Electronic Spin Resonance combined with Electronic Structure Calculation as a powerful tool for organic semiconductors characterization Carlos F.O. Graeff
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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
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24 Campi
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Nano at UNESP The main Campi of UNESP working on Nanoscience and Nanotechnology are:
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Synthesis; Composites; Sensors and catalyses; Nanoscale Electronics; Theoretical calculations Electron Microscopy
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Groups Working on Nano Laboratório Interdisciplinar de Eletroquímica e Cerâmica (LIEC-Araraquara)
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Laboratório de Materiais Avançados (Bauru) Grupo de Polímeros (Ilha Solteira) Laboratório de Compósitos e Cerâmicas Funcionais – LaCCeF (S.J.R. Preto)
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CEPID/FAPESP: Multifunctional Materials • US$ 25 million for 5 years (2013-2018) possibility to extent up to 2023
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Câmpus de Bauru
e Ciê n cia s Pá gina inicia l › Pós Gr a dua çã o › M e st r a d o/ D ou t ora d o › Ciê ncia e Te cn ologia de M a t e r ia is › H om e
Ciê n cia e Te cn ologia de M a t e r ia is Apr e se n t a çã o
o or m a s
a
is
O Programa de Pós-graduação em Ciência e Tecnologia de Materiais (POSMAT) tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da UNESP. O programa POSMAT conta atualmente com a participação de sete unidades da UNESP listadas abaixo. A criação dos cursos de Mestrado e Doutorado foi aprovada pela Capes em setembro de 2003. O programa foi avaliado com conceito 6 no período 2010-2012 pela Capes e desenvolve atividades em diversas linhas de pesquisa experimentais e teóricas em Ciência e Tecnologia de Materiais. O público alvo são os alunos egressos principalmente dos cursos de Ciências Exatas e Engenharias, tais como: Física, Química, Matemática, Computação, Engenharia de Materiais, Química, Elétrica, Civil, Mecânica, Produção ou outras áreas afins, que tenham experiência na área de Materiais. Pr ogr a m a : Ciência e Tecnologia de Materiais Gr a n de Ár e a n a CAPES: Multidisciplinar Ár e a : Materiais Con ce it o: 6 (triênio 2010-2013).
I N FORM AÇÕES SOBRE O PROCES SO SELETI VO 2 º / 2 0 1 5
• Multi campi Graduate College Un ida de s Pa r t icipa n t e s • 44 Supervisors Faculdade de Ciências - Bauru • 120 students
Faculdade de Ciências e Tecnologia - Presidente Prudente Faculdade de Engenharia - Bauru Faculdade de Engenharia - Ilha Solteira Instituto de Biociências - Botucatu Instituto de Química - Araraquara Campus Experimental - Sorocaba
çõe s
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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
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01/06/2015
Motivation Iridium Complexes • Efficient photoluminescent materials for OLEDs; • Oxygen sensing; • Catalytic applications
Open questions Photodegradation of Ir compounds
Basic principles of ESR 1 – Without application of magnetic field (Degenerate energy levels)
separation of degenerate levels 3 – Constant microwave radiation applied (E = hv) 2 – Application of magnetic field
4 – Energy absorption when hv = gµBH
Origin of ESR signal H≠0
http://www.intechopen.com/books/ferromagnetic -resonance-theory-andapplications/ferromagnetic-resonance
Photoinduced Charge Transfer Processes Two independent spin charge Separated Charges Singlet Extended Exciton Exciton Charge Transfer is Initiated
carriers
LUMO
HOMO
σ+ σ--.)* . (D D * + A)* A + D ------ A A
Two possible ESR signals
Materials
Host Matrices
Results
Remark: Line shape and ESR parameters are quite similar
independent of Ircomplex and matrix
Batagin-Neto, A. et al. J. Phys. Chem. A (2014), 118, 3717-3725
The centers experience very similar chemical environments
(Signal Amplitude Decay) x (Elapsed Time of Photoexcitation Interruption)
Goodamplitude Fit: by a second-order exponential with photoexcitation two components: a fast-temperature Signal dependence with elapseddecay time after interruption for the independent decay, τ , and a slow-temperature dependent decay, τ (T). A similar behavior 1 Remark: system FIrpic+PS in 0different temperatures (peak-to-peak difference between the was observed for all systems maximum and minimum of the central transition) LESR signal is associated with The decay photogenerated metastable paramagnetic suggests states 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 Our data:
Assumption:
of our data they shouldand be gIt is compatible with lower hyperfine coupling constant
LESR-induced paramagnetic delocalized values closer to the free electron , as observed.on the spins experience complex ligands weaker interactions with Ir
Paramagnetic Center Formation Considering that: it is located on the Ir-complex ligands. The similarity between the spectra of : Ir(ppy)3 and FIrppy
Phenylpyridine (ppy)
Unpaired Spins are located on similar ligands
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
Possibly:
by Charge Transfer followed by Charge Trapping
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 ligand decomplexation: Ir(ppy)2• ppy• • Negatively and positively charged distorted species: Ir(ppy)3-Nrot+ Batagin-Neto, A. et al. J. Phys. Chem. A (2014), 118, 3717-3725
Ir(ppy)3-Nrot-
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
Uncommon CT processes
Photoexcitation
Triplet transitions (MLCT MC)
Batagin-Neto, A. et al. J. Phys. Chem. A (2014), 118, 3717-3725
Metastable distorted structures
Paramagnetic States
Metastable Structures Thermally activated signal decay
From the fitting:
Ea values
τ1 ∼ 5−7 s
τ2 = τ2 (T) s
energy required for conformational relaxation: The large values of (τ) in the order of seconds its temperature dependent
TBP structures
ground-state (GS)
ESR signal quenching can be associated with structural relaxation small additional relaxation processes 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
[email protected]
01/06/2015
Mix conductor (ionic + electronic) Bioelectronic potential
M. d’ Ischia, et al., Angewandte Chemie 48, 3914-21 (2009).
Melanin’s EPR spectra: pH dependence (H2O suspensions)
EPR signal
CHIO, S.; HYDE, J. S.; SEALY, R. C. Archives Biochem. Biophys. (1982), 215(1), 100–106.
g-factor
Spin concentration
Signal linewidth
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 Comproportionation Reaction
Carbon centered Signal (CC) (~2.003)
??? MOSTERT, A. B. et al. J. Phys. Chem. B (2013), 117(17), 4965–4972.
Semiquinone signal (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-factors
• 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 Batagin-Neto, A. et al. PCCP (2015), 17, 7264-7274
Hyperfine interaction - DHI
Hyperfine interaction - DHICA
Hyperfine interaction Higher hyperfines:
CC signals: larger line width SQ signals: smaller line width
Most compatible structures g-factors and Aiso
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)
Thank you for your attention!
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01/06/2015
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01/06/2015