Ethynyl‐Tolyl Extended 2‐(2′‐Hydroxyphenyl)benzoxazole Dyes: Solution and Solid‐state Excited‐State Intramolecular Proton Transfer (ESIPT) Emitters

Abstract

Dual solution/solid‐state emissive fluorophores based on a 2‐(2′‐Hydroxyphenyl)benzoxazole (HBO) core bearing one or two ethynyl‐tolyl moieties at different positions were synthesized via an expedite two‐step synthetic procedure. HBO derivatives are known to display intense Excited‐State Intramolecular Proton Transfer (ESIPT) emission in the solid‐state but are mildly emissive in solution due to the detrimental flexibility of the excited‐state opening efficient non‐radiative pathways. The sole introduction of a rigid ethynyl moiety led to a sizeable enhancement of the fluorescence quantum yield in solution, up to a 15‐fold increase in toluene as compared to unsubstituted HBO dyes while keeping the high solid‐state fluorescence efficiency. The position of the substitution on the π‐conjugated core led to subtle fine‐tuning of maximum emission wavelengths and quantum yields. Moreover, we show that the ethynyl tolyl substituent at the para position of the phenol ring is a suitable moiety for an efficient stabilization of the corresponding emissive anionic HBO derivatives in dissociative solvents like DMF THF or EtOH. These observations were confirmed in CH3CN by a basic titration. For all dyes, the nature of the excited‐state involved in the fluorescence emission was rationalized using ab initio calculations.

Dual Solution-/Solid-State Emissive Excited-State Intramolecular Proton Transfer (ESIPT) Dyes: A Combined Experimental and Theoretical Approach

“…However, for dyes 2 , 3 , and 6 , the substitution by p - N , N ,-diethylaniline (NEt 2 ) groups leads to a dual E*/K* emission at 476/599, 470/565, and 475/603 nm, respectively. These observations indicate that the electrodonating characteristic of the NEt 2 group contributes to the stabilization of the energy level of E*, leading to a partial frustration of the proton transfer, which is a typical feature in such systems. ,, It is a clear indication that electron donors tend to decrease the acidity of the phenolic proton in the excited state (i.e., decrease of p K a *), leading to dual emission. Also, it is worth noting that this dual emission is observed in apolar toluene, highlighting the fact that ESIPT frustration is exclusively due to the electronic parameters and not the solvation effect in the protic environment.…”

“…In the neutral state, the pyridine moiety favors complete deprotonation of the excited phenol, leading to the impossibility of ESIPT to occur and the emission of the resulting phenolate. Indeed, it is reported that the phenolate moieties derived from HBO dyes can be highly emissive. , Moreover, the formation of the phenolate in neutral dichloromethane is consistent with the ab initio calculations (see below). To further evidence deprotonation in the excited state, the absorption and emission spectra of dye 8 in the presence of 0.1% base (NEt 3 ) were recorded, leading to similar maximum wavelengths (λ abs /λ em = 333/498 nm) as in neutral dichloromethane (Figure S75).…”

“…These optimized fluorophores have notably shown interesting random lasing properties in doped polymers . Despite the beneficial rigidification induced by the ethynyl-extended functionalization of the HBX core, the formation of an undesired benzofuran side product can be observed when the H-donating hydroxy group is ortho-substituted . This issue prompted us to find alternative ways to rigidify the HBX skeleton so as to increase the PLQY values in the solution state.…”

“…(a) Ethynyl-extended HBX derivatives, ,− (b) (hetero)­arylated HBO dyes (first series), and (c) alkyl/aryl pyridinium HBO dyes (second series) reported in this article.…”

Effect of Water Microsolvation on the Excited-State Proton Transfer of 3-Hydroxyflavone Enclosed in γ-Cyclodextrin

“…Generally, their photophysical properties can be easily modulated using many strategies [2,3] such as introducing electron-donating and withdrawing substituents, the heteroatom substitution, and the π-conjugation into the main core structure, to give the desirable absorption and emission spectra as well as large Stokes shift. The ESIntraPT molecules with such tunable photophysical properties including derivatives of salicylates [4][5][6], salicylideneanilines [7][8][9], flavones [10][11][12][13], benzazoles [14][15][16][17][18], and chalcones [19][20][21] have been reported and widely used in various applications ranging from chemical sensing to light-emitting diodes [22][23][24][25][26][27].…”