The Excited State Dynamics Of Methyl Dimethylaminobenzoate And Its Derivative

Combined application of femtosecond time-resolved fluorescence spectroscopy（fs-TRF）,femtosecond transient absorption spectroscopy（fs-TA）in conjunction with density functional theory（DFT）and time-dependent density functional theory（TDDFT）has been conducted to investigate the excited state dynamics of methyl dimethylaminobenzoate（MDMABA）and4-aminobenzoic acid（PABA）in different solvents including acetonitrile and water at varied pH conditions.The main results are as follows:We study the interplay and consequence of the interplay between intramolecular charge transfer（ICT）and intermolecular hydrogen（H）-bonding in MDMABA.Our result revealed a rapid ICT reaction proceeding with a time constant of⁰.5 ps.The fluorescence of the ICT state is strongly quenched,exhibiting a decay time of⁴9.7 ps,much faster than the² ns lifetime in acetonitrile.According to the study of the solvent kinetic isotope effect,this is identified to originate from an instantly enhanced strong solute–solvent H-bonding induced by the ICT reaction that allows decay of the ICT state to occur primarily through the internal conversion from the ICT to the S₀ state,eliminating the intersystem crossing（ISC）channel.The results have significant implication for understanding the excited state H-bonding dynamics and mechanism of photo-protection.Comparative fs-TRF and fs-TA measurements were conducted on PABA in acetonitrile and buffered water at different pH.Our data show that ISC to form long living triplet state is the major relaxation channel from the locally excited state of neutral PABA,occurring with⁶49.6ps time constant in pH7 buffer.In view of the damaging effect of the long living triplet state,this observation implies that the PABA type of molecule may cause DNA photo-damage when being used as an ingredient in water-based sunscreen product.Overall,the results reveal importance of the electron donating and accepting group as well as the solvent property in regulating strongly the excited state dynamics and deactivation pathway of the molecules under investigation.