Bright Frenkel Excitons in Molecular Crystals: A Survey

Abstract

We computed the optical properties of a large set of molecular crystals (∼2200 structures) composed of molecules whose lowest excited states are strongly coupled and generate wide excitonic bands. Such bands are classified in terms of their dimensionality (1-, 2-, and 3-dimensional), the position of the optically allowed state in relation with the excitonic density of states, and the presence of Davydov splitting. The survey confirms that one-dimensional aggregates are rare in molecular crystals highlighting the need to go beyond the simple low-dimensional models. Furthermore, this large set of data is used to search for technologically interesting and less common properties. For instance, we considered the largest excitonic bandwidth that is achievable within known molecular crystals and identified materials with strong super-radiant states. Finally, we explored the possibility that strong excitonic coupling can be used to generate emissive states in the near-infrared region in materials formed by molecules with bright visible absorption and we could identify the maximum allowable red shift in this material class. These insights with the associated searchable database provide practical guidelines for designing materials with interesting optical properties.

Feasibility of p-Doped Molecular Crystals as Transparent Conductive Electrodes via Virtual Screening

“…This is a consequence of having used a set for testing that was not built to have certain properties and that can be considered “unbiased” in this sense. Furthermore, we note that we have neglected, for simplicity, the effect of crystal packing on the absorption edge as spectral shifts in absorption edges are typically on the order of ∼0.1 eV (0.11 eV is the median value found among considering only bright excitons 62 ). Our analysis reveals that the oscillator strength of identified promising molecules, with a median being 0.12, is considerably smaller than those of molecules considered in ref ( 62 ), which result in small excitonic couplings.…”

“…Furthermore, we note that we have neglected, for simplicity, the effect of crystal packing on the absorption edge as spectral shifts in absorption edges are typically on the order of ∼0.1 eV (0.11 eV is the median value found among considering only bright excitons 62 ). Our analysis reveals that the oscillator strength of identified promising molecules, with a median being 0.12, is considerably smaller than those of molecules considered in ref ( 62 ), which result in small excitonic couplings. As reported in the SI , the median of the largest excitonic coupling in this database, considering an average dielectric constant 3, 114 is 0.0117 eV with a maximum being 0.0928 eV, implying that these structures have very narrow excitonic bandwidths and retain most of molecular characteristics.…”

High-throughput virtual screening for organic electronics: a comparative study of alternative strategies

“…In performing such analysis in a recent project, through screening the CSD, our group identified the maximum achievable excitonic bandwidth (∼1.16 eV) and spectral red-shift (∼0.6 eV) in excitonic materials. 239 In a similar analysis, a diverse set of conjugated molecules are evaluated to establish the relation between the exciton size and the size of the π-system. 419 This analysis suggests that the exciton binding energy of ∼0.3 eV can be considered as the lower limit of this property which is in accordance with the typical theoretical data reported in the literature.…”

“…Molecular arrangements in the crystal are also needed to study excitonic properties, as illustrated in a recent survey of ∼2200 crystals formed by molecules with bright lowest excited states for which the lowest excitonic band was characterised. 239 …”

“…Although there are datasets of organic molecular properties, 81 the field still lacks the availability of a large set of data for crystalline properties except for reviews of selected topics 237,238 from which it is possible to collate information on the optical spectra and luminescence of about ∼100 molecular crystals. This set is, however, sufficient to provide validation for the computation of the excitonic coupling 239 which is one of the key elements needed for the study of solid-state optical properties. The other key element is the evaluation of the local exciton phonon coupling 240,241 which is a property of the isolated molecule and is computable with a good accuracy.…”