Studies on Synthesis and Optoelectronic Properties of Azole-based Excited-State Intramolecular Proton Transfer (ESIPT) Materials

Abstract

Highly fluorescent molecules harnessing excited-state intramolecular proton transfer (ESIPT) process are very promising for many optoelectronic applications including future displays, new generation light sources, and bio-imaging agents because they can offer very unique photophysical properties compared to conventional fluorescent dyes. In this thesis, various multifunctional substituents are introduced into 2-(2-hydroxyphenyl) azole, which is the core unit of highly emissive ESIPT molecule HPI, to control the related optoelectronic properties and to establish structure-property relationship between them. Furthermore, utilizing the unique and beneficial properties of the newly designed molecules, novel and innovative concepts are provided, which can make a breakthrough for challenging issues in various optoelectronic applications including molecular pixel system, OLEDs, and fluorescent zinc sensors. In Chapter II, for the first step to realize innovative ESIPT optoelectronic applications, we developed a general molecular design strategy for a wide-range spectral tuning of azole-containing ESIPT materials based on two different approaches: conjugation length and substitution approach. A series of fifteen ESIPT molecules were designed, synthesized and comprehensively investigated for their photophysical properties by experimental methods as well as theoretical calculations. In Chapter III, we report a full-color molecular pixel system composed of RGB emitting ESIPT dyes, each of which has delicately tailored Stokes shift and independent emission capability completely free from energy transfer crosstalk between them. It is demonstrated that the whole range of emission colors enclosed within the RGB color triangle on the CIE 1931 diagram is predictable and conveniently reproducible from the RGB molecular pixels not only in the solution but also in the polymer film. It must be noted that mixing ratios to reproduce the desired color coordinates can be precisely calculated based on additive color theory according to their molecular pixel behavior. In Chapter IV, we report efficient thermally activated delayed fluorescence (TADF) from hydroxyl-substituted tetaphenyl imidazole derivatives into which electron donating and accepting groups are systematically introduced. Upon excitation, excited-state intramolecular proton transfer (ESIPT) of the imidazole molecules triggers spatial separation of HOMO and LUMO on the donor and acceptor fragments, respectively, resulting in an extremely small singlet-triplet energy. In Chapter V, we prepared a new fluorescent zinc sensor (HNBO-DPA) consisting of 2-(2-hydroxy-3-naphthyl)benzoxazole (HNBO) chromophore and a zinc-specific di(2-picolyl)amine (DPA) receptor and examined for zinc bioimaging. It is noteworthy that fluorescence response of the probe to zinc ions is conserved over a broad pH range with excellent selectivity due to its unique fluorescence response mechanism. The results obtained from the in vitro and in vivo imaging studies demonstrate the practical usefulness of the probe to detect zinc ions.