How Inter- and Intramolecular Processes Dictate Aggregation-Induced Emission in Crystals Undergoing Excited-State Proton Transfer

Dicyanorhodanine-Pyrrole Conjugates for Visible Light-Driven Quantitative Photoswitching in Solution and the Solid State

Small molecule photoswitches capable of toggling between two distinct molecular states in response to light are versatile tools to monitor biological processes, control photochemistry, and design smart materials. In this work, six novel dicyanorhodanine-based pyrrole-containing photoswitches are reported. The molecular design avails both the Z and E isomers from synthesis, where each can be isolated using chromatographic techniques. Inter- and intramolecular hydrogen bonding (H-bonding) interactions available to the E and Z isomers, respectively, uniquely impart thermal stability to each isomer over long time periods. Photoisomerization could be assessed by solution NMR and UV-vis spectroscopic techniques along with complementary ground- and excited-state computational studies, which show good agreement. Quantitative E → Z isomerization occurs upon 523 nm irradiation of the parent compound (where R = H) in solution, whereas Z → E isomerization using 404 nm irradiation offers a photostationary state (PSS) ratio of 84/16 (E/Z). Extending the π-conjugation of the pyrrole unit (where R = p-C6H4-OMe) pushes the maximum absorption to the yellow-orange region of the visible spectrum and allows bidirectional quantitative isomerization with 404 and 595 nm excitation. Comparator molecules have been prepared to report how the presence or absence of H-bonding affects the photoswitching behavior. Finally, studies of the photoswitches in neat films and photoinactive polymer matrices reveal distinctive structural and optical properties of the Z and E isomers and ultimately afford reversible photoswitching to spectrally unique PSSs using visible light sources including the Sun.

Dynamic Motions of Ligands around the Metal Centers Afford a Fidget Spinner-Type AIE Luminogen

A new type of aggregation-induced emission (AIE) luminogen containing a dimeric metal fragment and two or three phthalazine ligands is described, which shows dynamic motions of ligands around the metal centers in solution. Based on the variable-temperature and EXSY NMR spectroscopy data, X-ray crystallography structures, and computational results, three different pathways (i.e., reversible exchange with haptotropic shifts, circulation of ligands around the dimeric metal fragment, and walking on the spot of ligands on the metal centers) were considered for this dynamic behavior. Restriction of these dynamic processes in the aggregate forms of the compounds (in H2O/CH3CN solvent mixtures) contributes to their AIE. DFT calculations and NMR analysis showed that bright excited states for these molecules are not localized on isolated molecules, and the emission of them stemmed from π-dimers or π-oligomers. The morphologies and the mode of associations in the solvent mixtures were determined by using transmission electron microscopy (TEM) and concentration-dependent NMR spectroscopy. The computational results showed the presence of a conical intersection (CI) between the S0 and S1 excited state, which provides an accessible pathway for nonradiative decay in these systems.

Organic compounds for solid state luminescence enhancement/aggregation induced emission: a theoretical perspective

Organic luminophores displaying one or more forms of luminescence enhancement in solid state are extremely promising for the development and performance optimization of functional materials essential to many modern key technologies. Yet, the effort to harness their huge potential is riddled with hurdles that ultimately come down to a limited understanding of the interactions that result in the diverse molecular environments responsible for the macroscopic response. In this context, the benefits of a theoretical framework able to provide mechanistic explanations to observations, supported by quantitative predictions of the phenomenon, are rather apparent. In this perspective, we review some of the established facts and recent developments about the current theoretical understanding of solid-state luminescence enhancement (SLE) with an accent on aggregation-induced emission (AIE). A description of the macroscopic phenomenon and the questions it raises is accompanied by a discussion of the approaches and quantum chemistry methods that are more apt to model these molecular systems with the inclusion of an accurate yet efficient simulation of the local environment. A sketch of a general framework, building from the current available knowledge, is then attempted via the analysis of a few varied SLE/AIE molecular systems from literature. A number of fundamental elements are identified offering the basis for outlining design rules for molecular architectures exhibiting SLE that involve specific structural features with the double role of modulating the optical response of the luminophores and defining the environment they experience in solid state.

An aggregation induced emission chalcone fluorescent probe with large Stokes shift for biothiols detection

The homeostasis of biothiols is closely related to the health of organisms. In view of the important role of biothiols, a fluorescent probe (7HIN-D) for the detection of intracellular biothiols was developed based on a simple chalcone fluorophore 7HIN with "ESIPT + AIE" characteristics. The probe 7HIN-D was obtained by introducing a biothiols specific DNBS (2,4-dinitrobenzenesulfonyl) unit as a fluorescence quencher to the fluorophore 7HIN. The nucleophilic substitution reaction between biothiols and probe 7HIN-D will release the DNBS unit and the fluorophore 7HIN, which exhibits a "turn on" AIE fluorescence with a large Stokes shift of 113 nm. The probe 7HIN-D displays high sensitivity and good selectivity to biothiols, and the detection limits value of probe 7HIN-D for GSH, Cys and Hcy were 0.384 μmol/L, 0.471 μmol/L and 0.638 μmol/L, respectively. In addition, the probe has been successfully used for fluorescence detection of endogenous biothiols in living cells due to its excellent performance, good biocompatibility and low cytotoxicity.

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.

Understanding the solid state luminescence and piezochromic properties in polymorphs of an anthracene derivative

Luminescent molecular crystals have gained significant research interest for optoelectronic applications. However, fully understanding their structural and electronic relationships in the condensed phase and under external stimuli remains a significant challenge. Here, piezochromism in the molecular crystal 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA) is studied using a combination of density functional theory (DFT) and time-dependent TD-DFT. We investigate the effects that molecular packing and geometry have on the electronic and phonon structure and the excited state properties in this archetypal system. We find that the luminescence properties are red-shifted with the transition from a herringbone to a sheet packing arrangement. An almost continuous red-shift in the band gap is found with the application of an external pressure through the enhancement of π-π and CH-π interactions, and is a mechanism in fine tuning an emissive response. The analysis of the phonon structure of the molecular crystal suggests restriction of motion in the herringbone packing arrangement, with motion restricted at higher pressure. This is supported by the Huang-Rhys factors which show a decrease in the reorganisation energy with the application of pressure. Ultimately, a balance between the decrease in reorganisation energies and the increase in exciton coupling will determine whether nonradiative decay is enhanced or decreased with the increase in pressure in these systems.

A two-step MM and QM/MM approach to model AIEE of aryloxy benzothiadiazole derivatives for optoelectronic applications

Aryloxy-benzothiadiazole (ArO-Btz) derivatives show aggregation-induced enhanced emission (AIEE) in the solid-state and are promising candidates for optoelectronic applications. However, understanding the AIEE is a challenging task and is necessary for the rational molecular design of emitters. Therefore, in the present study, electron acceptors (-F, -CN, -NO₂, and -COOH) on the benzothiadiazole ring have been screened for emission in solution and aggregated phases. Herein, we report QM (DFT/TDDFT) and ONIOM (QM/MM) studies on the four ArO-Btz derivatives in comparison with the parent molecule with typical characteristics of AIEE, optoelectronic and non-linear optical properties. Starting from the optimized crystal structure of the parent compound, the structures of the designed clusters have been pre-optimized with MM and then with QM/MM to explore their absorption and emission in the solid phase. The results indicate that in the aggregated phase, the surrounding environment reduces intra-molecular rotations and molecular motion that lead to enhanced emission. Natural bond orbital (NBO) analyses reveal that the ground state structure is stabilized from electron delocalization and operative push-pull effects. Interestingly, nitro-benzothiadiazole exhibits prominent AIEE phenomena, with an emission wavelength beyond 700 nm in solution and in the cluster, reinforced by the magnification of its oscillatory strength by 100 times when aggregated. This dinitro-aryloxy-benzothiadiazole derivative is proposed as a near-infrared emitter for dye-sensitized solar cell, optoelectronic, and non-linear optical applications.

Emission Quenching in Tetraphenylfuran Crystal: Why This Propeller-Shaped Molecule Does Not Emit in the Condensed Phase

Due to their substantial fluorescence quantum yields in the crystalline phase, propeller-shaped molecules have recently gained significant attention as potential emissive materials for optoelectronic applications. For the family of cyclopentadiene derivatives, light-emission is highly dependent on the nature of heteroatomic substitutions. In this paper, we investigate excited state relaxation pathways in the tetraphenyl-furan molecule (TPF), which in contrast with other molecules in the family, shows emission quenching in the solid-state. For the singlet manifold, our calculations show nonradiative pathways associated with C-O elongation are blocked in both vacuum and the solid state. A fraction of the population can be transferred to the triplet manifold and, subsequently, to the ground state in both phases. This process is expected to be relatively slow due to the small spin-orbit couplings between the relevant singlet-triplet states. Emission quenching in crystalline TPF seems to be in line with more efficient exciton hopping rates. Our simulations help clarify the role of conical intersections, population of the triplet states and crystalline structure in the emissive response of propeller-shaped molecules.

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.

Protein confinement fine-tunes aggregation-induced emission in human serum albumin

Luminogens exhibiting aggregation-induced-emission characteristics (AIEgens) have been designed as sensitive biosensors thanks to their "turn-on" fluorescence upon target binding. However, their AIE mechanism in biomolecules remains elusive except for the qualitative picture of restricted intramolecular motions. In this work, we employed ab initio simulations to investigate the AIE mechanism of two tetraphenylethylene derivatives recently developed for sensitive detection of human serum albumin (HSA) in biological fluids. For the first time, we quantified the ab initio free energy surfaces and kinetics of AIEgens to access the conical intersections on the excited state in the protein and aqueous solution, using a novel first-principles electronic structure method that incorporates both static and dynamic electron correlations. Our simulations accurately reproduce the experimental spectra and high-level correlated electronic structure calculations. We found that in HSA the internal conversion through the cyclization reaction is preferred over the isomerization around the central ethylenic double bond, whereas in the aqueous solution the reverse is true. Accordingly, the protein environment is able to moderately speed up certain non-radiative decay pathways, a new finding that is beyond the prediction of the existing model of restricted access to a conical intersection (RACI). As such, our findings highlight the complicated effects of the protein confinement on the competing non-radiative decay channels, which has been largely ignored so far, and extend the existing theories of AIE to biological systems. The new insights and the multiscale computational methods used in this work will aid the design of sensitive AIEgens for bioimaging and disease diagnosis.

The study of intramolecular decay and intermolecular energy transfer for phosphorescent organic light-emitting devices

Due to the huge potential of organic light-emitting diodes (OLEDs) in optical display devices, the exciton utilization of devices should be elucidated comprehensively to achieve a high external quantum efficiency (EQE). In this study, theoretical calculations of intramolecular excited state decay and intermolecular excitation energy transfer (EET) were conducted to investigate the differences in EQE between the two studied systems. Compared to the PtOO7-based system (using PtOO7 as the guest and 26mCPy as the host), the greater EQE of the PtON7-based system (using PtON7 as the guest and 26mCPy as the host) was mainly governed by the stronger energy transfer efficiency, with a secondary role being played by the higher photoluminescence quantum yield of the emitter. We confirmed that the different triplet EET (TEET) rates mainly contribute to the difference in the energy transfer efficiency between two studied systems, where the higher TEET rate from 26mCPy to PtON7 can be attributed to the restrained structural deformation of PtON7 and the desirable energy gap in the energy transfer process. Our calculations indicated that the electronic structure can evidently affect the intramolecular excited state decay and intermolecular excitation energy transfer. In addition, considering the environmental effects on the emission spectra of the emitters, the simulated spectra were consistent with the experimental measurements, which indicated that our descriptions of electronic structures are accurate; furthermore, an effective description of the molecular environment should be obtained. Our computational protocol successfully explored the relationship between the electronic structures, intramolecular excited state decay, and intermolecular excitation energy transfer, which can provide a deep understanding for the design and development of high-quality OLEDs from a molecular perspective.

Role of Conical Intersections on the Efficiency of Fluorescent Organic Molecular Crystals

Organic molecular crystals are attractive materials for luminescent applications because of their promised tunability. However, the link between the chemical structure and emissive behavior is poorly understood because of the numerous interconnected factors which are at play in determining radiative and nonradiative behaviors at the solid-state level. In particular, the decay through conical intersection dominates the nonadiabatic regions of the potential energy surface, and thus, their accessibility is a telling indicator of the luminosity of the material. In this study, we investigate the radiative mechanism for five organic molecular crystals which display a solid-state emission, with a focus on the role of conical intersections in their photomechanisms. The objective is to situate the importance of the accessibility of conical intersections with regards to emissive behavior, taking into account other nonradiative decay channels, namely, vibrational decay, and exciton hopping. We begin by giving a brief overview of the structural patterns of the five systems within a larger pool of 13 crystals for a richer comparison. We observe that because of the prevalence of sheet like and herringbone packing in organic molecular crystals, the conformational diversity of crystal dimers is limited. Additionally, similarly spaced dimers have exciton coupling values of a similar order within a 50 meV interval. Next, we focus on three exemplary cases, where we disentangle the role of nonradiative decay mechanisms and show how rotational minimum energy conical intersections in vacuum lead to puckered ones in the crystal, increasing their instability upon crystallization in typical packing motifs. In contrast, molecules with puckered conical intersections in vacuum tend to conserve this trait upon crystallization, and therefore, their quantum yield of fluorescence is determined predominantly by other nonradiative decay mechanisms.

Recent advances in excited state intramolecular proton transfer mechanism-based solid state fluorescent materials and stimuli-responsive fluorescence switching

Substitutional change and controlling intra and intermolecular interactions of ESIPT molecules resulted in realizing multifunctional fluorescence properties.

Unraveling the Effects of Co-Crystallization on the UV/Vis Absorption Spectra of an N-Salicylideneaniline Derivative. A Computational RI-CC2 Investigation

This work aims at unraveling the effects of co-crystallization on the optical properties of an N-salicylideneaniline-derived molecular switch transforming between an enol and a keto form. This is achieved by way of a two-step multi-scale method where (i) the molecular geometry and unit cell parameters are optimized using a periodic boundary conditions density functional theory method and (ii) the optical properties are computed for a selection of clusters embedded in an array of point-charges that reproduce the crystal field electronic potential. The optical properties (vertical excitation energies and oscillator strengths) are obtained at the RI-CC2/def2-TZVPD level of approximation. This method allows us to decompose the effects of co-crystallization into (i) indirect effects, the geometry changes of the chromophore due to crystal packing with the coformer, and (ii) direct ones, the polarization due to the interacting coformer and to the crystal field. For the former effects, variations of a crucial torsion angle lead to modification of the π-conjugation and therefore to the decrease or increase of the excitation energies. About the latter, they are antagonistic: (i) the coformer is not directly involved in the excitations but its polarization decreases the excitation energies while (ii) the crystal field has the opposite effect. For the co-crystals with succinic and fumaric acids, combining these direct and indirect effects leads to a hypsochromic shift of the first absorption band with respect to the reference crystal, in agreement with experimental data.

A graphene oxide-aided triple helical aggregation-induced emission biosensor for highly specific detection of charged collagen peptides

Aggregation-induced emission (AIE) probes have emerged as promising "turn-on" sensing tools for DNA and proteins, and the AIE biosensors conjugated with graphene oxide (GO) have shown improved selectivity. Collagen is an essential structural protein in the human body, and its degraded products are involved in a plethora of severe diseases. Collagen has a high content of charged amino acids, while EOG represents one of the most abundant charged triplets in Type I collagen. We, herein, for the first time report the construction of a GO-aided AIE biosensor for the detection of charged collagen peptides. We have shown that an AIE fluorophore TPE conjugated with a triple helical peptide TPE-PRG possesses strong fluorescence due to the restriction of intramolecular rotation of TPE in the trimer state. The adsorption of the probe TPE-PRG by GO leads to efficient fluorescence quenching, while the addition of target collagen peptide EOG releases the probe peptide from the GO surface and recovers its fluorescence. We have demonstrated that the TPE-PRG/GO complex provides a highly specific "turn-on" sensing platform for the target collagen peptide with a typical charged amino acid-rich sequence. The assay has shown little interference from other biomolecules, and it can also effectively distinguish the target charged collagen peptide from its single amino acid mutant type. The development of robust analytical assays for charged collagen peptides could pronouncedly extend our capability to investigate the pathology of collagen diseases, showing great potential for their molecular diagnosis.

fromage: A library for the study of molecular crystal excited states at the aggregate scale

The study of photoexcitations in molecular aggregates faces the twofold problem of the increased computational cost associated with excited states and the complexity of the interactions among the constituent monomers. A mechanistic investigation of these processes requires the analysis of the intermolecular interactions, the effect of the environment, and 3D arrangements or crystal packing on the excited states. A considerable number of techniques have been tailored to navigate these obstacles; however, they are usually restricted to in‐house codes and thus require a disproportionate effort to adopt by researchers approaching the field. Herein, we present the FRamewOrk for Molecular AGgregate Excitations (fromage), which implements a collection of such techniques in a Python library complemented with ready‐to‐use scripts. The program structure is presented and the principal features available to the user are described: geometrical analysis, exciton characterization, and a variety of ONIOM schemes. Each is illustrated by examples of diverse organic molecules in condensed phase settings. The program is available at https://github.com/Crespo-Otero-group/fromage.

Deciphering the Mechanism of Aggregation-Induced Emission of a Quinazolinone Derivative Displaying Excited-State Intramolecular Proton-Transfer Properties: A QM, QM/MM, and MD Study

A combination of excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) has opened new opportunities to develop color-tunable luminescent materials with high quantum yield. Understanding the emission mechanism of these luminophores is essential for the molecular design and construction of a functional system. Herein, we report QM (MS-CASPT2//TD-DFT, MS-CASPT2//CASSCF) and ONIOM (QM/MM) studies on the fluorescence quenching and AIE mechanisms of 2-(2-hydroxy-phenyl)-4(3H)-quinazolinone with typical characteristics of AIE and ESIPT as an example. The computational results indicate that in the tetrahydrofuran solution, once being excited to the S₁ state, the molecule tends to undergo an ultrafast, barrierless ESIPT from enol to keto tautomer and then accesses a S₁/S₀ conical intersection in the vicinity of a C═C bond twisted intramolecular charge-transfer (TICT) intermediate, leading to a nonradiative decay from the excited to ground state. Hence, the TICT-induced nonadiabatic transition, which has been further confirmed by the on-the-fly trajectory surface hopping dynamics simulations, accounts for the fluorescence quenching in solution. In contrast, in the solid state, the nonradiative relaxation pathway via the C═C bond rotation is suppressed due to environmental hindrance, leaving the ESIPT-induced enol-keto tautomerization as the only excited-decay channel, thus the fluorescence is observably enhanced in the crystal.

Unraveling the Mechanisms of the Excited-State Intermolecular Proton Transfer (ESPT) for a D-π-A Molecular Architecture

Precise revealing the mechanisms of excited-state intermolecular proton transfer (ESPT) and the corresponding geometrical relaxation upon photoexcitation and photoionization remains a formidable challenge. In this work, the compound (E)-4-(((4H-1,2,4-triazol-4-yl)imino)methyl)-2,6-dimethoxyphenol (TIMDP) adopting a D-π-A molecular architecture featuring a significant intramolecular charge transfer (ICT) effect has been designed. With the presence of perchloric acid (35 %), TIMDP can be dissolved through the formation of a HClO₄ -H₂ O-OH(TIMDP)-N(TIMDP) hydrogen-bonding bridge. At the ground state, the ICT effect is dominant, giving birth to crystals of TIMDP. Upon external stimuli (e.g., UV light irradiation, electro field), the excited state is achieved, which weakens the ICT effect, and significantly promotes the ESPT effect along the hydrogen-bonding bridge, resulting in crystals of [HTIMDP]⁺ ⋅[H₂ O]⋅[ClO₄ ]^- . As a consequence, the mechanisms of the ESPT can be investigated, which distorted the D-π-A molecular architecture, tuned the emission color with the largest Stokes shift of 242 nm, and finally, high photoluminescence quantum yields (12 %) and long fluorescence lifetimes (8.6 μs) have achieved. These results not only provide new insight into ESPT mechanisms, but also open a new avenue for the design of efficient ESPT emitters.

Photochemical mechanism of 1,5-benzodiazepin-2-one: electronic structure calculations and nonadiabatic surface-hopping dynamics simulations

Due to the significant applications in bioimaging, sensing, optoelectronics etc., photoluminescent materials have attracted more and more attention in recent years. 1,5-Benzodiazepin-2-one and its derivatives have been used as fluorogenic probes for the detection of biothiols. However, their photochemical and photophysical properties have remained ambiguous until now. In this work, we have adopted combined static electronic structure calculations and nonadiabatic surface-hopping dynamics simulations to study the photochemical mechanism of 1,5-benzodiazepin-2-one. Firstly, we optimized minima and conical intersections in S0 and S1 states; then, we proposed three nonadiabatic decay pathways that efficiently populate the ground state from the Franck-Condon region based on computed electronic structure information and dynamics simulations. In the first pathway, upon photoexcitation to the S1 state, the system proceeds with an ultrafast excited-state intramolecular proton transfer (ESIPT) process. Then, the molecule tends to rotate around the C-C bond until it encounters keto conical intersections, from which the system can easily decay to the ground state. The other two pathways involve the enol channels in which the S1 system hops to the ground state via two enol S1/S0 conical intersections, respectively. These three energetically allowed S1 excited-state deactivation pathways are responsible for the decrease of fluorescence quantum yield. The present work will provide detailed mechanistic information of similar systems.

Consideration of Molecular Structure in the Excited State to Design New Luminogens with Aggregation-Induced Emission

Aggregation-induced emission (AIE) is a photoluminescence phenomenon in which an AIE luminogen (AIEgen) exhibits intense emission in the aggregated or solid state but only weak or no emission in the solution state. Understanding the mechanism of AIE requires consideration of excited state molecular geometry (for example, a π twist). This Minireview examines the history of AIEgens with a focus on the representative AIEgen, tetraphenylethylene (TPE). The mechanisms of solution-state quenching are reviewed and the crucial role of excited-state molecular transformations for AIE is discussed. Finally, recent progress in understanding the relationship between excited state molecular transformations and AIE is overviewed for a range of different AIEgens.

Exploring Potential Energy Surfaces for Aggregation-Induced Emission-From Solution to Crystal

Aggregation-induced emission (AIE) is a phenomenon where non-luminescent compounds in solution become strongly luminescent in aggregate and solid phase. It provides a fertile ground for luminescent applications that has rapidly developed in the last 15 years. In this review, we focus on the contributions of theory and computations to understanding the molecular mechanism behind it. Starting from initial models, such as restriction of intramolecular rotations (RIR), and the calculation of non-radiative rates with Fermi's golden rule (FGR), we center on studies of the global excited-state potential energy surfaces that have provided the basis for the restricted access to a conical intersection (RACI) model. In this model, which has been shown to apply for a diverse group of AIEgens, the lack of fluorescence in solution comes from radiationless decay at a CI in solution that is hindered in the aggregate state. We also highlight how intermolecular interactions modulate the photophysics in the aggregate phase, in terms of fluorescence quantum yield and emission color.

Mechanisms of fluorescence quenching in prototypical aggregation-induced emission systems: excited state dynamics with TD-DFTB

A recent implementation of time-dependent tight-binding density functional theory is employed in excited state molecular dynamics for the investigation of the fluorescence quenching mechanism in 3 prototypical aggregation-induced emission systems.

White-light generation from all-solution-processed OLEDs using a benzothiazole–salophen derivative reactive to the ESIPT process

ESIPT for white-light generation from all-solution-processed OLEDs.

Aggregation-caused quenching versus crystallization induced emission in thiazolo[5,4-b]thieno[3,2-e]pyridine (TTP) derivatives: theoretical insights

We report a QM (TD-DFT) and QM/QM' (ONIOM) study of the modulation of emission in a series of thiazolo[5,4,b]thieno[3,2-e]pyridine (TTP) derivatives [Huang et al., J. Mater. Chem. C, 2017, 14, 3456]. By computing the excitation energy transfer couplings and the Huang-Rhys (HR) factors, we rationalize the aggregation-caused quenching (ACQ) observed for the parent molecule and the crystallization-induced emission (CIE) observed for the derivatives presenting intra-molecular H-bonding. We also show that the CIE strategy relying on the rigidification of the arch-bridge-like stator should be considered with caution since it can promote the energy dissipation through vibrational motions.

Structural and Spectral Features of 4'-Substituted 2'-Hydroxychalcones in Solutions and Crystals: Spectroscopic and Theoretical Investigations

The article describes investigations of 2'-hydroxychalcone and its three derivatives bearing differently sized alkyloxy groups at position 4'. The compounds are investigated from the point of view of crystal structure, electronic absorption, fluorescence features in solutions and crystals using X-ray diffraction and electronic spectroscopy methods, and quantum chemistry calculations. In general, both in solutions and in the crystal phase, the influence of substituents on absorption spectra of chalcones was found to be insignificant. Exclusively in the case of 4'-(4-methoxybenzyloxy)-2'-hydroxychalcone, molecular packing influences the absorption features, which is because of the intermolecular interactions of substituent's phenyl ring and chromophore fragment of the neighboring molecules. The lack of fluorescence of the excited enol form of chalcones in solutions and crystals is mainly due to intersystem crossing and excited-state intramolecular proton transfer. Fluorescent properties of the phototautomer keto species formed by the proton transfer depend on molecular conformation. In solutions, the excited keto form is twisted and effectively deactivates nonradiatively due to conical intersection. In the crystal phase, the fixed planar geometry disables the conical intersection and the fluorescence of the keto form becomes detectable.