A theoretical study of the effect of electron delocalization on electron transfer reactions in small organic molecules

Abstract

The ionization potential and electron attachment energy are fundamental properties of molecules and is core to a range of physical processes such as those in photovoltaics, electrochemistry, spectroscopy, etc. Ionization energies are therefore properties worthwhile to have delicate control over for the purpose of finetuning molecules for specific goals. Here we have brought it to light that there are relationships between ionization energies and electron populations of small organic electron donors and acceptors. Electron population data was gathered with FALDI. For the electron donors Ph-XR2, where X = N or P and R = H, Me, Et or Pr, it was found that as the delocalization between phenyl and substituted group increases, the ionization potential decreases (electron removal becomes easier). For the acceptors Ph-RNO, where R = none, CH2, C2H4, or C2H2, as delocalization between phenyl and substituent increases electron attachment energy decreases further (electron attachment became more spontaneous). Furthermore, for these acceptor molecules, as phenyl ring electron population increases, electron attachment becomes less spontaneous. For acceptor molecules Ph-X, where X = F, Cl or Br, as the phenyl electron population increased, electron attachment became easier. All these relationships were near perfectly linear. This is the first time, to our knowledge, that such a link is found between ionization energies and electron populations. Also, physical phenomena such as conjugation, hyperconjugation, lone-pair electron resonance and Bent’s rule could be recovered and quantified with FALDI. Lastly, four donor-acceptor interfaces were built from donors Ph-NH2 and Ph-NPr2 and from acceptors Ph-NO and Ph-C2H4NO, to simulate the heterojunction in an organic photovoltaics. Marcus theory was implemented to successfully calculate forward (in presence and absence of light) and reverse charge transfer rates which are some of the factors that influence the efficiency of a solar cell. The quickest rate was two orders of magnitude faster than second best at 8.40E+13 e-/s for interface [Ph-NH2–Ph-C2H4NO]. The rates were decomposed into the coupling potential and thermodynamic driving force to find the origin of such a vast difference. This revealed that a greater transition dipole moment (on which the coupling potential is dependent) of the electronic transition caused this interface to completely excel in charge transfer relative to the others. FALDI analyses were performed on the interfaces to attempt to complete the chain between the chemistry of the molecules and the rate constants – in this case transition moment, therefore. There is good indication that the transition moment is linked to the FALDI terms, suggesting that all information about the transition moment is contained in them. FALDI therefore shows great potential for a new way of calculating the transition moment.