Photophysic and photochemistry of large [pi] system : photodecarbonylation, photocleavage and photoinduced large stokes shift

Abstract

Three kinds of large cyclic π molecular systems have been studied in this thesis and these molecules undergo intriguing photoinduced reactions such as photodecarbonylation, photodissociation and photophysical induced large Stokes-shifted fluorescence. The photoinduced decarboxylation reaction for diphenylcyclopropenone (DPCP) has been characterized by transient absorption (TA) spectroscopy from the femtoseconds to nanoseconds time regions and by time-resolved resonance Raman (TR3) spectroscopy from the picoseconds to nanoseconds time domains. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed and their results were used to help assign the vibrational modes of the intermediates and the transient absorption features acquired in the time-resolved spectroscopy experiments to identify and characterize the key excited states and intermediate species. The time-resolved spectroscopy, DFT and TD-DFT computational results in this thesis indicate that the reaction route of DPCP involves a multi-step, radical-driven homolysis photochemical reaction with the ketenlycarbene radical intermediate formed as a result of the photocleavage from a triplet precursor. However, the bis-p-anisylcyclopropenone (BACP), as an anisyl-substituted derivative of DPCP, undergoes an ion-driven heterolysis by generating a singlet zwitterion intermediate in a photodecarboxylation process instead of a radical-driven homolysis. The addition of two new dipole moment by altering substituents may give rise to the polarity increase of the molecular systems of interest, which then favors the formation of a zwitterion structure intermediate in the BACP photochemistry. Therefore, a precise control over the photodecarboxylation to go through either a homolytic or heterolytic pathway can be achieved by simply manipulating the substituent groups attached to the parent molecular structure. Photodissociation reactions for 3-diethylaminobenzyl (DEABn) photo-liable protecting groups (PPGs) lead to four different photoproducts which were studied first to provide benchmark information to help unravel the complex photochemistry of these interesting molecules. Combined results from time-resolved fluorescence (TRF), fs-TA and ns-TA elucidated a radical-driven reaction route in MeCN and an ion-driven route in a MeCN/H2O (1:1) mixed solution. Aside from the substituent groups, it also demonstrated solvent-determined reaction pathways with regard to heterolysis/homolysis processes. For the related DMATr PPGs, where three aryl rings are attached to the benzyl carbon, the radical intermediate was not obtained even in neat MeCN and the photodissociation reactions yield a relatively stable exited carbenium ion intermediate by adding water into the solvent system. An extraordinarily large Stokes shift in the fluorescence emission for dibenz[b,f]oxepin (DBO) has been discovered decades ago and has been known as a fascinating photophysical phenomenon but the mechanism that causes it remains unknown. The formation of the singlet excited state DBO was monitored by fs-TA and the disappearance of the double-bond characteristics of the reaction intermediate species was tracked by ns-TR3 experiments. Delocalization of the electrons and the formation of an 8π-electrons system in the seven-membered oxepin ring was found to give rise to transition from a bent ground-state geometry to planar excited states with a very large shift in fluorescence wavelength and hence this accounts for the extraordinarily large Stokes shift in the fluorescence emission for DBO.