Exploring an Emissive Charge Transfer Process in Zero-Twist Donor

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Exploring Emissive Charge Transfer Process in Zero-Twist Donor–Acceptor Molecular Design as Dual State Emitter Sunil Kumar, Punita Singh, Pushpendra Kumar, Ritu Srivastava, Suman Kalyan Pal, and Subrata Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01351 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 16, 2016

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The Journal of Physical Chemistry

Exploring Emissive Charge Transfer Process in Zero-Twist Donor– Acceptor Molecular Design as Dual State Emitter Sunil Kumara, Punita Singhb, Pushpendra Kumara, Ritu Srivastavab, Suman Kalyan Pala and Subrata Ghosha,* a

School of Basic Sciences, Indian Institute of Technology Mandi, Himachal Pradesh- 175001, India. b Physics of Energy Harvesting Division, National Physical Laboratory, New Delhi, India.

ACS Paragon Plus Environment


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ABSTRACT: The present work reports a new concept on how to diminish dark TICT states with the zero-twist D–A systems in order to design frameworks with dual solution and solid state emission property. The study began with theoretical calculations to understand the structural needs followed by the chemical synthesis of conceptually new two molecular designs, 1 and 2, with zero twist angle between electron donor and acceptor units linked through covalent bond, and finally their applications in OLED devices. Oxazole was used as acceptor in combination with phenothizene core as donor, and the effect of enhanced electron donation was studied using methyl and anisole donor groups. DFT studies indicated a partial segregation of HOMO-LUMO levels in molecular designs, and the photophysics of these planar charge transfer molecules have been investigated. Natural transition orbital (NTO) calculations were carried out to understand excited states transition character in these D–A molecules. Molecular level studies through single crystal analysis revealed the importance of steric factor in controlling other molecular parameters particularly short-range molecular forces. The synthesized compounds were eventually utilized in green emitting OLED devices as pristine emitting layer. Compound 2 showed better device efficiency than 1 in unoptimized devices largely due to the presence of anisole group which prevented stacking of molecules. Solution sate emission and electroluminescence data of fabricated devices using 1 and 2 pointed out that molecular modification helped to enhance emission efficiency of 2 without shifting the emission wavelength.


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1. INTRODUCTION Given their wide ranging applications in diverse fields starting from fluorescence bioimaging to semiconductor industries particularly in the area of solid state lighting and display technologies, organic emissive materials are in the forefront of the key interests of many researchers in academics as well as in industries.1-8 Organic light emitting diodes (OLEDs) are associated with exciting properties over conventional LEDs such as lower power consumption, color temperature tenability, fancy architectures, big area fabrication compatibility, wide viewing angle, high brightness, sharper image, better contrast ratio, transparent, and light weight. Out of these attracting properties, independency to print OLEDs on flexible substrates and on large area are primary factor for the growth and big selling point of this technology.9 The extensive research since the first report of OLED device in 198710 have led researchers to develop a huge library of organic emitters for OLED application and efforts are still on to explore more emitter systems and design strategies.11-23 To make OLEDs more energy efficient, both material design strategies and device technology have to be developed to maximum extent. At present, there is a growing interest on organic emitters having emission property in both solution and solid states.24-26 As often many solution phase emissive materials are found to be nonemissive in the solid state restricting their applications in multiple areas, the current state of art is to develop and document unique design strategies resulting in generation of dual state emissive materials.25,26 Aggregation induced emission (AIE) characteristic has been highly cherished mechanism to achieve alleviated solid state emission.27-29 But, there exists a scope to explore the underlying mechanism for solid state emission and thus, the studies investigating the



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photophysical insights of new molecular designs to report the structural–property relationships are required to establish solid state emission mechanism along with material development for various thin film applications.30 Many planer organic emitters suffer from aggregation caused quenching (ACQ) problem31 largely due to the face-to-face stacking and intermolecular interactions. In the last few decades, remarkable strategies have been developed and demonstrated by many researchers to deter close packing which include twist geometry,27,28,32 enhanced intermolecular charge transfer transitions,31

and bulky substitutions.33-37

But, so far there exists only limited reports where planar ACQ systems have been successfully tethered with solid state emission property through innovative molecular parameter tuning strategies.21-23 It is quite evident in literature that structural planarity, conformational rigidity and stacking modulator are the key requirements for a small molecule to be dual state (solution and aggregate) emitter.28 In this context, fundamental studies of new donor–acceptor (D–A) systems are also necessary to develop efficient organic emitters.38-40 Organic systems developed using D–A design strategy are known for their interesting photophysical properties but have the drawback of dark TICT (twist intramolecular charge transfer) state.41-42 Twist conformation of D–A pairs is accompanied with the twist intramolecular charge transfer (TICT) states which limit their solution state applications. The D–A molecules having localized molecular halves on respective parts within a planar geometry (Ψ= 0°, dihedral angle between D-A) has not been discussed and reported till date as it seems to be a theoretically destructive design for solid state emission.32 To localize charge densities on donor and acceptor units, an interruption of conjugation through a twist


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between the two has been envisioned,44,45 but the planar motifs are understood to devoid of this molecular half localization characteristic mainly due to elongated conjugation along the plane of the molecule. Thus, we were wondering whether we could design a zero twist (Ψ = 0) luminophore with ICT property46 for dual solution and solid emission property? Since the effect of organic donors and acceptors is different in different D–A pairs, DFT (Density functional theory) studies guided us to design required D–A architecture to fulfill our quest.47 With the help of DFT calculations48 and molecular level studies, we now report that butterfly shaped phenothiazine as a donor49 and oxazole as an acceptor50 can efficiently serve as a successful assembly for zero twist design with partial charge

localization property in novel D-π0-Ψ0-A

system (π0 = no extra pi-spacer). The adopted design strategy is highlighted in Figure 1. The allowed ICT, because of allowed orbital overlap in compounds 1 and 2 is responsible for the solution state fluorescence property.

Molecular level studies

through single crystal analysis revealed the detrimental effects of intermolecular short range forces in planar design, 1, on its solid state emission, and hence to intensify the solid state emission we introduced anisole group as bulky substitution to subtle stacked packing of 2. The new zero twist and rigid D-π0-Ψ0-A luminogens represent solution and solid dual state luminogens. The OLED device applications have been shown by fabricating green emitting devices using 1 and 2 as pristine emitting layers in unoptimized devices.



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Figure 1. Schematic representation of current design strategy adopted in D-π0-Ψ0-A design with zero twist and stacking modulator for dual state emitters.

The preliminary results from unoptimized devices clearly demonstrated that the molecular parameters particularly short range molecular forces deter device performance and have to be considered critically while designing new materials for OLED application.

2. EXPERIMENTAL SECTIONS 2.1. Synthesis Detailed synthesis procedures and spectroscopic characterization have been presented in the supplementary information. 2.2. Materials and Instrumentation All the starting reactants were purchased from available commercial sources (SigmaAldrich, Merck) and were used as obtained. Spectroscopy grade solvents were utilized to complete photophysical studies. 1H and

13

C NMR spectra were recorded on Jeol

ECX 500 MHz in CDCl3. UV–vis and fluorescence spectra were recorded on Simadzu UV-2450 and Cary Eclipse fluorescence spectrophotometer (Agilent Technologies)


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respectively. FluoroLog-3 spectrofluorometer was employed to evaluate the solid state absolute quantum yield of compounds. Quinine was used as reference dye to measure the solution quantum yields of compound 1 and 2. FT-IR spectra were recorded on a Perkin Elmer Spectrum 2 spectrophotometer. HRMS-ESI spectra were recorded on Bruker Maxis Impact HD instrument. Diffraction studies were performed on Agilent Technologies X-ray diffractometer and Bruker AXS KAPPA APEX II systems. DSC/TGA studies were completed using NETZCH STA449 F1 instrument. The electrochemical cyclic voltammetry (CV) was performed using a Metrohm Autolab electrochemical

workstation

in

0.1M

DCM

solution

of

tetrabutylammonium

hexafluorophosphate (Bu4NPF6). Pt wire and Ag/AgCl were used as the counter and reference electrodes respectively. Ferrocene couple was used as external standard. Life time measurements were carried out on ISS equipment chronosBH Fluorescence Lifetime Spectrometer. The EL spectrum has been measured with a high resolution spectrometer

(Ocean

Optics

HR-2000CG

UV-NIR).

The

current–voltage–

luminescence (I–V–L) characteristics have been measured with a luminance meter (LMT l-1009) interfaced with a Keithley 2400 programmable voltage– current digital source meter. 2.3. Device Fabrication Indium-tin oxide (ITO) coated glass substrate with a sheet resistance of 20 Ω/□, was used as anode which were patterned and cleaned by a conventional solution cleaning process. The substrates were further exposed to oxygen plasma to enhance the work function. All the layers were deposited by vacuum deposition at a base pressure of 1×10−6 Torr with a deposition rate of 0.1 Å/s. The doped films were deposited by co-



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evaporation process. The thicknesses of all the films were measured in situ by a quartz crystal thickness monitor. The size of each pixel was 4x4 mm2. All further measurements were performed at room temperature and under ambient atmosphere, without any encapsulation. Electroluminescence spectra (EL) and J-V-L characteristics (current density-voltage-luminescence) of each device were evaluated. α-NPD was used as hole transport layer, TPBi as hole blocking layer and Alq3 was used as electron transport layer. LiF/Al combination was used as cathode and deposited on the top of organic layers in all devices.

2.4. X-Ray Crystallography Compound 1 and 2 crystallized as light yellow needle like crystals utilizing slow evaporation crystallization method from THF:DCM mixture which were then used for X-ray diffraction analysis. X-ray crystallography data was collected at Agilent CCD diffractometer. CCDC numbers 1440823-1440824 were obtained for 2 and 1 from “The Cambridge Crystallographic Data Centre (CCDC)” by depositing structures at http://www.ccdc.cam.ac.uk/. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

2.4. Femtosecond Transient Absorption (TA) Studies Femtosecond tunable broad band TA spectrometer is based on a regeneratively amplified femtosecond Ti:Sapphire laser (wavelength ~ 800 nm, pulse width < 35 fs). The output light of energy 4 mJ/ pulse from the regenerative amplifier (Spitfire Ace, Spectra Physics) was divided into two channels for generating pump and probe


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pulses. The pump pulse at wavelengths 410 nm was obtained from the nonlinear optical parametric amplifier (TOPAS-Prime from Light Conversion). A white light continuum (WLC) probe was generated by focusing a small fraction of 800 nm beam on a sapphire crystal. The pump and probe pulses were overlapped spatially at the sample position. All the TA measurements were performed in a quartz cuvette of 2 mm path length. Typical pulse energy of 300 nJ was used for excitation at the sample position.

3. RESULTS AND DISCUSSIONS 3.1. Ground and Excited State Theoretical Calculations Spatial configuration of 1 and 2 was optimized using Gaussian 09 based DFT with B3LYP/6-31g (d) in the gas phase.48 Studied compounds were varied with two different donors e.g methyl and anisole. The optimized structures (see Fig. S1) were used to calculate the spatial distribution of HOMO and LUMO levels (See Fig. 2), transition energies (using TD-DFT) and natural transition orbitals51-55 (NTOs, see Fig. 2). The distribution of HOMO and LUMO energy levels are shown in Figure 2. The theoretical calculations (see Fig. S1) suggest that oxazole unit shares the plane of phenothiazine ring with no distortion between two units. Only distortion observed is the non-planarity of anisole ring with rest of the molecule. We assume this non-planarity of anisole group and butterfly shape of phenothiazine will help in preventing π-stacking of 2. Interestingly, both compounds show similar distribution of HOMO and LUMO levels



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Figure 2. Spatial distribution of HOMO-LUMO amplitudes and natural transition orbitals calculated for the first excited state in 1 and 2.

with a partial segregation of electron densities. Anisole ring did not contribute to any of the HOMO-LUMO levels because of high dihedral angle (96.8o) with phenothiazine plane. Phenothiazine contributes maximum to HOMO levels while oxazole unit keeps maximum of LUMO levels and thus a condition for active charge transfer (CT) process would be observed in these molecules.19,56 HOMO levels were assessed at 5.15eV, 4.99 eV and LUMO were at -1.35eV, -1.27 eV for 1 and 2, respectively. It is observed that anisole group shifted up both the HOMO and LUMO levels, which provides the insight of ease of electron oxidation process for 2 than 1. The ease of electron oxidation process is related to good hole transporting nature of the compound due to the formation of table cation radicals within the molecule. UV absorption transitions speculated using TD-DFT level studies suggest two major transition centering in region 209 – 305 nm and 370 – 390 nm (see Fig. S2 and S3) for both the compounds. The transitions at longer wavelength are due to charge transfer process in the molecules while the former transitions are π-π* transitions of the molecules. Effect of


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donor group on the transition energies was observed as transition peaks for 2 (299 nm, 391.6 nm) were red shifted from the peaks of 1 (296.8 nm, 377.9 nm). In order to understand the orbital transformation, which is quite helpful in providing a qualitative description of an electronic excitation, natural transition orbitals corresponding to charge transfer (CT) state were calculated (see Fig. 2). Natural transition orbitals for the first excited state of both compounds depicted the similar type of hole and particle distribution behavior. Holes are mainly localized over the phenothiazine unit, while electrons are distributed over the entire molecule with larger density towards acceptor part. It is clear from the orbital pictures (see Fig. 2) that although before irradiation, the electron density was localized on the phenothiazine donor, it got delocalized over the whole molecule with more electron density toward acceptor part on photoexcitation. This transfer of electron density from donor to acceptor depicted directional CT characteristic of molecules.51,52,55 To explore our molecular systems, a thorough study along the twist coordinate for ground state was carried out.30 The geometries of these molecules with different twist angle were optimized and the obtained energy plots for 1 and 2 have been shown in supporting information (see Fig. S4). It was found that, when the plane of acceptor is perpendicular to the plane of donor, which means a 0° to 90° twist destabilized both the molecules. Thus, we could expect that the geometric switching toward a twisted state is less likely to occur in these molecules on excitation. Further to confirm the formation of as expected planar ICT states, the configurations of the first excited state (S1) for 1 and 2 were optimized at b3LYP/6-31G (d) level. The frequencies were also computed at same level to confirm the absence of negative frequency. The optimized geometries for S1 states have been shown in Figure 3. The



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excited state geometries clearly reveal the planarization of 1 and 2 in S0 to S1 transition: the butterfly bent of the phenothiazine ring became totally planar while twist nature of excited S1 state was observed. Combining results from the above quantum chemical studies it could be concluded that the excitation energy is dissipated in attaining more planar geometry in the excited state and hence, a process to attain twist around single bond connecting donor and acceptor units, capable of forming dark TICT state, was not favoured. The interesting HOMO-LUMO distributions in zero–twist rigid D-π0-Ψ0-A design, NTOs for the first excitation and excited state (S1) geometries established charge transfer characteristic of 1 and 2 and promoted our quest to study the photophysics of such systems.

Compound 2

Compound 1 S1

S1

S0

S0

Figure 3. Optimized geometries in S1 state for 1 and 2. Side view of geometrical transition occurred from S0 to S1 electronic transition.

With these theoretical insights into new molecular architecture, we proceeded with their chemical synthesis. The designed compounds were synthesized in three steps using phenothiazine as starting material (Scheme 1, see SI for detailed procedures).


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In short, N-position of phenothiazine was methylated in THF using methyl iodide in the presence of potassium tert-butoxide (KtOBu) base. Whereas N-anisole phenothiazine was obtained through C-N coupling following Ullman reaction processs.57 Next, formylation was done using Vilsmeier and Haack reaction condition,58 and finally, the formyl group was converted to oxazole by refluxing with o-aminiphenol.59 3.2. Optical Properties Compounds 1 and 2 demonstrated absorption maxima at 297 nm and a broad peak at 304 nm respectively with a structured absorption in the 260 – 420 nm UV region (see Fig. 4). The less intense band in

low energy region 320-420 nm is due to the

intramolecular charge transfer (ICT), which is also evidenced in theoretical studies.60 Compound 2 have a slightly red shifted absorption than 1 signified the presence of donor group. The photoluminescence (PL) spectra of compounds in 5 µM

Scheme 1. Synthetic scheme showing the pathway to obtain 1 and 2 with ORTEP structures (50% ellipsoidal probability) of grown crystals.



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dichloromethane solutions were recorded (see Fig. 4) and it is clear that both compounds emit at 501 nm. This explained the absence of effect of side groups on emissive core which is same in both the molecules. Planar ICT process, as discussed in earlier section, in these emitters and the absence of effective intramolecular rotations contributed to impart good fluorescence property in solution state. Interestingly, this phenomenon was different from conventional ICT systems with a twist where a poor overlap of HOMO-LUMO charge densities is understood to be the reason for low or quenched fluorescence emission.41,42 Since a dependence of either absorption profile or emission profile or both on solvent polarity prove the presence of ICT band in molecules, optical properties in solvents of different polarities were investigated (see Fig. S5 and S6).61 When the solvent polarity was

Figure 4. UV-vis absorption profiles and Photoluminescence spectra of 1 and 2 collected for DCM solution (5uM).


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changed from THF to DMSO, no effect on the absorption profile was observed for both the molecules. But at the same time red shifts of 23 nm and 27 nm were observed for 1 and 2, respectively, in their PL spectra. These small red shift values as compared to TICT molecules are also proofs for smaller dipole moments of the excited state which is possible only in planar CT state.60 It is interesting to note that with the increase of solvent polarity, PL intensity did not quench as observed for conventional twisted D-A molecules.62 This signifies that ICT character of these molecules did not get affected with increase in polarity which is possible only if the CT state is planar and not twisted.62 Quantum yields also supported our assumption of planar ICT system as an increment in quantum yields was observed in DMSO (0.7 and 0.9) as compared to DCM (0.5 and 0.4) for compounds 1 and 2, respectively. Moreover, the enhancement of quantum yields in polar solvent could be related to the rigidification of conformation leading to loss of vibratory motions.26 The solid state PL spectra were recorded for both compounds and have been shown in Figure 5. Solid state PL maxima for 1 was broader and was found to center around 489 nm with a 11 nm blue shift from that in DCM solution. This represented the presence of intermolecular interactions, originating due to the specific molecular packing of planar molecular configuration of 1. While, compound 2 showed a structureless emission at 510 nm with a 9 nm red shift from DCM solution maxima. This small red shift is usual observation for powder state PL spectra and is due to the change in dielectric constant of medium from liquid to powder state. Further, aggregation nature of these compounds was investigated by performing photophysical studies in THF/water mixtures.28 Both the compounds



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revealed the absence of AIEE behavior which confirmed the absence of active rotary motions in the molecules (see Fig. 5 and Fig. S8). This gave an insight on the molecular rigidity of these newly developed zero twist molecules. Compound 1 showed a red shift of 36 nm in fluorescence emission

Figure 5. Solid state photoluminescence (PL) spectra 1 (excitation wavelength, λex = 310 nm) and 2 (b, λex = 377 nm). PL emissions of 1(λex= 297 nm) 2 (λex = 304 nm) with different water content in THF.

with increase in water content up to 80%. Upon the increase in water content up to 90%, a blue shifted emission at 502 nm along with decrease in emission intensity was noticed. Similarly, a red shift of 25 nm in emission maxima was observed for 2 up to


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60% water content. Further increment in water fraction to 80% resulted in blue shift in emission at 502 nm which further shifted to 489 nm when the water content was increased to 90%. As reported, the red shift in PL intensity is due to the relaxation of ICT state whereas the fluorescence decrement is actually due to the emission from the surface of formed bulky aggregates.63 At higher water content, molecules come out from the solvent and tend to aggregate which in turn create a non-polar microenvironment around the molecules which may decrease the fluorescence.19,26,63 3.3. Molecular level studies: To scrutinize the molecular packing and molecular design predicted by DFT studies, crystals were grown for 1 and 2 from THF:DCM solution using slow evaporation technique. Crystal data parameters are given in Supporting Information (see table S2, S3). Molecules in 2 adopted herring-bone type conformation (see Fig. 6a, b) with a P 21/n space group. As a proof to DFT studies, it is clear that oxazole and phenothiazine share the same conjugated plane with no twist angle between them, and anisole aromatic ring is out of the plane of rest of the conjugated molecular backbone of 2.



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Figure 6. Molecular packing diagrams (a, b and c) for 2 visualized along different axis; (d) Molecular packing of 1; and (e) short range contacts for molecule in red with neighbouring molecules (green) in 1.

Molecules align themselves in antiparallel geometry in adjacent layers with a partial πoverlap between aromatic units due to a slipped – away packing. Layer A and layer B (Fig. 6c) are separated by a distance of 3.7Å. A short range C–H4---π (anisole ring, 2.8 Å) contact between anisole aromatic ring and phenothiazine was observed between the molecules of same layers which is also helpful in providing molecular stiffness. It could be seen that anisole group helped to protrude the steric multitude and thus ensured minimum interaction between molecules. Similar type of randomness was absent in 1 mainly due to its almost planar nature. As shown in Figure 6d, molecules form parallel stacks in alternate columns (column A, B and C) and facing each other in each column with minimum distance of 3.3 Å. Such close


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interactions in planar molecule give rise to weak H-type aggregation. This specific molecular packing was responsible for blue shift in solid state emission.64 Also, in the case of 1 (Fig 6e), the single crystal data demonstrated the presence of nine short range contacts of a molecule with neighboring molecules [C13•••C5 (3.39 Å), C5•••C13

(3.39 Å), C15•••S1 (3.47 Å), H19•••S1 (2.82 Å), S1•••C15 (3.47 Å),

S1•••H19 (2.82 Å), C00O•••H19 (2.87 Å), H19•••C00O (2.87 Å) and H9•••H9 (2.26 Å]. These short range contacts are responsible to mediate non-radiative pathways for excited states.65-66 The intermolecular spacing was increased from 3.3 to 3.7 Å by anisole introduction into the molecular architecture and it also, successfully helped molecules to acquire emissive herringbone type packing with reduced number of short-range contacts. From the single crystal XRD data, it was excluded that the intermolecular distance was increased to 3.7 Å and also increased the solid state absolute quantum yield from 2.5% to 33%. Thus, the positive effect of anisole substitution on the molecular packing could not be underestimated.

3.4. Electrochemical and Thermal properties: We have explained that better electron donation from anisole group red shifted the fluorescence of the phenothiazine-oxazole unit than its methyl counterpart. Similarly, we expected the effect of anisole donor group on the oxidation and reduction behavior of 2 as compared to 1, and hence cyclic voltammetry studies were carried out to find the HOMO – LUMO energy levels. Dichloromethane solutions (5 mM) of each compound were scanned in potential range of -1.0 V to 1.2 V in conventional 3electrode voltammetry work station. Figure 7a shows the cyclic voltammograms of



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both the compounds. Both, 1 and 2 both showed a quasi-reversible oxidation peaks with oxidation onset at 0.84 V and 0.80 V respectively. Reversible nature of oxidation process is usually accompanied with stable cation radical formation process which is responsible for good hole transport property. Moreover a shift to smaller oxidation onset is visible for 2 than 1 due to better electron donation from donor anisole group. These observations were in agreement with the theoretical studies carried out on these molecules. Based on the oxidation onset values HOMOs were assessed at 5.11 eV and 5.06 eV for 1 and 2, respectively. No peaks corresponding to reduction behavior were observed in 0 V to -1.0 V potential window, thus electronic band gap (Eg) was calculated using UV-vis absorption onset. The Eg was calculated to be 2.72ev and 2.68eV which provided LUMOs at 2.39 eV and 2.38 eV for 1 and 2 respectively.

Figure 7. (a) Cyclic voltamograms and (b) thermogravimetric analysis (TGA) showing the mass loss (in percent) with temperature for of 1 and 2.


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As OLED devices demand good thermal stabilities for the developed light emitting materials, thus it is necessary to study the thermal behaviors of newly developed compounds.

Thermal stabilities and mass loss behavior of the present aromatic

heterocyclic compounds were measured using DSC-TGA graphs (see Fig. 7b). High melting points 190 oC and 210 oC with decomposition temperature, Td of 240 oC and 235 oC were observed for 1 and 2 respectively. No glass transitions were observed for both the compounds in heating scan. The good thermal properties suggested the potential candidature of 2 in the light emitting field.

3.5. Transient Absorption Studies In order to understand the excited state properties of compound 1 and 2 in more details, femtosecond transient absorption (TA) measurements were carried out in acetonitrile by using 410 nm laser source of 35 fs pulse duration. TA spectra of compounds 1 and 2 at different pump-probe delays (100 fs to 3 ps and 3 ps to 500 ps) are presented in Figure 8 and S9 (see SI), respectively. A broad positive absorption band was found from 450 to 700 nm with peak maximum at 550 and 525 nm for compounds 1 and 2, respectively. These absorption bands are attributed to the excited state absorption (ESA) of the compounds. Apparently, there is a gradual blue-shift in TA spectra for both the compounds up to 3 ps (no spectral shift thereafter). Similar shift is very common for stabilization of ICT states through either solvent or geometrical relaxation of compounds from the excited state67,68 To know more about the excited state processes, TA kinetics at the maximum of ESA band were measured for both the compounds. TA kinetics were fitted with a bi-exponential function, which



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(a)

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(b)

Figure 8. Excited state absorption of compound 1, (a) short time delays (100 fs to 3 ps) and (b) long time delays (3 to 500 ps). consist of one rise and one decay components (see Fig. 9). Acetonitrile being a fast dielectric relaxing solvent, possesses an average solvation time (sol) of 500 fs.69 Therefore, ultrafast lifetime component (τ1) of 667 fs is assigned to the solvent relaxation of the ICT states. However, as τ1 is higher than solvation time, the involvement of geometrical or vibrational (Franck-Condon states) relaxation cannot be ruled out. The time constant of 376 ps is associated with the decay of the excited state back to the ground electronic state. Ghosh et al. observed that the decay of the twisted ICT states back to the ground state occurs in about 6 ps.70 Relatively slower decay of the excited state suggests that ICT states forming in our compounds are not twisted.


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Figure 9. TA kinetics of compound 1 following 410 nm excitation and probe at 575 nm.

This observation supports our theoretical findings, which demonstrates that in both the compounds the butterfly bent of the phenothiazine ring becomes totally planar structure in the S1 state (after photoexcitation) ruling out the possibility of the formation of twisted ICT states.

3.5. Device Application Preliminary evaluation of developed D-π0-Ψ0-A based emitters for light emitting applications was brought out by fabricating multilayered fluorescent OLED devices and their efficiencies were compared. Compounds 1 and 2 were used as emissive layers in unoptimized preliminary OLED devices. Electroluminescence (EL) spectra and J-V-L characteristics (current density-voltage-luminescence) of each device were evaluated. Energy level diagram for adapted device structure is shown in Figure 7b. The used device configuration for Device 1 was; ITO/ α-NPD (30 nm)/1 (35 nm)/TPBi



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(20 nm)/Alq3 (10 nm)/LiF (1 nm)/ Al (150 nm). The EL spectrum recorded (see Fig. 10a) was in agreement with the PL spectrum obtained in solution state but, a very high turn on voltage (Von) of 9 V (See Fig. S10 for device data) with Commission International de L’Eclairage, CIE(x, y) coordinates 0.26, 0.49 was observed (see Fig. S12). Thus, the Device 1 was modified a little by including F4TCNQ as hole injection layer which provided Device 2: ITO/F4TCNQ (1 nm)/ α-NPD (30 nm)/1 (35 nm)/TPBi (20 nm)/Alq3 (10 nm)/LiF (1 nm)/ Al (150 nm). The EL spectrum was similar to Device 1. A low Von = 4V (CIEx,

y

= 0.28, 0.51) was observed for Device 2 with enhanced

luminance compared to Device 1 (see Fig. S11) with maximum current (CEmax) and power efficiency (PEmax) of 0.12 cd/A and 0.04 lm/W. Since 2 showed better fluorescence quantum yields due to non-planar structure, good OLED device properties could be expected. Thus Device 3 containing 2 as emitter was fabricated by adopting the

Figure 10. (a) EL spectra recorded for Device 1, 2 and 3; (b) Device architecture used for Device 2 and Device 3 with molecular level alignment


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structure ITO/F4TCNQ (1 nm)/ α-NPD (30 nm)/2 (35 nm)/TPBi (20 nm)/Alq3 (10 nm)/LiF (1 nm)/ Al (150 nm). As expected a low turn on voltage of 4V (CIEx, y = 0.32, 0.55) was achieved with enhanced device efficiencies (CEmax = 0.85 cd/A and PEmax = 0.24 lm/W) compared to device 2 (see Fig. S11). The CIE coordinates of Device 3 represent a pure colour emission, while CIE of Devices 1 and 2, both showed a blue component in emission (see Fig. S12). With the help of molecular level understanding we could relate this shift in colour purity in 2 to the less intermolecular interactions. Thus we assume that the zero twist D-π0- Ψ0-A emitter, 2, could provide an optimistic design strategy in the library of organic emitters developed from ACQ system with dual solution and solid state emission property.

4. CONCLUSIONS In summary, present work deals with conceptually new zero-twist molecular designs through DFT studies followed by their development through chemical synthesis, and finally applications in OLED devices. The main motivation was the fascinating segregation of HOMO-LUMO charge densities in these donor-acceptor (D-A) luminogens with zero twist. This new strategy provides a way to design luminogens with both solution and solid state emission properties. The emission of 1 and 2 in dichloromethane solutions was at similar wavelength which ascertained that there was no effect of molecular modification on the emissive property of 2. Similar speculation could be made on the basis of similar HOMO and LUMO electron densities and, hole



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and particle distribution in both the compounds. We also report the importance of steric bulkiness to avoid stacking through molecular level studies in a correlation with enhancement of solid state PL emission of 2 than 1. The incorporation of stacking modulator into the chemical architecture of 2 did not affect the planarity between donor and acceptor, and the ICT process remained active even in the solid state making 2 as potential dual state emissive material. Photophysical studies and single crystal XRD studies revealed the bright candidature of this D-π0-Ψ0-A design in 2. As expected, current and power efficiencies were increased when 2 was used as emissive layer in unoptimized OLED devices. Altogether, the present study demonstrates a novel design strategy to prevent planar solution emitter with zero twist angle between donor and acceptor units from concentration quenching in aggregate state through integration of suitable stacking controller into molecular backbone without affecting the active ICT process resulting in the generation of dual state emitter. Therefore, the DFT studies can be used as an efficient tool in predicting and designing organic materials to meet the structural requirements for OLED applications. We assume that an optimized device will help to improve the device efficiencies.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website. Detailed experimental procedures, theoretical results, photophysical studies data, OLED device data and characterization data for all compounds.


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AUTHOR INFORMATION Corresponding Authors * Email: [email protected]. Phone: 01905-300065

ACKNOWLEDGEMENTS The financial support for the DST (Grant No. SERB/F/2408/2012-13) is highly acknowledged. The authors are highly thankful to Director, IIT Mandi and AMRC (Advanced Material Research Centre) for research facilities. We are thankful to Dr Angshuman Roy Choudhury for his valuable suggestions during the preparation of manuscript. Sunil Kumar acknowledges University Grant Commission (UGC) for his fellowship. Punita Singh thanks the DST, India for financial aid (Grant no. SR/WOSA/CS-70/2012).

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314x140mm (96 x 96 DPI)


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Exploring an Emissive Charge Transfer Process in Zero-Twist Donor

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