Femtosecond dynamics in directly linked pyrenyl donor–acceptor systems: orbital control of optical charge transfer in the excited state

The ultrafast charge transfer (CT) dynamics in two directly linked pyrenyl donor–acceptor systems has been studied using femtosecond broadband pump–probe spectroscopy. This method allows to capture complex chemical dynamics by measuring the time-resolved spectral evolution of transient absorption and stimulated emission induced by femtosecond optical pumping. After electronic relaxation, an optical CT transition between two excited states was found in both systems. Although the excited state CT occurs adiabatically significant differences in the electronic coupling are deduced from the band intensities. The results are discussed in terms of an adiabatic state model and compared with predictions from orbital topology arguments.