Hui Li

Dr Rama M Shakaroun

Sophie M Guillaume
email: sophie.guillaume@univ-rennes1.fr

ORCID Jean-François Carpentier
email: jean-francois.carpentier@univ-rennes1.fr

Recent Advances in Metal-Mediated Stereoselective Ring-Opening Polymerization of Functional Cyclic Esters towards Well-defined Poly(hydroxy acid)s: From Stereoselectivity to Sequence-Control

Keywords: Ring-opening Polymerization, Lactone, Diolide, O-Carboxyanhydride (OCA), Poly(hydroxy acid), Poly(hydroxyalkanoate) (PHA), Sequence-Controlled Polymer, Stereoselective Catalysis, Yttrium

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Introduction

Poly(hydroxy acid)s, including poly(α,β,γ-hydroxy acid)s, namely PAHAs, PBHAsmore commonly referred to as poly(hydroxyalkanoate)s (PHAs) -, PGHAs, respectively, are a class of polyesters that find a variety of packaging, agricultural, biomedical and pharmaceutical applications, thanks to their biocompatibility and (bio)degradability. [ 1 ] Most ubiquitous examples include the commercially available poly(lactide) (PLA), poly(glycolide) (PGA), and poly(lactide-co-glycolide) (PLGA) as PAHA representatives, and poly (3-hydroxybutyrate) (PHB) with a PBHA pattern (Chart 1). The former PAHAs are industrially produced from readily available inexpensive, renewable resources (e.g. corn, sugar cane, sugar beet) by ringopening polymerization (ROP), while the latter PBHAs are prepared by fermentation from naturally occurring biomass-derived sugars (e.g. sucrose, glucose) using microorganisms that provide (only isotactic) PHAs. Poly(hydroxy acid)s feature different side-chain substituents along the backbone, thereby imparting chirality to each repeating units. The chemical (functional) nature of these side-groups and the successive enchainment of the chiral monomer units along the macromoleculesthat is the regularity of the configurations of adjacent stereocenters along the polymer chain -, both dictate/modulate the physical, mechanical and biological properties of the resulting polymer materials by tailoring the composition and architecture of the polymers. Stereoregular polymers basically feature either isotactic (in which neighboring stereocenters have the same configurations) or syndiotactic (in which neighboring stereocenters adopt alternating configurations) structures. Such stereoregular polymers are most desirable since they are usually crystalline and thus show improved thermal and mechanical properties as compared to those of atactic (lack of regularity in the neighboring stereocenters) analogues. Side-chain functionality along poly(hydroxyl acid)s' backbone is further highly desirable since, besides providing a way to influence the physico-chemical properties, it offers a handle to chemically and/or biologically modify the scope of the polymer materials through post-polymerization side-chain modification. Sequence-controlled poly(hydroxy acid)s with various functionalities and microstructures thus directly widen the scope of the resulting materials. [START_REF]Sequence-controlled polymers[END_REF] A c c e p t e d m a n u s c r i p t Chart 1. Poly(α,β,γ-hydroxy acid)s addressed in this review, namely PAHAs, PBHAs and PGHAs, and the corresponding cyclic ester monomers from which they can be synthesized by stereoselective metal-mediated ring-opening polymerization (ROP).

The homogeneous single-site metal-catalyzed living ROP of cyclic esters reaches high performances in terms of efficiency, i.e. activity, productivity, stereocontrol; it remains to date the best strategy toward the controlled synthesis of such well-defined stereoregular side-chain functionalized polyesters. [START_REF] Dechy-Cabaret | For leading reviews on metal-catalyzed ROP of cyclic esters[END_REF] Polyesters with controlled macromolecular features including tuned molar mass values, narrow dispersities, limited undesirable side reactions (typically interand intra-molecular transesterifications), chain-end fidelity, monomer/repeating unit functionality, controlled polymer topology and microstructure (i.e. tacticity) are thus commonly synthesized. In that topical field, group 3 metal (rare earth) complexes have demonstrated unique performances: first, in their distinctive ability to successfully enable the ROP of a large variety of cyclic esters, including stable, low-strained, and hence usually reluctant to ring-open cyclic esters, and also in their capability to stereoselectively fine tune via the ligand catalyst the polymer's microstructure when starting from racemic mixtures of chiral monomers. [4] In this quest for polyesters exhibiting targeted stereoregularity and for highly efficient stereoselective catalyst systems, the pioneering work tackled the ROP of racemic lactide (rac-LA) for the synthesis of isotactic polylactide (PLA). Crystalline isotactic PLLA, commercially produced from the ROP of enantiomerically pure (S,S)-LA, is arguably one of the most ubiquitous sustainable polymer alternative to common petrochemical-based plastics. Yet, in contrast to heterotactic PLA that is rather easily accessible, only a limited number of (always metal-based) catalyst/initiator systems enable the direct preparation of isotactic (sometimes stereocomplexed) PLA from the ROP of rac-LA. [START_REF] Ovitt | For selected references in the isoselective ROP of rac-LA[END_REF] Efforts on the design of ancillary ligands with fine-tuned substituents on their platform rewardingly provided a significant breakthrough in stereoselective ROP catalysis. This has been extended beyond LA to other chiral racemic cyclic ester monomers, among which in particular the family of -lactones and Ocarboxyanhydrides (OCAs). However, until very recent work, only syndiotactic or even atactic polymers derived from these latter monomers could be obtained.

In this minireview, we highlight some recent achievements on the metal-mediated stereoselective ROP of cyclic esters towards the synthesis of (functional) poly(hydroxy acid)s, focusing on (functional) and -lactones, diolide, and OCAs. Quite strikingly, not only syndiotactic and isotactic polymers can now be efficiently produced from racemic mixtures of these monomers, but also original copoly(hydroxyalkanoate)s with gradient or alternated patterns prepared from mixtures of chemically different, oppositely configurated enantiopure monomers. It is shown that those stereoselective patterning methodologies rely on the combination of both the stereocontrol imparted by the catalytic system, and especially of the steric and electronic factors imparted by the ligand substituents, and of the chemical nature of the monomer unit(s).

From syndiotactic to isotactic PBHAs, PGHAs and PAHAs

Extensive studies by our group and others have evidenced that the stereoselective ROP of chiral racemic β-lactones is best achieved with group 3/rare-earth catalysts via coordination-insertion routes. [4,6 ] Most typical and highly effective (pre)catalysts/initiators for this purpose are, arguably, yttrium complexes stabilized by dianionic, tetradentate diamino-or amino-alkoxybis(phenolate) ligands (Scheme 1). Such trivalent yttrium complexes bearing the tripodal {ON(X)O R',R" } [6] or salan-type {ON R N R O R',R" } [7,8] ligands show high activity in the ROP of a variety of cyclic esters, among which in particular the more reluctant β-lactones. These ROPs proceed readily under mild conditions, at room temperature, in a controlled fashion, delivering polymers with narrow dispersities (ÐM = Mw/Mn <1.3 typically) and predictable molar mass values; that is, molar masses dictated by the monomer-to-initiator ratio, the initiator being either the nucleophilic group of the yttrium complex itself (e.g. NH(SiMe2)2, OiPr) or an exogenous A c c e p t e d m a n u s c r i p t alcohol (most often isopropanol), added as a co-initiator or as a transfer agent when used in excess versus the yttrium precursor. [6b] Nonetheless, the most unique and valuable feature of these families of achiral Y{ON(X)O R',R" } and Y{ON R N R O R',R" } complexes is the ability to fine tune the stereocontrol of the polymerization by simply changing the nature of the substituents on the ligand: in the former series, onto the phenolate moietiesand most particularly the R' ortho-substituent, * the one which is obviously the closest to the active centerand, in the latter series, also onto the N atoms of the salan ligand framework. For achiral catalysts/initiators, microstructural NMR studies conducted on PHAs resulting from the ROP of different chiral racemic β-lactones, unsurprisingly showed that, just as for the even more investigated rac-LA, stereocontrol originates from a chain-end mechanism (CEM): that is, the chirality of each new incoming monomer is dictated by the chirality of the last monomer unit inserted into the growing polymer chain. In the vast majority of cases, CEM generates syndiotactic polymers, because the active species alternatively picks up monomers with opposite configuration to minimize steric tilting in the transition state. It was therefore not unexpected that the initial studies conducted on the benchmark cyclic esters, racemic β-butyrolactone (rac-BL Me ) [9] and rac-LA, revealed that the Y{ON(X)O R',R" } and Y{ON R N R O tBu2 } complexes generate syndiotactic-rich poly(3-hydroxybutyrate) (PHB = PBHA Me ) and heterotactic-rich polylactide (PLA); and also that the syndiotacticity level, as determined by the probability of racemic linkage (Pr; Pr = 1 for a perfectly syndiotactic polymer, Pr = 0 for an isotactic one, and Pr = 0.5 for an atactic one), apparently increases with the steric bulkiness of the ortho-R' substituent on the {ON(X)O R',R" } ligand framework. Whereas the latter trend is monotonous in the case of lactide (Pr = 0.55 (R' = Me), 0.81 (tBu), 0.91 (CMe2Ph), 0.95 (CMe2tBu), 0.95 (CPh3)), the "most crowded" complexes hence being the best, it is not the case for the ROP of rac-BL Me (Pr = 0.56 (R' = Me), 0.80 (tBu), 0.83 (CMe2C6H4(p-CF3)), 0.91 (CMe2Ph), 0.70 (CMe2tBu), 0.96 (CPh3)). In this latter ROP, an electron-rich aryl group on the R' substituent is also beneficial and necessary for achieving high syndiotacticity. As discussed in details in a recent feature article, [10] this was accounted for by the occurrence of attractive C-H…π(aryl) interactions between the aryl groups from the R' ortho-substituent and the acidic C-H of the methylene of the last inserted monomer unit within the growing poly(alkoxybutyryl) chain; such so-called "second-sphere" or "non-covalent" interactions (NCIs) [START_REF] Marshall | Such beneficial non-covalent interactions (NCIs) have also been proposed on the basis of DFT computations in rac-LA ROP promoted by Zn-β-diketiminate complexes[END_REF] are illustrated in Chart 2 (a).

* For synthetic reasons, the ortho-R' and para-R" substituents in these (pro)ligands most usually prepared from the parent disubstituted salicaldehydes are often identical; nonetheless, (pro)ligands having different R' and R" substituents have been prepared and the corresponding yttrium complexes revealed a much less important role of the remote para-R" substituent, although slight differences in terms of activity, and to an even lesser extent, of stereoselectivity have been noted; see ref [6b]. MLA R (c) with yttrium{ON(X)O R',R" } catalysts. [6b,10] In 2009, Coates and Thomas reported that the ROP of rac-BL Me and related β-lactones bearing alkyl side-groups higher than Me, using the yttrium-salan complex Y{ON Me N Me O tBu2 } also generates highly syndiotactic PBHAs (Pr = 0.90-0.94). [7] In 2018, Cui and collaborators revisited this chemistry by tuning the substituents on the N atoms of the salan ligand framework, and further using ytterbium(III) in addition to yttrium(III) complexes for the ROP of rac-BL Me . [8] Independently of the nature of the metal centerwhich essentially affects the activity, the larger yttrium metal being always the most active -, the tacticity of PHB is drastically impacted by the substituents on the bridging N atoms: cyclohexyl substituents lead to syndiotactic-enriched PHB (Pr up to 0.78) while phenyl groups induce the formation of isotactic-enriched PHB (probability of meso linkage, Pm = 1 -Pr, up to 0.77), and tert-butyl groups give atactic PHB (Pm ca. 0.5). The nature of the stereocontrol mechanism operative with these systems, CEM or enantiomorphic-site control mechanism (ESM), [START_REF]If the ancillary ligand in the catalyst is chiral and overrides the influence of the polymer chain-end (CEM), the mechanism of stereochemical direction is referred to as "enantiomorphic-site control" (ESM). In the former mechanism, a stereochemical error is propagated, while in[END_REF] was not established.

Although the tacticity values remain below 0.80, these latter results constitute a remarkable example that solely tuning the substituents from cyclohexyl (or methyl as per Coates and Thomas [7] ) to phenyl groups on the bridging N atoms of Y-salan complexes, can induce a switch of the polymerization selectivity from syndiotactic to isotactic. An open question that remains is whether this switch, and in particular the formation of isotactic-rich PHB, is driven by sterics or rather by the aryl nature of the N-phenyl substituents, which would be reminiscent of the above-mentioned C-H…π(aryl) NCIs.

Our group did confirm that syndio-stereocontrol in the ROP of 4-alkoxymethyleneβ-propiolactones (BPL OR s) with Y{ON(X)O R',R" } catalysts/initiators increases with the bulkiness of simple R' alkyl ortho-substituents on the ligand (Scheme 1). Yet, further studies conducted on this class of functional β-lactones with catalysts bearing on the ligand "electronically-active" substituents such as halogens, [10] or on another class of racemic functional β-lactones, namely β-malolactonates (MLA R s), [13] led to completely different trends in terms of stereocontrol and showed that the "bulky ligand = syndiotactic PHA" model is far too simplistic and suffers from many exceptions.

In fact, in the ROP of rac-MLA R s, highly syndiotactic polymers (PMLA R s) are produced with "uncrowded" Y{ON(X)O R',R" } catalysts/initiators bearing small R' orthosubstituents such as methyl or chloro (Pr > 0.95) (Scheme 1); bulkier substituents afford polymers with lower levels of syndiotacticity (Pr = 0.86 (tBu), 0.81 (CMe2Ph), 0.68 (CPh3)). [10,13] Even more surprisingly, in the ROP of rac-BPL OR s, Y{ON(X)O R',R" } catalysts/initiators bearing halogens as R' substituents (F, Cl, Br) afford highly isotactic PBPL OR s (Pm = 0.90->0.95; Scheme 1)! [10,14] This latter case of isoselectivity also calls for NCIs [START_REF] Marshall | Such beneficial non-covalent interactions (NCIs) have also been proposed on the basis of DFT computations in rac-LA ROP promoted by Zn-β-diketiminate complexes[END_REF] , where the halogen ortho-substituent on the ligand apparently induces attractive interactions with methylene hydrogens of the last inserted monomer within the growing polymer chain; this was supported by DFT computations (Chart 2, b). For the ROP of rac-MLA R s, while DFT computations fail to provide informative data so far, we suspect that, in contrast to other β-lactones, the alkoxycarbonyl side-group of the last inserted monomer unit in the growing PMLA R chains coordinates onto the metal center (through the O atom), thereby generating active species with a higher coordination number (Chart 2, c); this may therefore require less bulky (Me, Cl) R' ortho-substituents on the ligand for an optimal chain-end stereocontrol. Further studies with new families of β-lactones are ongoing in our laboratory to further explore these hypotheses and to shed light on the exact origins of these different, yet subtle stereocontrol modes. Anyway, the above results underpin the very high potential of yttrium complexes bearing such {ON(X)O R',R" } ligand platforms and their synthetic usefulness for preparing, nearly at will, PHAs with various levels of tacticity (vide infra).

The nearly perfect isoselective (Pm > 0.95) ROP of rac-BPL OR with the halogen(chloro)substituted yttrium catalyst is quite a unique example; indeed, no other racemic β-lactoneseven rac-BL Me , the simplest and by far the most studied one since the 1960s-could be polymerized in a highly isoselective manner so far with a molecular catalyst. [START_REF] Ajellal | The best isoselectivity in the ROP of racemic BL promoted by a molecular catalyst has been achieved with a Y-salan complex with Pm up to 0.77; see ref 8. b) Heterogeneous bis(borohydride)Nd surface species resulting from the grafting of Nd(BH4)3(THF)3 onto silica promotes the ROP of rac-BBL with Pm up to 0[END_REF] In 2018, Chen and Tang reported an exciting result, that is the synthesis of perfectly isotactic (Pm = 0.99) PHB via the yttrium-catalyzed ROP of a cyclic dimer of 3-hydroxybutyrate, namely the eightmembered racemic cyclic diolide (rac-DL) (Scheme 2). [16] DL can be readily derived from bio-A c c e p t e d m a n u s c r i p t sourced dimethyl succinate, either as the rac or meso diastereomer; it is a class of monomers that was unexplored in ROP till Chen's report. Considering the significantly increased sterics present in DL relative to β-BL Me , the authors anticipated a probable higher degree of stereochemical control in the ROP of DL, thereby potentially generating highly stereoregular PHB materials. In fact, ROP of rac-DL proceeds effectively with rare earth-based catalysts via a coordination/insertion route. The simple achiral catalyst system, obtained from the combination of La(N(SiMe3)2)3 with BnOH as a co-initiator (generating in situ the corresponding La-benzyloxide active species), proves effective but returns PHB with only a slight isotactic bias (Pm = 0.70-0.74). The Y{ON(O)O R',R" } systems (R', R" = tBu or CMe2Ph) are less active, especially the more bulky cumyl-substituted one, and also afford an isotactic bias (Pm = 0.76). It is noteworthy that, although modest, these yet noticeable Pm values obtained with achiral catalysts/initiators, indicate a distinctive propensity of the DL monomer to induce the formation of synthetic isotactic PHB. Highly isoselective ROP of rac-DL (Pm = 0.88-0.99) is achieved using yttrium catalysts associated to chiral salen ligands; the most effective one bears a very bulky ortho-trityl substituent (Y{CySalen R',R" } with R' = CPh3, R" = Me, Scheme 2), yet maintaining a significant polymerization activity (TurnOver Frequency > 20 mol(DL) mol(Y) -1 min -1 ) at room temperature. Used as a racemic mixture, complex Y{CySalen R',R" } produces essentially stereo-perfect, pure isotactic PHB ([mm], isotactic triad made up of two adjacent meso diads > 99%), with high molar mass (up to 154 kg mol -1 ), narrow dispersity (ÐM = 1.01-1.07), high crystallinity and, as a result, a high melting temperature (Tm) up to 171 °C.

When the yttrium-salen complex is used in enantiopure form, a perfect kinetic resolution polymerization of rac-DL is achieved, with the selective conversion of one particular enantiomer of the monomer (i.e. (R,R)-Y{CySalen R',R" } consumes exclusively (S,S)-DL, and vice-versa), even after extended reaction times, indicative of exclusive catalyst-site selectivity for the ROP. Based on this observation, the authors concluded that an ESM is operative for the stereoselective ROP process; in line with the abovementioned ability of achiral catalysts to generate PHB with a significant isotactic bias from the ROP of rac-DL, one can assume also that a synergistic combination between CEM and ESM controls most likely takes place in this reaction. [12b] When using a racemic catalyst for the polymerization, the resulting polymer is ). Remarkably, the high stereoselectivity of the rac-Y{CySalen R',R" } catalyst observed towards rac-DL does not apply to the ROP of rac-BL Me : the latter polymerization was not only sluggish but also non-stereoselective, producing atactic PHB. This clearly evidences the singularity of the diolide monomer and the plethora of opportunities for discovering new polymer materials that the ROP of rac-DL, of its meso diastereomer, or as well as of related cyclic dimers or trimers of other β-hydroxyacids, opens up. Overall, these recent results exemplify a paradigm shift in the stereoselective ROP of cyclic esters and the chemical synthesis of PBHAs. Scheme 2. Access to isotactic poly(β-hydroxybutyrate) (PHB) from the ROP of racemic diolide (DL) with chiral yttrium-salen complexes. [16] The aforesaid versatility of the yttrium complexes bearing {ON(X)O R',R" } ligand platforms was further recently documented by Chen's group. In their search for new, highperformance polymer materials that can be chemically depolymerized back to the monomer in a quantitative yield and purity, so as to establish a virtuous circular life-cycle, they studied the ROP of the five-membered γ-butyrolactone (GBL) and of related fused six-five bicyclic lactones, namely 3,4-trans-cyclohexyl and 4,5-trans-cyclohexyl fused γ-butyrolactone (GBL 3,4- T6 and GBL 4,5-T6 , respectively). [17] These rather stable monomers can be best polymerized under mild conditions with La(N(SiMe3)2)3/ROH or Y{ON(X)O R',R" }(CH2SiMe3)/ROH catalyst/initiator combinations. Most strikingly, with the latter systems, Chen and coworkers demonstrated that upon tuning the X capping moiety of the tripodal {ON(X)O R',R" } ligand, the stereoselectivity of the ROP of racemic GBL 4,5-T6 can be switched from syndiotactic (X = OMe) to isotactic (X = NMe2) (Scheme 3). [ 18] In fact, only the highly sterically crowded tritylsubstituted complex Y{ON(NMe2)O CPh3,Me } features a significant isoselectivity at room temperature (Pm = 0.77), all the other ones bearing a (slightly) less bulky CMePh2, cumyl (CMe2Ph) or tert-butyl ortho-substituent, no matter the X side-arm group, return slightly isobiased or atactic materials (Pm = 0.50-0.65). However, upon lowering the temperature down to -65 °C, the Y{ON(X)O R',R" } catalysts remain active (albeit with poor conversions for the bulkiest trityl-substituted one), and afford the corresponding poly(GBL 4,5-T6 )s (PGBL 4,5-T6 s) with more pronounced stereoselectivities, with Pm up to 0.95 for the trityl-substituted one with X = NMe2 and Pm down to 0.25 for the tBu-substituted one with X = OMe. Correspondingly, the sterically more hindered Y{ON(NMe2)O CMePh2/CPh3,Me } complexes afford crystalline isotactic PGBL 4,5-T6 with a Tm up to 171 °C. Microstructure statistical analysis by 13 C NMR of the carbonyl and methylene regions of the spectra, indicate that both heteroselectivity and isoselectivity originate from a CEM, although the reversal of stereoselectivity upon changing the X capping group could not be rationalized thus far. This nevertheless further demonstrates the high versatility and immense potential of this class of tunable catalysts/initiators. Scheme 3. Access to isotactic or heterotactic PGBL 4,5-T6 with full recyclability from the ROP of racemic trans-cyclohexyl-ring-fused γ-butyrolactone (rac-GBL 4,5-T6 ) with achiral yttrium complexes. [18] Besides poly(β-hydroxyalkanoate)s (PBHAs, i.e. PHAs), poly(α-hydroxyalkanoate)s (PAHAs) are another kind of topical materials that feature biocompatibility and biodegradability. Ubiquitous PAHAs include PLA, PGA, and PLGA, which have important commercial applications, notably as biomedical devices and packaging materials. [19] Yet, apart from these "blockbusters", the applications of PAHAs are somewhat restricted because of the short diversity of side-chain functionalities for modulating their physico-chemical properties.

To this aim, attention has been paid in the past decades to synthesizing PAHAs via the ROP of OCA R s to enrich the diversity of PAHAs and eventually enhance their properties and widen their applications. OCAs are activated monomers derived from α-hydroxy acids or amino acids, many of which are available in nature, and therefore possess rich side-chain functionalities. As pioneered and successfully demonstrated by different groups for the synthesis of PLA from L-OCA Lac , controlled ROP of OCA R s to produce PAHA R s (with concomitant release of CO2), can be efficiently mediated by organocatalysts and also, but to a lesser extent, by metal-based catalysts. [START_REF] Vaca | For reviews on OCAs polymerization[END_REF] However, regardless of the type of catalyst employed, stereoselective polymerization of OCA R s was unknown until very recently.

A breakthrough in that field was reported by the group of Wu in 2017. [21] Using the zirconium/hafnium-aminotris(phenolato) alkoxide catalysts/initiators previously reported by the group of Davidson for the hetero-stereoselective ROP of rac-LA, [22] they achieved for the first time the syndioselective ROP of different racemic OCA R s with Pr values in the range 0.80-0.95 (Scheme 4). The reactions could proceed at room temperature. Yet, the reaction rate and also the control of molar mass values prove to be significantly impacted by the nature of the initiating alkoxy group on the metal complex: for the benchmark ROP of 100 equiv. of rac-OCA Lac , while Zr-and Hf-isopropoxide complexes require 1.5 h and 24 h, respectively, for complete monomer conversion and return PLA with broad dispersities (ÐM = 1.57-1.63), both Zr-and Hf-lactate complexes require only 20 min and the polymers display narrower dispersity (ÐM = 1.12-1.23). These results hint at a much better initiation efficiency of the lactate complexes compared to their isopropoxide analogues, which may originates from the different coordination environments around the metal centers. NMR and MALDI-ToF MS studies confirm a coordination-insertion mechanism, with PAHA Lac macromolecules end-capped by the isopropoxy or lactate initiating group, respectively. The Hf complexes induce a slightly better syndiotacticity than the Zr analogues. If the Pr values remain modest for PLA (0.800.84), they reach up to 0.93-0.95 for PAHA R s derived from the bulkier rac-OCA Phe and rac-OCA Tyr(OBn) monomers; as expected, this is at the expense of reactivity as the ROP of the latter monomers requires 14 h and 50 h for 81% and 90% conversion of only 50 equiv. of OCA, respectively.

A c c e p t e d m a n u s c r i p t

Scheme 4. Access to syndiotactic poly(α-hydroxyalkanoate)s (PAHA R s) from the ROP of racemic O-carboxyanhydrides (OCA R s) with Zr-and Hf{NOOO tBu2 } complexes. [21] 13 C NMR analysis of the resulting polymer is consistent with a first-order chain-end stereocontrol. However, using the ROP of rac-OCA Lac as model reaction, the syndioselectivity decreases with temperature (Pr = 0.83 at 25 °C, 0.75 at -40 °C, 0.72 at -60 °C). This is inconsistent with a "regular" CEM in which only chirality of the growing polymer chain-end controls the configuration of the next incoming monomer. To account for this unusual behavior, the authors referred to a stereocontrol mechanism so-called "enhanced chain-end control", as initially suggested by Davidson and co-workers for the heteroselective ROP of rac-LA using the same complexes, in which both the growing chain-end and the chiral center of metal are important (Scheme 5). [22] This mechanism involves the interconversion of the two diastereomer forms in which the complex exists ((S)-(M)-Hf and (S)-(P)-Hf)), due, on one hand, to the chirality of the lactate initiating group and later of the growing polymer chain, and, on the other hand, to the axial chirality imparted at the metal center by the C3-symmetric ligand. [21] One of the two diastereomers is supposed to be more syndioselective and also more active than the other one. Because interconversion between the two diastereomers is slower at lower temperatures, the less active and less syndioselective species accumulates and eventually generates more stereo-defects. Although the actual occurrence of such "enhanced" CEM was Scheme 5. Illustration of the "enhanced chain-end control" mechanism possibly operative in the syndioselective ROP of racemic O-carboxyanhydrides (OCA R s) with the Hf{NOOO tBu2 } complex. [21] In 2018, Xie and Tong disclosed another powerful strategy to stereoselectively ringopen polymerize racemic OCA R s. [23] Using a catalytic system made of photoredox [Ni]/[Ir] catalysts coupled with a Zn complex supported by an achiral diaminophenolate liganda catalytic system previously elaborated for the non-epimerizing ROP of enantiopure OCAs -, they synthesized stereoblock PAHAs that are highly isotactic (Pm = 0.88-0.97), show high molar mass (Mn > 70,000 g mol -1 ) and very low dispersities (ÐM < 1.06) (Scheme 6). [24] The {NNO R }ZnEt complex is converted upon addition of an alcohol (BnOH) into a Zn-alkoxide species, that is the ROP active species which operates via coordination-insertion. The {NNO R }Zn(OR) species logically affects the control over the molar mass values, the best results being obtained with R electron-donating groups (tBu, OMe); it does also influence the stereoselectivity of the reaction and the bulky tBu leads to the overall highest performance (Pm = 0.930.97); in fact, replacing {NNO tBu }ZnEt by Zn(N(SiMe3)2)2 in the ROP of rac-OCA Phe returned, instead of high isotacticity, a polymer with a modest isotactic bias (Pm = 0.66-rather atactic vs. 0.97, respectively). Interestingly, the Ni complex accelerates the reaction and also the stereoselectivity (Pm = 0.85 in the absence of (bpy)Ni(COD) vs. 0.97), presumably because of increased reaction rates. Based on detailed kinetic studies, spectroscopic analyses, and quantum chemical simulations which suggest that the stereoselective step of the polymerization arises from steric interactions between the chiral OCA monomer and salicylidene moiety of the Zn catalyst, and the fact that all catalyst components are achiral, the authors proposed a chainend stereocontrol mechanism. This constitutes another rare example of CEM operative for the synthesis of isotactic/stereoblock polymers. Scheme 6. Access to isotactic stereoblock poly(α-hydroxyalkanoate)s (PAHAs) from the ROP of racemic O-carboxyanhydrides (OCA R s) with the {NNO tBu }ZnEt/BnOH/[Ni]/[Ir] system. [24] From syndiotactic homopoly(hydroxyalkanoate)s to alternated copoly(hydroxyalkanoate)s Alternating copolymers are of great interest, as demonstrated by the ubiquitous position of some natural polymers such as DNA and peptides/proteins in key processes of life. [START_REF]Sequence-controlled polymers[END_REF] It is rather easy to prepare alternating copolymers when using two very different comonomers such as epoxides and CO2 or CO, epoxides and anhydrides, or alkenes and CO, because the mechanism can simply not enchain two monomer units of the same type, or because it is a highly disfavored process. On the other hand, it is much more challenging to prepare alternating copolymers using two comonomers from a same chemical class, because they obviously follow the same insertion mechanism, with only minute details differentiating them.

The idea to use the stereoselective ability of a ROP catalyst to prepare alternating copolymers of slightly chemically different monomers was introduced in 2009 by Coates and Thomas. [7] They succeeded in the highly alternating ROP of chemically different chiral β-alkyllactones of opposite absolute configuration using a syndioselective catalyst. In this process, the catalyst Y{ON R N R O tBu2 } operates by only differentiating the monomers according to their absolute/relative configuration, so as to ultimately enchain units of opposite configuration regardless of the chemical nature of the side-chain. Because a 50:50 mixture of two different monomers with opposite configuration is used, this may eventually end up in the formation of a perfectly alternated copolymer. Even though the chemical differentiation of the two monomers selected in this initial work is minimale.g. the length of a side-alkyl chain in a β-lactone, or a functional group in a remote position from the chiral center and cyclic ester -, the degree of alternation of the resulting copolymer, as evidenced and quantified by NMR and mass spectrometry analyses, reflects essentially the syndioselective ability of the catalyst/initiator implemented. This seminal concept was first used by Thomas and Coates in the copolymerization of mixtures of non-functional β-lactones BL R s (Scheme 7). [7] The alternation degrees reach up to 90-92%. Our group then extended this process to functional β-lactones, namely alkyl β-malolactonates (MLA R s) varying by the nature of the side alkoxycarbonyl chain (Scheme 7). [13] The appropriate choice of the side functionalities can allow inducing a much more contrasted chemical differentiation by simple post-polymerization treatment. Hence, for instance, starting from a 50:50 mixture of (R)-benzyl and (S)-allyl βmalolactonate, copolymerization with a syndioselective Y{ONNO Cl2 }/HOiPr catalyst system and subsequent hydrogenolysis/hydrogenation gives rise to a unique, highly alternated (> copolymer successively enchaining hydrophilic β-malic acid and lipophilic propyl β-malolactonate units. [13] Similarly, the copolymerization of 50:50 mixtures of oppositely configurated 4-alkoxymethylene-β-propiolactones (BPL OR s) with Y{ON(X)O Cumyl2 }/HOiPr proceeds in a highly alternating fashion (up to 94%). [25] The concept was also successfully exploited by the group of Wu for the precise alternating sequence-controlled copolymerization of equimolar mixtures of oppositely configurated OCAs (D-OCA Phe and L-OCA Tyr(OBn) ; D-OCA Lac and L-OCA Phe ) with the syndioselective Hf{NOOO tBu2 }lactate complex (Scheme A c c e p t e d m a n u s c r i p t 8). [21] The alternation degrees are ca. 95% for mixtures of the two sterically crowded and welldiscriminated OCA Phe and OCA Tyr(OBn) ; however, the alternation drops to ca. 80% when the less discriminating OCA Lac is a component of the comonomers' mixture. Scheme 7. Access to highly alternated copoly(β-hydroxyalkanoate)s (PHAs) from the ROP of 50:50 mixtures of oppositely configurated β-lactones of a given chemical class with syndioselective Y{ON R N R O tBu2 }, Y{ONOO Cl2 }, or Y{ONOO Cum2 } complexes. [7,10,13,25] Scheme 8. Access to highly alternated poly(α-hydroxyalkanoate)s (PAHA R1R2 s) from the ROP of 50:50 mixtures of oppositely configurated O-carboxyanhydrides (OCA R1,R2 s) with a syndioselective Hf{NOOO tBu2 } complex. [21] From isotactic homopoly(hydroxyalkanoate)s to gradient copoly(α-hydroxyalkanoate)s

Using their isoselective {NNO tBu }ZnEt/BnOH/[Ni]/[Ir] system for the ROP of rac-OCA R s, Xie and Tong investigated the ROP of mixtures of enantiomerically pure but different OCA R s (Scheme 9). [24] Comonomers with opposite chirality are required for accessing copolymers with a distinctive, namely gradient, microstructure. Indeed, as expected from the use of an isoselective catalyst and of chemically different but closely similar monomers, copolymerization of two OCA R s that have the same chirality result in random copolymers (characterized by broad, unresolved 1 H and 13 C NMR resonances). On the other hand, the C NMR resonances) are recovered from the ROP of equimolar mixtures of (R)-OCA Phe with (S)-OCA OBn , (S)-OCA CO2Bn or (S)-OCA Lac , of (R)-OCA OBn with (S)-OCA CO2Bn ; reversing the chirality of each of the monomers in these latter mixtures does not affect either the polymerization results or the polymer microstructure. However, copolymerization of (R)-OCA CO2Bn with (S)-OCA Lac affords a random copolymer, because the two monomers have essentially the same reactivity.

These original results reveal that the isoselective character of a catalyst system can also be fruitfully exploited, namely for the production of controlled gradient microstructures from the ROP of mixtures of chemically different enantiopure monomers of a given class. Opposite configurations and also significantly differentiated reactivity are necessary for successful control of the gradient microstructure. Related investigations conducted in our group on the ROP of mixtures of oppositely configurated BPL OR monomers with the aforementioned isoselective halogen(chloro)-substituted yttrium catalyst Y{ON(X)O Cl2 } lead to similar observations, and will be reported in due course. [26] Scheme 9. Access to gradient poly(α-hydroxyalkanoate)s (PAHA R1R2 s) from the ROP of mixtures of various pairs of enantiomers of O-carboxyanhydrides (OCA R1,R2 s) with the {NNO tBu }ZnEt/BnOH/Ni/Ir system (see Scheme 6). [24] A c c e p t e d m a n u s c r i p t

Summary and Outlook

Precision synthetic poly(hydroxy acid)s, including stereoregular homopolymers and alternated copolymers, have been successfully designed from group 3 metal-catalyzed ROP of mixtures of chiral cyclic monomers (Table 1). Dianionic, tetradentate diamino-or amino-alkoxybis(phenolate) {ON(X)O R',R" } and {ON R N R O R',R" } yttrium complexes have demonstrated unique ability in the iso-and syndio-selective polymerization of (functional) -lactones (rac-BL R s, rac-MLA R s, rac-BPL OR s, rac-DL), γ-butyrolactones (rac-GBL R s), and Ocarboxyanhydrides (rac-OCA R s), providing the corresponding PHA R s, PGBL R s, and PAHA R s, respectively. The combination of the metal's ligand framework featuring appropriate R' substituents in ortho-position of the phenolates, with the functional cyclic ester monomer, is the key to a successful and specific stereocontrol. CEM is most often at play in these highly selective ROPs but the nature of the operative NCIs appears to be quite different: they may be either repulsive and driven by sterics, with bulky substituents favoring most often syndiotactic/alternated enchainments, as commonly thought, although more results evidence that isotactic polymerizations can also be achieved by this process; perhaps more interestingly, several recent results shed light on NCIs that are attractive in nature, involving aryl groups or halogens on the ligand and acidic hydrogens of the growing polymer chain. Unprecedented isotactic PHAs (PHB, PBPL OR s), PGHAs (PGBL R s), and PAHAs (POCA R s), and syndiotactic PHAs (PBL R s, PBPL OR s, PMLA R s) and PAHAs (POCA R s) have been thus prepared. Out of the many challenges that still exist in this ROP chemistry, which is clearly not as mature as some may think, a major one will be to understand and rationalize these NCIs, so as to be able to implement them for preparing "at will" syndio or isoselective systems. Obviously, this will require gathering more data, by extending the number of monomer-catalyst combinations that feature such marked "NCI effects" (which may well rely on serendipity at this stage), but also by extensive mechanistic studies, in particular preparation and structural studies of key intermediates evidencing NCI between monomer coordinated on (model) active sites. copolymers. The stereoselective patterning ROP methodology from racemic monomers, reviewed herein on poly(hydroxy acid)s, is highly efficient and, in principle, extendable to a range of diversely functionalized original polyesters. In this objective, an important challenge will be to design catalyst systems that exert a very strong stereoselective control (in particular syndioselective) but also minimal kinetic differentiation, so that they can be applied onto mixtures of (oppositely configurated) monomers with more significant chemical differentiation.

Syndiotactic

rac-BL R (R = Me, Et, nBu, CH2C4F9) crowded- Y{ON(X)O R',R" } PHB (Pr > 0.90) 6 (Y){ON Me N Me O tBu2 } PBL R (Pr = 0.90-0.94) 8 rac-MLA R (R = allyl, Bn) uncrowded- Y{ON(X)O R',R" } PMLA R (Pr > 0.
So far, admittedly, this methodology proved to be operative only on mixtures of monomers of a given class of β-lactones with minimal, peripheral modifications (i.e., the ester moiety of βmalolactonates, the ether moiety of 4-alkoxymethylene-β-propiolactones, the substituent of O-carboxyanhydrides); one obvious reason, which may remain an essential limitation of the methodology, is the different kinetic ROP regime (rates) between those classes of monomers.

Yet, one can confidently expect that some optimized catalyst systems should be able to operate, upon strict stereocontrol, the alternated copolymerization of mixtures of (oppositely configurated) monomers taken from these different classes of monomers. This is a key to access directly (i.e., without post-polymerization modification) original alternated copolymers.

These latest achievements in the ROP synthesis of stereoregular (co)polymers of functionalized poly(hydroxy acid)s, featuring a precise control of monomer(s) sequences combined with the regular distribution of functional and reactive side-chain groups along their backbone, enable to access highly precise polyesters previously synthetically difficult to obtain, and pave the way to the next generation of functional polyesters with original properties and applications. [START_REF]Sequence-controlled polymers[END_REF] The precise control of monomers sequences and the exact placement of functional side-chain groups along poly(hydroxyl acid)s' backbone, are currently main challenges for the next-generation of synthetic polymers. [START_REF]Sequence-controlled polymers[END_REF] Definitely, despite its maturity, stereoselective ROP has still much to be discovered and to offer!

  Scheme 1. Access to syndiotactic or isotactic poly(β-hydroxyalkanoate)s (PBHAs) from the ROP of racemic β-lactones with achiral yttrium complexes.[6][7][8][9][10]

Chart 2 .

 2 Illustration of putative "non-covalent interactions" (NCIs) at work in stereoselective ROP of rac-BL Me (a), rac-BPL OR (b), and of the multi-coordinated species in ROP of rac-

  predominately a mixture of (R)-PHB and (S)-PHB; formation of trace amounts of the stereoblock polymer [(R)-PHB]-b-[(S)-PHB] may result from prolonged exposure to the catalyst which slowly promotes transesterification (as evidenced by the broadening of ÐM values up to 1.20). Yet, the latter side reaction can be inhibited by limiting the reaction time and/or by reducing the catalyst amount in the feed (monomer loadings up to 1,600 mol(DL)

  s c r i p t not unambiguously evidenced, these results provide another example of a likely synergistic combination between CEM and ESM, as mentioned above.

  s c r i p t authors evidenced that gradient copolymers (characterized by distinct and well-resolved 1 H and 13

  stereoselective ROP inherent to these catalysts/initiators, unprecedented heterotactic-enriched PGBL R s and highly alternated PHA R1R2 s and PAHA R1R2 s copolymers have been profitably obtained from the ROP of equimolar mixtures of oppositely configurated -, β-lactones and OCAs of a given/same chemical class of comonomers, using the syndioselective Y{ON(OMe)O tBu2 }, and Y{ON R N R O tBu2 }, Y{ONOO Cl2 }, or Y{ONNO Cum2 }, and Hf{NOOO tBu2 } complexes, respectively. The multicomponent {NNO tBu }ZnEt/BnOH/Ni/Ir system uniquely affords either gradient or random PAHAR1,R2

Table 1 .

 1 Overview of highly stereoregular poly(hydroxy acid)s prepared from metal-catalyzed ROP of racemic monomers.

	Monomer	Catalyst	Polymer	Ref.


Fotis Kournoutas

Ioannis Konstantinos Kalis

Michaela Fecková

Sylvain Achelle

Mihalis Fakis
email: fakis@upatras.gr

The effect of protonation on the excited state dynamics of pyrimidine chromophores

Keywords: Pyrimidine chromophores, protonation, dynamics, fluorescence spectroscopy

 The protonation effect on the excited state dynamics is investigated  Two pyrimidine chromophores, bearing the A-(π-D)2 and A-(π-D)3 structure, are used  Time resolved fluorescence spectroscopy (fs and ns) is employed  Protonation using three different acids, CSA, TFA and AcOH is studied  The dynamics of the neutral chromophores become faster upon protonation with AcOH

Introduction

Organic fluorophores are important materials not only for their highly efficient fluorescence in the visible part of the spectrum, for use in lighting and imaging, but also because their fluorescence properties are highly dependent on the environmental conditions providing us with a sensitive and nondestructive way of sensing. The development of organic chromophores with suitable functional groups, capable of reacting physically or chemically with their environment and reporting these reactions through their fluorescence properties, via a remote access, is of tremendous scientific and technological interest. External impulses such as a change in polarity, [START_REF] Higginbotham | A fluorescent "2 in 1" proton sensor and polarity probe based on coresubstituted naphthalene diimide[END_REF][START_REF] Signore | Novel Coumarin Fluorescent Sensor to Probe Polarity Around Biomolecules[END_REF][3][START_REF] Mora | Role of solvent Hbonding and polarity on photophysical properties of a benzothiazole-based ratiometric amyloid fibril sensor[END_REF] viscosity, [START_REF] Zhou | Molecular Rotors as Fluorescent Viscosity Sensors: Molecular Design, Polarity Sensitivity, Dipole Moments Changes, Screening Solvents, and Deactivation Channel of the Excited States[END_REF][START_REF] Lee | Fluorescent Molecular Rotors for Viscosity Sensors[END_REF][START_REF] Lee | Pyrrolic molecular rotors acting as viscosity sensors with high fluorescence contrast[END_REF][START_REF] Raut | A homodimeric BODIPY rotor as a fluorescent viscosity sensor for membrane-mimicking and cellular environments[END_REF] pH, [START_REF] Qiu | An AIE and FRET-based BODIPY sensor with large Stoke shift: Novel pH probe exhibiting application in CO3 2detection and living cell imaging[END_REF][START_REF] Chen | Covalent Organic Framework Functionalized with 8-Hydroxyquinoline as a Dual-Mode Fluorescent and Colorimetric pH Sensor[END_REF][START_REF] Martineau | Semisynthetic fluorescent pH sensors for imaging exocytosis and endocytosis[END_REF][START_REF] Gotor | Optical pH Sensor Covering the Range from pH 0-14 Compatible with Mobile-Device Readout and Based on a Set of Rationally Designed Indicator[END_REF][START_REF] Matsui | UV-vis absorption and fluorescence spectra, solvatochromism, and application to pH sensors of novel xanthene dyes having thienyl and thieno[3,2-b]thienyl rings as auxochrome[END_REF][START_REF] Li | Benzimidazole-BODIPY as optical and fluorometric pH sensor[END_REF] or the presence of metal ions [START_REF] Achelle | Dipicolylamine Styryldiazine Derivatives: Synthesis and Photophysical Studies[END_REF][START_REF] Hadad | Metal Cation Complexation Studies of 4-Arylvinyl-2,6-di(pyridin-2-yl)pyrimidines: Effect on the Optical Properties[END_REF][START_REF] Wu | A three-dimensional Cd(II) metal-organic framework: a bifunctional luminescence sensor for benzaldehyde and Fe 2+ ions[END_REF][START_REF] Zhan | A water-stable europium-MOF as a multifunctional luminescent sensor for some trivalent metal ions (Fe 3+ , Cr 3+ , Al 3+ ), PO4 3ions, and nitroaromatic explosives[END_REF][START_REF] Guo | Highly sensitive and selective fluorescent sensor for visual detection of Cu2+ in water and food samples based on oligothiophene derivative[END_REF][START_REF] Denis | Chelating Rotaxane Ligands as Fluorescent Sensors for Metal Ions[END_REF][START_REF] Yang | Highly Selective Bifunctional Luminescent Sensor toward Nitrobenzene and Cu 2+ Ion Based on Microporous Metal-Organic Frameworks: Synthesis, Structures, and Properties[END_REF][START_REF] Yan | Two Lanthanide Metal-Organic Frameworks as Remarkably Selective and Sensitive Bifunctional Luminescence Sensor for Metal Ions and Small Organic Molecules[END_REF] can be transduced through fluorescence and become perceptible by naked eye.

More specifically, organic chromophores with nitrogen containing heterocycles such as pyridines, [START_REF] Achelle | Tuning the photophysical properties of push-pull azaheterocyclic chromophores by protonation: a brief overview of a French-Spanish-Czech Project[END_REF][START_REF] Kim | Brønsted acid-catalyzed α-halogenation of ynamides from halogenated solvents and pyridine-N-oxides[END_REF][START_REF] Broquier | Twisted Intramolecular Charge Transfer in Protonated Amino Pyridine[END_REF][START_REF] Xiong | Multi-Stimuli-Responsive Fluorescence Switching from a Pyridine-Functionalized Tetraphenylethene AIEgen[END_REF][START_REF] Wei | Influence of the protonation of pyridine nitrogen on pnicogen bonding: competition and cooperativity[END_REF][START_REF] Cariati | Solid state and solution fine tuning of the linear and J o u r n a l P r e -p r o o f nonlinear optical properties of (2-pyrene-1-yl-vinyl)pyridine by protonation-deprotonation reactions[END_REF] pyridazines, [START_REF] Achelle | Tuning the photophysical properties of push-pull azaheterocyclic chromophores by protonation: a brief overview of a French-Spanish-Czech Project[END_REF][START_REF] Li | Acid-Induced Multicolor Fluorescence of Pyridazine Derivative[END_REF][START_REF] Do | Patterning of pH Sensitive Fluorescent Bipyridazine Derivatives[END_REF] pyrimidines [START_REF] Achelle | Tuning the photophysical properties of push-pull azaheterocyclic chromophores by protonation: a brief overview of a French-Spanish-Czech Project[END_REF][31][START_REF] Kato | Series of carbazole-pyrimidine conjugates: syntheses and electronic, photophysical, and electrochemical properties[END_REF][START_REF] Muraoka | Systematic synthesis, comparative studies of the optical properties, and the ICT-based sensor properties of a series of 2,4,6-tri(5-aryl-2thienyl)pyrimidines with the D--A system[END_REF][START_REF] Sinkeldam | Multisensing Emissive Pyrimidine[END_REF] and pyrazines [START_REF] Achelle | Tuning the photophysical properties of push-pull azaheterocyclic chromophores by protonation: a brief overview of a French-Spanish-Czech Project[END_REF][START_REF] Xu | 4-Diphenylamino-phenyl substituted pyrazine: nonlinear optical switching by protonation[END_REF][START_REF] Muraoka | A series of 2,5-bis(5-aryl-2-thienyl)pyrazines with a linearshaped (D-pi)(2)-A system: synthesis and study of the optical properties including fluorosolvatochromism and proton-base-sensing[END_REF][START_REF] Schmitt | Diaryldistyrylpyrazines: Solvatochromic and Acidochromic Fluorophores[END_REF] are susceptible to protonation, resulting in new species with bathochromically shifted spectra. These heterocycles constitute electron accepting groups in D-π-A structures which result in efficient Intramolecular Charge Transfer (ICT) and are potential candidates for application in optoelectronics, sensors and non-linear optics. [START_REF] Lee | High Quantum Efficiency Blue Phosphorescent Organic Light-Emitting Diodes Using 6-Position-Modified Benzofuro[2,3-b]pyridine Derivatives[END_REF][START_REF] Li | Dual n-type units including pyridine and diphenylphosphine oxide: effective design strategy of host materials for high-performance organic light-emitting diodes[END_REF][START_REF] Lee | Triaryl boron derivatives of pyridine as electron transporting materials for blue phosphorescent organic light-emitting diodes[END_REF][START_REF] Hu | Design for a small molecule based chemosensor containing boron and pyridine moieties to detect HF[END_REF][START_REF] Xu | Thiophene-based pyridine derivatives: synthesis, crystal structures, two-photon absorption properties and bio-imaging applications in the near-IR region[END_REF][START_REF] Namboodiri | Enhanced two photon absorption cross section and optical nonlinearity of a quasi-octupolar molecule[END_REF][START_REF] Achelle | Incorporation of pyridazine rings in the structure of functionalized -conjugated materials[END_REF][START_REF] Achelle | Pyrimidine ring as building block for the synthesis of functionalized conjugated materials[END_REF][START_REF] Achelle | Luminescent materials incorporating pyrazine or quinoxaline moieties[END_REF][START_REF] Komatsu | Recent progress of pyrimidine derivatives for high-performance organic light-emitting devices[END_REF][START_REF] Lipunova | Functionalized quinazolines and pyrimidines for optielectronic materials[END_REF][START_REF] Nosova | Functionalized benzazines as luminescent materials and components for optoelectronics[END_REF] Protonation strengthens their electron accepting ability, enhancing the ICT.

In some cases, protonation is also used in order to prepare white light emitting materials to be J o u r n a l P r e -p r o o f used in lighting and displays by simultaneously utilizing the fluorescence of the neutral and protonated species. [START_REF] Achelle | Tuning the photophysical properties of push-pull azaheterocyclic chromophores by protonation: a brief overview of a French-Spanish-Czech Project[END_REF][START_REF] Liu | A novel approach towards white photoluminescence and electroluminescence by controlled protonation of a blue fluorophore[END_REF][START_REF] Romero-Nieto | Simple and Efficient Generation of White Light Emission From Organophosphorus Building Blocks[END_REF][START_REF] Li | Acid-induced Multicolor Fluorescence of Pyridazine Derivative[END_REF][START_REF] Tydlitát | Photophysical properties of acid-responsive triphenylamine derivatives bearing pyridine fragments: Towards white light emission[END_REF][START_REF] Achelle | Protonable pyrimidine derivative for white light emission[END_REF][START_REF] Achelle | Luminescence Behavior of Protonated Methoxy-Substituted Diazine Derivatives: Toward White Light Emission[END_REF][START_REF] Achelle | Carbazoleand triphenylamine-substituted pyrimidines: synthesis and photophysical properties[END_REF][START_REF] Wang | 2,6-Diphenylpyridine-based fluorophores:Synthesis, photophysical properties and effects of protonation[END_REF] It has been reported that energy transfer from the higher energy neutral to the lower energy protonated species takes place. [START_REF] Liu | A novel approach towards white photoluminescence and electroluminescence by controlled protonation of a blue fluorophore[END_REF][START_REF] Li | Acid-induced Multicolor Fluorescence of Pyridazine Derivative[END_REF][START_REF] Achelle | Luminescence Behavior of Protonated Methoxy-Substituted Diazine Derivatives: Toward White Light Emission[END_REF] Partial energy transfer, where the fluorescence of the neutral species is not totally quenched and both species emit light, can result in white light emission. [START_REF] Liu | A novel approach towards white photoluminescence and electroluminescence by controlled protonation of a blue fluorophore[END_REF] This method is of great importance, since white light is achieved by a single emitter existing simultaneously in two forms with supplementary emitting colors, while emission tunability can be simply achieved by altering the protonation conditions by employing different amounts of acid. Typically, in various other methods, white light emission is achieved by incorporating different chromophores, either chemically linked or self-assembled, having supplementary colors. [START_REF] Michaleviciute | Star-Shaped Carbazole Derivatives for Bilayer White Organic Light-Emitting Diodes Combining Emission from Both Excitons and Exciplexes[END_REF][START_REF] Mukherjee | Organic White-Light Emitting Materials[END_REF][START_REF] D'andrade | Efficient Organic Electrophosphorescent White-Light-Emitting Device with a Triple Doped Emissive Layer[END_REF][START_REF] Sun | Management of Singlet and Triplet Excitons for Efficient White Organic Light-Emitting Devices[END_REF] Notably, the biggest challenge towards this goal, is the careful choice of chromophores paying attention on their energetics as well as their mixing conditions. By means of protonation, the laborious procedures for mixing of several chromophores with different fluorescent colors to produce white light is not required. However, although white light emission upon protonation from chromophores containing azaheterocycles has been achieved and white organic light emitting diodes (WOLEDs) have been obtained using this strategy, the fundamental photophysical properties of these systems have been mainly studied by steady state spectroscopy while less attention has been given in their excited state dynamics. [START_REF] Kournoutas | Photophysical and Protonation Time Resolved Studies of DonorAcceptor Branched Systems With Pyridine Acceptors[END_REF] Such a study is crucial towards understanding the nature of the interactions among the neutral and protonated species as well as among the solute and different acids.

Very recently, our groups has studied the effect of protonation with acetic acid on the optical properties of pyridine-based chromophores, revealing a quenching of the neutral species fluorescence which was accompanied by a decrease of their excited state lifetime. [START_REF] Kournoutas | Photophysical and Protonation Time Resolved Studies of DonorAcceptor Branched Systems With Pyridine Acceptors[END_REF] In this work, the effect of protonation on the photophysical properties of two pyrimidine-based chromophores is addressed focusing on the excited state dynamics. For this reason time resolved fluorescence spectroscopy in the fs-ps and ps-ns timescales has been employed. The chromophores adopt the A-(π-D)2 and A-(π-D)3 topology where the diphenylamino fragment is used as electron donating group. The effect of protonation has been studied by using three acids namely camphorsulphonic J o u r n a l P r e -p r o o f (CSA), trifluoroacetic (TFA) and acetic acid (AcOH) revealing different effects in the excited state dynamics of the chromophores.

Experimental

Materials

The chromophores under study are shown in figure 1. They bear pyrimidine electron accepting group and diphenylamino electron donors. C2 adopts a pseudo-quadrupolar geometry of A-(π-D)2 type while C3 has geometry of the type A-(π-D)3. The synthesis of these compounds has been described previously. [START_REF] Achelle | V-Shaped 4,6-Bis(arylvinyl)pyrimidine Oligomers: Synthesis and Optical Properties[END_REF][START_REF] Fecková | 2,4-distyryl and 2,4,6-tristyrylpyrimidines: synthesis and photophysical properties[END_REF] Figure 1. Chemical structures of the herein studied pyrimidine molecules.

Steady state spectroscopy

A Jasco V-650 UV-Vis and a Horiba Fluoromax spectrophotometer have been used for the detection of the absorption and fluorescence spectra of the samples in solutions respectively.

Time resolved spectroscopy

The excited state dynamics has been studied in CHCl3 solutions of the pyrimidines, with or without protonation, in the fs-ps and ns timescale. Initially, a femtosecond time resolved upconversion (FU) system, described in details in the past has been used. [START_REF] Koutsoubelitis | Photophysics, Electronic Structure and Solar Cell Performance of a Donor-Acceptor Poly(N-dodecyl-2,7-carbazolealt-benzothiadiazole)[END_REF][START_REF] Karakostas | Energy Transfer within Self-Assembled Cyclic J o u r n a l P r e -p r o o f Multichromophoric Arrays Based on Orthogonally Arranged Donor -Acceptor Building Blocks[END_REF] The FU system is based on a Ti:Sapphire femtosecond laser (80 fs pulse duration, 80 MHz repetition frequency, 800 nm) which, after passing through a Second Harmonic Generator (SHG) crystal, is frequency doubled at 400 nm and is used for the excitation of the samples. The excitation power was below J o u r n a l P r e -p r o o f 2 mW while the samples were continuously rotating to avoid thermal effects. The Instrument's Response Function (IRF) was ~250 fs.

The ns dynamics have been detected via a Time Correlated Single Photon Counting (TCSPC)

technique, based on a Fluotime 200 spectrometer (Picoquant). [START_REF] Droseros | Steady State and Time Resolved Photoluminescence Properties of CuInS2/ZnS Quantum Dots in Solutions and in Solid Films[END_REF] The samples were excited by a ps diode laser emitting 60 ps pulses at 400 nm. The system's IRF was ~80 ps. The optical density of the samples was ~ 0.1 at the excitation wavelength. Fitting of the ns dynamics has been made by a single or bi-exponential function after taking into account the systems' IRF. The quality of the fitting was judged by reducing the χ 2 parameter and by visually inspecting the residuals. The excitation wavelength used at both techniques, as will be shown below, lays at the absorption band of the neutral molecules.

Results and discussion

Steady State spectroscopy

The absorption spectra of C2 in CHCl3 (10 -5 M) upon titration with CSA (pKa = 1.2), TFA (pKa = 0.3) and AcOH (pKa = 4.8) are shown in figure 2. As shown before, the protonation occurs on one of the nitrogen atoms of the pyrimidine ring which plays the role as a monobasic compound (pKa  1.1). [START_REF] Achelle | Tuning the photophysical properties of push-pull azaheterocyclic chromophores by protonation: a brief overview of a French-Spanish-Czech Project[END_REF][START_REF] Achelle | Carbazoleand triphenylamine-substituted pyrimidines: synthesis and photophysical properties[END_REF][START_REF] Achelle | V-Shaped 4,6-Bis(arylvinyl)pyrimidine Oligomers: Synthesis and Optical Properties[END_REF][START_REF] Fecková | 2,4-distyryl and 2,4,6-tristyrylpyrimidines: synthesis and photophysical properties[END_REF] After protonation of the first nitrogen atom, the basicity of the second one is decreased. This is because of the attractive inductive effect of the resulting quaternary nitrogen (pKa for the second nitrogen~ 6.3). 0 eq. 0.033 eq. 0.067 eq. 0.10 eq. 0.13 eq. 0.17 eq. 0.20 eq. 0.23 eq. 0.27 eq. 0.30 eq. 0.33 eq. C2 CHCl 3 /CSA (a) 300 350 400 450 500 550 600 650 700 750 800 850 (b) C2 CHCl 3 /TFA Wavelength (nm) 0 eq. 2.0 eq. 6.7 eq. 13.4 eq. 20.1 eq. 26.8 eq. 33.5 eq. 40.2 eq. 46.9 eq. 53.6 eq. 67.0 eq. Absorption Wavelength (nm) 0 eq. 5.9 10 3 eq. 18 10 3 eq. 30 10 3 eq. 59 10 3 eq. 118 10 3 eq. 177 10 3 eq. 236 10 3 eq. 354 10 3 eq. 472 10 3 eq. 590 10 3 eq. C2 in CHCl3 exhibits two absorption peaks at 436 nm and ~ 300 nm. Titration with CSA decreases the intensity of these absorption bands while a new red-shifted band emerges with a J o u r n a l P r e -p r o o f peak at 566 nm attributed to the protonated species (Figure 2a). [START_REF] Achelle | Tuning the photophysical properties of push-pull azaheterocyclic chromophores by protonation: a brief overview of a French-Spanish-Czech Project[END_REF] In addition, a second lowintensity absorption band at 350 nm appears. Two isosbestic points at 370 and 470 nm exist, clearly pointing to the existence of two distinct species i.e. the neutral and the acid generated protonated molecules. The absorption spectra upon addition of TFA, resemble those with CSA, with clear isosbestic points at 370 and 470 nm (Figure 2b). However, due to the important difference of pKa between pyrimidine and AcOH, a large amount of this acid is necessary to protonate the pyrimidine ring. The behavior upon titration with AcOH is therefore slightly different (Figure 2c). First, the low intensity band at 350 nm is not obvious. Second, a new redshifted absorption band appears at 560 nm, but its intensity is small and as a consequence no clear isosbestic point exists.

C2 CHCl 3 /AcOH (c)
The corresponding spectra for C3 are shown in figure S1 where it is obvious that the behavior is similar to that of C2, but the changes are more pronounced. In CHCl3, C3 exhibits an absorption peak at 414 nm, while after addition of only 0.067 eq. of CSA, this absorption band becomes negligible and the new red-shifted band, due to the protonated species, dominates the absorption.

The above also apply for TFA. After addition of AcOH, the changes are again more significant compared to C2. The new absorption band at 530 nm, although not as intense as after adding CSA, is enhanced compared to C2.

Figure 3 presents the fluorescence spectra of C2, upon addition of acid, taken after excitation at the isosbestic point, in order to eliminate the changes in fluorescence due to the decrease in absorption intensity. Upon addition of CSA, the intensity of the main band at 540 nm decreases i.e. the fluorescence of the neutral species is quenched (Figure 3a). Gradually, a second bathochromically shifted band at 690-700 nm emerges due to the protonated molecules while an isoemissive point is obvious at 655 nm. The intensity of the emission of protonated molecules seems to saturate after addition of 1.67 eq. The spectral behavior upon addition of TFA and AcOH is similar while in contrast to CSA, no isoemissive point exists and the red-shifted emission decreases upon adding high amounts of acid, probably due to an increase of polarity (Figures 3b and3c). Finally, figure 4 shows the 2D fluorescence maps of C2 in CHCl3 with 1 eq. CSA, 71 . 10 3 eq. of AcOH and 6.7 eq. of TFA respectively. The 2D maps show that upon excitation at J o u r n a l P r e -p r o o f approximately 440 nm, both emission bands are revealed covering almost the whole visible spectrum. For excitation at longer wavelengths, the single emission of the protonated species is only observed, which, for the case of CSA, is more intense than that of the neutral species. For the case of AcOH, the emission of the protonated species is not observed upon excitation at wavelengths longer than 500 nm. This is due to its very low intensity compared to the fluorescence of the neutral species. If the excitation wavelength is scanned from 460 nm to 600 nm and after increasing the slits of the spectrometer, the low intensity fluorescence band of the protonated species is revealed (figure S2). Figure S3 displays the fluorescence spectra of C3 upon protonation with CSA, TFA and AcOH while figure S4 presents the 2D spectra. The main conclusion is that, clearly, the fluorescence intensity of the protonated species for C3 is much lower than for C2.

Time resolved spectroscopy

Next, in order to shed more light in the interaction of the acid with the solute molecules, the fluorescence dynamics has been studied by means of the FU method. Note that for these J o u r n a l P r e -p r o o f experiments, excitation was made at 400 nm and not at the isosbestic point. This is, however, not important since in time resolved experiments we are only interested in the change of dynamics and not in the absolute intensity of the fluorescence. Figure 5 shows the early dynamics of the neutral species i.e. detected at the main fluorescence band. It is observed that CSA does not change the initial dynamics (figures 5a and 5b) for both molecules. Similar results are obtained for TFA (figure 5c and5d). On the other hand, the addition of AcOH results in a rapid decay, which mainly takes place within the first 20 ps after excitation (figures 5e and 5f). These results clearly constitute the first indication that AcOH interacts differently with the solutes compared to CSA and TFA. However, due to the short temporal range of the FU measurements, the effect of the acids on the excited state lifetime cannot be determined. Therefore, the dynamics were also detected in the ns timescale. 5.9 10 3 eq. 29.5 10 3 eq. 59 10 3 eq. 118 10 3 eq.

Time (ps)

C2 CHCl 3 /TFA C3 CHCl 3 /TFA C3 CHCl 3 /AcOH 0 eq. 5.9 10 3 eq. 29.5 10 3 eq. 59 10 3 eq. 118 10 3 eq. 295 10 3 eq. Note that the FU dynamics become faster upon addition of AcOH while they remain unchanged upon addition of CSA and TFA. The excitation wavelength was 400 nm.

The ns decays have been measured for both the neutral and the protonated species i.e. at the 520-540 and 670-700 nm bands. Figures 6 and7 show the results for C2 upon addition of CSA and AcOH respectively while Figure S5 shows the results for C2 with TFA. More specifically, in figure 6a, it is observed that the decays of the neutral species at 540 nm do not change with the addition of acid. Fitting of these decays, resulted in a single exponential process with a lifetime of J o u r n a l P r e -p r o o f Besides, the decays at 700 nm i.e. where the protonated species emit, are single exponential for small amounts of acid, with a lifetime of ~ 2.3 ns, meaning that the neutral and protonated species J o u r n a l P r e -p r o o f have the same lifetime. Note that excitation is made at 400 nm and thus, the neutral molecules are mainly excited in the time resolved experiments. Therefore, the fluorescence from the protonated species originates by an energy transfer mechanism. [START_REF] Kournoutas | Photophysical and Protonation Time Resolved Studies of DonorAcceptor Branched Systems With Pyridine Acceptors[END_REF] However, the decays become faster for CSA addition above 0.67 eq. (figure 6b), becoming bi-exponential with a short lifetime of 0.9 ns, whose amplitude increases by increasing the quantity of acid and a second component of 2.2-2.3 ns which is similar to the lifetime of the protonated species (Table 1). The short component of ~ 0.9 ns can be considered therefore as a quenching mechanism and can be ascribed to the stronger electron accepting ability of the protonated pyrimidine groups leading to enhanced ICT.

According to the energy gap law, the excited state potential of the protonated low energy species with strong ICT, experiences conical intersections with the ground state energy potential leading to the decrease of lifetime and fluorescence quenching. This decrease of the average lifetime of the protonated species can be also correlated with the saturation of the fluorescence intensity upon increasing the amount of acid.

Clearly, the behavior is different upon addition of AcOH (figure 7). The decays of the neutral species at 540 nm become significantly faster upon addition of AcOH pointing to an interaction among neutral C2 molecules and AcOH in the excited state. The lifetime decreases from 2.29 ns for 0 eq. to 0.89 ns for 118 . 10 3 eq. of AcOH acid (Table 2). Besides, the decays at 700 nm of the protonated species exhibit a similar behavior with the neutral ones, becoming faster with addition of AcOH. Figure S5 shows the ns decays for C2 after addition of 59 . 10 3 eq. of AcOH at various wavelengths across the neutral and protonated emission bands. The decays were found similar at all emission wavelengths showing negligible dependence on the detection wavelength. Intensity 0 eq. 5.9 . 10 3 eq, 30 . 10 3 eq, 59 . 10 3 eq, 88 .

10 3 eq, 118 .

10 3 eq,

λ det =540nm C2 CHCl 3 /AcOH (a)
Intensity Time (ns) 5.9 . 10 3 eq, 30 . 10 3 eq, 59 . 10 3 eq, 88 .

10 3 eq, 118 .

10 3 eq, λ det =700nm (b)

J o u r n a l P r e -p r o o f Finally, adding of TFA does not lead to a decrease of the lifetime of the neutral species which is again found 2.2-2.3 ns (Figure S6 and Table S1). However, when the amount of TFA exceeds 20.1 eq. a second decay mechanism with longer lifetime emerges. Its lifetime varies from 3.7 to 5.3 ns and its amplitude is less than 7 %. Table 2. Parameters of the nanosecond fluorescence dynamics for C2 in CHCl3 with different amounts of AcOH detected at 540 (main fluorescence band) and 700 nm (fluorescence band of the protonated species). The excitation wavelength was 400 nm.

Based on the above findings and especially on the excited state dynamics, it is concluded that CSA and TFA acids quench the fluorescence of the neutral C2 molecules via a different mechanism than AcOH. In the case of CSA and TFA, where the lifetime is not changed, the quenching is static while in the case of AcOH where the lifetime decreases, a dynamic quenching takes place. [START_REF] Lacowicz | Principles of Fluorescence Spectroscopy[END_REF] However, in the latter case, static quenching also plays a role as will be discussed in the following.

In order to more quantitatively study the dynamic quenching of C2 upon addition of AcOH, the Birks model [START_REF] Birks | Photophysics of Aromatic Molecules[END_REF][START_REF] Annable | a,a-Dinaphthylpropane: a model for polymer photophysics and a probe for the investigation of macromolecular dynamics?[END_REF] is applied. The corresponding kinetic scheme is shown in Figure S7. According to this, the neutral molecules decay towards the ground state by a rate constant 𝑘 𝑁 and towards the protonated species by 𝑘 𝑁𝑃 . Thus, the total decay rate constant of the neutral molecules is given by

𝑋 = 𝑘 𝑁 + 𝑘 𝑁𝑃 (1)
J o u r n a l P r e -p r o o f

The corresponding rate constants for the protonated molecules are 𝑘 𝑃 and 𝑘 𝑃𝑁 respectively. The fluorescence intensity decays of the neutral and protonated molecules are given by:

𝐼 𝑁 (𝑡) = 𝐴 1𝑁 𝑒𝑥𝑝(-𝜆 1𝛮 𝑡)+𝐴 2𝑁 𝑒𝑥𝑝(-𝜆 2𝛮 𝑡) (2)
𝐼 𝑃 (𝑡) = -𝐴 1𝑃 𝑒𝑥𝑝(-𝜆 1𝑃 𝑡)+𝐴 2𝑃 𝑒𝑥𝑝(-𝜆 2𝑃 𝑡)

The parameters 𝐴 1𝑁 , 𝐴 2𝑁 , 𝜆 1𝛮 and 𝜆 2𝛮 are obtained by fitting the FU (Figure 5e) and ns dynamics (Figure 7a) of the neutral molecules respectively and are shown in Table 3. Upon fitting the FU results, the long lifetime obtained by the ns decays was used as a constant parameter. On the other hand, the rise part of 𝐼 𝑃 (𝑡) (first component of equation 3) cannot be determined since it was not possible to obtain FU measurements at the protonated emission band. Therefore the model will be only applied for the neutral species leading to the calculation of the rate coefficient for the formation of the protonated ones 𝑘 𝑁𝑃 . The decay parameter 𝑋 is experimentally determined and defined as: [START_REF] Birks | Photophysics of Aromatic Molecules[END_REF][START_REF] Annable | a,a-Dinaphthylpropane: a model for polymer photophysics and a probe for the investigation of macromolecular dynamics?[END_REF] 𝑋 =

(𝐴 1𝑁 /𝐴 2𝑁 )•𝜆 1 +𝜆 2 𝐴 1𝑁 /𝐴 2𝑁 +1 (4)
and the calculated results are also presented in Table 3. Finally, the 𝑘 𝑁𝑃 is found following equation 1 where 𝑘 𝑁 is calculated by the decay of C2 without acid.

Acid (eq.)

𝐴 1𝑁 𝜆 1𝛮 (10 12 s -1 ) 𝐴 2𝑁 𝜆 2𝛮 (10 9 s -1 ) 𝑋 (10 9 s -1 ) 𝑘 𝑁 (10 9 s -1 ) 𝑘 𝑁𝑃 (10 F and F0 are the fluorescence intensities at the peak of the spectra after and before addition of acid. Similarly, τ and τ0 are the fluorescence lifetimes after and before addition of acid. Fitting the intensity Stern-Volmer plots in Figures 8a and8b, with the well-known equation

𝐹 0 𝐹 = 1 + 𝐾 𝑠 [𝑄],
where 𝐾 𝑠 is the association constant for static quenching and [𝑄] the concentration of the quencher (here in eq.) we obtained 𝐾 𝑠 =1.8 eq. -1 for CSA and 𝐾 𝑠 =0.18 eq. -1 for TFA meaning that 50% of the fluorescence intensity is quenched when the acid concentration is 0.55 eq. for CSA and 5.55 eq. for TFA. However, in the case of CSA, the intensity quenching plot seems to deviate from linearity, possibly indicating higher order static quenching effects (for this reason the plot was fitted for acid concentration up to 1.3 eq.). The quenching process upon adding AcOH has a more complicated feature. The reduction of the fluorescence lifetime indicates the existence of a dynamic (diffusional) mechanism. However, the plots of 𝐹 0 𝐹 and 𝜏 0 𝜏 do not have the same slope as expected for a purely dynamic quenching. More specifically, the intensity quenching plot ( 𝐹 0 𝐹 vs. acid) has a higher slope than the lifetime quenching one ( 𝜏 0 𝜏 vs. acid) and a slight nonlinear behavior (upward curvature). Therefore, the quenching process upon adding AcOH is a combination of both static (complex association) and dynamic (diffusional) mechanisms.

Therefore, the fitting of the intensity quenching curve is made with the equation

𝐹 0 𝐹 = (1 + 𝐾 𝐷 [𝑄])(1 + 𝐾 𝑠 [𝑄]
) where 𝐾 𝐷 is the dynamic quenching constant and [START_REF] Lacowicz | Principles of Fluorescence Spectroscopy[END_REF] Fitting initially the lifetime quenching curve in Figure 8c, we obtain 𝐾 𝐷 = 0.011 • 10 -3 eq. -1 . Then 𝐾 𝑆 can be easily found by the intensity quenching curve to be 𝐾 𝑆 = 0.022 • 10 -3 eq. -1 .

𝜏 0 𝜏 = (1 + 𝐾 𝐷 [𝑄]).
C3 exhibits a similar behavior regarding the dynamics i.e. the lifetime of the neutral species is unchanged upon adding CSA and TFA, while it decreases upon adding AcOH. The corresponding results are presented in figures S8-S11 and Tables S2, S3 and S4. Specifically, the decays of the neutral species display a ~2.30 ns component which is considered as the lifetime of the C3 J o u r n a l P r e -p r o o f molecules (Figure S8 and Table S2). However, in the case of CSA, although the average lifetime remains the same, the decays become bi-exponential, when the quantity of CSA exceeds 1.33 eq., with a 2.10 ns and a 2.5-4.5 ns components. The fact that the neutral lifetime remains almost unchanged when CSA is added, points to a static quenching as in the case of C2. Similarly to CSA, when adding TFA, the lifetime of the neutral species again remains unchanged, while a slower component also emerges at high amounts of acid (Figure S9 and Table S3). On the other hand, upon addition of AcOH, the excited state lifetime of the neutral molecules decreases from 2.32 to 0.86 ns (Figure S10 and Table S4), meaning that a dynamic quenching is operative as was also confirmed for C2. Finally, the decays of the protonated species, detected at 670 nm, also become accelerated with increasing the amount of acids. This is due to the enhanced ICT behavior of the protonated pyrimidines.

Conclusions

In conclusion, the photophysics of two pyrimidine chromophores, bearing the A-(π-D)2 and A-(π-D)3 structure, have been studied focusing on their excited state dynamics, upon protonation with three common acids i.e. CSA, AcOH and TFA in order to shed light on the interactions of solute and acid molecules. A quenching of the fluorescence of the neutral chromophores after addition of acid is revealed by steady state spectroscopy, while the generation of a red-shifted emission band, originating from the protonated species, is also observed. Upon addition of CSA and TFA, the excited state dynamics of the chromophores are not changed. This strongly shows that the interaction among the solute and acid molecules is operative in the ground state and the quenching is considered as static. On the other hand, the excited state dynamics of the neutral molecules becomes faster upon interaction with the AcOH molecules. This observation together with steady state measurements and the Stern-Volmer plots indicate that the interaction leading to the quenching, takes place both in the excited and ground state namely, the quenching has a dynamic and a static nature.

J o u r n a l P r e -p r o o f

Figure 2 .

 2 Figure 2. Absorption spectra of C2 in CHCl3 (10 -5 M) after addition of (a) CSA, (b) TFA and (c) AcOH.

Figure 3 .

 3 Figure 3. Fluorescence spectra of C2 in CHCl3 (10 -5 M) after addition of (a) CSA, (b) TFA and (c) AcOH. Excitation at 470 nm (isosbestic point of the absorption spectra).

Figure 4 .

 4 Figure 4. 2D fluorescence maps for C2 in CHCl3 (10 -5 M) with (a) 1 eq. CSA, (b) 6.7 eq. TFA and (c) 71 . 10 3 eq. AcOH.

Figure 5 .

 5 Figure 5. FU dynamics of C2 and C3 in CHCl3 detected at the peak of the neutral emission band, upon addition of various amounts of CSA (a) and (b), of TFA (c) and (d) and of AcOH (e) and (f).Note that the FU dynamics become faster upon addition of AcOH while they remain unchanged upon addition of CSA and TFA. The excitation wavelength was 400 nm.

Figure 7 .

 7 Figure 7. Fluorescence dynamics in the ns timescale for C2 in CHCl3 with different amounts of AcOH detected at (a) 540 (main fluorescence band) and (b) 700 nm (fluorescence band of the protonated species). Note that the lifetime of the neutral species decreases upon addition of AcOH which is in contrast to the case for CSA and TFA. The excitation wavelength was 400 nm.

Figure 8 .

 8 Figure 8. Stern Volmer plots for C2 in CHCl3 upon addition of (a) CSA, (b) TFA and (c) AcOH.F and F0 are the fluorescence intensities at the peak of the spectra after and before addition of acid. Similarly, τ and τ0 are the fluorescence lifetimes after and before addition of acid. Fitting the intensity Stern-Volmer plots in Figures8a and 8b, with the well-known equation

Table 1 .

 1 2.3 ns (Table1) which is attributed, therefore, to the lifetime of the neutral C2 molecules in CHCl3. The fact that the lifetime does not change upon addition of CSA means that interactions of the neutral C2 molecules with the acid molecules in the excited state are excluded. Parameters of the nanosecond fluorescence dynamics for C2 in CHCl3 with different amounts of CSA detected at 540 nm (main fluorescence band) and 700 nm (fluorescence band of the protonated species). The excitation wavelength was 400 nm.

		Intensity	1E-3 0.01 0.1 1	0.033 eq. 0.167 eq. 0.33 eq. 0.67 eq. 1.33 eq. 2.00 eq. 0 eq.	C2 CHCl 3 /CSA	λ det =540 nm	(a)
				2.33 eq.
			1
			0.1		λ det =700nm
		Intensity	1E-3 0.01	0.033 eq. 0.167 eq. 0.33 eq. 0.67 eq. 1.33 eq.
				2.00 eq
				2.33 eq.
			0	10	20		30
					Time (ns)
	Detection	Acid (eq.)	A1	τ1 (ns)	A2	τ2 (ns)	τavg (ns)
	wavelength
	540 nm	0				1	2.29	2.29
		0.033			1	2.3	2.3
		0.167			1	2.29	2.29
		0.33			1	2.29	2.29
		0.67			1	2.29	2.29
		1.33			1	2.3	2.3
		2.00			1	2.3	2.3
		2.33			1	2.3	2.3
	700 nm	0.033			1	2.34	2.34
		0.167			1	2.32	2.32
		0.33			1	2.29	2.29
		0.67	0.35	0.97	0.65	2.35	1.87
		1.33	0.57	0.89	0.43	2.27	1.48
		2.00	0.72	0.89	0.28	2.21	1.27
		2.33	0.74	0.9	0.26	2.19	1.23

(b)

Figure 6

. Fluorescence dynamics in the ns timescale for C2 in CHCl3 with different amounts of CSA detected at (a) 540 nm (main fluorescence band) and (b) 700 nm (fluorescence band of the protonated species). Note that the lifetime of the neutral species is not changed upon addition of CSA. The excitation wavelength was 400 nm.

Table 3 .

 3 Kinetic parameters for C2 upon addition of AcOH, following the Birks model.The Stern-Volmer plots[START_REF] Lacowicz | Principles of Fluorescence Spectroscopy[END_REF] for C2 upon addition of CSA, TFA and AcOH are given in figure8.

		6	F 0 /F
	F 0 /F	2 4	τ 0 /τ
		0.0		0.5	1.0	1.5	2.0
						CSA (eq.)
		Model	SterVolmer (Use r )
		Equation	1+A*x
		Reduced	0.02089
		Chi-Sqr
		Adj. R-Square	0.97831
				Value	Standard Error
		?$OP:F=1	A	1.81097	0.07938

Acknowledgements

M.F. thanks the Région Bretagne, France for funding of her Ph D.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.