Abstract
Photochromic molecules undergo reversible isomerization upon irradiation with light at different wavelengths, a process that can alter their physical and chemical properties. For instance, dihydropyrene (DHP) is a deep-colored compound that isomerizes to light-brown cyclophanediene (CPD) upon irradiation with visible light. CPD can then isomerize back to DHP upon irradiation with UV light or thermally in the dark. Conversion between DHP and CPD is thought to proceed via a biradical intermediate; bimolecular events involving this unstable intermediate thus result in rapid decomposition and poor cycling performance. Here, we show that the reversible isomerization of DHP can be stabilized upon confinement within a PdII6L4 coordination cage. By protecting this reactive intermediate using the cage, each isomerization reaction proceeds to higher yield, which significantly decreases the fatigue experienced by the system upon repeated photocycling. Although molecular confinement is known to help stabilize reactive species, this effect is not typically employed to protect reactive intermediates and thus improve reaction yields. We envisage that performing reactions under confinement will not only improve the cyclic performance of photochromic molecules, but may also increase the amount of product obtainable from traditionally low-yielding organic reactions.
Highlights
Dihydropyrene (DHP) belongs to the family of diarylethene photoswitches;[1,2] this switch consists of a large π-conjugated system[3] that can be converted to the open form cyclophanediene (CPD) using visible light
The 1H DOSY spectrum indicates that all signals observable by 1H NMR diffuse at the same rate (Figure S9); that is, the signals attributed to DHP and to cage 1 diffuse together through solution, which confirms the formation of the complex
Confinement within cage 1 serves as a protective environment for the reactive intermediate involved in this reaction, minimizing bimolecular events and improving fatigue resistance during photoswitching cycles
Summary
Dihydropyrene (DHP) belongs to the family of diarylethene photoswitches;[1,2] this switch consists of a large π-conjugated system[3] that can be converted to the open form cyclophanediene (CPD) using visible light This isomerization is reversible, as CPD can be converted back to DHP both photochemically and thermally.[4] Unlike other photochromic systems, such as azobenzenes and spiropyrans, DHP is a rare example of a compound where the colored form constitutes the thermodynamically more stable isomer.[5] One disadvantage of this molecular switch, is its instability and poor fatigue resistance. The observed fatigue is thought to arise from the reactivity of the biradical intermediate formed as a result of photocleavage of the central C−C bond in the singlet excited state of DHP.[6,7] To render photoswitching cycles more robust, we propose stabilizing the reaction under confinement,[8,9] by encapsulating DHP within the cavity of a coordination cage.[10−14]. To the best of our knowledge, this work constitutes a unique example in which stabilization of a reactive intermediate by a coordination cage is employed as a technique to improve the yield of a reversible reaction
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