Dimerization-Induced Solid-State Excimer Emission Showing Consecutive Thermochromic Luminescence Based on Acridine-Modified o-Carboranes

Abstract

Although excimer emission is a useful luminescent phenomenon for fabricating optical sensors and probes, it is difficult to apply excimer emission for film sensors due to critical concentration quenching in the solid state. Therefore, robust molecular designs for solid-state excimer emission are still being explored. One of the key examples is the previously reported acridine-ethynyl-o-carborane AcE1, which showed a bright solid-state excimer emission assisted by characteristic CcageH···N interactions. In this paper, we report the newly synthesized acridine-diehynyl-o-carborane AcE2 and comprehensively compare it to AcE1. Both compounds had the same crystalline packing mode based on dimer formation, resulting in an efficient π-overlapping area and solid-state excimer emission. Variable-temperature photoluminescence (VT-PL) measurements revealed the consecutive thermochromic luminescence of these compounds. Finally, on the basis of the easily accessible spray-coating method, we constructed the thermochromic luminescent sensors on quartz substrates. According to the mechanistic studies, it is demonstrated that the design strategy based on a dimer-induced solid-state excimer should have great potential for applications as a molecular thermometer.

Regulation of solid-state dual-emission properties by switching luminescence processes based on a bis-o-carborane-modified anthracene triad

“…We have previously reported the same results of emission band shifts upon heating. 11,29 From excimer emission, hypsochromic shifts were observed, and it was suggested that intermolecular interaction via p-p stacking should be weakened by the gradual expansion of crystal lattice. In contrast, bathochromic shifts were detected from CT emission and it was proposed that larger structural relaxation could be allowed in weakened crystal packing, where thermal motions at the substituents vigorously occur.…”

Controlling the Dual‐Emission Character of Aryl‐Modified o‐Carboranes by Intramolecular CH⋅⋅⋅O Interaction Sites

“…The peak assigned as the C cage À H stretching band shifted to the larger energy region, suggesting the formation of CH•••O interaction. While general hydrogen bonds shift the band to a smaller energy region, [26] the opposite shift is also known as an "improper hydrogen bond," which is sometimes observed in CH•••X hydrogen bonding systems. [31] From these results, the C cage H•••O interaction should be formed both in solution and solid states.…”

“…[25] We have recently shed light on this unique interaction to construct solid-state thermochromic materials possessing highly-ordered crystal structures assisted by intermolecular C cage H•••N interactions. [26,27] On the other hand, intramolecular interactions have not been sufficiently investigated except for pioneering works conducted by Endo and co-workers. [28] Those researches focused on 1-(methoxyphenyl)-o-carborane and the intramolecular C cage H•••O interaction was evaluated by NMR spectroscopy and quantum chemical calculations, while optical properties have not been investigated because of its small πconjugation.…”

“…The key feature is the weak acidic hydrogen atom on the cage‐carbon atom (C cage H), which can form noncovalent interactions with electron‐rich atoms [25] . We have recently shed light on this unique interaction to construct solid‐state thermochromic materials possessing highly‐ordered crystal structures assisted by intermolecular C cage H⋅⋅⋅N interactions [26,27] . On the other hand, intramolecular interactions have not been sufficiently investigated except for pioneering works conducted by Endo and co‐workers [28] .…”

Sonogashira coupling of the ethynyl monocarborane [CB11H11-12-CCH]−

“…[47][48][49][50][51][52] Sonogashira coupling of ethynyl boron clusters has only been demonstrated in single reports, and no optimization or generalization of the method have been established. [53][54][55][56] In this study, we report on the Sonogashira coupling of the ethynyl carborane [CB 11 H 11 -12-CCH] − with bromoarenes, affording aryl carboranyl alkynes (Fig. 2c).…”