Design of a New Mechanism beyond Thermally Activated Delayed

Subscriber access provided by UNIV OF SOUTHERN INDIANA

Article

Design of a new mechanism beyond thermally activated delayed fluorescence towards fourth generation organic light emitting diodes Hartmut Yersin, Larisa Mataranga-Popa, Rafa# Czerwieniec, and Yan Dovbii Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01168 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Design of a new mechanism beyond thermally activated delayed fluorescence towards fourth generation organic light emitting diodes Hartmut Yersin*, Larisa Mataranga-Popa, Rafał Czerwieniec, Yan Dovbii Institute for Physical Chemistry, University of Regensburg, Germany *[email protected] ORCID Hartmut Yersin: 0000-0003-3216-1370 ORCID Rafał Czerwieniec: 0000-0003-4789-6273

Abstract Usually, development of organic molecules with efficient thermally activated delayed fluorescence (TADF) focuses on minimizing the energy gap between the lowest singlet and triplet state. However, although this is crucial, it is not sufficient for optimizing the emitter´s molecular and electronic structure for OLED use. Here, we present a design strategy that does not lead us only to a new type of emitter, but also to a new exciton harvesting mechanism. This concept is realized (i) by drastically reducing the energy gap between the lowest singlet and triplet energy states of, (ii) by rigidifying the molecular structure to reduce inhomogeneity effects that usually induce long emission decay tails lying even in the ms time range, (iii) by maximizing the Franck-Condon factors that govern intersystem crossing (ISC), (iv) by shifting the charge transfer states, 1CT and 3CT, to become the lowest energy states applying polarity tuning, and (v) by providing energetically near-by lying states for spin-orbit coupling (SOC) and configuration interaction (CI) paths to speed-up ISC. Using this concept, we design an “almost zero-gap” compound showing E(1CT3CT) ≈ 16 cm-1 (2 meV). Thus, thermal activation is no longer a time delaying key problem at T = 300 K. Moreover, if the emitter is applied in an OLED, fast ISC will allow us to harvest all singlet and triplet excitons through emission from the lowest excited CT singlet state. This benchmark mechanism, the Direct Singlet Harvesting (DSH) Mechanism, offers the great advantage of a significant reduction of the overall emission decay time to the sub-s range. This is a shorter decay than found for TADF emission so far. Accordingly, this mechanism leads us to beyond TADF 1 ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

towards a new era in the design of OLED emitters and opens the way for reducing stability problems and roll-off effects. Introduction Application of organic molecules as emitters for efficient OLEDs has been made possible by the development of compounds that exhibit relatively small energy gaps E(S1T1) between the lowest singlet S1 (1CT) and triplet T1 (3CT) states.1-3 This allows to exploit the effect of thermally activated delayed fluorescence (TADF)4 in OLEDs to harvest up to 100 % of all generated triplet and singlet excitons through emission from the lowest singlet state, representing the singlet harvesting mechanism.5,6,7 Accordingly, a number of comprehensive and valuable TADF reviews appeared recently.8,9,10 Meanwhile, this approach is also characterized by an exceptional interdisciplinary research in the fields of chemistry, physics, and material sciences.1,2 For OLED applications, the emitters should exhibit an emission decay time as short as possible. This is important to minimize decomposition reactions in the excited state.11,12 However, for most TADF molecules known so far, the decay time is as long as many s. Moreover, mostly one finds additionally long decay tails even of several ms. In this investigation, we want to present design rules for new materials that show emission decay times in the sub-s range by optimizing essential emitter properties. This will not only lead us to a new type of short-lived emitter materials, but will also guide us to a new and promising exciton harvesting mechanism beyond the time-delaying TADF mechanism. The targeted key material should exhibit a series of structure/property motifs: (i) The energy separations between the lowest singlet and triplet excited states of charge transfer (CT) character should be minimized to a few meV (several cm-1). This can be achieved by designing molecular structures with spatially well separated frontier orbitals providing small exchange interaction and thus, small E(1CT3CT). However, the gap, or the HOMO-LUMO overlap, should not become zero, since in this case, the allowedness of the singlet 2 ACS Paragon Plus Environment

Page 2 of 19

Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fluorescence would vanish.7 (ii) The exchange interaction should not distinctly vary over the ensemble of the emitter molecules, otherwise marked inhomogeneity effects and pronounced variations of the energy gap E(1CT3CT) would result. As a consequence, long TADF decay tails would occur. Therefore, the targeted molecular structure should be rather rigid. (iii) The ISC processes between the low-lying singlet and triplet states, down-ISC and up-ISC, should be as fast as possible. The rate of ISC k(ISC) can be expressed by Fermi’s Golden Rule according to k(ISC) = (2π/ħ)ρ(FC)│1CT│HSO│3CT│2 with

1CT│HSO│3CT

the SOC matrix

element and ρ(FC) the Franck-Condon weighted density of states.13,14,15 However, for purely organic compounds, the SOC matrix element is usually very small even for small E(1CT3CT) gaps, since pure singlet and triplet states resulting from the same configuration (same HOMO-LUMO excitation) with equal occupation of the spatial orbitals do not show direct SOC.14,16,17 As a consequence, ISC processes would be largely forbidden. To solve this problem, only a relatively small coupling energy of 1 cm-1 (0.1 meV) or even less might be sufficient.15 This demand can be fulfilled by providing additional energy states that result from different configurations through suitable molecular design. These states should be close in energy to allow for distinct state coupling (direct and vibronic coupling). (iv) The ISC rate depends also on the Franck-Condon (FC) factors of the involved states, as is also expressed by Fermi´s Golden Rule. These FC factors have to be maximized. This is realized for couplings between the 1CT and 3CT states that usually have very similar geometries. (v) Both states, 1CT and 3CT, should be the lowest excited states or shifted to this energy situation. This can be achieved by adequately adapting the polarity of the environment. (Compare also refs 18 and 19.) In this study, we will show that these target properties can largely be realized. This will be demonstrated in two steps. First, we describe, as a guiding step, properties of compound 1 that fulfills essential requirements, but does not yet lead to fast ISC. (Compare the preliminary conference report.20) In a second step, we show how to speed-up ISC by adequate chemical substitution. Interestingly, the resulting milestone material, compound 2, 3 ACS Paragon Plus Environment


will lead us beyond TADF and could provide a new OLED harvesting mechanism, the direct singlet harvesting (DSH) mechanism.20,21 In particular, comparison of the photophysical properties of both compounds supports the validity of this new mechanism. Donor and acceptor separated and rigidified by two bridges Figure 1 shows the chemical structure of the guiding molecule compound 1. Two nonconjugated bridges separate and largely fix donor and acceptor with respect to each other. This structure type leads to a small overlap of the frontier orbitals as resulting from DFT calculations. (Figure 2)

Figure 1. Chemical structure of compound 1. Synthesis and chemical characterization are described in the Supplementary Information S-1.

4 ACS Paragon Plus Environment

Page 4 of 19

Page 5 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Contour plots of natural transition orbitals for the 1CT and 3CT state at the optimized 3CT geometry for compound 1 (at an iso-value of 0.05). Hole and electron correspond largely to HOMO and LUMO of the non-excited molecule, respectively. (See also SI-4)

TD-DFT calculations at the B3LYP/6-311G** level of theory22,23,24 allow us to estimate the energy gap to E(1CT(S1)  3CT(T1)) ≈ 50 cm-1 (6 meV) for the molecule at an optimized 3CT state geometry (in gas phase). Although the applied TD-DFT method frequently places the 1,3CT

states too low in energy, energy separations between states of the same configuration

are often well reproduced.25 (See also SI-4.) From the chemical structure, an even smaller gap would have been expected. Apparently, a residual overlap of HOMO and LUMO is induced by hyper-conjugation. As will be shown in the next section, reducing or even almost blocking the hyper-conjugation will further strongly diminish the gap. Figure 3 reproduces time-resolved emission spectra and the decay behavior measured at ambient temperature. The emission color is white-blue with a maximum at max = 476 nm. The emission decays with two components. (Figure 3c) The short component of (prompt) ≈ 270 ns is assigned to the prompt fluorescence, while the long component is classified as TADF. Adapted to these different decay times, we recorded time-resolved spectra with no time delay of t = 0 ns at a time window of t = 80 ns, giving the prompt fluorescence spectrum (Figure 3a), and with a time delay of t = 5 s at a time window of t = 30 s (Figure 3b) giving the TADF emission, respectively. Both spectra overlap exactly as is expected, since both emissions stem from the same 1CT(S1) state. This behavior is not only found for compound 1 dissolved in toluene, but also if 1 is doped in polystyrene. Obviously, the rather “rigid” molecular structure does not show the unfavorable inhomogeneity effects that lead to a red shifted TADF spectrum as compared to the prompt fluorescence for many other TADF molecules. Importantly, the molecular “rigidity” of compound 1 (and also for compound 2, see below) prevents the occurrence of long TADF decay tails, as observed for most TADF molecules discussed in the literature. Apparently, the new structure type fulfills the rigidity requirements as discussed above. 5 ACS Paragon Plus Environment


Figure 3. Time-resolved emission spectra (a) and (b) and emission decay plot (c) for compound 1 measured at T = 300 K, dissolved in degassed toluene at c ≈ 10-5 M, exc = 355 nm, pulse width 10 ns. The time-resolved spectra are recorded for the time ranges indicated. They are adapted to the prompt and delayed fluorescence ranges, respectively. This compound 1 displays usual TADF behavior apart from the absence of extra-long inhomogeneity-induced decay tails. det = 470 nm. For experimental details see SI-6.

The observed prompt fluorescence decay time of (prompt) ≈ 270 ns might appear to be relatively long, but it is in line with the small HOMO-LUMO overlap7,26 and the calculated small oscillator strength as resulting from TD-DFT calculations of f ≈ 0.00145. For Cu(I) complexes, such long prompt fluorescence decay times have also been reported.7,26 On the other hand, the TADF decay time of (TADF) ≈ 9 s is not unusually long. This indicates13,14,16,27 some wavefunction coupling (mixing) of states of other configurational 6 ACS Paragon Plus Environment

Page 6 of 19

Page 7 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


character to the 1CT and 3CT states by SOC17, configuration interaction (CI), and/or vibronic coupling.14,27 These quantum mechanical admixtures induce down- and up-ISC processes. Indeed, TD-DFT calculations show that just one higher lying triplet state (T2 state), i.e. three triplet substates, lies in the large energy range up to 9000 cm-1 (1.12 eV)28 above the

1,3CT

states (T2 lies 137 meV above S1, Table S6). Obviously, the discussed quantum mechanical mixings, although weak, provide sufficient allowedness for the occurrence of ISC processes and TADF emission. To describe quantum mechanical admixing, the │1CT and │3CT wavefunctions can be modified by adding a component of the triplet state wavefunction │T2 according to │1CT + a │T2 and │3CT + b │T2, respectively. In first order approximation, the coefficients are of the type a = 3CT),

1CT│HSO│T2/ ∆(T2 – 1CT) and b = 3CT│HCI│T2/ ∆(T2 –

wherein HSO and HCI are the SOC and the configuration interaction operator,

respectively, and ∆(T2 –

1,3CT)

are the corresponding energy separations for the undistorted

situation. HSO induces a short-range interaction that can couple states of different multiplicity, while HCI is based on long-range electron-electron interaction and can couple states with the same multiplicity. (For a quantitative discussion, the wavefunctions of the three triplet substates have to be used.) These modified wavefunctions, │1CT + a │T2 and │3CT + b │T2, if inserted into the expression of Fermi’s Golden Rule by replacing │1CT and │3CT, respectively, can relax the forbiddenness of ISC and thus, increase the rate k(ISC). As recently discussed in the literature14,15,27, a quantitative approach might additionally have to take vibronic coupling into account.14,27 In Figure 4, we summarize the results obtained for the guiding reference compound 1. According to TD-DFT calculations only one higher lying state of 3(CT/acceptor) character is found about 1100 cm-1 (137 meV, Table S6) above the

1,3CT

states in an energy range of

9000 cm-1 (1.12 eV)28. (SI-4) The coupling paths illustrated represent direct mixings according to SOC and CI. These (weak) mixings are responsible for the occurrence of ISC and TADF. Maybe also vibronic coupling routes14,27 might be of importance. 7 ACS Paragon Plus Environment


Figure 4. Schematic energy level diagram and quantum mechanical coupling (mixing) paths for compound 1. The data given result from measurements at ambient temperature of 1 dissolved in toluene and from theoretical TD-DFT calculations (SI-4). Only one higher lying state stemming from a configuration with a different occupation of spatial orbitals than that of 1,3CT is found in an energy range up to 9000 cm-1 (1.12 eV)28.

The guiding compound 1 represents already a valuable TADF molecule, but it does not yet show all the required properties as summarized above. The TADF decay time and ISC are still too long and the energy gap is not yet small enough. However, the observed electronic and TADF properties of compound 1 guide us to strategies for improvements. These will be discussed in the next section. Close to zero gap compound with fast ISC. Benchmark material Based on the properties observed for the TADF compound 1 that exhibits still rather slow ISC rates, we modified compound 1 by specific structure changes in order to achieve (i) a further drastic reduction of the energy gap E(1CT3CT) and (ii) to provide additional energy states for enhancing quantum mechanical state couplings for increasing ISC rates. Both requirements are realized by functionalizing the shorter one of the two bridges by a biphenyl substitution. (Figure 5) Accordingly, the residual hyper-conjugation between donor and acceptor is largely reduced. And the aromatic two-ring system provides several additional 8 ACS Paragon Plus Environment

Page 8 of 19

Page 9 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


low-lying energy states of different configurations, i. e. of different occupations of spatial orbitals than those of the

1,3CT

states. Thus, these additional states can lead to additional

quantum mechanical admixtures to the 1CT and 3CT state.

Figure 5. Chemical structure of compound 2. Synthesis, chemical characterization, and x-ray structure data are described in SI-2 and SI-3, respectively.

Theoretical studies and energy states Figure 6 displays contour plots for hole (HOMO) and electron (LUMO). The calculations are carried out at the B3LYP/6-311G** level of theory18,19,20 for the optimized geometry of the lowest 3CT state (gas phase).

Figure 6. Contour plots of natural transition orbitals for the 1CT and 3CT states at the optimized 3CT geometry for compound 2 (iso-value 0.05). Hole and electron correspond largely to HOMO and LUMO of the non-excited molecule, respectively. (See SI-4) 9 ACS Paragon Plus Environment


Page 10 of 19

Indeed, TD-DFT calculations based on the B3LYP functional provide a value of E(1CT3CT) ≈ 16 cm-1 (2 meV). At the MO6/def2-svp level of theory29, we obtained 10 cm-1 (1.2 meV). Thus, we can regard the calculated values, i. e. the calculated energy separations, as good estimates. Moreover, the calculated oscillator strength of the 1CT → S0 transition with f = 0.0011 is also (slightly) smaller than for the reference compound 1. Importantly, the TD-DFT calculations indicate the occurrence of six additional states (see Figure 9 below and Table S7) that involve the biphenyl bridge and that lie within a range of ≈ 9000 cm-1 (1.12 eV)28 above the

1,3CT

states. According to the calculations, the nearest state (T2 state) is only 0.2

eV (1600 cm-1 in terms of vertical transition energies) apart from the

1,3CT

states. (SI-4)

These six states consist of four triplets and two singlets. Accordingly, 14 states (2 singlets and 12 triplet substates) lie near to the energy of the

1,3CT

states. This is an important

message for potential quantum mechanical couplings and for speeding-up ISC rates as described in the previous section. Experimentally, it is difficult to register these states directly (except of the lowest one), since the allowednesses of the transitions from the triplet states to the electronic ground state are very weak with estimated radiative rates of the order of 1 s-1. Moreover, the TD-DFT calculations might place the CT states at lower energies than states that result from largely localized transitions. Therefore, the calculations, although displaying valuable trends, might not show correct energy separations between these localized states and the

1,3CT

states.

Such a behavior has to be taken into account, when discussing the experimental situation. Energy states and polarity It is well known that CT-transitions are strongly red shifted by increasing polarity.18,19,30 Therefore, it is highly instructive to investigate emission properties of compound 2 dissolved in toluene in a temperature range of 200 K ≤ T ≤ 300 K. In this range the solvent is fluid but


Page 11 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the polarity, described by the dielectric constant , changes distinctly from 2.57 (200 K) ≥  ≥ 2.38 (300 K).28

Figure 7. Emission spectra of compound 2 dissolved in degassed toluene (c ≈ 10-5 M) displayed at different temperatures. The dielectric constant

 describing the polarity is taken kr = PL/τ, with PL being the

from ref 31. The radiative rates are determined from photoluminescence quantum yield. The 0-0 transition of the 3(bridge/acceptor) → S0 phosphorescence is marked for comparison. This transition, which is largely localized to the bridge/acceptor molecular range (Table S7), does not show any distinct energy shift with polarity change. (SI-5)

Figure 7 displays the changes of emission properties of compound 2 resulting from temperature variation. The broad bands represent the 1CT → S0 transitions. With increasing polarity upon cooling, they red shift by 22 nm (1000 cm-1, 120 meV), whereby the radiative rate is, within limits of experimental error, constant. Interestingly, in this polarity range, the 00 energy of the 1CT → S0 transition (lying in the blue-side flank) can be shifted to be almost in resonance with the 0-0 transition being essentially of 3(bridge/acceptor) → S0 character (for an assignment see also below). Obviously, the proximity of the two states (small energy denominator) induces distinct mixing coefficients and thus, is very helpful for increasing quantum mechanical state mixtures and for speeding-up the ISC rates. The state coupling is largely independent of the sequence of the mixing states, if not in resonance. However, the 11 ACS Paragon Plus Environment


emission quantum yield depends on the sequence as seen in Figure 7. In a situation of a lower lying 3(bridge/acceptor) state than the

1,3CT

states (at low host polarity), population of

the localized state results in moderate emission quenching. The occurrence of a 3(bridge/acceptor) → S0 transition can be manifested by measuring the low-temperature (77 K) time-delayed emission (delay time t = 480 µs, time window ∆t = 900 ms), that is at a low-polarity host. The observed emission is well structured showing a progression of ≈ 1400 cm-1. (Figure S-17) The energy position of the 3(bridge/acceptor) → S0 spectrum is independent of the matrix polarity (Figure S-18), in contrast to the energy positions of the

1,3CT

states. We assign this experimentally identified state to be related to

the biphenyl substitution of the bridge, since the emission spectrum of a fractional molecular structure of compound 2, representing the fluorene unit, is similar and essentially shows the same leading progression. (SI-5) To summarize, according to the TD-DFT calculations (compare SI-4), the 3CT state represents the T1 state and the 3(bridge/acceptor) is the T2 state. However, experimentally we find that at a low-polarity environment (frozen toluene at T = 77 K) the state sequence is inverted. But with increasing polarity (fluid toluene, 200 K ≤ T ≤ 300 K), the 1,3CT states are sufficiently red-shifted and 1CT becomes the emitting state. Mono-exponential decay and fast 1CT - 3CT intersystem crossing


Page 12 of 19

Page 13 of 19

b

1000

log (intensity)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


420 ns

100

no long decay component

10 1 0

1

2

3

4 5 Time [s]

6

7

Figure 8. (a) Emission and absorption spectrum of compound 2 and photoluminescence quantum yields measured at T = 300 K dissolved in degassed/air saturated toluene at c ≈ 105 M. (exc) = 310 nm. (Note the efficient emission quenching under air saturation.) (b) Emission decay measured under same conditions (degassed) but with an excitation pulse width of 10 ns, (det) = 465 nm. Note no TADF decay tail is observed.

Figure 8 reproduces the absorption and time-integrated ambient temperature emission of compound 2 together with the decay behavior. The absorption spectrum indicates a weak and broad absorbance peaking at ≈ 370 nm with a molar extinction coefficient at the maximum of very roughly 150 M1cm1. We assign this absorption to the lowest S0 → 1CT transition. The absorbance below ≈ 330 nm is characterized by transitions to S2 and to higher lying localized singlet states. The broad emission spectrum represents the 1CT → S0 emission that decays strictly mono-exponentially over more than three orders of magnitude with a time constant of 420 ns. This decay time may be regarded as being too long for a 1CT fluorescence. But in comparison to the value found for compound 1 of ≈ 270 ns with a larger HOMO-LUMO overlap and thus, a higher oscillator strength than that of compound 2, the longer decay time seems to be well in line. Similarly large decay times are also reported for CT transitions of Cu(I) compounds.26 Moreover, no (additional) short and no long decay components are detectable. (Figure 8b) Therefore, we conclude that no short-lived fluorescence and no long-lived TADF occur. This is not unexpected, as the two 1CT and 3CT states with an energy separation of only ≈ 16 cm-1 (2 meV) are very close in energy as 13 ACS Paragon Plus Environment


Page 14 of 19

compared to the thermal energy available at T = 300 K of 210 cm-1 (26 meV). At ambient temperature both states are in a fast thermal equilibration, as will further be rationalized below. Accordingly, the emission observed may be assigned as an averaged decay determined by both states. Figure 9 shows a schematic energy level diagram for compound 2 dissolved in toluene for ambient temperature. By use of this diagram, we mainly want to illustrate coupling routes that can induce quantum mechanical perturbations of 1CT and 3CT by admixing different state character. Thus, according to Fermi’s Golden Rule, ISC can strongly speed-up. In an energy separation of up to 9000 cm-1 (1.12 eV), we find four triplet states (12 substates) and two singlet states that all result from configurations with different orbital occupation. They can admix their character to the 1CT state and the 3CT state via SOC and configuration interaction, respectively. Obviously, the lowest

3(bridge/acceptor)

state is of particular

importance, since its energy separation from the 1,3CT states is only of the order of 1600 cm-1 (200 meV) from theoretical calculations and of 102 to 103 cm-1 (12 to 120 meV) from experimental investigations depending on temperature and polarity of the environment (compare Figure 7). The coupling paths illustrated in Figure 9 represent direct mixings between the states. Additional coupling routes via vibronic coupling might further be of importance for speeding-up ISC.14,27


Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. Schematic energy level diagram and potential quantum mechanical coupling routes for the designed beyond-TADF material (compound 2, dissolved in a polar environment) with E(1CT(S1)  3CT(T1)) ≈ 16 cm-1 (2 meV)

Design of a New Mechanism beyond Thermally Activated Delayed

Recommend Documents