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Heavy-atom effect on optically excited triplet state kinetics.

ABSTRACT: In several fields of research, like e.g. photosensitization, photovoltaics, organic electroluminescent devices, dynamic nuclear polarization, or pulsed dipolar electron paramagnetic resonance spectroscopy, triplet state kinetics play an important role. It is therefore desirable to tailor the kinetics of photoexcited triplet states, e.g. by exploiting the intramolecular heavy-atom effect, and to determine the respective kinetic parameters. In this work, we set out to systematically investigate the photoexcited triplet state kinetics of a series of haloanthracenes by time-resolved electron paramagnetic resonance spectroscopy in combination with synchronized laser excitation. For this purpose, a procedure to simulate time traces by solving the differential equation system governing the triplet kinetics numerically is developed. This way, spin lattice relaxation rates and zero-field triplet life times are obtained concurrently by a global fit to experimental data measured at three different cryogenic temperatures.

SUBMITTER: Hintze C

PROVIDER: S-EPMC5695852 | biostudies-literature | 2017

REPOSITORIES: biostudies-literature

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Heavy-atom effect on optically excited triplet state kinetics.

Hintze Christian C Morgen Tobias O TO Drescher Malte M

PloS one 20171120 11

In several fields of research, like e.g. photosensitization, photovoltaics, organic electroluminescent devices, dynamic nuclear polarization, or pulsed dipolar electron paramagnetic resonance spectroscopy, triplet state kinetics play an important role. It is therefore desirable to tailor the kinetics of photoexcited triplet states, e.g. by exploiting the intramolecular heavy-atom effect, and to determine the respective kinetic parameters. In this work, we set out to systematically investigate th ...[more]

PMID: 29155822

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Project description: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 ?B97XD/aug-cc-pVTZ basis method. The substituents exhibited diverse ?- and ?-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 ?-electron donating in the first group while ?-electron withdrawing in the second one. Basically, U- and RO-?B97XD 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 ?-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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Heavy-atom effect on optically excited triplet state kinetics.

Publications

Heavy-atom effect on optically excited triplet state kinetics.

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