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Issue 33, 2009
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Photochromism of nitrobenzospiropyran in phosphonium based ionic liquids

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The photo-, thermo- and solvatochromic properties of 2,3-dihydro-1′,3′,3′-trimethyl-6-nitrospiro[1-benzopyran-2,2′-1H-indole] (BSP) and its photo-induced merocyanine isomer (MC) were investigated in phosphonium based ILs by UV-vis absorption spectroscopy. It was found that the kinetics and thermodynamics of the BSP ↔ MC equilibrium were sensitive to the nature of the anion. The MCλmax shifted from 560 nm to 578 nm when in solutions of [P1,4,4,4][tos] and [P6,6,6,14][dca], respectively. The BSP isomer was highly favoured at equilibrium in the ILs studied; Ke values observed were similar to non-polar solvents such as dichloromethane. The thermal relaxation of MC in all ILs is first order, and in comparison with aprotic polar solvents possessing comparable polarity (such as acetonitrile). Thermal relaxation rates of MC were monitored over a range of temperatures; at 293 K rates varied from 5.19 to 25.03 × 10−4 s−1. A non-linear relationship between Ke and k was observed; this contradicts what is expected for BSP in molecular solvents and suggests the isomers exhibit different molecular/solvation environments. The energetics of the thermal relaxation of MC in ILs were observed; activation energies ranged from 71 to 90 kJ mol−1 and all ILs exhibit negative activation entropies ranging between −72 and −8.2 J K−1 mol−1. A linear relationship between activation energy and entropy was observed.

Graphical abstract: Photochromism of nitrobenzospiropyran in phosphonium based ionic liquids

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Article information

23 Feb 2009
07 May 2009
First published
09 Jun 2009

Phys. Chem. Chem. Phys., 2009,11, 7286-7291
Article type

Photochromism of nitrobenzospiropyran in phosphonium based ionic liquids

R. Byrne, S. Coleman, K. J. Fraser, A. Raduta, D. R. MacFarlane and D. Diamond, Phys. Chem. Chem. Phys., 2009, 11, 7286
DOI: 10.1039/B903772A

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