Effects of Polar Substituents and Media on the Performance of N,N′‐Di‐tert‐Butoxycarbonyl‐Indigos as Photoswitches

Abstract

The N,N'-di-tert-butoxycarbonylindigo parent compound and four symmetrically substituted derivatives alternatively containing methoxy and nitro substituents in positions 5 and 6 were synthesized. The photochromic behavior was studied in solvents ranging in polarity from toluene to acetonitrile/water mixtures and characterized in terms of trans and cis isomer spectra, conversion quantum yields and thermal half-lives of the metastable cis isomers. The half-lives at 21°C varied from a few seconds (5-nitro in polar solvents) to several days (6methoxy in apolar media) and can be described by the substituents' Hammett constants. Spectroscopic and photochemical parameters, in contrast, are governed largely by the substituents' resonance effects according to a scheme well known from thioindigos and these are termed here favorable and unfavorable resonance effects (FRE and URE, respectively). The title photoswitches show little propensity to quenching by protons and may be operated in aqueous media.[a] Dr. B. Koeppe, V. R. F. Schröder Scheme 1. Structures of N,N'-di-tert-butoxycarbonyl-indigos. a) 1 a-e, h are studied in this work, 1 a, f, g were studied in Ref.[2b] b) Energy optimized structures (PCM-PBE0/6-31G(d)) of 1 a in PhMe (see also Figure S5 [9] ).

Molecular photoswitches in aqueous environments

“…16) is particularly photostable due to an efficient deexcitation pathway via an excited state proton transfer from the indole-NH onto the carbonyl moiety and consequently, cannot undergo E-Z isomerization. [223][224][225] However, when alkylated, 226 acetylated [227][228][229] or arylated 230,231 on the indole nitrogen, indigos become efficient photoswitches: irradiation of the thermally stable E isomer with orange-to-red light results in formation of the metastable Z form, and is accompanied by a hypsochromic shift of the absorption maximum (negative photochromism). 230 Both photoisomers are luminescent, the E weaker than the Z, and fluorescence QYs are significantly lower than in thioindigo, while photoisomerization QYs were comparable (vide infra).…”

“…The half-life can be tuned by altering the substituent pattern and could be rationalized in a Hammett analysis for both compound groups. 227,230 Alternatively, the thermal Z -E isomerization rate can be tuned by varying amounts of water in mono-arylated indigo. In particular, adding increasing amounts of water to a THF solution of a mono-arylated derivative deceases t 1/2 from 1000 s to only 5 s, while the thermal half-life of the corresponding bis-arylated indigo derivate stays unaltered.…”

“…Also, the QYs are lower in the presence of water for the E -Z but higher for the Z -E photo isomerization. 227 Thioindigo (Fig. 16) is the sulphur-analogue of parent indigo and was studied in various contexts, for instance in semiconducting films, 232 taking advantage of its redox switching 233 and modulation of electro-optical properties.…”

Red-Purple Photochromic Indigos from Green Chemistry: Mono-tBOC or Di-tBOC N-Substituted Indigos Displaying Excited State Proton Transfer or Photoisomerization

“…Schematic of MW-assisted synthesis of NtBOCInd and N,N'tBOCInd from indigo,1 H NMR of NtBOCInd in CDCl 3 ,13 C NMR of NtBOCInd in CDCl3, 1 H NMR of N,N'tBOCInd in CDCl 3 ,13 C NMR of N,N'tBOCInd in CDCl 3 , high-resolution mass spectrum of NtBOCInd, high-resolution mass spectrum of N,N'tBOCInd, orbital contours of the HOMO and LUMO in indigo, TDDFTsimulated spectra of NtBOCInd and N,N'tBOCInd in MCH and DMSO, normalized absorption and fluorescence emission spectra, absorption spectra of N,N'tBOCInd before and after irradiation, fluorescence emission spectra of trans and cis isomers of N,N'tBO-CInd in MCH, fluorescence decay and pulse instrumental response obtained for N,N'tBOCInd, determination of the rate constants, schematic representation of the experimental setup and picture of the irradiation system, and absorption spectra for trans−cis isomerization of N,N'tBOCInd in n-dodecane, MCH, 2MeTHF, and MeCN (PDF) h i b i t e d .…”

Visible‐Light Photoswitching by Azobenzazoles

“…In all cases,t he Z isomers of 1-3 are calculated to be more basic than the thermally stable E isomers (calculated pK a difference of 1.1 to 2.4 units)d ue to the formation of an intramolecular hydrogen bond (Figure 9), as observed for related systems. [14,42] The validity of this interaction was confirmed by atoms-in-molecule analysis,s ee SupportingI nformation, section S17. The calculated pK a values for Z-2H + and Z-3H + are sufficiently low that these species are not observed upon addition of TFA.…”