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Pure Appl. Chem., 2011, Vol. 83, No. 4, pp. 899-912

Published online 2011-03-14

Engineering supramolecular photoactive nanomaterials by hydrogen-bonding interactions

K. Yoosaf1, Abdelhalim Belbakra1, Anna Llanes-Pallas2, Davide Bonifazi3,2* and Nicola Armaroli1*

1 Molecular Photoscience Group, Istituto per la Sintesi Organica e la Fotoreattività, Consiglio Nazionale delle Ricerche (CNR-ISOF), Via Gobetti 101, 40129 Bologna, Italy
2 INSTM UdR di Trieste and Dipartimento di Scienze Farmaceutiche, Università di Trieste, Piazzale Europa 1, 34127 Trieste, Italy
3 Chemistry Department, University of Namur, Rue de Bruxelles 61, 5000 Namur, Belgium

Abstract: The photophysical properties of molecules containing anthracene, pyrene, or phenyleneethynylene chromophores bearing complementary triple H-bonding terminal units, namely, 2,6-di(acetylamino)pyridine (donor–acceptor–donor, DAD) and uracyl (acceptor–donor–acceptor, ADA) have been investigated as a function of solvent polarity. For asymmetric systems, presenting only one H-bonding unit, a solvatochromic effect is found, suggesting a charge-transfer character of the lowest electronic excited state. Systematic absorption and emission studies carried out as a function of temperature show that phenylene-ethynylenes having linear geometry and H-bonding functionalities at both ends undergo reversible self-aggregation in cyclohexane (CHX), leading to the formation of spherical nanoparticles, as evidenced by wide-field fluorescence microscopy (WFM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). A combination of an anthracene derivative bearing only one ADA terminal functionality and a linear phenylene-ethynylene derivative possessing two DAD terminal groups in CHX (2:1 molecular ratio) leads to the formation of vesicular nanostructures. The interaction of linear phenylene-ethynylenes possessing two terminal 2,6-di(acetylamino)pyridine functionalities with that bearing bis uracylic units gives origin to nanofibers, while the assembly of the former with bisuracylic units exhibiting bent geometry leads to the formation of helical nanofibers. The length of these fibers can be controlled by addition of the anthracene derivative having only one uracyl group which effectively blocks the extent of H-bonding, prompting the formation of shorter nanorods.