Stepwise Photoreactions and Photosalient Effects in Isostructural Donor-Acceptor Molecular Complexes with Tunable Optical Properties.

In the rapidly evolving field of molecular design, donor-acceptor complexes have garnered significant attention due to their unique optical, electronic, and photoreactive properties. This study explores the synthesis of donor-acceptor molecular complexes using electron-deficient tetrafluoroterephthalate and four electron-rich 4-vinylpyridine derivatives. Due to the similar chemical structures of 4-vinylpyridine derivatives (2tpy, 3tpy, 4spy, and 3F-4spy), three of the four complexes exhibit isostructural packing. These compounds display segregated arrangements and undergo [2 + 2] solid-state photoreactions even under ambient light, with stepwise structural evolution captured by single-crystal-to-single-crystal X-ray diffraction. Under UV irradiation, the crystals show single-step photoreactions along with distinct photosalient behavior. The charge-transfer nature of these complexes, which drives their photoreactivity under ambient light, is investigated through optical absorption studies and Raman spectroscopy. The vibrational signatures obtained from Raman experiments are further interpreted using TD-DFT calculations, offering detailed insight into the electronic structure and molecular interactions.