Dr Satoshi Suzuki
email: suzuki.satoshi.8v@kyoto-u.ac.jp

Dr Shunsuke Sasaki

] Amir

Sharidan Sairi

Riki Iwai

Ben Zhong Tang

Prof. Dr Gen- Ichi Konishi
email: konishi.g.aa@m.titech.ac.jp

A S Sairi

Dr B Z Tang

Principles of Aggregation-Induced Emission: Design of Deactivation Pathways for Advanced AIEgens and Applications

Twenty years ago, the concept of aggregation-induced emission (AIE) was proposed, and this unique luminescent property has attracted scientific interest ever since. However, AIE denominates only the phenomenon, while the details of its underlying guiding principles remain to be elucidated. This mini review discusses the basic principles of AIE based on our previous mechanistic study on the photophysical behavior of 9,10-bis(N,Ndialkylamino)anthracene (BDAA) and the corresponding mechanistic analysis by quantum chemical calculations. BDAA comprises of an anthracene core and small electron donors, which allows discussing the quantum chemical aspects of AIE. The key factor for AIE is the control over the non-radiative decay (deactivation) pathway, which can be visualized by considering the conical intersection (CI) on a potential energy surface. Controlling the conical intersection (CI) on the potential energy surface enables the separate formation of fluorescent (CI:high) and non-fluorescent (CI:low) molecules [control of conical intersection accessibility (CCIA)]. The novelty and originality of AIE in the field of photochemistry lies in the creation of functionality by design and in the active control over deactivation pathways. Moreover, we provide a new design strategy for AIE luminogens (AIEgens) and discuss selected examples.

Introduction

Over the last two decades, substantial attention has been focused on aggregation-induced emission (AIE) in materials science, analytical chemistry, and life sciences. [1] The unique luminescence behavior of AIE luminogens (AIEgens) has sparked the curiosity of various segments of the scientific community. After the discovery of prototypical AIEgens such as pentaphenylsilole [2] and tetraphenylethene (TPE), [3] research on the mechanisms of quenching in solution and emission in the solid state begun (Figure 1). AIE depends first and foremost on the control over non-radiative decay (deactivation pathways). The first proposal to control such non-radiative decay was based on the restriction of the intramolecular rotation (RIR) model. [4] In solution, non-radiative decay of biaryl compounds occurs by axis rotation, which is restricted in the solid state. In the solid state, this rotation is restricted. Therefore, many AIEgens exhibit multiple biaryl bonds and complex structures. The validity of the However, AIE is a general term for a phenomenon, and a variety of molecular systems and emission or quenching mechanisms exist (Figure 2). AIE phenomena are ambiguous such as RIR, E/Z isomerization, or the formation of J-type aggregates. [9,10] For example, the strong emission of 1-cyano-trans-1,2-bis-(4'methylbiphenyl)ethylene (CN-MBE) in the aggregated state is largely due to the planarization of the π-system and to the formation of J-type aggregates. [11] In the case of TPE, both RIM and planarization to expand the π-conjugation are thought as important factors for the observed AIE phenomena. [12] Other AIEgens such as BODIPY derivatives exhibit twisted intramolecular charge transfer (TICT) in solution, [1e,13] while Schiff-base derivatives exhibit excited state intramolecular proton transfer (ESIPT), [14] and cyclooctatetrathiophene derivatives are subject to restricted skeletal isomerization. [15] Figure 2. Selected representative AIEgens and a new class of AIEgens: 9,10bis(N,N-dialkylamino)anthracenes (BDAA) and their archetypical example 9,10-bis(piperidyl)anthracene (BPA) with its crystallographic structure.

10.1002/anie.202000940

Accepted Manuscript

Angewandte Chemie International Edition

This article is protected by copyright. All rights reserved.

To understand the relationship between intramolecular motion and AIE properties, Blancafort and Morokuma conducted independent quantum chemical investigations [16][17][18][19] in order to analyze diphenyldibenzofulvene [17] , tetraphenylsilole [18] , and phenyleneimide [19] AIEgens by restricted access to a conical intersection (RACI) model. RACI model explains AIE by difference of non-radiative decay rate in the different circumstances. The quantum chemical calculations predicted a largely distorted structure near the minimum energy conical intersection (MECI). In solution, the MECI is easy to access, while it becomes difficult to access in the solid state, and prohibited in the crystalline state (Figure 3). Kokado applied this method to TPE and further demonstrated experimentally the importance of isomerization. [20] The relationship between AIEgens and photophysical processes (Jabłoński diagram) is shown in Figure 4. Fluorescence quenching of AIEgens in solution must occur through an internal conversion process (S1→S0), whose efficiency is directly affected by the restrictions of intramolecular motions. Therefore, in order to rationally design AIEgens, one must design their internal conversion process so that it becomes fast only in solution but not in solid states. Recently, we discovered 9,10-bis(N,N-dialkylamino)anthracenes (BDAA), 9,10-bis(piperidyl)anthracene (BPA), as well as the corresponding naphthalene and pyrene analogues as a new class of AIEgens. [21][22][23] BDAA is a unique AIEgen that does neither contain isomerizable double bonds nor exhibit axial rotation and planarization of the π-system (Figure 2). However, the excited-state structures and luminescence properties of BDAA can be clearly explained by the RACI model. BDAA does not interact with nearby molecules in the excited state, but acts as an isolated molecule, which is advantageous for the elucidation of the AIE mechanism. Details of the features of BDAA will be described in the next chapter (vide infra).

"What is essential in the AIE mechanism?" In this review, we address this question and elucidate the AIE mechanism, as well as the necessary theoretical calculations and experiments in realizing the production of AIEgens based on this mechanism. In essence, we demonstrate a methodology of analyzing the photophysical properties of AIEgens in order to elucidate the mechanism of AIE by quantum chemical calculations, using BDAA as an example. [21][22][23] BDAA is an ideally suited fluorescent dye to characterize AIE. Our studies revealed that it is possible to discuss photophysical processes on the basis of the potential energy surface (PES), which is similar to thermal chemical reactions, and thus develop rational design strategies for AIEgens. [22,23] In thermal chemical reaction, transition states control kinetics of the reaction. On the other hand, kinetics of non-radiative decay is mainly determined by the conical intersection (CI). Thus, accessibility of conical intersection determines the non-radiative decay rates and concequently fluorescence quantum yield. Our approach to design of AIEgens is based on the concept that controling conical intersection accessibility (CCIA) enables to tune fluorescence quantum yield. We also discuss the classification of previously known AIEgens and the potential of new dyes with respect to governing the fluorescence on/off mechanism by controlling the CI. Finally, the impact of AIE research on photochemistry is described.

The non-radiative decay pathway can be explained on the PES.

If there exists a low conical intersection, the non-radiative decay is promoted in solution (Figure 3). This fast internal conversion leads to low fluorescence quantum yields (Φ fl ) in dilute solution.

In contrast, ring puckering is prohibited in the aggregated states and internal conversion is thus not preferred; therefore, the Φ fl in the aggregated states is enhanced. In the case of BDAA, a ring puckering type CI stabilized by rotation of amino groups promotes internal conversion. The Jabłoński diagram simply illustrates the interstate transition, and does accordingly not contain details about the photochemistry, given that it does not include nuclear coordinates (Figure 4).

Bis(N,N-dialkylamino)anthracenes

Our quest for acene-based AIEgens started in 2015 with the serendipitous discovery that the regioisomerism of bis(piperidyl)anthracenes (BPAs) switches their AIE/ACQ behavior. [21,[START_REF]We reported a type of push-pull fluorescent solvatochromic dyes. In the synthesis step of the anthracene analog, 9,10-BPA was obtained as a by-product. We purified the crude sample by silica gel column chromatography. The obtained by-product solution did not exhibit fluorescence. After solvent evaporation, the obtained solid showed strong green fluorescence under 365 nm UV[END_REF][START_REF]We also studied the conjugation between highly twisted amino groups and π-systems. 9,10-bis(diphenylamino)anthracene is a host molecule in OLEDs[END_REF] Figure 5 highlights the relationship between the photophysical properties of BPAs and their regioisomerism: the 1,4-(1,4-BPA) and 9,10-isomers (9,10-BPA) exhibit strong AIE, while the other isomers show ACQ. More interesting than the regioisomeric effects were the absence of any structural features commonly observed for other reported AIEgens (vide supra). BPAs possess neither extended π-systems with multiple free rotatable bonds, nor strong donor-acceptor pairs or isomerizable double bonds -they simply contain an anthracene ring and dialkylamino groups, whose Aryl-N bond rotation is severely hindered by peri-hydrogen atoms. Such structurally simple and Before discussing the photophysical details, several key features of 1,4-BPA and 9,10-BPA as AIEgens are summarized. Figure 6 illustrates the enhancement of their fluorescence intensity upon aggregation in a THF-water system. When the volume fraction of water reaches 90 vol%, both isomers form nanoaggregates (diameter = ca. 100 nm) with high Φ fl values that are comparable to those of polycrystalline solids (Figure 5). Apart from their quantum yields, their spectral shape and position are comparable, regardless of the conditions the solids are in (e.g., nanoaggregates, millimeter-sized crystallites, or milled dispersions in NaBr). Such robust fluorescence would be advantageous for potential imaging applications. Another attractive feature of these BPAs is their large Stokes shifts. After the emergence of super-resolution optical microscopy based on stimulated emission depletion (STED), numerous studies have engaged in the design of fluorescent dyes with large Stokes shifts, whereby crosstalk between the STED beam and the excitation is suppressed. [26] The Stokes shifts of solid 1,4-BPA (5500 cm -1 ) and 9,10-BPA (7400 cm -1 ) are comparable or larger than those of the aforementioned custom-designed dyes (Figure 7). 1,4-BPA and 9,10-BPA exhibit identical absorption maxima (ca. 400 nm) but fluorescence at very different spectral positions, which renders these dyes suitable candidates for potential applications in multicolor-color STED imaging using a single laser beam pair. [27] However, the fluorescence behavior of 9,10-BPA allows monitoring phenomena even more fundamental than aggregation. The fluorescence of the 9,10-BPA analogue 9,10bis(N,N-dimethylamino)anthracene exhibited sensitive response to the viscosity, η, of its environment (Figure 8). Its sensitivity toward η (x = 0.66) in the fluorescence intensity I(λ fl ) was estimated using the Förster-Hoffmann equation [28] log 𝐼 𝜆 𝐶 𝑥 log 𝜂 . As this value is comparable to that of representative molecular rotors such as 9-(dicyanovinyl)-julolidine (DCVJ; x = 0.53) [29] and Thioflavin T (x = 0.72), [30] we established synthetic routes to attach 9,10-BPA derivatives with different alkyl groups (BDAAs) to other macromolecular systems. [31] Hereafter, we show the photophysical properties common to the entire family of BDAAs, as all these exhibit outstanding AIE with comparable spectral parameters. To unlock the AIE mechanism of BDAAs one must firstly determine if the fluorescence properties of BDAAs can be discussed in the context of the RIR/RIV mechanisms, [6] similar to other AIEgens. If so, Φ fl values of BDAAs should be susceptible primarily to their internal conversion efficiency.

The enhanced fluorescence of BDAAs upon aggregation can be explained by their non-radiative transition rate. Table 1 demonstrates that aggregation largely suppresses the nonradiative transition rate, k nr , of the 9,10-isomer while its radiative transition rate remained at approximately k r = 5 10 7 s -1 both in solution and in aggregates. In addition, the k nr of the 9,10-isomer was higher in solution than those of other regioisomers, implying that 9,10-substitution promotes fast non-radiative transition processes that are restricted in the solid state. Although intersystem crossing is another potential non-radiative transition besides internal conversion, we can rule out this possibility as phosphorescence was absent in BDAAs, even at 77 K. These results rationalize that AIE phenomena of BDAAs was attributed to the suppression of internal conversion in their solid states: Indeed BDAAs may serve as an ideal example to study RIR / RIV effects upon aggregation since their solid-state fluorescence was almost free from other complicated electronic phenomena in solid state such as J-or H-aggregate formation. Their absorption/fluorescence spectral shapes in solid states remain unchanged from those in solution states (Figure 9), indicating the absence of excitonic interactions. Compared to the bulky structures of typical AIEgens with efficient solid-state fluorescence, [32] the structurally compact BDAAs feature relatively small interchromophoric distances in solids (Figure S1). Nevertheless, fluorescence of BDAAs were monomeric probably due to their large Stokes shifts with minimal structural relaxations, which can cause so-called exciton self-trapping, a process where local lattice deformations trap excitons into a single luminophore. [33] Table 1. Radiative (k r ) and non-radiative (k nr ) transition rates of each regioisomers in solution and colloidal suspensions (THF:H 2 O = 1:9).

THF solution

Colloidal suspension a k r [10 b The N,N-dipropylamine analogue of 9,10-BPA was used as the Φ fl of 9,10-BPA (0.02) is too small to derive reliably accurate rate constants.

Then, the discussion above clarified that internal conversion processes play a main role in the AIE phenomena of BDAAs. Therefore, a determination of which kind of intramolecular vibrations is responsible for AIE is necessary in order to develop the mechanistic study into a generalized design strategy. As noted previously, BDAAs contain, unlike the majority of AIE luminogens, neither isomerizable double bonds nor extended πsystems with multiple axial rotations. Nevertheless, in BDAAs, two Aryl-N axial rotations are present that might affect the energetics of the π-system. Namely, if Aryl-N bond rotations would drastically diminish the S 1 -S 0 gap, which is commonly known as the "free-rotor effect" of the Aryl-N bonds, fast nonradiative transitions would be promoted. [START_REF] Turro | Principles of Molecular Photochemistry: An Introduction[END_REF]35] However this is not case; rotations around the Aryl-N bond hardly decreased the S 1 -S 0 gap due to the presence of the peri-hydrogen atoms in anthracene, which prevent the rotation. Instead, the large Stokes shifts of BDAAs arise from the "umbrella motion" of the pyramidal dialkylamino groups (Figure 9). Since such a small inversion mode can take place even in the solid state, this mode is unlikely to cause AIE. Accordingly, it seems difficult to identify large-amplitude modes responsible for the AIE of BDAAs when we rely exclusively on the empirical perspective. In order to better understand the AIE mechanism, it is therefore necessary to undertake an ab initio approach. The ground state adopts two stable conformations (S0MIN) that are interconverted through the inversion of its NR 3 tetrahedra. After photoexcitation, each Franck-Condon (FC) state relaxes through planarization of one NR 3 , leading to the conformation shown as S1MIN1. This "umbrella motion" occurs also at the other NR 3 tetrahedron under concomitant tilting of these planar trigonal planes, which gives another stable conformation (S1MIN2). These structural relaxations decrease the S 0 -S 1 transition energy to the extent comparable with the experimental Stokes shifts. Any MECI was not sampled in this calculation and is hence not shown here.

As illustrated in Figure 3, the ab initio approach assumes that AIEgens dissipate their photoexcitation energy through CIs. If the S 1 /S 0 CI lies sufficiently low to be accessed in solution but involves large structural deformations, such molecules are usually subject to fast internal conversion in solution but fluorescent in the solid state due to CCIA. Accordingly, the key questions for BDAAs are (1) if they possess a suitably low-lying S 1 /S 0 CI for easy access, and (2) if so, how 9,10-substitution by dialkylamino groups lowers the S 1 /S 0 CI.

Our CASSCF(10e/8o)/6-31G(d) calculations performed on 9,10bis(N,N-dimethylamino)anthracene revealed that its S 1 /S 0 minimum energy CI (MECI) resides at a much lower level than the excited Franck-Condon state. [22] In addition, the structure at the MECI involves a large out-of-plane deformation of the anthracene ring with two upraised dialkylamino groups (Figure 10). Since such a large-amplitude motion is without doubt restricted in the solid state, the AIE of 9,10-bis(N,Ndimethylamino)anthracene can be rationalized in terms of this bending mode on the aromatic ring.

At first glance, this result seems counterintuitive to the stereotypical perception of classic polycyclic aromatic hydrocarbons (PAHs) as rigid planes. Thus, the second question must be addressed: how does the 9,10-substitution pattern of dialkylamines "soften" the anthracene ring in the excited state? Similar ring puckering has been observed in benzene, when benzene is excited with energies higher than 0.37 eV, which triggers the specific photophysical pathway "Channel 3". [36] In Channel 3, benzenes at the S 2 level are subject to fast internal conversion via the S 2 /S 1 CI and the S 1 /S 0 crossing seam with a Dewar-benzene structure. [37] Such a Dewar-benzene geometry was also found in our calculations for the MECI of benzene, which displays bonding between its bridgehead positions (Figure 10). In the same way, the S 1 /S 0 MECI of BDAA can be regarded as an analogue of Dewar benzene. Accordingly, we can expect that 9,10-substitution in BDAAs by strong electron donors amplifies the bonding nature between the BDAA bridgehead positions, which would stabilize the Dewar-benzene structure of the MECI. This hypothesis was confirmed by our natural orbital analysis for the S 1 /S 0 MECI of BDAAs. [22] In general, the AIE in BDAAs can be described as follows: dialkylamino groups at the 9,10-positions stabilize the S 1 /S 0 MECI of the anthracene ring by donating electrons to the bridge of a Dewar-benzene-like structure, which promotes dissipation of the photoexcitation energy via large-amplitude bending modes. Through these calculations and discussion, the AIE mechanism in BDAAs became clear. The next step was to generalize the insight into the design strategy of acene-based AIE luminogens (Figure 11). In short, the design strategy consists of two simple steps. To make classic PAHs AIE-active, firstly we must know their MECI, especially those with out-of-plane deformations, because their electronic states may be analogous to Dewar benzene. Fortunately, Maeda and co-workers have already reported the MECI for benzene, naphthalene, phenanthrene, anthracene, and pyrene. [38] The second step is to introduce dialkylamino groups at the positions where their aromatic rings deform the most in the MECI. During this step it will often be necessary to twist the Aryl-N bonds using steric hindrance, as twisted Aryl-N bonds can keep their Franck-Condon states and S 1 minima higher than those without steric hindrance, leaving the MECI more accessible. In the following parts, we briefly exemplify this design strategy by converting the two classic aromatic hydrocarbons naphthalene [22] and pyrene. [23] According to Maeda, [38] the most stable S 1 /S 0 MECI of naphthalene deforms in a way that pushes its 1-or 4-positions upwards. Therefore, we examined how dialkylamines at these positions would affect the photophysical properties of naphthalene in solution and in the solid states. [22] When one or two dialkylamine groups were introduced in the 1,4-position, the corresponding BDAAs exhibited moderate to strong fluorescence in solution (Figure 12). While we could not evaluate the Φ fl value of 1-(N,N-dimethylamino)naphthalene (DAN) crystals, 1,4-bis(N,N-dimethylamino) naphthalene (BDAN) exhibited enhanced fluorescence in solution (Φ fl = 0.59) and in polycrystalline solids (Φ fl = 0.90). As these two dyes exhibit fast internal conversions, [39] this might imply that the accessibility of their S 1 /S 0 MECI was improved simply by substitution with dialkylamine groups. This effect became much more pronounced when the dialkylamino groups were twisted via the methyl groups at the 2 Subsequently, we extended this design strategy to pyrene, which contains a bigger π-system compared to the two previous examples. At the same time, pyrene serves as an important building block for larger molecular systems. According to previous reports, [36] deformation around the 2-and 7-positions or the K-region (4-, 5-, 9-, and 10-positions) stabilizes the S 1 /S 0 MECI of pyrene more than deformation at the 1-, 3-, 6-, and 8positions. [38] To test our design strategy, we synthesized the four compounds shown in Figure 13. Although the Aryl-N bonds in 4,5-Py, 1-Py, and 1,6-Py are all sterically hindered both by adjacent functional groups and peri-hydrogen atoms, only 4,5-Py exhibits drastic AIE behavior with a prominent non-radiative transition rate (k nr = 57 10 7 s -1 ). Our calculations showed that the dilakylamino groups, which are in close proximity of each other, work cooperatively during the structural deformation, thus making the S 1 /S 0 MECI of 4,5-Py readily accessible. A comparison with ACQ-active 1,6-Py highlights the importance of these substituent positions.

Theoretical Insights into the AIE Mechanism

AIE can be understood as a response of the fluorescence quantum yield to the surrounding environment. An AIEgen leads to non-radiative decay in dilute solution, but not in aggregates. In some cases, a periodic electric field in e.g. crystals can enhance the fluorescence quantum yield. [40] However, AIE does not necessarily require crystallization and thus it is not feasible to consider that the enhancement of the fluorescence rate constant by the electric field from the surrounding molecules is the main reason for AIE. It seems intuitive to think that AIE is essentially due to a steric hindrance of the non-radiative decay by surrounding molecules. Thus, once non-radiative decay pathways on the PES are obtained, one can understand the working mechanism of the AIEgen. Non-radiative decay pathways can be obtained in the same way as pathways for thermal chemical reactions, except that nonradiative decay pathways contain transitions between different electronic states.

The PES is a function of the electronic energy, whose domains are nuclear coordinates. Thus, each minimum on the PES corresponds to an equilibrium structure, while each saddle point corresponds to a transition state. The minimum energy path connecting two different chemical species describes a static limit of a possible chemical reaction. Even though an actual chemical reaction at finite temperature deviates from the minimum energy path, the minimum energy path still affords a qualitative insight into the chemical reaction. Since the first successful determination of a full organometallic catalytic cycle, [41] quantum mechanical (QM) calculations have clarified numerous mechanisms of catalytic reactions and provided further insights to tune transition states to improve reaction rate and selectivity.

Although thermal chemical reactions can be discussed within the the ground state PES, photophysical processes and photochemical reactions contain transitions between multiple PESs. After photo-irradiation, an excited molecule is generated. Absorptions end up within negligibly short time compared to nuclear motion, and accordingly, such transitions are called vertical excitations. The excited molecule, which is in the higher singlet excited state (S n ) will rapidly relax into the lowest singlet excited state (S 1 ) by internal conversion. The S 1 state exhibits a longer lifetime than high-lying states before the fluorescence, intersystem crossing (ISC) to the lowest triplet state (T 1 ) state, or internal conversion to the singlet ground (S 0 ) state. As the fluorescence occurs from the relaxed structure, the fluorescence wavelength is usually longer than the absorption wavelength. This difference is called Stokes shift. Both ISC and internal conversion represent transitions between different states. Nonradiative transitions between two different spin states are called ISC, while non-radiative transitions between identical spin states are called internal conversions. Although ISCs to T 1 are slower (~ns) than internal conversions to S 0 (~ps), ISCs can be accelerated by large spin-orbit coupling involving relatively heavy elements. Once the T 1 state has been generated, it usually exhibits a very long lifetime (~ms). Thus, bimolecular photochemical reactions in the T 1 state are more likely to occure than in the S 1 state. Both types of non-radiative transition occur near the crossing seam of two PESs, and especially, a crossing seam between the same spin states is called conical intersection (CI). Thus, locating CIs on PESs, especially MECIs is important to understand how a molecule might engage in non-radiative decay. Accordingly, the photostability of many types of molecules has been investigated by identifying non-radiative decay pathways. [42] Possible products after passing through CIs can be located using the initial reaction direction (IRD) method. [43] Even though the static calculation of PESs does not give any insight into the dynamics, such as the excited-state lifetimes, it can potentially tell which kind of distortion (or vibrational mode) is responsible for the non-radiative decay. Just as transition states in thermal chemical reactions determine the reaction rate, tuning of the CI may determine non-radiative decay rate. Although some recent spectroscopic experiments have succeeded in observing CIs, [44] this approach is still far from making use of CIs to design functional molecules. AIE can be understood as a response of the fluorescence quantum yield to the surrounding environment. The fluorescence quantum yield is given as

Φ fl 𝑘 r 𝑘 r 𝑘 nr
As k r is determined by the transition dipole of the dye and thus not particularly sensitive to the surrounding environment, drastic changes of Φ fl are mainly derived from non-radiative decay rate k nr . If the CI is accompanied by a large distortion, stronger steric hindrance by surrounding molecules in the aggregate phase than in dilute solution can disturb the non-radiative decay. In such cases, Φ fl is enhanced by aggregation. This is the basic idea of the CCIA strategy. Several theoretical investigations have successfully rationalized AIE in terms of the CCIA strategy. [16,22] Some experimentalists have mentioned that the possibility of E-Z isomerization of some types of AIEgens may affect the Φ fl negatively. [45] The Φ fl is microscopically determined by k nr and thus fully independent from the product after internal conversion. However, the Φ fl for the entire system is determined by the mole fraction of the possible isomers. In case that only one isomer shows AIE, the total Φ fl might actually be affected by isomerization. Nevertheless, one has to take into consideration that the E/Z isomerization itself is independent of the AIE mechanism.

Structure of Typical CIs and Non-radiative Decay Pathways of AIEgens

CIs of large molecules typically results from local distortion. Thus, non-radiative decay pathways of AIEgens can be classified according to the deformation of their building blocks. The structures of the CIs of small molecules are classified into several types, which include ring puckering, torsions of double bonds, and ring openings. [46] In fact, some mechanism of AIEgens have been reported that are based on a coordinate double bond torsion and ring puckering. Theoretical investigations have suggested an involvement of double-bondtorsion-type CIs in AIEgens such as diphenyldibenzofulvene (DPDBF), [17,47] cyano-substituted distyrylbenzene (DBDCS), [48] and 2-(2-hydroxyphenyl)benzothiazole (HBT). [49] Ringpuckering-type CIs have also been found in DMTPS [18] and BDAA. [21] Based on the CCIA strategy, we can suggest three conditions that AIEgens should satisfy: 1.) the CI in solution should have low energy. This condition has to be satisfied to achieve low Φ fl in solution. Electronic control of the CI is not intuitive yet, but at least, it is possible as our BDAA succeeded in lowering the ringpuckered CI by conjugation of the amine. 2.) The CI should have large displacement in order to acquire sensitivity to the surrounding molecular environment. 3.) A small orbital overlap in aggregates to avoid ACQ. Conversely, large intermolecular orbital overlaps can lead to energy transfer pathways and negatively affect the Φ fl .

These design guidelines allow the molecular engineering of new AIEgens from small cores with known CIs. Starting from a nonfluorescent molecule with a known CI, AIEgens can be easily obtained by introducing bulky groups at certain positions.

Ring opening/closing-type CIs are not common in AIEgens unitl now. Although previous reports have suggested that the cyclization of TPE is easier than double bond twisting, it does not mean cyclization is the dominant mechanism of AIE in TPE. In any case, ring opening/closing-type CIs might be a novel candidate to achieve AIE. For example, coumarin exhibits a lowenergy CI with a ring-opening-type structure. [50] Modifications of certain positions in order to amplify the motion toward the CI might enable introducing AIE behavior in coumarin derivatives. Moreover, conrotation in 4n+2 electrocyclic reactions might also be applicable, provided that energy and structure are modified suitably. In this section, we have discussed the relationship between structures of typical CIs of small molecules and the non-radiative decay pathways of AIEgens. In many cases, CIs are governed by local deformation, and they thus represent good starting to determine the mechanism of AIE in terms of possible CIs of building blocks. In fact, many known non-radiative decay pathway of AIEgens can be classified in the same way as small molecules. Although it is still difficult to determine the structure of CIs by spectroscopy, it is already possible using computational methods. Several theoretical studies have already succeeded in explaining the basic mechanisms of AIE, and with advancing computational methods, it can be expected that it will be soon possible to predict the photophysical properties of a compound before it is synthesized.

Conclusions and Outlook

Aggregation-induced emission (AIE) requires control of the nonradiative decay, as our case study on the photophysical processes of BDAA and the mechanistic analyses by quantum chemical calculations have shown. [22] Based on this concept (published in the Angew. Chem. special issue on the occasion of the 20 th anniversary of AIE), [53] we have very recently introduced bridged stilbenes as a new type of AIE luminogens (AIEgens). The term "aggregation-induced emission" refers to the induction of luminescence upon formation of aggregates. However, this definition does not explain the non-radiative decay in solution. We thus propose that AIE should be explained on an "environment-responsive non-radiative decay" concept, which regards changes of quantum yields are derived from perturbation by surroundings. [53] Theoretical investigations have revealed that it is possible to discuss photophysical processes based on a potential energy surface (PES), similar to classical chemical reactions. Controlling the conical intersection (CI) enables the separate formation of fluorescent and non-fluorescent molecules. It should be noted that the process after passing through the CI has little effect on the AIE phenomenon itself. Although static calculations such as Fukui's intrinsic reaction coordinate (IRC) approach [51] do not describe non-adiabatic dynamics around the CI, it still provides a useful guide of possible reaction pathways. The novelty and originality of AIE is the creation of function by design and the active control of deactivation pathways (nonradiative decay pathways) in photophysical processes (Figure 4). Although current time-resolved absorption spectrum measurements cannot observe the behavior of AIE luminogens (AIEgens) in the excited state, it is interesting that quantum chemical calculations have already predicted transient structures during the internal conversion process. Therefore, the results of the calculations will be useful for understanding the measurement results of the chemical intermediates and transient species. In the near future, the molecular design of AIEgens using the control of conical intersection accessibility (CCIA) strategy is expected to evolve rapidly. The search of CIs by the artificial force induced reaction (AFIR) method is also possible and may be effective. [52] In this review, we discuss AIE in the photophysical process of single-molecule level. On the other hand, AIEgens, which exhibit a higher quantum yield in the high order structures such as eximers and J-aggregates, have also been reported. [10.11] In those case, we may need to include intermolecular interaction between two AIEgens. The difference between the potential energy surfaces for AIEgen monomer and dimer would tell what is important reaction coordinate in such cases.

Advanced light-emitting materials based on AIEgens and thermally activated delayed fluorescence (TADF), [54] upconversion luminescent materials based on triplet-triplet annihilation (TTA) [55] and singlet fission [56] are not new, neither phenomenologically nor mechanistically. [START_REF] Stokes Reported | AIE" effect in an inorganic salts system in 1853![END_REF] However, the design of such materials based on the fundamentals of photochemistry and state-of-the-art measurements and calculations is new. Although the thus obtained results are primarily fundamental in nature, they have an immediate impact on the development of advanced light-emitting materials. This approach is consistent with the philosophy of Confucius, who promotes "developing new ideas based on learning from the past." [START_REF]子曰、温故而知新、可以為師矣" in Japanese "温故知新 (onko-chishin)" by Confucius "孔子[END_REF] AIE is a phenomenon that, unlike TADF and upconversion, occurs due to internal conversions, which rely on spin control (Figure 4). AIE has been reported as a curiosity 20 years ago. [2] Since then, many researchers have investigated this phenomenon and numerous applications have been developed. In the near future, AIE will become an established phenomenon of functional materials that can be custom-tailored. As said by Confucius "a visit by a friend from afar is truly a delight", [START_REF]朋有り遠方より来 る、また楽しからずや[END_REF] let us enjoy and work together in AIE research. [START_REF] Feng | AIE research articles selected by Tutorial of MRS fall 2019[END_REF][61][62]

Satoshi Suzuki was born in

Figure 1 .

 1 Figure 1. Typical AIEgens and schematic illustration of the RIR mechanism.

Figure 3 .

 3 Figure 3. Schematic illustration of the conical intersection accessibility in dilute solution (left) and aggregates (right).

Figure 4 .

 4 Figure 4. Jabłoński diagram of a typical organic fluorophore

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  rigid D-π-D-type molecules should exhibit ACQ similar to other BPA isomers. This contradictory structure-property relationship of para-substituted BPAs motivated us to elucidate their underlying mechanism beyond the empirical discussion based on conventional organic physical chemistry.

Figure 5 .

 5 Figure 5. Photographs of the fluorescence of BPAs in the solid state and in solution.

Figure 6 .

 6 Figure 6. AIE behavior of (a) 1,4-BPA and (b) 9,10-BPA measured in THFwater. Fluorescence quantum yield (Φ fl ) values of their aggregates (diameter: ca. 100 nm; water fraction = 90 vol%) are noted for comparison. Adapted from ref. 21 with permission from the Royal Society of Chemistry ©2015.

Figure 7 .

 7 Figure 7. Stokes shifts of 1,4-BPA and 9,10-BPA in polycrystalline samples (solid lines) and in toluene solution (dashed lines). Diffuse-reflectance spectra were measured for polycrystalline samples dispersed in NaBr. Adapted from ref. 21 with permission from the Royal Society of Chemistry ©2015.

  Figure 8. Viscosity-sensitive fluorescence of 9,10-bis(N,Ndimethylamino)anthracene. (a) Viscosity-dependent fluorescence spectra and (b) the corresponding double-logarithmic plots. Adapted from ref. 22 with permission from the American Chemical Society ©2016.

Figure 9 .

 9 Figure 9. Schematic illustration of the PES of 9,10-bis(N,Ndimethylamino)anthracene obtained from DFT/TD-DFT calculations at the B3LYP/6-31+G(d) level of theory. The ground state adopts two stable conformations (S0MIN) that are interconverted through the inversion of its NR 3 tetrahedra. After photoexcitation, each Franck-Condon (FC) state relaxes through planarization of one NR 3 , leading to the conformation shown as S1MIN1. This "umbrella motion" occurs also at the other NR 3 tetrahedron under concomitant tilting of these planar trigonal planes, which gives another stable conformation (S1MIN2). These structural relaxations decrease the S 0 -S 1 transition energy to the extent comparable with the experimental Stokes shifts. Any MECI was not sampled in this calculation and is hence not shown here.

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Figure 10 .

 10 Figure 10. (a) Natural orbital (occupation number = 0.745) at the MECI calculated for benzene; (b) S 1 /S 0 MECI of 9,10-bis(N,Ndimethylamino)anthracene and its Lewis structure. The S 0 /S 1 MECI search was performed at the CASSCF(10e,8o)/6-31G(d) level of theory. Adapted from ref. 22 with permission from the American Chemical Society ©2016.

Figure 11 .

 11 Figure 11. Design of dialkylamino-substituted PAHs as new AIEgens.

  ,3-positions (DAN → DMe-DAN; BDAN → DMe-BDAN). DMe-DAN and DMe-BDAN exhibit faint fluorescence in solution (Φ fl = 10.1002/anie.202000940Accepted Manuscript 0.02-0.05), but bright solid-state fluorescence in polycrystalline solids (Φ fl = 0.73-0.78). In particular DMe-BDAN displayed a strong viscosity effect on its fluorescence (x = 0.27). Therefore, the steric hindrance around the Aryl-N bonds should be considered essential for the design of advanced AIEgens based on naphthalene.

Figure 12 .

 12 Figure 12. Comparison of the fluorescence quantum yields of 1dimethylaminonaphthalene, 1,4-bis(dimethylamino)naphthalene, 1dimethylamino-2,3-dimethylnaphthalene, and 1,4-bis(dimethylamino)-2,3dimethyl)naphthalene together with photographic images of their fluorescence in solution or in the polycrystalline state. Adapted from ref. 22 with permission from the American Chemical Society ©2016.

Figure 13 .

 13 Figure 13. (a) Molecular structure of N,N-dimethylamine-substituted pyrene derivatives; (b) x-ray crystallographic structure of 4,5-Py; (c) S 1 /S 0 MECI structure calculated at the CASSCF(12e,9o)/6-31G(d) level of theory.

Figure 14 .

 14 Figure 14. Schematic illustration of a typical potential energy surface (PES). Left: Low conical intersection (CI) causes competition between fluorescence and internal conversion through a CI. Middle: If a CI is sufficiently high, fluorescence becomes dominant. Right: In some cases, an ISC to the triplet state occurs, where phosphorescence ensures.

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  photochemistry of functional materials.

  6 s -1 ] k nr [10 6 s -1 ] k r [10 6 s -1 ] k nr [10 6 s -1 ]

	1,4-isomer 9,10-isomer b	25 50	76 250	24 53	30 109
	1,5-isomer	67	20	49	360
	1,8-isomer	48	20	31	170
	2,6-isomer	54	7.2	43	250

a Rate constants were calculated based on amplitude-averaged lifetimes

Table 2 .

 2 Classification of CIs and their corresponding compounds.

	Structure of the CI	Compound
	Double bond torsion	TPE,DPDBF, Distyreneanthracene,
		HBDI, HBT
	Ring puckering	Tetraphenylsilole, BDAA,
		cyclooctatetrathiophene
	Ring opening/closing	TPE

Acknowledgements

We would like to thank Dr. Ulrich Mayer for proofreading this review. This work is partially supported by the Grant-in-Aid for Scientific Research (B) (18H02045), Innovative Areas "π-System Figuration" (17H05145) and, JSPS Fellows (16J10324), and JSPS Overseas Research Fellowships from MEXT of Japan.

Keywords: aggregation induced emission (AIE) • bis(dialkylamino)anthracene • control of conical intersection accessibility (CCIA) • environment-responsive non-radiative decay • potential energy surface (PES)

Amir Sharidan Sairi was born in Selangor, Malaysia in 1994. He graduated from the Department of Polymer Chemistry, Tokyo Institute of Technology in March 2018, and is currently pursuing his Master's degree in the Department of Chemical Science and Engineering in the same university. His research includes functionalization and polymerization of AIEgens. Riki Iwai was born in Tokyo, Japan in 1997. He graduated from the Department of Polymer Chemistry, Tokyo Institute of Technology in 2019. Now, he belongs to the Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology. He is studying about stilbenes and aggregation-induced emission luminogen (AIEgen). Ben Zhong Tang received B.S. and Ph.D. degrees from South China University of Technology and , Department of Polymer Chemistry, Kyoto University, respectively. He conducted postdoctoral research at University of Toronto. He joined HKUST as an assistant professor in 1994 and was promoted to chair professor in 2008. He was elected to the Chinese Academy of Sciences (CAS) and the Royal Society of Chemistry(RSC) in 2009 and 2013, respectively. He is currently serving as Dean of the SCUT-HKUST Joint Research Institute. His research interestsinclude macromolecular chemistry, materials science, and biomedical theranostics. Gen-ichi Konishi was born in Rochester, NY in 1971. He recieved his B.S., M.S. from Osaka Prefecture University, Department of Applied Chemistry (supervisor, Prof. Kazuhiko Mizuno) and his Ph.D from Kyoto University, Department of Polymer Chemistry (supervisor, Prof. Yoshiki Chujo) in 2000. He then joined Shinshu University, Division of Physiology, School of Medicine (2000) and Kanazawa University, Department of Chemistry and Chemical Engineering, (2002) as an assistant professor. In 2006, he moved to an Associate Professor and a PrincipaI Investigator of Tokyo Institute of Technology. His current research interests are polymer science, photochemistry, physical organic chemistry, materials chemistry, and physiology (autonomic nervous system).

MINIREVIEW MINIREVIEW

Entry for the Table of Contents

Principle of aggregation-induced emission (AIE):

What is essential in the AIE mechanism? We address this question, by explanation of our case study of the photophysical processes of 9,10-bis(N,N-dialkylamino)anthracene and the mechanism analysis by quantum chemical calculations. The AIE phenomenon requires control of the non-radiative decay (deactivation) pathway, i.e., controlling the conical intersection (CI) on the potential energy surface enables the formation of fluorescent molecules (CI high) and non-fluorescent (CI low) molecules separately.


Elodie Patrice Berthod
email: patrice.berthod@univ-lorraine.fr

Elodie Conrath

Jean Lamour

Microstructure evolution in the bulk and surface states of chromium-rich nickel-based cast alloys reinforced by hafnium carbides after exposure to high temperature in air

Keywords: cast nickel alloys, hafnium carbides, high temperature, bulk microstructure, oxidation, hardness

Microstructure evolution in bulk and surface states of chromium rich nickel based cast alloys reinforced by hafnium carbides after exposure to high temperature air

Introduction

In the past decades the cast superalloys based on nickel and reinforced by carbides were among the more often used metallic materials for high temperature service [START_REF] Bradley | Superalloys: A Technical Guide[END_REF], in competition with cobalt-based alloys which are also often carbidesstrengthened [START_REF] Sims | Cobalt-Base Alloys[END_REF]. The resistance of these high temperature alloys against hot corrosion was the most often achieved by introducing several tens percents of chromium [START_REF] Kofstad | High Temperature Corrosion[END_REF]. Nowadays the most efficient nickel-based superalloys are the γ/γ' single crystal nickel-based superalloys for high temperature applications [START_REF] Donachie | Superalloys, A Technical Guide[END_REF] in which the reinforcing phase is the intermetallic compound Ni 3 Al or more generally (Ni, Fe, Co) 3 (Al, Ti Ta, Nb). Many of them contain several percents of chromium but the resistance against high temperature oxidation of these superalloys -when not coated -is essentially due to aluminium which allows forming a protective continuous alumina scale [START_REF] Kofstad | High Temperature Corrosion[END_REF][START_REF] Young | High Temperature Oxidation and Corrosion of Metals[END_REF].

For applications in which oxidation by gases are not the single mode of alloys' chemical deterioration but corrosion by molten substances may also occur, chromium must be present in significant quantities in the chemical composition of the alloys [START_REF] Kofstad | High Temperature Corrosion[END_REF][START_REF] Young | High Temperature Oxidation and Corrosion of Metals[END_REF]. These ones cannot be thus mechanically reinforced by Ni 3 Al compounds, the presence of which necessarily limit the chromium contents, but by solid solution hardening (not always sufficient) or by carbides again. The chromium carbidesfeaturing among the historically first used ones -are not stable enough in case of very high temperatures of service, typically 1200°C. Other frequently used carbides, as the tantalum ones, resist more efficiently at high temperature against coarsening and volume fraction decrease [START_REF] Berthod | Experimental and Thermodynamic Study of the Microstructural State at High Temperature of Nickel base Alloys containing Tantalum[END_REF]. Unfortunately TaC are not stable in nickel-based alloys in presence of high quantity of chromium, with as consequence a disappearance of the rare TaC carbides formed at solidification for the benefit of chromium carbides more stable than them (inversely to what generally observed in cobalt-based superalloys [START_REF] Michon | Application of thermodynamic calculations to study high temperature behavior of TaC-strengthened Co-base superalloys[END_REF][START_REF] Berthod | Experimental and thermodynamic study of the microstructure evolution in cobalt-base superalloys at high temperature[END_REF]). Other MC carbides, less common than the TaC ones, can be considered for reinforcing cast chromium-rich nickel-based alloys, as the HfC ones. Indeed, these carbides, which were earlier successfully obtained with a script-like eutectic shape in the interdendritic spaces [START_REF] Berthod | High temperature properties of several chromium-containing Cobased alloys reinforced by different types of MC carbides (M = Ta, Nb, Hf and/or Zr)[END_REF] in Cr-rich Co-based cast alloys in which they demonstrated an interesting stability at very high temperature in terms of volume fraction as well as of morphology, have recently shown the same shapes, locations and high volume fractions in Cr-rich Ni-based alloys [START_REF] Berthod | Hafnium carbides in cast chromium-rich refractory alloys. Part 1: Case of nickel-based alloys[END_REF]. 25wt.%Cr-containing nickel-based alloys and containing several tenths weight percents of carbon, 0.25 and 0.50 wt.%C, and several percents of hafnium, 3.72wt.% and 5.58wt.%Hf, were thus obtained with a potentially interesting microstructure, with only -or at least predominantly -HfC carbides. In the case of the alloy containing 0.25C and 3.72Hf, it was notably wished to respect the atomic equality between these two elements for favouring the exclusive appearance of HfC carbides. But the stability of these HfC carbides at very high temperature remains to be tested to start formulating hypotheses about the high temperature mechanical behaviour of such alloys. In parallel, since hafnium is the most often added to superalloys in much smaller quantities to improve the high temperature oxidation of superalloys in terms of oxidation rate or oxide scales adherence [START_REF] Bouhanek | High-temperature oxidation of singlecrystal Ni-base superalloys[END_REF], its presence in so high quantities may greatly influence the high temperature oxidation behaviour of these new alloys and this no necessarily in the best direction... In this work, the three 25wt.%Cr-containing Ni-based alloys recently obtained with significant, and even exclusive, presence of interdendritic HfC carbides were exposed for several tens of hours in a simple oxidant atmosphere, dry air, and at a chosen high temperature: 1200°C, typical temperature at which such alloys are supposed to be used in case of good behaviour. The objectives were to characterize with this single test, the microstructure changes, notably concerning the natures and morphologies of the HfC carbides, and the deterioration by hot oxidation of the samples' surface and even of the whole alloy in case of catastrophic oxidation.

Experimental

The studied alloys

In a recent work [START_REF] Berthod | Hafnium carbides in cast chromium-rich refractory alloys. Part 1: Case of nickel-based alloys[END_REF] three 40g-weighing compact ingots of Hf-rich nickelbased alloys were elaborated by High Frequency induction melting in an inert atmosphere of 300mbars Ar. The initially wished chemical compositions of the three studied alloys, named "NiHf1", "NiHf2" and "NiHf3", were respectively Ni(bal.)-25Cr-0.25C-3.72Hf, Ni(bal.)-25Cr-0.50C-3.72Hf and Ni(bal.)-25Cr-0.50C-5.58Hf (in wt.%).

The chemical compositions which were really obtained are Ni(bal.)-26Cr-0.25C-4.4Hf, Ni(bal.)-26Cr-0.50C-4.8Hf and Ni(bal.)-25Cr-0.50C-6.6Hf, as specified by Energy Dispersion Spectrometry (EDS) measurements (only Ni, Cr and Hf; C: too light element, content supposed respected).

The as-cast microstructures of the obtained alloys, observed with a JEOL JSM6010LA scanning electron microscope (SEM) in back scattered electrons mode (BSE), are presented in Figure 1. Their microstructures are all dendritic with the presence of HfC carbides (white) in their interdendritic spaces where they form a script-like eutectic compound with the austenitic matrix. The NiHf2 alloy also contains chromium carbides (darker than matrix). These microstructures were confirmed by X-Ray Diffraction using a Philips X'Pert Pro diffractometer (used wavelength: Cu Kα 1.5406 Angströms) [START_REF] Berthod | Hafnium carbides in cast chromium-rich refractory alloys. Part 1: Case of nickel-based alloys[END_REF].

The high temperature exposures and preparation of the metallographic samples

Parallelepipeds of about 10 mm × 10 mm × 3 mm were cut in the ingots the thermogravimetry tests. They were polished with papers enriched with SiC particles paper grade varying from 120 or 240 to 1200. Edges and corners were smoothed using the 1200-grit paper. They were exposed, in a resistive furnace, at 1200°C during 46 hours to a continuous flow (1.5L h -1 ) of dry industrial air (80%N 2 -20%O 2 ), after a heating phase achieved at +20°C/min and before a cooling phase realized at -5°C/min. Just after cooling and before any metallographic preparation, the surface of the thermally aged samples were subjected to X-ray diffraction (XRD, Philips X'Pert Pro diffractometer) to identify the oxides developed externally and covering the samples. After this preliminary characterization the aged samples were covered by a thin layer of gold (cathodic sputtering) to obtain an external surface electrically conductive. A thick nickel coating was then deposited by cathodic polarization of the sample in a Watt's bath heated at 50°C. The external oxide scale being thus protected against any mechanical stresses, each sample was cut in two parts which were then embedded in a cold resin mixture (resin CY230 + hardener HY956, ESCIL). The mounted samples were finally polished with the same successive SiCpapers as done above, but also ultrasonically cleaned and polished until reaching a mirror-like surface state as required for metallographic examinations.

Bulk microstructure observations and hardness measurements

The examination of the microstructures of the aged samples were realized using a SEM (JEOL JSM-6010LA) in BSE mode, at different magnifications. The particles were identified by EDS pinpoint measurements using the EDS device equipping the SEM, technique which was also used to specify the Cr and Hf contents in the matrix. Three ×1000 BSE micrographs were notably taken per aged sample to perform surface fraction measurements by image analysis, using the Photoshop CS software of Adobe.

Vickers indentations were carried out in the bulk using a Testwell Wolpert device (load: 30kg). An average value and a standard deviation one were then obtained for each of the aged samples.

The same characterizations, particles' identification, pinpoint EDS in matrix, image analysis of carbides and indentations, were additionally realized on the as-cast samples of the alloys, to allow comparisons.

2.4 Surface and sub-surface of the aged/oxidized alloys EDS measurements were performed with the same SEM on the thickness of the oxide scales seen in cross-section, to identify the present oxides and notably to verify if the scales are mainly composed of chromia. The same technique was used to specify the chromium (and hafnium) contents in extreme surface to verify whether the alloy is still chromia-forming and in the affirmative case if one can expect that the alloy will keep this chromia-forming behaviour a long time after these first 46 hours at this same temperature of 1200°C. EDS concentrations profiles were additionally realized to know the new distribution of chromium and of hafnium, and notably to know the depth over which the alloy is depleted in chromium.

Results

Evolution of the bulk microstructures; consequences on hardness

The initial microstructures and the ones in the aged condition are presented together in Figure 2. It appears that the hafnium carbides are still present, with globally the same morphology as initially. Chromium carbides have appeared in the NiHf1 and NiHf3 alloys in which they were not present initially while the chromium carbides which initially existed in the NiHf2 alloy seem having almost disappeared.

Concerning the HfC carbides again, one can also notice that there is a slight but visible decrease of their surface fractions.

To quantitatively value the observed changes image analysis was performed and the obtained results are graphically presented in Figure 3. The surface fraction of HfC carbides (Figure 3, A) has effectively decreased in the three alloys, especially in the NiHf3 one in which chromium carbides have appeared in such quantities that it is in this third alloy that there are now the most present. Chromium carbides (Figure 3, B) have also appeared in the NiHf1 alloy in which they were not present initially.

NiHf2 is the single alloy in which the surface fraction in chromium carbides has decreased when compared to the as-cast microstructure in which they were present.

The total surface fractions of carbides in the three alloys and their evolutions due to the high temperature exposure are presented in Figure 3 (C). There is no significant evolution of this total fraction during the high temperature stage neither for the NiHf1 alloy nor for the NiHf3 alloys. For these two alloys the surface fraction evolution of the hafnium carbides is more or less compensated by the chromium carbides' one. After high temperature exposure the NiHf3 alloy still contains more carbides than the Nif1 one. In contrast, at the same time there is for the NiHf2 alloy a significant decrease in total carbide surface fraction.

On another side it seems that the contents of the matrix in chromium and hafnium have evolved during the 46 hours spent at 1200°C (Figure 4). This is true for chromium for the NiHf2 and NiHf3 alloys for which the uncertainty bars are not overlapped but not evident for the NiHf1 alloy. The increase in chromium content in the bulk NiHf2 matrix is in good agreement with the partial disappearance of the chromium carbides existing initially while the decrease in matrix chromium content of the NiHf3 bulk well corresponds to the solid state precipitation of chromium carbides.

Concerning the Hf content in matrix the base results were very variable in a same sample, between 0 (values probably really obtained in the matrix) and values up to 1wt.% Hf probably due to the proximity of subjacent HfC carbides. This led to extremely high values of standard deviation and one can reasonably think that the real Hf content in matrix is very low in all cases. It is nevertheless interesting to see that the Hf content in matrix, which seemed initially higher in the Hf-richest NiHf3 alloy, has perhaps increased a lot while the HfC carbide fraction has significantly decreased in this same alloy.

Vickers indentations were performed on the as-cast samples and on the aged ones. The results are graphically presented together in Figure 5, to which are added hardness results measured in the same conditions for ternary alloys with the same chromium and carbon contents as the HfC-containing alloys studied here. One can see that for all alloys, for the Hf-containing ones as well as for the Hf-free ones, the {46h; 1200°C} stage induced a decrease in hardness. This is due to the decrease in chromium carbide fraction for the ternary alloys and to the decrease in hafnium carbides for the alloys of this study. For the latter ones, the solid state appearance and growth of chromium carbides was obviously not sufficient to counterbalance the hardness decrease due to the HfC partial loss.

Characterization of the oxidized states of the alloys

As demonstrated by X-ray diffraction before cross-section preparation the three samples are covered by essentially chromia, as illustrated by the diffractogram acquired on the NiHf2 oxidized surface given as example in Figure 6. XRD results also show that hafnium oxide is present too, but in small quantity. When examined in cross section (Figure 7) one can see that the oxide which has not been lost by spallation during cooling is essentially chromia, mixed here and there with small islands of HfO 2 (of the monoclinic type according to XRD the diffractogram) Obviously, oxidation tended to penetrate in alloy in some locations. In contrast with cast alloys containing chromium carbides and even tantalum carbides the interdendritic network of hafnium carbides did not disappear from the oxidation front. This is very different from what is usually observed during the high temperature oxidation of alloys containing chromium carbides and/or tantalum carbides, for which a carbide-free zone develops over a depth which increases with time as well as with temperature, due to the liberation of chromium and/or tantalum which thereafter diffuse towards the oxidation front. Here the HfC carbides close to the alloy surface did not disappear to liberate hafnium: either they stay unchanged on place or the hafnium carbides are transformed in hafnium oxides in the same location as they were initially.

Two EDS concentration profiles were acquired through the subsurface from the {alloy/external oxide} interface for each oxidized sample. One of them is presented per alloy as example in Figure 8. They all start in the bulk with Cr contents slightly higher than the chromium contents of the whole alloys since all chromium is contained in the matrix (no chromium in the HfC carbides). One can see that the chromium on extreme surface and the Cr-depleted depths may depend on the alloy. These two characteristics were noted for the two EDS profiles acquired per sample and the average values are graphically given in Figure 9. One can see first that the chromium content on extreme surface tends to be higher for the NiHf2 alloy than for the two other alloys. Second the chromium depletion is obviously deeper for the NiHf1 alloy than for the two alloys which have the higher carbon contents then the higher fractions of interdendritic carbides. Concerning hafnium, the EDS concentration profiles of which were also plotted in Figure 8, one can observe that its content is often extremely low (even not detected) except in some locations where the electron obviously probe shot either a HfO 2 oxide or a HfC carbides.

Discussion

These HfC-containing nickel-based alloys are thus not really stable at high temperature. Indeed, the microstructures formed during solidification do not correspond to the thermodynamic equilibrium at 1200°C and may necessarily evolve -as least a little -during the isothermal exposure which was applied to the alloy in this study. The surface fractions of the HfC carbides and of the chromium carbides present in the as-cast state have thus changed. A decrease in HfC surface fraction was noticed for the three Hf-containing alloys. Simultaneously one observed an increase (NiHf1 and NiHf3) or a decrease (NiHf2) in surface fraction of chromium carbides. These variations of carbides quantities were first qualitatively detected, and thereafter quantified by image analysis. Their consequences were here preliminarily noted on the room temperature hardness of the alloys. The loss of both HfC and chromium carbides for the NiHf2 alloy, and the replacement of a part of the hard HfC carbides (around 2900Hv50g, [START_REF] Samsonov | High Temperature Materials N°2: Properties Index[END_REF]) by softer chromium carbides (around 1500Hv50g, [START_REF] Samsonov | High Temperature Materials N°2: Properties Index[END_REF]) in the NiHf1 and NiHf3 alloy, explains the general loss in hardness due to high temperature exposure. However the global hardness hierarchy of these three alloys was not modified by comparison with the as-cast states. In contrast the morphology of the HfC carbides was not significantly changed, as it can be the case of the TaC carbides of other alloys, Co-based notably, which are generally much more fragmented by such exposure to high temperature with as results a general weakening of the mechanical properties at high temperature [START_REF] Michon | High temperature evolution of the microstructure of a cast cobalt base superalloy. Consequences on its thermomechanical properties[END_REF]. This should be not the case for these alloys since the script-like morphology of the interdendritic HfC carbides which are mixed with the periphery of the matrix dendrites in a eutectic compound is known to be favourable to creep-resistance notably.

Generally alloys and superalloys strengthened by interdendritic carbides of chromium and of tantalum, exposed to high temperature oxidation, are subjected to internal oxidation in CrTaO 4 oxides close to the alloy/chromia interface and also to the disappearance of these carbides over a depth increasing with time at a given temperature [START_REF] Berthod | Influence of the microstructural texture of cast superalloys on their hightemperature oxidation behaviour[END_REF]. In addition the inwards extension of this carbide-free zone is faster for a higher temperature [START_REF] Berthod | Etude cinétique de l'oxydation à haute température et de la volatilisation de la chromine pour des superalliages de fonderie renforcés par carbures. Partie II : Cas d'alliages à base de nickel contenant du tantale[END_REF] and this may lead to a depth not reinforced by carbides which can be of several tens of micrometers. It can be for example more than 70µm for a Co(bal.)-8Ni-30Cr-0.4C-6Ta (wt.%) after 100h in air at 1200°C [START_REF] Berthod | Etude cinétique de l'oxydation à haute température et de la volatilisation de la chromine pour des superalliages de fonderie renforcés par carbures. Partie II : Cas de superalliages à base de cobalt et influence de la finesse dendritique[END_REF]. In the present study internal oxides (HfO 2 but also chromia) have formed close to the extreme surface but no such carbide-free zone developed inwards. Thus, if the presence of internal oxides can threaten the mechanical resistance of the external alloy zone (which may be particularly solicited by tensile stress induced by flexural deformation for example) the strengthening by carbides of the just inner zone still exists.

Concerning the most external part of the oxides formed -the oxide scale covering the alloy -it appears that it is mainly composed of chromia. This protective oxide, unfortunately partly lost during the cooling, allowed the alloys oxidizing rather slowly and the presence of so high quantities in the very reactive element that is hafnium was not detrimental for the intrinsic good behaviour of the {Ni-25Cr-0.25 or 0.50C} base alloys, in contrast with what was observed for similar chromium-rich cobalt-based alloys also very rich in HfC carbides [START_REF] Conrath | Kinetics of high temperature oxidation of chromium-rich HfC-reinforced cobalt-based alloys[END_REF]. After 46 hours at 1200°C the chromium content in extreme surface is not lower than 22 wt.% this meaning that the chromia-forming behaviour of these nickel-based alloys will be surely maintained during an additional long time after these first 46 hours. One can remind that the chromium content in extreme surface is slightly higher for the NiHf2 alloy which was the single alloy out of the three ones which initially contained chromium carbides in its interdendritic spaces, as is to say rapidly available chromium reservoirs in the easy diffusion paths. However it is possible that the presence of hafnium carbides represented a hindrance to chromium diffusion since the Cr-depleted zone was the lowest in the two alloys in which the HfC carbides were the most present (NiHf2 and NiHf3).

Conclusions

Thus, the studied chromium-rich nickel-based alloys which contained rather dense interdendritic HfC networks were not really stable just after solidification but the greatest part of the HfC population was preserved during this rather long stage at high temperature. In contrast the chromium carbides that appeared in small quantities during solidification or the post-solidification solid state cooling in one of the alloys, and in higher quantities during the isothermal exposure in the two other alloys, showed that the chemical composition should be optimized to avoid the appearance of such carbides known to lower the solidus temperature (in TaCcontaining cobalt-based alloys for example) and then potentially detrimental for the high temperature properties. The refractoriness of the present alloys remains notably to specify, as well as their mechanical properties at high temperature as tensile behaviour or creep-resistance. These points will be the subjects of further investigations.

6.

NiHf1, as-cast

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .Fig. 5 .Fig. 7 .Fig. 8 .Fig. 9 .

 12345789 Fig. 1. Micrographs of the studied HfC-containing Nickel-based alloys (Scanning electron microscope, back scattered electrons mode)

Acknowledgements

The author wishes to thank P. Villeger for the X-Ray Diffraction runs.

7.