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Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap
Research
Fernando B. Dias
Cite this article: Dias FB. 2015 Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap. Phil. Trans. R. Soc. A 373: 20140447. http://dx.doi.org/10.1098/rsta.2014.0447
Department of Physics, University of Durham, Durham DH1 3LE, UK
Accepted: 9 March 2015 One contribution of 11 to a discussion meeting issue ‘Organic semiconductor spintronics: utilizing triplet excitons in organic electronics’. Subject Areas: chemical physics Keywords: thermally activated delayed fluorescence, delayed fluorescence, organic light-emitting diodes, reverse intersystem crossing, triplet–triplet annihilation Author for correspondence: Fernando B. Dias e-mail: [email protected]
Electronic supplementary material is available at http://dx.doi.org/10.1098/rsta.2014.0447 or via http://rsta.royalsocietypublishing.org.
FBD, 0000-0001-9841-863X The kinetics of thermally activated delayed fluorescence (TADF) is investigated in dilute solutions of organic materials with application in blue lightemitting diodes (OLEDs). A method to accurately determine the energy barrier (Ea ) and the rate of reverse intersystem crossing (kRisc ) in TADF emitters is developed, and applied to investigate the tripletharvesting mechanism in blue-emitting materials with large singlet–triplet energy gap (EST ). In these materials, triplet–triplet annihilation (TTA) is the dominant mechanism for triplet harvesting; however, above a threshold temperature TADF is able to compete with TTA and give enhanced delayed fluorescence. Evidence is obtained for the interplay between the TTA and the TADF mechanisms in these materials.
1. Introduction Thermally activated delayed fluorescence (TADF) has recently become one of the favorite methods to overcome the 25% limitation on the internal quantum efficiency of organic light-emitting diodes (OLEDs) due to spin statistics [1,2]. The ratio (1 : 3) between singlet and triplet excited states created from charge recombination in OLEDs imposes that only 25% of the charge recombination events will give origin to emissive singlet states, the rest is wasted by other non-radiative processes [3]. This imposes a serious limitation for the application of organic materials in displays and lighting devices [4]. Different methods have been used when trying to overcome this limitation, including the use of organic phosphors containing Ir(III), Pt(II) or other heavy metals, which can convert dark triplet states into emissive
2015 The Author(s) Published by the Royal Society. All rights reserved.
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Material synthesis has been reported previously [8,16]. Solutions (10−5 –10−4 M) of all materials were degassed using five freeze–thaw cycles. Absorption and emission spectra were collected
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2. Methods
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species with 100% efficiency due to the enhanced intersystem crossing via the heavy atom effect. These emitting complexes have to be dispersed in charge-transporting host materials with relatively higher energy triplet levels to avoid emission quenching. Finding appropriate host materials for emitters in the blue region is difficult. Also, the performance of blue-emitting phosphors is affected by substantial degradation, thus obtaining stable, long lifetime and deep blue-emitting phosphors has been difficult. Owing to these reasons, deep blue phosphorescent OLEDs with long operation lifetime have not yet been demonstrated [5]. Moreover, heavy metals are not abundant elements in nature, which will cause unnecessary stress on the fabrication costs of these devices. Using triplet–triplet annihilation (TTA) to up-convert non-emissive triplet states to emissive singlet states has also been attempted. In this case, the yield of singlet emissive states is highly dependent on the relative energy order of the excited singlet and triplet energy levels in the molecule, and the maximum total singlet yield is limited to 62.5%. OLEDs with TTA contribution as large as 40% have been demonstrated but the relative merit of this approach is still unsatisfactory [6,7]. The TADF mechanism uses the thermal energy to assist reverse intersystem crossing and promote the up-conversion of lower triplet states into more energetic but emissive singlet states [8]. The efficiency of the TADF mechanism is controlled by two main parameters, the energy splitting between the singlet and triplet states (EST ) and the efficiency of non-radiative pathways available for the excited singlet and triplet states to decay [9,10]. Minimization of EST has been achieved by designing materials that show intramolecular excited states with strong charge transfer character (ICT). The objective is to obtain minimal overlap between the highest occupied and lowest unoccupied molecular orbitals, HOMO and LUMO respectively, and thus achieve zero exchange energy and isoenergetic singlet and triplet states. Materials where the singlet and triplet states are separated by less than 0.1 eV have been synthesized in this way, and TADF (E-type fluorescence) emission has been observed with high efficiency in such emitters [11,12]. TADF emitters also have to be dispersed in host materials with relatively higher triplet levels, in order to avoid concentration quenching of fluorescence, and confine the excited states into the emitter molecule. This avoids the quenching of triplet states due to TTA. However, often host–guest interactions give origin to the formation of emissive exciplex states showing extended fluorescence lifetimes, and more complex photophysics [13,14]. Significant progress in recent years has been achieved on the design of materials that emit in the green region of the visible spectrum; however, TADF blue emitters are still scarce, and also red emitters with strong TADF contribution are not abundant. TADF is also finding application in other areas: for example, in bio-imaging applications due to the extended fluorescence lifetime and large Stokes-shift of these materials, which allows the tissue auto-fluorescence to be filtered and avoids the use of fluorescent probes based on heavy metals [15]. Further research work is therefore necessary to fully understand the mechanism and the way molecular structure affects the efficiency of triplet harvesting, via either TTA or TADF, in order to support the design of new emitters with stronger fluorescence yields and to cover the entire visible spectrum. For this objective the determination of fundamental kinetic parameters in a rapid and accurate manner is of paramount importance. Here, a kinetic method is developed to determine in an accurate and rapid way the energy barrier and the reverse intersystem crossing rate using kinetic analysis of the delayed fluorescence in TADF materials in dilute solution. Evidence is obtained for the interplay between the reverse intersystem crossing mechanisms via TTA and TADF in materials with singlet–triplet energy gaps much larger than the kT energy at room temperature.
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Figure 1 represents the energy diagram describing the kinetics of TADF in the long-time regime, i.e. after the prompt component of the fluorescence emission has decayed. Neglecting the prompt fluorescence component in this kinetic analysis is entirely justified since its lifetime is much shorter than the delayed fluorescence lifetime. The kinetic equations derived from figure 1, therefore, take into account only the singlet excited states that are formed from reverse intersystem crossing, and neglect the decay terms associated with the prompt component. The S1 and T1 populations are described in the long-time range by equations (3.1) and (3.2), dS1 = −kf S1 + kRisc T1 dt
(3.1)
dT1 = −(kRisc + kPH )T1 . dt
(3.2)
T1 (t) = T0 e−Xt
(3.3)
and
Solving (3.1) and (3.2) gives
and S1 (t) =
kRisc T0 e−Xt , kf − X
(3.4)
where X = kRisc + kPH . As expected for TADF, the delayed fluorescence decays with the triplet lifetime (see equation (3.3)). Equation (3.4) is used to determine the steady-state intensity of the delayed fluorescence, given by equation (3.5), where kf 0 represents the natural radiative decay rate constant and finst is an instrumental factor, +∞ T0 kRisc SS . (3.5) ITADF = finst kf 0 S1 (t)dt = finst kf 0 X(kf − X) 0 As kf X the above equation can be simplified to give SS = finst kf 0 ITADF
T0 kRisc . Xkf
(3.6)
From the above equation, it is clear that, in the case of TADF, the delayed fluorescence intensity SS versus excitation depends linearly on the excitation dose, i.e. in a log/log graph of ITADF dose, at constant temperature, the gradient is 1. This is in strong contrast to the delayed fluorescence appearing from TTA. In the TTA case, the variation of the steady-state delayed fluorescence intensity with excitation dose depends on the competition between the rate for triplet monomolecular decay, kPH , and the rate for diffusion collisional quenching of triplets, kTTA . When the former dominates, i.e. the triplets deactivate more quickly than the rate of SS shows a quadratic dependence on the triplet collisional diffusion (kTTA [T1 ] kPH [T1 ]), the ITTA excitation dose, equation (3.7). However, when TTA dominates, at higher triplet concentration,
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3. Kinetics of thermally activated delayed fluorescence
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using a UV-3600 double-beam spectrophotometer (Shimadzu), and a fluorescence spectrometer (Fluorolog-Jobin Yvon). Temperature-dependent measurements were acquired using a liquid nitrogen cryostat (Janis Research). Phosphorescence, prompt fluorescence (PF) and delayed emissions (DF) spectra and decays were recorded using nanosecond gated luminescence and lifetime measurements (from 400 ps to 1 s), using a high-energy pulsed Nd:YAG laser emitting at 355 nm (EKSPLA). Emission was focused on a spectrograph and detected in a sensitive gated iCCD camera (Stanford Computer Optics) with sub-nanosecond temporal resolution. Luminescence, PF and DF time-resolved measurements were performed by exponentially increasing gate and delay times; details can be found elsewhere [17].
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kSisc
S1
kf
N
N S O O
kPH (1) S0
Figure 1. Simplified energy diagram representing the up-conversion of triplet states (T1 ) to higher energy singlet states (S1 ). kRisc is the reverse intersystem crossing rate from T1 , and kf , kPH , represent the decay rates of singlet and triplet states to the −T S , and kisc are the intersystem crossing rate from S1 , ground state, respectively, including the internal conversion processes. kisc and the maximum reverse intersystem crossing rate, obtained from upper triplet levels to the S1 manifold. The chemical structure of compound (1) is shown on the right. (Online version in colour.)
SS dependence on the excitation dose turns to a linear regime (equation (3.8)), where f the ITTA represents the fraction of encounters between two triplets [18], SS = ITTA
and
fkTTA 2 T 2kPH 0
SS = 1 fT . ITTA 0 2
(3.7)
(3.8)
Going back to equation (3.6), there are two clear temperature regimes that can be defined for TADF. In the high-temperature limit (HTL), kRisc kPH and X ≈ kRisc , thus equation (3.6) further simplifies to T0 SS = finst kf 0 . (3.9) ITADF kf In this temperature limit, the intensity of the delayed fluorescence appearing from TADF is temperature independent, providing that T0 , kf 0 and kf are all not significantly affected by temperature in the interval used to follow the TADF. This is easily confirmed by following the intensity and fluorescence lifetime as a function of temperature. In the low-temperature limit (LTL), kRisc kPH and X ≈ kPH . In this regime, equation (3.6) is simplified to give T0 kRisc SS = finst kf 0 . (3.10) ITADF kPH kf Using the Arrhenius temperature dependence of the reverse intersystem crossing rate, kRisc = 0 e−Ea /RT , the energy barrier associated with TADF (E ) is determined directly from kRisc a equation (3.10). Usually the assumption is that this energy barrier is simply given by the S1 –T1 energy splitting (EST ), which is in principle, but not always, easily determined from the energy of the onset of the fluorescence spectra (S1 ) and phosphorescence spectra (T1 ). However, fluorescence from TADF emitters is usually broad and void of vibrational structure due to the strong charge transfer character of the lowest singlet excited state. This makes the S1 energy difficult to determine in an accurate manner. Moreover, phosphorescence is also usually observed at low temperature, and often the maximum of the phosphorescence emission spectrum shifts with temperature, which introduces another uncertainty on the determination of EST . Additionally, from measurements of the prompt and delayed fluorescence lifetime, kRisc is determined, and if Ea has been determined independently using (3.10), the maximum reverse intersystem crossing rate, i.e. the reverse intersystem crossing rate appearing from upper triplet −T can be also determined. levels in figure 1, kRisc
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k–T isc energy
4
–EST/RT kRisc = k–T isc e
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4. Results and discussion
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SS at room temperature Figure 3b shows the variation of the steady-state delayed fluorescence ITADF as a function of excitation dose, plotted in log/log scale, with a clear gradient 1, in agreement with equation (3.6) for the TADF mechanism. The temperature dependence of the delayed fluorescence of (1) in ethanol is shown in figure 4a. Clearly, the delayed fluorescence increases with temperature as expected for the TADF mechanism. The plot in figure 4b shows the HTL and LTL regimes, according with equations (3.9) and (3.10), and from the LTL regime the energy barrier for TADF is determined as Ea = 0.28 ± 0.02 eV. After determining the energy barrier for TADF, Ea = 0.28 ± 0.02 eV, and knowing the reverse intersystem crossing rate constant kRisc = (1.9 ± 0.2) × 102 s−1 , the maximum reverse intersystem −T = (1.4 ± 0.2) × 107 s−1 is also determined. Note that this method of crossing rate constant kRisc analysis does not rely on any complex fittings. In figure 4c, the corrected S1 energy is highlighted, ES1 = 2.91 ± 0.02 eV. The presence of a low-lying triplet charge transfer state, 3 CT, at energies immediately below the singlet charge transfer state, 1 CT, and above the triplet localized state, 3 LE, is often assumed to exist in many different studies on molecules that show intramolecular excited charge transfer states. In particular, this is seen with those that exhibit TADF [2,8,20]. Owing to the stronger coupling between the two triplet states, 3 LE and 3 CT, which facilitates the thermally assisted internal conversion, and owing to the small energy gap between 3 CT and 1 CT, which will promote reverse intersystem crossing, the presence of the intermediate 3 CT state enhances reverse intersystem crossing from 3 LE back to 1 CT. However, the energy gap between the 3 LE and 1 CT states is still the main effect that ultimately controls the recycling of triplets between 3 LE and 1 CT, i.e. if this energy gap is small ( 0.1 eV) the recycling 3 LE →1 CT →3 LE occurs in an efficient manner, but when the energy gap between 3 LE and 1 CT is much larger than 0.1 eV recycling is more unlikely to occur. The average number of recycling steps is easily determined from the ratio between the integrals of the delayed and prompt fluorescence, according to the equation below [10] (t)dt I TADF ¯ = n. (4.2) IPF (t)dt
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Figure 2a shows the absorption, fluorescence and phosphorescence spectra of (1) in ethanol dilute solution, and figure 2b shows the comparison between the steady-state fluorescence and the delayed fluorescence of (1) in ethanol. The fluorescence is broad and without vibrational resolution in clear contrast with the phosphorescence. This gives a strong indication that the fluorescence appears from a singlet excited state with strong charge transfer character, while the phosphorescence is from a more localized triplet excited state [8]. The steady-state fluorescence contains contributions from prompt and delayed components (IPF + ITADF ) and is in excellent agreement with the delayed fluorescence spectrum, showing that prompt and delayed fluorescence are originated from the same singlet excited state. From figure 2a, the energy splitting between the singlet and triplet states is estimated as 0.15 eV. Note that the error on this value can be large because of the uncertainty on the S1 energy, owing to the slow rising of the fluorescence onset. Previously a larger singlet–triplet energy gap was determined for this material using a different approach [8]. In figure 3a, the fluorescence decay of (1), followed over a time interval spanning nine decades, shows a clear bi-exponential decay with a fast component of 4.8 ± 0.1 ns, assigned to the prompt fluorescence component (IPF ), and a longer decay of 230 ± 5 µs, assigned to the delayed fluorescence component (ITADF ). From figure 3a, the reverse intersystem crossing rate constant is determined using equation (4.1) [9,10,19] to be kRisc = (1.9 ± 0.2) × 102 s−1 , ITADF (t)dt 1 . (4.1) kRisc = IPF (t)dt τTADF
5
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(b)
6
1.2 1/ethanol abs.
0.8
1/ethanol
fluor. fluor.
phosp.
delay. fluor.
0.4 2.78 eV
0
300
400 500 600 wavelength (nm)
700 400
500 600 wavelength (nm)
700
Figure 2. (a) Absorption, fluorescence and phosphorescence spectra of (1) in ethanol dilute solution, and (b) comparison between the steady-state fluorescence, containing contributions from prompt and delayed components (IPF + ITADF ), and the delayed fluorescence. (Online version in colour.) (b) 102 1
103
1/ethanol
emission integral (counts)
normalized intensity (arb. units)
(a)
prompt (4.8 ± 0.1 ns)
10–2 10–4
TADF (230 ± 5 µs) 10–6 10–8
102
10–10 –10 –9 10 10–8 10–7 10–6 10–5 10–4 10–3 10–2 10
10
1
102
10
time (s)
103
excitation dose (µJ)
Figure 3. (a) Time-resolved fluorescence decay of (1) in ethanol dilute solution, and (b) variation of the steady-state delayed fluorescence with excitation dose, showing that the delayed fluorescence is due to TADF. (Online version in colour.) (b) 6 DF-integral (counts)
intensity × 106 (cps)
1/ethanol 4
330 K
2 200 K 0 400
500 600 wavelength (nm)
700
(c)
104
1/ethanol Ea = 0.28 ± 0.02 eV
103 HTL
102 LTL
10
3.0
3.5 4.0 103/T (K–1)
4.5
normalized intensity (arb. units)
(a)
1.2
0.8
1/ethanol 2.63 eV phosp.
fluor.
0.4 2.91 eV 2.78 eV 0 400
500 wavelength (nm)
600
Figure 4. (a) Temperature dependence of the steady-state delayed fluorescence of (1) in ethanol dilute solution. (b) Variation of the delayed fluorescence integral with the reciprocal of temperature, according with equations (3.9) and (3.10), and determination of the energy barrier for the TADF. (c) Fluorescence and phosphorescence spectra of (1) with the S1 energy corrected for 2.91 eV, which is 0.28 eV above the T1 energy. (Online version in colour.)
For (1) the average number of recycling steps, 3 LE →1 CT →3 LE, is small, and no strong enhancement in the fluorescence intensity owing to TADF is observed. Next we move to investigate the delayed fluorescence mechanism in blue-emitting materials showing a large singlet–triplet energy gap. These materials are required for emission in the blue
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normalized intensity (arb. units)
(a)
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7
S O O
C6H13
(1)
(2) 1LE
1CT
3LE
C6H13
(3)
C6H13
ES1 = 3.75 eV
1CT
EST = 1.00 eV
3LE
ES1 = 2.91 eV EST = 0.28 eV
3LE
ET1 = 2.63 eV
(4)
S
1CT
ES1 = 3.43 eV EST = 0.47 eV
3LE
ET1 = 2.96 eV
ET1 = 2.65 eV
N
N
N
N
C6H13
C6H13
ES1 = 3.62 eV EST = 0.81 eV ET1 = 2.81 eV
Figure 5. Materials structure and state diagrams with indication of singlet (S1 ) and triplet (T1 ) energies. (Online version in colour.)
C6H13 C H 6 13
(2)
(c) 102
1.2 abs. 0.8
fluor.
0.4 0
(b) 1.2 del. fluor.
330 K
0.8 100 K
0.4 0 250
300
2/ethanol
2/ethanol DF integral × 106 (counts)
C6H13 C6H13
C6H13
int. × 106 (cps)
C6H13
norm. int. (arb. units)
(a)
350
400 450 500 wavelength (nm)
phosph.
10 Ea = 0.16 ± 0.05 eV
550
600
3.0
3.5 4.0 103/T (K–1)
4.5
Figure 6. Molecular structure of material (2). (a) Absorption and emission spectra of (2) in ethanol dilute solution. (b) Delayed fluorescence obtained in (2) as a function of temperature, weak phosphorescence is observed around 550 nm. (c) Temperature dependence of the delayed fluorescence, showing a clear TTA mechanism. (Online version in colour.)
region, where the TADF materials formed by relatively strong electron donor–electron acceptor units are more difficult to work, owing to the large Stokes-shift induced by the excited state charge transfer [21]. In fact, most TADF materials emit in the green region, and obtaining efficient TADF in the blue region is a very difficult task, which has not been fully achieved so far [20]. Figure 5 shows the chemical structure of the four materials used in this work, together with their state diagrams and singlet and triplet energies. Material (2), one of the blue emitters used in this work, is a pure non-charge transfer material, where only the 3 π π ∗ and the 1 π π ∗ transitions are involved on the population of the S1 excited state. The singlet–triplet energy gap in (2) is large, around 1.0 eV, and therefore TADF is not efficient. Material (2) is used purely as a reference for the TTA mechanism. The delayed luminescence observed in (2) is most probably entirely due to TTA (figure 6), and thus varies with the square of the excitation dose in agreement with equation (3.7), showing that in dilute solutions the monomolecular decay of the triplet state dominates over the triplet collisional diffusion, which gives origin to the delayed fluorescence. A smooth variation of the delayed fluorescence intensity is observed (figure 6c). In this regime, TTA is controlled by the rate of triplet collisional quenching, and thus the delayed fluorescence appearing as a result of TTA is affected by the continuous variation of the solvent viscosity with temperature; however, no threshold temperature for the observation of delayed fluorescence is detected, i.e. there is no well-defined temperature above which the delayed fluorescence intensity suddenly increases, as
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N
N
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C6H13 C6H13
C6H13
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S
(4) N
N
C6H13 C H 6 13
fluor./hexane
fluor./ethanol
(3)
8 fluor./hexane
phosp.
fluor./ethanol
(4)
phosp.
0.8
0.4
0
300
400 500 wavelength (nm)
600
300
400 500 wavelength (nm)
600
Figure 7. Molecular structures of materials (3) and (4). (a) Absorption, fluorescence and phosphorescence of (3) in hexane and ethanol. (b) Absorption, fluorescence and phosphorescence of (4) in hexane and ethanol. The phosphorescence is obtained in ethanol at 100 K. (Online version in colour.) is observed in TADF [8]. Moreover, the energy barrier determined by following the temperature dependence of the delayed fluorescence is 0.16 ± 0.02 eV and is in excellent agreement with the energy barrier for viscous flow in ethanol (0.15 eV) [21], clearly showing that the origin of the delayed luminescence is a diffusion-controlled mechanism. Interestingly, in (2) the delayed fluorescence spectrum does not match the steady-state fluorescence (figure 6a,b). This behaviour is not fully understood, and there is a possibility that the delayed luminescence in (2) is due to the formation of an excimer. However, this is very unlikely due to the low concentration used in these studies (less than 10−4 M), and the short fluorescence lifetime of (2), less than 10 ns, and also the extremely long lifetime of the delayed luminescence. For the interpretation of the experimental results in this work, however, the relevance of the results obtained with material (2) is directly related to the temperature dependence of the delayed luminescence, which shows that for mechanisms that are purely controlled by diffusion, e.g. the TTA and excimer formation mechanisms, the delayed luminescence should increase smoothly with temperature and show an energy barrier that is the barrier for viscous flow in ethanol (0.15 eV). Figure 7 shows the molecular structure of materials (3) and (4) with their absorption and fluorescence spectra in hexane and ethanol dilute solution. Both materials have no strong ICT character, which is confirmed by the lack of large Stokes-shifts, when comparing emissions in non-polar and polar solvents. The singlet and triplet energy gaps of (3) and (4) are determined directly from their fluorescence and phosphorescence spectra with good precision, (3) EST = 0.47 ± 0.05 eV and (4) EST = 0.81 ± 0.05 eV. In these materials, the emission grows steeply at the fluorescence spectra onset due to the weak-CT character of the S1 state, and no major uncertainty on the S1 energy exists. As expected, owing to the weak-CT character of (3) and (4), the singlet–triplet energy gaps are significantly larger than in the case of material (1). Therefore, the delayed fluorescence from (3) and (4) should appear exclusively from TTA and the temperature dependence should be similar to that observed for (2), i.e. no contribution from TADF should be observed. Clearly, looking at figure 8, this is not the case. Figure 8 shows the temperature dependence of the delayed fluorescence for materials (3) and (4). Instead of the expected temperature dependence observed for a mechanism controlled by diffusion, as is the case with TTA, and as is observed with material (2), the temperature dependence of the delayed fluorescence in (3) and (4) shows a well-defined threshold temperature above which the intensity of the delayed fluorescence increases dramatically. In material (4), for example, this regime occurs only in a very narrow interval above 280 K. Moreover, this threshold temperature scales with the magnitude of the singlet–triplet energy gap, and the energy barrier determined from the temperature dependence of the delayed fluorescence, according with equation (3.10), exactly matches the singlet–triplet energy gap determined from the fluorescence/phosphorescence spectra, in clear contrast with material (2). This behaviour
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(b) 1.2
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normalized intensity (arb. units)
(a) (3)
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0 300
400 500 600 wavelength (nm)
102
260 K
8
320 K
6
4
230 K
2 10 2.5 3.0 3.5 4.0 4.5 5.0 5.5 103/T (K–1)
0 300
400 500 600 wavelength (nm)
DF-integral × 106 (counts)
DF-integral × 106 (counts)
4
Ea = 0.47 ± 0.03 eV 103
104
(4)
(4) Ea = 0.83 ± 0.02 eV
103 280 K 102
10 2.5 3.0 3.5 4.0 4.5 5.0 5.5 103/T (K–1)
Figure 8. Temperature dependence of the steady-state delayed fluorescence of material (3) in figure 8a, and of material (4) in figure 8b, in ethanol dilute solution (a). Variation of the delayed fluorescence integral with the reciprocal of temperature, according with equations (3.9) and (3.10), and determination of the energy barrier for TADF (b). (Online version in colour.)
is thus assigned to the presence of a significant TADF component on the delayed fluorescence of materials (3) and (4). Intriguingly, the delayed fluorescence in both materials still shows a quadratic dependence on the excitation dose, which indicates that TTA is still the dominant mechanism that triggers the observation of TADF. For TADF to occur the excited triplet levels have to be populated, from which reverse intersystem crossing occurs back to the singlet manifold [20]. In materials with a small singlet– triplet energy gap, such as material (1), these upper triplet levels are populated using the thermal energy bath, and the delayed fluorescence varies linearly with the excitation dose. However, if the singlet–triplet energy gap is large, as it is in materials (3) and (4), the delayed fluorescence can only be obtained via the pure TTA mechanism, and in this case the delayed fluorescence intensity varies in a quadratic manner with the excitation dose. The behaviour observed in materials (3) and (4) shows that there is an intermediate situation where a mixed mechanism is operative. In this case, TTA is the dominant mechanism for the observation of delayed fluorescence, but above a threshold temperature, which varies with the magnitude of the singlet–triplet energy gap, TADF can compete with TTA and give origin to enhanced delayed fluorescence that is observed in a relatively efficient manner, even when the energy gap between the singlet and triplet states is large. The reason why this mechanism is active is still unclear, but the results strongly indicate that the presence of heteroatoms and the molecular structure of (3) and (4) facilitates the intersystem crossing and reverse intersystem crossing in these two molecules [8]. Note that both (3) and (4) show strong phosphorescence at low temperatures, which can be detected in a simple fluorimeter, using steady-state conditions and with no need of gated acquisition, i.e. at low temperature phosphorescence is able to compete strongly with internal conversion. In material (2), however, without heteroatoms no contribution from TADF is observed, even at high temperatures, and the phosphorescence is extremely weak at low temperatures. This observation, therefore, suggests that heteroatoms facilitate the coupling between the singlet and triplet states in (3) and (4) and are also responsible for the observation of TADF.
5. Conclusion In summary, the kinetics of the delayed fluorescence appearing as a result of triplet harvesting in organic blue emitters is studied in materials with increasing singlet–triplet energy gap. The kinetic analysis used in this study allows the determination of the energy barrier and the rate of reverse intersystem crossing in an accurate manner, even in molecular materials with strong charge
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material.
Acknowledgements. Thanks to Prof. Martin Bryce, who kindly provided the materials used in this work. Conflict of interests. I have no competing interests.
References 1. Im Y, Yeob J. 2014 Above 20% external quantum efficiency in thermally activated delayed fluorescence device using furodipyridine-type host materials. Chem. Mater. 26, 1413–1419. (doi:10.1021/cm403358h) 2. Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C. 2012 Highly efficient organic lightemitting diodes from delayed fluorescence. Nature 492, 234–238. (doi:10.1038/nature11687) 3. Baldo MA, O’Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, Forrest SR. 1998 Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 395, 151–154. (doi:10.1038/25954) 4. Forrest SR. 2004 The path to ubiquitous and low-cost organic electronic appliances on plastic. Nature 428, 911–918. (doi:10.1038/nature02498) 5. Baldo MA, Forrest SR, Thompson ME. 2005 Organic electrophosphorescence. In Organic electroluminescence (ed. ZH Kafafi), pp. 267–305. Boca Raton, CA: CRC Press. 6. Kondakov DY, Pawlik TD, Hatwar TK, Spindler JP. 2009 Triplet annihilation exceeding spin statistical limit in highly efficient fluorescent organic light-emitting diodes. J. Appl. Phys. 106, 124510. (doi:10.1063/1.3273407) 7. King SM, Cass M, Pintani M, Coward C, Dias FB, Monkman AP, Roberts M. 2011 The contribution of triplet-triplet annihilation to the lifetime and efficiency of fluorescent polymer organic light emitting diodes. J. Appl. Phys. 109, 074502. (doi:10.1063/1.3561430) 8. Dias FB, Bourdakos KN, Jankus V, Moss KC, Kamtekar KT, Bhalla V, Santos J, Bryce MR, Monkman AP. 2013 Triplet harvesting with 100% efficiency by way of thermally activated delayed fluorescence in charge transfer OLED emitters. Adv. Mater. 25, 3707–3714. (doi:10.1002/adma.201300753) 9. Berberan-Santos MN, Garcia JMM. 1996 Unusually strong delayed fluorescence of C70 . J. Am. Chem. Soc. 118, 9391–9394. (doi:10.1021/ja961782s) 10. Baleizão C, Berberan-Santos MN. 2007 Thermally activated delayed fluorescence as a cycling process between excited singlet and triplet states: application to the fullerenes. J. Chem. Phys. 126, 204510. (doi:10.1063/1.2734974) 11. Wu S, Aonuma M, Zhang Q, Huang S, Nakagawa T, Kuwabara K, Adachi C. 2014 Highefficiency deep-blue organic light-emitting diodes based on a thermally activated delayed fluorescence emitter. J. Mater. Chem. C 2, 421–424. (doi:10.1039/c3tc31936a) 12. Wang H, Xie L, Peng Q, Meng L, Wang Y, Yi Y, Wang P. 2014 Novel thermally activated delayed fluorescence materials—thioxanthone derivatives and their applications for highly efficient OLEDs. Adv. Mater. 26, 5198–5204. (doi:10.1002/adma.201401393) 13. Jankus V, Data P, Graves D, McGuinness C, Santos J, Bryce MR, Dias FB, Monkman AP. 2014 Highly efficient TADF OLEDs: how the emitter–host interaction controls both the excited state species and electrical properties of the devices to achieve near 100% triplet harvesting and high efficiency. Adv. Funct. Mater. 24, 6178–6186. (doi:10.1002/adfm.201400948) 14. Méhes G, Goushi K, Potscavage Jr WJ, Adachi C. 2014 Influence of host matrix on thermallyactivated delayed fluorescence: effects on emission lifetime, photoluminescence quantum yield, and device performance. Organic Elect. 15, 2027–2037. (doi:10.1016/j.orgel.2014.05.027) 15. Xiong X et al. 2014 Thermally activated delayed fluorescence of fluorescein derivative for timeresolved and confocal fluorescence imaging. J. Am. Chem. Soc. 136, 9590–9597. (doi:10.1021/ ja502292p) 16. Moss KC, Bourdakos KN, Bhalla V, Kamtekar KT, Bryce MR, Fox MA, Vaughan HL, Dias FB, Monkman AP. 2010 Tuning the intramolecular charge transfer emission from deep blue
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Data accessibility. The datasets supporting this article have been uploaded as part of the electronic supplementary
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transfer character, where the uncertainty on the determination of S1 energy is large. In blueemitting materials with significantly large singlet–triplet energy gap, and containing heteroatoms in their structure, the reverse intersystem crossing is facilitated and TADF is observed in competition with TTA, above a threshold temperature that depends on the magnitude of the singlet–triplet energy gap.
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to green in ambipolar systems based on dibenzothiophene s,s-dioxide by manipulation of conjugation and strength of the electron donor units. J. Org. Chem. 75, 6771–6781. (doi:10.1021/ jo100898a) Rothe C, Monkman AP. 2003 Triplet exciton migration in a conjugated polyfluorene. Phys. Rev. B 68, 075208. (doi:10.1103/PhysRevB.68.075208) Hertel D, Bassler H, Guentner R, Scherf U. 2001 Triplet-triplet annihilation in a polyfluorenederivative. J. Chem. Phys. 115, 10 007–10 013. (doi:10.1063/1.1415446) Baleizao C, Berberan-Santos MN. 2011 The brightest fullerene: a new isotope effect in molecular fluorescence and phosphorescence. ChemPhysChem 12, 1247–1250. (doi:10.1002/ cphc.201100156) Tao Y, Yuan K, Chen T, Xu P, Li H, Chen R, Zheng C, Zhang L, Huang W. 2014 Thermally activated delayed fluorescence materials towards the breakthrough of organoelectronics. Adv. Mater. 26, 7931–7958. (doi:10.1002/adma.201402532) Dias FB, Pollock S, Hedley G, Pålsson L, Monkman A, Perepichka II, Perepichka IF, Tavasli M, Bryce MR. 2006 Intramolecular charge transfer assisted by conformational changes in the excited state of fluorene-dibenzothiophene-S, S-dioxide co-oligomers. J. Phys. Chem. B, 110, 19 329–19 339. (doi:10.1021/jp0643653)
rsta.royalsocietypublishing.org
Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap
Research
Fernando B. Dias
Cite this article: Dias FB. 2015 Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap. Phil. Trans. R. Soc. A 373: 20140447. http://dx.doi.org/10.1098/rsta.2014.0447
Department of Physics, University of Durham, Durham DH1 3LE, UK
Accepted: 9 March 2015 One contribution of 11 to a discussion meeting issue ‘Organic semiconductor spintronics: utilizing triplet excitons in organic electronics’. Subject Areas: chemical physics Keywords: thermally activated delayed fluorescence, delayed fluorescence, organic light-emitting diodes, reverse intersystem crossing, triplet–triplet annihilation Author for correspondence: Fernando B. Dias e-mail: [email protected]
Electronic supplementary material is available at http://dx.doi.org/10.1098/rsta.2014.0447 or via http://rsta.royalsocietypublishing.org.
FBD, 0000-0001-9841-863X The kinetics of thermally activated delayed fluorescence (TADF) is investigated in dilute solutions of organic materials with application in blue lightemitting diodes (OLEDs). A method to accurately determine the energy barrier (Ea ) and the rate of reverse intersystem crossing (kRisc ) in TADF emitters is developed, and applied to investigate the tripletharvesting mechanism in blue-emitting materials with large singlet–triplet energy gap (EST ). In these materials, triplet–triplet annihilation (TTA) is the dominant mechanism for triplet harvesting; however, above a threshold temperature TADF is able to compete with TTA and give enhanced delayed fluorescence. Evidence is obtained for the interplay between the TTA and the TADF mechanisms in these materials.
1. Introduction Thermally activated delayed fluorescence (TADF) has recently become one of the favorite methods to overcome the 25% limitation on the internal quantum efficiency of organic light-emitting diodes (OLEDs) due to spin statistics [1,2]. The ratio (1 : 3) between singlet and triplet excited states created from charge recombination in OLEDs imposes that only 25% of the charge recombination events will give origin to emissive singlet states, the rest is wasted by other non-radiative processes [3]. This imposes a serious limitation for the application of organic materials in displays and lighting devices [4]. Different methods have been used when trying to overcome this limitation, including the use of organic phosphors containing Ir(III), Pt(II) or other heavy metals, which can convert dark triplet states into emissive
2015 The Author(s) Published by the Royal Society. All rights reserved.
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Material synthesis has been reported previously [8,16]. Solutions (10−5 –10−4 M) of all materials were degassed using five freeze–thaw cycles. Absorption and emission spectra were collected
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2. Methods
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species with 100% efficiency due to the enhanced intersystem crossing via the heavy atom effect. These emitting complexes have to be dispersed in charge-transporting host materials with relatively higher energy triplet levels to avoid emission quenching. Finding appropriate host materials for emitters in the blue region is difficult. Also, the performance of blue-emitting phosphors is affected by substantial degradation, thus obtaining stable, long lifetime and deep blue-emitting phosphors has been difficult. Owing to these reasons, deep blue phosphorescent OLEDs with long operation lifetime have not yet been demonstrated [5]. Moreover, heavy metals are not abundant elements in nature, which will cause unnecessary stress on the fabrication costs of these devices. Using triplet–triplet annihilation (TTA) to up-convert non-emissive triplet states to emissive singlet states has also been attempted. In this case, the yield of singlet emissive states is highly dependent on the relative energy order of the excited singlet and triplet energy levels in the molecule, and the maximum total singlet yield is limited to 62.5%. OLEDs with TTA contribution as large as 40% have been demonstrated but the relative merit of this approach is still unsatisfactory [6,7]. The TADF mechanism uses the thermal energy to assist reverse intersystem crossing and promote the up-conversion of lower triplet states into more energetic but emissive singlet states [8]. The efficiency of the TADF mechanism is controlled by two main parameters, the energy splitting between the singlet and triplet states (EST ) and the efficiency of non-radiative pathways available for the excited singlet and triplet states to decay [9,10]. Minimization of EST has been achieved by designing materials that show intramolecular excited states with strong charge transfer character (ICT). The objective is to obtain minimal overlap between the highest occupied and lowest unoccupied molecular orbitals, HOMO and LUMO respectively, and thus achieve zero exchange energy and isoenergetic singlet and triplet states. Materials where the singlet and triplet states are separated by less than 0.1 eV have been synthesized in this way, and TADF (E-type fluorescence) emission has been observed with high efficiency in such emitters [11,12]. TADF emitters also have to be dispersed in host materials with relatively higher triplet levels, in order to avoid concentration quenching of fluorescence, and confine the excited states into the emitter molecule. This avoids the quenching of triplet states due to TTA. However, often host–guest interactions give origin to the formation of emissive exciplex states showing extended fluorescence lifetimes, and more complex photophysics [13,14]. Significant progress in recent years has been achieved on the design of materials that emit in the green region of the visible spectrum; however, TADF blue emitters are still scarce, and also red emitters with strong TADF contribution are not abundant. TADF is also finding application in other areas: for example, in bio-imaging applications due to the extended fluorescence lifetime and large Stokes-shift of these materials, which allows the tissue auto-fluorescence to be filtered and avoids the use of fluorescent probes based on heavy metals [15]. Further research work is therefore necessary to fully understand the mechanism and the way molecular structure affects the efficiency of triplet harvesting, via either TTA or TADF, in order to support the design of new emitters with stronger fluorescence yields and to cover the entire visible spectrum. For this objective the determination of fundamental kinetic parameters in a rapid and accurate manner is of paramount importance. Here, a kinetic method is developed to determine in an accurate and rapid way the energy barrier and the reverse intersystem crossing rate using kinetic analysis of the delayed fluorescence in TADF materials in dilute solution. Evidence is obtained for the interplay between the reverse intersystem crossing mechanisms via TTA and TADF in materials with singlet–triplet energy gaps much larger than the kT energy at room temperature.
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Figure 1 represents the energy diagram describing the kinetics of TADF in the long-time regime, i.e. after the prompt component of the fluorescence emission has decayed. Neglecting the prompt fluorescence component in this kinetic analysis is entirely justified since its lifetime is much shorter than the delayed fluorescence lifetime. The kinetic equations derived from figure 1, therefore, take into account only the singlet excited states that are formed from reverse intersystem crossing, and neglect the decay terms associated with the prompt component. The S1 and T1 populations are described in the long-time range by equations (3.1) and (3.2), dS1 = −kf S1 + kRisc T1 dt
(3.1)
dT1 = −(kRisc + kPH )T1 . dt
(3.2)
T1 (t) = T0 e−Xt
(3.3)
and
Solving (3.1) and (3.2) gives
and S1 (t) =
kRisc T0 e−Xt , kf − X
(3.4)
where X = kRisc + kPH . As expected for TADF, the delayed fluorescence decays with the triplet lifetime (see equation (3.3)). Equation (3.4) is used to determine the steady-state intensity of the delayed fluorescence, given by equation (3.5), where kf 0 represents the natural radiative decay rate constant and finst is an instrumental factor, +∞ T0 kRisc SS . (3.5) ITADF = finst kf 0 S1 (t)dt = finst kf 0 X(kf − X) 0 As kf X the above equation can be simplified to give SS = finst kf 0 ITADF
T0 kRisc . Xkf
(3.6)
From the above equation, it is clear that, in the case of TADF, the delayed fluorescence intensity SS versus excitation depends linearly on the excitation dose, i.e. in a log/log graph of ITADF dose, at constant temperature, the gradient is 1. This is in strong contrast to the delayed fluorescence appearing from TTA. In the TTA case, the variation of the steady-state delayed fluorescence intensity with excitation dose depends on the competition between the rate for triplet monomolecular decay, kPH , and the rate for diffusion collisional quenching of triplets, kTTA . When the former dominates, i.e. the triplets deactivate more quickly than the rate of SS shows a quadratic dependence on the triplet collisional diffusion (kTTA [T1 ] kPH [T1 ]), the ITTA excitation dose, equation (3.7). However, when TTA dominates, at higher triplet concentration,
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3. Kinetics of thermally activated delayed fluorescence
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using a UV-3600 double-beam spectrophotometer (Shimadzu), and a fluorescence spectrometer (Fluorolog-Jobin Yvon). Temperature-dependent measurements were acquired using a liquid nitrogen cryostat (Janis Research). Phosphorescence, prompt fluorescence (PF) and delayed emissions (DF) spectra and decays were recorded using nanosecond gated luminescence and lifetime measurements (from 400 ps to 1 s), using a high-energy pulsed Nd:YAG laser emitting at 355 nm (EKSPLA). Emission was focused on a spectrograph and detected in a sensitive gated iCCD camera (Stanford Computer Optics) with sub-nanosecond temporal resolution. Luminescence, PF and DF time-resolved measurements were performed by exponentially increasing gate and delay times; details can be found elsewhere [17].
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kSisc
S1
kf
N
N S O O
kPH (1) S0
Figure 1. Simplified energy diagram representing the up-conversion of triplet states (T1 ) to higher energy singlet states (S1 ). kRisc is the reverse intersystem crossing rate from T1 , and kf , kPH , represent the decay rates of singlet and triplet states to the −T S , and kisc are the intersystem crossing rate from S1 , ground state, respectively, including the internal conversion processes. kisc and the maximum reverse intersystem crossing rate, obtained from upper triplet levels to the S1 manifold. The chemical structure of compound (1) is shown on the right. (Online version in colour.)
SS dependence on the excitation dose turns to a linear regime (equation (3.8)), where f the ITTA represents the fraction of encounters between two triplets [18], SS = ITTA
and
fkTTA 2 T 2kPH 0
SS = 1 fT . ITTA 0 2
(3.7)
(3.8)
Going back to equation (3.6), there are two clear temperature regimes that can be defined for TADF. In the high-temperature limit (HTL), kRisc kPH and X ≈ kRisc , thus equation (3.6) further simplifies to T0 SS = finst kf 0 . (3.9) ITADF kf In this temperature limit, the intensity of the delayed fluorescence appearing from TADF is temperature independent, providing that T0 , kf 0 and kf are all not significantly affected by temperature in the interval used to follow the TADF. This is easily confirmed by following the intensity and fluorescence lifetime as a function of temperature. In the low-temperature limit (LTL), kRisc kPH and X ≈ kPH . In this regime, equation (3.6) is simplified to give T0 kRisc SS = finst kf 0 . (3.10) ITADF kPH kf Using the Arrhenius temperature dependence of the reverse intersystem crossing rate, kRisc = 0 e−Ea /RT , the energy barrier associated with TADF (E ) is determined directly from kRisc a equation (3.10). Usually the assumption is that this energy barrier is simply given by the S1 –T1 energy splitting (EST ), which is in principle, but not always, easily determined from the energy of the onset of the fluorescence spectra (S1 ) and phosphorescence spectra (T1 ). However, fluorescence from TADF emitters is usually broad and void of vibrational structure due to the strong charge transfer character of the lowest singlet excited state. This makes the S1 energy difficult to determine in an accurate manner. Moreover, phosphorescence is also usually observed at low temperature, and often the maximum of the phosphorescence emission spectrum shifts with temperature, which introduces another uncertainty on the determination of EST . Additionally, from measurements of the prompt and delayed fluorescence lifetime, kRisc is determined, and if Ea has been determined independently using (3.10), the maximum reverse intersystem crossing rate, i.e. the reverse intersystem crossing rate appearing from upper triplet −T can be also determined. levels in figure 1, kRisc
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T1
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k–T isc energy
4
–EST/RT kRisc = k–T isc e
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4. Results and discussion
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SS at room temperature Figure 3b shows the variation of the steady-state delayed fluorescence ITADF as a function of excitation dose, plotted in log/log scale, with a clear gradient 1, in agreement with equation (3.6) for the TADF mechanism. The temperature dependence of the delayed fluorescence of (1) in ethanol is shown in figure 4a. Clearly, the delayed fluorescence increases with temperature as expected for the TADF mechanism. The plot in figure 4b shows the HTL and LTL regimes, according with equations (3.9) and (3.10), and from the LTL regime the energy barrier for TADF is determined as Ea = 0.28 ± 0.02 eV. After determining the energy barrier for TADF, Ea = 0.28 ± 0.02 eV, and knowing the reverse intersystem crossing rate constant kRisc = (1.9 ± 0.2) × 102 s−1 , the maximum reverse intersystem −T = (1.4 ± 0.2) × 107 s−1 is also determined. Note that this method of crossing rate constant kRisc analysis does not rely on any complex fittings. In figure 4c, the corrected S1 energy is highlighted, ES1 = 2.91 ± 0.02 eV. The presence of a low-lying triplet charge transfer state, 3 CT, at energies immediately below the singlet charge transfer state, 1 CT, and above the triplet localized state, 3 LE, is often assumed to exist in many different studies on molecules that show intramolecular excited charge transfer states. In particular, this is seen with those that exhibit TADF [2,8,20]. Owing to the stronger coupling between the two triplet states, 3 LE and 3 CT, which facilitates the thermally assisted internal conversion, and owing to the small energy gap between 3 CT and 1 CT, which will promote reverse intersystem crossing, the presence of the intermediate 3 CT state enhances reverse intersystem crossing from 3 LE back to 1 CT. However, the energy gap between the 3 LE and 1 CT states is still the main effect that ultimately controls the recycling of triplets between 3 LE and 1 CT, i.e. if this energy gap is small ( 0.1 eV) the recycling 3 LE →1 CT →3 LE occurs in an efficient manner, but when the energy gap between 3 LE and 1 CT is much larger than 0.1 eV recycling is more unlikely to occur. The average number of recycling steps is easily determined from the ratio between the integrals of the delayed and prompt fluorescence, according to the equation below [10] (t)dt I TADF ¯ = n. (4.2) IPF (t)dt
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Figure 2a shows the absorption, fluorescence and phosphorescence spectra of (1) in ethanol dilute solution, and figure 2b shows the comparison between the steady-state fluorescence and the delayed fluorescence of (1) in ethanol. The fluorescence is broad and without vibrational resolution in clear contrast with the phosphorescence. This gives a strong indication that the fluorescence appears from a singlet excited state with strong charge transfer character, while the phosphorescence is from a more localized triplet excited state [8]. The steady-state fluorescence contains contributions from prompt and delayed components (IPF + ITADF ) and is in excellent agreement with the delayed fluorescence spectrum, showing that prompt and delayed fluorescence are originated from the same singlet excited state. From figure 2a, the energy splitting between the singlet and triplet states is estimated as 0.15 eV. Note that the error on this value can be large because of the uncertainty on the S1 energy, owing to the slow rising of the fluorescence onset. Previously a larger singlet–triplet energy gap was determined for this material using a different approach [8]. In figure 3a, the fluorescence decay of (1), followed over a time interval spanning nine decades, shows a clear bi-exponential decay with a fast component of 4.8 ± 0.1 ns, assigned to the prompt fluorescence component (IPF ), and a longer decay of 230 ± 5 µs, assigned to the delayed fluorescence component (ITADF ). From figure 3a, the reverse intersystem crossing rate constant is determined using equation (4.1) [9,10,19] to be kRisc = (1.9 ± 0.2) × 102 s−1 , ITADF (t)dt 1 . (4.1) kRisc = IPF (t)dt τTADF
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(b)
6
1.2 1/ethanol abs.
0.8
1/ethanol
fluor. fluor.
phosp.
delay. fluor.
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400 500 600 wavelength (nm)
700 400
500 600 wavelength (nm)
700
Figure 2. (a) Absorption, fluorescence and phosphorescence spectra of (1) in ethanol dilute solution, and (b) comparison between the steady-state fluorescence, containing contributions from prompt and delayed components (IPF + ITADF ), and the delayed fluorescence. (Online version in colour.) (b) 102 1
103
1/ethanol
emission integral (counts)
normalized intensity (arb. units)
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prompt (4.8 ± 0.1 ns)
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TADF (230 ± 5 µs) 10–6 10–8
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10–10 –10 –9 10 10–8 10–7 10–6 10–5 10–4 10–3 10–2 10
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102
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103
excitation dose (µJ)
Figure 3. (a) Time-resolved fluorescence decay of (1) in ethanol dilute solution, and (b) variation of the steady-state delayed fluorescence with excitation dose, showing that the delayed fluorescence is due to TADF. (Online version in colour.) (b) 6 DF-integral (counts)
intensity × 106 (cps)
1/ethanol 4
330 K
2 200 K 0 400
500 600 wavelength (nm)
700
(c)
104
1/ethanol Ea = 0.28 ± 0.02 eV
103 HTL
102 LTL
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3.5 4.0 103/T (K–1)
4.5
normalized intensity (arb. units)
(a)
1.2
0.8
1/ethanol 2.63 eV phosp.
fluor.
0.4 2.91 eV 2.78 eV 0 400
500 wavelength (nm)
600
Figure 4. (a) Temperature dependence of the steady-state delayed fluorescence of (1) in ethanol dilute solution. (b) Variation of the delayed fluorescence integral with the reciprocal of temperature, according with equations (3.9) and (3.10), and determination of the energy barrier for the TADF. (c) Fluorescence and phosphorescence spectra of (1) with the S1 energy corrected for 2.91 eV, which is 0.28 eV above the T1 energy. (Online version in colour.)
For (1) the average number of recycling steps, 3 LE →1 CT →3 LE, is small, and no strong enhancement in the fluorescence intensity owing to TADF is observed. Next we move to investigate the delayed fluorescence mechanism in blue-emitting materials showing a large singlet–triplet energy gap. These materials are required for emission in the blue
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2.63 eV
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normalized intensity (arb. units)
(a)
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7
S O O
C6H13
(1)
(2) 1LE
1CT
3LE
C6H13
(3)
C6H13
ES1 = 3.75 eV
1CT
EST = 1.00 eV
3LE
ES1 = 2.91 eV EST = 0.28 eV
3LE
ET1 = 2.63 eV
(4)
S
1CT
ES1 = 3.43 eV EST = 0.47 eV
3LE
ET1 = 2.96 eV
ET1 = 2.65 eV
N
N
N
N
C6H13
C6H13
ES1 = 3.62 eV EST = 0.81 eV ET1 = 2.81 eV
Figure 5. Materials structure and state diagrams with indication of singlet (S1 ) and triplet (T1 ) energies. (Online version in colour.)
C6H13 C H 6 13
(2)
(c) 102
1.2 abs. 0.8
fluor.
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0.8 100 K
0.4 0 250
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2/ethanol
2/ethanol DF integral × 106 (counts)
C6H13 C6H13
C6H13
int. × 106 (cps)
C6H13
norm. int. (arb. units)
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350
400 450 500 wavelength (nm)
phosph.
10 Ea = 0.16 ± 0.05 eV
550
600
3.0
3.5 4.0 103/T (K–1)
4.5
Figure 6. Molecular structure of material (2). (a) Absorption and emission spectra of (2) in ethanol dilute solution. (b) Delayed fluorescence obtained in (2) as a function of temperature, weak phosphorescence is observed around 550 nm. (c) Temperature dependence of the delayed fluorescence, showing a clear TTA mechanism. (Online version in colour.)
region, where the TADF materials formed by relatively strong electron donor–electron acceptor units are more difficult to work, owing to the large Stokes-shift induced by the excited state charge transfer [21]. In fact, most TADF materials emit in the green region, and obtaining efficient TADF in the blue region is a very difficult task, which has not been fully achieved so far [20]. Figure 5 shows the chemical structure of the four materials used in this work, together with their state diagrams and singlet and triplet energies. Material (2), one of the blue emitters used in this work, is a pure non-charge transfer material, where only the 3 π π ∗ and the 1 π π ∗ transitions are involved on the population of the S1 excited state. The singlet–triplet energy gap in (2) is large, around 1.0 eV, and therefore TADF is not efficient. Material (2) is used purely as a reference for the TTA mechanism. The delayed luminescence observed in (2) is most probably entirely due to TTA (figure 6), and thus varies with the square of the excitation dose in agreement with equation (3.7), showing that in dilute solutions the monomolecular decay of the triplet state dominates over the triplet collisional diffusion, which gives origin to the delayed fluorescence. A smooth variation of the delayed fluorescence intensity is observed (figure 6c). In this regime, TTA is controlled by the rate of triplet collisional quenching, and thus the delayed fluorescence appearing as a result of TTA is affected by the continuous variation of the solvent viscosity with temperature; however, no threshold temperature for the observation of delayed fluorescence is detected, i.e. there is no well-defined temperature above which the delayed fluorescence intensity suddenly increases, as
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N
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C6H13
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S
(4) N
N
C6H13 C H 6 13
fluor./hexane
fluor./ethanol
(3)
8 fluor./hexane
phosp.
fluor./ethanol
(4)
phosp.
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0.4
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300
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Figure 7. Molecular structures of materials (3) and (4). (a) Absorption, fluorescence and phosphorescence of (3) in hexane and ethanol. (b) Absorption, fluorescence and phosphorescence of (4) in hexane and ethanol. The phosphorescence is obtained in ethanol at 100 K. (Online version in colour.) is observed in TADF [8]. Moreover, the energy barrier determined by following the temperature dependence of the delayed fluorescence is 0.16 ± 0.02 eV and is in excellent agreement with the energy barrier for viscous flow in ethanol (0.15 eV) [21], clearly showing that the origin of the delayed luminescence is a diffusion-controlled mechanism. Interestingly, in (2) the delayed fluorescence spectrum does not match the steady-state fluorescence (figure 6a,b). This behaviour is not fully understood, and there is a possibility that the delayed luminescence in (2) is due to the formation of an excimer. However, this is very unlikely due to the low concentration used in these studies (less than 10−4 M), and the short fluorescence lifetime of (2), less than 10 ns, and also the extremely long lifetime of the delayed luminescence. For the interpretation of the experimental results in this work, however, the relevance of the results obtained with material (2) is directly related to the temperature dependence of the delayed luminescence, which shows that for mechanisms that are purely controlled by diffusion, e.g. the TTA and excimer formation mechanisms, the delayed luminescence should increase smoothly with temperature and show an energy barrier that is the barrier for viscous flow in ethanol (0.15 eV). Figure 7 shows the molecular structure of materials (3) and (4) with their absorption and fluorescence spectra in hexane and ethanol dilute solution. Both materials have no strong ICT character, which is confirmed by the lack of large Stokes-shifts, when comparing emissions in non-polar and polar solvents. The singlet and triplet energy gaps of (3) and (4) are determined directly from their fluorescence and phosphorescence spectra with good precision, (3) EST = 0.47 ± 0.05 eV and (4) EST = 0.81 ± 0.05 eV. In these materials, the emission grows steeply at the fluorescence spectra onset due to the weak-CT character of the S1 state, and no major uncertainty on the S1 energy exists. As expected, owing to the weak-CT character of (3) and (4), the singlet–triplet energy gaps are significantly larger than in the case of material (1). Therefore, the delayed fluorescence from (3) and (4) should appear exclusively from TTA and the temperature dependence should be similar to that observed for (2), i.e. no contribution from TADF should be observed. Clearly, looking at figure 8, this is not the case. Figure 8 shows the temperature dependence of the delayed fluorescence for materials (3) and (4). Instead of the expected temperature dependence observed for a mechanism controlled by diffusion, as is the case with TTA, and as is observed with material (2), the temperature dependence of the delayed fluorescence in (3) and (4) shows a well-defined threshold temperature above which the intensity of the delayed fluorescence increases dramatically. In material (4), for example, this regime occurs only in a very narrow interval above 280 K. Moreover, this threshold temperature scales with the magnitude of the singlet–triplet energy gap, and the energy barrier determined from the temperature dependence of the delayed fluorescence, according with equation (3.10), exactly matches the singlet–triplet energy gap determined from the fluorescence/phosphorescence spectra, in clear contrast with material (2). This behaviour
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Figure 8. Temperature dependence of the steady-state delayed fluorescence of material (3) in figure 8a, and of material (4) in figure 8b, in ethanol dilute solution (a). Variation of the delayed fluorescence integral with the reciprocal of temperature, according with equations (3.9) and (3.10), and determination of the energy barrier for TADF (b). (Online version in colour.)
is thus assigned to the presence of a significant TADF component on the delayed fluorescence of materials (3) and (4). Intriguingly, the delayed fluorescence in both materials still shows a quadratic dependence on the excitation dose, which indicates that TTA is still the dominant mechanism that triggers the observation of TADF. For TADF to occur the excited triplet levels have to be populated, from which reverse intersystem crossing occurs back to the singlet manifold [20]. In materials with a small singlet– triplet energy gap, such as material (1), these upper triplet levels are populated using the thermal energy bath, and the delayed fluorescence varies linearly with the excitation dose. However, if the singlet–triplet energy gap is large, as it is in materials (3) and (4), the delayed fluorescence can only be obtained via the pure TTA mechanism, and in this case the delayed fluorescence intensity varies in a quadratic manner with the excitation dose. The behaviour observed in materials (3) and (4) shows that there is an intermediate situation where a mixed mechanism is operative. In this case, TTA is the dominant mechanism for the observation of delayed fluorescence, but above a threshold temperature, which varies with the magnitude of the singlet–triplet energy gap, TADF can compete with TTA and give origin to enhanced delayed fluorescence that is observed in a relatively efficient manner, even when the energy gap between the singlet and triplet states is large. The reason why this mechanism is active is still unclear, but the results strongly indicate that the presence of heteroatoms and the molecular structure of (3) and (4) facilitates the intersystem crossing and reverse intersystem crossing in these two molecules [8]. Note that both (3) and (4) show strong phosphorescence at low temperatures, which can be detected in a simple fluorimeter, using steady-state conditions and with no need of gated acquisition, i.e. at low temperature phosphorescence is able to compete strongly with internal conversion. In material (2), however, without heteroatoms no contribution from TADF is observed, even at high temperatures, and the phosphorescence is extremely weak at low temperatures. This observation, therefore, suggests that heteroatoms facilitate the coupling between the singlet and triplet states in (3) and (4) and are also responsible for the observation of TADF.
5. Conclusion In summary, the kinetics of the delayed fluorescence appearing as a result of triplet harvesting in organic blue emitters is studied in materials with increasing singlet–triplet energy gap. The kinetic analysis used in this study allows the determination of the energy barrier and the rate of reverse intersystem crossing in an accurate manner, even in molecular materials with strong charge
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Acknowledgements. Thanks to Prof. Martin Bryce, who kindly provided the materials used in this work. Conflict of interests. I have no competing interests.
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