Photochemical and thermal spiropyran (SP)-merocyanine (MC) interconversion

A dichotomy in dependence on viscosity

Jamie Whelan, Dalia Abdallah, Konrad Piskorz, James T.C. Wojtyk, Julian M. Dust, Jean Michel Nunzi, Shmaryahu Hoz, Erwin Buncel

    Research output: Contribution to journalArticle

    Abstract

    The current study extends our work with spiropyran-merocyanines (SP-MC) as molecular photoswitches by delving into the effects of viscosity. This has led to the interesting finding of a dichotomy in viscosity dependence. Solutions of SP [6′-nitro-1,3,3-trimethylspiro(indolino-2,2′-benzopyran)] in a wide range of ethylene glycol-methanol (EG-MeOH) media (3.59 to 17.9 M in EG) were irradiated 90 s (365 nm). The absorbance at 90 s of MC (532 nm) formed photolytically varied with solvent. The least viscous medium yielded the highest concentration of MC and yields declined with increasing viscosity. Once irradiation ceased each system achieved thermal equilibrium. Molecular dynamics studies of typical thermal reactions governed by electronic and steric factors show that the transition state is achieved primarily after solvent reorganization has occurred to accommodate the new structure. It follows that in such thermal reactions viscosity may not cause any hindrance to the motion of atoms in molecules because solvent has already rearranged. In contrast, photochemical excitations occur at much higher rates (10-15 s) than solvent reorganization, i.e. dielectric relaxation (10-10 to 10 -12 s). The viscosity dependence of photochemical MC formation suggests that a major geometrical change is required for excited SP to be converted to MC. The dichotomy in dependence on viscosity is confirmed by the thermal equilibration of SP and MC. The equilibrium constant for the process increases three-fold (from 0.0535 to 0.158) as the EG content of the medium increases. However, the forward rate constant (SP → MC) is almost invariant with EG content or viscosity. The process is viscosity independent. The increase in the equilibrium constant with EG concentration is a result of a decline in the reverse rate constant for MC cyclisation to SP. This is attributed to special stabilisation of the MC that increases with increasing EG concentration. The present study, to our knowledge, is the first to dissect viscosity from solvent stabilisation factors in SP-MC systems. Further, the study highlights the fundamental difference between photolytic and thermal processes, providing another avenue of control for these SP-MC photoswitches.

    Original languageEnglish (US)
    Pages (from-to)13684-13691
    Number of pages8
    JournalPhysical Chemistry Chemical Physics
    Volume14
    Issue number39
    DOIs
    StatePublished - Oct 21 2012

    Fingerprint

    dichotomies
    Viscosity
    viscosity
    Equilibrium constants
    stabilization
    Rate constants
    spiropyran
    Hot Temperature
    merocyanine
    Stabilization
    Benzopyrans
    Ethylene Glycol
    Dielectric relaxation
    Cyclization
    glycols
    ethylene
    methyl alcohol
    Methanol
    Molecular dynamics
    molecular dynamics

    ASJC Scopus subject areas

    • Physics and Astronomy(all)
    • Physical and Theoretical Chemistry

    Cite this

    Whelan, J., Abdallah, D., Piskorz, K., Wojtyk, J. T. C., Dust, J. M., Nunzi, J. M., ... Buncel, E. (2012). Photochemical and thermal spiropyran (SP)-merocyanine (MC) interconversion: A dichotomy in dependence on viscosity. Physical Chemistry Chemical Physics, 14(39), 13684-13691. https://doi.org/10.1039/c2cp42259j

    Photochemical and thermal spiropyran (SP)-merocyanine (MC) interconversion : A dichotomy in dependence on viscosity. / Whelan, Jamie; Abdallah, Dalia; Piskorz, Konrad; Wojtyk, James T.C.; Dust, Julian M.; Nunzi, Jean Michel; Hoz, Shmaryahu; Buncel, Erwin.

    In: Physical Chemistry Chemical Physics, Vol. 14, No. 39, 21.10.2012, p. 13684-13691.

    Research output: Contribution to journalArticle

    Whelan, J, Abdallah, D, Piskorz, K, Wojtyk, JTC, Dust, JM, Nunzi, JM, Hoz, S & Buncel, E 2012, 'Photochemical and thermal spiropyran (SP)-merocyanine (MC) interconversion: A dichotomy in dependence on viscosity', Physical Chemistry Chemical Physics, vol. 14, no. 39, pp. 13684-13691. https://doi.org/10.1039/c2cp42259j
    Whelan, Jamie ; Abdallah, Dalia ; Piskorz, Konrad ; Wojtyk, James T.C. ; Dust, Julian M. ; Nunzi, Jean Michel ; Hoz, Shmaryahu ; Buncel, Erwin. / Photochemical and thermal spiropyran (SP)-merocyanine (MC) interconversion : A dichotomy in dependence on viscosity. In: Physical Chemistry Chemical Physics. 2012 ; Vol. 14, No. 39. pp. 13684-13691.
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