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Dobrowolski JC, Karpińska G. Substituent Effect in the First Excited Triplet State of Monosubstituted Benzenes. ACS OMEGA 2020; 5:9477-9490. [PMID: 32363300 PMCID: PMC7191863 DOI: 10.1021/acsomega.0c00712] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/18/2020] [Accepted: 03/16/2020] [Indexed: 05/08/2023]
Abstract
The structure of 30 monosubstituted benzenes in the first excited triplet T1 state was optimized with both unrestricted (U) and restricted open shell (RO) approximations combined with the ωB97XD/aug-cc-pVTZ basis method. The substituents exhibited diverse σ- and π-electron-donating and/or -withdrawing groups. Two different positions of the substituents are observed in the studied compounds in the T1 state: one distorted from the plane and the other coplanar with a quinoidal ring. The majority of the substituents are π-electron donating in the first group while π-electron withdrawing in the second one. Basically, U- and RO-ωB97XD approximations yield concordant results except for the B-substituents and a few of the planar groups. In the T1 state, the studied molecules are not aromatic, yet aromaticity estimated using the HOMA (harmonic oscillator model of aromaticity) index increases from ca. -0.2 to ca. 0.4 with substituent distortion, while in the S1 state, they are only slightly less aromatic than in the ground state (HOMA ≈0.8 vs ≈1.0, respectively). Unexpectedly, the sEDA(T1) and pEDA(T1) substituent effect descriptors do not correlate with analogous parameters for the ground and first excited singlet states. This is because in the T1 state, the geometry of the ring changes dramatically and the sEDA(T1) and pEDA(T1) descriptors do not characterize only the functional group but the entire molecule. Thus, they cannot provide useful scales for the substituents in the T1 states. We found that the spin density in the T1 states is accumulated at the Cipso and Cp atoms, and with the substituent deformation angle, it nonlinearly increases at the former while decreases at the latter. It appeared that the gap between singly unoccupied molecular orbital and singly occupied molecular orbital (SUMO-SOMO) is determined by the change of the SOMO energy because the former is essentially constant. For the nonplanar structures, SOMO correlates with the torsion angle of the substituent and the ground-state pEDA(S0) descriptor of the π-electron-donating substituents ranging from 0.02 to 0.2 e. Finally, shapes of the SOMO-1 instead of SOMO frontier orbitals in the T1 state somehow resemble the highest occupied molecular orbital ones of the S0 and S1 states. For several planar systems, the shape of the U- and RO-density functional theory-calculated SOMO-1 orbitals differs substantially.
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Matsuoka H, Retegan M, Schmitt L, Höger S, Neese F, Schiemann O. Time-Resolved Electron Paramagnetic Resonance and Theoretical Investigations of Metal-Free Room-Temperature Triplet Emitters. J Am Chem Soc 2017; 139:12968-12975. [DOI: 10.1021/jacs.7b04561] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Affiliation(s)
- Hideto Matsuoka
- Institute
for Physical and Theoretical Chemistry, University of Bonn, Wegelerstr. 12, 53115 Bonn, Germany
| | - Marius Retegan
- Max Planck Institute for MPI for Chemical Energy Conversion, Stiftstr. 34−36, 45470 Mülheim an der Ruhr, Germany
| | - Lisa Schmitt
- Kekulé
Institute of Organic Chemistry and Biochemistry, University of Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany
| | - Sigurd Höger
- Kekulé
Institute of Organic Chemistry and Biochemistry, University of Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany
| | - Frank Neese
- Max Planck Institute for MPI for Chemical Energy Conversion, Stiftstr. 34−36, 45470 Mülheim an der Ruhr, Germany
| | - Olav Schiemann
- Institute
for Physical and Theoretical Chemistry, University of Bonn, Wegelerstr. 12, 53115 Bonn, Germany
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Richert S, Tait CE, Timmel CR. Delocalisation of photoexcited triplet states probed by transient EPR and hyperfine spectroscopy. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2017; 280:103-116. [PMID: 28579096 DOI: 10.1016/j.jmr.2017.01.005] [Citation(s) in RCA: 70] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/11/2016] [Revised: 01/05/2017] [Accepted: 01/07/2017] [Indexed: 05/22/2023]
Abstract
Photoexcited triplet states play a crucial role in photochemical mechanisms: long known to be of paramount importance in the study of photosynthetic reaction centres, they have more recently also been shown to play a major role in a number of applications in the field of molecular electronics. Their characterisation is crucial for an improved understanding of these processes with a particular focus on the determination of the spatial distribution of the triplet state wavefunction providing information on charge and energy transfer efficiencies. Currently, active research in this field is mostly focussed on the investigation of materials for organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs). As the properties of triplet states and their spatial extent are known to have a major impact on device performance, a detailed understanding of the factors governing triplet state delocalisation is at the basis of the further development and improvement of these devices. Electron Paramagnetic Resonance (EPR) has proven a valuable tool in the study of triplet state properties and both experimental methods as well as data analysis and interpretation techniques have continuously improved over the last few decades. In this review, we discuss the theoretical and practical aspects of the investigation of triplet states and triplet state delocalisation by transient continuous wave and pulse EPR and highlight the advantages and limitations of the presently available techniques and the current trends in the field. Application of EPR in the study of triplet state delocalisation is illustrated on the example of linear multi-porphyrin chains designed as molecular wires.
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Affiliation(s)
- Sabine Richert
- Centre for Advanced Electron Spin Resonance (CAESR), Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom.
| | - Claudia E Tait
- Department of Chemistry, University of Washington, Seattle, WA 98195, United States.
| | - Christiane R Timmel
- Centre for Advanced Electron Spin Resonance (CAESR), Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom.
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Ryno SM, Fu YT, Risko C, Brédas JL. Polarization Energies at Organic-Organic Interfaces: Impact on the Charge Separation Barrier at Donor-Acceptor Interfaces in Organic Solar Cells. ACS APPLIED MATERIALS & INTERFACES 2016; 8:15524-15534. [PMID: 27244215 DOI: 10.1021/acsami.6b02851] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
We probe the energetic landscape at a model pentacene/fullerene (C60) interface to investigate the interactions between positive and negative charges, which are critical to the processes of charge separation and recombination in organic solar cells. Using a polarizable force field, we find that polarization energy, i.e., the stabilization a charge feels due to its environment, is larger at the interface than in the bulk for both a positive and a negative charge. The combination of the charge being more stabilized at the interface and the Coulomb attraction between the charges results in a barrier to charge separation at the pentacene/C60 interface that can be in excess of 0.7 eV for static configurations of the donor and acceptor locations. However, the impact of molecular motions, i.e., the dynamics, at the interface at room temperature results in a distribution of polarization energies and in charge separation barriers that can be significantly reduced. The dynamic nature of the interface is thus critical, with the polarization energy distributions indicating that sites along the interface shift in time between favorable and unfavorable configurations for charge separation.
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Affiliation(s)
- Sean M Ryno
- Solar and Photovoltaics Engineering Research Center, King Abdullah University of Science and Technology , Thuwal 23599-6900, Kingdom of Saudi Arabia
- School of Chemistry and Biochemistry & Center for Organic Photonics and Electronics, Georgia Institute of Technology , Atlanta, Georgia 30332-0400, United States
| | - Yao-Tsung Fu
- School of Chemistry and Biochemistry & Center for Organic Photonics and Electronics, Georgia Institute of Technology , Atlanta, Georgia 30332-0400, United States
| | - Chad Risko
- Department of Chemistry & Center for Applied Energy Research, University of Kentucky , Lexington, Kentucky 40506-0055, United States
| | - Jean-Luc Brédas
- Solar and Photovoltaics Engineering Research Center, King Abdullah University of Science and Technology , Thuwal 23599-6900, Kingdom of Saudi Arabia
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Uvarov MN, Behrends J, Kulik LV. Higher triplet state of fullerene C70 revealed by electron spin relaxation. J Chem Phys 2015; 143:244314. [PMID: 26723678 DOI: 10.1063/1.4938417] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Spin-lattice relaxation times T1 of photoexcited triplets (3)C70 in glassy decalin were obtained from electron spin echo inversion recovery dependences. In the range 30-100 K, the temperature dependence of T1 was fitted by the Arrhenius law with an activation energy of 172 cm(-1). This indicates that the dominant relaxation process of (3)C70 is described by an Orbach-Aminov mechanism involving the higher triplet state t2 which lies 172 cm(-1) above the lowest triplet state t1. Chemical modification of C70 fullerene not only decreases the intrinsic triplet lifetime by about ten times but also increases T1 by several orders of magnitude. The reason for this is the presence of a low-lying excited triplet state in (3)C70 and its absence in triplet C70 derivatives. The presence of the higher triplet state in C70 is in good agreement with the previous results from phosphorescence spectroscopy.
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Affiliation(s)
- Mikhail N Uvarov
- Voevodsky Institute of Chemical Kinetics and Combustion, Siberian Branch of Russian Academy of Sciences, Institutskaya St. 3, 630090 Novosibirsk, Russia
| | - Jan Behrends
- Berlin Joint EPR Lab, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
| | - Leonid V Kulik
- Voevodsky Institute of Chemical Kinetics and Combustion, Siberian Branch of Russian Academy of Sciences, Institutskaya St. 3, 630090 Novosibirsk, Russia
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Tait CE, Neuhaus P, Anderson HL, Timmel CR. Triplet state delocalization in a conjugated porphyrin dimer probed by transient electron paramagnetic resonance techniques. J Am Chem Soc 2015; 137:6670-9. [PMID: 25914154 PMCID: PMC4569061 DOI: 10.1021/jacs.5b03249] [Citation(s) in RCA: 56] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
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The
delocalization of the photoexcited triplet state in a linear
butadiyne-linked porphyrin dimer is investigated by time-resolved
and pulse electron paramagnetic resonance (EPR) with laser excitation.
The transient EPR spectra of the photoexcited triplet states of the
porphyrin monomer and dimer are characterized by significantly different
spin polarizations and an increase of the zero-field splitting parameter D from monomer to dimer. The proton and nitrogen hyperfine
couplings, determined using electron nuclear double resonance (ENDOR)
and X- and Q-band HYSCORE, are reduced to about half in the porphyrin
dimer. These data unequivocally prove the delocalization of the triplet
state over both porphyrin units, in contrast to the conclusions from
previous studies on the triplet states of closely related porphyrin
dimers. The results presented here demonstrate that the most accurate
estimate of the extent of triplet state delocalization can be obtained
from the hyperfine couplings, while interpretation of the zero-field
splitting parameter D can lead to underestimation
of the delocalization length, unless combined with quantum chemical
calculations. Furthermore, orientation-selective ENDOR and HYSCORE
results, in combination with the results of density functional theory
(DFT) calculations, allowed determination of the orientations of the
zero-field splitting tensors with respect to the molecular frame in
both porphyrin monomer and dimer. The results provide evidence for
a reorientation of the zero-field splitting tensor and a change in
the sign of the zero-field splitting D value. The
direction of maximum dipolar coupling shifts from the out-of-plane
direction in the porphyrin monomer to the vector connecting the two
porphyrin units in the dimer. This reorientation, leading to an alignment
of the principal optical transition moment and the axis of maximum
dipolar coupling, is also confirmed by magnetophotoselection experiments.
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Affiliation(s)
- Claudia E Tait
- †Department of Chemistry, Centre for Advanced Electron Spin Resonance, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom
| | - Patrik Neuhaus
- ‡Department of Chemistry, Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, United Kingdom
| | - Harry L Anderson
- ‡Department of Chemistry, Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, United Kingdom
| | - Christiane R Timmel
- †Department of Chemistry, Centre for Advanced Electron Spin Resonance, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom
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Luzanov AV. Zero field splitting of triplet levels of conjugated molecules: A comparison of exact and approximate π-schemes. J STRUCT CHEM+ 2013. [DOI: 10.1134/s0022476613010010] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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Sugisaki K, Toyota K, Sato K, Shiomi D, Kitagawa M, Takui T. Quantum Chemical Calculations of the Zero-Field Splitting Tensors for Organic Spin Multiplets. PROGRESS IN THEORETICAL CHEMISTRY AND PHYSICS 2013. [DOI: 10.1007/978-94-007-4893-4_8] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
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