Pyrimidyl-substituted anthracene fluorophores: Syntheses, absorption spectra, and photophysical properties

Accepted Manuscript Pyrimidyl-substituted anthracene fluorophores: Syntheses, absorption spectra, and photophysical properties Antonio Santoro, Fabien Tuyèras, Grégory Dupeyre, Philippe P. Lainé, Ilaria Ciofini, Francesco Nastasi, Fausto Puntoriero, Sebastiano Campagna PII:

S0143-7208(18)30928-8

DOI:

10.1016/j.dyepig.2018.07.027

Reference:

DYPI 6881

To appear in:

Dyes and Pigments

Received Date: 24 April 2018 Revised Date:

15 July 2018

Accepted Date: 16 July 2018

Please cite this article as: Santoro A, Tuyèras F, Dupeyre Gré, Lainé PP, Ciofini I, Nastasi F, Puntoriero F, Campagna S, Pyrimidyl-substituted anthracene fluorophores: Syntheses, absorption spectra, and photophysical properties, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.07.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Pyrimidyl-substituted anthracene fluorophores: Syntheses, absorption spectra, and photophysical properties

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Antonio Santoro,a Fabien Tuyèras,b Grégory Dupeyre,b Philippe P. Lainé,b,* Ilaria Ciofini,c,* Francesco Nastasi,a,d Fausto Puntoriero,a,d and Sebastiano Campagnaa,d,*

Department of Chemical, Biological, Pharmaceutical and Environmental Sciences, University of

Messina, via Sperone 31, 98166 Messina, Italy

Univ Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR CNRS 7086, 15 rue J-A de Baïf, 75013

Paris, France c

PSL Research University, Institut de Recherche de Chimie Paris IRCP, CNRS-Chimie ParisTech, 11

rue P. et M. Curie, F-75005 Paris 05, France d

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Interuniversitary Research Center for Chemical Conversion of Solar Energy (Artificial

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Photosynthesis, SOLAR-CHEM, Messina node), via Sperone 31, 98166 Messina, Italy.

ACCEPTED MANUSCRIPT Abstract Twelve new polycyclic aromatic chromophores whose structures recall 9,10-substituted anthracene have been prepared and their absorption spectra, and luminescence properties (both in ethanol and dichloromethane solution at room temperature and in MeOH/EtOH (4:1 v/v)

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rigid matrix at 77 K) have been analyzed as well as pump-probe transient absorption spectroscopy and computational studies have been performed. The compounds have variously decorated pyrimidyl groups as substituents of the anthracene framework. The compounds are

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conveniently grouped in two series: 1–4 and 11, in which the pyrimidine nitrogen atoms are on the external side with respect to the anthracene framework (Ext-type compounds), and 5–10,

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where the pyrimidine nitrogen atoms are pointing towards the anthracene platform (Int-type compounds). Compound 12 contains both a pyrimidine substituent with “inner” nitrogen atoms (“Int”) and a pyrimidine with “outer” nitrogen atoms (“Ext”). All the new species are quite efficient luminophores (spectral range of emission maxima: 400-450 nm at 77 K; 415-520 nm

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at room temperature; emission lifetimes: 0.25-8.7 ns range; emission quantum yields ranging between 0.08 to 0.99, with a single exception), with their photophysical properties depending on the connection scheme of the pyrimidyl groups to the anthryl platform (i.e., Int-type vs Ext-

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type compounds). Luminescence originating from locally excited π−π* anthracene-based

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singlet states, delocalized states largely involving the pyrimidyl moieties, and charge transfer states has been identified. For the Int-type series, the initially-prepared excited state can deactivate via interconversion to a saddle-shaped conformation, opening the way to fast nonradiative decays.

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ACCEPTED MANUSCRIPT Introduction Design of new luminescent compounds is a research field of interest, for both fundamental and applicative reasons. Actually, studies of luminescence properties of new chromophores are relevant for the design of (i) systems for luminescence imaging in biological and medical

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fields,[1] (ii) innovative lightening systems (i.e., OLED),[2] (iii) molecular devices and/or machines whose machinery is driven or signaled by light.[3] Within the context of lightactivated molecular devices, also new systems for artificial photosynthesis and solar energy

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conversion in general can be included.[4,5]

Within this general frame, condensed polyaromatics are particularly interesting:[6-8]

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their lowest-energy excited singlet state is usually a π−π* level with relatively small structural distortions compared to the ground state, so that the excited states of condensed polyaromatics have a small Franck-Condon factor for radiationless decay and relatively intense fluorescence. A typical example of is 9,10-diphenyl-anthracene: this species exhibits its lowest-energy

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absorption feature, originating from population of its lowest-energy π−π* (1La) singlet state, in the 330 – 370 nm range (ε is about 12,000 M-1 cm-1 in ethanol, EtOH) and a structured emission with maximum at about 375 nm, with lifetimes in the nanosecond (ns) timescale and

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fluorescence quantum yield in the range of 0.80 – 0.98, depending on solvent and experimental

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conditions.[9] Moreover, it undergoes oxidation and reduction processes at relatively mild potentials (between +1.50 and -1.50 V vs SCE), and has relatively good chemical stability, both in the ground and excited states. More recently, it has also been demonstrated that 9,10diphenyl-anthracene and its derivatives are quite appealing for photon upconversion, and therefore for applications that can rely on such a process.[10] Here we report the synthesis, the characterization and the study of the absorption spectra and luminescence properties (both in fluid ethanol and dichloromethane solution at room temperature (RT) and in MeOH/EtOH (4:1 v/v) rigid matrix at 77 K) of twelve new 3

ACCEPTED MANUSCRIPT condensed polyaromatics whose structures are based on 9,10-substituted anthracene. The compounds, whose structural formulae are shown in Figure 1, bear variously decorated pyrimidyl groups as substituents (at 9 and 10 positions, except for two compounds that only bear a single pyrimidyl substituent) of the anthracene framework. The compounds are

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conveniently grouped in two series: compounds 1–4 and 11, in which the pyrimidine nitrogen atoms are on the external side with respect to the anthracene platform (also named Ext-type compounds), and compounds 5–10, where the pyrimidine nitrogen atoms are pointing towards

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the anthracene platform (also called Int-type compounds). Moreover, compound 12 contains

“outer” nitrogen atoms (“Ext”).

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both a pyrimidine substituent with “inner” nitrogen atoms (“Int”) and a pyrimidine with

Noteworthy, from former studies on similar compounds,[11,12] it is known that transitions and states having some charge-transfer (CT) character can be present in 1–12. The position of the nitrogen pyrimidine atoms with respect to the anthracene unit can determine the

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direction of such a CT character: in the Ext-type species, low-energy excited states with nonnegligible CT character involve anthracene-to-pyrimidine CT transitions,[11] whereas for the Int-type species, literature data suggests that the low-energy state can involve pyrimidine-to-

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anthracene CT levels.[12] The presence of (peripheral) end-capping pyrimidine substituents could also influence the direction of the CT character of low-energy transitions; thus, for

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example, it can be expected that the presence of strongly electron withdrawing substituents on the pyrimidine rings could invert the direction of the CT character. More importantly, strong electron-accepting or donating groups as substituents of the pyrimidyl moieties can increase the delocalization of relatively pure π−π* excited states mainly centered on the anthryl subunit towards the pyrimidyl rings, and the effect can be different for Ext- and Int-type species, due to their different geometries (i.e, rings coplanarity). Therefore, the main aim of the present work is to compare the photophysical properties, including the excited-state decay routes, of Ext-

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ACCEPTED MANUSCRIPT type and Int-type species, with the objective of deriving information for the design of efficient fluorophores. O

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Figure 1 – Structural formulae of the investigated compounds. 5

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To complete the characterization of these systems, four prototypical compounds of each series, schematically represented in Figure 2, were also investigated by the means of Density Functional Theory (DFT) and of its Time-Dependent counterpart (TD-DFT) to gain further

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insights into the properties of their ground and excited states, respectively.[13]

Finally, pump-probe transient absorption spectroscopy has been performed on most of the studied compounds, both in 1,2-dichloroethane (DCE) and EtOH solutions at RT: excited-

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state decay processes, including vibrational cooling within localized, delocalized, and CT

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states, and transitions between states, have been identified and kinetically characterized.

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Figure 2 – Compounds investigated from a computational viewpoint.

Results and Discussion

Syntheses. All mono-pyrimidine (R = H, ph) or monosubstituted di-(Int-pyrimidine)

derivatives (R = Int-pyrim) were synthesized (Scheme 1), starting from the key appropriate 9anthraceneboronate neopentyl esters, themselves obtained after Miyaura borylation of the adequate bromoanthracenes. Suzuki-Miyaura cross-coupling reactions were then performed by using either 2-chloropyrimidine / 5-bromopyrimidine (X = H; reference compounds), or 5bromo-2-iodopyrimidine (X = Br). In the latter case, Pd-catalyzed cyanation of the resulting 6

ACCEPTED MANUSCRIPT bromopyrimidines, followed by reduction of the cyano group, allowed isolation of the desired formylated pyrimidines. Nevertheless, it’s worth noticing that DIBAL-H reduction of cyanopyrimidyl anthracene derivatives systematically allowed unexpectedly low yields (about 30%) for reasons that remain to be clarified (an unusual gaseous evolution accompanies the

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reactant addition). The dissymmetric compound 12, bearing both Int- and Ext-pyrimidine moieties was also obtained according to this reaction sequence, but better yields were reached by modifying the Suzuki coupling reaction conditions, in comparison with those previously

Key : i) Pd(dppf)Cl2.CH2Cl2, KOAc, DME, 150°C, MW ; ii) Pd(PPh3)4, K2CO3,

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Scheme 1a

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employed.

PhMe/EtOH/H2O, reflux ; iii) Zn(CN)2, Pd(PPh3)4, DMF, 175 °C, MW; iv) DIBAL-H, CH2Cl2,

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–78 °C to RT; v) Pd(OAc)2, SPHOS, K3PO4, BuOH/H2O, reflux.

All symmetric dipyrimidine derivatives (both in Int- and Ext-series) were synthesized (Scheme 2), starting from the key 9,10-[di-(isopropyl)boronate]anthracene, itself obtained after lithiation of 9,10-dibromoanthracene. Similarly to the reaction sequence previously showed, SuzukiMiyaura cross-coupling reactions were performed by using either 2-chloropyrimidine / 5bromopyrimidine (X = H; reference compounds), or 5-bromo-2-iodopyrimidine / 5-bromo-2chloropyrimidine (X = Br or Cl). Subsequent Pd-catalyzed cyanation of the resulting 7

ACCEPTED MANUSCRIPT halopyrimidines was successfully performed in both series whereas isolation of the diformylated dipyrimidine failed in the Ext-series. It’s worth noticing that as described earlier, DIBAL-H reduction of bis(cyanopyrimidyl) anthracene 8 allowed a quite low yield (10%, i.e.

Key : i) 1) n-BuLi, Et2O, –78 °C, 2) B(OiPr)3, –78 °C to r.t. ; ii) Pd(PPh3)4, K2CO3,

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Scheme 2a

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about 32% per cyano group).

PhMe/EtOH/H2O, reflux or Pd(PPh3)4, K2CO3, DMF, 153°C, MW ; iii) Zn(CN)2, Pd(PPh3)4,

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DMF, 175 °C, MW; iv) DIBAL-H, CH2Cl2, –78 °C to r.t.

In order to circumvent the difficulty encountered in obtaining the diformylated pyrimidine

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derivative by reduction of the cyano groups carried by its precursor, alternative strategies were explored: direct Suzuki coupling of a formylated bromopyrimidine, deprotection of a diacetal dipyrimidine anthracene, Swern oxidation of the di(hydroxymethyl) dipyrimidine anthracene, in vain (see Scheme SI-1 drawn in the Supplementary Information), thus demonstrating the intrinsically unstable nature of the compound that probably deteriorates during the purification process (note that the change in luminescence behavior is most likely indicative of the effective obtaining of the desired product in the course of the procedures employed). For details on the

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ACCEPTED MANUSCRIPT synthesis of the newly functionalized halopyrimidines here described, see Supporting Information.

Absorption spectra. The absorption spectra data of 1–12, in ethanol and

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dichloromethane (DCM) at RT, are gathered in Table 1. The absorption spectra of 1, 2, and 3 (see Figure 3) are very similar to one another and are only weakly dependent on solvent; the absorption spectrum of 11, also belonging to the Ext-type series is also similar to those of 1–3

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(see Supporting Information, Fig. SI-1, for the absorption spectra of 11). In all cases the spectra are dominated by the typical absorption feature of anthracene derivatives, assigned to

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the lowest-energy spin-allowed π−π* transition producing the 1La state.[6-8] The separation between the vibrational peaks is about 1300 cm-1, typical of C=C stretching in aromatics. At wavelengths shorter than 280 nm, a more intense band is present (not shown), corresponding to the population of the 1Bu state of the anthracene moiety. The vibrational progression of the

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lower-energy absorption band and the presence of a more intense absorption band at higher energy than 280 nm are common to all the compounds here studied, although the vibrational progression is less prominent in some cases. From the data in Table 1, it results that for

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compounds 1–3 the substituents on the pyrimidine rings have negligible effects on the energies

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of the absorption electronic transitions. On the contrary, the absorption spectrum of 4 is sizably different from those of 1–3 (see Figure 3): it shows a less structured vibrational progression, and the red tail of the spectrum extends towards lower energies. This suggests that a chargetransfer (CT) contribution to the absorption band (probably involving the cyano group(s) as acceptor substituent(s)) could be present within the dominant π−π* band of 4. It should be taken into account, however, that such an electronic transition (and the corresponding excited state) could not necessary have a net CT nature, but may be a transition (and state) where the contribution of the pyrimidine rings to the orbitals involved is increased compared to a simple

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ACCEPTED MANUSCRIPT π−π* transition centered (mainly or exclusively) on the anthracene component, so leading to an extended delocalization. For convenience, such a somewhat more delocalized transition (and its corresponding excited state) will be called AP (anthracene-pyrimidine) /π−π* in the

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following.

Figure 3. Absorption spectra of 1–4 in DCM (top) and EtOH (bottom) solution. The spectra in ethanol are normalized.

The absorption spectra of 5, 6, and 9, all of them asymmetric species with respect to the anthracene platform and containing pyrimidine rings where nitrogen atoms are pointing towards the anthracene moiety (i.e., they are Int-type species), are shown in Figure 4. These spectra differ from those of 1–4 and also from one another. For 5 and 9, the spectra are 10

ACCEPTED MANUSCRIPT relatively broad, although still displaying some vibrational structure, and similar in both solvents used. Like for 4, also for these species a spin-allowed π−π* transition involving possibly more delocalized orbitals with respect to the localized spin-allowed anthracene-based π−π* transition (i.e., the AP/π−π* transition), with a non-negligible CT contribution could be

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responsible for the absorption feature in the studied spectral range. Indeed, the anthryl component is known to be able to behave both as an electron donor or as an electron acceptor,[9,11] and substituted-pyrimidine moieties can also be involved in CT-contributed

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states.[12]

Figure 4. Absorption spectra of 5, 6, and 9 in DCM (top) and EtOH (bottom, normalized) solutions.

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ACCEPTED MANUSCRIPT The absorption spectrum of 6 is significantly affected by the solvent (Figure 4). In ethanol, the spectrum is similar to those of 1–3, exhibiting the typical π−π* of anthracene. In DCM a low-energy contribution appears, indeed a clear shoulder at lower energy than 400 nm

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is present (Figure 3, top). This can be rationalized by assuming that in 6 the CT transition contributing to the absorption band at low energy involves the pyrimidine as the donor and the anthracene as the acceptor, as already proposed for similar pyrimidyl-anthracene

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assemblies:[12] EtOH can interact with the aldehyde groups by hydrogen bonding, removing electron density from the pyrimidine, ultimately moving the CT transition at higher energy (or

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apparently suppressing it at all). This effect would be absent in DCM.

Figure 5. Absorption spectra of 7 and 8 in DCM (top) and EtOH (bottom, normalized) solution.

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ACCEPTED MANUSCRIPT The absorption spectrum of 7 (Figure 5) is quite similar to that of 1, indicating the absence of a particular effect of the orientation of the pyrimidine nitrogen atoms with respect to the π−π* transitions producing the 1La state of the anthracene unit. The same is also valid for compounds 10 and 12 (Supporting Information, Figure SI-1). The spectrum of 8 (Figure 5)

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is much broader than that of 7, most likely because of a larger delocalization within the lowenergy transition, due to the presence of the electron withdrawing cyano substituent(s), however it is still similar to that of its counterpart compound 4 (compare with Figure 3 and

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data in Table 1). This suggests that the low-energy absorption transition band most likely

cannot be definitely ruled out.

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populates a delocalized AP/π−π* state, as in 4. However, some participation of a CT transition

Calculations of the vertical excitation energies for 1 (also called Ext-H, in a different terminology used to evidence the position of the pyrimidine nitrogen atoms with respect to the anthracene moiety and the nature of the para substituent), 4 (Ext-CN, according to the above

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terminology), 7 (Int-H), and 8 (Int-CN), also extended to analogous species in which the H- or CN-pyrimidyl substituents are replaced by chloride or bromide atoms (see data in Table 2),[13] show the presence of an intense and well-defined electronic transition of π−π∗

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character corresponding to a HOMO-LUMO excitation. The energy of such a transition is

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practically independent of the nature of the substituent of the pyrimidine units, in agreement with the experiments (Table 1). The only exception is constituted by the Int-CN compound 8, for which a sizeable red-shift is computed with respect to molecules belonging to the same class (i.e. of Int-R type). This behavior can be interpreted by analyzing both structural and electronic features of the compounds. In the ground state, all Ext-R systems show pyrimidine units practically perpendicular to the anthracene core, due to steric repulsion between the hydrogen atoms of these latter and the peri-H of anthracene. On the other hand, Int-R molecules present torsional angles between the anthracene core and the pyrimidine units 13

ACCEPTED MANUSCRIPT ranging from 68° (Int-CN) to 78.6° (Int-Br) and they are therefore expected to allow a better electronic coupling between the anthryl core and the pyrimidyl arm(s). In the case of the Inttype compounds, a slightly larger electronic coupling is therefore possible, thus resulting in a small – but sizeable – contribution stemming from the pyrimidine to the LUMO essentially

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only in the case of the CN substituent (see Supporting Information, Figure SI-2). A significant red-shift of the HOMO-LUMO excitation is thus predicted in 8 in agreement with its calculated more delocalized character, but this theoretical prediction disagrees with

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experimental data. This red-shift is nonetheless probably – as anticipated – overestimated at PBE0 level, the CAM-B3LYP values giving a better estimation of the shift with respect to the

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other compounds of the same series (Table 2).

Other discrepancies between computed and experimental absorption spectra are related to the presence in experimental spectra of CT contributions or net CT bands that calculations have difficulties to accurately take into account. As a matter of fact, the transition energies

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experimentally and theoretically observed for the Ext-type compounds are essentially identical to those observed for the Int-type ones. This, in spite of the circumstance that for some compounds, CT bands are evidently overlapped with the π−π∗ bands (or some CT character is

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present in the low-energy transition bands), resulting in the broadness of the lower-energy absorption band and/or the clear appearance of shoulders at lower energies than the pure

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π−π∗ bands (see the case of 6 in DCM). From a theoretical point of view, the lack of dependence of the absorption spectra on the nature of the substituents is justified since both the HOMO and the LUMO are mainly centered only on the anthracene with negligible direct contribution of the pyrimidine rings (for some example, see Supporting Information, Fig. SI2). In other words, pure anthracene-based π−π∗ are computed, a consequence of considering the pyrimidine and anthracene subunits as almost orthogonal one another, that is a too extreme simplification, particularly for the Int-type compounds. Experimentally, however, even when 14

ACCEPTED MANUSCRIPT the CT contribution is not negligible (according for example to the spectral shape), it appears as a shoulder on the red tail of the main absorption band, whose maximum does not change appreciably. It should be noted that for all compounds of both Ext-type and Int-type natures, low

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energy vibrational frequencies (in the range of 20 to 30 cm-1) are computed. These frequencies are related to skeleton deformations such as planarization of the pyrimidine subunits and saddle-shaped distortion of the anthracene core (for Int-type only). Such frequencies are

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anyway too low to become visible in the spectroscopic experiments discussed here. Because of

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their small frequency, they are also not effective in promoting fast radiationless decay.

Luminescence properties. For all the studied species, the photophysical data are independent of the excitation wavelength, so showing that the emissive state is produced with the same efficiency (probably unitary) from higher-energy excited states. The photophysical

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data of all the compounds are collected in Table 1.

The emission spectra of 1–12 at 77 K are roughly similar to one another and can be discussed together. For these compounds, the 77 K emission spectra are typical of π−π* singlet

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states of anthracene derivatives, with some CT character or with an extended delocalization in

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the case of species containing cyano-substituted pyrimidine rings, that is 4, 5, 8 and 9, whose emission spectra are in fact red-shifted.[9] Figure 6 shows the 77 K emission spectra of selected compounds (1, 5, 7 and 8). Other examples are shown in Supporting Information, Figure SI-3).

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Figure 6. Emission spectra of 1, 5, 7, and 8 in EtOH/MeOH 4:1 (v/v) matrix at 77 K.

Figure 7. Emission spectra of 1, and 4 in DCM (top) and in EtOH (bottom) at RT. 16

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On the contrary, the photophysical properties of 1–12 in fluid solution at RT exhibit a large variability. The emission spectra of 1 and 4, in ethanol and DCM at RT, are shown in Figure 7. The RT spectra of 2 and 3 are very similar to those of 1 (in both solvents) and can be

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attributed to a singlet π−π* state, although the vibrational progression is less clear than at 77 K. The same is also valid for 11 (see Table 1 and Supporting Information, Figure SI-4). Luminescence lifetime and quantum yields are also in line with the above attribution. Emission

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quantum yields are impressive (up to 0.99 for 1 in DCM, see Table 1), showing that these compounds are very powerful luminophores. Somehow different, among the otherwise

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similarly-behaved compounds 1–3 and 11, appears the behavior of 2 in EtOH, since in this condition both emission quantum yield and lifetime are reduced. A possible explanation could be an increased contribution – at least for the excited-state decay – of a CT state, which can be at low energy in 2 compared to 1, 3, and 11 because of the presence of the (inductive) electron-

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withdrawing chloro substituents; such a CT state would be stabilized in EtOH compared to DCM because of the polarity of the solvent.

The emission spectrum of 4 at RT in DCM is red-shifted compared to those of 1–3

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(Table 1, Figure 7). Because of the emission lifetime and quantum yield, the emission is still

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assigned to an excited state with a dominant π−π* nature but with a non-negligible CT contribution (a delocalized state, the above mentioned so-called AP/π−π* level, could be the emissive level). For 4 in EtOH, the red shift is significantly larger for the emission spectrum (which becomes much broader), and emission lifetime and quantum yield are extensively reduced compared to those of 1–3. This effect can be assigned to a probable formation of a less emissive, CT state that is strongly stabilized in polar solvents, so that the CT contribution is dominant in this condition.

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ACCEPTED MANUSCRIPT Figure 8 shows the emission spectra of 5 in various experimental conditions. Both in DCM and in EtOH the emission spectrum is broad and red-shifted in comparison with the 77 K spectrum (Table 1, Figure 6). As a consequence, the RT emission of 5 can be attributed to a state having a dominant CT contribution. Also in the light of the 77 K emission results, this

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suggests that the nitrile substituent plays an important role in determining the nature of the

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emissive level in 5.

Figure 8. Emission spectra of 5, 6, and 9 in DCM (top) and in EtOH (bottom) at RT.

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ACCEPTED MANUSCRIPT Figure 8 also shows the RT emission spectra of 6. The emission in ethanol can be assigned to the low-lying singlet π−π* state of anthracene derivatives, with a non-negligible CT contribution. The situation is largely different in DCM, where the emission spectrum shows a significant red-shift compared to that in ethanol, in agreement with the absorption spectra.

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The emission of 6 in DCM is therefore tentatively assigned to a CT state. As already discussed for the absorption spectrum, a likely hydrogen bonding interaction in alcoholic solvents could destabilize the pyrimidine-to-anthracene CT state that would be effective in 6, where the CT

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transition is proposed to be in opposite direction with respect to the (anthracene-topyrimidine(nitrile)) low-energy CT transition that would be operative in 5. The circumstance

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that the emissive excited state of 6 undergoes specific solvent interactions in alcoholic solvents is further supported by the emission properties in methanol at RT, which are similar to those in EtOH, whereas in acetonitrile solution 6 behaves analogously than in DCM solution (see Supporting Information, Fig. SI-5).

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The RT emission properties of compounds 7 and 8 (both Int-R species) are in line with the expectation on the basis of the emission properties of their Ext-R counterparts, that is 1 and 4, respectively. The spectra (see Table 1 and Figure 9) are in fact similar to the corresponding

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Ext-type species, except that they are red-shifted, in agreement with an increased electronic

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coupling between the pyrimidine and anthracene subunits due to more planar conformation of the Int-type species (see theoretical part, above). Therefore, the emission properties of 7 are assigned to a singlet π−π* state, although with some delocalization due to the presence of possibly coplanar pyrimidine rings (i.e., its emission can be approximated as an AP/π−π* emission, according to the definition of such a state used here). On the basis of the emission data (see Table 1), the quite close emission properties of 12 are assigned to a similar AP/π−π* state. An AP/π−π* state could also be the emissive state of 8 (Table 1), with the large red-shift compared to the emission of 4 due to the better co-planarity between anthryl and pyrimidyl 19

ACCEPTED MANUSCRIPT rings, and the red-shift compared to 7 attributed to the effect of the cyano substituents increasing the participation of the pyrimidyl rings on the delocalized state. Some CT contribution, however, is probably present, as suggested by a study of the solvatochromism of 8. Indeed, on moving from less polar to more polar solvents (solvents used: toluene, DCM,

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EtOH, acetonitrile, DMF, DMSO), the Stokes shift between absorption and emission maxima of 8 linearly increases (see Supplementary Information, Figure SI-5a). This can be justified on assuming a larger distortion of the excited-state potential energy curve with respect the

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ground-state one on increasing polarity, and can be rationalized with some CT contribution which increases with solvent polarity, or anyway that contributes to increasing the apparent

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Stokes shift.

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Figure 9. Emission spectra of 7 and 8 in DCM (top) and in EtOH (bottom) at RT.

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The emission spectrum of 9 in DCM (see Figure 8) is quite broad and extremely weak

(quantum yield < 10-4) and short-lived (0.4 ns) in comparison to the other compounds, in particular to 8, which could be considered a close compound (see Table 1). The situation is similar in ethanol, although the quantum yield is less reduced, and the spectrum is less broad (Figure 8, Table 1).[14] The emission energies, similar to those of 5 and 8 in both DCM and EtOH, suggest that a significant CT contribution is present in the emissive state of the asymmetric species 9. Anyway, possibly a further decay channel contributes to the excited

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ACCEPTED MANUSCRIPT state deactivation of 9, particularly in DCM, on the basis of the lifetime and quantum yields data in Table 1. The situation is similar for 10: although the emission spectra of this latter species are similar in energy and shape to those of 7, which can be considered its closest species - so allowing to assign the emissive properties of 10 to a delocalized, AP/π−π* level,

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analogously to 7 - the emission lifetime and quantum yields of 10 are significantly reduced, both in ethanol and in DCM, compared to those of 7 (Table 1, see Supporting Information, Fig. SI-4 for the emission spectra of 10 in both solvents). This suggests that a further decay

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channel for excited-state deactivation of 10 is present. Compound 12 exhibits emission spectral shape, energy, lifetimes and quantum yields (Table 1) quite similar to those of 7, so a

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delocalized AP/π−π* excited state is also considered to be the emissive level for 12. In this case, the additional decay channel that appears to be effective for 9 and 10 - and, on the basis of RT emission lifetime and quantum yields data in Table 1, even for 5 - does not seem to be

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operative (Table 1).

Transient absorption spectra. To clarify the excited-state properties of the new compounds we performed pump-probe femtosecond transient absorption spectroscopy (TAS)

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at RT for most of them, namely for 1–9. In all the experiments here described, the exciting

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wavelength is 400 nm, for technical reasons. A note of warning should be introduced at this stage: in the following (and also above),

for convenience we discuss the "pure" anthracene-centered 1La π−π* state as a different state from the so-called more delocalized AP/π−π* level. However, in the case where an AP/π−π* level is assumed to be responsible for the emission, it is not necessary a different state than the initially prepared π−π* level, but it could just be its relaxed conformation (in such a case, obviously, the geometry of the relaxed state should be more different from that of the ground state with respect to the case of emission apparently occurring from the initially prepared π−π* 22

ACCEPTED MANUSCRIPT level). So, the deactivation from the π−π* level to the more delocalized AP/π−π* would only be the relaxation from the Franck-Condon (FC) state to the relaxed state, probably coupled to some small nuclear rearrangement (i.e, increased planarization). To differentiate between transition (crossing) among different electronic states and nuclear reorganization within a

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single state is not straightforward, so we conventionally use the state transition terminology. Calculation cannot help since when extended delocalization with preparation of the so-called AP/π−π* level (or conformation) is associated with some charge-transfer contribution, as usual

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it is, this contribution is hardly evaluated by our theoretical methods. Even worse from a theoretical viewpoint is the case of what we refer to as CT states, that is states with a dominant

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CT contribution. Also in this case our state transition processes could be viewed as a further relaxation process occurring within the same electronic state, possibly connected with a more significant nuclear reorganization eventually connected with a charge transfer reorganization, partly involving also the first solvent coordination sphere.

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The initial transient spectrum of 1 (Figure 10top) in 1,2-dichloroethane (DCE) exhibits a broad transient absorption in the region 470-735 nm and a bleach at wavelengths shorter than 470 nm. This spectrum is a combination of stimulated emission (that for 1 in DCM peaks at

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417 nm) and of excited-state absorption. The transient spectrum undergoes small changes in

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about 1.3 ps, exhibiting a series of isosbestic points at about 500, 600, and 670 nm (see Fig. 10top). This process is attributed to molecular reorganization, most likely an enhanced planarization of the anthracene-pyrimidine framework. Then, the transient spectrum undergoes a further process with a time constant of 83 ps (Figure 10, middle panel), assigned to vibrational cooling,[15] finally followed by a longer-lived process exhibiting an isosbestic point at ∆A = 0 (Figure 10, bottom), which is not ended within the time window of our equipment (3.3 ns), and is safely assigned to the decay of the relaxed singlet, emissive π−π* state, in agreement with the emission lifetime (Table 1). Table 3 summarizes the proposed 23

ACCEPTED MANUSCRIPT processes and time constants in DCE. The behavior is qualitatively identical in ethanol (Table 4), where the main difference is a longer time constant for the vibrational cooling process (140

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ps).

Figure 10 – Transient absorption spectra of 1 in DCE. The various panels show successive steps, with top panel preceding the others; the time delays, in ps, from excitation pump are reported in the panels (only the initial and the last spectrum of each panel are explicitly indicated). Time constants for the various processes are gathered in Table 3. 24

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The transient absorption spectra of compounds 2 and 3 in DCE are similar to that of 1, although with different time constants (Table 3). Even in these cases, a fast molecular reorganization with a time constant of few picoseconds is followed by vibrational cooling,

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occurring in about 100 ps, and finally by the decay of the relaxed singlet emissive excited state, occurring in the ns timescale. However, whereas 3 exhibits a behavior qualitatively analogous to that of 1 even in ethanol (Table 4), 2 shows some differences in this solvent (see Figure SI-

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6 in the Supporting Information): the second process, with a time constant of 41 ps, is qualitatively different from the second process of 2 in DCE, and evidences a reduction of the

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bleach at about 470 nm and a decrease of the apparent transient absorption at 550 nm. This process is tentatively assigned to a transition from the initially-prepared π−π* state to a more delocalized state (the so-called, mixed AP/π−π* state, having some CT contribution), followed by a third process with a time constant of about 150 ps which leads to the emissive state. The

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150 ps process is assigned to vibrational cooling of the emissive state. This result agrees with the emission behavior of 2 in ethanol (see above) and further supports a AP/π−π* character for the emissive excited-state level of 2.

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Compound 4 in DCE exhibits a different transient spectrum evolution: the first process

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occurs in about 4.6 ps, followed by direct decay to the ground state (see Figure 11). The first process is spectroscopically similar to the second process of 2 in ethanol, with a moderate reduction of the bleach at about 470 nm and a decrease of the transient absorption at 550 nm, so it is assigned to a transition from the initially prepared state to a state with a mixed AP/π−π* character, which is the emissive state. Most likely, the 4.6 ps time constant also includes molecular reorganization. The absence of a clear vibrational cooling of the AP/π−π* level suggests that this latter state is formed with low vibrational energy, that is with a geometry

25

ACCEPTED MANUSCRIPT close to the minimum on its energy potential curve, or that vibrational cooling is within the

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time constant of the AP/π−π* level formation.

Figure 11. Transient absorption spectra of 4 in DCE. The various panels show successive steps, with top panel preceding the others; the time delays, in ps, from excitation pump are reported in the panels (only the initial and the last spectrum of each panel are explicitly indicated). Time constants are collected in Table 3.

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Figure 12. Transient absorption spectra of 4 in EtOH. The various panels show successive steps, with top panel preceding the others; the time delays, in ps, from excitation pump are shown in the panels (only the initial and the last spectrum of each panel are explicitly indicated). Time constants are collected in Table 4.

27

ACCEPTED MANUSCRIPT The transient spectrum of 4 in ethanol is quite different (Figure 12): the initial spectrum, similar to those of 1–4 in DCE, undergoes changes evidencing a first process that takes place with a time constant of about 4 ps. This process is practically identical to the first process of 4 in DCE, so it is suggested to be the formation of the delocalized AP/π−π* level

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from the initially prepared singlet π−π* state (or, according to the view of relaxation within a single state, it would be the relaxation from the FC state to the stabilized, delocalized AP/π−π* level). Successively, the peak at about 550 nm strongly decreases and two new bands, peaking

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at about 470 and 720 nm, appear. Simultaneously, two isosbestic points at around 520 and 640 nm are kept. These spectral changes occur with a time constant of about 25 ps. The so-formed

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transient spectrum then decays monotonically to the ground state. Since emission of 4 in ethanol has been attributed to a dominant CT state (see above), the process leading to the formation of the transient absorption at 470 and 720 nm is attributed to the formation of a CT state, that probably also includes vibrational cooling within the CT state. The evolution of the

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transient absorption spectrum can be interpreted as mainly due to the shift of the stimulated emission band, initially involving the π−π* state and therefore occurring at about 460 nm (see emission in DCM, Figure 7, top panel), to lower energies, as expected for the emissive CT

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state (see Table 1 and Figure 7, bottom).

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The data of 1–4 suggest that the different transient absorption spectral changes displayed by the various compounds in DCE or ethanol can be used to discriminate between: (i) emissive states originated from the initially prepared, relaxed singlet, π−π* levels

(locally-excited, LE, state): for such emissive states, very small changes are exhibited by the transient absorption spectra. A typical example is 1 in DCE (Figure 10); (ii) emissive states having dominant π−π* character, but with some increased delocalization (and maybe some CT contribution), that is the so-called AP/π−π* levels: the signature of such states in the transient absorption spectroscopy is a relatively moderate 28

ACCEPTED MANUSCRIPT increased absorption in the 430–490 nm region and a decreased absorption in the peak at about 550 nm. A typical example is 4 in DCE (see Figure 11). (iii) emissive CT levels: for such states a large transient absorption change takes place before the dominant emissive state is reached, with an intense increased absorption in the 430–

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490 nm region and decreased absorption in the peak at about 550 nm, a direct consequence of different position of the stimulated emission band in CT and π−π* emissive states. A typical example is 4 in ethanol (Figure 12).

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For convenience, discussion of the transient absorption spectroscopy of the Int-type compounds starts with the symmetric compounds 7 and 8. Such species are not photostable

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under intense laser pulse in ethanol, so the transient absorption spectroscopy has only been performed in DCE. Compound 7 in DCE shows a spectral evolution typical of processes leading to an emissive state having character of a relaxed AP/π−π* (type ii): in fact (Table 3), a fast process occurring in about 2 ps is followed by a second process with a time constant of

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19 ps, characterized by a moderate increased absorption in the 430–480 nm region and by a moderate decreased absorption in the 550 nm region, and by a third process of about 65 ps, finally followed by decay to the ground state with a time constant longer than 3 ns. The first

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process should be structural reorganization within the initially prepared π−π* state, the second

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would be the transition to the emissive state having an extended delocalized AP/π−π* character (or, in the alternative view, relaxation from the FC state), and the third would be equilibration via vibrational cooling within the emissive AP/π−π* state. This attribution agrees with the assignment of the emission of 7, formerly discussed. The emission properties of 8 in DCE have been assigned to a state having nonnegligible CT character. Actually, the first process recorded by TAS experiments (time constant, ca. 3 ps) is assigned to the formation of a state with AP/π−π* character, as indicated by the absorption increase in the 435–470 nm region (Figure 13a). Possibly, a structural 29

ACCEPTED MANUSCRIPT rearrangement is contained within the first process, as also happens for 4. The second process (time constant, 21 ps), characterized by a large spectral change with an isosbestic point at about 520 nm (Figure 13b), is assigned to formation of a state with a dominant CT contribution, probably coupled to some large structural reorganization (see also luminescence properties

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section). Then, a process (time constant, 70 ps) with negligible spectral changes takes place (Figure 13c), assigned to vibrational cooling within the CT state, finally followed by decay to

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the ground state, as indicated by isosbestic points with ∆A = 0 (Figure 13d).

30

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Figure 13. Transient absorption spectra of 8 in DCE. The various panels show successive steps, with top panel preceding the others; the time delays, in ps, from excitation pump are shown in the panels (only the initial and the last spectrum of each panel are explicitly indicated). Time constants are collected in Table 3. 31

ACCEPTED MANUSCRIPT For 6 in ethanol, the luminescence properties suggested that the emissive state is mainly an AP/π−π* level. As a consequence, it was expected that the transient absorption spectrum exhibits minor changes with time, in agreement with type (ii) behavior. Indeed, this is what is

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experimentally found (see Supporting Information, Figure SI-7): the transient spectrum initially produced is characterized by an intense transient absorption in the 440 – 600 nm region. The shape of the transient absorption spectrum evolves, the main change being an

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increased absorption at wavelengths shorter than 470 nm within few ps, occurring with a biphasic behavior. It can be concluded that the initially formed excited state decays to the

then decays to the ground state.

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AP/π−π* state by a biphasic process, probably also involving some vibrational relaxation, and

The transient absorption spectra and decays of 6 in DCE (Figure 14) are clearly different than those in ethanol: the initially-formed transient spectrum is essentially the same as

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in ethanol, but the spectrum evolves significantly towards a different situation, with the transient absorption maximum blue-shifted to the 440 – 500 nm region within about 20 ps. The decay is multiexponential, connected with a series of successive isosbestic points: a fast

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process, with a time constant of about 1 ps, is followed by a second process, with a time constant of ca. 3 ps. Within such a second process the main spectral changes are complete, so

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the formation of the CT state – responsible for emission in DCM – is probably connected with the second process (time constant, 3 ps), by production of a CT state from the initially prepared excited state. At longer timescales (10–60 ps) a further process leads to small spectral changes, probably due to vibrational cooling involving solvent dynamics within the CT state. Finally, a last longer decay is present, which corresponds to the emission lifetime of 6 in DCM.

32

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Figure 14. Transient absorption spectra of 6 in DCE. The various panels show successive steps, with top panel preceding the others; the time delays, in ps, from excitation pump are shown in the panels (only the initial and the last spectrum of each panel are explicitly indicated). Time constants are collected in Table 3.

33

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Figure 15. Transient absorption spectra of 5 in EtOH. The various panels show successive steps, with top panel preceding the others; the time delays, in ps, from excitation pump are shown in the panels (only the initial and the last spectrum of each panel are explicitly indicated). Time constants are collected in Table 4. 34

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Figure 15 shows the transient absorption spectra and decay of 5 in ethanol. The initially-formed spectrum is assigned to the anthracene-localized singlet π−π* state. The spectrum evolves until a transient absorption maximum in the region 430–480 nm is formed.

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The main spectral changes follow a multiexponential decay, with time constants of ca. 0.9 ps, 1.5 ps, and 10 ps (Table 3). These processes most likely include reorganization within the initially formed π−π* state, transition to the AP/π−π* state (or relaxation within the π−π* to

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reach a minimum with larger delocalization, that is the AP/π−π* conformation), and CT formation, also including some large structural reorganization. At longer times, a process of ca.

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70 ps is recorded, probably due to vibrational cooling. Finally, the transient absorption spectrum decays to the ground state on a timescale (1.3 ps) corresponding to luminescence lifetime (see Table 1).

The time-resolved transient absorption spectra of 5 in DCE are similar to those

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described for the same compound in ethanol, except for some difference in the spectral shape. Even in this experimental condition, the time-resolved transient absorption spectra of 5 show a series of isosbestic points for shorter times, which allow to isolate a series of processes with

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time constants of ca. 1.3 ps, 3 ps, and 15 ps (Table 3). The three processes, like in ethanol,

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should include reorganization within the initially formed π−π* state, relaxation to an AP/π−π* conformation, and successive CT formation. At longer times, the usual vibrational cooling within the CT state (ca. 60 ps) is followed by a final decay to the ground state (time constant, 1.4 ns), in agreement with luminescence lifetime. Compound 9 exhibits, both in ethanol and in DCE solutions, a behavior that looks similar to that of 5. Even in this case, a multiexponential decay takes place (Table 3), but the main changes, attributed to formation of the CT state, occur with a time constant of about 2.5 ps in DCE (Figure 16) and are not preceded by faster processes; the absence of faster 35

ACCEPTED MANUSCRIPT processes is probably due to (a) spectral changes associated with structural reorganization and formation of the intermediate state with mixed characters that are not large enough to be evidenced or (b) other type of potentially faster processes happening in a comparable time with the CT formation. At longer times, two successive processes are evidenced, with time constant

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of ca. 10 and 20 ps (Table 3), which can be attributed to vibrational cooling. The final relaxed state decays to the ground state with a time constant (370 ps), which is in line with the luminescence lifetime data (410 ps in DCM, see Table 1). The situation is quite similar in

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ethanol, with the only exception that the vibrational cooling is monoexponential (Table 3).

36

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Figure 16. Transient absorption spectra of 9 in DCE. The various panels show successive steps, with top panel preceding the others; the time delays, in ps, from excitation pump are shown in the panels (only the initial and the last spectrum of each panel are explicitly indicated). Time constants are collected in Table 3.

37

ACCEPTED MANUSCRIPT The combination of luminescence properties and transient absorption spectroscopy allows to obtain a satisfactory interpretation of the excited-state behavior of 1–9. Schematic representation of such a behavior for representative selected compounds is shown in Figure

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17.

Figure 17. Schematic representation of excited states and decay processes of representative compounds in DCE. G.S is the ground state. The figure considers a state transition approach, and therefore the presence of multiple states, for convenience. Alternatively, an approach

text.

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assuming various nuclear relaxation within a single state could be considered. For details, see

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However, looking at the luminescence data in Table 1, the luminescence lifetimes and quantum yields of the cyano-substituted, Int-type species 5 and (especially) 9 do not fit the

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general view, in particular considering the luminescence properties of their parent, symmetrical (with respect of the anthracene unit) compound 8. The most evident case is the emission quantum yields of 9, that is <10-4 whereas for 8 is 0.54. The data would suggest the presence of additional decay channels for the excited states of 5 and 9, as already mentioned in the former discussion. As also mentioned above, a similar comment could be made for another Int-type species, 10, when its properties are compared to those of its parent symmetrical species 7. A possible explanation can be offered by the following theoretical approach.

38

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Theoretical insights into excited-state properties In order to explore the relaxation of the vertical excited state leading to the emissive species, four compounds among the eight previously discussed at a theoretical level, namely the two

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unsubstituted Int and Ext isomers (Int-H, 7, and Ext-H, 1) and the CN-substituted ones (Int-CN, 8, and Ext-CN, 4) were studied in greater detail. For all species, TD-DFT structural optimization of the first excited state was performed in DCM at the CAM-B3LYP/6-31+G(d,p)

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level, in order to avoid any methodological artifacts that may be related to the description of charge transfer excited states using hybrid functional such as PBE0.

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Contrary to the ground state, for which only one stable conformation is found, all the four studied systems were found to show two stable conformations at the excited state: an axial one (essentially differing from that of the ground state in terms of planarization of the pyrimidine rings with respect to the anthracene platform) and a saddle-shaped one, actually

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corresponding to a large structural rearrangement as depicted in Figure 18 in the case of the Int-H molecule (7). Qualitatively similar structures were found for all other compounds. The relative stability of the saddle-shaped and axial structures is ruled by both the

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pyrimidine-anthracene connection scheme (Int or Ext) and by the nature of their peripheral substituents. Indeed, in the case of the Ext isomers the saddle-shaped conformer is always

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energetically disfavored by typically 8.7 (Ext-H) and 13.0 (Ext-CN) kcal/mol with respect to the corresponding axial one. On the other hand, in the case of the Int isomers, the saddleshaped configuration is only slightly higher in energy in the case of the H substituted molecule (1.4 kcal/mol for Int-H, that is 7) and practically isoenergetic to the axial conformer in the case of the Int-CN molecule (8), for which the difference in energy between the two conformers is of only about 0.2 kcal/mol.

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ACCEPTED MANUSCRIPT Therefore, based on energetic considerations it seems reasonable to assume that for the Int-type isomers only, and especially in the case of Int-CN (8), excited states of both the axial and the saddle-shaped forms could be formed, and eventually emission from both forms could be observed. In the case of the axial form, the emission would be a typical “anthracene-like”

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π−π∗ state (or in case a somewhat delocalized, AP/π−π* state), while in the case of the saddleshaped form, the transition should possess a more pronounced CT character. The emission

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energies computed for both conformers are reported in Table 5.

Axial S1 structure

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Saddle-shaped S1 Structure

Figure 18. Side (top) and top (bottom) views of the axial (left) and saddle-shaped (right) first excited state structures computed for Int-H (i.e., compound 7).

The excited-state formation of the saddle-shaped conformer from the axial geometry is

tentatively identified to the structural reorganization somehow connected with the CT formation, as suggested by emission spectroscopy and the temporal evolution of transient absorption spectra, occurring in some of the studied fluorophores. Of note, the saddle-shaped conformation corresponds to a first order saddle point at the ground state and to a minimum in 40

ACCEPTED MANUSCRIPT the triplet potential energy surface (PES) of all investigated compounds. However, this hypothesis disagrees with the computed vertical emission energies originating from relaxed singlet states of the hypothetical saddle-shaped conformation, collected in Table 5. The data in Table 5, in fact, would suggest that the saddle-shaped conformers should emit at very low

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energies, which is not the case. The conclusion is that, although in the case of Int-type species the excited-state axial-to-(saddle-shaped) structural relaxation is thermodynamically allowed, it is slow in comparison to the excited-state decay, so that the axial-to-(saddle-shaped) inter-

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conversion is usually not an effective excited-state decay process (therefore, the state responsible for the so-called CT emission in these compounds is not the saddle-shaped isomer).

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This also agrees with the fact that emission quantum yields and lifetimes of 7 and 8 are in line with those of the other compounds. Exceptions seem to be the compounds 5, 9 and 10: as mentioned above, the luminescence properties of which indicate that an additional decay

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channel is present for such compounds.

The axial-to-(saddle-shaped) excited-state interconversion A good reference for 5 and 9 is compound 8: all of these three species have cyano

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substituents on the pyrimidyl subunits, and belong to the Int-type series (see Figure 1). Their emission spectra, in all conditions, are similar in energy. However, the emission lifetimes and

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quantum yields of 5 and 9 are much smaller than those of 8, particularly in DCM. It can be noted that 5 and 9 are "asymmetrical" species, containing a single cyano-pyrimidine group. Like for 8 (see above), even for 5 and 9 the saddle-shaped conformation lies at a comparable energy as that of the axial conformation, so the axial-to-(saddle-shaped) inter-conversion is thermodynamically allowed. However, the process for 5 and 9 could be faster than for 8, since a single cyano-pyrimidine "arm" is involved, and this could make the structural reorganization less demanding in terms of activation energy for overcoming the nuclear barrier of the process.

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ACCEPTED MANUSCRIPT The axial-to-(saddle-shaped) inter-conversion process can also be active for 10: in fact, a reasonable model for 10 is compound 7, and although the emission spectra of the two compounds are very similar, in DCM (in EtOH the comparison cannot be made since the emission quantum yield of 7 is not available) both the emission lifetime and the quantum yield

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of 10 are strongly reduced compared to those of 7 (Table 1: lifetime is 1.8 ns and quantum yield is 0.16 for 10; the analogous values for 7 are 6.8 ns and 0.79, respectively). Compound 10 is also an "asymmetric" species, containing a single pyrimidine ring, so that the structural

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reorganization leading to the saddle-shaped conformation would be smaller than for the symmetric species 7, containing two pyrimidyl groups.[16] Compound 12 is also an

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“asymmetric” species, with only a single Int-type pyrimidyl group, suitable for axial-to-saddleshaped inter-conversion, like 10; however, the emission properties of 12 (Table 1), including lifetime and quantum yield, are close to those of 7, suggesting that the inter-conversion is not operative in 12.

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On the basis of the above considerations, we propose that the reduced lifetimes and quantum yields of 5, 9, and 10 with respect to those of the corresponding model species 8 and 7 are due to the formation of the non-emissive (or weakly emissive) saddle-shaped

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conformation from the emissive, axial level.[17] The situation is schematized in Figure 19. The axial-to-saddle-shaped interconversion would therefore act as a quenching process for the

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emission of 5, 9, and 10.

However, a quenching process leads to an analogous percentage of emission and

quantum yield reduction, compared to the model properties, on assuming that intrinsic radiative and radiationless decay rate constants (kr and knr) of the emissive levels of the models are unchanged in the quenched species. This is definitely not the case for 9, and only partly valid for 5 and 10. Actually, emission lifetimes and quantum yields are roughly similarly quenched in 5 and 10 in comparison with their models 8 and 7 in DCM, but not at all for 9. For

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ACCEPTED MANUSCRIPT this latter compound, the emission lifetime in DCM is reduced by about one order of magnitude in comparison with 8 (0.4 ns vs 5.6 ns, see Table 1), but the quantum yield is reduced by three orders of magnitude (<10-4 vs 0.54). This suggests that kr and knr of the quenched species are not the same as the corresponding kr and knr rate constants of the

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unquenched models. Neglecting these latter considerations, and therefore allowing a large approximation, however, the rate constants (kas) of axial-to-(saddle-shaped) inter-conversion processes in 5, 9 and 10 can be tentatively estimated using the equation kas = 1/τQ − 1/τ , in

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which τQ is the emission lifetime of the quenched species, 5, 9, and 10, and τU is the emission lifetimes of the respective unquenched models, 7 and 8. With such an approximation, in DCM

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the values of kas are 4.4 × 108 s-1 for 5, 2.3 × 109 s-1 for 9, and 4.0 × 108 s-1 for 10. By using an analogous equation based on quantum yields, kas = [(ΦU/ΦQ) – 1]/τU, in which ΦU and ΦQ are the emission quantum yields of the “unquenched” and “quenched” compounds, the calculated values of kas are 1.2 × 109 s-1 for 5, 9.6 × 1011 s-1 for 9, and 7.3 × 108 s-1 for 10. Whereas the

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estimated axial-to-saddle-shaped inter-conversion rate constants for 9 are clearly unreliable (too large difference among the two methods, that in principle should give the same result; However, an averaged value could still maybe be realistic), the values obtained for 5 and 10

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are in a someway acceptable, and the averaged and similar values of about 8 × 108 s-1 and 6 ×

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108 s-1 for the axial-to-(saddle-shaped) conversion rate constants in 5 and 10, respectively, can be proposed. The above discussion is based on the data in DCM, however similar results could be obtained in EtOH, so the axial-to-(saddle-shaped) deactivation is not limited to DCM.

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Figure 19. Schematization of the axial-to-(saddle-shaped) interconversion process. For details, see text. G.S. is the ground state. According to our hypothesis, kas would be about 8 × 108 s-1

Conclusions

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and 6 × 108 s-1 for 5 and 10, respectively.

Twelve new polycondensed aromatic chromophores have been prepared and their absorption

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spectra, luminescence properties (both in fluid ethanol and dichloromethane solution at room temperature and in MeOH/EtOH (4:1 v/v) rigid matrix at 77 K), pump-probe transient

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absorption spectroscopy, and computational studies have been performed. The new luminophores belong to a new family of compounds, the pyrimidyl-anthracene species, which exhibit quite interesting excited-state properties. In particular, whereas at 77 K the emission of all the compounds is dominated by the π−π* singlet state mainly centered on the anthracene moiety, at room temperature in fluid solution the emissive state can be the anthracene-based π−π* singlet state, a delocalized state largely involving contribution from the pyrimidyl group(s), or charge transfer states. The orientation of the nitrogen atoms of the pyrimidyl

44

ACCEPTED MANUSCRIPT substituents with respect to the anthracene framework (i.e., Int- or Ext-type compounds) as well as the substituents of the pyrimidyl groups are decisive in determining the nature of the emissive excited state of the compounds. In general, the Ext-type compounds exhibits better luminescence properties, but some exceptions are present. For some Int-type compounds,

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excited-state interconversion to a distorted saddle-shaped conformation can take place.

Because of their outstanding luminescence properties (in some case, quantum yields larger than 0.95 are observed) and their structural similitude with 9,10-diphenyl-anthracene,[8-10] the

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studied new class of fluorophores can be useful for fundamental and applicative purposes including the design of new luminophores for organic light-emitting diodes, luminescence

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imaging in biological and medical fields, and photon upconversion.

Acknowledgements

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SC, FP, AS, and FN would like to thank the Italian Ministero per gli Affari Esteri e la Collaborazione Internazionale, MAECI (Collaborative Research Projects of Large Relevance, Program 2017-2019), for funding support. PPL and IC are grateful to the French National

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Agency for Research (ANR) “programme blanc” (NEXUS project; No. BLAN07-1-196405) as well as to European Egide integrated action program “Germaine de Staël” (PAI-10612NL) for

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financial support. P.P.L. is also indebted to the French Ministry of Research for funding (ACI project No. JC4123). This project has also received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 648558) and from the French National Research Agency (ANR E-Storic).

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ACCEPTED MANUSCRIPT Supporting Information Supporting information available. It includes the description of materials and methods, seven additional figures for absorption spectra, luminescence properties, and computational details, detailed synthetic and characterization procedures, NMR spectra (proton NMR, carbon NMR,

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various bidimensional spectra), and mass spectra (including high-resolution spectra), for over

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180 figures and 122 pages.

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ACCEPTED MANUSCRIPT REFERENCES AND NOTES 1.

For some examples, see: (a) Tsien RY. Constructing and Exploiting the Fluorescent Protein Paintbox. Angew Chem Int Ed 2009;48:5612-26. (b) Rampazzo E, Boschi F, Bonacchi S, Juris R, Montalti M, Zaccheroni N, Prodi L, Calderan L, Rossi B, Becchi S,

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Sbarbati A. Multicolor Core/Shell Silica Nanoparticles for in vivo and ex vivo Imaging. Nanoscale 2012;4:824-30. (c) Ruiz-Sanchez, A. J.; Montanez, M. I.; Mayorga, C.; Torres, M. J.; De Cola, L.; Perez-Inestrosa, E. Dendrimer-Modified Solid Supports:

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Nanostructured Materials with Potential Drug Allergy Diagnostic Applications. Curr Med Chem. 2012;19(29):4942-54. (d) Stender AS, Marchuk K, Liu C, Sander S, Meyer MW,

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Smith EA, Neupane B, Wang G, Li J, Cheng J-X, Huang B, Fang N. Single Cell Optical Imaging and Spectroscopy. Chem Rev 2013;10;113(4):2469-527. (e) Guo Z, Park S, Yoon J, Shin I. Recent Progress in the Development of Near-Infrared Fluorescent Probes for Bioimaging Applications. Chem Soc Rev 2014;43:16-29.

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(a) Lamansky S, Djurovich P, Murphy D, Abdel-Razzaq F, Lee H-E, Adachi C, Burrows PE, Forrest SR, Thompson ME. Highly Phosphorescent Bis-Cyclometalated Iridium Complexes:  Synthesis, Photophysical Characterization, and Use in Organic Light

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Emitting Diodes. J Am Chem Soc 2001;123:4304-12. (b) Ladouceur S, Fortin D, ZysmanColman E. Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands. Inorg Chem 2011;50:11514-26. (c) Farinola GM, Ragni R.

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Electroluminescent Materials for White Organic Light Emitting Diodes. Chem Soc Rev 2011;40:3467-82. (d) Costa RD, Ortì E, Boling HJ, Monti F, Accorsi G, Armaroli N. Luminescent Ionic Transition-Metal Complexes for Light-Emitting Electrochemical Cells. Angew Chem Int Ed 2012; 51:8178-11. (e) Tang MC, Tsang DPK, Chan MM, Wong KMC, Yam VWW. Dendritic Luminescent Gold(III) Complexes for Highly Efficient Solution-Processable Organic Light Emitting Devices. Angew Chem Int Ed 2013;52:446-

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ACCEPTED MANUSCRIPT 9. (f) Wu C-L, Chang C-H, Chen C-T, Chen C-T, Su C-J. High Efficiency Non-Dopant Blue Organic Light-Emitting Diodes Based on Anthracene-Based Fluorophores with Molecular Design of Charge Transport and Red-Shifted Emission Proof. J Mater Chem C 2014;2:7188-00. (a) Balzani V, Credi A, Venturi M. Molecular Devices and Machines - Concepts and

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Perspectives for the Nanoworld. Weinheim: Wiley-VCH; 2008. (b) Balzani, V.; Ceroni, P.; Juris, A. Light-Powered Molecular Devices and Machines. In: Photochemistry and

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Photophysics – Concepts, Research, Applications. Weinheim: Wiley-VCH; 2014, p.245280.

(a) Hagfeldt A, Graetzel M. Light-Induced Redox Reactions in Nanocrystalline Systems.

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4.

Chem Rev 1995;95:49-68. (b) Graetzel M. Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells. Inorg Chem 2005;44:6841-51. (c) Gray HB. Powering the Planet with Solar Fuel. Nature Chem 2009;1,7. (d) Wasielewski MR. Self-Assembly Strategies for

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Integrating Light Harvesting and Charge Separation in Artificial Photosynthetic Systems. Acc Chem Res 2009;42:1910-21.

See, for example: (a) Balzani V, Moggi L, Manfrin MF, Bolletta F, Gleria M. Solar

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Energy Conversion by Water Photodissociation: Transition Metal Complexes Can Provide Low-Energy Cyclic Systems for Catalytic Photodissociation of Water. Science 1975; 89(4206):852-6. (b) Meyer TJ. Chemical Approaches to Artificial Photosynthesis. Acc

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Chem Res 1989;22:163-170. (c) Lewis NS, Nocera DG. Powering the planet: Chemical Challenges in Solar Energy Utilization. Proc Natl Acad Sci 2006;103:15729-35. (d) Youngblood WJ, Lee SHA, Kobayashi Y, Hernandez-Pagan EA, Hoertz PG, Moore TA, Moore AL, Gust D, Mallouk TE. Photoassisted Overall Water Splitting in a Visible LightAbsorbing Dye-Sensitized Photoelectrochemical Cell. J Am Chem Soc 2009;131;926-7. (e) Gust D, Moore TA, Moore AL. Solar Fuels via Artificial Photosynthesis. Acc Chem

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ACCEPTED MANUSCRIPT Res 2009;42:1890-8. (f) Berardi S, Drouet S, Francas L, Gimbert-Surinach C, Guttentag M, Richmond C, Stoll T, Llobet A. Molecular Artificial Photosynthesis. Chem Soc Rev 2014;43:7501-19. (g) Brennaman MK, Dillon RJ, Alibabaei L, Gish MK, Dares CJ, Ashford DL, House RL, Meyer GJ, Papanikolas JM, Meyer TJ. Finding the Way to Solar

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Fuels with Dye-Sensitized Photoelectrosynthesis Cells. J Am Chem Soc 2016;138:1308502, and refs. therein. 6

Jaffé HH, Orchin M. Theory and Applications of Ultraviolet Spectroscopy. New York:

Klessinger M, Michl J. Excited States and Photochemistry of Organic Molecules. New York: VCH; 1995.

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Wiley; 1964.

Turro NJ, Scaiano JC, Ramamurthy V. Principles of Molecular Photochemistry: An Introduction. New York: University Science Books; 2008.

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Montalti M, Credi A, Prodi L, Gandolfi MT eds. Handbook of Photochemistry. 3rd ed.

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Boca Raton: CRC Press; 2006.

10 (a) Schmidt TW, Castellano FN. Photochemical Upconversion: The Primacy of Kinetics. J Phys Chem Lett 2014;5:4062-72. (b) McCusker CE, Castellano FN. Efficient Visible to

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Near-UV Photochemical Upconversion Sensitized by a Long Lifetime Cu(I) MLCT Complex. Inorg Chem 2015;54:6035-42. (c) Gray V, Dzebo D, Lundin A, Alborzpour J,

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Abrahamsson M, Albinsson B, Moth-Poulsen K. Photophysical Characterization of the 9,10-Disubstituted Anthracene Chromophore and its Applications in Triplet–Triplet Annihilation Photon Upconversion. J Mater Chem C 2015;3:11111-21. (d) Ye C, Zhou L, Wang X, Liang Z. Photon Upconversion: from Two-Photon Absorption (TPA) to Triplet– Triplet Annihilation (TTA). Phys Chem Chem Phys 2016;18:10818-35, and references therein. (e) Zhong F, Zhao J. Phenyleneanthracene derivatives as triplet energy

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ACCEPTED MANUSCRIPT acceptor/emitter in red light excitable triplet-triplet-annihilation upconversion. Dyes & Pigments 2017;136:909-918. 11 Ciofini I, Adamo C, Teki Y, Tuyèras F, Lainé PP. Reaching Optimal Light Induced Intramolecular Spin Alignment within Photomagnetic Molecular Device Prototypes. Chem

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Eur J 2008;14:11385-05.

12 Credi, A.; Balzani, V.; Campagna, S.; Hanan, G. S.; Arana, C.; Lehn, J.-M. Photophysical Properties of a Dinuclear Rack-Type Ru(II) Complex and on its Components. Chem Phys

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Lett 1995;243:102-107.

13 Note that out of the eight compounds in Figure 2, only five have an experimental

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14 Even for 9 in DCM the emission spectrum, in spite of its rather unusual broadness, is independent of excitation wavelength, and lifetime is monoexponential and identical when recorded at various emission wavelengths.

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15 Throughout all the manuscript we shall use vibrational relaxation as synonymous of vibrational cooling. In both cases, they can contain contributions from inner and outer modes, that are sometimes named (small) structural reorganization and solvent dynamics,

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respectively. In general, nuclear reorganization processes within vibrational relaxation are assumed to be much smaller than those occurring in processes explicitly termed structural

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reorganization. However, such a distinction is often vague. 16 Within the frame of the multiple state approach, the emissive state of 10 is AP/π−π* in nature, according to luminescence data, whereas the emissive states of 5 and 9 are both CT levels. This tends to suggest that the nature of the emissive state is not essential for the occurring of the axial-to-(saddle-shaped) interconversion. 17 Axial-to-(saddle-shaped) interconversion leads to reduced lifetime and quantum yield of axial emission. The saddle-shaped excited state should eventually emit at quite lower

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ACCEPTED MANUSCRIPT energy (see calculated values shown in Table 5), however it most likely deactivates by fast radiationless decay. The fact that there is not any trace of another excited state in the transient absorption spectra of 5, 9, and 10 is in favor of ultrafast radiationless decay of the saddle-shaped conformers, whose decay to the ground state is probably faster than its

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photoinduced formation.

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Table 1. Absorption and luminescence data of the studied compounds.

Abs λ(nm)a

EtOH/MeOH 4:1 (v/v), 77 K

EtOH, room temperature

Luminescence

Abs

Luminescence

λ(nm)b

τ(ns)

Φ

assignment

λ(nm)a

λ(nm)b

τ(ns)

397

417

7.2

0.99

π−π*

394

417

6.4

2

396

421

6.2

0.78

π−π*

394

428

2.1

3

397

420

6.5

0.83

π−π*

395

420

4

397

464

5.8

0.57

AP/π−π*

395

5

385

492c

1.6

0.07

CT

382

6

384

531c

6.3

0.28

CT

381

7

389

439

6.8

0.79

AP/π−π*

8

394

500

5.6

0.54

CT

9

394

504

0.4

< 10-4

CT

10

393

442

1.8

0.16

AP/π−π*

11

397

414

6.9

0.51

12

394

436

6.9

0.78

Φ

assignment

λ (nm)b

τ (ns)

0.76

π−π*

404

8.5

0.22

AP/π−π*

406

5.7

0.89

π−π*

402

7.2

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Luminescence

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DCM, room temperature

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6.1

1.8

0.05

CT

432

5.5

510c

1.3

0.07

CT

430

6.8

438

2.3

0.25

AP/π−π*

406

10.0

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520

440

5.4

(d)

AP/π−π*

404

7.9

391

503

2.5

0.16

CT

444

5.7

390

520

0.25

0.04

CT

437

6.0

390

440

1.0

0.08

AP/π−π*

404

8.5

π−π*

394

441

3.5

0.40

π−π*

403

8.7

AP/π−π*

391

433

4.5

0.61

AP/π−π*

402

8.5

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(a) Only the maximum of the lowest-energy absorption band is reported. (b) Emission maximum or highest-energy emission feature, in case of structured emission. (c) A shoulder at lower energy is clearly present (see Figure 4). (d) This species undergoes photochemical reaction in ethanol so its emission quantum yield is difficult to calculate with confidence.

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Table 2: The most intense vertical transition energies (in nm) and oscillator strengths (au) computed at PBE0 and CAM-B3LYP levels on ground state optimized PBE0/6-31+G(d,p). H->L is the HOMO (H)to-LUMO (L) transition. CAM-B3LYP Character PBE0 λ (f) λ (f) Int-H (7) 394 (0.204) 366 (0.263) H->L Int-Cl 399 (0.248) 369 (0.311) H->L Int-Br 393 (0.236) 365 (0.303) H->L Int-CN (8) 443 (0.230) 383 (0.370) H->L Ext-H (1) 393 (0.270) 369 (0.267) H->L Ext-Cl 397 (0.231) 369 (0.296) H->L Ext-Br 397 (0.243) 369 (0.309) H->L Ext-CN (4) 396 (0.210) 369 (0.311) H->L

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Table 3. Decay time constants (in ps; uncertainties is 15%) obtained by transient absorption spectroscopy and assigned processes for 1-9 in DCE. proposed processa 1 2 3 4 5 6 7 8 9 2.7

1.7

vibrational cooling

83

110

90

formation AP/π−π*

of

1.3

the

4.7

vibrational cooling within the AP/π−π* state

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vibrational cooling within the CT state

15 60

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formation of the CT state

1.0

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state (conformation)

3.0

2.0

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1.3

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molecular reorganization

19

3.0

70

3.0

21

2.4

40

70

9.6; 23

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> 3300 > 3300 > 3300 1300 > 3300 > 3300 > 3300 410 decay to the > 3300 ground state from relaxed emissive state (a) In this table we used the hypothesis that 1-9 have multiple excited states. Within this approximation, formation of the AP/π−π* state includes molecular reorganization within the FC state for 4, 6, and 8. Alternatively, it can be viewed that molecular reorganization in these compounds leads to an equilibrated state having a significantly different conformation (and a subsequent different spatial electron distribution within the molecular framework) than the initially-prepared excited state. 54

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1.3

3.5

vibrational cooling

140

1.0

4.2

1.5

15

25

3.0

1.5

70

12

1500 1300 2300

210

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vibrational cooling within the AP/π−π* state

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formation of the CT state vibrational cooling within the CT state

0.9

150

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formation of the AP/π−π* state (conformation)

1.6

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molecular reorganization

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Table 4. Decay time constants (in ps; uncertainties is 15%) obtained by transient absorption spectroscopy and assigned processes for 1-6 and 9 in ethanol (compounds 7 and 8 are not photostable in this solvent under laser pulse). proposed process 1 2 3 4 5 6 9

> 3300

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decay to the ground state from relaxed emissive state

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2000

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Table 5. Computed vertical emission energies (in nm) from relaxed S1 CAM/B3LYP structures. CAM-B3LYP CAM-B3LYP λ (nm) Int-H-axial 469 Ext-H-axial 7 Int-H-(saddle-shaped) 693 Ext-H-(saddle-shaped) Int-CN-axial 525 Ext-CN-axial 8 Int-CN-(saddle-shaped) 760 Ext-CN-(saddle-shaped)

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λ (nm) 462 796 443 867

ACCEPTED MANUSCRIPT Pyrimidyl-substituted anthracene fluorophores: Syntheses, absorption spectra, and photophysical properties

Antonio Santoro, Fabien Tuyèras, Grégory Dupeyre, Philippe P. Lainé, Ilaria Ciofini,

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Francesco Nastasi, Fausto Puntoriero, and Sebastiano Campagna

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TOC Graphic

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Pyrimidyl-substituted anthracene fluorophores. Synthesis, absorption spectra, and photophysical properties

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Francesco Nastasi, Fausto Puntoriero, and Sebastiano Campagna

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Antonio Santoro, Fabien Tuyèras, Grégory Dupeyre, Philippe P. Lainé, Ilaria Ciofini,

Highlights

Twelve new aromatic luminophores, based on anthracene platform, have been prepared

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The 77 K emission comes from π−π* singlet state centered on anthracene moieties

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At RT, emission comes from π−π* delocalized states or charge transfer states

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Pump-probe transient absorption spectroscopy kinetically reveals decay routes

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For some species, axial-to-(saddle-shaped) conversion can drive radiationless decay

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