Electronic Spin Resonance combined with Electronic Structure Calculation as a powerful tool for organic

semiconductors characterization

Carlos F.O. Graeffgraeff@fc.unesp.br

http://www.asi.riken.jp/en/laboratories/departments/emd/emerg/spin-theory/

Outline

1) Materials Science and Nanotechnology in UNESP

2) Light-Induced Structural Change in Iridium Complexes Studied by Electron Spin Resonance

3) Electronic structure calculations of ESR parameters for melanin monomers

01/06/2015graeff@fc.unesp.br

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graeff@fc.unesp.br 01/06/2015

01/06/2015graeff@fc.unesp.br

Nano at UNESP

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01/06/2015graeff@fc.unesp.br

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1) Light-Induced Structural Change in Iridium Complexes Studied by Electron Spin Resonance

2) Electronic structure calculations of ESR parameters for melanin monomers

01/06/2015graeff@fc.unesp.br

Motivation

Open questions

Photodegradation of

Ir compounds

Iridium Complexes

• Efficient photoluminescentmaterials for OLEDs;

• Oxygen sensing;• Catalytic applications

Basic principles of ESR

http://www.intechopen.com/books/ferromagnetic-resonance-theory-and-applications/ferromagnetic-resonance

H ≠ 0

1 – Without application of magnetic field(Degenerate energy levels)

2 – Application of magnetic field

3 – Constant microwave radiation applied (E = hv)

4 – Energy absorption when hv = gµBH

Origin of ESR signal

separation ofdegenerate levels

Photoinduced Charge Transfer Processes

ADD*

Singlet ExcitonExtended Exciton

(D + A)*(Dσ+ + Aσ-)*

Charge Transfer is Initiated

D+. ------ A-.

Separated Charges Two independent spin charge carriers

Two possible ESR signals

HOMO

LUMO

Materials

Host Matrices

Results

Remark:

Line shape and ESR parameters are quite

similar

independent of Ir-complex and matrix

The centers experience very similar chemical

environments

Batagin-Neto, A. et al. J. Phys. Chem. A (2014), 118, 3717-3725

(Signal Amplitude Decay) x (Elapsed Time of Photoexcitation Interruption)

Signal amplitude dependence with elapsed time after photoexcitation interruption for thesystem FIrpic+PS in different temperatures (peak-to-peak difference between themaximum and minimum of the central transition)

Good Fit: by a second-order exponential decay with two components: a fast-temperatureindependent decay, τ0, and a slow-temperature dependent decay, τ1(T). A similar behaviorwas observed for all systemsRemark:

LESR signal is associated with photogenerated metastable paramagnetic

states

The decay suggests

that:

Delocalization of the LESR-spins on the complex ligands

In general, ESR spectra on the literature have: broad

line shapes and strong hyperfine interactions

paramagnetic centers close to the

Ir atoms

Delocalization of the LESR-spins on the complex ligands

of our data they should bedelocalized on the

complex ligandsLESR-induced paramagnetic

spins experienceweaker interactions with Ir

Our data:

Assumption:

It is compatible with lower hyperfine coupling constant and g-values closer to the free electron , as observed.

Paramagnetic Center Formation

The similarity between the spectra of :

Ir(ppy)3 and FIrppy

Considering that: it is located on the Ir-complex ligands.

Unpaired Spins are located on similar ligands

Phenylpyridine (ppy) Difluorophenylpyridine (2Fppy)

Paramagnetic Center Formation

Complex−Matrix Interactions:

Intense signals in (Ir(ppy)3+PS )and (Ir(ppy)3+PMMA )

complex −matrix interactions facilitate the paramagnetic center formation

by Charge Transfer followed by Charge Trapping

Possibly:

Batagin-Neto, A. et al. J. Phys. Chem. A (2014), 118, 3717-3725

Electronic Structure Calculations

Considering distinct complexes structures:

• Cationic and anionic ground state optimized species:

FIrpic− FIrpic+ Ir(ppy)3− Ir(ppy)3

+

• Optimized and Nonoptimized subproducts coming from liganddecomplexation:

Ir(ppy)2• ppy•

• Negatively and positively charged distorted species:

Ir(ppy)3-Nrot+ Ir(ppy)3-Nrot-

Batagin-Neto, A. et al. J. Phys. Chem. A (2014), 118, 3717-3725

Electronic Structure Calculations - Summary

The most suitable set of parameters for: Ir(ppy)3-Nrot−fac

Ir’s hyperfine interaction

Quartet structure of the LESR signal

Ligand Rotation – Distorted Structures

Photo-excitation

Triplet transitions(MLCT MC)

Metastabledistortedstructures

Uncommon CT processes

ParamagneticStates

Batagin-Neto, A. et al. J. Phys. Chem. A (2014), 118, 3717-3725

energy required for conformational relaxation:

Metastable Structures

Thermally activated signal decay

From the fitting: τ1 ∼ 5−7 s τ2 = τ2 (T) s

ESR signal quenching can be associated with structural relaxation processes

The large values of (τ) in the order of seconds

its temperature dependent

Ea values

TBP structures ground-state (GS)

small additional relaxation induced by the CT process

Thermally activated signal decay

Considering the enthalpy difference between ground state (GS) and TBP structures (ΔH), the energy required for conformational regeneration is supposed to be:

ETBP-GS = EGS-TBP − ΔH EGS-TBP ~ 0.3−0.5 eV

Compatible with our result!!!

Batagin-Neto, A. et al. J. Phys. Chem. A (2014), 118, 3717-3725

1) Light-Induced Structural Change in Iridium Complexes Studied by Electron Spin Resonance

2) Electronic structure calculations of ESR parameters for melanin monomers

01/06/2015graeff@fc.unesp.br

M. d’ Ischia, et al., Angewandte Chemie 48, 3914-21 (2009).

Mix conductor (ionic + electronic)Bioelectronic potential

Melanin’s EPR spectra:pH dependence

(H2O suspensions)

EPR signal

g-factor Signal linewidthSpin concentration

CHIO, S.; HYDE, J. S.; SEALY, R. C. Archives Biochem. Biophys. (1982), 215(1), 100–106.

Melanin’s EPR spectra:pH dependence

(pellets)

MOSTERT, A. B. et al. J. Phys. Chem. B (2013), 117(17), 4965–4972.

Melanin’s EPR spectra: pH dependence

MOSTERT, A. B. et al. J. Phys. Chem. B (2013), 117(17), 4965–4972.

Comproportionation Reaction

Carbon centered Signal (CC)

(~2.003)

???

Semiquinonesignal (SQ)

(~2.005)

Electronic structure calculations

Objective:

• Evaluate the origin of ESR signal in melanins monomers;

Methodology:

• Geometry optimization:Molecular dynamics (Amber99 force field)Semi-empirical method (PM6 – MOPAC2012)DFT approach (B3LYP/6-31G)

• Spin Hamiltonian parameters:g-factors: DFT/B3LYP and PBE0/6-31G**hyperfine constants: DFT/B3LYP and PBE0/EPRII)

Batagin-Neto, A. et al. PCCP (2015), 17, 7264-7274

Monomeric structures considered in calculations

• Anionic and cationic structures: HQ, IQ, QI• Radicalar structures: SQa, SQb and Ndef

g-factors

Anionic structures

Batagin-Neto, A. et al. PCCP (2015), 17, 7264-7274

g-factors

Cationic structures

Batagin-Neto, A. et al. PCCP (2015), 17, 7264-7274

g-factors

Radicalar structures

Batagin-Neto, A. et al. PCCP (2015), 17, 7264-7274

Two groups:

• g < 2.0040 (compatible with CC signal):- radicals: Ndef-DHI and Ndef-DHICA;- anions: HQ-DHI and HQ-DHICA;- cations: HQ-DHI, HQ-DHICA, QI-DHI and QI-DHICA;

• g > 2.0040 (compatible with SQ signal):- radicals: SQa-DHI, SQb-DHICA, SQa-DHI and SQb-DHICA;

- anions: IQ-DHI, IQ-DHICA, QI-DHI* e QI-DHICA*;- cations: IQ-DHI e IQ-DHICA;

* Intermediary values

g-factors

Batagin-Neto, A. et al. PCCP (2015), 17, 7264-7274

Hyperfine interaction - DHI

Hyperfine interaction - DHICA

Hyperfine interactionHigher hyperfines:

CC signals: larger line widthSQ signals: smaller line width

CC signal:- Ndef-DHI- Ndef-DHICA- HQ-DHI (anion)

SQ signal: • SQa-DHI• SQb-DHI• SQa-DHICA• SQb-DHICA• IQ-DHI (anion)• IQ-DHICA (anion)

Most compatible structures g-factors and Aiso

Thank you for your attention!

graeff@fc.unesp.br 01/06/2015

01/06/2015graeff@fc.unesp.br