Can the electronic energy spectra of bulk excitonic states be traced to parent molecular states in fluorene and its hetero-analogues? It depends on the mutual orientation of the respective transition dipoles in the crystal

TL;DR

This study investigates how intermolecular interactions influence electronic transition intensities in fluorene and dibenzofuran polymorphs, revealing that the orientation of transition dipoles affects whether bulk excitonic spectra resemble molecular spectra.

Contribution

It demonstrates that the mutual orientation of transition dipoles determines the spectral resemblance of bulk excitonic states to molecular states in these crystals.

Findings

Transitions with oblique dipoles resemble molecular spectra.

Parallel dipole transitions show surface state characteristics.

Bulk excitonic spectra are continuous unless B2 transition is at absorption onset.

Abstract

The effect of intermolecular interactions on intensity redistribution between individual electronic transitions in different polymorphs of fluorene and dibenzofuran was studied by transmittance electronic spectroscopy in the energy range from onset of absorption to the ionization energy (from ~4.0 eV to ~9.0 eV - Electronic transitions with transition dipoles at oblique angles to the crystallographic axes (A1 symmetry) in these crystals resemble molecular spectra. Transitions of B2 symmetry, which in these crystals have parallel transition dipoles, resemble molecular spectra for surface (substrate influenced) states. For bulk excitonic states the spectra of these transitions have a continuous, uniform intensity distribution in the whole energy range investigated, except for the case when the B2 transition is the onset of absorption.