Experimental Evidence for Localized Excitons in the Spectra of Charge-Transfer Complex Molecular Crystals

Abstract

The polarized absorption and emission of the anthracene—TNB complex crystal are shown to provide evidence for localization of low-energy excitation in this crystal. The absorption and fluorescence polarization ratios at 300°K are very different (5.6:1 and about 15:1, ‖c: ⊥c on 110) the former being close to an oriented gas value. The latter corresponds to a charge-transfer complex more closely aligned with the c axis of the crystal. The motion of the excited molecule is in accord with the behavior of a dipole in an electric field which is nearly along the c axis. Possible ramifications of this effect in photoconductive charge-transfer complexes are discussed. The influence of temperature on the luminescence spectrum of anthracene—TNB crystals is discussed, and apart from some artifacts due to cracking of crystals, the expected increase of emission intensity with decreasing temperature is observed.

Electronic Structure and Dynamics of the Excited State in CT Microcrystals As Revealed by Femtosecond Diffuse Reflectance Spectroscopy

“…Before going on with the detailed discussion, we have to mention whether the excited state is delocalized over many or several D and A moieties in the crystals as an exciton or localized on one D−A pair. Hochstrasser et al and Haarer et al considered that CT excited states of molecular crystals of weak D−A complexes are rather localized (self-trapped), because of its alternating molecular stacks, strong electron−phonon coupling, and also an appreciable Coulomb interaction between generated highly ionic component molecules. , Recently, we compared the transient absorption spectrum of a 1:1 CT crystal of durene−PMDA and that of 10 -3 M PMDA-doped durene crystal and found good agreement between the spectra . Here, we have further diluted PMDA to 10 -4 M in the durene crystal, but its transient absorption spectrum was still in accordance with that of the 1:1 CT crystal.…”

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

“…Another origin of the self trapping of CT triplet excitons may be a distortion of the lattice upon excitation 24 . Such a distortion is expected if the equilibrium configurations in the excited state and in the ground state of the complex are different 25 . A simple distortion with a strong effect on the triplet energy would be a decrease in the interplanar distance.…”

The Triplet State and Molecular Electronic Processes in Organic Molecules

“…The experiments of McGlynn and co-workers (251, 252) have been alluded to above. The phosphorescence of naphthalene, phenanthrene, and other hydrocarbons is reduced both in intensity and lifetime in complexes with 1,3,5-trinitrobenzene and other acceptors (54,55,56,153,247). Christodouleas and McGlynn (42) have found that complexing with trinitrobenzene increases the value of the ISC ratio by factors of 30 and 75 for naphthalene and acenaphthene, respectively.…”