Abstract
Twisting of the acene backbone out of planarity in twisted acenes leads to a variation in their optical and electronic properties. The effect of increasing twist angles on the properties of the photoexcited triplet states of a series of anthracene-based helically tethered twisted acenes is investigated here by Electron Paramagnetic Resonance (EPR) spectroscopy. Increasing signal intensities with increasing twist angles indicate increased intersystem crossing efficiencies for the twisted molecules compared to the untethered reference compound. Variations in the electron spin polarisation observed in the transient EPR spectra, in particular for the compound with the shortest tether, imply changes in the sublevel population kinetics depending on molecular geometry. Changes in the zero-field splitting parameters and in the proton hyperfine couplings for compounds with short tethers and therefore higher twist angles point towards a slight redistribution of the spin density compared to the parent compound. The experimental results can be explained by considering both an increase in twist angle and a related decrease in the dihedral angle between the phenyl side groups and the acene core. The observation of a clear excitation-wavelength dependence suggests preferential excitation of different molecular conformations, with conformers characterised by higher twist angles selected at higher wavelengths.
Highlights
A series of helically-locked tethered twistacenes based on an anthracene core was introduced,[7] in which the twist angle is determined by the length of the diagonal n-alkyl bridge (Fig. 1)
We investigate the influence of the twist angle on the properties of the photoexcited triplet state by Electron Paramagnetic Resonance (EPR) spectroscopy
The twisted acenes do demonstrate an increase in Intersystem Crossing (ISC) rate with increasing twist angle, as evidenced by the increased triplet yield detected by time-resolved EPR
Summary
A series of helically-locked tethered twistacenes based on an anthracene core was introduced,[7] in which the twist angle is determined by the length of the diagonal n-alkyl bridge (Fig. 1). The crystal structures of compounds with propyl- to hexyl-tethers linked at the 1,5-positions showed twist angles of 231, 301, 321 and 381, respectively.[7] Optical studies revealed a bathochromic shift of the b- and p-bands characteristic for curved aromatic systems,[8,9] as well as a relative increase in 21588 | Phys. We investigate the influence of the twist angle on the properties of the photoexcited triplet state by Electron Paramagnetic Resonance (EPR) spectroscopy. Transient EPR triplet state spectra contain information on the zero-field splitting interaction and the ISC mechanism.[13] In order to gain a more precise understanding of the influence of twisting of the acene core on the spin density distribution, proton hyperfine couplings are measured by Electron-Nuclear DOuble Resonance (ENDOR). In addition to the dependence of triplet state properties on the type of compound, the excitation wavelength dependence is investigated
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