Increased Charge Separation Rates with Increasing Donor–Acceptor Distance in Molecular Triads: The Effect of Solvent Polarity

Abstract

Distance-dependent electron transfer in donor–spacer–acceptor systems is accepted to occur via two distinct mechanisms, that is, by coherent superexchange or incoherent hopping. In general, the rate of electron transfer (k ET) decreases with increasing donor–acceptor distances, irrespective of the actual mechanism being responsible for the process. However, recently Wenger and his group showed that in the frame of the superexchange mechanism electron-transfer rates can pass a maximum when increasing the transfer distance. This manuscript presents an investigation of the forward electron transfer in a series of donor (N-methylphenothiazine)–photocenter (Ru­(II) bis­(terpyridine) complex)–acceptor (N-methylfulleropyrrolidine) triads that reveals the control of the electron-transfer rates by solvent variation to an extent that in acetonitrile an increasing electron-transfer rate is observed with increasing donor–acceptor distance, while in dichloromethane an increase in the separation causes the electron transfer rate to drop. This behavior is qualitatively rationalized based on a recently introduced model. Nonetheless, the quantitative mismatch between the results presented here and the theory indicates that nonexponential distance-dependent couplings will have to be considered in extending the theory.

Superexchange in the fast lane – intramolecular electron transfer in a molecular triad occurs by conformationally gated superexchange

“…For the first electron transfer (PTZ-3 P * -C 60 →PTZ from the neutral, optimized molecular structures. 2,3 For ET1, due to the same electron donor, electron acceptor and the same donor-acceptor distance, identical reorganization energy will be produced which was already shown in a previous study. 12 The calculated reorganization energy for ET2 as a function of temperature is summarized in Table S2-4.…”

“…The synthesis, steady-state emission spectra, electrochemical properties and photoinduced dynamics of 1-3 at room temperature have been reported elsewhere. [1][2][3] Time-Resolved Spectroscopy. Femtosecond (fs) transient absorption (TA) spectra were collected by using a previously reported home-built pump-probe laser system which is based on an amplified Ti:…”

“…Relaxation processes for 1-3 which refer to the model proposed at room temperature. 1,2 Terpyridine (tpy) ligand connected with N-methylphenothiazine (PTZ) and fullerene (C 60 ) are named as PTZ-tpy and tpy-C 60 , respectively. Excitation of Ru(tpy) 2 photosensitizer generates two differently distributed MLCT states, i.e.…”

“…E 00 is taken to be 2.07 eV. 2 For the second electron transfer step the radical pairs are C 60 •-/ PTZ •+ .…”

“…Plots of ln(k ET1 •T 1/2 ) vs. 1/T for the first electron transfer process ET1 in 1 (red square), 2 (blue triangle) and 3 (black circle) with the corresponding linear fit according to eqn (2)…”