Push-pull molecules represent a unique and fascinating class of organic p-conjugated materials. Herein, we provide a summary of their recent extraordinary design inspired by letters of the alphabet, especially focusing on H-, L-, T-, V-, X-, and Y-shaped molecules. Representative structures from each class were presented and their fundamental properties and prospective applications were discussed. In particular, emphasis is given to molecules recently prepared in our laboratory with T-, X-, and Y-shaped arrangements based on indan-1,3-dione, benzene, pyridine, pyrazine, imidazole, and triphenylamine. These push-pull molecules turned out to be very efficient charge-transfer chromophores with tunable properties suitable for second-order nonlinear optics, two-photon absorption, reversible pH-induced and photochromic switching, photocatalysis, and intercalation.

Sixteen model (donor-π-) 2 acceptor-π-donor [(D-π-) 2 A-π-D] molecules with an extraordinary T-shaped arrangement were designed and synthesized. Indan-1,3-dione was employed as a central acceptor with electron donors linked at the C-2, C-4, and C-7 positions. These push-pull molecules represent a first systematic modification of an indan-1,3-dione-fused benzene ring. The structures and properties of all target molecules were investigated by X-ray analysis, electrochemistry, UV/Vis absorption spectroscopy, differential scanning calorimetry, electric-field-induced second-harmonic generation (EFISHG) studies, and DFT calculations. A thorough evaluation of all of the gathered data has been performed,

a b s t r a c tFifteen new push-pull chromophores based on a proaromatic pyranylidene donor and its chalcogen analogues and various electron acceptor moieties were synthesized in a straightforward manner. These model molecules were designed and prepared to investigate the concept of proaromaticity as a tool to tune the fundamental properties of push-pull systems. All target chromophores with systematically varied structure were further investigated by electrochemistry, absorption spectra, and EFISH experi-ment in conjunction with DFT calculations. Employing structural variations such as chalcogen/acceptor replacement, extension of the p-system, and the position of substitution along the pyran ring, the HOMO-LUMO gap can be tuned within the range of 2.18 to 1.41 eV. A new and powerful electron withdrawing moiety, combining features of polarizable thiophene and successful indane-1,3-dione acceptor, cyclopenta[c]thiophen-4,6-dione (ThDione) has also been developed.