Manipulation of Charge-Transfer States by Molecular Design: Perspective from “Dynamic Exciton”

Abstract

Conspectus“Dynamic exciton”, an umbrella term concept in photochemistry, plays an important role in nature, science, and technology, especially in photoinduced and electrically induced electron transfer (ET) between a donor (D) and an acceptor (A). Typically, an exciton in molecular D–A systems is considered a locally excited (LE) state of a donor (D*) or an acceptor (A*) molecule, but let us extend the terminology of “exciton” to an integrated class of LE, charge-transfer (CT), and charge-separated (CS) states. The degree of CT (0–100%), spin multiplicity, and D–A interaction (i.e., electronic coupling) are pivotal factors in the “exciton”. Another important aspect of the “exciton” is strongly related to the “dynamic” aspect of the “exciton” by movement of atomic nuclei (i.e., vibration, rotation, and fluctuation) and their collective motions controlling behaviors of electrons and spins by the passage of time. The concept of “dynamic exciton” should cover a wide variety of the photochemical phenomena that are all essential for the energy conversion devices and processes including various kinds of living systems. In these, a huge amount of the nuclear motional modes may cooperatively be entangled to electronic orbitals. Thus, the idea behind “dynamic exciton” includes usage of the cooperation between nuclear motions and the spin–orbital, as electron–phonon coupling for innovative designs of the energy conversion materials and assemblies. For this, it is particularly important to examine how this wide variety of the motions plays roles on electronic couplings, intermediate geometries and mobilities, exciton energies, CT characters, and so on. We draw researchers’ attention to this aspect of the vibronic effect, like the entropy role by collective movement of side chains of the conjugated polymer to modulate the electronic coupling time-dependently at the charge delocalization and dissociation, which is reinforced three-dimensionally at the D/A domain interface of organic photovoltaics (OPVs). In addition, to realize the ultimate high-performance OPVs, the voltage loss must be diminished; i.e., the energy difference between the band gap and the open circuit voltage is caused in part by interfacial charge recombination via vibrational relaxation. Attaining efficient excited-state migration with prevention of the vibrational relaxation at the D/A interface would possibly overcome the voltage loss issues at the primary CT event, when we understand the roles of low-frequency disorder movements by the phonon on the vibrational relaxation in the solid state. Importantly, conversion of the excited state-to-CS state in organic OPVs is an opposite process to that of the CS state-to-excited state in organic light-emitting diodes (OLEDs). Specifically, intramolecular D–A linked systems and intermolecular D–A systems hold a prominent position in both OPVs and OLEDs, and an intrinsic similarity is seen in the mutual interplay among LE, CT, and CS states of such systems facilitated by the dynamic effects. These facts encourage us to work on the manipulation of CT states by logical molecular design, achieving efficient energy conversion in OPVs and OLEDs. Along with some examples of OPVs and OLEDs, here we introduce the “dynamic exciton” as a comprehensive, CT photochemistry in molecular D–A systems, eventually making innovations in electronics, energy, medicine/health care, and functional materials.