Substitution Effect on 2-(Oxazolinyl)-phenols and 1,2,5-Chalcogenadiazole-Annulated Derivatives: Emission-Color-Tunable, Minimalistic Excited-State Intramolecular Proton Transfer (ESIPT)-Based Luminophores

A TDDFT exploration on the excited-state intramolecular proton transfer in 2-(2'-hydroxyphenyl)-benzimidazole derivatives

Excited-state intramolecular proton transfer (ESIPT) reactions are one of the fundamental energy transformation reactions in catalysis and biological process. The combining ESIPT with the twisted intramolecular charge transfer (TICT) brings the richness of optical, photoelectronic performances to certain functional compounds. Delineating the mechanism of ESIPT + TICT reactions and further understanding why a specific functional group dominates are fundamentally crucial for the design and application of the functionally photoelectric materials. In this paper, six 2-(2'-hydroxyphenyl) benzimidazole (HBIgens) derivatives involved in ESIPT + TICT were investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations to have an insight into the photophysical and photochemical process in acetonitrile. The optimized geometries indicated that the intramolecular hydrogen bonds (-O-H···N-) were enhanced in the corresponding first singlet, which provided the fundamentally outstanding prerequisites of the ESIPT reactions. By further charge analysis, it is indicated that the introduction of substitutes to the different positions would determine the Stokes' shifts, and the electron-adopting p-cyanophenyl group mainly contributed to the TICT structure. Constraint scanning the potential energy curves of both ground and first singlet excited states, the electron-adopting N,N-diethylamino group on the meta position could enhance the barrier and inhibit the ESIPT reaction. Furthermore, the nucleus independent chemical shift (NICS(1)_ZZ) values of phenol groups indicate the relationship between the reversal aromaticity and the barrier of ESIPT, both of which were proved to be negatively correlated in the ESIPT reaction. It is concluded that not only both types and positions of substituents can tune the excited-state proton transfer behaviors in HBIgen derivatives, but also the aromatic rule can easily be applied to elaborate the ESIPT reaction.

Stimuli-Induced Fluorescence Switching in Azine-Containing Fluorophores Displaying Resonance-Stabilized ESIPT Emission

This article reports the synthesis, along with structural and photophysical characterization of 2-(2'-hydroxyphenyl)benzazole derivatives functionalized with various azaheterocycles (pyridine, pyrimidine, terpyridine). These compounds show dual-state emission properties, that is intense fluorescence both in solution and in the solid-state with a range of fluorescent color going from blue to orange. Moreover, the nature of their excited state can be tuned by the presence of external stimuli such as protons or metal cations. In the absence of stimuli, these dyes show emission stemming from anionic species obtained after deprotonation (D* transition), whereas upon protonation or metal chelation, ESIPT process occurs leading to a stabilized and highly emissive K* transition. With the help of extensive ab initio calculations, we confirm that external stimuli can switch the nature of the transitions, making this series of dyes attractive candidates for the development of stimuli-responsive fluorescent ratiometric probes.

Amination and Protonation Facilitated Novel Isoxazole Derivative for Highly Efficient Electron and Hole Separation

It is of great importance to understand the intrinsic relationship between phototautomerization and photoelectric properties for the exploration of novel organic materials. Here, in order to chemically control the protonation process, the aminated isoxazole derivative (2,2'-(isoxazolo[5,4-d]isoxazole-3,6-diyl)dibenzenaminium, DP-DA-DPIxz) with -N═ as the proton acceptor was designed to achieve the twisted intramolecular charge transfer (TICT) state which was triggered by an excited-state intramolecular proton transfer (ESIPT) process. This kind of protonation enhanced the intramolecular hydrogen bonding, conjugative effect, and steric hindrance effects, ensuring a barrierless spontaneous TICT process. Through the intramolecular proton transfer, the configuration torsion and conjugation dissociation of the DP-DA-DPIxz molecule was favored, which led to efficient charge separation and remarkable variations in light-emitting properties. We hope the present investigation will provide a new approach to design novel optoelectronic organic materials and shine light on the understanding of the charge transfer and separation process in molecular science.

Interplay between Dual-State and Aggregation-Induced Emission with ESIPT Scaffolds Containing Triphenylamine Substituents: Experimental and Theoretical Studies

We detail the synthesis of a series of fluorophores containing triphenylamine derivatives along with their photophysical, electrochemical, and electronic structure properties. These compounds include molecular structures derived from imino-phenol (anil) and hydroxybenzoxazole scaffolds originating from similar salicylaldehyde derivatives and display excited-state intramolecular proton transfer. We show that depending on the nature of the π-conjugated scaffold, different photophysical processes are observed: aggregation-induced emission or dual-state emission, with a modulation of the fluorescence color and redox properties. The photophysical properties are further rationalized with the help of ab initio calculations.

Excited-State Intramolecular Proton Transfer Dyes with Dual-State Emission Properties: Concept, Examples and Applications

Dual-state emissive (DSE) fluorophores are organic dyes displaying fluorescence emission both in dilute and concentrated solution and in the solid-state, as amorphous, single crystal, polycrystalline samples or thin films. This comes in contrast to the vast majority of organic fluorescent dyes which typically show intense fluorescence in solution but are quenched in concentrated media and in the solid-state owing to π-stacking interactions; a well-known phenomenon called aggregation-caused quenching (ACQ). On the contrary, molecular rotors with a significant number of free rotations have been engineered to show quenched emission in solution but strong fluorescence in the aggregated-state thanks to restriction of the intramolecular motions. This is the concept of aggregation-induced emission (AIE). DSE fluorophores have been far less explored despite the fact that they are at the crossroad of ACQ and AIE phenomena and allow targeting applications both in solution (bio-conjugation, sensing, imaging) and solid-state (organic electronics, data encryption, lasing, luminescent displays). Excited-State Intramolecular Proton Transfer (ESIPT) fluorescence is particularly suitable to engineer DSE dyes. Indeed, ESIPT fluorescence, which relies on a phototautomerism between normal and tautomeric species, is characterized by a strong emission in the solid-state along with a large Stokes’ shift, an enhanced photostability and a strong sensitivity to the close environment, a feature prone to be used in bio-sensing. A drawback that needs to be overcome is their weak emission intensity in solution, owing to detrimental molecular motions in the excited-state. Several strategies have been proposed in that regard. In the past few years, a growing number of examples of DSE-ESIPT dyes have indeed emerged in the literature, enriching the database of such attractive dyes. This review aims at a brief but concise overview on the exploitation of ESIPT luminescence for the optimization of DSE dyes properties. In that perspective, a synergistic approach between organic synthesis, fluorescence spectroscopy and ab initio calculations has proven to be an efficient tool for the construction and optimization of DSE-ESIPT fluorophores.

Spectroscopic and mechanistic insights into solvent mediated excited-state proton transfer and aggregation-induced emission: introduction of methyl group onto 2-(o-hydroxyphenyl)benzoxazole

2-(2-Hydroxy-5-methylphenyl)benzoxazole(HBO-pCH3), a solvatochromic benzoxazole-based probe, exhibited a typical dual fluorescence phenomenon, high fluorescence quantum yield, red-shifted emission and large Stokes’ shift via the ESIPT in solvents.