Characterization of the Phenanthrene-Tetrachlorophthalic Anhydride (P/TCPA) 1:1 Charge-Transfer Crystal: Spectroscopic and Structural Investigations

The 1:1 phenanthrene-tetrachlorophthalic anhydride (P/TCPA) charge-transfer complex crystalizes with monoclinic symmetry, space group P21, with two magnetically inequivalent stacks in the unit cell. The noncentrosymmetric space group is very unusual for CT-complexes. The optical emission spectra at low temperature are characterized by a strong CT phosphorescence and a weak CT fluorescence and delayed fluorescence.The S1 band lies at 22 800 ± 100 cm-1, the T1 band at 21 200 ± 100 cm-1. Above 15 K triplet excitons, moving along the stacks are revealed by ESR. They have a CT character of about 30%, coinciding with that of the shallow X-traps found by ODMR at low temperatures. A further trap, with zero-field-splitting (zfs) parameters of D = ± 0.0617, E = + 0.0116 cm-1 has a much larger CT character of 50% as found in the isolated complex in low-temperature glass [1]. A structural model is proposed.

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Mobile and Trapped Triplet States in Single Crystals of Charge Transfer Complexes

Zeitschrift für Naturforschung A ◽

10.1515/zna-1974-0817 ◽

1974 ◽

Vol 29 (8) ◽

pp. 1216-1228 ◽

Cited By ~ 2

Author(s):

H. Möhwald ◽

E. Sackmann

Keyword(s):

Charge Transfer ◽

Delayed Fluorescence ◽

Charge Transfer Complexes ◽

Zero Field ◽

Triplet States ◽

Zero Field Splitting ◽

Triplet Energy ◽

Molecular Arrangement ◽

Ct Complexes ◽

Triplet Excitons

Homogeneously doped crystals of charge transfer (CT-) complexes were grown by incorporating aromatic guest donors in host CT-crystals. The host crystals contained 1,2.4,5-tetracyanobenzene (TCNB) as acceptor and deuterated aromatic electron donors. By using such doped crystals CT complexes in a well defined configuration may be studied. The triplet states of the guest complexes were used as ESR spectroscopic probes in order to determine the molecular arrangement in the host lattice. The zero-field-splitting (ZFS) parameters, D and E, of the triplet energy traps were determined and the degree of electron derealization in the triplet state was calculated from these values. In some cases a very strong guest host interaction (multicomplex formation) was established. A method for the determination of CT-triplet energies is described (accuracy 200 cm-1) . The phosphorescence spectrum of the anthracene-TCNB complex was obtained from the delayed emission spectra of different anthracene doped CT-crystals. The vibronic structure is identical to that of anthracene, while the O-O-band of the complex is blue shifted by 600 cm-1. It is shown that the undoped anthracene-TCNB crystal exhibits P-type delayed fluorescence and that the triplet exciton diffusion in this crystal is nearly temperature independent. In the undoped biphenyl-TCNB crystal E-type delayed fluorescence originating from the thermal depopulation of the mobile triplet excitons is established. The remarkable differences of the two types of triplet excitons are interpreted in terms of the different polarity in the triplet states of the two CT-crystals.

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