a radiationless deactivation channel effectively competing with fluorescence. Solvent stabilization of the CT state increases the barrier for ESIPT and decreases the efficiency of the nonradiative channel. This results in the observed correlation between solvatochromism and an increase of fluorescence intensity in polar solvents.

Highlights

The rapid progress of modern technologies has relied on the design, creation, and implementation of new materials.Particular attention has been focused on novel organic chromophores as, unlike their inorganic counterparts, they do not contain rare, expensive, or unethically mined elements
We envisioned that the replacement of coumarin with ester 1 (Scheme 1) would lead to promising dyes containing a coumarin moiety fused with a seven-membered ring and core-modified [4]helicenes
Synthesis of Dye 3 Having a Coumarin Motif Fused to a Seven-Membered Ring and the Proposed Mechanism of This Transformation pubs.acs.org/joc

Summary

Introduction

The rapid progress of modern technologies has relied on the design, creation, and implementation of new materials.Particular attention has been focused on novel organic chromophores as, unlike their inorganic counterparts, they do not contain rare, expensive, or unethically mined elements. Coumarins are opportune fluorophores for many applications as the introduction and modification of substituents on this scaffold has been shown to enable comprehensive control of their photophysical properties.[1] Their special features, such as large fluorescence quantum yield, large Stokes shift, high absorption coefficient, and photostability, make them very desirable and allow them to be used as fluorescent probes,[2] as dyes for two-photon fluorescence microscopy,[3] in OLEDs,[4] and as constituents in energy and electron transfer systems.[5] Another important modification that allows for coumarins’ bandgap tuning is π-electron core expansion.[6] This allows the absorption and fluorescence to be red-shifted, a feature that is important for fluorescence imaging.[7] Multiple examples of coumarin chromophore modification have been published recently.[8] Among many scaffolds, the V-shaped biscoumarins, which can be synthesized by the Michael reaction of electron-rich phenols with esters of coumarin-3-carboxylic acids,[9] have attracted broader attention.[10] Independently, it was reported that coumarins possessing an electron-withdrawing group at position 3 undergo Michael addition with other nucleophiles to form dihydrocoumarins[11] or coumarins.[12] In both the latter case and in the case of phenol addition,[9] a second molecule of coumarin ester acts as an oxidant In light of these results, we have designed an intramolecular version of such a process. We envisioned that the replacement of coumarin with ester 1 (Scheme 1) would lead to promising dyes containing a coumarin moiety fused with a seven-membered ring and core-modified [4]helicenes

Methods

Results

Conclusion