Conformational Effects in the Fluorescence and Photochemistry of (9,10)Anthracenophanes (n = 4,5)

Article abstract of DOI:10.1021/ja00530a036

A molecular force field model has been used to predict the number of possible conformations of <2.4>(9,19)anthracenophane (I) and <2.5>(9,10)anthracenophane (II).The model predicts four conformers for I and six for II.Calculated heats of formation and observed X-ray structure data provide the most likely conformational arrangements of I and II in their ground states.These have the largest mean interchromophore separations.Absorption of light can lead to excited state intramolecular relaxation in which this separation is reduced.For I there are two recognizable relaxation paths.In the first, there is a relative translation of the two chromophores, in opposite directions, parallel to the long axis of the molecule.This leads to a reduction of fluorescence yield (to 0.68) and a reduction of decay time (to 63 ns) as well as a small red shift of the fluorescence band.In the second, there is a change of conformation of the butane bridge which brings the two chromophores closer together.It is accompanied by an activation barrier of 8.3 kcal mol^-1 and the new conformation has a fluorescence band with a large red shift.Photochemistry proceeds via this conformation with the formation of the photoisomer.The likely structure of this molecule has an arrangement of the butane bridge which has been found in the crystal structure of the photoisomer of 1,4-di(9-anthryl)butane.For II, the properties of the fluorescence can be rationalized on the basis of only two conformations, each with the same conformation of the pentane bridge.No photochemistry could be observed and no red-shifted fluorescence band, analogous to that of I, was found.Two rotational barrier would be involved to reach the conformation of the pentane bridge in the most likely molecular structure of the photoisomer and the rate is too low to be measurable.