Intramolecular Electronic Couplings in Class II/III Organic Mixed-Valence Systems of Bis(1,4-dimethoxybenzene)

Theoretical analysis of the absorption spectrum, electronic structure, excitation, and intramolecular electron transfer of D-A'-π-A porphyrin dyes for dye-sensitized solar cells

A series of porphyrin dyes with D-A'-π-A structure were designed and theoretically investigated by DFT and TD-DFT methods. Different electron-withdrawing auxiliary acceptor units were introduced into the dye molecule skeleton to shed light on how the type and position of auxiliary acceptors influence the photoelectric performance of the dyes. It was found that the introduction of electron-withdrawing units, BTD, TPZ, BTZ, PP and DPPZ, between the Zn-porphyrin core and the anchoring group had a significantly positive influence on the performance of the dye molecules. Also, more appropriate electron distribution in the molecular orbital and the lower HOMO-LUMO energy gap, more extensive absorption coverage, higher light-harvesting efficiency, lower orbital overlap, and more effective long-range intramolecular electron transfer (IET) process can be achieved as compared to the reference dye. Among these five electron-withdrawing units, BTD and TPZ had the effect leading to the greatest improvement and therefore, are the best candidates for auxiliary acceptors. The calculated results indicated that the longitudinal π-conjugation of the electron-withdrawing unit also had an obvious effect on the performance of the dye molecule, and NTD is expected to be a more effective auxiliary acceptor than BTD. The effects of the relative positions of the auxiliary acceptors in the dye molecular skeleton were also investigated. A comparative study of AX1-3 and AA1-3 showed that the introduction of BTD, TPZ and BTZ units between the donor part and the Zn-porphyrin core may have a negative impact on the performance of the dyes. Our studies are expected to provide new insight for the design and screening of high-efficiency porphyrin dyes for DSSCs applications.

Key Role of End-Capping Groups in Optoelectronic Properties of Poly-p-phenylene Cation Radicals

Poly-p-phenylenes (PPs) are prototype systems for understanding the charge transport in π-conjugated polymers. In a combined computational and experimental study, we demonstrate that the smooth evolution of redox and optoelectronic properties of PP cation radicals toward the polymeric limit can be significantly altered by electron-donating iso-alkyl and iso-alkoxy end-capping groups. A multiparabolic model (MPM) developed and validated here rationalizes this unexpected effect by interplay of the two modes of hole stabilization: due to the framework of equivalent p-phenylene units and due to the electron-donating end-capping groups. A symmetric, bell-shaped hole in unsubstituted PPs becomes either slightly skewed and shifted toward an end of the molecule in iso-alkyl-capped PPs or highly deformed and concentrated on a terminal unit in PPs with strongly electron-donating iso-alkoxy capping groups. The MPM shows that the observed linear 1/n evolution of the PP cation radical properties toward the polymer limit originates from the hole stabilization due to the growing chain of p-phenylene units, while shifting of the hole toward electron-donating end-capping groups leads to early breakdown of these 1/n dependencies. These insights, along with the readily applicable and flexible multistate parabolic model, can guide studies of complex donor-spacer-acceptor systems and doped molecular wires to aid the design of the next generation materials for long-range charge transport and photovoltaic applications.