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.