Fluorine-induced emission enhancement of tolanes via formation of tight molecular aggregates

Abstract

Incorporation of fluorine atoms into the tolane backbone constructed tight crystal packing structures to suppress molecular motions, which provided dramatic enhancement of photoluminescence efficiency in crystalline state.

Fluorinated Tolane Dyads with Alkylene Linkage: Synthesis and Evaluation of Photophysical Characteristics

“…Similar to the UV-vis absorption behavior of 2a, other dyads 2b-f also displayed absorption spectra with λabs at 321 and 329-331 nm. This analogy of the UV-vis absorption spectra for 2a-f with a parent monad 1a (λabs = 329 nm) [27] clearly indicated that the length of the alkylene linkage did not cause a significant change in the electronic structures in the solution state. To gain more information on the electronic transition in the absorption behavior, we also performed TD-DFT calculations at the same level of theory (CAM-B3LYP/6-31+G(d) with a CPCM for CH2Cl2) for dyads 2a and 2b as a representative example.…”

“…Thus, the reaction of 4-(methoxymethoxy)phenylacetylene (3), prepared from 4-bromophenol according to a reported procedure [33][34][35], with n-butyllithium in THF at 0 • C for 1 h, followed by the addition of octafluorotoluene (1.5 equiv.) proceeded well to afford the corresponding methoxymethoxy-substituted fluorinated tolane 4 in 82% yield through an addition-elimination process [27]. On treating 4 with concentrated hydrochloric acid in a mixed solvent (THF/MeOH v/v = 1/1) at 25 • C for 22 h, the methoxymethyl fragment was successfully removed to provide the corresponding hydroxy-substituted fluorinated tolane 5 in 86% yield.…”

“…Since a few years ago, we have extensively developed solid-state photoluminescent molecules based on the extended π-conjugated structure containing a fluorinated aromatic structure, which induces electron-density distribution owing to its large electronegativity and forms a tight crystal structure via van der Waals and hydrogen bonding interactions [22][23][24][25][26][27][28]. Among them, polyfluorinated tolane 1a [27] displayed highly efficient blue photoluminescence (PL) (Φ PL = 0.73) in crystalline states, although the PL efficiency was observed to be significantly low in dilute solution (Φ PL =~0.04) (Figure 1a). With deep investigations, interestingly, the PL behavior of 1a and the non-fluorinated analogue on the aromatic ring was found to change with an alteration of the molecular aggregated structures in the crystalline lattice; it was shown that fluorine atoms attached to an aromatic ring offering control over the molecular aggregated structure and photophysical behavior.…”

Systematic Studies on the Effect of Fluorine Atoms in Fluorinated Tolanes on Their Photophysical Properties

“…[ 20 , 21 ]. Our extensive efforts have led to the successful development of fluorinated tolane-based PL molecules [ 22 , 23 , 24 ]. From our previous results, fluorinated tolanes were found to possess both crystallization-induced emission enhancement (CIEE) as well as aggregation-induced emission enhancement (AIEE) characteristics ( Figure 1 d) [ 23 , 24 ].…”

“…Our extensive efforts have led to the successful development of fluorinated tolane-based PL molecules [ 22 , 23 , 24 ]. From our previous results, fluorinated tolanes were found to possess both crystallization-induced emission enhancement (CIEE) as well as aggregation-induced emission enhancement (AIEE) characteristics ( Figure 1 d) [ 23 , 24 ]. A deep investigation using X-ray crystallographic analyses has revealed that fluorinated tolane B in the crystalline state exhibits enhanced PL properties, while the non-fluorinated tolane A counterpart does not, owing to the tight and rigid molecular packing through several intermolecular hydrogen bonds, which are needed to suppress the non-radiative deactivation process.…”