Halogen-Bond Assisted Photoinduced Electron Transfer.

Bogdan Dereka,Eric Vauthey,Arnulf Rosspeintner,Ina Fureraj

doi:10.3390/molecules24234361

Abstract

The formation of a halogen-bond (XB) complex in the excited state was recently reported with a quadrupolar acceptor–donor–acceptor dye in two iodine-based liquids (J. Phys. Chem. Lett. 2017, 8, 3927–3932). The ultrafast decay of this excited complex to the ground state was ascribed to an electron transfer quenching by the XB donors. We examined the mechanism of this process by investigating the quenching dynamics of the dye in the S1 state using the same two iodo-compounds diluted in inert solvents. The results were compared with those obtained with a non-halogenated electron acceptor, fumaronitrile. Whereas quenching by fumaronitrile was found to be diffusion controlled, that by the two XB compounds is slower, despite a larger driving force for electron transfer. A Smoluchowski–Collins–Kimball analysis of the excited-state population decays reveals that both the intrinsic quenching rate constant and the quenching radius are significantly smaller with the XB compounds. These results point to much stronger orientational constraint for quenching with the XB compounds, indicating that electron transfer occurs upon formation of the halogen bond.

Highlights

Halogen bonds (XB) have been known of for a long time, their relevance to chemistry, biology and material sciences was realised relatively recently [1,2,3,4,5,6,7,8,9]
We recently reported on the observation of XB formation between an acceptor–donor–acceptor molecule (ADA, Figure 1) in the S1 electronic excited state and two iodine-based XB donors, perfluorinated isopropyl iodide (HFIP) and perfluorinated iodobenzene (IFB, Figure 1), acting as solvents, using time-resolved IR (TRIR) spectroscopy [33]
As discussed earlier [33,34], the larger shift of the fluorescence spectrum observed by going from the medium polar CHCl3 to the highly polar BCN reflects the dipolar character of the equilibrium S1 state as a consequence of excited-state symmetry breaking (ES-SB)

Summary

Introduction

Halogen bonds (XB) have been known of for a long time, their relevance to chemistry, biology and material sciences was realised relatively recently [1,2,3,4,5,6,7,8,9]. The halogen bond is often called the hydrophobic analogue of the hydrogen bond [5,10] It originates from the attractive interaction between the electron-rich nucleophilic part of the XB accepting molecule and the so-called σ-hole;. Sharing some similarities with H-bonds, halogen bonds exhibit distinct differences: they are hydrophobic and more directional than hydrogen bonds [12], and their strength and length can be tuned by changing the nature of the halogen [13]. The exploitation of these features appears to be a powerful tool in various areas of molecular sciences, including supramolecular chemistry, catalysis and crystal engineering. X-bonding interactions in the electronic excited state were considered in the context of solid-state, light-emitting materials [14,15,16]

Results

Discussion

Conclusion