Abstract
Narrow emission spectrum is highly desirable for organic light-emitting diodes (OLEDs) since it provides color purity required for displays. Recently, a novel class of compounds with exceptionally narrow emission spectrum — multiple resonance (MR) organic fluorophores — was suggested.1 These compounds consist of polycyclic hydrocarbon core with electron-donating (e.g., nitrogen) and/or electron-withdrawing (e.g., boron) heteroatoms. The latter ‘orient’ electron density at frontier orbitals so that it reaches maxima at distinct atoms and form nodes at others. This yields non-bonding character to frontier orbitals, suppressing electron–vibrational coupling and narrowing the emission spectrum. However, relationships between the molecular structure and electron-vibrational coupling, which are of great demand for design of MR fluorophores with ultra-narrow emission spectra, are poorly established. In this study, we performed theoretical investigation of electron-vibrational coupling in boron-free MR fluorophores. Specifically, we calculated the reorganization energies for hole/electron transfer and for optical excitation for various MR fluorophores, and traced the effect of changes in molecular structure on these quantities2. The molecular regions that undergo significant geometrical changes as a result of charge acquisition are shown to be correlated with the patterns of frontier orbitals, which in turn depend on molecular structure. We anticipate that the revealed relationships will be useful for the design of MR fluorophores for OLEDs with ultra-narrow emission spectra.