Investigation of delayed fluorescence phenomena in conjugated molecules using time-resolved laser spectroscopy

Abstract

This thesis reports structure-property relationships in a range of conjugated molecules where the nature of the delayed fluorescence (DF) is used as a powerful diagnostic tool in order to gain insight into the nature of the molecules. All the investigations revolve around using three spectroscopic methods; steady state, single-photon counting and particularly time-resolved nanosecond spectroscopy.
The rarely observed optically generated geminate electron-hole (e-h) pair recombination is designated as the origin of DF in Rhodamine 6G and its derivative ATTO-532 molecules. The DF shows a strong dependence on excitation energy, which is due to emission arising from higher energy excited dimeric states.
In addition, the complex excited-state nature of the polyspirobifluorene (PSBF) polymer is investigated in both dilute solutions and spin-coated films. According to investigations carried out in dilute solutions, solvent polarity and temperature dependent charge transfer (CT) state formation is observed as arising from the “inter/intrachain” interaction phenomena. The stabilisation of the intra- CT state at low temperatures exhibits the presence of both triplet-triplet annihilation (TTA) and monomolecular processes. These findings are used to interpret the up-conversion data of PSBF in thin films, clearly revealing that both TTA and thermally activated delayed fluorescence (TADF) are involved in indirect singlet generation. The same mixed contribution is also confirmed in anthracene based, small molecule, thin films. Consequentially, these findings highlight the investigated system as one of the desirable alternative molecular systems through which high efficiencies in organic light emitting device (OLED) applications can be attained.
Finally, an understanding of novel anthracene based novel acceptor molecule is developed through the use of a sensitizer based up-conversion experimental method, in which the question of how the side groups (having different electron affinities) affect the TTA efficiency and triplet energy transfer efficiency is clarified. Lastly, the investigations show that novel pyridine derivatives have solvent polarity and viscosity dependent excited state configurations, which are designated as twisted/wagged intramolecular charge transfer states. The DF, in this case, predominantly originates from monomolecular recombination of the geminately bound e-h pairs. It is believed that this can be a major loss mechanism for quantum yield in ICT systems.
Murat Aydemir, August, 2016