Stimuli‐Responsive Dual‐Color Photon Upconversion: A Singlet‐to‐Triplet Absorption Sensitizer in a Soft Luminescent Cyclophane

Abstract

Reversible emission color switching of triplet-triplet annihilation-based photon upconversion (TTA-UC) is achieved by employing an Os complex sensitizer with singlet-to-triplet (S-T) absorption and an asymmetric luminescent cyclophane with switchable emission characteristics. The cyclophane contains the 9,10-bis(phenylethynyl)anthracene unit as an emitter and can assemble into two different structures, a stable crystalline phase and a metastable supercooled nematic phase. The two structures exhibit green and yellow fluorescence, respectively, and can be accessed by distinct heating/cooling sequences. The hybridization of the cyclophane with the Os complex allows near-infrared-to-visible TTA-UC. The large anti-Stokes shift is possible by the direct S-T excitation, which dispenses with the use of a conventional sequence of singlet-singlet absorption and intersystem crossing. The TTA-UC emission color is successfully switched between green and yellow by thermal stimulation.

A 1,6-Diphenylpyrene-Based, Photoluminescent Cyclophane Showing a Nematic Liquid-Crystalline Phase at Room Temperature

“…DSC measurements clarified the phase transition behavior of cyclophane 1 (Figure 3). A phase transition from isotropic to nematic was observed at 57.6 • C on cooling, and further cooling results in a glass transition at −15.1 • C. In contrast with unsymmetric cyclophanes investigated in our previous studies [23][24][25], no transitions to crystalline phases appear on the heating DSC traces. As shown in Figure 4, no clear peaks were observed in the X-ray diffraction pattern obtained from cyclophane 1 at r.t., which coincides with the fact that cyclophane 1 exhibits the nematic LC phase at r.t.…”

“…The course of our previous studies on luminescent cyclophanes [22][23][24][25][26][27][28][29] suggested that the introduction of a cyclic structure into luminophores is a promising way to achieve mechanoresponsive luminescent materials [3,4,[30][31][32][33][34][35][36]. Luminescent cyclophanes having longer spacers have been found to show nematic LC properties.…”

“…Here, we demonstrate a 1,6-diphenylpyrene-based, unsymmetric cyclophane showing a stable nematic phase at room temperature and exhibiting strong photoluminescence from the condensed state. The observed sky-blue photoluminescence was dominated by the emission species ascribed to assembled luminophores rather than monomers.Crystals 2019, 9, 92 2 of 10 two luminophores in an individual cycle [23][24][25]. Furthermore, the unsymmetric cyclophanes exhibit kinetically trapped LC phases with nematic molecular order after rapid cooling from the nematic state.…”

“…Crystals 2019, 9, 92 2 of 10 two luminophores in an individual cycle [23][24][25]. Furthermore, the unsymmetric cyclophanes exhibit kinetically trapped LC phases with nematic molecular order after rapid cooling from the nematic state.…”

Linearly polarized photoluminescence from an asymmetric cyclophane showing thermo- and mechanoresponsive luminescence

“…Luminescent cyclophanes are intriguing compounds as they may exhibit unconventional photophysical functions that have never been achieved with the acyclic compounds. [10][11][12] The cyclic structure disturbs the ordered and close-molecular packing, which results in the formation of liquid-crystalline (LC) phases over wide temperature ranges. Even though a number of cyclophanes have been reported to show LC phases, 13,14 their luminescent properties remain unexplored.…”