jp036250u_si_001.pdf (823.9 kB)
Phenothiazine−Pyrene Dyads: Photoinduced Charge Separation and Structural Relaxation in the CT State
journal contribution
posted on 2003-11-13, 00:00 authored by Nursel Acar, Jana Kurzawa, Norbert Fritz, André Stockmann, Ciprian Roman, Siegfried Schneider, Timothy ClarkElectron donor−acceptor systems with phenothiazine linked directly to pyrene exhibit a dual emission in
moderately and very polar solvents. Steady state and time-resolved fluorescence spectroscopy provide evidence
that the “blue” and “red” emission bands originate from different species. Fluorescence excitation spectra
show a similar appearance when the emission is monitored either in the red or blue spectral range, but they
are slightly shifted against each other. This suggests that different isomers exist with a distinctly different
photophysical behavior. Semiempirical (AM1/CI) molecular orbital calculations with a continuum solvent
treatment have been used to establish the geometry of the two nearly isoenergetic stereoisomers and to calculate
the properties of their excited Franck−Condon states. Geometry optimization of various excited states provides
evidence for different internal relaxation coordinates for the phenothiazine-localized S1 state and the charge
transfer state S6, on one hand, and the pyrene-localized S2 (S3) state, on the other hand. The relaxed geometries
in the excited states of both isomers represent mirror images with identical properties. The different
photophysical behavior of the two isomers is most likely caused by the different potential energy curves, or
more precisely speaking, by different location and/or heights of the barrier along the reaction coordinates
from the locally excited to the geometry-relaxed CT states.