103rd CEMS Colloquium

Title

Electronic Properties of Molecular Systems in Condensed Phases: Interplays of Interaction Fields and Charge/Spin Transport

Abstract

Electrons are in the center of physical properties of molecular substances; it is hard to find out any specific physical properties irrelevant to energy and momentum of electrons. As long as the lightest effective mass of electrons realized on the carbon-based electronic conjugation reflecting extraordinary short chemical bonds, the design of conjugated molecules with explicit boundaries as well as their spatial arrangements secure the widest dynamic range in the modulation of electron effective mass. The keys in the modulation of translational and spin momentums of electrons are thus in the design of boundaries: inter-molecular spaces and interactions [1].

Herein we will establish a novel concept of intermolecular electron conjugation by revisiting thoroughly longitude and latitude in the development of “conjugation” in chemistry. Starting from precise design of organic molecules with well-confined intermolecular spaces, thermal fluctuation in condensed phases of the molecular systems will be controlled perfectly by wide-range/spatial alignment of intermolecular interactions as well as leading-edge energy dissipation theory, resulting in extraordinarily high density-of-states (DOS) in the molecular substances. A series of unique assessment techniques of opto- and magneto-electronic properties of molecular materials is now positioned as a center complex of the current research project, pioneering the unprecedented properties of molecular systems with the new intermolecular electronic conjugation.

We approach to the establishment of the new intermolecular electronic conjugation and its unprecedented physical properties as well with the following strategies.

I) “X”-conjugation beyond σ- and/or π-conjugation: Since very the beginning of electron π-conjugation in 1890s [2], “π” electrons have been always the key player in electron conjugation where the electronic states were stabilized by electron delocalization within a molecular skeleton. After the long interval of 50 years, electron delocalization over σ-bonding was established as σ-conjugation. These electron conjugations have been defined in terms of angular momentum of electrons in atomic orbitals of elements. Now we have a question: Are there any other electron conjugations? Revisiting the first definition of conjugation as energy gain in electron delocalization, we must have a room for the establishment of new electron conjugation in intermolecular spaces. By filling out the space with electron/electronic states, the new intermolecular conjugation must be found [3]. To achieve the extraordinary high DOS in condensed phases of organic molecules and hence the systems with the new electronic conjugation, we will pave the following steps of (1) shrinking the intermolecular spaces to the limit (0.3 nm), (2) precise and programmed alignments of intermolecular interactions with extremely wide dynamic range suppressing/controlling thermal fluctuation of molecules, and (3) loading electrons/spins onto molecules and actualizing new electronic states which contribute the overall high DOS. The benchmark value of intermolecular spaces (distances) has been deduced by the precise analysis of intercorrelation between local charge carrier mobility in condensed phase of a variety of conjugated molecules and their static packing structures (figure).

Correlation between intermolecular distances in conjugated molecular crystals and local electron mobility assessed by measurements. Several series of molecules gave evidently the divergence from the intercorrelation.

II) Toward unprecedented properties of the new intramolecular conjugation: ubiquitous nature of massless electrons suggests potential of conjugate molecular aggregates with electron mobility reaching up to 500 cm²V^–1s^–1. We will demonstrate the potentials of the new intermolecular-conjugated molecular materials as leading alternatives for the future electronic materials. We will lead to a paradigm shift in the research of organics from flexible/printable materials to the prospective platform to achieve ultimate opto- and magneto-electronic properties.

III) The primary factor impacting the DOS is apparently the gravitational density of the condensed phases of the molecular materials. The gravitational density is, however, reduced dramatically in the condensed phases of asymmetric molecular systems: aggregation of homochiral molecules. The empirical rule of homochiral/racemic molecular condensation was proposed by Wallach [4], and has been successively verified by the molecular scientists [5]. The rule is now unwavering with concrete basis of the large number of validation samples as 129 cases at 1991, reaching up to over 1000 in these days. This rule implies that an use of homochiral aggregates is evidently disadvantageous to promote high DOS in molecular systems, and thus to reduce effective mass of electrons in the systems. The limited degree of molecular freedom in homochiral aggregates is in contrast favorable to suppress vibrational coupling of electrons: reorganization energy and/or polaron binding energy on a molecule and/or a molecular lattice. An interplay of the negative and positive effects on the electronic and gravitational “densities” may lead the homochiral molecular systems with the higher DOS but the lower gravitational density; “anti-Wallach” rule now we are addressing by probing local electron mobility with the TRMC technique. Several series of molecular aggregates have been shown significant divergence from the general intercorrelation in the figure, suggesting some exceptional homochiral molecules can exhibit highly conductive pathways with the larger inter-pathway distances. The divergence has been, however, observed only by 6~7 cases which is far from securing “anti-Wallach” rules in electronic conjugation. To address this case, we have developed a new spectroscopy system of TRMC under circularly polarized excitation, and some preliminary results are discussed herein.

[1] J. Ma, S. Seki, et al., Nat. Commun. 10, 102 (2019).
[2] Thiele, J. “Zur Kenntnis der ungesättigten Verbindungen” Ann. der Chem. 1899, 306, 87.
[3] H. Shinokubo et al., Nat. Commun. 7, 13620 (2016); D. Sakamaki, S. Seki et al., Angew. Chem. Int. Ed. 56, 16597-16601 (2017); T. Kubo et al., J. Am. Chem. Soc. 2016, 138, 4665-4672 (2016)
[4] O. Wallach, “Zur Kenntniss der Terpene und der atherischen Oele”, Ann. Chem. 238, 78 (1887).
[5] C. P. Brock, B. Schweizer, J. D. Dunitz, J. Am. Chem. Soc. 113, 9811 (1991).