Time-resolved spectroscopic and computational study of photophysics and photochemistry of selected 2-(2'-hydroxyaryl)benzazole and 3',5'-dimethoxybenzoin derivatives

Abstract

In this thesis, the photophysical properties and photochemical reactions of 3',5'-dimethoxybenzoin and its derivatives as well as several novel compounds containing excited state intramolecular proton transfer (ESIPT) chromophores 2-(2’-hydroxyaryl)benzazole (HBO) were studied by time-resolved spectroscopic techniques such as femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman spectroscopy (ns-TR3). Theoretical calculations were also employed to help identify the transient species observed in the time-resolved absorption or Raman spectra and provide interpretation for the experimental results in terms of structures, electronic properties and energies.

The replacement of the phenol moiety of HBO by benzothiadiazol, quinoxalinol, phenanthrol and naphthol leads to the formation of compounds 1-4, respectively. Compared with the parent molecule HBO, compounds 1-4 have a larger π-conjugation framework and display red-shifted absorbance. Whereas traditional strategies to red-shift the fluorescent wavelength instead lead to blue-shifted emission for compounds 2-4. Baird’s rule and its complementarity were applied to interpret this unconventional behavior. Among the five compounds (HBO and 1-4), HBO shows an ultrafast and thorough ESIPT while compounds 2-4 have an equilibrium type of ESIPT with the observation of dual emission. Interestingly, only normal emission can be detected for compound 1 which suggests the prohibition of ESIPT. These differences of the ESIPT property of these compounds can be interpreted in terms of their structures, frontier orbitals and calculated potential energy surfaces. It was found that the introduction of an electron-withdrawing segment such as thiadiazol and pyrazine in the hydroxyphenyl ring apparently suppresses the proton-transfer in the excited states. Solvent effects on the fluorescence spectroscopic properties of these compounds can be concluded that the polar solvents restrain the ESIPT process and favor the normal emission. Moreover, fs-TA and ns-TA studies provide direct information of the transient species involved and reveal the overall reaction mechanisms of these compounds upon excitation. Long-lived triplet excited states were observed for all of these five compounds except for compound 3 and the ISC efficiency of compounds 1 and 2 are especially high. This study is helpful in understanding the ESIPT-structure relationships and in the design of novel heavy-atom-free organic triplet photosensitizers.

The photo-induced reaction mechanisms of 3',5'-dimethoxybenzoin (DMB) and its derivatives DMB fluoride and DMB chloride were also studied. Interestingly, these compounds undergo totally different reaction pathways in acetonitrile. For DMB, the α-cleavage occurred from the triplet state, leading to the formation of two radicals on the picosecond time scale. While for DMB fluoride, an efficient photo-deprotection-cyclization took place upon excitation and a biradical generated through a singlet character pathway was proposed to be the key intermediate. When the substituent is chlorine (DMB chloride), a long-lived α-keto cation (0.9 ms) was observed and then it underwent three steps (nucleophilic addition, proton transfer and isomerization) in the ground state to complete this novel photo-induced intramolecular chloride exchange reaction. Finally, it was worth noting that the solvent may has great influence on the reaction pathways as the heterolysis–solvolysis product (DMB) becomes predominant in a largely water containing solution of DMB fluoride.