Theoretical insights on the influence of Heteroatom substituents, polar solvents, and aggregations of luminescent materials

Abstract

Luminescent materials regarded as one of the most promising materials for light-emitting technology have been focused on their developments in modern society. Generally, the photophysical properties of luminescent materials are dependent on the nature of their structure, solvent used, and molecular packing in solid-state which are typically used as the key factors to modulate the emission spectra. Thus, understanding the effect of various factors on photophysical properties is indispensable in the molecular design of luminescent materials having suitable photophysical properties and improving the performance of light emitters. Computational calculations have striven to gain insights by applying computational techniques to understand the nature of chromophores in the excited state. Herein, the computational studies on photophysical properties of three selected systems consisting of (i) 3HF and its derivatives, (ii) 3HQ complexing with solvent molecules, and (iii) Pt(II) complexes were performed. Firstly, the heteroatom effect on the photophysical properties and excited-state intramolecular proton transfer processes (ESIntraPT) of 3HF and its derivatives were theoretically investigated both static and dynamic calculations. The ESIntraPT process of 3HSO is the fastest among 3HX in accordance with its highest intramolecular hydrogen bond strength, lowest PT barrier, and highest exothermic reaction. Nevertheless, after the ESIntraPT is complete, the structure of 3HSO is twisted leading to no observation of keto emission spectra in the experiment. Secondly, based on the static calculation, the investigation of polar solvents (DMSO, CH3OH, and H2O) effects on excited state proton transfer processes, both intramolecular hydrogen bond (Intra-HB) and intermolecular hydrogen bond (Inter-HB) conformers of 3HQ complexing with solvent molecules are attributed to their enol and keto emission peaks depending on type of solvent used particularly CH3OH and H2O but not for DMSO in which its keto emission might be from only 3HQ(DMSO)-intra leading to its lower intensity ratio of dual emission compared to those of CH3OH and H2O. This sensitivity of 3HQ with solvent used indicates that 3HQ can be potentially utilized as polarity-sensitive fluorescence ratiometric probes. Based on-the-fly dynamic simulations, ESIntraPT processes are possible for all Intra-HB conformers while ESInter double-PT processes are only plausible for 3HQ(CH3OH)-inter and 3HQ(H2O)-inter but not for 3HQ(DMSO)-inter. Moreover, ESInter double PT mechanisms of 3HQ(CH3OH)-inter and 3HQ(H2O)-inter conformers are stepwise judged from the time lag between the first and second proton transfers. Finally, the cooperativity effects and the changes in the photophysical properties of aggregates and excimers Pt(II) complexes were examined in which the cooperative effects in trimer excimers led to shortened Pt···Pt contacts as compared to the trimer aggregates. The main electronic transition changes from 3LC/3MLCT to 3MMLCT characters in aggregated species and the total interaction energy on trimer excimers are subtly controlled by the electrostatic and dispersion terms. Moreover, the synergistic effects are associated with a striking change in the photophysical properties when moving from very isolated unit to aggregate species, i.e., yielding red-shifted emission bands and larger photoluminescence quantum yields which are beneficial toward attaining very efficient near-infrared OLEDs. All obtained information from this dissertation would be very helpful in the molecular design of ESPT molecules for fluorescent probes and Pt(II) metal complexes for organic light-emitting diode applications.