R.
Keruckiene
a,
M.
Guzauskas
a,
L.
Lapienyte
a,
J.
Simokaitiene
a,
D.
Volyniuk
a,
J.
Cameron
b,
P. J.
Skabara
*b,
G.
Sini
*c and
J. V.
Grazulevicius
*a
aDepartment of Polymer Chemistry and Technology, Kaunas University of Technology, K. Barsausko St. 59, Kaunas, LT-50254, Lithuania. E-mail: juozas.grazulevicius@ktu.lt
bWestCHEM, School of Chemistry, University of Glasgow, Joseph Black Building, University Place, Glasgow, G12 8QQ, UK. E-mail: Peter.Skabara@glasgow.ac.uk
cLaboratoire de Physicochimie des Polymères et des Interfaces, CY Paris Cergy Université, EA 2528, 5 mail Gay-Lussac, Cergy-Pontoise Cedex 95031, France. E-mail: gjergji.sini@u-cergy.fr
First published on 2nd October 2020
Derivatives of trifluorobiphenyl and 3,6-di-tert-butylcarbazole were synthesised as potential components of emitting layers of OLEDs. Molecular design of the compounds was performed taking into consideration the hydrogen bonding ability of the fluorine atom and electron-donating ability of the carbazole moiety. Their toluene solutions exhibited very high triplet-energy values of 3.03 eV and 3.06 eV. Ionisation energies of the compounds in the solid-state were found to be in the range from 5.98 to 6.17 eV. Density functional theory (DFT) calculations using the ωB97XD functional, with the ω parameter tuned in the presence of the solvent, uncovered singlet–triplet energy splitting in good agreement with the experimental results. The materials were tested in the emissive layers of OLEDs, showing the ability to form exciplexes with complementary electron-accepting 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine. Using the synthesised compounds as exciplex-forming materials, highly efficient exciplex emission-based OLEDs were developed. In the best case, a high maximum current efficiency of 24.8 cd A−1, and power and external quantum efficiencies of 12.2 lm W−1 and 7.8%, respectively, were achieved.
In this work, two new compounds bearing trifluorophenyl and carbazole fragments are introduced. Molecular design of the compounds was performed taking into consideration the hydrogen bonding abilities of the fluorine atom and efficient electron-donating abilities of the carbazole moiety.21
The impact of the number of donor groups on the properties of the materials is discussed on the bases of results obtained by means of theoretical and experimental approaches. The compounds were shown to be capable of forming exciplexes with appropriate electron-accepting molecules. The exciplex forming molecular mixtures were tested as the emissive materials in OLEDs.
Morphological transitions and thermal stabilities of derivatives 1 and 2 were investigated by using DSC and TGA (Fig. S1, ESI†). Their thermal characteristics are shown in Table 1.
As both compounds were isolated after the synthesis and purification as crystalline materials, endothermic melting signals were observed in the 1st heating scans of DSC measurements. Crystallisation and subsequent melting signals were observed during the cooling and 2nd heating scans for compound 2. The higher melting and crystallisation temperatures of 2 could be attributed to its more symmetrical crystalline structure and overall higher molar mass. No further morphological transitions of sample 1 were detected during cooling and heating scans of DSC measurements, indicating its tendency to be transformed to a solid amorphous state. TGA experiments revealed the complete weight loss of compound 1, indicating its sublimation, and the impossibility to determine the temperature of onset of thermal degradation.
Fig. 1 Structures of 1 (left) and 2 (right) with dihedral angles 1, 2 and 3 denoted in red, black and blue, respectively. |
Dihedral 1 (°) | Dihedral 2 (°) | Dihedral 3 (°) | ||
---|---|---|---|---|
1 | ω-default/diethyl ether | 55.7 | 53.9 | — |
ω-CPCM | 89.5 | 81.5 | — | |
2 | ω-default/diethyl ether | 59.5 | 53.9 | 52.8 |
ω-CPCM | 90.4 | 80.4 | 81.5 |
Compound | Toluene solution/thin film | IECV (eV) | IEPE (eV) | ||||||
---|---|---|---|---|---|---|---|---|---|
λ Abs (nm) | λ FL (nm) | E optg (eV) | Stokes shift (cm−1) | S1:T1 (eV) | ΔEST (eV) | PLQY (%) | |||
λ Abs are wavelengths of absorption maxima; λFL are wavelengths of emission maxima; Stokes shift = λFl − λAbs; S1 is the singlet energy estimated as 1240/λFlu onset; T1 is the triplet energy estimated as 1240/λPH onset; ΔEST = T1 − S1; Eoptg is optical gap estimated as 1240/λAbs onset where λAbs onset is the wavelength of the onset of absorption; IEPE is the ionisation potential estimated by photoelectron emission spectrometry in air; IECV is the ionisation potential estimated by CV as IECV = Eonset ox vs. Fc + 5.1 eV.31,32 | |||||||||
1 | 297, 346/298, 347 | 354, 370/355, 366, 460 | 3.49 | 653/649 | 3.67:3.03/— | 0.64 | 13/1 | 5.15 | 5.98 |
2 | 297, 346/230, 345 | 352, 367/352, 374, 439 | 3.41 | 493/577 | 3.63:3.05/— | 0.58 | 28/1 | 5.28 | 6.17 |
Ionisation energy values (Table 3 and Fig. 3) of the solid samples of compounds 1 and 2 were estimated by photoelectron emission spectrometry. The values of ionisation energies were found to be comparable for both the compounds and were a little higher than those estimated by cyclic voltammetry. Small differences in the values of ionisation energy obtained by employing different methods can be explained by the different environments in the solution and the solid-state.
In order to obtain insights into the optical properties of compounds 1 and 2, TDDFT calculations were carried out on each of the optimised structures using geometries obtained with both default- and tuned ω-values. The results of these calculations are summarised in Table 4. When calculating S1 and T1 transitions, it is important that the nature of the excitation is described appropriately. In order to maximise the reverse intersystem crossing, there should be a change in the symmetry of the excited state, so typically in donor–acceptor compounds, the T1 excitation is a local excitation (LE) and S1 excitation is a charge transfer (CT) state.33 Natural transition orbitals (NTOs)34 for holes and electrons corresponding to the S1 and T1 states of compounds 1 and 2 are shown in Fig. 5 and 6, respectively, whilst the full list of TDDFT transitions for both compounds is presented in Fig. S4–S11 (ESI†). In order to further characterise these transitions, the spatial overlap (Λ) for all excitations has been reported.
T1 (eV) | T1 (nm) | Λ T1 | S1 (eV) | S1 (nm) | Λ S1 | ΔEST | ||
---|---|---|---|---|---|---|---|---|
a The most dominant individual transitions from TDDFT calculations are shown in parentheses. Full lists of these transitions, with molecular orbital diagrams, are presented in Fig. S4–S11 (ESI). | ||||||||
1 | Default ω | 3.21 (H−1 → L+1)a | 385.7 | 0.68 | 4.21 (H → L+1)a | 294.7 | 0.67 | 1.00 |
ω-CPCM | 3.17 (H−1 → L+2)a | 390.6 | 0.72 | 3.81 (H → L)a | 325.1 | 0.16 | 0.64 | |
2 | Default ω | 3.21 (H−2 → L+1)a | 386.2 | 0.72 | 4.24 (H → L+1)a | 292.6 | 0.67 | 1.03 |
ω-CPCM | 3.17 (H−3 → L+3)a | 390.8 | 0.61 | 3.66 (H → L)a | 338.7 | 0.20 | 0.49 |
Fig. 5 NTOs corresponding to S1 and T1 states for compound 1 obtained by means of the TDDFT method at the ωB97XD/6-31++G(d,p) level by using default ω (left) and ω-CPCM (right). λ = NTO eigenvalue. |
Fig. 6 NTOs corresponding to S1 and T1 states of 2 obtained by means of the TDDFT method at the ωB97XD/6-31++G(d,p) level by using default ω (left) and ω-CPCM (right). λ = NTO eigenvalue. |
The NTOs corresponding to the T1 states of compounds 1 and 2 (Fig. 5 and 6) indicate that both holes and electrons are globally localised on the electron-donating carbazole units, irrespective of the geometry used, highlighting the 3LE nature of these transitions. The calculated T1 energies using both default ω and ω-CPCM methods show good agreement with one another for both compounds and show generally similar spatial overlap for the transitions.
However, the calculated S1 energies are significantly different when using the separate methods. Calculations with the default ω show an increased S1 energy in both compounds, resulting in a large calculated ΔEST. The NTOs with the default ω have a generally large spatial overlap with occupied and unoccupied orbitals involved in the dominant transitions localised on the carbazole units. Therefore, the S1 state does not show the CT character that would be expected for a donor–acceptor compound.
When the ω-CPCM value is used for the TDDFT calculations, the S1 energy is significantly reduced and this can be explained by the increased CT nature of the transitions. The spatial overlap, Λ, is significantly reduced with the electron-NTOs of 1 and 2 (Fig. 5 and 6) localised on the electron-deficient 1,3,5-trifluorobenzene groups. The predicted ΔEST energy is significantly reduced and in good agreement with experimental values (see optical emission properties section), although it is too high for the material to be used alone as a thermally activated delayed fluorescence (TADF) candidate. Furthermore, the individual T1 and S1 energies show relatively good agreement compared to experimentally determined values, which is useful in the design of exciplex emissive layers.
The singlet excitations from the TDDFT calculations are listed in Table S1 (ESI†). As previously mentioned, the S1 transitions show local excitations when default ω is used but CT states are observed in both compounds when ω-CPCM is used. The nature of the excitation is important when comparing with the experimental absorption spectrum. Compound 1 shows the S1 excitation to have a small oscillator strength (f = 0.01), whilst that of the first local excitation, S3, is higher (f = 0.04). In compound 2, there is a similar trend where the S1 excitation (f = 0.02) also has a lower oscillator strength than the lowest energy local excitation S5 (f = 0.07). This suggests that the UV/Vis absorption spectrum will be dominated by the absorption of the carbazole unit. The lowest energy local excitations for compounds 1 and 2 occur at 313 and 312 nm, respectively, which is in good agreement with the experimental peaks at 297 nm for both compounds (Table 3).
The sensitivity of the S1 excitation to the environment in TDDFT calculations has been shown and therefore it is important that this is taken into consideration when estimating ΔEST, using the ω-tuned functional as an effective means of improving the accuracy of the estimation.
Fig. 7 PL spectra of toluene solutions and of solid films of compounds 1 and 2 recorded at room temperature (λexc = 330 nm) and PL and PH spectra recorded at 77 K. |
The PL spectra of thin films of compounds 1 and 2 were also vibronically structured in the same range of ca. 350–400 nm as those of their toluene solutions (Fig. 7). The PLQY values of the solid samples were found to be low (∼1%) due to aggregation-induced quenching.35 The origin of the low-energy band was studied more extensively. PL spectra of the non-evacuated and evacuated neat thin films were recorded at room temperature (Fig. S12a and c, ESI†). The PL spectrum of compound 1 was found to be sensitive to oxygen. The ratio of PL intensities (Ievac/Ipre-evac) was 1.9. The PL intensity change in the low-energy band of compound 1 confirms the impact of triplet states possibly through reverse intersystem crossing (RISC). PL decay curves of the solid sample of 1 were recorded (Fig. S12b, ESI†). They contained two components, i.e. a short-lived component in the ns range corresponding to a local excited (1LE) state at higher energies and a CT component at lower energies.
The PL spectrum of the solid sample of compound 2 was not significantly sensitive to oxygen. A slight increase of PL intensity was observed after evacuation (Fig. S12c, ESI†). PL decay curves of the solid sample of compound 2 (Fig. S12d, ESI†) contained two short-lived components in the ns range corresponding to 1LE emission. Slight differences in emission spectra and their sensitivity to oxygen in the solid samples of compounds 1 and 2, and in sensitivity of the PL spectra to oxygen, are apparently related to the different substitution patterns of the trifluorobiphenyl moiety. Taking into account the aggregation-induced quenching of emission of 1 and 2, it was decided to test the photophysical properties of these compounds in doped systems.
Compounds 1 and 2 formed sky-blue exciplexes with the electron acceptor 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), which is one of the most widely studied exciplex-forming acceptors (Fig. 8).36,37 The spectra of solid films of molecular mixtures of 1 and 2 with PO-T2T showed significant red shifts in comparison to the PL spectra of non-doped films of the compounds. PL spectra of both exciplex-forming systems were found to be broad and typical of CT. The exciplex-forming mixtures 1: PO-T2T and 2: PO-T2T exhibited rather low PLQY values of 4% and 2% in air, respectively. The PL spectra of exciplexes 1: PO-T2T and 2: PO-T2T peaked at the wavelengths of 489 and 470 nm (2.48 and 2.67 eV), respectively (Fig. 8a). This observation can be explained by the relatively high IDP values of compounds 1 and 2 5.98 eV for compound 1 and 6.17 eV for compound 2 (Fig. 3 and Table 3) taking into account the equation38hνmaxex ≃ IDP − EAA − EC (2.54 eV for 1: PO-T2T and 2.67 eV for 2: PO-T2T), where IDP is the ionization potential of the donor (compound 1 or 2), EAA is the electron affinity of the acceptor (3.5 eV for PO-T2T), and EC is the electron–hole Coulombic attraction energy. It should be noted that the trifluorobiphenyl moiety was mainly used to increase the ionization potential of compounds 1 and 2, resulting in blue-shifted emission of exciplexes formed between compound 1 (or 2) and PO-T2T.
Fig. 8 Normalised PL spectra with photo of emissive layers inset (a) and PL decay curves (b) of solid molecular mixtures of 1 or 2 with PO-T2T (λexc = 330 nm). |
The molecular mixtures 1: PO-T2T and 2: PO-T2T were characterised by PL decays in the μs region with shapes attributed to exciplex emission but not to monomer emission (Fig. 8b). The nanosecond-lived components of the decay curves represent prompt fluorescence, whilst the longer-lived components can be attributed to thermally activated delayed fluorescence. This assumption is in agreement with the PL decays of exciplexes 1: PO-T2T and 2: PO-T2T recorded at room temperature (295 K) and at the temperature of liquid nitrogen (77 K) (Fig. S13, ESI†). The intensity of the long-lived component is higher at 295 K than at 77 K, proving the TADF nature of the exciplex emissions. PL decay curves for the films of the mixtures of 1 with PO-T2T and of 2 with PO-T2T in air and under vacuum were recorded. The increasing intensity (shown by the arrows) of the delayed component of their emission under an inert atmosphere indicates a triplet contribution to the whole emission intensity (Fig. S14, ESI†). The TADF origin of the delayed fluorescence of the exciplex-forming systems 1: PO-T2T and 2: PO-T2T was additionally confirmed by recording the dependence of PL intensity on laser pump pulse. The slope values of the straight lines were found to be ca. 1 (Fig. S15, ESI†).39
Fig. 10 and Table 5 show the EL characteristics of the exciplex-based OLEDs with derivatives 1 and 2. The overall OLED characteristics were better with D2, which was based on the emissive layer of the exciplex system 2: PO-T2T. OLED D2 showed a higher brightness of 4100 cd m−2, while that of D1 reached 1250 cd m−2. Also, D2 showed higher maximum current (CE), power (PE) and external quantum (EQE) efficiencies of 24.8 cd A−1, 12.2 lm W−1 and 7.8%, respectively. The corresponding characteristics of D1 were found to be 19.7 cd A−1, 12.2 lm W−1 and 6.5% (Table 5). The efficiency roll-off of D2 was better than that of D1 possibly because of the higher thermal stability of compound 2 relative to that of compound 1 (Table 1). When brightness was increased from 100 cd m−2 to 1000 cd m−2, the current, power and external quantum efficiencies also increased apparently due to the better charge carrier balance within the light-emitting layer at higher applied voltages. It should be noted that EQE values of both devices did not correlate with the PLQY values of solid films of 1 and 2. A similar observation was reported by Monkman et al.,38 who also used exciplex-forming mixtures with low PLQYs of films to obtain OLEDs with a high EQE. To the best of our knowledge, these are the best characteristics so far obtained from exciplex systems containing a fluorinated acceptor.28
Device | λ max (nm) | V on (V) | Brightnessmax (cd m−2) | CE/PE/EQE (cd A−1/lm W−1/%) | CE/PE/EQE (cd A−1/lm W−1/%) | CE/PE/EQE (cd A−1/lm W−1/%) |
---|---|---|---|---|---|---|
Max | @100 cd m−2 | @1000 cd m−2 | ||||
D1 | 541 | 4.8 | 1250 | 19.7/8.4/6.5 | 9.9/5.5/3.3 | 8.5/2.7/2.8 |
D2 | 546 | 4.8 | 4100 | 24.8/12.2/7.8 | 13.2/7.8/4.2 | 24.3/10.7/7.7 |
Fig. 10c shows the electroluminescence (EL) spectra of D1 and D2. The EL spectra of the devices did not correlate with the PL spectra of 1: PO-T2T and 2: PO-T2T (Fig. 7). PL spectra were observed in the blue region (Fig. 7), while the EL spectra were in the green region (Fig. 10c). This result is quite intriguing since there is no such functional material in the device structure that could be characterized by a similar green emission. This observation may be explained by the formation of lower energy exciplexes, which were detected for other exciplex-forming mixtures such as mCP:PO-T2T.37 To prove this presumption, PL spectra of the films of the mixtures 1: PO-T2T and 2: PO-T2T were recorded before and after thermal annealing at ca. 120 °C trying to separate the lower energy exciplexes (Fig. S16, ESI†). Indeed, the annealed film of 1: PO-T2T showed a red-shifted PL spectrum peaking at 505 nm in comparison to the PL spectrum of the non-annealed film (489 nm). Meanwhile, the PL spectra of the non-annealed and annealed films of 2: PO-T2T were practically identical. This experimental result does not fully explain the differences between the PL and EL spectra of 1: PO-T2T and 2: PO-T2T (Fig. 8a, 10c and Table 5).
On the another hand, the EL spectra of devices D1 and D2 can be affected by exciplex emission of other exciplex-forming systems, especially knowing that exciplexes can be formed at the interfaces or even through spacers.39,40 In principle, at least several exciplex-forming systems, such as: mCP:PO-T2T (472 nm),41–43 TAPC:PO-T2T (550 nm),44 and TAPC:TPBi (442 nm),44 could be formed in the devices. In addition, TAPC is characterized by excimer (450 nm) and electromer (580 nm) emission,45 although none of these species emit in the green region. To determine the origin of EL emission, the device structure was simplified to the following one: ITO/HAT-CN (10 nm)/TCTA (40 nm)/compound 1 (4 nm)/compound 1: PO-T2T (24 nm)/PO-T2T (40 nm)/LiF/Al (device R1). This device R1 was used as the reference, and the layers of TAPC, mCP and TPBi were replaced by those of tris(4-carbazoyl-9-ylphenyl)amine (TCTA), compound 1, and PO-T2T, respectively. As a result, the EL spectrum of device R1 peaking at 480 nm was very similar to the PL spectrum of 1: PO-T2T (Fig. 7 and 10d). The simplified device R1 showed a relatively high turn-on voltage of 9.8 V and maximum EQE of 3.4% (Fig. S17, ESI†). Having the EL spectra of the device R1, it can be concluded that the EL spectra of devices D1 and D2 most probably resulted from overlapping of emissions of exciplexes of 1: PO-T2T (2: PO-T2T) and the TAPC-based exciplex TAPC:PO-T2T (or electromer of TAPC). As a result of such overlapping, green electroluminescence was obtained for devices D1 and D2. Similar EL behaviour was previously discussed elsewhere.46 Blue- and red-shifted EL spectra were well reproduced for other configurations of TAPC-free and TAPC-containing devices, respectively (Fig. S18, ESI†). These observations prove the presumption of overlapping of two or more EL species.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0tc02777d |
This journal is © The Royal Society of Chemistry 2020 |