Determining The Excited State Properties of Novel Aryleneethynylenes

Abstract

The first chapter introduces the area of phenyleneethynylenes. The parent of the novel systems presented here, 1,4-bis(phenylethynyl)benzene (BPEB), is an ex- tended π–conjugated system, known for its interesting structural, optical and elec- tronic properties. Derivatives of this system have demonstrated great potential in the field of organic electronics, particularly as components in OLEDs, PV cells and molecular wires. An introduction to the phenyeneethynylenes and fundamentals of various photophysical properties is presented, along with a brief overview of several synthetic methodologies available for the preparation of arylethynylenes. In Chap- ter 2 the equipment and methods used for the determination of the excited state properties of the compounds are discussed.
The preparations and excited state study of a series of thiophene–based aryle- neethynylene oligomers are reported in Chapter 3. This chapter begins with a short review of thiophene based molecules followed by a discussion of the synthetic pro- cedures available for the preparation of the various dibromoheteroarene cores. The work presented here has allowed comparisons to be drawn with the photophysical and structural properties of the known 2,5-bis(phenylethynyl)thiophene, upon mod- ification of the central thiophene core. Oligomers containing an oxidised thiophene (thiophene-1,1-dioxide) displayed significantly red–shifted absorption and emission bands compared to those of the parent and derivatives, indicating significant ICT in the excited state. Furthermore, the lifetimes of the thiophene-1,1-dioxide sys- tems were significantly longer than those of the non–oxidised derivatives, resulting in much higher rates of fluorescence decay (kf ). Quantum yields for this series of thienyl arylethynylenes varied from 3 % to 43 %. Several systems featuring pe- ripheral substituents (OMe and CN) were prepared to probe the effect of electron donating and withdrawing groups on the photophysical properties of the oligomers. Also, two of the oligomers prepared featured a break in the conjugation across the thiophene derivative and one of these molecules exhibited phosphorescence upon cooling to 77 K, allowing direct measurement of the triplet energy. Chapter 4 begins with a short review of the diazole heterocycles, to illustrate the potential applications of these systems. The novel oxygen-containing diazoles were compared to their sulfur–containing analogues, which have been reported pre- viously in the literature. The benzofurazan and benzothiadiazole–based oligomers have been appended with peripheral substituents in the form of electron–donating and electron–withdrawing groups (R = H, t–Bu, OMe, NH2, CO2Me and CN), in an attempt to monitor the effect of increasing electron donation and electron withdrawing effects on these already electron–deficient oligomeric systems. 2,5- bis(Phenylethynyl)-1,3,4-oxadiazole exhibited high energy absorption and emission maxima, as well as relatively narrow absorption and emission profiles, in contrast to the broad profiles of the other electron–deficient systems. The benzofurazan se- ries exhibited broad, unstructured emission profiles, yet all systems displayed two distinct absorption bands. Fluorescence quantum yields of these compounds were generally very high; for 4,7-bis(4-t-butylphenylethynyl)benzofurazan, φf = 0.96. All benzofurazan and benzothiadiazole arylethynylenes displayed fluorescence lifetimes
of several nanoseconds. Chapter 5 presents a comparison of the photophysical properties of selected
molecules from Chapters 3 & 4 and BPEB, as well as suggestions for future work. All novel systems have been purified and characterised using typical methods (1H and 13C NMR spectroscopy, accurate mass spectrometry, melting point, and elemen- tal analysis and crystallographic structure determination where possible) and their excited state properties have been probed using photophysical methods (including room and low temperature UV–vis absorption and fluorescence spectroscopy and fluoresence quantum yields and lifetime). Ab initio calculations have afforded opti- mised geometries which have been compared to molecular structures determined us- ing X–ray diffraction and vibrational spectroscopy. TD–DFT calculations have been used to rationalise the observed photophysical spectra, helping with assignment of bands in the fluorescence spectra, aiding the determination of states responsible for
absorption and emission.