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Multireference Computational Methods for Organic Electronics
A.Ya. Freidzon, A.A. BagaturyantsPhotochemistry Center, Russian Academy of Sciences
Industrial R&D Simulation Know-how
Small model objects
Accurate methods
Large-scale calculations of
large molecules
Fast methods
Basic research Theory Understanding the nature
Using accurate Using accurate methods in applied methods in applied
researchresearch
Organic Electronic DevicesOrganic Electronic Devices
• Photovoltaics– Light sensors (e.g., in photo cameras)– Solar cells
• Light-emitting devices
• Field-effect transistors
• Chemical sensors
Problem of efficiency and chemical stabilityProblem of efficiency and chemical stability
Processes in Organic ElectronicsProcesses in Organic Electronics• Light absorption
• Light emission
• Exciton recombination
• Charge separation
• Exciton transport
• Charge transport
• Chemical reactions– Intermolecular complexes in chemical sensors– Chemical degradation in excited or charged states
These processes are usually simulated using density functional theorydensity functional theory
Density Functional vs. Multireference MethodsDensity Functional vs. Multireference Methods
• Cheap and relatively fast
• Allows for large-scale calculations
• Allows for calculations of large molecules
• Easily automated and good for screening
• Relatively slow and expensive
• Requires focusing on only few molecules
• Moderate-size molecules can be calculated
• Unique custom calculations
So why multireference methods should So why multireference methods should be used in organic electronics?be used in organic electronics?
Density Functional vs. Multireference MethodsDensity Functional vs. Multireference Methods
N
N
Where is the hole?
Here?
Here?
Here?Somewhere
in the molecule…
Directly show states with
different hole localization
Why it is important?
Charge localization in the molecule influences charge mobility and chemical stability of the material
Density Functional Density Functional vsvs. Multireference Methods. Multireference Methods
• Overestimate the extent of charge or exciton delocalization
• Known issue with excited charge-transfer singlet states
• Underestimate triplet state energies wrt. excited singlets
• Correctly predict charge and exciton localization
• Correctly predict relative positions of excited states
• Accurate excited state energies
Why it is important?For predicting energy transfer pathways, emission efficiency,
and chemical stability of the material
What is Multireference Method?What is Multireference Method?Single-reference molecular wavefunctionSingle-reference molecular wavefunction is single
Slater determinant 0 (+ possible minor corrections)
= C00 + iCii
Multireference molecular wavefunctionMultireference molecular wavefunction is linear combination of Slater determinants i with comparable weights
(+ possible minor corrections)
= iCii + jCjj
Minor correction
Minor correction
HF, MP2, DFT, CC
MCSCF, MRMP, MRCC
Dominant contribution
Dominant contribution
Advantages of Multireference MethodsAdvantages of Multireference Methods
• Account for static correlation effects in (quasi)degenerate states
• Treats equally important states on equal grounds
• Not limited to single excitations
ButBut
• Not a black-box method
• Requires experience and professional skills
When Multireference Methods When Multireference Methods Should Be Used?Should Be Used?
• In the case of strict or quasi-degeneration of states– Partially filled f shell of lanthanides or d shell of transition
metals– Charge hopping
• In studying potential energy surfaces where (quasi)degeneration is expected– Many chemical and photochemical reactions fall within
this case
• When DFT gives definitely wrong results, and Coupled-Cluster methods are too expensive
Examples of Application of Examples of Application of Multireference MethodsMultireference Methods
Energy transfer pathways in lanthanide complexes
Accurate calculation of ligand-localized triplet states helps one to find the best antenna ligands
Examples of Application of Examples of Application of Multireference MethodsMultireference Methods
Phosphorescence rate constant in iridium complexes
Ir(Ppy)3
-1535.650
-1535.625
-1535.600
-1535.575
-1535.550
-1535.525
-1535.500
-1535.475
-1535.450
-1535.425
-1535.400
S0 3-MLCT 3-LC
S0
S1
T1
Calculated:kr = 1.71·106 s–1; r = 0.59 s
Examples of Application of Examples of Application of Multireference MethodsMultireference Methods
Charge hopping profiles in Bebq2 dimersHole hopping
0 .0
0 .1
0 .2
0 .3
0 .4
0 .5
0 .6
0 .7
0 .8
0 .9
1 .0
h o le o n L 2 m id p o in t 1 -2 h o le o n L 1 m id p o in t 1 -3 h o le o n L 3 m id p o in t 3 -4 h o le o n L 4
0 .0
0 .1
0 .2
0 .3
0 .4
0 .5
0 .6
0 .7
0 .8
0 .9
1 .0
h o le o n L 2 m id p o in t 1 -2 h o le o n L 1 m id p o in t 1 -3 h o le o n L 3 m id p o in t 3 -4 h o le o n L 4
Electron hopping
0 .0
0 .2
0 .4
0 .6
0 .8
1 .0
1 .2
1 .4
e le c tro n o n L 2 m id p o in t 1 -2 e le c tro n o n L 1 m id p o in t 1 -3 e le c tro n o n L 3 m id p o in t 3 -4 e le c tro n o n L 4
0 .0
0 .2
0 .4
0 .6
0 .8
1 .0
1 .2
1 .4
e le c tro n o n L 2 m id p o in t 1 -2 e le c tro n o n L 1 m id p o in t 1 -3 e le c tro n o n L 3 m id p o in t 3 -4 e le c tro n o n L 4
Understanding charge transfer mechanism in Bebq2 helps one to understand the same mechanisms in similar complexes, such as AlQ3
Examples of Application of Examples of Application of Multireference MethodsMultireference Methods
Chemical stability in a series of OLED hostsExcited state dissociation
energies
TerphCarb
0.4
1.6
DPTZCarb
1.9
0.7
Carbazolyl detachment is more probable
TerphTriphen
1.9
0.9
DPTZTriphen
2.1
1.3
Phenyl detachment is more probable
More stable
In all these cases single-reference methods (DFT or HF+MP2) fail
Conclusions• Multireference methods are not suitable for
screening
• However, they provide better values of important physical parameters
• And deeper insight into mechanisms of charge and energy transfer and chemical processes in organic electronics
Thank you for your attention!Thank you for your attention!
Personal acknowledgements
K.G. Komarova (PC RAS)A.A. Safonov (PC RAS)
S.V. Emelyanova (PC RAS)A.V. Odinokov (PC RAS)A.V. Scherbinin (MSU)
K.A. Romanova (KNRTU, Kazan)D.N. Krasikov (Kintech Lab)