Beyond Born-Oppenheimer Design of Thermally Activated Delayed Fluorescence Emitters. Tom Penfold

Transcription

1 Beyond Born-Oppenheimer Design of Thermally Activated Delayed Fluorescence Emitters Tom Penfold

2 Outline Introduction to Solving the Time-Dependent Schrödinger Equation What Problem are we trying to solve? How can we use theory? What are the different levels of theory 2 Solving the Time-Dependent Schrödinger Equation II How to use quantum dynamics The MDH Method Developing a Model Potential Energy Surface. Beyond Born-Oppenheimer Design of Thermally Activated Delayed Fluorescence Emitters The role of molecular vibrations in TADF The effect of Nonadiabatic Coupling Perspectives

3 Schematic for TADF S ΔE S-T k nr k isc k risc k risc k isc exp(-δe S-T /k b T) T k nr k f k ph S 0 Or

4 What are the problems? Rate calculated using Fermi Golden rule is far smaller than experimentally observed rates.

5 Clues from Experiment Switching TADF to RTP.. Ward, JS., et al. Chem. Comm. 52 (206):

6 Setting Up the Hamiltonian Ĥ HFI& =&0.2&cm *& Ĥ SOC =&2&cm *& Ĥ vib& &65&cm *& LE Pictorial Representation Matrix Representation

7 Dynamics ISC Dynamics a. Black: Hamiltonian as described. b. Red: Increased nonadiabatic coupling. c. Blue: Smaller energy gap. risc Dynamics a. Density Matrix simulations to address risc at 00 K. b. ISC and risc are connected. c. No nonadiabatic coupling -> Very slow rate. ChemPhysChem 7 (9),

8 Ĥ HFI& =&0.2&cm *& Ĥ SOC =&2&cm *& Ĥ vib& &65&cm *& LE What is the mechanism? Clearly two state are insufficient and so must move past first order perturbation theory. First step is a internal conversion between the triplet states. Allowed transition and therefore its fast. Second step:

9 The effect of the embedding environment (a) (b) k risc& (c) k risc& LE LE k risc& LE a! b! Changing the polarity shifts the states but not the LE.! M Etherington Nat Commun. 206; 7: 680.

10 What about D-A-D D-A (C s ) D-A-D (C 2v )! (A ) (A ) (B ) (A 2 ) (A 2 ) (B ) LE (A ) LE (B ) LE (A 2 )

11 How to think about the mechanism? Ĥ HFI& =&0.2&cm *& Ĥ SOC =&2&cm *& Ĥ vib& &65&cm *& LE - LE In reality the interchangeability depends upon the strength of the coupling. Slightly different mechanism Marian et al. [Journal of Physical Chemistry C 20 (206): ], but overall message- mixing and vibrations are important.

12 The extended picture of TADF. S k nr k isc ΔE S-T2 T 2 k risc T 2 ΔE S-T T k f k nr k ph T k ic/ric S 0 Kinetically, because the T-T2 conversion is so fast, its appears as a equilibrium and therefore the state model behaves in a similar fashion to the 2 state model. 2 This is not limited to states, there can be many more involved.

13 The effect of nonadiabatic coupling. Physical Chemistry Chemical Physics 9, (207)

14 k risc as a function of the energy gaps k risc shown as a function of the two energy gaps present in the state model. Unsurprisingly the rate is largest when the energy gap is smallest. As ΔE S-T is increased, the rate decreases. The opposite behavior is seen for ΔE T-T2, suggesting the importance of this energy gap, which is coupled by nonadiabatic coupling.

15 Non-Arrhenius TADF Ĥ HFI& =&0.2&cm *& Ĥ SOC =&2&cm *& Ĥ vib& &65&cm *& LE Non-adiabatic coupling results in mixingpopulation transfer without temperature. Physical Chemistry Chemical Physics 9, (207)

16 Regio- and Stereoselectivity The structure plays a big role and can be interconverted. 0. ev energy above S minimum is sufficient to convert. Nat Commun :4987

17 A different approach to design. LE LE (a) Present TADF Limited to small radiative rates. (b) Ideal TADF Large radiative rates achievable. Within the present approach, the nonadiabatic coupling mechanism doesn t lead to any significant advances. We propose an alternative approach

18 Considerations for TADF OLEDs Molecular Vibrations Aligning Energy Levels High Performing TADF OLEDs Effect of Embedding Environment Molecular Structure Need to marry detailed studies and high throughput!

19 Conclusions and Outlook Insight into Theory What Problem are we trying to solve? How can we use theory? What are the different levels of theory? 2 Insight into TADF Mechanism How to use quantum dynamics The MDH Method Developing a Model Potential Energy Surface. What needs to be done? The role of molecular vibrations in TADF The effect of Nonadiabatic Coupling Perspectives