Eisuke Ohta

Redox-responsive molecular helices with highly condensed π -clouds

Helices have long attracted the attention of chemists, both for their inherent chiral structure and their potential for applications such as the separation of chiral compounds or the construction of molecular machines. As a result of steric forces, polymeric o-phenylenes adopt a tight helical conformation in which the densely packed phenylene units create a highly condensed π-cloud. Here, we show an oligomeric o-phenylene that undergoes a redox-responsive dynamic motion. In solution, the helices undergo a rapid inversion. During crystallization, however, a chiral symmetry-breaking phenomenon is observed in which each crystal contains only one enantiomeric form. Crystals of both handedness are obtained, but in a non-racemic mixture. Furthermore, in solution, the dynamic motion of the helical oligomer is dramatically suppressed by one-electron oxidation. X-ray crystallography of both the neutral and oxidized forms indicated that a hole, generated upon oxidation, is shared by the repeating o-phenylene units. This enables conformational locking of the helix, and represents a long-lasting chiroptical memory.

Remarkable effects of terminal groups and solvents on helical folding of o-phenylene oligomers.

Although o-phenylene oligomers (OP(n)R) made of dimethoxyphenylene units are thought to be intrinsically dynamic due to π-electronic repulsion, we show that they fold into a regular helical geometry in CH(3)CN when they carry terminal groups such as CH(3), CH(2)OH, Br, CO(2)Bn, and NO(2). We evaluated their helical inversion kinetics via optical resolution of long-chain oligomers (e.g. 16- and 24-mers) by chiral HPLC. OP(24)Br at 298 K shows a half-life for the optical activity of 5.5 h in CH(3)OH/water (7/3 v/v) and requires 34 h for complete racemization. The perfectly folded helical conformers of OP(n)R, unlike their imperfectly folded ones, are devoid of extended π-conjugation and show a cyclic voltammogram featuring reversible multistep oxidation waves.

Butane-1,4-diyl dications stabilized by steric factors: electrochiroptical response systems based on reversible interconversion between dihydro[5]helicene-type electron acceptors and electron-donating 1,1′-binaphthyls

Incorporation in the dihydro[5]helicene framework prevents deprotonation of the title dications by steric factors, thus allowing their isolation as deeply colored stable salts. Based on the reversible interconversion with the electron-donating binaphthylic diolefins, they constitute a new class of electrochromic systems, in which C-C bond making/breaking is accompanied by two-electron transfer. Optically pure (R)-binaphthylic donors are interconvertible with the 1,4-dications with the R,R-configuration. The very large molar ellipticity makes it possible for them to be used as electrochiroptical response systems, by which the electrochemical input is transduced into two spectral outputs, i.e. UV-Vis and circular dichroism. Structurally related push-pull-type bis(quinonemethide)s also exhibit a similar multi-output electrochemical response.

Room-Temperature Phosphorescence of Crystalline Metal-Free Organoboron Complex.

The photoluminescence (PL) properties of a metal-free organoboron complex, bis(4-iodobenzoyl)methanatoboron difluoride (1BF2 ), were elucidated. At room temperature, 1BF2 emits blue fluorescence (FL) in nBuCl upon photoexcitation. In contrast, crystals of 1BF2 emit green PL comprised of FL and phosphorescence (PH). The room-temperature PH of crystalline 1BF2 is a consequence of 1) suppression of thermal deactivation of the S1 and T1 excited states and 2) enhancement of intersystem crossing (ISC) from the S1 to T2 or T1 . The results of X-ray crystallographic and theoretical studies supported the proposal that the former (1) is a result of intermolecular interactions caused by π-stacking in the rigid crystal packing structure of 1BF2 . The latter (2) is an effect of not only the heavy-atom effect of iodine, but also the continuous π-stacking alignment of 1BF2 molecules in crystals, which leads to a forbidden S1 →S0 transition and a small energy gap between the S1 and T2 or T1 .

Fluorescence Behavior Associated with a Possible Intercolumnar Charge- transfer Interaction in the Crystalline State of a Dyad Consisting of Mesitylene and 1,4-Dicyano-2-methylnaphthalene Subunits

ABSTRACT : Fluorescence (FL) properties of a novel donor–acceptor dyad, comprised of mesitylene and 1,4-dicyano-2-methylnaphthalene (DCMN) subunits connected by FL,Can ether linkage, were elucidated. The dyad in cyclohexane exhibits FL arising from an intramolecular exciplex. In the crystalline state, the dyad does not emit light from intra- and inter-molecular exciplexes but rather displays FL that is nearly equivalent to that of 2-methoxymethyl-substituted DCMN. However, the emission spectrum of the crystalline dyad contains a 361shoulder in the 0.39long wavelength region, suggesting that weak intercolumnar charge-transfer interactions take place between columns consisting of the mesitylene and DCMN subunits. An exciplex is an electronically-excited state complex comprised of two different molecules. 1 Mechanisms for inter- 2 and intra-molecular 3 formations and the photo-physical 4 and -chemical properties 5 of a number of exciplexes in solution have been studied. On the other hand, exciplexes in the crystalline states have gained much less attention. In a previous effort, we studied the fluorescence (FL) behavior of dyads containing a 1,4-dicyano-2-methylnaphthalene (DCMN) electron acceptor tethered to 4-methyl-, 4-methoxy-,

Unique Orbital Interactions in the Ground and Electronically Excited States of Biradicals Brought about by the Existence of “Twisted π-Space”

The six-membered cyclic 1,4-radical cation 2•+ forms a “twisted π-space” due to noncoplanarity of the cumyl radical 3• and cumyl cation 3+ originating from a chair conformation of the central cyclohexane ring system. The results of substituent effects on absorption wavelengths of 2•+ indicate that both through-space (TS) and through-bond (TB) orbital interactions operate in a “twisted π-space” of 2•+, resulting in the theoretically maximal orbital interactions between 3• and 3+. An operation of similar TS and TB orbital interactions is also suggested in the excited-state biradical 12••* by the results of substituent effects on thermoluminescence wavelengths.

Theoretical investigation on structure and electronic properties of Si-bridged π-conjugated systems

Density functional theory calculations were performed on silylene-bridged cyclic tri- and tetrathienylenes, 1b and 2b, respectively. The results show that in comparison to their respective unbridged counterparts 1a and 2a, 1b and 2b have lower LUMO energies but similar reorganization energies for single electron transfer between 1b,2b and their respective radical anions 1b•–,2b•–. The observations suggest that silylene bridging of π-conjugated systems can serve as a novel strategy for designing materials of n-type organic semiconductor.