Shogo Amemori

Polymer Phase‐Transition Behavior Driven by a Charge‐Transfer Interaction

Charge-transfer (CT) interactions are intermolecular interactions between p-electron-rich (donor molecules) and pelectron-deficient species (acceptor molecules). The high specificity of CT interactions enables the arrangement between a donor and an acceptor to be controlled, and the resulting CT complex shows a characteristic absorption band in the visible region; this absorption band provides information on the association of supramolecular complexes. Therefore, CT interactions are among the most powerful tools for the design of supramolecular complexes, such as organic crystalline materials with superconductivity or conductivity, low-molecular-weight organic gelators, preorganized building blocks for rotaxane, and supramolecular polymers. In many cases, the supramolecular CT complexes can be collapsed readily by heating, since a CT interaction provides a bond of modest strength unless a cavity, such as that of cucurbit[8]uril, is used to stabilize the CT complex. Herein, we demonstrate the precise and facile control of the lower critical solution temperature (LCST) of a polymer solution, that is, its phase-transition behavior, by the use of a CT interaction. The well-known phase-transition behavior associated with the LCST of a polymer solution occurs as a result of primitive molecular recognition between the polymer chains and solvent molecules (e.g., H2O). It has attracted broad interest with respect to the development of stimuli-sensitive materials because the solubility of the polymer chains changes drastically upon heating. However, one of the major drawbacks of this phenomenon is the restriction imposed by the required conditions (temperature, solvent, or pressure); as result, stimuli-sensitive materials are limited to those based on intrinsic LCST polymers, such as poly(N-isopropylamide) (PNIPAM) and poly(ethylene glycol) (PEG). Sophisticated supramolecular control of the LCST has been paid much attention, because such studies can uncover not only a fundamental perspective of the phenomenon but also potential applications in smart materials. As an outstanding example, Ritter and co-workers reported that a polymer bearing adamantyl moieties showed LCST behavior in the presence of methylated b-cyclodextrins (b-CD) in water owing to the dissociation of b-CD from the adamantyl group in the polymer chain upon heating and the subsequent association of adamantyl groups through a hydrophobic interaction. We also demonstrated readily adjustable LCST behavior based on hydrogen bonding between a polymer bearing urea units and an alcohol as an “effector” for polymer solubility. In both cases, the dissociation of supramolecular complexes at an increased temperature triggered a drastic change in solubility of the polymers. These findings inspired us to explore the applicability of other weak intermolecular interactions between polymers and effectors to clarify the molecular design of LCST polymer chains and external effectors. Herein, we demonstrate the first example of LCST behavior based on a CT interaction as the driving force (Figure 1). As the result of the appearance of a CT band in the UV/Vis absorption spectrum, a quantitative evaluation of the relationship between LCST behavior and the formation of supramolecular complexes was possible. Previously, the distinction between polymer–polymer interactions and polymer–effector (solvent) interactions was considered problematic in the common case of LCST behavior driven by hydrogen bonding.

Near-Infrared-to-Visible Photon Upconversion Sensitized by a Metal Complex with Spin-Forbidden yet Strong S0–T1 Absorption

Near-infrared (NIR)-to-visible (vis) photon upconversion (UC) is useful for various applications; however, it remains challenging in triplet-triplet annihilation-based UC, mainly due to the energy loss during the S1-to-T1 intersystem crossing (ISC) of molecular sensitizers. In this work, we circumvent this energy loss by employing a sensitizer with direct S0-to-T1 absorption in the NIR region. A mixed solution of an osmium complex having a strong S0-T1 absorption and rubrene emitter upconverts NIR light (λ = 938 nm) to visible light (λ = 570 nm). Sensitizer-doped emitter nanoparticles are prepared by re-precipitation and dispersed into an oxygen-barrier polymer. The obtained composite film shows a stable NIR-to-vis UC emission based on triplet energy migration (TEM), even in air. A high UC quantum yield of 3.1% is observed for this TEM-UC system, expanding the scope of molecular sensitizers for NIR-to-vis UC.

Increased vis-to-UV upconversion performance by energy level matching between a TADF donor and high triplet energy acceptors

A question at issue in triplet–triplet annihilation-based photon upconversion (TTA-UC) has been how to maximize the anti-Stokes shift, which requires minimization of the energy losses in intersystem crossing (ISC) of donors and consequent energy transfer (TTET) to acceptors. This is resolved by the energy level matching between a thermally activated delayed fluorescence (TADF) sensitizer and emitters with the highest triplet and singlet energy levels.

Metallonaphthalocyanines as triplet sensitizers for near-infrared photon upconversion beyond 850 nm

In triplet-triplet annihilation-based photon upconversion (TTA-UC), the utilization of near-infrared (NIR) light with a wavelength longer than 850 nm remains an outstanding issue. We realized this by employing metallonaphthalocyanines as triplet sensitizers; upon excitation of NIR light (856 nm), upconverted emission was observed in the visible range with remarkable photostability.

Near infrared-to-blue photon upconversion by exploiting direct S–T absorption of a molecular sensitizer

Triplet–triplet annihilation-based photon upconversion (TTA-UC) from NIR to blue light remains a great challenge. Here, we employ a direct singlet-to-triplet (S–T) excitation to circumvent an energy loss associated with the intersystem crossing (ISC) of triplet sensitizers. The TTA-UC based on the S–T absorption (STUC) allowed an efficient NIR (λ = 724 nm)-to-blue (λ = 462 nm) upconversion with a large anti-Stokes shift of 0.97 eV.

Gel thermoresponsiveness driven by switching of the charge-transfer interaction

A novel gel LCST system consisting of a pyrene containing acrylate network polymer and external effectors is demonstrated. The LCST behaviour was conducted by switching of the CT interaction between the gel and effector, which was readily tuned by effector concentration or molecular structure of the effector.

Hybridizing semiconductor nanocrystals with metal–organic frameworks for visible and near-infrared photon upconversion

Hybrid materials consisting of semiconductor nanocrystals and metal-organic frameworks (MOFs) were prepared for the first time to achieve photon upconversion based on triplet-triplet annihilation (TTA-UC) in the solid-state, which allowed TTA-UC with large anti-Stokes shifts in the visible and near-infrared regions.

Thermoresponsivity of polymer solution derived from a self-attractive urea unit and a self-repulsive lipophilic ion unit

The preparation of the polymers having a self-attractive urea unit via hydrogen bonding and a self-repulsive lipophilic ion unit via electrostatic interaction was carried out to achieve the thermoresponsivity of polymer solution. The solubility of the obtained copolymers depended on the balance between the attractive force induced by hydrogen bonding among the urea units and the repulsive force attributed to the dissociation of the ionic units. In aprotic and nonpolar solvents, the solubility of the polymers was changed from insoluble to soluble with the increasing content of the ionic units. Moreover, in acetonitrile and 1,2-dichloroethane, the UCST-type thermoresponsivity was observed. This result indicated that the UCST-type thermoresponsivity was induced by cleavage of the hydrogen bonds among urea units upon heating, and at the cloud point, the repulsive force among the ionic units overwhelmed the attractive force. From the result of this research, we would like to emphasize that the combination of a lipophilic ion pair and other self-attractive units can be a simple principle for the molecular design of UCST-type thermoresponsive polymers in any media.