Multi-functional dithiadiazolyl radicals - American Chemical Society

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Multi-functional dithiadiazolyl radicals: Fluorescence, electroluminescence and photo-conducting behavior in pyren-1#-yl-dithiadiazolyl. Yassine Beldjoudi, Mitchell Nascimento, Yong Joo Cho, Hyeonghwa Yu, Hany Aziz, Daiki Tonouchi, Keitaro Eguchi, Michio Matsushita, Kunio Awaga, Igor O. Osorio-Roman, Christos P. Constantinides, and Jeremy M. Rawson J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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Journal of the American Chemical Society

Multi-functional dithiadiazolyl radicals: Fluorescence, electro-luminescence and photo-conducting behavior in pyren1′-yl-dithiadiazolyl. Yassine Beldjoudi,¥ Mitchell A. Nascimento,¥ Yong Joo Cho,$ Hyeonghwa Yu,$ Hany Aziz,$ Daiki Tonouchi,¶ Keitaro Eguchi,¶ Michio M. Matsushita,¶ Kunio Awaga, ¶ Igor Osorio-Roman,¥ Christos P. Constantinides† and Jeremy M. Rawson*¥ ¥

Department of Chemistry & Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor, ON, N9B 3P4, Canada. Email: [email protected] $ Department of Electrical & Computer Engineering, Waterloo Institute of Nanotechnology, University of Waterloo, 200 University Ave. W., Waterloo, ON. N2L 3G1, Canada. ¶ Department of Chemistry & Integrated Research Consortium on Chemical Sciences (IRCCS), The University of Nagoya, FuroCho, Chikusa-Ku, Nagoya City, Aichi 464-8602, Japan. † Department of Chemistry, NC State University, 2620 Yarbrough Drive, Box 8204, Raleigh, NC 27695 KEYWORDS. Radical, dithiadiazolyl, fluorescence, electroluminescence, OLED, Mott Insulator, Photoconductivity, Pyrene.

ABSTRACT: The pyren-1′-yl-functionalized dithiadiazolyl radical, C16H9CNSSN (1) is monomeric in solution and exhibits fluorescence in the deep-blue region of the visible spectrum (440 nm) upon excitation at 241 nm. The salt [1][GaCl4] exhibits similar emission, reflecting the largely spectator nature of the radical in the fluorescence process although the presence of the radical leads to a modest quenching of emission (ΦF = 98% for 1+ and 50% for 1) through enhancement of non-radiative decay processes. TD-DFT on 1 coupled with the similar emission profiles of both 1+ and 1 are consistent with the initial excitation being of predominantly pyrene π−π* character. Spectroscopic studies indicate stabilization of the excited state in polar media with a fluorescence lifetime for 1 (τ = 5 ns) indicative of a short-lived excited state. Comparative studies between the energies of the frontier orbitals of pyren-1′-yl nitronyl nitroxide (2) (which is not fluorescent) and 1 reveal that the energy mismatch and poor spatial overlap between the dithiadiazolyl radical SOMO and the pyrene π manifold in 1 efficiently inhibits the non-radiative electron-electron exchange relaxation pathway previously described for 2. Solid-state films of both 1 and [1][GaCl4] exhibit broad emission bands at 509 and 545 nm respectively. Incorporation of 1 within a host matrix for OLED fabrication revealed electroluminescence with CIE coordinates of (0.205, 0.280) corresponding to a sky-blue emission. The brightness of the device reached 1934 cd/m² at an applied voltage of 16 V. The crystal structure of 1 reveals a distorted π-stacked motif with almost regular distances between the pyrene rings but alternating long-short contacts between dithiadiazolyl radicals. Conductivity measurements on a single crystal of 1 revealed photoconducting behavior.

Introduction In the last two decades, the emergence of new technologies based on efficient, cheap, and flexible organic materials has led to the development of a range of molecule-based materials such as organic light-emitting diodes (OLEDs),1 organic photovoltaic cells (OPV),2 memory cells3 and organic lasers.4 Amongst these materials poly-aromatic hydrocarbons (PAHs) have attracted attention due to the diverse nature of their materials properties including low bad gap semi-conductors, photoconductors and highly efficient fluorescence.5 The opportunity to tune the electronics of the PAH through incorporation of electron donating or withdrawing substituents or inclusion of functional substituents that can control the supramolecular arrangements of molecules in the condensed phase provide opportunities to optimize performance.6 For example, careful control of the π−π interactions between pyrene molecules via crystal

engineering has led to the emergence of conducting and photoconducting properties as the result of exciton migration through the lattice followed by charge separation at the molecule–electrode interface.7 More recently, hydrogen-bonded dynamic π-stacking assemblies derived from pyrene have been reported to present multifunctional properties, fluorescence, ferroelectricity and switching of electron transport properties.8 In addition, excimer fluorescence within pyrene derivatives has been reported to give rise to strong green emitters in condensed phases, as well as blue emitters desirable for OLED devices.9 Stable thiazyl radicals have been implemented in molecular materials chemistry as building blocks for neutral radical conductors (NRCs) exhibiting conductivities up to 102 S/cm,10 as building blocks in the design of some of the

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highest TC organic magnets11 and as paramagnetic ligands in coordination chemistry.12 In this context we were interested to explore the combination of a PAH with a paramagnetic radical as a move towards multi-functional materials in which the physical properties of the substituent PAH unit may be capable of interacting independently of, or indeed cooperatively with, such free radical substituents. In many cases the presence of a free radical has been shown to quench fluorophore emission through electron or energy transfer processes.13 Conversely free radicals have been proposed as an alternative strategy through which to enhance the quantum yield in OLEDs.14 In such cases the radical is itself the fluorophore and relaxation of the excited doublet state has been proposed to be 100% efficient whereas fluorescence from conventional diamagnetic fluorophores only occurs through the excited singlet state (25%) rather than the triplet (75%) where relaxation to the singlet ground state is spin forbidden. Peng et al. reported14 the triarylmethyl-carbazole radical was found to be a red emitter in solution with the fluorescence completely quenched in the solid state but found that doping the radical in a matrix of an OLED device offered both photoluminescence and electroluminescence in the red region of the visible spectrum. Recently we reported the synthesis of a dithiadiazolyl (DTDA) radical bonded to a phenanthrene PAH substituent which exhibited rather modest emission in solution (ΦF = 0.11).15 We found that this material was also emissive in composite films with both PMMA and PS, which augured well for potential applications in OLED devices in which the emissive material is typically blended with a hole transporter in the electroluminescent layer. In the current paper we describe the synthesis and characterization of pyren-1′-yl-DTDA (1, Scheme 1), which offers superior emission in solution to phenDTDA, and contrast it with the closely-related but non-emissive nitronyl nitroxide radical, 2 (Scheme 1).16 We use theoretical studies to rationalize the difference in emissive behavior of 1 and 2. We additionally describe the construction and emissive properties of a prototype OLED based on 1 as well as report the solidstate structure and photoconducting behavior of 1.

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netic properties.17 Significant exceptions comprise the work of Preuss who has developed a series of derivatives such as 2′-pyridyl, 2′-pyrimidinyl and benzoxazolyl-DTDA radicals as chelating ligands for coordination chemistry and recent studies by Haynes on complexes of the 4’pyridyl-DTDA system.12 As an extension of this chemistry we have recently begun to explore non-innocent substituents in order to incorporate additional functionality for the formation of multifunctional materials in which the electronics of the DTDA radical are coupled with the physical properties of the active substituent. The majority of DTDA radicals are prepared from the corresponding nitrile utilising a standard procedure in which the first step is reaction of the nitrile with Li[N(SiMe3)2] to form an intermediate amidinate.18 Using this approach we recently reported the preparation of the phenanthreneDTDA radical in which the fluorescence of the phenanthrene group was coupled to the redox-active DTDA ring.15 We found that redox switching of the DTDA ring from diamagnetic cation to paramagnetic radical led to a reduction of the quantum efficiency from 35 to 11%. As an extension of these studies we targeted the pyrene derivative 1 which we anticipated to have a higher quantum efficiency. Unfortunately the standard synthetic methodology failed due to reduction of the pyrene functional group by Li[N(SiMe3)2]. Similar behaviour was observed in the reaction of Li[N(SiMe3)2] with anthracene carbonitrile, affording the anthracene carbonitrile radical anion.19 To circumvent this redox process we utilized an alternative synthetic procedure (Scheme 2) which has previously been used to prepare sterically encumbered DTDAs such as 2,4,6(CF3)3C6H2CNSSN.20 Using this strategy, 1 was successfully prepared from 1-bromopyrene in 36% yield with crystals isolated by vacuum sublimation. In order to examine the effect of the redox state of the DTDA ring on the optical properties of the pyrene functionality we additionally prepared the diamagnetic salt [1][GaCl4] through reaction of intermediate [1]Cl with GaCl3 (Scheme 2). n-BuLi R Br

–78 °C, Et2O

Me 3SiN=C=NSiMe3 R Li

NSiMe3 R

RT, 24h, Et 2O

Li

Ag powder THF

N S

SCl2 (2.2 eq) RT, 24h

NSiMe3

R

Cl N S

N S R N S

N S R

Cl N S GaCl 3 THF

N S R

GaCl 4 N S

Scheme 2: Synthesis of 1 and [1][GaCl4] (R = 1′-pyrenyl) Scheme 1: Fluorophore-functionalized radicals 1 and 2 Results Previous studies on DTDA radicals have predominantly focused on simple aryl-functionalized derivatives (bearing alkyl, alkoxy, halo, cyano, nitro groups etc) in which functional groups have been implemented to direct the solid state structure to optimize their charge transport or mag-

Previous EPR and UV/vis spectroscopic studies on DTDA radicals reveal the dimerization energy is low (ca. 35 kJ.mol-1) such that at low concentration dissociation is essentially complete at room temperature, especially in dilute solution.21 For 1 a diagnostic 1:2:3:2:1 pentet was observed in the solution EPR spectrum consistent with a DTDA-based radical (Figure. S1).22 Electrochemical studies on 1 in solution reveal a reversible 1e- oxidation to form 1+ with a redox potential E1/2 = +0.60 V vs Ag/AgCl (Figure 2


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S2), similar to other DTDA radicals.23 Under reducing conditions a quasi-reversible 1e- reduction occurs at E1/2 ~ −1.6 V. This is comparable with other DTDA/DTDA− redox couples (~ −1.5 V vs Ag/AgCl)23 whereas pyrene itself only undergoes a one electron reduction to form the respective radical anion at – 2.13 V vs Ag/AgCl.24 Therefore both 1eoxidation and reduction processes associated with radical 1 are DTDA-based consistent with the DTDA SOMO lying energetically between the pyrene HOMO and LUMO. Solution Spectroscopic Properties: The optical properties of 1 and [1][GaCl4] were studied by absorption and fluorescence spectroscopies in MeCN and compared with those of the starting material, 1-bromopyrene (3). All three compounds were studied in very dilute solutions (≤10-6 M) to avoid excimer formation and the spectroscopic results summarized in Table S2. Solution UV/visible absorption spectra of all three compounds (4 × 10-7 M in MeCN) reveal an absorption profile dominated by the pyrene functionality. This comprises three well-defined sets of bands centered at 241, 277 and 340 nm which are associated with the S4 ← S0, S3 ← S0 and S2 ← S0 transitions.25 The effect of the redox state of the heterocyclic substituent appears to have little effect on the absorption spectra of 1 and 1+, although the transition intensities associated with the pyrene functional group do differ. The difference between 1 and 1+ is the additional electron generating a singly occupied MO (SOMO) in 1. This SOMO is localized on the DTDA ring and nodal at C. Previous work on a series of arylfunctionalized DTDAs has shown that substituents on the DTDA ring have very limited effects on (i) the spin distribution in DTDA radicals and (ii) the redox behavior of the DTDA/DTDA+ redox couple, reflecting limited electronic communication between the SOMO and the substituent.23 We conclude that the localized nature of the SOMO means that different electron-withdrawing/releasing groups do not significantly perturb the electronics of the SOMO. The corollary is that changes to the redox state of the DTDA ring likely have little effect on the substituent. Gas phase TD-DFT studies on both 1 and 1+ revealed similar bands in the 200 – 400 nm region but with slightly more variation in intensity and position than those observed experimentally (Fig. S4). We then examined the emission behavior of 1 and found that irradiation at 241, 279 and 340 nm all led to a broad intense emission at λmax = 440 nm. An intense emission occurred upon excitation at 241 nm with a quantum yield of 50% (referenced to diphenyl anthracene Φ = 0.88 in ethanol),26 (Fig. 1, Fig. S3 and Table S2). Relaxation can occur via a series of radiative and nonradiative processes and we can write a rate constant for the overall relaxation (kr) as: kr = kf + ki + kvib + …. = kf + knr

Eq. 1

where kf is the rate constant for fluorescent decay, ki is the rate constant for internal conversion, kvib is the rate constant for vibrational relaxation etc. The non-radiative processes can be incorporated within an effective knr which

accommodates all non-radiative decay processes. The quantum yield is therefore the rate constant for fluorescence in relation to the overall rate of decay to the ground state via all processes: ΦF = kf/(kf + knr) = kf · τ

Eq. 2

Data from fluorescence decay studies on 1 and 1[GaCl4] are summarized in Table 1. The fluorescence lifetimes for both 1 (5.0 ns) and 1+ (4.1 ns) are considerably shorter than pyrene itself (τ > 100 ns).27 The decrease in quantum efficiency of paramagnetic 1 in relation to closed shell 1+ reflects a more effective non-radiative decay process in 1. Calculations based on the fluorescence lifetimes and quantum yields reveal the fluorescence decay rates in both 1 and 1+ are nearly identical but the reduced quantum efficiency in 1 arises from the emergence of a substantially more effective non-radiative decay pathway (Table 1). Table 1. Excitation and emission parameters for 1 and 1[GaCl4] including relaxation times, radiative and nonradiative rate constants. Compound λexc (nm) λem (nm) τ (ns) kf (s-1) Φ knr (s-1) 1 241 440 5.0 1.0 × 108 0.50 1.0 × 108 1[GaCl4] 241 440 4.1 2.4 × 108 0.98 4.9 × 106

Figure 1. (Left) Excitation (black curve, λemission = 440 nm) and emission (red curve, λexc = 241 nm) profiles of 1 (4 × 10-7 M in MeCN) with (inset) the fluorescence of 1 under UV irradiation; (right) fluorescence decay of 1 following excitation at 340 nm (τ = 5.0 ns). To understand the effect the DTDA radical has on the quantum yield, we compared the emissive behavior of 1 with other PAH-based radicals. Previous studies have shown that nitroxide or nitronyl nitroxide radicals lead to efficient fluorescence quenching for both organic fluorophores16 and quantum dots.28 Green and coworkers found that efficient fluorescence quenching occurs when the radical-fluorophore interaction distances were in the 4 – 6 Å region.13a,b The remarkable efficiency of emission quenching by radicals has led to the emergence of dual fluorophore-radical compounds which have been applied successfully in bioanalysis and environment assays as radical traps and chemo-sensors. These operate via a nonemissive radical/fluorophore system whose emission can be ‘switched on’ when the radical character is quenched. This has been used in the detection of antioxidants at subµM concentrations.16,29 Alternatively Cu2+ ions have been 3



shown to switch on emission through metal coordination to the radical fluorophore in which strong antiferromagnetic metal-radical exchange coupling likely gives rise to a singlet ground state.30 In the current context, it is noteworthy that attachment of a nitronyl nitroxide radical directly to the pyrene fluorophore in the 1-position (radical 2) or via a heterocyclic or non-conjugated spacer have been shown to lead to complete quenching of emission.16,29c It was therefore surprising to observe incomplete quenching of fluorescence in 1, despite the radical being directly bonded to the fluorophore. This indicates other factors influence radiative and non-radiative relaxation processes other than the distance between the fluorophore and the radical center. The electrochemical, computational and EPR studies all reveal that the SOMO is of DTDA character with the pyrene π* orbitals higher in energy. TD-DFT studies (vide infra) indicate the initial excitation occurs within the pyrene manifold (D0 Dn, Fig 2). We recognize a range of possible non-radiative decay processes may occur (Fig. 2). In the electron transfer mechanism (Fig. 2a), after initial excitation the fluorophore β spin could then relax via intramolecular electron transfer to the lower lying radical β orbital, followed by a subsequent intramolecular electron transfer of the radical β-spin back to the pyrenyl β-HOMO, regenerating the ground state. In this case the electronpairing energy associated with placing an additional electron on the DTDA ring is significant and approximates to |Eox – Ered| ~ 2.2 V and would likely inhibit this as an effective non-radiative relaxation pathway. An alternative lower energy non-radiative relaxation pathway which has been established for fluorescence quenching by radicals involving electron exchange between the radical and the fluorophore (Fig. 2b).29 Initial excitation generates an open shell singlet excited state for the fluorophore, coupled to the doublet radical, generating a doublet configuration (Dn). Electron-exchange between the excited state fluorophore and the radical doublet affords a fluorophore triplet coupled to a radical doublet without a change in overall spin state (D1). The resultant fluorophore excited triplet (coparallel electrons) is stabilized over the initial excited singlet fluorophore configuration. Radiative decay from D1 to D0 within the pyrene fluorophore corresponds to a spinforbidden triplet-singlet relaxation and the alternative non-radiative decay process (D0  D1, Fig. 2b) via electron exchange is likely. If either of the two electron exchange processes is inefficient then non-radiative relaxation processes are switched off and fluorescence is likely. Both electron exchange and electron-transfer mechanisms involve transfer of electrons between pyrenyl and DTDAbased orbitals which become efficient when there is both good energy match and good spatial overlap between the orbitals involved. In order to examine the relative efficiencies of this decay process for 1 and 2, we undertook a series of computational studies. Theoretical studies: Time-dependent DFT (UB3LYP/6311G*+) calculations reproduce well the salient features of the UV/visible absorption spectrum of 1 (Figure S7). The calculated lowest energy transition at 689 nm corresponds to promotion of the unpaired α electron of DTDA π-

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character to the SOMO+1 orbital (of S-S σ* character), equivalent to the HOMO-LUMO transition for a closed-shell system. The oscillator strength for this transition is, however, very lowand so the lowest energy transition does not contribute significantly to the absorption profile. Several transitions with high oscillator strengths were computed in the vicinity of 241, 279, 340 nm and correspond to transitions which give rise to the observed fluorescence. An examination of each of these transitions revealed several configurations contribute to these transitions. Notably these transitions involve components from both α and βspins (i.e. ‘spin up’ and ‘spin down’ electrons) whose orbital wavefunctions are essen tially identical, i.e. these intense transitions can be consid(a) Electron transfer mechanism

(b) Electron exchange mechanism

Figure 2: Non-radiative decay mechanisms; (a) electron transfer and (b) electron-exchange. 4


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-ered to constitute a rearrangement of electron pairs associated with the π-systems of both pyrene and DTDA but which do not involve the radical SOMO. This is consistent with the similarity in absorption and particularly emission profiles for both 1 and 1+ (Figure S3).

2 1 Figure 3: One-electron frontier orbitals of the ‘spin-up’ α electrons of 1 (right) and 2 (left) based on UB3LYP/6311G*+ calculations. In order to decipher the discrepancy in behavior between the emissive DTDA radical 1 and non-emissive nitronyl nitroxide derivative 2, we undertook comparative theoretical studies to probe the efficiency of the D0  D1 electronexchange process (Fig. 2b, dashed red arrows). For 2 the α-HOMO comprises a localized NN-based orbital whereas the α-LUMO is predominantly pyrene based (Figure 3). Thus the α-HOMO/α-LUMO energy gap clearly reflects the smallest possible energy difference for α spin relaxation (3.12 eV for 2). In addition there is some limited spatial overlap of these two orbitals in the NN region, facilitating non-radiative relaxation (dashed orange arrow, Fig. 3). For 1 the α-HOMO is similarly radical-based (Figure 3) but the α-LUMO and α-LUMO+1 have significant DTDA S-S σ* character. The α-LUMO+2 orbital is the first truly pyrenebased MO. The resultant large energy gap (4.39 eV) between the α-HOMO and the α-LUMO+2 coupled with limited spatial overlap between these orbitals clearly disfavors the non-radiative decay process (dashed orange arrow, Figure 3). OLED fabrication Our recent studies revealed that composites of phenDTDA with both poly(methyl methacrylate) and poly(styrene) could be prepared in which fluorescence was maintained,15 augering well for testing in device applications. Preliminary studies also revealed that 1 formed composite films with PMMA and PS (Figure S11). OLED devices constructed from open shell organics are few.14,31 With 1 exhibiting superior solution fluorescence properties to phenDTDA, we prepared a prototype organic light-emitting diode OLED incorporating 1 as the emissive component. The design of the device structure implemented DFTcomputed orbital energies for the DTDA frontier MOs,

permitting the other materials for device fabrication to be selected from an existing library of components. The structure and relative energy levels of the different components of the OLED device are shown in Figure 4. The HOMO and LUMO (3.2 eV) of the hole transport material CBP (4,4′Bis(N-carbazolyl)-1,1′-biphenyl) and 1,3-bis(3,5-dipyrid-3yl-phenyl)benzene (BmpyPb) (2.7 eV) were obtained from the literature data based on UPS and optical measurements on thin solid films of CBP.32,33 For the emissive component 1 the energies of the frontier orbitals were determined by DFT comprising the SOMO (α – HOMO) (5.58eV) and SUMO (β-LUMO) (3.29 eV). The OLED comprised five thin film layers coated sequentially on a glass substrate (Figure 4, left). The ITO, which acts as a hole-injecting contact (anode), was coated with a layer of PEDOT:PSS (poly(3,4ethylenedioxy-thiophene):poly-styrene sulfonate) that serves as a hole transport layer, followed by a ~30 nm thick layer of CBP, doped at different ratios (0.5 wt % and 10 wt %) with 1, to serve as the electroluminescent layer (EL). On top of this layer, an electron transport and holeblocking layer of BmpyPb, (~40 nm thick), followed by LiF/Al (1 nm and 80 nm, respectively) were added together forming an electron-injecting contact (cathode).34 The layers were coated on ITO-coated glass substrates that were sonicated in acetone and isopropanol for 5 minutes each, in respective order, followed by CF4:O2 (1:3) plasma treatment for 5 minutes. The hole transport layer and the EL were made by spin-coating on the ITO substrates whereas the BmpyPb, LiF and Al were thermally evaporated at a rate of 1-2 Å/s at a base pressure of 5 × 10-6 torr.

Figure 4: (Left) Schematic diagram showing the layers and materials utilised in the OLED device (relative thicknesses not to scale). (Right) The device configuration showing the energy level of the different components in the OLED device as well the position of the carriers at the interfaces. Under forward bias (i.e. an external voltage that puts the ITO anode at a more positive electrical potential relative to the Al cathode), holes and electrons are injected from the anode and cathode, respectively, into the charge transport layers, and meet in the EL layer (1/CBP layer) where they recombine forming excitons that then decay radiatively producing light. Figure 5 shows the electroluminescence of 1 within an OLED device at 10 wt % loading of 1 in the EL layer, revealing a maximum emission at 492 nm.

5



Figure 5: EL spectrum of 1 in an OLED device at 10 wt % doping; (inset emission from the OLED). In solution, monomers of 1 are characterized by strong deep blue emission centred at 440 nm under photoexcitation, whereas solid state emission at 509 nm arises from the excimer due to the strong π−π overlapping of the pyrene moieties along the stacking direction (vide infra). The EL spectrum of 1 at 492 nm is therefore indicative of exciplex formation within the EL layer. The two sharp features around 390 and 420 nm are associated with a small degree of degradation of 1 during fabrication. Figure 6a shows the current density and luminescence vs voltage characteristics of the OLED, showing that significant current injection and luminance levels are reached at voltages in excess of 10 V. Although this voltage is somewhat higher relative to that observed with other pyrene-based OLEDs,35 it is known that dopant molecules can function as deep traps for charge carriers in the EL layer causing an increase in the driving voltage,35 suggesting that 1 participates in increasing the threshold voltage for EL. Notably the brightness increases as a function of applied voltage to reach almost 2000 cd/m² at 16 V, the upper limit of the measurements made during device testing. Figure 6b shows the EQE (external quantum efficiency) as a function of luminance, with a maximum EQE of 0.45% with respect to luminance (Fig. 6b) for a 10% doping of 1 in the EL layer. A plot of EQE as a function of current revealed a maximum EQE of 1.4% for a 10% loading of 1 (Fig. S18). The increase in luminance current efficiency characteristics of the OLEDs (with 0.5 wt % and 10 wt % dopant) as well as EQE behaviour show that the increase of concentration of 1 within the EL layer leads to a higher efficiency confirming that the electroluminescence of the device arises from 1.

Figure 6: (a) Current density-voltage (J-V) and luminescence-voltage (L-V) characteristics measured for the OLED device at 10 wt % of dopant; (b) Plots of the External Quantum Efficiency (EQE) as a function of luminance for the device at 0.5 wt % dopant and 10 wt % dopant.

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Solid-state structure: The crystal structure of 1 comprises two molecules in the asymmetric unit which form a cis-oid dimer with intra-dimer S···S contacts of 3.138(2) and 3.249(2) Å (Fig. 7a). Such multi-centre ‘pancake bonding’ interactions between radicals are common in these systems and afford a singlet ground state configuration.36 These dimers pack parallel to the crystallographic a-axis, affording a Peierls-distorted structure, similar to other DTDA radicals.15,37 The inter-dimer S···S distances along the stacking direction are 4.131(2) and 4.230(2) Å. Although the DTDA stacking is significantly distorted the distance between the pyrenyl rings along the stacking direction is much closer to regular with the closest C···C contacts in the range of 3.280(7) – 3.397(7) Å. Inter-stack contacts comprise electrostatically favorable Sδ+···Nδ− (3.189(5) - 3.374(4) Å) and dispersion driven S···S contacts (3.259(2) – 3.280(2) Å) leading to the formation of a layer-like structure in the bc-plane (Fig. 7b).

(a)

(b)

Figure 7. Crystal structure of the dimer (1)2 viewed (a) perpendicular to the π-stacking direction and (b) parallel to the π-stacking direction to emphasize inter-stack contacts. Solid state emission: Pyrenes have a strong tendency to form π−π interactions in concentrated solutions,38 within polymers39 or in solid state.40 Pyrene dimers are known to display relatively efficient fluorescence at longer wavelengths (usually 480 – 500 nm).41 As described earlier, in many cases radicals efficiently quench emission entirely.2830 In a smaller cross-section of systems such as triaryl methyl radicals, the fluorescence arises from excitation and relaxation of the unpaired electron.42 In such cases these radicals have been reported to fluoresce in solution or when doped into host matrices but the emission vanishes completely in neat solid materials.14,42 In 1 the process of π*-π* dimerization between radicals effectively quenches the sample paramagnetism which augured well for continued emission in the solid state. In order to test the solidstate photoluminescence of 1, solid films were prepared by drop-casting onto quartz slides under inert conditions. The solid state absorption profile of 1 (Fig. 8) is broad and comprises two bands with maxima at 375 and 523 nm. 6


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Previous solution UV/vis studies on 4-(2ʹ-pyridyl)-1,2,3,5dithiadiazolyl radical revealed the dithiadiazolyl dimer displayed a concentration dependent absorption band at 475 nm due to the HOMO–LUMO transition within the dimer21c and we assign the lower energy absorption at 523 nm to DTDA dimer based transitions and the higher energy absorption to transitions within the pyrene π-manifold. Confirmation of this assignment was undertaken through TD-DFT studies on singlet (1)2 which revealed a low energy absorption within the π manifold of the DTDA dimer (535 nm) and a higher energy set of transitions within the pyrene π-system (Fig. S7). From the experimental spectra we estimate the optical band gap to be ca. 2.09 eV for the DTDA dimer based on the absorption onset.43 Extrapolation of the higher energy absorption gives a second band gap for the pyrene unit around 2.5 eV. On irradiation in the solid state 1 exhibited a broad emission centered at 509 nm (Figure 8). The excitation spectrum giving rise to the 509 nm emission is associated with a series of absorptions with a maximum around 375 nm (3.30eV), falling in the region expected for pyrene-based excitations. The significant red-shift of both the excitation and emission bands in relation to monomeric 1 in solution is consistent with excimer-like emission from the pyrene group which is well known to form face-to-face aromatic π−π stacking interactions.9b (a)

ture of the DTDA moiety rather than the π-stack of the pyrenyl unit. This behavior is comparable to photo-conducting effects recently observed in thin films of the DTDA biradical, (4,4bis(1,2,3,5-dithiadiazolyl).45 This diradical was an insulator in the dark with a room temperature conductivity of approximately 10−9 S cm−1 but the conductivity was enhanced (on/off gain of 1.8 ×102) under excitation with a green laser (2.33eV, 532 nm, light intensity = 1.59 mW cm−2).

Figure 9: Photocurrent measured on a single crystal of 1 at room temperature (Constant applied voltage V = 50 V, White LED light Po = 30 mW cm-2).

Discussion

(b)

Figure 8. (left) Solid-state absorption and (right) excitation (black line, λem = 509 nm) and emission (red line, λexc = 375 nm) profiles for a thin film of 1. Solid-state transport properties: The transport properties of 1 were investigated on a single crystal connected with a pair of gold wires by gold paste. The resistivity of the crystal at room temperature was evaluated to be ρ = 3.5 × 109 Ω˖cm from the obtained resistance of R = 4.4 × 1012 Ω and the effective crystal size between the electrodes (ca. 0.5 × 0.2 × 0.01 mm). The high resistance of 1 is not unexpected for such strongly Peierls-distorted structures and comparable with other pristine DTDA derivatives.44 On the other hand, this crystal shows clear photo-conducting behavior upon light-irradiation with an LED lamp. The conductivity enhancement of 1.3 × 102, suggests photo-excitation of charge carriers and a resultant moderate carrier mobility in this crystal (Fig. 9). The LED lamp exhibits two maxima at λmax = 455 nm, 520 nm (Figure S17). Notably the lamp has no intensity below 420 nm precluding excitation within the pyrene π-manifold, whereas the 520 nm is well matched to the absorption of the DTDA dimer (λmax = 523 nm), indicating that the photoconducting response arises from excitation within the band struc-

Fluorophores bearing organic radicals such as nitroxide (N) or nitronyl nitroxide (NN) radicals directly attached to the fluorophore have been extensively studied as dual radical-fluorophore materials which can either be EPR active or display luminescence properties depending of the oxidation state of the N or NN moieties.28-30 On the other hand, other studies have utilized such compounds in the design of spin polarised materials with photo-tunable spin properties.46 Teki et al. extensively investigated the effect of photo-excitation on the spin state of the π system of anthracenyl-functionalized nitroxides, identifying the potential for exchange-coupling between the anthracenyl triplet excited state and the nitroxide radical (Scheme 4).47 In this context radicals such as 1 augur well for the development of multifunctional materials that can display both luminescent and photo-tunable magnetic properties.

Scheme 4: Spin alignment in the doublet ground state and the quartet excited state of an anthracenyl-functionalized nitroxide radical. In the ground state, the system behaves as a simple S = ½ paramagnet whereas in the photoexcited state ferromagnetic exchange between the anthracene triplet and the nitroxide doublet affords a quartet excited state.

7



The transport properties of pristine DTDA radicals and their heavier selenium DSDA analogues are well established.10,37 For those which adopt π-stacked structures,37 the presence of a Peierls distortion is manifested in shortintra-dimer S…S contacts and longer inter-dimer S…S contacts in which the band gap is typically large leading to poor semi-conductors with large activation barriers. Indeed the propensity for DTDA rings to adopt π-stacked structures can be considered as a supramolecular synthon which can be used to tailor the packing of the polyaromatic fluorophore (pyrene, phenanthrene15) units. This is important given the solid-state fluorescence transport properties depend not only on the chemical nature of the constituent molecules, but also on the solid state architecture. Thus while the DTDA radical may give rise to some reduction in fluorescence intensity in solution, radical dimerization in the solid state can be utilized as a structuredirecting group to facilitate formation of emissive excimers. The current electrochemical and computational studies on 1 reveal that the SOMO of the DTDA is electronically well separated from the frontier orbitals (HOMO and LUMO) of the pyrene framework such that (i) the redox behavior reflects addition of electrons to and removal of electrons from the DTDA heterocycle and (ii) there is a large energy gap between the pyrene frontier orbitals. Extending this to the solid state generates a multi-band structure in which there exists a composite set of pyrene and DTDA bands. Solid state UV/vis spectra on 1 reveals two ‘edges’; the first corresponding to a large band gap (2.5 eV) associated with the pyrene functionality with a narrower band gap (2.1 eV) arising from the DTDA distorted π-stack. Excitation at 375 nm (3.31 eV), in the middle of the pyrene band, gives rise to intense excimer emission indicative of pyrene-based exciton migration through the lattice. Notably there is negligible pyrene emission on excitation above 440 nm. Conversely lower energy photo-excitation (λ > 420 nm, precluding significant excitation in the pyrene band) offers a photoconducting response. We tentatively assign this to excitation within the DTDA manifold, based on (i) the limited excitation within the pyrene manifold at this wavelength; (ii) the strong absorption within the DTDA π-stack and (iii) previous studies showing photoconduction in the related DTDA radial upon excitation at long wavelength. Further studies are on-going in our laboratories to examine approaches to optically manipulate the electronics of these multi-band materials. Conclusions Radical 1 is a rare example of a fluorescent open shell organic molecule with deep blue emission in solution with a quantum yield of 50% and is, to our knowledge, the first example of a radical to exhibit solid state emission. Computational studies reveal the inefficient quenching of the emission appears linked to the low-lying nature of the radical SOMO which leads to poor energy-match and overlap between the pyrenyl-based π* excited state and the radical SOMO which partially switches off the electron exchange non-radiative decay mechanism. We have successfully fabricated and tested the first DTDA-based OLED device

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architecture and observed blue emission, albeit with high threshold voltages. In addition 1 exhibits a multifunctional response depending on the wavelength of irradiation. Lower energy irradiation reveals photoconducting behaviour whereas higher energy wavelengths drive an emissive response. The potential to tune the band structure (energy and band gap) of the PAH functional group in relation to the DTDA band offers future potential for generating photoexcited states through excitation within the DTDA band (low energy), within the PAH band (high energy) or to potentially drive charge-transfer transitions between bands. Further studies are underway. Experimental A.

Material and Methods

1-Bromopyrene was purchased from Alfa-Aesar Company. n-butyllithium, silver powder, sulfur monochloride and trichloro(1H,1H,2H,2H-perfluorooctyl)silane, were used as supplied (Sigma-Aldrich). Sulfur dichloride (SCl2) was prepared from sulfur monochloride (S2Cl2) according to the literature method.48 Dry solvents; tetahydrofuran (THF), diethylether (Et2O), acetonitrile (MeCN) (Sigma-Aldrich) were used without any further purification. Gallium trichloride (Strem Chemicals) was used as received. Poly(methyl methcrylate) (PMMA, 120,000 MW) was purchased from Sigma and vacuum dried at 50 °C for 18 h before use. Drop-cast polymer matrices employed a hydrophobic glass surface and Teflon former (Johnston Plastics, Toronto, Canada), machined in house. EPR spectra were recorded on a Bruker EMXplus EPR spectrometer at room temperature. All UV-visible spectroscopic studies were conducted on a G103A Agilent spectrophotometer. Fluorescence spectroscopy was carried out on a Varian fluorescence spectrometer using 1 cm path length quartz cuvettes equipped with an air-tight seal. Time-resolved fluorescence data were collected at room temperature with a streak camera system (Hamamatsu C4780 Streakscope). Cyclic voltammetry measurement were run using a CH Instruments electrochemical work station model CHI760E. Differential Scanning Calorimetric studies on 1 were performed on a Mettler Toledo DSC 822e with nitrogen (99.99%) used to purge the system at a flow of 60 mL/min. Single crystal XRD measurements were recorded on a Bruker D8 Venture diffractometer equipped with a Photon 100 area detector using Cu-Kα1 radiation (λ=1.54187 Å) at 170(2) K (see ESI for details). Time-dependent DFT calculations were carried out on the optimised gas-phase geometry using the unrestricted B3LYP functional and 6-311G*+ triple zeta basis set within Jaguar.49 AFM images were obtained using a Digital Instruments Multimode atomic force microscope in tapping mode. The measurements were carried out using Veeco type FESP cantilever with a nominal tip radius of 8 nm and a nominal force constant of 2.8 N/m. Atomic force microscopy (AFM) images were recorded using a Digital Instruments NanoScope IV, operating in tapping mode with a silicon tip (model TESPA, Bruker AFM tips). Images were collected with high resolution (1024 lines per scan) at a scan rate of 0.5 Hz. Digital pro8


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cessing and roughness calculations were made using the free SPM data analysis software Gwyddion 2.30. RMS roughness values from three different areas of a sample were averaged.

B.

Preparation of the dithiadiazolylium chloride salts 1[Cl]

1-bromo-pyrene (3.55 mmol, 1.00 g) was dissolved in diethyl ether (30 ml) and cooled to -78 °C. Butyllithium (10 M, 4.0 mmol, 0.4 ml) was added dropwise and the reaction left to stir for 1 h under argon atmosphere. Bis(trimethylsilyl)carbodiimide (3.55 mmol, 0.745 ml) was added to the mixture and the reaction was left to stir at room temperature for 24 h to afford a pale red solution. Subsequently SCl2 (7.82mmol, 0.5ml) was added dropwise with vigorous stirring to yield, after complete addition of SCl2, a yellow/green solid under an orange solution. After filtration, washing with diethyl ether and drying under vacuum, crude [1]Cl was isolated as a yellow/green powder contaminated with LiCl. Yield: 1.20 g. m/z = 317.2 (M+). The crude solid was sparingly soluble precluding NMR studies but the salt [1][GaCl4] prepared from [1]Cl (vide infra) was fully characterized spectroscopically. C.

Preparation of the dithiadiazolyl radical 1

A suspension of silver powder (0.151 g, 1.41 mmol) was stirred with [1]Cl (0.5 g, 1.41mmol) in THF under argon atmosphere. The reaction was left to stir for 12 h to afford a dark solution. The solid residue was filtered off and the solvent was evaporated in vacuo to afford a dark residue, which was purified via vacuum sublimation onto a cold finger (10-1 torr, 170 °C) to afford black crystals of the radical. Yield: 160 mg (36%). Mp (DSC) = 191-196 oC; MS (EI+): M+ = 317.3; Microanalytical data: Observed (calculated) C = 66.29 (66.85), H = 2.16 (2.97), N = 9.09 (9.17)%; EPR (X-band: dry THF, 298 K): g = 2.010, aN = 5.15G D.

The films were protected from moisture by another quartz slide. F.

Preparation of Hydrophobic Glass Substrates

Glass slides, measuring 2” × 3”, were cleaned by washing with water and isopropanol for 15 minutes each in an ultrasonic bath. The slides were then dried with a nitrogen stream and the surfaces oxidised with UV/ozone using UVO Cleaner Model No. 42A (Jelight Company Inc., Irvine, CA, USA). After surface oxidation, the slides were treated with trichloro(1H,1H,2H,2H-perfluorooctyl)silane to afford a self-assembled hydrophobic monolayer, utilizing a modified procedure from Sugimura et al., using diffusioncontrolled coating within a vacuum desiccator.50 G.

Preparation of thin films of 1:PMMA (1:500 w/w)

Dithiadiazolyl radical 1 (2.00 mg, 0.007 mmol) was stirred with PMMA (150 mg, 0.00125 mmol) in 3 mL of dry CH2Cl2 in a nitrogen-filled glovebox for 1 hour. The resultant solution (approx. 1.8 mL) was then deposited into a 1” × 1” Teflon mold attached to a hydrophobic glass substrate, and the solvent left to evaporate for 12 hours to yield an optically transparent film. Other films of 1:PMMA and 1[GaCl4]:PMMA were prepared in a similar fashion. ASSOCIATED CONTENT Supporting Information: Crystallographic information for 1 in cif format; a summary of the crystallographic studies; EPR and electrochemical studies on 1 in solution; UV/vis absorption and emission studies on 1 in solution and in the solid state; information on the TD-DFT calculations undertaken on 1 and 2; DSC studies on 1; details of the PMMA:1 and PS:1 polymer composites; details of the emission profile of the LD lamp employed in photoconductivity measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

Preparation of 1[GaCl4] AUTHOR INFORMATION

Gallium chloride (0.125g, 0.71mmol) was stirred with the solid chloride salt [1]Cl (0.25 g, 0.71mmol) in THF under argon atmosphere for 2 h. The solid residue was filtered off and the solvent evaporated in vacuo to afford a dark residue. Yield: 264 mg (72 %), MS(EI+): M+ = 317.2; 1H NMR (500 MHz, DMSO) δ 9.71 (s, 1H), 8.49-8.46 (m, 2H), 8.448.39 (m, 2H), 8.34-8.31 (m, 2H), 8.28 (d, 1H), 8.22(d, 1H); 13C NMR (126 MHz, DMSO) δ 175.0, 171.5, 167.0, 133.2, 132.6, 131.1, 130.4, 130.0, 129.5, 127.6, 126.9, 125.3, 124.9, 124.0, 123.8, 123.3, 120.7; Microanalytical data: Observed (calculated) C = 39.1 (39.5), H =1.7 (1.8) , N= 5.2(5.4). E.

Preparation of thin films for solid-state studies

Saturated solutions of 1 and 1[GaCl4] in THF were prepared and thin films prepared by drop-casting these saturated solutions onto the surface of quartz slides under inert conditions. After solvent evaporation, solid thin purple films of the radical 1 and cation salt 1[GaCl4] were formed.

Corresponding Author J.M. Rawson, Dept of Chemistry and Biochemistry, The University of Windsor, 401 Sunset Avenue, Windsor, ON, N9B 3P4, CANADA. E-mail: [email protected]

Present Addresses C. P. Constantinides, Dept of Chemistry, U. Michigan (Dearborn), 4901 Evergreen Rd, Dearborn, MI 48128. Y. Beldjoudi, Dept of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Il, 60090, USA.

Author Contributions All authors have given approval to the final version of the manuscript and there is no conflict of interest. Funding Sources We would like to thank the Canada Research Chairs Program for financial support (J.M.R.), the University of Windsor for a scholarship (Y.B.), NSERC for a CGS-M Scholarship (M.A.N.)

9



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and the JSPS Core-to-Core program for collaborative funding (J.M.R., M.M.M. and K.A.). Y.J.C. , H.Y. and H.A. acknowledge funding from NSERC.

243304. (e) Riedl, T.; Rabe, T.; Johannes, H.-H.; Kowalsky, W.; Wang, J.; Weimann, T.; Hinze, P.; Nehls, B.; Farrell, T.; Scherf, U. Appl. Phys. Lett. 2006, 88, 241116.

ACKNOWLEDGMENTS K.E., M.M.M. and K.A. acknowledge a Grant-in-Aid for Scientific Research and a Core-to-Core Program (A. Advanced Research Networks) from the JSPS. We would like to thank Jiawang Zhou (Northwestern University) for assistance with additional lifetime measurements on [1][GaCl4] and Dr. E. Heyer (U. Windsor for helpful discussions).

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The photo- and electro-luminescent 1’-pyrenyl-1,2,3,5-dithiadiazolyl radical shows variously photoconducting or light-emitting behavior in the solid state depending on the wavelength of irradiation. 82x44mm (300 x 300 DPI)


Multi-functional dithiadiazolyl radicals - American Chemical Society

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