| The relationship between the topology of electronic wavefunctions (orbitals) and optical properties in artificial multichromophores was investigated both theoretically and experimentally in our group.
Early theoretical studies on 1e24, 1e34 and 1e23 as well as steady state fluorescence and absorption measurements suggested that the electron transfer matrix element HLE,CT should be substantially different in these compounds because of the acceptor orbital topology which is determined by the substitution scheme. Indeed, 1e23 on the one hand and 1e24, 1e34 on the other hand show vastly different dynamics on the fs time scale. |
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The similarity of 1e24 and 1e34 indicates that sterical effects due to bulky substituents around an interaromatic s-bond have practically no influence on the initial CT dynamics. Instead, the dynamics can be profoundly changed by varying the position of an electron-withdrawing (or electron-donating) substituent on one of the aromatic subsystems. Such changes alter the orbital topology and thereby the overlap between the relevant D/A orbitals. As a result HLE,CT is altered and the CT process can be switched from the adiabatic to the nonadiabatic regime.
The results were a great triumph because they confirmed the predictions that were made based on semiempirical calculations in a publication in Chem. Phys. Lett. three years earlier.
Although the two-color pump-probe experiments on the pyrenyl biphenyl esters highlighted the strong dependence of the electronic coupling upon the substitution scheme the data analysis only allowed a kinetic interpretation as the nature and origin of the absorption bands could not be studied.
A major advancement was achieved by measuring the entire pump-probe spectrum in the near-UV to near IR-range. For the understanding of the charge transfer in the pyrenyl donor-acceptor this method provided new insight. In the directly linked D/A systems 1e4 and 1e3 we measured the transient absorption spectra of the charge transfer states.
Optical excitation in 1e4 and 1e3 populates the S2 state of pyrene which is properly described by the HOMO-LUMO transition. The S2 state is strongly coupled to a charge transfer state which represents a one-electron transition from the HOMO of pyrene into the LUMO of the benzoic acid ester. After relaxing into the CT state a Reverse Charge Transfer transition into a higher excited singlet is observed.
In a simple two state approximation, the oscillator strength for this transition is proportional to the square of the electronic D/A coupling HLE,CT, thereby largely determined by the topology of the donor and acceptor orbitals.
