Ultrafast electronic spectroscopy of simple molecular systems in the condensed phase

Ultrafast electronic spectroscopy was conducted on simple molecular systems in order to elucidate mechanisms involved in intra-electronic energy transfer as well as the role of solvent in reaction dynamics.; Intra-electronic energy transfer has been investigated for a series of 9-bromo-10-phenylethynyl-anthracene-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (ANBO) molecular cassettes. The ANBO series is a group of molecules with varying fluorescence spectrum dependent on the BODIPY derivative. A case study of ANBO2 was conducted since it is representative of the structures in the ANBO series. The relaxation time of the donor L_a bromo-anthracene-ethynylphenyl state of ANBO2 has been measured by both fluorescence upconversion and fluorescence detected two-photon absorption to be 630 fs. The acceptor (BODIPY) fluorescence time response of ANBO2 shows a biexponential rise (∼50 fs, 800fs), which demonstrates that two or more processes are occurring. The measured intra-electronic energy transfer rates for the ANBO series are not sensitive to the spectral overlap of the fluorescence of the donor and absorption of the acceptor portions of the molecules. The intra-electronic coupling mechanism for the ANBO series is proposed to be a non-uniform coupling of the higher manifold of vibrational levels of the S₁ BODIPY state with the L_a state of bromo-anthracene-ethynylphenyl. The observed response of ANBO2 was compared with quantitative predictions of Förster theory including a monopole representation of the transition charge interaction. No agreement was found. The through bond coupling and super exchange were also considered.; The photodissociation of HgBr₂ in solution was examined using femtosecond transient absorption spectroscopy. Wavepacket dynamics were observed for the HgBr radical after HgBr₂ photolysis. Whereas the photolysis of HgBr₂ in acetonitrile and ethanol show conventional damped wavepacket dynamics of HgBr, in dimethyl sulfoxide a chirped wavepacket with a nonexponential decay is observed. The chirped wavepacket in transient absorption response on the photolysis of HgBr₂, which is attributed to the modification of the HgBr potential as a result of solvation occurring on the time scale of the wavepacket motion, provides microscopic insight into the structural and dynamical properties of the solvation process.