Bridge-Length- and Solvent-Dependent Charge Separation and Recombination Processes in Donor–Bridge–Acceptor Molecules

Abstract

The photoinduced intramolecular charge separation (CS) and charge recombination (CR) phenomena in a series of donor–bridge–acceptor (D–B–A) molecules are intensively investigated as a means of understanding electron transport through the π-B. Pyrene (Pyr) and triarylamine (TAA) moieties connected via phenylene Bs of various lengths are studied because their CS and CR behaviors can be readily monitored in real time by femtosecond transient absorption (fs-TA) spectroscopy. By combining the steady-state and fs-TA spectroscopic measurements in a variety of solvents together with chemical calculations, the parameters that govern the CS behaviors of these dyads were obtained, such as the solvent effects on free energy and the B-length-dependent electronic coupling (V DA) between D and A. We observed the sharp switch of the CS behavior with the increase of the solvent polarity and B-linker lengths. Furthermore, in the case of the shortest distance between D and A when the electron coupling is sufficiently large, we observed that the CS phenomenon occurs even in low-polar solvents. Upon increasing the length of B, CS occurs only in strong polar solvents. The distance-dependent decay constant of the CS rate is determined as ∼0.53 Å–1, indicating that CS is governed by superexchange tunneling interactions. The CS rate constants are also approximately estimated using Marcus electron transfer theory, and the results imply that the V DA value is the key factor dominating the CS rate, while the facile rotation of the phenylene B is important for modulating the lifetime of the charge-separated state in these D–B–A dyads. These results shed light on the practical strategy for obtaining a high CS efficiency with a long-lived CS state in TAA–B–Pyr derivatives.

Long‐Lived Charge Separated States in Anthraquinone‐Phenothiazine Dyads: Synthesis and Study of the Photophysical Property by Using Transient Optical and Magnetic Resonance Spectroscopies

“…For instance, decreasing the electronic coupling matrix elements V is beneficial for longer CS state lifetime. From a molecular structural point of view, small V values can be obtained by using long and saturated linkers between the donor and acceptor [1b,9] . For instance, norbornane or bicyclo[2.2.2]octane have been used as this kind of linker.…”

Observation of Long-Lived Charge-Separated States in Anthraquinone–Phenothiazine Electron Donor–Acceptor Dyads: Transient Optical and Electron Paramagnetic Resonance Spectroscopic Studies

“…Charge separation (CS) and intersystem crossing (ISC) in electron donor–acceptor dyads have been the major topics in photochemistry for decades. − The study of CS in electron donor–acceptor dyads was usually focused on attaining long-lived CS states to mimic the natural photosynthetic center and for photocatalysis studies. ,− A long CS state lifetime is usually achieved by reducing the electronic coupling matrix elements between the CS state and the ground state ( V DA ). ,, Very often, it is stated as electronic coupling between the donor and acceptor although this is inappropriate. The method is to use a long, rigid, saturated linker between the donor and acceptor although this makes the synthesis difficult.…”

Molecular torsion controls the excited state relaxation pathways of multibranched tetraphenylpyrazines: effect of substitution of morpholine vs. phenoxazine

“…16,17 Indeed, their photophysical properties significantly depend on the various factors including the electronic nature of the donor and acceptor, planarity of the bridge, and nature of the ground and excited states. 16–22…”