Spiropyrans, experiment

Equation 3.22. This type of experiment will be discussed eventually for spiropyran and diarylethene chromophores in films of PMMA. Next, I compare reorientation observations after cis—>trans thermal isomerization of azobenzene to the theoretical developments in Section 3.2.3.2. 

In this experiment students create their own indicator of ultraviolet (UV) light exposure, a photochromic spiropyran, and evaluate different sunscreen products to determine their effectiveness in blocking UV light. 

In order to demonstrate the usefulness of the method, SERS spectra of indolino-spironaphthopyran (SPP, 4) and indolino-spironaphthoxazine (SPOX, 3) (see Scheme 7) were recorded at low concentration (10 7-10-8 A/) and their spectral characteristics analysed.48 Figure 8 shows the SERS spectrum of 4 in Ag colloids (spectrum a) along with the corresponding near-infrared Fourier transform (NIR-FT) Raman spectrum of the neat compound (spectrum b). Strong differences between the SERS and spontaneous Raman spectra are apparent. As stated above, these differences arise mainly from the particular orientation of the adsorbed spiropyran with to the metal surface in SERS experiments. 

This assumption is consistent with experimental data obtained recently by Campredon el al49 These authors trapped a biradical intermediate during electron spin resonance (ESR) experiments. Signals attributed to the radicals 61-64 were monitored when NO was added to solutions of spiropyrans 56-59. Similar results were obtained with spirooxazines 29 and 60 leading to radicals 65 and 66. Nevertheless there is no direct evidence indicating that the trapped biradicals are on the reaction path leading to the merocyanine. 

A fairly diastereoselective ring closure (1.6 1) was observed for an enantiomer-ically enriched spiropyran SP-3 (optical purity not reported) [48]. The oxygen atom of the benzopyran ring was shown, by NOE experiments, to prefer the methyl side rather than the propyl side. 

For the w-spiropyranes case, the w-p-xylene merocyanine has the faster fading rate compared with the w-decyl merocyanine. A priori one would expect that the to-decyl merocyanine would have a faster fading rate, taking into account that this molecule is bigger (24.1 A) than the w-p-xylene merocyanine (20.4 A), therefore it will experience more stress in the polymer matrix, so the recovery rate back to the spiropyrane form would be faster. An additional effect is obviously involved. 

In other experiments, the photon-gated transport of Fe(bpy)3 + at conical recessed nanopores, functionalized with a spiropyran (SP) moiety, has been described. Upon exposure to UV light, SP is converted in the presence of a weak acid to the protonated merocyanine, MEH+. MEH+ is converted back to SP by shining visible light on the nanopore orifice. The effect of the photon-generated charges on the diffusion-limited oxidation of Fe(bpy)3 + is significant. In the dark (i.e., with the surface attached molecule in the electrically neutral form, SP), for Fe(bpy)3 oxidation is ca. 8... 

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