Solvent polarity effects on photo-excited charge transfer intermediates of DAOC and its derivatives studied by spectroscopic methods and DFT calculations

Abstract

In developing a coumarin based fluorescent chemosensor, 7-diethyl-3-formylcoumarin (DAOC) was considered one of the most promising and applicable moiety. DAOC has a unique characteristic that the wavelength and quantum yields of its fluorescence are sensitive to the solvent polarity. Since it has a formyl group that is good electrophile at position 3, it easily reacts with other nucleophiles to transform into other structures. For example, DAOC reacts with amines or hydrazines to form fluorescent imine or hydrazone compound, which exhibits brighter fluorescence than the original DAOC in polar solvents. Several chemosensors specific to amines or hydrazine groups have been reported using these photophysical properties of DAOC. However, there is still a lack of explanation for why this fluorescence enhancement occurs. In this thesis, these photophysical characteristics of DAOC and its derivativess are addressed. I synthesized DAOC derivatives, DAOC-SEM and DAOC-AOZ through simple condensation reaction to study effects from the substituents on DAOC. The absorption, fluorescence spectrum, quantum yield, and fluorescence lifetime of the DAOC and its derivative are measured as varying the solvent polarity. I also performed DFT calculation to support these experimental data. Solvent polarity-fluorescence correlation of these compounds was then obtained, in which DAOC showed a distinctive feature compared to its derivatives: When the solvent polarity exceeds certain threshold (f(ε,n) = 0.160) , DAOC shows rapid decrease in fluorescence lifetime and quantum yields.This phenomenon is anticipated to be a quenching effect due to the rapid formation of twisted intramolecular charge transfer (TICT) in a polar solvent. DAOC that has a good electron withdrawing group is more favorable to form TICT state than other DAOC derivatives that has relatively poor electron withdrawing group.