Lui Brandon F, Tierce Nathan T, Tong Fei, Sroda Miranda M, Lu Hao, Read de Alaniz Javier, Bardeen Christopher J
Department of Chemistry, University of California, Riverside, CA 92521, USA.
Photochem Photobiol Sci. 2019 Jun 12;18(6):1587-1595. doi: 10.1039/c9pp00130a.
Donor-acceptor Stenhouse adducts comprise a new class of reversible photochromic molecules that absorb in the visible and near-infrared spectral regions. Unimolecular photoisomerization reactions are usually assumed to be insensitive to photochrome density, at least up to millimolar concentrations. In this paper, the photoisomerization kinetics of a third-generation donor-acceptor Stenhouse adduct molecule (denoted DASA) are examined over a range of concentrations. DASA switches efficiently at micromolar concentrations in both liquid solution and in polymers, but as the photochrome concentration is increased there is a dramatic inhibition of the photoisomerization. A kinetic study of both the reactant and photoproduct decays at varying concentrations and in different hosts indicates that the forward photoisomerization and the thermal backward reaction can change by factors of 20 or more depending on DASA concentration. Femtosecond transient absorption experiments show that the initial cis → trans step of the isomerization is not affected by concentration. It is hypothesized that long-range coulombic interactions interfere with the ground state electrocyclization stage of the isomerization, which is unique to the DASA family of photochromes. The physical origin of the inhibition of photoswitching at high photochrome concentrations must be understood if the DASA class of molecules is to be used for applications that require high photochrome concentrations, including photomechanical actuation.
给体-受体斯滕豪斯加合物构成了一类新型的可逆光致变色分子，它们在可见光和近红外光谱区域有吸收。单分子光异构化反应通常被认为对光致变色染料浓度不敏感，至少在毫摩尔浓度以下是这样。在本文中，研究了第三代给体-受体斯滕豪斯加合物分子（记为DASA）在一系列浓度范围内的光异构化动力学。DASA在微摩尔浓度下在液体溶液和聚合物中都能高效切换，但随着光致变色染料浓度的增加，光异构化受到显著抑制。对不同浓度和不同基质中反应物和光产物衰减的动力学研究表明，正向光异构化和热逆向反应会根据DASA浓度变化20倍或更多。飞秒瞬态吸收实验表明，异构化最初的顺式→反式步骤不受浓度影响。据推测，长程库仑相互作用干扰了异构化的基态电环化阶段，这是DASA类光致变色染料所特有的。如果要将DASA类分子用于需要高光致变色染料浓度的应用，包括光机械驱动，就必须了解高光致变色染料浓度下光开关抑制的物理起源。
