Optical Properties of Organic Molecules

Aromatic organic molecules exhibit a conjugated π-electron system giving rise to delocalized electrons which on the one hand can be excited by illumination of visible light and on the other hand can emit light in the visible energy range. This makes these molecules interesting for optoelectronic applications, such as OLEDs or organic solar cells. The typical absorption and emission processes are depicted in Fig. 1 as transitions between the singlet electronic ground state (S₀, also called HOMO, highest occupied molecular orbital) and the first electronic excited state (S₁, also called LUMO, lowest unoccupied molecular orbital).
A direct optical transition to the triplet state T₁ is forbidden, which can therefore only be populated by intersystem crossing, leading to a much longer lifetime compared to the singlet state. The electronic relaxation either occurs radiationless by internal conversion or radiatively by fluorescence or phosphorescene, respectively. Additionally, vibronic excitations take place during either absorption or emission, depicted as sublevels of the electronic states. Their population depends on the electronic- vibronic coupling or, in case of aggregated molecules called exciton-phonon coupling, described in the next section.

Due to a different charge distribution between the HOMO and the LUMO electronic excitations or relaxations lead to spatial deformations of the molecule that result in additional vibronic excitations. This deformation is represented by a change of the configuration coordinate Q in Fig. 2, which is in the simplest case of a diatomic molecule the distance between the nuclei. The bonding between the nuclei can be described by some potential as depicted in Fig. 2 from which the vibronic states, represented by the wavefunctions ψ, can be calculated for each electronic state S₀ and S₁.
According to the Frank-Condon picture electronic transitions take place much faster than any molecular deformation. Therefore a transition is represented by a perpendicular arrow keeping the configuration coordinate constant, leading to additional vibronic excitations, since the electronic excited molecule has not reached its relaxed geometry. A typical absorption spectrum of the free molecule is plotted in the upper left corner of Fig. 2, showing distinct peaks with nearly equidistant energy spacing that can be assigned to different vibronic states. This peak pattern is therefore called vibronic progression. The relative intensities and thus the strength of the exciton phonon coupling can be described by the Huang-Rhys parameter S.

Due to interactions in molecular aggregates, such as thin films or single crystals, the optical properties are modified. Compared to inorganic materials these interactions are small, mainly governed by the van-der-Waals interaction. Various effects, which are related to the structural properties of the aggregate system, such as

• optical anisotropy
• solvent shift (a red shift of the thin film spectrum compared to the monomer spectrum)
• Davydov splitting (a single molecular energy state is split up in two states when the crystal unit cell contains two spatially inequivalent molecules)
• exciton transfer (Frenkel-exciton, charge transfer exciton)

The optical spectra of thin films give therefore insight into the intermolecular coupling, which is relevant for transport properties and thus important for device applications. A further aspect is to monitor and control the film growth in order to obtain desired film properties. Optical methods are very well suited to study these properties in situ and in real-time to observe possible changes, such as:

• transition from the monomer to the dimer to the oligomer
• influence of the substrate onto the first monolayers
• orientational changes during growth
• following oxidation processes in real-time

For the optical characterization we employ different experimental setups:

• UV/vis spectroscopy
• differential reflection spectroscopy
• spectroscopic ellipsometry
• photoluminescence spectroscopy
• Raman spectroscopy