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

Abstract

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.

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(ii) complexes with 1-hydroxy-1H-imidazole-based ligands

“…8–10 The excitation of such a molecule in its normal form ( N ) with light results in the transfer of the electron density from the electron-donating group to the electron-accepting one, which in turn alters the acidity and basicity of the proton-donating and proton-accepting groups and promotes the proton to transfer (excited state intramolecular proton transfer, ESIPT) affording the tautomeric form ( T ) of the molecule (Scheme 1). 11–32 If the ESIPT process is barrierless, the only form to emit is the tautomeric form. This emission is associated with dramatic reorganization of the molecular structure, resulting in large Stokes shifts.…”

Dual-state emissionversusno emission by manipulating the molecular structures of cyanovinyl–benzofuran derivatives

“…Organic fluorophores whose emission is as effective in solution at low concentration as for aggregates, powders or crystals, now frequently termed dual-state emission (DSE) derivatives, have attracted much attention in the last decade. [1][2][3] The design of DSE molecules represents a fascinating challenge since it is necessary to circumvent the two antagonistic phenomena which are the quenching of emission for the solid state frequently called aggregationcaused quenching (ACQ) and conversely the emission essentially for the solid state, solid state luminescence enhancement (SLE), often defined by the term aggregationinduced emission (AIE) coined by B. Z. Tang et al [4][5][6] Often ACQ is observed for rigid and planar derivatives that present high luminescence in dilute solution while in the solid state the quenching of emission occurs due to active non-radiative pathways via phenomena such as cofacial π-π interactions and the presence of trap states which are always discussed.…”

Sterically Hindered 2‐(2’‐Hydroxyphenyl)benzoxazole (HBO) Emitters: Synthesis, Spectroscopic Studies, and Theoretical Calculations

“…In solution, proton transfer processes such as ESIPT might trigger a large panel of excited‐state dynamics, including deactivation through accessible conical intersection (CI) and hence strong fluorescence quenching. Several studies have consequently reported synthetic strategies to increase solution‐state fluorescence and engineer dual‐state emission ESIPT fluorophores displaying intense emission in both solution and solid [19] . For example, enhancing molecular rigidity in derivatives like imidazo[1,2‐ f ]phenanthridine [20] or 3,5‐ethynyl‐hydroxyphenyl‐benzazole [21–23] has appeared as a straightforward synthetic trick to reduce non‐radiative deactivations through the well‐known “twisting CI”.…”

“…Several studies have consequently reported synthetic strategies to increase solutionstate fluorescence and engineer dual-state emission ESIPT fluorophores displaying intense emission in both solution and solid. [19] For example, enhancing molecular rigidity in derivatives like imidazo[1,2-f]phenanthridine [20] or 3,5-ethynyl-hydroxyphenyl-benzazole [21][22][23] has appeared as a straightforward synthetic trick to reduce non-radiative deactivations through the well-known "twisting CI". A recent alternative includes the incorporation of pyridinium units onto the HBO scaffold to stabilize the K* species by resonance effects, leading to drastic enhancement of fluorescence intensity in solution.…”