Journal of the American Chemical Society vol:115 issue:13 pages:5702-5708
Excimer formation is reported for 1,2-di(1-pyrenyl)tetramethyldisilane (P2P), 1,3-di(1-pyrenyl)hexamethyltrisilane (P3P) and 1,4-di(1-pyrenyl)octamethyltetrasilane (P4P), while 1,6-di(1-pyrenyl)dodecamethylhexasilane (P6P) shows a sigma(Si-Si) --> pi* charge-transfer fluorescence. The H-1-NMR spectra of the bichromophores were measured and compared with the reference compounds 1-(trimethylsilyl)pyrene (Pl), 1-pyrenylpentamethyldisilane (P2), 1-pyrenylheptamethyltrisilane (P3), and 1-pyrenyltridecamethylhexasilane (P6). A strong shielding is observed for the aromatic protons of the bichromophoric compounds, which suggests that a fraction of the molecules are present in a conformation in which the two pyrene groups interact intramolecularly. These large shielding effects were not observed for the carbon analogues where the dimethylsilane unit is replaced by a CH2 group. The shielding of the aromatic protons of P3P in solution increases going from CDCl3 to deuterated cyclohexane, which can be explained by a better solvation of the pyrenyl moiety by chloroform. The tendency for the two pyrene groups to come together decreases going from P3P to P4P and P6P. Additional structural information concerning the conformation of the bichromophores in which the two pyrene groups interact could be gained using H-1 NMR. Two different ground-state conformations in which the two pyrene moieties interact are proposed for P3P and P4P. In the first conformation, the two pyrene groups adopt a symmetrical sandwich structure, while in the second, an asymmetrical, not completely overlapping structure is proposed. A symmetrical conformation in which the two pyrene groups are not parallel while the two H10 atoms point to each other is proposed for P2P. These suggestions were confirmed by time-resolved fluorescence measurements of the excimer emission and by the differences in the absorption spectra of the bichromophores and their reference compounds.