Computational Study on the Charge Transport and Optical Spectra of Anthracene Derivatives in Aggregates

A recent experiment [Angew. Chem. Int. Ed. 2017, 56, 722-727] found that a (1 : 9) blend film of two anthracene derivatives, 2-fluorenyl-2-anthracene (FlAnt) and 2-anthryl-2-anthracence (2 A), exhibit both efficient white light emission and high hole mobility, thus promising for organic light-emitting transistors (OLETs). Employing quantum chemistry at the polarizable continuum model (PCM) and the quantum mechanics/molecular mechanics (QM/MM) levels, we investigated the excited-state structures, optical spectra, band structure and the carrier mobility for FlAnt and 2 A from solution to aggregate phases. We suggest using the ratio of intermolecular excitonic coupling J and intramolecular excited state relaxation energy E to judge the bathochromic shift in optical emission in aggregates. For FlAnt, ρ=J/E is calculated to be less than 0.17, a critical value we identified earlier, and the spectra in solution and aggregate phases present quite similar features (blue emission). However, ρ is ∼0.5 for 2 A systems, and the calculated emission in the aggregate phase exhibits a remarkable bathochromic shift. In addition, the 0-0 emission is strongly suppressed in the herringbone stacking. These observations justify the experimental findings that (i) 2 A is blue emissive in solution but yellow-green in the aggregate phase, whereas FlAnt is always blue, and (ii) the blend of them show white emission. By using the "quantum nuclear tunneling" model we proposed earlier, we found the hole mobility for FlAnt and 2 A are 0.5 and 4.2 cm²  V^-1  s^-1 , respectively, indicating both are good hole transport materials.