Femtosecond broadband time-resolved fluorescence and transient absorption study on methyl-4-dimethylaminobenzoate and phenyleneethynylene gold(I) complexes

Abstract

Photo-induced intramolecular charge transfer (ICT) and luminescence are among the most significant processes in nature and have ample applications in chemistry, biology, and development of new materials. Methyl-4-dimethylaminobenzoate (DMABME) is a prototype molecule showing the ICT while a series of gold(I) oligo-phenyleneethynlene (PE) complexes provide rare example exhibiting ligand controlled luminescence upon photo-excitation. In spite of the interest and previous studies on these two types of systems, fundamental processes and factors governing the ICT in DMABME and the luminescence from the gold(I) PE complexes have remained unclear. With a purpose to address these issues, this thesis is conducted by integrating a range of time-resolved spectroscopic methods which combines the broadband time-resolved emission with transient absorption in timescale ranging seamlessly from femtosecond to hundreds microseconds for DMABME and the gold(I) PE complexes with excitation at varied selected wavelengths and solvent conditions. The study provides direct evidence for determining dynamics and deactivation pathways of the overall cascades of photo-excited DMABME and gold(I) PE complexes. The study of DMABME in acetonitrile provides unequivocal evidence for a common origin of La ππ* nature for both the ICT state and its precursor the locally excited state. Comparison of the time-resolved processes observed for DMABME in acetonitrile and in methanol unveils a remarkable effect of solvent hydrogen bonding (H-bonding) in altering strongly the dynamics and pathway of the excited state deactivation. It is identified for the first time that, in the protic solvent methanol, the H-bonding between solute and solvent promotes formation of an H-bonded ICT state which in turn opens up an effective channel of internal conversion to the ground state not available in acetonitrile. The time-resolved studies on gold(I) complexes containing oligo(p-PE), oligo(m-PE), and oligo(o-PE) ligands allow direct observation of the intersystem crossing (ISC) from emitting excited singlet to triplet state as well as to identify unambiguously the pathways for luminescence from these complexes. The results showed that, for the gold(I) oligo(p-PE) complexes, the rate of ISC is controlled largely by the extent of π-conjugation of the PE ligand in the excited singlet state. With an increase of ligand π-conjugation, the ISC rate drops by several orders of magnitude leading to a dramatic increase of the lifetime of prompt fluorescence (PF). This challenges the conventional view of heavy atom prompted ISC in luminescent transition metal complexes. Moreover, it is found that all the complexes investigated exhibit dual emission with the low energy component due to the phosphorescence from emitting triplet state and the high energy component the fluorescence, composed of PF and very long living delayed fluorescence produced through triplet-triplet annihilation (TTA) from the emitting triplet state. The efficiency of TTA is observed to vary with both the PE ligand length and the substitution pattern of the ligand, contributing a key factor for the varied luminescence behaviours displayed by these complexes. The work presented in this thesis demonstrates the power of time-resolved spectroscopy in unravelling the elementary steps and key factors for the dynamics and pathways of excited state processes. The results enable clearer pictures to be obtained for clarifying controversies on the ICT of the para donor-acceptor compounds and for a better understanding of the photophysics of luminescent gold(I) complexes which should prove useful in applications such as photo-catalysis and design of organic light-emitting diodes.