High‐Resolution Electronic Excitation and Emission Spectra of Pentacene and 6,13‐Diazapentacene Monomers and Weakly Bound Dimers by Matrix‐Isolation Spectroscopy

Abstract N‐Heteropolycycles are among the most promising candidates for applications in organic devices. For this purpose, a profound understanding of the low‐energy electronic absorbance and emission characteristics is of crucial importance. Herein, we report high‐resolution absorbance and fluorescence spectra of pentacene (PEN) and 6,13‐diazapentacene (DAP) in solid neon obtained using the matrix‐isolation technique. Accompanying DFT calculations allow the assignment of specific vibrationally resolved signals to corresponding modes. Furthermore, we present for the first time evidence for the formation of van der Waals dimers of both substances. These dimers exhibit significantly different optical characteristics resulting from the change of electronic properties evoked by the incorporation of sp2 nitrogen into the molecular backbone.


Introduction
Acenes, polyaromatic hydrocarbonsc omposed of linearly annulatedb enzene units, [1] receive much interest from theoretical and experimentals cientists. [2][3][4][5] The smallers pecies of this class have been intensely studied. [6][7][8][9][10][11][12][13][14] Synthetic approaches, based on the photochemical bisdecarbonylation of bridging a-diketones, the thermald eoxygenation of endoxide precursors or the thermalc leavage of covalentd imers, have made longer acenes accessible, [15][16][17][18] up to the on-surface generation of dodecacene. [19] Particular focus has been devoted to the application of large polyaromatic hydrocarbons [5,20,21] and acenes,i np articular [22][23][24][25][26] in organic electronics. Their performance in devices such as or-ganic field-effect transistors (OFETs), organicl ight-emitting diodes (OLEDs) or organic photovoltaic devices (OPV) depends on solid-state packing, reorganisation energy and intermolecular electronic coupling influencing the charge-carrier mobility. Limitingf actorsa re synthetic accessibility,s olubility,p hotostability and oxidative resistance. The improved electronic characteristicso ft he longer acenes come at the price of reduced stability,i mpeding application.P entacene still remains ap romising and widely used materiali nt he field of organic materials due to its relative stability combined with sufficientf ield-effect mobility. [27] It displays ar eorganisation energy as low as 59 meV. [28] Quasi-monolayer semiconductors on poly(amic acid) surfaces gave chargec arrierm obilities of 7.6 cm 2 V À1 s À1 in the linear region. [29] Substituted acenes gain importance in materials research. Twos ubstitution strategies exist:F irstly,e nd-ons ubstitutions relative to thelong axis of the acene tune the dipole characteristics with ap ush-pull substitution pattern. [30] Alternatively, side-chain substituents on the central ring (Anthony et al. [22] ) improvestability towards oxidation andprocessability. [31,32] Secondly,a(formal) substitution of one or more CH units by sp 2 nitrogen atoms createse lectron-deficient systems compared to the unsubstituted acenes. Energetic position of frontier orbitals and band gaps are easily tunable. [33,34] Combinationo ft hese two strategiesc reates stable, easily processable materialsw ith desired electronic and crystallisation characteristics. [35][36][37][38][39] Herein, we provide optical spectra of pure pentacene (PEN) and of its nitrogen-substituted analogue 6,13-diazapentacene (DAP,S cheme1)i ns olid neon using the matrix isolation technique. [40][41][42] This method allows the creationo fd iluted samples while minimising extrinsic effectss uch as solvation or aggregation. The substances trapped in solid noble gas matrices at low temperature (4 K) are analysed by standard spectroscopic methods (vis and fluorescence). At low concentrations, matrices serve as models for gas phase analyses that are not always feasible. Absorbance measurements on solid-state matrices of noble gases have, in turn, the advantage of avoiding hot bands. Narrowb and widths results implifying interpretation. This technique is powerful in terms of the simulation of gas phase analytics and the deconvolution of the vibronic states in the absorbance spectra and was hitherto appliedt oa series of higher acenes, including pentacene, in solid argon by Bettinger et al. [43] as well as tetracene anda zatetracenes in neon. [44] Here, we extend our studies to the pentacene analogues trapped in solid neon (Scheme 1).
The dependence of the absorbance characteristics of PEN from different matrix materials has previously been studied by Allamandola et al. alongside with its cationic form and the selective formation of the anion by using alkali metals as matrix dopants. [45] In this work, we presentf or the first time fluorescence spectra of (matrix-isolated) PEN monomers and assign all bands/signals in the electronic excitation and emission spectra with the help of quantum-chemical calculations. Even more importantly,weobtainedpositive prooffor the formation of weakly bound dimers and aggregates by series of experiments with variable concentrationsa nd annealing cycles. Last but not least and probably most importantly,w ed onots top with the analysiso fPEN,b ut report for the first time af ull matrix-isolation analysis (electronic excitation and emission spectra)o fDAP monomers, weakly bound dimers and aggregates, backed by the results of detailedq uantum-chemical calculations.

Comparison of UV/Vis methods
We have conducted comparative studies of UV/Vis measurements using three different methods:c ommon transmittance measurements in solution,s olid-stated iffuse reflectance measurements of the respective acene in am atrix of BaSO 4 (1:5) and opticalcharacterisation of the acene trapped in solid Ne at 4K.A ll absorbance spectra are summarised in Figure 1; Ta ble 1 summarises the most important absorbance bands of both species.
For both substances, the solid-state spectrum( showni n blue) is the most red-shifted, exhibiting an absorbance edge characteristic for semiconductor materials.
The solution-based absorbance spectra recorded in dichloromethane (green curves) show ad istinct vibronic progression startinga t1 6310 cm À1 for PEN and 17 330 cm À1 for DAP. These progression patterns are reproduced in the spectra of the matrix-isolated pentacenes (red curves). However,t he rigid character of the matrix combined with the low concentrations used gives much sharper bands.T he spectra reveal ad etailed fine structure that is not accessible with standard UV/Vis methods due to diverse band broadening mechanismss uch as interactions with the solvent, fluctuations in the molecular structure or aggregation.T he latter might be the reason fort he appearance of avery broad band, red shiftedtothe first intensive transition, around 14 500 cm À1 in the solution-based vis spectrum of DAP.

Pentacene
Following established procedures, [44] we varied the concentration of the analyte by regulating the flow rate of the matrix gas. An increasing neon flow rate leads to ad ecreasing concentration of the analyte in the resulting matrix. The concentration-dependent electronic absorbance spectrao fp entacene (PEN)t rapped in matrices of solid neon at 4K are shown in Figure 2a.
The absorbance of the pure solid exhibits the most red-shifted, broadest bands. With decreasing concentration,t he absorbance bands are blue-shifted, sharpened and structured. The most diluted matricese xhibit av ibronically resolved fine structurew ith afirst absorbance maximum at 18 410 cm À1 .
These data correspond well with previously reported studies on pentacenem atrices in variousm atrix materials, which assigned this band to a1 1 B 2u ! X 1 A g transition. [45] Our additional concentration-dependent spectra show as the only difference in the absorbance of the two most diluted matrices ad ecrease of the intensity of as houlder red-shiftedw ith respect to the first intense transition around1 8330 cm À1 with an increased flow of neon during deposition. This band can therefore most likely be assigned to ad imerics peciesw hose formation is statistically suppressed when using ah igherm atrix gas flow.T his explanation is supported by ac omplementary annealing experiment, which by atemporary increase of the particle mobility of the matrix should drive the statisticald istribution of species of the analyte towards the formation of energetically favoured, aggregated states such as dimers. After keeping the temperature of the matrix for 10 min at 10 K, the aforementioned red-shifted band is gaining in relative intensity and appears much more pronounced (Figure 2b), hintingt herefore at the formation of dimers, in line with the results of the concentration-dependent experiments.A t1 0K,t he neon atoms are near the sublimationp oint and very mobile, allowing thermodynamically favoured dimerisation processes to occur.Inprinci-ple, the formation of large aggregates is also possible,b ut in highly diluted matrices dimers are much more likely formed than higher aggregates. Additional experiments in which the matrix was heated up stepwise showed complete sublimation of the neon atomsa t1 2K (causing at emporary drop of the background pressure in the matrix chamber), leading to as imilar spectrum as obtained by deposition in the absence of matrix gas (see Supporting Information, Figure S5).
While the electronic absorbance spectra of the two most diluted matrices do not display further significant differences, the overall fluorescence intensity significantly decreases in the more concentrated samples,i ndependentf rom the excitation wavelength (see Figure 3f or an excitation wavelength of 514 nm). As the drastic loss of intensity is observable throughout the entire emission spectrum and not only at higher ener- [a] Additional bands below the first intensivet ransition assigned to dimerics pecies.
[b] Broad,r ed-shifted bands appearing in solution only and presumablya rising from solvent-driven aggregation or microcrystalline residuals. Figure 2. a) Electronic absorbance spectra of PEN in solid Ne after deposition for 5min with adeposition rate of 0.38 Hz s À1 .The concentration of the analyte has been screened by variation of the Ne flow rate in the range of 0mLmin À1 (leadingt oanon-diluted solid) to 5mLmin À1 .b )Electronica bsorbances pectrao fPEN in solid Ne after depositionf or 5min with aneon flow of 5mLmin À1 and adeposition rate of 0.38 Hz s À1 (blue) and after annealing at 10 Kf or 10 min (red). Both spectra havebeen recorded at 4K.
gies, at which it narrowly overlaps with the absorbance bands, ac oncentration-dependent quenching as ar esult of the absorbance of newly emitted photonsb ys urrounding molecules does not seem to be the predominant quenching mechanism. Instead, this observation may be explained by as inglet fission mechanism populatingt riplet states that undergo ad ifferent, radiationless relaxation mechanism. [46] Previous work on substituted pentacene dimers, [47] covalently linked dimeric species of substituted pentacene [48] and N-heteropentacene [49] have demonstrated the ability of acenes to undergo efficient singlet fission relaxation pathways in solution. Figure 4d isplays the fluorescences pectra resulting from the most dilutedm atrix recorded at different excitation wavelengths.I na ddition to the excitation-independento verall structure, we observe af ine structure of the emissionb ands, slightly more pronounced in the case of the lowest-energy excitation wavelength of 514 nm.
Particular attention with respect to aggregation should be paid to the photophysical behaviour of PEN after annealing. The electronic absorbance and emission spectra before and after annealing at 10 Kf or 10 min are showni nF igure 2b and Figure 4, each recorded at 4K.Quenching of the most energetic emission band system after annealing is only observed in the case of the highest-energy excitation wavelength of 476 nm, suggesting ad ifferent relaxation mechanism to be populated compared to lower-energy excitation with 496 nm or 514 nm light, for which the S 1 !S 0 transition seems to be the preferred relaxation route.
On the other hand, the second most energetic emission signal exhibitsa ni ntensity loss after annealing using the highest (476 nm) and lowest energy (514 nm) excitation wavelengths,b ut not for the mid-energy 496 nm excitation wavelength.
All fluorescences ignals sharpen after annealing of the matrices (insets of Figure 4). Features already observable before annealing afterwards appear as sharp distinct signals with FWHMso fl esst han 5cm À1 .A dditionally,a ll signals are split into am anifold, independento ft he excitation energy.C onsidering the unaltered positions of the bands before and after annealing, it seems likely that all bands are already present in the matrices after deposition, but detectable as their envelope only due to their larger FWHM. Ap ossible explanation for the detection of such an elevated number of emission bands with spacings as low as 20-30 cm À1 is am atrix effect. The slightly inhomogeneous encapsulation of the analyte molecules into the hexagonal denselyp acked neon atom lattice creates different lattice sites with slightly different interaction energies of the analyte with the surrounding medium. The band sharpening may then be ar esult of the occupation of more defined lattice sites after the reorganisation of the matrix upon increasing the particle mobility during the annealing. Ar elatede ffect may be structural distortion that is imposed during the codeposition with the noble gas-pentacene has ar elatively large backbonethat allowse asy deformation. Figure 5s hows the experimental absorbance and emission spectra of PEN alongside with the calculated spectra including the assignment of the most intense bands to the underlying  vibrational modes. Theo ptical spectra obtained from the Ne matrix can predominantly be attributed to the monomer as shown by comparison with the simulated vibrationally resolved electronic spectra (Table 2f or the contributing normal modes). The first calculated mode of the vibrational progression, with an energy as low as 265 cm À1 ,c orresponds to as ymmetric stretching mode (n s )o ft he molecule. The mode with the largest normalc oordinate displacement and as hift of 1410 cm À1 is most adequately described as scissoring [d(H 2 )] of the hydrogen atoms. However, the experimentally obtaineda bsorbance and fluorescences pectra do not entirely match the theoretically obtained ones. Some bands are not represented in the simulations (highlighted by asterisks in Figure 5).
Interestingly,c omparing the experimental spectra reveals that the same additional bands are not part of any obvious absorbance-fluorescenceb and pair.O nt he other hand, as uperpositionw ith as econd but shiftedm onomer spectrum results in the reproductiono ft he experimental spectra of pentacene (see Figure S4). As imilar occurrence in the matrix spectra was previously observed in the case of tetracene and assigned to van der Waals dimers. [44] We also assign herein the additional bands to PEN dimers, possibly in the form of Ha ggregation due to the blue shift in the absorbance, for which we are however not yet able to provide am atching structural proposal due to the computational demands.
6,13-Diazapentacene 6,13-Diazapentacene (DAP)was subjected to the same concentration-dependente xperiment (absorbance in Figure 6a,f luorescencei nF igure 7). As ystematic blue shift accompanied by ab and narrowing upon decreasing the concentration and a growth of am ost intensive absorbance signal at 17 664 cm À1 results. In contrast to PEN,a dditional red-shifted absorbance bands appear (highlighted by asterisks).
Upon annealing of the matrix (Figure 6b), the bands systematically disintegrate into sharp signals with small spacings as for PEN,i ncluding the red-shifteds maller bands, asar esult of as tructural relaxation induced by the increase of particlem obility.W eo bserve quenching of the absorbance spectrum with the exception of the additional red-shifted bands (inset of Figure 6b).
The comparison of the fluorescencei ntensities obtained with differente xcitation wavelengths before and after annealing of the most diluted matrix (Figure 8) gives as ignificantly differentp icture as in the case of the unsubstituted PEN.I n the case of DAP,p ronouncedq uenchingo ft he intensity to roughlyo ne half is observed for all appliede xcitation wavelengths. This is in line with previouss tudies on substituted pentacene derivatives with and withoutn itrogen substitution in the molecular backbone,s howings ome evidencef or an accelerateda nd therefore more competitive singlet-fissionrelaxation mechanism arising from stronger intermolecular interactions. [50] Note that one single band of the emission spectrum at 16 783 cm À1 (red-shifted by about8 80 cm À1 with respectt ot he 0-0 transition) is crucially gaining in intensity after annealing. This signalwill be discussed in more detail below.
The simulation of the vibrationally resolved electronic absorbance spectrumo fasingle DAP molecule without inclusion of environmental effects corroborates the experimental capability to obtain the electronic absorption spectrumo fasingle unperturbed DAP molecule. The vibrational progression is produced by an excitation into the first excited singlet state (S 1 ) (see Figure 9f or the spectrum and Ta ble 3f or major contributing modes,r espectively).T he first calculated mode of the vibrational progression, with an energy as low as 276 cm À1 ,c orresponds to as ymmetric stretching mode (n s )o ft he molecule. The mode with the largestn ormal coordinate displacement and ashift of 1193 cm À1 is most adequately described as ascissoring [d(H 2 )] of the hydrogen atoms. The S 1 state is character- ised as the 1 L w state, polariseda long the short axis and having al ess correlated electron-hole pair compared to its L s state (S 4 ) and its higher correlated exciton. [51,52] Similart ot he electronic absorbance spectrum,m ost features of the experimental fluorescences pectrum are equallyw ell reproduced by the simulated single unperturbed molecule spectrum ( Figure 9, Table 3).
However,s ome observed red-shifted bands (shifted by af ew hundred cm À1 ,highlightedbyasterisks in Figure 6a)are not reproduced by the single-molecule spectrum and could be assigned to the van der Waals dimer.C omparing the excitation energy and oscillator strength of the S 1 state of the energetically lowest minimum dimer structure found within at entative search (see Figure 10 and computational details) with the S 1 of the respective monomer indicates ad ecreased excitation energy from 22 036 cm À1 for the monomer to 21 480 cm À1 for the dimer,a sw ell as ad ecreased oscillator strength (dimer: 0.003, monomer:0 .111). Thus, the conformational space of dimers produces structuresd isplaying red-shifted low-intensity bands with respectt ot he monomer.T he calculated red shift of 556 cm À1 is in fair agreement with the experimental red shift of 880 cm À1 correspondingt ot he coupling constant J of the dimer.T his assignmenti ss upported by the observation of the fluorescenceb and at 16 783 cm À1 ,a ppearinga fter annealing, that has an exact analogue in the absorbance spectrum. We assign this band to aS 1 ! S 0 transition of an on-covalent dimer void of Stokes shift, in similarity to the monomer spectra. Table 2. Assignment A of the nine normalm odes (all totally symmetric, a g )w ith the respectivee nergy E and the largestn ormal coordinate displacement D with leading contributions to the shape/vibrational progressiono ft he vibrationally resolvede lectronic absorbancea nd fluorescence spectrao fPEN.

Conclusions
In summary,w ehave demonstrated the ability of the matrixisolation technique to produce high-resolution optical spectra (electronic excitation and emission) of monomeric pentacene derivatives well correlatedw ith quantum-chemical calculations on unperturbed single pentacenea nd 6,13-diazapentacene molecules free of environmental effects. The main vibrational modes leading to the progression patterns were assigned. The spectra give valuablei nsights into important vibrational modes of the ground state as wellast he excited state. Figure 6. a) Electronic absorbance spectra of DAP in solid Ne after deposition for 5min. with ad epositionrate of 0.19 Hz s À1 .The concentration of the analyte has been screened by variationo ft he Ne flow rate in the range of 0mLmin À1 (leadingt oa nu ndiluteds olid) to 5mLmin À1 .A sterisks denote bands assignedt od imeric species.Red asteriskh as ac orresponding band in the fluorescence spectrum after annealing (seeFigure8). b) Electronic absorbance spectra of DAP in solid Ne afterd eposition for 5min with aneon flow of 5mLmin À1 and adeposition rate of 0.19 Hz s À1 (blue) and after annealing at 10 Kf or 10 min (red). Both spectrahavebeen recorded at 4K.  19 Hz s À1 .The concentration of the analyte has beens creenedb yv ariation of the Ne flow rate in the range of 0mLmin À1 (leading toan undiluted solid) to 5mLmin À1 .R ed asterisks denoteaband assigned to ad imeric species having ac orresponding band in the absorbance spectra (seeFigure 6).
For both molecules, additional bands appear that could be assigned to van derW aals dimers on the basis of the response to variationsi nt he concentrationa nd to annealing experiments as well as quantum-chemical calculations. For pentacene, theseb ands in the absorbance and the fluorescence spectra that do not belong to the monomer S 1 state are assigned to H-type dimerisation leading to ab lue shift in the dimer absorbance (by 100 cm À1 )a nd aS tokes shift as ac onsequenceo fs tronge xcitonic coupling.6 ,13-Diazapentacene exhibits additional absorbance bands red-shifted to the 1 L w transition gaining in intensity upon annealing. In particular, ab and red-shifted by about 880 cm À1 from the 0-0 transition of the monomer is assigned to the 0-0 transition of the S 1 ! S 0 excitation of av an der Waals dimer,s upported by as ignificant increase of af luorescence signal at similare nergy upon annealing. Based on quantum-chemical calculations, ad imer structure is suggested with a0 -0 transition that is in fair agreement with the experimental value. Hence, in this work we provide for the first time spectroscopic evidencef or the dimers of pentacene and 6,13-diazapentacene, providing furtheru seful insights into the complex opticalp roperties of polyaromatic aggregates. [53] Thec hange of electronic characteristics upon insertion of sp 2 nitrogen into the molecular backbone is clearly reflected in the different spectralf eatures of the dimerics pecies.

Experimental Section
Experimental setup PEN was purchased from TCI Europe with ap urity of 99.999 %a nd used without further purification. DAP was synthesized according to literature procedures [54,55] and purified by recrystallisation from DMSO and sublimation. Matrix isolation experiments were conducted using standard techniques, in this form first presented by Pimentele tal. in 1954 [40] and further developedb yA ndrews, Maier et al. [56][57][58][59][60][61][62] We have describedt he setup of our matrix apparatus in detail elsewhere. [63] Preliminary calibration measurements have been conducted using as eparateq uartz apparatus to determine the evaporation rates as af unction of the applied voltage (see Supporting Information for am ore detailed description). The evaporation of the substances has been carried out with aw ater-cooled Knudsen-type effusion cell containing ag raphite tube inside ac eramic unit which was heated by applying ad efined voltage to a surrounding Ta coil. The substances were co-deposited with Ne (Air Liquide, 99.999 %) to create matrices on aR h-coated Cu surface which was held at 4Kusing ap ulse-tube cooler( Vericold) and ac losed-cycle helium cryostat (Leybold). During deposition,t he flow of the Ne gas was kept constant with af low controller (EL-FLOW,B ronkhorst).V is spectra were recorded with aB ruker Vertex 80v spectrometer with ar esolution of 1cm À1 ,w ith at ungsten lamp, aC aF 2 beam splitter and aS id iode detector. Fluorescence spectra were obtained with aS ymphony II charge-coupled device (CCD) detector( Horiba) using ab inning factor of 1a fter excitation with an Ar laser (Innova 90c-A3;Coherent).

Computational details
The calculation and subsequent simulation of vibrationally resolved electronic spectra (absorption and fluorescence) werep erformed using at ime-dependent approacho fa ni ndependent mode displaced harmonic oscillator modeld erived by Heller as implemented in the ORCA software package. [64,65] Kohn-Sham DFT using the B3LYP functional and the def2-TZVP basis was used for ground and excited state geometry optimisations with subsequent frequency calculations as well as for the calculation of excited states via linear-response time-dependent DFT (TDDFT) with the Ta mm-Dancoff approximation employed. [66][67][68] Calculations of the DAP dimer structures were performed using as tarting grid of multiple dimer conformations first optimized on aP BEh-3c level of theory. The minimum structuresw ere then refinedo nawB97X-D3/def2-TZVPP level of theory. Figure 9. Comparison of the experimental (Ne matrix at 4Kafterd eposition for 5min at ad epositionrate of 0.19Hzs À1 and an eon flow of 10 mL min À1 )a nd calculated (B3LYP-D3BJ/def2-TZVP) vibrationally resolvede lectronic absorbancea nd emission spectra of DAP.T he computational spectraw ere shifted by 2080 cm À1 (absorbance)a nd 2051 cm À1 (fluorescence) to match the 0-0t ransition in the experimental data. The most dominant vibrational modesa re numbered and assigned in Ta ble 3( see SupportingInformation for af ull list). Table 3. Assignment A of the nine normalm odes (all totally symmetric, a g )w ith the respectivee nergy E and the largestn ormal coordinate displacement D with leading contributions to the shape/vibrationalp rogression of the vibrationally resolved electronic absorbance and fluorescence spectra of DAP.