Lionel Delpech

Florence Sordes

Jean-Luc Sudres

La différence du vécu du cancer chez les personnes créatives. Une étude exploratoire The difference in cancer experience in creative people. An exploratory study

Keywords: Cancer, Créativité, Dépression, Qualité de Vie, Vécu Cancer, Cluster, Creativity, Depression, Quality of life

Creativity is described as allowing the individual to make compromises with their own environment or according to the context of their lives. Here, it may be understood as creating lives, ordering them. In case of illness, a creative person should adapt better. The purpose of this research is to demonstrate that a creative person lives differently with cancer. The sample consists of thirty-five men and women (61+11 years) with cancer and receiving chemotherapy. Evaluation tools are: Corsi Cubes, BFI-FR and TAS to determine creative cluster. In addition, participants passed the WHOQOL-26 and HADS-D to assess quality of life and level of depression. Two distinct groups of creativity were identified (λ = 0.28, p = .003). These profiles are significantly different in terms of their level of depression and 2/3 of the quality of life constructs (p between .001 and .032). Descriptive analyzes show that creative people have fewer depressive symptoms and better quality of life than the others. The creative patient seems to have a better experience of the disease, a phenomenon that may be useful for counseling in psycho-oncology.

France

Email : lioneldelpech@gmail.com Tél. : 06 73 11 34 51 Article reçu le 3 décembre 2019 ; accepté le 13 avril 2020

Résumé

La créativité est une capacité qui permet de composer avec son environnement. Si ce dernier est rendu difficile par la maladie, la personne créative devrait mieux s'y adapter. L'objectif de cette recherche est de démontrer que la personne créative vit différemment le cancer. L'échantillon se compose de trente-cinq hommes et femmes (61 + 11 ans) ayant un cancer et bénéficiant d'une chimiothérapie. Les outils d'évaluations utilisés sont : Les Cubes de Corsi, le BFI-FR et le TAS pour déterminer des groupes de créativité. De plus, les participants ont passé le WHOQOL-26 ainsi que le HADS-D pour évaluer la qualité de vie et le niveau dépression. Deux groupes distincts de créativité ont été identifiés (λ = .28 ; p = .003). Ces profils se différencient de manière significative avec leur niveau de dépression et par 2/3 des construits de la qualité de vie (p entre .001 et .032). Les analyses descriptives montrent que les personnes créatives ont moins de symptômes dépressifs ainsi qu'une meilleure qualité de vie que les autres. Le patient créatif semble avoir un meilleur vécu de la maladie, phénomène à explorer pouvant être utile pour l'accompagnement en psycho-oncologie.

Introduction

Le cancer désigne un terme générique pour un groupe de maladies. Aujourd'hui, nous savons qu'il existe plus d'une centaine de cancers pouvant affecter n'importe quel organe du corps humain. En 2018, il y a près de 382 000 nouveaux cas de cancer en France pour approximativement 157 400 décès, soit une tendance à la hausse selon l'Organisation Mondiale de la Santé (OMS) et l'Institut National du Cancer en 2019 qui prévoient une forte augmentation de cas dans les dix années à venir.

Dans cet article nous explorons les liens entre la créativité de la personne atteinte d'un cancer et son vécu de la maladie.

Rappelons que la créativité fait partie des capacités humaines qui ont participé à notre évolution [START_REF] Steidl | Les rouages de la créativité[END_REF]. Sans elle, il n'y aurait ni médecine ni technologie et encore moins d'art. De fait, nous n'aurions pas dépassé notre condition primitive. Même si nous ne sommes pas la seule espèce capable de créativité [START_REF] Iowa | Comparative Cognition Experimental Explorations of Animal Intelligence: Experimental Explorations of Animal Intelligence[END_REF], nous sommes celle qui a su l'utiliser de manière à modeler notre quotidien [START_REF] Zaidel | Art and brain: insights from neuropsychology, biology and evolution[END_REF]. La définition de créativité a évolué avec le temps et est encore débattue aujourd'hui. Toutefois il existe une définition consensuelle : la créativité est la capacité à réaliser une production qui soit à la fois nouvelle et adaptée au contexte dans lequel elle se manifeste [START_REF] Amabile | Motivation creativity in organizations: On doing what you love and lovieng what you do[END_REF]. Précisons que la production créative peut s'appliquer à de nombreux domaines. Par ailleurs, la nouveauté de cette production est à envisager sous plusieurs aspects : l'objet matériel ou abstrait peut ne présenter qu'une variation minime ou au contraire montrer une avancée importante. Si cette définition s'axe sur des attributs de la création en devenir, la créativité sert à l'adaptation, réconciliant individualité et environnement potentiellement problématique. Pour les psychanalystes tels que Winnicott [START_REF] Winnicott | Playing and Reality[END_REF], la créativité naît avec le phénomène transitionnel, période où la fantasmatisation de l'enfant se lie avec les expériences fonctionnelles. Elle a pour objectif d'amener à une construction identitaire en investissant l'aire transitionnelle, construction psychique réalisée par l'enfant pour faire face à des moments d'angoisses. Aussi, si l'homme vit, sa créativité lui permet d'exister. De ce point de vue, tout humain est créatif et, tout aussi admirable que puisse être l'Art dans sa globalité, ce dernier n'est finalement que l'un des media de la créativité. Bien sûr, nous pouvons estimer que le potentiel du cuisinier dominical préparant un brunch avec les restes de la semaine n'est pas du même niveau que celle de Philip Glass composant « Mad Rush ». Toutefois, ce qui différencie ces deux protagonistes n'est pas la capacité de créer en elle-même mais, entre autres, un rapport différent face à la portée espérée de leurs objectifs, à la motivation ou encore à la technique. Finalement, si chacun est créatif, ce qui permet d'estimer le potentiel d'une personne est soit la qualité subjective de la création réalisée, soit l'évaluation des capacités sous-tendant la créativité.

T. Lubart propose un modèle multivarié de ce concept qui repose sur la recherche expérimentale, des années soixante à nos jours [START_REF] Lubart | Psychologie de la créativité[END_REF]. Dans cette approche, la prise en compte de la créativité n'est plus cantonnée à son aspect cognitif traditionnel, notamment avec la pensée divergente et ses construits [START_REF] Kim | The Torrance Tests of Creative Thinking Already Overcome Many of the Perceived Weaknesses That Silvia et al. 's (2008) Methods Are Intended to Correct[END_REF]. Pour être plus précis, la créativité multivariée requiert une combinaison de facteurs relevant de l'individu : cognitifs, conatifs, émotionnels et environnementaux. Combinaison non identifiée et non standardisée dans le sens où ses facteurs peuvent se compenser, preuve de la flexibilité et de l'unicité de l'humain.

La créativité nous sert au quotidien pour composer avec un environnement relativement acceptable, mais quelles sont ses utilités dans un contexte aussi difficile que le cancer ?

Même si la médecine avance de jour en jour et explore de nouvelles pistes, les traitements actuels restent lourds de conséquence. Aussi, que ce soit le cancer lui-même ou encore son traitement, le vécu de la maladie altère les sphères physiologiques, sociales mais aussi psychologiques du patient. La maladie génère de multiples altérations de la sphère émotionnelle, dont une augmentation de la dépression [START_REF] Ogden | Psychologie de la sant: A Textbook[END_REF], ainsi que la qualité de vie [START_REF] Fischer | Psychologie de la santé, applications et interventions[END_REF]. La notion de qualité de vie est un concept multidimensionnel, synonyme de bien-être perçu, qui associe facteurs concrets et subjectifs pour une meilleure appréhension de la santé de l'individu. Elle correspond à la perception qu'a une personne de sa place dans l'existence. En oncologie, la qualité de vie a un rôle clé car elle permet de constater les bénéfices des soins non plus en termes seul de rallongement de l'espérance de vie mais aussi en termes de qualité.

Les objectifs de ce travail sont (a) d'identifier une typologie de la personne atteinte de cancer basée sur la créativité multivariée et (b) de déterminer si ces groupes ont un vécu différent de la maladie.

Notons qu'ici, le terme de vécu de la maladie est utilisé comme concept reliant les dimensions de la qualité de vie associées à la symptomatologie dépressive.

Méthode

Population

L'étude se base sur la participation de 35 personnes atteintes de cancer dit « solide » et suivant une chimiothérapie. Le groupe se constitue de 19 femmes et de 16 hommes ayant comme moyenne d'âge 61 ans plus ou moins 11 ans. La durée moyenne de traitement est de 15 mois.

Outils

Le potentiel créatif

Afin d'identifier la personne créative, nous avons retenu trois axes de la créativité sélectionnés selon leur pertinence, applicabilité et cohérence avec la théorie choisie :

-Axe cognitif : la Mémoire de Travail Visuo-Spatiale (MWVS). Elle est la capacité de stocker des informations visuelles et spatiales et d'encoder des informations verbales sous forme d'images visuelles. De plus, elle est la fonction exécutive qui explique le mieux la créativité [START_REF] Dorgans | imagerie mentale et fonctions exécutives: étude expérimentale des relations entre ces trois concepts chez l'homme adulte[END_REF]. Cette aptitude est évaluée par le test des cubes de Corsi. L'épreuve consiste à reproduire, dans l'ordre donné par l'observateur, une séquence de mouvement de pointage entre des cubes neutres. Les séquences s'enchaînent et se complexifient jusqu'à l'échec du participant. Plus le sujet effectue de séquences, meilleure est sa MWVS [START_REF] Corsi | Human memory and the medial temporal region of the brain[END_REF][START_REF] Kessels | The backward span of the Corsi Block-Tapping task ans association with the WAIS-III digit span[END_REF].

-Axe conatif : l'ouverture aux expériences. Traditionnellement, ce trait de personnalité est le plus à même à expliquer la créativité [START_REF] Mccrae | Creativity, divergent thinking, and openness to experience[END_REF]. Elle comprend plusieurs facettes allant de l'ouverture aux sentiments, aux nouvelles activités, à l'esthétisme ou encore aux valeurs. Cette capacité est évaluée par le Big Five Inventory français (BFI-FR) [START_REF] Plaisant | Validation par analyse factorielle du Big Five Inventory français (BFI-Fr). Analyse convergente avec le NEOPI-R[END_REF]. Constitué de quarante-cinq items (échelle de Likert en cinq points), ce test mesure les différences individuelles des cinq grandes dimensions de la personnalité (extraversion, agréabilité, conscience, névrosisme, ouverture). Seuls les items relatifs à l'ouverture ont été sélectionnés pour les besoins de ce protocole.

-Axe émotionnel : l'alexithymie, qui se définit comme le fait de ne pas pouvoir exprimer ses émotions. La créativité permet de protéger de l'alexithymie grandissante au cours de la vie ou encore face aux événements difficiles [START_REF] Botella | Alexithymia and Affect Intensity of Fine Artists[END_REF]. Son niveau est évalué par la Toronto Alexithymia Scale (T.A.S.) qui permet d'estimer trois facteurs en 20 items grâce à une échelle de Likert en cinq points. Ces facteurs sont la difficulté à identifier les émotions, à les décrire, ainsi que la pensée concrète (dénuée d'affect). Les études psychométriques du test montrent que cet outil est adéquat pour évaluer l'alexithymie [START_REF] Zimmermann | Qualités psychométriques de la version française de la TAS-20 et prévalence de l'alexithymie chez 264 adolescents tout-venant[END_REF].

Ainsi, nous considérons que la personne créative est dotée d'une bonne capacité de visualisation, se considère comme ouverte et peut exprimer ses émotions. Aussi, l'individu exprimant moins sa créativité a pour caractéristique de présenter au moins une altération au niveau des facteurs sélectionnés.

Le niveau de dépression

La Hospital Anxiety and Depression Scale (HADS) est un test qui évalue le niveau de dépression et d'anxiété en 14 items mesurés sur une échelle de 0 à 3 points [START_REF] Zigmond | The Hospital Anxiety and Depression Scale[END_REF]. Ici, seule la sous-échelle de dépression est gardée (HADS-D). Le test profite d'une validité satisfaisante et a été réalisé sur des populations variées [START_REF] Bjelland | The validity of the Hospital Anxiety and Depression Scale. An updated literature review[END_REF].

Niveau de qualité de vie

Le World Health Organisation Quality Of Life-26 (WHOQOL-26) est un outil qui, comme son nom l'indique, a été créé par l'Organisation Mondiale de la Santé. Il évalue la qualité de vie en quatre axes : la santé physique (fatigue, douleur, repos) ; la santé psychologique (image du corps, estime de soi et fonctions cognitives) ; les relations sociales (relation personnelle, soutien social et activité sexuelle) et l'environnement (ressources financières, sécurité, habitat, loisirs, transport). En plus de ces dimensions, le WHOQOL-26 propose une évaluation de la santé générale et de la qualité de vie globale. Le questionnaire s'effectue au moyen d'une échelle de Likert en cinq points. La validité de ce test est satisfaisante [START_REF] Saloppé | Validation du WHOQOL-bref en hôpital psychiatrique sécuritaire[END_REF].

Procédure

Après avoir donné son accord libre et éclairé, le participant passe l'épreuve des cubes de Corsi pour ensuite compléter un questionnaire. Celui-ci est composé d'une fiche signalétique renseignant sur les informations du patient, et des questionnaires susmentionnés.

Le tout s'effectue en 30 minutes, dans la chambre du participant et en présence de l'observateur.

Résultats

Clusters

Afin de trouver des groupes homogènes de participants, nous avons utilisé une analyse de classification hiérarchique en différentes étapes [START_REF] Rapkin | Cluster analysis in community research: Epistemology and practice[END_REF]. Après une conversion des scores en z scores, une analyse de classification de Ward a été réalisée. Le dendrogramme produit et la taille de l'échantillon ont permis de déterminer le nombre le plus favorable de groupes de participants. Par la suite, une analyse par nuée dynamique a été menée pour assigner les participants à l'un des groupes précédemment obtenus. De plus, nous avons réalisé une analyse discriminante de Wilks pour valider le classement.

Une solution à deux groupes a été identifiée (Figure 1). Aussi, la classification par nuée dynamique présente un modèle significativement fiable (λ = .28 ; p = .003).

Analyse de variance

Le test de normalité de Kolmogorov-Smirnov (tableau 1) a permis de déterminer l'utilisation d'une ANOVA ou d'un test de Kruskal-Wallis. L'analyse non paramétrique (tableau 2) dévoile une différence significative entre les groupes identifiés concernant le niveau de santé physiologique et psychologique perçue (z =.003 ; z = .004 respectivement) ainsi que le degré de dépression (z = .034). De plus, les analyses paramétriques (tableau 3) montrent que les groupes se distinguent significativement par leur niveau de perception de l'environnement et de qualité de vie globale (p = .022 ; p = .001, respectivement).

Une analyse descriptive (tableau 4) a été réalisée afin de comparer les scores moyens des deux groupes. Les PPC ont comme particularité de proposer des moyennes de scores toujours plus élevées, pour ce qui est des construits de la qualité de vie ainsi qu'un degré plus faible de dépression en comparaison avec les PCB ; différences assez prégnantes pour être majoritairement significatives.

Discussion

Ces résultats vont dans le sens de nos hypothèses. Chez les personnes atteintes de cancer, celles qui sont identifiées comme créatives ont un meilleur vécu de la maladie ; du moins en termes de dépression et des dimensions de la qualité de vie.

Bien que la méthode choisie ne permette pas de déduire une causalité, ce phénomène pourrait s'expliquer de différentes manières. Les personnes créatives ont, par définition, une tendance à la création. Au-delà de répondre à une problématique consciente ou non, ce produit original implique le flow qui correspond à un état d'absorption totale dans l'activité qui entraîne des sentiments intenses et profonds d'enchantement [START_REF] Csikszentmihalyi | Flow: The psychology of optimal experience[END_REF]. Selon Boyer-Labrouche [START_REF] Boyer-Labrouche | Manuel d'art-thérapie[END_REF], les bénéfices du flow contrebalancent le ressenti du poids des affects dépressifs et favorisent les améliorations psychologiques du sujet. Ainsi, les personnes productives pourraient se réguler psychologiquement, même dans un contexte de maladie.

Toutefois, il ne peut y avoir création sans créativité. Il est possible qu'un meilleur vécu de la maladie ne soit pas seulement dû aux implications de la production originale mais aussi au potentiel créatif.

La capacité à créer naît dans l'espace transitionnel, aire qui n'appartient ni au monde extérieur ni à la subjectivité de l'individu. Son objectif primaire est de lutter contre l'idée de n'être qu'un grain de poussière dans l'univers en prouvant notre existence [START_REF] Aubourg | Winnicott et la créativité[END_REF]. Serait-il possible que le combat perpétuel contre cette angoisse basique permette aux créatifs de mieux savoir jongler entre l'hétéronomie et l'autonomie ? Flexibilité potentiellement utile en société mais aussi dans un contexte de maladie. En effet, la période du diagnostic et du traitement enferme le patient dans un contexte aseptisé, strict, où tous les efforts sont déployés pour rester humain. Si le patient cède à l'hétéronomie, ce cadre peut l'amener à se voir comme un objet de soin, désindividualisé. À l'inverse, si le patient cède à son autonomie, il ne suivra le traitement que lorsqu'il le souhaite. Dans ces deux cas, le patient n'aurait pas la meilleure approche pour lutter contre la maladie. La personne créative, donc qui a plus de facilité à élaborer entre ces deux réalités, aurait plus de chance de créer un compromis adapté aux demandes de la structure de santé tout en étant en accord avec elle-même. Cette attitude aiderait le patient à ne pas sombrer dans des rôles inadaptés à sa santé et, potentiellement, lui permettrait in fine de devenir acteur de sa maladie.

Si la méthode sélectionnée met au jour ce phénomène, elle ne permet pas de l'expliquer outre mesure. Aussi, il serait intéressant de compléter ces informations par une étude mixte qui permettrait de lier les concepts entre eux, d'en déterminer les impacts, tout en enrichissant les données de manière qualitative.

Conclusion

Au-delà d'avoir proposé une méthode de l'identification de la personne créative, ce travail ouvre aussi la voie sur l'utilité de la créativité en psychologie de la santé. Les groupes identifiés montrent des niveaux significativement distincts de qualité de vie et de dépression ; différences en faveur des personnes créatives. Ce constat permet de se questionner sur l'existence d'un phénomène entre les variables étudiées. Si la créativité impacte bien le vécu

  Le premier groupe (n = 19, soit 54 % de l'échantillon d'origine) a été nommé « Personnes potentiellement créatives » (PPC). Il se caractérise par une forte capacité de MWVS, un niveau haut d'ouverture aux expériences et d'un faible degré d'alexithymie. À l'inverse, le second groupe (n = 16, soit 46 % de l'échantillon total) a comme particularité d'avoir un niveau faible de MWVS et d'ouverture ainsi qu'un niveau d'alexithymie élevé. Ce groupe a été surnommé « Personnes à la créativité bloquée » (PCB).

Figure :

 : Figure : Les deux profils de créativité fondés sur les niveaux de MWVS, d'ouverture aux expériences et d'alexithymie.

Tableau 1

 1 Test de normalité des dimensions du WHOQOL-26 et du HADS-D Test U de Mann-Whitney entre et les scores des construits du WHOQUOL-26 et du Analyse de variance (ANOVA) entre les construits suivant une loi normale du WHOQUOL-26 et les groupes de créativité

	p

Note1 : P.P.C. = Personnes potentiellement créatives ; PCB : Personnes à la créativité bloquée. Note2 : x¯=moyenne ; σ=écart type


Viktorija Andruleviciene

Karolis Leitonas

Dmytro Volyniuk

Gjergji Sini

Vidas Grazulevicius

Vytautas Getautis

TADF versus TTA emission mechanisms in acridan and carbazolesubstituted dibenzo[a,c]phenazines: towards triplet harvesting emitters and hosts

Carbazole or acridan-substituted dibenzo [a,c]phenazines (CzDbp and AcDbp, respectively) were synthesized and investigated exploiting the donor-acceptor-donor (D-A-D) architecture expecting thermally activated delayed fluorescence (TADF) in both cases. Unexpectedly, while experimental microseconds-lived TADF behaviour was observed for AcDbp efficiency of which was found to be dependent on environment, CzDbp exhibited nanosecond-lived fluorescence complemented by triplet-triplet annihilation (TTA). Theoretical calculations by means of the ωB97XD functional with optimally tuned range separation parameter ω, were performed for both molecules, supporting their experimentally established electrochemical, optical and photophysical properties. Using the same emitter and device structure, CzDbp as the bipolar host allowed to achieve by 12.5% better external quantum efficiency and better

Introduction

Materials exhibiting thermally activated delayed fluorescence (TADF) have attracted significant interest in the field of organic optoelectronics. [START_REF] Liu | All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes[END_REF] TADF based organic light emitting diodes (OLEDs) can achieve 100% internal quantum efficiency (IQE), which can be attained due to harvesting of triplet excitons through the reverse intersystem crossing (RISC).

Concerning TADF hosts based devices, the energy can be transmitted to the guest through the Förster energy transfer (FET) to reach 100% of its utilization. [START_REF] Zhang | Thermally activated delayed fluorescence host for high performance organic light-emitting diodes[END_REF][START_REF] Ban | Design of efficient thermally activated delayed fluorescence blue host for high performance solution-processed hybrid white organic light emitting diodes[END_REF] It is well known that very small singlet-triplet energy gap (ΔEST) is essential parameter to achieve efficient TADF effect via RISC. [START_REF] Yang | Recent advances in organic thermally activated delayed fluorescence materials[END_REF] Low ΔEST can be obtained using compounds with spatially separated donor (D) and acceptor (A) units with dihedral angle between D and A fragments approaching the orthogonality. [START_REF] Etherington | Revealing the spin-vibronic coupling mechanism of thermally activated delayed fluorescence[END_REF][START_REF] Méhes | Influence of host matrix on thermally-activated delayed fluorescence: Effects on emission lifetime, photoluminescence quantum yield, and device performance[END_REF] However, the TADF efficiency is determined not only by ΔEST but also by the spin-orbit coupling (SOC). The drawback of this architecture is consequently the smallness of SOC, which decreases in parallel with the decreasing ΔEST: due to the unification of the S1 and T1 character when ΔEST =0 (both become of CT nature), their coupling tends zero as stipulated by El Sayed's empirical rule. [START_REF] Lower | The triplet state and molecular electronic processes in organic molecules[END_REF] Organic materials emit prompt fluorescence even when ΔEST is too large for getting RISC However OLEDs based on prompt fluorescence with a theoretical external quantum efficiency (EQE) of 5% cannot meet the industrial requirements. TADF was observed even for compounds with relatively large ΔEST, witnessing the complexity of TADF mechanisms. [START_REF] Mayder | Thermally activated delayedfFluorescence in 1,3,4-oxadiazoles with π-extended donors[END_REF] Organic materials with large ΔEST have big potential to be used in OLED technology when they exhibit triplet-triplet annihilation (TTA) (also co-called triplet-triplet fusion) [START_REF] Qiao | Nonlinear optoelectronic processes in organic optoelectronic devices: Triplet-triplet annihilation and singlet fission[END_REF][START_REF] Zhou | Upconversion Luminescent Materials: Advances and Applications[END_REF], "hot excitons" [START_REF] Liu | Experimental evidence for "Hot Exciton" thermally activated delayed fluorescence emitters[END_REF], or upper level triplet-singlet intersystem crossing [START_REF] Xu | Highly efficient blue fluorescent OLEDs based on upper level triplet-singlet intersystem crossing[END_REF]. In this case, two low-energy triplet excitons can be fused to one high-energy singlet exciton. In case of TTA emitters, the theoretical IQE is 62.5%, corresponding to an EQE of 12.5% of OLEDs. [START_REF] Qiao | Nonlinear optoelectronic processes in organic optoelectronic devices: Triplet-triplet annihilation and singlet fission[END_REF] EQE values of over 10% were reported for TTA OLEDs [START_REF] Lee | Blue organic light-emitting diodes: current status, challenges, and future outlook[END_REF][START_REF] Kukhta | Deep-blue high-efficiency TTA OLED using para-and metaconjugated cyanotriphenylbenzene and carbazole derivatives as emitter and host[END_REF].

Both TADF and TTA materials can be used in OLEDs as hosts when they meet the requirements raised for host including high and balanced charge mobilities, wide transporting band gap with appropriate HOMO and LUMO energy levels for charge injection, high optical, thermal and electrochemical stability. [START_REF] Wex | Perspective on carbazole-based organic compounds as emitters and hosts in TADF applications[END_REF][START_REF] Bezvikonnyi | Diphenylsulfone-based hosts for electroluminescent devices: Effect of donor substituents[END_REF][START_REF] Hsin | 3% Lifetime elongation of blue TTA-OLED with assistant host[END_REF] Such hosts are used not only for overcoming of aggregation induced quenching of OLED emitters in solid state and for ensuring hole-electron recombination within the light-emitting layer but also for additional triplet harvesting within host. [START_REF] Song | Understanding and Manipulating the Interplay of Wide-Energy-Gap Host and TADF Sensitizer in High-Performance Fluorescence OLEDs[END_REF] If appropriate combination of TADF host and conventional fluorescent emitter are used, OLEDs (co-called hyperfluorescent OLEDs) with 100% theoretical EQE can be fabricated. [START_REF] Chen | Versatile Room-Temperature-Phosphorescent Materials Prepared from N-Substituted Naphthalimides: Emission Enhancement and Chemical Conjugation[END_REF][START_REF] Higuchi | High-efficiency white organic lightemitting diodes based on a blue thermally activated delayed fluorescent emitter combined with green and red fluorescent emitters[END_REF][START_REF] Khana | White hyperelectrofluorescence from solution-processable OLEDs based on phenothiazine substituted tetraphenylethylene derivatives[END_REF] While the favourable impact of TADF hosts is currently evident, knowledge on TTA OLED hosts is very limited. In this work, we partly aimed to focus on this issue. The development of efficient TADF and TTA hosts is not a routine task since TADF and TTA materials are characterized by highly twisted molecular structures usually inducing poor charge transporting properties.

The use of theoretical predictions before synthesis of potential TADF and TTA materials is essential for saving the developing time and for getting the required properties. Timedependent density functional theory (TDDFT) is a well-established tool to study S and T excited states of organic molecules. However, TDDFT calculations based on standard functionals can underestimate the excitation energies of the D-A molecules with the CT characteristics hence strongly impact the pertinence of theoretical diagrams of state energies.

Range-separated exchange (RS) density functionals with optimally tuned range separation parameter ω were found to perform better in overcoming this problem. [START_REF] Sun | Reliable prediction with tuned range-separated functionals of the singlet-triplet gap in organic emitters for thermally activated delayed fluorescence[END_REF] Here, we report on new D-A-D compounds with dibenzo[a,c]phenazine acceptor [START_REF] Boxi | Dibenzo[a, c]phenazine-based donor-acceptor (D-A) tetra branched molecules: fine tuning of optical properties[END_REF][START_REF] Zhou | Modulation of excited state property based on benzo[a, c]phenazine acceptor: three typical excited states and electroluminescence performance[END_REF] substituted at its C-2 and/or C-7 positions by electron donating carbazole or acridan moieties [START_REF] Xie | tert-Butyl substituted hetero-donor TADF compounds for efficient solution-processed non-doped blue OLEDs[END_REF][START_REF] Im | Molecular design strategy of organic thermally activated delayed fluorescence emitters[END_REF]. In contrast to low-molar-mass acceptor groups such as -CF3, -CN, [START_REF] Ivanauskaite | Structure-property relationship of blue solid state emissive phenanthroimidazole derivatives[END_REF], dibenzo[a,c]phenazine as the acceptor was selected expecting appropriate arrangements between planar dibenzo[a,c]phenazine units in solid films required for high electron mobility required for efficient OLED hosts [START_REF] Yamaguchi | A 1,4,5,8,9,11hexaazatriphenylenehexacarbonitrile (HAT-CN) transport layer with high electron mobility for thick organic light-emitting diodes[END_REF]. The compounds were designed as TADF materials, however, contrary to the expectations, TTA was observed for the developed dibenzo[a,c]phenazine derivatives. We should note that Tang et al [START_REF] Xie | Rational molecular design of dibenzo[a,c]phenazine-based thermally activated delayed fluorescence emitters for orange-red OLEDs with EQE up to 22.0%[END_REF] recently reported on dibenzo[a,c]phenazine derivatives with the different number of acridan moieties at C-3 and/or C-6 positions of dibenzo[a,c]phenazine moiety. These compounds as TADF emitters demonstrated conventional TADF favourable for the fabrication of efficient orange-red OLEDs. Neither evidence of TTA nor application as hosts in OLEDs were previously reported for dibenzo[a,c]phenazine derivatives to the best of our knowledge. Since the developed here compounds demonstrated relatively high mobilities of holes and electrons, they were used not only as triplet harvesting emitters but also as triplet harvesting hosts.

Theoretical calculations were performed using ωB97XD functional with optimally tuned range separation parameter ω in order to get insight on the optical, electrochemical and photophysical properties of the compounds.

Experimental section

Materials

In case of OLED fabrications, commercially available molybdenum oxide (MoO3), hexaazatriphenylenehexacabonitrile (HAT-CN), di(1-naphthyl)-N,N′-diphenyl (NPB), tris(4carbazoyl-9-ylphenyl)amine (TCTA), 1,3-bis(N-carbazolyl)benzene (mCP), 3,3-di(9H-carbazol-

9-yl)biphenyl (mCBP), bis(1-phenyl-isoquinoline-C2,N)(acetylacetonato)iridium(III) (Ir(piq)2(acac)), 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), 2,9- Bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (nBPhen), fluorolithium (LiF), 8-
Quinolinolato lithium (Liq) were purchased from Sigma-Aldrich or Ossila and used as received.

In case of synthesis, 9,10-Phenanthrenequinone (Aldrich), N-bromosuccinimide (Aldrich), ophenylenediamine (Aldrich), zinc chloride (Aldrich), 2-chloro-2-methylpropane (Aldrich), bis(tri-tert-butylphosphine)palladium(0) (Aldrich), sodium tert-butoxide, (Aldrich), 9Hcarbazole (Reakhim), 9,9-dimethyl-9,10-dihydroacridine (Center for Physical Sciences and Technology) were purchased as reagent grade chemicals and used as received. 2,7-Dibromo-9,10-phenanthrenequinone, 2,7-dibromodibenzo[a,c]phenazine (BDbp) and 3,6-di-tert-butyl-9H-carbazole were synthesized as reported earlier. [START_REF] Liu | A two-dimensional conjugated aromatic polymer via C-C coupling reaction[END_REF][START_REF] Unver | Effect of conjugated core building block dibenzo[a,c]phenazine unit on π-conjugated electrochromic polymers: Red-shifted absorption[END_REF][START_REF] Moonsin | Carbazole dendronised triphenylamines as solution processed high Tg amorphous hole-transporting materials for organic electroluminescent devices[END_REF] Thin layer chromatography was performed by using TLC plates covered with a silica gel matrix on aluminum backing (Aldrich).

Computational methodology

The density functional theory (DFT) [START_REF] Kohn | Self-consistent equations including exchange and correlation effects[END_REF] and time dependent density functional theory (TD-DFT) [START_REF] Gross | Time-dependent density-functional theory[END_REF][START_REF] Baurnschmitt | Treatment of electronic excitations within the adiabatic approximation of time dependent density functional theory[END_REF][START_REF] Casida | Molecular excitation energies to high-lying bound states from time-dependent density-functional response theory: characterization and correction of the time-dependent local density approximation ionization thresholdJ[END_REF] calculations for molecules were obtained employing the ωB97XD/6-31G(d,p) in gas phase and CPCM/ωB97XD/6-31G(d,p) in solvents where the value of the ω parameter was tuned by considering the effect of the media. Test calculations with the default ωB97XD and B3LYP functionals were also performed for the comparison of results. Up to 40 excited states were calculated by considering a band half-width at half-maximum of 0.3 eV obtaining the theoretical absorption spectra. The calculations of natural transition orbitals (NTOs) were carried out to visualize molecular orbitals of selected excited-states. The vertical ionization potentials (IP) were calculated as energy difference between neutral and cation radical species at the neutral state geometry. All calculations were performed with the Gaussian16 software.

Instrumentation

= + + +
Calculations using ωB97XD (ω default ), ωB97XD with ω tuned in gas (ω gas ) and B3LYP functionals were performed for the comparison of the obtained results, and the corresponding results are shown in the Annex I (Supporting information).

Geometry and Molecular orbitals

Theoretical calculations performed with ω tuned in solvents revealed that dihedral angles between Dbp and Ac fragments is 90° in AcDbp (Table 1). While analogous dihedral angle between Dbp and Cz moieties in CzDbp is affected by the ω meaning of ωB97XD functional. Table 1 indicates that the dihedral angle D-A obtained with the lower ω=0.0011 Å -1 is ∼76° and that calculated with the higher ω=0.

Ionization potentials and charge mobilities

The ionization potentials (IP) of investigated materials were estimated using photoelectron emission (PE) method in air (Fig. S1) and cyclic voltammetry (CV) (Fig. S2). The same IP PE value of 5.5 eV for AcDbp and CzDbp was found which is larger as compared to the IP CV values of 5.25 eV and 5.44 eV, respectively (Table 2). The difference stems from the stronger polarization effects in the solid state (PE measurements). Additionally, CV measurements reveal reduction peaks of the compounds which were used to estimate electron affinity (EA CV ) values of 3.12 eV for AcDbp and 3.11 eV for CzDbp (Table 2). The values of EA PE were obtained from the IP PE values and the optical band gaps (Eg, opt ), which were deduced from the edges of the absorption spectra of the vacuum deposited layers (Fig. S3).

The calculated EA PE value of AcDbp was by 0. In addition, the smaller electron mobility of CzDbp may result from the tert-butyl substituents of carbazole moieties. Tert-butyl substituents apparently increase the distance between neighboring CzDbp molecules in the solid films. The increased intermolecular distance may also reduce LUMO-LUMO overlapping between neighboring molecules causing difficulties for electron hoping, thus causing the lower electron mobility of CzDbp relative to that of AcDbp containing the same electron-accepting unit dibenzo[a,c]phenazine but unsubstituted donor moieties.

Thermal characteristics

AcDbp and CzDbp showed high thermal stability with 5% weight loss at 480 °C and 453 °C, respectively, as confirmed by thermogravimetric analysis (TGA) (Fig. 2b). The morphological transitions of the compounds were investigated by differential scanning calorimetry (DSC). Fig. 2c shows DSC heating and cooling curves for AcDbp. This compound was obtained after the synthesis and purification as the crystalline substance. It showed two endothermic melting signals at 387 °C and 427 °C during the first DSC heating scan. The cooling and the second heating scans revealed only a glass transition at 164 °C. In the heating and cooling scans of CzDbp no signals of melting, crystallization or glass transition were observed (Fig. S5).

Photophysical properties

Absorption. The experimental absorbance spectra of AcDbp and CzDbp were compared with the computed ones at the TD-DFT level with ω THF,TOL B97XD (Fig. 3). The position of theoretical absorption bands of AcDbp and CzDbp were in good agreement with the experimental ones. This observation can be related to the improved description of origin of excitations provided by the optimization of the ω. Analysis of the most intense transitions (Table S1) revealed that absorption band at 370 nm of AcDbp described transition S0→S9. In order to identify the nature of transition, natural transition orbitals (NTO) were calculated which showed that this absorption band of AcDbp is associated with the local electronic excitations in Dbp core (Fig. S6). Meanwhile, absorption band at 350 nm of CzDbp obtained at ω THF B97XD and ω TOL B97XD corresponds to the combination of electronic transitions towards two excited states of S8, S9 and S7, S9, respectively (Table S1). According to the NTO, S0→S8 and S0→S7 are dominated by the charge transfer from Cz units to Dbp core, with small contribution from the local Dbp excitations, and S0→S9 is associated only with the local electronic transitions in Dbp core (Fig. S6). S1). However, experimental spectra of the solutions in tetrahydrofuran (THF) and toluene (TOL) exhibit a weak CT tail in the region of 400-500 nm (Fig. 3). This suggests an increased oscillator strength for the CT states stemming due to shifted dihedral angle from 90° between Ac and Dbp. While the lowest energy transitions S1-S4 for CzDbp correspond to the CT between donors and acceptor giving a small intensity band in the range 450-600 nm (Table S1).

The larger absorption intensity of CT bands in experimental absorption spectra as compared to theoretical ones can be observed. This observation can be explained by the smaller dihedral angle D-A of molecules than the theoretical calculated ones (Fig. 3). The intramolecular differences may appear due to some degree of aggregation in the ground state in solutions. To check such assumption, the absorption spectra of the different concentrations of AcDbp and CzDbp in TOL and THF solutions revealed increase of intensity of CT bands when concentrations of the compounds in solutions increases (Fig. S7). Such differences revealed presence of aggregates in the solutions. The theoretical calculations confirmed that oscillator strength of the CT states depends on the size of dihedral angle D-A. The smaller dihedral angle resulted in the larger value of oscillator strengths at S1 state for AcDbp and CzDbp (Table S2). S1).

Emission

The measurements of the degassed solutions showed interesting differences: while solutions of AcDbp showed insignificant increase of PLQY, CzDbp demonstrated increase of PLQY by 16% for both the solutions, which, as we show below, could stem from TTA. To support this assumption, the plots of PL intensity versus excitation intensity (λexcitation=400 nm) (Fig.

S8

) were recorded for powders and THF solutions (0.5 mg/ml) of AcDbp and CzDbp using neutral filters according to the method described in ref. [START_REF] Grüne | Kinetic modeling of transient electroluminescence reveals TTA as an efficiency-limiting process in exciplex-based TADF OLEDs[END_REF]. Integrated PL intensities versus excitation intensities for powders and THF solutions of AcDbp and CzDbp are plotted in Fig. 4b. The linear fits of these dependences in the different excitation intensity ranges (9-100% (Fig. 4b) and 24-100 %( Fig. 4b inset)), revealed the slopes higher than unity displaying contribution of TTA emission [START_REF] Qiao | Nonlinear optoelectronic processes in organic optoelectronic devices: Triplet-triplet annihilation and singlet fission[END_REF][START_REF] Zhou | Upconversion Luminescent Materials: Advances and Applications[END_REF][START_REF] Lee | Blue organic light-emitting diodes: current status, challenges, and future outlook[END_REF][START_REF] Kukhta | Deep-blue high-efficiency TTA OLED using para-and metaconjugated cyanotriphenylbenzene and carbazole derivatives as emitter and host[END_REF][START_REF] Grüne | Kinetic modeling of transient electroluminescence reveals TTA as an efficiency-limiting process in exciplex-based TADF OLEDs[END_REF]. S2, and Fig. S11). However, for acridan-containing compound (AcDbp) the theoretical vertical ΔEST value is zero (Table S2, Fig. S11). The dichotomy between theory and experiment is also witnessed by the different nature of the corresponding lowest triplet state. While the experimental emissive triplet state is a local one of 2.35 eV (PH spectrum in Fig. 4a), the theoretical LE corresponds to T3 at 2.33 eV, T1 and T2 being of CT nature (Fig. S10,S11). Interestingly, the lowest energy absorption band in the experimental absorption spectrum of AcDbp is a CT one (Fig. 3). A recent report [START_REF] Rudnick | The influence of torsion on excimer formation in bipolar host materials for blue phosphorescent OLEDs[END_REF] indicated that THF becomes solid at 77K and geometric relaxation of the solvent is no longer possible. This effect was shown to blue-shift the PL maximum at 77 K by roughly 0.4 eV as compared to room temperature, which was traced back to the inefficient stabilization of the emissive CT state. Following these results, it is possible that CT in AcDbp is not entirely stabilized by solvent, thus resulting in emission from LE state. In the case of CzDbp, CT state could be stabilized by the electronic polarization of the adjacent CzDbp molecules [START_REF] Rudnick | The influence of torsion on excimer formation in bipolar host materials for blue phosphorescent OLEDs[END_REF], given that this compound was found to form aggregates (Fig. S7).

CzDpb
PL spectra and PL decay curves of the solutions of AcDbp and CzDbp in TOL and THF were recorded before and after degassing. As shown in Fig. 5 increased when the temperature was increased from ca. 220 K to 300 K (Fig. S14) Such increase of PL intensity with increasing temperature can be explained by TADF contribution [START_REF] Liu | All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes[END_REF]. However, the intensity of the decay profile of delayed fluorescence, thus contribution of delayed fluorescence (τTADF), decreases with an increase of temperature for the layer of nondoped AcDbp (Fig. 6b). Such PL decay behaviour under different temperatures for TADF compounds are not typical and it is rarely described/discussed in the literature [START_REF] Grüne | Kinetic modeling of transient electroluminescence reveals TTA as an efficiency-limiting process in exciplex-based TADF OLEDs[END_REF][START_REF] Wang | Triplet decay-induced negative temperature dependence of the transient Photoluminescence decay of thermally activated delayed fluorescence emitter[END_REF]. For example, J. Grüne at. el. demonstrated that negative temperature dependence of the transient electroluminescent decays at increasing temperature of exciplex-based TADF emitters are mainly related to contribution of TTA [START_REF] Grüne | Kinetic modeling of transient electroluminescence reveals TTA as an efficiency-limiting process in exciplex-based TADF OLEDs[END_REF] 3). Thus, population of the emissive singlet state via RISC from the triplet state is allowed. Such E values are in good agreement with corresponding values of previously reported TADF compounds. [START_REF] Kim | Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff[END_REF][START_REF] Bunzmann | Optically and electrically excited intermediate electronic states in donor:acceptor based OLEDs[END_REF] PL decay curves of the layers of CzDbp, in the temperatures ranging from 77 K to 300 K revealed only prompt fluorescence. No long-lived components assigned to phosphorescence or delayed fluorescence were detected (Fig. 6d,S13d). No phosphorescence was experimentally observed neither for the layer of non-doped CzDbp nor for that of doped 10 wt.% CzDbp in mCP. It should be noted that the intensity of the PL decay profile decreases with increasing of temperature for the non-doped CzDbp because of the TTA as it was mentioned above (Fig. 6d). For CzDbp, no delayed fluorescence was recorded. The reasons are additionally discussed in the section "Mechanistic considerations".

Electroluminescent properties

Compound AcDbp has potential as TADF host for red OLEDs taking into account its chargetransporting properties, energy of first triplet level, and ability to exhibit TADF. On the other hand, CzDbp showed ambipolar charge-transporting properties and higher PLQY than 4).

PHOLEDs A, B and C showed classical (piq)2Ir(acac) emissions with the peak wavelength of 623 nm (Fig. S16). Only the emission of phosphorescent emitter was observed in electroluminescence spectra of devices A and B, manifesting complete energy transfer from host to (piq)2Ir(acac). In the case of PHOLED C, additional weak emission in the range 350-500 nm is detected since the mCP host is not the best host for the selected red phosphorescent (piq)2Ir(acac). As it will be discussed below, no additional bands were observed when a more appropriate host was selected for the reference device (Fig. 8a,S20-24). exciton. [START_REF] Zhen | Achieving highly efficient fluorescent blue organic light-emitting diodes through optimizing molecular structures and device configuration[END_REF] However, despite the same emitter used, the slopes higher than unity were obtained for devices A and B at low current density (Fig. 7b,c). This result points to some contribution from the TTA mechanism, given that harvesting of 100% of triplets by means of the TTA manifests with the slope reaching the value of 2. [START_REF] Lee | Blue organic light-emitting diodes: current status, challenges, and future outlook[END_REF][START_REF] Chou | Efficient delayed fluorescence via triplet-triplet annihilation for deep-blue electroluminescence[END_REF][START_REF] Ganzorig | A possible mechanism for enhanced electrofluorescence emission through triplet-triplet annihilation in organic electroluminescent devices[END_REF] This observation indicates that electroluminescence of devices A and B is related to both emission of the (piq)2Ir(acac) and emission enhancement by TTA mechanism. It should be noted, that TTA efficiency of host CzDbp should be higher than that of host AcDbp. The corresponding slope (1.299) of device B is higher than that (1.191) of device A.

At high current density, the slopes of all devices A-C were lower than unity which is related

to the efficiency roll-off (Fig. 7b-d, S19). Efficiency roll-off of devices A and B was lower (the slopes are closer to the unity) than that of device C. This observation additionally highlights higher efficiency of the developed TADF/TTA hosts in comparison to the conventional mCP. a VON was taken at 10 cd/m 2 ; b VDriving was taken at 10 mA/cm 4). The differences between EQE values of device E and F-H are related to either their different charge-balance factors (γ), efficiency of exciton production (χ), PLQY or the outcoupling efficiency (ηout) according to the formula [START_REF] Chou | Efficient delayed fluorescence via triplet-triplet annihilation for deep-blue electroluminescence[END_REF] EQE=γ×PLQY×χ×ηout [START_REF] Liu | All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes[END_REF].

Since the thicknesses of all functional layers and device structure of OLEDs D-H was the same, their outcoupling efficiency should be similar. It should be noted that the layers 5wt. % solid solutions of (piq)2Ir(acac) in mCP, NPB or nBPhen (9, 15, or 7 %, respectively)

showed slightly higher PLQY values than 5wt. % solid solutions of (piq)2Ir(acac) in AcDbp and CzDbp (7 and 8%, respectively) (Fig. S25). The different PLQYs of (piq)2Ir(acac) can be related to higher polarity of donor-acceptor-donor type hosts AcDbp and CzDbp hosts than that of electron donating hosts mCP, NPB and electron-accepting host nBPhen. Thus, the higher maximum EQE of device E is not related to the higher PLQY of the light-emitting layer of 5wt. % solid solution of (piq)2Ir(acac) in CzDbp. It should be noted that the values of PLQYs recorded for (piq)2Ir(acac) dispersed in the different hosts are lower than the previously reported PLQY (20%) of the same emitter [START_REF] Su | Highly efficient red electrophosphorescent devices based on iridium isoquinoline complexes: remarkable external quantum efficiency over a wide range of current[END_REF]. Such differences were apparently obtained since the above PLQYs were taken under air conditions. In any case, the obtained PLQYs should not be used for the calculation of the theoretical maximum EQE for (piq)2Ir(acac)-based devices by formula [START_REF] Liu | All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes[END_REF]. It is known that, in case of phosphorescent emitters, low PLQYs (thus, disagreement between the theoretical and experimental maximum EQEs) can be obtained if quantum yields of singlet-triplet conversion (STC) efficiency of phosphorescent complexes are below 100 %, which is a typical case [START_REF] Tofighi | Fast triplet population in iridium(III) complexes with less than unity singlet to triplet quantum yield[END_REF]. Apparently, because of that, experimental EQE of 22.6 % was reached for (piq)2Ir(acac)-based devices despite experiment PLQY of 20% recorded for the emitter (piq)2Ir(acac) [START_REF] Su | Highly efficient red electrophosphorescent devices based on iridium isoquinoline complexes: remarkable external quantum efficiency over a wide range of current[END_REF][START_REF] Fan | Host and dopant materials for idealized deep-red organic electrophosphorescence devices[END_REF]. Therefore, one of the highest experimental EQE values reported for (piq)2Ir(acac) based devices which reached 22.6 % was selected for the comparison with EQE values of our devices [START_REF] Fan | Host and dopant materials for idealized deep-red organic electrophosphorescence devices[END_REF]. For example, device E demonstrated EQE of 15.9 % which is not far below of that observed for the previously published (piq)2Ir(acac) based devices. To provide comparison of characteristics of AcDbp and CzDbp based devices with the reference devices, the concentration of (piq)2Ir(acac) dopant of 5 wt. % in light-emitting layer was selected. It is predictively possible to improve the efficiency of AcDbp and CzDbp based devices when the concentration of the (piq)2Ir(acac) dopant or/and thicknesses of additional layers are optimised.

Both charge-balance factors and efficiency of exciton production can be responsible for high EQE of device E. The TOF experiment showed, that the balance of hole and electron transport in the host AcDbp should be even better than that in CzDbp. However, device D did not show higher maximum EQE value than device E (Fig. 8b). The balance hole and electron transport of devices F-H should be also good. Taking into account the similarities/differences of hole-electron balance, the high efficiency of device E is partly attributed to harvesting of triplet excitons not only within phosphorescent emitter (piq)2Ir(acac) but also within the host CzDbp due to its TTA/TADF properties. Thus, the higher efficiency of device E is partly attributed to the higher efficiency of exciton production in comparison to that of the similar/reference devices D, F-H. 3). This disagreement can be related to the possible loss mechanisms when the TADF molecules are optically excited as it was discussed in [START_REF] Dias | The role of local triplet excited states and d-a relative orientation in thermally activated delayed fluorescence: photophysics and devices[END_REF]. Devices I and J showed orange and yellow electroluminescence with CIE1931 colour coordinates of (0.513, 0.479) and (0.395, 0.578), respectively (Fig. S26, S27, Table 4). EL spectra were completely related to emissions of AcDbp or CzDbp indicating that hole-electron recombination took place within the light-emitting layers.

Mechanistic considerations

The triplet harvesting in AcDbp and CzDbp systems occurs by means of different either TADF or TTA mechanisms.

Static diagrams.

In order to understand these differences, we focus first on the TADF mechanism. Following Adachi [START_REF] Kobayashi | Contributions of a higher triplet excited state to the emission properties of a thermally activated delayed-fluorescence emitter[END_REF] and Gibson [START_REF] Gibson | The importance of vibronic coupling for efficient reverse intersystem crossing in thermally activated delayed fluorescence molecules[END_REF], energy diagrams representing only a selected set of excited states of AcDbp and CzDbp are shown in Fig. 9a,b. The energy of ). Based on these diagrams (Fig. 9a), interpretation of the delayed fluorescence in AcDbp seems to be straightforward: while mixing of 3 CT with 3 LE by means of IC makes the symmetry allowed 3 LE→ 1 CT rISC more efficient, the ΔEST( 3 LE-1 CT) values seem to be very different depending on the conditions, adopting a relatively large upper-limit value of 0.21 eV for AcDbp doped in mCP, but much smaller (0.04 eV) for the pure compound (Fig. 9a).

Consequently, TADF should be of different efficiencies for the layers of molecular dispersion of AcDbp in mCP and for the layer of pure compound, which seems to be incoherent with the close values of PLQY of 3 and 4 % respectively. A possible and partial explanation could stem from the practically orthogonal geometry of this compound: the prompt fluorescence is very inefficient, differences in TADF efficiency should be of secondary importance with respect of the total emission (prompt FL + TADF).

In the case of CzDbp , 1 CT and 3 CT energy levels (2.52 eV and 2.28 eV, respectively) were taken from the PL and PH spectra of the solutions in THF at 77 K. The experimental value of 3 LE for CzDbp is not accessible experimentally, given that T1 is of CT nature. One could consider, however, the value of 2.35 eV measured for the 3 LE of AcDbp as a first approximation for the 3 LE state of CzDbp in THF at 77 K, which seems coherent with the similar space localization of this state for both the compounds only on the Dbp core (Fig. S10). In this case, ΔEST ( 3 LE-1 CT)=0.17 eV deduced from Fig. 9b for CzDbp is smaller than 0.21 eV estimated for AcDbp in mCP, for which the TADF seems efficient, thus making difficult to explain the absence of delayed fluorescence in the emission of CzDbp. Dynamic diagrams. The diagrams and comparisons presented above lack of the impact of mixing in the energy levels. In order to better understand this effect, we remember some of the milestone ideas and insights: (i) El Sayed's rule indicates that rISC between electronic states of the similar nature such as π-π* CT states is forbidden, thus preventing the interconversion between pure 3 CT and singlet 1 CT excited states in TADF compounds. (ii)

The direct 3 LE-1 CT spin-orbit coupling was shown to be very small (merely several cm -1 )

[56], failing consequently to explain the experimental rISC rate constants. (iii) The important information on the TADF mechanism was obtained by finding that the vibronic coupling between S1 (T1) and S2 (T2) is mandatory to promote T1→S1 rISC. [START_REF] Gibson | The importance of vibronic coupling for efficient reverse intersystem crossing in thermally activated delayed fluorescence molecules[END_REF][START_REF] Chen | Nature of highly efficient thermally activated delayed fluorescence in organic light-emitting diode emitters: nonadiabatic effect between excited States[END_REF] (iv) Calculations taking into account the contribution of IC or rIC between several low-lying excited states provide the crucial insight that ISC or rISC processes cannot be explained in a static frame; dynamical processes through the dihedral-angle oscillations, which are by several orders of magnitude faster than ISC or rISC, were shown to be a key factor determining the TADF efficiency. [START_REF] Chen | Nature of highly efficient thermally activated delayed fluorescence in organic light-emitting diode emitters: nonadiabatic effect between excited States[END_REF][START_REF] Henry | Spin-orbit coupling in aromatic hydrocarbons. Analysis of nonradiative transitions between singlet and triplet states in benzene and naphthalene[END_REF][START_REF] Olivier | Nature of the singlet and triplet excitations mediating thermally activated delayed fluorescence[END_REF] Indeed, the dihedral-angle oscillations and deformations around the D-A bond at excited states allow exploration of ensemble of conformations reachable at room temperature.

In addition to these findings, we suggest that the impact of these oscillations and deformations is the mixing between states: the nature of T1 can change from pure CT to a mixed LE-CT state or vice-versa. This creates "new" excited states at intermediate energies between "static" 3 CT and 3 LE states, which are absent in the static-frame energy diagrams (deduced from the onset excitation energies). Fig. 10a,b show the dependence of the nature of "new" triplet state (T * ) from dihedral angle D-A size. These "dynamical" states allow molecules to profit from the LE contribution in T1 to make the T1→S1 transition "allowed" (satisfy to El Sayed's rule) (Fig. 10a), hence increase the intensity of the T1→S1 transition (spin-orbit coupling, SOC), but also increase the intensity of S1→S0 transition, hence the PLQY of fluorescence. On the other hand, these oscillations and deformations could result in the increased ΔEST (Fig. 10b), which is detrimental for rISC. Nevertheless, at some point (a given geometry), the combination of SOC and ΔEST becomes convenient for effective rISC. [START_REF] Samanta | Up-conversion intersystem crossing rates in organic emitters for thermally activated delayed fluorescence: Impact of the nature of singlet vs triplet excited states[END_REF] It is worth remembering the results from [START_REF] Samanta | Up-conversion intersystem crossing rates in organic emitters for thermally activated delayed fluorescence: Impact of the nature of singlet vs triplet excited states[END_REF] showing the drastic decrease of rISC rates when ΔEST approaches 0.2 eV and beyond.

The ensemble of the above results and ideas, [START_REF] Chen | Nature of highly efficient thermally activated delayed fluorescence in organic light-emitting diode emitters: nonadiabatic effect between excited States[END_REF][START_REF] Henry | Spin-orbit coupling in aromatic hydrocarbons. Analysis of nonradiative transitions between singlet and triplet states in benzene and naphthalene[END_REF][START_REF] Olivier | Nature of the singlet and triplet excitations mediating thermally activated delayed fluorescence[END_REF][START_REF] Samanta | Up-conversion intersystem crossing rates in organic emitters for thermally activated delayed fluorescence: Impact of the nature of singlet vs triplet excited states[END_REF]] are illustrated by means of a new diagram for AcDbp doped in mCP shown in Fig. 9c. In this figure, from right to left, the static frame with well-defined energies of 3 LE and 3 CT states (similarly to Fig. 9a) is followed by a "dynamic" frame, corresponding to dynamically created mixed 3 (LE-CT) states of intermediate energies between 3 LE-3 CT. Note, that dynamical energy levels below-or above the static LE (CT) and CT (LE) could also be expected; nevertheless, we show in the dynamical diagrams only the energy levels in between those of LE-and CT.

In this context, while different rISC pathways would be predicted in a static frame, depending on the relative energy order between 3 LE and 3 CT, in the dynamic frame a fundamentally identical pathway could be predicted: In the case of CzDbp the same mechanism should hold true (Fig. 9d), but the energy of the mid-gap mixed states created "dynamically" (in-between 3 LE-3 CT) would correspond to increased ΔEST as compared to the value of 0.17 eV deduced from the diagram shown in Fig. 9b. This effect seems opposite to what occurs in case of AcDbp, and the increase of ΔEST from 0.17 eV to 0.24 eV could explain the inefficiency (absence) of delayed fluorescence. [START_REF] Samanta | Up-conversion intersystem crossing rates in organic emitters for thermally activated delayed fluorescence: Impact of the nature of singlet vs triplet excited states[END_REF] Additionally, the theoretical (vertical) T 1 state for CzDbp is not a pure CT state, as shown in the experimental diagram of Fig. 9b,d, but a mixed 3 (LE-CT) one (Fig. S28). This means that the contribution of this state to the rISC by means of the second-order nonadiabatic coupling between 3 LE and 3 (CT-LE) would be less efficient than that between CzDbp and AcDbp (Fig. 4b). Thus evidencing that the TTA mechanism contributes to the triplet harvesting in both compounds [START_REF] Chou | Efficient delayed fluorescence via triplet-triplet annihilation for deep-blue electroluminescence[END_REF][START_REF] Ganzorig | A possible mechanism for enhanced electrofluorescence emission through triplet-triplet annihilation in organic electroluminescent devices[END_REF], when CzDbp as well as AcDbp plays the roles of both the synthesiser and the annihilator. This is in agreement with the higher slope than unity for brightness versus current density plots for devices A (1.191) and B (1.299)

containing AcDbp and CzDbp, respectively. In addition, this is coherent with the fact that the maximum wavelengths of the PL spectra of CzDbp recorded in degassed and in-air conditions are identical (Fig. 5c) and is consistent with the assumption that Sn, the new singlet state created by the interaction of two triplet molecules, relax to S1 before radiative transition towards the ground state. The increase of PLQY by16% after deoxygenation of the solution is in good agreement with the reported maximum theoretical value of 20%. [START_REF] Qiao | Nonlinear optoelectronic processes in organic optoelectronic devices: Triplet-triplet annihilation and singlet fission[END_REF][START_REF] Ieuji | Triplet-triplet upconversion enhanced by spin-orbit coupling in organic light-emitting diodes[END_REF] Concerning to the fast TTA supposed in the case of CzDbp, the experimental support comes from the fast TTA (in ns range; kTTA >10 9 M -1 •s -1 ) previously reported, for example, for perylene [START_REF] Ye | Annihilation versus excimer formation by the triplet pair in triplet-triplet annihilation photon upconversion[END_REF] and many other TTA materials [START_REF] Zhou | Upconversion Luminescent Materials: Advances and Applications[END_REF].

Conclusions

We TTA channel for triplet harvesting, the TADF mechanism was found to be inefficient in the case of CzDbp, due to a combination of large singlet-triplet splitting reaching 0.24 eV, and limited contribution from the mixing (second-order nonadiabatic coupling) between 3 LE and 3 (CT-LE). This is due to the relatively small dihedral angles in CzDbp resulting in mixing of LE and CT characters in T1, thus reducing the contribution of CT character to the T1-S1 rISC.

From a fundamental standpoint, our results demonstrate that triplet harvesting in donor acceptor molecules with small singlet-triplet splitting is not necessary related to TADF as it is widely accepted, but can proceed through eventually faster and competitive TTA mechanism.

"Dynamic" state-energy diagrams, which consider the dynamical CT-LE mixing through fast vibrational motions allow better understanding of the experimental results related to TADF mechanism, as compared to the static state-energy diagrams.

In view of the interesting properties of AcDbp and CzDbp, these compounds were used as hosts in red PHOLEDs. The device with CzDbp host exhibited superior parameters of electrophosphorescence allowing to achieve higher external quantum efficiency of 15.9 % as well as lower efficiency roll-off in comparison to those of the device based on the widely utilised mCP, mCBP, NPB and nBPhen hosts. External quantum efficiencies of 19.4% and 22.1% for doped yellow and orange devices were achieved when the compounds CzDbp and AcDbp were used as TADF/TTA emitters, respectively.

  0337 Å -1 is ∼61°. The larger dihedral angles in the case of AcDbp are due to the shorter (C-)H….Dbp distances as compared to CzDbp, in turn stemming from the larger C-C-C angle in the six-atom central ring in Ac as compared to five-atom central ring in Cz (~122° and ~108° respectively). The HOMO of AcDbp is distributed on the both peripheral Ac units, whereas the LUMO is localized exceptionally over the molecule Dbp core (Fig.1). The spatial separation of the HOMO and LUMO appears because of the perpendicular dihedral angles D-A. In the case of CzDbp, HOMO of CzDbp at ω TOL B97XD is localized on the Cz moieties extending to the Dbp core, while calculations at ω THF B97XD show HOMO localization on the donor fragments, with less contribution from the Dbp (Fig.1). As for LUMO of CzDbp, in both cases it is localized on the acceptor core.

Fig. 1 .

 1 Fig. 1. Geometry and plots of frontier orbitals of AcDbp and CzDbp at B97XD calculated with ω tuned in THF and TOL.

4 * 3 cm 2 /Fig. 2 .

 4322 Fig. 2. Electric field dependences of charge mobilities for the layers (a) of AcDbp and CzDbp, TGA curves of AcDbp and CzDbp (b) and DSC curves of AcDbp (c).

Fig. 3 .

 3 Fig. 3. Experimental and theoretical absorption spectra of AcDbp and CzDbp

.

  Fluorescence spectra and photoluminescence quantum yield (PLQY) of the solutions of AcDbp and CzDbp in the solvents of different polarity (ε TOL =2.38 and ε THF =7.50) were recorded (Table 3, Fig. 4a). Both the compounds exhibited structureless PL profiles, indicating CT nature of the emissive states. This observation is in line with the nature of the lowest absorption bands in solutions of AcDbp and CzDbp. The stronger solvatochromic effect was detected in the fluorescence spectra of the solutions of AcDbp compared to those of CzDbp because of the dominant CT nature in AcDbp, which is coherent with the corresponding D-A dihedral angle, hence the purer CT states in the case of AcDbp as compared to CzDbp. Solutions of CzDbp showed higher values of PLQY than solutions of AcDbp (Table3). The values of PLQY of the solutions in TOL and THF of AcDbp were 2% and 0.5% while those of the solutions of CzDbp were 37% and 28%, respectively. Again, these results are in line with the corresponding geometries, the orthogonal D-A geometry in AcDbp preventing from any efficient S1→S0 fluorescence in spite of the efficiency of the triplet→singlet conversion efficiency. Indeed, the theoretical results indicate a dark S1 state in the case of AcDbp (zero oscillator strength), as compared to 0.023 for CzDbp (Table

Fig. 4 .

 4 Room-temperature fluorescence spectra and photoluminescence as well as phosphorescence spectra recorded at 77 K of THF solutions of AcDbp and CzDbp (phosphorescence spectrum of the solution of Dbp in THF recorded at 77 K is presented as blue line) (a) and plots of integrated PL intensities versus excitation intensities for powders and THF solutions of AcDbp and CzDbp (b).The onsets of photoluminescence (PL) and phosphorescence (PH) spectra of the solutions ofAcDbp and CzDbp in THF recorded at 77 K were used to estimate singlet-triplet energy differences (ΔEST) (Fig.4a, Table3). The PL spectrum of AcDbp exhibited a structureless shoulder with the onset estimated at 2.55 eV (Fig.4a), indicative of CT nature of the emissive state. This observation is in line with the lowest energy band of the corresponding absorption spectra. PH spectrum of AcDbp showed characteristic vibrational structure, indicating that the lowest emissive T1 state at 77 K corresponds to the local excitation ( 3 LE) of Dbp-core. As for, Both PL and PH spectra of THF solution of CzDbp exhibited broad and structureless emission indicating CT nature of S1 and T1 excited states. Analysis of molecular orbitals of AcDbp and CzDbp indicates that for both the compounds, 1 CT and 3 CT occur due to transition from donor to acceptor, while3 LE transition takes place between orbitals delocalized over the Dbp core (Fig.S9, S10).

  and Fig. S12, the solution of AcDbp exhibited double exponential decay. The air equilibrated TOL and THF solutions showed lifetimes of long-lived component (τ2) of 362 ns and 420 ns, respectively. Meanwhile, under oxygen free conditions τ2 increased to 10946 ns for TOL solution and 2437 ns for THF solution suggesting a radiative PL stemming from triplet harvesting assumedly through TADF. The shapes of emission spectra of non-degassed and degassed solutions of the compounds were very similar showing that delayed fluorescence originated from the same CT state as prompt fluorescence (Fig. 5,S11). Intriguingly, the solutions of CzDbp in TOL and THF exhibited single exponential PL decays showing only prompt fluorescence (Fig. 5,S11). The solutions of CzDbp exhibited increase of PLQY by 16% after deoxygenation. This observation shows that the triplet states do play a role in the PL of CzDbp, which should consequently be realized through a channel different from TADF, being as fast as the prompt fluorescence.

Fig. 5 .

 5 Fig. 5. PL spectra (left) and PL decay curves (right) of air equilibrated and deoxygenated solutions of AcDbp (a-b) and CzDbp (c-d) in TOL

Fig. 6 .

 6 Fig. 6. PL spectra (left) and PL decay curves (right) of the layers of non-doped AcDbp (a-b) or CzDbp (c-d) recorded at different temperatures

AcDbp.

  Consequently, both AcDbp and CzDbp were use in PHOLED, in order to demonstrate their suitability as host materials for a commercial red phosphorescent emitter (piq)2Ir(acac). The analogous PHOLED based on well-known host mCP was also fabricated for comparison. All functional layers in the fabricated PHOLEDs were the same except lightemitting layers. Three devices with the configuration ITO/ MoO3 (0.6nm)/NPB (30 nm)/5 wt.% (piq)2Ir(acac): AcDbp or CzDbp or mCP (20 nm)/TPBi (70 nm)/LiF (0.8nm):Al fabricated using hosts AcDbp, CzDbp or mCP were named as devices A, B, or C, respectively (Fig. 7, Table

Fig. 7 . 1 ( 1 .

 711 Fig. 7. Equilibrium energy diagram (a) of devices A-C. IP PE and EA PE values of solid smples of AcDbp and CzDbp were taken for the design of the device structures (Table2). Linear fits to current density versus brightness plots for devices A-C (b-d).

  2 ; c CIE (x,y) was taken 1000 cd/m2 With the aim of additional optimization of device structure, five devices with the configuration ITO/HAT-CN(5 nm)/NPB(40 nm)/TCTA(10 nm)/mCBP(10 nm)/ 5wt. % (piq)2Ir(acac): AcDbp or CzDbp or mCBP or NPB or nBPhen (50 nm)/nBPhen (30 nm)/Liq(2 nm)/Al were fabricated using hosts AcDbp, CzDbp, mCBP, NPB, or nBPhen and named as devices D, E, F, G or H, respectively (Fig.8, Table 4). EL spectra recorded at the different voltages of PHOLEDs D-H were completely related to (piq)2Ir(acac) emissions (Fig.8a, S20-S24). Due to improved charge-injecting properties, (piq)2Ir(acac)-based devices D-H exhibited improved out-put electroluminescent characteristics. Device E fabricated using the developed host CzDbp, demonstrated the higher maximum EQE (15.9 %) in comparison to the reference devices F-H containing mCBP, NPB or nBPhen as hosts (Fig.8b, S20-S24, Table

Fig. 8 .

 8 Fig. 8. EL spectra (a) and EQE efficiencies versus brightness (b, c) characteristics for the devices D-J with AcDbp and CzDbp as host or emitter

Fig. 9 .

 9 Fig. 9. Diagrams of excited states energies of AcDbp (a) and CzDbp (b); diagrams illustrating the "dynamic" mechanism of mixing between LE and CT states through dihedral vibrations [56,57,58] in AcDbp (c) and CzDbp (d)

  3 (CT←→LE)1 CT. Considering now AcDbp doped in mCP, mixing of CT and LE characters in the same state is also accompanied by increase in the energy level somewhere in the mid-gap between 3 LE and 3 CT, resulting in reduced ΔEST as compared to the value of 0.21 eV deduced from the diagram shown in Fig.9athus making the TADF more efficient.

Fig. 10 .

 10 Fig. 10. Dependence of the nature of the first triplet state marked as T * of AcDbp (a) and CzDbp (b) on the D-A dihedral angle (obtained from TD-DFT calculations at ω THF B97XD).

3 LE and 3 (

 3 CT), again providing a possible explanation on the absence of delayed FL through the TADF channel in the case of CzDbp. The support to theoretical result related to the mixed3 (LE-CT) nature of T1 could rise from the shape of PL profiles of the films of pure CzDbp at very low temperatures (77-120-160 K, Fig.6c. A weak vibrational character is visible in PL spectra, that we assume originating from phosphorescence contribution related to a mixed 3 (LE-CT) emitting state.The above discussion provides possible explanations of the absence of triplet harvesting by means of TADF mechanism for CzDbp in THF and TOL solutions as well as in non-doped and doped in mCP layers. While, device J with CzDbp as emitter in mCBP demonstrating EQE of 22.1 % shows that triplet harvesting via TADF can occur, thus there are media in which CzDbp exhibit efficient TADF.Triplet-triplet annihilation. Our findings indicate that degassing of THF and TOL solutionsof CzDbp results in increase of PLQY by 16% of both solutions. This means that the triplet states do play a role in the PL of CzDbp, which should consequently be realized through a channel different from TADF being as fast as prompt fluorescence. One possible mechanism could be TTA, as shown in the section "Photophysical properties". Indeed, integrated PL intensities versus different excitation intensities give slope values higher than unity for

3 cm 2 /

 32 have synthesized two new D-A-D type molecules, CzDbp and AcDbp, and have characterized them by means of experimental and theoretical methods. The solid layers of both the semiconductors exhibit bipolar charge transport with drift mobilities exceeding 10 -Vs at high electric fields. The compounds are able to harvest triplet excitons by either triplet-triplet annihilation (TTA) or thermally activated delayed fluorescence (TADF) mechanisms.The experimental optical absorption spectra of AcDbp and CzDbp in different media indicate presence of very-low intensity CT-tails for both the compounds, the intensity of which increases with the increasing concentration of compounds. Photoluminescence spectra and decay curves of AcDbp revealed the presence of delayed fluorescence occurring through the TADF mechanism. In the case of CzDbp, no delayed fluorescence was observed, however, triplet exciton harvesting through the TTA mechanism was evidenced. The absence of delayed fluorescence is assumed to stem from a combination of two effects. Compared to TADF timescales observed for AcDbp (10 -3 s), faster triplet harvesting in CzDbp through TTA channel was evidenced through electroluminescence. The timescale is supposed to be similar to emission timescales of the prompt fluorescence. This leads to an increase of PLQY of the solutions by 16% after degassing. Independently from the faster (hence competitive)

7-bis(9,9-dimethylacridin-10(9H)-yl)dibenzo[a,c]phenazine (AcDbp) Compound

  Hz, J=2.9 Hz, 4H, ar.), 8.18 (dd, J=6.5 Hz, J=3.4 Hz, 2H, ar.), 8.84 (d, J=8.6 Hz, 2H, ar.), 9.42 (d, J=2.1 Hz, 2H, ar.).13 C NMR (101 MHz, CDCl3-d6, δ): 31.5, 36.1, 114.2, 120.8, 125.5, 126.5, 129.5, 130.2, 140.9. MS (APCl + ), m/z = 695 [M] + . Anal calcd for C50H38N4: C

	BDpb (0.35 g, 0.8 mmol) and 3,6-di-tert-butyl-9H-carbazole (0.56 g, 2.0 mmol) was
	dissolved in dry toluene under argon. Then, bis(tri-tert-butylphosphine)palladium(0) (0.008
	g, 0.016 mmol) and sodium tert-butoxide (0.38 g, 4.0 mmol) was added. The reaction
	mixture was heated at 90 °C for 24 h. After cooling to room temperature, the reaction mixture 86.42; H 5.51; N 8.06; found: C 86.56; H 5.24; N 8.19
	was added into water and was extracted with chloroform. The combined organic layers were
	washed with water and dried over anhydrous sodium sulfate. After filtration and evaporation,
	the crude product was purified by column column chromatography using hexane/ethylacetate
	(8/1) as eluent. Compound was obtained as yellow solids in 54% yield. IR (ν in cm -1 ): 3042
	2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)dibenzo[a,c]phenazine (CzDbp)

(C-H ar.), 2960 (C-H alk.), 2863 (C-H alk.), 1491 (C-H alk.), 1477 (C-H alk.), 1295 (C-N), 1263 (C-N).

1

H NMR (400 MHz, CDCl3-d6, δ): 1.43 (s, 36H, CH3), 7.44-7.51 (m, 8H, ar.), 7.75 (dd, J=6.6 Hz, J=3.4 Hz, 2H, ar.), 7.96 (dd, J=8.5 Hz, J=2.2 Hz, 2H, ar.), 8.15 (s, 4H, ar), 8.20 (dd, J=6.5 Hz, J=3.4 Hz, 2H, ar.), 8.72 (d, J=8.7 Hz, 2H, ar.), 9.61 (d, J=2.2 Hz, 2H, ar.). 13 C NMR (101 MHz, CDCl3-d6, δ): 32.1, 34.8, 109.4, 116.4, 123.7, 123.8, 124.0, 124.8, 128.6, 130.2, 130.2, 132.1, 138.1, 139.3, 142.5, 143.2. MS (APCl + ), m/z = 835 [M] + . Anal calcd for C60H58N4: C 86.29; H 7.00; N 6.71; found: C 86.29; H 6.86; N 6.98.

2,was synthesized according to the same procedure as described above for the synthesis of CzDbp, except that 9,9-dimethyl-9,10-dihydroacridine (0.24 g, 1.1 mmol) was used instead of 3,6-di-tert-butyl-9H-carbazole. The crude product was purified by column column chromatography using hexane/ethylacetate (8/1) as eluent. It was recrystallized from eluent. Compound was obtained as orange crystals in 43% yield. IR (ν in cm -1 ): 3059 (C-H ar.), 3033 (C-H ar.), 2954 (C-H alk.), 2924 (C-H alk.), 2853 (C-H alk.), 1475 (C-H alk.), 1449 (C-H alk.) 1325 (C-N), 1262 (C-N).

1

H NMR (400 MHz, CDCl3-d6, δ): 1.74 (s, 12H, CH3), 6.35-6.37 (m, 4H, ar.), 6.88-6.93 (m, 8H, ar.), 7.46-7.48 (m, 4H, ar.), 7.76 (dd, J=6.

[START_REF] Ban | Design of efficient thermally activated delayed fluorescence blue host for high performance solution-processed hybrid white organic light emitting diodes[END_REF]

3. Results and discussion 3.1. Synthesis As shown in Scheme 1, we

	CE-440 elemental analyser, Model 440 C/H/N. Thermogravimetric analysis (TGA) was moment of illumination, and tt is the transit time which was taken from the TOF transients. A
	performed on a TA Instruments Q50 apparatus at a heating rate of 20 ºC/min under nitrogen Keithley source meter 2400-C was utilized for recoding of the current density-voltage
	atmosphere. Differential scanning calorimetry (DSC) measurements were performed on a 100 characteristics. The current density-luminance characteristics were estimated using a
	TA Instruments Q2000 thermal analyser at a heating/cooling rate of 10 ºC/min under nitrogen calibrated silicon photodiode with the 6517B Keithley electrometer. Electroluminescence
	atmosphere. Optical and photophysical properties of solutions in THF, TOL and layers of the (EL) spectra were recorded by an Avantes AvaSpec-2048XL spectrometer. The current,
	synthesized compounds were recorded using a Perkin Elmer Lambda 35 and an Edinburgh power and external quantum efficiencies were estimated utilizing the current density,
	Instruments FLS980 spectrometers. Cyclic voltammetry (CV) measurements were carried out luminance, and EL spectra as reported earlier.[38]
	using Autolab potentiostat PGSTAT20 in a three electrode cell using platinum rod as a
	counter electrode, glassy carbon as working electrode and Ag/AgNO3 as the reference
	electrode. The experiments were carried out in dry dichloromethane solution containing 0.1
	M tetrabutylammonium perchlorate as electrolyte at room temperature under nitrogen designed and synthesized compounds with donor-acceptor-donor
	atmosphere at a scan rate 50 mV/s. The measurements were calibrated using the internal (D-A-D) architecture composed of dibenzo[a,c]phenazine (Dbp) acceptor core and
	standard ferrocene/ferrocenium (Fc/Fc + ). The ionization potentials (IPPE) of the layers of the peripheral aromatic donor moieties such as 9,9-dimethylacridine (Ac) and 3,6-di-tert-butyl-
	synthesized compounds were measured by the electron photoemission in air method.[37] carbazole (Cz). The intermediate compound BDbp was prepared through a condensation
	Hole drift mobility of the materials was estimated by a time-of-flight (TOF) method. The reaction between 2,7-dibromo-9,10-phenanthrenequinone and o-phenylenediamine. The
	samples were fabricated with structures of ITO/compound/Al. Commercial indium tin oxide Buchwald-Hartwig amination of BDbp with 3,6-di-tert-butyl-9H-carbazole or 9,9-dimethyl-
	(ITO) coated glass was used as a substrate, which was first cleaned chemically using a bath 9,10-dihydroacridine in the presence of bis(tri-tert-butylphosphine)palladium(0) as a catalyst
	of distilled water and acetone. Organic and Al films were sequentially vacuum-deposited afforded CzDbp and AcDbp, respectively. The target products were purified by column
	onto precleaned ITO-coated glass substrates under the vacuum of 2-5·10 -6 mBar using chromatography. Their chemical structures were confirmed by 1 H and 13 C nuclear magnetic
	vacuum equipment from Kurt J. Lesker in-built in an MB EcoVap4G glove box. In the TOF resonance (NMR), IR spectroscopies, mass spectrometry and elemental analysis.
	1 H NMR and 13 C NMR spectra were obtained of the solutions in deuterated chloroform experiments, charges were generated by a pulsed third-harmonic Nd:YAG laser EKSPLA
	(CDCl3-d6) with a Bruker Avance III spectrometer. The data are given as chemical shifts in δ NL300 working at a pulse duration of 3-6 ns and the wavelength of 355 nm. Electric fields
	(ppm) and tetramethylsilane was used as Elemental analysis was performed with an Exeter were applied by a Keithley 6517B electrometer. A digital storage oscilloscope Tektronix
	Analytical CE-440 elemental analyser, Model 440 C/H/N/. an internal standart. Mass spectra TDS 3032C was used to record TOF transients. The drift mobility was calculated by using
	were obtained on a Waters ZQ 2000 mass spectrometer. IR spectra were recorded with a the formula µ=d 2 /U·tt, where d is the layer thickness, and U the surface potential at the
	Vertex 70 Bruker spectrometer. Elemental analysis was performed with an Exeter Analytical

Scheme 1. Synthetic route for AcDbp and CzDbp. 3.2. Computational details

	Theoretical calculations were performed using ωB97XD functional with optimally tuned
	range separation parameter in toluene (TOL) and tetrahydrofuran (THF). Solvents effect was
	taken into account within the conductor-like polarizable continuum model (CPCM). The ω
	values were optimally tuned by minimizing a function J(ω), where εHOMO and εLUMO denote
	the energies of the HOMO and LUMO, IP and EA are the vertical ionization potential and
	electron affinity of the molecule, respectively:[39]

Table 1 .

 1 Inter

-fragment dihedral angles (degrees), energies of frontier orbitals and vertical ionization potential values for compounds AcDbp and CzDbp calculated at the DFT/6-31G**

ωB97XD ω THF =0.0011Å -1 ω TOL =0.0337 Å -1 ω THF =0.0011Å -1 ω TOL =0.0322 Å -1 AcDbp CzDbp

	dihedral angle	90.1	90.3	75.6	60.7
	HOMO, eV	-5.01	-5.34	-5.31	-5.56
	LUMO, eV	-2.30	-1.91	-2.31	-1.91
	IP teor , eV	4.98	5.28	5.28	5.49

Table 1 ,2).Table 2 .

 12 , but also from the fact that the optical gap values used in the calculation of EA PE are larger than the IP CV -EA CV gap by 0.6 eV and 0.64 eV of AcDbp and CzDbp, respectively, corresponding to the electron-hole binding energy in the first excited state. In order to extend theoretical investigation of the compounds, IP theor values were calculated. The analysis revealed, that experimental IP CV values of 5.25 eV and 5.44 eV well coincide with the values of IP theor obtained by ω TOL B97XD of 5.28 eV and 5.49, respectively (Ionization

	AcDbp	5.50	2.73	2.77	5.25	3.12	2.4•10 -3	1.2•10 -3
	CzDbp	5.50	2.97	2.53	5.44	3.11

[START_REF] Zhou | Modulation of excited state property based on benzo[a, c]phenazine acceptor: three typical excited states and electroluminescence performance[END_REF]

eV higher than that of CzDbp (2.77 eV and 2.53 eV, respectively). The difference between CV and PE values stem from not only the environmental polarization effectspotential, electron affinity and charge mobility data for AcDbp and CzDbp IP PE , eV Eg, opt eV EA PE , eV IP CV , eV EA CV , eV μh * , cm 2 /Vs μe * , cm 2 /Vs

Table 3 .

 3 Photophysical properties of AcDbp and CzDbp

		PLQY under air ,%*	PLQY degassed , %	S1,eV	T1,eV	ΔEST,eV
		THF/TOL/non-doped/mCP/mCBP	THF/TOL		THF
	AcDbp	0.5/2/3/4/8	0.6/3	2.55	2.35	0.20
	CzDbp	28/37/15/11/21	44/53	2.52	2.28	0.24
	* measured at air condition.

The experimental estimation of the singlet-triplet energy splitting (ΔEST) revealed values of 0.24 eV and 0.20 eV for CzDbp and AcDbp, respectively. The experimental ΔEST value for CzDbp is in good agreement with the theoretical vertical ΔEST values of 0.20 eV (THF) and 0.21 (TOL) (Table

Table 4 .

 4 EL parameters of the devices A-J

	Device	Emissive layer	a VON/	CEmax,	PEmax,	EQEmax,	CIE 1931

Table 4

 4

	). Experimental

  LE for AcDbp (2.35 eV) was taken from the PH spectrum of the solution in THF at 77 K.Below, we consider that this energy value remains roughly similar in the case of the films of pure and doped in mCP AcDbp. Note that 3 LE energy level of AcDbp (in THF at 77 K) is below1 CT and 3 CT energy levels of AcDbp doped in mCP (2.56 eV and 2.53 eV, respectively), but is between 1 CT and 3 CT of non-doped AcDbp (2.39 eV and 2.26 eV, respectively

3

Acknowledgements

This project has received funding from European Social Fund (project No 09.3.3-LMT-K-712-02-0106) under grant agreement with the Research Council of Lithuania (LMTLT)


Gérard Gouesbet
email: gouesbet@coria.fr

Leonardo André

Rayleigh limit of generalized Lorenz-Mie: axicon terms revisited

Keywords: generalized Lorenz-Mie theory, optical forces, Rayleigh limit

A systematic study of optical forces in the Rayleigh limit of the generalized Lorenz-Mie theory, GLMT in short, has been recently investigated [START_REF] Gouesbet | Generalized Lorenz-Mie theories, second edition[END_REF]. Although performed in the case of circularly symmetric Bessel beams, a general formulation for arbitrary shaped beams has been simultaneously obtained, both for longitudinal in the z-direction [START_REF] Gouesbet | Gradient, scattering and other kinds of longitudinal optical forces exerted by o¤-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory[END_REF] and for transverse forces in the x-and ydirections, see [START_REF] Gouesbet | Axicon optical forces and other kinds of transverse optical forces exerted by o¤-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory[END_REF], with corrections for the y-component of forces provided in [START_REF] Ambrosio | On transverse radiation pressure crosssections in the generalized Lorenz-Mie theory and their numerical relationships with the dipole theory of forces[END_REF]. In this framework, optical forces are expressed in terms of beam shape coe¢cients g m n;T M and g m n;T E (n from 1 to 1, m from ( n) to (+n), T M standing for "Transverse Magnetic" and T E standing for "Transverse Electric"). A remarkable result is that optical forces are then expressed in terms of BSCs associated with only (n = 1)-and (n = 2)-partial waves. This is deep contrast with the dipole theory of forces, e.g. [START_REF] Chaumet | Time-averaged total force on a dipolar sphere in an electromagnetic …eld[END_REF], [START_REF] Albaladejo | Scattering forces from the curl of the spin angular momentum of a light …eld[END_REF] in which optical forces are explicitly expressed in terms of the total electric …eld of the illuminating beam E, i.e. encompassing all partial waves from n = 1 to 1 of this beam, and asks the question to know what is the relationship between the Rayleigh limit of the GLMT and the dipole theory of forces (let us note what we here comply with the de…nition of total …eld in the dipole theory of forces denoting the illumination beam, in contrast with the point of view in the GLMT framework where total …eld instead denotes the summation of the incident and scattered …elds). Relying on numerical [START_REF] Ambrosio | On transverse radiation pressure crosssections in the generalized Lorenz-Mie theory and their numerical relationships with the dipole theory of forces[END_REF], [START_REF] Ambrosio | On longitudinal radiation pressure crosssections in the generalized Lorenz-Mie theory and their numerical relationship with the dipole theory of forces[END_REF] and on formal analyses [START_REF] Ambrosio | On the Rayleigh limit of the generalized Lorenz-Mie theory and its formal indenti…cation with the dipole theory of forces. I. The longitudinal case[END_REF], [START_REF] Ambrosio | On the Rayleigh limit of the generalized Lorenz-Mie theory and its formal identi…cation with the dipole theory of forces. II. The transverse case[END_REF], it has however been demonstrated that the 1 © 2021 published by Elsevier. This manuscript is made available under the CC BY NC user license https://creativecommons.org/licenses/by-nc/4.0/ Version of Record: https://www.sciencedirect.com/science/article/pii/S0022407321001849 Rayleigh limit of GLMT perfectly identi…es with the dipole theory of forces. This implies that the dipole theory of forces actually contains mainly super- ‡uous contributions, speci…cally all contributions from partial waves of order n > 2. These results furthermore imply that it is necessary to revisit a paper devoted to the existence of axicon terms in the framework of the Rayleigh limit of GLMT [START_REF] Gouesbet | Axicon terms associated with gradient optical forces in generalized Lorenz-Mie theory[END_REF]. To this purpose, it will be su¢ cient to limit ourselves to a discussion of longitudinal optical forces.

We start with scattering forces that we de…ne as forces proportional to the Poynting vector at the position of the scattering particle, denoted by the subscript P (this is here the most convenient de…nition although alternative points of view may be taken). In the Rayleigh limit of GLMT, the longitudinal component of the Poynting vector taken at P is found to read as, e.g. Eq.(66) in [START_REF] Gouesbet | Gradient, scattering and other kinds of longitudinal optical forces exerted by o¤-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory[END_REF]:

( f S z ) P = 2 Re[i(g 1 1;T M g 1 1;T E g 1 1;T M g 1 1;T E )] (1)
in which the normalization condition E 0 H 0 =2 = 1 has been used and which shows that scattering forces only depend on (n = 1)-partial waves. This implies that, when expressing the corresponding scattering forces in the dipole theory of forces, all partial waves of order n > 1 are super ‡uous and produce a null contribution as formally checked (after lengthy formal computations) in [START_REF] Ambrosio | On the Rayleigh limit of the generalized Lorenz-Mie theory and its formal indenti…cation with the dipole theory of forces. I. The longitudinal case[END_REF].

Let us now consider gradient forces that we de…ne as being proportional to the derivative of jEj 2 taken at P . In the case of circularly symmetric Bessel beams, this gradient is 0 in agreement with the fact that longitudinal gradient forces are found to be 0 as well in the Rayleigh limit of GLMT [START_REF] Gouesbet | Gradient, scattering and other kinds of longitudinal optical forces exerted by o¤-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory[END_REF]. However, it has been recognized soon after that this result was unsatisfactory and somehow inconsistent insofar as, according to the understanding gained by working in the Rayleigh limit of GLMT, it was clear that most of the partial waves involved in E were irrelevant to the computation of the gradient of jEj 2 . Inspired by Eq.1 above, it was then decided to evaluate this gradient by working with a restricted electric …eld e E in which only (n = 1)-partial wave contributions would be retained rather than working with the total …eld E. In the case of on-axis circularly symmetric Bessel beams, it has been found that the gradient of e E 2 was indeed zero as expected [START_REF] Gouesbet | Optical forces exerted by on-axis Bessel beams on Rayleigh particles in the framework of the generalized Lorenz-Mie theory[END_REF]. However, when applied to o¤-axis beams, the gradient of e E 2 generated axicon terms which were not zero, as it was expected at this time [START_REF] Gouesbet | Axicon terms associated with gradient optical forces in generalized Lorenz-Mie theory[END_REF]. The solution to this issue is that, in contrast with the case of scattering forces which are correctly predicted by using only (n = 1)-contributions, the evaluation of gradient forces and of the corresponding gradient of jEj 2 taken at P require to account both for (n = 1)-and for (n = 2)contributions. This may be demonstrated as follows. When establishing the identi…cation between the Rayleigh limit of GLMT and the dipole theory of forces, it was established that the longitudinal gradient of jEj 2 taken at P reads as [START_REF] Ambrosio | On the Rayleigh limit of the generalized Lorenz-Mie theory and its formal indenti…cation with the dipole theory of forces. I. The longitudinal case[END_REF]:

Acknowledgments/Funding. "National Council for Scienti…c and Technological Development (CNPq) (426990/2018-8, 307898/2018-0)"

(@ z jEj

2 ) P = 2k jE 0 j 2 Im(G) [START_REF] Gouesbet | Gradient, scattering and other kinds of longitudinal optical forces exerted by o¤-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory[END_REF] in which, see Eq.( 24) in [START_REF] Gouesbet | Gradient, scattering and other kinds of longitudinal optical forces exerted by o¤-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory[END_REF]:

But, it has been demonstrated that, in the case of circularly symmetric Bessel beams, see Eqs.( 28), ( 37) and (41) in [START_REF] Gouesbet | Gradient, scattering and other kinds of longitudinal optical forces exerted by o¤-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory[END_REF]:

which is to be compared with the fact that the gradient forces are found to be proportional to Im(G), e.g. Eq.( 27) in [START_REF] Gouesbet | Gradient, scattering and other kinds of longitudinal optical forces exerted by o¤-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory[END_REF]. Therefore, taking into account both (n = 1)-and for (n = 2)-contributions when evaluating (@ z jEj

2 ) P does not produce any axicon term, but a perfectly coherent formulation.