Formation of the Charge-Localized Dimer Radical Cation o 2-Ethyl-9,10-dimethoxyanthracene in Solution Phase

Abstract

Although dimer radical ions of aromatic molecules in the liquid-solution phase have been intensely studied, the understanding of charge-localized dimers, in which the extra charge is localized in a single monomer unit instead of being shared between two monomer units, is still elusive. In this study, the formation of a charge-localized dimer radical cation of 2-ethyl-9,10-dimethoxyanthracene (DMA), (DMA)(2)(center dot+) is investigated by transient absorption (TA) and time-resolved resonance Raman (TR3) spectroscopic methods combined with a pulse radiolysis technique. Visible- and near-IR TA signals in highly concentrated DMA solutions supported the formation of non-covalent (DMA)(2)(center dot+)by association of DMA and (DMA)(center dot+). TR3 spectra obtained from 30 ns to 300 mu s time delays showed that the major bands are quite similar to those of DMA except for small transient bands, even at 30 ns time delay, suggesting that the positive charge of noncovalent (DMA)(2)(center dot+) is localized in a single monomer unit. From DFT calculations for (DMA)(2)(center dot+), our TR3 spectra showed the best agreement with the calculated Raman spectrum of charge-localized edge-to-face T-shaped (DMA)(2)(center dot+), termed DT center dot+, although the charge-delocalized asymmetric pi-stacked face-to-face (DMA)(2)(center dot+), termed DF3(center dot)(+), is the most stable structure of (DMA)(2)(center dot+) according to the energetics from DFT calculations. The calculated potential energy curves for the association between DMA(center dot)(+) and DMA showed that DT center dot+ is likely to be efficiently formed and contribute significantly to the TR3 spectra as a result of the permanent charge-induced Coulombic interactions and a dynamic equilibrium between charge localized and delocalized structures.