Electron-Deficient Photosensitizers for Initiating Hole Transfer in Bioinspired Molecular Electrets
Molecular-level control of charge transfer is essential for organic electronics and solar energy conversion, as well as for comprehending a wide range of biological processes. Photoconversion efficiency is mainly hindered by interfacial charge recombination. In nature, charge recombination is suppressed by incorporating molecular electrets, systems with ordered electric dipoles. Local electric fields originating from molecular dipoles can thus aid in overcoming charge recombination while promoting the desired forward charge transfer. Protein alpha helices present the best example for natural molecular electrets, that is, they have intrinsic dipoles originating from the ordered amide and hydrogen bonds.
A principal challenge of polypeptide charge transfer systems composed of native amino acids is the inherent distance limitation for efficient transduction of electrons and holes. Along their backbones and hydrogen bonds, protein -helices mediate electron transfer via tunneling, the rate constants of which, ket, fall off exponentially with an increase in the length of the electron-transfer pathways, ret, i.e. ket∝exp(−β ret). Specifically, for protein -helices, β is about 1.3 Å-1. The inherent presence of competing nanosecond processes, such as fluorescence, internal conversion and intersystem crossing, therefore, places a practical limit of about 2 nm for attaining photoinduced charge transfer with acceptable efficiency in such bimolecular structures.
Because of the importance of long-ranger charge transfer via hole hopping, we focus on the development of suitable electron deficient sensitizer for initiation hole transfer. Hence, the photosensitizer must absorb away from the electron donor in the visible region of the spectrum, and have a carboxylic acid (or amine) functionality for covalent attachment to molecular electrets that are polypeptides themselves.
Therefore, my initial studies focused on characterization of non-native aromatic beta-amino acid residues, derived from anthranilic acid, as building blocks of hole-transfer molecular electrets. The aromatic moieties provide the means for attaining long-range hole transfer. Chemical modification of the distal position of the anthranilamides allows for adjusting the reduction potentials of oxidation over a range of one volt. My major contribution to this work was discovering what makes oxidized N-Acylanthranilamides stable. By correlating the electrochemical data with the spin-density distribution of the radical cation, I determined that for radical cations of the anthranilamide to be stable 1) their reduction potential must be at about 1.5 V or below vs. S.C.E., and 2) the electron spin density should not extend over the C-terminal amide. Indeed, this proved valuable for increasing our library of building blocks for molecular electrets that have practical feasibility.
A significant part of my studies encompass various aspects of synthesis of electron deficient photosensitizers for initiating the hole transfer in the anthranilamides, focusing on pyrene, nitropyrene and diketopyrollopyrolle chromophores. From my pyrene work I was able to shed light on the effect of the orientation of amide bonds on the electronic properties of polycyclic aromatic molecules. Incorporating an electron deficient diketopyrollopyrollo chromophore as an electron acceptor in donor-acceptor conjugates, where we use anthranilamides as donors, allowed us to determine the two important requirements for enhancing dipole effects on charge transfer and how to harness them. 1) The electric dipole has to be as close as possible to the charge-transfer systems. Indeed, incorporating the dipoles in the components of the charge-transfer systems presents the ideal case. 2) The media polarity should be lowered. While non-polar media impedes charge-separation processes, it also enhances the permiation of the dipole-generated electric fields. We demonstrated that the later of those two opposing effects can prevail. Changing the solvent from acetonitrile to toluene leads to a six-fold increase in charge separation rates when the electron transfers along the dipole. The same change in the solvent polarity completely shuts down the charge transfer when the electron is pushed to move against the dipole. These findings show rectification of photoinduced charge separation that is practically infinity and have important implications about the utility of dipole effects for electronic and energy materials and devices.
The most significant contributions from my doctoral research include: (1) the determination of the effects of location and strength of electron donating substituents on the stability of the radical cations of the non-native amino acids for building molecular electrets; (2) demonstrating that amide bonds are not just linker but they can significantly affect the properties of the moieties they are attached to; and (3) developing of electron-deficient and conjugating them with anthtanilamides, which showed us to demonstrate dramatic dipole effects on charge transfer.