Molecular packing, crystal to crystal transformation, electron transfer behaviour, and photochromic and fluorescent properties of three hydrogen-bonded supramolecular complexes containing benzenecarboxylate donors and viologen acceptors

Abstract

An investigation into the relationship between structures of three D–A supramolecular systems and their photoresponsive characteristics is presented. Compound 1 [H₂CPBPY]·[H₂BTEC] and compound 2 [HCPBPY]₂·[H₂BTEC]·5H₂O are prepared with the same starting materials at different pH values and characterized by single crystal X-ray diffraction, powder X-ray diffraction, UV-vis, IR and ESR spectra. Electron-deficient CPBPY moieties and electron-donating BTEC units are linked by hydrogen-bonds and π–π stacking interactions to form the donor–acceptor systems. Under light irradiation, photoactivated electrons can transfer from the donor to the acceptor units in both 1 and 2, giving rise to a long-lived charge-separated state and accompanying an interesting color changing phenomenon. Crystals of 2 allowed to dehydrate at elevated temperature undergo single-crystal to single-crystal transformation to yield 3 [HCPBPY]₂·[H₂BTEC], accompanied by a drastic change in structural arrangements. For the photocoloration character, compound 1 shows a faster photoresponsive rate than 2 and 3. For the fluorescence properties, compound 1 and 2 both exhibit photoluminescence emission centered at 400 nm, whereas 3 exhibits photoluminescence emission at 520 nm, showing a significant red shift of 120 nm. Their different photoactive characters can be attributed to the connecting, packing modes and the valence states of the functional D–A moieties.