Wire-like charge transport at near constant bridge energy through fluorene oligomers

Abstract

The study of photoinitiated electron transfer in donor-bridge-acceptor molecules has helped elucidate the distance dependence of electron transfer rates and behavior of various electron transfer mechanisms. In all reported cases, the energies of the bridge electronic states involved in the electron transfer change dramatically as the length of the bridge is varied. We report here, in contrast, an instance in which the length of the bridge, and therefore the distance over which the electron is transferred, can be varied without significantly changing the energies of the relevant bridge states. A series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene (FL(n)) bridges, and perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidation of PTZ via the FL(n) bridge. In toluene, the rate constants for both charge separation and recombination as well as the energy levels of the relevant FL(n)(+.) bridge states for n = 1-4 are only weakly distance dependent. After the initial photo-generation of (1)(PTZ(+.)-FL(n)-PDI(-.)), radical pair intersystem crossing results in formation of (3)(PTZ(+.)-FL(n)-PDI(-.)) that recombines to give (3.)PDI. The dependence of the (3.)PDI yield on an applied magnetic field shows a resonance, which gives the singlet-triplet splitting, 2J, of the radical ion pair. The magnitude of 2J directly monitors the contribution of coherent charge transfer (superexchange) to the overall electron transfer rate. These data show that charge recombination through FL(n) is dominated by incoherent hopping at long distances.

Fig. 1.

Structures of the compounds examined in this study.

Fig. 2.

Highest occupied molecular orbitals of for n = 2–4.

Fig. 3.

Absorbance spectra of PTZ–FL_n–PDI in toluene.

Fig. 4.

Transient absorption spectra of 2 after a 530-nm 130-fs laser excitation pulse.

Fig. 5.

Semilogarithmic plot of k_CS vs. distance (A) and plot of k_CS vs. 1/distance (B) for photoinduced charge separation in PTZ–FL_n–PDI in toluene. Error bars show ±1 standard deviation.

Fig. 6.

Semilogarithmic plot of rate constant vs. distance for charge recombination of PTZ^+·–FL_n–PDI^-· in toluene. Error bars show ± 1 standard deviation.

Fig. 7.

Plots of the relative yield of ^3*PDI vs. magnetic field strength for the indicated molecules in toluene.

Fig. 8.

Semilogarithmic plot of the spin–spin exchange interaction, 2J, vs. r_DA.