Polymeric Spiropyrans

In cyclohexane and heptane, the colored form of 6-nitroBIPS undergoes aggregation, giving absorption near 700 nm. An observed anomalous (nonexponential) thermal decoloration can be attributed to the deaggregation to the dimer (Xmax 620 nm) and monomer (/.max 580 nm),161 

The time-resolved resonance Raman spectrum of the colored species produced during a 30-ns laser pulse indicated that a short-lived intermediate, almost certainly the cis form, is a precursor to the stable trans form of the merocyanine.163 

---

Photoresponsive polymers can be obtained by introducing photochromic units, such as azobenzene or spiropyran groups, into the macromolecules of polymeric compounds. As described in Chapter 1 of this book, photochromic compounds can exist in two different states, such as two isomeric structures that can be inter-converted by means of a light stimulus, and the relative concentrations of which depend on the wavelength of the incident light. For instance, in azobenzene compounds, photochromism is due to trans-cis photoisomerization around the N=N double bond, while in spiropyran compounds photochromism involves interconversion between the neutral spiro form and the zwitterionic merocyanine form (Figure 1). 

Because spiropyrans (and especially BIPS) are relatively easy to prepare, a variety of compounds are available from which to deduce correlations (usually semiempirical Hammett-like equations) between structure and photochromic parameters in dilute fluid solutions. The photochromic behavior of a single dye in a variety of solvents can also be correlated with a solvent parameter such as. In applications, however, the dyes are almost always relatively concentrated in a bulk polymer or a polymeric binder film, and various other additives such as antioxidants, surfactants, fire retardants, plasticizers, and other colorants are also present. Under these conditions, the observed photochromic behavior is often very different from that predicted by the dilute fluid solution correlations. 

The immobilization of photoisomerizable host molecules onto an electrode surface can be used for the construction of novel ion-selective electrodes. A photoisomerizable calix[4]arene derivative was incorporated into a polymeric membrane on an electrode surface and the two different isomeric states of the host molecule provided responses selective to Li" or Na" ions depending on the state.Another photoisomerizable host molecule has been constructed from spiropyran and crown-ether subunits. Different binding affinities for Li were found depending on the isomeric state of the photoisomerizable component (26). The researchers suggested that this effect was caused by the coordination of the Li ion to the 0 of the zwitterionic merocyanine-form (Figure 7.29). The photocontrolled host molecules were immobilized in a polymeric film onto an electrode surface, resulting in a photochemically switchable ion-selective electrode. 

Though the field of molecular electronics is still in its infancy with respect to the porous layered materials many of these phases have yet to be considered as hosts for conducting polymeric materials. The properties of intercalated sufonyl-spiropyran have been examined to determine any novel photochromic behavior. The parent... 

Recently, we explored strategies for binding spiropyran moieties to structured brushes grafted from ETFE and Teflon (PTFE) surfaces in order to obtain light-sensitive structured polymer surfaces [18]. We focused on post-polymerization modification of grafted brush structures because this strategy increases the flexibility with respect to the optimization of the concentration of spiropyran moieties in the brushes. Moreover, the tedious synthesis and purification steps of spiropyran—monomer conjugates are circumvented. 

Recent efforts to report and to repair mechanical damage with mechanochemical reactions form the subject of the current chapter. We start with a brief discussion of the use of the spiropyran unit as a mechanophore for reporting strain. Spiropyran mechanochemistry inspired the development of another stress probe, the highly sensitive mechanoluminescent dioxetane, whose application as scission reporter in several types of polymeric materials is discussed. The chapter continues with a description of recent efforts to develop productive mechanochemistry, where initial scission leads to the formation of new bonds. Bond formation is either induced by the scission of covalent bonds, e.g. by the opening of rings, or bonds are formed under the action of a latent catalyst when it is activated by mechanochemical dissociation of a Lewis acid-base pair. These examples of productive mechanochemistry offer exciting possibilities to develop new modes of self-healing in... 

Samanta, S. Locklin, J. Formation of photochromic spiropyran polymer brushes via surface-initiated, ring-opening metathesis polymerization reversible photocontrol of wetting behavior and solvent dependent morphology changes. Langmuir 2008,24,9558-9565. 

Matsni K., Morohoshi T., Yoshida S. Photochromism of spiropyran in sol-gel glass and plasma-polymerized films. Proc. MRS Int. Meeting Adv. Mater. 1989 12 203-208 Matsni K., Nakazawa T. Fluorescence probes ofpyrene and pyrene-3-carboxaldehyde for the sol-gel process. Bnll. Chem. Soc. Jpn. 1990 63 11-16... 

---