Stimuli-Responsive Cyclopenta[ef]heptalenes: Synthesis and Optical Properties

• Published in: European journal of organic chemistry 2015-04, Vol.2015 (10), p.2271-2276
• Main Authors: Ghazvini Zadeh, Ebrahim H., Woodward, Adam W., Richardson, David, Bondar, Mykhailo. V., Belfield, Kevin D.
• Format: Article
• Language: English
• Subjects: Azulene; Cyclopenta[ef]heptalene; Dyes/pigments; Electron transfer; Natural products; Photoacid generator; Photochromism; Sensitizers; Stimuli response
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.201500059

We report the one‐pot synthesis, 1D and 2D NMR characterization, and UV/Vis study of a series of cyclopenta[ef]heptalenes 4a–c that exhibit strong stimuli‐responsive behavior, with a tunable energy gap as a result of perturbation of HOMO, LUMO, and LUMO+1 energies upon doping/dedoping with TFA/Et3N. The approach employed allows for the extension of conjugation at C‐4 of the cyclopenta[ef]heptalene skeleton from X = H (4a) to X = CN (4b) and X = 2‐thiophenyl (4c), resulting in longer absorption maxima and smaller optical energy gaps of the cyclopenta[ef]heptalenium cations 4a–c+. Additionally, in the presence of a UV‐activated (< 300 nm) photoacid generator (PAG), protonation of 4c can be indirectly achieved by intermolecular photoinduced electron transfer (PeT) from the excited state of 4c to the PAG, upon which the latter undergoes photodecomposition resulting in the generation of acid. This phenomenon facilitates the use of cyclopenta[ef]heptalenes 4a–c as visible sensitizers of commercial PAGs. Cyclopenta[ef]heptalenes 4a–c, prepared by annulation of 3‐formylguaiazulene (1), revealed a stimuli‐responsive behavior upon treatment with strong acids. 1D and 2D NMR spectroscopy indicated that protonation occurred on C‐6, while photoacid titrations demonstrated that the reported tricyclic systems can serve as efficient visible sensitizers of UV‐activated photoacid generators (PAGs).