Unusual thermo(photo)chromic properties of some mononitro- and dinitro- substituted 3′-alkyl indolospirobenzopyrans

• Published in: Dyes and pigments Vol. 121; pp. 57 - 72
• Main Authors: Abdullah, Ayse, Nevell, Thomas G., Sammes, Peter G., Roxburgh, Craig J.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2015
• Subjects: 6,8-Dinitro-N-methyl- and 6-mononitro-N-methyl-substituted spirobenzopyrans; Inductive (nitro-) and steric (alkyl-substituent) biasing control; NMR spectroscopy; Photochromism; Sigmoidal absorbance time plots; Synthesis; 6,8-Dinitro-N-methyl- and 6-mononitro-N-methyl-substituted spirobenzopyrans; Synthesis; Inductive (nitro-) and steric (alkyl-substituent) biasing control; Photochromism; NMR spectroscopy; Sigmoidal absorbance time plots
• ISSN: 0143-7208; 1873-3743
• DOI: 10.1016/j.dyepig.2015.04.027

Isomeric equilbria of dinitro-substituted indolospirobenzopyrans, possessing 3′-gem-methyl- or 3′-cyclohexyl-substitutents, have been investigated using 1H NMR spectroscopy at six temperatures, ranging from 298 K to 410 K; isomerisation processes in a methanolic solution have been monitored by spectrophotometry. For the mononitro-substituted compounds, equilibrium favoured the colourless spirocyclic isomers. Reversion of the coloured merocyanine isomers, generated by UV irradiation, followed first-order kinetics. For the dinitro-substituted compounds the coloured merocyanine isomers predominated. Following decolourisation by visible light photoirradiation, reversion towards the merocyanine structures were particularly slow, absorbances unusually increasing sigmoidally. UV. spectral observations for the gem-methyl- 1 and 3′-cyclohexyl-substituted systems 2 are qualitatively consistent with the simultaneous involvement of two relatively slow rate determining steps, possessing slowly forming and long-lived intermediates - postulated to be the oxygen protonated pyran-ring of the spirocyclic structure and TCC merocyanine isomer, the latter undergoing relatively slow isomerisation about the central β-alkenic bond to the TTC isomer. 1H NMR evidence for the simultaneous solution existence of the TTT and TTC merocyanine conformers of the dinitro- 3′-cyclohexyl- substituted indolospirobenzopyran 1a is presented: associated and unusually long, and sigmoidal, thermochromic zwitterion formation, following the photolysis of 3,3′-dimethyl- and 3′-cyclohexyl- substituted 6,8-dinitro-indolospirobenzopyrans (methanol at 21 °C and 40 °C), is observed. [Display omitted] •Photoirradiation of compounds 1 and 2 with visible light unusually produces sigmoidal increases in merocyanine concentrations.•Slow sigmoidal increases in conc's of merocyanines, attributed to two relatively slow rate determining steps.•Intermediates postulated to involve a pyran oxygen protonated benzopyran 14 and TTC merocyanine isomer 18.•Comparisons made to skeletally identical alkyl-substituted mono-nitro- congeners, which behave differently.•Differences in merocyanine formation rates between 3′-cyclohexyl- and 3′-gem-methyl- dinitro-substituted compounds discussed.