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.

中文翻译：

---

刺激响应的环戊二烯[ef]庚烯：合成和光学特性

我们报告了一系列环戊二烯[ef]庚烯4a-c的一锅合成、一维和二维核磁共振表征以及紫外/可见研究，这些环戊烯[ef]庚烯4a-c表现出强烈的刺激响应行为，由于使用 TFA/Et3N 掺杂/去掺杂后的 HOMO、LUMO 和 LUMO+1 能量。所采用的方法允许将环戊二烯骨架的 C-4 处的共轭从 X = H (4a) 扩展到 X = CN (4b) 和 X = 2-噻吩 (4c)，从而导致更长的吸收最大值环戊二烯鎓阳离子 4a-c+ 的光能隙更小。此外，在紫外线激活（< 300 nm）光酸产生剂 (PAG) 的存在下，4c 的质子化可以通过从 4c 的激发态到 PAG 的分子间光致电子转移 (PeT) 间接实现，后者发生光分解，产生酸。这种现象促进了使用环戊二烯 [ef] 庚烯 4a-c 作为商业 PAG 的可见敏化剂。