Photochromic derivatives of indigo: historical overview of development, challenges and applications

Abstract

The importance of indigo dyes is constantly increasing with the evolution of novel textile materials and photochromic material technologies. The aim of this review article is to provide a comprehensive overview of the development of photochromic indigo derivatives from the first report on the photochromic N,N'-diacetylindigo in 1954 until now. We begin with the list of historical milestones in the development of photochromic indigo derivatives. Further, we provide a brief description of the synthetic procedures utilised to obtain indigo and its derivatives, outline the structural peculiarities, photophysical and photochemical properties of indigo and proceed with the detailed discussion of the photochromic indigo derivatives. Finally, we highlight the photochromism of the structural isomers of indigo (isoindigo and indirubin) and provide an overview of prospective applications of indigo photoswitches.

Keywords: E–Z isomerization; indigoid dyes; photochemistry; photophysics; photoswitching.

Figure 1

Precursors used in the synthesis of indigo [4].

Figure 2

a) Intramolecular (a = 2.26 Å) and intermolecular (b = 2.11 Å) hydrogen bonds in indigo, b) crystal packing of indigo in the solid state obtained from the single-crystal X-ray diffraction data, CCDC 796873 [12], c) photos of indigo in the solid state and solutions of indigo in 1) DMSO, 2) DMF, 3) N-methyl-2-pyrrolidone.

Figure 3

Bond length in the indigo molecule obtained from the single crystal X-ray analysis [12], the typical bond lengths in organic compounds [15] and the main resonance structures of indigo.

Figure 4

The structure of the indigo chromophore (H-chromophore, highlighted in blue), asterisk indicates the molecule in the excited state.

Figure 5

Influence of substituents in the benzene rings on the color of indigo derivatives.

Figure 6

a) E–Z photoisomerization of indigo and b) photoinduced proton transfer in the excited state, asterisk indicates the molecules in the excited state.

Figure 7

Structures of indigo derivatives discussed in this review.

Figure 8

Photoswitching of N,N'-diacetylindigo (9a) in CCl₄ (c = 17.1 µM; cell length = 5.0 cm) irradiated with blue light (λ_irr = 350–510 nm): dotted line; irradiated with white light: dashed line; irradiated with yellow light (λ_irr > 495 nm): solid line; irradiated with orange light (λ_irr > 520 nm): dash-dotted line. In all cases irradiation time t = 5 min. Reprinted with permission from ref. [38]. Copyright (1965) American Chemical Society.

Figure 9

Photoisomerization of compound 18c upon irradiation with red light and schematic representation of the complexation of the Z-isomer with metal cations.

Figure 10

Schematic representation of indigo-type (left) and amide-type (right) resonances in N,N'-acetylindigo (9a).

Figure 11

Suggested intermediates for the double bond cleavage for the thermal relaxation of N,N'-diacylindigos: (A) a biradical transient species, (B) a dipolar transient species.

Figure 12

Zwitterionic resonance structures of Z-indigo.

Figure 13

Photos of crystalline N,N'-di(Boc)indigo 17a its solutions in 1) DMSO, 2) DMF, 3) N-methyl-2-pyrrolidone.

Figure 14

Structural isomers of indigo.

Figure 15

Photochromism of indirubin derivatives and supramolecular complexation of the E-isomers with Schreiner’s thiourea organocatalyst (STC).

Figure 16

Photoisomerization of the protonated isoindigo.