Syntheses of push-pull substituted non-symmetric bis(thienyl) ethenes (BTEs) possessing a central perfluorocyclopentene core are described. The substituent effects of anisole, phenole, and phenolate as well as pyridine, pyridinium, and N-methylpyridinium substituents, joined through their 3-or 4-positions to the central BTE core, respectively, cover the range from very strongly electron-donating [σ(4-phenolate) = À 1.00] to extremely strongly electron-withdrawing [σ(pyridinium-4-yl) = + 2.57] in the title mesomeric betaines. The different isomers possessing 4-yl/4-yl, 4-yl/3-yl and 3-yl/3-yl substituents represent different combinations of conjugated and cross-conju-gated partial structures and cause different spectroscopic properties. In addition, through-space conjugation between the 2-and 2'-position of the thiophenes can be observed which circumvents the charge-separation of through-bond crossconjugation. The BTE possessing the push-pull chromophore consisting of 3-anisole and 4-pyridinium substituents (24) displays the best extinction coefficients within the series of compounds described here (ɛ = 33.8/15.7 L/mol • cm), while the mesomeric betaine possessing an N-methylpyridinium-4-yl and a 4-phenolate substituent (29) displays considerable bathochromic shifts to λ max = 724 nm in its closed form.

Series of photochromic 1,2-bis(thienyl)ethenes possessing perfluorocyclopentene backbones, either hydrogen or methyl groups at the β-positions of the thiophenes, and a variety of substituents in their α'-positions were prepared, which cover the range from electron-donating to electron-withdrawing (Me, À CH 2 OH, À OTBS, À TMS, À Br, 1,3-dioxan-2-yl, pyridin-4-yl, À CH 2 OH, À COOH). As a linear free energy relationship the spectroscopic Hammett equation gives fair to excellent fits to the excitation energy of the absorption maxima of the ring-opened as well as the ring-closed forms of the BTEs, when Hammett substituent constants σ p were replaced by Brown's modified substituent constants σ p + and σ p À . Vice versa, hitherto unknown Hammett-Brown substituent constants can be estimated from the UV spectra. Furthermore, we compared the experimentally measured absorption maxima with values which we calculated by three different methods (DFT STEOM-DLPNO-CCSD/def2-TZVPP, TD-DFT ωB97X-D3/6-31G*, TD-DFT ωB97X-D3/6-311 + + G**).

In recent years, fluorescence microscopy has been revolutionized. Reversible switching of fluorophores has enabled circumventing the limits imposed by diffraction. Thus, resolution down to the molecular scale became possible. However, to the best of our knowledge, the application of the principles underlying superresolution fluorescence microscopy to reflection microscopy has not been experimentally demonstrated. Here, we present the first evidence that this is indeed possible. A layer of photochromic molecules referred to as the absorbance modulation layer (AML) is applied to a sample under investigation. The AML-coated sample is then sequentially illuminated with a one-dimensional (1D) focal intensity distribution (similar to the transverse laser mode TEM01) at wavelength λ 1 = 325 nm to create a subwavelength aperture within the AML, followed by illumination with a Gaussian focal spot at λ 2 = 633 nm for high-resolution imaging. Using this method, called absorbance modulation imaging (AMI) in reflection, we demonstrate a 2.4-fold resolution enhancement over the diffraction limit for a numerical aperture (NA) of 0.65 and wavelength (λ) of 633 nm.