We are currently investigating the influence of interchromophore coupling on the photophysics of bichromophoric systems. For this we are studying the 18-Crown-6 Pyrene bichromophore synthesized by the Lacour group (University of Geneva) shown in the Figure above. By binding cations of different sizes into the crown ether moiety we can effectifly alter the distance between the two Pyrene chromophores and therefore also their coupling. The unbound crown (left) shows next to a weak structured emission in the blue also a unstructured broad emission around 500 nm indicative of the Pyrene excimer. Upon binding Ba2+ to the crown (right) the excimer emission is completely quenched. When binding Mg2+ (middle) to the crown the excimer emission is only partially quenched.
The figure shows the merged fs-ns visibile transient absorption spectra of the three complexes. For the Ba2+-bound crown the spectral features of the locally excited S1 state emerge and decay completly in less than 1 ns. For the unbound crown, the S1 state quickly (< 10 ps) converts to the excimer due to the increased interchromophore coupling which then decays with a lifetime of around 30 ns. The Mg2+-bound crown is the middle ground between these two extremes. The interchromophore distance in not large enough to efficiently populate the excimer state as shown by a prolonged lifetime of the S1 state compared to the unbound crown. On the other hand, new bands reminiscent of the Pyrene radical ions emerge following the S1 decay, highlighting the possibility of symmetry breaking charge seperation (SB-CS), Py*-Py → Py•+-Py•– , i.e. electron transfer which can take place for less coupled chromophores contrary to excimer formation.
| FOR MORE INFORMATION | ||
  |
||
Contact:
Eric Vauthey
Physical Chemistry Department - Sciences II - University of Geneva
30, Quai Ernest Ansermet - CH-1211 Geneva 4 (Switzerland)
© All rights reserved by Eric Vauthey and the University of Geneva
Design and code by Guillaume Duvanel