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Substituent Effects on 3-Arylazoindole Photoswitches Nadja A. Simeth, Alfredo Bellisario, Stefano Crespi, Maurizio Fagnoni, and Burkhard König J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02973 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019
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The Journal of Organic Chemistry
Substituent Effects on 3-Arylazoindole Photoswitches Nadja A. Simeth,† Alfredo Bellisario,‡ Stefano Crespi, †,§,* Maurizio Fagnoni,§ and Burkhard König†,* †Institut
für Organische Chemie, Universität Regensburg, Universitätsstraße 31, 93040 Regensburg, Germany. of Physics, Università di Pavia, Via Bassi 6, 27100 Pavia, Italy. § PhotoGreen Lab, Università di Pavia, Via Taramelli 12, 27100 Pavia, Italy. ‡Department
*Email:
[email protected] *Email:
[email protected] Abstract The development and investigation of heteroazo switches flourished in recent years. Due to their specific photophysical and photochemical properties, they find versatile applications from material science to medicine. However, a deep mechanistic understanding is needed to be able to predict the properties of such azoswitches. Especially, the effect of different substituents on the azo chromophore is of great interest as they are often crucial to embed the molecular switch into a system of interest. Herein, we provide a detailed spectroscopic and computational study on the influence of substituents on 3-phenylazoindoles chosen as models. We will point out changes in absorption properties and analyze the photostationary state of the thermally labile Z isomers through computational means providing a general structure-property relationship guideline for the further use of these compounds.
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Introduction Photochromic compounds received a remarkably rising interest in the last two decades.1-3 They found various applications, for instance as molecular memory and logic devices,4-6 in lightgated chemical reactions,7, 8 material science,9, 10, 11 drug delivery,12, 13 and control of cell death.15, 16 Molecular photoswitching is based on bi-stable systems interconverting between two (or more) distinct isomeric forms by application of light as an external stimulus.16 In particular, azobenzenes are popular photochromes due to their large geometrical changes upon isomerization,17, 18 their synthetically versatile availability,19, 20 their high fatigue resistance,17, 18
and the tunability of their photophysical properties.21-24 Upon irradiation, they interconvert
from a thermodynamically stable E- to a meta-stable Z-isomer (Figure 1A). The reaction is reversible, and Z-to-E isomerization is triggered through irradiation with light of a longer wavelength or thermally. The incorporation of substituents on the molecular core has been frequently used to tune the properties of azobenzenes.2, 21, 25, 26 In contrast, a scaffold hopping approach27,
28
to heterocyclic aromatic systems was only considered very recently. An
increasing number of studies have focused on this aspect to broaden the selection of azo switches available. For instance, nitrogen-based heteroaryls like pyridine,29, 30 imidazole,31, 32 pyrazole,33, 34 purine35 and very recently, the indole scaffold36, 37 have been employed as arylazo switches. Recently, we became interested in phenylazoindole based photoswitches (PAIs), owing to their highly tunable properties depending on the position of the azo-function and the solvent employed.36 In particular, 5- (5-PAI, Figure 1B) and 6-phenylazoindoles (6-PAI, Figure 1B) behave similarly compared to azobenzene, regarding both their spectroscopic properties and the lifetimes () of the Z-isomer. In contrast, the spectrum of 2-phenylazoindole (2-PAI, Figure 1B) is bathochromically shifted, and the Z-isomer of the compound exhibits extremely short thermal lifetimes (
