Synthesis, Photophysical and Electronic Properties of New Red‐to‐NIR Emitting Donor–Acceptor Pyrene Derivatives

Abstract We synthesized new pyrene derivatives with strong bis(para‐methoxyphenyl)amine donors at the 2,7‐positions and n‐azaacene acceptors at the K‐region of pyrene. The compounds possess a strong intramolecular charge transfer, leading to unusual properties such as emission in the red to NIR region (700 nm), which has not been reported before for monomeric pyrenes. Detailed photophysical studies reveal very long intrinsic lifetimes of >100 ns for the new compounds, which is typical for 2,7‐substituted pyrenes but not for K‐region substituted pyrenes. The incorporation of strong donors and acceptors leads to very low reduction and oxidation potentials, and spectroelectrochemical studies show that the compounds are on the borderline between localized Robin‐Day class‐II and delocalized Robin‐Day class‐III species.


Introduction
The polycyclic aromatic hydrocarbon (PAH) pyrene is among the most widely studied chromophoresa nd possesses some unique properties such as intense blue emission and an exceptionally long-lived excited singlet state together with excimer and exciplex formation. [1] In addition to its photophysical properties, it has high chemical stability and charge-carrier mobility. Therefore, pyrene derivatives have been used in ab road range of applications in diverses cientific fields such as organic lightemitting diodes (OLEDs), organic field-effectt ransistors (OFETs) and organic photovoltaic cells (OPVs). [1,2] Furthermore, they have been used for sensing of temperature, [3] pressure [4] or pH, [5] or to detect guest molecules such as O 2 or NH 3 , [6] organic molecules, [7] or metals [1,8] and to construct covalent organic frameworks. [9] Its monomer and excimer fluorescence have also been used to study the properties such as dynamics or interactions of macromolecules [10] and lipids. [11] Its remarkably long fluorescencel ifetime of 354 ns, compared to other PAHs, makes pyrene exceptionallyw ell-suited for further applications such as the determination of cellular oxygen concentrations or reactive oxygen species (ROS) in biological systems. [12] Furthermore, 2-phenylethynylpyrenes have also been used as fluorescent labels for DNA. [13] To adjust the properties for as pecific application,e lectron donors and/or acceptors are often introduced onto aP AH core as they strongly modulate the frontier orbitals. Substituting PAHs with donors and acceptors gives derivatives with properties such as ap ermanent dipole moment, charge transfer (CT) excited states,s trong solvatochromism, environmentally influenced photophysics, the possibility for energy or electron transfer,a nd narrowed energy gaps. [14,15] Furthermore, such chromophores can absorba nd emit in the near-infrared (NIR) region, which is in demandfor applications such as bioimaging and cell recognition, as NIR light penetrates deeper into biological tissues, is less damaging than visible or UV light, and gives minimum interference from background autofluorescence by biomolecules. [16] In general,m aterials with high HOMO energies,s uch as the compound N,N'-diphenyl-N,N'bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD), are especially useful for hole transport. [17] Common p-donors that have been used in dyes to boost HOMO energies include amines,w ith al one pair on the nitrogen, such as diarylamino, diethylamino, dimethylamino or carbazolyl moieties. [18] Diarylamines are among the strongest p-electron donors and have been employed in diverse applications, [19] due to their outstanding physical, photochemical and electrochemical properties, and they are easy to synthesize and handle. [20] Am ethoxy group at the position para to the nitrogenn ot only increases the electron donating strength of diarylamines, but enables reversible oxidations. [20,21] Nevertheless, as ignificant drawback can be energy loss due to the rotational motion of the phenyl rings which can result in ar eduction in luminescence efficiency. [22] In addition, the possibility of rotation aroundt he NÀC(p) bond can lead to twisted intramolecular charget ransfer (TICT) excited states. [15] The compound 1,1,7,7-tetramethyl-julolidine is known to be an even stronger p-donor than diarylamines. The julolidinem oiety has been thoroughly studied since its discovery in 1892 by Pinkus, [23] and is used in aw ide range of dyes. [15,24] Its nitrogen lone pair is conformationally restricted to remainp arallel to the aromatic system, [25] in our case, the pyrene moiety, in both the ground and excited states, and our previouss tudies revealed as ignificantly enhanced electron donating effect on the pyrene core compared to diarylamines. [15] Pyrene exhibits ten peripheralr eactive positions, which can be classified into three sets of chemically inequivalent sites ( Figure 1a). Thus, the positiono fs ubstitution is very important and, typically,t he 1-, 3-, 6-and 8-positions are functionalized by electrophilics ubstitution reactions as the HOMO has its largestcoefficients at these positions (Figure1b). [1,26] Therefore, p-orbitals of substituents at the 1-, 3-, 6-, and 8-positions mix very efficiently with pyrene's HOMO/LUMO orbitals. However, unsymmetrical substitution at these positions is rather challenging due to the numerous possible isomers (Scheme 1). Niko, Konishi and co-workerss ynthesized D-p-A pyrene systems with donors and acceptors at the 1-,3-,6-and 8-positions, which displayed strong solvatochromism with emissions into the red region (557-648nm, in MeOH) and high quantum yields (f> 0.75). [27] Substituents at the 2,7-positions do not interact with the HOMO/LUMO orbitals of pyrene as they lie on the nodal plane ( Figure 1b). However,t hey can interact strongly with the HOMOÀ1a nd LUMO + 1o fp yrene that have nonzero contributions at these positions. We previously reported that the photophysical properties of pyrenes with substituents at the 2positions ignificantly differ from those with substituents at the 1-position. [28] Moreover, previouss tudies demonstrated that strong p-donors/acceptors are able to switch the energetic ordering of its HOMO/HOMOÀ1a nd LUMO/LUMO + 1, respec-tively,w hich results in greatlyi nfluenced photophysical and redox properties of these derivatives. [15,[29][30][31][32] Unsymmetrical substitution at these positions is straightforwardv ia iridiumcatalyzed CÀHb orylation, [33][34][35] and the introductiono fj ulolidine-like donors at the 2,7-positions (I, Scheme 2) even resulted in an unusualgreen luminescence. [15,[29][30][31][32]36] The K-region (positions 4,5,9,10,F igure 1) has large HOMO/ LUMO as well as HOMOÀ1/LUMO + 1c ontributions. Ottonelli and co-workers showed [37] that the contribution to the HOMO/ LUMO increases in the order:2 ,7 < 4,5,9,10 < 1,3,6,8, whilec ontributionst ot he HOMOÀ1/LUMO + 1i ncrease in the order 4,5,9,10 < 1,3,6,8 < 2,7. The K-region possesses alkene-like rather than aromatic character.H ence, reagents such as osmium tetroxide,w hich are known to react with alkene double bonds, selectively react with pyrene at the 4,5,9,10-positions. Müllen and co-workers reported ap rotocol for the asymmetrics ubstitution at theses ites and presented af ew examplesw ith emission into the orange region of the spectrum (613 nm in THF) (Scheme 1). [14,38] There are also reports on pyrenes that have donor and acceptor moieties on alternative positions, besides the typical three categories (vide supra). Hu and co-workers developed unsymmetric pyrenes with donors at the 1,3-positionsa nd acceptors at the 5,9-positionst hat emit in the blue-green region (Scheme 1). [39] Sutherland and co-workers combined the 2,7and 1,8-positionsw ith the K-region (V and VI,S cheme 2), respectively,a nd obtained an S 1 ! S 0 absorbance up to 900 nm, as it was their objective to obtain strongly redshifted absorptions with high molar absorptivities. However,t heir derivatives are not emissive and thus studies are missing on the influence on the excited state properties and how additional K-region substituents influence2 ,7-substituted pyrenes. [40,41] In our previous reports we showed that the influenceo fa second amine donor at the 2,7-positions on the occupiedo rbitals is larger than that of ac yano or even Bmes 2 acceptoro n the unoccupied orbitals and thus emission from the D-p-D derivative( I,S cheme 2) is furtherr edshifted than the D-p-A compound (II,S cheme 2). Therefore, our initialo bjective was to utilize two julolidine-like donors at the 2,7-positions and add additional acceptors to the pyrenec ore to obtain an ew class of D-p-A pyrene derivativew ith unparalleled photophysical properties such as red to NIRe missiona nd interesting redox behavior.M oreover,w ew ere interested to see how much more the S 1 state can be influenced by the addition of acceptors to the K-region.
For the acceptor unit, we chose n-azaacenes fused to the Kregion of pyrene that were reportedb yM ateo-Alonsoa nd coworkers and which possess strong p-accepting properties (VII and VIII,S cheme 2). [43] N-azaacenes have attracted much attention for their outstanding electronic properties and application in organic electronic devices and as anion radicals. [42] In particular,p yrene-fused azaacenes are even more stable than their N-azaacene analogues ande mit strongly in the blue-to-green region of the electromagnetic spectrum. [43,44] Results andDiscussion

Synthesis and structural characterization
The procedures used to synthesize the three key compounds are summarized in Schemes 3a nd 4. At the beginning of this project, we aimedf or derivative 7' (Scheme 3) and its analogue with ketone moieties at all four K-region positions (Scheme S1). The selective RuO 4 -catalyzed oxidation of pyrene at its 4-and 5-positions is the starting point for the synthesis route and gives the dione 1.T his one-step reactionw as reported by Harris and co-workersi n2 005 as an alternative to the toxic OsO 4 -catalyzed oxidation of pyrene. [47,48] In this regard, RuO 4 is generated in situ from RuCl 3 ·3 H 2 Oa nd NaIO 4 ,a nd the oxida-tion takes place in as olvent mixture of MeCN, CH 2 Cl 2 and H 2 O (1:1:1.25). In 1981, Sharpless and co-workersr eported that addition of MeCN to the RuO 4 -catalyzed oxidation greatly improvest he effectiveness of the reaction. However, this procedure suffers from severald isadvantages.T hus, the reaction can only be performed on as mall scale and, furthermore, there are variouss ide products such as dialdehydes or acids, which Nowicka et al. identified in detail, that complicate the workup and reduce the yield. [49] Bodwell and co-workersr ecently introduced an improved RuO 4 -catalyzed oxidation procedure to obtain 1,w hich is scalablea nd easest he workup.T his method includes the additive N-methylimidazole, and the solvent MeCN is replaced by THF. [50] In ordert op erform the Ir-catalyzed CÀHb orylation, [33][34][35] the ketoneg roups had to be protected as ad iketal to prevent unwanted reactions with the Ircatalyst. The lack of reactivity of oxidized pyrene in other metal-catalyzed reactions has been reported before. [51,52] Compound 2 was easily obtained by refluxing 1 in toluene with an excesso fe thylene glycol for 20 hi nt he presence of p-toluenesulfonica cid. The CÀHb orylation was performed according to our reportedp rocedure [33][34][35] andt akes place at the 2,7-positions selectively providing 3.
The Bpin moieties were further converted to azide moieties using ap rocedure reported by Chang and co-workers. [53] They converted arylboronic acids to arylazides in MeOH by using 1.50 equivalents of NaN 3 as the azide source and 5mol %o f Cu(OAc) 2 as the catalyst. Subsequent reduction of the azido species 4' with Pd/C andH 2 gave the corresponding primary amine 5'.T he reaction was stirred at room temperature until monitoring by IR spectroscopy confirmedt he complete disappearance of azide moieties (in the region 2000-2270 cm À1 )a nd the appearance of primary amine bandsa t3 450, 3363a nd 1620 cm À1 ( Figure S26). The amine moieties were alkylated in MeCN with 1-chloro-3-methylbutene in the presence of the base K 2 CO 3 ,a ccording to the approach used in our previously reported synthesis of I. [15] The alkylationr eactionw as complete after 48 ha nd gave the desired product 6' as ab right yellow solid (42 %). Unfortunately,t he final step to obtain the desired derivative 7' was unsuccessful. Our aim was to achieve ring closure and simultaneouslyd eprotect the acetal groups in order to regenerate the two ketonem oieties at the K-region. However,u sing ad iverse range of acids, the acetal protecting groups were removed first, which most likely generated a pyrene core that was too deactivated to permitthe electrophilic ring closure to take place. Therefore, we decided to change our julolidine-like donor moiety to ad iarylamine. The new synthesis route is depicted in Scheme 4. Compound 3 was transformed into the corresponding dibromo species 4 by ab romodeboronation reaction. [54] Thus, 3 and CuBr 2 were suspended in am ixture of THF/MeOH/H 2 O( 1:1:1) and the reactionm ixture was stirred at 90 8Cf or 4d.T he Buchwald-Hartwig amination, which was performed to obtain 5 from 4 using Pd 2 (dba) 3 ·CHCl 3 as the catalyst precursor and XPhos as the ligand, was achieved in ay ield of 58 %. The deprotectionof5 in atrifluoroacetic acid and water mixture( 6:1) is straightforward, andc ompound 6 was obtained in 81 %y ield. The cyclocondensation reactions between dione 6 and 2,3-diaminomaleonitrile to give 7,orb enzene-1,2-diamine to give 8,were performed in an ethanol/acetic acid mixture (1:1) at 80 8Cf or 15 ha ccording to the procedure of Mateo-Alonso and co-workers. [43,51,55] The solid-state structures of compounds 2, 5', 6',a nd 7 were determined via single-crystal X-ray diffraction ( Figure 2). In compound 7,t he biphenyl unit of the pyrene moiety exhibits typical aromatic CÀCb ond lengths ranging from 1.391(4) to 1.418(3) (bonds a, b, f, g, i,a nd j andt heir symmetrical equivalents, Figure 2a nd Table S2), as is also observed in pyrene. The c' and e' bonds on the unsubstituted site of the pyrene moiety are slightly longer (1.444(3) and 1.431(4) ), and the d' bond (1.355(3) )i stypical of aC =Cd ouble bond. This means that it can be viewed as ab iphenyl unit constrained to be planar by a-CH=CH-group. This has also recently been observed for many other 2-, and 2,7-substitutedp yrenes by Marder and co-workers. [15,29,30] Similarb ond lengths have also been reported for both sides of the pyrene moietyo f2 ,7-bis-(dianisylamino)pyrene, [32] the azaacene-free analogue of our compound 7.H owever,i n7 the c, d,a nd e bonds on the 4,5azaacene-substituted side of the pyrene moiety are all longer than the equivalent bonds on the unsubstituted side, the d bond (1.426(3) )b eing shorter than the c and e bonds (1.451(3)a nd 1.453(3) )( Ta ble S2). Hence,t he incorporation of the azaacenem oiety at the 4,5-positionsh as ab ond-lengthening effect, and the d bond can no longer be comparedw ith a C=Cd ouble bond but rather has aromatic character.I nterestingly,t he azaacene-moiety also effects the central bond h which links the two phenyl rings,a nd is slightly longer (1.435(3) )t han in the azaacene-free analogue compound (1.416(2) ). [32] As imilar, even more pronounced effect is observed in the unsymmetric compounds with acetals substituted at the 4,5positionso fp yrene, that is, compounds 2, 5',a nd 6'.W hile the unsubstituted side of the pyrene core still shows bond distances similar to those of pyrene or compound 7,t he 4,5-substituted side shows long bond distances for the c, d,a nd e bonds (1.515(2)-1.555(3) ), which are typical of CÀCs ingle bonds (Table S2). The central h bond is also further elongated (1.440(3)-1.447(3) )with respect to 7.
As is usually observed for aromatica mines, in compound 7 the NÀC(pyrene) bond lengths are significantly shorter  (2) ). Similar distances were also reported for the azaacene-free pyrene with amines substituted at the 2-and 7-positions. [32] The nitrogen atoms have a nearly ideal trigonal planar configuration with the sum of the C-N-C angles being between 359.0(6) and 360.0(3)8.T he interplanar angles between the NC 3 and pyrene planes ( 37.45(10) for N1 and 32.3(2)-33.2(4)8 for the disordered groups bonded to N2) are in as imilar range as those between NC 3 and the terminal phenylr ings (35.2(4)-47.5(2)8)( Ta ble S2). Again, this is in agreement with the NC 3 -pyrene angle (318)w hich was reported for the analogousa zaacene-free 2,7-substituted pyrene compound. [32] The methoxyphenyl groups R3 and R4 of 7 are strongly disordered and show ah igherd egree of rotational freedom than the groups R1 and R2. Indeed,t he interplanar angles between the NC 3 planesa nd the terminal rings vary between 38.2(4) and 47.5(2)8 for the disordered parts of R3 and between 35.2(4) and3 9.0(6)8 for those of R4. In addition, important intermolecular interactions involving the methoxyphenylg roups are only present for the R1 and R2 groups but are not observed for the R3 and R4 groups (Table S4).
In the crystal structure of 7,t he molecules form p-stacked dimers related by inversion symmetry.H ence, a p-stacking interaction with an interplanar separation of around3 . 41-3.47 is present between the pyrene core and the azaaceneg roup (Table S4). Dimers are arranged edge-to-face in asandwich-herringbonep acking, which is typicallyo bserved for pyrene itself and its derivatives( Figure 3). [56] Parallel and inverted molecules that are offset along the a axis exhibit C···C intermolecular interactions between their CN end groups. The molecular packing is furtherd etermined by the large steric demand of the amine moieties and the presence of tetrahydrofuran solvent molecules in the crystal lattice. IntermolecularC ÀH···N and H···H interactions are present between the R1 and R2 methoxyphenylg roups and the pyrene-4,5-azaacene core, while CÀ H···O interactions exist with the tetrahydrofuran molecule (Table S4). AH irshfeld surface analysis was performed in order to quantify the nature and type of intermolecular interactions in 7. [57] The disorder of the methoxyphenyl groups was not taken into account,b ut only the parts of highest occupancies were consideredi nt he analysis. Fingerprint analysisa nd its breakdown to the individual relative contributions, [58] shows a major contributionf rom H···H interactions (42 %), followed by a significant amount from C···H (22 %), N···H (15 %), and O···H (10 %) interactions ( Figures S44 and S45);5%C ···C interactions and around3 .6 %N ···C interactions are observed as well.

Photophysical and redox properties
The absorption spectra of derivatives 6, 7 and 8 are depicted in Figure 4a nd are generally similar to that of pyrene, in that the S 1 ! S 0 absorption is comparably weak with extinction coefficientso fe = 2700-4 000 m À1 cm À1 .H owever,f or 2,7-substituted pyrenes, these are the largeste xtinction coefficientsr eported so far.T hus,t he acceptorm oieties at the K-region increase how allowed this transition is. Furthermore, the S 1 ! S 0 absorption has as trong bathochromic shift in the order 8 < 7 < 6,a nd remarkably broad, covering ar ange of 6000 cm À1 ,w ith no vi-  brational progression,p roviding an indication of strongc harge transfer (CT) character.I np articular, dione 6 has av ery pronounced bathochromic shift (l max (abs) = 658 nm), which is significantly stronger comparedt ot he analogous dione VI reported by Sutherland and co-workers( Scheme 2). [40] However,t he donor moieties in their derivative are furthers eparated from the pyrene core by alkyne and phenyls pacers. [59] Hence, the donating ability through the 2,7-positions is reduced and, as a result, aw eaker CT character is obtained. Thus, derivative VI has as maller bathochromic shift with l max (abs) = 575 nm compared to derivative 6.I ti si nteresting to observe that the substituents at the 2,7-positionsi nt hese derivatives have such a significant effect on the S 1 ! S 0 absorption, which is unusual. The CT character of these derivatives is even more evident when comparing them to their analogues without the donor moieties at the 2,7-positions. Mateo-Alonso and co-workersr eported [45] that l max (abs) = 455 nm for VII,t he analogueo fo ur derivative 7,w hich demonstrates that the resulting CT character introduced via the additional donorsc auses ab athochromic shift of 3500 cm À1 of the S 1 ! S 0 absorption in our systems. Compound VIII,t he analogue of our derivative 8, has l max (abs) = 435 nm according to Sahoo et al. Hence,t he CT character introduced through our additionald onors at the 2,7-positions shifts the S 1 ! S 0 absorption by 3700 cm À1 . Compounds 6 and 7 are not emissive;h owever, 8 emits in the red to NIR region of the electromagnetic spectrum with l max (em) = 652 nm in toluene (Table 1), which is intriguing for pyrenes as they usuallye mit in the blue region and, to the best of our knowledge,s uch ar ed-shifted emission hasn ot been reported before for monomeric pyrenes. Even pyrenefused azaacenes reported previously do not have such ar edshifted emission. [55] Furthermore, compound 8 exhibits significant solvatochromism, as the emission shifts bathochromically with increasing solvent polarity from toluenet oC H 2 Cl 2 by 1051 cm À1 (48 nm), confirming the CT nature of the lowest energy excited state and as ignificant change in dipole momentb etween ground state ande xcited state. The large CT nature of the exciteds tate becomes more evident when comparing the emission of 8 with previously reported analogues that only possess ad onor or acceptorm oiety.D erivative VIII emits in the blue region with l max (em) = 471 nm and compound IV (unfortunately,t here is no report of an emission of compound III)e mits in the green region with l max (em) = 520 nm (both in CH 2 Cl 2 )w hile we observe a l max (em) = 700 nm for 8 in CH 2 Cl 2 . [32,46] Nevertheless, the non-radiatived ecay rates are significantly increased in CH 2 Cl 2 (k nr = 11 10 7 s À1 ), which is fully in line with the energy gap law as the reorganization energy is enhanced in polar solvents. [61] Therefore, the quantum yield is strongly decreased in the polar solvent. The apparent Stokes shifts are very large for pyrenes with values ranging from 3900 to 4900 cm À1 and are much larger compared to pyrenes that have D/A groups at the 2,7-or 1,3,6,8-positions only. [15,27,28,30] However,t he pyrene derivatives reported by Müllen and co-workerst hat have D/A moieties at the K-region exhibit even larger Stokes shifts ranging from 4500 to 5400 cm À1 . [14] Interestingly,t he radiative decay rates are rather slow (k r = 0.3-1.0 10 7 s À1 ), whichi saresult of the strong CT character.H owever, such slow radiative rate constants area lso typical of forbidden transitions (Strickler-Berg relation) and, thus, the lifetimes remain quite long (t 0 = 104-293ns) in this derivative,w hich is at ypical property of 2,7-substituted pyrenes. [28] There are not many reports on lifetimes of K-region substituted pyrenes,b ut they are typically in the range of (t 0 = 13-16 ns), which we recently reported. [62] Müllen and co-workers do not give lifetimesf or their K-region D/A derivatives; however, theS 1 ! S 0 absorptionsa re significantly more allowed with e > 7000 m À1 cm À1 and, therefore, shorter lifetimes than for 2,7-substituted pyrenes can be assumed. [14,63] Comparedt o compound 8,d erivative 7 has am ore pronounced CT nature, hence its energy gap is even smaller and, thus, it is possible that the non-radiative decay rates are largely increased, so that fluorescencebecomes too weak to detect.

Electrochemistry
In order to determinet he impact of the acceptor groupso n the K-region in combination with the donors at the 2,7-positions, cyclic voltammetry was performed.T he cyclic voltammograms are shown in Figure 5a nd the respective reduction and oxidation potentials are given in Ta ble 2. All derivatives exhibit one reversible reduction, whereas compound 6 has the lowest reduction potential with E 1/2 = À1.13 Va nd 8 the highest with E 1/2 = À1.84 Vv s. Fc/Fc + .H ence, the accepting strength of our derivatives is quite strong and decreasesi nt he order 6 > 7 > 8. Furthermore, all three derivatives can be reversibly oxidized twice;d erivative 8 is the easiest to be oxidized with E 1/2 = 0.17 and 0.39 V, and derivatives 6 and 7 have the same first (E 1/2 = 0.28) and second (0.45 V) oxidation potentials versus Fc/Fc + . Hence, our derivatives possess quite strongdonors and the donating strength in 8 is only minimally influencedb yt he addi- tional acceptora sc ompound III has oxidation potentials of E 1/2 = 0.14 and 0.38 Vv s. Fc/Fc + .Asimilar trend is observed in our derivative 6,a st he reduction potentiali sc omparable to that of the analogous derivative VI,w hich was reported by Sutherland [40] (Table3). Consequently,t he different donating abilities at the 2,7-positions do not influence the accepting effect of acceptorsa tt he K-region. This observation is also in line with the reduction potential of our derivative 7,w hich is very similar to that of compound VII which has no donors at the 2,7-positions (Table 3).
Upon oxidationt ot he respective monocations 7 + + and 8 + + ,a band rises in the NIR that is very broad,c overing ar ange of around7 000 cm À1 .T he observed plateau may be caused by two overlapping peaks of av ibronic progression. Thus, the peak maximum depends on the relative intensities of the first and second overtones. For 7 + + ,t he lowest energy band has a maximum atṽ IVCT max = 6400 cm À1 (1 563 nm, e = 15160 m À1 cm À1 ), but for 8 + + ,t he maximum isṽ IVCT max = 5350 cm À1 (1 869 nm, e = 28830 m À1 cm À1 )w ith some asymmetry in its shape ( Figure S1). The absorption spectra of the monocations 7 + + and 8 + + are very similar to that of compound III + + ,w hich was reported by Ito and co-workers, for which the lowest energy band has a maximum at 5260 cm À1 (1900 nm) and is also slightly asymmetric. [32] In general, delocalized (Robin-Dayclass-III) derivatives possess an asymmetrica nd narrow lowest energy band, while in localized (Robin-Day class-II) derivatives the lowest energy band resultsfrom an intervalence charge transfer (IV-CT) with a well-defined symmetric Gaussian shape. [64] For that reason, Ito and co-workersc oncluded that compound III + + is af ully delocalized Robin-Day class-III derivativeand assumed an electronic coupling V ofṽ IVCT max 2 = 2608 cm À1 . [32] However,o ur TD-DFT computations on the monocations 7 + + and 8 + + ,w ith as pecially adjusted functional (BLYP with 35 %e xact HF exchange [65] ), show that the lowest energy excitation is ar esult of the promotion of an electron from the b-HOMO to the b-LUMO orbital and these orbitals show the expected phase behavior for localized Robin-Dayc lass-II compounds (Figure 7). Further analysis of this lowest energy band enables the calculation of the electronic coupling V between the localized mixed valence states according to the Mulliken-Hush theory [66] (Equation (1)) where m ab is the transition moment( evaluated by integration of the IV-CT band), and the diabatic dipole moment difference Dm 12 was evaluated using the DFT calculated adiabatic dipole momentd ifference Dm ab . [67] According to these calculations, both derivatives possess al arge dipole moment change between their ground and excited states of Dm ab = 42 D( 7 + + )a nd 43 D( 8 + + ), which is typical for Robin-Day class-IIcompounds.

DFT and TD-DFT calculations
To rationalize the observed trends and properties we performed DFT and TD-DFT calculations. The ground state structures were first optimized in the gas-phase at the B3LYP/6-31 + G* level of theory.P revious studies [28] have shown that range-separated hybrid functionals are necessary to obtaina reliable pictureo ft he nature and relative energetic ordering of the excited states in pyrenes.W eh ave thus used the CAM-B3LYP functional for the subsequentTD-DFT calculations.
The nitrogen 2p z orbitalsi nb oth pyrene derivatives 7 and 8 mix very efficientlyw ith the HOMO-1 of the pyrene core. This leads to ad rastic destabilization of the HOMOÀ1( black, Figure 8) by ca. 1.51 eV in 7 and 1.82 eV in 8, which consequently switches the order of the HOMO and HOMOÀ1o rbitals. The pyrene-like HOMO (black, Figure 8), on the other hand, mixes with the acceptoru nit at the K-region. Hence, the pyrene bridge and the acceptor unit are fully delocalized and thus, this orbital (now HOMOÀ2) is stabilizedb ya round 0.54 eV in 7 and 0.08 eV in 8.Anew orbital (blue, Figure 8) of non-bonding character with large coefficients at the nitrogens of the dianisylamine donors is energetically positioned between the new HOMO andH OMOÀ2o rbitals, which was also observed for derivative IX.T he LUMOso fb oth 7 and 8 are greatly delocalized over the acceptoru nits and the pyrene bridge. Hence,t hese orbitals are considerably stabilized, with the LUMO of 7 stabilized by 1.08 eV,a nd of 8 by 0.52 eV compared to the one in pyrene. This pronounced stabilization of the LUMOs in 7 and 8 is reflected in our cyclic voltammetry studies by their remarkably low reduction potentials of   Figure 8) are not significantly affected in 7 (now LUMO + 2a nd LUMO + 3, respectively) and 8 (now LUMO + 1a nd LUMO + 2, respectively). The pyrene LUMO-like orbitali sd elocalized over the acceptoru nit and the pyrene core, while the pyrene LUMO + 1-like one is delocalized over the dianisylamine donors andt he pyrene core. The TD-DFT calculations show that the nature of the S 1 ! S 0 transition changes in such aw ay that it is no longer an early 50:50 weighted configuration interaction of HOMOÀ1!LUMO and HOMO!LUMO + 1a si np yrene (Tables 4a nd 5). In 7 and 8,t he S 1 ! S 0 transitions are nearly pure HOMO!LUMO transitions, which have significantC Tn ature (Figure 8). They are     strongly bathochromically shifted in the order 7 > 8 with low oscillator strengths, which matches well with the absorption maximaa nd extinction coefficients that were measured for this band (7: l max (abs) = 542 nm, e = 3100 m À1 cm À1 and 8: l max (abs) = 519 nm, e = 4000 m À1 cm À1 ).

Conclusions
We have reported the synthesis and structural characterization of new pyrene derivatives with donor moieties at the 2,7-positions and an acceptorg roup at the K-region. In general, unsymmetrically substituted pyrene derivatives remain rare due to their challengings ynthesis. The influence of the donors at the 2,7-positions and the acceptors at the K-region on the photophysical and electrochemical properties of the compounds is remarkable. The new derivatives possess very broad absorptions with maximaa t5 19-658 nm, and tails up to 800 nm, and compound 8 emits in the red to NIR region, which has not previously been reported for monomeric pyrenes to that extent. The intrinsic lifetimes remain rather long (t 0 = 104-293ns), which is at ypicalp roperty of 2,7-substituted pyrenes,w hereas the excited states of most K-region substituted pyrenes have lifetimes (t 0 )o fo nly around1 3-16 ns. Cyclic voltammetry studies reveal two reversible one-electron oxidations and one reversible reduction for all three derivatives 6, 7 and 8,w ith very low potentials showing the unique donating and accepting properties of our derivatives. Spectroelectrochemicalm easurements suggest as trong electronic coupling between the substituents at the 2,7-positionsf or 7 and 8.O ur DFT and TD-DFT calculations indicatet hat thesep roperties are the resulto ft he very strong donors and acceptors which couple very well with the pyrene orbitals, resulting in ar eordering of the occupied orbitals. Consequently,t he S 1 state of these derivatives hass trong CT nature giving rise to unparalleled properties.

Experimental Section
General considerations The catalyst precursors [Ir(COD)(OMe)] 2 [68] and Pd 2 (dba) 3 ·CHCl 3 [69] were prepared according to literature procedures, B 2 pin 2 was ag ift from AllyChem Co. Ltd. while other starting materials were purchased from commercial sources and used as received. Solvents used for synthesis were HPLC grade, further treated to remove trace water using acommercial solvent purification system from Innovative Technology Inc. and deoxygenated using the freezepump-thaw method.  4 ]w ere employed as supporting electrolytes. Compensation for resistive losses (iR drop) was employed for all measurements. Crystals suitable for single-crystal X-ray diffraction were selected, coated in perfluoropolyether oil, and mounted on MiTeGen sample holders. Diffraction data were collected on aB ruker X8 Apex II 4circle diffractometer with aC CD area detector using Mo-Ka radiation monochromated by graphite (2, 6', 11, 12)o rm ulti-layer focusing mirrors (5', 10). Diffraction data of 7 were collected on a Bruker D8 Quest 4-circle diffractometer with aC MOS area detector (Photon II) and multi-layer mirror monochromated Mo-Ka radiation. The crystals were cooled using Oxford Cryostream or Bruker Kryoflex II low-temperature devices. Data were collected at 100 K. The images were processed and corrected for Lorentz-polarization effects and absorption as implemented in the Bruker software packages. The structures were solved using the intrinsic phasing method (SHELXT) [70] and Fourier expansion technique. All non-hydrogen atoms were refined in anisotropic approximation, with hydrogen atoms "riding" in idealized positions, by full-matrix least squares against F 2 of all data, using SHELXL [70] software. In compound 11,t he coordinates of the hydrogen atom of the water molecule, which lies on at wo-fold rotation axis, were refined freely, but restraints were applied to the O-H and H-H distances. In compound 5' the coordinates of the hydrogen atoms bonded to nitrogen were refined freely.T he methoxyphenyl groups as well as the tetrahydrofuran solvent molecule are strongly disordered in compound 7.H ence, several restraints had to be applied in the refinement. Diamond [71] software was used for graphical representation. Hirshfeld surfaces were calculated and analyzed using the Crystal Explorer [72] program. Other structural information was extracted using Mercury [73] and OLEX2 [74] software. Crystal data and experimental details are listed in Ta ble S1. CCDC 1917153 (2), 1917154 (5'), 1917155 (6'), 1917156 (7), 1917157 (10), 1917158 (11), and 1917159 (12)c ontain the supplementary crystallographic data for this paper.T hese data are provided free of charge by The Cambridge Crystallographic Data Centre.

General photophysical measurements
All photophysical measurements were carried out under an argon atmosphere. All solution state measurements were performed in standard quartz cuvettes (1 cm x 1cmc ross section). UV/Vis absorption spectra were recorded using an Agilent 1100 diode array UV/Vis spectrophotometer.E xcitation, emission, lifetime and quantum yield measurements were recorded using an Edinburgh Instruments FLSP920 spectrophotometer equipped with a4 50 WX enon arc lamp, double monochromators for the excitation and emission pathways, and ar ed-sensitive photomultiplier (PMT-R928P) and a near-IR PMT as detectors. The measurements were made in rightangle geometry mode and all spectra were fully corrected for the

Fluorescence quantum yield measurements
Fluorescence quantum yields of the samples were measured using acalibrated integrating sphere (150 mm inner diameter) from Edinburgh Instruments combined with the FLSP920 spectrophotometer described above. For solution-state measurements, the longest wavelength absorption maximum of the compound in the respective solvent was chosen for the excitation. In order to exclude selfabsorption, the emission spectra were measured with dilute samples (ca. 0.1 OD at the excitation wavelength).

Fluorescence lifetimemeasurements
Lifetime measurements were conducted using the time-correlated single-photon counting method (TCSPC) on the FLSP920 spectrophotometer equipped with ah igh-speed photomultiplier tube positioned after as ingle emission monochromator.M easurements were made in right-angle geometry mode, and the emission was collected through ap olarizer set to the magic angle. Solutions were excited with either a3 15 (pulse width 932.5 ps), 376 (pulse width 72.6 ps) or a4 72 nm (pulse width 90.6 ps) pulsed diode laser at repetition rates of 1-5 MHz and were recorded at emission maxima. Decays were recorded to 10 000 counts in the peak channel with ar ecord length of at least 4000 channels. The band-pass of the monochromator was adjusted to give as ignal count rate of < 20 kHz. Iterative reconvolution of the IRF with one decay function and nonlinear least-squares analysis were used to analyze the data. The quality of all decay fits was judged to be satisfactory, based on the calculated values of the reduced c 2 and Durbin-Watson parameters and visual inspection of the weighted and autocorrelated residuals.

Spectroelectrochemical measurements
Spectroelectrochemical experiments in reflection mode were performed using an Agilent Cary 5000 Spectrophotometer in combination with ad esigned sample compartment consisting of ac ylindrical PTFE cell with an Infrasil wedge window with an angle of 0.58 and an adjustable three-in-one electrode (6 mm platinum disc working electrode, 1mmp latinum counter electrode and pseudo reference electrode). The potentials were adjusted with aG amry 600 potentiostat and all experiments were performed at room temperature under an argon atmosphere.