6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

During the past decade, fluorescent chemosensors have become an important research field of supramolecular chemistry and have attracted great attention because of their simplicity, high selectivity and sensitivity in fluorescent assays. In the design of new fluorescent chemosensors, exploration of new sensing mechanisms between recognition and signal reporting units is of continuing interest. Based on different photophysical processes, conventional sensing mechanisms including photo-induced electron transfer (PET), intramolecular charge transfer (ICT), metal–ligand charge transfer (MLCT), twisted intramolecular charge transfer (TICT), electronic energy transfer (EET), fluorescence resonance energy transfer (FRET), and excimer/exciplex formation have been investigated and reviewed extensively in the literature. This tutorial review will mainly focus on new fluorescent sensing mechanisms that have emerged in the past five years, such as aggregation-induced emission (AIE) and CN isomerization, which can be ascribed to fluorescence changes via conformational restriction. In addition, excited-state intramolecular proton transfer (ESIPT) has not been well reviewed yet, although a number of chemosensors based on the ESIPT mechanism have been reported. Thus, ESIPT-based chemosensors have been also summarized in this review.

New sensing mechanisms for design of fluorescent chemosensors emerging in recent years

In this paper we present the theory and implementation of analytic derivatives of time-dependent density functional theory (TDDFT) excited states energies, both in vacuo and including solvent effects by means of the polarizable continuum model. The method is applied to two case studies: p-nitroaniline and 4-(dimethyl)aminobenzonitrile. For both molecules PCM-TDDFT is shown to be successful in supporting the analysis of experimental data with useful insights for a better understanding of photophysical and photochemical pathways in solution.

Geometries and properties of excited states in the gas phase and in solution: theory and application of a time-dependent density functional theory polarizable continuum model.

The understanding of the dual fluorescence of certain aromatic systems has greatly advanced in recent years. The accompanying large charge separation has been shown to be linked to a twisted (or small overlap) arrangement of the chromophores. Recent theoretical models are able to describe the excited-state twisting of both single bonds (TICT compounds) and double bonds (olefins) in a unified picture. These models can help to elucidate the photophysical behavior of many organic, inorganic, and biologically relevant compounds, and their application to laser dyes and fluorescent probes provides a route to new “tailor-made” fluorescent materials. Applied to the primary processes of vision and photosynthesis, these models can lead to a deeper understanding of basic photobiological processes.

Charge Separation in Excited States of Decoupled Systems—TICT Compounds and Implications Regarding the Development of New Laser Dyes and the Primary Process of Vision and Photosynthesis

Recently, organic fluorescent molecules harnessing the excited-state intramolecular proton transfer (ESIPT) process are drawing great attention due to their unique photophysical properties which facilitate novel optoelectronic applications. After a brief introduction to the ESIPT process and related photo-physical properties, molecular design strategies towards tailored emission are discussed in relation to their theoretical aspects. Subsequently, recent studies on advanced ESIPT molecules and their optoelectronic applications are surveyed, particularly focusing on chemical sensors, fluorescence imaging, proton transfer lasers, and organic light-emitting diodes (OLEDs).

Advanced Organic Optoelectronic Materials: Harnessing Excited-State Intramolecular Proton Transfer (ESIPT) Process

Reinterpretation of the anomalous fluorescense of p-n,n-dimethylamino-benzonitrile

The fluorescence of 2-(2'-hydroxyphenyl)benzoxazole (HBO) is strongly Stokes-shifted due to intramolecular proton transfer in the excited singlet state by which the enol form ('E*) of HBO is converted into the keto form ('K*). At room temperature the decay of 'K* is dominated by a thermally activated radiationless transition. At lower temperatures fluorescence and intersystem crossing are the main deactivation processes. In the triplet state of the keto tautomer proton back-transfer to the enol triplet state, 3E*, takes place. At high triplet-state concentrations the transient decay is mainly governed by triplet-triplet annihilation. This process was used to determine the extinction coefficients of the triplet-triplet absorption. In contrast to other work the singlet ground-state absorption of the keto tautomer, 'K, was not observed.

Excited-state proton-transfer reactions in 2-(2'-hydroxyphenyl)benzoxazole. Role of triplet states.

Electron-transfer mechanism of fluorescence quenching in polar solvents. I. Dicyanobenzene as quencher

The enol-keto tautomerism of 2-(2′-hydrophenyl)benzoxazole (HBO) in its metastable triplet states has been studied in fluid, non-polar solution. After laser flash excitation the keto triplet state is populated with high efficiency. The establishment of a ketoenol equilibrium was observed between 55 and 220 K. At low temperatures the rate of this reaction is determined by tunnel effects. The barrier height of the proton transfer is in the triplet state much larger than in the singlet state.

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Intramolecular proton transfer and tunnel effects in the metastable triplet states of 2-(2′-hydroxyphenyl)benzoxazole studied by micro- and nanosecond laser flash photolysis

K. H. Grellmann

Papers

Reinterpretation of the anomalous fluorescense of p-n,n-dimethylamino-benzonitrile

Excited-state proton-transfer reactions in 2-(2'-hydroxyphenyl)benzoxazole. Role of triplet states.

Electron-transfer mechanism of fluorescence quenching in polar solvents. I. Dicyanobenzene as quencher

Intramolecular proton transfer and tunnel effects in the metastable triplet states of 2-(2′-hydroxyphenyl)benzoxazole studied by micro- and nanosecond laser flash photolysis

Mechanism of the photochemical cis⇌trans isomerization of thioindigo and 6,6'-diethoxy-thioindigo in solution