Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated.Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtos...

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Hydrogen Bonding in the Electronic Excited State

In this review we will explore recent advances in the design and application of excited-state intramolecular proton-transfer (ESIPT) based fluorescent probes. Fluorescence based sensors and imaging agents (probes) are important in biology, physiology, pharmacology, and environmental science for the selective detection of biologically and/or environmentally important species. The development of ESIPT-based fluorescence probes is particularly attractive due to their unique properties, which include a large Stokes shift, environmental sensitivity and potential for ratiometric sensing.

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Excited-state intramolecular proton-transfer (ESIPT) based fluorescence sensors and imaging agents

Solid state emitters based on excited state intramolecular proton transfer (ESIPT) have been attracting considerable interest since the past few years in the field of optoelectronic devices because of their desirable unique photophysical properties. The photophysical properties of the solid state ESIPT fluorophores determine their possible applicability in functional materials. Less fluorescence quantum efficiencies and short fluorescence lifetime in the solid state are the shortcomings of the existing ESIPT solid state emitters. Designing of ESIPT chromophores with high fluorescence quantum efficiencies and a long fluorescence lifetime in the solid state is a challenging issue because of the unclear mechanism of the solid state emitters in the excited state. Reported design strategies, detailed photophysical properties, and their applications will help in assisting researchers to overcome existing challenges in designing novel solid state ESIPT fluorophores for promising applications. This review highlights recently developed solid state ESIPT emitters with focus on molecular design strategies and their photophysical properties, reported in the last five years.

Excited-state intramolecular proton-transfer (ESIPT)-inspired solid state emitters

Fluorescent dyes have enabled much progress in the broad range of biomedical fields. However, many commercially available dyes suffer from small Stokes shifts, resulting in poor signal-to-noise ratio and self-quenching on current microscope configurations. In this work, we have developed a general method to significantly increase the Stokes shifts of common fluorophores. By simply appending a 1,4-diethyl-decahydro-quinoxaline (DQ) moiety onto the conjugated structure, we introduced a vibronic backbone that could facilely expand the Stokes shifts, emission wavelength, and photostability of 11 different fluorophores by more than 3-fold. This generalizable method could significantly improve the imaging efficiency of commercial fluorophores. As a demonstration, we showed that the DQ derivative of hemicyanine generated 5-fold signal in mouse models over indocyanine green. Furthermore, the DQ-modified fluorophores could pair with their parent molecules to conduct one-excitation, multiple emission imaging, allow...

A General Method To Increase Stokes Shift by Introducing Alternating Vibronic Structures

Most two-dimensional (2D) covalent organic frameworks (COFs) are non-fluorescent in the solid state even when they are constructed from emissive building blocks. The fluorescence quenching is usually attributed to non-irradiative rotation-related or π–π stacking-caused thermal energy dissipation process. Currently there is a lack of guiding principle on how to design fluorescent, solid-state material made of COF. Herein, we demonstrate that the eclipsed stacking structure of 2D COFs can be used to turn on, and tune, the solid-state photoluminescence from non-emissive building blocks by the restriction of intramolecular bond rotation via intralayer and interlayer hydrogen bonds among highly organized layers in the eclipse-stacked COFs. Our COFs serve as a platform whereby the size of the conjugated linkers and side-chain functionalities can be varied, rendering the emission colour-tuneable from blue to yellow and even white. This work provides a guide to design new solid-state emitters using COFs.

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Tuneable near white-emissive two-dimensional covalent organic frameworks.

Two novel donor–π bridge–acceptor compounds containing excited state intramolecular proton transfer (ESIPT) and non-ESIPT units based on triphenylamine–benzothiazole were synthesized via Suzuki coupling reaction. Their photophysical properties were studied in the solid state as well as in solutions with solvents of different polarities. The fluorophores showed absorption in the UV region and emission in the visible region in both solutions and solid state. The optical properties of these compounds are highly dependent on solvent polarity. Significant positive solvatochromism (∼30 nm absorption and ∼80 nm emission red shift in polar solvents) were observed for both the compounds. Large Stokes shift (∼15 000 cm−1), polarity sensitive optical properties and very high quantum efficiencies (∼90%) in solvents and the solid state are the striking features of the synthesized compounds. Intramolecular charge transfer (ICT) characteristics of the compounds were supported experimentally and computationally. Halochromism and the intermolecular charge transfer phenomenon were used for investigation of ESIPT emission for compound 9 over ICT emission.

AIE active triphenylamine–benzothiazole based motifs: ESIPT or ICT emission

Tuning or switching of the solid state luminescence of organic materials is an attractive target for both basic research and practical applications. In the present study, solid state emissive compounds with very high quantum efficiencies (ΦF up to 68%) were achieved by chemical alteration of the excited state intramolecular proton transfer (ESIPT) 2-2′-hydroxy benzothiazole (HBT) unit. Five ESIPT inspired compounds based on fluorene were synthesized via Suzuki coupling reaction. Their photophysical properties were studied by means of steady state absorption, emission spectra and a time resolved emission method in solid as well as in solution of different polarities. The fluorophores showed absorption in the UV region and emission in the visible region with large Stokes shift (∼232 nm). Efficient yellow emissive compounds showed very high quantum yields (ΦF = 55–68%) in the solid state, which are the highest quantum yields in the solid state to the best of our knowledge, for fluorene based ESIPT molecules. The fluorescence lifetime in the solid state is between 3.48–5.21 ns, while it is 5–10 fold less in chloroform (0.52–0.75 ns) solution. The optical properties of these compounds are sensitive towards the polarity of the medium. The structural properties, such as X-ray single crystal analyses, DSC and TGA were studied, and the lack of stacking and/or hydrogen bonding interactions around HBT motifs reveals enough room for ESIPT in the series of molecules even in their solid state. The DFT computations were performed to support experimental results and the calculations are well in line with the experimental results. These suggest high quantum efficiency ascribed to the large orbital energy difference between HOMOs and LUMOs of enol and keto forms transformed via ESIPT, and hence, singlet energy localization onto the keto form. The intra-molecular charge transfer nature between fluorene and HBT units plays a key role for the localization of energy on HBT motifs in their excited states.

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Highly emissive excited-state intramolecular proton transfer (ESIPT) inspired 2-(2′-hydroxy)benzothiazole–fluorene motifs: spectroscopic and photophysical properties investigation

Highly Fluorescent Liquid Crystals from Excited-State Intramolecular Proton Transfer Molecules

In bulk materials, positional isomers not only help in understanding how slight difference in molecular structure alters the crystal packing and optical properties, but also play a key role in developing new type of materials for functional applications. A detailed study on the photophysical properties of fluorene–HBT positional isomers in solution and in the solid state providing a molecular level understanding of the factors which influence fluorescence behavior is reported. Two molecules Ia and IIa were synthesized by Suzuki coupling reaction and their photophysical properties were compared to positional isomers Ib and IIb. Crystal structure analyses and density functional theory (DFT) computation studies were performed to understand structure–properties relation and the results reveal that changing substitution pattern has a marked influence on their packing modes and luminescence properties. Strong noncovalent interactions (π–π) in the solid state hamper the excited state intramolecular proton transf...

Vikas S. Padalkar

Papers

Excited-state intramolecular proton-transfer (ESIPT)-inspired solid state emitters

AIE active triphenylamine–benzothiazole based motifs: ESIPT or ICT emission

Highly emissive excited-state intramolecular proton transfer (ESIPT) inspired 2-(2′-hydroxy)benzothiazole–fluorene motifs: spectroscopic and photophysical properties investigation

Highly Fluorescent Liquid Crystals from Excited-State Intramolecular Proton Transfer Molecules

Optical and Structural Properties of ESIPT Inspired HBT-Fluorene Molecular Aggregates and Liquid Crystals.