Thermal and Photodegradation of Photochromic Spiroindolinenaphthooxazines and -pyrans:  Reaction with Nucleophiles. Trapping of the Merocyanine Zwitterionic Form

Abstract

Adducts of the zwitterionic (ZW) form of merocyanine (MC) of five photochromic spirooxazines (SO) and spiropyrans (SP) have, for the first time, been prepared photochemically or thermally through a trapping reaction of metastable MC with trimethylsilyl cyanide (TMSCN) in dichloromethane. The adducts (ZWAs) form easily and are obtained in almost quantitative yields. X-ray diffraction analysis of ZWA of spirooxazine 2 shows that in the adduct the planes of the indolyl and naphthyl moieties form a ca. 29° angle. The adduct crystallizes in the monoclinic space group P21/n with a = 22.663(4) Å, b = 8.234(1) Å, c = 14.455(2) Å, β = 104.72(1)°, V = 2637.4(7) Å3, and D calcd = 1.1476 Mg/m3 for Z = 4. Least-squares refinement of the model based on 1986 reflections (F > 4σ(F)) converged to a final R = 0.066 and R w = 0.068. ZWAs form through an unprecedented, clean, and regioselective Michael-type 1,6-addition of TMSCN to either a strongly polarized quinoidal MC form or its charge-separated zwitterionic limit structure. No evidence for the more common 1,2- and/or 1,4-addition has been found. The observed high reactivity of SOs and SPs with nucleophiles may contribute to the reported limited durability of these classes of photochromes.

Reactivity of tetrahydrochromeno[2,3‐b]indoles: chromic indicators of cyanide

“…Reaction kinetics (Table ) were measured by UV–vis with sample excitation using an arc‐lamp fitted with a 340 nm broad bandpass filter. Malatesta, et al . reported degradation of TMSCN adducts of SP leading to desilylated products; we also observed an increasing number of side‐products in the NMR for irradiations exceeding 20 min for chloroform solutions with arc lamp irradiation.…”

“…It has also been used to trap iminium ions and zwitterionic peroxide intermediates in the singlet oxygen reaction of electron‐rich systems . Malatesta and co‐workers formed adducts with zwitterionic MC isomers of spirooxazines and SP s in which the cyanide is attached to the spirocyclic carbon and the phenolate is O‐silylated. The adduct formed rapidly with UV irradiation by reaction with the zwitterion; there was no evidence for direct reaction with the SP .…”

Photoswitchable Nanoparticles for Triggered Tissue Penetration and Drug Delivery

“…To confirm particle shrinkage after irradiation (Figure 2 a), we produced NP H containing MC–CN, a similar but relatively stable MC quinoidal structure, which is a 1, 6-addition adduct of MC-C9 with trimethylsilyl cyanide ( Figure S3 ). 30 MC–CN NP H s were 59.4 nm in diameter, with a polydispersity of 0.04 ( Figure S3c ), similar to the size of MC NP H produced from SP NP H by UV-irradiation (47.2 nm with a polydispersity of 0.05, Figure 2 a). Direct nanoprecipitation of MC–CN resulted in 42.6 nm NP H s with a polydispersity of 0.11 ( Figure S3d ), a result consistent with the DLS measurements of MC NP H s.…”

Photochromic Materials by Postpolymerisation Surface Modification

“…1 Photoresponsive polymers incorporating the azo-group 1 (Figure 1) have found wide application, 2 so that light-modified biosensing, 3 self-assembly behavior, 4 and the delivery of bioactive molecules have all been reported. 5,6 Alternatively, spiropyrans 2a (Figure 1) 7,8 have been widely used in organic photochromic switches for their reversible decolouration–colouration 9 since UV photoirradiation gives the ring-opened, colored merocyanine form 2b , which may revert to the closed, colorless spiropyran form 2a under visible light or by thermal relaxation in the dark. 10−12 In addition to these photochromic changes, the conversion of the hydrophilic, zwitterionic, merocyanine form to the hydrophobic spiropyran form also causes a significant change in polarity, which is manifested as a change in surface wetting behavior.…”